Exploring common mode voltage stress and circulating currents in offshore wind turbine to MVDC collection grid interfaces

This work investigates common mode (CM) voltage stress and circulating ground currents in two proposed power electronic interfaces for integrating next-generation offshore wind turbines (WTGs) to medium voltage DC (MVDC) collection grids. In addition to the DC CM voltage imposed by the MVDC grid, the use of switching power electronics results in high frequency (HF) CM voltage applied to the HF isolation transformers used in the proposed approaches. While DC CM voltages stress transformer insulation, HF CM voltages also cause current to flow through parasitic capacitances between transformer primary and secondary windings, as well as between transformer windings and the grounded chassis where it then circulates back to the WTG grounded neutral. Detailed simulation results indicate that use of a per-pole interface architecture leads to lower and more balanced CM voltage stress on isolation transformers, which would reduce cost and increase reliability of the interface. A CM filter and transformer shield are applied as mitigation strategies; corresponding simulation results show a reduction in the common mode current in the WTG neutral in both per-phase and per-pole interfaces.