TABLE OF CONTENTS IEC 60781 presents an application guide for the calculation of short-circuit currents in low-voltage radial systems. The methods described there are identical to those as per IEC 60909-0 and as outlined in Chapter 3. The short-circuits are treated as far-from-generator short-circuits. This assumption is valid in the future as well, even with an increasing number of distributed generation units in low-voltage systems. New generation will be connected to the system in the case of
Photovoltaic installations by rectifier (six-pulse bridge or PWM-rectifier).
Small wind turbines, low-rated combined heat and power units and small hydro power plants by asynchronous generators (rotor fed).
Installations with synchronous generators are comparatively rare.
The following chapters describe the approach and special conditions for the calculation of short-circuit currents in low-voltage systems.
Depending on the type of protection against electrical shock in low-voltage systems all types of short-circuit, i.e., three-phase, double-phase with and without earth connection and single-phase-to-earth short-circuits can occur. The maximal short-circuit current depends on the impedances of the positive- and zero-sequence component. Reference is made to Figure 4.4. The ratio Z 2/ Z 1 can be set to 1, as low-voltage systems for the most part have no generation by synchronous generators. The three- phase short-circuit will lead in those cases to the maximal short-circuit current. Special attention must be given to the currents flowing through earth, as the phase angles of the impedances in the positive- and zero-sequence components differ a lot from each...
