Application CTand VT Wiring

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Landis+Gyr Application Note

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Landis+Gyr Inc. 2800 Duncan Rd. Tel: (800) 777-2774Lafayette, IN 47904-5012 Fax: (765) 742-0936

Product: MAXsys meters and other Transformer Rated MetersSubject: Information on the wiring of Instrument TransformersDate: 03/20/2005

Background Information

Warning this is to be used for information only. This is not arecommendation or instructions on how to wire InstrumentTransformers or Meters. Only trained Meter Technicians with agood understanding of Instrument Transformer Rated metersshould ever work on metering circuits and then they must always

follow their company’s safe work practices. Mistakes in wiringInstruments Transformers can cause damage to equipment anddeath to personnel.

To understand applications using instrument transformers it is important(necessary) to understand the polarity markings on the transformer as wellas following your company’s safety guide lines when wiring instrumenttransformer rated power instrumentation. You may have heard the term,with current in on polarity, you will get current out on polarity (they were

saying the signals would be the same). To help understand the polaritymarkings when looking at instrument transformers, you will find on thehigh side (primary) and on the low side (secondary) of the transformer a polarity mark. The best way to think about this, is the polarity marks aretwo points on the same wire, from the stand point the signal going in onpolarity will be the same signal you see coming out on polarity from aphase relation, the signals will be in phase with each other (look thesame), only the magnitude of the signal will change. If you think about justone VT and CT it maybe easer to see what happens. With the polarity

terminal of the VT (high side) connect to the Line and the polarity terminalon the CT (high side) facing the source and the polarity terminals on thelow side of the transformers connected to the “Vin” and “Iin” of the meterand with power flowing from the source to the load the meter will registerin the delivered direction (you could say the voltage and current are in thesame phase (direction). When the power starts flowing from the load to




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the source, the voltage signals remain the same however, now the current

is flowing into the non-polarity side of the transformer and now the currentsignal has now moved by 180 degrees and the meter starts registering inthe received direction.

With this basic understanding we can look at three possible ways to installInstrument Transformers and connect them to a three element meter.Please remember there are many ways to connect transformers to a meterand get the same results. We are only going to look at three installations.The first one is a 4 wire wye and the last two will be on 3 wire Delta. This

to help you understand the connections, it is NOT intended to be used as ameter wiring diagram. Warning because each company has its owngrounding practices for instrument transformers, no grounds are shown inany of the illustrations.




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Illustration 1 represents a full 3 element meter. This will be used as areference in illustrations 2-4.

Illustration 1

Va Vb Vc Ia Ib Ic Ic Ib Ia Va Vb Vc In In Out Return Out Return

Va Vb Vc Ia Ib Ic Ic Ib Ia Vc Vb Vc

In In In In In In Out Out Out Out Out Out




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Illustration 2 represents a 4 wire Wye installation.

With the connections below the delivered registers will increase whenpower flows from the source to the load. Note: with the VT’s beinginstalled on the source side of CT’s, they will not be seen as load to themetering system.

Illustration 2

V

T

V

T

C

T

C

T

V

T

C

T

To Va in meter

To Vc in meter

To (Vc out) meter or return

To Vb out meter or return

To Vb in meter

To Va out meter or return

To (Ia in) meter

To Ic out meter or return

To Ic in meter

To Ib out meter or return

To Ib in meter

To Ia out meter or return

SOURCE

LOAD

N A B C




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Illustration 3 represents 3 wire Delta installation. We use the samepolarities in this example as we did in illustration 2. When using a threeelement meter like the MAXsys meter you would only use the “A & C”elements. With the MAXsys meter you would leave “B” currents open andconnect “Vb in” to return (you can also leave the voltage connect open.

Illustration 3

V

T

V

T

C

T

C

T

To Va in meter

To Vc in meter

To (Vc out) meter or return

To Va out meter or return

To (Ia in) meter

To (Ic out) meter or return

To Ic in meter


SOURCE

LOAD

A B C




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Illustration 4 represents 3 wire Delta installation. We use the samepolarities in this example as we did in illustration 3, EXCEPT the polaritieson the “C” phase VT has been reversed, by reversing both the primary andsecondary connections we end up with the same signals. When using athree element meter like the MAXsys meter you would only use the “A & C”elements. With the MAXsys meter you would leave “B” currents open andconnect “Vb in” to return (you can also leave the voltage connect open).

Illustration 4

V

T

V

T

C

T

C

T

To Va in meter

To Vc out meter or return

To (Vc in) meter

To (Va out) meter or return

To Ia in meter

To Ic out meter or return

To Ic in meter


SOURCE

LOAD

A B C

Application CTand VT Wiring

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