Abstract
The current-carrying capability of dc lines is limited by their thermal and electric stress limits. Thus, the line current must be maintained within the permissible operational region to protect the lines from damage. In a dense dc grid, control over each line current cannot be achieved without including additional control devices. In this paper, a dual H-bridge current flow controller (2B-CFC) is used to manage the dc grid line power flow by providing dc voltage compensation in series with dc lines. A centralized hierarchical control system is proposed to coordinate the operation between multiple CFCs. A novel voltage-sharing control scheme is demonstrated. It is shown that such a scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, in addition, can be used to avoid control conflicts among active CFCs during communication failure. An experimental platform consisting of a three-terminal dc grid and small-scale 2B-CFC prototypes has been developed to validate the concepts. For completeness, the CFC performance has been analyzed for overload conditions and when no communication exists. Small-scale dc circuit breakers have been developed to study the CFC performance under a pole-to-pole fault.
Highlights
T HE breakthrough in the development of solid-state semiconductor valves in the 1970s has led to the installation of hundreds of point-to-point HVdc links around the globe
A 2B-current flow controller (CFC) consists of two electrically coupled H-bridges connected in series with dc lines
The results presented have clearly shown that the protection of the 2B-CFC is mainly determined by the response time of the bypass switches Q1 and Q2 and of the dc circuit breakers (DCCBs)
Summary
T HE breakthrough in the development of solid-state semiconductor valves in the 1970s has led to the installation of hundreds of point-to-point HVdc links around the globe. Cables are inherently suitable for underground or sub-sea transmission and pose a reduced visual and environmental impact compared to OHLs, OHLs are attractive owing to their high voltage and high power handling capabilities and lower cost [5], [6] Both dc cables and OHLs have intrinsic operational limits over the amount of power that can be transferred. Power flow controllers (PFCs) are power electronics based devices capable of providing dc line current control in MTdc grids [9]. Voltage source-based PFCs generate significant current harmonics in the valve winding, leading to further power losses. To eliminate these issues, the device has to be powered inside the dc grid [12]–[15]. Small-scale solid-state dc circuit breakers (DCCBs) have been implemented to assess the device performance under pole-to-pole dc faults, with the interaction between the DCCB and the 2B-CFC during fault conditions being analyzed
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