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A cascaded hybrid inverter with improved dc-link voltage control for grid connected systems

- Wanjekeche, T., Jimoh, A. A., Nicolae, Dan-Valentin

Authors: Wanjekeche, T. , Jimoh, A. A. , Nicolae, Dan-Valentin
Date: 2012
Subjects: Voltage-frequency converters , Converters , Electronic circuits
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/17312 , uj:15870 , ISBN: 9781467301589 , Ciatation: Wanjekeche, T.,Jimoh, A.A. & Nicolae, D.V. 2012. A cascaded hybrid inventer with improved dc-link voltage control for grid connected systems IEEE – ISIE 2012, 28–31 May, Hangzhou, China:1950-1956. DOI:10:1109/ISIE.2012.6237391
Description: Abstract: This paper presents investigation of a new Phase shifted PWM technique with improved harmonic suppression. A novel balance circuit for DC – link voltage balance of two three level legs connected back – to back is designed and tested. Combined with the individual voltage control, a complete voltage controller is developed for a cascaded nine level hybrid model with two cells. Robustness of the proposed algorithm under varying operating conditions and modulation indices is verified by simulation.
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A cascaded NPC/H-bridge inverter with simplifiied control strategy and minimum component count

- Wanjekeche, T., Nicolae, Dan-Valentin, Jimoh, A. A.

Authors: Wanjekeche, T. , Nicolae, Dan-Valentin , Jimoh, A. A.
Date: 2009
Subjects: Electric inverters , Harmonics (Electric waves) , Power electronics
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/22319 , uj:16188 , ISBN: 978-1-4244-3919-5 , Citation: Wanjekeche, T., Nicolae, D.V. & Jimoh, A.A. 2009. A cascaded NPC/H-bridge inverter with simplifiied control strategy and minimum component count. Proceedings of IEEE-Africon, 23-25 September, 2009, Kenya. DOI: 10.1109/AFRCON.2009.5308551
Description: Abstract: Please refer to full text to view abstract Please refer to full text to view abstract
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A new dc-dc double-boost typology for enhancing the voltage ratio

- Nicolae, Dan-Valentin, Van Rensburg, J. F. J.

Authors: Nicolae, Dan-Valentin , Van Rensburg, J. F. J.
Date: 2008
Subjects: Electric current converters , Voltage regulators
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/18600 , uj:16021 , ISBN: 978-0-88986-753-6 , Citation: Nicolae, D.V. & Van Rensburg, J.F.J. 2008. A new dc-dc double boost topology for enhancing the voltage ratio. The IASTED International Conference on Circuits and Systems, 18-20 August 2008, Kailua-Kona, USA:168-173.
Description: Abstract: In this paper anew boost topology is proposed. The circuit is similar with two parallel boost dc-to-dc converters, but the two inductors are charged in parallel and release energy in series, thus enhancing the voltage boost ratio. After a short analysis of the circuit, a comparative study with other classic boost converter (single boost and twocascade) is presented. The simulation results show a net improvement of the boost ratio for the new proposed topology.
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A novel 9-level multilevel inverter based on 3-level NPC/H-bridge topology for photovoltaic applications

- Wanjekeche, T., Jimoh, A. A., Nicolae, Dan-Valentin

Authors: Wanjekeche, T. , Jimoh, A. A. , Nicolae, Dan-Valentin
Date: 2009
Subjects: Harmonics , Photovoltaic power generation , Electric power generation
Language: English
Type: Article
Identifier: http://hdl.handle.net/10210/21278 , uj:16136 , Citation: Wanjekeche, T, Jimoh, A.A. & Nicolae, D.V. 2009. A novel 9-level multilevel inverter based on 3-level NPC/H-bridge topology for photovoltaic applications. International Review of Electrical Engineering, 4(5):769-777. , ISSN: 1827-6660
Description: Abstract: Please refer to full text to view abstract Please refer to full text to view abstract
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A Review of the state of the art of modulation techniques and control strategies for matrix converters

- Richards, C. G., Ehlers, P. J., Nicolae, Dan-Valentin, Monacelli, E., Hamam, Y.

Authors: Richards, C. G. , Ehlers, P. J. , Nicolae, Dan-Valentin , Monacelli, E. , Hamam, Y.
Date: 2012
Subjects: Matrix algebra , Modulation
Language: English
Type: Journal article
Identifier: http://ujcontent.uj.ac.za8080/10210/386726 , http://hdl.handle.net/10210/17968 , uj:15945 , ISSN:1974-6059 , Citation: Richards,C.G. et al. 2012. A review of the state of the art of modulation techniques and control strategies for matrix converters. International Review of Automatic Control, 5(3):298-307
Description: Abstract: The reliability and stability of the Matrix Converter has improved during the last years due to the enhanced control algorithms. The traditional direct transfer function control mode has been replaced by more complex – digitally implemented control methodologies. These methodologies allow for real time calculation of the optimal switching interval of each individual switch of the matrix converter. These new switching algorithms allow optimal performances, ensuring sinusoidal outputs at any desired power factor. This paper will first revise the underlying theory of matrix converters, then review the various control limitations and finally review the current control algorithms
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A study of improving the power factor of a three-phase induction motor using a static switched capacitor

- Jimoh, Adisa A., Nicolae, Dan-Valentin

Authors: Jimoh, Adisa A. , Nicolae, Dan-Valentin
Date: 2006
Subjects: Three-phase machines , Induction motors , Reactive compensation , Power factor correction
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/22367 , uj:16193 , ISSN: 1-4244-0121-6 , Citation: Jimoh, A.A. & Nicolae, D.V. 2006. A study of improving the power factor of a three-phase induction motor using a static switched capacitor. EPE-PEMC, 12th International Power Electronics and Motion Control, September, 2006, Portorož, Slovenia.
Description: Abstract: In this paper a static switched capacitor with an auxiliary three-phase stator winding, which is only magnetically coupled to the stator main winding, is explored for improving the starting and operating power factor of a three-phase induction motor. The scheme improves the power factor of the motor without compromising significantly on other performances. Important advantages of the scheme include preventing harmonics in the line current, and eliminating regeneration possibility as well as preventing high inrush currents at starting.
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A study of insulation failure in a high voltage current transformer

- Jimoh, A. A., Mahlasela, V. S., Nicolae, Dan-Valentin

Authors: Jimoh, A. A. , Mahlasela, V. S. , Nicolae, Dan-Valentin
Date: 2005
Subjects: Electric power transmission
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/22348 , uj:16190 , ISBN: 9075815085 , Citation: Jimoh, A.A., Mahlasela, V.S. & Nicolae, D.V. 2005. A study of insulation failure in a high voltage current transformer. EPE 2005 Conference, Dresden, Germany, 11-14 September 2005. DOI:10.1109/EPE.2005.219752
Description: Abstract: The ageing and deterioration of insulation in high voltage (HV) plants have been a source of concerns to utilities. Breakdown of insulation leads to failures of HV equipment. The ageing and eventual failure of insulation in a high voltage curTent transformer (JHVCT) is the subject of investigation in this paper. A lumiiped parameter model ofHVCT is developed. This model is used to study the influlence of deteriorating and failed iinsulation on the state variables of the HVCT. Somie possible scenarios that could lead to a CT failure are investigated in this paper. For all the scelarios considered, steady alid transient equations relating the state variables of the mlodel have been developed and analyzed. The objectives of these analyses are to establish the behavioural claracteristics of the state variables, establish the interactions between these variables, and investigate the possible generations of harmonics under the various scenarios of deteriorating and outright failure of insulation in the CT. The paper concludes with a discussion of the results obtained
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A study of the impact of broken neutral in a distribution system

- Xivambu, M., Jimoh, A. A., Nicolae, Dan-Valentin

Authors: Xivambu, M. , Jimoh, A. A. , Nicolae, Dan-Valentin
Date: 2006
Subjects: Electric wiring , Neutral density filters
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/22058 , uj:16153 , Citation: Xivambu, M., Jimoh, A.A. & Nicolae, D.V. 2006. A study of the impact of broken neutral in a distribution system. IASTED, September, 2006, Gaborone, Botswana.
Description: Abstract: In this paper a study of the impact of the neutral conductor in a distribution system is presented. The causes which determine faults of the neutral conductor together with its effects are presented. The paper proposes a solution which will minimize the impact of the neutral failure.
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Active power control for capacitive divider tapping method

- Nicolae, Dan-Valentin, Jimoh, Adisa A., Britten, Anton C.

Authors: Nicolae, Dan-Valentin , Jimoh, Adisa A. , Britten, Anton C.
Date: 2006
Subjects: Power systems , Captive divider systems , Tapping electric power
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/22404 , uj:16197 , ISSN: 1-4244-0121-6 , Citation: Nicolae, D.V., Jimoh, A.A. & Britten, A.C. 2006. Active power control for capacitive divider tapping method. EPE-PEMC, 12th International Power Electronics and Motion Control, 30 August-1 September, 2006, Portorož, Slovenia.
Description: Abstract: Capacitive divider systems for tapping electric power from high voltage transmission lines are known to exhibit certain detrimental problems of stability, sub-resonance harmonic oscillations, and ferroresonance when coupled with non-linear loads. On the other hand, they can improve reactive power flow, power factor and contribute to improved harmonic filtration of the larger power system. This paper, therefore, proposes a controlled power flow conditioner as an essential integral component of a capacitive divider system to control power transfer between the high voltage transmission line, capacitive divider system, and the load. The ultimate objective of this conditioner is to mitigate or minimize the attendant problems associated with coupling the system to the load, and optimize the derivable benefits to the larger network.
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Alternating current source to voltage source converter

- Case, M. J., Van Rensburg, J. F. J., Nicolae, Dan-Valentin

Authors: Case, M. J. , Van Rensburg, J. F. J. , Nicolae, Dan-Valentin
Date: 2005
Subjects: Voltage-frequency converters , Direct-current transmission
Language: English
Type: Conference proceedings
Identifier: http://ujcontent.uj.ac.za8080/10210/388929 , http://hdl.handle.net/10210/21562 , uj:16150 , Citation: Nicolae, D.V., Van Rensburg, J.F.J. & Case, M.J. 2005. Alternating current source to voltage source converter. IPST’05 Conference, Montreal, Canada, June 2005, Paper# 05IPST081-22c
Description: Abstract: A direct alternating current source to voltage source converter, which does not employ energy storage components, is presented. Conversion is done by a current steering mechanism in the form of a current transformer with two secondary windings. The direct conversion concept, basic circuit, simulation and experimental results of a reduced scale model are presented.
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An analysis of power quality of matrix converters when using a Fryze reference

- Richards, C. G., Nicolae, Dan-Valentin

Authors: Richards, C. G. , Nicolae, Dan-Valentin
Date: 2014
Subjects: Electric current power converters , Electric power systems
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/17680 , uj:15913 , ISBN: 978-1-4673-6487-4 , Citation: Richards, C.G. & Nicolae, D.V. 2014. An analysis of power quality of matrix converters when using a Fryze reference, ICHQP 2014, 16th International Conference on Harmonics and Quality of Power, 25-28 May Bucharest, Romania. DOI: 10.1109/ICHQP.2014.6842839
Description: Abstract Static power converters produce harmonics due to the nature of the conversion process. This paper will investigate the effect on the power factor of a sinusoidal and Fryze current reference on Matrix Converters. It has been shown in previous papers that a Fryze current ref is best. This paper will develop a mathematical model, based on the single sided Fourier expansion. Using this mathematical model it will be shown that even though a high harmonic content could be present in a Fryze controlled Matrix Converter; the power factor will have been improved
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An approach to quantify the technical impact of power quality in medium voltage distribution systems

- Dlamini, F. M., Nicolae, Dan-Valentin

Authors: Dlamini, F. M. , Nicolae, Dan-Valentin
Date: 2016
Subjects: Power factor correction , Total harmonic distortion , Resonance
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/212857 , uj:21036 , Citation: Dlamini, F.M & Nicolae, D.V. 2016. An approach to quantify the technical impact of power quality in medium voltage distribution systems.
Description: Abstract: This paper provides a useful guidance in the interpretation and analyses of the technical impact of power factor correction in a power system. It is intended to raise awareness in order to achieve correct measurements, calculations and the key dynamics of the power system when quantifying the impact of power quality (PQ) in a non-linear power system. This paper is therefore targeting the power producing utilities, municipalities and the bulk end users of electricity.
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An investigation on optimal current mode control for boost-type PWM rectifiers

- Richards, C. G., Ehlers, P. J., Nicolae, Dan-Valentin

Authors: Richards, C. G. , Ehlers, P. J. , Nicolae, Dan-Valentin
Date: 2009
Subjects: Harmonics , Power electronics , Reactive power (Electrical enginnering)
Language: English
Type: Abstract: Static power converters produce harmonics due to the nature of the conversion process. This paper will investigate the effect on the power factor of a sinusoidal and Fryze current reference on a PWM boost type rectifier. It has been shown in previous papers that a Fryze current ref is best. This paper will develop a mathematical model, based on the single sided Fourier expansion. Using this mathematical model it will be shown that even though a high harmonic content could be present in a Fryze controlled PWM boost type rectifier, the power factor has been optimally improved.
Identifier: http://hdl.handle.net/10210/21241 , uj:16131 , ISBN: 978-1-4244-2235-7 , Citation: Richards, C.G., Ehlers, P.J. & Nicolae, D.V. 2009. An investigation on optimal current mode control for boost-type PWM rectifiers. Proceedings of the IEEE PowerTech Conference, 26 June-02 July 2009, Bucharest, Rumania. DOI: 10.1109/PTC.2009.5282229
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Analysis of synchronous reluctance machine with magnetically coupled three-phase windings and reactive power compensation

- Ogunjuyigbe, A. S. O., Jimoh, A. A., Nicolae, Dan-Valentin, Obe, E. S.

Authors: Ogunjuyigbe, A. S. O. , Jimoh, A. A. , Nicolae, Dan-Valentin , Obe, E. S.
Date: 2010
Subjects: Synchronous machinery
Language: English
Type: Article
Identifier: http://hdl.handle.net/10210/18578 , uj:16017 , ISSN :17518660 , Citation: Ogunjuyigbe, A.S.O. et al. 2010. Analysis of syhnchronous reluctance machine with magnetically coupled three phase windings and reactive power compensation. IET Electric Power Applications, 4(4):291-303. , DOI:10.1049/iet-epa.2009.0134
Description: Abstract: Synchronous reluctance machine with simple salient rotor is known to have poor power factor because of its poor reactance ratio. This study presents the use of an auxiliary winding attached to a balanced capacitor to improve the effective reactance ratio as well as the performance characteristics of synchronous reluctance machine. A unique feature of this configuration is that although the machine runs at synchronous speed, the effective reactance ratio of the machine can be improved by appropriate sizing of the capacitor attached to the auxiliary winding, with the constraint of the ampere turn rating of the auxiliary winding. The operational characteristics of a simple salient rotor synchronous reluctance machine equipped with this configuration were investigated and shown to have a desirable performance. How the effective reactance ratio, power factor and airgap flux distribution depend on the size of capacitor attached to the auxiliary winding is determined using a simplified mathematical model of the system. Analytical results and experimental measurements are in good agreement, and demonstrate the effectiveness and benefits of the configuration
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Analysis, modeling and control of cascaded NPC/H bridge inverter for high power quality grid connection

- Wanjekeche, T., Nicolae, Dan-Valentin, Jimoh, A. A.

Authors: Wanjekeche, T. , Nicolae, Dan-Valentin , Jimoh, A. A.
Date: 2010
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/18314 , uj:15983 , Citation: Wanjekeche, T., Nicolae, D.V. & Jimoh, A.A. 2010. Analysis, modelling and control of cascaded NPC/HNPC/H bridge inverter for high power quality grid connection. In: Proceedings of the IASTED AsiaPes 2010, Thailand, Nov. 2010:74-79
Description: Abstract: The cascaded NPC/H- bridge inverter is first analyzed by establishing a new switching model based on a derived control law. From the developed switching model, a general model is obtained using state space technique. A new and accurate average model is developed using abc to dqo transformation technique. Finally small signal model is obtained by linearizing all the state variables around their quiescent operating points. The small signal model is used to study the effects of various control feedback variables on the dynamic performance of the Grid connected system. Feedback control scheme is designed to improve the dynamic response of the system. The system with the proposed control technique also achieves sinusoidal grid voltage and current output with low THD which is a preferred characteristic for any power injected to the grid. Simulation results are provided to validate the analytical models
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Analytical design and analysis of line-start permanent magnet synchronous motors

- Stoia, Dan, Cernat, Mihai, Jimoh, Adisa A., Nicolae, Dan-Valentin

Authors: Stoia, Dan , Cernat, Mihai , Jimoh, Adisa A. , Nicolae, Dan-Valentin
Date: 2009
Subjects: Electric motors, synchronous
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/21553 , uj:16149 , ISBN: 978-1-4244-3919-5 , Citation: Stoia, D. et al. 2009. Analytical design and analysis of line-start permanent magnet synchronous motors. IEEE-Africon, 23-25 September, 2009, Kenya. DOI: 10.1109/AFRCON.2009.5308177
Description: Please refer to full text to view abstract
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Another case of discrepancies when evaluating power theories using real data

- Cornelius, Jurgens J. J., Janse van Rensburg, J. F., Nicolae, Dan-Valentin

Authors: Cornelius, Jurgens J. J. , Janse van Rensburg, J. F. , Nicolae, Dan-Valentin
Date: 2014
Subjects: Electric power systems , Electric power transmission
Language: English
Type: Article
Identifier: http://hdl.handle.net/10210/17548 , uj:15899 , ISBN: 978-1-4673-6487-4 , Citation: Cornelius, J.J.J., Janse van Rensburg, J.F. & Nicolae, D.V. 2014. Another case of Discrepancies when evaluating power theories using real data. International Conference on Harmonics and quality of power, 25-28 May 2014, Bucharest, Romania. DOI:10.1109/ICHQP.2014.6842839 /9781497394874
Description: Abstract: Most non-sinusoidal power theories have been developed from theoretical techniques, as well as relying on hypothetical and experimental networks to highlight the advantages of each. The drawback of the power theories is that no single one has been universally accepted as a benchmark for other developments. This paper will however show this weakness by means of evaluating two power theories in the time domain, using real recorded data.
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Automating consumer's connections to the distribution network for enhanced performance

- Popoola, O. M., Jimoh, A. A., Nicolae, Dan-Valentin

Authors: Popoola, O. M. , Jimoh, A. A. , Nicolae, Dan-Valentin
Date: 2010
Subjects: Power electronics , Electric circuit breakers , Voltage detectors
Language: English
Type: Article
Identifier: http://hdl.handle.net/10210/18674 , uj:16029 , ISSN:1827-6660 , Citation: Popoola, O.M., Jimoh, A. & Nicolae, D.V. 2010. Automating consumer's connections to the distribution network for enhanced performance. International Review of Electrical Engineering, 5(5), October 2010.
Description: Abstract:Trial and error approach which involves service interruption is usually carried out to resolve unbalance on phases in power distribution system. This approach might improve the phase voltage and current but the resultant effect (change) does not last for too long; thereby reduces the market value in terms of quality and reliability of supply. To enhance the performance of the secondary distribution system there is a need for an automating technology. The aim of this paper is to introduce a method and technology developed for resolving imbalances in a secondary distribution system as a result of the uneven distribution of single phase load across a three phase power system. The technology developed is able to monitor, acquire/display collected data and perform self changing switching actions electronically. This action is in form of rearrangement or transfer of consumer loads for optimal performance of the distribution feeder. The proposed switching technology is based on open- transition switch that enables transfer or rearrangement of consumer loads in a three-phase system within milliseconds with supervisory control system. The following methodologies: System Simulator - Matlab (Simulink), Virtual Instrumentation-Lab VIEW and Hardware implementation were applied for the validation of the proposed technique
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Boost converter with improved transfer ration

- Nicolae, Dan-Valentin, Richards, C. G., van Rensburg, J. F. G.

Authors: Nicolae, Dan-Valentin , Richards, C. G. , van Rensburg, J. F. G.
Date: 2010
Subjects: Tapping tools , Boilers and boiler furnaces , Diodes , Fluid-power technologies
Language: English
Type: Conference proceedings
Identifier: http://hdl.handle.net/10210/18489 , uj:16005 , ISBN: 9781424453931 , Citation: Nicolae, D.V., Richards, C.G. & Van Rensburg, J.F.J. 2010. Boost converter with improved transfer ratio, The Sixth international power electronics conference (IPEC), Sapporo, Japan, June 21-24, 2010:76-81. DOI:10.1109/IPEC.2010.5543839 , DOI:10.1109/IPEC.2010.5543839
Description: Abstract: A variation on the classic boost topology is presented. The improved transfer ratio is obtained by storing energy in parallel inductors and releasing the energy in series. State-space analysis, signal flow graph and simulation validation are presented for a double inductor topology. This analysis is then compared with the experimental results on the same double boost topology.
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Brushless direct current motor efficiency characterization

- Van Niekerk, D., Case, M. J., Nicolae, Dan-Valentin

Authors: Van Niekerk, D. , Case, M. J. , Nicolae, Dan-Valentin
Date: 2015
Subjects: Brushless direct current motor , Unmanned aerial vehicles
Type: Article
Identifier: http://ujcontent.uj.ac.za8080/10210/388995 , http://hdl.handle.net/10210/16813 , uj:15451 , Citation: Van Niekerk, D., Case, M.J. & Nicolae, D.V. 2015. Brushless direct current motor efficiency characterization. ACEMP-OPTIM-Electromotion Joint Conference, 2-5 September 2015, Turkey:226-231 , 978-1-4763-7239-8
Description: Abstract: The use of a compact lightweight efficient propulsion system is one of the main technological requirements for a long endurance electric Unmanned Aerial Vehicle (UAV). Currently the most popular trend is to use a permanent magnet brushless direct current (BLDC) model motor. This type of motor is used because of its efficiency which can be greater than 90% when optimally driven. However the majority of manufacturers do not provide adequate performance response profiles throughout the specified motor applied voltage range. This paper presents a methodology to characterize the BLDC efficiency.
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