The so-called AFE (active front end) technique has rapidly become popular when the target is low harmonic levels. This article however challenges AFE as the most frequently used solution for harmonic dampening.
Should you care about harmonics?
Harmonics can create problems for you, for instance
- transformer overload and audible noise
- overheating in cables
- premature aging of serially installed equipment like PLCs and protection relays
- torque ripples from direct-on-line motors
- malfunction or even breakdown of electronic equipment
So the short answer is – YES. It is also worth considering requirements from the grid supplier (DNO) for maximum tolerable current distortion on the primary side of the transformer. So in a nutshell, you should care about harmonics – but there is no need to panic.
Why is there no panic?
Most premium drive manufacturers supply their drives with standard harmonic mitigation in the form of integrated DC or AC chokes. With this technique, the generated THDi (total harmonic current distortion) is dampened from as much as 100% (for drives without DC or AC chokes) down to a level below 40% THDi.
This level is sufficient to minimize harmonic problems in most cases, depending on the general load of the transformer and the share of drives on the transformer.
When a major portion of the load is in fact due to drives, then it is a good idea to perform a harmonics calculation.
For a general calculation, use a harmonics calculation tool such as the Danfoss VLT® Motion Control Tool MCT 31 available for free download here
For a more accurate result, you could hire a specialised power company to measure your current and voltage harmonics level.
One of the advantages of a DC choke over AC choke is that there is no voltage loss over the coils in the DC link. Drives equipped with an AC choke typically lose 3% of the supplied voltage. This loss leads to lower voltage on the motor terminal, thus generating a higher current.
AFE and power loss
Active Front End (AFE) drives were initially developed to regenerate power back to the grid for applications with heavy braking cycles, for instance in crane and hoist applications. Since an AFE drive uses an active rectifier, usually in the form of an IGBT bridge, harmonic current distortion is very low. However there are some disadvantages with the AFE technique:
In an AFE drive, the power electronics are placed in serial current flow. Serial connection means that if one power electronic component fails, for example in the filter, the entire drive stops, resulting in a stand-still.
An AFE drive contains twice the number of power electronics as a standard drive, plus an LCL filter, which does not exist in a standard drive. Twice the number of electrical components means there is twice the risk of a component failure.
Twice the power electronics also means greater power loss over the drive:
- A normal drive from a premium drive manufacturer has a heat loss of about 2% over the drive
- The heat loss from an AFE drive can easily exceed that of a premium drive
So if the extra harmonic dampening is not needed then don’t invest 150-200% of the price of a normal drive in buying a drive with lower efficiency. After all, the most important task of the drive in a pump application is to save energy.
Are there alternatives?
In short, yes! System mitigation using a VLT® Advanced Active Filter (AAF) is a preferable method to dampen harmonics in most cases. An AAF is easy to retrofit, can compensate multiple drives, and cleans up the entire transformer.
In contrast to the AFE drive, the AAF solution is installed in parallel with the current flow, meaning the current to the drives does not flow through the filter. Should any component in the AAF should fail, the current will continue to flow unhindered to the drives. So if a filter breakdown occurs, the harmonics level will of course rise, but the water treatment plant or pump station can still continue its business.
Another benefit of the AAF is the built-in “sleep mode” function. When the load level is low and thus the harmonic level becomes low (settable value in the AAF) the AAF enters sleep mode to conserve energy. For a comparable AFE solution, the extra losses due to harmonic dampening are constantly present. See below for calculation examples based on a fictional water treatment plant with a number of drives.