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MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

(OP)

I have an application where a ground fault relay on a main breaker is tripping during energization of several transformer downstream. This nusiance tripping has happened several times and it have been confirmed that there are no faults and that the nusiance tripping is being caused by the characteristics of transformer inrush. If transformers are energized seperately there is no tripping.

The relay is on a 1200A breaker, with the phase pickup settings set to 1200A. The ground fault relay is a residual type using the 1200A phase CT's. The pickup is currently set to 120A with a moderately inverse curve and time tile of 3.3 which equates to about .5s at maximum ground fault current. There is no instantaneous setting.

This nusiance tripping occurs when (4) downstream 2000kVA 4.16kV-480V transformers are being energized at the same time due to a particular switching scenario. It seems as if this is happening due to dis-similar CT performance during inrush cause by un-euqal currents, and aysymmetry of the inrush current.

I'm curious what methods in terms of settings others have implemented for such a case. One method I have heard of is setting the pickup of the residual overcurrent element close to the full load rating of the transformer and set a definite time delay long enough to allow the CT's to come out of saturation before the element operates. The time delay referenced is a function of a time constant determined from the X/R ratio a the point in the system.

I curious to hear how others have dealt with this issue?

Thanks for the help.

RE: MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

Just a stab at it, but are you closing all three phases at the same time?

RE: MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

(OP)
Cranky

Yes all three phases are being closed at the same time via a 5kV vaccum breaker which serves as the main breaker in a 4.16kV switchgear lineup.

RE: MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

Might not be feasible due to the relays and the existing configuration, but would adding a Zero Sequence Relay solve the problem?

RE: MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

120A pickup for earth fault protection corresponds to about 10% of the full load current of 4-transformers put together. Thus, the setting is not oversensitive to cause maloperation during inrush, though the recommendation is to have 15% as the minimum pickup for the residual connected earth fault protection, typically.
Increasing the time delay may not be a good idea as it amounts delaying the clearance in case of actual fault. However, it is possible to have two settings, one with sensitive setting and 1sec delay and the other highset with a small delay of 0.1sec.
I suggest you confirm that the CTs are protection class and not metering class. Next point is to check that the residual connection wiring is correct including the polarity of individual CTs.
If these are OK, then block the earth fault protection on sensing the 2nd harmonic (that is on inrush). The feature is available in many of the numerical protection relays.
Normally, 4.16kV system is resistance earthed, how about your system!!

RE: MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

(OP)
raghum

The system is solidly grounded. I will verify that the CT's are protection class as you mentioned.

I curious what is the reasoning behing the 15% as the minimum pickup for residual CT's? Is this just something that has proved to work over time is there some other underlying reason?

It sounds like blocking earth fault protection during inrush with 2nd harmonic detection. I gues also a zero sequence would work as mentioned above.

RE: MV Relay residual ground fault settings to avoid nusiance tripping during transformer inrush

rockman,
The 15% setting for residually connected earth fault protection is the minimum value and is some thing to do with the vectorial sum of errors in the three phase CTs that would appear at the relay terminals and could cause maloperation.

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