More and more utilities are implementing monitoring systems that can track power-quality conditions at substations and on distribution systems. These monitoring systems are integrating information from a variety of different intelligent electronics devices (IEDs) — power-quality monitors, SCADA RTUs, digital fault recorders, intelligent relays and reclosers, advanced metering and Power Factor Correction Control — and may be located at substations, on the distribution system or at customer service entrance locations. The result is an increasing amount of data describing the power system performance.

One of the most valuable applications of this monitoring information is intelligent analysis of fault conditions to help improve system reliability. This article focuses on some important applications for distribution monitoring that can improve reliability by better identifying and responding to faults. Several utilities are actively implementing these intelligent functions, and an Electric Power Research Institute (EPRI; Palo Alto, California, U.S.) initiative is organizing research efforts to develop a specification for fault analysis in power-quality monitoring systems.

Intelligent Applications

Most monitoring systems are used in a reactive mode. Monitoring results are used to review disturbance characteristics after a problem has occurred, or they are used to provide summaries of system conditions, such as power-quality and reliability indices. However, there are tremendous opportunities to use these monitoring systems in a proactive mode to actually improve system performance and identify problems before they result in equipment failures or extended outages.

Proactive applications try to derive important information from the vast amount of data collected in monitoring systems. There are many data-processing techniques that can be part of the information-development process. These can include rules-based expert systems, signal-processing techniques, inference engines and fuzzy logic systems. Usually a given objective will require a combination of these different technologies to be effective. This article concentrates on processing requirements that may be applied to a variety of different fault analysis applications. Understanding faults is the key to improving reliability.

Benchmarking

Benchmarking is a traditional application for monitoring systems, but it can also be a valuable tool in identifying areas that need attention. Objectives for system performance will depend on many parameters of the particular system:

• Urban versus rural systems

• Underground versus overhead systems

• Effect of different system topologies

• Effect of different voltage levels, source strength and number of circuits from a common bus

• Effect of lightning activity and other causes of system disturbances

• Effect of investment in maintenance and equipment.

Many other parameters may also be important. As the expected performance of the supply system is better understood, standards for performance can be developed (objectives specific to individual systems) and the actual performance information can be used to prioritize maintenance and expenditures on system performance improvement technologies.

United Illuminating, for instance, monitors all distribution substations and tracks voltage-sag performance on a monthly basis. Figure 1 illustrates a chart that is used within the company to track performance at individual substations. The chart shows voltage-sag performance at each substation over the past five years compared with performance in the last year. It also divides sags into events that are caused by transmission faults and by distribution faults. This helps focus the areas where performance improvement can be expected with maintenance or investments in the distribution system. This is only one example of using monitoring results and benchmarking effectively.

Fault Analysis Applications

Monitoring systems can actually help improve performance as well, if they can be used to reduce the number of faults or help respond to outages faster. This is the next generation of monitoring system applications. Some important applications include:

• Evaluation of protective device performance and identification of coordination issues

• Identification of recurrent fault conditions in the same location

• Fault location on radial distribution systems using fault-current characteristics and system electrical characteristics

• Identification of faults associated with galloping conductors

• Identification of incipient faults associated with cable splices and arresters

• Identification of fault causes — lightning versus trees versus animals — to help prioritize performance improvement strategies.

Fault Characteristics

Distribution system faults can have a variety of different characteristics. Faults can change characteristics during the period of the fault, such as changing from a phase-to-phase fault to a three-phase fault. The characteristics will depend on the fault location (equipment affected), the cause of the fault and the system characteristics. Significant research is underway to use the information in the fault waveforms to help understand what caused the fault and how equipment responded to it.

Figure 3 gives some examples of different fault conditions. Note that the most information about a fault is available in the current waveforms. Many power-quality monitoring systems focus on characterizing the voltages (voltage sags), but for most intelligent applications, the current waveforms and characteristics are highly important.

Evaluation of Protective Device Operation

The operation of protective devices to clear the fault can be seen clearly in the fault-current waveforms (Fig. 3). Evaluating the proper operation of protection equipment and identifying coordination problems can be a very important application of monitoring systems. Reclosers are used on most overhead distribution systems, so that temporary faults do not result in extended outages for customers. Multiple reclosing operations are often used to provide the best chance of clearing the fault condition automatically and for coordination with downline protective devices. The monitoring system can provide a performance check on the proper operation of reclosers for every fault condition and can flag abnormal conditions that require attention.

Checking for coordination problems can be a particularly valuable application of the monitoring system. Figure 2 illustrates an event experienced by a customer with a power-quality monitoring system. This case illustrates a lack of coordination between a substation breaker and a downline fuse. The branch fuse operates to clear the fault in a little more than one cycle (see the voltage waveforms in Fig. 2a). However, the substation breaker opens unnecessarily after three cycles because the trip setting was not coordinated properly with the fuse time-current curve. The result was an unnecessary momentary interruption for all the customers on the circuit (see the RMS voltage characteristic in Fig. 2b).

Locating Faults

Automatic fault location can be one of the most valuable applications of a monitoring system. Many other applications also can take advantage of the fault location function. There has been tremendous research on fault location algorithms, and recent applications have been successful at accurately calculating fault locations from fault-current characteristics and electrical circuit characteristics on radial distribution systems.

Tremendous benefits are possible if the monitoring system can identify the fault location and the information is available in real time over the Internet or a corporate intranet. Crews can be dispatched to the fault location much more efficiently, reducing system restoration times and improving the reliability of the overall system. Figure 4 illustrates the implementation of an automatic fault location capability within one power-quality monitoring system.

Of course, distribution systems are not just one radial circuit. There are usually many branches and fault location estimates will result in a number of possible fault locations, as indicated on the one-line diagram in Fig. 5. This illustrates the importance of integrating the fault location function with GIS systems to illustrate the possible locations and also with outage management systems that will provide information from customer calls about the section of the circuit that is out of service. Progress Energy Corp. (Raleigh, North Carolina, U.S.) has implemented such an integrated system and can often send a crew directly to the fault location.

Identifying Problem Conditions

Once the fault location algorithm has been implemented, many valuable capabilities can take advantage of this function. One important application is identifying locations on the system where there are recurrent fault problems, indicating a condition that might need attention, such as a tree problem, insulator issue or equipment malfunction. For instance, the system can check all fault conditions and alarm the company whenever there are multiple events within a specified period of time at approximately the same location. In Fig. 6, the spreadsheet summarizes the faults at one substation and illustrates that there were three faults in a one-month period that were all C-phase to ground and were calculated to be 5.2 miles to 5.5 miles (8.3 km to 8.8 km) from the substation. Even if these were only temporary faults, it indicates a possible problem that might be resolved with an investigation.

Another problem that can take advantage of fault location is “galloping conductors.” This problem occurs when there is a fault on a distribution system and the electromagnetic fields from that fault current cause the conductors somewhere closer to the substation to swing and possibly bring about a second fault after the first fault is cleared. This is a condition where spacers might be required or the conductor sag needs to be reduced to prevent the problem. The monitoring system can identify this multiple fault event with a combination of the timing and fault location characteristics.

Incipient Faults

It's great to identify the location of a fault that has caused a circuit interruption or a customer outage. It's even better if we can identify conditions that might result in a fault some time in the future and prevent that outage from occurring in the first place. There has been a lot of work to characterize incipient faults, and there are even relays that can trigger on incipient fault conditions, such as the voltage and current waveforms in Fig. 7. These are typical characteristics that can occur when moisture gets in a cable splice or an arrester resulting in a short self-clearing fault. Eventually, these conditions cause failure of the arrester or the splice and a fault condition that will result in an outage to customers. Detecting these conditions can allow a problem to be fixed before an outage actually occurs.

Summary

The result of all these advanced functions can be significantly improved reliability. Figure 8 illustrates the improvement in the Customer Average Interruption Duration Index (CAIDI) that has been achieved by Progress Energy since the implementation of feeder monitoring throughout its system. One of the main reasons for this improvement in CAIDI was faster response to faults, as a result of the fault location functionality. Progress Energy continues to identify new applications for the feeder monitoring system, and many other utilities are starting to adopt these advanced systems. The EPRI research in 2005 also will help identify applications that have the most promise for reliability improvement.

Mark McGranaghan is vice president of Consulting Services at EPRI Solutions, responsible for offering services to electric utilities and critical industrial facilities. These services include research projects, seminars, monitoring services, power system analysis, testing, failure analysis and design solutions. McGranaghan holds MSEE and MBA degrees. mmcgranaghan@eprisolutions.com

Scott Peele is a lead engineer for Progress Energy Corp., responsible for power-quality issues in the commercial, industrial and governmental section. A registered professional engineer in North Carolina, he has been involved with power quality for the last 19 years. Peele is a member of IEEE and has led and been an active member of various power-quality committees or working groups. His current work involves evaluating power-quality problems on electrical distributions system from 500 kV to 120 V. scott.peele@pgnmail.com

Marek G. Waclawiak is a system integrity, R&D team leader with the United Illuminating Co. in New Haven, Connecticut, U.S. He joined the company in 1985 after four years of distribution network design experience at the City Public Service of San Antonio in Texas, U.S. He is responsible for reliability, power quality, and distribution planning and standards. Waclawiak received the MSEE in 1976, and is a registered professional engineer and a member of the IEEE Distribution Voltage Quality Working Group. marek.waclawiak@uinet.com