CT Burden Test (Omicron CPC 100) 1

[buzzp]

I have a relatively simple test that is killing me. This is a long shot but maybe someone has had a similar problem.
We have successfully tested CTs with this set in the last couple of months. However, this time around we are getting high burdens (10 plus VA) when we should not be. We have some bushing type CTs (600:5) in our switchyard and we are injecting current from the switchyard where the secondary of the CT should be connected. We are using 5A's. There is nothing else connected to these leads except the protective relay (SEL 387). They do go through one or two terminal blocks. We have ohmed these leads out and get a reasonable 0.5 ohms to each other. However, when we try to use the CPC 100 we get high burdens. If the ohms is within reason (as seen by the ohmmeter), then you have to believe there is some impedance in the circuit which should not be to cause the high voltage that is causing the high VA. I am having a time trying to figure out where this could be coming from. I understand the wire has impedance associated with it but the previous tests used the same size and length of wire so this is ruled out.
Anyone have any suggestions?

 

[busbar]

Excerpt ANSI/IEEE C57.13.1-1981 …Field Testing of Relaying Current Transformers

To determine the external connected burden in volt-amperes, measure the voltage required to drive rated current through the connected burden. If both resistive and reactive components of the burden are desired, a suitable phase-angle meter can be connected. …

The following reminders have been found useful in obtaining correct burden data:
1) To represent in-service burden, the relays and other external devices must be on the correct tap.
2) Parallel current transformers should be disconnected.
3) Phase-to-neutral measurements in relay circuits can be high, particularly if ground relays with sensitive settings are involved.
4) Phase-to-neutral and phase-to-phase measurements of bus differential circuits can be high because of the impedance of the differential relay operating coil. …

 

[buzzp]

It turns out, the previous testing was done incorrectly so what I was comparing the recent data too was incorrect. Apparently the previous tester had some jumpers or something where there should not be. Thanks for the reply busbar.