Improved stepping inertia control method for wind turbine considering frequency response characteristics of synchronous machine

Abstract

The present invention discloses an improved method for stepping inertia control of a fan taking into account the frequency response characteristics of a synchronous machine. Aiming at the problem of secondary frequency drop caused by the traditional stepping inertia control of the fan in the speed recovery stage, the present invention fully considers the frequency response characteristics of the synchronous machine, and utilizes the advantages of fast, flexible and strong plasticity of the fan power output to improve its exit frequency modulation mode; when a frequency event occurs, the fan immediately increases the active output and maintains constant power for a certain period of time; when the frequency reaches the lowest point, the fan power decreases as the synchronous machine power increases, and together they make up for the load power shortage. While overcoming the secondary drop problem, the initial power support is further improved, thereby achieving the purpose of reducing the maximum frequency deviation.

Description

Technical Field

[0001] The invention belongs to the technical field of primary frequency modulation control strategy for fans, and in particular to an improved method for stepping inertia control of fans taking into account frequency response characteristics of synchronous machines. Background Art

[0002] The large-scale grid-connected wind power has reduced the inertia level of the power grid, and the frequency stability of the power grid faces huge challenges. Wind turbines usually operate in maximum power tracking mode, and their rotor speed is decoupled from the grid frequency, and they cannot respond to grid frequency changes by adjusting their output. In order to maintain the safe and stable operation of the power system, it is urgently necessary for wind turbines to have certain grid frequency auxiliary regulation functions. The frequency regulation method of wind turbines in MPPT mode in response to grid frequency changes is inertial control, which is essentially to support the grid frequency for a short time by absorbing or releasing rotor kinetic energy, thereby reducing the adverse effects of wind power grid connection on the system.

[0003] Traditional stepwise inertial control (SIC) immediately increases the constant output power when a disturbance occurs, and maintains it for a period of time until the rotor speed reaches its lower limit, and then quickly switches back to MPPT mode. Compared with frequency-based inertial control such as virtual inertial control and droop control, stepwise inertial control releases more rotor kinetic energy at the moment of load event, which can better improve the lowest frequency point.

[0004] Due to the limited kinetic energy of the fan's rotor, it is difficult for the fan to maintain high power output for a long time under inertial control. If the fan's rotor speed drops to its lower limit, the rotor's slip power will exceed the capacity of the rotor-side inverter. At this time, the fan is disconnected from the power system and the frequency modulation is exited to protect the rotor-side inverter. Therefore, the inertial control strategy must be terminated before the rotor reaches the minimum allowable speed, which will cause a sudden drop in the fan's active power output, causing a new round of system frequency drop during the speed recovery process, that is, a secondary drop in frequency.

[0005] Some studies have alleviated the problem of secondary frequency drop by changing the exit mode of SIC, but the current research on the exit mode of SIC often focuses on the wind turbine itself, while ignoring the mutual influence between the wind turbine and the synchronous generator set involved in frequency regulation. Some literature points out that by making full use of the advantages of fast, flexible and strong plasticity of wind turbine frequency regulation, better frequency regulation effect can be achieved through the coordination of wind turbines and synchronous machines. However, the method of directly obtaining the optimal output power curve through optimization algorithm and fitting it can only obtain a more idealized control strategy for given system conditions and disturbances, which is difficult to apply in practice. Summary of the invention

[0006] The object of the present invention is to provide an improved method for stepping inertia control of a fan taking into account the frequency response characteristics of a synchronous machine.

[0007] The technical solution to achieve the purpose of the present invention is as follows: A method for improving the stepping inertia control of a fan taking into account the frequency response characteristics of a synchronous machine comprises the following steps:

[0008] Step 1: Assume that the fan operates at a constant initial power P during a frequency modulation process. e0 Output, based on the grid frequency response model, the expression of synchronous machine output power P is derived m (t);

[0009] Step 2: Based on the P derived in step 1 m (t) expression, analyze the time t when the frequency reaches the lowest point n1 With the load increase P step , fan power P e0 Not relevant;

[0010] Step 3: Based on the P derived in step 1 m (t) expression, calculate the output power expression of the fan ΔP m (t);

[0011] Step 4: Based on the conclusion of step 2, use the ΔP derived in step 3 m (t) expression is improved to obtain the power output expression P of fan frequency regulation control e (t);

[0012] Step 5: Traverse and search to obtain wind speed v and load sudden increment P step The corresponding optimal fan initial power P e0_opt , whose value minimizes the maximum frequency deviation of the power grid;

[0013] Step 6: Estimate the load surge P using the initial frequency change rate of the power grid step ;

[0014] Step 7: Substitute the optimal fan initial power P obtained in steps 5 and 6 e0_opt and load sudden increment P step Substitute into the wind turbine power expression P obtained in step 4 e (t), the improvement of the fan step inertia control is achieved.

[0015] Furthermore, the simplified system frequency response model in step 1 regards the wind turbine as a variable load, so the frequency response expression of the system is:

[0016]

[0017] Where P dis the load power output, P e is the wind power output, F H is the output power ratio of the reheat turbine, H is the equivalent inertia time constant of the system, D is the damping coefficient, K m is the system unit regulating power, T R is the reheat time constant, R is the governor adjustment coefficient;

[0018] Assuming that the wind turbine always maintains a constant output power during the frequency regulation process, the relationship between its output power and time before the recovery phase is expressed as P e0 (t) = P MPPT +P up u(t), where P MPPT is the active output of the fan running in MPPT mode. The relationship between load power and time is expressed as P d (t) = P step u(t), u(t) is a unit step function. The output power of the synchronous machine in the time domain is derived as:

[0019]

[0020] Among them, k opt is the optimal torque gain coefficient, ω opt is the optimal torque corresponding to the maximum output power of the fan,

[0021]

[0022] Furthermore, in step 2, Laplace transform is used to transform P e (t) and P d After (t) is converted into a frequency domain expression and inserted into the frequency response expression of the system, we get:

[0023]

[0024] Then convert it into time domain expression:

[0025]

[0026] In the formula,

[0027] According to the frequency time domain expression, the time t at which the frequency reaches the lowest point caused by the load disturbance can be obtained. n1 With the load increase P step 、Sudden increase of wind turbine power P up Therefore, after the disturbance occurs, the time to reach the lowest frequency point remains unchanged. It is only necessary to n1 Then the output power strategy of the fan can be improved.

[0028] Furthermore, in step 3, the difference between the real-time output power of the synchronous machine and the active output of the synchronous machine when the frequency reaches the lowest point for the first time is calculated, that is,

[0029] Furthermore, in step 4, ΔP m (t) Substitute the improved control strategy of fan output power into:

[0030]

[0031] Where t0 is the time when the load event occurs, t n1 is the time it takes for the frequency to reach its lowest point once it drops, t off It is the time when the wind turbine switches to MPPT mode, which is determined by the operating status of the wind turbine. When the decreasing active power output and rotor speed meet When the fan switches back to the MPPT operation mode, the time point is t off ,ω W It is the real-time operating speed of the fan.

[0032] Furthermore, in step 5, the support amount P is changed up The simulation traversal is performed with the support amount ranging from 0.04pu to 0.1pu and the step length being 0.02pu. The support time, maximum frequency deviation and minimum speed are compared. up The larger the value, the longer the support time, the smaller the maximum frequency deviation, and the lower the minimum speed. Therefore, there is an optimal P up The minimum speed reaches the lower speed limit, the support time is the longest and the maximum frequency deviation is the smallest. The lower limit of the fan rotor speed is set to 0.7pu, and the optimal fan support amount P corresponding to different wind speeds and load sudden increments is obtained through simulation calculation and traversal search. up Specifically, by traversing the maximum frequency deviation of the power grid measured at constant wind speed (wind speed range is 7-12m / s, step size is 0.5m / s) and load sudden increment (0.06pu-0.2pu, step size is 0.02pu), the wind turbine power support value P that minimizes its value is obtained. up_opt .

[0033] Furthermore, in step 6, the load fluctuation amplitude is usually difficult to accurately detect, and the load change amount P is estimated according to the frequency change rate when the load event occurs (t=0). step :

[0034]

[0035] Among them, frequency, as an important power quality indicator of the power grid, can be accurately monitored in real time, and the time when the load event occurs can be estimated in conjunction with the frequency regulation dead zone.

[0036] Furthermore, step 7 will contain the parameters P obtained in steps 5 and 6. up_opt and P step Substituting it into the control strategy of the fan output power, an improved step inertia control that can optimize the frequency response is obtained. This improved method can make the fan and the synchronous machine cooperate well to complete the primary frequency modulation, which not only improves the minimum frequency of the system, but also avoids the phenomenon of secondary frequency drop.

[0037] A computer device comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein when the processor executes the computer program, the steps of the improved method for stepping inertia control of a fan taking into account the frequency response characteristics of a synchronous machine are implemented.

[0038] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps of the above-mentioned improved method for stepping inertia control of a fan taking into account the frequency response characteristics of a synchronous machine.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] (1) Give full play to the advantages of the fan in terms of rapid power regulation and the synchronous machine in terms of continuous support capacity, and through the coordination between the fan and the synchronous machine, effectively avoid the problem of secondary frequency drop during the fan frequency regulation process;

[0041] (2) By traversing the calculated and estimated parameters, the wind turbine can release more rotor kinetic energy at the beginning of the load event and reduce the frequency drop. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is the frequency response model of the present invention and is the model basis of the invention.

[0043] Figure 2 This is a control block diagram of the wind turbine SIC improvement method proposed by the present invention that takes into account the frequency response characteristics of the synchronous machine.

[0044] Figure 3 This is the frequency modulation output power curve of the synchronous machine in the present invention.

[0045] Figure 4 It is the system frequency deviation curve in the present invention.

[0046] Figure 5 This is the output power and speed curve of the fan in the present invention.

[0047] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. DETAILED DESCRIPTION

[0048] In order to solve the problem that the existing methods for improving the secondary frequency drop problem ignore the mutual influence between the fan and the synchronous machine, the present invention provides a fan SIC improvement method that studies the output change process of the synchronous machine in response to the frequency change after the load event occurs, and considers the frequency response characteristics of the synchronous machine on this basis. Through the coordination between the fan and the synchronous machine, the entire frequency modulation process gradually transitions from the power support dominated by the fan to the power support dominated by the synchronous machine, so as to give full play to the advantages of the fan in rapid power regulation and the synchronous machine in continuous support capacity, thereby effectively avoiding the secondary frequency drop problem in the fan frequency modulation process, and at the same time enabling the fan to release more rotor kinetic energy at the beginning of the load event and reduce the maximum frequency deviation.

[0049] Assuming that the wind turbine always maintains a constant output power during the frequency regulation process, the frequency response characteristics of the synchronous machine in the frequency modulation process are analyzed using a simplified system frequency response model, and the frequency response characteristics of the system are further obtained, thereby concluding that the time it takes for the same system to reach the lowest frequency point is constant; then, the expected active output drop mode curve expression of the wind turbine is calculated based on the obtained synchronous machine output power, and the active output process of the wind turbine is combined to obtain an improved control strategy; then, it is verified that under this control strategy, the rate of electric power decrease on the wind turbine is always no greater than the rate of electric power increase of the synchronous machine at this time; finally, the optimal values ​​of the parameters in the control strategy are obtained through simulation traversal and estimation analysis, and the effectiveness of the strategy is verified by simulation.

[0050] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0051] See also Figure 1 As shown in Figure 1, the wind turbine is regarded as a variable load and a simplified frequency response model of the system is established. The system includes an equivalent synchronous machine P m , an equivalent wind turbine P e and an equivalent load P d . According to the frequency response model, the frequency modulation output characteristics of the synchronous machine are analyzed, and the active output of the fan is improved according to the active output of the synchronous machine, so that the output power of the entire system will not be reduced due to the fan exiting the frequency modulation. Finally, the method is compared with the traditional SIC method and the case where the fan does not participate in the frequency modulation (i.e., MPPT control is adopted), which verifies the feasibility and superiority of the method proposed in the present invention.

[0052] Specifically, the method comprises the following steps:

[0053] In the improved strategy, it is assumed that the wind turbine always maintains a constant output power during the frequency regulation process, and the relationship between its output power and time before the recovery stage is expressed as P e0 (t) = P MPPT +Pup u(t), the load power variation over time is expressed as P d (t) = P step u(t), u(t) is a unit step function. The output power of the synchronous machine in the time domain is derived as:

[0054]

[0055] in,

[0056] Frequency response expression:

[0057]

[0058] Using Laplace transform, P e (t) and P d After (t) is converted into a frequency domain expression and inserted into the frequency response expression of the system, we get:

[0059]

[0060] Then convert it into time domain expression:

[0061]

[0062] In the formula,

[0063] According to the frequency time domain expression, the time t at which the frequency reaches the lowest point caused by the load disturbance can be obtained. n1 With the load increase P step 、Sudden increase of wind turbine power P step Therefore, after the disturbance occurs, the time to reach the lowest frequency point remains unchanged. It is only necessary to n1 Then the output power strategy of the fan can be improved.

[0064] Using P m (t) Calculate the difference between the real-time power of the synchronous machine and the output of the synchronous machine when it reaches the first frequency minimum point, that is, ΔP m (t) Substitute the improved control strategy of fan output power to achieve the purpose of jointly making up for the load power shortage:

[0065]

[0066] Among them, t off Determined by the operating status of the fan. When the decreasing active power output and the rotor speed meet When the fan switches back to the MPPT operation mode, the time point is toff The control block diagram is shown in Figure 2 .

[0067] Under this strategy, the output power of the entire system always maintains a trend of only increasing but not decreasing. Since the entire strategy is based on the assumption that the wind turbine always maintains a constant power output during the primary frequency modulation process, the power output of the wind turbine in the actual control strategy changes continuously with the increase of the synchronous machine power. Comparing the synchronous machine output power when the wind power decreases according to formula (5) and remains constant, as shown in Figure 3 As shown in Figure 1, the rate of decrease of the electric power reversely superimposed on the wind turbine is less than or equal to the rate of increase of the synchronous machine at this time. Therefore, the lowest point of the system frequency is always at time t n1 It can be achieved without causing a secondary drop in frequency.

[0068] From equations (1) and (5), it can be seen that the output curves of synchronous machines and fans are affected by many parameters. up The simulation was carried out with the support amount ranging from 0.04pu to 0.1pu and the step size being 0.02pu. The support time, maximum frequency deviation and minimum speed were compared. The results are shown in Table 1. up The larger the value, the longer the support time, the smaller the maximum frequency deviation, and the lower the minimum speed. Therefore, there is an optimal P up The minimum speed reaches the lower speed limit, the support time is the longest and the maximum frequency deviation is the smallest. The lower limit of the fan rotor speed is set to 0.7pu, and the optimal fan support amount P corresponding to different wind speeds and load sudden increments is obtained through simulation calculation and traversal search. up Specifically, by traversing the maximum frequency deviation of the power grid measured at constant wind speed (wind speed range is 7-12m / s, step size is 0.5m / s) and load sudden increment (0.06pu-0.2pu, step size is 0.02pu), the wind turbine power support value P that minimizes its value is obtained. up_opt .

[0069] Table 1 Influence of fan power support on simulation results

[0070]

[0071] In view of the problem that the amplitude of load fluctuation is usually difficult to accurately detect, the load change P is estimated based on the frequency change rate when the load event occurs (t = 0). step :

[0072]

[0073] Among them, frequency, as an important power quality indicator of the power grid, can be accurately monitored in real time, and the time when the load event occurs can be estimated in conjunction with the frequency regulation dead zone.

[0074] The obtained parameter P up_opt and P step Substituted into the control strategy of wind turbine output power, a simulation model was built in the MATLAB / Simulink environment for simulation. The conventional unit uses a 4.8MW steam turbine, and the wind power part uses two 0.6MW doubly-fed wind turbines, and the wind power penetration rate is about 20%.

[0075] The improved method proposed in the present invention is compared with the traditional SIC method and the case where the fan does not participate in frequency modulation (i.e., MPPT control is used). At a constant wind speed of 11m / s, 80s of simulation verification is used. In order to examine the responsiveness of the fan, a load of 0.12pu is added at 30s, and analysis is performed from three aspects: system frequency deviation, fan output work power, and fan rotor speed. To ensure fairness, the optimal parameters of the two SIC methods are obtained by traversal, that is, the parameter value that can minimize the maximum frequency deviation.

[0076] The simulation results are as follows Figure 4 and 5 As shown, when the load changes suddenly, compared with the traditional SIC method, the improved SIC method for fans designed by the present invention that takes into account the frequency response characteristics of synchronous machines, at the initial moment of frequency modulation, the fan outputs more active power, and then slowly reduces the active output to return to the operating state before frequency modulation. The improved method uses as much of the fan rotor kinetic energy as possible to raise the system frequency, rather than to restore the rotor speed. It has been verified that this method can make the fan and the synchronous machine cooperate better to complete a frequency modulation, which can not only effectively increase the minimum frequency of the system after a load event occurs, but also effectively avoid the secondary drop in frequency and keep the steady-state frequency unchanged.

[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An improved method for stepping inertia control of a fan considering the frequency response characteristics of a synchronous machine, characterized in that: The following steps are involved: Step 1: Assume that the fan operates at a constant initial power P during a frequency modulation process. e0 Output, based on the grid frequency response model, the expression of synchronous machine output power P is derived m (t); The simplified system frequency response model regards the wind turbine as a variable load, and the frequency response expression of the system is: Where P d is the load power output, P e is the fan power output, F H is the output power ratio of the reheat steam turbine, H is the equivalent inertia time constant of the system, D is the damping coefficient, K m is the system unit regulating power, T R is the reheat time constant, R is the governor adjustment coefficient; Assuming that the wind turbine always maintains a constant output power during the frequency regulation process, the relationship between its output power and time before the recovery phase is expressed as P e0 (t) = P MPPT +P up u(t), where P MPPT is the active output of the fan running in MPPT mode. The relationship between load power and time is expressed as P d (t) = P step u(t), u(t) is a unit step function; the output power of the synchronous machine in the time domain is derived: Among them, k opt is the optimal torque gain coefficient, ω opt is the optimal torque corresponding to the maximum output power of the fan, Step 2: Based on the P derived in step 1 m (t) expression, analyze the time t when the frequency reaches the lowest point n1 With the load increase P step 、Sudden increase of wind turbine power P up Not relevant; Using Laplace transform, P e (t) and P d After (t) is converted into a frequency domain expression and inserted into the frequency response expression of the system, we get: Then convert it into time domain expression: In the formula, According to the frequency time domain expression, the time t at which the frequency reaches the lowest point caused by the load disturbance can be obtained. n1 With the load increase P step 、Sudden increase of wind turbine power P up Therefore, after the disturbance occurs, the time to reach the lowest frequency point remains unchanged, and only the fixed time point, i.e., the time t of the lowest frequency point, needs to be n1 Then the output power strategy of the fan can be improved; Step 3: Based on the P derived in step 1 m (t) expression, calculate the output power expression of the fan ΔP m (t); Calculate the difference between the real-time output power of the synchronous machine and the active output of the synchronous machine when it reaches the lowest frequency point for the first time, that is, Step 4: Based on the conclusion of step 2, use the ΔP derived in step 3 m (t) expression is improved to obtain the power output expression P of fan frequency regulation control e (t); ΔP m (t) Substitute the improved control strategy of fan output power into: Where t0 is the time when the load event occurs, t n1 is the time it takes for the frequency to reach its lowest point once it drops, t off It is the time when the wind turbine switches back to MPPT mode, which is determined by the operating status of the wind turbine; when the decreasing active power output and rotor speed meet When the fan switches back to the MPPT operation mode, the time point is t off ,ω W is the real-time operating speed of the fan; Step 5: Traverse and search to obtain wind speed v and load sudden increment P step The corresponding optimal fan initial power P e0_opt , whose value minimizes the maximum frequency deviation of the power grid; Step 6: Estimate the load surge P using the initial frequency change rate of the power grid step ; Step 7: Substitute the optimal fan initial power P obtained in step 5 and step 6 e0_opt and load sudden increment P step Substitute into the wind turbine power output expression P obtained in step 4 e (t), the improvement of the fan step inertia control is achieved.

2. The improved method for stepping inertia control of a fan considering the frequency response characteristics of a synchronous machine according to claim 1 is characterized in that: Step 5: Change the fan power increment P up The simulation traversal is performed with the sudden increment range of 0.04pu~0.1pu and the step length of 0.02pu, and the support time, maximum frequency deviation and minimum speed are compared; P up The larger the value, the longer the support time, the smaller the maximum frequency deviation, and the lower the minimum speed; therefore, there is an optimal P up Make the minimum speed reach the lower speed limit, the support time is the longest and the maximum frequency deviation is the smallest; set the lower limit of the fan rotor speed to 0.7pu, and obtain the optimal fan power surge P corresponding to different wind speeds and load surges through simulation calculation and traversal search. up ; By traversing the maximum frequency deviation of the power grid measured at constant wind speed and load sudden increment, the wind turbine power sudden increment P that minimizes its value is obtained. up_opt .

3. The improved method for stepping inertia control of a fan considering the frequency response characteristics of a synchronous machine according to claim 1 is characterized in that: Step 6 estimates the load surge P according to the frequency change rate when the load event occurs. step :

4. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the steps of the improved method for stepping inertia control of a wind turbine taking into account the frequency response characteristics of a synchronous machine as described in any one of claims 1 to 3 are implemented.

5. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the improved method for stepping inertia control of a wind turbine taking into account the frequency response characteristics of a synchronous machine as described in any one of claims 1 to 3 are implemented.

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