Regulation and control method for multi-terminal flexible direct-current power transmission system

Abstract

The invention discloses a multi-terminal flexible DC power transmission system regulation and control method, and mainly solves the technical problem that the DC voltage deviation is large in the regulation and control process of an existing multi-terminal flexible DC power transmission system. According to the method, after unbalanced power of the disturbed multi-terminal flexible direct current power transmission system is acquired, the unbalanced power is distributed according to the power margin of each converter station, and the larger the power margin is, the larger the distributed unbalanced power is. And then unbalance power is added to a traditional droop controller, droop control curve translation is realized, and the direct-current voltage response characteristic is improved. On the basis, a direct-current voltage controller is additionally arranged on droop control, the power reference value increment is optimized, and no-difference adjustment of the direct-current voltage is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of DC power transmission, and particularly to a control method for a multi-terminal flexible DC power transmission system. Background Art

[0002] With a large number of new energy sources being connected to the existing power grid, the traditional AC power transmission technology has been difficult to adapt to the development of modern power industry. The multi-terminal flexible DC power transmission system (VSC-MTDC) based on voltage source converters (VSC) has gradually become an indispensable part of long-distance and large-capacity power transmission due to its advantages such as independent control of active power and reactive power, ability to supply power to weak AC systems or passive networks, no commutation failure problem, and easy construction of multi-terminal power transmission networks. Compared with the traditional two-terminal flexible DC power transmission system, the VSC-MTDC system expands the structure, allows more converter stations to be connected, and power can be coordinated and supplemented among converter stations, with stronger flexibility and reliability in new energy grid connection.

[0003] Compared with the traditional AC system that needs to consider three factors of frequency, voltage and power angle simultaneously in terms of stability, the multi-terminal flexible DC power transmission system (VSC-MTDC) only needs to consider the stability of DC voltage in terms of stability. Therefore, maintaining the stability of DC voltage becomes the core of the control of the entire VSC-MTDC system.

[0004] The known inter-station control strategies of the existing VSC-MTDC system can be roughly divided into three types: master-slave control, margin control, and DC voltage droop control. Among them, the droop control strategy is a multi-point control. The controller distributes active power by measuring the DC voltage, can absorb unbalanced power and stabilize the DC voltage according to the droop characteristics, and thus does not rely on communication between converter stations and has high operation reliability. For example, the patent document with the publication number CN112653176B and the patent name "A variable operating point droop control method for a multi-terminal flexible DC system" discloses determining the adjustment coefficient of the variable operating point droop control by using the local measurement information of the converter station and the preset target parameters, including the actually measured DC voltage, the actually measured exchanged active power, the DC voltage command value, and the initial active power command value, calculating the target operating point of the active power of the converter station in real time by using the calculation expression of the target operating point of the active power of the converter station, and continuously adjusting the active power command value of the converter station controller. Although this technical solution can achieve stable control of the DC voltage in the multi-terminal flexible DC system and maintain it within the specified limit range, however, the inventors of the present invention found that during the power regulation process of the above technology, there is a problem of relatively large DC voltage deviation, which is not conducive to the stable operation of the DC system.

[0005] The information disclosed in this background section is only for enhancing the understanding of the background of the present disclosure and should not be regarded as an admission or any form of implication that this information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0006] In view of at least one of the above technical problems, the present disclosure provides a control method for a multi-terminal flexible DC power transmission system, mainly solving the technical problem of large DC voltage deviation in the control process of the existing multi-terminal flexible DC power transmission system.

[0007] According to one aspect of the present disclosure, a control method for a multi-terminal flexible DC power transmission system is provided, which includes the following steps: (1) Establish a droop control that adjusts the output power of the converter station according to the change of the DC voltage and includes a first PI controller, obtain the unbalanced power generated by the multi-terminal flexible DC power transmission system under disturbance, and use the unbalanced power corresponding to the converter station as the feed-forward compensation amount of the droop control of the converter station for feed-forward compensation; (2) Adjust the increment of the active power reference value according to the DC voltage controller at the feed-forward compensation path until the DC voltage is restored to the reference voltage.

[0008] In some embodiments of the present disclosure, in the step (1), the unbalanced power is correspondingly feed-forward compensated to the active power reference value of the droop control.

[0009] In some embodiments of the present disclosure, in the step (1), the feed-forward compensation of the unbalanced power corresponding to each converter station is performed under the rated capacity of the converter station: ; Wherein, P is ’ is the actual value of the active power transmission of the i th converter station after feed-forward compensation; P isref is the reference value of the active power transmission of the i th converter station; ΔP i is the unbalanced power obtained by the i th converter station through the feed-forward compensation link; P ismax is the maximum rated capacity of the i th converter station; P is is the actual value of the active power transmission of the i th converter station.

[0010] In some embodiments of the present disclosure, in the step (2), the DC voltage controller includes a second PI controller.

[0011] In some embodiments of the present disclosure, the DC voltage controller further includes a voltage dead zone link.

[0012] In some embodiments of the present disclosure, the voltage dead zone range of the voltage dead zone link is ±2%.

[0013] One or more technical solutions provided in the embodiments of the present application have at least any of the following technical effects or advantages: Based on the traditional droop control, the droop curve is shifted by feedforward compensation for unbalanced power, which can effectively reduce the DC voltage deviation in the dynamic process of the system, realize the active power balance of the VSC-MTDC system in the dynamic process, and achieve the purpose of improving the DC voltage response characteristics; further, by adding a DC voltage controller to the droop control, the power increment calculated by the PI controller of the DC voltage deviation is superimposed on the droop control of the converter station, the power reference value increment is optimized, and by reasonably setting the response time of the two PI controllers, the two control objectives can be taken into account, and by changing the active power intercept of the droop curve, the DC voltage can be adjusted without difference. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 1 is a structural schematic diagram of a typical ring-shaped five-terminal VSC-MTDC system in an embodiment of the present application.

[0015] Figure 2 Schematic diagram of the control principle of a traditional droop controller in one embodiment of the present application.

[0016] Figure 3 1 is a control curve diagram of a traditional droop controller in one embodiment of the present application.

[0017] Figure 4 This is a control curve diagram of a droop controller under power balance cooperative control in one embodiment of the present application.

[0018] Figure 5 This is a control principle diagram of a droop controller under power balance cooperative control in one embodiment of the present application.

[0019] Figure 6 The simulation results of a five-terminal VSC-MTDC system under two control methods in one embodiment of the present application; wherein, (a) is the simulation result of the active power of the converter station VSC1, (b) is the simulation result of the active power of the converter station VSC2, (c) is the simulation result of the active power of the converter stations VSC3-VSC5, and (d) is the simulation result of the DC voltage of the system. DETAILED DESCRIPTION

[0020] The programs involved or relied on in the following embodiments are all conventional or simple programs in this technical field, and those skilled in the art can make conventional selections or adaptive adjustments according to specific application scenarios.

[0021] To better understand the technical solution of the present invention, the above technical solution will be described in detail below in conjunction with the accompanying drawings of the specification and specific implementation manners.

[0022] To solve the technical problem of large DC voltage deviation existing in the regulation process of the existing multi-terminal flexible DC transmission system, this example discloses a regulation method for a multi-terminal flexible DC transmission system. Based on a regulation strategy of coordinated regulation of power and DC voltage, it comprehensively considers the accuracy of the active power transmitted by the converter station and the stability of the DC voltage to achieve power balance coordinated control, so as to improve the DC voltage response characteristics in the dynamic process. See Figure 1 , taking a typical ring-shaped five-terminal VSC-MTDC system as an example, this multi-terminal flexible DC transmission system includes a total of five voltage source converter stations VSC1 to VSC5. The DC sides are connected in parallel through a DC network, and the AC sides are electrically connected to their respective AC power grids. Among them, the converter stations VSC1 and VSC2 on the DC side are connected in parallel through the DC network, and the converter stations VSC3 to VSC5 on the AC side are electrically connected to their respective AC power grids. The DC side is directly connected to the AC side, and P1 to P5 respectively represent the powers received by the five voltage source converter stations in VSC1 to VSC5.

[0023] When performing traditional droop control on the VSC-MTDC system, using the relationship curve between the DC voltage of the converter station and the active power, the output power of the converter station is regulated according to the change of the DC voltage, so as to realize the rapid distribution of unbalanced power and the stability of the DC voltage. See Figure 2 the schematic diagram of the traditional droop controller shown in Figure 3 and the control curve corresponding to the traditional droop controller shown in P s and P sref respectively represent the measured value and the reference value of the active power of the converter station; U dc and U dcref respectively represent the measured value of the DC side voltage and the voltage reference value; k is the droop coefficient; i dmax and i dmin respectively represent the upper limit value and the lower limit value of the active current component; ΔU dc is the change amount of the DC voltage; ΔP is the unbalanced power of the DC system.

[0024] Based on Figure 2 the schematic diagram of the traditional droop controller shown in the figure, when the system is in a steady state, the d-axis current reference value output by the PI controller set in the main controller of the converter station i dref is zero. Correspondingly, the relationship between the DC voltage and the active power is: U dc =U dcref +k ( P s -P sref ) (1).

[0025] From Figure 3 the control curve of the traditional droop controller shown in the figure, it can be seen that for the converter station adopting the traditional droop control, when its DC voltage changes, the actual value of the active power transmitted by the converter station will also change accordingly. Among them, the DC voltage change ΔU dc and the active power imbalance change ΔP The relationship between them is: ΔU dc =kΔP (2).

[0026] In the VSC-MTDC system, it is assumed that there are n ( n ≥ m ) converter stations, among which m m converter stations adopt droop control. When the system is disturbed by the unbalanced power, then the i (1 ≤ i ≤ m) The unbalanced power borne by the mth droop-controlled converter station is: (3).

[0027] Among them, k i is the droop coefficient of the i mth converter station, ΔP i is the unbalanced power borne by the i mth converter station. It can be seen from equation (3) that the unbalanced power borne by the converter station during the dynamic regulation process is inversely proportional to the droop coefficient, and the smaller the droop coefficient, the greater the unbalanced power borne by the converter station.

[0028] Furthermore, by combining equations (2) and (3), we can get: (4).

[0029] Wherein, Udc’ and Ps’ respectively represent the voltage and operating power values when the converter station is at the new equilibrium point, that is Figure 3 the operating point B1 shown.

[0030] In summary, when the system reaches a stable state again, the unbalanced power will cause the DC voltage and active power of the converter station to deviate from the reference values, and their changes satisfy Equation (4). Therefore, although the converter station using traditional droop control can absorb the unbalanced power during the dynamic regulation process, it is difficult to accurately control the transmission value of the active power, and it will also cause a large DC voltage deviation. The DC voltage deviation in the dynamic and steady-state processes of the system is not conducive to the stable operation of the system.

[0031] Therefore, in this embodiment, a power balance collaborative control method is disclosed, which includes the following steps: (1) Establish a droop control that adjusts the output power of the converter station according to the change in DC voltage and includes a first PI controller, obtain the unbalanced power generated by the multi-terminal flexible DC power transmission system due to disturbances, and use the unbalanced power corresponding to the converter station as the feed-forward compensation amount for the droop control of the converter station for feed-forward compensation.

[0032] Thus, the active power command value of the converter station is quickly adjusted during the dynamic process, that is, by injecting the unbalanced power as the feed-forward compensation amount into the droop control, so as to realize the translation of the droop control curve and reduce the DC voltage deviation during the dynamic process and after reaching the steady state.

[0033] See Figure 4 , in this embodiment, the power balance collaborative control strategy of the multi-terminal flexible DC power transmission system is reflected in Figure 4 the control curve shown, which includes two stages. The first stage is from point A i to point C i shown. During the A i -C i stage, the instantaneous value of the DC voltage is lifted and the droop curve is translated; the second stage is from point C i to point D i shown. The C i -D i stage is the same as the traditional droop control (A i -B i ) to absorb the unbalanced power of the system and stabilize the DC voltage.

[0034] Specifically, when the DC system is in the initial stable state, the converter stations VSC1 and VSC2 respectively operate stably at points A1 and A2, that is, the working voltage at this time is the voltage reference value U dcref, the active power is the active power reference value P sref , both are rated values, that is, reference values. When the DC system is disturbed by unbalanced power, see Figure 5 , feed forward the unbalanced power compensation to the active power reference value of the droop control, so as to realize the translation of the droop curve, see Figure 4 . At this time, the operating points of converter stations VSC1 and VSC2 are points C1 and C2 respectively, that is, the transient process operating points. Then, the droop control of the converter station starts to act, absorb the unbalanced power in the system, and stabilize the DC voltage. When the system reaches the steady state again, the stable operating points are near points D1 and D2, that is, the new steady state operating points.

[0035] Based on Figure 3 and Figure 4 for the effect comparison between traditional droop control and the power balance cooperative control in this example, it can be found that compared with traditional droop control, the power balance cooperative control disclosed in this example has a large reduction in DC voltage deviation and is approximately zero after the system dynamic adjustment, improving the operating stability of the system. In addition, the power balance cooperative control has low requirements for inter-station communication. Even if the inter-station communication of the converter station is interrupted, the system can still operate normally according to traditional droop control.

[0036] In addition, in this embodiment, considering that after introducing the feed forward compensation corresponding to the unbalanced power at the active power input of the droop control, if the unbalanced power is improperly distributed, it is easy to cause problems such as overload of the converter station and abnormal operation of the DC voltage. Therefore, in this embodiment, considering factors such as the rated capacity, active power reference value, and available power margin of each converter station, the unbalanced power of each converter station is reasonably distributed. Specifically, in order to reasonably adjust the active power command value of each converter station and realize the effective translation of the droop curve in the first stage (A i -C i ), this example calculates the power distribution coefficient considering the rated capacity of the converter station to realize the reasonable distribution of unbalanced power among each converter station: ; (5).

[0037] Among them, P is ’ is the actual value of the active power transmission of the i th converter station after feed forward compensation; P isref is the reference value of the active power transmission of the i th converter station; ΔP i is the unbalanced power obtained by the i th converter station through the feed forward compensation link; Pismax is the maximum rated capacity of the i th converter station; P is is the actual value of the active power transmission of the i th converter station.

[0038] Thus, the unbalanced power allocated to the converter station in the feed-forward compensation link is proportional to its power margin. The larger the power margin of the converter station, the more unbalanced power it is allocated. In addition, on the basis of reasonably distributing the unbalanced power, the peak value of the DC voltage rise will also be smaller, achieving relative suppression of the peak value of the DC voltage that first rises and then falls during the control process.

[0039] (2) According to the DC voltage controller at the feed-forward compensation path, correspondingly adjust the increment of the active power reference value until the DC voltage is restored to the reference voltage.

[0040] During the implementation of this embodiment, the inventor found that based on the above feed-forward compensation, when the system enters the steady state again, there is still a certain deviation between the actual value and the rated value of the DC voltage. Therefore, it is necessary to reasonably distribute the unbalanced power for feed-forward compensation and perform secondary regulation on the DC voltage to restore it to the rated value, that is, the second stage (C i -D i ) of the above power balance collaborative control.

[0041] In the second stage of the power balance collaborative control, in order to restore the DC voltage to the rated value, in this embodiment, refer to Figure 5 , superimpose the power increase amount calculated by the DC voltage controller corresponding to the DC voltage deviation to the corresponding droop control of the converter station, thereby changing the active power intercept of the droop curve and realizing the zero-error regulation of the DC voltage.

[0042] Specifically, in this embodiment, the DC voltage controller includes a second PI controller, and the additional power term ΔP k calculated by the DC voltage controller is: ΔP k = ( K p2 +K i2 / s ) ( U dcref -U dc ) (9).

[0043] Among them, K p2 is the proportional coefficient of the second PI controller; K i2is the integral coefficient of the second PI controller; s is the complex variable in the Laplace transform.

[0044] In this embodiment, to avoid the frequent operation of the secondary regulation controller and affect the normal operation of the droop control of the converter station, the DC voltage controller further includes a voltage dead zone link. In this example, the range of the voltage dead zone is set to ±2%. In addition, considering that the secondary regulation control of the DC voltage is a relatively secondary goal compared with the droop control, the parameters of the two PI controllers (the first PI controller and the second PI controller) are reasonably set to distinguish the prior and subsequent action response times. While satisfying the droop control to absorb the unbalanced power, the DC voltage is restored to the rated value by changing the active power intercept of the droop curve.

[0045] To verify the effectiveness of the multi-terminal flexible DC transmission system regulation method disclosed in this example, this example is based on PSCAD / EMTDC to construct Figure 1 the five-terminal VSC-MTDC system shown in the figure for simulation. The specific simulation parameters are shown in Table 1. Among them, the converter stations VSC1 and VSC2 adopt droop control, the converter stations VSC3 and VSC4 adopt constant active power control, and the converter station VSC5 adopts constant AC voltage control.

[0046] After the five-terminal VSC-MTDC system is established, at t = 3 s, the power command value of the converter station VSC3 is increased from 115 MW to 185 MW, and at t = 5 s, the power command value of the converter station VSC4 is decreased from 85 MW to 55 MW. The simulation results are shown in Figure 6 , CM1 represents the simulation result curve of the traditional droop control, and CM2 represents the simulation result curve corresponding to the regulation method disclosed in this example.

[0047] From Figure 6 it can be seen that due to the existence of system losses, there are small deviations between the actual values of the active power and DC voltage of the converter station and the rated values. When t = 3 s, the power command value of the converter station VSC3 is increased from 115 MW to 185 MW. At this time, there is a power deficit in the DC system and the DC voltage begins to drop. From Figure 6 subfigures (a), (b), and (d) of it can be seen that when the system first reaches the steady state, under the traditional droop control, the converter stations VSC1 and VSC2 absorb the unbalanced power according to the traditional droop control. The unbalanced powers ΔP1 and ΔP2 are 23.7 MW and 46.3 MW respectively, and the DC voltage change amount ΔU dc is 7.08 kV, and the DC voltage deviation rate is 1.77%. Under the power balance collaborative control strategy disclosed in this example, the DC voltage first rises and then drops. The unbalanced powers ΔP1 and ΔP2 are 23.7 MW and 46.3 MW respectively, and the DC voltage change amount ΔUdc is 0.14 kV, and the DC voltage deviation rate is 0.035%. It can be seen from the simulation results that under the two control strategies, the actual transmitted value of the active power of the converter station is equal before and after dynamic adjustment, which conforms to Figure 4 the control curve shown, and the DC voltage deviation under the control strategy of the present invention is much smaller than that of droop control, greatly improving the operation stability of the system. From Figure 6 (d) of it can be seen that when the control strategy of the present invention is adopted, when the system resumes stable operation after being disturbed, the DC voltage stabilizes at 400 kV, and the non-error regulation of the DC voltage can be realized; while when the traditional droop control is adopted, when the system resumes stable operation after being disturbed, the DC voltage stabilizes at 396 kV, and there is a change in the DC voltage before and after the disturbance.

[0048] Although some preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0049] Obviously, those skilled in the art can make various changes and modifications to the present disclosure without departing from the spirit and scope of its inventive concept. Thus, if these modifications and variations to the present disclosure fall within the scope of the claims of this application and their equivalent technologies, the present invention also intends to include these changes and modifications.

Claims

1. A control method for a multi-terminal flexible DC power transmission system, characterized in that, It includes the following steps: (1) Establish droop control that adjusts the output power of the converter station according to the change of DC voltage and includes a first PI controller, obtain the unbalanced power generated by the multi-terminal flexible DC transmission system under disturbance, and use the unbalanced power corresponding to the converter station as the feed-forward compensation amount of the droop control of the converter station for feed-forward compensation; (2) Adjust the increment of the active power reference value according to the DC voltage controller at the feed-forward compensation path until the DC voltage is restored to the reference voltage.

2. The control method of the multi-terminal flexible DC power transmission system according to claim 1, wherein In the step (1), the unbalanced power is correspondingly feed-forward compensated to the active power reference value of the droop control.

3. The control method for a multi-terminal flexible DC power transmission system according to claim 1, wherein In the step (1), the feed-forward compensation of the unbalanced power corresponding to each converter station is carried out under the rated capacity of the converter station: ； Among them, P is ’ is the actual value of the active power transmission of the i th converter station after feed-forward compensation; P isref is the reference value of the active power transmission of the i th converter station; ΔP i is the unbalanced power distributed by the i th converter station through the feed-forward compensation link; P ismax is the maximum rated capacity of the i th converter station; P is is the actual value of the active power transmission of the i th converter station.

4. The control method for a multi-terminal flexible DC power transmission system according to claim 1, characterized in that, In the step (2), the DC voltage controller includes a second PI controller.

5. The control method for a multi-terminal flexible DC power transmission system according to claim 4, wherein The DC voltage controller further includes a voltage dead zone link.

6. The control method for a multi-terminal flexible DC power transmission system according to claim 5, wherein The voltage dead zone range of the voltage dead zone link is ±2%.

Images