The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Thursday, 10 December 2015

Thoughts about inverters: Part 2

This post is best read after having read Part 1.

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 quoteSolar 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.

2 comments:

Hugh said...

Thanks for the interesting report - Especially interesting to me since I collect operational data for PowerSpouts and the various products for MPPT and grid tie, for publication as user manuals for Ecoinnovation. It's a pity that SMA no longer sell the windy boy or support the use of their inverters for wind or hydro applications.

Experience with grid tied inverters in "table mode" has mostly been disappointing up to now. Issues include the difficulites of programming the table itself. You seem to have been lucky in that respect because the default happens to fit your situation but this is pure fluke. Changing to a different flow will require reprogramming to get the best result and this may need to be done quite frequently on some sites. Wind curves need to rise in power with rising voltage, whereas hydros prefer higher voltage at lower power (when some jets are closed and the net head is higher), so the trend is not the same as for wind turbines.

I like the graphs but they only give some instant readings whereas what we really need is the time series data or the energy production figures. Does the solar tracking reach the sweet spot and stay there? Does it deliver the kWh of energy in the long run? If not then why not? Can the wind table mode get there quicker and stay there?

I understand your problem with the solar inverter pushing voltage upward to find the Voc and thereby possibly tripping the over-voltage protection. But modern grid tie inverters have such a wide operating window of voltage operation that it will be possible to operate without ever approaching the high voltage limit. It does need the right stator, or some packing of the stator in some cases, but it should not be an issue in a system where the site characteristics re correctly reported at the time of ordering the turbine.

It would be great to see how you got on with an Enasolar inverter rather than a sunny boy SMA inverter. You are a great help to others by pioneering solutions, and reporting your experiences.

Keep up the good work
Hugh

Bill said...

Hugh

Many thanks for your comments. I’ve had to spend quite a bit of time (as the date stamp of this comment shows, it is measured in weeks) really getting to grips with understanding how the inverter interacts with the SmartDrive, and not only the SmartDrive, but with the pelton coupled to it. The entire matter is inseparable from the other post I wrote, and on which you also commented: “The relationship between power, speed and and voltage”.

Rather than post a long comment here, I plan to write a new blog post aimed at bringing the many loose ends together under the title “Further thoughts on inverters”.

But to answer your specific questions above, you are so right that to properly compare mppt and turbine modes on the same installation, either time series data or energy production figures are needed. I have neither. The closest I have are some calculations of “whole system efficiency” for each of the two modes and those seem to show that turbine mode is no different from mppt mode.

Whether mppt mode identifies the “sweet spot” and holds to it, I cannot say. I didn’t have the tachometer installed at the time I was running in mppt mode. If I ever revert to mppt mode I will be sure to measure rpm at different power levels to see if the inverter finds the sweet spot, which for my installation should be at a speed of 1200 rpm.

Running in turbine mode interestingly, the system only hits this sweet spot speed at my maximum power output level (750W). At power levels below that level, the operating speed is quite a bit below optimal speed.

In your comment above, you anticipated this and suggested that reprogramming of the power curve for each flow would be the only way to get the best result at each flow level. I have my doubts about this and have tried to say why in my “Further thoughts” post referred to above. I won’t go into it here.

The one question of yours which I can answer categorically is when you ask if the wind table can get there (the sweet spot) quicker (than mppt mode) and stay there. The answer is very definitely Yes. It gets there within less than a minute as compared to mppt mode’s 15 - 20 mins. Once there, it holds it tightly: operating voltage deviates only +/- 3 volts compared with mppt mode’s +/- 20 volts.

One other difference, which is somewhat subjective, is that the turbine / alternator seems quieter in turbine mode. It lacks the high pitched whine from the alternator which is so intrusive in mppt mode. I attribute this whine to fine vibration of the stator poles and assume this is more prevalent when the inverter is constantly seeking the maximum power point by varying the operating voltage all the time. But this is a guess!

Thank you for stimulating me to go deeper in trying to understand all this. I’m grateful.