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Commentator

Join Date: Feb 2008
Posts: 59

Generator Plant Mgr

04/19/2009 6:16 AM

We have a stand alone (island) plant consisting of 15 1400kva .8 Pf 50 hz Cummins KTA50 units. Each one has its own 400v/11kv transformer feeding a common 11kv bus. Six (6) feeders are fed fom this bus radially. We contol voltage and frequency. There are many (250 - 500) kva transformers on each feeder. The total load being generated is 3200 kw. The transformer taps are set mid range in all cases. The power factor at the plant varies between .68 and .75 which is unacceptable. Will lowering the tap setting for all transformers help the power factor? Would installing capacitor banks on the 11 kv bus be a possibility? We are anticipating load increases this summer to full capacty of the plant.

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Guru
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#1

Re: Generator Plant Mgr

04/19/2009 12:06 PM

You can install capacitors any where on the load side of the transformer. If you are using variable speed drives you may have some problems. Is you load mostly motors?Is the 11kv distribution built overhead? Adding 1500 kvar will increase the pf to 0.92

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#2

Re: Generator Plant Mgr

04/19/2009 12:30 PM

Tackle the pf issue down stream of each transformer ie @400VAC ; pf correction capacitors are quite cheap once you have decided on the value of capacitance needed.Most suppliers will size the units you need. Sometimes multiple stages will be needed to give the appropriate level of correction in terms of the load applied.

Changing the tap setting will do absolutely nothing to solve the problem.

Good luck.

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Commentator

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#3
In reply to #1

Re: Generator Plant Mgr

04/19/2009 12:32 PM

Thanks for the reply. Yes, our load out here in Taji, Iraq is primarily motors, lighting, and air conditioner units. All feeder lines are overhead with cable transition to the distribution transformers. What about tap changes on the power transformers? As the plant gains load we would have to change the taps again, but if I remember right that's one way to raise the PF.

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#4
In reply to #2

Re: Generator Plant Mgr

04/19/2009 12:55 PM

Thanks, will we see an improvement at the generators in terms of efficiency (i.e. more kilowatts per gallon of fuel)? I would prefer to use low voltage caps but we have so many different loads the cost might prove to be too high. I know it's recommended to place capacitors as close as possible to the load but being able to switch banks in and out on the high voltage side would simplify ops and maint.

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Power-User

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#5
In reply to #4

Re: Generator Plant Mgr

04/19/2009 3:59 PM

hughes838: the fuel usage is rated to the load on the engine, the power factor is the load on the generator windings (overheating with poor PF). perry

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Guru
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#6
In reply to #3

Re: Generator Plant Mgr

04/19/2009 7:24 PM

You can install the caps on the 11kv feeder poles. I believe it is much cheaper that way. Raising the taps does not affect the power factor. I am listing a site that shows caps installed on poles. I've never used oil CB's in the installation, Just fused cutouts.

http://www.srpnet.com/electric/business/specs/pdfx/ohds/1200Capacitors12kV.pdf

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Commentator

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#7
In reply to #5

Re: Generator Plant Mgr

04/19/2009 10:39 PM

Perry, thanks for your comment. I'm sorry I wasn't more clear. What I meant was if I can generate more kw per kva my kw/gal of fuel should increase.

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#8
In reply to #4

Re: Generator Plant Mgr

04/20/2009 7:04 AM

Hi,

The optimum location for the capacitor(s) is directly on the end of the busbars in the 400VAC switch gear locations; putting a capacitor at each and every load is an expensive option which would not justify the extra cost. Therefore returning to the d.b based solution you will need either an unused Swtch fuse in the panel or make a new tapping onto the bars and connect the pf correction set-up via a free-standing Sw-fuse unit. By adopting this approach you will limit the investment to those loads which are collectively reducing the pf to such a low value i.e you can decide which are the worst examples and tackle them as you see fit.This arrangement gives you all the flexibility to manage the situation at the lowest cost.

Yes , you will see fewer litres of fuel used as the 'wattless' component consumes energy and gives you no useful work output. You will therefore have more useful power available and will ultimately recoup the cost of the installation over time. So this should even make your finance director a happy chap!

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#9
In reply to #8

Re: Generator Plant Mgr

04/20/2009 7:14 AM

You use an oil switch to break the load from the line to the capacitor bank. You can, should in my opinion, use an air gap, fused disconnect to isolate the capacitor bank when you have to work on them. RMS can afford the cap banks, oil switches and c/o as they will charge it all back to LOGCAP anyway.

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#10

Re: Generator Plant Mgr

04/20/2009 8:01 AM

It is not clear if you have the generators balanced, Voltage regulator and governor functions. Do not adjust these without knowing what you are doing!

-when the generators are synchronized the voltage on each generator will be identical (or you may have circulating currents between the generators), this is a voltage regulator / power factor controller function.

- the amperage on each on-line generator should be the same, this will also indicate the voltage output is correct and that each generator has it's share of the load, this is a governor function.

- most utilities use the generators to control power factor, this is set when a unit is on-line by adjusting the automatic voltage regulator (AVR does KVAR / power factor control when on-line) for the generator.

- if the bus voltage gets too high or too low, the automatic voltage regulators may have trouble matching bus voltage to synchronize.

- putting fixed capacitors on the bus, may cause over-excitation if the load is lost and the generators are still on-line with the capacitors. If there is

- the generators should be cleaned, generator breakers should be serviced, lubricated and cleaned more frequently in the high temperature environoment.

3200kw output may be 4 generators on-line? unless you have large motors to start which may need more WR2 (rotating mass) to start.

I would use the generator voltage regulators to adjust the power factor as the design provides for (0.8 power factor generators).

Can you check the power factor setting on the automatic voltage regulators ?

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#11

Re: Generator Plant Mgr

04/20/2009 8:27 AM

Can you give us volt and amp readings on all generators on-line with a typical load ?

Is there a bus volt, amp, power factor meter ? If so the readings ?

All readings in an instant is preferable.

Sometimes, meters don't add up, so before changing something, lets do the math.

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#12

Re: Generator Plant Mgr

04/20/2009 1:59 PM

Just my 2 cents...

a) Capacitors are generally preferred on utility supply and not on generator powered load.

In fact lot of generator suppliers advise switching OFF the capacitor banks when running on generators.

b) Lowering the bus voltage will increase the pf. ( but by lowering the tap of a step-up transformer you will increase the bus voltage) so it has to be other way round..you have to increase the tap of the stepup transformer.

c) Since you are expecting the increase in load ..you will notice the pf increasing as the load increases.

d) Diesel consumption is proportional to Kw (engine related) and not kva so increasing pf will reduce your transmission losses if the original transmission design is faulty but will not reduce the diesel consumption).

cheers.

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#13

Re: Generator Plant Mgr

04/20/2009 4:25 PM

Hughes838,

My understanding from your description is that you have several generators (generating at 400V) with a transformer 400V/11kV. The transformers secondaries are paralleled by a common bus. Several feeders are fed from this bus. Along this feeders there are several 250 kVA , 500 kVA transformers (I assume 11 kV/400V), that provide power to load ( whose power factor varies between 0.68 and 0.75). This is the power factor to be improved, by installing capacitor banks on the 400V (secondary) side.

On the 11kV side the power factor is given by the power factors on the primary sides of the 11000/400V transformers, which, in turn, depends partially on how much they are loaded ( 10%, 50%, etc). Changing the transformer taps on the secondary part will not influence very much the power factor on the primary side, but it definitely changes the secondary voltage. Changing taps on the primary part of these transformers will also change the ratio of the voltages, but also changes the power factor of the primary part, since the magnetization current component will have to be increased (the power factor goes down).

A modification of taps in both primary and secondary will lead to the following ( to keep turns/voltage ratio constant)

- increase: small increase in power factor, larger variations of secondary voltage between no-load and full load, increased efficiency of transformer.

- decrease: lower power factor, smaller variation in secondary voltage due to load, decrease in transformer efficiency

Therefore lowering the tap setting is not a good method of increasing the power factor.

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chaterpilar (1); CoronaCameraMan (2); hughes838 (3); Massey 726 (2); perry (1); tomad (1); vagabond (1); wareagle (2)

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