Introduction Figure 1. Lightning arc This article looks at the calculation of short circuit currents for bolted three-phase and single-phase to earth faults in a power system. A short circuit in a power system can cause very high currents to flow to the fault location. The magnitude of the short circuit current depends on the impedance of system under short circuit conditions.

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To illustrate its application, the methodology is applied to a study medium voltage network with a variety of distributed generation resources. Distributed generation resources are typically connected to the distribution network, at the low or medium voltage level. Therefore they contribute to the total fault level of the distribution grid. Hence, the basic requirement for permitting the interconnection of distributed generation resources is to ensure that the resulting fault level remains below the network design value.

The short-circuit currents is considered as the sum of an a. The Standard distinguishes between near-to-generation and far-from-generation short circuits. The methodology includes of a. In the present, two numerical methods for short-circuit calculations are used: superposition method and equivalent voltage source method. Superposition method gives the short-circuit current only in relation to one assumed amount of the load. Hence, it need not lead to maximum short-circuit current in the system.

For removing this lack, it was developed the equivalent voltage source method [2]. This source is defined as the voltage of an ideal source applied at the short-circuit location in the positive sequence system, whereas all other sources are ignored.

All network components are replaced by their internal impedances see Fig. Equivalent voltage source method In the calculation of the maximum short-circuit currents, the voltage factor c may be assumed equal to cmax, for any voltage levels see Tab.

According to Fig. If any distributed generation unit units is in the grid included, the resulting fault level is the sum of the maximum fault currents due to: the upstream grid, the various types of generators, the large motors connected to the distribution network. The calculation of contribution of the distribution generation units is not included in the IEC Only induction motors are dealt with, whereas the parameter values of synchronous generators provided in [4] are applicable to units of very large size [3].

X d , " X d - subtransient reactance of synchronous machine, K G - correction factor for generators connected directly to the grid. The value of t depended on the protection and fault.

It is needed only for breaking and thermal current calculations. Special case of generator used for variable speed wind turbines is doubly-fed induction generator DFIG. X G for the generator impedance [3].

Four distributed generation stations with total power cca 17 MW are connected to the medium voltage substation. They are three wind farms with six identical wind turbines and one small hydroelectric plant - SHP with three identical turbines.

The data about case study network are given in Table 2. Case study Tab.

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## Calculation of Short-circuit IEC 60909

The Redline version provides you with a quick and easy way to compare all the changes between this standard and its previous edition. The Redline version is not an official IEC Standard, only the current version of the standard is to be considered the official document. IEC specifies procedures for calculation of the prospective short-circuit currents with an unbalanced short circuit in high-voltage three-phase a. The currents calculated by these procedures are used when determining induced voltages or touch or step voltages and rise of earth potential at a station power station or substation and the towers of overhead lines. Procedures are given for the calculation of reduction factors of overhead lines with one or two earth wires. This edition constitutes a technical revision. The main changes with respect to the previous edition are: - New procedures are introduced for the calculation of reduction factors of the sheaths or shields and in addition the current distribution through earth and the sheaths or shields of three-core cables or of three single-core cables with metallic non-magnetic sheaths or shields earthed at both ends; - The information for the calculation of the reduction factor of overhead lines with earth wires are corrected and given in the new Clause 7; - A new Clause 8 is introduced for the calculation of current distribution and reduction factor of three-core cables with metallic sheath or shield earthed at both ends; - The new Annexes C and D provide examples for the calculation of reduction factors and current distribution in case of cables with metallic sheath and shield earthed at both ends.

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