Puppeteer

Author Topic: More panels!  (Read 5024 times)

glort

  • Hero Member
  • *****
  • Posts: 2021
    • View Profile
Re: More panels!
« Reply #60 on: April 16, 2018, 01:29:47 PM »

You have it Fundamentally right Bob.

I want to direct feed the solar power into a water heater.
I could run a battery with an inverter and feed that to the element, I could/ am doing as you say with a timer ( Voltage controlled relay) and just backfeed my meters.

I am looking for the sake of learning for a more easy and straight forward way.

The inverter/ battery method may be the most straight forward but the cost of an inverter that would last more than a month well outdoes the savings and then one would also need a battery(s) and charger.  If you look at the cost of off peak power, the chances of one getting a return on investment before these things need replacement IMHO is minimal.

I have seen on electronic forums some smart boffins build small electronic circuits and I'd be happy with that had I the smarts to follow and build them myself.
I am amazed how difficult a relatively simple thing like solar generated DC can be to put to work in such a simple application..... which is why I want to conquer the problem.

Bruce's idea of the voltage controlled relay I think was brilliant and is such a simple and effective soloution to what is often an expensive and overly complicated problem. I would like to go a step simpler though with the thought of an independent/ Offgrid PV water heater in mind.

I might get round to trying something with conventional relays tomorrow. I am using regular AC breakers on my solar input as isolators. By using double pole switches and breaking both the pos and neg together, I seem to be getting no arcing. I was also watching a YT vid last night where a guy put a large capacitor across the terminals of a relay running DC and removed all the arcing that way.
the conclusion I drew that a cap of around 1500 uf would be needed, maybe 2500UF for what I am doing.

I have some DPDT 250V conventional relays so I though I'd run both poles together to give me 20A capacity and put one relay, switched together on each pole from the panels.

As far as the voltage is concerned, while still reading and learning, I'm not sure that matters.  I am of the -belief- at this time that it is not the volts or the amps which matter but the total power. 
Ie, if I have 1000V @1A and a  240V, 1500W element, I'm still OK ( did I get that math right, 1000W? ) because the total power is still within the elements capacity.
The volts don't matter, it's the power ( watts) that are the thing you have to look at.  I'm still reading and learning on that but it's what I am lead to believe so far is relevant to a dumb load like a heating element.

Having a high voltage with solar would probably be an advantage because the heating power is going to fall off badly as soon as I go under the elements voltage.

Mike, I am looking into the SSR's and the snubbers.
Unfortunately the SSR's I have are only rated to 240V and I'd like to run around 360-380 so as to be able to switch back to an inverter when the heating is done and be in the sweet spot for the inverter. Also strings of 8 panels seem to fit my roof layouts and is easy to wire than smaller arrays paralleled .

My conventional relays are only rated to 250V as well but I don't think the voltage matters much on the switched side, only the coil side.
Still leaves me with having to run an AC 250V supply for the relay coils or work out a way of rectifying the DC to a stable voltage and see if the coils will work with that.

The other idea is to PWM the DC which would take away the arcing problem but how to PWM High voltage DC I'm drawing a blank with.

BruceM

  • Hero Member
  • *****
  • Posts: 2229
    • View Profile
Re: More panels!
« Reply #61 on: April 16, 2018, 04:42:23 PM »
Warning!  Danger!

DO NOT use AC rated breakers or switches with DC over 24VDC.   I've tried that experiment with only a 500 watt load and fried a lot of heavy duty and commercial grade switches on the very first OFF.  Arcing and melt down is the result.  Putting a capacitor on the output won't solve the problem.  That can help with inductive kick back but that's not the only issue here.

I know you like to experiment and see things for yourself so make sure you use the 300V DC rated breakers to be able to reliably shut down.  Don't exceed that rating at the rated amperage.  DC arcing is dependent on both voltage and current; very light loads won't arc as much on opening the contacts.  As voltage and current go up, the arcing gets very strong.  In things like DC driven rail/subways, they actually use compressed air to blow out the arc.  In HV DC rated switches and relays, they use permanent magnets to bend the arc plus other methods to quickly move the contacts far and wide apart to extinguish the arc.

Unless you have found a Buck  converter (simplest of PWM) to regulate (lower) the output voltage, you must keep your string series voltage under load to match the rated heating element (RMS) voltage (or less).  Electro-mechanical relays for HV DC aren't commonly available.  If you don't want to so some component level soldering to add a opto-isolated gate driver IC to control a surplus IGBT module (cheap on Ebay) to do your own solid state relay, then you'll have to search for a solid state relay rated well above your operating voltage and current.  AC relays can't be used...they will arc and fry.

I use 120V nominal DC for my home, and the no load voltage of my PV array of 5 panels in series is 220 volts on so on cold days.  On cold days under load, the voltage is about 20V higher than needed for my 146 volt battery charging.  There is really that much swing in output voltage based on temperature.  Normally, your PWM/MPPT charge controller hides that variation from you, but for direct DC use, you have to think about things a lot more.

You must match the loaded PV string voltage to the load unless you can find a PWM/switching power supply to take a higher voltage and down regulate .   Most large switching power supplies with 230VAC rated input can be operated with 350VDC (Max- no load PV voltage) input.  It might be possible to find a large one or put two or more in series to get a useful DC output.
The issue will be the PV string no-load voltage exceeding the max input voltage...that will kill them fast.  Direct from PV well pump drives are designed to handle that big voltage range on input, so are another possibility.

I can make a schematic for the opto-gate driver (8 pin dip)/IGBT module method which would give you a SS relay that is bulletproof and can tolerate 500VDC.  But it is really the sort of thing for a electronics technician/hobbyist.

For direct DC drive, here's an example.  US water heater elements  are typically 4800 watts at 230VAC (same as 230VDC).  That gives us an element resistance of 11 ohms. 
At 200V it would be 3636 watts and 13.2 amps.
At 120V it would be 1309 watts and 10.9 amps

So as long as you keep the full load voltage of the PV string at or below the rated voltage of the heating element, you are right in that there is a wide range of useful heating being done on unregulated direct DC.  Brief moderate overdrive of the element won't fry it.  But remember to allow for the 20% boost in voltage of the PV on very cold winter days.  I would guess that you might be best off using one or two parallel strings of (9) 250 watt panels in series depending on how fast you wanted to heat the water. A single series string will be under 2000 watts actual.   Your no load voltage would be roughly 396 so a 600V IGBT module  is a good match. 

Please note that you cannot use the existing water heater  thermostat to switch the DC power directly, it must be done through your solid state relay.  The input to the opto isolator would be about 10 ma of 5-12V; that could be switched through the thermostat.

I use 120VDC for cooking and and other resistive heating appliances every day, and operate all my computer gear on it as well.   I use HV mosfets for switching loads in cooking appliances, using the built in thermostats to only control the mosfet(s).  For simple things without an active switching thermostat such as crock pots, no modification is  needed.  Voltage regulation of the DC isn't really needed since power co. AC was never well regulated either. 

I hope this helps.  I was not impressed with the prices of large switching power supplies, though I did look into that.  For myself, I'd go direct PV to IGBT module approach.  With some 4000 series CMOS logic and a second IGBT module, you could automatically switch the PV array low side back to an inverter when the water tank temperature was maxed out.  I can help with that as well, again, just one 16 pin dip IC will do it.














« Last Edit: April 16, 2018, 05:22:06 PM by BruceM »

BruceM

  • Hero Member
  • *****
  • Posts: 2229
    • View Profile
Re: More panels!
« Reply #62 on: April 16, 2018, 04:52:28 PM »
PS, a DIY solid state relay ala opto gate driver and IGBT module is pretty simple.  A buck converter (PWM) is not something I would recommend you should try to tackle as a first electronics project. Frying the $50 IGBT module is the likely result, and an o-scope would be essential.  The same IGBT module could be used for either.

ajaffa1

  • Hero Member
  • *****
  • Posts: 563
    • View Profile
Re: More panels!
« Reply #63 on: April 16, 2018, 10:30:11 PM »
Hey Glort, Bruce is absolutely right about dc relays being dangerous, don`t do it!
My suggestion is to use a standard grid tie inverter. These only produce power when there is a 240 volt 50Hz supply connected to them, they produce nothing when there is a power cut. If you put your timer and the water heater thermostat between the grid and the invertor it will only produce electricity when there is a demand for hot water. Carefully matching the size of the invertor to the heater element should alleviate your issue with back feeding the grid.

BruceM

  • Hero Member
  • *****
  • Posts: 2229
    • View Profile
Re: More panels!
« Reply #64 on: April 17, 2018, 12:44:06 AM »
Great solution. Bob, ingenious!

What are your Aussie water heaters like, Glort?  Do you just one element or two, with essentially two thermostats, as we do here in the US?  Ours have the upper element on for fast recovery when tank gets too cold, the lower element is switched on when the tank is just a little cold.  Elements are each 4800 watt and mutually exclusive-  never both on.  Both elements are controlled by one "thermostat" module. 





glort

  • Hero Member
  • *****
  • Posts: 2021
    • View Profile
Re: More panels!
« Reply #65 on: April 17, 2018, 01:14:38 AM »

I hope this helps. 

Pfffft!
Yeah, Right!  :laugh:

Learned more in reading that one post than I have in several days and probably 8+ hours of searching around the Net!
Thank you again Bruce. You are a wealth of practical, realistic and very helpful knowledge.  I just can't find the sort of stuff you know on the net.

I have not had my panels up ( in number) long enough to observe the cold effect but it makes sense as I know the panels fall off in heat so logicaly they must do better cold.  What I have seen many times is the cloud edge effect where the voltage goes way hing when there is certain cloud conditions present.  From what I have seen the droplets act like as if they were magnifying glasses and increase some part of the spectrum that makes the panel voltage shoot up even though the total power isn't that high.

It has been that significant here I put one of the voltage monitoring relays on my main array connected to a floodlight. When the voltage goes too high, the light kicks in loading the inverter down and reducing the voltage. 
Wasteful, yes, but it's a whole load better than having the inverter resetting every 2 min due to the voltage being out of spec or turning off one string.

i'm not sure why it happens, my unloaded string Voltage is about half max allowable but I have seen this happen at least 6 times now and the rest of the time such as on clear sunny days the system runs perfect.
Clouds on a cold day may cause real problems!

Thank you for the heads up on the element voltage. I always though you had to stay within that but what I read was making me think different . Thanks for clarifying.
Again, just amazes me how difficult this DC thing can be.  I have also read things about the DC vs. AC argument and for this alone I can see why AC is the way to go.

I am aware of not using the thermostat with the High current DC. What I was going to do was use that as a switch with either 12V or AC to simply trigger a relay or contactor.  Easier said than done of course fining a suitable relay.

glort

  • Hero Member
  • *****
  • Posts: 2021
    • View Profile
Re: More panels!
« Reply #66 on: April 17, 2018, 01:47:49 AM »

Bob,

I had thought of using a GTI  as you said. I also mentioned it a while back for use with the 1 Leg on my 3 phase AC.  Measure the consumption with that and put a GTI on it that was as close to the same rating as possible to offset the consumption.  The trick with that would be switching it in and out with the reboot time on the inverter as the compressor Cycled. But that's another idea.

The GTI would of course work with the heater but I was ( initially anyway) hoping for a more straightforward and independent solution. When Bruce says he can't think of anything, then I know I have made a much tougher ask than I realized.

I was sure I saw something about cross wired relays or the like, I can still see the picture of the diagram in my head but I can't remember the way it was wired so maybe I am thinking of something else I was looking for.


Bruce,

Water heaters here come in single and Double element. I was looking on Dumbtree last night and found a couple of double element units.
Each element was 4800W.

The heater I have is 315L and has a single element of 3600W.  the largest normally made is 400L and they go down to 25L.  generally anything under 175L is single element and from there it's take your pick. If you fed one element from the mains and used that to run another element from a GTI as Bob mentioned, you sure could heat the water fast! Not as fast as my oil burning water heater where I have to try and keep the power down to 50 Kw but for a practical heater in a business or whatever, sure would have a good recovery time.

Heater elements here are largely square rather than the 1'1/4 screw like I believe they are in the states. some brands use the screw thread but the main home grown is a square plate with a rubber gasket that you squeeze in.  Had one leak once when the gasket failed and it rusted the threads out of the holding plate on the heater.  I cured that by drilling the holes out and then putting short bolts and nuts behind. got another 5 or so years out that thing before it literally exploded at the seams and had to be replaced.

I got a very lightly used unit cheap which was only 25L and replaced it. Was good in a way, Daughter was limited on shower time.  Coming here with the big heater she thought it was heaven. Nephew thought the same when he had a shower here once and actually had water pressure unlike at home. Thought it was a terrific luxury!

Elements usually go in 1200w increments. 1200, 2400, 3600, 4800.  As we are all 240V Single phase) that's a 5A spread.  There are 3 phase heaters but they are nearly non existent for home use. More an industrial and still rare-ish thing for change rooms and the like.
We also have gas storage units but they are becoming more rare as well being replaced by instant gas heaters.

For a time there was a ban on Electric storage heaters for new home builds in some parts but this got reversed with the advent of solar. I believe now you can have them IF you also have solar or there is no town gas available like around here in the many new estates they are throwing up with cardboard houses.

The double element thermostats are the key to what I would ultimately like to do in having the heater solar Direct powered then switch the power to a GTI when the tank was hot. The double element thermos fit the single heaters. I'd put one on mine and when the thermo switched to the top element, i'd use that to drive the relay to send power to the GTI. Simple in theory.
Bobs idea would be the same thing only simpler. With a GTI supplying the heater, once the thing clicks out the power goes to the grid anyway.

Still like to switch the DC somehow but maybe it's not possible without more expense and complication than I thought.
Wouldn't be the first time!   :laugh:

The other thing I could look at is getting a low voltage element. That means I'd have to run the panels in parallel which is a pain to wire when you have 16 or so of the things.

BruceM

  • Hero Member
  • *****
  • Posts: 2229
    • View Profile
Re: More panels!
« Reply #67 on: April 17, 2018, 05:30:40 AM »
Bob's GTI approach of only applying the AC to the inverter through the water heater thermostat switch seems to meet your goal of not backfeeding on that meter/circuit, if you limited the PV array to less power than the water heater element could handle.
With a double pole, double throw relay the inverter outout could also be directed elsewhere when the water heater was hot, this by simply having the coil of the DPDT relay fed by the WH thermostat,  if that is to be the highest priority.

I can imagine how after the luxury of bargain priced but working GTI's and a wide range of PV voltage, the details of direct DC seem harder.   

The range of heater element wattages does make it easier.  Your dual element units are exactly the same as the commonl US models.  Having two elements for two different sources could be quite handy.



glort

  • Hero Member
  • *****
  • Posts: 2021
    • View Profile
Re: More panels!
« Reply #68 on: April 17, 2018, 08:37:51 AM »

Didn't do much today with solar projects. It was a pretty overcast day from early on so testing some hairbrained ideas would have been inconclusive running low outputs from the panels.  Spent time unpacking all the new furniture we had delivered and filling the back Verandah with enough cardboard to make another house.

I did get to observe thanks to my insomnia the behaviour of my various arrays.
I was surprised to find the inverter for the west facing panels kick in at 7 am  even though it was cloudy then. By 7:30 the array was making power albeit just 31W from 4KW.
What did surprise me was the tally for the day from this array was a hair over 4 kw. It also tripped out twice through the day thanks I suspect to the cloud edge effect sending the volts High. It certainly wasn't overloading   The 5.5 kw array which is north facing at a shallower angle only managed 6.7 Kw.  The south facing array on the shed which is 13 and 5o managed 4.2 Kw from 3 Kw of panels.

 I picked up a reasonable amount more from my little test and temporary arrays even though they tripped out twice through the day thanks I suspect to the cloud edge effect sending the volts High. It certainly wasn't overloading anything, the 2 arrays are less than half the output the inverters they are connected to are capeable of and even yesterday in the sun they were fine and never went over 2kw Combined.
 I finished the day with a total of 24.5 Kwh which was a lot better than I expected.  Yesterday when it was sunny all day I managed 38 Kwh as a comparison.

Cloudy days are definitely the great equalizer in solar for tilt and orientation. Also highlight the shortfall with trying to increase efficiency of panels with tracking and adjustable tilt etc. Even my south facing array which is entirely the wrong direction here does well in sun or cloud thanks to the shallow angle.

I also noticed this morning that due to the lower sun angle and the vigorous growth of a tree I pruned when I first came here, it's shading my main array on the shed which could be why the gap is so close to other arrays with less ideal setups.  good gob some of that solar I made today went to charging the electric chainsaw so I can fix that problem tomorrow.  : 0) I'll give the annoying tree a real short back and sides  given how well it has bushed up since the last haircut and get the thing to bush out more at ground level where I want it for privacy rather than up high where it's a nuisance.

I reckon the tree would be shading my panels for about 90 min. If that 90 min is worth 3 Kwh from the 5.5Kw there, with what I pay for power, that $315  worth of generation a year at very least that damn tree is costing me!
Not to mention it's an eyesore.


Seems for the moment my DC water heater idea is overly ambitious for my limited skills.  The HWS is running off a phase already fed from the solar so I'll leave it as is atm. That phase is still in credit from the last meter read 8 weeks ago and slightly gaining probably due to the limited use of the AC of late.

I do have a couple of sets of Orphan panels ( 1.5kw ea) I was going to install low on the west house roof to power the HWS. Might sell them and put the funds towards a larger set and make them the 4th array when i'm organized.

Really would like to figure out how to switch this DC though as it is going to bug me now as I'm sure there has to be a simple way!  :laugh:

ajaffa1

  • Hero Member
  • *****
  • Posts: 563
    • View Profile
Re: More panels!
« Reply #69 on: April 17, 2018, 08:48:41 AM »
Hey Bruce, love your idea of DPDT relay to divert power elsewhere when the thermostat on the HW system opens. Perhaps Glort should buy some of those storage heaters and redirect current to them, this would give him unlimited solar hot water and heating at night with no fear of back feeding the grid. Adding an electric AGA to the system would be costly but also give him free cooking facilities (they are basically a glorified storage heater). Hope his wife doesn`t ever see this post or an Aga because she will want one and they are really expensive, there are probably cheaper alternatives available.

Bob

glort

  • Hero Member
  • *****
  • Posts: 2021
    • View Profile
Re: More panels!
« Reply #70 on: April 17, 2018, 11:05:04 AM »

Incredible!!

I was just looking at Aga's and Rayburns on Dumbtree before I came back here!
I was looking at the solid fuel ones though, didn't even realise there were electrics.

My Aunt and Uncle in Casino have a fuel stove, an Everhot I think it is.  I love that thing... so do they actually!
they cook on it and heat the house in winter and it also provides all their hot water thanks to the wetback.  Whenever I go there to visit ( have to go back soon!) in winter, they always tell me to have as long a shower a I like because there is plenty of hot water and plenty more where their oversupply came from.

Mrs and I have discussed putting one in when we re do the kitchen. Not keen on cutting wood for a heater but for a stove would be a different thing.

I have been thinking of a used electric water heater with home heating in mind pretty much as you suggest.  Have the main water heater warm up first then divert to the second heater which I warm up through the day then circulate the water through a radiator into the house at night. I could locate the radiator in front of the return air inlet for the ducted AC and just run it in vent mode.
I think i'd be better off with just running the AC though power wise. You basicaly get 4 times the heat for the power you put in although they do fall off the curve when it gets to about 3o Ambient.
  Going to take a LOT of energy to heat all that water.  One day might go back to the oil burner and look at controls for that to make it more practical for home heating like this.


I was reading up on Snubbers earlier but 99% of what I could find was AC related and the rest was like 24C Dc. Not much help.

What I was wondering was if having a constant small load would help with the arcing?
My idea which I'm sure is way too simple to have legs, was to have something like a pair of 25W globes in series ( for voltage handling) across the switched terminals of the relay so effectively there was power to the heater element all the time.... Just very limited and inconsequential.
If the circuit was not completely dead, would that give the power a path to follow and reduce/ eliminate the arcing when the relay switched? 

This seems to be what RC snubbers are essentially doing but as I said, what I found on high voltage/amperage DC was VERY limited. 

I was also thinking of a similar thing with using a GTI on the one leg of the AC I can't back feed.
If I put a lamp between the AC side of the inverter and the supply, would that allow the  inverter to sync with the grid and stay connected?
I would also connect the AC side of the inverter so when the compressor kicked in, a relay activated and gave the inverter a path of full current to the compressor.  When the compressor kicked out, relay opens, GTI is only feeding 25W back to that phase.

 I can't see a problem with the Load drop as it would be no different to switching off the AC power to a GTI and that doesn't blow them.  Alternatively, a DPDT couple be used to go to a 500W halogen bulb or something. Can't go back to the grid because that would require it to go to another phase and there would be the connection with the 25W globe which would blow the snot out of everything.
Unless..... I could rectify it back to DC and feed that back to the input side of another GTI on another phase..... Getting complicated and I probably don't need to worry about the loss of power so that part is very secondary to the switching in the first place.

ajaffa1

  • Hero Member
  • *****
  • Posts: 563
    • View Profile
Re: More panels!
« Reply #71 on: April 17, 2018, 11:33:08 AM »
Hey glort if you are considering and Aga or similar unit, try to find one that runs on heating oil. They have a gravity fed wick burner that will run real good on WVO, hell of a lot easier and cheaper than an electric version.

Just a thought, if you have spare electricity after heating your hot water, what about a hot tub? I`ve been lucky enough to have sat in one in the Swiss Alps in minus 30 degree temperatures after a long day of skiing. Very good for reviving parts that other beers can`t reach.

Bob

BruceM

  • Hero Member
  • *****
  • Posts: 2229
    • View Profile
Re: More panels!
« Reply #72 on: April 17, 2018, 04:11:44 PM »
Snubbers are simply a resistor and small capacitor with values selected to suppress the oscillations (aka ringing) from switching or the EMI emitted when diodes start and stop conduction.  There is a method to determine the values for optimum snubbing by trying different capacitor values while watching the ringing on the oscilloscope. A 50% reduction then lets the engineer calculate the optimal values.  This is only a modest reduction in the ringing or EMI, and if you use your trusty AM radio near any wire connected to your GTI, you will see that there is a whole lot still left.  The values used for this are small,  and excess capacitance just causes power loss in that the power transistors must overcome the capacitance when switching on. Values of 0.001 to 0.1 uF and R values of 1-100 ohms would be ballpark ranges.

For simple on-off controls that aren't pulsing rapidly as in a PWM controller (say at 50,000 times per second) they are often not used at all. 

When turning off an inductive load, there is also an inductive kick back which can cause some issues- if the low side (negative) is being switched, when the load is off, the voltage goes high.  It can be more than the switching transistor(s) are rated for.  Here a small capacitor can absorb some of that spike and keep the voltage within safe limits.  Selecting the transistors with voltage rating of near double the actual use will often provide margin to be able to ignore "snubbing".  The value for the cap is small- 0.01 to 1.0 uF.  AC or DC are handled differently, and there are several techniques commonly used for DC relay coils, a notorious inductive spike generator when switched off due to the very high inductance.  The techniques may include both a diode and snubber.  If the switching is infrequent, and the voltage rating of the transistors is high enough, no snubbing is needed. 

Slower switching, with limited slew rate also reduces this inductive spike problem and this is the technique I use for switching DC electric cooking elements along with picking transistors with a fair amount of head room- typically 250 volt rating for switching 120-140VDC.  I use no snubbing and limit the power transistor on/off times via high value resistor to gate...this can only be done when using MOSFETs rated for semi-linear operation.

AC relays, switches and breakers on DC typically fail on opening. DC rated relay contacts must be way tougher, they must open faster and further. Bouncing of contacts is normal so each open or close is actually a series of events...and even closing causes arcing.  DC also means that metal will always be transferred in one direction between contacts, which is very hard on the contact life.  Adding capacitance will  increase the current on closing of contacts but will reduce it on opening.  Arcing on opening will be helped by capactance since the difference in voltage seen at the contacts at opening will be smaller, and the current across the open contacts will be reduced somewhat.  So capacitance can improve things.  But I would not normally use AC relays, switches and breakers in and around the home on high voltage DC.  They WILL fail.

Even my 150VDC rated wall light switches fail over time.  Lights with regular daily use of say 10 times a day will start to fail (sound of arcing on switching) in about 5 years.  The old design rotary lamp switches also start to fail on DC in time, the ones most often used, again in about 3-5 years.  I have metal electrical boxes and all wiring in metal conduit, so there is no risk of fire.  The solid state switches (HV mosfet) in my cooking appliances have never failed...solid state switches are the way to go for DC.

DC mechanical switching is very different from AC; AC has spoiled us because the interruption of current flow every 8 or 10 milliseconds (for 60Hz or 50 Hz power) means arcs stop almost immediately, so all mechanical electrical contacts have a much easier and longer life. They are "value engineered" to just work well enough for AC.  The exception is rotary lamp switches which are still the same as the original DC version.  I also did find one double pole switch that has worked well for 120VDC at up to 3 amps ...but it is no longer in production, and I blew over $200 on testing others to find not one that would work.

"What I was wondering was if having a constant small load would help with the arcing?
My idea which I'm sure is way too simple to have legs, was to have something like a pair of 25W globes in series ( for voltage handling) across the switched terminals of the relay so effectively there was power to the heater element all the time.... Just very limited and inconsequential.
If the circuit was not completely dead, would that give the power a path to follow and reduce/ eliminate the arcing when the relay switched?"


The parallel small load would reduce switching current and voltage only very slightly. Not enough to allow use of AC relays and breakers.  Some AC breakers are said to work for 12/24V DC, but since the breakers sold by Midnight Solar are DC rated and approved (150 and 300V versions), it seems foolish to tempt fate.  Even with breakers, it is recommended that  for an off grid system, a DC rated fuse of high rated current be used between battery and breakers since a sudden dead short could cause instantaneous currents so high (before the breakers can open) that the breakers contacts will weld closed.

DC is much better (4x) than AC for shock safety, but greater caution needs to be taken regarding mechanical switching contacts.

"If I put a lamp between the AC side of the inverter and the supply, would that allow the  inverter to sync with the grid and stay connected?"

 Alas, no.  The inverter will immediately see an overvoltage when it tries to feed the circuit due to the resistance of the lamp.  Same as if you used a long run of tiny wire between the GTI and power panel.





merkland

  • Newbie
  • *
  • Posts: 12
    • View Profile
Re: More panels!
« Reply #73 on: April 17, 2018, 05:24:20 PM »
Hey glort if you are considering and Aga or similar unit, try to find one that runs on heating oil. They have a gravity fed wick burner that will run real good on WVO, hell of a lot easier and cheaper than an electric version.

ajaffa1
Are you speaking from experience? I have had an oil(Kerosene) Aga for some 47 years and, knowing how fussy and temperamental they can be over the fuel they will use, I am extremely dubious of them being able to use wvo as a fuel. The wick burner they use is entirely dependent on the fuel readily vapourising.

merkland.

glort

  • Hero Member
  • *****
  • Posts: 2021
    • View Profile
Re: More panels!
« Reply #74 on: April 18, 2018, 12:07:07 AM »

Thanks yet again Bruce.

It's clear that there are no straightforward solution's to this.

Your explanation with the lamp keeping the inverter engaged makes perfect sense.  One would have to have a dummy load and switch to the AC and then release the Dummy load. I feel as soon as a relay broke the connection in switching from one to the other the inverter would trip out.  Still, a 1 second delay would not be a difficult thing to engineer and the dummy load could be cheap and easy to set up.

Have you done any testing / investigation with switching both sides of DC load at the same time?

Speaking of Dummy Loads gives me yet another idea. Maybe I have been looking at this the wrong way. Instead of trying to turn the power off, maybe I should be thinking of just turning the heater off and diverting the power elsewhere so it has somewhere to go and will "absorb" the arc?

Looking at the relay spec sheets, they go got from NC to No in Ms as well. If I used a DPDT and Switched both sides of the power from the heater to a dummy load, that should limit the arc time because the power would then be absorbed by the Dummy load and the power needed to create the arc would be " Grounded" by the other load. 

I would think that like your light switches, the the relay life would still be shortened but If I got a year out of a $3 relay, I'd be more than happy with that and it would still be a financially viable option.

Any thoughts on that solution?