Contribution of a smart transformer in the local primary control of a microgrid

In order to enable an easy participation of microgrids in the electricity markets, the smart transformer (ST) concept has been developed. The ST controls the power exchange between a microgrid and the utility network by only controlling its microgrid side voltage, instead of the conventional arrangement where new set points are communicated to all microgrid elements. When the voltage-based droop (VBD) control is implemented in the DG units, loads and storage elements, all microgrid units automatically respond to this change of microgrid voltage by altering their power output or consumption. However, this reference value of power exchange is dependent on (day-ahead) predictions of both consumption and (renewable) power generation. Hence, when these predictions prove to be inaccurate, the ST will still control the power exchange, but with consequently large variations of the microgrid voltage from its nominal value. It is suggested to take the real-time microgrid voltage into account when determining the reference power of the ST. This is presented in this paper by extending the ST´s control strategy with a VBD control, such that the ST can contribute in the primary control. Simulations are included to analyze this primary control of the ST combined with VBD control of the other microgrid elements.