These days, more and more loads connected to AC power lines draw currents that are not sinusoidal. The typical example is the AC supply for a computer. When AC voltage is applied to the power supply’s diode bridge (see Figure 1), the voltage is rectified by the bridge, and the capacitor charges to near the peak of the rectified AC voltage. The result is a current waveform containing multiple harmonics (see Figure 2).
Harmonics increase the RMS (root-mean-square) current by as much as 50 percent and produce excessive heat in wires, contacts, fuses, and circuit breakers, which necessitates the added expense of using larger fuses and circuit breakers, and even heavier power lines. If the total harmonic current is large enough to distort the supply waveform, proper operation of the equipment can be compromised. Harmonics affect the power factor, which is the ratio of useful current to the total current. (If, for instance, the RMS current is 50 percent larger than the useful current, the power factor is 0.67.)
Obviously, current harmonics on the input to any unit are problematic. For low-power single-phase inputs, the ideal solution is electronic-power factor correction devices. These devices, which are available as power modules up to about 1 kW, force the input current though a pulse-width modulation (PWM) scheme to be sinusoidal and in phase with the input voltage. The design of these devices can be incorporated into any power supply; and in fact, there are many circuits available for doing so.
The approach above does not work for three-phase inputs, although three single-phase converters could be used in place of a three-phase input when the input is a WYE and the load is very symmetrical. The real problem comes into focus when the power level is much higher and the input is a DELTA. Without a neutral, three single-phase converters cannot be used. However, when rectifying three-phase power, the harmonics are less than what would occur in single-phase power rectification.
When a transformer is used on the three-phase input, the unit can be configured to have a multi-phase output. For instance, if the secondary of the transformer has a WYE and a DELTA, it would have the equivalent of six phases. A secondary with three sets of WYEs and DELTAs properly phase-shifted will have the equivalent of 18 phases. An 18-phase rectified output is referred to as having 36-pulse rectification since it would use 36 diodes (see Figure 3). With so many pulses, the choke becomes reasonably small and is easily realizable.
Not many devices have 18-phase, 36-pulse rectification capabilities. One that does is the BL+ 120 from Behlman Electronics (see Figure 4). The BL+ 120 is a 120 kVA frequency converter with a specially-wound input transformer that produces an 18-phase output that is rectified and filtered for 36-pulse rectification with the resultant DC supplied to a switching frequency inverter. It can produce low-distortion sine waves from 45 to 500 Hz, and up to 2000 Hz where required. Input voltage can range from 120/208 VAC to 277/480 VAC in either WYE or DELTA connection. The unit can be controlled manually or through an optional computer interface via RS-232, IEEE-488, USB, or Ethernet.
Problems caused by AC line harmonics are receiving more attention as a critical power quality concern with the growing percentage of electricity now passing through loads drawing non-sinusoidal currents. Clearly, today’s engineers must be aware of the negative impact of AC line harmonics in their systems, and of the solutions available to address problem.