Microgrid - Introduction

The simulated microgrid and its controller are shown in figures 1 and 2, respectively.

Figure 1: Schematic Representation of the Microgrid

Figure 2: Schematic Representation of the Microgrid Controller

The main components of the studied microgrid are as follows:

• A wind turbine generation system (WTGS) that delivers at any time instant t an output power P_Wt related to wind speed v_t [2]:

where P_W^max is the rated power, v_t is the wind speed at instant t, v_CI is the cut-in wind speed, v_CO is the cut-out wind speed and v_R is the rated wind speed.
The values used as part of the simulation for these parameters are presented in table 1.

The WTGS is connected to the microgrid via a 50 kVA transformer, with primary and secondary voltages of 6.6 kV and 548 V, respectively.

Table 1: Wind Turbine Power Curve Parameters

• A photovoltaic generation system (PVGS) delivering a maximum power of 12 kW at an irradiance E = 1000 W/m2 and a temperature T = 25 °C.
  The PVGS is connected to the microgrid through a 3-phase inverter and a 14.4 kVA transformer.
  The primary and secondary voltages of this transformer are respectively 6.6 kV and 208 V.
• A 60 kW lithium-ion battery energy storage system (BESS), connected to the microgrid through a 3-phase inverter and a 72 kVA transformer with primary and secondary voltages of 6.6 kV and 480 V respectively.
  The role of the BESS is to deliver or to absorb the difference between the load demand and the total power generated by the microgrid [3].
• The microgrid load demand is represented by three loads with different characteristics in terms of importance.
  The first load is a hospital, representing a critical load with a peak demand of 27 kW.
• The second one is a combination of a critical load demand of 25 kW and a demand response participation up to 9 kW.
  And finally, the last one is a fully sheddable residential load of 18 kW.
  It should be noted that a critical load refers to a load demand that needs to be met at all times, while part of a sheddable load demand can be disregarded when there is not enough power generation to serve the total load demand.
• The microgrid controller ensures the balance between load demand and power generation in the system.
  As shown in figure 2 above, the controller gathers all the information coming from the loads, the PV and wind generation systems, the BESS, and the main grid.
  A one-way communication can be seen between the critical load and the controller because the only action is to meet this demand once the command arrives.
  As for the other two loads, a two-way communication is seen, meaning that the controller can take actions to partially meet their energy demand if there is no sufficient power generation.

From the generation side, maximum power point tracking is implemented at both the PV solar array and the wind turbine, which enables an increase in the power delivered but also enhances the lifetime of the renewable generation system [4,5].
The controller manages when the BESS needs to be charged or to provide power based on two criteria: the state of charge (SoC) of the battery bank and the difference between load demand and renewable power generation.
Finally, the controller also enables when to connect or disconnect and when to buy or sell energy to the main grid utility.
An islanding breaker enables the connection/disconnection to the main grid.