Tema disertacije je aktivno potiskivanje niskofrekvencijskih struja smetnji primjenom upravljanih mrežnih sučelja energetskih pretvarača za napajanje glavnih i pomoćnih pogona u željezničkim vozilima. Široka primjena energetskih pretvarača u željezničkim vozilima, uz druge ključne nove tehnologije, omogućila je u proteklom razdoblju značajno poboljšanje dinamičkih karakteristika suvremenih vozila uz istodobno ostvarivanje ekonomičnijeg rada pogona kao i niz drugih prednosti proizašlih iz primjene glavnih pogona zasnovanih na vektorski upravljanim izmjeničnim reguliranim elektromotornim pogonima s asinkronim vučnim motorima i fleksibilnih pretvarača pomoćnih napajanja za osiguravanje energije svim ostalim pomoćnim podsustavima (rashladni sustavi pretvarača glavnog pogona, sustavi ventilacije, grijanja, klimatizacije, rasvjeta, informacijski i upravljački sustavi na vozilu…). Pri tome je ukupna instalirana snaga svih glavnih i pomoćnih pogona u vozilu znatno povećana, posebice uzmu li se u obzir i konfiguracije višestrukih vučnih jedinica (MTU – Multiple Traction Units) vrlo uobičajene u putničkom prometu, ali i u teretnom željezničkom prometu (povezivanjem lokomotiva u tzv. tandemske konfiguracije). Suvremeni pretvarači za napajanje glavnih i pomoćnih pogona, u pravilu su zasnovani na IGBT tehnologiji. Karakter rada takvih pretvarača, kod kojih se formiranje željenih iznosa napona i struja ostvaruje nekom od metoda upravljanja IGBT sklopkama (npr. modulacija širine impulsa – Pulse Width Modulation – PWM), uz istodobno povećanje jedinične i ukupne snage pretvarača na vozilu, uzrokovali su znatno povećanje sadržaja neželjenih viših harmonika i drugih smetnji u strujama pojne mreže. Sve struje smetnji sadržane u struji pojne mreže, uključujući i one nastale radom pretvarača glavnih i pomoćnih pogona, mogu svojim djelovanjem štetno utjecati na ispravnost rada telekomunikacijskih (TK) i signalno-sigurnosnih (SS) uređaja uz prugu, što se izravno odražava i na smanjenje sigurnosti željezničkog prometa. Posebice su opasni harmonici struja smetnji niskih frekvencija, jer se znatno teže filtriraju, pogotovo ako im je frekvencija u blizini radnih frekvencija (frekvencijskih pojasa) SS uređaja. Najbolji rezultati u potiskivanju struja smetnji u struji mreže koju vozilo razmjenjuje s aktivnom mrežom, postižu se kombiniranjem aktivnih i pasivnih mjera. Pri tome se posebna pažnja posvećuje provedbi aktivnih mjera u okviru upravljačko-regulacijske strukture mrežnih sučelja pretvarača za napajanje glavnih i pomoćnih pogona. Aktivnim potiskivanjem niskofrekvencijskih struja smetnji, primjenom upravljanih mrežnih sučelja energetskih pretvarača u željezničkim vozilima, nastoji se maksimalno iskoristiti raspoložive mogućnosti kako bi se ostvarilo što pouzdanije, ekonomičnije i energetski učinkovitije rješenje za potiskivanje struja smetnji sadržanih u struji pojne mreže. Pri postavljanju tehničkih zahtjeva na upravljačko-regulacijski sustav, kojim će se provesti aktivne mjere za potiskivanje struja smetnji poseban se naglasak stavlja na harmonike struja smetnji niskih frekvencija, pogotovo ako im je frekvencija u blizini radnih frekvencija SS uređaja te na identifikaciju točaka utiskivanja smetnji i njihovog širenja upravljačko-regulacijskom strukturom, kako bi se mogla provesti imunizacija sustava (uvođenjem odgovarajućih filtara na signale povratnih veza, odabirom odgovarajuće topologije regulacijske strukture i tipova regulatora pojedinih veličina, primjena adaptivnog/selektivnog predupravljanja …). Disertacija opisuje i predlaže algoritme upravljanja mrežnim sučeljima energetskih pretvarača za napajanje glavnih i pomoćnih pogona u željezničkim vozilima koji će aktivnim potiskivanjem struja smetnji osigurati da negativni utjecaji rada tih pretvarača na pojnu mrežu budu u dopuštenim granicama definiranim tzv. mrežnim pravilima i drugim relevantnim propisima. Za uspjeh predložene koncepcije aktivnog potiskivanja struja smetnji (poglavito osnovnih sklopnih harmonika) u struji mreže na razini vozila, koordiniranim djelovanjem na upravljačko-regulacijske strukture i pripadne PWM jedinice mrežnih sučelja više dislociranih pretvarača ključan je razvoj što robusnijih, a istodobno što točnijih sinkronizacijskih algoritama, kao osnove za formiranje zajedničke sinkronizacijske baze na osnovi estimiranog faznog kuta osnovnog harmonika napona mreže. Stoga je poseban naglasak u disertaciji stavljen na prikaz sinkronizacijske tehnike zasnovane na algoritmima sa zatvorenom petljom (npr. fazno zatvorena petlja ili PLL – Phase Locked Loop), koje su namijenjene za primjenu na jednofaznim sustavima napajanja. Primjenom predloženih algoritama upravljanja mrežnim sučeljima energetskih pretvarača za napajanje glavnih i pomoćnih pogona u željezničkim vozilima, pokazano je da se može osigurati da negativni utjecaji rada tih pretvarača na pojnu mrežu budu u dopuštenim granicama, unatoč relativno niskoj sklopnoj frekvenciji pretvarača s bipolarnim tranzistorima s izoliranom upravljačkom elektrodom. Razvijeni algoritmi potiskivanja niskofrekvencijekih struja smetnji detaljno su testirani ispitivanjima u laboratorijskim uvjetima i na stvarnom vozilu.
|Abstract (english)|| |
The objective of this thesis is active suppression of low-frequency interference currents by controlled grid interface of power converters in rail vehicles. Wide applications of power electronic converters installed onboard railway vehicles has been one of the key enabling technologies for significant improvement of their dynamic characteristics, energy efficiency and other benefits resulting from implementation of main propulsion system based on high-performance variable speed controlled AC traction drives and versatile auxiliary power supply converters providing energy for all auxiliary onboard systems (cooling system of the main propulsion, ventilation, heating, air-conditioning, lighting, on-board control and information systems…). Total power of traction drives and auxiliary power supply converters installed onboard modern railway vehicle has been significantly increased, particularly if multiple traction units for passenger and freight traffic are considered: multiple traction units composed from light rail vehicles for urban/suburban transport, or tandem configurations of locomotives. Operation of traction drives and auxiliary power supply power electronic converters installed onboard modern rail vehicles, dominantly based on IGBT switches, causes significantly higher content of line harmonics and interharmonic components in the line current. Interference currents, including those induced by the operation of onboard power converters can significantly influence the correct operation of telecommunication, train control systems and other railway signaling infrastructure along the tracks, having direct impact to the safety of the railway transport. Passive filtration techniques for suppression of low frequency interference line currents appeared to be insufficient, but are applied in combination with active mitigation techniques. Special attention is devoted to the correct implementation of active suppression techniques within the corresponding control structure of the grid interfaced converters of the traction drives and auxiliary power supply converters installed onboard. Active suppression of low-frequency interference currents, by implementation of properly controlled grid interface of power converters in rail vehicles in combination with passive mitigation techniques, results in reliable and energy efficient solutions, especially if cumulative suppression effects by coordinated action on a vehicle level are attained. The dissertation describes and suggests active suppression methods based on proper design of the grid interface of the power converter and its corresponding control system. In the thesis, particular focus is set on identification of sources of interferences currents, their propagation mechanisms, as well as identification of interference injection points within the control structure of the grid interface and corresponding mitigation techniques for minimization of negative effects. Development of more accurate and yet even more robust synchronization algorithms, presented in thesis, is crucial for successful implementation of the proposed concept of active suppression of switching harmonics by pulse-width modulation (PWM) carriers phase-shift control coordinated on a vehicle level. By implementation of dedicated block for estimation of DC-offset in the input signal, an enhanced quadrature-signal generator is attained, which in combination with standard or modified frequency locked loop (FLL) forms a core of the robust synchronization algorithm. Basic configuration can be also be upgraded by additional resonant parts and filters for stabilization of central frequencies for selected highorder harmonics of line voltage, in order to additionally improve robustness of the synchronization algorithm, particularly the noise immunity and the resiliency to the effects of high-order voltage/current harmonics, ensuring proper operation of the grid-interfaced converters even under very distorted overhead line voltage conditions. Single-phase versions of synchronization algorithms can easily be extended to the variants suitable in three-phase applications (currents/voltages signal-processing for measurement and control systems related to power-converters in general, particularly active power filters, grid-interfaced converters in distributed renewable energy generation units, power quality assessment). In order to implement the active suppression of low frequency interference currents on a vehicle level, few other necessary algorithms were proposed, enabling phase coordinated control of the PWM carriers of the onboard grid-interfaced converters without any dedicated infrastructure, i.e. employing only the regularly used measurements necessary used for normal operation of the traction drive converters and auxiliary power supply converters. Since all the converters installed onboard are fed by the same overhead-line voltage, measured for control purposes in all corresponding control systems, a common time-base necessary for the synchronization of all grid interfaces of the onboard converters can be derived from the estimated phase angle of the fundamental component of the line voltage. In order to minimize the noise impact and ensure maximal stability and operating precision in sometimes very polluted network environment (voltage sags/dips/swells due to pantograph bouncing and other overhead line voltage phenomena, severe harmonic distortion…) estimated phase angle of the fundamental component of the line voltage should be obtained by high-performance synchronization algorithm, resilient to the impact of high-order harmonics/subharmonics, dc-offset and other interferences while retaining good dynamic performance with respect to the steady-state and transient response tracking capabilities, settling times etc. Otherwise, due to rather high frequency modulation index, i.e. ratio between the switching frequency and fundamental frequency of the line voltage, instable common time base my produce adverse calculation outputs (e.g. synchrophasor components, necessary in alternative approach for determination of the phase angle of the PWM carrier), resulting in poor performance, sometimes even below the acceptance threshold level. Software net-mark algorithm is derived as software coded counterpart of hardware based zero-crossing detection solutions. The method is monitoring the outputs of the values of the estimated fundamental component of the line voltage and/or corresponding phase-angle, detecting the cyclic task when the polarity of these signals occurs. By simple mathematical expressions, the exact zero-crossing position within that cyclic task is obtained! Employing the excellent linearity of the estimated phase-angle (frequency of the line voltage is practically constant within the analyzed time range), the calculation precision for evaluation of the zero-crossing position within the cyclic task, can be further improved by employing simple proportional relations between the neighboring samples. This simple, computationally non-intensive algorithm is the core of the procedure for the precise measurement of the fundamental frequency, as well as for synchronization of cyclic tasks to the line frequency and phase-control of the PWM carriers for grid-interfaced converters attached to the same line voltage. Thesis proposes implementation of a parallel finite impulse response filter structure based on sliding mean value prototype (Sliding Mean Value tandem filters), in order to minimize the effects of intermodulation distortion, e.g. between the interference current and the fundamental component of the line current. Proposed Sliding Mean Value tandem filters are easy to implement and derive all their good features from the finite impulse response core they are based on – inherent stability, not demanding recursive form of the algorithm with respect to the CPU load regardless of the filter depth etc. Transport delay time of the proposed filter combination is shorter than for the series combination of the particular filters, while overall filtering characteristics in both sets of notch frequencies are improved with respect to the particular SMV filters. Proposed method can be very effectively applied for suppression of interference paths related to the voltage controller and its interaction with the synchronization output from the synchronization subsystem based on PLL (Phase Locked Loop). All necessary algorithms have to be implemented and executed in real-time, within rather short time frame limited by the period of the corresponding application program cyclic task. Thus, a selection of employed signal-processing and control algorithms have to take into consideration the total computing performance of the target embedded system based on the modest digital signal processor (DSP), using ultimately optimized application program code and partial emulation of high-precision math (necessary or even critical in proper implementation of some algorithms) to partially compensate the capabilities of modern digital controllers based on the latest generation of DSP/FPGA (Field Programmable Gate Array) solutions, which have significantly improved their CPU and overall system performance benchmarks recently. The required careful implementation of active/passive methods suppression techniques of low-frequency interference line currents was demonstrated in thesis on two examples. Quality evaluation of performances of the grid interfaces of the traction drive converters and auxiliary power supply converter, and their corresponding control system in particular, a series of tests for assessment of the line current frequency spectrum, resulting from the operation of the converter in typical exploitation conditions, were performed. Thorough tests started in laboratory conditions, where completed with extensive field tests performed on the vehicle, with satisfactory results. Dissertation is organized in eight chapters. The first chapter (“Introduction”) highlights the contexts of the research activities in the field of interference currents suppression (even demanding restrictions with respect to the permissible level of interference currents, even higher installed power of onboard converters – per single converter, single traction unit or total installed power per vehicle – multiple traction units, tandem configurations of locomotives, increase of railway traffic volume leading to staggering number of vehicles on the same power section…) with emphasis to the practical considerations related to the design of the most effective technical solution based on the application of appropriate active techniques by controlled gridinterfaced converters of onboard converters in combination with passive methods (application of interference currents passive filters in power circuitry…). Introductory part also announces scientific contributions of the dissertation, elaborated in the following chapters. Second chapter (“Topologies and control structures of the converters installed in railway vehicles“) brings an overview, based on the available information from the industry and academia, of the power circuit topologies of the converters used for traction drive converters and auxiliary power supply converters, particularly the grid-interfaced converters in such applications intended for AC overhead line voltage. Apart from the most widely used solutions, a topologies based mostly on multilevel power converters are also shortly referred to as state-of-the-art solutions which will rather soon take over the deserved dominating position. Third chapter preceding the central part of the dissertation (“Sources and non-active suppression techniques of low-frequency interference currents caused by operation of traction drive converters and auxiliary power supply converters”) brings the context of the interference generation phenomena (most common sources of interferences, relationships between the key factors in the process of generation of interference currents including mutual interaction of multiple vehicles and power facilities in the feeder stations, interaction mechanisms between the injected interference signals and control structure…). After initial part of the chapter, an overview of the most popular passive suppression methods applied within the modern low-frequency interference current suppression system solution is given, illustrated by practical examples referring to the traction converters and auxiliary power supply converters in railway applications. Finally the chapter concludes with insight to the active suppression techniques not based on the closed loop control algorithms like proper selection and implementation of modulation of IGBT switches, including adaptive phase-coordination of PWM carriers within converter, as well as phase coordination of carriers between identical grid interfaces of physically dislocated converters, identification and elimination/minimization of the most common causes of non-linearity effects in control of power converters (e.g. dead-time elimination etc.). Fourth chapter (“Active suppression of low-frequency interference currents caused by operation of traction drive converters and auxiliary power supply converters by application of control algorithms”), provides systematic approach to the relevant issues concerning proper design and implementation of the active suppression system concept by maximizing the usage of properly implemented appropriate control algorithms (sampling algorithms, filtering algorithms including parallel finite impulse response filter structures based on the for minimization of intermodulation distortion tuned to the symmetrical pairs of frequencies with respect to the fundamental frequency of the grid voltage, robust and accurate algorithms for synchronization of the power converter and estimation of the grid voltage characteristics, software net-mark as zero-crossing detector of the fundamental component of the input voltage, feedforward algorithms based on PLL outputs and narrow-band reconstruction of the actual value of the grid voltage, phase-coordinated control of PWM carriers of grid interfaced converters on a vehicle level, without dedicated hardware infrastructure…), minimizing the possibilities for interaction between the corresponding control structure and the power-stage of the converter and vice-versa. Fifth chapter (“Implementation of algorithms on a target embedded control system”) describes hardware and software related issues that have been solved within the research activities in order to implement the proposed set of algorithms within given constrains defined by limited computing capabilities (basic 16-bit integer arithmetic) of the target system in combination with rather short period of control cyclic task (<80μs). Implementation process for the grid interfaced converter of the main traction drive is illustrated separately from the grid interfaced converter of the auxiliary power supply. Sixth chapter („Experimental verification of the algorithms”) describes methods used for validation and assessment of results achieved by implemented control system of grid interfaced converters – by numerous tests in laboratory conditions employing appropriate test setup, as well as by comprehensive measurements of suppression level of interference currents, performed on a vehicle level during the ontrack tests. Due to volume limitations only characteristic operating modes of the system are illustrated. Final chapter („Conclusion”) contains concluding remarks about the research activities and achievements contained in the dissertation, indicating some possibilities for future activities and improvements.