When generators are operated in parallel condition,it necessary to be arrange for both real power ( kW ) and reactive load ( kVar ) sharing in proportion to their standard rating.Real load output from generator determined by torque applied to the shaft and this can only be controlled by controlling the prime mover driving the generator.
Reactive load is a function of generator excitation.Under parallel operation,adjustment to field excitation does not affect to output voltage.In general,an under-excited machine will generate at a more lagging power factor current.
The three phase sensing regulator operates in similar manner except that the voltage across the reactor carrying a current proportion to one phase current is added to the voltage of that phase.
Installation for generator in parallel
The current transformer and burden resistor used for parallel operation are fitted in the generator terminal box.The burden resistor is a 40 ohm with a variable tap adjustment which may be used to determine the amount of voltage droop with reactive load.
The current transformer consists of a wound toroid,the primary winding of which is formed by passing the ” W ” phase conductor through the transformer.Correct termination is important and the current transformer is labeled ” L ” and ” M ” for this purpose.
” W ” phase passes through the transformer such that marking ” M ” is electrically nearer to the generator terminal an the ” L ” marking nearer to the load.Secondary connections are connection ” L ” to AVR ( automatic voltage regulator ) terminal ” VR ” or ” R ” and connection ” M ” to ” V ” phase of the generator.The direct link from ” VR ” or ” R ” on the regulator to ” V ” phase on the generator being removed.
- Paralleled windings must not be confused with paralleled cables.
- On larger machines,to facilities cable handling,they may be more than one cable connecting the end of a particular winding to the associated generator terminal.
- Where such paralleled leads are used,all leads linking the winding to the terminal should pass through the current transformer.It is not permissible to fit the CT to one or two or more paralleled leads.