Method for suppressing stator current oscillation during fault ride-through and recovery of doubly-fed wind turbine generator systems

Abstract

The present invention relates to the field of wind turbine control technology, and more particularly to a method for suppressing stator current oscillations during fault ride-through and recovery of a doubly-fed wind turbine. The present invention addresses the fault ride-through and recovery processes of a doubly-fed wind turbine, coordinates the switching of a stator-side fault current limiter, and aims to minimize the amplitude of the stator current oscillation component of the doubly-fed wind turbine. The method solves for current optimization control reference values ​​of the rotor-side converter and the grid-side converter during the fault ride-through and recovery processes, thereby fully suppressing stator current oscillations of the doubly-fed wind turbine during both the fault ride-through and fault recovery processes.

Description

Technical Field

[0001] The present invention relates to the technical field of wind turbine control, and in particular to a method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine. Background Art

[0002] For wind turbines connected to the grid, they need to maintain operation for a period of time in the event of a grid failure to avoid premature shutdown of the wind turbines, thereby providing stable support to the grid during the fault. The wind turbine grid connection guidelines also provide relevant requirements for the fault ride-through capability of wind turbines.

[0003] For doubly-fed wind turbines, whose stators are directly connected to the grid, the primary challenge during fault ride-through is that voltage drops at the turbine terminals can induce significant flux and current oscillations in the stator. Suppressing stator current oscillations during fault ride-through can effectively improve fault ride-through capability. To this end, hardware protection solutions can be introduced to suppress stator current oscillations, such as installing a fault current limiter (FCL) on the stator side of the doubly-fed wind turbine. Furthermore, the converter control strategy of the doubly-fed wind turbine during fault ride-through can also affect its fault ride-through capability. By synergizing hardware protection devices with the converter control strategy during fault ride-through, the fault ride-through capability of the doubly-fed wind turbine can be further enhanced.

[0004] Furthermore, doubly-fed wind turbines are not only affected by the terminal voltage drop at the initial stage of fault ride-through, but also by the terminal voltage rise at the fault recovery stage, which can also cause stator current oscillations. To ensure safety and stability throughout the entire fault ride-through process, fault ride-through measures should be formulated taking into account the fault recovery process. Summary of the Invention

[0005] The present invention aims to address the aforementioned issues by proposing a method for suppressing stator current oscillations during fault ride-through and recovery of a doubly-fed wind turbine. This method addresses the fault ride-through and recovery processes of a doubly-fed wind turbine, coordinates the switching of a stator-side fault current limiter, and aims to minimize the amplitude of the stator current oscillation component of the doubly-fed wind turbine. The method then solves for current optimization control reference values ​​for the rotor-side converter and the grid-side converter during the fault ride-through and recovery processes, thereby fully suppressing stator current oscillations during both the fault ride-through and recovery processes of the doubly-fed wind turbine.

[0006] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0007] A method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system comprises the following steps:

[0008] S1: Detect the stator voltage of the doubly fed wind turbine generator set under the initial normal operating conditions and the stator voltage during the fault ride-through process. At the start of the fault ride-through process, activate the fault current limiter installed on the stator side of the doubly fed wind turbine generator set. Determine the reactive current output of the doubly fed wind turbine generator set during the fault ride-through process in accordance with the wind turbine generator set grid connection guidelines. During the fault ride-through process, the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set switch from the dual-loop PI control under the initial normal operating conditions to direct current control. Calculate the amplitude of the stator current oscillation component of the doubly fed wind turbine generator set during the fault ride-through process.

[0009] S2: Taking minimizing the amplitude of the stator current oscillation component of the DFIG wind turbine during the fault ride-through process as the optimization objective, taking into account the current capacity constraints of the rotor-side converter and the grid-side converter, as well as the reactive current output of the DFIG wind turbine during the fault ride-through process required by the wind turbine grid connection guidelines, the control reference values ​​of the rotor-side converter direct current control and the grid-side converter direct current control are determined;

[0010] S3: When it is detected that the stator voltage of the doubly fed wind turbine generator set has returned to normal operating conditions, the fault current limiter installed on the stator side of the doubly fed wind turbine generator set is cut out, and the doubly fed wind turbine generator set is adjusted to a unity power factor output, with minimizing the amplitude of the stator current oscillation component of the doubly fed wind turbine generator set during the fault recovery process as the optimization goal, taking into account the current capacity constraints of the rotor-side converter and the grid-side converter, and determining the control reference values ​​of the direct current control of the rotor-side converter and the direct current control of the grid-side converter; when it is detected that the stator current vector modulus value changes by less than 5% of its mean value within one power frequency cycle, the output of the outer loop integral link in the dual-loop PI control of the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set under the initial normal operating conditions is adjusted, and the direct current control is restored to the dual-loop PI control under the initial normal operating conditions.

[0011] Further as a preferred technical solution of the present invention, S1 comprises the following steps:

[0012] Detect the stator voltage V of the doubly fed wind turbine under the initial normal operating conditions s,n The stator voltage V during the fault ride-through of the doubly-fed wind turbine s,f At the beginning of the fault ride-through process, the fault current limiter installed on the stator side of the doubly fed wind turbine is put into operation. The impedance of the fault current limiter is Z FCL According to the wind turbine grid connection guidelines, the reactive current output of the doubly fed wind turbine during the fault ride-through process is

[0013]

[0014] Where |V s,n | is the stator voltage modulus value under the initial normal working condition of the doubly fed wind turbine generator set, |V s,f| is the stator voltage modulus during the fault ride-through process of the doubly-fed wind turbine generator set, and pu is the per-unit value;

[0015] The stator flux of the doubly fed wind turbine under the initial normal operating conditions is calculated based on formula (2):

[0016]

[0017] Where ψ s,n is the stator flux under initial normal working conditions, R s is the stator resistance, L s is the stator inductance, L m is the mutual inductance between the stator and the rotor, I r,n is the rotor current under the initial normal working condition, j is the imaginary unit, ω s is the synchronous electrical angular velocity;

[0018] During the fault ride-through process, the direct current control of the rotor-side converter and the grid-side converter of the doubly fed wind turbine are adopted. The steady-state stator flux of the doubly fed wind turbine during the fault ride-through process is calculated based on formula (3):

[0019]

[0020] Where ψ s,f is the steady-state stator flux of the doubly-fed wind turbine during the fault ride-through process, It is the control reference value of the direct current control of the rotor-side converter during the fault ride-through process;

[0021] The amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process|I s,DC,f |Based on formula (4), we can get:

[0022]

[0023] Further as a preferred technical solution of the present invention, S2 comprises the following steps:

[0024] The optimization objective is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process. The optimization objective is expressed as:

[0025] min|I s,DC,f |(5);

[0026] The control reference value of the direct current control of the rotor-side converter must meet the current capacity constraint of the rotor-side converter, that is:

[0027]

[0028] In the formula is the modulus of the control reference value of the direct current control of the rotor-side converter during the fault ride-through process, is the maximum current capacity of the rotor-side converter;

[0029] The control reference value of the direct current control of the rotor-side converter and the grid-side converter must meet the reactive current output of the doubly-fed wind turbine during the fault ride-through process required by the wind turbine grid connection guidelines, that is:

[0030]

[0031] Where Im represents the complex imaginary part function, It is the control reference value of direct current control of the grid-side converter during fault ride-through;

[0032] During the fault ride-through process of a doubly-fed wind turbine generator system, the rotor-side converter and the grid-side converter must meet the active power balance constraint, namely:

[0033]

[0034] Where Re represents the complex real part function, V r,f is the steady-state rotor voltage during the fault ride-through process, R g is the resistance of the grid-side converter filter circuit, R r is the rotor resistance of the doubly fed wind turbine, s is the slip of the doubly fed wind turbine, L r is the rotor inductance of the doubly-fed wind turbine, The modulus of the control reference value for direct current control of the grid-side converter during fault ride-through;

[0035] The control reference value of the direct current control of the grid-side converter must meet the current capacity constraint of the grid-side converter, that is:

[0036]

[0037] In the formula is the maximum current capacity of the grid-side converter.

[0038] Further as a preferred technical solution of the present invention, in said S3,

[0039] When it is detected that the stator voltage of the doubly fed wind turbine generator set has returned to normal operating conditions, the fault current limiter installed on the stator side of the doubly fed wind turbine generator set is switched off. During the fault recovery process, direct current control is adopted between the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set. During the fault recovery process, the steady-state stator flux of the doubly fed wind turbine generator set is calculated based on formula (10):

[0040]

[0041] Where ψ s,recis the steady-state stator flux of the doubly-fed wind turbine during fault recovery, It is the control reference value of the direct current control of the rotor-side converter during the fault recovery process;

[0042] The amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault recovery process |I s,DC,rec |Calculated based on formula (11):

[0043]

[0044] The optimization objective is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault recovery process. The optimization objective is expressed as:

[0045] min|I s,DC,rec |(12);

[0046] Adjust the doubly-fed wind turbine to unity power factor output. The control reference value of the direct current control of the rotor-side converter and the grid-side converter during the fault recovery process must satisfy equation (13):

[0047]

[0048] In the formula It is the control reference value of direct current control of the grid-side converter during fault recovery;

[0049] During the fault recovery process of a doubly-fed wind turbine generator system, the rotor-side converter and the grid-side converter must meet the active power balance constraint, namely:

[0050]

[0051] Where V r,rec is the steady-state rotor voltage during fault recovery, It is the modulus of the control reference value for direct current control of the grid-side converter during fault recovery.

[0052] Further as a preferred technical solution of the present invention, in said S3,

[0053] When it is detected that the stator current modulus value changes less than 5% of its mean value within a power frequency cycle, the outer loop output of the dual-loop PI control of the rotor-side converter and the grid-side converter of the doubly-fed wind turbine is set;

[0054] The integral link output G of the d-axis outer loop PI control of the rotor-side converter r,d Set to:

[0055]

[0056] Where k r,dis the proportional coefficient of the PI control of the d-axis outer loop of the rotor-side converter, i r,d is the integral coefficient of the PI control of the d-axis outer loop of the rotor-side converter, is the reference value for the stator active power control of the doubly-fed wind turbine;

[0057] The integral link output G of the q-axis outer loop PI control of the rotor-side converter r,q Set to:

[0058]

[0059] Where k r,q is the proportional coefficient of the PI control of the q-axis outer loop of the rotor-side converter, i r,q is the integral coefficient of the PI control of the q-axis outer loop of the rotor-side converter, is the stator reactive power control reference value of the doubly-fed wind turbine;

[0060] The output G of the integral link of the d-axis outer loop PI control of the grid-side converter g,d Set to:

[0061]

[0062] Where k g,d is the proportional coefficient of the PI control of the d-axis outer loop of the grid-side converter, i g,d is the integral coefficient of the PI control of the d-axis outer loop of the grid-side converter, V dc is the DC bus voltage of the doubly-fed wind turbine generator set, It is the reference value for the DC bus voltage control of the doubly-fed wind turbine generator set;

[0063] The integral link output G of the q-axis outer loop PI control of the grid-side converter g,q Set to:

[0064]

[0065] Where k g,q is the proportional coefficient of the PI control of the q-axis outer loop of the grid-side converter, i g,q is the integral coefficient of the PI control of the q-axis outer loop of the grid-side converter, It is the reactive power control reference value of the grid-side converter of the doubly-fed wind turbine.

[0066] The method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system proposed in the present invention has the following technical effects compared with the prior art by using the above technical solution:

[0067] (1) In the prior art, the fault current limiter is often considered as an additional impedance connected in series with the stator winding of a doubly fed wind turbine, and its role in limiting the stator current amplitude is considered only. The present invention further considers the interaction between the fault current limiter's own impedance and the stator current, and influences the stator current through converter control, so that it can cooperate with the fault current limiter impedance to minimize the stator current oscillation component.

[0068] (2) The prior art focuses more on the transient process of the doubly-fed wind turbine generator set after a voltage drop occurs during the fault ride-through process. In fact, the fault recovery process of the doubly-fed wind turbine generator set is also accompanied by a rapid rise in the terminal voltage. In order to enable the doubly-fed wind turbine generator set to resume normal operation as quickly as possible, the fault current limiter also needs to be switched out. Therefore, compared with the voltage drop stage, the doubly-fed wind turbine generator set will face a more serious stator current oscillation problem during the fault recovery stage. The converter control scheme designed in the present invention can effectively suppress the oscillation of the stator current of the doubly-fed wind turbine generator set during the fault recovery stage, and assist the doubly-fed wind turbine generator set in smoothly transitioning to normal operating conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 This is a flow chart of a method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system according to an embodiment of the present invention;

[0070] Figure 2 This is a topological diagram of a doubly-fed wind turbine generator system with a fault current limiter installed on the stator side according to an embodiment of the present invention;

[0071] Figure 3 This is a block diagram of direct current control of a rotor-side converter of a doubly-fed wind turbine generator system according to an embodiment of the present invention;

[0072] Figure 4 This is a block diagram of a direct current control of a grid-side converter for a doubly-fed wind turbine according to an embodiment of the present invention;

[0073] Figure 5 This is a stator current comparison diagram of a doubly-fed wind turbine generator set during fault ride-through according to an embodiment of the present invention;

[0074] Figure 6 This is a stator current comparison diagram during fault recovery of a doubly-fed wind turbine generator system according to an embodiment of the present invention;

[0075] Figure 7 This is a double-loop PI control block diagram of a doubly-fed wind turbine rotor-side converter under normal operating conditions according to an embodiment of the present invention;

[0076] Figure 8 This is a double-loop PI control block diagram of a grid-side converter of a doubly-fed wind turbine under normal operating conditions according to an embodiment of the present invention;

[0077] Figure 9This is a transient stator current comparison diagram of a doubly-fed wind turbine generator set according to an embodiment of the present invention when switching from direct current control to dual-loop PI control under conventional operating conditions. DETAILED DESCRIPTION

[0078] The present invention will be further explained below with reference to the detailed description of the accompanying drawings so that those skilled in the art can have a deeper understanding of the present invention and be able to implement it. However, the following reference examples are only used to explain the present invention and are not intended to limit the present invention.

[0079] See also Figure 1 A method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system comprises the following steps:

[0080] S1: Detect the stator voltage of the doubly fed wind turbine generator set under the initial normal operating conditions and the stator voltage during the fault ride-through process. At the start of the fault ride-through process, activate the fault current limiter installed on the stator side of the doubly fed wind turbine generator set. Determine the reactive current output of the doubly fed wind turbine generator set during the fault ride-through process in accordance with the wind turbine generator set grid connection guidelines. During the fault ride-through process, the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set switch from the dual-loop PI control under the initial normal operating conditions to direct current control. Calculate the amplitude of the stator current oscillation component of the doubly fed wind turbine generator set during the fault ride-through process.

[0081] See also Figure 2 The topology diagram of a doubly-fed wind turbine generator system with a fault current limiter installed on the stator side according to an embodiment of the present invention is shown. At the start of fault ride-through, after a stator voltage drop is detected, the switching switch connected in parallel with the fault current limiter is disconnected, and the fault current limiter is put into operation on the stator winding.

[0082] Specifically, the stator voltage V of the doubly fed wind turbine generator system under the initial normal operating conditions is detected. s,n The stator voltage V during the fault ride-through of the doubly-fed wind turbine s,f At the beginning of the fault ride-through process, the fault current limiter installed on the stator side of the doubly fed wind turbine is put into operation. The impedance of the fault current limiter is Z FCL According to the wind turbine grid connection guidelines, the reactive current output of the doubly fed wind turbine during the fault ride-through process is

[0083]

[0084] Where |V s,n | is the stator voltage modulus value under the initial normal working condition of the doubly fed wind turbine generator set, |V s,f | is the stator voltage modulus during the fault ride-through process of the doubly-fed wind turbine generator set, and pu is the per-unit value;

[0085] Specifically, in the embodiment of the present invention, the doubly fed wind turbine enters the low voltage ride-through state from 0.1s onwards. The stator voltage modulus of the doubly fed wind turbine under the initial normal working condition is 1pu, and the stator voltage modulus of the doubly fed wind turbine during the fault ride-through process is 0.2pu. Based on formula (1), the reactive current output of the doubly fed wind turbine during the fault ride-through process is calculated as follows: It is 1.6pu.

[0086] The stator flux of the doubly fed wind turbine under the initial normal operating conditions is calculated based on formula (2):

[0087]

[0088] Where ψ s,n is the stator flux under initial normal working conditions, R s is the stator resistance, L s is the stator inductance, L m is the mutual inductance between the stator and the rotor, I r,n is the rotor current under the initial normal working condition, j is the imaginary unit, ω s is the synchronous electrical angular velocity;

[0089] Specifically in the embodiment of the present invention, the parameter values ​​of the doubly fed wind turbine are: stator resistance R s is 0.00706pu, synchronous electrical angular velocity ω s is 1 pu, the mutual inductance between stator and rotor L m is 2.9pu, stator inductance L s is 3.07pu, rotor inductance L r is 3.056 pu, the resistance R g is 0.02pu, and the rotor current under the initial normal working condition is 0.3040-j0.3455pu. Based on formula (2), the stator flux ψ of the doubly fed wind turbine under the initial normal working condition is calculated as s,n It is 0.0000-j1.0020pu.

[0090] During the fault ride-through process, the direct current control of the rotor-side converter and the grid-side converter of the doubly fed wind turbine are adopted. The steady-state stator flux of the doubly fed wind turbine during the fault ride-through process is calculated based on formula (3):

[0091]

[0092] Where ψ s,f is the steady-state stator flux of the doubly-fed wind turbine during the fault ride-through process, It is the control reference value of the direct current control of the rotor-side converter during the fault ride-through process;

[0093] See also Figure 3The direct current control block diagram of the rotor-side converter of the doubly-fed wind turbine generator system according to an embodiment of the present invention is shown as follows: Figure 4 The block diagram of direct current control of the grid-side converter of a doubly-fed wind turbine according to an embodiment of the present invention is shown. During the fault ride-through process, the rotor-side converter and the grid-side converter of the doubly-fed wind turbine directly set the current control reference value.

[0094] Specifically, in the embodiment of the present invention, if the control reference value of the direct current control of the rotor-side converter during the fault ride-through process is set to the rotor current under the initial normal working condition (i.e., 0.3040-j0.3455 pu), the fault current limiter impedance Z FCL =0.2+j1 pu, the steady-state stator flux ψ of the doubly fed wind turbine during the fault ride-through process is calculated based on formula (3): s,f It is 0.1876-j0.4323 pu.

[0095] The amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process|I s,DC,f |Based on formula (4), we can get:

[0096]

[0097] Specifically in the embodiment of the present invention, the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process |I s,DC,f | is 0.3757pu.

[0098] S2: Taking minimizing the amplitude of the stator current oscillation component of the DFIG wind turbine during the fault ride-through process as the optimization objective, taking into account the current capacity constraints of the rotor-side converter and the grid-side converter, as well as the reactive current output of the DFIG wind turbine during the fault ride-through process required by the wind turbine grid connection guidelines, the control reference values ​​of the rotor-side converter direct current control and the grid-side converter direct current control are determined;

[0099] Specifically, the optimization objective is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process. The optimization objective is expressed as:

[0100] min|I s,DC,f |(5);

[0101] The control reference value of the direct current control of the rotor-side converter must meet the current capacity constraint of the rotor-side converter, that is:

[0102]

[0103] In the formula is the modulus of the control reference value of the direct current control of the rotor-side converter during the fault ride-through process, is the maximum current capacity of the rotor-side converter;

[0104] The control reference value of the direct current control of the rotor-side converter and the grid-side converter must meet the reactive current output of the doubly-fed wind turbine during the fault ride-through process required by the wind turbine grid connection guidelines, that is:

[0105]

[0106] Where Im represents the complex imaginary part function, It is the control reference value of direct current control of the grid-side converter during fault ride-through;

[0107] During the fault ride-through process of a doubly-fed wind turbine generator system, the rotor-side converter and the grid-side converter must meet the active power balance constraint, namely:

[0108]

[0109] Where Re represents the complex real part function, V r,f is the steady-state rotor voltage during the fault ride-through process, R g is the resistance of the grid-side converter filter circuit, R r is the rotor resistance of the doubly fed wind turbine, s is the slip of the doubly fed wind turbine, L r is the rotor inductance of the doubly-fed wind turbine, The modulus of the control reference value for direct current control of the grid-side converter during fault ride-through;

[0110] The control reference value of the direct current control of the grid-side converter must meet the current capacity constraint of the grid-side converter, that is:

[0111]

[0112] In the formula is the maximum current capacity of the grid-side converter.

[0113] Specifically, in the present invention, the optimization objective is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process (Formula (5)). Taking into account the constraints of Formulas (6) to (9), the optimized DC current control reference value of the rotor-side converter of the doubly fed wind turbine is 0.1681-j1.1569 pu, and the optimized direct current control reference value of the grid-side converter of the doubly fed wind turbine is -0.5530-j1.3943 pu. Figure 5The stator current comparison diagram of the doubly fed wind turbine during fault ride-through of an embodiment of the present invention is shown. It can be seen that after applying the optimized reference value of the converter direct current control during the fault ride-through period (0.1s to 0.45s), compared with the dual-loop PI control of the converter under conventional working conditions, the stator current oscillation of the doubly fed wind turbine during the fault ride-through period can be effectively suppressed.

[0114] S3: When it is detected that the stator voltage of the doubly fed wind turbine generator set has returned to normal operating conditions, the fault current limiter installed on the stator side of the doubly fed wind turbine generator set is cut out, and the doubly fed wind turbine generator set is adjusted to a unity power factor output, with minimizing the amplitude of the stator current oscillation component of the doubly fed wind turbine generator set during the fault recovery process as the optimization goal, taking into account the current capacity constraints of the rotor-side converter and the grid-side converter, and determining the control reference values ​​of the direct current control of the rotor-side converter and the direct current control of the grid-side converter; when it is detected that the stator current vector modulus value changes by less than 5% of its mean value within one power frequency cycle, the output of the outer loop integral link in the dual-loop PI control of the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set under the initial normal operating conditions is adjusted, and the direct current control is restored to the dual-loop PI control under the initial normal operating conditions.

[0115] Specifically, when it is detected that the stator voltage of the doubly fed wind turbine generator set has returned to normal operating conditions, the fault current limiter installed on the stator side of the doubly fed wind turbine generator set is switched off. During the fault recovery process, direct current control is adopted between the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set. During the fault recovery process, the steady-state stator flux of the doubly fed wind turbine generator set is calculated based on formula (10):

[0116]

[0117] Where ψ s,rec is the steady-state stator flux of the doubly-fed wind turbine during fault recovery, It is the control reference value of the direct current control of the rotor-side converter during the fault recovery process;

[0118] The amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault recovery process |I s,DC,rec |Calculated based on formula (11):

[0119]

[0120] The optimization objective is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault recovery process. The optimization objective is expressed as:

[0121] min|I s,DC,rec |(12);

[0122] Adjust the doubly-fed wind turbine to unity power factor output. The control reference value of the direct current control of the rotor-side converter and the grid-side converter during the fault recovery process must satisfy equation (13):

[0123]

[0124] In the formula It is the control reference value of direct current control of the grid-side converter during fault recovery;

[0125] During the fault recovery process of a doubly-fed wind turbine generator system, the rotor-side converter and the grid-side converter must meet the active power balance constraint, namely:

[0126]

[0127] Where V r,rec is the steady-state rotor voltage during fault recovery, It is the modulus of the control reference value for direct current control of the grid-side converter during fault recovery.

[0128] Specifically, in the example of the present invention, the optimization target is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault recovery process (Formula (12)). Taking into account the constraints of Formulas (6), (9), (13), and (14), the optimized direct current control reference value of the rotor-side converter of the doubly fed wind turbine is 0.2999-j0.3455 pu, and the optimized direct current control reference value of the grid-side converter of the doubly fed wind turbine is -0.5254-j1.4128 pu. Figure 6 The stator current comparison diagram of the doubly fed wind turbine during the fault recovery period of an embodiment of the present invention is shown. It can be seen that after applying the optimized reference value of the direct current control of the converter during the fault recovery period (0.55s to 1.25s), compared with the dual-loop PI control of the converter under conventional working conditions, the stator current oscillation of the doubly fed wind turbine during the fault recovery period can be effectively suppressed.

[0129] When it is detected that the stator current modulus value changes less than 5% of its mean value within a power frequency cycle, the outer loop output of the dual-loop PI control of the rotor-side converter and the grid-side converter of the doubly-fed wind turbine is set;

[0130] The integral link output G of the d-axis outer loop PI control of the rotor-side converter r,d Set to:

[0131]

[0132] Where k r,d is the proportional coefficient of the PI control of the d-axis outer loop of the rotor-side converter, i r,d is the integral coefficient of the PI control of the d-axis outer loop of the rotor-side converter, is the reference value for the stator active power control of the doubly-fed wind turbine;

[0133] The integral link output G of the q-axis outer loop PI control of the rotor-side converter r,q Set to:

[0134]

[0135] Where k r,q is the proportional coefficient of the PI control of the q-axis outer loop of the rotor-side converter, i r,q is the integral coefficient of the PI control of the q-axis outer loop of the rotor-side converter, is the stator reactive power control reference value of the doubly-fed wind turbine;

[0136] The output G of the integral link of the d-axis outer loop PI control of the grid-side converter g,d Set to:

[0137]

[0138] Where k g,d is the proportional coefficient of the PI control of the d-axis outer loop of the grid-side converter, i g,d is the integral coefficient of the PI control of the d-axis outer loop of the grid-side converter, V dc is the DC bus voltage of the doubly-fed wind turbine generator set, It is the reference value for the DC bus voltage control of the doubly-fed wind turbine generator set;

[0139] The integral link output G of the q-axis outer loop PI control of the grid-side converter g,q Set to:

[0140]

[0141] Where k g,q is the proportional coefficient of the PI control of the q-axis outer loop of the grid-side converter, i g,q is the integral coefficient of the PI control of the q-axis outer loop of the grid-side converter, It is the reactive power control reference value of the grid-side converter of the doubly-fed wind turbine.

[0142] For details about the present invention, please refer to Figure 7 The double-loop PI control block diagram of the rotor-side converter of a doubly-fed wind turbine generator system under normal working conditions according to an embodiment of the present invention is shown as follows: Figure 8 The double-loop PI control block diagram of the grid-side converter of a doubly-fed wind turbine according to an embodiment of the present invention under normal working conditions is shown. Figure 9The transient stator current comparison diagram of the doubly-fed wind turbine generator set of one embodiment of the present invention when switching from direct current control to dual-loop PI control under normal operating conditions is shown. It can be seen that when switching is performed at 1.5s, the output of the integral link of the outer loop PI control of the rotor-side converter and the grid-side converter is set by the method of the present invention. Compared with direct switching, the stator current oscillation after switching can be effectively suppressed, and a smooth transition from direct current control to dual-loop PI control under initial normal operating conditions can be achieved.

[0143] The present invention aims at the fault ride-through and fault recovery process of the doubly-fed wind turbine generator set, cooperates with the switching of the stator-side fault current limiter, optimizes the control strategy of the rotor-side converter and the grid-side converter, minimizes the stator current oscillation during the fault ride-through and fault recovery process, and improves the safety and stability of the entire fault ride-through process of the doubly-fed wind turbine generator set.

[0144] The specific implementation scheme described above further illustrates in detail the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above is only a specific implementation scheme of the present invention and is not intended to limit the scope of the present invention. Any equivalent changes and modifications made by any technician in this field without departing from the concept and principle of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system, characterized in that: The following steps are involved: S1: Detect the stator voltage of the doubly fed wind turbine generator set under the initial normal operating conditions and the stator voltage during the fault ride-through process. At the start of the fault ride-through process, activate the fault current limiter installed on the stator side of the doubly fed wind turbine generator set. Determine the reactive current output of the doubly fed wind turbine generator set during the fault ride-through process in accordance with the wind turbine generator set grid connection guidelines. During the fault ride-through process, the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set switch from the dual-loop PI control under the initial normal operating conditions to direct current control. Calculate the amplitude of the stator current oscillation component of the doubly fed wind turbine generator set during the fault ride-through process. S2: Taking minimizing the amplitude of the stator current oscillation component of the DFIG wind turbine during the fault ride-through process as the optimization objective, taking into account the current capacity constraints of the rotor-side converter and the grid-side converter, as well as the reactive current output of the DFIG wind turbine during the fault ride-through process required by the wind turbine grid connection guidelines, the control reference values ​​of the rotor-side converter direct current control and the grid-side converter direct current control are determined; S3: When it is detected that the stator voltage of the doubly fed wind turbine generator set has returned to normal operating conditions, the fault current limiter installed on the stator side of the doubly fed wind turbine generator set is switched off, and the doubly fed wind turbine generator set is adjusted to a unity power factor output, with minimizing the amplitude of the stator current oscillation component of the doubly fed wind turbine generator set during the fault recovery process as the optimization goal, taking into account the current capacity constraints of the rotor-side converter and the grid-side converter, and determining the control reference values ​​of the direct current control of the rotor-side converter and the direct current control of the grid-side converter; when it is detected that the stator current vector modulus value changes by less than 5% of its mean value within one power frequency cycle, the output of the outer loop integral link in the dual-loop PI control of the rotor-side converter and the grid-side converter under the initial normal operating conditions of the doubly fed wind turbine generator set is adjusted, and the direct current control is restored to the dual-loop PI control under the initial normal operating conditions; The step S1 includes the following steps: detecting the stator voltage V under the initial normal working condition of the doubly fed wind turbine generator set; s,n The stator voltage V during the fault ride-through of the doubly-fed wind turbine s,f At the beginning of the fault ride-through process, the fault current limiter installed on the stator side of the doubly fed wind turbine is put into operation. The impedance of the fault current limiter is Z FCL According to the wind turbine grid connection guidelines, the reactive current output of the doubly fed wind turbine during the fault ride-through process is Where |V s,n | is the stator voltage modulus value of the doubly fed wind turbine generator under the initial normal operating conditions, |V s,f | is the stator voltage modulus during the fault ride-through process of the doubly-fed wind turbine generator set, and pu is the per-unit value; The stator flux of the doubly fed wind turbine under the initial normal operating conditions is calculated based on formula (2): Where ψ s,n is the stator flux under initial normal working conditions, R s is the stator resistance, L s is the stator inductance, L m is the mutual inductance between the stator and the rotor, I r,n is the rotor current under the initial normal working condition, j is the imaginary unit, ω s is the synchronous electrical angular velocity; During the fault ride-through process, the direct current control of the rotor-side converter and the grid-side converter of the doubly fed wind turbine are adopted. The steady-state stator flux of the doubly fed wind turbine during the fault ride-through process is calculated based on formula (3): Where ψ s,f is the steady-state stator flux of the doubly-fed wind turbine during the fault ride-through process, It is the control reference value of the direct current control of the rotor-side converter during the fault ride-through process; The amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process|I s,DC,f |Based on formula (4), we can get:

2. The method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system according to claim 1, characterized in that: The S2 comprises the following steps: The optimization objective is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault ride-through process. The optimization objective is expressed as: my|I s,DC,f | (5); The control reference value of the direct current control of the rotor-side converter must meet the current capacity constraint of the rotor-side converter, that is: In the formula is the modulus of the control reference value of the direct current control of the rotor-side converter during the fault ride-through process, is the maximum current capacity of the rotor-side converter; The control reference value of the direct current control of the rotor-side converter and the grid-side converter must meet the reactive current output of the doubly-fed wind turbine during the fault ride-through process required by the wind turbine grid connection guidelines, that is: Where Im represents the complex imaginary part function, It is the control reference value of direct current control of the grid-side converter during fault ride-through; During the fault ride-through process of a doubly-fed wind turbine generator system, the rotor-side converter and the grid-side converter must meet the active power balance constraint, namely: Where Re represents the complex real part function, V r,f is the steady-state rotor voltage during the fault ride-through process, R g is the resistance of the grid-side converter filter circuit, R r is the rotor resistance of the doubly fed wind turbine, s is the slip of the doubly fed wind turbine, L r is the rotor inductance of the doubly-fed wind turbine, The modulus of the control reference value for direct current control of the grid-side converter during fault ride-through; The control reference value of the direct current control of the grid-side converter must meet the current capacity constraint of the grid-side converter, that is: In the formula is the maximum current capacity of the grid-side converter.

3. The method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system according to claim 2, characterized in that: In S3, when it is detected that the stator voltage of the doubly fed wind turbine generator set has returned to normal operating conditions, the fault current limiter installed on the stator side of the doubly fed wind turbine generator set is switched off. During the fault recovery process, direct current control of the rotor-side converter and the grid-side converter of the doubly fed wind turbine generator set is adopted. During the fault recovery process, the steady-state stator flux of the doubly fed wind turbine generator set is calculated based on formula (10): Where ψ s,rec is the steady-state stator flux of the doubly-fed wind turbine during fault recovery, It is the control reference value of the direct current control of the rotor-side converter during the fault recovery process; The amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault recovery process|I s,DC,rec |Calculated based on formula (11): The optimization objective is to minimize the amplitude of the stator current oscillation component of the doubly fed wind turbine during the fault recovery process. The optimization objective is expressed as: my|I s,DC,rec | (12); Adjust the doubly-fed wind turbine to unity power factor output. The control reference value of the direct current control of the rotor-side converter and the grid-side converter during the fault recovery process must satisfy equation (13): In the formula It is the control reference value of direct current control of the grid-side converter during fault recovery; During the fault recovery process of a doubly-fed wind turbine generator system, the rotor-side converter and the grid-side converter must meet the active power balance constraint, namely: Where V r,rec is the steady-state rotor voltage during fault recovery, It is the modulus of the control reference value for direct current control of the grid-side converter during fault recovery.

4. The method for suppressing stator current oscillation during fault ride-through and recovery of a doubly-fed wind turbine generator system according to claim 3, characterized in that: In said S3, when it is detected that the stator current vector modulus value changes by less than 5% of its mean value within one power frequency cycle, the output of the outer loop integral link in the dual-loop PI control of the rotor-side converter and the grid-side converter of the doubly-fed wind turbine generator set under the initial normal operating conditions is adjusted; The integral link output G of the d-axis outer loop PI control of the rotor-side converter r,d Set to: Where k r,d is the proportional coefficient of the PI control of the d-axis outer loop of the rotor-side converter, i r,d is the integral coefficient of the PI control of the d-axis outer loop of the rotor-side converter, P s ref is the reference value for the stator active power control of the doubly-fed wind turbine; The integral link output G of the q-axis outer loop PI control of the rotor-side converter r,q Set to: Where k r,q is the proportional coefficient of the PI control of the q-axis outer loop of the rotor-side converter, i r,q is the integral coefficient of the PI control of the q-axis outer loop of the rotor-side converter, is the stator reactive power control reference value of the doubly-fed wind turbine; The output G of the integral link of the d-axis outer loop PI control of the grid-side converter g,d Set to: Where k g,d is the proportional coefficient of the PI control of the d-axis outer loop of the grid-side converter, i g,d is the integral coefficient of the PI control of the d-axis outer loop of the grid-side converter, V dc is the DC bus voltage of the doubly-fed wind turbine generator set, It is the reference value for the DC bus voltage control of the doubly-fed wind turbine generator set; The integral link output G of the q-axis outer loop PI control of the grid-side converter g,q Set to: Where k g,q is the proportional coefficient of the PI control of the q-axis outer loop of the grid-side converter, i g,q is the integral coefficient of the PI control of the q-axis outer loop of the grid-side converter, It is the reactive power control reference value of the grid-side converter of the doubly-fed wind turbine.

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