Electric generation and delivery systems must be modern, reliable and able to handle diverse fuels and technologies. They also must be able to keep up with customer demand.
Distribution (the infrastructure and the processes that deliver electricity from high-voltage transmission lines to customers’ homes and businesses) continues to improve. Better tools and processes are being developed to manage the system, but several challenges remain.
Overall reliability is measured by the average number and duration of sustained outages on the distribution system. The System Average Interruption Duration Index (SAIDI) measures how many minutes the average customer experiences an interruption in electric service in a given year.
The System Average Interruption Frequency Index (SAIFI) represents the number of interruptions the average customer experiences in a year.
The Customer Average Interruption Duration Index (CAIDI) represents the average length of time it takes to restore service when an outage occurs.
Preventative vegetation management is critical to reliability and is one of the most proactive measures to reduce interruptions. Scheduled tree trimming is more cost effective and provides greater reliability than responding to overgrown vegetation. Cutting vegetation once it is entangled in a line requires more time and increases the risk of injury and customer outages.
Downed transmission lines and structure following the June 29, 2012, derecho in Ohio.
What AEP is doing
We must prevent outages to a practical extent and restore power safely and efficiently when outages do occur. We face challenges affecting our ability to maintain our existing 220,000-mile transmission and distribution network while also upgrading infrastructure to meet future demands and changes in the generation portfolio across the country. Our challenges include the age of our infrastructure, the threat of external interruptions, the need for new and future environmental regulations and the cost needed investments.
Rather than focusing on single-year numbers, AEP has begun using a three-year rolling average, which evens out weather-related outages. We believe this is a more meaningful measure that better reflects changes in the overall status of the system.
To improve the performance of the physical infrastructure of the grid, we created a Distribution Storm Hardening Strategy Team to recommend strategies that will the ease the impacts of severe weather events on our customers by making the infrastructure more resilient and easier to fix when it does break.
New design criteria to strengthen, or harden, the distribution system took effect in early 2014. We have elected to design new and replacement poles to withstand wind speeds and ice accumulation above and beyond the National Electrical Safety Code (NESC) requirement for our service territory. In the southern portion of our territory, where high winds are the primary driver of major storm damage, we have increased the system’s ability to withstand high winds from 60 mph to 90 mph. Along the Gulf Coast we continue to design facilities to withstand 150 mph winds.
AEP manages the trees and vegetation around power lines using a combination of performance-based (such as targeting low performing circuits) and cycle-based maintenance strategies. Maintaining a regular tree- trimming cycle is a significant expense that directly affects customer bills.
The importance of vegetation management