LOW ZEROSEQUENCE IMPEDANCES ON （低ZEROSEQUENCE阻抗）.pdfVIP

LOW ZERO-SEQUENCE IMPEDANCES ON GENERATORS Date: August 30, 2000 Revision:0 Authors: Michael Simpson and John Merrell INTRODUCTION When considering the AIC rating of switchboards and other components, it is important to remember the source of the fault current and the nature of that source. In a recent study performed by Power Systems Engineering, a generator-fed switchboard was determined to be underrated due to line-to- ground fault magnitudes. The engineer specifying the equipment had used three-phase fault calculations to select the ratings for the equipment. DISCUSSION Good design practice dictates that short circuit calculations be performed to ensure that the AIC rating specified for equipment is higher than the anticipated fault currents. Properly rated equipment is not only a code requirement but can impact the cost of a project. Equipment that is underrated must be replaced, and overrated equipment costs more than properly rated equipment. Short circuit calculations during the design phase require some assumptions and generalizations, which are validated or corrected when a detailed study is completed. The detailed study is usually completed prior to the review of equipment submittals. Underrated equipment identified in the study can be changed before orders are released or factories prepare the equipment for shipment. In the case study mentioned in the introduction, the detailed short circuit study was not completed until after the equipment had been installed. Since the engineer had used the positive sequence impedance of the generator for his calculations (or used generator decrement curve data, which is based on the positive sequence impedance), some of the equipment was underrated. As will be discussed below, the lower zero sequence impedance of a generator can result in line-to-ground fault currents that can be as high as one and a half times the phase fault currents. ANALYSIS To graphically illustr