OC Protection

8/19/2019 OC Protection

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OVER CURRENT RELAY

The relay

has

two

settings.

These

are

the

time

setting and the plug setting. The time setting

decides the operating time of the relay while

the plug setting decides the current required

for the relay to pick up.



The

current setting of relay

is expressed in percentage

ratio of relay pick up

current

to rated

secondary

current

of

CT.That means,

Current setting = Pick up current * 100

Rated secondary

current

of

CT

For example, suppose ,we want that, an

over

current

relay should operate when the system

current

just crosses

125%

of

rated

current.

If the relay is rated with 1 A, the normal pick upcurrent

of

the

relay

is

1

A

and

it

should

be

equal

to

secondary

rated

current

of

current transformer

connected to the

relay.

Then, the

relay

will

be

operated

when

the

current

of CT

secondary becomes more than or equal 1.25 A.

http://www.electrical4u.com/electric-current-and-theory-of-electricity/












































http://www.electrical4u.com/current-transformer-ct-class-ratio-error-phase-angle-error-in-current-transformer/

































As per definition,

Current setting

=

1.25

*

100

=

125 %

• 1

The

current

setting is

sometimes

referred

as

current

plug setting.

The

current

setting of over

current

relay is

generally ranged from 50% to 200%, in steps

of 25%.

For

earth

fault

relay

it

is

from

10%

to

70% in steps of 10%.













































Plug Setting Multiplier of Relay

Plug setting multiplier of relay

is referred as ratio of

fault

current

in

the

relay

to

its

pick

up

current.

Suppose we have connected on protection CT of ratio 200/1 A

and

current

setting is

150%.

Hence, pick up

current

of the relay is, 1 × 150 % = 1.5 A

Now, suppose fault

current

in the CT primary is 1000 A.

Hence, fault

current

in the CT secondary i.e. in the relay coil is,

1000 ×

1/200 = 5 A

Therefore PSM of the relay is, 5 / 1.5 =3.33


































































Time Multiplier Setting

Overcurrent

relays normally play the dual roles of

primary and

backup

protection.

Backup action requires a wait state.

By providing

the

additional

setting

feature

of

TMS,

this

can be achieved.

TMS is defined as the ratio T/Tm

, where T is the

desired relay

operating

time

for

a given

PSM

and

Tm

is

the

corresponding operating time at TMS=1.

Relay

operating

time

can

be

varied

by

varying

the

TMS

setting.



Calculation of Relay Operation Time

For calculating actual relay operating time, we

need to

know

following

• Current setting.

• Fault

current

level.

• Ratio of

current transformer.

• Time / PSM

curve.

• Time setting.


















The plug

‐setting

multiplier,

PSM,

is

defined

as

follows:

where Irelay

is the current through the relay operating coil and PS is the plug‐setting of the relay = pickup current.

The value

of

PSM

tells

us

about

the

severity

of

the

current as seen by the relay

A PSM less than 1 means that normal load current is

flowing. At

PSM

> 1,

the

relay

is

supposed

to

pick

up.

Higher values of PSM indicate how serious the fault is.

For example, let us consider a 1.0 A relay (i.e. a relay with

current coil

designed

to

carry

1.0

A

on

a continuous

basis)

whose plug has been set at 0.5 A, i.e. at 50%. Assume

that, for a certain fault, the relay current is 5.0 A. The

relay, therefore,

is

said

to

be

operating

at

a PSM

of

(5.0/0.5) = 10.



Inverse Time Over‐current Relay

Inverse time characteristic fits in very well, with the requirement that

the more

severe

a fault

is,

the

faster

it

should

be

cleared

to

avoid

damage to the apparatus



Inverse definite minimum time (IDMT) over ‐current relay

This is possibly the most widely used characteristic.

The characteristic

is

inverse

in

the

initial part, which tends to a definite minimum

operating time as the current becomes very high. The

operating

time

is

Very inverse time over ‐current relay

The inverseness

of this

characteristic

is

higher

than

that of the IDMT characteristic.

The operating time is

Extremely inverse

time

over

‐current

relay

The inverseness

of this characteristic is higher than

that of

the

very

inverse

characteristic.The operating time is



Guidelines for Setting Pickup Current for Phase Fault

Relays

• Pickup current

should

be

above

the

maximum

load

current.

•

Pickup current

should

be

below

minimum

fault

current.

• Pickup current should also be below the minimum

fault current

of

the

feeder

that

it

has

to

back

up.

• The back up relay also should provide time for

primary relay to operate. This time interval is known

as coordination time interval.



Application of IDMT Relays for Protection of a Distribution Feeder

The purpose of relay RB

is to provide primary protection to line BC.

The purpose of relay RA

is to provide primary protection to line AB

and

back‐up to line BC.

We start

the

setting

process

from

the

tail

end

of

the

system.

(a) Deciding the CT ratios and plug settings (pickup current):

(i) At relay B, the maximum load current, assuming 25% overload is:

80 A

+ (0.25

x 80

A)

= 100

A

Assuming 1 A relay to be used, the CT ratio can be selected to be 100 : 1. The plug setting PS can be done at 100%, i.e. PS = 1.0 A.



(ii) At relay A the maximum load current, assuming 25%

overload is

(160+80)+0.25(160+80)

=

300Assuming 1 A relay to be used, the CT ratio can be selected to

be 300 : 1. The plug

1)setting

can be done at 100%, i.e. PS = 1.0 A.

(b) Deciding the time‐multiplier settings: Starting from the

most remote

relay

RB

(i) Since RB

does not have to maintain selectivity with any

other relay, it can be made to operate the fastest. Thus the

TMS of

RB

can be

selected

as

0.1.

(ii) Now, to maintain selectivity between R A

and RB ,

the

following

constraint

must

be

met:



The

operating

time

of

RB

for

maximum

fault

just

beyond

bus

B

can

be

found from

For maximum fault at B, fault current = 3000 A on primary side which

becomes(3000/100) = 30 A secondary. Since plug setting is done at 1.0 A,

The TMS of RB

has already been set at 0.1. Substituting these values, we

get

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