· Phase-to-earth fault
The conditions
for a solid fault from line a to earth
· Phase-to-Phase fault
The conditions
for a solid fault between
_{ }
·
Phase-to-Phase-to-earth fault The conditions for a fault between lines b and c and earth are represented by the equations 1_{a }= 0 and V_{b}=Vc =0. From these equations it can be proved that:
2.3 Equivalent impedances for a power system.
When it is necessary to study the
effect of any change on Z= V^{2}/P Where:
Z
=
Equivalent positive and negative-sequence impedances P = three-phase short circuit power The equivalent zero-sequence of a system can be derived from the expressions of sequence components referred to for a single-phase fault, i.e. Ia1=Ia2=Ia3 = VLN/ (Z1 + Z_{2} + Z_{0})
Where: For lines and cables the positive and negative ímpedances are equal. Thus, on the basis that the generator ímpedances are not significant in most distribution-network fault studies, it may be assumed that overall Ζ_{2} = Z_{1}_{ }which simplifies the calculations. Thus, the above formula reduces to Ia = 3I_{0} = 3 VLN / (2Z_{1} + Zo), Where V_{LN}_{ }= line-to-neutral voltage and Zo= (3VLN / Ia) - 2Z1 ^{ } 3 Supplying the current and voltage signals to protection systems
In the presence of a fault the current transformers (CTs) circulate
current proportional to the fault current to the protection
equipment without distinguishing between the vectorial magnitudes of
the Sequence components.
Figure 10 Connection of sequence networks for a3ymmetrical faults a Phase-to-earth fault b Phase-to-phase fault c Double phase-to-earth fault Therefore, in the majority of cases, the relays operate on the basis of the corresponding values of fault current and / or voltages, regardless of the values of the sequence components. It is very important to emphasise that, given this, the advantage of using symmetrical components is that they facilitate the calculation of fault levels even though the relays in the majority of cases do not distinguish between the various values of the symmetrical components. |
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