What does purely inductive network actually mean?
When calculating network harmonic distortion of variable frequency drives (VFD) there are basically two ways differentiating in complexity and eventually also accuracy. One is a simple harmonic calculation while the other option is a network harmonic study. The harmonic calculation is based on inductive network [1]. So the valid question is: What does actually purely inductive network mean?
As we know from the theory about electric circuits: combination of an inductor and capacitor creates a resonance. Mathematically it is a system of second (or higher) order. On the other hand, a first order system does not have any resonance. And this is the case of purely inductive network.
Real, inductive and purely inductive network
A generic network consists of all three basic quantities of electric circuits:
— inductance L [H]
— resistance R [Ω]
— capacitance C [F]
If the capacitance is rather small it may be possible to neglect it in the harmonic calculation. By doing that the network reduces to an R-L combination which is a system of first order. We can characterize the grid by the values of R and L at the power frequency. Common inputs are the grid short circuit capacity and X/R ratio. From there you can easily calculate R and L.
Moreover, in some cases the resistance may be neglected as well (R→0). Voilà, here comes the purely inductive network.
Inductive network is always linear, i.e. the grid impedance linearly increases with frequency. That fact significantly simplifies the calculations. We just need to know the impedance at power frequency (50 Hz or 60 Hz) and can scale it for any other frequency.
Frequency scans in figure 2 show the difference between a generic R-L-C network (in this case with two visible resonance peaks) and an inductive network (R-L combination shown with dotted line, inductive only with full line).
Why is this simplification useful? Well, there are multiple reasons. First of all, inductive network is easy to calculate. You do not need any specialized network solver to calculate the harmonic distortion. Second reason might be a case where the grid data is not known in detail.
Of course, purely inductive network is an oversimplification. Every real electric grid has certain capacitive elements. In some cases the capacitances can be neglected while in other situations it is crucial to consider them. The decision shall be made based on adequate engineering judgement, practical experience, purpose of the study and the overall criticality.
Summary
Purely inductive network is a popular simplification of network representation. As the capacitance is neglected the network becomes linear and easy to handle. However, the engineer must know whether such simplified approach can be justified for the given case or not. Alternatively a network harmonic study shall be performed.
Do you need support with network harmonics? Contact us.
https://mb-drive-services.com/services/network-harmonic-studies/
References
[1] Harmonic calculation versus network harmonic study, https://mb-drive-services.com/harmonic-calculation-versus-network-harmonic-study/
[2] NEPLAN – Power System Analysis, https://www.neplan.ch/
