A Hybrid Active Damping Strategy for Improving the Adaptability of LCL Converter in Weak Grid

Abstract

In a weak grid, the line impedance variation will cause the resonant frequency of the LCL filter to shift towards lower frequencies, thus reducing the quality of the grid-connected current and affecting the power grid stability. To solve this problem, a hybrid active damping strategy with feedforward compensation is proposed for the neutral point clamped (NPC) LCL grid-connected inverter system. In order to reshape the output conductance of the grid-connected system, suppress the resonance spikes of the LCL filter and improve the adaptability of the grid-connected system to the weak grid. A first-order low-pass filter is designed in the grid-connected current loop, and an active damping control of grid-connected current based on a first-order high-pass filter is also proposed. Compared with the conventional capacitive current active damping, no additional sensors are required, and the use of a differential is avoided, which reduces the high-frequency noise. The use of passive resistors is reduced, which reduces the power loss of the grid-connected system. In addition, a point of common coupling (PCC) voltage feedforward strategy based on a low-pass filter is designed to suppress the background higher harmonics at PCC and improve the quality of grid-connected current. In this work, the robustness of the system is analyzed when the parameters of the LCL filter change. Finally, the virtual space vector modulation strategy is used to balance the neutral voltage of the DC bus. Simulation and experimental results show that the control strategy can effectively improve the adaptability of the system to the weak power grid, improve the quality of grid-connected current, and demonstrate strong stability. The THD can be decreased by 0.2% at least, and the improvements are more significant with larger line impedance; the THD is only 2.33% even at 10 mH line impedance.

SMC Algorithms in T-Type Bidirectional Power Grid Converter

“…Furthermore, ref. [5] demonstrated an NPC (Neutral Point Clamped) inverter with an LCL filter. The results indicated positive network current control quality, stability, and high-quality signal reception.…”

“…Figure 1 presents a simplified diagram of a bidirectional inverter in T topology, which cooperates with a DC voltage source through a capacitive filter. This source can, for example, constitute energy storage in a so-called microgrid with renewable energy devices [1][2][3][4][5][6][7].…”

“…The increasing number of different devices connected to the power grid means that the variability of power grid load is increasing. The number of so-called microgrids, i.e., local electrical grids, often equipped with energy storage and renewable energy sources such as photovoltaic panels [1][2][3][4][5][6][7], is growing. These structures mainly cooperate with the power grid, but they can also work in an island mode, i.e., disconnected from the main power grid [1][2][3][4][8][9][10][11].…”

“…In this work, a model of a structure, which includes a T-type three-level inverter connected to the power grid through an LCL filter [4][5][6][7][17][18][19][20], was proposed. In the literature, it is possible to find several examples of implementation of various three-phase inverter control algorithms that ensure improvement of the quality of transmitted energy.…”