Fixing transformer kt min case

[envelio-cb]

Fixing usage of transformer impedance correction factor K_T in min case of short-circuit calculation.
(See IEC 60909-0:2016 6.3.3)

[pawellytaev]

Hi @envelio-cb, thanks for the fix and the PR :)
I checked your code on my local machine (fixed merge conflicts), did some fixes to make the tests run through. Unfortunately I could not merge my changes directly to your branch via this PR, so I pushed it to my fork:
https://github.com/pawellytaev/pandapower/tree/fix/trafo_k_t_values
feel free to merge this branch into yours, then this PR can be reviewed, and merged

[envelio-cb]

@pawellytaev
Thanks for looking into it. Merged your fork.

[codecov]

Codecov Report

Attention: Patch coverage is 83.46369% with 740 lines in your changes missing coverage. Please review.

Project coverage is 76.01%. Comparing base (a277fbe) to head (6fb6a43).
Report is 1416 commits behind head on develop.

Files with missing lines	Patch %	Lines
...power/control/controller/DERController/PQVAreas.py	63.74%	91 Missing ⚠️
pandapower/converter/powerfactory/main_pf.py	0.00%	74 Missing ⚠️
...ntrol/controller/DERController/der_control_plot.py	0.00%	70 Missing ⚠️
pandapower/converter/powerfactory/gui.py	0.00%	45 Missing ⚠️
pandapower/auxiliary.py	80.00%	43 Missing ⚠️
pandapower/contingency/contingency.py	88.03%	31 Missing ⚠️
...power/converter/cim/cim2pp/convert_measurements.py	65.38%	27 Missing ⚠️
...lasses/connectivitynodes/connectivityNodesCim16.py	81.42%	26 Missing ⚠️
pandapower/control/controller/pq_control.py	69.87%	25 Missing ⚠️
pandapower/converter/powerfactory/validate.py	83.78%	24 Missing ⚠️
... and 52 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2168      +/-   ##
===========================================
- Coverage    76.99%   76.01%   -0.99%     
===========================================
  Files          213      282      +69     
  Lines        26450    32957    +6507     
===========================================
+ Hits         20366    25052    +4686     
- Misses        6084     7905    +1821     

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