Voltage control and grid synchronization of microgrids in real time

A system for synchronization of a utility grid and a microgrid, comprising:

• a utility grid including a grid side controller, wherein the utility grid is configured to operate at a utility grid voltage, a utility grid frequency and a utility grid phase angle;
• a grid side converter configured to transmit the utility grid voltage, the utility grid frequency and the utility grid phase angle to the microgrid;
• a diesel generator located in the microgrid in parallel with the utility grid;
• a diesel generator controller connected to the diesel generator, wherein the diesel generator controller is configured to calculate a power reference from the utility grid voltage and store the utility grid voltage, the utility grid frequency, and the power reference;
• a wind turbine located in the microgrid in parallel with the utility grid;
• a photovoltaic array located in the microgrid in parallel with the utility grid;
• a point of common coupling switchably connected between the utility grid and the microgrid;
• a synchro check relay connected to the point of common coupling;
• a switched capacitor bank connected between the synchro check relay and the microgrid;
• a plurality of loads connected to the microgrid;
• wherein, when operating in a grid connected mode:
• the microgrid is configured to match a microgrid voltage, a microgrid frequency and a microgrid phase angle to the utility grid voltage, the utility grid frequency and the utility grid phase angle respectfully, generate at least one of a first real power and a first reactive power, and transmit the at least one of a first real power and a reactive power to each of the plurality of loads;
• wherein the diesel generator is configured to operate in a speed droop mode;
• wherein, when operating in an island mode:
• the diesel generator controller is configured to provide the stored utility grid frequency and power reference to the diesel generator;
• the diesel generator is configured to operate at a constant angular speed to generate a second real and reactive power at a diesel generator frequency, which matches the stored power reference and stored utility grid frequency respectively;
• the diesel generator controller is configured to transmit the stored utility grid voltage and the diesel generator frequency as a wind turbine reference voltage and a wind turbine reference frequency respectively to the wind turbine and to transmit the stored utility grid voltage and diesel generator frequency to the photovoltaic array as a photovoltaic array reference voltage and photovoltaic array reference frequency respectively;
• wherein the diesel generator, the wind turbine and the photovoltaic array are configured to generate at least one of a first real power and a reactive power and transmit at least one of the first real power and the reactive power to each of the plurality of loads;
• wherein, when operating in a resynchronization mode:
• the switched capacitor bank is configured to be charged by the first real power produced by the microgrid;
• the diesel generator controller is configured to actuate a plurality of switches of the switched capacitor bank to adjust the reactive power such that the microgrid voltage, the microgrid frequency and the microgrid phase angle match the utility grid voltage, the utility grid frequency and the utility grid phase angle;
• and
• the synchro check relay is configured to actuate the switch to connect the microgrid to the utility grid.

A method for synchronizing a utility grid and a microgrid, comprising:

• when operating the utility grid and the microgrid in a grid connected mode:
• controlling the microgrid at a utility grid voltage, a utility grid frequency and a utility grid phase angle;
• generating, by a diesel generator of the microgrid, a preset reference power at the utility grid frequency by controlling a speed of a diesel generator in a speed droop mode;
• providing, by the utility grid and the microgrid, at least one of a first real power and a reactive power to each of a plurality of loads;
• then, when operating the microgrid in an island mode:
• operating the diesel generator at a constant speed based on the utility grid frequency to output a second real power and the reactive power at a reference frequency;
• operating the wind turbine in a decoupled d-q mode at the reference frequency with unity power factor to generate a third real power;
• generating, by a photovoltaic array, a fourth real power;
• providing the reactive power, the second real power, the third real power and the fourth real power to the plurality of loads;
• then, when operating the utility grid and the microgrid in a resynchronization mode:
• charging a switched capacitor bank connected to the point of common coupling with the second real power, the third real power and the fourth real power;
• and
• actuating, by a synchro check relay connected to the point of common coupling, a switch to connect the microgrid to the utility grid.