Harmonic distortion is created by electronic circuits that draw current in short pulses, such as variable-speed motor drives, personal computers, printers, electronic ballasts used in lighting applications, and many types of medical test equipment. Harmonics are especially a problem wherever there are a large number of nonlinear single-phase loads. Equipment efficiency is improved when electronic circuits draw current in short pulses, but this causes harmonic distortion on the power lines.
Nonlinear loads
There are two very common types of nonlinear loads that cause harmonics. The first type of nonlinear loads includes computers, copiers and other similar electronic 120 V devices that use switched-mode power supplies (SMPS). These loads are major contributors to neutral current in 208 Y/120 V systems. In addition, variable-speed motor drives use similar power supplies and operate at many voltages. The second type of single-phase nonlinear loads includes 277 V electronic lighting ballasts, which predominate in 480 Y/277 V distribution systems.
SMPS
An SMPS is a power supply for electronic devices that includes an internal control circuit that quickly switches the load current ON and OFF in order to deliver a stable output voltage. An SMPS may also be known as a switching power supply. SMPSs are commonly used in electronic equipment because they are inherently more efficient than older, linear power supply designs. However, an SMPS is much more complex and harder to design correctly.
The first task of the SMPS is to rectify the AC input. A typical single-phase SMPS is designed to use a full-wave bridge rectifier. The rectified DC voltage charges a smoothing capacitor that is used to even out the voltage. The diodes are forward biased only when the voltage exceeds a certain minimum value. Therefore, current only flows during the part of the cycle where the voltage exceeds the minimum value. The capacitor also draws current only at the voltage peak because it does not fully discharge between the ripples of the rectified DC power. During the rest of the sine wave, the capacitor draws no current. This results in a circuit that draws current only during the peak of each half cycle of the AC sine wave.
Harmonics can also cause “flat-topping” of the voltage waveform, lowering circuit peak voltage. Flat-topping occurs when many loads draw current in pulses at the maximum voltage level and overload the power source. When flat-topping occurs, the capacitor is not fully charged because of the lowered voltage. In severe cases of flat-topping or when flat-topping and voltage sags occur, computers or other electronic equipment can continually reset because of insufficient peak voltage to keep the capacitor charged.
Because the current waveform is significantly distorted, an SMPS circuit has a low power factor. The simplest types of SMPS have a power factor of about 0.6. A more sophisticated SMPS has circuits that force the input current to follow the sine waveform of the voltage input. For DC inputs, the rectification step is not needed.
The next step in SMPS operation is the inverter stage. The inverter changes the DC to AC by switching it ON and OFF at a high frequency. The frequency is usually selected to be above the audible range to minimize noise. The inverted AC can be isolated from the source by being sent through a relatively small high-frequency step-down transformer. If DC output is required, the transformer output is then rectified and filtered again. In other power supply designs, the step-down transformer may be ahead of the rectifier section. In either case, these loads are characterized as nonlinear because the input current is significantly distorted as compared to the ideal current waveforms.
Lighting ballasts
Lighting can account for up to 40% of energy use in many commercial buildings. This makes lighting a common target for energy efficiency initiatives. Energy-efficient fluorescent or high-intensity discharge (HID) lighting is frequently used to replace incandescent lighting.
Fluorescent and HID lamps require a ballast. A ballast is a controller responsible for providing the initial startup voltage and maintaining a constant current through a lamp. Older magnetic ballasts use power at the line frequency of 60 Hz. Modern electronic ballasts use diodes to convert the 60 Hz power supply to DC. The DC is then converted to high-frequency AC for use in the lamp. The frequency in electronic ballasts can be up to about 60 kHz. Ballasts can create harmonics in the power lines. In addition, ballasts can create a phase shift between current and voltage, resulting in a low power factor.
The electronic ballast industry has universally adopted standards that establish maximum allowable current THD. The standards are outlined in ANSI Standard C82.11. This ANSI standard put a limit of 32% current THD on lighting ballasts. The ANSI specification is quite comprehensive in that it puts limits on specific low-order harmonics (second and third), high-order harmonics (greater than 11th), triple harmonics and THD. Ballast manufacturers have implemented passive filtering to reduce harmonics, and many modern ballasts operate at 20% THD or less.
Note: The THD at a typical electrical wall outlet in the United States is about 3%.
