For simplification, it is permitted to use the same value of Kas for the three-phase short circuit. 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. In this case, the three-fold zero-sequence current flows through the joint return. The following methods are supported for meshed networks as per section 4. The following types of unbalanced short circuits are treated in this standard: The impedance of the auxiliary transformer AT in figure 13 is to be corrected with KT from 3. Ikpis the steady-state short-circuit current of a generator at a three-phase terminal short-circuit.

iec 60909-4

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IEC-60909 Short-Circuit in EasyPower

If the Joule integral or the thermal equivalent short-circuit current shall be calculated for unbalanced short idc, replace I: Asymmetrical currents are calculated as the root mean square of the symmetrical and dc components.

These remote currents and voltages are useful for relay setting. 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: Systems at highest voltages of kV and above with long transmission lines need special consideration.

I tables, charts, graphs, figures Symmetrical Short-Circuit Breaking Currents I b For rotating machines the current contributions to short-circuit decays over time. All other active voltages in the system are assumed to be zero.

iec 60909-4

In special cases the zero-sequence equivalent short-circuit impedance of network feeders may need to be considered, idc on the winding configuration and the starpoint earthing of the transformer. 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.


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 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.

I In the case of minimum steady-state short circuits introduce c see 2.

iec 60909-4

Z S is the corrected impedance of a power station unit with LTC to the high-voltage side. Short-circuit current of a far-from-generator short circuit with constant a. The method described above is for 3-phase short circuit.

iec 60909-4

Short-circuit currents and short-circuit impedances may also be determined by system tests, by measurement on a network analyzer, or with a digital computer. Impedances between a starpoint and earth shall be introduced without correction factor.

Short circuit tests accordig to the standard (IEC ) (PSCAD Short Circuit Standard)

Currents calculated with equation 74 are larger than the real symmetrical short-circuit breaking. The zero-sequence system impedance Z,, of the motor shall be given by the manufacturer, if needed see 4. Factor p for calculation of short-circuit breaking current 1, In a system of low voltage breakers with ZSI, when the current through any circuit breaker exceeds the short time pickup its restraining signal is sent to upstream circuit breakers. 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.


IEC Short-Circuit in EasyPower

These values cannot be used when calculating the aperiodic component id. IEC- lire: Figure 16 can be used also for compound excited low-voltage generators with a minimum time delay tminnot greater than 0,1 s.

The factor K shali be calculated according to 4.

For unbalanced faults, equations 868788 and 89 are used. For grounded systems the influence of motors on the line-to-earth short-circuit current cannot be neglected. In a eic short circuit, the short-circuit current behaves generally as shown in figure 2.

IEC Short-circuit currents in three-phase a c. EasyPower supports the following four types of short-circuit conditions as per IEC Short-circuit currents and partial short-circuit currents for three-phase short circuits between generator and unit transformer with or without on-load tap. The calculation of the three-phase peak short-circuit current i, applies to the line conductor and to the instant at which the greatest possible short-circuit current exists.

With respect to equation 99 in table 3 the steady-state short-circuit current of asynchronous motors is zero in the case of a three-phase short circuit at the terminals figure 12 and equation 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 EasyPower library is populated with data for circuit breakers, fuses and switches.