Fix bus/breaker topology in case of no connected terminal present

[jeandemanged]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

As reported on Slack by a PyPowSyBl user (many thanks):

Hello everyone, I might have found a bug when using network.get_bus_breaker_topology.
In my network there are 3 buses in one voltage level. The third is only connected via breakers to the other two. My expectation would be, that all 3 buses in network.get_bus_breaker_topology.buses have the same bus_id. But the ones only connected by breakers without additional elements are not linked to any bus whatsoever. Is that intended behaviour or a bug?
Here is a minimal example:

import pypowsybl as pp
import pandas as pd
network = pp.network.create_empty()
network.create_substations(id=['S1', 'S2'])
voltage_levels = pd.DataFrame.from_records(index='id', data=[
    {'substation_id': 'S1', 'id': 'VL1', 'topology_kind': 'BUS_BREAKER', 'nominal_v': 400},
    {'substation_id': 'S2', 'id': 'VL2', 'topology_kind': 'BUS_BREAKER', 'nominal_v': 400},
])
network.create_voltage_levels(voltage_levels)


network.create_buses(id=['B1_1', 'B1_2', 'B1_3', 'B2_1', 'B2_2'], voltage_level_id=['VL1', 'VL1', 'VL1', 'VL2', 'VL2'])
network.create_lines(id='LINE 1', voltage_level1_id='VL1', bus1_id='B1_2',
                     voltage_level2_id='VL2', bus2_id='B2_1',
                     b1=0, b2=0, g1=0, g2=0, r=0.5, x=10)
network.create_switches(pd.DataFrame.from_records(index='id', data=[
    {'voltage_level_id': 'VL1', 'id': 'DISC-BBS1-LOAD', 'kind': 'BREAKER', 'bus1_id': 'B1_1', 'bus2_id': 'B1_2', 'open':False},
    {'voltage_level_id': 'VL1', 'id': 'DISC-BBS2-LOAD', 'kind': 'BREAKER', 'bus1_id': 'B1_1', 'bus2_id': 'B1_3', 'open':False}
]))
network.get_bus_breaker_topology('VL1').buses

# Result
#  	name	bus_id
# id		
# B1_1		
# B1_2		VL1_0
# B1_3

Root cause of the issue is that the existing code assumes that every bus in bus/breaker view has at least one connected non-switch equipment.

What is the new behavior (if this is a feature change)?

We don't assume anymore that every bus in bus/breaker view has at least one connected non-switch equipment.

# Result
#  	name	bus_id
# id		
# B1_1		VL1_0
# B1_2		VL1_0
# B1_3		VL1_0

Does this PR introduce a breaking change or deprecate an API?

• No

[flo-dup]

Thanks for having sorted this out!

[sonarqubecloud]

Quality Gate passed for 'pypowsybl'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarqubecloud]

Quality Gate passed for 'pypowsybl'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[jonenst]

A good use case for
powsybl/powsybl-core#3107

you can't get from busbreakerbuses to their busviewbuses