connectableBus throws exception in topology calculated from node-breaker all switches open

[zamarrenolm]

• Do you want to request a feature or report a bug?
  Report a bug

• What is the current behavior?
  Exception is thrown when BusView of a Terminal is queried for calculatedBus in some situations.
  Specifically: the voltage level has topology kind of node-breaker and all switches are opened. In this situation no bus is calculated, and the search for a calculatedBus fails, throwing an exception.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
  A minimal example has been created as a unit test case in the branch issue_nb_connectableBus (module iidm-impl, class NodeBreakerTest, method issueConnectableBus). The following graph shows the export of the voltage level topology:

• What is the expected behavior?
  A bus should be calculated in the voltage level. If no bus is calculated by design, then null could be a valid return value from the method connectableBus.

• Please tell us about your environment:

  • PowSyBl Version: 3.0.0-SNAPSHOT
  • OS Version: *

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)
  It seems the current behavior is related to change in commit 0e25787 where the condition for throwing the exception was changed (fixed).

[miovd]

Yes, this seems like the same issue as #939 ... What do you think @geofjamg @mathbagu, should getConnectableBus() be allowed to return null if there is no bus in the voltage level/the whole voltage level is disconnected?

[geofjamg]

That would lead to a mess in existing code.
It would be simpler to return a fictitiou bus.

[zamarrenolm]

There are some points on the current code that already check for connectableBus returning null: iidm-converter-api BusFilter as an example (when the exported topology is the bus-branch).

[annetill]

Can we in a first approach retruns null and wait for the possible resulting problems ? I prefer to return a null than to introduce the notion of fictitious bus. For me, the definition of a bus is that it is connected, no ?