• Speed at which protection systems operate. A rapid decision to trip a breaker may prevent instability and permanent damage to lines or apparatus under fault conditions. However, disturbances and system dynamics may create electrical signals that emulate fault or overload conditions that can only be distinguished with sufficient analysis time. Consequently, a quick decision to trip may be required under certain conditions, but also may result in an improper decision under different dynamic conditions.
• Testing and maintenance practices. These can result in improper protection settings or inadvertent changes in protection logic. These have also caused large-scale blackouts. For example, even a cursory analysis of the August 2003 blackout shows several areas of concern with respect to protection system design, as integrated with system operations and communications. The loss of the first transmission line was caused by the correct operation of relays to clear a fault caused by the line sagging into trees. This resulted in heavier loading of parallel lines with the effect of subsequent loss of multiple lines due to faults and overload conditions. The lines associated with these events were properly protected and preserved and could have immediately been placed back in operation had operators had adequate knowledge and awareness of the dynamic events that were occurring.
• Systems to enhance awareness of operating conditions. New digital relays with advanced communications and information sharing capability coupled to control and information systems can decrease the probability of cascading failures as a consequence of multiple protection system operations.
• Proper settings of relays. Improper settings have resulted in cascading blackouts caused by the tripping of transmission lines under nonfault conditions. An improper setting of a “zone 3” impedance (distance) relay was a proximate cause of the November 9, 1965, Northeast blackout. The relay performed correctly based on its setting, but it had not been reset as system load grew. High load but nonfault electrical conditions caused the relay to operate. Emphasis should be given to remote monitoring of protective relay settings and improving maintenance and test procedures that mitigate the possibility of improper and insecure operation of relays.
• Addressing the “overreach” of protection systems. Overreaching distance protection, mainly in the form of zone 3 relays, has caused or contributed to many blackouts. Overreaching protection is generally applied as backup protection in the case of breaker failure in a distant substation. In other words, if a local protection system fails to detect a fault, surrounding substations “overreach” to detect the fault and eliminate fault current in-feeds to the local substation. Sensitive settings are required, and so the relays are prone to operate on nonfault conditions of overload, depressed voltage, or electromechanical swings among generators. There are several solutions to this problem, including redundant local relays, breaker failure relays, bus protection, and restrictions on the reach of impedance relays. NERC and the industry have addressed the backup relay problem in response to the 2003 blackout. Thousands of changes were made by North American power companies. (Reports are available at www.nerc.com.) Eternal vigilance, however, is required to ensure that relays respond only to short circuits.
The above approaches do not address all of the protection issues that can cause or exacerbate a cascading blackout. With millions of protective relays and protection schemes in place, undesirable or unnecessary operations cannot be prevented. However, fruitful areas of investigation and improvement include the following:
• Improvement in intelligent, digital relays allowing for self-evaluation and remote evaluation of settings and relay health to ensure reliable operation.
• Integration of protection systems with other control and operation systems to ensure that operators have full operational awareness as conditions change and deteriorate during a cascading event.
• Improved control philosophies and strategies for multiple contingency events occurring in close time proximity. Such improvements could address situations in which the proper operations of relays in response to changing conditions, when taken as a whole, can create unrecoverable instability in the power system.
• Methods to prioritize modernization of protection relays and schemes, including communications such as by fiber optics between stations.