From: John De Armond
Subject: Re: electric car grid overloads
Date: Tue, 02 Sep 2008 15:39:57 -0400
On Tue, 2 Sep 2008 15:26:53 +0000 (UTC), Cydrome Leader
>> With all that in mind, I guess buying your own PV system to charge
>> your car's batteries is a waste of money.
>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.
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
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 $4. (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+60=129kVA.
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.