Under Voltage Protection Working Principle 27

Under Voltage Protection Working Principle 27:

Under voltage fault protection is used to protect the alternator/generator/transformer winding from low voltage operation. Under voltage protection sense the phase to phase voltage of the generator/transformer using instrument transformer (Potential transformer). When the voltage drops below the rated voltage typically 85% (stage 2)-90% (stage 1) the under voltage protection will be activated.

Principle of Under voltage protection:

Three number of potential transformer normally installed in the generator LAVT panel (lighting arrester voltage transformer). They detect the voltage across the generator in real time. When the voltage across the generator winding drops the simultaneous voltage drop occurs in the PT output also. The reduced or dropped voltage activate the power system alarm or trip circuit.

Under Voltage wave form
Under Voltage wave form

Reason Under voltage protection:

i.e The output from the generator’s LAVT potential transformer will be given to the under voltage coil typically 110 Volts relay coil. In principle of U/V coil, which do not trip the circuit breaker when the voltage across the PT is high. When the voltage drops the preset value, the voltage coil operates the circuit breaker.

The generator under voltage protection consists of two stage tripping. Stage 1 trip command is given to grid circuit breaker and stage 2 trip command is given to generator circuit breaker. Most of the time in synchronous generator, the under voltage fault occurs from the grid due to earth fault and line short circuit. That’s why, the first stage will be given to grid circuit breaker.

At that same time, the under voltage fault occurs due to failed excitation, diode failure, under frequency or turbine low speed, failed PT fuse etc.

Under voltage relay setting:

Stage1: 90% of the rated voltage trip command to grid circuit breaker.

Stage2: 85% of the rated voltage, trip command to Generator circuit breaker.

ANSI code for under voltage protection: 27

Relay acted:

  • 27 relay opted
  • 86M grid mater trip opted-stage1
  • Annunciator under voltage alarm

Note: if the under voltage occurs due to excitation failure, then check your generator excitation system.


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