Virtual Damping Control Design of Three-Phase Grid-Tied PV Inverters for Passivity Enhancement
Zhiqing Yang, Chirag Ramgopal Shah, Tianxiao Chen, Jakob Teichrib, Rik W. De Doncker
Abstract
The interconnection stability of a grid-tied inverter system can be investigated not only based on classical control theory, e.g., Nyquist criterion, but also from the energy dissipation point of view. If a critical resonance occurs within the regions, where the system equivalent impedance behaves passively, i.e., has only nonnegative real parts, the system stability can be guaranteed. Thus, it is preferred to design systems with passive regions over a wide-frequency range. This article investigates passivity properties influenced by different control loops and operating points in the dq frame, which facilitates the stability assessment in the frequency domain. Based on this, a virtual damping control method is proposed to improve the stability by enhancing the system passivity. By implementing a damping transfer function in the d- and q-axis individually, nonpassive regions can be accordingly reduced over a specific frequency range. Asymmetrical designs can be considered in the d- and q-axis according to corresponding passivity properties, which is impossible to be realized through symmetric physical filter components. The design procedure is elaborated based on a 2 MW inverter system, which is based on a combined graphical and analytical approach. Verifications are conducted based on a 1 kW prototype including both simulations and experimental results. The method applies to all types of grid-tied inverters with multiple control loops. This article is accompanied by a video demonstrating the validations.