In the following notes a 3-phase short-circuit of zero impedance (the so-called bolted short-circuit) fed through a typical MV/LV distribution transformer will be examined. Except in very unusual circumstances, this type of fault is the most severe, and is certainly the simplest to calculate.

Fault-block mountains are formed by the movement of large crustal blocks when forces in the Earth’s crust pull it apart. Some parts of the Earth are pushed upward and others collapse down. Faults are simply cracks in the Earth’s crust.

 Large-scale normal faults are associated with structures called fault-block mountains. Fault-block mountains are formed as large blocks of crust are uplifted and tilted along normal faults.

Answer: Symmetrical faults are those faults which involve with all the three phase. it simply means that symmetrical faults affect all the three phases. On the other side, unsymmetrical faults are those faults in which either one or two phase involve. In unsymmetrical faults the three phase lines become unbalanced.

During the first half of a cycle, the fault current is at its largest magnitude – occurring at a moment when the voltage wave (not shown) is passing the reference axis. The asymmetry is brought on by the DC offset (as shown in Figure 1). So, for example, a 5Amp RMS AC current is the same as a 5Amp DC current.

If you know that symmetrical means that both sides of something are identical, then it should be easy to learn that asymmetrical means the opposite: the two sides are different in some way. Asymmetrical things are irregular and crooked, and don’t match up perfectly when folded in half.

The method of symmetrical components is used to simplify fault analysis by converting a three-phase unbalanced system into two sets of balanced phasors and a set of single-phase phasors, or symmetrical components. These sets of phasors are called the positive-, negative-, and zero-sequence components.

Symmetrical Components An unbalanced system of n phasors can be resolved into n systems of balanced phasors. These subsystems of balanced phasors are called symmetrical components. With reference to three-phase systems, the following balanced set of three components are identified and defined (Fig. 6.1).

Symmetrical components are most commonly used for analysis of three-phase electrical power systems. The voltage or current of a three-phase system at some point can be indicated by three phasors, called the three components of the voltage or the current.

When the system is unbalanced the voltages, currents and the phase impedances are in general unequal. The method of symmetrical components simplified the problems of the unbalanced three-phase system. It is used for any number of phases but mainly used for the three-phase system.

The three component variables V 1, V 2, V 0 are called, respectively, positive sequence, negative sequence and zero sequence. They are called symmetrical components because, taken separately, they transform into symmetrical sets of voltages.