power systems protection course - bau.edu.jo · calculate the plug setting and time multiplier...

Power systems Protection

course

Department of Electrical Power Energy Engineering Dr Audih alfaoury 1

Al-Balqa Applied University

Overcurrent Protective Relays

2 Dr Audih alfaoury

Part 6

Overcurrent relays

Is the simplest and cheaper type of protection used for lines,

transformers, generators and motor.

Overcurrent relays Types:-

Based on operating time characteristics, normally defined by the

time vs. current curve (or T-I curve), there are three main types :

Instantaneous

Time-dependent ( Definite time or Inverse time)

Mixed ( Definite time + Inverse time)

1.Instantaneous Overcurrent Relays(50,50N) These relays operate without time delay, so they are called

instantaneous units (operating time= 0.1 second).

The simplest form of these relays are the magnetic attraction

types, Solid- State, digital and numerical over-current relays .

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Instantaneous Overcurrent Relays operate without time delay, ( 0.1s)

The pickup current or threshold is adjustable **.

Pickup Current Setting

♣ Taps in the Relay Current Coil.

♣ Air-Gap Adjustment.

♣ Spring Adjustment.

Features of Electromechanical 50 Elements

♣ Suitable for AC and DC Systems.

♣ Operation Time Less Than 3 Cycles

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**smallest detectable value of current that will be operate the relay.

Instantaneous Curve (50)

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Operation zone

of relay

Un operation

zone due 3

cycles

2. Time Overcurrent Relays

Operate with a time delay.

Time delay is adjustable.

Pick up current is also adjustable.

There are five different types of time over-current relays

(depends on their time-current characteristic curves) :-

Definite time (DT) or (precise time).

Inverse-time :-

Moderately inverse

Inverse (Normal)

Very inverse

Extremely inverse

2.1 Definite-Time Overcurrent Relays (DT)

The definite-time relay operates with some delay this delay is

adjustable as well as the current threshold.

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Time over-current relays operate with a time delay

2.2 Inverses-Time

This type of relay have an operating time depending

on the value of the current, generally with an inverse

characteristic,(increasing time decreasing current or

voice versa)

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The operation time of is smaller as the current gets larger

The operation time

of the relay is smaller

as the current gets

larger (small value

of time compared with

current value).

Also have two settings:

1- pick-up current and

2- curve level.

In early electromechanical relays the curve is set by

means of a dial. Thus, the setting is called the “time dial

setting - TDS”.

The most common three

types of inverse curves

used are:

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o Standard inverse (SI)

o Very inverse (VI)

o Extremely inverse (EI).

IEEE does not specify coefficients in the standard curve equation. Thus, each manufacturer’s curve is

similar. But different IEC curves are standardized

Typical Inverse-Time Overcurrent Relay Elements:

1. Induction disc over-current relay.

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2. Shaded-Pole Induction 51 Element

Overcurrent Relay Setting

50 Elements: Pickup Setting

51 Elements Pickup setting

Time delay setting

definite time: time setting

inverse time: curve selection

Time-over-current relays (ANSI 51 relays) have two basic settings:

1- Pickup current and

2- Time delay settings.

The process for determining the time delay setting involves:

(1) Calculation of a time delay value in definite-time over-current

elements.

(2) Selection in inverse-time over-current elements of a time -

current curve from a family of curves.

Instantaneous over current (50) elements have only one setting: the pickup current.

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Note: The Selection of an Overcurrent Relay Time Curve According

to American Standard (ANSI)

2.3 Mixed Curves Over-Current Relays

Mixed curves have all the advantages of the different types of over-

current relays. As the over current elements are built as separate

units, we may implement the over-current protection principles using:

a) a combination of instantaneous and definite-time elements .

b) a combination of instantaneous and inverse-time elements.

c) a combination of instantaneous, definite-time and inverse-time

elements.

d) a combination of definite-time and inverse-time elements(IDMT).



Note: This type of characteristic is mainly used in digital relays.

2.3.3 Mixed Curves (Inverse-Time +Definite -Time)

IDMT –Characteristics

The most commonly used type of relay is the inverse

definite with minimum time lag relay (IDMT) in which

inverse characteristic plus definite time characteristic

are used. The operating time is approximately

inversely proportional to the fault current near pickup

value and become substantially constant slightly above

the pick up value

of the relay.

The characteristic

is shown in Fig.


Wattmetric and

shaded pole relays

of induction type can

be used to obtain

this characteristic.

Standard Time and Inverse definite minimum time

(IDMT) Relay Characteristics

In American standard the time over-current relay are

five different types of characteristic curves.

− CO-6 Definite minimum,

− CO-7 Moderately inverse,

− CO-8 Inverse,

− CO-9 Very inverse,

− CO-11 Extremely inverse,

The time level settings are selected so that all relays

operate in 0.2 sec at 20 times the tap setting.

These time-current characteristics are compared in Figure in next

slides.



Definite minimum

Moderately inverse

Inverse

Very inverse Extremely inverse

CO-8 (normal inverse).


CO-7 (moderate inverse).


Typical time curves for standard British Standard (BS142) and

IEC Standard over-current relay (normal inverse).

Time Multiples Setting TMS= 0.1-1


Typical IEC standard over-current relay(normal inverse).TMS=1.


Curve Equation

The typical time curves for CO-8 American over-current

relay(normal inverse) characteristics can be approximated by the

following equation.


The typical time curves for IEC standard overcurrent

relay(normal inverse) characteristics can be approximated by

the following equation.

Where :

TMS = Time multiplier setting

CTR = Current transformer ratio

PS = Plug setting

IF = Fault primary current


(PS) and (TMS) in Over-current Relays

The working principle of an inverse time over-current

relay is shown in figure below.

Plug Setting (PS). The current feeds a coil is controlled with

multiple taps

Time dial setting: Is the time that allows the adjustment

between contacts and the operating time

. The generated magnetic field

makes the disc rotate with a

speed proportional to the

current

The braking magnet lessens

the rotating speed and acts as

an opposing force to the

rotation. Varying the

magnetization, different

tripping curves can be

achieved.


Example:

Calculate the plug setting and time multiplier setting for an IDMTL

relay on the following network so that it will trip in 2.4 s .

(see Figure 1).The relay characteristic is shown in Figure 2.The C.T.

setting is 100/5 A and the fault current is 1000 A.

Figure 1


Answer:

Fault current = 1000 A

CT ratio = 100/5 A

The current into relay under fault conditions,

Choose plug setting100% this is 5 A Therefore, current into relay as

a multiple of plug setting during fault is 50 / 5 = 10

We require the relay to operate after 2.4 s as soon as this much

current starts flowing in the circuit. Referring to characteristic curves

below, read time multiplier setting where 10 times plug setting

current and 2.4 s cross, which is about 0.8. Accordingly,

The relay settings is: PS=5 A (100%) and TMS=0.8.

Alternatively, if the current plug setting is chosen as 125% (6.25

A), the fault current through the relay will be 50/6.25 = 8 A. The

graph shows that eight times plug setting to operate in 2.4 s, the time

multiplier should be about 0.7.


This technique is fine and give exactly required of the TM curve.

( )

51000 50A

100

Fop relay

II

CTR

If the desired setting falls between the curves, and not easy to

estimate the intermediate setting accurately as the scales of the

graph are log/log. Then the following procedure is recommended.

From the scale below go to the multiple of plug setting current and

read the second value corresponding to the 1.0 time multiplier

curve. Then divide the desired time setting by this figure. This will

give the exact time multiplier setting:

Example if the second value at 10 times = 3 then for ( at 8 times it

is about 3.33 or 3.4) Desired setting = 2.4.


3.4

8

2.43.4

Desired setting


***Note the PS is given as 200% and we need to find the PSM

Example 2:

Determine the time of operation of a relay of rating 5A, 2.2 sec

IDMTL and having a relay setting of 125%, TMS=0.6. It is

connected to a supply circuit through a C.T 400/5 ratio. The fault

current is 4000A.

Solution:

The operating current of the relay: 5×1.25=6.25A

Ip=IF=4000 A

If we select TMS=0.6 then the operation time is


From the curve, the operating time for PSM=8 is 3.2 sec (TMS=1)

( This means that the current in secondary CT in case of fault and if we set the PSM

to 8 then the relay will operate at 6.25 A and 3.2 second )

0.6( )3.2 1.92sec.

1.0( )

TMS

TMS

or From log to log curve (for

TMS=0.6)

Or from Figure scale figure (for TMS=1)


1.92 sec≈

Example 3:

If the rated current (pick up current) of a relay is 3A, and the time

dial setting(TDS) is 1.

(a) How long(time) does it take the relay to trip if the supply C.T is

rated at 400:5 A and the fault current is 480A? The type of the

OC relay is CO-8.

(b) Solve using the standard curve equation and compare the

results.

Solution:



CO-8 Characteristics

b) Alternative method using curve

equation

Note:

TDS(time dial setting)=TD(time delay)

Example 4:

For the relay R1 in the system shown determine the current tap

setting CTD. If the maximum three-phase fault current is 2400A and

the TDS=2.0.

Find the operating time if the relay type is CO-8 (inverse type).


Solution:

Load current at the busbar1

Since the current tap setting (CTS) of CO-8 relay available are

4,5,6,7,8,10 and 12A

Hence we choose CTS=4 (this is relay operation current in case of

fault)

Fault current If=2400 A


Dr Audih alfaoury 34

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