Relay ground fault setting for transformer feeder

[philly]

I'm looking for a rule of thumb or standard practice for setting the ground fault for a relay feeing a transformer using zero sequnce CT?

I have seen a rule of thumb for ground fault setting when using residual CT connection with the suggested pickup to be the minimum tap setting on Thr relay and the INST settikg set to 4x transformer fla at 0.2sec. The time delay of the relay is set as necessary to coordinate with other relays.

Does anyone know of any standard practice for ground pickup and INST setting with zero sequence CT's? I would think that you can set pickup to a minimum because of Zero sequence sensetivity?

 

[philly]

In doing some further looking around I've seen some recommendations for setting the INST at 10 A with a 20-30ms delay when using zero sequence. Some recommended pickups seemed to be same as INST which would essentially give a square relay curve shape. Has anyone seen similar?

 

[JoeStillman]

Why would the settings be different for zero sequence and residual?

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[Bugman1400]

A dedicated zero-seq CT is the same as using residual CT wiring and is the same as having the relay calc the residual current. The typical rule of thumb depends on the application. For substations, we set the ground pickup to 1/3 of the phase if there is sufficient ground current. In a resistive ground system, this method does not work and we typically set the pickup to 1/2 the max ground current. If this is a single feeder, you may be able to set the ground pickup to 1/10 the phase.

 

[philly]

Why would the settings be different for zero sequence and residual?

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Because the relay will have a minimum tap setting based off of the CT ratio. The CT ratio with residual connected CT's will be much higher than that of zero sequence CT's thus limiting how low you can set the pickup. For example a particular electromechanial relay I'm looking at has a minimum tap setting of 1.5. This results in 600A pickup for a 2000/5 residual CT and a 15A pickup of a 50/5 zero sequence CT is used. This is a big difference in available minimum pickups.

 

[philly]

A dedicated zero-seq CT is the same as using residual CT wiring and is the same as having the relay calc the residual current. The typical rule of thumb depends on the application. For substations, we set the ground pickup to 1/3 of the phase if there is sufficient ground current. In a resistive ground system, this method does not work and we typically set the pickup to 1/2 the max ground current. If this is a single feeder, you may be able to set the ground pickup to 1/10 the phase.

I'm curious is there is a method or reason behind the settings you mention or if these are just based off of experience?

 

[rian0201]

The principle as said above is that it should be set above "normal" unbalance. You could using the rules above. Also, check if there is no single phase switching involve.

There are others that use a fictitious fault impedance during ground faults to set their ground elements.

Lastly, the method of grounding also defines the setting.


Sent from Mars

 

[Bugman1400]

I'm curious is there is a method or reason behind the settings you mention or if these are just based off of experience?

Ground current is made up from 3 times I0 (3I0). Phase current is not.

If the available ground current does not variate much and there are no downstream devices to back up, 1/3 is a good number. As mentioned previously, the pickup should be above any expected imbalance of a grounded load. The imbalance for an un-grounded load will not affect ground current.