Use different symbols for different classes

[ceraolo]

LoadImpedancePQ and LoadPQVoltageDependence inside Electrical.Loads share the same symbols, which is inconvenient.
I propose making a change so that they are different from each other. Maybe LoadImpedancePQ can have a "Z" in its symbol, while LoadPQVoltageDependence a greek alpha?

[casella]

I'm fine with the letter Z for impedance loads, maybe we can leave the "regular" PQ load without any additional symbol, I understand PQ loads are the default choice in power system studies.

[ceraolo]

I understand PQ loads are the default choice in power system studies.

Not sure. Consider the load in ENTSOE model

Maybe in the LoadPQVoltageDependence icon we show the numerical value of alpha? this will add signicant info.
BTW, alha=beta=2 should automatically switch internal code into constant impedance (linear behaviour). If this is done, just one model will cover the behaviour of both LoadImpedancePQ and LoadPQVoltageDependence.

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A related problem (in case I can open a separate ticket). in PowerGrids we have PowerFlow.PQbus as well, that is connector-compatible with LoadImpedancePQ and LoadPQVoltageDependence, so a user would infer that all of them can be used in either transient and power flow studies.
What will this cause? I don't know. What I see that PowerLib does not complain, for instance, if in a PowerFlow study I add a LoadPQVoltageDependence, but I have no idea of what this will imply when I run the model.

I fear that a bullet-proof way to approach this would be using different connectors for power flow and transient studies, but this is overkill.
Maybe an error message should be issued when the library understands it has to perform a PowerFlow and sees the presence of models incompatible with these studies? But I don't know whether this is easy of difficult to implement.

I see in #54 that even though embedded power flow is successfully implemented, it will stay optional. So this issue will remain real.

[ceraolo]

I understand PQ loads are the default choice in power system studies.
Not sure. Consider the load in ENTSOE model

this is what I wrote an year ago on #54:

for transients, I've converted PQ nodes into constant impedance ones, as is very often done. This has the additional advantage to avoid severe convergence issues that arise when we simulate short circuits in a network containing constant P and Q nodes.

So, I had the impression that during transients studies constant impedance is more the norm than the exception.
Curious enough, this was also something that laid ground for the solution of #52 :-)

[ceraolo]

I add that Powergrids has many transformer models which, according to this ticket, should have different icons.

[casella]

I wonder if that won't make the diagrams too cluttered. I mean, you don't see the parameter names and much other information in the icon. Maybe seeing a transformer is enough?

[ceraolo]

I share the same doubt with you.

It would be an exception to the general rule that different models should have different icons. Have you seen any such exceptions anywhere?

Moreover, MSL for electric diagrams follow rather strictly the IEC rules for diagrams, which eases documentation.

I'll check whether standard symbols exist for these types. At least for TapChanger and PhaseShifter variants.

[casella]

OK. If there are standards, we should use them. You can also check what tools like Digsilent do.

[ceraolo]

It may also exist some redundancy.
For instance it seems to me that defining both TapChangerTarget and TapChargerInterval is overkill: there is just a tiny difference in the input parameters. Does PowerGrids really need both of them?

[casella]

I think this was a requirement from RTE.

[ceraolo]

I've checked with Italian CEI 3-18 that should be equivalent to IEC 60617
There I found a generic symbol for "transformer with variable coupling" which means tap-charger transformer.
I could not find (there, nor everywhere else) any symbol for a phase-shifting transformer.

Furthermore I don't see any reason to differentiate between TransformerWithTapChangerInterval and TransformerWithTapChangerTarget, as well as TransformerWithPhaseShifterInterval and TransformerWithPhaseShifterTarget.
If these models are not unified in pairs, for them I don't see any need to distinguish icons (for me they are the same model).

So, the icons to be defined should be five:

1. TransformerFixedRatioWithBreaker
2. TransformerWithTapChangerInterval and TransformerWIthTapChargerTarget
3. TransformerWIthTapChangerMax
4. TransformerWithPhaseShifterInterval and TransformerWithPhaseShifterTarget
5. TransformerWithPhaseShifterMax.

We should investigate for a suitable PhaseShift graphical indication.
Provisionally, the full set of transformers can appear as in the following screenshot.