Differential Relays

5/29/13 Differential Relays

xnet.rrc.mb.ca/janaj/differential_protection.htm 1/7

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DIFFERENTIAL PROTECTION

Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission

lines from the effects of internal faults.

Figure: Differential Protection of a Generator

In a differential protection scheme in the above figure, currents on both sides of the equipment are compared.

The figure shows the connection only for one phase, but a similar connection is usually used in each phase of the

protected equipment. Under normal conditions, or for a fault outside of the protected zone, current I1 is equal to

current I2 . Therefore the currents in the current transformers secondaries are also equal, i.e. i1 = i2 and no

current flows through the current relay.

If a fault develops inside of the protected zone, currents I1 and I2 are no longer equal, therefore i1 and i2 are not

equal and there is a current flowing through the current relay.

Differential Protection of a Station Bus

The principle of the differential protection of a station bus is the same as for generators.

The sum of all currents entering and leaving the bus must be equal to zero under normal conditions or if the fault

is outside of the protected zone. If there is a fault on the bus, there will be a net flow of current to the bus and the

differential relay will operate.



Figure: Single Line Diagram of Bus Differential Protection

Percentage Differential Relays

The disadvantage of the current differential protection is that current transformers must be identical, otherwisethere will be current flowing through the current relays for faults outside of the protected zone or even undernormal conditions. Sensitivity to the differential current due to the current transformer errors is reduced by

percentage differential relays.

Figure: Percentage Differential Relay

In percentage differential relays, the current from each current transformer flows through a restraint coil. Thepurpose of the restraint coil is to prevent undesired relay operation due to current transformer errors. Theoperating coil current | i1 - i2 | required for tripping is a percentage of the average current through the restraint

coils. It is given by



where k is the proportion of the operating coil current to the restraint coil current. For example if k = 0.1, the

operating coil current must be more than 10% of the average restraint coil current in order for the relay tooperate.

Differential Protection of Three Phase Transformers

Differential protection of three phase transformers must take into account the change in magnitude and phase

angle of the transformed current.

Transformers Connected Y-Y or Delta-Delta

In these two connections, the primary and secondary currents are in phase, but their magnitudes are different.The difference in the current magnitude must be balanced out by the current transformer ratios.

Figure: Differential Protection for a Y-Y Connected Transformer

If the transformer ratio is

The secondary currents of the current transformers are



During normal operating conditions or when the fault is outside of the protection zone,

Therefore, the ratios of the current transformers on the two sides of the power transformer must be

.

Sometimes standard current transformers with the ratios that satisfy the above equation are not available. In that

case auxiliary transformers between one of the current transformers and the relay are used.

Transformers Connected Y-D or D -Y.

The primary and secondary currents have different magnitudes and they also have 30 phase shift. Both, themagnitude and the phase shift must be balanced by appropriate ratio and connection of the current transformers.

The phase shift on a Y-D bank is corrected by connecting the C.T.s on the D in Y, and on the Y side in D .

Refer to the following drawing. The full load current on the 66 kV side is

The full load current on the 230 kV side is

The secondary currents in the current transformers on the 66 kV side then are

The magnitude of the currents coming out of the differential relay should be the same

From that, the current in the D arms of the D connected C.T.s should be

Ideally, the CTR on the 230 kV side of the transformer should be



The closest to that is the ratio

which is the ratio that will be used.. Using this ratio, the secondary current of the current transformers on the 230kV side is

The current through the operating coil of the differential relay is then

The average current through the current restraint coil is

From that, the current through the operating coil as a percentage of the restraint current under normal full load

conditions is

The percentage differential relays have settings for the allowable percentage difference. Examples of the

percentage values are 15%, 30%, 40%, etc. Any of these relays could accommodate the 0.46% operating coil

current without operating.



Connection of Differential Relays to a D -Y Connected Transformer.

Another problem that the differential relays used for transformer protection must overcome is the magnetizing

inrush current.



The inrush current occurs when a transformer is being energized. Since during the energization of the transformerthere is only current in and no current out, the inrush current appears to the differential relays as an internal fault.

The inrush current has some characteristic properties. Its magnitude may be as high as sixteen times the full load

current. It decays very slowly - from around ten cycles for small units to 1 minute for large units. The harmonic

content of the inrush current is different from normal load current and from fault currents. A typical waveform of

inrush current has a large fundamental frequency component, a significant d.c. component, and 2nd and 3rd

harmonic components. The 2nd harmonic component does not appear in the transformers under any other

conditions except during energization. Desensitizing of the differential relay to the inrush current involves the useof the second harmonic component to restrain the relay from operating.

(a)

(b)

Figure: Harmonic Restraint Circuit: (a) connection to current transformer (b) tripping circuit

Protective Relays Hydro Circuit Protection

Overcurrent Relays Directional OC Relays Voltage Relays Impedance Relays Pilot Protection

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Differential Relays