The diesel generator set runs at normal speed, but what causes the power generation frequency to be unstable? After many years of experience, after reading this, I was suddenly enlightened.

The diesel generator set runs at normal speed, but what causes the power generation frequency to be unstable? After many years of experience, after reading this, I was suddenly enlightened.

 

First, grasp the core premise: the diesel engine's actual measured speed is stable, but the indicated/output frequency jumps, indicating that there is no problem with the mechanical speed. The issues are concentrated on: abnormal speed sampling signals, instrument faults, electrical problems with the generator itself, excitation interference, measurement circuit faults, and are unrelated to the oil circuit/governor.

 

Ⅰ. Most common: Speed sampling (frequency sampling) signal distortion (high incidence in electronically regulated units)

1.1 Magnetic speed sensor (tachometer probe) failure
- Probe installation gap too large, loose, surface attracts iron filings;
- Internal coil aging and broken, output sine wave distortion; the controller receives fluctuating signals, misjudges unstable speed, and the frequency sampling jumps erratically; however, the actual diesel engine crankshaft speed measured visually/outside tachometer is stable.

1.2 Tachometer signal line poor connection, insulation damage to ground, poor shielding affected by inverter/contactor electromagnetic interference, sampling pulses jump, causing the electronic control system to incorrectly display frequency fluctuations.

1.3 Frequency detection channel inside the electronically regulated control board damaged, resulting in acquisition and calculation errors.

Ⅱ. Instrument/secondary measurement loop problems (unstable display only, actual true frequency normal)

2.1 Frequency meter on the unit control panel faulty, meter aging;

2.2 Voltage transformer (PT), frequency transmitter damaged, output signal fluctuates;

2.3 Loose terminal connections, poor contact, measurement circuit intermittently connected, meter needle/digital display jumps frequently;

2.4 Measuring the output power frequency with a multimeter is very stable; only the unit display jumps → purely a measurement instrument fault.

 

Ⅲ. Electrical Faults in the Generator Itself (True Frequency Fluctuations, Mechanical Speed Unchanged) In theory, the rotational speed of a synchronous generator is constant, so the frequency must also be constant. Such situations usually occur due to:

 

1. Short circuit between rotor poles or slight inter-turn short circuit, causing distortion of the rotor magnetic field, instantaneous fluctuation of the equivalent number of pole pairs, and small fluctuations in output power frequency; accompanied by unstable voltage and higher unit temperature rise.

 

2. Local insulation damage or leakage in the stator winding, causing abrupt changes in equivalent impedance under load, abnormal magnetic field coupling, and small output frequency jitter.

 

3. Slip rings or carbon brushes wear, bouncing, or poor contact, causing intermittent excitation current fluctuations, distortion of stator output waveform, making frequency measurement equipment determine unstable frequency.

 

Ⅳ. Excitation System Abnormalities Causing Waveform Distortion, Misinterpreting Frequency Instability

 

1. AVR (Automatic Voltage Regulator) failure or excessively frequent voltage regulation; excitation current repeatedly rises and falls quickly, deforming the output sine wave, high-precision frequency meters may detect frequency disturbances.

 

2. Soft breakdown or poor single-phase conduction of excitation rectifier module diodes, causing waveform spikes and disturbance in frequency measurement sampling.

 

3. Failure of excitation circuit capacitors or filtering components, causing harmonics to pile up and resulting in jumping frequency readings.

 

Ⅴ. Load Side Harmonics / Nonlinear Loads Causing Frequency Measurement Illusions

 

1. Loads with frequency converters, UPS, electric welding machines, or rectifiers generate a large amount of harmonics superimposed on the sine wave; ordinary digital frequency meters are affected by harmonics, causing the readings to jump continuously, while the actual fundamental frequency remains constant.

 

2. Severe three-phase load imbalance, with serious distortion in one phase, causing phase-specific frequency measurement fluctuations.

 

Quick Step-by-Step Diagnostic Method

1. Connect an external high-precision power frequency meter directly to measure the generator output: - External meter stable, local panel fluctuates: only repair speed sensing line/probe/panel instrument; - External meter also fluctuates: generator itself or excitation fault.

 

2. Run unloaded with all loads disconnected: - Frequency still drifts when unloaded: issue with probe, AVR, or generator winding; - Stable when unloaded, fluctuates under load: nonlinear load harmonics or poor contact of carbon brushes and slip rings under load.

 

3. Verify with an independent handheld tachometer to confirm the diesel engine crankshaft speed is constant throughout, completely ruling out governor system problems.

 

 

Simplified priority troubleshooting sequence:

 

1. Check the gap of the speed probe, clean iron filings, and tighten wiring;

2. Replace with a spare frequency meter/transmitter for comparison;

3. Check whether the carbon brushes and slip rings are bouncing or worn;

4. Test the AVR excitation and rectifier module;

5. Investigate load harmonics and add filters;

6. Use a megohmmeter to test the insulation of the generator stator and rotor, and check for inter-turn short circuits.