Standard Instrument PT Ratios

[mmt019]

I have a 15kV class switchgear rated 1200A 3-phase, 3-wire system, delta. THe utilization voltage will be changed from 6.9kV to 12.47kV. What PT ratio should be used? I will be using 2 PTs in an open delta configuration. Standard ratios that are available are 14,400/120V (120:1) and 12,000/120V (100:1). Most PTs are designed for 10% over nomimal utilization voltage, continuous rating. The PT ratio that was used at 6.9kV was 7200/120V (60:1). I do not think there are any disadvantages if I use 14,400/120V PTs. For example, on a 480/277V wye system, the standard PT ratio is 288/120V with nominal L-N voltage of 277V. In essence, do you goto the next higher PT primary voltage if your utlization voltage is between available primary PT voltages?

 

[scottf]

14400:120V will work fine.

Technically, the standard requires accuracy to be met between 90% and 110% rated voltage. However, the accuracy really doesn't drop off at the 90% point.

I would be much more concerned with possibly exceeding the 110% over-voltage rating versus operating slightly below the 90% rated voltage point.

To address your last question, my advise is to always go up in ratio.

With all of that said, 13200:120V is also a standard ratio and would be the best fit for what you describe.

 

[davidbeach]

But, why would you install new open delta PTs? The only advantage of open delta PTs is that they are cheap (not even in expensive, because that implies value for money), the disadvantages are many.

 

[mmt019]

Thanks for your replies. I realize that there is more than one answer for this. There are manufacturers of PTs that make a 104:1 (or 12480/120V) PTs if I need to be that accurate. But after further research, I discovered that 100:1, 120:1, OR 104:1 PTs all will work fine. Now david, to answer your concern, for metering voltage on a delta power system, two individual PTs are quite acceptable and is a standard. Yes, you can also use 3 PTs as well but it would be unusual to do so and would take up valuable instrument compartment space. Refer to Blondel's Theorem for a technical explanation of why 2 PTs work just as well. It states that "total power of a polyphase system is obtainable from n-1 wattmeter elements". This is true no matter how unbalanced your system may be. Additionally, you can apply this principle to the current elements, but I generally like to see 3 current elements in a delta configuration. Also, FYI, the existing equipment only had 2 PTs in the cabinet when this equipment was designed for use at 6.9kV. All I am doing is taking this equipment and using it at 12.47kV, so there is no use in trying to add a PT when it was designed with 2 PTs in there anyway.

 

[davidbeach]

Well, if all you're doing is metering, then it may not matter. But for protection the use of delta PTs rather than wye means you're throwing away valuable information.