Flexible Operation of VSC-HVDC and Phase Shifting Transformers for next generation of Transmission System Control Centres

摘要

Today, the application of VSC-based overlay HVDC point-to-point interconnectors becomes state-of-the art. Looking at Germany, there are up to four HVDC interconnectors under planning, one of them will have one converter within the control area of 50Hertz Transmission GmbH (50Hertz). In addition, 50Hertz is directly affected by the erection of some Phase Shifting Transformers (PST) at its tie lines to Poland and Czech Republic. The implementation of these controllable equipment devices creates new challenges in operation and planning. Consequently, a new operation and coordination concept is proposed comprising the day-ahead planning process as well as short term adaptation of PST and HVDC references. According to this HVDC links allow a fast and event triggered activation of corrective actions for significant disturbances, less severe events are followed by continuous set-point adaptations of both the Voltage Source Converter (VSC) and PSTs. Afterwards a new global optimum is re-established by the next regular schedule update. The schedule is based on a multi-objective coordination problem formulated for the day-ahead congestion forecast (DACF) determining PST angle set-points and HVDC link active power set-points. The intraday operation and real-time operation is realised by a multi-instance approach. Regarding significant AC disturbances (e.g. outages of highly loaded 400 kV AC lines), the VSC controllers identify critical AC disturbances and apply pre-calculated corrective actions which are aligned for each significant disturbance. These pre-calculated adaptations of active power are centrally evaluated and appropriate to resolve the disturbance-related congestions. The activation is triggered either by the operator or by a local automatic algorithm. PSTs are not involved in this close to real-time set-point adaptations due to the discrete mechanical switching which results in speed restriction. Beside these significant disturbances, discrepancies between the actual and the scheduled grid situation can occur. These deviations can be detected via real-time measurements by phasor measurement units (PMU) allowing both VSCs and PSTs to adapt their operation points continuously, but with respect to their different time constants. This approach enables a continuous participation in power flow changes, including unscheduled power flow conditions, activation of balancing power or less severe equipment tripping. The general approach presented in this paper is evaluated on the basis of phasor based simulations.