Hidden Risks of Low Generator Frequency

In a power system, a generator functions much like a heart, delivering power to the entire system. But if the generator frequency becomes too low, it is like an arrhythmic heartbeat, creating numerous problems for the power system and related equipment. Today, let's talk about the effects, causes, solutions, and preventive measures associated with low generator frequency to help everyone better address this issue and ensure stable operation of generator sets.

First, it is important to understand the kinds of problems a low frequency can bring. These effects concern not only the performance and lifespan of the generator itself but also the stability of the entire power system and the user's electricity experience.

When frequency drops, the generator's cooling system is the first to be affected. There are fans at both ends of the generator rotor, which normally deliver sufficient airflow for cooling. But as frequency decreases, the rotor speed slows down, the airflow from the fans decreases, much like how a household fan cools less effectively when spinning slowly. As a result, the temperatures of various generator components increase, especially the rotor windings. Higher temperatures increase resistance, which in turn causes more heating. If generator output is not reduced, the rotor windings may overheat and become damaged, affecting normal operation.

In addition to the cooling system, the generator's electromagnetic performance also worsens. To maintain constant terminal voltage, magnetic flux must be increased, but this easily causes saturation of the stator core and magnetic flux leakage. The leaked flux induces eddy currents in the frame components, generating local hot spots that, in severe cases, may spark. This not only damages components but may also cause fires, posing major hazards to the power system.

The rotational speed of plant motors is closely related to generator frequency. When frequency decreases, motor speed also drops, reducing output power, lowering working efficiency, and making it difficult for equipment to operate normally. For example, in factories where motors drive production equipment, slower motor speeds lead to reduced product quality and production efficiency. Moreover, long-term operation under low frequency causes overheating in motor windings due to increased current, shortening service life and increasing maintenance costs.

For users, excessively low frequency directly affects electrical safety and equipment operation. Lower frequency results in reduced voltage because the generator's induced electromotive force is related to rotational speed. When speed decreases, EMF drops and voltage follows. Precision instruments and electronic devices that require stable voltage may malfunction or suffer data loss under low-voltage conditions. In addition, low generator speed reduces the output of the coaxial exciter, affecting reactive power output and further lowering system voltage, impacting normal power usage.

For steam turbines connected to generators, low frequency is also a major concern. Reduced frequency increases blade pressure and may induce resonance. Resonance is extremely dangerous: it causes blades to vibrate excessively and may ultimately lead to blade fracture. Once a blade breaks, it can damage the turbine and trigger a chain reaction, harming the entire generator set and threatening the safety of the power system. Furthermore, turbine efficiency decreases with reduced frequency, affecting power generation efficiency and economic performance.

Low frequency may arise from various factors. Only by identifying the specific cause can the issue be targeted and resolved to ensure stable operation of the generator set.

The diesel engine provides power for the generator, and its operating condition directly affects generator frequency. If some cylinders in the diesel engine malfunction or stop working, power output decreases, generator speed drops, and frequency falls. Problems such as faulty injectors, worn piston rings, clogged low-pressure fuel lines, insufficient fuel supply from high-pressure pumps, stuck plungers, air in the fuel line, over-tightened connecting rod bolts, malfunctioning feed pumps, or non-injecting injectors can all reduce engine power and affect generator frequency.

The excitation system provides excitation current to the generator to maintain stable voltage and frequency. If the exciter speed does not match the generator speed, or if the rectifier diodes in the excitation circuit are damaged, excitation current becomes insufficient or unstable, affecting generator frequency. For example, if the exciter speed is lower than the generator speed, its output voltage drops, weakening the magnetic field and reducing frequency. Damaged rectifier diodes destabilize excitation current and cause frequency fluctuations.

Aside from diesel engine and excitation system issues, other factors may lead to low generator frequency. For instance, when grid frequency changes, if the generator cannot adjust quickly to synchronize with the grid, deviations occur. When the load exceeds the engine’s rated power, the engine cannot maintain rated speed, causing frequency to drop. Starting large equipment may create momentary overload, and if the governor responds slowly, generator frequency will decrease.

By following proper inspection steps and professional repair processes, the negative impact of low frequency on generator sets and the power system can be minimized. The following are detailed troubleshooting methods for low generator frequency.

When low frequency is detected, the first step is to check the diesel engine's performance. Start the diesel engine and bring its speed to around 700 r/min. Touch the exhaust pipes of each cylinder and observe whether exhaust temperatures are consistent. If cylinder temperatures are generally equal, the cylinders are functioning properly, and faulty individual cylinders can be ruled out.

Next, shut down the engine and check the fuel pump inlet filter screen for blockages. Perform a fuel pressure test on the feed pump and check pressure readings. If pressure is normal but the diesel drip in the high-pressure pump oil observation tube is abnormal, there may be internal leakage inside the high-pressure pump. Remove the side cover of the high-pressure pump and run the diesel engine to observe internal leakage. If no leakage is found, diesel may be entering the high-pressure pump housing through clearances in the feed pump plunger, causing insufficient fuel supply. Replace the feed pump, restart the engine, increase speed, and check whether rated speed is achieved and frequency returns to normal.

For excitation system faults, check whether the exciter speed matches the generator speed. Measure the exciter output voltage to determine its speed. If output voltage is too low, the exciter speed may be insufficient. Inspect the exciter transmission mechanism for slippage or damage. Check rectifier diodes in the excitation circuit by measuring forward and reverse resistance using a multimeter. Replace any faulty diodes to ensure stable excitation current.

In addition to diesel engines and excitation systems, check other components that may influence generator frequency. Inspect the engine governor to ensure proper operation and fuel control. Mechanical governors may require spring tension adjustment; electronic governors require sensor signal diagnostics and actuator checks. Inspect speed sensors, wiring connections, control module parameters, and software for faults. Clean and lubricate linkages in mechanical governors and replace worn parts. Check for clogged air filters, turbocharger malfunction, or air leaks in intake piping. If cylinder pressure is insufficient, measure compression pressure and repair or replace piston rings, valves, and gaskets as needed.

By applying scientific and effective preventive strategies, potential issues can be detected and resolved early to avoid equipment damage and production losses caused by low frequency. The following are specific preventive measures.

Regular Maintenance and Inspection: Regular maintenance is the key to preventing low generator frequency. All generator set components, including the diesel engine, generator, and excitation system, must be checked according to prescribed cycles. For example, inspect diesel engine cylinder operation regularly, identify and repair malfunctioning cylinders, inspect feed pumps and high-pressure pumps, and replace damaged parts promptly. Inspect rectifier diodes, speed sensors, and other excitation system components. Routine maintenance helps identify potential faults early and ensures stable generator operation.

Install Frequency Monitoring Equipment: Installing frequency monitoring equipment allows real-time observation of generator frequency, issuing alarms when deviations occur. Operators can promptly adjust diesel engine speed or excitation current when frequency drops below the rated value. Monitoring equipment also records frequency data for subsequent fault analysis.

Proper Load Allocation: Proper load allocation is essential to prevent frequency drops. Load must not exceed the engine's rated power. Excessive load prevents the engine from maintaining rated speed, resulting in low frequency. Avoid frequent startup of large-power equipment to prevent temporary overload. Use soft starters or variable-frequency drives to reduce impact on the generator.

Ensure Fuel and Air Supply Quality: High-quality fuel and air supply are crucial for generator performance. Replace fuel filters regularly to ensure clean fuel and prevent blockage. Use high-quality fuel and avoid fuel with excessive water or impurities. Clean or replace air filters regularly, ensure unobstructed air intake, and inspect turbochargers for proper operation.

Address Fault Hazards Promptly: Any issues potentially affecting generator frequency should be addressed immediately, for example, repairing malfunctioning diesel engine cylinders or replacing damaged rectifier diodes. Prompt handling prevents further deterioration and ensures stable generator operation.

Low generator frequency is a complex issue involving coordinated operation of multiple components and systems within the generator set. Low frequency causes serious adverse effects on the generator, plant motors, user equipment, and steam turbines, and therefore requires sufficient attention. By thoroughly understanding the causes, adopting effective troubleshooting methods, and implementing preventive measures, stable generator operation can be ensured, strengthening the safety and reliability of the power system.