Abstract
The impedance interactions between weak grid and LCL-filtered converter can bring severe harmonic resonance. Recently, frequency domain passivity theory has been applied to converter damping design. However, it is usually not easy to achieve full frequency passivity. What’s more, traditional passivity based damping design is not enough for system robustness. This paper first proposes a relaxed passivity based active damping design for capacitor current feedback. Then the system stability under grid impedance variation is also analyzed. It has been found that the wide variation of grid impedance can decrease system stability margin. As a result, critical oscillation can even happen in the passive region of converter output admittance, causing power quality problems. In order to further improve system stability, this paper proposes an enhanced passivity based impedance shaping method, which is based on hybrid damping and combined active damping. Effects of damping methods and damping parameters are also investigated. A step by step damping design procedure is formulated, which is easy to implement. Simulation and experimental results validate the effectiveness of proposed method.
Highlights
With the development of renewable energy system (RES), voltage source converters have been greatly accepted as the connection between RES and traditional power grid [1]–[5]
This paper mainly proposes an enhanced step by step damping method to ensure system stability margin under grid impedance variation
Kf to improve stability margin (3) Add high pass filter in capacitor current feedback to eliminate the additional non-passive region bought by point of common coupling (PCC) voltage feedforward
Summary
With the development of renewable energy system (RES), voltage source converters have been greatly accepted as the connection between RES and traditional power grid [1]–[5]. H. Liu et al.: Passivity Based Damping Design for Grid-Connected Converter With Improved Stability method does not need to build high order matrix. In order to simplify the system model, the PLL bandwidth is designed much lower than the current loop bandwidth and filter resonance frequency, its effect on system stability can be ignored [5], [27], [28]. For the PI current controller in dq frame, if the grid connected converter works in the high power factor, the system can be considered as decoupled SISO systems [29]. If the intersection point of converter and grid admittance is located in the non-passive region (shown as Fig. 7), the phase difference would beyond 180◦. Impedance interaction between converter and grid makes the whole system unstable
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