Design of photovoltaic grid-connected inverter based on LCL filter

1 Introduction

In recent years, renewable energy power generation has risen rapidly, and photovoltaic and wind power grid connection has become an important research direction. Inverters, as core components of grid connection, have become a new hot spot in the field of power electronics research.

In order to suppress the high-frequency harmonics generated by the high-frequency power switches in the inverter link, the grid-connected system introduces an LCL filter, which can eliminate the high-order harmonic components injected into the power grid. However, it has its shortcomings. As a third-order system, it is easy to produce resonance spikes, cause oscillations, and even affect the stability of the entire system. Therefore, it is necessary to improve the control strategy of the system, effectively suppress its resonance problem, and improve the power quality of photovoltaic grid-connected.

At present, domestic and foreign scholars have proposed many control strategies for LCL filters, which can be mainly divided into three types: current-based control strategy, virtual synchronous motor control strategy and direct power control strategy (DPC). This paper mainly optimizes the control strategy based on the research and analysis of the principle of direct power control strategy (DPC) and proposes an improved quasi-DPC control strategy, which not only retains the advantage of LCL filters in the ability to quickly reduce harmonics in the high-frequency band, but also fundamentally solves the resonance problem.

2 Direct Power Control Strategy

Compared with current-based and virtual synchronous motor control strategies, direct power control has many advantages such as higher power factor, lower THD (total harmonic distortion), excellent dynamic performance and simple structure. Since Tokuo Onhishi proposed applying direct power control to converter control in 1991, direct power control of inverters has received widespread attention from scholars at home and abroad.

Direct power control generally uses the switch vector table query method (L-DPC). The switch vector table is the core of system control, so studying the switch vector table is an important direction for studying direct power control. Some scholars have also proposed different space division and optimization switch table design methods. This improvement on the switch table greatly improves the system dynamic performance of direct power control. However, the L-DPC switching frequency is not fixed, which is not conducive to the design of the output filter.

To solve the above problems, people proposed constant frequency direct power control. Several methods based on constant frequency direct power control not only take into account the dynamics of power regulation, but also fix the switching frequency. However, this method cannot achieve direct control of grid-connected current. Later, someone proposed a direct power control method based on virtual flux linkage and active damping control, but the two control algorithms are difficult to implement and more complicated to control.

3 Control strategy of quasi-DPC photovoltaic grid-connected inverter

To this end, this paper proposes a control strategy for photovoltaic grid-connected inverters based on quasi-direct power control (DPC). The inner loop is controlled by the current loop to directly control the grid-connected current. The power loop is used as the outer loop to directly control the output power of the grid-connected inverter. In this way, the control performance of the system has both the advantages of current control and the excellent performance of direct power control. The control system includes the control link of the DC bus, the active power damping link, the link for controlling the power, and the current control link.

The structure of the system is shown in Figure 1.

According to the instantaneous power theory, the instantaneous active power and reactive power of the inverter grid-connected side are calculated. The grid-side complex power of the photovoltaic grid-connected inverter can be defined as:

In the above formula, us and is are the space vectors of the grid voltage and the grid-connected current respectively, and usα, usβ, isα, and isβ are the components of the grid voltage and current in the coordinate system respectively.

Compare the instantaneous active power p and reactive power q above with the set power reference values ​​p* and q*, and subtract the active damping work pd and qd to get the power error, and then send the error signal to the fuzzy power controller for analysis and adjustment, and then get the reference signal of the grid-connected current, and compare it with the actual detected grid-connected current signal, after repeated adjustment and transformation of the PR current controller, use SPWM modulation technology to control the opening and closing of the power switch tube of the inverter. The active power reference value p* of the system is obtained by multiplying the output of the DC side voltage controlled by PI regulation and the voltage of the DC bus. In order to achieve the operation of the unit power factor on the grid side, the reactive power reference value q* is set to 0.

3.1 Power Controller Design

Because the power output of the photovoltaic power generation system changes with the conditions of light, temperature, etc., it is difficult to achieve the ideal control effect by using conventional control of the power controller. Therefore, a fuzzy-based power loop control strategy is proposed. The fuzzy controller uses the fuzzy reasoning mechanism to adjust, optimize and control the controller parameters online.

3.2 Current Controller Design

The outer loop of the current controller is performed using repetitive PR control, which is used to control the current of the grid-connected system. The role of PR control is to enhance the control ability of the grid-connected current in the first cycle and improve the stability and dynamic performance of the system's repetitive control.

The improvement effect of repetitive PR control on the harmonic suppression characteristics of the system is basically not affected by parameters. As long as the compensation function of the repetitive control is designed reasonably, better harmonic suppression characteristics can be obtained.

4 Matlab\Simulink simulation verification and analysis of quasi-DPC control strategy system

The simulation results and analysis are shown in Figure 2 and Figure 3:

It can be seen from the above simulation results that the system has a faster dynamic response, and the grid-connected current can be stably controlled within the first cycle. When the system is stable, the sinusoidal waveform of the grid-connected phase current is very good, and the total harmonic distortion rate of the grid-connected current also meets the grid-connected requirements, and the current spectrum distribution is also relatively concentrated.

The relationship between the A-phase grid-connected current and voltage is shown in Figure 4:

As can be seen from Figure 4, the grid-connected current has the same frequency and phase as the corresponding grid voltage, achieving grid-connected operation of the photovoltaic inverter with unity power factor.

5 Conclusion

Based on the analysis of the principle of direct power control strategy, this paper proposes a design scheme of photovoltaic grid-connected inverter based on LCL filter. The outer power loop of the control system designed in this scheme adopts fuzzy control strategy, and the inner current loop adopts repetitive control strategy. This control method can take into account the advantages of current control and direct power control, which not only ensures the sinusoidality of grid-connected current waveform but also improves the dynamic performance of the system. The simulation test shows that the inverter effectively suppresses the resonance problem of LCL filter and improves the control ability of photovoltaic grid-connected voltage and current.