Distributed double-fed wind power generation system and voltage adaptive control method

Abstract

The present invention provides a distributed double-fed wind power generation system and a voltage adaptive control method. In the system, a nine-switch converter is connected between a double-fed wind power generator set and a common connection point, and the upper channels of two groups of three-phase output terminals of the nine-switch converter output a first voltage u for voltage compensation. vc , the lower channel outputs the second voltage u for current compensation cc ; The upper channel is connected to the line on the doubly-fed wind turbine generator side through the LC filter and the series transformer in turn, and the lower channel is connected to the line on the common connection point side in parallel after passing through the filter reactor; when the grid voltage is within the preset normal working range, the series transformer is short-circuited by the bypass switch; when the grid voltage is not within the preset normal working range, the bypass switch is disconnected, the series transformer is put into use, the machine end voltage remains normal, and through the second voltage u cc The DC side voltage of the nine-switch converter is kept stable. The invention can improve the voltage adaptability of the double-fed distributed wind power.

Description

Technical Field

[0001] The present invention relates to the field of wind power generation, and in particular to a distributed double-fed wind power generation system and a voltage adaptability control method. Background Art

[0002] Distributed wind power is built near load-concentrated areas, eliminating the need for long-distance power transmission and achieving local consumption. Distributed wind power solutions allow the development and utilization of discontinuous wind resources near loads, which serves as a supplement to centralized wind power and expands the area where land wind power can be developed.

[0003] Compared with centralized wind power, the voltage level of distributed access to the grid is low and the system impedance is high. If the characteristics of the grid make the grid voltage significantly disturbed with load changes. It is easy to overvoltage when lightly loaded, nonlinear loads will cause serious voltage harmonic distortion, and many inductive loads or short circuit faults will cause voltage drops. Due to the characteristics of local consumption, the penetration power of distributed wind power is large, which has a greater impact on the safety of the load center grid. Therefore, higher voltage adaptability is an inevitable requirement for the development of distributed wind power.

[0004] Doubly-fed wind turbines are widely used. The stator is connected to the grid, and the rotor is connected to the grid through a frequency converter, which only flows through the slip power. The stator voltage is directly determined by the grid voltage. The small-capacity frequency converter has limited ability to excite the rotor and cannot control the wind turbine under severe non-ideal grid voltage. This is also a problem that must be solved in distributed application scenarios for doubly-fed units.

[0005] Under non-ideal grid voltage conditions, the transient response of the doubly-fed wind turbine can be summarized as sudden changes in positive-sequence magnetic potential, negative-sequence magnetic potential, and harmonic magnetic potential. At present, the product strategy is to use the remaining capacity of the rotor inverter to control the above-mentioned transient magnetic potential under mild voltage fault conditions. Under severe voltage drop conditions, in order to avoid overvoltage or overcurrent of the rotor inverter, the rotor winding is short-circuited through the Crowbar circuit. At this time, the doubly-fed wind turbine is transformed into a squirrel cage machine, absorbing reactive excitation from the grid and aggravating the grid voltage drop. In the event of a high voltage fault, control systems with lower voltage withstand levels, such as the yaw and pitch control systems, need to switch to the USP power supply to prevent the high voltage of the grid from damaging the insulation of the equipment.

[0006] Existing research has studied the control methods of negative sequence magnetic potential and harmonic magnetic potential of doubly fed wind turbines from the perspective of fault voltage ride-through control strategy, by detecting transient magnetic potential and offsetting it through rotor excitation inverter control. The above research needs to increase the capacity of the rotor inverter to complete the control from the rotor side control strategy scheme. Large transient disturbances caused by sudden changes in the power grid state are inevitable at the time of voltage drop and recovery.

[0007] The existing DVR type solution maintains the stator stability during the grid voltage fault by series voltage compensation, thereby achieving the fault voltage ride-through of the wind turbine. This enables the wind turbine to achieve smooth fault voltage ride-through with minimal disturbance. Since this type of structure is accompanied by active flow during voltage compensation, maintaining the DC side voltage stability is the basis of voltage compensation. Existing studies have proposed a solution to install energy storage devices on the DC side, which is limited by the energy storage capacity and can only be charged under voltage drop conditions. The continuous operation time and compensation depth of the equipment are limited. Existing studies have proposed a solution to share the DC side with the rotor inverter, using the grid-side converter to balance the active flow during voltage compensation, but also transmit transient disturbances to the DC side of the rotor inverter, and at the same time, the rotor inverter capacity needs to be increased. Existing studies configure another set of inverters for the DC side, connect them in parallel to the grid to control the DC side voltage, and form a UPQC topology. This solution can achieve compensation for a wide range of grid voltage faults. Due to the large number of additional devices used, the cost is high. Existing research proposes to use a nine-switch converter as a commutation element to realize the UPQC function, reducing the back-to-back structure of 12 switch elements to 9 switches, simplifying the corresponding drive and protection circuits, and verifying the theoretical feasibility. However, there is a lack of research on the capacity configuration of the nine-switch converter.

[0008] The nine-switch converter simplifies the back-to-back converter by sharing the switch elements. At the same time, due to its constraints, there is a problem of high DC side voltage. Existing research applies the nine-switch converter to the rotor inverter of the doubly-fed wind turbine set, and uses the characteristics of the DC side voltage of the nine-switch converter to improve the transient magnetic potential control capability to a certain extent. In order to reduce the DC side voltage of the nine-switch converter, existing research introduces the SVPWM modulation method of the nine-switch converter, which can increase the DC voltage utilization rate by 15.5% under the traditional SPWM modulation method. Due to the common relationship of the switches, the cross-modulation of the upper and lower channels needs to be completed through the zero vector coordination, and the modulation method is complicated. Under the conditions of the same frequency and phase of the two channels introduced in the existing research, the DC voltage utilization rate can be further improved, and the single application scenario is single. Therefore, improving the voltage adaptability of doubly-fed distributed wind power has become a technical problem that needs to be solved urgently. Summary of the invention

[0009] In view of this, the present invention provides a distributed double-fed wind power generation system and a voltage adaptability control method to improve the voltage adaptability of the double-fed distributed wind power.

[0010] On the one hand, the present invention provides a distributed doubly-fed wind power generation system, comprising: a doubly-fed wind power generator set, a nine-switch converter, an LC filter, a filter reactor, a bypass switch and a series transformer; the nine-switch converter is connected between the doubly-fed wind power generator set and a common connection point, the nine-switch converter comprises three bridge arms, the three bridge arms are connected in parallel between DC capacitors, the three bridge arms constitute two groups of three-phase output ends, and the upper channels of the two groups of three-phase output ends output a first voltage u for voltage compensation vc The lower channels of the two groups of three-phase output terminals output a second voltage u for current compensation. cc ; The upper channels of the two groups of three-phase output ends are connected to the line on the side of the doubly-fed wind turbine generator set through the LC filter and the series transformer in turn, and the lower channels of the two groups of three-phase output ends are connected in parallel to the line on the side of the common connection point after passing through the filter reactor; the bypass switch is connected in parallel with the series transformer; when the grid voltage is within the preset normal working range, the series transformer is short-circuited by the bypass switch; when the grid voltage is not within the preset normal working range, the bypass switch is disconnected, the series transformer is put into operation, and the first voltage u vc The terminal voltage of the doubly-fed wind turbine generator set is kept normal, and the second voltage u cc Maintaining a stable DC side voltage of the nine-switch converter.

[0011] On the other hand, the present invention also provides a voltage adaptive control method for a distributed double-fed wind power generation system, the voltage adaptive control method is applied to the distributed double-fed wind power generation system, the voltage adaptive control method comprises: when the grid voltage is within a preset normal working range, the series transformer is short-circuited by the bypass switch; when the grid voltage is not within the preset normal working range, the bypass switch is disconnected, the series transformer is put into operation, and the first voltage u vc The terminal voltage of the doubly-fed wind turbine generator set is kept normal, and the second voltage u cc Maintaining a stable DC side voltage of the nine-switch converter.

[0012] The distributed double-fed wind power generation system and method of the present invention utilizes a relatively small voltage compensation circuit capacity to achieve the maximum zero voltage crossing capability of the wind turbine under rated conditions, and at the same time performs reactive compensation for voltage rise and drop conditions to promote voltage recovery. The unit can cross the full voltage condition and provide active and reactive power support; utilizing the characteristic that the three-phase three-phase inverter cannot output zero sequence, the third harmonic injection method is adopted to increase the DC voltage utilization rate to 1, which is easier to implement than the SVPWM technology, and can be implemented in conjunction with other modulation strategies; utilizing the difference in voltage compensation and current compensation voltage requirements under voltage drop conditions, the distribution of DC voltage in voltage compensation and current compensation modulation is dynamically adjusted. While meeting the DC voltage requirements of voltage compensation and current compensation, the DC side voltage is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0014] Figure 1 4 is a topological diagram of a distributed doubly-fed wind power generation system according to a first exemplary embodiment of the present invention.

[0015] Figure 2 FIG. 4 is an equivalent diagram of an operational circuit of a distributed doubly-fed wind power generation system according to an exemplary second embodiment of the present invention.

[0016] Figure 3 FIG. 4 is a control strategy diagram of a nine-switch converter in a distributed doubly-fed wind power generation system according to an exemplary third embodiment of the present invention.

[0017] Figure 4 1 is an active power control strategy and a modulation ratio distribution diagram of a nine-switch converter in a distributed double-fed wind power generation system according to a fourth exemplary embodiment of the present invention.

[0018] Figure 5 Schematic diagram of an SPWM modulation method for a nine-switch converter in a distributed double-fed wind power generation system according to a fifth exemplary embodiment of the present invention.

[0019] Figure 6 Schematic diagram of a SPWM third harmonic superposition method for a nine-switch converter in a distributed double-fed wind power generation system according to an exemplary sixth embodiment of the present invention.

[0020] Figure 7 1 is a waveform diagram of various parameters of a distributed doubly-fed wind power generation system before and after compensation according to a seventh exemplary embodiment of the present invention.

[0021] Figure 8 4 is a comparison diagram of harmonic components of a distributed doubly-fed wind power generation system according to an eighth exemplary embodiment of the present invention.

[0022] Fig. 9 1 is a waveform diagram of various parameters of a distributed doubly-fed wind power generation system during an asymmetric voltage drop compensation process according to a ninth exemplary embodiment of the present invention.

[0023] Fig.10 1 is a waveform diagram of various parameters of a distributed doubly-fed wind power generation system under a voltage symmetry fault according to the tenth exemplary embodiment of the present invention.

[0024] Fig.11 1 is a waveform diagram of various parameters of a distributed doubly-fed wind power generation system during a voltage deep sag according to an exemplary eleventh embodiment of the present invention.

[0025] Fig.12 1 is a waveform diagram of a third harmonic injection modulation method and a dynamic modulation ratio configuration for a distributed doubly-fed wind power generation system according to an exemplary twelfth embodiment of the present invention. DETAILED DESCRIPTION

[0026] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0027] It should be noted that the following embodiments and features in the embodiments may be combined with each other in the absence of conflict; and, based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without making any creative work are within the scope of protection of the present disclosure.

[0028] It should be noted that various aspects of the embodiments within the scope of the appended claims are described below. It should be apparent that the aspects described in the present invention may be embodied in a wide variety of forms, and any specific structure and / or function described in the present invention is merely illustrative. Based on the present disclosure, it should be understood by those skilled in the art that an aspect described in the present invention may be implemented independently of any other aspect, and two or more of these aspects may be combined in various ways. For example, any number of aspects described in the present invention may be used to implement the device and / or practice the method. In addition, other structures and / or functionalities other than one or more of the aspects described in the present invention may be used to implement this device and / or practice this method.

[0029] The meanings of the variables in the embodiments of the present invention are as follows:

[0030] u pcc , I pcc Respectively represent the grid voltage and grid current. g , I gRespectively represent the voltage and current of the doubly-fed generator set. s , I s Respectively represent the stator voltage and current of the doubly fed generator set. r Respectively represent the rotor current. e ,ω r I represents electromagnetic torque and rotor speed respectively. gsi : Grid-side converter current. U iv , U vc , I vc , P vc , Q vc They represent injected compensation voltage, compensation voltage, voltage compensation circuit current, voltage compensation active power, and voltage compensation reactive power respectively. cc , I cc , P cc , Q cc , S cc They represent the current compensation circuit voltage, compensation current, current compensation active power, current compensation reactive power, and current compensation capacity respectively. dc1 , U dc They represent the DC voltage of the double-fed rotor excitation inverter and the DC voltage of the nine-switch converter respectively. cf , I cf ,i L2 Respectively represent the capacitor voltage and capacitor current in the LC filter; the inductor current in the LC filter. Parameters with * represent the given value of the corresponding control quantity.

[0031] i dc_vc ,i cc_vc They represent the equivalent current flowing into the DC side of the nine-switch converter from the voltage compensation circuit and the equivalent current flowing out of the DC side of the nine-switch converter from the current compensation circuit. ra ,u rx ; U ram , U rxm ; U cm Respectively represent the upper and lower channel modulation signals, the upper and lower channel modulation signal amplitudes and the carrier signal amplitude. a , m x Respectively represent the modulation ratios of the upper and lower channels. θ represents the current compensation modulation reference signal in the dq synchronous rotating coordinate system and the dq reference system phase angle. 3ωt , A 3ωt : Third harmonic phase angle, third harmonic amplitude. u cc_3abc ,u cc_abc ,u cc_3ωt They respectively represent the current compensated third harmonic injection modulation signal, the initial current compensated modulation signal, and the third harmonic signal.

[0032] Figure 1FIG. 4 is a structural diagram of a distributed doubly-fed wind power generation system according to an exemplary first embodiment of the present invention. Figure 1 As shown, a distributed double-fed wind power generation system of the present invention includes: a double-fed wind power generator set, a nine-switch converter, an LC filter, a filter reactor, a bypass switch and a series transformer.

[0033] Among them, the nine-switch converter is a voltage source converter with three bridge arms connected in parallel between DC capacitors. Each bridge arm has three switch elements and two bridge arm output points. The three bridge arms constitute two groups of three-phase output terminals. The upper channel is used for voltage compensation, and the output voltage is the first voltage u vc The lower channel is used for current compensation, and the output voltage is the first voltage u vc The nine-switch converter is connected between the doubly-fed wind turbine generator set (DFIG) and the common connection point (PCC). The voltage compensation output passes through the LC filter and is connected to the line close to the DFIG side through a series transformer. The current compensation output passes through the filter reactor and is connected in parallel to the line on the PCC side.

[0034] The bypass switch is connected in parallel with the series transformer. Under normal voltage conditions, the series compensation circuit is short-circuited by the bypass switch to avoid power loss. When the voltage deviates, the bypass switch is disconnected and the voltage at the machine end is kept normal through series voltage compensation. The active power flow during the voltage compensation period will be sent to the grid through parallel current compensation to keep the DC side voltage stable. A Chooper circuit is set on the DC side of the nine-switch converter to consume the excess active power absorbed by the voltage compensation side.

[0035] In the distributed scenario, the PCC is connected to the local 35kV bus through a step-up transformer, and the local load is also connected to the 35kV bus. In addition, the 35kV is currently connected to the infinite power grid through transformers and circuit lines. Since the step-up transformer at the PCC is YD-11 connected, the DFIG and the nine-switch converter are all three-phase three-wire connected, so there is no zero-sequence component in the DFIG-UPQC system. Since the three-phase structure of the system is symmetrical, the neutral point voltage in the same grounding system is 0. Figure 1 The neutral point is connected with equal voltage, and the voltage source u vc Represents the voltage compensation output voltage, with voltage source u cc Represents the current compensation output voltage, converting the transformer compensation side circuit to the line side, the above system operation circuit is obtained as follows Figure 2 As shown. Since the injection transformer superimposes the compensation voltage on the voltage at the PCC, after conversion, u vc Will be with u pcc Series connection.

[0036] When the transformer ratio is k=1, Figure 1Transformer injection voltage u iv and i g They are

[0037]

[0038] For loop L 1 Write the KVL equation, and the voltage compensation relationship is:

[0039] u g (s) = u pcc (s)+u Cf (s) (2)

[0040] in

[0041] For 1 Write the KCL equation in points

[0042] i g (s)+i L2 (s) = i Cf (s) (4)

[0043] in

[0044] in u pcc When a deviation occurs, the output voltage u of the upper channel of the nine-switch converter is controlled vc Adjust the series injection voltage u iv , which can make the terminal voltage u g Stay steady.

[0045] For 2 Write KCL equations at the nodes

[0046] i Cf (s)-i L2 (s)+i cc (s) = i pcc (s) (6)

[0047] That is, g (s)+i cc (s) = i pcc (s) (7)

[0048] in

[0049] It can be seen that the wind turbine current and the compensation current are superimposed and injected into the PCC node. At the same time, by controlling the output voltage u of the lower channel of the nine-switch converter cc Adjustable compensation current i cc , thereby realizing current compensation control.

[0050] In voltage compensation, the voltage source uvc The current Il2 flows through to generate voltage compensation power P vc This power will generate an equivalent current i flowing into the DC side dc_vc , so that the DC side voltage increases. Voltage source u cc Current flowing i cc Power generated P cc , this power will generate an equivalent DC side outflow current i dc_cc . when i dc_vc with i dc_cc In dynamic balance, the DC side voltage Udc2 of the nine-switch converter is stable

[0051]

[0052] Through circuit model analysis, it is concluded that control u vc and u cc That is, voltage compensation and current compensation can be controlled. The present invention introduces a nine-switch converter u vc with u cc The control strategy and the overall compensation plan of the system.

[0053] According to the circuit model, the NSC-UPQC control strategy is designed as follows Figure 3 According to the circuit model, the NSC-UPQC control strategy is designed as follows Figure 3 shown. and is the modulation signal. After SPWM modulation, a pulse drive signal is generated to control the NSC converter. vc and u cc It is the voltage carried by the NSC output power pulse modulation wave. Figure 3 The right part in the middle is the NSC-UPQC circuit block diagram, and each variable is a complex frequency domain variable. The left part is the corresponding control strategy, and each variable is a time domain variable.

[0054] The voltage outer loop and current inner loop control are used in the voltage compensation control. To improve the tracking performance of the sine fundamental wave and harmonics, proportional resonance PR control is used. The voltage outer loop control is:

[0055]

[0056] The inner current loop is:

[0057]

[0058] The current compensation circuit obtains the active power reference value through DC voltage control.

[0059]

[0060] Under the conditions of specified active power and reactive power, the current compensation circuit is controlled in a closed loop to obtain the current compensation output voltage reference value u cc , which will not be described in detail here. cc and u vc After SPWM modulation, the nine-switch converter is driven to output the specified voltage. The active and reactive command values ​​of current compensation are also affected by the system compensation scheme, which will be introduced in detail in the following content.

[0061] The research on the capacity configuration of the nine-switch converter is an important part of its application research. In this case, the capacity configuration of the nine-switch converter is closely related to the series compensation and parallel compensation schemes. Theoretically, when the voltage is offset, the wind turbine current flows through the series transformer, and after multiplying with the compensation voltage, it generates voltage deviation power, which is sent to the power grid through current compensation to maintain the DC side voltage stability. The more serious the voltage deviation, the greater the power transferred by the nine-switch converter. When the grid voltage is 0, the nine-switch converter theoretically bears the full power of the wind turbine. In reality, the larger the capacity, the worse the economy, and optimized capacity configuration is very necessary. The present invention proposes a capacity configuration for a nine-switch converter, and analyzes the active and reactive power flows under different voltage deviation conditions and the control methods adapted thereto.

[0062] High voltage conditions are a test for the insulation level of the power system. The design supports a maximum of 1.3 high voltage conditions. The minimum support is 0 voltage ride-through, which is coordinated with grid protection and reclosing to improve system reliability. The voltage compensation capacity and current compensation capacity of the nine-switch converter are set to be less than 0.5. The specific parameters are as follows Figure 4 shown.

[0063] Voltage compensation power P vc The absolute value of increases with the increase of voltage deviation. At the same time, considering the capacity constraint of the nine-switch converter, vc The restriction is as shown in Formula 10.

[0064]

[0065] When a more severe voltage drop causes P vc When the upper limit is reached, the wind turbine output power P is limited. g Control P vc Satisfy the constraints. As shown in formula 11.

[0066]

[0067] In the figure, S 1 To limit the wind power area, S 2 is the voltage compensation power area, S 3 The area is the area of ​​power fed into the grid.

[0068] The current compensation circuit is directly connected to the PCC point. Under the constraint of the maximum current of the current compensation circuit, the current compensation capacity decreases as the grid voltage drops.

[0069]

[0070] According to the control strategy, the voltage compensation power P is converted into dvr The active power is sent to the grid to achieve DC side voltage stability. Consider the following three factors. The active and reactive power control of current compensation is set in combination with the current compensation capacity limitation under different grid voltage conditions. The goal of active power control is to control the current compensation power PCC and the voltage compensation power S cc Balance to maintain the DC side voltage stability of the nine-switch converter. As the voltage deviation increases, P vc Increase, and S cc When the voltage deviation exceeds a certain level, the current compensation cannot maintain the DC side voltage stability, and the nine-switch converter DC side Chopper circuit needs to consume the voltage compensation power S cc The active power control is shown in Equation 13.

[0071]

[0072] When 0.75<u pcc ≤1.3 At this time, all the wind turbine power is sent to the grid without loss. pcc =0.75, the line current becomes 1.3pu. The use of the chopper circuit also avoids line overload. Figure 4 (c) As shown in FIG. A4, the area in the figure represents the active power output by the current compensation, and A5, the area in the figure represents the active power consumed by the Chopper circuit.

[0073] Under voltage deviation conditions, appropriate reactive power compensation can help the voltage return to normal. The reactive power is set to change with voltage as shown in Formula 14.

[0074]

[0075] in u pcc Greater than 0.75 and less than 1.3, reactive power increases with the increase of voltage deviation. pcc =1.3 and u pcc =0.75, pcc2+qcc2=scc2, and the current compensation reaches the maximum capacity. When the voltage is lower than 0.75, the entire capacity is used for reactive power compensation. Figure 4 As shown in (d), the A6 area in the figure is the output reactive power. For the convenience of description, the voltage deviation degree is defined as formula 15 in combination with several key PCC point voltages proposed in the present invention.

[0076]

[0077] The high DC side voltage of the nine-switch converter will increase the DC side capacitance and the withstand voltage level of the switch elements. The present invention will analyze the traditional nine-switch converter SPWM modulation method, and design a third harmonic injection modulation method and a dynamic modulation ratio limiting strategy to reduce the DC side voltage. The nine-switch converter achieves independent control of the upper and lower channels by time-division multiplexing. Limited by the series structure of the switches, when the output point on the lower side of the bridge arm (hereinafter referred to as the lower channel) outputs a high level, its upper side output point (hereinafter referred to as the upper channel) must be a high level. Similarly, when the lower channel outputs a high level, the upper channel must output a high level. Therefore, when using the SPWM modulation method, it must be ensured that during the modulation process, the upper channel modulation signal u ra must be greater than the lower channel modulation signal u rx Therefore, it is necessary to bias the upper channel modulation signal positively and the lower channel signal negatively, and ensure u ra >u rx The traditional SPWM modulation method is shown in Figure 5. Uc represents the carrier signal.

[0078] According to the above modulation method, the DC bias belongs to the zero-sequence component, and the NSC converter cannot output it. The upper channel output modulation voltage amplitude U am , the lower channel output modulation voltage amplitude U am They are:

[0079]

[0080] Where U ram Indicates the amplitude of the upper channel modulation signal, U rxm Indicates the amplitude of the lower channel modulation signal, U cm Indicates the carrier signal amplitude, m a 、m x is the modulation ratio of the upper and lower channel signals, U dc is the DC side voltage of the nine-switch converter. The line voltage amplitudes of the upper channel and the lower channel are U abm , U xym , at modulation ratio m a +m x When ≤1, the maximum voltage utilization rate is only 0.866.

[0081]

[0082] The third harmonic injection modulation method of the nine-switch converter can increase the voltage utilization rate to 1 and reduce the DC voltage demand. When the grid voltage drops, the output voltage amplitude of the voltage compensation output increases and the current compensation output voltage amplitude decreases. By utilizing the voltage difference between the two, a strategy for dynamically adjusting the modulation voltage bias and modulation ratio limiting is designed to further reduce the DC voltage demand. In the SPWM modulation strategy of the nine-switch converter, the modulation signal amplitude can be reduced by superimposing the initial modulation signal with the corresponding third harmonic signal. The third harmonic also belongs to the zero-sequence component and does not affect the initial modulation voltage output by the NSC converter. When calculating the superposition of the fundamental wave and the third harmonic, it is found that when the third harmonic amplitude is 1 / 6 of the fundamental wave amplitude, the waveform amplitude after superposition is the smallest, which is 0.866 times the initial modulation wave amplitude. Therefore, in the case of no over-modulation, the initial modulation wave amplitude can be increased to 1.155 times, making the DC voltage utilization rate become 1. Furthermore, under the same other conditions, the third harmonic injection modulation method can reduce the DC side voltage by 13.4%.

[0083] The key to the third harmonic modulation method is to determine the amplitude and phase of the current third harmonic. Take the voltage modulation process of the current compensation circuit as an example. * cc_d and u * cc_q And the d-axis phase angle θ is the input modulation function. Figure 6 The third harmonic phase angle and amplitude are

[0084]

[0085] The third harmonic input modulation signal is

[0086] u cc_3abc =u cc_abc +u cc_3ωt (19)

[0087] where u cc_abc is the initial three-phase modulation signal, u cc_3ωt corresponds to the third harmonic signal. The detailed waveform of the modulation process is shown in Fig.12 When the grid voltage is normal, voltage compensation or harmonic voltage compensation is not required. vc Small, the current compensation circuit should be required to control the reactive power flow, requiring a higher voltage u cc Matches the grid voltage. When the voltage drops, the compensation voltage u vc Larger, and u cc As the grid voltage decreases. vc and u cc The voltage demand complementarity of the two is designed to change the modulation ratio limit with the voltage condition, so as to achieve a higher DC voltage utilization rate under various voltage conditions, that is,

[0088] m a +m x →1 (20)

[0089] The purpose of reducing the DC side voltage is achieved. The modulation ratio limit ma* is as follows

[0090]

[0091] The relationship between modulation ratio limiting and voltage is also Figure 4 As shown in (b).

[0092] When the DC side voltage is 1600V, the nine-switch converter can output a maximum open-circuit modulation voltage of

[0093]

[0094] when When , the corresponding voltage changes linearly in the above voltage range. Ignoring the ideal conditions such as the reactance drop, this DC voltage margin can reduce the DC side voltage by 31% under the dynamic modulation limiting strategy compared with the DC side voltage designed according to the maximum voltage demand.

[0095]

[0096] The voltage waveform corresponding to the dynamic modulation ratio limiting control is as follows Fig.12 shown.

[0097] In order to verify the performance of nine-switch converter in improving the voltage adaptability of distributed double-fed wind power and the proposed compensation strategy and modulation method of nine-switch converter, the Matlab\Simulink software design simulation platform was used to complete the verification. The key simulation parameters are shown in Table 1.

[0098] Table 1

[0099] Parameter name Actual value\unit Per Unit Value PCC rated voltage 690V 1 DFIG rated power 2.0MW 1 Rated speed 1.26 Rated electromagnetic torque -1 Rotor inverter DC voltage 1150V Nine-switch converter DC voltage 1600V Injection transformer ratio 1 Voltage compensation circuit capacity 1MVA 0.5 Current compensation circuit capacity 1MVA 0.5

[0100] Four grid voltage conditions are set, namely harmonic voltage condition, asymmetric voltage drop condition, mild voltage symmetrical rise and drop condition and deep voltage drop condition, to verify the nine-switch converter UPQC-DFIG system. DFIG is in rated working state. Nonlinear load is set to start local load at 0.5s, and the grid voltage u pcc Distortion occurs, and the machine-end voltage ug is distorted. In order to make a contrast before and after compensation, voltage compensation is started at 0.7s, and the compensation voltage Uiv is injected. The distortion of Ug is eliminated and the sine is restored. Figure 7 As shown. After harmonic content analysis, u pccThe main harmonics are the 5th, 7th and 11th harmonics, and the harmonic distortion rate THD is 9.53%. After compensation, the corresponding harmonic distortion rate THD in the machine-end voltage Ug is reduced to 0.52%, and the harmonic voltage compensation effect is obvious. Figure 8 shown.

[0101] exist Figure 7 In the case of harmonic distortion at the machine end, the stator current I s , rotor current I r , the grid-side converter current igc is distorted. The electromagnetic torque Te, the DC side voltage Udc1, and the active power (all active and reactive powers in the present invention are instantaneous powers) oscillate. After harmonic compensation, the DFIG-related distorted current returns to sinusoidal form, and the related oscillation phenomenon is eliminated. The harmonic voltage compensation process is positive, and the active and reactive power of the voltage compensation unit oscillate in a sinusoidal form, with an average value of 0. Through current compensation current control, the DC side voltage Udc2 of the nine-switch converter is stabilized at 1600V, and the current compensation circuit current icc is controlled to a sinusoidal waveform. After compensation, the torque oscillation of the DFIG wind turbine set is eliminated, and the overall UPQC-DFIG system current returns to sinusoidal form, presenting a high-impedance state to voltage harmonics.

[0102] At 0.5s, the 35kV busbar has an asymmetric voltage drop due to a phase-to-phase short circuit. Phase B drops to 0.67pu, phase A and phase C drop to 0.93pu, and the machine-side voltage ug drops accordingly. In order to make a comparison before and after compensation, voltage compensation starts at 0.7s, and the compensation voltage Uiv is injected. The distortion of Ug is eliminated and the sine is restored. Fig. 9 As shown. Under the condition of asymmetric voltage drop, the voltage change at the machine end mainly includes positive sequence voltage drop and negative sequence voltage. At this time, the stator current is, rotor current ir, and grid-side converter current igc are distorted and overcurrent occurs. The electromagnetic torque Te, DC side voltage Udc1, active power, and reactive power show typical double frequency oscillation. After voltage compensation, the DFIG-related distorted current returns to sinusoidal and the related oscillation phenomenon is eliminated.

[0103] During the asymmetric voltage drop compensation process, the compensation voltage Uiv contains positive-sequence and negative-sequence voltage components, so the voltage compensation power Pvc presents a double-frequency oscillation with a DC bias. The reactive power is a double-frequency oscillation with an average value of 0. In the DC side voltage closed-loop control of the nine-switch converter, the double-frequency component in the feedback voltage is filtered out so that the current of the current compensation circuit is controlled to be sinusoidal. Therefore, this DC side voltage Udc2 has a double-frequency disturbance corresponding to Pvc. Also due to the presence of negative-sequence voltage, the current compensation power Pcc oscillates at a double frequency. After compensation, the torque oscillation of the DFIG wind turbine is eliminated, and the current of the overall UPQC-DFIG system returns to a positive sine, presenting a high-impedance state to the negative-sequence voltage.

[0104] Set at 0.5s, the grid voltage u pcc The grid voltage u increases symmetrically to 1.3 pu and returns to normal at 0.7 s. pcc It drops symmetrically to 0.75pu and returns to normal in 1 second. After the voltage compensation detects the voltage deviation, it compensates in real time. When the voltage deviation occurs and is eliminated, the voltage ug at the machine end maintains amplitude stability and phase continuity through a small transient state, such as Fig.10 shown.

[0105] The above voltage deviation belongs to mild voltage deviation, and the modulation ratio is fixed at ma=0.3. DFIG only has a slight attenuation oscillation due to the transient impact of Ug at the moment of voltage compensation start and stop, which is different from normal operation. At the moment of voltage rise t=0.5s, the voltage compensation circuit outputs active power Pvc, causing the DC side voltage Udc2 of the nine-switch converter to drop temporarily. With the current compensation control, the DC side voltage recovers. At the moment of voltage recovery t=0.7, the voltage compensation stops. Due to the inertia of current compensation, the DC side voltage rises temporarily and then recovers. The voltage drop and recovery make the DC side voltage transient response opposite to it.

[0106] The current compensation output power Pcc is consistent with the voltage compensation absorption power Pvc, maintaining the stability of the DC side voltage Udc2. At the same time, according to the reactive power compensation control strategy, pcc When upcc is 1.3pu, the reactive power absorbed is 0.4pu. When upcc is 0.75, the reactive power emitted is 0.3pu. The overall active and reactive power Ppcc and Qpcc exchanged with the grid are consistent with the design.

[0107] Set at 0.5s, the grid voltage u pcc The voltage drops symmetrically to 0.2pu and returns to normal in 1s. The voltage compensation circuit injects the compensation voltage Uiv in real time. The terminal voltage ug maintains amplitude stability and phase continuity through a small transient when the voltage drops and recovers. Fig.11 shown.

[0108] Due to the large voltage drop depth, in order to avoid overloading the voltage compensation circuit, the DFIG unit is coordinated to perform load reduction control. According to formula (11), the wind turbine power Pg should be limited to 0.56pu. At this time, the voltage compensation circuit absorption power Pvc is 0.45pu. The above absorption power is consumed by the nine-switch converter DC unloading circuit, and Pdc is 0.45pu. Through the wind turbine load reduction control, the stator current Is, rotor current Ir, grid-side converter current Igsc, and electromagnetic torque are all reduced, and the accumulation of mechanical energy increases the speed rw (the speed change is related to the moment of inertia. In order to reflect the law of speed change, the simulated moment of inertia is smaller than the actual moment of inertia). While the wind turbine limits the output power Pg, the blade angle β is controlled to increase to reduce the captured wind energy, so that the speed gradually reaches a new steady state.

[0109] According to the reactive power compensation control strategy, at u pcc When the voltage is 0.2pu, the reactive power Qcc emitted by the current compensation circuit is 0.1pu, and the corresponding current compensation circuit current Icc amplitude is 1200A. The active power Pcc is 0. During the voltage drop, the active power Ppcc injected into the grid is 0.56*0.2=0.112pu. The reactive power Qpcc injected into the grid is 0.1pu, which is consistent with the design.

[0110] During the deep voltage drop, the modulation ratio ma is switched from 0.3 to 0.675. The details of the third harmonic injection modulation method and the waveform of the dynamic modulation ratio configuration introduced in the present invention under this working condition are as follows: Fig.12 As shown. Take the voltage reference signal Ucc' of the current compensation circuit as an example. Ucc_dq' is the waveform of the reference signal Ucc' in the dq coordinate system. The d-axis voltage decreases as the grid voltage drops, the q-axis voltage is 0, and there is no active power exchange. The waveform of Ucc_dq' is restored to the ABC coordinate system as Ucc_abc', and the corresponding third harmonic is Ucc_3t'. The superposition of the two results in the third harmonic injection modulation signal Ucc_3abc'. It can be seen that the amplitude of Ucc_3abc' is reduced from 1p.u. to 0.87pu

[0111] The voltage compensation modulation voltage and current compensation modulation voltage after the third harmonic injection are modulated by nine-switch SPWM, and the waveform is shown as Unsc'. When the grid voltage is normal, the bias of the upper channel voltage compensation voltage modulation signal is 0.7, and the bias of the lower channel current compensation voltage modulation signal is -0.3, and the corresponding modulation ratio amplitude limit is 0.3 and 0.7. After the voltage drops, the above voltage bias becomes 0.325 and -0.675, and the corresponding modulation ratio amplitude limit becomes 0.675 and 0.325. vc and u ccThe voltage waveform restored after filtering out the switching frequency of the output power pulse of the nine-switch converter. The last waveform Unsc' shows the detailed waveform of the dynamic modulation ratio configuration.

[0112] This embodiment adopts a nine-switch converter to improve the voltage adaptability of doubly-fed distributed wind power, and provides a configuration method using a smaller nine-switch converter capacity and a corresponding control strategy. Through the modulation method of third harmonic injection and dynamic modulation ratio allocation, the DC voltage utilization rate is improved and the DC side voltage of the nine-switch converter is reduced. Through simulation verification, the proposed scheme is consistent with the design in the voltage harmonic, voltage symmetrical\asymmetrical drop\rise conditions, and the UPQC-DFIG operating state is effectively improved. The voltage adaptability of the distributed wind turbine is effectively improved.

[0113] Improving the voltage adaptability of distributed wind power can promote its friendly interaction with the power grid. At the same time, expanding the topology and control strategy of the nine-switch converter, improving the frequency adaptability of distributed wind power and its adaptability to resistive power grids and even capacitive power grids can comprehensively improve the operating performance of distributed wind power. This will be the content of subsequent research.

[0114] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A distributed doubly-fed wind power generation system, characterized in that: include: A double-fed wind power generator set, a nine-switch converter, an LC filter, a filter reactor, a bypass switch and a series transformer; the nine-switch converter is connected between the double-fed wind power generator set and a common connection point, the nine-switch converter includes three bridge arms, the three bridge arms are connected in parallel between the DC capacitors, the three bridge arms constitute two groups of three-phase output ends, and the upper channels of the two groups of three-phase output ends output a first voltage u for voltage compensation vc The lower channels of the two groups of three-phase output terminals output a second voltage u for current compensation. cc ; The upper channels of the two groups of three-phase output ends are connected to the line on the side of the double-fed wind turbine generator set through the LC filter and the series transformer in turn, and the lower channels of the two groups of three-phase output ends are connected in parallel to the line on the side of the common connection point after passing through the filter reactor; the bypass switch is connected in parallel with the series transformer; When the grid voltage is within the preset normal working range, the series transformer is short-circuited by the bypass switch; when the grid voltage is not within the preset normal working range, the bypass switch is disconnected, the series transformer is put into operation, and the first voltage u vc The terminal voltage of the doubly-fed wind turbine generator set is kept normal, and the second voltage u cc Maintaining a stable DC side voltage of the nine-switch converter.

2. The distributed doubly-fed wind power generation system according to claim 1, characterized in that: A Chooper circuit is provided on the DC side of the nine-switch converter for consuming excess active power absorbed by the upper channels in the two groups of three-phase output terminals.

3. The distributed doubly-fed wind power generation system according to claim 2, characterized in that: The common connection point is connected to the regional 35kV busbar via a step-up transformer, the regional 35kV busbar is connected to the local load, and is connected to the power grid via a transformer and circuit lines.

4. The distributed doubly-fed wind power generation system according to claim 3, characterized in that: The first voltage u vc is the voltage carried by the output power pulse modulation wave of the nine-switch converter, The first voltage u is a pulse drive signal generated by SPWM modulation for controlling the nine-switch converter. vc The control adopts voltage outer loop and current inner loop control, among which the voltage outer loop control is: The current inner loop control is: in, is the given value of the inductor current in the LC filter; is the given value of the voltage of the doubly fed generator set, u pcc is the grid voltage, u cf is the capacitor voltage in the LC filter, i g is the doubly-fed unit current, is the first voltage u vc The given value of i L2 is the inductor current in the LC filter, G PR (jω) is the transfer function of proportional resonant PR control.

5. The distributed doubly-fed wind power generation system according to claim 4, characterized in that: The voltage compensation power P of the nine-switch converter vc The absolute value of increases with the voltage deviation, while satisfying the following restrictions: Among them, P vc is the voltage compensation active power, P g The power generated by the doubly-fed wind turbine generator.

6. The distributed doubly-fed wind power generation system according to claim 5, characterized in that: When the grid voltage drops, P vc When the preset upper limit is reached, the power P of the double-fed wind turbine generator set is limited. g Control PVC to meet the following constraints:

7. The distributed doubly-fed wind power generation system according to claim 6, characterized in that: The current compensation capacity of the nine-switch converter decreases as the grid voltage drops and satisfies: Among them, S cc is the current compensation capacity.

8. The distributed doubly-fed wind power generation system according to claim 7, characterized in that: The active power control of the nine-switch converter is specifically as follows: P cc is the current compensation active power, and P Chopper Indicates the active power consumed by the Chopper circuit.

9. The distributed doubly-fed wind power generation system according to claim 8, characterized in that: The relationship between the reactive power of the nine-switch converter and the voltage of the common connection point is as follows: Among them, Q cc Compensates the current for reactive power.

10. A voltage adaptive control method for a distributed double-fed wind power generation system, characterized in that: The voltage adaptive control method is applied to the distributed doubly-fed wind power generation system according to any one of claims 1 to 9, and the voltage adaptive control method comprises: When the grid voltage is within a preset normal operating range, the series transformer is short-circuited by the bypass switch; When the grid voltage is not within the preset normal working range, the bypass switch is disconnected, the series transformer is put into operation, and the first voltage u vc The terminal voltage of the doubly-fed wind turbine generator set is kept normal, and the second voltage u cc Maintaining a stable DC side voltage of the nine-switch converter.

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