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In view of the dependence of power generation at photovoltaic solar power plants on the level of intensity of solar radiation and cloud cover, their operation creates a number of problems in the power system. This article describes the problems of operation of such power plants of non-guaranteed capacity during their parallel operation as a part of the Unified Energy System of Ukraine. One of the measures of stabilizing the operation of power plants of non-guaranteed capacity is the use of systems of electric energy storage. The article describes the conditions of electrical connection, which ensure the possibility of combined operation of a system of electric energy storage and a photovoltaic solar power plant. The article presents the developed mathematical model of the combined operation of a photovoltaic solar power plant (PSPP) and a system of electric energy storage. We consider the daily mode of recharging from a PSPP and discharging batteries into the power system in order to preserve the excess of generated electricity at the PSPP, which earlier was lost due to the restriction on inverters caused by the overload with photovoltaic power. The model enables one to identify the key parameters of batteries – power and capacity, taking into account the physical and technical features of the operation of battery storage as to the conversion efficiency, the number of working cycles and the depth of possible discharge depending on the structure of PSPP equipment and solar radiation intensity. Using the developed model, we determined the values of power, charging and discharging capacities of a lithium-ion system for storing electrical energy, when it works together with a 10 MWAC photovoltaic solar power plant at different overload factors. The article presents some results of technical and economic assessment of the combined operation of a PSPP and a lithium-ion system for storing electrical energy. The results showed an increase in the power and capacity of a storage device with increase in the overload factor of PSPP, which leads to the growth of cost of electrical energy at their combined work. At the same time, the amounts and quality of electricity supplied increase. Keywords: mathematical model, photovoltaic solar power plant, system of electric energy storage, cost of electricity, power system

The auction allocation of state support quotas for renewable generation implementation that will change the existing incentive for their development by providing “green” tariffs to producers is based on competitive selection of the power plant projects with the least cost of electricity generation. The competitive advantage of solar photovoltaic power plant (PV stations) projects can be done by deciding what kind of equipment can provide the minimum cost of electricity generation during the period of operation. In the article, there is improvement of non-linear mathematical least-cost optimization model of the structure of PV-station equipment using DC coupled battery energy storage system (BESS) to store the excess electricity of photovoltaic modules (PV modules), which is lost on inverters when they are overloaded during the hours of the highest intensity of solar radiation. The article presents the modelling results of overall operation of PV station with fixed power of PV modules, as well as determines optimal power of inverter equipment and battery capacity. The model calculations have shown that when the DC equipment costs of the PV station are reduced by almost 62 % and DC equipment costs of BESS are reduced by 86.7 % of the actual value in 2020 at the power of 10 MW PV modules, the optimal power of inverter equipment decreases from 7.08 to 6.29 MW, and the storage capacity increases from 0.22 to 2.51 MWh. The use of BESS allows accumulating the amount of electricity produced by PV modules, which is lost with limitation on inverters, while the cost of their joint electricity production is decreased by 0.42 %.

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