Hybrid DC power flow calculation method and system including LCC-VSC series-parallel

Abstract

The present invention proposes a hybrid DC power flow calculation method and system containing LCC-VSC series-parallel connection, which relates to the technical field of power grids. The specific scheme comprises: obtaining system parameters of an AC-DC system containing LCC-VSC series-parallel connection, and constructing AC side and DC side network matrices; initializing AC side variables and DC side variables by using the AC side and DC side network matrices; based on the initialized AC side variables and DC side variables, performing iterative power flow calculation on an AC-DC hybrid system of a hybrid DC system model structure, updating the AC side variables and the DC side variables until an iteration stop condition is met, and taking the final AC side variables and the DC side variables as the result of the power flow calculation of the AC-DC hybrid system; the present invention can realize the power flow calculation of a hybrid DC transmission system under various scenarios, and provide a basis for safety analysis and planning of an AC-DC hybrid system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grids, and in particular relates to a hybrid DC power flow calculation method and system including LCC-VSC series-parallel connection. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] In the process of energy use, the efficient use of renewable energy plays a vital role. Since the inverse distribution characteristics of energy resources and load centers are very significant, renewable energy power generation bases such as wind power, hydropower and photovoltaics are mostly built in remote areas. Long-distance high-voltage direct current transmission systems are needed to transmit electricity, so as to realize the configuration of large-scale energy resources in the power grid; high-voltage direct current transmission systems have the advantages of large transmission capacity and low loss. As the proportion of renewable energy in the energy structure increases, they will play a greater role in energy transmission.

[0004] According to the different types of converters, high-voltage direct current transmission systems can be divided into line commutated converter-based high voltage direct current (LCC-HVDC) and voltage source converter-based high voltage direct current (VSC-HVDC). LCC-HVDC has the advantages of large transmission capacity and low cost, and is a very mature technology, but it consumes a lot of reactive power during operation and is prone to commutation failure. VSC-HVDC can achieve independent control of active and reactive power, without the risk of commutation failure and without the need for reactive power compensation, but has the disadvantages of high cost and large operating losses. Hybrid direct current transmission system makes full use of the advantages of the two high-voltage direct current technologies and becomes the main form of DC power grid in the future.

[0005] The influence mechanism between different converters in the hybrid DC transmission system is complex. In order to ensure the safe and stable operation of the hybrid DC transmission system, it is necessary to analyze its characteristics. The power flow calculation is the basis of the safety and stability analysis of the power system. Therefore, it is very necessary to study the power flow solution problem of the hybrid DC transmission system.

[0006] LCC and VSC converters mostly use quasi-steady-state models to derive power flow equations, and then complete the power flow solution of the AC / DC hybrid system based on the unified solution method and the alternating iterative solution method. At present, the research on the power flow solution algorithm for LCC-HVDC and VSC-HVDC models is relatively mature, but there is less research on the power flow solution of LCC-VSC hybrid DC transmission systems, and there is insufficient research on the steady-state model of the LCC-VSC series-parallel structure that appears in actual projects. Because the interaction between different converters in the hybrid DC system is complex, the original converter steady-state model cannot be used directly, and the series-parallel structure is diverse, making it difficult to solve the power flow. Summary of the invention

[0007] The interactions between different converters in a hybrid DC system are complex, the original converter steady-state model cannot be used directly, and the series-parallel structures are diverse, making it difficult to solve the power flow. To address this problem, the present invention provides a power flow calculation method and system for an AC / DC system containing a series-parallel hybrid DC of an LCC-VSC, which can realize the power flow calculation of a hybrid DC transmission system in a variety of scenarios and provide a basis for the safety analysis and planning of an AC / DC hybrid system.

[0008] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0009] A first aspect of the present invention provides a method for calculating a hybrid DC power flow including LCC-VSC series-parallel connection.

[0010] The hybrid DC power flow calculation method including LCC-VSC series-parallel connection includes:

[0011] Obtain system parameters of the AC / DC system containing LCC-VSC series-parallel connection, and construct AC and DC side network matrices;

[0012] Using the AC side and DC side network matrices, initialize AC side variables and DC side variables;

[0013] Based on the initialized AC side variables and DC side variables, iterative power flow calculation is performed on the AC / DC hybrid system of the hybrid DC system model structure, and the AC side variables and DC side variables are updated until the iteration stop condition is met, and the final AC side variables and DC side variables are used as the results of the AC / DC hybrid system power flow calculation;

[0014] Among them, one iteration in the power flow calculation is to sequentially perform power calculation of the converter injected into the DC system, power calculation of the DC network, power calculation of the converter injected into the AC system and power calculation of the AC grid.

[0015] Furthermore, the AC side variables include active power and reactive power injected into the AC system.

[0016] Furthermore, the DC side variables include bus voltage and DC current on the DC side of the converter.

[0017] Furthermore, the hybrid DC system model structure is specifically:

[0018] Taking LCC converter and VSC converter as the bottom layer, using two basic connection methods of LCC cascade and VSC parallel, multiple converters are combined into a converter station, and the converter stations are connected in series to form a DC terminal, thus constructing a converter-converter station-DC terminal model structure.

[0019] Furthermore, the power calculation of the converter injected into the DC system is based on the steady-state model of the VSC converter station to solve the active power on the tributary side of the VSC converter station.

[0020] Furthermore, the DC network power flow calculation is to solve the DC node voltage value according to the power balance equation and correction matrix under different control modes.

[0021] Furthermore, the power injected into the AC system by the converter is calculated as follows:

[0022] For the LCC converter, the LCC converter is equivalent to a PQ node in the AC network, and the power injected into the AC system is solved by the steady-state model of the LCC converter station;

[0023] For the VSC converter, the VSC converter is equivalent to a PQ or PV node in the AC network, and the power injected into the AC system is solved by the steady-state model of the VSC converter station.

[0024] Furthermore, the method further includes: if the DC variable exceeds the limit, switching the control mode;

[0025] If the trigger angle of the LCC converter exceeds the lower limit, it will be switched to the fixed minimum trigger angle control mode. In the non-layered access mode, the LCC converter cascaded with it will also be switched to the fixed minimum trigger angle control mode.

[0026] Furthermore, it also includes hierarchical access calculation, specifically:

[0027] When the LCC converter adopts a hierarchical access mode, the cascaded LCC converters are respectively connected to AC power grids of different voltage levels, and the power flow equation under the hierarchical access mode of the LCC converter is used to perform power flow calculation.

[0028] A second aspect of the present invention provides a hybrid DC power flow calculation system including LCC-VSC series-parallel connections.

[0029] The hybrid DC power flow calculation system containing LCC-VSC series-parallel connection includes a matrix construction module, a variable initialization module and an iterative calculation module:

[0030] The matrix construction module is configured to: obtain system parameters of the AC and DC systems including the LCC-VSCs connected in series and parallel, and construct AC-side and DC-side network matrices;

[0031] The variable initialization module is configured to: initialize AC side variables and DC side variables using the AC side and DC side network matrices;

[0032] The iterative calculation module is configured to: perform iterative power flow calculation on the AC-DC hybrid system of the hybrid DC system model structure based on the initialized AC-side variables and DC-side variables, update the AC-side variables and the DC-side variables until the iteration stop condition is met, and use the final AC-side variables and the DC-side variables as the result of the power flow calculation of the AC-DC hybrid system;

[0033] Among them, one iteration in the power flow calculation is to sequentially perform power calculation of the converter injected into the DC system, power calculation of the DC network, power calculation of the converter injected into the AC system and power calculation of the AC grid.

[0034] A third aspect of the present invention provides a computer-readable storage medium having a program stored thereon, which, when executed by a processor, implements the steps in the hybrid DC power flow calculation method containing LCC-VSC series-parallel connection as described in the first aspect of the present invention.

[0035] The fourth aspect of the present invention provides an electronic device, comprising a memory, a processor, and a program stored in the memory and executable on the processor, wherein when the processor executes the program, the steps in the hybrid DC power flow calculation method containing LCC-VSC series-parallel connection as described in the first aspect of the present invention are implemented.

[0036] One or more of the above technical solutions have the following beneficial effects:

[0037] Aiming at the problem that the diverse control modes and various complex series-parallel structures in the LCC-VSC hybrid DC transmission system make the power flow calculation difficult, the present invention proposes a power flow solution algorithm for an AC / DC hybrid system containing LCC-VSC series-parallel hybrid DC. In combination with actual projects, based on the hybrid DC transmission system model structure of the converter-converter station and the DC end, various converter series-parallel operation modes are flexibly expressed, and the power balance equations of various control modes under the above model structure are derived. At the same time, the LCC layered access mode is considered, which improves the applicability of the model in various control scenarios; based on the Newton-Raphson algorithm, an AC / DC hybrid system power flow solution algorithm containing a hybrid DC transmission system is designed, which effectively solves the power flow solution problem of the AC / DC hybrid system containing a hybrid DC transmission system, and provides a basis for the safety and stability analysis and planning of large-scale AC / DC hybrid power grids.

[0038] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0040] Figure 1 This is the structural diagram of the LCC steady-state model of the first embodiment.

[0041] Figure 2 This is a structural diagram of the VSC steady-state model of the first embodiment.

[0042] Figure 3 This is a single-pole schematic diagram of the Baihetan DC project in the first embodiment.

[0043] Figure 4 This is a schematic diagram of the structure of a hybrid DC power grid model in the first embodiment.

[0044] Figure 5 This is a simplified model structure diagram of hierarchical access for the first embodiment.

[0045] Figure 6 This is a flow chart of the algorithm for solving the alternating power flow of the AC / DC hybrid system in the first embodiment.

[0046] Figure 7 This is a schematic diagram of the approximate solution of the VSC converter in the first embodiment.

[0047] Figure 8 This is a structural diagram of the hybrid DC transmission system of the first embodiment. DETAILED DESCRIPTION

[0048] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0049] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0050] Embodiment 1

[0051] In one embodiment of the present disclosure, a hybrid DC power flow calculation method including LCC-VSC series-parallel connection is provided, comprising the following steps:

[0052] Obtain system parameters of the AC / DC system containing LCC-VSC series-parallel connection, and construct AC and DC side network matrices;

[0053] Using the AC side and DC side network matrices, initialize AC side variables and DC side variables;

[0054] Based on the initialized AC side variables and DC side variables, iterative power flow calculation is performed on the AC / DC hybrid system of the hybrid DC system model structure, and the AC side variables and DC side variables are updated until the iteration stop condition is met, and the final AC side variables and DC side variables are used as the results of the AC / DC hybrid system power flow calculation;

[0055] Among them, one iteration in the power flow calculation is to sequentially perform power calculation of the converter injected into the DC system, power calculation of the DC network, power calculation of the converter injected into the AC system and power calculation of the AC grid.

[0056] The implementation process of the hybrid DC power flow calculation method including LCC-VSC series-parallel connection in this embodiment is described in detail below.

[0057] 1. Steady-state model of LCC converter

[0058] In current practical projects, LCC-HVDC mostly uses a two-terminal system to achieve point-to-point power transmission, one side works in the rectification state, and the other side works in the inverter state. The DC side voltage and current and other parameters are easy to determine by solving the two ends together. In the hybrid DC transmission system, the DC side constitutes a DC grid, which has a flexible operation mode and cannot be solved together. Therefore, it is necessary to derive an independent solution method for the LCC converter.

[0059] The two ends here are DC ends, similar to DC nodes. The structure adopted by most LCC-HVDC transmission projects is to transmit from one DC end to another DC end. One plays a rectifier role and the other plays an inverter role. Therefore, the two sides can be solved jointly when solving; for example, DC current is the voltage at one end minus the voltage at the other end divided by the resistance value, and its DC network is a DC line; in a hybrid DC transmission system, the DC network can have multiple ends. At this time, there are multiple DC nodes and DC lines in the DC network, which is very complicated, so it cannot be solved jointly. Therefore, the independent solution mentioned here is to pass the corresponding variables to the LCC converter: DC voltage and DC current, and the converter can be solved.

[0060] The steady-state model of LCC is as follows Figure 1As shown in the figure, the two circles plus an arrow represent the converter transformer, in which the two circles represent the transformer and the arrow represents that the tap is adjustable; the LCC converter is connected to the AC grid through the converter transformer. The reference direction of the LCC converter operating in the rectification state is taken as positive, and its steady-state model is derived as follows:

[0061]

[0062]

[0063] P dc =U dc I dc (3)

[0064]

[0065] P s =P dc +P loss (5)

[0066] Q s =P s tanθ (6)

[0067] Among them, U dc and V ac E is the voltage of the DC bus and AC bus connected to the converter; ac is the AC voltage on the converter side of the converter transformer; I dc is the DC current on the DC side of the converter; N is the number of converter bridge groups; V drop is the voltage drop of the converter bridge; R and X are the impedance of the converter transformer; K and Tap are the ratio and tap of the converter transformer; α is the trigger angle of the converter; when working in the rectification state, it is the delayed trigger angle, and when working in the inverter state, it is the arc extinction angle; P dc Active power injected into the DC bus for the converter; P loss is the active power loss of the converter; P s and Q s is the active and reactive power injected into the AC system; θ is the power factor angle, and its calculation formula is:

[0068]

[0069]

[0070] Wherein, μ is the commutation overlap angle.

[0071] In this embodiment, the LCC model variables are divided into three categories: DC side variables, including U dc and I dc ; Converter variables, including α and Tap; AC side variables, including Ps and Q s First, the DC side variables are solved, and then the above formulas can be used to solve the converter variables and AC side variables in turn, thereby realizing the independent solution of the LCC converter.

[0072] 2. Steady-state model of VSC converter

[0073] The steady-state model of the VSC converter is as follows Figure 2 As shown, the VSC converter is connected to the AC grid through the commutation impedance, filter and converter transformer. The reference direction of the converter working in the rectification state is taken as positive, and its steady-state model is obtained as follows:

[0074]

[0075]

[0076] U c =U f +Z c I c (11)

[0077]

[0078] P dc =P c -P loss (13)

[0079] Among them, U s and U f are the complex voltages of the AC nodes connected to the converter and the filter nodes respectively; U c is the complex voltage on the converter valve side; I c is the AC complex current flowing to the converter; P s and Q s Inject active and reactive power of the DC system into the AC system; Q f is the reactive power absorbed by the filter; Z tf is the converter transformer impedance; Z c is the commutation impedance; P c is the active power injected into the converter; P dc Active power injected into the DC node by the converter; P loss is the active power loss of the converter.

[0080] In order to take into account the active power loss of the converter in the model, it is simulated as a polynomial function of the current, expressed as:

[0081] P loss =a+b|I c |+c|I c | 2(14)

[0082] Among them, a, b and c are the loss coefficients of the converter, which can be obtained by fitting the loss curve in actual engineering.

[0083] 3. Hybrid DC System Model Structure

[0084] In the hybrid DC transmission system, in order to improve the voltage level and transmission flexibility, various structures such as LCC cascade, VSC parallel and LCC-VSC series have emerged, such as Figure 3 The single-pole structure schematic diagram of the Baihetan DC project is shown. In order to express the series-parallel structure that appears in these actual projects, this embodiment proposes a hybrid DC grid model structure of converter-converter station-DC terminal.

[0085] The schematic diagram of the hybrid DC grid model structure of converter-converter station-DC terminal is as follows: Figure 4 As shown. The converter is the bottom layer in the hybrid DC grid structure, including LCC converter and VSC converter. The corresponding steady-state model is as described above. After the DC side variables of the converter (including DC voltage and DC current) are given, the independent solution of the converter can be completed, especially the power injected into the AC system by the converter. The specific solution process is in: "VIII. Algorithm for solving the flow alternation of AC / DC hybrid system containing hybrid DC transmission system" (3) Calculation of the power injected into the DC system by the converter; multiple converters form a converter station. Combined with the actual project, the converters form a converter station in two basic connection modes: LCC cascade and VSC parallel; finally, the converter station forms a DC end in series.

[0086] This embodiment is aimed at the flow calculation of a hybrid DC system containing LCC-VSC series-parallel. The basis of the flow calculation is to establish a steady-state model. When establishing a steady-state model, facing various complex series-parallel structures in the hybrid DC system, a single converter model cannot meet the needs of flow solution. For this reason, this embodiment proposes a converter-converter station-DC terminal model structure, which can represent various complex series-parallel connection methods, so that the proposed method can be free from scene restrictions and greatly improves the versatility. The structure proposed in this embodiment can realize the expression of various series-parallel forms in the hybrid DC power grid, greatly improving the versatility of the flow calculation method.

[0087] 4. Control Methods of Hybrid DC Transmission System

[0088] The basic control modes of LCC include fixed Pdc, fixed Idc, fixed Udc and fixed trigger angle control; VSC can realize decoupling control of active and reactive power, among which active control includes fixed Ps and fixed Udc control, and reactive control includes fixed Qs and fixed AC voltage amplitude control. By combining active control and reactive control, four basic control modes of VSC converter are obtained; the basic control modes of the converter are shown in Table 1.

[0089] Table 1 Converter control mode and AC node equivalent

[0090]

[0091] In a hybrid DC transmission system, it is necessary to coordinate the control methods of LCC and VSC. This embodiment introduces its coordinated control method based on the hybrid DC power grid structure of converter-converter station-DC terminal. Combining the basic control method of LCC and the basic control method of active power of VSC converter, the highest-level DC terminal has four terminal control modes: voltage terminal mode, current terminal mode, power terminal mode and trigger angle terminal mode. In order to maintain the stable operation of the DC system, at least one DC terminal needs to adopt the voltage terminal mode to maintain the voltage stability and power balance of the DC system.

[0092] After the terminal control mode of the DC terminal is defined, the station control mode of the converter station connected to this terminal needs to be the same as the terminal control mode, that is, voltage station mode, current station mode, power station mode and trigger angle station mode. Constant current control and fixed trigger angle control are control methods unique to LCC converters. Therefore, in the current station mode and trigger angle station mode, the converter station must adopt the LCC cascade connection method, and the corresponding LCC converter adopts fixed current control. dc and fixed α control; in power station mode, the LCC and VSC converters in the converter station adopt fixed power control. In voltage station mode, for the LCC cascade mode, the LCC converter adopts a fixed DC voltage control mode, and for the VSC parallel connection mode, one VSC converter station is selected to adopt a fixed DC voltage control mode, and the remaining VSC converters adopt a fixed power control mode.

[0093] 5. DC Power Grid Steady-State Model

[0094] The power flow model of the DC grid is the node power balance equation. For a hybrid DC system with n DC nodes, the power balance expression of the i-th node is:

[0095]

[0096] Among them, ΔP i is the power imbalance of node i; P Trefi is the injected active power of the DC terminal connected to node i, P Gi and PLi They are respectively the active power injected by the generator connected to the DC node and the reactive power absorbed by the load; ij is the element in the i-th row and j-th column of the DC network admittance matrix; U di and U dj is the DC voltage at DC node i and DC node j.

[0097] Therefore, in order to establish a DC power grid model, it is necessary to obtain the active power injected into the DC node by the DC end, which is determined by the control mode of the hybrid DC system; if the DC end adopts the power end mode, the converter station connected to the DC end adopts the power station mode, and the converter in the station also adopts fixed active power control. At this time, the power at the DC end is easy to obtain, which is the sum of the DC active power of the converter station. The sum of the active power of the converter station is the sum of the DC side active power of all converters in the station. The expression is:

[0098] P Trefi =∑P Sref (16)

[0099] P Sref =∑P Cref (17)

[0100] Among them, P Sref is the DC active power of the converter station at the DC end; P Cref It is the sum of the DC active powers of the converters in the converter station.

[0101] If the DC end adopts the current end mode, the connection mode of the converter station is LCC cascade. First, ensure that the current control instructions between the cascaded LCCs in the converter station are the same. This instruction value is the current instruction value I of the current end. Tref , the active power injected into the DC node by the DC terminal in the current terminal mode is:

[0102] P Trefi =U di I Trefi (18)

[0103] If the DC end adopts the firing angle end mode, the converter station in the DC end adopts the LCC cascade mode, and the LCC converter adopts the fixed firing angle control mode; according to the steady-state model of the LCC converter, it can be seen that if the firing angle of the LCC converter is fixed, its DC voltage and DC power satisfy the functional relationship:

[0104]

[0105] Among them, α ref is the trigger angle command value of the LCC converter. At this time, the current expression of the LCC converter can be obtained as:

[0106]

[0107] In normal operation, in the non-hierarchical access mode, the parameters of the cascaded LCC are the same, so the LCC operating state is known, the DC voltage is the same, and the total number of cascaded LCC converters is nLCC. The relationship between the DC voltage and current of the LCC converter and the DC voltage and current at the DC end can be obtained as follows:

[0108] U dc =U di / n LCC (twenty one)

[0109] I dc =I di (twenty two)

[0110] Substituting (20) into (19), the DC terminal current expression in the trigger angle terminal mode is obtained as follows:

[0111]

[0112] Then the power expression injected into the DC node is obtained as:

[0113]

[0114] If the DC end adopts the voltage end mode, the DC voltage is known and there is no need to write the power balance equation. The DC voltage value is the sum of the voltage reference values ​​of each converter station connected in series, and the expression is:

[0115] U Trefi =∑U Sref (25)

[0116] For the converter station, if the converter station adopts the LCC cascade mode, its voltage reference value is the sum of the voltage control instructions of each LCC converter in the station; if the converter station adopts the VSC parallel mode, it can be seen that one VSC converter needs to adopt constant DC voltage control, and the other VSC converters adopt constant active power control. At this time, the voltage reference value of the converter station is the voltage instruction value of the converter using constant DC voltage control.

[0117] 6. Solution of Hybrid DC Transmission System

[0118] The steady-state models of the converter and the DC power grid are established. The next step is to solve the power flow. This embodiment uses the Newton-Raphson algorithm to solve the DC power grid.

[0119] The model of the DC grid is the node power balance equation. For an n-node DC grid, there are m nodes connected to the DC end and operating in the voltage end mode. The remaining nm nodes can write the power balance equation, which is expressed in matrix form:

[0120]

[0121] Where A is the coefficient matrix of the power balance equation.

[0122] Based on the node power balance equation, the Newton-Raphson algorithm can be used to obtain the modified equation expression:

[0123]

[0124] Among them, the expression of the elements in the Jacobi matrix can be derived from the DC power grid steady-state model; when the DC terminal connected to the DC node adopts the power terminal mode, the expression is:

[0125]

[0126] When the converter station adopts the current terminal mode, the expression is:

[0127]

[0128] When the converter station adopts the firing angle end mode, the expression is:

[0129]

[0130] 7. Hierarchical Access Computing

[0131] In the above description, the hierarchical access mode of the LCC converter is not considered, and the control parameters and transformer parameters of the cascaded LCC converters remain consistent; when the LCC converter adopts a hierarchical access mode, the cascaded LCC converters are respectively connected to AC power grids of different voltage levels, and the control parameters and transformer parameters are different, which makes the solution more complicated. This embodiment derives the power flow equation under the hierarchical access mode of the LCC converter.

[0132] LCC hierarchical access method Figure 5 As shown, the high-end converter transformer and low-end circulating transformer of the receiving-end cascaded LCC converter are respectively connected to AC grids of different voltage levels. At this time, the LCC converter works in the inverter state, and generally operates in the form of fixed DC voltage control and fixed arc extinction angle control. When the cascaded converters connected in layers all adopt fixed DC voltage control, the solution method of the voltage end mode described above can still be used to solve. When a converter adopts fixed trigger angle control, the above solution method is no longer applicable, and the power flow equation needs to be re-derived.

[0133] When both converters at the receiving end of layered access adopt fixed trigger angle control, their DC voltage and DC current satisfy the functional relationship:

[0134]

[0135]

[0136] The subscripts H and L represent a high-end converter and a low-end converter, respectively.

[0137] The voltage value of the DC node is equal to the sum of the voltages of the two converters, and the current values ​​of the two converters are the same. Therefore, the functional relationship between the DC node and the DC current can be obtained as follows:

[0138]

[0139] From this, the node injection power expression can be obtained as:

[0140]

[0141] Using the above power flow expression, it can be deduced that when the DC terminal connected to the DC node i adopts a hierarchical access mode and both the high-end and low-end converters adopt fixed trigger angle control, the expression of the Jacobi matrix element when solving the DC system power flow is:

[0142]

[0143] When one of the two converters at the receiving end of the layered access adopts fixed trigger angle control and the other adopts fixed DC voltage control, taking the low-end converter adopting fixed trigger angle control as an example, the voltage-current function relationship between the two converters is:

[0144]

[0145] U dcL =U ref (37)

[0146] At the same time, the DC node voltage and current satisfy:

[0147] U i =U dcH +U ref (38)

[0148] I i =I dcH (39)

[0149] Combining (34)-(35), we can get the DC node voltage and current function relationship as follows:

[0150]

[0151] Therefore, the power expression injected into the DC node by the DC terminal is:

[0152]

[0153] Using the above power flow expression, it can be deduced that when the DC terminal connected to the DC node i adopts a hierarchical access mode and the low-end converter adopts a fixed trigger angle control, and the high-end converter adopts a fixed DC voltage control, the expression of the Jacobi matrix element when solving the DC system power flow is:

[0154]

[0155] When the high-end converter adopts fixed trigger angle control and the low-end converter adopts fixed DC voltage control, the solution method is consistent with formula (40), and the relevant parameters are replaced by the low-end converter parameters.

[0156] 8. Alternative solution algorithm for power flow of AC / DC hybrid system including hybrid DC transmission system

[0157] The idea of ​​the alternating solution method is to solve the AC system and the DC system independently. The flow chart of the alternating solution algorithm for the power flow of the AC / DC hybrid system including the hybrid DC transmission system is as follows: Figure 6 shown.

[0158] 1. First, read the data of the AC power grid and the DC power grid to form the AC side and DC side network matrix.

[0159] The data here are the structural parameters of the power grid, such as how many AC buses, how many DC buses, and the connection methods and impedances of the AC and DC lines; the network matrix is ​​the admittance matrix, the order is determined by the number of buses, and the elements are determined by the admittance.

[0160] 2. Then, initialize the AC variables and DC variables, specifically:

[0161] AC variables include bus voltage and phase angle. A flat start method is used. If the bus voltage is not specified, its initial value is set to 1 per unit and the phase angle is 0 degrees. The DC variable is the DC voltage. If the DC bus voltage is not specified, its initial value is set to 1 per unit.

[0162] 3. The single iteration process of power flow calculation of AC / DC hybrid system is divided into four steps: power calculation of converter injected into DC system, DC network power flow calculation, power calculation of converter injected into AC system and AC grid power flow calculation, specifically:

[0163] (1) Calculation of power injected into the DC system by the converter

[0164] The control power variable of the LCC converter is the DC side power, while the power control variable of the VSC converter is the AC side power. Therefore, it is necessary to calculate the VSC converter that adopts a fixed active power, that is, according to the steady-state model (9)-(14) of the VSC converter station, solve the active power on the branch side of the VSC converter station; the initial values ​​of the AC side voltage and reactive power are determined by the control method; if the VSC reactive power control method is fixed reactive power control, the reactive power on the AC side is the command value, the voltage amplitude is the rated voltage on the AC side, and the phase angle is 0°; if the VSC reactive power control method is fixed AC voltage amplitude control, the AC voltage amplitude is the command value, the phase angle is 0°, and the reactive power is 0MVar; it is kept unchanged during each iteration, and is updated according to the AC network solution result after a single iteration.

[0165] The update here depends on the control method of the VSC converter. In the steady-state model of the VSC converter, it can be seen that there are four variables where the AC system is connected on the left: active power, reactive power, AC voltage amplitude and AC voltage phase angle. If these four values ​​have a given control target, they will be fixed to the corresponding control instruction value. If not, they will be updated after each iteration. For these variables without given control targets, the initial values ​​are set as described above in the first iteration: if there is no given control target, the initial value of power is 0, the initial value of voltage amplitude is 1 per unit, and the phase angle is 0.

[0166] (2) DC network power flow calculation

[0167] That is, according to the power balance equation and correction matrix under different control modes, the DC node voltage value is solved. The detailed process has been described above.

[0168] Here, the DC network power flow calculation ultimately solves the DC voltage of each DC node; after the DC voltage is determined, the current power and so on can be obtained. It can be seen from formula (25) that each calculation obtains the correction amount of the DC bus voltage, that is, the DC voltage is corrected to meet the constraints of the steady-state model, so the calculation here refers to the calculation process of (25).

[0169] (3) Calculation of power injected into the AC system by the converter

[0170] After the DC network solution is completed, the DC side variables of each converter are known, and it is necessary to solve the power injected into the AC system, so as to complete the independent solution of the AC power grid. For the LCC converter, it is equivalent to a PQ node in the AC network. According to the LCC steady-state model (1)-(8), in order to solve the power injected into the AC system, it is necessary to determine the trigger angle and transformer tap of the LCC converter. The method adopted in this embodiment is to first set the converter trigger angle to the lower limit value, and the optimal transformer tap can be determined by (1). After determining the tap, the actual trigger angle of the converter is obtained to check whether it is within a reasonable range.

[0171] For the VSC converter, it is equivalent to a PQ or PV node in the AC network. The value of reactive power or AC voltage is determined by the reactive control method, while the active power injected into the AC system is determined by the active control method. If a fixed active power control is adopted, P is taken as the command value; if a fixed DC voltage control is adopted, it is necessary to perform a power flow calculation on the converter, that is, to solve the AC measured active power according to the active power on the DC side of the VSC converter. It can be seen from (9)-(14) that to solve the DC side power from the AC measured power, a simple power flow calculation is sufficient, while to solve the AC side power from the DC measured power, it is necessary to solve nonlinear equations, which is very complicated. In order to simplify the solution, an approximate solution method is adopted, as shown in the schematic diagram. Figure 7 As shown, let Pdc0 be the DC power of the converter, and take P acL 0.95 times P dc0 , P acH 1.05 times P dc0 , using the calculation method of the power injected into the DC system by the converter, the corresponding P dcL and P dcH , for P dcL and P dcH The curve between them is linearized and approximated to obtain P dc0 The corresponding P acnew , according to P dcnew and P dc0 The relationship between P acL or P acH Replace with P acnew , and then proceed to the next solution until P dcnew and P dc0 The error between them meets the requirements.

[0172] (4) AC system power flow calculation

[0173] The solution can be completed using the existing Newton-Raphson power flow solution algorithm; the power flow model of the AC power grid is the power balance equation, and the AC voltage is expressed in polar coordinates, as shown in the formula:

[0174]

[0175]

[0176] Among them, ΔP si and ΔQ si is the node power imbalance; P acGi and Q acGi The node AC power source generates active power and reactive power; PacLi and QacLi absorb active power and reactive power for the node load; U si is the voltage amplitude of AC node i; G ij and B ij is the real and imaginary part of the element in the i-th row and j-th column of the AC system node admittance matrix; θ ij is the phase angle difference between AC node i and AC node j.

[0177] After the solution is completed, the variables such as the AC side voltage and power of the converter are updated until the power convergence criterion is met or the maximum number of iterations is exceeded.

[0178] If the DC variable exceeds the limit, the control mode is switched; if the trigger angle of the LCC converter exceeds the lower limit, it is switched to the fixed minimum trigger angle control mode. In the non-layered access mode, the cascaded LCC converter is also switched to the fixed minimum trigger angle control mode. If the power of the fixed DC voltage converter in the parallel VSC exceeds the limit, the next priority converter is selected according to the priority to adopt the fixed DC voltage control, and the converter that exceeds the limit is changed to the fixed active power control.

[0179] To verify the effectiveness of the method proposed in this embodiment, a hybrid DC transmission system is added to the IEEE 39 node system to form an AC / DC hybrid system. The structure of the hybrid DC system is as follows: Figure 8 As shown, the sending end adopts LCC1 and LCC2 cascade to control the transmission power, and the AC sides of LCC1 and LCC2 converters are connected to the fourth AC bus; the receiving end adopts LCC-VSC series connection to control the DC voltage.

[0180] Using the DC model structure proposed in this embodiment, the hybrid DC transmission system includes two DC ends, namely the sending end and the receiving end; the sending end includes a converter station DC Station1, which adopts the LCC cascade operation mode; the receiving end includes two converter stations DC Station2 and DC Station3, of which DC Station2 adopts the LCC cascade operation mode and DC Station3 adopts the VSC parallel operation mode. The control mode of the converter is shown in Table 2.

[0181] Table 2 Control methods and parameters of converters in hybrid DC transmission system

[0182]

[0183] The power flow calculation converged after 7 global iterations, showing good convergence. The power flow calculation results are shown in Table 3. It can be seen that both the LCC converter and the VSC converter operate at the corresponding control command value level, and the sum of the DC currents of VSC1 and VSC2 is equal to the DC current of LCC3, satisfying the current relationship of the series circuit.

[0184] Table 3 Hybrid DC system power flow calculation results

[0185]

[0186] To summarize the results, in order to solve the problem that the power flow calculation is difficult due to the diverse control modes and various complex series-parallel structures in the LCC-VSC hybrid DC transmission system, a power flow solution algorithm for the AC-DC hybrid system containing LCC-VSC series-parallel hybrid DC is proposed; combined with actual projects, based on the hybrid DC transmission system model structure of converter-converter station and DC end, various converter series-parallel operation modes are flexibly expressed. The power balance equations of various control modes under the above model structure are derived, and the LCC hierarchical access method is considered at the same time, which improves the applicability of the model in various control scenarios. Based on the Newton-Raphson algorithm, an AC-DC hybrid system power flow solution algorithm containing a hybrid DC transmission system is designed, which effectively solves the power flow solution problem of the AC-DC hybrid system containing a hybrid DC transmission system, and provides a basis for the safety and stability analysis and planning of large-scale AC-DC hybrid power grids.

[0187] Embodiment 2

[0188] In one embodiment of the present disclosure, a hybrid DC power flow calculation system including LCC-VSC series-parallel connection is provided, including a matrix construction module, a variable initialization module and an iterative calculation module:

[0189] The matrix construction module is configured to: obtain system parameters of the AC and DC systems including the LCC-VSCs connected in series and parallel, and construct AC-side and DC-side network matrices;

[0190] The variable initialization module is configured to: initialize AC side variables and DC side variables using the AC side and DC side network matrices;

[0191] The iterative calculation module is configured to: perform iterative power flow calculation on the AC-DC hybrid system of the hybrid DC system model structure based on the initialized AC-side variables and DC-side variables, update the AC-side variables and the DC-side variables until the iteration stop condition is met, and use the final AC-side variables and the DC-side variables as the result of the power flow calculation of the AC-DC hybrid system;

[0192] Among them, one iteration in the power flow calculation is to sequentially perform power calculation of the converter injected into the DC system, power calculation of the DC network, power calculation of the converter injected into the AC system and power calculation of the AC grid.

[0193] Embodiment 3

[0194] The purpose of this embodiment is to provide a computer-readable storage medium.

[0195] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps in the hybrid DC power flow calculation method containing LCC-VSC series-parallel connection as described in the first embodiment of the present disclosure.

[0196] Embodiment 4

[0197] The purpose of this embodiment is to provide an electronic device.

[0198] An electronic device includes a memory, a processor, and a program stored in the memory and executable on the processor. When the processor executes the program, the steps in the hybrid DC power flow calculation method containing LCC-VSC series-parallel connection as described in the first embodiment of the present disclosure are implemented.

[0199] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A hybrid DC power flow calculation method containing LCC-VSC series-parallel connection, characterized in that: include: Obtain system parameters of the AC / DC system containing LCC-VSC in series and parallel connection, and construct AC and DC side network matrices; Using the AC side and DC side network matrices, initialize AC side variables and DC side variables; Based on the initialized AC side variables and DC side variables, iterative power flow calculation is performed on the AC / DC hybrid system of the hybrid DC system model structure, and the AC side variables and DC side variables are updated until the iteration stop condition is met, and the final AC side variables and DC side variables are used as the results of the AC / DC hybrid system power flow calculation; The hybrid DC system model structure is specifically as follows: taking LCC converters and VSC converters as the bottom layer, using two basic connection modes of LCC cascade and VSC parallel connection, multiple converters are formed into a converter station, and the converter stations are connected in series to form a DC terminal, thereby constructing a converter-converter station-DC terminal model structure; One iteration of the power flow calculation is to sequentially perform power calculation of the converter injected into the DC system, power calculation of the DC network, power calculation of the converter injected into the AC system, and power calculation of the AC grid; The flow of the alternating solution algorithm for the AC / DC hybrid system including the hybrid DC transmission system is as follows: First, the data of the AC power grid and the DC power grid are read to form the AC side and DC side network matrices; the data are the structural parameters of the power grid, and the network matrix is ​​an admittance matrix, the order of which is determined by the number of buses, and the elements are determined by the admittance; Secondly, initialize the AC variables and DC variables, specifically: AC variables include bus voltage and phase angle. Flat start mode is adopted. If bus voltage is not specified, its initial value is set to 1 per unit and phase angle is 0 degrees. DC variable is DC voltage. If DC bus voltage is not specified, its initial value is set to 1 per unit. The single iteration process of the power flow calculation of the AC / DC hybrid system is divided into four steps: power calculation of the converter injected into the DC system, DC network power flow calculation, power calculation of the converter injected into the AC system and AC grid power flow calculation, specifically: According to the steady-state model of the VSC converter station, the active power on the tributary side of the VSC converter station is solved; the initial values ​​of the voltage and reactive power on the AC side are determined by the control mode; if the VSC reactive control mode is fixed reactive power control, the reactive power on the AC side is the command value, the voltage amplitude is the rated voltage on the AC side, and the phase angle is 0°; if the VSC reactive control mode is fixed AC voltage amplitude control, the AC voltage amplitude is the command value, the phase angle is 0°, and the reactive power is 0MVar; it remains unchanged during each iteration, and is updated according to the AC network solution result after a single iteration is completed; The steady-state model of the VSC converter station is as follows: in, U s and U f are the complex voltages of the AC nodes connected to the converter and the filter nodes respectively; U c is the complex voltage on the valve side of the converter; I c is the AC complex current flowing to the converter; P s and Q s Inject active and reactive power from the DC system into the AC system; Q f is the reactive power absorbed by the filter; Z tf is the converter transformer impedance; Z c is the commutation impedance; P c is the active power injected into the converter; P dc Active power injected into the DC node for the converter; P loss is the active power loss of the converter; According to the power balance equation and correction matrix under different control modes, the DC node voltage value is solved; the DC voltage value is the sum of the voltage reference values ​​of each converter station connected in series, and the expression is: ； Let Pdc0 be the solved converter DC power, and take P acL 0.95 times P dc0 , P acH 1.05 times P dc0 , using the calculation method of the power injected into the DC system by the converter, find the corresponding P dcL and P dcH ,right P dcL and P dcH The curve between them is linearized and approximated to obtain P dc0 Corresponding P acnew ,according to P dcnew and P dc0 The relationship between P acL or P acH Replace with P acnew , and then proceed to the next solution until P dcnew and P dc0 The error between them meets the requirements; The solution is completed using the Newton-Raphson power flow solving algorithm; the power flow model of the AC power grid is the power balance equation, and the AC voltage is expressed in polar coordinates, as shown in the formula: Among them, Δ P si and Δ Q si is the node power imbalance; P acGi and Q acGi It generates active power and reactive power for the node AC power supply; PacLi and QacLi absorb active power and reactive power for the node load; U si For communication nodes i The voltage amplitude of G ij and B ij is the node admittance matrix of the AC system. i Line j real and imaginary parts of column elements; θ ij For communication nodes i and communication nodes j The phase angle difference; After the solution is completed, the AC side voltage and power variables of the converter are updated until the power convergence criterion is met or the maximum number of iterations is exceeded; If the DC variable exceeds the limit, the control mode is switched; if the trigger angle of the LCC converter exceeds the lower limit, it is switched to the fixed minimum trigger angle control mode. In the non-layered access mode, the LCC converter cascaded with it is also switched to the fixed minimum trigger angle control mode; if the power of the fixed DC voltage converter in the parallel VSC exceeds the limit, the converter with the next priority is selected according to the priority to adopt fixed DC voltage control, and the converter that exceeds the limit is changed to fixed active power control.

2. The hybrid DC power flow calculation method containing LCC-VSC series-parallel connection according to claim 1, characterized in that: The AC side variables include active power and reactive power injected by the AC system.

3. The hybrid DC power flow calculation method containing LCC-VSC series-parallel connection according to claim 1, characterized in that: The DC side variables include the bus voltage and DC current on the DC side of the converter.

4. The hybrid DC power flow calculation method containing LCC-VSC series-parallel connection according to claim 1, characterized in that: The calculation of the power injected into the DC system by the converter is based on the steady-state model of the VSC converter station to solve the active power on the DC side of the VSC converter station.

5. The hybrid DC power flow calculation method containing LCC-VSC series-parallel connection according to claim 1, characterized in that: The DC network power flow calculation is to solve the DC node voltage value according to the power balance equation and correction matrix under different control modes.

6. The hybrid DC power flow calculation method containing LCC-VSC series-parallel connection according to claim 1, characterized in that: The power injected into the AC system by the converter is calculated as follows: For the LCC converter, the LCC converter is equivalent to a PQ node in the AC network, and the power injected into the AC system is solved by the steady-state model of the LCC converter station; For the VSC converter, the VSC converter is equivalent to a PQ or PV node in the AC network, and the power injected into the AC system is solved by the steady-state model of the VSC converter station.

7. The hybrid DC power flow calculation method containing LCC-VSC series-parallel connection according to claim 1, characterized in that: Also includes: If the DC variable exceeds the limit, the control mode is switched; If the trigger angle of the LCC converter exceeds the lower limit, it will be switched to the fixed minimum trigger angle control mode. In the non-layered access mode, the LCC converter cascaded with it will also be switched to the fixed minimum trigger angle control mode.

8. The hybrid DC power flow calculation method containing LCC-VSC series-parallel connection according to claim 1, characterized in that: It also includes hierarchical access calculations, specifically: When the LCC converter adopts a hierarchical access mode, the cascaded LCC converters are respectively connected to AC power grids of different voltage levels, and the power flow equation under the hierarchical access mode of the LCC converter is used to perform power flow calculation.

9. A hybrid DC power flow calculation system containing LCC-VSC series-parallel connection, characterized in that: Includes matrix construction module, variable initialization module and iterative calculation module: The matrix construction module is configured to: obtain system parameters of the AC and DC systems including the LCC-VSCs connected in series and parallel, and construct AC-side and DC-side network matrices; The variable initialization module is configured to: initialize AC side variables and DC side variables using the AC side and DC side network matrices; The iterative calculation module is configured to: perform iterative power flow calculation on the AC-DC hybrid system of the hybrid DC system model structure based on the initialized AC-side variables and DC-side variables, update the AC-side variables and the DC-side variables until the iteration stop condition is met, and use the final AC-side variables and the DC-side variables as the result of the power flow calculation of the AC-DC hybrid system; The hybrid DC system model structure is specifically as follows: taking LCC converters and VSC converters as the bottom layer, using two basic connection modes of LCC cascade and VSC parallel connection, multiple converters are formed into a converter station, and the converter stations are connected in series to form a DC terminal, thereby constructing a converter-converter station-DC terminal model structure; One iteration of the power flow calculation is to sequentially perform power calculation of the converter injected into the DC system, power calculation of the DC network, power calculation of the converter injected into the AC system, and power calculation of the AC grid; The flow of the alternating solution algorithm for the AC / DC hybrid system including the hybrid DC transmission system is as follows: First, the data of the AC power grid and the DC power grid are read to form the AC side and DC side network matrices; the data are the structural parameters of the power grid, and the network matrix is ​​an admittance matrix, the order of which is determined by the number of buses, and the elements are determined by the admittance; Secondly, initialize the AC variables and DC variables, specifically: AC variables include bus voltage and phase angle. Flat start mode is adopted. If bus voltage is not specified, its initial value is set to 1 per unit and phase angle is 0 degrees. DC variable is DC voltage. If DC bus voltage is not specified, its initial value is set to 1 per unit. The single iteration process of the power flow calculation of the AC / DC hybrid system is divided into four steps: power calculation of the converter injected into the DC system, DC network power flow calculation, power calculation of the converter injected into the AC system and AC grid power flow calculation, specifically: According to the steady-state model of the VSC converter station, the active power on the tributary side of the VSC converter station is solved; the initial values ​​of the voltage and reactive power on the AC side are determined by the control mode; if the VSC reactive control mode is fixed reactive power control, the reactive power on the AC side is the command value, the voltage amplitude is the rated voltage on the AC side, and the phase angle is 0°; if the VSC reactive control mode is fixed AC voltage amplitude control, the AC voltage amplitude is the command value, the phase angle is 0°, and the reactive power is 0MVar; it remains unchanged during each iteration, and is updated according to the AC network solution result after a single iteration is completed; The steady-state model of the VSC converter station is as follows: in, U s and U f are the complex voltages of the AC nodes connected to the converter and the filter nodes respectively; U c is the complex voltage on the valve side of the converter; I c is the AC complex current flowing to the converter; P s and Q s Inject active and reactive power from the DC system into the AC system; Q f is the reactive power absorbed by the filter; Z tf is the converter transformer impedance; Z c is the commutation impedance; P c is the active power injected into the converter; P dc Active power injected into the DC node for the converter; P loss is the active power loss of the converter; According to the power balance equation and correction matrix under different control modes, the DC node voltage value is solved; the DC voltage value is the sum of the voltage reference values ​​of each converter station connected in series, and the expression is: ； Let Pdc0 be the solved converter DC power, and take P acL 0.95 times P dc0 , P acH 1.05 times P dc0 , using the calculation method of the power injected into the DC system by the converter, find the corresponding P dcL and P dcH ,right P dcL and P dcH The curve between them is linearized and approximated to obtain P dc0 Corresponding P acnew ,according to P dcnew and P dc0 The relationship between P acL or P acH Replace with P acnew , and then proceed to the next solution until P dcnew and P dc0 The error between them meets the requirements; The solution is completed using the Newton-Raphson power flow solving algorithm; the power flow model of the AC power grid is the power balance equation, and the AC voltage is expressed in polar coordinates, as shown in the formula: Among them, Δ P si and Δ Q si is the node power imbalance; P acGi and Q acGi It generates active power and reactive power for the node AC power supply; PacLi and QacLi absorb active power and reactive power for the node load; U si For communication nodes i The voltage amplitude of G ij and B ij is the node admittance matrix of the AC system. i Line j real and imaginary parts of column elements; θ ij For communication nodes i and communication nodes j The phase angle difference; After the solution is completed, the AC side voltage and power variables of the converter are updated until the power convergence criterion is met or the maximum number of iterations is exceeded; If the DC variable exceeds the limit, the control mode is switched; if the trigger angle of the LCC converter exceeds the lower limit, it is switched to the fixed minimum trigger angle control mode. In the non-layered access mode, the LCC converter cascaded with it is also switched to the fixed minimum trigger angle control mode; if the power of the fixed DC voltage converter in the parallel VSC exceeds the limit, the converter with the next priority is selected according to the priority to adopt fixed DC voltage control, and the converter that exceeds the limit is changed to fixed active power control.

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