Induced voltage on parallel transmission lines

[wbd]

Hello,

Is there a way of calculating what induced voltage would be present on an adjacent de-energized line that runs parallel to another line that is energized? both lines are 345kV.

 

[racookpe1978]

What do the measurements say?
How long are they in parallel, and how far apart are the two centers of the lines? (tower-tower distance)

Is the de-energized line grounded out on all phases and all lightening wires so seldom that there there is appreciable voltage buildup between the parallel section and the grounding point?

 

[wbd]

My understanding is that the de-energized line was in the process of being grounded when there was a large arc drawn.

 

[magoo2]

Yes, there is a method to calculate the resulting voltages in parallel 345 kV transmission lines. M. Harry Hesse published an IEEE paper on that subject about 40 years ago and his example was for two 345 kV lines. The concern is with capacitive coupling so the actual position of each conductor is important since you'll be calculating the potential coefficient matrix.

I've done this calculation using the matrix function in Excel and it works fine.

There's also information on this in the EPRI Red Book.

 

[odlanor]

I have seen induced current would be present on an adjacent de-energized line grounding by disconnecting switch that runs parallel to another line that is energized.
both lines are 345kV. Line energized with 890A loading with 35km in parallel with 50A induced current at de-energized grounding line. Ground line switch were changed

 

[bacon4life]

Isolated lines can easily have more than 10% of the adjacent line voltage from capacitive coupling. If the line is grounded at more than one point, 10's of amps of circulating current is common. Some arcing is thus expected while applying or removing grounds. If you just want ballpark numbers, Chapter 1 of most power system analysis textbooks show how to calculate both inductive coupling and capacitive coupling for parallel single phase lines.

 

[davidbeach]

One large company in this area that has lots of 500kV has, so I'm told, the equivalent of single pole breakers that they use in their ground leads so that they can make and break the ground current with breaker than with the ground sets. Long lines, paralleled with other in service lines, and grounded at both ends (terminal ground switches) can see thousands of amps for a ground fault on one of the paralleled in service lines.

 

[waross]

Beware of touch and step potentials near field grounding electrodes. The current to ground has been known to dry out the surrounding soil and raise the soil resistance to hazardous levels. I read a couple of accident reports concerning electric shock from ground electrodes due to high earth resistance around the ground electrodes and the current flowing.
One utility flagged a 50 foot square area so that no-one could approach within 25 feet or more of the temporary field ground rod.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racookpe1978]

So, where two or more very high lines are in parallel with each other, I understand from the above that the de-energized lines need to be grounded only once (to prevent the circulating currents) but need to be grounded almost as soon as the line is de-energized because the potential begins building up to dangerous levels almost immediately on shutdown?

Assume a thirty mile run, with the two lines in parallel for only 4 miles of the 30. The entire line becomes charged from the induced EMF, doesn't it?

 

[bacon4life]

The induced voltage is AC, so the length of time between isolating and grounding is not important. What matters is that the line is grounded before workers can switch from using hot line techniques to using deenergized techniques. Even with grounds in place they may still use rubber gloves etc.

When a line is initially isolated deenergized, the conductors may have a trapped DC charge. The trapped DC voltage may takes cycles to minutes to decay depending upon how much leakage occurs across insulators and through instruments transformers & surge arresters.

Yes, the entire line would have induced voltage. If you measured the voltage on the entire ungrounded 30 mile line, it would probably have a lower voltage than if you isolated just the 4 mile section and measured the voltage on just the parallel section.

 

[Marmite]

I'm not sure whether there is much value in trying to calculate a figure for the induced voltage as there are so many environmental and system variables that would affect the calculation at any given location and at any point in time. In reality the work methods employed have to err on the side of caution. It doesn't matter whether you experience a lethal voltage of 1000V, or 300V, the outcome is the same.
Attached is an example of what can happen when things go wrong.

Regards
Marmite

 

[waross]

My understanding is that there are two mechanisms involved.
One is the capacitive effect. The energized conductor and the de-energized conductor separated by an insulator, air, form one capacitor. The de-energized conductor and the ground form a second capacitor. The simplified circuit is two capacitors in series.
The second effect is magnetic induction. The lines of force caused by the current in the energized conductor cut the de-energized conductor and induce a voltage in the second conductor.
The capacitive effect depends mostly on spacing. The magnetic induction effect depends mostly on spacing and on the magnitude of the current in the energized line.

Long lines, paralleled with other in service lines, and grounded at both ends (terminal ground switches) can see thousands of amps for a ground fault on one of the paralleled in service lines.

This is in accord with my understanding of the magnetic induction effect.
We have mostly been considering the open circuit voltage. The current sourced by each type of induction will increase with the length of the line.



Bill
--------------------
"Why not the best?"
Jimmy Carter