Grid current regulation of a three-phase voltage source inverter with an LCL input filter

TLDR

LCL filters can reduce harmonic distortion at lower switching frequencies and with less inductance, but they require more complex control strategies and are rarely addressed in the literature. The study proposes a robust strategy to regulate the grid current from a three‑phase voltage source inverter connected through an LCL filter. The control scheme employs an outer‑loop grid‑current regulator and an inner‑loop capacitor‑current regulator, with a synchronous‑frame PI controller for the outer loop, and derives harmonic‑impedance expressions to analyze voltage‑distortion effects. Linear analysis, simulation, and experimental tests confirm the controller’s stability over a wide range of operating conditions.

Abstract

Many grid connected power electronic systems, such as STATCOMs, UPFCs, and distributed generation system interfaces, use a voltage source inverter (VSI) connected to the supply network through a filter. This filter, typically a series inductance, acts to reduce the switching harmonics entering the distribution network. An alternative filter is a LCL network, which can achieve reduced levels of harmonic distortion at lower switching frequencies and with less inductance, and therefore has potential benefits for higher power applications. However, systems incorporating LCL filters require more complex control strategies and are not commonly presented in literature. This paper proposes a robust strategy for regulating the grid current entering a distribution network from a three-phase VSI system connected via a LCL filter. The strategy integrates an outer loop grid current regulator with inner capacitor current regulation to stabilize the system. A synchronous frame PI current regulation strategy is used for the outer grid current control loop. Linear analysis, simulation, and experimental results are used to verify the stability of the control algorithm across a range of operating conditions. Finally, expressions for "harmonic impedance" of the system are derived to study the effects of supply voltage distortion on the harmonic performance of the system.