Enhanced Fast Frequency Support Schemes

The increased penetration of large-scale renewable energy sources may result in ac grids with low inertia, where a sudden generation shortfall could lead to a high rate-of-change-of-frequency (RoCoF). Under such conditions, high-voltage direct-current (HVdc) links are expected to provide fast frequency support to contain frequency deviations within statutory limits. This may be achieved by introducing supplementary loops to the conventional controllers of HVdc converters. However, the response time of typical frequency support schemes is limited by the ramp rate and magnitude of the additional active power through the converters. To overcome these limitations, a temporary overproduction with droop (TOD) control scheme is presented. TOD provides fast frequency support to disturbed ac grids through the injection of additional active power by HVdc converters. A three-terminal meshed HVdc grid implemented on a hardware-in-the-loop test-rig and modeled in Simulink is used to demonstrate the effectiveness of the scheme. Experimental and simulation results exhibit good agreement.