Excessive reactive power on a generator/transformer set 5

[PMACP]

Hi,

What could be the reason that leads a synchronous generator connected to a 15 kV network through a 0,4 kV/15 kV transformer to produce excessive reactive power?

This effect tends to occur during periods of reduced load on the MV network.

The generator must operate in a situation such that Q = 0.4 P. In fact, most of the time, the generator works on such regime. However, at certain periods, the reactive power generated goes far beyond the active power generated.

Some help?

 

[dpc]

Voltage regulator settings?


"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[PMACP]

dpc:

The generator works in accordance to Q=0,4P most of the time. Only in specific periods it produces excessive reactive power. Is that possible that this behaviour may result from AVR settings? Can you, please, be more specific?

Thanks

 

[waross]

What dpc said.
As the voltage of the grid increases, the generator becomes under excited for the higher voltage required to match the grid. The generator imports VARs from the grid to provide the extra excitation.
Assume that the sense voltage for the Automatic Voltage Regulator is taken from phases "A" and "B".
Install a Current Transformer on "C" phase and develop a voltage across a resistor. Active current will produce a voltage at 90 degrees to the voltage from "A" to "B".
However reactive current will produce a voltage that is in phase with the voltage from "A" to "B". This voltage may be used to bias the sense voltage to compensate for reactive current and grid voltage swings. The resistor is built in to many AVRs. You supply the CT
Google "paralleling generators"

http://www.kilowattclassroom.com/Generators/GenReg4.pdf

(See note 4)
4. A cross-current Compensation Current Transformer (CCT) is required when paralleling generators in order to
insure proper load sharing between units.

Alternately, use an AVR that has provisions for power factor control or VAR control.

http://www.google.com/search?hl=en&q=paralleling+generators&start=20&sa=N

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

 

[PMACP]

waross:

Thanks for your good tip (making more clear what dpc said).

Please, clarify me about an additional doubt. If the transformer was a different relationship, the problem may not occur?

Thanks

 

[racobb]

How old is the installation? Just wondering if it has worked for a long time until now, or has recently been commissioned. As the load becomes less on the grid, the voltage is probably rising (like waross said) and to maintain var control, the AVR must raise as well. Is it possible you are hitting the limit of your AVR under these circumstances? Most have a "window" of about 10% that must be set at the proper bandcenter to allow for the utility swings expected.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[waross]

Let's build an example.
The line voltage is 500 Volts.
The generator is started. As the excitation is increased, the voltage builds up.
When the generator output is 500 volts the generator is connected to the line.
If the line voltage now drops the generator will be overexcited and will export VARs.
If the line voltage rises as it may if the load on the line decreases, the generator will be under excited and will draw enough VARs or magnetizing current to match the higher line voltage.
If a transformer in the system is using an On Load Tap Changer this may well have an effect on the power factor of the generator. It depends.
The simplest solution is at the AVR.

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

 

[pwrengrds]

How do you operate the voltage regulator? It sounds like you are in auto with a shifting voltage through out the day.

If you are in auto and have a voltage regultor that has a pf or var mode you might try a different mode of operation. If you can put the voltage regulator in a VAR or PF mode it would regulate the VAR or PF to a constant.

You also might want to check out the var limiters on the regulator to make sure they are working.

 

[catserveng]

Try going to this site,

http://www.basler.com/html/dwntech.htm

and look for these two documents,

Parallel Operation with Networked Systems

Voltage Regulation vs VAR?Power Factor Control

These may help you better visulize some of the information offered above.

Maybe some more information about your system would help, like the size of your unit, how you're controlling real power, how you're currently controlling reactive power (if at all). Do you have a desired target, like do you want to maintain unity power factor, or are you required to provide some VARS into the system? Is you system a baseload or load following system?

As noted above, several current models of digital AVR's have built in VAR/PF controllers, just need to be activated and tuned/adjusted properly. Also many genset controllers offer integrated real and reactive power controls in one unit. There are also add on VAR/PF controllers like the Basler SCP250. Some switchgear manufacturers use a bias output from a PLC into an analog input of the AVR to control VAR/PF during parallel operation. So there a lots of ways to do it, all have pros and cons.

As asked above, is this a new site with startup problems, or a site that has been operating and now this is a new issue?

Sorry for all the questions, but it may help get you the info you need.

Hope that helps.

 

[PMACP]

Hi,

The installation has less than one year. The generator has 2000 kVA power and the transformer has also 2000 kVA. The generator is operated at 1500 - 1700 kVA.

The target is to ensure that the reactive power corresponds to 40% of active power.

In attach I'm presenting some data about power generated by the unit during two days. Note that the generator remains stoped during the night period.

The data also show some forced stops that ocurred.

Question:

a) Is that possible that the excitation system allows the existence of unbalanced active and reactive power generation between the phases?

b) Is that possible that a some moments the active power generator is lower than the reactive power? Why is that?

Thanks

 

[dpc]

Reactive power and active power are controlled completely differently and are, to a large extent, unconnected from each other.

Real power output is controlled by the governor of the prime mover. More kW in, more kW out.

Reactive power is controlled by the voltage regulator. More excitation, more reactive power produced.

It is possible to have negative real power (a motor) and produce huge amounts of reactive power (a synchronous motor). And vice versa.

The key to your situation, as everyone has been trying to tell you, is the configuration of your automatic voltage regulator. If the regulator is in automatic, in voltage-control mode, it is perfectly understandable that the reactive power is fluctuating quite a bit since it is swinging the reactive power in an attempt to control the bus voltage, which it may not even be able to do. If your goal is to maintain a constant production of reactive power, the voltage regulator needs to have an option for var and/or power factor mode.

If you review the Basler links that CatServeng gave you, I think you will get a better picture.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[nawao]

IMHO the droop setting of your AVR is too low. Try to set it at 4% droop, not less and let us know how far.

 

[PMACP]

Hi:

I'm now trying to obtain the set points od the regulators.

However, at this moment I know that there are two control cards. The first one is a AVR that trys to adjust voltages. The second one is a card that controls the power factor (cos fi). On that situatiom, is possible that the reactive problem occurs? Which card is the master?

PMAC

 

[catserveng]

Your voltage regulator is always going to try and maintain your voltage setpoint. If a power factor controller is installed, it usually will move the voltage reference value to maintain the power factor (or var) setpoint. The Power factor controller should have logic the enables it after the unit circuit breaker is closed and some other indication that the unit is paralleled.

Would you mind sharing who made the tail end and the AVR? There may be a specific quirk to that manufactuer someone can help you with.

 

[PMACP]

Hi,

Catserveng, according to your tip, I may conclude that when the netork voltage increases (low load time) the AVR tries to increase the voltage, no matter the reactive power setpoint? This means that reactive power may exceed the value defined by the PF regulator?

Thanks.

PMACP

 

[dpc]

There are (basically) two automatic control modes that can be used:

1. Voltage control mode - regulator attempts to maintain constant output or bus voltage.

2. VAR or Power Factor control. regulator attempts to maintain a constant power factor or var output.

#1 is standard and generally the default.

#2 is often an option.

You seem to want VAR control. If the regulator you have does not have the VAR/PF control option, it is never going to work the way you want it to.

If you could provide actual information on the generator and voltage regulator you are dealing with, more specific advice can be provided.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[racobb]

"Catserveng, according to your tip, I may conclude that when the netork voltage increases (low load time) the AVR tries to increase the voltage, no matter the reactive power setpoint? This means that reactive power may exceed the value defined by the PF regulator?"

IF it is set to control var/pf and IF the bus voltage changes, then the AVR will adjust as well to maintain the setpoints, UNLESS it reaches the limit of its control window. IF it does reach the limit then the generator will be either under or over-excited depending on which direction the bus voltage changed, and it will either be absorbing or producing vars.....and there we have - that old nagging lagging or neading leading thing again!



Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[waross]

I think that you have it sort of backwards.
Under stable conditions, vary the voltage setpoint of the AVR.
You will see the VAR production and the power factor of the generator change as the excitation is changed from under excited to over excited. You have changed your voltage setpoint relative to the grid voltage. Now if the AVR setpoint is fixed and the grid voltage is changed, the generator will be overexcited when the grid voltage is low and under excited when the grid voltage is high.
Please reread the posts by catserveng and dpc.

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

 

[racobb]

Please explain how that is different from what I just said Bill....I just don't get it. If the unit is already in parrallel, the avr will only adjust to control to the var/pf setpoints (if it has any) when the bus voltage changes and in the direction of the change.....and unless it has reached the limit of the voltage adjust range.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner. Liberty is a well armed sheep!
Ben Franklin

 

[waross]

Network at 400 Volts.
Generator at 400 volts.
No problem.
Network goes to 410 volts.
AVR sense sees a voltage higher than the setpoint (400V)and tries to lower the voltage to get it back to 400V.
The AVR is lowering the excitation, trying to keep the voltage down to 400V. It can't. Meanwhile, the prime mover is still inputting energy to the generator. This energy is still being converted into KiloWatts.
The generator is underexcited and so is drawing the VARs from the system so as to be able to generate Kilowatts. You get KiloWatts going out and VARs coming in.
You need VAR control, power factor control or VAR compensation. Uncompensated voltage control by the AVR will not work.
Note; If the excitation was held constant and the grid voltage increased, the situation would be poor.
When the AVR detects the increase in the grid voltage and reduces excitation to try to compensate for rising grid voltage the poor situation gets worse.

"Catserveng, according to your tip, I may conclude that when the netork voltage increases (low load time) the AVR tries to increase the voltage,

The AVR tries to decrease the voltage.

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