From 2010 to 2015, Indianapolis Power & Light (IPL) experienced an increase in secondary network events, resulting in cable fires and dislodged manhole covers. After conducting root cause analyses, the utility determined one of the main contributing factors to the increase in events was long-term exposure to excessive heat from the steam distribution system.
To help ensure the safety and reliability of the secondary network, the IPL engineering team was challenged to come up with a viable system that would proactively monitor the utility’s infrastructure from the threats posed by the steam distribution system.
Downtown Indianapolis, Indiana, is home to one of the largest steam distribution systems in the U.S. Unfortunately, the steam infrastructure is often located near IPL’s underground infrastructure and power cables. Because of the corrosive nature of steam and the age of the steam distribution system, many steam leaks have occurred over time. The affect steam leaks have on IPL’s underground infrastructure and cables can be damaging. The corrosive nature of steam prematurely ages cable and degrades the outer jacket. In addition, steam can deform conduit if it is exposed long enough to the leaks.
Traditionally, steam leaks in Indianapolis were identified by the steam company during its annual thermal survey, in which a thermal scan for hot spots was conducted on the downtown streets. Additionally, IPL crews would report any heat issues encountered during routine inspections. Between IPL’s inspections and the steam company’s thermal scans, heat issues could go undetected for a year or more. This process was not enough for IPL to proactively address the steam issues.
To become more proactive, IPL initially attempted to use a thermocouple attached to a flexible fiberglass rod for obtaining temperature readings deep in the duct banks during routine inspections. However, this proved not to be a viable solution. It was a time-consuming process and only provided a temperature reading at the time of the inspection. Any heat issues after an inspection would go undetected until the next inspection cycle. Because of the limitations of using the thermocouple in this manner, the IPL engineering team needed to devise a more sophisticated solution to actively monitor the utility’s infrastructure in real time.
In the later part of 2015, the IPL engineering team started exploring distributed temperature sensing (DTS) to address the steam issues. DTS systems have been used since the 1980s, primarily in the oil and gas pipeline industry, to detect leakage. DTS systems also have been used for dynamic cable rating of high-voltage transmission lines. If the DTS system proved successful for IPL’s application, IPL would be the first electrical utility to use a DTS system to monitor external temperatures in a network secondary grid.
DTS System Overview
A typical DTS system contains three components: a DTS controller, a standard multimode fiber-optic cable and a server to store the data. The DTS controller turns the fiber-optic cable into a linear sensor that has a spatial resolution of approximately 3.28-ft (1-m) increments with an accuracy of ±1.8°F (±1°C). To get the equivalent coverage, a thermocouple or thermal sensor would need to be installed about every meter, thus requiring thousands of sensors.
How the DTS controller measures the temperature is based on the principle of Raman scattering. Thermal effects induce lattice oscillations in the glass fibers, which, in turn, cause light to be scattered back to the DTS controller. The DTS controller interprets two components of the backscatter, stokes and anti-stokes, to determine the temperature at any given point along the fiber. The time it takes for the backscatter to reach the DTS controller is used to calculate the precise location of the thermal event.
In 2016, IPL partnered with Fiber Optic Pipeline Solutions (FiOPS) to devise an innovative system using DTS technology that would enable IPL to proactively monitor external high-temperature threats from the steam distribution system in real time. By June 2016, IPL and FiOPS installed and commissioned a DTS pilot to evaluate the effectiveness of the system.
The DTS pilot consisted of one DTS controller and approximately 7200 ft (2.2 km) of fiber-optic cable. For IPL’s application, the DTS controller has a maximum range of 19,685 ft (6 km) and can accommodate up to eight fiber-optic routes. The fiber-optic cable used in IPL’s pilot route consisted of six single-mode fibers and six multimode fibers with an outer jacket rated for 185°F (85°C). Because the fiber-optic cable contains both single-mode and multimode fibers, IPL has flexibility to use the fiber-optic cable for other purposes in addition to the DTS system.
For IPL’s application, the fiber-optic cable was installed in empty conduits without using innerduct. When permissible, the fiber-optic cable was installed in the conduit closest to the steam system. For a DTS system, the fiber-optic routes can be radial, can be looped back to the controller, or each end can be connected to a separate controller for maximum redundancy. For the pilot route, IPL chose to run the route radially, which the utility later learned was not a best practice.
Once the route was commissioned and on-line, the system started producing promising results almost immediately. A handful of hot spots were detected, some of which IPL and the steam company were not aware. In addition, within the first two weeks, the DTS system picked up the beginnings of a steam leak. By the end of July 2016, the temperature in this area had risen to more than 135F° (75.2C°) and was climbing steadily. The DTS system alerted IPL to the situation so the utility and steam company could address the issue in a timely manner before any damage was done to IPL’s infrastructure and cables.
In less than a month after seeing the promising results of DTS, IPL’s senior management gave the engineering team the go-ahead to install a DTS system throughout the secondary network to monitor threats from the steam system. As of December 2017, IPL has installed four radial fiber-optic routes consisting of approximately 37,000 ft (11,277 m), which is currently monitoring about 50% of the steam system as it relates to IPL’s infrastructure. In addition, IPL installed a second DTS controller at a substation near the downtown secondary grid for redundancy purposes. Data from the two DTS controllers is stored on two servers at separate locations.
Additional Use for DTS
On June 23, 2017, IPL experienced a secondary cable fire, which the DTS system was able to detect. The fire occurred in two manholes where the fiber-optic cable was installed. The DTS system captured the sudden increase in temperature. Unfortunately, the fiber-optic cable failed the following day, leaving the remainder of the route in the dark.
To compound the issue, IPL was working with the steam company on a couple of steam leaks toward the end of the route that had been damaged. Because of the damage, IPL was not able to give the steam company any updates on the temperature profile of the affected area, to confirm the steam company’s repairs had addressed the leaks.
Starting in the first quarter of 2018, IPL plans to install an additional four to six fiber-optic routes. In addition, because of the fire on June 23, the utility plans to turn the existing radial routes into a loop configuration or terminate both ends at each controller for redundancy purposes. Once the steam system has been covered, IPL will install additional fiber-optic cable to help monitor the secondary network and transformer vaults for thermal events not related to the steam distribution system.
The DTS system has proven to be a very useful tool for IPL to monitor its secondary grid proactively from thermal threats. Addressing thermal issues in a timely manner not only greatly reduces the damage to IPL’s infrastructure and cables but also the number of cable fires and dislodged manhole covers. ♦
Ken Jenkins is currently an engineer on the major underground projects team at Indianapolis Power & Light Co. His primary focus is designing and maintaining the downtown distribution network in Indianapolis, Indiana, and other large projects within the greater Indianapolis area. Jenkins received a BSEE degree from Indiana University-Purdue University Indianapolis in 2010. Prior to joining the major underground projects team in 2010, he was part of the IPL standards and code compliance team for two years.