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Regulacija in preizkušanje kaskadnega mostičnega razsmernika
ID GREGORIČ, JAN (Author), ID Vončina, Danjel (Mentor) More about this mentor... This link opens in a new window

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Abstract
Množična uporaba razsmernikov se je pojavila s komercialno uporabo polprevodniških elementov ter povečevanjem števila alternativnih virov električne energije. Med splošno populacijo je eden izmed najbolj poznanih ter hkrati tudi najpogostejših virov alternativne energije sončna energija, ki jo zajemamo s pomočjo fotovoltaičnih oziroma sončnih celic. Napetost, generirana s pomočjo fotovoltaike, je enosmerna, zato je treba med sončne celice in omrežje dodati razsmernik, ki enosmerno napetost pretvori v izmenično napetost želene amplitude in frekvence. Obstaja več vrst razsmernikov ter njihovih izvedb, vsaka pa ima svoje prednosti in slabosti. Delo, predstavljeno v tej nalogi, je opravljeno na kaskadnem mostičnem razsmerniku. Glavna prednost tega razsmernika je možnost doseganja visokih izhodnih napetosti z relativno nizko napajalno napetostjo, kar je omogočeno z zaporedno vezavo treh mostičnih vezij, katerih napajanje je izvedeno z galvansko ločitvijo. Glavni cilj te naloge je predelava obstoječe programske kode tako, da le ta omogoča regulacijo izhodnega oziroma bremenskega toka in ne le krmiljenja izhodne napetosti, kar je omogočala do sedaj. Pri tem je bilo treba pripraviti tako zaščitne funkcije razsmernika, ki preprečujejo poškodbo oziroma uničenje posameznih komponent, kot tudi algoritme za proženje mostičnih vezij. Predhodno pripravljeni štirje algoritmi proženja (poimenovani ALL, ONE, X2 in X3) so za krmiljenje izhodne napetosti zahtevali simetrično (ALL in ONE) oziroma asimetrično (X2 in X3) napajanje ter podatek o želeni vrednosti izhodne napetosti. V odvisnosti od napajalne in želene napetosti so bili proženi posamezni tranzistorji mostičnih vezij, vendar v samo delovanje ni bila vključena regulacija. Obstoječe algoritme proženja sem nadgradil (poimenovani ALL_tok, ONE_tok, X2_tok in X3_tok) tako, da je proženje tranzistorjev posameznega mostiča skladno z zahtevo regulatorja oziroma razliko med želeno in dejansko vrednostjo toka. Uporabljen je PIR regulator, sestavljen iz treh vzporednih vej – proporcionalne, integralne in resonančne. Resonančna veja je dodana vzporedno klasičnemu PI regulatorju z namenom popolne odprave pogreška pri spreminjajoči se sinusni želeni vrednosti, kar z uporabo PI regulatorja ni mogoče. Testiranje novih algoritmov proženja je potekalo z ohmsko-induktivnim bremenom in je bilo razdeljeno na dva dela. V prvem delu so bili z želeno amplitudo izhodnega toka 5 A testirani vsi štirje algoritmi proženja. Primerjava efektivnih vrednosti izhodnega toka z efektivnimi vrednostmi želenega toka nam da rezultate med 98,9% in 102,9%, kar pomeni do 3% odstopanje efektivne vrednosti izhodnega toka od efektivne vrednosti referenčne, želene vrednosti. Ta izračun nam ne da podatka o morebitnih višjih harmonskih komponentah v opazovanem signalu, zato sem opravil še harmonsko analizo izhodnega toka. Po izračunu THD (angl. Total Harmonic Distortion) faktorja z upoštevanjem prvih 2000 harmonskih komponent (do frekvence 100 kHz) se izkaže, da je tok z najnižjo stopnjo harmonske popačitve rezultat proženja z algoritmoma ONE_tok ter X2_tok. Vseeno se odločimo za nadaljnje testiranje algoritmov ALL_tok in ONE_tok, ki zaradi simetričnih obremenitev ločilnih transformatorjev omogočata prenos najvišje moči, obenem pa sta najbolj primerna za splošno uporabo zaradi simetričnih napajalnih napetosti vseh treh mostičev. Drugi del testiranja zajema preizkus izbranih dveh algoritmov pri prenosu višje moči, z želeno amplitudo izhodnega toka enako 10 A oziroma 15 A. Ponovno je bil izveden izračun THD izhodnega toka in napetosti, iz rezultatov izračuna pa lahko sklenemo, da je najprimernejši algoritem za uporabo ONE_tok, ki za razliko od algoritma ALL_tok dosega višje število izhodnih nivojev, kar se odraža v nižji harmonski popačitvi. Predstavljen je tudi problem pojavljanja druge harmonske komponente kot posledica valovitosti enosmerne napetosti na kondenzatorski banki. V zaključnem delu naloge je obdelana možna nadgradnja razsmernika. Tekom izdelave naloge so se pojavljale ideje, kaj bi bilo na obstoječem razsmerniku treba nadgraditi. Predelava kode za proženje tranzistorjev in nadgradnja iz bipolarne modulacije v unipolarno, nadgraditev zaščitnih funkcij razsmernika in regulacija pretoka energije v kondenzatorski paket so le vmesne, manjše nadgradnje do naslednjega mejnika v delovanju razsmernika. Kot zanimiva nadgradnja oziroma naslednji logični korak v izdelavi tega kaskadnega mostičnega razsmernika se mi zdi priključitev razsmernika v elektroenergetsko omrežje in uporaba razsmernika za prenos energije v omrežje. Kljub temu, da regulacija jalove moči pri malih virih električne energije ni potrebna, bi bila ob regulaciji delovne moči zanimiv izziv za nadaljnje delo.

Language:Slovenian
Keywords:kaskadni mostični razsmernik, tokovna regulacija, THD faktor, PIR regulator
Work type:Master's thesis/paper (mb22)
Organization:FE - Faculty of Electrical Engineering
Year:2021
PID:20.500.12556/RUL-134212 This link opens in a new window
COBISS.SI-ID:91403779 This link opens in a new window
Publication date in RUL:29.12.2021
Views:264
Downloads:47
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Secondary language

Language:English
Title:Control and testing of cascaded H-bridge inverter
Abstract:
Inverters gained their popularity when semiconductors became accessible to common population and when the number of alternative energy sources increased. One of the best-known and also one of the most popular types of alternative sources of energy is solar power. Solar energy is converted into electrical energy using photovoltaic (solar) cells. The voltage, generated using solar cells, is DC, so inverters are needed to connect solar cells to the grid. An inverter converts DC voltage into AC voltage with desired amplitude and frequency. There are many types of inverters, and everyone of them has their pros and cons. In this thesis I present some upgrades I did on the cascaded H-bridge inverter. The main advantage of this type of the inverter is capability of reaching relatively high output voltage with low input voltage. That is done with series connection of multiple H-bridge circuits. The supply voltages of the circuits are electrically isolated. The main goal of this thesis was to upgrade the program code of the inverter. Until now, the output of the inverter was controlled voltage. I upgraded the program code so the output of the inverter is now regulated current. I wrote the program code for overvoltage and overcurrent to prevent damages or destruction of the inverter. Also the program code for switching the transistors was upgraded. There were four previously written algorithms (named ALL, ONE, X2, X3) for controlling the output voltage. When using algorithms ALL and ONE, all three supply voltages had to be symmetrical, while using algorithms X2 and X3, the supply voltages had to be asymmetrical. The transistors were switching proportionally to the ratio of the supply and reference output voltage, but the voltage was not regulated. I upgraded the algorithms and named them ALL_tok, ONE_tok, X2_tok, X3_tok. The new algorithms ensure that the switching depends on the error, which is the difference between the reference and actual output current. I used the PIR controller made of three parallel parts – proportional, integral and resonant. The PI controller can’t achieve zero error when the reference output current is not constant. The resonant part is added parallel to the PI controller to minimize the error. Testing the new algorithms was done with the inductive-resistive load and was separated into two parts. In the first part, all four algorithms were tested with the reference amplitude of the output current equal to 5 A. The comparison between RMS (root mean square) value of the output current and RMS value of reference current gives us results between 98,9% and 102,9%, which means deviation less than 3% between mentioned RMS values. This calculation does not provide any information of possible higher harmonics in the output current, so I performed a harmonic analysis of output current. I calculated the THD (Total Harmonic Distortion) factor from the first 2000 higher harmonics - up to 100 kHz. It turned out that the current with the lowest harmonic distortion is a result of using algorithms ONE_tok and X2_tok. Nevertheless, we decided to continue with testing algorithms ALL_tok and ONE_tok. The first reason for choosing these two algorithms is that they enable transmitting the highest possible power due to symmetrically loaded isolating transformers. The second reason is the fact that these two algorithms are the most suitable for general usage, because they need symmetrical voltage supply. In the second part of the testing, I tested the two chosen algorithms with higher power transmission, with reference output current amplitude equal to 10 A and 15 A. Again, the THD factor of the output voltage and current was calculated and from the results we can conclude that the best algorithm for usage is algorithm ONE_tok. Compared to the algorithm ALL_tok, ONE_tok can produce a higher number of levels of the output voltage which reflects on the lower harmonic distortion. Also, the problem with the second harmonic is presented as the result of the unstable DC voltage on the capacitor package. In the last part of this thesis I presented possible future work on that inverter. While working on the thesis, I came up with some ideas about the possible upgrades of this inverter. Remaking the program code and upgrading it from bipolar to unipolar modulation, upgrading protective functions of the inverter and power control from grid to capacitor package are only some minor upgrades towards the next milestone in the inverter functionality. Another interesting upgrade or the next logical step in the process of upgrading this inverter is connecting the inverter to the grid and using it for an actual power transfer. Even though the reactive power does not need to be controlled when a small power source is connected to the grid, I think this in addition to the active power control, would be an interesting challenge for future work.

Keywords:cascaded H-bridge inverter, current control, THD factor, PIR controller

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