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Power Plant Generator Control
Power plant generator control mode between MVAR, voltage control and PF

Could you explain power plant generator control mode between MVAR , voltage control and PF?

Thank you.

tommy_yotsakorn,

>Could you explain power plant generator control mode
>between MVAR , voltage control and PF?

Very briefly, synchronous generators have excitation control systems that control the amount of power applied to the generator rotor (called the generator field). The amount of power applied to the generator field directly affects the generator terminal voltage; increase the power applied to the field and the generator terminal voltage will increase. Decrease the amount of power applied to the generator field and the generator terminal voltage will decrease. The excitation control system is often referred to as the "AVR" (Automatic Voltage Regulator).

When you are synchronizing a synchronous generator to a grid with other generators you (or the control system) will adjust the excitation being applied to the generator field to make the generator terminal voltage approximately equal to, but usually slightly greater than, the grid voltage. This is done for two reasons: First, to make the voltage difference between the contacts of the generator breaker
nearly zero (which means less mechanical power is required to close the contacts), and second, to try to ensure that when the generator closes there will be positive VArs (or MVArs) flowing out of the generator.

Once the generator breaker is closed and the generator and its prime mover is synchronized to the grid changing the amount of excitation applied to the generator rotor will have an effect on the amount of VArs (or MVArs) flowing in the generator stator. Let's say the amount of excitation is exactly equal to the amount required to make the generator terminal voltage exactly equal to the grid voltage. In this case the VArs (MVArs) flowing in the generator stator would be zero--none. And, the power factor, at the generator terminals would be 1.0. (VArs (MVArs) and Power Factor are related). Now, if you increased the excitation being applied to the generator field that would mean you were trying to increase the grid voltage--but you can't really have much of an effect on grid voltage with your generator set. So, what ends up happening is that more lagging VArs (MVArs) start flowing in the generator stator windings. This causes the Power Factor at the generator terminals to move in the Lagging direction, and to be something less than 1.0, say, 0.98 (Lagging). Keep increasing the excitation applied to the generator field and the amount of lagging VArs (MVArs) flowing in the generator stator windings will also increase in the lagging direction. AND, the Power Factor at the generator terminals will keep decreasing from 1.0, again in the lagging direction.

Now, starting from a 0 VAr (MVAr) and 1.0 Power Factor condition (where the amount of power being applied to the generator field is equal to the amount required to make the generator terminal voltage exactly equal to the grid voltage), if you decreased the amount of excitation applied to the generator field you would be attempting to decrease the grid voltage. But, again, your generator can't usually have much of an effect on grid voltage, so what happens is that Leading VArs (MVArs) start flowing in the generator stator windings. And, the Power Factor starts to decrease from 1.0 in the Leading direction, to say, 0.99, or lower.

If the generator operator is manually controlling the VAr (MVAr) readings using the excitation (AVR) control system, then the unit is being operated in Voltage control mode.

Most generator prime mover control systems (and some generator excitation control systems (AVRs)) also have two other modes of operation: VAr Control and Power Factor Control. These two modes of control allow the operator to set a reference point of either some number of VArs (or MVArs) or a Power Factor that they want the control system to maintain. And, when VAr control is activated with a particular setpoint, say, 5 MVArs (usually +5.00), the control system will automatically adjust the excitation (AVR) to automatically maintain +5.00 MVAr all day and all night (as long as VAr control is active). The Power Factor will vary if the operator varies load, but the amount of VArs flowing in the generator stator will remain at 5.00 (Lagging) regardless of the amount of Watts (or MW) being produced by the unit.

If the operator is told to maintain a certain Power Factor, regardless of load, then the operator would input a Power Factor reference, say, +0.97--which usually corresponds to a 0.97 Lagging Power Factor, and then enable (activate) Power Factor Control. And, the control system will automatically adjust the excitation being applied to the generator field to maintain a generator Power Factor of 0.97 Lagging, regardless of the amount of Watts (or MW) being produced by the generator.

So, the generator is either in Voltage control (where the operator has to manually adjust the excitation to maintain the desired VAr (MVAr) or Power Factor setpoint); or it is in VAr Control where the control system automatically adjust excitation to hold a particular VAr (or MVAr) setpoint regardless of Watts (or MW) or Power Factor; or the unit is is Power Factor Control where the control system automatically adjusts the excitation to hold a particular generator Power Factor regardless of the amount of Watts (or MW) or VArs (MVArs). The operations department has to choose which mode the generator is to be operated in--it can only be operated in Voltage control mode, or VAr control mode, or Power Factor Control mode. It's one, or the other, or the other, but no combination of any two or three.

Just to let you know--many grid operators around the world are asking, or requiring, generators/power plants to STOP using VAr or Power Factor Control. VAr or Power Factor Control can be destabilizing to the grid if there's a grid disturbance, and so they are asking or requiring (forbidding) generators/power plants to use VAr or Power Factor Control. Meaning that operators have to monitor the VArs or Power Factor and manually make adjustments as necessary.

This topic has been covered many times before on control.com. You can search the control.com Archives using the 'Search' field at the top of the page at the far right corner of the Menu bar. (Use the Search Help function, because the syntax of searching using the control.com Search function is not like most other search engines/functions. It is very powerful and fast, but the way you have to enter the word(s) is not like most modern-day search engines.)

Hope this helps!

"So, the generator is either in Voltage control (where the operator has to manually adjust the excitation to maintain the desired V Ar (MVAr) or Power Factor setpoint)"

Quoting from the reply by CSA,
1.Is the mode above a manual mode? I thought if we were in the mode, it will automatically adjusted the voltage to a preset voltage value by adjusting the excitation.

Please correct me if it is wrong.

gustavo_marcelo,

>(where the
>operator has to manually adjust the excitation to maintain
>the desired V Ar (MVAr) or Power Factor setpoint)"

It's "manual" VAr or Power Factor Control.

Just to be clear, when one is clicking on RAISE- or LOWER VOLTAGE when the generator breaker is open and the unit IS NOT synchronized to a grid, the actual generator terminal voltage WILL change. The VAr (or MVAr) or Power Factor meters WILL NOT change, but the generator terminal voltage meter will change.

When one is clicking on RAISE- or LOWER VOLTAGE when the generator breaker is closed and the unit IS synchronized to a grid, the generator terminal voltage may change (and it may change more on some units than others). But the most noticeable effects will be on the VAR (or MVAr meter) and the Power Factor meters. AND, that's what most operators are looking at when the unit is synchronized to a grid and they are clicking on RAISE- or LOWER VOLTAGE--the VAR (or MVAr) meter and/or the Power Factor meter.

Just like when the unit is not synchronized to a grid and the operator clicks on RAISE- or LOWER SPEED/LOAD the actual speed will change, and when the unit is synchronized to a grid and they click on RAISE- or LOWER SPEED/LOAD the load (Watts; MW) changes but the actual speed DOES NOT change--most operators do not realize in both cases they are changing the prime mover speed reference. When the unit is not synchronized to a grid, the actual speed changes. And, when the unit is synchronized to a grid the actual speed doesn't change--the load (Watts; MW) changes. Operators aren't looking at the turbine speed when the unit is synchronized and they are clicking on RAISE- or LOWER SPEED/LOAD--they are looking at the Watt (or MW) meter.

In the same way, when they are clicking on RAISE- or LOWER VOLTAGE when the unit is synchronized, they are almost ALWAYS looking at either the VAr (or MVAr) or Power Factor meter. (Occasionally, at some places in the world and depending on contractual requirements, they may be looking at generator terminal voltage, but that's not the norm.)

So, exciter Voltage Control Mode when the generator is synchronized to a grid is really manual VAr or Power Factor Control--since mostly the operators are only interested in the present VAr (or MVAr) reading or the present Power Factor reading. They usually have instructions from their operations supervisors to maintain either a certain VAr (or MVAr) reading or a certain Power Factor--and they do so, manually, by using the exciter's Voltage Control Mode.

If they want to control the VArs (or MVArs) or Power Factor automatically they will use VAr Control Mode or Power Factor Control Mode.

I fear the original poster's question may have actually been simpler. He may have been asking about the relationship between VArs and Power Factor when using Voltage Control.... That's very difficult to explain without drawings or graphics. It was said that VArs and Power Factor were related, and it was said that the Power Factor would vary if VAr control was enabled as load (Watts; MW) changed, and that VArs would vary if Power Factor control was enabled as load changed. But, explaining why is difficult.

And, MANY operators do not understand why, during the day, that VArs or Power Factor can vary even if load does not change. Many do not realize the grid voltage can vary greatly during the day, and that can have a large effect on VArs (or MVArs) and Power Factor during the day. And, that's also very difficult to explain. Most operators just concentrate on either the VAr (or MVAr) meter OR the Power Factor meter, and adjust the "voltage" (the voltage reference, actually) to maintain the desired value of reactive current or Power Factor, or they enable and use VAr Control or Power Factor Control to do so for them (to change the "voltage" (the voltage reference)).

So, I don't know if we've properly answered the original poster's question; I hope so.

No worry.It is a good explaination indeed.
A bonus to the original poster.