DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
Part I- Chapter 3: Insulation test techniques
3.2 Measurement of high voltages
Instructor: Dr. Jian Li
Lecture 7-1
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2 Measurement of high voltages
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Major approaches of HV measurement in laboratory
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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 systems
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Reference measuring systems
Approved measuring systems
Lecture 7-2
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.1 Peak voltage measurement by sphere gaps
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Two 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
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Breakdown voltage of air gap in uniform field is determined by gap
distance.
A quasi-uniform field between the two spheres
Lecture 7-3
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.1 Peak voltage measurement by sphere gaps
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Vertical sphere gap
1. Insulating support
2. Sphere shank
3. Operating gear, showing
maximum dimensions
4. High-voltage connection with
series resistor
5. Stress distributor, showing
maximum dimensions.
P. Sparking point of HV sphere
A. Height of P above ground plane.
B. Radius of space free from external
structures
X. 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.
Lecture 7-4
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.1 Peak voltage measurement by sphere gaps
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Clearance around the spheres
Lecture 7-5
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.1 Peak voltage measurement by sphere gaps
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Remarks on the use of the sphere gap
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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.
Lecture 7-6
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.2 Electrostatic voltmeters
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Electrostatic 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.
Lecture 7-7
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.3 Peak voltmeters
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Disruptive 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.
Lecture 7-8
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.3 Peak voltmeters
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A.C. peak voltage measurement by Chubb and Fortescue
(a) Fundamental circuit.
(b) Recommended actual circuit
1
Ic =
T
t2
∫t
1
C
i c ( t )dt =
T
t2
∫t
1
2Vmax C
dV =
T
Lecture 7-9
⇒
Vmax
Ic
=
2 fC
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.3 Peak voltmeters
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Peak voltmeter for AC measurements according to Davis,
Bowdler and Standring
Vd = Vm exp(−t / RC )
⇒ Vd ≈ [Vm + Vm exp(−T / RC )] / 2
Lecture 7-10
Vm =
Vd
T
1−
2 RC
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
1. Basic concept of voltage dividers
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Voltage dividing ratio is as:
k =V1 /V2 = (Z1+Z2 ) / Z2
Basic requirement for voltage dividers
‹ No voltage distortion
‹ Steady voltage dividing ratio
‹ Not to influence measured voltages
Lecture 7-11
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
2. High ohmic resistor voltage dividers
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Voltage dividing ratio k =(R1+R2 )/R2
To keep i << i2
Current flowing through voltage divider
„ Upper limits of 1 to 2 mA
„ Lower limit about 100 μA
Very low temperature coefficients.
„ Wire-wound metal resistors made from
Cu–Mn, Cu–Ni, and Ni–Cr alloys
„ Down 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.
Lecture 7-12
Mainly for
DC voltage
measurement
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
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HV resistor element
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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
X100-M, 100-kV standard resistor
Lecture 7-13
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
3. Capacitor voltage dividers
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Voltage dividing ratio k =(C1+C2 )/ C1
C1: 100~200 pF
Stray capacitors to surrounding objects and
ground influence the voltage dividing ratio.
Requirements for HV capacitors
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Independent of magnitude of voltage level
and no ageing effects
Very small temperature coefficient
Small effective inductivity
Lecture 7-14
Mainly for
AC voltage
measurement
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
4. 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
Lecture 7-15
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
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Resistor voltage dividers (for impulse voltages)
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HV resistor: 2000 – 20000 ohms
Resistor voltage dividing circuit
for impulse voltage measurement
R2 + R3 = Z
R4 = Z
k = n[( R1 + R2 )( R3 + R4 ) + R1 R2 ] / R2 R4
Equivalent circuit for resistor
voltage dividers
Lecture 7-16
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
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Capacitor voltage dividers (for impulse voltages)
Capacitor voltage dividing circuit with a
matched resistor at head terminal of cable
t = 0+ ,
t > 2τ ,
k1 = (C1 + C 2 ) / C1
k 2 = (C1 + C 2 + C0 ) / C1
Lecture 7-17
Capacitor voltage dividing circuit with
matched resistors at both terminals of cable
t = 0+ ,
t > 2τ ,
k1 = 2(C1 + C 2 ) / C1
k 2 = (C1 + C 2 + C3 + C0 ) / C1
= 2(C1 + C 2 ) / C1
DEPT OF HIGH VOLTAGE AND INSULATION ENG， CHONGQING UNIVERSITY
FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING
3.2.4 Voltage dividing systems
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Capacitor voltage dividers (for impulse voltages)
When R’ is a real resistor, the capacitor voltage becomes a
series-damped capacitor voltage divider.
t = 0+ ,
k1 = ( R1 + R2 ) / R2
t → ∞,
k 2 = (C1 + C 2 ) / C1
C1 R1 > C 2 R2
Lecture 7-18