Reducing drive harmonics: a look at the options
28 June 2011
Nick Brown, of ABB Ltd, explains the different options for reducing harmonics caused by AC drives.
Harmonics caused by AC drives can be mitigated using passive filters or active front-end drives. Passive filters cost less and can be effective under some circumstances; the challenge lies in establishing just what those circumstances are. If the required conditions for a passive filter are difficult to meet, the safest option is to install an active front-end drive.
Harmonics can cause problems on many electrical networks. The water industry is particularly affected, as many of its sites are in rural locations with weak networks that are particularly susceptible to harmonics.
Harmonics are waveforms in multiples of the standard network frequency. A 250 Hz waveform on a 50 Hz network, for example, is the 5th harmonic. The harmonic waveform represents energy that cannot be used by devices on the network and it may cause equipment to behave erratically. Motors can overheat or get noisy; meters give false readings; lights may start to flicker; and cable insulation can break down.
Even if none of these symptoms are experienced, users still have to comply with the G5/4 regulations regarding maximum harmonic content on the network, to ensure that other users are not affected by any harmonic waveforms generated.
Harmonics are caused by non-linear loads on the network, such as fluorescent lights, computers, welding supplies and AC drives. In the case of AC drives, there is reason to be extra careful, as an individual site can have many of these installed. When adding further AC drives at a site, the total harmonic load should always be considered.
Most drives have six-pulse diode rectifiers, which give relatively high levels of total harmonic distortion, about 40%. Twelve-pulse rectifiers were introduced some years ago, which reduce total harmonic distortion to about 12%. However, in recent years, their place in the market has gradually been taken over by active front-end drives. These have a total harmonic distortion of only 2 to 4%, providing a more fit-and-forget solution.
Treating harmonics in the drive
With an active front-end drive, harmonics are dealt with inside the drive and do not enter the network. ABB’s approach to active front-end drives involves using its drive control platform direct torque control (DTC) ‘back-to-front’. DTC is normally used to control the output to the motor terminals to achieve the required motor speed and torque. In the active front-end drive, it is used to rearrange the incoming waveform, ensuring that the output from the drive becomes a smooth waveform, free from harmonics.
Unlike passive filters, the active front end also coexists happily with generators on the network. Its disadvantage is cost, which is about 10 to 20% higher than a traditional arrangement with a drive and a filter.
As cost is a driving factor in many water industry projects, one solution is to use a six-pulse diode rectifier plus a passive filter. However, a fairly high level of harmonics may still exist on the network, even after a passive filter has been installed.
If a passive filter solution is to be successful, the characteristics of the network must be taken into consideration. The total harmonic voltage distortion of the non-influenced mains voltage has to be less than 2% and the ratio of short-circuit power to installed load must be at least 66. Under these conditions, the total harmonic distortion of the mains current of the AC drive is reduced to between 5 to 10% and the passive filter solution will be sufficiently effective. However, establishing whether these conditions exist requires a site survey to be undertaken.
Problems may also occur in networks that have an imbalance between the phases, as is the case in many rural networks. Under these conditions, the passive filter is less effective. The problems will be particularly severe at low load, around 20%, where the distortion is already high. In this range, distortion may increase from, say, 8 to 12%. At full load, the increase will be less pronounced, perhaps from 5 to 6%, but still significant.
Another drawback of the passive filter is the higher losses, particularly at reduced load. Comparisons between passive filters and low harmonic drives are frequently made at full load. However, a true comparison should be made at reduced load – after all, the reason the drive is installed is to enable operation at lower speed. The comparison should therefore be made around 75% load, which is closer to the actual operating point for most pump motors.
The increase in losses tends to be greatest in low power installations. For instance, a 22 kW six-pulse drive with a 43 A filter can have losses of 873 W, while a low harmonic drive of the same size has losses of 490 W, which means the losses of the filter solution are a full 78% higher. A 160 kW drive with a 324 A filter can have losses of nearly 5,753 W, while the low harmonic drive has losses of around 5,400 W, a difference of 6.5%.
The passive filter can also increase the voltage in the drive DC link, which will put additional stress on the capacitors in the drive. An allowance should be made for this by selecting a drive 10% larger if using a passive filter.
In addition, some passive filters are not effective across the whole harmonic spectrum but only at the lower end. Also, power factor can be very poor at low load; 0.5 at 20%, for instance, would not be unusual.
Easy installation with active filter
The active front-end drive is also easier to install, as you only have to consider the mains voltage, the pump absorbed power and the fault level of the network. For a passive filter, you have to consider all of the above plus the existing power factor correction. The filter also needs to be disconnected at low load, which means that there needs to be a discussion with the panel builder about additional circuitry to achieve this and how this should be designed.
In addition, a passive filter may be large and heavy and might be difficult to fit in the panel. A typical 6 A filter can weigh 19 kg while a 433 A filter weighs in at a hefty 225 kg. The cable length can be maximum three metres between the drive and the filter.
A generator supply complicates the issue further. Again, with an active front-end drive you only have to consider the mains voltage, the pump absorbed power and the fault level of the network. For a passive filter, you have to consider mains voltage, pump absorbed power, generator fault level, disconnection of passive filter when on generator, second harmonic prediction when on generator, as well as the load factor per scheme. It is not possible to operate the passive filter when on generator supply, so the panel builder has to install circuitry to disconnect the filters when the generators start up. However, disconnection of the filter will increase the level of harmonics on the network, which can cause the voltage regulator on the generator to trip.
A passive filter needs to be designed for each site and its design should reflect the conditions of the site at a specific time. If conditions change, the filter may have to be changed too. Passive filters should only be considered when there is a small network with a small number of pumps, no generator supply and consistent load conditions. Under these circumstances, the passive filter can provide a cost benefit. Otherwise, it is safest to go with a low harmonic drive.
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