The drive for harmonic balance
The presence of harmonics can drastically alter the quality of the power provided to electrical systems and may affect equipment on that supply. Peter Tomazic, Solution Consultant for the Systems and Solutions business of Rockwell Automation, explains how harmonics are generated, why they are a problem and discusses various engineered solutions that can mitigate their effects.
Harmonics are a form of invisible electrical ‘pollution’ that can affect electrical networks facility-wide, presenting an ever-increasing problem for industry. If not properly addressed, harmonics may result in unnecessary damage and disruption to plant equipment. Various harmonic standards exist that provide guidelines to ensure that the harmonic distortion generated by one user does not unduly affect the line voltage of neighbouring users, but the standards do not advise how to minimise the effects within a production facility itself. For this, the expertise of a drives solutions provider is needed to recommend the most cost-effective engineered solution for each application.
The principal problem associated with harmonic distortion is the increased supply current requirement it causes and the associated concerns of component overheating, nuisance tripping and subsequent equipment malfunction. Drive harmonics can also cause noise transfer to other loads and result in communication interference. These issues present a problem across a wide range of industries, from primary production through a broad spectrum of manufacturing processes, to service and utility providers.
Harmonics are generated by an AC variable speed drive when an AC power supply is rectified to DC. This type of load - often referred to as a non-linear load - also includes personal computers, servers, fluorescent tubes and air-conditioning units. As the use of these devices - especially VSDs - become more prevalent, harmonic distortion is becoming more widespread in a greater range of sectors throughout industry.
The sum of all harmonics
The waveform of the current drawn by linear and non-linear loads is very distinctive. Where the current drawn by a linear load is typically sinusoidal in shape, as shown in Figure 1a, a non-linear load such as a VSD creates a non-sinusoidal waveform, as shown in Figure 1b. The shape of this waveform is a direct result of the harmonics generated by the input of the VSD.
Each non-linear load will generate a number of harmonics. The harmonic ordinal denotes how many cycles of the harmonic current occur within one cycle of the fundamental. The fifth harmonic, for example, will have a frequency five times that of the fundamental current. It should be noted that harmonics generated by VSDs are always odd-numbered harmonics and that the third harmonic can generally be negated by a transformer with a delta secondary. The amplitude of the particular odd harmonic waveform also generally diminishes as the frequency increases.
The first, fifth, seventh and eleventh harmonics, typically generated by a non-linear load, VSD, are shown in Figure 2a. When all the harmonics from the fifth to the twenty-ninth are added, the summation appears as shown in blue in Figure 2b.
When the sum of the harmonics is combined with the fundamental, the resultant current curve - shown in red in Figure 2c - can be determined. This demonstrates how the non-linear current illustrated in Figure 1b is comprised. The resultant current drawn (shown here as 77.1 A), including the harmonic distortion (30.8 A), is higher than the fundamental (70.7 A) and this excess current draw - in this example, 6.4 A - is the cause of overheating, nuisance tripping and component malfunction.
Current harmonic distortion is commonly expressed by two ratios - total harmonic distortion (THD) and total demand distortion (TDD). THD is a ratio of the harmonic current over the fundamental current, whereas TDD is the harmonic current over the full-load capacity of the system. At full load the THD and TDD will be equal, but as the current drawn diminishes the THD will increase, while the TDD decreases.
Meeting the standards
Despite widespread experience of harmonic distortion throughout industry, there remains a great deal of misunderstanding regarding harmonic standards and how these can be met. A number of standards exist to address the side effects of harmonics, but their intent remains the same - to prevent one user from disrupting a neighbouring user’s line voltage. AS/NZS 61000 is the predominant standard used in Australasia and this is generally based on the international IEC 61000 standard. However, AS/NZS 61000 includes a caveat that states that drive harmonics must conform to local supply codes, and these occasionally reference additional standards, such as the American IEEE 519.
These various documents are intended to be interface standards, rather than equipment standards, and are not concerned with impacts within a facility - only the effects on other users sharing a common supply. The standards also refer to both current and voltage distortion. The principal differentiator between current and voltage distortion is that the current THD remains constant throughout a system, whereas the voltage THD will be greatest at the load and will diminish closer to the supply transformer.
Increasing the capacity of the system - by having a bigger transformer - will minimise the voltage distortion, but this will not affect the line current harmonics. Current harmonics create voltage distortion and so, if action is taken to mitigate current harmonics, then the voltage distortion will also be minimised. In a drive system comprising a transformer, an AC drive and a motor, line current harmonics will be present between the transformer and the AC drive. As such, this is where mitigation efforts need to be concentrated.
A number of solutions are available to mitigate the effects of current harmonics and these vary in effectiveness, cost and space requirements. The magnitude of the current THD will vary with load and with every application and, for a simple six-pulse drive without any mitigation or DC link choke, the harmonic current may be over 100 percent of the fundamental current. The use of either a VSD with a DC link choke, or adding a line reactor to the input of the drive, or a combination of both will make a significant difference and, on a six-pulse drive, may reduce the THD down to 25 percent. Similarly, a passive harmonic filter comprising inductors and capacitors can also be installed between the transformer and the drive and this will, typically, reduce the current THD down to five to eight percent.
While six-pulse drives are most commonly used, a 12-pulse drive generally generates less current harmonic distortion. An 18-pulse drive will be better still. While these options are expensive, a similar result can be achieved through a pseudo pulse solution. For applications with two motors equally loaded, two six-pulse drives can be engineered to produce a ‘delta-wye pseudo 12-pulse drive’, by using a phase-shifting transformer to change the phase shift of the second drive. With both drives running equally loaded, the effect at the supply of this configuration will be virtually identical to a single 12-pulse drive, with current THD around 10 percent.
While most AC drives’ front ends will only allow energy to flow in one direction, an active front end (regenerative AC drive) will allow energy to flow in both directions. The input configuration of an active front-end VSD means that far less current harmonics are generated as compared to a six-pulse VSD, giving a typical current THD in the three to five percent range.
One of the best harmonic mitigation solutions available is an active filter. This can be fitted upstream from a VSD or drive system. An active filter is an intelligent device that counter-injects currents to offset the measured harmonic currents. However, while capable of reducing THD down to the three to six percent range, this can be an expensive option.
In addition to the effectiveness of a harmonic mitigation solution, cost and the space requirements are also therefore important considerations. The effectiveness of the different options available is summarised in Figure 3a. Approximate comparative costs are shown in Figure 3b and the anticipated required footprint in Figure 3c. A comparison of these graphs quickly highlights that the most effective mitigation methods tend to be the most bulky and expensive; there is no single solution to fit all scenarios. Each application or system needs to be assessed to determine the most efficacious solution as a factor of effectiveness, size and cost.
In order to achieve harmonic compliance, there are a number of practical recommendations that need to be considered. To ascertain the effects of installing a given drive, it is necessary to analyse an entire installation from the transformer down. The point of common coupling needs to be identified and all cost-effective mitigation options for that location need to be appraised. Preliminary harmonic analysis is crucial to determine exactly what harmonic distortion is currently being generated and what THD will be expected with the addition of a new drive or system. With this information in hand, the most cost-effective solution can be chosen for the facility.
If current harmonics are an issue, a line reactor or DC link choke should ideally be added where possible to any unbuffered six-pulse drive or group of drives. On multiple drive systems, an active filter may often be a better alternative and, where appropriate, pseudo 12- and 18-pulse solutions can also provide a cost-effective solution. An active front end should be considered if the application requires regenerative operation and where harmonic compliance is an important issue.
Theoretical harmonic THDs can be predicted using spreadsheet tools, which can be downloaded from reputable drives manufacturers. The results these generate are only as good as the data inputted, however, and generally these are best utilised to predict harmonic distortion for new installations to enable mitigation measures to be incorporated early in the design stage. Moreover, new systems can be designed to isolate non-linear from linear loads - by feeding linear and non-linear loads from separate transformers, for example - and this can minimise the unnecessary harmonic interference on linear loads.
For existing installations, power monitors provide a more accurate assessment than spreadsheet tools. These can be used to take continuous online measurements, which enables exact THD information for both voltage and current to be measured directly.
Using these strategies, new installations can be designed with mitigation methods in place and, for existing drives systems, the effects of harmonic distortion can also be minimised if necessary. With harmonic distortion adequately assessed and addressed, supply current demands can be reduced. This will minimise component overheating, nuisance tripping and subsequent equipment malfunction, providing more efficient and reliable systems into the future.
By: Peter Tomazic, Rockwell Automation
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