Electrical Waveforms: Troubleshooting Power Quality Problems Using Electrical Waveforms by L. W. Brittian

Author:L. W. Brittian [Brittian, L. W.]
Language: eng
Format: azw3
Published: 2017-07-08T04:00:00+00:00

The reader should notice the steep (fast) rate of rise of the leading edge and the rate of decay of the trailing edge. The amount of potentially destructive energy applied to and then dissipated by the electrical system, can be visualized as the area under the waveform.

Oscillatory Transient

An oscillatory transient is a sudden change in the steady state condition, such as voltage or current. These changes can be either positive or negative or both in amplitude. Oscillatory transients typically decay to zero within a cycle and are known as decaying oscillations. These types of transients are produced when an inductor such as a transformer or motor, or some types of capacitive type load(s, such as power factor correction capacitor banks) are operated. An oscillatory transient can be a sudden rise at a frequency other than 60 HZ; above the steady state, condition (of either voltage or current) that swings (oscillates) in both the positive and the negative directions (polarity). These oscillations occur at a very high frequency.

Oscillatory transients with a dominate frequency that is less than 5 KHZ and last from 0.3 ms. to 50 ms. (miliseconds-1/1000 of one second) seconds are classified by IEEE as a low frequency transient. Low frequency oscillatory transients are associated with power factor correcting capacitor-switching operations by the electrical utility. Ok now did you make a note of that? Power factor correction capacitors are cut in and out of utility power distribution systems (and some large industrial power distribution systems) on a daily basis. Therefore, when you see this condition on a power monitor, you should know what it is by these variables.

An oscillatory transient with a dominate frequency of between 5 and 500 KHZ and a duration lasting in the tens of microseconds is classified as being a medium frequency transient. Medium frequency oscillatory transients are associated with back-to-back capacitor bank operation and cable (feeders) switching operations. This source of trouble can be eliminated by the use of a pre-charge circuit.

An oscillatory transient with a dominate frequency above 500 KHZ and a duration measured in microseconds, is considered to be a high frequency transient. The waveform below contains an oscillating transient type transient. Note that the waveform has both additive (positive) and subtractive (negative) regions of the deviations (spike-sag) from the steady state sinusoidal waveform.

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