On 12/04/2011 07:33, Home Guy wrote:
g wrote:


The grid can be seen as a pretty rigid beast. No small puny inverter
in the sub 1000kW class will much affect the grid voltage as a whole.
When voltage of the converter is attempted to be raised, current will
flow into the grid of course. The voltage increase will hardly be
measurable, as electrical characteristics of the grid will adjust
dynamically.

At any one time, there is a certain load on the grid as a whole. When
Mr. Homeowner adds 10Kw from some solar panels, some other power
generating systems connected to the grid will (have to) reduce their
output. As a result the voltage stays the same overall.

Here's the problem:

Many of the load devices you find in a typical home (primarily electric
motors that run cooling systems, air conditioners, fridges and freezers)
are not capable of regulating their input voltage.

So when a secondary electricity source comes on-line (like a small PV
system) then in order to push it's current into the local grid it will
have to *try* to raise it's output voltage in order to see some current
flow. It might only be a few volts, maybe less.

1) The actual voltage increase will relate to the ratio of grid
impedance vs local impedance, i.e. your local power consumers (fridges,
heaters etc) has a much higher impedance relatively, thus the grid will
"take" the majority of the generated power. The _only_ increase in
voltage you will see results from the voltage drop in the grid components.


But does that mean there will be a measurable net reduction in the
current being supplied by the high-voltage substation for that corner of
the city?

2) Pretty complex calculation, but yes, _somewhere_ one or more
generating pieces of machinery will reduce its output. Makes sense
intuitively, does it not?


Not if your typical load device in homes surround the PV system will
simply operate at a higher wattage.

3) You just set your PV system to operate at max power, the grid system
will balance out automatically. See 1) above


The only sort of load that can effectively be regulated by a slight
increase in local grid voltage are electric heaters. When you raise
their input voltage slightly, they will put out more BTU of heat, and if
their heat output set-point doesn't change, then their operational duty
cycle will change slightly.

4) The grid voltage does actually fluctuate a bit, depending on load.
Power companies have means of adjusting line voltages depending on load
fluctuations. The average subscriber never knows this.


The only way that a neighborhood PV system can actually supplement
municipal utility power is when the PV system is wired up as a dedicated
sole supply source for a few select branch circuits.

5) That will be a very inefficient way to utilize your PV system.

A simplified way is to look at the grid as a battery. When your PV
system generates more power than your local consumers, the surplus will
flow into the grid. At all other times the grid and the PV will both
supply the needed power to the local consumers.

The way I see it,
you have to feed certain select loads 100% from a PV system (ie -
disconnect them from the municipal energy source) if you're going to
make a meaningful contribution to the supply-side of a municipal or
city-wide grid.

6) Fairly close to impossible. How do you match local power consumers to
hit the 100% PV capacity?