As-built generator high resistance neutral grounding issue
As-built generator high resistance neutral grounding issue
(OP)
Hello,
After performing a re-analysis on an emergency diesel generator high resistance neutral grounding system (for single phase to ground protection), it was found that the resistive component of current was less than the total system charging current (i.e, the resistor kW < system charging kVA). Total ground current was however less than 10A. How could I determine the effect on the system if the neutral grounding resistor is unable to dissipate the total charging kVA? Thanks for your thoughts.
After performing a re-analysis on an emergency diesel generator high resistance neutral grounding system (for single phase to ground protection), it was found that the resistive component of current was less than the total system charging current (i.e, the resistor kW < system charging kVA). Total ground current was however less than 10A. How could I determine the effect on the system if the neutral grounding resistor is unable to dissipate the total charging kVA? Thanks for your thoughts.






RE: As-built generator high resistance neutral grounding issue
RE: As-built generator high resistance neutral grounding issue
If the ground fault current through the grounding resistor (presumably via a grounding transformer) is less than the system charging current, there is a risk of excessive voltage on the unfaulted phases during a ground fault due to repetitive restrike of the fault arc. For ungrounded systems, voltages impressed on the system can be many times the nominal phase-to-phase voltages. By sizing the effective ground resistance to equal or even a little less than the total system capacitance, the overvoltage will be less than about 2 or 2.5 times the normal voltage. So overvoltages still occur, but the magnitude is controlled. If you are tripping the generator on ground faults, I would not be overly concerned. If the generator is left on-line during a ground fault, You may want to consider a change in resistor size.
But in my experience, I have never seen any system require more than 10 A of ground current. The normal is around 5 A. So if your calculations are showing something larger than 10 A for charging current, I would be a little suspicious of the data used for the calculations.
The best reference I have on this topic is "Industrial Power System Grounding Design Handbook" by Dunki Jacobs, et al.
RE: As-built generator high resistance neutral grounding issue
dpc: this is a 4.16kV system where only 5kV cables, generator windings and 35mA/1000HP (charging current for 4kV, 1800rpm motors, ref. Baker) are used to determine charging current. Yes the system is grounded through a 10kVA distribution transformer (secondary resistance is about 4.2ohm). Re-strike could occur if Ifg is less than 10A, right? Generator is only alarmed on single phase fault. How could the effect of charging kVA which is larger than the resistor dissipation kW, be quantified?
Thanks all for your thoughts.
RE: As-built generator high resistance neutral grounding issue
RE: As-built generator high resistance neutral grounding issue
To refresh: Neutral current I_r is less than 3*I_c (i.e, resistor kW < system charging kVA, X_cg/R_n is less than 1.0) in an as-built system. From IEEE C62.92.1989 it can be seen that transient overvoltages will exceed 250% (lost control over magnitude as dpc rightly said).
What I'm wondering yet are more interpretations of what this means in view of the system being as-built. What happens to the resistor when system charging kVA > resistor kW? Under what ideal conditions have the transient overvoltage estimates (at most 250% for I_r = 3*I_c) been developed?
RE: As-built generator high resistance neutral grounding issue