Welcome to this session. My name is Poul Sorensen. I'm professor in wind power integration and control. And in this session, I will talk about good connection of wind turbines and wind power plants to the power system. The learning objectives of this session are the following. First, to be able to distinguish between the four different main types of electrical concepts that we have of modern wind turbines. And secondly, to describe a typical electrical layout of a wind power plant. And finally, to explain how wind turbines and wind power plants are typically connected to different voltage levels in the grid. And also how we can use DC as an alternative to AC connections. The first type of wind turbines Is called Type 1 and it is based on an asynchronous generator which connects the wind turbine to the grid. The advantage of this concept is that it is very simple and very robust. The asynchronous generator rotates with a rotor speed which is determined by the frequency in grid and it does not need to be controlled. This concept also includes a gear box because the rotational speed of the asynchronous generator is much faster than what we want to have in the rotor of the wind turbine. And we also are using a capacitor bank, and the reason for this is that the asynchronous generator is consuming reactive power. And if we are not compensating for this consumption of reactive power, then we may experience problems keeping the voltage at the terminals of the wind turbine. Finally, we have in most modern wind turbines, there is a built in transformer. This is transforming the voltage from the low voltage level to a medium voltage level in the grid where we connect the wind turbines. Type 2, is a simple extension of the Type 1 wind turbine, but this makes it possible to control the rotor speed of the Type 2 wind turbine. The idea is to use a variable rotor resistance, which is seen here, and by changing the resistance in the rotor, we can change the rotational speed of the wind turbine. This offers the opportunity to vary the speed typically with 10%, but there is also some heat loss in the variable rotor resistance which is not advantageous. The next concept Type 3 is utilizing the energy and is also much more controllable than the Type 2. This is connecting instead of variable resistance to the rotor we are connecting the rotor through back-to-back converter, and by this connection we can be able to utilize the energy which we were burning. In this resistor, and we can also control the speed in a much wither range, the rotor speed of the generator. And on top of that, we can also use this converter to control active and reactive power flow out of the wind turbine, and that is why we don't see the capacitor bank here anymore, because we don't need it. Because we can control the reactive power coming out of this state by controlling this back-to-back converter. Finally, we have the Type 4, where we have a full converter here, which is transmitting all the power. Stepping back to the Type 3, It's only about 30%, maxed 30% of the power that is passing through the converters. So the advantage is that the converter is much smaller here. Whereas, the main power is flowing this way through this data. But in the Type 4 case, all the power is going through the converter meaning that the converter must be larger. On the other hand, we have good isolation from the grid which is, can be an advantage in the case that we have disturbances from the grid side. The cost of this is added cost for our power electronics. The next slide shows that a typical layout of a wind power plant. There is a power collection system in the wind power plants which is collecting the power from all the wind turbines. And it is connecting them to the point of connection to the grid which is typically in the transformer station. And typically, at the lower voltage side of the transformer station, the point of connection can also, depending on which country we are in, it can also be in the high voltage side of the grid. Sometimes, it is necessary not only to have wind turbines but also to have some auxiliary equipment like reactors or SVCs which are serving to compensate for a reactive power. The wind turbines are capable of controlling reactive power. But maybe their capacity is not sufficient to what is required from the grid, and that is why in many cases we also use auxiliary equipment. Now we will talk about connection of wind turbines to the different voltage levels in the grid and the idea is that we want to, of course, connect to the wind turbine as close as possible to where we are building the wind turbine. And generally speaking, we can say that the low voltage levels, they are available in the vicinity. Like 0.4 kV whereas there can be a longer distances to the transmission. So we are only connecting at higher voltage levels if we need to, depending on the size of the wind turbine. Small wind turbines, they are typically connected to the low voltage gate, here. If we have larger groups of wind turbines we often connect them this way. We can connect them directly to the 10 kilowatt grid but we can also choose to build a dedicated radial cable where we are transmitting the power into the transformer station. So that we are disturbing less the consumers connected to the 10 kV grid here. And finally, having lots offshore, typically offshore, but it can definitely also be onshore wind farms, we will connect to the transmission grid at a voltage levels above 100 kilowatts. In some cases, when we go offshore, we are moving further and further away from land when we are connecting wind power plants or wind farms. And If we exceed say 100 or 200 kilometers distance then we have an issue with stable transmission of the power from the wind plant to the onshore grid. This cannot be done with conventional alternating current technology without compensations on the way to land. And this is because there is a large consumption of lost generation of reactor power in the cables. So we need to compensate for this generation. The alternative, which is used today already, is to transmit the power with direct current from the wind plant to the onshore. And to do that, we need the wind plant is still working with alternating currents. And then this alternating current is converted to direct current and transmitted to shore, and then it is converted back again to alternating current. Often, when we are connecting wind plants this way, the DC converter or the AC/DC connection system that we have here is serving not only one wind plant, but several wind plants, which often can have different owners also. So to summarize this lecture, what we have learned is we have been familiar with the four types of wind turbines that we have seen from an electrical point of view. We have also looked a typical electrical layout of a wind power plant or a wind farm. And finally, we have looked at how wind turbines and wind power plants are connected to the different voltage levels in the AC systems and also that we can use DC for transmission over long distances offshore.
