With a Great Trust in Technology Germany may go all Green in Power Generation

Posted by PITHOCRATES - April 7th, 2013

Week in Review

In 2003 one power plant went off line for maintenance in Ohio.  As their electrical load switched over to other power lines the extra current in them caused them to heat up and sag.  Coming into contact with some tall trees.  And the electric power flashed over to the trees.  This surge in current opened some breakers and transferred this electric load to other cables.  Overloading these lines.  More breakers opened.  More lines disconnected.  And with the electric load switching around it caused some electric generators to spin a little wildly.  So they disconnected from the grid as designed to protect themselves.

Eventually this cascade of failures would cause one of the greatest power outages in history.  The Northeast blackout of 2003.  Affecting some 55 million people.  And taking 256 power plants offline.  Apparently there was a software bug in the computer control system that didn’t warn them in time to rebalance the grid on other power sources before this cascade of failures began.  Once the event was over it took a lot of time to bring the power back online.  Three days before all power was restored.  Because you have to reconnect generators slowly and carefully.  As you are connecting generators together.  If these generators are not running in phase with each other fault currents can flow between them.  Damaging them and starting another cascade of failures.

So the electric grid is a very complex network of generators, cables, switches and computer control systems.  The more generation plants added to the grid the more complicated the switching and the computer controls.  Which makes having large-capacity power generation plants highly desirable.  For it reduces the complexity of the system.  And their large power capacity makes it easier for them to take on additional loads when another plant goes offline or a cable fails.  It provides a safe margin of error when trying to balance electric loads between available generation.  In Germany, though, the politics of green energy may take precedence over good engineering practices (see Linked Renewables Could Help Germany Avoid Blackouts by Paul Brown and The Daily Climate posted 4/5/2013 on Scientific American).

Critics of renewables have always claimed that sun and wind are only intermittent producers of electricity and need fossil fuel plants as back-up to make them viable. But German engineers have proved this is not so.

By skillfully combining the output of a number of solar, wind and biogas plants the grid can be provided with stable energy 24 hours a day without fear of blackouts, according to the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in Kassel.

For Germany, having turned its back on nuclear power and investing heavily in all forms of renewables to reduce its carbon dioxide emissions, this is an important breakthrough…

Kurt Rohrig, deputy director of IWES, said: “Each source of energy – be it wind, sun or biogas – has its strengths and weaknesses. If we manage to skillfully combine the different characteristics of the regenerative energies, we can ensure the power supply for Germany.”

The idea is that many small power plant operators can feed their electricity into the grid but act as a single power plant using computers to control the level of power…

The current system of supplying the grid with electricity is geared to a few large producers. In the new system, with dozens of small producers, there will need to be extra facilities at intervals on the system to stabilize voltage. Part of the project is designed to find out how many of these the country will need.

The project has the backing of Germany’s large and increasingly important renewable companies and industrial giants like Siemans.

If you are a heavy electric power consumer in Germany you might want to build your own power plant on site.  For if they go ahead with this they are going to create one complex and costly monster.  Which is why IWES and Siemens no doubt are on board with this.  For it would give them a lot of business in a recession-plagued Eurozone.  But the amount of switching and computer controls to make this work just boggles the mind.

Just imagine a night of high winds that shuts down all wind farms.  Which is something a wind turbine does to protect itself.  You can’t switch over to solar at night.  So you will have to switch that load over to the remaining power lines that are connected to active generation.  Heating those wires up.  Causing them to sag.  Perhaps flashing over to a tall tree.  If these lines disconnect from the grid will those small producers be able to pick up the demand?  Or will they disconnect to protect themselves from an overload?  Once the event is over how long would it take to bring all of these generation sources back in phase and back online?

If they move forward with this chances are that the Germans are going to learn a very painful and costly lesson about green energy.  It may make you look like you care but it won’t keep the lights on like a coal-fired or a nuclear power plant can.  Which they may learn.  The hard way.



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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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