6th Sem Electrical Engineering Department

Batch – B1 (2014 Batch)

Abhishek Choksi(140120109005)

High Voltage Engineering (2160904)

ALA PresentationOn

“Measurement of HVAC”

Prepared By: Guided By:

Prof. Grishma Pipaliya

Gandhinagar Institute Of Technology

MEASUREMENT OF HIGH AC VOLTAGES

Electrostatic Voltmeters Potential DividersSphere gapsRod-gape Peak Reading A.C. VoltmeterDigital Recording System

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.

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.

Sphere gaps

• Two adjacent metal spheres of equal diameters, whoseseparation distance is limited, form a sphere gap for themeasurement 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

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

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.

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