CAUSES OF ELECTRICAL POWER OUTAGES

Interruptions in the supply of electricity to customers can occur at any hour of the day or night and can last from fractions of a second to many hours or even days. Interruptions can be caused by disturbances to or malfunctions of any of the three components of the power system: generation, transmission, or distribution. They can also be caused by the unavailability of adequate resources to supply the customer load. These two attributes of reliability are characterized by NERC as security and adequacy.

Data shows that over 90% of customer outages are caused by problems originating on the local distribution system. Although generation and transmission-related outages are less common than those related to the distribution system, they often have much more serious consequences because of the number of customers affected and the duration of the outage.



Disturbances can be initiated by:

1) External events such as:

• Environmental factors, including wind, rain, lightening, ice, fire, floods, and earthquakes
• Accidents such as cars hitting poles
• Sabotage (sadly)

2) Internal events such as

• Insufficient resources
• Failure of equipment due to electrical or mechanical stresses
• Operating errors or decisions

Lack of resources can be due to:

1) Insufficient generation caused by

• Low load forecasts
• Shortages of fuel due to supply disruptions or delivery/transportation problems
• Opposition to the construction of required new generating capacity
• Failure of equipment due to electrical or mechanical stresses poor planning
• Excessive maintenance outages
• Regulatory actions restricting the operation of power plants
• Transmission constraints
• Generation being retired because it is noncompetitive in the new competitive market

2) Insufficient transmission or distribution caused by:

• Low load forecasts
• Opposition to the construction of required new transmission or distribution lines
• Failure of equipment due to electrical or mechanical stresses
• Poor planning
• Intentional outages required because of other infrastructure work, such as the widening of roads

The duration of the interruption will be affected by the severity of the disturbance, the power system facilities affected, the redundancy or reserve built into the system, and the preparedness of the involved operating entities to respond. Some interruptions are of very short duration because the disturbance is transient and the system self-corrects. Some interruptions, such as those caused by tornadoes or ice storms, damage significant portions of the system, requiring many days to restore service. When there are insufficient generation resources, the outages may be of a controlled and rotating nature. Their duration might be only during peak load hours.

The extent of the interruption will be determined by the initiating disturbances and the facilities affected. For example, cascading outages caused by a fault occurring when a system is operating above a safe level can involve many states, as can a widespread ice storm. Conversely, a distribution pole damaged by a car may affect only a few homes.

An increasingly important aspect of power system reliability is the quality of service or power quality. With the increasing importance of computers and new electronic communication procedures, imperfections in electric service become increasingly important to the customer. Such imperfections include:

• Momentary interruptions
• Voltages outside of acceptable limits
• Voltage dips of very short duration

Protection against power quality imperfections can often be handled by the consumer. Pressure is mounting, however, for the supplier to improve quality. This raises the question of the responsibility for such improvements in a deregulated power industry with separate companies providing distribution, transmission, and power supply services