Skip to Main Content
There have been 23 large blackouts which have occurred worldwide since 1965 each with a power loss ≥8,000MW. Blackouts usually occur through two stages: Stage one is related to voltage collapse/transient instability and the reason why most of the instability has occurred is because of the `cascading trips' by impedance relays due to load transfer initiated by a single disturbance and a heavy load superimposed to long distance/high impedance circuits. Stage two is that the transient instability may spread out to “blackout” due to `cascading trips' by impedance relays of transmission and out-of-step relays of generators operating during system instability. Two different strategies exist on how to handle system transient instability: The first strategy is from the North American NERC Planning Standards (1997), where the standards state that"Generation and transmission protection systems should avoid tripping for stable swings on the interconnected systems. "This means the operations of the protection tripping the generation and transmission systems are permitted during transient instability. The consequential results are blackouts. The second is from the rule of "Power System Security and Stability Criteria" in China: Firstly, avoid overload' cascading trips' of transmission lines by protection/automation technology to prevent transient instability. Secondly, avoid protection' cascading trips' of all the transmission lines and most of the generators even when instability occurs to prevent blackout, because re-synchronization of the system can be restored within a short time. Therefore, China (except Taiwan) has prevented large blackouts. This China system infrastructure and protection design is based on close examinations of blackouts that have occurred all over the world. The study concluded that "The cause of cascading blackouts was the Un-controlled power system infrastructure and protection". Thus for some 30 years of implementing "Cr- teria" - building proper AC/DC system infrastructure and correct system protection to keep system integrity, system transient instability may occasionally occur, but it could not cascade to widespread blackout.