Module 6 : Measurement of High Test Voltages
Lecture 30 : Introduction
High test voltages generated in laboratories require special techniques for their measurement. Each type of high voltage namely; ac (), dc (=) and impulse () can be suitably measured with more than one different techniques. The choice of the method of measurement in a laboratory is a matter of convenience, cost and the accuracy required for the measurement besides the safety. The complications in measurements increase with the magnitude of the voltage desired to be measured. The problems confine mainly with respect to the large structures of the device and the electric stress control techniques applied to prevent PB and flash overs.
The high voltage measurement techniques in electric power systems are different. These are not discussed in this course.
Different measurement techniques adopted for the three different types of voltages are given in Table 30.1
 
As it can be seen in this table, the earliest technique of sphere gap and the modern and most accurate technique of potential divider are applicable for the measurement of all the three types of voltages and both their polarities. The sphere gap technique, used widely for over a century, is not used any more. The disadvantages of this method are many. It does not provide a continuous measurement of the voltage and a flashover between the two spheres is essential. This measurement technique is not an accurate one. It may give error upto 3% in the measurement.
 The electrostatic voltmeter, suitable for the measurement of ac (rms)  and dc voltages, is one of the most accurate instrument. However, due to its size, cost and adjustment problems, it is not used very widely. For the measurement of ac power frequency voltage, the simplest way is to measure  the voltage on the primary side or at the low voltage side of the ac test transformer and calibrate the dial of the low voltage voltmeter by multiplication of the turns ratio to get the high output voltage. However, this technique could give rise error in two ways. First, in case of high capacitive loads, the leading current output from the test transformer results in higher actual voltage output than measured with the help of trunks ratio at the primary side, the well known Ferranti effect as shown in Fig. 30.1. Secondly, if the output voltage in not perfectly sinusoidal, the peak value is not times the rms value. To overcome this problem, measurement of peak value of the output voltage at the high voltage side is recommended.
 
Fig. 30.1 Phaser diagram of the output voltage with leading (capacitive) current
 
Table 30.1 High Test Voltage Measurement Techniques
Type of Voltage
Method or Technique
ac power frequency (i) Primary or low voltage measurement (rms)
(ii) Sphere gap (peak)
(iii) Electrostatic voltmeter (rms)
(iv) Series impedance and ammeter
(v) Potential dividers and oscilloscope
(resistive, capacitive or mixed)
(vi) Peak voltmeters with dividers
dc voltages (i) Sphere gap, either polarity
(ii) Series resistance and microammeter
(iii) Resistive potential divider
Impulse voltages
(li & si) either polarity and other high frequency high voltages 		(i) Sphere gap
(ii) potential dividers, oscilloscope
( capacitive, resistive & capacitive)
(iii) Peak voltmeters with series capacitors and potential dividers
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