Different Practices On System Neutral Earth. WHY?

[nkumarp]

We are facing problems in deciding on an optimum neutral earthing scheme for any typical industrial plant at 11 kV and at 415 V operating with their respective motors and genenerators.

The practices here are different in different regions. The British prefer a low resistance earth based on a premise that sufficient EF current flows to enable protection to clear the fault quickly, with the current limited to a level that would not pose rotation equipment core melt down ( i.e curents of 400 A to 1000 A cleared instantaneously)

The American & French prefer high or unearthed system with motor/generator core melt down prevention being more dominating factor at the expense of fault clearance and voltage rise posing insulation problems.

Has any one of you conducted a study comparing the schemes with clear recommendation on the best practice as well as a cost study since unearth system leads to additional cost in higher insulation requirement and down times as far as fault location is concerned

 

[busbar]

What are your/your client’s preferences? What expensive situations have you had that you would prefer to avoid?

On the North American continent there are valid arguments for high-resistance grounding on LV buses for starter- and drive-controlled motors. One review is JP Nelson, High Resistance Grounding of Low Voltage Systems: A Standard for the Petroleum and Chemical Industry at

http://www.neiengineering.com/papers/paper1JN.pdf

 

[jbartos]

Suggestion: Based on system grounding calculation results, an 11kV power supply is often the medium-resistance system grounding, since the high-resistance system grounding requires the ground charging current to be less or equal to 10A. This is often not the case for the 11kV power supply system. Therefore, the next feasible and reasonable system grounding is the medium-resistance system grounding, allowing about 200A ground fault current to alarm and trip.
480V/277V 3phase 4wire power distribution system has solidly grounded neutral since this is the only option for the multivoltage power supply.

 

[cuky2000]

The optimum neutral earthing scheme is a compromising between conflictive factors such as safety, service reliability, availability, maintainability, insulation, system protection, transient performance, cost and other factors.
The enclose site could help identify advantage and disadvantages for various grounding methods.

http://www.postglover.com/products/neutral/ng_comp.htm