Grounding in multipule locations on a transmission line and circulating current
Grounding in multipule locations on a transmission line and circulating current
(OP)
We will have three cranes installing some new steel at the top of some towers between deenergized transmission lines. The towers are all within a 15 mile stretch of the transmission line and some of the guys are worried about "Circulating Current" between the three sets of grounds that will be installed for the cranes to lift the steel thru the lines to the top of the tower. Can someone explain this type of "Circulating Current" to me? I wouldn't think a crane operator would put a crane up thru the lines even if they are deenergized without them being grounded which some of the guys are suggesting.






RE: Grounding in multipule locations on a transmission line and circulating current
Are there any energized transmission lines in parallel with the deenergized line? If so, the deenergized line will pick up both an induced voltage from the current flowing on the adjacent line and a capacitive voltage do to the voltage on the adjacent line.
Normally there would be both Shorts (that bond the 3 phase together) and Grounds (that go from the shorts to the grounding rod & crane). Without any shorts and grounds, the deenergized line could have >50 kV AC from capacitive coupling to nearby energized lines. If there are no nearby lines, there could still be a trapped DC charge on the line from when it was switched off. Unless the crane is fully insulated for the nominal line voltage, the line needs to be grounded. Even line trucks with fully insulated booms still ground the truck chassis.
When the first set of shorts & grounds is placed, the voltage at that location is reduced to 0 and instead a small charging current will flow through the shorts and grounds. At remote locations, there will be an induced voltage proportional to the current flowing in the adjacent energized lines.
When the second set of shorts & grounds is placed, it provides a path for currents to flow. The induction will cause currents to circulate through both sets of shorts and grounds proportional to the current in the adjacent energized line. The resulting currents can be large enough such that line crews can have trouble removing the shorts and grounds without causing large arcs.
RE: Grounding in multipule locations on a transmission line and circulating current
RE: Grounding in multipule locations on a transmission line and circulating current
As far as a ground loop, just because there is a circuit path, without a current source it won't do much will it.
Yea, we have people who have been told to expect 10kA fault currents at some substations. But they now expect 10kA available at every substation, and want ground mats to match.