An Adaptive Droop Control Method for Dynamically Correcting DC Voltage

Abstract

This invention proposes an adaptive droop control method for dynamically correcting DC voltage, comprising: establishing a mathematical model of the voltage source converter in a two-phase rotating coordinate system based on the physical model of the voltage source converter; calculating the available power margin and DC voltage variation of the converter station based on real-time measured active power and DC voltage; incorporating the real-time power margin and DC voltage deviation into the droop coefficient based on the traditional droop control strategy of the converter station, while optimizing the active power command value; and establishing an additional DC voltage stabilizer, which automatically adjusts the active power command value during power disturbances, thereby reducing the DC voltage variation, using the DC voltage variation as input. This invention rationally allocates the unbalanced power on the DC side and approximately achieves zero-error regulation of the DC voltage, effectively improving the stability of system operation. Simulation results show that this invention outperforms conventional droop control strategies and improved droop control strategies under different simulation conditions.

Description

Technical Field

[0001] This invention relates to the technical field of multi-terminal flexible DC transmission, and in particular to an adaptive droop control method for dynamically correcting DC voltage. Background Technology

[0002] With the continuous development of power electronics technology, the voltage source converter multi-terminal direct current (VSC-MTDC) system can independently regulate active and reactive power without problems such as commutation failure. It also offers flexible transmission methods, reliable power supply, and is suitable for grid connection of renewable energy and interconnection of multi-regional power grids.

[0003] Currently, VSC-MTDC system-level control strategies mainly include master-slave control, margin control, and droop control. The master-slave control strategy primarily uses one converter station as the master to control the DC voltage, while the other converter stations act as slaves to control active power. When the master station goes out of operation, the slave stations switch operating modes to become the master to stabilize the DC voltage. This control strategy requires high inter-station communication. The margin control strategy is an improvement on the master-slave control strategy, eliminating the need for inter-station communication. However, as the DC scale increases, it suffers from drawbacks such as voltage margin setting and slow control mode switching, and is therefore generally not used in engineering. The droop control strategy does not have the above-mentioned drawbacks. It adjusts the power output of the converter stations simultaneously through DC voltage, with multiple droop stations jointly absorbing the unbalanced power of the DC network. However, converter stations using droop control suffer from unreasonable power distribution and large DC voltage deviations, affecting the stable operation of the system. In the engineering practice of voltage source converter multi-terminal flexible DC system (VSC-MTDC), droop control is widely used for autonomous DC voltage regulation and power sharing due to the advantages of multi-station coordination. Its essence is to realize the redistribution of power flow, but it will cause the problem of DC voltage deviation.

[0004] Currently, many scholars have conducted extensive research on the DC voltage deviation problem caused by unreasonable power distribution between stations. The literature [Wu Jinlong, Liu Xinhe, Wang Xianwei, Yao Weizheng. Hybrid control strategy for DC voltage in multi-terminal flexible DC transmission system [J]. Power System Technology, 2015, 39(06):1593-1599.DOI:10.13335 / j.1000-3673.pst.2015.06.020.] proposes an improved droop control strategy, which combines margin control and droop control to achieve multi-level DC voltage stability control of VSC-MTDC system. However, the switching of multiple control modes may affect the normal operation of the system. The literature [Wang Yuhong, Chen Yong, Zeng Qi, Li Jian, Wang Biao, Yang Liwen. Improved droop control for VSC-MTDC[J]. High Voltage Engineering, 2018, 44(10):3190-3196.DOI:10.13336 / j.1003-6520.hve.20180619009.] proposes to introduce the DC voltage deviation into the droop coefficient, and to smoothly regulate the DC voltage by absorbing unbalanced power through multiple droop converter stations. However, it does not conduct further research on the reasonable allocation of unbalanced power. The literature [Liu Yingpei, Xie Sai, Liang Haiping, Wang Zhengping, Xing Zhikun, Zheng Lianyue. Coordinated control strategy for VSC-MTDC system applicable to new energy grid connection [J]. Electric Power Construction, 2018, 39(11):69-76.] proposes a method of sharing the active power of the main converter station controlled by constant DC voltage by multiple converter stations, so that the main converter station is not easy to reach full load, thereby maintaining the stability of DC voltage. However, the upper and lower limits of the droop coefficient are not limited, which can easily lead to abnormal operation of the system. The literature [Zeng Qi, Li Xingyuan, Zhang Likui. Improved and optimized droop control strategy for VSC-MTDC system considering operating losses and power margin [J]. High Voltage Engineering, 2016, 42(10):3117-3125.DOI:10.13336 / j.1003-6520.hve.20160926012.] considers the losses of the flexible DC system and the power margin of each converter station to correct the droop coefficient, and analyzes the characteristics of the proposed control strategy in detail. However, the DC voltage deviation is relatively large. Summary of the Invention

[0005] To address the technical problem of existing droop control methods causing the inherent DC voltage to deviate from the initial operating point when absorbing unbalanced power, this invention proposes an adaptive droop control method that dynamically corrects the DC voltage. Under the premise of ensuring the normal operation of the DC network, the droop coefficient is automatically corrected based on the real-time power margin of the converter station and the DC voltage deviation. At the same time, a DC voltage stabilizer is added to smooth the DC voltage during the dynamic adjustment process, so that the DC voltage can be restored to a state close to the rated value when the power of the converter station fluctuates slightly.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows: an adaptive droop control method for dynamically correcting DC voltage, comprising the following steps:

[0007] Step 1: Based on the physical model of the voltage source converter, establish the mathematical model of the voltage source converter in a two-phase rotating coordinate system, and determine the measured value P of the active power of the voltage source converter station. s Active power command value P sref Measured DC voltage U dc DC voltage command value U dcref ;

[0008] Step 2: Based on the measured active power value P of the voltage source converter station s and DC voltage command value U dcref Calculate the real-time available power margin P of the voltage source converter station. can and DC voltage change U can Utilizing the available power margin P can and DC voltage change U can Calculate the droop coefficient;

[0009] Step 3: Establish an additional DC voltage stabilizer. This stabilizer takes the DC voltage change as input and outputs an active power correction value, which is applied to the active power loop in the droop control. During power disturbances, it automatically adjusts the active power command value P. sref This enables error-free regulation of DC voltage.

[0010] Preferably, the mathematical model of the voltage source converter in the two-phase rotating coordinate system is as follows:

[0011]

[0012] In the formula, U sd and U sq These are the AC grid voltage vectors U and U, respectively. s d-axis and q-axis components; U d and U q The output voltage vector U of the voltage source converter are respectively c d-axis and q-axis components; i sd and i sq denoted as d-axis and q-axis components of the grid current, respectively, and ω is the synchronous rotational angular velocity of the grid voltage vector.

[0013] Preferably, the measured active power P of the voltage source converter station is obtained in real time by using a multimeter. s Measured DC voltage U dc Active power command value P sref DC voltage command value U dcref This is already set.

[0014] Preferably, at the cost of a slight change in the output power of the voltage source converter station, the DC voltage can be restored to a state close to the initial value, thereby approximately achieving error-free correction of the DC voltage.

[0015] Preferably, the available power margin P can and DC voltage change U can The calculation methods are as follows:

[0016]

[0017]

[0018] In the formula, P max U represents the maximum power that the voltage source converter station is allowed to transmit; dcmax and U dcmin These represent the maximum and minimum allowable DC voltage values ​​of the system, ΔU and ΔU, respectively. dc P represents the change in DC voltage. s U represents the measured active power of the voltage source converter station. dcref This indicates the command value for DC voltage.

[0019] Preferably, the droop coefficient is In the formula, α represents the voltage deviation factor, which is used to ensure that the DC voltage deviation is within the DC voltage limit.

[0020] Preferably, the voltage deviation factor is Where sgn represents the sign function;

[0021] When the DC voltage change ΔU dc When the voltage deviation factor α is small, it is close to 0.5, which is equivalent to reducing the proportion of DC voltage control in adaptive droop control, thereby improving the power distribution capability; when the DC voltage change ΔU dc When the value is large, the voltage deviation factor α also increases accordingly, improving the stability of DC voltage.

[0022] Preferably, in a VSC-MTDC system, there are N voltage source converter stations employing droop control. When a power disturbance ΔP occurs in the DC network, each voltage source converter station automatically finds a new equilibrium point along the droop curve. The unbalanced power ΔP borne by voltage source converter station i is then... i for:

[0023]

[0024] Where 1≤i≤N, ΔU dc k represents the change in DC voltage. i Let i be the droop coefficient of the voltage source converter station i;

[0025] The sum of the unbalanced power handled by each droop controller should equal the unbalanced power ΔP of the DC network, that is:

[0026]

[0027] but

[0028] The unbalanced power borne by each converter station is determined by its real-time power margin. When the real-time power margin of the converter station is large, the unbalanced power it bears is greater, which effectively avoids the problem of converter station overload caused by excessive unbalanced power in the DC network.

[0029] Preferably, the additional DC voltage stabilizer is implemented by taking the DC voltage change as input, continuously optimizing the active power command value during power disturbance, reducing the DC voltage change, and approximately achieving error-free DC voltage regulation.

[0030] Preferably, in the DC voltage change ΔU dc A voltage dead zone was added later; at the instant of power disturbance, the DC voltage change ΔU dc The droop controller is relatively large, with the DC voltage change ΔU as the metric. dc As input, during power disturbances, the DC voltage deviation is reduced by adjusting the active power command value, which is the active power command value P of the voltage source converter station. sref The update provides sufficient regulation; at the end of the dynamic regulation process, the DC voltage change ΔU dc It is almost zero and no longer participates in system regulation.

[0031] Compared with existing technologies, the beneficial effects of this invention are as follows: by introducing real-time power margin and DC voltage deviation of the converter station to automatically correct the droop coefficient, the unbalanced power of the DC network is rationally allocated. Simultaneously, a DC voltage stabilizer is designed to restore the DC voltage to near its rated value. Finally, a simulation model of a five-terminal DC transmission system is built in PSCAD / EMTDC and compared with two other control strategies. The simulation results confirm the effectiveness of the control strategy of this invention. Simulation results show that this invention outperforms conventional and improved droop control strategies under different simulation conditions. Furthermore, it minimizes the time required for the system to transition from transient to steady state. The optimized droop coefficient balances minimal changes in VSC power with optimal DC voltage regulation. Moreover, simulation results demonstrate that the DC voltage stabilizer effectively improves the DC voltage deviation problem during dynamic processes. Attached Figure Description

[0032] 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.

[0033] Figure 1 This is a block diagram illustrating the principle of optimized adaptive droop control for the additional DC voltage stabilizer in this invention.

[0034] Figure 2 This is a structural diagram of a five-terminal flexible DC system.

[0035] Figure 3 This is the physical model of a voltage source converter.

[0036] Figure 4 This is a block diagram of the VSC-MTDC dual-loop control.

[0037] Figure 5 This is a schematic diagram of a DC voltage droop controller.

[0038] Figure 6 This is a graph showing the change in the operating point for droop control.

[0039] Figure 7 for Figure 2 Simulation results of power boosting of VSC3 in medium voltage source converter station, where (a) is the active power of VSC1, (b) is the active power of VSC2, (c) is the active power of VSC3, VSC4 and VSC5, and (d) is the DC voltage of the system.

[0040] Figure 8 for Figure 2 Simulation results of power reduction in VSC4 of medium voltage source converter station, where (a) is the active power of VSC1, (b) is the active power of VSC2, (c) is the active power of VSC3, VSC4 and VSC5, and (d) is the DC voltage of the system.

[0041] Figure 9 for Figure 2 Simulation results of VSC2 disconnection from medium voltage source converter station, where (a) is the active power of VSC1, (b) is the active power of VSC2 and VSC3, (c) is the active power of VSC3, (d) is the active power of VSC4, and (e) is the DC voltage of the system. Detailed Implementation

[0042] 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.

[0043] An adaptive droop control method for dynamically correcting DC voltage, such as Figure 1 As shown, the steps are as follows:

[0044] Step 1: Based on the physical model of the voltage source converter, establish the mathematical model of the voltage source converter in a two-phase rotating coordinate system, and determine the measured value P of the active power of the voltage source converter station. s Active power command value P sref Measured DC voltage U dc DC voltage command value U dcref .

[0045] The flexible DC transmission system studied in this invention is as follows: Figure 2 As shown, in a five-terminal flexible DC system, the DC sides of five voltage source converter stations are connected in parallel via a DC network, while their AC sides are connected to their respective independent AC networks. The power transmitted by each voltage source converter station in this system is referenced to the positive direction of the injected AC network. All VSCs in the system have identical structures, and their steady-state physical models are as follows: Figure 3 As shown.

[0046] The mathematical model of a voltage source converter (VSC) in a two-phase rotating coordinate system is as follows:

[0047]

[0048] In the formula: U sd and U sq These are the AC grid voltages U s d-axis and q-axis components; U d and U q The output voltage U of VSC are respectively c d-axis and q-axis components; i sd and i sq denoted as d-axis and q-axis components of the grid current, respectively, and ω is the synchronous rotational angular velocity of the grid voltage vector.

[0049] The overall system control structure includes outer loop control and inner loop control. Outer loop control is divided into active power control and reactive power control, achieving tracking of the control quantity to a reference value. Inner loop control primarily tracks the current reference value output from the outer loop and converts it into a voltage signal, generating PWM pulses to control the switching on and off of power electronic devices in the voltage source converter station. The dual-loop control structure of VSC-MTDC is as follows: Figure 4 As shown. Constant DC voltage control is used to maintain voltage stability on the DC side and balance the active power of the DC system. When the load of the AC system changes, it causes frequency deviation. Constant AC frequency control can optimize the active power command value and achieve stable frequency control. Constant AC voltage control can achieve zero steady-state error tracking of AC voltage by controlling the AC voltage.

[0050] The basic principle of droop control is a control method based on the droop characteristic of the DC voltage in a voltage source converter station as it changes with active power. The structure of the droop controller is as follows: Figure 5 As shown. P sref and P s These represent the commanded and measured active power values, respectively; U dcref and U dc These represent the commanded DC voltage value and the measured DC voltage value, respectively; k is the droop coefficient; PI represents the proportional-integral controller.

[0051] according to Figure 5 The relationship between the DC voltage and active power of the droop controller in the control block diagram can be expressed as:

[0052]

[0053] As shown in equation (2), the magnitude of the droop coefficient k directly relates to the power distribution capability of the voltage source converter station. A larger droop coefficient k indicates stronger power control capability but weaker DC voltage stabilization capability. Conversely, a smaller droop coefficient k indicates stronger DC voltage rigidity, resulting in smaller DC voltage deviation under power disturbances but weaker power distribution capability. Therefore, current research on droop control focuses primarily on the selection of the droop coefficient. Generally, the droop coefficient of each converter station is inversely proportional to its capacity; that is, a larger capacity converter station handles more unbalanced power.

[0054] Assume a VSC-MTDC system has N voltage source converter stations employing droop control. When a power disturbance ΔP occurs in the DC network, each voltage source converter station automatically seeks a new equilibrium point along the droop curve. Let the DC voltage change at steady state be ΔU. dc Then the unbalanced power ΔP borne by voltage source converter station i (1≤i≤N) i for:

[0055]

[0056] Where, k i Let be the droop coefficient of voltage source converter station i.

[0057] To ensure power balance in the DC network, the sum of the unbalanced power handled by each droop controller should equal the unbalanced power ΔP of the DC network, i.e.:

[0058]

[0059] Combining equations (3) and (4), ΔU in equation (4) dc Substituting into equation (3), we get

[0060]

[0061] As can be seen from equation (5), the unbalanced power borne by the droop controller during dynamic adjustment is inversely proportional to its droop coefficient. That is, the larger the droop coefficient of the voltage source converter station, the smaller the unbalanced power it bears. Under normal circumstances, the droop coefficient of the voltage source converter station is inversely proportional to its rated capacity, thereby ensuring that the larger capacity converter station bears more unbalanced power. When the conventional droop control strategy is adopted, since the droop coefficient of each converter station is a fixed value, after a fault occurs, each converter station can only share the unbalanced power according to the fixed droop coefficient. When the fault is more serious, it will lead to excessive DC voltage, or even exceed the limit. The infeasibility of DC voltage error-free correction without power variation is analyzed theoretically, and an adaptive droop control strategy with quasi-error-free DC voltage correction is proposed.

[0062] like Figure 6 As shown, assuming the system initially operates at operating point 1, after being subjected to load disturbance, operating point 1 searches for an equilibrium point along a drooping curve. Let's assume the system reaches equilibrium and operates stably at point 2. At this point, the DC voltage change is ΔU. dc The power change of voltage source converter station 1 is ΔP1, and the power change of voltage source converter station 2 is ΔP2.

[0063] In a VSC-MTDC system, the output active power of the voltage source converter station can also be expressed as...

[0064]

[0065] In the formula, R ij P represents the line resistance between voltage source converter station i and voltage source converter station j. i1 U dci1 U dcj1 These represent the measured active power, measured DC voltage of converter station i, and measured DC voltage of converter station j, respectively.

[0066] according to Figure 6 It can be concluded that

[0067]

[0068]

[0069] Combining equations (6) and (7), we can obtain

[0070]

[0071] Combining equations (8) and (9), we can obtain

[0072]

[0073] From equation (10), we can conclude that ΔP1 equals 0, and from... Figure 6 It can be seen that due to the load disturbance, the system operating point changes, and ΔP1 is not equal to 0. Therefore, it can be concluded that when the output power of the voltage source converter station remains unchanged, the system operating point cannot move from point 2 to point 3, that is, it is impossible to achieve error-free correction of the DC voltage. However, at the cost of a slight change in the output power of the voltage source converter station, the DC voltage can be restored to a state close to its initial value, thus approximately achieving error-free correction of the DC voltage.

[0074] Step 2: Measured active power P of the voltage source converter station s and DC voltage command value U dcref Calculate the real-time available power margin P of the voltage source converter station can and DC voltage change U can Real-time available power margin P can and DC voltage change U can Calculate the droop coefficient.

[0075] The above demonstrates that it is impossible to achieve error-free DC voltage correction when the active power of the power source converter station remains constant. To address the DC voltage deviation problem, this invention incorporates the real-time power margin of the voltage source converter station and the DC voltage deviation value into the droop coefficient, defining the real-time available power margin of the converter station as P. can The change in DC voltage is U can Represented as

[0076]

[0077]

[0078] In the formula: P max This represents the maximum power that the voltage source converter station is allowed to transmit, which is set when the converter station is built; U dcmax and U dcmin These represent the maximum and minimum allowable DC voltage values ​​for the system, typically taken as 10% of the commanded DC voltage value. ΔU dc To achieve the DC voltage change in steady state, P sThis represents the measured active power value of the voltage source converter station, which was set when the converter station was built. (U) dcref This indicates the DC voltage command value, which can be measured using a multimeter in the converter.

[0079] The droop coefficient k of the adaptive droop control strategy can be expressed as:

[0080]

[0081] In the formula, α represents the voltage deviation factor, used to ensure that the DC voltage deviation is within the DC voltage limit. The voltage deviation factor α is defined as...

[0082]

[0083] From equation (14), it can be seen that when the DC voltage change ΔU dc When the voltage deviation factor α is small, it approaches 0.5, which is equivalent to reducing the proportion of DC voltage control in adaptive droop control, thereby improving the power distribution capability. When the DC voltage deviation ΔU dc When the value is large, the voltage deviation factor α also increases accordingly, improving the stability of DC voltage.

[0084] According to equation (14), the voltage deviation factor α is only related to the DC voltage change ΔU dc Yes, there is a relationship. Without considering transmission line impedance and inter-converter station errors, the DC voltage variation at each converter station is the same, and the voltage deviation factor α is considered equal. According to equation (12), the DC voltage variation is U. can The expression is the same for all converter stations. Substituting equation (13) into equation (5), we get...

[0085]

[0086] As can be seen from equation (15), under the adaptive droop control strategy, the unbalanced power borne by each converter station is determined by its real-time power margin. When the real-time power margin of the converter station is large, the unbalanced power it bears is larger, which effectively avoids the problem of converter station overload caused by excessive unbalanced power in the DC network.

[0087] Step 3: Establish an additional DC voltage stabilizer, which operates on the active power loop in the droop control. Taking the DC voltage change as input, the output is the active power correction value. During power disturbances, it automatically adjusts the active power command value, approximately achieving zero-error DC voltage regulation. The proportional-integral controller output provides the d-axis current reference value.

[0088] To address the issue that improved droop control can only regulate DC voltage after the voltage source converter station has stabilized, this invention designs a DC voltage controller to improve the DC voltage deviation problem during power regulation. The DC voltage change ΔU is used as the... dc As input, the DC voltage deviation is reduced by adjusting the power reference value during power disturbances. Considering equations (11), (12), (13), and (14), the optimized adaptive droop control with an additional DC voltage stabilizer is as follows: Figure 1 As shown. An additional DC voltage stabilizer is used to adjust the DC voltage change ΔU. dc The input is active power, and the output is active power correction. During power disturbances, the active power command value is automatically adjusted to achieve near-error-free regulation of DC voltage.

[0089] like Figure 1 As shown, to avoid frequent operation of the droop controller due to small fluctuations in DC voltage, the DC voltage change ΔU dc A voltage dead zone was added later. At the instant of power disturbance, the DC voltage change ΔU dc Typically large, it can be the active power command value P of a voltage source converter station. sref The update provides sufficient regulation. At the end of the dynamic regulation process, the DC voltage change ΔU dc It is almost zero and no longer participates in system regulation. According to Figure 1 After a power disturbance occurs, the DC voltage change is generally large, dΔU dc / dt can provide sufficient adjustment for updating the active power command value. When the DC voltage deviation ΔU dc When dΔU is 0, dc / dt is also 0, so it no longer participates in system regulation.

[0090] To verify the effectiveness of the control strategy proposed in this invention, a system was constructed in PSCAD / EMTDC as follows. Figure 2 The five-terminal DC transmission model is shown. VSC1 and VSC2 employ optimized adaptive control, VSC3 and VSC4 employ conventional droop control, and VSC5 employs constant AC voltage control. The AC side grid is an equivalent system with an AC voltage of 220kV. Furthermore, the simulation data of this invention is processed using per-unit values: per-unit value = measured value / reference value; the calculation results are clear, facilitating the judgment of the correctness of the calculation results, and the simulation images can also be clearly displayed. The active power reference value is 200MW, and the DC voltage reference value is 400kV. The system simulation parameters are shown in Table 1.

[0091] To fully illustrate the advantages of the control strategy proposed in this invention, a comparative study of the following control strategies is conducted.

[0092] 1) Control strategy 1 (CM1) is the adaptive droop control strategy proposed in this invention.

[0093] 2) Control strategy 2 (CM2) is a conventional droop control strategy.

[0094] 3) Control strategy 3 (CM3) is an improved droop control strategy based on the initial power margin of the converter station proposed in the literature [Zhu Ruike, Li Xingyuan, Wu Feng. Improved droop control strategy for VSC-MTDC system considering power margin [J]. Journal of Sichuan University (Engineering Science Edition), 2015, 47(03):137-143.DOI:10.15961 / j.jsuese.2015.03.019.].

[0095] Table 1 Main parameters of VSC-MTDC system

[0096]

[0097] At t=5s, the active power command value of voltage source converter station VSC3 increased from 0.575pu to 0.9pu, as shown in the simulation results. Figure 7 As shown in Table 2.

[0098] Table 2 Simulation results of DC voltage deviation

[0099]

[0100] like Figure 7 (a) and Figure 7 As shown in (b), due to system losses and differences in the initial droop coefficient, the actual active power of VSC1 and VSC2 in CM1, CM2, and CM3 is close to the power command value under the initial state, but slightly different. At t = 5s, the active power command value of VSC3 suddenly increases, which is equivalent to a power deficit in the DC grid, and the DC voltage gradually decreases.

[0101] Under CM2, while power balance in the DC transmission network can be achieved, the power imbalance distribution is not ideal, resulting in VSC1 operating near full load while VSC2 still has a significant power margin, and a large deviation in DC voltage. Under CM3, VSC1 and VSC2 employ improved droop control, with the droop coefficient corrected using the initial power margin value of the converter station, resulting in a more reasonable unbalanced power distribution. At this point, VSC1 still has a large available power margin to cope with network power fluctuations, and the DC voltage deviation is relatively small.

[0102] Under CM1, VSC1 and VSC2 employ adaptive droop control. The droop coefficient is automatically corrected based on the real-time power margin and DC voltage deviation of the converter station, effectively preventing overload caused by unbalanced power distribution. Figure 7 (d) It can be seen that the DC voltage deviation value of mode 1 is the smallest and closer to the DC voltage command value, which approximately achieves error-free regulation of DC voltage.

[0103] At t=4s, the active power command value of VSC4 decreased from 0.425pu to 0.075pu, as shown in the simulation results. Figure 8 As shown in Table 3.

[0104] Table 3 Simulation results of DC voltage deviation

[0105]

[0106] like Figure 8 As shown, after the power command value of VSC4 changes by 4 seconds, it is equivalent to a power surplus in the system, and the DC voltage gradually increases. In CM2, when a power surplus occurs in the DC network, VSC1 and VSC2 absorb the unbalanced power according to the pre-set droop coefficients, and the unbalanced power they bear are 0.115 pu and 0.235 pu, respectively, with a DC voltage deviation rate of 1.75%. In CM3, after correcting the droop coefficient through the initial power margin of the converter station, VSC1 has a larger power margin than VSC2, so the unbalanced power in the DC network is mainly borne by VSC1. The unbalanced power borne by VSC1 and VSC2 is 0.22 pu and 0.13 pu, respectively, with a DC voltage deviation rate of 0.49%.

[0107] In CM1, the power command values ​​of VSC1 and VSC2 are automatically corrected according to the droop coefficient calculated by formula (13), bearing unbalanced power values ​​of 0.18 pu and 0.17 pu respectively, with a DC voltage deviation rate of 0.21%. Figure 8 As can be seen from (a), (b) and (d), CM1 has a faster adjustment speed and the shortest time from transient to steady state.

[0108] Any MTDC system should satisfy the N-1 principle, meaning that the shutdown of any single converter station should not significantly impact the stability of the entire system. At t=3s, VSC2 shuts down, and the simulation results are as follows... Figure 9 As shown in Table 4.

[0109] Table 4 Simulation results of DC voltage deviation

[0110]

[0111] like Figure 9As shown, before the fault, VSC1 and VSC2 jointly maintained the power balance of the DC network. At t=3s, as VSC2 exited, its transmission power dropped to zero, and the unbalanced power caused the system DC voltage to drop rapidly. Under CM2, after VSC2 exited, VSC1, as the sole power balance point, quickly reached full load and switched to constant active power operation, losing its ability to control the DC voltage, and the DC voltage continued to drop. Under CM3, when VSC1 exited full load operation, VSC3 and VSC4, as backup stations, jointly bore the remaining unbalanced power on the DC network, effectively suppressing the continuous drop in DC voltage. At this time, the DC voltage deviation rate was 5.54%.

[0112] Under CM1, when VSC2 stops operating, the available power margin of VSC1 is insufficient to absorb the unbalanced power in the network. At this time, the unbalanced power in the network is shared by VSC1 and the backup station VSC3, with VSC1 and VSC3 sharing 0.1 pu and 0.4 pu respectively. VSC1 still has a certain power margin to stabilize the system DC voltage. After the system enters steady state, the DC voltage stabilizes at 394.68 kV with a deviation rate of 1.28%, which is the smallest DC voltage deviation compared to CM2 and CM3.

[0113] 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. An adaptive droop control method for dynamically correcting DC voltage, characterized in that, The steps are as follows: Step 1: Based on the physical model of the voltage source converter, establish the mathematical model of the voltage source converter in a two-phase rotating coordinate system, and determine the measured active power value of the voltage source converter station. Active power command value Measured DC voltage U dc DC voltage command value ; Step 2: Based on the measured active power values ​​of the voltage source converter station and DC voltage command value Calculate the real-time available power margin P of the voltage source converter station. can and DC voltage change U can Utilizing the available power margin P can and DC voltage change U can Calculate the droop coefficient; Step 3: Establish an additional DC voltage stabilizer. This stabilizer takes the DC voltage change as input and outputs an active power correction value, which is applied to the active power loop in the droop control. During power disturbances, it automatically adjusts the active power command value. This enables error-free regulation of DC voltage; The available power margin P can and DC voltage change U can The calculation methods are as follows: ； ； In the formula, P max U represents the maximum power that the voltage source converter station is allowed to transmit; dcmax and U dcmin These represent the maximum and minimum allowable DC voltage values ​​of the system, ΔU and ΔU, respectively. dc This represents the change in DC voltage. This represents the measured value of active power at the voltage source converter station. Indicates the command value of DC voltage; The droop coefficient is In the formula, α represents the voltage deviation factor, which is used to ensure that the DC voltage deviation is within the DC voltage limit. The voltage deviation factor is ;in, Represents a symbolic function.

2. The adaptive droop control method for dynamically correcting DC voltage according to claim 1, characterized in that, The mathematical model of the voltage source converter in the two-phase rotating coordinate system is as follows: ； In the formula, U sd and U sq These are the AC grid voltage vectors U and U, respectively. s d-axis and q-axis components; U d and U q The output voltage vector U of the voltage source converter are respectively c d-axis and q-axis components; i sd and i sq denoted as d-axis and q-axis components of the grid current, respectively, and ω is the synchronous rotational angular velocity of the grid voltage vector.

3. The adaptive droop control method for dynamically correcting DC voltage according to claim 2, characterized in that, The measured active power P of the voltage source converter station was obtained in real time by multimeter measurement. s Measured DC voltage U dc Active power command value P sref DC voltage command value U dcref This is already set.

4. The adaptive droop control method for dynamically correcting DC voltage according to claim 2 or 3, characterized in that, Suppose there are N voltage source converter stations using droop control in a VSC-MTDC system. When a power disturbance ΔP occurs in the DC network, each voltage source converter station automatically finds a new equilibrium point along the droop curve. Then, the unbalanced power ΔP borne by voltage source converter station i is... i for: ； Where 1≤i≤N, ΔU dc k represents the change in DC voltage. i Let i be the droop coefficient of the voltage source converter station i; The sum of the unbalanced power handled by each droop controller should equal the unbalanced power of the DC network. ,Right now: ； but ; The unbalanced power borne by each converter station is determined by its real-time power margin. The larger the real-time power margin of the converter station, the greater the unbalanced power it bears, effectively avoiding the problem of converter station overload caused by excessive unbalanced power in the DC network.

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