Neighboring system as black start source and restoration process based on the VSC-HVDC as tie line

Power system restoration takes place where power systems experience a blackout and might be splitted into several islands with limited number of black start units. The main challenge of the restoration process is to keep both frequency and voltage within allowable limits while restoring the network through energizing lines and transformers as well as load pickup. The load pickup scheme is determined by specific conditions to be fulfilled which are in particular lowest allowable frequency nadir and frequency rate of change. In a traditional power system restoration, these variables are a function of network inertia and characteristics of involved generators. This paper proposes the inclusion of VSC-HVDC systems into power system restoration. For this the VSC HVDC systems are equipped with an inertia emulating control system utilizing the electro-static energy stored in the HVDC DC shunt capacitors in an attempt to correct frequency behavior during load pickup. Thus, the restoration process can be accelerated since the variation of the transients can be reduced. In order to demonstrate the effectiveness of the proposed control scheme using VSC-HVDC tie lines as frequency supporting elements during restoration process, a large AC motor startup event is simulated in the early stage of a restoration process where network inertia is naturally low. A comparison between a large AC motor start-up during restoration process in a system with a hydro power plant as black start unit and that through VSC-HVDC system is made to show the adequacy and superiority of the VSC HVDC system both from the standpoint of converter control and transmission system operation.