Doubly fed and direct-drive hybrid wind power generation system and its control method

Abstract

This invention provides a doubly-fed and direct-drive hybrid wind power generation system and its control method. The direct-drive wind turbine generator set includes a first impeller, a permanent magnet direct-drive generator, a first generator-side converter, and a first grid-side converter connected in sequence. The doubly-fed wind turbine generator set includes a second impeller, a gearbox, a doubly-fed generator, a second generator-side converter, and a second grid-side converter connected in sequence. The stator of the doubly-fed generator, the second grid-side converter, and the first grid-side converter are connected in parallel. A nine-switch converter is connected between the wind turbine generator set and the step-up transformer. The midpoint of the upper arm of the nine-switch converter outputs a compensation voltage. After passing through an LC filter, the compensation voltage is injected by a transformer to superimpose the compensation voltage with the grid voltage. A bypass switch is used to short-circuit the injection transformer when the grid voltage is normal. The lower channel of the nine-switch converter outputs a compensation current, which is connected to the output lines of the first and second grid-side converters via a filter reactor.

Description

Technical Field

[0001] This invention relates to the field of wind power generation, and more specifically, to a doubly-fed and direct-drive hybrid wind power generation system and its control method. Background Technology

[0002] With my country's goal of "carbon peaking and carbon neutrality," "electricity substitution" technology, using clean energy as a key power source, will become an important pathway to achieving low-carbon development. Considering that centralized wind power is mostly located at the end of the grid, far from load centers, and incurring high transmission costs, decentralized wind power, which connects to the grid near load centers for local consumption and flexible investment and construction, will become another important direction for wind power development, possessing significant theoretical and engineering practical value. Due to diversified investment and asynchronous construction in decentralized wind power, there are scenarios where doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) operate in parallel. Furthermore, decentralized wind power projects are typically applied in low-to-medium wind speed areas around cities. Direct-drive turbines have better adaptability and higher power generation efficiency than doubly-fed turbines in low-wind-speed and low-speed scenarios, and can complement the advantages of doubly-fed turbines. Therefore, researching the parallel operation of DFIG-PMSG wind turbines is of practical significance. Decentralized wind power connections are close to loads, have lower voltage levels, smaller short-circuit capacity, and weak grid support. The start-up and shutdown of large-capacity rotating equipment, the use of nonlinear loads, and even grid faults can lead to power quality problems such as voltage rise, drop, imbalance, and harmonics in the grid. In distributed wind power scenarios, the higher wind power penetration capacity places higher demands on the fault voltage ride-through capability of wind turbines.

[0003] Low voltage ride-through (LVRT) schemes for DFIG units generally employ a combination of rotor-side Crowbar circuits and DC-side Chopper circuits in the excitation inverter for hardware support. Under severe grid voltage dips, the Crowbar circuit causes the DFIG's operating characteristics to shift towards a squirrel-cage induction machine, increasing reactive power consumption and hindering grid voltage recovery. Under grid voltage asymmetry and harmonic conditions, the direct grid connection of the DFIG stator will cause electromagnetic torque oscillations, and alternating loads will affect the lifespan of the mechanical structure. PMSG units have larger full-power inverters, and under grid voltage dips, transient differential power can be consumed through the converter's DC bus unloading circuit. The grid-side converter can also increase reactive power output, assisting in grid voltage recovery. Existing literature has proposed the above-mentioned LVRT schemes for PMSG units, but significant deviations exist in active and reactive power control under weak grid conditions.

[0004] In research on topology for improving fault voltage ride-through in wind turbines, existing literature has proposed using a dynamic voltage restorer (DVR) to achieve low-voltage ride-through in DFIG units, maintaining stable terminal voltage through series voltage compensation. Under grid voltage rise conditions, the DVR will continuously output active power, and its DC side cannot maintain the set voltage. Existing literature has extended the application of DVRs to comprehensive voltage sag management in modern industrial parks. Other literature has proposed installing energy storage devices on the DC side of the DVR, but this is limited to energy storage only during grid voltage dips, restricting its application scenarios. Existing literature has applied a unified power quality conditioner (UPQC) to flexible fault ride-through in wind power systems. Its typical structure combines a DVR and an active power filter (APF) in a back-to-back converter configuration. This topology can maintain the DC side voltage of the DVR through the APF, making it universally applicable to grid voltage rise and dip conditions, and simultaneously providing voltage compensation while achieving active filtering, reactive power compensation, and other current compensation functions. Existing literature has simplified back-to-back converters with 12 switching elements to 9 switching elements and studied the evolution and application of back-to-back converters. Existing literature has also applied nine-switch UPQCs to low-voltage ride-through scenarios in DFIG units, improving the DFIG unit's normal voltage ride-through capability through series voltage compensation and parallel current compensation. However, this research focuses primarily on the UPQC function, with limited research on the characteristics of the nine-switch converter.

[0005] Compared to back-to-back converters, the nine-switch converter reduces three switching elements, resulting in simplified drive and protection circuits, and consequently reduced switching losses and converter size. Existing literature has studied the nine-switch converter as a back-to-back converter for DFIG and PMSG units, with control strategies consistent with traditional strategies, but without exploring ways to improve fault voltage ride-through capability. Existing literature has also conducted in-depth research on the nine-switch converter replacing the grid-side converter in doubly-fed generator units, implementing a unified power quality controller, and using it as an excitation inverter. These studies expand the application scenarios of the nine-switch converter, but do not provide detailed solutions to the problem of high DC-side voltage inherent in the nine-switch converter itself. Existing literature has studied the SVPWM modulation method suitable for nine-switch converters, which is suitable for excitation control of rotating equipment. Compared to the SPWM modulation method, this modulation method can improve DC-side voltage utilization. Existing literature introduces a variable modulation ratio allocation method for a nine-switch converter. By utilizing the different output voltage requirements of the two midpoints of the bridge arm, two operating modes are set and different modulation ratios are configured. This method achieves variable modulation ratio, but the simple mode setting limits the effect of improving DC-side voltage utilization by optimizing the modulation ratio configuration. Summary of the Invention

[0006] The purpose of this invention is to provide a doubly-fed and direct-drive hybrid wind power generation system and its control method, which can optimize the ride-through performance of doubly-fed and direct-drive distributed wind power under conditions such as grid voltage drop, asymmetry, and harmonics.

[0007] In one aspect, a doubly-fed and direct-drive hybrid wind power generation system includes: a direct-drive wind turbine generator set, a doubly-fed wind turbine generator set, a step-up transformer, a nine-switch converter, an LC filter, a filter reactor, an injection transformer, and a bypass switch.

[0008] A direct-drive wind turbine generator set includes a first impeller, a permanent magnet direct-drive generator, a first machine-side converter, and a first grid-side converter connected in sequence; a doubly-fed wind turbine generator set includes a second impeller, a gearbox, a doubly-fed generator, a second machine-side converter, and a second grid-side converter connected in sequence.

[0009] The stator of the doubly fed generator and the second grid-side converter are connected in parallel with the first grid-side converter. The nine-switch converter is connected between the direct-drive wind turbine generator set and the doubly fed wind turbine generator set and the step-up transformer. The midpoint of the upper side of the bridge arm of the nine-switch converter is used as the upper channel output compensation voltage. After the compensation voltage is filtered by an LC filter, the compensation voltage is superimposed on the grid voltage by the injection transformer. The bypass switch is used to short-circuit the injection transformer when the grid voltage is normal, and disable the voltage compensation function.

[0010] The lower midpoint of the bridge arm of the nine-switch converter serves as the lower channel output compensation current, which is connected to the output lines of the first grid-side converter and the second grid-side converter via a filter reactor.

[0011] Furthermore, the nine-switch converter includes a controller for determining the logic state of the upper and lower channels of each phase arm in the nine-switch converter according to the SPWM modulation method, and then calculating the drive logic level of each switching element in each arm.

[0012] Furthermore, the reference value U of the compensation voltage dvr 'Calculated according to the following formula (1):

[0013] U dvr '=-ΔU 1abc -U -1abc -U nabc Formula (1)

[0014] Wherein, ΔU 1abc For positive sequence voltage deviation, U -1abc For negative sequence voltage and U nabc This is a harmonic voltage.

[0015] Furthermore, the reference value of the compensation current is calculated according to the following formula (2):

[0016] I apf '=ΔI p +ΔI q -I nabc Formula (2)

[0017] Where, ΔI P For active power compensation component, ΔI Q For reactive power compensation components and I nabc This is the harmonic compensation component.

[0018] Furthermore, the upper channel output compensation line voltage fundamental amplitude U of the nine-switch converter abm The maximum fundamental line voltage amplitude U of the lower channel output current compensation xym The results are obtained by calculating according to the following formulas (3) and (4):

[0019]

[0020]

[0021] Among them, U c For carrier signal, U dc U is the DC-side voltage of the nine-switch converter. ra U is the modulation signal for the upper channel of the nine-switch converter. rx This is the modulation signal for the lower channel of the nine-switch converter.

[0022] Furthermore, the controller is used to perform modulation based on the new upper channel modulation signal and the new lower channel modulation signal, wherein the upper channel modulation signal is superimposed with the third harmonic of the corresponding phase by k times to form the new upper channel modulation signal, and the lower channel modulation signal is superimposed with the third harmonic of the corresponding phase by k times to form the new lower channel modulation signal.

[0023] Furthermore, the modulation ratios of the upper and lower channels of the nine-switch converter are adjusted according to the grid voltage.

[0024] Furthermore, when the grid voltage is within the preset normal range, the upper channel of each phase arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively.

[0025] When the grid voltage deviates from the preset voltage threshold by -20% to 20%, the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively.

[0026] When the grid voltage deviates from the preset voltage threshold by 20% to 40%, increase the modulation ratio of the upper channel and decrease the modulation ratio of the lower channel.

[0027] When the grid voltage deviates from the preset voltage threshold by more than 40%, the lower channel of each phase arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 1 and 0, respectively.

[0028] Secondly, a control method for a doubly-fed and direct-drive hybrid wind power generation system is provided for the doubly-fed and direct-drive hybrid wind power generation system, the control method comprising:

[0029] The nine-switch converter outputs a compensation voltage on each phase bridge arm and performs closed-loop voltage control based on the difference between the compensation voltage reference value and the compensation voltage.

[0030] The nine-switch converter outputs a compensation current in the lower channel of each phase bridge arm, and performs current closed-loop control based on the difference between the compensation current reference value and the compensation current.

[0031] The modulation signal of the upper channel of each phase arm of the nine-switch converter is superimposed with the third harmonic of the corresponding phase by k times to form the new modulation signal of the upper channel. The modulation signal of the lower channel of each phase arm of the nine-switch converter is superimposed with the third harmonic of the corresponding phase by k times to form the new modulation signal of the lower channel.

[0032] The modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter are determined according to the grid voltage. The sum of the modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter is less than or equal to 1.

[0033] According to the SPWM modulation method, the nine-switch converter is controlled based on the new upper channel modulation signal, the new lower channel modulation signal, the modulation ratio of the upper channel, and the modulation ratio of the lower channel.

[0034] Furthermore, the determination of the modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter based on the grid voltage includes:

[0035] When the grid voltage is within the preset normal range, the upper channel of each phase bridge arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively.

[0036] When the grid voltage deviates from the preset voltage threshold by -20% to 20%, the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively.

[0037] When the grid voltage deviates from the preset voltage threshold by 20% to 40%, increase the modulation ratio of the upper channel and decrease the modulation ratio of the lower channel.

[0038] When the grid voltage deviates from the preset voltage threshold by more than 40%, the lower channel of each phase arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 1 and 0, respectively.

[0039] The doubly-fed and direct-drive hybrid wind power generation system and its control method of the present invention utilize the asynchronous characteristics of voltage compensation and current compensation on the DC-side voltage demand of the nine-switch converter. A strategy for dynamically allocating modulation ratio limiting based on the degree of voltage fault is designed. The UPQC with the nine-switch converter as the converter structure can improve the operating characteristics of the DFIG-PMSG hybrid distributed wind power system under grid voltage deviation, asymmetry, and harmonic conditions, and achieve flexible voltage fault ride-through. The SPWM modulation method of third harmonic injection of the nine-switch converter can improve the DC voltage utilization rate. The dynamic modulation ratio optimization strategy set by utilizing the asynchronous characteristics of voltage compensation and current compensation on inverter voltage demand can further reduce the demand on the DC-side voltage of the nine-switch converter. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a schematic diagram of the structure of a doubly fed and direct-drive hybrid wind power generation system according to Embodiment 1 of the present invention.

[0042] Figure 2 This is a diagram illustrating the overall control strategy of a doubly fed and direct-drive hybrid wind power generation system according to Embodiment 2 of the present invention.

[0043] Figure 3 This is the SPWM modulation method for a doubly fed and direct-drive hybrid wind power generation system according to Embodiment 3 of the present invention.

[0044] Figure 4 This is a schematic diagram of the third harmonic calculation method for a doubly fed and direct-drive hybrid wind power generation system according to Embodiment 4 of the present invention.

[0045] Figure 5 This is a schematic diagram of the modulation ratio optimization allocation of a doubly fed and direct-drive hybrid wind power generation system according to Embodiment 5 of the present invention.

[0046] Figure 6This is a waveform diagram of the third harmonic injection modulation of a doubly fed and direct-drive hybrid wind power generation system according to Embodiment Six of the present invention.

[0047] Figure 7 This is a schematic diagram of the first simulation results of the relevant electrical quantity changes of the doubly-fed and direct-drive hybrid wind power generation system according to the present invention.

[0048] Figure 8 This is a schematic diagram of the second simulation results of the relevant electrical quantity changes of the doubly-fed and direct-drive hybrid wind power generation system according to the present invention.

[0049] Figure 9 This is a schematic diagram of the compensation results for mild asymmetric drops using a nine-switch UPQC according to an embodiment of the present invention.

[0050] Figure 10 This is a schematic diagram of the compensation results for severe asymmetric drops using a nine-switch UPQC according to an embodiment of the present invention.

[0051] Figures 11a-11c This is a schematic diagram of the voltage harmonic compensation result of a nine-switch UPQC according to an embodiment of the present invention.

[0052] Figure 12 This is a flowchart of the control method for a doubly fed and direct-drive hybrid wind power generation system according to Embodiment 7 of the present invention. Detailed Implementation

[0053] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0054] It should be noted that, unless otherwise specified, the following embodiments and features can be combined with each other; and, based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] It should be noted that the following description covers various aspects of embodiments within the scope of the appended claims. It will be apparent that the aspects described herein can be embodied in a wide variety of forms, and any particular structure and / or function described herein is merely illustrative. Based on this invention, those skilled in the art will understand that one aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects set forth in this invention can be used to implement the device and / or practice the method. Furthermore, this device and / or method can be implemented using structures and / or functionalities other than one or more of the aspects set forth in this invention.

[0056] Figure 1This is a schematic diagram of the structure of a doubly-fed and direct-drive hybrid wind power generation system according to Embodiment 1 of the present invention. Figure 1 As shown, the nine-switch converter is an evolution of the back-to-back converter, with three switching elements per phase arm and one power pulse output from each of the two midpoints of the arm. This invention addresses the operational characteristics of distributed doubly-fed direct-drive hybrid wind power systems under different grid voltage conditions. It designs a UPQC system using a nine-switch converter as the converter element, achieving terminal voltage compensation and grid-connected current compensation through series voltage compensation and parallel current compensation. In the nine-switch converter UPQC and doubly-fed direct-drive hybrid wind power system, the stator and grid-side converter of the DFIG are connected in parallel with the grid-side converter of the PMSG. The nine-switch UPQC is connected between the wind turbine and the step-up transformer. The compensation voltage is output from the upper midpoint of the converter arm. After passing through an LC filter circuit, the compensation voltage is superimposed on the grid voltage by the injection transformer. A bypass switch can short-circuit the injection transformer when the grid voltage is normal, disabling the voltage compensation function. The compensation current is output from the lower midpoint of the converter arm and fed into the output line of the wind turbine's grid-side converter via a filter reactor.

[0057] When the grid voltage is not under ideal operating conditions, it often includes one or more combinations of positive-sequence overvoltage, positive-sequence undervoltage, negative-sequence, and harmonics. In this case, the compensation voltage U... dvr The positive sequence voltage deviation ΔU 1abc Negative sequence voltage U -1abc Harmonic voltage U nabc The negative value of the sum. Compensation current I apf Including active power compensation component ΔI P Reactive power compensation component ΔI Q Harmonic compensation component I nabc The negative value. When voltage compensation and current compensation work simultaneously, the difference in active power generated by voltage compensation can be fed into the grid by adjusting the active power compensation component, thereby stabilizing the DC side voltage of the nine-switch converter. When harmonics appear in the load current, harmonic elimination is achieved by outputting the negative value of the harmonic current. The overall control strategy is as follows: Figure 2 As shown. The nine-switch UPQC compensation voltage U dvr and compensation current I apf Controlling this is crucial for optimizing the operation of distributed hybrid wind power systems. Its reference value U... dvr 'and I apf 'They are respectively:

[0058] U dvr '=-ΔU 1abc -U -1abc -U nabc (1)

[0059] I apf '=ΔI p +ΔIq -I nabc (2)

[0060] The nine-switch converter is a voltage source converter, and the control of both the compensation voltage and compensation current requires modulation using a voltage reference signal. The upper channel is set at the midpoint of the upper side of each phase arm of the nine-switch converter, outputting the compensation voltage, whose modulation signal is U. ra The lower channel is located at the midpoint of the lower side of each phase arm, outputting the compensation current, and its modulation signal is U. rx . Will U ra Biased in the direction of positive voltage, U rx Biased in the direction of negative voltage to ensure U ra >U rx

[25] At this point, using the SPWM modulation method, the logic states of the upper and lower midpoints of each phase bridge arm can be obtained, and then the drive logic level of each switching element in the bridge arm can be calculated. The SPWM modulation method for a nine-switch converter is as follows: Figure 3 As shown.

[0061] According to the above modulation method, the upper channel output compensates for the fundamental amplitude U of the line voltage. abm The maximum fundamental line voltage amplitude U of the lower channel output current compensation xym They are respectively:

[0062]

[0063]

[0064] Among them, U c For carrier signal, U dc This represents the DC-side voltage of the nine-switch converter. Under no-modulation conditions, i.e., m... a +m x ≤1, the highest voltage utilization rate is only

[0065]

[0066] To ensure sufficient voltage output on the voltage compensation side and effective reactive power flow control on the current compensation side of a nine-switch UPQC, both the upper and lower channels of the nine-switch converter require a certain voltage output capability. Therefore, the DC-side voltage of a nine-switch UPQC is often relatively high. To reduce the DC-side voltage of the nine-switch converter, its application in a doubly-fed direct-drive hybrid distributed wind power system can be studied from two aspects: modulation methods and system control strategies.

[0067] In the SPWM modulation strategy of the nine-switch converter, the amplitude of the modulation signal is reduced by superimposing the initial modulation signal with the corresponding third harmonic signal, thereby increasing the modulation ratio while avoiding modulation distortion. The nine-switch converter is a three-phase, three-wire VSI converter. Its three-phase symmetrical circuit structure, lacking a neutral line, has a very high zero-sequence impedance, which naturally filters out the third harmonic component and restores the initial modulation wave. Taking the above channel as an example, the modulation signal U... ra The third harmonic, corresponding to a phase multiple of k, is superimposed to form a new modulation signal U. ra '. By solving U ra 'The minimum amplitude indicates that when k = 1 / 6, the amplitude of the original modulated signal can be reduced to...' This makes the voltage utilization rate from Increased to 1.

[0068] U ra =U ram cos(ωt) (6)

[0069] U ra '=U ram cos(ωt)+kU ram cos(3ωt) (7)

[0070] The third harmonic is injected into the DQ coordinate inverse transformation process of the embedded control strategy, based on the current dq axis voltage value U. d U q Calculate the amplitude U m and phase angle θ c The third harmonic quantity, calculated using real-time phase angle and amplitude, allows for optimal injection of the third harmonic when the fundamental amplitude and phase change. Modulation methods include... Figure 4 As shown.

[0071] Based on the design of a 1800V DC-side voltage for the nine-switch converter, the third harmonic injection technology can make the sum of the effective values ​​of the two-channel output line voltages of the nine-switch converter 1470V, which can simultaneously meet the DC voltage requirements for voltage compensation and current compensation.

[0072]

[0073] The topology of the nine-switch converter determines its modulation signal U. ra >U rx To meet the above constraints, the modulation signals of the upper and lower channels are DC biased and then limited, which means limiting the maximum modulation ratio of the corresponding channel.

[0074] The output voltage on the voltage compensation side of the converter increases as the grid voltage deviates from its rated value. On the current compensation side of the converter, the output voltage is adjusted near the grid voltage to control the bidirectional flow of reactive power.

[0075] From the perspective of wind power system fault ride-through operation, voltage compensation is more important than current compensation. Therefore, under voltage drop conditions, priority is given to ensuring the modulation ratio allocation for voltage compensation, while under normal grid voltage conditions, a larger modulation ratio range is allocated for current compensation. Based on the principle of differentiated allocation between the two, a dynamic allocation method for modulation ratio limiting as the grid voltage changes is designed to avoid configuring the DC-side voltage according to the maximum demand of both.

[0076] When the grid voltage is normal, the upper channel is closed, and the modulation ratio limit m is allocated. a =0.2, m x =0.8, providing sufficient output voltage for the current compensation side. At this point, the nine-switch converter implements current compensation functions, including reactive power compensation and harmonic current compensation. When the voltage deviation U dev Maintain the above modulation ratio between -20% and 20%; when the voltage deviation U dev Between 20% and 40%, as the voltage deviation increases, the upper channel modulation ratio m increases. a Reduce the lower channel modulation ratio m x This provides an output voltage to the voltage compensation side that matches the voltage drop conditions. At this time, the voltage compensation and current compensation functions of the nine-switch converter are simultaneously activated; when the voltage deviation exceeds 40%, the lower channel is shut down, and the modulation ratio limit is applied. a =1,m x =0. At this time, the nine-switch converter acts as a voltage compensation unit, supporting the distributed wind turbine, hence the constant voltage ride-through. The modulation ratio limiting function is:

[0077]

[0078] Modulation ratio m x m a With voltage deviation U dev Transformation relationship as follows Figure 5 As shown, where m a =1-m x Under voltage dip conditions, voltage compensation absorbs active power. When -20% ≤ U dev When the voltage compensation is less than 40%, the active power is fed into the grid through current compensation, and the overall system has no loss in active power; when 40% ≤ U dev When the voltage compensation active power is activated, the unloading circuit is started, and the voltage compensation active power is consumed through the unloading circuit to prevent overcurrent in the nine-switch converter and transmission line; the opposite is true when the grid voltage rises.

[0079] To verify the third harmonic injection modulation method and the dynamic allocation of the modulation ratio, the following was performed: Figure 6 simulation.

[0080] To verify the proposed study on optimizing the fault voltage ride-through capability of doubly-fed direct-drive distributed wind power using a nine-switch converter, simulations were conducted under three typical operating conditions: mild grid voltage dip, severe grid voltage dip, and grid voltage harmonics. The rated power of both the PMSG and DFIG was set to 2MW, the DC-side voltage of the back-to-back wind turbine converter was 1.2kV, and the DC-side voltage of the nine-switch converter was 1.8kV.

[0081] A simulation of a slight asymmetrical voltage drop in the power grid was conducted. The simulation involved a 30% asymmetrical voltage drop between phases A and B on the 35kV side of the transformer substation during a period of 0.4–0.6 seconds. The simulation results of the relevant electrical quantity changes in the hybrid wind power system are as follows: Figure 7 As shown. The voltage U at the parallel connection point of the doubly-fed direct-drive generator terminals. g,abc Asymmetrical drops began to occur at 0.4s, and the current I at the parallel connection point of the generator terminals... g,abc The amplitude of the DFIG rotor current I increases and fluctuates. r,abc During the fault, the waveform is severely distorted and the amplitude increases, affecting the service life of the rotor frequency converter. The active and reactive power jointly generated by DFIG-PMSG will generate double frequency fluctuations due to the grid asymmetry fault. The amplitude of the DC side voltage of DFIG and the DC side voltage of PMSG will increase and double frequency fluctuations will occur.

[0082] To investigate the impact of a mild symmetrical grid voltage fault on the electrical quantities of a doubly-fed induction generator (DFIG)-direct-drive hybrid wind power system, a simulation was designed where the three-phase voltage on the 35kV side of the transformer experiences a mild symmetrical drop to 70% within a period of 0.4–0.6 s. The simulation results of the relevant electrical quantity changes in the DFIG-direct-drive hybrid wind power system are as follows: Figure 8 As shown. Voltage U at the parallel connection point of the generator terminals. g,abc A slight symmetrical drop occurred between 0.4 and 0.6 seconds, and the current I at the parallel connection point of the generator terminal... g,abc The amplitude increases during the fault and fluctuates sharply at the beginning and end of the fault, DFIG rotor current I r,abc The amplitude increases during the fault, and also fluctuates sharply at the beginning and end of the fault. The active power P jointly generated by DFIG-PMSG G and reactive power Q G The DC-side voltage of DFIG and the DC-side voltage of PMSG fluctuate drastically during fault occurrence and clearance. dc2 Overvoltage during a fault can cause DC bus capacitor breakdown if no measures are taken.

[0083] To verify how the nine-switch UPQC improves the fault ride-through capability of the doubly-fed direct-drive hybrid wind power system under the condition of slight voltage asymmetry drop in the grid, a scenario was designed in which the three-phase voltages of the 35kV side of the transformer substation experienced slight voltage asymmetry drops within 0.4 to 0.6 seconds.

[0084] The voltage drops to 75% for phase A, 90% for phase B, and 85% for phase C within 0.8–1 s, simulating a short-circuit fault between phases B and C causing a 30% voltage drop at point PCC. The hybrid wind power system operates normally within 0.2–0.4 s, and this is compared with the subsequent grid voltage fault condition. The simulation results are as follows: Figure 9 As shown. By Figure 9 It can be seen that the grid connection point voltage U is within 0.4–0.6s and 0.8–1s. pcc,abc Asymmetry, after compensation by the DVR and APF units in the nine-switch UPQC, results in a terminal voltage U g,abc With terminal current I g,abc Maintaining normal operation, with a standard sinusoidal waveform, during voltage dips, the common grid connection point current I... pcc,abc The amplitude increases.

[0085] DFIG rotor current I r,abc DFIG-PMSG unit combined active power P G Unreactive Q G , grid connection point active power P grid Unreactive Q grid The overall active power P of the doubly-fed and direct-drive combined system remained basically stable. G It is 4MW, with reactive power Q G Maintaining a value of 0 Mvar, the active power P at the grid connection point grid For 4MW, reactive power Q grid The voltage is essentially 0 Mvar. The DC side voltage U of the DFIG is... dc1 PMSG DC side voltage U dc2 Due to a voltage asymmetry fault, a second harmonic fluctuation occurs, affecting the DC side voltage U of the nine switches. dc3 During a fault, an overvoltage of up to 20V occurs, accompanied by second-harmonic oscillations. A nine-switch UPQC can improve the ability of doubly-fed direct-drive hybrid wind power systems to achieve mild asymmetric fault ride-through capability.

[0086] To verify the effectiveness of the nine-switch UPQC for severe asymmetrical fault conditions, a fault condition was designed in which the grid voltage dropped by 80% between phases B and C within 0.4 to 0.7 seconds. The nine-switch UPQC and the unloading circuit were operated together, and the system simulation results are shown in Figure 10.

[0087] like Figure 10 It can be seen that due to the severe voltage drop, the unloading circuit is activated at 0.4s, the APF is deactivated, and I is deactivated during the fault period. APF,abc The value is 0; the nine-switch UPQC only operates as a DVR. The voltage U at the parallel connection point of the unit terminal... g,abc Current I at the parallel connection point of the machine terminal g,abc PCC point current I pcc,abcTo maintain normal operation, the DFIG rotor current I r,abc Slight fluctuations occurred at the beginning and end of the fault. DFIG-PMSG combined active power P G Maintaining at 4MW, reactive power Q G The value is basically 0 Mvar, fluctuating only slightly during fault occurrence and disconnection; the activation of the unloading circuit reduces the active power P at the grid connection point. grid Reduced to 2.6MW, unloading circuit P Crowbar Consumes 1.4MW, reactive power Q grid The DC side voltage U of the DFIG is 0 Mvar. dc1 An overvoltage of 80V occurred during fault clearing, which returned to normal after several cycles of fluctuation. The DC side voltage U of the PMSG... dc2 The voltage remained relatively stable. The DC-side voltage of the nine-switch system was maintained at 1800V, with only minor fluctuations occurring transiently at the end of a voltage dip. Under conditions of severe voltage asymmetry dips, the nine-switch UPQC operated in conjunction with the unloading circuit to improve the power quality of the doubly-fed direct-drive hybrid wind power system.

[0088] 3.4 Simulation of Comprehensive Voltage and Current Harmonic Control Operating Conditions

[0089] To verify the effectiveness of the nine-switch UPQC in mitigating voltage harmonics at the generator terminals and current harmonics at the grid connection point, a simulation was designed to inject 5th and 7th order current harmonics at the generator terminal parallel connection point and voltage harmonics containing 5th and 7th orders at the common grid connection point (PCC) within 0.3–0.5 seconds using a nonlinear load and a programmable power supply. To compare the compensation effect before and after the nine-switch UPQC was activated, the UPQC was set to be inactive during the 0.3–0.5 seconds period, and then activated at 0.5 seconds. The simulation results are as follows: Figure 11a As shown.

[0090] Depend on Figure 11a It can be seen that when the APF and DVR compensation units are activated at 0.5s, a clear contrast is formed in the waveform changes before and after compensation. Affected by voltage harmonics, the voltage U at the parallel point of the doubly-fed direct-drive generator terminal... g,abc The waveform is distorted. After UPQC is put into operation, I... pcc,abc The waveform returned to normal, and the current harmonic distortion rate (THD) decreased to 1.88%, demonstrating a significant compensation effect. The rotor current I of the DFIG... r,abc Under the influence of harmonic voltage and current, the waveform distortion was severe when UPQC was not activated. After 0.5s, the waveform distortion was improved. DFIG DC bus voltage U dc1 and PMSG DC bus voltage U dc2 There was a 6th harmonic oscillation 0.5s ago, which was suppressed after UPQC was applied. The DC bus voltage U of the nine switches... dc3An overvoltage of 20V occurred during operation, which then returned to normal.

[0091] Before and after startup of the nine-switch type UPQC, U g,abc Voltage harmonic analysis, such as Figure 11b As shown. Before 0.5s, the total harmonic distortion (THD) of the voltage was 20.18%. After 0.5s, with compensation enabled, the THD decreased to 1.99%. The common grid connection point current I... pcc,abc Current harmonic analysis, such as Figure 11c As shown, the total harmonic distortion (THD) of the current was 10.21% before 0.5s, and decreased to 1.88% after compensation was started.

[0092] The embodiments of this invention revolve around the control of a nine-switch converter. Based on the analysis of the DC-side voltage sharing relationship of the nine-switch converter, a third harmonic injection modulation method suitable for the nine-switch converter is proposed, improving the DC voltage utilization rate. Taking advantage of the asynchronous nature of voltage compensation and current compensation on the DC-side voltage demand of the nine-switch converter, a strategy for dynamically allocating modulation ratio limiting based on the degree of voltage fault is designed. Through simulation analysis of the above scheme under symmetrical voltage dips, asymmetrical voltage dips, and harmonic operating conditions, the feasibility of optimizing the operation characteristics of the nine-switch converter in a doubly-fed direct-drive hybrid distributed wind power system is verified.

[0093] Figure 12 This is a flowchart of a control method for a doubly fed and direct-drive hybrid wind power generation system according to Embodiment 7 of the present invention. Figure 1-Figure 1 The explanations and descriptions of the embodiments shown in Figure 1 can all be applied to this embodiment. For example... Figure 12 As shown:

[0094] In step S1201, the channel outputs compensation voltage on each phase bridge arm of the nine-switch converter, and performs voltage closed-loop control based on the difference between the compensation voltage reference value and the compensation voltage.

[0095] Step S1202: The lower channel of each phase bridge arm of the nine-switch converter outputs compensation current, and performs current closed-loop control based on the difference between the compensation current reference value and the compensation current.

[0096] Step S1203: The modulation signal of the upper channel of each phase arm of the nine-switch converter is superimposed with the third harmonic of the corresponding phase by k times to form a new modulation signal of the upper channel; the modulation signal of the lower channel of each phase arm of the nine-switch converter is superimposed with the third harmonic of the corresponding phase by k times to form a new modulation signal of the lower channel.

[0097] Step S1204: Determine the modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter according to the grid voltage. The sum of the modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter is less than or equal to 1.

[0098] Step S1205: According to the SPWM modulation method, the nine-switch converter is controlled based on the new upper channel modulation signal, the new lower channel modulation signal, the upper channel modulation ratio, and the lower channel modulation ratio.

[0099] Preferably, determining the modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter based on the grid voltage includes:

[0100] When the grid voltage is within the preset normal range, the upper channel of each phase bridge arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively.

[0101] When the grid voltage deviates from the preset voltage threshold by -20% to 20%, the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively.

[0102] When the grid voltage deviates from the preset voltage threshold by 20% to 40%, increase the modulation ratio of the upper channel and decrease the modulation ratio of the lower channel.

[0103] When the grid voltage deviates from the preset voltage threshold by more than 40%, the lower channel of each phase arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 1 and 0, respectively.

[0104] This embodiment utilizes the asynchronous nature of voltage and current compensation on the DC-side voltage demand of a nine-switch converter to design a strategy for dynamically allocating modulation ratio limiting based on the degree of voltage fault. Through simulation analysis of the above scheme under symmetrical, asymmetrical, and harmonic grid voltage drops, the feasibility of optimizing the operating characteristics of a doubly-fed direct-drive hybrid distributed wind power system using a nine-switch converter is verified.

[0105] In this case, the program code read from the storage medium can itself implement the function of any of the above embodiments, and therefore the program code and the storage medium storing the program code constitute part of the present invention.

[0106] Examples of storage media used to provide program code include floppy disks, hard disks, magneto-optical disks, optical disks (such as CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, DVD-RW, DVD+RW), magnetic tapes, non-volatile memory cards, and ROMs. Alternatively, program code can be downloaded from a server computer via a communication network.

[0107] Furthermore, it should be clear that not only can the program code read by the computer be executed, but also the operating system or other components operating on the computer can be instructed based on the program code to perform some or all of the actual operations, thereby realizing the function of any of the embodiments described above.

[0108] Furthermore, it is understood that the program code read from the storage medium is written to the memory set in the expansion board inserted into the computer or to the memory set in the expansion unit connected to the computer. Then, based on the instructions of the program code, the CPU or other components installed on the expansion board or expansion unit execute some and all of the actual operations, thereby realizing the function of any of the embodiments described above.

[0109] It should be noted that not all steps and modules in the above processes and system structure diagrams are mandatory; some steps or modules can be omitted as needed. The execution order of each step is not fixed and can be adjusted as required. The system structure described in the above embodiments can be a physical structure or a logical structure. That is, some modules may be implemented by the same physical entity, or some modules may be implemented by multiple physical entities, or they may be jointly implemented by certain components in multiple independent devices.

[0110] In the above embodiments, the hardware units can be implemented mechanically or electrically. For example, a hardware unit may include permanent dedicated circuitry or logic (such as a dedicated processor, FPGA, or ASIC) to perform the corresponding operation. The hardware unit may also include programmable logic or circuitry (such as a general-purpose processor or other programmable processor), which can be temporarily configured by software to perform the corresponding operation. The specific implementation method (mechanical, dedicated permanent circuitry, or temporarily configured circuitry) can be determined based on cost and time considerations.

[0111] The present invention has been shown and described in detail above with reference to the accompanying drawings and preferred embodiments. However, the present invention is not limited to these disclosed embodiments. Based on the above embodiments, those skilled in the art will know that more embodiments of the present invention can be obtained by combining the code review methods in the different embodiments. These embodiments are also within the protection scope of the present invention.

Claims

1. A hybrid wind power generation system combining doubly-fed and direct-drive systems, characterized in that, include: Direct-drive wind turbine generator sets, doubly-fed wind turbine generator sets, step-up transformers, nine-switch converters, LC filters, filter reactors, injection transformers, and bypass switches; A direct-drive wind turbine generator set includes a first impeller, a permanent magnet direct-drive generator, a first machine-side converter, and a first grid-side converter connected in sequence; a doubly-fed wind turbine generator set includes a second impeller, a gearbox, a doubly-fed generator, a second machine-side converter, and a second grid-side converter connected in sequence. The stator of the doubly fed generator and the second grid-side converter are connected in parallel with the first grid-side converter. The nine-switch converter is connected between the direct-drive wind turbine generator set and the doubly fed wind turbine generator set and the step-up transformer. The midpoint of the upper side of the bridge arm of the nine-switch converter is used as the upper channel output compensation voltage. After the compensation voltage is filtered by an LC filter, the compensation voltage is superimposed on the grid voltage by the injection transformer. The bypass switch is used to short-circuit the injection transformer when the grid voltage is normal, and disable the voltage compensation function. The lower midpoint of the bridge arm of the nine-switch converter serves as the lower channel output compensation current, which is fed into the output lines of the first and second grid-side converters via a filter reactor. The nine-switch converter includes a controller used to determine the logic states of the upper and lower channels of each phase bridge arm according to the SPWM modulation method, and then calculate the drive logic level of each switching element in each bridge arm. The reference value U of the compensation voltage... dvr 'Calculated according to the following formula (1): U dvr '=-ΔU 1abc -U -1abc -U nabc Formula (1) Wherein, ΔU 1abc For positive sequence voltage deviation, U -1abc For negative sequence voltage and U nabc For harmonic voltage, the reference value of the compensation current is calculated according to the following formula (2): I apf ' = ΔI p + ΔI q - I nabc Equation (2) Where, ΔI P For active power compensation component, ΔI Q For reactive power compensation components and I nabc For harmonic compensation components, the upper channel output compensation line voltage fundamental amplitude U of the nine-switch converter is... abm The maximum fundamental line voltage amplitude U of the lower channel output current compensation xym The results are obtained by calculating according to the following formulas (3) and (4): Among them, U c For carrier signal, U dc U is the DC-side voltage of the nine-switch converter. ra U is the modulation signal for the upper channel of the nine-switch converter. rx The modulation signal for the lower channel of the nine-switch converter is provided. The controller modulates the signal based on the new modulation signal for the upper channel and the new modulation signal for the lower channel. Specifically, the modulation signal for the upper channel is superimposed with k times the third harmonic of the corresponding phase to form the new modulation signal for the upper channel. Similarly, the modulation signal for the lower channel is superimposed with k times the third harmonic of the corresponding phase to form the new modulation signal for the lower channel. The modulation ratios of the upper and lower channels of the nine-switch converter are adjusted according to the grid voltage. When the grid voltage is within the preset normal range, the upper channel of each phase bridge arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively. When the grid voltage deviates from the preset voltage threshold by -20% to 20%, the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively. When the grid voltage deviates from the preset voltage threshold by 20% to 40%, increase the modulation ratio of the upper channel and decrease the modulation ratio of the lower channel. When the grid voltage deviates from the preset voltage threshold by more than 40%, the lower channel of each phase arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 1 and 0, respectively.

2. A control method for a doubly-fed and direct-drive hybrid wind power generation system, characterized in that, For the doubly-fed and direct-drive hybrid wind power generation system as described in claim 1, the control method includes: The nine-switch converter outputs a compensation voltage on each phase bridge arm and performs closed-loop voltage control based on the difference between the compensation voltage reference value and the compensation voltage. The nine-switch converter outputs a compensation current in the lower channel of each phase bridge arm, and performs current closed-loop control based on the difference between the compensation current reference value and the compensation current. The modulation signal of the upper channel of each phase arm of the nine-switch converter is superimposed with the third harmonic of the corresponding phase by k times to form the new modulation signal of the upper channel. The modulation signal of the lower channel of each phase arm of the nine-switch converter is superimposed with the third harmonic of the corresponding phase by k times to form the new modulation signal of the lower channel. The modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter are determined according to the grid voltage. The sum of the modulation ratio of the upper channel of each phase arm of the nine-switch converter and the modulation ratio of the lower channel of each phase arm of the nine-switch converter is less than or equal to 1. According to the SPWM modulation method, the nine-switch converter is controlled based on the new upper channel modulation signal, the new lower channel modulation signal, the modulation ratio of the upper channel, and the modulation ratio of the lower channel. The modulation ratio of the upper channel of each phase arm of the nine-switch converter is determined based on the grid voltage. The modulation ratio values ​​for the lower channel of the phase bridge arm include: When the grid voltage is within the preset normal range, the upper channel of each phase bridge arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively. When the grid voltage deviates from the preset voltage threshold by -20% to 20%, the modulation ratios of the upper and lower channels are 0.2 and 0.8, respectively. When the grid voltage deviates from the preset voltage threshold by 20% to 40%, increase the modulation ratio of the upper channel and decrease the modulation ratio of the lower channel. When the grid voltage deviates from the preset voltage threshold by more than 40%, the lower channel of each phase arm of the nine-switch converter is closed, and the modulation ratios of the upper and lower channels are 1 and 0, respectively.

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