On Tuesday, June 11, 2019 at 11:19:22 PM UTC-4, wrote:
On Tue, 11 Jun 2019 16:55:43 -0700 (PDT), trader_4
wrote:
On Tuesday, June 11, 2019 at 7:46:48 PM UTC-4, wrote:
On Tue, 11 Jun 2019 14:20:47 -0700 (PDT), trader_4
wrote:
On Tuesday, June 11, 2019 at 5:05:03 PM UTC-4, wrote:
On Tue, 11 Jun 2019 10:21:46 -0700 (PDT), trader_4
wrote:
On Tuesday, June 11, 2019 at 12:47:52 PM UTC-4, Terry Coombs wrote:
On 6/11/2019 8:32 AM, wrote:
Why is the ground rod at the service any better than the one at the
shop? I agree 4 is better than 2 but they are already bonded together
via the neutral in a 3 wire feeder. I doubt he wants to dig up the
yard. This was done this way for 90 years and we didn't pile up a lot
of bodies. The code change was mostly just to make the language
consistent with the rest of the code. Same with ranges and dryers. The
grandfather clause is still there for things done pre 96 adoption.
It does mean you have to be more careful with your bonding and
grounding tho.
the ground connection serves 2 purposes
1) a path for lightning
the ground rod is to provide a path for lightning etc.
this NEEDS to be a path to actual Earth ground because that is where lightning will go.
2) protection from shock due to equipment faults
this is to protect you from a shock if the hot wire shorts to the metal case inside an appliance. Think of a motor with all those windings of enameled wire. If the insulation should fail and connect the power to the metal case of the appliance, you can be shocked by touching the appliance. If the case is grounded
however,
when the short happens, a large fault current will flow and blow the breaker. For a LARGE fault current to flow, the ground must be low resistance. So you want a BOND WIRE between the neutral and ground in the system. This provides a path for the fault current to flow sufficient to blow the breaker if there is a ground
fault.
Often a ground rod alone is too high resistance.
#2 is an important safety feature, I would not skimp on this.
So if have only 3 wires between the buildings and you can't add the 4th wire, I think bonding the outbuilding neutral and ground together at the entrance to the outbuilding is the safer option compared to relying on a ground rod alone.
You can also add a ground rod there to help for lightning.
If you don't need 240V and need only 120V in the outbuilding, you can use the 3 wires as hot neutral and ground. That is 100% safe and legal but you give up having 240V.
mark
Â* Done is done , I have bonded the ground/neutral in the sub panel and
added a ground rod out in the shop . Doing without 240V is unacceptable
, I have 2 welders and an air compressor that all need 240V . And plans
are to install a 240V 3 phase motor with a VFD on the mill in the future
. Lightning is unlikely down in The Holler , but still possible .. This
is a "hobby" shop , but is very well equipped - pretty much a full
machine and welding shop .
--
Snag
Yes , I'm old
and crochety - and armed .
Get outta my woods !
That's what I would have done too, except probably adding a second ground
rod. IDK how you're going to install 3 phase eqpt though, without redoing
what you just did and a whole new service, for that matter?
You can get a VFD that is single phase in and 3 phase out
I see, well that solves that.
So here's another puzzling question. In that thread about solar panels
that has now run amok, Danny brought up the issue of preventing backfeeding
into the grid. AFAIK, that's built into the inverters, but how do they
do it? It would seem like the chicken and the egg problem. You need to
detect the grid power going off, but you are connected directly to it and
also powering it. So, how do they detect it? Obviously before connecting
they must first monitor the voltage, freq and phase and sync to it, but
once you connect, how do you then tell the other grid sources are gone?
I guess you could look for voltage drop, since with the grid down
you'd expect 99.9% there will be big voltage decrease. But is that
sufficient? And what do they really do inside those inverters?
Hypothetically, suppose there are 6 houses on a utility line that
have solar and that segment is connected to the whole distribution
system. The sun is shining, those houses are using significantly
less than the panels put out. I cut the line to the rest of the grid,
leaving just those 6 connected. What happens? Is the "grid" down?
How do those inverters detect it? Do they? What happens?
We got a pitch on these things a while ago. Basically a grid tie
inverter is clocked from the grid. No grid, no output.
But the question remains, the inverter is part of the grid, the inverters
along the block are part of the grid, in addition to some generators
somewhere. So, how does it know that the "grid" is down?
Hypothetically, suppose there are 6 houses on a utility line that
have solar and that segment is connected to the whole distribution
system. The sun is shining, those houses are using significantly
less than the panels put out. I cut the line to the rest of the grid,
leaving just those 6 connected. What happens? Is the "grid" down?
How do those inverters detect it? Do they? What happens?
That is an interesting question but theoretically each grid tied
inverter would lose clocking and shut down.
But they don't really lose clocking, because if you have six houses,
each was following the "grid" which is whatever sources, including
the other solar. So, they would then follow collectively what is
there which at the moment the segement is cut off, it 60 hz and A-OK.
So they would be following each other. But I think what happens is
that since they are each following the other and constantly trying
to match, it does lead to the freq starting to vary slightly.
I have posed this question to everyone I have talked to without an
answer. What happens if you disconnect from the grid, drop all of
your loads and connect a battery powered inverter. Will that clock
your grid tie and allow you to add loads until you overwhelm it?
Right, same idea. Would the grid tie inverter recognize that the
battery powered inverter isn't the "grid". Whatever the "grid"
even means. I guess the grid they are looking for would be
defined as having at least one power company generator source. But wait,
not sure even that's right. Power companies have solar and wind
today too. If a disconnected segment had power solar, it's like
the solar from home we're talking about. Wind, IDK, but that would
seem to have the same issues, I assume they are using inverters too
and not a generator where the rotation speed governs the freq.
They run at a
voltage that can be potentially slightly higher than the grid so if
there is any power left over, it flows backward into the grid. A smart
meter will see that and meter based on your local tariff agreement.
The old style would just run backward if net flow was out instead of
in. If you netted more use than you fed back in, no harm, just a
lower bill. If you showed a negative bill I guess you would be talking
to the PoCo.
That is how people used to steal power, plug the meter in upside down
it runs backward so there is a chance you would be starting in the
fraud department if they did not know about your solar.
The "plug in" collectors plug in just like your toaster and will boost
the power on that side of your service. These things are usually too
small to ever feed back into the grid if you have anything running at
all.
But from everything I see, a 10KW home solar system behaves exactly
like the plug in ones and needs to detect when to disconnect.
That is the way I understand it. Each inverter should detect loss of
grid and shut down. It would be interesting if you had something else
feeding the grid like your generator idea that the plan might fail. I
also doubt there is enough power going into the grid to sustain it if
you are not getting utility power, even if you have a lot of people
with a black roof.
I agree. In 99.9% of the cases of a segment coming disconnected from
the rest of the utility, you'd think there would be big, obvious
problems, starting with a big voltage drop. I also ran across a
study of whether this is such a big deal or not. In terms of being
a possible danger to linemen, the study concluded it really isn't,
that they take precautions and it's extremely unlikely they would
be injured. Higher chance of first responders, but even there very
low incidence. Even damage to eqpt that was connected was rated
very low. The biggest issue was that automatic reclosers on the
grid that try to close to restore power could be affected and not
close like they should.
This might work if you had one guy on a street full of solar power
systems with a generator back feeding the street to tickle everyone's
inverter, all residents being frugal and a line cut isolating that
street from the rest of the grid.
It would work until sundown anyway ;-)
I found this that explains a lot. We're on the right track of thinking
about what happens and that it's kind of a paradox as to how to detect
the loss of the "grid". They call this problem "islanding". This
talks about the ways they detect it, it's complicated and even includes
active action by inverters to deliberately try to destabilize the
grid so that it goes off kilter and all the other inverters can more
quickly detect it. It looks like they use voltage changes, freq changes
and power factor changes as possible parts of the algorithms and
that it's not a trivial problem. I think this gear is more for a larger
commercial array, but the same principles should apply to all inverters.
https://www.solectria.com/blog/anti-...-pv-inverters/