Smart Grids and Micro-Grids. Umashankar Subramaniam

are switching frequency and microgrid dc voltage, respectively. The commands “Batt – charge, V_dc–ctrl ” along with the power to be exchanged (I_{B(disc, ref)}, I_{B(cha, ref)}) with microgird are sent to the controller from the EMS of microgrid. The feedback signals inductor current (i_L), dc microgrid voltage (V_dc) are also input to the controller.

Table 2.3 Specifications of the boost converter interfacing dc microgrid and battery.

Parameter	Value
Nominal voltage (VB)	240 V
Rated capacity	35 Ah
Initial SOC	60%
Inductance (L) ESR (rL) of inductor	2.6 mH 0.005Ω
Output capacitor (C0)	2200 µF
DC load	400 Ω
DC microgrid voltage	400 V

Figure 2.5 shows the results for the case when V_dc–ctrl = 0, i.e., ESS should be charging or discharging depending on the status of the Batt – charge command. Initially, command V _dc–ctrl = 1, and the battery discharging current reference is set to 35 A. After 5 sec, the discharging current reference is changed to 17.5 A. After 10 sec, command Batt – charge = 1 and the reference of charging current is kept at 35 A, which was subsequently changed to 17.5 A at 15 sec. The controller was able to track the reference current, as evident from the waveform of Figure 2.5. It is also clear from the figure that the SOC was decreasing when the battery was discharging, while SOC was increasing for the case when the battery was discharging. A similar observation can be made about the battery voltage.

For the case, when it is required to maintain the dc voltage level of microgrid, the command V_dc–ctrl = 1 is received from the microgrid. The reference for dc microgrid voltage is set to 400V. To show the performance of controller under such condition, initially V_dc–ctrl = 0, Batt – charge = 0 and the battery discharging current reference (I_{B(disc, ref)} was set to 35 A. At t=5 sec, the V_dc–ctrl = 1 was sent to the controller. Figure 2.6 shows the performance of the controller. It shows that the converter was able to track the reference voltage.

       2.5.2 DC-AC VSI Interfacing AC-Microgrid

Depending on microgrid structure, grid-tied inverter interfaces the ESS (Energy Storage Systems) to the external ac gird or the ac bus of ac microgrid. Generally, 3-phase VSI based converter is used and a typical schematic is shown in Figure 2.7. The converter and its associated controller is supposed to perform the following functions:

       a. If the system is in grid-connected mode, the VSI is supposed to supply real and reactive power to the grid. In case the ESS is discharged, ac power has to be absorbed from the grid to charge the ESS.

       b. In islanded mode, the ESS is required to maintain the PCC voltage to the reference voltage.

Figure 2.5 Waveforms under the condition when battery-converter system is connected to microgrid.

Figure 2.6 Waveforms under the condition when battery-converter system is in islanded mode.

Figure 2.7 Three-phase inverter interfacing battery to grid.

Figure 2.8 Controller for inverter interfacing battery to grid.

In order to address the above requirements, the controller should have a provision to work under the mode dictated by EMS. One typical configuration of such a controller is shown in Figure 2.8. For an AC system, the variables to be regulated are a sinusoidal function of time and hence the closed-loop control system must have a sufficiently large bandwidth for tracking the variables. To transform variables to dc quantities for achieving zero steady-state error by using a PI controller and decoupling the regulation of real power and reactive power, the converter is controlled in the synchronous frame (d-q) [14].

      When Mode_command = 1 (the converter is in grid-connected mode), the battery is supposed to supply active or reactive power to the grid. The reference power (P_ref, Q_ref) that has to be supplied by the battery-converter system is obtained from the energy management system of the microgrid. The instantaneous active (P) and reactive powers (Q) supplied by the converter system can be expressed in a d-q frame as

      (2.17)

      (2.18)

      Where V_s, i_d, and i_q are PCC voltage on d-axis, output current on d-axis, and output current on q-axis, respectively. As the grid-connected converter act as current source, the reference grid current (i_d(ref), i_q(ref)) to be fed can be obtained using relation

(2.19)

      (2.20)

      The inner current loop is designed to track the reference current (id(ref), i_q(ref)) and it generates the PWM gating signal for switches of the converter. The current loop needs angle ρ ( to convert quantities from the d-q frame back to abc frame) information, which is obtained from the Phase-locked loop of the system. When the battery SOC is low, then the reference active power is negative (P_ref = -ve and Q_ref = 0) to charge the battery.

      When Mode_command = 0, the microgrid is