Core balance CT magnetic flux interference 2

[Gman1977]

Hi guys,

First time poster. Question is.... If the secondary of a CT is shorted out completely, S1 to S2, can current still flow in the secondary winding? My maths is saying yes but my brain is saying no. Sorry if this is a stupid question. The background to this question is......I have a new switchboard, the Phase CT's are mounted directly beside the Core balance CT. The protection relay is showing core balance current due to the proximity of the phase CT's. I have already proved this by moving the phase CT's further away from the Core balance... The effect decreases. Ideally I need to remove the phase CT's to prove 100% that they are interfering with the core balance but this is a big task due to the installation. My brain says that shorting the phase CT secondarys means there will be no flux generated but my maths says otherwise?

 

[davidbeach]

Current will absolutely flow in that zero resistance circuit.

 

[HamburgerHelper]

Loading down a CT reduces the amount of flux generated. You can see this by looking at the CT excitation curves. As you reduce the voltage generated on the secondary, the excitation current reduces as well.

 

[Gman1977]

But how is there current flowing? By shorting out the secondary it has become a closed loop.....My maths is saying that maximum current is there but my brain is saying zero.

 

[Gman1977]

My Engineering department want me to remove the phase CT's to 100% prove they are interfering with the core balance CT but this is impracticle, anyone have any experience of this, CT and switchgear manufacturer both say this shouldn't be an issue but it certainly seems to be.

 

[HamburgerHelper]

The induced voltage on the secondary is proportional to the derivative of the flux. You know that the induced voltage on the secondary will be very low due to you shorting the terminals so the flux has to be very low. A lot of current can flow but because it is shorted, not much flux is required.

If you want to look at the transformer model, shorting the secondary reduces the voltage across the excitation shunt to something very small.

 

[Gman1977]

Hi HamburgerHelper,

Thank you for your response. If I short out the Phase CT secondary's, the magnetic field produced by the secondary CT's will be greatly reduced, is this correct?

 

[HamburgerHelper]

Yes. Read page 30 in this pdf. I think this explains what is not making sense.

https://www.ieee.li/pdf/introduction_to_power_electronics/chapter_12.pdf

 

[Gman1977]

Thank you Hamburger Helper. That makes sense. I will be testing tomorrow, i'll let you know how it turns out.

 

[waross]

The resistance and impedance of a relay fed by a CT is very low. The current through a shorted CT is very close to the current when the relay is in the circuit.
I hope that you are not leaving the CTs open circuited.
That is not only bad practice but dangerous to both workers and equipment due to the high secondary voltages that may be developed.
Shorting unused CTs is so universally practiced that the manufacturers may not have considered the effect of an open circuit CT.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gman1977]

Hi Waross,

Don't worry, im well aware of the danger of open circuit CT's. My question is relating to the magnetic flux produced by a shorted CT. eg, is there any?

 

[HamburgerHelper]

Gman1977,

Here are some things on stray flux affecting protection systems. I don't know much about it but some people recognize it can have an impact something sensitive like differential. I don't know if they are applicable to your problem but here they are.

https://www.gegridsolutions.com/smartgrid/Mar07/article4.pdf

http://www.cce.umn.edu/documents/cp...trayfluxanditsinfluenceonprotectiverelays.pdf

If I were you and were trying to identify this without moving the CTs, I think you could show the problem by changing the burden of the CT, overexciting it with a higher voltage, shorting the secondary like you said, or show a change when a piece of iron or shield is introduced the two CTs that changes the flux paths.

 

[Gman1977]

Hi,

Just to let you know how testing went. We repositioned the Core Balance into the Main Bus Chamber which gave us 300mm clearance from the Phase CT's and this did improve matters even further but we still have the issue, just less pronounced. I am concerned also about the Core Balance CT type/installation. It is of Rectangular type and there is very little clearance between the edges of the CT and the L1/L3 busbars, the L2 Busbar is centred. Actually all 3 busbar edges are also very close to the CT. Looking over the documents that Hamburgerhelper linked to this doesn't seem like a very good physical installation.

 

[scottf]

Have you discussed this with the manufacturer of the core-balance CT?

There are a lot of ways to design/manufacture these type CTs. Some designs may not have the proper shielding/winding design based on your conductor configuration.

For clarity, what is the ratio of the core-balance CT and what is the actual primary current on the phase conductors?

 

[Gman1977]

Hi Scottf,

Discussions with CT manufacturer are ongoing as far as I know. They did originally state that there should be no issue with the proposed mounting, however we have shown through testing that that is not the case be moving the CT's to varying positions.
The CT's are 50/1A, Switchboard is 690V. We start to see Core Balance current at Protection relay around 1200A, increasing as we increase load using load bank. With original Core balance CT position, directly beside Phase CT's, we would see 57A core Balance current when 1500A @ 0.8pf, by moving the core balance 100mm this reduced to 0.4A. However, further increasing the load towards 2000A resulted in the core balance current increasing again. Todays testing involved a lot of work to enabled us to increase the distance to 300mm, this improved things further but not enough. I think we may be having also localised saturation of the CT due to the physical sizing, the outside busbars are around 2mm from the CT, CT is rectangular type. Cant seem to upload pic for some reason. Usually our core balance CT is mounted around the cables at the cable incoming position and everything is centred.

 

[scottf]

Yeah, sounds like you possibly have 2 different issues.

- "cross-talk" between your phase CT and the core-balance CT

- localized saturation issues based on conductor positioning and/or inadequate shielding/compensation techniques in the core balance CT design.