DC technology has gained considerable interest in the medium voltage applications due to the benefits over the AC counterpart. However, to utilize the full capacity of this development, the selection of a suitable power electronic converter topology is a key aspect. From the pool of voltage source converters (VSC's), it is unclear which topology is suitable for multi-megawatt applications at medium voltage dc (MVdc) levels. To address this, the paper proposes a selection guideline based on reliability and optimum redundancy levels of VSCs for MVdc applications. This will be combined with other functional factors such as operational efficiency and return-on-investment. Three candidate multi-level topologies namely three-level neutral point clamped converter (3L-NPC), modular multi-level converter (MMC) and cascaded 3L-NPC (which is being used for the first MVdc link in the U.K.) have been evaluated over two-level-VSC from $\pm$ 10 kV to $\pm$ 50 kV. Results show that with the increase of MVdc voltage level MMC shows better performance whereas at low MVdc voltage levels 3L-NPC is the prominent topology.

Power system interconnections using high-voltage direct-current (HVDC) technologies between different areas can be an effective solution to enhance system efficiency and reliability. Particularly, the multi-terminal DC grids, that could balance and ensure resource adequacy, increase asset utilization and reduce costs. In this paper, the technical feasibility of building DC grids using the line commutated converter based (LCC) and voltage source converter based (VSC) HVDC technologies are discussed. Apart from presenting the technical challenges of building LCC DC grids and LCC/VSC hybrid DC grids, the reliability modeling and analysis of these DC grids are also presented. First, the detailed reliability model of the modular multi-level converters (MMCs) with series connected high-voltage and low-voltage bridges are developed. The active mode redundancy design is considered for the reliability model. To this end, a comprehensive whole system reliability model of the studied systems is developed. The reliability model of each subsystem is modeled in detail. Various reliability indices are calculated using this whole system reliability model. The impacts of the redundancy design of the MMCs on these indices are presented. The studies of this paper provide useful guidance for DC grid design and reliability analysis.

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 damages. 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 scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, additionally, can be used to avoid control conflicts among active CFCs during communication failure. An experimental platform consisting on a 3-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.

The urgency for an increased capacity boost bounded by enhanced reliability and sustainability through operating cost reduction has become the major objective of electric utilities worldwide. Power electronics have contributed to this goal for decades by providing additional flexibility and controllability to the power systems. Among power electronic based assets, high-voltage dc (HVdc) transmission systems and flexible ac transmission systems (FACTS) controllers have played a substantial role on sustainable grid infrastructure. Recent advancements in power semiconductor devices, in particular in voltage source converter based technology, have facilitated the widespread application of HVdc systems and FACTS devices in transmission networks. Converters with larger power ratings and higher number of switches have been increasingly deployed for bulk power transfer and large scale renewable integration-increasing the need of managing power converter assets optimally and in an efficient way. To this end, this paper reviews the state-of-the-art of asset management strategies in the power industry and indicates the research challenges associated with the management of high power converter assets. Emphasis is made on the following aspects: condition monitoring, maintenance policies, and ageing and failure mechanisms. Within this context, the use of a physics-of-failure based assessment for the life-cycle management of power converter assets is introduced and discussed.

The valve-side single-phase-to-ground (SPG) faults in half-bridge modular multilevel converter (HB-MMC) based bipolar high-voltage direct-current (HVDC) systems induce some special fault consequences: overvoltage on the submodule (SM) capacitors in converter upper arms and non-zero-crossing currents in the grid-side AC circuit breaker (ACCB). In this letter, a protection strategy based on converter embedded double-thyristors is proposed to address these issues. The double-thyristors are installed in parallel with each SM in the lower arms of the converter. By triggering the double-thyristors, the SPG fault is converted into a three-phase short-circuit which ensures current zero-crossings in the grid-side ACCB. Moreover, as the voltages of converter ac buses are clamped by the ground through the triggered double-thyristors, the upper arm overvoltage is mitigated. The effectiveness of the proposed protection strategy has been verified in a bipolar HB-MMC HVDC link built in PSCAD/EMTDC.

The development of power electronics is driving high voltage (HV) and medium voltage (MV) applications based on dc technologies for power transmission and distribution, renewable energy integration, and ship and railway systems. DC circuit breakers (DCCBs) are needed to protect and improve the performance of dc or hybrid ac/dc systems during severe fault and/or abnormal operating conditions. This paper reviews the state-of-the-art of DCCBs for both HV and MV DC systems. The mechanisms and requirements of DCCBs are summarized. Various DCCB topologies and their merits and demerits are presented and critically reviewed. Future trends and requirements for DCCBs are described. This work is intended to provide a useful point of reference for this technology and to contribute to its future development.