Electrostatic Voltmeters Potential Dividers Sphere gaps Rod-gape Peak Reading A.C. Voltmeter Digital Recording System

1. 6th
Sem
Electrical Engineering Department
Batch – B1 (2014 Batch)
Abhishek Choksi
(140120109005)
High Voltage Engineering (2160904)
ALA Presentation
On
“Measurement of HVAC”
Prepared By: Guided By:
Prof. Grishma Pipaliya
Gandhinagar Institute Of Technology

2. MEASUREMENT OF HIGH AC
VOLTAGES
Electrostatic Voltmeters
 Potential Dividers
Sphere gaps
Rod-gape
Peak Reading A.C. Voltmeter
Digital Recording System

3. Electrostatic Voltmeters
• 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.
• High-precision-type electrostatic voltmeters have
been built for very high voltages up to 1000 kV and
not possible to measure lower than 50v.

4. Potential Dividers
 High ohmic resistor voltage dividers
 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
 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.

5. Sphere gaps
• 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.
• The arrangement is selected based on the relation between
the peak voltage, determined by sparkover between the
spheres, and the reading of a voltmeter on the primary or
input side of the high-voltage source. This relation should
be within 3% (IEC, 1973).
• Standard values of sphere diameter are 6.25, 12.5, 25, 50,
75, 100, 150, and 200 cm

6. Rod-gap
• Rod gap is used to measure the peak values of power
frequency alternating voltages, direct voltages 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 or
greater than one half of the gap spacing overhangs the
inner edge of the support.
• The breakdown voltages as found in different gap lengths
as well as any atmospheric conditions also. The
breakdown voltage for the same spacing and the
uncertainties associated with the influence of humidity,
rod gaps are no longer used for measurement of AC or
impulse voltages

7. Digital Recording System
• Digital measurement of hvac is becoming
popular in recent time.
• The low voltage o/p of capacitor divider is
fed to a sample and hold circuit which is
controlled by a microprocessor system.
• The o/p of sample and hold circuit is fed to
an analog to digital converter, the
propotional digital o/p of which is fed to the
microprocessor based system as an input.
• Microprocessor system controls the sample
and hold circuit and the A/D converter.

8. References
• International Conference on Science, Technology, Engineering &
Management
• Ravindra Arora, Wolfgang Mosch
• McGraw-Hill -> M. S. Naidu, V. Kamaraju
• Wikipedia -- https://en.wikipedia.org/wiki/HVAC