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　　Distributed inverter power supply parallel control technology and implementation

　　The parallel connection technology of inverter power supply is an important technical means to improve the reliability of inverter power supply and expand the power supply capacity. The current development trend of large-capacity inverter power supplies is to use new fully controlled high-frequency switching devices to form inverter power module units, and then expand the capacity by connecting multiple modules in parallel. This can improve the versatility and flexibility of the inverter power module, make system design, installation, and combination more convenient, while increasing the redundancy and reliability of the system. The parallel connection between AC power supplies is far more complicated than the parallel operation of DC power supplies. Due to its sine wave output, the parallel connection of inverter power supplies needs to meet five conditions, namely the same voltage, frequency, waveform, phase and phase sequence. Only in this way can the circulating current, Share the load power evenly to achieve the best operating status and truly realize parallel connection of inverter power supplies.

　　At present, inverter power supply parallel control methods are generally divided into five control strategies: centralized control, master-slave control, distributed control, 3C control and independent control without interconnection lines.

　　Among the various existing control methods, centralized control and master-slave control have certain applications in practical applications. However, since the failure of the parallel control circuit may cause the entire system to shut down, the application is subject to certain restrictions. 3C control is actually an improvement on distributed control, and there is a certain gap between interconnection-free control and practical applications, so distributed control has certain advantages.

　　Distributed control parallel control strategy

　　1. The concept of distributed control

　　Distributed control technology, also known as decentralized logic control, decentralizes and makes the control rights of each central link of the system independent, allowing each unit in the system to work independently. This control method can achieve true redundant parallel connection. When one module fails and exits, it will not affect the parallel operation of other modules; its good features such as high reliability, dispersed risks, and easy function expansion have been found in many fields. It has been widely used; it has become one of the main directions for the development of computer systems and is a relatively complete distributed intelligent control technology.

　　2. Control principle of distributed parallel connection

　　In the parallel control strategy of inverter power supply, both centralized control and master-slave control may cause the entire system to shut down due to partial circuit failure. The distributed parallel control strategy can solve this problem. The current and frequency signals of each power module are integrated in each inverter power supply to obtain the compensation signals of respective frequencies and voltages. This method can achieve a true N+1 operating mode.

　　The parallel control unit of each module detects the mains frequency and phase of the corresponding module and sends synchronization pulses to other power modules. When there is no mains power, the synchronization pulse is generated by a crystal oscillator. The phase-locked loop circuit of each inverter power supply is used to ensure its output voltage. The frequency and phase are synchronized with the synchronization bus pulse signal. The parallel control unit compares the load current of other module units with the load current of this unit, calculates the current deviation, and sends it to each inverter power supply unit as the compensation amount of the voltage command to eliminate the imbalance of the output current of each module. The current sharing and synchronization principle of the control integrated part is shown in Figure 1.

　　3. Parallel system control scheme

　　In the literature seen so far, distributed control parallel schemes mainly include active and reactive power control and average current instantaneous control schemes.

　　3.1. Active and reactive power control scheme

　　Active and reactive power are controlled in parallel, that is, power deviation is controlled in parallel. Through each inverter power supply unit, the deviation value of the active and reactive power output by the unit is detected to adjust the phase and amplitude of the output voltage of the inverter power supply unit to ensure that the active and reactive power output by each inverter power supply unit are equal. The purpose of current sharing.

　　Take two inverter power modules supplying power to the same load as an example for a simple analysis, as shown in Figure 2, where X is the line impedance and U0 is the parallel grid voltage.

　　It can be seen from equations (5) and (6) that the output active power of the inverter power supply mainly depends on the power angle, while the output reactive power mainly depends on the amplitude of the output voltage. Therefore, the inverter power supply can be changed by The output voltage amplitude is used to control the reactive power, and the phase is controlled to control the active power, thereby achieving current sharing of each output power module.

　　Through the above analysis, the characteristics of this control strategy can be concluded:

　　(1) Use three parallel control lines: active power line, reactive power line, frequency line;

　　(2) The status of each module is consistent, enabling true distributed redundant control;

　　(3) The parallel control line is a DC signal and has strong anti-interference ability;

　　(4) It belongs to the average control method and has poor dynamic response;

　　(5) The amount of calculation of active and reactive power is large.

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