Negative Sequence Protection of Generators against Unbalanced Load

When the current flowing through different phases of the generator is different then it is known as unbalanced loading. Unsymmetrical faults such as L-G, L - L, L-L - G, etc. in the system near the generating stations causes unbalanced loading to the generator.

Unbalanced loading on the generator mainly causes the rise to negative sequence currents which generates the negative sequence component of the magnetic field. 

The magnetic field produced due to the negative sequence component of the unbalanced current rotates in the opposite direction of the main fields and induces e.m.f's of double frequency in the rotor winding. 

Thus the unbalanced loading causes rotor heating. Unbalanced stator currents also cause severe vibrations and heating of stator.

A negative sequence protection scheme is used for unbalanced loading protection of generators.

Fig. shows the schematic arrangement for the protection of the generator against unbalanced loading.

The scheme consists of three current transformers one mounted in each phase.

The secondaries of the current transformer are connected in parallel. A relay is connected in parallel across the transformer secondaries.

Under normal operating conditions, equal currents flow through the different phases of the generator and their vector sum is zero.

Therefore, the sum of the currents flowing in the secondaries is also zero and no current flows through the operating coil of the relay.

However, if unbalance occurs, the currents induced in the secondaries will be different and the resultant of these currents will flow through the relay coil.

The operation of the relay will trip the circuit breaker to disconnect the generator from the system.

Another scheme of unbalanced current protection is shown in Fig.

In this case, two auxiliary current transformers having twin windings are connected to the line current transformers.

Under normal balanced load conditions; current Ia, Ib, and Ic flow in the direction shown.

Impedance Z1 and Z2 are connected across auxiliary current transformers T1 and T2, and a load impedance ZL is connected across the terminals 1-1'.

When the primary load current flows, the current through T1 will be (Ib – Ic) and that through T2 will be (Ia – Ib). The impedance Z1 and Z2 are chosen such that points L and N remain at the same potential i.e. voltage across LM and MN are equal and opposite.

Under unbalanced conditions, these voltages differ, and output is produced proportional to the negative phase sequence across 1-1' (Voltage E) so as to operate the relay.



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