Maximum power point tracking control method for ocean current power generation system and ocean current power generation system

Abstract

The invention discloses an ocean current power generation system maximum power point tracking control method and an ocean current power generation system, and the method comprises the steps: building horizontal axis ocean current power generation system models which comprise an ocean current turbine model, a permanent magnet synchronous motor model, a machine side converter model and a grid side converter model; the ocean current turbine model drives the permanent magnet synchronous motor model to generate power, the machine side converter model is connected to the generator side of the permanent magnet synchronous motor model and used for controlling the rotating speed of the ocean current turbine model, and the grid side converter model is used for transmitting and outputting electric energy generated by the permanent magnet synchronous motor model to a power grid; the rotating speed of the ocean current turbine is measured in real time, and the optimal torque is calculated; the optimal torque is approximate to the reference value of the electromagnetic torque, and the reference value isqr of the stator q-axis current is obtained; the reference value isqr of the stator q-axis current is compared with the actual value isq of the stator q-axis current, Vsq is obtained through superposition of a PI controller and a compensation value, the Vsq acts on the ocean current turbine, and the electromagnetic torque tracks the reference value of the ocean current turbine.

Description

Technical Field

[0001] The present invention relates to the technical field of ocean current power generation, and in particular to a maximum power point tracking control method for an ocean current power generation system and an ocean current power generation system. Background Art

[0002] With the rapid development of human society, fossil fuel consumption has aggravated global environmental pollution. Ocean current energy has attracted much attention due to its high power density, strong predictability and abundant reserves. Coastal countries have rich ocean current energy resources and high energy consumption. The development of ocean current energy is of great significance to reducing environmental pollution and fossil fuel consumption. In the development of ocean current energy, horizontal axis ocean current generators are more suitable for building large megawatt systems than vertical axis ocean current generators. This is crucial to promoting the development of ocean current energy and the transition to clean energy.

[0003] The ocean current energy reserves are huge, and the ocean current speed changes with time. Extracting ocean current energy with maximum efficiency under time-varying ocean current speed is one of the key technologies to reduce the operating cost of ocean current power generation systems. In order to maximize the conversion of ocean current energy into electrical energy, the maximum power point tracking (MPPT) control strategy is usually used, but the existing maximum power point tracking (MPPT) control strategy has the following defects:

[0004] (1) When the doubly fed induction generator (DFIG) is used as the target, the DFIG needs to be connected to the turbine through a gearbox, but the gearbox has a high failure rate, which leads to high operating costs. In addition, the DFIG is not suitable for the marine environment.

[0005] (2) The existing maximum power point tracking control strategy has little effect on improving the efficiency of the ocean current energy generation system (OCEGS). Summary of the invention

[0006] Purpose of the invention: In order to solve the defects of the existing maximum power point tracking control strategy, the present invention proposes a maximum power point tracking control method for an ocean current power generation system and an ocean current power generation system, which can greatly reduce the operating cost while maximizing the conversion of ocean current energy into electrical energy.

[0007] Technical solution: A maximum power point tracking control method for an ocean current power generation system, comprising the following steps:

[0008] Build a horizontal axis ocean current power generation system model, which includes: an ocean current turbine model, a permanent magnet synchronous motor model, a machine-side converter model and a grid-side converter model; the ocean current turbine model drives the permanent magnet synchronous motor model to generate electricity, the machine-side converter model is connected to the generator side of the permanent magnet synchronous motor model, and is used to control the rotation speed of the ocean current turbine model, and the grid-side converter model is used to transfer the electric energy generated by the permanent magnet synchronous motor model to the power grid;

[0009] Real-time measurement of the speed of ocean current turbines to calculate the optimal torque;

[0010] The optimal torque is approximately equal to the reference value of the electromagnetic torque, and the reference value i of the stator q-axis current is obtained. sqr ;

[0011] The reference value of the stator q-axis current i sqr The actual value of the stator q-axis current i sq Compare and superimpose the PI controller and compensation value to get V sq , V sq Acting on the ocean current turbine, the electromagnetic torque is made to track its reference value.

[0012] Furthermore, the real-time measurement of the rotation speed of the ocean current turbine and the calculation of the optimal torque are specifically as follows:

[0013] According to the following formula, the optimal torque is calculated:

[0014]

[0015] in,

[0016] In the formula, ρ w represents the density of seawater, R t represents the turbine blade radius, C pmax represents the maximum power factor, λ opt represents the optimal tip speed ratio, ω t represents the speed of the ocean current turbine at time t.

[0017] Furthermore, according to the following formula, the reference value i of the stator q-axis current is obtained: sqr :

[0018] T g =ni sq K

[0019] Where n represents the number of generator pole pairs and K represents the magnetic flux.

[0020] Furthermore, the reference value i of the stator q-axis current is sqr The actual value of the stator q-axis current i sqCompare and superimpose the PI controller and compensation value to get V sq , V sq Acting on the ocean current turbine to make the electromagnetic torque track its reference value, the specific operations include:

[0021] Get the information of ocean current turbines sd and i sq , the stator d-axis current i sd and the reference value i of the stator d-axis current sdr Compare and get result 1; the stator q axis current i sq and the reference value i of the stator q-axis current sqr Compare and get result 2;

[0022] Input result 1 into the PI controller to obtain result 3; add result 3 and compensation 1 to obtain the stator d-axis voltage V sd , the stator d-axis voltage V sd Acting on ocean current turbines;

[0023] Input result 2 into the PI controller to obtain result 4; add result 4 and compensation 2 to obtain the stator q-axis voltage V sq , the stator q-axis voltage V sq Acting on ocean current turbines.

[0024] Furthermore, the machine-side converter model and the grid-side converter model are interconnected via a DC bus.

[0025] Furthermore, a DC capacitor is provided between the generator-side converter model and the grid-side converter model.

[0026] The present invention discloses an ocean current power generation system based on a maximum power point tracking control strategy of optimal torque, comprising: an ocean current turbine, a permanent magnet synchronous motor, a machine-side converter and a grid-side converter; the ocean current turbine drives the permanent magnet synchronous motor to generate electricity, the machine-side converter is connected to the generator side of the permanent magnet synchronous motor and is used to control the rotation speed of the ocean current turbine, and the grid-side converter is used to transfer the electric energy generated by the permanent magnet synchronous motor to the power grid;

[0027] Among them, the machine-side converter adopts a maximum power point tracking control method of an ocean current power generation system as described in claims 1 to 6 to control the rotation speed of the ocean current turbine.

[0028] Furthermore, the grid-side converter adopts double closed-loop control to transfer the electric energy generated by the permanent magnet synchronous motor to the grid.

[0029] Furthermore, the machine-side converter and the grid-side converter are interconnected via a DC bus.

[0030] Furthermore, a DC capacitor is provided between the machine-side converter and the grid-side converter.

[0031] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0032] (1) The method of the present invention designs a maximum power point tracking control method for an ocean current power generation system based on an optimal torque method, so as to improve the efficiency of the horizontal axis ocean current power generation system in capturing ocean current energy under time-varying ocean current speeds, while ensuring the stability of the DC bus voltage and realizing the active and reactive decoupling control of the machine side and the grid side. The method of the present invention designs a voltage and current dual closed-loop control system; the method of the present invention is simulated and tested by Matlab / Simulink, and the simulation test results show the high efficiency of the method of the present invention and the ability to stabilize the DC bus voltage when facing time-varying ocean current speeds, thereby verifying its effectiveness and feasibility;

[0033] (2) When selecting a horizontal axis ocean current generator, both a permanent magnet synchronous generator (PMSG) and a doubly fed induction generator (DFIG) are options. They are both controlled by a converter, have the characteristics of variable speed operation and efficient power generation, and can be connected to the power grid, but the permanent magnet synchronous generator can be directly connected to the turbine, which is more suitable for the marine environment and reduces maintenance costs; therefore, the method of the present invention takes the horizontal axis ocean current power generation system based on the permanent magnet synchronous generator as the research object, and through the designed maximum power point tracking, the efficiency of the ocean current power generation system can be greatly improved;

[0034] (3) The method of the present invention has the advantages of high efficiency and no need for a current velocity meter. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of a horizontal axis ocean current power generation system;

[0036] Figure 2 This is the MPPT control block diagram of the tidal generator;

[0037] Figure 3 This is the control block diagram of the grid-side converter;

[0038] Figure 4 is the simulation test result; among them, Figure 4 (a) in the equation is the time-varying ocean current velocity, Figure 4 (b) in the figure is the turbine speed error;

[0039] Figure 5 is the simulation test result; among them, Figure 5 (a) is the power coefficient, Figure 5(b) in the figure is the DC bus voltage. DETAILED DESCRIPTION

[0040] The technical solution of this embodiment is now further described in conjunction with the accompanying drawings and embodiments.

[0041] Example:

[0042] This embodiment proposes a maximum power point tracking control method for an ocean current power generation system, which mainly includes the following steps:

[0043] Step 1: Construct a horizontal axis ocean current power generation system model, such as Figure 1 As shown, the horizontal axis ocean current power generation system model includes: an ocean current turbine model, a permanent magnet synchronous motor model, a machine-side converter model and a grid-side converter model.

[0044] In the horizontal axis ocean current power generation system, the ocean current turbine relies on the impact force of the ocean current to make the turbine rotate, and then converts it into a high speed to drive the permanent magnet synchronous motor to generate electricity. The machine-side converter is connected to the generator side of the permanent magnet synchronous motor and is mainly used to control the speed of the ocean current turbine to achieve maximum power point tracking, thereby maximizing the capture of wave energy.

[0045] The main control objectives of the grid-side converter are to maintain the DC bus voltage stability, control the AC input power factor to unity, and suppress the grid input harmonic current. Figure 3 As shown, this embodiment adopts dual closed-loop control, namely a DC voltage outer loop and a power inner loop, to achieve inverter grid-connected control.

[0046] The grid-side converter efficiently transfers the power generated by the generator to the grid by controlling the stability of the DC bus voltage. It is also responsible for the interface with the grid to ensure the quality and stable supply of power.

[0047] The generator-side converter and the grid-side converter are interconnected through the DC bus and work together to achieve smooth access to the grid. The generator-side converter controls the speed of the ocean current turbine to capture maximum energy, while the grid-side converter controls the DC bus voltage to ensure efficient output of power. Each model is now further explained.

[0048] The ocean current energy power extracted by the ocean current turbine model is expressed as:

[0049]

[0050] In the formula, ρ w represents the seawater density, β represents the propeller pitch angle, R t represents the turbine blade radius, C p Indicates the power factor, V c represents the ocean current speed, let λ t =ω t Rt / V c represents the tip speed ratio, ω t Indicates the turbine speed.

[0051] Power coefficient C p is β and λ t Function:

[0052]

[0053] Among them, λ i -1 =(λ t +0.08β) -1 -0.035(1+β 3 ) -1

[0054] The expression of the permanent magnet synchronous motor model in the dq coordinate system is:

[0055]

[0056] In the formula, i sd and i sq Represent the stator d-axis and q-axis currents respectively, R s represents the stator resistance, V sd and V sq represent the stator d-axis and q-axis voltages respectively, T t represents mechanical torque, J represents moment of inertia, L sd and L sq They represent the stator d-axis and q-axis inductances, respectively, and T g represents electromagnetic torque, n represents the number of generator pole pairs, and K represents magnetic flux.

[0057] The expression of the grid-side converter model in the dq coordinate system is:

[0058]

[0059] In the formula, i gd 、i gq represents the d-axis and q-axis currents of the grid-side converter, R g Represents resistance, L g Indicates inductance, V gd 、V gq represents the d-axis and q-axis voltages of the grid-side converter, E gd 、E gq Represents the d-axis and q-axis voltage of the power grid, V dc Indicates the DC bus voltage, i dc2 represents DC current, and C represents DC capacitance.

[0060] In this embodiment, the machine-side converter model adopts a maximum power point tracking control strategy based on optimal torque to control the torque of the ocean current turbine model to achieve maximum power output. By adjusting the torque of the ocean current turbine model in real time, the model is always operated near the maximum power point, thereby maximizing the energy capture efficiency and realizing the extraction of maximum ocean current energy under time-varying ocean current speed.

[0061] Now combined Figure 2 The maximum power point tracking control strategy based on optimal torque proposed in this embodiment is further explained.

[0062] Get the output of the ocean current turbine model sd and i sq , the stator d-axis current i sd and the reference value i of the stator d-axis current sdr Compare and get result 1; the stator q axis current i sq and the reference value i of the stator q-axis current sqr Compare and get result 2;

[0063] Input result 1 into the PI controller to obtain result 3; add result 3 and compensation 1 to obtain the stator d-axis voltage V sd , the stator d-axis voltage V sd Acting on the ocean current turbine model;

[0064] Input result 2 into the PI controller to obtain result 4; add result 4 and compensation 2 to obtain the stator q-axis voltage V sq , the stator q-axis voltage V sq Acting on the ocean current turbine model;

[0065] The torque corresponding to the maximum power point of the ocean current turbine model under different ocean current velocities is calculated, which is the optimal torque and is expressed as:

[0066]

[0067] in,

[0068] In the formula, ρ w represents the density of seawater, R t represents the turbine blade radius, C pmax Indicates the maximum power factor. In this embodiment, C pmax =0.411,λ opt represents the optimal tip speed ratio. In this embodiment, λ opt =7.95. The mechanical response speed of the ocean current turbine is much slower than the internal electromagnetic response speed of the permanent magnet synchronous motor, so T g ≈T topt ωt represents the speed of the ocean current turbine at time t.

[0069] In the above control, the reference value i of the stator q-axis current is sqr , expressed as:

[0070]

[0071] In actual operation, the optimal torque is calculated by measuring the turbine speed in real time. The optimal torque is close to the reference value of the electromagnetic torque, and the reference value of the stator q-axis current i is obtained. sqr ; Set the reference value of the stator q-axis current i sqr The actual value of the stator q-axis current i sq Compare and superimpose the PI controller and compensation value to get V sq , V sq Acting on the ocean current turbine, the electromagnetic torque is made to track its reference value, thereby achieving maximum power tracking.

[0072] Now we will use Matlab / Simulink to conduct simulation tests to verify its effectiveness and rationality. The rated power of the ocean current power generation system is 1.5MW, the radius is 10m, and the rated flow velocity is 2.83m / s. Figure 4 It can be seen that when the ocean current speed changes with time, the maximum error of the rotation speed can be controlled within 20%. This is because the MPPT control based on the optimal torque method indirectly controls the torque to track the optimal torque, which requires a certain response time. The density of seawater is more than 800 times that of air, and the dynamic response of large-capacity ocean current turbines is slow. Therefore, the optimal torque method indirectly controls the turbine speed and is more in line with the slow dynamic response characteristics of ocean current turbines. Under the maximum power point tracking control of the ocean current power generation system, the ocean current energy efficiently captured by the ocean current turbine will be converted into mechanical energy, and then converted into electrical energy through the transmission system and generator.

[0073] like Figure 1 As shown, for the grid-side converter, its main goal is to maintain the DC bus voltage at a rated value and transmit power to the grid. In this embodiment, a DC capacitor is provided between the machine-side converter and the grid-side converter, which can not only reduce the high-frequency ripple of the DC bus voltage, but also realize the decoupling control between the machine-side converter and the grid-side converter when the DC voltage is kept near the rated value, thereby improving the fault ride-through capability of the overall system. Figure 5 (a) in the figure shows the power coefficient curve, from which we can know that the ocean current generator can efficiently extract ocean current energy from the time-varying ocean current. Figure 5 It can be seen from (b) that under the control of the grid-side converter, the DC bus voltage can be maintained near its reference value, realizing decoupling control between the grid side and the generator side.

Claims

1. A maximum power point tracking control method for an ocean current power generation system, characterized in that: The following steps are involved: Build a horizontal axis ocean current power generation system model, which includes: an ocean current turbine model, a permanent magnet synchronous motor model, a machine-side converter model and a grid-side converter model; the ocean current turbine model drives the permanent magnet synchronous motor model to generate electricity, the machine-side converter model is connected to the generator side of the permanent magnet synchronous motor model to control the rotation speed of the ocean current turbine model, and the grid-side converter model is used to transfer the electric energy generated by the permanent magnet synchronous motor model to the power grid; Real-time measurement of the speed of ocean current turbines to calculate the optimal torque; The optimal torque is approximately equal to the reference value of the electromagnetic torque, and the reference value i of the stator q-axis current is obtained. sqr ; The reference value of the stator q-axis current i sqr The actual value of the stator q-axis current i sq Compare and superimpose the PI controller and compensation value to get V sq , V sq Acting on the ocean current turbine, the electromagnetic torque is made to track its reference value.

2. A maximum power point tracking control method for an ocean current power generation system according to claim 1, characterized in that: The real-time measurement of the rotation speed of the ocean current turbine and calculation of the optimal torque are specifically as follows: The optimal torque is calculated according to the following formula: in, In the formula, ρ w represents the density of seawater, R t represents the turbine blade radius, C pmax represents the maximum power factor, λ opt represents the optimal tip speed ratio, ω t represents the speed of the ocean current turbine at time t.

3. The maximum power point tracking control method of an ocean current power generation system according to claim 1, characterized in that: According to the following formula, the reference value i of the stator q-axis current is obtained: sqr : T g =ni sq K Where n represents the number of generator pole pairs and K represents the magnetic flux.

4. The maximum power point tracking control method of an ocean current power generation system according to claim 1, characterized in that: The reference value i of the stator q-axis current is sqr The actual value of the stator q-axis current i sq Compare and superimpose the PI controller and compensation value to get V sq , V sq Acting on the ocean current turbine to make the electromagnetic torque track its reference value, the specific operations include: Get the information of ocean current turbines sd and i sq , the stator d-axis current i sd and the reference value i of the stator d-axis current sdr Compare and get result 1; the stator q axis current i sq and the reference value i of the stator q-axis current sqr Compare and get result 2; Input result 1 into the PI controller to obtain result 3; add result 3 and compensation 1 to obtain the stator d-axis voltage V sd , the stator d-axis voltage V sd Acting on ocean current turbines; Input result 2 into the PI controller to obtain result 4; add result 4 and compensation 2 to obtain the stator q-axis voltage V sq , the stator q-axis voltage V sq Acting on ocean current turbines.

5. The maximum power point tracking control method of an ocean current power generation system according to claim 1, characterized in that: The machine-side converter model and the grid-side converter model are interconnected via a DC bus.

6. A maximum power point tracking control method for an ocean current power generation system according to claim 5, characterized in that: A DC capacitor is arranged between the machine-side converter model and the grid-side converter model.

7. A sea current power generation system based on a maximum power point tracking control strategy of optimal torque, characterized in that: include: A sea current turbine, a permanent magnet synchronous motor, a machine-side converter and a grid-side converter; the sea current turbine drives the permanent magnet synchronous motor to generate electricity, the machine-side converter is connected to the generator side of the permanent magnet synchronous motor and is used to control the rotation speed of the sea current turbine, and the grid-side converter is used to transfer the electric energy generated by the permanent magnet synchronous motor to the power grid; Among them, the machine-side converter adopts a maximum power point tracking control method of an ocean current power generation system as described in claims 1 to 6 to control the rotation speed of the ocean current turbine.

8. The ocean current power generation system based on the maximum power point tracking control strategy of optimal torque according to claim 7, characterized in that: The grid-side converter adopts double closed-loop control to transfer the electric energy generated by the permanent magnet synchronous motor to the grid.

9. The ocean current power generation system based on the maximum power point tracking control strategy of optimal torque according to claim 7, characterized in that: The machine-side converter and the grid-side converter are interconnected via a DC bus.

10. The ocean current power generation system based on the maximum power point tracking control strategy of optimal torque according to claim 9, characterized in that: A DC capacitor is arranged between the machine-side converter and the grid-side converter.

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