A Stable Control Analysis Method for a Direct-Drive Permanent-Magnet Wind Turbine Generator Set Connected to the Grid via VSC-HVDC

Abstract

The present invention belongs to the field of new energy power generation grid connection stability control, and relates to a grid connection stability control analysis method for a direct-drive permanent magnet wind turbine through VSC-HVDC. It solves the stability problems caused by the interaction between the direct-drive permanent magnet wind power system and VSC-HVDC; through the grid connection stability control analysis method for a direct-drive permanent magnet wind turbine with virtual inertia control, an interface dynamic model of the wind turbine through DC power transmission is established. Using the eigenvalue method, it is found that there is a low-frequency oscillation mode in this system, and the grid connection operation stability can be improved by increasing the virtual inertia time constant and the damping coefficient.

Description

Technical Field

[0001] The present invention belongs to the field of new energy power generation grid connection stability control, and relates to a method for analyzing the grid connection stability control of a direct-drive permanent magnet wind turbine via VSC-HVDC. Background Art

[0002] Since the 21st century, a large number of renewable energy sources such as photovoltaic and wind power have been connected to the grid. Especially with the increasing penetration rate of wind power, the operating characteristics of the grid have become more complex. The new type of subsynchronous oscillation caused by the interaction between wind power and the grid, that is, the subsynchronous control interaction (SSCI), seriously affects the safe and stable operation of the grid. On the other hand, the transmission of wind farms via high-voltage direct current (HVDC) transmission systems has become the main transmission method for large-scale wind power bases. Especially, the voltage source converter-based HVDC (VSC-HVDC) transmission technology has become an ideal solution for the grid connection and transmission of wind farms due to its fast and flexible control. The interaction between the wind farm controller and the VSC-HVDC controller is intertwined, making the interaction mechanism between the wind farm and VSC-HVDC more complex, and there is a problem of SSCI caused by the interaction between the wind farm and VSC-HVDC.

[0003] At present, there are few literatures studying the multi-band oscillation (MBO) problem of the grid connection and transmission model of direct-drive permanent magnet synchronous generators (D-PMSG) via VSC-HVDC. For the traditional D-PMSG grid connection and transmission system via VSC-HVDC, based on the eigenvalue analysis method, it is found that the system has low-frequency, sub / super-synchronous, and high-frequency multi-band oscillation modes. Summary of the Invention

[0004] A method for analyzing the grid connection stability control of a direct-drive permanent magnet wind turbine via VSC-HVDC is proposed. An interface dynamic model of the wind turbine via DC transmission is established. By using the eigenvalue method, the grid connection operation stability can be improved by increasing the virtual inertia time constant and damping coefficient.

[0005] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0006] A method for analyzing the grid connection stability control of a direct-drive permanent magnet wind turbine via VSC-HVDC, characterized by comprising the following steps:

[0007] S1. According to the interface analysis between the D-PMSG and the detailed model of VSC-HVDC, the interface matrix and the interface dynamic equation are derived;

[0008] S2. Establish the state equations of the entire system of D-PMSG connected to the grid and transmitted through VSC-HVDC based on virtual inertia control by using the state space model of the wind power grid-connected system;

[0009] S4. Analyze the eigenvalues obtained from the state equations. It is found through eigenvalue analysis that when the D-PMSG system with virtual inertia control is connected to the grid and transmitted through VSC-HVDC, there is a low-frequency oscillation mode in the system;

[0010] S7. Verify the correctness of the eigenvalue analysis results;

[0011] S5. Improve the grid-connected operation stability by increasing the virtual inertia time constant and damping coefficient.

[0012] The derivation method of the interface matrix is as follows: According to the dynamic models of the D-PMSG grid-side converter and the VSC-HVDC sending-end converter, set the dq rotating coordinate systems with the PCC-2 node voltage and the PCC-3 node voltage as the references respectively. There are two positive-sequence synchronous rotating dq coordinate systems, namely the d1q1 coordinate system and the d2q2 coordinate system. The angle between the d1 axis and the d2 axis is θ. The transformation between the two dq rotating coordinate systems is:

[0013]

[0014] In the formula, the transformation matrix T is the interface matrix between D-PMSG and VSC-HVDC.

[0015] The interface dynamic equation is:

[0016]

[0017] Y 1DV is the algebraic variable of the entire system; X1 is the interface state variable; A 1G 、A 1V 、B 1GV 、C 1G 、C 1V 、D 1GV are the corresponding coefficient matrices.

[0018] The dynamic equation of the entire system is:

[0019]

[0020] In the formula, X and Y are the state variables and algebraic variables of the entire system respectively, and there are:

[0021]

[0022] The system state matrix is obtained as:

[0023] A = A1 + B1C1

[0024] The eigenvalues of the system state matrix A are the oscillation modes corresponding to the power system.

[0025] The method for verifying the correctness of the eigenvalue analysis results is as follows: The frequency characteristics of the output impedance of the direct-drive permanent magnet wind power transmission system with virtual inertia control are used to verify the correctness of the eigenvalue analysis results.

[0026] The beneficial effects of the present invention are as follows: Aiming at the stability problem caused by the interaction between the direct-drive permanent magnet wind power system and the VSC-HVDC, a virtual inertia control method is adopted, which is based on the MPPT control of the wind turbine and introduces auxiliary power related to the proportional and differential components of the system frequency deviation, enabling the wind turbine to change its output when the system frequency fluctuates. This method can enable the unit to provide effective inertial support for the system. After the direct-drive permanent magnet wind turbine introduces virtual inertia control and is connected to the grid and transmitted through the VSC-HVDC, the grid connection stability of the direct-drive permanent magnet wind turbine transmission system through the VSC-HVDC is improved by appropriately increasing the virtual inertia time constant and damping coefficient. In this paper, the eigenvalue analysis method is used, and according to the frequency characteristics of the output impedance of the direct-drive permanent magnet wind power system, it is concluded that when considering virtual inertia control, the D-PMSG grid-connected transmission system through the VSC-HVDC may only have the risk of unstable operation in the low-frequency band. The traditional transmission system is optimized and controlled to improve the stability of the grid-connected transmission system. Description of the Drawings

[0027] Figure 1 is the flow block diagram of the present invention;

[0028] Figure 2 is the schematic diagram of the virtual inertia control of the wind turbine;

[0029] Figure 3 is the diagram of the direct-drive permanent magnet wind turbine grid-connected transmission system through the VSC-HVDC;

[0030] Figure 4 is the diagram of the relationship between the d1q1 coordinate system and the d2q2 coordinate system; Detailed Embodiments

[0031] The technical solution of the present invention will be further described below in conjunction with the drawings and through specific embodiments:

[0032] Embodiment 1

[0033] In the power system, the imbalance between the load and the power generation will cause changes in the grid frequency, and its principle can be simply described by the following equation:

[0034]

[0035] Where H is the system inertia time constant; Δω is the system angular frequency offset, Δω = ω - 1, where ω is the system angular frequency; ΔP is the system unbalanced power, ΔP = ΔP m -ΔP n , ΔP m and ΔP n are the fluctuations in the output of the system prime mover and the load power respectively; D is the damping coefficient. It can be seen from Equation (1) that the larger H is, the slower the system frequency changes under the same unbalanced power disturbance, and the more stable the system is.

[0036] On the basis of the MPPT control of the wind turbine, an auxiliary power P related to the proportional and differential components of the system frequency deviation is introduced v , so that the wind turbine can simulate the primary frequency regulation characteristics and inertial response characteristics of the traditional synchronous generator set under the virtual inertia control mode where the output power changes when the system frequency fluctuates. The principle is as Figure 2 shown. Figure 2 Among them, P m and are the mechanical power value of the prime mover and its reference value respectively, P L is the system load power, P w is the grid-connected power of the D-PMSG, and its reference value is the sum of the MPPT control output power P MPPT and the virtual inertia control auxiliary power P v , and the expression of P v is:

[0037]

[0038] In the formula, k p , k d are the proportional and differential control coefficients respectively.

[0039] After introducing the virtual inertia control, the frequency response equation of the system changes from Equation (1) to:

[0040]

[0041] Combined with Equation (2), we have:

[0042]

[0043] By comparing Equation (1) and Equation (4), it can be seen that the PD virtual inertia control improves the frequency stability of the system from both the system inertia and damping aspects.

[0044] When the direct-drive permanent magnet wind power system with virtual inertia control is connected to the grid and transmitted through the VSC-HVDC technology, there may be stability problems. This patent proposes an eigenvalue analysis method based on the dynamic model of the whole system for the stability problems generated when the direct-drive permanent magnet wind power system with virtual inertia control is transmitted through DC. AsFigure 3 Figure 0 shows the topology of the D-PMSG AC / DC hybrid transmission system via VSC-HVDC. The direct-drive permanent magnet wind turbine realizes MPPT control, virtual inertia control, and PMSG terminal voltage control through the machine-side converter, and controls the DC bus voltage and reactive power output through the grid-side converter. The converters in the VSC-HVDC part all adopt the space vector control strategy based on the grid voltage.

[0045] Among them, the dynamic models of the direct-drive permanent magnet wind power system with virtual inertia control and the VSC-HVDC system can be respectively described as follows:

[0046]

[0047]

[0048] In the formula, X D and Y D are respectively the state variables and algebraic variables of the D-PMSG unit and its corresponding controller; X V and Y V are respectively the state variables and algebraic variables of the VSC-HVDC and its corresponding controller.

[0049] Since the dynamic models of the D-PMSG grid-side converter and the VSC-HVDC sending-end converter are respectively set with the dq rotating coordinate system based on the PCC-2 node voltage and the PCC-3 node voltage, assuming there are two positive-sequence synchronous rotating dq coordinate systems, namely the d1q1 coordinate system and the d2q2 coordinate system, the angle between the d1 axis and the d2 axis is θ, and the transformation between the two rotating coordinate systems is as Figure 4 shown.

[0050] Further derivation gives the transformation between the two dq rotating coordinate systems as:

[0051]

[0052] In the formula, the transformation matrix T is the interface matrix between the D-PMSG and the VSC-HVDC. Transforming the physical quantities in the VSC-HVDC model based on the d1q1 coordinate system to the d2q2 coordinate system, the interface dynamic equation is further obtained as:

[0053]

[0054] In the formula, Y 1DV is the algebraic variable of the whole system; X1 is the interface state variable; A 1G , A 1V , B 1GV , C 1G , C 1V , D 1GV are the corresponding coefficient matrices, and there are

[0055] X1 ∈ (X D ∪ X V )(9)

[0056] X VR = TX VR0 X VR ∈ X V (10)

[0057] In the formula, X VR0 and X VR are the state variables of the VSC-HVDC sending converter under the d1q1 coordinate system and the d2q2 coordinate system respectively. By combining equations (5), (6), (7) and (10), the dynamic model of the whole system is further derived as follows:

[0058]

[0059] In the formula, X and Y are the state variables and algebraic variables of the whole system respectively, and there are:

[0060]

[0061] Eliminating the operating variable Y of the system, the system state matrix is further obtained as:

[0062] A = A1 + B1C1 (13)

[0063] Then the eigenvalues of A are the oscillation modes corresponding to this power system.

[0064] Establish Figure 3 the complete dynamic model of the system shown, and use a 0.69KV / 12MW D-PMSG to represent the direct-drive wind farm. Set the wind speed to 11m / s and the pitch angle β to 0. The d-axis reference current i d,ref of the machine-side controller is set to 0; the reactive power reference value Q s,ref of the grid-side controller is set to 0, and the DC voltage reference value U dc,ref is set to 1.0 (per unit value). The reactive power reference value Q s1,ref of the sending-end controller is set to 0.36 (per unit value), and the DC voltage reference value U dc1,ref is set to 1.0 (per unit value); the active power reference value P s2,ref of the receiving-end controller is set to 0.85 per unit value), and the reactive power reference value Q s2,ref is set to 0.13 (per unit value). The distance of the wind power grid connection, that is, the distance between the lines of PCC-2 and PCC-3, is set to 23km. By analyzing the system eigenvalues, it is found that the direct-drive permanent magnet wind power transmission system with virtual inertia control only has low-frequency oscillation modes.

[0065] The frequency characteristics of the output impedance of a direct-drive permanent magnet wind power transmission system with virtual inertia control are adopted to verify the correctness of the eigenvalue analysis results. Here, the output impedance under DC perturbation is taken as an example for analysis. Assuming that the system is within the power control loop bandwidth, it can be considered that the active power output of the direct-drive permanent magnet wind power transmission system is constant at P0, and the reactive power output is constant at Q0. At this time, the stator voltage vector of the direct-drive permanent magnet wind power transmission system can be expressed as

[0066]

[0067] The Taylor series of two variables is used to linearize Equation (14) and expand it to the second term near the steady-state point, and we can get:

[0068]

[0069] In the formula: F(·) represents the Taylor series expansion operation; represents the steady-state value of the Taylor series expansion at the operating point; represents the Taylor series expansion at the operating point The remainder value. After calculation, the output impedance matrix Z of the direct-drive permanent magnet wind power transmission system based on virtual inertia control out The DC component is

[0070]

[0071] Considering that the steady-state output reactive power is zero, Equation (16) can be simplified to

[0072]

[0073] From Equation (17), it can be seen that the d-d axis component Z of the DC component of the output impedance Z out shows a positive resistance characteristic, while the q-q axis component Z dd shows a negative resistance characteristic, verifying the correctness of the eigenvalue analysis results. The system only has a low-frequency oscillation mode. qq

[0074] Under weak grid conditions, the active loop control parameters T j and D in the virtual inertia control have an important impact on the stability of the direct-drive permanent magnet wind power transmission system. As they increase, the stability margin improves, but when T j is too large, it will deteriorate the dynamic response speed of power control. On the contrary, when D increases, both the dynamic overshoot and response time of the system decrease. Therefore, the grid-connected operation stability can be improved by increasing the virtual inertia time constant and damping coefficient.

[0075] ​For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention. Any reference signs in the claims should not be construed as limiting the claims involved.

[0076] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A stable control analysis method for a direct-drive permanent magnet wind turbine connected to the grid via VSC-HVDC, characterized in that, It includes the following steps: S1. Based on the interface analysis between the D-PMSG and the detailed model of the VSC-HVDC, the interface matrix and the interface dynamic equation are derived; S2. By establishing the state equation of the entire system of the D-PMSG connected to the grid and transmitted through the VSC-HVDC with virtual inertia control based on the state space model of the wind power integration system; S3. The eigenvalues are obtained from the state equation, and it is found through eigenvalue analysis that when the D-PMSG system with virtual inertia control is connected to the grid and transmitted through the VSC-HVDC, there is a low-frequency oscillation mode in the system; S4. Verify the correctness of the eigenvalue analysis results; S5. Improve the grid-connected operation stability by increasing the virtual inertia time constant and the damping coefficient; In the above S1, the derivation method of the interface matrix is as follows: According to the dynamic models of the D-PMSG grid-side converter and the VSC-HVDC sending-end converter, the dq rotating coordinate system is set with the PCC-2 node voltage and the PCC-3 node voltage as the reference. There are two positive-sequence synchronous rotating dq coordinate systems, namely the d1q1 coordinate system and the d2q2 coordinate system. The angle between the d1 axis and the d2 axis is θ. The transformation between the two dq rotating coordinate systems is: In the formula, the transformation matrix T is the interface matrix between the D-PMSG and the VSC-HVDC; The above interface dynamic equation is: Y 1DV is the algebraic variable of the whole system; X1 is the interface state variable; A 1G , A 1V , B 1GV , C 1G , C 1V , D 1GV are the corresponding coefficient matrices; In the above S2, the dynamic equation of the entire system is: In the formula, X and Y are the state variables and algebraic variables of the entire system respectively, and there are: The system state matrix is obtained as: A = A1 + B1C1.

2. The stable control analysis method for grid connection of a direct-drive permanent magnet wind turbine via VSC-HVDC according to claim 1, characterized in that: The characteristic roots of the above system state matrix A are the oscillation modes corresponding to the power system.

3. A stable control analysis method for a direct-drive permanent magnet wind turbine connected to the grid via VSC-HVDC according to claim 1, characterized in that: The method for verifying the correctness of the eigenvalue analysis results is: Use the output impedance frequency characteristics of the direct-drive permanent magnet wind power transmission system with virtual inertia control to verify the correctness of the eigenvalue analysis results.

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