CGMES import: use reference priority extension

[annetill]

Describe the current behavior

The reference priorities in CGMES are dedicated to angle reference bus modeling, not slack bus modeling.

Describe the expected behavior

I think that we have to fix the CGMES importer to fill for a generator for example:

    <iidm:extension id="G">
        <refpri:referencePriorities>
            <refpri:referencePriority priority="1" id="G"/>
        </refpri:referencePriorities>
    </iidm:extension>

Describe the steps

No response

Environment

No response

Relevant Log Output

No response

Extra Information

No response

[zamarrenolm]

Yes, CGMES import/export has to be updated after #2763. The previous issue #2747 could be closed and replaced by this one.

[zamarrenolm]

We have to be aware that if we use the CGMES referencePriority attribute to the set value for the IIDM ReferencePriority extension (I agree with it, I believe it is the correct way of handling it), then we will never be setting the SlackTerminal extension from CGMES.

That is, we will not be setting explicitly the slack (swing) bus, the bus used to initially balance the active and reactive power in the system during power flow calculation. We are already sending the participation factors of active power control, used in distributed slack, but we will not have a way to explicitly state which bus should be used as slack bus.

For other network formats (IEEE CDF, MATPOWER, PowerFactory, PSSE) the slack bus acts both as the angular reference and the node where active and reactive power is balanced.