CSA,
Another way to view this is by considering constant real power. At 1.0 PF the current required to transmit the power is lower than current required to transmit the same power by use of P=VI x p.f. Hence the lower the PF the higher the current needed for constant power.
According to Lenz's law, the current in the stator creates the electric torque which opposes the mechanical torque produced from the turbines. The higher the current, the higher the electrical torque hence more mechanical torque is required to generate the same power at a lower power factor.

Solomon

 

W

Ashish,

That's excellent; you've read the fine manual.

A synchronous generator's terminal voltage is a function of the amount of excitation applied to the rotor's windings. At synchronous speed (which is what most synchronous generators should be operated at) which should be stable and for all intents and purposes fixed, when the excitation is increased the generator terminal voltage will tend to increase, and when the excitation is decreased the generator terminal voltage will tend to decrease.

When the generator is NOT connected in parallel with other generators, the generator terminal voltage will absolutely increase or decrease in direct proportion to the amount of excitation applied. When the generator is being operated in parallel with other generators the terminal voltage doesn't usually appear to change very much because of the effect of the system voltage which appears to keep the generator terminal voltage relatively constant. (Depending on many external factors, the generator terminal voltage may actually be seen to increase or decrease, but not by the same amount as if the generator were NOT operating in parallel with other generators.)

It's generally intended to keep the generator terminal voltage relatively constant when the machine is operating in parallel with other generators. So, the amount of excitation is varied as necessary to maintain the generator terminal voltage setpoint. Usually, there are some PTs (Potential Transformers) used to provide the feedback to the circuit that's used to vary the excitation as required to maintain the actual generator terminal voltage equal to the setpoint.

When a synchronous generator is being operated in parallel with other generators on a large or infinite grid and one increases the excitation above the amount required to keep the generator terminal voltage exactly equal to the system voltage, then reactive current will begin to "flow" through the generator's stator windings. As the excitation is increased the Power Factor of the generator will begin to decrease from 1.0 in the lagging direction, and the VAr meter will increase above 0 in the lagging direction.

If the excitation is decreased below that required to keep the generator terminal voltage exactly equal to the system voltage the Power Factor of the generator will decrease from 1.0 in the leading direction, and the VAr meter will increase above 0 in the leading direction.

So, power factor and VArs are related to excitation. Different control loops can be created, using Power Factor sensors or VAr sensors, to control the Power Factor or VAr "flow" of the generator by varying excitation as required, "automatically." The operator sets a Power Factor setpoint, or a VAr setpoint, and the excitation control system automatically adjusts the excitation as required to maintain the desired Power Factor or VAr "flow."

When this is happening (excitation is being increased or decreased when the generator is being operated in parallel with other generators on a large or infinite grid), the generator terminal voltage may or may not appear to change by very much--again, that's a function of many factors. But when the excitation is exactly equal to the amount required to keep the generator terminal voltage equal to the voltage of the system which which it is synchronized then the Power Factor of the generator will be equal to 1.0 (Unity) and the VAr meter will be at 0.0 VArs.

Since the amount of excitation being applied to the generator rotor is being varied, there is an "inner loop" to this generator terminal voltage control mode, commonly referred to by most other manufacturers as Automatic Generator Voltage Control Mode, or Auto Mode, or AVR (Automatic Voltage Regulator) mode. When the excitation control system is in "Auto" mode, or what Brush appears to be referring to as Main control mode, the setpoint is generator terminal voltage and the feedback is generator terminal voltage.

The "inner loop" is the excitation current, which must be stable for the generator terminal voltage to be stable. In this loop, the setpoint might be either excitation voltage or excitation current, and the feedback would be either excitation voltage or excitation current, respectively.

Most excitation control systems have a method for only controlling the excitation terminal voltage or excitation current--the "inner loop." And most manufacturers call this the "Manual" control mode--what Brush appears to be calling Standby control mode.

In Manual control mode (or Brush's Standby control mode), the unit operator (a person) manually controls generator terminal voltage, or generator Power Factor, or VAr "flow" by adjusting the excitation terminal voltage setpoint or the excitation current setpoint.

Now, here's where it gets really dicey--especially on control.com. If the generator terminal voltage setpoint is not changed as a generator's load (Watts; KW; MW) are increased the Power Factor of the generator will "decrease" in the leading direction, and the VAr "flow" will increase in the leading direction. So, as a generator's real power increases, it's generally necessary to increase the excitation just to maintain a constant Power Factor or VAr "flow". There is some disagreement about the actual mechanics of how this happens amongst some posters here on control.com, but it is agreed that unless some action is taken as the generator is loaded the Power Factor and VAr meter will move to the leading directions.

Conversely, if a generator is operating at some load and the Power Factor is at 1.0 and the VAr meter is at 0.0 VArs, and the generator is unloaded, then the generator Power Factor will decrease in the lagging direction and the VAr meter will increase in the lagging direction. Again, the actual mechanics of how this happens in the generator is in contention, but it is agreed that this will happen.

Now, if a generator is being operated strictly on generator terminal voltage control mode (no Power Factor control or VAr control) and the system voltage changes, then the generator's Power Factor and VArs will change. And, system voltages do usually change during the course of a day, sometimes dramatically during the day depending on conditions and how the grid is "operated" or "controlled." So, this is another reason plants typically use either Power Factor control or VAr control, to make sure the generator's Power Factor or VAr "flow" stays relatively constant regardless of external factors (like changing system voltage).

Finally, some generator excitation systems have a method of reducing excitation automatically when a machine is being unloaded to prevent "excessive" lagging VArs from flowing, and thereby preventing the generator's Power Factor from becoming low in the lagging direction. This is usually called "VAr Shedding".

Hope this helps. It seems the Brush's Main Control Mode is the same as most other manufacturer's Auto excitation control mode (sometimes called the AC control mode), and the Brush's Standby control mode is the same most other manufacturer's Manual control mode (sometimes also called the DC Excitation control mode).

Warning: Be VERY careful about your electrical terms if you reply. (For example, don't refer to reactive "power". Reactive current is fine; reactive power is not--for one person on control.com.)

what is VAR?