A unidirectional voltage that rises rapidly to a maximum value and then falls to zero more or less rapidly without any appreciable superimposed oscillations. An impulse current is a unidirectional current with similar characteristics. Impulses are usually unwanted, being generated by fault conditions in electrical equipment or apparatus or by procedures such as switching on or off. A common source of large impulse voltages is a lightning stroke.
A typical waveshape is shown in Fig. a. The maximum voltage, V, of the impulse is the peak value. The wavefront is the rising portion OA and the wavetail is the falling portion ABC. The duration of the wavefront is the time, T1, required for the voltage to rise from zero to the peak value. The time to half value of the wavetail, T2, is the time required for the impulse to rise from zero, reach the peak value, and then decay to half the peak value. The waveshape in Fig. a is described as a T1/T2 impulse. The average rate of increase of voltage with time is the steepness of the wavefront. A typical impulse voltage that results from lightning is a 1/50 wave, where T1 and T2 are measured in microseconds; standard testing equipment uses a wave with these proportions produced by a surge generator.
An impulse voltage or current frequently causes flashover or puncture in electrical apparatus. If this occurs the impulse voltage collapses rapidly and is described as a chopped impulse voltage (Fig. b). Flashover or puncture can occur either during the wavefront or during the wavetail. The actual value of the impulse voltage at which flashover or puncture occurs during the wavefront is the impulse flashover voltage or the impulse puncture voltage. If flashover or puncture occurs on the wavetail the peak value of the impulse voltage is quoted. The time to flashover or the time to puncture is the time interval between the beginning of the impulse voltage or current and the instant at which the wave is chopped (point D in Fig. b).
(a) Waveshape of impulse voltage (b) Waveshape of chopped impulse voltage
In an insulator used in a.c. power transmission systems, the impulse voltage at which flashover or puncture occurs in the insulator is in general different to the voltage of the a.c. power at which failure occurs. The impulse ratio for flashover (or puncture) is the ratio of the impulse flashover voltage (or puncture voltage) to the peak value of the alternating voltage at which flashover or puncture occurs. Typical values for commonly used insulators are between 1.5 and 1.2.
- catalogue equinox
- catalysis
- catalyst
- catalyst poison
- catalytic activity
- catalytic coefficient
- catalytic converter
- catalytic cracking
- catalytic distillation
- catalytic hydrocracking
- catalytic reactor
- catalytic reforming
- catalytic rich gas process
- catalytic RNA
- cataphoresis
- cataracta
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- catarrhine
- catastrophe theory
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- catastrophic code
- catastrophic error propagation
- catastrophic evolution
- catastrophic failure
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