How does a grid harmonic filter work?
Recently we got a question in our chat group: How does a grid harmonic filter work? It sounds like a very basic question. However, since many people struggle to imagine the flow of harmonics in the system (it is fairly abstract thing) it may not be bad to write a short post. If it helps you to understand the functionality of a harmonic filter [1, 2] then it fulfilled the purpose.
Two worlds of harmonic filters
There are two very different types of harmonic filters [1]:
- Passive filters
- Active filters
These are totally different animals. Both of them aim to mitigate the unwanted harmonics but each has a specific approach.
Principle of passive filter
Passive filters are shunts that provide low impedance path for the harmonics. As we know from the basic theory of electrical circuits, the current prefers the path of lowest resistance. When taking about AC networks then more exact is to say lowest impedance.
Principle of active filter
Active filter is based on a different principle. It is a power electronic equipment that is sensing the present harmonic currents and generating currents of the same frequency but opposite phase.
How does a passive grid harmonic filter work?
As written in our previous posts, active filters are rarely seen in medium voltage range. The investment cost is still significant and frequency bandwidth limited. Thus majority of harmonic filters are of passive type. Let’s have a second look at their principle…
Harmonics are generated by non-linear sources or non-linear loads. In our case the primary source is the VFD. Depending on topology the VFD is a source of harmonic voltages or harmonic currents (in reality it might also be a combination of both but typically closer to one or the other type). Harmonics flow from the VFD (source) into the grid. The grid has certain impedance. Yet, exact characteristic may be very complex. However, the character is mainly inductive and we can imagine it as R-L combination [3]. This is obviously a simplification but for our explanation it fits well.
In a system with multiple paths the current splits indirectly proportion to the impedances. Path with highest impedance will conduct the smallest portion of current while the path with lowest impedance receives the largest portion. This principle is applied in passive harmonic filters. The filter shall create an alternative path with low impedance and “motivate” the harmonics to flow there instead of flowing into the grid. Remember that electric current always prefers the path of lowest impedance.
Distribution of harmonics
The principle of current distribution is schematically depicted in Figure 1. It is valid for a VFD that has a character of a current source for harmonics on the grid side (e.g. diode front end type of VSI or LCI). Nonetheless, a VFD with harmonic voltage source character (e.g. active front end VSI) would have similar behavior as well. Obviously, in the absence of a harmonic filter, the entire current incl. harmonics is injected into the grid. When a grid harmonic filter is present, the current has two paths: into the grid or into the harmonic filter. Current is distributed indirectly proportional to the impedances. Path with low impedance sees more current than path with high impedance. Most filters are designed (tuned) in a way to have low impedance for harmonics that shall not flow into the grid. The filter is a sink for those harmonics. The filter basically “shorts” the grid at specific frequencies that it is tuned to do so.
Grid harmonic filters can be tuned or untuned. They can consist of one of multiple branches. Regardless of the design the basic principle of a passive harmonic filter is always the same: create a sink for the harmonics generated by the VFD.
Frequency characteristic
To better understand the filter design it is useful to look at its frequency characteristic. That is a plot showing impedance versus frequency. A typical tuned passive filter is a bandpass filter (Figure 3 left). It has low impedance in a frequency range of the harmonics to be absorbed. When multiple harmonics shall be eliminated, multiple filter branches can be installed. Two parallel connected bandpass filters can be combined into one double tuned filter (Figure 3 middle). A bit different variant is so called untuned filter. A typical example is a high-pass filter with characteristic as shown in Figure 3 right.
This post provides a brief description of how a grid harmonic filter works. We just wanted to respond to a question of one of our readers with a short explanation. Of course, there are many more related topics not covered in this post, such as:
- What type of filter to select (which topology)?
- How to tune the filter?
- Reactive power compensation
- Best place to connect the filter
- Rules how to deal with grid resonance
- Filter and background harmonics
- Quality factor, damping and losses
- Performance during grid disturbances
- Filter protection
If you are interested in a deeper analysis then our dedicated webinars might be the right fit.
Summary
Principle of a passive harmonic filter is to offer an alternative path for harmonics. When the filter impedance at specific frequency of concern is significantly lower than the impedance of the grid, most of the current of that frequency naturally flows into the filter rather than into the grid. Filters can have one or more cut-off frequencies or can be untuned. The character of a filter is best described in a frequency-impedance plot.
Design of a filter shall respect several important design rules. These rules were not discussed in this short post. For more information subscribe to one of our premium plans or attend one of our webinars.
References
[1] Network harmonics: Harmonic filter, https://mb-drive-services.com/network-harmonics-harmonic-filter/
[2] Network harmonics: Mitigation methods, https://mb-drive-services.com/harmonic_mitigation_methods/
[3] What does purely inductive network actually mean? https://mb-drive-services.com/what-does-purely-inductive-network-actually-mean/
[4] How strong is a strong grid? https://mb-drive-services.com/how-strong-is-a-strong-grid/
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1 Comment
What factors shall be considered when designing a harmonic filter? - MB Drive Services · February 23, 2023 at 7:07 am
[…] [2] How does a grid filter work? https://mb-drive-services.com/how-does-a-grid-harmonic-filter-work/ […]
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