IEC Edition INTERNATIONAL. STANDARD. NORME. INTERNATIONALE. Short-circuit currents in three-phase a.c. systems –. Calculation as defined by IEC p. Equations for the various currents p. Examples of short-circuit current calculations p. 4 Conclusion. Index Terms—short circuit calculation, fault current, IEC. .. [1] “IEC Short-circuit currents in three-phase a.c. systems -.

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pdf. IEC Short-circuit currents in three-phase a c. systems - Calculation of CE1 IEC (Première édition - ) (First edition - ). First edition. This English-language version is derived from the original bilingual publication by leaving out all French-language pages. IHS Intra/Spex technology and images copyright (c) IHS Documents Similar To IEC pdf. Calculation of Short-circuit IEC

While using fuses or current-limiting circuit-breakers to protect substations, the initial symmetrical short- circuit current is first calculated as if these devices were not available. Your password has been changed. This phenomenon is not dealt with in this standard. I education ci military ci I other In all cases it is possible to determine the short-circuit current at the short-circuit location F with the help of an equivalent voltage source.

This is admissible, because the impedance correction factor KTfor network transformers is introduced. Despite these assumptions being not strictly true for the power systems considered, the result of the calculation does fulfil the objective to give results which are generally of acceptable accuracy. For balanced and unbalanced short circuits as shown in figure 3, it is useful to calculate the short-circuit currents by application of symmetrical components see 2.

When calculating short-circuit currents in systems with different voltage levels, it is necessary to transfer impedance values from one voltage level to another, usually to that voltage level at which the short-circuit current is to be calculated. For per unit or other similar unit systems, no transformation is necessary if these systems are coherent, i. The impedances of the equipment in superimposed or subordinated networks are to be divided or multiplied by the square of the rated transformation ratio t,.

Voltages and currents are to be converted by the rated transformation ratio t,. The equivalent voltage source is the only active voltage of the system. All network feeders, synchronous and asynchronous machines are replaced by their internal impedances see clause 3. In all cases it is possible to determine the short-circuit current at the short-circuit location F with the help of an equivalent voltage source.

Operational data and the load of consumers, tap- changer position of transformers, excitation of generators, and so on, are dispensable; additional calculations about all the different possible load flows at the moment of short circuit are superfluous. Figure 3 - Characterization of short circuits and their currents Figure 4 shows an example of the equivalent voltage source at the short-circuit location F as the only active voltage of the system fed by a transformer without or with on-load tap-changer.

All other active voltages in the system are assumed to be zero. Thus the network feeder in figure 4a is represented by its internal impedance Zot, transferred to the LV-side of the transformer see 3.

Shunt admittances for example, line capacitances and passive loads are not to be considered when calculating short-circuit currents in accordance with figure 4b. O 1 marks the positive-sequence neutral reference. The impedance of the network feeder and the transformer are related to the LV-side and the last one is also corrected with KT see 3.

This postulates that the electrical equipment has a balanced structure, for example in the case of transposed overhead lines. The results of the short-circuit current calculation have an acceptable accuracy also in the case of untransposed overhead lines.

Using this method, the currents in each line conductor are found by superposing the currents of the three symmetrical component systems: The following types of unbalanced short circuits are treated in this standard: For the purpose of this standard, one has to make a distinction between short-circuit impedances at the short-circuit location F and the short-circuit impedances of individual electrical equipment.

The positive-sequence short-circuit impedance z , at the short circuit location F is obtained according to figure 5a, when a symmetrical system of voltages of positive-sequence phase order is applied to the short-circuit location F, and all synchronous and asynchronous machines are replaced by their internal impedances. The values of positive-sequence and negative-sequence impedances can differ from each other only in the case of rotating machines.

The zero-sequence short-circuit impedance at the short-circuit location F is obtained according to figure 5c, if an a. When calculating unbalanced short-circuit currents in medium- or high-voltage systems and applying an equivalent voltage source at the short-circuit location, the zero-sequence capacitances of lines and the zero-sequence shunt admittances are to be considered for isolated.

The capacitances of lines overhead lines and cables of low-voltage networks may be neglected in the positive-, negative- and zero-sequence system.

Neglecting the -zero-sequence capacitances of lines in earthed neutral systems leads to results which are slightly higher than the real values of the short-circuit currents.

The deviation depends on the configuration of the network. Except for special cases, the zero-sequence short-circuit impedances at the short-circuit location differ from the positive-sequence and negative-sequence short-circuit impedances.

In this case, the three-fold zero-sequence current flows through the joint return. In the case of high-voltage feeders with nominal voltages above 35 kV fed by overhead lines, the equivalent impedance 2, may in many cases be considered as a reactance, i. The initial symmetrical short-circuit currents riQ,,, and riQm,, on the high-voltage side of the trans- former shall be given by the supply company or by an adequate calculation according to this standard.

In special cases the zero-sequence equivalent short-circuit impedance of network feeders may need to be considered, depending on the winding configuration and the starpoint earthing of the transformer. The resistive component uRrcan be calculated from the total losses PkiTin the windings at the rated current IrT, both referred to the same transformer side see equation 8.

For large transformers the resistance is so small that the impedance may be assumed to consist only of reactance when calculating short-circuit currents. The resistance is to be considered if the peak short-circuit current ip or the d. Zero-sequence impedance arrangements for the calculation of unbalanced short-circuit currents are given in IEC Additional information may be found in IEC For two-winding transformers with and without on-load tap-changer, an impedance correction factor KT is to be introduced in addition to the impedance evaluated according to equations 7 to 9: This correction factor shall not be introduced for unit transformers of power station units see 3.

If the long-term operating conditions of network transformers before the short circuit are known for sure, then the following equation 12b may be used instead of equation 12a. The impedance correction factor shall be applied also to the negative-sequence and the zero- sequence impedance of the transformer when calculating unbalanced short-circuit currents.

Impedances 2, between the starpoint of transformers and earth are to be introduced as 3 2, into the zero-sequence system without a correction factor. For three-winding transformers with and without on-load tap-changer, three impedance correction factors can be found using the relative values of the reactances of the transformer see 3. With these impedances the corrected equivalent impedances LK, z B K and ZCKshall be calculated using the procedure given in equation i i.

The three impedance correction factors given in equation 13 shall be introduced also to the negative-sequence and to the zero-sequence systems of the three-winding transformer. Impedances between a starpoint and earth shall be introduced without correction factor. NOTE Equivalent circuits of the positive-sequence and the zero-sequence system are given in IEC , table I , item 4 to 7 for different cases of starpoint earthing.

An example for the introduction of the correction factors of equation 13 to the positive-sequence and the zero-sequence system impedances of the equivalent circuits is given in 2.

The impedances and z o L of low-voltage and high-voltage cables depend on national techniques and standards and may be taken from IEC or from textbooks or manufacturer's data. For higher temperatures than 20 OC, see equation 3. The effective resistance per unit length RL of overhead lines at the conductor temperature 20 "C may be calculated from the nominal cross-section qn and the resistivity p: NOTE The following values for resistivity may be used: Short-circuit current-limiting reactors shall be treated as a part of the short-circuit impedance.

U, is the nominal voltage of the system; ur, is the rated voltage of the generator; ZGK is the corrected subtransient impedance of the generator; 2, is the subtransient impedance of the generator in the positive-sequence system: The following values for the fictitious resistances RGfmay be used for the calculation of the peak short- circuit current with sufficient accuracy.

The influence of various winding-temperatures on RGf is not considered. These values cannot be used when calculating the aperiodic component id. The effective resistance of the stator of synchronous machines lies generally much below the given values for RGf.

In this case the manufacturer's values for RG should be used. For the short-circuit impedances of synchronous generators in the negative-sequence system, the following applies with KG from equation 1 8: For the short-circuit impedance of synchronous generators in the zero-sequence system, the following applies with KG from equation 1 8: When an impedance is present between the starpoint of the generator and earth, the correction factor KG shall not be applied to this impedance.

The need for the calculation of minimum short-circuit currents may arise because of underexcited operation of generators low-load condition in cable systems or in systems including long overhead lines, hydro pumping stations.

In this case special considerations beyond the scope and procedure given in this standard have to be taken into account see for instance 2.

If synchronous motors have a voltage regulation, they are treated like synchronous generators.

If not, they are subject to additional considerations. T JSrT ; t, is the rated transformation ratio of the unit transformer: It is assumed that the operating voltage at the terminals of the generator is equal to UrG.

If only overexcited operation is expected, then for the calculation of unbalanced short-circuit currents the correction factor Ks from equation 22 shall be used for both the positive-sequence and the negative-sequence system impedances of the power station unit. The correction factor Ks shall also be applied to the zero-sequence system impedance of the power station unit, excepting, if present, an impedance component between the star point of the transformer and earth.

If underexcited operation of the power station unit is expected at some time for instance to a large extent especially in pumped storage plants , then only when calculating unbalanced short- circuit currents with earth connection see figures 3c and 3d the application of Ks according to equation 22 may lead to results at the non-conservative side.

Special considerations are necessary in this case, for instance with the superposition method. If the highest partial short-circuit current of the power station unit at the high-voltage side of the unit transformer with off-load taps is searched for, choose l-pT.

In the case of unbalanced short circuits, the impedance correction factor KSOfrom equation 24 shall be applied to both the positive-sequence and the negative-sequence system impedances of the power station unit. The correction factor Kso shall also be applied to the zero-sequence system impedance of the power station unit excepting, if present, an impedance component between the star point of the transformer and earth.

When calculating the partial short-circuit current I: Medium-voltage motors have to be considered in the calculation of maximum short-circuit current see 2. Low-voltage motors are to be taken into account in auxiliaries of power stations and in industrial and similar installations, for example in networks of chemical and steel industries and pump- stations. In the calculation of short-circuit currents, those medium-voltage and low-voltage motors may be neglected, provided that, according to the circuit diagram interlocking or to the process reversible drives , they are not switched in at the same time.

For the calculation of the initial short-circuit currents according to 4. The zero-sequence system impedance Z,,,, of the motor shall be given by the manufacturer, if needed see 4.

For simplification of the calculation, groups of motors including their connection cables may be combined to a single equivalent motor see motor M4 in figure 9. For these equivalent asynchronous motors, including their connection cables, the following may be used: For a short circuit at the busbar B in figure 9, the partial short-circuit current of the low-voltage motor group M4 may be neglected, if the condition I,,, I 0,Ol ICT3 holds. I,,, is the rated current of the equivalent motor M4.

The estimation according to equation 28 is not allowed in the case of three-winding transformers. Then they contribute only to the initial symmetrical short-circuit current I: They do not contribute to the symmetrical short-circuit breaking current I, and the steady-state short-circuit current I k. As a result, reversible static converter-fed drives are treated for the calculation of short-circuit currents in a similar way as asynchronous motors. The following applies: All other static converters are disregarded for the short-circuit current calculation according to this standard.

Regardless of the time of short-circuit occurrence, the discharge current of the shunt capacitors may be neglected for the calculation of the peak short-circuit current.

The effect of series capacitors can be neglected in the calculation of short-circuit currents, if they are equipped with voltage-limiting devices in parallel, acting if a short circuit occurs. In the case of high-voltage direct-current transmission systems, the capacitor banks and filters need special considerations, when calculating a.

Figure 1 gives schematically the general course of the short-circuit current in the case of a far-fiom- generator short circuit.

The symmetrical a. Single-fed short circuits supplied by a transformer according to figure 4, may a priori be regarded as far- from-generator short circuits if X, 2 Ur,, with XQ, calculated in accordance with 3. In the case of a near-to-generator short circuit, the short-circuit current can be considered as the sum of the following two components: In this case, the symmetrical short- circuit breaking current Ib is smaller than the initial symmetrical short-circuit current I,".

Normally, the steady-state short-circuit current Ik is smaller than the symmetrical short-circuit breaking current 1,. In a near-to-generator short circuit, the short-circuit current behaves generally as shown in figure 2. In some special cases, it could happen that the decaying short-circuit current reaches zero for the first time, some cycles after the short circuit took place.

This is possible if the d. This phenomenon is not dealt with in this standard. The decaying aperiodic component id. The type of short circuit which leads to the highest short-circuit current depends on the values of the positive-sequence, negative-sequence, and zero-sequence short-circuit impedances of the system. This figure is useful for information but should not be used instead of I calculation. This procedure is not allowed when calculating the peak short-circuit current i,.

In this case, it is necessary to distinguish between networks with and without parallel branches see 4. While using fuses or current-limiting circuit-breakers to protect substations, the initial symmetrical short- circuit current is first calculated as if these devices were not available.

From the calculated initial symmetrical short-circuit current and characteristic curves of the fuses or current-limiting circuit- breakers, the cut-off current is determined, which is the peak short-circuit current of the downstream substation. Short circuits may have one or more sources, as shown in figures 11, 12, and Calculations are simplest for balanced short circuits on radial systems, as the individual contributions to a balanced short circuit can be evaluated separately for each source figures 12 or RL is the line resistance for a conductor temperature of 20 OC, when calculating the maximum short-circuit currents.

Xk may be neglected. In the case of figure 4, for instance, to the LV side. For the examples in figures 1Ib and 1 IC, the initial symmetrical short-circuit current is calculated with the corrected impedances of the generator and the power station unit see 3. The short-circuit impedances for the examples in figures 1l b and 1I C are given by the following equations: Example figure 11b: Example figure 1IC: The generator impedance shall be transferred to the high-voltage side using the rated transformation ratio t,.

Each branch short- circuit current can be calculated as an independent single-source three-phase short-circuit current in accordance with equation 29 and the information given in 4.

The initial short-circuit current at the short-circuit location F is the phasor sum of the individual partial short-circuit currents see figure Within the accuracy of this standard, it is often sufficient to determine the short-circuit current at the short-circuit location F as being the sum of the absolute values of the individual partial short- circuit currents.

In general, the calculation according to 4. For the calculation of the partial short-circuit current I L 2 feeding into the short-circuit location F2, for example at the connection to the high-voltage side of the auxiliary transformer AT in figure 13, it is sufficient to take: The total short-circuit current in F1 or F2 figure 13 is found by adding the partial short-circuit current IL,,,, caused by the medium- and low-voltage auxiliary motors of the power station unit. The impedance ZrS1in equation 38 or 43 is used to determine the partial short-circuit current ILT in figure 13 for the short circuit in F3.

The impedance of the auxiliary transformer AT in figure 13 is to be corrected with KT from 3. The total short-circuit in F1 or F2 figure 13 is found by adding the partial short-circuit current ILrnv, caused by the medium- and low-voltage auxiliary motors of the power station unit. The impedances in systems connected through transformers to the system, in which the short circuit occurs, have to be transferred by the square of the rated transformation ratio.

If there are several transformers with slightly differing rated transformation ratios trT1trT During the initial stage of the short circuit, the negative impedance is approximately equal to the positive-sequence impedance, independent of whether the short circuit is a near-to-generator or a far-from-generator short circuit.

The equations 46 and 47 are given for the calculation of and in figure 3c: The initial line-to-earth short-circuit current Iil in figure 3d shall be calculated by: For a synchronous generator use Rm see 3. The peak short-circuit current i, at a short-circuit location F, fed from sources which are not meshed with one another, in accordance with figure 12, is the sum of the partial short-circuit currents: Example figure It is only necessary to choose the branches which carry partial short-circuit currents at the nominal voltage corresponding to the short-circuit location and branches with transformers adjacent to the short-circuit location.

Any branch may be a series combination of several impedances. As long as RIX remains smaller than 0,3 in all branches, it is not necessary to use the factor 1, It is not necessary for the product 1, Method c is recommended in meshed networks see IEC When using this method in meshed networks with transformers, generators and power station units, the impedance correction factors KT,KG and Ks, respectively Kso, shall be introduced with the same values as for the 50 Hz or 60 Hz calculations.

For simplification, it is permitted to use the same value of Kas for the three-phase short circuit. The factor K shall, be calculated according to 4. For simplification, it is permitted to use the same value for IC as for the three- phase short circuit. The factor K shali be calculated according to 4. For simplification, it is permitted to use the same value for as for the three- phase short circuit. Depending on the product f. NOTE For some near-to-generator short circuits the value of id.

The values of p in equation 70 apply if synchronous machines are excited by rotating exciters or by static converter exciters provided, for static exciters, the minimum time delay tminis less than 0,25 s and the maximum excitation voltage is less than 1,6 times rated load excitation-voltage.

The factor p may also be obtained from figure He has more than 18 years of professional experience in planning and design of electrical power systems. Free Access. Summary PDF Request permissions.

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