Imperial College London

ProfessorTimothyGreen

Faculty of Engineering

Director of the Energy Futures Laboratory (EFL)
 
 
 
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Contact

 

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

 
 
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Assistant

 

Miss Linnea Luuppala +44 (0)20 7594 5865

 
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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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320 results found

Chaffey GP, judge PD, Green TC, Energy Requirements for Modular Circuit Breakers in Multiterminal HVDC Networks, HVDC 2016, International Conference on. CSEE, 2016.

CONFERENCE PAPER

Judge P, Chaffey G, Merlin MMC, Clemow P, Green Tet al., Dimensioning and modulation index selection for the hybrid modular multilevel converter, IEEE Transactions on Power Electronics, ISSN: 1941-0107

The Hybrid MMC, comprising a mixture of fullbridgeand half-bridge sub-modules, provides tolerance to DCfaults without compromising the efficiency of the converter to alarge extent. The inclusion of full-bridges creates a new freedomover the choice of ratio of AC to DC voltage at which theconverter is operated, with resulting impact on the converter’sinternal voltage, current and energy deviation waveforms, allof which impact the design of the converter. A design methodaccounting for this, and allowing the required level of deratingof nominal sub-module voltage and up-rating of stackvoltage capability to ensure correct operation at the extremes ofthe operating envelope is presented. A mechanism is identifiedfor balancing the peak voltage that the full-bridge and halfbridgesub-modules experience over a cycle. Comparisons aremade between converters designed to block DC side faultsand converters that also add STATCOM capability. Resultsindicate that operating at a modulation index of 1.2 gives agood compromise between reduced power losses and additionalrequired sub-modules and semiconductor devices in the converter.The design method is verified against simulation results and theoperation of the converter at the proposed modulation index isdemonstrated at laboratory-scale.

JOURNAL ARTICLE

Judge PD, Chaffey G, Clemow P, Merlin MMC, Green TCet al., Hardware testing of the alternate arm converter operating in its extended overlap mode, International High Voltage Direct Current 2015 Conference (HVDC2015)

CONFERENCE PAPER

Merlin MMC, Soto-Sanchez D, Judge PD, Chaffey G, Clemow P, Green TC, Trainer DR, Dyke KJet al., The extended overlap alternate arm converter: a voltage source converter with DC fault ride-through capability and a compact design, IEEE Transactions on Power Electronics, ISSN: 1941-0107

The Alternate Arm Converter (AAC) was one ofthe first modular converter topologies to feature DC-side faultride-through capability with only a small penalty in powerefficiency. However, the simple alternation of its arm conductionperiods (with an additional short overlap period) resulted in(i) substantial 6-pulse ripples in the DC current waveform,(ii) large DC-side filter requirements, and (iii) limited operatingarea close to an energy sweet-spot. This paper presents a newmode of operation called Extended Overlap (EO) based onthe extension of the overlap period to 60â—¦which facilitates afundamental redefinition of the working principles of the AAC.The EO-AAC has its DC current path decoupled from the ACcurrent paths, a fact allowing (i) smooth DC current waveforms,(ii) elimination of DC filters, and (iii) restriction lifting on thefeasible operating point. Analysis of this new mode and EO-AAC design criteria are presented and subsequently verifiedwith tests on an experimental prototype. Finally, a comparisonwith other modular converters demonstrates that the EO-AACis at least as power efficient as a hybrid MMC (i.e. a DC faultride-through capable MMC) while offering a smaller converterfootprint because of a reduced requirement for energy storagein the submodules and a reduced inductor volume.

JOURNAL ARTICLE

Xiang X, Zhang X, Chaffey G, Gu Y, Green Tet al., The isolated resonant modular multilevel converters with extreme step-ratio for MVDC application, 18th IEEE Workshop on Control and Modeling for Power Electronics, COMPEL 2017, Publisher: IEEE

The dc-dc conversion will play an important role in multi-terminal dc networks and dc grids. This paper presents two isolated resonant modular multilevel converters (IRMMCs) to fulfill the large step-ratio conversion for medium voltage dc (MVDC) networks. The conventional resonant modular multilevel converters (RMMCs) suffer the common problems of non-isolation and high current stress, which are solved in the proposed IRMMCs. They not only inherit the beneficial features of inherent sub-module (SM) voltage-balancing and soft-switching operation from RMMCs, but also develop multi-module configurations to neutralize the current ripples on both sides of the dc-links. The theoretical analysis is verified by a set of full-scaled simulations for different application examples in MVDC collection and distribution. The results demonstrate the proposed IRMMCs and its derived configurations have good potential for operation as large step-ratio MVDC transformers.

CONFERENCE PAPER

Xiang X, Zhang X, Chaffey G, Merlin M, Green Tet al., A New Modulation Method for Resonant Modular Multilevel DCDC Converter with Flexible Ratio Operation and Inherent Balance Capability in HVDC Application, 2016 International High Voltage Direct Current Conference (HVDC 2016)

CONFERENCE PAPER

Chaffey G, Green TC, 2017, Low speed protection methodology for a symmetrical monopolar HVDC network

Protecting a future HVDC network from short circuit DC faults is presently expected to require extensive implementation of fast HVDC circuit breakers, allowing for rapid fault isolation. The requirement for circuit breakers may, however, be reduced or removed depending on the protection methodology, resulting in a slower post-fault recovery. The expected fault response and the subsequent network recovery is dependent on the circuit breaker, network, and converter configurations. This paper examines a low speed protection methodology for a symmetrical monopolar HVDC system, a configuration which requires pole to pole balancing following a pole to ground fault. The fault clearance and network recovery timings are therefore evaluated for both pole to pole and pole to ground faults on several case study networks.

CONFERENCE PAPER

Gu Y, Bottrell N, Green TC, 2017, Reduced-Order Models for Representing Converters in Power System Studies, IEEE Transactions on Power Electronics, ISSN: 0885-8993

CCBY A reduced-order model that preserves physical meaning is important for generating insight in large-scale power system studies. The conventional model-order reduction for a multiple-timescale system is based on discarding states with fast (short-timescale) dynamics. It has been successfully applied to synchronous machines, but is inaccurate when applied to power converters because the timescales of fast and slow states are not sufficiently separated. In the method proposed here, several fast states are at first discarded but a representation of their interaction with the slow states is added back. Recognizing that the fast states of many converters are linear allows well-developed linear system theories to be used to implement this concept. All the information of the original system relevant to system-wide dynamics, including non-linearity, is preserved, which facilitates judgments on system stability and insight into control design. The method is tested on a converter-supplied mini power system and the comparison of analytical and experiment results confirms high preciseness in a broad range of conditions.

JOURNAL ARTICLE

Guo J, Liang J, Zhang X, Judge PD, Wang X, Green TCet al., 2017, Reliability Analysis of MMCs Considering Submodule Designs with Individual or Series-Operated IGBTs, IEEE TRANSACTIONS ON POWER DELIVERY, Vol: 32, Pages: 666-677, ISSN: 0885-8977

JOURNAL ARTICLE

Judge PD, Merlin MMC, Green TC, Trainer DR, Vershinin Ket al., 2017, Thyristor-Bypassed Sub-Module Power-Groups for Achieving High-Efficiency, DC Fault Tolerant Multilevel VSCs, IEEE Transactions on Power Delivery, Pages: 1-1, ISSN: 0885-8977

JOURNAL ARTICLE

Martinez Sanz I, Judge PD, Spallarossa CE, Chaudhuri B, Green TCet al., 2017, Dynamic Overload Capability of VSC HVDC Interconnections for Frequency Support, IEEE Transactions on Energy Conversion, ISSN: 0885-8969

CCBY In future power systems reduced overall inertia caused by an increased dominance of asynchronous generation and interconnections would make frequency control particularly challenging. As the number and power rating of Voltage Source Converter (VSC) HVDC systems increases, network service provision would be expected from such systems and to do so would require overload capacity to be included in the converter specifications. This paper studies the provision of frequency services from Modular Multilevel Converter (MMC)-based VSC HVDC interconnections using temperature-constrained overload capability. Overload of the MMC-based HVDC system is achieved through controlled circulating currents, at the expense of higher losses, and subject to a control scheme which dynamically limits the overload available in order to keep the semiconductor junction temperatures within operational limits. Two frequency control schemes that use the obtained overload capacity to provide frequency response during emergency conditions are investigated. The controllers & #x0027; performance is demonstrated in the context of the future Great Britain transmission grid through a reduced equivalent test system. Simulation results show that even modest temperature margins which allow overload of MMC-based HVDC systems for a few seconds are effective as a primary frequency reserve and also reduce the loss of infeed requirements of such interconnections.

JOURNAL ARTICLE

Pipelzadeh Y, Chaudhuri NR, Chaudhuri B, Green TCet al., 2017, Coordinated Control of Offshore Wind Farm and Onshore HVDC Converter for Effective Power Oscillation Damping, IEEE TRANSACTIONS ON POWER SYSTEMS, Vol: 32, Pages: 1860-1872, ISSN: 0885-8950

JOURNAL ARTICLE

Pipelzadeh Y, Moreno R, Chaudhuri B, Strbac G, Green TCet al., 2017, Corrective Control With Transient Assistive Measures: Value Assessment for Great Britain Transmission System, IEEE TRANSACTIONS ON POWER SYSTEMS, Vol: 32, Pages: 1638-1650, ISSN: 0885-8950

JOURNAL ARTICLE

Wang B, Wang X, Bie Z, Judge PD, Wang X, Green TCet al., 2017, Reliability Model of MMC Considering Periodic Preventive Maintenance, IEEE TRANSACTIONS ON POWER DELIVERY, Vol: 32, Pages: 1535-1544, ISSN: 0885-8977

JOURNAL ARTICLE

Xiang X, Zhang X, Chaffey G, Green Tet al., 2017, An Isolated Resonant Mode Modular Converter with Flexible Modulation and Variety of Configurations for MVDC Application, IEEE Transactions on Power Delivery, ISSN: 0885-8977

The dc tap or dc transformer will play an important role in interfacing different voltages of dc links in dc grids. This paper presents an isolated resonant mode modular converter (RMMC) with flexible modulation and assorted configurations to satisfy a wide variety of interface requirements for medium voltage dc (MVDC) networks. The transformer-less RMMC, as introduced in the literature, implemented a restricted modulation scheme leading to a very limited range of step-ratio and the diode rectifier resulted in unidirectional power flow. Both of these limitations are removed in this proposal and galvanic isolation has also been added. Moreover, this new RMMC approach can serve as a building block for variety of configurations. Two such derived topologies are given, which inherently balance the voltage and current between different constituent circuits and realize the high power rating conversion for very low or very high step-ratio application. The theoretical analysis is validated by a set of full-scale simulations and a down-scaled experimental prototype. The results illustrate that this isolated RMMC and its derivatives have promising features for dc taps or dc transformers in MVDC applications.

JOURNAL ARTICLE

Zhang X, Xiang X, Green TC, Yang Xet al., 2017, Operation and Performance of Resonant Modular Multilevel Converter With Flexible Step Ratio, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, Vol: 64, Pages: 6276-6286, ISSN: 0278-0046

JOURNAL ARTICLE

Beddard A, Sheridan CE, Barnes M, Green TCet al., 2016, Improved Accuracy Average Value Models of Modular Multilevel Converters, IEEE TRANSACTIONS ON POWER DELIVERY, Vol: 31, Pages: 2260-2269, ISSN: 0885-8977

JOURNAL ARTICLE

Beddard A, Wang W, Barnes M, Green PR, Beddard A, Green TCet al., 2016, Impact of Parameter Uncertainty on Power Flow Accuracy in Multi-terminal Systems, IEEE-Power-and-Energy-Society General Meeting (PESGM), Publisher: IEEE, ISSN: 1944-9925

CONFERENCE PAPER

Bloemink JM, Green TC, 2016, Required VSC Efficiency for Zero Net-Loss Distribution Network Active Compensation, 7th IEEE International Symposium On Power Electronics for Distributed Generation Systems (PEDG), Publisher: IEEE, ISSN: 2329-5759

CONFERENCE PAPER

Cao W, Wu J, Jenkins N, Wang C, Green Tet al., 2016, Operating principle of Soft Open Points for electrical distribution network operation, APPLIED ENERGY, Vol: 164, Pages: 245-257, ISSN: 0306-2619

JOURNAL ARTICLE

Cao W, Wu J, Jenkins N, Wang C, Green Tet al., 2016, Benefits analysis of Soft Open Points for electrical distribution network operation, APPLIED ENERGY, Vol: 165, Pages: 36-47, ISSN: 0306-2619

JOURNAL ARTICLE

Chaffey G, Judge PD, Merlin MMC, Clemow PR, Green TCet al., 2016, DC Fault Ride Through of Multilevel Converters, IEEE Energy Conversion Congress and Exposition (ECCE), Publisher: IEEE, ISSN: 2329-3721

CONFERENCE PAPER

Judge PD, Merlin MMC, Green TC, Trainer DR, Vershinin Ket al., 2016, The augmented trapezoidal alternate arm converter: a power-group augmented DC fault tolerant voltage source converter, 2nd International Conference on High Voltage Direct Current

Achieving DC fault tolerance in modularmultilevel converters requires the use of a significantnumber sub-modules which are capable of generatinga negative voltage. This results in an increase in thenumber of IGBT devices in the current path, resultingin an increase in conduction losses within the converter.This paper introduces a thyristor augmented multilevelstructure called a Power-Group, offering both negativevoltage capability as well as a low conduction loss state,which can be used to construct high efficiency DC faulttolerant converters. When combined with the AlternateArm Converter topology the power-group concept hasbeen found to result in a converter topology that exhibitsvery low power-losses, high quality AC and DC currentwaveforms, while also achieving DC fault tolerance.

CONFERENCE PAPER

Pereda J, Green TC, 2016, Direct Modular Multilevel Converter With Six Branches for Flexible Distribution Networks, IEEE TRANSACTIONS ON POWER DELIVERY, Vol: 31, Pages: 1728-1737, ISSN: 0885-8977

JOURNAL ARTICLE

Spallarossa CE, Merlin MMC, Green TC, 2016, Augmented Inertial Response of Multi-level Converters using Internal Energy Storage, IEEE International Energy Conference (ENERGYCON), Publisher: IEEE, ISSN: 2164-4322

CONFERENCE PAPER

Xiang X, Merlin MMC, Green TC, 2016, Cost analysis and comparison of HVAC, LFAC and HVDC for offshore wind power connection

Low frequency AC (LFAC) has been proposed as a means to avoid some of the large converter station costs of high voltage DC (HVDC) while delivering some of the benefits in terms of better cable utilization and its technical feasibility has been established. It is said to offer lower costs than HVDC or conventional high voltage AC (HVAC) for a range of intermediate distances, with HVDC becoming cheaper over long distances. However, the basis for identifying the distance range and extent of cost saving has not been established. Here, cost estimate methodologies are extended for LFAC. A difficulty is the absence of commercial schemes that can provide practical examples of costs. In this paper, costs are broken down into constituent terms and estimates are made from the most similar equipment from other schemes. The capacity limits and power losses associated with subsea cables are analyzed for low frequency cases. For a given power transfer and for each distance, a choice of operating voltage, cable size and number of parallel circuits is made in order to find the lowest route cost. This yields cost as a function of distance that is a non-linear and discontinuous function. The cost curves for LFAC are compared with HVDC and HVAC options. The results for current cost estimates show that LFAC has a range of route length over which it is the lowest cost option and but this range narrows and eventually ceases to exist for higher power transfer ratings.

CONFERENCE PAPER

Zhao Z, Yang P, Guerrero JM, Xu Z, Green TCet al., 2016, Multiple-Time-Scales Hierarchical Frequency Stability Control Strategy of Medium-Voltage Isolated Microgrid, IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol: 31, Pages: 5974-5991, ISSN: 0885-8993

JOURNAL ARTICLE

Bottrell N, Green TC, 2015, Investigation into the Post-Fault Recovery Time of a Droop Controlled Inverter-Interfaced Microgrid, IEEE 6th International Symposium on Power Electronics for Distributed Generation Systems (PEDG), Publisher: IEEE, Pages: 223-229, ISSN: 2329-5759

CONFERENCE PAPER

Chaffey G, Green TC, 2015, Reduced DC Circuit Breaker Requirement on Mixed Converter HVDC Networks, PowerTech, 2015 IEEE Eindhoven, Publisher: IEEE

CONFERENCE PAPER

Chaffey G, Green TC, 2015, Directional Current Breaking Capacity Requirements for HVDC Circuit Breakers, IEEE Energy Conversion Congress and Exposition, Publisher: IEEE, Pages: 5371-5377, ISSN: 2329-3721

CONFERENCE PAPER

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