PSCC & PFC Measurement Accuracy Using MFT

[JezNZ]

Hi,

I am curious as to the experience of others in using multifunction testers (primarily designed for insulation resistance measurements) for measuring prospective short-circuit currents with regards to accuracy.

Typically in my company we use the Megger range of MFT (such as MFT-1835) to measure the short-circuit currents on the LV terminals of a new transformer or main switchboard installation.

Recently I have noticed the measured values to be significantly deviated from the theoretical short-circuit current used in the design (design including HV impedances) by upto 50%. This is at the point where it gives me concerns as to the adequacy of the HV protection to operate in all scenarios on a fault between the LV terminals an the first LV protective device.

Can anyone shed any light on similar experiences or recommendation to use a dedicated test unit (I'm not aware of what dedicated units might be used)?

Thanks

 

[thermionic1]

Is the goal to measure or verify the nameplate short circuit impedance (%Z or Zk) of the transformer to calculate the fault current? If so, you would need to perform an open and short circuit impedance test. Impedance Tests

I doubt you would get anywhere close with a small instrument like what is being used - I'm guessing that it uses a DC source, which may deliver the DC winding resistance, but that's not the Zk. Care should also be taken when attempting a xfmr resistance test, due to high voltages generated by inductive kickback.

There are some tests that are performed in the field in conjunction with Tan Delta / Power Factor tests that can come somewhat close to the nameplate impedance, but are used for determining any mechanical issues in the transformer. Doble has a leakage reactance accessory and Omicron & Megger have something similar.

Sometimes a primary injection test is performed to prove stability of differential protection. In this case a reduced 3 phase source is connected to the HV side of Xfmr, with the secondary shorted out to create current flow. Say 480V on a 138kV HV / 12.47kV LV. I suppose these measurements could be carried out with this method, however one also has to consider that the %Z is given at some nominal temperature and this would need to be compensated for in the final calculations.

 

[IrishPower]

I have not used it, but perhaps something like the Megger NIM1000 is along the lines of what you are looking for?

 

[JezNZ]

Hi thermionic1,

The goal is to measure the actual short circuit available fault current, inclusive of system impedance. The Megger MFT do this as a live test, automatically by drawing a range of load currents up to several amps and measuring the voltage changes of the source and applying a simple V_nom/Z_meas = PSCC formula.

For reference, by LV I'm referring to <1kV, this question applies to our testing of industrial and distribution transformers in the range 0.5-2MVA (11 or 22kV primary, 415V secondary).

Hi IrishPower,

It does look that may be a more suitable instrument, if there is not some other way of improving our measurement accuracy. I suspect the low current draw of the MFT to be the limitation of accuracy on these size distribution/industrial transformers.