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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