When Thinking of Distribution Automation, What Enters the Mind are thoughts of pole-mounted overhead circuits, with overhead switches and remote terminal units (RTUs). Many electric utilities have developed overhead distribution automation (DA) schemes. However, it is unique to approach a completely underground distribution system with this in mind. Back in 2006, NSTAR Electric & Gas (Boston, Massachusetts, U.S.) set out to implement underground automation in its vast underground distribution system, beneath the city streets of Boston.

The majority of Boston's distribution system is underground. It consists of more than 30,000 manholes and 3000 miles (4828 km) of 4-kV and 15-kV underground distribution circuits. NSTAR has thousands of underground switches located in manholes. Access to switches is a major issue when most of the distribution equipment is located beneath busy city streets and sidewalks. Restoration efforts tend to be slower and more complicated than on a traditional overhead system. NSTAR wanted to enhance the reliability and restoration efforts of its underground circuits. These were the motivating factors to begin the process of designing, building and implementing a new underground DA system.

STARTING OBSTACLES

While eager to attack the challenges, NSTAR soon realized introducing automation to the underground would not be easy. Many obstacles were identified and had to be overcome. Some of the major hurdles had to do with the wet subsurface environment. The environment inside a manhole is less than habitable; building completely submersible RTU cabinets and controls was necessary. Additionally, finding a way to communicate with subsurface RTUs by using the existing overhead supervisory control and data acquisition (SCADA) communications infrastructure was both a priority and a challenge. Finally, making the products safe, durable and user-friendly were key initial requirements.

To start, NSTAR engineered a game plan to identify the wants and needs. Brainstorming sessions that entailed drawing on white boards were held in conference rooms. As the design unfolded, so did a list of wants and needs. Soon after, meetings were held with targeted stakeholders in engineering, operations and SCADA. Group input was gained from underground lineworkers, field supervisors, trainers and troubleshooters.

The group also visited potential automation sites on existing switch locations. Their focus was to determine the feasibility of installing products in the wet, unforgiving subsurface environment. Space was a major concern. Each location seemed to be different, which led to a design that consisted of components rather than one complete, unitized RTU and controller.

The component approach allowed for easier installation, faster repairs and more-manageable maintenance. NSTAR's collaborative approach to the design provided valuable insight from those who would be impacted most by underground DA. This approach also gained buy-in to the new concept, which made the implementation more successful.

“LOVE THAT DIRTY WATER”

“Well I love that dirty water. Oh Boston, you're my home …” is the refrain of a very popular song in Boston. When the Boston Red Sox baseball team wins a home game, The Standells' hit “Dirty Water” blares. It is one of Fenway Park's proud traditions.

The same love, however, does not apply when underground lineworkers must enter manholes. Lineworkers and underground equipment battle tidal holes near the harbor, runoff from rain and snow, heat from nearby high-pressure steam lines and everything else found on the streets of a large city. It all ends up inside the manhole as manhole muck.

The harsh New England weather and ocean-side environment means equipment submersibility is a must. The products had to meet the National Electrical Manufacturers Association 6P or IP 68 rating or greater. NSTAR engineers consulted with folks who build submarines for the U.S. Navy and used many of those vendors to ensure the utility had top-notch submersible products. Once they were identified, the initial design stage progressed to building one automation control package for a test pilot. That is when NSTAR engaged Microsol Inc. (Seymour, Connecticut, U.S.), a leader in RTU technology. Microsol's input was pivotal in the process of designing and building the pilot automation package.

CONTROLLER SCHEME

A great deal of time and effort were invested in designing the controller. Since NSTAR work rules prohibit switching while a worker is inside a manhole, the controller was designed with a cord long enough to lift the controls well outside of the manhole. This controller design was the result of a collaborative effort between engineers and lineworkers. The results have been very positive.

The controller has a switch allowing either local or remote control of the switches in the manhole. When in the remote mode, the SCADA dispatcher controls switching. Before line-workers enter a manhole with an automated switch, they are required to switch the controller to the local mode, which disables SCADA control and shifts the control to the lineworker.

The controller is designed to not be illuminated when in remote mode. Once switched to local, it illuminates and reveals the switch position (open, closed, trip). NSTAR dispatchers and lineworkers have several ways to identify which manholes have switches with DA control. The SCADA dispatch screens and maps are clearly marked to identify DA equipment. In addition, lineworkers are required to call dispatch before lifting a manhole cover. They must first log in with dispatch to gain clearance before entry. They also must obtain permission to switch from remote to local. An alarm is signaled on the SCADA screens when control shifts from remote (SCADA control) to local (lineworker control). Upon logging out of a manhole, lineworkers are required to shift control back to SCADA by switching the controller back to the remote mode.

An additional step was taken by installing a bright yellow sign on the wall of the manhole entrance that clearly explains the automation and work standard reference. Plus, it is difficult to miss the control box and the cable mounted and hanging just inside the manhole entrance. While the engineers were working on the design challenges, the design team had to address how to get an overhead radio signal to penetrate the surface and communicate with the subsurface RTU location.

HOW DID THEY DO THAT?

Many skeptics asked, “How will you get an overhead signal into a manhole?” NSTAR decided first to attempt to use the existing overhead radio and SCADA system rather than build another communication platform. Testing signal strength was critical. Signal-strength tests were conducted above and inside many manholes that housed key underground switches targeted for automation. Yankee ingenuity came into play when creating a basic testing device consisting of an antenna, a microwave-data-systems (MDS) radio from General Electric Multilin (Markham, Ontario, Canada), a battery and a laptop.

Once a location registered a reasonable signal, the manhole location and switch were added to the list of potential DA locations. Next, a test box, or test switch, was built to conduct long-term signal-strength testing. The test switch was a mock switch connected to SCADA. The inside of a submersible test box held a radio and an RTU that was programmed with a SCADA test address. The test box was polled by SCADA and monitored for weeks. The result was a 98% polling success rate.

Many obstacles were encountered while signal testing, such as the skyline, vehicle traffic, snow banks and delivery trucks parked overhead. Even NSTAR vehicles were in this last category. All the while the signal strength remained strong and consistent. Overcoming such obstacles proved that adequate signal strength below the surface was indeed attainable. This proved using the overhead SCADA radio platform would work.

SECRETS TO SUCCESS

Several primary factors led to this project's success. Not the least of these factors is slotted manhole covers. NSTAR increased its subsurface signal penetration by changing from solid manhole covers to slotted ones. NSTAR's 900-MHz radio system is also anchored by a strong signal emitted from one of the tallest buildings in the Boston skyline. The Prudential Tower, commonly known as “The Pru,” holds the main antenna for the NSTAR radio system serving Boston. It is supported by two other antennae locations surrounding Boston, providing a triangulated signal pattern that blankets the city.

If the signal strength requires a boost, NSTAR introduces an overhead repeater. Repeaters are installed on either existing streetlight poles or padmounted gear in the area near the targeted underground switch.

The signal-strength testing program took the guesswork out of installations. This program was aimed at existing switches targeted for automation to determine signal strength and reliability. NSTAR's work with Microsol also provided key inputs into signal-testing equipment selection and automation package design.

PROTOTYPE GOES UNDERGROUND

By December 2006, the first prototype was designed, built and ready for installation. NSTAR's first underground automation site was located a few streets away from Fenway Park. The existing switch was a 4-kV vacuum-tie switch between two circuits. The manhole location was a good choice and had ample room to install the DA package.

The first unit had solid covers and consisted of an RTU cabinet, battery box, control box and a termination box. The key was having components that were easy to install and connect. NSTAR installed the lead-acid battery separate from the RTU cabinet. This eliminated the possibility of electrical contacts or controls igniting any small amount of gas generated by the battery. A separate battery compartment also made it easier to install, maintain and replace batteries.

Soon after the prototype underground DA unit was installed, it was called into action. Late one night in February 2007, the dispatchers used the underground SCADA-controlled DA switch in a series of switching steps following a major outage. It was an immediate hit with the dispatchers and troubleshooters.

IMPROVED SWITCHING AND RESTORATION TIMES

Lineworker feedback has been very positive. Once a line-worker logs in with dispatch and receives permission to access and remove a manhole cover, the lineworker sees the controller hanging in or near the chimney opening. The controller is connected to the RTU by a long cord. The lineworker can easily lift the control above the manhole using a stick or gloved hand. Once above the hole, the lineworker switches the controller from remote to local. Once in local mode, the controls disable SCADA control and the position of the switch is displayed. Knowing the switch position is a huge benefit for both safety and restoration efforts.

Without automation, the lineworker would have to enter the hole to identify the switch position. Oftentimes, the manhole would be filled with water and the switch submerged. Valuable time and effort were spent pumping down manholes to access switches. By adding automation to the switch, the switch position is displayed on the controller and on the SCADA screens, proving to be valuable when trying to restore power or when switching is necessary.

DESIGN EVOLUTION

Since the first prototype, NSTAR's underground automation package has gone through some significant design improvements based on experience with the pilot package. One immediate change was the use of see-through hinged covers on the submersible containers. Seeing inside without having to remove covers, while standing inside a nasty, wet environment, was a big help. It makes installation, acceptance testing and troubleshooting a breeze. All of the connectors were keyed and sized differently to ensure they are installed in the correct location. The mounting brackets were increased in size and changed to a keyhole mount for easier installation. This also made it easy to remove components for repair or maintenance. Since the first installation, unknown issues have been encountered several times during acceptance testing. In these cases, lineworkers remove the quick disconnects, bring the RTU back to the radio room and diagnose the problem. Then the RTU is easily and quickly reinstalled before completing the acceptance test.

One of the greatest design enhancements has been to the handheld controller. It became more durable, easier to read and universal. To become universal, the controls were redesigned to accept switches from more than one manufacturer. The goal was to have one control device to control switches made by several manufacturers.

Overall, the underground DA control package design continues to be scrutinized and evolves as better ways are found to handle underground automation.

WHAT IS NEXT?

NSTAR installed its first underground DA control package in December 2006. Since then, the utility has been on a DA roll. In 2007, NSTAR's goal was to design, build, install and perform acceptance tests on seven additional underground DA control packages. However, before rolling out the new DA line, many behind-the-scene tasks had to be accomplished first. NSTAR redesigned and modified its SCADA screens to accept underground DA. Design, construction and work method standards were written. And, all stakeholders were trained. Dispatch procedures and acceptances test procedures were created.

Early 2007 was very busy with new product training and new product rollout activity. Once the benefits were explained and became apparent, underground lineworkers and troubleshooters warmed to the concept of underground DA. In 2008, NSTAR installed an additional 18 underground DA units, and the target for 2009 could exceed 20 units.

Underground DA is here to stay. The benefits and enhancements impact reliability and outage restoration efforts, as well as improve the safety and performance of underground workers. The sky is the limit in Boston's underground.

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Gavin Curley (gavin.curley@nstar.com) is the lead reliability engineer for NSTAR Electric & Gas. He currently handles system reliability and underground automation. With NSTAR, Curley has been involved in overhead circuit design and performance, root-cause failure analysis, project management, and distribution design and construction standards. Previously, he worked with Florida Power Corp. He has BSEE and MBA degrees and more than 20 years of utility experience.

John A. Kingston (john.kingston@nstar.com) is a lead engineer with NSTAR Electric & Gas. With NSTAR for 21 years, Kingston's experience ranges from overhead line supervisor to underground line supervisor to lead engineer in Distribution Technical. Kingston holds a BSME degree from Wentworth Institute of Technology and a MBA degree from Northeastern University.