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The amount of electricity generation from renewable energy resources (RES) has been increasing significantly all over the globe. However, traditional power grid management is challenged when a large amount of intermittent and unpredictable RES-based generation units are integrated into the power network. This can lead to more severe grid frequency fluctuation events. In this paper, a variable speed drive (VSD) based motor load is utilized as a frequency responsive load to support grid frequency stability. A primary frequency control scheme is proposed and applied to the VSD-based motor load, which incorporates the sophisticated rotating speed feedback controller. Additionally, the proposed frequency responsive VSD-based load is modeled and simplified. As a result, a droop-like response can be achieved with multiple VSD load units. The effectiveness of the proposed model and control scheme is evaluated by experimental studies performed in a multi-converter-based hardware testbed (HTB).

In this letter, the phenomenon is found that there are non-minimum phase and “undershoot” behavior in grid-tied VSC, even if the outer loop is not applied. Based on the small-signal model, the mechanism that q-axis-related non-minimum phase behavior reduces VSC's stability and the “undershoot” behavior are analyzed in the letter. It is found that the q-axis current control dynamics of weak grid-tied VSC introduce an open-loop unstable zero when the reactive power is supplied from the AC system, which is the key factor for the non-minimum phase behavior.