[BUG] Source impedance vary with ShortCircuitVoltageScaling

[nitbharambe]

Describe the bug

One cause of #1341

The source impedance related attribute sk does not align with the source input: skss_max and skss_min which are corresponding to max and min voltage correction factor. Its does not get scaled by c at the moment.

Script for hand calculation and replication of bug:

import math 


# Note that s_base is irrelevant to calculations and only help in explaination. 
# Results should be the same whatever s_base is chosen.
s_base = 1e6 
u_rated_source_node = 100e3
z_base_source_node = u_rated_source_node * u_rated_source_node / s_base

def calculate(skss, c):
    # calculation within PGM
    z_k_pu = 1.0 / (skss / s_base)
    z_k_ohms = z_k_pu * z_base_source_node
    i_f = c * u_rated_source_node / (z_k_ohms * math.sqrt(3))
    print(f"c={c} PGM i_f result: {i_f}")
    print(f"c={c} PGM z_k_pu result: {z_k_pu}")
    print(f"c={c} PGM z_k_ohms result: {z_k_ohms}")
    print(f"\n")

    # Hand calculation
    i_f = skss / (u_rated_source_node * math.sqrt(3))
    print(f"c={c} Hand calculation result: {i_f}")
    z_k_ohms = c * u_rated_source_node / (i_f * math.sqrt(3))
    z_k_pu = z_k_ohms / z_base_source_node
    print(f"c={c} Hand calculation z_k_pu result: {z_k_pu}")
    print(f"c={c} Hand calculation z_k_ohms result: {z_k_ohms}")
    print(f"\n")

    

calculate(skss=900e6, c=1.0)
calculate(skss=1000e6, c=1.1)

Gives

c=1.0 PGM i_f result: 5196.152422706632
c=1.0 PGM z_k_pu result: 0.0011111111111111111
c=1.0 PGM z_k_ohms result: 11.11111111111111


c=1.0 Hand calculation result: 5196.152422706633
c=1.0 Hand calculation z_k_pu result: 0.001111111111111111
c=1.0 Hand calculation z_k_ohms result: 11.111111111111109


c=1.1 PGM i_f result: 6350.852961085885
c=1.1 PGM z_k_pu result: 0.001
c=1.1 PGM z_k_ohms result: 10.0


c=1.1 Hand calculation result: 5773.5026918962585
c=1.1 Hand calculation z_k_pu result: 0.0011
c=1.1 Hand calculation z_k_ohms result: 11.0

Note that the equations should be:

ikss = c * u_rated / (sqrt3 *  z_k_ohms)
skss = sqrt3  * u_rated *  ikss

By definition in IEC 60909, the voltage scaling applies to calculating ikss using a specific prefault voltage. conversion to skss, ie. grid strength is not on the basis of prefault voltage but on the rated voltage at the conencted node.

Implementation

Hence we should scale y_ref depending on the c factor either at source, main_core / main_model or at short circuit solver.

The results obtained for minimum short circuit scaling stays the same. Only the maximum scaling calculations are affected.