See the unseen: Power data diagnostics

Using reports from relay events can help diagnose power system problems, increase system reliability, lower maintenance costs, and reduce potential for system damage and outages. Today's microprocessor-based relays collect and save event reports during faults and other problematic conditions. The event report and sequential events recorder (SER) capability of advanced relays enable protection e...

06/01/2008


Using reports from relay events can help diagnose power system problems, increase system reliability, lower maintenance costs, and reduce potential for system damage and outages.

Today's microprocessor-based relays collect and save event reports during faults and other problematic conditions. The event report and sequential events recorder (SER) capability of advanced relays enable protection engineers to gain a better understanding of faults and disturbances on transmission line systems through analysis of event reports. This analysis frequently leads to better line parameters, more accurate fault locating for complex faults, and improved understanding of electric power system operations.

Event reports help determine fault resistance and explain otherwise inexplicable events (see graph). They also aid in testing and troubleshooting relay settings and protection schemes that protect costly equipment critical to consumer and industrial services.

Event record detective

The importance of event report records was demonstrated at the city of Conway, AR, 20 miles north of Little Rock. Conway's city-owned power system includes two feeders from its substation, one supplying an industrial customer, and the other supplying the balance of the small city's business and residential customers. Technology from Schweitzer Engineering Laboratories Inc. (SEL) of Pullman, WA, helped detect what might have become a much larger problem.

“The industrial customer had detected what they considered a voltage problem,” says Emery Perry, an SEL field application technician called in to consult on the situation. “Yet when technicians from the Conway Electric Department went to inspect the substation, they noticed that there was no trip on the breaker,” Perry says. The technicians at Conway Electric received training on SEL equipment, enabling them to take advantage of event report records in the SEL relays. In the next step, technicians decided to check the event history in the SEL relays to check for relevant event reports on the feeder in question.

They found that existing event reports seemed to indicate a voltage issue. So they consulted Perry to confirm what seemed to be an unusual problem.

“From the oscillograph readings, it appeared that the problem could be in the source—and in this case the source was a transformer,” Perry says.

Hardware, software involved

Conway Electric Department uses SEL-351S-7 and SEL-351S-6 protection and breaker control relays and SEL-551 overcurrent/reclosing relays in its distribution substations. SEL-351S-7 Relay is used on the feeder main, with the SEL-551 as backup. SEL-351S-6 Relays provide feeder protection and reclosing.

Perry says event records there showed a decrease in C-phase current (IC) and C-phase voltage (VC), while A-phase and B-phase currents increased. “This occurred on two feeders. I suggested testing their transformer, which was a tap changer. When we inspected it, the rotary tap changer contacts were severely worn. [See photo.]

“What was causing the problem was that the C-phase on the utility transformer was opening up, and the industrial customer was in single phase,” Perry says. “It wasn't a complete opening; actually, the moving contacts in the transformer were arcing. That's why the event report oscillograph showed the collapsed C-phase voltage and the collapsed C-phase current, just as if you were removing the voltage off of the load C-phase. There was a three-phase system within the industrial facility that was actually single phasing, which could cause equipment damage. And, of course, the arcing of the transformer caused damage to the contacts.” Although transformers are generally among the most dependable components in an electrical system (failure rates are estimated at a minimal 76 failures per 10,000 years of transformer life), when failures occur, they can result in costly outages, considerable downtime, and expensive replacements, he explains.

Damage avoided

“In this case, I would say that early analyses of the event record helped prevent a possible transformer fire and a sustained power outage, and also possible damage to the industrial plant equipment from a continued single-phasing condition,” Perry says. Each time an SEL relay generates a standard event report, it also generates a corresponding event summary. Event description includes relay/terminal identification; event type, date, and time; fault location; and other information. Event summaries can be viewed using the front-panel LCD or front-panel serial port of the SEL-351S.

Date and time of each transition are available in event reports, downloadable to any PC, he says. The chronology helps technicians determine the order and cause of events so that comprehensive troubleshooting is facilitated. With an appropriate setting, the relay will automatically send an event summary in ASCII text to one or more serial ports each time an event report is triggered.

In addition to storing event summaries and full-length event reports, SEL relays can track the pickup and dropout of protection elements, control inputs, and contact outputs.






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