Arc Suppression Switching between Mains/Off-grid (Interlocked Contactors)

[UV-PWRD]

Wasn't sure where to post this.

I can currently switch power source instantly in my shed from mains power to a 48v system using interlocked contactors, however when switching from mains to off-grid specifically, I'm getting some arc flash on the contacts in the mains contactor. I presume because as the contactors switch, the load side is already energised by the off-grid supply, but I see this is common for contactors regardless.


Consider the following image, ignoring the arc suppressor for now and the lack of wiring showing the interlocking, rest assured they are interlocked.



I was considering using a passive arc suppressor (capacitor and resistor) or 2 (one for active one for neutral) on the mains contactor , but my concern is current flow across it to the mains supply side when I'm running on the off-grid supply. Or am I misunderstanding how the Arc suppressor works and no current will flow because it's a high resistance path once the contactors are fully switched?

Thanks

 

[Warpspeed]

Introduce some slight delay during changeover. So it disconnects from one source, pauses, then connects to the alternative source.
And it needs to do it when changing in both directions.

I suspect you may already have a delay switching from mains to 48v, but not when switching back the other way.
Contactors are fairly slow, typically tens of milliseconds maybe one or two mains cycles.
It all needs to be thought through.

 

[UV-PWRD]

Introduce some slight delay during changeover. So it disconnects from one source, pauses, then connects to the alternative source.
And it needs to do it when changing in both directions.

I suspect you may already have a delay switching from mains to 48v, but not when switching back the other way.
Contactors are fairly slow, typically tens of milliseconds maybe one or two mains cycles.
It all needs to be thought through.

Have considered this and did order a little single din timer also so I can break/make the triggering circuit, just interested in the arc suppression setup.

As above though, the arc is most noticable when switching FROM mains to Off-grid. If there's an arc happening the other way I'm not seeing it and the contacts are not showing any degradation it.

 

[Warpspeed]

Try switching manually.
Turn one source off, wait one second, switch over to the other source.
If that fixes the arcing problem, it will give you a direction to explore a bit more.

 

[UV-PWRD]

Try switching manually.
Turn one source off, wait one second, switch over to the other source.
If that fixes the arcing problem, it will give you a direction to explore a bit more.

Can do, thoughts on the arc suppressors in this this example?

Is my understanding about flow/resistance correct?

 

[RCinFLA]

Your drawing leaves out some details.

I going to assume you have ganged breakers between a 48v battery-based inverter and AC mains coming in to immediately switch between inverter AC output and grid.

I am also assuming the inverter is not a grid interactive inverter, just battery sourcing a stand-alone inverter.

The issue in this case is the inverter's AC output and the grid AC output have their sinewave phasing random to each other.

Any load, like an AC motor or device with a power transformer for example, does not like to have its AC power source to make a sudden discontinuity jump in AC sinewave phasing. It can be like driving your car down the street in forward then throwing your transmission suddenly in reverse.

Depending on the random phasing, going from grid to inverter supplying an AC motor load could cause the AC motor to kick back a large surge current into the out-of-sync inverter AC output that could damage the inverter.

If you allow a time gap of no AC sourcing, allowing motor to stop, before switching on the inverter it can prevent the back surge. You may then have a motor starting issue as an air compressor or water pump may require more starting current because they have head pressure built up causing harder start up. If that happens then the time gap needs to be longer to allow the head pressure to drop.

 

[Warpspeed]

Can do, thoughts on the arc suppressors in this this example?

Is my understanding about flow/resistance correct?

Arc (on opening) and spark (on closing) suppressors can be made to work reasonably well only with dc.

An ac arc, as in an arc welder, is usually created by breaking a highly inductive circuit, and is difficult if not imposiible to suppress electrically once the arc forms.
An arc welder is only about roughly 50 volts, and you can make about a two inch long arc with that before it extinguishes.
Plasma is highly conductive, and the arc grows, and it does not need a very high voltage for the arc to become continuous if there is sufficient current available to feed it.

 

[UV-PWRD]

Quick update, chatted with my elec engineering mate, he believes it's a control circuit issue due to timing of the coils movement.

I'm collecting an interposing relay this afternoon and will drive the coils from that and we'll see how we go.

 

[bolek_bolek]

Better listen to the contacts clicking to see how long (if there is) the switching delay between break and make , both ways. And bear in mind that the mains is much stronger than the inverter when it comes to surge currents. Going from mains to inverter under high (or inductive) load will ramp the current according to the inverter's power limit. Restarting the load after switching back to mains will be much faster due to the very high current available.

 

[UV-PWRD]

Update for anyone that was interested, in short interposing relay fixed it.

Long version.

Previous control logic, electronically interlocked contactors as you'd expect had one contactors coil being driven off the aux contactor on the other coil, so one cannot be opened while the other is closed - I'm using LS Metasol 65a units.

However, it seems the timing of this was an issue, because the aux contactor does not appear to operate until the contactor that it's attached to has fully completed its travel, meaning, in one direction there appeared to be a few ms of overlap between them when a few cheeky electrons could flow - bit of an issue when you have 2 unique AC sources not in sync etc.

I knew I needed to briefly delay the switching or build in some cleaner make/break logic and this interposing relay has done that. They are switching very nicely now and still very fast (clocks stay on etc) and of course no visible arcs. I have a replacement contactor on the way also to swap out the one I witnessed arcing.

So, this relay (AC coil so I can drive it from a smart plug via Home Assistant etc)

Finder 55 Series Miniature General Purpose Relay 7A 4CO 240V AC Coil with PB plus LED plus Mech Ind

Finder 55 Series Miniature General Purpose Relay 7A 4 Changeover 240V AC Coil with Push Button plus LED plus Mechanical Indicator

www.nhp.com.au

Plus this base to make it din mountable and just nicer to use.

Finder 90 Series Base for 5534 Relay 9902 LED

Finder 90 Series Base for 5534 Relay 9902 LED

www.nhp.com.au

Thanks all.

 

[rogerheflin]

You likely also need some sort of feedback to confirm that the relay disconnects (is not a stuck/spot welded) as this sort of relay's common failure mode is to stick closed.

 

[UV-PWRD]

Thanks, I'll keep that in mind. Home Assistant tells me which power source is in use and I can see where the load is being supplied from. I can automate a notification if the state of the switch (that drives the relay) does not match the source flow.

I'm not worried about the relay failing, so long as it fails fully open or fully closed. Each contactor has its own coil supply so unless there's no power on both supplies, I always have power - I also have a generator circuit in addition to all this.

 

[rogerheflin]

Thanks, I'll keep that in mind. Home Assistant tells me which power source is in use and I can see where the load is being supplied from. I can automate a notification if the state of the switch (that drives the relay) does not match the source flow.

I'm not worried about the relay failing, so long as it fails fully open or fully closed. Each contactor has its own coil supply so unless there's no power on both supplies, I always have power - I also have a generator circuit in addition to all this.

The issue is if one relay fails closed, and you attempt to disengage it (and it is stuck) and then engage the other relay, then something (probably the inverter) is going to break because out-of-sync typically causes larger currents If your are lucky the one of the breakers will trip, if you are unlucky the inverter will let the smoke out before the breaker trips.