PECO installation enhances metering and control of distribution feeders in the literature of the electric utility industry, there is nowhere a more romantic and mysterious designation than "silent sentinels" to scribe protective relays. When the phrase was coined at Westinghouse in the 1940s, the new generation of relay was being designed to provide protection using its time-current characteristic. This characteristic was much heralded because it allowed the protection engineer to set-in the current lexicon, to program-the relay to operate faster for primary faults and slower for secondary faults. It was thus possible for each relay to back up other relays on the system, which gave rise to the notion of primary and secondary zones of protection. If the relays did not actually communicate with each other, they at least stood at the ready to help if a primary relay failed, representing a kind of static, non-verbal form of communication.

With the development of intelligent electronic devices, the relay was able to not only sense a fault, but to pinpoint its location. In the evolutionary cycle so common to utility engineering, the new devices were soon designed to provide monitoring and control as they engaged in authentic communications within the system: one substation could now "talk" to another. It was within this framework that PECO Energy, seeking to enhance the flexibility of its control system, initiated a monitoring and control system for its distribution circuits, which were rated at 34, 13, 4 and 2.4 kV.

The system, which came on line in 1997, had been developed by ABB to control distribution feeders and automatic reclosers that sectionalize the lines. During the development phase of the project, a study was made to determine the most beneficial and least costly method to gather field information that would be used by DMACS. Having had experience with supervisory control and data acquisition (SCADA) applications on its transmission system, along with a few pilot projects involving SCADA with distribution, PECO was familiar with the use of remote terminal units (RTUs) that were part of these SCADA systems. Since the RTU required direct wiring to all monitored data points, significant time and cost were required to install this wiring. In a typical 13-kV feeder compartment, for example, an average of 26 individual control and metering wires would have to be run from the feeder compartment to the RTU. Motor operators would be added to the SB-1 control switches and analog transducers would be installed to acquire amp, watt and VAR values on each feeder. With a bus of six compartments, all of these requirements amounted to a significant installation. The SCADA control philosophy called for both automatic and manual control, requiring that costly motor operators be added to the existing control points for breaker operation, fast trip and auto reclose.

Defining the System PECO wanted an alternative to the existing protocol that could interface with the DMACS, while still providing the required features and performance of RTU-based SCADA. The various automation systems that were offered varied from a complex mini computer supervisory system to PC-based process control schemes. Evaluating the proposals that had been received from several companies, PECO developed the criteria that would define the future system:

- There would be no single point of failure that would result in a loss of protection on substation equipment. - The speed and throughput of the system was specified such that monitoring and control would be maintained. - The system would have to process and confirm a remote breaker control operation within 2 seconds. - All metering and status information would have to be sampled, processed and reported within 10 seconds.

Since the goal was to provide control and metering through both the DMACS master system and a local graphical user interface, PECO was able to reduce SCADA costs further by relaxing the requirement of motor-operated SB-1 control switches. In the end, these switches would not be needed at all. Primary local control would be implemented via the substation computer, and backup local control would be implemented via the relay front panel. The proposal called for the demolition of the existing relays, controls and wiring. A big concern among station operators regarding the retirement and removal of breaker control handles was alleviated after the operators had the opportunity for training and practice on the new system.

Equipment Monitoring At the time that the RTU alternatives were being evaluated, PECO was in the process of completing a major review of its maintenance programs. During this review, reliability centered maintenance principles were applied to optimize inspection and maintenance intervals. This optimization process required that information be available on a continual basis regarding the condition of specific equipment. This need for real-time-condition data on substation equipment was identified as a key element to reduce substation maintenance costs. Existing inspection methods were limited and could only obtain information that was related to a specific point in time. By continually monitoring the condition of a particular device, PECO can get a clearer picture of the device's performance. In addition, visual inspection has been shown to actually mask a developing problem. Conversely, a real-time system is capable of identifying, and thereby slowing down, deteriorating performance by predicting a pending failure. For example, the maintenance that is performed on transformer oil systems would benefit from this kind of monitoring since the practice of routinely breaking systems down for inspection can actually introduce leaks into the system. A real-time-condition monitoring system can prevent unnecessary maintenance by extending the time before invasive inspection is required.

PECO has had good success with programmable logic controllers (PLC) as field data logging devices, which are capable of gathering data from several locations. When employed as a transformer monitor, the PLC can record data for tap oil temperature, tap changer temperature, tap position, transformer load, cooling system status and nitrogen pressures. From these parameters, additional information can be calculated for cooling system efficiency and tank versus tap changer temperature differentials. In addition, alarm points warning of developing failures may be generated from the data.

When PECO has used the PLC as a circuit breaker monitor, the utility has employed sensors to gather data for air and gas pressures, temperature, compressor status and breaker position. As with the transformer monitor, the breaker monitor calculates additional data points from the sampled data, which include compressor efficiency, gas-leak detection and gas density. It is also possible to create alarm conditions/triggers for any of the data points. Currently, this technology is being used to gather data from transformers and circuit breakers to supplement existing SCADA alarms.

The PECO SI System A request for proposals was prepared that contained both protection and monitoring requirements for vendors who would be responsible for a turnkey installation at 87 substations. The substation integration (SI) system that was finally accepted was multifaceted, which not only provided monitoring and control for distribution-class switchgear but also provided condition monitoring on 69-kV to 500-kV transformers and breakers.

PECO ultimately decided to contract with a general system integrator to provide the SI system with the understanding that the contractor would have the freedom to choose "best in class" when selecting equipment and components for installation. Automated Control Concepts, Inc. (ACC) of Neptune, New Jersey, U.S., was selected as the bidder best suited to provide an overall balanced system.

The distribution protection system is comprised of relays and communications processors provided by Schweitzer Engineering Laboratories, Inc. of Pullman, Washington, U.S. The protection scheme is based on a unique combination of redundant relays that provide numerous enhanced protection features while still meeting the cost constraints for the project. The primary relay is the SEL-251, which is an intelligent microprocessor-based relay that is a direct replacement for the previously installed CO-8 electromechanical relays. Since the SEL-251 is a three-phase relay, the three single phase and ground CO-8 relays are removed during the SI conversion process. One of the features of a digital relay is event reporting, which gives detailed information on the feeder conditions that initiated the circuit breaker trip. The backup relay is the SEL-501-2, which is also a microprocessor-based relay that provides three-phase and ground overcurrent protection. Despite its limited functionality compared to the SEL-251, the SEL-501-2 has on-board controls that enable the operator to trip or close a breaker directly from the relay, which compensates for the lack of control on the primary relay.

There are actually two over current relays contained with the SEL-501-2 package, typically labeled as the X and Y units. This feature allowed PECO to reduce the number of relays on a feeder compartment from two to one and a half, where two adjacent compartments share a single backup relay. The primary and backup relays work in tandem to protect a single feeder with both relays sensing the same current during a fault. However, a small delay has been introduced into the backup relay's trip action to allow the primary relays to react first and to minimize confusion when reconstructing the circumstances that caused the relay operation.

To provide for SCADA communications, all of the relays are wired to SEL-2020 communication processors. To ensure redundancy, there are actually several sets of communication processors that make up the "lower tier" and the "upper tier." The lower tier concentrates the raw data from the relays for delivery to the upper tier, which gathers the data and transmits them to the host SCADA systems. Although this system of upper and lower tier communication processors might appear to be unnecessarily complex, it provides the required redundancy while still being able to meet the criteria governing the speed of communications. In the case of connections between the relays and the communication processors, the primary and backup relays speak to two separate devices, ensuring that redundancy is maintained and, also, that the requirement is met for no single point of failure.

A local computer, the SI Controller (Fig. 3), acts as the graphical user interface for local substation control. This computer employs Wonderware Intouch as the supervisory control software. Graphic screens, which mimic the controls and meters, display the current status of the substation. An "office" grade computer was selected for this application rather than an expensive hardened industrial computer because the computer is not a critical element of the protection equipment. Experience has demonstrated that this grade of computer is acceptable for such an application. The computer is mounted in a dedicated SI Controller cabinet with a high-volume fan installed to provide cooling for the electronic components. In a particularly hot substation, an air conditioning unit may be added to the Controller. Since all normal substation operations are made via the DMAC's SCADA master system, the substation computer is provided as a backup control point to be used when the master system is unavailable.

With the PLCs as the backbone of the equipment-monitoring portion of this project, PECO packaged the PLC and the associated appurtenances in a weatherproof NEMA 4X-type enclosure, which was used for both the transformer and circuit breaker monitor. The software configuration determines the specific personality of the unit. The equipment monitors are easily retrofitted to the existing substation equipment with most of the sensors being of the type that either clip on or are surface mounted. There have been no equipment outages required for the monitor and sensor installations, which were further simplified through the use of spread-spectrum radio modems to communicate to the computer in the substation control house. Since there was no need to trench new communication cables or fiber optics to the equipment, the method helped substantially reduce installation costs over traditional monitoring methods.

Each of the equipment monitors is configured to sample data in 15-minute intervals for transmittal to the local SI Controller where additional processing and alarming is performed. The data are then packaged after 24 hours and made available for retrieval by a central computer.

The data that are acquired via the equipment monitors are retrieved nightly from each substation for storage in a large centralized database. Numerous analyses and reporting functions occur at this computer, which include a general reporting of all points that have exceeded pre-established criteria. Also reported are analog data trending and comparisons of like equipment for different substations. This reporting function is a major component in PECO's maintenance programs.

The entire SI system has been designed to treat each component as a module or an object. When a design for a new substation is initiated, the quantities of relays, communications and the equipment are identified. The corresponding software and hardware modules are then integrated to create the complete system; a process that can be compared to an a la carte menu. The first phase of the SI system installation for seven substations was completed in July 1998. The next group of six substations is expected to be completed by the end of 1998.

Conclusions The PECO SI system has provided a mechanism to improve reliability and quality of service. The new SCADA and protective relay functions provide critical data to system operators who use the DMACS for all routine substation operations. The data collected by the equipment monitors provide key information on the daily performance of the substation equipment

Glenn A. Pritchard graduated from Clemson University in 1990 with a BSEE and is completing the requirements for an MSEE at Penn State University. He is a registered professional engineer in Pennsylvania. He has been with PECO Energy for seven years, working in the Engineering Services Department of the Power Delivery Business Unit. Pritchard specializes in finding new applications for existing and developing technologies.