CT Testing to Determine Secondary Winding Resistance & Thermal Rating 2

[JezNZ]

Hi All,

I have a current 11kV private network expansion project wherein my company is performing design & build works.

Separately a consultant has been contracted by the client to perform the Fault Study and develop the Protection Settings. They also produced the concept design for the project.

The concept design identified expansion of an 11kV switchboard. The expectation was that the CTs would be provided 200/1 like-for-like (5P10 15VA)) with the existing switchboard tiers and this was expressed to the OEM and procured as such.

Later, the consultant has identified a concern with the CTs and requested to 1. Confirm the thermal rating of the CTs at >= 180%, 2. Perform a CT adequacy check.

Unfortunately, the CT manufacturer (Indonesian) has declined to either confirm the thermal rating or to supply any datasheet information ostensibly claiming IP and the risk of IP theft from Chinese manufacturers.

I can't perform a CT adequacy check without the secondary winding resistance, but I figured I can directly measure this from the CT, is this really feasible/practical? Are there any precautions I should take with regards to the primary of the CT?

Then following, it possible to undertake testing of the CT to determine the thermal rating? My guess is no...

Thanks

 

[RRaghunath]

You can measure the CT secondary winding resistance by applying DC power at secondary terminals, keeping primary terminals open.
Following the DC resistance measurement test, do demagnetise the CT, if you intend to measure its knee point voltage, magnetising current etc.
Thermal rating test is not conducted at site, to my knowledge.

 

[scottf]

If this is a wound-type CT with a primary winding, the primary winding is likely the limiting factor in terms of thermal rating...both continuous thermal rating factor and short-time thermal ratings (normally 1 second rating).

There is not really a field test that can be done to determine either of those ratings. However, both of these ratings should be on the nameplate.

In reading your post, it sounds like maybe you want to know if they unit is good for continuous current at 1.8xInom and the marked rating factor is less than that. If so, then practically speaking, the only option is to get that info from the manufacturer or perform a heat-run test, which would be running the primary current through the primary winding with the secondary winding short-circuited or connected to rated burden (depends on standard) and then measure the heat-rise. This can be done by taking primary and secondary resistance measurements and ploting change with time and computing back the temp-rise. Most utilities would not have the set-up needed for this test.

 

[JezNZ]

Hi scottf,

At present we have the new switchboard in the workshop of a local switchboard manufacturer who managed the order and import, not on-site, so we do have the ability to undertake some testing at the factory.

You are correct that I'm looking for the continuous thermal rating. I will have the opportunity to inspect later this week, I will look for this on the nameplate.

Thanks

 

[JezNZ]

Hi,

I've been to inspect the CT's and have found that the secondary winding resistance is marked.

However the there is no thermal rating in % of Inom, only a thermal insulation class and rating "Ins. Class Y Temp. Cat. -5/50degC"

Is there a way to convert this to an equivalent % of Inom continuous current rating?

Picture of CT nameplate attached (a bit difficult to read).

 

[DiscoP]

I don't think that information relates to the continuous rating of the CT, it is more to do with the insulation rating.

The nameplace is showing a short term themral current rating 65kA for 3 sec, but not a continuous rating.

If it doesn't say anything on the nameplate, then I think the IEC standard says that the continous rating is the rated primary current.

 

[JezNZ]

So an interesting result.

I had a read of 61869-2 and it confirmed that otherwise unmarked CTs have an Icth equivalent to primary rating.

Then shortly after I finally got a response from the CT OEM (as opposed to the switchboard OEM) who I tracked down and they stated they are rated to 150%!

Unfortunately not suitable for application, but annoying that they are not marked for their Icth in accordance with the IEC requirements.