A review of microgrid development in the United States – A decade of progress on policies, demonstrations, controls, and software tools

Microgrids have become increasingly popular in the United States. Supported by favorable federal and local policies, microgrid projects can provide greater energy stability and resilience within a project site or community. This paper reviews major federal, state, and utility-level policies driving microgrid development in the United States. Representative U.S. demonstration projects are selected and their technical characteristics and non-technical features are introduced. The paper discusses trends in the technology development of microgrid systems as well as microgrid control methods and interactions within the electricity market. Software tools for microgrid design, planning, and performance analysis are illustrated with each tool's core capability. Finally, the paper summarizes the successes and lessons learned during the recent expansion of the U.S. microgrid industry that may serve as a reference for other countries developing their own microgrid industries

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

Development of a smart transformer to control the power exchange of a microgrid

A smart transformer enables to control the power exchange between a microgrid and the utility network by controlling the voltage at the microgrid side within certain limits. The distributed generation units in the microgrid are equipped with a voltage-based droop control strategy. This controller reacts on the voltage change, making the smart transformer an element that controls power exchange without the need for communication to other elements in the microgrid. To build a smart transformer, several concepts are possible. In a smart transformer with continuous turns ratio, hereafter referred to as continuous smart transformer, the transformer's microgrid-side voltage can be controlled without voltage steps and the accuracy of the voltage control can be very high. The voltage control of a smart transformer with discrete turns ratio, hereafter referred to as discrete smart transformer, is less accurate, as the output voltage is regulated between several discrete values. In this paper, the development of a continuous and discrete smart transformer will be elaborated. Their validity will be proven by implementing these smart transformers in an experimental test setup. Also, some concepts to improve the control accuracy will be proposed