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#169404 - 10/04/07 01:16 AM polyphase metering single phase CT
johno12345 Offline
Member

Registered: 09/13/05
Posts: 93
Loc: United Kingdom
Hi,

Brief background:
We have a 1Mw 11Kv - 400V transformer for our site (customer owned) with the metering CTs on the HV ring main unit.

We have had a company in to make recomendations on how we can save money on our fuel bills.

They recently monitored our electricity supply and came up with a recomendation that we shift load from the phase 1 to phase 2 (which is slightly less loaded).

They recommend moving approximately 20Kw over which will save £1000/month.

I thought this sounded a bit odd but they said that the CTs are usually only installed on phase 1. Has anyone else heard of this, it seems a suspiciously easy way to save money? Surely CTs arent that expensive when our average demand is in excess of 400kW

Thanks
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#169427 - 10/04/07 05:44 PM Re: polyphase metering single phase CT [Re: johno12345]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8530
Loc: SI,New Zealand
Hi johno,
I really can't see the point in moving loads from one phase to another, unless the installation is wildly out of balance.

Only 1 CT?, what went wrong there?, with only 1 CT you're not seeing the whole picture.
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#169444 - 10/05/07 06:25 AM Re: polyphase metering single phase CT [Re: Trumpy]
johno12345 Offline
Member

Registered: 09/13/05
Posts: 93
Loc: United Kingdom
Hi,

Aparently it is common practice for the poco to only have a CT on L1 and they just multiply it by 3. We have some capacity to move some loads over to L2 which should reduce our bill but not consumption. it all seems a little cock-eyed to me \:\)
_________________________
I took my time, I hurried up, The choice was mine, I didn't think enough

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#169448 - 10/05/07 09:33 AM Re: polyphase metering single phase CT [Re: johno12345]
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
If you had a CT on just one HV phase, it would register for more than one LV phase anyway, as the 11kV primary is delta and the 240/415 secondary is wye.

I agree, the whole idea sounds completely cock-eyed to me. I've never heard of just metering one phase of the supply.

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#169453 - 10/05/07 10:08 AM Re: polyphase metering single phase CT [Re: pauluk]
adamh Offline
Member

Registered: 10/04/07
Posts: 13
Loc: UK
I'm not really into transformer theory, but it seems likely that the transformer would load each leg of a delta primary equally so you would only need to meter one leg to get an accurate reading.

Is it possible that you would gain something in transformer efficiency by balancing the load?

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#169527 - 10/07/07 09:59 AM Re: polyphase metering single phase CT [Re: adamh]
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Yes, sometimes they only measure one phase. Not a good system think, but as Paul points out, since this is a delta, the load from two phases on the secondary is sort of added on the primary. CT's are very cheap. So are meters. Maybe HV CT's are not that cheap?

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#169528 - 10/07/07 12:50 PM Re: polyphase metering single phase CT [Re: C-H]
RODALCO Offline
Member

Registered: 12/08/05
Posts: 862
Loc: Titirangi, Akld, New Zealand
That statement sounds strange to me.

Normally 11 kV / 400 supplies, if metered at the HV side, are metered in the 2 wattmeter configuration which generally meters the Red and Blue phases on the 3 phase delta side of the TX.

According to Blondells metering theoreem any poly phase circuit will be accurately measured with n-1 metering element.

Where n are the amount of phases, in this case 3 minus 1 equals 2 metering elements.

The VT is normally 3 phase and provides 110 Volts for the meter, of which the yellow phase is earthed for safety.
th CT's are often 5 Ampères and sometimes 1 Ampère in case of long wire looms between and HV supply and metering point.

I think that the person is confused with the occasional cowshed, or irrigation pump supply with a polyphase motor on 400 Volts.

These often get metered with one single phase meter direct or a single phase CT meter and CT on supplies over 100 Ampères.
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The product of rotation, excitation and flux produces electricty.

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#169550 - 10/08/07 01:04 AM Re: polyphase metering single phase CT [Re: RODALCO]
johno12345 Offline
Member

Registered: 09/13/05
Posts: 93
Loc: United Kingdom
Hi,

I dont understand transformer theory at all so it is all a learning experience for me \:\)

I now have the data that was monitored:

Main incomer average demand 375kw
Main incomer total 62884 kWh

Phase 1 reading 64130 kWh
Phase 2 reading 59427 kWh
Phase 3 reading 65098 kWh

Poco meter reading 64420 kWh

They state that as the phase 1 reading is so close to the poco meter reading, they have a ct on phase 1 and are multiplying by 3. They state that the difference is within the tolerance of the 2 meters

The poco meter does state 11kv/110v on it. I will check it later.

I may be more confused than ever \:\)

Thanks
_________________________
I took my time, I hurried up, The choice was mine, I didn't think enough

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#169579 - 10/09/07 03:29 AM Re: polyphase metering single phase CT [Re: johno12345]
RODALCO Offline
Member

Registered: 12/08/05
Posts: 862
Loc: Titirangi, Akld, New Zealand
The POCO meter will also measure the losses of the TX, because it measures at the primary ( 11kV) side.

From the 3 readings obtained you can see there is minor imbalance over the 3 phases. The second phase is the lightest loaded. For the 375 kW load that is pretty good, i have seen a lot worse imbalances.

A 1 MVA TX will have an Fe loss and Cu loss of a couple of kW / hr. depending upon loading, which will account for the difference in readings.

The utility metering is done correctly. it looks to me that the building owner (landlord) tries to save some money on CT's and only measure one phase, and multiply the meter readings by 3, in stead of three CT's and polyphase meter.

What type of load do you have ?

Motors or mixed, eg lighting , airco, computers etc.

You could perhaps save a little bit by shifting some single phase loads from the first ( metered ) phase to the second or third phase in your case, although big unbalances on TX's are not the best practise.
_________________________
The product of rotation, excitation and flux produces electricty.

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#169624 - 10/10/07 12:55 AM Re: polyphase metering single phase CT [Re: RODALCO]
johno12345 Offline
Member

Registered: 09/13/05
Posts: 93
Loc: United Kingdom
Hi,

We(the building owner) dont have any metering CTs permanently installed so we just have the poco CT for their meter which is the only one that we get billed on.

I had wondered about the tx losses, it gets pleasently warm and people gather around it in winter for the warm draft!

Our loading is mainly 3 phase, large motors (90kw is the largest I spotted) a moderate server room, lots of discharge lighting some A/C and a couple hundred PCs. 50% of the load goes into our gas furnaces and cooling plant which is all motors and pumps.

We are hoping to save £100/month by moving 20kw over. This will probably be the single phase office consumer units which have about 20kw of electric heating so it should average out as the heaters cycle off and on.

Thanks \:\)
_________________________
I took my time, I hurried up, The choice was mine, I didn't think enough

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