## 5 Simulation Results  
MATLAB/Simulink is used for simulation studies. The objective is to investigate the potential benefits of an independently controlled SC unit in the dynamic regulation of the DC nanogrid voltage. Ideally, this should not affect the ability of the SC unit to mitigate the high frequency and fast changing currents in the battery. In order to test this feature, the solar converter operates in MPPT while the power demanded from the DC nanogrid varies between no-load and full-load. From t = 0 s to 0.05 s, there is no power demanded from the DC nanogrid, and the storage unit(s) absorbs all the power provided by the RES, around 200 W. Then a full-load impedance of 5.2 \(\Omega\) is connected to the DC nanogrid to be supplied by both the RES and storage unit(s), both at full power. This operating condition remains until 0.05 s when the load is removed. This corresponds to the largest normal disturbance to be faced by the DC nanogrid and will be used to compare the performance of the proposed control scheme with the conventional one with a HESS. The parameters of the system are as stated in previous Sections. (The battery and SC were represented by ideal DC voltage sources of 24 V and 28.4 V, respectively.)  
Figure 10 shows the results of the key system waveforms for the conventional scheme. Initially at no-load, the DC nanogrid voltage (top screen) is about 49.25 V. The solar converter operates in the MPPT mode with a boost inductor current (average) of around 7.35 A, bottom screen. The average current absorbed (negative sign) by the battery is 8.4 A while that of the SC is zero, as expected. Following the load step, the SC current increases very fast reaching a peak value of 10 A, while the  
Figure 9: DC nanogrid power electronics interfaces.  
battery's increases more slowly, as desired, until it supplies all the HESS current. The DC bus voltage reaches the steady-state value of 46.75 V in 17 ms. A similar behavior, in terms of current magnitudes and settling times, can be observed when the load is removed at t = 0.3 s and the HESS has to, again, absorb rated power.  
Figure 11 shows the performance of the system, for the same load transients, with the proposed control scheme. The values of the waveforms in the steady-state for the no-load condition are virtually the same as for the conventional HESS control scheme. Following the full-load step variation, one can see that the DC bus voltage decreases much more slowly than with the conventional HESS control approach. This is due to the way the SC converter is controlled, injecting a higher current and for a longer time to slow down the reduction of the DC bus voltage. At the same time, as expected, it slowed down the increase of the battery current. This was done indirectly, via the DC bus voltage, without any signals exchanged between the control loops of the battery and SC converters. Just before t = 0.3 s, the values of the waveforms obtained with the proposed scheme are very similar to those obtained with the conventional HESS scheme. Then, following a step load rejection, to no-load, the proposed scheme provides a similar improvement in the DC bus voltage variation and in the battery current variation. It is worth mentioning that besides using the proposed control scheme, one should also make sure that the SC is large enough to provide/absorb the required compensating energy without significant changes in its voltage level and that the boost inductor of the SC converter will not saturate with the increased current. Considering the relatively short action time of the proposed control scheme for the SC converter, it should not be necessary to increase the current ratings of the semiconductor switches.  
Figure 10: Simulation results for the conventional approach when the load changed from no-load to 5.2 \(\Omega\) (full nanogrid load) and back to no-load.