Review of technologies for DC grids – power conversion, flow control and protection

Abstract

This article reviews dc transmission technologies for future power grids. The article emphasizes the attributes that each technology offers in terms of enhance controllability and stability, resiliency to ac and dc faults, and encourage increased exploitations of renewable energy resources (RERs) for electricity generation. Discussions of ac/dc and dc/dc converters reveal that the self-commutated dc transmission technologies are critical for better utilization of large RERs which tend to be dispersed over wide geographical areas, and offer needed controllability for operation of centralized and decentralized power grids. It is concluded that the series power flow controllers have potential to restrict the expensive isolated dc/dc converters to few applications, in which the prevention of dc fault propagation is paramount. Cheaper non-isolated dc/dc converters offer dc voltage tapping and matching and power regulation but they are unable to prevent pole-shifting during pole-to-ground dc fault. To date hybrid dc circuit breakers target dc fault isolation times ranging from 3ms to 5ms; while the resonance-based dc circuit breakers with forced current zeros target dc fault clearance times from 8ms to 12.5ms.

A modified Cuk DC-DC converter for DC microgrid systems

“…Thus, the losses in modified Cuk DC-DC power converter is lower than the conventional ones. In (16,17) show that for the same output voltages, the power loss in modified Cuk DC-DC power converter is lower than the conventional boost DC-DC power converter.…”

“…Figure 2 shows that a DC-DC converter is used as an interface between the DC grid and DC energy sources and between the DC grid and battery energy storage. Various DC-DC power converters for these purposes have been proposed in the literature [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Though the DC-DC converter can be isolated or nonisolated, the nonisolated DC-DC converter is preferred because of power loss consideration.…”

DC Fault Protection Algorithms of MMC-HVDC Grids: Fault Analysis, Methodologies, Experimental Validations, and Future Trends

“…When the MMC HVDC grid is subject to short-circuit faults on the transmission lines, all the sub-module (SM) capacitors discharge rapidly, leading to a high fault current. Due to the low inertia of the DC networks, the DC fault propagation speed is much faster than the AC fault [6]. To protect the converters, double thyristor switches can be connected in parallel with the half-bridge sub-modules to isolate the fault lines by turning on the firing signals [7].…”

Non-Unit Protection for HVDC Grids: An Analytical Approach for Wavelet Transform-Based Schemes

“…HVDC grids are considered as the natural evolution of existing point-to-point transmission links that enable the integration and better utilization of massive amounts of offshore energy. Moreover, they offer a plethora of attractive features such as: enhanced controllability, flexibility and redundancy, improved reliability and security, lower investment costs, etc [1]. However, as with any developing technology, the HVDC grid concept has its challenges, with the requirement for fast and discriminative protection typically being the most prominent.…”