Measurement of three phase current 1

[simpsonmichael]

I am discussing with a colleague the measurement of three phase current in a distribution network. The contractor is proposing using two current transformers and the outputs passed via a 3 pos selector switch & panel meter to show the current in the system. I argue that for accuracy there should be three current transformers. Can anyone give me an advantage, apart from cost, for using only two current transformers. Also any other comments would be appreciated.

 

[ScottyUK]

Is it a three-wire or four-wire system, i.e. is there a neutral conductor? If there are only three wires then the vector sum of any two currents gives the third. If there's a neutral then you need three CTs.


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If we learn from our mistakes I'm getting a great education!

 

[simpsonmichael]

It is a three wire system with no neutral. Using summation for the third phase is this more accurate than actually measuring it?

 

[ScottyUK]

No, but it's not significantly worse either for most applications. If it's a revenue-class metering installation on a power plant or a transmission substation then use three CTs. If it's general-purpose metering at low voltage then use two CTs because the error will be so small that you'd need some reasonably good instruments to find it. What accuracy class are you working to?


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If we learn from our mistakes I'm getting a great education!

 

[simpsonmichael]

The CTs are 40VA, 7500/A, Class 1. Ammeter is Class 1.5

 

[stevenal]

For summation to work, your meter must be smart enough to add the currents vectorily. If you are using simple magnitude indicating ammeters, you might want three CTs.

 

[ScottyUK]

Or some complex switching from the old days. {wink]


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

With the selector switch, you just wire the sum of the two CTs backward through the third position. No fancy wiring, no fancy switches. And if the meter is only current magnitude, probably any analog meter, no real need to run the sum backward either.

 

[ScottyUK]

Is that a standard switch over there David? They *used* to be standard over here, but finding replacements is getting tricky.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

I believe you can still order them from Electro-Switch. In fact, look at page 15 of

http://www.electroswitch.com/electroswitchesandrelays/pdf/util.pdf

(a huge pdf), they even show the two CT application.

 

[ScottyUK]

Excellent! Just need to find a European distributor.


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If we learn from our mistakes I'm getting a great education!

 

[simpsonmichael]

Thanks for the advice guys, very useful.