Analysis and treatment of system power failure caused by inverter harmonics

The root cause of inverter harmonics

　　The core of the frequency converter is to convert the industrial frequency AC power into AC power with adjustable frequency and voltage through the switching action of power electronic devices to achieve speed control of the motor . However, it is the fast switching characteristics of these power electronic devices that cause the distortion of the current waveform, thereby generating harmonics. Taking the common AC-DC-AC frequency converter as an example, its rectification link usually uses diodes or thyristor rectifiers. During the rectification process, since the input current is a non-sinusoidal wave, in addition to the fundamental wave (50Hz), it also contains a series of higher harmonics. The frequencies of these harmonics are usually integer multiples of the fundamental frequency, such as the 5th, 7th, and 11th harmonics.

　　Mechanism of Harmonics Causing System Power Failure

　　Affects the quality of power supply voltage

　　The harmonic current generated by the inverter produces a voltage drop on the grid impedance, causing the power supply voltage waveform to be distorted. This voltage distortion is manifested as the voltage waveform is no longer a standard sine wave, but has irregular shapes such as spikes and depressions. For other electrical equipment, this distorted voltage is equivalent to a kind of "pollution" and will seriously affect its normal operation. For example, for some precision instruments and equipment with extremely high voltage quality requirements, voltage distortion may cause increased measurement errors and even damage the equipment. In a three-phase four-wire power supply system, the third and integer multiple harmonic currents of 3 cannot offset each other in the neutral line, which will cause excessive current in the neutral line, causing overheating of the neutral line and increasing the risk of electrical fires.

　　Interference with the stability of the power system

　　Harmonics can interfere with the stability of the power supply system and cause resonance. Resonance occurs when the harmonic frequency generated by the inverter is close to or equal to the natural frequency of the power supply system. Resonance can cause the current and voltage in the system to increase sharply, far exceeding the normal operating values. This overcurrent and overvoltage can cause great damage to equipment in the power supply system, such as transformers and capacitors. For example, resonance may cause transformer windings to overheat, accelerate insulation aging, and even cause short-circuit faults. At the same time, resonance can also affect the power factor of the power system, reduce the transmission efficiency of electric energy, and increase the loss of the power grid.

　　Impact on motors and other equipment

　　Harmonics not only affect the power supply system, but also have adverse effects on the motors and other equipment connected to the inverter. Harmonic currents will generate additional copper and iron losses in the motor windings, causing the motor to heat up severely and reduce efficiency. If the motor is operated in a harmonic environment for a long time, the insulation performance will gradually decrease, shortening the service life of the motor. In addition, harmonics will also cause the vibration and noise of the motor to increase. This is because the harmonic magnetic field generated by the harmonic current interacts with the fundamental magnetic field, generating additional electromagnetic force, which causes the motor vibration to intensify. Vibration not only affects the running stability of the motor itself, but may also cause damage to the mechanical equipment connected to it.

　　Fault diagnosis method

　　Harmonic detection

　　Accurately detecting the harmonics generated by the inverter is a key step in diagnosing power supply faults. At present, the commonly used harmonic detection methods are based on Fourier transform and based on instantaneous reactive power theory. The method based on Fourier transform converts the time domain signal into a frequency domain signal by performing discrete Fourier transform on the current or voltage signal, thereby analyzing the harmonic components contained in the signal and their amplitude and phase. The method based on instantaneous reactive power theory uses the instantaneous reactive power relationship of the three-phase circuit to quickly and accurately detect the harmonic current. In practical applications, a professional harmonic analyzer can be used to monitor the current and voltage in the power supply system in real time, obtain key parameters such as harmonic content and total harmonic distortion (THD), and provide data support for fault diagnosis.

　　Fault phenomenon analysis

　　Comprehensive analysis based on actual fault phenomena is also an important means of diagnosis. When a power failure occurs in the system, observe the operating status of related equipment. For example, if the motor has abnormal heating, vibration, or increased noise, and the power supply voltage fluctuates or distorts, it is likely that the fault is caused by the inverter harmonics. In addition, checking the operation of the protection devices in the system, such as whether the overcurrent protection, overvoltage protection and other devices are frequently operating, can also help determine the cause of the fault.

　　Troubleshooting measures

　　Harmonic suppression technology

　　Install filters: Filters are commonly used devices for suppressing harmonics, and can be divided into passive filters and active filters. Passive filters are composed of capacitors, reactors, and resistors, and filter harmonic currents by tuning to specific harmonic frequencies. It has a simple structure and low cost, but the filtering effect is greatly affected by the impedance of the power grid, and can only filter harmonics of a specific frequency. Active filters detect harmonic currents in real time and generate compensation currents of equal magnitude and opposite direction to offset the harmonic currents. Active filters have the advantages of good filtering effect, fast response speed, and dynamic compensation, but the cost is relatively high. In practical applications, appropriate filters can be selected according to the specific conditions of the system, or a combination of passive filters and active filters can be used to achieve the best harmonic suppression effect.

　　Optimize the control strategy of the inverter: By improving the control algorithm of the inverter, such as using the space vector pulse width modulation (SVPWM) method in the pulse width modulation (PWM) technology, the generation of harmonics can be effectively reduced. SVPWM technology can make the waveform of the output voltage closer to a sine wave and reduce the harmonic content. In addition, the carrier frequency of the inverter can be adjusted to change the distribution characteristics of the harmonics, and the harmonic frequency can be moved to a higher frequency band to reduce the impact on low-frequency equipment. However, the increase in carrier frequency will increase the switching loss of the inverter, so it is necessary to balance the harmonic suppression effect and the inverter efficiency.

　　System Optimization

　　Reasonable selection and arrangement of equipment: When designing the power supply system, the parameters of equipment such as transformers and capacitors should be reasonably selected to avoid resonance between the system's natural frequency and the inverter's harmonic frequency. For example, according to the load characteristics and the capacity of the inverter, select the appropriate transformer ratio and short-circuit impedance. At the same time, the location of the equipment should be reasonably arranged to shorten the cable length, reduce the line impedance, and reduce the impact of harmonics during transmission.

　　Use isolation transformer: Installing an isolation transformer between the inverter and the power supply can effectively isolate the conduction of harmonics generated by the inverter to the power supply system. The primary and secondary sides of the isolation transformer transfer energy through electromagnetic induction, without direct electrical connection , which can prevent harmonic current from entering the power supply system. In addition, the isolation transformer can also balance the three-phase voltage, suppress common-mode interference, and improve the stability of the power supply system.

　　Strengthen maintenance and management

　　Regular inspection and maintenance: Establish a regular inspection and maintenance system, use professional instruments to inspect inverters, filters and other equipment, and promptly discover hidden faults of equipment. Check whether the capacitors, inductors and other components of the filter are damaged, and whether the power electronic components of the inverter have signs of overheating, aging, etc. Repair or replace any problems found in a timely manner to ensure the normal operation of the equipment.

　　Develop an emergency plan: Develop a comprehensive emergency plan so that when a power system fails, measures can be taken quickly to reduce losses. The emergency plan should include emergency shutdown procedures, troubleshooting procedures, and backup power startup methods when a failure occurs. At the same time, regularly practice the emergency plan to improve the staff's ability to respond to sudden failures.

　　System power failures caused by inverter harmonics seriously affect the stability and safety of industrial production. By deeply understanding the root causes of harmonics and the mechanism of faults, adopting scientific fault diagnosis methods, and implementing effective treatment measures, harmonics can be effectively suppressed, the stable operation of the power supply system can be guaranteed, and reliable power support can be provided for the sustainable development of industrial production.