ice said:
Solidly grounded systems generally fail phase to ground first and often don't trip until the fault goes phase to phase. Lots of damage occurs during the phase to ground fault time.
D!NNY said:
What kind of damage, i thought resistance grounding limits the fault current so there will be no damage and continuous service under fault condition
This part of my comment is about solidly grounded systems - not about HRG. It is about why one sould want to pick an HRG over a solidly grounded system
ice said:
Ungrounded systems are subject to arcing re-striking ground faults. These faults subject the system to severe overvoltage transients.
D!NNY said:
In Ungrounded system for single phase to ground faults currents are low and cannot form arc as there is no closed loop for the power or it is dependent on the fault resistance,
when do these happen?? can you give me more explanation on this one?
Yes, ungrounded systems can have Arcing, Re-striking, Ground faults. Following from ieee242, p238-239:
8.2.5 Ungrounded systems
Ungrounded low-voltage systems (see Figure 8-5a and Figure 8-5b) employ ground detectors
(e.g., lamps or voltmeters connected from each phase to ground) to indicate a ground fault.
These detectors show the existence of a ground on the system and identify the faulted phase,
but do not locate the ground, which could be anywhere on the entire system. The system
operates with the ground fault acting as the system ground point. The ground-fault current
that flows is the capacitive charging current of the system, generally only a few amperes.
If this ground fault is intermittent or allowed to continue, the system could be subjected to
possible severe overvoltages to ground, which can be as high as six or eight times phase voltage. Such overvoltages can puncture insulation and result in additional ground faults.
These overvoltages are caused by repetitive charging of the system capacitance or by
resonance between the system capacitance and the inductances of equipment in the system.
D!NNY said:
I heard this (ice - arcing GF) is one of the disadvantages of resistance grounding but how??
No, the NGR is selected to supply the capacitive charging current so this won't happen.
ice said:
So, one selecst an HRG to limit the damage of solidly grounded system GF faults and not be subject to the Over Voltage transients of ungrounded system GF faults. This is the normal reasoning for using an HRG on 13.8KV systems. And they are set to trip on GF.
D!NNY said:
Got it so HRG is used to limit the fault current for limited amount of time, not to damage any equipment using tripping. But why is HRG in 13.8kV can't limit the fault for higher duration!
I'm getting out of my area here. The 13.8KV systems I work with are not considered HRG, as mb pointed out. The 50A of fault current is too high for an HRG and too low for a Low Resistance grounded system. I think the designers picked it because it was as low of a GF current as they could go and still differentiate between true GFs and switching transients.
Why not continuous? Well, again not my area, so I'm guessing:
Safety: Step and touch potentials are higher with 13.8KV faults than 480V faults, they don't want to leave them on
Money: A continuous 400KW resistor is HUGE compared to a 10sec rated resistor
ice