Calculation and prevention of short circuit currents in high voltage grids

Types of short circuit currents

Three-phase electric power is a kind of poly-phase system and it is a method of AC electric power generation, transmission, and distribution. It is the most popular and the most commonly used method by electric power grids worldwide to transfer power. It is also used to power heavy loads and large motors.

A three-phase system is normally more economical than an equivalent single-phase system or two-phase system at the same voltage level, since it uses less conductor material to transmit electrical power. The three-phase system was independently invented by Galileo Ferraris, Mikhail Dolivo-Dobrovolsky and Nikola Tesla in the late 1880s.

The most occurring types of shunt faults are:

1. Phase-to-earth (80% of faults): Phase-to-earth is the most occurring fault. This type of fault occurs when one conductor falls to ground or contacts the neutral wire. It could also be the result of trees falling on top of the lines.(See Figure 2-1 (a))
2. Phase-to-phase (15% of faults): Phase-to-phase fault is the result of two conductors being short-circuited. For example: a bird could sit on one line somehow coming in contact with other, or a tree branch could fall on top of two of the power lines. (See Figure 2-1 (b))

   

   
   

3. Phase-to-phase-to-earth: This can be a result of a tree falling on two of the power lines, or other causes. See Figure 2-2 (a).
4. Three-phase (only 5% of initial faults): It is the least occurring fault, this type of fault can only occur by a contact between the three power lines in various forms. See Figure 2-2 (b).

   

   
   

Series faults can occur along the power lines as the result of an unbalanced series impedance condition of the lines in the case of one or two broken lines for example.

In practice, a series fault is encountered, for example, when lines (or circuits) are controlled by circuit breakers (or fuses) or any device that does not open all three phases. One or two phases of the line (or the circuit) may be open while the other phases or phase is closed.”