Publications
424 results found
Chaudhuri B, Ramasubramanian D, Matevosyan J, et al., 2024, Rebalancing needs and services for future grids: system needs and service provisions with increasing shares of inverter-based resources, IEEE Power and Energy Magazine, Vol: 22, Pages: 30-41, ISSN: 1540-7977
The primary objective of electricity grids is to reliably meet the electricity demand at a minimum cost. This objective can be broken down into a set of needs that are met through services. These services are procured by mandating them either in grid codes or via market mechanisms. While grids in different countries/regions share common features in terms of needs and services, there are variations arising in physical, regulatory, and policy contexts. With the increased use of inverter-based resources (IBRs), such as wind and solar photovoltaic (PV) power and battery energy storage systems (BESSs), grids are undergoing changes that are altering the balance between needs and services. This balance is crucial in managing changes that will ensure that grids will continue to be able to meet demands. As increasingly more synchronous machines (SMs) are replaced by IBRs, the services inherently provided by the remaining SMs are dwindling, thus requiring the IBRs to contribute where they can.
Zhu Y, Green T, Zhou X, et al., 2024, Impedance Margin Ratio: a New Metric for Small-Signal System Strength, IEEE Transactions on Power Systems, ISSN: 0885-8950
The high and growing penetration of inverter-based resources (IBR) in power systems challenges the way that system strength is assessed. It has been noticed that the standard indicator of system strength, short-circuit ratio (SCR), is not fully effective in anticipating the sub/super-synchronous oscillation phenomena that can arise from interactions of the control system of an IBR and the rest of the system. In this paper, system strength is first viewed from two perspectives, small-signal system strength and large-signal system strength, which is a natural separation considering the markedly different response of an IBR in normal operation and under faults. For assessing the onset of small-signal instability, a new metric for small-signal system strength is proposed and named Impedance Margin Ratio (IMR). IMR is the ratio between the allowed variation of the impedance of an IBR and the original value of the impedance at a particular point of interconnection. IMR is assessed at the oscillatory frequency of the dominant mode or taken as the minimum value across several modes. It is shown that as a localised frequency-domain indicator, IMR can successfully reveal weak points in the system towards various causes of oscillations.
Ducoin E, Gu Y, Chaudhuri B, et al., 2024, Analytical design of contributions of grid-forming & grid-following inverters to frequency stability, IEEE Transactions on Power Systems, ISSN: 0885-8950
Most of the new renewable generation in power systems is connected through Grid-Following inverters (GFL). The accompanying decline of fossil-fuelled synchronous generation reduces the grid inertia. As these two trends progress, instabilities become more likely. To allow more renewables onto the grid, the use of combinations of GFL and Grid-Forming inverters (GFM) has been proposed, however, it is unclear how to parametrise these inverters for system objectives. This paper tackles the issue of parametrizing each GFM and GFL to ensure frequency trajectories at all buses, expressed in terms of frequency deviation, Rate of Change of Frequency and settling time, are stable, recognising that local frequencies can deviate substantially from the Center of Inertia (COI). The procedure to achieve this comprises simple closed-form equations, and yields the required values of droop slopes, GFM filter bandwidth and GFL Phase-Locked Loop bandwidth. These equations are derived from an analytical formulation of swing equations for GFM and GFL which are combined to describe the behaviour of not only the COI but also each bus. The detailed EMT simulations of a modified IEEE 14-bus network demonstrate that the simplifying assumptions made in the analysis are justified by the close correspondence between simulation and analytical projections.
Fan S, Xiang X, Gu Y, et al., 2024, Optimal circulant modulation for submodule voltage ripple minimization with inherent balancing capability in modular multilevel dc-dc converters, IEEE Transactions on Power Electronics, Vol: 39, Pages: 784-798, ISSN: 0885-8993
The modularity of the modular multilevel dc-dc converters (MMDCs) makes it as a competitive candidate in medium voltage applications but brings the submodule (SM) voltage balancing issue. This paper proposes an optimal circulant modulation method for minimizing the SM voltage ripples with inherent balancing capability proven at the same time, which allows smaller SM capacitors and avoids the high-frequency communication for SM voltage balancing. Firstly, the optimal switching pattern is strictly derived providing a general method to theoretically minimize the SM capacitor voltage ripple. Then the switching matrix of the optimal circulant modulation is formulated by introducing the generalized-circulant matrix. It verifies the circularity and full-rank feature of the optimal switching matrix, which promises the uniformity of SM actions and the inherent balancing of SM voltages. Finally, full-scale simulations and down-scaled experiments are both provided with the isolated LLC -based MMDC model and prototype. The results show that the proposed optimal circulant modulation can reduce the SM capacitor voltage ripple by 37% compared with the existed method, and it also promises the inherent SM voltage balancing and the SM uniformity.
Li Y, Green TC, Gu Y, 2023, Descriptor state space modeling of power systems, IEEE Transactions on Power Systems, Pages: 1-13, ISSN: 0885-8950
State space is widely used for modeling power systems and analyzing their dynamics but it is limited to representing causal and proper systems in which the number of zeros does not exceed the number of poles. In other words, the system input, output, and state can not be freely selected. This limits how flexibly models are constructed, and in some circumstances, can introduce errors because of the addition of virtual elements in order to connect the mismatched ports of subsystem models. An extension known as descriptor state space (also known as implicit state space, generalized state space, singular state space) can model both proper and improper systems and is a promising candidate for solving the noted problems. It facilitates a modular construction of power system models with flexible choice of ports of subsystems. Algorithms for mathematical manipulation of descriptor state space models are derived such as preforming inverse, connection, and transform. Corresponding physical interpretations are also given. Importantly, the proposed algorithms preserve the subsystem states in the whole system model, which therefore enables the analysis of root causes of instability and mode participation. Theoretical advances are validated by example power systems of varied scales including inductor or capacitor systems, and modified IEEE 14-bus, 68-bus, and 118-bus generator-inverter-composite systems.
Li Y, Green TC, Gu Y, 2023, The intrinsic communication in power systems: a new perspective to understand synchronization stability, IEEE Transactions on Circuits and Systems Part 1: Regular Papers, Vol: 70, Pages: 4615-4626, ISSN: 1549-8328
The large-scale integration of converter-interfaced resources in electrical power systems raises new threats to stability which call for a new theoretical framework for modelling and analysis. In this paper, we present the intrinsic analogy of a power system to a communication system, which is here called power-communication isomorphism. Based on this isomorphism, we revisit power system stability from a communication perspective and thereby establish a theory that unifies the heterogeneous power apparatuses of power systems and provides a bridge between electromagnetic transient (EMT) and phasor dynamics. This theory yields several new insights into power system stability and new possibilities for stabilization. In particular, we demonstrate that a system of 100% converter-interfaced resources can achieve stable synchronization in small-and large-signal sense under grid-following control which was commonly considered impossible.
Fan S, Xiang X, Li C, et al., 2023, Coprime polynomial based dual-circulant modulation for inherent submodule voltage balancing in MMDC, IEEE Transactions on Industrial Electronics, Vol: 70, Pages: 10134-10145, ISSN: 0278-0046
The modular multilevel dc-dc converters (MMDCs) have attracted much interest in medium voltage dc applications, but have to face the main issue of balancing submodule (SM) capacitor voltages. This paper proposes a dual-circulant modulation and proves it possessing the inherent balancing capability for arbitrary operation cases. High-speed communication for real-time sensor data transfer could be avoided, which reduces implementation costs and also enhances the system reliability. Two sets of circular switching patterns are preset and combined to complete the full constrains on SM voltages. The associated polynomial of circulant matrix is introduced and the coprime of polynomials demonstrates the full-rank feature of the extended switching matrix that promises the inherent balancing for any operation cases. Then the switching patterns are reallocated and optimized to keep the SM uniformity and reduce the capacitor voltage ripple at the same time. Full-scale simulations on MMDC models and down-scaled experiments on prototypes are both presented, which validates the inherent voltage balancing capability and the optimization of SM capacitor voltage ripple with the proposed dual-circulant modulation.
Gu Y, Green T, 2023, Power system stability with a high penetration of inverter based resources, Proceedings of the Institute of Electrical and Electronics Engineers (IEEE), Vol: 111, Pages: 832-853, 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.
Kang C, Kirschen D, Green TC, 2023, The Evolution of Smart Grids, PROCEEDINGS OF THE IEEE, Vol: 111, Pages: 691-693, ISSN: 0018-9219
Sheehan C, Green T, 2023, ChargeUp! Data Swap: Using data from battery swapping e-motorcyclesin Nairobi to assess impacts and plan infrastructure, ChargeUp! Data Swap: Using data from battery swapping e-motorcyclesin Nairobi to assess impacts and plan infrastructure, Publisher: Energy Futures Lab
The dearth of available data on e-motorcycle usage in Africancities is a significant challenge in impact studies of e-motorcycledeployment. The ChargeUp! project aimed to fill this research gapusing operational data from e-motorcycles and battery swap stationsin Nairobi to perform modelling and analysis to determine severalkey outputs. This project included the analysis of: e-motorcycle trips;battery swapping demand; battery charging energy consumption;swap battery charging related emissions for a high renewables andhigh fossil energy mix scenarios; charging related electricity costsfor different tariff scenarios; the effect of a co-ordinated chargingscenario on emissions and tariffs; optimal battery ratios and requirednumbers of swap stations; and a methodology to determine optimalregions for battery swap stations based on trip data.
Zhu Y, Zhang Y, Green T, 2023, Injection amplitude guidance for impedance measurement in power systems, IEEE Transactions on Power Electronics, Vol: 38, Pages: 6929-6933, ISSN: 0885-8993
Impedance measurement techniques based on small-signal injections in ac power system have been well developed but a gap in the analysis is that the selection of an injection amplitude is based only on experience. This letter develops an analytical process to analyze the noise existing in power systems and thereby determines the minimum injection amplitude according to an allowable error. The process is based on knowledge of stochastic processes and statistics, where the Monte Carlo method is also employed to simplify the process. A power hardware-in-the-loop system with a grid-following inverter is employed in experimental verification of the results.
Zhang Y, Li Y, Gu Y, et al., 2023, Consideration of control-loop interaction in transient stability of grid-following inverters using bandwidth separation method, 2023 11th International Conference on Power Electronics and ECCE Asia (ICPE 2023 - ECCE Asia), Publisher: IEEE, ISSN: 2150-6086
Grid-following inverters have been widely adopted as a grid interface for renewable energy resources, but ensuring their transient stability has been a growing challenge in part because of a lack of clarity on how interactions between the inverter’s various outer control loops impact transient response. In this paper, the nonlinear dynamics and transient stability of grid-following inverters are investigated, where not only the phase-locked loop (PLL) but also the DC-link voltage control (DVC) and the AC terminal voltage control (TVC) are taken into account. The use of bandwidth separation method is proposed, though which the original differential equations can be simplified, and their order can be reduced, providing that the separation between their bandwidths is sufficiently large. Analysis of the interaction between the DVC and the PLL is developed, proving that this interaction negatively impacts stability. Furthermore, optimal combinations of bandwidth of PLL and DVC are identified under various types of grid faults. The impact of TVC on transient stability is analyzed. All of the results and the conclusions drawn from them are verified by simulation.
Sheehan C, Green T, 2023, Kenya Charging Forward: A brief assessment of Kenya’s e-mobility policy landscape and proposed changes, Publisher: Energy Futures Lab
The transport sector’s share of Kenya’s total greenhouse gas (GHG)emissions has been projected to grow from 11% in 2015 to 14.7%by 2030 in a business as usual (BAU) scenario [1]. In line with theKenyan Government’s goal of reducing total GHG emissions by 32%relative to BAU in 2030 [2], it has begun several initiatives to startmitigating its transport emissions, including opportunities relatedto growing its nascent e-mobility sector. The private sector, with arange of around 25 new e-mobility companies [3], has been providingsolutions through the supply of various electric vehicles aimed atserving the local market, while also leading the way in deployingcharging and battery swapping infrastructure.
Ducoin E, Gu Y, Chaudhuri B, et al., 2023, Swing equation modelling of GFL inverter and comparison of its damping and inertia with GFM inverter, The 19th International Conference on AC and DC Power Transmission, Publisher: IET, Pages: 108-114
The increasing integration of inverter-based renewable generation into the network as Grid-Following inverters (GFL), combined with the reduction in traditional Synchronous Generation (SG) leads to a reduction in inertia and to instabilities. To alleviate this, researchers have proposed using a mix of GFL and Grid-Forming controllers (GFM). However, there is a lack of simple system-wide analytical models. This paper proposes a swing equation to describe the transient frequency response of GFL that has droop regulation of real and reactive power. The swing equation for GFL with droop has an inertia and a damping term as do SG and GFM swing equations. Simulations show that the use of swing equations for GFM and GFL is accurate to study the transient frequency stability following a loss of infeed. GFM and GFL damping are equivalent, and their sum allows prediction of frequency deviation. However, although SG and GFM inertia terms limit the initial Rate of Change of Frequency (RoCoF), the GFL inertia term is found not to contribute towards limitation of the initial RoCoF. Instead, the GFL RoCoF isseen to follow adjacent GFM RoCoF and worsens slightly when the GFL Phase Locked Loop (PLL) bandwidth is reduced.
Collins C, Clemow P, Green T, 2023, Experimental study on the influence of MMC control design on the propagation of AC grid-unbalance to a DC network, IET International Conference on AC and DC Power Transmission, Publisher: IET
The Modular Multilevel Converter (MMC) has become the leading technology for delivering High Voltage DC (HVDC) power transmission, due to its scalability, harmonic quality and ability to ride through AC network faults. However, unbalanced AC-Grid conditions create harmonics at twice the fundamental AC-Grid frequency that can propagate to the DC-side as a result of the unbalance in the arm energy. These harmonics may risk excitation of network resonances as well as the maloperation of connected assets, and thus control action is needed to suppress such harmonics. Several different control solutions have been proposed in the literature and the aim of this paper is to provide a comparison of the performance of a sample of these techniques on a 1.2 kV/12 kVA lab-scale MMC demonstrator and cable emulator under unbalanced AC-Grid conditions. The sample of control methods chosen showcases a range of design complexity from basic direct modulation techniques with no DC current control to advanced energy based controllers. Experimental testing of these controllers under the same test condition not only validates the operation of each individual controller but also allows like-for-like comparison of their relative performance. It was found that the controllers tested were capable of significantly suppressing double line-frequency components on the DC -bus compared to the base case, however the more complex controllers had the additional benefit of being able to tune the transient response.
Fan S, Xiang X, Gu Y, et al., 2023, Optimal Circulant Modulation for Submodule Capacitor Ripple Minimization and Inherent Voltage Balancing in Modular Multilevel DC Converters, Pages: 352-357
Modular multilevel dc-dc converters (MMDCs) have attracted much interests in medium voltage dc applications, but it has to face the main issue of balancing submodule (SM) capacitor voltages. Conventionally, the SM balancing demands monitoring and sorting the real-time SM capacitor voltages. It highly relies on the high-speed bidirectional communication between submodules and central controller, which increases the implementation costs and decreases the system reliability. This paper proposes the optimal circulant modulation with inherent balancing capability as well as the minimized the SM capacitor voltage ripple. Firstly, the optimal SM switching pattern is derived according to the SM ripple expression cycle-by-cycle. Then the full-rank feature of the optimal switching matrix is proven, which promises the inherent balancing of SM capacitor voltages. Finally, the ripple optimization and inherent balancing capability of the proposed optimal circulant modulation are validated by experiments on an LLC-based MMDC prototype.
Bravo P, Pereda J, Merlin M, et al., 2022, Modular multilevel matrix converter as solid state transformer for medium and high voltage substations, IEEE Transactions on Power Delivery, Vol: 37, Pages: 5033-5043, 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.
Li Y, Hadjileonidas A, Green TC, 2022, Comparative analysis of transient stability of grid-forming and grid-following inverters, The 3rd IEEE International Power Electronics and Application Conference, Publisher: IEEE, Pages: 296-301
The increasing dominance of renewable energy sources in the modern power grids, leads to the installation of powerelectronic inverters as they are required to interface these DC and variable frequency sources to the grid. These inverters can provideadditional flexibility to the control of the power grid and are distinguished into grid-forming (GFM) and grid-following (GFL) types.However, their complex control structures give rise to new threats to system’s stability. This article investigates the transient stabilityof these two inverter types. They are studied when connected to an infinite bus and subject to large disturbances, such as three-phasefaults or line disconnections. This investigation begins with an analysis of the various inverter configurations and proceeds to derivethe systems’ swing equations. The analysis is validated using time-domain simulations and stability is studied for a range of operatingconditions and control settings. It is revealed that GFM inverters suffer greater impacts from faults in strong grid conditions whichcontrasts to GFL which suffer large impacts from faults in weak grid conditions. Furthermore, it is shown that the inverters’ virtualinertia and damping coefficients play an important role to their transient stability and ability to ride-through faults.Keywords — GFM, GFL, synchronization, transient stability
Li Y, Gu Y, Green T, 2022, Revisiting grid-forming and grid-following inverters: a duality theory, IEEE Transactions on Power Systems, Vol: 37, Pages: 4541-4554, 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.
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, Vol: 37, Pages: 3423-3433, 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.
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, Pages: 1-6
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.
Collins CT, Green TC, 2022, DC power filter design for a neutral-point clamped hybrid multilevel converter, 2022 International Power Electronics Conference (IPEC-Himeji 2022- ECCE Asia), Publisher: IEEE, 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.
Jansen M, Duffy C, Green T, et 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.
Zhu Y, Gu Y, Li Y, et al., 2022, Impedance-based root-cause analysis: comparative study of impedance models and calculation of eigenvalue sensitivity, IEEE Transactions on Power Systems, Vol: 38, Pages: 1642-1654, 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.
Hou X, Sun K, Zhang N, et 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.
Fan S, Xiang X, Sheng J, et al., 2022, Inherent SM voltage balance for multilevel circulant modulation in modular multilevel DC-DC converters, IEEE Transactions on Power Electronics, Vol: 37, Pages: 1352-1368, ISSN: 0885-8993
The modularity of a modular multilevel dc converter (MMDC) makes it attractive for medium-voltage distribution systems. Inherent balance of submodule (SM) capacitor voltages is considered as an ideal property, which avoids a complex sorting process based on many measurements thereby reducing costs and enhancing reliability. This article extends the inherent balance concept previously shown for square-wave modulation to a multilevel version for MMDCs. A switching duty matrix dU is introduced: it is a circulant matrix of preset multilevel switching patterns with multiple stages and multiple durations. Inherent voltage balance is ensured with a full-rank dU . Circulant matrix theory shows that this is equivalent to a simplified common factor criterion. A nonfull rank dU causes clusters of SM voltage rather than a single common value, with the clusters indicated by the kernel of the matrix. A generalized coprime criterion is developed into several deductions that serve as practical guidance for design of multilevel circulant modulation. The theoretical development is verified through full-scale simulations and downscaled experiments. The effectiveness of the proposed circulant modulation in achieving SM voltage balance in an MMDC is demonstrated.
Fan S, Xiang X, Zhao Y, et al., 2022, Dual-circulant Modulation for Inherent Submodule Voltage Balancing in MMDC, Pages: 33-38
The modular multilevel dc-dc converter (MMDC) has attracted much interest in medium voltage dc applications, but it has to face the main issue of balancing submodule (SM) capacitor voltages. This paper proposes a dual-circulant modulation and mathematically proves it possessing the inherent balancing capability for arbitrary operation cases. Two sets of circular switching patterns are preset and combined to complete the full constrains on SM voltages for balancing, and then reallocated to keep the advantage of SM uniformity. The associated polynomial of circulant matrix is introduced and the coprime of polynomials demonstrates the full-rank feature of the extended switching matrix that promises the inherent balancing. Experiments are conducted on prototype to validate the inherent voltage balancing capability of the dual-circulant modulation.
O'Malley M, Bowen T, Bialek J, et al., 2021, Enabling power system transformation: system operator research agenda for bulk power system issues, IEEE Power and Energy Magazine, Vol: 19, Pages: 45-55, ISSN: 1540-7977
The primary objective of a power system is to safely provide reliable energy services to society at an affordable cost. In many countries, this objective has been supplemented by another: meeting the energy demand with sustainable resources, which has culminated in the energy transition to low-carbon and zero-carbon energy systems. This transition, occurring rapidly around the world, is characterized by the increasing penetration of variable renewable energy (VRE), inverter-based resources (IBRs), and distributed energy resources (DERs).
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.
Trask A, Wills K, Green T, et 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.
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