Generator Transformer HV Voltage Selection

The High Voltage side of a GSU (Generator Step Up) transformer is chosen based upon how and where it is connected to the grid and its voltage(s), just as the low voltage side is determined by the rating of the generator.

Generally, the generator will generated the medium voltage of 6.6KV and feed it in to the step up transformer. The step up transformer will then convert the voltage to 220KV or 400KV depend upon the nominal voltage of the grid used.

The HV winding of Generator Transformer (Unit transformer) is Star Connected, with Off Load Tap Changer. Vector group of this transformer has to match with Vector Group of other Transformers in the switch yard of the power station.

The Generator end Voltage is regulated from the AVR of Generator and this depends up on Power to be shared with other Power Station on the Grid. Star connections are necessary to fix voltage of system with respect to ground and operation of earth fault relays. The Neutral point of transformer is solidly earthed.

This is contrary to Receiving Transformers of Distribution Transformers where in the HV side is Delta Connected and mostly has On load Tap Changer to regulate voltage to consumers. Consumer end winding of such transformers is star connected for the same reasons as GT.

Sometime there are coupling Transformers between two Power Systems or Sub-grids. In this case if the power is to be exchanged between tow systems in either directions then these are Star/Star (Yy0). If voltage ratio between two system is less than or equal to 2 then Star Connected Auto Transformers are used.

"...The Generator end Voltage is regulated from the AVR of Generator...", no problem.

"...and this depends up on Power to be shared with other Power Station on the Grid...", big problem unless you meant to say "reactive power". Generally speaking, most people use the word "power" to mean "real power" (Watts) hence the clarification, whereas reactive power (VARs) always requires both words.

It's easy to remember, real power comes from the fuel and is controlled by the throttle, and automatically via the governor. Reactive power comes from the magnetic field in the air gap and is controlled by the excitation system, and automatically via the AVR (Automatic Voltage Regulator).

They operate in quadrature with very little effect on each other under normal operating conditions, unless the generator is very large relative to all the other generation on the grid, or the impedance between the generator and the grid is very large relative to all the other impedances.

"Reactive Power" ..... Problem here. Reactive Power will always flow, it will be capacitive if Sending end "Power Station" voltage is lower than Receiving End "Consumers" and Inductive if Sending End "Power Station" is higher than "Receiving End"

Please read below to realise why for Active Current to flow from Sending End to Receiving End, the Voltage at Sending End shall be more than Receiving End.

When Operator at Sending End (Power Station) raises the speed (Frequency) of the Prime Mover to share greater portion of Load of Grid, the Active Current will only increase, if the Station Voltage at Power Station is more than that at Load End. If it is not more - then it will result in Power Factor of Power Station to go Capacitive - think again why? ..... for the following reason:

The Armature Reaction of Capacitive Current is Re-magnetising. This results in increase in Voltage of The Generating Set - THIS WILL LET THE FLOW OF ACTIVE CURRENT (AS FREQUENCY OF THE GRID HAS TO STAY AT A FIXED AVERAGE LEVEL).

We also know that Increase in Capacitive Current of Synchronous Alternator shall result in drop in field current (BY ACTION OF AVR), thus chance of loos of coupling (magnetic coupling) between Rotor and Stator - resulting in hunting and heating of pole pieces - a instability - and operation of Loss of Excitation relay.

HENCE FOR STABILITY OF OPERATION OF A POWER STATION RUNNING ON A POWER GRID - EVEN FOR ACTIVE CURRENT TO FLOW THE SENDING END VOLTAGE SHALL BE ALWAYS MORE THAN RECEIVING END - (Thanks for reminds me, freshening memories of my days in Engineering College, 43 years (1968-69) before, my Professor of Electrical Transmission and Distribution drawing the vector diagram on the Black Board, explaining that SENDING END VOLTAGE TO BE MORE THAN RECEIVING END FOR LOAD TO FLOW).

Best regards,

Ramesh Kapur