What do computers, arc welders, and variable speed drives have in common? They are nonlinear loads that feed off the electrical utility. As such, they can produce harmonics that distort the supply voltage and current in the shared ac line, causing signal interference, shorter equipment life, and even total burnout.
As power electronics and electrical machinery saturate industry, reducing harmonics becomes a real priority. But traditional methods of relief, such as attaching filters onto nonlinear loads or changing the topology of individual component rectifiers, can be bulky and expensive. IEEE 519- 1992, a set of recommendations to minimize harmonics, favors more elegant schemes. For example, the initial design should use transformers and the nonlinear loads themselves to minimize harmonics at the point of common coupling.
Imperfect harmony
Typical 3-phase, non-linear loads like adjustable speed drives and uninterruptible power supplies generate harmonic currents according to the following equation:
h = kq ± 1 k = 1,2,3...
where h is the harmonic order and q is the converter pulse number. Therefore, a six-pulse converter draws harmonics in the order of the 5th, 7th, 11th, 13th, and so on. Singlephase loads, including PCs, add triplen harmonics (for instance, the 3rd, 9th, and 15th) and also contribute to problems.
There are two schools of thought for harmonic mitigation:
Component level. You can add "outboard" components to the drive, like sequence filters, broadband filters, shunt filters, ac line reactors, and harmonic traps. But this approach, while effective at reducing harmonics, can be bulky and excessive.
Built-in components, like ac line reactors, dc reactors and 12, 18, 24- pulse converter sections, are less physically cumbersome. Nevertheless, component level solutions tend to be costly.
System level. Ideal for new construction and a viable option for existing facilities, these solutions typically avoid extra material cost by using typical equipment. System solutions initially require thorough harmonics data analysis.
Give precedence to system solutions. Consider component solutions as instruments for fine tuning a system configuration.
Three system solutions are introduced here:
• Distributing linear and non-linear loads
• Virtual 12, 18, 24-pulse
• Mixing non-linear single and three-phase loads
Distributing linear and non-linear loads between multiple service transformers is a simple approach, even though it might complicate choosing the point of common coupling. Equally distributing nonlinear loads, however, can significantly reduce the ratio of the short circuit current to the demand load current at each service transformer. From the viewpoint of the utility and IEEE 519, the point of common coupling will be where all of your facility's service transformers share power. Additional impedance from extra transformers and wires will lower the voltage distortion at that point.
Other factors to consider for distributing non-linear loads include:
• Cable size, a little talked about expenditure for installing non-linear loads, can have a significant impact on the total cost of the system. The required cable cross section decreases as the power factor of the equipment increases.
• To reduce the effects of harmonic distortion on sensitive laboratory and monitoring equipment, consider placing an isolation transformer at their power feed.
• Multiple panels and easy access to transformers will facilitate proper load distribution throughout a site.
• For retrofit applications, the power factor improves with the addition of non-linear loads. For instance, if a motor with a power factor of 0.85 is fitted with an adjustable speed drive with a power factor of 0.95, the combination draws less current than if the motor was run directly across the line.
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Virtual 12, 18, 24-pulse converters achieve results similar to authentic 12, 18, 24- pulse converters. A virtual 12- pulse converter is integrated into the system by configuring existing equipment and standard electrical elements. A true 12-pulse, however, attaches onto the system as a separate, self-contained unit.
The idea here is to balance out, and therefore neutralize, the three power source phases. Transformers with a 1:1 phase shift may reduce harmonics in the order of the 5th, 7th, 11th, 13th, and so on. Exactly which harmonics are affected in that range depends on the setup.
A direct connection between 6-pulse converters ensures equal loading for the 12-pulse component configuration, which is necessary to nullify the 5th and 7th harmonic currents. A virtual 12-pulse under the worst case scenario of one converter taking the full load still experiences only half of the harmonic currents that would be present if the converters were fed from the same source.
Adding another phase-shifted transformer and 6-pulse converter with an equal converter load produces a virtual 18-pulse system. One converter and load connects to the ac line, and appropriately phase-shifted transformers feed the other two. Virtual 24, 30-pulse systems follow similar procedures, but there is rarely a need to go beyond a virtual 12 or 18- pulse system.
In order for phase shifting transformers to cancel current harmonics for 6-pulse converters, a general rule is to divide the number of loads by 60 to find the minimum phase shift.
Mixing non-linear single and three-phase loads, while infrequently used as a stand-alone harmonics solution, deserves consideration in system design. As usual, building around this systemic principle requires intensive data analysis, but the improved harmonic results require no extra equipment.
Non-linear loads (especially PCs), and scattered three phase loads such as adjustable speed drives, are often found in industrial distribution networks and commercial facilities. We might assume that removing the adjustable speed drives from the system would improve voltage distortion at the point of common coupling, but this isn't so. Conversely, adding large three-phase nonlinear loads to a single- phase rich environment actually reduces the voltage total harmonic distortion by vector addition of harmonic currents.
In a system with both single and three-phase non-linear loading, the 5th harmonic dominates the total harmonic distortion voltage at the transformer. (Only single-phase loads draw the 3rd harmonic.) Because the 5th harmonic is in counter-phase between single and three-phase loads (almost perfectly opposed) it is significantly decreased. Though the 7th and higher order harmonic currents are still present, mixing the loads adds negligibly to those harmonics. What results is an overall lower total harmonic distortion at the point of common coupling.
The same principle can also be applied when the primary side of the service transformer is rich in 5th harmonics - probably accumulated from single-phase loads at other sites. Accordingly, the secondary side, which feeds your facility, inherits the distortion of the 5th harmonic. Adding three-phase non-linear loads will counter the effects and reduce the total harmonic distortion at the point of common coupling. The larger the load on the transformer, the more significant the results.
Keep cool
Definitely watch out for transformer heating in conjunction with voltage distortion when evaluating system harmonics. To determine the chance of heat degradation on a service transformer, you can use the IEEE 519 recommendations for total demand distortion limits, or ANSI C57.110 for transformer derating.
The total harmonic voltage distortion and total demand distortion per IEEE 519 are measured at the point of common coupling. Transformer derating per ANSI C57.110 is calculated to include transformer information (K-factor, eddy losses) and load information (total horsepower, total harmonic distortion of input current).
A general rule is to do a harmonic analysis on a transformer when nonlinear loading hits 20%. This "default" test level could be pushed as high as 60% if you add certain system components; just be sure no equipment associated with that transformer is easily affected by voltage distortion.
Erica Herdey is Product Specialist with Danfoss Drives, Rockford, Ill.