Metering PT Reading different values on all three ph-ph voltages

[AmitDarwhekar]

Hi All,

I have a 4200:120 open delta metering PT, which gives following voltage measurements when the voltage is measured phase to phase on the secondary side of the PT: Vab = 118.8 (4160), Vbc = 119.4 (4180), and Vca = 120(4200). I know that the voltages typically differ marginally on all three phases but they all should be more or less the same. Has anyone encountered this issue before where phase-phase readings are different on all three phases?

Just to give you guys background, I have a Basler DECS-100 voltage regulator that controls the voltage. A +/-3V bias signal is provided from Woodward Easygen 3200 controller to the regulator. When I monitored the bias signal, I noticed that once the voltage reaches 4160V on phase AB, the bias signal stays fairly steady in the range of 53.12% to 53.26%. According to Woodward tech. support Easygen only looks at the phase AB voltage for controlling the bias signal. Since I am reading 4160V on phase AB, Easygen does not try to bring the voltage down even though my average voltage is higher than my actual voltage set point. Further, presently I don't have any load on the generator.

I am fairly new when it comes to troubleshooting transformer related issues. I would appreciate few hints from where I can start troubleshooting this issue.

Thanks and Regards,

Amit

 

[pwrengrds]

and the problem is?? If your problem is the 1.2 volt difference in the PT volts perhaps you have to much time on your hands.

 

[ScottyUK]

What accuracy class are the VTs?
How balanced are the loads?
What accuracy class is the meter?

Find something else to worry about!


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

 

[LSpark]

They look near enough balanced to me, a 1.2V imbalance on one of the circuits I'm responsible for wouldn't concern me.

The difference could be caused by VT inaccuracies (what class are the VTs), genuine imbalance due to the load or a high resistance joint/loose connection on the VT wiring.

The first two are difficult to do anything about and the last can be checked with a multimeter. Start as near top the VT as possible, take a set of voltage reading then gradually move towards the meter. I've had VT links not fully home before now as well as loose connections in terminal rails.

 

[waross]

Step one: swap the PTs and see if the difference moves with the PTs.
Step two: Is this a two pole generator? Check the air gap for uniformity.
Step three: Live with it.
PS if the over voltage is a concern, change your sensing to the high phase.

Bill
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"Why not the best?"
Jimmy Carter