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Power-User

Join Date: Nov 2011
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Power Quality Meter CT 180 Degree Phase Shift

12/03/2020 3:38 AM

Hi I'm using a system of 3 PQMs to give me a MW output from a generator to compare against the MW input from 2 x 11kV incomers. With some simple maths I want to be able to see whether as a net figure am I importing or exporting power. PQM on the generator works well and displays MW. The PQMs on the incomers are giving silly figures and looking to the currents they would appear to be 180 deg out of phase. The PQM to the generator has been in for many years, to the incomers they are new. I noticed the input to the generator PQM from the cts had been wired in reverse for each phase the S1 connections of the cts had been commoned and the S2s were the ct outputs to the PQM rather than the expected S1s. Following suit I wired the incomer PQMs in the same way with S1s being commoned. If I simply swap the wiring over to the conventional on just the incomer PQMs so the S2s are commoned will this resolve the 180 degree phase shift issue?

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Guru

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#1

Re: power quality meter CT 180 deg phase shift

12/03/2020 3:54 AM

Are you trying to wire a 3 phase generator into a split phase supply?

https://www.janitza.com/installation-of-current-transformers.html

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Power-User

Join Date: Nov 2011
Posts: 286
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#2
In reply to #1

Re: power quality meter CT 180 deg phase shift

12/03/2020 4:49 AM

No. Its 11kV 3 phase. All measurements are at 11kV.

The 11kV generator is paralleled with the 11kV incomers.

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Guru

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#3

Re: Power Quality Meter CT 180 Degree Phase Shift

12/03/2020 9:09 AM

If you reverse all three CT phases (reverse S1 and S2 for all 3 CTs), it will reverse the total power flow indicated.

P = I1V1 + I2V2 + I3V3.

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Power-User

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#4
In reply to #3

Re: Power Quality Meter CT 180 Degree Phase Shift

12/03/2020 9:23 AM

Hi

So to be clear the cts on the bars will remain in situ unchanged. My connections to the current inputs of the PQM will be reversed. The PQM current reading is 180 degrees out of phase at the moment so am I right in thinking 'making this negative' is in effective pulling it back by 180 degrees?

Your theory backs up the reverse power we are seeing.

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Guru

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#5
In reply to #4

Re: Power Quality Meter CT 180 Degree Phase Shift

12/03/2020 9:54 AM

Yes, reversing the wiring on all three will reverse the power.

Do not remove any connections while there is power on the bus. If the current transformers are not securely connected to the power meter (or shorted together), high voltage will result when the bus is energized.

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Power-User

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#6
In reply to #5

Re: Power Quality Meter CT 180 Degree Phase Shift

12/03/2020 10:00 AM

Hi thanks for getting back.

I'm going to get my electricians onto now to swap the wiring.

Thanks also for the warning of disconnecting energised cts. I'm aware of it but it might help anyone else who reads the thread. Thinking where these cts are located on the switchboard it would be big job to repair + loads of downtime.

cheers

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