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GENERATION OF HIGH VOLTAGES AND CURRENTS
Transcript of GENERATION OF HIGH VOLTAGES AND CURRENTS
Needed in insulation tests on capacitors and cable.
Impulse generator charging units require high dc voltages about 100-200kV.
>> electronic valve rectifiers used
>> output current : 100mA
Electrical eng. and applied physics required high voltages in several applications
GENERATION OF HIGH VOLTAGES AND CURRENTS
High impulse voltage are required for testing purpose
> to simulate overvoltages that occurs in power systems due to lightning or switching action
+ power frequency AC
+ high frequency
+ switching/lightning impulses
High generation of amperes
> needed for short circuit testing of switchgear and surge diverters testing
VOLTAGE DOUBLER RECTIFIER
simple voltage doubler
cascaded voltage doubler
GENERATION OF HIGH AC VOLTAGES
-Normally, transformer is used for test purposes. Only test voltage of <200kw is suitable.
-If only a single unit of transformer is used, it will be
complicated in construction
not cost effective
Series and Parallel Resonant Set
-Harmonic distortion can be produced in the transformer and it must be filtered before supplying it to the test circuit.
series resonant set
can be used to achieve
a pure 50Hz waveform
Parallel resonant set
can be used to minimize the max. power requirement of the low voltage winding of the test supply by
improving the power factor
GENERATION OF IMPULSE VOLTAGES
Standard Impulse Waveshape
-Lightning and switching surges cause a steep build up of voltage on transmission lines and other electrical apparatus called
-Normally, it has a rise time of 0.5μs to 10μs and decay time to 50% of the peak value of 30μs to 200μs.
1.2/50μs, 1000kV wave
Impulse Voltage Generator
-It provides impulse voltage waveshapes into the test object.
-The load in the cicuit may consist of R, L and C i.e. Transformer.
-The peak voltage output of the generator must be variable and changes with the impedance of the test object and the required waveshape.
>>>It has a charging component, producing the wavefront of the waveshape.
>>>It has a discharging component, producing the wavetail of the waveshape.
Multistage Impulse Generator
-It is used to obtain large impulse voltage. Therefore, a relative size of High Voltage transformer can be kept small and costs less.
-It charges a number of capacitor in parallel through a rectifier and discharges them in series. This results in:-
nominal output voltage
charging voltage x
the no. of stages
Marx Impulse Generator Circuit:-
GENERATION OF IMPULSE CURRENTS
Typical Impulse Current Wave
Lightning discharge involve both high voltage impulses and high current impulse.
Protective gear like surge diverters/collectors have to discharge the lightning currents without damage.
Thus the generation of impulse current waveforms of high magnitude (>100kA peak) is necessary.
Circuit For Producing Impulse Current Wave
A bank of capacitors connected in parallel are charged to a specific value and are dischagre through a series R-L circuit.
C represents a bank of capacitors connected in parallel which are charged from a d.c. source to a voltage up to 200kV.
R represents the dynamic resistance of the test object and the resistance of the circuit and shunt.
L is an air cored high current inductor.
An impulse current generator has a total capactance of 8 μF. The charging voltage is 25kV. If the generator to give an output current of 10kA with 8/20μs waveform, calculate:
a) The circuit inductance.
b) The dynamic resistance in the circuit.