Overvoltage suppression method for the sending-end grid of a hybrid offshore wind power bipolar flexible direct current system

Abstract

The present invention discloses a method for suppressing overvoltage in the sending-end grid of a hybrid offshore wind power bipolar flexible direct current (DC) system. For a hybrid offshore wind power bipolar flexible direct current (DC) system in which the sending-end negative-pole converter uses a diode rectifier, the present invention achieves system overvoltage suppression in the event of a sending-end grid fault through coordinated control of the sending-end positive-pole MMC converter, the receiving-end negative-pole MMC converter, and the receiving-end positive-pole MMC converter. Compared with existing solutions, the control method proposed in the present invention can suppress overvoltage levels to a limited extent in the event of a sending-end grid fault, ensuring the safe and stable operation of the system, and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a method for suppressing overvoltage in a sending-end power grid of a hybrid offshore wind power bipolar flexible direct current transmission system. Background Art

[0002] As the voltage level and transmission capacity of flexible direct current (HVDC) transmission systems gradually increase, bipolar flexible direct current (HVDC) transmission systems, due to their high flexibility and reliability, have attracted increasing attention. In large-capacity offshore wind farm bipolar flexible direct current transmission systems, the offshore converter stations using the MMC topology are large in size and weight, resulting in high construction and transportation costs, which is not conducive to the grid parity of offshore wind power. Furthermore, when a fault occurs in the AC grid at the sending end of the offshore wind farm HVDC transmission system, its operational performance will be severely affected. The overvoltage caused by the fault not only threatens the safe and stable operation of the entire system, but also places high demands on the overvoltage and insulation levels of equipment and lines, significantly increasing construction costs. Therefore, research on fault overvoltage suppression strategies for offshore wind farm HVDC transmission systems is of great significance.

[0003] For AC / DC converters of the same capacity and voltage level, the volume of a diode solution is less than 50% of that of an MMC solution. Therefore, in a bipolar flexible DC transmission system, replacing some MMC rectifiers with diode rectifiers can significantly reduce the weight and cost of the offshore converter platform. To address these issues, a feasible design solution is as follows: an MMC converter is used at the positive pole of the sending end to maintain the stability of the AC grid, while a diode rectifier is used at the negative pole of the sending end to assist in power transmission. This solution optimizes construction costs while ensuring safe and stable system operation.

[0004] For hybrid MMC-diode bipolar flexible DC transmission systems, when a fault occurs in the sending grid, using traditional control strategies can result in severe overvoltage in the sending grid. Furthermore, because the faulted phase diode cannot reach its conduction threshold, the sending grid fault also reduces the output power of the negative diode converter, which in turn causes overvoltage in the DC bus of the positive MMC, seriously affecting the stable operation of the system. Currently, there is little research on methods for suppressing overvoltage in the sending grid of hybrid offshore wind farm-bipolar DC transmission systems using diodes. There is an urgent need to propose a method for suppressing overvoltage in the sending grid of hybrid offshore wind farm-bipolar flexible DC transmission systems to ensure the safe and stable operation of the system. Summary of the Invention

[0005] The purpose of the present invention is to overcome the overvoltage problem that exists in a hybrid offshore wind power bipolar flexible direct current transmission system under a sending-end grid fault. This invention provides a method for suppressing overvoltage in the sending-end grid of a hybrid offshore wind power bipolar flexible direct current system. Through the coordinated control of the sending-end positive pole MMC converter, the receiving-end negative pole MMC converter, and the receiving-end positive pole MMC converter, the system overvoltage is suppressed under a sending-end grid fault, and the power is balanced between the positive and negative poles. To achieve the above-mentioned purpose of the invention, this method adopts the following technical solutions:

[0006] A method for suppressing overvoltage in the power grid at the sending end of a hybrid offshore wind power bipolar flexible direct current transmission system. The topology of the hybrid offshore wind power bipolar flexible direct current transmission system includes: a positive MMC converter at the sending end, a negative diode converter at the sending end, a positive MMC converter at the receiving end, and a negative MMC converter at the receiving end. The method is characterized in that:

[0007] The control system used to implement the method includes: a sending-end positive pole MMC converter control system, a receiving-end positive pole MMC converter control system, and a receiving-end negative pole MMC converter control system;

[0008] The sending-end positive pole MMC converter control system adopts a sending-end AC grid voltage control outer loop and a current control inner loop, wherein the reference value of the sending-end AC grid positive sequence voltage is calculated based on the amplitude of the negative sequence voltage; when the transmission system is operating normally, the reference value of the positive sequence voltage is the rated value; when a fault occurs in the sending-end AC grid, the positive sequence voltage reference value is adjusted based on the amplitude of the negative sequence voltage, thereby suppressing AC overvoltage during the fault process; when the fault line is removed, the positive sequence voltage reference value is restored to the rated value; in addition, the sending-end positive pole MMC converter control system also includes a sending-end positive pole negative sequence current control module, which adopts a resonant controller to suppress the negative sequence current during the fault process;

[0009] The receiving-end positive MMC converter control system adopts the positive DC bus voltage and reactive power control outer loop and the current control inner loop to maintain the positive DC bus voltage stability and provide the necessary reactive power support to the receiving-end AC power grid;

[0010] The receiving-end negative MMC converter control system adopts an outer loop for negative DC bus voltage and reactive power control and an inner loop for current control; a negative DC bus voltage reference value is given based on the active power of the positive and negative MMC converters, and the control target is to keep the active power of the negative MMC converter equal to that of the positive MMC converter; when a sending-end AC grid fault occurs, the fault phase voltage will be lower than the diode conduction threshold, at which point the negative MMC DC bus voltage reference value will be reduced, achieving balanced distribution of output power between the positive and negative poles.

[0011] Further: In the control system of the positive MMC converter at the sending end, the reference value of the AC grid voltage at the sending end, the d-axis component U, and the q-axis component U are calculated according to the following method: gdref and U gqref :

[0012]

[0013] Among them, |U g- | is the negative sequence voltage amplitude of the power grid, U gdn is the d-axis rated voltage.

[0014] Further: In the negative MMC converter control system at the receiving end, the negative DC bus voltage reference value U is calculated according to the following method: dc2ref :

[0015] U dc2ref =U dcn2 -F PIP (s)(P g1 -P g2 )

[0016]

[0017] Among them, U dcn2 is the negative DC bus rating, F PIP (s) is the transfer function of the PI controller, k pp is the proportionality coefficient, k ip is the integral coefficient, P g1 is the MMC active power at the positive electrode of the receiving end, P g2 is the active power of the negative MMC at the receiving end.

[0018] The beneficial effect of the present invention is that by adopting the technical solution of the present invention, coordinated control of power between the positive and negative poles of the hybrid offshore wind power bipolar flexible direct current system can be achieved, and the overvoltage of the system can be effectively suppressed when a fault occurs in the sending-end power grid, thereby ensuring the safe and stable operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a typical topology diagram of the hybrid offshore wind power bipolar flexible direct current transmission system of the present invention.

[0020] Figure 2 The present invention is a flow chart of the operation of the hybrid offshore wind power bipolar flexible direct current transmission system during the fault occurrence stage and after the fault line is cut off when the method for suppressing overvoltage in the sending-end power grid of the system is adopted.

[0021] Figure 3This is a schematic diagram of a specific example of the receiving-end positive and negative MMC converter control method of the present invention. The diagram includes: 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; and 11 - receiving-end negative bridge arm voltage calculation module.

[0022] Figure 4 This is a system schematic diagram of a specific example of the control method of the sending-end positive MMC converter of the present invention.

[0023] Among them, 12-sending end positive pole voltage reference value calculation module, 13-sending end positive pole voltage control module, 14-sending end positive pole positive sequence current control module, 15-sending end positive pole negative sequence current control module, 16-sending end positive pole Park inverse transformation module, 17-sending end positive pole internal circulation control module, 18-sending end positive pole bridge arm voltage calculation module. DETAILED DESCRIPTION

[0024] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] First, combine Figure 1 and Figure 2 , the operation process of the hybrid offshore wind power bipolar flexible direct current transmission system during the sending-end power grid fault process and after the fault line is removed when the technical solution of the present invention is adopted is explained.

[0026] When a fault occurs in the sending-end AC grid, the fault phase voltage of the sending-end AC grid decreases, and the non-fault phase voltage increases, resulting in overvoltage. At this time, the negative-sequence component of the sending-end grid voltage increases significantly. In the control scheme proposed in the present invention, the negative-sequence voltage is used as a feedback value to adjust the AC voltage reference value of the sending-end positive MMC. Therefore, when a fault occurs, the AC voltage reference value of the sending-end positive MMC will immediately decrease, effectively suppressing the overvoltage of the sending-end grid. In addition, the significant drop in the fault phase voltage of the sending-end grid will cause the corresponding phase voltage of the sending-end negative diode converter to be lower than the conduction threshold and unable to conduct, thereby causing a decrease in the negative active power transmission capacity. In the control scheme proposed in the present invention, the DC bus voltage reference value of the receiving-end negative MMC is adjusted by the transmission power of the positive and negative poles. Therefore, in the fault stage, the power transmission capacity of the negative diode converter can be improved by reducing the negative DC bus voltage to ensure power balance between the positive and negative poles. When the fault line of the sending-end power grid is removed, the negative-sequence voltage value of the sending-end AC power grid becomes zero, the AC voltage reference value of the sending-end positive-pole MMC is restored to the rated value, and the sending-end power grid voltage is also restored to the rated value; at the same time, the DC bus voltage value of the receiving-end negative-pole MMC will also increase to the normal operating level, ensuring the balanced distribution of output power between the positive and negative poles, and the system returns to normal operation.

[0027] In the method for suppressing overvoltage of the sending-end grid of the hybrid offshore wind power bipolar flexible direct current transmission system of the present invention, the control system of the positive and negative MMC converters at the receiving end is implemented as follows: Figure 3 As shown, the receiving-end positive pole MMC converter control system includes a receiving-end positive pole DC bus voltage and reactive power control module 1, a receiving-end positive pole current control module 2, a receiving-end positive pole Park inverse transformation module 3, a receiving-end positive pole internal circulating current control module 4, and a receiving-end positive pole bridge arm voltage calculation module 5; the receiving-end negative pole MMC converter control system includes a receiving-end negative pole DC bus voltage reference value calculation module 6, a receiving-end negative pole DC bus voltage and reactive power control module 7, a receiving-end negative pole current control module 8, a receiving-end negative pole Park inverse transformation module 9, a receiving-end negative pole internal circulating current control module 10, and a receiving-end negative pole bridge arm voltage calculation module 11.

[0028] like Figure 3 As shown, in the method for suppressing overvoltage in the sending-end grid of the hybrid offshore wind power bipolar flexible direct current transmission system of the present invention, the control method for the positive pole MMC converter at the receiving end includes the following steps:

[0029] The positive DC bus voltage U dc1 Controlled by PI controller to follow the reference value U dc1ref The output of the controller passes through the limiting link and serves as the reference value of the positive d-axis current of the receiving end I gdref1 ; For reactive power Q g1Controlled by PI controller to follow the reference value Q g1ref The output of the controller passes through the limiting link and serves as the reference value of the positive q-axis current of the receiving end I gqref1 ;

[0030] The positive current control module 2 of the receiving end is used to control the positive d and q axis currents I gd1 and I gq1 Use PI controller to control it to follow the reference value I gdref1 and I gqref1 The output of the controller is used as the reference differential mode voltage U of the positive MMC at the receiving end. difdq1 ;

[0031] The receiving end positive pole Park inverse transformation module 3 is used to reference the differential mode voltage U of the receiving end positive pole MMC. difdq1 Perform Park inverse transformation to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc1 The angle used in the Park inverse transform is the AC grid phase θ g ;

[0032] The internal circulation current control module 4 of the positive electrode of the receiving end is used to control the internal circulation current I cabc1 The resonant controller is used for control, and the output of the internal circulating current controller of the positive electrode of the receiving end is used as the reference common mode voltage U of the positive electrode MMC of the receiving end. comabc1 ;

[0033] The receiving end positive pole bridge arm voltage calculation module 5 is used to calculate the positive pole reference differential mode voltage U difabc1 and the reference common mode voltage U comabc1 , the reference voltage U of the upper and lower bridge arms of the positive terminal MMC is obtained by calculation prefabc1 with U nrefabc1 The positive MMC converter at the receiving end is controlled through the valve control module.

[0034] like Figure 3 As shown, in the method for suppressing overvoltage in the sending-end grid of the hybrid offshore wind power-bipolar flexible direct current transmission system of the present invention, the method for controlling the negative pole MMC converter at the receiving end includes the following steps:

[0035] The DC bus voltage reference value calculation module 6 of the receiving end is used to calculate the active power P of the positive and negative MMCs of the receiving end. g1 and P g2 , calculate the negative DC bus voltage reference value U dc2ref , the specific calculation method is as follows:

[0036] U dc2ref =U dcn2 -F PIP (s)(P g1 -Pg2 )

[0037]

[0038] Among them, U dcn2 is the negative DC bus rating, F PIP (s) is the transfer function of the PI controller, k pp is the proportionality coefficient, k ip is the integral coefficient, P g1 is the MMC active power at the positive electrode of the receiving end, P g2 is the active power of the negative MMC at the receiving end;

[0039] The negative DC bus voltage U dc2 Controlled by PI controller to follow the reference value U dc2ref The output of the controller passes through the limiting link and serves as the reference value of the negative d-axis current of the receiving end I gdref2 ; For reactive power Q g2 Controlled by PI controller to follow the reference value Q g2ref The output of the controller passes through the limiting link and serves as the reference value of the negative q-axis current of the receiving end I gqref2 ;

[0040] The receiving end negative electrode current control module 8 is used to control the receiving end negative electrode d and q axis currents I gd2 and I gq2 Use PI controller to control it to follow the reference value I gdref2 and I gqref2 The output of the controller is used as the reference differential mode voltage U of the negative MMC at the receiving end. difdq2 ;

[0041] The receiving end negative pole Park inverse transformation module 9 is used to adjust the receiving end negative pole MMC reference differential mode voltage U difdq2 Perform Park inverse transformation to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc2 The angle used in the Park inverse transform is the AC grid phase θ g ;

[0042] The internal circulation current control module 10 of the negative electrode of the receiving end is used to control the internal circulation current I cabc2 The resonant controller is used for control, and the output of the internal circulating current controller of the negative electrode of the receiving end is used as the reference common mode voltage U of the negative electrode MMC of the receiving end. comabc2 ;

[0043] The receiving end negative pole bridge arm voltage calculation module 11 is used to calculate the negative pole reference differential mode voltage U difabc2 and the reference common mode voltage U comabc2, the reference voltage U of the upper and lower bridge arms of the negative MMC at the receiving end is obtained by calculation prefabc2 with U nrefabc2 The negative MMC converter at the receiving end is controlled through the valve control module.

[0044] In the method for suppressing overvoltage of the sending-end grid of the hybrid offshore wind power bipolar flexible direct current transmission system of the present invention, the control system of the sending-end positive pole MMC converter is implemented as follows: Figure 4 As shown, it includes a sending-end positive pole voltage reference value calculation module 12, a sending-end positive pole voltage control module 13, a sending-end positive pole positive sequence current control module 14, a sending-end positive pole negative sequence current control module 15, a sending-end positive pole Park inverse transformation module 16, a sending-end positive pole internal circulation control module 17, and a sending-end positive pole bridge arm voltage calculation module 18.

[0045] like Figure 4 As shown, in the method for suppressing overvoltage in the sending-end grid of the hybrid offshore wind power bipolar flexible direct current transmission system of the present invention, the control method for the positive pole MMC converter at the sending end includes the following steps:

[0046] The sending-end positive pole voltage reference value calculation module 12 is used to calculate the sending-end AC grid voltage reference value d and q axis components U according to the following method: gdref and U gqref :

[0047]

[0048] Among them, |U g- | is the negative sequence voltage amplitude of the power grid, U gdn is the d-axis rated voltage.

[0049] The d and q axis voltages U gdqs The PI controller is used to control the reference value U gdref and U gqref (U gqref Set to 0), the output of the controller passes through the limiting link and serves as the reference value I of the positive electrode d and q axis current gdref3 and I gqref3 ;

[0050] The positive-sequence current control module 14 at the sending end is used to control the positive-sequence current I gd3 and I gq3 Use PI controller to control it to follow the reference value I gdref3 and I gqref3 The output of the controller is used as the DC component U of the reference differential mode voltage of the positive MMC at the sending end. difdq3+ ;

[0051] The sending end positive negative sequence current control module 15 is used to control the current I gdq3 Control is performed and its reference value is given to zero to achieve the suppression of negative sequence current. The output of the resonant controller is used as the double frequency pulsating component U of the MMC reference differential mode voltage. difdq3- , the controller is implemented as follows:

[0052]

[0053]

[0054] Among them: F R100 (s) is the transfer function of the resonant controller, k g is the gain coefficient of the resonant controller, ω c is the cut-off frequency. In the embodiment, the resonant frequency is selected to be ±100 Hz and the cut-off frequency is selected to be 12 Hz. difd3- 、U difq3- Corresponding to U difdq3- The d-axis and q-axis components of .

[0055] The positive pole Park inverse transformation module 16 of the sending end is used to reference the differential mode voltage U of the positive pole MMC of the sending end. difdq3 Perform Park inverse transformation to obtain the reference differential mode voltage U in the stationary three-phase coordinate system difabc3 , the angle used by the Park inverse transform is the reference phase θ r Among them, U difdq3 The calculation method is as follows:

[0056]

[0057] Among them: U difd3+ 、U difq3+ Corresponding to U difdq3+ The d-axis and q-axis components of .

[0058] The internal circulation control module 17 of the positive electrode of the sending end is used to control the internal circulation I of the positive electrode MMC of the sending end. cabc3 The resonant controller is used for control, and the output of the internal circulating current controller of the positive electrode of the sending end is used as the reference common mode voltage U of the positive electrode MMC of the sending end. comabc3 ;

[0059] The sending end positive pole bridge arm voltage calculation module 18 is used to calculate the positive pole reference differential mode voltage U difabc3 and the reference common mode voltage U comabc3 , the reference voltage U of the upper and lower bridge arms of the positive MMC at the sending end is obtained by calculation prefabc3 with U nrefabc3 The positive MMC converter at the sending end is controlled through the valve control module.

[0060] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. Method for suppressing overvoltage in the sending-end grid of a hybrid offshore wind power bipolar flexible direct current system. The topology of the hybrid offshore wind power bipolar flexible direct current transmission system includes: The positive MMC converter at the sending end, the negative diode converter at the sending end, the positive MMC converter at the receiving end, and the negative MMC converter at the receiving end are characterized by: The control system used to implement the method includes: a sending-end positive pole MMC converter control system, a receiving-end positive pole MMC converter control system, and a receiving-end negative pole MMC converter control system; The sending-end positive pole MMC converter control system adopts a sending-end AC grid voltage control outer loop and a current control inner loop, wherein the reference value of the sending-end AC grid positive sequence voltage is calculated based on the amplitude of the negative sequence voltage; when the transmission system is operating normally, the reference value of the positive sequence voltage is the rated value; when a fault occurs in the sending-end AC grid, the positive sequence voltage reference value is adjusted based on the amplitude of the negative sequence voltage, thereby suppressing AC overvoltage during the fault process; when the fault line is removed, the positive sequence voltage reference value is restored to the rated value; in addition, the sending-end positive pole MMC converter control system also includes a sending-end positive pole negative sequence current control module, which adopts a resonant controller to suppress the negative sequence current during the fault process; The receiving-end positive MMC converter control system adopts the positive DC bus voltage and reactive power control outer loop and the current control inner loop to maintain the positive DC bus voltage stability and provide the necessary reactive power support to the receiving-end AC power grid; The receiving-end negative MMC converter control system adopts a negative DC bus voltage and reactive power control outer loop and a current control inner loop; the negative DC bus voltage reference value is given according to the active power of the positive and negative MMC converters, and the control target is to keep the active power of the negative MMC converter equal to that of the positive MMC converter; when a fault occurs in the sending-end AC grid, the fault phase voltage will be lower than the diode conduction threshold, at which point the negative MMC DC bus voltage reference value will be reduced, achieving balanced distribution of output power between the positive and negative poles; In the control system of the positive MMC converter at the sending end, the reference value of the sending end AC grid voltage d and q axis components U are calculated according to the following method: gdref and U gqref : Among them, |U g- | is the negative sequence voltage amplitude of the power grid, U gdn is the d-axis rated voltage; 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 : U dc2ref =U dcn2 -F PIP (s)(P g1 -P g2 ) Among them, U dcn2 is the negative DC bus rating, F PIP (s) is the transfer function of the PI controller, k pp is the proportionality coefficient, k ip is the integral coefficient, P g1 is the MMC active power at the positive electrode of the receiving end, P g2 is the active power of the negative MMC at the receiving end.

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