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Many studies show the advantages of direct current (DC) systems over alternating current (AC) systems for integrating renewable energy sources into the grid. However, protection design is still one of the key challenges, which delays the fast implementation of DC grids. High-voltage direct current (HVDC) is a well-established technology that resulted in several commercial installations. On the contrary, medium-voltage direct current (MVDC) is a technology in an earlier research and development stage with mostly research-based installations and only a few commercial ones. A research gap is present in many aspects of MVDC protection. This paper aims to review, compare and discuss the similarities and differences between HVDC and MVDC protection, and to propose potential strategies for future research and development in order to accelerate the development and integration of MVDC grid, During writing and publication, Ilka Jahn was with RWTH Aachen UniversityQC 20240705

With the increasing maturity and flexibility of power electronics-based voltage conversion techniques, DC grids, and distribution systems have gained significant interest. These systems offer advantages such as improved power quality, efficiency, and flexibility. Medium-voltage DC (MVDC) applications, including shipboard, railway systems, distribution networks, and microgrids, are emerging as critical areas of interest. To integrate MVDC systems with existing power grids, MV AC/DC conversion techniques are crucial. Moreover, the lack of mature protection strategies and equipment, particularly DC circuit breakers (DCCB), poses a significant challenge to the development of MVDC systems. Therefore, this thesis aims to address two primary challenges in the field: the improved topologies of MV AC/DC conversion techniques for interfacing MVDC systems with power grids and the development of high power density DCCB for MVDC systems. The traditional modular multilevel converter (MMC) is widely used for medium voltage (MV) AC/DC conversion due to its modularity, scalability, and reliability. However, the presence of numerous semiconductor devices and capacitors in MMCs results in challenges such as low power efficiency and density. To enhance the performance of MMCs, this thesis proposes several novel hybrid MMC (HMMC) topologies, including the three-level HMMC, flying capacitor HMMC, and hybrid-leg MMC. These topologies aim to leverage the advantages of both conventional multilevel converters and MMCs. By replacing the low-voltage (LV) submodule (SM) in MMCs with a simple high-voltage (HV) switch, higher efficiency, a smaller footprint, and lower cost can be achieved. The HV switch operates at line frequency, simplifying device-switching and addressing the challenges of series-connected devices. The introduction of additional HV switches enables alternative connections compared to traditional MMCs, reducing the number of required SMs. Consequently, there is a significant red

The development of DC grids faces challenges from DC fault protection. The conventional DC circuit breaker (DCCB) employs metal-oxide varistor (MOV) to isolate the faulted line, in which the fault isolation process is coupled with the energy dissipation process. In this study, a clamping type DCCB (CTCB) using internal capacitors to clamp the converter voltage is proposed. Thanks to the proposed configuration, fault isolation and energy dissipation are decoupled, resulting in a fast fault isolation and low energy dissipation compared to the conventional DCCB. The working principle of the proposed CTCB is presented and verified in a DC grid simulation model. A comparison is made with the traditional DCCB. The fault isolation time can be reduced by 34.5%. The dissipated energy can be reduced by 17.4%. The energy dissipation power can be reduced by 76.2%.

This paper deals with the development of an accurate finite-element model of an arrester to investigate the electrothermal and mechanical stress during dc current interruption. The comprehensive analysis performed on a ZnO surge arrester is supported by experiments during high-voltage dc circuit breaker current interruption. The performed experimental analysis comprises three sequential 26 kV/10 kA direct current interruption tests carried out within a period of one hour. The dynamic temperature and current distribution of the surge arrester columns during current interruption are measured. The finite-element simulation results are in good agreement with the test results. The influence of the surge arrester temperature on the current distribution among the surge arrester columns is analyzed. The impact of the surge arrester temperature on ZnO electrical characteristics and mechanical stress inside the surge arrester are also investigated. The surge arrester finite-element model can be used with full success for parameter optimization of the surge arresters to prevent possible failures when dc circuit breakers performed multiple interruptions in short period of time., taverne, Intelligent Electrical Power Grids

Enhanced active resonant (EAR) dc circuit breakers (DCCBs) are a promising set of recently proposed DCCB concepts. As other DCCBs, EAR DCCBs still require a fast mechanical switch. The requirements on the actuator of the mechanical switch depend on the DCCB concept and the dc grid and are derived here for an EAR DCCB. Thomson-coil actuators (TCAs) can open and close mechanical switches sufficiently fast to satisfy the requirements. This work studies experimentally a TCA system with active damping for an off-the-shelf industrial vacuum interrupter used as mechanical switch in an EAR DCCB. The prototype is explained in detail, and extensive measurement results are presented, showing that active damping must be perfectly timed to be effective. A novel Thomson-coil (TC) driver is proposed and studied experimentally, which operates the TCA more efficiently by recycling energy during the actuation. Moreover, the novel TC driver reduces the capacitive storage by 50% and allows for faster recharging with lower charging current. Finally, the autoreclosing and proactive commutation operation of the TCA system and the interruption capability of the prototype EAR DCCB are demonstrated experimentally., QC 20220322

DC circuit breakers (DCCBs) are one of the key components to facilitate large meshed HVDC grids. Driven by the needs for achieving high speed, low loss and low cost, various DCCB technologies have been proposed for HVDC applications. Unlike AC circuit breakers (ACCBs), some DCCBs provide a variety of functions, such as proactive opening or fault current limiting (FCL), mainly attributed to the high controllability of the power electronic switches used in such DCCBs. To enable these functions, the intelligent electronic devices (IEDs) are expected to provide signals steering these functions in addition to a trip command. Interoperability between IEDs and DCCBs from different vendors is considered feasible, but expected to be more complex than their AC counterparts, due to the different functions provided by various DCCB technologies. It is therefore crucial to understand which of the functions are essential to fulfil the requirements imposed in HVDC grid protection, and to standardise the interfaces between the IEDs and DCCBs to achieve multivendor interoperability between IEDs and DCCBs provided by different vendors. This paper first classifies the DCCB functions into minimally required and auxiliary ones based on reviewing the existing literature. Then, standardised interfaces between the IEDs and DCCBs are proposed to enable both types of DCCB functions. An example of such IED is implemented in PSCAD/EMTDC to demonstrate that the proposed interfaces are adequate to enable both minimally required and auxiliary functions using a four-terminal test system. Auxiliary functions of hybrid DCCBs, such as proactive opening, fault current limiting, fast reclosing and reopening, breaker failure internal detection and repeated O-C-O operation are demonstrated by simulations., QC 20190819

In recent decades, the electrical power system has evolved into a new phase, in which the renewable energy resources are massively integrated into the grid. This change is mainly inspired by global policies that intend to reduce greenhouse gas emissions and decrease the society’s reliance on fossil fuels by replacing them with sustainable energy sources. The good examples are the European Network of Transmission System Operators for Electricity (ENTSO-E) that intends to integrate a high degree of renewables in Europe’s energy system, and the West-East Electricity Transmission Project that delivers wind energy from the northwest to the southeast of China. One important technology used to connect renewable energy resources is the high voltage direct current (HVDC) system based on the voltage source converter (VSC). Aside from the simple point-to-point HVDC link, the multi-terminal HVDC (MTDC) system is another option to connect these remote energy resources. In the MTDC system, the generation units are usually unsynchronized turbines that are interfaced with powerelectronic- based converters. As such, the responses of theMTDC system after faults occur are drastically different fromthe conventional AC systems that are based on synchronized generators. Since the development of an MTDC system is an important process, the research on the matter must be carried out. In an electrical power system, the transient events refer to a system’s response shortly after disturbances occur, such as the generation loss, the load shedding, the transmission line tripping, and the fault. This thesis focuses on the MTDC system’s protection based on the system’s transient events after faults. Due to the low impedance of the DC system and the low inertia of the HVDC converter, a fault in the DC system can spread quickly throughout both the DC and AC sides. Usually, the transient behavior of the HVDC system must be observed within severalmilliseconds, and it is a challenge to simulate the tra, Intelligent Electrical Power Grids

One of the major challenges of DC circuit breakers is the required fast mechanical actuator. In this paper, a Thomson coil actuator system for a vacuum interrupter is designed. Active damping is used to decelerate the moving contacts. Challenges are discussed, especially concerning the power supply needed for the Thomson coil actuator. The design philosophy is explained and FEM simulation results are presented. The results indicate that a wide range of combinations of drive circuit capacitance and voltage fulfill the requirements for armature acceleration. However, active damping requires a very careful selection of drive circuit voltage and timing of applied damping., QC 20211025

The electronic circuit breaker used in DC microgrids are required to work within their safe operating areas bounded by temperature, voltage and current limits. Traditional approach managed to protect these switches through rapid current cut-off operations at over-load or fault situations, but failing to avoid the disturbance induced by transient current surges or noises which are not harmful to the grid operations. Aiming to increase the quality of circuit breaker operations and furthermore improve its reliability, this paper proposed a SiC MOSFET based DC circuit breaker based on the variable time-delay protection scheme. The cutoff operations only take place after proper delay time, which are precisely catered according to the transient thermal properties of SiC devices and the properties of DC loads. The proposed scheme has been implemented with hardware prototype and experimentally verified under different fault situations.