Substation Metering

[bazzer1962]

For efficient three phase metering in a substation,what is the minimum number of ct and pt required.

 

[ScottyUK]

You must have entered the contest for the most badly worded question of the week. You might even win, although this week there have been some other strong contenders. You give us virtually no information, so the only sensible answer you'll get is 'it depends'.

What quantities are you trying to meter?

Are there any single phase loads?


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

 

[chadwiseman]

Assume there are some single phase loads from the 3 phase substation. I seem to recall that two (2) CT's can be used, however I cannot find a reference to suppport this. Also, I cannot get any inidication on how many PT's are required. Any ideas?

 

[racobb]

Is it a 3 or 4 wire bus that you need to meter?

Alan

 

[Eleceng01]

I believe that ScottyUK is right when he says "it depends." You really need to consider Blondel's theorem & it's errors to determine the answer.

Blondel's theorem states that "If energy is supplied to any system of conductors through N wires, the total power in the system is given by the algebraic sum of the readings of N wattmeters, so arranged that each of the N wires contains one current coil (transformer), the corresponding voltage coil being connected between that wire and some common point. If this common point is on one of the N wires, the maeasurement may be made by the use of N-1 wattmeters."

So if you research, you will find 2-element, 2-1/2 element, & 3 element meters. The element is the number of PTs & CTs where for example, a 2-1/2 element is 2 voltages & 3 currents.

The 2-element & 2-1/2 element will introduce errors if the system is not symmetrical/balanced but it depends on your reason for metering & if you can live with these errors.

There are numerous manaufacturers of industrial grade meters. I personally like the SEL-734 for the cost & features.