Configuration of Generator Protections and Their Respective Functions

• GoWatron provides a full range of synchronous generator protection solutions. This page outlines the standard configuration of generator protection systems and the corresponding functions of each ANSI device number, covering differential protection, stator ground fault protection, loss-of-field protection, reverse power protection, negative-sequence protection, and other widely applied generator protection functions.
• Manufactured by GoWatron, the GWPR300-GRE Generator Rotor Earth Fault Protection Relay, GWPR300-GB Generator Backup Protection Relay, and GWPR300-GD Generator Differential Protection Relay are designed to work together to form a comprehensive generator protection system. Each relay addresses specific fault conditions and integrates complete protection functions required for generators. Combined, they deliver full protection against various operational faults and abnormal conditions in synchronous generators.

I. Main Protections

(ANSI 87G) Generator Differential Protection

• Serves as the primary protection against phase-to-phase short circuits in generator stator windings. It compares currents measured at the generator terminals and neutral point. The protection zone covers internal stator windings and connecting leads extending from the generator terminals to the generator outlet circuit breaker. It cannot detect faults outside the outlet circuit breaker, inter-turn faults or single-phase-to-ground faults. When an internal phase-to-phase short circuit occurs, the protection rapidly trips the generator outlet circuit breaker, activates field de-excitation and initiates unit shutdown to limit damage to windings and the stator core.

(ANSI 87G/59N) Inter-Turn Short Circuit Protection

• Detects inter-turn short-circuit faults within the same phase of generator stator windings. For double-star connected generators, a split-phase transverse differential scheme is normally adopted; for single-star connected generators, zero-sequence voltage-based inter-turn protection is deployed. Inter-turn faults usually feature relatively low fault current in the initial stage. If not cleared promptly, such faults may develop into phase-to-phase short circuits and cause severe insulation failure of stator windings.

(ANSI 64G) 100% Stator Ground Fault Protection

• Adopts composite criteria of fundamental-frequency zero-sequence voltage and third-harmonic voltage, covering all single-phase-to-ground faults along stator windings from generator terminals to the neutral point. It prevents continuous burning of the stator core caused by capacitive ground fault current and avoids insulation deterioration that may further evolve into severe phase-to-phase short circuits.

II. Backup Protections and Abnormal Operating Condition Protections

(ANSI 51V) Compound Voltage Restrained Overcurrent Protection

• Acts as backup protection for phase-to-phase faults. It detects remote internal phase-to-phase short circuits inside the generator as well as phase-to-phase faults on external busbars at generator terminals, providing backup tripping upon main protection failure. With undervoltage and negative-sequence voltage restraint logic, it improves sensitivity for unsymmetrical faults and reliably prevents unintended operation under normal load current.

(ANSI 64R) Rotor Ground Fault Protection

• Integrates rotor one-point ground fault and rotor two-point ground fault functions. Rotor one-point ground protection continuously monitors the insulation condition of the excitation winding to earth. A single-point ground fault generates no short-circuit current, but indicates degraded rotor insulation. This protection mainly outputs alarm signals to remind Operation and Maintenance staff to take timely measures and prevent fault escalation. Rotor two-point ground protection responds to short-circuit faults occurring at two separate points of the excitation circuit. The resulting magnetic field imbalance triggers severe unit vibration and may seriously damage the rotor shaft and bearings. The protection initiates unit shutdown once measured values exceed the setting.

(ANSI 40) Loss-of-Field Protection

• Responds to loss-of-field conditions caused by excitation system faults, including excitation circuit disconnection, accidental tripping of the field breaker, excitation regulator faults and other failures. After loss of field, the generator operates asynchronously and continuously absorbs large amounts of reactive power from the grid, which may lead to stator overheating, terminal voltage drop and power system oscillation. The protection applies impedance criteria to distinguish loss-of-field conditions from system oscillations for selective tripping.

(ANSI 46) Negative-Sequence Overcurrent Protection

• Responds to negative-sequence currents induced by unsymmetrical short-circuit faults, unbalanced three-phase loads and open-phase operation. The negative-sequence magnetic field generates eddy currents in the rotor body and damping windings, resulting in rotor overheating and thermal damage. This protection generally adopts an inverse-time I²t characteristic: the higher the unbalance level, the shorter the permissible operating time, with graded alarm and tripping functions.

(ANSI 49S) Stator Overload Protection

• Handles sustained symmetrical three-phase overload of the generator. It monitors continuous stator load current exceeding rated value to avoid prolonged temperature rise that accelerates stator insulation aging. It differentiates short-term allowable overload from persistent overheating conditions, and is normally configured with an alarm stage and a time-delayed tripping stage.

(ANSI 49E) Excitation (Rotor) Overload Protection

• Monitors excitation current staying above the rated limit for extended periods. It prevents burnout of rotor windings under conditions such as prolonged forced excitation or excitation regulator malfunctions. It considers both the allowable duration of short-term forced excitation and long-term thermal limit constraints.

(ANSI 59) Overvoltage Protection

• Primarily addresses rapid terminal voltage rise caused by generator load rejection. Excessive overvoltage may puncture the stator main insulation. This risk is more prominent for hydro-generators, whose governors have relatively slow response and are prone to overspeed and overvoltage during load rejection. When voltage reaches the setting, the protection executes time-delayed tripping, field de-excitation and unit shutdown.

(ANSI 81U) Underfrequency Protection

• Monitors sustained low-frequency unit operation. Low-frequency operation may excite resonance and damage steam turbine blades. This protection is mandatory for turbo-generators. Multiple frequency setpoints together with time-delay logic are configured to prevent maloperation.

(ANSI 78) Out-of-Step Protection

• Responds to synchronous oscillation and out-of-step conditions between the generator and power grid. Continuous power angle swing generates periodic pulsating torque that may damage shaft components. The protection can distinguish stable oscillation from out-of-step conditions and disconnect the generator at an appropriate instant once out-of-step is detected.

(ANSI 32P) Reverse Power Protection

• Monitors motoring operation of turbo-generators after closure of the main steam valve. Under motoring conditions, the steam turbine loses steam cooling, and windage heating of blades may lead to rapid component damage. The protection supports both alarm and tripping logic; large units generally adopt programmed reverse power tripping.

(ANSI 24) Overexcitation Protection (V/Hz Protection)

• Monitors excessive voltage-to-frequency ratio. It prevents magnetic flux density in generator and main transformer cores from exceeding design limits, so as to avoid core overheating and accelerated insulation aging.

III. Non-Electrical Protections

(ANSI 63) Bearing Oil Level Protection

• Monitors abnormal rise or fall of generator bearing oil level. Low oil level may lead to insufficient bearing lubrication and bearing bush burnout; excessively high oil level may cause oil leakage and insulation contamination. Upon detecting abnormalities, the protection issues alarm or tripping commands according to operating conditions.

(ANSI 38/49) Temperature Protection (Stator Windings, Core and Bearings)

• ANSI 38 corresponds to bearing temperature monitoring, and ANSI 49 corresponds to winding temperature monitoring. It monitors overtemperature of stator windings, stator core and generator bearings to prevent insulation aging and bearing bush damage caused by overheating. Alarm setpoints and high-temperature tripping setpoints are configured.

(ANSI 63) Hydrogen Pressure/Humidity/Purity Protection (For Hydrogen-Cooled Units)

• Monitors low hydrogen pressure, reduced hydrogen purity and excessive humidity. It mitigates explosion risks caused by insufficient hydrogen purity and prevents moisture from eroding stator insulation.

(ANSI 63) Cooling System Failure Protection (For Water-Cooled or Air-Cooled Units)

• Monitors loss of cooling water flow and cooling fan faults. Loss of cooling medium reduces the generator’s heat dissipation capacity and leads to rapid unit overheating.

Note: Per industry practice, main protections for short-circuit faults adopt fast tripping to clear severe internal faults. Backup protections consist of phase-fault backup protection as well as various abnormal operating condition protections. Non-electrical protections operate based on mechanical/physical signals such as temperature, pressure and flow, independent of electrical quantities.