Publications
424 results found
Gu Y, Li Y, Yoo H-J, et 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.
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.
Xiang X, Zhang X, Zhu Y, et 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
Gu Y, Liu J, Green TC, et 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.
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.
Zhang X, Tian M, Xiang X, et 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.
Liu Y, Green TC, Wu J, et 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.
Gu Y, Li Y, Yoo H-J, et 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
Zhang X, Xiang X, Green T, et 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.
Judge P, Green T, Merlin M, et 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.
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.
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
Mendonca T, Bottrell N, Green T, 2019, Incorporating Ancillary Service Costs in Distributed Energy Resources Management Systems, IEEE PES Innovative Smart Grid Technologies Europe (IEEE ISGT-Europe), Publisher: IEEE, ISSN: 2165-4816
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- Citations: 1
Bashar E, Wu R, Ran L, et al., 2019, A Method to Contain the Temperature Rise of a Press-Pack Thyristor during a Short Circuit Protection Operation, 11th Annual IEEE Energy Conversion Congress and Exposition (ECCE), Publisher: IEEE, Pages: 3311-3317, ISSN: 2329-3721
Mendonca T, Bottrell N, Green T, 2019, Simplified Voltage Sensitivity Based Curtailment Arrangement for Active Network Management, IEEE Milan PowerTech Conference, Publisher: IEEE
Pipelzadeh Y, Chaudhuri NR, Chaudhuri B, et al., 2018, Coordinated Control of Offshore Wind Farm and Onshore HVDC Converter for Effective Power Oscillation Damping, IEEE-Power-and-Energy-Society General Meeting (PESGM), Publisher: IEEE, ISSN: 1944-9925
Pipelzadeh Y, Moreno R, Chaudhuri B, et al., 2018, Corrective Control With Transient Assistive Measures: Value Assessment for Great Britain Transmission System, IEEE-Power-and-Energy-Society General Meeting (PESGM), Publisher: IEEE, ISSN: 1944-9925
Rousis AO, Chairiman, Pipelzadeh Y, et al., 2018, Voltage Support from Distribution Level Resources in South-East England, IEEE-Power-and-Energy-Society General Meeting (PESGM), Publisher: IEEE, ISSN: 1944-9925
Olvera JP, Green T, Junyent-Ferre A, 2018, Using Multi-terminal DC Networks to Improve the Hosting Capacity of Distribution Networks, IEEE PES Innovative Smart Grid Technologies Conference Europe (ISGT-Europe), Publisher: IEEE, ISSN: 2165-4816
Wylie J, Judge PD, Green TC, 2018, Modular Multi-level Converter for Medium Voltage Applications with Mixed Sub-module Voltages within Each Arm, IEEE PES Innovative Smart Grid Technologies Conference Europe (ISGT-Europe), Publisher: IEEE, ISSN: 2165-4816
Xiang X, Zhang X, Chaffey G, et al., 2018, Analysis on circulating current frequency of chain-link modular multilevel DC-DC converters for low step-ratio high-power MVDC applications, IEEE ECCE 2018, Publisher: IEEE
The direct chain-link modular multilevel dc converter has raised great interest recently in medium/high- voltage dc-dc conversion due to the high power device utilization and lower power losses than the front-to-front configurations of modular multilevel converter. This paper introduces a single-phase chain-link modular multilevel buck-boost converter for medium voltage dc applications and presents an analysis methodology on its internal circulating current frequency. Its connection and comparison with the classic dc-ac modular multilevel converter are given first, and its dc and ac components are analyzed in the respective equivalent circuits. Then, the derivation methodology for the proper circulating current frequency with lowest internal reactive power is provided, which could minimize the current stress and decrease the power losses. Also, this method can be directly applied in the derivative topologies and further configurations to satisfy various conversion requirements. The theoretical analysis is verified by a set of full-scaled simulations and further verified against experimental tests on a down-scaled prototype.
Xiang X, Zhang X, Chaffey G, et al., 2018, A modular multilevel DC-DC converter with a compact sub-module stack suited to low step-ratios, IEEE Transactions on Power Delivery, Vol: 34, Pages: 312-323, ISSN: 0885-8977
The voltage ratings for dc transmission and distribution are not currently standardized because cable technology is advancing rapidly and therefore low step-ratio dc-dc converters at multi-megawatt scale are crucial technology for combining point-to-point dc links into multi-terminal networks. This paper presents a modular multilevel dc-dc converter configuration with only a single stack of half-bridge sub-module (SM) and a resonant tank. The power processed through the stack is a small fraction of the throughput power if the step-ratio is small. Detailed analysis shows the required semiconductor ratings are much smaller compared to the front-to-front MMC and modular converters derived from standard dc-dc topologies. Further, the requirement for SM capacitor energy storage is also significantly less than those converters. The theoretical analysis and the operating principles have been verified by full-scale simulation examples and experiments with down-scaled prototype. The SMs are operated with an adjustable phase-shift to create a high frequency excitation across the resonant tank for further volume reduction. The SM capacitors self-balance without the need for switching pattern adjustments and the resonant operation provides soft-switching for all power switches to increase efficiency.
Xiang X, Zhang X, Luth T, et al., 2018, A compact modular multilevel DC-DC converter for high step-ratio MV and HV use, IEEE Transactions on Industrial Electronics, Vol: 65, Pages: 7060-7071, ISSN: 0278-0046
In multi-terminal dc networks or future dc grids, there is an important role for high step-ratio dc-dc conversion to interface a high voltage network to lower voltage infeeds or offtakes. The efficiency and controllability of dc-dc conversion will be expected to be similar to modular multi-level ac-dc converters. This paper presents a modular multilevel dc-dc converter with a high step-ratio for medium voltage and high voltage applications. Its topology on high-voltage side is derived from the half-bridge single-phase inverter with stacks of sub-modules replacing each of the switch positions. A near-square-wave current operation is proposed which achieves near-constant instantaneous power for single-phase conversion, leading to reduced stack capacitor and filter volume and also increased the power device utilization. A controller for energy balancing and current tracking is designed. The soft-switching operation on the low-voltage side is demonstrated. The high step-ratio is accomplished by combination of inherent half-bridge ratio, sub-module stack modulation and transformer turns-ratio, which also offers flexibility to satisfy wide-range conversion requirement. The theoretical analysis of this converter is verified by simulation of a full-scale 40MW, 200 kV converter with 146 sub-modules and also through experimental testing of a down-scaled prototype at 4.5 kW, 1.5 kV with 18 sub-modules.
Sang Y, Junyent Ferre A, Xiang X, et al., 2018, Analysis and Control of a Parallel DC Collection System for Wind Turbines with Single Active Bridge Converters, IEEE ENERGY CONVERSION CONGRESS & EXPO
Gu Y, Bottrell N, Green TC, 2018, Reduced-order models for representing converters in power system studies, IEEE Transactions on Power Electronics, Vol: 33, Pages: 3644-3654, ISSN: 0885-8993
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 nonlinearity, 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.
Judge PD, Merlin MMC, Green TC, et al., 2018, Thyristor-Bypassed Submodule Power-Groups for Achieving High-Efficiency, DC Fault Tolerant Multilevel VSCs, IEEE TRANSACTIONS ON POWER DELIVERY, Vol: 33, Pages: 349-359, ISSN: 0885-8977
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- Citations: 20
Sanz IM, Judge PD, Spallarossa CE, et al., 2018, Effective Damping Support through VSC-HVDC Links with Short-Term Overload Capability, IEEE PES Innovative Smart Grid Technologies Conference Europe (ISGT-Europe), Publisher: IEEE, ISSN: 2165-4816
Damping service provision through VSC-based HVDC links has been extensively covered in the literature. However, little or no attention has been paid to the available range of active and reactive power modulation when the HVDC link is already operating at rated capacity. In these conditions some overload capability is usually assumed, ignoring the physical constraints imposed by the safe operating area of the IGBT modules in the converter. This paper presents, in a unified framework, the provision of damping support from VSC-HVDC links equipped with additional control for short-term overload capability. The performance of a Model Predictive Control (MPC) damping controller that accounts for the extended P/Q operating area of the converter is analysed. Case studies are presented to show that the extracted short-term overload capability can significantly improve the damping support from VSC-HVDC links. Simulation results also include the impact of damping control action on the junction temperatures of the IGBT modules of the converters, quantifying the effect of this service on the semiconductor temperature dynamics.
Lachichi A, Junyent-Ferre A, Green T, 2018, Optimal Design of a LCL Filter for LV Modular Multilevel Converters in Hybrid ac/dc Microgrids Application, 44th Annual Conference of the IEEE Industrial-Electronics-Society (IECON), Publisher: IEEE, Pages: 3973-3978, ISSN: 1553-572X
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- Citations: 6
Martinez Sanz I, Judge P, Spallarossa C, et al., 2017, Dynamic overload capability of VSC HVDC interconnections for frequency support, IEEE Transactions on Energy Conversion, Vol: 32, Pages: 1544-1553, ISSN: 0885-8969
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 that 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' 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.
Mendonca TRF, Collins ME, Pinto MF, et al., 2017, Decentralisation of power flow solution for facilitating active network management, 24th International Conference & Exhibition on Electricity Distribution (CIRED) 12-15 June 2017, Pages: 1669-1672
© 2017 The Institution of Engineering and Technology. All rights reserved. The rapid growth in connections of distributed energy resources (DER) is leading to constrains in distribution networks that were not designed with this in mind. The required traditional reinforcement is expensive and so network operators and utilities have sought to use active network management (ANM) as a more effective approach. However, ANM as often proposed relies on a centralised data acquisition and control scheme, which is a challenge if the numbers of DER across the distribution network are very large. In order to avoid these undesirable features, efforts have been directed into decentralising ANM. This study presents a decentralised power flow solution that reduces centralised calculations and facilitates ANM. The formulation of the algorithm and the approximations it rests on are described. Simulation results demonstrate that errors in estimating voltages and power flows due to non-local changes in power flow are as small as 0.1%.
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