I. Simplified Internal Topology Diagram of SD700 Three-Phase 660VAC Series Inverter Cabinet Unit ( 6-Pulse, 12-Pulse ) II. Explanation of Harmonic Suppression Level After Adding EMC Filters and Harmonic Reactors Inverter speed control technology is a high-tech technology integrating automatic control, microelectronics, power electronics, and communication technologies. With its excellent speed control and energy-saving performance, it has been widely used in various industries. Due to its soft-start capability, it can reduce mechanical shock to equipment and motors, extending their service life. With the rapid development of science and technology, inverters, with their energy-saving, reliable, and efficient characteristics, have been applied to various fields of industrial control, such as water supply, fan and air conditioning equipment, process control, elevators, and machine tools, ensuring adjustment accuracy, reducing the labor intensity of workers, and improving economic efficiency. However, because the switching characteristics of inverters constitute a non-linear load, the resulting harmonic interference is becoming increasingly significant, especially in high-power inverter speed control systems. The main types of interference from frequency converters are: First, frequency converters commonly use nonlinear rectifier devices such as thyristors or rectifier diodes, which generate harmonics that conduct interference to the power grid, causing voltage distortion (THDv is used to represent voltage distortion rate; the THDv caused by harmonics from frequency converters is around 10-40%), affecting the power supply quality of the power grid. Second, the output section of frequency converters generally uses switching devices such as IGBTs. When the transistors commutate, while outputting energy, they will generate strong high-frequency electromagnetic radiation interference on the output cable, affecting the normal operation of surrounding electrical appliances. I. What are harmonics? The fundamental cause of harmonics is nonlinear loads. When current flows through a load, it is not linearly related to the applied voltage, forming a non-sinusoidal current, thus generating harmonics. Harmonic frequencies are integer multiples of the fundamental frequency. According to the analysis principle of the French mathematician Fourier, any repeating waveform (including square waves, triangular waves, etc.) can be decomposed into sinusoidal components containing the fundamental frequency and a series of harmonics that are multiples of the fundamental frequency. Harmonics are sinusoidal waves, each with a different frequency, amplitude, and phase angle. Harmonics can be classified into even and odd orders. The 3rd, 5th, and 7th harmonics are odd, while the 2nd, 14th, 6th, and 8th harmonics are even. For example, if the fundamental frequency is 50Hz, the 2nd harmonic is 100Hz, and the 3rd harmonic is 150Hz. Generally speaking, odd harmonics cause more and greater harm than even harmonics. In a balanced three-phase system, due to symmetry, even harmonics are eliminated, leaving only odd harmonics. For a three-phase rectified load, the resulting harmonic currents are 6n±1 harmonics, such as 5th, 7th, 11th, 13th, 17th, and 19th harmonics. Frequency converters mainly generate the 5th and 7th harmonics. A schematic diagram of the harmonic definition is shown in Figure 1. II. What are the hazards of harmonics and electromagnetic interference to the power grid and other systems and equipment? 1. Harmonics distort the voltage and current waveforms of the power grid, causing local resonance and amplifying harmonics, increasing line losses, and lowering grid voltage. 2. Harmonics generate additional harmonic losses in electrical components (including frequency converters), reducing the power factor of electrical equipment and affecting its efficiency and lifespan. 3. Harmonics increase the losses of power transformers in the power grid, directly affecting their capacity and efficiency; they also increase transformer noise, causing localized overheating, insulation aging, shortened lifespan, and even damage. 4. Harmonics can be conducted through the power grid to other electrical equipment, affecting the normal operation of many electrical devices. The instantaneous voltage distortion caused by these harmonics can interfere with the normal operation of the internal software or hardware of electrical equipment in the power grid, such as causing relay protection devices to malfunction. 5. The presence of harmonics can cause a decrease in the efficiency of asynchronous motors, increased noise, heat generation leading to insulation aging, and shortened lifespan. 6. Electromagnetic radiation interference can disrupt control signals (computer or PLC), detection signals (instruments or sensors), and weak electrical signals such as communication signals passing near the inverter's output cable. In severe cases, this can lead to inaccurate instrument readings, failure to receive correct detection and control signals, or communication disruptions in the control system. Generally, the impact of inverters on systems with large power grid capacity is not very significant, which is why harmonics are often overlooked by most users. However, for high-power inverters or systems with relatively small power grid capacity, harmonic interference cannot be ignored. Comprehensive and effective measures should be taken to ensure the stable operation of the system. III. What measures should be taken to suppress harmonics and electromagnetic interference for high-power frequency converters? From the internal topology diagram of the SD700 660 high-power frequency converter above, it can be seen that PE's SD700 series high-power frequency converters employ built-in AC harmonic reactors, EMC input filters, and output dv/dt filters. Specifically: 1. The built-in AC reactor, placed at the front end of the rectifier bridge, achieves the following two functions: Firstly, it protects the rectifier bridge from instantaneous fluctuations in grid voltage and suppresses the impact of surge current on the frequency converter. Some converters use DC reactors placed on the DC bus, but this approach leaves the rectifier bridge without adequate protection. Secondly, the AC reactor can filter out grid harmonics and increase impedance, smoothing the current sine wave and improving power factor and efficiency. Therefore, PE's high-power frequency converters employ built-in AC reactors, which can improve the reliability and efficiency of the frequency converter and the entire power grid system. 2. The built-in EMC active filter can track and compensate for harmonics with varying frequency and amplitude, and its compensation characteristics are unaffected by grid impedance. Its basic principle is to detect the harmonic current from the object being compensated, and then generate a compensation current spectrum equal in magnitude but opposite in polarity to cancel the harmonics generated by the original line harmonic source. This results in the grid current containing only the fundamental component, significantly reducing motor heating and energy loss caused by higher-order harmonics, while also minimizing grid disturbance. The total harmonic voltage (THDv) is less than 8%, fully meeting the requirements for industrial applications and EN61000-6-2 electromagnetic interference immunity standards. It is superior to traditional LC passive filters, overcoming some inherent drawbacks of passive filters, such as susceptibility to overload and burnout. Furthermore, passive filters are uncontrollable; therefore, over time, component aging or changes in grid load can alter the resonant frequency, leading to a decrease in filtering effectiveness. More importantly, passive filters can only filter one harmonic component (e.g., some filters can only filter the third harmonic). If different harmonic frequencies need to be filtered, different filters must be used, which increases equipment investment. Therefore, PE's SD700 series high-power frequency converters all have built-in EMC active filters, thus ensuring long-term stable performance of the entire system. 3. The built-in dv/dt output filter filters out interference signals such as phase-to-phase electromagnetic radiation and coupling noise during the transmission process of the frequency converter output. This meets standards such as UNE-EN 61800-3, reducing interference to surrounding control and detection signals. The cable length from the frequency converter to the motor is allowed to reach a maximum of 300 meters, ensuring the safety and stability of the entire system. The inverter bridge A-phase output waveform of the frequency converter is shown in the figure below: If the output waveform of the frequency converter is magnified, it can be seen that the angle of the waveform is not a 90-degree right angle. However, a sharp overpulse voltage will be generated at the output of the frequency converter. This voltage is much higher than the motor output. To solve this problem, Power Electronics has developed the gate resistor of IGBTs to ensure that the peak pulse does not exceed the limit. Rg gate resistor controls the "capacitance" of the load to be consistent with that of the IGBT. Actual measured waveform of the frequency converter output without dv/dt output filter under 200A load. Actual measured waveform of the frequency converter output with dv/dt output filter under 200A load: POWER ELECTRONICS Power Electronics frequency converters integrate a dV/dt output filter. This filter consists of a coil connected in series at the output, and it performs the function of filtering peak pulses.