Connecting Together Solar Panels Output and AC-DC Rectifier Output??

[emaq]

I want combine the power supplied by solar panels and power from a AC to DC rectifier. For that, is it possible to connect the solar panels output (210V, 6A DC) and a full-wave rectifier output (260V RMS) together?

I think the the operating point of 7 solar panels connected in series (7 x 30V = 210V) will move to the high voltage side (set by rectifier output) and current supplied will drop to almost zero. Is there a cheap and dirty solution to combine the power?

 

[KlausST]

Hi,

260V RMS means 365Vp.

it won´t hurt, if you connect the solar panels to this voltage with at least one series diode.

But I doubt it will help.
I assume there is a capacitor at the DC rail. This makes the solar cell to work almost never.

What´s your goal?
Use as much solar power as possible and thus draw only a minimum from mains? Then I assume you need a more intelligent solution.

Klaus

 

[emaq]

Hi,
What´s your goal?
Use as much solar power as possible and thus draw only a minimum from mains? Then I assume you need a more intelligent solution.
Klaus

Yes, that is the goad. Would it be possible with the following circuit?

 

[KlausST]

Hi,

Your circuit doesn't change anything.
As long as the mains voltage is higher than the PV voltage ... the mains power will be used.

Btw: why is AC input 230V RMS..... and 260V RMS after rectifier?

Klaus

 

[emaq]

Its actually a modified boost converter/regulator. The original circuit is without the secondary DC source and winding. When FET is ON the inductor gets charged by the rectifier output and when OFF the inductor acts as a current source to keep the output voltage constant which in turn is controlled by the switching frequency (PWM).

I thought with the above circuit when FET is ON the transformer core gets charged by both the rectifier output and DC source. The other end of both windings is shorted to ground while ON... so it does not matter which DC voltage source is higher and when OFF there are two current sources in parallel... I don't know may be it might not work this way.

I have actually measured 260V RMS after rectifier with a voltmeter... for which I don't know the reason... theoretically it shouldn't be.

 

[c_mitra]

I want combine the power supplied by solar panels and power from a AC to DC rectifier. For that, is it possible to connect the solar panels output (210V, 6A DC) and a full-wave rectifier output (260V RMS) together?

I think the the operating point of 7 solar panels connected in series (7 x 30V = 210V) will move to the high voltage side (set by rectifier output) and current supplied will drop to almost zero. Is there a cheap and dirty solution to combine the power?

Of course you can. But the solar panels will only charge the input caps when the rectified (but unfiltered) voltage goes below 210V. Not the best way to use the solar panel.

If you use a diode in series with the solar panels (wise decision) thing will not change much but it all depends on your load. If the load is less than 1.2kW go ahead- solar panels will take over when the AC-DC rectifier is down. In the meantime it will share some 25% of the load...

 

[emaq]

the solar panels will only charge the input caps when the rectified (but unfiltered) voltage goes below 210V

There is no input cap in the above circuit. The transformer core magnetic field will be developed by the two winding currents when FET is ON irrespective of the voltage difference of the two sources... and the contribution of each source will depend on the number of turns.

I really need some professional input on the modified circuit.

 

[FvM]

I don't see any specification for the coupled inductors or "transformer" you added to your design. There are parameters like inductance, coupling factor, saturation current describing it. The operation principle seems questionable in any case. At first sight, it's obvious that the rectified mains input will drive current into the solar panel, surely an unwanted effect.

How about a simple series diode for the solar panel blocking reverse current and joining both sources at the bridge rectifier output?

It's a bad idea to connect the boost inductor to either of the voltage sources without a filter capacitor blocking pwm-frequent current.

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E.g. like below

 

[emaq]

How about a simple series diode for the solar panel blocking reverse current and joining both sources at the bridge rectifier output?

Yes, that was the first idea I had of joining both sources at the rectifier output... no doubt it is a switching circuit... and yes, there should be a diode.

With this configuration my understanding is...
FET ON: the two sources of different voltage will see a common impedance with respect to ground... the solar panels will drive only minor current because of higher voltage at its output pushing its operating point to a level of minimum current (I/V curve).
https://www.pveducation.org/sites/default/files/PVCDROM/Solar-Cell-Operation/Images/IV-VOC.gif
FET OFF: The inductor becomes a current source charging the capacitor at the output side... only small current flows from both sources.

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Would it be possible with the following circuit?

 

[c_mitra]

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Would it be possible with the following circuit?
View attachment 138984

Basically (looking at the primary side), you are adding two sources- rectified AC (unfiltered) and the solar panel DC (no need to filter). I presume the rectified DC at the input will be 100Hz (say).

Consider the switch closed (transformer is getting current from the primary side): The solar panel will supply power when the unfiltered DC has come down to about 200V or so. Therefore the solar panel will see a AC load (100Hz). It will deliver about 30% of the total power.

Consider the AC power source off: the solar panels will now supply the full power to the load.

Consider the solar panel turned off (down for maintenance): the unfiltered DC will not turn on OR off reliably because of the AC component.

Full wave rectified DC is equivalent to DC +AC.

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I have actually measured 260V RMS after rectifier with a voltmeter... for which I don't know the reason... theoretically it shouldn't be.

You need to put a small capacitor across the multimeter leads. Most modern digital multimeters are high impedance devices but they are rather slow. Perhaps they will take 50-100 ms to take a reading and if the voltage changes during the "sample and hold" time, the result will be rather unpredictable.

 

[KlausST]

Hi,

I have some doubts:

you are adding two sources

Adding sounds like: V1 + V2 = Vtotal....
In reality it somewhere inbetween ADDing and ORing two voltages.

It will deliver about 30% of the total power.

While a rectified a 230V AC is about 56% of the time higher than 210V DC...means in best case 44% of time the power is delivered by the PV. No way to come beyond this 44% with your circuit.
But the PV acts like a current source (not like a voltage source) and the current is limited by the amount of sunlight..therefore I assume the PV voltage will drop. Especially with no capacitor.

Additionally if you don't use a current mode SMPS technology I assume the voltage peaks of the rectified AC will be emphasized in power delivery ratio.

Full wave rectified DC is equivalent to DC +AC.

A rectifed DC will give DC, a rectified AC will give DC and AC.

the solar panel DC (no need to filter)

According EMC regulations I assume you need a filter. And because of the "current source" behaviour of the PV.

You need to put a small capacitor across the multimeter leads.

This is true for a pure DC voltmeter without internal filters (I don't know if they exist).
But RMS voltmeters should take care about varying input voltages at least in the range of mains frequencies.
Usually they should take care of at least the 20th overtone of mains frequency.

Klaus

 

[dick_freebird]

What is your goal in all of this? To simply have two sources
of current? Or do you want more, such as to have either
source properly charge a battery bank, and have a well
controlled voltage whichever the source is?

 

[emaq]

What is your goal in all of this?

My goal is to get as much power from solar panels as possible even when AC power is available and to drive a 1 KW load using boost converter/regulator circuit.
I have the boost converter/regulator working only with AC, but I want to "modify" it to make a "hybrid" circuit without much modification and cost.

 

[FvM]

My goal is to get as much power from solar panels as possible even when AC power is available.

Only possible if you have a separate MPPT controlled DC/DC converter for the solar panel. If a boost converter for the AC input is required depends on the intended output voltage, not yet specified in this thread.

 

[c_mitra]

My goal is to get as much power from solar panels as possible even when AC power is available and to drive a 1 KW load...

What is the peak and typical capacity of the solar panel? Is the load constant?

 

[dick_freebird]

I think it would be best to have the AC - DC feed at a
voltage below the panels' peak power output voltage,
then a simple diode-OR (or more elaborate MOSFET
switched) would do the steering job. The downstream
converter / inverter presumably is good for somewhat
below the panels' minimum output voltage when producing
useful power, so AC-DC a little lower still ought to play OK.

Of course this now requires that the AC-DC conversion
be something more than a full wave bridge and a filter
cap, but that (FWB+C) is a pretty lousy, high ripple
current approach when you want a lot of current out of it.

 

[emaq]

Only possible if you have a separate MPPT controlled DC/DC converter for the solar panel

That's what I want to avoid... no external DC converter for solar panels and no batteries.

What is the peak and typical capacity of the solar panel? Is the load constant?

What do you mean by capacity?... solar panels characteristics are defined by I/V curve...
https://www.pveducation.org/sites/default/files/PVCDROM/Solar-Cell-Operation/Images/IV-VOC.gif
right now I have 30V x 7 = 210V, 6A available around peak power and it depends on the operating point on I/V curve... yes, the load is constant.

 

[c_mitra]

In post #3, it appears that 230VAC becomes 260VDC RMS is confusing. IF this is measured actually with the solar panel ON and supplying full voltage (210VDC) then it may be fine.

But if you disconnect the solar panel you should get about 230VDC(RMS)- if not, then something I do not understand.

When both (AC and solar panel) are on, the solar panels take the load about 30% of the time (see post #11) but the power share will be more.

You can use a mosfet switch and control the load share depending on the PV voltage OR the available light.

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right now I have 30V x 7 = 210V, 6A available around peak power and it depends on the operating point on I/V curve... yes, the load is constant.

I guess you will need an "intelligent" converter that will extract max power from the solar panel and any balance power (if needed) will be taken from the mains. With 20% headroom, the solar panel may not be able to run the load continuously but that again depends on the light available.

The voltage converter may also consume some of the headroom (say 5-10%) that you need to factor in the estimates.

 

[emaq]

I think it would be best to have the AC - DC feed at a
voltage below the panels' peak power output voltage

Yes, it seems thats the only solution available at this moment.

 

[c_mitra]

Yes, it seems thats the only solution available at this moment.

A trivial solution may be to use a half-wave rectifier for the mains but just ensure that the input filter capacitors can hold off the load without the solar panel.