Wind Turbine Requirements
The Feed-in-tariff (FIT) was introduced by the government in April 2009 to encourage the installation of renewable energy systems. The FIT provides three mechanisms to benefit from electricity generation:
- A generation tariff: a payment is made for every kilowatt hour (kWh) of electricity generated by the turbine regardless of where it is used. This payment varies by turbine size.
- An export tariff: electricity which is generated but not consumed on site can be exported to the electricity grid. A payment of 3.1p/kWh is made for all exported generation.
- Reduced bills: where electricity produced by the turbine is consumed on site, this will reduce the electricity used from the grid and therefore decrease the annual electricity bills.
When a wind turbine operates whist connected to the electricity distribution system this is classified as 'Distributed Generation' or sometimes called 'Embedded Generation'. The vast majority of 50 – 500kW wind turbines operate in this way and therefore require a suitable grid connection design.
The electricity supply grid was designed before embedded generation became commonplace. The pylons and wires of the ‘Super Grid’ carry electricity from the large electricity generators around the country at 275,000V (275kV) or 400,000V (400kV).
The Distribution Network Operator (DNO) extracts electricity from this extra high voltage network and distributes it to consumers via a series of transformers and its own distribution wiring. Domestic and small industrial users typically take their electricity as a 230V single phase supply or a 400V three-phase supply.
Wind turbines in the 50 – 500kW range will require a 3-phase supply. The connection voltage will almost certainly be 400V at the lower end of this range but depending upon the site situation an 11kV connection may be necessary.
The main requirement is that there are 11kV distribution lines in the general area, as shown in adjacent image. Connection to higher voltage parts of the network is likely to be too expensive for 50 – 500kW wind turbines.
Protection of people & equipment
Embedded generation poses a new risk to the electricity network in that, in the event of mains failure, a section of the network could 'island' with electricity provided only by the wind turbine. This causes two potentially huge problems. First is a simple safety issue for personnel working to repair any fault as they may expect lines to be 'dead' which are actually still energised by the wind turbine.
Second is a more subtle issue of phase synchronisation. If the wind turbine were to operate on its own it would likely produce a voltage cycle that would become out of phase with the grid on reconnection. When the grid came back on line the two systems would 'fight' each other. Naturally the huge generators of the grid would 'win' and power surges would occur within the wind turbine systems causing severe damage and equipment failure.
Grid Connection Standards
Fortunately there are two grid connection standards that ensure the safety of people and equipment. These are termed 'G83' and 'G59'. Both standards require synchronisation with the grid voltage cycle and loss of main detection with automatic disconnection within 0.5 seconds. The G83 standard is the simpler of the two and allows for 'type approval' of equipment usually avoiding an on-site safety check.
This reduces costs but the G83 standard only allows for an output of up to 16Amps per phase. It is, therefore, very common for small wind turbines but insufficient for larger machines.
The grid connection standard for wind turbines in the 50 – 500kW range is termed 'G59'. Equipment to perform the functions required in the G59 standard is available for connection at both 400V and 11kV. However, the connection process is slightly more complicated in that the DNO will need to approve the connection design, G59 settings and earthing arrangements as well as perform an on-site 'witness test' to ensure that the standards have been met.
Redcotec are specialists in completing the required design and electrical drawings required by the DNO based on your specific site circumstances, network information obtained through a 'Network Assessment' and the electrical details of the favoured turbine(s).
- The first step is usually to complete a 'Network Assessment' to determine what size of wind turbine can be economically installed on the site and provide an estimate of likely costs.
- Once the size of the turbine has been agreed with you, we can approach the DNO for a formal grid connection offer. It is only at this point that a full electrical design needs to be completed.
We have designed this staged process in order to get you the grid connection approvals required as quickly as possible but also keep you in full control of costs at all times.