Virtual AC power transmission network construction method and system for power grid MTDC

Abstract

The invention provides a virtual AC power transmission network construction method and system for a power grid MTDC, and the method comprises the steps: generating N physical nodes according to a common coupling point PCC of an N-end modular multilevel converter MMC, constructing a virtual node o, determining a voltage amplitude and a phase angle of the virtual node o according to the operation state of the physical nodes, each physical node forms a power relation through the virtual node o; connecting each physical node with the virtual node o through a virtual line reactance xio so as to construct a power transmission relation between each physical node and the virtual node o and realize dynamic power distribution and mutual aid among the physical nodes; an ideal transformer with the transformation ratio kio is constructed to achieve voltage matching of the MMC at each end, and the transformation ratio kio is the ratio of the reference voltage of the MMC at the i-th end to the reference voltage of the MMC at the N-th end. According to the method, virtual node center star topology construction is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic power systems, and in particular to a method and system for constructing a virtual AC transmission network for a multi-terminal DC (MTDC) power grid. Background Art

[0002] With the large-scale integration of renewable energy and the dynamic changes in load characteristics, megacities are facing systemic risks from severe imbalances in the temporal and spatial distribution of sources and loads. While the multi-terminal direct current transmission system (MMC-MTDC), with its modular multilevel converter (MMC) core architecture, has become a key technical solution for interconnecting regional power grids due to its compact footprint, topology reconfiguration flexibility, and AC fault isolation capabilities, it suffers from fundamental flaws in its traditional master-slave control architecture. In a shared DC bus network, active power regulation at each converter station relies strictly on centralized commands from a dispatching control center (DCC), which has an update cycle of up to 15 minutes, leading to dual loss of control in dynamic scenarios.

[0003] On the one hand, when sudden changes in renewable energy output or load cause power fluctuations, the system's inertial response delays can cause deviations in tie-line power. For example, some literature has pointed out that sudden load changes can lead to imbalanced active power from renewable energy sources, interactions between renewable energy station control strategies and flexible DC converter stations, and abnormal operating conditions. On the other hand, when communication between DCC and MTDC stations is interrupted, continued use of historical instructions can cause active power flows to deviate from the optimal operating trajectory, reducing the efficiency of energy exchange between the MTDC terminals. More seriously, under extreme operating conditions (such as DC voltage exceeding limits or AC system frequency deviation exceeding ±0.2Hz), the rigid instruction constraints of master-slave control can trigger power imbalance protection, causing the multi-terminal system to degrade from parallel operation to islanding mode. Some literature has explored the capacitance characteristics of voltage source converters (VSCs) to mitigate short-term power fluctuations from renewable energy sources. However, this strategy requires high-speed communication and is difficult to implement in a VSC-MTDC within a short period of time. Therefore, there is an urgent need to build a new control architecture with topology self-reconstruction capability, which can adaptively achieve power mutual assistance in short-time-scale coordinated control. Summary of the Invention

[0004] The present invention provides a method and system for constructing a virtual AC transmission network for a mobile distributed control (MTDC) power grid. This method addresses the shortcomings of existing converter stations in their insufficient short-term power regulation capabilities, implements a virtual node central star (VNCS) topology, and optimizes virtual impedance parameters to achieve efficient multi-terminal power coordination, ensuring stable operation even in extreme situations such as communication interruptions.

[0005] The present invention provides a method for constructing a virtual AC transmission network for a MTDC (Medium-Scale Direct Current) power grid, comprising:

[0006] Generate N physical nodes based on the common coupling point PCC of the N-end modular multilevel converter MMC, and construct a virtual node o. The voltage amplitude and phase angle of the virtual node o are determined by the operating state of the physical node. Each physical node is power-connected by the virtual node o.

[0007] The virtual line reactance x is connected between each physical node and the virtual node o. io Connected to build a power transmission relationship between each physical node and the virtual node o, and realize dynamic power allocation and mutual assistance between each physical node;

[0008] Construct a transformation ratio of k io ideal transformer to achieve voltage matching of each end MMC, with a transformation ratio k io is the ratio of the reference voltage of the i-th MMC to the reference voltage of the N-th MMC, i is 1 to N-1, and N is a positive integer greater than or equal to 2.

[0009] According to a method for constructing a virtual AC transmission network for a power grid MTDC provided by the present invention, the voltage amplitude of the virtual node o is determined by the initial voltage amplitude of the N-terminal MMC sent by the DCC after the end of the previous instruction cycle and before the start of the next instruction cycle. The calculation formula is:

[0010]

[0011] Among them, V o represents the voltage amplitude of the virtual node o, V j 0 represents the initial voltage amplitude of the j-th terminal MMC, and N represents the number of terminals of the MMC.

[0012] According to a method for constructing a virtual AC transmission network for a power grid MTDC provided by the present invention, the phase angle of the virtual node o is determined by the initial phase angle value of the N-terminal MMC sent by the DCC after the end of the previous instruction cycle and before the start of the next instruction cycle, and the allocation coefficient. The calculation formula is:

[0013]

[0014] Among them, δ o represents the phase angle of the virtual node o, δ j 0 represents the initial phase angle value of the j-th end MMC, N represents the number of ends of the MMC, k j is the distribution coefficient of the j-th end MMC.

[0015] According to a method for constructing a virtual AC transmission network for a MTDC power grid provided by the present invention, k j The allocation satisfies the following conditions:

[0016]

[0017] in, and They represent the minimum and maximum values of the virtual phase angle considering the safety margin, S surj Indicates the remaining capacity of the j-th MMC.

[0018] According to a method for constructing a virtual AC transmission network for a MTDC power grid provided by the present invention, S surj The calculation formula is:

[0019]

[0020] Among them, S Nj is the rated capacity of the j-th MMC, p jref and q jref They represent the active power instruction and reactive power instruction sent by DCC to the j-th MMC respectively.

[0021] According to a method for constructing a virtual AC transmission network for a MTDC power grid provided by the present invention, the initial phase angle δ of the virtual node o is o 0 The value range of is:

[0022]

[0023] in, is the maximum phase angle of the MTDC that transmits active power to the AC grid, To obtain the minimum phase angle of the MTDC that absorbs active power from the AC grid, the initial phase angle δ of the virtual node o is determined by the following formula: o 0 The value range of is:

[0024]

[0025] According to a method for constructing a virtual AC power transmission network for a power grid MTDC provided by the present invention, the power p transmitted by each physical node to the DC power grid connected to the MMC is i satisfy:

[0026]

[0027] Among them, V i represents the voltage of the MMC at the i-th end, δ i Represents the phase angle of the MMC at the i-th end.

[0028] According to a method for constructing a virtual AC transmission network for a MTDC power grid provided by the present invention, the virtual line reactance x io The calculation formula is:

[0029]

[0030] Among them, V j 0 represents the initial voltage amplitude of the j-th terminal MMC, δ j 0 represents the initial phase angle value of the j-th end MMC, represents the initial phase angle value of the i-th end MMC, N represents the number of ends of the MMC, k j is the distribution coefficient of the j-th end MMC, is the active power instruction of the MMC at the i-th end in one instruction cycle.

[0031] The present invention also provides a virtual AC transmission network construction system for a MTDC power grid, comprising:

[0032] A first building module is configured to generate N physical nodes based on a common coupling point PCC of an N-end modular multilevel converter MMC, and to build a virtual node o, wherein a voltage amplitude and a phase angle of the virtual node o are determined by an operating state of the physical node, and each physical node is power-connected by the virtual node o;

[0033] The second building block is used to connect each physical node to the virtual node o through a virtual line reactance x io Connected to build a power transmission relationship between each physical node and the virtual node o, and realize dynamic power allocation and mutual assistance between each physical node;

[0034] The third building block is used to construct a transformation ratio k io ideal transformer to achieve voltage matching of each end MMC, with a transformation ratio k io is the ratio of the reference voltage of the i-th MMC to the reference voltage of the N-th MMC, i is 1 to N-1, and N is a positive integer greater than or equal to 2.

[0035] The present invention also provides an electronic device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the program, the method for constructing a virtual AC transmission network for a grid MTDC as described above is implemented.

[0036] The present invention also provides a non-transitory computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements any of the above-described methods for constructing a virtual AC transmission network for a grid MTDC.

[0037] The present invention also provides a computer program product, comprising a computer program, wherein when the computer program is executed by a processor, the computer program implements any of the above-mentioned methods for constructing a virtual AC transmission network for a grid MTDC.

[0038] The present invention provides a method and system for constructing a virtual AC transmission network for a mobile multi-connected direct current (MTDC) power grid. This method introduces virtual nodes o to reconstruct the power path and compensate for converter losses, solving the complex calculation problem of time-varying line resistance under a purely physical topology. Simultaneously, a distribution coefficient based on dynamic optimization of the remaining capacity is used to achieve reasonable allocation of converter station mutual aid capacity. By constructing a virtual impedance network and an ideal transformer model, dynamic decoupling and adaptive allocation of multi-terminal power flows are achieved, resolving the problems of dynamic response lag and power imbalance caused by communication interruptions in traditional MMC-MTDC systems that rely on periodic instructions from a dispatching center. The virtual network can be constructed and put into operation within milliseconds, enabling adaptive power regulation of a multi-terminal flexible direct current (FDC) system and rapid response to dynamic scenarios such as sudden changes in renewable energy output or load. This topology exhibits excellent scalability and can adapt to the ever-changing source-load conditions and network topology in metropolitan power grids, making it applicable to N-terminal flexible direct current (FDC) systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0040] Figure 1 This is one of the flow charts of the method for constructing a virtual AC transmission network for a power grid MTDC provided by the present invention;

[0041] Figure 2 It is a structural diagram of a four-terminal MMC-MTDC system in the virtual AC transmission network construction method for grid MTDC provided by the present invention;

[0042] Figure 3 It is a schematic diagram of a virtual node center star topology structure in the virtual AC transmission network construction method for power grid MTDC provided by the present invention;

[0043] Figure 4 This is the second flow chart of the method for constructing a virtual AC transmission network for a power grid MTDC provided by the present invention;

[0044] Figure 5 (a) is a diagram showing the experimental test results of Case 1 of the method for constructing a virtual AC transmission network for a power grid MTDC provided by the present invention; (b) is a diagram showing the experimental test results of Case 2 of the method for constructing a virtual AC transmission network for a power grid MTDC provided by the present invention;

[0045] Figure 6 It is a structural diagram of a virtual AC transmission network construction system for power grid MTDC provided by the present invention. DETAILED DESCRIPTION

[0046] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0047] The following combination Figure 1 The present invention describes a method for constructing a virtual AC transmission network for a MTDC, comprising:

[0048] Step 101: Generate N physical nodes according to the common coupling point PCC of the N-end modular multilevel converter MMC. The voltage amplitude and phase angle of the physical nodes are inherited from the corresponding PCC point.

[0049] Constructing a virtual node o introduced as a reference point, wherein the voltage amplitude and phase angle of the virtual node o are determined by the operating state of the physical node, and each physical node is power-connected by the virtual node o;

[0050] Step 102: connect each physical node to the virtual node o through a virtual line reactance x io Connected to build a power transmission relationship between each physical node and the virtual node o, and realize dynamic power allocation and mutual assistance between each physical node;

[0051] Step 103: Construct a transformation ratio k io ideal transformer to achieve voltage matching of each end MMC, with a transformation ratio k iois the ratio of the reference voltage of the i-th MMC to the reference voltage of the N-th MMC as a reference value, i is 1 to N-1, and N is a positive integer greater than or equal to 2.

[0052] For an N-terminal multi-terminal direct current (MTDC) transmission system, active power has N-1 degrees of freedom. The remaining degrees of freedom are used to maintain a constant DC voltage. Therefore, N-1 equivalent AC lines can be constructed. The last degree of freedom is defined as a reference node, forming a virtual transmission network that interconnects all physical nodes. Considering the difficulty of simulating losses when interconnecting purely physical nodes, a virtual node o is introduced as a reference point to form the MTDC.

[0053] This embodiment provides a virtual AC transmission network topology and design method for a megacity MTDC power grid, which is used to implement adaptive power regulation of a multi-terminal flexible DC system in a short time in subsequent control design. Figure 2 The figure shows a four-terminal multi-terminal flexible DC system consisting of three areas (A, B, and C). Each area is connected to the DC line via a modular multilevel converter (MMC), forming an interconnected DC grid. The three areas are connected to the DC network through the MMCs, forming a multi-terminal interconnected structure. The four-terminal flexible DC system forms a strongly coupled system with the AC ring network, meaning that the frequencies are essentially the same. The entire multi-terminal flexible DC system uses master-slave control. MMC1, MMC2, and MMC3 use PQ control to adjust the converter station output power, while MMC4 uses DC voltage control to stabilize the DC bus voltage.

[0054] Figure 2 The blue dotted box in the figure represents the DC grid of the multi-terminal flexible DC system. The equivalent virtual node center star topology is as follows: Figure 3 As shown in Figure 1, the four nodes correspond to the PCC points of the four-terminal converter stations. These nodes are connected to the virtual node O through four lines with ideal transformers and reactors in series, forming a virtual interconnection network.

[0055] This embodiment introduces a virtual node o to reconstruct the power path and compensate for the loss of the converter, thereby solving the problem of complex calculation of time-varying line resistance under a purely physical topology. At the same time, through the allocation coefficient based on dynamic optimization of the remaining capacity, the reasonable allocation of the mutual aid capacity of the converter station is achieved. By constructing a virtual impedance network and an ideal transformer model, dynamic decoupling and adaptive allocation of multi-terminal power flows are achieved, solving the problems of dynamic response lag and power imbalance caused by communication interruption in the traditional MMC-MTDC system that relies on periodic instructions from the dispatching center. The virtual network can be constructed and put into use within a millisecond time scale, realizing adaptive power regulation of the multi-terminal flexible direct current system and quickly responding to dynamic scenarios such as sudden changes in new energy output or sudden changes in load. The topology has good scalability and can adapt to the ever-changing source-load conditions and network topology in mega-city power grids, and can be applied to N-terminal flexible direct current systems.

[0056] Based on the above embodiment, the voltage amplitude of the virtual node o in this embodiment is determined by the initial voltage amplitude of the N-terminal MMC sent by the DCC after the end of the previous instruction cycle and before the start of the next instruction cycle. The calculation formula is:

[0057]

[0058] Among them, V o represents the voltage amplitude of the virtual node o, V j 0 represents the initial voltage amplitude of the j-th terminal MMC, and N represents the number of terminals of the MMC.

[0059] In view of the small fluctuation of the voltage amplitude of each MMC, this embodiment uses the average of the voltage amplitudes measured at each converter station as the voltage amplitude of the virtual node o.

[0060] Based on the above embodiment, the phase angle of the virtual node o in this embodiment is determined by the initial phase angle value of the N-end MMC sent by the DCC after the end of the previous instruction cycle and before the start of the next instruction cycle and the allocation coefficient. The calculation formula is:

[0061]

[0062] Among them, δ o represents the phase angle of the virtual node o, δ j 0 represents the initial phase angle value of the j-th end MMC, N represents the number of ends of the MMC, k j is the allocation coefficient of the j-th MMC, which determines the amount of emergency power support of the j-th MMC under the power angle disturbance condition, and it satisfies:

[0063]

[0064] Considering the differences in the actual operating conditions and rated capacities of each MMC, this embodiment introduces a distribution coefficient k j , which is used to perform weighted distribution of the power flow caused by the phase angle changes of each MMC.

[0065] Partition coefficient k j The configuration follows the following principle: the MMC with higher remaining capacity has a higher allocation coefficient k j The larger the value, the stronger the power support capability can be provided in emergency situations. This method not only realizes the dynamic allocation of power, but also improves the flexibility and reliability of the system, ensuring the efficient and coordinated operation of each converter station under complex working conditions.

[0066] Based on the above embodiment, in this embodiment, k j The allocation satisfies the following conditions:

[0067]

[0068] in, and They represent the minimum and maximum values of the virtual phase angle considering the safety margin, S surj Indicates the remaining capacity of the j-th MMC.

[0069] This embodiment aims to improve the emergency power support capability of the interconnected power grid based on the actual operating conditions of each converter station and considers the dynamic response of each converter to the power angle change. j Make a design.

[0070] On the basis of the above embodiment, in this embodiment, S surj The calculation formula is:

[0071]

[0072] Among them, S Nj is the rated capacity of the j-th MMC, p jref and q jref Remaining capacity is the physical measure of the current converter station's ability to deliver power.

[0073] On the basis of the above embodiment, in order to ensure the accurate flow of power between nodes in this embodiment, the designed virtual reactance xio must be positive, which requires the initial phase angle δ of the virtual node o to be o 0 The value range of is:

[0074]

[0075] in, is the maximum phase angle of the MTDC that transmits active power to the AC grid, is the minimum phase angle of the MTDC that absorbs active power from the AC grid.

[0076] At the same time, in order to prevent the virtual reactance from approaching zero, this embodiment sets a 10% safety margin to ensure the stability and reliability of the system. That is, the initial phase angle δ of the virtual node o is determined by the following formula: o 0 The value range of is:

[0077]

[0078] On the basis of the above embodiment, in this embodiment, after the virtual node o is designed, each physical node is connected by the virtual node o, and the power p transmitted by the i-th physical node (i=1, 2, 3...N-1) to the DC grid connected to the MMC is i satisfy:

[0079]

[0080] Among them, V i represents the voltage of the MMC at the i-th end, δ i Represents the phase angle of the MMC at the i-th end.

[0081] On the basis of the above embodiment, in this embodiment, after obtaining the upper layer communication instruction p i * Then, the design method of virtual node o is brought into formula (8) to obtain the virtual line reactance x between each physical node and virtual node o. io , virtual line reactance x io The calculation formula is:

[0082]

[0083] Among them, V j 0 represents the initial voltage amplitude of the j-th terminal MMC, δ j 0 represents the initial phase angle value of the j-th end MMC, represents the initial phase angle value of the i-th end MMC, N represents the number of ends of the MMC, k j is the distribution coefficient of the j-th end MMC, is the active power instruction of the MMC at the i-th end in one instruction cycle.

[0084] Figure 4 The following shows the construction process of the virtual network topology structure in the embodiment of the present invention. Its working principle is as follows:

[0085] 1. Sending periodic instructions from the upper scheduling control center

[0086] After the previous instruction cycle ends and before the next instruction cycle begins, the upper-level dispatching and control center collects the operating status and power flow information of each converter station and sends a network construction flag to the virtual connection construction system to activate and execute the network construction system.

[0087] 2. Acquisition of initial voltage amplitude and initial phase information of each node

[0088] After the network construction system is activated, the upper-level dispatching and control center immediately sends the parameters of each node converter station (such as rated capacity, rated voltage, etc.) and node status information (such as the power command value of each converter station in the command cycle, the voltage amplitude and phase angle value of each node).

[0089] 3. Construction of virtual node o

[0090] After obtaining the voltage amplitude and phase value of each node, the system determines the voltage amplitude of the virtual node o according to formula (1). o The determination process also involves the allocation parameter k j Optimization. Calculate the remaining capacity in the instruction cycle based on the converter station parameters sent by the upper dispatch center, construct the objective function, and combine the range constraint and normalization constraint of the safety margin to form an optimization model, such as formula (4). Perform linear parameter optimization to obtain the allocation coefficient k of each converter station. j , and then according to formula (2) the phase angle value δ of the virtual node o is obtained o .

[0091] 4. Calculation of virtual line parameters

[0092] When obtaining the voltage amplitude V of the virtual node o o and phase angle δ o Then, according to the power command value of the upper scheduling control center in this command cycle, the virtual reactance xio and the ideal transformer k are calculated by combining formula (9). io , and then obtain the topological structure of the virtual communication network.

[0093] 5. Investment in virtual networks

[0094] After the virtual interconnection network is successfully constructed, the topology information is sent to the control system. The control system monitors the voltage and phase angle changes of each node in real time, calculates and sends the inner loop current reference value to the converter station MMC based on the value of each virtual reactance, and adjusts the flexible DC network control structure, so that the flexible DC system has the ability to respond to short-term power flow mutual assistance.

[0095] This example demonstrates the effectiveness of the VNCS disclosed in this invention through hardware-in-the-loop testing using a TMS320F28379D control board. The DC voltage control of the four-terminal flexible DC system's main DC grid circuit and MMC2, as well as the construction of the virtual interconnected network topology, were simulated and executed in the Typhoon 604 simulator. Virtual AC transmission network control for MMC1, MMC2, and MMC3 was implemented on three independent TMS320F28379D control boards, with key parameters listed in Table 1.

[0096] Table 1 Main parameters of experimental test

[0097]

[0098] After the simulation system simulates the upper-level dispatching control center issuing the network construction flag instruction, the experimental conditions and VCNS network construction results are shown in Table 2.

[0099] Table 2 Experimental conditions and VCNS network construction results

[0100]

[0101] The test environment for building the virtual network used MATLAB R2022b software, and the PC was equipped with an AMD Ryzen 75700G CPU and 32GB of RAM. The entire build process was completed within 81ms, meeting the requirements of actual projects.

[0102] Based on the above experimental conditions and parameter configurations, the present invention designs two power angle disturbance test cases to evaluate the dynamic response performance of the multi-terminal flexible DC system under the VCNS network topology.

[0103] Case 1: The power angle of Region A in the urban power grid continuously changes

[0104] Under the set experimental conditions, the urban grid area A experienced a process in which the power angle continuously decreased by 0.087rad in one instruction cycle, and then the power angle recovered. The phase angle and power changes of each converter station node and the phase angle value change of the virtual node are shown in the following figure. Figure 5 (a) shows the experimental conditions. This experimental condition simulates the situation where the load in urban grid area A exceeds the power generation capacity due to the access of new energy or load fluctuation. Since this area is directly connected to the converter station MMC1, the phase angle of node 1 decreases immediately, while the phase angles of MMC2 and MMC3 remain unchanged, which in turn causes the phase angle of virtual node o to decrease. According to formula (8), the power output to MMC1 increases, while the power delivered to the DC grid by MMC2 and MMC4 decreases, and the power delivered to MMC3 decreases. The power change rate is obtained according to formula (9):

[0105]

[0106] Since the line virtual reactance satisfies the relationship: 4o >x 3o >x 2o , resulting in the power change satisfying: Δp4<Δp3<Δp2. Figure 5 The experimental results in (a) verify the above analysis.

[0107] Case 2: Continuous jumps in the power angle of Region C in the urban power grid

[0108] Under the set experimental conditions, the phase angle and power changes of each converter station node and the phase angle value change results of the virtual node are as follows: Figure 5 As shown in (b). Since converter stations MMC2 and MMC4 both belong to urban grid region C, phase angle changes in region C will affect the phase angle changes of both converter stations simultaneously. Within one instruction cycle, the phase angle of MMC2 continuously decreases by 0.044 rad, while the phase angle of MMC4 continuously decreases by 0.052 rad, and then the power angle recovers. Similarly, from formula (8), it can be concluded that the phase angle of virtual node o increases, the power output to MMC1 reverses, the power delivered to the DC grid by MMC2 and MMC4 decreases, and the power output to MMC3 decreases. Figure 5 (b) The experimental results of 5 verify the above analysis.

[0109] In addition, the above experimental results have been measured and calculated, and the system response time under power angle disturbance is less than 81ms, which meets the requirements of the flexible DC system for rapid power regulation in a short time.

[0110] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0111] First, in view of the technical problems existing in the above-mentioned prior art and the difficulty in solving them, some creative technical effects are achieved after solving the problems. The specific description is as follows:

[0112] (1) By constructing VNCS, the present invention can realize adaptive power regulation of multi-terminal flexible direct current systems in a short period of time, quickly respond to dynamic scenarios such as sudden changes in renewable energy output or sudden changes in load, effectively solve the power imbalance problem caused by communication delays or interruptions in the traditional master-slave control architecture, and significantly improve the stability and reliability of the system.

[0113] (2) The present invention uses the allocation coefficient k in the design method of the virtual node jEndowed with dynamic properties, the allocation coefficient can adaptively change according to the remaining capacity of the converter station, so that the entire topology can be dynamically adjusted after each upper-layer instruction cycle, ensuring that converter stations with high remaining capacity can provide stronger power support and avoid system failures caused by single-point overload.

[0114] (3) In terms of loss, the present invention introduces a virtual node o to provide a loss flow path, avoiding the complex calculations caused by the time-varying resistance in traditional methods. In traditional methods, purely physical nodes require resistors to be connected in series when considering losses, and the time-varying nature of resistors makes the topology complex and difficult to calculate. In contrast, the introduction of the virtual node o simplifies the loss processing process, making the topology of the entire system clearer and the calculations easier.

[0115] Second, the technical solution of this invention overcomes technical bias: in today's short-term coordinated control strategies for multi-terminal flexible DC systems, engineers primarily employ flexible grid-level control strategies, such as improved voltage droop control and frequency modulation strategies. For even shorter timescales, on the order of seconds, energy storage devices are employed to coordinate with the flexible DC grid. This invention considers the topology of the flexible DC grid and leverages the adaptive power flow regulation of traditional AC tie lines, reducing the investment in additional equipment while improving response speed.

[0116] Third, the topology proposed in the present invention is designed based on the power angle drive of the converter stations at each end. In the urban power grid, each sub-grid is connected through a 500kV loop, and the frequency changes are basically synchronized. The virtual interconnection network control based on frequency difference drive (frequency synchronization control) will no longer be applicable. Therefore, the topology proposed in the present invention can further propose a virtual interconnection network control method based on power angle drive. By establishing an accurate virtual interconnection network model, the dynamic characteristics of the power angle between nodes can be captured in real time, forming an active power-phase angle coupling relationship, thereby realizing the MTDC converter station adaptively adjusting the mutual aid power according to the change of the local power angle.

[0117] Fourth, the virtual AC transmission network topology proposed in the present invention has good scalability and can adapt to the ever-changing source-load conditions and network topology in metropolitan power grids. VNCS breaks the dependence of traditional topology on physical nodes by introducing virtual nodes and equivalent AC lines. When adding a new converter station or adjusting the capacity of a converter station, it is only necessary to update the parameters of the virtual node without the need for large-scale transformation of the entire system. This scalability enables the system to flexibly adapt to the development needs of future power grids, such as adding new energy access points or adjusting the location of load centers. When adding a new converter station to a metropolitan power grid, it is only necessary to adjust the parameters of the virtual node to seamlessly connect it to the existing network.

[0118] The expected benefits and commercial value of the technical solution after transformation are as follows: First, by constructing a virtual node center star topology (VNCS), the multi-terminal flexible DC system can achieve adaptive power regulation in a short period of time, quickly respond to dynamic scenarios, and effectively solve the power imbalance problem under the traditional control architecture, thereby significantly improving the stability and reliability of the system and reducing equipment losses and maintenance costs caused by power fluctuations. Second, the allocation coefficient introduced in the virtual node design can be dynamically adjusted according to the remaining capacity of the converter station, ensuring that converter stations with high remaining capacity provide stronger power support in emergency situations, avoiding system failures, and improving the overall operating efficiency and equipment utilization of the power grid. In terms of loss processing, the introduction of virtual node o simplifies the calculation process, making the system topology clearer, reducing the complex calculations caused by time-varying resistance, and reducing the difficulty and cost of operation and maintenance. In addition, the scalability of the present invention enables the system to flexibly adapt to the development needs of the future power grid, such as adding new energy access points or adjusting the location of load centers, reducing the large-scale investment in future power grid transformation. Finally, the present invention overcomes the biases of the existing technology and adopts an innovative control strategy, reducing the investment in additional equipment while improving the response speed, providing an efficient and economical solution for multi-terminal flexible DC systems. These advantages make the present invention have broad market prospects and significant economic benefits in the application of metropolitan power grids.

[0119] It is worth noting that, because the flexible DC network itself does not have the ability to transmit reactive power, the reactive power required by each AC grid must be provided by local equipment. Based on this consideration, the present invention does not include reactive power simulation in the design of the simulated virtual AC network.

[0120] Actual test data confirms that the implementation of this topology demonstrates significant performance improvements in multi-terminal flexible DC systems in terms of power regulation, system stability, loss management, and scalability. Under varying power angle disturbances, the system rapidly responds and adjusts power, effectively avoiding power imbalances caused by communication delays. These experimental results demonstrate that the technical solution of this invention represents a significant technological advancement, addresses existing issues, and possesses significant practical value.

[0121] The following describes a virtual AC transmission network construction system for a power grid MTDC provided by the present invention. The virtual AC transmission network construction system for a power grid MTDC described below and the virtual AC transmission network construction method for a power grid MTDC described above can be referenced to each other.

[0122] like Figure 6 As shown, the system includes a first building block 601, a second building block 602 and a third building block 603, wherein:

[0123] The first building module 601 is configured to generate N physical nodes according to the common coupling point PCC of the N-end modular multilevel converter MMC, and construct a virtual node o. The voltage amplitude and phase angle of the virtual node o are determined by the operating state of the physical node, and each physical node is power-connected by the virtual node o.

[0124] The second building module 602 is used to connect each physical node to the virtual node o through a virtual line reactance x io Connected to build a power transmission relationship between each physical node and the virtual node o, and realize dynamic power allocation and mutual assistance between each physical node;

[0125] The third building module 603 is used to build a transformation ratio k io ideal transformer to achieve voltage matching of each end MMC, with a transformation ratio k io is the ratio of the reference voltage of the i-th MMC to the reference voltage of the N-th MMC, i is 1 to N-1, and N is a positive integer greater than or equal to 2.

[0126] This embodiment introduces a virtual node o to reconstruct the power path and compensate for the loss of the converter, thereby solving the problem of complex calculation of time-varying line resistance under a purely physical topology. At the same time, through the allocation coefficient based on dynamic optimization of the remaining capacity, the reasonable allocation of the mutual aid capacity of the converter station is achieved. By constructing a virtual impedance network and an ideal transformer model, dynamic decoupling and adaptive allocation of multi-terminal power flows are achieved, solving the problems of dynamic response lag and power imbalance caused by communication interruption in the traditional MMC-MTDC system that relies on periodic instructions from the dispatching center. The virtual network can be constructed and put into use within a millisecond time scale, realizing adaptive power regulation of the multi-terminal flexible direct current system and quickly responding to dynamic scenarios such as sudden changes in new energy output or sudden changes in load. The topology has good scalability and can adapt to the ever-changing source-load conditions and network topology in mega-city power grids, and can be applied to N-terminal flexible direct current systems.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for constructing a virtual AC transmission network for a MTDC power grid, characterized in that: include: Generate N physical nodes based on the common coupling point PCC of the N-end modular multilevel converter MMC, and construct a virtual node o. The voltage amplitude and phase angle of the virtual node o are determined by the operating state of the physical node. Each physical node is power-connected by the virtual node o. The virtual line reactance x is connected between each physical node and the virtual node o. io Connected to build a power transmission relationship between each physical node and the virtual node o, and realize dynamic power allocation and mutual assistance between each physical node; Construct a transformation ratio of k io ideal transformer to achieve voltage matching of each end MMC, with a transformation ratio k io is the ratio of the reference voltage of the i-th MMC to the reference voltage of the N-th MMC, i is 1 to N-1, and N is a positive integer greater than or equal to 2.

2. The method for constructing a virtual AC transmission network for a power grid MTDC according to claim 1, characterized in that: The voltage amplitude of the virtual node o is determined by the initial voltage amplitude of the N-terminal MMC sent by the DCC after the end of the previous instruction cycle and before the start of the next instruction cycle. The calculation formula is: Among them, V o represents the voltage amplitude of the virtual node o, V j 0 represents the initial voltage amplitude of the j-th terminal MMC, and N represents the number of terminals of the MMC.

3. The method for constructing a virtual AC transmission network for a power grid MTDC according to claim 2, characterized in that: The phase angle of the virtual node o is determined by the initial phase angle value of the N-end MMC sent by the DCC after the end of the previous instruction cycle and before the start of the next instruction cycle, and the allocation coefficient. The calculation formula is: Among them, δ o represents the phase angle of the virtual node o, δ j 0 represents the initial phase angle value of the j-th end MMC, N represents the number of ends of the MMC, k j is the distribution coefficient of the j-th end MMC.

4. The method for constructing a virtual AC transmission network for a power grid MTDC according to claim 3, characterized in that: k j The allocation satisfies the following conditions: in, and They represent the minimum and maximum values of the virtual phase angle considering the safety margin, S surj Indicates the remaining capacity of the j-th MMC.

5. The method for constructing a virtual AC transmission network for a power grid MTDC according to claim 4, characterized in that: S surj The calculation formula is: Among them, S Nj is the rated capacity of the j-th MMC, p jref and q jref They represent the active power instruction and reactive power instruction sent by DCC to the j-th MMC respectively.

6. The method for constructing a virtual AC transmission network for a power grid MTDC according to claim 4, characterized in that: The initial phase angle δ of the virtual node o o 0 The value range of is: in, is the maximum phase angle of the MTDC that transmits active power to the AC grid, To obtain the minimum phase angle of the MTDC that absorbs active power from the AC grid, the initial phase angle δ of the virtual node o is determined by the following formula: o 0 The value range of is:

7. The method for constructing a virtual AC transmission network for a MTDC power grid according to claim 3, characterized in that: The power p transmitted by each physical node to the DC grid connected to the MMC i satisfy: Among them, V i represents the voltage of the MMC at the i-th end, δ i Represents the phase angle of the MMC at the i-th end.

8. The method for constructing a virtual AC transmission network for a MTDC power grid according to claim 3, characterized in that: Virtual line reactance x io The calculation formula is: Among them, V j 0 represents the initial voltage amplitude of the j-th terminal MMC, δ j 0 represents the initial phase angle value of the j-th end MMC, δ i 0 represents the initial phase angle value of the i-th end MMC, N represents the number of ends of the MMC, k j is the distribution coefficient of the j-th end MMC, is the active power instruction of the MMC at the i-th end in one instruction cycle.

9. A virtual AC transmission network construction system for a power grid MTDC, characterized in that: include: A first building module is configured to generate N physical nodes based on a common coupling point PCC of an N-end modular multilevel converter MMC, and to build a virtual node o, wherein a voltage amplitude and a phase angle of the virtual node o are determined by an operating state of the physical node, and each physical node is power-connected by the virtual node o; The second building block is used to connect each physical node to the virtual node o through a virtual line reactance x io Connected to build a power transmission relationship between each physical node and the virtual node o, and realize dynamic power allocation and mutual assistance between each physical node; The third building block is used to construct a transformation ratio k io ideal transformer to achieve voltage matching of each end MMC, with a transformation ratio k io is the ratio of the reference voltage of the i-th MMC to the reference voltage of the N-th MMC, i is 1 to N-1, and N is a positive integer greater than or equal to 2.

10. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the method for constructing a virtual AC transmission network for a power grid MTDC according to any one of claims 1 to 8 is implemented.

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