Generator unit transformer tap setting 1

[HamidEle]

We are connecting 2 co-generators into the utility. The generator output voltage is 13.8KV, the transformer primary voltage is 35kV, which is also Utility voltage. At 35kV level, generators are supplying power to the customer plant. 2 generators are running in parallel with the grid. I wonder how we can set the generator tap changers, on-load or off-load in order to maintain the stable voltage at 35KV distribution system? 2 generators are 30MW. Can governors control the primary voltage of the generator(s)? What is the normal practice in this industry? Any inputs would be much apprecited.

 

[waross]

Hello Hamid. You may want to print this up and put it over your desk for awhile. Similar questions are often asked by good people who are working with generators for the first time.
"The governor controls the power input to the generator and the power output of the generator.
The voltage setting controls the flow of reactive power into or out of the generator."

In your instance the voltage setting of interest would be the voltage setting at the high voltage side of the transformer at the point of connection to the grid.
This voltage may be set or adjusted by either or both of two methods. The generator voltage may be adjusted or the transformer taps may be adjusted.
Normal practice: It depends.
One factor is the terms of the contract with the utility.
Two extremes of contract terms may be:
A> Dumping excess power onto the grid. The price may be slightly more than the cost of production and there is little incentive to maximize output. The operators at your plant may find it most economical overall to run the plant at less than maximum output.
B> Peaking. Rates for peaking power may be very lucrative and the head office may demand that the operators export every kWHr possible to maximize profits.
An actual contract may include both rates.
Another possibility is voltage control. Some co-gen contracts pay for KVARs. An instance of this that I am familiar with is a city that was originally supplied by diesel generation. As the National grid evolved and grew, a transmission line was constructed to feed the city and the diesel generators were mothballed.
As the load increased over the years low voltage issues during peak demand periods developed. The old diesels were put back in service. They were run at low governor settings so as to output minimum kW and extend the life of the old engines but were run at high excitation levels to export the maximum KVARs to raise and control the voltage.
Another issue is station services.
One method of control is to set the generator voltage so that the correct voltage is supplied to the station services and use the transformer taps to adjust the voltage at the point of connection to the grid.
Peaking: Although the governor controls kW output, The maximum allowable kW output is at maximum voltage. If a very high rate is paid for peaking power, the generator may be run at the maximum allowable voltage setting and the transformer taps used to match the grid voltage. In cases such as this, there would be other means installed to control the voltage to station utilities.
In the real world the station operating method may be a compromise.
Most contracts to supply power to the grid will address power factor or KVAR issues.

Bill
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