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electric car grid overloads

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Posted by misterfact@yahoo.com on September 2, 2008, 2:19 pm
glad to hear that plugging-in millions of electric cars into the grid
during the day and night won't contribute to grid overloads and power

glad to hear that there is an endless supply of cheap coal and natural
gas for the power companies!

glad to hear that electric rates will always stay about the same as
they are now!

With all that in mind, I guess buying your own PV system to charge
your car's batteries is a waste of money.

Posted by Cydrome Leader on September 2, 2008, 3:26 pm

The night is when there is plenty of electricity to spare.

The arc furnace mini mill here ( Finkl & Sons ) goes nuts on nights and
weekends when they can use all the electricity they can. They have some ok
pics on their site www.finkl.com

During the day, they stick with forging and electricity is for the local
businesses and residential areas, which they're actually surrounded by.

Posted by Neon John on September 2, 2008, 7:39 pm
 On Tue, 2 Sep 2008 15:26:53 +0000 (UTC), Cydrome Leader

That's almost as simplistically wrong as the OP.  What a major electricity
consumer like a mini-mill does really has no relevance to residential users.  

Perhaps a larger concern than generating capacity is the ability to distribute
the energy.  Let's work through a small example

If your house/apartment is connected to a pole pig (pole-mounted transformer),
go take a look at it.  It'll almost surely have a 2 digit number on the side.
That's its rating in kVA.  A typical pig is 20kVA.  It'll have from 1 to about
4 homes connected to it.  A little math shows that 20,000 VA /240 volts = 83
amps.  You also know that normal residential electrical service is 200 amps.

Two things at work.  One, the utility knows from past experience that houses
use but a tiny fraction of that capacity for long.  Two, utility ratings are
very under-stated.  A 20kVA pig can handle 40kVA indefinitely, albeit with a
shorter life.  

A second limiting factor is the primary service.  7,200 volts is a typical
primary voltage.  The 20kVA transformer will have a 5 amp fuse in the primary
line.  That will allow 36kVA to pass continuously and twice that for a short

All this sizing is based on historical load patterns.  A massive changeover
from another energy source to electricity upsets that sizing.  With oil as
high as it is, there is great incentive to change over to heat pumps in areas
where there is no natural gas.  

My 2.5 ton heat draws a measured 3kW in heat mode when the outside temperature
is 15 degrees, just before the blast coils kick in.  It draws 72 amps at 240
volts when the blast coils are on.  That's 17.3kW.  Let's say that there are 4
houses on the pig, each with that same heat pump.  (It's small for a typical
house but good enough for this example).  17.3 * 4 = 69kW.  That 20kVA pig is
almost 4X overloaded.

In the normal course of business, utilities keep an eye on feeder loads,
either by periodic surveys or if they're more modern, SCADA.  If they see a
feeder becoming overloaded, they schedule a crew out to find the load and
upgrade as necessary.  It is not unusual to change out a whole feeder from
7,200 to 14,400 volts to accommodate the larger load.

This is in the normal course of things.  But suppose an abrupt change happens.
Say, the price of heating oil tops $.  (Oh wait, it has :-( Everyone in the
neighborhood cuts a deal with an HVAC contractor to change over to heat pumps.
My block did exactly that when I converted my apartments over to gas heat.  We
got a super deal.

The first thing that will happen is on the first very cold night when the
blast coils have to supply most of the heat, the primary fuse on that pig will
blow.  Since this will be happening all over town, it probably won't get
attention very fast.

The utility's normal response to this is to plug in a larger fuse and then
schedule a pig upgrade.  That'll be a long schedule if the same problem is
occurring all over town.  In the meantime, everyone keeps their fingers
crossed that a) the pig can handle the load, b) the feeder can handle the load
and c) the substation can handle the load.

Now let's toss in some EVs.  An EV with enough range to interest non-believers
will have at least 60kWh onboard and probably more.  Most companies who are
claiming to be developing lithium traction packs talk about 4 hour recharge
times.  Many say that slower charging is unhealthy for the batteries.  So
let's go with 4 hours.

60kWh / 4 hours = 15kW.  At 240 volts, that's 62.5 amps.  If all 4 houses on
that pig got an EV all at once, when the chargers turned on in the middle of
the night, the pig's load would be 60kVA, assuming 100% charge efficiency and
a PF of 1.  It's three times overloaded JUST by the EV chargers.  If we toss
in 4 heat pump loads at 69kW total, the total pig load would be 69+609kVA.
That 20kVA pig will not be happy, nor will the primary fuse and probably not
the feeder fuse or recloser.  Scatter this same situation around the
neighborhood and suddenly the substation is overloaded and goes down.

This isn't theoretical.  I've lived through just such as situation in the
lovely (NOT!) little town of Royalton, PA in the early 80s.  This little berg
of 4,000 people had its own little borough electric company.  A substation
sitting on the main highway fed the town with Met-Ed power.

Problem was, the system was set up before many people had ACs.  A heat wave
hit that area in '81 which resulted in most folks running out and buying
window units.  The result was, when the temperature rose above a certain point
and everyone got home from work, turned on the AC, the TV and the stove (at
5:30 pm, plus or minus a few minutes), a phase fuse on the substation
transformer primary would blow.  That dropped my line voltage to about 70
volts. The borough electrician (who was also the garbage man) would amble over
in an hour or two and install a new fuse.

70 volts isn't enough to run anything but it certainly is enough to burn out
electric motors and ballasts.  The result was, if I wanted to go out to eat or
something after work, I could either risk the power not dropping or else open
the main breaker to keep the AC and refrigeration compressors and fluorescent
lamp ballasts from burning out.  I'd get to come home to a screaming hot

The borough claimed that they couldn't afford to install a larger substation
(so why were they in the power utility business?).  I drove by there a couple
of years ago and saw that the same old sub was still in service.  Try to
imagine very many EVs in that environment.

Those of us who work in the utility industry are scared sh*tless at the
prospect of mass conversion to either EVs or heat pumps or both.  Especially
in the leftist states of the northeast where obstructionists have had their
way and have blocked new generation and transmission capacity for years.

Down here in the Tennessee Valley, we're in pretty good shape so my concern is
mostly academic.  Folks in the blue states should be VERY worried.

John De Armond
See my website for my current email address
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
It isn't Global Warming.... It's Jerry Falwell arriving in hell.

Posted by Cydrome Leader on September 2, 2008, 8:42 pm
Actually it does. It shows there trends in electrical consumption, and
nights are when there is excess capacity available, even on already taxed
and underbuilt infrastructure.

I'm familiar with the concepts you mention. It's the same as how the sum
or current for branch circuits in a breaker panel are almost always going
to exceed the capacity of the main breaker.

Not that it matters, but 20kVA is very small, and not even seen around
here. I am seeing what looks like 100kVA units being upped to 167 in lots
of residential areas.

What's stupid about current hybrid vehicles is there's no way to even
take advantage of the fact it's electric. You can't plug a prius unto a 15
amp outlet and let it complete some of a charge overnight. You have to
feed it gas to charge up, which is obviously going to be less efficient
that just plugging the thing in for a while.

It's all so senseless.

It's unlikely that out of nowhere, everbody is going to have installed
larger electrical service to charge their cars. Take a city, like Chicago.
There's more cars out on the streets than in garages. Those ones aren't
going to get plugged into anything, day or night.

The oddest voltage I ever measured at an outlet was 25 volts. It was so
strange I had to test with multiple meters. I'm still not sure how that
even happened.

While this is a pretty sad hick town example, all utilities claim they
can't upgrade, no matter how rich they are. That's how they operate.

Maybe the small town should all run their ACs at once and finally burn
that substation out.

I'd be scared if it was raining hammers and wrenches all day, but it's
simply not going to happen, and if there is a trend towards it, it's going
to be slow. In fact, a "mass conversion" to EV is about as silly as a
saying there will be a "mass conversion to new cars".

Posted by Chris Hill on September 3, 2008, 1:30 pm
 On Tue, 2 Sep 2008 20:42:40 +0000 (UTC), Cydrome Leader

Not so obvious.  Coal isn't that efficient either, it is just cheap.
Besides that, the prius batteries aren't designed to be fully charged
anyway, and they are small enough that the benefit would never pay for
itself from plugging it in without adding batteries.

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