The differential protection used for power transformers is based on Merz-Price circulating current principle. Such protection provides protection against internal phase-to-phase and phase-to-earth faults and is generally used for transformers of rating exceeding 2 MVA. This protection system as applied to power transformers is fundamentally the same as that for generators but with certain complicating features not encountered in the generator application.
Figure 9.3 shows the Merz-Price protection system as applied to a star/delta power transformer. The overload protection is provided by the use of overload fuses in the pilot wires between the three pairs of CTs. When the overload blows a fuse one of the pair of CTs is disconnected from the relay which receives the current from the remaining CTs. The balance is thus disturbed and the relay operates. Thus a backup protection is provided.
On energisation of transformer the transient inrush of magnetising current flowing into the transformer may be as large as 10 times full-load current and it decays relatively slowly. Since the large magnetising current flows only in primary windings, it causes difference in CTs output and makes the differential protection of transformer to operate falsely. In order to prevent operation of differential protection due to inrush of magnetising current, the “kick fuses” are provided across the relay coils, as shown in Fig. 9.3. These fuses are of the “time-limit” type with an inverse characteristic and do not operate in short-time duration of the switching-in-surge.
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Under sustained fault conditions, however, the fuses operate (blow off) and the full current flows through the relay coils and operate the protection system. This scheme affects the relay setting and to avoid this, arrangements are sometimes made to have the “kick fuses” in circuit only during the period of switching-in. This problem can also be overcome by using a relay with an inverse and definite minimum time characteristic instead of an instantaneous type.
The circulating current protection system also provides protection against inter-turn faults. Short circuit between turns on the same phase alters the turn-ratio of the power transformer and so causes imbalance between CT pairs and makes the protection system to operate. However, the change in the turn- ratio of the transformer must be considerable so as to cause flow of sufficient current into the relay to operate. Such short circuits are dealt with more efficiently by Buchholz protection.
There are two basic requirements which are to be satisfied by differential relay connections. The differential relay should not operate on overload or external faults, and it must operate on severe internal faults.
CT Connections:
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There is an inherent phase displacement between phasors representing voltage induced in high voltage windings and low voltage windings having same marking letter and corresponding neutral points, in case of star-delta transformers. Hence the load currents on the hv side are displaced in phase with respect to load currents of corresponding phase on the Iv side.
The power transformers are grouped, according to the phase displacement, as follows:
Group 1 (Star-star) – Zero phase displacement
Group 2 (Star-star) – 180° phase displacement
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Group 3 (Delta-star) – 30° lag phase displacement
Group 4 (Delta-star) – 30° lead phase displacement
In the circulating current differential protection, the phase displacement in line currents on the two sides, introduces phase difference in the secondary currents of CTs on two sides. The result is that even if CTs of proper turn-ratio are used a differential current will flow through the relay operating coil under normal operating conditions and cause the relay operation.
The correction for phase difference is affected by appropriate connections of CTs. The connections of CTs should be such that the resultant currents fed into the pilot wires from either side are displaced in phase by an angle equal to the phase shift between the primary and secondary currents. As a general rule, the CTs on any star winding of a power transformer are connected in delta and that the CTs on any delta winding are connected in star. Table 9.1 shows the type of connections to be used for CTs in order to compensate for the phase difference in the primary and secondary currents of power transformer.
CT Ratios:
It is noteworthy that the currents in the primary and secondary windings of a power transformer are not equal and, therefore, use of identical CTs (of same turn-ratio) will give differential current and operate the relay even under no load condition. The difference in the magnitude of primary and secondary currents is compensated by using CTs of different turn-ratios. The turn-ratio of CTs used on the two sides of a power transformer should be such that their secondaries have equal currents during normal operation. Thus in star-star transformer with a ratio of 33 kV/132 kV, the CTs must have a turn-ratio of 132/33.
Problems Associated with the Application of Differential Protection to Power Transformers:
Simple differential protection system is inadequate because of its following inherent drawbacks:
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1. Difference in Length of Pilot Wires on Either Side of Relays:
This difficulty is overcome by inserting balancing resistances in series with the pilot wires. These are adjusted on site to get equipotential points on pilot wires.
2. Different CT Characteristics:
Unless saturation is avoided, the difference in CT characteristics due to different ratios being required in circuits of different voltages may cause appreciable difference in the respective secondary currents on occurrence of through faults. This problem is aggravated in the case of power transformers due to unequal ratio CTs being used on either side of the power transformer. A source of ratio error which results in circulating currents under through-fault condition is the unequal burden imposed on the CTs due to unequal lead length.
3. Tap Changing:
In most of the transformers there is provision for tap changing so that the voltage can be varied as per requirement to keep the system voltage within limits. During tap changing voltage ratio differs from the original one and affects the operation of the differential relay. This problem is overcome by using biasing coil (i.e., percentage differential relay protection).
4. Magnetising Inrush Current:
Under normal operating conditions, the magnetising current is very small. However, when a transformer is energized after it has been taken out of service, the magnetising inrush current can be extremely large (as large as 10 times full-load current) for a short period (5 to 10 cycles, sometimes as large as 4 to 6 seconds). Since magnetising current flows only through primary winding, it causes difference in CTs output and makes the relay to operate, which is not desirable.
There are a number of ways of overcoming the above problem. Firstly, the relay may be provided with a setting larger than the maximum inrush current; secondly, the time setting may be made long enough for the magnetising current to subside (fall below the primary operating current) before the relay operates. But for EHV transformers these simple remedies are not incompatible. The third alternative is the use of second harmonic restraint relay.