Selection of DC Voltage ControllingStation in an HVDC Grid

Abstract: In the recent power system expansion driven by growing energy demand, more attention is beingput towards integration of large-scale renewable resources. The HVDC transmission system on topof, or in complement to, existing AC power has been considered as a relevant solution for integratingremote energy resources as well as cross-border power trading. This thesis considers an approachwhere more than two HVDC stations are connected in the DC side to form a meshed HVDC grid.In an HVDC grid, the real-time mismatch of power injection can be compensated by the DC voltagecontrolling converter(s). Similar to frequency in the AC grid, the DC voltage deviation is a localindication of power mismatch in an HVDC grid. The capability of a converter to control DC voltageand contribute in power sharing impacts the secure operation of an HVDC grid. The selection ofDC voltage controlling station becomes even more critical when a present DC voltage controllingstation is tripped and the system operator has to assign a new one.This thesis proposes a real-time quantitative evaluation of HVDC converters in an HVDC gridto select the proper DC slack converter. This real-time evaluation considers the strength of theconnecting AC grid as well as the converter's on-line capacity margin as the selection metrics.Strength of the AC grid is evaluated in real-time by estimation of grid Short Circuit Capacity(SCC). Low computational requirement, low operational complication and acceptable accuracy ofestimated parameters have been considered as performance metrics for the selection of a suitablealgorithm. This thesis shows that the Recursive Least Square (RLS) algorithm can be very ecientlyused for the real-time estimation of SCC.The concept of on-line ranking of the converter's capability in controlling DC voltage has beenevaluated through dierent scenarios for the CIGRE B4 DC grid test system. To evaluate theperformance of the concept, a real-time co-simulation platform has been used to mimic the aspectsof the system. This platform includes OPAL-RT to model the power system, ABB's industrialHVDC controllers and corresponding ICT (Information and Communication Technology) systems.The results show that the proper selection of the slack station can improve the AC system responseand DC voltage drops during the disturbances.