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No-Outage Inspection Cornerby Don A. Genutis, Group CBS

Figure 1 Hot Fuse Connection

Figure 2 Unshielded Medium-Voltage Jumper Cable

Top Five Switchgear Failure Causesand how to avoid them

O

ver the years, we have all gained valuable field

experiences that provide insight into how and

why equipment fails. I have created the following

list of top five general causes for switchgear failures and how

they can be prevented, based upon my particular experiences.

I hope that this information can sparkcreative thinking from

our readers to prevent future switchgear failures due to their

own unique field observations.

Loose connectionsLoose and faulty connections cause an increase of resis-

tance at that localized point. Te increased resistance causesincreased heat in accordance with Ohms law, P = I2R . Teincrease in heat will escalate until complete thermal fail-ure of the connection occurs or the nearby insulation failsresulting in a fault. One major insurance carrier estimatesthat approximately 25 percent of all electrical failures occurdue to loose connections.

Te solution to avoiding these types of failures is to per-form regular infrared inspections of all switchgear. Infrared

viewing ports should be installed and medium-voltageswitchgear should have ports that also pass ultraviolet lightso that corona cameras can be used to inspect for corona andsurface partial discharge activity. Figure 1 shows a thermalimage of a loose switchgear connection that could lead tofuture failure if not repaired.

Insulation BreakdownA whole series of articles could be written about these

types of problems, but we will focus just on the most com-mon culprits here. Low-voltage insulation is fairly simpleand is not subject to the same voltage stresses as medium-

voltage switchgear insula-tion. Keeping low-voltageinsulation failures in checkmostly involves keepingthe insulation dry andclean and ensuring clear-ances are adequate.

Medium-voltage insu-lation systems are muchmore complex due to thegreater voltage stressesthat exist. Areas withinthe switchgear that areoverstressed will initiallyfail over a small portion ofthe insulation but will then

escalate over time until complete failure occurs. Te mostlikely areas for these problems to occur are:

a. Jumper cables Unshielded jumper cables are usedto connect switches to transformers in unit substationdesign; connect potential transformers, control powertransformers, and surge arresters to the bus; and connecttransformer coils and taps together. Anytime these cablescome in contact with ground, other phases, or even other

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Figure 5 Te upper left photo shows good insulation with hydrophobic surface qualities that progressively worsen until the insulation

becomes hydrophilic in the lower right photo due to voltage stresses

Figure 3 Switchgear Bus Support Barrier

Figure 4 ermination Failure

b. Bus Most switchgear designs use simple insulationbarriers to separate compartments and to support thebus. Tese barriers often have small air gaps between thebus and insulation. Te localized voltage stress is greatenough to cause the air to break down and this partialbreakdown will lead to eventual failure (see figure 3).

types of insulation used to support them, they will likelydevelop stresses at those points that will lead to futurefailure (see figure 2).

c. Cable terminations Cable terminations are difficult tobuild properly. Tese components transition the shieldedcable that has nearly perfect inherent voltage stresscontrol to a connection to the bus. Many things can gowrong if great care is not taken during the terminationconstruction, and the proper voltage stress relief detailsare not realized. Additionally, the portion of the termi-nation nearest to the connection essentially consists ofunshielded insulation, so care must be taken to ensure

adequate clearances in that area. All of these locationwithin the termination can create localized partial insulation breakdown that will lead to failure(see figure 4).

Localized partial insulation failures are known as partiadischarges. Fortunately, the partial discharge activity createdetectable signals which are the early warning signs of fu-ture complete insulation failure. Te solution to preventingmedium-voltage switchgear failures begins with utilizingthe hand-held partial discharge detector equipped with anultrasonic sensor to detect surface insulation defects and atransient earth voltage sensor to detect internal insulationdefects.

WaterWater intrusion or immersion due to natural disasters o

accidents can create instant short circuits, long term insula-tion damage, and long term metallic component corrosionamong other complications. Medium-voltage switchgeathat is exposed to high humidity conditions will absorbmoisture, and voltage stresses will attack the hydrophobicinsulation surfaces which were designed to inhibit moisture

absorption (see figure 5).

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Figure 6 Severe Breaker Damage Causedby Improper Rack-In Operation

Figure 7 Severe Damage from Arcing Ground Fault

For medium-voltage switchgear, using the partial dis-charge detector described above will prevent long termmoisture-related insulation faults while the infrared cameracan detect abnormal heating of corroded connections.

Breaker RackingRacking in a closed circuit breaker onto an energized

bus can quickly cause severe personal injury or death andimmediate severe equipment damage (see figure 6). Addi-tionally, the breaker may not always line up properly or mayencounter other difficulties as it is being racked, and theseproblems can cause a sudden severe fault. Unfortunately,the traditional act of breaker racking requires personnel tomanually perform this task directly in harms way.

Te solution to this problem is to always make sure thatmechanical and electrical interlocks are functional and allbreaker and cell components are properly inspected andserviced. o ensure personnel safety, strong considerationshould be given to employment of a remote circuit breakerracking device such as shown in figure 6.

Te solution to this problem requires an outage andmanually testing the ground fault protection system bycurrent injection. Just as important is to pay close attention

to ensure that the equipment is properly installed. Sensorpolarities must be tested when applicable and the neutralground connection must be located in the correct positionso that the sensors will detect fault currents properly. A listof recommended test procedures can be found in the NEAAS and MS standards.

Mr. Genutis received his BSEE from CarnegieMellon University. He was a NEA Certifiedechnician for 15 years and is a Certified Coronaechnician. Dons technical training and educationare complemented by twenty-five years of practical

field and laboratory electrical testing experience.Don serves as President of No-Outage Electricalesting, Inc., a Group CBS affiliate that focuses onnew inspection technologies performed while the

equipment remains in service.

Faulty Ground Fault ProtectionUnlike the items above, a defective ground fault protective

device will not create a fault itself. However, it will not offerprotection from an arcing ground fault which is a commonfailure mode in 480Y/277V solidly-grounded switchgear.Tese types of faults are very destructive but do not drawhigh enough currents to trip breakers or cause fuses to openand can persist until catastrophic failure of the switchgearoccurs (see figure 7).