CT change and accuracy

[veritas]

An existing 3.3kV switchboard is to be replaced with a new one. There is a 400/1 existing CT with an EF setting of 20A (digital relay). The manufacturer of the new swbrd has replaced this CT with a 1000/1, Class 0.05PX200R5 (IEC 60044.1). An interposing CT is now required in order to achieve the 20A pickup as before.

I have two concerns here:

1) A 20A EF setting is now 2% of rated compared to 5% before. Is the requirement on the relay not more onerous now to detect a lower current signal amidst the surrounding noise (power station environment). Are spurious trips more likely? What about relay filtering? Relay used is SEPAM T20.

2) The increased CTR implies operating at a lower range of the CT mag curve. I have been told that the % error here is not covered by the IEC standard for Class PX CT's. In other words the standard specifies that at the top end (loosely speaking from about 2 to 3 times rated current to kneepoint voltage, Ek)the maximum mag current is 50mA.

However, at the lower range of the CT (below rated primary current)where I expect my EF operation to be as the maximum EF is only 500A, the %error is undefined. It is normally assumed that the CT is linear up to the Ek but there is no guarentee of this as accuracy and consequently errors are not tied down by the IEC spec.

Is this true?

Thanks in advance.

 

[RRaghunath]

Veritas,
Class PX CT for earth fault protection! It sounds strange.
Is there any reason behind that! Can it be that the protection is not simple EF but REF one!
More info will be helpful.
Generally speaking, if the earth fault current is limited to 500A, you can have a setting of not exceeding 150A (about 1/3rd of available current). If it is REF, that is a different issue and you would like to have as low a setting as practicable to have max winding coverage, with the CT parameters and the stabilising resistor selected.

 

[veritas]

raghun

For some or other reason all the protection CT's have been specified as Class PX. I was not privy to any of the design decisions and have only been approached to do the protection settings.

Yes, it definitely is EF not REF. This does raise an interesting point as for Class PR CT's, say 10%, then composite error is tied down by the spec to no more than 10% across the whole operating range of the CT. Apparently this is not so with Class PX CT's which is why I posed the question.

Any ideas?

Kind regards.

 

[RalphChristie]

The reason for specifying Class PX CTs, are to ensure that the kneepoint and anklepoint voltages are the same. Class PX CTs are used in differential schemes, thus the composite errors will be much smaller than normal Class PR CTs, even at lower ranges.
You can use a much lower setting with a PX CT than with a PR CT, unfortunately I can not say how low. I would not go very much lower than 50A on the primary side. (1000/1 CTs used in a residual scheme)

In my opinion you might run into nuisance trips with such a low setting (20A) on a residual scheme, even with Class PX CTs.

I would:

Add a sensitive earth fault scheme. Extra core balance CT, extra relay. You can use a very low setting (1% of CT-ratio), use a longer time delay, and detect small earth faults.
For bigger earth faults you can still rely on the the residual scheme for faster tripping, even with a higher current setting.

Regards
Ralph


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[veritas]

Ralph,

Your comments are appreciated and noted. Thanks.

The question I really have is that for a Class PR CT standard accuracy classes are 5% and 10%. This means that over the whole CT operating range the composite error does not exceed 10% say. This is practically calculated as the ratio Iexc/Itotal, where Iexc is the current drawn by the magnetisation circuit and I total is Iprim/CTR. The current in the secondary relay circuit is then Itotal - Iexc.

Consequently the ratio Iexc/Itotal =< 10%. Now for a Class PX CT a typical mag current is say 50mA at a kneepoint voltage of 200V say. The % error would be around 0.5% or less.

So the real question is - does the Class PX specification gaurantee that the error is =< 5% over the whole operating range?

Thanks in advance once again.

 

[randyman]

The class PR has same limits of error as the standard class P CT which is as follows;
class error @ 100%Ip error @ limiting factor
5P, 5PR ±1% ±5%
10P, 10PR ±3% ±10%

with errors at rated burden. The error below 100% should hold to 1% or 3% for lower levels to some point where the errors may start to increase. The standard does not define operation below 100% so there realy are no guarantees, which is why you would rely on characteristic curves. The major difference between P and PR is that a PR is a gapped core for remanence control. Inherently, a gapped cored tends to have higher exciting losses than that of an non-gapped core.

The PX is one that has a defined characteristic, with as a minimum, has a defined knee point voltage. The knee point current and/or winding resistances can also be specified. By specifying the knee point you can actually obtain an error better than 1%, but defined at the knee point. The turns error for this type must be ±0.25%.

The best way to decide which is best is by evaluating the characteristic curves themselves. In some cases you can get good performance without the added expense of specifying a special design.