Using Acoustic Emissions for Corona Detection

Transcription

1 Using Acoustic Emissions for Corona Detection MARCO ANTÔNIO MARTINS RENNO Transmineira Power Transmission Co. Av. do Contorno, 7962 Belo Horizonte MG BRAZIL marco.renno@transmineira.com.br ERIK L. BONALDI, LEVY ELY L. OLIVEIRA, JONAS G. BROGES DA SILVA, GERMANO LAMBERT-TORRES PS Solutions Rua Cel. Francisco Braz, 185 Itajuba MG BRAZIL {erik, levy, jonas, germano}@pssolucoes.com.br Abstract: - Transmission lines are projected so that the loss by corona effect is small enough in normal weather, because the loss by corona effect reduces the efficiency of the lines. The flashover voltage critical is approximately 10% higher than the operating voltage. It is acceptable, even though some loss by corona effect occurs under bad climatic conditions. Typically, the loss by corona effect, under bad conditions of climate, will be 10 times greater than under acceptable conditions of climate. This article presents the development of a system (hardware and software) for the early detection of corona in insulators chains of transmission lines or substations. Key-Words: - Corona effects, Transmission lines, Measuring system, Signal processing, Acoustic emissions. 1 Introduction Electrical discharges produced by corona effect can take different forms, depending on the specific conditions of operation, as the polarity of the field and the geometric configurations of the electrode. In the schematic configuration of needle-type dish, positive bias causes field due to corona discharges begin silently and in the form of pulses, then proceed to a discharge in the form of plume and will evolve in such a way to achieve the discharge by sparks, and could even reach discharges by arcing in the most extreme cases. For a negative field and the same geometric schematic, the discharge electrodes evolve differently with the increase of the field. The corona can be detected by a number of methods. The simplest methods are by sight and sound. When the corona effect occurs, produces visible light and, at times, is seen as a bluish glow around the electrodes. But, as the corona emits very low radiation (mainly in the ultraviolet band), it is difficult to detect the effect of corona by vision. The corona can only be seen to the naked eye in darkness almost absolute. The other method of detection of corona effect is by the sounds that are produced [1]. The audible noise produced by corona effect can be described as popping or hissing sounds caused by the expanding gas explosion. This noise can often be heard by the human ear. High voltage insulators are usually fully closed [2]. When the transmission line is energized it is not possible to have access to their constituent components, due to the high degree of dangerousness that this row represents in its vicinity and by the need to maintain in operation the components of this line [3]. For this special condition, one should throw hand of noninvasive equipment to perform the inspection and make its possible effect corona maintenance. There are 3 forms of possible maintenance: Corrective maintenance: it is the one that maintains the equipment or production unit, i.e., it is concerned about the fact that the services are provided in the shortest possible time in order to allow for the immediate resumption of operations, within the required levels of quality and safety. The intervention occurs with the equipment stopped, after the occurrence of a failure. Preventive maintenance: it is the one that supports the corrective maintenance, through application of methodology, or yet, is responsible for the intervention that may interrupt or not the production of a programmed way. The intervention occurs on a ISBN:

2 scheduled basis based on some criterion of use or time. Predictive maintenance: is conceptualized as the which has the purpose to monitor the operating parameters of the equipment and provide for its flaws, for intervention at the appropriate time. It can also be considered as an evolution of preventive, examined in a comprehensive way. The speech is scheduled on the basis of the assessment of some parameter indicative of the condition of the equipment. The system presented in this article project is inserted in the context of predictive maintenance. This type of maintenance makes interference at the right time is the one that tends to settle today, in most industries that have maintenance department developed enough to keep the production volume within sufficient parameters for the company to generate profits [4]. The predictive maintenance program should be established with extreme attention and need to have the correct information hand: the operation of equipment, the environmental conditions in which they work and the aging process of each component etc. It is still important to know how the machine or equipment, that if you want to keep, you can suffer damage, namely their failure modes. With the above data, in hand, you can check what parameters or variables those are of interest to predictive maintenance. There are a few methods available in the market for detection of corona effect in electrical insulators. Among them the detection of current pulses corona effect can be accomplished by means of electrical measuring instruments. This method in high-voltage power lines is not ideal due to the need for proximity detection equipment and the insulator (complex installation and maintenance due to the high level of danger and the influence of the magnetic field generated by the transmission line detection system). Therefore, the methodology to be used in the project will be the distance ultrasonic inspection. 2 Types of Corona Discharges Effect of corona discharge with negative polarization propagates with the ionization of gas that occurs via collisions between molecules, in a sequential manner. Already in positive polarization, it depends most of the photoionisation for their propagation [5, 6]. Discharges can be classified in four different ways: Silent Discharge (burst pulse corona): as its name indicates, it does not produce any kind of noise or luminescence. The ionized region is electrically charged and the electrode repels the ions of the same electrical signal, producing so called "electric wind". It occurs in regions where the electric field takes values high. The discharge current is there, but its intensity is very small. However, if the electrode surface features small curvature, the electric field is uniform and the surface density of load increases considerably, as well as the intensity of the electric field. In these places the ionization intensifies, making the electric wind more intense. If ionization is even higher, the gas begins to illuminate and in spots where the radius of curvature is very small, appears brightness (corona). The area adjacent to the gas lit surface layer is called corona effect, the rest of the region of discharge is called black current region. Radiant (or Plume) Discharge: If the tension of the electrodes increase, the corona effect takes the form of a luminescent plume, in the form of intermittent radial beams. Not enough to establish a contact between the electrodes. Discharges by sparks: further increasing the tension between the electrodes, it produces sudden considerable gas ionization, due to the creation of channels for driving. In these channels of ionization, the electric current is a much lower resistance to its passage in the previous cases. In this way, the intensity of the discharge current in these channels is quite high. The sudden current in these channels produces the sudden expulsion of gas generating a shock wave, which is perceived by the observer by the characteristic noise that sparks produce. The process also generates a substantial brightness in the discharge channel, which is pierced through the gas by the flow of charged particles; however, the trail travelled is highly unstable due to the multiple collisions of particles that constantly change position, thus producing a sinuous channel arborescent form. The passage of the spark produces an electric potential (potential disruptive). In the air, when the intensity of the electric field reaches values of 30 kv/cm (under normal pressure and with 20 mm diameter electrodes), spark is produced. The pressure of the gas and the shape of the electrodes influence notably on the values of the disruptive potential, this is the so-called effect of tips, resulting in the decrease of the values of the disruptive potential. For a certain distance between electrodes, with the gas to 1 atm, the strain under which produce the spark and corona effects are different, the first being greater than the second. However, the breakdown voltage, the disruptive potential is much more sensitive to decrease the distance between the electrodes that the voltage to the corona effect. In ISBN:

3 this way it is possible to find a critical distance such that, for a distance between the electrodes is smaller than the critical distance, no longer is possible the existence of the corona effect and only produces the spark discharge. Driving and the contact between the electrodes are established in this case. Arc Discharge: In the arc, the intensity of electric current is too high; however, the voltage between the electrodes is small. The temperature of the gas is too high and the electrodes heat considerably. In arc, the particles are accelerated to high speeds so that they reach the electrodes with violence, producing physical deformations in them and generating new electrons by thermionic emission. The gas can achieve in the discharge channel temperatures of around 5000 degrees Celsius. 3 Factors affecting the Corona Effect This section presents some factors that influence the intensity of the corona effect. Effect of system voltage: electric field strength in the space between the wires depends on the potential difference between the conductors. If the potential difference is high, the intensity of the electric field will also be high and therefore the loss by corona effect will also be high. Effect of air density: loss by corona effect is inversely proportional to the air density factor. The loss by corona effect of a high-voltage transmission line through a mountainous region may be greater than that of a similar system in a flatter area, due to the lower air density value to high altitudes. Effect of the radius of the conductor: If the radius of the conductor is high, the field strength on the surface is smaller and therefore the loss by corona effect is smaller. For the same power capacity, an ACSR conductor has radius larger than a conventional copper conductor. Therefore, transmission lines with the ACSR conductor have a loss by corona effect less than the lines with copper conductors. For conductors encapsulated, the effective radius is greater and, therefore, the corona loss is smaller. Effect of increased temperature of the conductor due to the charge current: the current of conductor increases the temperature of the conductor and, therefore, it leads to a reduction, in an indirect way, the corona loss. The loss by corona effect is greater if the conductor temperature is lower and this is due to the fact that at low temperatures small droplets of water form during fog and damp climates. With a high current in the conductor this condensation decreases due to increased temperature and thus decreases the loss by corona effect. Effect of snow and ice: snow and ice layer transmission lines conductors cause very high losses by corona effect and emission of radio interference. Effect of corona discharges in conductors with surface covered by snow and the electric field strength not to exceed 30kV/cm consists primarily of ticks in negative semi-cycles and a pulsed corona discharge in positive half-cycle. Corona discharges affect the shape of snow layers and even repel in regions close to default. Loss effect of corona in old and new conductors: new conductors in the loss by corona effect are greater due to scratches, burrs, etc. With the aging of the conductor, the corona effect decreases. Effect of supply voltage: If the voltage is high, the effect of corona loss will be high. In transmission lines, low-voltage corona effect is negligible due to the inadequacy of the electric field to maintain a self-sustaining ionization. Effect of dirt and dust: in the presence of dirt and dust, the required voltage gradient is less to keep the discharge. The flashover voltage criticizes is reduced due to dust and dirt and therefore the loss by corona effect is greater. Effect of adverse weather conditions (pollution, rain, etc.): interference of the environment, such as: pollution and rain tend to increase the losses by corona effect in the lines. Effect of conductor configuration: three-phase overhead transmission lines can be positioned both in vertical and in horizontal configuration setting. The electric field intensity on the surface of the conductor is greater than the intensity of the electric field in the other conductors. Therefore, the flashover voltage critical to these median conductors will be less than for the other two conductors and, therefore, there will be a loss by corona effect larger conductor. If the conductors are placed in the same level, the average electric field intensity for each conductor will be the same. Since the ground has the same potential surface, the distribution of the electric field is affected by the presence of the earth. Corona loss is less if the leads are placed in greater heights, but not high enough for significant changes in air density surrounding the conductors. 4 Main Problems in an Insulator The failure of an insulator can occur either inside the material (drilling) or through the air that surrounds (external download). Its design must be such as to ensure a balanced distribution of potential and, consequently, the gradient in the air, with the aim of ensuring adequate discharge voltages. Hence their peculiar shapes. ISBN:

4 In addition to these requirements, must still meet the other no less important, that is the nonproduction, even after long periods of operation, undesirable radio interference. This is usually caused by insulators in tiny dots of electrical disruption to the air: corona effect, which will be discussed in more detail later. The spikes thus produced cause currents of high frequencies, radiating energy similar to a walkie-talkie. It is a problem that should be eliminated by own design and surface finish of the insulators. It requires extreme robustness insulators still, so that they can resist the delicate, handling, not always in the warehouses and works. It should be durable when in service, reducing to a minimum the number of replacements over the years and resist well to thermal shocks to which they are submitted by local weather conditions. Their surfaces must have finishing able to resist well to time exposures, even in an atmosphere of high degree of pollution in which there is presence of sulphur oxides and other reagents. Therefore, we can say that the major flaws of an insulator are: Failure inside the material (drilling); Fails due to air that surrounds the insulator (external download); Failure to meet the performance requirements and use as necessary; Production of radio interference; Failure by corrosion (rust due to insulator, pins, acid corrosion, among others). 5 The Developed System 5.1 The Ultrasound Strategy When the ultrasound equipment is used as predictive maintenance tool in the industrial plant, it turns out that the background noise (sander, welding machine, etc.) are normally little intense compared to ultrasound signals that are detected, such as the effects of electrical discharges, leaks and poorly lubricated bearings, which are very obvious. In operation, the ultrasound has an excellent signal to noise proportional. In this way, the equipment can reject a high level of background noise, focusing on desired frequency range of sounds. A typical industrial environment may contain levels of excess background noise. The noise audible range is 100 db, while only 15 db of noise is around 40 khz. Waves of low frequency sounds and audible tend to travel large distances, reflecting themselves into walls, equipment, etc., before completely diminish. These reflections add to background noise in an audible range. On the contrary, the ultrasound is usually softened completely before it has a possibility of reflection. This fast attenuation tends to keep low ultrasound background levels, even in the worst environments. Because of the low level of background ultrasonic noise in industrial environments, it is possible to increase the breadth of small ultrasonic signals produced by defects in the insulator to present useful information. Therefore, isolating the desired signal from the noise produced in the operating environment has in hand the desired information for this system. The joint of a tool of ultrasonic flaw detection with a data acquisition system and a computer form the basis for this project. By making use of detection technique described above, it can be said that the system will use the advantage of being able to be used in open field plus the advantage of microcomputer allow full-time monitoring, reporting possible errors on insulators at any time in which these occur. Highly directional nature of ultrasound also enables to identify problems in specific areas. 5.2 The Hardware of the System The analog signal is picked up by the system developed by means of ultrasonic measurement and acquired by a purchase module under the command of microcomputer. The acquired signals are handled by digital signal processing algorithms and feature extraction. The parameters taken from the sign are registered in trend curves. The extracted parameters are monitored and compared with levels of configured alarms in the system. In the event of infringement of the alarm levels, these events are recorded and flagged for what actions are taken. The ULD-40 is the corona sensor. It is equipped with a piezo-ceramic ultrasonic sensor type and with a cone of amplification that gives the ULD-40 an exceptional sensitivity [7]. The ULD-40 is an instrument that transforms the ultrasonic pressure in audible signals. The role of the ULD-40 consists in capturing these sounds and sends them on a scale audible. The ULD-40 allows the location and identification of corona effects and arches that can be found in any type of high voltage installation, simply by scanning around the suspected area. The sensor of the ULD and the amplifier are positioned in front of the appliance. A parabolic external sensor is always accessible, which allows the user to detect defects at long distance. The parabolic sensor is connected to ISBN:

5 the side of the ULD and is equipped with a laser sight, which allows you to find exactly the ultrasonic emission source. The parabolic antenna is removable and has a small size, which facilitates the use of the ULD in the field and offers access in any type of facility. In the case of corona, the ULD-40 has an ultrasonic level indication in 10dB or greater indicates that there is some potential problem in insulator: pollutant in the insulator, insulator damaged, some object on the insulator etc.. The system to be installed must ensure that the signal does not come from some other piece of equipment, such as a bolt or cable end etc. The latter elements are non-critical sources of corona. Figure 1 shows the hardware of the system. Fig. 2 - Main screen of the software. The software developed allows you to adjust some parameters, which are: 1. Signal acquisition: this field allows you to set the time interval in which each acquisition occurs. To perform a single Instant acquisition field must be filled in with the value 0 (zero) minutes. 2. Signal level: this field displays the value of the audio signal intensity in RMS (db). This field cannot be changed; the value displayed is generated from the collected signal. 3. Level of alarm: this field allows you to set up the levels of attention and emergency alarms. These fields must be adjusted by the user according to the expertise of the same in relation to the emission level and the condition of the insulator. 4. General configuration: these fields allow the setting of sampling frequency and number of samples to be hold in signal acquisition. The relationship between the number of samples and the sampling frequency is the time of signal acquisition. Fig. 1 - Photo of the hardware of the system. 5.3 The Analysis Software The acquisition software has a graphical simple interface and with easy operation (this interface is written in Portuguese). Figure 2 presents the main screen of the software. 6 Some Illustrative Results For carrying out the laboratory tests was used an ultrasound transmitter connected to a conventional signal generator, allowing the variation of amplitude and frequency of the transmitted signal. The transmitter was thrilled with a voltage of 6Vp and frequencies were changed according to the following values: 35 khz (below the nominal range), 38 khz (nominal track lower threshold), 40 khz (center frequency), 48 khz (nominal upper range) and 51 khz (above the nominal range). The ULD-40 was used without parable and located 1 meter from the transmitter. Figure 3 shows some results. ISBN:

6 (a) for performing collections and act in the control system and automation of the parabola. The targeting system of the parable is controlled via serial port using software for an accurate pointing of the point to be examined. This solution was possible, since the ultrasound measurement equipment feature an output in the range of audible frequencies (20 to 20 khz) through the ultrasound frequency offset. Acknowledgement The authors would like to thank CNPq, CAPES, and FAPEMIG - Brazilian research funding agencies, for the research scholarships, which supported this work. Also, they would like to express their gratitude to ANEEL R&D. (b) Fig. 3 Results for signals of: (a) 40 khz and (b) 48 khz. In Figure 4, an example of the trend curve is shown to the effective values of the audio signal. With this graphic it is possible to evaluate the ultrasonic emission behavior of the standoff over time. Fig. 4 Trend curves produced by the systems. 7 Conclusion The fails in insulations are important factors related to wear and tear, loss of efficiency and reduction of the lifetime of an electrical network. Currently, modern maintenance is based on condition monitoring of machinery and equipment. It is made In order to reduce cost and unavailability. The developed system fits in this type of practice and allows remote acoustic inspections with great precision. This system is easy to use and requires no training. It was designed to work in harsh environments. The place of installation of this equipment is in external environments. The software is responsible References: [1] G. Lambert-Torres, G.M. Ribeiro, C.I.A. Costa, A.P. Alves da Silva, and V.H. Quintana, Knowledge Engineering Tool for Training Power-Substation Operators, IEEE Trans on Power Systems, Vol. 12, No. 2, 1997, pp [2] H.G. Arango and G. Lambert-Torres, Spatial Electric Load Distribution Forecasting using Simulated Annealing, WSEAS Transactions on Systems, Vol. 3, No. 1, 2004, pp [3] E. Bonaldi, L.E. Borges da Silva, G. Lambert- Torres and L.E.L. de Oliveira, A Rough Sets Based Classifier for Induction Motors Fault Diagnosis, WSEAS Transactions on Systems, Vol. 2, No. 2, 2003, pp [4] L.A.P. Peres, L.A.H. Nogueira and G. Lambert-Torres, Diesel Power Generation on Rural Areas in Brazil and Effective Opportunities for Renewable Sources, In: Advances in Systems Science: Measurements, Circuits and Control, por N.E. Mastorakis and L.A.P. Peres, WSEAS Press, pp , [5] J.-S. Chang and P.A. Lawless, T. Yamamoto, Corona discharge processes, IEEE Trans on Plasma Science, Vol. 19, No. 6, 1991, pp [6] U. Corbellini and P. Pelacchi, Corona losses in HVDC bipolar lines, IEEE Trans on Power Delivery, Vol.11, No. 3, 1996, pp [7] Information on products/partial-discharge-detector/uld/ ISBN: