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Topic Title: VSD harmonics and power measurement
Topic Summary: Billing for kWh correct?
Created On: 26 May 2010 08:50 AM
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 26 May 2010 08:50 AM
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Makeabrew

Posts: 2
Joined: 26 May 2010

Hello. I'm not an electrical engineer so you may have to forgive my ignorance. I've taken some power readings from a VSD that is powering a submersible borehole pump motor. The power meter I used is a Fluke 1735 and it showed a non-sinusoidal current waveform and significant harmonics (THD was 60%) on the supply to the VSD.

The power factor was shown as 0.82 but I now know this is the true (or total) power factor and not the displacement power factor.

The Fluke meter displays kVA and kW. It is the kW that I am interested in as this is what we are billed for.

The issue I have is that the kWh meter that is owned by the electricity company does not take into account the harmonics (I think...). When we get the bill it shows our power factor is 0.98.

My question is: are the electricity company measuring kVA and displacement power factor and multiplying the two to get kW. And if this is the case, is this calculation correct?

I'm not able to accurately compare what we are billed with to the power measurements I have taken as this site is not a half-hourly site and the VSD runs at different speeds and is off for a proportion of the day. We therefore get a bill each month (some of which are estimated).

I am looking at potentially replacing the pump for a more efficient uint but I need to know we will see the predicted reduction in our bills. Any help greatly appreciated and if I have got the wrong end of the stick, then please correct me!
 29 May 2010 09:47 PM
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Backintime

Posts: 282
Joined: 11 April 2007

I have an old fluke harmonic meter. From the readings I got, with a distorted current, the TPF is normally a lot worse than the DPF.

The electricity utility company appears to be giving you a good deal that they use the DPF and kW to bill you.

No matter what the billing calculation is, if you replace the motor with a more efficient one - and with VSD control - you should get quite a bit of energy savings. Whether you can achieve a payback in less than 5 years will depend on the operational profile of the motor. If it runs 24-7 all year round, you could achieve a payback in two years.

Regards

Marty
 30 May 2010 08:49 AM
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PQD

Posts: 5
Joined: 09 May 2010

The ratio between TPF (True PF, including harmonics) and DPF (displacement PF, fundamental frequency) is estimated (when THDv is not too high) by the formula TPF = DPF/sqrt(1+THDi^2).

Some utilities charge on total kW and some on fundamental (and some don't really know - mainly if they still use electro-mechanical meter). However, you cannot combine kVA which include harmonics with DPF. There is also kVA for each harmonic. kW (h1) = kVA (h1) * DPF.
 02 June 2010 08:16 AM
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Makeabrew

Posts: 2
Joined: 26 May 2010

Thanks for your replies.

It sounds like I need to look at the type of billing meter the electricity company have installed and then ask them what they are billing us on.
 03 June 2010 11:21 AM
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PQD

Posts: 5
Joined: 09 May 2010

Good luck. They probably don't know.
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