Electrical system design and analysis of a geothermal organic Rankine cycle power plant

Organic Rankine Cycle (ORC) technology is increasingly used to convert low- and medium-temperature heat, like geothermal brine, into electricity. The electrical design of ORC plants currently lacks systematization, particularly for Electrical Auxiliary Systems (EAS) integrating power electronics. This thesis proposes a systematic EAS design approach, developed and verified during an internship at Exergy International S.r.l. The methodology covers dimensioning the generator, transformers, cables, Variable Speed Drives (VSDs), and earthing network. It includes ETAP -based verifications (load flow, short-circuit, harmonics, protection coordination, motor starting, and stability) in compliance with IEC and IEEE standards. The approach is applied to the GEX300 United Downs Deep Geothermal Power Plant (UK), a 3.6 MVA binary plant connected to a 33 kV grid. The key findings are the following. The harmonic analysis at the point of common coupling yields a total voltage distortion of 0.057% and current distortion (TDD) of 3.78%, below the 5% limit (IEEE 519-2022, ENA G5/5). The auxiliary transformer K-factor remains 1.00-1.07, requiring no thermal derating or passive filters. IEC 60909 short-circuit calculations show that the two 400 V main switchboards are highly stressed: peak current for 12-LVB-01 reaches 99.2 kA (within the 100 kA limit, but with a narrow 0.8% margin), while 14-LVB-01 reaches 107.1 kA, exceeding its limit. The analyses confirm compliance with applicable power-quality and grid-code requirements.