High Voltages Seen on CT's

When I've ever done this, it was with induction disc protective relays, and the CTs were grounded AT EACH CT, as is prescribed, at the first point the CT leads are accessible. This connection was used to sum main feeder phase currents with a cross-tie breaker, and had no residually connected elements. These circuits have been in operation since 1978, with no problem. (solidly grounded system)

Rixter is correct - CT circuits MUST BE continuous. You probably have a poor connection somewhere so that a CT is being open circuited (or at least sees a high-impedance). Remember that in a circuit, Ohm's Law must be obeyed. Consider a CT circuit that is designed to have very low impedance. If you open the circuit, current goes to zero, so voltage tries to go toward infinity. That's what causes an open-circuited CT to fail, that the overvoltage exceeds the capability of the insulation and it burns up, etc.

Also, your CT circuit should have a single-point ground. Don't leave it ungrounded (that can lead to excessive voltage on the CT circuit) and don't have multiple grounds (can cause your protection not to function because the current circuit may bypass critical devices). Check the wiring against the schematics to see that it is correct, and measure the continuity all the way around your circuit from connection to connection, including the resistance of the relay components themselves. My guess would be that you have a bad connection someplace, possibly at the point the ground connection is made to the CT circuit.

Just to be clear, one ground per CT, no multiple grounds on a single CT circuit. You have two CTs summing, you have two grounds, one at each CT. A ground for every current source.

No, not if the wiring from both CTs comes together to a common point or device terminal. Draw it out for yourself. If you have a ground at 2 CTs, then the circuit may have the possibility that wire damage someplace bypasses part of the circuit. You need just ONE point of grounding on the CT circuit. For example, if you have 3 phase CTs that share a common "neutral" wire, you also have only ONE point grounded, typically where the CT leads are tied together (wye-point). Here's an example drawing:

That is industry standard practice for CT (and VT) circuits.

That is wrong, each CT is a power source, each one needs a local ground. I’m not talking about phase groups of CTs, where in practice there are often 3 separate grounds for each of the CTs, a 6 point terminal block with shorting bars and jumpers.

When you are summing two separate phase group CTs, that is where people get confused thinking there can be some sort of ground loop if there are 2 grounds on the wye connection.

You don’t want someone to be working on the circuits trying to figure out where the ground is, or where the common ground might be. Ground at each CT, standard practice.

No, it works the same. I do consulting for a large electric utility, and when we sum CTs on a single phase of a bus (such as from two breakers in a breaker-and-a-half scheme, or a partial differential scheme in a main-tie-main switchgear bus), there is still only one ground point for the commoned CTs. What I'm describing is "common practice."

Please send a diagram showing what you are proposing for grounding, if you think it's different.

Well, you wouldn’t be the first consultant to have established a standard that everyone blindly follows, without fully examining the possible consequences in operation. Every CT secondary winding has a ground connection made at the first place in the equipment that the secondary wires are accessible, no exceptions for any sort of relaying scheme.

Not following this standard practice exposes your end user to potentially operate one of the CTs ungrounded, a clear safety concern. There is no down side to this, there can be no ground loop concerns as there are two power sources, each with its own ground.

You need a test case.

1. Transfo with separate earths on LV & HV. Say these ground grids are each the usual 0.5 ohm specification.
2. Differential relay protecting transfo has LV CTs bonded to LV earth. HV CTs bonded to HV earth.
3. 30 kA earth fault on HV side flows thro' 0.5 ohm earth, notionally 0.5 x 30kA = 15 kV source.
4. HV & LV Cts are connected by 100 feet of #6 AWG copper, which is 13 square millimetres & greater than 6 sq.mm I remember usually required for 5 amp CT wiring.
5. #6 annealed copper is 0.395 ohm/1000 ft. Say 0.04 ohm for 100 ft.
6. On one side you have 0.5 ohm and 30kA current source - in parallel you have 0.04 ohm in series with 0.5 ohm of second.
7. The consequence is #6 tries to carry almost half of the 30 kA fault current.
8. #6 AWG copper has fusing current of 668 amps.
9. Wiring regulations give cable short circuit withstand time t = k²s²/I² seconds where s = 13 sq.mm, k =115 for 70'C to 160'C rise with copper conductor, I = 15 kA. t = 10 milliseconds - that is less than relay operate time + breaker time say 50 ms.
10. So the #6 insulation will melt even if the copper does not fuse.

It seems there is a choice between earthing interconnected LV & HV CTs only at one side, say LV, and relying on the CT to busbar insulation integrity or accepting the melting of insulation or copper of #6 AWG.

10 ms for differential relays seems high, plus 50ms for interruption is also on high side, old school equipment, but that is total clearing time, not I2R of fault current applied to your CT wires.

Also ignored is saturation of the CTs. I’ve never seen #6 for CT leads, #8 is tough enough to work with, but 100ft is a typical value for 20 to 50MVA transformer in my limited experience. I think I’ve used #10AWG in those cases, but not for insulation protection, but for burden & transformation accuracy concerns.

OK, take #10 AWG, which is 5.3 sq.mm, the cable S/C withstand time is at 12 kA, reducing a bit for increased.

115² x 5.3²/[12² x 10⁶] seconds = 2580 x 10^-6 seconds = 2.58 milliseconds.

- that is shorter than any clearance time.

I took #6 because we do not know if this is "old school equipment" -it could well be. Could be 5 amp CTs , for which 6 sq.mm was often specified minimum for CT wiring. So I doubled that as a max, to see what 15 kA would do to it.

My clearance time was 20ms for transfo differential relay + 30ms for breaker, the 10ms is the withstand time of #6 at 15 kA.

I am trying to "fully examining the possible consequences in operation" as you wrote.

Your assertion is that the secondary wiring of CTs must always be earthed locally. If you have a transfo then it is quite possible the HV side has one earth mat and the LV another - they may be independent competing companies owning the kit who do not trust or want to help one another. So you are connecting the CTs for a differential relay to different earths. Hence my proposing two ground mats of 0.5 ohm resistance to Earth.

I agree that Most CTs and relays do not need a secondary connection to remote CTs , so a local earth at the CTs is the safest practice. However, for differential CTs, a single secondary earth connection avoids false tripping on an external fault due to unwanted currents in CT circuit and the possibility of fusing cables and injuries.

You will never see tens of thousands of amperes on relaying class CT secondaries due to saturation, but this is not the point. The point is that people expect local grounds on CT secondaries, industry standard, and not a problem with differential relay operation for decades. Your 0.5 ohm difference in ground resistance is plausible, but again, the currents you predict are way too high, and the ground potential does not at all affect the currents that operate the differential relay, so misoperation is not a problem. If those relays had residually connected elements, then you might have a case, might.

Each CT is its own power source, what is your practice with separate power sources? Ground it once, right?

I was not supposing thousands of amps circulating from CTs windings, but the effect of an "out of zone" earth fault at secondary of power transfo when the CTs secondaries on each side are bonded to different earth mats & secondary earth mat is displaced by current flowing to an earth fault. Sorry that was not clear, a diagram would have helped.

I note that OP Polerz has reported that fizzle was due to "poor earthing connection at star point" - which looks like a single earth on secondary circuit of differential CTs.

What is your practice when transfo differential CTs are connected Δ on power transfo star side and Y on its Δ side?

All the 87 relays I’ve ever applied were connected wye, regardless of transformer winding. The relay took care of phase shift, and winding ratios. For straight differentials alway wye.

Not sure just how you would ever provide safety grounds on delta CT, tough one.

My mistake, looking back the CTs are delta connected on the wye winding, so I wasn’t paying attention as well as I thought when building the wiring diagrams for the relaying. So my question still stands, don’t know how a safety ground is applied to delta CTs...

Further research brings this little gem to light, http://www.pes-psrc.org/kb/published/reports/I13presentation.pdf

Where it shows the IEEE Power Engineering Society has agreed that grounding at first point of use is better than at first point of access to the wires. This does allow some uniformity for situations like delta CTs, which are grounded through the protective relay windings. Also, you can inventory and test for multiple or unintentional grounds from a convenient location. So as long as everyone plays by the same rules, it should work out. See Annex D at the end for survey results.

Thanks for useful reference, which gives the accepted practice for VT & CT as a single earth, typically at the relay or meter panel. Provision for testing of the relay/cabling insulation, disconnected from CTs, would demand shorting links & grounding facility at the first accessible safe location near CTs, to be used before any disconnection - which was your assertion of good practice.

Interesting that document uses term VT - I thought USA liked PT!

rwilliams, this document shows exactly what I was describing earlier about a single point of grounding for a CT circuit when multiple CTs are paralleled. Pages 39 and 40 (Ring bus/breaker-and-a-half and Multiple use of CTs) shows parallel CTs into a single protection scheme, with a single ground point. I didn't see any place in these CT schemes where it showed giving each CT its own, separate ground point. This is what my employer and all the design firms I have worked with follow. Or were we just miscommunicating?

This is counter to practices I have accepted as standard since the 1970s. Each method has its advantages, but most important to correct function of the equipment is that everyone understands the method employed, to avoid costly mistakes. Having a safety ground on each set of summing CTs does not introduce any error, however it does make it less convenient to check the equipment for extra ground points. The new PES standard adds convenience, but does leave exposed the chance that a safety ground might be disconnected. As long as everyone understands the methods, then the exposure is minimized.

I am assuming you have a high impedance differential scheme (relay 2 terminal) rather than biased scheme (relay 3 terminal)

Q1 - So long as you actually have an earth on the CT circuit, any capacitive current should pass to earth. Have you checked continuity of earth with substantial current (0.5 amp)? It is possible you have a fault on HV side which is producing spikes which couple to hot side of CT - If earthed side of CT coil was next to live busbar, I would not expect such effect.

Q2 - Multiple earths will just cause trouble.

Q3 - Have you actually checked continuity [correct resistance of relay & any ballast resistance in series]? Have you checked relay functions at set current? High Z schemes usually have a voltage dependent resistor across the relay circuit (is it OK?).

Q4 - seems obvious, but are you sure CTs are correct polarity? If wrong, both CTs would feed current to relay at far greater than rating, which may have burn it out before any contact operation was noted. Or you may have a short in panel wiring due to overvoltage, limited current means it can just "fizzle" (you have not indicated 5 amp or 1 amp CTs). Have you insulation tested the circuit with CTs & relay disconnected?

I would suggest you use ears and a battery power AM radio to localise the noise you hear "back of panel". There may be a bad connection or short at terminals or wire.

My understanding of the interconnection for a High Z relay is that if one CT is shorted, the other will be shorted - I do not understand your sentence " Also note that if there were an open circuit, the high voltages would appear when shorting out one of the CT sets".

Any detailed info on the CTs and the connection diagram?

One still comes across the moronic concept of putting fuses in CT circuits from time to time though this is becoming increasingly rare.

Thanks for sharing the findings at site. It helps us.

"the currents cancel and the protection relay see's no current" are you even sure this is the right term. as i see in the diagram you have this for your differential protection. the current will not cancel out but is kept or monitored at a specific level of difference. and the relay will be triggered once there is a huge differential current between the two CT's. secondly, i am with Rixter that you have some termination problem causing high voltage induction in your circuit. CT circuit will register voltage readings once there is an open circuit and may eventually led to its failure or damaged. you may also experience intermittent tripping of your protection due to this cause.