Design Review - Growatt SPF 5000 ES Grid Backup & Neutral Bonding

[Struc]

@fmeili1 - I don't want to seem rude, but you have 3 posts here. I'm not going to recount everything that has been said 10 times in this and other threads. Please do some of your own reading on this inverter and other inverters that bond the neutral and ground.

 

[RCinFLA]

The '5kW' Solar Edge and Growatt autotransformers are actually 2500 VA autotransformers. They can take 2500 VA of continuous imbalance between 120vac phases. Solar Edge (discontinued produce) has some heat sink fins on back of case so they claim 3000 VA imbalance max,

Problem with having inverter with more 240vac power than 2500 watts (like 5KVA Growatt 240vac inverter) is it is possible to have 5kVA or greater of imbalanced 120 vac phase load that will overload auto transformer. 240vac loads don't put any current through autotransformer. Perfectly balanced 120vac phase loading does not put any current load on autotransformer.

There are two issues that need to be prevented. First is never lose the neutral while 240vac is still present. This will give you a floating neutral in breaker panel subjecting your 120vac loads to 0 to 240vac depending on 120vac phase side loading balance. Ignore this and you may be buying a replacement refrigerator or other 120vac appliances. Never put a breaker on the neutral line from autotransformer. If neutral breaker opens you will have floating neutral.

Second is allowing transformer to overheat due to excessive 120vac phase load imbalance. There is two ways to prevent these issues. You have to shutdown the 240vac from inverter either by detecting transformer temperature or measuring transformer neutral current to break the 240vac from inverter. This can be either a solenoid deactivated 240vac dual breaker on output of inverter or remote control shut down of inverter. Both need a controller to make the autotransformer sensing and generate the remote control output to control the breaker solenoid or inverter remote control shutdown. Victron autotransformer uses the solenoid system with both neutral current and transformer temp sensing.

The 240vac breaker Victron uses is a solenoid deactivated breaker with their designed controller board. Solar Edge has a 'K' type thermocouple on autotransformer core that connects to their inverter to remotely shut down their inverter if transformer gets too hot.
Growatt autotransformer has nothing, it is a fire waiting to happen.

The Victron uses a CHiNT breaker with the 'S9' solenoid add on module. It has a 24v solenoid. There is also a 48v solenoid version.



Autotransformer's can take some overload for 5 to 20 minutes depending on how much overload imbalance current so minimum acceptable solution is to shut down 240vac from inverter based on a temperature controller which are readily available. Thermocouple based temp sensors are more reliable than thermistor based temp sensors. Shutdown if transformer gets above 50 to 60 degs C.

 

[automatikdonn]

Can you confirm or correct what I have found after researching this topic?

Main (grid) panel, has G/N bond. Each sub panel must not be bonded.

Separate power source panels, a generator load distribution panel, for instance, should have G/N bond. And a solar inverter is the same, so a separate ground rod, with a G/N bond at the primary distribution panel.

This is what I found from NEC discussions on the topic, I am not stating as fact, just observation. What do you think?

I love your circuit design btw, planning to copy it.

This is the final video in the series.

 

[automatikdonn]

Here's the first suitable match I found on eBay.
2x 240V primary windings, 2x 120V secondary windings, 3000VA.
However, I'm not sure secondary windings in series for 120/240V split-phase could deliver 3000VA at 120V. Certainly at least 1500VA.
If secondary windings are put in parallel it will deliver 3000VA at 120V only, no isolated 240V.
Maybe look for 6000VA if you want to load with 3000VA 120V one moment, 3000VA 240V the next.

Daykin Transformer 3000va 240/480-120/240v TF3000EP | eBay

Find many great new & used options and get the best deals for Daykin Transformer 3000va 240/480-120/240v TF3000EP at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Here is a supplier of listed isolation transformers

LarsonElectronics.com

LarsonElectronics.com Description

www.larsonelectronics.com

 

[copec]

I didn’t see this gentleman’s project posted, but it’s how I would do it if I were using it with a grid feed-through:

https://ebay.to/3IkxwMl

 

[Taulli1]

I didn’t see this gentleman’s project posted, but it’s how I would do it if I were using it with a grid feed-through:

https://ebay.to/3IkxwMl

This is a fantastic video.

 

[Stuartholmau]

I was trying to get this very question answered by the people at Signature Solar just this past week. The person I emailed back and forth with was unhelpful and never answered my question about how to wire AC in for the on-grid scenario.

One way I think you could do this is to wire grid power directly into the inverter (AC in) and then wire the inverter (AC out) to the breaker panel (attached to a circuit breaker), where the transformer also would be wired (attached to its own circuit breaker). That is, put the inverter in between the meter and the main breaker panel. The problem with this approach is that the inverter likely is not big enough to run all the circuits on the house.

If you figure this out, I hope you will post it here as I would love to know how to do it with a sub-panel.

I own a farm with two houses, the primary house ran with an old MPP48PIP inverter charger which is similar to the Growatt and my solution for this problem was to use the AC in for battery charging only and divert external AC supply via MBB breaker. This worked very well until we were flooded this February. After spending a little time I found the product that takes care of everything and that was a Victron MultiplusII with built in charge controller. Now my generator starts automatically and the power transfer is done completely by the Multiplus II itself.

The down side of the Multiplus is its cost. However, when I reinstall the off grid plant to the second house, I will stick with Victron.

 

[automatikdonn]

I didn’t see this gentleman’s project posted, but it’s how I would do it if I were using it with a grid feed-through:

https://ebay.to/3IkxwMl

Isolation transformer is probably the way to go. The content in this thread is for people looking for the cheapest possible solution and mainly to fully answer the OP's question.

It is what I will be using for my 50KW project.

 

[B-Mod]

I have two of the growatt auto transformers. I built the control cabinet automatikdonn posted here, with a few tweaks, and I added over temp protection. Does anyone know what temp I should set the controller to? What is too hot?

 

[automatikdonn]

I have two of the growatt auto transformers. I built the control cabinet automatikdonn posted here, with a few tweaks, and I added over temp protection. Does anyone know what temp I should set the controller to? What is too hot?

I don't know what temp it should be set at, but I would love to see your design for the thermal protection part of this setup. !!! Way to go

 

[??????]

@B-Mod - some previous work has been done on this, see post #207 - tl;dr is that nobody (currently - no pun intended) knows, it was tested up to 95C/200F and did not trip. Most thermistors of similar spec seem to be about 150C/300F, so the best guess at time of writing is that it's about that, which imo is still quite high for my personal comfort. My proposed implementation uses a continuous sensor so I'll be able to change it in software as knowledge improves.

 

[Larry Oswald]

Having thought through the whole Auto-Transformer thing A LOT, I've come up with what I think is the best, and KISS, solution:

Don't use an auto-transformer. Use an isolation transformer. Take in 240V from L1/L2 of the inverter and convert to isolated 120/240V split-phase. Bond the center-tap (neutral) to ground, same ground as main panel. Transformer VA rating must be sufficient for full inverter output and full pass-through from grid. It just works.

I haven't read through all pages and all 261 previous posts of this thread, just some pages. But I did all of a couple other threads on the topic, and found multiple safety and equipment hazards.

I may yet add an auto-transformer to my system, but for other purposes. Not to establish a neutral; the inverters are stacked 120V each. Auto-transformer would be for load balancing, and to create missing phase if all inverters except one master area allowed to sleep for power savings. It would be disconnected when on-grid.

It was pointed out by the NCSolar guy that the entire load goes THROUGH an isolation transformer and only the IMBALANCE between the two 120VAC phases goes THROUGH the auto transformer. Huge difference in the demand (and heating) of whichever transformer. If the two phases are balance there should be 'no' heating inside the auto transformer.

 

[Hedges]

Yes, auto-transformer is the low-loss way to support a bit of 120V load, without much loss when it isn't doing anything for a 240V load.
But, we've figured out major issues with using auto-transformer for 240V inverter that is also fed from grid. Those can be avoided, and some manufacturers have figured it out, but not all.

In other news, I've discovered driving the secondary windings of a transformer doesn't always work well. Armed with that knowledge, I can do a better job of repurposing an isolation step-down transformer as an auto-transformer. But I'm going to have to work harder to get step-up working right.

3-Phase SMA Sunny Island (15kW) + Sunny TriPower (30kW)

Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...

diysolarforum.com

 

[Larry Oswald]

Yes, auto-transformer is the low-loss way to support a bit of 120V load, without much loss when it isn't doing anything for a 240V load.
But, we've figured out major issues with using auto-transformer for 240V inverter that is also fed from grid. Those can be avoided, and some manufacturers have figured it out, but not all.

In other news, I've discovered driving the secondary windings of a transformer doesn't always work well. Armed with that knowledge, I can do a better job of repurposing an isolation step-down transformer as an auto-transformer. But I'm going to have to work harder to get step-up working right.

3-Phase SMA Sunny Island (15kW) + Sunny TriPower (30kW)

Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...

diysolarforum.com

I have a defective 3KW isolation transformer with a low 95VAC output (other half good 115VAC) with grid 220VAC as input 1->1 connected. I am using the two split windings PRIMARY as an auto-transformer. So far for low single loads and two at a time (fridge, freezer, 1/2HP water pump) it is working well. Might try the coffee maker later. That is an emergency device for sure. My EAsun 3.6KW only produces 230 (or 240) floating output. The transformer split is nice 115VAC (or 120) My clampon meter is not accurate enough for good current data.

 

[Hedges]

95VAC and 15VAC?
The split windings you're driving are intended as primary? That should be satisfactory, although the transformers I've tested so far still have current shoot up a bit at nominal voltage (just enough material used to avoid going excessively into saturation.) I plan to use higher voltage windings, e.g. 240V winding driven with 120V, expect lower current and loss that way.

For reasons I don't understand, my Harbor Freight Ames 1000A meter, on the 600/60A scale (0.01A resolution) doesn't register AC current into transformer windings. Even though that is 0.13Arms.

It registers multiple amps AC just fine, also 10's of mA DC. I think AC uses current transformer, DC uses Hall effect.

I can see the current on scope with a flexible Tektronix current probe (Rogowski coil) for a calculated 15VA, and clamp DC reading on battery gave me power consumption reading (7W more with transformer than without). How accurate the meter is with ripple on DC, I don't know.

When I have more transformers on hand to play with I'll try A/B tests using 120 vs. 240V windings driven, and 240V primary vs. secondary. Expectation is driving a winding at half its intended voltage will show much nicer sine wave current, as will intended primary vs. backfeeding secondary.

Efficiency of inverter driving reactive load probably varies with brand, so can't do that myself (except for using a StatPower MSW inverter). Maybe can compare notes with somebody else testing a different brand sine wave inverter.

 

[ioanych]

I've bought new Growatt SPF 5000 ES two months ago. In FilterGuy papers i've found that 'Growatt has updated the SPF 300TL LVM to do the dynamic NG- bonding internally'. But how this dynamic NG-bonding is implemented ? Will it tie ground to neutral, or make common neutral ?

 

[automatikdonn]

Has one found a din rail mounted temp sensor yet? I have been looking and turning up with a zero.

I plan to do some space heater based testing of the auto transformers to see what they do, what temps they get to, etc.

 

[B-Mod]

Has one found a din rail mounted temp sensor yet? I have been looking and turning up with a zero.

I plan to do some space heater based testing of the auto transformers to see what they do, what temps they get to, etc.

I cut one into the face of my control cabinet. Highest temp I have seen was maybe 92 degrees F, most of the time it is 85 or less. This is installed in my basement where it is about 55 to 65 degrees F. I used a Dwyer TSS2, is a dual temp as I have 2 auto transformers, or a single temp is TS2.

 

[automatikdonn]

In an effort to give people some other options for a more simple setup, I built a significantly more compact version of the the safety circuit I built this prototype yesterday. It does not use any contactors at all - however it lacks the capability to shutdown the input to the AT in the event of failure. The output side is handled by a shunt trip breaker.

I also wanted to point out these two NEC sections - the TLDR is you *must* connect your autotransformer to the grid neutral if you have a grid supply in passthrough mode.

210.9 Circuits Derived From Autotransformers
Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

215.11 Circuits Derived From Autotransformers
Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

 

[Hedges]

I also wanted to point out these two NEC sections - the TLDR is you *must* connect your autotransformer to the grid neutral if you have a grid supply in passthrough mode.

210.9 Circuits Derived From Autotransformers
Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

215.11 Circuits Derived From Autotransformers
Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

*must* only If a circuit is derived from autotransformer.

I think it would be better to switch center-tap of autotransformer, so only derived when not fed from grid.

While in pass-through mode, autotransformer is not needed to establish neutral. When disconnected from grid, it is needed.

If neutral of branch circuit is hardwired to neutral of grid, that provides neutral/ground bonding (for fixed, not mobile applications.)
When inverter disconnects output L1 & L2 from grid and starts producing floating 240V output, connect center tap of transformer to neutral.
This could be accomplished inside inverter with 3PDT relay. Two poles connect L1 & L2 to grid in one position. Third pole connects N to autotransformer in other position.

(Either inverter is always connected to L1 & L2, or gets connected in other position, perhaps depending on whether it is grid interactive or an off-grid inverter.)

This avoids auto-transformer trying to rebalance grid and carrying current even when you have no loads (as Ian or someone showed), it avoids violating NEC because it doesn't derive a circuit while on grid, and it establishes a neutral (still ground bonded) when off grid.

Is there a way to control an external relay to accomplish this with GroWatt? Signaling relay programmable to show "on grid/off grid"? Tap into control signal of ground bonding relay? Tap into (isolated) grounding terminal of ground bond relay?

Ground bond relay connects neutral to ground when off grid. If screw has been removed to defeat dynamic bonding, that relay terminal (and PCB ring around screw hole) is open circuit when on-grid, connected to neutral off-grid. So just get a SPST relay, 120V coil, and wire coil from L1 to screw hole (but isolated from ground)?

For fail-safe (avoid lost neutral), use 3PDT relay. One position connects loads L1 & L2 to grid. Other position connects loads L1 & L2 to load panel on output of GroWatt and connects auto-transformer center-tap to neutral. Autotransformer hard-wired to load panel, no breaker. Load panel fed by GroWatt through breaker (sized for auto-transformer).

I think that is safe, but significantly limits 240V power (including two 120V loads on opposite legs) which can be powered on that loads panel with autotransformer. You can have a separate 240V-only loads panel.

The Victron autotransformer gets around current limit of autotransformer with a 3-pole breaker and monitor circuit. Breaker passes up to 100A through to loads. Autotransformer centertap is monitored by current transformer, and temperature is monitored. If either limit is violated, 3rd pole is remote tripped, tripping the other two poles. The Victron has dynamic bonding of neutral to ground. It does not dynamically switch neutral.

I think it does not comply with the NEC rules quoted above (derives neutral for loads and does not connect to neutral of grid.) Perhaps I could rewire it so it does.

https://www.victronenergy.com/upload/documents/Datasheet-Autotransformer-32A-and-100A-EN.pdf