Position/speed sensor-less control of wind energy conversion systems based on Rotor-Tied Doubly-Fed induction generator systems

Thesis (PhD)--Stellenbosch University, 2019.ENGLISH ABSTRACT: The doubly-fed induction generator (DFIG) is amongst the most popular wind turbine generator in South Africa. This is partly due to the fact that its backtoback power converters are partially rated. More precisely, they are rated at 30% of the generator rated power. A new DFIG topology has been proposed recently. That is the rotor-tied doubly-fed induction generator (RDFIG). In this topology, the rotor side is connected to the grid while the stator side is connected to the power converter. It has been shown that this topology holds the advantage of higher effeciency compared to the standard DFIG topology. High accuracy in all the measurements is required for the optimum operations of wind energy conversion systems (WECSs). The measurement of the rotor position/speed is amongst the most important measurements when it comes to implement any control system for the WECS. The conventional method of measuring the rotor position/speed is to use an electronic/mechanical sensor (encoder or resolver). This measurement involves the use of long cables and in a harsh environment, this can lead to faulty operations of the WECS. In this thesis, several slip speed estimators for sensor-less control of RDFIGbased WECSs are developed and implemented. The proposed slip speed estimators are based on the association of different sliding mode observers and the PLL estimator. The association of the PLL estimator improves the estimation performance by reducing the noise created by the sliding control control functions. Also, the proposed PLL estimator helps in avoiding a phase shift of π in super-synchronous operating conditions. In addition, in this thesis, several sliding mode observers were developed in order to improve the estimation performance. The proposed sliding mode observers were satisfactory for all the operating conditions of the RDFIG-based WECSs. The robustness of the proposed slip speed estimators is validated experimentally under various operating conditions. A 5.5-kW custom-designed gridconnected RDFIG test-bench based on a National Instrument (NI) PXIe-8115 controller is used. The proposed slip speed estimators solve the problem linked to the failure of the electromechanical sensors. The overall sensor-less control strategy provides an alternative to the sensor-based control of the RDFIGs. Also, the proposed sensor-less vector control strategy can be used as a back-up in case the electromechanical sensor fails.AFRIKAANSE OPSOMMING: Die dubbelgevoerde induksiegenerator (DFIG) is een van die gewildste windturbinegenerators in Suid-Afrika. Dit is deels as gevolg van die laer kapasiteit omsetters wat die DFIG benodig. 'n Nuwe DFIG-topologie is onlangs voorgestel: Die rotorgebonde DFIG (RDFIG). In dié topologie word die masjien se rotor aan die netwerk gekoppel en die stator aan laer kapasiteit omsetters. Dit is bewys dat hierdie topologie 'n hoër masjieneffektiwiteit het in vergelyking met die standaard-DFIG. Hoë akkuraatheid van metings word vereis vir die optimale beheer van windenergie-omsettingstelsels (WECS). Die meting van die rotorposisie en - spoed is van die belangrikste metings wanneer dit kom by die beheerstelsel van 'n WECS. Die konvensionele metode van rotorposisie- en rotorspoedmeting is deur middel van 'n kodeerder. Dié meting behels die gebruik van lang kabels in 'n fel omgewing, wat kan lei tot die foutiewe werking van die stelsel. In hierdie verhandeling word verskeie glipspoedafskatters vir die sensorlose beheer van 'n RDFIG-gebaseerde WECS ontwikkel en geïmplimenteer. Die voorgestelde glipspoedafskatter is gebaseer op die assosiasie van verskillende glymodus-observeerders en 'n PLL-afskatter. Die assosiasie van die PPLafskatter verbeter die afskattingprestasie deur die geruis van die glymodusbeheerstelsels te verminder. Die voorgestelde PPL-afskatter help ook om 'n faseskuif van π te vermy tydens super-sinchroonoperasie. In hierdie verhandeling word verskeie glymodusobserveerders ontwikkel om die afskattingsprestasie te verbeter. Die voorgestelde glymode-observeerders presteer bevredigend vir alle toestande van die RDFIC-gebaseerde WECS. Die kragtigheid van die voorgestelde glipspoedafskatters se geldigheid word onder verskeie toestande eksperimenteel getoets en bewys. 'n 5.5 kW netwerkgekoppelde RDFIG toetsbank gebaseer op 'n National Instrument PXIe-8115-beheerder word gebruik. Die probleem van gefaalde elektromeganiese sensors word deur die voorgestelde glipspoedafskatters opgelos. Die sensorlose beheerstrategie gee ook 'n alternatief vir tradisionele sensor-gebaseerde beheer van die RDFIG's.Master

Unified approach for synthesis and analysis of non-isolated DC-DC converters

Transformational techniques unifying synthesis of two-state DC-DC converters and analytical synthesis techniques allowing generation of all possible converters meeting a certain criteria already exist. The analysis of a family of converters derived from a single converter cell has also been uni ed. Current waveforms generated by the family of converters were shown to be related. However, a concept or basic building blocks that facilitate uni ed synthesis, analysis, prediction of current waveforms and assignment of switch states over a very wide range of DC-DC converters is still lacking. This study will propose three 3-terminal basic building blocks and one 3-terminal lter block. It will be shown that between them, they are suf cient for realizing all non-isolated DC-DC converters excluding those with coupled inductors. The various DC-DC converters fall into those realized through cascade, stacked, stacked plus cascade, interleaved/paralleled or differential connection of the basic building blocks. A systematic approach for evaluating input-output current gains will be presented. Moreover, a basic building block will be shown to have xed switching states for proper operation. This gives rise to the generation of a unique set of current waveforms at the three terminals irrespective of where a basic building block is embedded. It has been shown that the effort and time needed to design DC-DC converters can be reduced as switching device stresses can be estimated without the need for tedious rst principle derivations.http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=6287639am2022Electrical, Electronic and Computer Engineerin

Modelling and control of doubly-fed induction generator systems in wind turbine applications

Thesis (MEng)--Stellenbosch University, 2017.ENGLISH ABSTRACT: The need to reduce the world's carbon foot print has led to a significant rise in wind energy generation. The doubly-fed induction generator (DFIG) is one of the most popular wind turbine generators partly due to its low rated backto- back converter. A constant DC-link voltage in the doubly-fed induction generator system's back to back converter allows for bidirectional power flow of the rotor power. Hence, effective control of the DC-link voltage is necessary. The presence of the switching elements in the back-to-back converter creates harmonics in the systems. LCL and L filters are mostly used to mitigate the harmonics. DFIGs are mainly connected to the grid, however, they can be used in stand-alone mode in isolated rural areas, where there are low loads with no grid connection. DFIGs in the stand-alone mode have to be controlled such that they provide voltage and frequency stability at varying load conditions and changing wind speeds. In this thesis, the power control of the grid-connected DFIG systems in wind turbine applications is presented. Power factor regulation is conducted since it helps in the reduction of the costs linked to the capacitor bank. Maximum power point tracking is also investigated. DC-link voltage control is analysed whereby the grid-side converter is controlled as a voltage-source converter. A comparative analysis of the LCL filter and L filter for switching frequencies below 5 kHz is done and described in this thesis. This is important for systems with low sampling frequencies. Furthermore, in this thesis, the control of a stand-alone DFIG together with simulation result, is presented. Experimental results are also given to demonstrate the effectiveness of the developed rotor-side control algorithm on a DFIG test bed.AFRIKAANSE OPSOMMING: Die behoefte om die wêreld se koolstof spoor te verminder het gelei tot 'n groot toename in die windopwekking van energie. Die dubbel-gevoerde induksie generator is een van die mees gewilde wind-turbine opwekkers, gedeeltelik weens die lae kapasiteit rug-aan-rug omsetter. Die konstante GS-bus spanning in die induksie generator sisteem se rug-aan-rug omsetter maak twee-rigting vloei van die rotordrywing moontlik. Daarom is doeltreffende beheer van die GS-bus spanning nodig. Die teenwoordigheid van die skakelelemente in die rug-aan-rug omsetter skep harmoniek in die sisteme. LCL en L filters word meestal gebruik om die harmoniek te verminder. Dubbel-gevoerde induksie generators word meestal aan die network gekonnekteer, maar kan ook in alleen-staan modus gebruik word vir verafgeleë landelike gebiede met lae las en sonder network konneksies. Hierdie alleenstaande generators moet so beheer word dat hulle stabiele spanning en frekwersie onder verskillende lastoestande en windspoede kan verskaf. In hierdie tesis word die beheer van die drywing in die network-verbinde Dubbel-gevoerde induksie generator sisteme in die windturbine toepassings bespreek. Regulering van die arbeidsfaktor word toegepas aangesien dit bydra tot die verlaging van kostes wat gekoppel is aan die kapasitorbank. Die maksimum drywingspunt opsporing word ook ondersoek. Die GS-bus spanningsbeheer word geanaliseer waar tyders die netwerk-kant se omsetter as 'n spanningsbron beheer word. 'n Vergelykende ontleding van die LCL en die L filters vir skakel frekwensies onder 5 kHz is gedoen en in hierdie tesis beskryf. Dit is belangrik, veral in sisteme met lae monster frekwensies. Die beheer van 'n alleenstaande Dubbelgevoerde induksie generator en die simulasie resultate word beskryf. Die resultate van die eksperimente word gegee om die doeltreffendheid van die ontwerpte rotor kant beheer algoritme op n Dubbel-gevoerde induksie generator toetsbed demonstreer