By Mark Faulkner, Eaton Cutler-Hammer, and Roy Middleton, American Electric Power
Beneath our feet, under the concrete and side streets of most major cities, there exists a myriad of cables and power equipment supporting AC power distribution systems. This distribution network consists of interlaced loops or grids supplied by two or more power sources so that the loss of any one source does not interrupt power to the load. This redundant power delivery system provides the highest possible level of reliability over all other types of distribution systems.
Because of the unique features of underground network systems-such as the subterranean environment, the always-energized secondary side and the enclosed spaces of the equipment-unique demands and specialized training are required for the utility. This article explores the current practice of vault visits vs. an external communication method and the benefits, savings and technology behind this innovation.
Vaults and Safety
Maintenance of a network system requires routine operational switching of the network circuits. A common procedure many utilities follow before opening a network circuit is to check the status of spot network protectors. With increased security concerns today, access to many buildings and vaults is becoming more difficult. It is not uncommon to wait hours for a building security officer to arrive and escort a network mechanic to the building vault. Remote status checks of protectors can accelerate this process.
A recommended safety practice that many utilities follow is to switch network equipment from outside the vault. Some utilities rope the network protector handle to the “Open” position while standing on the sidewalk out of harm’s way. Other utilities open a network protector by opening the associated substation breaker and letting the protector open due to the relay’s automatic reaction. Doing so, however, requires unnecessary operation for the remaining protectors on the same circuit and additional time for the substation switching procedures. This practice often creates additional work for mechanics who have to visit the vaults on the opened feeder to ensure network protector status. The mechanics then enter the various vaults to manually verify the protector position.
There are obvious potential hazards when entering an enclosed space with the possibility of electrical fault or failure. To avoid having to send utility personnel into vaults, remote hard-wired monitoring systems have been developed. However, even though the advantages of communications and monitoring systems are enormous, history has shown that hard-wired systems are economically out of reach for smaller utilities due to installation and system complexities.
Throughout the 80-plus years since the introduction of AC distribution networks, the most common method of monitoring network protectors has been to “go down and check it out.” To do that, several steps must be taken in advance to prepare for a vault visit. Figure 1 lists the most common preparations.
Some of the activities listed in Figure 1 would result in a congested sidewalk and could present pedestrian and utility worker hazards. Most utilities presently send two- or three-man crews to inspect a network protector. The cost of this labor-intensive method often drives utilities to limit vault visits, denying them the benefits of frequent network data collection and maintenance activities. The typical set-up time for site preparation is approximately four to six hours per site. If a wireless solution is employed, vault preparation is not needed and the network protector status and data collection can be acquired from a utility vehicle parked nearby.
Generally, when a mechanic or engineer enters a vault, he or she must undergo a sequence of events, listed in Figure 2. Typically, the most common activities for routine check-ups are collecting data, checking network protector status, and modifying/verifying relay set points. Reducing or eliminating many of the time-intensive tasks required of these personnel enables them to be allocated to more pressing issues.
Today, there is a better way to keep tabs on the network and improve the entire process. The wireless method allows instant monitoring on the network without having to spend the time to enter enclosed spaces, eliminating the labor issues that arise and the potential for personal injury.
Wireless Solutions To Traditional Methods
The need for accurate information from data collection is essential in maintaining and making accurate load flow studies to meet the needs of existing and future customers. However, the network subterranean environment is not a friendly place to collect profile data. Eliminating safety obstacles such as ladders, vault grates and water accumulation on the vault floor is just one benefit of the wireless tool.
Wireless systems are based on the application of intelligent relays in conjunction with a wireless module-operating box inside the vault, which transmits data, status and control through radio frequency technology to a laptop computer.
The wireless system from Eaton Cutler-Hammer consists of a communicating master relay (called the MPCV) daisy chained to a wireless module in the vault. The wireless signal can be received up to 130 feet outside the vault by a laptop computer and Powernet software.
Wireless systems display the data that the relay is seeing and provide better accuracy than many external device measurements. Additionally, remote sensors can be added inside the vault to alert users of existing conditions. Standard monitoring parameters and sensor data are described in Figure 3.
By continually monitoring information from the relays, maintenance crews will have the information they need to conduct performance-based maintenance. They will be able to focus their attention on known problems, which will increase the efficiency of smaller utility maintenance staffs, thereby making better economical sense. The resources will be focused on performing needed maintenance tasks rather than random, routine inspections.
American Electric Power’s Experience
In 2004, Eaton joined with American Electric Power (AEP) of Columbus, Ohio, for its initial wireless pilot project. As with all utilities, AEP was looking for ways to reduce cost while improving system performance and safety.
The Eaton communication system relies on the Industrial Communications chip (INCOM) that resides in the current MPCV relay. This integrated monitoring protection and control communications solution was developed specifically for power distribution and industrial applications and has been successfully implemented and used since 1996. The INCOM chip employs Frequency Shift Key (FSK) technology and features the benefit of daisy chaining devices with inexpensive shielded twisted pair cable.
Eaton’s communications master relay, the MPCV, is currently transmitting wirelessly out of the vault approximately 130 feet from the module. The AEP crew can monitor, calibrate, and control the network protector while in the utility van from street side.
The Eaton wireless system presented several benefits to the utility. It was easy to install, and its ability to daisy chain up to 10 MPCVs allowed AEP to position the unit in the most advantageous position for maximum wireless transmission.
The actual wireless module, WBT-MINT, was easy to install because the system only uses two wires for communication. These wires are brought into the network protector via a watertight epoxy connector and are connected directly to the MPCV.
The wireless module had an immediate payoff when installed at an AEP location in South Bend, Ind., where two spot networks were monitored. Five network protectors were involved. One network protector on the system had previously been diagnosed with an inability to automatically open when the substation breaker was opened. AEP had been unable to determine the root cause. However with the module installed, data acquisition and calibration screens could easily be accessed from street side.
Upon review of the relay settings, it was discovered that a non-sensitive relay curve was actively causing the network protector’s non-response upon light reverse current. The wireless system made it easy to review and change set point data without the large time investment required the traditional way. The relay was re-calibrated and, during the next trip check, the network protector responded accordingly.
This was not the only benefit realized at this location. At the same spot, abnormally high network voltages were displayed on one network protector. In this situation, the crew had to enter the vault to verify and investigate the problem. The investigation revealed a loose ground connection inside the breaker. Finding and diagnosing a problem such as this would have been difficult without a monitoring system and the ability of the crews to check on the location. Had this problem gone unnoticed, it would have resulted in improper operation of the protector that could affect service reliability.
The savings of the wireless method is evident when compared to traditional crew methods. Figure 4 reflects typical savings.
In conclusion, a wireless communication system creates a safe environment and reduces the overall operating expense of network protector maintenance, while also providing users with more data to make performance-based maintenance decisions. For those utilities with an existing SCADA or centralized communication station, the ability to add remote access for crews at a site adds an additional dimension of safety, assurance and man-hour savings. à¢®à¢®
Mark Faulkner is network protectors product manager for Eaton Cutler-Hammer.
Roy Middleton is network operations coordinator for American Electric Power.