This function calculates the oil test modifier
for 33/10kV, 66/10kV and 132kV transformers and tapchangers.
See e.g. section 6.11 on page 64 in CNAIM (2017).
Numeric. Refers to the moisture level in the
transformer oil. Moisture levels are measured in ppm.
A setting of "Default" will result in the best possible result.
acidity
Numeric. Refers to the acidity level in the
transformer oil. Acidity levels are measured in (mgKOH/g).
A setting of "Default" will result in the best possible result.
bd_strength
Numeric. Refers to the breakdown strength.
Breakdown strength is measured in kV.
A setting of "Default" will result in the best possible result.