SMA inverters

[Art]

Hi. Thanks for the interesting web site, and info about your trials with the SMA grid-tie inverters. I wonder if you could elaborate on your statement "There are some severe limitations presented by the SMA inverter, which was not designed for wind use," I'd also like to know how you change the inverter settings-are they user-accessible or do you have to have special software and computer connection to reprogram them? What is the purpose of Upv start? The SMA web-page doesn't seem to offer any help on these items! Would you also clarify for me-does the turbine need to be approved in order to be grid connected, or just the inverter? Hope the trials go well!

Art

[Ian]

Hi Art - I will try to answer some of your questions but I do not want to be seen as answering on behalf of Navitron so please bear that in mind...


The "severe limitations"  come from the fact that SMA grid tie inverters were designed for photovoltaic (PV) panels (which is why they were called Sunny Boy inverters). Now PV panels produce power in a fairly predictable way. Without going too deep into the electronics, you could reasonably say that at a certain panel voltage, the panel could deliver a predictable current. The inverter needs to know this as it needs a point of reference by which to "load" the circuit and convert the power to mains AC. It also needs to know when to take the full output. In the case of PV panels, there is assumed to be a start voltage when the panel can deliver reasonable (useful)  currents and when the panel hits a known higher voltage it is capable of delivering full current at maximum power for the panel. This is pretty close to a straight line "curve" so the inverter just needs to know the start and maximum voltages to properly control the panel for maximum output.

Wind generators do not have power curves like PV panels. Take a look at the power curves on the Navitron site - they are no-where near straight. As SMA (and I suspect just about all other inverter suppliers) have not designed an inverter just for wind power, they took a Sunny Boy and added a slightly different mode of operation and they are calling this inverter "Windy Boy". In reality, it is just an upgradre of the firmware - the inverter hardware stays the same. All that they have done is make some allowances for the power curves - but it does make a big difference to the maximum output you can get from your generator. The downside is that, according to SMA, the "Generator Provider" is supposed to programme the settings into the Windy Boy relative to the power characteristics of their generators.

Just to build a little more on here... It is necessary to know what power a wind generator can deliver or the wrong things will happen. If a generator can deliver more than an inverter is taking, then the spin speed will increase as will voltage (this is not too bad a situation as the inverter should then start to take more power but it might take time to do this - say as it syncronises with the mains - whilst the generator is spinning its guts out). This is a "bad thing". However - if the inverter tries to take more power than the generator can provide - then the generatoir will stall and no power will be delivered. This is also a "bad thing".

It IS possible to change these settings but you have to be in the "installation" mode under "engineer" settings. SMA do not like Joe Public fiddling around with the settings themselves.

With regard to your last question - No the turbine does NOT have to be approved (I assume you mean G83 certified). The inverter would have to be G83 certified if you want to "sell" power back to the utility and it would be advisable for it to be tested to this standard anyway - but it is not a requirement if you just want to supplement your own power requirements.

I hope this all made sense!

Regards
Ian

[Ivan]

Yes, I agree with everything that Ian has said.

The limitations I feel the SMA unit has are :

1. requires rectified/smoothed input (so we need to provide this as part of the wind turbine kit)

2. must not be exposed to voltage over 600v (and does not have over-voltage protection built in, despite being designed for input voltages up to 500v)

3. SMA technical support does not seem to know how the Windy Boy works!! Strange, I know.

4. Wind turbine needs to exceed Upv (250v minimum), even though once it has exceeded this voltage it will carry on grid-feeding until voltage drops below the minimum (can be programmed as low as 198v)

5. The Windy Boy 'tests' the grid for several minutes, even when it is manually set to the lowest timeframe. This does NOT suit gusty conditions, where the wind drops to nothing every 10minutes or so, as much of the generating time.

There are various other things. The Windy Boy is not that bad once you get used to it, but I feel that SMA could have a far superior device if they spent a few weeks finishing off the development of this product!

ivan