## 6 Experimental Results  
A DC nanogrid with a RES, two energy storage units and a variable load was assembled in the laboratory. An Agilent Solar Array Simulator (SAS) (E4350B) was used to emulate a solar array. It was programmed to provide a maximum power of 213 W with 7.35 A and 29 V at rated solar irradiance. Maxwell supercapacitor modules (BMOD0165 P048) of 165 F with a rated voltage of 48 V were employed for the energy storage units. One is used as a battery and the second is used as a SC. As in the simulation studies, the \"battery voltage\" was adjusted to 24 V and the SC's to 28.4 V. Regarding the power electronics interface, a Semikron \"MiniSkiiP 8 Three-phase 1200 V Power board\", with a _MiniSkiiP 83 AC_ power module and a _SKHI61_ IGBT driver, was used in this study. The values of passive components, control parameters, and so on, used in the experimental set-up are those presented in Section 4. The control schemes are implemented with a dSPACE DS-1103 rapid prototyping system with a 20 us time step. The dSPACE code was generated by Simulink C coder by means of Real-time interface (RTI), which converts the control diagram from MATLAB/Simulink into dSPACE code. This does not provide the fastest and the shortest code, but it is sufficient in experimental proof-of-concept tests. Eight parallel switchable 44 \(\Omega\) resistors were used for realizing the full DC nanogrid load impedance. A picture of the experimental set-up is shown in Figure 12.  
Having demonstrated the improved performance of the proposed scheme over the conventional HESS control scheme by means of simulations, experimental results are provided to demonstrate the feasibility of the proposed control scheme. The focus of this Section is on the impact of different system parameters obtained using the proposed design approach. The first case, which was also presented in the simulation studies, concerns a SC interface with a droop factor \(R_{dSCI}\) = 0.0145, 20 times lower than the battery's for providing a peak current of 20 times that of the battery for a given DC bus voltage variation. It employs a HPF with a time constant \(\tau_{F}=3.7\) ms, computed to give a settling time of \(T_{s}=0.22\) s. The key waveforms of the system are shown in Figure 13 for a no-load to full-load (5.2 \(\Omega\)) variation and back to no-load. At no-load, the DC bus voltage is initially at 49.2 V and it decreases slowly to 46.8 V at full-load in the steady-state. (The screen shows 0.4 V/div.) The load current increases in a step-like manner from 0 A reaching 8.45 A in steady-state  
Figure 11: Simulation results for the proposed approach when the load is changed from no-load to 5.2 \(\Omega\) (full nanogrid load) and back to no-load. (1.3 A/div). The SC (inductor) current, initially at 0A, increases very fast following the load step, reaching a peak value of about 20 A. Then, it decreases exponentially reaching 0 A after about 0.24 s. The battery (inductor) current changes slowly from -8 A, when it absorbs the full PV power at no-load, to 8 A, when it provides to the DC bus the same amount of power as the PV converter, at full-load (4 A/div). The PV (inductor) current remains virtually constant since it operates with MPPT with rated solar irradiance. The experimental waveforms match very well with the simulation ones. In the second case, a larger droop factor (\(R_{dSCI}=0.0289\)\(\Omega\)), which is 10% of the battery's, is used. Thus, the SC converter should provide a lower contribution during transient conditions. The time constant of the HPF is calculated for the same value (\(T_{s}=0.22\) s), resulting in \(\tau_{F}=6.9\) ms. Figure 14 shows the transient response of the system, with the abovementioned parameters, for the no-load to  
Figure 12: Picture of the experimental set-up assembled in the laboratory. Figure 13: Experimental result for the proposed control scheme with \(R_{dSCI}=0.05\)\(R_{dB}\) and designed for \(T_{s}=0.22\) s, as the load changed from no-load to full-load (5.2 \(\Omega\)) and back to no-load.