Eliminating harmonics from the facility power system

odically occur in a facility’s electrical system can cause equipment malfunction, data distortion, transformer and motor insulation failure, overheating of neutral buses, nusiance tripping of circuit breakers, and solid-state component breakdown. The cost of power quality problems can be enormous. Equipment replacement can cost tens of thousands of dollars. Downtime, however, can run in the millions of dollars. Power quality problems take many forms. Voltage sags, transients, and spikes are probably the most understood of these forms. Harmonic distortion, though, can occur from adjustable-frequency drives and other non-sinusoidal loads typically where ac/dc conversion is present. In any facility, the voltage supplied by a power system is generally not a pure sine wave. Rather, it usually possesses some amount of distortion, which has a fundamental frequency and harmonics at that frequency, (see box, “Harmonic basics”). Distortion comes from various sources, particularly equipment with power-circuitry devices that draw current in a nonlinear fashion. Non-linear devices are those that switch the current on and off, such as transistors, diode bridges, and SCRs. Two major classes of equipment that contain these devices and produce current harmonics are: • Internal power supplies, such as computers, copiers, and electronic ballasts. • Any type of static power converter, such as an uninterruptible power supply, dc drives, or adjustable-frequency controllers (AFC). An AFC has a converter section, which converts ac line power to dc, and an inverter section, which converts dc to adjustable frequency ac. Both contain non-linear devices in their power circuitry, and therefore produce harmonics on the input and output lines. Input line harmonics are caused solely by the converter section and are usually referred to as line-side harmonics. Output line harmonics are caused solely by the inverter section and are called loadside harmonics. They are completely isolated from each other. Thus, load-side harmonics only affect the equipment driven by the AFC, while line-side harmonics affect the whole power system. Standard three-phase PWM drives typically use a six-diode rectifier bridge in the converter section, Figure 1. These diodes draw current non-linearly, in a 6-step converter waveform, Figure 2. The high levels (amplitude) of 5th and 7th harmonics Eliminating harmonics from the facility power system