Drives & Controls Services, Inc. · • 3 - 250 KVA transformers should have a full load of 903...

Drives & Controls Services, Inc.OVER 30 FIELD REPRESENTATIVES WORLD

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Drives & Controls Services, Inc.

•HIGH VOLTAGE CIRCUITS•LOW VOLTAGE CIRCUITS•LIGHTING CIRCUITS

•Boom was pulled back and pinned afteran extended outage•Dragline power tripped at the substation•Suspected Sync Motor Stator failure•Primary 200 amp fuses were blown on the7200/480 transformers

480 VOLT PHASE to PHASE to GROUND BLOWS 8KV FUSEON PRIMARY OF TRANSFORMER

•Old cable was replaced•Dragline power still tripped withground fault•Measured a ground fault on primaryof “C Phase” transformer

•Replaced grounded transformer with spare•Dragline power still tripped with ground fault•Measured a ground fault on primaryof “C Phase” on the “new transformer”

•Acquired bank of three transformers from anearby mine•Power was applied to dragline•200 amp primary fuse blew again•Found secondary wiring error

•On a schematic it looks simple

•Not so much in reality

“B” to “C” PhaseShort Circuit

•All leads were disconnected and isolatedagain•Rang out each lead to the phase of thebreaker it was feeding and megged toground•We found that one of the “B” phase leadshad been connected to “C” phasetransformer

•2 weeks ago another of thesetransformers had a catastrophic failure•Causing a fire in the High Voltage room

•The “B” phase transformer explodedblowing the lid off and breaking a primarybushing

•A flash and then smoke was observed bythe operator on his camera system

•The electricians were called and killedpower at the substation•Operator and oiler put out the fire with afire extinguisher•All three transformers were replaced with anew “matched” set•The primary high voltage cable wasreplaced

•An AC ground alarm was detected in theoperator cab

•Boom lights are sometimes part of the480 volt circuit•All 480 volt circuits begin at the 3phase buss in the high voltage room•From there they are connected to thererespective breaker panels•The first step is to isolate the primarycircuit of the ground

•The dragline power was disconnectedlocked, tagged and grounded at thesubstation•The incoming fuses to the 7200 / 480transformer bank were removed

•The next task was to locate the cablewith the ground fault•The third cable disconnected had theground•It was then traced hand over handthrough the cable tray•It led to the boom lighting breaker panelmounted on the dragline front wall

•Boom lighting breaker panel fedthe boom lighting ballast box

•The lighting ballast primary fuseswere remove one at a time until theground went away•The ground was then isolated to onelighting circuit on the boom

GROUND FAULT WASFOUND AT THE LAMPSOCKET

•High temperature in the high voltageroom•3 transformers for the main buss and 3for the auxiliary buss•The two end transformers on the mainbuss were much hotter

• 3 - 250 KVA transformers should have afull load of 903 amps or 301 amps perphase•This buss feeds the GF Exciters, MFExciters•All motor blowers, 4 house fans, 8exhaust fans, 6 bleeder fans and theboom lights

• An estimated average load of 700 to800 amps•With the M-G sets off we measured “A”278 amps, “B” 238 amps and “C” 300amps

• We performed a windings test on bothbanks of the transformers by injecting120 volts ac on the primary andmeasuring the output•We then swapped transformers to getthe best windings match per set oftransformers

• Now the current in the main buss wasnear zero with the M-G sets and all fansand blowers off•The mine has since replaced all of themain and auxiliary transformers withnew “matched” transformers