Fault Ride Methods for Hybrid Offshore Wind Power Bipolar Flexible DC Transmission Systems

Abstract

This invention discloses a fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system. Specifically designed for hybrid offshore wind power bipolar flexible DC transmission systems using diode converters in the sending-end negative converter, this method, through the coordinated control of the sending-end positive MMC converter, the receiving-end positive MMC converter, the receiving-end negative MMC converter, and the wind turbine, enables fault ride-through of the hybrid offshore wind power bipolar flexible DC transmission system without the need for energy-consuming resistors. This method offers significant economic benefits and promising application prospects.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system. Background Technology

[0002] As the voltage levels and transmission capacities of flexible DC transmission systems gradually increase, bipolar flexible DC transmission systems have attracted increasing attention due to their high flexibility and reliability. In large-capacity offshore wind farm bipolar flexible DC transmission systems, offshore converter stations using the MMC topology are large in size and weight, resulting in high construction and transportation costs, which is detrimental to grid parity for offshore wind power. Furthermore, when a fault occurs in the receiving-end AC grid, traditional flexible DC transmission systems use DC power dissipation devices to consume surplus power for fault ride-through. Large-capacity DC power dissipation devices are costly and require separate valve halls. Therefore, exploring lightweight design methods for offshore converter stations, optimizing system fault ride-through methods, and improving the economics of bipolar DC transmission schemes are important research directions in this field.

[0003] In bipolar flexible DC transmission systems, replacing some MMC rectifiers with diode rectifiers can significantly reduce the weight and cost of offshore converter platforms. However, the black start problem of offshore wind farms and the stability of the offshore AC grid need to be considered. To address these issues, a feasible design scheme is as follows: an MMC converter is used at the sending-end positive terminal to maintain the stability of the sending-end AC grid, while a diode rectifier is used at the sending-end negative terminal to assist in power transmission. This scheme optimizes construction costs while ensuring the safe and stable operation of the system. Currently, there is little research on fault ride-through methods using diode schemes in offshore wind power bipolar flexible DC transmission systems. There is an urgent need to propose a hybrid fault ride-through method for offshore wind power bipolar flexible DC transmission systems that can achieve AC grid fault ride-through without the need for energy-consuming resistors. Summary of the Invention

[0004] The purpose of this invention is to overcome the problem that existing studies have not considered the fault ride-through control of hybrid offshore wind power bipolar flexible DC transmission systems, and to provide a fault ride-through method for hybrid offshore wind power bipolar flexible DC transmission systems. During the system fault ride-through process, there is no need to put in energy-consuming resistors, and the system fault ride-through can be achieved through the coordinated control of the hybrid bipolar flexible DC system and the wind turbine.

[0005] To achieve the above-mentioned objectives, this method adopts the following technical solution:

[0006] A fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system, wherein the topology of the hybrid offshore wind power bipolar flexible DC transmission system includes: a sending-end positive MMC converter, a sending-end negative diode converter, a receiving-end positive MMC converter, and a receiving-end negative MMC converter; characterized in that...

[0007] The control system used to achieve the fault ride-through method includes: a sending-end positive MMC converter control system, a receiving-end positive MMC converter control system, and a receiving-end negative MMC converter control system.

[0008] The sending-end positive MMC converter control system employs an outer loop for sending-end AC grid voltage control and an inner loop for current control. The reference value of the sending-end AC grid voltage is given based on the positive DC bus voltage. During normal operation of the hybrid offshore wind power bipolar flexible DC transmission system, the positive DC bus voltage fluctuates around its rated value, and the reference value of the sending-end AC grid voltage is given as the rated voltage. The control objective of the system is to maintain the stability of the sending-end AC grid voltage. When a fault occurs in the receiving-end AC grid, the positive DC bus voltage increases. When the positive DC bus voltage exceeds the control threshold, the reference value of the sending-end AC grid voltage is adjusted to a first multiple M of the rated voltage. At this time, the control objective of the system is to reduce the sending-end AC grid voltage, causing the wind turbine to engage its own energy-consuming resistors and assisting the hybrid bipolar flexible DC transmission system in completing fault ride-through operation. The first multiple M is less than or equal to 0.3 and greater than or equal to 0.1.

[0009] The receiving-end positive MMC converter control system employs an outer loop for positive DC bus voltage and reactive power control, and an inner loop for current control. During normal system operation, the positive DC bus voltage reference value is set to its rated value, and the control objective of the system is to maintain a constant positive DC bus voltage. When a fault occurs in the receiving-end AC grid, the receiving-end AC grid voltage drops significantly. When the receiving-end AC grid voltage falls below a control threshold, the positive DC bus voltage reference value is set to a second multiple H of the rated value; this second multiple H is less than or equal to 1.2 and greater than or equal to 1.1.

[0010] The receiving-end negative MMC converter control system employs an outer loop for negative DC bus voltage and reactive power control, and an inner loop for current control. When the hybrid offshore wind power bipolar flexible DC transmission system is operating normally, the negative DC bus voltage reference value is given based on the wind farm's output power and the negative MMC converter's active power. The control objective of the system is to maintain the negative MMC converter's active power at half the wind farm's output power. When a fault occurs in the receiving-end AC grid, the receiving-end AC grid voltage drops significantly. When the receiving-end AC grid voltage falls below the control threshold, the negative DC bus voltage reference value is set to the rated value. At this point, since the sending-end AC grid voltage has already been reduced through the sending-end positive MMC control strategy, the sending-end negative diode converter has been turned off, and the receiving-end negative MMC no longer transmits power.

[0011] Preferably, the first multiple M is 0.2; the second multiple H is 1.15.

[0012] Furthermore: In the sending-end positive MMC converter control system, the sending-end AC grid voltage reference value d and q-axis component U are calculated according to the following method. gdref and U gqref :

[0013]

[0014] U gqref =0

[0015] Among them, U gdn U is the rated voltage of the sending-end AC grid. dc1 U is the positive DC bus voltage. dcn1 The positive DC bus voltage is the rated value, M is the first multiple, and H is the second multiple.

[0016] Furthermore: In the receiving-end positive MMC converter control system, the positive DC bus voltage reference value U is calculated according to the following method. dc1ref :

[0017]

[0018] Among them, U dcn1 U is the rated value of the positive DC bus voltage. ga U gb U gc These are the effective values ​​of the three-phase AC grid voltage at the receiving end, U gn This is the rated value of the AC grid voltage at the receiving end.

[0019] Furthermore: In the receiving-end negative MMC converter control system, the negative DC bus voltage reference value U is calculated according to the following method. dc2ref :

[0020]

[0021]

[0022] Among them, U dcn2 The rated value of the negative DC bus voltage, F PIP (s) is the transfer function of the PI controller, k pp k is the proportionality coefficient. ip P is the integral coefficient. wind P represents the output power of the wind farm. g2 For the active power of the receiving-end negative MMC, U ga U gb U gc These are the effective values ​​of the three-phase AC grid voltage at the receiving end, U gn This is the rated value of the AC grid voltage at the receiving end.

[0023] The beneficial effects of this invention are: by adopting the technical solution of this invention, fault ride-through operation of hybrid offshore wind power bipolar flexible DC transmission system can be achieved without the need for the energy-consuming resistor of the flexible DC system, thus solving the problem of difficult fault ride-through of hybrid offshore wind power bipolar flexible DC transmission system, and has significant economic benefits and good application prospects. Attached Figure Description

[0024] Figure 1 This is a typical topology diagram of the hybrid offshore wind power bipolar flexible DC transmission system of the present invention.

[0025] Figure 2 This is a flowchart illustrating the fault ride-through and fault recovery process of the hybrid offshore wind power bipolar flexible DC transmission system using the fault ride-through method of the present invention.

[0026] Figure 3 This is a schematic diagram of a specific example system for the control method of the receiving-end positive and negative MMC converter of the present invention. The modules are: 1- Receiving-end positive DC bus voltage reference value calculation module; 2- Receiving-end positive DC bus voltage and reactive power control module; 3- Receiving-end positive current control module; 4- Receiving-end positive Park inverse conversion module; 5- Receiving-end positive internal circulating current control module; 6- Receiving-end positive bridge arm voltage calculation module; 7- Receiving-end negative DC bus voltage reference value calculation module; 8- Receiving-end negative DC bus voltage and reactive power control module; 9- Receiving-end negative current control module; 10- Receiving-end negative Park inverse conversion module; 11- Receiving-end negative internal circulating current control module; 12- Receiving-end negative bridge arm voltage calculation module.

[0027] Figure 4 This is a schematic diagram of a specific example system for the control method of the sending-end positive MMC converter of the present invention.

[0028] Among them, 13-Sending end positive electrode voltage reference value calculation module, 14-Sending end positive electrode voltage control module, 15-Sending end positive electrode current control module, 16-Sending end positive electrode Park inverse transformation module, 17-Sending end positive electrode internal circulating current control module, and 18-Sending end positive electrode bridge arm voltage calculation module. Detailed Implementation

[0029] To describe the present invention more specifically, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. In the embodiments of the present invention, the first multiple M is 0.2, and the second multiple H is 1.15.

[0030] First, combined Figure 1 and Figure 2 This paper describes the fault ride-through and fault recovery process of a hybrid offshore wind power bipolar flexible DC transmission system when using the technical solution of this invention.

[0031] When a fault occurs in the receiving-end AC grid, the voltage of the receiving-end AC grid decreases, limiting the output power of both the positive and negative MMC converters at the receiving end. This causes surplus power to accumulate on the DC bus, resulting in an increase in the voltage of both the positive and negative DC buses. When the voltage of the positive DC bus rises to 1.09 times its rated value, the AC voltage command of the sending-end positive MMC converter decreases to 0.2 times its rated value, causing the voltage of the sending-end AC grid to drop rapidly. The AC side voltage of the sending-end negative diode converter falls below the conduction threshold, causing the negative diode converter to turn off. Through the control of the receiving-end negative MMC converter, the voltage of the negative DC bus is reduced to its rated value. At the same time, due to the rapid decrease in the sending-end AC grid voltage, the wind turbine will activate its own energy-consuming resistor, significantly reducing the overall power output of the wind farm. The surplus power problem of the positive DC bus is also resolved, and the voltage of the positive DC bus is maintained at 1.15 times its rated value. After the AC grid at the receiving end returns to normal, the voltage of the positive DC bus returns to its rated value, the AC voltage command of the positive MMC converter at the sending end is adjusted to its rated value, the AC grid voltage at the sending end returns to normal, the negative diode converter is turned on again, and the system returns to its pre-fault operating state.

[0032] In the fault ride-through method of the hybrid offshore wind power bipolar flexible DC transmission system of the present invention, the implementation of the control method of the positive and negative MMC converters at the receiving end is as follows: Figure 3As shown, it includes: receiving end positive DC bus voltage reference value calculation module 1, receiving end positive DC bus voltage and reactive power control module 2, receiving end positive current control module 3, receiving end positive Park inverse transformation module 4, receiving end positive internal circulating current control module 5, receiving end positive bridge arm voltage calculation module 6, receiving end negative DC bus voltage reference value calculation module 7, receiving end negative DC bus voltage and reactive power control module 8, receiving end negative current control module 9, receiving end negative Park inverse transformation module 10, receiving end negative internal circulating current control module 11, and receiving end negative bridge arm voltage calculation module 12.

[0033] like Figure 3 As shown, in the fault ride-through method of the hybrid offshore wind power bipolar flexible DC transmission system of the present invention, the receiving-end positive MMC converter control method includes the following steps:

[0034] Using the receiving-end positive DC bus voltage reference value calculation module 1, the positive DC bus voltage reference value U is calculated according to the following method. dc1ref :

[0035]

[0036] Among them, U dcn1 U is the rated value of the positive DC bus voltage. ga U gb U gc These are the effective values ​​of the three-phase AC grid voltage at the receiving end, U gn The rated value of the AC grid voltage at the receiving end;

[0037] Using the positive DC bus voltage and reactive power control module 2 at the receiving end, the positive DC bus voltage U is controlled. dc1 Controlled by a PI controller to make it follow the reference value U dc1ref The controller output, after passing through a limiting circuit, serves as the reference value I for the positive d-axis current at the receiving end. gdref1 For reactive power Q g1 Controlled by a PI controller to make it follow the reference value Q g1ref The controller output, after passing through a limiting circuit, serves as the reference value I for the positive q-axis current at the receiving end. gqref1 ;

[0038] Using the receiving-end positive current control module 3, the d-axis and q-axis currents I of the receiving-end positive terminal are controlled. gd1 and I gq1 A PI controller is used for control, making it follow the reference value I. gdref1 and I gqref1 The controller output serves as the reference differential voltage U of the positive MMC terminal. difdq1 ;

[0039] Using the Park inverse transformation module 4 at the positive terminal of the receiving end, the differential mode voltage U of the MMC reference at the positive terminal of the receiving end is... difdq1 Performing the inverse Park transform yields the reference differential voltage U in the stationary three-phase coordinate system. difabc1 The Park inverse transform uses the AC grid phase θ. g .

[0040] Using the receiving-end positive electrode internal circulating current control module 5, the receiving-end positive electrode MMC internal circulating current I is controlled. cabc1 A resonant controller is used for control, and the output of the circulating current controller inside the positive terminal of the receiving end is used as the reference common-mode voltage U of the MMC at the positive terminal of the receiving end. comabc1 ;

[0041] Using the receiving-end positive arm voltage calculation module 6, based on the receiving-end positive reference differential voltage U difabc1 and reference common-mode voltage U comabc1 The reference voltage U of the upper and lower arms of the MMC at the receiving end was obtained through calculation. prefabc1 with U nrefabc1 The positive MMC converter at the receiving end is controlled via a valve control module.

[0042] like Figure 3 As shown, in the fault ride-through method of the hybrid offshore wind power-bipolar flexible DC transmission system of the present invention, the receiving-end negative pole MMC converter control method includes the following steps:

[0043] Using the receiving end negative DC bus voltage reference value calculation module 7, based on the wind farm output power P wind Active power P at the receiving end negative electrode MMC g2 Calculate the reference value U of the negative DC bus voltage. dc2ref The specific calculation method is as follows:

[0044]

[0045]

[0046] Among them, U dcn2 The rated value of the negative DC bus voltage, F PIP (s) is the transfer function of the PI controller, k pp k is the proportionality coefficient. ip P is the integral coefficient. wind P represents the output power of the wind farm. g2 For the active power of the receiving-end negative MMC, U ga U gb U gc These are the effective values ​​of the three-phase AC grid voltage at the receiving end, U gn The rated value of the AC grid voltage at the receiving end;

[0047] Using the negative DC bus voltage and reactive power control module 8 at the receiving end, the negative DC bus voltage U is controlled. dc2 Controlled by a PI controller to make it follow the reference value U dc2ref The controller output, after passing through a limiting circuit, serves as the reference value I for the d-axis current of the receiving end negative electrode. gdref2 For reactive power Q g2 Controlled by a PI controller to make it follow the reference value Q g2ref The controller output, after passing through a limiting circuit, serves as the reference value I for the q-axis current of the receiving end negative electrode. gqref2 ;

[0048] Using the receiving end negative electrode current control module 9, the d-axis and q-axis currents I of the receiving end negative electrode are controlled. gd2 and I gq2 A PI controller is used for control, making it follow the reference value I. gdref2 and I gqref2 The controller output serves as the reference differential voltage U of the receiving-end negative MMC. difdq2 ;

[0049] Using the Park inverse transformation module 10 at the receiving end negative terminal, the MMC reference differential voltage U at the receiving end negative terminal is... difdq2 Performing the inverse Park transform yields the reference differential voltage U in the stationary three-phase coordinate system. difabc2 The Park inverse transform uses the AC grid phase θ. g .

[0050] Using the internal circulating current control module 11 of the receiving end negative electrode, the internal circulating current I of the receiving end negative electrode MMC is controlled. cabc2 A resonant controller is used for control, and the output of the circulating current controller inside the negative terminal is used as the reference common-mode voltage U of the negative terminal MMC. comabc2 ;

[0051] Using the receiving-end negative arm voltage calculation module 12, based on the receiving-end negative reference differential voltage U difabc2 and reference common-mode voltage U comabc2 The reference voltage U of the upper and lower arms of the MMC at the receiving end was obtained through calculation. prefabc2 with U nrefabc2 The control of the receiving-end negative MMC converter is achieved through a valve control module.

[0052] In the fault ride-through method of the hybrid offshore wind power bipolar flexible DC transmission system of the present invention, the implementation of the sending-end positive MMC converter control method is as follows: Figure 4As shown, it includes a positive terminal voltage reference value calculation module 13, a positive terminal voltage control module 14, a positive terminal current control module 15, a positive terminal Park inverse transformation module 16, a positive terminal internal circulating current control module 17, and a positive terminal bridge arm voltage calculation module 18.

[0053] like Figure 4 As shown, in the fault ride-through method of the hybrid offshore wind power bipolar flexible DC transmission system of the present invention, the sending-end positive MMC converter control method includes the following steps:

[0054] Using the positive terminal voltage reference value calculation module 13, the d-axis and q-axis components of the AC grid voltage reference value U at the sending end are calculated according to the following method. gdref and U gqref :

[0055]

[0056] U gqref =0

[0057] Among them, U gdn U is the rated voltage of the sending-end AC grid. dc1 U is the positive DC bus voltage. dcn1 This is the rated value of the positive DC bus voltage.

[0058] Using the positive terminal voltage control module 14, the d-axis and q-axis voltages U are controlled. gdqs Controlled by a PI controller, it follows the given reference value U. gdref and U gqref (U gqref When set to 0), the controller output, after passing through a limiting circuit, serves as the reference value I for the positive d-axis and q-axis currents. gdref3 and I gqref3 ;

[0059] Using the positive terminal current control module 15, the d-axis and q-axis currents I of the positive terminal are controlled. gd3 and I gq3 A PI controller is used for control, making it follow the reference value I. gdref3 and I gqref3 The controller output serves as the reference differential voltage U of the MMC at the sending end. difdq3 ;

[0060] Using the Park inverse conversion module 16 at the sending end positive terminal, the MMC reference differential voltage U at the sending end positive terminal is... difdq3 Performing the inverse Park transform yields the reference differential voltage U in the stationary three-phase coordinate system. difabc3 The Park inverse transform uses the reference phase θ as the angle. r .

[0061] Using the internal circulating current control module 17 of the positive electrode at the sending end, the internal circulating current I of the positive electrode MMC at the sending end is controlled. cabc3 A resonant controller is used for control, and the output of the circulating current controller inside the positive terminal of the sending end is used as the reference common-mode voltage U of the MMC at the positive terminal of the sending end. comabc3 ;

[0062] Using the positive arm voltage calculation module 18 at the sending end, based on the reference differential voltage U at the sending end... difabc3 and reference common-mode voltage U comabc3 The reference voltage U of the upper and lower arms of the MMC at the sending end was calculated. prefabc3 with U nrefabc3 The positive MMC converter at the sending end is controlled through a valve control module.

[0063] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. It will be apparent to those skilled in the art that various modifications can be made to the above embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system, wherein the topology of the hybrid offshore wind power bipolar flexible DC transmission system includes: A sending-end positive MMC converter, a sending-end negative diode converter, a receiving-end positive MMC converter, and a receiving-end negative MMC converter; characterized in that: The control system used to achieve the fault ride-through method includes: a sending-end positive MMC converter control system, a receiving-end positive MMC converter control system, and a receiving-end negative MMC converter control system. The sending-end positive MMC converter control system employs an outer loop for sending-end AC grid voltage control and an inner loop for current control. The reference value of the sending-end AC grid voltage is given based on the positive DC bus voltage. During normal operation of the hybrid offshore wind power bipolar flexible DC transmission system, the positive DC bus voltage fluctuates around its rated value, and the reference value of the sending-end AC grid voltage is given as the rated voltage. The control objective of the system is to maintain the stability of the sending-end AC grid voltage. When a fault occurs in the receiving-end AC grid, the positive DC bus voltage increases. When the positive DC bus voltage exceeds the control threshold, the reference value of the sending-end AC grid voltage is adjusted to a first multiple M of the rated voltage. At this time, the control objective of the system is to reduce the sending-end AC grid voltage, causing the wind turbine to engage its own energy-consuming resistors and assisting the hybrid bipolar flexible DC transmission system in completing fault ride-through operation. The first multiple M is less than or equal to 0.3 and greater than or equal to 0.

1. The receiving-end positive MMC converter control system employs an outer loop for positive DC bus voltage and reactive power control, and an inner loop for current control. During normal system operation, the positive DC bus voltage reference value is set to its rated value, and the control objective of the system is to maintain a constant positive DC bus voltage. When a fault occurs in the receiving-end AC grid, the receiving-end AC grid voltage drops significantly. When the receiving-end AC grid voltage falls below a control threshold, the positive DC bus voltage reference value is set to a second multiple H of the rated value; this second multiple H is less than or equal to 1.2 and greater than or equal to 1.

1. The receiving-end negative MMC converter control system employs an outer loop for negative DC bus voltage and reactive power control, and an inner loop for current control. When the hybrid offshore wind power bipolar flexible DC transmission system is operating normally, the negative DC bus voltage reference value is given based on the wind farm's output power and the negative MMC converter's active power. The control objective of the system is to maintain the negative MMC converter's active power at half the wind farm's output power. When a fault occurs in the receiving-end AC grid, the receiving-end AC grid voltage drops significantly. When the receiving-end AC grid voltage falls below the control threshold, the negative DC bus voltage reference value is set to the rated value. At this point, since the sending-end AC grid voltage has already been reduced through the sending-end positive MMC control strategy, the sending-end negative diode converter has been turned off, and the receiving-end negative MMC no longer transmits power.

2. The fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system according to claim 1, characterized in that: In the sending-end positive MMC converter control system, the sending-end AC grid voltage reference value d and q-axis component U are calculated according to the following method. gdref and U gqref : U gqref ＝0 Among them, U gdn U is the rated voltage of the sending-end AC grid. dc1 U is the positive DC bus voltage. dcn1 The positive DC bus voltage is the rated value, M is the first multiple, and H is the second multiple.

3. The fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system according to claim 1, characterized in that: In the receiving-end positive MMC converter control system, the reference value U of the positive DC bus voltage is calculated according to the following method. dc1ref : Among them, U dcn1 U is the rated value of the positive DC bus voltage. ga U gb U gc These are the effective values ​​of the three-phase AC grid voltage at the receiving end, U gn This is the rated value of the AC grid voltage at the receiving end.

4. The fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system according to claim 1, characterized in that: In the receiving-end negative MMC converter control system, the negative DC bus voltage reference value U is calculated according to the following method. dc2ref : Among them, U dcn2 The rated value of the negative DC bus voltage, F PIP (s) is the transfer function of the PI controller, k pp k is the proportionality coefficient. ip P is the integral coefficient. wind P represents the output power of the wind farm. g2 For the active power of the receiving-end negative MMC, U ga U gb U gc These are the effective values ​​of the three-phase AC grid voltage at the receiving end, U gn This is the rated value of the AC grid voltage at the receiving end.

5. The fault ride-through method for a hybrid offshore wind power bipolar flexible DC transmission system according to claim 1, characterized in that: The first multiple M is 0.2; the second multiple H is 1.15.

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