Solar PV frequency support via synthetic inertia : technical perspectives

Abstract

The reduction in grid rotational inertia is due to the integration of photovoltaic units in place of synchronous generators. This rotational inertia is responsible for damping the frequency oscillations caused by the grid disturbance. Due to this reduction, the rate of change of frequency (ROCOF) has increased, limiting the time available for corrective action. Response speed is limited by dependency on reserves during primary frequency control (PFC). However, synthetic inertial response (IR) and energy storage systems (ESS) have improved the response speed. This thesis evaluates those strategies, individually and combined, in a 2.01 MW grid-connected PV plant simulated in MATLAB/Simulink. Six scenarios compare PFC alone, PFC+IR, and PFC+IR+ESS at initial operating points of 0.30 pu and 0.85 pu, including a de-loaded mode. The model consists of a boost converter, a DC link, an inverter, a grid model, a lithium-ion battery, and a bidirectional DC/DC converter. The performance is evaluated through reaction time, response time, and settling time. The addition of IR in the PFC reduces the response time by 53.8% and the reaction time by 44.4% at a de-loaded 0.85 pu under under-frequency conditions. Over-frequency tests show a 37% to 38% reduction in response times and a 28% to 31% reduction in reaction times at both operating points (0.3 pu and 0.85 pu). In most cases, settling time increased due to post-disturbance overshoot. Rated battery capacity is limiting the ESS contribution. De-loading is a prerequisite for under-frequency support. The response time is improved by synthetic inertia. However, battery sizing limits the ESS's effectiveness. Future work should examine hybrid storage systems (capacitor-battery), variable irradiance, and hierarchical plant controllers.