injection of CT primary current during comissioning is necessary? 1

[odlanor]

The drawback of CT primary current during comissioning is that they are time consuming and expensive to organise;

Wiring errors between VT’s/CT’s and relays, or incorrect polarity of VT’s/CT’s may not be discovered until Primary injection testing;

This hazard is much reduced where digital/numerical relays are used, since the current and voltage measurement/display facilities that exist in such relays enable checking of relay input values against those from other proven sources.
Many connection/wiring errors can be found in this way, and by isolating temporarily the relay trip outputs, unwanted trips can be avoided.

1-Under these circumstances, the primary injection tests during comssioning may be omitted ?
2- With primary injection tests during comissioning can some secondary injection teste be omitted?

 

[rasevskii]

Odlanor:

Before the above primary tests are considered, all the secondary testing and calibrations have to have been completed successfully.

rasevskii

 

[odlanor]

smallgreek
One nice little trick is to use a 3 phase current test set (modern DC coupled amplifiers, whose neutral is isolated from earth)...
what is modern DC coupled amplifiers?

Our TCs are connected in Yy with primary neutral grounded at CT location. Secondary cable are four wires , with neutral grounded at CT location.

 

[odlanor]

smallgreek (2)
One nice little trick is to use a 3 phase current test set (modern DC coupled amplifiers, whose neutral is isolated from earth)...what is modern DC coupled amplifiers?

Our TCs are connected in Yy with primary neutral grounded at CT location. Secondary cable are four wires , with neutral grounded at CT location.

Inject Ia (L1) 1A <0Ib (L2) 2A <-120Ic (L3) 3A <120...
I guees this is a three-phase secondary injection, isnt it?

 

[odlanor]

smallgreek (3)
One nice little trick is to use a 3 phase current test set (modern DC coupled amplifiers, whose neutral is isolated from earth)...what is modern DC coupled amplifiers?

Our TCs are connected in Yy with primary neutral grounded at CT location. Secondary cable are four wires , with neutral grounded at CT location.

Inject Ia (L1) 1A <0Ib (L2) 2A <-120Ic (L3) 3A <120...I guees this is a three-phase secondary injection, isnt it?

It is more simple inject single phase an= 1A, bn=2A, cn=3A and reading same result at relay phase correspondent.
What do you think?

 

[DTR2011]

Hi Mr. odlanor,

A modern relay test set does not use a transformer, other than the power supply. It's neutral is isolated from earth. The CT circuit you describe is what I am used to seeing.

The ABC, 123 method I described can also be used in the 1A IEC world, simply divide by 10 (amplitudes), if you are concerned about excessive current on your relays.

As I said, this method is very comprehensive, as it is the vectoral sum, ie a ground fault. You are checking the phasing and polarity of all connections from the CT secondary terminal block, all the way through any test switches to the relay. In my opinion, single phase testing is not acceptable, especially since the relay test equipment is available.

As to primary current testing, this is preferred and I found a very interesting case on ABB ELK gear in 2000, where there was a tiny grounding problem betweeen the oil filled cable bushing to the GIS gear itself, which caused the gear to become a 1:1 transformer. Very strange and at 3AM, at least we found it prior to energizing.

For those on the East side of the puddle, the Voltage Method of CT testing has been an approved IEEE method for many years. Again, we use dead tank devices, especially in power transformers, so it is impossible to push current though the transformer.

In the voltage method (100:1 CT), say 100 Voltage is applied to the secondary of the CT, 1 volt is induced on the primary. Our IEEE standards for a C800 CT state that the CT shall have its kneepoint voltage greater than the lower class (C400), up to 800V. So we inject voltage again on the CT secondary and measure the voltage/current relationship, to plot the excitation curve. Simple small wires are connected across either the breaker bushings (per phase) or the transformer bushings (H1-H0), for example to measure the induced voltage for ratio and polarity.

 

[odlanor]

Smalgreek(4)
13.8kV bus duct ABC phases(4000A) go through Core-Balance (Zero Sequence) CT.
How do you test Core-Balance (Zero Sequence) CT’s ?

 

[DTR2011]

Hi Mr. Odlanor,

Do you have a single line diagram? It would help to show you where to connect. In theory, if you are testing L1 phase with respect to ground, you could pick up the CB CT. I would have to check what you have and what is available to connect to in able to confirm this.

I am assuming this is the Lowest voltage SWGR from your other post, and I am beginning to get posts mixed up.

I have seen from this excellent forum that the more information you supply the fine folks here, the better quality answer you get.

Where is this installed?

 

[odlanor]

smallgreek,
both post refers to same problem.
each 2x60MVA-13.8kV generators are connected to 13.8-13.8/500kV stepup transformer. Generator are grounded by high impedance.
Seletivity of stator protection 59N is made by 67N installed at Core-Balance (Zero Sequence).
CT-600V has 520mm diameter with 3 phases crossing it. Each phase is a tube insulated for 24kV-4000A.
It is the biggest Core-Balance CT I know.
I want to check operation of ( 67N and 59N ), not CT.

Yes, this forum is excellent to give technical information.
I am just know with the responsability to aprove the comissioning manual for protection.
This forum has helped me to make decisions.

 

[DTR2011]

Hello,

I'm a bit confused by the statement "Seletivity of stator protection 59N is made by 67N installed at Core-Balance (Zero Sequence)."

59N is a voltage sensing function and 67N is a current-current or current to voltage polarized function. Generally what I see core balance CT's used for is sensitive ground fault protection on motors.

That being said, I wanted a single line diagram in order to see how we can connect to the system for testing.

"I want to check operation of ( 67N and 59N ), not CT." I am assuming this statement means via some kind of "live test", rather than a secondary test. As I interpret this, I would check the operation of the relays via secondary injection and a relay test set.

I believe ScottyUK had mentioned methods of using the generator itself as the voltage source, which I have done in the past. If this is the only way (determined if we get 1 line), then you simply place one earth cable on the circuit and the other phases are open circuit, which provides the healthy voltages for the 59N function. Sometimes it is difficult to get the machines up and running, if you require 13.8kV backfeed for starting motors, lube oil, etc. Do you have some kind of blackstart 13.8kV feed to energize the balance of plant equipment?

Thanks

 

[odlanor]

smallgreek,
see attachment

 

[odlanor]

smallgreek,
see attachment