As-built generator high resistance neutral grounding issue

[brainsalad]

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.

 

[odlanor]

is 10A the total current of a fault-to-ground or is neutral generator current contribution to the fault?

 

[dpc]

I would be curious as to how the system charging current was determined this time around. This is notoriously difficult to compute due to insufficient data.

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.

 

[brainsalad]

odlanor: the total current at the point of the fault I_fg = sqrt(I_r^2 + 3I_c^2) is less than 10A, but my I_r (current through resistor) is < 3I_c (total charging current).

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.

 

[odlanor]

see thread238-322186

 

[brainsalad]

odlanor: Thanks very much that was the correct guide indeed.

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?

 

[dpc]

The purpose of the high-resistance grounding is to limit overvoltage while also limiting fault current. What happens to the resistor is of secondary importance. Nothing will happen to the resistor if it is adequately rated for current and voltage. Or maybe I didn't understand your question.