Publication: Medida de descargas parciales en radiofrecuencia
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2014-02
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2014-03-28
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Abstract
La detección de la actividad de Descargas Parciales (DPs) es de gran
importancia para establecer cuál es el estado del aislamiento eléctrico de un
equipo y poder aplicar un mantenimiento basado en la condición del mismo,
condition-based maintenance. El método más común de medida de DPs es el
eléctrico, en el que se miden pequeños cambios en la corriente que se fuga a
tierra cuando se producen DPs en el equipo. En este sentido, se utilizan los
esquemas propuestos en la norma (IEC-60270, 2000) en los que, se conecta
a tierra un objeto de ensayo de DPs, considerado como una capacidad, a
través de un condensador de acoplamiento. En el camino a tierra, se instala
un equipo de medida de corriente y se aplica alta tensión al circuito, para
adquirir la propagación del pulso. A partir de las medidas del pulso de DP,
se puede identi car el tipo de fuente y hacer una clasi cación de las medidas
en descargas internas, super ciales y corona.
En los últimos años, con la extensión de las subestaciones en aislamiento
de gas, Gas-Insulated Substation (GIS), se han popularizado los métodos de
medida de la radiación en ultra-alta frecuencia, Ultra-High Frequency (UHF),
emitida por la DP mediante el uso de antenas. Estas adquisiciones, se realizan
en instalaciones de GIS debido a que en este tipo de subestaciones con gas
a presión las emisiones de DPs pueden tener energía a frecuencias del orden
de giga-hercios. En la actualidad, se está dando el salto a la utilización de
esta misma tecnología en transformadores y líneas de alta tensión. Al ser
una técnica novedosa, de hecho sus procedimientos de medida se publicarán
previsiblemente en 2015 en la norma (IEC-62478, pendiente de publicación),
quedan muchos aspectos por conocer y resolver, y algunos se abordan en esta
tesis doctoral. Como aproximación, por un lado, es necesario establecer la
relación que existe entre el pulso de radiación que mide la antena y el tipo de
DP. Para ello, hay que aislar los distintos tipos de pulsos, utilizando objetos
de ensayo que generen sólo un tipo de DP y, a continuación, caracterizar el pulso en el tiempo y en frecuencia para encontrar una correlación entre
el tipo de descarga, su espectro de potencia en frecuencia y su forma de
onda en el tiempo. Además, cuando se realiza la medida de los pulsos de
DPs en Radiofrecuencia (RF) en el rango de muy alta frecuencia, Very-High
Frequency (VHF), y UHF (30 3000 MHz), es de vital importancia separar
las posibles fuentes de interferencia electromagnética de la radiación emitida
por la fuente de DPs. Estas fuentes son: radio en frecuencia modulada,
Frequency Modulated (FM), radio difusión sonora digital, Televisión (TV),
sistema global para las comunicaciones móviles, Global System for Mobile
Communications (GSM), delidad inalámbrica, Wireless-Fidelity (Wi-Fi),
etc.
Por otro lado, también se pueden aplicar las técnicas de medición de DPs
en VHF y UHF para la monitorización del estado del aislamiento de uno de
los activos más importantes de una red eléctrica, como son los transformadores
de potencia. La mayor parte de los transformadores de gran potencia
tienen un depósito denominado tanque o cuba en el que se alojan las
columnas del material electromagnético, los arrollamientos y el sistema de
aislamiento, todos ellos embebidos en aceite. El sumergir todos estos elementos
en aceite, tiene como n reducir la temperatura y servir como aislante
eléctrico para evitar fallos del sistema de aislamiento de los transformadores
de potencia. Además, separan los arrollamientos del equipo, en cuyo sistema
de aislamiento pueden haber imperfecciones cuyo origen es debido a su
construcción, puesta en servicio o al funcionamiento del transformador. En
estas imperfecciones, pueden originarse DPs que pueden dar lugar al fallo en
el equipo si se produce una rotura del aislamiento y, en consecuencia, una
descarga total.
Para poder medir la actividad de DPs, es de vital importancia conocer
cómo se propagan electromagnéticamente dentro del tanque de un transformador.
Este recinto, se comporta como una cavidad resonante en el que se
excitan ondas estacionarias que activan unas determinadas frecuencias de
resonancia dependientes de la geometría del mismo. Una vez conocido el espectro
electromagnético dentro del tanque, se estudiará la medida del mismo
efecto desde fuera. De esta manera, se pueden conocer el efecto de su blindaje
sobre las frecuencias que atenúa, así como las frecuencias que deja pasar.
En esta tesis, se aborda el estudio de la radiación electromagnética emitida
por las DPs, su parametrización y la selección de antenas para la correcta
medida de fuentes de DPs. Además, se considera el efecto de la distancia en
su adquisición y se comparan las respuestas del sistema de medida utilizado
compuesto por un osciloscopio con la de un analizador de espectros comercial
PDS-100. Por otro lado, se identi can los modos de resonancia propios de
un modelo de cuba de transformador, así como de su in uencia en la propagación electromagnética de las DPs. Para esto, se preparan y seleccionan
los objetos de ensayo que experimentalmente reproducen la emisión de DPs internas, super ciales y corona. Por último, se realiza un programa que permite
el post-procesado de las señales adquiridas para el estudio posterior de
los resultados obtenidos que darán lugar a interesantes conclusiones a partir
de los mismos. -----------------------------------------------------------------------------------------------
The detection of partial discharges (PD) activity is crucial to establish which is the status of insulation systems in electrical equipments and to ensure an e ective condition-based maintenance of electrical assets. The electrical measurements of PD is the most widely method used to detect possible failures. With this method, it is possible to measure short uctuations in the current leaked to ground when PD are produced in equipments. In this sense, the proposed schemes by the (IEC-60270, 2000) standard are used. In which, a current measuring equipment is connected to a partial discharge test object, considered as a capacity, and high-voltage is applied to the circuit to acquire pulse propagation through a coupling capacitor connected to ground. From partial discharge pulse measures, it can be identify the type of pulse, as well as the classi cation of the measures in internal discharges, surface and corona could be done. During the last years, the extended use of Gas-Insulated Substation (GIS) have led to the increase of popularity of methods for measuring the partial discharge emission in radio-frequency by antennas. These measurements are taken in GIS due to the fact that, in this kind of subsatations with pressurized gas, PD emissions can send out energy at frequencies in the order of giga-hertz. Currently, it is being made the leap to the use of the same technique in power transformers and power lines. As it is a new technique, in fact their measurement procedures will be published in (IEC-62478, not set published) standard in 2015, there are many aspects to be known about it and to be solved; therefore, some of them are addressed in this thesis. As an approach to this novel technique, on the one hand, it is necessary to establish the relationship between radiation pulse measured by the antenna and the type of partial discharge. This requires to isolate the di erent kinds of pulses using test objects that only generate one type of partial discharge, and then characterize the pulse in time and frequency domains so as to nd a correlation between the kind of discharge, its power frequency spectrum, and its waveform in time domain. Furthermore, when the partial discharge measurement is performed in RF at the VHF and UHF bands (30 3000 MHz) through the use of antennas, it is crucial to separate electromagnetic interferences from the electromagnetic radiation emitted by the partial discharge sources. These noise sources are: FM radio, television, GSM, Wi-Fi, etc. On the other hand, PD measurement techniques by using antennas can also be applied in insulation condition monitoring on one of the most important assets in a power grid as is the power transformer. Most of high-power transformers have a reservoir called tank in wich the columns of the electromagnetic material, the insulation system and the windings are found. All of them are inmerse in oil, that is used to reduce the temperature inside the tank and serve as an electrical insulator to prevent transformer insulation failures. This insulation systems, that separate the winding transformer, might have imperfections due to their construction, commisioning or caused by the transformer operation. In these weak points of the insulation system PD may be occur, it could trigger the failure of the equipment when the insulation is broken that provoke a total discharge. In this sense, it is crucial to know how electromagnetic waves caused by the PD are propagated inside a transformer tank. A tank behaves like a resonant cavity which allows the excitement of stationary waves which activate certain resonant frequencies depending on its own geometry. Known the characteristic of the electromagnetic propagation inside the tank, the same e ect will be studied from outside the cavity. By this way, the frequencies that are attenuated by the shielding can be detected. This thesis deals with the study of the electromagnetic radiation emitted by PD, its parametrization and the selection of proper antennas for measuring PD sources. Moreover, the distance e ect on the PD acquisition is also considered and the responses of the two acquisition systems, compounds by the oscilloscope and a commercial spectrum analyzer PDS-100 respectively, will be compared. On the other hand, will be obtained the identi cation of the resonance modes characteristic of a transformer tank model and its in uence on the electromagnetic emission of the PD. With this aim, test objects that reproduce experimentally PD are prepared and selected. This kind of PD are internal, surface and corona types. Finally, it is executed a program that allows the post-processing of the acquired signals for further study of the results obtained that will lead to interesting conclusions from them.
The detection of partial discharges (PD) activity is crucial to establish which is the status of insulation systems in electrical equipments and to ensure an e ective condition-based maintenance of electrical assets. The electrical measurements of PD is the most widely method used to detect possible failures. With this method, it is possible to measure short uctuations in the current leaked to ground when PD are produced in equipments. In this sense, the proposed schemes by the (IEC-60270, 2000) standard are used. In which, a current measuring equipment is connected to a partial discharge test object, considered as a capacity, and high-voltage is applied to the circuit to acquire pulse propagation through a coupling capacitor connected to ground. From partial discharge pulse measures, it can be identify the type of pulse, as well as the classi cation of the measures in internal discharges, surface and corona could be done. During the last years, the extended use of Gas-Insulated Substation (GIS) have led to the increase of popularity of methods for measuring the partial discharge emission in radio-frequency by antennas. These measurements are taken in GIS due to the fact that, in this kind of subsatations with pressurized gas, PD emissions can send out energy at frequencies in the order of giga-hertz. Currently, it is being made the leap to the use of the same technique in power transformers and power lines. As it is a new technique, in fact their measurement procedures will be published in (IEC-62478, not set published) standard in 2015, there are many aspects to be known about it and to be solved; therefore, some of them are addressed in this thesis. As an approach to this novel technique, on the one hand, it is necessary to establish the relationship between radiation pulse measured by the antenna and the type of partial discharge. This requires to isolate the di erent kinds of pulses using test objects that only generate one type of partial discharge, and then characterize the pulse in time and frequency domains so as to nd a correlation between the kind of discharge, its power frequency spectrum, and its waveform in time domain. Furthermore, when the partial discharge measurement is performed in RF at the VHF and UHF bands (30 3000 MHz) through the use of antennas, it is crucial to separate electromagnetic interferences from the electromagnetic radiation emitted by the partial discharge sources. These noise sources are: FM radio, television, GSM, Wi-Fi, etc. On the other hand, PD measurement techniques by using antennas can also be applied in insulation condition monitoring on one of the most important assets in a power grid as is the power transformer. Most of high-power transformers have a reservoir called tank in wich the columns of the electromagnetic material, the insulation system and the windings are found. All of them are inmerse in oil, that is used to reduce the temperature inside the tank and serve as an electrical insulator to prevent transformer insulation failures. This insulation systems, that separate the winding transformer, might have imperfections due to their construction, commisioning or caused by the transformer operation. In these weak points of the insulation system PD may be occur, it could trigger the failure of the equipment when the insulation is broken that provoke a total discharge. In this sense, it is crucial to know how electromagnetic waves caused by the PD are propagated inside a transformer tank. A tank behaves like a resonant cavity which allows the excitement of stationary waves which activate certain resonant frequencies depending on its own geometry. Known the characteristic of the electromagnetic propagation inside the tank, the same e ect will be studied from outside the cavity. By this way, the frequencies that are attenuated by the shielding can be detected. This thesis deals with the study of the electromagnetic radiation emitted by PD, its parametrization and the selection of proper antennas for measuring PD sources. Moreover, the distance e ect on the PD acquisition is also considered and the responses of the two acquisition systems, compounds by the oscilloscope and a commercial spectrum analyzer PDS-100 respectively, will be compared. On the other hand, will be obtained the identi cation of the resonance modes characteristic of a transformer tank model and its in uence on the electromagnetic emission of the PD. With this aim, test objects that reproduce experimentally PD are prepared and selected. This kind of PD are internal, surface and corona types. Finally, it is executed a program that allows the post-processing of the acquired signals for further study of the results obtained that will lead to interesting conclusions from them.
Description
Keywords
Descargas parciales, Aislamiento eléctrico, Antenas