A DC voltage adaptive control method for a three-terminal flexible DC distribution system

Abstract

This invention discloses an adaptive DC voltage control method for a three-terminal flexible DC distribution system, belonging to the field of power technology. The method includes the following steps: constructing a three-terminal flexible DC distribution system; analyzing the deviation value of the DC voltage in the three-terminal flexible DC distribution system; determining the magnitude of the DC voltage deviation value; and controlling the change and voltage value of the DC voltage in the three-terminal flexible DC distribution system. This invention superimposes an active power command correction controller onto the constant power control mode (PQ mode) of the flexible DC converter, enabling the flexible DC converter to dynamically adjust the output active power according to the instantaneous state of the DC voltage, thereby improving the DC voltage recovery speed.

Description

Technical Field

[0001] This invention relates to the field of power technology, and in particular to a method for adaptive DC voltage control of a three-terminal flexible DC distribution system. Background Technology

[0002] Flexible DC distribution systems incorporate flexible DC transmission technology and employ multi-terminal flexible DC converters, enabling routine interconnection of power grids across different regions, voltage levels, and phases. This overcomes the disadvantages of open-loop operation in traditional distribution systems. It significantly improves the power supply reliability and the accuracy and flexibility of power flow regulation, effectively guaranteeing electricity users' demands for high-quality, highly reliable power.

[0003] However, when a fault occurs on one side of the grid in a flexible DC distribution system, the sudden change in active power in that faulty grid disrupts the original active power balance of the multi-terminal system, causing the DC voltage of the flexible DC distribution system to rise or fall. This, in turn, affects the safe operation of the normal grid and the flexible DC converter. Therefore, it is urgent to study DC voltage control methods for flexible DC distribution systems to quickly restore the DC voltage of the system. Summary of the Invention

[0004] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description and other accompanying drawings.

[0005] The purpose of this invention is to overcome the above-mentioned shortcomings and provide a DC voltage adaptive control method for a three-terminal flexible DC power distribution system. This invention superimposes an active power command correction controller onto the constant power control mode (PQ mode) of the flexible DC converter, enabling the flexible DC converter to dynamically adjust the output active power according to the instantaneous state of the DC voltage, thereby improving the DC voltage recovery speed.

[0006] To achieve the above objectives, the technical solution of the present invention is: a DC voltage adaptive control method for a three-terminal flexible DC power distribution system, the method comprising the following steps: A three-terminal flexible DC distribution system is constructed, comprising distribution network 1, distribution network 2, distribution network 3, and flexible DC converters T1, T2, and T3; wherein, flexible DC converter T1 is located at V... dc Both flexible DC converters T2 and T3 operate in PQ mode; Analyze the deviation of the DC voltage in the three-terminal flexible DC distribution system and set the rated value of the DC voltage for the three-terminal flexible DC distribution system. And detect the DC voltage feedback value of the three-terminal flexible DC distribution system. In order to analyze the deviation of DC voltage in a three-terminal flexible DC distribution system; Determine the magnitude of the DC voltage deviation in the three-terminal flexible DC distribution system, and determine whether the absolute value of the DC voltage deviation in the three-terminal flexible DC distribution system is greater than the DC voltage deviation threshold of the three-terminal flexible DC distribution system; The change in DC voltage of the three-terminal flexible DC distribution system is controlled. If the absolute value of the deviation of the DC voltage of the three-terminal flexible DC distribution system is greater than the deviation threshold of the DC voltage of the three-terminal flexible DC distribution system, the active power command correction controller is activated to control the change in DC voltage of the three-terminal flexible DC distribution system and restore it to the normal value.

[0007] This invention superimposes an active power command correction controller onto the constant power control mode (PQ mode) of a flexible DC converter, enabling the converter to dynamically adjust its output active power according to the instantaneous state of the DC voltage, thus improving the DC voltage recovery speed. Furthermore, this method effectively reduces the risk of system instability caused by voltage fluctuations, further enhancing the operational reliability of the three-terminal flexible DC distribution system. Through real-time monitoring and dynamic adjustment, not only can precise control of the DC voltage be achieved, but the system's high efficiency performance can also be maintained under various operating conditions. This method is particularly suitable for complex and variable power environments, providing important technical support for the construction and optimization of future smart grids. Simultaneously, its adaptive characteristics give the system a stronger ability to cope with emergencies, thereby significantly improving the safety and stability of the overall distribution network.

[0008] In some implementations, the active power command correction controller includes a DC voltage comparison unit, a proportional-integral control unit, and a correction coefficient unit. The DC voltage comparison unit is used to determine the DC voltage feedback value. Whether the voltage deviates from the threshold, the proportional-integral (PI) control unit is used to eliminate errors. The correction coefficient unit multiplies the output of the PI control unit by a correction coefficient to further adjust the output of the active power command correction controller. If faster DC voltage regulation is needed, the coefficient of the correction coefficient unit can be increased, and vice versa. The DC voltage comparison unit can capture subtle changes in DC voltage deviation in real time and transmit the comparison result to the PI control unit. Based on the received signal and preset control parameters, the PI control unit calculates an appropriate adjustment amount to ensure that the system remains stable during dynamic changes. The correction coefficient unit further corrects the result calculated by the PI control unit, ultimately generating the active power correction amount. Through fine adjustment of the output command, precise control of DC voltage is achieved. This process not only improves the system's response speed but also effectively reduces the risk of voltage fluctuations caused by external interference, thereby further optimizing the overall performance of the three-terminal flexible DC distribution system.

[0009] In some implementations, V dc The Q mode includes an outer loop for DC voltage and an inner loop for current. In the DC voltage outer loop, and These are the rated DC voltage and the DC voltage feedback value, respectively. PI stands for Proportional-Integral Controller. and Q These are the reactive power reference value and the reactive power feedback value, respectively. PI stands for proportional-integral controller. In the inner current loop, and i d These are the d-axis current inner loop reference value and current feedback value, respectively. PI stands for proportional-integral controller. ωL The coefficient representing the d-axis current decoupling. and i q These are the q-axis current inner loop reference value and the current feedback value, respectively. PI stands for Proportional-Integral Controller. ωL The coefficient representing q-axis current decoupling, e d and e q These represent the grid voltage disturbances on the d-axis and q-axis, respectively. dq / abc indicates converting the dq-axis output to an abc three-phase output. s a , s b and s c .

[0010] The VdcQ mode design effectively improves system stability and response speed. The DC voltage outer loop monitors the deviation between the voltage feedback value and the rated value in real time and uses a proportional-integral controller for rapid adjustment, ensuring the DC voltage remains within the set range. Simultaneously, the dynamic balance between the reactive power reference and feedback values ​​relies on precise calculations by the proportional-integral controller to achieve optimized reactive power control. In the current inner loop, the decoupling design of the d-axis and q-axis currents is crucial. By introducing a decoupling coefficient, the mutual influence between the d-axis and q-axis currents can be effectively eliminated, thereby improving control accuracy. Furthermore, the introduction of grid voltage disturbances enhances the system's adaptability to grid fluctuations, making the output more stable and reliable.

[0011] In some implementations, the PQ mode includes a PQ outer loop and a current inner loop; In the outer ring of PQ, and P These are the active power reference value and the active power feedback value, respectively. PI stands for Proportional-Integral Controller. and Q These are the reactive power reference value and the reactive power feedback value, respectively. PI stands for proportional-integral controller. In the inner current loop, and i d These are the d-axis current inner loop reference value and current feedback value, respectively. PI stands for proportional-integral controller. ωL The coefficient representing the d-axis current decoupling. and i q These are the q-axis current inner loop reference value and the current feedback value, respectively. PI stands for Proportional-Integral Controller. ωL The coefficient representing q-axis current decoupling, e d and e q These represent the grid voltage disturbances on the d-axis and q-axis, respectively. dq / abc indicates converting the dq-axis output to an abc three-phase output. s a , s b and s c .

[0012] In practical applications, this control method effectively improves system stability and response speed. By precisely adjusting the parameters of the proportional-integral controller, accurate control of active and reactive power can be achieved, thus meeting the needs under different operating conditions. Furthermore, the design of the inner current loop fully considers the decoupling relationship between the d-axis and q-axis, giving the system strong anti-interference capabilities when facing grid voltage disturbances. The introduction of the conversion stage further enhances the accuracy and reliability of the output signal, providing a solid guarantee for the efficient operation of the entire power distribution system.

[0013] In some implementations, in a three-terminal flexible DC distribution system, flexible DC converter T1 absorbs active power into distribution network 1, while flexible DC converters T2 and T3 transmit active power into distribution network 2 and distribution network 3, respectively.

[0014] In some implementations, the change in DC voltage of the three-terminal flexible DC distribution system is controlled. If the absolute value of the DC voltage deviation exceeds a threshold value, the active power command correction controller is activated to control the change in DC voltage and restore it to its normal value. Specifically: In a three-terminal flexible DC distribution system, a fault occurs in distribution network 1, and the active power absorbed by the flexible DC converter T1 into distribution network 1... P T1 The active power of flexible DC converters T2 and T3 is reduced. P T2 and P T3 Unchanged, that is, P T1 + P T2 + P 3 < 0, DC bus voltage drops. When Below When the active power command correction controller is activated, its output positive adjustment is superimposed onto the active power reference values ​​of flexible DC converters T2 and T3 to reduce the active power output of flexible DC converters T2 and T3, and the DC voltage. Gradually rise, until equal .

[0015] In some implementations, the change in DC voltage of the three-terminal flexible DC distribution system is controlled. If the absolute value of the DC voltage deviation exceeds a threshold value, the active power command correction controller is activated to control the change in DC voltage and restore it to its normal value. Specifically: After the fault in distribution network 1 of the three-terminal flexible DC distribution system is cleared, the active power absorbed by the flexible DC converter T1 into distribution network 1... P T1 The active power of flexible DC converters T2 and T3 increases. P T2 and P T3 When the output remains unchanged, that is, P T1 + P T2 + P When 3>0, the DC bus voltage increases. Higher than When the active power command correction controller is activated, its output negative adjustment is superimposed onto the active power reference values ​​of flexible DC converters T2 and T3 to increase the active power output of flexible DC converters T2 and T3, and the DC voltage. Gradually decrease until equal .

[0016] In some implementations, the rated DC voltage of the three-terminal flexible DC distribution system It is 20 kV.

[0017] In some implementations, the DC voltage deviation threshold of the three-terminal flexible DC distribution system is the rated DC voltage value of the three-terminal flexible DC distribution system. 1%.

[0018] In some implementations, distribution network 1 includes line 11, line 12, line 13 and load 1; distribution network 2 includes line 21, line 22, line 23 and load 2 and load 3; distribution network 3 includes line 31, line 32, line 33 and load 4.

[0019] In summary, the beneficial effects of this invention are: This invention adds an active power command correction controller to the constant power control mode (PQ mode) of the flexible DC converter, enabling the flexible DC converter to dynamically adjust the output active power according to the instantaneous state of the DC voltage, thereby improving the DC voltage recovery speed.

[0020] Furthermore, this method can effectively address different types of power system faults. By dynamically adjusting the output power of the flexible DC converter, it can quickly restore the DC voltage to its rated value, thereby reducing the impact of faults on system operation. In practice, this method not only improves the system's anti-interference capability but also significantly reduces the risk of equipment damage caused by voltage fluctuations. Through intelligent management of power flow between distribution networks, the overall performance of the three-terminal flexible DC distribution system is further optimized. Simultaneously, this method possesses strong scalability, allowing for flexible adjustment of control parameters according to actual needs, making it suitable for larger-scale power networks. This characteristic provides a solid technical foundation for the future development of smart grids, enabling the system to maintain high efficiency, stability, and reliability in complex and ever-changing operating environments.

[0021] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure.

[0022] Undoubtedly, such and other objects of the present invention will become more apparent after the following detailed description of the preferred embodiments, which are illustrated in various accompanying drawings and figures.

[0023] To make the above and other objects, features and advantages of the present invention more apparent and understandable, one or more preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0024] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0025] In the accompanying drawings, the same parts use the same reference numerals, and the drawings are schematic and not necessarily drawn to actual scale.

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only one or more embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on such drawings without creative effort.

[0027] Figure 1 This is a topology diagram of the three-terminal flexible DC power distribution system of the present invention; Figure 2 For traditional V dc Q-mode structure diagram; Figure 3 This is a structural diagram of the traditional PQ model; Figure 4 This is a schematic diagram of a three-phase short-circuit fault occurring in distribution network 1 according to the present invention; Figure 5 This is a structural diagram of the improved PQ mode of the present invention; Figure 6 This is a schematic diagram of the power flow direction of the three-terminal flexible DC power distribution system of the present invention; Figure 7 The instantaneous AC voltage value when a three-phase short-circuit fault occurs in distribution network 1 (comparison test 1); Figure 8 The output current of the flexible DC converter T1 (comparison test 1); Figure 9 The active power output of flexible DC converters T1, T2 and T3 is compared (comparative test 1). Figure 10 This is the instantaneous value of DC voltage (comparison test 1); Figure 11 The output current of the flexible DC converter T1 (comparison test 2); Figure 12 The active power output of flexible DC converters T1, T2 and T3 is compared (2). Figure 13 This is the instantaneous value of DC voltage (comparison test 2). Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0029] Furthermore, in the description of this invention, it should be understood that the terms "center," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0030] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral unit; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. However, specifying a direct connection indicates that the two main bodies are not connected through a transitional structure, but rather formed as a whole through a connecting structure. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0031] In this invention, unless otherwise expressly specified and limited, the first feature "on" or "below" the second feature may be in direct contact with the first and second features, or indirect contact through an intermediate medium. In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0032] This invention provides a method for adaptive DC voltage control in a three-terminal flexible DC power distribution system. The method includes the following steps: A three-terminal flexible DC distribution system is constructed, comprising distribution network 1, distribution network 2, distribution network 3, and flexible DC converters T1, T2, and T3; wherein, flexible DC converter T1 is located at V... dc Both flexible DC converters T2 and T3 operate in PQ mode; Analyze the deviation of the DC voltage in the three-terminal flexible DC distribution system and set the rated value of the DC voltage for the three-terminal flexible DC distribution system. And detect the DC voltage feedback value of the three-terminal flexible DC distribution system. In order to analyze the deviation of DC voltage in a three-terminal flexible DC distribution system; Determine the magnitude of the DC voltage deviation in the three-terminal flexible DC distribution system, and determine whether the absolute value of the DC voltage deviation in the three-terminal flexible DC distribution system is greater than the DC voltage deviation threshold of the three-terminal flexible DC distribution system; The system controls the change in DC voltage of the three-terminal flexible DC distribution system. If the absolute value of the DC voltage deviation exceeds the DC voltage deviation threshold, the active power command correction controller is activated to control the change in DC voltage and restore it to its normal value.

[0033] like Figure 1 As shown, the three-terminal flexible DC distribution system consists of three AC power grids (distribution network 1, distribution network 2, and distribution network 3) interconnected via three-terminal flexible DC converters (T1, T2, and T3). The control strategy for the three-terminal flexible DC converters is provided by the converter station, such as... Figure 2 As shown, the flexible DC converter T1 operates in VdcQ mode to maintain a constant DC bus voltage; as Figure 3 As shown, flexible DC converters T2 and T3 operate in PQ mode, providing constant power. The grid connection points for flexible DC converters T1, T2, and T3 are PCC1, PCC2, and PCC3, respectively. In the three-terminal flexible DC distribution system, flexible DC converter T1 absorbs active power from distribution network 1, while flexible DC converters T2 and T3 supply active power to distribution networks 2 and 3, respectively.

[0034] Distribution network 1 includes line 11, line 12, line 13, and load 1. The line impedances of distribution network 1 are as follows: Z line11 , Z line12 , Z line13 Distribution network 2 includes lines 21, 22, and 23, and loads 2 and 3. The line impedances of distribution network 2 are respectively... Z line21 , Z line22 , Z line23 Distribution network 3 includes line 31, line 32, line 33, and load 4. The line impedances of distribution network 3 are as follows: Z line31 , Z line32 , Z line33 The rated line voltage of distribution network 1 and distribution network 2 is 10 kV, the rated line voltage of distribution network 3 is 20 kV, and the rated DC voltage of the three-terminal flexible DC distribution system is 20 kV.

[0035] exist Figure 2 In the middle, V dc The Q mode includes an outer loop for DC voltage and an inner loop for current. In the DC voltage outer loop, and These are the rated DC voltage and the DC voltage feedback value, respectively. PI stands for Proportional-Integral Controller. and Q These are the reactive power reference value and the reactive power feedback value, respectively. PI stands for proportional-integral controller. In the inner current loop, and i d These are the d-axis current inner loop reference value and current feedback value, respectively. PI stands for proportional-integral controller. ωL The coefficient representing the d-axis current decoupling. and i q These are the q-axis current inner loop reference value and the current feedback value, respectively. PI stands for Proportional-Integral Controller. ωL The coefficient representing q-axis current decoupling, e d and e q These represent the grid voltage disturbances on the d-axis and q-axis, respectively. dq / abc indicates converting the dq-axis output to an abc three-phase output. s a , s b and s c .

[0036] exist Figure 3 In the PQ mode, there are two loops: the PQ outer loop and the current inner loop. In the outer ring of PQ, and P These are the active power reference value and the active power feedback value, respectively. PI stands for Proportional-Integral Controller. and Q These are the reactive power reference value and the reactive power feedback value, respectively. PI stands for proportional-integral controller. In the inner current loop, and i d These are the d-axis current inner loop reference value and current feedback value, respectively. PI stands for proportional-integral controller. ωL The coefficient representing the d-axis current decoupling. and i q These are the q-axis current inner loop reference value and the current feedback value, respectively. PI stands for Proportional-Integral Controller. ωL The coefficient representing q-axis current decoupling, e d and e q These represent the grid voltage disturbances on the d-axis and q-axis, respectively. dq / abc indicates converting the dq-axis output to an abc three-phase output. s a ,s b and s c .

[0037] In this invention, when a three-phase short-circuit fault occurs in distribution network 1, such as Figure 4 As shown. Assume that flexible DC converter T1 is currently absorbing active power from the grid, while flexible DC converters T2 and T3 are outputting active power to the grid. Due to a voltage drop in distribution network 1, its active power decreases sharply. The active power absorbed by flexible DC converter T1 decreases, while if flexible DC converters T2 and T3 continue to output at their original power, the three-terminal flexible DC distribution system will experience an active power deficit, causing a DC voltage drop in the system. At this time, Figure 2 The PI output of the outer loop of the DC voltage converter will saturate, causing the outer loop of the DC voltage converter to shut down. At this time, the DC voltage of the three-terminal flexible DC distribution system will become uncontrolled.

[0038] Since the DC voltage drop in a flexible DC distribution system is caused by the imbalance of active power in the system, this invention proposes an improved PQ mode structure to solve the above problem.

[0039] In this invention, such as Figure 5 As shown, the active power command correction controller includes a DC voltage comparison unit, a proportional-integral control unit, and a correction coefficient unit. The DC voltage comparison unit is used to determine the DC voltage feedback value. Whether it deviates from the threshold, the proportional-integral control unit is used to eliminate the error, and the correction coefficient unit multiplies the output of the proportional-integral control unit by the correction coefficient to further adjust the active power command correction controller output. If faster DC voltage regulation is required, the coefficient of the correction coefficient unit can be increased, and vice versa.

[0040] Specifically, in the active power command correction controller, and These are the rated DC voltage and the DC voltage feedback value, respectively. PI stands for Proportional-Integral Controller. k It is the active power correction factor, whose function is to adjust the output of the active power command correction controller, thereby affecting the recovery speed of the DC voltage. k >0. k The larger the value, the larger the output of the active power command correction controller, and the faster the DC voltage recovery, but the greater the DC voltage overshoot. Conversely, kThe smaller the value, the smaller the output of the active power command correction controller, the slower the DC voltage recovery speed, but the smaller the DC voltage overshoot. S1 is a status switch; when the DC voltage drops beyond a certain range, S1 automatically closes to adjust the active power output of the flexible DC converter. In this invention, S1 automatically closes when the DC voltage deviates from the rated value by ±1%. In this invention, the change in DC voltage of the three-terminal flexible DC distribution system is controlled. If the absolute value of the DC voltage deviation of the three-terminal flexible DC distribution system is greater than the DC voltage deviation threshold, the active power command correction controller is activated to control the change in DC voltage of the three-terminal flexible DC distribution system and restore it to the normal value; specifically: In a three-terminal flexible DC distribution system, a fault occurs in distribution network 1, and the active power absorbed by the flexible DC converter T1 into distribution network 1... P T1 The active power of flexible DC converters T2 and T3 is reduced. P T2 and P T3 Unchanged, that is, P T1 + P T2 + P 3 < 0, DC bus voltage drops. When Below When the active power command correction controller is activated, its output positive adjustment is superimposed onto the active power reference values ​​of flexible DC converters T2 and T3 to reduce the active power output of flexible DC converters T2 and T3, and the DC voltage. Gradually rise, until equal .

[0041] After the fault in distribution network 1 of the three-terminal flexible DC distribution system is cleared, the active power absorbed by the flexible DC converter T1 into distribution network 1... P T1 The active power of flexible DC converters T2 and T3 increases. P T2 and P T3 When the output remains unchanged, that is, P T1 + P T2 + P When 3>0, the DC bus voltage increases. Higher than When the active power command correction controller is activated, its output negative adjustment is superimposed onto the active power reference values ​​of flexible DC converters T2 and T3 to increase the active power output of flexible DC converters T2 and T3, and the DC voltage. Gradually decrease until equal .

[0042] To more clearly illustrate the power regulation process of the active power command correction controller, a power distribution diagram of a three-terminal flexible DC distribution system is provided, such as... Figure 6 As shown. In Figure 6 middle, P 配电网1 This represents the active power flowing into distribution network 1. P 配电网2 This represents the active power flowing into distribution network 2. P 配电网3 This represents the active power flowing into distribution network 3. u dc This represents the instantaneous value of DC voltage. P T1 , P T2 ,and P T3 These represent the active power output of flexible DC converters T1, T2, and T3, respectively (with output active power defined as positive and input active power as negative). P T1 <0 indicates that the flexible DC converter T1 absorbs active power. P T2 >0, P T3 >0 indicates that flexible DC converters T2 and T3 output active power. Figure 6 In the middle, the flexible DC converter T1 absorbs active power from the grid ( P T1 <0), both flexible DC converters T2 and T3 output active power to the grid ( P T2 >0, P T3 >0).

[0043] When distribution network 1 is fault-free, the active power of the three-terminal flexible DC distribution system is balanced, that is... P T1 + P T2 + P T3 =0, the system DC voltage remains unchanged.

[0044] When a fault occurs in distribution network 1, the active power absorbed by the flexible DC converter T1 into the grid... P T1 The active power of flexible DC converters T2 and T3 is reduced. P T2 and P T3 If unchanged, then P T1 + P T2 + P 3 < 0, causing the DC bus voltage to drop, i.e. v dc Below At this time, the positive adjustment value output by the active power command correction controller is superimposed on the active power reference value of flexible DC converters T2 and T3, thereby reducing the active power output of flexible DC converters T2 and T3 and achieving the purpose of raising the DC bus voltage.

[0045] After the fault in distribution network 1 is cleared, the active power absorbed by the flexible DC converter T1 into the grid... P T1 The active power of flexible DC converters T2 and T3 increases. P T2 and P T3 If the output remains unchanged, then P T1 + P T2 + P 3>0, the DC bus voltage increases, that is v dc Higher than At this time, the negative adjustment amount output by the active power command correction controller is superimposed on the active power reference value of the flexible DC converter T2 and the flexible DC converter T3, thereby increasing the active power output of the flexible DC converter T2 and the flexible DC converter T3, and achieving the purpose of reducing the DC bus voltage.

[0046] To verify the effectiveness of the proposed DC voltage adaptive control method, two sets of simulation comparison tests were conducted in Matlab / Simulink software. In the simulation tests, flexible DC converter T1 was set to absorb active power, while flexible DC converters T2 and T3 were set to output active power. The simulation results are as follows: Figures 7 to 13 As shown.

[0047] exist Figures 7 to 13 In the diagram, the vertical axis is defined as follows: u represents the instantaneous value of AC voltage in kV, i represents the instantaneous value of AC current in kA, and P represents the output active power of the flexible DC converter in MW. dcThe horizontal axis represents the instantaneous value of DC voltage, in kV; the horizontal axis t represents time, in seconds.

[0048] Comparative Test 1 (without using the method proposed in this invention): like Figure 7 As shown, a three-phase short-circuit fault occurs in distribution network 1 at 0.6 s, and the fault is cleared at 0.9 s. The fault point is as follows. Figure 4 As shown.

[0049] During the period from 0.6s to 0.9s, from Figure 7 It can be seen that the AC voltage drops instantaneously. When the flexible DC converters T2 and T3 do not employ the DC voltage adaptive control method of this invention, such as Figure 8 As shown, the outer loop of the DC voltage of the flexible DC converter T1 is saturated and it stops operating. At this time, the flexible DC converter T1 is in short-circuit current-limiting mode and outputs a constant current. Figure 9 As shown, the active power absorbed by the flexible DC converter T1 decreases, while the active power output by the flexible DC converters T2 and T3 remains unchanged. This results in a continuous drop in DC voltage during the period from 0.6s to 0.9s. Figure 10 As shown.

[0050] After the fault in distribution network 1 was cleared in 0.9s, the power absorbed by the flexible DC converter T1 increased and the DC voltage slowly recovered. However, the DC voltage still did not recover to the rated value (20 kV) until 1.3s.

[0051] Comparative Test 2 (using the method proposed in this invention): When both flexible DC converters T2 and T3 employ the DC voltage adaptive control method proposed in this invention, their control diagrams are as follows: Figure 5 As shown.

[0052] like Figures 11 to 13 As shown, at the instant the fault occurs (0.6s), flexible DC converter T1 enters current-limiting mode, and its absorbed active power begins to decrease. Meanwhile, the active power of flexible DC converters T2 and T3 remains unchanged, causing the DC voltage to drop. Subsequently, when flexible DC converters T2 and T3 detect that the DC voltage is below 1% of its rated value, they initiate DC voltage adaptive control, reducing their respective output active power. The rate of DC voltage drop is significantly slower than... Figure 10 Around 0.7 seconds, the DC voltage began to recover, and by 0.9 seconds when the fault was cleared, it had recovered to 20 kV. After the fault was cleared, the flexible DC converters T2 and T3 adaptively adjusted their output active power. After a brief fluctuation, the DC voltage stabilized at the rated value of 20 kV.

[0053] The results of comparative tests 1 and 2 show that the system's performance during fault periods is significantly improved after adopting the DC voltage adaptive control method proposed in this invention. Specifically, when a three-phase short-circuit fault occurs, in the system without this method, the flexible DC converter T1 exits normal operation due to DC voltage outer loop saturation, resulting in a continuous drop in DC voltage and a long recovery time, failing to quickly return to its rated value after the fault is cleared. In contrast, in the system using the method of this invention, flexible DC converters T2 and T3 can quickly respond to changes in DC voltage and effectively slow down the voltage drop rate by adaptively adjusting the output power, while rapidly recovering to the rated value after the fault is cleared. This demonstrates that the method of this invention not only maintains system stability during fault periods but also significantly shortens the recovery time after a fault, thereby improving the reliability and dynamic performance of the entire three-terminal flexible DC distribution system.

[0054] It should be noted that many specific details have been set forth in the above description in order to provide a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

Claims

1. A method for adaptive DC voltage control in a three-terminal flexible DC distribution system, characterized in that, The method includes the following steps: A three-terminal flexible DC distribution system is constructed, comprising distribution network 1, distribution network 2, distribution network 3, and flexible DC converters T1, T2, and T3; wherein, flexible DC converter T1 is located at V... dc Both flexible DC converters T2 and T3 operate in PQ mode; Analyze the deviation of the DC voltage in the three-terminal flexible DC distribution system and set the rated value of the DC voltage for the three-terminal flexible DC distribution system. And detect the DC voltage feedback value of the three-terminal flexible DC distribution system. In order to analyze the deviation of DC voltage in a three-terminal flexible DC distribution system; Determine the magnitude of the DC voltage deviation in the three-terminal flexible DC distribution system, and determine whether the absolute value of the DC voltage deviation in the three-terminal flexible DC distribution system is greater than the DC voltage deviation threshold of the three-terminal flexible DC distribution system; The change in DC voltage of the three-terminal flexible DC distribution system is controlled. If the absolute value of the deviation of the DC voltage of the three-terminal flexible DC distribution system is greater than the deviation threshold of the DC voltage of the three-terminal flexible DC distribution system, the active power command correction controller is activated to control the change in DC voltage of the three-terminal flexible DC distribution system and restore it to the normal value.

2. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, The active power command correction controller includes a DC voltage comparison unit, a proportional-integral control unit, and a correction coefficient unit. The DC voltage comparison unit is used to determine the DC voltage feedback value. Whether it deviates from the threshold, the proportional-integral control unit is used to eliminate the error, and the correction coefficient unit multiplies the output of the proportional-integral control unit by the correction coefficient to further adjust the active power command correction controller output. If faster DC voltage regulation is required, the coefficient of the correction coefficient unit can be increased, and vice versa.

3. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, V dc The Q mode includes an outer loop for DC voltage and an inner loop for current. In the DC voltage outer loop, and These are the rated DC voltage and the DC voltage feedback value, respectively. PI stands for Proportional-Integral Controller. and Q These are the reactive power reference value and the reactive power feedback value, respectively. PI stands for proportional-integral controller. In the inner current loop, and i d These are the d-axis current inner loop reference value and current feedback value, respectively. PI stands for proportional-integral controller. ωL The coefficient representing the d-axis current decoupling. and i q These are the q-axis current inner loop reference value and the current feedback value, respectively. PI stands for Proportional-Integral Controller. ωL The coefficient representing q-axis current decoupling, e d and e q These represent the grid voltage disturbances on the d-axis and q-axis, respectively. dq / abc indicates converting the dq-axis output to an abc three-phase output. s a , s b and s c .

4. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, PQ mode includes a PQ outer loop and a current inner loop; In the outer ring of PQ, and P These are the active power reference value and the active power feedback value, respectively. PI stands for Proportional-Integral Controller. and Q These are the reactive power reference value and the reactive power feedback value, respectively. PI stands for proportional-integral controller. In the inner current loop, and i d These are the d-axis current inner loop reference value and current feedback value, respectively. PI stands for proportional-integral controller. ωL The coefficient representing the d-axis current decoupling. and i q These are the q-axis current inner loop reference value and the current feedback value, respectively. PI stands for Proportional-Integral Controller. ωL The coefficient representing q-axis current decoupling, e d and e q These represent the grid voltage disturbances on the d-axis and q-axis, respectively. dq / abc indicates converting the dq-axis output to an abc three-phase output. s a , s b and s c .

5. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, In a three-terminal flexible DC distribution system, flexible DC converter T1 absorbs active power into distribution network 1, while flexible DC converters T2 and T3 transmit active power into distribution network 2 and distribution network 3, respectively.

6. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, To control the change in DC voltage of the three-terminal flexible DC distribution system, if the absolute value of the DC voltage deviation exceeds the DC voltage deviation threshold, the active power command correction controller is activated to control the change in DC voltage and restore it to the normal value; specifically: In a three-terminal flexible DC distribution system, a fault occurs in distribution network 1, and the active power absorbed by the flexible DC converter T1 into distribution network 1... P T1 The active power of flexible DC converters T2 and T3 is reduced. P T2 and P T3 Unchanged, that is, P T1 + P T2 + P 3 < 0, DC bus voltage drops. When Below When the active power command correction controller is activated, its output positive adjustment is superimposed onto the active power reference values ​​of flexible DC converters T2 and T3 to reduce the active power output of flexible DC converters T2 and T3, and the DC voltage. Gradually rise, until equal .

7. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 6, characterized in that, The system controls the change in DC voltage of the three-terminal flexible DC distribution system and restores it to its normal value. If the absolute value of the DC voltage deviation of the three-terminal flexible DC distribution system exceeds the DC voltage deviation threshold, the active power command correction controller is activated to control the DC voltage of the three-terminal flexible DC distribution system; specifically: After the fault in distribution network 1 of the three-terminal flexible DC distribution system is cleared, the active power absorbed by the flexible DC converter T1 into distribution network 1... P T1 The active power of flexible DC converters T2 and T3 increases. P T2 and P T3 When the output remains unchanged, that is, P T1 + P T2 + P When 3>0, the DC bus voltage increases. Higher than When the active power command correction controller is activated, the negative adjustment output of the active power command correction controller is superimposed onto the active power reference values ​​of flexible DC converters T2 and T3 to increase the active power output of flexible DC converters T2 and T3, and the DC voltage. Gradually decrease until equal .

8. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, DC voltage rating of three-terminal flexible DC distribution system It is 20 kV.

9. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, The DC voltage deviation threshold of the three-terminal flexible DC distribution system is the rated value of the DC voltage of the three-terminal flexible DC distribution system. 1%.

10. The DC voltage adaptive control method for a three-terminal flexible DC distribution system according to claim 1, characterized in that, Distribution network 1 includes line 11, line 12, line 13 and load 1; distribution network 2 includes line 21, line 22, line 23 and load 2 and load 3; distribution network 3 includes line 31, line 32, line 33 and load 4.

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