Update doc

Signed-off-by: p-arvy <[email protected]>

docs/optimizer/preprocessing.md

@@ -18,7 +18,7 @@ Note that for lines considered as non-impedant and connecting two different nomi
 
 ## Impedance of transformers
 
-In the calculations of the ACOPF (see [AC optimal powerflow](acOptimalPowerflow.md)), the transformers with an impedance (specified in `ampl_network_branches.txt`) considered as null (see [Zero-impedance threshold](#zero-impedance-threshold))  **are treated as lines**. Then, the transformation ratios/phase shifts are ignored, as well as the impedance specified in the tap changer table `ampl_network_tct.txt`.
+In the calculations of the ACOPF (see [AC optimal powerflow](acOptimalPowerflow.md)), the impedance specified in tap changer table `ampl_network_tct.txt` is ignored for transformers with an impedance (specified in `ampl_network_branches.txt`) considered as null (see [Zero-impedance threshold](#zero-impedance-threshold)). Note that the transformer ratios and phase shiftings are considered anyway.
 
 For phase shifters transformers considered as impedant, the reactance values from the tap changer table (in `ampl_network_tct.txt`) replace the reactance specified in `ampl_network_branches.txt`. The resistance is then calculated proportionally. 
 For the ratio tap changers, the impedance stays as specified in `ampl_network_branches.txt`. **Please notice there is no specified handling for cases where resistance and/or reactance is negative or if there is both a ratio tap changer and a  phase shift transformer on the same branch.**