Consulting-Specifying Engineer - October 2008 - (Page 71)

Codes & Standards amps at the fault. It is calculated by: ISC SYMMETRICAL = Secondary Voltage 3 x Zt Next, determine the motor load of the system. Motor load contribution usually is approximated by using a percentage of the transformer full load current. In general, 50% motor load is assumed. The asymmetrical motor contribution is calculated by: Asymmetrical Motor Contribution = 5 x Full Motor Load Current We also have to calculate the symmetrical motor contribution of the system. The asymmetry factor depends on the motor design of the system; the factor typically is approximated as 1.25. Therefore, the symmetrical motor contribution is determined by: Asymmetrical Motor Contribution 1.25 Utility per unit Reactance (Xu pu) = Base kVA Utility Short Circuit kVA Transformer per unit Reactance (Xt pu) = (%X) x Base kVA 100 x Transformer kVA (%R) x Base kVA Transformer per unit Resistance (Rt pu) = 100 x Transformer kVA Component per unit Reactance (Xc pu) = X(Ω) x Base kVA 1,000 x kV2 Component per unit Resistance (Rc pu) = R(Ω) x Base kVA 1,000 x kV2 Symmetrical Motor Contribution = per unit Total Impedance (Zt pu) = The total symmetrical short circuit is calculated by: Total I SC SYMMETRICAL = I SC SYMMETRICAL + Symmetrical Motor Contribution The asymmetrical factor is then determined by tables based upon the X/R ratio calculated below. This enables calculation of the asymmetrical short circuit current, using the following equation: Total Reactance (Xt) X Ratio = Total Resistance (Rt) R I SC ASYMMETRICAL = I SC SYMMETRICAL X Asymmetrical Factor 2 2 Rt pu + Xt pu I SC SYMMETRICAL = Base kVA 3 x kV x Zt pu Asymmetrical Motor Contribution = 5 x Full Load Motor Current Symmetrical Motor Contribution = Asymmetrical Motor Contribution 1.25 Total I SC SYMMETRICAL = I SC SYMMETRICAL + Symmetrical Motor Contribution Total I SC ASYMMETRICAL = I SC ASYMMETRICAL + Asymmetrical Motor Contribution Total Reactance (Xt ) X Ratio = Total Resistance (R pu ) R t pu Per-unit calculations Since we’ve discussed the base method in detail, we won’t discuss the per-unit method in depth. The per-unit method is used for complex calculations and also is from the Bussmann Mfg. book. The same assumptions are made in the per-unit method as in the base method, and both methods have similar calculation steps. The per-unit method equations are: I SC ASYMMETRICAL = I SC SYMMETRICAL X Asymmetrical Factor Total I SC ASYMMETRICAL = I SC ASYMMETRICAL + Asymmetrical Motor Contribution Consulting-Specifying Engineer • OCTOBER 2008 71

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Consulting-Specifying Engineer - October 2008 Contents Viewpoint Letters News M/E Roundtable Cities Under One Roof Eight Design Tips for Integration Is 'Green' Really New? Down the Drain Product of the Year Awards Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - October 2008

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