Measuring Capacitors /Peak Voltage Measurement According to Chubb and Fortescue

Measuring Capacitors / Peak Voltage Measurement According to Chubb and Fortescue

• As opposed to sphere gaps, the circuit suggested by Chubb and Fortescue in 1913 is capable of measuring the peak value of a high AC voltage continuously and accurately. Figures A and B shows the circuit with its current and voltage curves.

(a) Circuit

(b) Current and voltage curves.

• A charging current I, given by the rate of change of the applied voltage (t), flows through the high voltage capacitor C and is passed through two anti parallel rectifiers V₁ and V₂ to ground.
• The arithmetic mean value I₁ of current i₁ in the left hand branch is measured with a moving coil instrument.
• As shown below, this current is proportional to the peak value U of the high voltage provided that certain conditions are fulfilled.

Figure A

If the behavior of the rectifiers is assumed ideal, then for the conducting period of one has : 

i_l = i = c du
/dt

for t = 0 to T/2

I_l =

If the voltage is symmetrical
with reference to the zero line :

U (T/2) – u(0) = 2u     ……(2)

And with T = 1/f, one
obtains

U = I_I 1 / 2 fc     …(3)

• In the derivation of this expression , it is not assumed that u (t) is a sinusoid, but when semiconductor rectifiers are used, only one maximum per half period can occur. 
• The use of synchronous mechanical rectifiers or controllable rectifiers (oscillating contacts, rotating rectifiers) allows correct measurement of AC voltages with more than one maximum per half-period. Oscillographic monitoring of high voltage shape is necessary and is usually done by observing the current i₁, which is allowed to have only one crossover in each half-period. 
• The half period circuit is shown in Figures C and D. In this circuit the measuring capacitor C_m is charged to the peak value U of the lower arm voltage u (t) of the capacitive divider. 
• The resistor which discharges the capacitor C_m is meant to follow reductions of the applied voltage. 
• The choice of time constant for this discharge process is determine by the desired response of the measuring system. In general one chooses,   

R_m C_m < 1 Second, and    …(4)

R_m C_m >> 1 / f                  …(5)

Figure B

• The resistor R₂ is necessary to minimize charging of C₂ by the current flowing through the rectifier V_m. The value of R₂ must be chosen in such a way that the DC voltage drop across R₂ which causes DC charging of C₂ remains as small as possible. 
• In this case one must have <<R_m. On the other hand, the capacitive divider ratio should not be affected much by, which requires  R₂ >> 1 / ω C₂.
• With all these conditions, the relation between the peak value of the high voltage and the indicated voltage U_m.is given by :

U = C₁ + C₂
/ C₂ U_m

• The working principle of the “Impulse Peak Voltmeter” of the UBC high voltage test set is based on this technique, even through a more elaborate circuit is employed.
• In this control box, there is also an AC voltmeter which
• The measuring instrument ammeter reads the magnitude of charge per cycle operates on the same principle but indicates peak value divided by √2(U /√2)  instead of peak value. or mean value of current 

I_C = C (dU / dt)

Figure C