Capacitances Potential Dividers

Capacitances Potential Dividers

Q. Explain With neat diagrams, why are capacitance voltage dividers preferred for high ac voltages measurement ?

Q. With a neat sketch explain trigatron spark gap used in impulse generators.

• Are ideal for measurements fast rising voltage and pulses. Capacitances resistances is independent of frequency if their leakage resistances is high enough to be neglected. Usually dividers are connected to the source voltage to rough long leads which introduce lead inductances and residual resistances. The low voltage capacitor C should be non inductive. 
• The coupling between high and low voltage arms of the divider be purely capacitive. Hence a low voltage arm should contain one capacitor only, two or more capacitors in parallel must be avoided because of appreciable inductance that would be introduced.
• A low resistances in parallel to C₂ would load the low voltage arm of the divider too heavily and decrease the output voltage with time. Since 1 and 2 from a potential divider and R = Z. The voltage input to the cable will be half of the voltage across the capacitors C_2. These halved voltage towards the open end of the cable (CRO end) and gets doubled after reflection. That is 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. The reflection takes place at Rand 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 ratios changes from

V₁ / V₂  = C₁ + C₂ / C₁ of high frequency to

C₁ + C₂ + C_d at low frequency C_d is comparable.

An additional damping resistance is usually connected in the lead on the high voltage side as shown in Figure A.

The performance of the divider can be improved. The damped capacitor divider acts for high frequency as a resistive dividers and for low frequency as a capacitive dividers.

i.e. can be used over a wide range of frequency.

Figure A

Figure B

Figure C

Use : =

Area for recording of slow or fast transient voltage.

1. Simple method. Compensated damping.
2. Stray inductance and capacitances connected by C₄ and R₄
3.  C₁ have high valve high length, high inductance and conform oscillatory circuit to test capacitance and hence to damp oscillation resisters are connected in series.
4.  C₂ should be single. If parallel combinations are used then produces error due to mutual inductance.

V₁ = C₁
 C₂ / C₁. V₂

=1 + C₂ / C₁

Merits of Capacitances Potential Dividers :

1. Simple in construction
2. Free from heating effects.
3. Comparatively free from effects of stray magnetic field and can be easily shunted 

Demerits of Capacitances Potential Dividers :

1. High cost.
2. Not suitable for measurements with test object of very low capacitances.
3. Error due to resistance of capacitance.

Mixed potential dividers : 

• R-C elements are connected in series or parallel. One method is to connect capacitance in parallel with each R’ elements and is successfully employed for voltage dividers of rating 2Mega volt and above. 
• Better construction is to use R-C series element connection as shown in Figure E and are made for 5 Mega volt with response timer less than 30 nano seconds. 
• The low voltage arm R₂ is given “L peaking” by connecting a variable inductances L in series with R₂ and for correctly designed voltage dividers L peaking will not be necessary.

Figure D

Advantages of Capacitances Potential Dividers :

1. Less numbers of capacitors are required.
2. Cost is less.

Disadvantages of Capacitances Potential Dividers :

1. Power loss I’R.
2. Leakage current.
3. Inductance effect.