Smart Grids and Micro-Grids. Umashankar Subramaniam

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      The coupling between V_sd and V _sq is eliminated by a decoupling feed-forward compensation. One possible control law is given by

      (2.31)

      Where i_d(ref), i_q(ref) are reference for inner current loop and v_vd, and v_v are the feedback control input which is expressed as

      (2.32)

      Where e_vd = (V_{sd ( ref )} – V_sd), e_vq = (V_sq(ref) – V_sq), K_pv,d, K_iv,d, K_pv,q, and K_iv,q are parameters/gains of PI controller for d-axis current and q-axis voltage controller. The parameter of PI controller can be easily selected. For d-q transformation in islanded system, the reference angle ρ (and thus ) is decided independently.

       2.5.2.3 Typical Case Study in MATLAB-Simulink

A MATLAB-Simulink model of the three-phase inverter system shown in Figure 2.7 is developed with the specification listed in Table 2.4. The filter parameter are selected based using following equations [16, 17]

      (2.33)

      (2.34)

Table 2.4 Specifications of battery-converter system for AC microgrid.

Parameter	Value
Nominal voltage (VB)	700 V
Rated capacity	150 Ah
Initial SOC	60%
inverter side inductance (L) and ESR (rL) of inductor	1mH, 0.005Ω
Filter capacitor (Cf)	20 µF
AC load	PL=1000 W QL =100 VAR
AC microgrid voltage (Vphase(rms)) grid side inductance Lg	220 V300 µH

(2.35)

      Where P is the rated output active power of one phase for three-phase full-bridge inverter, V_g is the phase voltage, V_dc is the dc link voltage f_sw is switching frequency, K_a = 0.2,

      The control is implemented in the Simulink platform. The commands “Mode_command” along with the power to be exchanged (P_ref, Q_ref), with microgird are sent to the controller from the EMS of the microgrid. The feedback signals PCC voltage (V_sx), inverter current (i_Lx), grid voltage (V_gx) are also input to the controller.

Figure 2.9 shows the results for the case when Mode_command = 1, i.e. battery converter system is in grid-connected mode. Initially, the battery was having SOC=60% and Q_ref was set to 0 value. P_ref was set to 10kW, which was changed to 5kW at t=3.5 sec. At t=2.5 sec, Q_ref was set to 1kVAr, subsequently changed to zero value at t=5 sec. At t=7 sec, P_ref was set to -1kW so that the power can be drawn from the grid to charge the battery. From the waveform, it is evident that the controller could track the reference power as stated by the EMS of the microgrid. Also, during the time up to 7 sec, the battery SOC and battery voltage were decreasing as ESS was discharging to supply the power to the grid.

To show that the controller would work for the case when the battery converter system is in islanded mode, i.e., Mode_command = 0 is received from the EMS, the battery converter system is made to maintain the PCC voltage. Initially, the system was operated in grid-connected mode and was supplying 10k W of real power to the grid. At t=1 sec, the grid was disconnected and reference voltage to be maintained at PCC is given to the controller. At 1.5 sec, the battery converter system is again connected back to the grid. Figure 2.10 shows the result for this case. From the waveforms, it is clear that the converter was able to maintain the PCC voltage even for the case when the grid was disconnected and again connected back to the grid.

Figure 2.9 Performance of the controller when the battery converter system is in grid connected mode.

Figure 2.10 Waveform of PCC voltage, real and reactive power exchanged fed for the condition when the battery converter system switches between grid-connected mode and islanded mode.

2.6 Conclusion

      With the high demand for integration and implementation of renewable energy sources (RES) to the microgrid due to climate change policies, the requirement of ESS and its control is expanding. Hence, the need and availability of ESS and its category is briefly explained with the assessment. Moreover, power converters play an incredibly significant role in the integration of RES and ESS to the microgrid. The categories, needs and applications of power electronics converters were also presented. To discuss the control and design of battery integrated dc and ac microgrid system, the MATLAB-Simulink model is developed. Different case studies prove the fact that the controller can perform satisfactorily.

References

      1. Rahimi, A., Zarghami, M., Vaziri, M., Vadhva, S. (2013, September). “A simple and effective approach for peak load shaving using Battery Storage Systems”. In 2013 North American Power Symposium (NAPS) (pp. 1–5). IEEE.

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