Working principle, function, and setting calculation for the generator 100% stator earth fault protection.

1. Generator 100% Stator Earth Fault Protection Principle
        Traditional stator earth fault protection (e.g., fundamental zero-sequence voltage protection) covers only 80–95% of the stator winding, leaving a dead zone near the neutral point. 100% stator earth fault protection combines dual principles for full coverage:

(1) Third-Harmonic Voltage Principle (Neutral Zone Protection)
        Mechanism: Under normal operation, magnetic field distortion in the air gap generates third-harmonic voltages (U3N at neutral, U3T at terminal). The ratio satisfies:

|U_3N|/|U_3T|≈1

Fault Response:
        Earth fault near neutral: U_3N decreases,increases → Ratio U_3T drops significantly.
        Protection Criterion:|U_3N|/|U_3T|<K_set(Typical setting: K_set=0.3−0.5).

(2) Low-Frequency Injection Principle (20 Hz Injection, Full Coverage)
        Method: Injects a 20 Hz voltage signal via the neutral earthing transformer or dedicated coupler.
Fault Detection:
        Normal: High impedance → Small injection current (I_inj).
        Earth fault: Stator insulation resistance (R_f) drops → I_inj increases.
        Criterion: Monitors I_inj or computes R_f:

R_f=U_inj/I_inj(Trip if R_f<R_set )

2.generator 100% stator earth fault protection Functions
        Full Winding Coverage: Eliminates dead zone (5–20% near neutral), protecting 100% of the stator.
        High Sensitivity : Detects high-resistance faults (up to several kΩ), preventing fault escalation.
        Dual Redundancy : Third-harmonic + injection principles act as mutual backups.
        Backup Protection : Provides backup for 95% stator earth fault protection.

3. generator 100% stator earth fault protection Setting Calculations
(1) Third-Harmonic Voltage Protection
        Ratio Setting:

        K_set=K_rel×(|U₃N|/|U_3T|)_min

        K_rel: Security margin (1.2–1.5).
        (|U_3N|/|U_3T|)_min: Minimum measured ratio under normal operation (typically 0.5–1.0). 
        Typical Setting: K_set =0.3−0.5.

(2) Low-Frequency Injection Protection
Insulation Resistance Setting:

        R_set= R_ins.min/K_rel

        R_ins.min : Minimum allowable stator insulation resistance (usually 5–10 kΩ).
        K_rel: Security margin (1.5–2.0).
        Typical Setting: R_set=1−5kΩ.
Time Delay: 0.5–1.0 s (avoids transient disturbances).

(3) Fundamental Zero-Sequence Voltage Protection (95% Coverage, Coordination)
Setting:

        U_0.set=K_rel×U_0.max

        U _0.max: Maximum unbalanced voltage during normal operation (5–10 V secondary).
        K_rel: Security margin (1.3–1.5).
        Typical Setting: U_0.set=5−15V (secondary).

Engineering Considerations
        1.Third-Harmonic Scheme Applicability: Performance varies with load/power factor; verify across all operating conditions.
        2.Injection Scheme Advantage: Unaffected by operating state; requires dedicated injection equipment.
        3.Setting Validation: Measure U_3N/U_3T ratio and fundamental U₀ during commissioning.
        4.Coordination: 100% protection delays (0.5–1 s) coordinate with instantaneous 95% protection.

Example Setting Procedure:
        1.Measure:U_3N/U_3T at no-load/full-load; obtain minimum ratio 0.6.
        2.Set third-harmonic protection: K_set=1.3×0.6=0.48→0.5.
        3.Set injection protection: R_set=3kΩ (per manufacturer).
        4.Set zero-sequence voltage:U_0.set=1.3×8V=10.4V→10V

Generator 100% Stator Earth Fault Protection Summary

Conclusion: The synergy of third-harmonic monitoring and active low-frequency injection enables dead-zone-free, high-reliability stator earth fault protection, essential for large generators.

Our company’s GWZC-9886 Generator Backup Protection Relay features 100% Stator Ground Protection. Welcome inquiries and procurement!