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Feeding solar power back into municipal grid: Issues and finger-pointing - Page 5

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Posted by Home Guy on April 12, 2011, 2:08 pm
 
harry wrote:


Our regional and municipal electricity distributors are not pointing the
finger at the capability or specs of residential service panels or
neighborhood distribution / stepdown transformers as the reason why they
won't let small-scale (less than 10kw) roof-top PV systems to connect to
the grid.

They are saying that the local sub-station doesn't have the "capability"
to allow for a handful (or perhaps even a single) small-scale PV systems
to be hooked up and they would need to "upgrade" the sub-station in some
way.

For more about this, see here:

http://www.greenpowertalk.org/archive/index.php/t-13885.html

And less relevant, here:

http://www.canadianenergylaw.com/2010/06/articles/electricity/metering-discrepancy-first-major-snag-in-the-rollout-of-fit-and-microfit-projects/

All the arguments put forward here about why homeowner-operated PV
systems (with nameplate rating under 10kw) are not being allowed to
connect to the grid through their own bi-directional revenue meter have
not addressed the issue as to how the connection of such a PV system can
possibly affect or influence the operation of the regional municipal
sub-station supplying power (at 20 kv?) to the neighborhood in
question.  The sub-station is "insulated" from direct exposure to any
individual home by at least 1 step-down distribution transformer (in our
case, a ground-mounted distribution transformer supplying maybe 20 homes
- our electrical service runs underground - not on poles in our
neighborhood).

There may not yet even be a single residential PV system that's been
connected to the grid for the area being served by the sub-station in
question.

Posted by bud-- on April 12, 2011, 3:44 pm
 
On 4/11/2011 4:31 PM, daestrom wrote:

This is a fatal flaw in your argument. Transformers are not infinite
sources. A utility transformer might supply a fault current 20x the
rated current (for a "5% impedance" transformer). (While a transformer
will supply a fault current larger than the rated current that is not
likely with PV. PV is basically a constant current source.)


Using a real transformer houses will have far less available fault current.


Cite where 100kA is required.


I agree that is very likely. One reason is that a higher rating is not
necessary.

(SquareD, if I remember right, has a rating of 20kA downstream from both
the main and branch circuit breaker.)

I doubt many Canadian house panels have fuse protection, or are
different from US panels with circuit breaker protection rated around 10kA.


The interrupt rating required goes up with the service current rating.
For a house, the utility is not likely to have over 10,000kA available
fault current. The transformers become too large, many houses are
supplied with longer wires and higher resistance losses, and the system
is much less safe.

I believe it would take a rather massive amount of PV installations to
cause a problem. The PV installations would all have to be on the
secondary of the same utility transformer. The transformer is then not
likely to support the PV current back to the grid. If the fault current
is 20x the transformer full load current, and the PV current is equal to
the transformer full load current, the PV supply would increase the
fault current by about 5% (assuming the inverter doesn't shut down). If
there were too many PV installations the utility could put fewer houses
on a transformer. Seems like a problem that is not that hard to handle
for the utility, at least until PV generation becomes rather common.

--
bud--

Posted by m II on April 12, 2011, 3:58 pm
 


On 4/11/2011 4:31 PM, daestrom wrote:

This is a fatal flaw in your argument. Transformers are not infinite
sources. A utility transformer might supply a fault current 20x the
rated current (for a "5% impedance" transformer). (While a transformer
will supply a fault current larger than the rated current that is not
likely with PV. PV is basically a constant current source.)


Using a real transformer houses will have far less available fault current.


Cite where 100kA is required.


I agree that is very likely. One reason is that a higher rating is not
necessary.

(SquareD, if I remember right, has a rating of 20kA downstream from both
the main and branch circuit breaker.)

I doubt many Canadian house panels have fuse protection, or are
different from US panels with circuit breaker protection rated around 10kA.


The interrupt rating required goes up with the service current rating.
For a house, the utility is not likely to have over 10,000kA available
fault current. The transformers become too large, many houses are
supplied with longer wires and higher resistance losses, and the system
is much less safe.

I believe it would take a rather massive amount of PV installations to
cause a problem. The PV installations would all have to be on the
secondary of the same utility transformer. The transformer is then not
likely to support the PV current back to the grid. If the fault current
is 20x the transformer full load current, and the PV current is equal to
the transformer full load current, the PV supply would increase the
fault current by about 5% (assuming the inverter doesn't shut down). If
there were too many PV installations the utility could put fewer houses
on a transformer. Seems like a problem that is not that hard to handle
for the utility, at least until PV generation becomes rather common.

--
bud--

-----------------
Perhaps re-read ( or just read ) the last few posts. Your objection is
mostly agreement with items already covered.

Can you cite the percent impedance of the transformers  or the code rules
you discuss?




mike


Posted by m II on April 12, 2011, 4:37 pm
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On 11-04-12 09:58 AM, m II wrote:

Nothing.

 Forgery reported to nntp provider

mike






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Posted by bud-- on April 13, 2011, 3:15 pm
 On 4/12/2011 10:58 AM, m II wrote:

-----------------
|Perhaps re-read ( or just read ) the last few posts. Your objection is
|mostly agreement with items already covered.

Perhaps you should take reading lessons. Maybe you and harry could get
group rates.

- You said "considering the street transformer as an infinite current
supply" which no one does.
- As a result your calculation is meaningless.
- You said Canadian house panels were protected with fuses. I disagree.
Perhaps a cite?
- You said "any approved O/C device in a panel these days is rated at
100kA". I asked for a cite - still missing.

- Daestrom said adding PV systems to residences could result in an
available fault current larger than the rating of existing service
panels. It is certainly an interesting point, but not likely for reasons
stated.

I did agree with daestrom that most US house panels are likely to have a
10kA IR.


|Can you cite the percent impedance of the transformers

5% impedance would be common

| or the code rules you discuss?

I didn't discuss code rules.


Your 'newsreader' is incompetent at treating sigs.

--
bud--

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