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An aircraft warning beacon Pickering, George Robert 1978

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AN AIRCRAFT WARNING BEACON by GEORGE ROBERT PICKERING B.A.Sc, University of Toronto, 1976 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER. OF, APPLIED. SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF ELECTRICAL ENGINEERING We accept this thesis as conforming to the required s tandard THE UNIVERSITY OF BRITISH COLUMBIA September, 1978 (c) George I Robert Pickering, 1978 In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make it f ree ly ava i lab le for reference and study. I further agree that permission for extensive copying of th is thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of this thes is for f inanc ia l gain sha l l not be allowed without my writ ten permission. n . f E l e c t r i c a l E n g i n e e r i n g Department of ° to The Univers i ty of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 29 September 1978 Abstract The Canadian Ministry of Transport recommends that powerline crossings on river valleys be marked by white flashing lights. In remote areas a power supply for these lights can be difficult to obtain. A warning beacon system which uses the leakage current of semi-conductive glaze insulators to power a capacitive discharge flashtube is described. Consideration is given to the relevant characteristics of flash tubes and semiconductive glaze insulators. Tests which have been conducted to confirm the viability of the system are described. i i i TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i i i LIST OF TABLES i v LIST OF ILLUSTRATIONS v ACKNOWLEDGEMENT v i I. INTRODUCTION 1 II. CAPACITIVE DISCHARGE FLASHTUBES 4 III. SEMICONDUCTIVE GLAZE INSULATORS 8 IV. TRANSFORMER - DESIGN CONSIDERATIONS H V. TRANSFORMER - MODEL 24 VI. BEACON POWER SUPPLY AND CONTROL CIRCUITS 32 6.1 Beacon Power Supply Circuit 32 6.2 Single Flashtube Control Circuit 32 6.3 Control Circuit for Three Flashtubes 37 VII. OBSERVED BEACON PERFORMANCE 41 VIII. CONCULSIONS AND SUGGESTIONS FOR FURTHER WORK 44 APPENDIX A. STRAY CAPACITANCE IN THE TRANSFORMER 45 APPENDIX B. MAGNETIZING IMPEDANCE OF THE TRANSFORMER 50 APPENDIX C. CONTROL CIRCUIT WAVEFORMS 52 REFERENCES 56 XV List of Tables Page Table I. Power Available into a Battery Pack. 42 Table II. Battery Pack Charge Characteristic. 42 V . List of Illustrations Figure Page 3.1 6-Unit Semicon String Model on a 230 kV Line 10 4.1 Bobbin for Primary Winding 14 4.2 Lamination Strain Calculation 16 4.3 Photograph of Transformer Core Assembly 18 4.4 Photograph of Transformer 19 4.5 Transformer Connection to 230 kV Line 21 4.6 Model for Transient Analysis of the Circuit of Figure 4.5 22 4.7 Transient Voltage at Node H in Figure 4.6 23 5.1 Stray Capacitance in the Transformer 26 5.2 Transformer Model 27 5.3 Power Delivered to a Resistive Load Using a Single Semicon String 28 5.4 Power Delivered to a Resistive Load Using Two Semicon Strings 30 5.5 Current Distribution for R^ = 22 M°, 31 6.1.1 Beacon Power Supply 33 6.2.1 Single Flashtube Control Circuit 34 6.2.2 Waveforms of Control Circuit 36 6.3.1 Sychronizing Signal Generator for Three Single Flashtube Units 38 6.3.2 Three Lamp Control Circuit 40 7.1 Battery Pack Charge Characteristic 43 A. l Stray Capacitances 46 B. l Transformer Magnetizing Impedance 51 C. l Discharge Capacitor Charging Circuit 53 vi' ACKNOWLEDGEMENT I wish to thank Dr. A.D. Moore, for his assistance throughtout the course of my stay at the University of British Columbia. Valuable advice on the construction of the transformer was pro-vided by Mr. H. Burgess of the British Columbia Transformer Company Limited. Mr. R.A. Briggs of Cor-Mag Company supplied the material used in the trans-former core. Thanks are also due to Mr. A. Reed of the British Columbia Hydro and Power Authority for his advice and his assistance in arranging for testing of the prototype. The technical staff of the University of British Columbia deserves a great deal of credit. In particular I would like to thank Mr. D. Daines and Mr. D. Fletcher for constructing the prototype, and Mr. A. MacKenzie for his assistance in testing the prototype and in dealing with bureaucratic red tape. The manuscript was typed by Mrs. S.Y. Hoy. I would also like to thank Dr. H.W. Dommel for reading the thesis, and for the use of his computer program. This research was supported in part by NRC grant A-3357, and by a grant from the British Columbia Hydro and Power Authority. 1. I. INTRODUCTION It i s common practice for small aircraft to use river valleys as navigation aids, especially during periods of low v i s i b i l i t y . The hazard introduced by power lines crossing the valleys is well recognized. At present these obstructions are marked, in most cases, by painting the towers on each side of the valley, and by suspending reflective spheres from the catenary. To increase the v i s i b i l i t y of the lines the Canadian Ministry of Transport [1] recommends that high intensity white flashing lights be mounted on the towers at crossings. Three lights, one located at the top of the tower, one at the level of the lowest point of the catenary, and one midway between, serve to provide an indication of the position of the power cables. As stated in the Ministry's recommendations [1] The white lights shall flash sequentially; f i r s t the middle light, then the top light and last, the bottom light. The off interval between the top light and the bottom light shall be twice as long as the interval between the middle light and the top light. Interval between the end of one sequence and the beginning of the next shall be about 10 times the interval between the middle and top light. Each light unit of the system shall flash at a rate of 40 to 60 per minute; 60 per minute preferable. One of the major problems encountered when attempting to develop such a warning beacon system i s providing a source of power for the lights. Where a low voltage distribution network i s nearby this problem does not arise, but many river crossings exist in remote areas where no such network is available. Several methods have been developed to solve this problem, a l l consisting of a way of obtaining power from the high voltage line i t -self. The amount of power required depends upon the type of beacon desired. 2. A typical unit approved by the Federal Aviation Authority of the United States consumes in excess of 200 W. The xenon flashtube units used on aircraft operate at approximately 25 W. The current flowing in a power line w i l l induce a current i n any nearby conductive loop. This principle has been used by H.J. Dana [2] to provide power for a mid-catenary neon warning light. In this system an iron core is clamped around the conductor. A many-turn secondary on the core is connected to a neon tube, which produces a steady red light. It would appear that this technique could also be used to power a flashing light system. However, since the current in the line fluctuates over a wide range, the amount of power available is not well defined. L'Institut de recherche de 1'Hydro-Quebec (IREQ) has developed a system which obtains power from the high voltage line using electrostatic coupling between the overhead ground wire and the line [3,4]. Overhead ground wires are strung above the power lines to provide lightning protec-tion. Capacitance exists between the overhead ground wire and the lines, the value of which depends upon the physical dimensions involved and the atmospheric conditions. For the line configuration studied by IREQ, this capacitance i s 6.5 pF/m. To collect the energy coupled to the overhead ground wire by this capacitance, the wire is elec t r i c a l l y isolated from the towers, and connected to a step-down transformer. IREQ has obtained 5 kW/km from a 735 kV line, and has used a single span to power a beacon containing two 100 W incandescent lamps. The amount of power available from a capacitive system depends upon the line voltage rather than the current, and should thus fluctuate less than the power obtained by inductive coupling. 3. Overhead ground wires are only used in very few places on the power lines of the British Columbia Hydro and Power Authority, so that the use of a system of the type developed by IREQ would require considerable modification of the existing lines. The system described in this thesis, which was developed in cooperation with the British Columbia Hydro and Power Authority, uses the leakage current of semiconductive glaze insulators to supply power for a xenon flashtube warning beacon. This leakage current depends upon the line voltage and the atmospheric conditions. Because the characteristics of the insulators and flashtubes have a significant i n f l u -ence on the design of the warning beacon system, they w i l l be considered before the system i t s e l f is described. 4. II. CAPACITIVE DISCHARGE FLASHTUBES A flashing light can be obtained from a flashtube, or by switch-ing an incandescent lamp on and off. Capacitive discharge flashtubes are superior to flashed incandescent lamps for use as warning beacons due to their higher luminous efficacy, higher effective intensity, and longer l i f e . Luminous efficacy is the total visible luminous flux output (in lumens) divided by the total power input (in watts) [5]. Xenon flashtubes produce between 10 and 50 lumens per watt, with a typical value of 35 lm/W. Incandescent lamps normally produce approximately 10 lm/W. At low levels of illumination, the effective intensity of a flash-ing light i s given by [6] Si I ( t ) d t XE a + ( t 2 - t ) where I(t) is the light intensity at time t, (lm), a is the Blondel-Rey constant normally taken to be 0.2 seconds, t.j., t<^ (seconds) are chosen so as to maximize I . For a capacitive discharge lamp, t^ and are the beginning and end of the discharge and ( t 2 ~ t i ^ -*-s m u c h less than 0.2 s. Thus, for such a lamp K t ) d t t i - l A flashed incandescent lamp usually has a duration (t2~t^) greater than 0.2s, Consequently, given a discharge lamp and a flashed incandescent lamp with the same value of integrated intensity, the discharge lamp w i l l be more effective. 5. Incandescent lamps have a short l i f e which is reduced considerably by the continuous heating and cooling of the element resulting from switch-ing the unit on and off. Flashtubes are subject to two main failure mechanisms. Cracking or crazing of the wall of the tube can occur i f the current density of the arc i s too high. Blackening of the tube caused by sputtering of the cathode material due to bombardment by high velocity atoms of the f i l l gas w i l l also occur. Both of these failure mechanisms depend upon the applied energy per flash; i t has been found that reducing the energy by half results in a ten-8 fold increase in the l i f e of the tube. Lifetimes of 10 flashes are possible, so that a warning beacon operating at a rate of one flash per second could last longer than three years. Inert gases are normally used in flashtubes, since their outermost atomic orbits are f u l l of electrons, making them readily excitable. Of the inert gases, xenon i s most often used because i t has the largest c o l l i s i o n cross-section and the lowest excitation and ionization potentials, as well as an output spectral distribution which most closely approximates daylight. There are three main c r i t e r i a to be considered when applying a flashtube. The f i r s t two, flash duration and anode voltage are interrelated. Flash durations of less than 10 us are possible. However, to obtain signif-icant light output from such a flash, a high anode voltage (perhaps 15 kV) would be required, resulting in a high current density and attendant problems in tube design. In addition, a shift in the spectral distribution of the light output towards ultraviolet would occur. For consistent operation at low anode voltages (i.e. about 200 V), low f i l l pressure is necessary. 6. This results in low energy-handling capability, low luminous efficacy, and -. a spectral shift towards infrared. Practical figures for warning beacon applications are anode voltage between 300 V and 2 kV and flash duration of 100 ys to 10 ms. The third criterion is triggering method. Flashtubes are operated with anode voltages well below self-ionization voltage levels, which are typically greater than 500 V per inch of arc length. Three methods are commonly used for i n i t i a t i n g the arc: external shunt triggering, internal shunt triggering, and series injection triggering. External shunt triggering consists of applying a high voltage (greater than 4 kV) pulse to a wire or conductive film on the outside of the flashtube. This ionizes some of the f i l l gas atoms, i n i t i a t i n g the arc. The pulse is usually generated by dis-^ charging a small capacitor through the primary of a pulse transformer, the secondary of which is connected to the trigger terminal of the tube. Inter-nal shunt triggering differs only i n that the trigger pulse is applied to a third electrode inside the flashtube. This allows a lower voltage pulse to be used, but increases the complexity of the tube. In series injection triggering, the secondary of the trigger transformer is placed in series with the anode of the flashtube. The trigger pulse raises the anode voltage above the self-ionization voltage of the tube. The other circuit components must withstand the trigger voltage, and the pulse transformer secondary must be capable of carrying -S f. the discharge current. Series injection triggering does not apply high voltage to the outside of the tube as i n external shunt triggering, and thus possible blackening of the glass is avoided. 7. The system described herein uses external shunt triggering and an anode voltage of approximately 350 V. This allows the use of readily avail-able, reasonably priced flashtubes. 8. III. SEMI CONDUCTIVE GLAZE INSULATORS When a contaminated insulator becomes wet, i t s breakdown voltage is greatly reduced. It has been found that the surface may be kept dry i f i t i s maintained at a temperature a few degrees above ambient. This can be done by coating the insulator with a conductive glaze. The current flow-ing through the glaze provides the desired heating effect. Modern insulators of this type use an antimony-doped stannic oxide glaze, and are referred to as "semiconductive glaze insulators" (or "semicons"). One such insulator has been developed by the Canadian Porcelain Company Limited in cooperation with the Hydro-Electric Power Commission of Ontario [7]. It has a deep b e l l shape which shields much of the surface from direct wetting, enabling the drying effect of the semiconductive glaze to be effective even during heavy rain. This insulator has a 60 Hz with-stand voltage of 30 kV per unit when heavily contaminated, which is approxi-mately double that of a conventional glaze insulator. The shorter strings which may thus be used allow the design of more compact transmission lines, as well as the upgrading of existing lines to higher voltage while maintain-ing adequate ground clearance. Standard glaze insulators have a 60 Hz impedance of approximately 85 MQ, which i s due to a pin to cap capacitance of about 30 pF. A b e l l -shaped, semiconductive glaze insulator has, in addition to pin to cap capacitance, a resistive path along i t s surface, and capacitance between . . the inside and outside surfaces. This distributed R-C network may be re-presented approximately by a resistor and a capacitor in parallel. 9. Since the resistance of the semiconductive glaze has a negative temperature coefficient, low resistance insulators are prone to thermal runaway. If the resistance of the glaze is too high, the desired heating effect i s lost. These considerations limit useful insulators to d.c. re-sistances between 20 and 100. Mfl.' For this range of d.c. resistances, the parallel resistance and capacitance of the lumped model l i e between 13 Mft and 60 MQ, and 110 pF and 80 pF respectively. When the insulators are connected together to form a string, capacitance between units and capacitance from each unit to ground must be considered. By measuring the voltage distribution i n a string of insulators using a calibrated sphere gap, Rau [8] has been able to develop the model .. for a six unit string.shown in Figure 3.1. R^  and represent the semicon insulators. The currents shown were obtained by computer analysis using a source voltage of 132.8 kV (230 kV line to l i n e ) , and R =35 Mfi, C = 80 pF. 10-1k I — v W 41.52 /65° 6 187.8 sin( 1 2 0 r r t ) | ( kV ) . 0.80^8° 0 . 1 8 Z 9 0 0 0..85Z98 0 * X ZZZ3 35 M. y0.02Z98° 0. 71/4° "1 C p y S O X 3.7 _ 0 ^ 5 Z 9 A : 2 A .0.0 5 Z96° • R p 35 M Q.6JZ-1 0 1 6.19Z88 0 X 4.8 0.19Z86",, 0.56 Z 8 9° J . Z T 1 . 9 8 .' i 0.06 Z9A° •Rp 3 5 M 0.58^Z-4° X 6.2 Z I 1 . 6 8 >io.06Z92° • R P "3 5 M 0.1AZ85 1 . — 6 ' 6 0.^61 Z86' 0.5AZ-7J C p j 8 0 0.12 Z84° i0.05Z91° '35 M 0 . 5 7 Z 8 3 0 C n ± 8 0 0.56^-5° "a X 8.6 currents, in mA rms . capac i tances in pF • res is tances in ohms 3 5 M 0.59 Z85° C„± 8 0 10.81 LU 2° Figure 3.1-Model For a 6-un i t Semicon S t r i n g On a 230kV Line 11. IV. TRANSFORMER - DESIGN CONSIDERATIONS In order to use the leakage current of a semiconductive glaze insulator string to power a warning beacon, some means of collecting the current and transforming i t to a usable level i s required. The low voltage end of the semicon string i s normally connected to the tower, allowing the leakage current to flow to ground. Collecting the current may be accomplish-ed by inserting a number of standard glaze insulators between the semicon string and the tower, and tapping off at the low voltage end of the semicons. The number of standard glaze insulators required is determined by the maxi-mum expected voltage at the tap-off point. A transformer was designed to convert the leakage current to a higher level at a lower voltage. 20 kV was chosen as the maximum primary voltage. This i s the normal operating voltage across each semiconductive glaze insulator, and therefore produces less than a twenty per cent disturb-ance in the voltage distribution across a six unit string. In addition, i t is high enough to allow a useful amount of power to be obtained, but not so high as to unduly complicate the transformer design. For this primary volt-age, two standard glaze insulators are required between the semicon string ... and the tower. Grain-oriented s i l i c o n steel (type M-4) was used to make the core of the transformer. This material has high permeability at high flux density in the rolling direction, as well as low loss. It has a thickness of 0.011 inches and is coated with an insulating layer of magnesium s i l i c a t e , thus allowing many laminations to be used in a core without giving rise to eddy current problems. 12. For sinusoidal excitation, the voltage-flux density relationship for a c o i l i s K i = NA co B m where V m is the peak value of the applied voltage (volts) N i s the number of turns A is the cross-sectional area of the flux path (m2) co i s the angular frequency of the applied voltage (radians/s) B m is the peak flux density (Tesla) M-4 type s i l i c o n steel enters saturation at B m ~ 1.7 Tesla. At this flux density level, loss i s approximately 0.77 W per pound of steel. For a c o i l with a core of magnetic material, A=aAj_ where A-j_ is the cross-sectional area of the core and a is the "stacking factor" which accounts for insula-tion and air space between the laminations of the core. For M-4 type steel a=0.966 when the laminations are subjected to a pressure of 10 p s i . Machine-wound toroids should approach this pressure, and since this was the type of core considered for the transformer, a value of a=0.95 was used i n the ca l -culations. Because the iron loss of the transformer i s directly proportional to the amount of steel used, i t i s desirable to minimize A^. However, i f Ai i s too small, an impractical number of turns (N) would be required. With a cross-section of one square inch, the number of primary turns should be Vm 3 N = — t 5 = 72 x 10 turns. aA^ co B m This value of N was thought to be reasonable. 13. The primary was wound with 32 AWG magnet wire, since this was about the smallest size that could be handled conveniently using the avail-able c o i l winding equipment. The insulation on this wire is rated to with-stand 800 V. In order to allow for imperfect winding, flows in the insulation, and possible overvoltages, an operating voltage of 50 V between adjacent windings (25 V per layer) was chosen. To obtain this value, the primary was wound on six two-segment bobbins. At a primary voltage of 20 kV, the voltage per turn is 0.39V. Thus to obtain 25V per layer, the layers must be composed of about 63 turns. Each of the 12 segments of the c o i l must have 6000 turns, so that approximately 95 layers are required. On the basis that 32 AWG wire could be wound 103 turns per inch, bobbins of the dimensions shown in Figure 4.1 were constructed. A slot beneath the central flange was provided so that the two segments of the c o i l could be joined at the inside of the wind-ing. The two segments were then wound in opposite directions, thus giving two coils connected in series without the insulation problems inherent i n running a lead from the inside layer of the winding to the outside. A toroidal shape for the core was chosen for several reasons. In order to take advantage of the high permeability of M-4 type steel, the magnetic flux must be aligned with the r o l l i n g direction of the material, and the reluctance of any air gaps must be minimal. A toroid wound from a continuous strip of steel f u l f i l s both of these requirements. The lower limit on the size of the core is fixed by the necessity of f i t t i n g the windings onto i t . A toroid provides the greatest enclosed area for a given amount of material, and one with an inside diameter of • seven inches was found to be large enough for the coils used. Since the density of s i l i c o n steel i s 7.65 gm/cm , the weight of the core is 6.6 pounds, 15. and thus a loss of 5.1 Watts at a flux density of 1.7 Tesla was expected. After any cold working of grain-oriented material, a stress-relief anneal i s required to restore i t s magnetic properties. Since the temperatures used in the annealing process would destroy both the c o i l forms and wire i n -sulation, i t is important that the insertion of the core into the coils involve no cold working. The stress produced in the material by winding i t into a seven inch inside diameter toroid may be calculated as follows. Con-sider a small section of the inside layer and l e t r^ = radius to inside of layer ±2 = radius.:tOL.outside of layer r l + r 2  rn = —~2 9 = the angle subtended by the section (see Figure 4.2) . Before winding, the length of the segment was r ^ 0 . After winding, the outside oftthe segment has a length of ^ 6 . s t r a i n = change in length original length r o 0 - r 6 2 n n r 6 n V r i r 2 + r l r^ = 3.5 inches r^ = 3.511 inches -3 Therefore strain = 1.569 x 10 The modulus of e l a s t i c i t y for M-4 type s i l i c o n steel in the r o l l i n g direc-6 3 tion i s 19 x 10 psi. Therefore the stress is 31.5 x 10 psi. Since the 3 yield strength is 48 x 10 psi, this stress is within the elastic limits F igu re 42 - L a m i n a t i o n St ra in C a l c u l a t i o n •17. of the material, and consequently no stress-relief anneal should be required after winding. A toroid of 7 inch inside diameter and 91 layers (i.e. 1 inch of magnetic material) was wound from a continuous strip of 1 inch wide M-4 type steel supplied by the Cor-Mag Company of Burlington, Ontario. After the toroid had been annealed, the primary c o i l forms were mounted in a c i r c l e , and the core was wound through the forms. Two secondary windings were added to the transformer after the core had been wound through the primary. These were wound using 24 AWG magnet wire. The f i r s t has 1018 turns providing a peak voltage of 400V, which can be used to charge either the discharge capacitor of the warning beacon or a high voltage battery pack. The second winding has taps at 76, 89, 102, 115, 127, 140 and 153 turns, and is used to tune the magnetizing impedance of the transformer to approximately unity power factor. Figure i: 4.3 is a photograph; of the completed core assembly. To provide sufficient dielectric strength between the various parts of the transformer, the core and coils are immersed in o i l . For this purpose a steel can surmounted by a standard 35 kV bushing was constructed. A terminal block mounted above the level of the o i l allows connections to the low-voltage windings to be made. The core is mounted in the can using "U"-shaped aluminum standoffs lined with plexiglass (these can be seen i n Figure 4.3). The plexiglass provides el e c t r i c a l isolation between the core and the can, thus reducing the effect of stray capacitance between the primary and the core. Figure 4.4 is a photograph of the completed trans-former. Figure 4.4 Photograph of Transformer. 20. Figure 4.5 shows the manner in which the transformer is connected to the line. Two strings of semiconductive glaze insulators connected in parallel are shown since this i s the configuration used in the f i n a l proto-type. Because the capacitance of standard glaze insulators is much less than that of semiconductive glaze insulators, the transformer may be subject-\ ed to transient high voltage levels i f the line is switched on at a peak of the voltage waveform. The behaviour of the transformer under these con-ditions i s d i f f i c u l t to predict, but i t s interwinding capacitance should reduce the level of the transient. Thus a worst case value of the over-voltage can be obtained by representing the transformer as an open cir c u i t under transient conditions. The resulting circuit model is shown in Figure 4.6. The voltage waveform at node H which results when the line i s switched on at peak voltage was obtained by computer analysis and is shown in Figure 4.7. The peak level of 77 kV is about 3 times the normal operating voltage, indicating that some sort of protection for the transformer is required. A spark gap is mounted on the transformer can to provide this protection. 21 -o -o 730 Legend . -semicon : i nsu la to r y/\ standard y f \ i n s u l a t o r Figure A .5-Trans former Connection To 230 kV Line 1k 1 v V 187.8 cos( ^2Cm^ (kV) ^T2.3^35M ' ^ 8 0 | 3 - 7 ±11.98 ZT1.6S Z1.H i:2.A ^ 3 5 M ' J 8 0 J 4 ^•35M ^180. J_ ^•35M T 8 0 T 35M 1 8 0 35M " ~ ^ 8 0 35M ~ ^ T 8 0 ^2.A 35 M ^ 8 0 r:i.98 35M l\l80 Z I1 .68 ^•35 M 1.H node H r: 31 r 8 0 35M ~ ~ ^ 8 0 35M " ~ ^ 8 0 c a p a c i t a n c e s in p F r e s i s t a n c e s in o h m s 1 . X "1 X 4.8 1 X 6.2 1 X 6.6 Figure 4 .6-Mode l For Transient Analys is of the Circuit of F igure 4.5 ro Vol tage at node H (kV) 0.0 0.1 0.2 0.3 OA Time (JJS) Figure 47-Trans ient Voltage at Node H in Figure 4.6 24. V. TRANSFORMER - MODEL In order to predict the performance of the transformer, a series of measurements were made to determine a circuit model for i t . The model .. is applicable in the steady state when the voltages are below the saturation level of the transformer. Standard short circuit tests gave values for the total copper loss and leakage inductance. The leakage inductance was assumed to be .• evenly divided between the two windings, while the copper loss was s p l i t in proportion to their d.c. resistances. Because of the low core loss and high magnetizing inductance, stray capacitance has a significant influence on the open cir c u i t input impedance of the transformer. In addition to the interwinding capacitance, capacitances between the primary and core, primary and can, and core and can must be considered. Under the assumption of perfect coupling and neg-l i g i b l e core loss, i t can be shown (Appendix A) that the effective steady state capacitance is c „ = c . + C ^ eff t 3 C 4(1+ ~ ) C 0 where is the total interwinding capacitance CQ is the capacitance from primary to core is the capacitance from core to can is the capacitance from primary to can Since the secondaries operate at a potential close to ground, the capacit-ance between the core and the secondaries i s included in C^. By measuring 25. the interwinding capacitance of a two-segment c o i l like those used to form the primary of the transformer, C was found to be 42 pF. The other stray capacitances were measured with a capacitance meter. The results are shown in Figure 5.1. Using these values (CQ= 134 pF, (^ =109 pF, C2=98 pF) gives C ££=101 pF. ef f Standard open cir c u i t tests were used to determine the magnetiz-ing impedance. It can be shown (Appendix B) that 2u)2C f l Z 2 ± / - [Wc f 2R 2Z 4- 4Z 2R 2 (to2Z - :to2R2 + c^Z 2C 2 R2) ] L _ ef f m eff m m m ef f m m 2 [co2Z2 - io2R2 + c o ^ C 2 R2 ] m eff m where is the magnetizing inductance (H) Rm i s the real part of the magnetizing impedance (ft) Z i s the magnitude of the magnetizing impedance (ft) to i s the angular frequency of the applied voltage (rad./s ) V 2 R i n Rm " P. m V. i n where V. is the applied voltage in I. is the input current in P^n is the input power Figure 5.2 shows the complete transformer model. This model was used in conjunction with the insulator string model of Figure 3.1 to predict the power that could be obtained from a 230 kV line into a resistive load (R^). Figure 5.3 is a graph of the power output versus load resistance obtained by computer analysis. The maximum value of approximately 5.8 W is / / / / / • / / / __ / r / . / -can-13ApF Av\ /Core • • C g M l / secondaries CIZ l09pF Figure 5.1 -St ray C a p a c i t a n ce i n t he Transformer o-R1 A / V 1 0 ka 190 k / l 124 kH Figure 5.2-Transformer Model ^4 28. Power (W) 5.74 5.72 5.70 5.68 -I :—|- 1 r-J i I-22 24 -i J_ 26 28 30 32 Figure 5.3 - Power Delivered To a Resistive Load Using a Single Semicon String 29, insufficient to supply a warning beacon. To obtain more power, two semi-conductive glaze insulator strings may be connected in parallel between the line and the transformer primary. Figure 5.4 shows the power obtained in this case, and Figure 5.5 shows the current distribution for 11^ =22 Mft. The available power of approximately 18.5 W Is sufficient to run a beacon of the type used on aircraft. 30. Figure 5.4- Power Delivered To a Resistive Load Using Two Semicon Strings 1 k I —w -0.18/90 + 2.8/65 187.8 cos( 120rrt)" 0 0.73/9.2° 35 M 0.77/99 4-0.02/99 0.63/3.7 ~ 2 A >35M 10.0 4/97° ^  XT 80 0.54/-2.6 -< yl98 4-0.05/94° 0.67/94 — » 1 j 8 0 057/87 35M j^T30 0.48/-8.5 y 1.68 I 0.05/91° ^35M 0.51/81' 0.44/-16 =1.14 10.04/89° ^35M 80 0.45/-13 0.46/74 3«o 35M c a p a c H a n c e s in p F r e s i s t a n c e s in ohms c u r r e n t s in mA rmS 0.48/77 jO.12/106 = 31 = 31 4-0.18/90* ^-0.19/88° Z4.8 X ll X 0.20/86 6.2 0.16/ 86 "1- 0.16/88° j -8.6 0.73/9.2° 0.77/99° I ^ 1 »1 -^ 37 Z:2.3^ 35M y 8 0 "]* ' 4-0.02/99 0.63/3.7° y2.4 >^35M 0.04/97' o.^/ss^ 67/94° * I -r80 T 0.54 Z-2.6 = 1.98 |o.05/94° >35M 0.20/86' 4.8 0.48/-8.5 _1.68 j 0.05/91° ^>35M = 1.14 10.04/89° 3^5 0.57/87° — h >• X 0.16/86°~ 0.51/81° ^ — ^ a s _ _ i 8 0 i 0.16/88° * 0.4 4/-16 | 0.46/74 >35M =80 0.45/-13 1I a6 35M 0.48/77 80 10k 1.3/29 900 H 900 H 190k 0.92/14° 0.33/14° 4-0.77/104° 4-0.44/-76 i " 1 »124kH >22M Figure 5.5-Current Distribution For R = 22 M i l 32. VI. BEACON POWER:SUPPLY AND CONTROL CIRCUITS 6.1 Beacon Power Supply Circuit As mentioned above, the secondary of the transformer can be used to charge either the discharge capacitor of the warning beacon or a high voltage battery pack. A battery pack not only provides standby power in the event of a line outage, but also increases the amount of power which can be extracted from the line. This is because the combination of line and semicon insulator string approximates a constant current source (due to the high. :\impedance of the insulators). Since the average voltage on the discharge capacitor i s lower than the voltage of an appropriate battery pack, more power w i l l be delivered to the pack than to the capacitor. For these two reasons only the battery pack power supply w i l l be considered ,: here. Figure 6.1.1 shows the power supply c i r c u i t . A full-wave bridge r e c t i f i e r is used to charge the battery pack which i s made up of 16 12V sealed lead acid ("gel cell") batteries. These batteries have the advan-tages of standard lead acid batteries without the attendant maintenance problems. Inductor L^ increases the conduction angle of the r e c t i f i e r . L 2 is used to tune the magnetizing impedance of the transformer to approx-imately unity power factor. The zener diodes prevent overcharging of the batteries. 6.2 Single Flashtube Control Circuit Figure 6.2.1 shows the control circuit used with a single flash-:"-, tube. Programmable unijunction transistor Q-^  i s used to form a relaxation CO CO J0\ HI v V — i 0.1 220 6A fuse SCR, 3.3 11mH 168V — T, Cj^60 )V, v. '3.9M 24V-E-I r •M-•220 k ->>100k b.7k ^150 k \A'7Jix'W> 0.22 —I 150 k • 1M 0.022 ^47k •w-220 k TT \ •10 k Q 3 i S39 S « 3 mH 220 k SCR 0.22 2^Z ^220k >47k Z X >3.3 capacitances in pF resistances in ohms Figure 6.2.1-Single Flashtube Control Circui t •35. oscillator with a period adjustable around the nominal value of 1 second. When fi r e s , SCR^ and the monostable (C^J 1^) a r e triggered. Figure 6.2.2 shows the resulting waveforms. When the monostable resets f i r e s , trigger-ing SCR^ which fires the flashtube. It can be shown (Appendix C) that • .v , ' V B . a/B t a n _ 1 ( - f ) x) peak current x = — e a P 3LX air i i ) f i n a l capacitor voltage v c^ = [e + 1] i i i ) energy delivered by battery E = C V £ v c p / 1 2 R where 8 = / -—— - a z , a = - —rz— and R i s the total resistance of L]_C zL^ the charging circuit (i.e. battery output resistance, series resistances of L-^  and C) . The energy lost in charging capacitor C i s E T = E - i C,v2  L cp E T = C v [ V n - i v ] L cp B cp This compares favourably with a linear c i r c u i t in which the energy lost is i C v 2^ and the fi n a l capacitor voltage i s v C p = V B, requiring a higher voltage battery. The use of an SCR allows charging the capacitor immediately before the flashtube is triggered, which is advantageous because the capacitors desgined for use with flashtubes are quite lossy. The snubber circuit ( R l ' C l ) P r e v e n t s triggering of SCR-^  when the flashtube-discharges dapacitor C. 36. 37. 6.3 Control Circuit for Three Flashtubes The Ministry of Transport recommendations on warning beacons c a l l for three lights flashing i n sequence. Two approaches to extending the single flashtube control circuit to a circuit for three flashtubes are given below. The f i r s t has the advantage of redundancy, while the second minimizes the parts count. Redundancy i n a warning beacon system is advantageous when main-tenance and monitoring of the system's performance i s d i f f i c u l t , as i s the case when the beacon i s located in a remote area. Three complete single flashtube units, one supplied by each phase of the transmission line, pro---vide maximum redundancy. In the event of a failure in one of the units, or an outage on a single phase of the line, two flashtubes remain operating. To synchronize the three tubes so that they flash in accordance with the MOT regulations, a central control circuit i s coupled to the relaxation oscillators of each of the single flashtube control circuits. These relaxa-tion oscillators are set for a period somewhat longer than 1 second, and a trigger signal i s capacitively coupled to the gate of the PUT (Q^ i n Figure 6.2). This trigger signal forces the PUT to fi r e earlier than normal. If the central control circuit f a i l s , each flashtube reverts to single unit operation. Figure 6.3.1 shows the central control c i r c u i t . To minimize the parts count of the beacon system, a l l three flash-tubes can be powered by a single battery pack. The battery pack can be charged by a single transformer fed by six semicon insulator strings in parallel on a single phase, or by three transformers (one on each phase) 100 Hz Oscillator •r10 counter (CD4017) Cl ca r ry 0 1 2 3 4 5 6 7 8 9 •^ 10 counter (CD4017) . Cl 0 1 2 3 4 5 6 7 8 9 L ^ 7 to gate of Ql for middle l amp to gate of QL| for top lamp to gate of Q for bottom lamp Figure 6 .3 .1-Synchron iz ing S igna l Generator For Three Single F lash tube Uni ts CO 00 and a three phase rectifier. Figure 6.3.2 shows the required control circuit. Its operation is similar to that of the two described above. 180 V ^ _ Figure 6.3.2-Three Lamp Control Circuit 41. VII. OBSERVED BEACON PERFORMANCE A series of tests of a single flashtube warning beacon prototype have been carried out at the Research and Development Laboratory of the British Columbia Hydro and Power Authority. These tests used a step-up transformer providing 80 kV as a power source. This is the line to ground voltage of a 138 kV line. A suitable semicon insulator string for use at this voltage consists of four units. Since the voltage across the primary of the beacon system step-down transformer is approximately equal to the voltage across each unit of the insulator string, the string can be reduced to three units when connected to the transformer without increasing the volt-age across the individual units. Alternatively, the f u l l four-unit string may be retained, providing f u l l insulation strength in the event of a trans-former failure. Table I summarizes the power available from single and double strings of three and four units. The differences between predicted and observed power for four-unit strings may be explained by the fact that the predictions were made using a higher line voltage and six-unit strings and a resistive load instead of a rectifier and battery load. The prototype beacon consumes 10.2 J of energy per flash, and thus requires two insulator strings in parallel to remain operating. To observe the charge characteristics of the battery pack in this system, a ten percent charge was established on the batteries, and the system was connected to two four-unit semicon strings in parallel. With the flashtube operating, the batteries were observed to charge over a peri-od of time. Table II and Figure 7.1 show the results. 42. Table I - Available Power Semicon Insulator Configuration Transformer Primary Voltage (kV) Direct Current Flowing Into A 200V Battery (mA) Power ( w ) Predicted Available Power (W) 1 4-unit string 14 23 4.6 5.8 2 4-unit strings in parallel 22 76 15 18.5 1 3-unit string - 31.5 6.3 .. 2 3 -uni t s trings in parallel 25 129 26 Table II - Battery Charge Elapsed Time(Hr:Min) Transformer Primary Voltage (kV) Charging-Current(mA) Battery Voltage(V) 0:0 20.5 84.5 196.5 22:35 22.6 84.0 208.8 27.50 - - 209.5 30:10 22.3 82.0 209.1 56:57 23.0 81.8 214 76:30 23.0 82 218 102:00 23.0 80 220 172:40 23.0 79 221 199:10 23.0 79 221 222:00 23.0 79 221 43. Battery Voltage] (V) 2201 i 1 1 — — i r T 1 — — i 1 1 — ~ T 215 210 205 200 195 -4 1 L -1 I , L J L__ I 0 AO 80 120 160 200 240 C h a r g e Time (hrs.) F igure 7.1- Battery Pack Charge Characteristic 44. VIII. CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK It has been demonstrated that sufficient power can be obtained from the high voltage line through semiconductive glaze insulators to operate a xenon flashtube warning beacon. Whether the light output of the beacon is sufficient to satisfy the Ministry of Transport has yet to be determined. Performance of the system on a line (as opposed to i n the labora-tory) needs to be checked. In addition, tests at low temperatures should be carried out, since the leakage current of the insulators w i l l be lower at reduced temperatures. The effectiveness of charging current supplied to the batteries w i l l also be less at lower temperatures. Consideration should be given to increasing the value of the i n -ductor in series with the battery pack (L^ in Figure 6.2) to a point where i t would prevent discharge of the battery through the flashtube i n the event of false triggering. This would increase the r e l i a b i l i t y of the sys tem. APPENDIX A - STRAY CAPACITANCE IN THE TRANSFORMER Let C = capacitance/radian from c o i l to core V C = core voltage C^ = capacxtance from core to ground C c = capacitance/radian from c o i l to ground C = t o t a l interwinding capacitance L = inductance/radian of c o i l (see Figure A.l) d i d0 = difference i n current i n 2 parts of the c o i l d0 apart = jwc(v(e) - v c ) + jo)Ccv(e) ju)Ciy 0 = (•2T7 0 [jcoc(v(e,) - v Q ) ] de i . e . sum of currents.in C flows through Cn V c - ~ , f 2TT (v(e) - v )de o c v(e) de - 2TTV ; ] o c [2.TT V = c v(e)de C ^ T T C v(e)de assuming perfect coupling de d 2 i Figure A.1 - Stray Capac i tances 47. 2TT = - O J 2 (C + C J L ' 0 ' i(e)de 0 = -co 2 (C + C ) LA c where A = 1 (6 )d9 , a constant J 0 „dl 2 d0 d l d0 d l de = - w z L ( C + C r ) A 6 + const = - j u ) C V c + jcoV(O) (C + G_) 6=0 -co 2 L ( C + C C )A6 - jcoCV c 1(6) = -co 2 L ( C + C C ) A 2" ~ jooCV c + B B a constant I (2TT) = I m + B = I m + co 2 L(C + C c ) A - ^ - 2 + JCOCVc2TT 1(6) = I m + ^C+CC^ [(2T T ) 2 - e 2 l + j<oCV c[2^e] 277 A = i(e)de J 0 o -r co 2 L(C+Cp)A;8 'Tf 3 , . _ T 0 2 A = 2.-rrIm + |p + jcoCVc2Tf^ = 2TfLm+jcoCVc2-rr2 i r 3 w 2 L ( C + C c ) 1(6) = l m [ 1+ ^ 2 U C + C C ) (4TT 2 -e 2 ) . + [ 2 I T _ E + A 2 L ( C + C c ) ( 4 T r 2 - 6 2 ) j 1- | TT 3 W 2L(C+C C) 1- j TT 3 0) 2L(C+C C) V(6) - j co(C+C c ) = d * ( 9 ) + jcoCV c 48. (6) jto(C+C ^ y - [[-^L(C +C C)9] >yJ"CV c2^ _ +  C + C ^ 1. | ^ 2L(C+C P) CVr C+Cc (6) 2Trea)2LC 1- -j ir 3co 2L(C+C c) [ . M + TTVC] V c = CX+2TTG J o 2 TT v ( e ) d e v = c 4T71+U)2LC2 (C^TrOtl- |ir3di2L(C+Cc) ] 4TT 3OJ 2LC 2 . I ( C ^ C ) [1- |rr 3a) 2L(e+C c) ] j a ) C 4TT3CJLCI m ( C ^ i r C ) [1- |ir&o 2L(C+C c)] + 4iTltu>2LC2 V(:6) j ^ , 8 : 2Treco2LC 1- f r r 3 w 2L(C+C c) 4TT1+ULC CG2.+2TTC) [1- |ir 3a) 2L(C+C G) ] + 4 i r l t ( j j 2 L C 2 V(2TT) i n Z. = j 4TT2IO2LC i n " 1- Jrr 3u> 2L(C+C c) ( C ^ i r C ) [1- -|rr3co2L(C+Cc) ] + 4 7 r ^ 2 L C 2 - 4 i T 1 + a ) 2 L C 2 uCtCC^irC) [ 1 - -|rr3a)2L(C+Cc) ]+4T7 l fa) 2LC 2] m J 4ir2a)L(C1+2TrC) (C-L+21TC) [ 1 - | r r 3 c o 2L(C+C C) I+AT T WLC 2 L 0 = 4TT2L C Q = 2TTC C 2 = 2trCc u)L, 0 Z i r i J o j2 L n(c n+t2) , ^ 2LnC 0 2 3 4<Ci+e 0) z. = in xn 1+jcoC Z. J t in i in JwL0 l-w2L, 0 C +-t c 0 +c 2 Co 4(1+^- )) o that the effective open circuit input capacitance is = r + e0 + C2 c0 eff ^t 3 Ci 4(1+ — ) c0 APPENDIX B - MAGNETIZING IMPEDANCE OF THE TRANSFORMER Figure B.l shows a circuit maodel of the magnetizing impedance the transformer. Z. =f ~~ + jtoC „ + - 7 - r I in L Rm eff jcoLm J = T J^m - ^ 2LmCe££Rm + % 1 1 I jwL mR m J A „ Z 2 S | Z . | 2 = in (R m-a) 2L mC e f fR M F+co 2L 2 »2I2Z2-.+ .Z2[.R2 - 2co 2L mC e f fR m + C ^ R 2 ] - co^R 2 L m [u,2Z2 - co 2^ + ^Z 2C^ f fR m ] - L m(2co 2C e f f R mZ 2) + .Z*R* = 0 2 a i 2 C e f f R m Z 2 ± ^ A e f f l f c 1 ' - *Z24 (co 2Z 2-a 1 2 4 +^Z 2c; f £R f f l)  m — 12 72_,„2 R 2 . ;,4 72r 2 p 2 i 51. Figure B.1 - Transformer Magnet i z ing Impedance ~ • 52. APPENDIX C - CONTROL CIRCUIT WAVEFORMS The cir c u i t which charges the discharge capacitor may be repre-sented by an underdamped RLC circuit with a step input (see Figure C.l). The differential equation for the current i s : d 2 i , R di , 1 + -^~ i F + - L T C - 1 = 0 solving: dt 2 ^1 U L 2 _J_ R _L. 1 r> r + " L l r + "Lie = ° R / , R c-2 1 2L-, ~ v 2Li ' LC i - A R a A / 1 2 l e t a ~ 2 L ] ~ ' ' LVC- " -° then r = ai± ig ott i = e [A cosgt + B sinf3t] = 0 A = 0 t=0 rr- = ae a t B singt + ge a tB:cosgt dt V 3L^ V B at . Dt_ l = -77— e smBt 53.. 'B 0 R L1 6 FigureC.1 - D ischarge C a p a c i t o r Charg ing Circui t 54. the peak current occurs when d ^ = 0 at i.e. when e (asingt + gcosgt) = 0 tangt = t - 4 - t a n " 1 ( - -§-) a P 6L X at t = — r — , i becomes negative. In the actual circuit (Figure 6.2) this i s p prevented by SCR.. , so that i remains at zero for t > TT idt at . „ , e smgtdt v = c BCL VB . e a t(asinBt - gcosgt)j + c o n s t < [ g ] a 2 + B 2 at t = 0, v c= 0 -»• cons t. = 1 a t v c = V B [ — - e (asingt-gcosgt)+l] at t = air vc = vc p = V B te B + H the energy delivered by the battery: '.fir E = « V B l d t 0 B a t e s i n S t d t a t e ( a s i n g t - BcosS t ) a 2 + B 2 air C V B [e p + 1] CV vr 56. REFERENCES 11 Canadian Ministry of Transport, Standards Obstruction Markings, Amend-ment No. 2 (June 6, 1976), p.22. 2. Dana, Homer J., "An Aviation Hazard Light for Mid-Span Operation on Power Transmission Lines", AIEE Transactions on Power Apparatus and  Systems, (December 1960), p. 911. 3. Berthiaume, R.; Blais,R., Dery M., Tapping the Overhead Wire on Trans- mission Lines Produces a 20 kW, 60 Hz Power Supply, Canadian Electrical Association, (March 1977). 4. Berthiaume, R., Study on the Fixed Light Beacon and Its Associate  Supply, Canadian Ele c t r i c a l Association, (September 1977). 5. Amglo Corporation, Flashtube Engineering Manual, Amglo Corporation, Rosemont, Illinois', (1974). 6. "IES Guide for Calculating the Effective Intensity of Flashing Lights", Illuminating Engineering, Volume LIX, No. 11 (November 1964), p. 747. 7. Nigol, 0.; Reichman, J.; Rosenblatt, G., "Development of New Semi-conductive Glaze Insulators", IEEE Transactions on Power Apparatus  and Systems, Volume 93 (March/April- 1974), p. 614. 8. Rau, N.S., personal communication. 9. Spiegel, M.R., Mathematical Handbook of Formulas and Tables, Schaum's Outline Series, McGraw-Hill (1968), p. 85. 

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