UNIT-4 of HVE

of 18

  • date post

    02-Jun-2018
  • Category

    Documents

  • view

    222
  • download

    0

Embed Size (px)

Transcript of UNIT-4 of HVE

  • 8/10/2019 UNIT-4 of HVE

    1/18

    High Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringUnitUnitUnitUnit----IVIVIVIV

    Deependra SinghDeependra SinghDeependra SinghDeependra Singh95

    UNIT-IV

    MEASUREMENT OF HIGH VOLTAGES

    37. INTRODUCTION

    In industrial testing and research laboratories, it is essential to measure the voltages

    and currents accurately, ensuring perfect safety to the personnel and equipment. Hence a

    person handling the equipment as well as the metering devices must be protected against

    over-voltages and also against any induced voltages due to stray coupling. Therefore, the

    location and layout of the devices are important. Secondly, linear extrapolation of the devices

    beyond their ranges is not valid for high voltage meters and measuring instruments, and they

    have to be calibrated for the full range.

    Electromagnetic interference is a serious problem in impulse voltage and current

    measurements, and it has to be avoided or minimized. Therefore, even though the principles

    of measurements may be same, the devices and instruments for measurement of high voltages

    and currents differ vastly from the low voltage and low current devices. Different devices

    used for high voltage measurements may be classified as in Tables 7.1 and 7.2.

    Table 7.1: High Voltage Measurement Techniques

    Type of Voltage Method or Technique(i) Series Resistance Micro-ammeter

    (ii)Resistance Potential Divider

    (iii)Generating VoltmetersDC Voltages

    (iv)Sphere & spark gaps

    (i) Series Impedance ammeters

    (ii)Potential Dividers (R or C type)

    (iii)Potential Transformers

    (Electromagnetic or CVT)

    (iv)Electrostatic Voltmeters

    AC Power Frequency

    (v)Sphere Gaps

    (i) Sphere gaps

    (ii)potential dividers with a cathode ray

    oscillo-graph (capacitive, or resistive

    dividers)

    A.C. High Frequency Voltages, Impulse

    Voltages, and other rapidly changing

    voltages(iii)Peak voltmeters

  • 8/10/2019 UNIT-4 of HVE

    2/18

    High Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringUnitUnitUnitUnit----IVIVIVIV

    Deependra SinghDeependra SinghDeependra SinghDeependra Singh 96

    Table 7.2: High Current Measurement Techniques

    Type of Current Method or Technique

    (i) Resistive shunts with milli-ammeter

    (ii)

    Hall effect generators(a)

    Direct Currents(iii) Magnetic Links

    (i) Resistive shunts(b)Alternating currents (Power

    Frequency) (ii) Electromagnetic current transformers

    (i) Resistive shunts

    (ii) Magnetic potentiometers or Rogoswki coils

    (iii) Magnetic Links

    (c)High frequency AC, impulse

    and rapidly changing currents

    (iv)

    Hall effect generators

    REFERENCE:REFERENCE:REFERENCE:REFERENCE:

    HVE KAMARAJU NAIDU; 7.1; P-157-158

  • 8/10/2019 UNIT-4 of HVE

    3/18

    High Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringUnitUnitUnitUnit----IVIVIVIV

    Deependra SinghDeependra SinghDeependra SinghDeependra Singh97

    38. ELECTROSTATIC VOLTMETER-PRINCIPLE, CONSTRUCTION AND

    LIMITATION

    Coulombs law defines the electrical field as a field of forces, and since electrical

    fields may be produced by voltages, the measurement of voltages can be related to a force

    measurement. In 1884 Lord Kelvin suggested a design for an electrostatic voltmeter based

    upon this measuring principle. If the field is produced by the voltage V between a pair of

    parallel plane disc electrodes, the force F on an area A of the electrode, for which the field

    gradient E is the same across the area and perpendicular to the surface, can be calculated

    from the derivative of the stored electrical energy Weltaken in the field direction (x). Since

    each volume elementAdxcontains the same stored energy ( ) 2/2AdxEdWel = , the attracting

    force dxdWF el/= becomes

    2

    2

    2

    22V

    S

    AAEF

    == , (3.7)

    Where = permittivity of the insulating medium and S = gap length between the parallel

    plane electrodes.

    The attracting force is always positive independent of the polarity of the voltage. If

    the voltage is not constant, the force is also time dependent. Then the mean value of the force

    is used to measure the voltage, thus:

    ( )2...20

    2

    0

    2 2)(

    1

    2)(

    1smr

    TT

    VS

    Adtt

    TS

    AdttF

    T

    == , (3.8)

    where T is a proper integration time. Thus, electrostatic voltmeters are r.m.s.-indicating

    instruments.

    The design of most of the realized instruments is arranged such that one of the

    electrodes or a part of it is allowed to move. By this movement, the electrical field will

    slightly change which in general can be neglected. Besides differences in the construction of

    the electrode arrangements, the various voltmeters differ in the use of different methods of

    restoring forces required to balance the electrostatic attraction; these can be a suspension of

    the moving electrode on one arm of a balance or its suspension on a spring or the use of a

    pendulous or torsional suspension. The small movement is generally transmitted and

    amplified by a spotlight and mirror system, but many other systems have also been used. If

    the movement of the electrode is prevented or minimized and the field distribution can

    exactly be calculated, the electrostatic measuring device can be used for absolute voltage

  • 8/10/2019 UNIT-4 of HVE

    4/18

    High Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringUnitUnitUnitUnit----IVIVIVIV

    Deependra SinghDeependra SinghDeependra SinghDeependra Singh 98

    measurements, since the calibration can be made in terms of the fundamental quantities of

    length and forces.

    The paramount advantage is the extremely low loading effect, as only electrical fields

    have to be built up. The atmospheric air, high-pressure gas or even high vacuum between the

    electrodes provide very high resistivity, and thus the active power losses are mainly due to

    the resistance of insulating materials used elsewhere. The measurement of voltages lower

    than about 50 V is, however, not possible, as the forces become too small.

    The measuring principle displays no upper frequency limit. The load inductance and

    the electrode system capacitance, however, form a series resonant circuit, thus limiting the

    frequency range. For small voltmeters the upper frequency is generally in the order of some

    MHz.

    In spite of the inherent advantages of this kind of instrument, their application for

    H.V. testing purposes is very limited nowadays. For D.C. voltage measurements, the

    electrostatic voltmeters compete with resistor voltage dividers or measuring resistors, as the

    very high input impedance is in general not necessary. For A.C. voltage measurements, the

    r.m.s. value is either of minor importance for dielectric testing or capacitor voltage dividers

    can be used together with low-voltage electronic r.m.s. instruments, which provide acceptable

    low uncertainties. Thus the actual use of these instruments is restricted and the number of

    manufacturers is therefore extremely limited.

    REFERENCE:REFERENCE:REFERENCE:REFERENCE:

    High Voltage Engineering Fundamentals Kuffel, Zaengl, Kuffel; 3.2; P-94-96

  • 8/10/2019 UNIT-4 of HVE

    5/18

    High Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringUnitUnitUnitUnit----IVIVIVIV

    Deependra SinghDeependra SinghDeependra SinghDeependra Singh99

    39. GENERATING VOLTMETER

    PRINCIPLE & CONSTRUCTION

    Similar to electrostatic voltmeters the generating voltmeter, also known as the rotary

    voltmeter or field mill, provides a lossless measurement of D.C. and, depending upon the

    construction, A.C. voltages by simple but mainly mechanical means.

    The principle of operation is explained by Fig. 3.11. An adequately shaped, corona-

    free H.V. electrode excites the electrostatic field within a highly insulating medium (gas,

    vacuum) and ground potential. The earthed electrodes are subdivided into a sensing or pick-

    up electrode A, a guard electrode G and a movable

    electrode M, all of which are at same potential.

    Every field line ending at these electrodes binds free

    charges, whose density is locally dependent upon

    the field gradient E acting at every elementary

    surface area. For measurement purposes, only the

    elementary surface areas dA = aof the electrode A

    are of interest. The local charge density is then, (a)

    = E(a),with the permittivity of the dielectric.

    If the electrode M is fixed and the voltage V(or field-distributionE(a)) is changed, a current i(t)

    would flow between electrode A and earth. This current results then from the time-dependent

    charge density, (t,a), which is sketched as a one-dimensional distribution only. The amount

    of charge can be integrated by

    == AA

    daatEdaattq ),(),()( ,

    Where A is the area of the sensing electrode exposed to the field. This time-varying charge is

    used by all kinds of field sensors, which use pick-up electrodes (rods, plates, etc.) only. If the

    voltage V is constant, again a current i(t) will flow but only if M is moved, thus steadily

    altering the surface field strength from full to zero values within the covered areas. Thus the

    current is