Overhead High Voltage Power Lines, Lightning Rod, Grounding Conductor, Ground Rods, Flashover and Underground Duct Bank

Posted by PITHOCRATES - August 29th, 2012

Technology 101

Electricity always wants to Take the Path of Least Resistance to Ground

Have you ever noticed bright-color globes on overhead high voltage power lines?  Do you know why they are there?  Because it’s hard to see those wires.  Which could be a problem for ships with tall masts traveling a river where these wires cross.  Or to low-flying aircraft.  Which is why you see them around airports.  And many hospitals.  Why?  Helicopters.  So when helicopter pilots are bringing critically injured patients to a hospital they will be able to see these bright-color globes and take evasive action to avoid flying into these wires.

Of course, not everything takes evasive action to avoid these lines.  One thing in particular tries its hardest to purposely hit these overhead high voltage power lines.  Lightning.  Why?  For the same reason you get a static electric shock after sliding over your cloth seats to get out of your car.  It creates a potential difference between you and your car.  So as your hand approaches your car handle to close your door a little spark jumps between you and your car.  To rebalance that unbalanced charge.  And send those ‘stripped’ electrons back home.  Which is a how a lightning strike occurs.  Only with the clouds being a much, much larger butt sliding across a car seat.  And anything sticking out of the ground being your finger.

Electricity wants to flow to the ground.  But if it flows straight to ground it can’t do much work for us.  So we try to prevent that from happening.  Which can be a struggle as electricity always wants to take the path of least resistance.  Instead of turning a motor it would much rather flow directly to the ground.  And it sometimes happens.  And when it does it can be dangerous.  For if the same amount of energy that can accelerate a subway train is shorted directly to ground there will be arcs and sparks and smoke and even a little welding as that electric discharge melts metal and ionizes the gas into an explosion of heat and noise.

So Overhead Cabling is Simpler and More Convenient to work with and Requires Fewer Power interruptions

Now these are the last things you want to happen to our electric grid.  Explosions of ionized gases and molting metal.  Because they tend to interrupt the flow of electricity in the power lines to our homes and businesses.  And thanks to work started by Benjamin Franklin we can do something to try and prevent this.  After Franklin made his wealth he became a scientist.  Because it interested him.  He studied the new field of electricity.  And he proved that lightning was in fact electricity.  So he invented the lightning rod.  To attract that lightning and help it go where it wants to go.  To the ground.  Instead of hitting the structure below the lightning rod.  And starting it on fire.

If you look at our overhead high voltage transmission lines you will notice a set of three wires.  Supported horizontally from a tower.  Or two sets of three wires supported vertically from a tower.  These are the high voltage transmission lines.  Above these lines you will see smaller lines.  At the very top of the transmission tower.  These wires are the lightning rods for the power lines below them.  They either terminate to the metal transmission towers.  Or there is a grounding wire running from these wires down a nonconductive pole to the earth.  At the base of the tower these conductors terminate to ground rods driven into the earth.  In the case of a metallic tower there are conductors connecting the base of the tower to ground rods.  So if lightning strikes at these grounding conductors or towers it will take the path of least resistance to go where it wants to go.  Along these grounding conductors to earth.

Low flying aircraft, tall ships and lightning?  Seems like overhead transmission lines give us a lot of trouble.  Wouldn’t it be smarter to bury these lines?  Yes and no.  While it is true it would be difficult for a plane, ship or lightning to hit a buried power line there are other considerations.  Such as infrastructure cost.  Overhead conductors need towers on small plots of land evenly spaced underneath.  Underground conductors need a trench, conduits, manholes, sand, rebar, concrete, etc., wherever the conductors go.  Also, overhead wires are bare.  Because they are in the open air separated from other conductors.  Conductors underground need insulation to prevent short circuits between phases.  Because the three cables of a 3-phase circuit are pulled into one conduit.  And these cables touch each other.  So the insulation, conduit, concrete and sand make it difficult to ‘tap’ a feeder to feed, say, a new substation.  Requiring power interruptions, excavating, cutting and splicing to tap an underground feeder.  Whereas tapping a bare overhead conductor requires none of that.  They can simply attach the new substation feeders to the live overhead wires.  Then close a switch in the new substation to energize it.  So overhead cabling is simpler and more convenient to work with.  And some voltages simply make overhead lines the only option.

For a Given Current you can use a Smaller Conductor in the open Air than you can use Underground

Current flows when there is a voltage differential.  The greater the voltage difference is the greater the current flow.  In 3-phase AC power generators push and pull an alternating current through a set of three cables.  Think of the reciprocating gasoline engine.  Where the up and down motion of the piston is converted into useful work.  Turning the wheels of a car.  When the current is equal in each of the three cables the 3-phase circuit is balanced.  Which means when current is moving away from the power plant on one cable it is returning to the power plant on another cable.  In North America a complete cycle of current on one conductor happens 60 times a second.  During that second voltage rises and falls as the current flows.   Think of three pistons going up and down.  The crankshaft turns at the same speed for all three pistons.  But the pistons don’t go up and down at the same time.  As it is in a three-phase feeder.  Current leaves the power plant in one conductor.  When it’s one-third of the way through its cycle current leaves in the second conductor.  When the first current is two-thirds of the way through its cycle, and the second current is one-third of the way through its cycle, current leaves in the third conductor.

Current and voltage are both zero twice in each cycle.  Just like the speed of a piston is zero twice a cycle (at the top and the bottom of its stroke).  But it’s never zero at the same time in more than one conductor.  In fact, the voltage is never the same in any two conductors at the same time.  Which means there is always a voltage differential between any two of the three conductors in a 3-phase circuit.  So a current will always flow between two phase conductors if they come into contact with each other.  And if the voltage is high enough the current will arc across the air gap (or flashover) between two conductors.  If they get too close to each other.  And the higher the voltage of these feeders the greater the distance required between the phase conductors to prevent any flashover.  On some of the highest voltage feeders (765 kilovolt) the conductors are more than 50 feet apart.  With one conductor in the middle and one on either side 50 feet away that’s 100 feet minimum distance required for a three-phase 765 kV feeder.  To put these underground would require a very wide trench.  Or cables with very, very thick insulation.  Requiring large conduits.  Deep and wide trenches.  And great cost.

Cables in open air have another advantage over underground cables.  High currents heat cables.  If a cable gets hot enough it can fail. There are only two ways to prevent this heat buildup.  Use thicker cables.  Or cool the cables.  Which can happen with overhead cabling.  The open air can dissipate heat.  Conductors in an underground duct bank have no air blowing across these cables to cool them.  Which means for a given current load you can use a smaller conductor in the open air than you can use in an underground duct bank.  Bigger cable means bigger costs.  On top of all the other additional costs.  And the inconvenience of excavating, cutting and splicing to make a tap.  So despite the risk of a ship, aircraft or lightning hitting our electric grid going overhead just makes more economic sense that going underground.  Because they are less costly.  And are easier to work on.  For replacing a failed overhead cable is a lot easier than replacing a failed underground cable.  Especially if you can’t pull the old cable out.  And don’t have a spare duct to pull a new cable in.  If that happens then you have to install new duct bank before you pull in new cable.  Which will be more expensive than the cable itself.

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Electric Grid, Voltage, Current, Power, Phase Conductor, Neutral Conductor, 3-Phase Power, Transmission Towers and Corona Discharge

Posted by PITHOCRATES - August 15th, 2012

Technology 101

The Electric Grid is the Highways and Byways for Electric Power from the Power Plant to our Homes

Even our gasoline-powered cars operate on electricity.  The very thing that ignites the air-fuel mixture is an electric spark.  Pushed across an air-gap by a high voltage.  Because that’s something that high voltages do.  Push electrons with such great force that they can actually leave a conductor and travel through the air to another conductor.  Something we don’t want to happen most of the time.  Unless it’s in a spark plug in our gasoline engine.  Or in some movie prop in a cheap science fiction movie.

No.  When we use high voltage to push electrons through a conductor the last thing we want to happen is for the electrons to leave that conductor.  Because we spend a pretty penny to push those electrons out of a power plant.  And if we push the electrons out of the conductor they won’t do much work for us.  Which is the whole point of putting electricity into the electric grid.  To do work for us.

The electric grid.  What exactly is it?  The highways and byways for electric power.  Power plants produce electric power.  And send it to our homes.  As well as our businesses.  Power is the product of voltage and current.  In our homes something we plug into a 120V outlet that draws 8 amps of current consumes 960 watts.  Which is pretty big for a house.  But negligible for a power plant generator producing current at 20,000 volts.  For at 20,000 volts a generator only has to produce 0.48 amps (20,000 X 0.48 = 960).  Or about 6% of the current at 120V.

Between our Homes and the Power Plant we can Change that Current by Changing the Voltage

Current is money.  Just as time is money.  In fact current used over time helps to determine your electric bill.  Where the utility charges you for kilowatt hours (voltage X current X time).  (This would actually give you watt-hours.  You need to divide by 1000 to get kilowatt hours.)  The electric service to your house is a constant voltage.  So it’s the amount of current you use that determines your electric bill.  The more current you use the greater the power you use.  Because in the power equation (voltage X current) voltage is constant while current increases.

Current travels in conductors.  The size of the conductor determines a lot of costs.  Think of automobile traffic.  Areas that have high traffic volumes between them may have a very expensive 8-lane Interstate expressway interconnecting them.  Whereas a lone farmer living in the ‘middle of nowhere’ may only have a much less expensive dirt road leading to his or her home.  And so it is with the electric grid.  Large consumers of electric power need an Interstate expressway.  To move a lot of current.  Which is what actually spins our electrical meters.  Current.  However, between our homes and the power plant we can change that current.  By changing the voltage.  Thereby reducing the cost of that electric power Interstate expressway.

The current flowing through our electric grid is an alternating current.  It leaves the power plant.  Travels in the conductors for about 1/120 of a second.  Then reverses direction and heads back to the power plant.  And reverses again in another 1/120 of a second.  One complete cycle (travel in both directions) takes 1/60 of a second.  And there are 60 of these complete cycles per second.  Hence the alternating current.  If you’re wondering how this back and forth motion in a wire can do any work just think of a steam locomotive.  Or a gasoline engine.  Where a reciprocating (back and forth) motion is converted into rotational motion that can drive a steam locomotive.  Or an automobile.

The Voltages of our Electric Grid balance the Cost Savings (Smaller Wires) with the Higher Costs (Larger Towers)

An electric circuit needs two conductors.  When current is flowing away from the power plant in one it is flowing back to the power plant in the other.  As the current changes direction is has to stop first.  And when it stops flowing the current is zero.  Using the power formula this means there are zero watts twice a cycle.  Or 120 times a second.  Which isn’t very efficient.  However, if you bring two other sets of conductors to the work load and time the current in them properly you can remove these zero-power moments.  You send the first current out in one set of conductors and wait 1/3 of a cycle.  Then you send the second current out in the second set of conductors and wait another 1/3 cycle.  Then you send the third current out in the third set of conductors.  Which guarantees that when a current is slowing to stop to reverse direction there are other currents moving faster towards their peak currents in the other conductors.  Making 3-phase power more efficient than single-phase power.  And the choice for all large consumers of electric power.

Anyone who has ever done any electrical wiring in their home knows you can share neutral conductors.  Meaning more than one circuit coming from your electrical panel can share the return path back to the panel.  If you’ve ever been shocked while working on a circuit you switched off in your panel you have a shared neutral conductor.  Even though you switched off the circuit you were working on another circuit sharing that neutral was still switched on and placing a current on that shared neutral.  Which is what shocked you.  So if we can share neutral conductors we don’t need a total of 6 conductors as noted above.  We only need 4.  Because each circuit leaving the power plant (i.e., phase conductor) can share a common neutral conductor on its way back to the power plant.  But the interesting thing about 3-phase power is that you don’t even need this neutral conductor.  Because in a balanced 3-phase circuit (equal current per phase) there is no current in this neutral conductor.  So it’s not needed as all the back and forth current movement happens in the phase conductors.

Electric power travels in feeders that include three conductors per feeder.  If you look at overhead power lines you will notice they all come in sets of threes when they get upstream of the final transformer that feeds your house.  The lines running along your backyard will have three conductors across the top of the poles.  As they move back to the power plant they pass through additional transformers that increase their voltage (and reduce their current).  And the electric transmission towers get bigger.  With some having two sets of 3-conductor feeders.  The higher the voltage the higher off the ground they have to be.  And the farther apart the phase conductors have to be so the high voltage doesn’t cause an arc to jump the ‘air gap’ between phase conductors.  As you move further away from your home back towards the power plant the voltage will step up to values like 2.4kV (or 2,400 volts), 4.8kV and13.2kV that will typically take you back to a substation.  And then from these substations the big power lines head back towards the power plant.  On even bigger towers.  At voltages of 115kV, 138kV, 230kV, 345kv, 500kV and as high as 765kV.  When they approach the power plant they step down the voltage to match the voltage produced by its generators.

They select the voltages of our electric grid to balance the cost savings (smaller wires) with the higher costs (larger towers taking up more land).  If they increase the voltage so high that they can use very thin and inexpensive conductors the towers required to transmit that voltage safely may be so costly that they exceed the cost savings of the thinner conductors.  So there is an economic limit on voltage levels  As well as other considerations of very high voltages (such as corona discharge where high voltages create such a power magnetic field around the conductors that it may ionize the air around it causing a sizzling sound and a fuzzy blue glow around the cable.  Not to mention causing radio interference.  As well as creating some smog-causing pollutants like ozone and nitrogen oxides.)

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