Unlocking the Potential: Grid Connection of Wind Power

Table of Contents

1. Introduction
2. Types of Wind Turbines
   1. Type 1: Asynchronous Generator
   2. Type 2: Variable Rotor Resistance
   3. Type 3: Back-to-Back Converter
   4. Type 4: Full Converter
3. Electrical Layout of Wind Power Plants
4. Connecting Wind Turbines to the Grid
   1. Connectors for Small Wind Turbines
   2. Connectors for Groups of Wind Turbines
   3. Connectors for Offshore Wind Farms
5. Alternative Transmission: DC Connection
6. Conclusion

Wind Turbines and the Connection to Power Systems

In this session, we will discuss the integration and control of wind turbines in power systems. We will explore the different types of wind turbines, the electrical layout of wind power plants, and how wind turbines are connected to the grid. Additionally, we will delve into the use of direct current (DC) as an alternative to alternating current (AC) for connections.

Types of Wind Turbines

Type 1: Asynchronous Generator

The first type of wind turbine is based on an asynchronous generator, which directly connects the wind turbine to the grid. This concept is simple and robust, as the rotational speed of the generator is determined by the grid frequency and does not require additional control. A gearbox is used in this type to match the rotor speed of the generator to the desired speed of the wind turbine. A capacitor bank is also included to compensate for the reactive power consumption of the generator, ensuring stable voltage at the wind turbine terminals. Most modern wind turbines of this type also have a built-in transformer to step up the voltage to a medium voltage level for grid connection.

Type 2: Variable Rotor Resistance

Type 2 wind turbines build upon the concept of type 1 but introduce the ability to control the rotor speed. This is achieved by using a variable rotor resistance, which allows for varying the rotational speed of the wind turbine. However, this type of turbine has the disadvantage of heat loss in the variable rotor resistance.

Type 3: Back-to-Back Converter

Type 3 wind turbines utilize energy that would otherwise be dissipated as heat in a resistor. Instead of using a resistor, the rotor is connected through a back-to-back converter. This connection allows for the utilization of the energy and provides better control over the speed of the generator. This converter also enables control over active and reactive power flow, eliminating the need for a capacitor bank.

Type 4: Full Converter

Type 4 wind turbines employ a full converter that transmits all the power generated. Unlike type 3 turbines, where only about 30% of the power passes through the converter, type 4 turbines route all power through the converter. This design allows for a smaller converter and provides better isolation from disturbances in the grid. However, the cost of power electronics is higher in this configuration.

Electrical Layout of Wind Power Plants

Wind power plants consist of multiple wind turbines connected to a power collection system. This system collects the power from all the turbines and connects them to a point of connection with the grid, usually in a transformer station. The point of connection can be at a low voltage level if the wind turbines are small or directly at the 10 kilovolt grid for larger groups of turbines. In some cases, a dedicated radial cable is built to transmit power to the transformer station, reducing interference with consumers connected to the 10 kilovolt grid. Offshore wind farms, on the other hand, are typically connected to the transmission grid at voltage levels above 100 kilovolts.

Connecting Wind Turbines to the Grid

The connection of wind turbines to the grid depends on the voltage levels available in the vicinity and the distance to the transmission grid. Smaller wind turbines are usually connected to low-voltage grids, while larger groups of turbines can be connected directly to the 10 kilovolt grid or through a dedicated radial cable. For large offshore wind farms located far from land, connection to the onshore grid is challenging due to the consumption and generation of reactive power in the cables. In such cases, the power is transmitted through direct current (DC) to compensate for these challenges. The wind plant works in alternating current (AC), which is then converted to DC for transmission and subsequently converted back to AC onshore.

Alternative Transmission: DC Connection

When connecting wind plants offshore at significant distances from land, conventional AC transmission becomes impractical due to reactive power issues. To overcome this, wind plants utilize direct current (DC) transmission. The wind plant operates with AC, which is converted to DC for transmission and converted back to AC onshore. This allows for stable power transmission even over long distances. The DC converter or high-voltage direct current (HVDC) connection system used in these cases often serves multiple wind plants with different owners.

Conclusion

In this session, we have explored the different types of wind turbines and their electrical concepts, discussed the layout of a wind power plant, and examined how wind turbines are connected to different voltage levels in the grid. We have also learned about the use of DC as an alternative for power transmission, specifically for offshore wind farms located far from land. The integration and control of wind turbines in power systems play a crucial role in harnessing wind energy efficiently and effectively.