Small scale renewable generation unlocking an era of peer-to-peer energy trading and internet of energy

Millions of people worldwide suffer from lack of reliable electric energy supply. Energy justice scholarship has noted that small scale decentralized renewable energy offers a unique opportunity to democratize local energy provision, increasing the access to and affordability of electricity for those who are currently on the margins of centralized energy provision systems. This, in turn, is believed to result in critical human development benefits at the local level.There is a strong drive in the industrial and academic community toward the deregulation and decentralization of power systems to enable wider deployment of medium and small-scale renewable energy resources such as wind and solar systems. In a centralized power system, the flow of electric power is unidirectional from generators to consumers. In a decentralized system of distributed generators and consumers that are also able to produce energy (prosumers), power flow is no more unidirectional, and so are payments. To this end, peer-to-peer (P2P) energy trade concept aims to provide the business model and technical infrastructure enabling prosumers to trade their produced energy with one another in addition to (or instead of) trade with the utility. This eventually will realize a power grid structure based on the concept of Internet of Energy (IoE) where electric power becomes a commodity tradable in an open market. Implementation of this concept is made possible by the ongoing migration of traditional power grids from centralized systems to more decentralized networks so as to accommodate renewable and distributed energy resources (DERs) as well as Smart Grid infrastructure.The generic P2P energy trading system can be represented as a four-layer architecture. The basic (physical) layer is the power grid layer followed by communications layer, a control layer, and business layer at the highest level. These interoperable layers control the whole P2P trading process whereas peers can be prosumers, electric vehicle (battery) owners, microgrids, or regions of the power system. Much of the attention in the literature has been dedicated to developing appropriate structures of the communications and business layers in a bid to realize P2P trading without expensive alterations to the existing AC grid physical infrastructure. Therefore, researchers and innovators focus on developing platforms that run different forms of trading processes among peers taking in account grid security, economic incentives, and system operator requirements. Various criteria for peers to select who to trade with are possible, including ‘least power loss’, ‘highest reliability’ or ‘most environmental’.  As it is very hard to trace actual power flows in a power grid and reward prosumers for their energy production, energy market regulators normally issue Renewable Energy Certificates (RECs) as an incentive for system operators to purchase renewable energy from DER owners. For instance, in the US each DER owner is issued 1 REC, which is tradable with utilities, for each 1MWh of energy produced (REC is named differently in different countries). Likewise, it is the common theme between the various P2P energy trade platforms whether in operation or under development to manage energy trade between peers and third parties by means of trading RECs, or equivalent tokens, efficiently and securely.Some of such platforms utilize third party for transactions auditing, while the more advanced platforms are built on Blockchain technology to realize near-instant decentralized payment and auditing of transactions without the need for a third party.</p

Analysis and design of a modular multilevel converter with trapezoidal modulation for medium and high voltage DC-DC transformers

Conventional dual active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique where staircase approximated trapezoidal voltage waveforms are produced; thus alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. Therefore, the analyzed converter is a modular multi-level converter operated in a new mode with no common-mode dc arm currents as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft switching characteristics will be shown to be comparable to those of the two-level dual active bridge. Experimental results from a scaled test rig validate the presented concept

Analysis and design of a modular multilevel converter with trapezoidal modulation for medium and high voltage dc-dc transformers

Conventional dual active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique where staircase approximated trapezoidal voltage waveforms are produced; thus alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. Therefore, the analyzed converter is a modular multi-level converter operated in a new mode with no common-mode dc arm currents as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft switching characteristics will be shown to be comparable to those of the two-level dual active bridge. Experimental results from a scaled test rig validate the presented concept

Generalized switching function model of modular multilevel converter

This paper presents a generalized switching function model of the modular multilevel converter (MMC) that can be used instead of MMC electromagnetic transient simulation model for full-scale simulations of high-voltage dc (HVDC) and flexible ac transmission systems (FACTS). The proposed method is computationally more efficient and numerically stable than its electromagnetic transient simulation counterpart, and it is applicable for wide range of studies, including ac and dc network faults. The proposed switching function model is packaged in a graphical form to suit various simulation platforms such as Simulink and PSCAD. The validity of the presented model is confirmed using simulation and its scalability has been demonstrated on MMC with 301 cells per arm, considering power reversal during normal operation and dc short circuit fault

Ride-through capability of grid-connected brushless cascade DFIG wind turbines in faulty grid conditions—a comparative study

Doubly-fed induction generator (DFIG)-based wind turbines (WTs) are reported to suffer reliability problems due to the presence of slip-rings, brushes, and the gearbox. These disadvantages encouraged several research groups to investigate the viability of employing single or double-frame brushless cascade DFIGs (BCDFIGs) in grid-connected multimegawatt WTs especially offshore and in solar chimney power plants. In this regard, this paper tackles three main issues. Initially, the expected reduction in gear ratio when BCDFIGs are used instead of DFIGs is examined from a steady state perspective. A reduction would lead to less frequent maintenance and an improved return on investment. Next, a detailed comparison between DFIG-WTs and BCDFIG-WTs under unbalanced grid voltage is presented. Finally, the extent to which a multimegawatt BCDFIG-WT is grid code compliant in terms of fault ride-through capability is studied. This is illustrated by comparing and quantifying the response of a DFIG-WT and a BCDFIG-WT to a severe three-phase voltage dip. Simulation and experimental results indicate promising behavior for BCDFIGs during disturbances

The transition arm multilevel converter — a concept for medium and high voltage DC-DC transformers

This paper proposes a dc/ac converter topology where each phase leg comprises a conventional series switch valve in one arm and a series connection of half-bridge chopper cells in the other arm. Operation with trapezoidal modulation is analyzed where the half-bridge modules have a reduced footprint and silicon area compared to standard modular multilevel converter modules. Furthermore, no arm inductance is required. The utilization of the proposed converter topology in medium and high voltage front-to-front DC-DC converter topologies is analyzed. Simulations investigate key features of the converter in comparison to other published topologies.Scopu

Quasi two-level operation of modular multilevel converter for use in a high-power DC transformer with DC fault isolation capability

DC fault protection is one challenge impeding the development of multi-terminal DC grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and DC fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode DC component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a DC to DC transformer where two converters operating in the proposed mode are coupled by an AC transformer for voltage matching and galvanic isolation. The proposed DC transformer is shown to be suitable for high-voltage high-power applications due to the low switching frequency, high efficiency, modularity, and reliability. The DC transformer facilitates DC voltage regulation and near instant isolation of DC faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach