Thoughts about inverters: Part 1

Those Powerspouts which are connected to a national electricity supply are unusual amongst small water turbines in that they interface with the grid through an inverter. I don't know of any other make of water turbine which connects in this way. Small wind turbines more often do.

In the UK, as in other countries, there are strict regulations about connecting a privately owned generating plant to the national electricity network.  In the UK, these regulations are written down in the document: Engineering Recommendation G83 Issue 2 (August 2012).  It is commonly referred to as just G83/2.

In the original version of this document which was called G83/1 and was issued in September 2003, a useful distinction was made (which has been dropped in G83/2) between micro hydros connecting via an inverter and those connecting directly to the grid.  The former were designated Type A, the latter Type B:

There is a clever thing about Type A connection and it is hinted at in the diagram above. It is that a lot of clever electronics have been squeezed into one box. These electronics, both hardware and software, perform two main functions: 

• converting power from dc to ac 
• managing the grid connection according to the requirements of G83/2.  

The development work for these conditioning and controlling functions has been perfected by companies competing in the huge global market for inverters for the photovoltaic industry.  An inverter is, therefore, a sophisticated bit of kit whose price has been forced down by fierce market competition.  For what it is, it's a bargain.

The market for Type B connections is, by comparison with the solar market, tiny. Within this small market it is difficult to develop a grid connection package cheaply: economies of scale are absent and also there is such a variety of rotating generators available for type B installations (induction motors-as-generators, synchronous alternators, 3 phase, single phase) that standardisation is impossible.  Each has to be specially made for its location.  The price is high.

So all in all, Powerspout's use of a standard PV inverter is an elegant and economic solution to satisfying the complicated regulations of grid connection. It is surprising that other small hydro manufacturers have not followed the same route.  

There is, however, a not-so-clever thing about using an inverter: the electricity generated has to be changed first from ac to dc, and then back again to ac, - and at each conversion power is lost, making Type A installations intrinsically less efficient.  Lower efficiency means lower productivity, - quite significant lower productivity over the entire life span of an installation, and that in turn means a return on investment which is not as good. Perhaps this is the reason why others have not followed the same route.

The G83/1 document of 2003 foresaw that inverters used in Type A hydro systems would "normally be an adaptation of a PV inverter".  Today in 2015, it is evident that as a prediction this phrase wasn't precisely correct: the inverters recommended for use with Powerspouts are not adapted PV inverters but standard ones. They operate in the same maximum power point tracking (MPPT) mode that was designed for solar inverters.  EcoInnovation provide on their website 'compatibility tests' for several different inverters and all of them, they say, should be operated in MPPT mode as if they were handling power coming from an array of solar cells.

This adherence to MPPT mode for a Powerspout is something I have wondered about.  There is no doubt that it works and there is every reason to expect, theoretically, that it should control the speed of the pelton to the point where maximum power will be extracted.  But in the two years of running my turbine in MPPT mode, I have noticed that the way the inverter controls the turbine is not always all that it could be. In particular, the control of dc voltage at different levels of power output has given me problems.

Just recently, I have obtained a Windy Boy inverter.  As the name suggests, this was intended for interfacing a wind turbine to the grid.   Its electronic architecture is absolutely identical to the Sunny Boy but the way the inverter is programmed suits it better to a rotating generator rather than a photo-diode. The mode it operates in is 'turbine mode'.

The two modes, MPPT and turbine, are both programmed into all Sunny and Windy Boy 1200's. If you have the right computer connection cable it is possible to re-configure which mode your inverter will perform in. Not having this specialist cable, nor the expertise for the job of re-configuring my existing SunnyBoy, I was happy to find a second hand Windy Boy on Ebay which was already programmed in "turbine mode".

In the second part of this post, I want to try to explain as simply as possible my understanding of how an inverter controls the voltage output from a Powerspout, and illustrate how the two modes end up causing the package of inverter plus turbine to behave quite differently.