Grounding in multipule locations on a transmission line and circulating current 1

[k26kv]

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.

 

[bacon4life]

Is this for three cranes at the same site, one crane at three sites at different times, or three cranes at three different sites simultaneously?

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.

 

[k26kv]

We had about a 15 mile stretch, 3 cranes at different towers within that 15 mile range. At each of the crane sites we installed grounds to create the equipotential work zone. The ground switches were also closed at both ends of the line. We were not working on the lines just installing cell site antenna brackets at the top of the tower. We were worried mostly about the one crew that was going to change out a set of bells that got shot out and that they would also be installing their grounds to create their equipotential zone but that they would be the only ones that would be "hands on" the conductor at any time. The guys at the bell change out site put the high voltage amp meter on the line at their location before they applied their grounds and only got a reading of 1 amp. No arc putting grounds on or taking them off. Things went well, I just need to research this a little more so that I more fully understand it. Our safety unit just tossed out that we needed to beware of circulating current but they did not provide any direction as to what should be done and couldn't give us any numbers that we might be dealing with out in the field. It was frustrating. Like I said before I will be researching this more so that I can understand it better. Thank You for your reply.

 

[cranky108]

It is typical for people who don't know there stuff to throw out hurdles like that. It makes them appear to know something. If you ask them if the measured 25 amps is typical, and see what they say, it will tell you quickly what they really know.

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.