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The following are harmonic solutions that are commercially available products or combinations of products for reducing harmonic currents and minimising harmonic voltage distortion on a power system.
These harmonic solutions are divided into three major categories:
Harmonics in power systems result in increased costs and downtime. Get help from Eaton.
Individual load harmonic solutionsThere is also a category of loads that correct the harmonics within the device and are often called low distortion loads. This clearly adds cost to the device, but they are often selected in lieu of detailed analysis and sizing considerations for individual harmonic solutions or for system solutions. There has been a significant move toward these devices, especially power supplies for data centre servers where all servers previously used harmonic-rich (>80% THD) switched-mode power supplies and now nearly all use PF-corrected power supplies that are almost perfectly sinusoidal or linear, as you can see in the figure below.
These include:
System solutions for harmonic correctionThe following harmonic solutions use a system approach to correct the harmonics for groups of loads for commercial and industrial power systems:
Power factor solutions that address harmonicsBecause harmonics and power factor are closely related, we will highlight the following harmonic solutions related to power factor:
Finally, it’s important to note that most non-active rectifiers on the front end of VFDs, battery chargers and other converters use diodes and therefore naturally draw a current that is “in-phase” with the voltage. They have a high displacement (60 Hz) power factor and don’t typically require additional PF correction.
The decision to apply one harmonic solution over another is typically an economic one but is also highly dependent upon the effectiveness of the solution. Table 1 – Comparison of harmonic solution options by corrective equipment shows the general effectiveness of various harmonic solutions. For each solution, the resulting typical ITHD is shown.
For example, a line reactor is certainly much less expensive than an active filter, but a typical line reactor will only reduce the current harmonics to approximately 35% while an active filter will reduce the current distortion to less than 5%. The active filter will ensure that harmonic problems will most likely be eliminated while correcting the power factor, if needed.
Solution type |
Effectiveness |
Significant advantages |
Significant disadvantages |
Individual load solutions |
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| AC line reactors | ITHD <35% |
Inexpensive 6-pulse standard drive/rectifier reduces harmonic current distortion from 80% down to about 35%–40% |
May require additional compensation |
| DC choke for drives | ITHD <35% |
Slightly better than AC line reactors for 5th and 7th harmonics | Not always an option for drives Less protection for input semiconductors |
| Isolation transformers | ITHD <35% | Offers series reactance (like reactors) and provides electrical isolation for some transient protection | No advantage over reactors for reducing harmonics unless used in pairs for phase shifting |
| Drive dedicated (broadband) blocking filters | ITHD <8% or 12%, depending on type | Turns 6-pulse into 18-pulse equivalent at reasonable cost | Higher cost Requires one filter per drive |
| 12-pulse drives | ITHD <15% | Reasonable cost for substantial reduction in voltage and current harmonics | Impedance matching of phase-shifting sources is critical to performance |
| 18-pulse drives | ITHD <5% | Excellent harmonic control for larger drives (>100HP) Ensures Std IEEE 519-2014 compliance |
Higher cost |
| 24-pulse drives (MV) | ITHD <3% | Excellent cost benefit for large drives (> 1,000 HP) Insures Std IEEE 519-2014 compliance |
Higher cost Requires special safety knowledge for MV systems |
| Active front-end drives | ITHD <3% | Ensures Std IEEE 519-2014 compliance Can be used on regenerative loads |
Higher cost Increased physical size of drive |
| Active front-end UPS | ITHD <3% | Ensures Std IEEE 519-2014 compliance Can be used as bi-directional inverter for utility demand response programmes |
Can have leading PF at light loads No magnetic decoupling of input/output |
| Low distortion loads (lighting, computers, etc.) | ITHD depends on load type | Can purchase loads specified to have a predetermined harmonic level | Generally higher cost – especially for redundant loads |
System solutions |
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| Passive harmonic filter | ITHD <15% | Reduces most prevalent harmonics (typically 5th) to acceptable level Provides PF correction support Avoids resonance by selecting “tuned” frequency |
Typically requires system knowledge and application study/analysis |
| Parallel active harmonic filter | ITHD <5% | Excellent cancellation for 2nd to 50th harmonic currents Cannot be overloaded Easy to specify |
Typically higher cost |
| Series active harmonic filter | VTHD <5% | Excellent power conditioning for removing source voltage harmonics | Typically higher cost Does not reduce current harmonics |
| Neutral blocking filter | 3rd harmonic is reduced to <10% | Eliminates the 3rd harmonic current from load Relieves system capacity |
High cost May increase voltage distortion at loads |
| Harmonic mitigating transformers | ITHD <10% possible with transformer combinations | Substantial (50–80%) reduction in harmonics when used in combinations of two or more Not prone to resonance issue (like harmonic filters and capacitors) Good for new construction |
Harmonic cancellation dependent on load balance More difficult to justify cost as a retrofit solution |
| Transformer de-rating |
N/A | Typically most reasonable (cost) solution | Does not remove harmonics Reduces capacity of power system |
| K-factor transformers | N/A | Offers series reactance (like line reactors) and provides electrical isolation for some transient protection | No advantage over reactors for reducing harmonics unless used in pairs for phase shifting |
| Oversized neutrals | N/A | “Live-with” high 3rd harmonics | All downstream panels and shared neutrals must be oversized Transformer windings and neutral must be sized for high harmonics |
Power factor solutions that address harmonics |
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| Passive filters | ITHD <15% | Avoid resonance versus standard capacitors | Must be designed specific to system and loads |
| Active filters | ITHD <5% | Can select harmonic correction, PF correction or both Easy to specify |
Highest cost |
| AFE drives and UPS | ITHD <3% | Can design loads to have unity PF | High cost |
Eaton’s Paul Olis discusses harmonic risks and solutions, such as clean power drives, to reduce energy costs and protect systems on FlowControlNetwork.com.
Harmonic solutions FAQSee mathematical explanation of phase-shifting cancellation for harmonics using transformers and 6-pulse drives to achieve a 12-pulse system.
Do you have additional questions or just want to communicate with someone well-versed in harmonics? We have experts ready to answer any questions you may have.
