13.8 kV Distribution Grounding

[kmh1]

My question concerns system grounding of a 13.8 kV distribution system where there are no qualified electrical maintenance personnel on site. The main incoming transformer is a 25kV/13.8 kV 5000 kVA delta/wye unit where the secondary neutral bushing has been left disconnected. The system supplies several smaller unit substations and pole mount transformers via a combined underground/overhead distribution system. There are no potential transformers or voltmeters on the 13.8 kV system. Primary and secondary protection on the main transformer is provided using fuses. My feeling is that a low resistance grounding resistor is required on the main transformer in order to allow for faults to be detected and cleared by the secondary fuses. Am I correct? What should the resistor current be rated at?

 

[dpc]

The fuses won't help much for a ground fault unless your system is solidly-grounded. The resistance ground would limit fault current too much to allow the fuses to operate. Remember that the fuses must be large enough to pass load current.

Without something to trip, ground fault protection can't really function.

But I agree with your assessment that the 13.8 kV system should be grounded. This would probably require addition of some type of 13.8 kV breaker or recloser to allow you the ability to automatically clear a fault. With a breaker, you could implement low resistance or high resistance grounding, utilizing appropriate relaying to detect a ground fault and trip the breaker.

You might be able to get some special fuses (S&C) that can be blown pyrotechnically when a ground fault is detected.

 

[kmh1]

A circuit breaker and neutral ground resistor would be a relatively expensive proposition for this installation. To meet electrical code minimum requirements, line to ground PT's and a ground fault detector could also be installed. The problem I see is is knowing whether someone would be around to act to isolate and correct the problem.

 

[busbar]

Having any ungrounded system without ground detection and particularly no one qualified to quickly act on it can be a disaster at any voltage, much less at 13.8kV. This is the case from simple equipment {insulation} preservation, service continuity and equipment and personnel safety.

Transient and resonant overvoltage is usually murder at that voltage. For low-range fuse clearing, search on low-resistance grounding in the IEEE Green Book [Std 142-1991] At very least, a high-resistance-grounding retrofit may be a minimum step, without the benefit of fuse clearing.

A worthwhile reference on tradeoffs {or adding fuel to the fire} may be ANSI/IEEE C62.92.4-1991 Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution

Also Beeman chapter 6.

 

[jghrist]

Is there a reason, such as a need to limit Ø-grd fault current because of equipment limits, not to solidly ground the neutral?

 

[kmh1]

No, there are no equipment fault current limitations in this instance. I am starting to think that perhaps the ground connection to the neutral bushing was simply forgotten. All remote substations have an interconnecting ground conductor within the multiconductor cable jacket so any system ground fault should be cleared by the line fuse if a solid ground connection to the neutral is made.

 

[alehman]

Solid grounding is very common for 13.8kV distribution. You will need to evaluate ground fault currents and interrupting capacities of your equipment prior to grounding the neutral.