A Survey/Compilation of (Power) Transformer Percent Impedances (%Z)

Transformer impedance is one of the important information that is needed for the transmission line modeling for power flow, short circuit and other various power system simulations, and protection coordination. However, unlike the transmission line that has a, more or less, standard design, thus easy to compute and obtain the equivalent model, modeling of the transformer is almost impossible without the percent impedance (%Z) information. Unfortunately, % impedance value is hard to generalize because it varies for every transformer model and ratings. Due to various parameters affecting the value of %Z, it is also hard to come-up even with a typical value. Even for transformers with same capacity and voltage ratings, %Z could still vary to some range.

This survey is an initial approach in making a study for good % impedance estimation for grid modeling (if information from the actual transformer is not available) based on the analysis of the surveyed actual % impedance that we personally gathered.

Below is the table of transformers that we, so far, gathered showing its essential ratings including the percent impedance or %Z;

MVA rating	Cooling System	LV (In kV at nominal tap)	HV (In kV at nominal tap)	(Hz)	% Impedance	Cnxn	Loca- tion	Standard	Made by	Date Made
32/44.8/64	OA/FOA/FOA	13.2	230	60	13.63 at 64 MVA	Y-D	Ph	ANSI- C57-12	Mitsubishi	1976
70/100/125	OA/FA/FA	13.8	230	60	9.52 at 100 MVA	Y-D	Ph	ANSI- C57	ItalTraFo	1974
90/123/150	ONAN/ ONAF1/ ONAF2	13.8	230	60	14.68 at 150 MVA	Y-D	Ph	ANSI- C57	Siemens	1998
70/100	OA/FA	13.8	230	60	17.57 at 100 MVA	Y-D	Ph	ANSI- C57- 12-1973	Pauwels Trasfo	1993
47.5	ONAN	13.8	138	60	11.73 at 47.5 MVA	Y-D	Ph	ANSI- C57- 12-1973	Fuji Electric	1973
12	ONAN	4.16	13.8	60	9.61 at 12 MVA	-	Ph	ANSI- C57- 12-1973	Fuji Electric	1973
1.8	ONAN	0.48	13.8	60	10.61 at 1.8 MVA	-	Ph	ANSI- C57- 12-1973	Fuji Electric	1973
13.333/ 15.333	OA/FA	13.8	69	60	4.97/5.72 at 13.33/15.33 MVA	Y-D	Ph	ANSI- C57- 12-1973	Meiden	1979