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 HVAC voltages. Even though D.C. tests on A.C cables is convenient and economical, these suffer from the fact that the stress distribution within the insulating material is different from the normal operating condition.

In industry it is being used for electrostatic precipitation of ashing in thermal power plants, electrostatic painting, cement industry, communication systems etc. HVDC is also being used extensively in physics for particle acceleration and in medical equipment's (X-Rays).

The most efficient method of generating high D.C. voltages is through the process of rectification employing voltage multiplier circuits. Electrostatic generators have also been used for generating

high D.C. voltages.

According to IEEE standards 4-1978, the value of a direct test voltage is defined by its arithmetic mean value and is expressed mathematically as where T is the time period of the voltage wave having a frequency f = 1/ T. Test voltages generated using rectifiers are never constant in magnitude. These 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 the maximum and minimum values of voltage i.e.,

and 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 a particu-lar equipment as the requirement on voltage shape may differ for different applications.

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