Topology and control method of resonant DC converter for DC transmission in offshore wind farms

Abstract

The invention relates to a topological structure and control method of a DC transmission resonant DC converter for an offshore wind farm, and belongs to the field of wind power technology. The resonant DC converter includes a plurality of resonant DC converter modules, which adopt an input-output parallel structure, one end of which is connected to a low-voltage side bus, and the other end of which is connected to a high-voltage side bus. The resonant DC converter module includes an inverter and a booster; the booster includes three boost modules, which adopt an input parallel output series structure. The control method is: subtract the actual value of the voltage reference value of the DC bus on the input side from its actual value, obtain the output current reference value of the inverter through PI, divide this value by 3, and use it as the resonant current reference value of each boost module; subtract the actual value of the resonant current of each boost module from this value, and then add them after passing through a low-pass filter and PI respectively, and compare them with a triangular wave with a frequency of the switching frequency after PI to generate the drive of the inverter, so as to realize the current sharing, boosting and stable control of the DC voltage on the input side.

Description

Technical Field

[0001] The invention relates to a converter, in particular to a topological structure and a control method of a DC power transmission resonance type DC converter of an offshore wind farm, belonging to the technical field of wind power. Background Art

[0002] With the continuous reduction of wind power generation costs, the continuous maturity of power electronics technology and the improvement of control algorithms, the development of offshore wind power will be an inevitable trend.

[0003] Traditional offshore wind power uses AC grid connection, which requires power frequency transformers for voltage boosting. These transformers are generally installed in the nacelle or at the bottom of the tower. However, due to the large size and bulkiness of the transformer, installation and maintenance are difficult, costly, and cause large losses, which reduces the overall efficiency and reliability of the system. When the transformer is placed in the nacelle, the nacelle volume and yaw power must be increased, and higher mechanical stress will be imposed on the tower. These shortcomings restrict the development of offshore wind power.

[0004] As an alternative to AC grid, DC grid has received more and more attention recently. The operating frequency of the DC boost converter used in it is much higher than the frequency of the power frequency transformer. The advantage of using DC grid is that it does not require any AC reactive power compensation in the grid, and there is no synchronization problem inherent in AC grid. The DC transmission grid-connected system of offshore wind farm will be an important transmission method for offshore wind farms in the future. In this grid-connected system, whether it is a doubly fed wind turbine or a permanent magnet direct drive wind turbine, its converter system includes a front-end AC / DC part and a back-end DC / DC DC boost part (called DC grid-side converter GSC), and the DC grid-side converter GSC is directly connected to the DC high-voltage grid. As one of the key components in the DC transmission system, GSC has the following functions: when the wind turbine is working normally, GSC realizes the DC boost function, transmits the power generated by the wind turbine to the DC grid, and maintains the voltage stability on its low-voltage side. It can be seen that GSC can realize DC high-gain transformation, so there is no need for power frequency transformer, small footprint, low loss, and can effectively solve the bottleneck of flexible DC transmission system of offshore wind farm.

[0005] GSC is a high-power DC converter. At present, DC converters can be divided into isolated and non-isolated types. Isolated DC converters mainly include forward, flyback, half-bridge and full-bridge (DAB) structures. Among them, the DAB boost converter is a well-known topology using an intermediate frequency transformer. However, the design of a high-power intermediate frequency transformer with a large turns ratio has certain inherent limitations in terms of isolation and magnetic coupling effects. Some literatures use diode-capacitor cascade, switch-capacitor cascade, and capacitor clamp modules to achieve the boost effect, but the efficiency is low, and under high-power conditions, the voltage stress of the switch tube is too high. There are also topologies such as staggered parallel and cascade, which are all hard-switched non-isolated circuit topologies and have the disadvantage of low efficiency; there are also literatures that use modular working methods, the circuit topology is complex and difficult to control; or use LLC circuits, but the gain effect is not enough. Summary of the invention

[0006] The main purpose of the present invention is to provide a high voltage gain, modular, high power offshore wind farm DC transmission resonant DC converter topology structure and control method for use as a DC grid-side converter (GSC) to address the deficiencies of the above-mentioned prior art.

[0007] In order to achieve the above purpose, the present invention discloses a topological structure of a DC transmission resonant DC converter for an offshore wind farm, comprising a plurality of resonant DC converter modules, an input-side DC bus and an output-side DC bus. The input sides of all the resonant DC converter modules are connected in parallel and connected to the input-side DC bus; the output sides of all the resonant DC converter modules are also connected in parallel and connected to the output-side DC bus; the output-side DC bus is connected to a DC power grid.

[0008] The resonant DC converter module includes an inverter and a booster; one end of the inverter is connected to the input side DC bus, and the other end is connected to the booster; the booster includes three boost modules: a first boost module, a second boost module, and a third boost module; the three boost modules adopt an input parallel and output series structure, that is, the input sides of the three boost modules are connected in parallel, the output side positive pole of the first boost module is connected to the positive pole of the output side DC bus, and its negative pole is connected to the output side positive pole of the second boost module; the output side negative pole of the second boost module is connected to the output side positive pole of the third boost module, and the output side negative pole of the third boost module is connected to the negative pole of the output side DC bus.

[0009] The inverter is a full-bridge inverter circuit, including four switch tubes S1 to S4 and their corresponding four anti-parallel diodes D s1 ~D s4; Its function is to invert the voltage of the DC bus on the input side into a high-frequency square wave and control the voltage gain and resonant current.

[0010] The boost module is composed of a resonant circuit, a high-frequency transformer and a voltage-doubling rectifier circuit; one end of the resonant circuit is connected to the output of the inverter, and the other end is connected to the primary coil of the high-frequency transformer; the secondary coil of the high-frequency transformer is connected to the voltage-doubling rectifier circuit;

[0011] The resonant circuit includes a resonant capacitor, a resonant inductor and an excitation inductor; one end of the resonant capacitor is connected to one end of the output side of the inverter, and the other end is connected to the resonant inductor; the other end of the resonant inductor is respectively connected to the excitation inductor and one end of the primary coil of the high-frequency transformer; the other end of the excitation inductor is connected to the other end of the primary coil of the high-frequency transformer, and the connection point of the two is connected to the other end of the output side of the inverter.

[0012] The voltage doubler rectifier circuit includes two rectifier diodes and two output capacitors; the two rectifier diodes are connected in series, and the connection point between the two is connected to one end of the secondary coil of the high-frequency transformer; the two output capacitors are also connected in series, and the connection point between the two is connected to the other end of the secondary coil of the high-frequency transformer.

[0013] Furthermore, ideally, the parameters of the resonant inductor, resonant capacitor, and excitation inductor in the three boost modules are the same, but in fact, there are inherent deviations in the parameters of the products, which will cause unbalanced resonant currents in the three boost modules, and therefore resonant current equalization control needs to be implemented.

[0014] To this end, the control method of the topological structure of the DC transmission resonant DC converter of an offshore wind farm of the present invention adopts the following steps:

[0015] Step 1: Set the voltage reference value V of the DC bus at the input side iref Subtract its actual value V i , through the PI controller, the output current reference value i of the inverter is obtained ref , controlling the DC bus voltage at the input side to be stable;

[0016] Step 2: Set the output current reference value i of the inverter ref Divided by 3, the resonant current reference value i of each boost module is Lr-ref ;

[0017] Step 3: Set the resonant current reference value i of the boost module Lr-ref Subtract the actual value i of the resonant current of each boost module Lrj, respectively filtered through a low-pass filter to obtain a resonant current sharing deviation value of each boost module;

[0018] Step 4, adding the resonant current equalizing deviation value of each boost module after passing through their respective PI controllers, and sending it to the PWM module through the PI controller; at the same time, comparing it with the triangular wave with a frequency of the switching frequency, generating a drive signal for the switch tubes S1~S4 of the inverter, controlling the resonant current of the three boost modules and the voltage of the input side DC bus, realizing the functions of resonant current equalizing control, voltage stabilization control of the input side DC bus and boosting.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1) The DC converter module adopts an isolated boost structure and uses a resonant circuit and a voltage doubler rectifier output parallel structure to achieve high voltage gain. It has the advantages of high voltage gain, small size, high power density, safety and reliability.

[0021] 2) The present invention has simple control and high resonant current balancing accuracy, and can achieve natural current balancing among the boost modules while achieving voltage stability control of the DC busbar on the input side, fully meeting the requirements of the DC transmission system of offshore wind farms for the grid-side DC converter, and can effectively solve the bottleneck of the DC transmission system of offshore wind farms. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the topological structure of the resonant DC converter for DC transmission in an offshore wind farm of the present invention.

[0023] Figure 2 This is an example diagram of the structure of a multi-terminal flexible DC transmission grid-connected system for an offshore wind farm.

[0024] Figure 3 Schematic diagram of the topological structure of the resonant DC converter module of the present invention.

[0025] Figure 4 It is a schematic diagram of the circuit structure of the resonant DC converter module of the present invention.

[0026] Figure 5 This is the main working waveform diagram of the resonant DC converter module of the present invention.

[0027] Figure 6 This is the working state 1 (t0-t1) of the resonant DC converter module of the present invention.

[0028] Figure 7 This is the working state 2 (t1-t2) of the resonant DC converter module of the present invention.

[0029] Figure 8This is the working state 3 (t2-t3) of the resonant DC converter module of the present invention.

[0030] Fig. 9 This is the working state 4 (t3-t4) of the resonant DC converter module of the present invention.

[0031] Fig.10 This is a block diagram of the resonant current balancing control of the present invention.

[0032] Fig.11 It is the voltage and current output by the inverter of the present invention.

[0033] Fig.12 is the resonant current of the three boost modules of the present invention.

[0034] Fig.13 are the resonant current and the excitation current of the first boost module of the present invention.

[0035] Fig.14 It is the input side DC bus voltage and the output side DC bus voltage of the resonant DC converter module of the present invention.

[0036] Among them, 1-inverter; 2-boost; 21-first boost module; 22-second boost module; 23-third boost module; 211-resonant circuit of the first boost module; 212-voltage doubling rectifier circuit of the first boost module; 221-resonant circuit of the second boost module; 222-voltage doubling rectifier circuit of the second boost module; 231-resonant circuit of the third boost module; 232-voltage doubling rectifier circuit of the third boost module; 6-input side DC bus; 8-output side DC bus. DETAILED DESCRIPTION

[0037] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0038] like Figure 1 As shown, the topological structure of the resonant DC converter for DC transmission of an offshore wind farm of the present invention includes n resonant DC converter modules (hereinafter referred to as "modules"), an input-side DC bus 6 (a low-voltage DC bus) and an output-side DC bus 8 (a high-voltage DC bus), and adopts an input-output parallel structure (IPOP): the input side of each module is connected in parallel and connected to the input-side DC bus 6; their output sides are also connected in parallel and connected to the output-side DC bus 8. Figure 1 In, V i is the input voltage of the resonant DC converter of the present invention, V o is the output voltage of the resonant DC converter of the present invention; i1, i2, ..., i n are the input currents of the 1st, 2nd, ...nth modules respectively, i o1 、i o2 、…ion They are the output currents of the 1st, 2nd, ...nth modules respectively.

[0039] Figure 2 The figure shows a new multi-terminal DC transmission grid-connected system for an offshore wind farm. The resonant DC converter of the present invention serves as a DC grid-side converter (GSC) in the system, realizing high-gain DC boosting, while achieving low-voltage side voltage stability, and transmitting the generated power of the wind power generation system to the DC grid.

[0040] like Figure 3 As shown, each resonant DC converter module includes an inverter 1 and a booster 2; one end of the inverter 1 is connected to the input side DC bus 6, and the other end is connected to the booster 2; the booster 2 includes three boost modules: a first boost module 21, a second boost module 22, and a third boost module 23; the input sides of the three boost modules 21, 22, and 23 are connected in parallel and connected to the other end of the inverter 1. The output side positive pole of the first boost module 21 is connected to the positive pole of the output side DC bus 8, and its negative pole is connected to the output side positive pole of the second boost module 22; the output side negative pole of the second boost module 22 is connected to the output side positive pole of the third boost module 23, and the output side negative pole of the third boost module 23 is connected to the negative pole of the output side DC bus 8.

[0041] Inverter 1 is a full-bridge inverter circuit, which is used to invert the DC voltage of the input DC bus 6 into a high-frequency square wave and control the voltage gain and resonant current. It includes four switch tubes S1 to S4 and their corresponding four anti-parallel diodes D S1 ~D S4 and four parasitic junction capacitances C S1 ~C S4 Among them, the switch tubes S1 and S3 are controlled by one driving signal, and the switch tubes S2 and S4 are controlled by another driving signal.

[0042] like Figure 4As shown, the first boost module 21 is composed of a resonant circuit 211, a high-frequency transformer T1 and a voltage-doubling rectifier circuit 212; the second boost module 22 is composed of a resonant circuit 221, a high-frequency transformer T2 and a voltage-doubling rectifier circuit 222; the third boost module 23 is composed of a resonant circuit 231, a high-frequency transformer T3 and a voltage-doubling rectifier circuit 232; the three boost modules 21, 22, 23 adopt an input parallel and output series structure (IPOS), that is, one end of the resonant circuits 221, 221, 231 is connected in parallel and connected to the output of the inverter 1, and the other ends thereof are connected to the high-frequency transformers 211, 212 and 213, respectively. The primary coils of the transformers T1, T2 and T3 are connected; the secondary coils of the high-frequency transformers T1, T2 and T3 are respectively connected to one end of the voltage doubler rectifier circuits 212, 222 and 232; the output side positive pole of the voltage doubler rectifier circuit 212 is connected to the positive pole of the output side DC bus 8; the output side negative pole of the voltage doubler rectifier circuit 212 is connected to the output side positive pole of the voltage doubler rectifier circuit 222; the output side negative pole of the voltage doubler rectifier circuit 222 is connected to the output side positive pole of the voltage doubler rectifier circuit 232; the output side negative pole of the voltage doubler rectifier circuit 232 is connected to the negative pole of the output side DC bus 8, so as to realize triple voltage boosting.

[0043] The resonant circuit 211 includes a resonant capacitor C r1 , resonant inductor L r1 and the magnetizing inductance L m1 ; Resonant capacitor C r1 One end of the inverter is connected to the output side of the inverter 1, and the other end is connected to the resonant inductor L r1 Connected; resonant inductor L r1 The other end is connected to the excitation inductance L m1 Connected to one end of the primary coil of the high-frequency transformer T1; the excitation inductor L m1 The other end of is connected to the other end of the primary coil of the high-frequency transformer T1, and the connection point of the two is connected to the other end of the output side of the inverter 1.

[0044] Similarly, the resonant circuit 221 includes a resonant capacitor C r2 , resonant inductor L r2 and the magnetizing inductance L m2 ; Resonant capacitor C r2 One end of the inverter is connected to the output side of the inverter 1, and the other end is connected to the resonant inductor L r2 Connected; resonant inductor L r2 The other end is connected to the excitation inductance L m2 Connected to one end of the primary coil of high-frequency transformer T2; excitation inductance L m2 The other end of is connected to the other end of the primary coil of the high-frequency transformer T2, and the connection point of the two is connected to the other end of the output side of the inverter 1.

[0045] Likewise, the resonant circuit 231 includes a resonant capacitor Cr3 , resonant inductor L r3 and the magnetizing inductance L m3 ; Resonant capacitor C r3 One end of the inverter is connected to the output side of the inverter 1, and the other end is connected to the resonant inductor L r3 Connected; resonant inductor L r3 The other end is connected to the excitation inductance L m3 Connected to one end of the primary coil of high-frequency transformer T3; excitation inductance L m3 The other end of is connected to the other end of the primary coil of the high-frequency transformer T3, and the connection point of the two is connected to the other end of the output side of the inverter 1.

[0046] Resonant inductor L r1 , L r2 and L r3 is a coupled inductor.

[0047] The voltage doubler rectifier circuit 212 includes two rectifier diodes D o1 , D o2 and two output capacitors C o1 , C o2 ; Two rectifier diodes D o1 , D o2 The connection point of the two is connected to one end of the secondary coil of the high-frequency transformer T1; D o1 The other end (i.e., the positive electrode of the output side of the voltage doubler rectifier circuit 212) is connected to the capacitor C o1 and connected to the positive pole of the DC bus 8 on the output side; D o2 The other end (i.e., the negative electrode of the output side of the voltage doubler rectifier circuit 212) is connected to the capacitor C o2 and connected to the positive electrode of the output side of the voltage doubler rectifier circuit 222; two output capacitors C o1 , C o2 They are also connected in series, and the connection point of the two is connected to the other end of the secondary coil of the high-frequency transformer T1.

[0048] The voltage doubler rectifier circuit 222 includes two rectifier diodes D o3 , D o4 and two output capacitors C o3 , C o4 ; Two rectifier diodes D o3 , D o4 The connection point of the two is connected to one end of the secondary coil of the high-frequency transformer T2; D o3 The other end (i.e., the positive electrode of the output side of the voltage doubler rectifier circuit 222) is connected to the capacitor C o3 and connected to the negative electrode of the output side of the voltage doubler rectifier circuit 212; D o4The other end (i.e., the negative electrode of the output side of the voltage doubler rectifier circuit 222) is connected to the capacitor C o4 and connected to the positive electrode of the output side of the voltage doubler rectifier circuit 232; two output capacitors C o3 , C o4 They are also connected in series, and the connection point of the two is connected to the other end of the secondary coil of the high-frequency transformer T2.

[0049] The voltage doubler rectifier circuit 232 includes two rectifier diodes D o5 , D o6 and two output capacitors C o5 , C o6 ; Two rectifier diodes D o5 , D o6 The connection point of the two is connected to one end of the secondary coil of the high-frequency transformer T3; D o5 The other end (i.e., the positive electrode of the output side of the voltage doubler rectifier circuit 232) is connected to the capacitor C o5 and connected to the negative electrode of the output side of the voltage doubler rectifier circuit 222; D o6 The other end (i.e., the negative electrode of the output side of the voltage doubler rectifier circuit 232) is connected to the capacitor C o6 One end of the capacitor is connected to the negative pole of the DC bus 8 on the output side; two output capacitors C o5 , C o6 They are also connected in series, and the connection point of the two is connected to the other end of the secondary coil of the high-frequency transformer T3.

[0050] The working process of each resonant DC converter module of the DC transmission resonant DC converter for an offshore wind farm of the present invention is as follows:

[0051] like Figure 5 As shown, from top to bottom, they are the drive signals V of the switch tubes S1 to S4. gS1 With V gS3 、V gS2 With V gS4 (indicated by dotted lines in the figure), the four anti-parallel diodes D corresponding to the switch tubes S1~S4 S1 ~D S4 The terminal voltage V DS1 ~V DS4 , resonant inductor current i Lrj (j=1,2,3) and the excitation inductance current i Lmj (j=1,2,3), rectifier diode current i Don (n=1,2,3,4,5,6) waveform.

[0052] In one switching cycle (t0-t8), the resonant DC converter module has 8 working states. Figure 5As shown, in the first half cycle (t0-t4), the drive signal V of the switch tubes S2 and S4 is gS2 、V gS4 is low level; in the second half cycle (t5-t8), the drive signal V gS1 、V gS3 For simplicity, the focus is on analyzing the four working states (t0-t4) of the resonant DC converter module in the first half of the switching cycle, as follows:

[0053] Working state 1 (t0~t1): Figure 6 As shown, suppose that before time t0, D S1 , D S3 If the magnetizing inductance L is on, the switch tubes S1 and S3 are turned on at t0, and the voltages at both ends are 0, thus achieving zero voltage switching (ZVS). m1 ~L m3 The current to the right is positive, so the resonant current is positive in this working state. The primary side voltage of the high-frequency transformer T1~T3 is positive at the top and negative at the bottom, which leads to the rectifier diode D on the secondary side of the high-frequency transformer T1~T3. o1 , D o3 , D o5 On, magnetizing inductance L m1 ~L m3 The voltages at both ends are clamped by the secondary side voltages of high frequency transformers T1 to T3, so they cannot participate in resonance. r1 ~L r3 and the magnetizing inductance L m1 ~L m3 The currents of the magnetizing inductor current i Lmj (j=1,2,3) increases linearly, and the resonant inductor current i Lrj (j=1,2,3) increases sinusoidally. Flowing through the rectifier diode D o1 , D o3 , D o5 The current values ​​are respectively the resonant inductor current i Lrj (j=1,2,3) and the excitation inductance current i Lmj (j=1,2,3), when the difference between the two is 0, that is, when the two are equal, this working state ends. During working state 1, switch tubes S1 and S3 are continuously turned on, and the rectifier diode D o2 , D o4 , D o6 Deadline.

[0054] Working state 2 (t1-t2): Figure 7 As shown, at time t1 i Lmj =i Lrj(j=1,2,3), at this time, no current flows through the secondary side of high-frequency transformers T1~T3, so the rectifier diode D o1 ~D o6 The current is 0, the rectifier diode achieves zero current shutdown (ZCS), and the voltage of the output DC bus 8 is controlled by the output capacitor C o1 ~C o6 In this working state, the resonance changes from two-element resonance to three-element resonance. At this time, L mj , L rj With C rj (j=1,2,3) all have series resonance.

[0055] Working state 3 (t2~t3): Figure 8 As shown, at the end of the previous stage, the drive signal V gS1 、V gS3 becomes low level, so the switches S1 and S3 are in the off state. In order to prevent short-circuit damage caused by the conduction of the upper and lower bridge arms of the full-bridge inverter circuit, a dead time is added between the conduction and cutoff of the upper and lower bridge arms of one bridge arm. At this stage, the resonant current i Lrj (j=1,2,3) are all parasitic capacitances C of switches S1 and S3. S1 , C S3 The parasitic capacitance C of the switch tubes S2 and S4 is charged. S2 , C S4 In a state of continuous discharge. The resonant current i Lrj In this stage, the sine will decrease, because L mj (j=1,2,3) No resonance occurs, so the excitation current i Lmj (j=1,2,3) decreases linearly. In this stage, the excitation current i Lmj Greater than the resonant current i rmj , rectifier diode D o2 , D o4 , D o6 On, D o1 , D o3 , D o5 At time t3, the switch tubes S2 and S4 drive the signal V gS2 、V gS4 is low, which prepares for the subsequent zero voltage switch-on (ZVS). S1 , C S3 Charging completed, C S2 , C S4 Discharge is finished.

[0056] Working state 4 (t3~t4): Fig. 9 As shown, the resonant current i Lrj(j=1,2,3) is negative, the anti-parallel diode D of the switch tubes S2 and S4 S2 , D S4 The voltage at the terminals of the switch tubes S2 and S4 is 0, and the excitation inductance L mj (j=1,2,3) is clamped, D o2 , D o4 , D o6 Continue the previous state and continue conducting, D o1 , D o3 , D o5 At time t4, the switch tubes S2 and S4 drive the signal V gS2 、V gS4 It becomes a high level, the switch tubes S2 and S4 are turned on, and enter the next state, and the previous half-cycle working process is completed.

[0057] The four working states of the second half cycle are similar to the four working states of the first half cycle.

[0058] Through the above analysis, the voltage gain G of the resonant DC converter module of the present invention can be obtained as follows:

[0059]

[0060] Where V i is the voltage of the DC bus 6 on the input side, V o is the voltage of the DC bus 8 at the output side, k j is the resonant circuit excitation inductance L of the jth boost module mj With the resonant inductor L rj The ratio of k j =L mj / L rj ;f nj is the normalized frequency of the resonant circuit of the jth boost module, and f nj =f s / f rj , where f s is the switching frequency of inverter 1, f rj is the resonant frequency of the resonant circuit of the jth boost module, Q j is the quality factor of the resonant circuit of the jth boost module, j=1,2,3.

[0061] f rj Calculate as follows:

[0062]

[0063] Q j Calculate as follows:

[0064]

[0065] In the formula, ω rj is the resonant angular frequency, and ω rj =2πf rj ; R ac For load R o The equivalent load resistance on the primary coil side of the high-frequency transformer is R ac =8R o / π 2 .

[0066] From formula (1), we can see that the voltage gain G is related to the normalized frequency f n , inductance ratio k and quality factor Q, so the switching frequency f is appropriately selected. s , resonant frequency f r , inductance ratio k and quality factor Q, the optimal voltage gain G can be obtained. Conversely, if the desired voltage gain G is known, the corresponding optimal switching frequency f can be calculated s , resonant frequency f r , inductance ratio k and quality factor Q, thereby determining the parameters of corresponding components such as resonant inductor, resonant capacitor, excitation inductor and rectifier diode.

[0067] Furthermore, ideally, the resonant inductance L in the three boost modules 21, 22, and 23 is rj , resonant capacitor C rj , Excitation inductance L mj (j=1,2,3) and other devices have the same parameters, but in fact, there are inherent deviations in the parameters of the products, which will cause the resonant current i of the three boost modules 21, 22, 23 to Lr1 、i Lr2 and i Lr3 Unbalanced, so resonant current equalization control is required.

[0068] For this reason, Fig.10 As shown, the control method of the topological structure of the DC transmission resonant DC converter of an offshore wind farm of the present invention adopts the following steps:

[0069] Step 1: Set the voltage reference value V of the input side DC bus 6 iref Subtract its actual value V i , through the PI controller, the output current reference value i of inverter 1 is obtained ref , control the voltage stability of the DC bus 6 on the input side;

[0070] Step 2: Set the output current reference value i of inverter 1 to ref Divide by 3 to obtain the resonant current reference value i for each boost module. Lr-ref ;

[0071] Step 3: Set the resonant current reference value i of the boost module Lr-ref Subtract the actual value i of the resonant current of the first boost module 21 Lr1 , filtered by a low-pass filter, the resonant current equalization deviation value i of the first boost module 21 is obtained Lr10 ; Set the resonant current reference value i Lr-ref Subtract the actual value i of the resonant current of the second boost module 22 Lr2 , filtered by a low-pass filter, the resonant current equalizing deviation value i of the second boost module 22 is obtained. Lr20 ; Set the resonant current reference value i Lr-ref Subtract the actual value i of the resonant current of the third boost module 23 Lr3 , filtered by a low-pass filter, the resonant current equalizing deviation value i of the third boost module 23 is obtained. Lr30 ;

[0072] Step 4: Lr10 、i Lr20 、i Lr30 After being added by their respective PI controllers, they are sent to the PWM module through the PI controller; at the same time, the frequency is the switching frequency f s The triangle wave is compared to generate the drive signal V of the switch tubes S1 to S4 of inverter 1. gS1 、V gS2 、V gS3 、V gS4 (Where V gS1 With V gS3 Same, V gS2 With V gS4 Same, V gS1 、V gS3 With V gS2 、V gS4 complementary), controlling the resonant current i of the three boost modules 21, 22, and 23 Lr1 、i Lr2 、i Lr3 The voltage of the DC bus 6 on the input side is used to realize the resonant current balancing control and the voltage stability of the DC bus 6 on the input side.

[0073] In order to verify the function of the resonant DC converter of the present invention, a simulation analysis is performed on a resonant DC converter module with a rated power of 500kW. i is 1150VDC, simulating the output voltage of the front-end AC / DC part of the MW-class wind turbine converter system; its output side voltage V o The voltage is the same as the DC grid voltage, which is 10 kV DC. Other simulation parameters are shown in Table 1.

[0074] Table 1 Resonant DC converter simulation model parameters

[0075] name parameter <![CDATA[Resonant capacitor C r1 ~C r3 (nF)]]> 2945 <![CDATA[Resonant inductor L r1 ~L r3 (μH)]]> 86 <![CDATA[Excitation inductance L m1 ~L m3 (μH)]]> 172 <![CDATA[Turn ratio of high-frequency transformer N1~N3]]> 1:1

[0076] Fig.11 The output voltage and current waveforms of the inverter 1 are shown. Fig.12 are the three resonant currents i Lr1 、i Lr2 and i Lr3 ,from Fig.12 It can be seen that the three resonant currents achieve current sharing. Fig.13 is the resonant current i of the first boost module 21 Lr1 With the excitation current i Lm1 . Fig.14 is the input voltage V of the 500kW resonant DC converter in this example. i , output voltage V o The waveform from Fig.14 It can be seen that the voltage increases from 1150 V to 10 kV. The above simulation results verify the effectiveness of the present invention.

Claims

1. A topological structure of a DC power transmission resonant DC converter for an offshore wind farm, comprising a plurality of resonant DC converter modules, an input-side DC bus and an output-side DC bus, wherein the input sides of all the resonant DC converter modules are connected in parallel and connected to the input-side DC bus; the output sides of all the resonant DC converter modules are also connected in parallel and connected to the output-side DC bus; the output-side DC bus is connected to a DC grid; The resonant DC converter module includes an inverter and a booster; one end of the inverter is connected to the input side DC bus, and the other end is connected to the booster; the booster includes three boost modules: a first boost module, a second boost module, and a third boost module; the three boost modules adopt an input parallel connection and an output series connection structure, that is, the input sides of the three boost modules are connected in parallel, the output side positive pole of the first boost module is connected to the positive pole of the output side DC bus, and its negative pole is connected to the output side positive pole of the second boost module; the output side negative pole of the second boost module is connected to the output side positive pole of the third boost module, and the output side negative pole of the third boost module is connected to the negative pole of the output side DC bus; The inverter is a full-bridge inverter circuit, including four switch tubes and their corresponding four anti-parallel diodes; Its function is to invert the voltage of the DC bus on the input side into a high-frequency square wave, and control the voltage gain and the resonant current; The boost module is composed of a resonant circuit, a high-frequency transformer and a voltage-doubling rectifier circuit; one end of the resonant circuit is connected to the output of the inverter, and the other end is connected to the primary coil of the high-frequency transformer; the secondary coil of the high-frequency transformer is connected to the voltage-doubling rectifier circuit; The resonant circuit includes a resonant capacitor, a resonant inductor and an excitation inductor; one end of the resonant capacitor is connected to one end of the output side of the inverter, and the other end is connected to the resonant inductor; the other end of the resonant inductor is respectively connected to the excitation inductor and one end of the primary coil of the high-frequency transformer; the other end of the excitation inductor is connected to the other end of the primary coil of the high-frequency transformer, and the connection point of the two is connected to the other end of the output side of the inverter; The voltage doubler rectifier circuit includes two rectifier diodes and two output capacitors; the two rectifier diodes are connected in series, and the connection point between the two is connected to one end of the secondary coil of the high-frequency transformer; the two output capacitors are also connected in series, and the connection point between the two is connected to the other end of the secondary coil of the high-frequency transformer; The control method of the topological structure of the DC transmission resonant DC converter of the offshore wind farm adopts the following steps: Step 1: Set the voltage reference value of the DC bus at the input side V iref Subtract its actual value V i , through the PI controller, the output current reference value of the inverter is obtained i ref , controlling the DC bus voltage at the input side to be stable; Step 2: Set the output current reference value of the inverter i ref Divide by 3 as the resonant current reference value of each boost module i Lr-ref ; Step 3: Set the resonant current reference value of the boost module i Lr-ref Subtract the actual value of the resonant current of each boost module i Lrj , respectively filtered through a low-pass filter to obtain a resonant current sharing deviation value of each boost module; Step 4, adding the resonant current equalizing deviation value of each boost module after passing through their respective PI controllers, and sending it to the PWM module through the PI controller; at the same time, comparing it with the triangular wave with a frequency of the switching frequency, generating driving signals for the four switching tubes of the inverter, controlling the resonant current of the three boost modules and the voltage of the input side DC bus, realizing resonant current equalizing control, voltage stabilization control of the input side DC bus and boosting function.

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