Under voltage [27]/Over voltage [59] Relay:

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In this post, we can learn the working and configuration of the Numerical Under voltage [27] /Over voltage [59] protection relay.

These relays measure either phase-to-phase (Ph-Ph) or phase-to-neutral (Ph-N) fundamental frequency RMS voltage depending on the input voltage setting. If the value of measured voltages deviates from the setting values then these relays will give a trip indication.

The Example Numerical under/over voltage relay is shown in the below figure.

Numerical Under/Over Voltage Relay
Under/Over Voltage Relay

The following protection functions are used in this relay.

  1. Under voltage Protection [27]
  2. Over Voltage Protection [59]
  3. Voltage Transformer Supervision (60VTS)

1. Under Voltage Protection:

Under voltages occur due to several reasons like any faults on the system; increase in the amount of loading, loss of an incoming transformer, etc. During normal operating conditions, the voltage is adjusted within acceptable voltage limits by transformer On Load Tap Changers (OLTC) and Generator automatic voltage regulators (AVR).

The under-voltage setting is usually 80 percent of the normal operating voltage. If the voltage falls below this level for the set amount of time then the tripping command is issued by the relay and hence the system is isolated. The time setting is used to avoid tripping due to any transient disturbances.

Usually, motors stall at below 80 percent of their rated voltage. An under-voltage element can be set to trip motor circuits once fall below 80 percent so that on the restoration of supply an overload is not caused by the simultaneous starting of all the motors.

2. Over Voltage Protection:

System overvoltages can damage the insulation of components. Overvoltages occur due to sudden loss of load, improper working of tap changer, Generator AVR malfunction, Reactive component malfunctions, etc.

The Overvoltage setting is usually 110 to 130 percent of the normal operating voltage depending on the system requirement. If the voltage rises above this level for the set amount of time then the tripping command issued by the relay and hence the system is isolated. The time setting is used to avoid tripping due to any transient disturbances.

3. Voltage Transformer Supervision (60VTS):

Voltage Transformer or Potential Transformer Supervision is used to detect the failure of PT. If the PT itself fails due to any reason the Under voltage protection should not operate. Otherwise, it leads to the isolation of the health system by unwanted tripping.

The VT supervision function monitors the healthiness of Potential transformers and gives an alarm in case of PT fails to measure voltage.

It is rare to fail PTs themselves, the common problem is the failure of protective fuses connected in series with PTS.

The VTS function operates in two modes.



When a PT failure occurs in one or two phases, the voltage levels become unbalanced. A large level of Negative Phase Sequence [NPS] voltage is detected it is around 30 percent of the nominal voltage for one or two PT failures.

A similar condition also occurs due to the presence of system faults. To differentiate between the two conditions, the VTS function also uses NPS currents. However, it is noticed that NPS currents also generate due to load imbalance. This problem can be overcome by careful selection of settings; it is usual practice to keep the maximum setting of NPS current 10 percent of the nominal value current for detecting PT fuse failure in one or two phases.

Zero Phase Sequence [ZPS] voltages and currents can’t be used here as it is difficult to differentiate between a VT failure and a Phase-Phase fault.


If a failure occurs in all 3 Phases of a Voltage Transformer, then there will be no NPS or ZPS voltage to work with. However, the Positive Phase Sequence [PPS] Voltage will fall below the expected minimum measurement levels.

The PPS voltages also fall if any nearby 3-phase failure occurs. To differentiate from this need to consider the PPS currents also. The PPS current setting value must remain above the minimum load level BUT below the minimum fault level.

If PT MCB trips for any reason this condition is also detected as 3-phase failure detection.

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