What factors shall be considered when designing a harmonic filter?
Most modern variable frequency drives (VFD) have so high power quality that they comply with the corresponding standards without the need for additional harmonic filtering. Nevertheless, there are cases where a filter is recommended or necessary. The consequent questions are: How to make a proper design? What factors to consider when designing a harmonic filter?
Trend to go filterless
A general trend in modern AC drives is to eliminate the need for a grid harmonic filter. The motivation behind it is rather clear:
- Optimized hardware cost¹
- Reduced footprint²
- Increased robustness during grid faults³
¹ This is not always granted. Eliminating a grid filter reduces the hardware cost if there is no need to use a more expensive type of VFD.
² Filter typically has lower power density than the VFD. It is comparably large when talking about MVA per m³. For example, 10 MVAr filter may have similar footprint (or better volume) like 50 MW VFD.
³ See explanation further in the article. Filter branches are poorly damped and thus sensitive to external excitation.
Tailor made filter design
State of the art VFD topologies can usually satisfy the international standards on power quality and harmonic control without the need for a filter. More information on harmonic mitigation can be found in []. Despite of that, there are few cases where filter is still needed. Examples are installations with very weak network, poorly damped resonances in low frequency range (below 5 kHz), presence of sensitive equipment connected to the same grid etc. The remedy might be an additional grid harmonic filter. This filter is to a large extent tailor made for the specific site. That is an advantage and a risk at the same time. Why?
Challenges when designing a harmonic filter
As the VFDs usually come along as ‘filterless’, the grid filter is neither integrated as part of the product nor available as a pre-engineered option (few exceptions may exist but in most cases the statement is true). Instead, the filter is made to fit the project requirements. This approach provides flexibility on one hand but also presents potential risks on the other hand. There are few factors that are particularly important when designing a harmonic filter. We briefly introduce them in this post. Do you wish the full text article? That is easy – just purchase one of our premium plans.
1. Existing harmonic pollution
Besides the harmonics generated by the VFD, the filter may be exposed to harmonics from other loads that are present in the grid. These are harmonics already present in the grid before installing the VFD or harmonics for non-linear equipment that is added later. Let’s call them background harmonics. The best way is to measure the background harmonics before performing final design of the harmonic filter. If no measurement is available, a “qualified guess” shall be made.
2. Various grid configurations
Grid is a complex system. Most grids are operated in various configurations depending on actual needs. The filter shall perform well in all these scenarios. The challenge is that depending on current grid configuration the grid resonance can shift to lower or higher frequency. The filter shall then be designed in such a way that it works satisfactory with each grid configuration. The more configurations exist the more challenging is the filter design and the more time consuming is the network study.
3. Future grid expansion
Apart from the different grid configurations at present one shall also consider the configurations in the future. MV drive system has an expected lifetime of at least 20 years. Thus, it is important to think ahead and ensure the compatibility with the future grid. The power grid is changing quite dramatically and it can be assumed that the future grid can look very different from what it is today.
4. Location of a filter
There are several options where to connect the grid filter. The choice depends on VFD topology, grid connecting voltage, power rating or manufacturer’s preference/experience.
Principally we can think of:
- Filter on HV side (transformer primary)
- Filter on LV side (transformer secondary)
- Filter connected through a dedicated tertiary winding
The location of the filter impacts the filter design, performance during transients, effectiveness of harmonic filtering etc. For more information purchase one of our premium plans or a dedicated webinar.
5. Grid faults
When designing a harmonic filter, one key parameter are filter losses. As we want to minimize them the filter is naturally poorly damped. This can be a challenge during grid faults. The fault may cause severe oscillations of the filter that decay very slowly. That is a challenge for the control, especially when using an active front end (AFE) drive.
6. Filter protection
Custom designed harmonic filters can have various topologies – ranging from a simple LC or RLC filter up to complex multi-branch structures. An adequate protection is thus very important. The protection concept shall cover different fault scenarios and provide sufficient protection to individual filter components. Needless to say – personal safety is of course the top priority.
This blog post just scratches the surface…
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References
[1] Network harmonics: Harmonic filter, https://mb-drive-services.com/network-harmonics-harmonic-filter/
[2] How does a grid filter work? https://mb-drive-services.com/how-does-a-grid-harmonic-filter-work/
[3] Undervoltage ride through, https://mb-drive-services.com/ride_through/
