VFD harmonic spectrum
Introduction
After the theoretical introduction [1] we jump to variable frequency drives as sources of harmonics. This post is about VFD harmonic spectrum. As already mentioned, VFDs belong to non-linear devices with non-sinusoidal currents and/or voltages that they draw from the grid or inject into the grid. The evaluation of harmonic levels shall be done in early stage. Preliminary calculations of harmonic distortion shall be done and if necessary additional mitigation methods shall be used. We will discuss the mitigation methods in a separate article as it is a topic on its own.
The negative effects of harmonics were discovered shortly after first power electronic based frequency converters were installed, i.e. more than 40 years ago. Nowadays the phenomena are well known and in detail described in the literature (e.g. [2]). Manufacturers of VFD can provide the user with VFD harmonics. There is a variety of commercial software tools to calculate network harmonics for any system. The VFD topologies available on the market in these days usually comply with international harmonic standards that define harmonic limits. However, the compliance shall always be checked in early phase. Adaptations and changes at later stage would be more costly and shall be avoided.
The harmonic spectrum of VFD depends on two main factors:
– VFD topology
– System parameters
For active front end type of rectifier the VFD harmonics also depend on modulation technique.
VFD topology and harmonics
The first important differentiation depends on the way of frequency conversion. Most VFDs are indirect frequency converters, i.e. they have a dc link. The group of direct frequency converters is much smaller and contains cyclo-converters (seldom used) and matrix converters (will be discussed separately). Therefore, we focus on the main two families of VFDs:
– Current Source Inverters (CSI)
– line commutated
– self commutated
– Voltage Source Inverters (VSI)
– diode front end (DFE), i.e. line commutated
– active front end (AFE), i.e. self commutated
Current source inverters use either self commutated rectifier (with transistors, GTO thyristors or IGCT thyristors) or a grid commutated thyristor rectifier (classical thyristor, also called SCR). Current source inverters consist of a controlled rectifier supplying a fairly large reactor in the dc link.
Voltage source inverters are supplied either from a diode rectifier or from active (self commutated) rectifier. Voltage source inverters commonly consist of an uncontrolled diode rectifier supplying a capacitor bank in the dc link. If regenerative braking or power factor control on grid side is required, then active front end converter is used.
This differentiation is important when analytically analyzing the network harmonics.
Figure 1 shows only the grid side converter section as this is relevant for harmonics injected into the grid.
Another important differentiation depends on the type of semiconductor used in rectifier or the switching frequency, respectively. Line commutated devices (diodes and thyristors) switch with the grid frequency. Self commutated devices (IGCT thyristors, IGBT or IEGT transistors) operate with higher switching frequency independent on the grid frequency.
Line commutated rectifiers generate characteristic harmonics as integer multiples of the pulse number plus minus one and line frequency (equation 1 in Introduction). Self commutated rectifiers often use pulse pattern based modulation with selective harmonic elimination.
The above explanation is very fundamental for understanding the the VFD harmonic spectrum:
–> line commutated rectifiers (thyristor based current source inverter and diode front end voltage source inverter have characteristic harmonics defined by the pulse number. 12-pulse thyristor rectifier has dominant 11th and 13th harmonic. 24-pulse diode rectifier shows dominant 23rd and 25th harmonic in its spectrum.
The harmonic cancellation is achieved by multiple rectifier bridges supplied from a phase shifting transformer.
–> self commutated rectifiers do not have direct relation between rectifier pulse number and harmonic spectrum. Phase shifting transformer eliminates certain harmonics while other harmonics are eliminated by appropriate modulation. So called optimized pulse pattern (OPP) is frequently used. It is a selective harmonic elimination method where the pulse patterns are offline pre-calculated, typically using the quarter period symmetry. In other words, the control switches the semiconductors on and off at pre-defined angles in each period. Usually there are several optimized pulse patterns to choose from. Their selection is defined project specifically considering the switching frequency and losses, specified limits for harmonic distortion, possible grid resonance etc.
To reduce harmonic distortion of a diode or thyristor based (line commutated) rectifier it is standard practice to increase the rectifier pulse number. In medium voltage VFDs a 6-pulse diode rectifier practically does not exist. 12-pulse diode rectifier on the hand is quite common. However, also this one might not satisfy the limits of the local or international standards regarding harmonic distortion, such as e.g. IEEE 519. Therefore, 18-pulse, 24-pulse, 30-pulse or 36-pulse rectifiers can be offered by most manufacturers.
The cancellation of specific harmonic orders is achieved by multi-pulse rectifier in combination with phase shifting transformers [3]. It is well-known method, but we will anyway briefly explain it in another article of this series. Note that in ideal system the non-characteristic harmonics do not exist. However, real systems are not ideal. The standards recommend to consider certain percentage (5-20%) of non-characteristic harmonics.
Grid voltage distortion
Figure 2 shows different level of current distortion depending on the rectifier pulse number. How does it translates into voltage distortion? It depends on the grid impedance. The lower the grid impedance the less voltage distortion will be caused by harmonic currents. In simple considerations the grid strength is characterized by its minimum and maximum short circuit power at the point of common coupling (that is the point where harmonic performance shall typically be guaranteed). High short circuit power means low grid impedance, i.e. strong grid. Vice versa, low short circuit power points towards a weaker grid.
Grid characterization with just a short circuit power is a simplification.
How does distorted grid voltage look like? The total harmonic distortion (THD) is a quantity to define overall distortion. Two voltage waveforms might have the same distortion, yet their signals will look slightly different as they will contain different frequencies. Below are few examples of distorted voltage waveforms in time domain.
Short intermediate summary:
Line commutated rectifiers have a fix relation between the grid harmonic spectrum and rectifier pulse number along with corresponding phase shifting transformer.
Self-commutated rectifiers use phase shifting transformers as well (besides 6-pulse active rectifiers) plus modulation technique with selective harmonic elimination.
For example, 12-pulse diode rectifier will show typical 12-pulse spectrum while 12-pulse active rectifier generates a spectrum corresponding to 24-pulse diode rectifier or (probably) even higher pulse number.
Now when we understand the VFD harmonic spectrum (harmonic content) we shall differentiate one more important thing: the character of harmonics:
All current source inverters (CSI) and voltage source inverters (VSI) with diode rectifier generate current harmonics. The rectifier in combination with stray impedance of input transformer provide the current source character (Do not confuse the inverter type and character of grid harmonics).
Voltage source inverters with active rectifiers generate voltage harmonics.
VFDs with current source harmonic behavior inject current harmonics into the grid. The voltage distortion depends on the grid impedance.
VFDs with voltage source harmonic behavior produce voltage harmonics. The currents driven by these harmonic voltages depend on the impedance of the path of current.
The character of the harmonic source (harmonic current source/harmonic voltage source) is particularly important when performing network harmonic studies. Very good software calculation tool for electric networks is for example NEPLAN. There is a specific module for harmonic analysis. We have used that tool for several projects and can definitely recommend it.
References
[1] Network harmonics: Introduction, https://mb-drive-services.com/net_harm-introduction/
[2] Guide to harmonics with AC drives, https://library.e.abb.com/public/bc35ffb4386c4c039e3a8ec20cef89c5/Technical_guide_No_6_3AFE64292714_RevF_EN.pdf
[3] VFD transformers: Multi-winding design, https://mb-drive-services.com/vfd_transformer_design/
[4] NEPLAN – Powerful analysis tool for electric networks, https://www.neplan.ch/neplanproduct/en-electricity/
