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Short Circuit Current Calculations Using Symmetrical Components

Ralph Fehr, Ph.D., PE and John Huang, PE

In practice, three-phase faults account for less than 5% of the short-circuit faults on a typical power system. The vast majority of short-circuit faults, about 70%, are line-to-ground faults. Line-to-line faults account for about 15% of short-circuit faults, with double line-to-ground faults making up the remainder. Of these fault types, only the three-phase fault is balanced. The asymmetrical voltages and currents produced by unbalanced faults preclude the use of single-phase analysis as can be done under balanced conditions. Fortunately, a powerful tool exists for analyzing unbalanced systems the method of symmetrical components.

This course explains the theory and origin of symmetrical components, using an intuitive method of derivation which likens the phasor components to force vector components. The mathematics needed to convert between sequence and line quantities is presented in a clear, concise way. A highly-reliable and methodical approach for developing sequence networks is also presented, including a novel approach of producing the zero-sequence network by modifying the negative- sequence network. Circuit models for the four different short-circuit faults that can occur on a three-phase system three-phase, line-to-ground, double line-to-ground, and line-to-line are presented, including means of modeling fault impedance. A brief but complete review of the per-unit system is included, including worked examples. The per-unit material is followed by examples of calculating both balanced and unbalanced short-circuit fault currents in the per-unit system, using the circuit models presented earlier.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

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