DQ impedance-decoupled network model-based stability analysis of offshore wind power plant under weak grid conditions

Instability phenomena of an offshore wind power plant (OWPP) may occur in a wide frequency range, due to impedance interactions between control loops of grid-connected inverters (GCIs) and long transmission cable (LTC) networks. This paper presents a dq impedance-decoupled network modelling method of OWPP for stability analysis. DQ impedance frequency responses of GCIs and LTCs are first measured by frequency scanning method, which are fitted as transfer function matrices based on matrix fitting algorithm. Then, the GCIs are modelled as Norton equivalent circuits. In addition, per-unit-length electrical parameters, e.g., resistance, inductance and capacitance, of the LTCs are extracted from the measured dq impedance frequency responses of a specific LTC, based on which dq impedance-decoupled two-port network models of the LTCs are established. DQ impedance-decoupled network model of the whole OWPP is then established based on connection relationships of these GCIs and LTCs. Finally, generalized Nyquist criterion (GNC) is performed in all the dq impedance-decoupled subsystems, and the subsystems where the GNC is not satisfied are identified as instability sources. Compared with conventional impedance-based stability analysis methods of OWPP, the proposed dq impedance-decoupled modelling method is able to facilitate the application of GNC and further instability source identification based on partitioning the whole OWPP into several decoupled subsystems.The effectiveness of the dq impedance-decoupled network modelling method for stability analysis is validated in a typical type-IV permanent magnet synchronous generator-based OWPP under weak grid conditions based on time-domain simulation results in Matlab/Simulink platform and real-time verification results in OPAL-RT platform.