A utility focuses investments to achieve measurable improvements in reliability
In this Day and Age of Digital Everything, Customers Perceive Momentary Outages, or “blinks,” as equally disruptive as short outages. With this in mind, Public Service Electric & Gas Co. (Newark, New Jersey, U.S) embarked on a targeted, date-driven approach to improve system reliability.
To do this, PSE&G had to develop a methodology to markedly improve its system average interruption frequency index (SAIFI) and momentary average interruption frequency index (MAIFI), where possible. PSE&G's current 13-kV loop design, operating with the tie reclosers normally open, makes for high MAIFI scores, which needed to change.
Historically, utilities have attempted to improve reliability by trimming trees, applying lightning protection or installing branch fuses, the impact of which is difficult to measure. Today, thanks to technology like the advanced loop scheme, PSE&G can focus its investments and achieve measurable improvements in reliability.
TRADITIONAL LOOP SCHEME
Distribution controls on PSE&G's 13-kV system include the use of circuit breakers at the substation, feeder and tie reclosers on the line, and the use of simple overload devices for protective relay functions. All serve the purpose of isolating a fault on the distribution line with minimal impact to customers.
Traditional design typically includes the circuit breakers at the substation, at a feeder recloser about halfway down the circuit and at a tie recloser at the end of the circuit. The purpose of the tie recloser is to provide an alternate source of power to a distribution circuit in the event the primary source of power is lost.
As an example, assume a fault occurs in the first section of the feeder out of the station. In this case, the station circuit breaker trips to clear the fault. An automatic restoration attempt (a reclose attempt) by the breaker occurs approximately 10 seconds later. If this is not successful because of a permanent fault, then the tie recloser closes 50 seconds later. The operation of the tie recloser picks up the customers in the second section of the feeder out of the station. If the fault is still present, the feeder recloser opens.
Assuming an even distribution of customers along the feeder, and that the station breaker's reclose attempt is unsuccessful, half of the customers on the initial feeder out of Station A will experience a momentary outage and half of the customers on the initial feeder will experience an extended outage. If the station breaker's reclose attempt is successful, then all of the customers on the feeder will experience a momentary outage.
This traditional loop scheme is deployed throughout PSE&G's service territory and has been very successful in making PSE&G an industry leader in reliability scorecard measures. However, PSE&G's electric delivery organization is not stopping there. It has been challenged to develop a more advanced scheme to improve reliability even more.
ADVANCED LOOP SCHEME
PSE&G recently deployed an innovative distribution control scheme, leveraging intelligent devices throughout the network. Each device communicates with two neighboring devices. Each device creates information and collects it from neighboring devices to make decisions. Then the data that result from local calculations and message subscriptions are sent to other neighboring devices for further localized decision making.
The investigation of this concept began in January 2008, with implementation occurring that summer. Although the scheme is still based on circuit breakers and reclosers, the use of fiber-optic communications and advanced recloser controllers have brought the distribution loop design to a new level.
All of the breakers and reclosers at the West Caldwell and Laurel Avenue stations have been equipped with state-of-the-art microprocessor controllers. The devices are all tied together by a fiber-optic link. Although the logic employed is complex, the end result is quite simple. Every fault is isolated by the adjacent reclosers while fully minimizing momentary and extended outages.
With regard to the traditional loop scheme example given previously, assume a fault occurs between FR1 and FR2 on the West Caldwell circuit. In this case, FR1 senses the fault and begins the tripping sequence. Prior to the opening of FR1, FR1 tells the tie recloser to close. Only after the tie recloser closes do FR1 and FR2 trip to isolate the fault. This sequence only takes milliseconds. An automatic restoration attempt by FR1 occurs approximately 10 seconds later. If this is not successful, thus a permanent fault, then the section between FR1 and FR2 is out until it can be repaired.
As a comparison to the traditional design, again assume customers are evenly distributed along the feeder. In this case, because more feeder reclosers are deployed, only 16% of West Caldwell customers are located between any two protective devices. So, in the advanced loop design, no customers will see a momentary outage, and only the customers on the faulted section will experience an extended outage.
Using the traditional scheme as the base case, the advanced loop scheme means a 100% reduction of customers experiencing a momentary outage and a 66% reduction of customers experiencing an extended outage. Further, if the reclose attempt by FR1 is successful, the advanced loop scheme will have disturbed 84% fewer customers with a momentary outage. When compared to those of the traditional scheme, the numbers of the advanced loop scheme are eye-catching. PSE&G plans to continue deploying the advanced loop scheme.
IMPACT ON FIELD TECHNICIANS
Implementation planning was tough for PSE&G's relay technicians, because they had never used this type of relay controller or this application. The logic used is similar to that used on high-speed transmission relaying. The traditional sequential operation of sectionalizing devices does not easily translate to the multiple, simultaneous operations of the advanced scheme.
Fiber-optic communications opens a whole new world. In the past, PSE&G would never have considered closing in on a fault and then clearing one. Now, with the fast-acting microprocessor relays, the utility is able to perform logic checks and, using one intelligent device, make the right decision in a matter of cycles. Despite being challenged by the new intelligent devices, the PSE&G team learned how to test and validate operations, and completed the job on time.
Supervisory control and data acquisition (SCADA) redundancy at a pole-top unit is unheard of and, in most cases, cost prohibitive. Having already invested in fiber between two stations, PSE&G decided it was prudent to increase the SCADA reliability.
As it is for many utilities, a single copper-based frame-relay circuit from the telephone company is PSE&G's communications path back to SCADA from the station. By joining forces with its information technology department, PSE&G developed a plan to allow a logical ring topology. This created a redundant path for SCADA in each station and at the pole top to pass through either frame-relay circuit. A physical ring also was created in the field to maintain Ethernet communications in the event of a single switch or fiber failure.
Lastly, the recloser controllers were still serial-based units. None of what has been discussed here could be accomplished without a small, rugged managed switch and port server at each unit. This design also allowed engineering access to the pole tops over the same fiber network used for distributed network protocol traffic.
Using SCADA for commissioning offers many benefits. In this case, having the engineering access to troubleshoot and correct minor startup issues, and evaluate the first events by quickly and easily downloading event sequences, was a valuable contributing factor to actually meeting the extremely aggressive schedule.
PSE&G has developed a data-driven investment strategy that mitigates poorest-performing circuit loops based on SAIFI improvements. The utility no longer focuses on the most recent problem, but determines how to get the biggest metric improvement for its money. Corporate SAIFI targets are difficult to improve; however, by leveraging technology and focusing on customers affected, PSE&G will impact results in a positive manner.
Richard Wernsing (Richard.Wernsing@pseg.com) is the reliability-centered maintenance expert at Public Service Electric & Gas Co. (PSE&G). Wernsing has acquired extensive knowledge and technical expertise over the past 40 years at PSE&G in the delivery, transmission and production of electricity, computer and telecommunications support. Most recently, he has developed a computerized maintenance management system that provides condition and critical assessment on all major inside plant equipment transformers, breakers and relays.