Capacitance Potential Dividers
Capacitance potential dividers are more complex than the resistance type. For measurement of impulse voltages not exceeding 1 MV capacitance dividers can be both portable and transportable. In general, for measurement of 1 MV and over, the capacitance divider is a laboratory fixture. The capacitance dividers are usually made of capacitor units mounted one above the other and bolted together. It is this failure which makes the small dividers portable. A screening box similar to that described earlier can be used for housing both the low voltage capacitor unit C2 and the matching resistor if required.
The low voltage capacitor C2 should be non-inductive. A form of capacitor which has given excellent results is of mica and tin foil plate, construction, each foil having connecting tags coming out at opposite corners. This ensures that the current cannot pass from the high voltage circuit to the delay cable without actually going through the foil electrodes. It is also important that the coupling between the high and low voltage arms of the divider be purely capacitive. Hence, the low voltage arm should contain one capacitor only; two or more capacitors in parallel must be avoided because of appreciable inductance that would thus be introduced. Further, the tappings to the delay cable must be taken off as close as possible to the terminals of C2. Fig. 4.21 shows variants of capacitance potential dividers.
Figure: 4.13 Capacitor dividers (a) Simple matching (b) Compensated matching(c) Damped capacitor divider simple matching For voltage dividers in FIG. (b) and (c), the delay cable cannot be matched at its end.
A low Resistor in parallel to C2 would load the low voltage arm of the divider too heavily and decrease the output voltage with time. Since R and Z form a potential divider and R = Z, the voltage input to the cable will be half of the voltage across the capacitor C2. This halved voltages travels towards the open end of the cable (CRO end) and gets doubled after reflection. That is, the voltage recorded by the CRO is equal to the voltage across the capacitor C2. The reflected wave charges the cable to its final voltage magnitude and is absorbed by R (i.e. reflection takes place at R and since R = Z, the wave is completely absorbed as coefficient of voltage reflection is zero) as the capacitor C2 acts as a short circuit for high frequency waves. The transformation ratio, therefore, changes from the value:
However, the capacitance of the delay cable Cd is usually small as compared with C2.For capacitive divider an additional damping resistance is usually connected in the lead on the High voltage side as shown in FIG. 4.14 (c). The performance of the divider can be improved if damping resistor which corresponds to the a periodic limiting case is inserted in series with the individual element of capacitor divider. This kind of damped capacitive divider acts for high frequencies as a resistive divider and for low frequencies as a capacitive divider. It can, therefore, be used over a wide range of frequencies i.e. for impulse voltages of very different duration and also for alternating voltages.
Fig. 4.22 shows a simplified diagram of a resistance potential divider after taking into Considerations the lead in connection as the inductance and the stray capacitance as lumped capacitance. Here L represents the loop inductance of the lead-in connection for the high voltage arm. The damping resistance Rd limits the transient overshoot in the circuit formed by test object, L, Rd and C. Its value has a decided effect on the performance of the divider. In order to evaluate the voltage transformation of the divider, the low voltage arm voltage V2 resulting from a square wave impulse V1 on the hv side must be investigated. The voltage V2 follows curve 2 in Fig. 4.15 (a) in case of a periodic damping and curve 2in Fig. 4.15 (b) in case of sub-critical damping. The total area between curves 1 and 2 taking into consideration the polarity is described as the response time.
With subcritical damping, even though the response time is smaller, the damping should not be Very small. This is because an undesirable resonance may occur for a certain frequency within the passing frequency band of the divider. A compromise must therefore be realized between the short rise time and the rapid stabilization of the measuring system. According to IEC publication No. 60 a maximum overshoot of 3% is allowed for the full impulse wave, 5% for an impulse wave chopped on the front at times shorter than 1 micro sec. In order to fulfill these requirements, the response time of the divider must not exceed 0.2 micro sec. for full impulse waves 1.2/50 or 1.2/5 or impulse waves chopped on the tail. If the impulse wave is chopped on the front at time shorter than 1 micro sec the response time must be not greater than 5% of the time to chopping.
