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GENERATION OF HIGH VOLTAGES AND HIGH CURRENTS
There are various applications of high D.C. voltages in industries, research medical sciences etc. HVDC transmission over both overhead lines and underground cables is becoming more and more popular. 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 are 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 a sink 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 equipments (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 Vd 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 Vd i.e., δV/Vd.
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 shape may differ for different applications.
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