A multi-point direct-current voltage difference-free coordinated control method for a VSC-MTDC system

Abstract

The application provides a multi-point direct-current voltage zero-deviation coordinated control method for a VSC-MTDC system, and aims to solve the technical problem of direct-current voltage deviation caused by the traditional droop control method used in a converter station to absorb unbalanced power. According to the power margin of the converter station, the application reasonably allocates the unbalanced power, shifts the droop curve, and realizes zero-deviation regulation of the direct-current voltage. The control system collects the active power value of the non-droop converter station, calculates the power disturbance amount borne by the droop converter station, judges whether the power disturbance amount is equal to 0, according to the power balance allocation method, takes the power disturbance amount as a feedforward compensation amount, allocates the power disturbance amount to each droop converter station according to the power margin, shifts the droop curve, judges whether the direct-current voltage exceeds the set range, and changes the control mode of the fixed active power converter station to the droop control to participate in the unbalanced power regulation. The application can realize quasi-zero-deviation regulation of the direct-current voltage, optimizes the power flow distribution of the system, and improves the operation stability of the system.

Description

Technical Field

[0001] This invention relates to the technical field of droop control, and more particularly to a method for zero-error coordinated control of multi-point DC voltage in a VSC-MTDC system. Background Technology

[0002] With the continuous adjustment of the energy structure, the penetration rate of high-capacity power electronic devices in the power grid is gradually increasing, showing a trend of power electronization in the four fields of power generation, grid, load, and energy storage. In terms of power transmission systems, voltage source converter based multi-terminal high voltage direct current (VSC-MTDC) is one of the important development directions of long-distance power transmission technology in the future, and is gradually becoming the best choice for grid connection of new energy sources.

[0003] Currently, the inter-station control strategies applied to VSC-MTDC systems can be broadly categorized into three types: master-slave control, voltage margin control, and DC voltage droop control. Master-slave control and voltage margin control are single-point control methods, where only a single converter station participates in the power regulation of the DC system at any given time, resulting in poor dynamic response characteristics of the DC voltage. DC voltage droop control can utilize the power regulation capabilities of multiple converter stations, achieving rapid distribution of unbalanced power along their respective droop curves, and exhibits better DC voltage response characteristics, but it suffers from significant DC voltage deviation.

[0004] To address the DC voltage deviation problem associated with droop control in converter stations, scholars both domestically and internationally have conducted extensive research, and the main control methods currently fall into three categories. The first method involves improving the droop coefficient to achieve a reasonable distribution of unbalanced power and reduce DC voltage deviation. References: [Wang Yuhong, Chen Yong, Zeng Qi, et al. Improved droop control for VSC-MTDC[J]. High Voltage Engineering, 2018, 44(10): 3190-3196.], [Luo Yongjie, Li Yaohua, Wang Ping, et al. Research on adaptive droop control strategy for DC voltage in multi-terminal flexible DC transmission system[J]. Proceedings of the CSEE, 2016, 36(10): 2588-2599.], [Liu Yingpei, Xie Sai, Liang Haiping, Xie Qian. Adaptive droop control of VSC-MTDC system considering voltage error between converter stations[J]. Journal of Electrical Engineering, 2020, 35(15): 3270-3280.], [Zhang Yuanshi, Wang Liwei and Li Wei. Autonomous DC line power flow regulation using adaptive droop control in HVDC grid[J]. IEEE Transactions on Power Delivery, [2021, 36(6):3550-3560.] By adaptively adjusting the droop coefficient, the deviation of DC voltage is effectively reduced. However, the deviation between the actual active power transmission value and the reference value of the converter station persists, and the DC voltage deviation also persists, which is not conducive to the stable operation of the system. The second method is to achieve error-free regulation of DC voltage by changing the operation mode of the converter station. The literature [Yuan Zhichang, Wu Zhili, Jin Qiang, et al. Frequency stability control of VSC-MTDC interconnection system with secondary regulation of DC voltage [J]. Automation of Electric Power Systems, 2018, 42(23): 9-13+19.] proposes to superimpose the DC voltage error onto the active power loop through the PI controller, and use the zero steady-state error characteristic of the PI controller to achieve error-free regulation of DC voltage. Reference [Zhao Xiaobin, Shao Bingbing, Han Minxiao. Joint control strategy for multi-terminal flexible DC transmission system based on N-1 rule [J]. Electric Power Construction, 2017, 38(11): 19-25.] proposes a joint control strategy combining master-slave control and droop control, which achieves error-free regulation of DC voltage by changing the control mode. Reference [Li Zhou, Li Yazhou, Lu Yuping, et al. Active power balance coordination control strategy for multi-terminal flexible DC grid [J]. Automation of Electric Power Systems, 2019, 43(17): 117-124.] proposes a multi-point voltage coordination control strategy, which minimizes the dynamic power deviation of converter stations while improving the performance of DC voltage.However, the essence of the above control strategy is to turn the drooping station into a constant DC voltage control station, thus losing the advantage of multi-station coordinated absorption of unbalanced power in droop control. The third method is to achieve the purpose of error-free DC voltage regulation by adjusting the active power reference value. The literature [Li Zhou, Li Yazhou, Zhan Roupei, He Yan and Zhang Xiaoping. AC grids characteristics oriented multi-point voltagecoordinated control strategy for VSC-MTDC[J]. IEEE Access, 2019, 7: 7728-7736.] achieves the translation of the droop curve by collecting the unbalanced power of the DC system and superimposing it on the active power loop. The literature [Liu Haoyu, Liu Chongru, Zheng Le, Wang Qunqiao. Cooperative optimization droop control of VSC-MTDC system with quasi-error-free DC voltage correction [J]. Automation of Electric Power Systems, 2022, 46(06): 117-126.] demonstrates that after a power disturbance in the DC system, each converter station cannot restore the DC voltage to its rated value while maintaining constant output power. Therefore, a coordinated control strategy is proposed that minimizes both converter station power variation and DC voltage deviation, achieving "pseudo" error-free DC voltage regulation. However, the DC voltage deviation is also relatively large. Summary of the Invention

[0005] To address the technical problem of DC voltage deviation caused by the traditional droop control method used in converter stations to absorb unbalanced power, this invention proposes a multi-point DC voltage zero-error coordinated control method for VSC-MTDC systems. This method can achieve quasi-zero-error regulation of DC voltage, optimize the power flow distribution of the system, and improve the operational stability of the system.

[0006] To achieve the above objectives, the technical solution of this invention is as follows: a multi-point DC voltage error-free coordinated control method for a VSC-MTDC system, which rationally allocates unbalanced power according to the power margin of the converter station, shifts the droop curve, and realizes error-free DC voltage regulation; the steps are as follows:

[0007] Step 1: The control system collects the active power values ​​of the non-droop converter stations at time t and time t+Δt, and calculates the power disturbance ΔP borne by the i-th droop converter station.

[0008] Step 2: Determine if the power disturbance ΔP is equal to 0. If it is, return to Step 1; otherwise, proceed to Step 3.

[0009] Step 3: According to the power balance allocation method, the power disturbance ΔP is used as the feedforward compensation amount and allocated to each droop converter station according to the power margin. Quasi-error-free regulation of DC voltage is achieved by shifting the droop curve.

[0010] Step 4: Determine if the DC voltage exceeds the set range. If not, return to Step 1; if yes, proceed to Step 5.

[0011] Step 5: Change the control mode of the constant active power converter station to droop control to participate in unbalanced power regulation.

[0012] Preferably, the control system includes a multimeter, a data transmission module, and a calculation module. The multimeter is connected to the data transmission module, and the data transmission module is connected to the calculation module. The multimeter collects the active power value of the non-droop converter station and sends it to the calculation module through the data transmission module.

[0013] Preferably, the power disturbance ΔP is calculated as follows:

[0014] The DC voltage is the same at all converter stations;

[0015] Under steady-state conditions, the relationship between DC voltage and active power at the converter station is as follows: Among them, U dc and U dcref These represent the measured DC-side voltage and the reference voltage, respectively, where k is the droop factor and P... s and P sref These represent the measured active power and the reference active power of the converter station, respectively.

[0016] Suppose that the VSC-MTDC system has N converter stations, of which 1 to m converter stations use traditional droop control to absorb unbalanced power in the VSC-MTDC system and stabilize DC voltage; m+1 to n converter stations use constant active power control; and n+1 to N converter stations use constant AC voltage control.

[0017] The sum of active power reference values ​​of 1~m droop converter stations ;in, Let be the reference value of active power for the i-th droop converter station, 1≤i≤m;

[0018] The sum of power reference values ​​for m+1 to n constant active power converter stations ;in, Let m+1 be the reference value of active power for the b-th constant active power converter station, where m+1≤b≤n;

[0019] The sum of active power transmission values ​​of n+1 to N constant AC voltage converter stations ;in, Let n+1 be the active power transmission value of the j-th constant AC voltage converter station, where n+1≤j≤N;

[0020] In the initial steady state, when a power disturbance ΔP occurs in the DC system, the m-1 converter stations absorb the unbalanced power of the network using their own droop characteristics, and the stable operating point will also shift accordingly; when the VSC-MTDC system reaches steady state again, the unbalanced power ΔP borne by the i-th droop converter station will... i DC voltage deviation ΔU dc The relationship is According to the law of conservation of energy, the sum of the changes in active power at each droop converter station should equal the power disturbance ΔP, i.e. ;

[0021] therefore, ; ;

[0022] DC voltage deviation ΔU dc It is proportional to the power disturbance ΔP and the droop coefficient k of m droop converter stations. i The sum of the reciprocals is inversely proportional; the droop coefficient k i This determines the amount of unbalanced power that the converter station will bear during dynamic adjustment.

[0023] Preferably, by rationally allocating the unbalanced power of the DC system and injecting the unbalanced power into the active power reference value of the converter station, error-free regulation of the DC voltage can be achieved.

[0024] Preferably, in the initial steady state, when the DC system is subjected to an unbalanced power disturbance, a feedforward compensation amount is provided to each droop converter station, the magnitude of which is equal to the unbalanced power ΔP. i At this point, the stable operating point of the VSC-MTDC system changes from the initial stable operating point to the operating point instantaneously reached by the system under feedforward compensation power, achieving a shift in the droop curve and satisfying the requirements. Among them, P isref and These are the reference values ​​of active power before and after adjustment for the i-th droop converter station;

[0025] The droop converter station absorbs unbalanced power based on its droop characteristics. The VSC-MTDC system can operate stably and recover to the vicinity of the stable operating point of the rated voltage without changing the output power, thus achieving error-free regulation of DC voltage.

[0026] Preferably, in order to reasonably distribute the unbalanced power in the DC system to each droop converter station, the available power margin of the droop converter station is introduced into the power allocation coefficient to quickly adjust the active power reference value.

[0027] Preferably, the method for incorporating the available power margin of the droop converter station into the power allocation factor is as follows:

[0028] ;

[0029] Among them, P isref ´ represents the optimized active power reference value for the i-th droop converter station; , These represent the active power transmission values ​​of the constant active power converter station and the constant AC voltage controlled converter station at time t+Δt, respectively. , Δt represents the active power transmission value of the constant active power converter station and the constant AC voltage controlled converter station at time t, respectively; Δt is the sampling time.

[0030] Preferably, the setting range is set according to the available power margin of the droop converter station in the VSC-MTDC system used.

[0031] Preferably, DC voltage deviation control is introduced into the constant active power converter station. When the DC voltage exceeds (U dc l U dc h When operating within the specified range, the constant active power converter station will switch to droop control, working in conjunction with the droop converter station to absorb the remaining unbalanced power in the system and undertake the task of stabilizing the DC voltage; among which, U dc l U dc h These represent the lower and upper limits of the set range, respectively; the DC voltage deviation droop coefficient is... ;

[0032] Where, k b P is the DC voltage deviation droop coefficient. bmax This is the maximum capacity of a converter station with a fixed active power rating; Let be the active power of the b-th constant active power converter station.

[0033] Preferably, the constant active power converter station adopts DC voltage deviation droop control. After the drooping converter station loses its ability to control the DC voltage, the remaining unbalanced power in the VSC-MTDC system is denoted as ΔP. * When the VSC-MTDC system reaches equilibrium again, the active power of the converter station with DC voltage deviation droop is: Among them, P j To ensure the active power of the converter station is controlled by deviation after the system stabilizes, k b Let be the droop coefficient of the b-th constant active power converter station. This is the reference value for the active power of the b-th constant active power converter station.

[0034] The beneficial effects of this invention are as follows: Unbalanced power is injected as feedforward compensation into traditional droop control, achieving quasi-error-free DC voltage regulation by shifting the droop curve; the feedforward compensation amount for each converter station is rationally set according to the power margin of the converter station; to avoid excessive unbalanced power leading to full-load operation of the droop-controlled converter station, deviation control is introduced into the constant active power converter station to coordinate with the droop-controlled converter station to absorb the remaining unbalanced power; finally, a five-terminal VSC-MTDC system is established based on PSCAD / EMTDC for simulation. Simulation results show that this invention can achieve quasi-error-free DC voltage regulation, optimize the power flow distribution of the system, and improve the operational stability of the VSC-MTDC system. This invention allocates unbalanced power in the converter station system based on the power margin of the converter station, which can effectively avoid the situation where some converter stations are overloaded while other converter stations still have power margin. By feeding forward to compensate the active power reference value and shifting the droop curve, it can approximately achieve error-free regulation of DC voltage. By introducing deviation droop control to converter stations with constant active power, it improves the system's operational stability when the unbalanced power is too large or when the converter station is out of service. Attached Figure Description

[0035] 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 some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a flowchart of the present invention.

[0037] Figure 2 This is a structural diagram of the VSC-MTDC system.

[0038] Figure 3 The figures are characteristic curves of DC voltage droop control in traditional droop control, where (a) is DC-side converter station VSC1 and (b) is DC-side converter station VSC2.

[0039] Figure 4 This is a schematic diagram of a DC voltage droop controller.

[0040] Figure 5 The characteristic curves of the optimized coordinated control strategy of the present invention are shown, where (a) is DC-side converter station VSC1 and (b) is DC-side converter station VSC2.

[0041] Figure 6 The above are the control characteristic curves of the VSC-MTDC system of this invention.

[0042] Figure 7 The simulation results for the power of the VSC-MTDC system are added, where (a) is the active power of converter station VSC1, (b) is the active power of converter station VSC2, (c) is the active power of converter stations VSC3, VSC4 and VSC5, and (d) is the DC voltage of the system.

[0043] Figure 8 The simulation results for power reduction of the VSC-MTDC system are shown, where (a) is the active power of converter station VSC1, (b) is the active power of converter station VSC2, (c) is the active power of converter stations VSC3, VSC4 and VSC5, and (d) is the DC voltage of the system.

[0044] Figure 9 The simulation results of the converter station shutdown of the present invention are shown in the figure, where (a) is the active power of converter station VSC1, VSC2 and VSC5, (b) is the active power of converter station VSC3, (c) is the active power of converter station VSC4, and (d) is the DC voltage of the system. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] like Figure 1 As shown, a multi-point DC voltage zero-error coordinated control method for a VSC-MTDC system is presented. This is a novel optimized coordinated control strategy that rationally allocates unbalanced power based on the power margin of the converter station, shifting the droop curve to approximately achieve zero-error DC voltage regulation. Simultaneously, deviation droop control is introduced to converter stations with constant active power to prevent the drooping station from losing its ability to control DC voltage when fully loaded. Finally, simulations are performed using a PSCAD / EMTDC five-terminal VSC-MTDC system, and the simulation results verify the effectiveness of the proposed control strategy. The specific steps of this invention are as follows:

[0047] Step 1: The control system collects the active power values ​​of the non-droop converter stations at time t and time t+Δt, and calculates the power disturbance ΔP borne by the i-th droop converter station.

[0048] The five-terminal VSC-MTDC system architecture is as follows: Figure 2As shown, VSC1 to VSC5 represent five voltage source converter stations. The DC-side converter stations VSC1 and VSC2 are connected in parallel via a DC network, while the AC-side converter stations VSC3 to VSC5 are connected to their respective AC power grids. The DC and AC sides are directly connected.

[0049] Analogous to the primary frequency regulation characteristics of traditional generators, droop control eliminates the need for inter-station communication. Utilizing the characteristic curve between DC voltage and active power, it achieves rapid distribution of unbalanced power and stable control of DC voltage. DC voltage - Active power (U dc -P s The control characteristics and controller structure are as follows: Figure 3 and Figure 4 As shown. Figure 3 and Figure 4 Middle,U dc and U dcref P represents the measured and reference values ​​of the DC-side voltage, respectively. s P isref and P imax These represent the measured, reference, and rated active power values ​​of the converter stations, respectively. k is the droop coefficient, and i=1,2, indicating drooping converter stations VSC1 and VSC2. dc ’ The DC voltage value of the system after the converter station absorbs unbalanced power, ΔU dc Let ΔP1 and ΔP2 represent the changes in DC voltage of the system, respectively. A1 and A2 represent the initial stable operating points of converter stations VSC1 and VSC2, respectively. B1 and B2 represent the points where converter stations VSC1 and VSC2 reach stable operation again after absorbing unbalanced power. C1 and C2 represent the stable operating points where converter stations VSC1 and VSC2 recover to their rated voltage without changing their output power. To simplify the analysis, this invention assumes that the DC voltage of all converter stations is equal.

[0050] In steady state, by Figure 3 The relationship between DC voltage and active power at the converter station can be obtained as follows:

[0051] (1);

[0052] Assume a DC system has N converter stations. Stations 1 to m employ traditional droop control to absorb unbalanced power in the VSC-MTDC system and stabilize the DC voltage. Stations m+1 to n employ constant active power control, and stations n+1 to N employ constant AC voltage control. Constant active power control meets the AC system's requirements for stable and high-quality active power input, while constant AC voltage control absorbs fluctuating active power and provides stable AC voltage support for renewable energy stations.

[0053] The sum of the active power reference values ​​P of 1~m droop converter stations r for

[0054] (2);

[0055] In the formula, Let be the reference value of active power for the i-th droop converter station, 1≤i≤m.

[0056] The sum of power reference values ​​P for m+1~n constant active power converter stations t for

[0057] (3);

[0058] In the formula, Let m+1 be the reference value of active power for the b-th constant active power converter station, where m+1≤b≤n.

[0059] The sum of active power transmission values ​​P of n+1 to N constant AC voltage converter stations w for

[0060] ;

[0061] In the formula, Let be the active power transmission value of the j-th constant AC voltage converter station, where n+1≤j≤N.

[0062] In the initial steady state, the system operates in state 1. When a power disturbance ΔP occurs in the DC system, the m-1 converter stations absorb the unbalanced power of the network using their own droop characteristics, and the stable operating point also shifts accordingly. When the VSC-MTDC system reaches steady state again, it operates in state 2. Let the DC voltage deviation be ΔU. dc ,Depend on Figure 3 It can be seen that the unbalanced power ΔP borne by the vertices converter station i (1≤i≤m) i DC voltage deviation ΔU dc The following relationship exists

[0063] (4);

[0064] According to the law of conservation of energy, the sum of the changes in active power at each drooping station should equal the power disturbance ΔP, i.e.

[0065] (5);

[0066] By combining equations (4) and (5), we can obtain

[0067] (6);

[0068] (7);

[0069] It can be seen from equation (6) that the DC voltage deviation ΔU dc It is directly proportional to the power disturbance ΔP and inversely proportional to the sum of the reciprocals of the droop coefficients of m-1 droop stations. This indicates that when the DC system power fluctuates, all converter stations will work together to absorb the unbalanced power according to their own droop curves. However, the resulting DC voltage deviation will affect the normal operation of the system.

[0070] As can be seen from equation (7), the droop coefficient determines the amount of unbalanced power borne by the converter station during dynamic adjustment. If all converter stations adopt the same droop coefficient, then all converter stations will share the unbalanced power equally. If each converter station adopts a different droop coefficient, then the converter station with a smaller droop coefficient will bear more unbalanced power, and the converter station with a larger droop coefficient will bear less unbalanced power.

[0071] The control system mainly includes a multimeter, a data transmission module, and a calculation module in the PSCAD software. The multimeter primarily collects the active power values ​​of the non-droop converter station and transmits them to the calculation module via the data transmission module. Power disturbances are generally generated at the non-droop converter station; therefore, the power disturbance can be obtained by subtracting the active power values ​​collected from two non-droop converter stations within a sampling period.

[0072] Step 2: Determine if the power disturbance ΔP is equal to 0. If it is, return to Step 1; otherwise, proceed to Step 3.

[0073] The power imbalance is mainly caused by power fluctuations in renewable energy stations. This invention uses the power fluctuations of constant active power converter stations to replace the power fluctuations of renewable energy stations. Since the purpose of this invention is to restore the DC voltage of the converter station to approximately its rated value after it experiences power disturbances, through additional control strategies, achieving near-error-free DC voltage regulation, it is essential to obtain accurate values ​​for the power disturbance.

[0074] Step 3: According to the power balance allocation method, the power disturbance ΔP is used as the feedforward compensation amount and allocated to each droop converter station according to the power margin. Quasi-error-free regulation of DC voltage is achieved by shifting the droop curve.

[0075] To address the issue of large DC voltage deviation associated with traditional droop control in converter stations, this invention proposes a multi-droop station collaborative optimization control strategy that can approximately achieve error-free DC voltage regulation. By rationally allocating the unbalanced power of the DC system and injecting it into the active power reference value of the converter station, error-free DC voltage regulation can be approximately achieved.

[0076] The characteristic curve of the optimized coordinated control strategy is as follows: Figure 5 As shown. Where, Udc ’’ P1'' and P2'' are the DC voltage of the converter station under feedforward compensation power, respectively; D1 and D2 are the active power values ​​of converter station VSC1 and VSC2 under feedforward compensation power, respectively; and D1 and D2 are the instantaneous operating points reached by the system at converter station VSC1 and VSC2 under feedforward compensation power, respectively.

[0077] In the initial steady state, the system operates in state 1. When the DC system is subjected to unbalanced power disturbance, a feedforward compensation of magnitude ΔP is provided to each droop station. i At this point, the system's stable operating point shifts from A. i Change to D i This achieves the translation of the drooping curve, P isref and These are the reference values ​​of active power before and after the adjustment of the converter station, respectively, which meet the requirements.

[0078] (8);

[0079] The converter station then absorbs the unbalanced power based on its droop characteristics, and the system eventually operates stably at C. i Near the point, near-zero DC voltage regulation is achieved. Compared with traditional DC voltage droop, the active power transmitted by the converter station remains unchanged, while the DC voltage deviation is approximately zero, significantly improving system stability.

[0080] To rationally allocate unbalanced power in a DC system to each drooping station, this invention incorporates the available power margin of the converter station into the power allocation coefficient, rapidly adjusting the active power reference value. The specific method is as follows:

[0081] (9);

[0082] In the formula, P isref ´ represents the optimized active power reference value for the droop converter station. , These represent the active power transmission values ​​of the constant active power converter station and the constant AC voltage controlled converter station at time t+Δt, respectively. , Δt represents the active power transmission values ​​of the constant active power converter station and the constant AC voltage controlled converter station at time t, respectively; Δt is the system sampling time.

[0083] Under this power allocation strategy, the control system only needs to collect the active power value of the non-droop converter stations, and only updates the active power reference value to the droop stations when the DC system power flow changes. According to formula (9), if the active power transmission value of the non-droop stations does not change during the sampling time, the numerator of the formula is 0, so the active power reference value of the droop stations will not change. In other cases, the droop converter stations only need to operate stably based on the most recently updated active power reference value. Furthermore, this power allocation strategy has extremely low requirements for inter-station communication, because this power allocation strategy requires the active power transmission value of the non-droop stations, so it needs to be transmitted through the communication data channel in the actual project, which does not increase the burden on the communication system and has low requirements for communication. Even when the communication between the converter stations is interrupted, the droop stations can still operate normally according to the traditional droop control method.

[0084] Step 4: Determine if the DC voltage exceeds the set range. If not, return to Step 1; if yes, proceed to Step 5.

[0085] The DC voltage deviation droop control proposed in this invention determines whether the control mode of a constant active power converter station should switch to droop control based on whether the DC voltage exceeds a set operating range. The set range is determined based on the available power margin of the drooping station in the five-terminal model used.

[0086] Step 5: Change the control mode of the constant active power converter station to droop control to participate in unbalanced power regulation.

[0087] When the above-mentioned coordinated control strategy is adopted, if a fault such as excessive system power imbalance or converter station shutdown occurs, exceeding the adjustment range of all droop converter stations, the droop converter stations will operate at full load and switch to constant active power operation, losing the ability to control DC voltage. To avoid the above situation, this invention introduces DC voltage deviation control to constant active power converter stations. When the DC voltage exceeds (U... dc l U dc h When operating within the range, U dc l U dc h These represent the lower and upper limits of the set range, respectively. The constant active power converter station will switch to droop control, working in conjunction with the droop converter station to absorb the remaining unbalanced power in the system and undertake the task of stabilizing the DC voltage. The DC voltage deviation droop coefficient is set to...

[0088] (9);

[0089] In the formula, k bP is the DC voltage deviation droop coefficient. bmax This refers to the maximum capacity of a converter station with a fixed active power. Let be the active power of the b-th constant active power converter station. This indicates the lower limit of the set DC voltage operating range.

[0090] A constant active power converter station employs DC voltage deviation droop control. After the drooping converter station loses its ability to control the DC voltage, the remaining unbalanced power in the DC system is denoted as ΔP. * When the VSC-MTDC system reaches equilibrium again, the active power of the converter station with DC voltage deviation droop is...

[0091] (10);

[0092] In the formula, P j Once the system stabilizes, deviation control of the converter station's active power will be implemented.

[0093] for Figure 2 The five-terminal VSC-MTDC system shown employs optimized droop control for VSC1 and VSC2, DC voltage deviation droop control for VSC3 and VSC4, and constant AC voltage control for VSC5. The control characteristic curve of the VSC-MTDC system is as follows: Figure 6 As shown. From Figure 6 It can be seen that when the DC voltage exceeds the set range, converter station VSC3 and converter station VSC4 will switch to droop control, which will work with the original droop station to absorb the unbalanced power in the system and improve the stability of the system.

[0094] This invention establishes a system in PSCAD / EMTDC as follows: Figure 2 The five-terminal VSC-MTDC system shown is illustrated in Table 1, with specific simulation parameters. Three simulation examples are used to compare three control strategies and verify the effectiveness of the proposed control strategy.

[0095] Control Strategy 1 (CM1): Traditional droop control;

[0096] Control Strategy 2 (CM2): Optimize coordinated control;

[0097] Control Strategy 3 (CM3): Optimized Coordination Control + Deviation Droop Control.

[0098] Table 1 Simulation parameters of VSC-MTDC system

[0099]

[0100] At t=3s, the active power command value of converter station VSC3 increases from 115MW to 185MW. The simulation results are as follows: Figure 7 As shown. By Figure 7 It can be seen that due to losses in the VSC-MTDC system, the active power of the converter station and the DC voltage of the system differ slightly from their rated values. Let ΔP1-ΔP5 represent the change in active power of the converter station from VSC1 to VSC5, and ΔU dc This represents the DC voltage deviation. At t=3s, the DC system exhibits a power surplus. Under control strategy CM1, converter stations VSC1 and VSC2 absorb the unbalanced power according to traditional droop control. When the DC system reaches steady state again, ΔP1 and ΔP2 are 23.5MW and 46.5MW respectively, and the DC voltage deviation ΔU... dc The voltage is 7.09 kV, with a deviation rate of 1.77%. Under the control strategy CM2, ΔP1 and ΔP2 are 16 MW and 54 MW respectively, and the DC voltage deviation ΔU dc The voltage is 0.14kV with a deviation rate of 0.035%, achieving near-zero error correction of the DC voltage. Simultaneously, the system allocates more unbalanced power to converter stations with larger power margins. Therefore, converter station VSC2 handles more unbalanced power than converter station VSC1, preventing VSC1, with its smaller power margin, from being fully loaded. Since the DC voltage does not exceed the system's set operating range, the simulation results under control strategy CM3 are the same as those under control strategy CM2.

[0101] At t=3s, the active power command value of converter station VSC3 drops from 115MW to 45MW. The simulation results are as follows: Figure 8 As shown. At t=3s, the DC system experiences power loss, caused by... Figure 8 It can be seen that under the control strategy CM1, ΔP1 and ΔP2 are 23.5MW and 46.5MW respectively, and the DC voltage deviation ΔU dc The voltage is 7.03 kV, with a deviation rate of 1.76%. Under control strategies CM2 or CM3, ΔP1 and ΔP2 are 42 MW and 28 MW respectively, and the DC voltage deviation ΔU dc The voltage is 0.11kV, and the deviation rate is 0.028%. At this time, the power margin of converter station VSC1 is greater than that of converter station VSC2, so converter station VSC1 bears more unbalanced power than converter station VSC2.

[0102] At t=3s, converter station VSC2 exits operation, and the simulation results are as follows. Figure 9 As shown. By Figure 9It can be seen that under control strategy CM1, converter station VSC1, as the sole power balance point, quickly reaches full load and switches to constant active power operation, losing its ability to control DC voltage, and the DC voltage continues to drop. Under control strategy CM2, since converter stations VSC3 and VSC4 are still controlled in constant active power mode and do not participate in power regulation, ΔP2 and ΔP3 are both 0. Under control strategy CM3, when the DC voltage drops to 390 kV, converter stations VSC3 and VSC4 switch from constant active power to droop control, working with converter station VSC1 to absorb unbalanced power and undertake the task of stabilizing DC voltage. After the DC system enters steady state, ΔP1, ΔP3, and ΔP4 are 26MW, 41MW, and 33MW respectively, and the DC voltage deviation ΔU dc The voltage was 14.49 kV with a deviation rate of 3.62%. Meanwhile, converter station VSC1 was not at full load and still possessed the ability to stabilize the DC voltage, significantly improving system stability.

[0103] The simulation results show that when the converter station uses traditional droop control, the DC voltage deviation is large when subjected to unbalanced power disturbances. However, when the power control strategy of this invention is adopted, the DC voltage deviation is approximately zero, achieving quasi-error-free regulation of the DC voltage.

[0104] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for multi-point DC voltage zero-error coordinated control of a VSC-MTDC system, characterized in that, Based on the power margin of the converter station, unbalanced power is reasonably allocated, and the droop curve is shifted to achieve error-free regulation of DC voltage. The steps are as follows: Step 1: The control system collects the active power values ​​of the non-droop converter stations at time t and t+Δt, and calculates the unbalanced power borne by the i-th droop converter station. ; Step 2: Determine the unbalanced power Check if it equals 0. If yes, return to step one; otherwise, proceed to step three. Step 3: According to the power balance allocation method, the unbalanced power is... As a feedforward compensation, it is allocated to each droop converter station according to the available power margin, and the quasi-error-free regulation of DC voltage is achieved by shifting the droop curve; Step 4: Determine if the DC voltage exceeds the set range. If not, return to Step 1; if yes, proceed to Step 5. Step 5: Change the control mode of the constant active power converter station to droop control to participate in unbalanced power regulation; The unbalanced power The calculation method is as follows: The DC voltage is the same at all converter stations; Under steady-state conditions, the relationship between DC voltage and active power at the converter station is as follows: Among them, U dc and U dcref These represent the measured DC-side voltage and the reference voltage, respectively, where k is the droop factor and P... s and P sref These represent the measured active power and the reference active power of the converter station, respectively. Suppose that the VSC-MTDC system has N converter stations, of which 1 to m converter stations use traditional droop control to absorb unbalanced power in the VSC-MTDC system and stabilize DC voltage; m+1 to n converter stations use constant active power control; and n+1 to N converter stations use constant AC voltage control. The sum of active power reference values ​​of 1~m droop converter stations ;in, Let be the reference value of active power for the i-th droop converter station, 1≤i≤m; The sum of power reference values ​​for m+1 to n constant active power converter stations ;in, Let m+1 be the reference value of active power for the b-th constant active power converter station, where m+1≤b≤n; The sum of active power transmission values ​​of n+1 to N constant AC voltage converter stations ;in, Let n+1 be the active power transmission value of the j-th constant AC voltage converter station, where n+1≤j≤N; In the initial steady state, when a power disturbance ΔP occurs in the DC system, the m converter stations absorb the unbalanced power of the network using their own droop characteristics, and the stable operating point will also shift accordingly; when the VSC-MTDC system reaches steady state again, the unbalanced power ΔP borne by the i-th droop converter station will... i DC voltage deviation ΔU dc The relationship is According to the law of conservation of energy, the sum of the changes in active power at each droop converter station should equal the power disturbance ΔP, i.e. ; therefore, ; ; DC voltage deviation ΔU dc It is proportional to the power disturbance ΔP and the droop coefficient k of m droop converter stations. i The sum of the reciprocals is inversely proportional; the droop coefficient k i This determines the amount of unbalanced power that the converter station will bear during dynamic adjustment; The method for incorporating the available power margin of the droop converter station into the power allocation factor is as follows: ； Among them, P isref ´ represents the optimized active power reference value for the i-th droop converter station; , These represent the active power transmission values ​​of the constant active power converter station and the constant AC voltage controlled converter station at time t+Δt, respectively. , These represent the active power transmission values ​​of the constant active power converter station and the constant AC voltage controlled converter station at time t, respectively; Δt is the sampling time; P isref This is the reference value of the active power before adjustment for the i-th droop converter station.

2. The VSC-MTDC system multi-point DC voltage zero-delay coordinated control method according to claim 1, characterized in that, The control system includes a multimeter, a data transmission module, and a calculation module. The multimeter is connected to the data transmission module, and the data transmission module is connected to the calculation module. The multimeter collects the active power value of the non-droop converter station and sends it to the calculation module through the data transmission module.

3. The VSC-MTDC system multi-point DC voltage zero-delay coordinated control method according to claim 1 or 2, characterized in that, By rationally allocating the unbalanced power of the DC system and injecting the unbalanced power into the active power reference value of the converter station, error-free regulation of DC voltage can be achieved.

4. The VSC-MTDC system multi-point DC voltage zero-delay coordinated control method according to claim 3, characterized in that, In the initial steady state, when the DC system is subjected to an unbalanced power disturbance, a feedforward compensation amount is provided to each droop converter station, the magnitude of which is the unbalanced power ΔP. i At this point, the stable operating point of the VSC-MTDC system changes from the initial stable operating point to the operating point instantaneously reached by the system under feedforward compensation power, achieving a shift in the droop curve and satisfying the requirements. Among them, P isref and These are the reference values ​​of active power before and after adjustment for the i-th droop converter station; The droop converter station absorbs unbalanced power based on its droop characteristics. The VSC-MTDC system can operate stably and recover to the vicinity of the stable operating point of the rated voltage without changing the output power, thus achieving error-free regulation of DC voltage.

5. The VSC-MTDC system multi-point DC voltage zero-delay coordinated control method according to claim 4, characterized in that, The setting range is determined based on the available power margin of the droop converter station in the VSC-MTDC system used.

6. The VSC-MTDC system multi-point DC voltage zero-delay coordinated control method according to claim 5, characterized in that, DC voltage deviation control is introduced into constant active power converter stations. When the DC voltage exceeds (U... dc l U dc h When operating within the specified range, the constant active power converter station will switch to droop control, working in conjunction with the droop converter station to absorb the remaining unbalanced power in the system and undertake the task of stabilizing the DC voltage; among which, U dc l U dc h These represent the lower and upper limits of the set range, respectively; the DC voltage deviation droop coefficient is... ; Where, k b U is the DC voltage deviation droop factor. dcref P represents the DC-side voltage reference value. bmax This is the maximum capacity of a converter station with a fixed active power rating; Let be the active power of the b-th constant active power converter station.

7. The VSC-MTDC system multi-point DC voltage zero-delay coordinated control method according to claim 6, characterized in that, The constant active power converter station adopts DC voltage deviation droop control. After the droop converter station loses its ability to control the DC voltage, let the remaining unbalanced power in the VSC-MTDC system be ΔP. * When the VSC-MTDC system reaches equilibrium again, the active power of the converter station with DC voltage deviation droop is: Among them, P j To ensure the active power of the converter station is controlled by deviation after the system stabilizes, k b Let be the droop coefficient of the b-th constant active power converter station. This is the reference value for the active power of the b-th constant active power converter station.

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