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?

 

[inpran]

Odlanor,
I suppose this is related more to experience than a standard practise. A colleague of mine has never come accross an Open CT connection in his entire carrer and he argues that Primary Injection might be omitted and a simple measurement of loop resistance for CT and ensuring that no short-circuits in VT secondary circuitary should be sufficient.
I do not know about the IEEE world, but none of the IEC standards or the standards in my country (Indian Standards) specifies anything about primary injection. But I have always felt more comfartable when primary injection is done. It is the best feel good factor about your system. I keep primary injection as the last test and hence this way ensure that CT circuits are closed and VTs aren't shorted (just in case anybody else have opened them for any testing / checking purpose).
As for the secondary injection tests, I think almost all of them are related to relay testing which cannot be avoided.

 

[rasevskii]

If you do not do primary injection, you have not completed the commissioning, and you have not done a reliable and safe job.

How many times have I and others come across mistakes, reversals, shorted secondaries, wrong phases, wrong ratios, etc. in completed work by others, sometimes years afterward.

If I was the Client, primary testing would be mandatory.

rasevskii

 

[Bramb]

I fully agree with rasevskii.

Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.

 

[mgtrp]

I come from an environment where primary injection is mandatory (IEC as the prevailing standards body), but now live/work in North America where it is not (IEEE as the prevailing standards body). Errors in the CT wiring are situation normal (including open circuits, but mainly incorrect polarity).

For the most part, errors are discovered by measuring/reading the spill currents etc after commissioning, or for differential circuits, when the equipment trips off once load starts to come up.
Errors in CT wiring for restricted earth fault are generally found when inadvertent trips occur for remote earth faults.

My preference - to complete the primary injection tests, in order than unexpected "surprises" are found prior to commissioning. That being said, most inadvertent operations seem to have a lot less impact than you might think.

 

[lz5pl]

My 2 cents too:
We are doing secondary injection from the terminal block of first marshalling cubicle in the switchyard (closest to the CT's). We check (by clamp meter) where currents are flowing, and correct reading on relay protection displays and measuring instruments. That way all wiring, from these terminals up to correct setting of CT-ratio in numerical relay protections and other IED's is checked.
But cables between CT's and marshalling cubicle are not loaded and we check them manually by Ohmmeter.
Finally, usually couple of days before energizing, we do primary injection, verifyng all wiring and also correct CT-ratio, when CT's are switchable. After that test I warn all other contractors that any person found to touch terminal blocks will be shot on spot
We are in IEC-world, but I think that there is no strict requirement here about primary injection tests. It is more a matter of good engineering practise. Actually field test of CT-ratio are required, but not with complete wiring after CTs.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[odlanor]

rasevskii,

If I was the client, primary testing would be mandatory ...
You are not the client. The client wants the plant is operating before the deadline to sell the energy in the black market!
mgprp is more to reality.

mgrp,
I need to find out a very efficient CT injection secondary tests for differential circuits, polarity, ground sensor CT.
Do you have any tips?

 

[odlanor]

lz5pl (Electrical),
I forgot you! good tips!

 

[DTR2011]

Mr odlanor,

I would suggest you look into Omicron CT Analyzer. It is my experience that in the IEC world, CT test have often been performed with current. The Omicron device uses the voltage method.

Here in the USA, we have been using the voltage method for many years, as we often utilize busing CT's on breakers and more important, transformers. It is impossible to test via current through a transformer.

As time is a bit scarce for me, please see

http://www.omicronusa.com/en/products/pro/primary-testing-diagnosis/ct-analyzer/

I assume you are in IEC world, so omicron.at if you prefer.

Quick, safe and comprehensive tests of all CT types, per any standard you require.

If I seem biased, it is because I have been using this brand of equipment since 1998 and have always been pleased.

Now off to your other thread...

 

[rasevskii]

Odlanor:

Yes you are quite right in saying that if the Client does not want to do primary tests, then, if you are being paid by that client, you have to do what they say. But...if something goes wrong after the commissioning (spurious trips, etc)have a good explanation available at the next site meeting.

Beware of "unexpected surprises" as mqtrp has said.

rasevskii

 

[odlanor]

question of the client for
mqtrp,
If I eliminate the primary injection tests,
could your unexpected "surprises" damage any equipment of power plant?

 

[mgtrp]

I've seen plenty of false trips, but never damage to equipment. Then again, I've never closed onto a faulted piece of equipment with a mis-connected protection system.

 

[DTR2011]

If you test with the voltage method on CT's, you still connect to the primary, just with much smaller leads. Voltage is applied to the Ct secondary and induced on the primary. After that, a burden test (or secondary injection as I call it) is performed back to the protection device. This is fairly standard practice in the IEEE world.

On a VT it is primary injection from HV to LV, but the cables are much smaller.

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

Inject

Ia (L1) 1A <0
Ib (L2) 2A <-120
Ic (L3) 3A <120

The vector sum on the neutral is a very familiar 1.732A. Measure with clamp on.

In the US, most CTs are in breaker or transformer cabinet, so this is a convenient injection point.

 

[pwrengrds]

I seem to have missed what the question was exactly due to lack of definition. Please forgive if I am missing the mark...

Primary injection on what? Are we talking about low voltage gear (<1kV) where primary injection is a standard course of action? Or... are we talking about high voltage >1kV where CT's and secondary injection are used?

The only time I have done primary injection being a standard is with low voltage gear such as molded case or power circuit breakers. With some power breakers it is possible to do secondary injection testing, though when I do it I come up with some way of testing the primary at the same time.

This thread seems to be talking about medium voltage, where in my world, PT/CT ratio and directional testing then, secondary injection is standard. Followed by going on line then checking very carefully, that the expected currents are seen in each relay or meter.

 

[odlanor]

pwrengrds,
Good question.
Our question comes from a complex hydroelectric of two power plants interconnected by 500kV SF6-GIS .
Each plant has six step-up transformer 500/13.8/13.8kV-400MVA-Ynd1d1. Each phase of 13.8kV busduct segregated phase - for derivation has two 60MVA generator and derivation of the auxiliary system.
CTs and VTs are enclosed in 13.8 kV busduct, 13,8kV-switchgear circuit breaker(SF6) and 500kV GIS.
So, there is no convenient primary injection point.

 

[pwrengrds]

Primary injection is generally reserved for low voltage breakers.

For high voltage, the instruments are tested for ratio, polarity, saturation, grounding, then ringing them out. For CT's I personally like the 1,2,3 amp, ABC method mentioned above, that is done from the same point that the ratio test was done, at that point you do not have to disconnect the CT's, then check the single point grounding, burden testing, and turn it on. As soon as you get enough primary current to see it check the metering or relaying to verify that it's working properly, typically I would expect that to be done a couple of times at various current levels.

The commissioning agent should provide yellow highlighted drawings of the CT wiring done when when the injection was done to as proof it was done correctly. Same for the PT circuits and for the trip and close circuits.

This is what I would do if I was doing it, or if I was in charge of seeing it was done. I believe its fairly standard through the industry.

 

[odlanor]

smallgreek, pwrengrds

...CTs and VTs are enclosed in 13.8 kV busduct, 13,8kV-switchgear circuit breaker(SF6) and 500kV GIS. So, there is no convenient primary injection point.

And we are talking about 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;

 

[DTR2011]

Hi odlanor,

I have commissioned 550kV, 345kV, 138kV & 34.5kV GIS (ISO Phase and non ISO phase) and in my experience, there is always a way to get into the bus bar. Generally, this is found in the grounding switch or fast acting ground switch, which has a removable link to ground. It is ther to remove for testing. You typically remove the link and connect the polarity side to this. I am sure this is available on ABB German & Swiss gear and some (Areva/Alstom) gear I have commissioned.

You may want to contact the vendor of the gear if you have questions about where to connect. I believe the problem you perceive is the same one we commissioning / maintenance folks deal with all of the time. "How the heck do I connect to this gear, without ripping it apart?"

With the voltage method of testing VT's and CT's, you are connecting to the primary via the methods I described above. These devices are transformers, so its a matter of apply a test signal and measure the results. In both cases, you are measuring a known reference (apply) for phase and magnitude. The Instrument transformers do their transforming and you either measure that on the primary or secondary connection (different for VT vs CT). What is measured is the magnitude and phase angle. Simple math gives the ratio and the polarity is either ~0 deg (correct) or ~180 deg (incorrect).

If you need more information on how this is done, let us know. I have tested these devices in a pinch using "old time" methods which include a variac and lantern batteries and a few meters and yielded satisfactory results. The modern test methods, are simply nicely packaged versions of this.

I have worked as an Application Engineer for 2 test equipment OEMs in the past and have a pretty good handle on this if you are curious.

 

[ScottyUK]

Yes, it is definitely possible to inject current onto the busbar via the earthing switch on ABB's ELK gas insulated gear (Swiss origin).

I haven't used the voltage injection method but your description makes sense and it could be a useful in certain scenarios. I wonder why it is not favoured over here in Europe?


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If we learn from our mistakes I'm getting a great education!

 

[rasevskii]

Odlanor:

If you post a one or three-line diagram of the plant you are involved with (remove the name of the site and the OEM, etc) we can suggest a way of primary testing using the generators as current sources via the Gen step up transformers and earthing switches on the GIS.

I am assuming, perhaps wrongly, that this is a new installation not yet in service and you have to prepare a test method which can be actually done.

I was involved in primary tests for a large hydro plant in which new upgraded protections had been installed, at the 6KV, 15KV and 500 KV levels. This was not, however GIS but existing open air switchyard. A complete program had to be developed, listing all switch operations and devices. The existing generators were each used as a current source for each units tests. The tests included current, earth fault, and open circuit, protection and metering primary tests for each unit

It can be done but it is a lot of work just preparing the program, but goes smoothly and safely if well thought out and a good team is available.

The excitation systems are run in manual only, with extra precautions such as an emergency excitation trip button at the test operators desk, in case something goes wrong.

rasevskii