How does automatic generation control (AGC) maintain system frequency?

AGC continuously adjusts generator outputs to maintain frequency and scheduled interchange between control areas. It works in two modes: Area Control Error (ACE) mode balances generation with load plus interchange, and frequency bias mode distributes responsibility among areas. AGC uses measured frequency deviation and tie-line flow error to compute required generation changes. Response time is seconds to minutes. Fast-responding units (hydro, gas turbines) provide regulation; baseload units may not participate.