Investigation Into a Proposed HVDC Series Tap Topology
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Investigation into a Proposed HVDC Series Tap Topology by André Hartshorne Thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Engineering at Stellenbosch University Supervisors: Prof Hendrik Du Toit Mouton Department Electrical and Electronic Engineering March 2016 Stellenbosch University https://scholar.sun.ac.za Declaration By submitting this thesis electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the owner of the copyright thereof (unless to the extent explicitly otherwise stated) and that I have not previously in its entirety or in part submitted it for obtaining any qualification. Signature: . André Hartshorne Date: . Copyright © 2015 Stellenbosch University All rights reserved Stellenbosch University https://scholar.sun.ac.za Abstract HVDC Transmission has found widespread application through-out the world. The typical HVDC Transmission system consists of two terminals and provides point-to-point large scale power delivery. HVDC Transmission systems can be extended to become multi-terminal sys- tems. Introducing series taps is the easiest way to achieve this. Introducing a HVDC Series Tap into a HVDC Transmission system presents several challenges. These include the additional voltage drop present on the line, switching noise introduced by the use of a power electronic converter and providing galvanic isolation between the line potential and the ground potential. Several topologies have been proposed but none has been accepted as the industry standard implementation. This thesis discusses the design of a HVDC Series Tap utilising an air-core transformer. A scale model of the proposed HVDC Series Tap is constructed and implemented on a HVDC Transmission Line model. Experimental results are presented. iii Stellenbosch University https://scholar.sun.ac.za Uittreksel Hoogspanning Gelykstroom Transmissie word tans wêreldwyd toegepas. Die algemene Hoog- spanning Gelykstroom Transmissiestelsel bestaan uit twee terminale en word gebruik om dry- wing van punt tot punt te verskaf. Hoogspanning Gelykstroom Transmissiestesels kan uitgebrei word om uit meer as twee terminale te bestaan. Die eenvoudigste manier om hierdie doel te bereik is om serie aftap punte by te voeg. Om ’n Hoogspanning Gelykstroom Serie Tap tot ’n Hoogspanning Gelykstroom Transmissie- stelsel by te voeg bied verskeie uitdagings. Hierdie uitdagings sluit die teenwoordigheid van ’n addisionele spanningsval oor die lyn, skakelruis wat weens die gebruik van skakelelektronika ontstaan en die voorsiening van galvaniese isolasie tussen die lynpotensiaal en grondpotensi- aal in. Verskeie topologieë is al voorheen voorgestel, maar geen word aanvaar as die industrie standaard nie. Hierdie tesis bespreek die ontwerp van ’n Hoogspanning Gelykstroom Serie Tap wat van ’n lugkerntransformator gebruik maak. ’n Model op skaal is van die voorgestelde Hoogspanning Gelykstroom Serie Tap gebou en op ’n model van ’n Hoogspanning Gelykstroom Transmissie- stelsel geïmplementeer. Eksperimentele resultate word voorgelê. iv Stellenbosch University https://scholar.sun.ac.za Acknowledgements The author would like to thank the following people for their contribution towards this project: • Professor Mouton for his guidance, patience and encouragement. • Professor Botha for assisting with the numerical simulation of the air-core transformer. • Friedel Rauch for the illustration of a basic power system. • Lucas Janse van Vuuren for providing the photos of the Cahora-Bassa Line. • Jan van Bosch for his support. • Ronelle Hauptfleisch for her generous help with language editing. • My wife and family for their endless support. v Stellenbosch University https://scholar.sun.ac.za Contents Abstract iii Uittreksel iv Acknowledgements v Nomenclature x List of Figures xv List of Tables xx 1 Introduction 1 1.1 Background ...................................... 1 1.2 Thesis Objectives ................................... 4 1.3 Thesis Outline ..................................... 5 2 Literature Review 6 2.1 Power Transmission & Distribution ......................... 6 2.2 HVDC Transmission ................................. 7 2.2.1 Introduction .................................. 7 2.2.2 Historic Origins ................................ 8 2.2.3 Comparison of HVDC and conventional transmission ........... 8 2.2.4 Workings of a HVDC transmission system . 12 2.2.5 Configurations or Types ........................... 13 2.2.6 Converter types and Switching devices ................... 18 2.2.7 HVDC Control ................................ 21 2.2.8 Harmonics ................................... 23 2.2.9 Notable HVDC Projects ........................... 24 2.2.10 Modern Trends ................................ 24 2.3 The Cahora-Bassa HVDC Transmission System . 26 3 HVDC Terminals Model Hardware Design 30 vi Stellenbosch University https://scholar.sun.ac.za CONTENTS vii 3.1 Introduction ...................................... 30 3.2 HVDC Terminals Model Specifications ....................... 30 3.3 Model System Design ................................. 32 3.3.1 Overview ................................... 32 3.3.2 System Level Design ............................. 32 3.3.3 Transformer Tests ............................... 34 3.3.4 System Level Simulation ........................... 37 3.3.5 Conclusion ................................... 39 3.4 Hardware Design ................................... 40 3.4.1 Overview ................................... 40 3.4.2 Thyristor Bridge Design ........................... 41 3.4.3 Thyristor Losses & Thermal Design ..................... 46 3.4.4 Thyristor Snubbers .............................. 47 3.4.5 Thyristor Gate Driver ............................ 49 3.4.6 Smoothing Reactor Design .......................... 50 3.4.7 Line Filters .................................. 56 3.4.8 Transformer Banks .............................. 57 3.4.9 Controller Board ............................... 59 3.4.10 Voltage Measurement Board ......................... 59 3.4.11 Current Measurement Board ......................... 60 3.4.12 Optic Fibre Driver Board .......................... 61 3.5 Results ......................................... 62 3.6 Summary ....................................... 64 4 HVDC Terminals Model Controller Design 65 4.1 Introduction ...................................... 65 4.2 Rectifier Controller .................................. 65 4.2.1 Overview ................................... 65 4.2.2 Determine transfer function ......................... 66 4.2.3 Design of controller .............................. 67 4.3 Inverter Controller .................................. 69 4.3.1 Overview ................................... 69 4.3.2 Determine transfer function ......................... 69 4.3.3 Design of controller .............................. 70 4.4 System Simulation & Implementation ........................ 72 4.4.1 System simulation .............................. 72 4.4.2 Practical System Implementation ...................... 74 4.5 Summary ....................................... 76 5 HVDC Series Tapping Options 77 Stellenbosch University https://scholar.sun.ac.za CONTENTS viii 5.1 Introduction ...................................... 77 5.2 Series Tap Requirements ............................... 77 5.3 Previously Proposed Taps & Applicable Technologies . 79 5.4 Evaluated topologies ................................. 80 5.4.1 Option 1 .................................... 80 5.4.2 Option 2 .................................... 80 5.4.3 Option 3 .................................... 82 5.4.4 Option 4 .................................... 82 5.5 Comparison ...................................... 84 5.6 Conclusion ....................................... 86 6 Air-core Transformer Analysis & Simulation 88 6.1 Introduction ...................................... 88 6.2 Theoretical Overview ................................. 88 6.2.1 Self Inductance & Mutual Inductance .................... 88 6.2.2 Air-core Transformer Model ......................... 89 6.2.3 Pashen’s Law ................................. 90 6.3 Physical Description ................................. 91 6.4 Mathematical Description .............................. 92 6.5 Numerical Analysis .................................. 93 6.5.1 Overview of the simulation .......................... 93 6.5.2 Simulation process and assumptions explained . 94 6.6 Results ......................................... 96 6.7 Conclusion ....................................... 97 6.8 Summary ....................................... 98 7 First Series Tapping Option Analysis 99 7.1 Introduction ...................................... 99 7.2 Operation ....................................... 99 7.3 Simulation .......................................101 7.4 Conclusion .......................................103 7.5 Summary .......................................104 8 Second Series Tapping Option Analysis 105 8.1 Introduction ......................................105 8.2 Converter Analysis ..................................105 8.2.1 Current source to Voltage source stage analysis . 107 8.2.2 Isolation stage analysis ............................111 8.3 Simulation Results ..................................112 8.4 Summary .......................................116 Stellenbosch University https://scholar.sun.ac.za CONTENTS ix 9 Construction Of Series Tap Model 117 9.1 Introduction ......................................117 9.2 Design, Simulation & Implementation .