Similar to bus protections, transformers are protected by differential relays. Inter-winding faults (short circuits) and ground faults within power transformers can be detected by this protection scheme.
Failure to detect these faults and quickly isolate the transformer may cause serious damage to the device.
Remember that a differential relay is basically an instantaneous overcurrent relay that operates on the difference of current flowing into and out of the protected zone.
For transformers the differential protection (Figure 1) is basically the same as that for a bus but there are certain differences that we will look more closely at.
These differences are a direct result of three characteristics or a transformer:
1. A transformer has a turns ratio so the current in is not really equal to the current out. The current transformers are not likely exactly matched to the transformer turns ratio so there will always be an unbalance currentin the operating coil of a transformer differential relay.
2. Transformers require magnetising current. There will be a small current flow in the transformer primary even if the secondary is open circuited.
3. A transformer has an inrush current. There is a time period after a transformer is energized until the magnetic field in the core in alternating symmetrically. The size and the length of this inrush depends on the residual field in the core and the point in the ac cycle the transformer is re-energized.
In large transformers in might be ten or twenty times the full-load current initially and it might take several minutes to reduce to negligible values.
Transformer differential relays haverestraint coils as indicated in Figure 1. The value of the operate current has to be a certain set percentage higher than the current flowing in the restraint coils. For this reason transformer differential relays are said to percentage-differential relays.
Referring again to Figure 1, you will notice that when the transformer is first energized, there will not be any current flowing in CT2. The CT1 secondary current I1s flows through both the restraint and operate coils and prevents operation unless the current is very high.
The restraint coils also prevent relay operation due to tap-changes, where the ratio of transformer input to output current can continuously vary.
When transformers are first energized there is over-fluxing (saturation) of the core and the large inrush energizing current has a distorted waveform. This waveform is described as having high second harmonic content.
The transformer differential relays make use of this known fact and add in extra restraint when it detects this second harmonic. This extra feature prevents the transformer from tripping due to magnetizing current when being energized, but does not add any time delay.
Because the differential relay will not operate with load current or faults outside the protected zones (through faults), it can be set to operate at a low value of current thereby giving rapid operation when a fault occurs. There is no need to time delay the operation of the relay and therefore a fast acting type of relay can be used.
Resource: Science and Reactor Fundamentals – Electrical – CNSC Technical Training Group