How are wind farms connected to the grid 2

1. Wind turbine generates electricity at a voltage below 1000 volts.

2. Voltage is stepped up at the wind turbine to a collector system, usually 34.5 kV or so.

3. A number of collector systems are gathered and that power is then transformed at a transmission station (115, 230, or 500 kV in the western US).

4. Done

So, to summarize, wires and transformers, just like any other generator. Really nothing too special in getting renewables up on the grid.

tlrols

Thanks for the information.

What are the collector systems you mention made up of? Are these single units, or are these several units involved in thsese collector systems?

At the transmission stations, how is the wind farm frequency synced with the grid? Does it convert it to DC first?

There are literally dozens of permutations of grid connection schemes for wind power now, maybe even hundreds. It will all depend on the type of turbine, the size, the proximity to a grid connection, the amount of wind available, the maintenance issues that users will or will not want to deal with etc. etc., it goes on and on. As a gross generality, there are 3 different types (classes?) of wind generation schemes: AC Induction, AC Synchronous and DC generators.

Small grid connected systems tend to be most cost effective with DC because they can generate at a very wide range of wind speeds, but they need an inverter to create AC. That inverter synchs up with the grid. DC becomes impractical above a certain size (5kW, 10kW maybe?) because they either have too much maintenance (brushes and commutators) or they become too expensive (permanent magnet).

AC Induction generators are basically just AC induction motors which are run up over speed by the turbine so that they become generators. Induction generators range from 5kW to megawatt sizes. The connection to the grid itself is what regulates the output frequency, but then mechanical speed control of the turbine is necessary so as to not create too much negative torque. So you forgo the brush / commutator electrical maintenance and add in mechanical maintenance for the speed control systems. Depending on size, these will feed a local transformer to step up to the grid voltage where they are connecting, or really large ones may actually generate at the grid voltage.

AC Synchronous generators are used when there is a possibility of having to run "off grid", because they can be externally excited (whereas the Induction machines need the grid for winding excitation). They can also be run at much slower speeds, reducing the amount of speed control necessary. Other than that, they are connected to the grid in the same way as Induction generators. They are not as popular because they are more expensive, but if you want "island mode" generation (off-grid) then they are really the only choice other than DC.


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To expand further on jraef's comments, there are several variations of induction generators. jraef described the basic induction generator system, which was standard a number of years ago, and is still common on smaller machines. Most larger (>1MW or so) turbines these days use a variation called a doubly-fed induction generator. These have wound rotors and produce power from both the stator and rotor. By connecting the rotor (via slip rings and brushes) to an AC/DC/AC converter, a range of variable speed operation is possible which improves efficiency of energy extraction from the wind. Some use electromagnetic coupling to the rotor to avoid brushes.

Permanent magnet synchronous generators are starting to become more common. Most of these do not operate at synchronous speed, but rather supply their output via a AC/DC/AC converter. See thread626-272241

Alan
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