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Yüksek gerilim labaratuvarlarının tasarımı ve deney devrelerinin incelenmesi

Design of high voltage laboratories and analysis of test circuits

  1. Tez No: 39135
  2. Yazar: MEHMET BAYRAK
  3. Danışmanlar: Y.DOÇ.DR. ÖZCAN KALENDERLİ
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1993
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 125

Özet

ÖZET Uygulamada yüksek gerilimin söz konusu olduğu yerlerde kullanılan malzeme ve aygıtlar araştırma ve geliştirmede, liretim sonrası ve kullanım aşamalarında elektriksel davranışlarının, güvenilirliklerinin ve ömürlerinin belirlenmesi amacıyla yüksek gerilim deney lerine tabi tutulurlar. Deneyler, yüksek gerilim labo- r at uvarl arında veya isletmede öncelikle standardlara uygun koşullarda belirli deney devreleri ile gerçekleş tirilir. Yüksek gerilim deney devreleri yüksek gerilim üretme, ölçme ve denetim düzenlerinden oluşur. Bu dü zenler elde edilecek gerilim türüne göre yapısal ayrı lıklar gösterir. Yüksek alternatif gerilimler, genellikle yüksek gerilim transformatörleri ile, bazı durumlarda rezonans devreleri ile üretilir. Yüksek doğru gerilimler genel olarak yüksek alternatif gerilimlerin doğrul tulması ve birçok durumlarda katlanması suretiyle, cok nadir du rumlarda elektrostatik generator ler yardımıyla üretilir. Yüksek darbe gerilimleri, kondansatörlerin direnç ve kondansatörlerden oluşan devre üzerinden boşalmasıyla ol uçur. Yüksek alternatif gerilimlerle yapılan hemen hemen bütün deneylerde, yüksek gerilim değerinin mümkün olduğu kadar doğru ölçülmesi gerekir. Bu da ancak ölçmenin yüksek gerilim tarafında yapılmasıyla olur. Bu sebeple yüksek alternatif gerilimlerin ölçülmesi için birçok ölçme yöntemleri geliştirilmiştir. Yüksek doğru geri limler genellikle yüksek değerde dirence seri bağlı mil i ampermetre veya voltmetre ile veya elektroststik gerilim ölçü aletleriyle ölçülür. Darbe gerilimlerinin tepe değerleri, ölçü elektrotları yi a veya gerilim bölü cüsü ile birlikte kullanılan ölçü aletleriyle ölçülür. Bu çalışmada, yüksek gerilimlerin üretilmesi, ölçülmesi ve deney devrelerinin yapısal özellikleri ayrıntılı olarak incelenmiştir. Deney devrelerinin incelenmesi, devre el emeni arının değerleri ve yerleri değiştirilerek, PSpice paket programı ile bilgisayarla yapılmıştır. vi

Özet (Çeviri)

SUMMARY DESIGN OF HIGH VOLTAGE LABORATORIES AND ANALYSIS OF TEST CIRCUITS High voltage -test equipment is generally used in a> Research laboratories, b> Routine testing laboratories. The work carried out in research laboratories varies considerably from the establishment to the other, and the type of equipment needed varies accordingly. A general high voltage laboratory may include equipment for all classes of tests. The routine testing is concerned with testing equipment such as transformers, switchgear, bushings and cables, etc., and is often carried out on dhe factory premises, and the efficiency and reliability of the pro duct are the main factors. The high voltage equipment is required to study the insulation behaviour under all conditions which the apparatus is likely to encounter. Tests are also made with voltages higher than the normal working voltage to determine the factor of safety over the working conditions and to ensure that the working margin is neither too high nor too low. The conventional forms of high voltage in use can be divided into the following classes: a> Alternating voltage. b > Di r ect vol t age. c> Transient voltage. High alternating voltages are required in labora tories for experiments and a. c. tests as well as for most of the circuits for the generation of high direct and impulse voltages. Test transformers generally used for this purpose have considerably lower power rating and frequently much larger transformer ratios than power transformers. The primary current is usually supplied by regulating transformer fed from the mains supply or, in special cases, by synchronous generators. Most tests and experiments with high alternating voltages require precise knowledge of the value of the voltage. This demand can normally only be fulfilled by measurements on the high voltage side of the supply; viivarious techniques for the measurement, of high alternating voltages have been devised for this purpose. The shape of u for high alternating voltages will often deviate considerably from the sinusoidal. In high voltage engineering, the peak value U and the root- mean-square value 1 -İ4 rms X T 2 u = 4 4r- f u (t>dt a> rms x J are of particular importance. For high voltage tests the quantitiy U/^HTis defined as the test voltage. Transformers for generating high alternating test voltages: usually have one end of the high voltage win ding earthed. For numerous circuits for the generation of high d.c. and impulse voltages, however, transformers with completely isolated windings are required. To generate voltages above a few hundred kV single state transformers are rarely used; for economical and technical reasons one employs instead a series connec tions of the high voltage windings of several transfor mers. In such a cascade arrangement, the individual transformers must be insulated for voltages correspon ding to those of the lower stages. The excitation windings of the transformers of all stages except the lowest will operate at high potential. Test transformers, especially in cascade connec tion, represent spatially extended networks capable oscillation. Harmonics of the primary voltage and the magnetization current may excite natural oscillations at various frequencies, and this can lead to considerable distortion of the secondary voltage. Fig. 1 is used to demonstrate the possibility of a very considerable voltage enhancement on the secondary side of a test transformer by series resonance with a capacitive load. This effect can be used for the gene ration of high alternating test voltages; to extend the tuning range, the short circuit inductance of the test transformer is then augmented by a separate high voltage inductor. The series resonant circuit formed by the inductance and the capacitance of the test object may be excited by a transformer of relatively low secondary voltage. Resonant circuits are particularly advantagues when the test object has a high capacitance, for instan ce, a high voltage cable. The special advantage of such vma circuit is that the output voltage deviates little from a sinusoidal and that, due to the characteristics of the series resonant circuit, almost complete compen sation of the reactive power required for the test ob j ect. ?[ T Ci u2 =i=c a) Ci»Ca = C ^r U2 b) Fig- 1. Working performance of test transformers a> Circuit diagram b> Equivalent circuit c > Phas or di agr am The Tesla transformer, named after its inventor, also belongs to the class of resonant circuits. The circuit comprises a primary and a secondary oscillatory circuit in loose magnetic coupling. Periodic discharge of the primary capacitor via a spark gap will excite high frequency oscillations, typically in the frequency range 10 to 10 Hz. Depending upon the chosen circuit data and the transformation ratio of secondary to pri mary winding, voltages of more than 1 MV have been generated with Tesla transformers. There are numerous application for high direct voltages in the laboratory, such as insulation testing of arrangements with high capacitance, e.g. capacitors or cables, and fundamental investigations in discharge physics and dielectric behaviour. The most common generation medhods of high direct voltages employ rectification methods of hugh alterna ting voltages, often using voltage multiplication; electrostatic generators are also in use. The d. c. test voltage is defined as the arithmetic mean value IX1 T dt Periodic fluctuations of the direct voltage between the peak value U and the minimum value Um in are given in terms of ripple amplitude; «ŞU = >/2 Impulse voltages are required in high voltage tests to simulate the stresses due to external and internal over voltages, and also for fundamental inves tigations of the breakdown mechanisms. They are usually- generated by discharging high voltage capacitors through switching gaps onto a network of resistor and capacitors whereby voltage multiplier circuits are often used. Figure 2 shows the two most important basic circuits, denoted“circuit a”and“circuit b”used for the generation of impulse voltages. U«0) F Rd r-0 Cs R, cb u(t) (UO) i-OO =}= cs U u(t) circuit b Fig. 2. Basic diagrams of impulse voltage circuits In high voltage technology a single, unipolar voltage pulses is termed an impulse voltage, three important examples are shown in Fig. 3 with reference to the possible characteristic parameters. The time depen dence, as well as the duration of the impulse voltage, depend upon the method of generation. Measurement of high voltages in high voltage laboratories is done utilizing the following methods: a> Voltage dividers b> Sphere gaps c> High voltage electrostatic voltmeters d> Potential transformers.0,9 U-f o) b) front C) peak Fig. 3. Examples of impulse voltages a> Rectangular impulse voltage b> Wedge-shaped impulse voltage c> Double exponential impulse voltage The first method, using a voltage divider, allows measurement of the rms and peak values of voltage at both power and higher frequencies. It also allows measuring peak voltage during impulse tests. A voltage divider reduces the high voltage to be measured to a level which can be measured safely using commonly avaible instruments Cusually between 100 V and 200 V>. Three basic types of voltage divider are used: a> Capacitive b> Resistive c> Mixed. The capacitive voltage divider insures adequate frequency response over the power frequency range from IS Hz to lOO kHz. The voltage divider consists of one or several high voltage capacitors in series with a low voltage capacitor. The ratio between the capacitances of the high voltage and low voltage capacitors is also the voltage division ratio of the divider. The capacitance of the voltage divider is selected so that it does not load the voltage source excessively. At voltages above 1 MV, the problem of stray capacitance becomes si gni fici ant. In this case the voltage divider should be calibrated in place in order to achieve maxi mum accuracy. The typical values of capacitances of high voltage dividers are between 100 and 10O0O pF. A resistive voltage divider is usually employed for the measurement of direct voltages. If, however, XIthe ripples on the d. c. voltage are to be recorded as well, a resistive/ capacitive voltage divider will then be more suitable. The mixed voltage dividers consist of a combina tion of capacitive and resistive elements in series or parallel configuration. This type of divider is much more universal than the purely capacitive type. It pro vides acceptable frequency response for both A. C. and impulse voltages. The main feature of these dividers is good response characteristic shows a short rise time and is highly damped, which is important especially when measuring chopped wave impulses. The second method, using a sphere gap, is limited to measurement of peak voltage only. It can be used both at power frequency and during impulse tests. If the voltage measured is purely sinusoidal, the rms values can be calculated from the results obtained using a sphere gap. of; Sphere gaps can be used to measure the peak value a> Alternating voltages b> Impulse voltages c> Direct voltages. The main advantage of a sphere gap lies in its simplicity. The calibration curves for standard sphere gaps are avaible in all countries and are universally accepted. They can also be used as primary standards to calibrate other systems. The drawbacks of sphere gaps are large space requirements, relatively low accuracy and susceptibility to interferences in wave shape. The sphere gaps consist basically of two metal spheres of the same diameter in vertical or horizontal configura tion. The diameters of the spheres are standardized in IEC publication 52 and in IEEE std. 4-1978. The spheres are made in the following diameters: 2-5-6. 25-10-12. 5-1 S 25-50-75-1 OO-l 50 and 200 cm. The sphere gaps can be used for voltages up to 2.5 MV. For higher voltages, other measurement techniques are preferable. The third method, utilizing a high voltage elec trostatic voltmeter, is used to measure rms value of high voltages usually in a range of up to 50 kV or at most, to lOO kV. It is used where very low capacitances of the measuring circuit are required and mainly for power frequency tests. Electrostatic voltmeters are not suitable for measuring transient voltages or peak values of distorted power frequency voltages. xiiUsefulness of the SPICE, Simulation Program with Integrated Circuit Emphasis, circuit analysis program is demonstrated in high voltage engineering education, where assembling the actual circuit might be very bulky; time consuming and costly, or hand calculation technique could be very complicated. SPICE can be used for most high voltage systems in the laboratory, making use of time much more effective, ensuring therefore personal safety in the laboratory. The dimensions and equipment of a high voltage laboratory are primarily determined by the magnitude of the voltage to be generated. A second important feature is the intended application e.g. for teaching purposes, as a testing or research laboratory. The layout of a high voltage laboratory is an efficient testing facility. Laboratory arrangements differ very much from a single equipment to multi d. c., a. c. and impulse arrangements in different testing programmes. Each laboratory has to be desined individu ally considering the type of equipment to be tested, the avaible space, other accessories needed for the tests, the storage space required, etc. Earthing, control gear and the safety precautions require most careful conside ration. xiii

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