Flexibility has become a requirement for modern power systems dominated by renewable generation sources. It can be extracted from different assets, ranging from demand response to fast generating units. This paper proposes an investment model that finds an optimal mix of transmissionlevel non-generation flexible assets: battery energy storage (BES), thyristor-controlled series compensators (TCSC), and transmission lines. The role of BES is to offset renewable generation in time, but its power converter is additionally utilized to provide voltage regulation by injecting/withdrawing reactive power. TCSC is used to alter power flows and increase existing lines' capacity, while new power lines are used to increase bulk power transfer. The proposed planning model uses a linearized AC OPF and employs Benders' decomposition to develop an iterative procedure for obtaining the optimal solution. The presented case study illustrates usefulness of the model for different BES costs and investment policies.

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Battery energy storage systems (BESS) and renewable energy sources are complementary technologies from the power system viewpoint, where renewable energy sources behave as flexibility sinks and create business opportunities for BESS as flexibility sources. Various stakeholders can use BESS to balance, stabilize and flatten demand/generation patterns. These applications depend on the stakeholder role, flexibility service needed from the battery, market opportunities and obstacles, as well as regulatory aspects encouraging or hindering integration of storage technologies. While developed countries are quickly removing barriers and increasing the integration share of BESS, this is seldom the case in developing countries. The paper identifies multiple case opportunities for different power system stakeholders in Croatia, models potential BESS applications using real-world case studies, analyzes feasibility of these investments, and discusses financial returns and barriers to overcome.

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