By Anwarul I. Sifat in Technical Notes —

Under Voltage Load Shedding

Under Voltage Load Shedding
There are two basic types of automatic UVLS schemes that utilities have installed. Both types involve the installation of undervoltage relays at key utility substations. These relays must measure the transmission system voltage and are typically installed at the primary of distribution substations that are located close to key transmission substations. Figure shows a typical utility installation of both undervoltage (27) and under frequency (81) relays

It’s a protection and stability control scheme used in power systems to prevent voltage collapse during severe disturbances.

When a major contingency occurs, i.e. a large generator trips, a transmission line goes out, or there’s heavy reactive power deficiency, which results system voltage sag. If the voltage keeps declining, loads (especially induction motors) draw more current, which worsens the situation. That feedback loop can push the system toward voltage instability or collapse.