Umashankar Subramaniam - Smart Grids and Micro-Grids

(2.30)

The coupling between V sdand V sqis 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 vare 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,qare 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.

A MATLAB-Simulink model of the three-phase inverter system shown in Figure 2.7is 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.

(2.35)

Where P is the rated output active power of one phase for three-phase full-bridge inverter, V gis the phase voltage, V dcis the dc link voltage f swis 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 2.9shows 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 refwas set to 0 value. P refwas set to 10kW, which was changed to 5kW at t=3.5 sec. At t=2.5 sec, Q refwas set to 1kVAr, subsequently changed to zero value at t=5 sec. At t=7 sec, P refwas 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 2.10shows 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.

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.

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