This master's thesis investigates the impact of MW-scale battery energy storage systems (BESS) on 110/20 kV distribution transformer substations in the Elektro Gorenjska distribution system. As the share of renewable energy sources grows and because of conditions on electric energy markets, battery storage systems are becoming an increasingly common element of distribution systems. However, their market-driven operation can have significant effects on the infrastructure to which they are connected.
A simulation tool was developed in Python, based on real 15-minute operational data for eleven regional transformer substations for the year 2024. The tool simulates BESS operation at rated powers of 5 MW and 10 MW across three scenarios: without storage, with unconstrained arbitrage operation, and with dynamic transformer load limitation, and verifies compliance with the n-1 criterion under normal operation, transformer outage, and line outage conditions. In addition to transformer and line loading, the tool analyses the effect of the BESS on voltage regulation via the on-load tap changer (OLTC), including line drop compensation.
Results show that connecting a BESS has a measurable impact on both transformer loading and the annual frequency of voltage regulator tap changes. The magnitude of this impact depends on the BESS power rating, the transformer rated capacity, and the loading characteristics of each individual substation. It is found that dynamic load limitation effectively ensures safe network operation with minimal impact on the economic performance of the storage system. The results provide distribution system operators with a methodological basis for assessing the suitability of BESS connections at individual substations prior to issuing grid connection approvals.
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