Due to the rapid revolution in trading strategies, market environment is very different from the past, and the market intervention in national stock exchanges has been taken seriously again . However, very few studies discussed the rule-based trading suspension for individual stocks in the past, and most of them only focused on the impact of trading halt on market performance. For this reason, this study in addition to measures impact of market performance, another major analysis focuses on the differences between individual and institutional investors order behavior under different halt conditions. We try to understand whether the current halt mechanism achieves the purposes of reducing the information asymmetry and the abnormal volatility. The market performence empirical results show that transitory trading halt can reduce the overreaction of re-opening, but the halt of follow-up 5 minutes of the liquidity decreased, and volatility increased. Next, we find the individual investors order aggressive tend to be conservative in the period of suspension. In contrast, Institutional investor behavior will tend to be more positive with higher information asymmetry and will not be affected by the trading halt. Moreover, from follow-up 20 minutes individual-institutional transaction VWAP ratio, we find that the trading halt will improve the performance of individual investors transactions.

This thesis analyses the controlled switching methods applied to modern SF6 type circuit breakers for the purpose of reducing switching overvoltages. Main emphasis is placed on controlled switching methods applied at extra high voltage level, since the cost of failures caused by switching overvoltages is highest in this voltage level. After a brief introduction about circuit breakers in general, switching overvoltages and controlled switching methods are analysed. Also a case study about controlled switching of an unloaded overhead line is provided, and success of controlled switching method is evaluated.

Sulphur hexafluoride gas is the dominant insulation and interruption material in high voltage gas insulated substation. Its usage remains a concern of transmission system operators owing to the global warming potential of the gas. The work carried out in this thesis aims to find the environment-friendly materials that can replace SF6. These candidates are required to have a strong dielectric strength for high voltage busbar insulation and well arc extinguishing capability necessary for high voltage circuit breaker.A range of alternative insulation types including CF3I gas and its mixture, high pressure air and solid insulating foam are considered as substitute of SF6. Theoretical studies on the dimensions of busbars used in substations are carried out for these options. The dimension of the dielectric system and its ampacity of respect system are calculated using heat transfer models considering their boiling point and proper working pressure which is related with the dielectric strength of some gas.On the other hand, SF6 gas circuit breaker is extremely popular on the medium and high voltage power networks owning to its effective arc extinguishing performance. It would be ideal if a substitute material could be found for SF6 as an interruption material. Biodegradable oil PTFE ablation, other gas candidates including N2, CF3I are investigated as possible replacement of SF6 through literature study.The usage of vacuum circuit breaker is eventually capable to operate in high voltage transmission system. Simulations have been carried out with software ATP/EMTP to investigate the influence of different characteristics of vacuum circuit breaker including chopping current level, the dielectric strength of vacuum gap and the opening time. And then the probability of overvoltages when vacuum circuit breakers installed is studied by statistical study in MATLAB.

The master thesis is focused on the calculation of electrodynamic forces acting on the contacts of the moulded case circuit breaker Siemens 3VA5 from OEZ Letohrad. This work is divided into ten chapters. In the introductory chapter it is described why it is important to deal with the calculation of the electrodynamic forces and the design of the current path. The second chapter describes the problem of the origin and effect of electrodynamic forces on individual parts of the electrical device. The third chapter describes the parameters, properties and design of the 3VA5 Siemens circuit breaker. The fourth chapter describes the Finite Element Methods. The fifth chapter describes how to create a 3D current path model in Solidworks. The sixth to ninth chapter is focused on the stationary calculation of moments in the program Ansys Maxwell by means of Magnetostatic Analysis and on the calculation of the total repulsive force between contacts with the influence of ferromagnetic materials. The last chapter is focused on dynamic calculation of moments and forces acting on contacts in a certain time interval.

DC circuit breakers must be able to arrest overcurrent conditions to prevent electrical equipment and wiring from causing building fires or other hazards from occurring. With more DC renewable sourced structures such as Cal Poly’s DC House, an inexpensive and reliable protection system is necessary to ensure safe energy transfer to the loads. One method of protecting a system is preventing excessive amounts of current to be drawn by the load when the surrounding components are rated at a lesser value. DC circuit breakers act as a monitoring system and barely presents an effect on the voltage or power. With most DC circuit breakers on the market being mechanical, the response time to an overload condition is limited to the speed the contacts can disconnect. The examination of response timing and overcurrent limiting is explored in this thesis when using a solid state based DC circuit breaker. The system is designed to handle 600 W, where the operating voltage is 48 V and the maximum allowable current is 12.5 A. The solid state DC circuit breaker has the capability of arresting excessive currents within 30 µs and can be reset through a single pole single throw switch.

No description available.

The vehicle industry must reduce emissions to lessen their environmental impact. Using electric motors for propulsion and batteries for energy storage decreases the pollution and greenhouse gases emitted by heavy commercial vehicles such as trucks and buses. As they often travel long distances, a large energy storage is needed, and it needs to be possible to recharge it quickly. The fast recharging requires high power chargers with high voltage and high current, and that in turn necessitates a suitable overcurrent protection system to avoid damageif a fault would occur. Possible technologies for this overcurrent protection system are investigated in this thesis, and the different solutions are evaluated based on important metrics. A solid state breaker is found to be a promising technology that limits the current quickly and reliably. The evaluation including computer simulations shows that the technology is feasible, but some aspects of the circuit design need to be executed carefully like the inductance, the sampling frequency of the current measurement, and the design of the snubber circuit.

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 reduction in the number of semiconductor devices, capacitor energy storage, and power losses. Furthermore, an average model is developed for the three-level HMMC to illustrate the additional power flow path between the AC and DC sides, as well as the reduced SM capacitor energy storage requirement. As a result, the proposed HMMCs exhibit substantial potential to replace traditional MMCs, offering higher efficiency and power density. Unidirectional high-voltage (HV) and medium-voltage (MV) rectifiers are essential for applications where power flows exclusively from the AC to the DC side. Examples of such applications include HVDC transmission, front-end converters for electric vehicle (EV) charging stations, and data centers. Therefore, hybrid modular multilevel rectifiers (HMMRs) are proposed for these unidirectional AC/DC applications. Instead of utilizing active devices for HV switches, the HMMR employs HV diode to achieve step-up HMMR, step-down HMMR, and flying capacitor HMMR configurations. As diodes are passive devices that do not require gate driver units, the HMMR design becomes simpler, resulting in cost and volume savings. Additionally, voltage sharing among the HV diode stack becomes more manageable as concerns regarding gate signal mismatch are eliminated. However, it is important to note that diodes lack current interruption capability. This limitation requires further investigation, particularly in non-unity power factor (PF) operations, which may impose restrictions on the operational range of the rectifiers. In terms of medium voltage (MV) DC circuit breakers (DCCB), this paper introduces the concept and design procedure of a high-power-density, modular, and scalable power electronic interrupter (PEI) for MV hybrid circuit breakers (HCB). The analysis includes trade-offs and limiting factors of various components within a single PEI module. A prototype of a 12 kV, 1 kA breaking-capable PEI is constructed, and new staged turn-off strategies are proposed to ensure the balanced distribution of metal-oxide varistor (MOV) energy. The developed PEI achieves a peak power density of 7.4 kW/cm$^3$, much higher than the solution based on the IGBT modules. After integrating the developed PEI into a full-scale HCB, the breaking capability of the developed PEI and the effectiveness of the staged turn-off strategy are validated. Furthermore, the scalability of the HCB is evaluated, which can simplify the design process from a low-voltage HCB to a higher-voltage version. For series-connected devices in SSCB or HCB configurations, the conventional gate driver structure necessitates an individual gate driver unit, fiber-optic, and isolated power supplies for each device. This design increases cost and volume, particularly for this single-pulse application. To address this issue, two new single gate driver structures are proposed to reduce component count and system complexity. The first solution, namely the MOV-coupled structure, employs a metal-oxide varistor (MOV) for the turn-off path. On the other hand, the transformer-coupled structure combines the auxiliary power and gate signal, enabling both simultaneous and staged turn-off schemes. Moreover, the cascaded high- and lower-voltage transformer structure simplifies insulation design and demonstrates improved scalability. These proposed gate driver structures aim to streamline the system, reduce component numbers, and simplify control for series-connected devices, leading to cost savings and improved overall performance. / Doctor of Philosophy / The advent of modern power electronics has paved the way for the implementation of medium-voltage (MV) DC systems, which offer advantages such as improved power quality, efficiency, and flexibility. However, the development of advanced AC/DC power conversion techniques and MVDC distribution system equipment, particularly MV DC circuit breakers (DCCBs), poses significant challenges for future MVDC systems. While the modular multilevel converter (MMC) is considered one of the best solutions, it suffers from a large number of devices and submodules (SMs). To overcome this limitation, novel topology concepts are introduced by combining high-voltage (HV) switches with low-voltage SMs, which leverage the benefits of both MMC and conventional multilevel converters. Several Hybrid MMC (HMMC) topologies, such as the three-level HMMC, flying capacitor HMMC, and hybrid-leg MMC, have been proposed. The introduction of additional HV switches enables different configurations over one line cycle, reducing the number of SMs and achieving higher power density and efficiency compared to the traditional MMC. Moreover, for unidirectional power flow, the hybrid modular multilevel rectifiers (HMMRs) can be derived by replacing the HV switch with HV diodes. This modification further reduces cost and volume compared to bidirectional converters. However, the non-unity power factor operation is different from the HMMC version, and more investigation is carried out in this work. As for MV DCCBs, the concept and design procedure of a compact, modular, and scalable power electronic interrupter (PEI) for MV hybrid circuit breakers (HCBs) are discussed. Additionally, two single gate driver structures are proposed to simplify the gate driver design, leading to a significant reduction in component count and cost. These advancements in topology solutions, MV DCCBs, and gate driver structures hold promise for the development of efficient and cost-effective MVDC systems.

Advancements in power semiconductor and power converter technology have enabled new low-voltage (LV) and medium-voltage (MV) direct current (DC) distribution systems for a variety of applications. Power electronics converters and DC circuit breakers (DCCBs) are the key components of a DC system and are hence the focus of this work. The combination of growing power density requirements and higher voltages can result in enhanced electric field (E-field) intensities, leaving the system vulnerable to partial discharges (PDs). The manifestation of such PD events gradually degrades the insulation system of the equipment, reducing its lifetime and ultimately leading to total insulation failure. Therefore, inception E-field based insulation design guidelines are developed to help achieve zero-PD operation of power electronics systems with considerations for internal as well as external (surface) E-field distribution. Additionally, surface E-field mitigation methods are experimentally investigated using representative PCB coupons to provide suitable solutions for low air pressure applications. Consequently, E-field management methods consisting of geometry-based techniques are proposed for PCB-based systems to mitigate E-field magnitudes in areas of the system that are prone to peak stresses (e.g. surface interconnections and triple junctions, conductor discontinuities, critical airgaps etc.). Successful design examples are provided including that of a 16 kV rated PCB-based DC bus and a 540 V, 100 kW aircraft generator rectifier unit operating at up to 50,000 ft cruising altitudes. DC circuit breaker (DCCB) technology, though crucial to ensure the safety of DC systems, is still in the early stages of development. As protection devices, their reliable operation is paramount and the selection and sizing of their components are not trivial. In this regard, comprehensive design guidelines are developed for the DC solid-state circuit breaker (SSCB) to ensure that its functional requirements can be met. System analyses and modeling are performed to understand the interactions between the various components, i.e. solid-state device, metal oxide varistor (MOV), and their impact on the breaker operation. A 2.5 kV, 400 A SSCB prototype is designed and verified with experimental results to validate the design approach. Traditional MOV based voltage clamping circuits (VCC) used in solid-state circuit breakers (SSCBs) impose a high interruption voltage on the main solid-state device. The voltage burden arises from the material properties of the MOV which fixes its clamping voltage at a value more than twice its maximum continuous dc voltage rating. A novel and reliable VCC termed as the electronic MOV (eMOV) is proposed to decouple the peak clamping voltage of the MOV from the nominal dc voltage of the system aiming to improve the voltage suppression index (V SI = Vpk/Vdc) of the VCC, thereby reducing the peak system voltage and allowing easier insulation design. By virtue of the proposed circuit, a lower voltage rated device can be used for the main switch enabling higher system efficiency and power density. In all, this work aims to address insulation system design for power electronics converters and systems, ultimately to eliminate PD under specified working voltage conditions for improved electrical safety and insulation lifetime. The implications of high-density integration, unsuitable ambient conditions and higher system voltages are considered to develop a suitable design and assessment methodology for practicing engineers. Techniques to mitigate/ manage E-Field inside and outside (surface) solid dielectric are proposed to attain the above goal. Additionally, design guidelines are formulated for DC SSCBs which are essential to the safety of DC distribution systems and an enhanced VCC is proposed for the same to limit its clamping voltage for easier insulation design. / Doctor of Philosophy / The recent advancements in power conversion technology have promoted the development and use of DC distribution networks for a variety of applications (e.g. electric ships, aircrafts, electric vehicle charging stations etc.). The insulation system of typical power electronics equipment consists of multiple solid insulating media (e.g. PCB dielectric, potting material, conformal coat etc.) separated by air gaps in the assembly. The combination of higher operating voltages, power density targets and unfavorable ambient conditions (e.g. low air pressure) can pose a risk to the insulation system of the equipment, if not addressed. The electric field (E-Field) stresses at certain vulnerable areas can exceed breakdown values of the corresponding media, initiating localized electrical discharge events also called as partial discharges (PD). Internal discharges generally occur in the vicinity of material defects, conductor discontinuities or sharp geometric features, while surface discharges may occur along exposed conductor metallizations on insulator surfaces (at the interface of multiple media) or critical air gaps in the assembly. PD events, while not posing any imminent threat, can degrade the surrounding area over time to reduce the operating life of the system and in some cases may cause catastrophic failures. Therefore, irrespective of location, such PD events must be eliminated to improve the overall system lifetime and reliability. Therefore, the main focus of this work is to develop insulation design guidelines and methodologies to achieve zero-PD operation of power converters and DC circuit breakers (DCCBs), both of which are key components of DC systems. A generalized design guideline is proposed to help with the insulation design of power electronics systems. Design techniques are developed to reduce E-field magnitude at critical areas to avoid over-designing the insulation system. Successful converter-level design examples are provided to validate the proposed approaches. DCCB technology is still in the early stages of development. As a protection device, its reliable operation is paramount and the selection and sizing of its components are not trivial. Therefore, in addition to the above insulation design methodology, comprehensive design guidelines are developed for the solid-state device and voltage clamping circuit (VCC) of the DC solid-state circuit breaker (SSCB), to ensure that its functional requirements can be met. Additionally, a novel VCC is proposed for the same to limit its fault interruption voltage for easier insulation design. Both SSCB and VCC prototypes are built and successfully demonstrated in a fault current breaking application. Overall, this dissertation provides a reference for the design and assessment of next generation power electronics converters and DC circuit breakers, to address, specifically, the challenges to their insulation systems.

No description available.