How to calculate value of Neutral grounding resistors 3

[rajderkar]

I would like to know how to calculate Neutral grounding resistor values for 6 MW (7.5 MVA), 11 Kv alternator. What is the standard practice? Can anybody tell about site where i can get more design information?

 

[busbar]

Basic information in IEEE C62.92.2-1989 …Application of Neutral Grounding in Electrical Utility Systems Part II—Grounding of Synchronous Generator Systems

Closely related is IEEE C37.101-1993 …Generator Ground Protection

 

[busbar]

Resistor rating information may be found in ANSI/IEEE Std 32-1972 Requirements, Terminology, and Test Procedure for Neutral Grounding Devices

 

[jbartos]

Suggestion: The calculation of system grounding schemes tend to be proprietary since it is the very important calculation and a lot depends on its outcome. To rephrase it, by a wrong system grounding many things can go wrong.

 

[RajT]

Answer as follows.

Since you have opted to use NERs then I assume that you wish to keep the e/f current low inorder to protect the connected equipment from damage during earth faults.

The alternator rating is 7.5MVA at 11kV which will give the alt full load current a value of 394amps.

The NER is sized such that the e/f current value is not to exceed the Alt Ifl value and in practice it is sized such that the e/f current is limited to say between 10-20% of alt Ifl.
In your case I would size the NER to limit the current to
say 40amps.

Therefore using V=IR
R = 11000/^3.40= 159 ohms.
The NER would further be specified to have a 10secs rating at 40 amps and a continous rating at say 5amps (to allow for any circulating currents between gen sets)

The CT (fitted to the ground side of NER) ratio should be 50:1/5 and connected to device 51G fitted on each gen incomer CB.

The 51G on the gen CB should be set to discriminate with the largest outgoing feedr on the switchboard.
Finally, inorder to achieve close coordination between 51G on the gen CB and the out going feeders, the feeders should also be fitted with a core balance CT connected to respective 51G on each CBs, noting that core balance CT will give you good sensitive e/f protection often set as low as 2-5 amps.

 

[jbartos]

Suggestion: The grounding scheme calculations are presently not standardized in IEEE, IEE, ANSI, IEC, BS, DIN, CIGRE, CENELEC, EN, etc. standards to the best of my knowledge. It would certainly be very helpful to have some standards available, just like for battery sizing, for example.