Capacitance Voltage Transformer

Capacitance Voltage Transformer 

The high voltage measurement is possible by utilizing the concept of resonance. The setup used for it is often known as capacitance voltage transformer. The capacitance voltage transformer is shown in Figure A its equivalent circuit is shown in Figure B.
It consists of 

C₁ – High voltage capacitance (value is in few thousand pico farads)

C₂ – Standard low voltage capacitance

M – Electrostatic voltmeter

L – Variable inductor (to achieve resonance condition)

T – Resonant transformer or matching transformer

Figure A

Figure B

V^’_M = Secondary voltage of transformer referred to primary

Z_T = Equivalent impedance of transformer referred to primary,

Z_T = √R²_T
 + X²_T

V_ZT = Voltage drop in equivalent impedance of transformer 

V_hv = High voltage to be measured.

V
_hv = V’ _M + V_ZT + V_C1

and V_C2 = V’ _M + V_ZT

∴ V _hv = V_C2 + V_ZT also
I_c1 = I_m + I_c2

The Hv side of transformer is rated up to 30 kV and LV side is rated up to 500 volt. 

The inductor L is varied such that resonance condition is obtained 

At resonance, Total inductive reactance of the circuit = Total capacitive reactance of the circuit

X_L = X_C

2 πf (L + L_T) = 1 / 2 πf (C₁ + C₂)

L = Inductance of variable inductor

L_T = Equivalent inductance of transformer referred to hv side (primary side) 

The phasor diagram is drown at resonance condition. The secondary circuit is assumed to be resistive so V^’_M and I_M are in phase 

• The voltage drops Is Ry and I X4 are added at the tip of V as shown. 
• Thus V_c2 is obtained.

• The voltage V_c1 is added at the tip of V_c2 to obtain V_hv i.e. the high voltage to be measured 

∴ OA = V_hv.