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

 

Ms Beata Cypcar +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
to

323 results found

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, Green, 2017, Reduced-Order Models for Representing Converters in Power System Studies

Matlab codes of reduced-order models for representing power electronic converters in power system analyses.

SOFTWARE

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, Chaffey G, Merlin MMC, Clemow PR, Green TCet al., 2017, Dimensioning and Modulation Index Selection for the Hybrid Modular Multilevel Converter, IEEE Transactions on Power Electronics, ISSN: 0885-8993

CCBY The Hybrid MMC, comprising a mixture of full-bridge and half-bridge sub-modules, provides tolerance to DC faults within compromising the efficiency of the converter to a large extent. The inclusion of full-bridges creates a new freedom over the choice of ratio of AC to DC voltage at which the converter is operated, with resulting impact on the converter's internal voltage, current and energy deviation waveforms, all of which impact the design of the converter. A design method accounting for this, and allowing the required level of de-rating of nominal sub-module voltage and up-rating of stack voltage capability to ensure correct operation at the extremes of the operating envelope is presented. A mechanism is identified for balancing the peak voltage that the full-bridge and half-bridge sub-modules experience over a cycle. Comparisons are made between converters designed to block DC side faults and converters that also add STATCOM capability. Results indicate that operating at a modulation index of 1.2 gives a good compromise between reduced power losses and additional required sub-modules and semi-conductor devices in the converter. The design method is verified against simulation results and the operation of the converter at the proposed modulation index is demonstrated at laboratory-scale.

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, ISSN: 0885-8977

OAPA Achieving DC fault tolerance in modular multilevel converters requires the use of a significant number of Sub-Modules (SMs) which are capable of generating a negative voltage. This results in an increase in the number of semiconductor devices in the current path, increasing converter conduction losses. This paper introduces a thyristor augmented multilevel structure called a Power-Group (PG), which replaces the stacks of SMs in modular converters. Each PG is formed out of a series stack of SMs with a parallel force-commutated thyristor branch, which is used during normal operation as a low loss bypass path in order to achieve significant reduction in overall losses. The PG also offers negative voltage capability and so can be used to construct high efficiency DC fault tolerant converters. Methods of achieving the turn-on and turn-off of the thyristors by using voltages generated by the parallel stack of SMs within each PG are presented, while keeping both the required size of the commutation inductor, and the thyristor turn-off losses low. Efficiency estimates indicate that this concept could result in converter topologies with power-losses as low as 0.3% rated power, whilst retaining high quality current waveforms and achieving tolerance to both AC and DC faults.

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

Merlin MMC, Judge PD, Chaffey G, Wylie J, Green TCet al., 2017, Soft-switching of the director switch in the alternate arm converter using blocked sub-modules

© 2017 IEEE. The presence of Director Switches (DS) in its arms enables the Alternate Arm Converter (AAC) to generate smooth current waveforms while using a reduced number of Sub-Modules (SMs) when compared to the Modular Multilevel Converter. To maximise the power efficiency of the AAC and simplify the DS design, it is highly beneficial to soft-switch the DS. This paper presents a method which reliably ensures their soft-switching operation using the blocked state of SMs during the non-conducting part of the cycle of an arm. Experimental results demonstrate the effectiveness of the proposed method over a wide range of operating conditions.

CONFERENCE PAPER

Merlin MMC, Soto Sanchez D, Judge PD, Chaffey G, Clemow P, Green TC, Trainer DR, Dyke KJet al., 2017, 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: 0885-8993

CCBY The Alternate Arm Converter (AAC) was one of the first modular converter topologies to feature DC-side fault ride-through capability with only a small penalty in power efficiency. However, the simple alternation of its arm conduction periods (with a short overlap period) resulted in (i) substantial 6-pulse ripples in the DC current waveform, (ii) large DC-side filter requirements, and (iii) limited operating area close to an energy sweet-spot. This paper presents a new mode of operation called Extended Overlap (EO) based on the extension of the overlap period to 60 & #x00B0; which facilitates a fundamental redefinition of the AAC working principles. The EO-AAC has its DC current path decoupled from the AC current paths, a fact allowing (i) smooth DC currents, (ii) elimination of DC filters, and (iii) restriction lifting on the feasible operating. Analysis of this new mode and EO-AAC design criteria are presented and subsequently verified with tests on an experimental prototype. Finally, a comparison with other modular converters demonstrates that the EO-AAC is at least as power efficient as a hybrid MMC (with DC fault ride-through capability) while offering a smaller converter footprint because of a reduced requirement for energy storage in the submodules and a reduced inductor volume.

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

Wang B, Wang X, Wang X, Shao C, Judge PD, Green TCet al., 2017, An Analytical Approach to Evaluate the Reliability of Offshore Wind Power Plants Considering Environmental Impact, IEEE Transactions on Sustainable Energy, ISSN: 1949-3029

IEEE The accurate quantitative reliability evaluation of off-shore wind power plants (OWPPs) is an important part in planning and helps to obtain economic optimization. However, loop structures in collector systems and large quantities of components with correlated failures caused by shared ambient influences are significant challenges in the reliability evaluation. This paper proposes an ana-lytical approach to evaluate the reliability of OWPPs considering environmental impact on failures and solve the challenges by protection zone models, equivalent power unit models and common cause failure (CCF) analysis. Based on investigation of the characteristics of OWPP and related failures mechanisms, the components are divided into three CCF subsets. With the aid of the protection zone model and equivalent power unit model merged with CCF, the faulty collector system state eval-uation is applied to reduce the computational burden. The case studies present the necessity and improved per-formance of merging CCF analysis into modeling via the comparison with other two simplified methods. A sensi-tivity analysis is also carried out to account for inaccu-racy of failure data. The results show that the assump-tion of independent failures in the conventional method might lead to over-optimistic or over-pessimistic evalua-tion depending on the CCF style.

JOURNAL ARTICLE

Xiang X, Zhang X, Chaffey GP, Green TCet 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

CCBY 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

Xiang X, Zhang X, Chaffey GP, Gu Y, Green TCet al., 2017, The isolated resonant modular multilevel converters with large step-ratio for MVDC applications

© 2017 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

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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