The application of frequency converter in the test bench of doubly-fed generator

1 Introduction
Energy and the environment are the two major problems to be solved by human beings, and an energy structure dominated by clean and renewable energy will become an inevitable development in the future. As a clean and renewable energy, wind energy has been paid more and more attention by countries all over the world. Its reserves are huge. The global wind energy is about 2.74×109mw, of which the available wind energy is 2×107mw, which is 10 times larger than the total amount of water energy that can be developed and utilized on the earth. With the development of the global economy, the wind energy market has also developed rapidly. In the past five years, the world wind energy market has grown at a rate of 40% every year. In 1997, the installed capacity of wind power in the world was only 7,000 MW, and in 2007, it was 90,000 MW. It is expected that in the next 20 to 25 years, the world wind energy market will increase by 25% every year. With the advancement of technology and the development of environmental protection, wind power generation will be able to compete with coal-fired power generation in business.
China’s wind energy reserves are huge and widely distributed, with about 253 million kilowatts of wind energy reserves on land alone. During the “Tenth Five-Year Plan” period, China’s grid-connected wind power has developed rapidly. As of June 2006, the country’s total installed wind power capacity reached 1.26 million kilowatts, ranking 10th in the world and 3rd in Asia, becoming one of the major markets for wind power development after Europe, the United States and India.

2 Power drive system scheme
(1) There are two main schemes for the power drive system of the current popular variable-speed pitch wind turbines:
●One is speed-up gearbox + wound asynchronous motor + double-fed power Electronic converter;
●One is direct drive low-speed permanent magnet generator + full-power inverter without gearbox.
(2) Two compromise schemes with great development potential are also introduced
●One is permanent magnet synchronous motor with low-speed integrated gearbox + full-power inverter;
●One is permanent magnet synchronous motor with high-speed gearbox + full power inverter.
2.1 High-speed asynchronous generator double-fed system (dfig)
The high-speed asynchronous generator double-fed system is mainly composed of a speed-up gearbox + wound asynchronous generator + double-fed inverter. The typical power range of abb generator is 600 ~ 5000kw, as shown in Figure 1.

The characteristic of dfig is that the generator speed can be changed in two directions above and below the synchronous speed. Assuming that the impeller speed variation range of 1.5mw wind turbine is about 10~20r/min, usually 15r/min corresponds to the motor synchronous speed, so the speed variation range is ±1/3 of the rated speed of the motor, and the power of the corresponding inverter is only the motor power 1/3 of . If you want to simplify the mechanism and use direct drive, the rated speed of the motor should also be 15 r/min. Since the stator of the asynchronous motor is connected to the 50hz power grid, the number of pole pairs of the motor is required to be 200, which is difficult to achieve. Therefore, this solution must use a speed-up gearbox. , with high-speed asynchronous motor (usually 6-pole motor). The speed ratio of the speed-up gearbox is large, the load is heavy, the fluctuations with the wind speed are large and frequent, the high cost and easy fatigue damage are the main shortcomings of this scheme. In addition, the brushes and slip rings of the wound asynchronous motor will also affect the system performance. reliability and increased maintenance workload.
2.2 Low-speed permanent magnet synchronous generator direct drive system (pmdd)
The low-speed permanent magnet synchronous generator direct drive system is mainly composed of low-speed permanent magnet synchronous generator + full power inverter, as shown in Figure 2. The typical power range of abb generator is 600 ~ 5000kw.

The characteristic of pmdd is that there is no speed-up gearbox, and the impeller directly drives the low-speed generator rotor, which eliminates the weak link of dfig, greatly improves reliability and reduces maintenance workload. Since the stator windings of the generator are not directly connected to the grid, but are connected through the frequency converter, the rated speed of the motor can be reduced to reduce the number of motor poles to a reasonable value. The disadvantage is that the low-speed motor is bulky, the insulation level of the stator winding is high, and the inverter needs to transmit the full power of the generator, so the price of the motor and the inverter are higher than dfig.
2.3 Permanent magnet wind power generation system with integrated low-speed gearbox
The wind power generation system integrates the low-speed gearbox into the permanent magnet generator, which makes the structure of the system more compact, usually the number of poles is greater than 20, and the rated speed of the motor is generally 120-450 r/min, which has more reliable and longer service life . The typical power range of abb generator is 1 ~ 5mw, and the structure is shown in Figure 3.

2.4 Permanent magnet wind power generation system with high-speed gearbox
The mechanical structure of the system is basically the same as that of the double-fed type, without the drawbacks caused by the winding motor slip ring, and the generator is light in weight and high in power generation efficiency. Usually the number of poles of the motor is 6 or 8, and the speed of the generator Generally 1000 ~ 2000r/min, the typical power range of abb inverter is 1 ~ 5mw, the structure is shown in Figure 4.

3 abb wind power inverter
At present, abb transmission company mainly has two types of products used in wind power generation systems, one is the frequency conversion product acs800-67 applied to the double-fed generator system, and the other is applied to the permanent magnet synchronous motor without gearbox (direct drive system). The frequency conversion product acs800-77, here mainly introduces the frequency conversion product acs800-67.
3.1 Control principle
The acs800-67 wind power inverter is mainly used with induction generators with rotor windings and slip rings, and is connected between the rotor of the doubly-fed generator and the power grid. Circuit diagram and control principle^[1]shown. The crowbar in the picture can be used to prevent the overvoltage of the DC bus when the grid is abnormal (such as grid voltage loss or grid short circuit). There are two crowbars to choose from.
(1) Passive crowbar
The passive crowbar measures the DC bus voltage, if the DC voltage exceeds 1210v, the crowbar is triggered and the drive unit can be removed from the grid immediately.
(2) Active crowbar
For applications that require the drive to remain on the grid during grid voltage transients, an active crowbar must be used to support the grid by generating capacitive reactive power. The crowbar can be turned on or off according to the influence of the grid voltage on the rotor-side converter, ensuring that the drive unit can work normally even when the grid voltage changes rapidly.
3.2 Technical Features
acs800-67 also has the following technical features:
(1) Long life design
The selection of internal components and system configuration of the inverter are designed according to the 20-year service life, especially the DC bus capacitors use film capacitors to replace the original electrolytic capacitors, which have longer life and good low temperature resistance. The cooling fan has the function of speed regulation, which can prolong its service life;
(2) Suitable for use in harsh environments
Both the inverter cabinet and the module have built-in heaters, and are equipped with temperature and humidity sensors to resist low temperature and high humidity environments. All circuit boards are provided with anti-corrosion coating, and the protection level of the cabinet is ip54, which ensures the reliable operation of the inverter in harsh environments;
(3) High-end configuration, compact design
The frequency converter has input lcl filter, output filter du/dt, incoming line contactor and DC fuse as standard configuration, communication adapter and Ethernet adapter as optional configuration. The compact design concept makes it the smallest volume among the inverters of the same power, and is suitable for being placed in the generator compartment;
(4) Low voltage ride through capability
During severe grid failure, such as short circuit or instantaneous power failure, active or passive crowbar hardware can be used to provide support to the grid to ensure that the motor is still on the grid;
(5) Excellent controllability
Since the rectifier unit adopts IGBT controllable rectification, the DC bus voltage is pumped up, so the voltage of the motor rotor can be controlled up to 750v, the speed range of the fan is wider, and the current of the rotor is lower; the power factor of the generator can reach ± ​​0.9, or even Higher, it all depends on the motor design, the inverter is not a bottleneck for this;
When the rotor voltage is close to 0v, the frequency converter is also fully controllable and can be switched in and out at any point within the speed range.
Even when the fan is stationary, it can support the power grid by sending reactive power from the rectifier unit;
(6) Perfect protection function
With multiple protection functions, such as overcurrent, grounding, fan overspeed and stall protection, it provides complete protection for the motor rotor and inverter.
4 Application cases
Sichuan Leshan Wind Turbine Factory uses acs800-67 frequency converter to build an experimental platform for double-fed wind turbines. The wind turbine is simulated by a DC motor, that is, the rotor of the double-fed generator is driven by a DC motor. The schematic diagram of the system connection is shown in Figure 5.
The technical data are as follows:

(1) Generator data:
Stator: rated voltage 690v; rated current 1095a; rated frequency 50hz; generating power 0～1310kw; synchronous speed 1500r/min; power factor 0.87;
Rotor: open circuit voltage 1955v; current 372a; rated speed 1513 r/min; power generation -50 ~ 250kw;
(2) Inverter model: acs800-67-0480/0770-7; rated input current 400a, rated output current 645a, speed regulation range ±30%.
4.1 Synchronous operation
Before the doubly-fed wind power generation system is put into the power grid, it must first perform synchronous operation, so that the stator voltage of the generator is consistent with the grid voltage in amplitude, frequency and phase.Synchronous operation steps and waveforms such as reference[1]shown.
4.2 Power Generation Operation
Figure 6 shows the generator working in the under-synchronized state (rotor speed is 1300r/min), the given torque is 70% of the rated rotation, and the power factor is 1, the generator stator u-phase flux and current and rotor u-phase current waveform diagram.

In Figure 6: Waveform 1 is the rotor u-phase current (rotor iu[%];
Waveform 2 is the stator u-phase magnetic flux (stator u flux[%]);
Waveform 3 is the y-axis component of the stator current in the xy stationary coordinate system (stator iy[%]).
According to the internal electromagnetic theory of the motor, the u-phase magnetic flux of the stator lags the u-phase voltage by 90° electrical angle.
It can be known from the coordinate transformation (3/2) of the three-phase winding of the motor to the two-phase winding (the amplitude of the rotation vector is always kept unchanged during the transformation process), the current component of the stator current on the x-axis leads the y-axis current component by 90° electrical angle , and the x-axis current component is in phase with the stator u-phase current, that is, the stator u-phase current leads the y-axis current component by 90° electrical angle.
It can be seen from FIG. 6 that the curve 2 leads the curve 3 by 180° electrical angle, that is, the stator u-direction magnetic flux is out of phase with the stator y-axis current component. From the relationship between the internal electromagnetic theory of the motor and the 3/2 coordinate transformation, it can be found that the u-phase voltage of the stator leads the stator u-phase current by 180° electrical angle, that is, the stator u-phase voltage and current are completely in reverse phase, the motor works in the power generation state, and the power factor is 1.
Figure 7 shows the generator power waveforms on the stator side and rotor side when the generator works in an under-synchronized state (rotor speed is 1300r/min), the given torque is 70% of the rated rotation, and the power factor is 1.

In Figure 7: Waveform 1 is the rotor-side power generation (kw); Waveform 2 is the stator-side power generation (kw).
As can be seen from Figure 7, since the rotor speed is lower than the synchronous speed of the stator magnetic field rotation (1500r/min), for the rotor side, the electric power absorbed from the grid provides the generator with an excitation magnetic field, and the power consumed by the rotor side is 119.607kw at this time. . For the stator side, it is in the working state of power generation, converting the kinetic energy of the rotor into electrical energy and outputting it to the power grid. The power generated at this time is -914.044kw. Therefore, the total power generation at this time is -914.044+119.607=794.437kw.
Figure 8 shows the generator working in the super-synchronous state (rotor speed is 1800r/min), the given torque is 100% of the rated rotation, and the power factor is 1, the generator stator u-phase flux and current and rotor u-phase current waveform diagram.

In Figure 8: Waveform 1 is the rotor u-phase current (rotor iu[%];
Waveform 2 is the stator u-phase magnetic flux (stator u flux[%]);
Waveform 3 is the y-axis component of the stator current in the xy stationary coordinate system (stator iy[%]).
It can be seen from Figure 8 that curve 2 leads curve 3 by 180o electrical angle, that is, the u-direction magnetic flux of the stator and the y-axis current component of the stator are out of phase. From the relationship between the internal electromagnetic theory of the motor and the 3/2 coordinate transformation, it can be found that the u-phase voltage of the stator leads the stator u-phase current by 180° electrical angle, that is, the stator u-phase voltage and current are completely in reverse phase, the motor works in the power generation state, and the power factor is 1.
Figure 9 shows the generator power waveforms on the stator side and rotor side when the generator works in the supersynchronous state (rotor speed is 1800r/m), the given torque is 100% of the rated rotation, and the power factor is 1.

In Figure 9: Waveform 1 is the rotor-side power generation (kw); Waveform 2 is the stator-side power generation (kw).
As can be seen from Figure 9, since the rotor speed is higher than the synchronous speed of the stator magnetic field rotation (1500r/min), for the rotor side, the power provided by the rotor side is -261.454kw when sending electric power to the grid. For the stator side, in the working state of power generation, the kinetic energy of the rotor is converted into electrical energy and output to the grid. The power generated at this time is -1298.24kw. Therefore, the total power generation at this time is -1298.24-261.454=-1559.694kw, The rated power of the generator is reached.
Figure 10 shows the generator working in the super-synchronous state (rotor speed is 1800r/min), the given torque is 100% of the rated rotation, and the power factor is 0.91 (the reactive power given[即参数组24.01local react p ref]is 52%), the waveforms of the generator stator u-phase flux and current and rotor u-phase current. In Figure 10: Waveform 1 is the rotor u-phase current (rotor iu[%]; waveform 2 is the stator u-phase magnetic flux (stator u flux[%]);

Waveform 3 is the y-axis component of the stator current in the xy stationary coordinate system (stator iy[%]).
It can be seen from Figure 10 that the stator u-phase voltage and current are no longer completely inverse, but lead the current by an electrical angle of 155°, and the power factor is 0.91. Therefore, by adjusting the reactive power, the grid-side power factor can be adjusted.
5 Conclusion
To sum up, wind power, as one of the most promising new energy sources in the world in the 21st century, will surely receive more and more attention. The wind power frequency conversion product acs800-67 developed and produced by abb has been successfully applied all over the world, and has been widely recognized by customers in China, and is running in various wind farms in China, which is very important for my country to fully utilize wind energy and develop clean energy. played a positive role in promoting.
About the Author
Li Shijie Male Doctor, majoring in power electronics and power transmission, engaged in the research, popularization and promotion of frequency converters.
references
[1] Li Shijie. Application of abb inverter in wind power generation industry. Inverter World, 2008 (3)