# IEC 60909-1:2002 pdf download

IEC 60909-1:2002 pdf download.Short-circuit currents in three-phasea.c. systems – Part 1: Factors for the calculation of short-circuitcurrents according to lEC 60909-0.

The meaning of the voltage factor c is illustrated for a simple model of a radial network in 2.1.4. Furthermore, results of extended calculations given in 2.2.5 and 2.3.4 demonstrate the possible deviations of calculations with the equivalent voltage source at the short-circuit location against the worst-case values found with a special procedure using the superposition method.

2.1.2 Calculation methods

In principle, there are two methods for the calculation of the initial symmetrical short-circuit current at the short-circuit location (IEC 60909-0, figures 1 and 2):

• the superposition method, derived from Helm holtz’s or Thevenin’s principle;

• the method using the equivalent voltage source at the short-circuit location (see 2.1.3).

Examples for the superposition method are given in 2.2 and 2.3. There the results of the superposition method are compared with the results found with the method using the equivalent voltage source at the short-circuit location.

If a certain load flow in an existing network is known, then it is possible to calculate the initial symmetrical short-circuit current with the superposition method, but this method gives the short-circuit current only related to the load flow presupposed. Therefore, it does not necessarily lead to the maximum short-circuit current. The reason is that for one short-circuit location there are as many short-circuit currents as load-flow conditions without exceeding the boundary conditions of voltages and currents during normal operation, even if the same operational voltage at the short-circuit location is given.

To overcome this problem and to find the worst-case load flow that leads to the maximum short-circuit current at one short-circuit location, a special method was developed by varying the operation conditions [9], [13], [26j1). Further information is given in 2.2.5 and 2.3.4.

2.1.3 Equivalent voltage source at the short-circuit location and voltage factor c

The procedure for the calculation of the initial symmetrical short-circuit current using the equivalent voltage source at the short-circuit location is described in IEC 60909-0. This method, which is normally based only on the rated data of electrical equipment, is an essential simplification compared to the superposition method or a transient calculation, because also in this case it is necessary to know the load-flow conditions before the short circuit.

Using this simplified procedure, the equivalent voltage source cU iJ at the short-circuit location is the only active voltage in the positive-sequence system. All network feeders, synchronous machines and asynchronous motors are short-circuited behind their internal (subtransient) reactances (IEC 60909-0, 3.6.1). All the shunt capacitances and the shunt admittances (loads), except those of the motors, are to be omitted in the positive- and the negative-sequence system (IEC 60909-0, 2.3). Capacitances of the zero-sequence system are to be considered in general. The zero-sequence capacitances are to be omitted in low-voltage systems and in high-voltage effectively grounded systems (i.e. earth fault factor S 1,4). Special considerations are necessary in high-voltage networks with longdistance lines and, of course, in the case of isolated neutral or resonant earthed networks (IEC 60909-0, 1.1). An example for the application of the calculation using the equivalent voltage source at the short-circuit location F is given in IEC 60909-0, figure 4.

The short-circuit voltage of network transformers may in special cases reach values up to 35 %; the rated voltages of these transformers (two-winding or three-winding transformers) may be considerably different from the nominal voltages of the connected networks and in many cases the transformers are equipped with on-load tap changers. Therefore, impedance correction factors are introduced for the calculation of the impedances of network transformers (IEC 60909-0, 3.3.3).

NOTE The note given in 8.3.2.2 of IEC 60909:1988 [281, did not give any help for the user, when carrying out practical calculations with network transformers, if one of the conditions given there is valid. Therefore, it became necessary to develop an impedance correction factor for network transformers (see 2.3).

2.2 Impedance-correction factors when calculating the short-circuit impedances of

generators, unit transformers and power-station units

2.2.1 General

One of the main criteria for the rating of electrical equipment is the maximum short-circuit current and, in many cases, also the maximum partial short-circuit current.