High DC Voltage Measurement
The general objectives of D.C voltage measurements are :
- To measure the arithmetic mean value of the voltage , typically with an error of not more than 3%.
- To measure the ripple amplitude, typically with an error of not more than 10% of the actual ripple amplitude or not more than 1 % of the arithmetic mean value of the DC voltage, whichever is larger.
To fulfill the above error requirements , the measuring system has to satisfy certain specifications
(a) The voltage ratio of the voltage divider is stable and known with an error of to not more than 1%. In the case of high impedance systems where it may not be possible to comply with this specification, an overall error slightly exceeding 3% may have to be accepted.
(b) The current drawn from the high voltage source at full voltage is not less than 0.05 mA
(c) The frequency response of the system used for measuring ripple voltage is adequate and known within 10% for frequencies from the fundamental of the ripple frequency up to five times this frequency.
Measurements of High D.C. Voltage Measurement :
Potential dividers :
- Across low resistance arm R2 electrostatic voltmeter or high impendence voltmeter is connected.
- Due to sudden changes in voltages such as switching operations, flashover of the test objects or source short circuits, flashover or damage may occur to divider elements due to stray capacitances across elements. To avoid these transient voltages voltage controlling capacitors are connected across the elements.
With potential dividers high voltage = (R1 + R2 / R2) V2
Where V2 – DC voltage applied low voltage R2
Potential dividers are made with 0.05% accuracy up to 100kV, with 0.1% accuracy up to 300kV and with better than 0.5% accuracy for 500kV.