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Harmonic Impedance Loci – Help or Hindrance?

I have a bit of an open question for those of you that do complex harmonic studies. Do you think that harmonic impedance loci provided by DNOs and TSOs are actually useful and beneficial, or are they more trouble than they are worth? Would a series of frequency dependent profiles for specific outage cases a better approach?

On a large system when you are assessing up the 100th harmonic, it is pretty common to get a harmonic violation or two, but sometimes we can get more and these can often be significant. When this occurs, this can be a problem if the violation is reasonably significant and over the planning, or compatibility limit. 

If the background frequency dependent profile has been provided in the form of an impedance polygon or loci, then finding out what is causing the problem is much harder, as the polygon represents all the various operating envelopes. The violation could be due to a specific outage case that’s unlikely, or multiple violations could be the result of different outage cases; but these get obscured by the generic nature of the polygon.

You can of course, identify where on the polygon the problem is occurring, but this only gives so much information and doesn’t indicate what specific case or cases are causing the violation without a lot of further work. When a significant violation occurs, there is then the challenge of to how to proceed, as it either needs further rounds of clarifications with the DNO / TSO to improve the clarity on the polygon or to proceed down designing a filter.

If instead the DNO / TSO have provided a set of specific frequency dependent profiles, you can see if there is a specific outage case that is causing the problem and then design around it, or accept a constraint should that specific outage occur.

The other problem with impedance polygons, relates to how the DNO / TSO creates the polygons in the first place; as this is not an exact science. The polygon creation typically needs a very experienced engineer mapping the RX values into a complex series of spreadsheets and then trying to split them into harmonic envelopes by hand, or to use some sort sophisticated script to perform some form of curve fitting around the various frequency plots. In both cases, this potentially leads to overly conservative polygons that may have been fitted around some outliers, creating overly conservative cases. 

The advantage of the harmonic impedance polygon is that it covers a lot of intermediate cases and potential future scenarios, bounded inside the envelope. But if you look at this practically, i am not sure how much benefit it really is. If there were a lot of system changes, the DNO / TSO would (should) issue updated impedance polygons. 

Overall I see a number of disadvantages to using the impedance polygon / loci approach, and not many advantages. The use of frequency dependent profiles against specific outage cases is more convenient for everyone. It is much quicker and easier to produce for a DNO / TSO than a set of harmonic polygons, and easier for the consultant to process – so it seems like a win-win, or have I missed something?