CT Secondary Test Current Injection 2

[dvscrobe14]

I have noticed in the field two ways of injecting CT secondary test current, when testing the integrity of the entire CT circuit that is connected to a CT, once all the burden elements have been individually tested. One way is to break away the leads at the CT terminal board and connect them to a test set. Then, the open CT would be shorted down and grounded. The other way is to leave the CT circuit intact and "piggyback" the test leads onto the circuit, thus creating a parallel path for test current to flow. Typically, .5A, 1.0A and 1.5A would be pushed on the phases and then going to each of the circuit elements to check that the current magnitudes are reading correctly and are on the correct phases. Also, the neutral current would be checked which would be .86A in this case. A lot of my coworkers say that piggybacking is not a good idea but no one can tell me what would happen if you did it that way.

 

[oldfieldguy]

"Piggybacking" works if you have a good secondary circuit. I have employed it for years. The benefits include having a way to perform testing where access to CT terminals may be difficult and the fact that you have one less chance of improperly terminating a conductor after testing.

The voltage your test set must provide to push the currents at levels you indicate will (should) be well below the CT saturation voltage. The CT winding will basically act as a very high impedance, absorbing almost no current.

The entire remaining current will flow through the secondary circuit through your devices. Measuring voltage at a given current level gives you the actual circuit burden for you secondary circuits.

IF you are unable to 'push' current through the secondary due to any of several problems (things you may find in commissioning, following circuit modifications, etc.) the voltage at the CT terminals will rise. You then know you have an issue to resolve.

old field guy

 

[pwrengrds]

Piggy backing CT cicuits works fine if you have a good CT with core iron that won't saturate at low voltages.

Problems come in when the CT starts to saturate at the voltage you're using to push current, usually less than 10 volts. If the CT won't saturate at > 100 volts (C100 or greater) the AC impedance of the CT will cause current to go to the load and not backfeed the CT. If the CT saturates then the current pushing out to the loop isn't correct (because it's going back to the CT) and creates reading errors.

 

[dvscrobe14]

Thanks for replies. I was told on another forum that some current is going thru the CT and can be found by taking the voltage of the CT circuit that is across the CT and looking up what the excitation current is on a CT data sheet, that graphs secondary excitation current to secondary voltage. I was wondering that if some excitation current is actually going through the CT, their wouldn't be a path for current to flow on the primary side.