Paul M. Eldridge writes:

Let's say the average electric oven draws 3.0 kW (my convection oven
happens to be 5.5 kW). We might expect the typical household oven to
operate at full power for the first ten minutes or so, then cycle on
perhaps one third of the time thereafter (i.e., an average of one
minute on for every two minutes off).

Suppose you *did* have an oven with electronic variable power control,
where temperature controlled the "on" time of a triac. This oven would
still operate at full current until it came up to operating temperature,
and then sit at 33% duty cycle after that. The only difference between
this and what you have now is that the on/off cycle repeats every 1/120
second, rather than every couple of minutes. But that makes no real
difference to the utility, which is looking at the load averaged over
100000 ovens.

In fact, the electronic control wastes a bit of power in the switching
element, and consumes slightly *more* power than the non-electronic
oven. The triac control also distorts the utility waveform into
something that is less of a sine wave, which the utility also will not
like (the power factor gets worse, so they need higher current capacity
for the same billable watts).

If all 100,000 ovens were energized at the same time, we would expect
this load to be 300 MW.

Right - whether or not they have electronic controls.

We're assuming, of course, that as each of
these ovens come up to temperature, the actual load at a 33% duty
cycle, would be closer to 100 MW, and since these ovens are not all
turned on at the same time, a coincidental peak of 100 MW is probably
within spitting distance of the mark.

Again, true with or without electronic controls.

If, however, each of these
ovens were equipped with variable wattage controls and, again,
assuming a 33% duty cycle, our coincidental peak should drop to just
33 MW.

This makes no sense. The 33% duty cycle has already been factored into
the drop from 300 MW to 100 MW. You can't divide by 3 *again*. You
need that 100 MW to keep all of the ovens at operating temperature.

On a typical winter's day, Nova Scotia Power's peak falls between
1,500 to 2,000 MW, so a 67 MW reduction in provincial demand would
represent a peak savings of perhaps as much as 3 to 5 per cent. In
theory, it would exceed the province's total installed wind capacity
of some 60 MW (which, assuming a 40 per cent annual capacity factor, I
take it might be closer to just 25 MW). Even if we were to cut the
number of ovens in operation by half, the impact on a utility such as
Nova Scotia Power is not exactly insignificant.

This is all based on the assumption that you can somehow run all these
ovens with electronic controls on 1/3 the average power you would need
with conventional switching controls. That's nonsense - they need just
as much energy, on average, to heat the same contents to the same
temperature for the same time.

The only time it makes sense to use dimmer-like electronic power control
is when the temperature swings with conventional controls are too large.

Dave