JBinCA
Electrical
- Jun 25, 2005
- 98
Hi,
On a recent project, I was asked to check operation of a panelmount kW meter at the last minute. Since we didn't have a manual defining the test, I attempted to draw out the phasors of the meter and devise a test of my own.
The connections to the meter (per the diagram on the side) are are as follows (system is 12kV 3 wire / ungrounded):
open delta - open delta PTs with B phases grounded.
A and C phase CT inputs.
Since the meter is a 2 element meter, a single phase test is possible with one PT and one CT input being injected from the relay test set. This would yield 1/2 of the calculated primary kw (V * I * sqrt (3) * PTR * CTR). This single element test did produce 1/2 the indicated power of the dual element test.
The problem I can't explain is the phase angles involved. Since the meter is "looking at" phase to phase voltages, it seems to me that unity pf would occur with some phase angle between the voltage and current of a particular elemet. Testing proved that unity pf occured with voltage and current in phase (maximum kW reading at given voltage and current magnitude occured when voltage and current were in phase).
As I analyze things, A - N voltage (on primary) should be in phase with A phase current (on primary) at Unity pf. It follows that A-B voltage should lead A phase current by 150 deg on the secondary at unity pf.
Admittedly, it's been a long time since I've regularly had to draw out phasors. I'm certain the error is with my analysis and that the meter is operating properly.
I would appreciate it if someone is able to tell me where I went wrong or suggest an online reference that they believe breaks down phasors for kW (MW) metering satisfactorily.
Best Regards,
JBinCA
On a recent project, I was asked to check operation of a panelmount kW meter at the last minute. Since we didn't have a manual defining the test, I attempted to draw out the phasors of the meter and devise a test of my own.
The connections to the meter (per the diagram on the side) are are as follows (system is 12kV 3 wire / ungrounded):
open delta - open delta PTs with B phases grounded.
A and C phase CT inputs.
Since the meter is a 2 element meter, a single phase test is possible with one PT and one CT input being injected from the relay test set. This would yield 1/2 of the calculated primary kw (V * I * sqrt (3) * PTR * CTR). This single element test did produce 1/2 the indicated power of the dual element test.
The problem I can't explain is the phase angles involved. Since the meter is "looking at" phase to phase voltages, it seems to me that unity pf would occur with some phase angle between the voltage and current of a particular elemet. Testing proved that unity pf occured with voltage and current in phase (maximum kW reading at given voltage and current magnitude occured when voltage and current were in phase).
As I analyze things, A - N voltage (on primary) should be in phase with A phase current (on primary) at Unity pf. It follows that A-B voltage should lead A phase current by 150 deg on the secondary at unity pf.
Admittedly, it's been a long time since I've regularly had to draw out phasors. I'm certain the error is with my analysis and that the meter is operating properly.
I would appreciate it if someone is able to tell me where I went wrong or suggest an online reference that they believe breaks down phasors for kW (MW) metering satisfactorily.
Best Regards,
JBinCA