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EE2303 - Transmission & Distribution Page | 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI- 621213.

QUESTION BANK DEPARTMENT: EEE SEMESTER – V SUBJECT CODE: EE2303 SUBJECT NAME: Transmission & Distribution

UNIT - 1

PART-A

1. Why all transmission and distribution systems are 3 phase systems?

A 3 phase a.c circuit using the same size conductors as the single phase circuit can carry

three times the power which can be carried by a 1 phase circuit and uses 3 conductors for the 2

phases and one conductor for the neutral. Thus a 3 phase circuit is more economical than a 1

phase circuit in terms of initial cost as well as the losses. Therefore all transmission and distribution

systems are 3 phase systems.

2. Why the transmission systems are mostly overhead systems?

Because of the cost consideration, the transmission systems are mostly overhead systems.

3. Why all overhead lines use ACSR conductors?

ACSR conductors comprises of hard drawn aluminium wires stranded around a core of

single or multiple strand galvanized steel wire. They provide the, necessary conductivity while the

steel provides the necessary mechanical strength. Has less corona loss. The breaking load is high

and has less weight.

4. Why overhead line conductors are invariably stranded?

They are stranded to make them flexible during erection and while in service.

5. State the advantages of interconnected systems. ?

Any area fed from one generating station during overload hours can be fed from another

power station and thus reserved capacity required is reduced, reliability of supply is increased and

efficiency is increased.

6. Define resistance of the transmission line?

It is defined as the loop resistance per unit length of the line in a single phase system. In 3

phase system it is defined as the resistance per phase.


7. What are the advantages of high voltage ac transmission?

The power can be generated at high voltages.

The maintenance of ac substation is easy and cheaper.

8. Mention the disadvantages of high voltage ac transmission?

An ac line requires more copper than a dc line.

The construction of an ac line is more complicated than a dc transmission line.

Due to skin effect in the ac system the effective resistance of the line is increased.

9. Mention the limitations of using very high transmission voltage?

The increased cost of insulating the conductor.

The increased cost of transformers ,switch gears and other terminal apparatus.

10. Mention the terminal equipments necessary in HVDC system?

Converters, mercury arc valves and thyristor.

Due to absence of charging currents.

11. Mention the equipments that supply reactive power in HVDC converter stations?

AC filters Static shunt capacitors Synchronous condensers StaticVAR compensators.

12. Why dc transmission is economical and preferable over ac transmission for large

Distances only?

Because with larger distances, the saving in cost of dc overhead lines become greater than

the additional expenditure on terminal equipment.

13. Define Sag?

In order to permit safe tension in the conductors, they are not fully stretched but are allowed

to have a dip or sag.

The difference in level between points of support and the lowest point on the conductor is

called sag.

14. Conductor sag should be kept to a minimum. Why?

To reduce the conductor material required

To avoid extra pole height.

15. What are the factors affecting the sag?

1. Weight of conductor per unit length

2. Loading due to ice and wind

3. Span length


4. Surrounding temperature

5. Conductor tension

16. Define safety factor.

S=Breaking stress/Working stress

17. What is breaking stress?

If tension is increased beyond the ultimate strength, mechanical failure of conductor occurs.

This ultimate stress is called as breaking stress.

Stress in Kg=kg/meter square*Area of conductor

(PART- B)

1.Explain about FACTS and its Controllers with neat diagram?

Flexible A.C. Transmission system is a new technology by which capacity of transmission

lines can be enhanced. The essential ingredients in FACTS are the rapid and precise switching in

and out of capacitor banks. This is made possible by the use of solid state switches such as

thyristors. This can influence the impedance of a line and power can be routed within a system and is

between system.

SERIES COMPENSATION (TCSC)

• Increase power transmission capability.

• Improve system stability.

• Reduce system losses.

• Improve voltage profile of the lines.

• Optimize power flow between parallel lines.

Thyristor-controlled series capacitors:

(TCSC) is also a type of series compensator, can provide many benefits for a power system including

controlling power flow in the line, damping power oscillations, and mitigating sub synchronous

resonance.

The TCSC concept is that it uses an extremely simple main circuit. The capacitor is inserted

directly in series with the transmission line and the thyristor-controlled inductor is mounted directly in

parallel with the capacitor. Thus no interfacing equipment like e.g. high voltage transformers is

required. This makes TCSC much more economic than some other competing FACTS technologies.

Thus it makes TCSC simple and easy to understand the operation.


World’s first 3 phase, 2 X 165 MVAR, TCSC was installed in 1992 in Kayenta substation,

Arizona. It raised the transmission capacity of transmission line by 30%, but it was soon realized that

the device is also a very effective means for providing damping of electromechanical power

oscillations. A third possible application of TCSC emerged from the on site observations that it can

provide series compensation without causing the same risk for sub-synchronous resonance (SSR) as

a fixed series capacitor. World’s first TCSC for subsynchronous resonance (SSR) mitigation was

installed in Stode, Sweden in 1998, by ABB. Specifically this period makes a valiant period for TCSC

and makes the researchers to turn on to TCSC.

The main purpose of this paper is to furnish a concise study of TCSC in simple way. Section

II, brings out the operation of TCSC along with numerical equations. Section III gives an impedance

characteristics curve of a TCSC device and specifies the range of inductance and capacitance region.

In section IV and V deals, condition for single & multiple resonance points theoretically and evaluate

by simulation. Also finds a suitable value of inductance and capacitance. Section VI analyzes the

different waveforms in the capacitive region of TCSC with Simulink model. Finally it carries some of

additional benefits of TCSC device along with power system in last section.

II. OPERATION OF TCSC

The basic operation of TCSC can be easily explained from circuit analysis. It consists of a series

compensating capacitor shunted by a Thyristor controlled reactor (TCR). TCR is a variable inductive

reactor XL (figure 2) controlled by firing angle α. Here variation of XL with respect to α is given by


For the range of 0 to 90 of α, XL(α) start vary from actual reactance XL to infinity. This controlled

reactor is connected across the series capacitor, so that the variable capacitive reactance (figure 3) is

possible across the TCSC to modify the transmission line impedance. Effective TCSC reactance

XTCSC

With respect to alpha (α) is,

III. IMPEDANCE CHARACTERISTIC

Figure 4 shows the impedance characteristics curve of a TCSC device. It is drawn between effective

reactance of TCSC and firing angle α


Net reactance of TCR, XL(α) is varied from its minimum value XL to maximum value infinity. Likewise

effective reactance of TCSC starts increasing from TCR XL value to till occurrence of parallel

resonance condition XL(α) = XC, theoretically XTCSC is infinity. This region is inductive region.

Further increasing of XL(α) gives capacitive region, Starts decreasing from infinity point to

minimum value of capacitive reactance XC.

Thus, impedance characteristics of TCSC shows, both capacitive and inductive region are

possible though varying firing angle (α).

From 90 < α < αLlim

Inductive region.

αLlim < α < αClim

Capacitive region

Between αLlim < α < αClim

Resonance region

While selecting inductance, XL should be sufficiently smaller than that of the capacitor XC.

since to get both effective inductive and capacitive reactance across the device.

Suppose if XC is smaller than the XL, then only capacitive region is possible in impedance

characteristics. In any shunt network, the effective value of reactance follows the lesser reactance

present in the branch. So only one capacitive reactance region will appears.

Also XL should not be equal to XC value; or else a resonance develops that result in infinite

impedance – an unacceptable condition. Note that while varying XL(α), a condition should not allow

to occur XL(α) = XC.

UNIFIED POWER FLOW CONTROLLER (UPFC)

A Unified Power Flow Controller (or UPFC) is an electrical device for providing fast-acting

reactive power compensation on high-voltage transmission networks.

The UPFC is a versatile controller which can be used to control active and reactive power flows in a

transmission line.

The concept of UPFC makes it possible to handle practically all power flow control and

transmission line compensation problems, using solid state controllers, which provide functional

flexibility, generally not attainable by conventional thyristor controlled systems.


Single line diagram of UPFC

UPFC Construction

The UPFC consists of two voltage source converters; series and shunt converter, which are

connected to each other with a common dc link.

Series converter or Static Synchronous Series Compensator (SSSC) is used to add controlled

voltage magnitude and phase angle in series with the line.

Shunt converter or Static Synchronous Compensator (STATCOM) is used to provide reactive

power to the ac system, beside that, it will provide the dc power required for both inverter

Each of the branches consists of a transformer and power electronic converter

The two voltage source converters share a common dc capacitor

The energy storing capacity of this dc capacitor is generally small. Therefore, active power drawn

by the shunt converter should be equal to the active power generated by the series converter.

The reactive power in the shunt or series converter can be chosen independently, giving greater

flexibility to the power flow control. The coupling transformer is used to connect the device to the

system.

The series inverter is controlled to inject asymmetrical three phase voltage system, vse, of

controllable magnitude and phase angle in series with the line to control active and reactive power

flows on the transmission line. So, this inverter will exchange active and reactive power with the line.

The reactive power is electronically provided by the Series inverter and the active power is

transmitted to the dc terminals. The shunt inverter is operated in such


a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage

across the storage capacitor Vdc constant.

So, the net real power absorbed from the line by the UPFC is equal only to the losses of the two

inverters and their transformers. The remaining capacity of the shunt inverter can be used to

exchange reactive power with the line so to provide a voltage regulation at the connection point.

2.Draw and explain the structure of modern power systems with typical voltage level

The transmission and distribution of electrical energy s important part of power system. The

transmission system of an area is called grid. The different grids are inter connected through tie lines

to form a regional grid. The regional grid are further inter connected to form a nation grid.

The maximum generation voltage is 33 Kv and in India it is 11 kV .the amount of power that has to be

transmitted is very large and if this power is to be transmitted at 11 kv the line current and power

losses would be very large. Therefore the voltages are steeped up to a higher levels say 66kv 110kv,

132 kV, 220/230kv and 400kv. At the receiving end substation the voltages are stepped down to 66,

33,11kv. The secondary transmission system forms a link between main receiving end substation and

the secondary substation.

At the secondary sub station the voltage are further reduced to 11, 3.3kv and is fed to

primary distribution system.

The distribution sub station consists of a ste3p down transformer which are usually placed

on the road side. In large multi stored building the distribution transformer are located within the

building itself. These transformer steps down voltage to 440v .the 400v distribution line are laid along

the road and services connections to the consumers are tapped off form the distributors.Every power

syst6em need not necessarily have all parts shown in fig. in some case there is only one4 level of

transmission and the secondary transmission do0es not exist. In such case the feeders directly take

off from the main sub station .some large consumers are supplied at 132 kv and 66 kv directly.

The figure given below shoes the structure of the power system network.


3.(i)What are the different kinds of DC links? Draw relevant diagrams(8)

(ii)Compare EHVAC and HVDC transmission (8)

D.C.(Advantages) A.C.

Line construction is simple and cheaper Line construction complex

Power transmitted per conductor is more

hence lesser number of conductor is

required.

More conductors required.

Charging currents is totally absent. Hence

no distance limit

Charging current increases with

length of line, which imposes a

limit on distance

Current is uniformly distributed Due to skin effect current is concentrated to

perifery

Reactive power compensation is not

required. Line operation at unity p.f

charging currents are absent. Line drop is

purely resistive.

Long distance transmission is possible only

if reactive compensation is done.

Corona loss in proportional to (f+25).

Hence for D.C. corona loss in less.

Corona loss is more

Switching surges are less Switching surges are more


Stability problems do not arise since it is in

a synchronous operation of the generators

Due to stability problem distance is limited.

Contribution of D.C. Line to short circuit

current is less.

In AC system short current is more

Power control is fast and accurate since

there is less inertia due to the absence of

rotating parts

Power control is slow

Unlimited power can be transmitted

Disadvantages

The converters required at both ends are

very expensive. Hence D.C lines are

economically viable only of the distance of

the line is more?

Investment in terminal equipment is less.

The converters absorb reactive power which

must be supplied locally. Since D.C. blocks

the transmission of reactive VA. the

receiving end must be capable of supplying

the whole of reactive component of power

required by load.

No such problem

Suitable for point to point transmission only Suitable for inter connected system

4.Derive an expression for sag calculation in transmission line

(i) When the supports are at equal heights(8)

catenary Method a span of conductor with two supports at the same level and separated by a

horizontal distance L. Let ‗0‘ be the lowest point on the catenary curve and ‗l‘ be the length of the

conductor between two supports. Let ‗w’ be the weight of the conductor per unit length, ‗T‘ be the

tension of the conductor at any point ‗P’ in the direction of the curve, and H be the tension at origin 0.

Further, let ‗s’ be the length of the curve between points 0 and P, so that the weight of the portion s is

ws. Tension T can be resolved into two components, T the horizontal component and T,, thc vertical

component. Then, for equilibrium,

Thus, the portion OP of the conductor is in equilibrium under the tension T at P. the weight

ws acting vertically downward, and the horizontal tension H.In the triangle shown in Figure 2, ds


represents a very short portion of the conductor, in the region of point P When s is increased by ds

the corresponding x and y are increased by dx and dy, respectively.Hence,

Since

Then

Therefore

Integrating on both side

Therefore,


Or

For approximately

Or

Integrating on both side

If the lowest point of the curve is taken as the orgin,when X=0,Y=0

Since by the series cosh0=1,therefore


(ii)When the supports are at un equal heights(8)

Consider a span L between two supports, whose elevations differ by a distance h.

Let the horizontal distance from the lowest point of the curve to the lower and the higher support.

(d1 and d2 sag can be found as )

And

Therefore

Or

Or

Since

Therefore


By adding the above two equation

In this equation,

If X1 is negative, the lowest point(the point 0) of the imaginary curve lies outside the actual span.

mahalakshmi 1.pdf · question bank department: ... what are the advantages of high voltage ac...

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