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Feeding solar power back into municipal grid: Issues and finger-pointing - Page 15

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Posted by g on April 11, 2011, 4:48 pm
 
On 11/04/2011 07:28, Home Guy 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.

If enough homeowners are added to the grid where they are able to affect
the overall power consumption of the grid up to some significant
percentage, it is really important that the inverters are accurate wrt
voltage output and frequency stability.



Posted by Home Guy on April 12, 2011, 2:33 pm
 
g wrote:


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.

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?

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

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.

But in the case of an AC compressor, the fact that it might be getting a
slightly higher input voltage because a neighboring house is feeding PV
power into the local grid won't mean that the AC compressor will reduce
it's current consumption from the municipal utility supplier because of
the extra current coming from a neighbor's roof-top solar array.  It
just means the motor will use BOTH sources of current and (I suppose)
run a little hotter but in the end not do any extra cooling work in the
process (it's rotational speed won't change).

Same theory would hold true for lighting (incandescent especially).  If
you raise the input voltage, you'll get more light output - the bulb
will simply consume all the juice it would normally get from the utility
in addition to that being supplied by the neighborhood PV system.

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

Posted by bud-- on April 12, 2011, 3:56 pm
 On 4/12/2011 9:33 AM, Home Guy wrote:

A devastating analysis.

I am sure when the utilities read it they will stop paralleling
generators, since that just causes the amount of electricity used to go
up from what would be used by isolated systems.

--
bud--

Posted by m II on April 12, 2011, 4:04 pm
 


On 4/12/2011 9:33 AM, Home Guy wrote:

A devastating analysis.

I am sure when the utilities read it they will stop paralleling
generators, since that just causes the amount of electricity used to go
up from what would be used by isolated systems.

------------------

Careful!
Sarcasm does not work well in a text medium, at all!

People cannot see your facial expression and people are never sure unless it
is totally ridiculous.

I agree with your point but it is made very poorly in a text only medium.



mike


Posted by Home Guy on April 12, 2011, 11:07 pm
 bud-- full-quoted:


How many utilities connect the output of new parallel generating sources
to the 120/208 connection side of a grid, instead of at the sub-station
high-voltage side?

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