3. Excitation System

The purpose of the excitation system is to provide direct current to the rotating field winding of the generator in order to produce a magnetic field. The system is designed to allow accurate and sensitive control of the voltage supplied to the field winding, thus controlling the output voltage of the generator as well as the power factor and reactive demand of the system. 

The excitation system is a static excitation system, in which thyristor rectifiers are provided with AC power from the excitation transformers connected to the main phase isolated bus (PIB). The thyristor gate pulse signals are controlled by the pulse generator unit in combination with the automatic voltage regulator (AVR), and are varied in proportion to the load on the generator. The excitation control system consists of two AC-AVR’s for automatic operation and two DC-AVR’s for manual operation. 

Changeover between automatic and manual control can be made by operating the changeover switch on the excitation control cubicle (ECC), located in an air conditioned room on the turbine mezzanine floor. The changeover switch is marked MAN, ADJ and AUTO. The ADJ position of the switch allows the PT and CT circuits for both AC AVR channels to be closed before effecting changeover to automatic control. 

Selection of AVR-1 or AVR-2 can be made by operating the pushbuttons on the Unit Control Console. These pushbuttons are marked AVR-1 and AVR-2, and near them are pushbuttons for AVR-1 LOCK and AVR-2 LOCK, and RAISE/LOWER pushbuttons for varying the generator voltage in either mode of control. 

Excitation Transformers

Three single phase excitation transformers are located on the ground floor. Each single phase transformer is made up of a 1500kVA, 20/3kV:900V excitation transformer and a 20/3kV:100V auxiliary transformer. The neutral bushings on the high voltage side of these transformers are connected together by Busbars in a star configuration. 

Two low voltage bushings are located on each transformer, one for the line side and the other for the neutral side. The Delta connection of the secondary windings for the excitation transformer is made in the transformer itself, while the Delta connection for the auxiliary transformer is made in a junction box mounted on the fireproof wall of the excitation transformer.

Each single phase transformer is a self-cooled type with a radiator, the oil level and oil temperature of which can be monitored. Each transformer is fitted with a pressure relief vent which is designed to operate in the event of a transformer internal fault.

The main rectifiers are thyristor diodes combined with a forced-air cooling system and are housed in the excitation power rectifier cubicle, EPR. The 24 thyristor rectifying elements are connected in three-phase full-wave configuration and are housed in trays in the cubicle. There are sufficient Thyristors to allow the generator to operate continuously at rated output with up to 20% of the diodes out of operation. Failure of any thyristor will initiate an alarm in the UCR.

An initial excitation system is provided in order to supply direct current to the generator field during start-up. It consists of an initial excitation transformer and rectifier stack, housed in the initial excitation transformer rectifier cubicle. The excitation power rectifier cubicle and the initial excitation transformer cubicle are located on the turbine mezzanine floor below the generator.