The faults, involve power conductor or conductors-to-ground or short circuit between conductors and are termed as shunt faults. Such faults are characterized by increase in current and fall in voltage and frequency.
On the other hand, a plain break in one or two conductors in a 3-phase circuit leads to an open-conductor or series fault. When fuses are used in 3-phase circuits, the fuses in one or two phases may blow off due to overload, the fault incurred is known as open-conductor fault. Series faults are characterized by increase in voltage and frequency and fall in current in the faulted phases.
The series faults are classified as:
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(i) One conductor open and
(ii) Two conductor open faults.
Such faults also disturb the symmetry in one or two phases and are, therefore, unbalanced faults. The unbalanced conditions so developed can be analyzed by the method of symmetrical components.
(i) One Conductor Open Fault:
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A 3-phase line with conductor a broken leading to one conductor open condition is shown in Fig. 5.26.
The terminal conditions at the fault point give following equations –
In terms of symmetrical components these conditions can be represented as –
The above Eqs. (5.32) and (5.33) suggest a parallel connection of sequence networks, as shown in Fig. 5.27.
(ii) Two Conductors Open Fault:
A 3-phase line with conductors b and c broken leading to two conductors open condition is shown in Fig. 5.28.
The terminal conditions at the fault point give the following equations –
In terms of symmetrical components these conditions can be represented as –
The above Eqs. (5.36) and (5.37) suggest a series connection of sequence networks, as shown in Fig.5.29.
