Hybrid Energy Storage System Micro gridsIntegration for Power Quality Improvement UsingFour-Leg Three-Level NPC Inverter and Second-Order Sliding Mode Control

Abstract

Rising demand for distributed generation based on renewable energy sources (RES) has led to several issues in the
operation of utility grids. The micro-grid is a promising solution to solve these problems. A dedicated energy
storage system could contribute to a better integration of RES into the micro-grid by smoothing the renewable
resource’s intermittency, improving the quality of the injected power and enabling additional services like voltage
and frequency regulation. However, due to energy/power technological limitations, it is often necessary to use
hybrid energy storage systems (HESS). In this paper, a second-order sliding mode controller is proposed for the
power flow control of a HESS, using a four-leg three-level neutral-point-clamped (4-Leg 3L-NPC) inverter as the
only interface between the RES/HESS and the micro-grid. A 3-D space vector modulation and a sequencedecomposition-based ac-side control allow the inverter to work in unbalanced load conditions while maintaining a
balanced ac voltage at the point of common coupling. DC current harmonics caused by unbalanced load and the
NPC floating middle point voltage, together with the power division limits, are carefully addressed in this paper.
The effectiveness of the proposed technique for the HESS power flow control is compared to a classical PI control
scheme and is proven through simulations using a 4-Leg 3L-NPC inverter.