Frequency Trajectory Planning Based Strategy for Improving Frequency Stability of Droop-Controlled Inverter Based Standalone Power Systems

摘要

Inverter-based standalone power systems (ISPSs), with limited inertia/damping, are prone to significant changes in frequency indicators encompassing frequency deviation and rate of change of frequency (RoCoF), which can easily exceed the pertinent relay thresholds and cause power outages. Limited by estimating the disturbance and system inertia/damping deficiency, existing controls cannot ensure system frequency stability. To overcome this issue, a frequency trajectory planning (FTP) based strategy is developed in this article to improve frequency stability of droop-controlled ISPS. This frequency-indicator-oriented control relates system frequency with several pre-defined planning parameters, decoupling from system's disturbances and inertia/damping deficiency that are difficult to evaluate. During normal operation or when the disturbance level is insignificant, the inverter operates in standard droop mode since the system intrinsic inertia/damping features suffice to regulate the frequency. In the event of a large disturbance, an FTP block is triggered by detected frequency indicators, and a marginally safe frequency trajectory is planned according to the requirement of the grid code. In this case, by tracking the planned frequency trajectory, the inverter provides suitable inertia/damping support needed by the system, thereby guaranteeing frequency stability of the ISPS. Finally, simulation and experimental results proved the effectiveness and advancement of the FTP based strategy.