--------

---------------

Grid Connect Systems - Overview

Wind turbines are a variable power source due to the nature of the wind. Therefore a storage/buffer is required. Traditionally batteries are used to store excess power, which can be made available during times of low turbine output. Batteries are cumbersome, expensive, require maintenance, and careful use if they are to provide a long lifespan. The idea of feeding wind turbine power directly into the household mains, with any excess being sold to the National Grid for profit, is very attractive, although such systems are expensive and often tricky to set up.

To grid-feed your electricity - you will first need an 'export meter' (forget ideas of spinning the meter backwards - 95% of meters will have a system built-in to prevent this). An export meter is relatively cheap (£80 supplied and fitted by Western Power), and you will be able to sell your electricity for over 8p/kWh, once a buy-back contract has been signed with a utility company such as Western Power. There is also the opportunity to collect the ROCs - which are worth 4p/kWh, although there is a lot of paperwork, and it is considered not worth the time required for small-scale production.

Selling your Electricity

There are very few regulations regarding connecting a renewable power generation source to the National Grid, according to Ofgen (whose job it is to make up the rules). However, the utility companies will generally require that the G83 advisory standard is observed if you want them to purchase your surplus electricity. Note that the G83 certificate is not necessary for grid-connection equipment, as long as you do not want to sell your surplus.

We have been experimenting with a couple of systems suitable for grid-connection:

SYSTEM 1: 2kW High Voltage Grid-Connect System under Development

We have taken our 220v wind turbine and married it to the SMA Windy Boy 2500 Grid-tie Inverter. There are some severe limitations presented by the SMA inverter, which was not designed for wind use, but modified from a photovoltaic grid-tie inverter, but at present this is the only G83 certified unit available for wind use.

Despite the limitations by the SMA equipment, we managed to get some quite reassuring results on the first day of testing - the turbine produced up to 350W into the grid, and trickled 12-20watts throughout much of the day. Given the very light winds experienced (5m/s measured at the local weather station), this seemed pretty encouraging. We now need to wait until we have some windy weather to see how well the system works at higher wattage.

These are the important WB settings that need to be used:

T-Start               - 5s
T-Stop               - 180s
Upv Start           - 250v
Udc Wind Start  - 200v
Udc Wind Max   - 320v
Betriebsart          - Turbine

UPDATE: Our friend Richard pointed out that (in direct contradiction to SMA technical data), you can use Udc Wind Start at voltages below Upv Start. This means that we can set the wind turbine to continue to operate as voltage drops below Upv Start, extending the time and wind speed range which allows grid-feeding. This made a big improvement in the amount of power we have managed to feed into the grid. Still not heavy winds.....!

This is how we connected the Grid-feed System:

The 3phase output of the turbine is taken to the Navitron Controller/ Dump unit, which rectifies the 3-phase and prevents it exceeding 300v. The output is then fed into the WB2500, which feeds mains electricity direct to the grid.

Note that the Navitron Controller unit is not designed specifically for use with the SMA WB2500, and therefore we cannot guarantee that the reaction speed is fast enough to prevent over-voltage damage to the SMA WB2500, which can occur if the input DC voltage exceeds 600V. If replicating this system it is necessary to include some kind of failsafe over-voltage protection device, just in case the turbine voltage should exceed 600v.

****UPDATE 24th May 2006******
Finally we have had some wind!! The turbine seemed to perform well - with output up to around 1kW into the grid. Winds were not particularly high, but high enough to provide some data. We revised Udc Wind Max to 300v, which may provide us with a little extra power, without having the dump loads kickin. Also, we bypassed the dump load to check the maximum open-circuit voltage before the SMA unit connects. So far the maximum recorded in a heavy gust was 385v - well within the SMA's capabilities, without relying on dump loads. All in all, the system looks very promising.

SYSTEM 2: Low voltage Dutch Grid-Connect System for 200W - 1kW turbines

This system fills the gap which exists for low voltage wind grid-connect applications. The grid-connection inverter has been specifically designed for our turbines. The performance is excellent as it is not a modified product.

The tests were carried out on a 500W 24v navitron turbine. On the first day of testing, the dutch manufacturer recorded a maximum of 425watts from the turbine to the inverter, and 0.9kWh of energy fed into the mains in the first day.

The Dutch Grid-tie inverters can handle 250W each, so two work in parallel in this application (see picture below). The system is relatively new, so at present it is not G83 certified. This is something that we will address later, if sufficient demand exists.