A Wind Power Frequency Regulation Control Method Applicable to High Wind Power Penetration Levels

Abstract

The present invention provides a wind power frequency modulation control method applicable to a high wind power penetration level, which relates to the field of wind power grid connection and includes the following steps: continuously monitor the grid frequency and calculate the instantaneous grid frequency. When a disturbance occurs in the power system resulting in the grid frequency deviation exceeding the set dead zone range; measure the fan speed, and when the fan rotor speed ω r is greater than the allowable minimum speed ω min at this time, start the virtual inertia control module of the doubly-fed wind turbine generator set, and the fan actively participates in frequency modulation; calculate the active power increment of the wind turbine generator set; calculate the equivalent active power increase in the time domain; calculate the active power reference value of the fan; after a time Δ t , the active power output of the fan smoothly switches to the maximum power tracking output power; the speed gradually returns to the initial state. By adding a virtual inertia control link to the rotor side converter controller of the wind turbine generator set, when the grid frequency exceeds the limit due to a disturbance, the fan rotor quickly absorbs and releases kinetic energy to participate in the frequency modulation of the power system, resulting in a faster frequency modulation response and better effect.

Description

Technical Field

[0001] The present invention relates to the field of wind power grid connection, and particularly to a wind power frequency modulation control method applicable to a high wind power penetration level. Background Art

[0002] With the exhaustion of reserves of non-renewable energy sources such as petroleum and the worsening of environmental problems, in recent years, new energy power generation has developed rapidly due to its advantages of cleanness, low carbon, and high efficiency. Traditional thermal power is being gradually replaced, and the power system framework in China is gradually transforming into a new power system with new energy as the main body. Among them, wind power has developed particularly rapidly, mainly because doubly-fed wind turbines have the advantages of decoupled control of active and reactive power, maximum power tracking control, and small volume. However, its grid connection through a power electronic converter decouples the rotor speed from the grid frequency, resulting in the "hiding" of the advantage that the fan has rich rotational kinetic energy and the lack of frequency response ability. In addition, doubly-fed wind turbines usually operate in the maximum power tracking mode and cannot provide active power reserve for the power system. With the continuous increase of the wind power penetration level, the stability of the grid frequency will inevitably face huge challenges.

[0003] Currently, to solve the problem of ensuring the stability of the power system frequency after large-scale wind power grid connection, additional control links are mainly used to enable the fan to actively participate in grid frequency modulation. Among them, the most common virtual inertia control methods include virtual inertia control, droop control, and step-type virtual inertia control. The fan simulates the inertia response and primary frequency modulation characteristics of a synchronous generator through virtual inertia and droop control; the step-type virtual inertia control uses the rotor to quickly absorb and release kinetic energy to participate in system frequency modulation. Compared with the first two control strategies, the frequency modulation response is faster and the frequency modulation effect is better. However, the speed recovery after frequency response is accompanied by an active power mutation, and the phenomenon of secondary impact on the grid frequency still exists generally. Especially at a high wind power penetration level, the fan speed recovery will cause a greater active power deficit in the grid, resulting in a more serious secondary frequency drop. To alleviate the secondary impact of the fan speed recovery on the grid frequency, the existing wind turbine speed recovery strategy adopts a constant power method, driving the speed recovery through a small amount of active power reduction, but it cannot take into account both the degree of secondary frequency drop and the speed recovery time: too small active power reduction makes the difference between the electromagnetic power output by the fan and the input mechanical power small, slowing down the rotor speed recovery; on the contrary, too large active power reduction will cause an instantaneous drop in the system active power, resulting in a secondary drop in the grid frequency. In a high wind power grid-connected power system, the secondary frequency drop is even lower than the first frequency drop. To sum up, how to eliminate the secondary frequency drop problem caused by the wind turbine speed recovery after frequency response needs to be solved urgently. Summary of the Invention

[0004] The present invention provides a wind power frequency modulation control method applicable to a high wind power penetration level, so as to solve the problem of secondary frequency dip caused by the recovery of the wind turbine speed during frequency response in the prior art.

[0005] To solve the above technical problems, the present invention provides a wind power frequency modulation control method applicable to a high wind power penetration level. A wind power frequency modulation control method applicable to a high wind power penetration level is characterized in that it includes the following steps:

[0006] S1: Monitor the grid frequency in real time and calculate the instantaneous grid frequency. When the grid frequency deviation caused by the disturbance in the power system exceeds the set dead zone range, execute step S2;

[0007] S2: Measure the wind turbine speed, and when the wind turbine rotor speed ω r is greater than the allowable minimum speed ω min , start the virtual inertia control module of the doubly-fed wind turbine, and the wind turbine actively participates in frequency modulation;

[0008] S3: Start the virtual inertia control module of the doubly-fed wind turbine;

[0009] S4: Calculate the active power increment of the wind turbine;

[0010] S5: Calculate the equivalent active power increase in the time domain;

[0011] S6: Calculate the active power reference value of the wind turbine;

[0012] After a time Δt, the active power output of the wind turbine smoothly switches to the maximum power tracking output power; the speed gradually returns to the initial state.

[0013] Preferably, the dead zone range is 49.98 Hz - 50.02 Hz.

[0014] Preferably, in S3, collect the wind turbine rotor speed at the disturbance moment, and calculate the maximum output power P Tlim (ω0) and the maximum power tracking output power P MPPT (ω0), and the calculation formulas are as follows:

[0015] P Tlim (ω0) = T lim ω0 (1)

[0016]

[0017] In formulas (1) and (2) above, ω0 is the wind turbine rotor speed corresponding to the disturbance occurrence moment; T lim is the torque protection limit value, and k g is a calculation constant regarding the wind turbine characteristic parameters.

[0018] Preferably, the value of T lim is 1.07 pu; the value of k g is 0.512.

[0019] Preferably, the calculation formula in S4 is as follows:

[0020] ΔP DFIG = P Tlim (ω0) - P MPPT (ω0)(3)

[0021] In formula (3), ΔP DFIG is the active power change amount for the doubly-fed wind turbine to participate in system frequency regulation.

[0022] Preferably, in S5, by means of a first-order decreasing function with respect to time, the calculated active power change amount of the wind turbine is coupled with time to obtain the equivalent active power increase amount ΔP in the time domain. The calculation formula is as follows:

[0023]

[0024] In formula (4), t0 is the moment when the disturbance occurs; Δt is the duration of the power grid frequency support stage.

[0025] Preferably, in S6, the calculated active power increase amount is superimposed on the maximum power tracking output power of the doubly-fed wind turbine to obtain the active power reference value of the wind turbine unit. The calculation formula is as follows:

[0026]

[0027] In formula (5), P MPPT is the maximum power tracking output power.

[0028] Beneficial effects brought by the present invention:

[0029] (1) By adding a virtual inertia control link to the rotor-side converter controller of the wind turbine unit, when the power grid frequency exceeds the limit due to disturbance, the wind turbine rotor quickly absorbs and releases kinetic energy to participate in the power system frequency regulation, with faster frequency regulation response and better effect;

[0030] (2) The control method of the present invention, during the power grid frequency support stage, the power function based on the torque limit is a function of time and rotational speed. On the premise that the wind turbine releases the same energy, the wind turbine can provide more power support in the initial stage of the disturbance, further increasing the lowest point of the system frequency and starting the rotational speed recovery earlier;

[0031] (3) The present invention starts the speed recovery at the moment when the virtual inertia response of the wind turbine begins. On the premise that the speed of the wind turbine returns to the initial state at the same moment, with the help of a time-varying power function, the active power increment of the wind turbine is smoothly reduced to zero within a set time, so that the active power output of the wind turbine is smoothly reduced and restored to the maximum power tracking mode, thereby achieving smooth speed recovery while eliminating the secondary frequency dip. In addition, to a certain extent, the negative impact of mechanical fatigue on the wind turbine body is reduced.

[0032] (4) The present invention verifies through simulation that adopting this method can further optimize the frequency response ability of wind turbine generators under various wind power penetration levels, raise the lowest point of the grid frequency. At the same time, the simulation also verifies that adopting this method can eliminate the problem of secondary frequency dip of the grid frequency and the problem of mechanical fatigue during the speed recovery process of wind turbine generators. Description of the Drawings

[0033] Figure 1 is a flowchart of a wind power frequency modulation control method proposed by the present invention applicable to high wind power penetration levels;

[0034] Figure 2 is a schematic diagram of the active power change trajectory curve of a doubly-fed wind turbine in the speed domain;

[0035] Figure 3 is a schematic diagram of the simulation model of the embodiment of the present invention;

[0036] Figure 4 is a schematic diagram of the structure of a doubly-fed wind turbine in the embodiment of the present invention;

[0037] Figure 5 is the simulation result of the embodiment of the present invention with a wind power penetration rate of 20%. Among them, Figure (a) is the grid frequency change curve, Figure (b) is the active power output change curve of the doubly-fed wind turbine, Figure (c) is the speed change curve of the doubly-fed wind turbine, Figure (d) is the active power increment change curve of the wind turbine in the optimization method, Figure (e) is the change curve of the torque difference between the high-speed shaft and the low-speed shaft of the wind turbine, and Figure (f) is the active power output change curve of the doubly-fed wind turbine in the speed domain;

[0038] Figure 6 is the simulation result of the embodiment of the present invention with a wind power penetration rate of 40%. Among them, Figure (a) is the grid frequency change curve, Figure (b) is the active power output change curve of the doubly-fed wind turbine, Figure (c) is the speed change curve of the doubly-fed wind turbine, and Figure (d) is the active power increment change curve of the wind turbine in the optimization method. Detailed Embodiment

[0039] Glossary:

[0040] Phase-locked loop: A negative feedback control system that uses the voltage generated by phase synchronization to tune a voltage-controlled oscillator to generate a target frequency.

[0041] In the embodiments, each symbol represents:

[0042]

[0043]

[0044] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments.

[0045] A wind power frequency modulation control method applicable to a high wind power penetration level. When the grid frequency exceeds the dead zone due to a disturbance, the wind turbine provides inertial response by increasing the active power to the maximum active power output under the torque limit, and performs speed recovery by means of a time-varying power function.

[0046] Please refer to Figure 1 , and the specific process is as follows:

[0047] S1: Real-time monitor the grid frequency, and calculate the instantaneous grid frequency using a phase-locked loop. When a disturbance occurs in the power system resulting in the grid frequency deviation exceeding the set dead zone range, execute step S2; otherwise, repeat step S1.

[0048] In one embodiment, the dead zone range is 49.98 Hz - 50.02 Hz. Therefore, when the grid frequency deviation is less than or equal to 49.98 Hz, execute step S2.

[0049] S2: Measure the rotor speed of the wind turbine, and when the rotor speed ω r is greater than the minimum speed ω min , start the virtual inertia control module of the doubly-fed wind turbine, and the wind turbine actively participates in frequency modulation; otherwise, the wind turbine does not participate in the system frequency modulation.

[0050] S3: Start the virtual inertia control module of the doubly-fed wind turbine:

[0051] Specifically, collect the rotor speed of the wind turbine at the moment of disturbance, and calculate the maximum output power P Tlim (ω0) under the torque limit protection of the wind turbine corresponding to this speed and the maximum power tracking output power P MPPT (ω0), and the calculation formulas are as follows:

[0052] P Tlim (ω0) = T lim ω0 (1)

[0053]

[0054] In the above formulas (1) and (2), ω0 is the rotor speed of the wind turbine corresponding to the moment of disturbance; T limis the torque protection limit value. In one embodiment, T lim takes a value of 1.07 pu; k g is a calculation constant regarding the characteristic parameters of the wind turbine. In one embodiment, k g takes a value of 0.512; and the maximum allowable output of the wind turbine is equal to the maximum power corresponding to the torque protection limit to ensure the safe and stable operation of the doubly-fed wind turbine and protect the safety of the wind turbine body.

[0055] When the grid frequency exceeds the limit due to disturbances, the active power of the doubly-fed wind turbine is increased from the output power P MPPT (ω0) before the disturbance to the maximum output power P Tlim (ω0) under the torque limit protection, so that more rotor kinetic energy is released into the grid, effectively raising the lowest point of the frequency.

[0056] S4: Calculate the active power increment ΔP of the wind turbine DFIG ,

[0057] In one embodiment, based on the formula in S3, calculate the active power change of the wind turbine participating in the power system frequency regulation. The calculation formula is as follows:

[0058] ΔP DFIG = P Tlim (ω0) - P MPPT (ω0) (3)

[0059] In the above formula (3), ΔP DFIG is the active power change of the doubly-fed wind turbine participating in the system frequency regulation.

[0060] S5: Calculate the equivalent active power increase ΔP in the time domain;

[0061] In one embodiment, by means of a first-order decreasing function with respect to time, couple the calculated active power change ΔP DFIG of the wind turbine with time to obtain the equivalent active power increase ΔP in the time domain;

[0062] The calculation formula is as follows:

[0063]

[0064] In the above formula (4), t0 is the moment when the disturbance occurs; Δt is the duration of the grid frequency support stage.

[0065] Normally, before the secondary frequency response starts, the wind turbine speed returns to the initial state. Considering that the system secondary frequency start time is about 20 - 30 s after the disturbance, so in this application, Δt takes a value of 26 s.

[0066] S6: Calculate the active power reference value of the wind turbine:

[0067] In one embodiment, the calculated active power change ΔP is superimposed on the maximum power tracking output power of the doubly-fed wind turbine to obtain the active power reference value of the wind turbine, and the calculation formula is as follows:

[0068]

[0069] In the above formula (5), P MPPT is the maximum power tracking output power. Moreover, when calculating the effective power reference value P ref of the wind turbine, to prevent the wind turbine from overloading and reduce mechanical fatigue, the calculated active power reference value P ref needs to be limited by the active power rate limiter and the maximum active power limiter.

[0070] After the wind turbine starts the virtual inertia response, through the action of the decreasing function of time in formula (4), the active power increment of the wind turbine gradually decreases to zero within the set time, so that the active power output of the wind turbine smoothly decreases and returns to the maximum power tracking state, that is, the active power output of the wind turbine smoothly switches to P MPPT , the rotational speed gradually returns to the initial state. At the same time, the secondary frequency dip is eliminated, and the mechanical fatigue problem is alleviated to a certain extent.

[0071] Please refer to Figure 2 , Figure 2 which is a schematic diagram of the power change trajectory curve of the doubly-fed wind turbine in the rotational speed domain of this application. The process of the wind turbine participating in the power system frequency modulation is as shown by the curve A-B-C-D-A in Figure 2 . When the power grid frequency is normal, the doubly-fed wind turbine operates in the maximum power tracking mode, and its active power output is the maximum power tracking power P MPPT (ω0), corresponding to point A in Figure 2 ; when a disturbance occurs in the power grid resulting in the frequency dropping beyond the dead zone, the virtual inertia control of the wind turbine starts, and the active power output of the doubly-fed wind turbine increases to the maximum active power P Tlim (ω0) under the torque limit (corresponding to the A-B stage), so that more rotor kinetic energy is released into the power grid, effectively increasing the lowest frequency point; since the active power of the wind turbine in this application is a function of time and rotational speed and changes with time and rotational speed, the active power output of the wind turbine smoothly decreases and returns to the maximum power tracking mode (corresponding to the B-C-D stage), and then the active power of the wind turbine returns to the state before the disturbance along the maximum power tracking curve, thus achieving a smooth recovery of the rotational speed while eliminating the secondary frequency dip.

[0072] The beneficial effects of the present invention will be elaborated in detail below in combination with specific simulation examples.

[0073] In this application, by adding a virtual inertia control loop to the rotor-side converter controller, leveraging the advantages of the fast energy absorption and release function of the wind turbine rotor, frequency response services are provided quickly to improve the lowest point of the grid frequency.

[0074] Please refer to Figure 3 , to verify the rationality and effectiveness of a wind power frequency modulation control method applicable to a high wind power penetration level provided by this application, a power system model with different wind power penetration rates is built using the EMTP-RV software platform. This system includes 4 synchronous generator sets, 1 asynchronous motor, a static load with a capacity of 240 MW, and an aggregated wind farm. Among them, the structural schematic diagram of the doubly-fed wind turbine generator is as Figure 4 shown.

[0075] In this application, when the wind speed is 9 m / s and 120 MW of synchronous units are offline, the frequency modulation characteristics of the doubly-fed wind turbine under the following three control strategies will be compared and analyzed in scenarios with wind power penetration rates of 20% and 40% respectively.

[0076] (1) Maximum power point tracking control (MPPT);

[0077] (2) Constant power speed recovery strategy (existing method);

[0078] (3) Frequency modulation strategy applicable to high wind power penetration level (optimized method).

[0079] To better analyze and compare with the existing technical methods, it is set in the experiment that: in the existing method and the optimized method, the doubly-fed wind turbine releases the same rotor kinetic energy to participate in the grid frequency regulation; and the wind turbine speed returns to the initial state at the same time in both methods. After debugging, in this simulation analysis, the frequency support duration Δt of the optimized method is set to 26 s, and the active power reduction amount during the speed recovery stage of the existing method is set to 0.15 pu.

[0080] When the wind power penetration rate is 20%, it is set that the 4th synchronous unit is offline at 40 s, the active power of the power system is lost by 120 MW, and the grid frequency drops. As can be seen from Figure 5 (a), when the doubly-fed wind turbine generator adopts the maximum power point tracking control, it does not participate in the grid frequency modulation, its active power output and speed remain unchanged, and the maximum deviation of the grid frequency is 0.806 Hz. When the existing method is adopted, the maximum grid frequency deviation is reduced to 0.578 Hz. The main reason is that the wind turbine releases a certain amount of rotational kinetic energy to the grid, and the active power output of the wind turbine increases from 63 MW to 108 MW. Compared with the existing method, when the optimized method is adopted, the maximum grid frequency deviation is further reduced to 0.566 Hz, an increase of 0.012 Hz. This is mainly because the optimized method releases more active power in the early stage of the disturbance, as shown in Figure 5 (b) and 5(f).

[0081] From Figure 5 (c), it can be seen that in the early stage of the disturbance, the fan speed in the optimization method drops faster and starts to recover at 53 s. Since the speed of the existing method converges slowly, the speed recovery starts at 60 s, and the speeds of both methods return to the state before the disturbance at 116 s. In addition, the existing method adopts a constant-power speed recovery strategy, and the active power is reduced by 20 MW at 60 s, resulting in a secondary drop in the grid frequency, with a drop value of 0.466 Hz. The optimization method uses a time-varying power function, and the active power output of the doubly-fed fan decreases smoothly over time and returns to the maximum power tracking mode, effectively eliminating the secondary frequency drop.

[0082] From Figure 5 (e), it can be seen that at the initial stage of the disturbance, due to the rapid increase in the active power of the fan in both the existing method and the optimization method, both methods inevitably have similar mechanical problems. However, during the speed recovery process, the torque change of the fan in the optimization method is gentle, which improves the mechanical fatigue problem caused by the sudden change in the active power of the fan to a certain extent.

[0083] When the wind power penetration rate is increased to 40%, from Figure 6 (a) and Figure 6 (b), it can be seen that in the existing method, the first drop value of the grid frequency is 0.963 Hz, and the second drop value is 1.082 Hz. Therefore, the maximum deviation of the grid frequency is 1.082 Hz. Compared with the results of a 20% wind power penetration rate, the secondary frequency drop is more serious. The main reason is that at a 40% wind power penetration rate, the active power reduction for driving the speed recovery is 41 MW, which is significantly higher than the active power reduction at a 20% penetration rate. In addition, when the doubly-fed fan adopts the optimization method, the maximum deviation of the grid frequency is 0.942 Hz, which is improved by 0.140 Hz compared with the existing method, and the frequency curve changes gently, without obvious secondary frequency drop. This is mainly because the optimization method uses a time-varying power function to smooth the sudden change in active power during the speed recovery stage.

[0084] The above simulation results show that with the continuous increase in the wind power penetration level, especially in the scenario of a 40% wind power penetration rate, the phenomenon of secondary frequency drop in the existing method is very serious and much lower than the first frequency drop. In addition, on the premise that the fan releases the same energy and the rotor speed recovery time is the same: adopting the method proposed in the present invention can effectively solve the problem of power system frequency stability under high wind power penetration, further increase the lowest frequency point, while eliminating the secondary frequency drop during the fan speed recovery process and reducing the mechanical fatigue of the fan.

[0085] A wind power frequency modulation control method applicable to a high wind power penetration level provided by this application improves the existing wind turbine speed recovery method. With the aid of an optimized power function based on torque limit, it enables the wind turbine to provide more power support at the initial stage of disturbance, raises the lowest frequency point, and starts the speed recovery earlier. Secondly, the speed recovery is carried out with a time-varying power function, and the active power output of the wind turbine smoothly decreases and returns to the maximum power tracking mode, effectively eliminating the secondary frequency dip caused by the sudden change of the active power of the wind turbine during the speed recovery process, and alleviating the mechanical fatigue problem of the wind turbine during the speed recovery to a certain extent.

[0086] The above are only the embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims

1. A wind power frequency modulation control method applicable to a high wind power penetration level, characterized in that: It includes the following steps: S1: Monitor the grid frequency in real time and calculate the instantaneous grid frequency. When the grid frequency deviation caused by the disturbance in the power system exceeds the set dead zone range, execute step S2; S2: Measure the fan speed and, when the rotational speed ω of the fan rotor r is greater than the allowable minimum rotational speed ω min , start the virtual inertia control module of the doubly-fed wind turbine generator set, and the fan actively participates in frequency regulation; S3: Start the virtual inertia control module of the doubly-fed wind turbine; First, collect the rotational speed of the wind turbine rotor at the moment of disturbance, and calculate the maximum output power P under the torque limit protection corresponding to this rotational speed Tlim (ω0) and the output power P of maximum power tracking MPPT (ω0), and the calculation formula is as follows: P Tlim (ω0) = T lim ω0 (1) In formulas (1) and (2), ω0 is the rotational speed of the wind turbine rotor corresponding to the moment when the disturbance occurs; T lim is the torque protection limit value, and k g is a calculation constant related to the characteristic parameters of the wind turbine; S4: Calculate the active power increment of the wind turbine; ΔP DFIG = P Tlim (ω0) - P MPPT (ω0) (3) In formula (3), ΔP DFIG is the active power change amount for the doubly-fed wind turbine to participate in system frequency regulation; S5: Coupling the calculated active power change of the wind turbine with time by means of a first-order decreasing function with respect to time, so as to obtain the equivalent active power increase ΔP in the time domain. The calculation formula is as follows: In formula (4), t0 is the moment when the disturbance occurs; Δt is the duration of the grid frequency support stage; S6: Calculate the active power reference value of the wind turbine; In the above S6, the active power reference value of the wind turbine unit is obtained by superimposing the calculated active power increase on the output power of the maximum power tracking of the doubly-fed wind turbine. The calculation formula is as follows: In formula (5), P MPPT is the output power of maximum power point tracking After a time Δt, the active power output of the wind turbine smoothly switches to the output power of the maximum power tracking; the rotational speed gradually returns to the initial state.

2. The wind power frequency modulation control method applicable to a high wind power penetration level according to claim 1, wherein: The dead zone range is 49.98Hz - 50.02Hz.

3. The wind power frequency regulation control method applicable to high wind power penetration levels according to claim 1, wherein: The T lim takes a value of 1.07 pu; the k g takes a value of 0.512.

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