Fault Current versus Distance

Background: A major utility noted that they were having far more splice failures on their 34.5 kV distribution lines within a few miles of the substation than they were toward the end of their circuits. The primary reason is the available system fault current diminishes with distance from the sources.

In an attempt to quantify this, representative fault current calculations were made on a “typical” 34.5 kV circuit. I’ll call it “typical” because no circuit details or transformer nameplate data were available. We do know that the utility uses both 336.4 kcmil (Linnet) and 636 kcmil (Grosbeak) ACSR conductors on their 34.5 kV circuits and it is assumed that both are loaded to their 75°C maximum operating temperatures. The transformer used for these calculations was arbitrarily selected as 34.5 kV, 95.0 MVA, X/R=5.0, 5%Z (which may or may not be representative of the actual transformer).

Short circuit studies normally begin with a line diagram showing all loads and potential sources of fault current. (During a symmetrical fault, induction motors will contribute only during the asymmetric portion of fault current but synchronous motors may contribute 4 – 6 times their full load current to all fault locations). Capacitors may also be a factor under some conditions. Protective devices are not normally included in the line diagram.

Worst case short circuits are normally based on bolted 3 phase fault conditions in which all three phases are “bolted” together to obtain a zero impedance fault. This results in maximum thermal and mechanical stress in the system and typically assumes infinitely available fault current from the primary source.

In this case, we are only interested in the available short circuit current at the location of a line splice versus distance from the transformer, based entirely on the conductor resistance, reactance, and voltage at the fault (splice) location. Following are the graphical results of point-to-point analysis of both conductors from 1/8 mile to 10 miles from the transformer.

Conclusions: While this exercise may or may not be truly representative of a particular utility system, it does illustrate fault current magnitude relative to distance from a transformer source, regardless of the transformer type and location. In this case, fault currents are higher on the larger Grosbeak conductor because its total impedance is less than that of Linnet.

Although the slopes and magnitudes would change for other source and conductor combinations, the results would be similar.

Degraded, high resistance splices, connectors, damaged conductors, etc. would be less able to withstand the higher fault currents both electrically and mechanically so it would be expected that failure rates would also reduce with distance.

Waymon P. Goch
March 14, 2012

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The Truth About Infrared Inspections

Hangin’ Your Repairs on Temperature?
By, Kimberly Lewis
HOT/SHOT Infrared Inspections, Inc.

Infrared technology has been heavily utilized by the utility industry for over 30 years and yet it is still vastly misunderstood.  In a perfect world, utility engineers want to qualify the results of infrared inspections and use the temperature data collected from anomalies (a connector or other component that is found operating at a higher than normal temperature for system conditions) as the sole source for deciding the severity of a problem and consequently, the urgency for repair.  While it is a natural tendency given the need to prioritize repairs, especially when there are limited man hours and repair budgets, unfortunately the entire picture cannot be revealed by infrared alone.

So, what is the problem with using temperature as the sole indicator of an anomaly’s severity?  It all starts with a concept called the “Hypothetical Fault Cycle” (see diagram below).

A. At Point “A” the anomaly is rising in temperature: no deterioration visible if inspected

B. Internal arcing and deterioration starting

C. A weak weld develops and temperature starts to drop

D. The weld breaks and the temperature rises again

E. More deterioration occurs and welding takes place

F. Due to welding, resistance has decreased considerably and temperature drops drastically

G. At this point, there is low temperature and high deterioration if inspected at this time

Thermal problems can be detected at any time during the failure cycle and thus, one can never be sure if the anomaly was found when it is just starting to rise in temperature such as at point B or if the anomaly had just re-welded itself as at point F.  If the temperature is relatively low, then the problem could just be starting OR it could be about to spike severely.  Sufficient evidence exists to substantiate that once a connector begins heating, it will eventually reach a point of thermal runaway, although it may live for hundreds of additional cycles.  Some thermal anomalies will actually repeat the welding cycle over and over and may not fail for a long time, while others could appear as a minor problem and yet, fail in the next hour.  While use of infrared is a valuable tool, especially because it is fast, and does not require contact with the energized component, it is clear temperature alone really is not a sufficient indicator of imminent component failure.  Once a thermal anomaly is detected, it is prudent to schedule a repair, as the connector is likely to fail, but temperature alone cannot accurately predict when.

There are a great many factors in determining when a thermal problem will fail and unfortunately, none of them are exact.  One must consider the loading of the line at the time of detection, the weather conditions, the criticality of the particular line and the temperature of the problem.   In addition, utilities should consider the history of the line and/or problem component.  Priorities should be determined on the basis of safety and reliance on critical systems, not solely on temperature of an anomaly.  For example, a line with a hot connector crossing a freeway or parking lot where a mechanical failure thereof would have a high risk of contacting a person or vehicle and could  possibly contribute to a wreck or electrocution, should have higher priority than a similar line in a rural setting.  Likewise, a line serving a hospital or a water pumping station, where failure might result in more than just the inconvenience of a few households ought to have a higher priority.

The bottom line is that while there is no scientific way to qualify “time to failure” of thermal anomalies, infrared is a cost effective tool in quickly determining the location of thermal problems on transmission, distribution and substation systems.  If you ask me – I say treat every problem like it could fail tomorrow, then you won’t be surprised if it does!

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Connectors - The Weak Link

When considering increased conductor temperatures, numerous issues are of concern, particularly with the dynamic effects on electrical connectors when suspended overhead aluminum conductors are operated at high temperatures, specifically above 93°C (200°F). Click here to read the complete article.

Alabama Power Repairs Hot Splice In 8 Minutes and 23 seconds!

Following annual line surveys at Alabama Power, with the discovery of some hot (aged) splices found with IR Thermography, a decision was made to utilize the advantage of a helicopter to quickly and efficiently install ClampStar units over the hot splices to restore their electrical and mechanical integrity.  An EHV Live-Line crew from Haverfield Aviation was brought in for the installations, along with some additional work installing some marker balls.  A couple of the splices were located in areas where it would have required significant time and funding to access with a bucket truck.

Installing ClampStar CSF-1302-048
Installing ClampStar CSF-1302-048

The conductor was 1033.5 ACSR, and being energized at 230kV, corona shields were installed on the CSF-1302-048 ClampStar units.  These were the first ClampStar units this particular crew had seen, and personnel from Classic Connectors, Inc., were on site to answer any questions the crew might have.  The additional photos which follow show how easily the ClampStar units were installed.  Complete with a very thorough conductor cleaning, Haverfield installed the CSF-1302-048, complete with corona shields, in 8 minutes and 23 seconds!  A number of Alabama Power personnel on hand all had the same statement - "We should have been doing this for years!"

SMUD Uses Helicopter to Install ClampStar Unit After IR Inspection Identifies Hot Splice

Jim Wilhelm, Tower Patrol T&D Assets, Distribution Services Lineman for Sacramento Municipal Utility District said “Installation was difficult due to the movement of the helicopter, but allowed us to do this over a deep gorge, not accessible from a bucket truck. Had we not had ClampStar, this would have required a minimum of an 8 hour outage on the line, purchasing wheel (backup) power, letting the line down to the ground and changing out the splice and adding about 150’ of conductor in the “old” conventional manner of repair.

It would have been even easier if we could have worked it with the skid board on the chopper, but because it was the middle phase, it required the work be done suspended 60 feet below the aircraft. It is significantly more difficult to hover steady in this position, as the pilot has no close reference point on which to fix his position.”

Installing ClampStar from 60' below the chopper
Installing ClampStar from 60' below the chopper

“With ClampStar, not only did we save many thousands of dollars and many man-hours but got the extra benefit of having a superior connection, much better than we would have if we had put yet another connector up in place of the one that had already failed!” Click to see photos.

New "CSS" Style ClampStar

Due to popular demand, CCI has developed the New “CSS” style ClampStar units for use on Deadends and Suspension Clamp applications!  Made with a single conductor leg, the new unit installs in minutes, the same as the traditional ClampStar units that are installed over splices.

ClampStar CSS Unit
ClampStar CSS Unit Installed on a Deadend

Several applications have given rise to the need for this new design. While the standard “CSF” ClampStar units can be installed on deadends and suspension clamps, it is more difficult than installing them over splices, as the torque nuts must be tightened from above. This new “CSS” design allows the installer to approach from below, with the fasteners positioned downward as well, allowing the same easy access to tightening them as that of a traditional ClampStar over a splice. Read More......

Line Crews Repair Failing Splices with No Power Interruption!

May 01, 2010, Transmission & Distribution World, By Mike Dario, Los Angeles Department of Water and Power

Corrosive salt water and high winds in Los Angeles' beach areas were wreaking havoc on the Los Angeles Department of Water and Power's automatic quick sleeves. Linemen were noticing more failures of these sleeves, which are spring-loaded with jaws that grip the wire. To solve the problem, linemen had to support the wire with a hoist and grips, and install a temporary jumper to maintain electrical Read more....

Los Angeles Department of Water & Power corrects failing splices in record time!

On a recent Tuesday morning at 10:00, two linemen from LADWP, Mike Traweek and Sammy Sempelsz lifted off the ground in their new Altec bucket truck to begin the first round of ClampStar® installations. They completed the first 6 installations in 55 minutes, moved the equipment about ½ mile down the street to the next location, waited a few minutes for someone to move a vehicle that was parked in the way, and installed the next 6 in only 25 minutes. Mike said, “I guess we just needed to get used to them a little bit”.

Crew Foreman, Mike Dario said replacing these 12 splices on energized 795 conductor would normally have taken 7-8 hours in these locations, and basically would have been an all-day job doing it the old-fashioned way with jumpers and come-alongs and it certainly would have been more dangerous. Beginning at 10:00 AM and having both jobs, at two separate locations, completed before lunch, including travel time, Dario was very pleased to have these finished quickly allowing his crew to move onto another project.

Be sure to view the installation photos in the Classic Connectors Photo Gallery.

ClampStar makes line up-rates possible!

Major Breakthrough With New Cost Saving Connector Technology Propels Classic Connectors, Inc. And Their Revolutionary Product “ClampStar®” Into Electric Utilities Must-Have Product For 2010

For the past several years, utilities have been under considerable pressure to operate aging power lines and equipment at higher levels, forcing them to transport large amounts of energy over an aged and antiquated system. Many of the components that make up the power grid are approaching their 30−50-year useful lives. Utilities therefore are constantly searching for cost-effective alternatives while maintaining technical and safety requirements as well as reliability. Enter “ClampStar®”! READ MORE....

Dominion VA Power installs ClampStar in Grandy, NC

The unit was installed on a 477 ASCR conductor, 34.5kV L-L, 19.9kV L-N, over an automatic splice.

Including the time for set up, and covering/uncovering the adjacent conductors, the install took 25:19. Once the ClampStar® was loaded in the bucket, the actual install of placing the unit over the splice and tightening the bolts took about 3 minutes.