Power coordination control method for hybrid offshore wind power bipolar direct current transmission system

Abstract

The application discloses a kind of hybrid offshore wind power bipolar flexible transmission system power coordination control methods.For sending end negative converter adopts diode rectifier hybrid offshore wind power bipolar flexible transmission system, by adjusting the active power flexible distribution between bipolar by negative MMC DC bus voltage of receiving end.Compared with the existing method, the scheme can coordinate the power distribution between bipolar, maintain the stable operation of offshore ac grid, and has good application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a power coordination control method for a hybrid offshore wind power bipolar flexible direct current transmission system. BACKGROUND

[0002] With the gradual increase of the voltage level and transmission capacity of the flexible direct current transmission system, the bipolar flexible direct current transmission system has drawn more and more attention due to its high flexibility and reliability. The flexible direct current transmission technology based on the modular multilevel converter (MMC) topology has the advantages of low manufacturing difficulty, low system loss and high waveform quality, and has a very good application prospect in long-distance power collection and transmission.

[0003] However, in the bipolar flexible direct current transmission system of a large-capacity offshore wind farm, the offshore converter station adopting the MMC topology structure has a large volume and weight, and the construction and transportation cost is high, which is not conducive to the parity on-grid of offshore wind power. Therefore, exploring the light design method of the offshore converter station and improving the economy of the bipolar direct current transmission scheme is an important research direction in the field at present.

[0004] For the single-pole direct current transmission system of an offshore wind farm, a topology structure is proposed in which a diode converter is used in the offshore converter station and an MMC converter is used in the onshore converter station. When this topology is used, the offshore converter cannot provide a starting power supply for the offshore wind farm, and cannot maintain the stability of the offshore alternating current grid, and an additional starting power supply needs to be provided for the wind farm. In view of this problem, the following solutions are currently proposed: one is to connect to the land through an additional alternating current submarine cable; two is to parallel an auxiliary converter on the direct current side of the offshore converter station; three is to provide a starting power supply by adding energy storage equipment. The additional cost generated by the above three schemes is still high, so there is no application in actual engineering at present.

[0005] When the offshore converter station adopts a bipolar topology, replacing part of the MMC rectifier device with a diode rectifier device can significantly reduce the weight and cost of the offshore converter platform. The Chinese patent application with the publication number CN113472001A discloses a hybrid bipolar direct current transmission system for the sending end of offshore wind power, but the document only gives the overall framework and control target of the system, and does not give the specific converter topology and the implementation method of the control strategy of different converters, and it does not consider the inter-pole power coordination control method of the bipolar system, which cannot guarantee the balanced distribution and flexible adjustment of power between the two poles during system operation. At present, there is still little research on the power coordination control strategy of the diode scheme for the offshore wind power bipolar direct current transmission system, and it is urgent to propose a power coordination control method for a hybrid offshore wind power bipolar flexible direct current transmission system to optimize the engineering construction cost under the condition of ensuring the safe and stable operation of the system. SUMMARY

[0006] The application aims to overcome the problem that the existing hybrid offshore wind power bipolar flexible DC power transmission system cannot realize balanced distribution and flexible adjustment of power between the two poles, and provides a hybrid offshore wind power bipolar flexible DC power transmission system power coordination control method.

[0007] In order to achieve the above application purpose, the method adopts the following technical scheme:

[0008] The hybrid offshore wind power bipolar flexible DC power transmission system topology comprises 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; the AC sides of the sending end positive MMC converter and the sending end negative diode converter are connected in parallel to an offshore wind farm AC power grid; the AC sides of the receiving end positive MMC converter and the receiving end negative MMC converter are connected in parallel to an onshore AC power grid; the DC side of the sending end positive MMC converter is connected to the DC side of the receiving end positive MMC converter through a positive sea cable and a neutral sea cable; the DC side of the sending end negative diode converter is connected to the DC side of the receiving end negative MMC converter through a negative sea cable and a neutral sea cable; characterized in that,

[0009] The control system used by the power coordination control method comprises 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;

[0010] The sending end positive MMC converter control system adopts AC grid voltage and frequency control outer ring and current control inner ring, and the control target is to maintain the stability of the offshore AC grid voltage;

[0011] The receiving end positive MMC converter control system adopts DC bus voltage and reactive power control outer ring and current control inner ring, and the control target is to maintain the positive DC bus voltage at the rated value;

[0012] The receiving end negative MMC converter control system adopts DC bus voltage and reactive power control outer ring and current control inner ring, and the control target is to adjust the negative DC bus voltage according to the wind farm output power, so as to adjust the power flowing into the negative diode converter.

[0013] Further, in the receiving end negative MMC converter control system, the control of the receiving end negative MMC converter is realized according to the following method:

[0014] First, according to the wind farm output power P wind , the receiving end negative MMC active power P g2 , the negative DC bus voltage reference value U dc2ref; For the negative DC bus voltage U dc2 Controlled by a PI controller to make it follow the reference value U dc2ref The output of the PI controller, after passing through a limiting circuit, serves as the reference value I for the d-axis current of the receiving end negative terminal. 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 Then, for the negative terminal d-axis current I... 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 Then, through the inverse Park transformation, the reference differential voltage U in the stationary three-phase coordinate system is obtained. difabc2 Next, the internal circulating current I of the receiving-end negative electrode MMC is analyzed. 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 Finally, the differential mode voltage U is referenced at the negative terminal of the receiving end. 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.

[0015] Furthermore, for the receiving-end negative MMC converter, the negative DC bus voltage reference value U is calculated according to the following method. dc2ref :

[0016] U dc2ref =U dcref -F PIP (s)(0.5P wind -P g2 )

[0017]

[0018] Among them, U dcref For the negative DC bus rated value, 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 The active power of the receiving-end negative MMC.

[0019] The application has the beneficial effect that by adopting the technical scheme of the application, the power distribution between the bipolar of the hybrid offshore wind power bipolar HVDC transmission system can be coordinated, the stable operation of the offshore AC power grid is maintained, and the application prospect is good. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A typical topology diagram of the hybrid offshore wind power bipolar HVDC transmission system of the application, wherein the receiving end positive pole, the receiving end negative pole and the sending end positive pole converter adopt MMC converters, and the sending end negative pole converter adopts a diode converter.

[0021] Figure 2 A specific example structure diagram of the MMC converter in the hybrid offshore wind power bipolar HVDC transmission system, wherein the SM (N) The SM is a sub-module in the MMC; and N is the sub-module serial number.

[0022] Figure 3 A specific example structure diagram of the diode converter in the hybrid offshore wind power bipolar HVDC transmission system.

[0023] Figure 4 A specific example system principle diagram of the receiving end positive and negative pole MMC converter control method of the application. Wherein, 1-receiving end positive pole voltage sampling module, 2-receiving end positive pole current sampling module, 3-receiving end positive pole Park transformation module, 4-receiving end positive pole power calculation module, 5-receiving end positive pole DC bus voltage and reactive power control module, 6-receiving end positive pole current control module, 7-receiving end positive pole Park inverse transformation module, 8-receiving end positive pole internal circulating current control module, 9-receiving end positive pole bridge arm voltage calculation module, 10-receiving end negative pole DC bus voltage reference value calculation module, 11-receiving end negative pole voltage sampling module, 12-receiving end negative pole current sampling module, 13-receiving end negative pole Park transformation module, 14-receiving end negative pole power calculation module, 15-receiving end negative pole DC bus voltage and reactive power control module, 16-receiving end negative pole current control module, 17-receiving end negative pole Park inverse transformation module, 18-receiving end negative pole internal circulating current control module, 19-receiving end negative pole bridge arm voltage calculation module, 20-receiving end phase-locked loop module.

[0024] Figure 5 A specific example system principle diagram of the sending end positive pole MMC converter control method of the application. Wherein, 21-sending end positive pole voltage sampling module, 22-sending end positive pole current sampling module, 23-sending end positive pole Park transformation module, 24-sending end positive pole voltage control module, 25-sending end positive pole current control module, 26-sending end positive pole Park inverse transformation module, 27-sending end positive pole internal circulating current control module, 28-sending end positive pole bridge arm voltage calculation module. DETAILED DESCRIPTION

[0025] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] The implementation of the control method for the positive and negative MMC converters at the receiving end of the hybrid offshore wind power bipolar flexible DC transmission system in this invention is as follows: Figure 4 As shown, the module includes: a receiving-end positive voltage sampling module 1, a receiving-end positive current sampling module 2, a receiving-end positive Park conversion module 3, a receiving-end positive power calculation module 4, a receiving-end positive DC bus voltage and reactive power control module 5, a receiving-end positive current control module 6, a receiving-end positive Park inverse conversion module 7, a receiving-end positive internal circulating current control module 8, a receiving-end positive bridge arm voltage calculation module 9, a receiving-end negative DC bus voltage reference value calculation module 10, a receiving-end negative voltage sampling module 11, a receiving-end negative current sampling module 12, a receiving-end negative Park conversion module 13, a receiving-end negative power calculation module 14, a receiving-end negative DC bus voltage and reactive power control module 15, a receiving-end negative current control module 16, a receiving-end negative Park inverse conversion module 17, a receiving-end negative internal circulating current control module 18, a receiving-end negative bridge arm voltage calculation module 19, and a receiving-end phase-locked loop module 20.

[0027] like Figure 4 As shown, the control method for the receiving-end positive MMC converter of the hybrid offshore wind power bipolar flexible DC transmission system of the present invention includes the following steps:

[0028] The three-phase voltage U of the MMC AC power grid is acquired through the receiving-end positive voltage sampling module 1. gabc Positive DC bus voltage U dc1 The positive terminal positive current sampling module 2 collects the three-phase current I of the positive MMC AC power grid. gabc1 and internal circulation I cabc1 .

[0029] Using the receiving-end phase-locked loop module 20, based on the three-phase voltage U of the AC power grid gabc Obtain the grid voltage phase θ g .

[0030] Using the receiving-end positive terminal Park conversion module 3, the positive terminal MMC AC grid three-phase voltage U gabc and three-phase current I gabc1 Performing the Park transformation yields the corresponding voltage vector U in the synchronously rotating dq coordinate system. gdq and current vector I gdq1 The Park transform uses the grid voltage phase θ as the angle. g .

[0031] Using the positive terminal power calculation module 4 at the receiving end, based on the voltage vector Ugdq and the current vector I gdq1 , the positive active and reactive power P g1 and Q g1 are calculated.

[0032] The positive DC bus voltage U dc1 at the receiving end is controlled by the positive DC bus voltage and reactive power control module 5 to follow the reference value U dc1ref , and the output of the controller, after passing through a limiting link, is used as the reference value I gdref1 of the positive d-axis current at the receiving end; the reactive power Q g1 is controlled by a PI controller to follow the reference value Q g1ref , and the output of the controller, after passing through a limiting link, is used as the reference value I gqref1 of the positive q-axis current at the receiving end.

[0033] The positive d-axis and q-axis currents I gd1 and I gq1 at the receiving end are controlled by the positive current control module 6 to follow the reference values I gdref1 and I gqref1 , and the output of the controller is used as the reference differential mode voltage U difdq1 of the MMC at the receiving end.

[0034] The reference differential mode voltage U difdq1 of the MMC at the receiving end is subjected to Park inverse transformation by the positive Park inverse transformation module 7 to obtain the reference differential mode voltage U difabc1 in the static three-phase coordinate system, and the angle used in the Park inverse transformation is the phase θ g of the AC power grid.

[0035] The positive internal circulating current I cabc1 is controlled by the positive internal circulating current control module 8 using a resonant controller, and the output of the positive internal circulating current controller is used as the reference common mode voltage U comabc1 of the MMC at the receiving end.

[0036] The reference differential mode voltage U difabc1 and the reference common mode voltage U comabc1 of the MMC at the receiving end are used by the positive bridge arm voltage calculation module 9 to calculate the reference voltages U prefabc1 and U nrefabc1 of the upper and lower bridge arms of the MMC at the receiving end, and the valve control module is used to control the MMC converter at the receiving end.

[0037] As shown in Figure 4As shown, the method for controlling the negative MMC converter of the receiving end of the hybrid offshore wind power bipolar HVDC power transmission system in the application comprises the following steps:

[0038] The three-phase voltage U gabc of the MMC AC power grid is collected by the negative voltage sampling module 11 of the receiving end dc2 The three-phase current I gabc2 of the negative MMC AC power grid is collected by the negative current sampling module 12 of the receiving end cabc2 .

[0039] The three-phase voltage U gabc and the three-phase current I gabc2 of the negative MMC AC power grid are Park-transformed by the negative Park transformation module 13 of the receiving end to obtain the corresponding voltage vector U gdq and the current vector I gdq2 in the synchronous rotating d-q coordinate system, and the angle used in the Park transformation is the phase θ g of the AC power grid.

[0040] The active and reactive power P g2 and Q g2 of the negative pole are calculated by the negative power calculation module 14 of the receiving end according to the voltage vector U gdq and the current vector I gdq2 .

[0041] The negative DC bus voltage reference value U dc2ref is calculated by the negative DC bus voltage reference value calculation module 10 of the receiving end according to the wind farm output power P wind and the active power P g2 of the negative MMC, and the specific calculation method is as follows:

[0042] U dc2ref = U dcref -F PIP (s)(0.5P wind -P g2 )

[0043]

[0044] Wherein, U dcref is the rated value of the negative DC bus, F PIP (s) is the transfer function of the PI controller, k pp is the proportional coefficient, k ip is the integral coefficient, P wind is the wind farm output power, and P g2 is the active power of the negative MMC of the receiving end.

[0045] The negative DC bus voltage and the reactive power control module 15 are used to control the negative DC bus voltage U dc2 The PI controller is used for control, so as to follow the reference value U dc2ref The output of the controller is used as the reference value I gdref2 of the negative d-axis current after the limiting link, and the reactive power Q g2 The PI controller is used for control, so as to follow the reference value Q g2ref The output of the controller is used as the reference value I gqref2 of the negative q-axis current after the limiting link.

[0046] The negative current control module 16 is used to control the negative d-axis current I gd2 and the q-axis current I gq2 The PI controller is used for control, so as to follow the reference value I gdref2 and I gqref2 The output of the controller is used as the reference value U difdq2 of the negative MMC reference differential mode voltage.

[0047] The negative Park inverse transformation module 17 is used to perform Park inverse transformation on the negative MMC reference differential mode voltage U difdq2 to obtain the reference differential mode voltage U difabc2 in the static three-phase coordinate system, and the angle used for the Park inverse transformation is the AC power grid phase θ g .

[0048] The negative internal circulating current control module 18 is used to control the negative internal circulating current I cabc2 The output of the negative internal circulating current controller is used as the reference value U comabc2 of the negative MMC reference common mode voltage.

[0049] The negative bridge arm voltage calculation module 19 is used to calculate the reference voltage U difabc2 and U comabc2 of the upper and lower bridge arms of the negative MMC according to the negative reference differential mode voltage U prefabc2 and the reference common mode voltage U nrefabc2 , and the valve control module is used to control the negative MMC converter.

[0050] The implementation of the sending positive MMC converter control method of the hybrid offshore wind power bipolar HVDC system in the application is as follows: Figure 5As shown, it includes a positive terminal voltage sampling module 21, a positive terminal current sampling module 22, a positive terminal Park conversion module 23, a positive terminal voltage control module 24, a positive terminal current control module 25, a positive terminal Park inverse conversion module 26, a positive terminal internal circulating current control module 27, and a positive terminal bridge arm voltage calculation module 28.

[0051] like Figure 5 As shown, the control method for the sending-end positive MMC converter of the hybrid offshore wind power bipolar flexible DC transmission system in this invention includes the following steps:

[0052] The three-phase voltage U of the sending-end MMC AC grid is collected by the sending-end positive voltage sampling module 21. gabcs The positive terminal current sampling module 22 collects the three-phase current I of the positive terminal MMC AC power grid. gabc3 and internal circulation I cabc3 .

[0053] Using the positive terminal Park converter module 23, the three-phase voltage U of the positive terminal MMC AC grid is converted. gabcs and three-phase current I gabc3 Performing the Park transformation yields the corresponding voltage vector U in the synchronously rotating dq coordinate system. gdqs and current vector I gdq3 The Park transform uses the reference phase θ. r .

[0054] Using the positive terminal voltage control module 24, 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 ;;

[0055] Using the positive terminal current control module 25, 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 ;

[0056] Using the Park inverse conversion module 26 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. difabc3The angle used in the Park inverse transformation is the reference phase θ r .

[0057] The sending positive internal loop current control module 27 is used to control the sending positive MMC internal loop current I cabc3 The sending positive internal loop current controller is controlled by a resonant controller, and the output of the sending positive internal loop current controller is used as the sending positive MMC reference common-mode voltage U comabc3 ;

[0058] The sending positive bridge arm voltage calculation module 28 is used to calculate the reference voltages U difabc3 and U comabc3 of the upper and lower bridge arms of the sending positive MMC according to the sending positive reference differential-mode voltage U prefabc3 and the reference common-mode voltage U nrefabc3 , and the valve control module is used to control the sending positive MMC converter.

[0059] The above description of the embodiments is for the purpose of facilitating the understanding and application of the present application by those of ordinary skill in the art. Those skilled in the art can easily make various modifications to the above embodiments, and apply the general principles described herein to other embodiments without having to go through creative labor. Therefore, the present application is not limited to the above embodiments, and any improvements and modifications made to the present application by those skilled in the art based on the disclosure of the present application should be within the scope of protection of the present application.

Claims

1. A power coordination control 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: The system comprises 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; the AC sides of the sending-end positive MMC converter and the sending-end negative diode converter are connected in parallel to the AC grid of an offshore wind farm; the AC sides of the receiving-end positive MMC converter and the receiving-end negative MMC converter are connected in parallel to the onshore AC grid; the DC side of the sending-end positive MMC converter is connected to the DC side of the receiving-end positive MMC converter via a positive submarine cable and a neutral submarine cable; the DC side of the sending-end negative diode converter is connected to the DC side of the receiving-end negative MMC converter via a negative submarine cable and a neutral submarine cable; characterized in that: The control system used in the power coordination control 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 adopts an outer loop for AC grid voltage and frequency control and an inner loop for current control, with the control objective being to maintain the stability of the offshore AC grid voltage. The receiving-end positive MMC converter control system adopts an outer loop of DC bus voltage and reactive power control and an inner loop of current control. The control objective is to maintain the positive DC bus voltage at the rated value. The receiving-end negative MMC converter control system adopts an outer loop of DC bus voltage and reactive power control and an inner loop of current control. The control objective is to adjust the negative DC bus voltage according to the output power of the wind farm, thereby adjusting the power flowing into the negative diode converter. In the receiving-end negative MMC converter control system, the receiving-end negative MMC converter is controlled according to the following method: According to 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 ; For the negative DC bus voltage U dc2 Controlled by a PI controller to make it follow the reference value U dc2ref The output of the PI controller, after passing through a limiting circuit, serves as the reference value I for the d-axis current of the receiving end negative terminal. 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 Then, for the negative terminal d-axis current I... 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 Then, through the inverse Park transformation, the reference differential voltage U in the stationary three-phase coordinate system is obtained. difabc2 Next, the internal circulating current I of the receiving-end negative electrode MMC is analyzed. 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 Finally, the differential mode voltage U is referenced at the negative terminal of the receiving end. 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.

2. The power coordination control method for a hybrid offshore wind power bipolar flexible DC transmission system according to claim 1, characterized in that: For the receiving-end negative MMC converter, the negative DC bus voltage reference value U is calculated according to the following method. dc2ref : U dc2ref =U dcref -F PIP (s)(0.5P wind -P g2 ) Among them, U dcref For the negative DC bus rated value, 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 The active power of the receiving-end negative MMC.

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