The structure and function of modern inverter system and its effect on power factor correction

Due to the continuous improvement of performance requirements, especially the increasing call for "green" power supply, modern inverter systems have higher requirements for power factor correction and current harmonic suppression. This article briefly introduces the application of power factor correction in modern inverter power supplies. Several inverter configuration schemes with PFC function are analyzed and compared. The analysis results show that the two-stage inverter with single-stage isolated PFC circuit has higher reliability, higher efficiency and lower cost.
1 Composition and structure of modern inverter power supply system
With the development of control technology in various industries and the improvement of operating performance requirements, the electrical equipment in many industries does not directly use the AC power provided by the general AC power grid as the power source, but transforms it in various forms to obtain the required power form. The modern inverter system is a power supply system that realizes the inverter function through a combination of rectification and inversion circuits. In addition to the rectification circuit and the inversion circuit, the inverter system must also have a control circuit, a protection circuit and an auxiliary circuit. The basic structure of the modern inverter system is shown in Figure 1. Figure 1 Basic structure diagram of inverter system The functions of various parts of modern inverter system are as follows: 1. Rectifier circuit: The rectifier circuit uses rectifier switching devices, such as semiconductor diodes, thyristors (silicon controlled rectifiers) and self-turning off switching devices, to convert AC into DC. In addition, the rectifier circuit should also have the function of suppressing current harmonics and adjusting power factor. 2. Inverter circuit: The function of the inverter circuit is to convert DC into AC, that is, by controlling the operating frequency and output time ratio of the inverter circuit, the frequency and amplitude of the inverter's output voltage or current can be flexibly changed according to people's wishes or the requirements of equipment operation. 3. Control circuit: The function of the control circuit is to generate and adjust a series of control pulses as required to control the conduction and shutdown of the inverter switch tube, so as to cooperate with the inverter main circuit to complete the inverter function. 4. Auxiliary circuit: The function of the auxiliary circuit is to convert the input voltage of the inverter into a DC voltage suitable for the working needs of the control circuit. For AC grid input, power frequency voltage reduction, rectification, linear voltage regulation and other methods can be used, and of course DC-DC converters can also be used. 5. Protection circuit: The functions to be realized by the protection circuit mainly include: input overvoltage and undervoltage protection; output overvoltage and undervoltage protection; overload protection; overcurrent and short circuit protection; overheating protection, etc. 2 Analysis of power factor and harmonic interference problems of inverter power supply system For the rectifier link (AC-DC) of the inverter, the traditional method still uses uncontrolled rectification to convert the AC power provided by the general AC grid into DC through rectification. Although the uncontrolled rectifier circuit is simple and reliable, it will draw high peak current from the grid, causing distortion of both the input current and AC voltage. In other words, the input pre-stage circuit of the voltage-stabilized power supply of a large number of electrical equipment is actually a peak detector. The charging voltage on the high-voltage capacitor filter shortens the conduction angle of the rectifier by three times, and the current pulse becomes a narrow pulse of a non-sinusoidal wave, thus generating a highly distorted harmonic peak interference at the input end of the grid, as shown in Figure 1.2. (a) Distortion of current and voltage at the grid input end (b) Spectrum of each harmonic component in the peak current Figure 2 Grid voltage and current distortion and harmonic interference component diagram at the input end of a traditional rectifier circuit

It can be seen that the application of a large number of rectifier circuits causes the power grid to supply severely distorted non-sinusoidal currents. Fourier analysis of this distorted input current shows that it contains not only fundamental waves, but also abundant high-order harmonic components. These high-order harmonics flow back into the power grid, causing serious harmonic pollution and reducing the power factor at the input end, which will cause huge waste and serious harm. The main hazards of input current harmonics are:
(1) Reduce the efficiency of power generation, transmission and utilization, cause electrical equipment to overheat, generate vibration and noise, and cause insulation aging, shorten service life, and even cause failure or burnout.
(2) It can cause local parallel resonance or series resonance in the power system, amplify the harmonic content, and cause capacitors and other equipment to burn out.
(3) It causes additional harmonic errors in measuring instruments. Conventional measuring instruments are designed and work on sinusoidal voltage and current waveforms, so they can guarantee their accuracy when measuring sinusoidal voltage and current. However, when these instruments are used to measure non-sinusoidal quantities, additional errors will be generated, affecting the measurement accuracy.
(4) Harmonics can also cause relay protection and electric devices to malfunction, causing confusion in power metering.
Modern inverter power supply systems have higher requirements for power factor correction and current harmonic suppression. In order to reduce the noise generated by the harmonics at the input end of the AC-DC circuit and the harmonic pollution generated to the power grid, so as to ensure the power supply quality of the power grid and improve the reliability of the power grid; at the same time, in order to improve the input power factor and achieve energy saving, many countries and international academic organizations have formulated standards and regulations for limiting power system harmonics and power equipment harmonics. For example, the Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC) and the International Conference on Large Electric
Systems (CIGRE) have all introduced their own recommended harmonic standards, the most influential of which are IEEE519-992 and IEC1000-3-2. China has also formulated regulations and national standards for limiting harmonics in 1984 and 1993 respectively. Therefore, in modern inverter power supply systems, power factor correction circuits are an indispensable and important part. Power factor correction can be divided into passive power factor correction technology (Passive PFC) and active power factor correction technology (Active PFC). Passive power factor correction technology uses passive devices, such as resonant filters composed of inductors and capacitors, to achieve the PFC function; active power factor correction technology uses active devices, such as switch tubes and control circuits to achieve the PFC function. Modern inverter power supply systems mostly use active power factor correction technology, which can correct the input current into a sine wave in phase with the input voltage and increase the power factor to close to 1.
3 Inverter structure scheme with PFC function
There are usually three types of inverter structure schemes with power factor correction function: three-level structure scheme I, three-level structure scheme II and two-level structure scheme.
1. Three-level structure scheme I. Its structure is shown in Figure 3. The first stage is a 50Hz power frequency transformer, which is used to achieve electrical isolation function, thereby ensuring the safety of power supply equipment and avoiding dangers from high-voltage feeders. The second stage is a power factor correction circuit, which is used to force the line current to follow the line voltage, make the line current sinusoidal, improve the power factor, and reduce the harmonic content. Its output is a high-voltage DC of about 400V. The third level is the DC-AC module, which is used to realize the inverter function, that is, by controlling the working frequency and output time ratio of the inverter circuit, the frequency and amplitude of the inverter's output voltage or current can be flexibly changed according to people's wishes or the requirements of equipment operation. Figure 3 Main circuit diagram of three-level structure scheme I

This is an earlier adopted solution, and the technology is relatively mature. Its main advantages are simple circuit structure and easy implementation. The main disadvantages are that the power is converted through three stages, which reduces the reliability and efficiency of the inverter; the power frequency isolation transformer is large, heavy, and consumes a lot of materials; the output of the PFC stage, that is, the input of the DC-AC is a high-voltage direct current of about 400V, which limits many applications that require the inverter stage to have a low-voltage input. For example, many important inverter application fields such as railway inverters and aviation inverters require 110V sinusoidal alternating current output. If this configuration scheme is adopted, not only is the reliability difficult to guarantee, but the efficiency of the inverter will be further reduced, generally not exceeding 80%.
2. Three-stage configuration scheme II. Its structure is shown in Figure 4. The first stage is the PFC stage, and its structure and function are the same as the PFC circuit in the three-stage configuration scheme I. The second stage is the DC-DC stage, which is used to adjust the PFC output voltage and achieve electrical isolation. The third stage is the DC-AC module, and its structure and function are the same as the DC-AC circuit in the three-stage configuration scheme I. This is a solution that is currently used more frequently and is the best choice for medium and large power applications. Figure 4 Main circuit diagram of three-stage configuration scheme II The main advantage of this solution is that the bulky power frequency transformer is removed; each stage has its own control link, which makes the circuit have good performance; the DC-AC input voltage can be adjusted according to the different requirements of the inverter output, which is suitable for various power occasions and has improved efficiency compared with the three-stage configuration scheme I. The disadvantage is that each stage requires a set of independent control circuits, which increases the number of devices and the complexity of the control circuit; because the electric energy also undergoes three-stage conversion, the reliability and efficiency of the inverter are still unsatisfactory. 3. Two-stage configuration scheme. In view of the shortcomings of the above two schemes, a two-stage configuration scheme has been proposed. This scheme combines the first two stages in the three-stage configuration scheme II into one stage, so that the PFC and DC-DC stages share the switch tube and control circuit (as shown in Figure 5), and obtains an adjustable PFC output DC voltage through a high-frequency transformer to achieve electrical isolation, as shown in Figure 5. This scheme maintains the advantages of the three-stage configuration scheme II and improves the shortcomings of the three-stage configuration scheme II. In short, high reliability, high efficiency and low cost are the most significant advantages of this inverter configuration scheme. Figure 5 Typical single-stage PFC converter circuit diagram Figure 6 Main circuit diagram of two-stage inverter configuration scheme 4 Conclusion After comparing the configuration schemes of these three inverters, it is not difficult to find that their inverter parts have exactly the same structure and function. The only difference is the rectification link, that is, different methods are used to generate (adjustable) DC voltage after isolation and power factor correction as the input of the inverter stage. Since the single-stage PFC circuit combines the PFC stage and the DC-DC stage together, the energy is only processed once, and the input PFC and output voltage regulation functions can be completed with one controller, so it is very suitable for the front-stage rectification link of the inverter power supply. The inverter using a single-stage PFC circuit has higher reliability, higher efficiency and lower cost. Therefore, the two-stage inverter technology with a single-stage PFC circuit has become a hot topic in the field of power electronics. Although the single-stage PFC circuit has the above advantages, it has to withstand higher voltage stress and more power loss than the traditional two-stage PFC converter. These problems are particularly prominent when the switching frequency is high, affecting the reliability of the converter and the further increase of the switching frequency, and also limiting its application in high-power occasions. To this end, various soft switching technologies have been proposed in recent years, such as zero current switching (ZCS), zero voltage switching (ZVS), zero voltage conversion-pulse width modulation (ZVT-PWM), zero current conversion-pulse width modulation (ZCT-PWM), etc., which effectively solve these problems and make the single-stage PFC circuit have a broader application prospect in the inverter power supply system.