A single-phase, rectified input, switching power supply will have a sharp reduction in current at the beginning of a sag, followed by an increase in current to a value greater than the original current levels, until the sag has ended or the equipment trips off line.
One of the most common type loads in a commercial facility is the single-phase, rectified input, switching power supply found in computers, fax machines, printers and other electronic equipment. They are also called “switching power supplies.” That’s because they have circuits that alternately close switches (transistors) on either side of a transformer winding to make alternating current from direct current so it moves through a transformer, reducing the voltage to levels used by the power converters. Earlier switching power supplies operated at a few KHz, while newer ones operate at 100 KHz and higher, making for smaller transformers and smaller power supplies. But this switching has nothing to do with the PQ problems and its associated characteristics. It is the stage before the switching circuitry that is of concern.
The first stage is to convert 120 VAC into roughly 160 VDC. This is done by a full-wave bridge rectifier charging a storage capacitor (see Figure 1). The capacitor voltage is not quite the square root of two times the RMS of the input voltage, as there are losses in the system, plus energy is drained from the capacitor by the rest of the power supply circuitry. The diodes conduct current only when the AC voltage level is larger than the capacitor’s stored voltage. Hence, you get the impulse of current in the middle of the sine wave, as shown in Figure 2. As the power supply becomes more heavily loaded, the conduction-angle width and peak-current magnitude increase to provide more energy, and vice versa.
The current will go down dramatically when the voltage first begins to sag, as the input voltage is now significantly less than the voltage on the storage capacitor. As the power supply consumes the capacitor’s stored energy, it draws current again and gets larger and larger if the sag continues. This gives a basically constant power input and output from the supply, despite the varying voltage input levels. Figure 1 shows this quite clearly.
Besides the classic current waveform, the current harmonic spectrum is a clue to the presence of rectified input power supplies. As previous articles mentioned, harmonic current is best measured in amps, not percent of fundamental amps. Figure 3 shows the downward staircase pattern of magnitude of the harmonic current for the 3rd, 5th, 7th, 9th, 11th and 13th harmonic, typical of such power supplies. Note that it consists exclusively of odd harmonics, with appreciable levels of the 3rd and 5th harmonic, relative to the fundamental. Having a couple hundred of these devices in a commercial facility can result in significant flat-topping of the voltage waveforms, as well as the possibility of overheated neutrals in wye circuits, where these odd triplen (3rd, 9th, ...) harmonics become additive, instead of canceling out. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.