A strategy for optimal and selective utilization of multiple interline DC power flow controllers in VSC-HVDC grids

Inserting interline dc power flow controllers (IDC-PFCs) promote the control flexibility of the DC power flow in meshed voltage source converter (VSC)-based high voltage direct current (HVDC) grids. Moreover, the steady-state security of the VSC-HVDC grids can be enhanced significantly when the IDC-PFCs are optimally placed and tuned. Therefore, it is proposed to apply minimum spanning tree graph theory to specify safe subgraph and unsafe subgraphs (which are prone to getting overloaded) of the VSC-HVDC grids and provide graphical insights on the static security conditions. The concept of the current variation index (CVI) is introduced to find the optimal locations of the IDC-PFCs within the identified unsafe subgraphs with maximized current alteration capability. The location with the highest value of CVI is considered the optimal placement scenario. As the focus would be on unsafe subgraphs, the optimal placement process would be done with the minimum computational burden. Then, static mixed-integer power injection models are proposed to model multiple optimally-placed IDC-PFCs and develop mixed-integer security-constraint dc optimal power flow (MI-SC DC-OPF) formulations. The proposed MI-SC DC-OPF would simultaneously activate the minimum required number of most effective optimally-placed IDC-PFCs and specify optimum references for them according to the operating point of the VSC-HVDC grid. The effective performances of the proposed placement strategy and selective/optimal operation method are evaluated on CIGRE B4 and IEEE 57-bus VSC-HVDC girds by performing static analysis.