Harmonic calculation versus network harmonic study
As our regular reader you already have a solid knowledge about network harmonics caused by non-linear power electronic systems. It does not surprise you that network might have resonance(s) – a topic that needs to be addressed. Also, you are familiar with harmonic footprint of different types of VFDs. In addition, you understand the most relevant harmonic mitigation methods. Basic design of harmonic filters is familiar to you. And finally, you have an idea about software tools for deeper network analysis. Consequently, it is time to talk about the difference between harmonic calculation and network harmonic study.
Network harmonics series
If you have missed any article on this topic feel free to go through the whole network harmonics series [1] that is archived. It is much easier to follow the explanation if you remember the things explained in the previous posts. We also make couple of references with hyperlinks for your convenience and better navigation.
Difference between harmonic calculation and network harmonic study
First note that the wording ‘harmonic calculation‘ and ‘network harmonic study‘ are not any official terms defined in standards or so. It is something that we are using. Maybe you call these disciplines slightly different. That is why we start with a brief explanation of their respective meaning.
i. Harmonic calculation
Harmonic calculation refers to a basic harmonic assessment. This calculation is relatively simple. Instead of complex relationships it uses a simplified approach. One of the most common assumptions is an inductive network. It means that grid impedance is purely inductive or it is a resistive-inductive combination. In any case, capacitive components are neglected. It is therefore a first order system that does not have any eigenfrequency meaning there is no electric resonance.
Harmonic calculation often does not require special software programs. It can be performed with standard office tools, such as e.g. Excel. The initial harmonics might come from a time domain simulation or they can be derived analytically. Ideally, these harmonics were validated with real site measurements.
ii. Network harmonic study
Network harmonic study (grid harmonic study, network harmonic analysis) can be viewed as extended harmonic analysis. The power system is analyzed in much greater detail. A more sophisticated software tool is used for to perform calculations such as load flow and of course harmonic analysis. Importantly, the real impedance of the grid is taken into account. By doing that, phenomena such as grid resonance can be analyzed.
Network harmonic study requires a dedicated software that supports modeling of the grid according to a single line diagram of the plant.
When is a harmonic calculation sufficient
Following points typically allow to consider simplified harmonic calculation:
— Grid can be assumed as (purely) inductive
— There are no elements with higher capacitance (such as power cables or capaitor banks)
— Power system is relatively simple and straight-forward
— Harmonic distortion shall be calculated at one point (e.g. PCC)
— Grid short circuit power is much larger than the installed power of all non-linear loads
— Project is in early stage and grid details are anyway not known
When a network harmonic study is recommended
As opposite to previous case or an extension of it, a network harmonic study is recommended when:
— Network is rather complex and extensive
— Multiple grid configurations shall be considered
— Multiple grid points shall be evaluated
— Grid includes power cables of non-negligible lengths
— Presence of capacitor banks (filters, power factor compensation)
— Grid contains considerable harmonics from existing non-linear load
— There are grid resonances that may be excited by VFD harmonics
— There are multiple harmonic sources, eventually with different character
— Grid short circuit power is relatively low compared to the installed power of VFDs
— VFD has a broadband grid side harmonic spectrum
— One of the goals is to perform a sensitivity analysis
As already stated, a network harmonic study requires bigger time investment. On the other hand, if a grid model must be created anyway, e.g. for other disciplines such as load flow or short circuit study, then the harmonic analysis can largely reuse the model and the additional effort become less of a burden.
Short circuit ratio
Engineers often use short circuit ratio, i.e. ratio between short circuit power of the grid and total installed VFD power, as an indicator to evaluate harmonic distortion. For example, if you use a VFD with 24-pulse rectifier and the ratio is over 20 then you will most likely fulfill all common harmonic standards such as IEEE 519 or IEC 61000-2-4. On the other hand, low ratio is a warning sign to look at the things closely.
Similarly, one can use this ratio also as indicator whether a harmonic calculation is sufficient or a network harmonic study is recommended. Just remember not to stick to that number only, but consider the overall picture and use your judgement and experience.
Accuracy and safety margins
Although harmonic calculation is more simple, it does not mean that it is completely inaccurate or too optimistic. In fact, whether the calculation is rather optimistic or tends to be pessimistic is mainly related to the methodology. It is the responsibility of the person performing the calculation or study to justify whether certain simplification is allowed or not.
Let’s take a small example:
Our job is to calculate harmonic distortion of a 24-pulse DFE drive connected through an input transformer to a medium voltage grid. The point of common coupling is directly at the primary terminals of the converter transformer. Grid is characterized just by minimum and maximum short circuit power.
This is one of the most basic configuration ever. The system is so simple that a network harmonic study probably does not make sense. Instead, we would just perform a quick harmonic calculation. However, two guys may calculate the harmonics and get slightly different results. Yes, you guess right. It is not just about how you calculate but also what inputs you use. For 24-pulse DFE you can take more of an ideal spectrum with dominant 23rd and 25th harmonics and then their multiples. Everything below the order of 23 is zero.
However, you may also consider certain residual harmonics of a 6-pulse pattern as per recommendation from standards. Obviously, the results will be different. Not because of the harmonic calculation, but because of different inputs.
(For more details on VFD harmonic spectrum refer to previous article)
Grid impedance scan
One of key differences between the harmonic calculation and network harmonic study is the representation of a grid impedance.
In case of harmonic calculation the impedance is linear, i.e. directly proportional to the frequency. Impedance at power frequency is calculated from a short circuit power. Minimum value of short circuit power is typically considered as it leads to highest grid impedance and highest distortion. That is true at least for DFE drives where harmonics have a character of a current source.
Example:
VFD is connected through an input transformer to a 33 kV / 50 Hz grid. Minimum short circuit capacity at the point of common coupling is 480 MVA. Assuming an inductive grid the impedance at 50 Hz is 2.269 Ω. For the 5th harmonic the corresponding impedance is simply 5 ⋅ 2.269 Ω = 11.245 Ω and so on.
Network harmonic study models the grid more precisely. It allows to consider elements such as power cables, harmonic filters or reactive power compensation equipment. All these items introduce certain capacitance. And as we know from high school, combination of inductive and capacitive elements creates a resonance.
Example of a grid impedance scan is provided in figure 3. The characteristics with a resonance consider approx. 7 km of a power cable. As the cable capacitance has certain tolerance and may slightly differ based on cable manufacturer, two values had been considered. However, the purpose is not to dig into the specific values. We wanted to show the effect of non-linear impedance with a resonance peak. In such case a harmonic calculation is not recommended and a network harmonic analysis would be preferred.
By the way, as discussed recently in the article on active harmonic mitigation – what the manufacturers don’t tell you, the frequency scan is performed up to 5 kHz (harmonic order 100) instead of just 2.5 kHz defined by standards. The engineer proactively looks into higher frequencies to make sure that a relevant resonance is not missed.
A resonance is often seen as something dangerous. But the fact is that practically any grid has a resonance (most likely several resonances). In the end, what is relevant is whether the VFD can excite the resonance or not and how well is the resonance damped. Resonances at high frequencies, well above the harmonic footprint of the VFD, are not critical. Also, the system damping mostly increases with frequency (skin effect etc). When the grid resonance drops into a range where VFD produces harmonics, the system shall be analyzed more carefully. An excitation from VFD is possible. Further on, lower resonance frequencies tend to be less damped.
Summary
In this article we have explained the major differences between a harmonic calculation and a network harmonic study. Both methods are widely used to assess harmonic distortion, compliance with standards etc. Harmonic calculation is relatively easy while network harmonic study requires a bit larger time investment. The selection of one or the other method depends on project specific conditions. A rough guideline to select the right approach was introduced. There will be a follow-up article focusing closer on network harmonic study.
References
[1] Network harmonics – entire series, https://mb-drive-services.com/category/net-harmonics/
[2] VFD harmonic spectrum, https://mb-drive-services.com/network_harmonics_vfd_harmonic_spectrum/
[3] Harmonic mitigation methods, https://mb-drive-services.com/harmonic_mitigation_methods/
[4] Software tools for network analysis, https://mb-drive-services.com/software-tools-for-network-analysis/
[5] NEPLAN – Smarter tools, https://www.neplan.ch/
