Discontinuous transmission and continuous transmission system

The discontinuous variable speed power generation system is also called a two-speed asynchronous power generation system. Its wind wheel does not change continuously with the change of wind speed, but when the change of wind speed reaches a certain value, the speed changes. It can be realized by using two generators with different speeds or dual-winding dual-speed induction generators or dual-speed pole amplitude modulation induction generators. When the wind speed reaches the cut-in wind speed (usually 3m/s) and continues for 5-10 minutes, the control system sends out a signal and the wind turbine generator starts to start. At this time, the small generator works and is in a low-power generation state; when the detected average wind speed is much higher than the start-up wind speed, it will switch to the large generator to work and enter the state of high-power generation, so as to improve the efficiency of wind energy utilization.

The continuous variable speed power generation system mainly relies on power electronic equipment to make the output electric energy frequency constant, so as to realize the optimization of variable speed operation. At present, the main generators used in this system are permanent magnet synchronous direct drive generators, doubly-fed generators and brushless doubly-fed generators.

The permanent magnet synchronous direct drive generator system uses permanent magnets to replace the rotor excitation magnetic field, and does not require an external excitation power supply. The variable-speed and constant-frequency strategy is implemented on the stator side, and the variable-frequency and variable-voltage power output from the generator is converted into AC power with the same frequency and voltage as the grid by controlling the power electronic equipment. As shown in Figure 1, the generator output is rectified by a rectifier, filtered by a capacitor, and then converted into a constant voltage and constant frequency electric energy fed into the grid by an inverter.

Discontinuous transmission and continuous transmission system
Figure 1 – Block diagram of direct drive wind power generation system

Due to the direct drive structure, the wind wheel is directly coupled with the permanent magnet synchronous generator rotor, eliminating the need for speed increasing gearboxes, improving system reliability and overall machine efficiency, reducing operating noise and reducing maintenance costs. However, due to the low speed and high torque of the direct drive generator, the generator design is difficult, the number of poles is large, the volume is large, the cost is high, and the transportation is difficult.

The double-fed asynchronous generator set (DFIG) has two sets of windings, a stator and a rotor. The stator winding is directly connected to the power grid, and the rotor winding is connected to the power grid through a power electronic converter to provide an excitation current whose frequency, amplitude, phase, and phase sequence can be changed, as shown in Figure 2.

Discontinuous transmission and continuous transmission system
Figure 2 – The composition of a doubly-fed asynchronous generator set

Suppose the rotor speed of the asynchronous generator itself is n, and n1 is the synchronous speed of the asynchronous generator corresponding to the grid frequency of 50 Hz. According to the principle of the motor, when a three-phase symmetrical alternating current is connected to the rotor winding, a rotating magnetic field will be generated in the air gap of the motor. The speed n2 is related to the current frequency ƒ2 and the number of pole pairs p of the motor, n2=60ƒ2/p. Therefore, if the stator winding output electric energy frequency ƒ1 is maintained at a constant 50Hz, only the rotor current frequency ƒ2 can be adjusted to adjust the speed n2, and the phase sequence is to change the direction of the rotor magnetic field rotation, so as to maintain n1=n±n2 as a constant and realize constant frequency control.

The doubly-fed asynchronous generator system can effectively realize variable-speed and constant-frequency operation when the wind speed and generator speed change, and it can also flexibly control the active power and reactive power of the generator. Its high speed, small torque, light weight, small volume and small converter capacity. However, because a multi-stage gearbox is used to drive a brushed double-fed asynchronous generator, there is mechanical wear between the brushes and the slip ring, and the operation and maintenance cost of the gearbox is relatively high.

Brushless doubly-fed generators are emerging AC generators in recent years. There are two sets of windings with different levels on the stator: one set is a control winding, which is connected to the grid through a frequency converter, usually as an excitation winding; the other set is a power winding, which is directly connected to the grid as a generating winding. The stator and rotor structures are specially designed so that the magnetic fields generated by the two sets of stator windings can only be indirectly coupled through the rotor to realize the mutual transfer of energy and jointly maintain the stable operation of the generator. Suppose the number of pole pairs of the power winding is pp, the frequency of the power frequency power supply is ƒp, the number of pole pairs of the control winding is pc, the output frequency of the inverter is ƒc, and the rotor speed is nr. In steady state operation, the relationship is as follows

Discontinuous transmission and continuous transmission system
Frequency calculation formula for power frequency power supply

In the formula, the ± sign means that the phase sequence of the external power supply of the two windings is the same or opposite. It can be seen from the above formula that when the rotation speed nr changes with the wind speed, the output frequency ƒc of the inverter can be controlled to keep the output electric energy frequency ƒp of the power winding unchanged, thereby realizing variable-speed and constant-frequency power generation. Compared with the doubly-fed asynchronous generator, the brushless doubly-fed generator has no brushes and slip rings, and has a simple structure, which reduces the cost and improves the operational reliability, but the manufacturing process is more complicated.