Off grid house bonding sanity check

[jaggedben]

...

I thought of another analogy for this. If the generator is a valid place for the N-G bond then the solar array would also be a valid place. But this would be ridiculous. ...

Well, it's ridiculous in the first place because the PV array isn't going to have a neutral. Beyond that it's ridiculous practically, but not code wise in the context of this discussion.

 

[ggunn]

Therein lies the problem. There is no switching at the OUTPUT of the inverter. The inverter calls for generator and then takes power from the generator into itself. It may pass generator power on to the loads but that is only incidental. The inverter does not switch BETWEEN itself and anything else. The inverter is always the "source".

This thread is about the place of the N-G bond, GES etc. on an off grid system. Some postulated that it maybe should be at the generator.

I thought of another analogy for this. If the generator is a valid place for the N-G bond then the solar array would also be a valid place. But this would be ridiculous. The generator, solar array, some water wheel, are all auxiliary sources that feed into the inverter (or battery system) but the inverter is the one thing that is always there, always running, always required.

Ah, so. Is the generator DC?

 

[ggunn]

Well, it's ridiculous in the first place because the PV array isn't going to have a neutral. Beyond that it's ridiculous practically, but not code wise in the context of this discussion.

'Sno biggie; we go off on ridiculous tangents all the time.

 

[Birken Vogt]

Ah, so. Is the generator DC?

Some are. Some are not. Why does it matter?

 

[ggunn]

Some are. Some are not. Why does it matter?

It matters because if it is DC the generator can cycle off and on without interrupting the inverter, but if it is AC unless there is a way to synch the generator with the inverter it won't match phase with it when it starts up. I am assuming that the generator won't run unless it is needed and unless there is a way to synch it, it seems to me that the generator has to be the master when it is running and there will be a glitch when it starts up and connects.

Please do not misunderstand me; I don't know the system and I am trying to learn about it.

 

[pipe_bender]

If you bond at the generator or inverter your bonding in equipment that may need serving and or replacing, if you loose the one and have to ship it back to civilization for replacement you dont loose you G-N bond wile you run the other.
Also if you bond in the main panel its easy to find, and easier for an inter-system bond to your massive off grid HAM radio setup that we know you have in your off grid house.

 

[Birken Vogt]

It matters because if it is DC the generator can cycle off and on without interrupting the inverter, but if it is AC unless there is a way to synch the generator with the inverter it won't match phase with it when it starts up. I am assuming that the generator won't run unless it is needed and unless there is a way to synch it, it seems to me that the generator has to be the master when it is running and there will be a glitch when it starts up and connects.

Please do not misunderstand me; I don't know the system and I am trying to learn about it.

I thought I responded to this further back but maybe I didn't.

When the inverter senses that the batteries are getting low, it calls for generator start.
The generator starts and the inverter gives it a minute to warm up.
[Most] inverters will sync the frequency and voltage output to the generator phase at this point. If you are standing next to it you can hear it happening. You could see it on a meter too but I have never tried that.
At this point they close a switch and parallel generator output to inverter output and there is barely a flicker.
There is sometimes a small glitch because all the load is placed on the generator at once and its AVR and governor have to react but it is minor.

The point is that the generator is an optional component under full command of the inverter and is not the place for the bond. Frequently it is also located far away. And as pipe_bender said, it may get taken out and serviced. Also there may be a MTS or something out there so a second generator can be used or portable plugged in.

This is what I do for a living and I get sort of passionate about it. I've seen too many bad installs and I don't want any more.

Story time: A couple of years ago we took out one generator and installed another (off grid system just like above) and the new one's electronics caused it to refuse to start and do all kinds of weirdness. After much painful troubleshooting I discovered the only bond had been in the old gen, the new one didn't have a bond and it didn't like the N-G voltage floating like that. I installed a bond at the first main distribution panel at the inverter/battery shed and all was well with the world.

 

[ggunn]

I thought I responded to this further back but maybe I didn't.

When the inverter senses that the batteries are getting low, it calls for generator start.
The generator starts and the inverter gives it a minute to warm up.
[Most] inverters will sync the frequency and voltage output to the generator phase at this point. If you are standing next to it you can hear it happening. You could see it on a meter too but I have never tried that.
At this point they close a switch and parallel generator output to inverter output and there is barely a flicker.
There is sometimes a small glitch because all the load is placed on the generator at once and its AVR and governor have to react but it is minor.

The point is that the generator is an optional component under full command of the inverter and is not the place for the bond. Frequently it is also located far away. And as pipe_bender said, it may get taken out and serviced. Also there may be a MTS or something out there so a second generator can be used or portable plugged in.

This is what I do for a living and I get sort of passionate about it. I've seen too many bad installs and I don't want any more.

Story time: A couple of years ago we took out one generator and installed another (off grid system just like above) and the new one's electronics caused it to refuse to start and do all kinds of weirdness. After much painful troubleshooting I discovered the only bond had been in the old gen, the new one didn't have a bond and it didn't like the N-G voltage floating like that. I installed a bond at the first main distribution panel at the inverter/battery shed and all was well with the world.

Thanks for the information. What happens if/when the generator starts up and it is, say, 180 degrees out of phase with the inverter? Does the inverter gradually come into synch with the generator before adding it to the AC bus, or does it suddenly jump into phase? Also, there is perhaps a semantic difference between what we were saying; an inverter converts DC to AC and cannot take AC as an input, so the switching that adds the generator to the AC bus is necessarily on the inverter output. However, from a component standpoint the generator is apparently another input to the inverter unit.

A few years ago I was involved in an experimental project that combined about half a MW of PV with a couple of large DC generators that fed into the inputs of two inverters in parallel with the PV array. When the sunlight diminished at the end of the day or because of heavy cloud cover, the generators started up and the inverters continued to run. That's why I asked if the generator was DC.

 

[tortuga]

Thanks for the information. What happens if/when the generator starts up and it is, say, 180 degrees out of phase with the inverter? Does the inverter gradually come into synch with the generator before adding it to the AC bus, or does it suddenly jump into phase?

It definitly does not jump, it 'slowly' syncs, my *guess* is the 'island capable' or grid forming inverters emulate the behavior of a traditional synchronous generator and a digital syncroscope, a possible simple method is to use a Phase-Locked Loop (PLL) circuit to read the generator output at all times.
They could have a inductor and or capacitor or both bank capable of emulating 'grid inertia' with droop control.
Its an interesting topic in the Utility world they are dealing with the same issue as electronic generation from any inverter no matter how big or small does not provide rotational inertia.
More reading on it here:

https://docs.nrel.gov/docs/fy20osti/73856.pdf

 

[Birken Vogt]

Thanks for the information. What happens if/when the generator starts up and it is, say, 180 degrees out of phase with the inverter? Does the inverter gradually come into synch with the generator before adding it to the AC bus, or does it suddenly jump into phase?

I am 95% sure they come into phase because right before the relay click, you can hear the buzz of the inverter start to warble, I think it is adjusting to match phase.

Regardless, from field experience with generator-utility transfer, most single phase and even three phase ATSs do not care about inphase transfer by default, you have to turn it on. So many systems out there are jumping phase every day and nobody seems to know or care.

Also, there is perhaps a semantic difference between what we were saying; an inverter converts DC to AC and cannot take AC as an input, so the switching that adds the generator to the AC bus is necessarily on the inverter output. However, from a component standpoint the generator is apparently another input to the inverter unit.

Off grid battery inverters definitely do work both directions. I remember early Outback inverters where it was not yet officially supported, they had solar string inverters out in the field and would push power in reverse through the inverters and charge the batteries, had to add a relay to cut off the string inverters because batteries would grossly overcharge if you let them.

A few years ago I was involved in an experimental project that combined about half a MW of PV with a couple of large DC generators that fed into the inputs of two inverters in parallel with the PV array. When the sunlight diminished at the end of the day or because of heavy cloud cover, the generators started up and the inverters continued to run. That's why I asked if the generator was DC.

I only know about small size inverters but the original topic of this thread was where to put the N-G bond and some said the generator, but the generator in these system is only an auxiliary component off to one side and ought not to be bonded for this reason.

 

[ggunn]

Off grid battery inverters definitely do work both directions. I remember early Outback inverters where it was not yet officially supported, they had solar string inverters out in the field and would push power in reverse through the inverters and charge the batteries, had to add a relay to cut off the string inverters because batteries would grossly overcharge if you let them.

I get that, but what I meant was that inverter circuitry has DC input and AC output. Reverse flow AC to DC goes through rectifier circuitry. AC input to AC output does not go through inverter circuitry although it may go through the inverter's ancillary circuits. The semantic difference is what we are referring to as "the inverter".

 

[jaggedben]

...Also, there is perhaps a semantic difference between what we were saying; an inverter converts DC to AC and cannot take AC as an input, so the switching that adds the generator to the AC bus is necessarily on the inverter output. However, from a component standpoint the generator is apparently another input to the inverter unit.
...

I get that, but what I meant was that inverter circuitry has DC input and AC output. Reverse flow AC to DC goes through rectifier circuitry. AC input to AC output does not go through inverter circuitry although it may go through the inverter's ancillary circuits. The semantic difference is what we are referring to as "the inverter".

I don't believe that's correct. The basic power electronics in an inverter can generally be used in either direction and for battery inverters they are. That is, an inverter can be a rectifier (though not vice versa, necessarily). Outback used to refer to their products with the phrase "inverter/charger".

 

[ggunn]

I don't believe that's correct. The basic power electronics in an inverter can generally be used in either direction and for battery inverters they are. That is, an inverter can be a rectifier (though not vice versa, necessarily). Outback used to refer to their products with the phrase "inverter/charger".

Like I say, I get that, but inversion refers to conversion of DC to AC. Even if the same circuitry is used for both inversion and rectification, the generator must be connected to the AC side. Either that or the generator output goes through a rectification step before it connects to the DC side, which makes it essentially a DC generator. There is no DC component to an AC waveform, or at least there shouldn't be.

 

[Birken Vogt]

The semantic difference is what we are referring to as "the inverter".

It is the box on the wall that has terminals for the load, generator, and batteries.

 

[tortuga]

Like I say, I get that, but inversion refers to conversion of DC to AC. Even if the same circuitry is used for both inversion and rectification

Yeah the sol-ark 'Inverter' is just what they call a multi function piece of equipment run by one computer.
On the PCB would be a 48V DC buss isolated from the other 500V DC bus.
If we did a tear down we'd probably discover the main components are a
computer (CPU),
rectifier,
two transfromerless inverters,
a digital PLL syncroscope
as well as switchable capacitors and inductor banks
relays,
all controlled by said computer.

The DC output is probably switch mode regulated 48 nominal with a wide range like 43-63V for charging.
The PV DC input to the inverter section is probably 160 - 500 VDC.
The DC input to the 2nd inverter is probably 48V nominal.
None of that is field serviceable by a typical electrician or PV technician so its simply 'the inverter'

 

[JoshuaCobb]

Ah, so. Is the generator DC?

Well explained you’re absolutely right that the inverter remains the true source in an off-grid setup, so the N-G bond should be established at the inverter, not at auxiliary sources like the generator or solar array. Thanks

 

[ggunn]

It is the box on the wall that has terminals for the load, generator, and batteries.

There's more than just an inverter in that box, so yeah, there's the semantic difference I was referring to.