## 1 Introduction
### Grid Integration Challenges of Variable Renewable Energy Sources and Storage Systems  
Integrating the PV technologies into the EU grid system will address the need for a more flexible electricity grid [62]. At present, the integration of PV power into the electricity grid is a challenge due to the variable nature of power generation, since the existing grids and their capacities were established to comply with less or non-variable energy sources, predictable load peaks, and dispatchable power generation. Nowadays, the flexibility of the grid is an indispensable economic-technical aspect that is required to handle the network constraints caused by PV generation during the peak hours of the demand [63; 64; 17; 33; 65]. One of the most important steps is to evaluate existing rules to determine whether new approaches to design, planning, and operation are needed for higher penetrations of PV technologies. Real market examples show that the EU countries do not have a unified approach to this situation. For example, the Spanish Government has developed a regulation for deploying a net-metering system that will allow restraining energy demand on the system. This concept focuses not only on generation, but also on generation and demand management [66; 67]. In contrast, the Czech and Polish transmission system operators (TSOs) have installed phase-shifting transformers (PSTs) at their borders with Germany to prevent the loop flows of northern German VRE power meant for Bavaria and Austria flooding their networks. The interesting thing about the situation is that Austria buys wind power to fill its pumped hydropower storage systems, and ideally 35% of the electricity flowing from Germany to Austria passes through the Czech Republic [67; 68]. [PERSON] et al. [40] showed by a UK network example that the addition of 5 GW of storage in the case of a 60 GW average grid demand reduces the transmission expansion needs by 20% using 24-h storage [39]. The examples help to understand that it is necessary for each country to develop their own policies and system operations to achieve the safe system flexibility and reliability needed for successful PV power integration. For a given power system, the planning consists of an inherently complex set of activities that are undertaken by multiple jurisdictions and groups. The PV technology can be accommodated by integrating the planning of system performance, generation, and transmission. To achieve this, institutions and markets need to be designed to allow access to physical capacity [67]. Energy storage is practically equivalent to grid enhancements in a country because this should be integrated into the network and/or the entire transformer area. This means that not only the transmission capacity and interconnection should be analyzed, but also the network development features (also local smart grid, low- and medium-voltage developments, virtual power plant concepts, transmission reliability margin) and the costs for each country. Based on the energy storage capacities, power capacities, discharge time, and capacity factor values, it is possible to calculate the role of energy storage in the energy mix. In this way, the declining role of peak power plants or the load-reducing role of VRE can be examined/calculated, too. This makes it possible e.g., to determine some reduction of interconnection values. Due to increased VRE, local energy storage is a security issue for each country, which also serves the interests of the market. Due to the heterogeneous installations and weather dependence of PV energy within a country, an opposite energy flow can occur in the local electricity network. This problem can only be effectively managed by a local storage system to optimize the electrical parameters of the entire transformer area. The main purpose of the new storage systems is to keep the voltage within the permitted range by charging and discharging according to the local VRE systems [67; 69; 70].