Budapest University of Technology and Economics Faculty of Electrical Engineering and Informatics Department of Electric Power Engineering Gergely György BALÁZS Controlling of Four-Quadrant Converters Especially in Electric Vehicles Ph.D Thesis Booklet Supervisor: Dr. István SCHMIDT Department of Electric Power Engineering Budapest, 2012
Introduction, objectives The electrical energy is the most valuable and the most versatilely used energy source in our technically developed World. The energy reverses of the Earth are getting exhausted, therefore the utilization of the energy efficiency and environmentally friendly alternative energy sources are an important role in order to make the valuable electrical energy available with sufficient quantity and quality. Therefore each research and rationalization purpose becomes important. Several disturbing consumers are connected to the alternating voltage networks. These non-linear consumers inject harmonic current into the network, consequently the ideal sinusoidal waveform of the AC network becomes strongly distorted. Therefore each solution should be explored that can reduce the undesired effects of the distorted network. The adequate control of switching mode converters can be one of the solutions to solve this issue. The doctoral dissertation focuses on issues which can arise in vehicular applications. The first objective of the dissertation was, to define a simulation framework that enables studying the harmonic penetration of grid-connected active rectifiers and the interaction between the non-linear loads and the active rectifiers with fast simulation methods. Another objective of the dissertation was to create a new control strategy for active rectifiers that enables us to reduce the harmful effects of the network distortion and compare it with other widely used strategies. Research methodology Simulation studies: The double domain simulation method was used for examining the harmonic penetration of the active rectifiers. This method is a link between the frequency- and time domain models. Those parts of the system are calculated in the frequency domain, that parameters are given in the frequency domain. On the other hand the time variant parts of the model that can be characterized by differential equations - are implemented in time domain. Therefore the parts of the AC network are calculated in frequency domain with frequency-dependent parameters, while the grid-connected converters and drive systems are implemented in time domain. I created a flexible, modular model for the time domain part of the double domain simulation. I modelled DC-link frequency converters equipped with single- or three-phase grid connected line-side converter, and the line- and motor-side converters are voltage-source inverters. Moreover I created squirrel cage induction machine and permanent magnet synchronous motor models. The implemented parts and these connections are shown in Fig.1. The frequency converter and the motor models were built based on the characterized differential equations in per-unit quantities. To reduce the simulation time I made simplifications. The whole drive system was modelled with linear differential equations with constant coefficients. The three-phase quantities were considered by Park vectors. 2
Fig.1: Modular drive system models Tests and measurements: In my opinion the value of a doctoral dissertation is enhanced if the results of the research is presented not only with analytical or simulation methods but also with experimental results in a realized system. Therefore a single-phase test system was created to examine the new methods, its core is a four-quadrant bridge-type converter. A board was built for the converter control. The test system enables to examine the new method of the second thesis and to compare it with other widely used current control strategies. In real conditions I analysed the improvement of the power quality if a grid-connected converter is operated by the new current control strategy. Analytical studies: I performed analytical studies to compare the current control strategies of the grid-connected converters. The consumed currents of the converter were compared while it was operated by different current control strategies. Despite the complicated mathematical derivation, I strived to receive simple results. In my opinion the detailed derivation and the short logical steps help the understanding. The definition of the total voltage harmonic distortion and the effective value were used in the derivation. Initially, it was assumed that the consumed AC power mean values of the grid-connected converters in each current strategy are equal. I studied analytically the fundamental and root mean square (RMS) currents of the strategies. The derivation was detailed by assuming single-phase grid-connected converter operating in rectifier mode. Moreover the same results can be received if the converter is operating in inverter mode or if three-phase network connected converter is assumed. Finally I received expressive coefficients, depending only on the total harmonic distortion of the AC network. 3
New scientific results First thesis In this thesis a simulation method is described that enables to examine DC-link, fourquadrant, bridge-type network-friendly AC grid connected converters. I created a more reliable model for the time-domain simulation part of the double-domain simulation. The subthesis is studying simplification methods to reduce the simulation time. Thesis 1: For double domain simulation methods, I created a modular time domain model of squirrel cage induction machine and permanent magnet synchronous machine fed by a modern frequency converter. The model is implemented with per unit quantities and differential equations. The model enables us to examine the harmonic penetration of network-friendly locomotives and photovoltaic inverters (PVs) - based on the similarity of the line-side converter. Sub-thesis 1.1: I proved with simulation results that the motor-side converter and the motor can be considered as a current generator for the harmonic penetration of the DC-link frequency converters. Publications connected to this thesis: [S2]-[S16] Second thesis The thesis is dealing with a new current control strategy, can be applied for grid-connected inverters operating in inverter mode. Simulation and measured results are presented the beneficial effects of the method. The sub-theses detail the practical applications and the limits of the new method. Thesis 2: I created a new current control strategy for network-friendly line-side converter operating in inverter mode and connected to harmonic distorted grid. According to the principle, at inverter mode the generated fundamental current of the converter connected to single-phase grid is in antiphase with the grid voltage fundamental and proportional to it while the harmonics of the generated current are in phase with the harmonics of the grid voltage, and proportional to them. If the converter is connected to three-phase grid, at inverter mode the generated fundamental current vector is opposite with the grid voltage fundamental vector and proportional to it, while the harmonic vectors of the generated current are in direction with the harmonics of the grid voltage and proportional to them. Therefore the system behaves as a fundamental generator and harmonic consumer. The proportionality factors of the fundamental and harmonic currents are the same. In singlephase case, I proved with measurements and simulation results that the voltage distortion can be reduced with the new strategy. Sub-thesis 2.1: I worked out, how the principle can be applied in grid-connected converters equipped with current controller and pulse width modulation (PWM) controller. In singlephase case the voltage waveform of the converter AC terminals (u h ) are measured by voltage sensors, then its fundamental (u h1 ) is computed by a fundamental former device, after it the difference of the two signals is developed ( u h =u h -u h1 ), finally the defined signal is subtracted from the fundamental signal (u h1 - u h ). This signal should feed the waveform determining input of the current reference signal former of the converter. If the converter is connected to a 4
three-phase network, the subtracted signal (u h1 - u h ) should be defined in each phase of the converter, similarly to the single-phase case. Sub-thesis 2.2.: I determined that if the new principle is applied in line-side converter of high-power drive systems equipped with switching elements enabling low switching frequency, it is worth filtering the high frequency harmonics of the grid voltage because the PWM voltage source inverter is not able to follow the high frequency component of the control signal. It is worth to perform the filtering at the waveform determining input of the current reference generator. Publications connected to this thesis: [S1], [S4]-[S7] Third thesis This thesis is dealing with the comparison of different current control strategies. Three methods are compared, two are widely-used and the third is the new strategy described in Thesis 2. This thesis defines coefficients that depend only on the total harmonic distortion of the AC voltage network. The coefficients enable to compare the RMS and fundamental currents of the strategies. The sub-thesis gives an advice which strategy should be applied in rectifier and in inverter mode of the four-quadrant converter. The sub-thesis details how the strategy switch should be implemented. Thesis 3: I defined coefficients to compare three control strategies for grid-connected network-friendly converters. The first enforces current proportional to the grid voltage waveform, the second enforces current proportional to the grid voltage waveform fundamental and the third is the new strategy described in Thesis 2. These coefficients compare the RMS and fundamental currents of the different strategies and depending only on the total harmonic distortion of the network and dependent form the load. Sub-thesis 3.1: I determined that it is worth applying different current control strategies for four-quadrant bridge-type line-side converters in inverter and in rectifier mode. The switching between the strategies can be performed by the sign of the DC voltage controller output or by the direction of the current mean value that flows in the DC-link. Publications connected to this thesis: [S1], [S4], [S7], [S8], [S11], [S14], [S16] Practical applications of the theses Harmonic penetration of network-friendly converters can be analysed by the models and simulation environment that were described in the first thesis. In the dissertation two practical applications were presented: locomotives equipped with active rectifier and PV inverters. The simulation environment gives an opportunity to study the harmonic penetration of other DClink frequency converters, e.g. the harmonic penetration of renewable resources power plants. In the future I would like to examine the filter circuits of PV inverters by using the built simulation environment. It is worth to apply the new principle that was presented in Thesis 2 for distorted network connected converters, in applications where there is no opportunity to measure the grid current, just the AC-side current of the converter. This current control strategy can be utilized in many applications, e.g.: 5
- AC voltage network connected network-friendly locomotives, - to produce the DC-link voltage for applications that is connected to a common DClink (e.g. for drive systems of robots, cranes, splitting-mills, etc.), - renewable resources drive systems, e.g. in line-side converters of domestic power plants, - line-side converters of DC voltage energy distribution systems. If the line-side converter possess network-friendly behaviour, equipped with semiconductor switching elements and has an AC voltage sensor, the new current control strategy can be easily applied by modifying the analogue or digital control circuit. For four quadrant line-side converters it is worth to implement a switching device that was described in Sub-thesis 3.1. Therefore the distortion of the grid can be effectively reduced if in rectifier mode the converter is operated with a current reference signal proportional to the grid voltage, while in inverter mode the new strategy of Thesis 2 is applied. My own publications S1. Balázs Gergely György, Horváth Miklós, Schmidt István: Control Devices and Control Methods for Converter Devices. (in Hungarian) Case number: P 12 00452, Submitted in Hungary, Hungarian patent, Submitted at: 30.07.2012. S2. G. Gy. Balázs, I. Schmidt, P. Kiss: Harmonic Penetration Analyses for DC-Link Frequency Converter Drive Systems by Considering the Motor-Side Converter as an Ideal Current Generator International Conference on Renewable Energies and Power Quality (ICREPQ 13). Bilbao, Spain, 20.03.2013-22.03.2013. Paper 573. (Accepted for publication, each conference paper will be published in the Renewable Energy and Power Quality Journal (ISSN 2172-038X)) S3. G. Gy. Balázs, P. Kiss: Effect on Power Quality of Numerous PV Inverters Connected to the Same LV Network in a Suburban Area International Conference on Renewable Energies and Power Quality (ICREPQ 13). Bilbao, Spain, 20.03.2013-22.03.2013. Paper 477. (Accepted for publication, each conference paper will be published in the Renewable Energy and Power Quality Journal (ISSN 2172-038X)) S4. Gergely György Balázs, Miklós Horváth, István Schmidt: New Current Control Method for Grid-Connected Inverter of Domestic Power Plant. EPE-PEMC 2012 15th International Power Electronics and Motion Control Conference. Novi-Sad, Serbia, 03.09.2012.-06.09.2012. pp. 1-5. Paper 482. S5. G G Balázs, M. Horváth, I. Schmidt, P. Kiss: Examination of New Current Control Methods for Modern PWM Controlled AC Electric Locomotives. 6th IET Power Electronics, Machines and Drives Conference 2012. Bristol, UK, 27.03.2012.- 29.03.2012. pp. 1-5. Paper c.2.4 (0276). (ISBN: 9781849196161) 6
S6. G G Balázs, P Kiss, M Horváth, I Schmidt: Application of the Double Domain Simulation in Comparison of New Current Control Methods for PWM Controlled AC Electric Locomotives. 15th IEEE International Conference on Harmonics and Quality of Power. Hong Kong, China, 17.06.2012.-20.06.2012. pp. 1-6. Paper 0094. S7. Balázs Gergely György: New Current Control Strategies for Network-Friendly Locomotive Line-Side Converter: (in Hungarian) II. Mechwart András Ifjúsági Találkozó. Budapest, Hungary, 09.05.2012. pp. 1-2. (ISBN: 978-963-9299-18-4) S8. G Gy Balázs, I. Schmidt, M. Horváth: Analytical and Simulation Comparison of Sinusoidal and Resistive Modulation Strategies for Network-Friendly Three-Phase Grid-Connected Inverters. Periodica Polytechnica-Electrical Engineering 55: 11 p. (2011) S9. Balázs Gergely György: MFrom a Locomotive to a Racecar Electric Vehicle Projects (in Hungarian) I. Mechwart András Ifjúsági Találkozó. Budapest, Hungary 13.04.2011. S10. P. Kiss, G. Gy. Balázs, A. Dán, I. Schmidt: The Application of the Double Domain Simulation with PWM Controlled Locomotives: 14th IEEE International Conference on Harmonics and Quality of Power 2010. Bergamo, Italy, 26.09.2010.-29.09.2010.pp. 1-6. S11. G Gy Balázs, M. Horváth, I. Schmidt: Features of Sinusoidal and Resistive Current Loads in Harmonic Distorted Networks. EPE-PEMC' 2010. 14th International Power Electronics and Motion Control Conference Ohrid, Macedonia, 06.09.2010.-08.09.2010. pp. 1-6. Paper 205. S12. Balázs Gergely György: HExamination of Network-Friendly Rail Vehicle Drives. (in Hungarian) VIK60 - Erősáram a fiatalok szemével: Ünnepi konferencia a VIK 60 éves fennállása alkalmából. Budapest, Hungary, 07.03.2010. S13. Balázs G Gy, Horváth M, Schmidt I: Network pollution reduction by using single phase network friendly converters. (in Hungarian) Elektrotechnika 103:(3) pp. 12-14. (2010) S14. Balázs G Gy, Horváth M, Schmidt I: Comparison of Sinusoidal and Resistive Modulation Strategies for Single Phase Four Quadrant Converters. Electrotechnica Electronica Automatica 58:(3) pp. 108-113. (2010) S15. Balázs G Gy, Schmidt I: Network Friendly Operation of Alternating Voltage Fed Vehicles. (in Hungarian) Elektrotechnika 102:(1) pp. 9-12. (2009) S16. Balázs G Gy, Schmidt I, Horváth M: Control Methods of Alternating Voltage-Fed Vehicles' Modern Line-Side Converters 2nd International Youth Conference on Energetics. Budapest, Hungary, 04.06.2009.-06.06.2009. Budapest: pp. 1-7. Paper 447. (ISBN: 9789634209836) 7
Publications not directly connected to the thesis: S17. dr Vincze Gyuláné, Balázs Gergely György: Electric Vehicles (in English and Hungarian) Nemzeti Fejlesztési Ügynökség, 132 pages. 2012 S18. Gergely György Balázs, Zalán Kohári, Norbert Elek: Konzept einer elektrischen Fahrzeug mit Radnabenmotorantrieb. Audi Production Award 2010. Ingolstadt Germany, 12.10.2010.-13.10.2010. pp. 1-11. S19. Szentmiklóssy B, Balázs G Gy, Kerekes S: Ultracapacitors as a Primary Energy Storage Device in Public Transport. IYCE 2007. International Youth Conference on Energetics. Budapest, Hungary, 2007.05.31-2007.06.02. (BME) Budapest: pp. 1-5. Paper 193. (ISBN: 978-693-420-908-0) S20. Szentmiklóssy B, Balázs G Gy, Kerekes S: Trolley-bus without Overhead-Line. (in Hungarian) XII. Fiatal Műszakiak Tudományos Ülésszaka: FMTÜ 2007. Cluj, Romania, 16.03.2007.-17.03.2007. pp. 177-180. Paper 43. (ISBN: 973-8231-67-1; 978-973-8231-67-2) S21. Balázs G Gy, Szentmiklóssy B, Kerekes S: Examination of Ultracapacitor Behaviour: Measured and Simulation Results. (in Hungarian) XII. Fiatal Műszakiak Tudományos Ülésszaka: FMTÜ 2007. Cluj, Romania, 2007.03.16-2007.03.17. pp. 15-18. Paper 8. (ISBN: 973-8231-67-1; 978-973-8231-67-2) S22. Balázs G Gy, Szentmiklóssy B, Kerekes S: Long Term and Direct Current Behaviour of the Ultracapacitor by Comparing Measured and Simulated Results. IYCE 2007. International Youth Conference on Energetics. Budapest, Hungary, 31.05.2007.- 02.06.2007. (BME) Budapest: pp. 1-6. Paper 149. (ISBN: 978-693-420-908-0) 8