SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE

SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE
SINGLEPHASE OVERHEAD PS201001EN FUNCTIONAL SPECIFICATION FOR SINGLEPHASE OVERHEAD TYPE
TYPE V4H SINGLEPHASE VACUUM RECLOSER PS280012EN FUNCTIONAL SPECIFICATION FOR

TYPE V4L SINGLEPHASE VACUUM RECLOSER PS280013EN FUNCTIONAL SPECIFICATION FOR

Single-Phase Nonlinear Loads

This section will characterize the more common types of single-phase nonlinear loads, including switch-mode power supplies, electronic lighting ballasts, variable speed heat pumps and air conditioners, and battery chargers for electric vehicles.

Switch-Mode Power Supplies

SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE A variety of ac electronic equipment is powered internally by dc. In the early days of power supply design, the common single-phase power supply was linear. In Fig. 1-1, a schematic of a linear power supply, the rectifier stage utilizes a transistor (not shown) which operates in its active region. Comparatively speaking, the linear power supply has high power losses (overall efficiency in the 30-60% range). In addition, the transformer represents a significant weight to the power supply.

SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE

Fig. 1-2 is a schematic of a switch-mode dc power supply. The dc voltage change is attained through dc-to-dc converter circuits which employ transistors or MOSFETS to perform the switching (on or off). Unlike the linear power supply, the solid-state devices do not have to operate in their active region, resulting in reduced power losses. In this design, the 60 Hz transformer has been eliminated, resulting in reduced weight and improved overall efficiency (in the 70-90% range).

With a switch-mode power supply, the ac voltage is converted to dc through diode rectifiers. The regulated dc output is attained by means of high frequency (e.g., 20-100 kHz) switching. Although not shown, a high frequency isolation transformer is used. This SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE t ransformer is much lighter and smaller than the one used in the linear power supply. The current draw is controlled by the voltage appearing across the low voltage capacitor. As a result, the current wavefom appears like that shown in Fig. 1-3, in which the current has a zero value until some minimum capacitor voltage is reached, and then rises to a peak value and back to zero again. As contrasted with the linear power supply, the current appearing on the ac side lasts for only a portion of each half-cycle. As a result, the characteristic is nonlinear and the device is a SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE s ource of harmonics.

SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE

Most single-phase converters produce all the odd harmonics at about twice the amount of their three-phase converter counterparts. In addition, single-phase converters have significant amounts of third harmonic content, which the three-phase converters do not have. An approximate expression for determining the amount of each current harmonic present is given by the following equation:

SINGLEPHASE NONLINEAR LOADS THIS SECTION WILL CHARACTERIZE THE MORE

where: n = harmonic order (3, 5, 7, etc.)

Thus, one would expect to find about 67% third harmonic, 40% fifth, 29% seventh, etc. This equation underestimates the % of each harmonic in most cases. The current THD, which is the weighted or rms assessment of all harmonics, approaches and exceeds 100% for many of these devices.

Recent measurements[from Grady] indicates the current waveform for a color television has a current THD of 121% and a third harmonic content of 84%. Similar measurements for a desktop computer with a laser printer indicate a current THD of 140% and a third harmonic content of 91%.

Electronic Ballasts for Fluorescent Lighting

Fluorescent lamps with magnetic ballasts are three to four times more efficient than incandescent lamps. Electronic ballasts for fluorescent lamps result in a further 20-30% in efficiency over the magnetic ballasts. Most of the older high-loss magnetic ballasts can no longer be sold in the United States. Like the switch-mode power supply, electronic balllasts employ high frequency switching, usually in the 20-45 kHz range. When electronic ballasts were first introduced, the current THD was around 100% .

A relative newcomer is the compact fluorescent lamp (CFL) with electronic ballast. Generally found in the range of 15 to 27 W, CFLs are used to replace 40 to 100 W incandescent lamps. Many utilities’ energy conservation programs encouraged their customers to install these more efficient lamps and later discovered they introduced a significant source of harmonics. Pileggi et al concluded that a feeder with just 2 or 3 compact fluorescents per household would lead to excessive voltage distortion [1]. Obviously, the amount of current distortion from such a device can vary.

Following complaints resulting from high neutral currents in three-phase commercial and industrial environments, ANSI established limits on current harmonics. The maximum current THD limit is 32% and the triplens (third harmonic and odd multiples of third) are limited to 30% per ANSI C82.11-1993.

Variable Speed Heat Pumps and Air Conditioners

Some newer heat pump and central air-conditioning designs incorporate variable speed control of the compressor, blower and fan motors. Using electronically-commutated motors (ECM) makes a variable speed heat pump look like a large switch-mode power supply (e.g., 3 kW or more). Conventional heat pumps have a current THD of 13% with around 9% third harmonic content; the newer ECM designs have current THD values of 123% with the third harmonic content of 85%.

Because of the large load, there is concern that it would only take relatively few installations of this type of heat pump to cause voltage distortion problems. In [2], the authors found the voltage distortion on a distribution feeder reached 10% when the penetration rate for these variable-speed drives reached 10%. Though a 10% THDv would be deemed unacceptable by most of us, it does give us a sense of how much of a particular nonlinear load would cause problems on a residential feeder.

Electric Vehicle Battery Chargers

Many utilities are presently experimenting with electric vehicles (EVs). At issue is the fact that the technology exists to produce low distortion chargers. However, without any harmonic limit to meet, manufacturers will opt for a lower cost, higher distortion design. Considering that the single-phase chargers represent a 240 V load of around 6 kW, the potential for problems is genuine, particularly at the residential level.

Some of the least expensive battery chargers have current THD values of 91%. Based on [2] and the assumption that the EV charger load is twice that of the variable-speed drive, the penetration rate would only have to be 5% before the voltage THD reaches 10%. Obviously, to meet the recommended THD limit of 5%, the penetration would have to be less than 5% or the equipment harmonic limit would have to be lower.

References:

Power Electronics, Mohan, Undeland and Robbins, John Wiley and Sons, 1995

Grady, W.M., Presentation from the 1996 T&D Conference

Pileggi, D.J. et al, “The Effect of Modern Compact Fluorescent Lights on Voltage Distortion”, IEEE paper 92 SM 509-0

Thallam, R.S. et al, “Estimating Future Harmonic Distortion Levels Due to Single Phase Adjustable-Speed Drive Air Conditioners, IEEE ICHPQ 1992

ANSI C82.11-1993, American National Standard for Lamp Ballasts: High Frequency Fluorescent Lamp Ballasts

DJ Ward 1/02/98

Tags: characterize the, characterize, singlephase, loads, nonlinear, section