Transmission Lines - stored energy

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jesse varela

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Does anyone know how determine how much charge remains on a transmission line once the circuit has been opened but not brought to ground? I have a 3 mile long 138kV (nominal) transmission line that was opened up and never grounded. Many senior coworkers believe that the voltage will remain on the line for a very long time due to inherant capacitance,but according to a simulation program i tried, it appears that the voltage rapidly decays to less than 2kV after several minutes. I'm not 100% with the simulation program and i used a "pi equivalent" model to show the transmission line. Anyone have any experience with this?

regards,
jesse
 
I am thinking you can model this as an RLC equivalent circuit, and if you can calculate the equivalent capacitance, inductance, and resistance, you can calculate the time required for discharge. I am not familiar with the "Pi equivalent" model. Can you elaborate on this? I'll see if anyone in my employer's transmission department might know anything about this.
 
jesse varela said:
Does anyone know how determine how much charge remains on a transmission line once the circuit has been opened but not brought to ground? I have a 3 mile long 138kV (nominal) transmission line that was opened up and never grounded. Many senior coworkers believe that the voltage will remain on the line for a very long time due to inherant capacitance,but according to a simulation program i tried, it appears that the voltage rapidly decays to less than 2kV after several minutes. I'm not 100% with the simulation program and i used a "pi equivalent" model to show the transmission line. Anyone have any experience with this?

regards,
jesse
Click to expand...

For those who do not know, capacitance of a transmission is the result of potential difference between conductors, causing them to charge in the same manner as plates of a capacitor when there is a potential difference between them. The capacitance of parallel conductors is constant, and depends on the size and spacing of condcutors.

Now to answer the question. Power lines less then 50 miles in length have a negligible capacitance effect, and thus usually neglected. Longer lines of higher voltage, it becomes increasingly more important to consider. Therefore, a 3 mi line, would be negligilble. Your co-workers may be thinking of the longer lines.

There are formulas for determining the capacitance, that are quite rigoruous to perform by hand.
 
Static

Static

We refer to it as static, on the railroad transmission lines. Even when off and grounded there can be quite a charge when in close proximity to a substation.
We are talking 138KV 25cycle
 
Once the circuit is opened, you will have a static DC charge on the line. Since its DC, I don't think the RLC model would apply. I think the only thing that would matter is the series resistance between the lines, and to ground. I would also expect that to vary with the voltage on the lines. As the voltage drops, there is probably less conductance to short out the charge. (That is, the insulators designed for several KV would be almost perfect at a lower voltage.)

I would guess the only safe way to be sure the charge is gone is to ground the lines. But I'm not a utility engineer and I don't really know for sure.
 
geometry of the lines?

geometry of the lines?

jesse,

do you have the geometry of the lines, such as the spacing between the lines, bundling, and any other information about the line/pole configuration.

i agree with kingpb, that the capacitance is negligible for power system engineers, but there still is some charge there. i think that it would be fun to put this amount of charge into perspective since i wrote a MATLAB program to do such a thing for a HW assignment last semester :)

thanks.

matt
 
I?m assuming your talking about a 60hz AC transmission line.
If you start with a 138KV line, then I think you would have anywhere between zero and (1.41 x 138KV =) 195.2KV on the line when you open the line.

However, taking into consideration the often quoted rule of thumb that electricity jumps an inch per 10,000V, you will not be left with the voltage on the line at the instant of the break because a giant electric arc will continue to affect the remaining voltage on the line until the switch is sufficiently open.

I don?t believe that the classic capacitor formulas apply to wires that are spaced as far apart as typical primary lines are spaced. Instead, the wires would act more like simple charged objects. Meaning they don?t hold much energy.

How long the charge lasts, is of course dependant on the insulators and the dialectic (air)
The dust, pollen, bird feces, dead squirrels and rain will affect these characteristics drastically. Some of the foreign matter on the insulators would likely act like semiconductor material and rectify some of the RF that is picked up, thus recharging the wires to a small degree.

If you take an AM radio around various power lines, arcing on the insulators will manifest itself in the form of RFI and this will very greatly. It follows that the time a charge is held by a line would very greatly.

I would be more concerned with induced voltage in 3 miles of floating line then in residual charge.
 
capacitance and spacing

capacitance and spacing

I don?t believe that the classic capacitor formulas apply to wires that are spaced as far apart as typical primary lines are spaced. Instead, the wires would act more like simple charged objects. Meaning they don?t hold much energy.
Click to expand...

Could you extend on this?

Thanks.

Matt
 
johnny watt said:
I would be more concerned with induced voltage in 3 miles of floating line then in residual charge.
Click to expand...

Hi JW
Do you have any knowledge about induced voltage on other parallel metallic items (pipeline, fences, vehicles, etc) running along the HV line?
 
In my original post, I was thinking of the transient analysis of opening the switch. I think when people are talking static charge, that is more steady state type of analysis. However, I do not know how long it would take to develop a static charge on the disconnected transmission conductors.
 
Natfuelbilll said:
Hi JW
Do you have any knowledge about induced voltage on other parallel metallic items (pipeline, fences, vehicles, etc) running along the HV line?
Click to expand...

It's a common occurence. I worked for a utility who built a 25kv distribution line parallel to a 345 kv transmission line for a couple of miles. We had a 480 volt 3 phase transformer bank and service at the end of the lateral. With the cutouts open on the 25kv line, there was over 500 volts available at the service when no load was applied. Nobody closed the service disconnect, but I assume the disappeared when a load was applied.

I've seen metal conduit in buildings erected adjacent to transmission lines becomes energized until grounded.

Fences will energize if they aren't properly grounded also.

Jim T
 
mpross said:
Could you extend on this?

Thanks.

Matt
Click to expand...
S= εA/c
That?s the formula you see in all the textbooks
I think it can be used on close wires like a 300 ohm ribbon cable. The wires can be treated like plates and would a give result close to the actual.

I don?t think the formula works on large spacings like powerlines.
 
equation for power lines

equation for power lines

jw,

yeah that eqn is not the one in mind for the capacitance of power lines.

the eqn for average capacitance to neutral

c = (2*pi*epsilon) / ln(Dm/Rb) [F/m]

is the one that i used in class... where of course Dm is the GMD between phase conductors, and Rb is the GMR of each phase bundle with the true radius being used.

mp
 
Not sure if the original question has been answered, but if the purpose is to work on the overhead line, then "The Lineman's and Cableman's Handbook" by Thomas M. Shoemaker and James E. Mack gives the instuctions for Protective Grounds in Section 27. Among other things, each section of the de-energized line must be protectively grounded. Due to the many configurations of transmission lines and proximity of other circuits on these and other distribution lines, all portions of the line to be worked on must be effectively Protectively Grounded to insure the linemans safety. Corporate procedures also would tag and test the de-energized line before work can commence.
However, this book does not give any way of calculation residual and capacitive voltages that I have yet seen.
 
Static

Static

You can have AC voltage induced on the line if there is a nearby AC line. Inductive coupling.

You can have static charge build up on the line from wind (DC charge).

The Pi and T models only apply for AC operations in steady state conditions.

You can also have leakage (resistive couling) from another pwoer line on the same structure.

Standard utility practice is "unless it is grounded, it isn't dead."
 
beanland said:
Standard utility practice is "unless it is grounded, it isn't dead."
Click to expand...
"Work it like it's hot, even if it's dead", is the present motto of many line construction outfits. Seems silly, when you can see macs hanging, but I'm sure it's saved many lives.
 
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