Fault calculation method and system for power distribution network containing distributed inverter power supply

Abstract

The invention provides a fault calculation method and system for a power distribution network containing a distributed inverter power supply, and the method comprises the steps: carrying out the complete modeling of a control system, building a coupling model of current inner loop control and a filter circuit, representing the nonlinear characteristics of a control process and the coupling characteristics between state variables, and carrying out the calculation of a fault. An inverter power supply grid-connected system is described as a mathematical form of a discrete function, state variables of the system in the fault process are calculated by solving discrete segments of the state variables of the system, finally, the provided method is used for calculating the state variables of the system before and after the fault in different scenes of single-machine grid connection and multi-machine grid connection, and through simulation comparison, the state variables of the inverter power supply grid-connected system are calculated. And the feasibility and effectiveness of the method are verified. According to the method, the technical problems of high limitation, difficult analysis and the like, and limited applicability and feasibility are solved.

Description

Technical Field

[0001] The present invention relates to the field of power system control and protection, and in particular to a fault calculation method and system for a distribution network containing distributed inverter power supplies. Background Art

[0002] The traditional AC power grid is supported by synchronous power supply as the main force, which has the characteristics of strong inertia. In the electromagnetic transient stage, the change of generator speed can be ignored and it is equivalent to a voltage source that remains unchanged before and after the fault. For the new power system, the power supply form has undergone a subversive change. Its fault response is controlled by the control strategy and topology of power electronic equipment, and has significant nonlinearity, weak inertia, low damping, high harmonics and other characteristics. The fault analysis method and short-circuit current calculation method based on the linear superposition principle and the simplified equivalent of constant power supply are no longer applicable. At the same time, the traditional relay protection principle uses the steady-state quantity of power frequency to design the protection criterion. The fault characteristics of the new power system blur the characteristic differences between faults and non-faults, and between faults within and outside the zone, which seriously deteriorates the action performance of the traditional protection principle. The adaptability of traditional protection has been challenged. The response of the power electronic converter depends not only on the power supply side, but also on its supporting control system algorithm, which is characterized by fast response speed and strong controllability. Inverter power supplies are divided into grid-following inverter power supplies and grid-building inverter power supplies according to different control methods. Grid-following inverter power supplies are oriented by phase-locked loops and are widely used in distribution networks. However, their weak power supply characteristics lead to weak inertia and poor stability. In order to ensure the stable operation of new power systems, domestic and foreign scholars have proposed a variety of grid-building control strategies that can achieve synchronous grid connection without relying on phase-locked loops. Among them, droop control based on active power synchronization and virtual synchronous generator control are the most widely used. Limited by the voltage support capacity and current inrush capacity of power electronic equipment, the fault characteristics of inverter power supplies have undergone fundamental changes. The excitation control method used by the doubly fed wind turbine during the fault is an important factor in determining the short-circuit current characteristics. The prior art analyzes the fault characteristics of new energy sources of full power converter type and partial power converter type. When a three-phase short circuit occurs, the short-circuit current injected into the grid by new energy sources is very small, generally not exceeding 1.5 times the rated current. In some existing schemes, simulations have verified that when the control system damping is large or the controller adopts anti-saturation control mode, the fault current fluctuation of the grid containing photovoltaic power sources is serious. Some existing studies believe that the control strategy of the DC bus unloading circuit on the photovoltaic power supply side has no effect on the transient steady-state process of the fault, while the negative sequence current suppression strategy will affect the drop depth of the positive sequence voltage at the generator end after the fault. At present, a large number of studies have been carried out in the field of fault modeling and solution, which describe the fault response of grid-connected inverter power supplies to varying degrees.

[0003] In the existing literature "A Practical Calculation Method for System Short-Circuit Current Taking Flexible DC into Account", factors such as controller saturation and equipment limiting are ignored, the current inner loop and the primary circuit are combined to be equivalent to an inertia link, and the system state variables after the fault are described by analytical method. Although this method can analyze the characteristics of the fault current through expressions, the neglect of nonlinear links will cause the results to deviate from the actual situation.

[0004] The existing literature "A Robust Low-Voltage-Ride-Through Strategy for Grid-Forming Converters Based on Reactive Power Synchronization" and the existing literature "Three-Phase Short-Circuit Fault Characteristics of Direct-Drive Permanent Magnet Synchronous Wind Turbines" use the control link output as the input of the converter Thevenin model and analytically calculate the steady-state fault current based on the symmetrical component method, but this existing technology can only reflect the power frequency short-circuit current in the steady-state stage of the fault.

[0005] The existing literature "Analysis of Inverter Fault Transient Current Taking into Account the Dynamic Characteristics of the Phase-Locked Loop" is based on the component analysis of the fault transient components. It establishes analytical expressions for the voltage and current instructions at the inverter terminal in the time domain. It can more accurately express the fault transient characteristics of the inverter. However, since the fault characteristics present high-order nonlinearity, it is very difficult to solve the differential equation.

[0006] The existing literature "Short-circuit analysis models for unbalanced inverter-based distributed generation sources and loads" and the existing literature "Calculation of three-phase short-circuit current in transmission systems with high-ratio inverter-type power sources connected to the grid" derive analytical expressions for power frequency fault currents through the second-order differential equations of the system, but they also have the defect of being unable to extract the multi-frequency characteristics of the short-circuit current.

[0007] The existing literature "Transient fault current analysis of IIRESs consideringcontroller saturation" models the full-power inverter power supply and obtains the transient response model by solving the differential equation corresponding to the control link. It considers the effects of the phase-locked loop and controller saturation on the short-circuit current characteristics respectively, but only considers the existence of a single nonlinear link and cannot accurately describe the nonlinear characteristics of power electronic equipment.

[0008] The existing invention patent application document "A method for determining the fault current of a droop-controlled inverter" with publication number CN110162889A, the existing method includes: including pre-establishing an equivalent model of a grid-connected system including a droop-controlled inverter under an asymmetric fault type, and then obtaining a full-current relationship of the fault current of the droop-controlled inverter under an asymmetric fault type based on the equivalent model of the grid-connected system. In the subsequent fault current calculation, the full-current relationship of the fault current is combined and a fault current iteration algorithm is adopted, so as to eliminate the strong coupling between power and voltage, and obtain the full-scale time fault current. In this way, the fault current characteristics (including transient characteristics) of the droop-controlled inverter at any time under the asymmetric fault type can be obtained. However, the aforementioned prior art focuses on the control method of the inverter. The prior art can only realize the grid-connected calculation of a single inverter power supply, and it is difficult to adapt to the fault calculation of the inverter power supply grid-connected system under various control strategies.

[0009] The response of the power electronic converter depends not only on the power supply side, but also on the control system algorithm that supports it, which is characterized by fast response speed and strong controllability. Commonly used fault calculation research methods include simulation experiment method, symmetrical component method, and analytical method. However, most of the existing analytical methods focus on the power frequency component characteristics of transient processes, and rarely consider the full-band time domain characteristics. At the same time, there is the problem of difficulty in solving high-order nonlinear processes, and they are not applicable.

[0010] In summary, the existing technology has disadvantages such as strong limitations and difficulty in analysis, as well as technical problems of limited applicability and feasibility. Summary of the invention

[0011] The technical problem to be solved by the present invention is: how to solve the disadvantages of the prior art such as strong limitations, difficulty in analysis, and limited applicability and feasibility.

[0012] The present invention adopts the following technical solutions to solve the above technical problems: A fault calculation method for a distribution network containing a distributed inverter power supply includes:

[0013] S1. Analyze the coupling relationship between the output current of the grid-connected inverter power supply and the control system, and couple the control system and the filter circuit of the grid-connected inverter power supply to make the grid-connected inverter power supply equivalent to a controlled current source;

[0014] S2. Perform power flow calculation according to the structure of the control system, the inverter output current of the previous step, and the fault condition to obtain the system node voltage distribution of the current step;

[0015] S3, perform coordinate transformation operation to transform the PCC point voltage signal of the control system into the dq coordinate system;

[0016] S4. By using the discretization segmentation method, the state quantity of the control system is solved for each grid-connected inverter power supply in the distribution network to obtain the inverter outlet voltage signal;

[0017] S5, convert the inverter output voltage signal to the abc coordinate system for power flow calculation;

[0018] S6. Solve the filter circuit to obtain the output current of the grid-connected inverter power supply for the next step of long power flow iteration calculation. The output current of the inverter power supply can be obtained by alternately solving.

[0019] The present invention realizes the whole process solution of the system state variables. By solving the state variables of the control system in discrete segments, the nonlinear characteristics of the control process and the coupling characteristics between the state quantities can be characterized, thereby improving the calculation accuracy. Compared with the traditional fault calculation method based on the linear superposition principle and the simplified equivalent of the constant power supply, the present invention is suitable for a new distribution network system containing multiple power electronic inverters, and has strong feasibility and high calculation accuracy.

[0020] In a more specific technical solution, S1 includes:

[0021] S11, control the output current of the inverter with the current inner loop, and output the voltage reference value u of the grid-connected inverter power supply cdref ;

[0022] S12, using equivalent virtual impedance, coupling the current inner loop to the primary circuit to obtain the Thevenin equivalent circuit model of the current inner loop;

[0023] S13. Combining the impedance circuit model of the current inner loop, the Norton equivalent circuit of the grid-connected inverter during the fault period is obtained.

[0024] The current inner loop impedance circuit model constructed by the present invention can directly express the relationship between the primary circuit current and the control system current reference value, and can directly solve the fault steady-state current.

[0025] In a more specific technical solution, in S11, the voltage reference value u is determined using the following logic: cdref :

[0026]

[0027] In the formula, u d and u q are the d-axis and q-axis components of the grid-connected point voltage respectively; i d and i q are the d-axis and q-axis components of the filter output current respectively; i cd and i cq are the d-axis and q-axis components of the output current of the inverter power port respectively; i cdref and icqref are the d-axis and q-axis components of the inverter power port output current reference value; Lf is the filter inductance; K iI and K iP are the PI loop parameters of the current inner loop.

[0028] The present invention models the control system of the inverter, and through the voltage-current relationship between the current inner loop and the inverter filter circuit, the inverter power supply is equivalent to a controlled current source model, which simplifies the solution process of the power electronic equipment and can simplify the complexity of fault calculation.

[0029] In a more specific technical solution, in S12, the equivalent virtual impedance in the current inner loop is obtained by processing using the following logic:

[0030]

[0031] In the formula, G I It represents the control delay, and F represents the current feedforward gain.

[0032] In a more specific technical solution, in S13, the following logic is used to represent the equivalent impedance of the impedance circuit model:

[0033]

[0034] Where F is the current feed gain, G I To control the delay to be equal to 1, Z e1 , Z e2 Represents equivalent impedance.

[0035] In a more specific technical solution, in S3, the following Park transformation logic is used to perform a coordinate transformation operation:

[0036]

[0037] In the formula, u a ,u b ,u c and u d ,u q are the voltages before and after coordinate transformation, respectively.

[0038] In a more specific technical solution, S4 includes:

[0039] S41. Through the discretization segmentation method, the control system and the external circuit are interactively solved according to the following logic:

[0040]

[0041] In the formula, I max ,I N are the maximum output current and rated current of the inverter respectively; UPCC is the actual voltage value of PCC;

[0042] S42, numerically integrate and solve the inner current loop, wherein the PI link of the inner current loop is represented by the following logic:

[0043]

[0044] For the limiting part in the control link, the following logic is used to perform limiting operation to obtain the inverter port voltage output by the control system:

[0045]

[0046] The present invention can take into account the nonlinear characteristics of the control system and factors such as the limited current flow of the equipment through discrete segmented solution, and can completely express the control switching process under different states. In addition, the numerical integration method is applicable to inverter power grid-connected systems under different control modes.

[0047] The present invention uses a discrete linearization method to express the state variables of the control system, and there is no error caused by ignoring nonlinear links such as PI limiting. The discretization and segmented solution method of the state variables proposed by the present invention takes into account all links of the control system and can describe the dynamic response process of the inverter power grid-connected system under various control modes.

[0048] In a more specific technical solution, in S5, the following inverse Park transformation logic is used to perform coordinate transformation:

[0049]

[0050] In the formula, u d ,u q and u a ,u b ,u c Indicates the voltage before and after the inverse Park transformation.

[0051] The present invention characterizes the nonlinear characteristics of the control process and the coupling characteristics between state quantities by establishing a coupling model of the inverter power supply control system and the filter circuit; then constructs a discrete function model of the inverter power supply grid-connected system, and obtains the system transient quantity in the fault process by discrete segmented solution, without the need to analytically solve a group of high-order differential equations, with low computational complexity and strong feasibility.

[0052] In a more specific technical solution, step S6 includes:

[0053] S61. Calculate the output current of the grid-connected inverter power supply using the following logic:

[0054]

[0055] S62. Use the following logic to solve the filter circuit:

[0056]

[0057] In a more specific technical solution, the fault calculation system of the distribution network including the distributed inverter power supply includes:

[0058] The inverter power supply equivalent module is used to analyze the coupling relationship between the output current of the grid-connected inverter power supply and the control system, and couple the control system and the filter circuit of the grid-connected inverter power supply to make the grid-connected inverter power supply equivalent to a controlled current source;

[0059] The node voltage distribution calculation module is used to perform power flow calculation according to the structure of the control system, the inverter output current of the previous step and the fault condition to obtain the system node voltage distribution of the current step. The node voltage distribution calculation module is connected to the inverter power equivalent module;

[0060] A coordinate change module is used to perform coordinate transformation operations to convert the PCC point voltage signal of the control system into a dq coordinate system. The coordinate change module is connected to the inverter power supply equivalent module;

[0061] The inverter outlet voltage signal calculation module is used to solve the state quantity of the control system for each grid-connected inverter power supply in the distribution network through a discretization segmentation method to obtain the inverter outlet voltage signal. The inverter outlet voltage signal calculation module is connected to the coordinate change module;

[0062] The output voltage coordinate system conversion module is used to convert the inverter output voltage signal to the abc coordinate system for power flow calculation. The output voltage coordinate system conversion module is connected to the inverter output voltage signal calculation module;

[0063] The alternating solution module is used to solve the filter circuit and obtain the output current of the grid-connected inverter power supply for the next long power flow iteration calculation. The output current of the inverter power supply can be obtained by alternating solution. The alternating solution module is connected to the output voltage coordinate system conversion module.

[0064] Compared with the prior art, the present invention has the following advantages:

[0065] The present invention realizes the whole process solution of the system state variables. By solving the state variables of the control system in discrete segments, the nonlinear characteristics of the control process and the coupling characteristics between the state quantities can be characterized, thereby improving the calculation accuracy. Compared with the traditional fault calculation method based on the linear superposition principle and the simplified equivalent of the constant power supply, the present invention is suitable for a new distribution network system containing multiple power electronic inverters, and has strong feasibility and high calculation accuracy.

[0066] The current inner loop impedance circuit model constructed by the present invention can directly express the relationship between the primary circuit current and the control system current reference value, and can directly solve the fault steady-state current.

[0067] The present invention models the control system of the inverter, and through the voltage-current relationship between the current inner loop and the inverter filter circuit, the inverter power supply is equivalent to a controlled current source model, which simplifies the solution process of the power electronic equipment and can simplify the complexity of fault calculation.

[0068] The present invention can take into account the nonlinear characteristics of the control system and factors such as the limited current flow of the equipment through discrete segmented solution, and can completely express the control switching process under different states. In addition, the numerical integration method is applicable to inverter power grid-connected systems under different control modes.

[0069] The present invention uses a discrete linearization method to express the state variables of the control system, and there is no error caused by ignoring nonlinear links such as PI limiting. The discretization and segmented solution method of the state variables proposed by the present invention takes into account all links of the control system and can describe the dynamic response process of the inverter power grid-connected system under various control modes.

[0070] The present invention characterizes the nonlinear characteristics of the control process and the coupling characteristics between state quantities by establishing a coupling model of the inverter power supply control system and the filter circuit; then constructs a discrete function model of the inverter power supply grid-connected system, and obtains the system transient quantity in the fault process by discrete segmented solution, without the need to analytically solve a group of high-order differential equations, with low computational complexity and strong feasibility.

[0071] The present invention takes into account the circuit relationship of the inverter power grid-connected system, couples the filter current and the current inner loop of the control system, and realizes the decoupled expression of the current d-axis and q-axis. At the same time, the present invention is based on the globality of the distributed inverter power grid-connected system. The entire calculation process is from local equivalence to global solution, and system-level calculation can be performed. In summary, the present invention is more applicable to inverter power fault calculations using different control strategies and system-level fault calculations involving multiple inverter power grids.

[0072] The present invention solves the technical problems existing in the prior art such as strong limitations, difficulty in analysis, and limited applicability and feasibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 A schematic diagram of basic steps of a fault calculation method for a distribution network including a distributed inverter power supply according to Embodiment 1 of the present invention;

[0074] Figure 2 The topology diagram of the inverter power grid-connected system of Embodiment 1 of the present invention;

[0075] Figure 3This is a block diagram of the pre-power control of Embodiment 1 of the present invention;

[0076] Figure 4 This is a block diagram of the voltage outer loop of Embodiment 1 of the present invention;

[0077] Figure 5 This is a block diagram of the active power outer loop of Embodiment 1 of the present invention;

[0078] Figure 6 Schematic diagram of an equivalent circuit model of a current inner loop control and filtering circuit according to Embodiment 1 of the present invention;

[0079] Figure 7 It is a schematic diagram of a Thevenin equivalent circuit model of the current inner loop control coupled with the filter circuit according to Embodiment 1 of the present invention;

[0080] Figure 8 Schematic diagram of a Norton equivalent circuit model of a current inner loop control coupled with a filter circuit according to Embodiment 1 of the present invention;

[0081] Fig. 9 Schematic diagram of discrete segment calculation principle of embodiment 1 of the present invention;

[0082] Fig.10 This is a schematic diagram of discrete segmented computing data flow processing according to Embodiment 1 of the present invention;

[0083] Fig.11a This is a dq axis current waveform diagram when the fault point voltage drops to 0.7 pu according to Embodiment 2 of the present invention;

[0084] Fig.11b This is a waveform diagram of the A-phase current when the fault point voltage drops to 0.7 pu in Example 2 of the present invention;

[0085] Fig.11c This is a three-phase current waveform diagram when the fault point voltage drops to 0.7 pu according to Embodiment 2 of the present invention;

[0086] Fig.12a This is a dq axis current waveform diagram when a serious fault occurs in the system according to Embodiment 2 of the present invention;

[0087] Figure 12b This is a waveform diagram of the A-phase current when a serious fault occurs in the system according to Embodiment 2 of the present invention;

[0088] Fig.12c A three-phase current waveform diagram when a serious fault occurs in the system according to Embodiment 2 of the present invention;

[0089] Fig.13 This is a schematic diagram of the system structure including two inverter power supplies according to Embodiment 3 of the present invention;

[0090] Fig.14adq axis current waveform diagram of IIDG1 according to Embodiment 3 of the present invention;

[0091] Fig.14b dq axis current waveform diagram of IIDG2 according to Embodiment 3 of the present invention;

[0092] Fig.15 This is a fault current waveform diagram of embodiment 3 of the present invention;

[0093] Fig.16a This is a dq axis current waveform diagram of IIDG1 when a fault occurs after the system of Embodiment 3 of the present invention operates normally for 3 seconds;

[0094] Fig.16b This is a dq axis current waveform diagram of IIDG2 when a fault occurs after 3s of normal operation of the system in Example 3 of the present invention;

[0095] Fig.17 This is a fault current waveform diagram when a fault occurs after the system of Embodiment 3 of the present invention operates normally for 3 seconds;

[0096] Fig.18 This is a schematic diagram of a typical distribution network structure according to Embodiment 4 of the present invention;

[0097] Fig.19a This is a fault current waveform diagram of IIDG1 when the voltage at the fault position drops to 0.7 pu in Example 4 of the present invention;

[0098] Fig.19b This is a fault current waveform diagram of IIDG3 when the voltage at the fault location drops to 0.7 pu in Example 4 of the present invention. DETAILED DESCRIPTION

[0099] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0100] Example 1

[0101] like Figure 1 As shown in FIG. 1 , the fault calculation method of the distribution network containing distributed inverter power sources includes the following basic steps:

[0102] Step S1, analyzing the coupling relationship between the output current of the power electronic inverter and the control system, coupling the control system of the inverter with the filter circuit, and making the inverter equivalent to a controlled current source;

[0103] like Figure 2 As shown, in this embodiment, the output current of the voltage source inverter (VSI) in the system topology is i c , voltage is u c , the grid-connected current is i and the voltage is u.

[0104] In this embodiment, the equivalent resistance, inductance and capacitance of the filter are R f , L f , C f , the equivalent resistance and inductance of the fault point are R s , L s .

[0105] Filter circuit model

[0106] In this embodiment, the mathematical model of the filter circuit model is a differential equation group represented by the filter inductor current and the filter capacitor voltage as state variables, that is:

[0107]

[0108] Among them, u f is the fault point voltage, which is determined by the fault condition. The inverter output voltage u c Determined by the control system output.

[0109] Pre-Power Control Model

[0110] In this embodiment, the output power of the grid-connected inverter power supply adopts a vector decoupling control method, and its output power equation is:

[0111]

[0112] Among them, P and Q are the active and reactive power output of the photovoltaic power source respectively; u d and u q are the d-axis and q-axis voltage components of the grid connection point respectively; i d and i q They are the output d-axis and q-axis current components respectively.

[0113] In this embodiment, the grid-forming inverter power supply presents voltage source characteristics, and synchronization is achieved by adding a front-stage control link of the output power, and an AC voltage can be independently established.

[0114] like Figure 3 As shown, in this embodiment, the front power control link includes but is not limited to: an active frequency synchronization loop and a reactive power control loop with frequency modulation function, which can generate the phase and amplitude of the inverter power grid connection point voltage. Figure 3 , P ref and Q refare the reference values ​​of the inverter output active power and reactive power respectively; K p is the active power-frequency droop coefficient; is the reference value of the inverter output reactive power; K q is the reactive power-voltage coefficient; ω, ω ref Input virtual rotor angular velocity and reference angular velocity to the converter; U m and U ref are the rated voltage amplitude and reference voltage amplitude of the grid connection point respectively; is the output phase of the active power loop;

[0115] At the same time, various variables in the control system are established in the dq rotating coordinate system, and interact with the state variables of the next circuit in the abc three-phase stationary coordinate system, which is manifested as a high degree of coupling between the state variables, and the medium (park transformation) at the interaction end presents nonlinear characteristics.

[0116] Outer loop control model

[0117] like Figure 4 As shown, in this embodiment, the grid-type inverter power supply controls the voltage of the common coupling point (PCC) through the voltage outer loop, and the current reference value outputted by the grid-type inverter power supply is used as the input of the current inner loop. The mathematical model of the voltage outer loop is expressed as follows:

[0118]

[0119] Among them, i cdref and i cqref is the reference value of current, u dref and u qref is the voltage reference value of PCC, K uI and K uP They are the integral coefficient and proportional parameter of the voltage outer loop respectively.

[0120] like Figure 5 As shown, in this embodiment, the grid-following inverter power supply adopts a power outer loop control strategy, and the mathematical model of the power outer loop is expressed as:

[0121]

[0122] Among them, K dl and K dP They are the integral coefficient and proportional parameter of the power outer loop respectively.

[0123] Current inner loop impedance circuit model

[0124] In this embodiment, the current inner loop controls the inverter output current and outputs the voltage reference value u of the inverter power supply. cdref , its mathematical model is expressed as the following formula (5):

[0125]

[0126] Among them, u d and u q are the d-axis and q-axis components of the grid-connected point voltage respectively; i d and i q are the d-axis and q-axis components of the filter output current respectively; i cd and i cq are the d-axis and q-axis components of the output current of the inverter power port respectively; i cdref and i cqref are the d-axis and q-axis components of the inverter power port output current reference value; Lf is the filter inductance; K iI and K iP are the PI loop parameters of the current inner loop.

[0127] like Figure 6 As shown, in this embodiment, the current inner loop control and the filter circuit are jointly represented in the form shown in the figure, and the actual values ​​and reference values ​​of the state quantities voltage and current in the model in the dq coordinate system are both complex vectors.

[0128] Thevenin equivalent circuit model of grid-connected inverter power supply

[0129] like Figure 7 As shown, in this embodiment, ucdq is the dq axis component of the inverter power port voltage, and the current inner loop is coupled to the primary circuit in the form of virtual impedance to obtain its Thevenin equivalent circuit model. Among them, the equivalent virtual impedance in the current inner loop is expressed as the following formula (6):

[0130]

[0131] Among them, G I It represents the control delay, and F represents the current feedforward gain.

[0132] In this embodiment, when the control delay is ignored, G I is approximately equal to 1. If at this time Φ is equal to Φ ref , that is, the system frequency is considered to be the rated frequency, then G I Virtual reactance of item -jΦL f G I The dq axis coupling resistance can be compensated.

[0133] Norton equivalent circuit model of grid-following inverter

[0134] In this embodiment, during a fault, the current reference value in the dq coordinate system can be regarded as a constant value determined by the LVRT control.

[0135] like Figure 8As shown, in this embodiment, the Norton equivalent circuit of the grid-connected inverter during the fault period can be obtained by combining the current inner loop impedance circuit model. For the grid-connected inverter, voltage orientation is required through a phase-locked loop, which is generally equivalent to a controlled current source. When the fault period occurs, the voltage outer loop is disconnected, and the current reference value of the inner loop is output by the LVRT control link. Therefore, the LVRT control and the current inner loop impedance circuit model are combined.

[0136] In this embodiment, the equivalent impedance is expressed as the following formula (7).

[0137]

[0138] It can be seen that when the current feed gain F is equal to 1, the control delay G I When Z is equal to 1, e1 =-Z e2 , at this time Z N is 0. At this time, the inverter directly enters the low voltage ride-through mode during the fault period, and the fault current can be solved according to the Norton equivalent circuit model.

[0139] In this embodiment, the complex vector of the fault current can be expressed as:

[0140]

[0141] In this embodiment, when nonlinear links such as limiting in the control system are not considered, the dynamic process of the outer loop is ignored, and it is assumed that the system frequency remains unchanged during the fault period, then:

[0142]

[0143] The above formula is the transfer function relationship between the output current of the grid-connected inverter and the current reference value. According to the voltage drop at the grid connection point, the analytical expression of the fault current can be derived, which can qualitatively describe the change process of the fault current to a certain extent. However, since the controller saturation and other links are ignored in the analytical process, the result will have a certain error during the fault transient period. In order to improve the calculation accuracy of the transient quantity, this paper proposes a discrete segmented solution method based on the above equivalent model. Using the above equivalent model, the inverter is equivalent to a controlled current source.

[0144] Low Voltage Ride Through Control Model

[0145] In this embodiment, after a fault occurs, when the grid connection point voltage drops below 0.9 pu, the voltage outer loop is disconnected, and the current inner loop is directly regulated through low-voltage ride-through (LVRT) control.

[0146] In this embodiment, for a grid-type power supply, during a fault, LVRT control is implemented by a front power converter, which is specifically expressed as follows:

[0147]

[0148] Among them, P ref0 is the reference value of the inverter output active power before the fault occurs, U PCC and U N are the actual voltage and rated voltage of the grid connection point, respectively, i max is the maximum current allowed to be output by the inverter, and K is the reactive power compensation coefficient, which can be taken as 1.5, for example.

[0149] Step S2: Calculate the power flow according to the system structure, inverter output current and fault conditions to obtain the node voltage distribution;

[0150] In this embodiment, the distribution of the system node voltages can be obtained through power flow calculation according to the system parameters and the inverter output current in the previous step.

[0151] Step S3, converting the voltage signal to a dq coordinate system;

[0152] In this embodiment, the signal in the control system is a DC signal, and the voltage signal at the PCC point must be converted to the dq coordinate system through coordinate transformation. The voltage before and after the coordinate transformation is u a ,u b ,u c and u d ,u q The coordinate transformation method is Park transformation, and the transformation method is as follows (11):

[0153]

[0154] Step S4, solving the state quantity of each inverter control system in the distribution network by discrete segmentation to obtain the inverter outlet voltage;

[0155] In this embodiment, during the fault period, the current reference value in the dq coordinate system can be regarded as a constant value determined by the LVRT control, as shown in the following equation (12):

[0156]

[0157] Among them, I max ,I N are the maximum output current and rated current of the inverter respectively; U PCC is the actual voltage value of PCC.

[0158] In this embodiment, considering the full frequency domain mathematical model of the system, the system state variables in a certain period of time can be described by a differential algebraic equation group, and the expression of its time domain solution is:

[0159]

[0160] Among them, A k is the state matrix of the kth period, is the starting time of the kth time period. At the moment of time period switching, the state variables of the system will not mutate, that is, they satisfy:

[0161]

[0162] in, is the end time of the kth period.

[0163] In this embodiment, when the system state x(t0) of the first time period is taken as the starting point, the subsequent time periods are solved, and the expression of the system state variable at time t (in the kth time period) is:

[0164]

[0165] In summary, the method of solving in time periods can be applied to solving systems of differential equations.

[0166] For systems containing inverter power supplies, the dynamic response of state variables is dominated by differential algebraic equations and a large number of piecewise discrete functions, which makes analytical solutions difficult. However, the instantaneous non-mutation characteristics of state variables are still satisfied, and time-segment solutions are still applicable. In the transient process, the state variable of the system is the current, and the voltage is an algebraic variable. When the system's operating conditions change, the voltage may mutate. Therefore, in fault calculations, the voltage mutation at the moment of the fault can be used as the basis for solving the system's state variables. At the moment of the operating condition change, the system node voltage is first calculated, and this is used as a fixed solution condition to solve the inverter output current.

[0167] like Fig. 9 As shown, in this embodiment, in order to describe the strong coupling and nonlinear characteristics between the state variables in the control link of the inverter power grid-connected system, this paper proposes a discretized piecewise analytical method to solve the differential equations of the system state variables in a time-sharing and piecewise manner. The control system and the external circuit are interactively solved by the discretized piecewise method. The idea of ​​the discretized piecewise solution method can be found in Fig. 9 .

[0168] In this embodiment, x(t) and x[k] are the instantaneous value of the system state variable at time t and the discrete value corresponding to the kth point, uf(t) and uf[k] are the instantaneous value of the fault point voltage at time t and the discrete value corresponding to the kth point, and the differential algebraic equation group of the system is expressed as:

[0169]

[0170] The discretized numerical integral equation of the system is expressed as:

[0171]

[0172] Among them, B, C, and D are the coefficient matrices of the corresponding variables.

[0173] The above-mentioned discretization segmented solution method takes into account the nonlinearity of the control system, establishes a nonlinear differential algebraic equation group as shown in equation (16), and then discretizes the control process as shown in equation (17), and uses the numerical integration method to solve the state variables within the discretization step range. It can be seen that this method is not affected by the initial value of the system. During the initialization period, each state variable is directly set to zero. After the system state is constructed, the state variables during normal operation and stability are obtained. Fault and disturbance conditions are set in the corresponding time series. When a fault or disturbance occurs in the system, the system variables directly enter the fault or disturbance stage.

[0174] like Fig.10 As shown, in this embodiment, for the discrete segmented calculation process, taking the grid-connected inverter power supply system as an example, the specific process of the fault data flow processing process is shown in Fig.10 .

[0175] It should be noted that this calculation method can not only solve the state variables of the grid-connected inverter power supply system proposed in this paper, but can also be applied to other control strategies, and can characterize the dynamic change process of the system state variables during steady state and disturbance.

[0176] At the same time, for the fault calculation of the multi-machine system, the interactive coupling problem between multiple machines is focused on in the application process of the discrete segmentation method. During the fault period, each inverter power supply enters the low voltage ride-through control stage and outputs the fault current according to the voltage of the grid connection point. Therefore, the change of the system node voltage determines the fault response of each inverter power supply, that is, the node voltage is an important medium for the interactive coupling between multiple inverter power supplies and the system. The present invention first solves the voltage of the inverter power grid connection point according to the fault conditions and system architecture in the small interval of the discrete segment, and then applies the numerical integration method in the control system of each unit to solve the state variables. This continuity allows the method to take into account the interactive coupling problem between multiple machines and realize the solution of the state variables of the nonlinear control system.

[0177] In this embodiment, the current inner loop is numerically integrated and solved, and its PI link can be expressed as the following formula (18):

[0178]

[0179] In this embodiment, the limiting part in the control link is considered and the limiting is performed according to the following formula (19):

[0180]

[0181] In this embodiment, the inverter port voltage value output by the control system is finally obtained.

[0182] Step S5, converting the voltage signal to the abc coordinate system;

[0183] In this embodiment, since power flow calculation is required, the output current of the filter circuit must be solved in the abc coordinate system, and the inverter port voltage signal output by the control system is converted to the abc coordinate system through coordinate transformation. The voltage before and after the coordinate transformation is u d ,u q and u a ,u b ,u c The coordinate transformation method is the inverse Park transformation, and the transformation method is as follows (20):

[0184]

[0185] Step S6, solving the filter circuit to obtain the inverter power supply output current;

[0186] In this embodiment, the output current of the filter circuit satisfies the following formula (21):

[0187]

[0188] The solution process is carried out as follows:

[0189]

[0190] In this embodiment, the output current of the inverter at this moment is used as the input of the next step of long medium power flow calculation. The output current of the inverter power supply can be obtained by alternately solving.

[0191] Example 2

[0192] In this embodiment, the effects of the present invention are described below by means of specific embodiments:

[0193] In this embodiment, the fault current of the inverter power supply is solved in discrete segments according to the grid connection point voltage. When the system fails after running normally for 3 seconds, the voltage at the fault location drops by 0.7 pu and 0.2 pu. Two cases are calculated.

[0194] like Figures 11a to 11c As shown in the figure, in this embodiment, the fault calculation and simulation of the grid-connected inverter power supply are carried out. In order to verify the correctness of the method proposed in this paper, a simulation model of the grid-connected power supply system is built in PSCAD. The filter circuit and control system are Figure 1 Design. The rated power of the inverter is 1.2MW, the rated voltage of the grid point is 10kV, the frequency is 50Hz, and the maximum current allowed to pass through the equipment is twice the rated current. The main parameters in the system are shown in Table 1 below:

[0195] Table 1 System parameters

[0196]

[0197]

[0198] (1) When the voltage at the fault point drops to 0.7 pu, the control system enters the LVRT mode. The fault current calculated by the method in this paper and the simulation results are shown in Figure 2. Figures 12a to 12c .

[0199] like Figures 12a to 12c As shown, in this embodiment, it can be seen from the simulation and calculation results that when the voltage drops to 0.7pu, the reactive current increases rapidly, and according to the LVRT strategy, the active current decreases. Since the voltage drop is shallow, the active current can be restored to the rated state in a short time, and the reactive current is the main component of the fault current.

[0200] (2) When a serious fault occurs in the system and the voltage at the fault point drops to 0.2 pu, the control system enters the limiting mode. The fault current calculated by the method in this paper and the simulation results are shown in Figure 2. Figures 12a to 12c .

[0201] From the simulation and calculation results, it can be seen that when a serious fault occurs in the system, the short-circuit current increases suddenly. At the moment of the fault, the active current decreases rapidly and the reactive current increases suddenly, providing reactive power for the system and maintaining the grid connection point voltage. While the current increases rapidly, due to the current limit of the equipment, the inverter output current is controlled to be stable at the upper limit of the limit range. When the current limit link is started, the current is stabilized at the limit value in a short time.

[0202] Example 3

[0203] like Fig.13As shown, in this embodiment, the system failure calculation and simulation are performed for more than two inverters. In the actual system, due to the synchronization stability problem, most of the inverters adopt the grid-following control, and there is only one grid-forming inverter. Considering the situation where the system contains both grid-forming inverters and grid-following inverters, see Fig.13 Taking a system containing two inverter interfaced distributed generation (IIDG) as an example, fault calculation and analysis are performed.

[0204] like Fig.14a and Fig.14b As shown, in this embodiment, IIDG1 is a grid-forming inverter power supply, with a rated current of 0.55 kA and a maximum current allowed to pass through that is 2 times the rated current. IIDG2 is a grid-following inverter power supply, with a maximum current allowed to pass through that is 1.2 times the rated current.

[0205] In this embodiment, A, B, and C represent bus A, bus B, and bus C, respectively; f represents the fault point; E s is the system equivalent potential. Rv1 and Lv1 are the equivalent resistance and inductance of line L1 between bus A and bus B, Rs1 and Ls1 are the equivalent resistance and inductance of bus B from the fault point, Rs2 and Ls2 are the equivalent resistance and inductance of the fault point from the system power supply (bus C), and the line parameters are shown in Table 1 below:

[0206] Table 1: System line parameters

[0207] Parameter / Unit Numeric Parameter / Unit Numeric <![CDATA[R v1 / Oh]]> 1.8 <![CDATA[L v1 / H]]> 0.4 <![CDATA[R s1 / Oh]]> 1.525 <![CDATA[L s1 / H]]> 0.25 <![CDATA[R s2 / Oh]]> 3.31 <![CDATA[L s2 / H]]> 0.74

[0208] Fig.14b The fault current of the medium grid power source IIDG2 is solved by discrete segments according to the grid connection point voltage. When the system fails after running normally for 3s, the output current waveform of the inverter power supply under different fault conditions is as follows.

[0209] like Fig.15 As shown, in this embodiment, when the system runs normally for 3 seconds, Fig.14a and Fig.14b The voltage at the fault location shown drops to 0.7 pu. Fig.15 , are the d-axis and q-axis current waveforms of the two inverter power supplies;

[0210] like Fig.16a , Fig.16b as well as Fig.17As shown in the figure, in this embodiment, due to the shallow voltage drop, the two inverters did not reach the limit. After the fault occurred, the reactive current of the two inverters increased rapidly to support the grid connection point voltage. The inverter outputs different active currents and reactive currents according to the capacity limit and the grid connection point voltage drop. In this embodiment, the system fails after 3s of normal operation, and the voltage at the fault point drops to 0.2pu.

[0211] Depend on Fig.16a , Fig.16b as well as Fig.17 It can be seen from the simulation and calculation values ​​that when a serious fault occurs in the system, the two inverters detect the grid voltage, reach the current limit of the equipment according to the low voltage ride-through strategy, and quickly reach a stable state. The current of the inverter flowing to the fault location quickly transitions from the normal operating state to the fault current limiting state.

[0212] According to the above analysis, the proposed method can accurately describe the change process of system state variables before and after the fault, with extremely small error, high accuracy and strong applicability.

[0213] Example 4

[0214] like Fig.18 As shown, in this embodiment, calculation and simulation are performed for a typical distribution network fault containing multiple inverter power supplies. For the case where there are multiple IIDGs in the actual distribution network, the IIDG should be equivalent to a controlled current source model when it fails, and its output has strong nonlinearity. The traditional linear equivalent model is no longer applicable. Therefore, the proposed discrete segmented calculation method is used to analyze the fault of the distribution network containing multiple IIDGs. Without loss of generality, Fig.18 A specific analysis is conducted on a typical distribution network containing multiple IIDGs.

[0215] In this embodiment, the rated capacity of IIDG1, IIDG2, and IIDG4 is 2MW, the rated capacity of IIDG3 is 5MW, and the maximum current allowed to pass through the equipment is 1.5 times the rated current. Ln represents the length of line n (n=1, 2, 3, 4), D1 and D2 represent load 1 and load 2 respectively, ZS is the system equivalent impedance, and the meanings of the remaining symbols are consistent with Figure 16. RL and LL are the resistance and inductance per kilometer of the line respectively, and the specific parameters are shown in Table 3 below.

[0216] Table 3 System architecture parameters (typical distribution network)

[0217]

[0218] In this embodiment, for a multi-machine system, only the simulated waveforms and calculated waveforms of the output currents of some IIDGs before and after the fault are displayed, as well as the simulated values ​​and calculated values ​​of the output currents of each IIDG at several time points after the fault occurs.

[0219] like Fig.19a and Fig.19b As shown, in this embodiment, the system fails after running normally for 3 seconds, and the voltage drops to 0.7pu at the fault location as shown in FIG. 19 .

[0220] Table 4 below shows the simulated and calculated values ​​of the d-axis and q-axis currents output by each IIDG at multiple time points after a fault occurs, with the calculated values ​​in brackets.

[0221] Table 4 IIDG dq axis current

[0222]

[0223]

[0224] Since the fault severity is not high, the output current of all inverter power supplies has not reached the upper limit. Among the IIDGs with the same capacity, IIDG1, which is closest to the fault location, outputs the largest reactive current under low voltage ride-through control, and IIDG4, which is farthest from the fault location, outputs the smallest reactive current.

[0225] It can be seen from the simulation and calculation results that the proposed method is also applicable to the distribution network system containing multiple IIDGs, with extremely small errors and calculation accuracy generally reaching the fourth decimal place. At the same time, the calculation method in this paper is implemented by programming in Matlab. The calculation time of this example is less than 0.5s, with fast calculation speed and high efficiency.

[0226] In summary, the present invention realizes the whole process solution of the system state variables. By solving the state variables of the control system in discrete segments, the nonlinear characteristics of the control process and the coupling characteristics between the state quantities can be characterized, thereby improving the calculation accuracy. Compared with the traditional fault calculation method based on the linear superposition principle and the simplified equivalent of the constant power supply, the present invention is suitable for a new distribution network system containing multiple power electronic inverters, with strong feasibility and high calculation accuracy.

[0227] The current inner loop impedance circuit model constructed by the present invention can directly express the relationship between the primary circuit current and the control system current reference value, and can directly solve the fault steady-state current.

[0228] The present invention models the control system of the inverter, and through the voltage-current relationship between the current inner loop and the inverter filter circuit, the inverter power supply is equivalent to a controlled current source model, which simplifies the solution process of the power electronic equipment and can simplify the complexity of fault calculation.

[0229] The present invention can take into account the nonlinear characteristics of the control system and factors such as the limited current flow of the equipment through discrete segmented solution, and can completely express the control switching process under different states. In addition, the numerical integration method is applicable to inverter power grid-connected systems under different control modes.

[0230] The present invention uses a discrete linearization method to express the state variables of the control system, and there is no error caused by ignoring nonlinear links such as PI limiting. The discretization and segmented solution method of the state variables proposed by the present invention takes into account all links of the control system and can describe the dynamic response process of the inverter power grid-connected system under various control modes.

[0231] The present invention characterizes the nonlinear characteristics of the control process and the coupling characteristics between state quantities by establishing a coupling model of the inverter power supply control system and the filter circuit; then constructs a discrete function model of the inverter power supply grid-connected system, and obtains the system transient quantity in the fault process by discrete segmented solution, without the need to analytically solve a group of high-order differential equations, with low computational complexity and strong feasibility.

[0232] The present invention takes into account the circuit relationship of the inverter power grid-connected system, couples the filter current and the current inner loop of the control system, and realizes the decoupled expression of the current d-axis and q-axis. At the same time, the present invention is based on the globality of the distributed inverter power grid-connected system. The entire calculation process is from local equivalence to global solution, and system-level calculation can be performed. In summary, the present invention is more applicable to inverter power fault calculations using different control strategies and system-level fault calculations involving multiple inverter power grids.

[0233] The present invention solves the technical problems existing in the prior art such as strong limitations, difficulty in analysis, and limited applicability and feasibility.

[0234] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fault calculation method for a distribution network containing distributed inverter power sources, characterized in that: The method comprises: S1, analyzing the coupling relationship between the output current of the grid-connected inverter power supply and the control system, coupling the control system and the filter circuit of the grid-connected inverter power supply to make the grid-connected inverter power supply equivalent to a controlled current source; S2. Perform power flow calculation according to the structure of the control system, the inverter output current of the previous step, and the fault condition to obtain the system node voltage distribution of the current step; S3, performing a coordinate transformation operation to transform the PCC point voltage signal of the control system into a dq coordinate system; S4. By using a discretization segmentation method, for each grid-connected inverter in the distribution network, solve the state quantity of the control system to obtain an inverter outlet voltage signal; S5, converting the inverter output voltage signal to the abc coordinate system for power flow calculation; S6. Solve the filter circuit to obtain the output current of the grid-connected inverter power supply for the next step of iterative calculation of the power flow. The output current of the inverter power supply can be obtained by alternately solving.

2. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 1, characterized in that: The S1 includes: S11, control the output current of the inverter with the current inner loop, and output the voltage reference value u of the grid-connected inverter power supply cdref ; S12, using an equivalent virtual impedance, coupling the current inner loop to a primary circuit to obtain a Thevenin equivalent circuit model of the current inner loop; S13. Combining the impedance circuit model of the current inner loop, processing to obtain the Norton equivalent circuit of the grid-connected inverter during the fault period.

3. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 2 is characterized in that: In S11, the voltage reference value u is determined by using the following logic: cdref : In the formula, u d and u q are the d-axis and q-axis components of the grid-connected point voltage respectively; i d and i q are the d-axis and q-axis components of the filter output current respectively; i cd and i cq are the d-axis and q-axis components of the output current of the inverter power port respectively; i cdref and i cqref are the d-axis and q-axis components of the inverter power port output current reference value; Lf is the filter inductance; K iI and K iP are the PI loop parameters of the current inner loop.

4. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 2, characterized in that: In S12, the equivalent virtual impedance in the current inner loop is obtained by processing using the following logic: In the formula, G I It represents the control delay, and F represents the current feedforward gain.

5. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 2, characterized in that: In S13, the following logic is used to represent the equivalent impedance of the impedance circuit model: Where F is the current feed gain, G I To control the delay to be equal to 1, Z e1 , Z e2 Represents equivalent impedance.

6. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 1, characterized in that: In S3, the coordinate transformation operation is performed using the following Park transformation logic: In the formula, u a ,u b ,u c and u d ,u q are the voltages before and after coordinate transformation, respectively.

7. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 1, characterized in that: The S4 includes: S41, using the discretization segmentation method, the control system and the external circuit are interactively solved according to the following logic: In the formula, I max ,I N are the maximum output current and rated current of the inverter respectively; U PCC is the actual voltage value of PCC; S42, numerically integrate and solve the current inner loop, wherein the PI link of the current inner loop is represented by the following logic: The limiting part in the control link is limited by the following logic to obtain the inverter port voltage output by the control system:

8. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 1, characterized in that: In S5, the following inverse Park transformation logic is used to perform coordinate transformation: In the formula, u d ,u q and u a ,u b ,u c Indicates the voltage before and after the inverse Park transformation.

9. The fault calculation method for a distribution network containing distributed inverter power sources according to claim 1, characterized in that: The step S6 comprises: S61. Calculate the output current of the grid-connected inverter power supply using the following logic: S62, using the following logic to solve the filtering circuit:

10. A fault calculation system for a distribution network including a distributed inverter power supply, characterized in that: The system comprises: An inverter power supply equivalent module is used to analyze the coupling relationship between the output current of the grid-connected inverter power supply and the control system, and couple the control system and the filter circuit of the grid-connected inverter power supply to make the grid-connected inverter power supply equivalent to a controlled current source; A node voltage distribution calculation module is used to perform power flow calculation according to the structure of the control system, the inverter output current of the previous step length and the fault condition to obtain the system node voltage distribution of the current step length, and the node voltage distribution calculation module is connected to the inverter power supply equivalent module; A coordinate change module, used for performing coordinate transformation operation, converting the PCC point voltage signal of the control system into a dq coordinate system, the coordinate change module being connected to the inverter power supply equivalent module; An inverter outlet voltage signal calculation module is used to solve the state quantity of the control system for each grid-connected inverter power supply in the distribution network through a discretization segmentation method to obtain an inverter outlet voltage signal, and the inverter outlet voltage signal calculation module is connected to the coordinate change module; An output voltage coordinate system conversion module is used to convert the inverter output voltage signal to the abc coordinate system for power flow calculation, and the output voltage coordinate system conversion module is connected to the inverter output voltage signal calculation module; The alternating solution module is used to solve the filter circuit to obtain the output current of the grid-connected inverter power supply for the next step of the power flow iteration calculation. The output current of the inverter power supply can be obtained by alternating solution. The alternating solution module is connected to the output voltage coordinate system conversion module.

Images