DIFFERENTIAL PROTECTION

Differential protection

Differential protection application

Differential Relay – Operating Principle

PLANT TO BE PROTECTED

I1 I2

I1 = I2 Healthy; no trip| I1 - I2 | > Limit Fault; trip

“Balanced beam” type of relay – difference current trips beam; through current restrains it.Modern relays do the comparison by microprocessor

Differential Relay Coil Connection

Differential Relay – Transformer Application

• Transformation ratio has to be taken into account. Trip if |aI1 – I2| > Limit, where a is trf. ratio (VP/VS)

• Three-phase delta-star or star-delta. The phase shift and 3 have to be taken into account

• Allow for inrush currents – suppress tripping (2nd harmonic restraint).

• Allow for ratio changes due to tap-changing (may be 10-15%)

Transformer Inrush Current-Full DC Offset

Differential Relay Set-up for Delta-Star Transformer

CTs in delta

CTs in star

DeltaStar

Transformer Differential Protection Example

• 33/11 kV, star-star transformer. Rating = 25 MVA

• Standard CT ratios available are 200:5, 400:5, 600:5, 800:5, 1000:5, 1600:5, 1800:5

• Allow for 15-20% unbalance

Transformer Differential Protection Example .. 2

• Full–load current = 437 A/ph (33kV side), =1312 A/ph (11kV side)• Allow 150% overload. Choose 600:5 CT for 33kV. • Transformation ratio = 3. Choose 1800:5 CT for 11kV

side• Set HV normal current In = 450 A say• Set bias to 20%.• If delta-star transformer, allow for 3 (in LV CT ratio for

old relays – Idelta = Iline/ 3; taken care of in setting for modern relays)

Busbar Differential Protection Scheme

Limitations with OC Protection in Transformers

• A simple overcurrent and earth fault relay will not provide adequate protection for winding earth faults.

• Need some earth fault protection.• Degree of earth fault protection is very

much improved by the application of unit differential or restricted earth fault systems.

Restricted Earth Fault Protection

Restricted Earth Fault - How it Works

• On the HV side, the residual current of the 3 line CT’s is balanced against the output current of the CT in the neutral conductor.

• For the LV side, earth faults occurring on the delta winding may also result in a level of fault current of less than full load. HV overcurrent relays will not provide adequate protection.

• A relay connected to monitor residual current will provide restricted earth fault protection since the delta winding cannot supply zero-sequence current to the system.

Restricted Earth Fault - How it Works .. 2

• Both windings of the transformer can thus be protected separately with restricted earth fault.

• Provide high speed protection against earth faults over the whole of the transformer windings.

• Relay used is an instantaneous high impedance type.

High Impedance Differential Scheme

High Impedance Scheme - How it WorksThe CTs generate currents that create a high voltage across the relay (which has a high burden).

CTs must generate 2 times the trip voltage to work on an internal fault

If one only CT saturates for through fault, not enough voltage is generated to trip relay.

High Impedance Scheme - Requirements

• CTs must have identical turns ratios, knee-point voltages 2 times relay operating voltage, high accuracy, low reactance (‘PX’ class)

• High internal faults can generate high voltage across relay - some form of voltage dependent resistor (“metrosil”) is required to avoid this.

High Impedance Scheme -

Example … 1

High Impedance Scheme - Example … 2