Harmonic filters
The continuation of our “Network harmonics” series [1] presents an article about AC harmonic filters. We explain the purpose of harmonic filter. Different types of harmonic filters are shown. Several design considerations are discussed. Main items are illustrated with figures and pictures. Grid side filters are covered in this post. Filters on the motor side belong to different series and are not scope of this article.
Purpose of harmonic filter
The purpose of grid harmonic filter is to:
1. Reduce harmonic distortion in the grid
This is the basic function of filter to create a path for harmonics and reduce harmonic distortion in the grid. The filters are sometimes called suction circuits as they provide low impedance path for specific harmonics that shall be minimized or eliminated. For this purpose the filter might have one branch or several branches tuned for various frequencies of harmonics. The topology and design of harmonic filter depend on several factors, such as VFD rectifier topology, grid characteristic, standards for harmonics to be fulfilled etc.
Figure 1 shows exemplary current waveforms of a current source type VFD (LCI is this case). On the left side is the input current of the LCI rectifier. It has typical rectangular shape (with some ripple as the smoothing reactor is not infinite) and corresponding “6-pulse” spectrum. There are two such rectifiers supplied from 30° phase displaced transformer windings creating a 12-pulse system. Therefore, the filter branch is tuned to absorb 11th and 13th harmonic (see middle section). Finally on the right hand side the current drawn from the grid is shown. It is practically sinusoidal. Don’t get confused by the spectrum below in red. The scale of y-axis is zoomed to see something. The distortion is negligible.
2. Reduce losses
Harmonics create additional losses in components such as transformers, overhead lines and cables. Minimizing the harmonics helps to reduce overall losses in the system. Especially by placing the filter system as close to harmonic source as possible, additional losses due to harmonics can be mnimized.
3. Provide reactive power
This is the reactive power compensation function. It is also called power factor correction (PFC). Note that this function is not always needed. For example VFDs with diode rectifier (DFE) usually have power factor above 0.95 for almost entire speed/load range. In this case the reactive power compensation is not needed. In fact, it is even not desired as overcompensation of the grid is generally not allowed by utilities. Some equipment inside an industrial plant may be operated with leading (capacitive) power factor to compensate other loads with lagging (inductive) power factor. However, the plant as whole shall generally not be overcompensated.
Need for harmonic filter
The trend in VFD market is clearly towards filter-less solutions, i.e. variable frequency drive systems that do not require filters. Drive systems without harmonic filter are simpler from engineering and integration point of view, cheaper, less complex and overall more straight forward. However, in some cases a harmonic filter becomes unavoidable. Therefore the question: when is a harmonic filter needed?
A. VFD producing excessive amount of harmonic distortion
Some VFD topologies may inject too much harmonics into the grid and filter is required to comply with applicable standards regarding harmonics (e.g. IEEE 519, IEC 61000, GB/T 14549).
B. Low short circuit capacity of the grid
In case of weak grid with low short circuit capacity even “grid friendly” VFD may not fully comply with harmonic standards and additional filter is needed. For initial assessment the ratio between grid short circuit power and VFD installed power is often used (R = Ssc/Sdrive). For example, if this ratio is larger than 15, it is expected that the VFD complies with harmonic limits defined in international standards. Drive systems with low ratio need a closer analysis to ensure smooth integration of a VFD into the system.
C. Network parallel resonance interacting with VFD harmonics
If the network parallel resonance coincides with one of the harmonic orders of the VFD a significant harmonic distortion may occur (or may not – depending on excitation level and system damping). In such case an additional filter might be needed to damp the parallel resonance and provide alternative path for harmonics. Long medium voltage cables on the grid side connection (several kilometers) may indicate a parallel resonance issue.
D. Filter needed for harmonics and power factor correction
This case is well-known from load-commutated inverter (LCI) type of VFD. The rectifier consists of phase controlled thyristors. At lower motor speeds the rectifier operates with larger firing angle. The firing angle is approximately equal to the angle between fundamental voltage and current. At low speed of LCI the rectifier firing angle significantly delays the current against the voltage resulting in poor power factor. Most LCI drives are designed with 12-pulse rectifier and input filter with its dual function: harmonic filtering and power factor correction.
Design of harmonic filter
When designing a harmonic filter several considerations shall be taken into account. The factors influencing the design are:
– VFD harmonic spectrum
– VFD operation modes
– Grid characteristics
– Applicable harmonic standards / grid codes
– Background harmonics
– Fault cases
Types of harmonic filters
Various criteria can be used to classify harmonic filters. This list provides some practical classification:
According to the “working” (resonance) frequency of the filter we can distinguish
– Single-tuned filters
Filters are tuned to specific single frequency.
– Double-tuned filters
Harmonic filter is tuned for two different frequencies.
– De-tuned filters
Filter is not tuned to a specific harmonic. Instead, it is typically used as broad-band high-pass filter. The purpose is not to reduce specific harmonic. It shall damp the parallel resonance and create path for any harmonic that could eventually coincide with the parallel resonance.
Note that single-tuned filter can still eliminate two or more harmonics. The characteristic of a single-tuned filter with a high-pass resistor can be designed in a way that the filter eliminates both 5th and 7th harmonic (or 11th and 13th harmonic – numbers just as example).
In some applications, such as LCI drives, a filter system with several filter branches is used.
Integration of harmonic filter
The filter system needs to be integrated into the plant. Again, several options are possible. Shall the filter be installed indoors, outdoors or combined? Where is the filter connected?
– between input isolation transformer and VFD
– on primary side of input isolation transformer
– through dedicated tertiary winding on input isolation transformer
Each of these solutions has its advantages and drawbacks. The choice depends on the global system design, power and voltage rating and also experience of manufacturer.
(a) Filter on the primary side has the advantage that in case of several VFDs connected to the same point of common coupling (PCC) only one filter can be used for all. However, this design becomes impractical if the voltage level on the primary side is too high (costly solution) or if the filter installed power becomes larger (dangerous transients when switching on/off large filter branches). Also, design of filter on the primary side needs to consider harmonics from other sources than just the VFD(s).
(b) Filter between the input transformer and VFD is a good solution if the VFD has just one 3-phase input (one secondary winding, i.e. basic 2-winding transformer). For multi-winding designs there is a clear disadvantage as each transformer secondary would need its own filter branch.
(c) Filter connected through a dedicated transformer (tertiary) winding is fairly decoupled from the grid and is usually more efficient in its filtering function. The voltage level can be selected independent on grid voltage. The disadvantage is higher complexity and cost of the VFD input transformer and stricter requirements on the transformer in general (impedance matrix).
Summary
This article presented basic information about AC grid-side harmonic filters associated with variable frequency drive systems. The purpose and need for harmonic filter were explained. Various types of filters were presented and most important design considerations had been shared.
As mentioned in the introduction, the trend is towards systems that do not need input harmonic filters. The aim is to reduce the complexity and system integration. However, there will always be some cases where the harmonic filter becomes necessary or at least recommended. For such cases the considerations above apply.
We hope that the information here is useful.
Any questions or comments regarding this article or our blog in general? Write us an email or use the contact form below. Best regards from the editorial office!
References
[1] Network harmonics series, https://mb-drive-services.com/category/net-harmonics/
[2] Power consulting – ABB, https://new.abb.com/power-consulting
[3] T. Hruby, S. Kocman, “Using broadband passive harmonic filters for harmonic mitigation in AC drives”, 16th International Conference on Harmonics and Quality of Power (ICHQP), Bucharest, Romania, 2014
[4] NEPLAN – Power System Analysis tool, https://www.neplan.ch/
