Entering the automated distribution zone
MED goes underground with new commercial development
Published In: Intelligent Utility Magazine July/August 2010
IN A PERFECT WORLD, THE DISTRIBUTION SYSTEM HAS intelligent equipment that easily handles two-way power flow and receives distributed generation like renewables. Operators have advanced voltage control integrated with smart meters to manage energy, reduce losses and lower customer bills. Distribution quickly heals itself from assaults by nature and equipment failures while limiting outages to as few customers as possible. Power surges no longer destroy expensive equipment.
What's a dream at most utilities is becoming a reality at others and being built out in various stages.
''Substation automation is getting to the point where operators can view status, performance and loading of all major substation equipment and control it for improved relability, efficiency and asset management,'' said Mark McGranaghan, director of distribution research at the Electric Power Research Institute (EPRI). ''The penetration of substation automation is between 40 to 60 percent and growing. Going out to automated distribution circuits, (it's) probably 10 to 15 percent of the circuits in the country.''
Underground distribution driving automation
A major force driving automation is underground distribution, especially for large, new commercial developments where outages can cause serious financial losses. While more expensive than overhead distribution, the aesthetic advantages and fewer faults of underground are cursed with the problems of pinpointing faults when they do happen.
Take the case of Murfreesboro, Tenn., one the fastest-growing cities in the country, with a population that mushroomed from 46,000 in 1990 to more than 100,000 today. Murfreesboro Electric Department (MED), a city-owned utility serving more than 50,000 customers, was asked to provide extensive underground service for a major city development project.
''A few years back, the town fathers decided to build a new gateway into Murfreesboro - a new access highway off the interstate called Medical Center Parkway - with plans to do a business park, hospital, hotel and convention center, a 100-acre mall and a lot of restaurants, retail and commercial buildings. And they wanted all electric service underground,'' said Mark Kimbell, MED's chief engineer. ''We wanted a good way to protect it, isolate it quickly if we had a problem and a way to keep as many customers on as we could.''
Kimbell pointed out that the area is a very busy one, so traffic is a problem if there is an outage. ''If traffic signals go down, we couldn't get into the area quickly. We looked at all the factors and decided we would invest in automation. We've spent upward of two million dollars on it to the present day,'' he said.
MED was one of the earliest utilities in Tennessse to implement automated distribution on this scale, over six miles of underground infrastructure. The system has been operational for three years and Kimbell characterized performance as ''great, outstanding.''
Two of the utility's 10 substations are automated over several circuits that generally cover the new commercial area.
''In general, there are some advantages to pulse closing in terms of not exposing the circuit to multiple faults when you are reclosing,'' McGranaghan said. ''That helps other customers by not exposing them to multiple voltage sags and it is less damaging to transformers due to a reduced fault current exposure. But there are a lot of companies that make automated switch gear that are part of these automated systems for reconfiguration. There's a lot of competition in this market right now.''
Restoring power quickly
MED started with underground, expanded to overhead and now has about half of each with approximately 10 percent of its entire distribution automated. The utility currently uses radio communications but is working on a fiber loop to speed up response time.
''We had a fairly large tornado here a little over a year ago that destroyed both feeds of a 46-kV transmission to a substation,'' Kimbell said. ''With automation, we could have restored power to 5,000 to 6,000 customers within a minute. Without automation, it took several hours. Because of that we are looking at automating transmission lines to all of our 46-kV substations. We are also looking to expand automated distribution to the rest of our commercial areas.''
He added: ''Before, we would have a circuit go out to 2,000 customers and we would have to rely on someone calling in to say we have trouble. Then we would have to go to that spot and it could take a long while to ride out the circuit to find the problem. With automation it narrows it down. We break down the circuit to about five or six 100-amp sections. You can concentrate more people to find a fault quicker and fix it. I would say it's about 75 percent faster to find a fault.''
Managing the distribution system connection
McGranaghan takes a broader view of smart grid development: ''Automated distribution is absolutely critical because that's the connection to the customer, and the smart grid is all about making the customer integrated with pricing, being able to use demand response to control loads and allowing customers to provide generation. It's not possible unless we manage the distribution system connection to the customer.''
Leading the industry toward a more intelligent automation is the Institute of Electrical and Electronics Engineers (IEEE) Smart Distribution Working Group and its task force on volt/VAR control.
The group is working to advance the design guidelines for the optimization of smart overhead and underground distribution, and looking at how volt/VAR control needs to change.
The march toward automated distribution appears inevitable. As protection equipment ages, it will naturally be replaced with digital relays with capabilities that can be integrated with automation systems. Meanwhile, utilities are building automation from both ends of the line. Some are concentrating on advanced metering and are building outward. Others are focusing efforts on advanced control systems for reconfiguring substations and circuits and waiting for the latest meters.