High Voltage Engineering

Transcription

1 High Voltage Engineering Course Code: EE 2316 Prof. Dr. Magdi M. El-Saadawi saadawi1@gmail.com 1

2 Contents Chapter 1 Introduction to High Voltage Technology Chapter 2 Generation of High Voltages and Currents Chapter 3 Measurement of High Voltages and Currents Chapter 4 Breakdown Mechanism of Gases, Liquid and Solid Materials 2

3 Chapter 3 Measurement of High Voltages and Currents 3.1. Introduction 3.2. Measurement of High Direct Current Voltages High Ohmic Series Resistance with Microammeter Resistance Potential Dividers for d.c. Voltages Generating Voltmeters 3.3. Measurement of High A.C. and Impulse Voltages Series Impedance Voltmeters Capacitance Potential Dividers and Capacitance Voltage Transformers Electrostatic Voltmeters Peak Reading a.c. Voltmeters Spark Gaps Potential Dividers 3.4. Measurement of High A.C. and Impulse Currents Measurement of High Direct Currents Measurement of High Frequency and Impulse Currents Cathode Ray Oscillographs for Impulse Measurements 3.5. Solved Examples 3

4 Table 3.1 High voltage Measurement Techniques 4

5 Table 3.2 High Current Measurement Techniques 5

6 3.3 Measurement of High A.C. and Impulse Voltages Measurement of high a.c. voltages employ conventional methods like series impedance voltmeters, potential dividers, potential transformers, or electrostatic voltmeters. But their designs are different from those of low voltage meters, as the insulation design and source loading are the important criteria. When only peak value measurement is needed, peak voltmeters and sphere gaps can be used. Often, sphere gaps are used for calibration purposes. Impulse and high frequency a.c. measurements invariably use potential dividers with a cathode ray oscillograph for recording voltage waveforms. 6

7 3.3.1 Series Impedance Voltmeters For power frequency a.c. measurements the series impedance may be a pure resistance or a reactance. In H.V. a capacitor is preferred as series reactance because: Resistances involve power losses, For high resistances, the variation of resistance with temperature is a problem, and The residual inductance of the resistance gives rise to an impedance different from its ohmic resistance. High resistance units for HV have stray capacitances and have an equivalent circuit as shown. 7

8 3.3.1 Series Impedance Voltmeters The entire resistor unit then has to be taken as a transmission line equivalent, for calculating the effective resistance. Also, the ground or stray capacitance of each element influences the current flowing in the unit, and the indication of the meter results in an error. The equivalent circuit of a high voltage resistor neglecting inductance and the circuit of compensated series resistor using guard and timing resistors is shown in Figs. 3.5a and b respectively 8

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10 3.3.1 Series Impedance Voltmeters In Fig. 3.5b stray ground capacitance effects can be removed by shielding the resistor R by a second surrounding spiral R s which shunts the actual resistor but does not contribute to the current through the instrument. By tuning resistors R a the shielding resistor end potentials may be adjusted with respect to the actual measuring resistor so that the resulting compensation currents between the shield and the measuring resistors provide a minimum phase angle. 10

11 3.3.2 Capacitance Potential Dividers and Capacitance Voltage Transformers 11

12 To avoid the drawbacks pointed out earlier, a series capacitor is used instead of a resistor for a.c. high voltage measurements. The schematic diagram is shown The current I c through the meter is: I c = jωcv where, Capacitance Potential Dividers C = capacitance of series capacitor, ω = angular frequency, and V= applied a.c. voltage. 12

13 Capacitance Potential Dividers Series capacitance voltmeters were used with cascade transformers for measuring rms values up to 1000 kv. The series capacitance was formed as a parallel plate capacitor between the high voltage terminal of the transformer and a ground plate suspended above it. The meter was usually a μa moving coil meter and the overall error was about 2%. 13

14 Objective: Measuring high A.C voltage using capacitive voltage dividers Components: An electrostatic voltmeter or a high impedance V.T.V.M. (vacuum-tube voltmeter) or an oscilloscope A standard compressed air or gas condenser, C 1 A large loss condenser (mica, paper,..). C 2 A long cable for connecting the HV source to the meter Wiring: as shown in Fig. 3.7 Procedure: Capacitance Potential Dividers 14

15 Capacitance Potential Dividers Procedure: Measure the value of C m Read the values of C 1 and C 2 Take the required H.V. measurement cautions Wire the circuit components as shown in Figure Connect the H.V. source to the connected circuit Read the voltmeter reading V 2 Calculate the Value of V 1 using Repeat the experiment and take the average 15

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17 Capacitance Voltage Transformer - CVT It is similar to series capacitance voltmeter A matching transformer is connected between the load or meter M and C 2 The value of the tuning choke L is chosen to make the equivalent circuit of the CVT purely resistive or to bring resonance condition: L= inductance of the choke, L T = equivalent inductance of the transformer referred to h.v. side. 17

18 Capacitance Voltage Transformer - CVT The voltage ratio becomes: (neglecting the voltage drop I m X e which is very small compared to the voltage V C1 ) where V Ri is the voltage drop in the transformer and choke windings Prof. Dr. Magdi El-Saadawi 18

19 Capacitance Voltage Transformer - CVT The advantages of a CVT are: simple design and easy installation, can be used both as a voltage measuring device for meter and relaying purposes. frequency independent voltage distribution along elements as against conventional magnetic potential transformers which require additional insulation design against surges, and provides isolation between the high voltage terminal and low voltage metering. The disadvantages of a CVT are: the voltage ratio is susceptible to temperature variations, and the problem of inducing ferro-resonance in power systems. Prof. Dr. Magdi El-Saadawi 19

20 3.3.5 Spark Gaps A uniform field spark gap will always have a spark over voltage within a known tolerance under constant atmospheric conditions. Hence a spark gap can be used for measurement of the peak value of the voltage, if the gap distance is known. Normally, only sphere gaps are used for voltage measurements. In certain cases, uniform field gaps and rod gaps are also used, but their accuracy is less Sphere gap breakdown is independent of the voltage waveform and hence is suitable for measuring the peak value of all H.V. types: d.c., a.c. and impulse voltages of short rise times (rise time > 0.5 μs). 20

21 3.3.5 Spark Gaps Sphere gaps can be arranged either Vertically with lower sphere grounded, or horizontally with both spheres connected to the source voltage or one sphere grounded. The two spheres used are identical in size and shape. Spheres are generally made of copper, brass, or aluminum; the latter is used due to low cost. One sphere is grounded and the other is connected to the HV source A series resistance is usually connected between the source and the sphere gap to: limit the breakdown current The standard diameters for the spheres are as shown in 10/28/ tables: 2,5,6.25,10,12.5,15,25,50,75,100,150, and 200 cm.

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24 3.3.5 Spark Gaps Procedure: The voltage to be measured is applied to the sphere The distance or spacing S between them is decreased until the spark occur Ground the spheres to discharge the electrical charges Take the distance S between spheres and compute the value of the measured voltage from the tables Repeat the experiment and take the average 24

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27 Factors Influencing the Sparkover Voltage of Sphere Gaps nearby earthed objects, atmospheric conditions and humidity, irradiation, and polarity and rise time of voltage waveforms. 27

28 (i) Effect of nearby earthed objects 28

29 (ii) Effect of atmospheric conditions Humidity effect increases with the size of spheres and is maximum for uniform field gaps, and Sparkover voltage increases with the partial pressure of water vapor in air, and for a given humidity condition, the change in sparkover voltage increases with the gap length.

30 (iii) Effect of Irradiation Illumination of sphere gaps with ultra-violet or x- rays aids ionization in gaps easy. يساعد فى تسهيل عمليات التأين The effect of irradiation is pronounced واضح for small gap spacings. Hence, irradiation is necessary for smaller sphere gaps of gap spacing less than 1 cm for obtaining consistent values. 30

31 (iv) Effect of polarity and waveform It has been observed that the sparkover voltages for positive and negative polarity impulses are different. Experimental investigation showed that for sphere gaps of 6.25 to 25 cm diameter, the difference between positive and negative d.c. voltages is not more than 1%. For smaller sphere gaps (2 cm diameter and less) the difference was about 8% between negative and positive impulses of 1/50 μs waveform. 31

32 Rod Gaps A rod gap may be used to measure the peak value of power frequency and impulse voltages. The gap usually consists of two 1.27 cm square rod electrodes square in section at their end and are mounted on insulating stands so that a length of rod equal to or greater than one half of the gap spacing overhangs the inner edge of the support. The arrangement consists of two hemispherically capped rods of about 20 mm diameter as shown in Fig The accuracy of the above relation is better than 20% and, therefore, provides better accuracy even as compared to a sphere gap. 32

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