Control strategy and parameter selection method for new energy warp-constructed network type flexible direct delivery system

Abstract

The invention discloses a control strategy and parameter selection method for a new energy via-structure network type flexible direct delivery system. The method comprises the following steps: establishing a small-disturbance stability analysis model of the new energy via-structure network type flexible direct delivery system; the small disturbance stability of the new energy via-network flexible direct delivery system along with power change is analyzed based on a root locus method, a dominant control link influencing an oscillation instability mode of the new energy via-network flexible direct delivery system is identified based on a participation factor analysis method, and an overall design scheme of a control strategy of the new energy via-network flexible direct delivery system is determined; designing a voltage outer loop control structure of the modular multilevel converter; and designing a modular multilevel converter voltage outer loop parameter calculation method, and calculating modular multilevel converter voltage outer loop proportional control link parameters according to a given AC voltage maximum deviation requirement. According to the invention, on the basis of considering the AC voltage control capability, stable operation of the new energy via-network flexible direct-current delivery system in a full-power range is ensured, and the system delivery capability is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of stable control of flexible direct current systems, and in particular relates to a control strategy and parameter selection method for a new energy network-type flexible direct current transmission system. Background Art

[0002] The flexible direct current system (flexible direct current) is a direct current transmission system that uses fully controlled power electronic converters. It has the advantages of no risk of commutation failure, the ability to supply power to passive systems, and the ability to achieve active and reactive decoupling control. It is widely used in large-scale new energy long-distance transmission scenarios. The traditional flexible direct current system adopts a grid-following control strategy and is highly dependent on phase-locked loops for synchronization. Since the proportion of synchronous machines in new energy stations is relatively low, they present the characteristics of weak AC systems with low inertia and weak damping. The use of a grid-following flexible direct current system to transmit new energy is prone to stability problems. In comparison, the flexible direct current (grid-following flexible direct current) that adopts a grid-building control strategy does not rely on phase-locked loop synchronization, can provide voltage / frequency support for new energy stations, and is an effective way to achieve stable transmission of new energy.

[0003] However, the new energy grid-type flexible direct current transmission system is a 100% power electronic system, and its stable operation ability is greatly affected by the control system. The traditional grid-type flexible direct current adopts a voltage and current dual-loop control structure, which shows weak / negative damping characteristics in multiple frequency bands, and is prone to broadband oscillation and even instability. To this end, domestic and foreign scholars have carried out relevant research. For example, Chinese patent CN118399466A discloses a grid-type control strategy for a flexible direct current system using a power / voltage outer loop single-loop control structure, which solves the oscillation suppression problem of a dual-loop control system. However, this control strategy lacks a current inner loop and cannot effectively suppress fault current. In response to this problem, Chinese patent CN118399466A discloses a flexible direct current grid-type control method and system without an inner loop with a current limiting function, which avoids overcurrent of the converter by limiting the voltage modulation signal output by the control system. Since the response speed of the outer loop is significantly slower than that of the current inner loop, the above-mentioned grid-type control strategy based on the power / voltage outer loop single-loop structure may reduce the dynamic response effect of the system under disturbance. Chinese patent CN115051404A discloses a simplified voltage outer loop and a conventional current inner loop cascaded network control strategy, which avoids system oscillation problems by simplifying the voltage outer loop and uses the current inner loop to achieve the current limiting function of the converter bridge arm. However, the invention does not provide a control parameter selection method. If the control parameters are not selected properly, the system may experience AC voltage over-limit problems under power disturbances. Summary of the invention

[0004] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a control strategy and parameter selection method for a new energy network-type flexible direct current transmission system.

[0005] The technical solution of the present invention is: a control strategy and parameter selection method for a new energy network-type flexible direct current transmission system, comprising the following steps:

[0006] A. Establish a small disturbance stability analysis model for the new energy network-type flexible direct current transmission system;

[0007] B. Analyze the small disturbance stability of the new energy grid-type flexible direct current transmission system with power changes based on the root locus method, identify the dominant control link affecting the oscillation instability mode of the new energy grid-type flexible direct current transmission system based on the participation factor analysis method, and determine the overall design scheme of the control strategy of the new energy grid-type flexible direct current transmission system;

[0008] C. Design a modular multilevel converter voltage outer loop control structure. The voltage outer loop realizes the constant AC voltage control function based on the proportional control link. The rest of the control system structure is consistent with the conventional control system.

[0009] D. Design a method for calculating the voltage outer loop parameters of a modular multilevel converter, and calculate the parameters of the voltage outer loop proportional control link of the modular multilevel converter according to the given AC voltage maximum deviation requirements.

[0010] Furthermore, step A establishes a small disturbance stability analysis model for the new energy grid-type flexible direct current transmission system. The specific process is as follows:

[0011] First, establish an analytical model of new energy stations and collection lines;

[0012] Then, an analytical model of the new energy network-type flexible direct current transmission system is established;

[0013] Finally, based on the above analytical model, a small disturbance stability model of the new energy network-type flexible direct current transmission system is established.

[0014] Furthermore, the new energy station adopts an equivalent aggregation model, and the collection line model is modeled using a P-type equivalent circuit.

[0015] Furthermore, in step B, the small disturbance stability of the new energy grid-type flexible direct current transmission system with power changes is analyzed based on the root locus method. The specific process is as follows:

[0016] First, it is set that at the initial moment, the new energy network-type flexible direct current transmission system transmits 0 p.u. active power;

[0017] Then, the active power transmitted by the new energy through the grid-type flexible direct current transmission system is uniformly increased to 1 p.u.;

[0018] Then, based on the state matrix A of the new energy network-based flexible direct current transmission system, the eigenvalues ​​of the system under different transmission powers are obtained λ=[λ1,λ2,...,λ n ] T, get the root locus of the system changing with power;

[0019] Finally, based on the above root loci, the small disturbance stability of the new energy grid-type flexible direct current transmission system is analyzed.

[0020] Furthermore, in step B, the dominant control link affecting the oscillation instability mode of the new energy grid-type flexible direct current transmission system is identified based on the participation factor analysis method. The specific process is as follows:

[0021] Firstly, the participation factor matrix of the new energy grid-type flexible direct current transmission system is calculated;

[0022] Then, according to the analysis of participation factors, it is considered that the voltage outer loop controller of the modular multilevel converter is the dominant control link that causes the system oscillation instability mode;

[0023] Finally, the overall control strategy plan for the new energy network-type flexible direct current transmission system is determined.

[0024] Furthermore, the overall control strategy of the new energy network-type flexible direct current transmission system is as follows:

[0025] The conventional current inner loop controller is retained to realize the fault current limiting function, and a new voltage outer loop controller is designed to ensure that the new energy grid-type flexible direct current transmission system can operate stably in the full power range.

[0026] Furthermore, step C designs a modular multilevel converter voltage outer loop control structure, and the specific process is as follows:

[0027] Firstly, the control strategy of the voltage outer loop controller of the modular multilevel converter adopts constant AC voltage control;

[0028] Then, the input signal of the modular multilevel converter voltage outer loop controller is the dq axis component v of the MMC converter transformer grid side voltage pd 、v pq , and the grid-side dq axis voltage reference value of the MMC converter transformer

[0029] Furthermore, in step C, the voltage outer loop realizes the constant AC voltage control function based on the proportional control link, and the specific process is as follows:

[0030] The voltage outer loop controller of the modular multilevel converter realizes constant AC voltage control based on the proportional controller, and the dq axis voltage deviation signal After being amplified by the proportional controller, the current inner loop dq axis current reference value is generated

[0031] Furthermore, the remaining control system structure in step C is consistent with the conventional control system, and the specific process is as follows:

[0032] The voltage outer loop controller of the modular multilevel converter is cascaded with the conventional current control. The structure of the rest of the MMC control system remains unchanged. The WFMMC voltage outer loop controller converts the current inner loop dq axis current reference value It is input to the inner current loop as the output signal.

[0033] Furthermore, step D designs a method for calculating the voltage outer loop parameters of the modular multilevel converter, and calculates the parameters of the voltage outer loop proportional control link of the modular multilevel converter according to the given AC voltage maximum deviation requirement. The specific process is as follows:

[0034] First, the WFMMC voltage outer loop controller parameter is the proportional coefficient of the proportional controller;

[0035] Then, the above-mentioned proportionality coefficient is calculated based on the maximum allowable deviation rate of the given AC voltage.

[0036] The beneficial effects of the present invention are as follows:

[0037] The present invention solves the technical problems of insufficient AC voltage control capability, oscillation suppression and current limiting difficulties in the new energy network-type flexible direct current transmission system.

[0038] The present invention can ensure the stable operation of the new energy grid-type flexible direct current transmission system in the full power range while taking into account the AC voltage control capability, thereby improving the system transmission capacity.

[0039] The present invention retains the conventional current inner loop control structure, and can use the conventional current inner loop proportional-integral (PI) control link to limit the fault current when a power grid fault occurs, thereby avoiding overcurrent locking and damage of the converter.

[0040] The control structure and parameter selection method of the present invention are simple and easy to implement in actual engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of the structure of the direct-drive wind power grid-type flexible direct transmission system in the present invention;

[0042] Figure 2 It is a simplified equivalent model schematic diagram of the direct-drive wind power grid-connected flexible direct transmission system in the present invention;

[0043] Figure 3 It is a schematic diagram of the method flow of the present invention;

[0044] Figure 4 It is a schematic diagram of the structure of the GSC constant DC voltage control and current inner loop control in the present invention;

[0045] Figure 5It is a schematic diagram of the structure of the GSC phase-locked loop in the present invention;

[0046] Figure 6 It is a schematic diagram of the conventional voltage outer loop and current inner loop control structure of WFMMC in the present invention;

[0047] Figure 7 It is a schematic diagram of the structure of the WFMMC circulating current suppressor in the present invention;

[0048] Figure 8 It is the root locus of the direct-drive wind power grid-connected flexible direct transmission system in the present invention as the power changes;

[0049] Fig. 9 It is the participation degree of each control link in the oscillation instability mode 1 of the direct-drive wind power grid-type flexible direct transmission system in the present invention;

[0050] Fig.10 A schematic diagram of the voltage outer loop control structure based on proportional control designed for the present invention;

[0051] Fig.11 It is the electromagnetic transient simulation result of the system power transmission when the WFMMC in the present invention adopts double closed-loop AC voltage control;

[0052] Fig.12 It is the electromagnetic transient simulation result of the system power transmission when the WFMMC in the present invention adopts AC voltage proportional control;

[0053] Fig.13 This is the verification result of the steady-state voltage control capability of the AC voltage proportional control in the present invention; DETAILED DESCRIPTION

[0054] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and embodiments:

[0055] like Figures 1 to 13 As shown, the control strategy and parameter selection method of the new energy network type flexible direct current transmission system include the following steps:

[0056] A. Establish a small disturbance stability analysis model for the new energy network-type flexible direct current transmission system;

[0057] B. Analyze the small disturbance stability of the new energy grid-type flexible direct current transmission system with power changes based on the root locus method, identify the dominant control link affecting the oscillation instability mode of the new energy grid-type flexible direct current transmission system based on the participation factor analysis method, and determine the overall design scheme of the control strategy of the new energy grid-type flexible direct current transmission system;

[0058] C. Design a modular multilevel converter voltage outer loop control structure. The voltage outer loop realizes the constant AC voltage control function based on the proportional control link. The rest of the control system structure is consistent with the conventional control system.

[0059] D. Design a method for calculating the voltage outer loop parameters of a modular multilevel converter, and calculate the parameters of the voltage outer loop proportional control link of the modular multilevel converter according to the given AC voltage maximum deviation requirements.

[0060] Step A establishes a small disturbance stability analysis model for the new energy network-type flexible direct current transmission system. The specific process is as follows:

[0061] First, establish an analytical model of new energy stations and collection lines;

[0062] Then, an analytical model of the new energy network-type flexible direct current transmission system is established;

[0063] Finally, based on the above analytical model, a small disturbance stability model of the new energy network-type flexible direct current transmission system is established.

[0064] The new energy station adopts an equivalent aggregation model, and the collection line model is modeled using a P-type equivalent circuit.

[0065] In step B, the root locus method is used to analyze the small disturbance stability of the new energy grid-type flexible direct current transmission system with power changes. The specific process is as follows:

[0066] First, it is set that at the initial moment, the new energy network-type flexible direct current transmission system transmits 0 p.u. active power;

[0067] Then, the active power transmitted by the new energy through the grid-type flexible direct current transmission system is uniformly increased to 1 p.u.;

[0068] Then, based on the state matrix A of the new energy network-based flexible direct current transmission system, the eigenvalues ​​of the system under different transmission powers are obtained λ=[λ1,λ2,...,λ n ] T , get the root locus of the system changing with power;

[0069] Finally, based on the above root loci, the small disturbance stability of the new energy grid-type flexible direct current transmission system is analyzed.

[0070] In step B, the dominant control link affecting the oscillation instability mode of the new energy grid-type flexible direct current transmission system is identified based on the participation factor analysis method. The specific process is as follows:

[0071] Firstly, the participation factor matrix of the new energy grid-type flexible direct current transmission system is calculated;

[0072] Then, according to the analysis of participation factors, it is considered that the voltage outer loop controller of the modular multilevel converter is the dominant control link that causes the system oscillation instability mode;

[0073] Finally, the overall control strategy plan for the new energy network-type flexible direct current transmission system is determined.

[0074] The overall control strategy of the new energy network type flexible direct current transmission system is as follows:

[0075] The conventional current inner loop controller is retained to realize the fault current limiting function, and a new voltage outer loop controller is designed to ensure that the new energy grid-type flexible direct current transmission system can operate stably in the full power range.

[0076] Step C designs the voltage outer loop control structure of the modular multilevel converter. The specific process is as follows:

[0077] Firstly, the control strategy of the voltage outer loop controller of the modular multilevel converter adopts constant AC voltage control;

[0078] Then, the input signal of the modular multilevel converter voltage outer loop controller is the dq axis component v of the MMC converter transformer grid side voltage pd 、v pq , and the grid-side dq axis voltage reference value of the MMC converter transformer

[0079] In step C, the voltage outer loop realizes the constant AC voltage control function based on the proportional control link. The specific process is as follows:

[0080] The voltage outer loop controller of the modular multilevel converter realizes constant AC voltage control based on the proportional controller, and the dq axis voltage deviation signal After being amplified by the proportional controller, the current inner loop dq axis current reference value is generated

[0081] The remaining control system structure in step C is consistent with the conventional control system, and the specific process is as follows:

[0082] The voltage outer loop controller of the modular multilevel converter is cascaded with the conventional current control. The structure of the rest of the MMC control system remains unchanged. The WFMMC voltage outer loop controller converts the current inner loop dq axis current reference value It is input to the inner current loop as the output signal.

[0083] Step D designs a method for calculating the voltage outer loop parameters of a modular multilevel converter, and calculates the parameters of the voltage outer loop proportional control link of the modular multilevel converter according to the given AC voltage maximum deviation requirement. The specific process is as follows:

[0084] First, the WFMMC voltage outer loop controller parameter is the proportional coefficient of the proportional controller;

[0085] Then, the above-mentioned proportionality coefficient is calculated based on the maximum allowable deviation rate of the given AC voltage.

[0086] Specifically, in step A, an analytical model of a network-building flexible direct current transmission system is established. The specific process is as follows:

[0087] Firstly, the grid-type flexible direct current transmission system consists of MMC, receiving-end MMC and DC lines.

[0088] Then, since the grid-type flexible DC receiving-end MMC usually adopts a constant DC voltage control strategy, when the receiving-end power grid is a strong power grid and the DC voltage control bandwidth is small, the receiving-end MMC can maintain a constant DC voltage.

[0089] Finally, a simplified analysis is performed. The analytical model of the grid-type flexible direct current transmission system established by the present invention equates the receiving-end MMC and the DC circuit to a constant voltage source, retaining the dynamic characteristics of the MMC. The MMC analytical model is composed of the MMC main circuit analytical model and the control system analytical model. The MMC control system mainly includes a voltage outer loop controller using constant AC voltage control, a current inner loop controller, and a circulating current suppression controller, wherein the voltage outer loop controller adopts a conventional control structure based on a proportional-integral (PI) controller.

[0090] Specifically, in step A, a small disturbance stability model of the whole system is established, as follows:

[0091]

[0092] In the formula, Δx is the system linearization state variable; Δu is the system linearization input variable; Δy is the system linearization output variable; A is the system state matrix; B is the system input matrix; C is the system output matrix; and D is the system direct transmission coefficient.

[0093] Specifically, the participation factor matrix of the new energy grid-type flexible direct current transmission system in step B is:

[0094] P=[P1,P2,...,P n ] T ,

[0095] Among them, P i =[p 1i ,p 2i ,...,p ni ] T =[φ 1i ψ i1 ,φ 2i ψ i2 ,...,φ ni ψ in ] T ;

[0096] In the formula, φ ki is the right eigenvector Φ i The kth term of ik is the left eigenvector Ψ iThe kth item of .

[0097] Specifically, the participation factor in step B is p ki .

[0098] Specifically, the WFMMC new voltage outer loop controller parameter in step D is the proportional coefficient k of the proportional controller u ;

[0099] The proportionality coefficient k u Calculate based on the maximum allowable deviation rate δ of a given AC voltage;

[0100] The proportionality coefficient k u The calculation formula is

[0101] Embodiment 1

[0102] based on Figure 1 The direct-drive wind power grid-type flexible direct transmission system shown is used to analyze and explain the method of the present invention. Figure 1 The direct-drive wind farm shown is composed of multiple direct-drive wind turbines, which are connected to the 35kV busbar through back-to-back converters and machine-end transformers in turn, and are then boosted to 110kV by busbar transformers and connected to the grid-type flexible direct current transmission system through collector lines.

[0103] Figure 2 The simplified equivalent model of the direct-drive wind power grid-connected flexible direct current transmission system is presented. Since the direct-drive wind turbine machine-side converter (MSC) and the grid-side converter (GSC) are decoupled due to the DC capacitor voltage stabilization, the interaction between the direct-drive wind turbine and the MSC and the flexible direct current transmission system can be ignored. To simplify the analysis, the direct-drive wind turbine and the MSC are equivalent to a constant power source, and the direct-drive wind farm aggregation model is established using the single-machine multiplication method.

[0104] The grid-type flexible direct current transmission system consists of a MMC (Wind Farm Connected MMC, WFMMC) connected to a direct-drive wind farm, a grid-side MMC (GSMMC) and a DC line. Since GSMMC usually adopts a constant DC voltage control strategy, when the receiving grid is a strong grid and the DC voltage control bandwidth is small, the receiving MMC can maintain a constant DC voltage, so the receiving MMC and the DC circuit can be equivalent to a constant voltage source.

[0105] like Figure 2As shown in the figure, WFMMC directly sets the angle of coordinate transformation of the fixed AC voltage control to output AC voltage with constant frequency and amplitude, thus providing stable voltage and frequency support for the direct-drive wind farm. and WFMMC valve side current Performing Parker transformation, we can get and The dq-axis component u pd 、u pq 、i vd 、i vq , where ω0 is the angular frequency of the power frequency.

[0106] WFMMC constant AC voltage control usually adopts voltage and current dual-loop control structure. The voltage outer loop uses PI controller according to the PCC point voltage dq axis reference value To u pd 、u pq Control and output WFMMC valve side dq axis current reference value The inner current loop further uses a PI controller to achieve closed-loop tracking of the AC current.

[0107] At the same time, WFMMC is usually equipped with a circulating current suppressor to suppress the circulating current in the bridge arm. Performing Parker transformation, we can get The dq axis component i cird and i cirq Circulation suppressor with i cird and i cirq is the bridge arm circulating current feedback signal, according to the dq axis circulating current reference value The PI controller is used to suppress the circulating current in the bridge arm.

[0108] To improve the circulation suppression effect, Usually, they are set to 0. The GSC control system mainly includes phase-locked loop and Parker transformation, constant DC voltage control and current inner loop control. The phase-locked loop tracks the bus voltage of the direct-drive wind farm. Phase, output GSC control system coordinate transformation phase θ PLL According to θ PLL Current on GSC valve side and Performing Parker transformation, we can get and The dq axis component i od 、i oq 、u td 、u tqThe DC voltage control uses a PI controller to control the DC voltage reference value. For DC voltage E dc Control and output the d-axis GSC valve side current reference value The current inner loop uses a PI controller to adjust the current reference value of the GSC dq axis valve side Achieve closed-loop tracking of GSC valve-side current, Typically set to 0 for unit power operation.

[0109] The collector circuit is modeled using a π-type equivalent circuit, R line , L line , C line The total resistance, inductance and ground capacitance of the collector line are calculated respectively. m Mechanical power injected into the direct drive fan, C dc is the DC capacitor of the back-to-back converter of the direct-drive wind turbine, L f is the GSC valve side series filter reactor, i line is the collector line current.

[0110] The method flow diagram of the present invention is as follows Figure 3 As shown, it mainly includes the following steps:

[0111] A. Establish a small disturbance stability analysis model for direct-drive wind power grid-connected flexible direct transmission system;

[0112] B. Based on the root locus method, the small disturbance stability of the direct-drive wind power grid-connected flexible direct current transmission system is analyzed with power changes. Based on the participation factor analysis method, the dominant control link of the system oscillation instability mode is identified, and the overall design scheme of the control strategy of the direct-drive wind power grid-connected flexible direct current transmission system is determined.

[0113] C. Design the WFMMC voltage outer loop control structure. The voltage outer loop realizes the constant AC voltage control function based on the proportional control link. The rest of the control system structure is consistent with the conventional control system.

[0114] D. Design the WFMMC voltage outer loop parameter calculation method, calculate the WFMMC voltage outer loop proportional control link parameters according to the given AC voltage maximum deviation requirements, and ensure that the grid-connected point voltage steady-state deviation meets the system requirements within the full power operation range.

[0115] The specific modeling process of the small disturbance stability analysis model of the direct-drive wind power grid-connected flexible direct transmission system described in step A is as follows:

[0116] Firstly, an analytical model of direct-drive wind farm and collection line is established.

[0117] like Figure 2 As shown in the figure, the main circuit analytical model of the direct-drive wind farm can be described as:

[0118]

[0119] In the formula, k T1 =0.69 / 33 is the transformer ratio at the machine end, u od 、u oq is the dq-axis component of the GSC valve-side voltage.

[0120] Figure 4 The GSC constant DC voltage control, current inner loop and phase-locked loop structure are shown. The analytical model of the direct-drive wind farm control system can be described as:

[0121]

[0122] In the formula, They are the proportional and integral coefficients of the PI controller in the GSC constant DC voltage control. It is the state variable of the integral link in the GSC constant DC voltage control; are the proportional and integral coefficients of the PI controller in the GSC current inner loop, They are the state variables of the integral link of the dq axis channel of the inner loop of the GSC current respectively; are the proportional and integral coefficients of the PI controller in the phase-locked loop, They are the integral link state variables of the phase-locked loop PI controller respectively; They are the dq axis voltage modulation signals output by the GSC control system respectively.

[0123] The analytical model of the collector line can be described as:

[0124]

[0125] In the formula, k T2 =33 / 110, k T3 =110 / 210 are the transformation ratios of bus transformer T2 and converter transformer T3 respectively, i ld 、i lq is the dq-axis component of the collector line current.

[0126] Secondly, an analytical model of the grid-type flexible direct current transmission system is established.

[0127] The analytical model of the WFMMC main circuit can be described as:

[0128]

[0129] In the formula, i dc is the DC current output by WFMMC; u cdc is the DC component of the submodule capacitance, u cac1d 、u cac1qThey are the dq axis components of the submodule capacitance fundamental frequency, u cac2d 、u cac2q They are the double frequency dq axis components of the submodule capacitance respectively; are the base frequency dq axis voltage modulation signals output by the WFMMC control system, are the circulating current suppression dq axis voltage modulation signals output by the WFMMC control system; R arm is the equivalent resistance of the WFMMC bridge arm, L arm is the WFMMC bridge arm smoothing reactor, and N is the number of bridge arm series sub-modules.

[0130] WFMMC conventional voltage outer loop and current inner loop control structure is as follows Figure 6 As shown, its analytical model can be described as:

[0131]

[0132] In the formula, are the proportional and integral coefficients of the WFMMC voltage outer loop PI controller, They are the state variables of the integral link in the dq axis channel of the WFMMC voltage outer loop; are the proportional and integral coefficients of the WFMMC voltage inner loop PI controller, are the state variables of the integral link in the dq axis channel of the inner loop of the WFMMC voltage; L T3 are the equivalent reactances of converter transformer T3 respectively.

[0133] The structure of WFMMC circulating current suppressor is as follows Figure 7 As shown, its analytical model can be described as:

[0134]

[0135] In the formula, are the proportional and integral coefficients of the PI controller of the WFMMC circulating current suppressor, They are the state variables of the integral link in the dq axis channel of the WFMMC circulating current suppressor.

[0136] Finally, by linearizing the above analytical model at the steady-state operation point, we can obtain the small disturbance stability model of the whole system:

[0137]

[0138] In the formula, Δx is the system linearization state variable; Δu is the system linearization input variable; Δy is the system linearization output variable; A is the system state matrix; B is the system input matrix; C is the system output matrix; and D is the system direct transmission coefficient.

[0139] The root locus method described in step B is used to analyze the small disturbance stability of the direct-drive wind turbine grid-type flexible direct delivery system with power changes. The specific analysis process is as follows:

[0140] At the initial moment, the direct-drive wind power grid-type flexible direct current transmission system transmits 0 p.u. active power, and then the active power transmitted by the system is uniformly increased to 1 p.u. Based on the system state matrix A, the eigenvalues ​​of the system under different transmission powers are obtained λ=[λ1,λ2,...,λ n ] T , plotting the system characteristic value versus power curve, we can get the following Figure 8 The root locus of the system shown varies with power.

[0141] The eigenvalue of mode 1 moves to the right half plane of the complex plane as the system power increases, and the system damping gradually weakens. When the system power increases to 0.77pu, the root trajectory of mode 1 crosses the imaginary axis and enters the right half plane, and the system will experience small disturbance oscillation instability.

[0142] The specific process of identifying the leading control link of the oscillation instability mode 1 of the direct-driven wind power grid-connected flexible direct current transmission system based on the participation factor method described in step B is as follows:

[0143] First, according to the system state space matrix A, the direct-drive wind turbine grid-type flexible direct delivery system participation factor matrix P = [P1, P2, ..., P n ] T , where P i =[p 1i ,p 2i ,...,p ni ] T =[φ 1i ψ i1 ,φ 2i ψ i2 ,...,φ ni ψ in ] T , where φ ki is the right eigenvector Φ i The kth term of ik is the left eigenvector Ψ i The kth item of the oscillation instability mode 1 of the system is selected as the participation factor item P λ1 =[p 1,λ1 ,p 2,λ1 ,...,p n,λ1 ] T , and draw as Fig. 9 The bar chart of the degree of participation of each state variable in mode 1 is shown. Fig. 9The results show that the control link that participates most in mode 1 in the direct-drive wind farm is the phase-locked loop of the GSC, and the control link that participates most in mode 1 in the WFMMC is the voltage outer loop.

[0144] There are differences in the control structures of different new energy units. The present invention designs a stable control strategy for the new energy network-type flexible direct current transmission system. Therefore, it is believed that the MMC voltage outer loop controller is the dominant control link that causes the system oscillation instability mode.

[0145] Finally, the overall control strategy of the new energy grid-type flexible direct current transmission system is determined. The conventional current inner loop controller is retained to realize the fault current limiting function, and a new voltage outer loop controller is designed to ensure that the new energy grid-type flexible direct current transmission system can operate stably in the full power range.

[0146] The WFMMC voltage outer loop control structure designed in step C is as follows: Fig.10 As shown, the dq axis voltage reference value at PCC point is and PCC point dq axis voltage u pd 、u pq The error signal is proportional to the After amplification, the dq axis current reference values ​​are generated respectively The voltage outer loop controller based on proportional control is cascaded with the current inner loop, and the current inner loop adopts a conventional structure based on a PI controller.

[0147] In order to ensure that the AC voltage of the direct-drive wind farm can meet the voltage deviation requirements within the full power range, step D provides a WFMMC voltage outer loop parameter calculation method, the specific process is:

[0148] Calculate the voltage outer loop control parameters of WFMMC based on proportional control according to the given AC voltage maximum deviation requirement;

[0149] The voltage outer loop control parameter of the WFMMC based on proportional control is the proportional coefficient of the proportional controller

[0150] The proportionality factor Calculate based on the maximum allowable deviation rate δ of a given AC voltage;

[0151] The proportionality coefficient k u The calculation formula is

[0152] based on Figure 2 The direct-drive wind power grid-type flexible direct transmission system shown is built with relevant models in PSCAD / EMTDC electromagnetic transient simulation software, and the control effect of the present invention is demonstrated by comparing simulation results.

[0153] Working condition 1

[0154] At the initial moment, the WFMMC voltage outer loop adopts the conventional control structure based on the PI controller, and the system transmits 0.7pu active power. At 0.5s, the output active power of the direct-drive wind farm is stepped to 0.77pu.

[0155] Fig.11 The dynamic response simulation results of the system's active power transmission are shown. After the system is disturbed at 0.5s, the active power transmitted by the system rapidly increases to 0.77pu, and then gradually oscillates and diverges.

[0156] Working condition 2

[0157] The initial moment WFMMC voltage outer loop adopts a control structure based on a proportional controller The system transmits 0.7 pu of active power, and at 0.5 s the output active power of the direct-drive wind farm jumps to 1 p.u.

[0158] Fig.12 The dynamic response simulation results of the system's active power transmission are demonstrated. After the system is disturbed at 0.5s, the active power transmitted by the system rapidly increases to 1p.u., and then stably transmits 1p.u. of power.

[0159] The simulation results of working conditions 1 and 2 can prove that the method of the present invention can effectively solve the small disturbance stability instability problem existing in the traditional direct-drive wind power grid-type flexible direct transmission system, and ensure the small disturbance stability of the system within the full power range.

[0160] Working condition three

[0161] The initial moment WFMMC voltage outer loop adopts a control structure based on a proportional controller The system transmits 0p.u. active power. At 0.4s, the system transmits active power from 0p.u. to 1p.u., and at 1.1s Increase to 50, at 1.6s Increased to 100.

[0162] Fig.13 Shows the effective value of the voltage at the PCC point u p Simulation results of dynamic response. After the system transmits active power from 0p.u. to 1p.u. in 0.4s, u p After a short overshoot process, it increases to 114.9kV, and the voltage deviation rate is about 4.6%; at 1.1s Increase to 50, then u p Rapidly reduced to 112kV, the voltage deviation rate is about 1.8%; at 1.6s Increase to 100, then up The voltage deviation rate is about 0.9% when it is quickly reduced to 111kV. The following table shows the different Comparison results of the theoretical calculated value and the simulation value of the maximum deviation rate of the PCC point voltage under the condition.

[0163]

[0164] At this point, the control strategy and parameter selection method of the new energy grid-connected flexible direct current transmission system proposed in the present invention have been explained.

Claims

1. The control strategy and parameter selection method of the new energy network-type flexible direct current transmission system is characterized by: The following steps are involved: A. Establish a small disturbance stability analysis model for the new energy network-type flexible direct current transmission system; B. Analyze the small disturbance stability of the new energy grid-type flexible direct current transmission system with power changes based on the root locus method, identify the dominant control link affecting the oscillation instability mode of the new energy grid-type flexible direct current transmission system based on the participation factor analysis method, and determine the overall design scheme of the control strategy of the new energy grid-type flexible direct current transmission system; C. Design a modular multilevel converter voltage outer loop control structure. The voltage outer loop realizes the constant AC voltage control function based on the proportional control link. The rest of the control system structure is consistent with the conventional control system. D. Design a method for calculating the voltage outer loop parameters of a modular multilevel converter, and calculate the parameters of the voltage outer loop proportional control link of the modular multilevel converter according to the given AC voltage maximum deviation requirements.

2. The control strategy and parameter selection method of the new energy network-based flexible direct current transmission system according to claim 1 is characterized by: Step A establishes a small disturbance stability analysis model for the new energy network-type flexible direct current transmission system. The specific process is as follows: First, establish an analytical model of new energy stations and collection lines; Then, an analytical model of the new energy network-type flexible direct current transmission system is established; Finally, based on the above analytical model, a small disturbance stability model of the new energy network-type flexible direct current transmission system is established.

3. The control strategy and parameter selection method of the new energy network-type flexible direct current transmission system according to claim 2 is characterized by: The new energy station adopts an equivalent aggregation model, and the collection line model is modeled using a P-type equivalent circuit.

4. The control strategy and parameter selection method of the new energy network-type flexible direct current transmission system according to claim 1 is characterized by: In step B, the root locus method is used to analyze the small disturbance stability of the new energy grid-type flexible direct current transmission system with power changes. The specific process is as follows: First, it is set that at the initial moment, the new energy network-type flexible direct current transmission system transmits 0 p.u. active power; Then, the active power transmitted by the new energy through the grid-type flexible direct current transmission system is uniformly increased to 1 p.u.; Then, based on the state matrix A of the new energy network-based flexible direct current transmission system, the eigenvalues ​​of the system under different transmission powers are obtained λ=[λ1,λ2,...,λ n ] T , get the root locus of the system changing with power; Finally, based on the above root loci, the small disturbance stability of the new energy grid-type flexible direct current transmission system is analyzed.

5. The control strategy and parameter selection method of the new energy network-based flexible direct current transmission system according to claim 1 is characterized by: In step B, the dominant control link affecting the oscillation instability mode of the new energy grid-type flexible direct current transmission system is identified based on the participation factor analysis method. The specific process is as follows: Firstly, the participation factor matrix of the new energy grid-type flexible direct current transmission system is calculated; Then, according to the analysis of participation factors, it is considered that the voltage outer loop controller of the modular multilevel converter is the dominant control link that causes the system oscillation instability mode; Finally, the overall control strategy plan for the new energy network-type flexible direct current transmission system is determined.

6. The control strategy and parameter selection method of the new energy network-based flexible direct current transmission system according to claim 5 is characterized by: The overall control strategy of the new energy network type flexible direct current transmission system is as follows: The conventional current inner loop controller is retained to realize the fault current limiting function, and a new voltage outer loop controller is designed to ensure that the new energy grid-type flexible direct current transmission system can operate stably in the full power range.

7. The control strategy and parameter selection method of the new energy network-based flexible direct current transmission system according to claim 1 is characterized by: Step C designs the voltage outer loop control structure of the modular multilevel converter. The specific process is as follows: Firstly, the control strategy of the voltage outer loop controller of the modular multilevel converter adopts constant AC voltage control; Then, the input signal of the modular multilevel converter voltage outer loop controller is the dq axis component v of the MMC converter transformer grid side voltage pd 、v pq , and the grid-side dq axis voltage reference value of the MMC converter transformer 8. The control strategy and parameter selection method of the new energy network-based flexible direct current transmission system according to claim 1 is characterized by: In step C, the voltage outer loop realizes the constant AC voltage control function based on the proportional control link. The specific process is as follows: The voltage outer loop controller of the modular multilevel converter realizes constant AC voltage control based on the proportional controller, and the dq axis voltage deviation signal After being amplified by the proportional controller, the current inner loop dq axis current reference value is generated 9. The control strategy and parameter selection method of the new energy network-based flexible direct current transmission system according to claim 1 is characterized by: The remaining control system structure in step C is consistent with the conventional control system, and the specific process is as follows: The voltage outer loop controller of the modular multilevel converter is cascaded with the conventional current control. The structure of the rest of the MMC control system remains unchanged. The WFMMC voltage outer loop controller converts the current inner loop dq axis current reference value It is input to the inner current loop as the output signal.

10. The control strategy and parameter selection method of the new energy network-based flexible direct current transmission system according to claim 1 is characterized by: Step D designs a method for calculating the voltage outer loop parameters of a modular multilevel converter, and calculates the parameters of the voltage outer loop proportional control link of the modular multilevel converter according to the given AC voltage maximum deviation requirement. The specific process is as follows: First, the WFMMC voltage outer loop controller parameter is the proportional coefficient of the proportional controller; Then, the above-mentioned proportionality coefficient is calculated based on the maximum allowable deviation rate of the given AC voltage.

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