1、 Analysis of Key Factors Affecting Frequency Stability
Frequency instability is usually manifested as fluctuations in the output frequency around the rated value, commonly known as "traveling wave" or "surge". The root cause can be traced back to the speed fluctuations of diesel engines, which are caused by a series of mechanical, fuel, electrical, and control system problems.
1. Fuel supply system malfunction
Fuel supply is the energy source for diesel engines to perform work, and the stability of the supply directly determines the stability of the engine speed.
Oil circuit problem: Low pressure oil circuit blockage and dirty diesel filter can limit fuel flow, resulting in poor fuel supply. Mixing air or leakage in the fuel pipeline can cause fuel supply interruption or intermittent. Both of these situations result in unstable fuel pressure obtained by the fuel injection pump, leading to uneven fuel injection in each cylinder cycle and fluctuations in the instantaneous power output of the engine, thereby causing periodic fluctuations in speed and frequency.
Fuel injection system malfunction: The internal plunger of the high-pressure fuel pump is worn and stuck, the fuel supply of each sub pump is uneven and excessive, and the fuel injector is poorly atomized or blocked, all of which can cause inconsistent explosive power of combustion in each cylinder. This unbalanced combustion will cause periodic pulsation of the crankshaft rotational torque, directly manifested as unstable speed, leading to fluctuations in power generation frequency.
2. Failure of speed control system
The governor is the core component that maintains the constant speed operation of diesel engines. Its function is to automatically adjust the fuel supply according to load changes, in order to offset the impact of load changes on the speed.
Mechanical governor failure: The elasticity of the speed regulating spring has weakened due to long-term fatigue, resulting in a decrease in speed regulating sensitivity; Wear or jamming of the flying hammer and lever mechanism; Damage to one-way thrust bearings can cause the governor to respond slowly or act inaccurately, making it difficult to correct speed deviations in a timely manner.
Electronic speed regulation system failure: For electronic speed regulation systems commonly used in modern units, speed sensor signal distortion, controller (such as ECU or dedicated speed regulation module) calculation errors, and actuator (such as electric or electro-hydraulic actuator) response lag or jamming can all lead to speed regulation failure. In addition, although the automatic voltage regulator (AVR) mainly controls voltage, its abnormal coordination with the speed control system may indirectly affect frequency stability.
2、 Frequency stability requirements in different application scenarios
The application scenarios of diesel generator sets are extremely wide, ranging from household backup power sources to national key infrastructure. Different scenarios require differentiated and stringent requirements for frequency stability accuracy, dynamic response speed, and sustained stability capability.
1. Data center and communication hub
This is one of the scenarios with the highest requirements for power quality. The switching power supply and precision clock circuit inside IT equipment such as servers, storage devices, and network switches are extremely sensitive to input frequency.
3、 Diagnosis and maintenance strategies for frequency instability
Based on the above analysis, it is crucial to establish a systematic fault diagnosis and preventive maintenance system.
Preliminary judgment: When frequency instability occurs, first observe the operating status of the diesel engine. If accompanied by abnormal smoke exhaust color (such as black smoke) and uneven sound ("traveling car" sound), the problem is likely to be in the fuel system or engine body. If the engine runs smoothly but with frequency fluctuations, it is important to focus on checking whether the generator excitation system (such as AVR), governor electrical components, and load are balanced.
Segmented investigation:
Check the fuel circuit: eliminate the air in the circuit, inspect and clean the fuel filter and fuel pump filter screen, and ensure smooth fuel supply from the fuel tank to the fuel injection pump.
Testing the speed control system: Check whether the mechanical speed controller connecting rod is flexible and whether the spring has failed; Check whether the speed sensor signal of the electronic governor is normal and whether the actuator action is in place.
Calibration of fuel injection pump and injector: This is a highly specialized maintenance task. The fuel injection pump assembly should be sent to the professional pump calibration center to check the fuel supply uniformity and advance angle of each cylinder, and to conduct atomization quality and injection pressure tests on the fuel injectors.
Electrical system inspection: Check the excitation circuit of the generator, measure the residual voltage of the exciter, inspect the AVR control board and its connecting circuits to ensure stable excitation current.
Mechanical condition inspection: Check the tightening of the foundation bolts of the unit, inspect the alignment of the coupling between the engine and generator, and eliminate obvious sources of mechanical vibration.
Preventive maintenance: Develop a regular maintenance plan, including replacing fuel and air filters, cleaning the fuel tank, inspecting the governor mechanism, and conducting regular no-load and on load tests to monitor frequency response characteristics. For units in critical locations, an online monitoring system should be equipped to monitor real-time parameters such as frequency, voltage, and speed, achieving predictive maintenance.
