EG4 18KPV frying furnace transformer when PV power is low

Unicorn1618

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Nov 7, 2023
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Nashua NH
10X 370 watt PV panels, 18KPV, 2X Powerpro batteries, totally off grid.
Battery state of charge ~70%.
When the sun is coming up in the morning and the solar panels are putting out less than 100 watts, the inverter shows that it is charging the batteries and sending power to the house. When turning on a 900 watt hair dryer, the dryer goes to full power for a few milliseconds, then the fan speed drops down very low. The hair dryer is new and works fine at all other times.

Somehow, at essentially the same time, the furnace is either on or starting up, and the 24V winding in the 120 to 24VAC transformer in the furnace fries. The transformer has a built in 2A circuit breaker and there is a dedicated inline 2A circuit breaker. Neither trip.

This has happened 3 times.

I'm assuming there is some issue with the 18KPV output voltage drooping when a high load appears when the PV panels are putting out low power.

I have changed the PV startup voltage from the default 140V to 350V and set the inverter to "charge first", but have not tested these changes yet (11/7/23).

Any input/advice/suggestions are welcome. I sincerely hope someone can help with this!
Thanks in advance.
 
You should look through the logs at the time this is happening, to me it sounds like some kind of voltage imbalance but only with low PV.
Can you outline your whole setup.
 
I agree this is what it sounds like. But what is the solution? There doesn't seem to be a way to control how the inverter deals with low PV power.

My "whole setup" is really simple. 10X 370 watt panels in series going into the PV1 input on the 18KPV, two PowerPro batteries in parallel going into the BAT1 input, and the AC output goes into a 70A breaker on the house's breaker panel. This breaker has a mechanical interlock with the 100A main breaker on the panel, so the PV system is effectively a backup power system to my house.
 
10X 370 watt PV panels, 18KPV, 2X Powerpro batteries, totally off grid.
Battery state of charge ~70%.
When the sun is coming up in the morning and the solar panels are putting out less than 100 watts, the inverter shows that it is charging the batteries and sending power to the house. When turning on a 900 watt hair dryer, the dryer goes to full power for a few milliseconds, then the fan speed drops down very low. The hair dryer is new and works fine at all other times.

Somehow, at essentially the same time, the furnace is either on or starting up, and the 24V winding in the 120 to 24VAC transformer in the furnace fries. The transformer has a built in 2A circuit breaker and there is a dedicated inline 2A circuit breaker. Neither trip.

This has happened 3 times.

I'm assuming there is some issue with the 18KPV output voltage drooping when a high load appears when the PV panels are putting out low power.

I have changed the PV startup voltage from the default 140V to 350V and set the inverter to "charge first", but have not tested these changes yet (11/7/23).

Any input/advice/suggestions are welcome. I sincerely hope someone can help with this!
Thanks in advance.
Are you sure it is the 24V side of the transformer getting blown?

That side is isolated from ac power, so it shouldnt be affected by inverter issues.
 
Are you sure it is the 24V side of the transformer getting blown?

That side is isolated from ac power, so it shouldnt be affected by inverter issues.
I'm not sure which side is blown, but there is ac input and nothing output. I will take one apart to see, but either way, the thing stops working without blowing the 2A breaker. My thought was that the voltage droops and the regulator on the PCB demands more current to maintain power and something burns up.
 
I'm not sure which side is blown, but there is ac input and nothing output. I will take one apart to see, but either way, the thing stops working without blowing the 2A breaker. My thought was that the voltage droops and the regulator on the PCB demands more current to maintain power and something burns up.
The lv fuse/breaker will be on the 24v side.

If you have one pulled out, ohm each side.
I bet the high voltage side is open.
 
Somehow, at essentially the same time, the furnace is either on or starting up, and the 24V winding in the 120 to 24VAC transformer in the furnace fries. The transformer has a built in 2A circuit breaker and there is a dedicated inline 2A circuit breaker. Neither trip.

This has happened 3 times.
So you replaced a transformer in your furnace three (3) times and it has a breaker in front of the primary winding and another one at the secondary and they are both 2A? I can honestly say I've seen a few cooked low voltage transformers, but generally it's been somewhat more spectacular than just dying, and if properly engineered should pull out the primary fuse if you drastically overload the secondary, before you cook any of the coils.

The 24v stuff should all be for controls, maybe some DC for the igniter. You really shouldn't need 240 watts, it should draw next to nothing.

"Totally Off Grid" = You have ZERO power to anything and the inverter cranks on as the unit starts to provide power, and you don't have any batteries as a buffer?
 
I'm not sure which side is blown, but there is ac input and nothing output. I will take one apart to see, but either way, the thing stops working without blowing the 2A breaker. My thought was that the voltage droops and the regulator on the PCB demands more current to maintain power and something burns up.
Also, if only problem that happens is the furnace transformer fails, you could install a 50VA or 75VA transformer.
A standard 40VA transformer cannot output 2A long enough to blow a fuse or trip a breaker if it is not a dead short.
50VA can handle 1.8ish 75VA can handle nearly 3
 
So you replaced a transformer in your furnace three (3) times and it has a breaker in front of the primary winding and another one at the secondary and they are both 2A? I can honestly say I've seen a few cooked low voltage transformers, but generally it's been somewhat more spectacular than just dying, and if properly engineered should pull out the primary fuse if you drastically overload the secondary, before you cook any of the coils.

The 24v stuff should all be for controls, maybe some DC for the igniter. You really shouldn't need 240 watts, it should draw next to nothing.

"Totally Off Grid" = You have ZERO power to anything and the inverter cranks on as the unit starts to provide power, and you don't have any batteries as a buffer?
I have 2 14kwh batteries and the solar panels powering the 12kw inverter. I've given up trying to figure out what's going on and am instead putting a dedicated battery to power the hair dryer, then it recharges slowly so there is no spike from the hair dryer.
 
Theses odd ball problems are what worry me when buying stuff. Spend hard earned $ and creates problems. I look forward to seeing your solution. Good luck
 
This may help:

A breaker only trips when the current amps are higher for a set period of time. This is to protect the insulation of the wiring to prevent fire. There are specific curves for tripping such as B,C and D. So it is not surprising from what you are saying that the breaker is intact.

A fuse on the other hand inline of a circuit can blow quickly or more slowly depending on the model you install. This protects the motors, etc.

The batteries may be charged enough, but it is also important that the battery cabling be able to supply the necessary surge amps to the inverter to supply the start up loads.

I did have an Outback Radian inverter which is a low frequency transformer inverter with a 900 amp battery bank and it would start a 5 HP saw motor no problem.

The "900 watt" hairdryer draws up to 7.5 amps at 120 volts for high heat which may or may not include the blower. The induction startup for this should only be like a total of 10 amps maybe.

The thing to do would see if you have a heat gun, another fan, a window AC or space heater and see what happens then. Something is wrong because the load is not large.
 
My problem is still how you blow up a 24v "Transformer". A transformer is two coils of wire, it would have a 120v winding and a 24v winding. A surge will not burn up a transformer unless it's ridiculous. In fact a transformer would be the least likely thing to croak ever over dirty power. Pure DC maybe , but even a square wave wouldn't affect it. IC's, relays, solid state devices, but not a transformer. Transformers that get hit by lightning blow up, spectacularly. It has to really get pretty darn hot to cook a winding, like feeding a 120v winding 240v for an extended period, until it gets hot enough to melt the enamel on the wire. In my sordid youth I wired a 12vct backwards once. It popped the breaker (15a) and got pretty warm. It still worked fine after correcting the wiring mistake. Turning on a blow dryer should drop voltage then recover. Turning it off might raise it then recover, but it ain't gonna do anything that would bother a transformer

Now you could get a small isolation transformer and put it inline in front of the AC feed to the furnace, but I'm thinking the transformer is not a transformer.
 
The "900 watt" hairdryer draws up to 7.5 amps at 120 volts for high heat which may or may not include the blower. The induction startup for this should only be like a total of 10 amps maybe.
The thing about heaters is that they often use nichrome (Nickle / Chromium) wire. It is not a pure resistive load. It's resistance increases with temperature, so there can be a current surge at startup that is independent of any inductive load associated with the fan motor.

Technically, this is not a true inductive load, since it is not storing up magnetic energy which then must be dissipated.
 
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Pure DC maybe
In this case the inverter has a 70% charged battery and the panels are producing a low amount of power. The inverter would be storing the power into the batteries. You flip on the hair drier and suddenly the system is being asked to pull power from the batteries and produce AC. It sounds like it it not transitioning well and the hair drier is experiencing a voltage sage. Now if there is a transient DC voltage being generated that could burn out a transformer. It would be interesting to look at this with a scope, but if this inverter is consistently burning out the 24 volt transformer it probably has some firmware issues and is not producing proper AC power.

A little research seems to indicate that there may be protective fuses buried in the windings of these small thermostat transformers. Knowing that doesn't really help solve the problem.
 
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Does your furnace by any chance have a direct 24V AC terminal connection so that you do not have to connect a 120V to 24 VAC transformer to make the connection?
 
About 5 years ago I did a poor job of tapeing a splice in power line to the propane unit of a split HVAC system. I had done the splice at least 5 years before the problems began. Hot momentarily shorting to nuetral but not long enough to trip breaker - I found out when the 24 volt transformer stopped working, then the replacement stopped working then I went looking. I never tried to figure out what inside the transformer failed.
I would suggest the OP consider the possibility of the neutral getting energized momentarily from something, perhaps the hair dryer.
 
In this case the inverter has a 70% charged battery and the panels are producing a low amount of power. The inverter would be storing the power into the batteries.
The charge controller is putting high voltage onto the high voltage bus. The inverter takes the high voltage to supply loads.

What may be reversing is batteries may be taking some energy from the high voltage bus and storing it. When hair dryer turns on, the inverter draws more power, and the battery has to switch from storing power to supplying power. Seems like a capacitor problem on the low voltage battery bus, or the high voltage bus.
 
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