Designing a Diversion Water Heater Load For AUX 1 using an SSR - AC or DC?

[ClassicCrazy]

Tim,
I think you may find Will's project on diversion project interesting - he has done it a different way using an Arduino and has it all well documented.
http://midniteftp.com/forum/index.php?topic=2511.0
I think another approach to what Will has done would be to use a Raspberry Pi and Node Red which I have just started trying to learn.
Using a micro computer instead of the built in Midnite Aux gives a lot more control capabilities and monitoring too.
Larry

[WillEert]

I had problems using the AUX 2 PWM that I could not resolve. That is not to say they can't be resolved - just that I couldn't.
1) PWM settings: Under varying conditions the PWM did not turn on when it should have. If one adjusted the  offset to always have it turn on then it would not turn off.
2) I was using AC diversion from my inverters. The Classic PWM is very fast and it made my lights flicker like crazy. Also the inverters did not sound happy. I understand now there is a function that permits this PWM frequency to be slowed somewhere in the wind side of the Classic.
3) I really wanted the WBJr which I could not use with the PWM on AUX 2.

So I went to a diversion system using waste not Hi on AUX 1. This system did not work that well for me , particularly on sun / cloud days.

So I went to my own PWM diversion control which I run at 15HZ. Also I dedicate an inverter to diversion and charging. The advantage of this is that you can use a small generator when charging while your house load inverters carry the electrical loads. This system is working very well for me. It is not hard to build and get running the early Arduino UNO version. This system works very well in all conditions.
I wish the Classic was a bit more aggressive with it's algorithm that increases load. I have two Classics each one connected to very similar arrays. I can see that sometimes one array makes quite a bit more power than the other one. Sometimes I turn my stove on to add load for a few moments and to force the Classic to produce more. This works but I cannot figure how to do this automatically. I do consider that the Classic is the best CC on the market and that Midnites support and method of doing business is also the best.
Sometimes I wonder if the new inverter charger will have a diversion system built into it.

Will

[TimBandTech]

I had problems using the AUX 2 PWM that I could not resolve. That is not to say they can't be resolved - just that I couldn't.
1) PWM settings: Under varying conditions the PWM did not turn on when it should have. If one adjusted the  offset to always have it turn on then it would not turn off.
2) I was using AC diversion from my inverters. The Classic PWM is very fast and it made my lights flicker like crazy. Also the inverters did not sound happy. I understand now there is a function that permits this PWM frequency to be slowed somewhere in the wind side of the Classic.
3) I really wanted the WBJr which I could not use with the PWM on AUX 2.

So I went to a diversion system using waste not Hi on AUX 1. This system did not work that well for me , particularly on sun / cloud days.

So I went to my own PWM diversion control which I run at 15HZ. Also I dedicate an inverter to diversion and charging. The advantage of this is that you can use a small generator when charging while your house load inverters carry the electrical loads. This system is working very well for me. It is not hard to build and get running the early Arduino UNO version. This system works very well in all conditions.
I wish the Classic was a bit more aggressive with it's algorithm that increases load. I have two Classics each one connected to very similar arrays. I can see that sometimes one array makes quite a bit more power than the other one. Sometimes I turn my stove on to add load for a few moments and to force the Classic to produce more. This works but I cannot figure how to do this automatically. I do consider that the Classic is the best CC on the market and that Midnites support and method of doing business is also the best.
Sometimes I wonder if the new inverter charger will have a diversion system built into it.

Will

OK Will. I'm going to take your word for it on MS.

The dirty AC power is possibly caused by a randomly switched SSR as opposed to the zero crossing variety. Which did you select?

It surely does get complicated and could depend quite a lot on the inverter too. I'd love to see a clean AC solution. 

[bee88man]

I use a stand-alone solution for AC diversion on a grid-tied system. Can hold exporting to zero joules.
Arudino based with current direction senseing at main breaker and burst rate cycle to match surplus operates zero-crossing triac or SSR.

Not by my design or code but is a well proven method.

Yes, I have LA battery, but battery is only a back-up, and Classic bulk is set at battery float setting and never switches out of bulk all day everyday.
Also have multi-stage diversion in case early stages become filled diverversion seamlessly shifts to the next line and so on...

I can see where off-grid cannot use this approach, though.

[TimBandTech]

I use a stand-alone solution for AC diversion on a grid-tied system. Can hold exporting to zero joules.
Arudino based with current direction senseing at main breaker and burst rate cycle to match surplus operates zero-crossing triac or SSR.

Not by my design or code but is a well proven method.

Yes, I have LA battery, but battery is only a back-up, and Classic bulk is set at battery float setting and never switches out of bulk all day everyday.
Also have multi-stage diversion in case early stages become filled diverversion seamlessly shifts to the next line and so on...

I can see where off-grid cannot use this approach, though.

Where you are grid tied I don't think the noise issue will show up as strongly.
Over the course of a second there are 120 zero crossings; each half cycle of the AC voltage signal.
Conceivably an algorithm that syncs to the 60 Hz could be divying up a heavy load to approx. 0.001 of the actual load power as duty cycle in time intervals of one second. So a 4kW heat element could be incrementing at 40 watt increments; plenty good in terms of gradation and I think in terms of time response on a PV system. Thinking of the battery bank as a capacitor it is filtering quite deeply. Still, the electronics should be filtering properly too. Part of the AC problem is having excess inverter headroom to handle both the diversion load and unforeseen house loads. Generally I think when those house loads are running the battery voltage (the metric that determines traditional diversion) should be low, so the diverter will be off anyhow. Inverters have the ability to generate headroom power for short periods of time beyond their steady state output. Given the possibilities of transient overload during inductive loading it could be meaningful to hold off the diverter until the power is clean; beyond its own brownout effect. This problem is lessened in a grid tied system where the grid power is bound to fill the gap.

PV array power versus inverter power becomes a major design issue to this AC style diversion system. With 12 kW of inverter and 7 kW of PV I think the system could work with 7kW of diversion load. With 6kW of inverter and 7kW of panel the system cannot work with 7kW of diversion load; at least not at 100% duty.  It is a shame to throw away PV power in an offgrid system and the option to harvest it completely is quite difficult. Still, it is a worthy cause. On a series of sunny days the potential power that is thrown away (given a high days of autonomy system) is substantial. This really argues for diversion load that matches PV power with duty cycle style loading. Also house loads that can be scheduled to work during sun hours can preserve the health of a battery bank. Refrigeration, water pumping, cooking, laundry. It's a tall order, but more practical than not.

In the future the best system will likely tap the high voltage available from the PV array directly, but for now the MPPT algorithms don't like this solution. Well, when they aren't sipping someone else might as well be. The ultimate solution lays within the MPPT algorithm. It knows when there is excess power doesn't it? I've never written one, but this seems terribly logical. To hell with the battery drop style of algorithm. The MPPT can theoretically tell you exactly how much power to consume while keeping the charge controller algorithm spot on.

[bee88man]

My grid-tied works well enough to hold the utility metre at a very slight near motionless quiever.

If a off-grid could be made to show current flow attepted as that of grid...the search would be over...

[gcleve]

A big thank you to Matrix for posting the summary info about this topic of Diversion Water Heater Load. This is super helpful:
http://midniteftp.com/forum/index.php?topic=3881.msg37894#msg37894

I have been down the rabbit hole on this topic and have done almost nothing else but read this forum for a full week. 

From what I can tell there doesn't yet seem to be a definitive conclusion to the point about avoiding micro cycling your batteries. Could anyone contribute to help resolve this?

To expand the discussion:
The problem with battery micro cycling is mentioned (with no analysis) here: https://youtu.be/A9Meq8wLbME

TimBandTech has done an amazing job of unpicking some of the related issues here:
http://midniteftp.com/forum/index.php?topic=3881.msg37894#msg37894
But the thread does not conclude with a definitive solution for others to use

Some suggestions are made that perhaps the battery cycling issue can be avoided by configuring the settings appropriately:
http://midniteftp.com/forum/index.php?topic=3457.msg33090#msg33090

So I'm left with a few possible conclusions:
1. Most people are letting their batteries micro cycle and don't know about it, or don't care enough to do anything about it; and
2. A few people with expert knowledge are managing to fine tune the configuration settings on AUX2 on WASTE NOT active high (including the DvrtCnt TWEAKS setting) to successfully avoid micro cycling.
or;
3. Micro cycling cannot be avoided at all.

Could anyone contribute to help form (or point to) a definitive answer on this topic?

Thank you!


Grant