Secondary injection tester going into overload

[bspace123]

Hello

I am currently trying to test a protection relay per the attached schematic. Basically, the relay has two sets of CT's connected together in parallel with the relay, which are mounted on different feeder cables. The idea is that when feeder A is reading xx Amps and feeder B is reading yy Amps, the relay will see xx - yy Amps.

The feeder B CT's are connected via 2.5mm2 4C+E cable. The feeder A CT links have been opened and this is where I have connected my test set.

When I inject per the schematic shown, my test set goes into overload. The DC resistance between A/A- B/B- and C/C- is ~13 ohms and I am only injecting 100mA. This should not overload the test set? When I short out A-/B-/C-/N- the test set works fine.

Why are these CT's overloading the test set?

Schematic link: Link

 

[thermionic1]

Typically, when testing a relay one would want to completely isolate the device for the AC&DC circuits. A popular example would be the Westinghouse / ABB FT test switch - Areva/Alstom has something similar. When all of the switches are open, the CT's are shorted. There is an accessory test paddle that is used that connects the test set to the relay. Depending on the test set, 4 or 6 current wires are used to connect to the Polarity & non polarity terminals entering the relay. If the test set has 4 wires, the common neutral is made up by placing small jumpers from the sing test set neutral to the remaining phases. I see your relay is using a separate In input. Remember to mind the polarity of the phase neutrals out and the Neutral polarity. Depending on the test set design, your test set neutral may or may not be isolated from earth. If not isolated and the CT ground is still in place, the current will split between the test set mains earth connection the the CT earth connection.

FWIW, when we perform "loop checks" (DC resistance) on say a 3000:5 circuit, somewhere around an ohm of DC resistance is measured on a single CT/relay circuit. You haven't mentioned 1A or 5A CT secondaries. Either way, 13 Ohms of DC resistance is a lot to ask from a modern relay test set - its even a greater impedance at mains frequency.

I strongly suspect that if you jumper the neutrals together and inject only into the relay you will have success. Please be sure either the primary circuit is isolated and / or the CT secondaries are short circuited. Extremely dangerous & lethal voltages can appear across the secondary of an open CT under load.