Power smooth control method for energy-storage-free offshore wind power hydrogen production system

Abstract

The invention relates to a power smooth control method for an energy-storage-free offshore wind power hydrogen production system. According to the method, a specially designed second-order filter is connected behind a fan P-omega curve, and corresponding filter parameters can be designed according to the characteristics of the electrolytic cell. By introducing the filter, the fan can keep good maximum power tracking performance in a low-frequency band, and meanwhile, the power fluctuation in a high-frequency band can be inhibited. According to the method, a large amount of rotor kinetic energy stored in the fan rotor is used for balancing power between the electrolytic cell and the fan, voltage fluctuation of the direct-current bus is restrained, and stable operation of the fan is ensured.

Description

Technical Field

[0001] The present invention relates to the field of motor transmission control, and in particular to a power smoothing control method for an offshore wind power hydrogen production system without energy storage. Background Art

[0002] Offshore wind energy resources are more abundant and stable than onshore wind energy. Offshore wind farms can use sustained strong winds to provide a stable power supply, thus overcoming the instability and intermittency of wind energy, which is crucial for the power stability requirements in the hydrogen production process. Secondly, offshore wind power hydrogen production can give full play to the advantages of marine resources, avoiding the land resource and environmental constraints that onshore wind power may face, making large-scale wind power hydrogen production projects possible at sea.

[0003] In order to solve the volatility of wind power generation, most offshore wind power hydrogen production systems are connected to the power grid, which requires laying long-distance submarine cables, which is not only expensive and complex to maintain, but also has the problem of power loss during transmission. In contrast, the off-grid system directly converts wind energy into the electricity required for hydrogen production, which improves the efficiency of wind energy utilization. More importantly, the independence of the off-grid system makes it more flexible and suitable for deep-sea wind farms far away from land, and can provide a sustainable solution for deep-sea energy development. Therefore, the off-grid offshore hydrogen production system has obvious advantages in terms of economy, efficiency and flexibility.

[0004] Although the off-grid offshore wind power hydrogen production system has broad prospects, its widespread application still faces some key challenges, one of which comes from the volatility of wind power generation and the strict operating restrictions on the power change rate of the electrolyzer. The randomness and intermittency of wind power generation make it difficult to stabilize the output power, and the electrolyzer has strict requirements on the power rise and fall rate due to the dynamic characteristics of its internal chemical reaction. If the power changes too quickly, it may cause drastic fluctuations in the temperature and pressure inside the electrolyzer, which will affect the purity of hydrogen, system efficiency and equipment life, and even bring safety hazards. In order to ensure safe operation, factories generally limit the power change rate of the electrolyzer according to the situation. Off-grid offshore wind power cannot rely on the power grid to compensate for the energy shortage, so how to effectively smooth the wind power output, balance the energy between wind power and electrolyzer, and meet the operating needs of the electrolyzer are issues worthy of attention at present.

[0005] The use of new materials to manufacture electrolyzer system components or the development of new electrode catalytic materials can improve the properties of the electrolyzer, accelerate the electrochemical reaction rate, and improve the reaction efficiency. However, this method requires expensive materials and advanced hydrogen production technology and has not yet been widely used in business. Another common solution is to equip wind turbines with energy storage devices to offset the volatility of wind energy by controlling the energy storage system to absorb or release energy. However, energy storage systems require high costs in terms of purchase, maintenance, and replacement, and require complex control logic and energy management strategies, which increases the complexity of system design and operation. Most of the existing research on wind power hydrogen production systems focuses on its economic efficiency, while there is less research on the control strategy of the wind turbine itself. In addition, most systems use energy storage to smooth wind power without considering the use of the large amount of kinetic energy stored in the wind turbine rotor.

[0006] Therefore, how to smooth the fan output power without using a storage device and meet the power change rate requirements of the electrolyzer is the key to improving system stability. Summary of the invention

[0007] In order to overcome the problems existing in the related art, the disclosed embodiment of the present invention provides a power smoothing control method for an offshore wind power hydrogen production system without energy storage.

[0008] The technical solution comprises the following steps:

[0009] S1, based on the fitting function of the optimal wind turbine power and rotor speed and the wind turbine dynamic equation, establish the transfer function of the wind turbine electromagnetic power and wind speed under the maximum power tracking state;

[0010] S2, introducing an improved second-order filter to smoothly control the wind turbine power, wherein the second-order filter includes a two-pole-one-zero structure, and establishing a transfer function between the electromagnetic power and wind speed of the wind turbine after the filter is introduced;

[0011] S3, by adjusting the second-order filter parameters a, b, c to suppress high-frequency power fluctuations and maintain the response of low-frequency power fluctuations, where the turning frequency ω1 is less than ω3, the value of ω3 is set according to the electrolyzer power difference slope limit, and ω2 is between ω1 and ω3, satisfying the trade-off between power fluctuation smoothing and wind energy capture efficiency;

[0012] S4, through the second-order filter output smoothed fan power reference value, so that the fan power change and the electrolyzer power change rate limit adaptation, significantly reducing the fluctuation of the DC bus voltage.

[0013] Air flow can generate wind energy. According to Betz theory, wind rotor blades can capture part of the wind power, as shown in the following formula:

[0014]

[0015] Where P m is the mechanical power of the wind wheel; ρ is the air density, S = πR 2 is the swept area of ​​the wind rotor blade; R is the blade radius; v is the wind speed; C p (λ,β) is the wind energy utilization coefficient; λ is the tip speed ratio; β is the pitch angle; ω r is the rotor speed.

[0016] Furthermore, assuming that the transmission system is a rigid system and ignoring friction, the mechanical dynamic characteristics of the permanent magnet direct-drive wind turbine system can be expressed as:

[0017]

[0018] J is the total equivalent moment of inertia of the wind turbine, transmission system and generator; T m is the mechanical torque of the fan, T e is the electromagnetic torque of the fan. In general, the electrical dynamics of the converter are very fast, so it can be considered that the electromagnetic power output by the converter is equal to its power command, as shown in the following formula:

[0019]

[0020] Furthermore, the wind turbine uses an MPPT controller to give a power command of:

[0021]

[0022] In the formula, K opt =0.5C pmax πρr 5 / λ opt 3 , C pmax is the maximum wind energy utilization coefficient, λ opt It is the optimal tip speed ratio of the fan.

[0023] Furthermore, when the fan is in a steady state, it can be considered that ΔP e0 =ΔP m0 =3K opt ω r0 3 , and for MPPT control, there is always C in steady state p0 ′=0, combining the above formula and linearizing it, and then performing Laplace transform, the transfer function of the electromagnetic power and wind speed of the wind turbine can be obtained as follows:

[0024]

[0025] Furthermore, it can be seen from the above formula that the wind turbine system controlled by MPPT can be equivalent to a first-order low-pass filter for wind speed fluctuations, and its cut-off frequency is related to the moment of inertia of the wind turbine, the characteristic parameters of the wind turbine and the steady-state speed.

[0026] Furthermore, the expression of the second-order filter is:

[0027]

[0028] Among them, a, b, and c are filter parameters. The second-order filter adopts a two-pole-one-zero architecture. Compared with the first-order low-pass filter, this architecture enhances the ability to smooth high-frequency power fluctuations while not weakening the system's ability to smooth low-frequency power fluctuations. The second-order filter has three turning frequencies, corresponding to ω1=a, ω2=ab / c, and ω3=b. The configuration principles of the filter parameters a, b, and c are as follows:

[0029] 1) To fully demonstrate the differentiation of SOF, the turning frequency ω1<<ω3. The value of ω1 is small to attenuate the power fluctuation in the low frequency band. However, too small ω1 will cause the overall output power amplitude to be excessively attenuated, resulting in reduced benefits of wind turbines, so a trade-off needs to be made based on the wind energy capture efficiency.

[0030] 2) ω3 is responsible for filtering out high-frequency harmonics, so the value of ω3 should not be too large. Assuming that the power slope change rate of the electrolytic cell is 0.1 pu / s, the cutoff frequency ω3 can be set to 0.1×2π≈0.628 rad / s.

[0031] 3) ω2 needs to be between ω1 and ω3. It is worth noting that ω2 needs to maintain a certain bandwidth with ω3. When c increases, the contribution of the slope c·s of the numerator to the amplitude-frequency characteristic also increases, which will cause the gain of the entire system to increase in the mid- and high-frequency bands.

[0032] Furthermore, after introducing the second-order filter, the transfer function of the system becomes:

[0033]

[0034] Compared with MPPT control, the transfer function of the second-order filter can significantly attenuate high-frequency wind speed disturbances and maintain the gain in the low-frequency band unchanged. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is the power smoothing control block diagram of the machine-side converter;

[0036] Figure 2 is the amplitude-frequency characteristic of the second-order filter SOF(s) under different parameters;

[0037] Figure 3is the amplitude-frequency characteristic of the ratio of electromagnetic power to wind speed Gf(s) under different SOF parameters;

[0038] Figure 4 It is a model of hydrogen production system of offshore wind power on isolated island without energy storage;

[0039] Figure 5 It is the experimental simulation of actual wind speed;

[0040] Figure 6 It is a comparison of electromagnetic power of wind turbines with different control strategies at actual wind speed;

[0041] Figure 7 It is a comparison of the rotor speeds of different control strategies under actual wind speed;

[0042] Figure 8 It is the comparison of DC bus voltage under different control strategies at actual wind speed; DETAILED DESCRIPTION

[0043] The present invention is described in detail below in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention and implement it, but this is not intended to limit the scope of protection of the present application.

[0044] Air flow can generate wind energy. According to Betz theory, wind turbine blades can capture part of the wind power, as shown in equations (1) and (2):

[0045]

[0046] Where P m is the mechanical power of the wind wheel; ρ is the air density, S = πR 2 is the swept area of ​​the wind rotor blade; R is the blade radius; v is the wind speed; C p (λ,β) is the wind energy utilization coefficient; λ is the tip speed ratio; β is the pitch angle; ω r is the rotor speed.

[0047] Furthermore, assuming that the transmission system is a rigid system and ignoring friction, the mechanical dynamic characteristics of the permanent magnet direct-drive wind turbine system can be expressed as:

[0048]

[0049] J is the total equivalent moment of inertia of the wind turbine, transmission system and generator; T m is the mechanical torque of the fan, T e is the electromagnetic torque of the fan. In general, the electrical dynamics of the converter are very fast, so it can be considered that the electromagnetic power output by the converter is equal to its power command:

[0050]

[0051] Furthermore, the wind turbine uses an MPPT controller to give a power command of:

[0052]

[0053] In the formula, K opt =0.5C pmax πρr 5 / λ opt 3 , C pmax is the maximum wind energy utilization coefficient, λ opt It is the optimal tip speed ratio of the fan.

[0054] Furthermore, after small signal linearization, equation (3) can be obtained:

[0055]

[0056] ΔP m is the mechanical power increment output by the wind turbine; Δω r is the mechanical speed increment of the system; ΔP e is the electromagnetic power increment output by the fan; the subscript 0 indicates the steady-state value, the same below;

[0057] Furthermore, from equations (1) and (2), we can know that the mechanical power of the fan P m is the rotor speed ω r The function of wind speed v. After linearizing equation (1), we can get:

[0058]

[0059] Where ΔP mb =ΔP m / P m0 ; Δω rb =Δω r / ω r0 ; Δv b =Δv / v0;λ0=rω r0 / v0;C p0 =C p (λ0); C p0 ′=dC p (λ) / dλ| λ0 ;

[0060] Furthermore, by linearizing equation (5), we can obtain:

[0061]

[0062] Furthermore, in steady state, it can be considered that ΔP e0 =ΔP m0 =3K opt ωr0 3 , and for MPPT control, there is always C in steady state p0 '=0, combining equations (6), (7) and (8), and performing Laplace transform, the transfer function of the electromagnetic power of the wind turbine and the wind speed can be obtained as follows:

[0063]

[0064] Furthermore, it can be seen from formula (9) that the wind turbine system controlled by MPPT can be equivalent to a first-order low-pass filter for wind speed fluctuations, and its cut-off frequency is related to the moment of inertia of the wind turbine, the characteristic parameters of the wind turbine and the steady-state speed.

[0065] Analysis shows that the power-wind speed characteristics of the traditional MPPT control wind turbine are completely dependent on its own parameters and cannot adapt to the complex and changing wind speed environment. Therefore, it is necessary to introduce a new control method to smooth the output power of the wind turbine.

[0066] The present invention proposes a rotor kinetic energy power smoothing strategy based on an improved second-order filter, such as Figure 1 shown.

[0067] The expression of the second-order filter is:

[0068]

[0069] Among them, a, b, and c are filter parameters. The second-order filter adopts a two-pole-one-zero architecture. Compared with the first-order low-pass filter, this architecture enhances the ability to smooth high-frequency power fluctuations while not weakening the system's ability to smooth low-frequency power fluctuations. Its Bode plot amplitude-frequency characteristics are as follows: Figure 2 As shown. It can be seen that the second-order filter has three corner frequencies, corresponding to ω1=a, ω2=ab / c, ω3=b. The configuration principles of the filter parameters a, b, c are as follows:

[0070] 1) To fully demonstrate the differentiation of SOF, the turning frequency ω1<<ω3. The value of ω1 is small to attenuate the power fluctuation in the low frequency band. However, too small ω1 will cause the overall output power amplitude to be excessively attenuated, resulting in reduced benefits of wind turbines, so a trade-off needs to be made based on the wind energy capture efficiency.

[0071] 2) ω3 is responsible for filtering out high-frequency harmonics, so the value of ω3 should not be too large. Assuming that the power slope change rate of the electrolytic cell is 0.1 pu / s, the cutoff frequency ω3 can be set to 0.1×2π≈0.628 rad / s.

[0072] 3) ω2 needs to be between ω1 and ω3. It is worth noting that ω2 needs to maintain a certain bandwidth with ω3. When c increases, the contribution of the slope c·s of the numerator to the amplitude-frequency characteristic also increases, which will cause the gain of the entire system to increase in the mid- and high-frequency bands.

[0073] Furthermore, after introducing the second-order filter, equation (6) becomes:

[0074]

[0075] Furthermore, ΔP f is the reference power obtained after the wind turbine MPPT power is filtered by a second-order filter, and its small signal disturbance expression is:

[0076]

[0077] Furthermore, by combining equations (9), (11), and (12) and performing Laplace transform, the transfer function of the system after the filter is introduced is:

[0078]

[0079] The amplitude-frequency characteristics of this formula are as follows Figure 3 As shown, it can be seen that compared with MPPT control, the transfer function of the second-order filter can significantly attenuate high-frequency wind speed disturbances and maintain the low-frequency gain unchanged.

[0080] To further illustrate the effects of the embodiments of the present invention, the following experiment was conducted.

[0081] The present invention verifies the performance of the proposed control scheme through Matlab / Simulink and builds Figure 4 The simulation model shown. Six 2MW permanent magnet direct-drive wind turbines are connected in parallel at the AC end and connected to the offshore centralized electrolyzer hydrogen production system via cables. The hydrogen production system consists of three alkaline electrolyzers with a rated power of 4MW; the electrolyzer power distribution method adopts an average distribution strategy, and the efficiency can be maximized when the system power is high; the power change rate is limited to 0.1pu / s; the filter parameter values ​​are set to a=0.06, b=0.628, c=0.3.

[0082] The waveform of wind speed is as follows Figure 5 As shown, the electrolyzers in this experiment are equipped with a power compensation controller by default.

[0083] according to Figure 6 It can be seen that the use of power smoothing control can effectively filter out the high-frequency fluctuations of power, making the power trend smoother.

[0084] according to Figure 7Since power smoothing control essentially uses rotor kinetic energy to compensate for energy shortage, the fluctuation range of its rotor speed is larger than that of MPPT control.

[0085] according to Figure 8 The fan output power is well matched with the power consumption of the electrolyzer under limited operation. Compared with MPPT control, the fluctuation amplitude of the DC bus voltage is reduced, and there will be no large fluctuation when the wind speed changes greatly.

[0086] It can be seen from the experimental results that the proposed power smoothing controller can well balance the power of the fan and the electrolyzer and reduce the DC bus voltage fluctuation.

Claims

1. A power smoothing control method for an offshore wind power hydrogen production system without energy storage, characterized in that: The method comprises the following steps: S1, based on the fitting function of the optimal wind turbine power and rotor speed and the wind turbine dynamic equation, establish the transfer function of the wind turbine electromagnetic power and wind speed under the maximum power tracking state; S2, introducing an improved second-order filter to smoothly control the wind turbine power, wherein the second-order filter includes a two-pole-one-zero structure, and establishing a transfer function between the electromagnetic power and wind speed of the wind turbine after the filter is introduced; S3, by adjusting the second-order filter parameters a, b, c to suppress high-frequency power fluctuations and maintain the response of low-frequency power fluctuations, where the turning frequency ω1 is less than ω3, the value of ω3 is set according to the electrolyzer power difference slope limit, and ω2 is between ω1 and ω3, satisfying the trade-off between power fluctuation smoothing and wind energy capture efficiency; S4, through the second-order filter output smoothed fan power reference value, so that the fan power change and the electrolyzer power change rate limit adaptation, significantly reducing the fluctuation of the DC bus voltage.

2. The fan power smoothing control method according to claim 1, characterized in that: In step S3, the amplitude-frequency characteristic of the second-order filter is adjusted by adjusting parameters a, b, and c to ensure good suppression effect on power disturbances above the set cutoff frequency ω3 and maintain the gain in the low-frequency band; ω3 is responsible for filtering out high-frequency harmonics, so the value of ω3 should not be too large; assuming that the power slope change rate of the electrolytic cell is 0.1pu / s, the cutoff frequency ω3 can be set to 0.1×2π≈0.628rad / s.

3. The wind turbine power smoothing control method according to claim 1, characterized in that: In step S3, in order to fully demonstrate the discrimination of SOF, the turning frequency ω1<<ω3; the value of ω1 is small to attenuate the power fluctuation in the low frequency band; but too small ω1 will cause the overall amplitude of the output power to be excessively attenuated, resulting in a reduction in the profit of the wind turbine, so it is necessary to make a trade-off based on the wind energy capture efficiency.

4. The wind turbine power smoothing control method according to claim 1, characterized in that: In step S3, ω2 needs to be between ω1 and ω3, and ω2 needs to maintain a certain bandwidth with ω3. When the parameter c increases, the contribution of the slope c·s of the numerator to the amplitude-frequency characteristic also increases, which will cause the gain of the entire system to increase in the mid- and high-frequency bands.

5. The wind turbine power smoothing control method according to any one of claims 1 to 4, characterized in that: The frequency response characteristics of the second-order filter can ensure effective application in electrolyzer systems with limited power change rate.

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