TABLE OF CONTENTS IEC 60909-3 describes methods and procedures for the calculation of currents during two separate simultaneous single-phase line-to-earth short-circuits (s.-c.) at different locations of the system, which are called double earth-fault in the context of this section. The double earth-fault is not identical to a double-phase short-circuit, where two phases have a short-circuit at the same location. Furthermore, the branch short-circuit currents flowing through earth are dealt with.
In order to calculate the short-circuit currents I ? kEE in the case of a double earth-fault, the mutual impedance in the positive- and zero-sequence component between the two short-circuit locations is needed.As the mutual impedance comparatively difficult to determine, the double earth-fault can only be analysed in a simple manner for special system configurations. The following cases are considered:
Both short-circuit locations are on the same line.
The short-circuit locations are on different lines.
Single-fed line.
Double-fed line.
In the case of a single-fed (radial) line, both short-circuit locations are on the same line and the double earth-fault is identical to a double-phase short-circuit without earth connection, as seen from the feeding point of the line. In the case of a double-fed single-circuit line, the voltage line-to-earth is changing significantly between the two short-circuit locations, whereas the line-to-line voltages are remaining almost unchanged. Details are outlined in [3].
In general, the short-circuit current in the case of a double earth-fault with short-...
