My project deals with the Neutral point clamped converter. The three-level neutral point clamped (NPC) converter is one of the most common multilevel converter applied in high-power and in medium-voltage applications. One of the main benefits of having a three-level converter is that there are twice as many switches compared to the two-level inverter. Another type of NPC converter is the 3L-Active-NPC that provides a greater degree of freedom. The 3L-ANPC overcomes the disadvantage of the 3L-NPC where it experiences unequal distribution losses among its switches. This literature review discusses these converters, their applications in research and standards applied in the converters.
Review of Sources
The NPC converter has a standard for connecting to different power sources such as converter for wind turbine, limitation of capacity point common coupling, and a wave of switched-mode power supply. The IEEE Application Guide for IEEE Std 1547 identifies the frequency, voltage, and current caused by the wave effect. The standard is that the frequency should control the rotor winding but should be at a stable speed and not change when the wind turbine movement changes every time. The standard also identifies the limitation that should be at 60 Hz or 50 Hz at voltages of 120v for single-phase connections and 210 for three-phase connections [1].
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The investigations on the applications of three-level NPC converter where it is used along with the diode converter to feed an open-winding permanent magnet generator synchronous generator (PMSG) systems were done by L. Chen, H. Nian and Y. J. Zhou [2]. The open winding PMSG system was expected to have several advantages such as simpler system configuration, less switch losses, and the multilevel modulation effect. It was expected that the use of the NPC converter and diode converter along with the open-winding PMSG system would realize advantages of having both systems such as multi-modulation effect. The simulation that was carried out validated the theoretical investigations regarding the advantages of the open winding PMSG system.
Tiefu Zhao, S. Bhattacharya and A. Q. Huang investigated the operation of series and shunt converters with 48-pulse series-connected three-level NPC converter for unified power flow controller (UPFC) [3]. The 48-pulse series-connected 3L-NPC was chosen as it had a near-sinusoidal voltage. The investigators wanted to investigate the control and operation of series and shunt converters when applied on a 48-pulse NPC for UPFC application. The complete simulation model was implemented in Matlab/Simulink and the practical reactive and real power operational boundary in a 3-bus power system was investigated. The results from the simulation validated efficient control under both dynamic and steady-state operating conditions.
The 5-level active NPC (ANPC) with an optimum PWM strategy was investigated on real-time solution for total harmonic distortion (THD) by J. Li, Y. Liu, S. Bhattacharya and A. Q. Huang [4]. The aim of the study was to investigate whether the strategy of the ANPC with an optimized PWM would realize a minimized THD. The switching angles in the optimum PWM were calculated through a real-time algorithm approach and control scheme of balancing floating capacitors was proposed. The results of the investigation verified the performance of the proposed strategies with the main result being minimized THD.
The three-phase three-level active NPC converters that is used in high power systems was investigated by C. Attaianese, M. Di Monaco and G. Tomasso [5]. The aim of the investigation was to construct a comparison between the classical structured NPC converter and the newly emerging Active NPC converter. One of the analysis that was carried out was the numerical analysis of losses distribution among power devices through PWM techniques. It was expected that the ANPC would not experience unequal losses like that seen in the NPC. The experimental results indicated a case losses distribution for the ANPC topology. The data provided an insightful analysis of the three-phase three-level ANPC converter.
R. K. Behera and S. P. Das carried out an experimental investigation for a three-level NPC ac-dc converter system [6]. The system was identified that it could be applied in a drive topology that required both ac to dc and dc to ac conversion for control of speed and torque. The space vector modulation (SVM) technique was applied to the ac-dc converter systems, for the input current control, and dc-link voltage control. The experimental investigations were carried out through the use of PC based control by making use of National instruments (NI) data acquisition system for controlling the converter. The simulation and laboratory prototype confirmed validity of the current control approach.
P. Marcin, Z. Krzysztof, Z. Zbigniew and G. Maciej investigated a 3-terminal high power medium voltage converter that could be used as a solution for coupling grids to improve grid parameters [7]. The four-level three-phase NPC converter and three-level NPC and series connection using a three two-level three-phase converters that were supplied with an MV isolation transformer. The results of the study showed that the prototype converter could be used to realize different states of operations when connected in different ways.
Y. Li, L. Shi and Y. Li investigated a control strategy for a rectifier through a 3-level NPC converter to propel a high speed maglev train [8]. The study was relevant because it presents a novel application of the NPC converter. The investigators sough to analyze the neutral-point voltage and to thus present a new SVPWM modulation strategy that can be achieved through an NP voltage balance. The investigators were able to verify the performance of the rectifier controller by investigating various elements such as neutral point voltages, power factor, and the current harmonics and the results verified the system of the hardware-in-the-loop (HIL) system.
Detailed Review of Sources
[5] The prior art article titled “Three-phase Three-level active NPC converters for high power systems” by C. Attaianese, M. Di Monaco and G. Tomasso sought to investigate the process of implementing active NPC converters. Throughout their investigation, the researchers were able to identify some of the problems associated with using NPC converters and identified solutions and implementation strategies that could be beneficial for use in the current project dealing with NPC converters. The first challenge that the group experienced was the conversion of power from AC to DC. Power transformation from AC to DC required several steps and there were power losses experienced throughout the transformation. In order to mitigate the challenges, the researchers added an insulated-gate bipolar transistor (IGBT) that worked as the switch to control the wave and to minimize power losses. From the study, converting three-phase AC power to DC power required 6 IGBTs at each phase. The researchers also identified that the reason for power losses was caused by paths that depended only on the direction that was undertaken by the phase current [5]. The second challenge identified was as a result of conversion challenges by the converter from AC to DC, from DC to AC or from DC to DC. The NPC can experience losses throughout the voltage conversion and saving power can be realized by voltage control techniques. A capacitor was connected to the NPC circle and the neutral point was made at a zero potential in order to maximize power transfer. The analysis of the graph from the experiment showed that three-phase three-level active NPC converter worked efficiently and power losses was minimized. The results and analysis carried out by the investigators would be applicable for the project because it presents various challenges and solutions of working with NPC converters. By making use of the solutions presented, maximum power transfer will be made possible throughout the operation of the NPC.
[8] The article that was titled “A novel control strategy of rectifier for NPC back-to-back converter in HIL system” by Yaohua Li, Yang Li, and Limming Shi presented a new idea of control 3-level neutral-point clamped (NPC) for the rectifier. The hardware-in-the-loop (HIL) system could be control through the strategy of a rectifier for NPC in a back-to-back converter. One of the benefits of having such a system was to avoid lost control in the NPC converter. Additionally, the NPC converter could be used to realize a high-speed maglev train which requires high-speed transmission for working. The transmission of the model could be simulated by a speed-circuit controller in Simulink. The authors also mention the use of real-time interface (RTI) software that was more efficient. The components of the RTI system include interface circuit boards and a simulation computer [8]. The RTI was efficient as it was able to convert the AC to DC and DC to AC quite efficiently. The advantage experienced with the use of the converter was used to overcome some of the challenges that were experience previously with using the NPC by itself. The simulation results were tabulated and also revealed DC voltage in each of the capacitor. The RTI was also advantageous because it resulted in power factor correction and control of power factor. The power factor is dependent on real power divided by apparent power and the RTI was able to control real power by adding a correctional voltage and current. The results of the data showed that the RTI can be used to keep the current and voltage at stable levels. The analysis of the research can play a big role in the current project by indicating the need to use the RTI to conduct the project. Additionally, the strategies that were implemented can be used to improve the voltage and current profiles of the NPC converter throughout the Simulink App.
[1] The standard research that was titled “IEEE Application Guide for IEEE Std 1547™, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems” disused about the different parts of the neutral-point clamped (NPC) converter. Some of the applications that were discussed were in its application on a wind turbine, a wave of switched-mode power supply, limitation of capacity point common coupling(PCC). First, according to standard research, the wind turbine can contact with a rotate motor that calls wind generation that converts mechanical power to electrical power. Based on the research “The DR operator of a single-unit 1.1 MW wind turbine intends to operate as an independent power producer” [1]. Additionally, the Standard research discussed the wave effect to frequency, voltage, and current. The frequency controls the rotor winding but needs a stable speed that cannot happen by wind turbine changes in movement every time. The use of gears can control the speed effectively. There is also a limitation in the frequency and voltage where it should be matched at 60 Hz or 50 Hz, voltage to a 120V single-phase or a 210V three-phase. Understanding the limitations with regard to speed control, the frequency and voltage can be used to further understand the project when further implementing the NPC converter on a wind turbine.
References
[1] IEEE Application Guide for IEEE Std 1547(TM), IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems," in IEEE Std 1547.2-2008 , vol., no., pp.1-217, 15 April 2009. doi: 10.1109/IEEESTD.2008.4816078.
[2] L. Chen, H. Nian and Y. J. Zhou, "Investigation on open-winding PMSG system fed by three-level NPC converter and diode converter," International Conference on Renewable Power Generation (RPG 2015) , Beijing, 2015, pp. 1-5. DOI: 10.1049/cp.2015.0439.
[3] Tiefu Zhao, S. Bhattacharya and A. Q. Huang, "Operation of series and shunt converters with 48-pulse series-connected three-level NPC converter for UPFC," 2008 34th Annual Conference of IEEE Industrial Electronics , Orlando, FL, 2008, pp. 3296-3301. DOI: 10.1109/IECON.2008.4758488
[4] J. Li, Y. Liu, S. Bhattacharya and A. Q. Huang, "An optimum PWM Strategy for 5-level active NPC (ANPC) converter based on real-time solution for THD minimization," 2009 IEEE Energy Conversion Congress and Exposition , San Jose, CA, 2009, pp. 1976-1982. doi: 10.1109/ECCE.2009.5316229
[5] C. Attaianese, M. Di Monaco and G. Tomasso, "Three-Phase Three-Level active NPC converters for high power systems," SPEEDAM 2010 , Pisa, 2010, pp. 204-209. doi: 10.1109/SPEEDAM.2010.5542195
[6] R. K. Behera and S. P. Das, "Space vector modulation for a three-level NPC ac-dc converter system: An experimental investigation," 2010 International Conference on Power, Control and Embedded Systems , Allahabad, 2010, pp. 1-5. doi: 10.1109/ICPCES.2010.5698654
[7] P. Marcin, Z. Krzysztof, Z. Zbigniew and G. Maciej, "3-terminal high power medium voltage grid coupling converter," 2015 9th International Conference on Compatibility and Power Electronics (CPE) , Costa da Caparica, 2015, pp. 340-345. doi: 10.1109/CPE.2015.7231098
[8] Y. Li, L. Shi and Y. Li, "A novel control strategy of rectifier for NPC back-to-back converter in HIL system," 2012 15th International Conference on Electrical Machines and Systems (ICEMS) , Sapporo, 2012, pp. 1-4.
