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Iec 60949 Pdf Free Download
62 Theoretical background Relative quantities On the contrary, the unit of a relative quantity is One or 1 p.u. by definition, i.e., Relative quantity = quantity/reference quantity Because the four quantities voltage, current, impedance and power required for system calculations are linked to each other, two reference quantities only are required to specify a relative system of units. Voltage and power are usually chosen for this purpose. This system is called the per-unit system. As reference voltage either the phase-to-phase or the phase-to-earth voltage can be chosen. If the power of 100 MVA is selected as reference quantity, the system is called the p.u.-system on 100 MVAbase. Table 2.2 gives the definitions in the p.u.-system. It should be observed that the phase-to-phase voltage is chosen as reference voltage. In case the phase-to-earth voltage is selected as reference voltage, the current i has to be calculated as per [28] as indicated by ( 1) in Table Semirelative quantities In the semirelative system of units only one quantity is freely chosen as the reference quantity. If the voltage is chosen, the %/MVA system is obtained, which is outstandingly suitable for network calculations because the values of the equipment Table 2.2 Definitions of quantities in physical, relative and semirelative units Ohm-system %/MVA-system p.u.-system Physical units Semirelative units Relative units No reference One reference quantity Two reference quantities quantity Voltage U u = U = U U B U B 100% u = U = U U B U B 1 Current I i = I U B =I U B i = I U B /S B =I U B /S B 1 MVA ( 1) remark see text i = I U B /S B =I 3U B /S B 1 Impedance Z z = Z/U 2 B =Z 100/U 2 B z = Z S B /U 2 B =Z S B/U 2 B 1 %/MVA Power S s = S =S 100% MVA s = S/S B =S/S B 1 applies also to P and Q
224 Chapter 10 Effects of short-circuit currents 10.1 General Calculation methods for the thermal and electromagnetic effects of short-circuit currents are dealt with within IEC , which is applicable to short-circuit currents in d.c. auxiliary installations in power plants and substations and IEC , related to three-phase a.c. systems a.c. systems Thermal effects and thermal short-circuit strength The thermal withstand capability (thermal short-circuit strength) of equipment is determined by the maximal permissible conductor temperature prior to the shortcircuit, the duration of the short-circuit and the short-circuit current itself. The maximal permissible temperature of conductors under normal operating conditions and in the case of short-circuits, e.g., as per DIN VDE 0276, is summarised in Table Figures are given for a short-circuit duration of T k = 5 s. It is assumed that no heat transfer is taking place during the short-circuit duration (adiabatic heating). Skin- and proximity-effects are neglected, the specific caloric heat of the conductor and insulation is constant and the relation resistance-to-temperature is linear. Special consideration is to be taken for conductors in a.c. installations with cross-section above 600 mm 2, as the skin-effect has to be taken into account. Additional requirements according to IEC 60986:1989 and IEC 60949:1988 for cables and isolated conductors are to be met Conductors and equipment The analysis is based on the calculation of the thermal equivalent short-time current I th Tk Q 0 ik 2 (t) dt I th = = (10.1) R T k R T k 2ff7e9595c
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