Voltage Transformer Burden Measurement

[anonymous]

This is my first post on here, I'm sorry if this is a stupid question or if this has been asked before. I did some searching and couldn't find anything.

NETA 7.10.2 B.5 requires that voltage circuit burdens are measured at VT transformer terminals. This test is listed as optional in MTS but not in ATS. I've asked a number of experienced testers and none of them could give me a straight answer on how to do this test. Should the test result be in ohms or VA? If it's ohms, is it enough to just put an ohmmeter on the secondary circuit? This seems wrong to me, since I expect most of the impedance will be reactive (inductive, as in meter coils) and an ohmmeter will give a DC resistance. If it's VA, how do you calculate it? Can I measure DC resistance and then give a test result as a range, like so many VA at PF=0.1, so many VA at PF=1.0?

Is there a voltmeter-ammeter method for this? For example (assuming a 14400-120 VT), disconnect the VT from the circuit, feed 120VAC to the secondary circuit, ammeter in series on one leg, voltmeter between the two legs. Volts x amps = VA. Would this work? Is this overkill? (Either way, NETA says "at the transformer terminals," where feeding 120V to the secondary would be unsafe).

Most of the VTs we've worked on have had high accuracy/burden ratings, like 0.3W-Z (12.5-200 VA) on a substation with one or two relays on the VT circuit, where I'd guess the actual burden would be more like 1 VA or less. The last thing I want to do is inject any kind of current into a voltage sensor on a protective relay. So how important is this test, really? Is the purpose just to make sure there are no shorts?

[Kalbi_Rob]

This is my first post on here, I'm sorry if this is a stupid question or if this has been asked before. I did some searching and couldn't find anything.

NETA 7.10.2 B.5 requires that voltage circuit burdens are measured at VT transformer terminals. This test is listed as optional in MTS but not in ATS. I've asked a number of experienced testers and none of them could give me a straight answer on how to do this test. Should the test result be in ohms or VA? If it's ohms, is it enough to just put an ohmmeter on the secondary circuit? This seems wrong to me, since I expect most of the impedance will be reactive (inductive, as in meter coils) and an ohmmeter will give a DC resistance. If it's VA, how do you calculate it? Can I measure DC resistance and then give a test result as a range, like so many VA at PF=0.1, so many VA at PF=1.0?

Is there a voltmeter-ammeter method for this? For example (assuming a 14400-120 VT), disconnect the VT from the circuit, feed 120VAC to the secondary circuit, ammeter in series on one leg, voltmeter between the two legs. Volts x amps = VA. Would this work? Is this overkill? (Either way, NETA says "at the transformer terminals," where feeding 120V to the secondary would be unsafe).

Most of the VTs we've worked on have had high accuracy/burden ratings, like 0.3W-Z (12.5-200 VA) on a substation with one or two relays on the VT circuit, where I'd guess the actual burden would be more like 1 VA or less. The last thing I want to do is inject any kind of current into a voltage sensor on a protective relay. So how important is this test, really? Is the purpose just to make sure there are no shorts?

The only stupid question, is the question never asked.

From Omicron:

"Burden measurement
Why measure?
As the connected burden has a strong influence on the IT's performance, the exact
operating burden must be known for correct IT operation. The measurement can
determine the influence of cables and connections on the burden impedance. As
the burden defines and / or affects the accuracy of ITs, the burden values should be
known and the specified burden should not be exceeded or undercut.
The burden measurement can also indicate wrong connections or failures in the
connection, preventing a CT from open circuit operation and a VT from short circuit
operation.
How does it work?
The burden is connected to the measuring device instead of the IT. The burden is
then measured with a complex impedance measurement (with magnitude and
phase). The burden value is displayed in VA and as impedance. The VA rating always
refers to the nominal secondary voltage or current."

That being said, when performing CT burden the secondary amperage is injected (typically 5A or 1A) and the voltage drop is measured to determine the burden in VA.
When performing VT burden, the secondary voltage is injected (typically 120V) and the current (meter inline) is measured to determine the burden in VA.

The purpose of testing burden is to verify that no high resistance connections and resistance of the wiring does not cause increased load on the instrument transformer (IT) which exceeds the burden rating of the IT. When this happens, the accurate operating region of the IT will be exceeded, and could lead to premature saturation of the IT for extended periods of time, thus causing possible premature failure and of course inaccurate readings at the meter/relay.

[anonymous]

Thanks for the detailed reply.

When performing VT burden, the secondary voltage is injected (typically 120V) and the current (meter inline) is measured to determine the burden in VA.

The purpose of testing burden is to verify that no high resistance connections and resistance of the wiring does not cause increased load on the instrument transformer (IT) which exceeds the burden rating of the IT.

Does this mean the reading should include secondary fuses and wiring all the way up to the VT secondary? Or is it acceptable to do this at a test switch downstream of the fuses for convenience?

[Kalbi_Rob]

Thanks for the detailed reply.



Does this mean the reading should include secondary fuses and wiring all the way up to the VT secondary? Or is it acceptable to do this at a test switch downstream of the fuses for convenience?

There is no documentation stating where this shall be taken, but it would be recommended to take the reading from the connection point on the secondary of the VT. Since, you want to include all resistances within the circuit to ensure the burden is acceptable. That being said, if you take it at a lower point and the burden is close to the maximum burden, I would retake the test at the terminals of the VT secondary, but more than likely you will not be anywhere near the maximum burden allowed IF engineered correctly.