A hybrid microgrid system with improved battery energy storage, comprising:
- a hybrid AC/DC microgrid comprising an AC sub-grid and a DC sub-grid, said AC sub-grid comprising a variable speed wind generator and a diesel generator, and said DC sub-grid comprising an array of photovoltaic solar panels;
- and
- a triple function battery energy storage system connected via a bi-directional inverter to the AC sub-grid or the DC sub-grid, the inverter controlled to transfer power between the battery energy storage system to the AC sub-grid or the DC sub-grid by a controller providing switch control signals to the inverter and one or more of said variable speed wind generator, diesel generator, and array of photovoltaic solar panels, said triple function battery energy storage system comprising a dual loop decoupled control system implementing a droop-based control method and a grid-feeding strategy;
- wherein said dual loop decoupled control system is implemented in a Rotational Reference Frame dq system, where d axis corresponds to active power control and q axis corresponds to reactive power control;
- wherein the droop-based control system uses P-w and Q-V droops, where w is AC sub-grid voltage, V is AC sub-grid frequency, P is active power, and Q is reactive power;
- wherein the control system is configured to track power references for active power, Pref and reactive power, Qref, where power references are defined as
- P
- ref
- =
- P
- *
- -
- m
- p
- (
- ω
- -
- ω
- *
- )
-
- Q
- ref
- =
- Q
- *
- -
- n
- q
- (
- V
- -
- V
- *
- )
- (
- 1
- )
- m
- p
- ≥
- P
- max
- -
- P
- min
- ω
- *
-
- n
- p
- ≥
- V
- max
- -
- V
- min
- V
- *
- (
- 2
- )
- wherein P* is a received active power reference value, Q* is a received reactive power reference value, ω* is a voltage rated set point of the AC sub-grid, V* is a frequency rated set point of the AC sub-grid, Pmax is a maximum active power value, Pmin is a minimum active power value, Vmax is a maximum AC sub-grid frequency value, Vmin is a minimum AC sub-grid frequency value, and mp and nq are droop gains for power sharing;
- and
- wherein the control system is configured to control the active power and reactive power absorbed and injected by the inverter with the hybrid AC/DC microgrid;
- and
- wherein said control system is adapted to reduce frequency fluctuations and power fluctuations in the hybrid AC/DC microgrid during a change in wind speed or solar irradiance by setting the active power reference value, P*, equal to a power balance mismatch calculated by subtracting a total amount of power supplied to the hybrid AC/DC microgrid from a total load power drawn from the hybrid AC/DC microgrid, and setting the reactive power reference value, Q*, equal to zero, to thereby control the battery energy storage system inverter to absorb or inject the amount of power balance mismatch between load and generation.