I'm beginning to wonder if the allowed voltage range for grid-tie
inverters (+/-10%) is too
sensitive in some networks and contributes more harm than benefit.
This is close to home.
We actually have a significant number of customers who've had trouble
with grid voltage
causing their systems to go offline at various times. I think most or
all are SCE customers.
So I've been having problems with the inverter at my own house going
offline due to high
utility line voltage. We actually have 4 other systems in the
neighborhood, with two more
going in in the next couple months. All of them are about equally
affected, even though
each house is feed by a different transformer. I did a bit of research
and found that some
of the grid hardware in the neighborhood is very old---some of the
oldest in existence---
dating back as far as 1892. You can read about the San Antonio Power
Plant (hydro)
and the Pomona substation online.
Going back to the 1950's, most of the local neighborhood sub-
transmission in the US is
at a voltage around 12 kV or more, with transformers for the
120/208/240 or 277/480.
But some older neighborhoods that haven't been upgraded (and some
campuses) use
an intermediate system, usually 2400/4160 volts. My neighborhood is
still mostly fed by
the old 2400/4160v network.
Generally, the utility voltage at my house is in the mid 120's---
around 125 vac per phase.
But at times, the voltage goes up higher---two weeks ago I saw 129 to
130V per phase.
That voltage was high enough to put all of the inverters that I
checked offline for much
of the day.
So I've had to call in "voltage trouble" complaints to the utility at
least a dozen times
over the past 5 years in response to seeing inverters offline and line
voltage about
8% above nominal.
This periodic voltage problem has been going on for years, typically
worst in summer.
I upgraded with new service panels and heavier feeders at each of the
houses to do
my part to help. The circuits are now sized so that voltage drop in
every case from the
inverter terminals to the utility entrances are all under 1% at full
solar output.
Six months ago, utility crews replaced some transformers around the
neighborhood
(some appeared to be original), yet they didn't move any customers
from the old 4kV
supply to the newer 12 kV supply.
I've read that the utility's operating guidelines call for keeping the
supply voltage
within +/- 5% of nominal voltage at the customer's service terminals.
If the utility actually stays within 5%, and if solar inverters allow
for voltages to vary
up to +/- 10% from nominal, then things _should_ be okay. However, in
practice, I see
the inverters going offline right at 129 to 130V per phase, not 132V
(+10%).
In response to the last trouble call, I talked with the crew that came
out. I think their
response helps explain the problem... my issue seems to go back to the
substation.
Apparently other customers on another feeder from the same substation
complain
about low voltage---around 108-110V per phase. So they raise the voltage
regulator at the substation and everyone goes up 5%. It's easy to see
how the drop
on a 4kV feeder could be 1% on one feeder and 10% on another----
example:
a load of 865 kW (~400 homes with A/C) or 120 amps at 4kV 3 ph, in 2
miles of #2
copper is a voltage drop of 10%... yet 300 kW going 3000 ft gives a
voltage drop
under 1%. If voltage drop is 10% on one feeder, but only 1% on a
different feeder...
it's tricky for the operator to keep everyone reliably within 5% of
nominal.
In my neighborhood, SCE technicians apparently view it as normal
operating
procedure to supply 126 or 129 volts per phase.
So it seems there are several possible remedies:
1) ignore the problem and hope I don't lose too many kWh's due to grid
overvoltage
2) I could install transformers to lower the voltage to the inverters
by ~5%----so far,
grid voltage in my neighborhood has *always* measured at or above
nominal.
3) if it weren't for the UL and IEEE standards, solar inverters could
be set with values
that made them compatible with prevailing utility operating
conditions, perhaps
+/- 15% of nominal voltage.
4) at a cost of many millions of $$$, SCE could completely replace the
local
distribution system with new 12kV facilities, even though annual load
growth
on the old wires is trivial or possibly negative...
5) at a more modest cost, SCE could make strategic upgrades to lower
the load
and voltage drop on the most stressed parts of the old network
How have other installers dealt with utility voltage problems?
/wk
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