Control of Grid-Interactive Inverters as Used in Small Distributed Generators

Small distributed generation (DG) systems provide standby service during utility outages and, when operated during peak load hours, potentially reduce energy costs. The connection of a generation system to the utility grid requires a robust phase locked loop (PLL) and continuous detection of utility grid status (such as outage, over-/under-voltage and over-/under- frequency). This paper details a flexible control strategy to operate a small/micro single-phase DG in both stand-alone and grid-connected modes. The described system is effectively an extended line-interactive uninterruptible power supply (UPS). In particular, a high-performance PLL for single-phase inverters is proposed. The PLL uses a transport delay method to generate a virtual quadrature signal, thereby emulating a balanced three- phase system. Furthermore, an orthogonal filter is incorporated to improve the PLL performance when the grid voltage is distorted. To achieve zero steady-state error and to implement selective harmonic compensation, resonant controllers are proposed, for both the voltage regulation in stand-alone control mode and the current regulation in grid-connected control mode. Simulation and experimental results from a 3.5 kW prototype with a fuel cell as the primary source are included to prove the effectiveness of the proposed methods.