Imperial College London

Professor Tim Green, FREng

Faculty of Engineering

Co-Director of the Energy Futures Laboratory (EFL)
 
 
 
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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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375 results found

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

Green T, Liu H, Li B, Wen W, Zhang J, Ning Z, Chen Let al., 2020, Ultra-fast Current Differential Protection with High-sensitivity for HVDC Transmission Lines, International Journal of Electrical Power and Energy Systems, ISSN: 0142-0615

Journal article

Leterme W, Judge PD, Green TC, 2020, Analysis of DC-side fault response of MMCs with controlled fault blocking capability for different transmission line types, 2020 22nd European Conference on Power Electronics and Applications (EPE'20 ECCE Europe), Publisher: IEEE, Pages: 1-9

MMCs with controlled fault blocking capability retain control of their currents during a dc-side fault, thereby reducing the required interruption capabilities for switchgear. To design the dc-side control to achieve this capability, it is important to take into account the interaction between the converter control and the transmission line during a short-circuit on the transmission line. This paper uses a dc-side equivalent model to assess the interactions of the converter control for two types of converters, i.e., full-bridge and hybrid, with two main types of transmission lines, i.e., cable and overhead line, during dc-side faults. In general, the dc-side voltages and currents for a full-bridge MMC connected to an overhead line show more oscillatory behavior compared to an MMC connected to a cable. Furthermore, a hybrid MMC connected to an overhead line may provide a more damped dc-side fault response due to its limited negative voltage capability.

Conference paper

Mian S, Judge P, Junyent Ferre A, Green Tet al., 2020, A Delta-Connected Modular Multilevel STATCOM with Partially-Rated Energy Storage for Provision of Ancillary Services, IEEE Transactions on Power Delivery, ISSN: 0885-8977

Journal article

Shao W, Wu R, Ran L, Jiang H, Mawby PA, Rogers DJ, Green TC, Coombs T, Yardley K, Kastha D, Bajpai P, Zhou Let al., 2020, A Power Module for Grid Inverter With In-Built Short-Circuit Fault Current Capability, IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol: 35, Pages: 10567-10579, ISSN: 0885-8993

Journal article

Sanchez-Sanchez E, Junyent-Ferre A, Prieto-Araujo E, Gomis-Bellmunt O, Green Tet al., 2020, Modelling and experimental validation of a laboratory-scaled HVDC cable emulator tested in an MMC-based platform

© 2020 EPE Association. Typical Modular Multilevel Converter (MMC)-based HVDC systems don't include capacitors directly connected to the DC-side in parallel with the HVDC lines. The dynamics of the HVDC line voltage depend on the equivalent capacitance of the HVDC system, which in turn depends on the length of the lines. Recent work in the literature has shown that this can become problematic for HVDC line voltage control in the case of short links. This paper describes the operation of a laboratory-scaled MMC-based HVDC short link platform devised to study this phenomenon. Special emphasis is put on the design and the scaling of a frequency-dependent cable emulator. Experimental results are compared with a benchmark high-voltage simulated model.

Conference paper

Li Y, Yunjie G, Yue Z, Junyent-Ferre A, Xin X, Green TCet al., 2020, Impedance circuit model of grid-forming inverter: visualizing control algorithms as circuit elements, IEEE Transactions on Power Electronics, 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

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, 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

Li Y, Gu Y, Green TC, 2020, Interpreting frame transformations in AC systems as diagonalization of harmonic transfer functions, IEEE Transactions on Circuits and Systems I: Regular Papers, Vol: 67, Pages: 2481-2491, ISSN: 1549-8328

Analysis of ac electrical systems can be performed via frame transformations in the time-domain or via harmonic transfer functions (HTFs) in the frequency-domain. The two approaches each have unique advantages but are hard to reconcile because the coupling effect in the frequency-domain leads to infinite dimensional HTF matrices that need to be truncated. This paper explores the relation between the two representations and shows that applying a frame transformation on the input-output signals creates a direct equivalence to a similarity transformation to the HTF matrix of the system. Under certain conditions, such similarity transformations have a diagonalizing effect which, essentially, reduces the HTF matrix order from infinity to two or one, making the matrix tractable mathematically without truncation or approximation. This theory is applied to a droop-controlled voltage source inverter as an illustrative example. A stability criterion is derived in the frequency-domain which agrees with the conventional state-space model but offers greater insights into the mechanism of instability in terms of the negative damping (non-passivity) under droop control. Therefore, the paper not only establishes a unified view in theory but also offers an effective practical tool for stability assessment.

Journal article

Collins C, Green T, 2020, Comparative analysis of an MV neutral point clamped AC-CHB converter with DC fault ride-through capability, IEEE Transactions on Industrial Electronics, Vol: 67, Pages: 2834-2843, ISSN: 0278-0046

The AC side cascaded H-bridge converter with a two-level main bridge has previously been proposed as a fault tolerant converter for HVDC. This paper explores the benefits of replacing the two-level bridge with a neutral point clamped three-level (NPC) bridge for MVDC applications and defines the optimum operating conditions for this case. By modifying the topology to include an NPC main bridge, the peak stack voltage during normal operation is decreased considerably which results in a 58% reduction in the required number of sub-modules (SM), thereby significantly increasing efficiency. However, this sacrifices the fault ride-through capability as the stacks are no longer able to support the AC voltage and thus two new SM topologies are proposed. The proposed topologies function as single full-bridges with two capacitors in parallel during normal operation. Under fault conditions, the SMs divide into two series connected full-bridges to enable DC fault ride-through. Reverse-blocking IGCTs or ultra-fast mechanical switches are used to bypass the IGBTs which are unused in normal operation and therefore the topology maintains a high efficiency. Simulation results are shown, and the proposed topologies are compared with more conventional designs in terms of efficiency, energy storage requirement and device count.

Journal article

Qiao Y, Zhang X, Xiang X, Yang X, Green Tet al., 2020, Trapezoidal Current Modulation for Bidirectional High-Step-Ratio Modular DC–DC Converters, IEEE Transactions on Power Electronics, Vol: 35, Pages: 3402-3415, ISSN: 0885-8993

Modular dc-dc converter (MDCC) has been proposed for high step-ratio interconnection in dc grid applications.To further optimize the performance of MDCC, this paperpresents a trapezoidal current modulation with bidirectionalpower flow ability. By giving all the sub-module (SM) capacitorsan equal duty to withstand the stack dc voltage, their voltages arebalanced without additional feedback control. Moreover, basedon soft-switching performance and circulating current analysis,three-level and two-level operation modes featured with highefficiency conversion and large power transmission, respectively,are introduced. The control schemes of both modes are designedto minimize the conduction losses. Besides, the SM capacitorvoltage ripples with different switching patterns are comparedand the option for ripple minimization is presented. A full-scalecase study is provided to introduce the design process and deviceselection of the MDCC. The experimental tests based on a downscaled prototype are finally presented to validate the theoreticalanalysis.

Journal article

Bashar E, Rogers D, Wu R, Ran L, Jennings M, Green TC, Mawby Pet al., 2020, A new protection scheme for an SSSC in an MV network by using a varistor and thyristors, IEEE Transactions on Power Delivery, Pages: 1-12, 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

Sang Y, Junyent Ferre A, Green TC, 2020, Operational principles of three-phase single active bridge DC/DC converters under duty cycle control, IEEE Transactions on Power Electronics, Vol: 35, Pages: 8737-8750, ISSN: 0885-8993

Single Active Bridge (SAB) DC/DC converters are attractive options for unidirectional DC/DC conversion in future medium power generation applications, offering galvanic isolation and a diode-based output side. SAB DC/DC converters must be controlled by adjusting the active bridge switching duty cycle, unlike Dual Active Bridge (DAB) converters where the phase-shift angle control is normally used. This paper presents a comprehensive analysis of the operational principles of three-phase SAB (3p-SAB) DC/DC converters that arise from different duty cycle operation ranges. Moreover, the converter performances such as transformer harmonic currents are analyzed and are compared between different input and output DC voltage conditions. Finally, an experimental validation using a small scale prototype is presented.

Journal article

Zhao Z, Yang P, Bottrell N, Lai LL, Green TCet al., 2020, Dynamic modeling, sensitivity assessment, and design of VSC-based microgrids with composite loads, Journal of Power Electronics, Vol: 20, Pages: 245-259, ISSN: 1598-2092

© 2019, The Korean Institute of Power Electronics. Microgrids are seen as useful for increasing the flexibility of distribution networks and integrating large amounts of distributed generations. Ensuring the dynamic stability of power converter-dominated microgrids that is robust to a range of load conditions is a significant challenge and essential for ensuring reliability. Induction motor (IM) loads are widespread and have substantial impacts on the dynamic behavior and stability characteristics of low-inertia microgrids. The stability assessment and design of microgrids considering composite loads have not been sufficiently addressed in the current literature, where static loads are commonly used to simplify the modeling and analysis. In this paper, the dynamic stability of voltage source converter-based microgrids is investigated, considering composite loads as dynamic element. A complete state-space model of the microgrid with both IM load and static load is developed. Participation factor analysis is conducted to identify the contribution of the composite loads to the dominant oscillatory modes of the microgrid. Furthermore, sensitivity assessment of the dominant eigenvalues is presented to further identify the appropriate ranges of variations in the control parameters, operating conditions and operating points of the microgrid. It is shown that composite loads in a realistic microgrid significantly affect the dominant oscillatory modes and, consequently, the system stability margin. Ignoring the composite load dynamics in microgrid stability studies may lead to misleading analytical results. Simulations based on MATLAB/Simulink and experimental tests based on a laboratory prototype microgrid are implemented to demonstrate the theoretical analysis.

Journal article

Xiang X, Qiao Y, Gu Y, Zhang X, Green Tet al., 2020, Analysis and criterion for inherent balance capability in modular multilevel DC-AC-DC converters, IEEE Transactions on Power Electronics, Vol: 35, Pages: 5573-5580, ISSN: 0885-8993

Modular multilevel dc-ac-dc converters (MMDAC) have emergedrecently for high step-ratio connectionsin medium voltage distribution systems.Extended phase-shiftmodulation has been proposed and was found to create the opportunity for inherent balance of SM capacitor voltages. This letter presents fundamentalanalysis leading toclear criterion for the inherent balancecapability in MMDAC. A sufficient and necessary condition,with associated assumptions,to guarantee this capability isestablished. Using the mathematics of circulant matrices, this condition is simplified to a co-prime criterion which gives rise to practical guidance for the design of an MMDAC. Experimentson down-scaled prototypesand simulations on full-scale examples both provide verification of the analysis and criterion for the inherent balance capability of MMDAC.

Journal article

Pozo C, Limleamthong P, Guo Y, Green T, Shah N, Acha S, Sawas A, Wu C, Siegert M, Guillén-Gosálbez Get al., 2019, Temporal sustainability efficiency analysis of urban areas via data envelopment analysis and the hypervolume indicator: Application to London boroughs, Journal of Cleaner Production, Vol: 239, Pages: 1-14, ISSN: 0959-6526

Transitioning towards a more sustainable society calls for systematic tools to assess the sustainability performance of urban systems. To perform this task effectively, this work introduces a novel method based on the combined use of Data Envelopment Analysis (DEA) and the hypervolume indicator. In essence, DEA is applied to (i) distinguish between efficient and inefficient urban systems through the identification of best practices; and to (ii) establish improvement targets for the inefficient urban systems that, if attained, would make them efficient. Meanwhile, the hypervolume indicator is employed in conjunction with DEA to evaluate how urban systems evolve with time. The capabilities of this approach are illustrated through its application to the sustainability assessment of London boroughs between 2012–2014. Results reveal that most boroughs tend to perform well in terms of the indicators selected, with 20–25 of the 32 boroughs found efficient depending on the year. Regarding the temporal assessment, a global improvement in sustainability performance was found, with a strong relationship between the boroughs’ performances and their locations. The method proposed opens new pathways of social and environmental research for the application of advanced multi-criteria decision-support tools in the assessment and optimisation of urban systems.

Journal article

Judge PD, Chaffey G, Wang M, Dejene FZ, Beerten J, Green TC, Van Hertem D, Leterme Wet al., 2019, Power-system level classification of voltage-source HVDC converter stations based upon DC fault handling capabilities, IET Renewable Power Generation, Vol: 13, Pages: 2899-2912, ISSN: 1752-1416

To date, numerous concepts for converter station designs for use in voltage source converter (VSC)-based high-voltage direct current (HVDC) systems have been proposed. These differ not only in converter circuit topology, sub-module design, and control scheme but also in AC-or-DC switchgear and other auxiliary equipment. In the main, the existing literature categorises these converter stations according to just the converter circuit technologies and controls. However, for the development of network codes and to enable systematic network studies, a system-focused and technology-independent classification is needed. As such a classification does not yet exist, this study proposes a new framework, which categorises VSC station designs according to their capabilities during a DC-side fault and the method by which post-fault restoration may be achieved, given that these are the main differentiating factors from a system perspective. The classification comprises six converter station types and three time-intervals through which to fully characterise a design. Many well-known forms of converters are used as case studies, and simulation results are used to exemplify the classification framework. The outcome is a generic and technology-independent way of characterising converter station designs that is useful in wider power-system analysis but also for putting proposed converter stations into context.

Journal article

Leterme W, Judge PD, Wylie J, Green TCet al., 2019, Modeling of MMCs with controlled DC fault blocking capability for DC protection studies, IEEE Transactions on Power Electronics, Vol: 35, Pages: 5753-5769, ISSN: 0885-8993

The fault current characteristics in dc systemsdepend largely on the response, and hence also the topology,of the ac-dc converters. The presently used ac-dc convertertopologies may be categorized into those with controlled oruncontrolled fault blocking capability and those lacking suchcapability. For the topologies of the former category, genericmodels of the dc-side fault response have not yet been developedand a characterization of the influence of control and sensordelays is a notable omission. Therefore, to support accurate andcomprehensive dc system protection studies, this paper presentsthree reduced converter models and analyzes the impact of keyparameters on the dc-side fault response. The models retainaccurate representation of the dc-side current control, but differin representation of the ac-side and internal current controldynamics, and arm voltage limits. The models were verifiedagainst a detailed (full-switched) simulation model for the casesof a full-bridge and a hybrid modular multilevel converter, andvalidated against experimental data from a lab-scale prototype.The models behave similarly in the absence of arm voltage limits,but only the most detailed of the three retains a high degree ofaccuracy when these limits are reached.

Journal article

Gu Y, Li Y, Yoo H-J, Nguyen T-T, Xiang X, Kim H-M, Junyent Ferre A, Green Tet al., 2019, Transfverter: imbuing transformer-like properties in an interlink converter for robust control of a hybrid ac-dc microgrid, IEEE Transactions on Power Electronics, Vol: 34, Pages: 11332-11341, ISSN: 0885-8993

In a hybrid ac-dc microgrid, stiff voltage sources may appear in either the dc or ac subgrids which gives rise to multiple operation modes as power dispatch changes. This creates a challenge for designing the interlink converter between the ac and dc subgrids since the different modes require different interlink controls. To solve this problem, this paper proposes the concept of a transfverter inspired by how transformers link ac grids. Like a transformer, a transfverter can react to the presence of stiff voltage sources on either the dc or ac side and reflect the “stiffness” and voltage stabilizing capability to the other side. A back-to-back converter with droop control is used as the underlying technology to implement this concept. A novel optimization method called model bank synthesis is proposed to find control parameters for the interlink converter that offer the best controller performance across the different microgrid modes without requiring mode-changing of the controller. The effectiveness of the proposed solution is validated through both simulation and experiments.

Journal article

Mendonca TRF, Green TC, 2019, Distributed active network management based on locally estimated voltage sensitivity, IEEE Access, Vol: 7, Pages: 105173-105185, ISSN: 2169-3536

Two challenges need to be addressed in designing active network management (ANM) fordistribution networks that use non-firm connection agreements for quicker and cheaper connections ofdistributed energy resource (DER). First is the replacement of scripted actions based on priority lists byreal-time selection of actions offered as ancillary services and judged on efficacy and cost. Second is theneed to decentralize or distribute ANM decision making to avoid unrealistic communication and computationburdens as the number of controllable devices increases. This paper proposes a distributed form of ANMfor radial networks, based on local estimation of the voltage sensitivities to offered adjustments of real orreactive power and then uses message passing between local controllers to arrive at near-optimum choicesof actions. To manage a voltage constraint, the minimum volume (or cost) of ancillary services is found byselecting services from DERs with highest voltage sensitivity to the service offered. A method of sensitivityestimation for individual nodes is extended to all terms of the inverted Jacobian matrix. The accuracy of thisapproximation is discussed and explored in a case-study network. The format of message passing from onelocal controller to another is described. Simulations demonstrate that the proposed distributed ANM closelyapproaches the solution found by a centralized optimal power flow. It is confirmed that the use of locallyestimated voltage sensitivity to identify the most effective DER can minimize the volume of power flowadjustment service that the ANM needs to manage voltage and thermal constraints.

Journal article

Xiang X, Zhang X, Zhu Y, Chaffey G, Gu Y, Green Tet al., 2019, The Resonant Modular Multilevel DC Converters for High Step-ratio and Low Step-ratio Interconnection in MVDC Distribution Network, IECON 2019 - 45th Annual Conference of the IEEE Industrial Electronics Society

Conference paper

Gu Y, Liu J, Green TC, Li W, He Xet al., 2019, Motion-induction compensation to mitigate sub-synchronous oscillation in wind farms, IEEE Transactions on Sustainable Energy, Vol: 11, Pages: 1247-1256, ISSN: 1949-3029

This paper presents a comprehensive solution to mitigate the sub-synchronous oscillation (SSO) in wind farms connected to series-compensated transmission lines. The concept of motion-induction amplification (MIA) is introduced to reinterpret the physical root cause of the negative resistance in doubly-fed induction generators (DFIGs). Based on this new interpretation, a novel control scheme called motion-induction compensation (MIC) is proposed to counteract the MIA effect. The MIC control eliminates the negative resistance in DFIGs across the entire frequency range, and makes the Type-III (DFIG) generator behave like a Type-IV generator in dynamics. The proposed solution provides wide-range SSO damping and also shows excellent robustness against model and measurement errors.

Journal article

Lachichi A, Junyent Ferre A, Green T, 2019, Comparative optimization design of a modular multilevel converter tapping cells and a 2L-VSC for hybrid LV ac/dc microgrids, IEEE Transactions on Industry Applications, Vol: 55, Pages: 3228-3240, ISSN: 0093-9994

The paper presents the performance of the modular multilevel converter tapping cells associated with an ac filter in term of efficiency and power density in a hybrid LV ac/dc microgrid application and compares it to the performance of the conventional topology used in LV application, i.e., the Two-Level voltage source converter (2L VSC). A bi-objective optimization based on the Genetic Algorithm is hence developed, providing details on designing the components of the LCL filter, the MMC and the 2L VSC. The MMC reaches an efficiency of 99.4% when the main dcgrid is left floating. However, due to its modularity and scalability, offering multiport connections option, the MMC tapping cells has the disadvantage of low power density. Exploring the filtering capability of the equivalent arm inductance of the MMC seen from the ac grid side, optimization design results show that higher switching frequencies allow a significant volume reduction of the inductive components of the MMC/LCL filter while higher switching frequencies have little impact on the switching losses of the MMC. This has the benefit of reducing the overall footprint of the converter and encouraging the use of the MMC in LV application.

Journal article

Zhang X, Tian M, Xiang X, Pereda J, Green T, Yang Xet al., 2019, Large step ratio input-series-output-parallel chain-link DC-DC converter, IEEE Transactions on Power Electronics, Vol: 5, Pages: 4125-4136, ISSN: 0885-8993

High-voltage and high-power dc-dc conversion is key to dc transmission, distribution and generation, which require compact and efficient dc transformers with large step ratios. This paper introduces a dc-dc converter with the input-series-output-parallel (ISOP) arrangement of multiple high step ratio sub-converter units. Each sub-converter unit is an isolated modular dcdc converter with a stack of half-bridge cells chopping the dc down to low voltage level. The transformer provides galvanic isolation and additional step ratio. The converter achieves a large step ratio due to the combination of the series-parallel configuration, the modular cells, and the isolation transformer. The proposed dc-dc converter is analyzed in a 30 kV to 1 kV, 1 MW application to discuss the operation performance, trade-offs, power efficiency and selection of components. Finally, the converter is validated through a laboratory down-scaled prototype.

Journal article

Liu Y, Green TC, Wu J, Rouzbehi K, Raza A, Xu Det al., 2019, A new droop coefficient design method for accurate power-sharing in VSC-MTDC systems, IEEE Access, Vol: 7, Pages: 47605-47614, ISSN: 2169-3536

This paper proposes a new droop coefficient design method with the aim of improving the power-sharing accuracy among the converters in a multi-terminal dc (MTDC) system. The proposed droop coefficient design method works by adjusting the droop coefficient and can realize an arbitrary power-sharing ratio among all the converters in an MTDC system. This method does not rely on a communication network and therefore has the potential for higher reliability than the alternative methods. Mitigating the impact of the variation of dc line resistances on the power-sharing is discussed. Simulation of a four-terminal MTDC system is carried out by using PSCAD/EMTDC. The experimental results under a scaled-down four-terminal dc grid platform demonstrate the effectiveness of the proposed method.

Journal article

Gu Y, Li Y, Yoo H-J, Nguyen T-T, Xiang X, Kim H-M, Junyent-Ferre A, Green Tet al., 2019, Transfverter: imbuing transformer-like properties in an interlink converter for robust control of a hybrid ac-dc microgrid

Matlab/Simulink models and codes for getting the results in the paper with the same title on IEEE Transactions on Power Electronics, DOI: 10.1109/TPEL.2019.2897460

Software

Zhang X, Xiang X, Green T, Yang X, Wang Fet al., 2019, A push-pull modular multilevel converter based low step-up ratio DC transformer, IEEE Transactions on Industrial Electronics, Vol: 66, Pages: 2247-2256, ISSN: 0278-0046

A non-isolated dc-dc converter with chain- links of sub-modules (SMs) is proposed in this paper. The converter can perform as a dc transformer with low step- up ratio. To demonstrate the step-up dc-dc conversion, the converter is operated in resonant conversion mode, achiev- ing soft-switching and inherent-balancing of SM capacitor voltages. The circuit configuration is presented and the operation principle is analyzed in the paper. Experimental results based on a downscaled prototype validate the per- formance of the proposed converter as a low step-up ratio dc transformer. The converter also exhibits a good linearity covering a broad range.

Journal article

Judge P, Green T, Merlin M, Trainer D, Vershinin Ket al., 2019, Thyristor/Diode-bypassed sub-module power-groups for improved efficiency in modular multilevel converters, IEEE Transactions on Power Delivery, Vol: 34, Pages: 84-94, ISSN: 0885-8977

The half-bridge Modular Multilevel Converter(MMC) is a Voltage Source Converter (VSC) with high effi-ciency, controllability and modularity. The topology is weak toDC side faults unless bipolar sub-modules are used, but thisresults in decreased efficiency. Power-Groups (PGs), a thyristoraugmented multilevel structure, have been proposed as a wayto reduce the power-loss increase arising from achieving DC-fault-tolerance. This paper investigates whether the PG conceptcan also achieve significant efficiency improvements in VSCs thatare not required to be DC fault tolerant. A Single Sub-ModuleVoltage (SSMV) method of controlling the turn-on/turn-off ofthe thyristor assembly within each PG structure is presentedand the differences with the previously detailed Dual Sub-ModuleVoltage (DSMV) technique are described. Two thyristor-based PGstructures for use in non-DC-fault-tolerant MMCs are proposed,one using SSMV and the other using DSMV. A comparisonis made considering the required semiconductor device count,the impact on thyristor snubber design, and the overall power-losses achieved. A further, simplified, variant using a diodebypassed PG structure is presented which results in power-loss reductions during rectifier mode only. Results show thatpower-loss reductions of∼20-25% can be achieved by using theproposed PG structures to augment a half-bridge MMC.

Journal article

Judge PD, Green TC, 2019, Modular Multilevel Converter with Partially Rated Integrated Energy Storage Suitable for Frequency Support and Ancillary Service Provision, IEEE Transactions on Power Delivery, Vol: 34, Pages: 208-219, ISSN: 0885-8977

© 1986-2012 IEEE. Grid scale energy storage systems (ESSs) have received significant interest in recent years due to their ability to reduce/defer investment in transmission/distribution networks, as well their ability to act as primary reserve sources and provide emergency support to the transmission system. This paper investigates the dual purposing of an HVdc-Scale modular multilevel converter (MMC), allowing it to also act as an ESS. This has potential application in primary frequency response provision and other services such as de-coupled power oscillation damping. In the proposed topology, a certain percentage of sub-modules (SM) within the MMC have their capacitor interfaced through a dc-dc converter to an energy storage element (ESE), formed of a battery or ultracapacitor. By applying appropriate control, energy can be exchanged from the ESE to the main SM capacitor of each ESE-SM, and from there to either the ac or dc bus. It was found that for some operating points, an injection of circulating current was required to facilitate exchanging energy with the ESE-SMs. Analysis shows that, for instance, an additional power injection to ac or dc terminal of 0.1 per unit (10%) is possible with only 4% of the SMs replaced by full-bridge ESE-SMs, and no additional SMs added.

Journal article

Lachichi A, Junyent-Ferre A, Green T, 2019, LCL Filter Design Optimization for LV Modular Multilevel Converters in Hybrid ac/dc Microgrids Application, 3rd International Conference on Electrical Sciences and Technologies in Maghreb (CISTEM), Publisher: IEEE, Pages: 945-949

Conference paper

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