question

kenyon avatar image
kenyon asked

Common Neutral in Domestic Situation

Hi All,

I need some solid engineering input on this question of making input and output inverter/charger neutrals common. I have seen the Common Neutral thread which is for a different application. This is a static application situation - a problem I have been battling with for over 6 years on all my projects.

It is essential to state the two key problem causes and why they are not solvable:

1. Our municipal grid has been degrading severely over the last 30 years with poor maintenance causing neutral disconnections and ground-neutral bonding issues on the utility/transformer side. This is getting worse due to utility having little finance and incompetent maintenance staff.

2. House wiring standards are not adhered to in many site cases meaning joins on neutral in ceilings/conduits for multiple electrical circuits. Or missing neutrals. To fix this (compliance issue) takes up a huge amount of time which customers are mostly not willing to pay for so I have found myself working hours, days, weeks and months for free to sort out these issues. Why is this a problem? Apart from the obvious electrical problems and neutral overloading, in my country customers cant afford to go off-grid so we split the house wiring into "inverter powered" and "grid powered" circuit (btw: we have daily power outages everywhere). Which means inevitably we will have a neutral fault or a common neutral somewhere in the wiring due to these non-compliant situations. So it may be worth common-ing inverter and grid neutral in the main distribution to "draw" the connection off house wiring - just a thought as the worst thing is to power a circuit without a neutral - our utility conditions has done us the honor of doing the "hands-on" test for us - to the detriment of equipment or lots of SPDs.

So my question: Is there any electrical compromise I can look into? - to stop me from sponsoring customer's compliance because of the nasty 80% of electricians who cheat and make lots of money.

Maybe some specific protective device or solution including common neutral, a specific SPD? some RCD that works -

* a 30mA RCD is not an option - electricians bypass them because of continuous tripping.

* off grid not an option as customers can't afford it.

* redoing site wiring not an option because customers can't/ don't want to afford it

* me quitting the business is a less than ideal situation as with 90% unemployment rate in my country other lines of work are just as difficult.

So to all the engineers out there who like solving problems please HELP! :D

P.S. I'm an Electrical/Electronics Engineer with more knowledge in Electronics than Electrical Power - I have fair experience with the effect of surges and earthing issues in electronics ;)

Kenyon

MultiPlus Quattro Inverter Chargerwiringac coupling
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On the inside of the multi and multi ii are diagrams. Both show the ac in and ac out neutral are common.

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Hi Keith,

Thanks for this. However, the neutral input gets disconnected by the double pole contactor. A lot (but not all) of neutral related inverter "problems" come when the grid switches on and off (while this neutral is disconnected). I suspect that surges induced by the grid switching is what causes neutral overloading by activating SPDs in all the various switch-mode power supplies every house has these days - i have not quite figured out how yet.
The particular site in question (prompting me to come back here again :P) has checked and rechecked, meggered and loop tested circuits so I am 99.9999% sure that there are no grid powered devices with neutrals "crossed" over to the inverter system and there is no reason for earth to sink back to my inverter station before the grid point.

Food for thought.

I'm working up a new thread to take this to the next level - "what is the maximum residual current a multiplus can handle before it "pops"?"

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3 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Bad situation, but I can’t think of many safe options for you.


Perhaps the introduction of a seperate isolated charger from the grid to the batteries and then running all loads off the Multi?

At least then loads will have a reliable neutral, customer can keep battery size and solar size under control, and a trickle charge coming into the battery from the grid when available.


No chance then of a dangerous fault coming through to the loads, and keeps the price down.

Chargers can be stacked up in parallel as budget allows.

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Thanks Guy and Charlie,

This is an expensive option in my country but something I pondered a few years ago...I'm certainly going to pursue this again. its shame to loose some efficiency in the process but the benefit is probably worth it.

Its a shame... the over loaded grid could benefit from a bit of feed in too. Wish we could get our grid infrastructure sorted out.

Thanks, cheers.

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On the common neutral issue - because it is still a question. In a situation where wiring code is 100% OK is there anything electrically wrong about connecting AC in and AC out neutrals together? IS there any detailed electrical explanation on this - my assumption is that technically in AC the neutral is just a reference? When does neutral current become a thing? Is there any reading you can refer me to on this?

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Your municipal grid sounds awfully like the one in Zimbabwe.
You should not connect the neutral of AC-IN to the neutral of AC-OUT. Doing so would bypass the neutral contact of the back-feed relay. When inverting you'll end up with two neutral-earth bonds, the one in the inverter/charger and the municipal one.
There is only supposed to be one neutral-earth link in a system.

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hi wkirby,

yes when bonding in and out neutrals it is necessary to "uncheck" the ground relay option in VE.config.

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Consider the technologies applied ie power electronics transformerless inverters vs isolated transformer inverters

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@Guy Stewart (Victron Community Manager)


Unfortunately, the Skyla chargers are too expensive and restricted in capacity especially at 48 V end ( MAX 2.5 KW)


In one case we used two nos of Hybrid inverters white label brand very popular in africa (which has Inverter, MPPT and AC charger all in one box)

One inverter connected to House as supply system with Some PV attached but disconnected from Grid and another inverter connected to grid and PV attached ( to work as a charger) but no loads on AC output This method normally allows 100 % load during day time and around 70 % load during the night without excessively draining batteries


The efficiency of the whole system is horrible - GRID to AC charger to battery to the inverter but basically makes a UPS system for the whole house


I have tried other solutions such as DC power supplies, generic Chargers ( even Generator powered DC chargers- called as telecom generator) -

The charger always becomes the weak link in the system and prone to failure over sustained use



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Anil Ghatikar avatar image
Anil Ghatikar answered ·

@Kenyon

Here in Nigeria, we have the exact situation -

The safest and easiest option I normally advise friends and family is to

use a Quattro with AC out 1 connected to critical loads TV Fridge, FAN and one some lights ( separate DB - directly wired) and then feed the Main DB with AC out 2

AC OUT 1 has 30 MA ELCB ( type B or F) which trips mainly on equipment insulation failure - very rare

AC out 2 has 300 ma ELCB ( Auto reclosing Type A /AC) which fails on any serious-earth faults - very common including rainy days ( lightning ) or water in outdoor fittings

When the grid is available the neutral to all loads is from the grid ( one common point)

when the grid fails Quattro disconnects AC out 2 and provides the power to AC out 1 including local neutral through a grounding contactor

The input 1 AC IN 1 of Quattro is protected by an SPD and voltage protection device Or stabilizer as per the local conditions of grid

and AC IN 2 is normally wired to the generator

I am aware of at least 10 installations where the users are happy about the improvement of reliability and safety of the whole installation


Depending on the consumer load ( max load) and installed PV power you can even write assistants to use AC Input ignore and then force AC out 2 to ON status to exclusively use solar during the day time



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Hi Anil, This is such an obvious solution i'm asking myself why didn't i consider this? :P Thank you!

I think one of the main reasons I probably have not taken this route yet (other loads to AC out 2) is because a lot of my installations are a long distance from the main DB so cost of cabling adds up rather quickly. (especially on the 16kVA +).

A relay may or may not be an option. I have been right on the point of changing all my (remaining AC in and out tied neutral) installations to "isolated" neutrals when this last weekend a 5kVA quattro gave up - off grid - grid was disconnected, about 200W of loads were running then !overload, !lowbattery and !high DC ripple alarms all at the same time. disconnecting all AC cables and powering on the inverter results in a overload shutdown - so the output is blown... Ground fault? Ground and Neutral fault? There were no electrical storms anywhere near, faulty Quattro? - will see.

Anyway I have taken to the science world to see what the people studying this say (researchgate.net, ieee and nrel)

PS. I have found "stabilizers" are not effective, unless correcting long term transients.

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Charlie Johnson avatar image
Charlie Johnson answered ·

The recommendation by @Guy Stewart (Victron Community Manager) is spot on. This will eliminate all of the safety concerns leaving only cost of a universal battery charger as the impediment to putting this plan in place.

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