3.2 measurement of high voltages e and insulation...
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FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-1
3.2 Measurement of high voltages
Part I- Chapter 3: Insulation test techniques
Instructor: Dr. Jian Li
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-2
3.2 Measurement of high voltagesMajor approaches of HV measurement in laboratory
Measurement of peak values of AC, DC, and both types of impulse high voltages by using sphere gaps.Measurement of effective values of AC and DC high voltages by using electrostatic voltmeters.Measurement of AC, DC, and impulse voltages by using voltage dividing systems.Measurement of AC high voltages by using peak voltmeters.Optical measurement systems of high voltages.
Basic classification of HV measurement systemsReference measuring systemsApproved measuring systems
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-3
3.2.1 Peak voltage measurement by sphere gapsTwo adjacent metal spheres of equal diameters, whose separation distance is limited, form a sphere gap for the measurement of the peak value of either AC, DC or both types of impulse voltages.Basic mechanisms of HV measurement by sphere gaps
Breakdown voltage of air gap in uniform field is determined by gap distance.A quasi-uniform field between the two spheres
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-4
3.2.1 Peak voltage measurement by sphere gaps
1. Insulating support2. Sphere shank3. Operating gear, showing
maximum dimensions4. High-voltage connection with
series resistor5. Stress distributor, showing
maximum dimensions.P. Sparking point of HV sphereA. Height of P above ground plane.B. Radius of space free from external
structuresX. Item 4 not to pass through this
plane within a distance B from P. Note: The figure is drawn to scale for a 100-cm sphere gap at radius spacing.
Vertical sphere gap
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-5
3.2.1 Peak voltage measurement by sphere gapsClearance around the spheres
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-6
3.2.1 Peak voltage measurement by sphere gapsRemarks on the use of the sphere gap
To avoid excessive pitting of the spheres, protective series resistances may be placed between test object and sphere gap.
For AC and DC voltages, the value of the protective resistor may range from 0.1 to 1 MΩ.For impulse voltages, it should not exceed 500 ohms and its inductance should be smaller than 30 μH.
Influenced by irradiation and humidity.
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-7
3.2.2 Electrostatic voltmetersElectrostatic voltmeters are related to measurement of electrical field force generated by voltages between a pair of parallel plane disc electrodes.Electrostatic voltmeters are RMS-indicating instruments for AC and DC voltages.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. High-precision-type electrostatic voltmeters have been built for very high voltages up to 1000 kV.
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-8
3.2.3 Peak voltmetersDisruptive discharge phenomena within electrical insulation systems or high-quality insulation materials are in general caused by the instantaneous maximum field gradients stressing the materials.The necessary calibration procedure and the limited accuracy of the sphere gap are hindering its daily application and call for more convenient methods.Peak voltmeters are based on a simple but accurate method for the measurement of peak values of AC voltages, proposed by Chubb and Fortescue.
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-9
3.2.3 Peak voltmetersA.C. peak voltage measurement by Chubb and Fortescue
(a) Fundamental circuit.(b) Recommended actual circuit
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FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-10
3.2.3 Peak voltmetersPeak voltmeter for AC measurements according to Davis, Bowdler and Standring
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FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-11
3.2.4 Voltage dividing systems1. Basic concept of voltage dividers
Voltage dividing ratio is as:k =V1 /V2 = (Z1 +Z2 ) / Z2
Basic requirement for voltage dividersNo voltage distortionSteady voltage dividing ratioNot to influence measured voltages
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-12
3.2.4 Voltage dividing systems2. High ohmic resistor voltage dividers
Voltage dividing ratio k =(R1+R2 )/R2
To keep i << i2
Current flowing through voltage dividerUpper limits of 1 to 2 mALower limit about 100 μA
Very low temperature coefficients.Wire-wound metal resistors made from Cu–Mn, Cu–Ni, and Ni–Cr alloysDown to about 10-5/K
Distributed stray capacitance to ground causes a strongly non-linear voltage distribution along a resistor column and overstresses individual elements during breakdown of a test object.
Mainly for DC voltage measurement
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-13
3.2.4 Voltage dividing systemsHV resistor element
Wire-wound resistors are not only very expensive to produce, but also quite sensitive to sudden voltage drops.
▲Sketch of cross-section of an HV resistor element
100-M, 100-kV standard resistor
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-14
3.2.4 Voltage dividing systems3. Capacitor voltage dividers
Voltage dividing ratio k =(C1+C2 )/ C1
C1: 100~200 pFStray capacitors to surrounding objects and ground influence the voltage dividing ratio.Requirements for HV capacitors
Independent of magnitude of voltage level and no ageing effectsVery small temperature coefficientSmall effective inductivity
Mainly for AC voltage measurement
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-15
3.2.4 Voltage dividing systems4. Generalized impulse voltage measuring circuit
1. Voltage supply. 2. Lead to test object. 3. Test object. 4. Lead to voltage divider. 5. Voltage divider. 6. Signal or measuring cable. 7. Recording instrument. 8. Ground return
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-16
3.2.4 Voltage dividing systemsResistor voltage dividers (for impulse voltages)
HV resistor: 2000 – 20000 ohms
Resistor voltage dividing circuit for impulse voltage measurement
Equivalent circuit for resistor voltage dividers
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FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
EP
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Lecture 7-17
3.2.4 Voltage dividing systemsCapacitor voltage dividers (for impulse voltages)
Capacitor voltage dividing circuit with a matched resistor at head terminal of cable
Capacitor voltage dividing circuit with matched resistors at both terminals of cable
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FUNDAMENTALS OF HIGH VOLTAGE ENGINEERINGD
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Lecture 7-18
3.2.4 Voltage dividing systemsCapacitor voltage dividers (for impulse voltages)
When R’ is a real resistor, the capacitor voltage becomes a series-damped capacitor voltage divider.
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