Generation of High DC Voltages

• Testing of cables : HVDC is used for testing HVAC cables of long lengths as these have very large capacitance and would require very large values of currents if tested on alternating voltages.
• In industries it is being used for.

1. Electro-static precipitation of ashing in thermal power plants.
2. Cement industry
3. Communication systems.
4. Electro-static painting etc.

• It is also being used extensively in physics for particle acceleration and
• In medical equipments like X- rays.

Generation of DC voltages :

• The most efficient method of generating high DC voltage is through the process of rectification employing voltage multiplier circuits.
• Electro-static generators have also been used for generating high DC voltages. According to IEEE standard 4-1978, the value of direct test voltage is defined by its arithmetic mean value V_d and is expressed mathematically as

• T is the time period of voltage wave having a frequency f = 1/T.
• Test voltages generated by use of these rectifiers are never constant in magnitude but deviate from the mean value periodically and this deviation is known as ripple.
• The magnitude of the ripple voltage denoted by 𝛿V is defined as half the difference between maximum and minimum values of voltage i.e.

𝛿V = 1/2 [V_max – V_min]

• Ripple factor is defined as the ratio of ripple magnitude to the mean value V_d i.e.
  𝛿V/V_d.
• The test voltages should not have ripple factor more than 5 % or as specified in a specific standard for particular equipment as the requirement on voltage may differ for various applications .