2.3 Voltage Sag: Fundamental Principle of Protection

Utility Distribution system sag Performance Evaluation:

The utility protection scheme plays an important role in the voltage sag and momentary interruption performance.

There are two possible locations for faults on the distribution systems.

a. On Parallel Feeder

The voltage at the end-user facility can be due to 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.
The voltage sag performance for a specific sensitive equipment having the minimum ride through voltage of can be calculated as:
E(parallel) = N1Ep1 + N3Ep3
Where, N1 and N3 are the fault performance data for SLG (Single Line to Ground) and 3PF (3-Phase Fault) faults in faults per miles per month, and Ep1 and Ep3 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.

b. On Same Feeder

Here the expected voltage sag magnitude at the end-user location is computed as a function of fault location on the same feeder. 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.
The voltage sag performance for a specific sensitive equipment having the minimum ride through voltage of can be calculated as:
E(same) = N1Es1 + N3Es3
Where, Es1 and Es3 are the total circuit miles of exposure to SLG and 3PF 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 addition of expected voltage sag performance on the parallel and the same feeders.

From above figure, it is clear that, as we move from right to left i.e., consumers end towards Source, the cost of the Protection Devices increases.
Equipment manufacturers should have voltage sag ride-through capability curve available to their customers so that an initial evaluation of the equipment can be performed.
The company procuring new equipment should establish a procedure that rates the importance of the equipment. If the equipment is critical in nature, the company must make sure that adequate ride-through capability is included when the equipment is purchased.
Equipment should at least be able to ride through voltage sags with a minimum voltage of 70 percent. The relative probability of experiencing a voltage sag to 70 percent or less of nominal is much less than experiencing a sag to 90 percent or less of nominal.

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