Here is another simple controller circuit to utilise excess PV output.
The associated Arduino program (rename attached pv_program.txt pv_program.pde) can be configured for a single load, two equal load or to step through two binary weighted ones (e.g. 1kw & 2kW). The scheme can easily be extended to more loads.
It is still a work in progress and the program may well be refined as Summer approaches and more sun is available for testing. The purpose of posting at this stage is to act as a catalyst to encourage further developments in this field.
The current being exported is sensed using a single CT on the meter tail, enabling it to be used with installations where the PV system is wired into the consumer unit, rather than being wired separately.
It will be fooled by large phase controlled loads however.
While basing it on an Arduino is totally OTT in computational terms, it has the advantage that the hardware is largely ready built and its accessable to more people than a cheaper PIC based circuit would be. The cost overall is still very reasonable. More contributions for improving the system are also likely to be forth coming.
The circuit only responds to the current being exported. When this current exceeds a preset maximum threshold (somewhat greater than 1 load level) the load will be increased by one step. When the current falls below a minimum the load will be reduced one step. The advantage of doing it this way means that it only has to sense the two thresholds accurately rather than a whole range, the disadvantage is that it can take a little time to step through the load settings.
The choice of current transformer is important. For best sensitivity it needs to generate an output of around 8v pk-pk when suitably loaded at the max threshold power. A SCT-013-000 is suggested, I used SCT-013-030 (was on Ebay) but dismantled it to remove the internal load resistor. The zener diode is added to clamp the high voltages obtained during high levels of consumption.
In order to distinguish between exported and consumed currents a phase reference is required. This is obtained directly from the transformer. The smoothing capacitor on the power supply to the Arduino needs to be increased for half wave operation, adding the extra diode means everything can be built on a simple plug-in shield.
As the signals from both the phase and CT swing beyond the supply voltage, these must come high impedance sources so they can be safely clamped by the ESD protection diodes within the Arduino.
25A SSR's are freely available on Ebay they all seem to be a standard design. At 10A the datasheets indicate that some form of heat sinking will be needed.
The usual safety disclaimer applies.
Mains electricity is potentially dangerous so if you do not understand what you are doing - leave well alone.
Please feel free to contribute any comments/suggestions/improvements.
Component list
Processor Arduino/Arduino Nano
Current Xformer SCT-013-000
SSR(s) 25A ones available on Ebay
Power supply 8v Bell transformer
D1 1A Rectifier 1N4001 etc.
ZD1 11v 1w zener (eg BZV85C11V from Maplin)
C1 470 uF
R1,R2 47k
R3 1k for 1kw & 750w load
470 for 2kW
