Yüksek darbe gerilimleri altında gazlarda kısmi boşalma akımlarının zaman-frekans analizi
Yüksek darbe gerilimleri altında gazlarda kısmi boşalma akımlarının zaman-frekans analizi
Dosyalar
Tarih
2021
Yazarlar
Arıkan, Kemal
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Lisansüstü Eğitim Enstitüsü
Özet
Enerjiye olan ihtiyacın nüfusa ve teknolojik gelişmelere paralel olarak arttığı dünyada, enerjinin kesintisiz ve ucuza tedarik edilmesi ülkeler açısından büyük önem arz etmektedir. Enerji sistemlerinde bulunan elektriksel cihazların ömürleri yalıtım arızalarına bağlı olarak oldukça kısa olabilmekte ve bu da enerji tedariğinde sürekli kesintilere ve büyük maddi kayıplara neden olmaktadır. Bu nedenle yüksek gerilim altında çalışan enerji iletim sisteminde bulunan aygıtların elektriksel özellikleri tarihsel süreçte bilim insanları tarafından sürekli bir araştırma konusu olmuştur. Yüksek gerilim altında çalışan cihazların yalıtımları sistemden kaynaklı kısa devre arızaları gibi veya yıldırım gibi dış etkilerden kaynaklı anlık aşırı yüksek gerilimlere maruz kaldığı için zamanla yalıtkanlık özelliklerini kaybederler. Elektriksel yalıtkanlar katı, sıvı veya gaz olabilmektedir. Yalıtkanların zamanla yalıtkanlık özelliklerini kaybetmelerinde kısmi boşalmalar önemli ölçüde rol oynamaktadır. Kısmi boşalmalar iletken yüzeylerin yalıtkana temas ettiği noktalarda sürekli elektriksel boşalmalar olarak kendini gösterir. Kısmi boşalma bir elektriksel yalıtkanı tam olarak köprülemeyen boşalmalardır. Bu sürekli elektriksel boşalmalar yalıtkanı zamanla yaşlandırarak yalıtkanlık özelliğinin kaybolmasına neden olurlar. Bu nedenle kısmi boşalmaların fiziksel doğası çok iyi anlaşılması gereken bir konudur. Kısmi boşalmalar gazlarda, katı yalıtkanlar ve sıvı yalıtkanlarda yüzeysel boşalmalar, iç kısmi boşalmalar, korona ve ağaçlanma gibi farklı şekillerde oluşurlar. Gaz ortamında gerilim uygulanan iletkenlerin pürüzlü olduğu yerlerde kısmi boşalmalar 1 ns'den daha kısa süreli akımların oluşmasına neden olur. Yüksek gerilim cihazlarının yalıtkanlık testleri cihazların ömürlerinin belirlenmesi bakımından çok önemlidir. Bu testler uluslararası standartlara göre gerçekleştirilirler. Kısmi boşalma testleri de bu testlerden birisidir. Kısmi boşalmalarının ölçülmesine yönelik uluslararası standart IEC 60270'tir. Kısmi boşalmaların tespiti birçok farklı yöntemle gerçekleştirilmektedir (kimyasal, akustik, elektriksel vb.) bu yöntemler tez giriş bölümünde bulunan literatür özetinde detaylı olarak bulunmaktadır. Bu tez kapsamında gaz yalıtımlı sistemlerde kullanılan bazı gazların farklı polariteli kesik yıldırım darbe gerilimleri altında kısmi boşalma deneyleri gerçekleştirilerek kısmi boşalma akım eğrileri elde edilmiştir. Test sisteminde kapalı bir kap içine farklı basınçlarda gaz ve gaz karışımları konularak, test kabının içinde bulunan, arasında belirli bir açıklık olan küresel elektrot ve sivri uçlu-düzlem elektrot sistemine farklı polariteli kesik yıldırım darbe gerilimleri uygulanmıştır. Elektrot konfigürasyonu değiştirilmemiştir. Deneylerde saf SF6 gazı ve %1 oranında SF6 gazı %99 oranında N2 gazı karışımı kullanılmıştır. Ölçüm devresi IEC 60270 standardına göre kurulmuştur. Gaz basınçları 1 bar, 2 bar ve 3 bar olacak şekilde değiştirilerek farklı basınçlarda kısmi boşalma akım eğrileri elde edilmiştir. Her bir deney konfigürasyonunda en az xx üç deney yapılmıştır. Kesik yıldırım darbe gerilimi pozitif ve negatif polariteli olarak farklı polaritelerde test elektrotlarına uygulanmıştır. Kesik yıldırım darbe gerilimlerinin üretilmesi 1 MV, 50 kJ Marx tipi üreteç ile sağlanmıştır. Kısmi boşalma akımları elektrot konfigürasyonuna bağlı bir ölçüm empedansı üzerinden dijital bir osiloskoptan okunarak bilgisayara kaydedilmiştir. Ölçüm sistemi, osiloskop ve bilgisayar ölçülen kısmi boşalma akımlarına elektromanyetik olarak girişimin engellenmesi için ekranlanmıştır. Elde edilen akım grafiklerinden kısmi boşalma akımlarının genlik, frekans, oluşma zamanları gibi birtakım fiziksel özellikleri ve bu özelliklerin basınç, gaz türü, darbe gerilimi polaritesi gibi değiştirilen deney parametreleriyle arasındaki ilişkiler incelenmiştir. Bu incelemeler için öncelikle elde edilen akım grafiklerinin zaman-frekans analizleri yapılmıştır. Zaman-frekans analizi Kısa Süreli Fourier Dönüşümü temel alınarak gerçekleştirilmiştir. Zaman-frekans analizi gerçekleştirilen akım verilerinin örnekleme frekansları 1 GHz'dir. Bu analizlerin yapılmasında MATLAB paket bilgisayar programı kullanılmıştır. Bu analizler sonucunda her bir deneyden elde edilen kısmi boşalma akım eğrilerinin akım piklerinin frekans bilgileri elde edilmiştir. Daha sonra ise elde edilen frekans bilgilerine göre kısmi boşalma akım genlikleri arasında tespit edilen ilişkiler açıklanmıştır. Ayrıca elde edilen frekans bilgilerine göre gaz türü, gaz basıncı, kısmi boşalma oluşum zamanı gibi parametrelerin kısmi boşalma akımlarının frekansları üzerindeki etkileri tespit edilmiştir. Son olarak 1, 2 ve 3 bar basınçta saf SF6 gazı üzerinde pozitif polariteli darbe gerilimi uygulanarak elde edilen kısmi boşalma akım grafiklerinin ortalama değer, standart sapma, çarpıklık ve basıklık değerleri hesaplanarak bu değerlerin kısmi boşalma akımlarının frekans bilgileri ile arasındaki ilişki incelenmiştir.
Today, demand for energy increases in parallel with growing population and technological developments in the World. Thus, it is very important for countries to supply energy with cheap costs and uninterruptedly. The lifespan of electrical devices in energy systems is short owing to the fact that insulation faults under high voltages, consequently this situation causes energy interruption and too much financial lost. For these reasons, the electrical and dielectric properties of high voltage apparatus have been a constant research topic by scientists for a long time. The insulation of high voltage apparatus, lose their electrical insulating properties over time as they are exposed to external effects such as instantaneously extremely high voltages by lightning or internal effects such as short circuit faults. The insulating materials in high voltage apparatus can be solid, liquid or gas. On the other hand, partial discharges act major role in the loss of insulating properties of insulation materials in the long run. The insulating and breakdown voltage strength of insulation layer are affected and weakened by consecutive partial discharge events. Partial discharges defined in IEC 60270 standard as local discharges which occur in insulation. Partial discharges do not electrically bridge the whole insulating media. Partial discharges are separated by formation place such as internal or external. External partial discharges occur outside the energy apparatus. For example, external partial discharges occur on the surface of overhead energy lines. The internal partial discharges occur inside the electrical devices. The internal partial discharges form in the voids or extending cavities in the insulation. Moreover, the internal partial discharges occur on the conductive surfaces extending in solid, liquid or gas insulations. Partial discharges in gas insulated systems are usually local discharges at a distance of less than 1 mm. Partial discharges on surface defects such as small protrusions cause current pulses which has very short rising time less than 1 ns. These current pulses cause corona sparks. The very short rise time of partial discharges lead to electromagnetic radiation in gas room. Due to the intense corona or micro discharges lead to expansion of ionized gas channel then afterwards breakdown occurs with acoustics pressure wave. Partial discharge also emitted light from excited molecules and chemical components. Partial discharges can be identified by electrical, chemical, acoustical and electromagnetic methods. Due to the adverse effects of partial discharges for insulators, continuous condition monitoring of them should be performed. Furthermore, in order to produce high voltage equipments that can withstand lightning voltage and internal over voltages, it is necessary to carry out insulation tests of the devices. These tests should be performed in laboratory environment with shielding rooms to prevent interference to measuring cables. xxii Lightning impulse (LI) voltages can be both positive and negative polarities. These voltages rise their peak values rapidly, however fall to zero values more slowly. Lightning voltages have very high amplitudes (~kV) and they are short-term voltages (~μs). There are three types of lightning impulse voltages, which are standard lightning impulse, lightning impulse chopped on the tail and lightning impulse chopped on the front. Lightning impulse voltages are produced by Marx generators and they are measured by resistive or capacitive voltage dividers. High amplitude impulse currents are measured by low resistant measurement resistors or measurement coils traditionally. Partial discharge current signals are stochastic and non-stationary signals by their physical nature therefore, it is difficult to analyze and compare them with each other. To analyze such signals; time domain analysis, frequency domain analysis and time-frequency domain analysis methods can be used. If statistical values of signals such as mean value, standard deviation, skewness and kurtosis are to be calculated, time domain analysis method can be used. On the other hand, time domain analysis method is not the best method in too many applications, because some important informations are hidden in frequency domain of the signal. There are different transform methods for time-frequency analysis such as Short-Time Fourier Transform, Wavelet Transform. Short-Time Fourier Transform (STFT) is simple and effective method to analyse signals together in time and frequency domain. The Short-Time Fourier Transform is better than the Fourier Transform for non-stationary signals. It gives interesting information about energy spectrum of each frequency in the signal. In this thesis, experiments were conducted in a high voltage laboratory. A sealed test vessel was filled with different dielectric gases and gas mixtures. With the scope of this study two different types of dielectric gases which are pure SF6 (Sulfur hexafluoride) and 1% SF6 + 99% N2 mixture were used. The pressure of the filled pure SF6 gas and 1% SF6 + 99% N2 mixture were changed in the test vessel by 1, 2 and 3 bar. Two different types of electrodes which are spherical and needle on plate electrode were used for test. Positive and negative polarity chopped lightning impulses were applied on spherical electrode which is the upper electrode. The lower electrode which is needle on plate was grounded. The spherical electrode has diameter of 5 cm. The distance between the two electrodes is 5 cm. The needle on the plate electrode has length of 1 cm and diameter of 1 mm. Lightning impulse voltages were generated by a Marx type generator which has 1 MV output voltage and 50 kJ energy capacity. Partial discharge currents were measured on a measurement resistance in a measurement circuit via coaxial cable. All measurement data was recorded with a 120 MS (Mega sample)/s digital oscilloscope which has 12- bit vertical resolution. The sampling frequency of the digital data is 1 GS (Giga sample)/s. Without changing the electrode configuration in the experimental set up, by using two different types of dielectric gases at 1, 2 and 3 bar pressures both positive and negative polarity lightning impulse voltage was applied to each insulator type (pure SF6 and 1% SF6 + 99% N2) and the impulse voltages and partial discharge currents were recorded. At least three experiments were done for each experiment condition. Current waveforms which were recorded by the oscilloscope, after applying the impulse voltage to the electrodes were given as input data to the MATLAB to find the frequencies of partial discharge current peaks in the waveform. First, a data range inside the current waveform was chosen for Short-Time Fourier Transform analysis, then Short-Time Fourier Transform was applied to this selected range. In the STFT analysis, sampling frequency was 1 GHz and window size was selected as 16. Window type was selected as Hanning, and FFT number was selected as 512. Short-Time Fourier Transform parameters such as window size, FFT number, window type and sampling frequency were kept constant for all digital data from experiments. The spectrograms were obtained after STFT analysis of whole experimental data. A spectrogram is an energy spectrum composed of different colors which represents different intensities of energies. Bold colors are the most intense energy areas. All frequencies, occurrence time and amplitudes of partial discharge currents were recorded for each experiment. For each experiment, pressure of the gas, frequency, occurrence time and amplitude of PD current were given in charts in the thesis. Moreover, the changes of average amplitudes and average frequencies of partial discharges under different gas pressures and different LI polarities were given in a bar graph for each gas environment. Also, frequencies and amplitudes of PDs were compared one by one for different gases environments under different polarities and pressures in bar graphs. According to the results, both under positive polarity and negative polarity lightning impulse, the frequency is showing decreasing tendency while the amplitudes of partial discharge currrents are increasing. In general, amplitudes of partial discharges occur in 100% SF6 under negative polarity LI voltage are higher than the under positive polarity in the same ambiance. This situation confirms the polarity effect of partial discharge. Contrarily, amplitudes of partial discharges occur in 1% SF6 + 99% N2 under positive polarity LI voltage tended to be higher than the under negative polarity LI voltage in the same ambiance. Moreover, there is a relationship between PD amplitudes and occurrence time of PDs. The PDs which occur in the early period before the breakdown voltage have low amplitudes, but the PDs which occur nearly before the breakdown voltage have high amplitudes. In general, PD frequencies in 1% SF6 + 99% N2 gas mixture are higher than PD frequencies in the 100% SF6 gas. This situation confirms the fact that SF6 gas is more sensitive in non-uniform fields. Furthermore, the PD current amplitudes are low in 100% SF6 under different pressures thus, it can be said that SF6 is damping the PD current amplitudes and frequencies. From the analysis results, PD current amplitudes in 2 bar pressure are higher than in the 1 and 3 bar pressures. SF6 gas has abnormal breakdown phenomenon between this pressure ranges. To find a relationship between the PD frequency and statistical values of the PD current data, mean value, standard deviation, skewness and kurtosis were calculated for 100% SF6 under positive polarity LI voltage. The frequencies of PD current peaks are decreasing with the increasing mean value, standard deviation, skewness and kurtosis. Under 100% SF6 gas and positive polarity LI voltage at 2 bar pressure statistical values are higher. Both skewness and kurtosis are positive for all cases, the reason is many PD amplitudes are smaller than the average amplitude value. This method mentioned in the thesis can be used for examining partial discharge phenomenon in different gas mixtures in the future.
Today, demand for energy increases in parallel with growing population and technological developments in the World. Thus, it is very important for countries to supply energy with cheap costs and uninterruptedly. The lifespan of electrical devices in energy systems is short owing to the fact that insulation faults under high voltages, consequently this situation causes energy interruption and too much financial lost. For these reasons, the electrical and dielectric properties of high voltage apparatus have been a constant research topic by scientists for a long time. The insulation of high voltage apparatus, lose their electrical insulating properties over time as they are exposed to external effects such as instantaneously extremely high voltages by lightning or internal effects such as short circuit faults. The insulating materials in high voltage apparatus can be solid, liquid or gas. On the other hand, partial discharges act major role in the loss of insulating properties of insulation materials in the long run. The insulating and breakdown voltage strength of insulation layer are affected and weakened by consecutive partial discharge events. Partial discharges defined in IEC 60270 standard as local discharges which occur in insulation. Partial discharges do not electrically bridge the whole insulating media. Partial discharges are separated by formation place such as internal or external. External partial discharges occur outside the energy apparatus. For example, external partial discharges occur on the surface of overhead energy lines. The internal partial discharges occur inside the electrical devices. The internal partial discharges form in the voids or extending cavities in the insulation. Moreover, the internal partial discharges occur on the conductive surfaces extending in solid, liquid or gas insulations. Partial discharges in gas insulated systems are usually local discharges at a distance of less than 1 mm. Partial discharges on surface defects such as small protrusions cause current pulses which has very short rising time less than 1 ns. These current pulses cause corona sparks. The very short rise time of partial discharges lead to electromagnetic radiation in gas room. Due to the intense corona or micro discharges lead to expansion of ionized gas channel then afterwards breakdown occurs with acoustics pressure wave. Partial discharge also emitted light from excited molecules and chemical components. Partial discharges can be identified by electrical, chemical, acoustical and electromagnetic methods. Due to the adverse effects of partial discharges for insulators, continuous condition monitoring of them should be performed. Furthermore, in order to produce high voltage equipments that can withstand lightning voltage and internal over voltages, it is necessary to carry out insulation tests of the devices. These tests should be performed in laboratory environment with shielding rooms to prevent interference to measuring cables. xxii Lightning impulse (LI) voltages can be both positive and negative polarities. These voltages rise their peak values rapidly, however fall to zero values more slowly. Lightning voltages have very high amplitudes (~kV) and they are short-term voltages (~μs). There are three types of lightning impulse voltages, which are standard lightning impulse, lightning impulse chopped on the tail and lightning impulse chopped on the front. Lightning impulse voltages are produced by Marx generators and they are measured by resistive or capacitive voltage dividers. High amplitude impulse currents are measured by low resistant measurement resistors or measurement coils traditionally. Partial discharge current signals are stochastic and non-stationary signals by their physical nature therefore, it is difficult to analyze and compare them with each other. To analyze such signals; time domain analysis, frequency domain analysis and time-frequency domain analysis methods can be used. If statistical values of signals such as mean value, standard deviation, skewness and kurtosis are to be calculated, time domain analysis method can be used. On the other hand, time domain analysis method is not the best method in too many applications, because some important informations are hidden in frequency domain of the signal. There are different transform methods for time-frequency analysis such as Short-Time Fourier Transform, Wavelet Transform. Short-Time Fourier Transform (STFT) is simple and effective method to analyse signals together in time and frequency domain. The Short-Time Fourier Transform is better than the Fourier Transform for non-stationary signals. It gives interesting information about energy spectrum of each frequency in the signal. In this thesis, experiments were conducted in a high voltage laboratory. A sealed test vessel was filled with different dielectric gases and gas mixtures. With the scope of this study two different types of dielectric gases which are pure SF6 (Sulfur hexafluoride) and 1% SF6 + 99% N2 mixture were used. The pressure of the filled pure SF6 gas and 1% SF6 + 99% N2 mixture were changed in the test vessel by 1, 2 and 3 bar. Two different types of electrodes which are spherical and needle on plate electrode were used for test. Positive and negative polarity chopped lightning impulses were applied on spherical electrode which is the upper electrode. The lower electrode which is needle on plate was grounded. The spherical electrode has diameter of 5 cm. The distance between the two electrodes is 5 cm. The needle on the plate electrode has length of 1 cm and diameter of 1 mm. Lightning impulse voltages were generated by a Marx type generator which has 1 MV output voltage and 50 kJ energy capacity. Partial discharge currents were measured on a measurement resistance in a measurement circuit via coaxial cable. All measurement data was recorded with a 120 MS (Mega sample)/s digital oscilloscope which has 12- bit vertical resolution. The sampling frequency of the digital data is 1 GS (Giga sample)/s. Without changing the electrode configuration in the experimental set up, by using two different types of dielectric gases at 1, 2 and 3 bar pressures both positive and negative polarity lightning impulse voltage was applied to each insulator type (pure SF6 and 1% SF6 + 99% N2) and the impulse voltages and partial discharge currents were recorded. At least three experiments were done for each experiment condition. Current waveforms which were recorded by the oscilloscope, after applying the impulse voltage to the electrodes were given as input data to the MATLAB to find the frequencies of partial discharge current peaks in the waveform. First, a data range inside the current waveform was chosen for Short-Time Fourier Transform analysis, then Short-Time Fourier Transform was applied to this selected range. In the STFT analysis, sampling frequency was 1 GHz and window size was selected as 16. Window type was selected as Hanning, and FFT number was selected as 512. Short-Time Fourier Transform parameters such as window size, FFT number, window type and sampling frequency were kept constant for all digital data from experiments. The spectrograms were obtained after STFT analysis of whole experimental data. A spectrogram is an energy spectrum composed of different colors which represents different intensities of energies. Bold colors are the most intense energy areas. All frequencies, occurrence time and amplitudes of partial discharge currents were recorded for each experiment. For each experiment, pressure of the gas, frequency, occurrence time and amplitude of PD current were given in charts in the thesis. Moreover, the changes of average amplitudes and average frequencies of partial discharges under different gas pressures and different LI polarities were given in a bar graph for each gas environment. Also, frequencies and amplitudes of PDs were compared one by one for different gases environments under different polarities and pressures in bar graphs. According to the results, both under positive polarity and negative polarity lightning impulse, the frequency is showing decreasing tendency while the amplitudes of partial discharge currrents are increasing. In general, amplitudes of partial discharges occur in 100% SF6 under negative polarity LI voltage are higher than the under positive polarity in the same ambiance. This situation confirms the polarity effect of partial discharge. Contrarily, amplitudes of partial discharges occur in 1% SF6 + 99% N2 under positive polarity LI voltage tended to be higher than the under negative polarity LI voltage in the same ambiance. Moreover, there is a relationship between PD amplitudes and occurrence time of PDs. The PDs which occur in the early period before the breakdown voltage have low amplitudes, but the PDs which occur nearly before the breakdown voltage have high amplitudes. In general, PD frequencies in 1% SF6 + 99% N2 gas mixture are higher than PD frequencies in the 100% SF6 gas. This situation confirms the fact that SF6 gas is more sensitive in non-uniform fields. Furthermore, the PD current amplitudes are low in 100% SF6 under different pressures thus, it can be said that SF6 is damping the PD current amplitudes and frequencies. From the analysis results, PD current amplitudes in 2 bar pressure are higher than in the 1 and 3 bar pressures. SF6 gas has abnormal breakdown phenomenon between this pressure ranges. To find a relationship between the PD frequency and statistical values of the PD current data, mean value, standard deviation, skewness and kurtosis were calculated for 100% SF6 under positive polarity LI voltage. The frequencies of PD current peaks are decreasing with the increasing mean value, standard deviation, skewness and kurtosis. Under 100% SF6 gas and positive polarity LI voltage at 2 bar pressure statistical values are higher. Both skewness and kurtosis are positive for all cases, the reason is many PD amplitudes are smaller than the average amplitude value. This method mentioned in the thesis can be used for examining partial discharge phenomenon in different gas mixtures in the future.
Açıklama
Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2021
Anahtar kelimeler
Yüksek gerilim,
High voltage,
Zaman-frekans analizi,
Time-frequency analysis