When an inverter starts accepting power from a Powerspout and feeding to the grid, an electrical circuit is completed. In this circuit, the source of power is the SmartDrive alternator and the inverter is the load (resistance)*.
In common with all circuits, the resistance in the circuit will determine the current. But in this circuit, unlike in other circuits where the source of power is a source which gives constant voltage, eg: a battery, a change in resistance here will not only determine the current but also the voltage.
The reason for this is to be found in the behaviour of permanent magnet alternators (PMA's). With a SmartDrive PMA, the voltage it puts out is affected by two factors: rotational speed and load. Thus:
- the voltage output is directly proportional to the speed of revolution (rpm).
- the voltage output per revolution (v/rpm) is inversely proportional to the load in the circuit.
So here we have a circuit where the load (the inverter) is variable and can set the voltage by changing the resistance it places on the circuit. As an aside, we should note that since one determinant of voltage is rpm, the inverter also has some control over the speed of the turbine.
The question to be answered now is: by what process of logic does the inverter decide what load it places on the circuit ? And the answer is there are two control techniques which are possible, variously named and described as follows:
- MPPT mode (maximum power point tracking) aka: Iterative / adaptive / intelligent load control, - primarily designed for optimising output from PV. To quote : Solar cells have a complex relationship between temperature and total resistance which produces a non-linear output efficiency. It is the purpose of the MPPT system to sample the output of the PV cells and apply the proper resistance (load) to obtain maximum power for any given environmental conditions. Different methods are used to find the optimum combination of voltage and current which will provide maximum power. In the "perturb and observe" method, the controller adjusts the voltage from the array by a small amount and measures the resulting power; if the power increases, further adjustments in that direction are tried until power no longer increases. This method can result in oscillations of power output. From Wikipedia (abridged)
- Turbine mode aka table mode: primarily designed for optimising output from a rotating generator. In this method: The inverter regulates the input current by reference to generator voltage by using a 'look up' table. This table, which can also be represented as a curve, defines the relationship which gives best ac power output for any prevailing DC input voltage. The table, and also the curve, can be programmed by the user to best suit it to the particular turbine and alternator being used. From SMA WindyBoy literature (abridged)
As mentioned in Part 1, both of these control algorithms are to be found in SMA WindyBoy and SunnyBoy 1200 inverters. Although I'm not familiar with other inverters, eg the EnaSolar range, I believe you can choose either mode in these inverters too.
In the 2½ years I've been operating my Powerspout, I've been keeping a record of the operating dc voltage, (sometimes called the MPPv or Vmpp, ie the voltage at the maximum power point). For most of that time, I have had a SunnyBoy operating in MPPT mode as the grid interface, but for the past month I've been using a WindyBoy in turbine mode. The turbine curve programmed into it is the default, factory one without any optimisation by me.
The difference in the way the two modes function is very clearly seen in the plot below of MPPv against ac power out to the grid. I should add that all data points were taken using the same 42 pole stator: 60-7s-2p-star (which has a v/rpm of 0.509 v when tested in open circuit conditions).
It can be seen that:
- MPPv trends down as ac power rises for the SunnyBoy, whilst the opposite is true for the WindyBoy
- the scatter of MPPv for the SunnyBoy is wide, narrow for the WindyBoy
From these observations, it can be deduced that a WindyBoy holds the dc voltage much more constant and at a lower level than a SunnyBoy. The plot also shows clearly why I had the problem I had last year when using a SunnyBoy, - the problem of MPPv rising to such a high level at low ac power output levels that it began to knock against the V Clamp's dumping threshold set at 378 v. This was what prevented continuing generation at low water flows: - so much power got dumped, it wasn't worthwhile continuing. See earlier blog post here.
This year, I used a reduced core stator to get around the problem. But it would appear that if I use a WindyBoy in turbine mode at low flow times of year, the issue will not arise.
I never thought I'd hear myself say this: I can't wait for next summer's low flows to check this out !
*(addendum written 27/1/2016) Like all over simplifications, this statement, that the inverter is the load, compromises truth. All grid connected generators run in parallel with each other so that properly speaking the load is provided by the sum total of consumer load on the grid. It follows that the inverter needs to behave as an 'open window' to the grid, transforming (from dc to ac) as much power to the grid as possible with minimal power being lost within the inverter. Nevertheless, the characteristics of the inverter (its impedance, its capacitance and its resistance) at any point in time have an effect on the SmartDrive output, and so this simplistic statement stands, but purely as a means of gaining understanding of how inverter and PMA interact.