Columbo No. 1 - PF Cap Switching Transient

Having covered the basic laws governing how power quality events propagate through the electrical system, this column’s focus will shift now to “who done it,” or identifying the source of the power quality (PQ) events. Since transients are the first item in the IEEE 1159 list of PQ phenomena, they are first up. Despite the common folklore that lightning-induced transients are the chief nemesis, the transient resulting from the switching of power factor correction capacitors is usually the most common transient event recorded at the point-of-common-coupling (PCC), alias the service entrance. Based on changes in load, the electric utility (or sometimes the facility itself) will switch in/out power factor capacitors to help maintain the power factor closer to unity, and to support the voltage level. These banks of capacitors are switched either at a certain time of day, or when the current levels exceed a certain value. An uncharged capacitor is like a balloon attached to the line of an air-pressure tool. When the valve is opened to balloon, the balloon is the path of least resistance, so it will fill with air first. The result will be a decrease in air pressure to the tool, which may make it slow down or stop performing. The same situation applies to the capacitor on the electrical circuit. When the capacitor is switched in, the voltage will decrease as some of the current is diverted to charge the capacitor up to the supply voltage level. This occurs quite rapidly, because dv/dt (change in voltage relative to the time) is based on the capacitor’s impedance value, which is low at 60Hz. This will result in a negative transient. (Remember that a negative transient subtracts energy from the sine wave curve. It is not important whether it points up or down on the axis.) Once the capacitor is charged, it becomes part of a circuit that has lots of inductance in the wiring (and some resistance). This circuit will resonate slightly, because the inductance isn’t “happy” with the current being changed instantaneously. The result will be an oscillation about the normal sine wave shape, until the system resistance damps it out. The first positive transient peak following the initial negative transient may go as high as two times the normal value for that part of the sine wave. The frequency of oscillation is dependent on the system impedance, but is typically between 400 Hz and 2,000 Hz, lasting 1/4 to 1 cycle long. Some of the newer capacitor back systems are designed to switch in at the zero crossing points, where there is no voltage, hence, no need for the sudden charge of the discharged capacitor. However, there are still plenty of those that can come on at any point in the sine wave. The transients may cause the protection circuits on adjustable speed drives and other electronics to shut down. Surge suppression devices may kick in on the peak of the first positive transients. In summary, some of the key characteristics of a PF cap-switching event, as illustrated in Figures 1 and 2, are: • A negative transient followed by a positive transient (possibly two times). • Decaying oscillation lasting 1/4 to 1 cycle at 400 to 2,000 Hz. • Typically occurs at fixed time of day, such as 6 a.m. BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.

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