Induced Voltages on Un-Energized Distribution Lines from Transmission Lines 1

[hd2000]

Hi does anyone have any experience or tips on how to work on e.g. 11kV Distribution lines running close/parallel to eg. 220kV Transmission lines?

Whenever we have to work on distribution lines even with multiple earth connected our linesmen still feel induced voltages when wearing HV gloves, at time leading to outages on large Transmission line.

Any experience on the best way to work?

 

[magoo2]

Work it like it is energized. The induced voltage can be capacitively coupled and the magnitude of that voltage depends on the spacing between the 2 lines. You can also get inductive coupling where the 2 lines are parallel to each other and the voltage depends on how much current flow is taking place in the transmission line. We just tell our crews to treat it like it is already energized and then they don't get surprized.

 

[Jebb]

How close is the 220kV line? Are the linemen using dual point grounding (i.e. installing grounds one pole away, on each side of the work zone) and personal grounds at the work location? This method, with the dual point grounds as close as possible to the work zone, should reduce the induced voltage to an acceptable level.

 

[squeeky]

Hi hd2000,

I'm no expert. We have to work between two grounded points. We have special clips that attach to the conductors via your link stick. They have quick release to get them off. Other end is connected to a temporary earth rod. While installing OHL in Zambia, the wind and dust induced voltages in the line. Lightning strikes as far away as far10 kms also caused people to get shocks. You cannot get a shock if it is at ground potential.

 

[hd2000]

True, our utility has never done live line work and we are looking into it nowadays.

The 220KV line is roughtly about 20m or so metres away but running parallel to the 11kV line. We apply earths at the isolation point (about 400m away) and then also apply earths both side the point of work (10m each side) so yep dual point grounding.

 

[ExtraHighVoltage]

The induced voltage depends upon the clearance between the lines and there are different methods available to compute the same. Please be specific with the requirements.

 

[stevenal]

You are describing "bracket" grounding. Might want to try equipotential (worksite) grounding instead. See

http://www.hubbellpowersystems.com/literature/encyclopedia-grounding/pdfs/07-0801-09.pdf.

 

[Marmite]

Transmission lines are considered one of the major sources of induction. The current in the energised HV conductor causes a magnetic field that can induce dangerous voltages and currents in conductive materials that are in proximity to the energised line. The induced voltage in the de-energised circuit is a function of:
o the current in the higher voltage line
o the distance separating the two lines
o the nature of the dielectric, ambient conditions, moisture, heat, wind, etc.

There is no point in trying to calculate the value of the induced voltage due to the variable nature of all of the above factors. The common approach is to establish whether a risk exists and then introduce additional precautions to mitigate against it. For example, an initial inspection of the line route or examination of a map indicating all other powerlines adjacent to the line to be isolated must be conducted. This is to determine the likelihood of the existence of any induced voltages. Hazardous induced voltages may be present on the lower voltage line due to it running in close proximity to a live conductor or adjacent transmission line at any point along its length. The danger may not be visible from the worksite.

There is a distinct possibility that a voltage detector used to prove that the line is dead prior to the application of earths may give a false live indication.

If there is an earth switch available on the feeder at a ground mounted substation, then that should be the first earth applied and it should remain closed throughout the work, unless it is required to be opened for testing.
Working earths must be placed both sides, and as close as possible, to the workers who are working on the apparatus.
Ensure that the earth grounding leads are bonded at a single earthing point, or rod, if possible to create an equipotential work zone.
Apply and remove portable earths using an approved and rated insulated stick.
When operating on the structure, connect mobile equipment (e.g. cranes) to the portable earthing point on the structure to create an equipotential zone.
Wear a minimum of 500 V rated gloves with approved wrist length mechanical protective gloves to avoid touch potential which may occur when working around mobile plant and other conductive apparatus.
If any part of the conductor being worked on is disconnected in any way, a temporary jumper must be installed to bridge out the break in the conductor. If this is not done, the worker touching both ends of the disconnected conductor will act as a bridge and may receive a shock if an induced voltage is present in the isolated conductor.
Never allow a situation where a person could become part of the ‘circuit’, a bond must always exist across a break in a conductor where access to both ends is possible.


Regards
Marmite

 

[Jebb]

Are the grounds solidly connected (i.e. clean, bright, & tight) to the conductors at the grounding point. I've seen issues where the grounds were not effective in draining stray/induced voltage because the Al conductor had some surface corrosion that was not removed. It doesn't take much to make a poor connection. Be sure to take a look at stevenal's link to the Hubbell Power Systems. I should give you some insight into your problem.

 

[hd2000]

Thank you guys, Stevenal & Marmite that is the exact info I was looking for. Next time we will make sure to create an "equipotential zone" by making sure there is a "common" with our portable earths & treat the work as "live".

Thanks!!!