Publications
424 results found
Bottrell N, Lang P, Green T, 2017, Algorithm for soft open points to solve thermal and voltage constraints in low-voltage distribution networks, 24th International Conference & Exhibition on Electricity Distribution (CIRED), Pages: 1567-1570
© 2017 The Institution of Engineering and Technology. All rights reserved. Thermal and voltage constraints in the low-voltage network (0.4 kV) may become more prevalent if increase in load or increase in distributed generation occur. Typically, constraints in the distribution network are solved by replacing the transformer, replacing the feeders, or installing a new substation. One solution to avoid these costly reinforcements is to utilise power electronics. The UK Power Networks project called Flexible Urban Networks-Low Voltage explored the use of connecting multiple feeders with power electronics. This study reports on the design of the control algorithm and demonstrates the operation by presenting field trial results.
Rousis AO, Pipelzadeh Y, Green TC, et al., 2017, Benefits of distributed power generation for voltage support in GB transmission system: Case study on the south-east region, IET International Conference on Resilience of Transmission and Distribution Networks (RTDN 2017), Publisher: Institution of Engineering and Technology
Distributed Generation (DG) has lately attracted great interest by energy market players due to its potential for independent provision of active and reactive power. High penetration of renewable energy sources can, however, adversely affect the power system. As such, the power system needs to be reinforced or expanded to be able to successfully cope with potential issues. It has been argued, though, that support from DG could form part of the solution as a means of driving down network reinforcement costs. In this context, this work particularly focuses on the potential of DG to support the voltage of the transmission system by providing reactive power when requested to. The south-east region of National Grid's transmission system in Great Britain is used as a case study to demonstrate the benefit from utilization of DG. The main contribution of this paper is to specifically demonstrate that distributed allocation of reactive power sources can alleviate imminent voltage instability issues, while minimising transmission reinforcement costs.
Wylie J, Merlin MC, Green TC, 2017, Analysis of the Effects from Constant Random and Wear-out Failures of Sub-modules within a Modular Multi-level Converter with Varying Maintenance Periods, 19th European Conference on Power Electronics and Applications (EPE ECCE Europe), Publisher: IEEE, ISSN: 2325-0313
Xiang X, Zhang X, Chaffey G, et al., 2017, An Isolated Resonant Mode Modular Converter with Flexible Modulation and Variety of Configurations for MVDC Application, IEEE Transactions on Power Delivery, Vol: 33, Pages: 508-519, 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.
Judge P, Chaffey G, Merlin MMC, et al., 2017, Dimensioning and modulation index selection for the hybrid modular multilevel converter, IEEE Transactions on Power Electronics, Vol: 33, Pages: 3837-3851, 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.
Xiang X, Zhang X, Chaffey G, et al., 2017, The isolated resonant modular multilevel converters with extreme step-ratio for MVDC application, 2017 IEEE 18TH WORKSHOP ON CONTROL AND MODELING FOR POWER ELECTRONICS (COMPEL), 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.
Merlin MMC, Judge PD, Chaffey G, et al., 2017, Soft-switching of the director switch in the alternate arm converter using blocked sub-modules, 2017 IEEE 18th Workshop on Control and Modeling for Power Electronics, COMPEL 2017
© 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.
Lachichi A, Junyent-Ferre A, Green T, 2017, Power Converters Design for Hybrid LV ac/dc Microgrids, 6th IEEE INTERNATIONAL CONFERENCE on RENEWABLE ENERGY RESEARCH and APPLICATIONS (ICRERA 2017), Publisher: IEEE, Pages: 850-854, ISSN: 2377-6897
Merlin MMC, Soto-Sanchez D, Judge PD, et 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, Vol: 33, Pages: 3898-3910, 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.
Sang Y, Junyent-Ferre A, Green TC, 2017, Transformer Design in a Medium Voltage DC/DC Converter for a DC Collection Network, 19th European Conference on Power Electronics and Applications (EPE ECCE Europe), Publisher: IEEE, ISSN: 2325-0313
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.
Wang B, Wang X, Wang X, et al., 2017, An Analytical Approach to Evaluate the Reliability of Offshore Wind Power Plants Considering Environmental Impact, IEEE Transactions on Sustainable Energy, Vol: 9, Pages: 249-260, ISSN: 1949-3029
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.
Zhang X, Xiang X, Green TC, et al., 2017, Operation and performance of resonant modular multilevel converter with flexible step ratio, IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol: 64, Pages: 6276-6286, ISSN: 0018-9421
Resonant modular multilevel converters (RMMCs)have been proposed for high voltage dc-dc applications. Using dif-ferent modulation strategies, RMMC operates in different modesand achieves flexible step ratio. To provide a comprehensive studyof RMMCs, this paper presents the modulation method achievinga wide range of step ratio with the ability of inherent-balancing.The conditions for guaranteeing the inherent-balancing abilityare provided. The operation principle and performance of theRMMC are presented in this paper, which have been exploredin a case study. The experimental results are obtained from abench-scale setup, which have verified the theoretical analysis.
PIpelzadeh Y, Moreno R, Chaudhuri B, et 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
In this paper, the efficacy and value of using corrective control supported by transient assistive measures (TAM) is quantified in terms of the cost savings due to less constrained operation of the system. The example TAM is a rapid modulation of the power order of the high-voltage direct current (HVDC) links in the system so as to improve transient stability during corrective control. A sequential approach is used for the offline value assessment: a security constrained economic dispatch (SCED) module (master problem) determines the optimal generation dispatch, HVDC settings, and the corrective control actions to be used post-fault (generation and demand curtailed) so as to minimize the operational costs while ensuring static security. The transient stability module (slave problem) assesses the dynamic stability for the operating condition set by the SCED and, if needed, applies appropriate TAM to maintain the system transiently stable. If this is not possible, the master module uses a tighter set of security constraints to update the dispatch and other settings until the system can be stabilized. A case-study on the Great Britain system is used to demonstrate that corrective control actions supported by TAM facilitate significantly higher pre-fault power transfers whilst maintaining N-2 security.
Xiang X, Merlin MMC, Green TC, 2016, Cost Analysis and Comparison of HVAC, LFAC and HVDC for Offshore Wind Power Connection, IET 12th International Conference on AC andDC Transmission, Publisher: IET
Low frequency AC (LFAC) has been proposed as a means toavoid some of the large converter station costs of high voltageDC (HVDC) while delivering some of the benefits in terms ofbetter line or cable utilization and its technical feasibility hasbeen established. It is said to offer a lower costs than HVDC orconventional high voltage AC (HVAC) for a range ofintermediate distances, with HVDC becoming cheaper overlong distances. However, the basis for identifying the distancerange and extent of cost saving has not been established. Here,cost estimate methodologies are extended for LFAC. Adifficulty is the absence of commercial schemes that canprovide practical examples of costs. In this paper, costs arebroken down into constituent terms and estimates are madefrom the most similar equipment from other schemes. Thecapacity limits and power losses associated with subsea cablesare analyzed for low frequency cases. For a given powertransfer and for each distance, a choice of operating voltage,cable size and number of parallel circuits is made in order tofind the lowest route cost. This yields cost as a function ofdistance that is a non-linear and discontinuous function. Thecost curves for LFAC are compared with HVDC and HVACoptions. The results for current cost estimates show that LFAChas a range of route length over which it is the lowest costoption and but this range narrows and eventually ceases to existfor higher power transfer ratings.
Chaffey GP, Green TC, 2016, Low speed protection methodology for a symmetrical monopolar HVDC network, 13th IET International Conference on AC and DC Power Transmission 2017., Publisher: IET
Protecting a future HVDC network from shortcircuit DC faults is presently expected to require extensive imple-mentation of fast HVDC circuit breakers, allowing for rapid faultisolation. The requirement for circuit breakers may, however, bereduced or removed depending on the protection methodology,resulting in a slower post-fault recovery. The expected faultresponse and the subsequent network recovery is dependenton the circuit breaker, network, and converter configurations.This paper examines a low speed protection methodology fora symmetrical monopolar HVDC system, a configuration whichrequires pole to pole balancing following a pole to ground fault.The fault clearance and network recovery timings are thereforeevaluated for both pole to pole and pole to ground faults onseveral case study networks.
Beddard A, Wang W, Barnes M, et al., 2016, Impact of Parameter Uncertainty on Power Flow accuracy in multi-terminal systems, IEEE Power and Energy Society General Meeting 2016, Publisher: IEEE, ISSN: 1944-9925
Accurate power flow in a MT system can be achievedwith droop controllers. However, almost all publications haveassumed that the DC voltage, DC current and DC cableresistances can be measured with 100% accuracy. In this paper, anovel power flow solver is developed which enables the user toanalyse the impact of these parameters on power flow accuracy.The developed Parameter Uncertainty Power Flow Solver (PUPFS)is shown to be able to accurately calculate the power flowerror for hundreds of parameter uncertainty scenarios in lessthan a second. The PU-PFS is employed to investigate the impactof parameter uncertainty on a potential MT system and theresults show that realistic measurement errors (0.2%) can resultin significant power flow error (>150MW). Finally, the paperassesses the key factors which influence the power flow accuracyresulting in a number of important conclusions.
Judge PD, Merlin MMC, Green TC, et 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.
Chaffey GP, judge PD, Green TC, 2016, Energy Requirements for Modular Circuit Breakers in Multiterminal HVDC Networks, HVDC 2016, International Conference on. CSEE, 2016.
Pipelzadeh Y, Ray Chaudhuri N, Chaudhuri B, et al., 2016, 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: 1558-0679
Damping contribution from wind farms (WFs) islikely to become a mandatory requirement as part of the gridcodes. For remote offshore WFs, connected through a voltagesource converter (VSC)-based direct current (DC) link, the mostconvenient option for the onshore transmission system operator(TSO) is to modulate the reactive power at the onshore VSCwithin their own jurisdiction. In this paper, we show thatsupplementary control through the onshore VSC alone, althoughattractive for TSOs, could result in unacceptable voltage variationsin the onshore power grid. On the other hand, modulation ofactive power output of the wind turbine generators (WTG) aloneturns out to be inadequate due to the limited overload capabilityof the WTGs and the on- and offshore VSCs. Coordinated controlover both onshore VSC and aggregated WF output overcomes theabove limitations and is shown to be effective for power oscillationdamping. A homotopy approach is used to design the coordinatedcontroller, which can be implemented locally (at offshore WF andonshore converter site) using a decentralized architecture. Casestudies on two test systems show that the proposed controlleryields similar system dynamic response as supplementary controlthrough the WF alone.
Wang B, Wang X, Bei Z, et al., 2016, Reliability model of MMC considering periodic preventive maintenance, IEEE Transactions on Power Delivery, Vol: 32, Pages: 1535-1544, ISSN: 0885-8977
Abstract:Periodic preventive maintenance (PPM) is a significant measure to ensure reliable operation of the modular multilevel converter (MMC), but to date has always been neglected in the reliability evaluation. A mathematical reliability model of MMC considering PPM is proposed in this paper. The model is derived rigorously by the reliability function and finally described by the indices equivalent failure rate and the forced outage rate. Two operation modes related to redundant submodules are considered and described by the standby model and k-out-of-n model, respectively. Employing this model, we can analyze the sensitivity of reliability to redundancy and maintenance interval and provide some reliability indices for planning, which is especially valuable for the offshore cases. The necessity of the considered PPM and the effectiveness of the proposed model are verified by case studies. A method to include PPM in reliability modeling is provided.
Xiang X, Zhang X, Chaffey G, et al., 2016, 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)
Chaffey GP, Judge PD, Merlin MMC, et al., 2016, DC Fault Ride Through of Multilevel Converters, 2016 IEEE Energy Conversion Congress and Exposition (ECCE)
Modular Multilevel Converters (MMC) can providesignificant advantages for power transmission applications, how-ever there are remaining challenges trading off DC fault response,losses and controllability. Alternative multilevel converter topolo-gies using combinations of full bridge and half bridge submodulesor series switches allow for competitive efficiency whilst retainingcontrol over the DC fault current. Several possible fault responsesare analysed to evaluate appropriate converter control actions.Experimental results from a 60 submodule 15 kW demonstratorare presented to validate the DC fault performance of the fullbridge MMC, the mixed stack MMC and the alternate armconverter. It is shown that each can control the current intoa low impedance DC fault, and there no requirement to blockthe semiconductor devices.
Guo J, Liang J, Zhang X, et al., 2016, 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
The half-bridge-based modular multilevel converter (MMC) has emerged as the favored converter topology for voltage-source HVDC applications. The submodules within the converter can be constructed with either individual insulated-gate bipolar transistor (IGBT) modules or with series-connected IGBTs, which allows for different redundancy strategies to be employed. The main contribution of this paper is that an analytical method was proposed to analyze the reliability of MMCs with the consideration of submodule arrangements and redundancy strategies. Based on the analytical method, the relative merits of two approaches to adding redundancy, and variants created by varying the submodule voltage, are assessed in terms of overall converter reliability. Case studies were conducted to compare the reliability characteristics of converters constructed using the two submodule topologies. It is found that reliability of the MMC with series-connected IGBTs is higher for the first few years but then decreases rapidly. By assigning a reduced nominal voltage to the series valve submodule upon IGBT module failure, the need to install redundant submodules is greatly reduced.
Beddard A, sheridan CE, Barnes M, et al., 2016, Improved Accuracy Average Value Models of Modular Multilevel Converters, IEEE Transactions on Power Delivery, Vol: 31, Pages: 2260-2269, ISSN: 0885-8977
Modular multilevel converters (MMCs) have become the converter topology of choice for voltage-source converter-high-voltage direct-current systems. Excellent work has previously been conducted to develop much needed average value models (AVM) for these complex converters; however, there a number of limitations as highlighted in this paper. This paper builds on the existing models, proposing numerous modifications and resulting in an enhanced MMC-AVM, which is significantly more accurate and which can be used for a wider range of studies, including DC faults.
spallarossa C, merlin M, Green TC, 2016, Augmented inertial response of Multi-Level Converters using internal energy storage, 2016 IEEE International Energy Conference (ENERGYCON), Publisher: IEEE
During the recent years, the number of High Voltage Direct Current (HVDC) links, used for accommodating renewable energy sources and for interconnecting different power systems, has increased significantly. Apart from enhancing system stability, HVDC links can be used to provide ancillary services, such as frequency support. This paper investigates the potential of the energy storage capability of Modular Multi-level Converters (MMCs) to contribute to the frequency response. MMCs provide fast released energy through their cells capacitor that may be used to improve the system inertial response during the fault transient after a loss of generation event. To assess the effect of the converter energy storage capability, a mixed AC and DC transmission platform is developed in PowerFactory. It consists of a multi-machine system including a point-to-point MMC based HVDC link. A generation outage is applied to investigate the contribution of the MMC energy storage capability to the frequency recovery.
Pereda Torres J, green T, 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
This paper presents a complete analysis of a direct ac-to-ac modular multilevel converter (direct MMC) applied in medium voltage distribution networks through the soft-openpoint concept. The direct MMC is capable of bidirectional power flow between two feeders at any power factor, even when the feeders have different nominal voltages and operate with a phase shift angle or unbalanced voltages. The converter has six branches, each one composed of full H-bridges cells connected in series to generate a multilevel voltage waveform, to share the blocking voltage of the power switches and to have fault tolerant operation.This paper presents a suitable control scheme and provides a discussion about the capabilities and limitations of the converter, the capacitor voltage balance control, the efficiency and the power loss mitigation at various operation points. Simulation results and power loss calculations are presented for a threephase 11 kV 16 MVA direct MMC with 10 H-bridge cells per branch. The direct MMC is simulated in a distribution network to demonstrate the features of the converter and control under various operation conditions, including grid faults.
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
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Cao W, Wu J, Jenkins N, et al., 2015, Benefits analysis of Soft Open Points for electrical distribution network operation, Applied Energy, Vol: 165, Pages: 36-47, ISSN: 1872-9118
Soft Open Points (SOPs) are power electronic devices installed in place of normally-open points in electrical power distribution networks. They are able to provide active power flow control, reactive power compensation and voltage regulation under normal network operating conditions, as well as fast fault isolation and supply restoration under abnormal conditions. A steady state analysis framework was developed to quantify the operational benefits of a distribution network with SOPs under normal network operating conditions. A generic power injection model was developed and used to determine the optimal SOP operation using an improved Powell’s Direct Set method. Physical limits and power losses of the SOP device (based on back to back voltage-source converters) were considered in the model. Distribution network reconfiguration algorithms, with and without SOPs, were developed and used to identify the benefits of using SOPs. Test results on a 33-bus distribution network compared the benefits of using SOPs, traditional network reconfiguration and the combination of both. The results showed that using only one SOP achieved a similar improvement in network operation compared to the case of using network reconfiguration with all branches equipped with remotely controlled switches. A combination of SOP control and network reconfiguration provided the optimal network operation.
Cao W, Wu J, Jenkins N, et al., 2015, Operating principle of Soft Open Points for electrical distribution network operation, Applied Energy, Vol: 164, Pages: 245-257, ISSN: 1872-9118
Soft Open Points (SOPs) are power electronic devices installed in place of normally-open points in electrical power distribution networks. They are able to provide active power flow control, reactive power compensation and voltage regulation under normal network operating conditions, as well as fast fault isolation and supply restoration under abnormal conditions. Two control modes were developed for the operation of an SOP, using back-to-back voltage-source converters (VSCs). A power flow control mode with current control provides independent control of real and reactive power. A supply restoration mode with a voltage controller enables power supply to isolated loads due to network faults. The operating principle of the back-to-back VSCs based SOP was investigated under both normal and abnormal network operating conditions. Studies on a two-feeder medium-voltage distribution network showed the performance of the SOP under different network-operating conditions: normal, during a fault and post-fault supply restoration. During the change of network operating conditions, a mode switch method based on the phase locked loop controller was used to achieve the transitions between the two control modes. Hard transitions by a direct mode switching were noticed unfavourable, but seamless transitions were obtained by deploying a soft cold load pickup and voltage synchronization process.
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