Primary Services, Power Redundancy, Double-Ended Primary Switchgear and Backup Generators

Posted by PITHOCRATES - July 3rd, 2013

Technology 101

The Higher the Currents the Thicker the Conductors and the Greater the Costs of Electrical Distribution

If you live by a hospital you’ve probably noticed something.  They never lose their power.  You could lose your power in a bad storm.  Leaving you sitting in your house on a hot and humid night with no air conditioning.  No lights.  No television.  No nothing.   And across the way you see that hospital lit up like a Christmas tree.  As if no storm just blew through your neighborhood.  Seemingly immune from the power outage afflicting you.  Why?  Because God loves hospitals.

Actually the reason why their lights never seem to go out has more to do with engineers than God.  And a little thing we call power redundancy.  Engineers know things happen.  And when things happen they often cause power outages.  Something we hate as we’ve become so accustomed to our electric-powered world.  But for us it is really more of an annoyance.  For a hospital, though, it’s not an annoyance.  It’s a life safety issue.  Because doctors and nurses need electric power to keep patients alive.  So engineers design ‘backup plans’ into a hospital’s design to handle interruptions in their electric service.  But first a brief word on power distribution.

Nikola Tesla created AC power transmission and put an end to Thomas Edison’s DC power dreams.  The key to AC power is that the alternating current (AC) allows the use of transformers.  Allowing us to step up and step down voltage.  This is very beneficial for the cost in electric power distribution is a factor of the size of the current carrying conductors.  The higher the currents the thicker the conductors and the greater the costs.  Because power is the product of voltage and current, though, we can reduce the size of the conductors by raising the voltage.  Power (P) equals voltage (E) times current (I).  Or P = I * E.  So for a given power you can have different voltages and currents.  And the higher the voltage the lower the current.  The smaller the conductors.  And the less costly the distribution system.

Neighborhoods typically get a Radial Feed so when we Lose our Power our Neighbors Lose their Power

Generators at power plants produce current at a relatively low voltage.  This power goes from the generators to a transformer.  Which steps this voltage up.  Way up.  To the highest voltages in our electric distribution system.  So relatively small conductors can distribute this power over great distances.  And then a series of substations filled with transformers steps the voltage down further and further until it arrives to our homes at 240 volts.  Delivered to us by the last transformer in the system.  Typically a pole-mounted transformer that steps it down from a 2,400 volt or a 4,800 volt set of cables on the other side of the transformer.  These cables go back to a substation.  Where they terminate to switchgear.  Which is terminated to the secondary side of a very large transformer.  Which steps down a higher voltage (say, 13,800 volt) to the lower 2,400 volt or a 4,800 volt.

We call the 240 volt service coming to our homes a secondary service.  Because it comes from the secondary side of those pole-mounted transformers.  And we can use the voltage coming from those transformers in our homes.  Once you get upstream from these last transformers we start getting into what we call primary services.  A much higher voltage that we can’t use in our homes until we step it down with a transformer.  Some large users of electric power have primary services because the size of conductors required at the lower voltages would be cost prohibited.  So they bring in these higher voltages on a less costly set of cables into what we call primary switchgear.  From that primary switchgear we distribute that primary power to unit substations located inside the building.  And these unit substations have built-in transformers to step down the voltage to a level we can use.

There are a few of these higher primary voltage substations in a geographic area.  They typically feed other substations in that geographic area that step it down further to the voltage on the wires on the poles we see in between our backyards.  That feed the transformers that feed our houses.  Which is why when we lose the power in our house all of our neighbors typically lose their power, too.  For if a storm blows down a tree and it takes down the wires at the top of the poles in between our back yards everyone getting their power from those wires will lose their power.  For neighborhoods typically get a ‘radial’ feed.  One set of feeder cables coming from a substation.  If that set of cables goes down, or if there is a fault on it anywhere in the grid it feeds (opening a breaker in the substation), everyone loses their power.  And they don’t get it back until they fix the fault (e.g., replace cables torn down by a fallen tree).

Hospitals typically have Redundant Primary Electrical Services coming from two Different Substations

Now this would be a problem for a hospital.  Which is why hospitals don’t get radial feeds.  They get redundant feeds.  Typically two primary services.  From two different substations.  You can see this if a hospital has an overhead service.  Look at the overhead wires that feed the hospital.  You will notice a gap between two poles.  There will be two poles where the wires end.  With no wires going between these two poles.  Why?  Because these two poles are the end of the line.  One pole has wires going back to one substation.  The other pole has wires going back to another substation.  These two different primary services feed the main primary switchgear that feeds all the electric loads inside the hospital.

This primary switchgear is double-ended.  Looking at it from left to right you will see a primary fusible switch (where a set of cables from one primary service terminates), a main primary circuit breaker, branch primary circuit breakers, a tie breaker, more branch primary circuit breakers, another main primary circuit breaker and another primary fusible switch (where a set of cables from the other primary service terminates).  The key to this switchgear is the two main breakers and the tie breaker.  During normal operation the two main breakers are closed and the tie breaker is open.  So you have one primary service (from one electrical substation) feeding one end (from the fusible switch up to the tie breaker).  And the other primary service (from the other electrical substation) feeding the other end (from the other fusible switch up to the tie breaker).  If one of the primary services is lost (because a storm blows through causing a tree to fall on and break the cables coming from one substation) the electric controls will sense that loss and open the main breaker on the end that lost its primary service and close the tie breaker.  Feeding the entire hospital off the one good remaining primary service.  This sensing and switching happens so fast that the hospital does not experience a power outage.

This is why a hospital doesn’t lose its power while you’re sitting in the dark suffering in heat and humidity.  Because you have a radial feed.  While the hospital has redundancy.  If they lose one primary service they have a backup primary service.  Unlike you.  And in the rare occasion where they lose BOTH primary services (such as the Northeast blackout of 2003) hospitals have further redundancy.  Backup generators.  That can feed all of their life safety loads until the utility company can restore at least one of their primary services.  These generators can run as long as they can get fuel deliveries to their big diesel storage tank.  That replenishes the ‘day tanks’ at the generators.  Allowing them to keep the lights on.  And their patients alive.  Even while you’re sitting in the dark across the street.  Sweating in the heat and humidity.  With no television to watch.  While people in the hospital say, “There was a power outage?  I did not notice that.”

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Inrush Current, Redundancy, Electric Grid, High Voltage Transmission Lines, Substations, Generators and Northeast Blackout of 2003

Posted by PITHOCRATES - August 22nd, 2012

Technology 101

In Electric Generators and Motors there is a Tradeoff between Voltage and Current

If you have central air conditioning you’ve probably noticed something when it turns on.  Especially at night.  The lights will momentarily dim.  Why?  Because a central air conditioner is probably the largest electric load in your house.  It draws a lot of current.  Even at 240V volts.  And when it switches on the inrush of current is so great that it sucks current away from everything else.  This momentary surge of current exceeds your electrical panel’s ability to keep up with it. Try as it might the panel cannot push out enough current.  It tries so hard that it loses its ‘pushing’ strength and its voltage fades.  But once the air conditioner runs the starting inrush of current settles down to a lower running current that the panel can easily provide.  And it recovers its strength.  Its voltage returns to normal.  And all the lights return to normal brightness.

If you have ever been stopped by a train at a railroad crossing you’ve seen another example of this voltage-current tradeoff.  As a diesel-electric locomotive starts moving you’ll see plumes of diesel exhaust puffing out of the engine.  Why?  The diesel engine drives a generator.  The generator drives electric traction motors that turn the engine’s wheels.  These traction motors are like turning on a very large air conditioner.  The inrush of current sucks current out of the generator and makes the voltage fall.  The load on the engine is so great that it slows down while it struggles to supply that current.

To prevent the engine from stalling more fuel is pumped into the engine to increase engine RPMs.  Like stomping on the accelerator in a car.  Causing those plumes of diesel exhaust.  As the wheels start turning the current in the motor windings creates a counter electromotive force (the electric field collapses on the windings inducing a current in the opposite direction).  Which resists the current flow.  Current falls.  And the voltage goes back up.  If the engine is pushed beyond its operating limits, though, it will shut down to protect itself.  Bring the locomotive to a standstill wherever it is.  Even if it’s blocking all traffic at a railroad crossing.

Generators have to be Synchronized First before Connecting to the Electric Grid

The key to reliable electric power is redundancy.  To understand electrical redundancy think about driving your car.  Your normal route to work is under construction.  And the road is closed.  What do you do?  You take a different road.  You can do this because there is road redundancy.  In fact there are probably many different ways you can drive to work.  The electric grid provides the roads for electric power to travel.  Bringing together power plants.  Substations.  And conductors.  Interconnecting you to the various power plants connected to the grid.

Electric power leaves power plants on high voltage overhead transmission lines.  These lines can travel great distances with minimal losses.  But the power is useless to you and me.  The voltage is too high.  So these high voltage lines connect to substations.  Typically two of these high voltage feeders (two cable sets of three conductors each) connect to a substation.  Coming out of these substations are more conductors (cable sets of three conductors each) that feed loads at lower voltages.  In between the incoming feeders and the outgoing feeders are a bunch of switches and transformers.  To step down the voltage.  And to allow an outbound set of conductors to be switched to either of the two incoming feeders.  So if one of the incoming feeders goes down (for maintenance, cable failure, etc.) the load can switch over to the other inbound cable set.

Redundant power feeds to these substations can come from larger substations upstream.  Even from different power plants.  And all of these power plants can connect to the grid.  Which ultimately connects the output of different generators together.  This is easier said than done.  Current flows between different voltages.  The greater the voltage difference the greater the current flow.  Our power is an alternating current.  It is a reciprocating motion of electrons in the conductors.  Which makes connecting two AC sources together tricky.  Because they have to move identically.  They have to be in phase and move back and forth in the conductors at the same time.  Currents have to leave the generator at the same time.  And return at the same time.  If they do then the voltage differences between the phases will be zero.  And no current will flow between the power plants.  Instead it will all go into the grid.  If they are not synchronized when connected there will be voltage difference between the phases causing current to flow between the power stations.  With the chance of causing great damage.

The Northeast Blackout of 2003 started from one 345 kV Transmission Line Failing

August 14, 2003 was a hot day across the Midwest and the Northeast.  People were running their air conditioners.  Consuming a lot of electric power.  A 345 kV overhead transmission line in Northeast Ohio was drawing a lot of current to feed that electric power demand.  The feeder carried so much current that it heated up on that hot day.  And began to sag.  It came into contact with a tree.  The current jumped from the conductor to the tree.  And the 345 kV transmission line failed.  Power then switched over automatically to other lines.  Causing them to heat up, sag and fail.  As more load was switched onto fewer lines a cascade of failures followed.

As lines overloaded and failed power surged through the grid to rebalance the system.  Currents soared and voltages fell.  Power raced one way.  Then reversed and raced the other way when other lines failed.  Voltages fell with these current surges.  Generators struggled to provide the demanded power.  Some generators sped up when some loads disconnected from the grid.  Taking them out of synch with other generators.  Generators began to disconnect from the grid to protect themselves from these wild fluctuations.  And as they went off-line others tried to pick up their load and soon exceeded their operating limits.  Then they disconnected from the grid.  And on and on it went.  Until the last failure of the Northeast blackout of 2003 left a huge chunk of North America without any electric power.  From Ontario to New Jersey.  From Michigan to Massachusetts.  All started from one 345 kV transmission line failing.

In all about 256 power plants went off-line.  As they were designed to do.  Just like a diesel locomotive engine shutting down to protect itself.  Generators are expensive.  And they take a lot of time to build.  To transport.  To install.  And to test, start up and put on line.  So the generators have many built-in safeguards to prevent any damage.  Which was part of the delay in restoring power.  Especially the nuclear power plants.  Restoring power, though, wasn’t just as easy as getting the power plants up and running again.  All the outgoing switches at all those substations had to be opened first before reenergizing those incoming feeders.  Then they carefully closed the outgoing switches to restore power while keeping the grid balanced.  And to prevent any surges that may have pulled a generator out of synch.  It’s a complicated system.  But it works.  When it’s maintained properly.  And there is sufficient power generation feeding the grid.

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