VOLTAGE SAG PREDICTIONS BASIC INFORMATION

Voltage sags associated with fault clearing have many predictable characteristics. It is possible to predict the sag magnitude for individual faults by calculating the voltage drop at the critical load.

Predicting how long the voltage sag will last requires an estimate of the total clearing time for the overcurrent protective device. The waveform of voltage sags is somewhat predictable from analysis of recorded voltage sag data available and with the aid of transient network analysis.

However, it is very important to estimate how often voltage sags will upset sensitive electrical equipment.

Predicting characteristics for one sag caused by a specific fault at a specific location is straightforward. Prepare an accurate electrical model of the system, apply the fault, and calculate the voltage sag magnitude at the critical load.

Use the protective device characteristics to estimate sag duration. Compare the sag characteristics with the sensitive equipment capability to determine if the process will have an outage.

Predicting sag characteristics a sensitive load will see during several years of operation requires a probabilistic approach. It is impossible to predict exactly where each fault will occur, but it is reasonable to assume that many faults will occur.

The most accurate predictions require sag calculations for every possible fault on the electrical system and estimating each fault’s frequency of occurrence.

The overall sag frequency is the sum of the individual frequencies. A practical approach is to locate boundaries on the electrical system where specific sag magnitudes are possible; then estimate the fault frequency in the boundary.