The creation and delivery of power is one basic infrastructure expected by all modern industrial societies. Power distribution basics started in the US in the early 1900s when the first AC grid was established.
Since 12 independent power grids were completed in the lower 48 states in the mid-1950s, reliability has been a customer concern. Parts of these earlier grids have been updated, and four of the US power grids are tied to similar power grids in Canada. In this context, reliability concerns itself with an understanding of why there are power failures.
A power surge in one part of the extended grid may bring down other sections when they are shut down for self-protection. This action is a self-protection mechanism for the grid which preserves the equipment, but extends the outage to more customers. As one grid becomes larger and more interconnected with other grids, higher power demands, and perhaps larger surges, may be expected.
Shutdown self-protection, intrinsic design margins, and redundant paths become keys to maintaining reliability.
Major power outages
Table I presents a summary of major power outages over the last 45 years. Because this is a time dependent table, one can determine if the power reliability is improving or declining over the years.
Because the evolution of the power grid is dynamic, one must be careful when drawing conclusions. The items in the following tables show only the major power outages in the US & Canada, Europe, and the rest of the world (International). These failures are typically known by date, weather conditions, grid conditions, cause or causes, locations of failures, extent of failures in population, and duration of the power outage.
Table I – A Summary of Major Grid Failures
|Years||US & Canada||Europe||International|
|1965-1988||3 fails||3 fails||1 fail|
|1989-1994||2 fails||None||1 fail|
|1995-1999||8 fails||1 fail||7 fails|
|2000-2005||11 fails||7 fails||15 fails|
|2006-2009||33 fails||16 fails||20 fails|
|Totals:||57 Total||27 total||44 total|
The definition of a major grid failure is:
- an unplanned event (down for maintenance or rotating blackouts don’t count) that
- affects at least 1000 customers (in this study the number is 30,000 minimum) for
- a total downtime of at least 1,000,000 customer hours.
Power outage frequency
Few major incidents were reported in the 1960s, 1970s, and early 80s. The last 20 years reflects many more outages, and so was divided into smaller groups.
Reporting in developed countries (US & Canada and Europe) could not be responsible for this increase.
Perhaps, in some of the international entries the change in reporting methods may have added to the increase in reported incidents over the last 45 years.
Twelve power grids exist across the US & Canada, and some parts of Mexico.
In Western Europe, one large grid exists, and several smaller grids connect most countries. Most of the rest of the world has small selfcontained power grids within their country, and are not connected to other grids. It is less likely that power outages beyond the US & Canada or Europe could meet the three criteria for major outages.
In fact, if one counts all outages in the US & Canada, there were 76 events in the 1995 to 1999 years, and 140 from 2000 to 2005 . The general trend of Table I reflects increasing major outages since 1970, as well as all outages.
Table II shows a basic breakdown of the high level causes. In some cases, there could be multiple interacting events. For example, a windstorm may take down one segment of transmission lines, and that leads to a surge elsewhere which takes down a power grid temporarily.
Table II – A Breakdown of Major Grid Failures by Cause
|Main Causes||US & Canada||Europe||International|
|Snow Storm||13 fails||3 fails||3 fails|
|Summer Storms||11 fails||2 fails||1 fail|
|High Winds||8 fails||1 fail||3 fails|
|Unknown||3 fails||8 fails||11 fails|
|Substation||3 fails||2 fails||7 fails|
|Transmission Line Failures||3 fails||2 fails||4 fails|
|Lightning strike||2 fails||None||None|
|Heat wave||2 fails||2 fails||3 fails|
|Other||4 fails||7 fails||12 fails|
|Total:||57 fails||27 fails||44 fails|
This type of event is reflected only as “high winds” in Table II (i.e. greater than 60 MPH). These events most often occur in the Spring and Fall months as weather patterns change.
The biggest single cause listed in Table II is snow storms (this includes ice storms) in the US & Canada occurring mainly from December to March. The second biggest item is summer storms. The combination of high winds with lightning and heavy rains may cause an outage.
Hurricanes are next at eight events, mostly along the Gulf Coast. Six hurricane events occurred in just three years (2003 to 2005), and this group reflects an irregular hurricane cycle for powerful storms. This cycle has multiple nodes with repeat times of 22 years, 35 years, and 50 years. The years from 1970 to 1995 were a period when the annual number of hurricanes was low, and only one caused an outage.
One hurricane in 2008 caused a major outage, and the hurricane trend appears to be for fewer and less powerful storms in the near future . The number of hurricanes in years 1965-2002, and 2006-2009 produced only two major power outages in those 41 years.
High winds were next in number.
This was important in the US, but not as important in Europe or International regions, though one cause of a power failure in Egypt was listed as “sandstorm”.
This last incident, like the July 2006 New York City outage, and the August 2006 London outage, can be attributed to multiple events. A multi-day heat wave pushed power consumption of the grid to near its limit. A minor incident then brought the high stressed grid down.
The total of all the entries in the “other” category is important, but the events themselves are unique. The same is true of the “other” category for Europe and International.
Reference: Major Power Outages in the US, and around the World – James McLinn Rel-Tech Group (This article is part of the IEEE Reliability Society 2009 Annual Technology Report)