Reducing AFD-Caused Harmonics at Partial Load
Harmonic currents are component parts of a distorted sine wave current. Figure 1 uses a simple single-phase example to illustrate the operation of a capacitor-filtered rectifier circuit.
In this case, the capacitor is charged to the peak line voltage. The capacitor discharges as current is supplied to the load. Line current flows to recharge the capacitor only while the peak of the line voltage waveform is greater than the capacitor voltage. Only the source resistance limits the peak current.
A three-phase rectifier is generally similar to the single-phase example, but there are three voltage sources, six diodes, and six pulses of current per cycle. The source impedance is mostly inductive and additional inductance is often added inside the drive or in series with the source. The total impedance shapes both the height and width of the current pulses.
Figure 2 shows a simplified line current waveform for a typical three-phase drive, including the fundamental and the 5th and 7th harmonic current components. The nonsinusoidal, or "distorted," current waveform is the sum of its component parts.
REASONS FOR LIMITING HARMONICSIEEE Standard 519-1992 explains the reasons for limiting harmonics and recommends limits to be applied in various situations. The harmonic currents drawn by a load cause extra heating in all of the power distribution equipment that supplies the load. Harmonic voltages are generated by the action of the harmonic currents flowing in all of the impedances in the system, from the utility generating and transmission system and the substation and distribution transformers to the branch circuit protective devices and wiring.
Harmonic voltages cause additional harmonic currents to flow in equipment that does not ordinarily draw harmonic currents. Harmonics can interfere with the operation of some sensitive equipment.
HARMONIC MITIGATION ALTERNATIVESLine chokes and dc bus chokes are the most common harmonic mitigation alternative. The terms choke, reactor, and inductor are generally used interchangeably to mean a circuit component that provides a desired value of inductance. Inductors offer very little resistance to continuously flowing dc current, but the flowing current causes a magnetic field that stores energy and opposes any increasing or decreasing current. Either line chokes or dc bus chokes may be provided by the drive manufacturer as a built-in standard feature or made available as a built-in optional feature. Line chokes are also available as accessory items. (See Figure 3.)
Note that two bus chokes are shown in Figure 3. Providing chokes in both the positive and negative sides of the bus is a key element of the protective functions (which will be described at the end of this article).
SWINGING CHOKE: WHAT IS IT?The swinging choke is an inductor that has an inductance value that is inversely proportional to its operating current. Over a substantial portion of the normal operating current range, the inductance decreases as the current in the choke increases. A conventional, or linear, choke has a fixed inductance value that changes very little as the operating current varies in the normal operating range. (See Figure 4.)
As it is applied, the ABB swinging choke is comparable to a 5 percent linear choke in terms of the effective impedance at 100 percent rated current, but comparable to a 3 percent linear choke in terms of the copper windings and iron core used.
What does a swinging choke do? Used as a line choke or dc bus choke in an adjustable-speed drive (AFD), the swinging choke reduces harmonic current just as a linear choke does. The main objective of using a swinging choke is to provide further harmonic reduction when the drive is operating below its rated horsepower. This further harmonic reduction is provided with no increase in drive cost, physical size or weight compared to a similar drive equipped with a conventional choke.
Let's compare the advantages of a swinging choke vs. a conventional choke:
Some design advantages of a built-in choke are gained internally. Incorporating an ac line or dc bus choke in the basic design of the drive allows the designer to utilize the choke to the maximum advantage. The choke offers benefits in the following areas of design:
Some of the internal design benefits of a built-in choke are passed directly to the customer; others help make it possible to provide a built-in choke as a standard feature at a competitive price.
CUSTOMER BENEFITSCustomers can benefit by a built-in swinging choke. It, for one, limits harmonic distortion, as a choke reduces the level of harmonics generated by each drive. Some benefit is gained by reducing the level of harmonics generated by each piece of equipment connected to the user's power system, even when an analysis shows that the system can tolerate some harmonic generating equipment without taking additional measures to limit the harmonics.
It also limits harmonic currents, as this reduces the drive's total root mean square (RMS) input current. The swinging choke ensures that the drive's input current will never exceed the output current supplied to the motor. The rated input current marked on the drive's nameplate is the same as the rated output current. This means that there is no need to oversize the branch circuit wiring, disconnecting means and protection, to comply with the National Electrical Code. (See Figure 5.)
Limiting the harmonics generated by an individual piece of equipment frees power system capacity for adding future equipment. When each piece of new equipment includes this simple, cost-effective, built-in feature, the user reduces the risk of needing to retrofit harmonic limiting measures in the future.
In regard to harmonic mitigation objectives, IEEE 519 addresses two issues:
When a harmonic study is undertaken, the demand current and harmonic current contributions of adjustable-speed drives should be based on the maximum continuous brake horsepower (bhp) required to drive the connected equipment. For fans and pumps, this would be the BHP at the maximum flow design point.
There are swinging choke advantages. Since the design point bhp is nearly always less than the nameplate horsepower of the selected motor, the harmonic contribution of the drive can nearly always be based on an operating point that is less than the maximum rated output power. Since the harmonic limiting effectiveness of the swinging choke increases when the operating point is less than maximum power, a drive with a swinging choke has an advantage over a similar drive with a comparable linear choke.
Other protective functions include electromagnetic compatibility. In addition to reducing harmonics, chokes provide other benefits in ensuring that the drive will operate in its installation environment while neither causing nor experiencing electromagnetic interference (EMI).
EMI is any interference with normal equipment operation caused by abnormal energy entering equipment either by conduction though wiring connections or by radiated wave reception. Radiated EMI is also called radio frequency interference (RFI). Chokes help filter out any high frequency noise that might otherwise be emitted from the drive through the power lines.
Chokes also help protect the drive from voltage transients on the power lines. Chokes are particularly effective in protecting the drive from current surges that can occur when the utility switches power factor correction capacitors.
In regard to ground fault and short-circuit protection, chokes contribute to a drive's ground fault and output short circuit protection design. Chokes limit the rate of rise and maximum prospective value of fault current. Effective April 25 of this year, UL 508 requires the drive's maximum short circuit current rating to be marked on the nameplate. The 2005 edition of the National Electrical Code includes a similar requirement. Built-in chokes help make it possible to provide a 100,000-A output short circuit rating as a standard feature at a competitive price.
Dual dc bus chokes and ac line chokes provide similar benefits. All of the benefits described are provided by either dc bus chokes or ac line chokes. When dc bus chokes are provided, two chokes are required, one in the positive side and one in the negative side. Dual dc chokes are required to provide impedance between the ac line and any path to ground. Impedance in the ground path is needed for ground fault and EMI protection.
For more information, refer to ABB publications Fact File TD2 EN, "Reducing harmonics caused by variable speed drives," and Technical Guide No. 6, "Guide to Harmonics with AC Drives." Contact at ABB is Peter Walter, email@example.com. Also refer to U.S. Patent 6,774,758, available at www.uspto.gov and a Microsoft Excel spreadsheet harmonic analyzer tool is available at www.abb-drives.com. Click ABB Product Documentation in the list of Popular Pages and scroll down to -14 Software.
Publication date: 07/31/2006