4.4 Power Outages

Most of us take electricity for granted. However, when the power goes out, it becomes headline news!

What is a Power Outage?

A power outage (also called a blackout, power failure, or power cut) is the planned, or more often unwanted, loss of electrical power to an end user. Power outages can occur throughout the grid, for many reasons. Power outages often impact a single neighbourhood or local community but can potentially be city-wide or even province-wide or beyond.

Power Outage Causes

The electricity system in Canada is expansive, and although the grid is engineered to provide safe and reliable service, at times issues can arise. Some of the external causes of power outages are shown below.

• Interruptions can be caused by electrical distribution equipment being exposed to abnormal environments, such as road salt spray, humidity, corrosion, vibration, fire, or flooding.

• Adverse weather conditions such as extreme rain, ice, snow, wind, extreme temperatures, freezing rain or frost can all cause damage to distribution infrastructure.

• The energy released during a lightning strike is usually mitigated through specialized equipment.

However, lightning strikes can sometimes cause major damage.

• In urban areas, trees are sometimes allowed to get too close to power lines, creating safety and reliability issues. Electrical utilities usually have robust tree-trimming programs to manage this problem.

• Other foreign objects can cause power outages, including events such as animal contact (squirrels and birds), vehicle accidents, unsafe excavations, vandalism, and even objects such as metallic balloons.

Defective Equipment

Defective equipment includes failure of electrical distribution system components primarily due to aging. Large capital investments keep these failures in check by replacing aging equipment.

Loss of Supply

Loss of supply can occur due to problems on the provincial transmission system.

Scheduled Outages

Scheduled outages are planned in advance for construction or preventative maintenance. This is the only category that utilities can fully control.

Unknown

Sometimes an outage will occur and even after investigation, no obvious cause can be found. Power gets restored and the event is registered as unresolved.

Quantifying Power Outages

Utilities are very good at analyzing, interpreting, and reacting to outage data. Considerable effort goes into understanding the root causes of why the power goes out, the trends over time, and the customer impacts. This knowledge becomes the foundation for grid maintenance and capital programs.

Utilities use standardized terminology and metrics to describe and track the performance of the grid and describe power outages.

Momentary outages

From time to time, the power can go out briefly. These are called momentary outages. They are often the outcome of the grid protection system doing what it has been designed to do, by isolating failed equipment and restoring power to the remainder of the customers.

System Average Interruption Frequency Index (SAIFI)

The System Average Interruption Frequency Index (SAIFI) is the average number of interruptions per customer in a given year. The total number of customers who experienced at outage is divided by the total number of customers served.

System Average Interruption Duration Index (SAIDI)

The System Average Interruption Duration Index (SAIDI) is the average duration of all interruptions in a given year per customer. The total length of all interruptions is calculated and then divided by the total number of the utility’s customers.

Customer Average Interruption Duration Index (CAIDI)

The Customer Average Interruption Duration Index (CAIDI) is the average duration of all interruptions in a given year per impacted customer. The total length of all interruptions is calculated and then divided by the total number of the utility’s customers that were impacted by them. It can also be thought of as average time to restore service.

Feeders Experiencing Multiple Interruptions (FEMI)

Feeders Experiencing Multiple Interruptions (FEMI) measures how many interruptions each feeder or primary circuit experiences. This measurement helps utilities focus on improving the worst performing feeders.

Customers Experiencing Multiple Interruptions (CEMI)

Customers Experiencing Multiple Interruptions (CEMI) measures how many outages individual customers have experienced. This allows focused utility work on improving service to those customers with higher CEMI scores.

Mutual Assistance

Mutual assistance is when utilities help each other on an urgent but non-profit basis when responding to large-scale power outages. Support is typically offered in the form of experienced crews and additional equipment.

There are many regional mutual assistance arrangements between utilities. One example of a larger outage arrangement is the North Atlantic Mutual Assistance Group (NAMAG) which exists for the benefit of 36 million electricity customers in the north-eastern U.S. and neighbouring parts of Canada.

Mutual assistance pre-planning includes:

• A process for identifying and prioritizing events as they occur

• Analysis of anticipated support requirements

• Pre-organization of actual support arrangements

• Pre-arranged contracts to cover terms and conditions and to support international travel

Knowledge Check

Momentary outages are often the outcome of the grid protection system doing what it has been designed to do, by isolating faulted equipment and restoring power to the remainder of the customers.

The System Average Interruption Frequency Index (SAIFI) is the average number of interruptions per customer in a given year.

The System Average Interruption Duration Index (SAIDI) is the average duration of all interruptions in a given year per customer.

The Customer Average Interruption Duration Index (CAIDI) is the average duration of all interruptions in a given year per impacted customer. It can also be thought of as average time to restore service.

Feeders Experiencing Multiple Interruptions (FEMI) measures how many interruptions each feeder or primary circuit experiences. This measurement helps utilities focus on improving the worst performing feeders.

Customers Experiencing Multiple Interruptions (CEMI) measures how many outages individual customers have experienced. This allows focused utility work on improving service to those customers with higher CEMI scores.