Generator grounding is basically connecting the generator’s neutral point to the earth in a controlled manner so that the fault current has a path to flow. Unlike normal system grounding, the generator needs own dedicated grounding because its behaviour during the faults is different from transformers as the internal reactance is lower and the fault current rises very fast while the AVR reacts instantly to the unbalance. If the grounding is improper, the generator faces the risk of overvoltage, shock hazard, unstable AVR operation leading to overheating and insulation breakdown.
Table of Contents
Requirement of dedicated grounding
Internal Emf and time graded reactance: A generator produces its own voltage using excitation, therefore the internal Emf drives huge current which is only limited by the sub transient reactance which increases with time sub transient to transient to steady state. This causes huge electrical and thermal stress. A shared grounding cannot limit this current.
Dangerous rotor and stator stress during fault: During a ground fault, violent electromagnetic forces are at play in the stator winding, it causes the rotor to heat up, overstressing the field winding and insulation. Hence the grounding system has to be suitably designed to limit these stresses.
Neutral shift: As generator produces their own flux, therefore during a ground fault, the neutral point shifts dramatically. A dedicated grounding ensures the neutral stays stable, voltage on the healthy phases remains within limits.
Back feeding faults: A shared grounding may feed plant and grid fault current, return through the generator’s neutral which can damage the winding of the generator, cause unnecessary tripping, even induce torsional stress. A dedicated grounding isolates the generator from external ground return currents.
Types of Generator grounding
Solid grounding: In the solid generator grounding, the neutral of the generator is connected directly to the earth, which creates a low impedance path for the fault current. The low impedance results in very high ground fault current, which is equal or greater than the three phase short circuit current of the generator. The advantage of the solid generator grounding is that it results in faster decisive tripping by the protection relays.
But, the high current produces mechanical and thermal stress in the stator winding, bus duct and terminal which is detrimental for large generators. Hence the solid grounding is used only in some small alternators, emergency gensets and machines which can withstand these stresses because of insulation and physical clearance.
Low resistance grounding (LRG): Low resistance generator grounding connects the neutral of the generator to earth through a resistor typically between 1 to 10 Ω. This clamps the ground fault current to a much manageable level between 50 to 300 A for medium and large generators. Limiting the current minimizes the stator core damage and excessive heating of the windings.
The low resistance generator grounding also enables the essential relays to detect the current, which is of lower value compared to solid grounding but non-destructive. This type of generator grounding is most popular in industrial generators above 1 MVA rating.
High resistance grounding (HRG): High resistance generator grounding limits the ground fault current to very low levels typically below 10 A with the use of a high value neutral grounding resistor. At this level of fault current, the stator damage becomes minimal, while allowing the generator to operate while the fault is being located.
This type of generator grounding also prevents the transient overvoltage as it stabilizes the potential of the neutral avoiding dangerous shifts seen in ungrounded systems. As the fault current is very low, conventional overcurrent protection wont work the same way and hence specialized relay like 64G and neutral overvoltage 59 N are used to detect the fault. This type of generator grounding is preferred when continuity of operation is the priority.
Zigzag transformer grounding: When the generator grounding is not physically available, which is common in older machine or delta connected stator winding or retro fit situation, the zigzag winding configuration creates a low impedance zero sequence path, which is an artificial neutral used for effective grounding of the generator. A resistor is connected between the neutral and ground for controlling the ground fault current similar to LRG and HRG system. The zig zag method of generator grounding enables proper detection of ground faults with support for the sensitive relays and limits the transient overvoltage.
Common mistakes in generator grounding
Grounding the generator neutral directly: Grounding the generator neutral directly results in extremely high ground fault because the sub transient resistance of the generator is very low. This results in high mechanical forces on the winding of the generator and severe heating.
Using same pit for neutral and body earthing: If the body and the neutral grounding shares same grounding electrode, then the resistance of the electrode rises during fault which results in high touch potential on the generator’s body.
Neutral grounding resistor bypass: Bypassing the neutral grounding resistor during maintenance converts the system into solidly grounded system, allowing damaging fault current levels. Thus the protection relay calibrated for NGR grounding becomes ineffective, leading to no tripping of the circuit thus severely stressing the insulation during earth faults.
This article is a part of the Safety and Earthing page, where other articles related to topic are discussed in details.
