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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Northeasters, Convection Heating, Thunderstorms, Electricity, Electric Charge, Capacitors, Lightning and Lightning Rods

Posted by PITHOCRATES - February 8th, 2012

Technology 101

A Couple of Centuries ago when a Winter Storm Approached we Stocked Up on Wood for our Cast-Iron Heating Stoves

A study of prevailing weather conditions can predict tomorrow’s weather.  Once you’ve learned some basic weather phenomenon.  Weather generally moves from west to east.  Where cold fronts meet warm fronts we can get storms, tornados, rain, sleet, snow, etc.  And if a swirling northeaster buries a town under snow people in a town northeast of this town can expect the same.  Even though the winds are blowing in the opposite direction.

Today in the worst of winter’s weather we can stay warm and snug at work.  And at home.  Amazing when you consider some of our work places have a lot of exterior glass walls.  Glass curtain walls.  Which really transmit the cold.  Of course, even these rooms can be toasty rooms.  Ever wonder how?  Take a look at the floor under the window.  What do you see?  Fin-tube radiation heating registers.  Copper pipes with metal fins soldered to them.  We pump heating hot water through the copper pipe.  And when we do these fin-tube radiators heat the air as it moves through those fins.  As air heats it expands and gets thinner.  Becoming lighter than the cold air.  And rises.  As it moves up it pulls the cold air below through those heated fins.  Heating the cold air.  Where it, too, expands and gets thinner.  And rises.  Creating a heating convection current.  Heating the room.  And the window.  By washing it with warm air.  All without using a fan to move the air.  Heating units that do have fans and move the air are more for circulating the air to prevent the build of carbon dioxide (produced as we breathe).  While fin-tube heating does the lion’s share of heating our buildings.

So when they predict a winter storm we really don’t worry much about staying warm inside.  Of course, it wasn’t always like this.  A couple of centuries ago when we saw a winter storm was moving our way we made sure we had enough wood available.  To burn in our cast-iron heating stoves.  Where we burned our heating fuel in the room we heated.  And vented the products of combustion out through the chimney.  A big difference to using heating hot water and fin-tube radiators.  But the same principle nonetheless.  These wood-burners heated the cold air and created a heating convection current.  Just like those fin-tube radiators.

During Thunderstorms Clouds act like Charging and Discharging Capacitors

In the summertime when a cold front runs into a warm front it often generates some big thunderstorms.  And some dangerous lightning.  Which has started many building fires throughout history.  Especially churches with tall spires.  Which seemed to be magnets for lightning.  Which they were.  In a way.  Because thunder storms are electrical storms.  Which is why we have lightning.  But first a little about electricity.

Electricity flows between a positive and a negative charge.  The greater the difference in charges the greater the flow of electricity.  A battery can store a charge.  A battery has both a positive (plus) and a negative (minus) terminal.  You charge a battery by applying a voltage across these terminals.  The higher the voltage and/or the longer the charge the more energy is stored in the battery.  When we connect a light to a battery it completes the circuit between the plus and minus terminals.  And electricity flows through the light and illuminates it.  The light will stay lit until the battery runs out of charge.  Or until we open the circuit.  Depending on the voltage or amount of stored charge you may see sparks at the point where the circuit opens or closes.  The charge being strong enough to jump a small air gap just before the circuit is closed.  Or just after it opens.

A capacitor can also hold a charge.  What we used to call a condenser.  Which is a couple of plates separated by an insulator.  When we apply a voltage across the plus and minus terminals the plates charge.  The insulator keeps them from discharging internally.  The bigger the capacitor (i.e., the bigger the surface area of the plates) the bigger the stored charge.  After you charge a capacitor it will hold that charge.  It will dissipate slowly over time.  Or quickly if you short out the plus and minus terminals.  And if you discharge a capacitor quickly you’re going to see some sparking.  As the charge jumps the air gap just before the circuit is closed.  The bigger the capacitor the bigger the sparking.  Funny story.  I saw a kid cutting out the capacitor from an old television set.  The kind your parents had.  With a big glass cathode ray picture tube that used high voltage to move a scanning electron beam to excite (i.e., make glow) the phosphorous coating on the inside of the picture tube.  High voltage and a capacitor mean only one thing.  A very BIG stored charge.  No one turned on that TV for a long time.  But that capacitor held its charge.  As this kid quickly learned.  The hard way.  As he cut the wire going to the plus terminal his un-insulated side cutters touched the metal of the TV chassis.  Which was, of course, grounded.  So you had the plus terminal of a highly charged capacitor coming into contact with the minus terminal of said capacitor (via the grounded TV chassis).  It was like the Fourth of July in the back of that TV.  Threw that poor kid back on his butt.  Funny.  We all had a good laugh.  He was no worse for wear.  Except, perhaps, needing a new pair of undershorts.

All right, back to those electrical storms.  And lightning.  In a nutshell, those ugly black storm clouds are like capacitors.  As the atmosphere churns up these warm and cold weather fronts as they collide something happens.  They charge.  Like a capacitor.  With one plate being on the top of the cloud.  And the other plate being on the bottom of the cloud.  As the charge grows on the bottom of the cloud it induces an opposite charge in the ground below.  The old ‘opposites attract’.  So if a larger and larger minus charge is building up in the bottom of the cloud it attracts (i.e., induces) a larger and larger plus charge on the surface of the earth beneath the cloud.  Until the charges grow so great that they jump the air gap.  But this is no capacitor discharging.  The amount of energy in a lightning strike is so great it can melt sand into glass.  And anything that can do that can play havoc with trees.  And tall buildings.  Igniting a lot of fires along the way.  And killing a lot of people.  Until, that is, we started using lightning rods on our buildings.  Sharp pointed pieces of metal above the highest surfaces of the building.  We attach these rods to conductors running down the sides of the building to ground rods driven below the surface of the earth.  Providing a ‘path of least resistance’ for that charge to discharge through while causing minimal damage to the building.

Ben Franklin gave us Weather Forecasting, Convection Heating and Lightning Rods as well as the United States

Fascinating information, yes?  What’s even more fascinating is that we can trace these developments back to one point in time.  More fascinating still, we can trace them back to one man.  A curious fellow.  With a fascination for scientific experimentation.  Who went by the name of Benjamin Franklin.  Who pioneered weather predicting when a swirling northeaster hit Philadelphia with winds blowing in from the northeast.  Curiously, though, this storm had not yet ravished Boston.  In direct line with those winds.  But the storm moved on to Boston AFTER Philadelphia.  It was Franklin who observed that the northeaster was a counterclockwise spinning storm that moved northeast.  The winds in Philadelphia and Boston were only the top part of that spinning storm.  And weather forecasting was born.

Convection heat goes back to the Philadelphia stove.  What we later called the Franklin stove.  Franklin didn’t discover convection currents.  Or the stove using convection currents.  But he used the available knowledge to make a practical heating stove.  It wasn’t perfect.  But subsequent improvements made it the standard for indoor heating for about a century or two.

Ben Franklin did not discover electricity.  But electricity fascinated him.  And he discovered that lightning was electricity (yes, he actually flew a kite in a storm).  His experimentation gave us the first battery.  The first capacitor.  The standard of using ‘plus’ and ‘minus’ for electrical charges.  The conservation of charge (you can’t create or destroy an electrical charge.  You can only move it around).  The battery.  The capacitor.  Insulators.  Conductors.  Grounding.  All of the fundamentals of electrical circuits we use to this day.  And let us not forget that one other thing.  The effect of points on electrical charges (pointy metallic things help charges jump air gaps).  Which, of course, led to the lightning rod.  This after he set up the U.S. postal service and printed his newspapers and Poor Richard’s Almanac.  But before his political and diplomatic service.  And role as a key Founding Father.  Being the only one to sign the Declaration of Independence, the Treaty of Paris and the U.S. Constitution.  The document that started the Revolutionary War.  The document that ended it.  And the document that created the United States of America.  A busy man that Franklin was.  And a great man.

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