When to go for an active front end drive?
Abstract
Grid side of the VFD can be either Diode Front End (DFE) or Active Front End (AFE). The later one is more expensive but brings several additional options such as regenerative operation of power factor control. This blog post is a polemic on the subject when AFE is beneficial and when not.
Introduction
Variable frequency drives (VFD) can be classified according to several different criteria. With respect to grid side hardware of the VFD the two main categories are: Diode Front End (DFE) and Active Front End (AFE).
AFE is more expensive on the hardware level but allows additional technical features. However, not every application or system can fully benefit from those features. In the next paragraphs the difference in performance between DFE and AFE drives will be described. The additional benefits and drawbacks of AFE solution are mentioned. Finally, a non-binding recommendation is given on where AFE drive brings benefits and where DFE is a preferred choice.
Advantages of active front end drive
AFE drive variants have two well-known advantages:
- Support of regenerative braking (bi-directional power flow)
- Control of input power factor
The first point, regenerative braking, is often the decisive factor to choose AFE type of a VFD. For applicationswith frequent braking and high braking power the regenerative braking is the best option. Control of reactive power and input power factor is often overestimated in usual drive systems. However, there are applications where the reactive power control can bring a significant advantage.
Besides those two factors, there are additional benefits when going for an AFE type of drive. They are described in following paragraphs.
A. Regenerative braking
Applications requiring frequent braking can largely benefit from AFE type of VFD. The active rectifier allows power flow in both directions: from grid through VFD to the motor and from motor through VFD back into the grid. Regenerative braking can bring substantial energy savings when appearing regularly. The precondition is that the grid allows such regeneration:
Power regeneration within an industrial network is usually not a problem since one drive is regenerating power while other loads consume that power. However, a net surplus of energy supplied to the utility grid may not be allowed by the grid operator (regenerated power to be consumed within the plant).
B. Support of dynamic applications
The primary task of an AFE is to actively control the dc link voltage. Thus, the dc link is dynamically balanced. This feature, combined with regenerative braking described above, is suitable for dynamic applications with sudden load changes and possible quick power reversal. AFE is superior to a DFE with braking chopper. It supports the dc link with both sudden load increase or load reduction.
C. Better performance during grid faults
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D. Simpler transformer design
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E. Transformerless configuration
Transformerless drive is a specific AFE configuration eliminating the need for an input transformer. The VFD is directly connected to the grid without any coupling transformer. Such setup may reduce the system cost (absence of the main transformer) and improve the overall efficiency (no transformer losses). It also reduces the overall footprint of the drive system. This point may be very important in case of space restrictions, such
as e.g. in a brownfield installation.
However, there are some limitations as well. Not every AFE model supports a transformerless configuration. When it is supported, it only works for a specific grid voltage, similar to the motor nominal voltage. Moreover, the input transformer is not only used for adaptation of voltage. It inherently limits the fault currents or harmonic distortion. The transformerless configuration therefore needs an input reactor to provide at least partial decoupling from the grid. Such reactor obviously requires some space and generates power losses.
F. Reduction of grid harmonics
AFE solution reduces harmonics primarily by the modulation, e.g. using a suitable pulse pattern. Depending on the number of switching instances per period, certain number of harmonic orders can be eliminated. The elimination depends on the modulation type, switching frequency and of course AFE topology. 3-level NPC will not produce the same harmonics as MMC based AFE. Generally we can’t say that AFE always have lower harmonic distortion than DFE. The statement might be true in low voltage drives with so called “ultra-low harmonic” drives. It is because low voltage drives have higher switching frequency and the DFE solution is often just 12-pulse. In contrast, at medium voltage the switching frequency of AFE may be more restricted and the DFE variant can be 24-pulse or 36-pulse. Thus, the superiority of one or the other solution is not granted.
G. Reactive power control
The option of reactive power control is sometimes overestimated when putting arguments for an AFE drive. A diode rectifier (DFE) drive reaches power factor of approx. 0.95 inductive at full load. At partial load the power factor of DFE further increases. That is already an improvement compared to a direct on-line asynchronous machine with a typical power factor 0.88 to 0.92. An extra improvement due to AFE exists but it is not the primary argument when deciding for AFE. Finally, for most utilities power factor 0.95 is sufficient and the consumer is not punished for the small amount of reactive power consumed. There are however cases where the reactive power control becomes much more relevant:
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Drawbacks of active front end
Advantages as well as drawbacks are always relative and cannot/shall not be generalized. AFE offers range of benefits and additional technical features. Some of the potential drawbacks include higher purchase price of the VFD (usually also on the system level as the simpler transformer does not offset the extra cost of AFE compared to DFE), slightly more losses in the normal motoring operation (meaning lower efficiency and recooling equipment with higher capacity) or higher acoustic noise of the transformer due to modulated voltage.
These drawbacks are normally fully outweighed by the advantages and possibilities that AFE offers, at least when used in the right application.
Performance of DFE and AFE based drives
The type of rectifier affects the performance of the drive system and also the design of other system components, particularly the transformer. A brief comparison of AFE and DFE drive systems is listed in Table 1.
Table 1: Comparison of active front end and diode front end based drive systems
Summary
The advantages and drawbacks of AFE type of VFD, in detail described in previous paragraphs, are summarized in Table 2.
Table 2: Advantages and drawbacks of active front end drive
Conclusion
AFE type of VFD enables additional functionality of the drive system. Most frequently mentioned benefits are regenerative braking and reactive power control. In fact, there are even more benefits, including active control of dc bus voltage (beneficial for high dynamic applications or fault ride through), flexibility to adjust the harmonic spectrum injected into the grid or a simpler design of input transformer. Whether AFE is the right choice or not
depends on how much of this extra functionality can be utilized in the specific application/project.
