A flexible control system and method based on power flow conversion

Abstract

The present invention relates to a flexible control system and method based on electric energy flow conversion. The system includes new energy power generation equipment such as wind power or solar power generation, boosting facilities, a first transformer, a first converter, a second converter, a second transformer, and a power grid. The new energy power generation equipment outputs an AC voltage, which is boosted to 220kV AC by the boosting facility, and then converted from AC to DC by the first converter, and then converted from DC to AC by the second converter, and then connected to the power grid through the second transformer. The present invention smoothes out the volatility, intermittency and other problems of renewable energy power generation, reduces its impact on the power grid, and realizes flexible control of changes in the power grid's electric energy flow.

Description

Technical Field

[0001] The present invention relates to the technical field of renewable energy power generation, and in particular to a flexible control system and method based on electric energy flow conversion. Background Art

[0002] my country is rich in wind and solar energy resources. New energy power generation such as wind power generation and solar power generation has been developed on a large scale, and the power generation capacity has increased rapidly. With the rapid development of new energy power generation technology, the support of the state's new energy power generation policy and the active promotion of local governments, new energy power generation has become an important form of power generation in the future power system. my country's new energy resource bases are mainly distributed in remote areas such as the eastern coastal areas and the western inland areas. At present, electricity is usually transmitted through high-voltage direct current or alternating current. However, after the new energy power generation is connected to the grid, due to its own intermittent and volatile nature, the power grid current changes frequently in multiple points and time periods, which impacts the power grid and affects the power quality of the power grid. Therefore, with the continuous increase in the capacity of new energy grid connection, the future power network should have the ability to absorb large-scale renewable energy power generation, and the transmission system should be able to effectively isolate the adverse effects of new energy power generation on the AC power grid.

[0003] Flexible DC transmission technology has become a suitable power transmission method for connecting new energy to the grid due to its significant advantages in the field of new energy grid connection. When new energy supplies power to the AC system through flexible DC technology, the frequency connection between new energy and AC grid will be decoupled, and the frequency change of the grid will be undamped. When new energy fluctuates, its frequency stability is very low, and connecting to the grid will cause unstable operation of DC and AC systems. Therefore, the flexible DC transmission system needs to use a control method with good dynamic performance to improve the stability and reliability of new energy grid connection.

[0004] How to smooth out the volatility and intermittency of renewable energy generation through the comprehensive application of DC power grid and low-voltage flexible AC / DC distribution equipment, minimize its impact on the power grid, effectively reduce the current large-scale problems of wind and solar power abandonment, achieve effective development and utilization of renewable energy, realize flexible regulation of power grid power flow changes, and solve the impact and influence of power flow changes on the power grid are issues that need to be urgently addressed in this field. Summary of the invention

[0005] In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a flexible control system and method based on power flow conversion, which specifically adopts the following technical solutions:

[0006] A flexible control system based on power flow conversion includes: a new energy power generation device 11, a boosting facility 12, a first transformer 13, a first current conversion device 14, a second current conversion device 15, a second transformer 16, and a power grid 17;

[0007] The new energy power generation equipment 11 outputs an AC voltage, which is boosted to 220 kV AC by a booster 12, converted to DC by a first converter 14, and then converted to AC by a second converter 15, and then connected to a power grid 17 by a second transformer 16;

[0008] The first converter device 14 includes a positive converter and a negative converter; the positive converter includes a positive transformer 21, a positive circuit resistor 22, a positive circuit reactor 23, and a positive VSC 24; the negative converter includes a negative transformer 25, a negative circuit resistor 26, a negative circuit reactor 27, and a negative VSC 28;

[0009] The second commutation device 15 includes a second commutation VSC81, a second commutation circuit reactance 82, a power calculation module 83, a phase-locked loop 84, a third dq conversion module 85, a DC voltage control module 86, a second current control module 87, a fourth dq conversion module 88, and a third SPWM control module 89.

[0010] The present invention also relates to a flexible control method based on power flow conversion, which is used in the flexible control system based on power flow conversion as described above, and comprises the following steps:

[0011] S1. The error between the AC voltage Up and the AC terminal voltage reference value Upref is proportionally integrated, and the result is used as the modulation ratio M, and the set frequency fref given by the control system to jointly generate the control target values ​​Uc1a, Uc1b, and Uc1c of the three-phase voltage fundamental component;

[0012] S2. According to the control target values ​​Uc1a, Uc1b, and Uc1c, the SPWM method is used to control the on and off of the IGBT in the positive electrode VSC24;

[0013] S3. The frequency setting value Wref of the AC voltage Up given by the control system is integrated, and the result Qs is output as a synchronous phase signal;

[0014] S4. After the dq conversion of the AC current Ip and the AC voltage Up, Upd, Upq, Ipd, and Ipq are output, where Upd is the d-axis component of the AC voltage Up, Upq is the q-axis component of the AC voltage Up, Ipd is the d-axis component of the AC current Ip, and Ipq is the d-axis component of the AC current Ip;

[0015] S5. The system gives a d-axis component setting value Updr of the AC voltage Up and a q-axis component setting value Upqr of the AC voltage Up, and processes the two by operation to obtain Ipdr and Ipqr, where Ipdr is the d-axis component setting value of the AC current Ip, and Ipqr is the q-axis component setting value of the AC current Ip, and then processes the two by operation to obtain Eds and Eqs, where Eds and Eqs are the d-axis and q-axis components of the first target AC voltage, respectively;

[0016] S6. The Eds and Eqs are subjected to dq transformation to output control target values ​​Uc2a, Uc2b, and Uc2c. According to the control target values ​​Uc2a, Uc2b, and Uc2c, the SPWM method is used to control the on and off of the IGBT in the negative electrode VSC28;

[0017] S7.Ur outputs the synchronous phase Qr through the phase-locked loop 84; after the AC voltage Ur and the current Ior are transformed into dq, Urd, Urq, Iord, and Iorq are output, where Urd is the d-axis component of the AC voltage Ur, Urq is the q-axis component of the AC voltage Ur, Iord is the d-axis component of the AC current Ior, and Iorq is the q-axis component of the AC current Ior;

[0018] S8. The DC voltage Udc, the power Qg calculated according to the voltage Ur and Ir, the Udc setting value Udcr given by the system, the Qg setting value Qgr, Urd, and Urq given by the system are processed together to obtain the setting values ​​Iordr and Iorqr of Iord and Iorq;

[0019] S9. Iord, Iorq, Iordr, and Iorqr are processed together to obtain d-axis and q-axis components Edr and Eqr of the second target AC voltage;

[0020] S10. Perform dq transformation on Edr and Eqr to output control target values ​​Uc3a, Uc3b, Uc3c. According to the control target values ​​Uc3a, Uc3b, Uc3c, the SPWM method is used to control the on and off of the IGBT in the second commutation VSC81.

[0021] The technical solution of the present invention has achieved the following beneficial effects: for the grid connection of new energy power generation, the conversion of AC to DC and DC to AC is realized through the conversion equipment, the active power is distributed according to the various setting values ​​given by the control system, the DC voltage stability is enhanced, the active power absorption capacity is improved, the influence of the fluctuation of reactive power is reduced, the volatility and intermittency of renewable energy power generation are smoothed, the impact of renewable energy power generation on the power grid is reduced, and the flexible regulation of the power flow changes of the power grid is realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural schematic diagram of the flexible control system based on power flow conversion of the present invention.

[0023] Figure 2 It is a schematic structural diagram of the first current conversion device of the present invention.

[0024] Figure 3 FIG. 4 is another schematic diagram of the structure of the first current conversion device of the present invention.

[0025] Figure 4 It is a structural schematic diagram of the first control module of the present invention.

[0026] Figure 5 It is a schematic diagram of the structure of the second control module of the present invention.

[0027] Figure 6 It is a structural schematic diagram of the voltage control module of the present invention.

[0028] Figure 7 It is a structural schematic diagram of the current control module of the present invention.

[0029] Figure 8 It is a schematic structural diagram of the second current conversion device of the present invention. DETAILED DESCRIPTION

[0030] The present invention will be further described below in conjunction with the accompanying drawings. The following examples are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the scope of protection of the present invention. It should be pointed out that the following detailed description is exemplary and is intended to provide further explanation of the present invention.

[0031] Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those generally understood by those of ordinary skill in the art to which the present invention belongs. It should be noted that the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0032] See attached Figure 1As shown, it is a schematic diagram of the structure of the flexible control system based on power flow conversion of the present invention. The flexible control system based on power flow conversion includes a new energy power generation equipment 11 such as wind power or solar power generation, a boosting facility 12, a first transformer 13, a first converter 14, a second converter 15, a second transformer 16, and a power grid 17. The new energy power generation equipment 11 outputs an AC voltage, which is boosted to 220 kV AC by the boosting facility 12, and then converted from AC to DC by the first converter 14, and then converted from DC to AC by the second converter 15, and then connected to the power grid 17 through the second transformer 16. The boosting facility 12 can be integrated in the new energy power generation equipment 11.

[0033] See attached Figure 2 , is a schematic diagram of the structure of the first converter device 14 of the present invention. The first converter device 14 is composed of two sets of AC / DC converter networks that can be independently operated and controlled, including two sets of independent positive converters and negative converters. The positive converter includes a positive transformer 21, a positive circuit resistor 22, a positive circuit reactor 23, and a positive VSC (voltage source converter) 24. The negative converter includes a negative transformer 25, a negative circuit resistor 26, a negative circuit reactor 27, and a negative VSC 28.

[0034] The positive and negative converters are grounded, and the grounding lead is drawn from the connection point of the positive converter and the negative converter on the DC side. The grounding point clamps the potential. Under normal operating conditions, the system adopts a bipolar with metal return line single-end grounding operation mode, and the unbalanced current of the two poles returns through the metal return line; when a single pole fails, the fault pole converter valve is locked and shut down, and all or part of the power of the fault pole is transferred to the normal pole. The normal pole and the metal return line are converted to a single pole with metal return line operation mode, and the working current returns through the metal return line.

[0035] On the AC side of the first converter device 14, the AC power is connected to the positive pole VSC 24 and the negative pole VSC 28 respectively through the positive pole transformer 21 and the negative pole transformer 25 connected in parallel, and the positive pole VSC 24 and the negative pole VSC 28 are independently controlled.

[0036] On the DC side of the first converter device 14, the metal return layer provides a current path, and the positive and negative DC networks can respectively form independent DC current paths with the metal return line, and the two-pole DC circuits can operate independently.

[0037] Since the reactance value of the circuit reactance is much greater than the resistance value of the circuit resistance, the circuit resistance can be ignored. The reactance value of the positive circuit reactance 23 is Xc1, the AC terminal voltage of the positive circuit reactance 23 is Up, and the voltage at the connection point between the positive circuit reactance 23 and the positive VSC24 is Uc1, then:

[0038] Uc1=Up-Qs1 Xc1 / Up (1)

[0039] In the formula (1), Qs1 is the reactive power flowing into the positive electrode VSC 24.

[0040] When the converter is connected to a new energy grid or a passive AC grid, the reactive power fluctuation on the AC grid side will be transmitted to the positive and negative converters. If the positive and negative converters use different control methods, the reactive power of one pole will be kept constant. The fluctuation of reactive power ΔQs will be fully transmitted to the other converter, and the fluctuation of reactive power on the AC side can be expressed as:

[0041] ΔQs=ΔQs1=Qs′1-Qs1 (2)

[0042] Where: ΔQs1 is the reactive power fluctuation of a pole converter; Qs′1 is the reactive power transmitted to a pole converter after the reactive power fluctuation. The reactive power fluctuation transmitted to Up will be fully transmitted to this pole without affecting the independent converter of the other pole.

[0043] Assume that the Up voltage fluctuation is ΔUp, the expression is:

[0044] ΔUp=Up′-Up (3)

[0045] Where: U′ is the AC voltage at Up after reactive power fluctuation.

[0046] At this time, the voltage control electrode can adjust the voltage Uc′1 at the outlet of the VCS AC side by changing the modulation ratio M, thereby changing the voltage drop on the positive reactance and maintaining the Up reference voltage Upref. The expression is:

[0047] Upref=Uc′1-(Qs1+ΔQs1)Xc1 / Up=Up (4)

[0048] Thus, stable control of the Up voltage can be achieved.

[0049] See attached Figure 3 As shown, the first inverter device 14 of the present invention further includes a first control module 31 and a second control module 32 .

[0050] The first control module 31 is shown in the attached Figure 4As shown, it includes a first proportional integral module 41, a three-phase sine wave module 42, and a first SPWM control module 43. The error between the AC terminal voltage Up and the AC terminal voltage reference value Upref is input to the first proportional integral module 41, and the modulation ratio M is obtained through proportional integral calculation. The modulation ratio M and the set frequency fref are input to the three-phase sine wave module 42, and the three-phase sine wave module 42 is set to be three-phase symmetrical. The three-phase sine wave module 42 outputs the control target values ​​Uc1a, Uc1b, and Uc1c of the three-phase voltage fundamental wave components to the first SPWM control module 43. The first SPWM control module 43 uses a sinusoidal pulse width modulation method to control the on and off of the IGBT in the positive electrode VSC24, thereby realizing the control of the positive electrode VSC24.

[0051] The second control module 32 is shown in the attached Figure 5As shown, it includes an integration module 51, a first dq conversion module 52, a voltage control module 53, a current control module 54, a second dq conversion module 55, and a second SPWM control module 56. Wref is the frequency setting value of the AC voltage Up, which is generally the power frequency. Qs is the phase obtained by integrating Wref input to the integration module 51, and the phase Qs is respectively input to the first dq conversion module 52, the voltage control module 53, the current control module 54, and the second dq conversion module 55. Updr is the d-axis component setting value of the AC voltage Up, and Upqr is the q-axis component setting value of the AC voltage Up, both of which are input to the voltage control module 53. Ip is the AC current at the connection point between the negative circuit reactance 27 and the negative VSC28. The AC voltage Up and the AC current Ip are input to the first dq conversion module 52, and after dq conversion, Upd, Upq, Ipd, and Ipq are output, where Upd is the d-axis component of the AC voltage Up, Upq is the q-axis component of the AC voltage Up, Ipd is the d-axis component of the AC current Ip, and Ipq is the d-axis component of the AC current Ip. Upd and Upq are input to the voltage control module 53, and Ipd and Ipq are input to the current control module 54. The voltage control module 53 outputs Ipdr and Ipqr, where Ipdr is the d-axis component setting value of the AC current Ip, and Ipqr is the q-axis component setting value of the AC current Ip, and both are input to the voltage control module 54. The voltage control module 54 outputs Eds and Eqs, where Eds and Eqs are the d-axis and q-axis components of the first target AC voltage, respectively. Eds and Eqs are input to the second dq conversion module 55, and the second dq conversion module 55 outputs the control target values ​​Uc2a, Uc2b, and Uc2c of the three-phase voltage fundamental components to the second SPWM control module 56. The second SPWM control module 56 uses a sinusoidal pulse width modulation method to control the on and off of the IGBT in the negative pole VSC28, thereby realizing the control of the negative pole VSC28.

[0052] See attached Figure 6 , the voltage control module 53 includes a first first-order inertia link 61, a second first-order inertia link 62, a first inductance 63, and a second inductance 64. After Upd and Updr are differentiated at the first node, they are input to the first first-order inertia link 61, and after Upq and Upqr are differentiated at the second node, they are input to the second first-order inertia link 62. The first first-order inertia link 61 outputs Ipdr, which is input to the second inductance 64, and the output of the second inductance 64 is connected to the second node. The second first-order inertia link 62 outputs Ipqr, which is input to the first inductance 63, and the output of the first inductance 63 is connected to the first node.

[0053] See attached Figure 7The current control module 54 includes a second proportional integral module 71 and a third proportional integral module 72. After the difference between Ipd and Ipdr is calculated at the third node, the difference is input to the second proportional integral module 71, and after the difference between Ipq and Ipqr is calculated at the fourth node, the difference is input to the third proportional integral module 72. The second proportional integral module 71 and the third proportional integral module 72 output Eds and Eqs respectively.

[0054] See attached Figure 8, is a schematic diagram of the structure of the second commutation device 15 of the present invention, comprising a second commutation VSC81, a second commutation circuit reactance 82, a power calculation module 83, a phase-locked loop 84, a third dq conversion module 85, a DC voltage control module 86, a second current control module 87, a fourth dq conversion module 88, and a third SPWM control module 89. The DC voltage Udc output by the first commutation device (14) is input to the second commutation VSC81 and the DC voltage control module 86. There is a second commutation circuit reactance 82 between the second commutation VSC81 and the second transformer 16, and the voltages Ur and Ir between the second commutation circuit reactance 82 and the second transformer 16 are input to the power calculation module 83, and the power Qg is calculated and output to the DC voltage control module 86. Ur is also input to the phase-locked loop 84 and the third dq conversion module 85. The phase-locked loop 84 outputs the phase Qr, which is respectively output to the third dq conversion module 85, the DC voltage control module 86, the second current control module 87, and the fourth dq conversion module 88. The current Ior between the second commutation VSC 81 and the second commutation circuit reactance 82 is input to the third dq conversion module 85, and the third dq conversion module 85 outputs Urd, Urq, Iord, and Iorq, wherein Urd is the d-axis component of the AC voltage Ur, Urq is the q-axis component of the AC voltage Ur, Iord is the d-axis component of the AC current Ior, and Iorq is the q-axis component of the AC current Ior. Urd and Urq are input to the DC voltage control module 86, and Iord and Iorq are input to the second current control module 87. The respective setting values ​​Udcr and Qgr of Udc and Qg are input to the DC voltage control module 86, and the DC voltage control module 86 outputs Iordr and Iorqr, which are the setting values ​​of Iord and Iorq respectively, and inputs them to the second current control module 87. The second current control module 87 outputs Edr and Eqr, which are the d-axis and q-axis components of the second target AC voltage respectively. Edr and Eqr are input to the fourth dq conversion module 88, and the fourth dq conversion module 88 outputs the control target values ​​Uc3a, Uc3b, and Uc3c of the three-phase voltage fundamental wave components to the third SPWM control module 89, and the third SPWM control module 89 uses a sinusoidal pulse width modulation method to control the conduction and shutdown of the IGBT in the second commutation VSC81, thereby realizing the control of the second commutation VSC81.

[0055] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A flexible control system based on power flow conversion, characterized in that: The system comprises: a new energy power generation device (11), a voltage boosting facility (12), a first voltage transformation device (13), a first current conversion device (14), a second current conversion device (15), a second voltage transformation device (16), and a power grid (17); The new energy power generation equipment (11) outputs an AC voltage, which is boosted to 220 kV AC by a boosting facility (12), converted from AC to DC by a first converter (14), then converted from DC to AC by a second converter (15), and then connected to a power grid (17) via a second transformer (16); The first converter device (14) comprises a positive converter and a negative converter; the positive converter comprises a positive transformer (21), a positive circuit resistor (22), a positive circuit reactance (23), and a positive VSC (24); the negative converter comprises a negative transformer (25), a negative circuit resistor (26), a negative circuit reactance (27), and a negative VSC (28); The second commutation device (15) comprises a second commutation VSC (81), a second commutation circuit reactance (82), a power calculation module (83), a phase-locked loop (84), a third dq conversion module (85), a DC voltage control module (86), a second current control module (87), a fourth dq conversion module (88), and a third SPWM control module (89); The first commutation device (14) further comprises a second control module (32); the second control module (32) comprises an integration module (51), a first dq conversion module (52), a voltage control module (53), a current control module (54), a second dq conversion module (55), and a second SPWM control module (56); the second SPWM control module (56) controls the on and off of the IGBT in the negative electrode VSC (28); The frequency setting value Wref of the AC voltage Up is input to the integration module (51), and the integration module (51) transmits the phase Qs to the first dq conversion module (52), the voltage control module (53), the current control module (54), and the second dq conversion module (55); the AC current Ip and the AC voltage Up at the connection point between the negative circuit reactance (27) and the negative electrode VSC (28) are input to the first dq conversion module (52), and after dq conversion, Upd, Upq, Ipd, and Ipq are output, wherein Upd is the d-axis component of the AC voltage Up, Upq is the q-axis component of the AC voltage Up, Ipd is the d-axis component of the AC current Ip, and Ipq is the d-axis component of the AC current Ip. components; the Upd and Upq are input to the voltage control module (53); the Ipd and Ipq are input to the current control module (54); Updr is a d-axis component setting value of the AC voltage Up, and Upqr is a q-axis component setting value of the AC voltage Up, both of which are input to the voltage control module (53); the voltage control module (53) outputs Ipdr and Ipqr, wherein Ipdr is a d-axis component setting value of the AC current Ip, and Ipqr is a q-axis component setting value of the AC current Ip, both of which are input to the voltage control module (54); the voltage control module (54) outputs Eds and Eqs, wherein Eds and Eqs are the d-axis and q-axis components of the first target AC voltage, respectively. axis component; the Eds and Eqs are input to the second dq conversion module (55), the second dq conversion module (55) outputs the control target values ​​Uc2a, Uc2b, Uc2c of the three-phase voltage fundamental wave component to the second SPWM control module (56), and the second SPWM control module (56) controls the on and off of the IGBT in the negative electrode VSC (28); The DC voltage Udc output by the first commutation device (14) is input to the second commutation VSC (81) and the DC voltage control module (86); a second commutation circuit reactance (82) is provided between the second commutation VSC (81) and the second transformer (16); voltages Ur and Ir between the second commutation circuit reactance (82) and the second transformer (16) are input to the power calculation module (83), power Qg is calculated and output to the DC voltage control module (86); Ur is output to the phase-locked loop (84) and the third dq conversion module (85); the phase-locked loop (84) outputs phase Qr to the third dq conversion module (85), the DC voltage control module (86), the second current control module (87), and the fourth dq conversion module (88); the second commutation VSC (81) and the second commutation circuit reactance (8 2) is input into the third dq conversion module (85), and the third dq conversion module (85) outputs Urd, Urq, Iord, and Iorq, wherein Urd is the d-axis component of the AC voltage Ur, Urq is the q-axis component of the AC voltage Ur, Iord is the d-axis component of the AC current Ior, and Iorq is the q-axis component of the AC current Ior, Urd and Urq are input into the DC voltage control module (86), and Iord and Iorq are input into the second current control module (87); the setting values ​​Udcr and Qgr of Udc and Qg, respectively, are input into the DC voltage control module (86), and the DC voltage control module 86 outputs the setting values ​​Iordr and Iorqr of Iord and Iorq to the second current control module (87); the second current control module (87) outputs the d-axis component of the second target AC voltage The a-axis and q-axis components Edr and Eqr are transmitted to the fourth dq transformation module (88), the fourth dq transformation module (88) outputs the control target values ​​Uc3a, Uc3b, Uc3c of the three-phase voltage fundamental components to the third SPWM control module (89), and the third SPWM control module (89) controls the on and off of the IGBT in the second commutation VSC (81).

2. The flexible control system based on power flow conversion according to claim 1 is characterized in that: The positive converter and the negative converter are grounded, and the grounding lead is led out from the connection point of the positive converter and the negative converter on the DC side. The grounding point is used to clamp the potential, and the unbalanced current of the two poles returns through the metal return line; On the AC side of the first converter device 14, the AC power is connected to the positive pole VSC (24) and the negative pole VSC (28) respectively through the positive pole transformer (21) and the negative pole transformer (25) connected in parallel; On the DC side of the first converter device 14, the metal return layer provides a current path, and the positive and negative DC networks can respectively form independent DC current paths with the metal return line.

3. The flexible control system based on power flow conversion according to claim 1 is characterized in that: The first current conversion device (14) further comprises a first control module (31); the first control module (31) comprises a first proportional integral module (41), a three-phase sine wave module (42), and a first SPWM control module (43); An error between the AC terminal voltage Up and the AC terminal voltage reference value Upref is input to the first proportional-integral module (41), and the first proportional-integral module (41) outputs a modulation ratio M; the modulation ratio M and a set frequency fref are input to the three-phase sine wave module (42); the three-phase sine wave module (42) outputs control target values ​​Uc1a, Uc1b, Uc1c of the three-phase voltage fundamental wave components to the first SPWM control module (43); the first SPWM control module (43) controls the on and off of the IGBT in the positive electrode VSC (24).

4. The flexible control system based on power flow conversion according to claim 1 is characterized in that: The voltage control module (53) comprises a first first-order inertia link (61), a second first-order inertia link (62), a first inductance (63), and a second inductance (64); the Upd and Updr are input to the first first-order inertia link (61) after being differentiated at a first node; the Upq and Upqr are input to the second first-order inertia link (62) after being differentiated at a second node; the first first-order inertia link (61) outputs the Ipdr to the second inductance (64), and the output of the second inductance (64) is connected to the second node; the second first-order inertia link (62) outputs the Ipqr to the first inductance (63), and the output of the first inductance (63) is connected to the first node.

5. The flexible control system based on power flow conversion according to claim 1 is characterized in that: The current control module (54) comprises a second proportional-integral module (71) and a third proportional-integral module (72); the Ipd and Ipdr are input into the second proportional-integral module (71) after being differentiated at a third node; the Ipq and Ipqr are input into the third proportional-integral module (72) after being differentiated at a fourth node; the second proportional-integral module (71) and the third proportional-integral module (72) respectively output the Eds and Eqs.

6. A flexible control method based on power flow conversion, used in the flexible control system based on power flow conversion as claimed in any one of claims 1 to 5, characterized in that: The following steps are involved: S1. The error between the AC voltage Up and the AC terminal voltage reference value Upref is proportionally integrated, and the result is used as the modulation ratio M, and the set frequency fref given by the control system to jointly generate the control target values ​​Uc1a, Uc1b, and Uc1c of the three-phase voltage fundamental component; S2. According to the control target values ​​Uc1a, Uc1b, and Uc1c, the SPWM method is used to control the on and off of the IGBT in the positive electrode VSC (24); S3. The frequency setting value Wref of the AC voltage Up given by the control system is integrated, and the result Qs is output as a synchronous phase signal; S4. After the AC current Ip and the AC voltage Up are transformed into dq, the outputs are Upd, Upq, Ipd, and Ipq, where Upd is the d-axis component of the AC voltage Up, Upq is the q-axis component of the AC voltage Up, Ipd is the d-axis component of the AC current Ip, and Ipq is the d-axis component of the AC current Ip; S5. The system gives the d-axis component setting value Updr of the AC voltage Up and the q-axis component setting value Upqr of the AC voltage Up, and processes the two by operation to obtain Ipdr and Ipqr, where Ipdr is the d-axis component setting value of the AC current Ip, and Ipqr is the q-axis component setting value of the AC current Ip, and then processes the two by operation to obtain Eds and Eqs, where Eds and Eqs are the d-axis and q-axis components of the first target AC voltage, respectively; S6. Performing dq transformation on the Eds and Eqs to output control target values ​​Uc2a, Uc2b, and Uc2c, and controlling the on and off of the IGBT in the negative electrode VSC (28) by using the SPWM method according to the control target values ​​Uc2a, Uc2b, and Uc2c; S7. Ur outputs the synchronous phase Qr through the phase-locked loop 84; after the AC voltage Ur and the current Ior are transformed into dq, Urd, Urq, Iord, and Iorq are output, where Urd is the d-axis component of the AC voltage Ur, Urq is the q-axis component of the AC voltage Ur, Iord is the d-axis component of the AC current Ior, and Iorq is the q-axis component of the AC current Ior; S8. The DC voltage Udc, the power Qg calculated according to the voltage Ur and Ir, the Udc setting value Udcr given by the system, the Qg setting value Qgr, Urd, and Urq given by the system are processed together to obtain the setting values ​​Iordr and Iorqr of Iord and Iorq; S9. Iord, Iorq, Iordr, and Iorqr are processed together to obtain the d-axis and q-axis components Edr and Eqr of the second target AC voltage; S10. Perform dq conversion on Edr and Eqr to output control target values ​​Uc3a, Uc3b, Uc3c. According to the control target values ​​Uc3a, Uc3b, Uc3c, an SPWM method is used to control the on and off of the IGBT in the second commutation VSC (81).

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