overcurrent relays2011-2

7/31/2019 Overcurrent relays2011-2

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Over current protection

• Protection against excess current was

naturally the earliest protection system to

evolve

• Over current protection, on the other hand, is

directed entirely to the clearance of faults,

although with the settings usually adopted

some measure of overload protection may beobtained



Over current relays are the most common form of

protection used to deal with excessive currents due to

Faults in power systems Over current relays use the current level to detect the

presence of faults and operate circuit breakers

Over current relays are primarily intended to operate

under faults They should not be installed purely as a means of

protecting systems against overloads

The setting of an over current relay is usually a

compromise in order to cope with both over currents and

overloads



The operating characteristic of an over current relay can be presented as a

plot of the operating time vs. the current.

Over-current relays

This figure represents the operating time

for an independent delay time over

current relay.

It will operate always at the same time for

currents over the pick up setting This relays are defined by the pick up

current, as number of times the normal

current, and the operating time

Coordination of different protections of

this type is achieved by time delaying andpick up setting

It must be a minimum of 0,3 sec. to permit

operating of the first breaker

t

i In n*In

t 0

Relé tiempo independ.

50 (ANSI)



Types of over current relays

Instantaneous

Definite time

Inverse time



Definite-Current Relay

Definite-current relay operateinstantaneously when the current

reaches a predetermined value

Coordination of definite-current relays is

based on the fact that the fault current

varies with the position of the faultbecause of the difference in the impedance

between the fault and the source

The relay located furthest from the

source operate for a low current value

The operating currents are progressively

increased for the other relays when

moving towards the source



Relay Basics

• Electromechanical Relays – Magnets, coils, transformers, springs, etc.

• Static or Electronic –

Transistors, diodes, opamps• Also magnets, coils, transformers, etc.

– Can duplicate E/M relay functions

• Digital or Microprocessor

– Computers!• Also magnets, coils, transformers, etc.

– Can duplicate E/M relay functions



Relay Basics

•

Non-directional – Detect fault in any direction

– Operate when quantity exceeds pickup value

• Directional

– Use voltages, currents, and angles to determine

fault direction

• Trip for faults in front of relay

•Block for faults behind relay

– Operate when quantities exceed pickup value and

correct direction is determined



T i m e ,

S e c

o n d s

Multiples of PU Current

1 21 10

Inverse

Definite Time

Modern Relay



Definite time over current relay



Feeder Protection

1) Over Current Protectiona) Time delayed non directional (51)

b) Time delayed directional (67)

c) Instantaneous (50)

2) Ground Over Current Protection

a) Time delayed non directional (51N)

b) Time delayed directional (67N)

c) Instantaneous (50N)



Instantaneous (50) protection

• Used for detecting high magnitude fault current

• Same time delay regardless of fault magnitude or distance

•

Co-ordination with down stream section cannot be maintained.



Protection for 3 Phase System

a) Three Over Current One Earth

Current relayb) Two Over Current One Earth

Current relay

Ia

Ib

Ic

In=Ia+Ib+Ic

Ia

Ib

Ic

In=Ia+Ib+Ic



E/F O/C O/C O/C

Earth Fault

E/F O/C O/C



E/F O/C O/C

Phase Fault

O/CE/F O/C O/C



Overcurrent Protection (50,51)

A

B

C

132 kV /33 kVA

B

C

Relay(s)Relay(s)

50/5150/51

50/51N51GN



Earth fault Protection (50N,51N)

A

B

C

132 kV /33 kVA

B

C

Relay(s)Relay(s)

50/5150/51

50/51N51GN

Grading between 51GN & 51N necessary



Thanks for Good Listening Any Questions?

overcurrent relays2011-2

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