Control strategy and simulation research of distributed power grid-connected inverter

　　Distributed power generation has a growing market share due to its many advantages such as local power generation to serve users, cleanliness and environmental protection. The development of microelectronics technology has created a platform for the practical application of inverter technology. The birth of microprocessors has met the development requirements of inverter technology, enabling advanced control technologies such as vector control technology, multi-level conversion technology, repetitive control, and fuzzy technology. Advanced control algorithms such as logic control have been well applied in the inverter field. Entering the 21st century, inverter technology is developing towards higher frequency, greater power, higher efficiency, and smaller size. This design adopts a DC-DC-AC structure that can effectively improve efficiency. It uses high-frequency DC boost technology to make the inverter grid-connected device smaller and its safety performance greatly improved. Aiming at the experimental problems of dynamic systems, it is proposed to use the parameter estimation function of Simulink to enable the theoretical model to estimate numerical parameters based on experimental data, so that the theoretical model is fully close to the actual experimental environment.

　　1 Design of distributed power grid-connected inverter system

　　1.1DC-DC converter

　　The DC-DC converter is a circuit that first changes the DC voltage into an AC voltage through the switching action of the semiconductor valve device, and then changes it into a DC voltage with different polarity and voltage value after rectification. What is going to be explained here is the coupling through the transformer in the middle. DC indirect conversion circuit. The frequency of the DC-DC converter can be selected arbitrarily when converting DC voltage into AC voltage. Therefore, using a high-frequency transformer can reduce the size and weight of magnetic components such as transformers and inductors and smoothing capacitors. Nowadays, with the advancement of semiconductor valve devices, power supplies with an output power of more than 100W actually use switching frequencies in the range of 20 to 500kHz, and MHz-level high-frequency converters are also under development and research. Furthermore, by increasing the conversion frequency, the capacitance of the smoothing capacitor can be reduced, allowing the use of highly reliable components such as ceramic capacitors. Moreover, this article illustrates the principle of operation by using examples such as bipolar power transistors, IGBTs, MOSFETs and other controllable on-off devices as semiconductor valve devices that convert DC voltage into AC voltage. The most commonly used ones are MOSFETs.

　　1.2 DC bus voltage PID controller design

　　As the DC bus voltage of 400V must have a certain degree of stability, it should not fluctuate with changes in load or battery voltage. Therefore, the concept of feedback must be used. The elements of feedback theory include three components: measurement, comparison, and implementation. The variable of interest is measured, compared with the expected value, and this error correction is used to adjust the response of the control system. Since the PID controller can achieve error-free regulation, has excellent dynamic steady-state characteristics, and is convenient and flexible in parameter tuning methods, the PID control algorithm is selected for the voltage control of the DC bus in the inverter grid-connector.

　　In the closed-loop control system, the regulator is placed under pure proportional action, and the proportional coefficient of the regulator is gradually changed from small to large to obtain a transition process of constant amplitude oscillation. The proportional coefficient at this time is called the critical proportional coefficient Ku, and the time interval between two adjacent wave peaks is called the critical oscillation period Tu.

　　Critical proportion method steps:

　　(1) Set the integral time of the regulator to the maximum (TI=∞), set the differential time to zero (TD=0), set the proportional coefficient KP appropriately, operate the balance for a period of time, and put the system into automatic operation.

　　(2) Gradually increase the proportional coefficient KP to obtain a constant amplitude oscillation process, and record the critical proportional coefficient Ku and critical oscillation period Tu values.

　　(3) Based on the Ku and Tu values, use empirical formulas to calculate the values of various parameters of the regulator, namely KP, TI and TD.

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