3/23/2012 CHAPTER 2 MEASUREMENT OF HIGH VOLTAGE AND CURRENTS • 2.1 MEASUREMENT OF HIGH DIRECT VOLTAGES • 2.2 MEASUREMENT OF HIGH ALTERNATING VOLTAGES • 2.3 MEASUREMENT OF IMPULSE VOLTAGE & CURRENT 1
3/23/2012 Introduction � There are difficulties in measurement of such high voltages. � The measurement technique should be able to control high electric fields . � Sometimes the heat dissipation within the measuring circuits may also have to be controlled. 2
3/23/2012 Introduction � Generally the mean values of dc voltage and peak value of ac and impulse voltages are of practical interest. � There are certain measuring techniques which are applicable to all the three types of voltages (ac, dc and impulse). � There are others which are specific to a particular type of voltage. � The measuring methods will be classified according to the types of voltages to be measured. 3
3/23/2012 � Measurement of high dc voltages as in low voltage measurements, is generally accomplished by extension of meter range with a large series resistance � The net current in the meter is usually limited to one to ten microamperes for full-scale deflection. � a resistance potential divider with an electrostatic voltmeter is sometimes better when high precision � Spark gaps such as sphere gaps are gas discharge devices and give an accurate measure of the peak voltage � the measurement is affected by the atmospheric conditions like temperature, humidity, etc 4
3/23/2012 Con’t… High voltage Measurement Techniques Type of voltage Method (a) D.C voltages 1) Series resistance microammeter 2) Resistance potential divider 3) Generating voltmeters 4) Sphere and other spark gaps (b) A.C voltages 1) Series impedance ammeters 2) Potential dividers 3) Potential transformers 4) Electrostatic voltmeters 5) Sphere gaps (c) Impulse voltages 1) Potential dividers (cathode ray oscillograph) 2) Peak voltmeters 3) Sphere gaps 5
3/23/2012 Con’t… High current Measurement Techniques Type of current Method (a) Direct current 1) Resistive shunts with miliammeter 2) Hall effect generators 3) Magnetic links (b) Alternating currents 1) Resistive shunts 2) Electromagnetic current transformers (c) Impulse current 1) Resistive shunts 2) Magnetic potentionmeters / Rogowski coils 3) Magnetic links 4) Hall effect generators 6
3/23/2012 Measurement Of High Direct Voltages 2.1.1 Series Resistance Microammeter � An impedance can be used in series with a micro (or milli-) ammeter for the measurement of high voltages � The current through the instrument will be proportional to the applied voltage � If the impedance is a resistor, the current is in phase with and faithfully represent the voltage � The current I flowing through R is measured by micro ammeter, where the source voltage is given by V = I R 7
3/23/2012 Measurement Of High Direct Voltages Series Resistance Microammeter 8
3/23/2012 Measurement Of High Direct Voltages � The voltage drop in the meter is insignificant as the impedance of the meter is only few ohms. � A protective device like a paper gap or zener diode with a suitable series resistance as a protection against high voltage. � The ohmic value of the series resistance R is chosen such that a current of one to ten microamperes is allowed for full-scale deflection. � The resistance is constructed from a large number of wire wound or thin film resistor in series. � The voltage drop in each resistor element is chosen to avoid surface flashovers and discharges 9
3/23/2012 Measurement Of High Direct Voltages � The limitations in the series resistance design are: (i) Power dissipation and sources loading (ii) Temperature effect and long time stability (iii) voltage dependence or resistive elements (iv) Sensitivity to mechanical stresses � Series resistance meters are built for 500 kV d.c. with an accuracy better than 0.2%. 10
3/23/2012 Measurement Of High Direct Voltages 2.1.2 Potential Dividers � A potential divider consists of two impedances, Z1and Z2, connected in series, to which the voltage to be measured is applied. � The components that constitute the impedances are referred to as the high-and low-voltage arms of the divider 11
3/23/2012 Measurement Of High Direct Voltages � Connection between the low-voltage arm and the measuring instrument must be made through a shielded coaxial cable to avoid the adverse effects of stray capacitance between that connection and the high-voltage arm � High-voltage dividers generally consists of either resistors or capacitors, but sometimes a combination of resistors and capacitors, in either series or parallel is used 12
3/23/2012 Measurement Of High Direct Voltages Resistance Potential Dividers � A resistance potential divider with an electrostatic or high impedance voltmeter is shown. � Usually employed for the measurement of direct voltages • P – Protective device • ESV – Electrostatic voltmeter 13
3/23/2012 Measurement Of High Direct Voltages � The high voltage magnitude is given by � Where V2 is the dc voltage across the low voltage arm R2. � With sudden change in voltage, such as switching operations, flash over of the test objects or source short circuits, damage may occur to the divider elements. � To avoid these transient voltages, voltage controlling capacitors are connected across the elements. 14
3/23/2012 Measurement Of High Direct Voltages Exercise 1 It is required to measure 100 kV peak DC voltage using a suitable voltage divider and a suitable low voltage meter. If a 100 volt DC voltmeter is available, design a suitable voltage divider and calculate the values of the elements needed. Assume that the current I flowing through the circuit is 1mA. 15
3/23/2012 Measurement Of High Direct Voltages Sphere Gap � Sphere gaps technique are reliable only for certain gap configurations. � Normally, only sphere gaps are used. In certain cases uniform field gaps and rod gaps are also used, but their accuracy is less. � IEC-Publ. 52-1960, BS 358 shows the tabulated breakdown voltages for standard conditions and various sphere diameters as a function of the gap spacing. 16
3/23/2012 Measurement Of High Direct Voltages � The actual breakdown voltage Vd at air density d may be found from the tabulated value, Vdo by the following formula; V d = k V do (kV) where V do = 24.22 S + 6.08 √S for uniform field at standard conditions of temperature T=20 0 C and pressure p =1013 milibars or 760mmHg S is the gap length in cms. 17
3/23/2012 Measurement Of High Direct Voltages � Sphere gap can be arranged either Horizontal with both spheres connected to the sources a) voltage or one sphere grounded Vertically with lower sphere grounded b) 18
3/23/2012 Measurement Of High Direct Voltages � The voltage to be measured is applied between the two spheres and the distance spacing S between them gives a measure of the spark over voltage. � Standard values of sphere diameter are specified. � Standard table lists standardized disruptive voltages from the results of large number of international experiments. 19
3/23/2012 Tabulated value of breakdown voltage (BS 358) 20
3/23/2012 Measurement Of High Direct Voltages Exercise 2 Determine the breakdown voltage for air gaps of 5 mm and 15 mm lengths under uniform field and standard atmospheric conditions. Also, determine the voltage if the atmospheric pressure is 750 mm Hg and temperature 35 ° C. 21
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