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Vietnam Energy Forum

Innovative Protection Schemes Using Pulse Closing® Technology

 - Distribution faults can damage equipment, reduce reliability, and adversely affect a utility’s bottom line. Some existing fault-management strategies can even cause more damage by multiplying the energy applied to a system when testing a line for faults after an event. Better switching and protection techniques are available to increase customer reliability and reduce the impact of persistent outages on customers.

 

By Jason Lander

Utilities around the world are transitioning to advanced, point-on-wave switching schemes with S&C’s IntelliRupter® PulseCloser® Fault Interrupter. With the use of PulseClosing® Technology, which uses 95% less energy than fault current when testing for faults, IntelliRupter® fault interrupters are designed to rapidly close and open interrupter contacts on individual poles to test for the continued presence of a fault after the initial fault interruption.

PulseClosing Technology dramatically reduces the amount of force used during fault-testing and significantly lessens momentary outages for customers on the main feeder. This requires accurate voltage and current sensing. The IntelliRupter fault interrupter senses voltage on both sides of the interrupter on each pole, and its current sensors are accurate to within ±0.5%, leading to protection accuracy better than ±2%. This article reviews three different protection techniques utilities can use to increase customer reliability by reducing the impact of persistent outages on customers. These techniques are Intelligent Fuse Saving, the PulseFinding™ Fault Location Technique, and Loop Restoration.

Intelligent Fuse-Saving

In conventional fuse-saving schemes, the upstream protective device operates to save the fuse located on the spur line. With intelligent fuse-saving, IntelliRupter fault interrupters can detect fault-current levels and then adjust the operating time based on whether the device can operate faster than the fuse. This eliminates any unnecessary blinks along the line that happen with conventional fuse-saving strategies when conventional reclosers can’t beat fuses. Customers downstream of the IntelliRupter fault interrupter then do not experience a momentary outage.

Coordination with fuses on spur lines is also important because the combined line exposure beyond all fuses on these lines is typically far greater than for the main lines. Effectively applied fuse-saving overcurrent protection can reduce outage frequency for customers downstream. Part of the problem, historically, is the difficulty achieving effective fuse-saving with single-use devices equipped with conventional time-current characteristic (TCC) curves.

The conventional fuse-saving practice has an inherent trade-off: Sustained outage improvement comes at the expense of increased momentary-outage activity. These challenges have led some utilities to abandon the practice of fuse-saving and instead migrate to a fuse-blowing philosophy, meaning fast-trips on reclosing devices are turned off. When using the fuse-blowing practice, the trade-off is reversed: Reduced momentary activity comes at the expense of more frequent sustained outages. Customers located downstream from a tap fuse experience a sustained interruption for every fault, even for the faults that would have been temporary had they been given a chance to be cleared by the upstream recloser tripping. The utility then incurs the cost-of-service calls to replace the blown fuse.

 

Figure 1: (a) Partial-range fuse-saving TCC curves and (b) effective first-trip protection.

As seen in Figure 1(a), the shape of the “intelligent” fuse-saving curve is designed to conform to the shape of the specified downline fuse curve as closely as possible to minimise interference with even smaller downline transformer fuses and to maximise the current range over which fuse-saving is achieved. Figure 1(b) shows the effective first-trip protection that combines the fuse-saving curve over the range it is active with the delayed curve that is active for higher-fault currents.

This implementation of intelligent fuse-saving over a partial range of available fault currents, combined with only tripping the faulted phases, can still achieve all the benefits of fuse-saving for low-magnitude fault currents.

PulseFinding Fault Location Technique

A practical engineering consideration that limits the use of conventional reclosers in series of three to five devices is that all the reclosers and relays must be properly coordinated from one end of the loop to the other and in both directions, albeit one direction at a time. Coordinating many devices in series, in high fault-current areas, can be challenging, especially when sensing and timing tolerances are considered.

An innovative IntelliRupter fault interrupter application helps users overcome protection-coordination limitations when installing multiple-series feeder fault interrupters downstream of a substation breaker. This is called the PulseFinding Fault Location Technique. The PulseFinding technique enables multiple IntelliRupter fault interrupters to overcome these limitations by sharing the same coordination characteristics. After the devices have tripped open, PulseClosing Technology can restore service up to the faulted segment. If this was done with a conventional recloser, the network would be subjected to extensive fault current, known as the fault-multiplier effect, which the use of PulseClosing Technology avoids.

IntelliRupter fault interrupters can be easily installed downstream of an existing recloser by using the PulseFinding technique and having multiple devices share the same curve to operate the technique. This also helps engineers simplify the task of coordinating fault interrupters by eliminating the need to change settings.

Using the PulseFinding technique with IntelliRupter fault interrupters improves feeder reliability by working with existing assets for feeder segmentation. In the case of a feeder with existing assets, IntelliRupter fault interrupters’ protection settings are configured the same as an existing upstream recloser or breaker. When these devices trip (because they are configured with the same TCC curve), the IntelliRupter fault interrupter (using the PulseFinding technique) will begin to look for the fault and restore service to the unaffected segments, resulting in SAIFI (outage-frequency) and SAIDI (outage-duration) improvements for persistent faults. See Figure 2.

Figure 2: The PulseFinding technique enables multiple-series IntelliRupter fault interrupters to isolate the fault.

Loop Restoration

In a loop-restoration scheme, once a persistent fault has been successfully cleared, looped feeders automatically restore service by closing a normally open device to a nearby feeder. Proper time-current coordination of series fault interrupters is also required to ensure initial and subsequent faults are only isolated by the closest upstream fault interrupter. These loop schemes are relatively simple to apply and eliminate the need for communications between devices.

Figure 3 shows the circuit topology for a three-recloser loop and a five-recloser loop, each with a normally open tie recloser. Simple reliability calculations for loop systems assume there is a constant fault-incidence rate in all feeder segments, equal segment lengths, even distribution of customers, and a constant restoration time throughout the system. SAIFI and SAIDI reliability indices for radial feeders without the mid-line and tie-point protective devices are used as the baseline. The benefit of a three-device loop system over two radial feeders is a 50% reduction in SAIFI and SAIDI. Expanding to a five-device loop improves the reliability indices to 33% of their baseline values.

Figure 3: Three-recloser loop, relative SAIDI = 0.500 PER UNIT, and five-recloser loop, relative SAIDI = 0.333 PER UNIT.

Many utilities believe transitioning feeders from radial to loop restoration is an easy and seamless process, but there can be hidden costs and complexities during the transition. IntelliRupter fault interrupters can address these transition challenges by reducing long-term costs and achieving maximum feeder reliability.

When an IntelliRupter fault interrupter is applied at the tie point in an otherwise conventional recloser loop system, a timer is started upon loss of voltage on either side of the tie. When the timer expires, the IntelliRupter fault interrupter uses a pulse to test the line. Whereas conventional loop systems require the tie device to lock out on the first trip to avoid multiple sags for the fault, as shown in Figure 4, it may be beneficial to issue additional pulses over a period to give the fault a longer chance to disappear.

The additional pulses detect whether the fault is still present without causing any further line disturbances. At the end of the sequence, if the fault persists, the tie device locks out without ever having closed into the fault or disturbing the unfaulted feeder. If the fault is cleared at any time during the test sequence, then the tie device closes in and further reconfiguration can proceed as usual.

The rest of the loop-system setup and operational characteristics remain the same as for a conventional loop. Overcurrent coordination of all devices in the loop is required, and a return-to-normal condition requires manual intervention. Even greater benefits are seen when all the devices can support PulseClosing Technology.

Figure 4: A recloser loop using PulseClosing Technology at the normally open tie.

When effectively implementing PulseClosing Technology, fault interrupters give distribution-protection engineers increased flexibility and functionality to design innovative protection systems that improve distribution-system reliability, while also reducing the stress on distribution equipment during fault-testing.

Advanced protection schemes, such as Intelligent Fuse Saving, the PulseFinding Technique, and Loop Restoration, eliminate some of the most significant drawbacks and limitations of conventional schemes. Advanced protection schemes, along with the high-accuracy current sensing of the fault interrupters, allow for more devices to be used in series, thus improving reliability while reducing the voltage sags and equipment stress associated with conventional reclosers.

For further information please contact S&C at sandc.com.

 

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