AVR panel in generating stations 2

[hassankh]

We have a thermal power plant with a multi-megawatt generator of 15 kV voltage. It is connected to the grid through a 15/400 kV transformer. Definitely, the generator has its own AVR system. But I wonder if it is also necessary to have an AVR panel for the 15/400 kV transformer?

In transmission substations, transformers are equipped with tap changer and AVR. But this is a "generating station" where the (generated) voltage is regulated by the generator's AVR.
Also note that the transformer is also used at times when the generator is shut down, to transform 400 kV voltage from grid to supply the house load of the power plant.

Thank you in advance

 

[crshears]

Hello hassankh, you have gone a good ways in answering your own question...

Depending on how tight the voltage tolerance is for the house load supplied when the generator is off line, you either may or may not require on load tap changing facilities in the generator step up transformer.

It may be possible to use a GSU transformer with off load/off potential tap changing, provided that either [a] the generators can carry the local load while the transformer taps are being changed or there is an alternate means of supplying the load, or [c] if the house load is interruptible for the short duration of the outage...

Beware that when the generator is on line, depending on voltage regulating schemes and/or operating set-ups, there is a distinct possibility that the GSU tap control system and the generator AVR may fight each other, and it may be necessary to place and leave the GSU [on-load] taps on manual, adjusting them as needed.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

Often the transformer tapchanger is required to meet the reactive despatch while keeping the generator terminal voltage at an acceptable level. As crshears notes, it is quite common to leave the GSU tapchanger in manual control and make trim adjusments on the AVR. Operation at the neutral tap position can often leave the AVR running well below its rated current (and the generator producing well below its reactive capability) with the V/Hz limiter active to prevent over-fluxing in the generator. This is especially common when the grid voltage is a little high. Tapping down the GSU transformer acts to drive the generator terminal voltage down, and in response the AVR increases the field current in order to restore the machine terminal voltage. In doing so it increases the reactive output of the set.