Three-phase observer-based low distortion grid current controller using an LCL output filter

In literature on shunt active power filters [1-4] and grid current controllers [5-6], much effort is done to achieve low current distortions. Some topologies incorporate only an inductor between utility grid and inverter, and this may not suffice to meet regulatory standards on harmonics emission, like IEEE 519 or IEC 61000-3-2 [7]. To reduce the effect of PWM harmonics, more complex output filters are used, such as LC or LCL topologies [8-9]. These enhanced output filters allow lower switching frequencies. In this paper, a three-phase application is built and tested. After the hardware design of the output stage for a prototype PWM inverter, the control loop is explained briefly. It consists of a Kalman observer to monitor the three-phase grid voltage (including distortion and unbalance), and the LCL output stage. The 20-state observer vector is used for state feedback. To control the DC link voltage, a fast control loop is designed, for operation as an Active Front End (AFE), or 'active rectifier', or to provide reactive power, and secondly, a slow controller is treated, which is useful for harmonic grid currents injection. Finally, many measurements show that this observer/controller operates very well (ITHD < 1%), even under severely unbalanced three-phase voltage systems (VTHD=3%).