The Difference Between Off-Grid Wind Turbine and On-Grid Wind Turbine

Wind power generation can be divided into off-grid wind power generation and on-grid wind power generation.

Off-grid wind turbine mainly refers to: independent of the power grid operation, single capacity is small, do not need to connect the national electricity can generate electricity. He is mainly completely rely on wind power generation or solar photovoltaic panels to generate electricity, greatly saving the national electricity, suitable for the population is also relatively wide.

Off-grid wind power generation is small in scale, and can solve power supply problems in remote areas through energy storage devices such as batteries or by combining with other energy generation technologies (such as wind/hydropower complementary system, wind power/diesel combined power supply system).

The off-grid wind power generation system has the advantages of low cost, flexible application and easy maintenance, so it is very suitable for pastoral areas, forest areas, communication base stations, weather stations, islands and border guard posts and other areas that can not be effectively covered by the power grid.

Although the current off-grid wind power is developing rapidly and has a broad prospect, the related technical research work is relatively insufficient, and the research aspects of improving the wind energy conversion rate and improving the reliability of the generator system are still not in-depth and perfect. Because the characteristics of the traditional off-grid wind power generation system do not match the characteristics of the wind turbine, the Maximum power point tracking (MPPT) cannot be automatically realized when the wind speed changes, resulting in a low utilization rate of wind energy. The existing MPPT technology from the perspective of electronic control, through the converter and other electronic devices to control the characteristics of the generator system, can effectively realize MPPT. However, the electronic control device contains a large number of electronic devices, which are very easy to be damaged when facing bad weather in the field environment, thus leading to the failure of the whole generator system. Therefore, the reliability of the off-grid wind power generation system based on the electronic control MPPT method is relatively low. Therefore, research on MPPT technology with high reliability is an urgent need for the development of off-grid wind power.

Different Types of off-grid wind turbine systems and Their Application status According to the types of generators included, off-grid wind turbine systems can be divided into three types: asynchronous generator based off-grid wind turbine systems, permanent magnet synchronous generator based off-grid wind turbine systems and switched reluctance generator based off-grid wind turbine systems.

Off-Grid Wind Turbine System Based on Asynchronous Generator 

The off-grid wind turbine system based on asynchronous generator uses the asynchronous generator as the energy conversion device. Asynchronous generators can be divided into three types: cage asynchronous generators, winding asynchronous generators and doubly-fed asynchronous generators. 

Cage asynchronous generators are widely used in traditional off-grid wind turbine systems because of their low cost, high reliability and robustness. However, because of its own characteristics, the cage-type asynchronous generator can only run at single speed or double speed with two generators, but cannot run at variable speed. Therefore, cage-type asynchronous generators cannot achieve MPPT, resulting in a low wind energy conversion rate, so they are gradually replaced by permanent magnet synchronous generator systems. 

Although the generator system has a wider speed range than the cage generator system, it can only operate with limited variable speed, which makes it impossible to achieve MPPT. Because of the complex structure, the winding asynchronous generator system has not been widely used in off-grid wind power generation field. 

In order to reduce the capacity of power converter, doubly-fed asynchronous generator system is widely used in the field of wind power generation. By controlling the frequency of the slip, the doubly-fed asynchronous generator system can realize the doubly-fed speed regulation, so as to operate in a wide range of speed, and can easily implement MPPT control. However, doubly-fed induction generator system is only used in on-grid large-capacity wind power systems, but not in off-grid wind power systems because of its complex structure, high cost and complex control methods.

 

Off-Grid Wind Turbine System Based on Permanent Magnet Synchronous Generator 

Permanent magnet synchronous generator is excited by permanent magnet, so there is no excitation loss and no reversing device, so it has the advantages of high efficiency and long life. Compared with other generators of the same power, permanent magnet synchronous generators are only 1/3 or 1/5 of their size and weight. These advantages make the off-grid wind turbine system based on permanent magnet synchronous generator gradually replace other types of off-grid wind turbine system and become the absolute mainstream in the market. 

There are many types of permanent magnet synchronous generators that can be used in the system. According to the structure, it can be divided into: radial magnetic field inner rotor motor, radial magnetic field outer rotor motor, axial magnetic field double stator groove motor, axial magnetic field double rotor groove motor and stator balance single axial magnetic field motor and other types.

 

Off-Grid Wind Turbine System Based on Switched Reluctance Generator 

In recent years, some scholars have proposed an off-grid wind turbine system based on switched reluctance generator. Switched reluctance generator has the advantages of simple structure, strong overload capacity, good dynamic and static performance and high reliability, which is very suitable for the field environment of wind power generation. However, the control of switched reluctance generator system is more complex, and its power density is less than that of permanent magnet generator, so switched reluctance generator system is not widely used in off-grid wind power generation field. 

 

The on-grid wind turbine is connected to the power grid and has a large single capacity, which has been developed to the level of single megawatt. This kind of on-grid wind generator is mainly used for industrial electricity by the State Electric Power Administration. This kind of on-grid wind power generator is mainly used for national power generation and can be sold for money. It belongs to the industrial electricity that can be used only through the license certification of the state electric power Administration. 

On-grid wind power is a large-scale wind farm with a capacity of several megawatts to several hundred megawatts, consisting of dozens or even hundreds of thousands of turbines. Wind farms connected to the grid can be compensated and supported by the large power grid, and more fully develop the available wind resources, which is the main development direction of wind power generation at home and abroad. In an increasingly open electricity market, the cost of wind power generation will continue to fall, and if indirect benefits such as environmental factors are taken into account, wind power is also economically attractive.

 

The difficulty of wind power grid connection is a problem that troubles the industry. How to achieve wind power grid connection simply and easily?

First, it is necessary to establish the basic point of view of orderly, rhythmic, scoped and scattered development. Second, we should focus on the system and mechanism innovation of power dispatching; Three is to deal with the problem of long interval walk. The above three aspects are indispensable, among which the change of scheduling rules is one of the key keys.