becoming mandatory as a consequence of energy legislation enacted in 2005.
A key feature of the FERC-approved reliability standards is a performance table showing planning and operating criteria for normal operations (Category A) and three categories of disturbances.4 For single or multiple outages, the following apply:
5. Category B, events such as a short circuit causing loss of a single element or component in the system (i.e., an N-1 event with outage of a single generator, transmission line, or transformer).5 The power system must remain stable (no cascading) and within thermal and voltage limits. Loss of load or curtailment of firm transfer (i.e., sales of energy that have been agreed upon by contracts) is not allowed. For operations, the system must be readjusted within 30 minutes to withstand another outage.
6. Category C, certain related (non-independent) events causing outages of multiple elements. An outage of two circuits of a multiple circuit is one example. Similar performance to Category B is required except that planned/controlled load shedding and/or firm transfer curtailment are allowed. Cascading must be prevented.
7. Category D, extreme events resulting in multiple elements removed or cascading out of service. Selected events may be evaluated for risks and consequences.
To date, NERC standards have given little consideration to scenarios in which multiple facilities are destroyed by terrorists. In the future it may be prudent to design and operate bulk power systems to withstand multiple outages (Category D) that have some likelihood or history of occurring, or that are vulnerable to well-thought-out terrorist attacks. Such a standard would likely be expensive to implement and might reduce transfer capacity until additional facilities are added, but some movement in this direction is probably warranted.6
For Category D events, controls may be applied to prevent or mitigate cascading and massive loss of load. These are sometimes termed safety nets. For example, underfrequency load shedding is universally applied for controlled or uncontrolled separations (islanding). Undervoltage load shedding may be applied in areas where voltage collapse is a concern. These and other automatic controls attempt to restore equilibrium conditions within the electric power system or portions thereof. Loss of components due to malicious attacks would also cause imbalances and, if necessary, such controls would also be activated to mitigate the detrimental effects.
According to the NERC performance table, actions such as reduced power transfers and canceling of planned outages (e.g., for maintenance) may occur during abnormal conditions such as storms or forest fires. Similar actions should be taken during elevated terrorist threats resulting in a DHS red alert status.
The U.S.-Canadian power system currently consists of four large regions (see Chapter 2) within which all connected generators operate synchronously. Asynchronous connections between the regions are accomplished with DC tie lines or back-to-back AC-DC-AC converters (asynchronous links). Large synchronous regions evolved for economic power transfers and for the mutual support inherent with AC transmission.7 Under some operating conditions, however, large synchronous interconnections are vulnerable to large cascading failures when certain faults occur.8 (For examples, see Table 1.1.) Upgrades of AC transmission capability to improve the strength of the existing interconnections, the selective addition of advanced controls, and power electronics-based equipment, and other solutions such as prioritized modernization of power plant and substation equipment, including emergency control and protection are urgently needed.9
A critical component of the bulk power system is the design and layout of transmission substations and switch-yards. Substations are designed for reliability, flexibility of operation (including access), and cost. Substations provide the ability to safely switch equipment out of service during either scheduled or unscheduled outages while maintaining service. Several substation configurations have evolved to
4See Table 1 at http://www.nerc.com/pub/sys/all_updl/standards/rs/TPL-001-0.pdf.
5Simulations for N-1 planning/operating criteria often involve a rarely occurring three-phase fault at a critical location with outage of a key line or transformer during peak load or transfer. The three-phase fault “umbrella” events are more severe than many multiple outages, especially those occurring during less stressed (off peak) operating conditions.
6Recall also that an N-2 event is defined as one in which the system would continue to operate reliably without two elements. Note, however, that there is no requirement for the frequent N-2 event of a short circuit with line outage, and with simultaneous outage of a parallel line or line with common termination because of a protective relay mis-operation. Storms, fires, airplanes, and terrorists may also cause loss of parallel lines on the same right-of-way. However, moving to N-n reliability standards in which n is larger than 2 should only be undertaken after a careful quantitative probabilistic assessment of costs and benefits.
7For example, for an outage in one line, power automatically shifts to other parallel lines in a fraction of a second. With DC links, special controls are needed.
8One theoretically possible approach to containing the extent of such outages would be to reduce the size of synchronous regions. For example, the large Eastern and Western interconnections could be restructured into regions similar in size to the Quebec and the Texas interconnections. This would require breaking up these two large interconnections into smaller ones connected by asynchronous links. Such a change would prevent the propagation of disturbances across very large areas. However, this approach would have serious limitations. It would undermine the kind of automatic support now provided by a large interconnected AC grid when large loads or generators are tripped. Further, asynchronous links are expensive.
9Such control equipment may include selective conversion to asynchronous links, such as a link proposed between Ontario and Michigan that might have reduced the extent of the August 14, 2003, blackout.