The term ACE (area control error) is used to describe the instantaneous difference between a balancing authority’s net actual interchange flow and the scheduled interchange flow, taking into account the effects of frequency and metering error. The term flat tie line control is used when only tie line flows are closely monitored in consideration of the actual interchange flow. The term flat frequency control is used when only frequency is carefully controlled. When both tie line flow and frequency are carefully controlled by AGC (automatic generation control), the term is called tie line bias. Tie line bias allows the balancing authority to maintain its interchange schedule and respond to interconnection frequency error. The AGC system is part of the energy management system (EMS). (Note: the computer program tools used by system operators that make up the EMS, System Control Centers and Telecommunications).
Tie line bias is carefully monitored and reported for all tie lines. Bias is the accepted standard operating constraint for controlling ACE. Carefully monitoring and adjusting tie line flow helps keep the interconnected system stable.
