Grid-connected transient stability improvement control method for grid-connected system of double-fed wind turbine generator

Abstract

This invention discloses a control method for improving the transient stability of doubly-fed induction generator (DFIG) wind turbine systems connected to the grid, belonging to the fields of wind power grid connection technology and power system stability control technology. First, a reduced-order model of the system is established to accurately reflect the dynamic behavior during low-voltage ride-through. Based on this model, the attraction domain boundary and maximum power angle constraint are obtained, and then adaptive current reference control is proposed: the rotor d-axis current limit is calculated based on the maximum power angle constraint and the grid voltage, and compared with the original reference generated by maximum power point tracking. The smaller value is taken as the command to achieve adaptive limiting of the rotor current. Simultaneously, transient pitch collaborative control is proposed: the rotor speed is monitored in real time, and when the speed exceeds the safe upper limit, a transient load reduction power command is calculated and the pitch angle is rapidly increased to reduce mechanical torque. This invention can significantly enhance the fault ride-through capability and grid-connected operation reliability of DFIG wind turbines during transient processes such as voltage dips.

Description

Technical Field

[0001] This invention relates to the fields of wind power grid connection technology and power system stability control technology, and in particular to a method for improving grid-connected transient stability control of doubly fed wind turbine grid connection systems. Background Technology

[0002] As the global energy structure shifts towards cleaner and lower-carbon energy, the penetration rate of new energy sources, represented by wind power, in the power system continues to increase. Doubly-fed induction generators (DFIGs) have become the mainstream model due to their mature technology and controllable costs. However, the large-scale grid connection of wind turbines alters the dynamic characteristics of the power system, making transient stability issues during grid faults, especially low-voltage ride-through (LVRT), increasingly prominent. This directly affects the safe and reliable operation of the grid and the efficient absorption of wind power.

[0003] Against this backdrop, improving the stability of doubly-fed induction generator (DFIG) wind turbines during transient processes such as grid voltage dips, ensuring they do not disconnect from the grid or worsen grid voltage, has become a key technical requirement for guaranteeing the safe and stable operation of high-proportion renewable energy power systems. Currently, numerous studies both domestically and internationally have proposed solutions to the transient instability problem of DFIG wind turbines during low-voltage ride-through. For example, one solution proposes a transient voltage control strategy based on lookup tables, using parameters such as grid connection point short-circuit capacity and voltage sag index to improve the LVRT capability of the DFIG. However, this method relies on pre-set lookup table data, and the control parameters are difficult to dynamically and adaptively adjust according to the real-time operating status of the system, resulting in insufficient flexibility in dealing with complex and ever-changing fault scenarios. Another solution proposes using a high / low voltage fault ride-through collaborative control algorithm and parameter optimization method after hardware protection is disconnected, which can suppress overvoltage and overcurrent during symmetrical voltage changes, but it does not fully consider the power balance problem during electromechanical transient processes. Another approach proposes a transient stability control scheme based on real-time power angle trajectory fitting, which uses extended phase trajectory for instability judgment and combines extended equal area criterion to determine trip control. However, this method focuses on system-level emergency control and does not adequately consider the continuous control and protection coordination of the wind turbine itself.

[0004] Therefore, how to provide a transient stability enhancement control method that can adapt to changes in the power grid state and coordinate the balance between electromagnetic power and mechanical power is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a control method for improving grid-connected transient stability of doubly-fed induction generator (DFIG) wind turbine systems, overcoming the technical shortcomings of existing technologies in dealing with low-voltage ride-through of DFIG wind turbines, thereby enhancing the dynamic response capability and operational reliability of the unit to transient events such as grid voltage dips.

[0006] To achieve the above objectives, this invention provides a method for improving grid-connected transient stability control of doubly-fed induction generator (DFIG) wind turbine grid-connected systems, comprising the following steps: Step S1: Establish a reduced-order system model for transient stability analysis. The reduced-order model is based on the time scale separation principle. It ignores the stator resistance and fast electromagnetic transients, and uses the grid connection point voltage to characterize the stator dynamics. At the same time, it ignores the current loop dynamics, and retains the dynamics of pitch control, drive train, DC link, voltage control and phase-locked loop. Step S2: Perform instability mechanism analysis based on the reduced-order model of the system, obtain the boundary of the system's attraction domain after the fault, and determine the maximum allowable work angle constraint; Step S3: Implement adaptive current reference control, including: calculating the stable upper limit of the grid-connected active current based on the maximum allowable power angle constraint and the real-time grid voltage, and deriving the corresponding rotor current. Axis current limit This limit is compared with the original rotor current reference value generated based on the maximum power point tracking strategy. The two values ​​are compared, and the smaller value is selected as the final rotor current control command. ; Step S4: Implement transient pitch coordinated control, including: real-time monitoring of the rotor speed of the doubly-fed wind turbine. When the rotor speed Exceeding the preset safe speed limit At that time, the transient load reduction power command is calculated based on the speed difference, and the transient load reduction power command is converted into a corresponding transient pitch angle adjustment signal to increase the pitch angle. .

[0007] Preferably, the system order reduction model established in step S1 is specifically represented as follows: ; In the formula, The phase difference between the voltage at the grid connection point and the grid. ω is the output angular frequency of the phase-locked loop, and ω is the rated angular frequency of the power grid. , These are the proportional and integral coefficients of the phase-locked loop PI controller, respectively. , Input current to the AC side of the grid-side converter Axial components, , For the stator and rotor winding resistance, For grid-side line resistance, , For the equivalent self-inductance of the stator and rotor phases, For grid-side line inductance, , For grid voltage Axial components, , AC input voltage of grid-side converter Axial components, This is the bus capacitor voltage. For bus capacitors, , Rotor current Axial components, , Rotor voltage Axial components, For slip frequency, For the mutual inductance between the stator and rotor equivalent windings, Stator current Axial components, For the moment of inertia of the motor, For mechanical torque, For electromagnetic torque, For the integral channel output of the pitch angle controller, The pitch angle is the time constant of the actuator. This is a reference value for the pitch angle.

[0008] Preferably, the adaptive current reference control in step S3 further includes: Step S31: Based on the reduced-order model of the system, determine the maximum allowable power angle of the system after the fault through attraction domain analysis. ; Step S32: Based on the maximum power angle With grid voltage Calculate the stable upper limit of the grid-connected active current and derive the rotor d-axis current limit. ; Step S33: Rotor Axis current limit The original rotor current reference value generated by the maximum power point tracking strategy The comparison is performed, and the minimum value selection strategy is used to generate the final rotor current control command. .

[0009] Preferably, the maximum power angle after the fault in step S31 Determined in the following manner: ; In the formula, This is the maximum permissible power angle after a system fault, calculated using the system's attraction domain.

[0010] Preferably, in step S32, the rotor Axis current limit Calculated using the following formula: ; In the formula, For the stator equivalent self-inductance, For the mutual inductance between the stator and rotor equivalent windings, For grid-side line inductance, This is the grid voltage. This represents the maximum permissible power angle of the system after a fault.

[0011] Preferably, the final rotor current control command in step S33 The generation method is represented as follows: ; In the formula, The original rotor current reference value is generated based on the maximum power point tracking strategy. The rotor calculated based on stability constraints Shaft current limit.

[0012] Preferably, the transient load reduction power command in step S4 for transient pitch coordinated control Calculated in the following way: ; In the formula, To control the gain, To preset the upper limit of safe speed, The generator's angular velocity; Furthermore, the transient pitch angle adjustment signal is based on the transient load reduction power command. Generates the pitch angle for controlling the propeller angle. Increase.

[0013] Preferably, the startup logic for transient pitch coordinated control is as follows: when At that time, transient load reduction power command Transient pitch control does not start; when hour, Furthermore, the absolute value increases with the increase of the speed deviation, triggering transient pitch control, which reduces the wind energy utilization coefficient by increasing the pitch angle. This reduces the input mechanical torque.

[0014] Preferably, the specific method for establishing the system reduced-order model based on the time scale separation principle in step S1 includes: ignoring the stator resistance and the stator-side fast electromagnetic transients, and directly characterizing the stator dynamics with the grid connection point voltage; ignoring the rotor-side converter current loop dynamics and grid-side converter current loop dynamics that can quickly track commands; and retaining the pitch angle control, transmission chain, DC bus voltage control, and phase-locked loop dynamics as the dominant slow dynamics.

[0015] Preferably, the method provided by the present invention is implemented using this system, including: The order reduction model building module is used to build a system order reduction model for transient stability analysis. The instability mechanism analysis module is used to perform instability mechanism analysis based on the reduced-order model of the system, obtain the boundary of the system's attraction domain after the fault, and determine the maximum allowable power angle constraint. The adaptive current reference control module is used to calculate the rotor voltage based on the maximum allowable power angle constraint and the real-time grid voltage. The shaft current limit is set and compared with the original rotor current reference value generated by the maximum power point tracking strategy, and the smaller value is selected as the final rotor current control command. The transient pitch control module is used to monitor the rotor speed in real time. When the rotor speed exceeds the preset safe speed limit, it calculates the transient load reduction power command based on the speed difference and converts it into a pitch angle adjustment signal to increase the pitch angle.

[0016] Therefore, the grid-connected transient stability improvement control method for doubly-fed wind turbine grid-connected systems using the above-described structure has the following beneficial effects: (1) The reduced-order model established by the present invention can accurately reflect the main dynamic behavior of the original system in transient processes such as low voltage ride-through. Moreover, the model has a low order, which facilitates attraction domain analysis and controller design, and provides a reliable mathematical tool for stability quantification.

[0017] (2) The adaptive current reference control method proposed in this invention does not directly force the injection of demagnetizing current, but dynamically limits the rotor current command based on the attraction domain boundary, which can minimize the disturbance to the system operating state, realize the smooth switching of transient reference current, and effectively improve transient voltage stability.

[0018] (3) The transient pitch control strategy proposed in this invention is triggered only when the risk of transient overspeed occurs. It achieves electromechanical power rebalancing by rapidly reducing mechanical torque, suppressing rotor overspeed and reducing fault recovery impact, without affecting the MPPT operating efficiency under normal operating conditions.

[0019] (4) The present invention achieves a synergistic effect through adaptive current control and transient pitch control, which simultaneously suppresses transient instability from both electrical and mechanical perspectives, significantly enhancing the fault ride-through capability and grid-connected operation reliability of the doubly fed wind turbine under severe faults such as deep voltage drops.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the grid-connected system structure of the doubly fed wind turbine generator involved in this invention; Figure 2 This is a schematic diagram of the domain of attraction (DOA) after a system failure in this invention; Figure 3 This is a schematic diagram of the transient pitch control structure in this invention; Figure 4 This is a comparison chart of the output responses of the full-order model and the reduced-order model when wind speed changes in an embodiment of the present invention; Figure 5 This is a simulation result diagram of a system using conventional control in an embodiment of the present invention; Figure 6 The diagram shows the system simulation results of the control method of the present invention in an embodiment of the present invention. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0024] Example This embodiment takes a doubly fed wind turbine grid-connected system with a rated power of 2000kW, a grid rated voltage of 690V, and a rotor radius of 55.5m as an example, and describes in detail the transient stability improvement control method proposed in this invention with reference to the accompanying drawings.

[0025] 1. Reduced-order electromechanical transient model of a doubly-fed induction generator (DFIG) wind turbine grid-connected system Figure 1 This is a structural diagram of a doubly-fed induction generator (DFIG) wind turbine grid-connected system. Figure 1 middle, Stator voltage Axial components; , Rotor current Axial components; , AC input current for grid-side converter (GSC) Axial components; , The equivalent self-inductance of the stator and rotor phases; The mutual inductance between the stator and rotor equivalent windings; This is the output angular frequency of the phase-locked loop (PLL). For PLL output phase, , These are the proportional and integral coefficients of the phase-locked loop PI controller, respectively. , These are the proportional and integral coefficients of the PI controller in the GSC voltage control loop, respectively. For grid-side filter inductance; For grid-side filter capacitors; and The sum of these is the equivalent impedance between the inverter's grid connection point and the power grid.

[0026] Depend on Figure 1 The diagram of the doubly-fed induction generator (DFIG) grid-connected system shown can be processed to obtain a full-order model of the system. Because the full-order model contains multi-timescale dynamics and complex coupling relationships, it exhibits high-order, nonlinear, and strongly coupled characteristics, posing a significant challenge to transient stability analysis. Therefore, this invention employs a time-scale separation principle to reduce the order of the model. Specifically: stator resistance and fast electromagnetic transients are ignored, and the stator dynamics are characterized by the grid connection point voltage; simultaneously, the current loop dynamics that can quickly track commands are ignored, while the dominant slow dynamics such as pitch control, drive train, DC link, voltage control, and phase-locked loop are retained. The final reduced-order model suitable for transient stability research is as follows: ; In the formula, This is the integral channel output of the wind turbine pitch angle controller; The pitch angle is the time constant of the actuator; This represents the moment of inertia of the motor; the physical meanings of the other variables are consistent with the conventional definitions in the field of power system analysis.

[0027] This embodiment verifies the effectiveness of the order reduction model through simulation comparison. For example... Figure 4 As shown, under the condition that the wind speed jumps from 8 m / s to 13.3 m / s, the output response of the reduced-order model is basically the same as that of the original full-order model, and both of them become unstable under the same wind speed conditions. This indicates that the reduced-order model has the same stability boundary as the original system and can accurately reflect the main dynamic performance of the system in transient processes such as sudden changes in wind speed.

[0028] 2. Adaptive Current Reference Control When the grid voltage drops, traditional control strategies typically employ active rotor current injection to accelerate stator flux decay for demagnetization control. However, this method weakens the unit's power support to the grid and may cause rotor overspeed by exacerbating torque imbalance. Therefore, this invention proposes a rotor current adaptive control strategy that balances transient stability with grid support requirements. This embodiment specifically includes the following steps: Step S31: Determination of the maximum power angle constraint after the fault When both the grid-connected converter and the generator-side converter use PLL phase tracking, their phases are consistent. According to... Figure 1 The structural diagram and reduced-order model of the system shown can be used to derive the steady-state relationship between the grid-connected dq-axis current and the rotor current, where the grid-connected active current... The only rotor shaft current control.

[0029] Based on the reduced-order model of the grid-connected system, the stable equilibrium point (SEP) and its domain of attraction (DOA) boundary after a fault can be determined, such as... Figure 2 As shown. The maximum allowable power angle after a system fault is calculated using DOA. : ; Step S32: Determining the upper limit of rotor d-axis current To ensure that the system state remains within the DOA (Depth of Arrival) after a fault, this invention applies an attraction domain constraint to the grid-connected active current component, thereby obtaining the upper limit of the grid-connected active current. This is then combined with the grid connection in step S31. The steady-state relationship between shaft current and rotor current yields the limit value of rotor d-axis current: ; Step S33: Adaptive generation of rotor current reference During normal operation, the rotor d-axis current reference Calculated using Maximum Power Point Tracking (MPPT) power control. After LVRT triggering, this invention does not directly force the injection of additional demagnetizing current, but instead employs an adaptive reference generation method with stable constraint limiting. ; This strategy dynamically adjusts the rotor current reference value by tracking the grid status in real time, effectively suppressing the grid-connected active current during deep voltage drops, thereby improving the system's transient voltage stability.

[0030] 3. Transient pitch control strategy During LVRT, the rotor-side converter is typically constrained by current limiting and reactive power priority, resulting in a limited output electromagnetic torque. Significantly decreased, while the mechanical torque on the wind turbine side... It approximately retains its original value for a short period of time. According to the rotor motion equation, when The rotor will be accelerated, and the rotational speed will increase. Continued to rise. When Approaching the upper limit In such cases, overspeed protection actions or even mechanical damage are highly likely to occur. To address this, this invention proposes a transient pitch control strategy that intervenes only when a transient overspeed risk occurs and automatically exits after the fault is cleared.

[0031] The specific control structure is as follows: Figure 3 As shown in the figure, The transient load reduction power calculated for transient pitch control is negative, indicating the power that the wind turbine needs to reduce. Its expression is as follows: ; in, To control the gain.

[0032] The control logic is: when hour, ,but The transient pitch module does not start, and the unit operates entirely according to the original MPPT logic; when hour, ,and The larger, The larger the pitch angle, the more the transient pitch module is activated, rapidly increasing the pitch angle. To reduce the wind energy utilization coefficient This reduces the input mechanical torque and suppresses rotor overspeed. The strategy achieves adaptive reduction under different fault severity levels and wind conditions.

[0033] 4. Simulation Verification This embodiment verifies the effectiveness of the proposed cooperative control strategy through simulation comparison. The simulation conditions are set as follows: Time grid inductance Leap to (Simulated grid voltage deep drop). Figure 5 The system response using traditional control methods is shown, revealing that the system eventually becomes unstable. Figure 6 The system response using the cooperative control strategy (adaptive current reference control + transient pitch control) proposed in this invention is demonstrated. The system maintains stable operation under the same severe fault, the rotor speed is effectively limited within a safe range, and the grid voltage is successfully restored.

[0034] Simulation results fully demonstrate that the adaptive current reference control and transient pitch control proposed in this invention can work together to significantly enhance the stability and fault ride-through capability of the doubly fed wind turbine grid-connected system under severe transient conditions such as parameter mutations.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for improving grid-connected transient stability control of doubly-fed induction generator (DFIG) wind turbine grid-connected systems, characterized in that, Includes the following steps: Step S1: Establish a reduced-order system model for transient stability analysis. The reduced-order model is based on the time scale separation principle. It ignores the stator resistance and fast electromagnetic transients, and uses the grid connection point voltage to characterize the stator dynamics. At the same time, it ignores the current loop dynamics, and retains the dynamics of pitch control, drive train, DC link, voltage control and phase-locked loop. Step S2: Perform instability mechanism analysis based on the reduced-order model of the system, obtain the boundary of the system's attraction domain after the fault, and determine the maximum allowable work angle constraint; Step S3: Implement adaptive current reference control, including: calculating the stable upper limit of the grid-connected active current based on the maximum allowable power angle constraint and the real-time grid voltage, and deriving the corresponding rotor current. Axis current limit This limit is compared with the original rotor current reference value generated based on the maximum power point tracking strategy. The two values ​​are compared, and the smaller value is selected as the final rotor current control command. ; Step S4: Implement transient pitch coordinated control, including: real-time monitoring of the rotor speed of the doubly-fed wind turbine. When the rotor speed Exceeding the preset safe speed limit At that time, the transient load reduction power command is calculated based on the speed difference, and the transient load reduction power command is converted into a corresponding transient pitch angle adjustment signal to increase the pitch angle. .

2. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 1, characterized in that, The system order reduction model established in step S1 is specifically represented as follows: ； In the formula, The phase difference between the voltage at the grid connection point and the grid. This is the output angular frequency of the phase-locked loop. The rated angular frequency of the power grid. , These are the proportional and integral coefficients of the phase-locked loop PI controller, respectively. , Input current to the AC side of the grid-side converter Axial components, , For the stator and rotor winding resistance, For grid-side line resistance, , For the equivalent self-inductance of the stator and rotor phases, For grid-side line inductance, , For grid voltage Axial components, , AC input voltage of grid-side converter Axial components, This is the bus capacitor voltage. For bus capacitors, , Rotor current Axial components, , Rotor voltage Axial components, For slip frequency, For the mutual inductance between the stator and rotor equivalent windings, Stator current Axial components, For the moment of inertia of the motor, For mechanical torque, For electromagnetic torque, For the integral channel output of the pitch angle controller, The pitch angle is the time constant of the actuator. This is a reference value for the pitch angle.

3. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 1, characterized in that, The adaptive current reference control in step S3 further includes: Step S31: Based on the reduced-order model of the system, determine the maximum allowable power angle of the system after the fault through attraction domain analysis. ; Step S32: Based on the maximum power angle With grid voltage Calculate the stable upper limit of the grid-connected active current and derive the rotor d-axis current limit. ; Step S33: Rotor Axis current limit The original rotor current reference value generated by the maximum power point tracking strategy The comparison is performed, and the minimum value selection strategy is used to generate the final rotor current control command. .

4. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 3, characterized in that, Maximum power angle after fault in step S31 Determined in the following manner: ； In the formula, This is the maximum permissible power angle after a system fault, calculated using the system's attraction domain.

5. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 3, characterized in that, In step S32, the rotor Axis current limit Calculated using the following formula: ； In the formula, For the stator equivalent self-inductance, For the mutual inductance between the stator and rotor equivalent windings, For grid-side line inductance, This is the grid voltage. This represents the maximum permissible power angle of the system after a fault.

6. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 3, characterized in that, The final rotor current control command in step S33 The generation method is represented as follows: ； In the formula, The original rotor current reference value is generated based on the maximum power point tracking strategy. The rotor calculated based on stability constraints Shaft current limit.

7. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 1, characterized in that, The transient load reduction power command in step S4 for transient pitch coordinated control Calculated in the following way: ； In the formula, To control the gain, To preset the upper limit of safe speed, The generator's angular velocity; Furthermore, the transient pitch angle adjustment signal is based on the transient load reduction power command. Generates the pitch angle for controlling the propeller angle. Increase.

8. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 7, characterized in that, The startup logic for transient pitch coordinated control is as follows: when At that time, transient load reduction power command Transient pitch control does not start; when hour, Furthermore, the absolute value increases with the increase of the speed deviation, triggering transient pitch control, which reduces the wind energy utilization coefficient by increasing the pitch angle. This reduces the input mechanical torque.

9. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 1, characterized in that, The specific methods for establishing the system reduced-order model based on the time scale separation principle in step S1 include: ignoring stator resistance and stator-side rapid electromagnetic transients, and directly characterizing stator dynamics with grid connection point voltage; ignoring rotor-side converter current loop dynamics and grid-side converter current loop dynamics that can quickly track commands; and retaining pitch angle control, drive train, DC bus voltage control, and phase-locked loop dynamics as the dominant slow dynamics.

10. The method for improving grid-connected transient stability control of a doubly-fed wind turbine grid-connected system according to claim 1, characterized in that, The method is implemented using this system and includes: The order reduction model building module is used to build a system order reduction model for transient stability analysis. The instability mechanism analysis module is used to perform instability mechanism analysis based on the reduced-order model of the system, obtain the boundary of the system's attraction domain after the fault, and determine the maximum allowable power angle constraint. The adaptive current reference control module is used to calculate the rotor voltage based on the maximum allowable power angle constraint and the real-time grid voltage. The shaft current limit is set and compared with the original rotor current reference value generated by the maximum power point tracking strategy, and the smaller value is selected as the final rotor current control command. The transient pitch control module is used to monitor the rotor speed in real time. When the rotor speed exceeds the preset safe speed limit, it calculates the transient load reduction power command based on the speed difference and converts it into a pitch angle adjustment signal to increase the pitch angle.

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