Stand-alone: incorporating "floating mode" with PWM and MPPT controllers

[julmou]

Good morning,

From PVsyst Help:

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PV array new charging strategies: modern controllers are now able to regulate the charging current using PWM techniques. When attaining the maximum charging voltage, they will limit the current for keeping a constant voltage (floating mode). In a solar system, this is very useful for compensating the load current if any: the PV array will provide the required load current, ensuring a null current in the battery.

From André Mermoud in a previous post:

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You are right: it is not very logical that the battery is used when there is sufficien PV power for feeding the user needs.

However if you think about the real system in operation: when the battery is full, it has to be disconnected.

This means that the PV array is directly connected to the user's circuit, through the Power converter.  If the user's needs are lower than the PV production, how does the MPPT-DC converter behave ?  

In a Grid-connected system, the Power limitation (which is a fixed value) is achieved by a displacement of the operating point on the I/V curve. A MPPT-DC converter should be able to detect the output needs level;  probably an overpower (with respect to user)  will increase the output voltage. The device should be able to detect this voltage increase and displace its operating point on the I/V curve accordingly, Which MPPT converters are equipped with such a control system ?  I don't know !

When I wrote this simulation behaviour (25 years ago! ) I was pretty sure that such a control doesn't exist. This is now perhaps possible  (this should be mentioned on the datasheets), and perhaps the simulation should be able to treat this case.   

 

All solar chargers sold on the market now are either PWM or MPPT.

We know that, with PWM controllers, when the battery reached the defined maximum SOC or voltage, instead of disconnecting the array, the controller will modulate the width of the pulses in order to maintain a constant voltage on the battery that is not too high and stay below the Gassing region. This modulation will also limit the charging current into the battery. In the end, this "floating mode" prevents the PV array to be disconnected, and this one will be able to provide just the current needed for the load => we maximise the use of the PV array (direct use) and limit the use of the stored energy (thus increasing battery lifetime).

Similarly, we know that on normal solar inverters, if we have a grid limitation, the inverter is able to displace the operating point on the I/V curve, so that the power delivered stay below the grid limitation. MPPT solar chargers (DC converter) are now also able to adjust I/V operating point when the battery is full, so that the PV array can keep running and supply just the right amount to meet the load: search Google for "mppt float voltage" or "mppt float charge" to see that it is now a well-known and common feature.

 

So my question is just: is there a planned update/upgrade to incorporate PWM floating mode and/or MPPT floating mode into Stand-Alone systems?

Julien

[André Mermoud]

You are right, this is a really necessary development in the present time.  

This is on our roadmap, but I don't know when we will have the possibility of developing this feature. Probably several months.

This requires some new parameters definitions in the controller, and to analyze all the converters of the database for identifying which ones propose this feature.

Then we will have to implement this in the simulation.