Module 6 : Measurement of High Test Voltages
Lecture 33 : Potential Dividers, their types and applications

Principle of Potential Dividers
For the measurement of High Voltages of any type, the best technique is by dividing the voltage. The HV potential divider arms could be pure capacitive, pure resistive or a combination of the two. The essential requirement is that the wave shape to be measured is correctly reproduced on the oscillograph with a known voltage reduction ratio. The general potential divider technique is shown in the following Figure 33.1.
Fig 33.1      Voltage Divider principle

Z1  - the high voltage arm of the divider. It could be pure C, pure R or combination of R and C as per requirement.

  • Values of Z1 and Z2  are chosen such that the potential across Z2   is around 100 V
  • The voltage division ratio can be given as;
                
    For the circuit in Fig 33.1       
    where
                  Z"2 =  terminal impedence of oscilloscope
 
For the measurement of high test voltages the technique of potential dividers is most accurate. It provides a continuous measurement of all the types of applied high voltages i.e. dc, ac and impulse. However, the suitability of the type of divider depends upon not only the type of voltage but also upon the range of magnitude of the voltage to be measured.
The types of potential dividers or basically the construction of the HV arm and their suitability are described in the following:
1.   Pure Capacitance Voltage Dividers:
The HV arm of the capacitive voltage dividers, usually having their capacitance in pF range, are compressed gas or   vacuum capacitors. The nF range HV capacitors are built in stack or in series a number of capacitors made out of polypropylene or paper filled with oil. In the absence of stray capacitances to earth with such HV capacitors, these provide desired exact value of low capacitance and small dimensions of the HV capacitive arm. The value of low voltage arm of the divider is normally chosen in µF range. Compressed gas (SF6)  filled HV capacitors are built even above 1000 kV (rms) rating. These can also be used as standard capacitors. In Fig 33.2, a photograph and a sketch of such a capacitor are shown.   The value of inductance of the long lead (in mts) connecting the low voltage electrode inside such a capacitor causes problems in giving a good response of the divider. Necessary constructional precautions are therefore taken in their design.

                             

Fig 33.2  (a)  Standard (compressed gas) capacitor for 1000 kV rms (Micafil, Switzerland)
(b) Cross-section of a compressed gas capacitor. 1. Internal HV electrode. 1'. External HV electrode. 2. Low-voltage electrode with guarding, 3. Supporting tube. 4. Coaxial connection to l.v. sensing electrode. 5. Insulating cylinder.

 

Pure capacitance voltage dividers are suitable for the measurement of power frequency and impulse high voltages. However, the cost of the HV capacitors is high and their cost and size increases with their voltage rating.

 
2.   Pure Resistance Potential Dividers
The high potential drop across a resistive arm by simple Ohm's law is used to reduce the voltage to a measurable quantities, circuits for such measurement are shown in Fig 33.3 (a) and (b)

Fig 33.3  Measurement of high dc and ac voltages by means of
(a)  ammeter in series with resistor R;
(b) voltage divider R1, R2 and voltmeter of negligible current input. OP, Over-voltages protection.

 

Such pure resistive arm or divider can be used for the measurement of dc, ac as well as impulse voltages. The wave forms of ac and impulse voltages could also be recorded with the help of an oscilloscope. However, while measuring very high voltages, the resistive arm inductance and stray capacitance cause large error in the accuracy of measurement and hence in response of the measuring device.
The wire-wound resistors used earlier were made of constantan, Cu & Ni, manganin, cu, Mn, Ni or Gr and also of nichrome. To overcome the error caused due to inductance of the wire wound resistors, thin film resistors are now used. A conducting element thin film is deposited on the surface of a glass or ceramic rod or tube. The value of the desired resistance is obtained by varying the thickness of the thin film. Such films could be of carbon, metal oxide; stannic and antimony oxides. Nickel-Chromium thin film is obtained by their evaporation in vacuum. Resistive voltage dividers are produced both screened (also known as shielded) and unscreened. Since the effect of stray capacitance is not uniform, it causes a non-uniform potential gradient along its length.
 

3.   Mixed Resistive and Capacitive Dividers

 
The problem caused by the stray capacitance in resistive voltage divider is over come by placing actual capacitors in parallel with the resistor units of the HV arm. This gives rise to a mixed resistive and capacitive divider used for highest voltage range of impulse voltage measurement. These are also known as 'damped capacitive divider' or RC divider. These were first introduced by Elsner in 1939, [6.1 ] . The simplified equivalent circuit of such a divider is shown in Fig 33.4

Fig 33.4    Simplified equivalent circuit for parallel-mixed resistor-capacitor dividers.

 
In this Figure R' represents the resistance of single element of the HV arm where a number of them are in series. C'p represents the capacitance across each of such element and C'e, is the stray capacitance to earth of the element. A photograph of such a  4.5 MV voltage divider is shown in Fig 33.5
 
Fig 33.5 Damped capacitor voltage divider for 4.5-MV impulse voltage, outdoor
 
 
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