Mitigating harmonics in electrical systems

03/14/2014


Figure 2: This diagram shows the connection and polarity arrangement of a typical delta-wye transformer.Harmonic mitigating transformers

In a standard delta-wye transformer, zero-sequence currents flow through the secondary wye winding and are coupled into the primary delta winding where they are trapped (see Figure 2). These zero-sequence currents can cause excessive heating and voltage distortion. Harmonic mitigating transformers can be implemented in pairs to mitigate 5th, 7th, and higher-order harmonic currents by taking advantage of the transformer phase shifts relative to each other, to cancel a significant amount of the harmonic current at these higher frequencies.

One type of harmonic mitigating transformer uses a zig-zag configuration. The zig-zag transformer is configured by winding half of the secondary turns of one phase of the transformer on one leg of the 3-phase transformer, with the other half of the secondary turns on an adjacent phase (see Figure 3).

Figure 3: This diagram shows the connection and polarity arrangement of a typical delta-wye zig-zag type transformer.

Note that harmonic mitigating transformers are not a panacea for the elimination of harmonics in an electrical system. Mitigation of 5th, 7th, and higher order harmonic currents requires the installation of multiple transformers with a 30-deg relative phase shift between the two, connected to a common bus in an electrical distribution system. Also, when mitigating these higher level harmonic currents by this means, balance of loads between the transformers is required. Figure 4: This diagram shows a parallel connection of a harmonic mitigating transformer and a typical delta-wye transformer.As shown in Figure 4, one transformer is a delta-zigzag configuration harmonic mitigating transformer with a 0-deg phase shift, and the second transformer is a delta-wye with a 30-deg phase shift.

Voltage distortion is normally greatest at the point where the equipment is connected to the distribution system. Therefore, to attain maximum benefit, harmonic mitigating transformers should be installed as close as practical to the load that they feed.

Installation of a non-phase-shift harmonic mitigating transformer provides an effective treatment of triplen (3rd, 9th, 15th, and so on) harmonic currents that are generated by loads connected to the transformer. Triplen harmonic currents are treated in the secondary windings of the transformer due to the transformer’s low zero-sequence impedance.

When a standard or K-rated delta-wye transformer is installed in an electrical distribution system, the addition of a non-phase-shift harmonic mitigating transformer offers an economical solution for treating higher order harmonic currents. The 30-deg phase-shift created between the standard or K-rated delta-wye transformer and harmonic mitigating transformer provides treatment of 5th, 7th, 17th, and 19th order harmonic currents to the extent of the balance of the load between the two transformers. In this configuration, the harmonic currents are canceled in the common electrical bus that feeds the transformers. Close coordination between the construction and location of the two transformers must be executed, as the impedance values of the transformers should be identical to receive the maximum mitigation of these higher-order harmonic currents.

Figure 5: This diagram shows a conceptual arrangement of an active harmonic filter as a parallel device.

Active harmonic filter (AHF)

The concept of an active filter is to produce harmonic components of the fundamental current waveform that are out of phase with—and thus cancel the harmonic components generated from—the nonlinear loads. Figure 5 conceptually illustrates how the harmonic current generated by the AHF is injected into the system to cancel harmonics from a VFD load. The AHF is installed as a parallel device and is scalable, making it a highly effective device that cancels multiple order harmonics in the distribution system. This method addresses harmonics from a systemic point of view and can save significant cost/space in many applications, with performance levels that can meet a TDD 5% target.

The active harmonic filter uses a current transducer to actively monitor the load current in real time to react to changes in load. Some AHFs are designed to also inherently synchronize the line current with the voltage to approach unity displacement power factor. The system typically performs fast Fourier transforms to calculate the amount of harmonics present for each harmonic order in the load current to determine the amplitude of the first 30 to 50 orders. The system logic processor filters out the fundamental frequency, and then directs the power converter to inject the phase-inverse of only the harmonic currents back into the circuit for cancellation of the harmonic content.

Figure 6: This diagram shows a typical implementation of an active harmonic filter in a motor control center.The benefits of AHFs include:

  • Dynamic adjustment for virtual real-time correction of the nonlinear current
  • Synchronization of the current and voltage waveforms
  • Adjustment using a feedback loop to prevent leading power factor.

AHF equipment is available for implementation at the PCC of the facility to the utility, for connection to a distribution bus within 3-phase power distribution systems inside facilities, and within distribution and control equipment, such as motor control centers (see Figure 6).



The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Doubling down on digital manufacturing; Data driving predictive maintenance; Electric motors and generators; Rewarding operational improvement
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Research team developing Tesla coil designs; Implementing wireless process sensing
Commissioning electrical systems; Designing emergency and standby generator systems; Paralleling switchgear generator systems
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me