02 differential protection

8/20/2019 02 Differential Protection

1/23

Mansoor Asif

Lecturer

Electronics, Power and Control Group

Electrical Engineering DepartmentSEECS, NUST


2/23

Differential Protection


3/23

1-Introduction

• Protects equipment from fault occurring within theequipment

• Principle

–

The difference of current (magnitude, phase) entering andleaving the equipment is measured

– If the difference exceeds a predetermined value, tripoutput is issued

• Only suitable for equipment's where sending and

receiving end of the equipment are physically near

• Used for equipment protection

– Transformer, Busbar e.t.c.

School of Electrical Engineering andComputer Science, NUST


4/23

2-Dot markings


• A, B and C are mutually coupled coils

According to Lenz’s law current would flow in coil B in such a direction

So that the flux generated by it oppose the flux generated due to A

• Current enters the dot mark onthe primary coil

• Current should leave the dotmark on the secondary coils

•

If currents are made to entertwo dot marked coils, theirrespective fluxes would add up


5/23

3-Simple Differential Protection

AKA Merz-Price Scheme


• Current entering and leaving the equipment is

stepped down with help of CT’s

• During the normal load flow current in the

spill path would be zero

•

Current would just keep circulating in the“pilot wire”

When current enters the dot marks on the primary side of CT, it should leave

the similarly marked dot mark on the secondary side and vice versa


6/23

3.1-Behavior during external faults


• During external faults or through faults the

current entering and leaving the equipment

remains unchanged

• Therefore Diff Protection scheme should

remain stable

• Should not generate trip signal


7/23

3.2-Behavior during internal faults


• Fault current If,int/n will flow through spill path

• Minimum internal fault current that would

generate a trip signal would be


8/23

3.3-Simple Differential Protection,

Double End Fed: Behavior during

internal faults



9/23

4-Zone of protection of differential

relay


• Any fault occurring within the two CT’s iswithin zone of differential protection

Differential protection

Should restrain during external faults/through faults

Differential protection

Should trip during internal faults


10/23

5- Actual Behavior of a Differential

Protection Scheme


IS1 & IS2 Should be lagging instead i.e.

Above the horizontal axis

CT Errors

Ratio Error

Phase Angle Error

Spill current build up as through fault current increases


11/23

5.1-Through fault and stability ratio


Higher the stability ratio, better is the systems ability to discriminate between

internal and through faults


12/23


13/23

5.2.1-Differential Protection Scheme

considering Equivalent Circuit of CT



14/23

6-Percentage Differential Relay

• Simple differential relay can be made more

stable if

– Restraining torque proportional to the through

fault current can be generated

– Operating torque remains proportional to the spill

current



15/23




16/23




17/23




18/23

6.1-Block diagram of Percentage

Differential Relay


• The relay has two settings

– Pick up setting

– Slope setting


19/23

7-Earth Leakage Protection


• Due to insulation failure the chassis ofequipment may become live

• As the chassis is connected to earth, so

current will start flowing into earth• Earth current too small for OC relay to operate

• Earth leakage relays or current balance relays

are used to detect earth faults• Can save the personnel when they come in

contact with live unearthed chassis.


20/23

7.1-Earth Leakage Protection for a

Single-Phase load



21/23

7.1-Earth Leakage Protection for a

Three-Phase load



22/23

Problem



23/23

Problem

School of Electrical Engineering andComputer Science NUST

02 differential protection

Documents