Transformer Impedance Considerations

During Program 10, DSTAR commisioned a series of whitepapers on technological topics of common interest. The goal of this project was to deliver an overview treatment of three key technology areas that are relevant to modern distribution systems. The issues under consideration were:

  • The use of broaband powerline carrier technology for power equipment communication
  • The availability and application of sub-cycle electronic switching equipment
  • Impedance considerations for three-phase pad-mount transformers design

The impedance of three-phase pad-mounted transformers is governed primarily by the ANSI Standard C57.;12.26 that specifies an impedance of 5.75% for transformers of size 750-2500 kVA. Application of any standard is wholly voluntary, unless adopted by a governmental body, but some manufacturers are reluctant to offer lower impedance designs and charge a premium for these transformers. Smaller utilities, such as cooperatives, purchase a relatively small number of large transformers and, as such, do not have the buying power of large IOUs. Therefore, many smaller utilities do not have sufficient market leverage to convince manufacturers to make “nonstandard” impedance transformers. Although there are some practical design limits to manufacturing lower impedance transformers, there appears to be significant margin between this limit and the recommendation of 5.75%. Figure 1 shows the relationship between transformer kVA size and transformer impedance based on sample data from a large IOU located in Southeastern US. It is clear from the impedance discontinuity between 500 kVA and 750 kVA in this figure that the 5.75% specified for transformer size 750 kVA and above is artificially high.


The majority of this investigation was outsourced to an external vendor, HVolt, Inc. Based on analysis conducted by GE Energy Consulting and HVolt, it was concluded that the impedance of 5.75% was adopted in the 1930’s and 1940’s as a balance between limiting fault current and maintaining reasonable voltage regulation. The choice of this impedance level was also driven by manufacturability and the integration of the transformer with other substation components such as switchgear. With an additional 60 to 70 years of product development experience, it is reasonable to expect that this impedance specification is no longer the necessity it once was, but a guideline that should be considered in the context of each specific transformer application.

The outline of the 37-page final report is shown below.


The final report for this project can be downloaded from P10-11