As the number of high voltage (HVDC) installations is increasing, there is a growing need for interconnection to create more efficient grids. To tackle the main challenges in HVDC grids, a multiport DC-DC converter, often referred to as DC hub, is considered a solution. This paper provides a methodology for the clearing of AC faults within the DC hub and the protection of the system during a fault. A methodology is also presented for the safe connection or disconnection of additional ports from the DC hub without affecting the operation of the other ports. A design based on modular multilevel converters (MMC) is chosen due to their inherent advantages and its control operation is described. A DC hub was modelled in Matlab/Simulink and was tested in a case study including the interconnection of three DC lines in symmetric monopolar configuration, operating at different voltage levels.

A practical issue faced by today's ac grid is the rapidly growing power demand on its aging infrastructure. One possibility to maximize the capacity of the existing infrastructure is to refurbish the ac links to operate under dc conditions using modular multilevel converters. In this paper, the idea is applied to restructure an actual medium voltage distribution system. Further, a systematic reconfiguration strategy is proposed to maintain high power delivery capacity even during (n-1) contingency. Contingency analysis is carried out for faults in different system components of the distribution grid. Towards this goal, novel concepts such as reconfigurable switch, dc link converter bypass and flexible dc to ac operational transition are proposed.

This paper presents a breaker arrangement concept, the Multi-Line Breaker (MLB), for the protection of multi-terminal high voltage dc (MTdc) networks. Based on the design of a hybrid breaker, the MLB is an economically attractive solution for the protection of multiple dc lines in nodal connection using a single main breaker path. By using commutation units, the MLB directs the fault current through the main breaker in a unidirectional way, irrespective of the fault location. Hence, this study presents the design requirements for the MLB, regarding both hardware and control, and evaluates its operation within a grid. For this reason, a four-terminal half-bridge MMC-based MTdc grid in radial configuration was used and pole-to-ground dc fault conditions were investigated. The dc fault response of the grid with one MLB at the central node is compared to the respective response of the grid when one hybrid breaker is employed at each dc line. The simulations show that the MLB is feasible and that the overall MTdc grid fault response for the two protection systems is very similar. As a result, the design advantages of the MLB make it a promising solution for the dc fault isolation in MTdc grids.

This paper studies the dc fault development stages and analyzes the fault dynamics in a point-to-point multilevel modular converters (MMC)-based dc connection. First, the effect of the dc grid configuration on the normal operation was investigated by comparing an asymmetric monopole with metallic return and a symmetric monopole. Then, the main parameters that affect the dc fault response of a grid, such as the fault type, impedance and converter blocking, were examined. Compared to previous studies, which are based on simulation results, the analysis is performed hereby both in theory, deriving the equations that describe the dc fault stages, as well as using experimental results obtained in the designed laboratory setup. The setup consists of two MMC terminals connected to two ac sources representing independent ac grids. These terminals are connected using a simple dc link based on pi-section equivalent of dc cables. The obtained results, which verified the theoretical analysis, can be used to determine the protection function thresholds of the MMC, as well as to estimate the developed stresses on dc lines during fault conditions and to define the design requirements for dc breakers.

Due to the increase of power electronic-based loads, the maintenance of high power quality poses a challenge in modern power systems. To limit the total harmonic distortion in the line voltage and currents at the point of the common coupling (PCC), active power filters are commonly employed. This paper investigates the use of the multilevel modular converter (MMC) for harmonics mitigation due to its high bandwidth compared with conventional converters. A selective harmonics detection method and a harmonics controller are implemented, while the output current controller of the MMC is tuned to selectively inject the necessary harmonic currents. Unlike previous studies, focus is laid on the experimental verification of the active filtering capability of the MMC. For this reason an MMC-based double-star STATCOM is developed and tested for two representative case studies, i.e., for grid currents and PCC voltage harmonics. The results verify the capability of the MMC to mitigate harmonics up to the thirteenth order, while maintaining a low effective switching frequency and thus, low switching losses.

As the number of high voltage direct current (HVDC) installations is increasing, there is a growing need for interconnection to create more efficient grids. To tackle the main challenges in HVDC grids, a multiport DC-DC converter, often referred to as DC hub, is considered a solution. This paper provides a thorough review of different DC hub design approaches and examines the modularity of existing DC-DC converter designs to allow the connection of more ports. A design based on multilevel modular converters (MMC) is chosen due to their inherent advantages and its control operation is described. A DC hub was modelled in Matlab/Simulink and was tested in a case study including the interconnection of three DC lines in symmetric monopolar configuration, operating at different voltage levels. The simulations showed that the DC hub can maintain controllability under different operating conditions and can assist the power flow control of an HVDC grid.

Grid faults are common in power systems and can have a severe impact on the operation of the converters in the system. In this paper, the operation of a Modular Multilevel Converter (MMC)-based Static Synchronous Compensators (STATCOM) is investigated during grid faults. The study focuses on the challenging internal control of the converter to allow the independent control of the energy levels of each arm, with the goal to maintain internal balancing of the MMC during contingencies. Extensive experimental results highlight the need for a sophisticated internal control. Moreover, the experimental analysis verifies that, by using the proposed control structure, the MMC can effectively ride through a fault on the AC side without tripping, while injecting the necessary positive and negative sequence reactive current levels according to the most recent grid codes.

An integrated design approach for the reactors used in multi-terminal HVdc (MTdc) grids based on the Modular Multilevel Voltage Source converters (MMC-VSC) technology is proposed in this paper. Arm reactors and dc limiting reactors are used to limit the rate of rise of currents in case of dc faults to protect the converter valves and allow more time for the dc breakers to isolate the faulty line within a grid. A mathematical model of the MMC and the dc grid is used for the analysis for the dc fault analysis and the reactor design. The reactor design is evaluated using a radially connected 3-terminal MTdc network. This analytical model is then used to investigate the most important dc fault protection design parameters, such as arm inductors and dc limiting reactors when using solid-state dc breakers. The main objective of the design procedure is to minimize the cost and mass of the required inductors, while maintaining control of the 'healthy' part of the dc grid at all times, during a dc fault.