Chris,
SCRs are capable of creating voltage spikes that cause noise well into
the radio frequencies. But with TWACS there is an inductor in series
with the SCR that is tuned such that the pulse caused when it fires is
0.5 ms to 1.0 ms wide with very little high frequency content. Because
the frequency content is very low, it difficult to filter out. Ferrite
toroids are of no help and large capacitors will prevent the utility
from reading the meter too. The pulse location is near the zero crossing
so it makes for a dirty looking waveform for about 8 seconds while the
meter is sending data. Never-the-less, it's actually easy to make a
reliable frequency measurement by shifting the measurement threshold off
zero far enough that the TWACS signal doesn't interfere. The Enpase
folks should be able to do that with a firmware change.
Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com
t: 541-568-4882
On 7/8/2012 4:47 PM, Chris Mason wrote:
Kent,
I haven't any direct experience with the smart meters tripping
inverters, but the issue of SCRs and generators is well known. I
suspect you understand the problem, but to recap it, the problem
revolves around the way SCRs make a sine wave.
A motor driven generator controls the alternator output by increasing
or decreasing the DC excitation level. It does that by sensing the
"zero crossing", or point in the sine wave at which the voltage is
zero. This happens 120 times a second on a 60Hz system.
When the source of the electricity is electronic, the waveform will
sometimes be messy around the zero crossing. That gives the voltage
control circuit problems to detect the zero crossing We see this a lot
when trying to drive a UPS from a generator, it can drive the
generator crazy.
I suspect your problem is the same. The SCR in the meter disturbs the
waveform and the inverter thinks it is a faulty waveform.
I would try a low pass filter between the meter and the inverter. A
properly sized capacitor ought to remove any noise above 60hz and
present a clean waveform to the inverter.
On Sun, Jul 8, 2012 at 7:06 PM, Kent Osterberg <[email protected]
<mailto:[email protected]>> wrote:
Wrenches,
There weren't very many replies to my question about
grid-connected inverter compatibility with utility remote meter
reading systems so I'll explain my recent experience with hope
that this information may be of assistance sometime.
Last fall I installed an Ephase system for a customer. Soon after
it was on line he complained that the inverters were dropping off
line frequently. Since we had a lot of cloudy weather in northeast
Oregon all winter, it wasn't easy to see what was going on. In the
spring we had some clear days and noticed that the inverters were
dropping off line several times a day at exactly the same time
every day. It didn't take very long for our suspicion to turn to
the utilities new remote meter reading equipment. Because our
weather has been so cloudy it took a while to find clear days
where the utility could provide assistance. But with their help by
reading the meter on demand and temporarily disabling the meter
communications we were able to finally prove the remote meter
reading system is tripping the inverters offline for five minutes
every time the meter sends a data packet.
Recently the remote meter reading system has expanded to more
areas so we have now observed this problem with both Enphase M190
and M215 inverters. The system we've observed that is in town is
seeing many more dropouts, so we think every kWh meter on the same
service transformer and maybe some beyond it are causing trouble too.
The remote meter reading equipment is made by Aclara and uses
their TWACS technology. This isn't new technology and with more
remote meter reading systems going in all the time there will be
more occurrences of incompatibility. Certainly more between
Enphase and TWACS. And possibly more between other inverters and
other communications systems.
At present I'm really dissatisfied with Enphase's support on this
issue. All they've done is point out that only a few systems have
been affected and put us in the position of telling customers that
they'll just have to live with the lost production time. Until
Enphase comes up with a fix, I won't install any more Enphase
inverters.
--
Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com <http://www.bluemountainsolar.com>
t: 541-568-4882 <tel:541-568-4882>
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Chris Mason
President, Comet Systems Ltd
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Cell: 264.235.5670
Skype: netconcepts
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