I use frequency shifting by a MultiPlus 48/3000/35-50 (firmware v430, off grid) to control electrical heaters via an AC Thor controller. The idea is to maximize PV power consumption. Layout:
My MultiPlus setup is the following (full settings attached):
Absorption voltage 55.50 V (absorption voltage of set MPPT = 57.4 V)
Float voltage 55.10 V (MPPT float 55.2 V)
PV inverter support assistant settings:
*) The solar converter will start reducing its output power at 50.10 Hz.
Output power will be reduced to minimum when the frequency is 51.00 Hz.
The converter will disconnect when the frequency is higher than 53.00 Hz.
*) Total installed PV inverter power is 1500 Watts.
Total installed PV panel power is 1500 Watts.
The issue is that the MultiPlus frequently gets stuck at the maximal value of the frequency control range, i.e. 51 Hz. So not at the "disconnect" value of 53 Hz, which seems to be reset according to what is written in the documentation of the PV inverter assistant (at a voltage equal to Float V - 5.2 V).
An example of this issue is shown in the graph below. At time = 0 h (midnight), the system is already stuck at 51 Hz from the previous day (green curve), despite the fact that the battery voltage is 49 V or lower (blue curve), which is well below the charger absorption voltage setting of 55.5 V (dotted blue line). I had switched off the AC Thor loads the previous day, to avoid battery draining.
In the morning I remotely reset the MultiPlus to get the frequency back at 50 Hz.
At around 12:00 pm, the system did what it should: when the battery voltage increased above +/- 55.5 V (blue curve), the frequency shifted. I switched on the loads again, and AC power was sent to the heaters by the AC Thor controller (red curve). I have two sequential heaters of different power controlled by the AC Thor, explaining the variation in AC power consumption.
One can also see that at the end of the day, around t =18 h , the system again was stuck at 51 Hz (and so power consumption maximal at 1500 W), despite the fact that there was not enough PV power available and that the battery voltage reached values as low as 48 V. This is far below the absorption (and float) voltage set for the MultiPlus charger, so one would expect the frequency to be shifted back to 50 Hz. The battery was drained to 80 %, at which time the AC loads were switched off automatically by a Multiplus relay that I programmed for this purpose.
The problem is of course this behaviour drains the battery unnecessarily.
Weirdly enough, the issue does not always happen. Many times it just works as it should (see bottom graph). I did not find any regularity, except that it seems to happen on days with alternating sunshine/clouds, and after the Multiplus returned from the "disconnect" frequency, 53 Hz in my case. Similar behaviour has been reported a few times before on this forum (mostly in the context of frequency shifting for AC coupled PV charging) but a definitive solution was not given.
Can this be solved by changing the settings somehow? I tried many charger voltage/ frequency range settings already, but always the issue pops up. Or is this inherent to the way the frequency shifting is programmed?
Multiplus settings: MP_settings.txt
A good (sunny) day - example of how it should work: