Introduction to the Generator II

6. Cubicles

The following cubicles contain equipment used in the operation of the generator:

This cubicle houses the gauges, switches, meters, filters and valving used during generator operation and cooling. 

Generator Potential Transformer Cubicle

The generator potential transformer cubicle houses two sets of 3 single phase transformers which are connected together in star and delta configurations and also incorporate high voltage fuses.

Three of the transformers, PT-1, are used to step down the generator output voltage from 20000V to 110V, as well as providing power for the AVR-1 excitation control system, meters in the UCR, protection relays in the URR and synchronising instruments.

The remaining three transformers, PT-2, step down the generator output voltage from 20000V to 110V, which is then applied to AVR-2 excitation control system and also to the residual voltage protection relay. 

Neutral Earthing Transformer Cubicle

The neutral earthing transformer cubicle houses a single phase earthing transformer and a neutral earthing resistor. The generator windings are connected together to form a star-point. The star-point is connected in series with the primary of the neutral earthing transformer, the secondary of which is loaded by a high capacity resistance bank.

As long as the generator three-phase load is balanced, the neutral current to earth is zero. If the generator three-phase load becomes unbalanced, due to a fault in one phase, a current will flow in the primary of the transformer, inducing a voltage in the secondary winding.

These voltages will cause the turbine master trip solenoids to energise the turbine trips, and hence a “TURBINE TRIP” alarm will be initiated in the UCR. The impedance of the transformer prevents a large neutral current from flowing, thus avoiding damage to the generator windings. 

Excitation Control Cubicle

Contains equipment used to monitor and control the excitation current to the generator rotor field winding. 

Excitation Power Rectifier Cubicle

The Excitation Power rectifier cubicle has three sections, each section containing trays with four Thyristors. The AC output from the excitation transformers is rectified and applied to the generator rotor. 

Field Circuit Breaker Cubicle

The field circuit breaker cubicle contains the field circuit breaker, two shunts, one connected to the rotor temperature transmitter and the other to the millivolt-to-milliampere converter, and a discharge resistor that dissipates the energy in the rotor winding when the field circuit breaker opens. 

Initial Excitation Transformer Rectifier Cubicle

The initial excitation transformer cubicle contains a magnetic contactor, a 90kVA self air cooled transformer, which steps down the AC voltage from 415V to 70V, and rectifiers.