# Evaluating Cybersecurity Risks of Bulgaria's Energy Sector: Focus on PV and HVAC-R
## 4 PV Plants with Energy Storage Capability  
Increased energy storage capacity seems to be the most probable scenario for solving the energy imbalance problem. Both pumped-hydro power plants and batteries are considered. The big investors already direct their attention to dedicated battery storage that will let them speculate about significant electricity price changes. The fastest growing battery storage capacity is in small (typically less than 100 kWp) PV installations with hybrid inverters. There are no statistics for such installations in Bulgaria, but based on surveys among companies with a combined 80% share of the photovoltaic market, between 10,000 and 50,000 hybrid inverter PV plants with power between 5 and 100 kWp have been installed over the last 2 years. There are economic, psychological, and normative reasons that determine this pace. The average price of electricity for residential consumers in Bulgaria is 110.5 EUR/MWh, which is approximately equal to the average annual exchange price. There is very strong external political pressure to bring residential consumers into the electricity market, just as commercial consumers had been a decade ago. As illustrated, this should not lead to a serious price hike, but the general populace is very suspicious of such changes and perceives the measure with extreme disapproval and serious concerns. These concerns act as an incentive to install one's own residential PV power plant. In certain rural and suburban areas, the electricity for residential consumers is of poor quality. This is mainly due to overbuilding and network congestion. Users in these areas are also forced to install their own power plants to ensure independence from the centralized power supply. The latest amendments to the Territorial Development Act require that every newly built residential building has renewable energy sources with installed capacity not less than half of the provided electrical power by the grid. The provided electrical power is the maximum power that the building can consume. It is controlled by technical (automatic circuit breakers) and administrative (15 min load check) measures. As already mentioned, the only practical renewable resource available in the country is solar energy. The structure diagram of a PV system with a hybrid inverter is shown in Figure 8.  
Due to the heavy bureaucratic procedure, many residential users and small- and medium-sized business owners prefer to build photovoltaic power without injecting energy into the grid, as shown in Figure 9.  
Additional current transformers (red) are connected to the inverter, measuring the current at the point of common coupling to the grid. The inverter is operated in a way that prevents energy from being injected into the grid. The energy from the PVs is used to feed the load and to charge the battery, or it is not generated at all. A PV plant operating according to the scheme in Figure 9 has a significantly simplified construction and grid connection procedure, as statutory consent is replaced by tacit consent. Since this setup is for residential users and small businesses, it is very important that the inverter allows phase imbalance when operating in a non-injecting mode. When the load consists of single-phase  
Figure 8: Structure diagram of a PV system with hybrid inverters. consumers, it is natural to have working modes in which one of the phases is significantly more loaded or significantly more underloaded than the others, and the inverter must be able to cover the asymmetrical load without injecting energy into the grid. The typical load profile of a Bulgarian residential consumer is given in Figure 10. Based on the author's measurements, where common residential loads are taken into account, as the maximum and minimum values for a month were averaged, an area of the most probable change in the household load was identified. Figures for the winter and summer months, December and July, respectively, are shown.