At sites with harmonic content on the electrical network, users normally try to prevent more harmonic content being added when installing further ac drives. Even if the site has no obvious problems from harmonics, harmonics mitigation is frequently necessary to comply with the Energy Networks Association’s engineering recommendation G5/4. However, just adding filters does not guarantee that the installation will be successful. An installation with passive filters will only comply if the total harmonic distortion onsite is within acceptable limits.
Frequently, passive filters are added to new panels used for controlling ac drives. The specifier typically requests low-harmonics and the panel builder then delivers a system fitted with a passive harmonic filter, as this is the lowest cost option. Neither has done anything wrong, but the result may be a system that fails to meet the requirements and adds to the total harmonic distortion on the site, perhaps to the extent where harmonics then become a problem. Typically, this will include overheating cables, electronic displays and lighting that flicker, tripping circuit breakers and false meter readings. And if harmonics are exported to the public network, the company may be fined, or even disconnected, by its electricity supplier. If passive filters are the preferred option there is really no substitute for a skilled pre-installation survey.
The passive filter only works within specified limits, typically to 2%. If you don’t know what those limits are, the safest bet is to use an active front-end drive. This operates by creating a mains current waveform that is free from harmonics in the first place, instead of trying to filter out the harmonics after they have been created.
Preventing harmonics
Most drives in the market use plain diode bridges. These are simple and robust, but have one main drawback in that they produce high levels of harmonics. Diode rectifiers can be used in multiples with special transformers to give 12-pulse, 18-pulse or 24-pulse rectifiers.
At sites where total harmonic distortion is low, the harmonics created by a drive may not be a problem. But if the levels are building up, mitigating action needs to be taken. Fitting passive filters is the normal course of action, but if this proves inadequate, it may be necessary to consider either an active filter or an active front-end drive.
A passive filter, in its simplest form, comprises an inductor in series with a capacitor bank. It works by presenting low impedance at high frequencies that absorbs harmonic frequencies. However, at some lower frequencies, amplification can occur. The active filter compensates the harmonics generated by nonlinear loads by generating the same harmonic components in the opposite phase. External active filters are most suited to multiple small drives.
Active front-end drives have become more common in recent years. This method involves the use of an active rectifier-converter - usually having the same components as the motor side inverter-converter - to generate the DC voltage. My company’s approach to active front-end drives involves using the Direct Torque Control (DTC) platform to modify the incoming waveform.
Normally, DTC is used to control the output to the motor terminals. In the active front-end, it arranges the incoming waveform to ensure that the output from the drive becomes a smooth waveform, free from harmonics. The main benefit is a virtually sinusoidal supply current. In some applications bi-directional power flow is possible, making the unit regenerative.
Enter the ‘advanced harmonics filter’
Not all vendors have a full range of harmonics mitigating equipment. Problems can arise if a company tries to sell passive filters for applications where active front end drives would be more suitable. There are modified variants of the passive filter, such as the ‘advanced harmonics filter’, but these are essentially passive filters that do not offer a realistic alternative to the active front-end drive. Being passive filters, they can only be used under certain conditions; for instance, with a maximum background distortion of 2%. Unless a pre-installation survey is carried out, the user has no way of knowing whether the conditions are met.
Passive filters may also have other limitations, such as inability to operate across the full load range and unacceptably large heat losses. Passive filters use much more energy than an active front end drive; indeed, this can be as much as 50% more at full load. At partial load, the difference becomes even greater as the energy consumption of the active front end drive drops off while the energy consumption of the passive filter stays about the same throughout the load range. Some modified filter designs can have even higher losses than a standard filter.
Also, passive filters may not be able to handle the complete spectrum of harmonic frequencies, and the power factor may change with the load. If the load changes, this can cause the power factor of the whole system to change. When used with a generator supply, passive filters require particular attention as the generator may not be able to manage the reactive power. It is also important to bear in mind that a passive filter is designed for a site at a particular moment in time and will require re-design if the site changes.
Passive filters are also restricted by their large size; frequently, filters can be so big and heavy that they are impossible to wheel in. A further restriction is the cable length, which can be a maximum of three metres between the filter and the drive.
The active front-end drive is always better in terms of size and heat dissipation. The cost is typically 10-20% higher compared with passive filters but then it will resolve a harmonics problem, because no harmonics are added. Although, on first sight, the passive filter may appear more attractive, it needs to be used with a lot more caution than the ‘fit-and-forget’ approach possible with an active front-end drive. The passive filter may be cheaper, but the bargain may include additional problems.
In general, a single drive up to 37kW meeting EN 61000-3-12 will not need any additional filtering on a 400V network. A large number of independent small drives are best filtered by an active filter, while a few larger drives will, in most cases, be best served by an active front-end drive approach.
Passive filters should only be used in applications where a site survey indicates that it is safe to do so. And if this is indeed the case, then they will certainly provide a cost-effective alternative.
Frank Griffith is a drives consultant engineer at ABB