Performance of Relaying during Wide-Area Stressed Conditions

Recent wide-area electrical disturbances have clearly demonstrated the vulnerability of the interconnected power system when operated outside its intended design limits and have shown that protective relay systems are very often involved in major wide area perturbations. This important topic was recently addressed by the IEEE Power System Relaying Committee (PSRC). The working group C12 has created a report on Performance of Relaying during Wide-Area Stressed Conditions. The report addresses key aspects of relaying schemes during wide-area disturbance conditions such as: performance; equipment rating; settings and coordination; dependability vs. security; and maintenance. It describes key stressed system conditions that affect conventional protection schemes, analyzes field experiences under stressed conditions, and suggests proven methods and solutions to improve protection performance and minimize disturbance propagation.

The report includes analysis of wide area phenomena affecting protection, such as voltage instability, collapse and excursions, angular and small-signal instability, high equipment loadings and power transfers, frequency excursions and high system unbalance.

The behavior of different protection functions under the stressed operating conditions listed above has been analyzed. Sample examples from the report are described below.

Of f-nominal frequency affect s relay measurement for all types and designs of relays. Microprocessor-based relays are typically designed to measure fundamental frequency components in their input signals. Straightforward phasor estimation algorithms such as the generic Fourier algorithm work well under nominal system frequency. If the frequency changes, the measurement becomes less accurate in a manner similar to measuring circuits of analog relays. To remain accurate under off-nominal frequency conditions, microprocessor-based relays either apply a variable sampling frequency scheme (frequency tracking), or apply a constant sampling frequency but compensate mathematically the measured phasors for the difference between the nominal and actual system frequencies (frequency compensation). Both methods, although implemented differently, are quite similar: they measure the actual system frequency and adjust either the sampling clock or the raw phasor measurements for the difference in frequency.

As experienced during a number of disturbances, transmission protective relays that may operate for non-fault conditions such as:

• Power swing/Angular instability
• Load encroachment
• Frequency deviation
• Voltage instability
• Combination of the above

Problems are exacerbated by the significant pressure to increase the transfer limits to serve the increasing demand. For example, an increased load jeopardizes security of the distance function, having increased resistive coverage such as memory-polarized (expanding) mho, self-polarized mho under long reach settings, or quadrilateral functions if the resistive reach stretches too far.

Using load encroachment characteristics, blinders, or quadrilateral functions with less aggressive resistive coverage goals solves the problem, and allows retaining dependability of protection under excessive load. In conclusion, modern microprocessor relays provide solutions for the conditions above.

Protective devices applied for generator and system backup protection have often tripped during a system disturbance. These relays need to be applied properly so that they protect the generator, but help preserve the system integrity by not tripping unnecessarily during a disturbance. For example, a loss-of-field (LOF) relay can trip on a recoverable transient swing that may enter its operating zone. Different power swing scenarios should be run to find out how long a stable power swing locus is likely to remain in the LOF operating zone. Initially leading generator power factors, slow/no voltage regulator response, low system impedance, and close-in three-phase faults cleared almost at critical clearing time are factors contributing to the worst stable swing conditions. The time delay for this relay should be set depending on the worst-case results to avoid operation during stable swings. The settings of LOF relays built in static exciters should be reviewed to ensure that they coordinate with the corresponding LOF relay protections.

The design and maintenance of protection schemes have a significant effect on the overall performance during wide area disturbances and other abnormal system conditions. Different protection solutions offer different advantages and disadvantages that need to be considered during the protection scheme design and maintenance process. In general, the art of protective relaying is a constant balance between capacity to detect abnormal conditions in a protected asset, and the ability to restrain from operation under all the other conditions. Considered separately, dependability and security of protection are easy targets. It is the necessity to satisfy both requirements simultaneously that makes protective relaying a challenging technical field. The protection function faces both security and dependability problems when pushed beyond their regular design limits, particularly under severe system-wide conditions, calling for more sophisticated relaying methods. Such methods in turn are more difficult to set and verify. Moreover, they do not provide the same high level of performance, but rather avoid impairing the protection system response too much. A solution could be to increase the security of protection design in the areas vulnerable to blackouts or during stressed conditions.

From recent major events it also appears that a large number of protection system failures are caused by human errors such as wiring, inadequate design, errors during maintenance or VT switching, incorrect settings and use of type test results as opposed to comprehensive field and system simulation testing

Implementing review and certification of the processes, as well as adequate testing procedures, can reduce the number and severity of human errors causing outages.

It is recommended to read the report that could be found on the PSRC committee web site: http://www.pes-psrc.org.