AVR - thread238-107396

[Mark GT]

thread238-107396
A reply from dpc says "So even a very small voltage error signal will cause the regulator to drive the excitation to maximum (or minimum) trying to get the voltage input back to the setpoint, but the generator just can't do it. "
Can anyone elaborate?

 

[itsmoked]

You have to take dpc's entire statement not one sentence of it.

However, consider the situation of a generator tied to the power grid or another larger generator. The single generator is not large enough to change the grid voltage in most cases. So even a very small voltage error signal will cause the regulator to drive the excitation to maximum (or minimum) trying to get the voltage input back to the setpoint, but the generator just can't do it. So the generator excitation will swing from min to max as the voltage varies. For these situations, you must switch from voltage control to Var or PF control. In these modes, the setpoint is the kVAR output of the machine or its output power factor. Now the voltage can vary (within limits) and the regulator will be happy and making excitation adjustments to maintain a constant generator power factor or VAR output.

What is your specific issue? What situation is your generator working in?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

With a paralleled generator changing the excitation will change the reactive current.
If the grid voltage drops, and the excitation stays the same, the reactive current will usually increase. (Exception: If the generator is operating under-excited, the reactive current may drop, and then as the grid voltage drops further the reactive current will increase.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mark GT]

A reply from dpc says "So even a very small voltage error signal will cause the regulator to drive the excitation to maximum (or minimum) trying to get the voltage input back to the setpoint, but the generator just can't do it. "
Can anyone elaborate?

HiI do not really understand why does a small voltage error causing the AVR to drive the excitation to max or min.

Can anyone elaborate the meaning of docs post.

 

[waross]

Islanded: A small drop in the voltage (due to increased load) will cause the AVR to increase the excitation and so bring the voltage back up to the set point.
Parallel: A small voltage drop (Grid voltage drops) will cause the AVR to increase the excitation to try to bring the voltage back up to the set point. One generator cannot change the voltage of the grid. (One generator may make a very small change but not enough to bring the voltage back to the set-point and to satisfy the AVR.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

If something, a system, a generator, (anything!), is trying to control an output variable but, in-fact, has no control of the output variable it will normally go all the way in the direction the error points.

Does this make sense to you Mark?

Example: A valve is looking at the water level in a tank. The tank has a hole in the bottom too big for the supply the valve is controlling to fill the tank back up to the desired level. The valve is going to be wide open ALWAYS but it won't ever affect the level it's trying to control to.

A generator working against a LARGE power network is in the same situation. Whatever the network voltage is one generator is not going to change it. It simply doesn't have the authority to force a change. So, the regulator controlling the field which is attempting to control the output voltage is going to go to a maximum or minimum depending on the voltage setting it's asked to control to. If the grid is 20kV and the generator voltage control is set to 20.001kV the regulator will go full ON trying to drag the entire network up one more volt.

In some cases this could fry the regulator or burnout the field as it will just sit pegged the entire time. It could and likely would trip the unit off line.

In this situation the generator control is changed to an output variable it does have control over. VARs, or power, or current, all variables that can be controlled against a rigid fixed voltage.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[crshears]

Caveat: An AVR's ability to control generator terminal voltage when in parallel to the grid is directly proportional to the impedance of the path to the grid; in cases where the generator is behind a higher impedance path to the larger grid, it will, within limits, be capable of controlling the generator terminal voltage to its setpoint.

CR

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