Direct Current (DC) transmission and distribution technologies have evolved in recent years. They offer superior efficiency, current carrying capacity, and response times as compared to conventional AC systems. Further, substantial advantages are their natural interface with many types of renewable energy resources, such as photovoltaic systems and battery energy storage systems at relatively high voltage, and compliance with consumer electronics at lower voltages, say, within a household environment. One of the core building blocks of DC-based technologies, especially at medium voltage levels, is power electronic systems technology. This cannot be emphasized enough as these units process, convert, and regulate all DC power and provide intelligence and sensing as electric power grids evolve. These advantages have led to a rise in the utilization and applications of DC in modern power systems. This includes high voltage DC transmission systems, DC distribution grids, DC microgrids, electric vehicle charging infrastructure, and the maritime industry. However, there are still substantial challenges to the operation of these systems. Examples include a lack of standards for DC based power infrastructure and DC system protection. This book presents the state of the art in medium voltage DC systems research and development, covering grid architecture, power converter design, transformers, control and protection for both traditional and mobile DC applications such as all-electric ships. This text, the first of its kind, provides essential information for researchers and research-oriented engineers working for academia, a manufacturer or utility, who wish to broaden or update their knowledge of medium voltage DC systems and associated equipment.
Inspec keywords: power electronics; distributed power generation; power transformers; power grids; power convertors
Other keywords: DC-DC power convertors; power transformers; power distribution faults; power distribution control; power convertors; renewable energy sources; distributed power generation; power distribution protection; power grids; power electronics
Subjects: Transformers and reactors; Power electronics, supply and supervisory circuits; General electrical engineering topics; Power convertors and power supplies to apparatus; Control of electric power systems; General and management topics; Distributed power generation; Power systems
The electrical energy supply system changed in many developed countries from a top-down grid architecture with large-scale central power stations towards a decentralized system with many medium- and small-scale distributed power generators. The architecture of the distribution grid must evolve from the classical radial grid structure towards a multi-terminal grid to better interconnect prosumers. The potential efficiency gains, material, and cost savings that can be realized by using DC technology in solid-state substations and distribution grids are so substantial that a radical change towards DC technology cannot be ignored. Electrical distribution grid structures have to change when electrical power production changes from high-power central power stations towards more decentralized small-scale power plants that may include a large fraction of volatile, renewable power generators.
The realization of MVDC systems represents an extremely rich area for research, particularly when it comes to pushing technologies, such as WBG power semiconductors and packaging of increasingly commercially available multi-chip modules into larger systems. While any MVDC compatible PEC can be designed and built for a specific application, the real challenge comes to viability is in the maturing of truly plug and play, modular approaches to the build-up of PEC solutions across a wide application space. This is especially challenging to the MVDC environment where there is little experience with regard to practicalities such as impacts of this environment to creepage and clearance constraints.
Typical distribution networks (DN) are predominantly AC and radial in nature. The voltage of a long-distance high voltage (HV) network is stepped down to a medium voltage (MV) level at a substation located a few tens of kilometres outside an urban area. Power demand of the downstream network is delivered to an inner-city substation using multiple parallel operating 3-phase AC links. Considering the critical function of this distribution link, adequate redundancy is employed to maintain the required power capacity during (n1) contingencies, which refers to the operating condition with single component failure in the system.
Development and commercialization of medium-voltage (MV), multi-megawatt DC-DC converters, that is, so-called DC (electronic) transformers, is a key component to realize flexible, interconnected MVDC grids. These DC transformers can provide an interface between high- and MV, as well as medium- and low-voltage distribution grids and loads. In addition, they can control continuously the power flow between arbitrary grid segments. Nevertheless, in such environments, the DC transformer has to be able to fulfil all functions of a classical AC transformer. AC transformers not only transform the voltage and provide the required insulation (protection against lightning strikes) but also play an important role in the coordination of fault currents with protection gear.
The power system landscape is rapidly evolving. The global push to reduce greenhouse gas emissions is driving change into the way electric power is generated, stored and delivered. Perhaps one of the biggest technological shifts being witnessed by the classical AC power grid is the utilization of DC power, across the full spectrum of generation, transmission and distribution levels. High-voltage DC(HVDC) transmission first emerged in the 1950s owing to its potential benefits such as low-loss delivery of bulk power over long distances and enhanced control of system power flows. The number of HVDC installations has steadily risen over the years, accompanied by advancements in semiconductor and converter technology, with the last decade or so seeing explosive levels of global growth. With the promise of realizing similar benefits for lower voltage and power applications, medium-voltage DC (MVDC) systems are now starting to emerge at the distribution level. As the grid continues to evolve it is anticipated that MVDC systems will play a key role in the reinforcement of legacy AC distribution networks, as well as to increase system efficiency and reliability.
Constant power loads (CPLs) exhibit negative incremental impedance characteristics contributing to destabilizing effects in power systems. Power electronic converters and motor drives, when tightly regulated, behave as CPLs. For the case of a motor drive, the inverter drives the motor and tightly regulates the speed to achieve constant speed. Assuming a linear relationship between torque and speed, motor torque will remain constant resulting in constant power consumption by the motor. For CPLs, the instantaneous value of the impedance is positive (V/I>0), but the incremental impedance is always negative (dV/dI<0). In the literature, the latter is referred to as negative incremental impedance instability.
The performance and operation of medium frequency and medium voltage inductors and transformers in DC-DC converter applications, several fundamental concepts must first be defined and understood. Numerous textbooks have been published on this topic and so this review chapter will emphasise the relevant topics and physics from an intuitive perspective while referencing detailed derivations and theoretical descriptions described elsewhere.
This chapter presents a general introduction to the fundamental theory and various aspects of stability issues in MVDC systems. The objective is to provide an overview of system stability analysis and physical reasoning to facilitate the design of the circuits, control, and protection of MVDC systems.
This Paper reviewed circuit breaker technologies for MVDC system protection, including MCBs, SSCBs and HCBs. Medium voltage direct current (MVDC) systems act as a layer of infrastructure between transmission and distribution to facilitate the installation of renewable resources and DC loads like wind farms, solar farms, electrical vehicles (EVs), etc. With reduced stages of power conversion, MVDC systems potentially feature higher efficiency compared to conventional AC systems. However, MVDC systems also propose new challenges, especially coming from the fault protection.
Marine vessels tend to change from full mechanical propulsion to electrical propulsion. With the development of high-voltage and high-current semi-conductor devices, DC electrical systems on marine vessels become economically feasible and competitive to conventional AC systems. Low voltage DC (LVDC)has been applied to small commercial marine and naval vessels. Medium voltage DC (MVDC) may be implemented on large commercial and naval vessels requiring high power. FOR utility power systems, the DC transmission and distribution systems have the main advantages of high power transfer capability over long distance, low loss, and ease of energy storage integration.
The power conversion architectures in multiple promising MVDC applications are elaborated in this paper. MVDC electrical distribution system architectures are informed by the topological implementations of MVAC to MVDC PECs and isolated MVDC to low voltage power electronic transformers and the approach to short circuit fault mitigation.