Engineering guide for onshore power plants and marine diesel-electric vessels: brushless excitation, AVR reactive droop, circulating kVAr and ANSI 32, 40 and 51V relays.
← All technical guides · Category: Power-Plant & Electrical · Updated: · 9 min read
| Fault / Trip Condition | Primary Electrical Signature | Root Cause on Gas / DF / Marine Sets | Immediate Corrective Action |
|---|---|---|---|
| kVAr Circulating Current / Power Factor Split | Same bus voltage, but one generator shows high current at a low lagging power factor while the other shows low current or a leading power factor | AVR voltage setpoint mismatch, reversed droop CT connections, or unequal reactive droop settings between AVRs | Match no-load terminal voltages closely before synchronising, check droop CT polarity, and set the same reactive droop on all sets |
| ANSI 32 Reverse Power Trip | Breaker trips during unloading or after a gas trip when the set starts to motor (draw power from the bus) | Governor speed reference not lowered cleanly during unloading, a sluggish fuel actuator, or closing the breaker with the incoming set running slow | Close the breaker only with the incoming set running slightly fast (synchroscope turning slowly in the “fast” direction), and set reverse power pickup and time delay to the engine maker’s and class requirements |
| ANSI 40 Loss of Excitation / Field Trip | Generator draws heavy leading reactive current (−kVAr) from the bus, terminal voltage sags and the rotor may slip poles | Open or shorted rotating diode on the brushless exciter, AVR power-stage failure, or a tripped excitation field breaker | Isolate the generator, disconnect the rectifier bridge leads, and test each rotating diode (forward and reverse) and the surge suppressor with a suitable diode tester |
| Active Power (kW) Hunting in Parallel | Rhythmic kW and current swings between generators while bus frequency fluctuates | Noise on the isochronous load-sharing communication line, aggressive governor PID gains, or cylinder misfire / GVU gas pressure oscillation | Check load-sharing line termination and screening, check cylinder EGT spread, and switch temporarily to speed-droop mode to isolate the cause |
On a paralleled AC busbar, terminal voltage and frequency are common to every connected alternator. If Generator A's Automatic Voltage Regulator (AVR) has a slightly higher internal voltage setpoint than Generator B's, Generator A attempts to raise the bus voltage while Generator B attempts to lower it. Because the bus voltage cannot take two values at once, a circulating reactive current (I_circ) flows between the two stator windings, lagging by nearly 90 degrees in the over-excited machine and leading in the under-excited machine.
Reactive droop (quadrature current compensation) prevents this by feeding a current transformer (CT) signal from one phase into the AVR sensing circuit. As lagging reactive current rises, the AVR lowers its voltage reference slightly, by the droop percentage set at rated reactive load, so paralleled generators share reactive load in proportion to their ratings.
Modern medium-speed dual-fuel, spark-gas and marine auxiliary alternators usually use brushless excitation: the AVR feeds DC current into a stationary exciter field winding, inducing three-phase AC in the rotating exciter armature, which is rectified by a shaft-mounted rotating diode bridge (usually six diodes) and fed directly into the main rotor field.
If one rotating diode fails open-circuit, the AVR increases exciter field current to maintain stator voltage, overheating the exciter and introducing a characteristic ripple on the DC excitation current. If a diode fails short-circuit or the surge suppressor varistor breaks down, excitation collapses and trips the generator on ANSI 40 (Loss of Excitation) or under-voltage.
Active power (kW) is controlled by the engine governor, whereas reactive power (kVAr) and power factor are controlled by the generator AVR. If one generator carries high amps at a low lagging power factor while the other shows a high or leading power factor, their AVR voltage setpoints or quadrature droop settings are mismatched, driving circulating reactive current between the stators.
Before shutdown, a rise in AVR exciter field current at normal load and a ripple in the exciter field current point to an open-circuit rotating diode. Once the engine is stopped and locked out, open the exciter inspection covers, disconnect the diode leads from the bridge, and check forward voltage drop and reverse blocking on each diode with a diode tester, comparing the readings across the bridge.
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