1

CHAPTER 01

FAULT ANALYSIS OF ELECTRICAL POWER SYSTEM

1.1 Fault in a power systemFault is an aspect of something that is wrong or not perfect1.2 Types of Fault:The Fault in a 3-phase system can be classified into two main categories viz,1) Symmetrical Faults.2) Unsymmetrical Faults1) Symmetrical Fault: The Fault which gives rise to symmetrical fault current is called a symmetrical fault. The symmetrical fault occurs when all the three conductors of a 3-phase like are brought together simultaneously into short circuit condition. 2) Unsymmetrical Fault: The Fault which gives rise to unsymmetrical fault current is called an unsymmetrical fault. Various types of unsymmetrical Fault occur in power system. They are-

a) Shunt type fault.i) Single line to ground(L-G) fault.ii) line to line (L-L)fault.iii) Double line to ground (L-L-G)fault. b) Series type fault i) Open conductor (one or two conductor open) fault

So power system faults can be broadly divided into two categories. A) Short Circuit Fault B) Open Circuit Fault.

A) Short Circuit Fault: The following definition of a short circuit is taken form an IEEE standard. “An abnormal connection (including an arc) of relatively low impedance , whether made accidentally or intentionally, between two points of different potential is called short circuit. Note: The term fault or short circuit fault is used to desired a short – circuit”.Symmetrical faults, L-G faults, L-L faults, L-L-G faults are included in this type of fault.

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B) Open Circuit Faults: Open circuit fault is the series type unsymmetrical fault. An IEC (International Electrotechnical Committee) – Standard has the following definition of a series fault. “A fault for which the impedances of each of the three phases are not equal, usually caused by the interruption of one or two phases is called series type fault”. 1.3 Causes of Fault in a power system:1.3.1 Causes of Short Circuit: It may cause due to internal or external effects.

a) Internal effects:i) Breakdown of equipment or transmission line.ii) Deterioration of insulation in generator, transformer etc.iii) Aging of insulation.iv) Impedance design.v) Improper installation.vi)

b) External effects:

i) Insulation failure due to lighting surges.ii) Overloading of equipment causing excessive heating.iii) Mechanical damage by public.

1.3.2 Causes of Open Circuit:Open Circuit faults can be caused due to

i) Broken conductorii) Circuit breaker malfunctions in one or several phases.

1.4 Consequences of Fault Effects of short Circuit Fault:

The heavy current due to short circuit causes excessive heating which may result in fire or explotion.

Power system components caring abnormal currents, get our heated, with consequence reduction in the life span of their insulation.

The low voltage created by the fault has a harmful effect on he service rendered by the power system.

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Power flow is severely restricted or even completely blocked, while the short circuit lasts.

As a consequence of blockage of power flow, power system areas can lose synchronism. The longer a faults lasts the more is the possibility of loss of synchronism.

Effect of Open Circuit Faults Abnormal system operation. Danger to personal. Voltage tends to rise well beyond acceptable values in certain parts of the system

with possibility of insulation failure and development of short circuit faults.

1.5 Fault Clearing and System protectionNeeds of Fault Clearing:When a failure occurs on any parts of the system it must be quickly detected and cleared due to the following reasons.

a) If the fault is not cleared quickly, it may cause unnecessary interruption of source to the customers.

b) Rapid clearance of the fault prevents the effects of the fault from spreading into the system.

Fault Clearing System:A fault clearing system consists of a relay protection system and a circuit breaker. In case of a fault, the task of the circuit breaker is to clear the fault and the task of the relay protection system is to detect the fault.Circuit Breaker:A circuit breaker is a piece of equipment which can

a) Make or break a circuit either manually or by remote control under normal condition.b) Break a circuit automatically under fault condition.c) Make a circuit either manually or by remote control under fault condition.

Protective Relays:A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the detective element from the rest of system.The relay protection system can be further divided into

a) Transducers

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b) Wiringc) Trip circuitd) Relays

i) Magnitude relaysii) Directional relaysiii) Impedance relaysiv) Differential relaysv) Pilot relays

1.6 Necessity of fault analysis By fault analysis we can determine bus voltages and line currents during various types of faults.

i) Need of symmetrical fault analysis

Usually the 3-φ symmetrical short circuit yields the lowest fault current. Hence relay settings are usually based upon three –phase symmetrical fault because it is desirable to protect a system for the minimum fault current.

ii) Need of unsymmetrical fault analysis

Since a breaker must interrupt the largest short current that can possible exist, the size of a breaker is determined by the largest possible fault current. The greater current usually occurs for either the L-L or L-G fault.

5

CHAPTER 02SYMMETRICAL THREE – PHASE FAULTS

2.1 Introduction The current flowing when a generator is short circuited is similar to that flowing when an altering voltage is suddenly applied to a resistance and an inductance in series. However there are important differences, because the current in the armature effects the rotating field. The short circuit currents and the reactance of synchronous machines are defined by the following equations, which apply to an alternator operating at no load before the occurrence of a 3- phase fault at its terminals.

| I | = ¿ E∨¿g

Xd¿ (2.1)

|I’|= ¿ E∨¿g

X 'd¿ (2.2)

|I’’|= ¿ E∨¿g

X ' 'd¿ (2.3)

Where | I | = Steady state current, rms value | I’ | = Transient current , rms value excluding dc component Xd = Direct – axis symmetrical reactance. X’d= Direct – axis transient reactance. X’’d= Direct – axis subtransient reactance. |Eg| = rms voltage from one terminal to neutral at no load.2.2 The bus Impedance Matrix in fault Calculation

We extend our study of fault calculation to a general networks. However , let us proceed to the general equations by starting with a specific network with which is given bellow in figure-

6

Fig. 2.1 Reactance diagram with sub transient reactance and sub transient internal voltages. Reactance values are marked in per unit. If this network is single phase equivalent of three phase system and we are choose to study a fault at bus 4. Vf is the voltage at bus 4 before the fault occurs.

Fig 2.2 : Circuit of fig 2.1 with admittance marked in per unit and a 3- phase fault on bus 4 of the system simulated by Vf and –Vf in series.A 3-φ fault at bus 4 is simulated by the network of fig. 2.2 ,where the impedance values of fig. 6.1 have been chanced to admittances. The generated voltages Vf and –Vf in series constitute the short circuit. Generated voltage Vf alone in this branch would cause no current in the branch. With Vf and –Vf in series in the branch is a short circuit , and the branch current is If’’. Admittance rather than impedances have been marked in per unit on this diagram. If E a

’’ , Eb’’,

Ec’’ and Vf are short – circuited, the voltage and current are those due only to –Vf. Then the only

7

current entering at node from a source is that from –Vf and –If’’ into node 4. Since there is no current in this branch until the insertion of –Vf. The node equations in matrix form for the network with –Vf the only sources are

[ 000

−I f' ' ] = j [−12.33 0.0 4.0 5.0

0.0 −10.83 2.5 5.04.05.0

2.55.0

−17.83 8.08.0 −18.0 ] [ V 1

∆

V 2∆

V 3∆

−V f] (2.4)

When the superscript ∆ indicates that the voltages are due only to –Vf. The ∆ sign is chosen to indicate the bus admittance matrix. The bus admittance matrix of the network of fig. 2.2 we obtain the bus impedance matrix. The bus voltage due to –Vf are given by-

[ V 1∆

V 2∆

V 3∆

−V f] = Zbus [ 0

00

−I f' ' ] (2.5)

And so

If’’=

V f

Z44 (2.6)

V 1∆ = -If

’’Z14 = - Z14

Z44 Vf

V 2∆= -

Z24

Z44 Vf

V 3∆= -

Z34

Z44 Vf (2.7)

When the generator voltage -Vf is short circuited in the network of fig. 2.2 and Ea’’ , Eb

’’, Ec’’ and

Vf are in the circuit, the voltages and currents everywhere in the network are those existing before the fault. By the principle of the voltages existing after the fault occurs. Usually the faulted network is assumed to have been without loads before the fault. In such a case no current is flowing before the fault, and all voltages throughout the network are the same and equal to V f. This assumption simplifies our work considerably, and applying the principle of superposition givesV1 = Vf + V 1

∆= Vf –If’’ Z14

V2 = Vf + V 2∆= Vf –If

’’ Z24

8

V3 = Vf + V 3∆= Vf –If

’’ Z34 (2.8)V4 = Vf – Vf = 0These voltages exist when sub- transient current flows and Zbus has been formed for a network having sub-transient values for generator reactance. In general terms for a fault on bus k, and neglecting prefault currents,

If = V f

Zkk (2.9)

And the post fault voltage at bus n is

Vn = Vf - Znk

Zkk Vf (2.10)

Using the numeric values of equation (2.4), we invert the square matrix Ybus of that equation and find

Zbus = j[0.1488 0.0651 0.0864 0.09780.0651 0.1554 0.0799 0.09670.08640.0978

0.07990.0967

0.1341 0.10580.1058 0.1566] (2.11)

Usually Vf is assumed to be 1.000 per unit, and this assumption for our faulted network

If’’ =

1j 0.1566 = - j6.386 per unit

V1 = 1- j 0.0978j 0.1566 = 0.3755 per unit

V2 = 1- j 0.0967j 0.1566 = 0.3825 per unit

V3 = 1- j 0.1058j 0.1566 = 0.3244 per unit

Current in any part of the network can be found from the voltages and impedance. For instance, the fault current in the branch connecting nodes 1 and 3 flowing toward node 3 is

I13’’ =

V 1−V 3

j 0.25 =

0.3755−0.3244j0.25 = - j0.2044 per unit

Ia’’ = Ea

' '−V 1

j 0.3 =

1−0.3755j 0.3 = - j2.0817 per unit

Other currents can be found in a similar manner, and voltages and currents with the fault on any other bus are calculated just as easily from thr bus impedance matrix. Equation (2.9) is simply an application of “Thevnin’s theorem, and we recognize that quantities on the principal diagonal of the bus impedance matrix are the Thevnin impedances of the network for calculating fault current at the various buses.

9

CHAPTER 03SYMMETRICALCOMPONENTS

3.1 Fundamentals of Symmetrical ComponentsSymmetrical components allow unbalanced phase quantities such as currents and voltages to be replaced by three separate balance symmetrical components.In three - phase system the phase sequence is defined as order in which they pass through a positive maximum. Consider the phasors representation of a three - phase balance current shown in the following fig-

Ib1 Ic

2

10

Ic

1 Ib2

Ia1 Ia

2

Ib1 Ic

2

Fig: (a) Fig: (b)

Fig: (c)Fig: 3.1 Representations of Symmetrical Components.By convention, the direction of rotation of the phasors is taken to be counterclockwise. The three phasors are written as Ia

1=Ia1¿0 °=Ia

1

Ib1=Ia

1¿240 °=a2Ia1 (3.1)

Ic1=Ia

1¿120 °=aIa1

The order of the phasors is abc . thie is designated the positive phase sequence. When the order is acb as in fig. (b) ,it is designate the negative phase sequence. The negative phase sequence quantities are represented asIa

2=Ia2¿0 °=Ia

2

Ib2=Ia

2¿120 °=aIa2 (3.2)

Ic2=Ia

2¿240 °=a2Ia2

When analyzing certain types of unbalanced faults, it will be found that a thirt set of balance phasors must be introduced. These phasors, known as the zero phase sequence, are found to be in phase with each other which is shown in fig (c) and they are represents as Ia

0 = Ib0 = Ic

0 (3.3)Consider the three phase unbalance currents Ia ,Ib and Ic shown in fig. we are seeking to find the symmetrical components of the current such thatI a=I a

0+ I a1+ I a

2

11

I b=I b0+ I b

1+ I b2 (3.4)

I c=I c0+ I c

1+ I c2

According to the definition of the symmetrical components as given by equation (3.1) &(3.2) and (3.3) we can rewrite (3.4) all in terms of phase a components .

I a=I a0+ I a

1+ I a2

I b=I a0+a2 I a

1+aI a2 (3.5)

I c=I a0+a Ia

1+a2 I a2

Or

[ Ia

Ib

I c] = [1 1 1

1 a2 a1 a a2][ I a

0

I a1

I a2] (3.6)

Or I abc=A . I a012 (3.7)

Or I a012=A−1 . I abc (3.8)

Here , A is symmetrical components transformation matrix and is

A= [1 1 11 a2 a1 a a2] (3.9)

And

A−1= 13 [1 1 1

1 a a2

1 a2 a ] (3.10)

Substitute A−1 in equation (3.8), we have

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[ Ia0

Ia1

Ia2] =

13 [1 1 1

1 a a2

1 a2 a ] [ Ia

Ib

I c] (3.11)

In a three phase system the sum of the line currents is equal to the currents In in the return path through the neutral. Thus I a+ Ib+ I c=I n (3.12)And we get, I n=3 I a 0 (3.13)Similarly expression exist for the symmetrical components in terms of unbalance voltages are

[V a0

V a1

V a2]=1

3 [1 1 11 a a2

1 a2 a ] [V a

V b

V c] (3.14)

3.2 SEQUENCE NETWORKS OF A LOADED GENERATOR

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Fig 3.2 represents a 3-φ synchronous gsnerator with neutral grounded through an impedance Zn. The synchronous machine generates balanced 3-φ internal voltages and is represented as a positive – sequence set of phasors

Eabc=[ 1a2

a ] Ea (3.15)

The machine is supplying a 3-φ balanced load. Applying Kirchhoff’s voltage law to each phase we obtain

V a=Ea−Z s Ia−Zn I n

V b=Eb−Z s Ib−Zn I n

V c=E c−Z s I c−Zn I n

In compact form, we haveV abc=Eabc−Zabc I abc (3.16)Transforming the terminal voltages and currents phasors into their symmetrical components results inAV a

012=AEa012−Zabc A I a

012 Multiplying by A−1,we getV a

012=Ea012−A−1 Zabc AI a

012 = Ea

012−Z012 Ia012 (3.17)

Since the generated emf is balanced, there is only positive –sequence voltage, i.e.

Ea012=[ 0

Ea

0 ] (3.18)

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Substituting for Ea012 and Z012 in equation (3.17) ,we get

[V a0

V a1

V a2] = [ 0

Ea

0 ] - [Z0 0 00 Z1 00 0 Z2

] [ Ia0

Ia1

Ia2] (3.19)

V a0=0−I a

1 Z0 V a

1 = Ea−I a1 Z1 (3.20)

V a2=0−I a

1 Z2

The sequence networks are-

Fig (a) positive-sequence ;(b) negative- sequence; (c) zero- sequence

CHAPTER 04

15

UNSYMMTRICAL FAULT

4.1 Introduction Most of the faults that occur on power systems are unsymmetrical faults, which may consist of unsymmetrical short circuits, unsymmetrical faults through impedance, or open conductors.Any unsymmetrical fault causes unbalanced currents to flow in the system, so the method of symmetrical components is very useful in an analysis to determine the currents and voltages in all parts of the system after the fault occurrence of the fault.

4.2 Sequence Impedance:This the impedance of an equipment or component to the current of different sequences.the impedance offered to flow of positive sequence currents is known as the positive sequence impedance and denoted by z1. The impedance offered to the flow of zero sequence currents is known as the negative sequence impedance and denoted by z2. When zero sequence current flow, the impedance known as zero sequence impedance and denoted by z0 .

4.2.1 Sequence Impedance of Transmission Line:For transmission line positive sequence and negative sequence impedance are the same.Negative sequence impedance can be derived by using canon formula. The sequence Impedance of transmission line is 3 times greater than positive and negative sequence impedance. [2]

4.2.2 Sequence Impedance of Synchronous Machine:As positive sequence reactance Xd’,Xd”,Xd are used. Negative sequence reactance is close to positive sequence sub transient reactance.Zero sequence reactance is approximated to leakage reactance.

4.2.3 Sequence Impedance of Transformer:

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For transformer z0=z1=z2=zl where zl is the leakage reactance. The zero sequence equivalent circuit of various transformer bank is shown in figure 4.1

17

Figure: 4.1 Zero sequence equivalent circuit

4.3 Single Line to Ground Fault

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Fig. 4.2 single line to ground fault on phase a at the terminals of an unloaded generator. The circuit diagram for Single Line to Ground Fault on an unloaded Y- connected generator with its neutral grounded through a reactance is shown in fig. 4.1 where phase a is one on which the fault occurs the relations to be developed for this type of fault will apply only when the fault is on phase a, but this should cause no difficulty since the phase are labeled arbitrarily and any phase can be designated as phase a. The boundary conditions at the fault point are-Va = Zf Ia

Ib = 0Ic = 0

Then the symmetrical components of currents are-

[ Ia0

Ia1

Ia2] =

13 [1 1 1

1 a a2

1 a2 a ][ I a

00 ] (4.1)

I a0 = I a

1 = I a2=

13 Ia (4.2)

Substituting I a1 forI a

2 and I a0 in equation (3.19) we obtain

[V a0

V a1

V a2] = [ 0

Ea

0 ] - [Z0 0 00 Z1 00 0 Z2

] [ Ia0

Ia1

Ia2] (4.3)

orV a

1 = Ea−I a1 Z1

V a2=−I a

1 Z2

19

V a0=−I a

1 Z0 (4.4)Phase a voltage in term of symmetrical components is Va = V a

0+V a1+V a

2

= -Z0 I a1+Ea-Z1 I a

1-Z2 I a1

= Ea- (Z0+Z1+Z2)I a1

Zf Ia = Ea- (Z0+Z1+Z2)I a1

3ZfI a1 = Ea- (Z0+Z1+Z2)I a

1

Ea= (Z0+Z1+Z2)I a1

I a1 =

Ea

Z0+Z1+Z2+3 Z f

The fault current is

Ia = 3I a1 =

3 Ea

Z0+Z1+Z2+3 Z f

If the three sequence network of the generator are connected in series as shown in fig. 4.2 we see that the current and voltages resulting there from satisfy the equations above, for the three sequence impedance are then in series with the voltages Ea.

Fig 4.3 sequence network connections for single line to ground fault.

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4.4 Line To Line Fault

Fig 4.4 Line To Line Fault between phase b and c.

The circuit diagram for a line to line fault through impedance Z f between phase b and c on an unbalanced, Y-connected generator with its neutral grounded through a reactance is shown in fig 4.2. the boundary conditions at the point are-

Vb – Vc=Zf Ib (4.5)

Ib = -Ic (4.6)

Ib+Ic = 0 (4.7)

Ia = 0

Then the symmetrical components of current are-

[ Ia0

Ia1

Ia2] =

13 [1 1 1

1 a a2

1 a2 a ][ 0I b

−I b] (4.8)

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∴ I a0 = 0 (4.9)

I a1 =

13 (a−a2)I b (4.9)

I a2 =

13 (a2−a)I b (4.10)

∴ I a1 = - I a

2 (4.11)

From equation (3.19) we obtain

[V a0

V a1

V a2] = [ 0

Ea

0 ] - [Z0 0 00 Z1 00 0 Z2

] [ Ia0

Ia1

Ia2] (4.12)

∴ V a1 = Ea−I a

1 Z f V a

2=I a1 Z2

V a0=−I a

0 Z0

Phase b and c voltage in terms symmetrical components are-

V b=V a0+a2 V a

1+aV a2

V c=V a0+a V a

1+a2 V a2

V b−V c=(a2−a ) (V a1−V a

2)

= Z f I b

Substituting for V a1 and V a

2 we get

(a2−a ) [ Ea−( Z1+Z2 ) Ia1 ]=Z f Ib

I a1 =

Ea

Z f +Z1+Z2

22

The phase currents are-

[ Ia

Ib

I c] = [1 1 1

1 a2 a1 a a2] [ 0

I a1

−I a1]

The fault current is-

I b=−I c=(a2−a) I a1

Or I b=− j √ 3 I a1

Again the symmetrical components of voltage are given by-

[V a0

V a1

V a2] =

13 [1 1 1

1 a2 a1 a a2][V a

V b

V c]

∴V a1=V a

2

Fig 4.5 sequence network connections for line to line fault.

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4.5 Double Line To Ground Fault

Fig 4.6 Double line to Ground fault.

The circuit diagram for a double line to ground fault on phase’s b and c through impedance Zf to ground on an unloaded, Y-connected generator with its neutral to grounded through a reactance is shown in fig 4.4. the boundary conditions at the fault point are-

V b=V c=Z f ( I b+ I c) (4.13)

I a=I a0+ I a

1+ I a2=0 (4.14)

Phase b and c voltage in terms symmetrical components are-

V b=V a0+a2 V a

1+a V a2

V c=V a0+a V a

1+a2 V a2

Since Vb = Vc , hence

V a1=V a

2

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Substituting for the symmetrical components of currents, we get

V b=Z f (I a0+a2 I a

1+aI a2+ I a

0+a I a1+a2 I a

2)

=Z f ¿

=3Z f I a0

Again

V b=V a0+a2 V a

1+aV a2

3Z f I a0 = V a

0+a2V a1+aV a

2

= V a0+(a2+a)V a

1

= V a0−V a

1

Now

[V a0

V a1

V a2] = [ 0

Ea

0 ] - [Z0 0 00 Z1 00 0 Z2

] [ Ia0

Ia1

Ia2]

∴V a0=−I a

0 Z0

V a1 = Ea−I a

1 Z1 V a

2=I a2 Z2

3Z f I a0=−I a

0 Z0−Ea+ I a1 Z1

I a0 ( Z0+3Z f )=−Ea+ I a

1 Z1

I a0=

Ea−I a1 Z1

Z0+3 Z f

Again , V a1=V a

2

Ea−I a1 Z1=−I a

2 Z2

I a2=

−Ea−I a1 Z1

Z2

We can also find that

25

I a1=

Ea

Z1+Z2(Z0+3 Z f )Z0+Z2+3 Z f

And the fault current is

I f =I b+ I c=3 I a0

Fig 4.7 sequence network connections for double line to ground fault.

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Chapter: 05

Fault simulation

5.1 Introduction:

Fault Analysis is an important tropic in power system. By simulating various types of fault at various bus we can determine the line current and bus voltage during the fault. Fault should be cleared as soon as possible, otherwise excessive current will flow through line, it can make line damage, voltage at various bus will fluctuate and system will be unstable. So ,it is very important to clear faulty section from rest of the system as soon as possible. In design of protective system, we require circuit breaker rating. By fault analysis we can determine circuit breaker rating.

In this chapter, fault analysis of various power system will be demonstrated by computer program.

5.2 Computer Program For Fault Analysis :

I have made a computer program by MATLAB software that can simulate any type of fault of any power system. It can be used simulate fault at any type at any bus of a power system containing any number of bus. It may contains tap changing transformer , capacitive effect of transmission line is considered.

In convention fault analysis ,for sake of simplicity pre fault bus voltage is consider 1 per unit. In actual system pre fault voltage is not equal to 1 p.u. Pre fault voltage depend on load condition .

27

This pre fault voltage can be determined by load flow studies. In my program there has a option to consider load effect for pre fault voltage. This program use gauss-seidel method to determine pre fault bus voltage. You may neglect load effect also. In this case pre fault voltage assume 1 per unit. The program is shown in next page. You can give data input at MATLAB Editor or at command window as your wish.

Problem1:

The one line diagram of a simple power system is shown in figure 5.1. The neutral of each generator is grounded through a current limiting reactor of .25/3 per unit on 100 MVA base. The system data expressed in per unit on a common 100 MVA base is tabulated below. The generator are running at no load condition. Simulate various type of fault at various bus.

28

Figure 5.1:single line diagram of problem 1

Figure 5.2:positive sequence network of problem 1

29

Figure5.3: negative sequence network of problem 1

Simulation result of this problem is shown in next page

Problem 2: The 11 bus power system of an electric utility company is shown in figure 5.4.The positive and zero sequence reactance of generator , line and transformer in per unit on a 100 MVA base is tabulated below .The Y- delta x-mer bank between bus 11 and 7 is grounded through a reactor of reactance .08 per unit. Consider no load effect. Simulate various type of fault at various bus.

30

Figure 5.4:single line diagram of problem 2

31

Table: data for problem 2

Figure5.5: Zero sequence network of problem 2

Simulation result is shown in next page

32

Problem 3:

Simulate different type of fault on the power system shown in figure 5.6. Necessary data are given in table below. Consider load effect.

Figure5.6: A five bus power system.

33

Table: data for problem 3

Simulation result is shown in next page

Chapter: 06 Circuit Breaker6.1 Arc Phenomenon:

When as short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionize the air or vapourise and ionize the oil. The ionized air or vapour acts as conductor an arc is struck between the contacts. The p.d. between the contacts is quite small and is just sufficient to maintain the arc. The arc provides low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists.

34

During the arcing period, the current flowing between the contacts depends upon the arc resistance The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors:

(i) Degree of ionization- the arc resistance increases with the decrease in the number of ionized particles between the contacts.

(ii) Length of the arc- the arc resistance increases with the length of the arc i.e., separation of contacts.

(iii) Cross-section of- the arc resistance increases with the decrease in area of X-section of the arc.

6.2 Principles of Arc Extinction

Before discussing the methods of arc extinction, it is necessary to examine the factors responsible for the maintenance of arc between the contacts. These are:

(i) p.d. between the contacts

(ii) ionized particles between contacts

Taking these in turn,

(i) When the contacts have a small separation, the p.d. between them is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that p.d. becomes inadequate to maintain the arc. However, this method is impracticable in high voltage system where a separation of many metres may be required.

(ii) The ionized particles between the contacts tend to maintain the arc. It the arc path is deionised, the arc extinction will be facilitated. This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts.

6.3 Methods of arc Extinction:

There are two methods of extinguishing the arc in circuit breakers viz.

1. High resistance method. 2. Low resistance or current zero method

1. High resistance method. In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished. The principal disadvantage of this method is that enormous energy is dissipated in the arc. Therefore, it is employed only in d.c. circuit breakers and low-capacity a.c. circuit breakers.

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(i) Lengthening the arc. The resistance of the arc is directly proportional to its length. The length of the arc can be increased by increasing the gap between contacts.

(ii) Cooling the arc. Cooling helps in the deionisation of the medium between the contacts. This increases the arc resistance. Efficient cooling may be obtained by a gas blast directed along the arc.

(iii) Reducing X-section of the arc. It the area of X-section of the arc is reduced, the voltage necessary to maintain the arc is increased. In other words, the resistance of the arc path is increased. The cross-section of the arc can be reduced by letting the arc pass through a narrow opening or by having smaller area of contacts.

(iv) Splitting the arc. The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series. Each one of these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts.

2. Low resistance or Current zero method. This method is employed for arc extinction in a.c. circuits only. In this method, arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from restriking inspite of the rising voltage across the contacts. All modern high power a.c. circuit breakers employ this method for arc extinction.

In an a.c. system, current drops to zero after every half-cycle. At every current zero, the arc extinguishes for a brief moment. Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage known as restriking voltage. If such a breakdown does occur, the arc will persist for another half-cycle. If immediately after current zero, the dielectric strength of the medium between contacts is built up rapidly than the voltage across the contacts, the arc fails to and the current will be interrupted. The rapid increase of dielectric strength of the medium near current zero can be achieved by:

(a) causing the ionized particles in the space between contacts to recombine into neutral molecules.

(b) Sweeping the ionized particles away and replacing them by un-ionised particles

Therefore, the real problem in a.c. arc interruption is to rapidly deionise the medium between contacts as soon as the current becomes zero so that the rising contact voltage or restriking voltage cannot breakdown the space between contacts. The de-ionisation of the medium can be achieved by:

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(i) lengthening of the gap. The dielectric strength of the medium is proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, higher dielectric strength of the medium can be achieved.

(ii) high pressure. If the pressure in the vicinity of the arc is increased, the density of the particles constituting the discharge also increases. The increased density of particles causes higher rate of de-ionisation and consequently the dielectric strength of the medium between contacts is increased.

(iii) Cooling. Natural combination of ionized particles takes place more rapidly if they are al-lowed to cool. Therefore, dielectric strength of the medium between contacts is increased.

(iv) Blast effect. If the ionized particles between the contacts are swept away and replaced by unionized particles, the dielectric strength of the medium can be increased considerably. This may be achieved by a gas blast directed along the discharge or by forcing oil into the contact space.

6.4 Important Terms:

The following are the important terms much used in the circuit breaker analysis:

(i) Arc Voltage. It is the voltage that appears across the contacts of the circuit breaker during the arcing period.

As soon as the contacts of the circuit breaker separate, an arc is formed. The voltage that appears across the contacts during arcing period is called the arc voltage. Its value is low except for the period the fault current is at or near zero current point. At current zero, the arc voltage rises rapidly to peak value and this peak voltage tends to maintain the current flow in the form of arc.

(ii) Restriking voltage. It is the transient voltage that appears across the contacts at or near current zero during arcing period.

At current zero, a high-frequency transient voltage appears across the contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission lines of the system. This transient voltage is known as restriking voltage (Fig. 19.1). The current interruption in the circuit depends upon this voltage. If the restriking voltage rises more rapidly tan the dielectric strength of the medium between the contacts, the arc will persists for another half-cycle. On the other hand, if the

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dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted.

(iii) Recovery voltage. It is the normal frequency (50 Hz) r.m.s. voltage that appears across the contacts of the circuit breaker after final arc extinction. It is approximately equal to the system voltage.

When contacts of circuit breaker are opened, current drops to zero after every half cycle. At some current zero, the contacts are separated sufficiently apart and dielectric strength of the medium between the contacts attains a high value due to the removal of ionized particles. At such an instant, the medium between the contacts is strong enough to prevent the breakdown by the restriking voltage. Consequently, the final arc extinction takes place and circuit current is interrupted. Immediately after final current interruption, the voltage that appears across the contacts has a transient part (See Fig. 19.1). However, these transient oscillations subside rapidly due to the damping effect of system resistance and normal circuit voltage appears across the contacts. The voltage across the contacts is of normal frequency and is known as recovery voltage.

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6.5 Classification of Circuit Breakers:

There are several ways of classifying the circuit breakers. However, the most general way of classification is on the basis of medium used for arc extinction. The medium used for arc extinction is usually oil, air, sulphur hexafluoride (SF6) or vacuum. Accordingly, circuit breakers may be classified into:

(i) Oil circuit breakers which employ some insulating oil (e.g., transformer oil) for arc extinction.

(ii) Air-blast circuitbreakers in which high pressure air-blast is used for extinguishing the arc.

(iii) Sulphur hexafluroide circuit breakers in which sulphur hexafluoride (SF6) gas is used for arc extinction.

(iv) Vacuum circuit breakers in which vacuum is used for arc extinction.

Each type of circuit breaker has its own advantages and disadvantages. In the following sections, we shall discuss the construction and working of these circuit breakers with special emphasis on the way the arc extinction is facilitated.

Oil Circuit Breakers:

In such circuit breakers, some insulating oil (e.g., transformer oil) is used as an arc quenching medium. The contacts are opened under oil and an arc is struck between them. The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of gaseous hydrogen at high pressure. The hydrogen gas occupies a volume about one thousand times that of the oil decomposed. The oil is, therefore, pushed away from the arc and an expanding hydrogen gas bubble surrounds the arc region and adjacent portions of the contacts (See Fig. 19.2). The arc extinction is facilitated mainly by two processes. Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionisation fo the medium between the contacts. Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path. The result is that arc is extinguished and circuit current interrupted.

Advantages. The advantages of oil as an arc quenching medium are:

(i) It absorbs the arc energy to decompose the oil into gases which have excellent cooling properties.

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(ii) It acts as an insulator and permits smaller clearance between live conductors and earthed components.

(iii) The surrounding oil presents cooling surface in close proximity to the arc.

Disadvantages. The disadvantages of oil as an arc quenching medium are:

(i) It is inflammable and there is a risk of a fire.

(ii) It may form an explosive mixture with air

(iii) The arcing products (e.g., carbon) remain in the oil and its quality deteriorates with successive operations. This necessitates periodic checking and replacement of oil.

Type of Oil Circuit Breakers:

The oil circuit breakers find extensive use in the power system. These can be classified into the following types:

(i) Bulk oil circuit breakers which use a large quantity of oil. The oil has to serve two purposes. Firstly, it extinguishes the arc during opening of contacts and secondly, it insulates the current conducting parts from one another and from the earthed tank. Such circuit breakers may be classified into:

(a) Plain break oil circuit breakers (b) Arc control oil circuit breakers.

In the former type, no special means is available for controlling the arc and the contacts are directly exposed to the whole of the oil in the tank. However, in the latter type, special arc control devices are employed to get the beneficial action of the arc as efficiently as possible.

(ii) Low oil circuit breakers, which use minimum amount of oil. In such circuit breakers, oil is used only for arc extinction; the current conducting parts are insulated by air or porcelain or organic insulating material.

Air-Blast Circuit Breakers:

These breakers employ a high-pressure air-blast as an arc-quenching medium. The contacts are opened in a flow of air-blast established by the opening of blast valve. The air-blast cools the arc and sweeps away the arcing products to the atmosphere. This rapidly increases the dielectric strength of the medium between contacts and prevents from re-establishing the arc. Consequently, the arc is extinguished and flow of current is interrupted.

Types of Air-Blast Circuit Breakers:

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Depending upon the direction of air-blast in relation to the arc, air-blast circuit breakers are classified into:

(i) Axial-blast type in which the air-blast is directed along the arc path as shown in Fig. 19.8 (i).

.

(ii) Cross-blast type in which the air-blast is directed at right angles to the arc path as shown in Fig. 19.8 (ii).

(iii) Radial-blast type in which the air-blast is directed radially as shown in Fig. 19.8 (iii).

Sulphur Hexaflouride (SF6) Circuit Breakers:

In such circuit breakes, sulphur hexaflouride (SF6) gas is used s the arc quenching medium. The SF6 is an electro-negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker arc opened in a high pressure flow of SF6 gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. The SF6 circuit breakers have been found to be very effective for high power and high voltage service.

Construction. Fig. 19.11 shows the parts of a typical SF6 circuit breaker. It consists of fixed and moving contacts enclosed in a chamber (called arc interruption chamber) containing SF6 gas. This chamber is connected to SF6 gas reservoir. When the contacts of breaker are opened, the valve mechanism permits a high pressure SF6 gas from the reservoir to flow towards the arc interruption chamber. The fixed contact is hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF6 gas to let out through these holes after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF6 gas is costly, it is reconditioned and reclaimed by suitable auxiliary system after each operation of the breaker.

Working. In the closed position of the breaker, the contacts remain surrounded by SF6

gas at a pressure of about 2.8 kg/cm2. When the breaker operates, the moving contact is pulled

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apart and an arc is struck between the contacts. The movement of the moving contact is synchronized with the opening of valve which permits SF6 gas at 14 kg/cm2 pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the breaker operation (i.e., after arc extinction), the valve is closed by the action of a set of springs.

Advantages. Due to the superior arc quenching properties of SF6 circuit breakers have many advantages over oil or air circuit breakers. Some of them are listed below:

(i) Due to the superior arc quenching property of SF6, such circuit breakers have very short arcing time.

(ii) Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.

(iii) The SF6 circuit breaker gives noiselss operation due to its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker.

(iv) The closed gas enclosure keeps the interior dry so that there is no moisture problem.

(v) There is no risk of fire in such breakers because SF6 gas is non-inflammable.

(vi) There are no carbon deposits so that tracking and insulation problems are eliminated.

(vii) The SF6 breakers have low maintenance cost. Light foundation and minimum auxiliary equipment.

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(viii) Since SF6 breakers re totally enclosed and sealed from atmosphere, they are particularly suitable where explosion hazard exists e.g., coal mines.

Disadvantages:

(i) SF6 breakers are costly due to the high cost of SF6.

(ii) Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for his purpose.

Applications. A typical SF6 circuit breaker consists of interrupter units each capable of dealing with currents upto 60 k A and voltages in the range of 50-80 kV. A number of units are connected in series according to the system voltage. SF6 circuit breakers have been developed for voltages 115 kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.

Vacuum Circuit Breakers (VCB):

In such breakers, vacuum (degree of vacuum being in the range from 10-7 to 10-5 torr) is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. For example, when contacts of a breaker are opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than that obtained with other circuit breakers.

Advantages. Vacuum circuit breakers have the following advantages:

(i) They are compact, reliable and have longer life.

(ii) There are no fire hazards.

(iii) There is no generation of gas during and after operation.

(iv) They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite open position.

(v) They require little maintenance and are quiet in operation.

(vi) They can successfully withstand lightning surges.

(vii) They have low arc energy.

(viii) They have low inertia and hence require smaller power for control mechanism.

Applications. For a country like India, where distances are quite large and accessibility to remote areas difficult, the installation of such outdoor, maintenance free circuit breakers should prove a

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definite advantage. Vacuum circuit breakers are being employed for outdoor applications ranging of applications in rural areas.

6.6 Protective Relays:

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the faulty circuit.

A typical relay circuit is shown in Fig. 21.1. This diagram shows one phase of 3-phase system for simplicity. The relay circuit connections can be divided into three parts viz.

(i) First par is the primary winding of a current transformer (C.T.) which is connected in series with the line to be protected.

(ii) Second part consists of secondary winding of C.T. and the relay operating coil.

(iii) Third part is the tripping circuit which may be either a.c. or d.c. It consists of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts.

When a short circuit occurs at point F on the transmission line, the current flowing in the line increases to an enormous value. This results in a heavy current flow through the relay coil, causing the relay to perate by closing its contacts. This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of they system. In this way, the relay ensures the safety of the circuit equipment from damage and normal working of the healthy portion of the system.

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Chapter: 07

Protection of Alternator and Transformer

7.1 Differential Protection of Alternators:

The most common system used for the protection of stator winding faults employs circulating-current principle . In this scheme of protection, currents at the two ends of the protected section are compared. Under normal operating conditions, these currents are equal but

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may become unequal on the occurrence of a fault in the protected section. The difference of the currents under fault conditions is arranged to pass through the operating coil of the relay. The relay then closes its contacts to isolate protected section from the system. This form of protection is also known as Merz-Price circulating current scheme.

Schematic arrangement. Fig. shows the schematic arrangement of current differential protection for a 3-phase alternator. Identical current transformer pairs CT1 and CT2 are placed on either side of each phase of the stator windings. The secondaries of each set of current transformers are connected in star; the two neutral points and the corresponding terminals of the two star groups being connected together by means of a four-core pilot cable. Thus there is an independent path for the currents circulating in each pair of current transformers and the corresponding pilot P.

The relay coils are connected in star, the neutral point being connected to the current-transformer common neutral and the outer ends one to each of the other three pilots. In order that burden on each current transformer is the same, the relays are connected across equipotential points of the three pilot wires and these equipotential points would naturally be located at the middle of the pilot wires. The relays are generally of electromagnetic type and are arranged for instantaneous action should be cleared as quickly as possible.

7.2 Stator inter-turn Protection:

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Merz-price circulating- current system protects against phase-to- ground and phase-to-phase faults. I does not protect against turn-to-turn fault on the same phase winding of the stator. It is because the current that this type of fault produces flows in a local circuit between the turns involved and does not create a difference between the currents entering and leaving the winding at its two ends where current transformers are applied. However, it is usually considered unnecessary to provide protection for inter-turn faults because they invariably develop into earth-faults.

In single turn generator (e.g. large steam-turbine generators), there is no necessity of protection against inter-turn faults. However, inter-turn protection is provided for multi-turn generators such as hydro-electric generators. These generators have double-winding armatures (i.e. each phase winding is divided into two halves) owing o the very heavy currents which they have to carry. Advantage may be taken of this necessity to protect inter-turn faults on the same winding. Fig. shows the schematic arrangement of circulating-current and inter-turn protection of a 3- phase double wound generator. The relays RC provide protection against phase-to-ground and phase to-phase faults whereas relays R1 provide protection against inter-turn faults.

Fig. shows the duplicate stator windings S1 and S2 of one phase only with a provision against inter-turn faults. Two current transformers are connected on the circulating-current principle. Under normal conditions, the currents in the stator windings S1 and S2 are equal and so will be the currents in the secondaries of the two CTs. The secondary current round the loop then is the same at all points and no current flows through the relay R1. If a short-circuit develops between adjacent turns, say on S1, the currents in the stator windings S1 and S2 will no longer be equal. Therefore, unequal currents will be induced in the secondaries of CTs and the difference of these two currents flows through the relay R1. The relay then closes its contacts to clear the generator from the system.

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7.3 Protection of Transformers:

Transformers are static devices, totally enclosed and generally oil immersed. Therefore, chances of faults occurring on them are very rare. However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessitates to provide adequate automatic protection for transformers against possible faults.

Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay equipment is required. However,the probability of faults on power transformers is undoubtedly more and hence automatic protection is absolutely necessary.

Common transformer faults. As compared with generators, in which many abnormal conditions may arise, power transformers may suffer only from:

(i) open circuits

(ii) overheating

(iii) Winding short-circuits e.g. earth-faults, phase-to-phase faults and inter-turn faults.

An open circuit in one phase of a 3-phase transformer may cause undesirable heating. In practice, relay protection is not provided against open circuits because this condition is relatively harmless. On the occurrence of such a fault, the transformer can be disconnected manually from the system.

Overheating of the transformer is usually caused by sustained overloads or short-circuits and very occasionally by the failure of the cooling system. The relay protection is also not provided against this contingency and thermal accessories are generally used to sound an alarm or control the banks of fans.

Winding short-circuits (also called internal faults) on the transformer arise from deterioration of winding insulation due to overheating or mechanical injury. When an internal fault occurs, the transformer must be disconnected quickly from the system because a prolonged arc in the transformer may cause oil fire. Therefore relay protection is absolutely necessary for internal faults.

7.5 Protection systems for transformers:

For protection of generators, Merz-Price circulating-current system is unquestionably the most satisfactory. Though this is largely true of transformer protection, there are cases here

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circulating current system offers no particular advantage over other systems or impracticable on account of the troublesome conditions imposed by the wide variety of voltages, currents and earthing conditions invariably associated with power transformers. Under such circumstances, alternative protective systems are used which in many cases are as effective as the circulating-current system. The principal relays and systems used for transformer protection are:

(i) Buchholz devices providing protection against all kinds of incipient faults i.e. slow-developing faults such as insulation failure of windings, core heating, fall of oil level due to leaky joints etc.

(ii) Earth-fault relays providing protection against earth-faults only.

(iii) Overcurrent relays providing protection mainly against phase-to-phase faults and overloading.

(iv) Differential system (or circulating-current system) providing protection against both earth and phase faults.

The complete protection of transformer usually requires the combination of these systems. Choice of a particular combination of systems may depend upon several factors such as (a) size of the transformer (b) type of cooling (c) location of transformer in the network (d) nature of load supplied and (e) importance of service for which transformer is required. In the following sections, above systems of protection will be discussed in detail.

7.6 Buchholz Relay:

Buchholz relay is a gas-actuated relay installed in oil immersed transformers for protection against all kinds of faults. Named after its inventor, Buchholz, it is used to give an alarm in case of incipient (i.e. slow-developing) faults in the transformer and to disconnect the transformer from the supply

in the event of severe internal faults. It is usually installed in the pipe connecting the conservator to the main tank as shown in Fig. 22.11 It is a universal practice to use Buchholz relays on all such oil immersed transformers having ratings in excess of 750 k VA.

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Construction. Fig. 22.12 shows the constructional details of a Buchholz relay. It takes the form of a domed vessel placed in the connecting pipe between the main tank and the conservator. The device has two elements. The upper element consists of a mercury type switch attached to a float. The lower element contains a mercury switch mounted on a hinged type flap located in the direct path of the flow of oil from the transformer to the conservator. The upper element closes an alarm circuit severe internal fault.

Figure: Buchholz relay

Operation. The operation of Buchholz relay is as follows:

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(i) In case of incipient faults within the transformer, the heat due to fault causes the decomposition of some transformer oil in the main tank. The products of decomposition contain more than 70% of hydrogen gas. The hydrogen gas being light tries to go into the conservator and in the process gets entrapped in the upper part of relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and close the contacts of mercury switch attached to it. This completes the alarm circuit to sound an alarm.

(ii) It a serious fault occurs in the transformer, an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer.

Advantages

(i) It is the simplest form of transformer protection.

(ii) It detects the incipient faults at a stage much earlier than is possible with other forms of protection.

Disadvantages

(i) It can only be used with oil-immersed transformers equipped with conservator tanks.

(ii) The device can detect only faults below oil level in the transformer. Therefore, separate protection is needed for connecting cables.

7.7 Applying Circulating-current system to Transformers:

Merz-Price circulating-current principle is commonly used for the protection of power transformers against earth and phase faults. The system as applied to transformers is fundamentally the same as that for generators but with certain complicating features not encountered in the generator application. The complicating features and their remedial measures are briefed below:

(i) In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, therefore, the use of identical transformers (of same turn ratio) will give differential current and operate the relay even under no load conditions.

The difference in the magnitude of currents in the primary and secondary of power transformer is compensated by different turn ratios of CTs. It T is the turn-ratio of power transformer, then turn-ration of CTs on the i.v. side is made T times that of the CTs on the h.v.

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side. Fulfilled this condition, he secondaries of the two CTs will carry identical currents under normal load conditions. Consequently, no differential current will flow through the relay and it remains inoperative.

(ii) Thereis usually a phase difference between he primary and secondary currents of a 3-phase power transformer. Even if CTs of the proper turn-ration are used, a differential current may flow through the relay under normal conditions and cause relay operation.

The correction for phase difference is effected by appropriate connections of CTs. The CTs on one side of the power transformer re connected in such a way that the resultant current fed into the pilot wires are displaced in phase from the individual phase currents in the same direction as, and by an angle equal to, the phase shift between the power-transformers primary and secondary currents. The table below shows the type of connections to be employed for CTs in order to compensate for the phase difference in the primary and secondary current of power transformer.

S. No.

Power transformer connections Current transformer connectionPrimary Secondary Primary Secondary

1 Star with neutral earthed

Delta Delta Star

2 Delta Delta Star Star3 Star Star with neutral earthed Delta Delta4 Delta Star with neutral earthed Star Delta

Thus referring to the above table, for a delta/star power transformer, the CTs on the delta side must be connected in star and those on the star side in delta.

(iii) Most transformers have means for tap changing which makes this problem even more difficult. Tab changing will cause differential current to flow through the relay even under normal operating conditions.

The above difficulty is overcome by adjusting the turn-ration of CTS on the side of the power transformer provided with taps.

(iv) Another complicating factor in transformer protection is the magnetizing in-rush current. Under normal load conditions, the magnetizing current is very small. However, when a transformer is energized after it has been taken out of service, the magnetizing or in-rush

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current can be extremely high for a short period. Since magnetizing current represents a current going into the transformer without a corresponding current leaving, it appears as a fault current to differential relay and may cause relay operation.

In order to overcome above difficulty, differential relays are set to operate at a relatively high degree of unbalance. This method decreases the sensitivity of the relays. In practice, advantage is taken of the fact that the initial in-rush currents contain prominent second-harmonic component. Hence, it is possible to design a scheme employing second-harmonic bias features, which, being tuned to second-harmonic frequency only, exercise restrain during energizing to prevent maloperation.

While applying circulating current principle for protection of transformers, above precautions are necessary in order to avoid inadvertent relay operation.

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Chapter: 08

Protection of Bus bars and Line

8.1 Introduction:

Busbars and lines are important elements of electric power system and require the immediate attention of protection engineers for safeguards against the possible faults occurring on them. The methods used for the protection of generators and transformers can also be employed, with slight modifications, for the busbars and lines. The modifications are necessary to cope with the protection problems arising out of greater length of lines and a large number of circuits connected to a busbar. Although differential protection can be used, it becomes too expensive for longer lines due to the greater length of pilot wires required. Fortunately, less expensive methods are available which are reasonably effective in providing protection for the busbars and lines. In this chapter, we shall focus our attention on the various methods of protection of busbars and lines.

8.2 Bus bar Protection:

Busbars in the generating stations and sub-stations form important link between the incoming and outgoing circuits. If a fault occurs on a busbar, considerable damage and disruption of supply will occur unless some form of quick-acting automatic protection is provided to isolate the faulty busbar. The busbar. The busbar zone, for the purpose of protection, includes not only the busbars themselves but also the isolating switches, circuit breakers and the associated connections. In the event of fault on any section of the busbar, all the circuit equipments connected to that section must be tripped out to give complete isolation.

The standard of construction for busbars has been very high, with the result that bus faults are extremely rare. However, the possibility of damage and service interruption from even a rare bus fault is so great that more attention is now given to this form of protection. Improved relaying methods have been developed, reducing the possibility of incorrect operation. The two most commonly used schemes for busbar protection are:

(i) Differential protection (ii) Fault bus protection

(i) Differential protection. The basic method for busbar protection is the differential scheme in which currents entering and leaving the bus are totalized. During normal load condition, the sum of these currents is equal to zero. When a fault occurs, the fault current upsets the balance and produces a differential current to operate a relay.

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Fig. 23.1 shows the single line diagram of current differential scheme for a station busbar. The busbar is fed by a generator and supplies load to two lines. The secondaries of current transformers in the generator lead, in line 1 and in line 2 are all connected in parallel. The protective relay is connected across this parallel connection. All CTs must be of the same ration in the scheme regardless of the capacities of the various circuits. Under normal load conditions or external fault conditions, the sum of the currents entering the bus is equal to those leaving it and no current flow through the relay. If a fault occurs within the protected zone, the currents entering the bus will no longer be equal to those leaving it. The difference of these currents will flow through the relay and cause the opening of the generator, circuit breaker and each of the line circuit breakers.

(ii) Fault Bus protection. It is possible to design a station so that the faults that develop are mostly earth-faults. This can be achieved by providing earthed metal barrier (known as fault bus) surrounding each conductor throughout its entire length in the bus structure. With this arrangement, every fault that might occur must involve a connection between a conductor and an earthed metal part. By directing the flow of earth-fault current, it is possible to detect the faults and determine their location This type of protection is known as fault bus protection.

Fig. 23.2 show the schematic arrangement of fault bus protection. The metal supporting structure or fault bus is earthed through a current transformer. A relay is connected across the secondary of this CT. Under normal operating conditions, there is no current flow from fault bus to ground and the relay remains inoperative. A fault involving a connection between a conductor and earthed supporting structure will result in current flow to ground through the fault bus, causing the relay to operate. The operation of relay will trip all breakers connecting equipment to the bus.

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8.3 Protection of Line:

Differential Pilot-Wire Protection:

The differential pilot-wire protection is based on the principle that under normal conditions, the current entering one end of a line is equal to that leaving the other end. As soon as a fault occurs between the two ends, this condition no longs holds and the difference of incoming and outgoing currents is arranged to flow through a rela** which operates the circuit breaker to isolate the faulty line. There are several differential protection schemes in use for the lines. However, only the following two schemes will be discussed:

1. Merz-Price voltage balance system

2. Translay scheme

1. Merz-Price voltage balance system. Figure shows the single line diagram of Merz-Price voltage balance system for the protection of a 3-phase line. Identical current transformers are laced in each phase at both ends of the line. The pair of CTs in each line is connected in series with a relay in such a way that under normal conditions, their secondary voltages are equal and in opposition i.e. they balance each other.

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Under healthy conditions, current entering the line at one-end is equal to that leaving it at the other end. Therefore, equal and opposite voltages are induced in the secondaries of the CTs at the two ends of the line. The result is that no current flows through the relays. Suppose a fault occurs at point F on the line as shown in Figure. This will cause a greater current to flow through CT1 than through the pilot wires and relays. The circuit breakers at both ends of the line will trip out and the faulty line will be isolate.

Figure shows the connections of Merz-Price voltage balance scheme for all the three phases of the line.

Advantages

(i) This system can be used for ring mains as well as parallel feeders.

(ii) This system provides instantaneous protection for ground faults. This decreases he possibility of these faults involving other phases.

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(iii) This system provides instantaneous relaying which reduces the amount of damage to over-head conductors resulting from arcing faults.

Disadvantages

(i) Accurate matching of current transformers is very essential.

(ii) If there is a break in the pilot-wire circuit, the system will not operate.

(iii) This system is very expensive owing to the greater length of pilot wires required.

(iv) In case of long lines, charging current due to pilot-wire capacitance effects may be sufficient to cause relay operation even under normal conditions.

(v) This system cannot be used for line voltages beyond 33 kV because of constructional difficulties in matching the current transformers.

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Chapter: 09 Protection Against Overvoltage

9.1 Introduction

There are several instances when the elements of power system (e.g generators, transformers, transmission lines, insulators etc.) are subjected to over voltages i.e. voltages greater than the normal value. these over voltages on the power system may be caused due to many reasons such as lightning, the opening of a circuit breaker, the grounding of a conductor etc. Most of the over voltages are not of large magnitude but may still be important because of their tude but may still important because of there effect on the performance of circuit interputing equipment and protective devices. An applicable number of these over voltages are of

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sufficient magnitude to cause insulation breakdown of the equipment in the power system. They're fore, power system engineers always device ways and means to limit the magnitude of the over voltages produced and to control their effects on the ages produced and to control their effects on the operating equipment. In this chapter, we shall con fine our attention to the various causes of over voltages on the power system with special emphasis on the protective devices used for the purpose.

9.2 Voltage Surge

A sudden rise in voltage for a very short duration system is known as a voltage surge or transient voltage.

Transients or surges are of temporary nature and exist for a very short duration (a few hundred s) but they cause over voltages on the power system. They originate from switching and from other causes but by far the most important transients are those caused by lightning striking a transmission lone. When lightning strikes a line, the surge rushes along the line, just as a flood of water rushes along a narrow valley when the retaining wall of a reservoir at its head suddenly gives way. In most of the cases, such surges may cause the line insulators (near the point where lightning has struck) to flash over and may also damage the nearby transformers, generators or other equipment connected to the line if the equipment is not suitable protected.

Figure 9.1: surge voltage

Figure 9.1 shows the waveform of a typical lightning surge. the voltage build-up is taken along y-axis and the time along x-axis. It may be seen that lightning introduces a steep-fronted wave. The of the cases, this build-up is comparatively rapid, being of the order of 1-5 s. Voltage surges are generally specified in terms of rise time t1 and the time t2 to decay to half of

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the peak value. For example, a 1/50 s surge is one which reaches its maximum value in 1s and decays to half of its peak value is 50 us.

9.3 Causes of over voltages

The over voltages on a power system may be broadly divided into two main categories viz.

1. Internal causes

(i) Switching surges (ii) Insulation failure

(iii) Arcing ground (iv) Resonance

2. External causes i.e. lightning

Internal causes don not produce surges of large magnitude. Experience shows that surges due to internal causes hardly increase the system voltage to twice the normal value. Generally, surges due to internal causes are taken care of by prodding proper insulation to the equipment in the power system. However, surges due to lightning are very severe and may increase the system voltage to several times these surges may cause considerable damage. In fact, in a power system, the protective devices provided against over voltages mainly take care of lightning surges.

9.4 Internal causes of over voltages

Internal causes of over voltages on the power system are primarily due to oscillations set up by the sudden changes in the circuit conditions. This circuit change may be a normal switching operation such as opening of a circuit breaker, or it may be the fault condition such as grounding of a line conductor. In practice, the normal system insulation is suitably designed to withstand such surges. We shall briefly discuss the internal causes of over voltages.

1. Switching Surges. The over voltages produced on the power system due to switching surges. A few cases will be discussed by way of illustration:

Case of an open line. During switching operations of an unloaded line, traveling waves are set up which produce over voltages on the line. As an illustration, consider, consider an unloaded line being connected to a voltage source as shown in Fig. 24.2.

When the unloaded line is connected to the voltage sources, a voltage wave is set up which travels along the line. On reaching the terminal point A, it is reflected back to the supply end without change of sign. This causes voltage doubling i.e. voltage on the line becomes twice the normal value. If Er.m.s. is the supply voltage, then instantaneous voltage which the line will

have to withstand will be 2√2 E . This over voltage is of temporary nature. It is because the line

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losses attenuate the wave and in a very short time, the line settles down to its normal supply

voltage E. Similarly, if an unloaded line is witched off, the line will attain a voltage of 2√2 E for a moment before settling down to the normal value.

9.5 Lightning:

An electric discharge between cloud and earth, between clouds or between the charge centers of the same cloud is known as lightning.

9.6 Protection against Lightning:

(i) Earthing screen

(ii) Overhead wires

(iii) Lightning arresters or surge diverters

9.7 The Earthing Screen:

The power stations and sub-stations generally house expensive equipment. These stations can be protected against direct lightning strokes by providing earthing screen. It consists of a network of copper conductors (generally called shield or screen) mounted all over the electrical equipment in the sub-station or power station. The shield is properly connected to earth on atleast two points through a low impedance. On the occurrence of direct stroke on the station, screen provides a low resistance path by which lightning surges are conducted to ground. In this way, station equipment is protected against damage. The limitation of this method is that it does not provide protection against the travelling waves which may reach the equipment in the station.

9.8 Overhead Ground Wires:

The most effective method of providing protection to transmission lines against direct lightning strokes is by the use of overhead ground wires as shown in Fig. 24.7. For simplicity, one ground wire and one line conductor are shown. The ground wires are placed above the line conductors at such positions that practically all lightning strokes are intercepted by them (i.e.

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ground wires). The ground wires are grounded at each tower or pole through as low resistance as possible. Due to their proper location, the ground wires will take up all the lightning strokes instead of allowing them to line conductors.

9.9 Lightning Arresters:

A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground.

Figure 9.2:Basic surge absorber

Fig. 9.2 (i) Shows the basic form of a surge diverter. It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in Fig. 9.2 (ii).

Action. The action of the lightning arrester or surge diverter is as under:

(i) Under normal operation, the lightning arrester is off th line i.e. it conducts no current to earth or the gap is non-conducting.

(ii) On the occurrence of overvoltage, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground. In this

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way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.

(iii) It is worthwhile to mention the function of non-linear resistor in the operation of arrester. As the gap sparks over due to overvoltage, the arc would be a short-circuit on the power system and may cause power-follow current in the arrester. Since the characteristic of the resistor is to offer high resistance to high voltage (or current), it prevents the effect of a short-circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.

Two things must be taken care of in the design of a lightning arrester. Firstly, when the surge is over, the arc in gap should cease, if the arc does not go out, the current would continue to flow through the resistor and both resistor and gap may be destroyed. Secondly. I R drop (where I is the surge current) across the arrester when carrying surge current should not exceed the breakdown strength of the insulation of the equipment to be protected.

9.10 Surge Absorber:

A surge absorber is a protective device which reduces the steepness of wave front of a surge by absorbing surge energy.

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Figure 9.3: surge absorber

Chapter: 10

SWITCHING OVERVOLTAGES

10.1 Introduction :

Among the most common reasons for dielectric failures in an electric system, aside from lighting strikes, are the over voltages produced by the switching that is normally required for the ordinary operation of the electrical network.

Switching over voltages can be produced by closing an unloaded line, by opening an isolating switch, or by interrupting low currents in inductive or capacitive circuits where the possibility of restrikes exists.

Switching over voltages are probabilistic in nature and their appearances in a system depend mainly upon the number of faults that must be cleared on a line and on how frequently

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routine switching operations are performed on a particular system. This implies that not only opening operations that are intended for interrupting a short circuit current are responsible for switching over voltages; but also the many routine operations that are performed, sometimes daily, in a system. These routine operations are fully capably of producing over voltage effects by virtue of them altering the system configuration.

As it has been said repeatedly, overvoltages in transimission and distribution sysmes cab not be totally avoided, but their effects can be minimized. Generally the occurrence and the magnitude of the over voltage can be limited by the use of appropriate measures such as the use of series or parallel compensation, closing resistors, surge suppressors; such as metal oxide varistpors, or sunbbers containing combinations of resistors and capacitors, and in some cases by simply following basic established procedures for the proper design and operation of a system.

It is Appropriate at this point to emphasize that although circuit breakers participate in the process of over voltage generation they don not generated these voltages, but rather these voltages are generated by the system. Circuit break surges. They can do so either by timing controls or by incorporation additional hardware such as closing resistor as an integral part of the circuit breaker design.

10.2Contacts closing :

The simple closing of a switch or a of a circuit breaker cab produce significant overvoltages in an electric system. These overvoltages are due to the system adjusting itself to an emerging different configuration of components as a result of the addition of a load impedance. Furthemore, there are changes that are trapped in the lines and in the equipment that is connected to the system and these charges now must be re-distributed within the system.

In additin, and whenever the closure of the circuit occurs immediately after a circuit breaker opening operation the trapped charges left over from the preceding opening cab significacantly contribute to the increase in the magnitude of the overvoltages that may appear in the system. It is important to note that of the overvoltages that may appear in the system. It is important to note that preceding opening cam significantly contribute to the increase in the magnitude of the overvoltages that may appear in the system. It is important to note that of the overvoltages will be produced by the fast reclosing of in most cases the highest overvoltage swill b produced by fast reclosing of a line. It should also be realized that the higher magnitudes of the over voltage produced by the closing or the reclosing operation of a circuit breaker will always be observed at the open and end of the line.

Although the basic expressions describing the voltage distribution across the source and the line re relatively simple, defining the effective impedance that controls the voltage

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distribution within the elements of the circuit is rather that controls the voltage distribution within the elements of the circuit is rather difficult and generally can only be adequately handled with the aid of a computer.

Because of the complexity of the problem no attempt will be made here to provide a quantitative solution. The aim of this chapter will be to describe qualitatively the voltage surges phenomena that take place during a closing or a reclosing operation, and during some special cases of current interruption. The upper limits of over voltages that have been obtained either experimentally or by calculation will be quoted but only as general guidelines.

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10.3 Closing of a Line :

A cable that is being energized from a transformer represents the simplest case of a switching operation as is shown in figure 10.1 (a).For the sake of simplicity, the transformer has been represented by its leakage inductance; while the cable is represented by its capacitance. As a result of this simplification the equivalent circuit cab take the from of the circuilt illustrated in figure 10.1 (b).

The transient voltage, shown in figure 10.1 (c),oscillates along the line at a relatively low single frequency and has an amplitude that reaches a peak value approximately equal to twice the value of the system voltage that was present at the instant at which the closure of the circuit took place.

Although the above described circuit may be found in some very basic applications, in actural practice in is more likely to expect that a typical system will consist of a one or more long interconnected overheat lines, as depicted in figure 10.2 (a). The equivalent circuit, and the transient response of this system. is shown in figure 10.2 (b) and (c) respectively. The transient response. As cab be seen in the figure, is determined by the combined impedance of the transformer

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Figure:10.1 Representation of the simplest case of closing into a line. (a) Single line schematic, (b) Equivalent circuit and (c) Transient surge.

Figure 10.2 :Switching surge resulting from energizing a complex system. (a) Single line schematic of the system, (b) Equivalent circuit and (c) Surge voltage.

that is feeding the system and by the total surge impedance of the connected lines. The total surge impedance, as it cab be recalled, is equal to the surge impedance of each individual line divided by the number of connected lines.

The overvoltage factor for the source is given by the following equation.

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k s=1

cos2πf √LCIX s

Zsin2πf√LCI

Where:

f= Power frequency

L= Positive sequence inductance per length of line

C= positive sequence capacitance pear length of line

I = line length

Xs=Short circuit reactance of source

Z= surge impedance of the line

It is evident, by simply observation of the above equation, that a higher power frequency overvoltage factor can be expected as a result of the following occurrences:

1. When the length of the lines increase

2. when the source reactance increases

3. When the source reactance increases increased number of connected lines and

4. when the power frequency is increased , which means that the overvoltage is higher in a 60 Hz system than in a 50 Hz one.

The overvolate factor for the transient response portion of the phenomeana is not as easy to calculate manually and a simple formula as in the preceding paragraph is just not available. However, it is possible to generalize and it can be said that the overvoltage factor for the transient response is proportional to:

1. the instantaneous voltage difference between the source voltage and the line voltage as the contacts to of the circuit breaker close,

2. the damping impedance of the lines connected at the source side of the circuit and

3. the terminal impedance of the unloaded line/lines being energized side of the

In any case what is important to remember is:

1. When switching a number of lines the amplitude factor of the overvoltage is always reduced as the size of the system increases, and

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2. The reduction of the amplitude factor is not due to the damping effects of the system but rather to the superposition of the individual responses each having a different frequency.

10.4 Reclosing of a Line

Since in order to improve the stability of the system it is desirable to restore service as quickly as possible, it is a common operating practice to recluse a circuit breaker a few cycles after it has interrupted a fault.

If the interrupted fault happens to be a single phase to ground fault, then it is possible that a significant voltage may remain trapped in the unfaulted phases. This happens because the three phases represent a capacitor tht has been switched off at current zero and therefore, because of the inductive nature of the system, this coincides with the instant where a maximum voltage is present in the line.

Since the closing of the contacts may take place at any point in the voltage wave, it could then be expected that when reclosing the circuit, the circuit breaker contacts may close at the opposite polarity of the trapped change, which, when coupled with the voltage doubling effect produced by the traveling wave, leads to the possibility of a overvoltage across the contacts that cab reach a magnitude as high as 4 per unit.

10.5 Contact opening

The opeining of a circuit was previously discussed in the context of interrupting a large magnitude of current where that current was generally considered to be the result of a short circuit. However, there are many occasions where a circuit breaker is required to interrupt currents that are in the range of a few amperes to several hundred amperes, and where the loads as characterized as being either purely capacitive or purely inductive.

The physics of the basic interrupting process; that is the balancing of the are energy is no different whether the interrupted currents are small or large However, since lower currents will contribute less energey to the are is natural to expect that interrupting these lower currents would be a relatively simpler task; but, this is not always the case because, as it will be shown later, the very fact that the currents are relatively low in comparison to a short circuit current promotes the possibility of restrikes occurring across the contacts during interruption. Those restrikes cab be responsible for significant increases in the magnitude of the recovery voltage.

According to standard established practice, a restrict is defined as being an electrical discharge that occurs one quarter of a cycle or more after the initial current interruption. A resignation is defined as a discharge that occurs not later than one eight of a cycle after current zero.

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Figure 10.3: Recovery voltage resulting from the switching capacitor banks

10.6 Interruption of small capacitive Currents:

The switching of capacitor banks and unloaded lines requires that the circuit breaker interrupts small capacitive currents. These currents are generally less than ten amperes for switching unloaded lines and most often less than one thousand amperes for switching off capacitor banks.

Interruption, as always, takes place at current zero and therefore the system voltage, for all practical purposes is at its peak. This as it should be recalled makes current interruption relatively easy but, again as it was said before, this gap between the circuit breaker contacts is very short and consequently, a few milliseconds later as the system recovery voltage appears across the circuit circuit breaker tow withstand the recovery voltage.

At the time when current interruption takes place theline to ground voltage stroted in the capacitor in a solidly grounded circuit is equal to 1.0 per unit.

The source side, in the other hand will follow the oscillation of the power frequency voltage and therefore in approximately one half of a cycle the voltage across the contacts would reach its peak value, but, with a reversal of its polarity. At this time then the total voltage across

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the contacts reaches a value of 2.0 per unit which corresponds to the algebraic sum of the capacitor voltage charge and the source voltage as is shown in figure 10.3

If the circuit has an isolated neutral connection then the voltage trapped in the capacitor, for the first phase to clear, has a line to ground value of 1.5 per unit and the total voltage across the contacts one half of a cycle later will then be equal to 2.5 per unit.

Restries can be thought as being similar to a closing operation where the capacitor is suddenly reconnected to the source, and therefore it is expected that there will be a flow of an inrush current which due to the inductance of the circuit absence of any damping effects will force the voltage in the capactitor to swing with respect to the instantaneous system voltage to a peak value that is approximately equal to the intital value at which it started but with a reversed polarity. If the restrike happens at the peak of the system voltage, twhen the capacitor voltage will attain a charge value of 3.0 per unit under these conditions, if the high frequency inrush current is interrupted at the zero crossing, which some circuit breakers are capable of doing so, then the capacitor will be left with a charae corresponding to a voltage of 3.0 per unit and one half of a cycle later there will be a voltage of 4.0 per unit applied across the circuit breaker contacts. It the sequence is repeated, the capacitor voltage will reach a 5.0 per unit value, as is illustrated in figure 4.4. theoretically, and if damping is ignored, the voltage across the capacitor cab build up according to a series of 1, 3, 5, 7,…….. and so on without limit.

10.7 Interruption of Inductive Load currents:

When a circuit breaker that has an interrupting capability of several tens of kiloamperes is called upon to itnerrup inductive load currents that are generally in the range of a few tens to some hundereds of amperes, as for example in the case of are furnace swithching, those currents are interrupted in a normal fashion, that is at current zero. However, and again due to the high interruption with the interruption of small capacitive currents.

At the time of interruption the gap between the contacts may be very short, and since the voltage is at its peak, then in many cases the small gap may not be sufficient to withstand the fulmagnitude of the revovery voltage which begins to appear across the contacts immediately following the interruption of the current. As a consequence the are may restike resulting in a very steep volage chang and in significant overvoltages.

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Figure10.4: Voltage escalation due to restrikes during a capacitance switching operation.

However, because of the randomness of the point at which the restikes take place and due to the ingerent damping of the circuit, it is very unlike that the upper limit of these ovedrvoltages will exceed a value of 2 per unit.

There are however special cases that arise when a circuit breaker has ex-generated by a reignition or a restike. Whenever the high frequency current is interrupted the normal power frequency recovery voltage reappears across the contacts and in some cases it is possible that a restike may occur again.

During the interval between the tw reignitions the contact have moved thus increasing the gap distance and therefore a higher breakdown voltage is to be expected. Nevertheless, during this interval more magnetic energy is accu available to trigger a breakdown which would occur at a voltage that is higher than the previous one. This process may repeat itself as successive reignitions occur across a larger gap and at increased magnetic energy levels, and

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Figure10.5: Voltage surges caused by successive reignitions when interrupting low inductive currents.

therefore, at higher mean voltage levels resulting in a high frequency series of volatage spikes such as those shown in Figure 10.5.

Because of the statistical nature of this phenomenon it is not possible to establish an upper limit for the overvoltage; however, it is advisable to be aware of the potential risk and to use protective devices such as surge arrestors.

It is commonly believed that only vacuum circuit breakers are cappble of chopping currents. However, this is not the case, all types of circuit breakers cab chop. Nevertheless, what is different is that the instantaneous current magnitude at which the chopping occurs varies among the different type of interrupting mediums and indeed it is higher for vacuum interrupters.

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Figure10.6: Typical Current Chopping. (a) Equivalent circuit, (b) Chopped current across the breaker and (c) Transient voltage across the breaker.

In theory, when current chopping occurs the current is reduced instantaneously from a small finite value to zero, but, in reality this does not happen so suddenly simply because of the inductance that is present in the circuit and as it is well known, current cab not change instantaneously in an inductor. It is therefore, to be expected that some small finite element of time must elapse for the transfer of the magnetic energy stored in the load the transfer of the magnetic energy that is trapped in the system inductance.

At the instant when current chopping occurs the energy stored in the load inductance is transferred to the load side capacitance and thus creating a condition where overvolatage scab be generated. In figure 4.6 (a) the simplified equivalent circuit is shown and in (b) the voltage and current relationships are illustrated.

Referring to the equivalent circuit the energy balance equations can be written as:

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12

CEm2 =1

2CEc

2+ 12

LI02

and the overvoltage factor K is given by:

D=Em

Es=√( I0

E s )( LC )+ Ec

E s

Where:

Em = Overvoltage peak

E0 = Peak voltage at supply side

Ec= Capacitor voltage at instant of chop

E0= Instantaneous value of chopped current.

LC = Surge impedance of the circuit

As it cab be seen, the magnitude of the overvoltage factor K is highly dependent upon the instantaneous vale of the chopping current.

10.8 Current Chopping in circuit Breakers other than vacuum :

For air, oil, or SF6 interrupters, the are instability that leads to current chopping is primarily controlled by the capacitance of the system. the effects of the system capacitance on the chopping level are illustrated in figure 10.7 . The effects of the capacitance on vacuum interrupters is also included in this figure for comparison purposes.

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Figure10.7: Current Chopping Level as Function of Systme Capacitance for Minimum oil Circuit Breakker (Mocb), SF6 Gas Circuit Breakers (GCB), Air Blast Circuit Breakers (ABCB), and Vacuum Circuit Breakdrs (VAC)

Oil Circuit Breakers (MOCB), SF6 Gas Circuit Breakers (GCB), Air Blast Circuit Breakers (ABCB), and Vacuum Circuit Breakers (VAC).

For gas or oil circuit breakers the approximate value of the chipping curren is given by the formula

I0 = λ√CL

Where:

= Chopping number

The following are typical values for chopping numbers:

For Minimum Oil circuit breakers 7 to 10 104

For Air Blast circuit breakers 15 to 40 104

For SF6 circuit breakers 4 to 17 104

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The values of the system capacitance cab be assumed to be in the range of 10 to 50 nano-farads.

10.9 Current Chopping in Vacuum Circuit Breakers:

In contrast to other types of circuit breakers the current instability in vacuum interrupters is not strongly influenced by the capacitance of the system (see figure 10.7), but is dependent upon the material of the vacuum contacts and by the action of the anode spot created by the vacuum are. There is no chopping number for Vacuum interrupters but, instead the chopping current itself cab be specifed as follows:

For Copper-Bismuth contacts current chopping 5 to 17 Amperes

For chrome copper contacts current chopping 2 to 5 Amperes

10.10 Virtual Current Chopping

Virtual current chopping in reality is not a true chopping phenomenon but rather its is the normal interruption of a fast transient current. Virtual chopping is a phrase that has been coined to describe the condition illustrated in the simplified circuit shown in figure 10.8

Referring to this figure, the power frequency currents are shown as IA, IB and Ic Assuming that for example, a current reignition occurs shortly after the interruption of the power frequency current in phase a, the reignition current ia will then flow to ground through the line to ground capacitance Cg in the load side of the breaker in in phase A and the components ib and ic flow in phases B and C due to the coupling of their respective line to ground capacitances.

The high frequency transient current produced by the regnition superimposes itself on the power frequency; furthermore, the high frequency current could be larger in magnitude that the power frequency current and therefore it can be larger in magnitude than the power frequency current and therefore it cab force curren zeroes at times other than those expected to occur normally with a 50 or 60 Hz current.

At it has been stated before there a some types of circuit breakers which are capable to interrupt thse high frequency currents, and therefore it is possible to assume \that in some cased the circuit breaker may clear the circuit breaker may clear the circuit at a current zero corsiing tha thas been forced bythe high frequency current and power frequency current. When this happens, as far as the load is concerned, it looks the same as if the power frequency current has been chpped since a sudden current zero has been forced.

Since the high frequency current zeroes will occur at approximately the same time in all there phases the circuit breaker may interrupt the currents in all there phases simultaneously thus

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giving rise to a very complicated sequence of voltag transients that may even include reignitions in all three phases.

Considering that, when compared in a "normal" current chopping, we find that the instantaneous value of current, from which the load current is forced to

Figure10.8: Virtual current chopping (a) Circuit showing the flow of the induced currents. (b) Relationships between the three phase currents

Zero is significantly higher but, also that the surge impedance is somewhat lower, then the line to ground over voltage could be assumed to be at about the same order of magnitude as the overvoltages that are generated by the conventional current chopping; however, in heworst case, if the neutral is un grounded one half of the reignition current would return through each of the other two phases being twice their corresponding line to ground overvoltage.

10.11Controlling Overvoltages :

Circuit breakers themselves do not generate over voltages, but they do initiate them by changing the quiescent conditions of the circuit. As it has been stated before the switching over voltages are the result of two over voltage components the power frequency over voltage, and the transient over voltage component. Limiting the magnitude of the first is usually sufficient to within acceptable limits. However, this does not exclude the possibility

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of using appropriate measures to additionally limit the magnitude of the over voltage by limiting the transient response.

Among the measures that can be taken to reduce the magnitudes fo the power frequency overvoltages are:

(a) Provide polarity controlled closing

(b) Add closing and or opening resistors across the breaker contacts

(c) Provide a method combining polarity control and closing resistor

(d) Add parallel compensation

(e) Reduce the supply side reactance

The transient overvoltage factor can be controlled by:

(a) removing the trapped charges from the line sysnchronized closing which be accomplished either by closing a voltage zero of the supply side or by matching the polarity of the line and the supply side.

(c) Synchronized opening which optimized the contact gap at current zero using pre-insertion resistors

From all the listed alternatives only resistors can be considered to be an integral part of a circuit breaker. The practice of including closing resistors as part of a circuit breaker is relatively common for circuit breakers intended for applications at voltages above 123 kV.

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Reference:1. Element of Power System Analysis, Fourth EditionWilliam D. Stevenson2. Power System AnalysisHadi Saadat3.Power System Analysis And Design ,Fourth EditionGlover,Sarma,Overbye4.Principal of Power SystemV.K.Mehta,Rohit Mehta5.High Voltage Circuit BreakersRuben D.Grazon6.Switchgear Protection And Power SystemSunil S.Rao