Understanding Multipliers

This may come as a huge shock to some, but there are still a number of (so called) power quality investigators out there that don't know, when it comes to situations other than unity, what figure to enter in the little box usually labeled "multiplier".

Take the following; A recorder is installed in a substation with 300:5 CTs. The recorder has 5A clamps on the input. Strangely enough, I've seen databases with anything from 300 through to 7500 (yip, 300 X 5 X 5). There appears to be a belief that the clamp has a ratio too. I've yet to see a decent recorder that has this problem.

The first rule is, the clamp into the recorder is always to be read as 1:1 meaning that when 5A is injected, 5A is read. Second rule is a CT of 300:5 means that for 300A primary current means the output is 5A (I know I am stating the obvious, but some are still unaware of this).

So, with 300A primary current means the recorder will see 5A or 1/60th of the primary current. To correct this one needs to multiply by the inverse of this i.e. 60 and this is the figure to enter in the little magic box.

Although the above does make for a bit of interesting reading, it becomes pretty obvious what the mistakes are. But life is not that easy when the difference is a little more subtle. I've seen things like 0.8 entered as "this reflected the true power as opposed to VA". Let's not even think of going down this road!

Exactly the same principles apply to VTs. The strange one is I have seldom seen mistakes made on the voltage multiplier, probably because there isn't the same amount of confusion present with voltage inputs as there isn't a clamp range to consider.

There is one thing to be careful of; I have seen many cases where the current input is 'multiplied' by wrapping the conductor being measured around the current sensor a few times. If, for example, the line being measured is wrapped around the clamp 5 times, then the multiplier must be set to 0.2 to make the input again read correctly.

A Matter of Timing  >>

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© 15.11.03