Do you need a multi-user copy? Our prices are in Swiss francs CHF. We accept all major credit cards American Express, Mastercard and Visa , PayPal and bank transfers as form of payment. Preview Abstract IEC is applicable to the calculation of short-circuit currents in low-voltage three-phase AC systems, and in high-voltage three-phase AC systems, operating at a nominal frequency of 50 Hz or 60 Hz.
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For example, edition numbers 1. Information relating to this publication, including its validity, is available in the IEC Catalogue of publications see below in addition to new editions, amendments and corrigenda. On-line information is also available on recently issued publications, withdrawn and replaced publications, as well as corrigenda.
Please contact the Customer Service Centre see below for further information. Short circuit location F 1 in figure Short-circuit location F 1 in figure 9.
The object of the IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work.
International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization for Standardization ISO in accordance with conditions determined by agreement between the two organizations. Any divergence between the IEC Standard and the corresponding national or regional standard shall be clearly indicated in the latter.
The IEC shall not be held responsible for identifying any or all such patent rights. However, a technical committee may propose the publication of a technical report when it has collected data of a different kind from that which is normally published as an International Standard, for example "state of the art". Technical reports do not necessarily have to be reviewed until the data they provide are considered to be no longer valid or useful by the maintenance team.
This technical report shall be read in conjunction with IEC The committee has decided that the contents of this publication will remain unchanged until This technical report aims at showing the origin and the application, as far as necessary, of the factors used to meet the demands of technical precision and simplicity when calculating short-circuit currents according to IEC Thus this technical report is an addition to IEC It does not, however, change the basis for the standardized calculation procedure given in IEC NOTE References are given in some cases to offer additional help, not to change the procedure laid down in the standard.
The variations during operation in a three-phase a. Therefore, it is difficult to find the special load flow that leads either to a maximum or to a minimum short- circuit current at the different locations of the network. In a given system, there are as many different short-circuit current magnitudes as there are possible different load-flow conditions for every location.
Normally, extreme load-flow cases are not empirically known. This method, described in IEC , is an approxi- mation method without special conditions of operation. The aim of this standard is to find the maximum short-circuit currents with sufficient accuracy, mainly taking into account safety aspects and as far as possible economical aspects. During the planning stage of a network, the different future load-flow conditions are unknown.
These factors c are given in table 1 of IEC The introduction of a voltage factor c is necessary for various reasons IEC , 1. The meaning of the voltage factor c is illustrated for a simple model of a radial network in 2. Furthermore, results of extended calculations given in 2.
Examples for the superposition method are given in 2. 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 necessa- rily 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 , ,  1.
Further information is given in 2. 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. All network feeders, synchronous machines and asynchronous motors are short-circuited behind their internal subtransient reactances IEC , 3.
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 , 2. 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. Special considerations are necessary in high-voltage networks with long- distance lines and, of course, in the case of isolated neutral or resonant earthed networks IEC , 1.
An example for the application of the calculation using the equivalent voltage source at the short-circuit location F is given in IEC , figure 4. Corresponding to c max or c min special conditions are introduced for the calculation of the maximum and minimum short-circuit currents see IEC , 2.
The introduction of the voltage factor c aims at finding a short-circuit current, for instance, a maximum short-circuit current, as near as possible to the real value. Using the impedance correction factors IEC , 3.
The positive-sequence system of the model in figure 1a is given in figure 1b. One example is the internal voltage E " behind the subtransient reactance X d" of a synchronous machine figures 4 and 6. It can be shown  that this is also the most unfavourable case, even in comparison with results of meshed networks and long-distance transmissions up to about km.
If in a network this assumption for a normal system is not fulfilled, and the highest voltage during operation does differ more than above, then a higher factor c max may be necessary, see equation Therefore, impedance correction factors are introduced for the calculation of the impedances of this electrical equipment IEC , 3.
Therefore, impedance correction factors are introduced for the calculation of the impedances of network trans- formers IEC , 3. NOTE The note given in 8. Therefore, it became necessary to develop an impedance correction factor for network transformers see 2. It is necessary to introduce impedance-correction factors KG see 2.
The correction factors for generators and power-station units are given in IEC , 3. These calculations are normally carried out only once during the construction of the power-station unit. The impedance-correction factors given in IEC , 3.
The factor c max shall be taken from IEC , table 1 in accordance with the voltage U rG when using IEC , equation 18 and in accordance with the voltage U nQ at the high-voltage side of the unit transformer when using IEC , equations 22 and Additional investigations in meshed networks have shown that these correction factors are also adequate when calculating short-circuit currents at different locations of the network see 2.
Special considerations are necessary for the correction factors when calculating minimum short-circuit currents, because the special boundary conditions for the different power-station units must be known. These conditions, for instance, are given by the maximum extent of underexcited operation, the minimum active power of thermal power stations during long-term operation or the maximum reactive power overexcited or underexcited of units in hydro- pumping stations as well as by special devices for the limitation of the torque angle.
Further- more, attention shall be given to the fact that even during low load conditions in a network, the number of power-station units operating with partial load within the underexcited region is usually minimized.
Therefore, a rough estimation for minimum short-circuit currents may be found using the instructions given in IEC , 2. Figure 4a gives the equivalent circuit diagram positive-sequence system of a generator. The terminal voltage U G of the generator is controlled and therefore constant during operation before the short circuit. Using the superposition method similar to figure 4 , both the currents I S figure 6a and I "kSU b are derived as follows.
The investigations for a large number of power-station units with on-load tap-changer have shown that, similar to the case of generators directly connected to the network, the short- " circuit current I kS S reaches its maximum in many cases if the power-station unit is operated at its rated point before the short circuit , , , .
Details are given in . Only in the remaining seven cases the maximum short-circuit current is to be expected after underexcited operation. Figure 7 gives one example from these investigations with maximum short-circuit current from the generator after operation at rated conditions before the short circuit.
The maximum short- " circuit current I kS S is reached for the lowest value U Q at the high-voltage side of the unit transformer with on-load tap-changer. It was assumed that it is sufficient to take U nQ as the lowest value during normal operation. It is assumed that the operating voltage at the terminals of the generator is equal to U rG.
The simplified impedance correction factor in equation 31 leads to a sufficient approximation " for the maximum partial short-circuit current I kS, IEC curve 2 in figure 8.
Safety and economical aspects are sufficiently met, if KS is used. In IEC , 5. This situation may change in special cases, for instance, where gas-turbine units are additionally connected to the auxiliary busbar at other than during emergency situations.
The impedances of the busbar connections between generator, unit transformer and auxiliary transformer are neglected. It is presupposed that U G is equal to U rG. The calculation according to IEC , 4. The results of figure 11 are valid whether operation before the short circuit is in the overexcited or underexcited region.
In special cases, it may be possible that the generator of a power-station unit only works in the overexcited region during its lifetime. Then the results of figure 12 are found using the same boundary conditions as in figure The maximum value of the partial short-circuit current is found if the operating voltage U Q before the short circuit reaches its maximum.
Fundamental investigations are given in . Investigations are given in  and . This is given for historic reasons.
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