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Frequency control improvement in an autonomous power system: An application of virtual synchronous machines

This work addressed the problem of frequency control improvement in a 3⊘/400V/50Hz autonomous power system, consisting of two diesel generators of 26kW nominal power, a fixed-speed wind generator of 20kW nominal power, two resistive loads of 20kW each, and an energy storage system consisting of a voltage source converter (VSC) with a 1000V dc-bus voltage. The model of the system was developed in Simulink-SimPowerSystems library. Simulations were conducted for a wind speed profile of 20s and considering the connection of the wind generator, and connection/disconnection of one resistive load. Frequency excursions of 0.1pu were observed in simulations when the system operates with only one genset. To support frequency control, a virtual synchronous machine (VSM) was proposed as a solution. To implement the VSM, the line currents of the VSC were controlled by the technique of input-output linearization. The reference for the q-axis was set to zero and the reference for the d-axis current was used to implement the VSM. For the testing conditions mentioned before, the VSM was capable of emulate the inertial response and the prime mover of the disconnected genset. It is also possible to have a better performance since parameters like the inertia of the VSM can be easily adjusted.