Optimization method and system containing distributed new energy power distribution network load flow calculation

Abstract

The invention discloses an optimization method and system for load flow calculation of a power distribution network containing distributed new energy, and relates to the field of reactive power regulation configuration of a power distribution network containing a new energy field station, and the optimization method comprises the steps of issuing a short-time-scale power generation instruction in a grid-connected preparation stage of a grid-connected inverter, and carrying out a load flow simplified calculation strategy in a long-time scale. The grid-connected preparation stage comprises the steps of analyzing output characteristics of a new energy grid-connected converter, constructing a static impedance model, obtaining key parameters such as voltage, current and phase of each device connected to a power distribution network node, changing active and reactive output, obtaining multi-dimensional and multi-group data, receiving a power generation instruction of an inverter, and starting a grid-connected operation initial stage. According to a static model containing a distributed new energy power distribution network, forward-backward substitution power flow calculation of PQPV nodes is carried out, operation of a grid-connected converter can be controlled more accurately, meanwhile, overall actual active output and reactive output of new energy equipment in a district can be deduced reversely, and dispatching and planning of reactive voltage in the district are facilitated.

Description

Technical Field

[0001] The present invention relates to the field of reactive power regulation configuration of a new energy station distribution network, and in particular to an optimization method and system for power flow calculation of a distributed new energy distribution network. Background Art

[0002] In recent years, large-scale intermittent clean energy such as photovoltaic power stations and wind farms have been integrated into the power grid at a high rate, and the distribution network system has evolved towards a large and complex trend. Regarding the reactive power planning of the distribution network containing new energy stations, the uncertainty, indirectness and instability of new energy power generation itself make the power demand of the distribution network and the changing characteristics of power load more difficult to predict, making the problem of reactive power and voltage fluctuations in the distribution network gradually prominent.

[0003] In traditional power systems, problems such as system active power and frequency are usually solved by unified centralized control. However, the characteristics of reactive power that are not suitable for long-distance and large-scale transmission and voltage regulation that are not suitable for global control determine the principles that need to be met for reactive power and voltage regulation in power systems. With the increasing proportion of new energy access, it is difficult to match the reactive power and voltage regulation of the power system with the actual local operation of new energy power plants. At this stage, based on the sub-area control of the plant, the two main methods of external reactive equipment regulation and optimized flow calculation methods are used to solve the problems of reactive power and voltage stability of distributed new energy distribution networks.

[0004] Taking energy storage as an example, the external reactive device regulation method usually adopts the comprehensive operation control strategy of the external access energy storage system, cuts in from the perspective of the active and reactive power of the system connected to the energy storage system, improves the power system flow, improves the node voltage quality, reduces the grid voltage deviation, and controls the system transmission network loss. In this type of technology, most of them are based on reducing the node voltage deviation. Although the control effect is good, the reactive planning cost is high, and it is necessary to consider the continuous switching of the energy storage system, which poses certain challenges to the control algorithm.

[0005] The optimization power flow calculation method is based on the construction of the distribution network power flow model, and the active power and reactive power of the distribution network are considered as constraints for calculation. However, under the condition of microgrid, the multiple iterations of the power flow calculation method still take up too much time, and cannot accurately measure the reactive output of the distribution network under the new energy station to achieve the purpose of station-level voltage regulation. Therefore, it is necessary to propose a strategy for the rapid implementation of the active distribution network power flow calculation containing new energy stations, to achieve a more accurate calculation of the active and reactive output of new energy equipment, so as to achieve the purpose of better dispatching and transmitting reactive power, and to rely on the hierarchical control of the equipment in the new energy station to realize the principle of "layered and partitioned, local balance" of reactive power at the station level. Summary of the invention

[0006] In view of the above existing problems, the purpose of the present invention is to realize the power flow calculation of the distribution network containing new energy stations more accurately and quickly, simplify the calculation complexity of conventional power flow algorithms, and improve the accuracy of the reactive power output measurement of the station. Specifically, the present invention proposes a simplified optimization strategy for the power flow calculation of a distribution network containing distributed new energy. By analyzing the output characteristics of the new energy grid-connected converter, the distribution network is simplified into a whole composed of static devices and motor components. By constructing a static model, the distributed new energy distribution network is simplified once and planned as a device impedance model for the segmented area. Then, based on this model, assuming that the active and reactive outputs of the root nodes of the new energy grid-connected converter equipment connected to the feeder are known, the key parameters such as voltage, current, and phase of each device connected to the distribution network node can be obtained. By changing the known output at the new energy station level and performing the same power flow calculation, the electrical parameters of the feeder node equipment under different parameters can be obtained, and multi-dimensional data can be obtained, which is convenient for the edge gateway of the new energy station to issue instructions to the new energy grid-connected converter equipment to generate electricity. This step responds quickly to realize the power flow analysis stage of the distribution network containing new energy stations. Within the time scale of the grid-connected converter equipment control algorithm responding to the power generation command issued by the edge gateway, the forward-backward substitution method of the PQ\PV node power flow calculation is performed based on the static model of the distributed renewable energy distribution network. This can more accurately control the operation of the grid-connected converter, and at the same time, the overall actual active and reactive output of the renewable energy equipment in the area can be reversed, which is convenient for the scheduling and planning of reactive voltage in the area.

[0007] In order to solve the above technical problems, an optimization method for power flow calculation of distributed renewable energy distribution network is proposed, including:

[0008] It includes three stages of grid-connected power generation operation of distributed new energy distribution network, including grid-connected preparation stage, grid-connected initial operation stage and grid-connected normal operation stage; in the grid-connected preparation stage, the output characteristics of the new energy grid-connected inverter are evaluated, the system static model is constructed, the impedance matrix is ​​formed, and the power flow calculation of the known reactive power generation indicators within a short time scale is performed; in the initial stage of grid-connected operation, the information and instructions of the output voltage and current of the inverter equipment are received, and the grid-connected program is started; in the normal operation stage of the grid-connected power flow analysis based on the LCL type three-phase grid-connected inverter access feeder node, based on the PQ\PV switchable node, the system power flow calculation within a long time scale is performed, and instructions are issued to the inverter equipment to realize its normal and efficient grid-connected operation, and at the same time, the reactive output of the equipment is reversed to facilitate station scheduling.

[0009] As a preferred solution of the optimization method of power flow calculation of a distributed renewable energy distribution network described in the present invention, the three stages include: the grid preparation stage performs a short-time scale fast power flow calculation and analysis at the renewable energy station level, the initial stage of grid operation and the normal grid operation stage perform a long-time scale fast and stable grid power flow calculation and analysis;

[0010] The grid connection preparation stage includes data preprocessing through the LCL type three-phase grid-connected inverter device, modeling the output impedance of the grid-connected inverter of the new energy station power generation equipment, and obtaining the device parameters of the internal capacitor and inductor of the LCL type three-phase grid-connected inverter;

[0011] Extract the voltage and current at the grid connection point, use harmonic linearization, establish the relationship between the positive and negative sequence voltage and current harmonic components at the grid connection point through the inverter control structure, obtain the analytical expression of the positive and negative sequence impedance of the inverter, and plan the impedance model construction of the grid-connected inverter equipment in the segmented area;

[0012] The impedance data of the grid-connected inverter in the new energy station is analyzed and calculated according to the time series, and the impedance basis of the grid-connected line is considered. The inverter output impedance model is constructed according to whether the phase-locked loop is considered.

[0013] As a preferred solution of the optimization method for power flow calculation of a distributed renewable energy distribution network described in the present invention, the grid connection preparation stage also includes constructing a unified model between active power, reactive power and device output impedance based on the root node of the feeder connected to the renewable energy station, and calculating the active power and reactive power based on the short time scale of the time series and the voltage and current output reference value of the corresponding device connected to the distribution network node;

[0014] After obtaining the impedance model of each inverter device, the reference value instructions of the output voltage and current of each renewable energy power generation equipment in each area are received according to the known active power and reactive power of the reactive power output dispatch center of the station containing distributed renewable energy, and the instructions are sent to the communication interface of each grid-connected inverter to instruct the internal grid-connected control algorithm of the grid-connected inverter to execute;

[0015] According to the calculated positive and negative sequence impedance values ​​of each device output and the known active and reactive power, the fast power flow calculation is as follows:

[0016] Combined with the radial characteristics of the distribution network, the node hierarchical method is used to describe it. The active and reactive power of the nodes are known, and the voltage and current of the grid-connected inverter equipment of new energy are calculated. In the AC power system, the complex power S of each node i Including active power P i and reactive power Q i , and the relationship between voltage and current is as follows:

[0017]

[0018] Among them, S i is the complex power of node i, P i is the active power of node i, Q i is the reactive power of node i, j is the imaginary unit, Vi is the voltage at node i, Y ik is the admittance matrix element between node i and node k, is the voltage V at node k k The conjugate complex number of , N is the total number of nodes;

[0019] Current I ik According to Ohm's law, we can get the following from the node voltage and branch impedance:

[0020] I ik =Y ik (V i -V k )

[0021] Among them, Y ik is the admittance from node i to node k, V i and V k are the voltages at node i and node k respectively;

[0022] By changing the known output value of the active distribution network containing new energy stations, repeatedly obtaining the device parameters of the internal capacitor and inductor of the LCL type three-phase grid-connected inverter, obtaining the electrical parameters of the feeder power-saving equipment under different parameters, and obtaining multi-dimensional data based on the active and reactive output indicators and the grid-connected output voltage and current of the LCL type three-phase grid-connected inverter.

[0023] As a preferred solution of the optimization method for power flow calculation of a distributed renewable energy distribution network described in the present invention, the initial stage of grid-connected operation includes calculating the voltage and current output reference values ​​of each grid-connected node and the power generation instructions issued by the active and reactive power dispatching center of the active distribution network containing renewable energy stations according to the grid-connected preparation stage, and the grid-connected inverter devices in different areas receive the power generation instructions and perform grid-connected startup control;

[0024] The grid-connected startup control is to initialize the LCL inverter system and detect whether the system state is correct. When the system state is incorrect, it is determined whether a startup failure occurs. When a startup failure occurs, the software operation process is terminated to determine whether there is a problem with the system hardware and the drive module. When no startup failure occurs, it returns to the system initialization stage to continue execution and record an error log;

[0025] When the system status is correct, check whether the inverter grid-connected condition is normal. When the inverter grid-connected condition detection is passed, the LCL type three-phase inverter grid-connected startup process is carried out, including the boost circuit startup, MPPT grid-connected control algorithm and the normal operation process of grid-connected power generation.

[0026] As a preferred solution of the optimization method of power flow calculation of a distributed renewable energy distribution network described in the present invention, the initial stage of grid-connected operation also includes: in the MPPT grid-connected control algorithm, the inverter receives the grid-connected operation active power and reactive power generation indicators sent by the reactive power dispatching center in the station, and uses the current indicators as the input power limit of the inverter DC side;

[0027] During the grid-connected startup phase and subsequent normal operation, the inverter keeps real-time monitoring of the system status. When the system is in normal status, MPPT grid-connected control power generation is implemented;

[0028] When the system is in an abnormal state, the driver is blocked to detect whether a startup failure occurs. When a startup failure occurs, the software operation process is terminated to determine whether there is a problem with the system hardware or the driver module;

[0029] When no startup failure occurs, the system returns to the system initialization phase to continue execution and record an error log.

[0030] As a preferred solution of the optimization method of power flow calculation of a distributed renewable energy distribution network described in the present invention, the normal operation phase of the grid connection includes importing the initial parameters of the network, including load parameters and parameters of each branch, initializing the grid connection parameters of the distributed power source and the reactive initial value of the PV node:

[0031] Q i (1) =Q i min +(Q i max -Q imin ) / 2

[0032] Among them, Q imax , Q imin are the upper and lower limits of the node reactive power;

[0033] Active distribution network topology analysis: the electrical network is layered according to the electrical distance from the root node to form a node layered matrix; the power flow calculation and analysis based on the forward and backward method of PV and PQ switchable nodes are performed. In the first iteration, the voltage of the starting node is set to the voltage of all nodes, and the current of all branches is set to 0, and k=1; the injection current of distributed power sources of different node types is forward calculated and superimposed with the injection current of the load node; the voltage of each node is back-calculated, and the PQ, PI, and PQ (V) nodes can be obtained according to the voltage calculation formula of each node:

[0034]

[0035] Among them, U i , U jare the voltages of node j and its upper node i, respectively, Z ij is the impedance of the branch with i and j as the first and last nodes;

[0036] The output voltage and current of renewable energy generation are constrained for convergence judgment. The voltage amplitude at the PV type DG node should meet the Among them, U pv is the node voltage at the i-th PQ node obtained in this calculation, represents the voltage amplitude calculated in this iteration, It represents the known voltage amplitude calculated in the previous iteration, and δ represents a small positive number close to 0. It is judged whether the convergence condition is met. If it is met, the voltage of each node is output and iterated k times. If it is not met, the forward calculation is performed to enter the next iteration. At the same time, k=k+1 is set to judge whether the reactive value of the PV node is out of bounds. If it is out of bounds, the injected reactive power of the PV node is corrected, and the injected current of each type of distributed power source is recalculated. If it is not out of bounds, the PV node is converted to a PQ node.

[0037] As a preferred solution of the optimization method for power flow calculation of a distributed renewable energy distribution network described in the present invention, the normal grid-connected operation stage also includes outputting the data of power flow calculation of the forward-backward substitution method as a power generation instruction, transforming the face-to-face relationship between the distribution network and the microgrid into a face-to-point relationship, and sending the power generation instruction to all LCL-type three-phase grid-connected inverter devices in the segmented area. When the inverter receives the power generation instruction from the dispatching center, it performs internal control output adjustment.

[0038] Another object of the present invention is to provide an optimization system for power flow calculation of a distribution network containing distributed new energy. The present invention aims to improve the stability and efficiency of the new energy grid-connected inverter before grid connection, grid connection startup and normal operation; optimize the operating state of the distribution network through accurate power flow calculation to ensure system safety, stability and efficiency; realize refined management and scheduling of distributed power sources to improve the utilization rate of new energy; and adapt to the complex distribution network structure containing distributed energy by optimizing the power flow calculation method to improve the overall performance of the distribution network.

[0039] As a preferred solution of the optimization system for power flow calculation of a distributed renewable energy distribution network described in the present invention, it is characterized by comprising a grid connection preparation module, a grid connection initial operation module and a grid connection normal operation module;

[0040] The grid connection preparation module receives the power generation equipment data of the new energy station, evaluates the output characteristics of the new energy grid-connected inverter, builds a system static model, forms an impedance matrix, builds an output impedance model of the inverter, calculates the active and reactive output reference values ​​within a short time scale, and performs power flow calculation within a short time scale;

[0041] The grid-connected operation initial module receives the reference value calculated by the grid-connected preparation module and the power generation instruction of the dispatching center, starts the grid-connected program, receives the output voltage and current information and instructions of the inverter equipment, and performs grid-connected startup control;

[0042] The grid-connected normal operation module imports network parameters and distributed power supply parameters, performs system power flow calculations over a long time scale, and outputs the calculation results as power generation instructions to the inverter device, and the inverter adjusts the output according to the instructions.

[0043] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and is characterized in that when the processor executes the computer program, the steps of the optimization method for power flow calculation of a distributed new energy distribution network are implemented.

[0044] A computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of the optimization method for power flow calculation of a distributed new energy distribution network are implemented.

[0045] The beneficial effects of the present invention are as follows: the present invention improves the stability and grid-connected operation efficiency of the new energy grid-connected inverter before grid connection, grid connection startup and normal operation stages; by simplifying the power flow calculation, the power generation startup process of the distribution network containing distributed new energy is optimized, and the distribution network connected to the distributed power source is refined. The management and scheduling, improve the utilization rate of its new energy; by optimizing the power flow calculation method, it adapts to the complex distribution network structure containing distributed energy and improves the overall performance of the distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0047] Figure 1 An overall flow chart of an optimization method for power flow calculation of a distributed renewable energy distribution network provided by an embodiment of the present invention.

[0048] Figure 2 A flowchart of the grid connection preparation phase including power flow calculation of a distributed renewable energy distribution network is provided for one embodiment of the present invention.

[0049] Figure 3 A flowchart of the initial stage of grid-connected operation including power flow calculation of a distributed renewable energy distribution network is provided for one embodiment of the present invention.

[0050] Figure 4 A flowchart of the normal operation phase of grid-connected power distribution network including power flow calculation of distributed renewable energy is provided for one embodiment of the present invention.

[0051] Figure 5 A system solution module diagram of an optimization system for distributed renewable energy distribution network power flow calculation provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0052] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0053] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0054] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it an embodiment that is mutually exclusive with other embodiments, either individually or selectively.

[0055] The present invention is described in detail with reference to schematic diagrams. When describing the embodiments of the present invention, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

[0056] At the same time, in the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "upper, lower, inner and outer" are based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first, second or third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0057] In the present invention, unless otherwise clearly specified and limited, the terms "install, connect, connect" should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection or an integral connection; it can also be a mechanical connection, an electrical connection or a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0058] Example 1, reference Figure 1-Figure 4 , which is the first embodiment of the present invention, and provides an optimization method for power flow calculation of a distribution network containing distributed new energy, comprising:

[0059] S1: The present invention implements a fast power flow calculation process for each access node of each renewable energy power generation equipment in each area of ​​a distribution network containing renewable energy stations based on known active and reactive outputs.

[0060] Reference Figure 2 ,S11: In the grid connection preparation stage, data preprocessing is performed. Relying on the more widely used LCL type three-phase grid-connected inverter equipment, a model is constructed for the output impedance of the grid-connected inverter of common new energy site power generation equipment such as photovoltaic and wind power, and the device parameters such as internal capacitance and inductance of the LCL type three-phase grid-connected inverter are obtained.

[0061] The grid-connected inverter output impedance model needs to consider the grid-connected inverter control phase-lock angle. First, the frequency characteristics of the phase-locked loop are not considered, and the inverter output impedance model is established. At this time, the phase-lock angle θ p Given as an ideal.

[0062] According to the grid-connected inverter structure, the circuit equations of the inverter output voltage, the voltage and current at the common connection point PCC in the abc coordinate system are as follows:

[0063]

[0064] Where s represents the complex frequency in the Laplace transform of the transfer function, C f Indicates the capacitance of the filter capacitor of the LCL type three-phase grid-connected inverter, L 1 , L 2 Indicates the bridge-side filter inductance and grid-side filter inductance of the LCL three-phase grid-connected inverter. To suppress the inherent resonance of the LCL inverter, a filter capacitor is connected in series with a resistor, and its resistance is R d ,u ak 、u bk 、u ck 、u ag 、u bg 、u cg 、i ag 、i bg 、i cgThey are the three-phase voltage on the inverter side and the three-phase voltage and current at the grid connection point. According to the symmetrical component method, they can be converted into positive, negative and zero sequence expressions:

[0065]

[0066] Among them, u i1 、u i2 、u in 、u 1 、u 2 、u 0 、i 1 、i 2 、i 0 They are respectively the positive, negative and zero sequence components of the voltage and current at the inverter side and the grid connection point. The harmonic linearization method is used to establish the relationship between the positive and negative sequence and the harmonic components of the current through the above-mentioned grid-connected inverter control structure and other links, and the expression of the grid connection point current of phase a in the three-phase symmetrical system in the time domain is defined as follows:

[0067]

[0068] Among them, i 1 is the positive sequence fundamental current amplitude of the power grid, i p 、i n are the current amplitudes of positive and negative sequence harmonics, ω 1 is the fundamental angular frequency (100πrad / s), ω p ,ω n are the angular frequencies of the positive and negative sequence harmonics, is the initial phase angle of the corresponding component.

[0069] After dq coordinate transformation of the grid-connected current, we can get i d (t) and i q (t), after Fourier transform, we get its frequency domain expression:

[0070]

[0071] Among them, I p ,I n It is expressed as the amplitude of positive and negative sequence current harmonics, f represents the frequency, j is the imaginary unit, f p The frequency of the positive sequence impedance, f n The frequency of the negative sequence impedance, f 1 Indicates the fundamental frequency, I 1 Indicates the fundamental current amplitude of the power grid, represents the initial phase angle of a certain harmonic component, They are the frequency domain expressions of positive and negative sequence current harmonics, respectively:

[0072]

[0073] Where e represents the natural index, Indicates the positive and negative sequence phase angle of this harmonic.

[0074] Let K dq is the dq axis cross decoupling coefficient, and the dq axis DC output of the grid-connected inverter voltage and current is D 0 , Q 0 If PI control is used as the control method to eliminate the net difference of grid-connected current, voltage, dq-axis output DC quantity, its expression is H i (s) = k p +k i / s, where k p represents the proportional parameter of the PI controller design, k i Represents the integral parameter of the PI controller design. Therefore, the expression of the output voltage in the frequency domain can be obtained:

[0075]

[0076] If the dq axis results are known, the output voltage values ​​of the three phases abc can be inferred. Taking phase a as an example, U ak In the frequency domain it is expressed as:

[0077]

[0078] Among them, I 1 Indicates the fundamental current amplitude of the power grid, is the initial phase angle of the component corresponding to the fundamental wave, It means that PI control is used as the control method to eliminate the net difference of grid-connected current, voltage, dq axis output DC quantity, which means

[0079] S12: Extract the voltage and current at the grid connection point, adopt the harmonic linearization method, establish the relationship between the positive and negative sequence voltage and current harmonic components at the grid connection point through the inverter control structure and other links, so as to obtain the analytical expression of the positive and negative sequence impedance of the inverter, and construct the impedance model of the grid-connected inverter equipment planned for the segmented area, which can be used for the analysis and calculation of the impedance data of the grid-connected inverter in the new energy station according to the time series. On the basis of considering the line impedance on the grid side, the inverter output impedance model is constructed according to whether the phase-locked loop is considered;

[0080] According to the relationship between positive and negative sequence harmonic voltage and current, the positive and negative sequence output impedance Z is derived. pp (s), Z nn (s) are as follows:

[0081]

[0082] When considering the PLL phase-locked loop, when the grid voltage has positive and negative sequence harmonics, the angle obtained by phase locking is not only the positive rotation angle θ obtained by the fundamental frequency voltage. 1 , including the angle disturbance Δθ caused by the harmonic voltage, the phase-locking angle can be expressed as θ P =θ 1 +Δθ, where θ 1 =2πf 1 . Sample the three-phase voltage at PCC, taking phase a as an example, its expression in the time domain is:

[0083]

[0084] Among them, u 1 is the positive sequence fundamental voltage amplitude of the power grid, u p 、u n are the voltage amplitudes of the positive and negative sequence harmonics, ω 1 is the fundamental angular frequency (100πrad / s), ω p ,ω n are the angular frequencies of the positive and negative sequence harmonics, is the initial phase angle of the corresponding component. After Fourier transformation, the voltage of phase a is expressed in the frequency domain as:

[0085]

[0086] in, They are respectively expressed as the grid positive-sequence fundamental voltage amplitude, positive-sequence harmonic voltage amplitude, and negative-sequence harmonic voltage amplitude.

[0087] Similarly, the frequency domain expressions of phase b and phase c voltages can be obtained.

[0088] If the PLL is controlled by a PI controller, its transfer function is H PLL (s)=(k pPLL +k iPLL / s) / s, where k pPLL represents the proportional parameter of the PI controller design, k iPLL Represents the integral parameter of the PI controller design.

[0089] According to the above derivation, the dq axis current components can be obtained, and the expressions are as follows:

[0090]

[0091] in, Referring to the above derivation method, the impedance expression can be obtained as follows:

[0092]

[0093] Where D 0 , Q0 ,ω 1 The expression is as follows:

[0094]

[0095] S13: Construct a unified model between the active power, reactive power and equipment output impedance based on the root node of the feeder connected to the new energy station, and calculate the active and reactive output based on the short time scale of the time series and the voltage and current output reference value of the corresponding equipment connected to the distribution network node;

[0096] After obtaining the impedance model of each inverter device, the reference value instructions for the output voltage and current of each renewable energy power generation equipment in each area are received based on the known active power and reactive power of the reactive power output dispatch center of the station containing distributed renewable energy, and the instructions are sent to the communication interface of each grid-connected inverter to instruct the internal grid-connected control algorithm of the grid-connected inverter to execute. The following is a fast power flow calculation in a short time scale according to the calculated positive and negative sequence impedance values ​​of each device output and the known active and reactive power:

[0097] Considering the radial characteristics of the distribution network, a node hierarchical method is used to describe the original data of the distribution network. The following format is used:

[0098] Branch parameter matrix BranchM:

[0099] {branch connected to node 1, branch connected to node 2, branch impedance parameters}

[0100] Node parameter matrix NodeM:

[0101] {node number, active load, reactive load}

[0102] Given the active and reactive power of a node, the following calculation method is used to calculate the voltage and current of the new energy grid-connected inverter equipment:

[0103] In the AC power system, the complex power S of each node i (including active power P i and reactive power Q i ) is related to voltage and current as follows:

[0104]

[0105] Among them, S i is the complex power of node i (unit: kVA), P i is the active power of node i (unit: kW), Q i is the reactive power of node i (unit: kVAr), V i is the voltage of node i (unit: pu or kV), j is the imaginary unit, Y ikis the admittance matrix element between node i and node k (unit: S, Siemens), is the conjugate complex number of the voltage at node k, and N is the total number of nodes.

[0106] Current I ik It can be obtained from the node voltage and branch impedance by Ohm's law:

[0107] I ik =Y ik (V i -V k )

[0108] Among them, Y ik is the admittance from node i to node k, V i and V k are the voltages at node i and node k respectively.

[0109] S14: By changing the known output value of the active distribution network containing the new energy station and repeating the calculation of step S13, the electrical parameters of the feeder power-saving equipment under different parameters can be obtained, and multi-dimensional data based on the active and reactive output indicators and the grid-connected output voltage and current of the LCL-type three-phase grid-connected inverter can be obtained in advance.

[0110] S2: In the initial stage of grid-connected operation, the voltage and current output reference values ​​of each grid-connected node and the power generation instructions issued by the active and reactive power dispatching center of the active distribution network including the new energy station are calculated according to the above steps. The grid-connected inverter equipment in different areas receives the power generation instructions and performs grid-connected startup control.

[0111] The LCL inverter system is initialized, the system status is detected to be correct, and the inverter grid-connected condition is detected to be normal. If the detection of whether the system status is correct fails, it is determined whether a startup failure has occurred. If a startup failure has occurred, it is necessary to end the software operation process to determine whether there is a problem with the system hardware, driver and other modules; if no startup failure has occurred, return to the system initialization stage to continue execution and record the error log.

[0112] If all the above processes can be passed, the LCL type three-phase inverter grid-connected startup process will be carried out, including the boost circuit startup, MPPT grid-connected control algorithm and the normal operation process of grid-connected power generation.

[0113] In the MPPT grid-connected control algorithm, the inverter receives the active power and reactive power generation indicators for grid-connected operation sent by the reactive power dispatching center in the station, and it uses this indicator as the limit of the input power on the DC side of the inverter.

[0114] During the grid-connected startup phase and subsequent normal operation, the inverter needs to monitor the system status in real time. If it is normal, the above power generation process will be implemented; if it is abnormal, the drive will be blocked to detect whether a fault has occurred. If a startup failure is determined, the software operation process needs to be terminated to determine whether there is a problem with the system hardware, drive and other modules; if no startup failure is determined, return to the system initialization phase to continue execution and record the error log.

[0115] S3: During the normal operation of the grid connection, the following operations are performed over a longer time scale:

[0116] S31: The active and reactive power dispatching center performs power flow calculation by forward push-back method, and calculates the specific and accurate reference values ​​of active and reactive power generation of each grid-connected node by calculating the convergence of its power flow;

[0117] The power flow calculation of the distribution network containing new energy stations during the normal operation of the grid connection stage is to transform the original one-way power flow into a multi-source weak loop network with uncertain power flow direction. The face-to-face relationship between the distribution network and the microgrid is transformed into a face-to-point relationship. At this time, since the new energy equipment is in a grid-connected power generation state, the active power and reactive power output to the root node are known, so the power flow calculation and analysis can be re-performed to more accurately constrain the output voltage and current of the new energy power generation. Based on the branch-oriented forward and back substitution method, the PQ node is calculated and processed, and the parameters of the branch path are used as iterative variables to avoid the complex calculation process of the Jacobian matrix. It has good numerical stability, small storage volume, fast convergence speed, and overcomes the limitation of the R / X ratio of the distribution network, improving the speed and accuracy of the actual station instruction issuance.

[0118] The steps for calculating the power flow of the distribution network including new energy stations are as follows:

[0119] Import the initial parameters of the network, including load parameters and parameters of each branch.

[0120] Parameter initialization: Initialize the distributed power grid-connected parameters and the reactive power initial values ​​of the PV nodes.

[0121] Q i (1) =Q i min +(Q i max -Q imin ) / 2

[0122] Among them, Q imax , Q imin It is the upper and lower limits of node reactive power.

[0123] The electrical network is layered according to the electrical distance from the root node.

[0124] In the first iteration, the voltage of the starting node is first set to the voltage of all nodes, and the current of all branches is set to 0, and k=1.

[0125] The injected current of distributed generation at different node types is calculated forward and superimposed with the injected current of the load node.

[0126] Back-calculate the voltage of each node. The PQ, PI, and PQ(V) nodes can be obtained according to the voltage calculation formula of each node:

[0127]

[0128] Among them, U i , U j are the voltages of node j and its upper node i, respectively, Z ij is the impedance of the branch with i and j as the first and last nodes, which is already Figure 2 The method shown is obtained in the static characteristics of the inverter for renewable energy power generation and grid connection.

[0129] Convergence judgment is performed. The voltage amplitude at the PV type DG node should meet Among them, U pv is the node voltage at the i-th PQ node obtained in this calculation, represents the voltage amplitude calculated in this iteration, It represents the known voltage amplitude calculated in the previous iteration, and δ represents a small positive number close to 0. It is judged whether the convergence condition is met. If it is met, the voltage of each node is output and iterated k times. If it is not met, the forward calculation is performed to enter the next iteration, and k=k+1 is set at the same time.

[0130] When the boundary is crossed, the injected reactive power of the PV node is corrected and the injected current of each type of distributed power source is recalculated; when the boundary is not crossed, the PV type node is converted to a PQ type node.

[0131] The specific implementation method of the expansion model of a specific node is described as follows:

[0132] Construct the model shown in the PQ node:

[0133]

[0134] Among them, C i is the set of sub-branches of branch i, P i ,P Li ,P Di are the active power at the head end of branch i, the active power loss of the line, and the active power loss of the load, respectively. Li ,Q Di are the line reactive loss and load reactive loss of branch i respectively; P jis the active power at the head end of branch j; Q i is the reactive power at the head end of branch i; Q j is the reactive power at the head end of branch j; k is the number of iterations, and k+1 is the next iteration. j is the equivalent resistance of branch j; X j is the equivalent reactance of branch j; V j is the node voltage amplitude of branch j.

[0135] The voltage phasor can be expressed as

[0136]

[0137] The voltage amplitude is calculated as

[0138]

[0139] Construct the model shown in the PV node:

[0140] The grid-connected inverter of the new energy power generation equipment is connected to the feeder as a PV node. Assuming there are n of them, all at the end of the network, let Z = R + jX be the network equivalent impedance matrix at the PV node, where Z is the system impedance. Let the current correction amount injected at the PV node be ΔI (n × 1) = ΔC + jΔD, where ΔC is the real part and ΔD is the imaginary part. Let the voltage increment at the PV node be ΔU (n × 1) = ΔE + jΔF, where ΔE is the real part and ΔF is the imaginary part: Then:

[0141] ΔU=ZΔI

[0142] Where ΔU is the voltage increment, ΔI is the current increment. Decompose the above formula into real and imaginary parts:

[0143]

[0144] Among them, R and X are the real and imaginary parts of the network equivalent impedance matrix at the PV node; ΔC and ΔD represent the real and imaginary parts of the injection current correction at the PV node.

[0145] Since the voltage amplitudes of the nodes in the distribution network are very small when the distribution network is operating normally, and the voltage amplitude of the PV node is constant during the iteration process, it can be considered that:

[0146]

[0147] Among them, U pv is the PV node voltage obtained during the iteration process, U mb is the given PV node voltage amplitude, that is, U mb =U mb∠0°, Re{} means taking its real part, and Im{} means taking its imaginary part. Thus, we can get:

[0148]

[0149] The power correction amount injected at the PV node can be obtained as for:

[0150]

[0151] Among them, ΔI * is the conjugate of ΔI, j is the imaginary unit, and The real and imaginary parts of the active power and reactive power correction values ​​injected into the PV node are obtained as follows:

[0152]

[0153] Since the active power injected into the PV node is constant, that is:

[0154] ΔP=ΔC·Re{U pv}+ΔD·Im{U pv}=0

[0155] Im{U pv = -ΔD -1 (ΔC·Re{U pv})

[0156] The reactive power correction amount injected at the PV node is obtained as:

[0157] ΔQ=X -1 Re{U pv -U mb}·Re{U pv}

[0158] Among them, X -1 Indicates the reciprocal of the system reactance value.

[0159] The value of the real part of the voltage at each node in the distribution network is approximately equal to 1. Therefore, the reactive power correction amount injected at the PV node is mainly determined by the network equivalent reactance matrix and voltage deviation at the PV node. The reactance matrix can be transformed according to the content of the aforementioned implementation method to obtain the PV node flow calculation and correction analysis.

[0160] S32: Send instructions to each LCL-type three-phase grid-connected inverter device in the segment area, and its internal voltage and current control can ensure that the active and reactive power scheduling requirements of the new energy station are met within a shorter time sequence.

[0161] When the inverter receives the power generation command from the dispatching center, it adjusts its internal control output. For details, see Figure 3The normal operation part of the inverter facilitates the reverse calculation of the active and reactive output at the new energy station level, and facilitates the implementation of power scheduling at the station level.

[0162] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

[0163] Embodiment 2, the second embodiment of the present invention, is different from the first two embodiments in that:

[0164] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, and other media that can store program codes.

[0165] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0166] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering or, if necessary, processing in another suitable manner, and then stored in a computer memory.

[0167] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, a plurality of steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0168] Example 3, reference Figure 5 , which is the third embodiment of the present invention, and provides an optimization system for power flow calculation of a distributed new energy distribution network, including a grid-connected preparation module 10, a grid-connected operation initial module 20 and a grid-connected normal operation module 30;

[0169] The grid connection preparation module 10 receives the data of the power generation equipment of the new energy station, evaluates the output characteristics of the new energy grid-connected inverter, builds a system static model, forms an impedance matrix, builds an output impedance model of the inverter, calculates the active and reactive output reference values ​​within a short time scale, and performs power flow calculation within a short time scale;

[0170] The grid-connected operation initialization module 20 receives the reference value calculated by the grid-connected preparation module 10 and the power generation instruction of the dispatching center, starts the grid-connected program, receives the output voltage and current information and instructions of the inverter equipment, and performs grid-connected startup control;

[0171] The grid-connected normal operation module 30 imports network parameters and distributed power supply parameters, performs system power flow calculations over a long time scale, and outputs the calculation results as power generation instructions to the inverter device, and the inverter adjusts the output according to the instructions.

[0172] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. An optimization method for power flow calculation of a distributed renewable energy distribution network, characterized in that: include, It includes three stages of grid-connected power generation operation of distributed renewable energy distribution network, including grid-connected preparation stage, grid-connected initial operation stage and grid-connected normal operation stage; In the grid connection preparation stage, the output characteristics of the new energy grid-connected inverter are evaluated, a system static model is constructed, an impedance matrix is ​​formed, and power flow calculations with known reactive power generation indicators within a short time scale are performed; In the initial stage of grid-connected operation, the information and instructions of the output voltage and current of the inverter equipment are received to start the grid-connected procedure; During the normal grid-connected operation phase, based on the flow analysis of the LCL-type three-phase grid-connected inverter connected to the feeder node, the system flow calculation within a long time scale is performed based on the PQ and PV switchable nodes, and instructions are issued to the inverter equipment for grid-connected operation, while the reactive output of the equipment is inferred.

2. The optimization method for power flow calculation of a distributed renewable energy distribution network according to claim 1, characterized in that: The three stages include: the grid connection preparation stage, which involves rapid calculation and analysis of power flows at the new energy station level on a short time scale; the initial grid connection operation stage and the normal grid connection operation stage, which involve rapid and stable grid connection operation on a long time scale; The grid connection preparation stage includes data preprocessing through the LCL type three-phase grid-connected inverter device, modeling the output impedance of the grid-connected inverter of the new energy station power generation equipment, and obtaining the device parameters of the internal capacitor and inductor of the LCL type three-phase grid-connected inverter; Extract the voltage and current at the grid connection point, use harmonic linearization, establish the relationship between the positive and negative sequence voltage and current harmonic components at the grid connection point through the inverter control structure, obtain the analytical expression of the positive and negative sequence impedance of the inverter, and plan the impedance model construction of the grid-connected inverter equipment in the segmented area; The impedance data of the grid-connected inverter in the new energy station is analyzed and calculated according to the time series, and the impedance basis of the grid-connected line is considered. The inverter output impedance model is constructed according to whether the phase-locked loop is considered.

3. The optimization method for power flow calculation of a distributed renewable energy distribution network according to claim 2, characterized in that: The grid connection preparation stage also includes constructing a unified model between active power, reactive power and equipment output impedance based on the root node of the feeder connected to the new energy station, and calculating the active and reactive output based on the short time scale of the time series and the voltage and current output reference value of the corresponding equipment connected to the distribution network node; After obtaining the impedance model of each inverter device, the reference value instructions of the output voltage and current of each renewable energy power generation equipment in each area are received according to the known active power and reactive power of the reactive power output dispatch center of the station containing distributed renewable energy, and the instructions are sent to the communication interface of each grid-connected inverter to instruct the internal grid-connected control algorithm of the grid-connected inverter to execute; According to the calculated positive and negative sequence impedance values ​​of each device output and the known active and reactive power, the fast power flow calculation is as follows: Combined with the radial characteristics of the distribution network, the node hierarchical method is used to describe it. The active and reactive power of the nodes are known, and the voltage and current of the grid-connected inverter equipment of new energy are calculated. In the AC power system, the complex power S of each node i Including active power P i and reactive power Q i , and the relationship between voltage and current is as follows: Among them, S i is the complex power of node i, P i is the active power of node i, Q i is the reactive power of node i, j is the imaginary unit, V i is the voltage at node i, Y ik is the admittance matrix element between node i and node k, is the conjugate complex number of the voltage at node k, and N is the total number of nodes; Current I ik According to Ohm's law, we can get the node voltage and branch impedance: I ik =Y ik (V i -V k ) Among them, Y ik is the admittance from node i to node k, V i and V k are the voltages at node i and node k respectively; By changing the known output value of the active distribution network containing new energy stations, repeatedly obtaining the device parameters of the internal capacitor and inductor of the LCL type three-phase grid-connected inverter, obtaining the electrical parameters of the feeder power-saving equipment under different parameters, and obtaining multi-dimensional data based on the active and reactive output indicators and the grid-connected output voltage and current of the LCL type three-phase grid-connected inverter.

4. The optimization method for power flow calculation of a distributed renewable energy distribution network according to claim 3, characterized in that: The initial stage of grid-connected operation includes calculating the voltage and current output reference values ​​of each grid-connected node and the power generation instructions issued by the active power distribution network active and reactive power dispatching center including the new energy station according to the grid-connected preparation stage, and the grid-connected inverter equipment in different areas receives the power generation instructions and performs grid-connected startup control; The grid-connected startup control is to initialize the LCL inverter system and detect whether the system state is correct. When the system state is incorrect, it is determined whether a startup failure occurs. When a startup failure occurs, the software operation process is terminated to determine whether there is a problem with the system hardware and the drive module. When no startup failure occurs, it returns to the system initialization stage to continue execution and record an error log; When the system status is correct, check whether the inverter grid-connected condition is normal. When the inverter grid-connected condition detection is passed, the LCL type three-phase inverter grid-connected startup process is carried out, including the boost circuit startup, MPPT grid-connected control algorithm and the normal operation process of grid-connected power generation.

5. The optimization method for power flow calculation of a distributed renewable energy distribution network according to claim 4, characterized in that: The initial stage of grid-connected operation also includes, in the MPPT grid-connected control algorithm, the inverter receives the grid-connected operation active power and reactive power generation index sent by the reactive power dispatching center in the station, and uses the current index as the input power limit of the inverter DC side; During the grid-connected startup phase and subsequent normal operation, the inverter keeps real-time monitoring of the system status. When the system is in normal status, MPPT grid-connected control power generation is implemented; When the system is in an abnormal state, the driver is blocked to detect whether a startup failure occurs. When a startup failure occurs, the software operation process is terminated to determine whether there is a problem with the system hardware or the driver module; When no startup failure occurs, the system returns to the system initialization phase to continue execution and record an error log.

6. The optimization method for power flow calculation of a distributed renewable energy distribution network according to claim 5, characterized in that: The normal operation phase of the grid connection includes importing the initial parameters of the network, including load parameters and parameters of each branch, initializing the grid connection parameters of the distributed power source and the initial reactive power value of the PV node: Among them, Q imax , Q imin are the upper and lower limits of the node reactive power; Active distribution network topology analysis: the electrical network is layered according to the electrical distance from the root node to form a node layered matrix; the power flow calculation and analysis based on the forward and backward method of PV and PQ switchable nodes are performed. In the first iteration, the voltage of the starting node is set to the voltage of all nodes, and the current of all branches is set to 0, and k=1; the injection current of distributed power sources of different node types is forward calculated and superimposed with the injection current of the load node; the voltage of each node is back-calculated, and the PQ, PI, and PQ (V) nodes can be obtained according to the voltage calculation formula of each node: Among them, U i , U j are the voltages of node j and its upper node i, respectively, Z ij is the impedance of the branch with i and j as the first and last nodes; The output voltage and current of renewable energy generation are constrained for convergence judgment. The voltage amplitude at the PV type DG node should meet the Among them, U pv is the node voltage at the i-th PQ node obtained in this calculation; determine whether the convergence condition is met. If so, output the voltage of each node and iterate k times. If not, perform forward calculation to enter the next iteration. At the same time, let k=k+1 to determine whether the reactive value of the PV type node is out of bounds. If so, correct the injected reactive power of the PV node and recalculate the injected current of each type of distributed power source; if not, the PV type node is converted to a PQ type node.

7. The optimization method for power flow calculation of a distributed renewable energy distribution network according to claim 6, characterized in that: The normal grid-connected operation stage also includes outputting the data of the power flow calculation of the forward-backward substitution method as a power generation instruction, transforming the face-to-face relationship between the distribution network and the microgrid into a face-to-point relationship, and sending the power generation instruction to all LCL-type three-phase grid-connected inverter devices in the sub-area. When the inverter receives the power generation instruction from the dispatching center, it performs internal control output adjustment.

8. A system using the optimization method for power flow calculation of a distributed renewable energy distribution network as claimed in any one of claims 1 to 7, characterized in that: It includes the grid connection preparation module, the grid connection operation initial module and the grid connection normal operation module; The grid connection preparation module receives the power generation equipment data of the new energy station, evaluates the output characteristics of the new energy grid-connected inverter, builds a system static model, forms an impedance matrix, builds an output impedance model of the inverter, calculates the active and reactive output reference values ​​within a short time scale, and performs power flow calculation within a short time scale; The grid-connected operation initial module receives the reference value calculated by the grid-connected preparation module and the power generation instruction of the dispatching center, starts the grid-connected program, receives the output voltage and current information and instructions of the inverter equipment, and performs grid-connected startup control; The grid-connected normal operation module imports network parameters and distributed power supply parameters, performs system power flow calculations over a long time scale, and outputs the calculation results as power generation instructions to the inverter device, and the inverter adjusts the output according to the instructions.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of an optimization method for power flow calculation of a distributed new energy distribution network according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of an optimization method for power flow calculation of a distributed renewable energy distribution network according to any one of claims 1 to 7 are implemented.

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