A shot in the dark

Investigations of recent blackouts indicate that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960s and 1970s. The operation of today´s power system with generation frequently being remote from the load centers has made the power system very dependent on the utility transmission system. When transmission lines trip, voltage drops at the load center, whereas frequency may remain normal until a complete system collapse occurs. Utilities have begun to recognize this problem and are installing UVLS programs. Also the industrial customers that have in-house generation that operates in parallel with the utility need to recognize the problem. These industrial cogeneration customers should consider the use of undervoltage separation schemes in addition to their existing underfrequency schemes to address the voltage collapse scenario. This article proposes undervoltage separation logic schemes that can be easily installed within digital relays to enhance the security of an undervoltage separation to prevent false operation due to slow-clearing system faults. The article also indicates the required point of installation of these relays so that they properly measure system voltage.