Comparisons of MVAC and MVDC systems in dynamic operation, fault protection and post-fault restoration

One of the most significant obstacles preventing the large-scale application of direct-current (DC) technology in medium voltage (MV) distribution networks is their fault protection. The existing AC relay protection needs to be changed or redesigned to protect the future overlay MVAC and MVDC distribution networks. Therefore, a comprehensive understanding of the dynamic and fault behavior and post-fault restoration strategies of MVAC and MVDC systems are critically important. Moreover, a comparison of MVAC and MVDC systems during a fault will also contribute to designing the protection systems of hybrid MV AC/DC systems. In this paper, the challenges of protecting DC faults of MVDC systems and possible solutions are first introduced. Then, the fault characteristics and post-fault restoration of MVDC and MVAC distribution systems are compared and investigated through case studies. Time-domain simulations have been conducted in PSCAD/EMTDC. The work in this paper will be valuable for the protection design for future hybrid MV AC/DC systems

Toward Fault Adaptive Power Systems in Electric Ships

Shipboard Power Systems (SPS) play a significant role in next-generation Navy fleets. With the increasing power demand from propulsion loads, ship service loads, weaponry systems and mission systems, a stable and reliable SPS is critical to support different aspects of ship operation. It also becomes the technology-enabler to improve ship economy, efficiency, reliability, and survivability. Moreover, it is important to improve the reliability and robustness of the SPS while working under different operating conditions to ensure safe and satisfactory operation of the system. This dissertation aims to introduce novel and effective approaches to respond to different types of possible faults in the SPS. According to the type and duration, the possible faults in the Medium Voltage DC (MVDC) SPS have been divided into two main categories: transient and permanent faults. First, in order to manage permanent faults in MVDC SPS, a novel real-time reconfiguration strategy has been proposed. Onboard postault reconfiguration aims to ensure the maximum power/service delivery to the system loads following a fault. This study aims to implement an intelligent real-time reconfiguration algorithm in the RTDS platform through an optimization technique implemented inside the Real-Time Digital Simulator (RTDS). The simulation results demonstrate the effectiveness of the proposed real-time approach to reconfigure the system under different fault situations. Second, a novel approach to mitigate the effect of the unsymmetrical transient AC faults in the MVDC SPS has been proposed. In this dissertation, the application of combined Static Synchronous Compensator (STATCOM)-Super Conducting Fault Current Limiter (SFCL) to improve the stability of the MVDC SPS during transient faults has been investigated. A Fluid Genetic Algorithm (FGA) optimization algorithm is introduced to design the STATCOM\u27s controller. Moreover, a multi-objective optimization problem has been formulated to find the optimal size of SFCL\u27s impedance. In the proposed scheme, STATCOM can assist the SFCL to keep the vital load terminal voltage close to the normal state in an economic sense. The proposed technique provides an acceptable post-disturbance and postault performance to recover the system to its normal situation over the other alternatives

A dual-bridge hybrid DC circuit breaker

Various DC circuit breakers (DCCBs) have been widely proposed for the DC fault protection of high-voltage direct-current (HVDC) grids. In recent years, hybrid DCCBs (HCBs) have been paid significant attentions due to their features of low power losses and fast dynamic response. However, several aspects regarding the design of HCB should be further addressed. For instance, the requirement of deploying a large surge arrester to dissipate the large fault current energy should be further addressed and the strategy to perform zero-voltage switching (ZVS) of semiconductor devices during the post-fault restoration processes should be investigated. In this paper, a dual-bridge hybrid DC circuit breaker (DB-HCB) with freewheeling diode branches is proposed to address the above issues. The operation principle of the proposed DB-HCB for pole-to-ground and pole-to-pole faults is presented. Compared with other HCBs, the capacity of the surge arrester is obviously reduced, so that the capital cost and volume of the proposed DB-HCB is decreased. Moreover, the ZVS is implemented during the post-fault restoration processes. Simulation results in PSCAD/EMTDC are given to validate the effectiveness of the proposed DB-HCB

Research and development of diagnostic algorithms to support fault accommodating control for emerging shipboard power system architectures

The U.S. Navy has proposed development of next generation warships utilising an increased amount of power electronics devices to improve flexibility and controllability. The high power density finite inertia network is envisioned to employ automated fault detection and diagnosis to aid timely remedial action. Integration of condition monitoring and fault diagnosis to form an intelligent power distribution system is anticipated to assist decision support for crew while enhancing security and mission availability. This broad research being in the conceptual stage has lack of benchmark systems to learn from. Thorough studies are required to successfully enable realising benefits offered by using increased power electronics and automation. Application of fundamental analysis techniques is necessary to meticulously understand dynamics of a novel system and familiarisation with associated risks and their effects. Additionally, it is vital to find ways of mitigating effects of identified risks. This thesis details the developing of a generalised methodology to help focus research into artificial intelligence (AI) based diagnostic techniques. Failure Mode and Effects Analysis (FMEA) is used in identifying critical parts of the architecture. Sneak Circuit Analysis (SCA) is modified to provide signals that differentiate faults at a component level of a dc-dc step down converter. These reliability analysis techniques combined with an appropriate AI-algorithm offer a potentially robust approach that can potentially be utilised for diagnosing faults within power electronic equipment anticipated to be used onboard the novel SPS. The proposed systematic methodology could be extended to other types of power electronic converters, as well as distinguishing subsystem level faults. The combination of FMEA, SCA with AI could also be used for providing enhanced decision support. This forms part of future research in this specific arena demonstrating the positives brought about by combining reliability analyses techniques with AI for next generation naval SPS.The U.S. Navy has proposed development of next generation warships utilising an increased amount of power electronics devices to improve flexibility and controllability. The high power density finite inertia network is envisioned to employ automated fault detection and diagnosis to aid timely remedial action. Integration of condition monitoring and fault diagnosis to form an intelligent power distribution system is anticipated to assist decision support for crew while enhancing security and mission availability. This broad research being in the conceptual stage has lack of benchmark systems to learn from. Thorough studies are required to successfully enable realising benefits offered by using increased power electronics and automation. Application of fundamental analysis techniques is necessary to meticulously understand dynamics of a novel system and familiarisation with associated risks and their effects. Additionally, it is vital to find ways of mitigating effects of identified risks. This thesis details the developing of a generalised methodology to help focus research into artificial intelligence (AI) based diagnostic techniques. Failure Mode and Effects Analysis (FMEA) is used in identifying critical parts of the architecture. Sneak Circuit Analysis (SCA) is modified to provide signals that differentiate faults at a component level of a dc-dc step down converter. These reliability analysis techniques combined with an appropriate AI-algorithm offer a potentially robust approach that can potentially be utilised for diagnosing faults within power electronic equipment anticipated to be used onboard the novel SPS. The proposed systematic methodology could be extended to other types of power electronic converters, as well as distinguishing subsystem level faults. The combination of FMEA, SCA with AI could also be used for providing enhanced decision support. This forms part of future research in this specific arena demonstrating the positives brought about by combining reliability analyses techniques with AI for next generation naval SPS

Reliability and cost-oriented analysis, comparison and selection of multi-level MVdc converters

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, selection of a suitable power electronic converter topology is a key aspect. From the pool of voltage source converters (VSCs), 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 UK) have been evaluated over two-level-VSC from 10 kV to 50 kV. Results show that with the increase of MVdc voltage level MMC shows better performance whereas at low MVdc levels 3L-NPC is the prominent topology

Operational Planning and Optimisation in Active Distribution Systems for Flexible and Resilient Power

The electricity network is undergoing significant changes to cater to environmental-deterioration and fuel-depletion issues. Consequently, an increasing number of renewable resources in the form of distributed generation (DG) are being integrated into medium-voltage distribution networks. The DG integration has created several technical and economic challenges for distribution network operators. The main challenge is basically the problem of managing network voltage profile and congestion which is caused by increasing demand and intermittent DG operations. The result of all of these changes is a paradigm shift in the way distribution networks operate (from passive to active) and are managed that is not limited only to the distribution network operator but actively engages with network users such as demand aggregators, DG owners, and transmission-system operators. This thesis expands knowledge on the active distribution system in three specific areas and attempts to fill the gaps in existing approaches. A comprehensive active network management framework in active distribution systems is developed to allow studies on (i) the flexibility of network topology using modern power flow controllers, (ii) the benefits of centralised thermal electricity storage in achieving the required levels of flexibility and resiliency in an active distribution system, and (iii) system resiliency toward fault occurrence in hybrid AC/DC distribution systems. These works are implemented within the Advanced Interactive Multidimensional Modelling Systems (AIMMS) software to carry out optimisation procedure. Results demonstrate the benefit provided by a range of active distribution system solutions and can guide future distribution-system operators in making practical decisions to operate active distribution systems in cost-effective ways

AN INVESTIGATION OF AN ENERGY DIVERTING CONVERTER FOR HVDC APPLICATIONS

Wind power generation in Europe has experienced an unprecedented expansion fuelled by a very favourable regulatory framework promoted to fight climate change. It is currently the second largest power generation source accounting for 17% of the total energy mix and in 2016 it covered an impressive 10.4% of the total energy demand. With faster wind speeds and better availability, offshore wind farm developments have also experienced a surge in recent years. There are 12.7 GW of cumulative installed capacity with the hot spot located in the North Sea. The grid integration of offshore wind farms has evolved to meet the requirements of recent projects, much larger in power capacity and located farther offshore. High voltage direct current (HVDC) connections using state of the art multilevel voltage source converters are now the industry standard for distant wind farms, with transmission capacities of up to 1 GW. The scale of the projects and frequent grid weakness at the onshore locations challenge transmission system operators which need to ensure the entire grid stability. Grid codes have evolved to regulate such interconnections, with a set of well specified requirements which need to be fulfilled. One such requirement is the fault ride-through capability, which defines the need for the HVDC interconnector to remain connected during onshore grid faults. A Dynamic Braking System (DBS) is a power electronics device that provides fault ride-through capability to the HVDC interconnector by absorbing the excess energy injected to the link for the duration of the fault. This energy is commonly dissipated in a resistive element. In this way the DC over-voltage is avoided and the operation of the connected wind farms is kept undisturbed. There is a lack of knowledge in the design and implementation of such devices. Therefore four concepts put forward by industry and other researchers are studied in this work. The rating of the different components in each circuit is investigated as the basis for the comparison. Taking into account the modular structure of AC/DC converters in HVDC stations it makes commercial sense to reuse the same modules as building blocks for the DBS. With modular structures, a good balancing of the total energy stored in the converter and its distribution among the different modules is one of the key elements. Modular DBS circuits can synthesize multilevel voltage waveforms, allowing for advanced power modulation strategies. Two novel strategies are developed in the thesis and an accurate mathematical modelling is performed to ensure that the energy balance conditions are met for all points of operation. An overall control strategy for each of the four circuits is also developed and presented in the thesis. A good coordination of the protective actions of the DBS and the main HVDC converters is important to ensure that no negative interactions occur. An operation strategy based on over-voltage thresholds is developed in the thesis. Accurate simulation models of the HVDC link integrating the DBS and controls are also implemented to give the required degree of confidence in the overall system behaviour. These are finally validated by a laboratory scaled-down test platform, where the control actions and the different converters are implemented in real hardware, and the correct coordination of all the elements during a fault event is experimentally tested. The main drawback of the DBS solution usually highlighted in literature is its cost. The option of adding some extra functionality to better justify the economic investment is explored in this thesis, resulting on a multifunctional circuit named Energy Diverting Converter (EDC). Two proposals including active filtering and HVDC tapping are developed in this thesis, for which two patent applications have been filed

State of the Art Review of High Voltage Insulation Monitoring

The devastating effects of global warming and climate change are now well understood and there is broad unity that fundamental changes are needed. This is clearly addressed in the United Nations Sustainable Development Goals. The main perpetrator contributing to global warming and climate change is how we consume energy, which will need to transition from fossil fuels to renewable energy. The mass integration of renewable energy sources aimed to mitigate the effects of global warming, will greatly alter how we generate, transmit and consume energy. If we combine this with the large shift in load consumption, due to the integration of electrical vehicles, there is no doubt that the electrical transmission system will be subjected to major changes in future decades. The existing transmission grid is an aged and mature system, with large parts being installed all the way back in 60s and 70s, thus nearing the end of service. The existing grid has continuous performance issues and the knowledge on fault and ageing mechanisms are still insufficient. A thorough assessment of the current state of the grid is necessary in order to properly gauge its ability to cope with mass integration of HV systems, predominantly HVDC. A key part in assessing the current state of the grid while simultaneously increase its resilience is the utilization of high voltage monitoring methods, as they are key to prevent and predict transmission faults. Due to the increased requirement of long distance high capacity transmission, especially in submarine conditions, the knowledge and monitoring of cables will be of high importance. Compared to AC technology, DC have been regarded as niche and specialist field, thus have been allocated far less attention and research, hence the knowledge and technology of DC is still limited. This thesis will assess the state of the art of high voltage monitoring while simultaneously explore its role towards achieving the UN Sustainable Development Goals. Keywords: UN Sustainability Goals, Partial Discharges, Tan Delta, SF6, XLPE, High Voltage MonitoringMasteroppgave i energiENERGI399MAMN-ENER