Public Power - November/December 2007 - (Page 46)

FOR ENGINEERS Analyzing Autotransformer Applications By Mark Feller Many municipal utilities use three-phase autotransformers to change delivery point voltage to transmission voltage for delivery of power to the distribution substations in their community. A typical three-phase autotransformer consists of grounded wye-connected tapped phase windings and a delta-connected winding called the tertiary, which may or may not be loaded. After the basics such as MVA ratings, voltage levels and taps, loss evaluation constants, and accessories have been specified, the engineer needs to dig a little deeper into the electrical characteristics affecting both system and transformer protection. Some of these characteristics make circuit analysis a bit more of a challenge than for a two-winding transformer and there are a few additional considerations when analyzing the autotrans- The autotransformer at Wadsworth’s delivery point substation is 45/60/75MVA 138-k-69kV. Photo by Jay Kline former application. First, ground fault current magnitudes are directly impacted by the impedances associated with the tertiary winding and cannot be accurately calculated without knowledge of those impedances. The impedances normally do not appear on the transformer nameplate. Upon request, the transformer manufacturer should be able to provide positive sequence short circuit impedance values for the high voltage to low voltage, high voltage to tertiary voltage, and low voltage to tertiary windings. Calculations performed with only these positive sequence impedances yield a good estimate for ground faults in many cases. Measured zero sequence series impedances can also be provided by the manufacturer; however, this must be requested when the transformer is ordered. The zero sequence impedances generally range from 75 to 90 percent of the positive sequence values and can be estimated accordingly for the transformers typically found in municipal utilities, if measured zero-sequence values are not available and if a better estimate of actual ground fault current is needed. Reasonable estimates are also possible based on published information such as that found in table 7.4 of Analysis of Faulted Power Systems by Paul M. Anderson, John M. Wiley & Sons, 1995. An excellent reference for the practical aspects of power transformer application, including impedance data, other technical data, and the relative costs of design variables is the Westinghouse/ABB catalog section 48-500 technical data and price list bulletins. These apparently are no longer published. If you find copies in your files, make sure you save them. They contain valuable information that to my knowledge appears nowhere else. A second area of interest is the polarization of traditional zero-sequence directional ground fault relaying applied on the transmission lines connected to these transformers. To polarize the ground relays, a current transformer (CT) is often mounted on the tertiary winding. In special cases a neutral CT can be used, but a detailed analysis is required to ensure that the neutral current does not reverse direction under varying system conditions. The tertiary mounted CT is less prone to current reversal, but if any branch of the converted impedance network has a negative value, this should be compared to system impedance under all system switching or contingency conditions to verify that the net result is positive for proper polarization in all cases. Newer microprocessor-based relays PUBLIC POWER 46 NOVEMBER-DECEMBER 2007

Table of Contents Feed for the Digital Edition of Public Power - November/December 2007

Public Power - November/December 2007 Contents Washington Focus 10 Questions Connecting with the Customer Through Automation AMR or AMI? The High Court’s Antitrust Thunderbolts California’s Push for Demand-Response Plug-In Partners Get Plugged In Wiring Up for Double-Digit Growth For Engineers Safety Community Broadband For Governing Boards DEED Customer Service Hometown Connections Index to Advertisers Parting shot

Public Power - November/December 2007

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