Search

Using second-life batteries (SLBs) to build battery energy storage systems (BESS) yields substantial environmental and economic benefits. The cascaded H-Bridge (CHB) converter has emerged as an attractive candidate to integrate SLBs into the electrical grid, allowing the unbalanced power distribution among its sub-modules (SMs) with high efficiency and a low estimated cost. However, capacity differences among SLBs pose further challenges for the control system in meeting the BESS power constraints, while balancing the state-of-charge (SoC) of SLBs. This work proposes a dual-stage model predictive control (DS-MPC) strategy to balance the SoC of SLBs using a delta-connected CHB (DCHB) converter. The formulation of the proposed DS-MPC strategy is based on a discrete-time SoC dynamic model, which considers the SM modulating signals and the DCHB circulating current reference in the rotating synchronous dq-frame as control inputs. In this way, the proposed DS-MPC strategy obtains optimal charging and discharging currents for each SLB-SM by solving two sequential optimizations, which include maximum current ratings and converter modulation constraints, ensuring the safe operation of the BESS. Experimental results that verify the effectiveness of the proposed DS-MPC strategy are provided for a DCHB converter with nine SMs connected to Lithium-ion SLB packs of different capacities.

In grid-connected power converters, active and reactive power references are typically provided for calculating current and/or voltage references. This is generally achieved by using the standard instantaneous power theory (IPT). However, when a converter is required to operate with different per-phase power levels, the IPT cannot be applied to fully describe this inter-phase power interaction. To address this limitation, this work presents an interpretation of the inter-phase power imbalance problem, which enables the representation of the system as a combined electrical interaction between the unequal per-phase active powers on the inverter side and balanced power on the grid side. To achieve this, a Clarke transformation based on sequence order rotation is proposed. Thus, per-phase quadrature $\alpha \beta$-components can be obtained, allowing the instantaneous per-phase unbalanced power on the inverter side to be analytically derived. Based on this representation, an analytical expression of the instantaneous zero-sequence voltage (ZSV) is obtained. This ZSV allows one to extract unbalanced per-phase power on the inverter side while injecting balanced power on the grid side. A case study showing experimental results from a 10 kVA cascaded H-bridge converter applied to a scaled-down medium-voltage (MV) photovoltaic system are presented.

This study aims to prepare the energy sector for uncertainty using a foresight tool known as weak signals. Weak signals (subtle signs of emerging issues with significant impact potential) are often overlooked during strategic planning due to their inherent predictive uncertainty. However, the value does not lie in precise forecasting but in broadening the consideration of future possibilities. By proactively monitoring and addressing these otherwise neglected developments, stakeholders can gain early awareness of threats and opportunities and enhance their resilience, adaptability, and innovation. A panel of technology experts identified eight weak signals in this study: 1) growing mistrust and local grid security measures, 2) consumer reactions to overly prescriptive policies, 3) long-term forecasting errors for thin-margin projects, 4) emergence of variable power industries, and 5) establishment of intercontinental transmission precedence; including three potential ‘wild cards’ requiring proactive mitigation: 6) escalating electrical generation dependence on continued imports, 7) a new threat surpassing climate change, and 8) mass deployment of low-emissions technology triggering a runaway loss of social license. Political factors were the predominant source of uncertainty, as decisions can suddenly transform the energy landscape. Economic, technological, and social factors followed closely behind, generally through the emergence of new industries and behavioural responses. While environmental and legal factors were less frequent, stakeholders should still adopt a holistic approach, as the signals were found to be highly interconnected. Organisations should also assess their local context when applying these findings and continuously update and respond to their own list of weak signals.

This paper proposes a steady-state power distribution derivation method for voltage source converter (VSC)-based multi-terminal HVDC (MTDC) systems and dc grids under mixed power/voltage (P/V) and current/voltage (I/V) droop control. P/V and I/V droop control are two commonly used control schemes, which can be combined to achieve co-regulation of powers & currents in MTDC systems and dc grids. The proposed method can be used to estimate the power distributions under different scenarios for MTDC systems and dc grids based on VSCs with mixed P/V and I/V droop control. After determining the initial operating point based on an estimation-correction algorithm, redistributed power due to power disturbances, current changes or converter outages is analyzed in detail considering converter overload. An excess power reduction strategy is further proposed to avoid violation of power limits after converter outage. The accuracy of the proposed method is validated through multiple scenarios in a modular multilevel converter (MMC)-based four-terminal dc grid. The comparison between the proposed method and other approaches in the current literature further demonstrates the advantages of proposed power distribution derivation method.

In the operation of battery energy storage systems based on the cascaded H-bridge converter, it is beneficial to balance the state of charge of batteries in different submodules within the converter phase-arm. This is achieved by distributing the active power among the submodules. Although multiple methods have been proposed for this purpose, they face the challenge of rendering optimal active power distributions that maximize balancing speed while meeting power constraints in the battery energy storage system. To overcome this challenge, a model predictive control scheme is developed in this paper. The proposed method is remarkably robust against parametric uncertainties (battery voltage, capacity, etc.), as evidenced by its ability to tolerate a substantial 50% uncertainty in the parameters, resulting in a mere 0.05% steady-state error. Furthermore, because the predictive control can be executed at a low frequency, the computational burden is comparable to other existing methods.

Background: The Asclepion of Epidaurus is one of the first healing environments in the world. Descendants of Asclepius, specifically medical students, have been singularly deprived of any information concerning this legacy. This article illuminates the role of Asclepion of Epidaurus and examines the view of medical students upon the subject and the possible benefits of this knowledge in their medical education. Methods: The participants were 105 senior-year students from the Athens Medical School, who attended a multi-media assisted lecture related to the structure and the role of the Asclepion of Epidaurus. Afterwards, they answered anonymously a questionnaire of 12 pairs of opposite adjectives in order to describe their view regarding the meaning of Asclepion. The method used in the evaluation of their answers was that of semantic differential. Results: The attitude of the students towards the meaning of Asclepion was positive, showing interest and excitement about a powerful, though unfamiliar piece of knowledge. Conclusion: Today’s novice doctors have welcomed the concept of Asclepion as essential knowledge for the service they will be called to fulfill. The potential benefits of the Asclepian ideals in medical education and ethos are thoroughly discussed. © 2022, The Author(s).

Political decarbonisation commitments and outcompeting renewable electricity costs are disrupting energy systems. This foresight study prepares stakeholders for this dynamic, reactive change by examining visions that constitute a probable, plausible and possible component of future energy systems. Visions were extrapolated through an expert review of energy technologies and Australian case studies. ‘Probable–Abundant’ envisages a high penetration of solar and wind with increased value of balancing services: batteries, pumped hydro and transmission. This vision is exemplified by the South Australian grid, where variable and distributed sources lead generation. ‘Plausible–Traded’ envisages power and power fuel exports given hydrogen and high-voltage direct-current transmission advances, reflected by public and private sector plans to leverage rich natural resources for national and intercontinental exchanges. ‘Possible–Zero’ envisages the application of carbon removal and nuclear technologies in response to the escalating challenge of deep decarbonisation. The Australian critical minerals strategy signals adaptations of high-emission industries to shifting energy resource values. These visions contribute a flexible, accessible framework for diverse stakeholders to discuss uncertain energy systems changes and consider issues from new perspectives. Appraisal of preferred futures allows stakeholders to recognise observed changes as positive or negative and may lead to new planning aspirations.

Arm inductances and the operation of two arms of a phase leg in an alternate arm converter (AAC) can lead to hard-switching of the director switches (DSs) during steady-state operation, increasing voltage stresses across the DSs. The DS voltage stress also depends on the magnitude of the interrupted arm current, which, in turn, depends on the output active/reactive power. Hard-switching of the DSs limits the operating region of the AAC. This article characterizes the zero-current switching (ZCS) capability of the AAC and proposes extended ZCS operation in steady state for HVDC applications. This is achieved by coordinating the AAC operating point and the transformer on-load tap changer (OLTC). The coordinated operation utilizes the tap positions of the HVDC transformer effectively to extend the ZCS operating region with no added effort on circulating current control. Results based on an AAC-HVDC transmission system demonstrate the effective coordination of the transformer OLTC and the converter operating point to achieve extended ZCS performance.

The increasing importance of cascaded multilevel converters (CMCs), and the sub-category of modular multilevel converters (MMCs), is illustrated by their wide use in high voltage DC connections and in static compensators. Research is being undertaken into the use of these complex pieces of hardware and software for a variety of grid support services, on top of fundamental frequency power injection, requiring improved control for non-traditional duties. To validate these results, small-scale laboratory hardware prototypes are often required. Such systems have been built by many research teams around the globe and are also increasingly commercially available. Few publications go into detail on the construction options for prototype CMCs, and there is a lack of information on both design considerations and lessons learned from the build process, which will hinder research and the best application of these important units. This paper reviews options, gives key examples from leading research teams, and summarizes knowledge gained in the development of test rigs to clarify design considerations when constructing laboratory-scale CMCs.

The ongoing transformation of power systems driven by the need for more interconnected networks will eventually lead multiterminal dc (MTDC) grids to become more commonplace in future electricity networks. Addressing interoperability challenges associated with multiple converter topologies for future MTDC implementations is a paramount requirement. This paper introduces a multi-converter MTDC (MC-MTDC) system combining both state-of-the-art and emerging modular voltage source converter (VSC) topologies. The developed MC-MTDC system, (i) demonstrates operation of the modular multilevel converter (MMC) and the alternate arm converter (AAC) in different MTDC configurations, and (ii) sets the basis for further development of MTDC configurations with diverse modular VSC topologies. The developed MC-MTDC system will also facilitate accurate testing and validation of multi-converter interoperability using real-time electromagnetic transient (EMT) models. The design parameters of the AAC-based terminals are determined ensuring compliance with existing MMC-based dc power systems. Detailed analysis and a complete set of results based on real-time implementation for different operating scenarios including steady-state, transient, and contingencies, verify multi-converter interoperability and associated control functions in different MTDC configurations.

The modular multilevel converter (MMC) is currently one of the power converter topologies which has attracted more research and development worldwide. Its features, such as high quality of voltages and currents, high modularity and high voltage rating, have made the MMC a very good option for several applications including high-voltage dc (HVdc) transmission, static compensators (STATCOMs), and motor drives. However, its unique features such as the large number of submodules, floating capacitor voltages, and circulating currents require a dedicated control system able to manage the terminal variables, as well as the internal variables with high dynamical performance. In this paper, a review of the research and development achieved during the last years on MMCs is shown, focusing on the challenges and proposed solutions for this power converter still faces in terms of modeling, control, reliability, power topologies, and new applications.

This paper demonstrates the interoperability of an emerging alternate arm converter (AAC) with the state-of-the-art modular multilevel converter (MMC) in high-voltage direct current (HVDC) systems based on a hybrid VSC-HVDC system. The paper also showcases the parameter derivation of the hybrid HVDC system and its detailed control structure. The study provides preliminary steps towards detailed analysis of AAC interoperability in complex hybrid dc grid configurations. A detailed set of results based on the 800 MVA hybrid voltage source converter (VSC)-HVDC system showcases the interoperability performance of the AAC under different operating scenarios and verifies its associated control functions.

A hybrid HVDC transmission system based on a line-commutated converter (LCC) and an alternate arm converter (AAC) is developed in this article. The basic operation principles and control methods of the proposed LCC-AAC hybrid HVDC system are demonstrated following with appropriate system design and parameter selection. In addition, this paper proposes a set of control schemes for riding through ac faults and handling dc fault. The operation and performance of both converters in such a hybrid HVDC configuration are verified through a detailed set of results based on a 3000 MVA LCC-AAC hybrid HVDC system including steady state operation, ac-grid harmonics & dc-side ripple analysis, reference tracking (active power change at LCC station, reactive power change at AAC station) and system performance under ac-grid faults without blocking converter at both terminals of the HVDC system. Also, the response of the hybrid system under dc fault presents potential dc fault handling capability compared with the hybrid system based on LCC and modular multilevel converter (MMC) with half-bridge submodules (HBSMs). The hybrid LCC-AAC HVDC transmission system lays preliminary steps for research on complex mixed converter multiterminal dc configurations in future dc super grids.

Low losses are a major advantage of the modular multilevel converter (MMC) in high-power applications. Losses on the MMC depend on a number of parameters such as switching frequency, voltage balancing algorithm, the topology of the sub-module (SM). This paper investigates the impact of different power factors in the losses of different MMC SMs with a focus on HVDC applications. Considering 27 different SM topologies, we demonstrate that the operation mode of the converter (inverter / rectifier) and the power factor affect not only the average losses but also the minimum and maximum losses within a period due to the voltage balancing requirements.

Benchmark models provide a common platform that facilitates comparison of different methods and strategies that can be further validated in a real representation of the power system. Nevertheless, when using RMS test cases, transients present in modern power systems may be neglected. Modern energy conversion systems require an enhanced level of detail in order to properly represent their behavior. Moreover, due to the steady increase of non-synchronous generation, new operation and management strategies need to be tested to ensure power system security. One way to overcome these constraints is to use EMT real-time digital simulations which allows to obtain high precision results in a timeframe suited to a specific study. This work presents an EMT real-time model for integration studies of advanced energy conversion systems that can be flexibly expanded to higher renewable penetration. The model, based on the Simplified Australian 14-Generator test system, has been expanded to represent in an improved way the dynamics of power system components as well as the high penetration of non-synchronous power electronics generation. Transient simulations are undertaken to illustrate system's response due to non-synchronous generation and when faults are applied. This work also addresses the current gap in benchmark models for real-time simulations.

The proportion of power electronics interfaced generation connected to electricity grids continues to increase annually. Likewise, the complexity of the power system and all its relevant assets (communication, IT, market operations etc) has also expanded. All of these developments necessitate new tools and methods in ensuring a stable and reliable power system. To increase simulation reliability, networks are modeled across multiple software where they are studied. This also provides a common platform to facilitate comparison of different methods and strategies. EMT-type models are more accurate and can provide more details during transient simulations when compared to RMS-type models. This work develops an EMT real-time model using the HYPERSIM simulation software, based on the existing Simplified Australian 14-Generator power system model in PSS/E. The network with its topology and components is converted from the RMS domain to the EMT one. The benchmark model is verified by static load flow calculations and transient fault analysis to illustrate the validity of the model.