Mesurement Of High Voltages & High Currents. Unit 4. High Voltage Measurement Techniques. Sphere Gaps. Applicatios : Voltage Measurement (Peak) - Peak values of voltages may be measured from 2 kV up to about 2500 kV by means of spheres. Arrangements:
Fig C :Breakdown voltage characteristic of sphere gaps
For d.c. Voltages,
If the capacitance varies linearly with time and reaches its peak value Cm is time Tc/2 and again reduces to zero linearly in time Tc /2, the capacitance is given as
If the capacitance C varies sinusoidally between the limits C0 and (C0 + Cm) then
C = C0 + Cm sin ωt
and the current ‘i' is then given as, i(t) = im cos ω t , where im = VCmω
Here ω is the angular frequency of variation of the capacitance.
I(t) = n/30 CmV(t)
Absolute Electrostatic Voltmeter
where V1,V2 ,... ,Vn represent the rms value of the fundamental, second... and nth harmonics.
I1=ωCV1 , I2=2ωCV2 , ……In=nωCVn
Chubb Frotscue Method:
(Chubb Frotscue Method Continued…)
Digital Peak Voltmeter:
Digital Peak Voltmeter continued….
Peak voltmeter with Potential divider:
V2 is reduced
where, B-Magnetic Flux density
d-Thickness of the metal plate
R-Hall Coefficient (depends on Material of the plate & temperature)
R is small for metals and High for semiconductors
θ = α Bl
α = A constant of the cyrstal which is a function of the wave length of the light.
B = Magnetic flux density due to the current to be measured in this case.
l = Length of the crystal.
Extended series impedance with inductance neglected
C – capacitor
D1,D2 – Diodes
OP – Protective devices
I – indicating meter
V(t) – voltage waveform
Ic(t) – capacitor current waveform
T – period
Ub = kd Ub0
HALL GENERATORS FOR D.C CURRENT MEASUREMENTS
Figure. a:- Measuring spheres
Fig.b:Breakdown voltage characteristic of sphere gaps
Figure. d: Sphere gap
When the gap distance is increased, the uniform field between the spheres becomes distorted, and accuracy falls.
Figure . C:- Breakdown voltage characteristics