Hi Everyone.
We have 132KV GIS, by practice if any drop in substation voltage we used to tap down the generator step up transformer (GSUT) which in turn increase that particular synchronous generator MVAR subsequently that generator connected GI substation voltage also will be improved.

My doubt is by reducing the tap in GSUT how it influencing with generator MVAR?

 

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I win!

I correctly guessed there would be a tap changer question from @curiouscat.

VERY QUICKLY, let’s say the generator terminal voltage is exactly equal to the voltage of the electric power system on the GENERATOR side of the GSU (Generator Step-Up transformer)—the input side of the GSU. (In this condition the power factor at the generator terminals is 1.0, or unity—meaning that no reactive current is flowing in the synchronous generator stator windings, meaning the VAr magnitude is 0 MVAr.) And then the transformer tap is adjusted such that the electric power system voltage on the input side of the GSU (at the generator output terminals) is higher than it was before (for example 100 volts higher)—AND the synchronous generator excitation was NOT adjusted/changed.

This would mean the synchronous generator power factor would move to the LEADING direction and reactive current would flow into the synchronous generator stator windings from the electric power system, the power factor magnitude would change a value less than 1.0 in the LEADING direction and the MVAr reading would increase to a value more than 0 in the leading direction. (The Mark* turbine control system shows the LEADING value of MVAr as being a negative number.)

In this case the operator might be instructed to increase the excitation to return the power factor to 1.0 and the MVAr value to zero. This would have an affect on the output voltage of the GSU and in many cases it would be done to have some affect on the electric power system on the output side of the GSU. There are several possible scenarios where a tap changer could be used to enhance electric power system operation and/or stability under certain conditions or circumstances. Without knowing the design criteria for the electric power system interface it’s impossible for us to say with any certainty what the tap changer is supposed to do and when it’s supposed to be used. I have read of synchronous generators that rarely, if ever, have their excitation systems adjusted/modulated manually or in some cases automatically but the electric power systems were in need of some serious support for unknown reasons.

Anyway, that’s all I got. If you’re working at a power plant with tap changers there is probably a document that describes when and how the tap changer would be used to achieve a desired affect on the electric power system on the output of the GSU. You need to find that document and read it and re-read it several times to gain a better understanding of how and what it was intended to do.

I will try to make an analogy to what happens when the energy flow-rate into the synchronous generator prime mover is increased while the machine is synchronized to an electric power system with other machines. You mentioned a load angle. That is one way the change in torque being applied to a synchronous generator can be visualized and even quantified to an extent. As was agreed if the torque from the prime mover increases it will TRY to increase the speed of the generator rotor but the speed can’t change or the frequency of the machine would change and all machines synchronized together must produce power at the same frequency. That increased actually changes the amount of the “twist” being applied to the generator rotor coupling shaft. The more torque applied to try to make the shaft speed increase the more “twist” that the coupling experiences, and if measured in degrees of angle it can be considered the torque angle of the machine. (The actual torque angle is really just a theoretical measurement that is used to choose some design synchronous generator criteria as well as when choosing a coupling for connecting the prime mover to the generator rotor.

If at a steady operating output power the generator terminal voltage is exactly equal to the voltage of the system on the output side of the generator breaker then the power factor of the machine is 1.0, or unity. If the generator excitation is increased the generator terminal voltage increase and the generator will be trying to also increase the system voltage on the other side of the generator breaker. BUT the electric power system will actually try to resist any increase of its voltage. And the DC power being applied to the synchronous generator rotor windings actually causes reactive current to flow out of the generator stator windings to the electric power system and to the electric power system. This results in LAGGING MVArs flowing out to the electric power system and a power factor less than 1.0 in the LAGGING direction. (The GE Mark* considers LAGGING VArs to be positive VArs.)

 

If you reduce the tap on the GSUT, that decreases the output voltage at the generator's terminals. It prompts the generator's AVR to increase excitation. It increases the MVAR output. This increased reactive power output can help improve the overall voltage at the connected GIS substation, enhancing voltage stability and operational reliability.