Reactive power control system and method

Abstract

The invention discloses a reactive power control system and method. The reactive power control system comprises an energy management and control system, a regional central station and a flexible direct current converter, the energy management and control system is used for managing at least one regional central station; the regional central stations are configured corresponding to power supply intervals, and each regional central station is used for managing at least one flexible DC converter located in the power supply interval; the flexible direct current converter is a flexible direct current converter in a special power supply system and is connected to a power grid, and the energy management and control system controls the flexible direct current converter to output reactive power to the power grid through the regional central station. According to the technical scheme provided by the invention, the residual capacity of the flexible DC converter is utilized to provide reactive compensation for the power grid, the construction cost of dynamic reactive equipment is effectively reduced through the method, and the voltage stability of the power grid is improved.

Description

Technical Field

[0001] This application relates to the field of power technology, and in particular, to a reactive power control system and method. Background Art

[0002] In large cities, residential electricity consumption has become the main body of the electricity load. The power load, especially the motor load represented by air conditioner compressors, has been increasing year by year, which poses a severe challenge to the voltage stability of the power grid. Voltage is an important indicator to measure the stability of the power grid. The stalling and blocking of induction motor loads after a fault are the main reasons for the delayed recovery of the system voltage (FIDVR), and in severe cases, it may even lead to voltage collapse, posing a major threat to the voltage stability of the power grid. Voltage reactive power regulation is the most direct and effective method to solve the problem of voltage instability. Ensuring that the power grid has sufficient reactive power reserve has become the key to the stable operation of the power system.

[0003] The most widely used reactive power compensation devices include the automatic voltage regulator (AVR) of generators, static reactive power compensation devices such as capacitor banks (CB), and dynamic reactive power compensation such as static var compensators (SVC) and static synchronous compensators (STATCOM). However, large cities generally have receiving-end power grids, and the distance between the generating units and the load center is relatively far. The action rate of capacitor banks is relatively slow (in minutes), which does not match the time scale of dynamic voltage reactive power. The cost of dynamic reactive power compensation devices is relatively high, and they are usually centralized compensation, unable to achieve global voltage control. Summary of the Invention

[0004] In view of the above technical problems, this application provides a reactive power control system and method, and the technical solutions are as follows:

[0005] According to the first aspect of this application, a reactive power control system is provided. The system includes: an energy management and control system, a regional central station, and a flexible DC converter.

[0006] The energy management and control system is used to manage at least one regional central station.

[0007] The regional central station is configured corresponding to a power supply interval, and each regional central station is used to manage at least one flexible DC converter located in the power supply interval.

[0008] The flexible DC converter is a flexible DC converter in a dedicated power supply system and is connected to the power grid. The energy management and control system controls the flexible DC converter to output reactive power to the power grid through the regional central station.

[0009] According to the first aspect of this application, a reactive power control method is provided. The method includes:

[0010] The flexible DC converter reports its own status to the regional central station.

[0011] The regional central station aggregates the status reports from each flexible DC converter and reports the aggregation results to the energy management and control system;

[0012] Based on the aggregation results reported by each regional central station, the energy management and control system calculates the reactive power demand for each regional central station using a prediction model and sends the overall reactive power demand for each regional central station to the corresponding regional central station respectively;

[0013] The regional central station determines the reactive power demand for each flexible DC converter according to the overall reactive power demand and issues the reactive power demand for each flexible DC converter to the corresponding flexible DC converter respectively;

[0014] The flexible DC converter injects reactive power into the power grid according to the received reactive power demand.

[0015] The technical solution provided by this application uses a dedicated power supply system, such as an urban rail traction power supply system, to compensate the reactive power of the power grid. Specifically, the remaining capacity of the flexible DC converter can be used to provide reactive power compensation for the power grid. By this method, the construction cost of dynamic reactive power equipment can be effectively reduced, and the voltage stability of the power grid can be improved.

[0016] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit this application. In addition, any embodiment in this application does not need to achieve all the above effects. Description of the Drawings

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in this application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

[0018] Figure 1 is a schematic diagram of the architecture of the distributed power supply flexible DC traction power supply system of this application;

[0019] Figure 2 is a schematic diagram of the relationship between the active power and reactive power capacity of the flexible DC converter provided by this application;

[0020] Figure 3 is a schematic diagram of the architecture of the reactive power control system provided by this application;

[0021] Figure 4 is a schematic diagram of the relationship between the converter coordinate system and the common coordinate system provided by this application;

[0022] Figure 5 is a schematic diagram of the process of the reactive power control method provided by this application;

[0023] Figure 6 is a schematic diagram of the basic grid structure of the load center for testing in this application;

[0024] Figure 7 is a schematic diagram for comparing the slip rate change curves of motor loads under centralized and hierarchical control architectures;

[0025] Figure 8 is a comparison of the grid voltage change curves under centralized and hierarchical control architectures. Detailed implementation manners

[0026] To enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be described in detail below in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art shall fall within the protection scope of this application.

[0027] For urban rail transit, a dedicated power supply system is generally required to convert grid power distribution into electrical energy that meets the operating requirements of urban rail. This system is called the urban rail traction power supply system. In recent years, flexible power transmission technology has begun to be gradually applied to urban rail transit, resulting in a flexible DC traction power supply system (flexible DC traction power supply system). The flexible DC traction power supply system uses a flexible converter and can achieve decoupled control of active and reactive power, with high controllability. Based on the above background, this application proposes to use the flexible DC traction power supply system of urban rail to perform reactive power compensation on the grid. Specifically, the remaining capacity of the flexible converter can be used to provide reactive power compensation for the grid, effectively reducing the construction cost of dynamic reactive power equipment and improving the voltage stability of the grid through this method.

[0028] The flexible DC traction power supply system includes a centralized power supply system and a distributed power supply system. Figure 1 The following shows the system architecture of a typical distributed power supply system: A subway line can be divided into multiple power supply intervals, and each interval has several flexible converters. The 10 kV distribution network supplies power to all the flexible converters in the power supply area through the main substation. Through the flexible converter, 10 kV alternating current is converted into 1500 V / 750 V direct current to supply the train.

[0029] Compared with the centralized power supply system, in the distributed power supply system, each flexible converter is directly connected to the grid without passing through the main converter, which enables each flexible converter to provide reactive power compensation for the grid alone, thus providing a large-capacity, low-loss distributed reactive power source for the grid.

[0030] In the specific embodiments of the present application, further consideration will be given to how to optimize the reactive power compensation between the traction network and the power grid:

[0031] First, analyze the reactive power capacity output by the flexible DC converter. The output power constraint of the flexible DC converter is:

[0032]

[0033] Where S max is the rated capacity of the flexible DC converter, and P 0 and Q 0 are its active power and reactive power respectively.

[0034] When the train suddenly accelerates or decelerates, if the active power of the flexible DC converter changes by ΔP, its reactive power capacity will also change by ΔQ accordingly:

[0035]

[0036] Because the power change is not large, the high-order terms can be ignored. Then the relationship between the reactive power capacity and the change in active load can be approximated as:

[0037]

[0038] This indicates that the maximum reactive power capacity of the flexible DC converter is related to its active load, and the relationship between its output active power and reactive power capacity is as Figure 2 shown.

[0039] When the train is coasting, the active load of the flexible DC converter is low, so the reactive power that can be output is high, that is, the light-colored area in Figure 2 . If a small number of trains accelerate or decelerate, the active load of the flexible DC converter increases, but the change in reactive power capacity is very small, that is, the intermediate-color area in Figure 2 . However, when multiple trains accelerate or brake simultaneously, the flexible DC converter is at a high load level, the reactive power capacity is very low, and it is severely affected by the change in active power, that is, the dark-colored area in Figure 2 . At this time, the change in the output reactive power ability of the flexible DC converter is large, which will have a greater impact on the effect of voltage reactive power control.

[0040] To solve the problem of the flexible DC traction power supply system participating in the power grid voltage control, the present invention proposes a hierarchical dynamic voltage reactive power control architecture, as Figure 3 shown. This architecture consists of three layers: the Energy Management and Control System (EMCS) layer, the regional central station layer, and the flexible DC converter layer.

[0041] Among them, the regional central station layer includes M regional central stations 200, corresponding to M power supply intervals, and the M regional central stations are uniformly managed by the energy control system 100; each flexible converter 300 located in the flexible converter layer is naturally grouped according to the power supply interval division, and the flexible converters 300 in each power supply interval are uniformly managed by the regional central station 200 of that interval and communicate with the energy control system 100 through the regional central station 200.

[0042] Energy control system 100:

[0043] Responsible for the global communication and coordination of the flexible DC traction power supply system. In the energy control system, a global dynamic voltage and reactive power control model is established using model predictive control (MPC). Model predictive control is a closed-loop control that can effectively resist system disturbances and overcome system uncertainties and is widely used in the voltage and reactive power control of power grids. Through model predictive control, the optimal control problem with constraints can be solved and it is applicable to linear and non-linear systems.

[0044] The following will illustrate the specific implementation of a prediction model adopted in this application:

[0045] First, a small-signal model is established for the q-axis of the voltage-source converter. During the modeling process, the current inner loop is replaced by a first-order delay link, and the state equation of the converter is obtained as:

[0046]

[0047] Where T d and T q are the d-axis and q-axis inertia time constants respectively, k p and k i are the PI parameters of the reactive power control loop, Δi q and x b are state variables, and (ΔQ ref -ΔQ) is the input variable.

[0048] A linear relationship between the controlled voltage and the reactive current output of the converter is established using the sensitivity coefficient:

[0049] ΔV i,d =-s i,j Δi qj

[0050] Where ΔV i,d is the controlled voltage, s i,j is the voltage-reactive current sensitivity coefficient, and Δi qj is the reactive current output of the flexible converter.

[0051] Combining the state equation and the output equation gives the state - space equation, and discretizing it using the zero - order hold method results in the discrete state - space equation:

[0052]

[0053] where the state variable is Δx = [Δi q1 , Δx b1 ,..., Δi qL , Δx bL T ,

[0054] the control variable is Δu = [ΔQ ref1 - ΔQ 1 ,..., ΔQ refL - ΔQ L T ,

[0055] the output variable is Δy = [ΔV 1,d ,..., ΔV N,d T , and the coefficient matrices A d , B d , C d are defined as follows:

[0056]

[0057] Define the output matrix and the control matrix of the prediction model as:

[0058] ΔU = [Δu(k), Δu(k + 1),..., Δu(k + N c - 1)] T

[0059] ΔY = [Δy(k + 1), Δy(k + 2),..., Δy(k + N p )] T

[0060] where N p and N c are the prediction horizon and the control horizon respectively, Δu(k + i) is the (k + i)-th input, and Δy(k + i) is the predicted output at time k + i.

[0061] Then, according to the state - space equation, the relationship between the output matrix and the control matrix can be recursively obtained as:

[0062] ΔY = FΔx(k)+ΦΔU

[0063] where the coefficient matrices F and Φ are calculated as follows:

[0064] ​​​

[0065] The objectives of dynamic voltage control include two parts:

[0066] 1) The node voltage tracks its reference value;

[0067] 2) The VSC output reactive power is made as small as possible;

[0068] Therefore, the objective function can be expressed in the following form:

[0069] minJ = ΔY T QΔY + ΔU T RΔU

[0070] where Q and R are the output deviation weight matrix and the control cost weight matrix respectively.

[0071] During the dynamic voltage control process, it is required that the VSC output reactive power does not exceed the capacity limit. Therefore, we have:

[0072] ΔU ≤ [ΔQ max (k), ΔQ max (k + 1),..., ΔQ max (k + N c - 1)] T

[0073] The control model is arranged into the structure of quadratic programming (QP):

[0074] minJ = ΔU T (Φ T QΦ + R)ΔU + 2Δx(k) T F T QΦΔU + Δx(k) T F T QFΔx(k)

[0075] where Δx(k) is the state of the VSC at time k, and ΔU is the reactive power command sequence of the VSC to be solved.

[0076] In the application stage, according to the state of the VSC traction power supply system at the current time k (including the active / reactive power of the VSC and its voltage and current, etc.), the control model is solved by the interior point method to obtain the control sequence ΔU. Among them, when the energy management and control system is connected to multiple regional central stations, the above-mentioned state Δx(k) of the VSC traction power supply system specifically refers to the aggregation result of the states of each power supply section, and the output result includes the control action sequences for each regional central station respectively.

[0077] In a specific embodiment of the present application, after the energy management and control system obtains the control action sequence ΔU for the current moment k, only the first control action is sent to the regional central station for execution. At the next moment k + 1, the latest VSC-HVDC converter state Δx(k + 1) is used as the initial state to solve again, that is, dynamic voltage and reactive power control is realized through rolling optimization and feedback correction.

[0078] In addition, in a specific embodiment of the present application, trajectory sensitivity is used to linearize the relationship between the grid voltage and the reactive current output by the VSC-HVDC converter, eliminating the nonlinearity of the system.

[0079] Regional central station 200:

[0080] Since the VSC-HVDC converters in the power supply area have the same AC bus (main distribution substation), the VSC-HVDC converters in the same power supply area can be regarded as a whole for control during the dynamic voltage and reactive power control process. Furthermore, the overall reactive power demand for a certain power supply area is calculated by the energy management and control system, rather than the reactive power demand for a single VSC-HVDC converter.

[0081] The regional central station in each power supply area is responsible for the coordination between the energy management and control system and the VSC-HVDC converter. That is to say, the reactive power demand for each region calculated by the energy management and control system will be sent to the corresponding regional central station respectively, and the regional central station will then distribute the overall reactive power demand of the power supply area to each VSC-HVDC converter in the power supply area.

[0082] In addition, the regional central station is also responsible for aggregating the states of all VSC-HVDC converters in the region and sending the aggregated result to the energy management and control system for solution. By dynamically aggregating multiple VSC-HVDC converters into a whole, the dimension of the control model is reduced and the solution speed is accelerated.

[0083] To aggregate the reactive power control model, the most crucial thing is to aggregate the control quantity ΔQ ref -ΔQ and the state quantity Δi q First, aggregate the state quantity Δi q Convert the converter coordinate system to the common coordinate system. The relationship between the converter coordinate system and the common coordinate system is shown in Figure 4 As shown, the q-axis current is:

[0084] i q = cosα 1 i c1q + sinα 1 i c2q + cosα 1 i c1d + sinα 2 i c2d

[0085] where \(i_d\) cd and \(i_q\) cq are the d-axis and q-axis currents of the VSC-HVDC converter respectively.

[0086] Assuming that the active current remains unchanged during the reactive power control process, the aggregated result of the small-signal q-axis current is:[[]]

[0087]

[0088] Secondly, aggregating the control variables \(\Delta Q\) ref and \(-\Delta Q\), and ignoring the power loss on the transmission line, we have:[[]]

[0089] \(\Delta Q = \Delta Q_1\) 1 +\Delta Q_2 2

[0090] After obtaining the aggregation of the control variables and the state variables, the final aggregated model can be obtained. The structure of the aggregated model is exactly the same as the reactive power control structure of a single converter, and the calculation method of the aggregated parameters is:[[]]

[0091]

[0092] where the matrices A, B, and C are defined as follows:[[]]

[0093] A = [\(\cos\alpha\) 1 \(\cos\alpha\) 1

[0094]

[0095] Since the control model established at the energy management and control system layer is based on the aggregated model, the reactive power demand solved is for the entire power supply area. Therefore, an important role of the regional central station is to allocate and calculate the reactive power demand of each VSC-HVDC converter. Since parameters such as the length, gradient, and passenger flow of different electric sections in the same energized section are different, the load rates of different VSC-HVDC converters are different, resulting in inconsistent reactive power capacities. To maximize the utilization rate of the reactive power of the VSC-HVDC converter, when the regional central station allocates the reactive power responsibility, it can be allocated according to the reactive power capacity of each VSC-HVDC converter, so that the reactive power responsibility of each VSC-HVDC converter is proportional to its own reactive power capacity. Of course, the above is only a specific reactive power responsibility allocation method provided by this application and should not be construed as a limitation of this application.[[]]

[0096] After the regional central station calculates the reactive power demand for each VSC-HVDC converter based on the current overall reactive power demand and generates a reactive power control instruction, it issues them to each VSC-HVDC converter respectively. Each VSC-HVDC converter outputs reactive power according to the received instruction and uploads its latest status to the regional central station for processing in the next cycle.[[]] ​

[0097] Figure 5 The figure shows a schematic flow diagram of a reactive power control method implemented based on the above reactive power control architecture. The method may include the following steps:

[0098] S101, the flexible DC converter reports its own status to the regional central station;

[0099] S102, the regional central station aggregates according to the status reported by each flexible DC converter;

[0100] S103, the regional central station reports the aggregation result to the energy management and control system;

[0101] S104, the energy management and control system calculates the reactive power demand for each regional central station by using a prediction model according to the aggregation results reported by each regional central station;

[0102] S105, the energy management and control system separately sends the overall reactive power demand for each regional central station to the corresponding regional central station;

[0103] S106, the regional central station determines the reactive power demand for each flexible DC converter according to the overall reactive power demand;

[0104] S107, the regional central station separately sends the reactive power demand for each flexible DC converter to the corresponding flexible DC converter;

[0105] S108, the flexible DC converter sends reactive power to the power grid according to the received reactive power demand.

[0106] It should be noted that Figure 5 only one group of interactions between the "flexible DC converter - regional central station - energy management and control system" is shown. In a complete reactive power control architecture, one regional central station may be connected to multiple flexible DC converters and play the roles of aggregating status (S102) and distributing reactive power demand (S106); one energy management and control system may also be connected to multiple regional central stations and comprehensively calculate the reactive power demand for each regional central station according to the status of multiple regional central stations (S104). For specific details, please refer to the previous description and will not be elaborated here.

[0107] In addition, from the time dimension, S101 to S108 only show the processing method for one cycle. In each cycle, the energy management and control system calculates the reactive power demand for the current state according to the current state, and distributes it to each flexible DC converter through each regional central station respectively. After S108 is executed, in the next cycle, the flexible DC converter uploads its updated own state to the regional central station, and returns to execute S101, that is, dynamic voltage and reactive power control is achieved through rolling optimization and feedback correction. In practical applications, the above cycle can be configured according to requirements, such as 100 ms, 200 ms, 500 ms, etc. This application does not need to limit specific values.

[0108] If a centralized reactive power control architecture is adopted, the energy management and control system needs to communicate with all traction stations and solve the global control model, which will bring significant communication delays and long calculation times. In addition, the update of control instructions is not timely, making it unable to respond well to the rapid changes in the power system, thus causing grid voltage oscillations.

[0109] To verify the advantages of the hierarchical control scheme proposed in this application, tests were carried out based on the actual scenarios of a load center power grid in Beijing and a certain line of the Beijing Subway. The basic architecture of the load center power grid is as Figure 6 shown. At the 1st second, the capacity of all capacitor banks in the power grid was halved, resulting in a voltage sag. During the test process, the flexible DC traction power supply system adopted two methods: traditional centralized control and hierarchical control. In the centralized control scheme, the energy management and control system collects all information and directly determines the reactive power instructions for each flexible DC converter. Due to communication and calculation delays, the control period is set to 1 s. In the hierarchical control scheme, the energy management and control system establishes and solves the global predictive control model every 100 ms, and the regional central station is responsible for distributing the overall reactive power responsibility and sending the results to each flexible DC converter.

[0110] Table 1 shows a comparison of some test results between the centralized reactive power control architecture and the hierarchical dynamic voltage and reactive power control architecture provided in this application:

[0111] Centralized control Hierarchical control Voltage average recovery time 11.36s 0.96s Average solution time 552ms 48ms

[0112] Figure 7 shows the comparison of the slip rate change curves of the motor load under the centralized and hierarchical control architectures; Figure 8 shows the comparison of the grid voltage change curves under the centralized and hierarchical control architectures.

[0113] It can be seen that, compared with the centralized reactive power control architecture, the hierarchical dynamic voltage and reactive power control architecture provided by this application can reduce the model solving time from hundreds of milliseconds to dozens of milliseconds. At the same time, the average grid voltage recovery time is reduced from more than 10 seconds to about 1 second, achieving global voltage recovery on a second time scale after disturbance. It is fully capable of eliminating the problem of delayed voltage recovery after grid faults, maintaining system stability, and can be effectively applied to solve the problem of insufficient dynamic reactive power reserve in large urban load center areas, reducing the construction cost of dynamic reactive power compensation equipment, and improving the voltage stability of the power system.

[0114] From the description of the above embodiments, those skilled in the art can clearly understand that this application can be implemented by means of software plus a necessary general hardware platform. Based on such an understanding, the technical solution of this application, in essence, or the part that makes contributions to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods described in various embodiments or some parts of the embodiments of this application.

[0115] The systems, devices, modules, or units illustrated in the above embodiments can be specifically implemented by computer chips or entities, or by products with certain functions. A typical implementation device is a computer, and the specific form of the computer can be a personal computer, a laptop computer, a cellular phone, a camera phone, a smart phone, a personal digital assistant, a media player, a navigation device, an email transceiver device, a game console, a tablet computer, a wearable device, or a combination of any several of these devices.

[0116] Each embodiment in this specification is described in a progressive manner. The same or similar parts among the embodiments can be referred to each other, and the key point of each embodiment is to illustrate the differences from other embodiments. The system embodiments described above are only illustrative. The modules described as separate components may or may not be physically separated. When implementing the solution of this application, the functions of each module can be realized in the same or multiple software and / or hardware. It is also possible to select some or all of the modules according to actual needs to achieve the purpose of the solution of this embodiment. Those of ordinary skill in the art can understand and implement it without creative work.

[0117] The above are only specific implementation manners of this application. It should be noted that for those of ordinary skill in the art, without departing from the principle of this application, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of this application.

Claims

1. A reactive power control system, characterized in that: The system includes: energy management and control system, regional central station, flexible DC converter; The energy management and control system is used to manage at least one regional central station; The regional central station is configured corresponding to the power supply interval, and each regional central station is used to manage at least one flexible DC converter located in the power supply interval; The flexible DC converter is a flexible DC converter in a dedicated power supply system and is connected to a power grid. The energy management and control system controls the flexible DC converter to output reactive power to the power grid through a regional central station.

2. The system according to claim 1, characterized in that The dedicated power supply system is an urban rail traction power supply system.

3. The system according to claim 1, characterized in that The energy management and control system determines the reactive power demand of the power grid for the flexible DC converters through a preset prediction model according to the status of each flexible DC converter.

4. The system according to claim 3, characterized in that The energy management and control system periodically obtains the current state of each flexible DC converter, and calculates the reactive power demand of the power grid for the flexible DC converter through the prediction model.

5. The system according to claim 3, characterized in that When the output of the prediction model is a control action sequence, the energy management and control system only sends the first control action in the sequence to the regional central station.

6. The system according to claim 1, characterized in that The energy management and control system is also used to linearize the relationship between the grid voltage and the reactive current output by the flexible DC converter using trajectory sensitivity.

7. The system according to claim 3, characterized in that The states of the flexible DC converters are provided in aggregate form according to the power supply intervals. The regional central station is specifically used to obtain the status of each flexible DC converter in the power supply area, and send the aggregated status to the energy management and control system after aggregation processing.

8. The system according to claim 3, characterized in that The reactive power demand of the flexible DC converter is: the overall reactive power demand for each power supply section; The regional central station is specifically used to distribute the overall reactive power demand for the power supply section to each flexible DC converter in the power supply section.

9. The system according to claim 7, characterized in that The regional central station allocates reactive power according to the reactive power capacity of each flexible DC converter, so that the reactive power responsibility of each flexible DC converter is proportional to its own reactive power capacity.

10. A reactive power control method based on the system according to any one of claims 1 to 9, characterized in that: The method includes: The flexible DC converter reports its status to the regional central station; The regional central station aggregates the status reported by each flexible DC converter and reports the aggregation result to the energy management and control system; The energy management and control system calculates the reactive power demand for each regional central station using the prediction model based on the aggregation results reported by each regional central station, and sends the overall reactive power demand for each regional central station to the corresponding regional central station; The regional central station determines the reactive power demand for each flexible DC converter based on the overall reactive power demand, and sends the reactive power demand for each flexible DC converter to the corresponding flexible DC converter respectively; The flexible DC converter sends reactive power to the grid according to the reactive power demand received.

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