Network harmonics: Mitigation methods
Today’s post focuses on harmonic mitigation methods. Let us review the most common, well proven methods of minimization of harmonic distortion on the grid side.
Previous articles presented a general view on harmonics, their origin and effects [1]. The grid side harmonic spectrum of VFDs had been described based on rectifier topology and character of dc link [2]. As the presence of harmonics in the electric grid is undesirable we will look at some methods how to minimize the harmonic distortion (‘grid pollution’) and mitigate technical issues.
What are the most common and most effective techniques to reduce harmonics injected into the supply grid?
Harmonic mitigation methods
1. Increase of rectifier pulse number in combination with phase shifting transformers
2. Connecting the VFD to higher voltage level
3. Use of active front end rectifiers with selective harmonic elimination
4. Use of passive harmonic filters
5. Use of active harmonic filters
1. Increased pulse number of diode or thyristor rectifier
This is one the most popular harmonic mitigation methods. Pulse number of DFE type VFDs determines the harmonic spectrum as already explained in [2]. Basic building block is a 6-pulse rectifier bridge supplied from a three-phase system. Combining multiple 6-pulse bridges in series and parallel connection and supplying each bridge from a dedicated transfomer winding with suitable phase displacement [3, 4] leads to increased pulse number.
– Multi-pulse diode rectifier
6-pulse diode rectifier practically does not exist in medium voltage VFDs due to large harmonic distortion (with exception of special cases, e.g. supply directly from on-boad generator). Minimum DFE pulse number is normally 12-pulse. Such rectifier consists of two 6-pulse bridges. The diode rectifier is supplied from a three-winding transformer. Secondary windings of the transformer are displaced by 30° electrical. Such transformers are generally called phase shifting transformers. To achieve a 30° phase displacement is easy. Conventional solution is to use one secondary winding star (wye) connected and other secondary winding connected in delta (e.g. Yy0d11). Another option is to use two extended delta secondary windings, one displaced by +15° and other one by -15° referred to the primary winding.
– Principle of phase shifting and harmonic cancellation
The principle is fairly straight forward. Characteristic harmonics are determined by simple formula
h = k⋅p ± 1
where h is the characteristic harmonic order, k is an integer and p is the pulse number of the rectifier.
According the amplitude law the magnitude of harmonic is inverse proportional to the harmonic order. It means that the lowest order characteristic harmonics are those with largest magnitude and therefore an effort is made to eliminate them.
Let’s take the easiest example of a 12-pulse rectifier already described. It consists of two 6-pulse rectifier bridges, each supplied from a separate winding.
Simple explanation (not 100% correct, bur easy to understand):
The windings have relative displacement of 30° for the fundamental frequency (normally 50 or 60 Hz). For the 6th harmonic the displacement is 6 ⋅ 30° = 180°. Therefore, the harmonics are in opposite phase and cancel (compensate) each other. In ideal case of perfect cancellation there is no 5th and 7th harmonic seen on transformer primary side.
More correct explanation – see phasor diagram above:
While the 12-pulse behavior with 30° phase displacement might be somewhat intuitive, diving a bit deeper it might be confusing. When drawing a vector diagram note that the 5th harmonic has a negative sequence, i.e. rotates in opposite direction than the fundamental. 7th harmonic is of a positive sequence. The 5th and 7th harmonics are not entirely eliminated. However, the residual magnitudes are just a fraction of the magnitudes present in a 6-pulse system.
Note that not only 5th and 7th harmonics are cancelled. The same principle applies for all odd multiples of 6th harmonic (18th, 30th, 42nd etc) on the DC side and corresponding harmonic orders on the AC side (17th, 19th, 29th, 31st, 41st, 43rd etc).
Analogously, rectifiers with higher pulse number and suitable multi-winding phase-shifting transformers [3] can eliminate more harmonic orders and achieve lower grid distortion.
* The above explanation refers to diode rectifiers as the most common VFD rectifiers. Same principles apply also for thyristor rectifiers bridges (e.g. LCI drives).
2. Connecting VFD to higher voltage level
Another way to minimize the harmonic distortion is to connect the VFD directly to a higher voltage level (if available, of course). Larger plants might have more than just one voltage system, e.g. 11 kV and 33 kV. In such case it is beneficial to connect the VFD transformers directly to the higher voltage level.
That grid is expected to be stronger meaning it has higher short circuit power and lower impedance. The voltage distortion for harmonics of current source type (diode and thyristor rectifiers) is a product of grid impedance and current harmonics. The lower the impedance the lower voltage distortion.
3. Use of active front end rectifiers with selective harmonic elimination
In [2] we explained the harmonics of active front end (AFE) rectifiers with self-commutated semiconductors. Note that the rectifier pulse number does not directly say AFEanything anything about harmonic spectrum. Many AFE rectifiers are just 6-pulse. However, they shall not be compared with 6-pulse diode rectifier. First of all, most AFE rectifiers supply a voltage source inverter and from harmonic perspective they behave as harmonic voltage surce (comparison: diode rectifiers have harmonic current source behavior). Second, low order harmonics, such as 5th, 7th, 11th, 13th etc are normally eliminated or minimized by suitable modulation. So called ‘optimized pulse pattern’ (OPP) is very popular. It is a selective harmonic elimination method. Typically the manufacturers have several standardized OPPs available and the most suitable one is selected based on project requirements and grid characteristic (min/max short circuit power and eventually also frequency dependent plots revealing potential resonances).
AFE rectifiers also exist with higher pulse numbers, e.g. 12-pulse and 18-pulse AFE. In such case two principles are used:
– Certain harmonic orders eliminated by phase shifting technique (e.g. 12-pulse configuration with 30° phase displacement –> minimization of 5th and 7th harmonic and their odd multiples).
– Other (higher) harmonic orders eliminated by the pulse pattern (OPP).
Such multi-pulse AFE rectifiers have superior harmonic performance while maintaining reasonable switching frequency.
4. Use of passive harmonic filters
Generally the intention is to avoid external harmonic filters when possible and solve the harmonic issue directly at the source (rectifier). However, sometimes the filter becomes unavoidable.
In what cases might harmonic filter be needed:
– VFD topology does not support higher rectifier pulse number
– presence of grid resonance that could be excited by VFD harmonics
– very strict requirements on grid power quality allowing almost no harmonic injection
– filter is used not only for harmonic filtering, but also for power factor compensation
– solution with filter is overall more economical
For example, if there is an indication of grid resonance at relatively high frequency that could still be excited from the VFD then it might be difficult/not practical to modify the rectifier (e.g. parallel resonance close to 47th harmonic -> it is easier to provide small filter branch for resonance damping rather than building a 96-pulse rectifier).
There are various topologies of passive filters:
– single tuned filter
– double tuned filter
– high-pass filter
– de-tuned filter
– filter system (multi- branch type)
The filters might be with or without a damping resistor. By “tuning” is meant the adjustment of filter characteristic for specific resonance frequency. Detailed design of passive harmonic filter requires knowledge of the supplying grid (resonances, existing harmonics from other sources than VFD etc). A network study is recommended as part of filter fine design. There are several software tools for network modeling. NEPLAN is one of them [5].
We prepare a dedicated article focusing specifically on passive harmonic filters. You will learn more about design principles, frequency characteristics and other aspects.
5. Use of active harmonic filter
Active filter is power electronic based equipment. It produces (injects) harmonics of same frequency and magnitude like those in the grid, but with opposite phase, hence cancelling them.
Active filters are sometimes used in low voltage systems. In medium voltage area the use of active filters as harmonic mitigation method is still rather limited. It is relatively expensive solution and usually has some limitation in regards to harmonic orders, resp. bandwidth of active filter (e.g. only 5th, 7th, 11th and 13th harmonics can be cancelled). As such solution is at present very rare in medium voltage and especially at higher power, we do not go deep into details and just mention that active filters exist.
Summary
Five harmonic mitigation methods were presented. Multi-pulse diode rectifiers present clearly the majority of medium voltage VFD installed base. Active rectifier offer additional possibilities, but come with price adder and slightly higher losses. such solutions make sense when additional benefits of active rectifiers can be utilized as well: regenerative braking or reactive power compensation. External passive filters had been frequently used in the past. Nowadays the trend is to use rectifier topologies that produce low harmonic distortion and avoid external filters. It reduces the engineering effort and complexity of installation. Exceptions are topologies that need the filter not only for harmonic filtering, but also for reactive power compensation.
The selection of harmonic mitigation method depends on the application, VFD topology or characteristics of the supplying grid. Some method can be combined, e.g. multi-pulse rectifier with phase shifting transformer and modulation scheme for active rectifiers. The last method, active filter, is not very common in medium voltage drive applications due to relatively high cost and limitation of bandwidth.
Most medium voltage variable frequency drives, regardless if diode front end (DFE) or active front end (AFE), fulfill all the international standards regarding harmonics, such as IEEE 519-2014, IEC 61000-3-6 or GB/T 14549. Product not fulfilling these standards would be difficult to market and sell. However, attention must be paid in situations with poorly damped network resonance in frequency range where VFD injects harmonics.
References
[1] Network harmonics: Introduction, https://mb-drive-services.com/net_harm-introduction/
[2] Network harmonics: VFD harmonic spectrum, https://mb-drive-services.com/network_harmonics_vfd_harmonic_spectrum/
[3] VFD transformers: Multi-winding design, https://mb-drive-services.com/vfd_transformer_design/
[4] VFD transformers: Clock number and vector group, https://mb-drive-services.com/clock-number-and-vector-group/
[5] NEPLAN – Powerful analysis tool for electric networks, https://www.neplan.ch/neplanproduct/en-electricity/
