Imperial College London

Professor Tim Green, FREng

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

Professor of Power Engineering
 
 
 
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Contact

 

+44 (0)20 7594 6171t.green Website CV

 
 
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Location

 

Energy Futures Lab, Electrical Eng BuildingElectrical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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400 results found

Gu Y, Green T, 2022, Power system stability with a high penetration of inverter based resources, Proceedings of the Institute of Electrical and Electronics Engineers (IEEE), ISSN: 0018-9219

Inverter-based resources (IBRs) possess dynamics that are significantly different from those of synchronous-generator-based sources and as IBR penetrations grow the dynamics of power systems are changing. This article discusses the characteristics of the new dynamics and examines how they can be accommodated into the long-standing categorizations of power system stability in terms of angle, frequency, and voltage stability. It is argued that inverters are causing the frequency range over which angle, frequency, and voltage dynamics act to extend such that the previously partitioned categories are now coupled and further coupled to new electromagnetic modes. While grid-forming (GFM) inverters share many characteristics with generators, grid-following (GFL) inverters are different. This is explored in terms of similarities and differences in synchronization, inertia, and voltage control. The concept of duality is used to unify the synchronization principles of GFM and GFL inverters and, thus, established the generalized angle dynamics. This enables the analytical study of GFM-GFL interaction, which is particularly important to guide the placement of GFM apparatuses and is even more important if GFM inverters are allowed to fall back to the GFL mode during faults to avoid oversizing to support short-term overload. Both GFL and GFM inverters contribute to voltage strength but with marked differences, which implies new features of voltage stability. Several directions for further research are identified, including: 1) extensions of nonlinear stability analysis to accommodate new inverter behaviors with cross-coupled time frames; 2) establishment of spatial–temporal indices of system strength and stability margin to guide the provision of new stability services; and 3) data-driven approaches to combat increased system complexity and confidentiality of inverter models.

Journal article

Jansen M, Duffy C, Green T, Staffell Iet al., 2022, Island in the Sea: The prospects and impacts of an offshore wind power hub in the North Sea, Advances in Applied Energy, Vol: 6, Pages: 1-14, ISSN: 2666-7924

Europe's offshore wind capacity is increasing rapidly, with larger turbines installed further from shore. TenneT proposed an innovative concept, the North Sea Wind Power Hub, in which several farms are connected to an artificial island which has interconnection to surrounding countries. This aims to reduce overall costs, but studies have so far evaluated hypothetical designs rather than the specific system operators’ proposal, and focused primarily on construction costs rather than ongoing system-wide impacts. Here we develop a bottom-up capital cost estimate to compare the Power Hub with conventional point-to-point connection for offshore wind farms. We quantify its consequential impacts on regional electricity prices, the value of wind generation, fossil fuel generation and carbon emissions in 2030 using a pan-European electricity market model. We find that the Power Hub is cheaper if more than 10 GW of wind is built, as the avoided cost of multiple converter platforms offsets the cost of building an artificial island. It has profound impacts on electricity markets across Europe, especially in Britain and Ireland if they gain more interconnection to continental Europe. This impacts the economic viability of wind and gas power stations particularly, and saves 10Mt CO2 per year through reduced curtailment.

Journal article

Zhu Y, Gu Y, Li Y, Green Tet al., 2022, Impedance-based root-cause analysis: comparative study of impedance models and calculation of eigenvalue sensitivity, IEEE Transactions on Power Systems, ISSN: 0885-8950

Impedance models of power systems are useful when state-space models of apparatus such as inverter-based resources (IBRs) have not been made available and instead only black-box impedance models are available. For tracing the root causes of poor damping and tuning modes of the system, the sensitivity of the modes to components and parameters are needed. The so-called critical admittance-eigenvalue sensitivity based on nodaladmittance model has provided a partial solution but omits meaningful directional information. The alternative whole-system impedance model yields participation factors of shunt-connected apparatus with directional information that allows separate tuning for damping and frequency, yet do not cover series-connected components. This paper formalises the relationships between the two forms of impedance models and between the two forms of root-cause analysis. The calculation of system eigenvaluesensitivity in impedance models is further developed, which fills the gaps of previous research and establishes a complete theory of impedance-based root-cause analysis. The theoretical relationships and the tuning of parameters have been illustrated with a three-node passive network, a modified IEEE 14-bus network and a modified NETS-NYPS 68-bus network, showing that tools can be developed for tuning of IBR-rich power systemswhere only black-box impedance models are available.

Journal article

Hou X, Sun K, Zhang N, Teng F, Zhang X, Green TCet al., 2022, Priority-driven self-optimizing power control scheme for interlinking converters of hybrid AC/DC microgrid clusters in decentralized manner, IEEE Transactions on Power Electronics, Vol: 37, Pages: 5970-5983, ISSN: 0885-8993

Hybrid AC/DC microgrid clusters are key building blocks of smart grid to support sustainable and resilient urban power systems. In microgrid clusters, the subgrid load-priorities and power quality requirements for different areas vary significantly. To realize optimal power exchanges among microgrid clusters, this paper proposes a decentralized self-optimizing power control scheme for interlinking converters (ILC) of hybrid microgrid clusters. A priority-driven optimal power exchange model of ILCs is built considering the priorities and capacities in subgrids. The optimization objective is to minimize the total DC-voltage/AC-frequency state deviations of subgrids. To realize the decentralized power flow control, an optimal-oriented quasi-droop control strategy of ILCs is introduced to not only achieve a flexible self-optimizing power flow management, but also provide an ancillary function of voltage support. Consequently, as each of ILCs only monitors the local AC-side frequency and DC-side voltage signals, the whole optimal power control of the wide-area microgrid clusters is achieved in a decentralized manner without any communication link. Thus, the proposed control algorithm has the features of decreased cost, increased scalability, reduced geographic restrictions and high resilience in terms of communication faults. Finally, the proposed method is validated by case studies with four interconnected microgrids through hardware-in-loop tests.

Journal article

Bravo P, Pereda J, Merlin M, Neira S, Green T, Rojas Fet al., 2022, Modular multilevel matrix converter as solid state transformer for medium and high voltage substations, IEEE Transactions on Power Delivery, ISSN: 0885-8977

The use of power converters as solid state transformer is an attractive solution to modernize the power network, but this solution has not been fully addressed for MV and HV substations. This paper presents a customized and simple control for the Modular Multilevel Matrix Converter (M3C), specially conceived for its operation on synchronous ports, which is the case of AC substations. The control allows to transfer bidirectional active power, generate/absorb reactive power and provide ancillary services. The converter is compared to the back-to-back Modular Multilevel Converter (B2B-MMC) where the key performance indicators to carry out the comparison are power efficiency, number of semiconductor devices, passive components required, footprint, voltage cell balance, fault blocking capability and stress of components. The simulation results show the features, performance and attractiveness of the M3C topology in a 33/11 kV, 16 MW substation under different operating conditions, including grid faults and dynamic operation. The M3C presents similar efficiency and performance than the B2B-MMC, but it uses less semiconductor devices, passive components and total cell capacitor energy than the B2B-MMC, reducing cost and footprint. The experimental results show the performance of the M3C under less ideal conditions including a substation transformer saturation and power step response.

Journal article

Li Y, Gu Y, Green T, 2022, Revisiting grid-forming and grid-following inverters: a duality theory, IEEE Transactions on Power Systems, ISSN: 0885-8950

Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters. They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure. In this article, a new perspective based on duality is proposed to create new insights. It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form. Analysis shows that the grid-forming and grid-following inverters are duals of each other in several ways including a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltage-following current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) inner-loop controllers: output impedance shaping and output admittance shaping; and e) grid strength compatibility: strong-grid instability and weak-grid instability. The swing equations are also derived in dual form, which reveal the dynamic interaction between the grid strength, the synchronization controllers, and the inner-loop controllers. Insights are generated into cases of poor stability in both small-signal and transient/large-signal. The theoretical analysis and simulation results are used to illustrate cases for single-inverter systems, two-inverter systems, and multi-inverter networks.

Journal article

Li Y, Gu Y, Green T, 2022, Mapping of dynamics between mechanical and electrical ports in SG-IBR composite grids, IEEE Transactions on Power Systems, ISSN: 0885-8950

Power grids are traditionally dominated by synchronous generators (SGs) but are currently undergoing a major transformation due to the increasing integration of inverter-based resources (IBRs). The state space method with transparent apparatus models can be readily used. However, models of IBRs are usually not disclosed by manufacturers. Alternatively, the port-based approach represents dynamics by input-output transfer functions without exposing internal states. These transfer functions at various ports are normally configured with a particular focus: an SG-dominated grid is traditionally analyzed in a mechanical-centric view which ignores fast electrical dynamics and focuses on the torque-speed dynamics, whereas the emergent IBR-dominated grid usually takes an electrical-centric view which focuses on the voltage-current interaction. In this article, a new perspective called the port-mapping method is proposed to combine these two views. Specifically, the mechanical dynamics are mapped to the electrical impedance seen at the electrical port; and the electrical dynamics are also mapped to the torque coefficient seen at the mechanical port. The bidirectional mapping gives additional flexibility and insights to analyze the sub-system interactions in whole-system dynamics and guide the tuning of parameters. Application of the proposed method is illustrated in three cases with increasing scales.

Journal article

Singh N, Collins C, Green T, 2022, Fast fault current via STATCOM with electronic tap-changer, IET Developments in Power System Protection (DPSP 2022), Publisher: Institution of Engineering aand Technology

Reducing levels of fault current infeed as inverter-based resources (IBR) displace synchronous machines are a concern forsystem operators. The reduction undermines the ability of a conventional protection system to identify and locate faults in aneffective manner. A possible route to avoid elevated ratings is to recognise that the increased current is generally needed at timesof decreased voltage such that a tap-change transformer could be used to change how the voltage-current rating of an IBR ispresented to the network. This paper proposes a thyristor-based electronic tap-changer (eTC) fitted to the coupling transformerof a STATCOM. The thyristors of the tap-changer can be commutated rapidly by using the STATCOM to drive phase currentsto zero briefly. The case-study circuit demonstrates delivery of substantial fault current contribution (FCC) of up to 4 pu at thepoint of common coupling (PCC) in less than half a cycle (10 ms) after detection of three- and single-phase faults.

Conference paper

Collins CT, Green TC, 2022, DC Power Filter Design for a Neutral-Point Clamped Hybrid Multilevel Converter, Pages: 2679-2686

The neutral point clamped AC-side cascaded H-bridge (NPC-AC-CHB) converter has been proposed as a low volume alternative to the Modular Multilevel Converter (MMC) for medium voltage DC networks. It is known that due to the slow switching of the main bridge large current distortion is observed on the DC-side and thus require a DC-side filter, usually given as a passive LC filter in the literature. However, design and dimensioning of the required DC filters has not been addressed in the literature. This paper considers the design of a passive LC filter topology and shows that such a solution would requires a very high DC capacitance to achieve a suitably low harmonic current on the DC-side. This large DC filter undermines the stated low volume benefits of the NPC-AC-CHB and therefore the DC filter topology must be reconsidered. Thus this paper investigates the use of both tuned passive filters and active filters to reduce the DC filter passive component size. It is shown that both designs provide significant improvements over the LC filter case, but the active filter provides the greatest reduction. Finally, simulation results are presented for verification of the design analysis.

Conference paper

O'Malley M, Bowen T, Bialek J, Braun M, Cutululis N, Green T, Hansen A, Kennedy E, Kiviluoma J, Leslie J, Li Y, Matevosyan J, McDowell J, Miller N, Pettingill P, Ramasubramanian D, Robinson L, Schaefer C, Ward Jet al., 2021, Enabling Power System Transformation GloballyA System Operator Research Agenda for Bulk Power System Issues, IEEE POWER & ENERGY MAGAZINE, Vol: 19, Pages: 45-55, ISSN: 1540-7977

Journal article

Sheehan CS, Green TC, Daina N, 2021, A simulation approach to analyse the impacts of battery swap stations for e-motorcycles in Africa, 2021 IEEE AFRICON, Publisher: IEEE, Pages: 1-6

Electric motorcycles are being introduced in some African countries to combat the negative environmental impacts from the rapid growth in the use of traditional internal combustion engine motorcycle taxis. However, the electricity systems in many of these countries are strained, with generation and/or distribution capacity at their limits, leading to regular power outages that could impact the charging of these e-motorcycles. These fragile grids may be put under further strain by additional e-motorcycle charging. Commercial motorcycle taxi drivers may not be willing to wait for extended periods to charge during their shift. The use of battery swapping stations could mitigate these issues. However, modelling of their system impacts is required to fully understand their potential. This paper presents a hybrid model to simulate the key operational processes of battery swapping stations and their energy systems, allowing various configurations and scenarios to be investigated for the specific context of e-motorcycles in Africa. The configuration parameters include the numbers of batteries and charging slots, the charging power, and the addition of solar PV and static battery energy storage capacity. Power outages can be modelled for various scenarios. A test case of a battery swap station in Nairobi, Kenya, was used to showcase and validate the model. The results demonstrated how the various sub-models performed and interacted with each other, and clearly showed what impact the chosen BSS configuration would have on the grid.

Conference paper

Trask A, Wills K, Green T, Staffell I, Auvermann O, Coutellier Q, Muuls M, Hardy J, Morales Rodriguez D, Martin R, Sivakumar A, Pawlak J, Faghih Imani SA, Strbac G, Badesa Bernardo Let al., 2021, Impacts of COVID-19 on the Energy System, Impacts of COVID-19 on the Energy System

This Briefing Paper explores the impactthe COVID-19 pandemic had on the UK’senergy sector over the course of thefirst government-mandated nationallockdown that began on 23 March 2020.Research from several aspects of theIntegrated Development of Low-carbonEnergy Systems (IDLES) programme atImperial College London is presented inone overarching paper. The main aim isto determine what lessons can be learntfrom that lockdown period, given theunique set of challenges it presented inour daily lives and the changes it broughtabout in energy demand, supply, anduse. Valuable insights are gained intohow working-from-home policies,electric vehicles, and low-carbon gridscan be implemented, incentivised, andmanaged effectively.

Report

Mian S, Judge P, Junyent Ferre A, Green Tet al., 2021, A delta-connected Modular Multilevel STATCOM with partially-rated energy storage for provision of ancillary services, IEEE Transactions on Power Delivery, Vol: 36, Pages: 2893-2903, ISSN: 0885-8977

This paper proposes a delta-connected Modular Multilevel STATCOM with partially rated storage (PRS-STATCOM), capable of providing both reactive and active power support. The purpose is to provide short-term energy storage enabled grid support services such as inertial and frequency response, either alongside or temporarily instead of standard STATCOM voltage support. The topology proposed here contains two types of sub-modules (SM) in each phase-leg: standard sub-modules (STD-SMs) and energy storage element sub-modules (ESE-SMs) with a dc-dc interface converter between the SM capacitor and the ESE. A control structure has been developed that allows energy transfer between the SM capacitor and the ESE resulting in active power exchange between the converter and the grid. Injecting 3rd harmonic current into the converter waveforms can be used to increase the amount of power that can be extracted from the ESE-SMs and so reduce the required ESE-SMs fraction in each phase-leg. Simulation results demonstrate that for three selected active power ratings, 1 up, 2/3 pu, & 1/3 pu, the fraction of SMs that need be converted to ESE-SMs are only 69%, 59% & 38%. Thus, the proposed topology is effective in adding real power capability to a STATCOM without a large increase in equipment cost.

Journal article

Fan S, Xiang X, Sheng J, Gu Y, Yang H, Li W, He X, Green Tet al., 2021, Inherent SM Voltage Balance for Multilevel Circulant Modulation in Modular Multilevel DC-DC Converters, IEEE Transactions on Power Electronics, ISSN: 0885-8993

Journal article

Green T, Xiang X, Gu Y, yang Q, Zhang X, Li W, He Xet al., 2021, Resonant modular multilevel DC-DC converters for both high and low step-ratio connections in MVDC distribution systems, IEEE Transactions on Power Electronics, Vol: 36, Pages: 7625-7640, ISSN: 0885-8993

DC transformers based on power electronics are keyitems of equipment for medium voltage dc (MVDC) distributionsystems. Both high and low step-ratio dc-dc conversions arerequired to interface dc links at different voltages to form anintegrated dc distribution system. The transformer-coupledresonant modular multilevel dc-dc converter (RMMC) is wellsuited to high step-ratio connection between a MVDC network anda low voltage dc (LVDC) network but it is not suitable in low stepratio conversion for linking two MVDC networks with similar butnot identical voltages. This paper presents a circuit evolution of thehigh step-ratio transformer-coupled RMMC into its low step-ratiotransformer-less RMMC counterpart. These two RMMCsshare thesame structure and the same resonant process within the resonantSM stack (RSS) giving rise to the same operational advantages. Also,the circuit design experience can be readily transferred from one tothe other. From these two base RMMC circuits, a family of RMMCswith further configurations is elaborated that provide a wider varietyof connection optionsfor MVDC distribution systems. In this circuitfamily, each high step-ratio transformer-coupled RMMC has a lowstep-ratio transformer-less RMMC counterpart and one can betransformed into the other via the circuit evolution processpresented. The theoretical analysis for both high and low stepratio RMMCs has been verified through full-scale simulations ofmedium voltage examples and further verified through downscaled experiments on laboratory prototypes.

Journal article

Perez-Olvera J, Green TC, Junyent-Ferre A, 2021, Self-learning Control for Active Network Management

Active network management (ANM) using power electronic devices will become an essential tool for distribution network operators to deal with the variability of a large number of low-carbon technologies. To enable ANM, this paper proposes a control scheme based on deep reinforcement learning, as an alternative to traditional optimisation. The algorithm uses only a small number of network measurements and can learn approximations of optimal control actions, identified in offline simulations, via a neural network. Once trained, the control scheme chooses power converter set-points that can, for instance, even out loadings on different substations in real-time without the computational burden of high-level optimisation. The performance of the proposed control algorithm is validated against the optimal power flow (OPF) using data from real low-voltage networks. The results show that the solution and benefits are comparable to those obtained by the OPF.

Conference paper

Xiang X, Fan S, Gu Y, Ming W, Wu J, Li W, He X, Green TCet al., 2021, Comparison of cost-effective distance for LFAC with HVAC and HVDC in connections of offshore and remote onshore wind energy, CSEE Journal of Power and Energy Systems, Vol: 7, Pages: 954-975, ISSN: 2096-0042

For cost-effective connection of large-scalelong-distance wind energy, low frequency alternating current(LFAC) transmission scheme (16.7 Hz or 20 Hz) has beenproposed as an alternative to the conventional high voltagealternating current (HVAC) transmission scheme (50 Hz or 60 Hz)and recently popular high voltage direct current (HVDC)transmission scheme (0 Hz). The technical feasibility of LFACsystem has been demonstrated but the basis for identifying thedistance ranges for which LFAC would be preferable to HVACand HVDC has not been established and the dependence of thisrange on factors such as power transfer rating, voltage rating andcable/line type has not been investigated. This paper presents anin-depth analysis for the overall cost of LFAC system and thenprovides an extensive comparison with HVAC and HVDC, toexplore the distance ranges over which LFAC is cost-effectiveover both HVAC and HVDC in connections of offshore andremote onshore wind energy. The results demonstrate that LFACsystem does possess ranges in the intermediate distance for whichit is more cost-effective than both HVAC and HVDC, and itsoverall cost advantage is generally larger in the overhead line(OHL) connection of remote onshore wind energy than the cableconnection of offshore wind energy.

Journal article

Zhu Y, Gu Y, Li Y, Green Tet al., 2021, Participation analysis in impedance models: the grey-box approach for power system stability, IEEE Transactions on Power Systems, Vol: 37, Pages: 343-353, ISSN: 0885-8950

This paper develops a grey-box approach to small-signal stability analysis of complex power systems that facilitates root-cause tracing without requiring disclosure of the full details of the internal control structure of apparatus connected to the system. The grey-box enables participation analysis in impedance models, which is popular in power electronics and increasingly accepted in power systems for stability analysis. The Impedance participation factor is proposed and defined in terms of the residue of the whole-system admittance matrix. It is proved that, the so defined impedance participation factor equals the sensitivity of the whole-system eigenvalue with respect to apparatus impedance. The classic state participation factor is related to the impedance participation factor via a chain-rule. Based on the chain-rule, a three-layer grey-box approach, with three degrees of transparency, is proposed for root-cause tracing to different depths, i.e. apparatus, states, and parameters, according to the available information. The association of impedance participation factor with eigenvalue sensitivity points to the re-tuning that would stabilize the system. The impedance participation factor can be measured in the field or calculated from the black-box impedance spectra with little prior knowledge required.

Journal article

Aunedi M, Wills K, Green T, Strbac Get al., 2021, Net-zero GB electricity: cost-optimal generation and storage mix, Great Britain's electricity generation capacity mix for net-zero carbon emissions, Publisher: Energy Futures Lab

Report

Carmichael R, Gross R, Hanna R, Rhodes A, Green Tet al., 2021, The Demand Response Technology Cluster: accelerating UK residential consumer engagement with time-of-use tariffs, electric vehicles and smart meters via digital comparison tools, Renewable and Sustainable Energy Reviews, Vol: 139, ISSN: 1364-0321

Cost-effectively decarbonising the power sector and household energy use using variable renewable energy will require that electricity consumption becomes much more flexible and responsive to constraints in supply and the distribution network. In recent years residential demand response (DR) has received increasing attention that has sought to answer, based on current evidence, questions about how much consumers will engage with DR. This paper critically reviews the evidence base for residential consumer engagement with DR and draws out several important limitations in it. We argue for a more action- oriented focus on developing practical strategies to enable and unlock greater loadshifting and consumer engagement with DR within a changing technology and regulatory context. A number of recommendations are put forward for accelerating UK consumer engagement with DR, presented under three broad strategies: (a) promote awareness of smart tariffs, smart meters and storage and automation behind-the-meter devices as mutually-supportive components within a common ‘DR technology cluster’; (b) deliver targeted support for adoption of electric vehicles and other storage and automation technologies; (c) enable and support informed adoption of DR-enabling products and services through ‘smarter’ digital comparison tools (DCTs), data portability, and faster, simpler switching. The interdependency between components within this DR technology cluster delivers efficiency but also poses a risk that one delayed component (e.g., smart metering) will hold-up policy and industry support for other components. The urgency of decarbonisation goals makes it necessary to push forward as many of these elements as possible rather than the pace being set by the slowest.

Journal article

Oulis Rousis A, Tzelepis D, Pipelzadeh Y, Strbac G, Booth CD, Green TCet al., 2021, Provision of Voltage Ancillary Services Through Enhanced TSO-DSO Interaction and Aggregated Distributed Energy Resources, IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, Vol: 12, Pages: 897-908, ISSN: 1949-3029

Journal article

Heath T, Barnes M, Judge PD, Chaffey G, Clemow P, Green TC, Green PR, Wylie J, Konstantinou G, Ceballos S, Pou J, Belhaouane MM, Zhang H, Guillaud X, Andrews Jet al., 2021, Cascaded- and modular-multilevel converter laboratory test system options: a review, IEEE Access, Vol: 9, Pages: 44718-44737, ISSN: 2169-3536

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.

Journal article

Green T, Liu H, Li B, Wen W, Zhang J, Ning Z, Chen Let al., 2021, Ultra-fast current differential protection with high-sensitivity for HVDC transmission lines, International Journal of Electrical Power and Energy Systems, Vol: 126, ISSN: 0142-0615

Protection of transmission lines is the most important defense against faults in multi-terminal HVDC system. Up until now, no single protection can balance the operation speed and sensitivity needed for all fault types. To solve this problem, a novel current differential protection for HVDC transmission line is proposed in this paper. The intrinsic properties of a HVDC transmission line are analyzed first. Then, essence of ideal current differential protection is revealed and fundamental relations between line models and performance degradation of existing differential protections are discussed. Based on this analysis, a novel current differential protection scheme, including fault detection and faulty pole selection, is developed, and Bergeron model is used to describe the electrical wave propagation process along line, resulting in only simple computations are needed in each calculation cycle. Finally, a simulation model of Wudongde ± 800 kV HVDC project in China is established in PSCAD/EMTDC for verification. By discussing diversified faults and transient processes, simulation results prove that regardless of fault type, distance and transition impedance, the proposed differential protection can respond correctly. Main contribution of this paper is reducing the operation time of differential protection from hundreds of milliseconds to several milliseconds.

Journal article

Ge P, Zhu Y, Green TC, Teng Fet al., 2021, Resilient secondary voltage control of islanded microgrids: an ESKBF-based distributed fast terminal sliding mode control approach, IEEE Transactions on Power Systems, Vol: 36, Pages: 1059-1070, ISSN: 0885-8950

This paper proposes a distributed secondary voltage control method based on extended state Kalman-Bucy filter (ESKBF) and fast terminal sliding mode (FTSM) control for the resilient operation of an islanded microgrid (MG) with inverter-based distributed generations (DGs). To tackle the co-existence of multiple uncertainties, a unified modelling framework is proposed to represent the set of different types of disturbances, including parameter perturbation, measurement noise, and immeasurably external variables, by an extended state method. Kalman-Bucy filter is then applied to accurately estimate the state information of the extended DG model. In addition, based on the accurate estimation, a fast terminal sliding mode (FTSM) surface with terminal attractors is designed to maintain the system stability and accelerate the convergence of consensus tracking, which significantly improves the performance of secondary voltage control under both normal and plug-and-play operation. Finally, case studies are conducted in both MATLAB/Simulink and an experimental testbed to demonstrate the effectiveness of the proposed method.

Journal article

Li Y, Yunjie G, Yue Z, Junyent-Ferre A, Xin X, Green TCet al., 2021, Impedance circuit model of grid-forming inverter: visualizing control algorithms as circuit elements, IEEE Transactions on Power Electronics, Vol: 36, Pages: 3377-3395, ISSN: 0885-8993

The impedance model is widely used for analyzing power converters. However, the output impedance is an external representation of a converter system, i.e., it compresses the entire dynamics into a single transfer function with internal details of the interaction between states hidden. As a result, there are no programmatic routines to link each control parameter to the system dynamic modes and to show the interactions among them, which makes the designers rely on their experience and heuristic to interpret the impedance model and its implications. To overcome these obstacles, this paper proposes a new modeling tool named as impedance circuit model, visualizing the closed-loop power converter as an impedance circuit with discrete circuit elements rather than an all-in-one impedance transfer function. It can reveal the virtual impedance essence of all control parameters at different impedance locations and/or within different frequency bandwidths, and show their interactions and coupling effects. A grid-forming voltage-source inverter (VSI) is investigated as an example, with considering its voltage controller, current controller, control delay, voltage/current dq-frame cross-decoupling terms, output-voltage/current feedforward control, droop controllers, and three typical virtual impedances. The proposed modeling tool is validated by frequency-domain spectrum measurement and time-domain step response in simulations and experiments.

Journal article

Bashar E, Rogers D, Wu R, Ran L, Jennings M, Green TC, Mawby Pet al., 2021, A new protection scheme for an SSSC in an MV network by using a varistor and thyristors, IEEE Transactions on Power Delivery, Vol: 36, Pages: 102-113, ISSN: 0885-8977

To control power flow and manage fault level in meshed MV networks, back-to-back voltage source converters (B2B-VSCs) are being used. However, their high cost and relatively low efficiency are of concerns. Partially rated series compensators, such as SSSCs or UPFCs, are desired but come with the challenge of protecting the device during grid faults. Their potential of use has been limited in comparison with the fully rated back-to-back converters. This paper proposes a new system topology including thyristor crowbars and a varistor to protect the SSSC in an MV network and improve the reliability and flexibility of the network operation. Using the proposed method, the time required for isolating the series compensator from the grid is reduced from at least 20 ms, corresponding to the interruption time of conventional circuit breakers, down to 3 μs in the worst case in addition to the grid fault detection delay. The performance is evaluated by simulation. A small-scale single-phase prototype operating at 230 V/16 A is tested in order to demonstrate the concept.

Journal article

Gu Y, Li Y, Green TC, Zhu Yet al., 2021, Impedance-based whole-system modeling for a composite grid via embedding of frame dynamics, IEEE Transactions on Power Systems, Vol: 36, Pages: 336-345, ISSN: 0885-8950

The paper establishes a methodology to overcome the difficulty of dynamic frame alignment and system separation in impedance modeling of ac grids, and thereby enables impedance-based whole-system modeling of generator-converter composite power systems. The methodology is based on a frame-dynamics-embedding transformation via an intermediary steady frame between local and global frames, which yields a locally defined impedance model for each generator or converter that does not rely on a global frame but retains all frame dynamics. The individual impedance model can then be readily combined into a whole-system model even for meshed networks via the proposed closed-loop formulation without network separation. Compared to start-of-the-art impedance-based models, the proposed method retains both frame dynamics and scalability, and is generally applicable to various network topologies (meshed, radial, etc) and combinations of machines (generators, motors, converters, etc). The methodology is used to analyze the dynamic interaction between generators and converters in a composite grid, which yields important findings and potential solutions for unstable oscillation caused by PLL-swing coupling in low-inertia grids.

Journal article

Xiang X, Gu Y, Chen K, Astolfi A, Green Tet al., 2021, On the dynamics of inherent balancing of modular multilevel DC-AC-DC converters, IEEE Transactions on Power Electronics, Vol: 36, Pages: 34-40, ISSN: 0885-8993

Modular multilevel dc–ac–dc converters (MMDACs) serve as an enabler for dc distribution systems. The modular multilevel structure enables flexible voltage transforms, but raises issues over balancing of the submodule (SM) capacitor voltages. This letter focuses on the dynamics of inherent balancing of MMDACs under circulant modulation. We provide an invariance-like result using a variant of Barbalat's Lemma and prove that the SM capacitor voltages converge to the kernel of the circulant switching matrix, which is the intersection of the invariant sets for each switching state. We further interpret the balancing dynamics as a permuted linear time-invariant system and prove that the envelop of the balancing trajectories is governed by the eigenvalues of the permuted state-transition matrix. This result extends previous full-rank criterion for inherent balancing in a steady state and provides new insight into the dynamic behavior of MMDACs.

Journal article

Wang J, Xu W, Gu Y, Song W, Green TCet al., 2021, Multi-Agent Reinforcement Learning for Active Voltage Control on Power Distribution Networks, Pages: 3271-3284, ISSN: 1049-5258

This paper presents a problem in power networks that creates an exciting and yet challenging real-world scenario for application of multi-agent reinforcement learning (MARL). The emerging trend of decarbonisation is placing excessive stress on power distribution networks. Active voltage control is seen as a promising solution to relieve power congestion and improve voltage quality without extra hardware investment, taking advantage of the controllable apparatuses in the network, such as roof-top photovoltaics (PVs) and static var compensators (SVCs). These controllable apparatuses appear in a vast number and are distributed in a wide geographic area, making MARL a natural candidate. This paper formulates the active voltage control problem in the framework of Dec-POMDP and establishes an open-source environment. It aims to bridge the gap between the power community and the MARL community and be a drive force towards real-world applications of MARL algorithms. Finally, we analyse the special characteristics of the active voltage control problems that cause challenges (e.g. interpretability) for state-of-the-art MARL approaches, and summarise the potential directions.

Conference paper

Xiang X, Zhang X, Gu Y, Chaffey G, Green Tet al., 2020, Analysis and investigation of internal AC Frequency to minimize AC current magnitude and reactive power circulation in chain-link modular multilevel direct DC-DC converters, IEEE Transactions on Circuits and Systems Part 1: Regular Papers, Vol: 67, Pages: 5586-5599, ISSN: 1549-8328

Chain-link modular multilevel direct dc-dc converters (CLMMCs) have attracted much interest recently in for dc power systems because they achieve higher device utilization, lower power losses and they are physically more compact than the alternative front-to-front modular multilevel dc-ac-dc converters (FFMMCs). The CLMMCs rely on circulating an internal ac current to manage energy balance of the sub-module (SM) stacks but this current inevitably contributes to extra current stresses for circuit components and can also lead to excess reactive power circulation within the converter. This paper presents a circuit analysis that there exists a frequency value for the internal ac components that may minimize the current stresses and avoid excessive reactive power circulation. For illustration, the circuit analysis is applied to one of the base formats, the buck-boost CLMMC as an example. The key relationship between the CLMMC and the standard dc-ac modular multilevel converter is explored and established. The equivalent circuits for their dc and ac components with consideration of SM capacitance and SM voltage ripples are created and analyzed in detail, and a full derivation is provided for the specific ac frequency. From this example, this analytical method is extended and applied to other base formats of CLMMCs. The theoretical analysis and results are verified by a set of full-scale simulation examples and down-scaled experiments on a laboratory prototype.

Journal article

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