Reducing harmonics with IEEE 519 practices, procedures
12-pulse converter front end
In this configuration, the front end of the bridge rectifier circuit uses 12 diodes instead of 6. The advantages are the reduction of the 5th and 7th harmonics to a higher order where the 11th and 13th become the predominant harmonics. This will minimize the magnitude of these harmonics, but will not eliminate them.
The disadvantages are higher cost and special construction, as it requires either a delta-delta and delta-Wye transformer, "zig-zag" transformer, or an autotransformer to accomplish the 30-deg phase shifting necessary for the proper operation of 12-pulse configuration. This configuration also affects the overall drive system efficiency rating because of the voltage drop associated with the transformer/s. Figure 5.2 illustrates the typical elementary diagram for a 12-pulse converter front end. The dc sides of both 6-pulse bridge rectifiers are connected in parallel for higher current (Figure 5.2) and connected in series for higher voltage. Typical values of harmonic distortion of the current drawn by a 12-pulse converter are given in Table 5.1.
18-pulse converter front end
An 18-pulse converter front end topology is composed of either a 3-phase to 9-phase isolation transformer or a lower cost patented design of 3-phase to 9-phase autotransformer, to create a phase shift of ±20 deg necessary for the 18-pulse operation, and a 9-phase diode rectifier containing 18 diodes (two per leg) to convert 9-phase ac to dc. Figure 5.3 shows the block diagram of the 18-pulse system. Similar to 12-pulse configuration, 18-pulse also has disadvantages of higher cost and special construction.
Nine-phase, 18-pulse converters not only have low harmonic distortion in the ac input current, but they also provide a smoother, higher average value of dc output.
In addition, since the characteristic harmonics for 18-pulse configuration are 18n ± 1 (where n is an integer 1, 2, 3,...), it virtually eliminates the lower order noncharacteristic harmonics (5th, 7th, 11th, and 13th). A typical harmonic performance of 18-pulse configuration is shown in Table 5.1.
Active filters are now relatively common in industrial applications for both harmonic mitigation and reactive power compensation (such as electronic power factor correction). Unlike passive L-C filters, active filters do not present potential resonance to the network and are unaffected by changes in source impedance.
Shunt-connected active filters (parallel with the nonlinear load) as shown in Figure 8 are the common configuration of the active filter. The active filter is composed of the isolated gate bipolar transistors (IGBT) bridge and dc bus architecture similar to that seen in ac PWM drives. The dc bus is used as an energy storage unit. The active filter measures the "distortion current" wave shape by filtering out the fundamental current from the nonlinear load current waveform, which then is fed to the controller to generate the corresponding IGBT firing patterns to replicate and amplify the "distortion current" and generate the "compensation current," which is injected into the load in anti-phase (180 deg displayed) to compensate for the harmonic current. When rated correctly in terms of "harmonic compensation current," the active filter provides the nonlinear load with the harmonic current it needs to function while the source provides only the fundamental current.
Active filters are complex and expensive products. Also, careful commissioning of active filters is very important to obtain optimum performance, although "self-tuning" models are now available. However, active filters do offer good performance in the reduction of harmonics and the control of power factor. Their use should be examined on a project-by-project basis, depending on the application criteria.
Active front end
"Active front ends" (AFE), also known as "sinusoidal input rectifiers," are available from a number of ac drive and uninterruptible power supply (UPS) system companies to offer a low input harmonic footprint.
A typical configuration of the ac PWM drive with active front end is shown in Figure 9.
As shown in the figure, a normal 6-pulse diode front end is replaced by a fully controlled IGBT bridge, an identical configuration to the output inverter bridge.
The dc bus and the IGBT output bridge architecture are similar to that in standard 6-pulse ac PWM drives with diode input bridges.
The operation of the input IGBT input bridge rectifier significantly reduces lower order harmonics compared to conventional ac PWM drives with 6-pulse diode bridges (<50th harmonic). However, it inherently introduces significant higher order harmonics, above the 50th. In addition, the action of IGBT switching introduces a pronounced "ripple" at carrier frequencies (~2-3 kHz) into the voltage waveform, which must be attenuated by a combination of ac line reactors (which also serve as an energy store that allows the input IGBT rectifier to act as a boost regulator for the dc bus) and capacitors to form a passive (also known as clean power) filter. As compared to conventional 6-pulse ac PWM drives of same rating, AFE drives have significantly higher conducted and radiated electromagnetic interference (EMI_ emissions, and therefore special precautions and installation techniques may be necessary when applying them. AFE drives are inherently "four quadrant" (they can drive and brake in both directions of rotation with any excess kinetic energy during braking regenerated to the supply), offer high dynamic response, and are relatively immune to voltage dips. The true power factor of AFE drives is high (approximately 0.98-1.0). The reactive current is usually controllable via the drive interface keypad.
Power system design
Use the following equation to determine if a resonant condition on the distribution could occur:
hr = Resonant frequency as a multiple of the fundamental frequency (= fr/f1)
kVAsc = Short circuit kVA at the point of study
kVARc = Capacitor kVAR rating at the system voltage
There is a possibility of a resonance condition if hr is equal or close to characteristic harmonics (for example 5th or 7th).
- Nikunj Shah is with design engineering, low-voltage drives, Siemens Industry Inc. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
- With increased use of nonlinear loads, the issues of power supply harmonics are more noticeable than ever.
- Industrial facilities should include a system evaluation, including a harmonic distortion analysis, while planning facility construction or expansion.
- Vendors of nonlinear loads devices, such as variable frequency drives, can provide help to comply with IEEE 519.
How are you controlling and monitoring industrial system designs and their effects on utility distribution systems? Are you paying fines to your utility or controlling harmonics?
 IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, ANSI/IEEE Std. 519-1992.
 Industrial and Commercial Power Systems Analysis, ANSI/IEEE Std. 399-1990.
 Ned Mohan, Tore M. Undeland and William P. Robbins, Power Electronics: Converter, Applications and Design, Wiley, 3rd edition, 2002.
 Kevin lee, Derek A. Paice and James E. Armes, "The Wind Mill Topology - Evaluation of Adjustable Speed Drive Systems", IEEE IAS Magazine, Mar/Apr 2009.
 ABS Guidance Notes on Control of Harmonics in Electrical Power Systems, American Bureau of Shipping, 2006
 Electric Power Distribution for Industrial Plants, ANSI/IEEE Std. 141-1986.
 IEEE Guide for Application and Specification of Harmonic Filters, IEEE Std 1531-2003.