automatic test set for instrument transformers 3

[Gestur3]

Hello

Im of the opinion that you should know how to test ct and VT with old school methods.

But when testing huge substations it is nice to have automatic test sets.

What are the opinion about different test sets? Good experience and of Course problems and limitations.

I have tested some different makes/models:

OMICRON CT analyzer, great instrument but expensive and alot of pushing buttons to get from A to B. Uses voltage methode, which is ok with me
but some people dislike. Sometimes gives fail result on perfect good CT cores, usually redo the test and everything is ok(Litle disturbing)

NDB Technologies CTTx2/CTTx5 easy and fast test set. Limit in core resistance measurement to 20 ohm! Also maximum 2kV for knee point.
(Also voltage methode)

OMICRON CPC100, semi automatic test set CT and VT and more(Like a big Sverker). Alot of rewiring to get one core from A-B.
Great instrument. Feels litle outdated, maybe a new version coming?

Br Gestur

 

[DTR2011]

While I agree that knowing how to test the ratio & polarity of an IT using old school methods is
important, getting the job done in a reasonable time with proper documentation is important as well.

In the USA and other areas, multi-ratio CT's are common, if not the norm. With this comes more, if not different
CT testing requirements - sometimes to the point of being way overboard.

There are several manufacturers of automated CT test sets available. I have used most of them.

My vote is the Omicron CT analyzer, with the multi ratio switch box. In less than one minute,
all ratios, polarities, winding resistances and excitation curves are tested. Most important,
if there is a mistake in the connections, the user is immediately made aware of this. There
are several modes in the test set, where it can act like a simple voltage / current source with
associated metering, so diagnostics can be made easily. The test set also has the advantage if being able
to test the excitation characteristics at a much safer (and lower) voltage level.

Again, in the USA, we have most CT's with ANSI "C" classifications, with the highest being C800. When testing
with other CT test sets for excitation, people tend to apply up to 1200-1500 VAC, with 1-2A, on a C800 CT. With one slip of a wire
someone could easily be killed.

The last batch of CT's I tested were in a transformer. 12 of them. It was - 6 Deg. F on the day I tested. I rented the Omicron
and had all CT's complete in less than three hours. Using other test sets, which can take 7-11 minutes per CT, I would have been outside, freezing all
day!!

 

[scottf]

The Omicron CT Analyzer is by far the best automatic test set on the market in my opinion.

We use a number of them in our instrument transformer factories. As mentioned above, the Multi-Ratio add-on box makes it ideal for testing protection CTs in a substation environment, but it is just as good at performing metering accuracy tests as well.

The only limitation we have run into is it has problems testing CTs with "partial turn compensation"/"zero-flux compensation". These are somewhat rare and would typically in 600V low-ratio metering class designs. I've been told that Omicron is working on an solution to this issue. It may already be available.

We've tested some of the automated VT test sets, but they are all relatively slow and don't save us much/any time over conventional factory test methods. However, I suspect that would be well-suited for field tests.

 

[lz5pl]

It depends of what is your scope of works - only CT/VT testing or also general plant commissioning. For plant commissioning I would suggest CPC100. CT-analizer is good if you are only in CT/VT testing, but with CPC you can do almost any test in the switchgears.

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

 

[Gestur3]

Hello IZ5pl,

We test CT/VT and protection(OMICRON 356) and control, we used CPC100, for VT it is ok from my perspective(Ratio and polarity) but for CT it takes to much times compared to NDB or CT analyzer.
Our customer usually want Polarity, Ratio, megger test, winding resistance, burden and mag curve.

Anyone tested ISA Test STS5000? Is it a copy of CPC100 or is it automatic like CT analyzer? Looking for an instrument to test CT and VT(70-400kV) with higher voltage than the 300V
I get from my Sverker 750.

 

[111R]

Raytech released an automated CT test set recently. I do not have experience with it, but have used other products from them that were all solid and simple to use.

I have experience with the Omicron CPC100, Vanguard EZCT-2000, and Megger MC1605. The Omicron is by far the most versatile, but is more bulky and slower to run tests on. The Vanguard works well and has good software. The Megger is slower and less reliable.

 

[randyman]

When using portable CT test sets keep one thing in mind - what are you accomplishing with it?? I will tell you that the IEEE C57.13 and IEC 61869 standards require testing be performed at rated current at rated power frequency. Manufacturers are mandated to assure extreme accuracy and many have traceability to NIST, NRC-Canada or some other standards lab.

With that being said I can tell you this about the Omicron CT Analyzer - it is small, compact and easy to use AND is a good tool for troubleshooting and commissioning. However, it does not perform test at rated current or rated power frequency and therefore does not comply with the standards. It constructs a typical B-H characteristic of the CT and from that everything else is extrapolated. And to my knowledge it has no traceability, but that may have changed over the past few years. It also has some limitations.

Omicron does have a lager CT set (on a 2 wheel dolly) that will work at rated power frequency up to some current level. And there are other makes out there as well which are good for qualifying/checking protection class CTs. The biggest problem I find is many do not have large enough power supply to properly excite a CT core. Qualifying metering class CTs is another topic.

As a CT manufacturer we do not use any portable sets to qualify our finished products but do use them for troubleshooting in the field.

 

[collies99]

Protection Class CTs are much easier to test than high accuracy clasd revenue metering CTs.

I like specialized automated test set much better than old school method. You still need your old school basic theory.
When it comes time to do pre-commissioning work which may total more than 500 individual CT's, Raytech and Vanguard would be my equipment of choice.

Vanguard makes easy soft copy to boot. It comes down to personal preference and each to his own.

 

[DTR2011]

"When using portable CT test sets keep one thing in mind - what are you accomplishing with it??" Great statement. I believe the OP was asking about field testing of large numbers of IT's in a substation environment. What can be done in a practical amount of time is to verify the ratio, polarity, excitation, winding resistance and some kind of test on the secondary insulation (megger, etc)? Additionally, one must balance local safety (lock out, tag out / hold card) rules, environmental conditions, cost, scheduling and available test supply power.

In my world (contractor - utility / heavy industrial commissioning), it is #1 to satisfy the customers specifications / requests, #2 to fulfill my own (trust / distrust) in the (design / application) of the circuit. #3 if there is a problem (most likely in wiring) to identify the problem, have a way to rectify the problem and #4 to document the results in an efficient and meaningful manner.

With all of that said, there are practical limitations to anything that is performed in the field. Additionally, I believe it really depends on what part of the world the work is being performed. The best example would be in what I call the "dead tank" world, meaning the CT's on (outdoor) breakers and transformers are normally located physically within the device itself. I'll pick a power transformer as an example. Unless there are steps taken during the construction of power transformer (halt transformer construction activities so commissioning / relay tech can enter transformer), the only way to test a CT in an assembled power transformer is the voltage method - one is not going to pass power frequency current through a transformer winding to access the CT primary any other way. The voltage method, has been approved in the IEEE C57.13 Field Testing Guide for as long as I can remember IEEE C57.13.1-1981.

Getting back to the OP questions, what is good test equipment and why? What is not so good and why? I believe the Omicron CT Analyzer is the best because it is the lightest and quickest. My company owns a Vanguard EZ CT2000C. I avoid using it because it is slow, heavy and when I have an issue it offers limited control of a manual test. One could argue that the Vanguard (or others using similar test methods) is "better" because it tests excitation tests with "high" voltage and thus is also testing the secondary insulation. As mentioned in the OP, megger tests are often required and I can say that it is much quicker to use a separate megger than the built in megger in the Vanguard (or others). One could also argue that if OSHA (national safety) were on site and witnessed a single person lugging a 73lb test set around by themselves and applying 1200+ Volts @ 1+ amps, there would be some kind of discussion. Given that the equivalent Omicron and Vanguard are roughly the same price, that's my opinion. All of the other modern field CT voltage method test sets are basically the same general design as the Vanguard. My company has had both the AVO/Megger and Adwell versions in the past and they have been abandoned for reliability reasons. I believe it is also important to select a test equipment manufacture that can support the product locally and in the native language that you speak. My second choice would be OMICRON CPC-100, especially if I have other testing in substation. The CPC offers the additional benefit of being able to test via voltage or current method. I made a test box to manually change the taps on a multi ratio CT. Yes CPC is much bigger and heavier, but there are many things it can do, it has automated excitation tests and the reporting is easy. Years ago, I had a Programma Sverker 750. With a few switch boxes and a 1200:120 VT, I could accomplish much of the same (without fancy reports) in a smaller, lighter, less expensive package.

Some years ago, I had an Engineer question the CT Analyzer, as it performs certain tests (excitation) differently. I was provided the attached KEMA report, which passed the scrutiny of the utility protection and metering departments.

 

[stevenal]

...the only way to test a CT in an assembled power transformer is the voltage method - one is not going to pass power frequency current through a transformer winding to access the CT primary any other way

. Why not? If the unused windings and CTs are shorted and the current used is within the winding's rating, I fail to see an issue.

 

[prc]

Setevenal,it is not easy to get short circuit voltage at site to send the rated current of transformer through windings. To overcome this, some manufacturers provide a test winding in Bushing CTs. It is a one turn primary on CT core with a rating of 10A. During normal operation this test winding will be kept open with one end grounded. During CT testing, transformer winding terminals will be kept open, while feeding current through primary one turn of BCT.

 

[SHR1969]

I am "old school" when it comes to testing CT's. I like hands on testing of ratio error and saturation tests, etc. All these new automatic CT testers make me question how the test is done and what can I expect from the results? I expect the automatic tester to do the tests in accordance with the IEEE standard, nothing less. I expect the saturation test to be performed at rated frequency, in my case 60 Hz. I need an insulation test done in accordance with the IEEE standard. As long as the automated test set manufacturer states that they test the CT's in accordance with IEEE C57.13.1 then I am good to go with something new.