A
simple over current protection relay can also be provided for the protection of
the transformer But the operating over current will always depend upon the
system source impedance and may vary in case of large transmission network.
Hence, for any fault in the transformer, this protection may not operate
reliably. Further, for over current protection, time delay characteristics
needs to be chosen to discriminate secondary side protection. This delayed operation
may further damage the transformer during faults. Hence, a separate unit
protection is required to be provided for the transformer which will operate
instantaneously and will not depend upon source impedance. For this purpose,
Differential protection is provided for the protection ofthe transformer.
High
speed protection is provided for faults on both the HV and LV windings of
transformer by a biased differential relay (D). The relay operates on the basic
differential principle that HV and LV CT secondary currents entering and
leaving the zone of protection can be balanced under load and through fault
conditions, whereas under internal fault conditions balance will be lost and a
differential current will cause the relay to trip. The zone of protection is
clearly defined by the CT locations and, as the protection is stable for
through faults, it can be set to operate without any intentional time delay.
The
figure 2.0 illustrates the simple differential protection circuit for single
phase. Under normal load condition, the secondary CT current of HV & LV
windings are balanced, hence the operating current through the relay is almost
zero. This remains true for even through fault condition also. In case of
internal fault, direction of currents of HV & LV CT secondary will change
and add up to operate the relay. In order to operate the Differential
protection satisfactorily, it is necessary to balance the HV & LV CT
Secondary currents, under normal and through fault conditions, both in phase
and magnitude. In old electromechanical relays this was achieved by providing
Interposing (IP) CTs. The purpose was to match the HV & LV CT Secondary
current by selecting proper ratios of IP CTs. Further, correct operation of the
differential protection requires that the transformer primary and secondary
currents, as measured by the relay, are in phase. In case of phase difference,
the relay will see through fault current as unbalance current and will operate.
Hence, phase correction must be implemented. In applying the principles of
Differential protection to transformers, variety of consideration needs to be
taken into account. These include
I.
Correction for possible unbalance of currents of
HV and LV CTs ( Ratio correction)
II.
Correction for possible phase shift across
transformer windings (Phase correction)
III.
The effects of variety of neutral earthing and
winding arrangement (Zero sequence filtering)
IV.
Effect of magnetizing inrush during initial
charging (2nd Harmonic setting)
V.
Possible over fluxing during over voltage /
under frequency conditions (V/F setting)
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