Asymmetric fault ride-through control method for improving power quality at grid connection point of doubly-fed wind turbine generator system

Abstract

The present invention discloses an asymmetric fault ride-through control method for improving the power quality of a doubly-fed wind turbine generator set at the grid-connected point. The control method involves giving instructions for active current and reactive current of a rotor-side converter and a grid-side converter of a doubly-fed wind turbine generator set; the control current instruction obtained by the rotor-side converter is sent to a rotor current loop controller, thereby suppressing the double frequency fluctuation of the active power output on the stator side of the doubly-fed wind turbine generator set. The control current instruction obtained by the grid-side converter is sent to a grid-side current loop controller, thereby suppressing the double frequency fluctuation of the total active power output of the doubly-fed wind turbine generator set. The method makes full use of the capacity of the rotor-side and grid-side converters in the doubly-fed wind turbine generator set to suppress the double frequency fluctuation of the active power on the basis of meeting the minimum requirements of the grid-connected guidelines, and provides active current to the grid to the maximum extent, thereby improving the fault ride-through capability of the doubly-fed wind turbine generator set and the power quality of the grid-connected point.

Description

Technical Field

[0001] The present invention relates to wind power generation technology, and in particular to an asymmetric fault ride-through control method for improving the power quality of a double-fedinduction generator (DFIG) grid connection point, belonging to the field of new energy power generation technology. Background Art

[0002] Since most of my country's wind power resources are concentrated in remote areas and have weak connections with the main power grid, the probability of asymmetric short-circuit faults in the power grid is higher than that of symmetrical short-circuit faults in the power grid. When an asymmetric fault occurs in the power grid, the active power output of the generator and the active power output of the system will fluctuate at a double frequency. If no measures are taken, the power quality at the generator grid connection point will be seriously affected. At the same time, as the requirements for wind farm grid connection become more and more stringent, the low voltage ride-through capability of wind turbines under asymmetric faults has also been included in the assessment scope of the grid connection guidelines. Therefore, in order to improve the low voltage ride-through capability and grid connection point power quality of doubly fed wind turbines under asymmetric grid faults, it is necessary to further study the method of suppressing the double frequency fluctuation of the total output active power of doubly fed wind turbines under asymmetric grid faults. At present, some scholars have conducted research on the control strategy of DFIG systems under asymmetric grid faults, such as the following published literature:

[0003] (1) Zhang Di, Wei Yanjun, Yang Zongfeng, Ding Hao, Qi Hanhong. Control strategy of rotor-side converter of doubly-fed wind power system based on sliding mode variable structure control under unbalanced grid voltage[J]. Transactions of the Chinese Society of Electrotechnical Engineering, 2016, 31(17): 121-131.

[0004] (2) Yao Jun, Du Hongbiao, Zhou Te, Li Qing. Method for suppressing total output active power fluctuation of doubly-fed wind power system under unbalanced and harmonically distorted grid voltage [P]. Chongqing: CN103997064A, 2014-08-20.

[0005] Reference (1) uses a DC power control method based on sliding mode variable structure, and adopts a power optimization compensation control strategy to directly compensate the power setting of the rotor-side converter, so as to effectively suppress the double frequency fluctuation of the active power of the stator input grid, and to a certain extent improve the fault ride-through capability of the permanent magnet direct-drive wind power system. However, this strategy does not take into account the requirements of the grid-connected code for the reactive current of the wind turbine during a fault. In addition, due to the existence of the grid-side converter, the proposed control strategy cannot suppress the double frequency fluctuation of the total output active power of the wind turbine. Reference (2) suppresses the total active power fluctuation of the doubly-fed wind power system by adding a series grid-side converter, and improves the power quality and stability of the grid to which the doubly-fed wind power system is connected. However, this method requires additional hardware, making the doubly-fed wind power system complex and increasing the cost. In addition, this method does not take into account the requirements of the new grid-connected code for negative sequence reactive current when an asymmetric fault occurs.

[0006] During the asymmetric fault of the power grid, the appearance of negative sequence voltage and negative sequence current in the power grid will inevitably lead to the double frequency fluctuation of the active power output of the doubly fed wind turbine, thereby reducing its low voltage ride-through capability and the power quality of the system grid connection point. Therefore, without adding additional hardware equipment and meeting the requirements of "GB / T 19963.1-2021 Technical Specifications for Wind Farm Access to Power System", it is of great practical significance to make full use of the capacity, controllability and coordinated control capabilities of the converter in the doubly fed wind turbine to study the double frequency suppression method of the total output active power of the doubly fed wind turbine under the asymmetric fault of the power grid, so as to enhance the low voltage ride-through capability of the doubly fed wind turbine and the power quality of the grid connection point. Summary of the invention

[0007] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to propose an asymmetric fault ride-through control method for improving the power quality at the grid-connected point of a doubly-fed wind turbine. The method makes full use of the rotor-side and grid-side converter capacities in the doubly-fed wind turbine to suppress the double frequency fluctuation of active power on the basis of meeting the minimum requirements of the grid-connected guidelines, and to provide active current to the grid to the maximum extent, thereby improving the fault ride-through capability of the doubly-fed wind turbine and the power quality at the grid-connected point.

[0008] The technical solution of the present invention is achieved in this way:

[0009] An asymmetric fault ride-through control method for improving the power quality of a doubly-fed wind turbine generator grid-connected point, the control method involves giving instructions for active current and reactive current of a rotor-side converter of the doubly-fed wind turbine generator;

[0010] (A) Calculation of positive and negative sequence active current instructions of the rotor-side converter of the doubly-fed wind turbine during an asymmetric grid fault and positive and negative sequence reactive current commands The specific calculation steps are:

[0011] A1) During the fault period, the grid guidelines require the injection of positive and negative sequence reactive currents. The rotor side reactive current expression is:

[0012]

[0013] In the formula, are the positive and negative sequence components of the terminal voltage of the doubly-fed wind turbine in the positive and reverse dq synchronous rotating coordinate system; ω 1 is the synchronous angular frequency; L s and L m are the equivalent inductance and equivalent mutual inductance of stator winding respectively; K + , K - is the dynamic positive and negative sequence reactive current proportionality coefficient;

[0014] A2) In order to suppress the active power cosine value P on the stator side scos2 The fluctuation of P scos2 The following equation should be satisfied:

[0015]

[0016] A3) Using the rotor side reactive current expression obtained in step A1), calculate the relationship between the dynamic positive and negative sequence reactive current proportionality coefficients when the equation in step A2) is satisfied:

[0017]

[0018] A4) Solve the relationship according to steps A1), A2) and A3) to obtain the positive and negative sequence reactive current instructions on the rotor side for:

[0019]

[0020] A5) In order to suppress the stator side active power sinusoidal quantity P ssin2 The fluctuation of P ssin2 The following equation should be satisfied:

[0021]

[0022] A6) Using the equation relationship in step A5), calculate the positive and negative sequence active current instructions on the rotor side The relationship is:

[0023]

[0024] A7) Since the capacity of the rotor-side converter is limited, based on the rotor-side positive and negative sequence reactive current instructions obtained in step A4), the constraint condition for determining whether the doubly-fed wind turbine generator system can output active power under this operation condition is:

[0025]

[0026] In the formula, I rmax The maximum current allowed for the rotor-side converter of the doubly-fed wind turbine generator system;

[0027] A8) If the constraint conditions in A7) are met, a certain reactive power is outputted using the residual current margin on the rotor side. Based on the relationship between the positive and negative sequence active currents on the rotor side obtained in step A6), the positive and negative sequence active currents and reactive current instructions on the rotor side are calculated as follows:

[0028]

[0029] In the formula,

[0030] If the constraints in A7) are not met, the doubly-fed wind turbine generator set cannot output active power, and the positive and negative sequence active current and reactive current instructions on the rotor side are calculated as follows:

[0031]

[0032] A9) The control current instruction obtained in step A8) By sending it into the rotor current loop controller, the double frequency fluctuation of the active power output on the stator side of the doubly fed wind turbine can be suppressed.

[0033] Furthermore, the control method also involves giving instructions for active current and reactive current of the grid-side converter of the doubly-fed wind turbine;

[0034] (B) Calculation of positive and negative sequence active current instructions of the grid-side converter of the doubly-fed wind turbine during asymmetric grid fault and positive and negative sequence reactive current commands The specific calculation steps are:

[0035] B1) In order to suppress the double frequency fluctuation of the total output active power, the grid-side output active power sinusoidal quantity P gsin2 and cosine P gcos2 The following relationship should be satisfied:

[0036]

[0037] In the formula, are the positive and negative sequence components of the grid voltage dq axis in the positive and reverse dq synchronous rotating coordinate system;

[0038] B2) Based on the grid-side output active power sinusoidal quantity P obtained in step B1) gsin2 and cosine P gcos2 The relationship between the positive and negative sequence active current instructions of the grid-side converter is calculated and positive and negative sequence reactive current commands for:

[0039]

[0040] In the formula, and U dc are the DC bus voltage reference and feedback, k p and k i are the proportional coefficient and integral coefficient of the PI controller of the active current command calculation module of the grid-side converter, and s is the Laplace operator;

[0041] B3) The control current instruction obtained in step B2) By sending it into the grid-side current loop controller, the double frequency fluctuation of the total output active power of the doubly-fed wind turbine can be suppressed, and the power quality of the grid-connected point of the doubly-fed wind turbine can be improved.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] The present invention provides positive and negative sequence dq axis current given instructions for the rotor side and grid side converters of the doubly fed wind turbine generator set on the basis of fully considering the converter capacity and the unit operating conditions, so that the doubly fed wind turbine generator set can suppress the double frequency fluctuation of active power on the basis of meeting the minimum requirements of the grid guidelines, and provide active current to the grid to the maximum extent, thereby improving the fault ride-through capability of the doubly fed wind turbine generator set and the power quality at the grid connection point. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a structural diagram of a doubly-fed wind turbine connected to a power system.

[0045] Figure 2 This is a block diagram of the control method of the doubly-fed wind turbine generator set described in the present invention.

[0046] Figure 3 This is a calculation flow chart of the current instruction calculation module of the rotor-side converter of the doubly-fed wind turbine set of the present invention.

[0047] Figure 4 This is a calculation flow chart of the current instruction calculation module of the grid-side converter of the doubly-fed wind turbine of the present invention.

[0048] Figure 5 This is a simulation waveform diagram of the operating characteristics of a doubly-fed wind turbine when the voltage imbalance at the grid connection point is 54% and the voltage positive sequence component drops to 0.47pu.

[0049] Figure 6 It is a simulation waveform diagram of the double-fed wind turbine generator set when the voltage unbalance degree at the grid connection point is 54% and the voltage positive sequence component drops to 0.47pu when the control method of the present invention is adopted.

[0050] Figure 7 It is a simulation waveform diagram of the double-fed wind turbine generator set when the voltage unbalance degree at the grid connection point is 40% and the voltage positive sequence component drops to 0.55pu when the control method of the present invention is adopted. DETAILED DESCRIPTION

[0051] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0052] Figure 1 This is a structural diagram of a doubly-fed wind turbine with a total capacity of 50MW connected to the power system. The doubly-fed wind turbine is connected to the large power grid through a public connection point.

[0053] Figure 2 The control block diagram of the doubly-fed wind turbine generator system of the present invention under an asymmetric grid fault includes the following control objects: a rotor-side converter 1, a grid-side converter 2, a rotor-side converter current command calculation module 3, a grid-side converter current command calculation module 4, a rotor current loop control module 5, and a grid-side current loop control module 6.

[0054] The present invention provides an asymmetric fault ride-through control method for improving the power quality of the grid-connected point of a doubly-fed wind turbine, which is used to improve the fault ride-through capability of the doubly-fed wind turbine and improve the power quality of the grid-connected point, and suppress the double frequency fluctuation of the total output active power of the doubly-fed wind turbine during the fault process. The method involves giving instructions for the active current and reactive current of the grid-side converter and the rotor-side converter of the doubly-fed wind turbine. The specific implementation steps are as follows:

[0055] (A) Positive and negative sequence active current instructions of the rotor side converter (RSC) of the doubly-fed wind turbine during an asymmetric grid fault and positive and negative sequence reactive current commands Calculation, the specific calculation steps are:

[0056] A1) During the fault period, the grid guidelines require the injection of positive and negative sequence reactive currents. The rotor side reactive current expression is:

[0057]

[0058] In the formula, are the positive and negative sequence components of the terminal voltage of the doubly-fed wind turbine in the positive and reverse dq synchronous rotating coordinate system; ω 1 is the synchronous angular frequency; L s and L mare the equivalent inductance and equivalent mutual inductance of stator winding respectively; K + , K - is the dynamic positive and negative sequence reactive current proportionality coefficient;

[0059] A2) In order to suppress the active power cosine value P on the stator side scos2 The fluctuation of P scos2 The following equation should be satisfied:

[0060]

[0061] A3) Using the rotor side reactive current expression obtained in step A1), calculate the relationship between the dynamic positive and negative sequence reactive current proportionality coefficients when the equation in step A2) is satisfied:

[0062]

[0063] A4) Solve the relationship according to steps A1), A2) and A3) to obtain the positive and negative sequence reactive current instructions on the rotor side for:

[0064]

[0065] A5) In order to suppress the stator side active power sinusoidal quantity P ssin2 The fluctuation of P ssin2 The following equation should be satisfied:

[0066]

[0067] A6) Using the equation relationship in step A5), calculate the positive and negative sequence active current instructions on the rotor side The relationship is:

[0068]

[0069] A7) Since the capacity of the rotor-side converter is limited, based on the rotor-side positive and negative sequence reactive current instructions obtained in step A4), the constraint condition for determining whether the doubly-fed wind turbine generator system can output active power under this operation condition is:

[0070]

[0071] In the formula, I rmax The maximum current allowed for the rotor-side converter of the doubly-fed wind turbine generator system;

[0072] A8) If the constraint conditions in A7) are met, the residual current margin on the rotor side can be used to output a certain reactive power. Based on the relationship between the positive and negative sequence active currents on the rotor side obtained in step A6), the rotor side converter current instruction module 3 (see Figure 3) Calculate the positive and negative sequence active current and reactive current instructions on the rotor side as:

[0073]

[0074] In the formula,

[0075] If the constraint condition in A7) is not met, the doubly-fed wind turbine generator set cannot output active power, and the rotor-side converter current instruction module 3 calculates the rotor-side positive and negative sequence active current and reactive current instructions as follows:

[0076]

[0077] A9) The control current instruction obtained in step A8) By sending it to the rotor current loop control module 5, the double frequency fluctuation of the active power output on the stator side of the doubly-fed wind turbine generator set can be suppressed.

[0078] (B) Positive and negative sequence active current instructions of the grid side converter (GSC) of the doubly fed wind turbine during asymmetric grid fault and positive and negative sequence reactive current commands Calculation, the specific calculation steps are:

[0079] B1) In order to suppress the double frequency fluctuation of the total output active power, the grid-side output active power sinusoidal quantity P gsin2 and cosine P gcos2 The following relationship should be satisfied:

[0080]

[0081] In the formula, are the positive and negative sequence components of the grid voltage dq axis in the positive and reverse dq synchronous rotating coordinate system;

[0082] B2) Based on the relationship between the sine and cosine quantities of the grid-side output active power obtained in step B1), the grid-side converter current instruction calculation module 4 (see Figure 4 ) Calculate the positive and negative sequence active current instructions of the grid-side converter (GSC) and positive and negative sequence reactive current commands for:

[0083]

[0084] In the formula, and U dc are the DC bus voltage reference and feedback, k p and k iare the proportional coefficient and integral coefficient of the PI controller of the active current command calculation module of the grid-side converter, and s is the Laplace operator;

[0085] B3) The control current instruction obtained in step B2) By sending it to the grid-side current loop control module 6, the double frequency fluctuation of the total output active power of the doubly-fed wind turbine can be suppressed, and the power quality of the grid-connected point of the doubly-fed wind turbine can be improved.

[0086] Effect description of the present invention:

[0087] Figure 5 The following is a simulation waveform of the operating characteristics of the DFIG unit when the voltage positive sequence component drops to 0.47pu and the imbalance is 54%. If the RSC does not adopt the corresponding control strategy during the fault continuation stage, the stator output active power and total active power of the DFIG system will have large double frequency fluctuations, which will not only greatly reduce the power quality of the grid-connected system, but also cause the wind turbine to be cut off. Figure 6 The simulation waveform diagram is when the voltage positive sequence component drops to 0.47pu and the unbalance degree is 54% and the suppression method proposed by the present invention is used. It can be seen from the figure that this method can suppress the double frequency fluctuation of the stator side active power and the total active power when meeting the minimum reactive current required by the grid guidelines, and can output a smaller active power by using the current residual capacity. Figure 7 The simulation waveform diagram is when the voltage positive sequence component drops to 0.55pu and the unbalance degree is 40% and the suppression method proposed by the present invention is used. It can be seen from the figure that this method can suppress the double frequency fluctuation of the stator side active power and the total active power when the minimum reactive current required by the grid guidelines is met. Due to the limitation of the converter capacity, active power cannot be generated at this time.

[0088] Finally, it should be noted that the above examples of the present invention are merely examples for illustrating the present invention, and are not intended to limit the embodiments of the present invention. Although the applicant has described the present invention in detail with reference to the preferred embodiments, for those of ordinary skill in the art, other different forms of changes and modifications can be made based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. An asymmetric fault ride-through control method for improving the power quality of the grid-connected point of a doubly-fed wind turbine generator system. Features: The control method involves giving instructions for active current and reactive current of a rotor-side converter of a doubly-fed wind turbine generator set; (A) Calculation of positive and negative sequence active current instructions of the rotor-side converter of the doubly-fed wind turbine during an asymmetric grid fault and positive and negative sequence reactive current commands The specific calculation steps are: A1) During the fault period, the grid guidelines require the injection of positive and negative sequence reactive currents. The rotor side reactive current expression is: In the formula, are the positive and negative sequence components of the terminal voltage of the doubly-fed wind turbine in the positive and reverse dq synchronous rotating coordinate system; ω 1 is the synchronous angular frequency; L s and L m are the equivalent inductance and equivalent mutual inductance of stator winding respectively; K + , K - is the dynamic positive and negative sequence reactive current proportionality coefficient; A2) In order to suppress the active power cosine value P on the stator side scos2 The fluctuation of P scos2 The following equation should be satisfied: A3) Using the rotor side reactive current expression obtained in step A1), calculate the relationship between the dynamic positive and negative sequence reactive current proportionality coefficients when the equation in step A2) is satisfied: A4) Solve the relationship according to steps A1), A2) and A3) to obtain the positive and negative sequence reactive current instructions on the rotor side for: A5) In order to suppress the stator side active power sinusoidal quantity P ssin2 The fluctuation of P ssin2 The following equation should be satisfied: A6) Using the equation relationship in step A5), calculate the positive and negative sequence active current instructions on the rotor side The relationship is: A7) Since the capacity of the rotor-side converter is limited, based on the rotor-side positive and negative sequence reactive current instructions obtained in step A4), the constraint condition for determining whether the doubly-fed wind turbine generator system can output active power under this operation condition is: In the formula, I rmax The maximum current allowed for the rotor-side converter of the doubly-fed wind turbine generator system; A8) If the constraint conditions in A7) are met, a certain reactive power is outputted using the residual current margin on the rotor side. Based on the relationship between the positive and negative sequence active currents on the rotor side obtained in step A6), the positive and negative sequence active currents and reactive current instructions on the rotor side are calculated as follows: In the formula, If the constraints in A7) are not met, the doubly-fed wind turbine generator set cannot output active power, and the positive and negative sequence active current and reactive current instructions on the rotor side are calculated as follows: A9) The control current instruction obtained in step A8) By sending it into the rotor current loop controller, the double frequency fluctuation of the active power output on the stator side of the doubly fed wind turbine can be suppressed.

2. According to claim 1, an asymmetric fault ride-through control method for improving the power quality of a doubly-fed wind turbine generator grid connection point, Features: The control method also involves giving instructions for active current and reactive current of the grid-side converter of the double-fed wind turbine; (B) Calculation of positive and negative sequence active current instructions of the grid-side converter of the doubly-fed wind turbine during asymmetric grid fault and positive and negative sequence reactive current commands The specific calculation steps are: B1) In order to suppress the double frequency fluctuation of the total output active power, the grid-side output active power sinusoidal quantity P gsin2 and cosine P gcos2 The following relationship should be satisfied: wherein, are the positive and negative sequence components of the dq-axis of the grid voltage in the positive and reverse dq synchronous rotating coordinate systems; B2) Based on the grid-side output active power sinusoidal quantity P obtained in step B1) gsin2 and cosine P gcos2 The relationship between the positive and negative sequence active current instructions of the grid-side converter is calculated and positive and negative sequence reactive current commands for: In the formula, and U dc are the DC bus voltage reference and feedback, k p and k i are the proportional coefficient and integral coefficient of the PI controller of the active current command calculation module of the grid-side converter, and s is the Laplace operator; B3) The control current instruction obtained in step B2) By sending it into the grid-side current loop controller, the double frequency fluctuation of the total output active power of the doubly-fed wind turbine can be suppressed, and the power quality of the grid-connected point of the doubly-fed wind turbine can be improved.

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