Standard Instrument PT Ratios
Standard Instrument PT Ratios
(OP)
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?






RE: Standard Instrument PT Ratios
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.
RE: Standard Instrument PT Ratios
RE: Standard Instrument PT Ratios
RE: Standard Instrument PT Ratios