Wind power grid-connected inverter control method based on proportional complex integral regulator

In order to omit the complex control method and cost in phase lock loop (PLL) in existing renewable energy grid-connected inverters, a double closed loop control method based on proportional complex integral (PCI) control for power source input grid-connected inverter is proposed in this paper. According to the topology of the grid-connected inverter, the control strategy is determined, which adopts the PCI control as inner current control loop and the voltage control with power source as outer control loop. The controller design process is obtained by deriving the mathematic model of this inverter under αβ stationary frame reference. At first, the PCI inner current loop is analyzed and calculated to obtain the fast response and wide bandwidth. The gain of the open loop transfer function after compensation is infinite at grid frequency (50 Hz) to realize the zero static error of grid current to keep the system stable. Besides, the bandwidth is chosen as 700 Hz according to the switching frequency to satisfy the system requirement, which can effectively improve the steady-state and dynamic performances of inner current loop. Secondly, the direct current (DC) voltage outer loop is adopted, which contains a compensation term, a current close loop and a transfer function from active power to the square of DC voltage. The compensation term is determined by considering the feature of this system. In order to eliminate the phase delay of PCI inner current loop, the crossing frequency is set to a lower value. By designing this compensation term, the stability of DC voltage in power source system and the reliable operation of the inverter system for grid-connection are realized respectively. A simulation model based on Matlab/Simulink is built to examine the correctness of the control algorithm. The input source is the power source. And the output of the inverter is connected directly to the grid with three-phase inductance. The transient time at start is approximately 0.1 s, and the total harmonic distortion (THD) of grid-side current is about 2.04%. When the input power increases to the rated power, the response time of current is 0.2 s and the THD is about 1.18%. The simulated results show the availability of this proposed method for the control of the grid-connected inverter. On the basis of the theory and the simulation, the digital controller is implemented by using TMS320F2812 digital signal processing (DSP) and combined with variable speed wind power system. At start, the response of the output current is quick to be stable with 4 cycles. And the overshoot is lower and decreases the shock to power system. When the input power changes, the current loop response is fast to be stable with 2 cycles and hardly overshoots. Meanwhile, the DC outer loop realizes the stability of voltage with 0.2 s, obtaining the good steady-state and dynamic performance. The results also show that the inverter can satisfy the requirement of grid to keep the power factor above 0.97 and the current harmonic about 2.13%. The simulation and the experiment results illustrate that the proposed PCI control method for power source input inverter is available and effective.