Confusing behavior in connectivity check

[SimonRubenDrauz]

Describe the bug
I found an example where the ext grid is set out of service by the connectivity check during the hydraulic calculation. The problem: It seems that it is still used as slack in the heat transfer calculation. This can be shown by setting the ext_grid out of service beforehand and restarting the pipeflow. It raises a heat transfer calculation did not converge.

To Reproduce
Use the test case test_connectivity_heat4 in test_inservice.py. Set the ext_grid 2 out of service manually. The pipeflow for the heat transfer does not converge anymore.

Error message
File "C:\Users\sdrauz\git\pandapipes\pandapipes\pipeflow.py", line 111, in pipeflow
raise PipeflowNotConverged("The heat transfer calculation did not converge to a "
pandapipes.pipeflow.PipeflowNotConverged: The heat transfer calculation did not converge to a solution.

Expected behavior
In both use cases the results should be the same

Python environment (please complete the following information):

• OS: Windows 10
• pandapipes version: 0.8.5
• Package Version

---

alabaster 0.7.13
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