DIFFERENTIAL PROTECTION
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.
Percentage Differential Relays
The disadvantage of the current differential protection is that current transformers must be identical, otherwise there will be current flowing through the current relays for faults outside of the protected zone or even under normal 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. The purpose of the restraint coil is to prevent undesired relay operation due to current transformer errors. The operating 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 to operate.
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, the
magnitude 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 230 kV 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 transformer there 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 use of the second harmonic component to restrain the relay from operating.
(a)
Figure: Harmonic Restraint Circuit: (a) connection to current transformer (b) tripping circuit
