Traditionally, electric supply reliability and power quality have been viewed as separate concerns. However, in today's digital environment, the two are closely interconnected. Reliable power depends on effective system protection, which in turn depends on quality power. In this article, I'll explain how protective relay performance interacts with power quality (and vice versa) in the areas of voltage, phase balance, harmonics, and frequency. As you'll see, protective relays discriminate more easily between fault and nonfault conditions when power quality is good.
Voltage Levels
Protective relays are not effective in protecting equipment against fast overvoltage transients (short swells and spikes). Minimum circuit breaker tripping time plus relay operating time limit the effectiveness for durations less than 100 ms. In these cases, properly rated surge arresters will be more effective.
For longer voltage swells, there are two types of overvoltage protection systems available. The pure overvoltage type protects equipment against excessive dielectric stress. A slower overexcitation type (often called a volts per hertz or Volts/Hz relay) protects equipment from overheating. This overheating is caused by excessive flux levels that occur when flux escapes from the designed carrying path(s).
The Information Technology Industry Council (ITIC) curve is widely used to determine acceptable magnitude- and duration-levels for sags and swells for IT equipment. In Fig. 1, the solid red lines represent the ITIC curve, and the broken blue lines represent typical overvoltage and overexcitation protection curves (including breaker clearing times). As you can see, the response times for the protective relays are well beyond the short time limits of the ITIC curve, which is why they can't protect against fast transients.
One way to minimize sag durations and increase power quality is to pay special attention to fault-clearing speeds. This helps because short circuits depress voltages through a power system far beyond the region of the faulted equipment. The clouded area in Fig. 1 shows the effective areas for short-circuit operation. The sag portion of the ITIC curve is flat between .02 sec and 0.5 sec, so it might appear that protection speeds faster than 0.5 sec provide little benefit. However, reducing the fault clearing from 0.5 sec to 0.1 sec (the fastest practical clearing with a 5-cycle breaker) noticeably improves power quality.
We can now understand the limitations of using the ITIC curve as a general indicator of acceptable power quality levels. The recently developed standard SEMI F47, which sets sag tolerance requirements for semiconductor processing equipment, tolerates a lower voltage than the ITIC curve — up to 0.2 sec. This demonstrates the benefit of keeping clearing times lower than 0.2 sec.
You can improve fault-clearing times by using communications systems assistance for subtransmission (and sometimes feeder) protection instead of relying on lower-cost, time-delayed tripping. In some cases, deliberately sacrificing full coordination of protection zones also may increase power quality.
Now let's consider voltage spikes. Protection systems can't guard against voltage spikes, but they can be affected by them. Fig. 2 shows how a voltage spike caused by lightning affects different sets of protection on a 500kV system. The spike causes two noticeable effects: a near miss of the line protection system and an undesirable trip at the 5MB1 bus protection.
The near miss occurs as the current drawn by the surge suppressor (visible as a spike in the transmission line's residual current 94IN) momentarily pulses the forward-fault detector in the 5L94 protection zone. Fortunately, the forward-fault detector resets a few milliseconds before receiving a permissive-trip signal from the remote terminal, avoiding an undesirable line trip. (Notice the desirable single-phase trip of the B-phase circuit about 20 cycles after the first voltage transient. In this case, a second lightning strike actually short-circuits the line.)
The undesirable trip at the 5MB1 bus protection is caused by the brief conduction of the gapped surge arrester in the 5MB1's differential protection zone. The conduction is long enough for the high-speed bus protection to trip even though the fault does not persist. One solution would be to replace the insecure protection with a slightly slower type. Another more expensive solution would be to replace the gapped 500kV surge arresters with a more modern MOV type.
Phase Balance
Protective relays often use system voltage or current imbalances as indicators of fault conditions. This means that imbalances created by nonfault conditions may affect protection systems as well. Some causes of power-system imbalance include imbalanced loads; non-symmetrical power-system components, such as untransposed transmission lines; and temporary imbalanced stresses, such as those caused by geomagnetic induced currents (GIC).
Negative- and zero-sequence overcurrent relays are most adversely affected by system imbalances. The pickup settings of these functions must allow for steady-state imbalances. Power-system imbalances, in fact, often create a minimum threshold for the sensitivity of a protection system.
One benefit of modern multifunction digital relays is the opportunity to detect and signal (via alarms) steady-state system imbalances that approach trip thresholds. At one utility, for example, engineers set the alarms at 50% of the distribution feeder's negative- and zero-sequence overcurrent relays' pickup settings. They also are considering similar alarms for the utility's transmission-line protection systems after high steady-state, negative-sequence currents (approximately 12% of positive-sequence currents) recently tripped a 230kV transmission line.
Harmonics
The Institute of Electrical and Electronics Engineers' (IEEE) Standard 519-1992 defines acceptable degrees of harmonic waveform distortion in high-quality power. However, distortion levels in many power systems may exceed those defined by the standard, and protective relays respond to these distortions in many different ways. No single response is desirable in all instances of the same distortion type.
In some cases, relays are routinely exposed to excessively distorted signals. For example, significant levels of third harmonics are often observed under normal conditions in zero-sequence currents and voltages. Relays measure residual quantities to detect fundamental-frequency imbalances that may indicate power-system short circuits. Typically, relays that measure zero-sequence quantities are specifically designed to reject harmonics, particularly third harmonics.
In other cases, harmonics in phase currents or voltages may be harmful to equipment. For instance, excessive harmonics in the power system can threaten the health of shunt capacitor banks. This is because capacitor banks tend to absorb harmonics, which may cause eventual failure of the banks due to overload. If phase overcurrent relays are insensitive to harmonics, then they may not adequately protect the banks. But even if the relays do sense harmonics, they may overprotect or underprotect, depending on their specific response.
Without special investigations, the response of protective relays to distorted harmonic waveforms is usually unknown. Fig. 3, however, shows some possible responses. The figure illustrates three different measuring principles for a waveform with 30% third harmonic, which is imposed on a fundamental frequency with zero-phase shift. The true rms measurement will be about 4% higher, and the peak measurement will be 30% higher, than the fundamental-frequency measurement. Many modern relays measure only the fundamental-frequency phasor.
In several cases, transmission-line protective relays have undesirably operated in response to harmonics caused by GICs, system resonances, or other factors. In most of these cases, modifying the filtering or replacing the relay with a more tightly tuned device improved the performance. Presently, relays designed to protect capacitors, transformers, and generators from excessive harmonic currents are not widely available. In the absence of such protection, power quality monitors are sometimes used to provide alarms or automatic switching in instances where harmonics may threaten power-equipment health.
Frequency Deviations
Smaller power systems may experience wider frequency deviations than large integrated power systems, which normally operate close to nominal frequencies. Sometimes, though, small islands may separate from large power systems and suffer significant frequency excursions.
A common solution to protect systems from excessive frequency deviations (and to protect generators against damage from such deviations) is to apply relays that are specially designed to operate at certain frequency settings. Unfortunately, system-frequency changes can adversely impact the performance of other digital and analogue relays. For example, a frequently observed problem in the early designs of distance relays was the inability to retain their specified operating characteristics. Adaptive sampling techniques can make modern designs more tolerant of frequency changes. Unfortunately, relay response to frequency deviations is not always specified in instruction manuals.
Conclusion
Poor power quality blurs the line between fault and nonfault conditions, making it difficult for relays to operate reliably. More information about relay performance under poor power quality conditions will enable engineers to design more secure and dependable protection systems. Furthermore, additional standards are needed to set limits for acceptable power quality. No universally acceptable tolerances for frequency excursions, reliability, or phase balance presently exist. Harmonic limits are well-defined, but the ITIC curve is limited in its application.
The good news is that many protective relay applications and settings can provide improved power quality by reducing sag durations, improving supply reliability, and protecting loads against unacceptable voltages, frequencies, and phase imbalances.
This article is based on “Protective Relay Impacts on Power Quality — and Vice Versa,” by Charles F. Henville, which appeared in the 2001 IEEE Power Engineering Society Summer Meeting Proceedings. ©2001 IEEE.
Charles Henville is a specialist protection engineer at BC Hydro in Burnaby, British Columbia, Canada. He can be reached at [email protected].
