## 2 The Site and Microgrid Description
### Description of Microgrid Structure  
PV modules were assembled on the roof-top of a passive solar residential and commercial building. The roofs of both buildings were north facing with an inclination angle of 20\({}^{\circ}\) for the residential and 12\({}^{\circ}\) for the commercial building. The selected roof angles are not the optimum angle for PV power plant design but were constrained by the buildings' design. A photo of the PV module array is given in Figure 2.  
Figure 1: Location of Alice indicated in; (**a**) climatic map [18] and (**b**) 21 years average global horizontal irradiance distribution of South Africa [19]. However, the PV array in each building was divided into east and west wing. On the residential building, a total of 20 modules (190 W\({}_{\text{p}}\)) occupy both wings of the roof; amounting to a peak power generating capacity of 3.8 kW. In the PV array, two modules were connected in series to give 10 PV parallel string (5 per wing) generator. On the commercial building, each wing on the roof contains 15 sets of 250 W PV modules with 3 modules connected in series to give a 5 PV parallel string. This results in a 3.8 kW peak power generator in each wing. The connections of the PV array are guided by the maximum operating voltage (Voc) of the charged controllers in the generators. Both generators in each building made up the PV power plant in this study. The generator in the residential building is off-grid while that in east and west wind of the commercial building are grid-assisted and hybrid PV systems respectively. The system component balance of the PV power plant is given in Figure 3.  
Figure 3: \(Cont\). Figure 2: Rooftop mounted PV system in the SolarWatt Park. In the context of this study, an off-grid (stand-alone) system uses solar energy as the sole source of electricity, whereas a grid-assisted system supplements solar energy with the national grid to provide electricity. A robust and effective load sizing, which matches utility consumption and the system component of the off-grid system was conducted to ensure an uninterrupted power supply. It should be noted that the load sizing of both buildings is outside the scope of this study. The selected load in the commercial building is powered by the grid-assisted system during peak solar energy period and backed up by the national grid when the battery power is low. Other loads in the commercial building are powered by the hybrid system and surplus solar energy generated is fed into the national grid. Figure 3: Wire diagram of the component balance of system in (a) off-grid, (b) hybrid, and (c) grid-assisted microgrid systems at SolarWatt Park. In terms of operation, a hybrid system is a combination of off-grid and grid assisted systems with the ability to feed and draw energy to and from the national grid. Based on the design of the systems, each system comprises a charge controller, inverter and battery bank. As indicated in Figure 3, the grid-assisted and hybrid systems use a Microcare LCD MPPT 100 A charge controller, whereas FLEX Max 80 was used in the off-grid system. However, both charge controllers use a continuous MPPT algorithm to search for maximum available power in the PV array to charge their respective batteries. Like most MPPT charge controllers, the system charge controllers ensure optimum battery performance over a long period with three fundamental operation modes; Boost, Equalize, Float and Sleep mode. In addition, a bidirectional inverter in each of the systems is used to convert DC power from the charge controller (PV array or battery) to the consumable AC power of most appliances. In other words, the inverters can operate in uninterruptible power supply (battery charger) and inverter mode with the hybrid system's inverter equipped with grid connectivity function. The application of the inverters due to their multiple functions is predetermined by the user, following specified settings in the inverters. Therefore, all inverters were set to inverter mode since charge controllers are in-cooperated in the systems.