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Zero Sequence CT on MV Solid ground system

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transky

Electrical
Nov 16, 2010
3
Hello every one,

Just wondering if anyone has seen 50/5 C10 zero-sequence CTs on a solidly-grounded medium voltage switchgear and MCC feeders? I am worried about the CT saturation in the event of ground fault, and burning the relay CT input coils since I have a ground fault current of about 30kA. I would like to replace them with bigger zero-sequence CTs, say 500/5 C100, instead of using residual connection of phase CTs or relay internally calculated Io current.

Is this the right way to go?

For the switchgear feeders I will be using GE ITI 143 ZSCTs they appear to fit with out any problem.

But, I am having trouble in finding an appropriate ZSCT of higher rating which can fit into MV MCC buckets. Any suggestions on style and make are appreciated.

Thanks.
 
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With so much difficulties in hand, why should you provide ZCT. A residually connected earth fault protection should be adequate for the general purpose earth fault protection.
 
Do you have some specific reasons for excluding the use of residual connected CT's for measuring the ground fault current? I'm not seeing the need for a ZSCT in your application.

Still, you don't need to limit your search to the ZSCT pages of the catalog. You just mentioned the 143 series ZSCT as your choice. The 143-501 is exactly what you asked for, a 143 series 500:5 C100 CT.
 
Wow, 30kA of ground-fault current. I don't know what type of equipment you have, but please keep in mind that a typical vacuum contactor in a MV MCC has an IC of only 4-8kA. (Note: A MV contactor with fuses can have an IC of up to 50kA.) If called upon to interrupt 30kA, things will go south on you pretty quickly. I agree with krisys, residually connected CTs should work fine.

Better yet, why not install a NGR on the transformer neutral. They are inexpensive and should be your first line of defense against serious ground faults. (Note: 95% of all faults initiate as ground faults.)
I'd recommend a 25 - 50A low-resistance NGR with (say) a 30s rating. LittelFuse-Startco make a NGR relay, cw CTs and sense resistor modules. See the link below.
Once a NGR is installed, every CB and motor-starter should be equipped with zero-sequence CTs and relays. Most modern 50/51 relays are equipped with sensitive GF inputs.
Good luck.

 
I tend to typically see 50:5 ZSCT's used for ground fault in MV MCC's and for MV Switchgear I typically see the residual connection of phase CT's used for detection.

I'm not sure if one method is better than the other but from what I've heard the ZSCT can be set much more sensitive.

Are there any guidelines for setting regarding ground fault pickup/delay with ZSCT? I have never seen any. For residual connections I have seen that you typically set it at the lowest allowable pickup (with value depending on size of phase CT)with a time delay necessary for coordination with other devices or equipment.
 
rockman7892 said:
I'm not sure if one method is better than the other but from what I've heard the ZSCT can be set much more sensitive.

On systems with low fault currents (e.g., impedance grounded), zero-sequence CTs can be used with much lower ratios than the phase CTs allowing for greater sensitivity. They are also less prone to erroneous ground fault readings during high phase fault currents - residually connected phase CTs with similarly low ratios would need high accuracy classes and thermal ratings to achieve the same sensitivity.

On systems with high fault currents, sensitivity usually isn't a problem. In this case, a residual connection of the CTs used for phase overcurrent protection provides all the sensitivity needed; lower ratio CTs aren't needed, and the additional expense can be spared.

The OP's system has 30kA of fault current which is quite high. This is why other posters are questioning the need for the zero-sequence CTs or suggesting the installation of an NGR to reduce the fault current.

Cheers,
mgtrp
 
rockman7892

Most of the MV experience that I have had has been with low-resistance systems, typically 50A or less. With a low-resistance system sensitive zero-sequence CTs are the norm (ie 2000:1). At the MV MCC level (ie motor starter) I usually set the GF pickup to 1A with a 1s TD, so far with no problems. The 1s TD is required if your motor is equipped with a surge cap. I expect that the 1s TD could be reduced if there are no surge caps, but then the NGRs are good for 10 - 30s fault duration, so no issue with 1s.
In the olden days, prior to digital relays, a low-resistance system used to be > 100A, and as high as 400A, but I digress....

Regards
 
GroovyGuy

Yes on MV MCC's I usually see GF pickups of around 5A with some time delay. I never thought about the motor surge caps, but I guess your right, this could cause some leakage current.

It sounds like if the system was not impedance grounded than the residual connection of the phase CT's in the MV MCC would be fine and a ZSCT would not be needed.

Is there any rule of thumb for setting pickup settings on MV SWGR feeder breakers which have ZSCT's?
 
As GroovyGuy says, for the Medium Voltage MCC motor feeders, if we use residual current for the earth fault detection, in the solidly grounded system, there is a risk of motor contactor attempting to break the possible higher current exceeding the contactor's rated value. I have seen some installations where they give a sufficient time delay (say one second) for the earth fault relay. So if there is any severe earth fault current, the fuse blows.
For the lower earth fault currents, (say, earth leakage currents of few amperes, but sustained) the earth fault relay would open the contactor. This would avoid the contactor damage.
 

If the MV system is Impedance grounded with ground fault limited to 400A then I suspect that having the earth fault protection open the contactor is not really an issue since the 400A will be will within the contactors interrupting capability?

Interesting comment about sustained lower earth fault currents of a few amps damaging the contractor. Why would this small leakage current only be a concern with a contactor and not other equipment throughout the system? wouldn't the same issue and philosophy apply elsewhere in a system?

Would residual CT's not be capable of detecting this small amount of leakage current?

Are there any good papers out there that compare the sensibility of residual CT's vs ZSCT's and proper application of each?
 

Sorry, I could not convey the message properly. This is a simple and economical technique to protect the contactor, in the high earth fault regime.

If there is a large fault current, the fuse will clear the fault with in no time. Irrespective of whether the current is from earth fault or from the phase fault. If you slightly delay the trip from the earth fault relay (say 0.4 to 1.0 s), the fuse will clear the fault before the contactor attempts to open during the high current fault. If the contactor opens for such high current faults it will damage.

Experience has shown that the majority of earth faults start with a small leakage current. No other relays can sense this small earth fault current. Even if you slightly delay the fault clearing time the damage will be limited. Thus in order to distinguish between the high and low magnitude of earth fault current this techniques is found to be useful.

I agree that this is not very accurate, but a practical approach. Hope I have made it clear.

I wish some one like Davidbeach can share their point of view.
 
Since you ask...

A residual connection of the phase CTs is the appropriate means of detecting ground faults on a solidly grounded system. On a high-resistance grounded system you're going to need a core balance (zero sequence, etc.) CT if you want to get meaningful currents into the relay. What to do on a low-resistance grounded system depends on a number of factors; it may be that low ampacity circuits can get by with the residual connection and high ampacity circuits need the CBCT.

It's been long enough since I last had to work with fuse/contactor combinations that I'm not going to comment on that aspect.
 
I'm having doubts that a low current ground fault of a few amps would exist for any significant time on a solidly grounded system.

I agree with davidbeach, the residual connection is appropriate for detecting ground faults on a solidly grounded system.

Contactor/fuse combinations seem capable of handling faults fine when the control power uses a CPT and is lost as soon as the phase-phase short occurs. In some 480V/600V testing, we found the components survived better if the contactor was allowed to open when the fault was detected instead of attempting to hold the contactor closed.
 
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