Development of High Voltage Solid State Crowbar System for High Power Microwave Tube Protection A Project Report Submitted in partial fulfilment of the requirements for the Degree of Master of Engineering in Electrical Engineering by Manu Varkey Under the guidance of Dr. Vinod John Department of Electrical Engineering Indian Institute of Science Bangalore 560 012 JUNE 2012 Acknowledgements I wish to extend my deep sense of gratitude to my project guide, Dr. Vinod John, Professor, Department of Electrical Engineering, Indian Institute of Science, Bangalore, for his most valuable help and support in carrying out this project work and also for his concern shown during intricate situations. His invaluable guidance, constant support and encouragement has helped me explore a wide range of ideas and develop a broader vision in my project area. I thank all the faculty members of IISc for the splendid courses taught in a simple and inspiring way. The course work that I completed at IISc have helped me tremendously in broadening my knowledge in the eld of Electrical Engineering. I would also like to extend my sincere gratitude to all the sta members of the Electrical Engineering Department, IISc, for helping me with the formalities and the logistics of my M.E. I thank the Government of India, for nancial support during my study in this institute. I am grateful to my parents and friends without whose valuable aid I would not have accomplished this work as was planned to be. I thank the free software community for developing various software packages which were used towards the completion of this project. Especially I would like to extend my gratitude for the developers of Scilab, KiCad, Inkscape, Gimp, Texlive and GNU-Linux. These software packages were of great help during the course of the project work and in the completion of this report. Above all I would like to acknowledge the great tradition of scientic enquiry and collaboration that made this work possible. i Abstract Microwave tubes in the event of arc faults can draw too much current and can get damaged due to excessive power dissipation. It can be protected in such an event by diverting the fault current away from the device. Such a protective mechanism that is connected in parallel with a device so as to protect it in the event of a short circuit is called a crowbar. This project deals with the design of a solid state crowbar system for protecting high voltage high power microwave tubes against arc faults. The solid state switch is implemented using a series thyristor string. A gate drive circuitry is designed for ring the thyristor devices so as to ensure high di=dt during crowbar operation and to conform to the isolation voltage requirement. The control circuitry is implemented on an FPGA. A thermal model is developed for modelling the transient thermal behaviour of SCR modules. Simulation models are also developed for studying crowbar operation. ii Contents Acknowledgements i Abstract ii 1 Introduction 1 1.1 General outline . 1 1.2 Major project tasks . 3 2 Thermal design 4 2.1 Formulation of the thermal model . 4 2.2 Simulation models . 5 2.3 Simulation . 8 2.3.1 Device model estimation . 8 2.3.2 Crowbar operation . 8 3 Thyristor gate drive 13 3.1 General outline . 13 3.2 Converter current controller . 14 3.3 Gate drive card . 16 3.3.1 Regulated power supply circuit . 16 3.3.2 Unregulated power supply and gate triggering circuit . 17 3.3.3 Thyristor turn-on detection circuit . 20 3.4 Converter circuit . 20 4 FPGA controller 22 4.1 General structure . 22 4.2 Interface circuit . 22 5 Testing 25 5.1 Observations . 25 6 Conclusions 29 6.1 Summary of present work . 29 6.2 Suggestions for future work . 29 iii CONTENTS iv References 32 A Program listings 33 A.1 FPGA code . 33 A.2 Curve tting code . 36 B Converter current controller 38 B.1 Schematic . 38 B.2 Bill of materials . 40 B.3 Testing procedure . 40 B.4 PCB layout . 40 C Thyristor gate drive card 42 C.1 Schematic . 42 C.2 Bill of materials . 44 C.3 Testing procedure . 44 C.4 PCB layout . 45 D Physical setup 48 List of Figures 1.1 General outline of the system to be protected . 2 1.2 General outline of the crowbar system . 2 2.1 Heat ow diagram . 5 2.2 Thermal model of the thyristor module . 6 2.3 Transient thermal model of the thyristor module . 6 2.4 Short circuit current calculation simulation model . 7 2.5 Power dissipation calculation simulation model . 8 2.6 Temperature estimation simulation model . 9 2.7 Zth vs time curve original and tted . 9 2.8 Short circuit current . 10 2.9 Load energy dissipation . 11 2.10 Thyristor power dissipation . 11 2.11 Temperature estimation . 12 3.1 General outline of the isolated gate drive . 14 3.2 Converter current controller circuit . 15 3.3 Gate drive card regulated power supply circuit . 17 3.4 Gate drive card unregulated power supply and gate triggering circuit . 18 3.5 Gate drive card thyristor turn-on detection circuit . 20 3.6 Converter circuit . 21 4.1 FPGA controller block diagram . 23 4.2 FPGA interface circuit . 24 5.1 Current waveform at the output of converter . 26 5.2 Gate signals generated by current controller . 26 5.3 Thyristor gate current . 27 5.4 Turn-on signals of two thyristor modules before compensation . 27 5.5 Turn-on signals of two thyristor modules after compensation of 4*50ns . 28 5.6 Anode-Cathode voltage of two thyristor modules during turn-on . 28 6.1 Proposed temperature measurement circuit . 31 B.1 PCB layout of converter current controller - Front . 41 v LIST OF FIGURES vi B.2 PCB layout of converter current controller - Back . 41 C.1 PCB layout of converter current controller - Front . 46 C.2 PCB layout of converter current controller - Back . 47 D.1 Current controller circuit - Physical setup . 48 D.2 Converter circuit - Physical setup . 49 D.3 Gate drive card - Physical setup . 49 D.4 Physical circuit setup . 50 Chapter 1 Introduction 1.1 General outline Crowbar is a protective mechanism that is connected in parallel with a device so as to protect it in the event of a short circuit. On occurrence of the short circuit within the device, the crowbar diverts the current from the device preventing its destructive failure. Crowbars may be implemented using electro-mechanical relays or using solid state switches. The protection of high power microwave tubes requires the device current to be diverted away from the device within a time-span of a few microseconds. This cannot be achieved by using electro-mechanical relay based crowbars because of their slower speed of operation. Hence the use of solid state crowbar system, which allows faster diversion of current away from the device is warranted. The general outline of the system to be protected is described in Figure 1.1. When a short-circuit1 occurs at the load, the current transducer (CT) senses this information and sends a signal to the controller which res the crowbar and send a trip signal to the circuit breaker so as to trip the main supply. The crowbar when red provides a low impedance path in parallel with the load and diverts current away from the load. This limits the energy dissipation in the load, thereby protecting the load from destructive failure. The general outline of the crowbar system to be used for protection is described in Figure 1.2. The solid state switch is implemented using a series string of SCRs. Since device characteristics are not perfectly matched, the thyristor string alone cannot ensure uniform voltage sharing across the devices during blocking state and during dynamic conditions. Therefore to ensure static voltage sharing, a resistor string is connected in parallel to the thyristor string. In a conventional thyristor string, dynamic voltage sharing is ensured by providing RC snubbers across the devices. But for crowbar ap- plications an RC snubber is preferably not used, as it can delay the turn-on process. Instead dynamic voltage sharing is ensured by delaying the gating pulses to the SCRs by the individual device turn-on times. Also an SCR device has an inherent di=dt lim- itation imposed by the device physics. This limit arises because during triggering, the 1For ex. an arc fault in the microwave tube 1 Chapter 1. Introduction 2 Figure 1.1: General outline of the system to be protected Figure 1.2: General outline of the crowbar system Chapter 1. Introduction 3 anode current is conned to regions around the gate and gradually spread to surround- ing regions. If anode current rises too sharply2, then the device can get damaged due to hot-spots produced by high current densities surrounding the gate structure[6]. To limit di=dt in the crowbar branch, an inductor Ls is introduced in series. Since the thyristor gates are at dierent voltages, an isolated gate drive is required. The triggering signal from the controller to gate drive and the diagnostic information from gate drive to controller are transferred over optical bre links. Since the timing of the gate trigger pulses are critical and the various signal are to be generated in parallel, an FPGA is used to implement the logical section of the controller. The current transducer that is used is a hall eect sensor so as to obtain good dynamic response. 1.2 Major project tasks The major project tasks can be broken down as follows.