diff protection, primary injection test

[worldtraveler]

Dear engineers,
I need your opinion.... Is it possible to test ''stability'' of line differential Protection on 36kV cable on way that we inject, for example, 100A on one phase (to ground) and connect this phase to ground on other side? So that we close loop through ground. Cable lenght is around 2km. Will be ground resistance to big problem to make this test.
We have fibre optic cable beetwen relays on both sides and we would like to check that CT's are connected correctly and of course that fiber optic cable is ok.

Thanks all,

 

[DTR2011]

I would think a 3 phase load box and a suitable 3 phase source would be a better solution. I've rented these in the past, with 3 ph 480V as a source. I calculated the secondary current I needed to select the kW size of load box. I forget how many kW I used, as this was 15 years ago. I don't think it was terribly expensive and the cooling units kept the load stable. Generator service companies generally have these on hand as well.

Load Bank

 

[ScottyUK]

It's entirely possible to use a mobile generator as an injection source if you arrange direct access to the alternator field and drive it with a lab-type benchtop variable DC power supply. This arrangement gives you an infinitely variable primary injection source from zero to rated output voltage and current. Obviously make sure the generator can handle the required injection current - for a cable diff test you probably won't need a great deal of current and a fairly small set will suffice.

 

[worldtraveler]

thanks for replay...
I am thinking between two possibilities.
One is with 3ph source like Diesel generator and connect around 400V on one side with 3ph short circuit on another side.
Another is with Omicron CPC100 set, to inject for example 200 A AC in one phase and connect it to ground on other side to get closed loop. Rated power, according manual, for 200A is 1300VA. My concern is will the ground resistance be too high (or have some other influence) to perform this test. I have done similar test before but in small distance (around 30meters) but now I have a 2km long path.
Is it possible to get some ground resistance data and how?

 

[ScottyUK]

I wasn't thinking of 400V from the generator, but the option exists to boost the drive voltage if you needed to.

How good are the earth electrodes at the ends of the cable?

 

[stevenal]

I agree your ground impedance would be a problem. Why don't you use a second phase for the return?

We simply parallel the new line into service. If it trips, no outage occurs and the event report is available for diagnosing the problem. If it doesn't trip, the load current can be examined for polarity, phase, or magnitude issues. It does help to use a smart relay.

 

[DTR2011]

You would have to check the cable impedance if you want to use the Omicron vs the Omicron Output (compliance) voltage.

As alluded to, connecting 400V through an unknown impedance (cables + earth) may cause a few surprises you may not want during a commissioning test. Perhaps a lower source voltage 220?

Primary Injection is generally something where the expected results are know, prior to testing.