Thursday, 19 January 2017

GENERATION OF IMPULSE VOLTAGES AND CURRENTS

IMPULSE VOLTAGE:

An impulse voltage is a unidirectional voltage which, without appreciable oscillations, rises rapidly to a maximum value and falls more or less rapidly to zero Fig. 3.1. The maximum value is called the peak value of the impulse and the impulse voltage is specified by this value. Small oscillations are tolerated, provided that their amplitude is less than 5% of the peak value of the impulse voltage. In case of oscillations in the wave shape, a mean curve should be considered.

If an impulse voltage develops without causing flash over or puncture, it is called a full impulse voltage; if flash over or puncture occur, thus causing a sudden collapse of the impulse voltage, it is called a chopped impulse voltage. A full im-pulse voltage is characterized by its peak value and its two time intervals, the wave front and wave tailtime intervals defined below:

The wave front time of an impulse wave is the time taken by the wave to reach to its maxi-mum  value starting from zero value. Usually it is difficult to identify the start and peak points of the wave and, therefore, the wave front time is specified as 1.25 times (t 2 – t 1), where  t 2  is the time for the wave to reach to its 90% of the peak value and  t 1  is the time to reach 10% of the peak value. Since ( t 2 –  t 1) represents about 80% of the wave front time, it is multiplied by 1.25 to give total wave front time. The point where the line  CB intersects the time axis is referred to be the nominal starting point of the wave.

The nominal wave tail time is measured between the nominal starting point  t 0  and the point on the wave tail where the voltage is 50% of the peak value i.e. wave fail time is expressed as (t 3 –  t 0 ).
The nominal steepness of the wave front is the average rate of  rise of voltage between the points on the wave front where the voltage is 10% and 90% of the peak value respectively.
The standard wave shape specified in BSS and ISS is a 1/50 micro sec. wave  i.e.  a wave front of 1 micro sec. and a wave tail of 50 micro sec. A tolerance of not more than  ±50% on the duration of the
wave front and 20% on the time to half value on the wave tail is allowed. The wave is completely specified as 100 kV, 1/50 micro sec. where 100 kV is the peak value of the wave.

The wave shape recommended by the American Standard Association is 1.5/40 micro sec. with permissible variations of 0.5 micro sec. on the wave front and  ±10 micro sec. on the wave tail. Here wave front time is taken as 1.67 times the time taken by the wave to rise from 30% to 90% of its peak
value and wave tail time is computed as in BSS or ISS i.e . it is given as (t 3 – t 0 ) Fig. 3.1.

Impulse Flash Over Voltage:


Whenever an impulse voltage is applied to an insulating medium of certain thickness, flash over may or may not take place. If out of a total of say ten applications of impulse voltage about 5 of them flash over then the probability of flash over with that peak voltage of the impulse voltage is 50%. Therefore, a 50 per cent impulse flash over voltage is the peak value of that impulse flash over voltage which causes flash over of the object under test for about half the number of applications of impulses. However, it is to be noted that the flash over occurs at an instant subsequent to the attainment of the peak value. The flash over also depends upon the polarity, duration of wave front and wave tails of the applied impulse voltages.

Impulse Puncture Voltage:


The impulse puncture voltage is the peak value of the impulse voltage which causes puncture of the material when puncture occurs on the wave tail and is the value of the voltage at the instant of puncture when puncture occurs on the wave front.

Impulse Ratio for Flash Over:


The impulse ratio for flash over is the ratio of impulse flash over voltage to the peak value of power frequency flash over voltage.

The impulse ratio is not a constant for any particular object, but depends upon the shape and polarity of the impulse voltage, the characteristics of which should be specified when impulse ratios are quoted.

Impulse Ratio for Puncture:


The impulse ratio for puncture is the ratio of the impulse puncture voltage to the peak value of the power frequency puncture voltage.

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