5 kV Generator Grounding 1

[cbark]

We are adding a 2.5 MVA, 4.16 kV generator to an existing system. The 4.16 kV system is fed from a 12.47 kV-4.16 kV, 5 MVA transformer with a 400 A neutral grounding resistor. How should the generator neutral be grounded (high resistance, low resistance, etc.)? What IEEE standards would apply to determine the appropriate grounding method?

 

[dpc]

This standard is applicable: C37.101-2006

The grounding choice will depend on how important selective coordination is for your system. High-resistance grounding could be used but this will generally not coordinate with ground overcurrent relaying that may be existing, meaning the generator will trip off for any ground fault on the 4160 V system. Also, if the generator is operating connected to the transformer, the fault current into a generator fault current will be 400 A, even with high-resistance generator grounding.

Low-resistance grounding would offer the opportunity to coordinate with the 4160 V ground overcurrent relays on the system. The downside would be somewhat increased damage in the event of an internal GF in the stator of the machine.

IEEE (I think Pulp and Paper) committee has also put forth a hybrid grounding scheme that combines high-resistance and low-resistance grounding methods and switching between the two.

 

[odlanor]

hybrid grounding scheme is applied to 40MVA and above.
For 2,5MVA you can use ungrounded. A generator without neutral is better for physical arrangement.
You only need to guarantee generator only for loads of system. Otherwise you will have problem supplying ungrounded loads with generator desconnected.

 

[Fuse4]

Dear all

More standards:

C62.92.3 Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part III—Generator Auxiliary Systems

C62.92.3 Guide for the Application of Neutral Grounding in Electrical Utility Systems Part II—Grounding of Synchronous Generator Systems