Analysis and Control VSC-HVDC under Unbalanced AC Conditions

In VSC-HVDC configuration, the core of the controller is the inner current controller. The capability of current controller determines the VSC-HVDC performance under unbalanced AC conditions. In this paper three different control systems with different current controllers for VSC-HVDC have been investigated and compared by PSCAD/EMTDC under unbalance conditions of the grid voltage. The simulation results show that the current controller designed by only considering positive-sequence components (VCC1), the negative-sequence current appears in system, and the DC-link voltage is affected by an oscillation at double the power frequency; by feeding the controller only the positive-sequence components as well as forward feeding negative-sequence components of grid voltage (VCC2), the response is improved, negative-sequence current and DC-link voltage ripple all decrease; the dual current controller (DVCC) where both voltages and currents are separated into positive and negative-sequences components namely VCC3 has a good performance to deal with unbalanced system.