There are two possible locations for faults on the distribution systems, i.e., on the same feeder and on parallel feeders. An area of vulnerability defining the total circuit miles of fault exposures that can cause voltage sags below equipment sag ride- through capability at a specific customer needs to be defined.

**The computation of the expected voltage sag performance can be performed as follows: **

**Faults on Parallel Feeders: **

Voltage experienced at the end-user facility following a fault on parallel feeders can be estimated by calculating the expected voltage magnitude at the substation. The voltage magnitude at the substation is impacted by the fault impedance and location, the configuration of the power system, and the system protection scheme.

Figure 2.10 illustrates the effect of the distance between the substation and the fault locations for 3LG and SLG faults on a radial distribution system. The SLG fault curve shows the A-B phase bus voltage on the secondary of a delta-wye-grounded step-down transformer, with an A phase-to-ground fault on the primary.

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**The voltage sag performance for a specific sensitive equipment having the minimum ride-through voltage of vs can be computed as follows: **

E parallel (vs) = N_{1 }x Ep_{1} + N_{3} x Ep_{3}

Where, N_{1} and N_{3} are the fault performance data for SLG and 3LG faults in faults per miles per month, and Ep_{1} and Ep_{3} are the total circuit miles of exposure to SLG and 3LG faults on parallel feeders that result in voltage sags below the minimum ride- through voltage vs at the end-user location.

**Faults on the Same Feeder: **

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In this step the expected voltage sag magnitude at the end-user location is computed as a function of fault location on the same feeder. Note that, however, the computation is performed only for fault locations that will result in a sag but will not result in a momentary interruption, which will be computed separately. Examples of such fault locations include faults beyond a downline recloser or a branched fuse that is coordinated to clear before the substation recloser.

The voltage sag performance for specific sensitive equipment with ride-through voltage vs is computed as- E same (vs) = N_{1} x Es_{1} + N_{3} x Es_{3}.

Where, E_{1} and Es_{3} are the total circuit miles of exposure to SLG and 3LG on the same feeders that result in voltage sags below vs at the end-user location. The total expected voltage sag performance for the minimum ride through voltage vs would be the sum of expected voltage sag performance on the parallel and the same feeders, i.e., E parallel (vs) – E same (vs). The total expected sag performance can be computed for other voltage thresholds, which then can be plotted to produce a plot similar to ones in Fig. 2.10.

The expected interruption performance at the specified location can be determined by the length of exposure that will cause a breaker or other protective device in series with the customer facility to operate. For example, if the protection is designed to operate the substation breaker for any fault on the feeder, then this length is the total exposure length.

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The expected number of interruptions can be computed as- E_{int} = L_{int} x (N_{1} + N_{3})

Where, L_{int} is the total circuit miles of exposure to SLG and 3LG that results in interruptions at an end-user facility.