Double-fed wind turbine generator braking method

Abstract

The invention discloses a doubly-fed wind turbine generator braking method, which belongs to the technical field of wind turbine generators, and comprises the following steps of: performing energy treatment in a normal power supply state of a power grid: after blades on a wind turbine generator capture wind energy, starting to operate a doubly-fed generator, and directly transmitting energy generated by a stator of the doubly-fed generator to the power grid at the moment; and transmission of electric energy to a power grid is realized. Through power failure of the power grid, the output power of the converter is basically consistent with the power of the chopper resistor. When a power grid is powered down, a unit can still provide 40% of a rated load, a variable pitch system can quickly receive a safety angle, safe shutdown of the unit is guaranteed, good robustness and dynamic and static performance are achieved, meanwhile, a control algorithm is simple and easy to engineer, high practical value is achieved, and the purposes that a part of energy is actively consumed through hardware of a converter, and the energy consumption is reduced are achieved. Certain buffering is provided for a variable-pitch system of the unit, blades of the unit can be received at a safe angle in time, and the purpose of safe shutdown of the unit is facilitated.

Description

Technical Field

[0001] This invention belongs to the field of wind turbine technology, and in particular relates to a braking method for a doubly fed wind turbine. Background Technology

[0002] With the continuous increase in the capacity of single wind turbine units, and in order to increase the swept area and power generation, the length of a single blade reaches 110m and the weight reaches 40 tons. When the unit fails or the grid loses power, the unit converter suddenly reduces the load to zero. Due to the large inertia of the blades, the pitch system cannot adjust the blade angle to a safe angle in a short period of time. Especially under high wind conditions, this can cause the wind turbine to overspeed severely, which may lead to the wind turbine tower collapsing.

[0003] Currently, the industry's solutions to wind turbine braking problems mainly focus on pitch system research. There are two main approaches: First, to address the issue, under high wind conditions, the blade pitch angle is controlled in advance to improve wind energy utilization efficiency and ensure turbine safety. Second, to quickly bring the blade pitch angle to a safe position, the pitch system capacity is increased (requiring larger battery packs and pitch converters) to provide sufficient pitch torque, significantly increasing turbine costs. However, these methods either increase overall turbine costs or reduce wind resource utilization, greatly diminishing the product's market competitiveness. Therefore, we propose a doubly-fed induction generator (DFIG) wind turbine braking method. This method utilizes the converter's own hardware to actively consume some energy, providing a buffer for the turbine's pitch system, enabling timely retraction of the turbine blades to a safe angle, thus facilitating safe turbine shutdown. Summary of the Invention

[0004] The purpose of this invention is to provide a braking method for a doubly-fed wind turbine, which has the advantages of actively consuming a portion of the energy by utilizing the converter's own hardware to provide a certain buffer for the turbine's pitch system, timely bringing the turbine blades to a safe angle, and facilitating safe shutdown of the turbine, thereby solving the aforementioned technical problems.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A braking method for a doubly-fed wind turbine generator, the steps of which are as follows: Step 1: Energy processing under normal grid power supply conditions: After the blades on the wind turbine capture wind energy, the doubly-fed generator starts to operate. At this time, the energy generated by the stator of the doubly-fed generator is directly transmitted to the grid, realizing the transmission of electrical energy to the grid. At the same time, the energy generated by the rotor of the doubly-fed generator is converted and processed by the converter connected to it, and then its energy is also sent to the grid, ensuring the effective utilization of wind energy to electrical energy and the stable transmission to the grid. Step Two: Energy Handling During Grid Power Outages: After the wind turbine blades capture wind energy, the doubly-fed generator (DFIG) is in operation. The energy generated by the stator is first transferred to the DC bus via the grid-side converter (LCONV), converging energy onto the DC bus. Next, a chopper assembly processes the energy from the stator that has converged onto the DC bus, dissipating this energy and preventing uncontrolled energy accumulation during grid power outages. For the rotor, the generated energy is first transferred to the DC bus via the generator-side converter (GCONV), completing the energy transfer and convergence. Finally, a chopper assembly dissipates the energy from the rotor that has converged onto the DC bus, thus achieving proper energy management during grid power outages.

[0006] Preferably, the chopper component is a bidirectional DC / DC chopper circuit, which controls the energy consumption rate by adjusting the duty cycle of the switching devices, so that the DC bus voltage is maintained within a preset safe range.

[0007] Preferably, the switching device of the bidirectional DC / DC chopper circuit is an IGBT, which is controlled to turn on and off by a pulse width modulation (PWM) signal to achieve rapid energy consumption of the DC bus.

[0008] Preferably, in the step of directly feeding the energy of the doubly fed generator stator to the power grid, the AC output of the stator is kept in the same frequency and phase as the power grid to achieve synchronous grid connection.

[0009] Preferably, the converter is a back-to-back converter structure consisting of a machine-side converter (GCONV) and a grid-side converter (LCONV). The rotor energy is sequentially converted into DC power by the machine-side converter (GCONV) and then into AC power by the grid-side converter (LCONV) before being connected to the power grid.

[0010] Preferably, in the step of sending the energy of the doubly-fed generator stator to the DC bus through the grid-side converter (LCONV) after the grid fails, the grid-side converter (LCONV) switches to rectification mode to convert the AC power output from the stator into DC power and output it stably to the DC bus.

[0011] Preferably, in the step of sending the rotor energy of the doubly-fed generator to the DC bus through the generator-side converter (GCONV) after the grid power failure, the generator-side converter (GCONV) stops its excitation regulation function and only serves as an energy conversion channel to convert the AC power on the rotor side into DC power and transmit it to the DC bus.

[0012] The beneficial effects of this invention are: 1. This invention achieves a power output of approximately the same as the chopper resistor power during grid outages. Even when the grid fails, the generator can still provide 40% of its rated load, facilitating a rapid return to a safe pitch angle by the pitch control system and ensuring safe shutdown. It exhibits good robustness and dynamic and static performance. Furthermore, the control algorithm is simple, easy to engineer, and has significant practical value. It effectively utilizes the converter's own hardware to actively consume a portion of the energy, providing a buffer for the generator's pitch control system and enabling timely return of the generator blades to a safe pitch angle, thus contributing to safe shutdown. Attached Figure Description

[0013] The advantages of the present invention, both above and / or other aspects, will become clearer and more readily understood through the following detailed description taken in conjunction with the accompanying drawings, which are merely illustrative and do not limit the invention, wherein: Figure 1 This is a schematic diagram of the energy flow of a doubly-fed generator unit during a strong wind before a power grid outage, according to one embodiment of the present invention. Figure 2 This is a schematic diagram of the energy flow of a doubly-fed generator unit during strong winds after a power grid outage, according to one embodiment of the present invention. Figure 3 This is a block diagram of a machine-side converter control according to an embodiment of the present invention; Figure 4 This is a schematic diagram of a braking mode detection logic according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the feedback power calculation logic of one embodiment of the present invention; Figure 6 This is a schematic diagram of torque command switching logic according to an embodiment of the present invention; Figure 7 An embodiment of the present invention controls the angle. A schematic diagram of the switching logic; Figure 8 This is a schematic diagram of a grid-side converter control block according to an embodiment of the present invention; Figure 9 This is a schematic diagram of the grid-side current waveform after a power outage, according to one embodiment of the present invention. Figure 10 This is a schematic diagram of the generator-side current waveform after a power grid outage, according to one embodiment of the present invention. Figure 11 This is a schematic diagram of the active power and chopper resistor power after a power grid outage, according to one embodiment of the present invention. Detailed Implementation

[0014] In the following description, embodiments of the doubly-fed wind turbine braking method of the present invention will be described with reference to the accompanying drawings.

[0015] Figure 1-11This invention illustrates a braking method for a doubly-fed wind turbine generator set, comprising the following steps: Step 1: Energy Processing under Normal Grid Supply: After the wind turbine blades capture wind energy, the doubly-fed induction generator (DFIG) starts operating. At this time, the energy generated by the stator of the DFIG is directly transmitted to the grid, realizing the transmission of electrical energy to the grid. Simultaneously, the energy generated by the rotor of the DFIG is converted and processed by the connected converter before being sent to the grid, ensuring the effective utilization of wind energy to electrical energy and the stable transmission of energy to the grid. Its energy flow is as follows: Figure 1 As indicated by the arrow; Step Two: Energy Handling During Grid Outages: After the wind turbine blades capture wind energy, the doubly-fed induction generator (DFIG) is in operation. The energy generated by the stator is first transferred to the DC bus via the grid-side converter (LCONV), converging energy onto the DC bus. Next, a chopper assembly processes the energy from the stator that has converged onto the DC bus, dissipating this energy and preventing disorderly energy accumulation during grid outages. For the rotor, the generated energy is first transferred to the DC bus via the generator-side converter (GCONV), completing the energy transfer and convergence. Finally, a chopper assembly dissipates the energy from the rotor that has converged onto the DC bus, thus achieving rational energy management during grid outages. The energy flow is as follows: Figure 2 As indicated by the arrow.

[0016] Example 2 is basically the same as Example 1, except that: the chopper component is a bidirectional DC / DC chopper circuit, which controls the energy consumption rate by adjusting the duty cycle of the switching device to keep the DC bus voltage within a preset safe range. The switching device of the bidirectional DC / DC chopper circuit is an IGBT, which is controlled to turn on and off by a pulse width modulation (PWM) signal to achieve rapid energy consumption of the DC bus. In the step of directly sending the energy of the doubly fed generator stator to the grid, the AC power output by the stator is consistent with the frequency and phase of the grid to achieve synchronous grid connection.

[0017] Example 3 is essentially the same as Example 1, except that the converter is a back-to-back converter structure consisting of a generator-side converter (GCONV) and a grid-side converter (LCONV). The rotor energy is sequentially converted to DC power by the generator-side converter (GCONV) and then inverted to AC power by the grid-side converter (LCONV) before being fed into the grid. In the step where the energy of the doubly-fed generator stator is sent to the DC bus via the grid-side converter (LCONV) after a grid power outage, the grid-side converter (LCONV) switches to rectification mode, converting the AC output from the stator into DC power and stably outputting it to the DC bus. In the step where the rotor energy of the doubly-fed generator is sent to the DC bus via the generator-side converter (GCONV) after a grid power outage, the generator-side converter (GCONV) stops its excitation regulation function and only acts as an energy conversion channel to convert the AC power on the rotor side into DC power for transmission to the DC bus. Basic Principles of Doubly Fed Generator Control Mathematical models of flux linkage and voltage of a doubly-fed induction generator in a synchronous rotating coordinate system: (1) (2) In the above formula, Generator stator resistance; Rotor resistance; Stator voltage d and q components; Rotor voltage d and q components; Stator current d and q components; Rotor current d and q components; Stator flux linkage d and q components; Stator flux linkage d and q components; Generator stator winding leakage inductance; Generator winding mutual inductance; Generator rotor winding leakage inductance; Grid angular frequency; Slip frequency; Under stable grid conditions, using grid voltage orientation and neglecting stator resistance, we can obtain the following from equations (1) and (2): (3) In the formula, , Let be the phase voltage amplitude. Equation (3) is the control equation for a doubly-fed induction generator operating in constant power mode. Its basic control block diagram is as follows: Figure 3 : Figure 3 The dashed line in the middle represents the phase-locked loop for grid voltage and stator voltage. The three-phase voltage of the power grid; The three-phase voltage of the power grid; Total three-phase current; Rotor three-phase current, The electric angular velocity of the doubly-fed motor rotor; Stator three-phase current; the islanding detection function block is not the focus of this patent and will not be elaborated upon; the braking mode detection function logic diagram is as follows. Figure 4 : The unit enters braking mode if all three conditions are met simultaneously: the islanding flag (Flag_island) equals 1, the generator speed (speed) is greater than the rated speed (speed_rate), and the converter serious fault flag (Flag_island) is not equal to 1. Otherwise, the unit will shut down normally. The feedback power calculation logic control diagram is as follows: Figure 5 : If the braking mode flag is set to 1, the feedback active and reactive power are selected from the active and reactive power calculated by the stator current and stator voltage; otherwise, the active and reactive power are selected from the total current and stator voltage. The formulas for calculating the feedback stator power Ps, Qs, Pt, and Qt are as follows: (4) (5) Torque switching logic as follows Figure 6 As shown: If the braking mode flag is set to 1, the torque command value will be reduced to 40% of the rated value (generally set as a parameter); otherwise, it will be reduced to 0, and the unit will be shut down.

[0018] Control Angle The switching logic is as follows Figure 7 As shown: If the braking mode indicator is set to 1, control the angle. Switching to the angle obtained by stator voltage phase-locked loop Otherwise, switch to the angle obtained by phase-locked loop from the grid voltage. .

[0019] The basic principle of grid-side converter control is as follows: Figure 8 As shown: When the grid voltage is normal, the grid-side control principle is no different from conventional control, and since this is existing technology, it will not be elaborated on here. In braking mode, the grid-side control angle also needs to be switched to the stator voltage phase-locked angle, and the switching logic is as follows. Figure 7 As shown, since grid-side converters generally do not detect stator voltage and speed, the stator voltage phase-locked angle and braking mode flag need to be transmitted to the grid-side converter via fast communication.

[0020] from Figure 9 and Figure 10 It can be seen that even when the grid is de-energized at 6 seconds, the rotor current and grid-side current can still continue to operate, and their magnitudes are still within the range that the converter can withstand.

[0021] from Figure 11 It can be seen that the wind turbine is running at full power 6 seconds before the grid power is cut off after the rated 6 seconds. The output power of the wind turbine and the power of the chopper resistor are basically the same. When the grid power is cut off, the unit can still provide 40% of the rated load, which is conducive to the pitch system to quickly reach the safe angle and ensure the safe shutdown of the unit.

[0022] In summary, this doubly-fed induction generator (DFIG) braking method ensures that the converter output power and chopper resistor power are essentially the same when the grid fails. Even during a grid outage, the unit can still provide 40% of its rated load, which helps the pitch system quickly reach a safe angle, ensuring safe shutdown. It exhibits good robustness and dynamic and static performance. Furthermore, the control algorithm is simple, easy to engineer, and has significant practical value. It effectively utilizes the converter's own hardware to actively consume some energy, providing a buffer for the unit's pitch system and enabling timely repositioning of the turbine blades to a safe angle, thus contributing to safe shutdown.

Claims

1. A braking method for a doubly-fed wind turbine generator according to claim 1, characterized in that, The chopper component is a bidirectional DC / DC chopper circuit. By adjusting the duty cycle of the switching devices, the energy consumption rate is controlled, so that the DC bus voltage is maintained within a preset safe range.

2. The braking method for a doubly-fed wind turbine generator according to claim 5, characterized in that, The switching device of the bidirectional DC / DC chopper circuit is an IGBT, which is controlled to turn on and off by a pulse width modulation (PWM) signal to achieve rapid energy consumption of the DC bus.

3. The braking method for a doubly-fed wind turbine according to claim 3, characterized in that, In the step of directly feeding the energy of the doubly fed generator stator to the power grid, the AC output of the stator is kept in the same frequency and phase as the power grid, so as to achieve synchronous grid connection.

4. The braking method for a doubly-fed wind turbine generator according to claim 4, characterized in that, The converter is a back-to-back converter structure consisting of a machine-side converter (GCONV) and a grid-side converter (LCONV). The rotor energy is sequentially converted into DC power by the machine-side converter (GCONV) and then into AC power by the grid-side converter (LCONV) before being connected to the power grid.

5. A braking method for a doubly-fed wind turbine generator according to claim 5, characterized in that, After a power grid failure, in the step of sending the energy of the doubly-fed generator stator to the DC bus through the grid-side converter (LCONV), the grid-side converter (LCONV) switches to rectification mode to convert the AC power output from the stator into DC power and output it stably to the DC bus.

6. A braking method for a doubly-fed wind turbine generator according to claim 6, characterized in that, After a power grid failure, in the step of sending the rotor energy of the doubly-fed generator to the DC bus through the generator-side converter (GCONV), the generator-side converter (GCONV) stops its excitation regulation function and only serves as an energy conversion channel to convert the AC power on the rotor side into DC power and transmit it to the DC bus.

Images