X International Symposium on Lightning Protection th
th
9 -13 November, 2009 – Curitiba, Brazil
DEVELOPMENT OF AVOLTAGE IMPULSE GENERATOR MODEL AND A BUILDING MOCKUP WITH A LPS FOR INSTRUCTIONAL PURPOSES Hélio E. Sueta1, Miltom Shigihara2, Luis E. Caires3, José Aquiles B. Grimoni4, Clóvis Y. Kodaira5 1,2,3,4,5
Institute of Electrotechnics and Energy of USP, Brazil –
[email protected]
[email protected];
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[email protected] Abstract – The present work shows a development of a educational kit composed by a high voltage impulse generator (Marx Generator type) in reduced size model and a mockup simulating a building. This small structure has also a Lightning Protection System in the reduced size model. This educational kit is used to simulate the lightning discharge in a building.
1 INTRODUCTION This work presents the development of a simplified reduced scale model of a voltage impulse generator for educational purposes to be used in a mockup building with lightning protection system (LPS). It was not considered in this work any technical characteristics concerning the adjust of the electrical parameters of the voltage impulse generator, such as the front time and half tail time, energy, voltage peak, wave velocity in the discharge channel, distance between the cloud (generator) to the ground and channel current. So the results do not validate any inference of the experiment results related to the actual phenomena. Then, this experiment was conducted exclusively with the aim of showing visually the lightning phenomena on the air termination system of the mockup, in assistance of broadcasting the subject “Structures Lightning Protection” and increasing students’ motivation on a such important matter. 2 DEVELOPMENT OF THE HIGH VOLTAGE GENERATOR A high voltage generator can be assembled in several ways with several devices described according to this topic. Probably the simplest high voltage generator is composed by a double voltage circuit assembled with diodes and capacitors. Figure 1 illustrates a typical circuit of this kind generator:
Fig. 1 – Typical circuit example of the voltage multiplier circuit composed by capacitors and diodes. This circuit suits a quadruplier voltage.
In the Figure 1, V1 is an alternate voltage source, C1 to C4 are capacitors and D1 to D4 are diodes. The output voltage (between points 1 and 2) can reach some kilovolts and it depends on the input voltage V1. The output voltage is continuous and it is limited to the characteristics of circuit components such as capacitors and diodes dielectric breakpoints. The double voltage circuit is known as peak to peak detector. Another type of generator can be assembled with an oscillator circuit connected to a step up transformer such as flyback or spark coil (used in car ignition). The transistors Q1 and Q2 illustrate the simplified way in Figure 2 that oscillate by voltage pulses transmitted to the low voltage LV side and it produces a high voltage to the HV side (between the points 1 and 2). Whereas the use of integrated circuit (IC) with oscillation function in replacement of transistors is more appropriate. So commonly, the 555 IC is used because it is easier to set the functions than in transistors.
Generator. An another kind of high voltage generator is a Tesla Coil and its circuit is illustrated in Figure 5. These two circuits are used to rise the output voltage of the flyback to dozen kilovolts values, sometimes it can reach hundreds kilovolts when using a Tesla coil. The output voltage limitation of these circuits depends on devices such as resistors and capacitors because these devices have to withstand few kilovolts.
Fig. 2 – Typical circuit example using two transistors.
The fundamental 555 chip operation and its application are verified in [1]. The 555 IC depending on the configuration can behave as monostable, bistable and astable multivibrators and its depends on the configuration. The pulses is generated by 555 chip in the astable multivibrator mode with regular and defined frequency, i.e., the chip operate as oscillator. In the monostable configuration the 555 IC operate as a timer and in the bistable configuration operate as switcher. The circuit composed by oscillator (based on 555 IC) illustrated in Figure3 and the flyback or spark coil also are used to generate high voltage with some kilovolts peak pulses waveform.
Fig. 4 – Example of a high voltage circuit generator Marx Generator type.
Fig. 5 – Example of a high voltage circuit generator Tesla Coil type.
Fig – Typical example of oscillator with 555 chip.
In the circuit of Figure 3 the pulses frequency is adjusted by R2, R3 and C1, the transistor Q has a switch function and its apply pulses to the low side LV of TR transformer. The high voltage pulses are generated in the 1 and 2 points of the set up voltage transformer TR in the HV side. This circuit based on oscillator and flyback can be used to simulate a spark gap such as lightning discharge whereas the voltage peak is less than 20 kV, so the breakdown distance between the eletrodes reaches approximately 2 cm since the air dielectric breakdown is possible at a 1 mm/kV rate. However this difficulty is solved by using one more stage after the flyback stage using a Marx generator. It is commonly used in the high voltage laboratories. The Figure 4 shows an example of the circuit of Marx
The Tesla coil works by charging capacitor C1 followed by a discharge in the gap with the air breakdown. When the air breakdown occurs the energy of the capacitor is transferred from the primary LV side of the coil TR to the secondary HV side. The TR coil must have a low magnetic coupling and resonance between the primary and secondary sides. The input charge of the C1 can be made by a TX1 neon light transformer that generates approximately 5 kV with 30 mA current. The generator Marx Generator type is formed by a set of electrical resistances, capacitors and gaps. It is named “one stage” the assembled group formed by: one capacitor, two electrical resistances and one gap. The operation is based on the fact that of having homogeneously charged all the parallel connected capacitors and ideally discharging, via the gaps, all the capacitors then series connected. The output voltage of the electric air breakdown, in this case, is the sum of the voltage of each stage. Thus, if we have, e.g. ten stages, being each stage charged with 4 kV, the sum of the output voltage will be of 40 kV. This value is enough to breakdown the dielectric air to a distance of 4 cm. The distance between the electrodes of the air gap must be in set such way that it does not compromise the functioning of the capacitors and electrical resistances.
For the present project, some premises had been taken in consideration: cost, simplicity and availability of components. The developed voltage impulses generator uses an electronic circuit to generate the pulses, in integrated circuit 555 and a switching stage is applied to one flyback, this device is commonly and widely used in PC monitors. In this aspect, the idea of reusing resources was one of the basis of the present project. With this assembling it was possible to generate high voltages that were connected to a capacitors cascade circuit, in a similar form than those high voltage impulse generators used in high voltage laboratories. This assembling generates dozen “kilovolts” and, in such a way, was possible to get an electric arc in order of 8 centimeters, simulating a lightning discharge in reduced size model. The Figure 6 shows the prototype of the mounted set.
LPS (Lightning Protection System) designed with the Mesh Method as air terminations systems and the external down-conductors systems was formed by copper wires, the Grounding system was made of an aluminum leaf installed in the base of the mockup. The model of the LPS was made in accordance with the recommendations of the Brazilian ABNT standard for lightning protection of structures, the NBR5419:2005 [2]. All the LPS were mounted in reduced size. Figure 7 shows the mockup, Figure 8 shows details of the air termination system and one of the down-conductors system in reduced scale and Figure 9 shows the partial vision of the bottom of the mockup corresponding to the Grounding system.
Fig. 7 – Illustration of the upper view of the mockup.
AIR TERMINATION SYSTEM
DOWN-CONDUCTOR SYSTEM
Fig. 6- Prototype of high voltage impulse generator in reduced model.
Concerning real scale generators, in the High-Voltage Laboratory of the Institute of Electrotechnics and Energy of the USP there is a generator that supplies high voltage impulses about 1.6 MV (voltage peak), with possibilities of adjusting of the front time, as well the tail time, beyond other parameters. 3 THE LPS IN THE BUILDING MOCKUP The mockup used was the one developed and conceived for the future amphitheater of the IEE/USP. It consists, basically, in a construction relatively small and low with a
Fig. 8 – Details of the air termination system and downconductor system.
With this reduced size assembly, with generation of electric arcs, it was possible to awake the interest of graduation students and technician courses for the lightning protection of structures matter. 5 CONCLUSIONS
Fig. 9- Grounding system.
4 THE EXPERIMENT Completed the generator circuit assembling, having the mockup set with its LPS it was possible to visually carry through simulations of the attachment point by locating the generator output conductor (electrode) (simulating one of the tips of the discharge for the cloud) in points next to the structure and verifying if the arc is closed in one of the components of the LPS or in some point of the structure not protected for the LPS. With this mounted set, as it illustrates Figure 10, it is possible to obtain repetitive voltage impulses of approximately 80 kV, that in air it corresponds approximately to a dielectric breakdown with electric arcs of 8 centimeters of length.
We think that next steps for increasing the quality of the experiments and allowing students to explore more on the concepts of building’s LPS and grounding systems are: - assembling scale different models of grounding grids using real soil with different resistivity; - designing and assembling a new source (generator) that could allow controlling the waveform (shapes, level and times), the energy and the frequency; - use materials in the mockup simulating a building, with the similar electrical characteristics that the material used in real buildings; - development of a system allowing measurement and visualization of the waves generated by the system. Acknowledgments For their help while developing the experiment, we would like to express our thanks to the students of the “Escola Politécnica da USP”: Derik Ogata, Eric Omine, Guilherme Campos and Helder dos Santos. We would also like to express our thanks to MSc. Ricardo Santos d’Ávila. 6 REFERENCES [1] A. S. Sedra and K. C. Smith, Microelectronics Circuits, Oxford University Press, 1997.
[2] ABNT – Associação Brasileira de Normas Técnicas, “ABNT NBR 5419 – Proteção de estruturas contra descargas atmosféricas”, 2005, in portuguese.
Fig. 10 – Complete prototype of the set.
