Grounding current suppression method for direct-current power transmission system with positive and negative poles arranged in addressing manner

Abstract

The invention relates to the technical field of direct current power transmission and unbalanced current suppression, in particular to a grounding current suppression method for a positive and negative electrode addressing arrangement direct current power transmission system, which comprises the following steps of: acquiring an actual detection value of a positive electrode current and an actual detection value of a negative electrode current; if the actual detection value of the positive current is larger than the actual detection value of the negative current, the positive converter station is switched to a mode I, and the negative converter station is switched to a mode II, and if the actual detection value of the positive current is smaller than the actual detection value of the negative current, the positive converter station is switched to the mode II, and the negative converter station is switched to the mode I; a constant direct-current voltage control strategy is adopted in the mode I, and a current unbalance suppression control strategy is adopted in the mode II; through the above strategy, the number of the sub-modules which are put into work of the corresponding bridge arm is adjusted, so that the output voltage of the bridge arm is close to the instruction value under the current imbalance suppression control strategy and the rated direct current voltage under the constant direct current voltage control strategy.

Description

Technical Field

[0001] The present invention relates to the technical field of HVDC transmission and unbalanced current suppression, and particularly to a method for suppressing grounding current in a DC transmission system with positive and negative poles located separately. Background Art

[0002] Currently, in-depth research work has been carried out on the topology schemes of new energy DC external transmission systems at home and abroad. Under the background of "dual carbon" in China, as the energy bases in the western and northern regions of China are rich in wind and solar resources, building large-scale new energy bases in the desert, gobi and wasteland areas is the main development direction, and it is an irresistible trend to transmit flexible DC power from large-scale new energy bases in the desert, gobi and wasteland areas.

[0003] Considering the scenario where multiple types and large-scale renewable energies such as wind and solar are distributed over a wide area, the Jinshang project adopts a high-low valve site-separated construction scheme. However, the overhead line used for site-separated connection has a high DC fault incidence rate and poor economy. In response to this, some scholars have proposed a construction method with positive and negative poles located separately, which can utilize the original grounding lines of the DC converter station to form positive and negative poles located separately, reducing the DC fault incidence rate and improving the economy. However, most of the time, the power of wind and solar energy in each area is not consistent. If the new energy power of the positive and negative poles is inconsistent, in-ground current will be generated, affecting surrounding communication equipment and corroding underground metal pipelines. Summary of the Invention

[0004] Aiming at the above-mentioned existing technologies, the present invention provides a method for suppressing grounding current in a DC transmission system with positive and negative poles located separately, mainly solving the technical problems existing in the above background art.

[0005] To achieve the above object, the technical solution of the embodiment of the present invention is realized as follows:

[0006] A method for suppressing grounding current in a DC transmission system with positive and negative poles located separately, the method comprising the following steps:

[0007] Obtain the actual detected value of the positive current at the positive pole converter station, and obtain the actual detected value of the negative current at the negative pole converter station;

[0008] If the actual detected value of the positive current is greater than the actual detected value of the negative current, the positive pole converter station switches to Mode I, and the negative pole converter station switches to Mode II. If the actual detected value of the positive current is less than the actual detected value of the negative current, the positive pole converter station switches to Mode II, and the negative pole converter station switches to Mode I. In Mode I, a constant DC voltage control strategy is adopted, and in Mode II, a current imbalance suppression control strategy is adopted;

[0009] Based on the fixed DC voltage control strategy or the current imbalance suppression control strategy, adjust the number of sub-modules in the corresponding arm that are put into operation, so that the arm output voltage approaches the command value under the current imbalance suppression control strategy and the rated DC voltage under the fixed DC voltage control strategy respectively.

[0010] Optionally, the current imbalance suppression control strategy specifically includes:

[0011] Perform PI calculations on the actual detected value of the positive current and the actual detected value of the negative current respectively to obtain the difference ΔU dvp and the difference ΔU dcn ;

[0012] Based on the rated DC voltage and the difference ΔU dcp , obtain the final positive DC voltage command value, and based on the rated DC voltage and the difference ΔU dcn , obtain the final negative DC voltage command value.

[0013] Optionally, calculate the difference ΔU using the following formula dcp and the difference ΔU dcn :

[0014]

[0015] where K P is the proportional coefficient, K1 is the integral coefficient, I dcp is the actual detected value of the positive current, I dcn is the actual detected value of the negative current, and s is the complex frequency domain variable in the Laplace transform.

[0016] Optionally, calculate the final positive DC voltage command value and the final negative DC voltage command value through the following formula:

[0017]

[0018] where U dcN is the DC voltage rated value.

[0019] Optionally, the current imbalance suppression control strategy further includes: Based on U dcp , adjust the number of sub-modules in the corresponding arm that are put into operation, so that the arm output voltage approaches the U dcp .

[0020] The current imbalance suppression control strategy further includes: Based on U dcn , adjust the number of sub-modules in the corresponding arm that are put into operation, so that the arm output voltage approaches the U dcn .

[0021] Optionally, the fixed DC voltage control strategy includes: Using the DC voltage rated value UdcN As the DC reference voltage U deref , by adjusting the number of sub-modules put into operation in the bridge arm, the output voltage of the bridge arm is made to approach the DC reference voltage.

[0022] The beneficial effects of the present invention are as follows: By obtaining the actual detection values of the positive-pole current and the negative-pole current in real time, when the actual detection value of the positive-pole current is greater than the actual detection value of the negative-pole current, it indicates that there is an excess trend in the positive-pole side current. At this time, the control logic triggers a mode switch: the positive-pole converter station switches to Mode I, and the negative-pole converter station switches to Mode II. Conversely, if the actual detection value of the positive-pole current is less than the actual detection value of the negative-pole current, the positive-pole converter station switches to Mode II, and the negative-pole converter station switches to Mode I;

[0023] In the constant DC voltage control strategy in Mode I, with the rated DC voltage as the target, the output voltage of the bridge arm is stabilized at the level corresponding to the rated value by adjusting the number of sub-modules put into operation in the bridge arm, thereby maintaining the stability of the DC voltage. In the current imbalance suppression control strategy in Mode II, according to the voltage command value output by the PI controller, the number of sub-modules put into the target bridge arm is dynamically adjusted, and the DC current of the corresponding pole is adjusted by changing the bridge arm voltage, so that the positive and negative pole currents tend to be balanced under the action of the voltage difference, and finally the effective suppression of the grounding current is realized. On the premise of ensuring the safe and stable operation of the system, the magnitude of the in-ground current can be effectively reduced, the interference to the surrounding communication equipment and the corrosion of the underground metal pipelines can be reduced, which has a significant effect on the practicability of the positive and negative pole separate-site construction scheme. Description of the Drawings

[0024] Figure 1 is the basic control strategy diagram of the positive and negative pole separate-site arranged DC transmission system provided by the present invention;

[0025] Figure 2 is the flowchart of the grounding current suppression method for the positive and negative pole separate-site arranged DC transmission system provided by the present invention;

[0026] Figure 3 is the control diagram of the current imbalance suppression process provided by the present invention;

[0027] Figure 4 is the simulation diagram of the current imbalance suppression control provided by the present invention. Detailed Embodiments

[0028] The technical solution of the present invention will be further elaborated in detail below in conjunction with the accompanying drawings of the specification and specific embodiments. Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention in this application are only for the purpose of describing specific embodiments and are not intended to limit the present invention. In the following description, the expression "some embodiments" is described, which describes a subset of all possible embodiments. However, it should be understood that "some embodiments" may be the same subset or different subsets of all possible embodiments and may be combined with each other without conflict.

[0029] In the following description, a large number of specific details are given to provide a more thorough understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present invention, some technical features well known in the art are not described.

[0030] It should be understood that the present invention can be implemented in different forms and should not be construed as limited to the embodiments presented herein. On the contrary, providing these embodiments will make the disclosure thorough and complete and will fully convey the scope of the present invention to those skilled in the art. And the purpose of the terms used herein is only to describe specific embodiments and is not a limitation of the present invention. When used herein, the singular forms "a", "an" and "the" are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms "comprising" and / or "including", when used in this specification, determine the presence of the described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups. When used herein, the term "and / or" includes any and all combinations of the related listed items.

[0031] It should be further noted that when an element is referred to as "fixed to" another element, it can be directly on the other element or there can also be an intermediate element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be an intermediate element at the same time. The terms "vertical", "horizontal", "inner", "outer", "left", "right" and similar expressions used in this application are only for the purpose of illustration and do not represent the only implementation.

[0032] In order to thoroughly understand the present invention, detailed structures will be presented in the following description to illustrate the technical solution proposed by the present invention. The optional embodiments of the present invention are described in detail as follows. However, in addition to these detailed descriptions, the present invention can also have other implementations.

[0033] In the sandy, gobi and desert areas, in the absence of conventional power supply support, new energy converters or flexible DC are required as network-forming power supplies. The advantage of using flexible DC to form a network is that the MMC converter has a larger capacity and a simpler control method compared to the converters of new energy units, and has a stronger control ability for the AC voltage at the PCC point. When the output power of new energy units fluctuates, the control ability of the MMC can be used to achieve frequency control at the PCC point, which is beneficial to system stability.

[0034] For the MMC converter under the ±800 kV UHV voltage level, the same pole adopts the form of series connection of high and low valve groups. To avoid the competition for the control rights of system frequency and voltage among valve groups, coordination is required. Currently, the methods applicable to this topology are: selecting one valve group to adopt V / f control, and the remaining valve groups to adopt constant P / Q control. This method can ensure uniform current sharing on the AC side by adjusting the power command values of the constant P / Q control valve groups to reasonably distribute the power among valve groups. However, in the case of co-location of positive and negative poles, the ratio of the sending end to the network-forming capacity is only 3:1. For the structure of grouped networking and separate-site arrangement of positive and negative pole converter stations, the positive and negative pole converter valves each establish an AC power grid, and there is no problem of competition for the control rights of the AC system between positive and negative pole valve groups. The number of valve groups forming the network is the same as that of the network, and the ratio of the network-forming capacity is 1:1.

[0035] Therefore, under the topology proposed in this application, two valve groups at one pole of the sending end of the flexible DC converter station adopt a differential control strategy to adapt to different operation requirements: one of the valve groups adopts V / f control, that is, voltage / frequency control, to ensure the voltage stability and frequency stability of the AC port of the converter by maintaining the proportional relationship between the AC side voltage and frequency; the other valve group adopts constant P / Q control (constant active power / constant reactive power control), and accurately controls the active power and reactive power output by this valve group according to the system dispatching instructions or operation requirements to achieve flexible adjustment of the power injection at the sending end. In addition, at the sending end of the flexible DC converter station, high valve network-forming type control and low valve network-following type control are used in cooperation, and at the receiving end, constant DC voltage and sub-module capacitor voltage control are adopted. The basic control block diagram is as Figure 1 shown.

[0036] Currently, the new energy units and MMC converters for grouped networking are arranged in the sandy, gobi and desert areas with a diameter of about 100 kilometers. The characteristics of wind and light are basically the same, and the light intensity and wind speed will not vary too much but cannot be exactly the same. When the new energy powers of the positive and negative poles are inconsistent, an in-ground current will be generated, interfering with surrounding communication equipment and corroding underground metal pipelines. To reduce the in-ground current, this application provides a method for suppressing the grounding current of a DC transmission system with separate-site arrangement of positive and negative poles. Please refer to Figure 2 , and the method includes the following steps:

[0037] S1. Obtain the actual detection value of the positive pole current at the positive pole converter station, and obtain the actual detection value of the negative pole current at the negative pole converter station;

[0038] S2. If the actual detected value of the positive - pole current is greater than the actual detected value of the negative - pole current, the positive - pole converter station switches to Mode I and the negative - pole converter station switches to Mode II. If the actual detected value of the positive - pole current is less than the actual detected value of the negative - pole current, the positive - pole converter station switches to Mode II and the negative - pole converter station switches to Mode I. In Mode I, a constant DC voltage control strategy is adopted, and in Mode II, a current imbalance suppression control strategy is adopted;

[0039] S3. Based on the constant DC voltage control strategy or the current imbalance suppression control strategy, adjust the number of sub - modules put into operation in the corresponding arm to make the arm output voltage approach the command value under the current imbalance suppression control strategy and the rated DC voltage under the constant DC voltage control strategy respectively.

[0040] Specifically, the actual detected value of the positive - pole current and the actual detected value of the negative - pole current are obtained in real - time through current sensors installed in the positive - and negative - pole converter stations. When the actual detected value of the positive - pole current is greater than the actual detected value of the negative - pole current, it indicates that there is an excess trend in the positive - pole side current. At this time, the control logic triggers a mode switch: the positive - pole converter station switches to Mode I and the negative - pole converter station switches to Mode II. On the contrary, if the actual detected value of the positive - pole current is less than the actual detected value of the negative - pole current, the positive - pole converter station switches to Mode II and the negative - pole converter station switches to Mode I.

[0041] In the constant DC voltage control strategy, with the rated DC voltage as the target, the number of sub - modules put into operation in the arm is adjusted to make the arm output voltage stable at the level corresponding to the rated value, thereby maintaining the stability of the DC voltage. In the current imbalance suppression control strategy, according to the voltage command value output by the PI controller, the number of sub - modules put into the target arm is dynamically adjusted. By changing the arm voltage, the DC current of the corresponding pole is regulated to make the positive and negative pole currents tend to balance under the action of the voltage difference, and finally the effective suppression of the grounding current is achieved.

[0042] In an optional embodiment, please refer to Figure 3 , the current imbalance suppression control strategy specifically includes;

[0043] The actual detected value of the positive - pole current and the actual detected value of the negative - pole current are respectively subjected to PI calculation to obtain the difference ΔU dcp and the difference ΔU dcn ;

[0044] Among them, the difference ΔU dcp and the difference ΔU dcn are calculated by the following formula:

[0045]

[0046] Among them, K Pis the proportionality coefficient, K1 is the integral coefficient, I dcp is the actual detected value of the positive - pole current, I dcn is the actual detected value of the negative - pole current, and s is the complex - frequency domain variable in the Laplace transform.

[0047] According to the actual detected value of the positive - pole current and the actual detected value of the negative - pole current, calculate the difference ΔU dcp and the difference ΔU dcn ;

[0048] Based on the rated DC voltage and the difference ΔU dcp , obtain the final positive - pole DC voltage command value. Based on the rated DC voltage and the difference ΔU dcn , obtain the final negative - pole DC voltage command value;

[0049] Furthermore, calculate the final positive - pole DC voltage command value and the final negative - pole DC voltage command value through the following formula:

[0050]

[0051] where, U dcN is the rated value of the DC voltage;

[0052] Based on U dcp , adjust the number of sub - modules in the corresponding bridge arm that are put into operation to make the output voltage of the bridge arm approach the U dcp , based on U dcn , adjust the number of sub - modules in the corresponding bridge arm that are put into operation to make the output voltage of the bridge arm approach the U dcn .

[0053] Specifically, after obtaining the actual detected value of the positive - pole current and the actual detected value of the negative - pole current, input them into the corresponding proportional - integral (PI) controller respectively. The PI controller, based on the dynamic characteristics of the DC power transmission system, performs fast - response regulation of the proportional link and static - error elimination of the integral link on the input current signal. The proportional link generates a corresponding voltage compensation amount according to the instantaneous value of the current deviation at present, quickly suppressing the unbalanced trend of the current; the integral link ensures the zero - error regulation of the current deviation during long - term operation by accumulating historical deviations. After being calculated by the PI controller, the differences ΔU dcp and the difference ΔU dcn are output respectively, which reflect the voltage adjustment direction and amplitude required for the current system to balance the positive and negative currents.

[0054] After obtaining the difference ΔU dcp and the difference ΔU dcn , for the positive - pole converter station, based on the rated DC voltage and the difference ΔU dcp generate the final positive - pole DC voltage command value; similarly, based on the rated DC voltage and the difference ΔUdcn , the final negative DC voltage command value is generated. This process is essentially based on the dynamic regulation of the PI controller, with a steady-state deviation compensation based on the rated value superimposed, so that the voltage command value not only includes a rapid response to real-time current fluctuations but also takes into account the rated operating conditions requirements of the system for long-term operation. The final voltage command value is converted into a control signal for the number of sub-modules put into the arm of the modular multilevel converter (MMC). By adjusting the number of sub-modules in the arm that are in the working state, the output voltage of the arm is changed, and then the DC current of the corresponding pole is adjusted, so that the positive and negative pole currents gradually approach balance under the action of the voltage difference, achieving effective suppression of the grounding current in the DC transmission system.

[0055] It can be understood that in the DC transmission system composed of modular multilevel converters, there is a direct physical relationship between the working state of the sub-modules in the arm and voltage and current regulation. Each sub-module is essentially an independently controllable voltage source, and its internal capacitor is charged to the rated voltage during normal operation. The output voltage of a single arm is equal to the sum of the voltages of all the sub-modules put into it, and each phase of the converter consists of two arms, the upper and the lower. When it is necessary to adjust the DC current of a certain pole (such as the positive pole or the negative pole), by changing the number of sub-modules put into the arm of the corresponding converter station, the output voltage of the arm of that pole can be directly changed. For example, increasing the number of sub-modules put into the positive pole converter station will increase the voltage of the positive pole arm, and then raise the potential of the positive pole side of the DC voltage; conversely, reducing the number of sub-modules put into it will lower the potential of the positive pole side. This potential change is transmitted through the DC line to the whole system, forming a voltage difference with the negative pole side. And the magnitude of the DC current is essentially determined by the voltage difference between the positive and negative poles and the equivalent impedance of the system. Therefore, adjusting the number of sub-modules in the arm is actually to dynamically regulate the potentials of the two poles of the DC voltage by changing the arm voltage, thereby changing the voltage difference between the two poles and finally realizing the regulation of the DC current.

[0056] In the scenario of suppressing current imbalance, when the positive pole current is greater than the negative pole current, it indicates that there is an excess of energy injection on the positive pole side. At this time, through mode switching, the negative pole converter station enters the current imbalance suppression mode: using the PI controller to calculate the command value for reducing the voltage of the negative pole arm, by reducing the number of sub-modules put into the negative pole arm, the potential of the negative pole side is reduced, the voltage difference between the positive and negative poles is increased, forcing the positive pole current to flow towards the negative pole until the current is balanced; conversely, if the negative pole current is in excess, the number of sub-modules put into the negative pole arm is increased, the potential of the negative pole side is raised, the voltage difference is reduced, and the current deviation is suppressed. During the whole process, the sub-module, as the smallest unit of voltage regulation, realizes the continuous control of the arm voltage through discrete quantity adjustment, and then converts the voltage signal into a precise regulation of the DC current.

[0057] In an optional implementation manner, the fixed DC voltage control strategy includes: taking the rated value U of the DC voltage dcNAs the DC reference voltage U deref , by adjusting the number of sub-modules put into operation in the bridge arm, the output voltage of the bridge arm is made to approach the DC reference voltage.

[0058] Furthermore, the constant DC voltage control strategy can be selected when the DC control loop is in a steady state, and switched to constant current control in the event of a DC fault. Constant current control means setting the current reference value to 0, comparing it with the actually detected fault current, and then performing an operation through a PI controller to output a control signal to adjust the converter, making the DC current approach 0, effectively limiting the fault current, avoiding damage to system equipment caused by excessive current, and achieving fault ride-through.

[0059] To verify the effectiveness of this solution, it is set that the new energy areas 3 and 4 corresponding to the negative pole respectively generate rated powers of 1250 MW, and the new energy areas 1 and 2 corresponding to the positive pole respectively generate rated powers of 990 MW. The conventional control strategy is adopted before 4.5 s, and the current imbalance suppression control is put into operation at 4.5 s. The simulation results are as Figure 4 shown.

[0060] It can be Figure 4 seen that before 4.5 s, under the conventional control, the DC currents of the positive and negative poles are not equal, and there is a maximum in-ground current of 0.629 kA. After switching to the current imbalance suppression strategy at 4.5 s, the DC currents of the positive and negative poles can be balanced, and the in-ground current is reduced to 0, thus preventing the generation of in-ground current, reducing the interference to surrounding communication equipment and the corrosion of underground metal pipelines, which proves the effectiveness of the control strategy.

[0061] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. The protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. Method for suppressing grounding current of DC transmission system with positive and negative electrode sites arranged separately, characterized in that, The method includes the following steps: Obtain the actual detected value of the positive-pole current at the positive-pole converter station and the actual detected value of the negative-pole current at the negative-pole converter station; If the actual detected value of the positive-pole current is greater than the actual detected value of the negative-pole current, the positive-pole converter station switches to Mode I and the negative-pole converter station switches to Mode II. If the actual detected value of the positive-pole current is less than the actual detected value of the negative-pole current, the positive-pole converter station switches to Mode II and the negative-pole converter station switches to Mode I. In Mode I, a constant DC voltage control strategy is adopted, and in Mode II, a current imbalance suppression control strategy is adopted; Based on the constant DC voltage control strategy or the current imbalance suppression control strategy, adjust the number of sub-modules put into operation in the corresponding bridge arm so that the output voltage of the bridge arm approaches the command value under the current imbalance suppression control strategy and the rated DC voltage under the constant DC voltage control strategy respectively.

2. The method for suppressing the grounding current of a DC transmission system with separated positive and negative electrode locations according to claim 1, wherein The current imbalance suppression control strategy specifically includes; The actual detected value of the positive electrode current and the actual detected value of the negative electrode current are respectively subjected to PI calculation to obtain a difference ΔU dcp and the difference ΔU dcn ; Based on the rated DC voltage and the difference ΔU dcp , the final positive DC voltage command value is obtained. Based on the rated DC voltage and the difference ΔU dcn , the final negative DC voltage command value is obtained.

3. The method for suppressing the grounding current of a DC power transmission system with separated positive and negative electrode locations according to claim 2, wherein The difference ΔU is calculated using the following formula dcp and the difference ΔU dcn : Among them, K P is the proportionality coefficient, K1 is the integral coefficient, I dcp is the actual detected value of the positive electrode current, I dcn is the actual detected value of the negative electrode current, and s is the complex frequency domain variable in the Laplace transform.

4. The method for suppressing grounding current of a DC power transmission system with separated positive and negative electrode locations according to claim 3, characterized in that, Calculate the final positive-pole DC voltage command value and the final negative-pole DC voltage command value through the following formula: where U dcN is the DC voltage rating.

5. The method for suppressing the grounding current of a DC transmission system with separated positive and negative electrode sites according to claim 4, wherein The current imbalance suppression control strategy further includes: Based on U dcp , adjusting the number of sub-modules put into operation in the corresponding arm to make the arm output voltage approach the U dcp .

6. The method for suppressing grounding current of a DC power transmission system with separated positive and negative electrode locations according to claim 5, characterized in that The current imbalance suppression control strategy further includes: based on U dcn , adjusting the number of sub-modules put into operation in the corresponding arm to make the arm output voltage approach the U dcn .

7. The method for suppressing the grounding current of a DC power transmission system with separated positive and negative electrode locations according to claim 6, characterized in that, The fixed DC voltage control strategy includes: using the rated value U of the DC voltage dcN as the DC reference voltage U deref , and by adjusting the number of sub-modules in the bridge arm that are put into operation, making the output voltage of the bridge arm approach the DC reference voltage.

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