Negative sequence overvoltage protection is used for protection of service main, motor circuits, sensitive loads for conditions such as reverse phase rotation (reverse phase sequence), unbalanced phase voltage and unbalanced phase angle. A perfectly balanced three phase voltage source will only have positive sequence voltage. Source with slightly unbalanced voltage will have majority positive sequence voltage and non-negligible negative and zero sequence quantities. Source that has perfect phase voltage but opposite phase sequence (Example ACB rotation instead of ABC or CW rotation instead of CCW) will have no positive and zero-sequence voltage and only negative sequence voltage. Three phase voltage vectors with non-ideal phase angles will also produce negative and zero sequence components. Negative sequence relays use these principles to activate protection of power circuits.
Case Study: Nuisance breaker trip due to voltage phase angle variation
Negative sequence voltage is also used in calculating voltage unbalance using the ‘True definition’ or ‘IEC Definition’. Magnitude of negative sequence voltage thus affects the voltage unbalance. Induction motors for example are affected by voltage unbalance and should be protected using a negative sequence or voltage unbalance relay.
Case study: Negative sequence over voltage trip due to phase angle variation
The relation between three phase voltages Va, Vb, Vc and the sequence components V0 [zero sequence], V1 [positive sequence], V2 [negative sequence] is given by the following equation:
Example 1: Consider an ideal three phase voltage with ABC rotation.
Results indicate that Ideal three phase voltage waveform will only have positive sequence component with no zero or negative sequence component.
Example 2: Consider a situation where A phase voltage is lost indicating a single-phase condition.
Results indicate that for an open phase or single-phase condition, negative sequence voltage rises significantly. Single phase condition if not detected and isolated can lead to motor damage. Negative sequence overvoltage relays can be used to detect and isolate motor circuits from damaging effects of single phasing. Note that any open phase condition after the relay monitoring location will not be detected. Example of this would be an open fuse downstream from negative sequence relay. A current unbalance relay or negative sequence current relay will be needed if such protection is also desired. Current unbalance relay however has the drawback that it can only detect unbalance after the motor starts.
Calculator for sequence voltage is provided in this article.
Example 3: Consider a situation where utility phase voltage is reversed. This can happen during initial commissioning or when lines are damaged after a storm or accident on lines. For this example, let’s assume B and C phases are swapped. Assume the reference phase rotation is A-B-C.
Results indicate that for a phase reversal condition, negative sequence voltage rises to a value equal to phase voltage.
Negative Sequence Protection Settings
Most sensitive device to voltage unbalance in any facility will be induction motors. Voltage unbalance equation using ‘True Definition’ is:
Voltage unbalance using true definition does account for both voltage and phase angle variation and is more representative of motor heating than NEMA definition of unbalance. It is possible to have difference in the calculated % unbalance using the two methods. Where possible use true definition.
When a three-phase motor is fed with unbalanced voltage, motor current several times the voltage unbalance percentage will flow. For example, motor fed with 5% voltage unbalance could have 20-30% current unbalance. NEMA has developed derating curves for motor operating under unbalanced voltage conditions. Curve assumes motor is delivering name plate rated load.
From the curve it can be seen that any motor should be able to tolerate up to 1% voltage unbalance without derating. For 3% unbalance, motor should be de-rated to 0.9. Operation of motor above 5% voltage unbalance is not recommended.
Link to: Current Unbalance Calculator
Usually, service entrance protection relays are set to trip between 5-10% voltage unbalance while feeder motor circuits may be set more sensitively at 3-5%. Delay of 10-120 seconds may be used for tripping circuit breaker.
Link to: Voltage Unbalance Calculator