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<font face="Verdana">There are many points here that really need to be
made. I could write for days on this subject, but I'll just make a few
points in anticipation that it will generate some open and accurate
discussion. Here's my 10 cents, in no particular order:<br>
<br>
Right off the bat, don't get me wrong. Utilities are often obstinate,
stubborn, lethargic, and obstructionist. <br>
<br>
Utilities do have a very difficult and challenging job. There are
many, many variables that are out of their control which have a
dramatic affect on their service level. Loads, generation sources,
weather, varmints, equipment failure, aged infrastructure, regulations,
politics, etc, etc, etc. There are probably 30 more factors that
affect their ability to provide reliable power at a reasonable price.<br>
<br>
The utilities carry a HUGE burden. If anything goes wrong, and
equipment or personnel are harmed, they are THE DEEP POCKETs. So,
regardless if they are truly at fault or not, they bear the burden of
defending themselves. This inherent liability adds measurable expense
to their cost of doing business. It also makes them a bit paranoid and
slow to react.<br>
<br>
Most utilities are obliged to maintain voltage within ANSI-A range
(+/-5% of nominal). Occasional excursion into range B (+/-10%) is
allowed.<br>
<br>
There are two voltage "categories", service voltage, and utilization
voltage. Service voltage is the voltage supplied by the utility to the
PCC (typically where the connection is made at the meter). Utilization
voltage is the voltage on the local premises where the actual load is
connected. The premises wiring system and local loads and generation
dictate the difference between the utilization voltage and the service
voltage. This is exactly what William has correctly addressed by
limiting local drops to 1% or less. The utility can only control (and
is held responsible for) the voltage to the PCC.<br>
<br>
UL1741 now defers to IEEE1547 for utility protective functions
(voltage, frequency, islanding, synchronization, harmonics, etc.)
Inverters are required to "cease exportation of power" if the utility
voltage goes outside -12% and +10% of Vnom. This requirement is for
the PCC, however, voltage sensing at the inverter is allowed. This is
an important point, and is a blessing and a curse. It is an
accommodation the utilities made in order to allow connection of DG to
their grid.<br>
<br>
The utility can grant permission to use "alternate setpoints" for
voltage or frequency trip points. In fact, per IEEE1547, installations
over 30kW MUST incorporate adjustable setpoints in the inverter to
allow "tweaking" of these setpoints. If the inverter has not been
evaluated for alternate setpoints, it cannot be aggragated into an
installation which generates more than 30kW peak. <br>
<br>
In the case of continued nuisance overvoltage trips, If the overvoltage
trip points are raised, this may become an inverter protection issue.
Some inverters may tolerate +20% line voltage without risk of internal
failure. Others may not be tolerant of anything over +10% on a regular
basis. No matter what inverter you are using, there is some upper
limit to the tolerable continuous line voltage.<br>
<br>
Others with utility experience should weigh in here. Matt L., John B.
???<br>
<br>
<br>
</font>
<pre class="moz-signature" cols="72">See Ya!
Marv
Enphase Energy
707 763-4784 x7016</pre>
<br>
<br>
William Korthof wrote:
<blockquote cite="mid:48F27499-88D1-4F5C-B760-25FFD01347B1@eesolar.com"
type="cite">I'm beginning to wonder if the allowed voltage range for
grid-tie inverters (+/-10%) is too
<br>
sensitive in some networks and contributes more harm than benefit. This
is close to home.
<br>
<br>
We actually have a significant number of customers who've had trouble
with grid voltage
<br>
causing their systems to go offline at various times. I think most or
all are SCE customers.
<br>
<br>
So I've been having problems with the inverter at my own house going
offline due to high
<br>
utility line voltage. We actually have 4 other systems in the
neighborhood, with two more
<br>
going in in the next couple months. All of them are about equally
affected, even though
<br>
each house is feed by a different transformer. I did a bit of research
and found that some
<br>
of the grid hardware in the neighborhood is very old---some of the
oldest in existence---
<br>
dating back as far as 1892. You can read about the San Antonio Power
Plant (hydro)
<br>
and the Pomona substation online.
<br>
<br>
Going back to the 1950's, most of the local neighborhood
sub-transmission in the US is
<br>
at a voltage around 12 kV or more, with transformers for the
120/208/240 or 277/480.
<br>
But some older neighborhoods that haven't been upgraded (and some
campuses) use
<br>
an intermediate system, usually 2400/4160 volts. My neighborhood is
still mostly fed by
<br>
the old 2400/4160v network.
<br>
<br>
Generally, the utility voltage at my house is in the mid 120's---
around 125 vac per phase.
<br>
But at times, the voltage goes up higher---two weeks ago I saw 129 to
130V per phase.
<br>
That voltage was high enough to put all of the inverters that I checked
offline for much
<br>
of the day.
<br>
<br>
So I've had to call in "voltage trouble" complaints to the utility at
least a dozen times
<br>
over the past 5 years in response to seeing inverters offline and line
voltage about
<br>
8% above nominal.
<br>
<br>
This periodic voltage problem has been going on for years, typically
worst in summer.
<br>
I upgraded with new service panels and heavier feeders at each of the
houses to do
<br>
my part to help. The circuits are now sized so that voltage drop in
every case from the
<br>
inverter terminals to the utility entrances are all under 1% at full
solar output.
<br>
Six months ago, utility crews replaced some transformers around the
neighborhood
<br>
(some appeared to be original), yet they didn't move any customers from
the old 4kV
<br>
supply to the newer 12 kV supply.
<br>
<br>
I've read that the utility's operating guidelines call for keeping the
supply voltage
<br>
within +/- 5% of nominal voltage at the customer's service terminals.
<br>
<br>
If the utility actually stays within 5%, and if solar inverters allow
for voltages to vary
<br>
up to +/- 10% from nominal, then things _should_ be okay. However, in
practice, I see
<br>
the inverters going offline right at 129 to 130V per phase, not 132V
(+10%).
<br>
<br>
In response to the last trouble call, I talked with the crew that came
out. I think their
<br>
response helps explain the problem... my issue seems to go back to the
substation.
<br>
Apparently other customers on another feeder from the same substation
complain
<br>
about low voltage---around 108-110V per phase. So they raise the
voltage
<br>
regulator at the substation and everyone goes up 5%. It's easy to see
how the drop
<br>
on a 4kV feeder could be 1% on one feeder and 10% on another----
example:
<br>
a load of 865 kW (~400 homes with A/C) or 120 amps at 4kV 3 ph, in 2
miles of #2
<br>
copper is a voltage drop of 10%... yet 300 kW going 3000 ft gives a
voltage drop
<br>
under 1%. If voltage drop is 10% on one feeder, but only 1% on a
different feeder...
<br>
it's tricky for the operator to keep everyone reliably within 5% of
nominal.
<br>
In my neighborhood, SCE technicians apparently view it as normal
operating
<br>
procedure to supply 126 or 129 volts per phase.
<br>
<br>
So it seems there are several possible remedies:
<br>
1) ignore the problem and hope I don't lose too many kWh's due to grid
overvoltage
<br>
<br>
2) I could install transformers to lower the voltage to the inverters
by ~5%----so far,
<br>
grid voltage in my neighborhood has *always* measured at or above
nominal.
<br>
<br>
3) if it weren't for the UL and IEEE standards, solar inverters could
be set with values
<br>
that made them compatible with prevailing utility operating conditions,
perhaps
<br>
+/- 15% of nominal voltage.
<br>
<br>
4) at a cost of many millions of $$$, SCE could completely replace the
local
<br>
distribution system with new 12kV facilities, even though annual load
growth
<br>
on the old wires is trivial or possibly negative...
<br>
<br>
5) at a more modest cost, SCE could make strategic upgrades to lower
the load
<br>
and voltage drop on the most stressed parts of the old network
<br>
<br>
<br>
How have other installers dealt with utility voltage problems?
<br>
<br>
/wk
<br>
<br>
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