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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