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Neutral Grounding Design Procedure


Generator Grounding Method



Distr. Trans. With Grounding Trans. Secondary Resistor


Generator System

General Comments

Inherently high fault Ground fault current Ground fault current current permits use of restricted to that very low; required for relaying; grounded-neutral No current for arresters if selective relaying;

Provides permanent ground source; Ground fault current restricted to that required for relaying; Ungrounded-neutral arresters usually required


Low Voltage Bus (No low voltage feeders)

X1 3

Ungrounded-neutral arresters required

Ungrounded-neutral arresters required Generally not applicable but may be used if selective ground relaying of machines is not required or if special relaying is used to obtain selectivity

No direct exposure Satisfactory but Preferred to lightning; fault current high

Ground current for selective relaying required Low Voltage Bus (Overhead low voltage feeders)

Direct exposure to lightning; Ground current for selective relaying required; Preferred where lightning esposure is severe;

Required for 4-wire

Preferred where lightning exposure is slight;

May be preferred for very large machines because of desire to restrict fault currrent

Not suitable due to lack of relaying selectivity

Sufficient machines to systems provide permanent ground

Low Voltage Bus (Cable Feeders)

No direct exposure to lightning; ground current for selective relaying required; Sufficient machines to provide permanent grounding; desirable to keep dynamic voltages as well as fault current low

Preferred where primary objective is to keep dynamic overvoltage on cable network to minimum

Not suitable due to Preferred where primary objective lack of relaying selectivity is to keep fault current to a minimum

Single Machine (Low voltage feeders)

Cannot depend on machine neutral for permanent ground

Unit System

Generator neutral desirable to detect machine faults before synchronizing No direct exposure to Satisfactory but lightning high ground fault No selective relaying current required

Does not provide Does not provide Does not provide Preferred in permanent ground permanent ground permanent ground addition to reactor or distribution transf. Grounding of generator neutral

Satisfactory but high ground fault current


Step-Up Through No direct exposure to Required where auto- Impedance trans has no tertiary; undesirably high if Auto-Transformer lightning; used with autotrans, having no No selective relaying tertiary required;

Generator neutral need not be grounded if auto- trans should be connected to common trans has tertiary and ground point. X0/X1 < 10

Generator and auto-

Impedance undesirably high if used with autotrans, having no tertiary

Figure 1 Summary of Generator Grounding Applications


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