Wednesday, 18 January 2017

GENERATION OF HIGH VOLTAGES | AC AND DC VOLTAGE GENERATION

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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