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How to Mitigate Power Cable Capacitance

03/04/2018 10:37 AM

In one of the electrical installations receiving 22kV supply, they are facing low pf (leading) on no load / light load conditions. The 500kVA transformer is fed through a 185 sq.mm. A2XFY cable of length around 500m. when the transformer is feeding the lighting load alone (~3kW) the supplier's meter is recording low power factor (~0.4) leading due to the cable capacitance and they are penalized for this. how to compensate this capacitance and improve pf.

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#1

Re: How to mitigate power cable capacitance?

03/04/2018 11:25 AM
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#2

Re: How to mitigate power cable capacitance?

03/04/2018 1:46 PM

See-for instance:

http://www.hilkar.com/ironcoreshuntreactors.html

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#3

Re: How to Mitigate Power Cable Capacitance

03/04/2018 11:53 PM

Thanks for the solutions. someone has suggested transposition of conductors in the middle of cable length. will it really help?. any one has faced such situation.

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#4
In reply to #3

Re: How to Mitigate Power Cable Capacitance

03/05/2018 2:52 AM

No, it won't.

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#5

Re: How to Mitigate Power Cable Capacitance

03/05/2018 2:56 AM

It sounds as though the transformer is over-sized by a factor of 100 or so. It might be economic to feed the lighting load from elsewhere such as a smaller transformer, or even a local generator, rather than the <...500kVA transformer...>. Local economic conditions, which are commercially sensitive and about which the forum knows nothing, will determine the best solution.

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#6

Re: How to Mitigate Power Cable Capacitance

03/05/2018 8:41 AM

I have never heard of a utility charging for leading power factor presented by a customer, you are helping to support their system voltage, at some risk to your own.

Theoretically, they should be paying you for your support. If they are charging you, they do not know how to read their meter.

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#7

Re: How to Mitigate Power Cable Capacitance

03/05/2018 9:24 AM

Thanks for the inputs.
1) As PWSlack rightly pointed out, as a temporary measure the consumer is running a small generator for the light load.

2) The utility co. penalize low power factor (<0.85) as per the tariff order where it is not mentioned as lag. The consumer has raised his concern on this matter to the utility.

Originally they have planned for OH lines but due to the constraints in the site, they have gone for HT cable. They have used old HT cable bought from another co. will this be a cause.

Regards

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#9
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Re: How to Mitigate Power Cable Capacitance

03/05/2018 2:08 PM

No, it won't.

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#8

Re: How to Mitigate Power Cable Capacitance

03/05/2018 11:27 AM

interesting topic here and not something i ever considered

Curious to hear the results

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#10

Re: How to Mitigate Power Cable Capacitance

03/06/2018 12:13 PM

Do not understand the problem. A typically billing arrangment consists of:

1. kWHs, 2. A peak kVA charge, and 3. A penalty for low PF.

The penalty for low PF (3) applies only during the peak KVA occurrence.

So, why are you bothered with a low PF at a 3kVA demand, where your peak demand is likely 100 - 200kVA??

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#11
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Re: How to Mitigate Power Cable Capacitance

03/06/2018 1:29 PM

Quite.

The answer to that question will make fascinating reading.

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#12

Re: How to Mitigate Power Cable Capacitance

03/07/2018 12:41 PM

The light load does present a problem. The capacitance inside the stepdown transformer is apparently very significant. That being said, a cable 500m in length is also significant. Separate the cables from each other if you can. Power lines in the air do not suffer from this as much because they are separated by a distance that makes it tough to couple.

If the light load is purely resistive, the capacitance is coming from the power transformer and is therefore their problem, not yours.

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#13

Re: How to Mitigate Power Cable Capacitance

03/11/2018 2:13 PM

In my opinion, the transformer load will be 3 kW at night only[in order to supply only the light]

However, about of 21 kVAr will be the 22 kV cable capacitive reactive power. The transformer -of 500 kVA -let's say 22/0.4 kV 4% short-circuit impedance,1.85% no-load current and 1.1 kW no-load losses- requires 9.18 kVAr no-load inductive power. Then the remaining capacitive power will be 21-9=12 kVAr.

I agree with you NotUrOrdinaryJoe, the transformer presents a capacitance but since the no-load reactive power is inductive I don't think this capacitance is a problem.

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#14
In reply to #13

Re: How to Mitigate Power Cable Capacitance

03/12/2018 12:41 PM

Well, if not the transformer, perhaps excess unswitchable power factor correcting capacitors nearby? There have been waves of hocus pocus dealers selling small capacitor banks to be installed near every motor in the plant from time to time. The sellers were very convincing and they spoke of large line losses inside the plant which was nonsense. They knew that I wasn't about to waste my time doing a huge network analysis to prove them wrong but I did catch their mistakes in their presentation.

People like to treat the problem as if the supply is perfect and the load is singular, but it isn't even close to the reality. By the way, those local power factor correction capacitors were to be wired in permanately while the motors were switched. Not wise at all, and more expensive than the simple approach.

The moral of my story is this: Make sure you don't have a bunch of small power factor correction capacitor banks installed permanately. You wouldn't be the first to have this problem. It has been too many years for me to work out the details of your calculations. From a practical point of view, one needs to understand where all the line capacitors are and how they are connected.

By the way, placing the capacitors near the motor may raise the voltage impressed to a point where the insulation is at risk. If the combination is reasonate at the powerline frequency, the actual tank circuit voltage could be 6 to 10 times the line voltage. That is not healthy for the insulation.

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#15
In reply to #14

Re: How to Mitigate Power Cable Capacitance

03/13/2018 10:00 AM

Building on your comments, switching power factor capacitors with the motor has been practiced for decades, people have been lazy to replace fuses, I have found.

It is indeed the most efficient and conservative method to apply power factor correction. No special controls are needed. The maximum kVAR you can apply for a motor is typically listed on NEMA frame motor nameplates, to avoid overvoltage and over excitation conditions.

The downside is that there are many small units to maintain, fuse protection does age over time, and you need to adjust the overloads if you connect the capacitor to the motor terminals, rather than on the line side of the overload relay. Also, when a less efficient motor is replaced with a more efficient one, the capacitor rating must be reviewed.

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