Using a low voltage current transformer in high side ( 13200 V ) power 2

[COMAPRO]

A friend of mine is using a low voltage current transformer to measure the current is passing through a 15 KV insulated cable in the high side of a power transformer. He states that it is working ok and he is saving a lot of money in the instalation because instead of using the right 13200 Volts current transformer,he is using the cheap low voltage current transformer , which by the way has not blown out yet.

Do you guys agree with that ?

 

[busbar]

Is the cable shielded or otherwise?

 

[davidbeach]

The standard bushing CT used in 15kV class switchgear has 600V insulation, and several manufacturers of CTs give installation notes for use of 600V CTs on INSULATED conductors through 15kV. The key is that the conductor being used is already insulated. Trying to use a 600V bar type CT on 15kV class gear would be a horrific mistake, but properly applied 600V donut CTs on INSULATED 15kV conductors are not an issue, not even a violation of the manufacturer's instructions.

 

[jbartos]

Suggestion: Generally, it is necessary to have designed a correct insulation system fitting the voltage level. The BILL requirement should fit to the voltage level. The 600V CT may have BIL=10kV and the insulated cable much larger, e.g. 110kV BIL.

 

[COMAPRO]

thank you guys for your promtly reply.busbar , to my knowledge a 15 KV insulated cable always has a wire to be grounded . Davidbeach , according to your opinion my friend s action is ok but what do you mean by "15 KV class gear " ?( I am not an electrical engineer)and you jbartos , if your quoting "600V CT may have a BIL= 10 KV" , does it mean it is not right to place in a 13.2 KV system ?

 

[jbartos]

Suggestion to the previous posting: Often, the insulation system consists of insulation layers that are one over the other. In such case a plain addition of BILs approximately suffices, e.g. on 15kV cable of BIL = 120kV is added to 600V CT transformer BIL=130kV. Then, the total 130kV BIL is supposed to be equal or exceed the ANSI Std 57.13-1978 BIL for CT under 15kV level. Checking this level in the Standard on page 20 in Table 7 "Basic Impulse Insulation Levels for Current Transformers," the 13.8kV Nominal System Voltage (kV) row reveals the BILL and Full Wave Crest kV of 110kV or 95kV.
Usually, 95kV is preferred and 110kV is the luxury BIL required by some Utilities.

 

[busbar]

For shielded MV cables, use of 600V window CTs is generally acceptable where the CTs are placed “downstream” of the termination where voltage stress is low. {Note the shield-grounding lead should be routed ‘back through’ the CT opening.} In IEEE C37.20.2 MV metalclad equipment, often 600V CTs are positioned over extended insulation located at the ‘throats’ of primary disconnects for ndrawout MV circuit breakers, being integral to the fixed cradle portion on the switchgear.

Related discussion at “PTB #85”

http://www.powellelectric.com/pdf/ptb/booklet 1-91.pdf

page 138. Supporting ‘bare-air clearance’ info at energy.tycoelectronics.com/rrg/raychem_rrg/98.pdf

 

[jbartos]

Comment on the previous comment: The CTs do not necessarily have to be placed over shielded conductors. In that case, the conductor shields do not have to be routed through the CT windows.

 

[twosockets]

Comment on the previous comments: If the insulated conductor has a concentric neutral (shield) that is referenced at both ends, the shield MUST be routed back through the window of the CT to cancel any neutral or stray currents that may exist in the shield.

Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com

 

[jbartos]

Comment on the previous posting: Yes that is understood. However, at some point the concentric neutral is separated from the conductor insulation and, there are just separated conductors with insulation run to terminals.

 

[twosockets]

jbartos,

You are correct. Unfortunately, several of the higher voltage installations that I have seen have the concentric neutral passing through the window of the 600 volt CT in only one direction. Apparently some forget that it (concentric neutral) can, and many times does, carry current!

My comments were only to emphasize the point.

A similar situation occurs when the metering potential connection is made after the metering CTs, especially in pad mount transformers. For convenience, the termination is often done on the load side of the CTs, but, the conductor must be passed backwards through the CT to cancel out the metering potential current needed to power the metering equipment. Failing to do so renders the installation technically illegal in most U.S. jurisdictions. I am aware of one large energy theft case that was thrown out of court because of this condition! The judge ruled that the installation was technically illegal.

Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com

 

[jbartos]

To the previous posting: Agreed.

 

[stevenal]

Separating the concentric or shield prior to entering the CT window is not okay. The insulation in this region is at a potential somewhere between ground and phase, and may exceed the the 600V CT insulation. If the semicon is left on, now you have an unmetered quantity. Do it as Busbar stated.

 

[jbartos]

Suggestion to the previous posting: It depends on the peel off conductor configuration, e.g. spacing in the space. The BIL still will remain, e.g. 120kV. Therefore, the CT may be installed there.

 

[stevenal]

I don't care if you over insulate to 2MV BIL. Halfway up an insulator stack (or terminator) at the 1MV BIL level you will find a potential that is half the phase to ground voltage. Even if no leakage is present, you still have a capacitive voltage divider. The issue isn't the BIL of the cable and terminator insulation, it is the BIL of the CT and where it lies in the potential gradient. Yes, the presense of the CT will alter this gradient. Anyone know how to model this? Keep it close to the shield end and ensure an even air gap, and you might be okay. Simplest to just route the concentric or shield adapter back through as Busbar and Twosockets said. Tried and true.

 

[gordonl]

I agree with Stevenal, it would be quite unadvised to place a 600V CT on the terminator or bare insulation. Let the 15KV insulation take the stress not the 600V CT.

 

[stevenal]

thread765-75985

 

[jbartos]

Comment: The further apart the MV insulated conductors are the less influence of the neighboring insulated phase conductor is; especially, when it is insulated. This is possible to model in terms of concentric equipotential circles. See Reference:
Reference:
James Clerk Maxwell "A Treatise on Electricity and Magnetism", Volume One, Dover Publications, Inc., 1954 (Slightly modified reprint of 1891 publication).
Fig. II Art. 119

 

[jbartos]

Suggestion: Even if the CT potentially degrades the external insulation of the insulated conductors, there still will be 10kV BIL from one phase insulated conductor CT and another 10kV BIL from the other insulated conductor CT.
Now, if the phase conductors are without shield, there will be some deformation of the electrical equipotentials; however, the insulation will withstand it. There will not be short. The shield is added to conductors as an additional measure to make the electrical equipotential concentric to avoid electrical stresses and displacement of equipotential circles. The insulation is rated between the inner insulation diameter and outer insulation diameter, i.e. insulation thickness. E.g. Inslulation level 100% may be compared to insulation level 133% by insulation thickness, not by any extra shields.

 

[stevenal]

It's not so much a question of insulation failure of the cable. A capacitor passes AC power even when the dielectric is intact. It is a question of insulation failure of the 600V CT and the secondary wiring. We carefully terminate the conductor to insure the potential is graded evenly from phase to neutral and ensure a long leakage path. Strip the concentric back from the base of the terminator, and you've changed that grading pattern. Now grounded (per code)metallic conductors with 600 V insulation and no sheds is introduced into the field. I'm sure it can be modelled, but the concentric circles no longer work. A lot of work and potential error to avoid looping it back. And looping it back takes less time since less cable stripping is involved. What's the point?