CT Testing - Primary Injection and Seconday Core Checking

[mikelangel78]

Hello again everyone.

I'm studing the CT testing issue, because a I've to prepare the tests to go on site.

I have to test a Multi-core CT: 400A/1-1-1

secondary N1: metering
secondary N2: protection
secondary N3: protection

The secondary nix of the CT is connected to a marshalling box that is installed in the CT steel structure and from that the circuits goes to the relay and metering in a control house.

For perfom the test I must use a OMICROM CPC100.

The first step that is mentioned in the test procedure is:

Seconday Cores Identification

1 - Checking that the secondary grounding is present in each core;
2 - Checking that all the CT seconday circuits are closed;
3 - Inject 200A in the primary side of the CT, start in fase 0 (phase system here is 0, 4, 8) and the expected current in the seconday side should be 0,5A.
4 - Verifying that the secondary core in test, for example N1 (metering,) is the correct one. According to the protocol, this should be done by shorting the correspondent terminals in the seconday boxe of the CT, and checking that the current in the phase and in the neutral of the remaing cirucit (marshalling box and in the control house) will disapper.

From what I understand from the point 4), if I shorting core N1 in the CT terminal box, the current disapper in the arrival on the marshalling box and neve came to the control house where the meter is. I don't understand why the current never reachs the marshalling boxe by doing this jumper in the CT secondary terminals. I tought that part of the current stays in the loop made in the CT terminal box and part of the current will flow to the marshalling box. Could some one explain me that?

Thank you

 

[davidbeach]

Strictly speaking, you're correct, "some" current will still take the loop I tithe control house instead of the jumper, but for all practical purposes the branch through the jumper will get a share that will measure indistinguishable for the total current.

 

[mikelangel78]

Ok. So in pratical terms when I place a multimeter in the terminals (related with the N1 secondary) in the marshalling box the current will mark 0 Amps because most of the current will circulate in the loop made in the correspondent secondary at the CT terminal box. Then when I remove the shunt in the CT terminal box, the current will mark 0,5 Amps in the marshalling box connected to N1. And at that stage any shunt made in the others CT secondary terminal box (N2 and N3) shouldn't origin the disapper of the current in the N1 terminals at the marsahling box, I'm I correct?

Thank you

 

[mikelangel78]

Could please someone confirm my bellow conclusion/assumption?

Thank you in advanced

 

[pwrengrds]

It sounds like you have 3 CT circuits to check. In no particular order.

Check the CT ratio. It should be per the nameplate

Check the polarity of the CT, this is important if reading power, differential, or distance relaying.

Check the single grounding point by lifting the ground and meggering or ohm check the circuit.

Ring out the circuit. Make sure the CT is wired per the drawings. Note, to do this you must know what's in the circuit. Inject current (sounds like your doing primary injection bus secondary is ok also) measure current at each device and make sure each device is programmed for the right CT ratio so it reads correctly, check that the device is reading what current is being injected correctly. While your at it open the test switches, if they exist, and make sure they short around the device and the CT isn't open circuited when the switches are opened.

Shorting each one at a time and seeing if current decreases isn't a bad idea in this case. The current should decrease by the amount of current going through the shorting wire.

The burden can be checked if needed, Inject full secondary current at the CT and measure the voltage it takes to push the current through the loop.

Make sure that ring lugs are used in the CT circuit, that's so it's harder for the circuit to be open circuited and the wires won't fall out if loose. Make sure all wires are tightened down.

 

[mikelangel78]

Hello,

I've uploaded the diagram in order to be more clear in my doubts.

CT:
400A/1-1-1-1

Step 1:
Injection of 200A in the primary of phase "0"
Confirm that in the terminals X1:25/28 of the marshalling box I will have 0,5A
What should be the value of the current between X1:25/29?

Step 2:
Short-circuiting the secondary N1 (1S1-1S2) in the CT terminal box
Confirm that in the terminals X1:25/28 of the marshalling box I will have 0 A
What should be the value of the current between X1:25/29?

Thank you