Jim Burke, ABB-Electric
For many utilities, deregulation has resulted in reduced manpower, less experienced manpower, lower maintenance budgets, lower capital budgets and less conservative loading practices. All of these changes will most likely negatively impact distribution system reliability. The ability of the customer to monitor their power quality (PQ) cheaply and easily introduces a very important incentive to the utility to make sure that the reliability and PQ values they report to commissions and customers are correct.
Historically, if a utility wanted better reliability it would design a better system, purchase better products and more conservatively rate the components. In some cases, these changes had a major effect, and in other cases, very little. In all instances, it meant a considerable increase in cost.
Deregulation has meant a severe curtailing of capital investments at many utilities and has been accompanied by customers’ demands for better reliability and PQ. One approach, which allows utilities to compensate for the reduced reliability caused by restricted capital outlays, is the use of distribution software. Software available today allows utilities to reduce outage time, place protective equipment to reduce interruptions and design systems that are inherently more reliable at virtually no difference in cost.
Outage management systems (OMS)
Sophisticated outage management systems (e.g., CADOPS) in use today allow the utility to restore service to the customer more quickly and efficiently than in years past. Another benefit to these systems is that they allow the utility to obtain more accurate data related to the outage and also use this data to perform predictive reliability studies. Data is entered immediately as each outage occurs. This data can be tailored by the utility to keep track of various types of equipment, the manufacturer of the equipment, causes of outages, weather, crew size, etc. Individual feeder histories can be maintained allowing the utility to compare the performance of similar or different design and configuration.
Predictive reliability analysis
Predictive reliability software offers the utility a very powerful tool for analyzing system existing reliability problems and making plans for the future. This software (e.g., Relinet) is capable of performing accurate analysis on large systems to allow utility engineers to pinpoint where the problem is and then determine what should be done to obtain the best possible increase in reliability at the lowest cost. The biggest problem with these studies is the determination of failure rates. Failure rates are generally obtained from various industry publications.
There are many problems for any utility using data they did not personally develop. Some of the concerns with universal data rates are:
- Over how many years was the data developed?
- Is the equipment failure rate data skewed by preferences for one or more particular manufacturers of that equipment?
- How much does geography have to do with the failure rates (lightning, etc.)?
- Do equipment failure rates include failures resulting from poor installations?
- What does the failure rate include? For example, does a cable failure rate include dig-ins, animals, etc.?
- Is age a factor for these failure rates?
- Is the failure rate different for different levels of voltage?
- Is overload an important factor in these failure rates?
- Are restoration times for storm or non-storm outages?
- How large a crew was used for each type of restoration?
- What percent of customers were restored prior to complete restoration?
Blindly using failure rates produced by others can create solutions where there is no problem and vice versa. In these days of performance-based rates (PBR) as well as budget constrained planning, incorrect data can be a disaster. An OMS can assist the utility in tracking accurate equipment failure rates. All data associated with a particular outage and the device most likely responsible for the outage is stored in the database and may be retrieved by a reliability analysis application for future study. Actual failure rates, combined with equipment manufacturer’s data, enable a far more accurate reliability analysis. As additional data is collected, the reliability analysis becomes progressively more accurate.
New products and designs
When engineers discuss the subject of better distribution reliability, it usually centers around subjects like automation and custom power devices and not around fundamental design considerations like voltage level, product quality or lengths of mains and laterals. This section addresses hardware issues that can increase reliability (and PQ) without major capital outlays.
One way to improve PQ is by using faster devices. The longer the fault clearing time, the higher probability that the sags will cause all the surrounding sensitive loads to misoperate. The use of current limiting fuses, for example, not only decreases the duration of the sag, but also the magnitude since it severely limits the fault current level. Feeder relays are fast (about 1 cycle) but most of our breakers are 5 cycle devices giving a minimum interrupt time of 6 cycles. Vacuum breakers are much faster and could be made faster still. An even easier way to increase speed is to use more interruption devices so they can be set much more sensitively. Use of more line reclosers is a good example of this approach.
Reclosers probably offer the lowest cost and most efficient way to increase reliability for the typical feeder. In the author’s opinion, they are very underutilized by the industry and have even more value in these days of PQ. Reclosers improve MAIFI, CAIDI, SAIFI, etc. and even help out on sags since they reduce the duration of the fault. Some of the requirements for reclosers installed for future distribution designs should be considered today since no one wants to purchase equipment today which will be obsolete in the near future. Some items to keep in mind are:
Faster interruption. The addition of a recloser to a feeder automatically allows faster interrupting for faults out on the feeder, due to the increased sensitivity of the trip settings. Reclosers are also faster than breakers in clearing time (breaker with relay about 6 cycles and recloser about 3 cycles).
Fault evaluation. Reclosers of the future, due to their inherent ability to record, should be able to allow the distribution engineer to evaluate the effectiveness of coordination practices (was the fuse saved on the first shot or not?). Fault location, although much more difficult at the distribution level, can still be helpful, especially for long lines.
Independent phase operation. Studies show that most faults (>75 percent) are temporary on overhead systems and from phase to ground. The ability to operate the phases independently allows two significant improvements in the area of reliability improvement:
Since residential loads are connected phase to ground, interruption of only one phase reduces interruptions by two-thirds.
Sensitive loads in the industrial/commercial sector normally trip out on PQ disturbances due to the sensitive controls on the device and not due to the device itself (adjustable speed drives). These controls are normally connected single phase, so the probability of the fault being on the same phase as the control is only 0.33 percent.
Monitoring. Feeder monitoring of the distribution feeder, with a commercial monitor, is difficult due to the cost of installation of the monitor. The fact that the recloser is already installed with this capability saves the utility well over $30,000 per installation. With the advent of PBR and public utility commission reliability concerns, it will become more important than ever that the recloser be able to accurately measure and calculate reliability indices, PQ disturbances (sags, surges, harmonics) and load levels.
Directionality. The advent of distributed resources (DR) and the need for automatic loop designs means that reclosers will have to be directional to accommodate future requirements. The ability of the recloser to automatically update its settings, based on the changing DR environment, would be a useful addition since the proliferation of DR could make manual changes time-consuming and expensive.
Aside from major design changes, which have the effect of essentially changing reliability to all customers, there are a number of methods that can change reliability to a specific group of users on the same feeder on even specific users. The following is a brief summary of these practices.
System design. System designs can be altered locally to increase PQ and reliability without much investment. Some of these practices include: reduce the number of reclosers; decrease arrester lead lengths; use secondary arresters; and use open tie arresters.
Quality products. All products are not created equal. An easy way to increase customer reliability is to use better products for those customers paying for better reliability. Failure rates between manufacturers can vary dramatically while price usually does not.
Equipment loading. Equipment deteriorates faster as loading increases. Loading levels of equipment are being significantly increased taking life out of the equipment and ultimately reducing reliability. Customers paying increased rates for better reliability should be assured that on-site equipment will never be allowed to reach an overloaded condition.
Reclosing. Reclosing can be utilized to increase overall reliability of a section of feeder. While reclosers are generally only used for long lines, they can also be utilized to sectionalize and reconfigure a looped system to attain significant increases in reliability.
Utilities will have to rethink the manner in which they address reliability. Emphasis must be placed on creating scenarios, which can provide measurable improvements in reliability at very small increments of cost per customer. Computer software will be the primary weapon that will allow the utility to identify the problem and access the most cost-effective solution.
Finally, customers having sensitive loads and who consider disturbances like sags as bad as interruptions, will have to “bite the bullet” and pay for higher quality power. Utilities should offer increased PQ, but at a price.
Burke is an executive consultant with ABB-Electric Systems Consulting, Raleigh, N.C. He may be contacted at 919-856-3311 or e-mail email@example.com.