High voltage EngineeringHigh voltage Engineering

1) Impulse voltage1) Impulse voltage

2)Methods of generation of impulse 2)Methods of generation of impulse voltage. voltage.

IMPULSE VOLTAGEIMPULSE VOLTAGE

An impulse voltage is a unidirectional voltage which without appreciable oscillations An impulse voltage is a unidirectional voltage which without appreciable oscillations rises rapidly to a maximum peak value and falls more or less rapidly to zero.rises rapidly to a maximum peak value and falls more or less rapidly to zero.

Wave Front: The wave font time of an impulse wave is the time taken by the wave to Wave Front: The wave font time of an impulse wave is the time taken by the wave to reach to its maximum value starting from zero. It requires 1us to reach the peak reach to its maximum value starting from zero. It requires 1us to reach the peak value.value.

Wave tail: The nominal wave tail is measured between the nominal starting point O Wave tail: The nominal wave tail is measured between the nominal starting point O and the point on the wave tail where the voltage is 50% of the peak value. It requires and the point on the wave tail where the voltage is 50% of the peak value. It requires 50us to reach the peak value.50us to reach the peak value.

Causes for the generation of impulse voltagesCauses for the generation of impulse voltages

Impulses on the transmission and the distribution system are frequently Impulses on the transmission and the distribution system are frequently caused by two kinds of transient voltages whose amplitude may greatly caused by two kinds of transient voltages whose amplitude may greatly exceed the peak value of the normal operating ac voltages.exceed the peak value of the normal operating ac voltages.

Impulses are caused either by:Impulses are caused either by:

1} Lightning strokes.1} Lightning strokes.

2} switching phenomena. 2} switching phenomena.

Lightning strokesLightning strokes

Lightning strokes are caused by lightning phenomena originated by lightning strokes Lightning strokes are caused by lightning phenomena originated by lightning strokes hitting the phase wire of overhead lines or the busbar of outdoor substation.hitting the phase wire of overhead lines or the busbar of outdoor substation.

The amplitudes are very high usually 1000kv and current up to 100 kA.The amplitudes are very high usually 1000kv and current up to 100 kA.

Impulse voltages with front duration varying from less than one up to a few tens of Impulse voltages with front duration varying from less than one up to a few tens of microseconds are in general considered as lightning impulses.microseconds are in general considered as lightning impulses.

Switching phenomenaSwitching phenomena These are caused due to opening and closing of the circuit breaker or due These are caused due to opening and closing of the circuit breaker or due

to the failure of the electronic equipment.to the failure of the electronic equipment.

It requires 250 us to reach to the peak value and 2500us to reach the zero It requires 250 us to reach to the peak value and 2500us to reach the zero value.value.

Their amplitudes are always related to the operating voltage and the shape Their amplitudes are always related to the operating voltage and the shape is influenced by the impedances of the system.is influenced by the impedances of the system.

The rate of voltage rise is usually slower, but it is well that the wave shape The rate of voltage rise is usually slower, but it is well that the wave shape can also be very dangerous to different insulation system .can also be very dangerous to different insulation system .

The figure illustrates the slope of a switching impulse where T2is the time The figure illustrates the slope of a switching impulse where T2is the time to half valueto half value

The time to peak Tp is the time interval between the actual origin and the The time to peak Tp is the time interval between the actual origin and the instant when the voltage has reached its maximum value.instant when the voltage has reached its maximum value.

Different methods used for the generation of impulse Different methods used for the generation of impulse voltagesvoltages

Single stage impulse generator.Single stage impulse generator.

Three stage impulse generator.Three stage impulse generator.

High voltage pulse generator.High voltage pulse generator.

11 High voltage pulse generator using spark gaps.High voltage pulse generator using spark gaps.

22 High voltage pulse generator using energy storage components and High voltage pulse generator using energy storage components and transformers.transformers.

33 High voltage pulse generator using a non linear capacitor.High voltage pulse generator using a non linear capacitor.

44 High voltage pulse generator using parallel and series mosfets.High voltage pulse generator using parallel and series mosfets.

Single stage generator circuit Single stage generator circuit

The capacitor is slowly charged from a dc source until a spark gap ‘G’ The capacitor is slowly charged from a dc source until a spark gap ‘G’ breaks down.breaks down.

The resistance R1,R2 and capacitance c2 forms the wave shaping network The resistance R1,R2 and capacitance c2 forms the wave shaping network R1 will primarily damp the circuit and control the front time T1.R1 will primarily damp the circuit and control the front time T1.

R2 will discharge the capacitor and control the wave tail .R2 will discharge the capacitor and control the wave tail .

The capacitor C2 represent the full load i.e. the object under test.The capacitor C2 represent the full load i.e. the object under test.

One terminal of the capacitor is solidly grounded and the polarity of the One terminal of the capacitor is solidly grounded and the polarity of the output voltage can be changed by changing the dc supply charging voltage.output voltage can be changed by changing the dc supply charging voltage.

The most significant parameter of a single stage impulse generator is the The most significant parameter of a single stage impulse generator is the maximum stored energy which is given bymaximum stored energy which is given by

½ C1 {Vo max} ½ C1 {Vo max}

The efficiency of this single stage impulse generator is given as The efficiency of this single stage impulse generator is given as

Efficiency =Vp/VoEfficiency =Vp/VoWhere, Where, Vp is the peak value of the output voltage andVp is the peak value of the output voltage andVo is the input voltage .Vo is the input voltage .

Multi stage impulse generatorMulti stage impulse generator The Marx generator is very effective in delivering ultra short impulses at The Marx generator is very effective in delivering ultra short impulses at

very intense power level.very intense power level.

The Marx generator is capable of delivering voltage pulses of several The Marx generator is capable of delivering voltage pulses of several hundred kilovolts at a duration of several nanoseconds to ten’s of hundred kilovolts at a duration of several nanoseconds to ten’s of nanoseconds.nanoseconds.

The time require to reach the peak value is about 20 picoseconds.The time require to reach the peak value is about 20 picoseconds.

After the generator is started the total discharge capacitance C1 may be After the generator is started the total discharge capacitance C1 may be given asgiven as

nn

1/C1 1/C1 ΣΣ 1/C1’ 1/C1’

Where n = number of stagesWhere n = number of stages

The conventional Marx circuit is shown in the figure.The conventional Marx circuit is shown in the figure.

The capacitor is charged in parallel through resistor ,the spark gap switch is The capacitor is charged in parallel through resistor ,the spark gap switch is connected in series to add the individual capacitor voltage at the output connected in series to add the individual capacitor voltage at the output terminals.terminals.

The impulse capacitors are charged through to the charging voltage Vo The impulse capacitors are charged through to the charging voltage Vo through the high charging resistor.through the high charging resistor.

When all the gaps breakdown the capacitor c are connected in series so When all the gaps breakdown the capacitor c are connected in series so that the voltage at the end is increased to a great value.that the voltage at the end is increased to a great value.

The distance between the spark gap is such that the first gap G1 is slightly The distance between the spark gap is such that the first gap G1 is slightly less then than that of G2 and so on.less then than that of G2 and so on.

The wave front control resistance may have three different locations The wave front control resistance may have three different locations

1}Entirely within the generator.1}Entirely within the generator.

2}Entirely outside the generator.2}Entirely outside the generator.

3}Partly within the generator and partly outside the generator.3}Partly within the generator and partly outside the generator.

The first two arrangement are not used as they require much space and a The first two arrangement are not used as they require much space and a single resistor has to withstand the entire voltage even though for a short single resistor has to withstand the entire voltage even though for a short duration of time.duration of time.

The compromise between the two is the third arrangement as shown in the The compromise between the two is the third arrangement as shown in the figure and thus both the damping of the oscillation and space economy are figure and thus both the damping of the oscillation and space economy are taken care of.taken care of.