4.16kV solidly grounded generators

[rockman7892]

I am working on a project on which the existing MV distribution system has (4) 4.16kV 1650kW backup generators connected through paralleling Switchgear. Each one of these generators is solidly grounded.

I have always been under the impression that MV generators should be impedance grounded in order to minimize damage to the generators in the case of a ground fault. Are there particular applications or instances where MV generators can be solidly grounded?

In addition to that the normal 4.16kV system (utility source) is an impedance grounded system so this system will be impedance grounded during normal operation and solidly grounded during emergency operation. I believe this will have an impact on ground fault coordination as the relays will be set for the normal impedance grounded condition and will have different levels of fault current for the solidly grounded operation.

The other issue I see is the potential saturation of the zero sequence CT's on the 4.16kV distribution switchgear when operating on generators however these are electronic relays and can use the calculated residual ground current as a backup?

On a side note, this facility has a 10MVA transformer with a dual voltage secondary winding that can be manually switched from 13.8kV to 4.16kV. When switching operating voltage of this transformer there is an automatic switch that is interlocked to change the impedance of the grounding resistor when switching secondary voltage from 13.8kV to 4.16kV. Is there a specific reason for changing grounding impedance when operating at the lower 4.16kV voltage?

 

[waross]

Changing grounding impedance.
That may be so that the maximum fault current will be the same on either voltage. This may be done to facilitate relay pickup settings.
Generator grounding, a suggestion.
Impedance ground each generator with 10 times the impedance of the system ground.
Example. System ground current = 10 Amps
With generators connected the ground current may vary between 11 amps and 14 Amps.
Set your relays to trip on 10 or more Amps.
Set your generator relays to trip on one Amp or more.
Or
Monitor the voltage across each generator grounding impedances and trip at the calculated voltage.

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

 

[RRaghunath]

IEEE recommends impedance grounding of MV systems. Thus, it is good to ground the backup generator neutrals as well through impedance. However, if the generators are designed to work in parallel, only one generator neutral shall be grounded at any given point of time.
It is preferred to maintain the grounding impedance connected to generator neutral same as that connected to incoming grid transformer neutral. This ensures the protection system will see the same amount of fault current whether the system is supplied y the grid or by the backup generators.

Rompicherla Raghunath

 

[dpc]

Generally some type of impedance grounding is recommended for medium-voltage industrial systems to reduce damage from line-to-ground faults. Solid grounding of nedium voltage generators is not recommended. However, if the generators are serving long utlity-type distribution system, solid grounding is often used to provide adequate ground fault current for fault detection and to allow for line-neutral loads.