Metering PT Reading different values on all three ph-ph voltages
Metering PT Reading different values on all three ph-ph voltages
(OP)
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
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





RE: Metering PT Reading different values on all three ph-ph voltages
RE: Metering PT Reading different values on all three ph-ph voltages
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!
RE: Metering PT Reading different values on all three ph-ph voltages
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.
RE: Metering PT Reading different values on all three ph-ph voltages
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