SATURATION OF ct TESTED FROM REMOTE RELAY ROOM?

[ppaya]

I was asked to test CT's in new substation from breakers itself (OK with me) and from relay room(this is part I don't quite understand why)???
I am not sure if there is such a practice since I haven't seen one in my 10 years of field service. Any thoughts guys?

 

[PHovnanian]

What sort of tests did they want run from the relay room?

I don't think its out of line to do some tests from the relay room to verify correct wiring of the complete CT circuit, voltage drops within specs, etc. If the CT circuit impedance is too high, it will saturate even if the CT tests OK at the breaker.

 

[oldfieldguy]

A good commissioning test regimen is going to do both of these. I typically did ratio and ratio and saturation tests of the CT's on the breaker when it was set on its pad, verifying that they met specifications and that he wiring was correct.

Then, when the wiring from the breaker to the relay room was completed, I verified that field wiring was terminated to the correct terminals in the breaker and at the switchboard.

Then, I injected current at the breaker terminal strip and checked for proper indication at any metering and relaying at eh switchboard. Finally, while that known current was being injected, I measured the terminal voltage at the breaker terminal strip as a check of circuit burden to compare with expected values.

Once you know the saturation voltage of the CT's, you can test them from the relay room. The saturation voltages measured from the relay room should be essentially identical to the voltages measured at the breaker terminals. This keeps you from having to walk out to the breaker in the future.

old field guy

 

[ppaya]

OldFieldGuy:
if you apply voltages from relay room, I don't quite understand what is it that you achieve with this, for following reasons:

1. between current links of the CT in entry Panel in the Relay Room and actual CT in the Yard is usually only wire (tech-cable). Applying voltages remotelly or localy should yield same result for saturation

2. Correct wiring can be proved by injectiong current not voltage

3. Some CT's (such as ones I work on this Site) need to be injected with up to 1200V for saturation, where control wires are rated lower

Can you comment?

 

[oldfieldguy]

ppaya--

You're right on #1 IF (big if) there are no problems in the wiring. So you go to what you expect the correct terminals to be, and you inject voltage. the current should show saturation at essentially the same level as your tests back at the breaker. BUT if the terminals do NOT go to your CT's, you won't replicate your previous figures. You're basically verifying correct wiring by a redundant test.

On #2, again you're essentially correct, but in between the CT at the circuit breaker and the relay or meter, there are multiple connections. You want to make sure that your wiring throughout your system is correct. The current injection test can do that for you as well.

For #3, many of the high-ratio CT-s are multi-ratio. You can do your saturation test at lower ratios, but realize that the high voltage will appear on the full-ratio terminals. If this voltage exceeds your limits, then a full saturation curve cannot be obtained in field conditions. I have saturated CT's in the field to the 800-volt level by using lower ratios, but I won't do anything past that.

old field guy

 

[jghrist]

BUT if the terminals do NOT go to your CT's, you won't replicate your previous figures.

What if the terminals go to the wrong phase or are connected in the wrong polarity? The saturation test will not show this.

but in between the CT at the circuit breaker and the relay or meter, there are multiple connections.

So, are you certain that the things connected to these multiple connections can withstand the voltage of a saturation test?

 

[scottf]

oldfieldguy-

I question your assertion that performing an excitation test on lower ratios of multi-ratio CTs will impose a higher voltage over the full winding.

For instance, for a 2000:5A MR CT, if you apply 400 V to the 1200:5A ratio (X1-X3) there will not be a higher voltage on the full winding, i.e. X1-X5. If 400V is applied to X1-X3, then the voltage measured from X1-X5 would also be 400V, because there would be no current flow between the X3 and X5 terminals.

 

[jghrist]

I question your assertion that performing an excitation test on lower ratios of multi-ratio CTs will impose a higher voltage over the full winding.

Actually, it will. The winding acts like an autotransformer. This is the reason that you can't use part of the winding in high impedance differential ralays unless you carefully evaluate the full winding voltage.

 

[oldfieldguy]

jghrist--

You know and **I** know that the typical insulation class for CT wiring and terminal hardware is 600 volts. However, I have tested saturation on C800 CT's for a bunch of years and never had insulation break down.

In my experience, the idea of testing saturation from in the relay house is an expedient to keep from having to stand out in the weather at the breaker, but it can provide useful data.

As for verifying polarity and phasing, that should have been done in the original commissioning tests. If I were investigating improper operation, I'd be redoing those tests, but we typically did the periodic saturation testing on CT's in GCB service where they were more exposed to weather and rusting of the iron caused saturation changes, or the physical movement of OCB action caused the mounting plates to shift. Both these will change saturation downward. None of them will change polarity or phasing.

ppaya--

I agree that injecting current from the CT terminals in the breaker is the best determinant of the sufficiency of the entire current loop, but then we go back to that 'expedient' thing.

scottf--

This one's too easy to play with on the bench. Get a multiratio CT. Inject a voltage in any part of the winding. Measure the voltage across other terminals. If you inject a voltage from x1 to x5, then your measurements for x1-x2 plus x2-x3 plus x3-x4 plus x4-x5 WILL add up to the voltage x1-x5. Guaranteed.

CT's are fun things to play with on the bench and you can show your technicians a lot of interesting things with them that apply to any transformer.

old field guy

 

[davidbeach]

scottf, remember, the excitation branch is distributed around the core, just like the windings. If your statement

then the voltage measured from X1-X5 would also be 400V, because there would be no current flow between the X3 and X5 terminals

were actually true then an open circuited CT would be quite benign as there is no current flow. You and I both know that an open circuited CT is far from benign.

 

[ppaya]

Thanks everybody.
I can sum up what I heard from all of you by quoting OldFieldGuy:
:In my experience, the idea of testing saturation from in the relay house is an expedient to keep from having to stand out in the weather at the breaker, but it can provide useful data"

If this is the only reason for doing this test, I would refrain from doing this test remotely. So many things that can go wrong with it, and test is very limited with answers.