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Excessive reactive power on a generator/transformer set
5

Excessive reactive power on a generator/transformer set

Excessive reactive power on a generator/transformer set

(OP)
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?

 

RE: Excessive reactive power on a generator/transformer set

Voltage regulator settings?  
 

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

RE: Excessive reactive power on a generator/transformer set

(OP)
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

RE: Excessive reactive power on a generator/transformer set

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

RE: Excessive reactive power on a generator/transformer set

(OP)
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
  

RE: Excessive reactive power on a generator/transformer set

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

RE: Excessive reactive power on a generator/transformer set

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

RE: Excessive reactive power on a generator/transformer set

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.

RE: Excessive reactive power on a generator/transformer set

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.

RE: Excessive reactive power on a generator/transformer set

(OP)
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

RE: Excessive reactive power on a generator/transformer set

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

RE: Excessive reactive power on a generator/transformer set

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.

RE: Excessive reactive power on a generator/transformer set

(OP)
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

RE: Excessive reactive power on a generator/transformer set

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.

RE: Excessive reactive power on a generator/transformer set

(OP)
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

RE: Excessive reactive power on a generator/transformer set

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

RE: Excessive reactive power on a generator/transformer set

"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

RE: Excessive reactive power on a generator/transformer set

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

RE: Excessive reactive power on a generator/transformer set

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

RE: Excessive reactive power on a generator/transformer set

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.

Quote:

"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

RE: Excessive reactive power on a generator/transformer set

Bill,

Please re-read my posts.  I specified in each that the unit must be equipped with and must be set for var/pf control for this to work.  Not pure voltage control.

I believe that you missed that part....
Thanks

Alan

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

RE: Excessive reactive power on a generator/transformer set

I apologize. I did miss that part.
Yours
Bill

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

RE: Excessive reactive power on a generator/transformer set

Thanks Bill....just wanted to be sure I was not missing something important.  It has appeared that most of the posts are essentially saying the same basic stuff...Alan

The major point that I wanted to make is that the AVR will have a sliding window of about 10% that must be adjusted at a level that will allow for the nromal utility voltage swings or it will run out of regulation range.  This can be especially true if they set it up on a "normal" non-peak day and then is run during a peak time when the utility has reduced the voltage for load control.  I have seen units that can't even parrallel when that happens.  The same can happen when the load on a feeder drops and voltage rises during non-peak times of the day or month.

BTW...Love the handle for the member ivegotgas....that's a hoot!

Alan

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

RE: Excessive reactive power on a generator/transformer set

It looks like you have gotten very good answers to most of your questions from those above.

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

As far as I know, the answer is NO.  The excitation system acts to increase the magnetic field of the rotor, which increases the voltage (very simple explanation), the balance is mostly affected buy the connected loads.  Some newer design generators, with what is called "square laminations" by one manufacturer, has an inherent votlage imbalance, and in networked systems canhave some strange readings at times, but not usually a problem, except if you're unfamiliar with it it may cause some head scratching.

As well explained above, increase excitation from the AVR to the pilot excitor (I'm assuming this is what you have due to size and age you have described)and the terminal voltage will rise if operating in a standalone condition.  If you are networked, or paralleled, the network pretty much defines the system voltage, unless your generator has enough capacity to change the system.  So your AVR can't work very well for controlling voltage.  Most power factor controllers for your size unit work by becoming active when told the unit is in parallel, then biasing the voltage adjustment reference setpoint.  If the unit is under-excited and consuming VAR's, the controller raises the reference setpoint, causing the AVR to increase excitation, in turn causing the generator to consume less VAR's, or produce VAR's into the system.  If the unit is over-excited and producing excess VAR's, then the controller reduces the setpoint, the AVR lowers the excitation level, and so on.

Along with this is the dynamic adjustments, the AVR has stabililty settings to determine how well it responds to transients in the system, and most power factor controllers also have some dynamic adjustments to determine their response capabilities.  In my experience most VAR/PF control systems get setup during periods of stable operation, then when system disruptions or transients occur, the VAR/PF controller may get into a dynamic conflict with the AVR, causing some instability, usually seen as large VAR swings.

So I guess if you want to be sure your system is functioning correctly, I'd try monitoring the field output of the AVR along with the other parameters you have.  Then you can see the actual system response to some of the apparent disturbances your data showed.

Hope that helps, Mike

RE: Excessive reactive power on a generator/transformer set

A man after my own heart!  Good description.

Alan

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

RE: Excessive reactive power on a generator/transformer set

You guys are right on: regulator problems.

My guess is your AVR does not have a PF/KVAR regulator.
If not, setting the droop higher might help.

We paralleled for years without PF/VAR regulators: we just set the droop higher. That is, more current, less voltage. As long as you stay in the capability curve of the generator, you will be fine. But those were big gensets.

You might have a stiff, moving, grid vs. a smaller genset. In that case, you will need to add a PF/VAR regulator or have someone manually adjust the AVR while watching a PF meter. And maybe change some components or your AVR. Some AVR's cannot accept a voltage reference, so you may need to add a motor-operated pot, for example.

Good luck
Steve  

RE: Excessive reactive power on a generator/transformer set

Fully agreed with what BSE1980 said:
"My guess is your AVR does not have a PF/KVAR regulator.
If not, setting the droop higher might help.
We paralleled for years without PF/VAR regulators: we just set the droop higher. That is, more current, less voltage. As long as you stay in the capability curve of the generator, you will be fine"
Set the droop higer and your problem will be solved.

RE: Excessive reactive power on a generator/transformer set

I have worked lots of sites where running in voltage droop between 3 and 6% worked fine while paralleled to grid, and a lot of sites it didn't work worth a darn.

Each site will have different dynamics, including voltage sag and swell, transients, and imbalance.  Voltage droop is a simple method, but isn't a cure all.  Neither is a VAR/PF controller, have seen sites where they caused their own problems.  Some sites do well with active power factor control, mostly "stiff systems" in my experience.  Some softer systems have done better in VAR control, some sites did fine with droop even though a controller was installed, we just didn't activate it.

While I'm sure operating in droop has worked for many people, based on the OP's information, just increasing the droop and calling it good may not solve his issues.

Best bet is to find someone experienced in DG systems to take a look at your system, see what you do have and how it's installed, check it tuning/adjustments, and get some experienced eyeballs on your system to make sure their ins't additional contributors to the issues you're seeing.

Hope that helps, Mike

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi,

The control cards that are installed on the generator are presented on attach. There is a A V R card and a cosfi/Var card regulated to cos fi control.

I don't know until this moment the set points of the cards. Do someone recognize those control cards? How do they the control?

Assume that the voltage on the grid increases. On that circunstances, the AVR card try to decrease the excitation of the generator in order to bring the voltage level to the adjusted value (as waross said before). However, decreasing the excitation imply lower generation of reactive power and, as a result, the cos fi tends to change unlesse the active power is also reduced. The active power is controlled by the power governator and, as long I know, it remains constant. So, on such circusntances, what may happen?

Thanks
 

RE: Excessive reactive power on a generator/transformer set

Can you tell us the manufacture of the controllers? Then those of us who are not familiar with those particular controllers may search for instructions on-line.
Thanks.

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

RE: Excessive reactive power on a generator/transformer set

"Assume that the voltage on the grid increases. On that circunstances, the AVR card try to decrease the excitation of the generator in order to bring the voltage level to the adjusted value"

If the voltage on the grid increases, proper var/pf control would raise the excitation on the gen set to compensate, unless it is at its upper limit.

Alan

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

RE: Excessive reactive power on a generator/transformer set

Is this an FKI AVR and Controller?

I have only worked on one of these on a shipboard system to parallel the ships's service generator to shore power.  My experience with this controller was not good.  Maybe my ignorance or unfamiliarity.

But we installed a Basler AVR and an SCP250 PF/VAR controller and the system worked fine after that, no reported problems after about 3 years.

You may want to see if someone from the forum from Europe may have more experience or information on your controller.

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi,

I have now some more data. My measuremnts show me that the reactive power that is supplied by the generator to the transformer are different on each phase. Moreover, the reactive power, at some periods, are capacitive on two phases and inductive on the other one.

Between the generator and the transformer there is some ancillary services (ie, the generator supplies some loads corresponding to ancillary services). This means that only a part of the generated power (the one that is not absorved by ancillary services) is injected on the transformer and then goes to the main grid. I checked that the ancillary services are reasonably balanced and so, the unbalanced reactive powers do not result from those loads.

The transformer has a voltage of 400 V on the side that is connected to generator. However, most the time it is working on higher voltage levels (420-440 V). Is that possible that the transformer may produce the referred effect on the reactive power due, for example, to satutation effects? The transformer has a wye with neutral - delta connection. The Wye winding is connected to the generator (with neutral connected).

Thanks

RE: Excessive reactive power on a generator/transformer set

Try floating the neutral. Leave the grounding connections on the generator. Normally a wye primary (power in) must have the neutral connected to the source. The exception is the wye primary:delta secondary. The delta secondary holds the phase relations ships of the wye and keeps the neutral point stable. Connecting the wye neutral and supplying balanced voltages forces the secondary (high side in this case) to have equal voltages and equal phase angles. However, if the high side voltages are not equal, you will have phase shifts on the high side. Now when you connect that to a transformer that is constrained to equal voltages and angles, you get circulating current transferring energy from one phase to another. There are a lot of ways to explain what the phase angle shifts do to the kW flow and the KVAR flow.
For instance, power flow depends on phase angle. When you open the governor to generate more kW, the relative angle of the rotor in relation to the utility phase angles advances until the power flow matches the new governor/throttle setting. When you have phase angle errors on the parallel system, you get different power transfer on different phases depending on the phase angle errors. The net power will still be governed by the throttle setting on the prime mover but one phase may be exporting more or less energy than the other phases. Even though the line currents may be moderate, the circulating current may be high.
If the voltage unbalance exceeds the %Z voltage of the transformer or bank by very much you may have overloaded transformers with little or no actual load. On a three transformer bank you may measure the circulating current in the connecting jumpers. In a sealed, three phase, high voltage transformer it is virtually impossible to measure the circulating current but disproportionate transformer heating is a symtom.
As I suggested, lift the neutral connection. Measure the voltage between the generator neutral and the transformer neutral. If this voltage is significant when compared to the %Z voltage of the transformer, unbalanced high side voltage and circulating currents are suspect.
Or, check the current in the neutral. If there is little current, move on to other solutions. If there is a heavy current on the neutral, suspect unbalanced high side volktages and proceed to disconnect the neutral and check further.

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

RE: Excessive reactive power on a generator/transformer set

(OP)
Bill,

Hi. thank you for your very good tips. I think some of the problems I have are in fact related to what you said. However i need some more details if you can give them to me.

a) Have you some written information on that subject (some book, or a paper for example)?

b)You said that I should "floating the neutral". I have some current on the neutral conductor (50A while the phases are near 2000 A). This means that the currents supplied by the generator to the transformer are quite balanced! However, the ancillary services have some single phase loads, so I need to keep the neutral connected (or to create an alternative). Besides that discussion, at this moment I need to understand those "strange" unbalanced powers. That is why I need some more information.


c) you said. "However, if the high side voltages are not equal, you will have phase shifts on the high side. Now when you connect that to a transformer that is constrained to equal voltages and angles, you get circulating current transferring energy from one phase to another."

Well, because we are talking about a delta winding (MV side), those circulating currents only appear on that side. Is that true? Is there some way to catch some evidence of that phenomena on the LV side of the transformer?

d) You said "There are a lot of ways to explain what the phase angle shifts do to the kW flow and the KVAR flow.
For instance, power flow depends on phase angle. When you open the governor to generate more kW, the relative angle of the rotor in relation to the utility phase angles advances until the power flow matches the new governor/throttle setting. When you have phase angle errors on the parallel system, you get different power transfer on different phases depending on the phase angle errors. The net power will still be governed by the throttle setting on the prime mover but one phase may be exporting more or less energy than the other phases."

Well, in fact I have some unbalences on the values of active power. However, the more important unbalances takes place on reactive power. In some periods it occurs that one phase is absorving reactive power while the others are injecting it on the transformer. Is that possible? And, why are the unbalances more significant on reactive power? Also it is important to stress that in some periods the total reactive power increases significantly and it becomes greater than the active power.

e) You said: "On a three transformer bank you may measure the circulating current in the connecting jumpers. In a sealed, three phase, high voltage transformer it is virtually impossible to measure the circulating current but disproportionate transformer heating is a symtom."

I have no equipment to measure the currents on the HV side (due to voltage level). So I have a problem. However the transformer is, in fact, heating more than what is supposed to. You suggest me to "lift the neutral connection and measure the voltage between the generator neutral and the transformer neutral." Which could be the risks of doing that? Also you suggest me to check the current in the neutral. If there is little current, I should move to other solutions.

Which do you believe to be a significant current in terms of phase currents for instance?

f) Finally, what about a  possible situation of transformer saturation?

Thanks
 

RE: Excessive reactive power on a generator/transformer set

Can you measure the primary voltages?
Can you disconnect the generator and measure the voltages at the 400 volt side of the transformer? (Use the transformer as as a potential transformer to determine if the primary circuit is unbalanced.)
With the neutral disconnected from the generator the delta winding should keep the neutral in place.
Your neutral current looks like about 3.6% of full load current.
Circulating current is limited by the transformer impedance so let's hope that the 50 A represents unbalanced loads and not circulating current.
Circulating currents caused by voltage unbalance are a classic case of actions and reactions. They tend to lessen the effect that causes them. Once the neutral is disconnected treat it as energized. There should not be much voltage between the generator neutral and the floating transformer neutral, but in the event of a ground fault on your 400V system the voltage on the floating neural may rise to lethal levels.
Floating the neutral allows the phase angles to align with any phase errors on the utility side.

Yes, saturation may be an issue. Is it possible to change the taps on the transformer? 440V is 10% high for a 400V transformer. +5% would be right on for 420V and more forgiving of 440V. (or maybe -5% depending on how you look at tap ratings.)

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

RE: Excessive reactive power on a generator/transformer set

In order to avoid unbalanced voltage on the primary side of the transformer, could you try feeding the auxiliary services from a separate transformer 15kV/0.4 kV delta-wye of adequate kVA. In this case you can leve floating the neutral on the primary as waross sugested

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi

"Can you measure the primary voltages?"

Yes, I have some values. I'm sending you an attach with some violtage, current and power values. As well I made some comments about the regime of operation of the generator-transformer set.


"Can you disconnect the generator and measure the voltages at the 400 volt side of the transformer? (Use the transformer as as a potential transformer to determine if the primary circuit is unbalanced.)"

Well, in fact I did it but during times when the generator is not deliberately disconnected from the transformer. at those periods, the transformer remaisn conncted to the grid in order to supply some ancillary services.


"Your neutral current looks like about 3.6% of full load current."

You can see some more detailed values on attached file.

 

"Yes, saturation may be an issue. Is it possible to change the taps on the transformer? 440V is 10% high for a 400V transformer. +5% would be right on for 420V and more forgiving of 440V. (or maybe -5% depending on how you look at tap ratings.)"

yes, it is possible, but I need to understand what happened.

Do you thing that primary overvoltages greater than 5% will create a saturation situation?

RE: Excessive reactive power on a generator/transformer set

(OP)
nawao:

in fact I can't do it. I don't have an available transformer to do that. Note that the ancillary services current may achieve 300 A per phase.

Thanks

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi again

Is that possible that the generator works with symetrical active and reactive power on each phase and than the transformer presents unbalanced reactive powers on each phase? Is that a evidence of saturation?

Thanks

RE: Excessive reactive power on a generator/transformer set

That's an interesting question. I'll have to think about it. If the voltage is high on one phase of the utility, and the transformer goes into saturation on that phase, the transformer will draw more magnetizing amps or VARs from the line. I'm not sure what you would see on the generator instrumentation.
If voltage is high on one phase of the utility side, then the generator will be under excited on that phase and will be importing VARs. The PU current could be higher than normal on the utility side with the difference or loses producing heat in the transformer.  

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

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi:

Bill: "That's an interesting question. I'll have to think about it."

Thank you. I wait for your opinion.

Now, can you tell me if there is some problem resulting from connecting a measurement equipment to the voltage transformers on the HV side of the power transformer. I'm thing to connect my measurement equipment to the terminals of a power relay. Please see the attach.

If I want also measure the currents, may I put my measuremnt circuit in serie?

Thanks

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi,

I have now some data about MV voltages:  

Time    UAB    UBC    UAC
7:00    16026    16065    16126
8:00    16050    16066    16120
9:00    15837    15871    15924
10:00    15897    15913    15958
11:00    15805    15838    15912
12:00    15882    15898    15987
13:00    16018    16045    16113
14:00    15799    15829    15880
15:00    15880    15880    15978
16:00    15922    15937    16018
17:00    16026    16059    16101
18:00    15969    16005    16075
19:00    15885    15843    15952
20:00    15945    15987    16011
21:00    16084    16087    16186
22:00    16194    16191    16259
23:00    16072    16083    16165
0:00    16162    16201    16266

Note that the tap is regulated for 15375 V. It has Ucc=6%, and it is a ynD05 with 2000 kVa of power.

There are some unbalances on voltages on the HV side of the transformer. Is that sufficient to produce problems?

The voltage is permanently above the regulated voltage. Problems?

Thanks.
 

RE: Excessive reactive power on a generator/transformer set

There are some errors in the data you earlier supplied; you cannot add the values as you did in the columns R, V and Z.
Definitely, the AVR is not doing its job at all.
Did you check the following setting procedure in the card M50FA400A?

SETTING PROCEDURE WHEN MACHINE IS
CONNECTED WITH THE MAINS
After connection to the mains, check the correct operation
of current transformer (TA2).
Rotate the potentiometer P1 clockwise: the reactive power
(when BRI is in VAR position) must increase, or the power
factor (lagging) must reduce. If there is a different
behaviour, reverse the wires from the current transformer
TA2 and check if it is connected to the correct phase.
 

RE: Excessive reactive power on a generator/transformer set

(OP)
Nawao,

Hi. Can you, please, explain why I can't add the values? I measured the power per phase. So, The total reactive power (for instance) should be the sum of the reactive power of each phase. Where is my mistake?

Thanks

RE: Excessive reactive power on a generator/transformer set

(OP)
Nawao,

I forguet to say that in last post. The AVR only works when the machine starts. After it is connected to the main grid, the AVR is stopped and the control is made by the cos Fi regulator. Sorry for forguet say you that.

This means that the voltage is not controlled, right?

RE: Excessive reactive power on a generator/transformer set

Is not right. Both cards work together. The procedure I'm talking about corresponds to the reactive power/power factor regulator M50FA400A.  The voltage must be always controlled  

RE: Excessive reactive power on a generator/transformer set

(OP)
nawao:

The guy that isntalled the system said to me that the AVR card was set to 400 V. Also he said to me that this card only is used in order to do the excitation of the generator before the paralell. After the paralell, the AVR card doesn't any contro (He said), and only the cos fi card acts.

I agreed with your opinion that both cards should act at same time... however the guy said that is not the case. So, in your opiniom is impossible the kind of control the guy told me?


Please, can you give me more details about why I can't add the values you refereed in your previous post? I measured the power per phase. So, The total reactive power (for instance) should be the sum of the reactive power of each phase. Where is my mistake?

 

RE: Excessive reactive power on a generator/transformer set

Sorry, the data is correct. I mistake lines to phasors.
Regarding what the guy says; forget about it.
Just applie the settings procedure and let us know the results.

 

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi,

nawao, note that the card M50FA400A is regulated in order to control de power factor, not the reactive power directly. That is, the card trys to maintain a specific cos fi not a constant value of reactive power. So, when the voltage goes up the generated reactive power tends to increase and, as a consequence, the cos fi tends to decrease. The card should reduce the excitation of the generator in order to reduce the reactive power generation, right?

Therefore, the voltage should be reduced.

However, because there is the obligation of injecting a value of reactive power corresponding to 40% of the generated active power, the generator cannot work importing reactive power. So, it can't reduce the voltage levels on its terminals... That is, if the AVR was working, we can control the voltage level but it will occur that the reactive power goes from exported by the generator to imported... right?



 

RE: Excessive reactive power on a generator/transformer set

No 400V generator tied into the larger system should be trying to do voltage control, it can't.  If it tries, one of the symptoms is outrageous var flows.  The voltage will be what the voltage will be and your generator can't do anything about it, but if it is in voltage control mode it will try to produce or absorb vars to bring the voltage into range.

It is also impossible to do both voltage control and power factor control simultaneously.  Trying to do both can easily result in a situation where the voltage controller is trying to absorb vars while the power factor controller is trying to produce vars.  Pick one.

RE: Excessive reactive power on a generator/transformer set

"nawao, note that the card M50FA400A is regulated in order to control de power factor, not the reactive power directly"
The card has both possibilities; the switch BRI allows COSfi or VAR control. Read the attached document again.

RE: Excessive reactive power on a generator/transformer set

(OP)
Hi,


nawao, I read the datasheet, and I know what you said. However, the card is regulated to cos fi control, and i need to know what happens on that situation before do some other control. On other and, davidbeach said (previous post) that is not p+ossible to control the voltage.... what you think anout that?

Thanks.
 

RE: Excessive reactive power on a generator/transformer set

PMACP,
This procedure verifies if the card and the associated current transformer are working properly.
Seems to me that you are not familiar with this kind of problems. In this case ask assistance to a qualified technician and perform the procedure.

SETTING PROCEDURE WHEN MACHINE IS
CONNECTED WITH THE MAINS
After connection to the mains, check the correct operation
of current transformer (TA2).
Rotate the potentiometer P1 clockwise: the reactive power
(when BRI is in VAR position) must increase, or the power
factor (lagging) must reduce. If there is a different
behaviour, reverse the wires from the current transformer
TA2 and check if it is connected to the correct phase.
 

RE: Excessive reactive power on a generator/transformer set

(OP)
nawao:

I did the procedure. The card appears responding as expected.

Note that the excessive reactive generation only occurs at some specific periods of the week. In fact it tends to occur at saturday near 12:00-14:00 when the voltage on the grid increases. During the rest of the time the system works well.

As well, it is important to stress that the reactive power tends to appear unbalanced and, sometime there is a phase importing vars and the others exporting it

Assuming that the card is operating correctly, which shoul be its behaviour when the voltage level of the grid increases? if the voltage increases, the reactive power generated tends to increase and, assuming a constant value of active power, the cos fi tends to decrease. So, the card will act in order to decrease the excitation of the machine in order to reduce reactive power generated. Correct? So, why that is not happening? In fact, when the voltage goes beside a specific value, the generated reactive power increases so much.

Is there some possibility of transformer saturation?

 

 

RE: Excessive reactive power on a generator/transformer set

Quote:

As well, it is important to stress that the reactive power tends to appear unbalanced and, sometime there is a phase importing vars and the others exporting it
I expect that the same load shedding that is causing the grid voltage to rise is also causing the phase voltages to become unbalanced.
Why, good question. It may be a sticking voltage regulator,it may be a faulty capacitor switching mechanism or a faulty capacitor in a stepped capacitor bank, it may be a large phase to neutral load still in place on the grid, it may be a combination.
It has been shown how the voltage relationship between the generator voltage and the grid voltage influences the magnitude and direction of VAR flow. If the voltage relationship varies from phase to phase (due to unbalanced grid phase voltages), so also may VAR production and direction vary.

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

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