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.

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High Voltage DC (HVDC) grids provide an efficient solution for the transmission of bulk power over long distances between the energy production and consumption centers. Up to now, most of the implemented dc connections are point-to-point, creating dc links to strengthen the existing, predominantly ac, electricity grid. However, to optimize the use of assets and energy harvest, multi-terminal HVdc grids are envisioned. Based on recent research and industrial trends, the Multilevel Modular Converter (MMC) technology will be the building block for the realization of these grids. Yet, as the grid structure becomes more complex, the protection of HVDC grids poses one of the most important challenges. This dissertation proposes different ways to isolate and ride through dc faults, while maintaining controllability of the converters that can offer ancillary services to their respective ac grid, having as a main objective to reduce the downtime of the grid and the time in which assets are not utilised. More specifically: The characteristics of dc faults and the main parameters which affect the fault response are investigated. A dc breaker optimized design is proposed which allows equipment sharing, offering bidirectional isolation capability for multiple lines at the same time. In case MMC with fault-blocking capability is used, e.g. Full-bridge MMC, a methodology for dc fault ride-through is proposed. Moreover, a dc current controller is presented, ensuring that the MMC can continue its controlled operation towards the respective ac grid. Once the dc side is isolated, the MMC operation as STATCOM is studied. More specifically, a control structure is proposed to maintain internal balancing of the converter, while ensuring Low-Voltage-Ride-Through (LVRT). The capability of the MMC to operate as Active Power Filter (APF) is also investigated. A selective harmonics detection and control method is presented and experimentally veri_ed for the mitigation of high-order current and voltage harmonics up to 13th order. Each of the aforementioned topics is dealt with in a respective Chapter of this dissertation.@en

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.

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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.

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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.

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While the scientific community recognizes the benefits of DC power transfer, the distribution network operators point out the practical and economic constraints in refurbishing the existing AC network at a medium-voltage level. Some apprehensions like reliability, cost of ownership, and safety in adopting a universal DC distribution may merit considerable attention, particularly considering the long operational experience with the existing mature AC system. This paper introduces the novel concept of reconfigurable DC links as a flexible backbone integrated within the future AC distribution grids. Benefits such as hardware reconfiguration for a modular AC–DC cable operation to achieve fault redundancy, control reconfiguration for flexibility and grid-supporting ancillary services, network reconfiguration for system level distribution loss minimization and load redistribution, and fault reconfiguration for improving the grid availability are discussed. The vision, around which the concepts developed in this paper revolve around, is to present a viable way of gradual transition from AC to hybrid AC–DC to finally a universal DC system.@en

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.

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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.

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It is anticipated that with the thrust towards use of clean energy resources such as electric vehicles, future distribution grids will face a steep increase in power demand, forcing the utility operators to invest in enhancing the power delivering capacity of the grid infrastructure. It is identified that the critical 5–20 km medium voltage (MV) underground ac distribution cable link, responsible for bulk power delivery to the inner urban city substation, can benefit the most with capacity and efficiency enhancement, if the existing infrastructure is reused and operated under dc. Quantification of the same is offered in this paper by incorporating all influencing factors like voltage regulation, dc voltage rating enhancement, capacitive leakage currents, skin and magnetic proximity effect, thermal proximity effect and load power factor. Results are presented for three different ac and dc system topologies for varying cable lengths and conductor cross-sections. The computed system efficiency is enhanced with use of modular multilevel converters that have lower losses due to lower switching frequency. A justified expectation of 50–60% capacity gains is proved along with a generalized insight on its variations that can be extrapolated for different network parameters and configurations. Conditions for achieving payback time of 5 years or lower due to energy savings are identified, while the socio-economic benefits of avoiding digging and installing new cable infrastructure are highlighted. The technical implications of refurbishing cables designed for ac to operate under dc conditions is discussed in terms of imposed electric fields, thermal profile and lifetime. A novel opportunity of temperature dependent dynamic dc voltage rating to achieve additional capacity and efficiency gains is presented.@en

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.

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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.

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This paper proposes a post-fault control technique for H-bridge multilevel modular converters (MMC), which in combination with a restoration framework, can achieve fast operation recovery in multi-terminal direct current (MTDC) networks under different dc fault types. The studied network consists of four voltage-source converters (VSC) for high voltage dc (HVDC) transmission. A meshed MTDC grid topology is used for the connection of two asynchronous grids with two offshore wind farms (OWFs). The effect of different dc fault types on the grid restoration time is evaluated. Following the proposed restoration steps, the grid is able to restore its operation within 34 ms, if a pole-to-pole fault occurs. To accelerate the discharging process of the grid during a pole-to-ground fault, a control technique is proposed that allows momentarily the connection of the two dc pole cables through the MMC-VSC switch valves. In this way, the dc grid is discharged faster and thus, the operation can be faster restored. The stresses to which the switches of the submodules are subjected are also analysed in this paper. The study showed that, with the proposed strategy, an H-bridge MMC-based network can restore its operation after a pole-to-ground dc fault within 158 ms without the need for expensive dc breakers.@en

This paper proposes a new modeling approach for multi-terminal high voltage direct current (MTdc) grids which use the modular multilevel voltage source converter (MMC-VSC) technology. In this paper, primary focus is given to the mathematical analysis of the MMC operation and control, which coupled with the state-space model that describes the dc grid, can accurately simulate the normal operation and dc fault response of a dc grid with low computational requirements. A 3-terminal MTdc network with half-bridge MMC converter stations in radial configuration is used as a case study for the evaluation of the model and the obtained results are compared to a dynamic switching model implemented in Matlab/Simulink both for normal operation as well as in case of a positive pole-to-ground fault. The study showed that the model can accurately estimate the most important parameters needed for the design of an MTdc grid and can simulate fast dynamics without the need for computationally heavy software implementation.@en

This paper derives the mathematical expressions for computation of the load impedance magnitude, such that rated cable current is imposed at the sending end, with sending end voltage as the reference phasor. Thereby, the variation in
maximum power transferred to the receiving end of a medium voltage cable link is described for varying link length, conductor cross-sectional area and load power factor. The percentage error in transmitted power computation due to simplification
by neglecting the cable capacitance is quantified. The merit of the developed theory for future use is highlighted.@en

Lesnicar and Marquardt introduced a modular multilevel converter (MMC) topology back in 2003. Although this topology has received a great deal of attention in recent years by both the research community and industry, hitherto no steady-state model has been developed which accurately captured all the relevant power losses while being computationally light. Hence, the aim of this paper is to introduce a fast MMC loss model which captures the key sources of power losses in steady-state operation. The proposed model was compared to a loss model developed by Marquardt. Both models presented similar results under the same assumptions. However, the proposed model captures the switching losses more realistically and considers the temperature of operation of the electronics, as well as the losses of the inductors and cooling system, in the overall efficiency of the MMC. To validate these new additions, the proposed steady-state model was compared to a dynamic model. Once again, the proposed model was able to capture the different sources of power losses. Nonetheless, results demonstrated that the balancing strategy greatly influences the efficiency of the MMC. Therefore, information regarding the envisioned control strategy is necessary to accurately calculate the efficiency curve of the MMC.@en

Refurbishing existing ac distribution cables to operate under dc conditions can offer several advantages in terms of capacity enhancement, efficiency and flexibility in power and voltage control, among others. In this paper, technical aspects such as insulation ageing, capacity and efficiency enhancement are explored. A novel idea of dynamic cable voltage rating based on the temperature dependent electric field is developed. The proposed algorithm can be incorporated in the dc link converters to obtain additional efficiency and capacity gains. Finally, challenges in the presented concept are highlighted.@en

The aim of this paper is to present the current status of the offshore wind industry and to identify trends in Offshore Wind Projects (OWPs). This was accomplished via a thorough analysis of the key characteristics – commissioning country, installed capacity, number of turbines, water depth, project area, distance to shore, transmission technology and investment cost – of the commissioned and under construction European OWPs. Furthermore, the current status of the several countries outside of Europe was also investigated. The analysis revealed that the European offshore wind power grew on average 36.1% yearly since 2001. Currently, there are 7748 MW installed and 3198 MW under construction distributed among 76 OWPs situated in European waters. These projects are spread among ten countries, with the highest share of offshore projects belonging to the northern European countries. The UK has 46% of the total installed European offshore wind capacity with 26 projects, Germany ranks second with 16, while Denmark is third with 13 projects. These countries constitute 88% of the European offshore capacity. The analysis also showed that, although the installed capacity of the OWPs is growing, the projects׳ area is not increasing at the same pace due to the release of turbines with higher rated capacities which allow projects to increase their power nameplate without proportionally increasing the number of turbines. The average distance to shore and the water depth are both increasing throughout the years. Although the average investment cost per project is rising with the higher distances to shore and water depths, the multi-GW plans of the northern European and Asian countries indicate that the industry will continue to grow. The European Union targets of having 40 GW of offshore wind capacity deployed by 2020 in Europe and 150 GW by 2030 may represent plausible scenarios since the required growth is below the European.@en