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Generator And Pf Correction


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#1 BServEngr

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Posted 29 June 2006 - 05:00 AM

We have a 500 kVA Diesel gen set to feed the essential loads( HVAC, elevators,some lights, Water pumps).
Should I connect the PFC capacitor panel to the Gen supply Distribution panel or to the Main distribution panel? Is there any harm in connecting the PFC capaciotrs to gen supplied panel?

Any clarifications would be of great help.
Thanks.


#2 chaterpilar

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Posted 23 July 2006 - 03:34 AM

The PFC should be used very carefully in the case of generator and precise calculations done to decide the vars required.

Any overcompensation will be harmful to alternator and also negate the use.

0.95 should be the target at normal running load, and of course the the capacitors should be interlocked with the loads.

Chaterpilar

#3 kimngoc

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Posted 14 September 2006 - 07:40 AM

QUOTE(chaterpilar @ Jul 23 2006, 03:34 AM) View Post

The PFC should be used very carefully in the case of generator and precise calculations done to decide the vars required.

Any overcompensation will be harmful to alternator and also negate the use.

0.95 should be the target at normal running load, and of course the the capacitors should be interlocked with the loads.

Chaterpilar


Hi, my name is Ngoc.
My plant have capacitors, now we want to turn it on. But I don not know the range correction power factor when we turn on capacitor. You said that "0.95 should be the target at normal running load". Why you think so that 0.95 is the target at normal running load? If I increase pf greater than 0.95 in my plant what problem will happen?
Best regard,
Kimngoc.

#4 subhash

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Posted 27 September 2006 - 12:41 AM

Hallo I am subhash ,
ours is the continious process industry we are running both 1600 kva capacity generators
load nature is @ 60 % load is resistive heaters we are using for air heating of HVAC system
due to frequent switching of heaters through pid cycle of PLC we can not run DG set above
50 % as there is peak of inrush current and which causes voltage veriation problem,+/- 10 to 20V
also there is tremondus veriation in PF due such load nature ( from 0.90 to Lead 0.50) frequently.
Is there any solution to maintain alternator pf close to designe level that we can utilise our full capacity.
please suggest

#5 marke

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Posted 27 September 2006 - 08:00 AM

Hello Kimngoc

Sorry for the slow reply. - I missed your post.

If you correct to a power factor of 1, you will have the minimum current flow from the supply. At a power factor of 1, you also have the greatest potential of resonance between the capacitors and the load. If the load is high, this will not be a problem.
Adding capacitors will cuase stepped changes in power factor due to the capacitors steps used. If you have a small number of large steps, then the power factor will step by large amounts. Many small steps will allow closer control, but icrease the cost.
The figure of "better than 0.95" is the common target and allows for reasonable steps in capacitance. The supply curent at 0.95 is only a little higher than the current with a power factor of 1.0

If you over correct, the power factor begins to drop down below 1.0 and the current increases. There is no advantage in over correcting and the potential for resonance of the capacitors with the supply inductance increases if the load is over corrected.

Generally, if there is a power factor penalty, it is applied to loads with a power factor of less than 0.95 so that is a target set by the power suppliers.

Best regards,

#6 marke

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Posted 27 September 2006 - 08:11 AM

Hello subash

If your powerfactor is going very capacitive, it will not be reducing or minimising the supply current and this will not help our voltage regulation.

There are two ways to apply power factor correction, either bulk correction with an automatic controller, or static correction with the capacitors controlled with the individual loads. I would sugest that you apply static coreection to loads that are frequently switching. Bulk correction will lift the average correction, but will not track rapidly changong loads. This is fine where you are avoiding a maximum demand tarrif which is basedon an integrating meter. In you case, you are concerned with instantaneous loads on the generators.

If you are switching very quickly, you may need to consider zero voltage switching solid state contactors on the capacitors.

I would suggest that you try to keep the power factor in the 0.95 to 1.0 region.

If you are having voltage stability problems due to current transients, you probably have an issue with the AVR and excitation systems on the generators.

You should avoid the use of single phase peak reading AVRs on generator systems that have overcurrent transients caused by the switching of loads such as motors. The best form of AVR to use is a three phase averaging AVR where there are curent transients and/or harmonics present.
Self excited systems run out of excitation on small overload transients and effectively current limit causing the voltage to drop. PMG (Permanent Magnet Generator) based excitation will provide a much higher overload capacity and transient ride throughs.

Best regards,

#7 subhash

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Posted 27 September 2006 - 08:39 PM

Hi
Thanks for early reply
our is the PMG ALTERNAATOR with one of the good quality three phase sensing AVR but due to regular switching of load (load is different heaters which we are using for air heating application in our HVAC system) load switching is for @ 4 sec each it is requirement of our process to put temp,within narrow band.we cant eliminate this,due to such loading on our alternator we are facing problem of continious load which causes veriation in voltage say @ 10 to 20V and we are not able to load our DG above 50% of its capacity.Is there any solution to play with this very veriable load and avoied voltage veriation problem.
is a line reactors are suitable and economical for 1600 kva x 2 nos DG set
pl suggest
Regards,


#8 waross

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Posted 11 October 2006 - 02:19 AM

Hello subash;
I think that you have two problems and I would suggest that both problems should be corrected.
The first problem is your voltage swings and the time proportional control scheme.
This is very hard on the generator bearings.
Years ago, I encountered a generator problem. A new diesel generator had the crankshaft bearings fail in just a few weeks. The engine was repaired under warranty, only to fail again in a few weeks. After the third failure, the load was analyzed. It was found that an office building supplied by the generator was using time proportional control of the electric heating. The control was changed to a simple thermostat and there were no more crankshaft bearing failures.
There are a few differences;
The set I encountered was switching about every 4 cycles rather than every 4 seconds.
However, the electric heating load was a much smaller proportion of the total load.
It is normal for diesel engines in your size range to run 30,000 hours between scheduled main bearing replacement. That is, the sets are running well but are taken out of service at 30,000 hours and and a major overhaul including replacing the main bearings is preformed.
I doubt very much that your engines will reach 30,000 hours before failure.
I don't want to write an article on heating controls, but I strongly suggest that you consider changing your control scheme to one that does not abuse the engines.
If you measure the voltage that the AVR supplies to the field at various load levels, you will quickly see why there are voltage swings when the heavy load switches. This is normal for a few cycles until the AVR can respond. A smoother control scheme will reduce and possibly eliminate the voltage swings.

Power factor.
The fact that the power factor is going leading indicates that you are over correcting the power factor.
The over correction will make the voltage swings worse.
The heating load is buffering the action of the capacitors. I suspect that you have 90% leading under heavy load, and 50% leading under light load.
Disconnect some capacitors, possibly as much as half of the capacitors.
It is bad practice to allow the load on a generator to go leading. Avoid it if possible. The leading current supplies magnetizing current and drives the generator output voltage up. The AVR will compensate as much as it is able, but a point will be reached where the AVR is no longer able to control or limit the terminal voltage of the generator.
There is no reason to want a leading power factor and it causes increased current in addition to possible high voltage problems.
Yours

#9 Rzn

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Posted 12 October 2006 - 02:27 AM

Hi, smile.gif
Sorry for wrong post. Actually I am new in this forum and was not known about reply.
The answer of your question is that, there are two ways for correcting power factor, "Static correction" and "Bulk Correction". In the static correction PF improve on the motor terminals and in bulk correction, PF improve at main distribution panels.
Now in your case, if you connect the PFC panel near main distribution panel, there will be two advantages, first it will reduce the line losses between main distribution panel to Gen distribution panel and second, cable size between two distribution panels will also be reduced. It is like "Static correction".
Inductive load demands two types of powers, Active power and Reactive power. Capacitors provide the reactive power to the load in PFC system. If capacitors are connected near load, the cable size and line losses would be reduced between supply cable from circuit to main distribution panel. So my suggestion is that you should connect the PFC panel near main distribution panel.


Rizwan

#10 joshua

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Posted 05 April 2007 - 08:15 AM

I have a question regarding Power Factor Correction with Generator. The following reading was recorded at the DB located at the Gen Set Room at full load :
Gen Set Rated At 225KVA

Red Phase : KW 23.04, PF 0.920, KVA 25.65, A 102.2
Yellow Phase : KW 34.25, PF 0.964, KVA 35.50, A 151.8
Blue Phase : KW 32.25, PF 0.969, KVA 33.39, A 138.6

Later i took some reading towards the load side DB and got the following readings : PF ; 0.728, KW ; 4.50 ( the readings was taken at an island resort, the load side was the staff quaters)

And another load side reading was : 0.622, KW ; 7.92 ( different block)

I was wondering, by correcting the power factor, would there be a significant savings in diesel consumption in this case as the overall consumption at full load was 94.54KVA ? The generator set was rated at 225KVA in meaning only 42.01% of overall capacity of the generator was being utilised. Any benefits by correcting the PF in this case other than an increase in supply capacity ?

Thank u..

#11 marke

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Posted 08 April 2007 - 05:39 AM

Hello Joshua

The important point is the power factor seen by the generator.

If the power factor at the generator terminals is 0.95 or better, there will be reduced losses in the alternator. If the power factor correction is connected near the generator, it will work fine. Measuring the power factor on the load side of the correction will always give you the load power factor (uncorrected). If you move the correction closer to the load, you will reduce the losses in the cables between the generator and the load. If the cable losses are significant, then there is an advantage. If the cable losses are very small, there is little advantage.

In you case, it appears that the power factor seen by the generator is OK. Adding additional power factor near the load, will improve the power factor near the load, but could overcorrect the supply, increasing the current seen by the Alternator. Be careful not to over correct!!

Best regards,

#12 joshua

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Posted 09 April 2007 - 04:20 AM

QUOTE(marke @ Apr 8 2007, 01:39 PM) View Post

Hello Joshua

The important point is the power factor seen by the generator.

If the power factor at the generator terminals is 0.95 or better, there will be reduced losses in the alternator. If the power factor correction is connected near the generator, it will work fine. Measuring the power factor on the load side of the correction will always give you the load power factor (uncorrected). If you move the correction closer to the load, you will reduce the losses in the cables between the generator and the load. If the cable losses are significant, then there is an advantage. If the cable losses are very small, there is little advantage.

In you case, it appears that the power factor seen by the generator is OK. Adding additional power factor near the load, will improve the power factor near the load, but could overcorrect the supply, increasing the current seen by the Alternator. Be careful not to over correct!!

Best regards,


Thank u very much Mr. Marke... So in this case, it would be highly recommended not to do the PF correction right ? There should not be any significant savings in fuel consumption ?

thanks alot for your prompt answer. appreciate it alot..

#13 marke

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Posted 09 April 2007 - 07:21 AM

Yes, that is correct, the power factor as seen by the generator is pretty good and there is little to be gained.

You could shift the existing correction closer to the actual loads so that you reduce the current in your distribution cable, but the gains from doing this will depend on the losses in the cables. - if it is'nt broken, dont fix it!!
Best regards,

#14 A.D.Thirumoorthy

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Posted 09 August 2007 - 11:57 PM

QUOTE(BServEngr @ Jun 29 2006, 10:30 AM) View Post

We have a 500 kVA Diesel gen set to feed the essential loads( HVAC, elevators,some lights, Water pumps).
Should I connect the PFC capacitor panel to the Gen supply Distribution panel or to the Main distribution panel? Is there any harm in connecting the PFC capaciotrs to gen supplied panel?

Any clarifications would be of great help.
Thanks.




#15 A.D.Thirumoorthy

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Posted 10 August 2007 - 12:07 AM

QUOTE(BServEngr @ Jun 29 2006, 10:30 AM) View Post

We have a 500 kVA Diesel gen set to feed the essential loads( HVAC, elevators,some lights, Water pumps).
Should I connect the PFC capacitor panel to the Gen supply Distribution panel or to the Main distribution panel? Is there any harm in connecting the PFC capaciotrs to gen supplied panel?

Any clarifications would be of great help.
Thanks.



Hello,
The desiding factor in alternator loading is the capacity of the Engine coupled with
the alternator.So whatever the Power Factor is maitained we should be carefull not to overload
the Engine.Normally people trying to improve the Power Factor ignore the loading on the Engine
which is ultimate factor in loading a Generator. Actually the improvement in PF to the optimum (UPF) reduces the losses in the Distribution and reduces the current loaded to the Alternator.

Thank you




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