The full control system of a grid-connected current-controlled voltage-source inverter (CC-VSI) has been designed and implemented on a field-programmable gate array (FPGA). Various control functions and implementation methods are described and discussed. The practical viability of the system is evaluated in an experimental setup, where a VSI supplies 30？kW into the local grid at 400？V. A phase-locked loop (PLL) is used for grid phase tracking and evaluated for simulated abnormal grid conditions. Power factor is kept at unity, and the implemented control system is stressed with step responses in the supplied active power. A moving-average filter is implemented to reduce the effects of noise and harmonics on the current control loops. A coupling between active and reactive power flow is observed for the step responses but may be ignored in this context. The proposed system is fully comparable with more conventional microprocessor-based control systems. 1. Introduction The voltage source inverter (VSI) has become increasingly used for grid connection of renewable energy sources, such as wind power turbines or solar cells. Improved ratings in semiconductor components like the insulated-gate bipolar transistor (IGBT) are gaining market for more and more powerful applications where only the thyristor-controlled current-source converter has been used earlier. The VSI is dominating the market of distributed generation and may be controlled by either voltage or current feedback. Current-control (CC) is favoured due to its excellent dynamic characteristics and its inherent overcurrent protection. There are three major groups of current-control modulations for VSIs, namely, hysteresis control [1], predictive current control [2], and ramp comparison [3]. A survey on current-control methods is presented in [4]. Depending on the control strategy, the power converter control system has traditionally been designed with microcontrollers or complex programmable logic devices (CPLDs). However, increased complexity and more real-time data analysis have called for the development of digital signal processors (DSPs). The DSPs have very good programmability and can easily manage conditional code. Due to their ability to work with floating point arithmetics, they can handle complex mathematical functions well. Alternatively, the application-specific integrated circuit (ASIC) may be used, where all the functions are designed into a fixed integrated circuit. The ASIC has to implement all functions in gates and very complex conditional programs may result in very poor gate
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