Electric Power, Alternating Current, Transformers, Magnetic Flux, Turns Ratio, Electric Panel and Circuit Breakers

Posted by PITHOCRATES - February 6th, 2013

Technology 101

AC Power is Superior to DC Power because it can Travel Farther and it Works with Transformers

Thanks to Nikola Tesla and his alternating current electric power we live in the world we have today.  The first electric power was direct current.  The stuff that Thomas Edison gave us.  But it had some serious drawbacks.  You needed a generator for each voltage you used.  The low-voltage of telephone systems would need a generator.  The voltage we used in our homes would need another generator.  And the higher voltages we used in our factories and businesses would need another generator.  Requiring a lot of power cables to hang from power poles along our streets.  Almost enough to block out the sun.

Another drawback is that direct currents travel a long way.  And spend a lot of time moving through wires.  Generating heat.  And dropping some power along the way due to the resistance in the wires.  Greatly minimizing the area a power plant can provide power to.  Requiring many power plants in our cities and suburbs.  Just imagine having three coal-fired power plants around your neighborhood.  The logistics and costs were just prohibitive for a modern electric world.  Which is why Thomas Edison lost the War of Currents to Nikola Tesla.

So why is alternating current (AC) superior to direct current (DC) for electric power?  AC is more like a reciprocating motion in an internal combustion engine or a steam locomotive.  Where short up & down and back & forth motion is converted into rotation motion.  Alternating current travels short distances back and forth in the power cables.  Because they travel shorter distances in the wires they lose less power in power transmission.  In fact, AC power lines can travel great distances.  Allowing power plants tucked away in the middle of nowhere power large geographic areas.  But there is another thing that makes AC power superior to DC power.  Transformers.

The Voltage induced onto the Secondary Windings is the Primary Voltage multiplied by the Turns Ratio

When an alternating current flows through a coiled wire it produces an alternating magnetic flux.  Magnetic flux is a measure of the strength and concentration of the magnetic field created by that current.  When this flux passes through another coiled wire it induces a voltage on that coil.  This is a transformer.  A primary and secondary winding where an alternating current applied on the primary winding induces a voltage on the secondary winding.  Allowing you to step up or step down a voltage.  Allowing one generator to produce one voltage.  While transformers throughout the power distribution network can produce the many voltages needed for doorbells, electrical outlets in our homes and the equipment in our factories and businesses.  And any other voltage for any other need.

We accomplish this remarkable feat by varying the number of turns in the windings.  If the number of turns is equal in the primary and the secondary windings then so is the voltage.  If the number of turns in the primary windings is greater than the number of turns in the secondary windings the transformer steps down the voltage.  If the number of turns in the secondary windings is greater than the number of turns in the primary windings the transformer steps up the voltage.  To determine the voltage induced onto the secondary windings we divide the secondary turns by the primary turns.  Giving us the turns ratio.  Multiplying the turns ratio by the voltage applied to the primary windings gives us the voltage on the secondary windings.  (Approximately.  There are some losses.  But for the sake of discussion assume ideal conditions.)

If the turns ratio is 20:1 it means the number of turns on the primary windings is twenty times the turns on the secondary windings.  Which means the voltage on the primary windings will be twenty times the voltage on the secondary windings.  Making this a step-down transformer.  So if you connected 4800 volts to the primary windings the voltage across the secondary windings will be 240 volts (4800/20).  If you attached a wire to the center of the secondary coil you can get both a 20:1 turns ratio and a 40:1 turns ratio.  If you measure a voltage across the entire secondary windings you will get 240 volts.  If you measure from the center of the secondary and either end of the secondary windings you will get 120 volts.

The Power Lines running to your House are Two Insulated Phase Conductors and a Bare Neutral Conductor

This is a common transformer you’ll see atop a pole in your backyard.  Where it is common to have 4800-volt power lines running at the top of poles running between houses.  On some of these poles you will see a transformer mounted below these 4800-volt lines.  The primary windings of these transformers connect to the 4800-volt lines.  And three wires from the secondary windings connect to wires running across these poles below the transformers.  Two of these wires (phase conductors) connect to either end of the secondary windings.  Providing 240 volts.  The third wire attaches to the center of the secondary windings (the neutral conductor).  We get 120 volts between a phase conductor and the neutral conductor.

The power lines running to your house are three conductors twisted together in a triplex cable.  Two insulated phase conductors.  And a bare neutral conductor.  These enter your house and terminate in an electric panel.  The two phase conductors connect to two bus bars inside the panel.  The neutral conductor connects to a neutral bus inside the panel.  Each bus feeds circuit breaker positions on both sides of the panel.  The circuit breaker positions going down the left side of the panel alternate between the two buss bars.  Ditto for the circuit breaker positions on the right side.

A single-pole circuit breaker attaches to one of the bus bars.  Then a wire from the circuit breaker and a wire from the neutral bus leave the panel and terminate at an electrical load.  Providing 120 volts to things like wall receptacles where you plug things into.  And your lighting.  A 2-pole circuit breaker attaches to both bus bars.  Then two wires from the circuit breaker leave the panel and attach to an electrical load.  Providing 240 volts to things like an electric stove or an air conditioner.  Then a reciprocating (push-pull) alternating current runs through these electric loads.  Driven by the push-pull between the two bus bars.  And between a bus bar and the neutral bus.  Which is driven by the push-pull between the conductors of the triplex cable.  Driven by the push pull of secondary windings in the transformer.  Driven by the push-pull of the primary windings.  Driven by the push-pull in the primary cables connected to the primary windings.  And all the way back to the push-pull of the electric generator.  All made possible thanks to Nikola Tesla.  And his alternating current electric power.



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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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