Negative Sequence Relay:
Negative Sequence Relay is used to protect the alternator/generator from unbalance loading and negative sequence component. The negative sequence current flows in the generator winding due to phase to phase short circuit. The negative sequence currents cause the overheating of generator/alternator/motor/transformer’s winding. Negative sequence relays are generally used to give protection to generators and motors against unbalanced currents.
Negative Sequence Relay Operation:
Before understanding about the Negative sequence relay, you must know what is negative sequence. Negative sequence is nothing but A balanced three-phase system with the opposite phase sequence as the original sequence which means the set of the three phasors are equal in magnitude, spaced 120° apart from each other and having the phase sequence opposite to that of the original phasors. Read more.
Under balanced condition, the generator delivers equal currents in all three phase either star connected or delta connected loads. Here the vector sum of negative sequence current is zero, hence net unbalance current is zero. here, The unbalance may occur due to circuit breaker operation, circuit breaker failures or system faults. Hence the negative sequence current starts flowing in the stator winding, which creates additional flux in the air gap which rotates in opposite direction to that of rotor synchronous speed. This flux induces currents in the rotor body, wedges, retaining rings at twice the line frequency. Due to this mismatch, the rotor body temperature starts increasing. But if the magnitude of these negative sequence current is high, then this opposite direction potentially affects the normal operating direction of the turbine. It leads to serious damage to the turbine-generator set.
Generally, negative sequence relays are having two stage of tripping. The first stage tripping is given to the grid circuit breaker as the grid is infinite loads, the second stage of tripping will be given to the generator circuit breaker.
ANSI code for negative sequence relay: 51_2 – Negative Sequence Time Overcurrent
- 51_2 – Negative Sequence Time Overcurrent opted
- 86M Grid mater trip operates 110kV circuit breaker
- 86M Gen master trip (if the negative sequence is uncontrolled)