Industry News Display
2014-05-20 Control Engineering
Technology Update: Industrial facilities should include a system evaluation, including a harmonic distortion analysis, while planning facility construction or expansion. Vendors of nonlinear loads, such as variable frequency drives, can provide services and recommend equipment that will reduce harmonics to comply with IEEE 519.
By: Nikunj Shah
With increased use of nonlinear loads, power supply harmonics are more noticeable than ever. Controlling and monitoring industrial system designs and their effects on utility distribution systems are potential problems for the industrial consumer, who is responsible for complying with the IEEE 519 recommended practices and procedures. Industrial facilities should include a system evaluation, including a harmonic distortion analysis, while planning facility construction or expansion. Vendors of nonlinear loads, such as variable frequency drives, can provide services and recommend equipment that will reduce harmonics to comply with IEEE 519 guidelines.
Generally, at any point of common coupling (PCC), the measured value of total harmonic voltage distortion should not exceed 5% and that of any individual harmonic voltage distortion should not exceed 3% of the fundamental value of the line voltage. Normally, in typical applications, the harmonics are measured up to 25th order, but in critical applications, those are measured up to 50th or 100th order.
There are many harmonic mitigation methods available for individual applications (for example, per drive) and for "global mitigation" (such as a common harmonic mitigation solution for a group of nonlinear equipment). A particular type of harmonic mitigation solution can be used depending upon the application and desired level of attenuation to meet the limits given in IEEE 519.
Control of harmonics, IEEE 519-1992 Guidelines
IEEE 519 was initially introduced in 1981 as an "IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters." It originally established levels of voltage distortion acceptable to the distribution system for individual nonlinear loads. With the rising usage of industrial nonlinear loads, such as variable frequency drives, it became necessary to revise the standard.
The IEEE working groups of the Power Engineering Society and the Industrial Applications Society prepared recommended guidelines for power quality that the utility must supply and the industrial user can inject back onto the power distribution system. The revised standard was issued on April 12, 1993, updating the 1992 version of IEEE 519 that established recommended guidelines for harmonic voltages on the utility distribution system as well as harmonic currents within the industrial distribution system. According to the standard, the industrial system is responsible for controlling the harmonic currents created in the industrial workplace. Since harmonic currents reflected through distribution system impedances generate harmonic voltages on the utility distribution systems, the standard proposes guidelines based on industrial distribution system design.
In 2004, an IEEE working group named "519 Revision Task Force (PES/T&D Harmonics WG)" was created to revise the 1992 version of IEEE 519 (Recommended Practices and Requirements for Harmonic Control in Electric Power Systems) and develop an application guide IEEE 519.1 (Guide for Applying Harmonic Limits on Power Systems). A revision to IEEE 519 includes the changes based on the significant experience gained in the last 20 years with regard to power system harmonics, their effects on power equipment, and how they should be limited. In addition, this document contains certain material dedicated to the harmonization of IEEE and other international standards where possible.
The application guide IEEE 519.1 contains significant rationale for and numerous example scenarios of the limits recommended in IEEE 519 and provides procedures for controlling harmonics on the power system along with recommended limits for customer harmonic injection and overall power system harmonic levels.
Both documents, revised IEEE 519 and the application guide IEEE 519.1, are considered complementary.
Evaluation of system harmonics
To prevent or correct harmonic problems that could occur within an industrial facility, an evaluation of system harmonics should be performed if:
- A plant is expanded and significant nonlinear loads are added
- Power factor correction capacitor banks or line harmonic filters are added at the service entrance or in the vicinity
- A generator is added in the plant as an alternate stand-by power source
- The utility company imposes more restrictive harmonic injection limits to the plant.
Often, the vendor or supplier of nonlinear load equipment, such as variable frequency drives, evaluates the effects that the equipment may have on the distribution system. This usually involves details related to the distribution system design and impedances, similar to performing a short circuit study evaluation.
[This article online, below, explains various harmonic mitigation methods and provides 7 more online figures for connections of passive harmonic filter, 12 pulse converter front end, 18 pulse converter front end, active filter, and active front end, along with a values table of harmonics corrections for different types of front ends.]
Methods for Harmonic Mitigation
A majority of large power (typically three-phase) electrical nonlinear equipment often requires mitigation equipment to attenuate the harmonic currents and associated voltage distortion to within necessary limits. Depending on the type of solution desired, the mitigation may be supplied as an integral part of nonlinear equipment (such as an ac line reactor or a line harmonic filter for ac pulse width modulation (PWM) drive) or as a discrete item of mitigation equipment (such as an active or passive filter connected to a switchboard). There are many ways to reduce harmonics, ranging from variable frequency drive designs to the addition of auxiliary equipment. A few of the most prevailing methods used today to reduce harmonics are explained below.
Delta-delta and delta-Wye transformers
This configuration uses two separate utility feed transformers with equal nonlinear loads. This shifts the phase relationship to various 6-pulse converters through cancellation techniques. A similar technique is also used in the 12-pulse front end of the drive, which is explained below.
An isolation transformer provides a good solution in many cases to mitigate harmonics generated by nonlinear loads. The advantage is the potential to "voltage match" by stepping up or stepping down the system voltage, and by providing a neutral ground reference for nuisance ground faults. This is the best solution when using ac or dc drives that use silicon controlled rectifiers (SCRs) as bridge rectifiers.
Use of reactors is a simple and cost-effective method to reduce the harmonics produced by nonlinear loads and is a better solution for harmonic reduction than an isolation transformer. Reactors or inductors are usually applied to individual loads such as variable speed drives and available in standard impedance ranges, such as 2%, 3%, 5%, and 7.5%.
When the current through a reactor changes, a voltage is induced across its terminals in the opposite direction of the applied voltage, which consequently opposes the rate of change of current. This induced voltage across the reactor terminals is represented by the equation below.
e = Induced voltage across the reactor terminals
L = Inductance of the reactor, in Henrys
di/dt = Rate of change of current through reactor in Ampere/second
This characteristic of a reactor is useful in limiting the harmonic currents produced by electrical variable speed drives and other nonlinear loads. In addition, the ac line reactor reduces the total harmonic voltage distortion (THDv) on its line side as compared to that at the terminals of the drive or other nonlinear load.
In electrical variable speed drives, the reactors are frequently used in addition to the other harmonic mitigation methods. On ac drives, reactors can be used either on the ac line side (called ac line reactors) or in the dc link circuit (called dc link or dc bus reactor) or both, depending on the type of drive design and/or necessary performance of the supply.
The ac line reactor is used more commonly in the drive than the dc bus reactor, and, in addition to reducing harmonic currents, it provides surge suppression for the drive input rectifier. The disadvantage of using reactors is a voltage drop at the terminals of the drive, approximately in proportion to the percentage reactance at the terminals of the drive.
In large drives, both ac line and dc bus reactors may be used, especially when the short circuit capacity of a dedicated supply is relatively low compared to the drive kVA or if the supply is susceptible to disturbances. Typical values of individual frequency and total harmonic distortion of the current waveform of a 6-pulse front end without and with integral line reactors are given in Figure 5.
Passive harmonic filters (or line harmonic filters)
Passive or line harmonic filters (LHF) are also known as harmonic trap filters and are used to eliminate or control more dominant lower order harmonics, specifically 5th, 7th, 11th, and 13th. They can be used either as stand alone parts integral to a large nonlinear load (such as a 6-pulse drive) or for a multiple small single-phase nonlinear load by connecting it to a switchboard. A LHF is composed of a passive L-C circuit (and also frequently resistor R for damping) that is tuned to a specific harmonic frequency that needs to be mitigated (for example, 5th, 7th, 11th, 13th, etc.). Its operation relies on the "resonance phenomenon" which occurs due to variations in frequency in inductors and capacitors.
The resonant frequency for a series resonant circuit, and (in theory) for a parallel resonant circuit, can be given as:
fr = Resonant frequency, Hz
L = Filter inductance, Henrys
C = Filter capacitance, Farads
The passive filters are usually connected in parallel with nonlinear load(s) as shown in Figure 4, and are "tuned" to offer very low impedance to the harmonic frequency to be mitigated. In practical application, above the 13th harmonic, their performance is poor; therefore, they are rarely applied on higher-order harmonics.
Passive filters are susceptible to changes in source and load impedances. They attract harmonics from other sources (such as from downstream of the PCC); therefore, this must be taken into account in their design. Harmonic and power system studies are usually undertaken to calculate their effectiveness and to explore the possibility of resonance in a power system due to their proposed use. Typical values of individual frequency and total harmonic distortion of the current waveform of a 6-pulse front end with integral LHF are given in Figure 5.
See next page for more ways to control harmonics, diagrams, and references.
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.