Method for calculating three-phase asymmetric power flow of middle-low voltage distribution network containing doubly-fed wind turbine generator

Abstract

The application discloses a kind of low voltage distribution network three-phase asymmetric power flow calculation methods containing doubly-fed wind turbine, comprising: 1 based on back-to-back converter steady-state constraint, the sequence component equivalent circuit of doubly-fed wind turbine DFIG is constructed, and the internal node power balance equation of unit under stator voltage asymmetry is established;2 using the node power balance equation when stator voltage is asymmetric, the internal flow of DFIG is calculated, and the output power of unit stator node is obtained;3 the unit is equivalent to PQ power supply, considering the influence of neutral point voltage, the three-phase asymmetric node power balance equation of distribution network is established, the power flow of low voltage distribution network is calculated, and the three-phase voltage of grid-connected point is obtained;4 according to the difference between DFIG grid-connected point and DFIG stator sequence voltage, the convergence of alternating iteration is judged, and if it is not converged, the internal flow of unit is recalculated and cyclic iteration is carried out.The application considers the influence of three-phase asymmetric node voltage of low voltage distribution network on the internal flow of DFIG, and improves the accuracy of power flow calculation of doubly-fed wind turbine into low voltage distribution network.

Description

Technical Field

[0001] This invention belongs to the field of grid-connected power flow calculation and optimized scheduling of doubly-fed wind turbines, specifically involving a method for calculating three-phase unbalanced power flow in medium and low voltage distribution networks containing doubly-fed wind turbines. Background Technology

[0002] With the increasing scarcity of fossil fuels and the growing environmental pollution, the large-scale grid connection of distributed renewable energy sources poses a severe challenge to the voltage stability and power balance of the distribution network. Therefore, improving the distribution network's capacity to accommodate wind power is of significant practical importance and urgency. Doubly fed wind turbines, with their flexible control characteristics of rotor-side converters, can not only precisely adjust the amplitude and phase of the excitation current to achieve continuous adaptation across a wide range of turbine speeds, thereby maximizing the capture of wind energy resources at different wind speeds and improving power generation efficiency, but also achieve decoupled independent control of active and reactive power, rapidly responding to distribution network dispatch commands, effectively smoothing wind power fluctuations, and supporting distribution network voltage stability. They play an irreplaceable role in promoting the efficient grid connection of distributed wind power and enhancing the operational flexibility of the distribution network.

[0003] Current research on the use of Doubly Fed Intake (DFIG) wind turbines in distribution network power flow calculations, both domestically and internationally, largely focuses on the power balance and renewable energy absorption levels of the distribution network. It treats DFIG wind turbines as equivalent to controllable power sources operating symmetrically in three phases, neglecting the three-phase voltage asymmetry at nodes caused by numerous asymmetrical three-phase loads in the distribution network. This fails to account for the interference of this asymmetry on the internal operating conditions of the DFIG wind turbines, resulting in simplified symmetrical power flow models that yield power flow distribution results within the DFIG wind turbines that significantly differ from actual operating conditions. Therefore, a novel iterative three-phase asymmetrical power flow calculation method for medium- and low-voltage distribution networks containing DFIG wind turbines is needed to improve the accuracy of power flow calculations in such networks. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the above-mentioned technologies and propose a method for calculating the three-phase unbalanced power flow in medium and low voltage distribution networks containing doubly-fed induction generator (DFIG) wind turbines. This method aims to fully consider the impact of stator voltage asymmetry on the steady-state power flow of DFIG units, thereby improving the accuracy and precision of power flow calculation for DFIG wind turbine units and providing strong support for the optimized scheduling of energy storage systems.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: The present invention provides a method for calculating three-phase unbalanced power flow in medium- and low-voltage distribution networks containing doubly-fed wind turbines, characterized by comprising the following steps: Step S1: Based on the steady-state constraints of the back-to-back converter, construct the sequence component equivalent circuit of the doubly fed wind turbine DFIG, and establish the internal node power balance equation of the DFIG when the stator voltage is unbalanced based on the relationship between the sequence component power and the total power. Step S2: Based on the internal node power balance equation of DFIG, the internal power flow of DFIG under stator voltage asymmetry is solved by the Newton-Raphson iterative method, thereby calculating the optimal three-phase output power of DFIG stator; Step S3: Based on the optimal three-phase output power of the DFIG stator, the DFIG is equivalent to a PQ power supply, and the three-phase load in the medium and low voltage distribution network is processed with equivalent admittance to calculate the three-phase unbalanced power flow of the medium and low voltage distribution network with the neutral point grounded through the arc suppression coil, taking into account the influence of the neutral point voltage, so as to obtain the three-phase voltage of the DFIG grid connection node. Step S4: Convert the three-phase voltage of the DFIG grid-connected node into positive, negative, and zero-sequence voltages, and determine whether the overall convergence condition is met based on the difference between the voltage and the positive, negative, and zero-sequence voltages of the DFIG stator. If the condition is met, it means that the calculation result of the three-phase unbalanced power flow of the medium and low voltage distribution network containing the doubly fed wind turbine is obtained; otherwise, return to step S2 to recalculate.

[0006] The method for calculating three-phase unbalanced power flow in a medium- and low-voltage distribution network containing a doubly-fed wind turbine, as described in this invention, is characterized in that step S1 is performed as follows: Step S1-1: Construct the equivalent circuit of the negative sequence component of DFIG, including: stator node s, virtual node m, rotor node r, converter node g, and transformer k; Step S1-2: Construct the relationship between the total power of any internal node of the DFIG and the positive-sequence power and negative-sequence power using equation (1): (1) In equation (1), P total , Q total These represent the total active power and total reactive power of any internal node in the DFIG, respectively. P (1) , Q (1) These are the positive-sequence active power and positive-sequence reactive power of the corresponding internal nodes in the DFIG, respectively. P (2) , Q (2) These are the negative-sequence active power and negative-sequence reactive power of the corresponding internal nodes in the DFIG, respectively. Step S1-3: Construct the positive-sequence power balance equation for the virtual node m in the positive-sequence equivalent circuit of DFIG using equation (2): (2) In equation (2), Δ P m(1) and Δ Q m(1) These are the positive-sequence active power imbalance and positive-sequence reactive power imbalance of virtual node m, respectively. P ms(1) and Q ms(1) These are the positive-sequence active power and positive-sequence reactive power flowing from virtual node m to stator node s, respectively. P mr(1) and Q mr(1) These are the positive-sequence active power and positive-sequence reactive power flowing from virtual node m to rotor node r, respectively. Q mm(1) This represents the positive-sequence reactive power of the excitation circuit in the DFIG. Step S1-4: Construct the negative-sequence power balance equation of the virtual node m in the DFIG negative-sequence equivalent circuit using equation (4): (4) In equation (4), Δ P m(2) and Δ Q m(2) These are the negative sequence active power imbalance and negative sequence reactive power imbalance of virtual node m, respectively. P ms(2) and Q ms(2) These are the negative-sequence active power and negative-sequence reactive power flowing from virtual node m to stator node s, respectively. P mr(2) and Q mr(2) These are the negative-sequence active power and negative-sequence reactive power flowing from virtual node m to rotor node r, respectively. Q mm(2) This refers to the negative sequence reactive power of the excitation circuit in the DFIG. Step S1-5: Construct the total power balance equation for converter node g using equation (6): (6) In equation (6), Δ P g and Δ Q g These represent the total active power imbalance and the total reactive power imbalance at converter node g, respectively. P rm(1) and P rm(2)These are the positive-sequence active power and negative-sequence active power flowing from rotor node r to virtual node m, respectively. P gs(1) and P gs(2) These represent the positive-sequence active power and negative-sequence active power flowing from converter node g to stator node s, respectively. Q gs(1) and Q gs(2) These represent the positive-sequence reactive power and negative-sequence reactive power flowing from converter node g to stator node s, respectively. Q g,set The reactive power setpoint for converter node g; Step S1-6: Construct the total reactive power balance equation of stator node s using equation (8): (8) In equation (8), Δ Q s The stator reactive power imbalance of the DFIG; Q DFIG,set The stator reactive power setting value for DFIG; Q sm(1) and Q sm(2) These are the positive-sequence reactive power and negative-sequence reactive power flowing from stator node s to virtual node m, respectively. Q sg(1) and Q sg(2) These are the positive-sequence reactive power and negative-sequence reactive power flowing from stator node s to converter node g, respectively. Step S1-7: Construct the torque balance equation of DFIG using equation (12): (12) In equation (12), Δ T This refers to the torque imbalance. P em(1) and P em(2) These are positive-sequence electromagnetic power and negative-sequence electromagnetic power, respectively. P wt Capture power for the wind turbine; s This refers to the slippage rate.

[0007] Furthermore, step S2 is performed as follows: Step S2-1: Define the current iteration number for calculating the three-phase unbalanced power flow in a medium- and low-voltage distribution network containing doubly-fed wind turbines as... x and initialize x= 1; Initialize the stator node s in DFIG at the 1st xThe positive, negative, and zero-sequence voltage values ​​in the next iteration are respectively , , ; In the initialization of DFIG, all internal nodes except the stator node s are initialized in the 1st... x The initial value matrix of the positive sequence voltage amplitude in the next iteration is: The initial value matrix of the positive sequence voltage phase angle is: ; Initialize virtual node m in the first... x The initial value of the negative sequence voltage amplitude in the next iteration is... The initial value of the negative sequence voltage phase angle is ; The three-phase nodes and neutral point of the medium- and low-voltage distribution network are set at the [number]th [number]. x Voltage amplitude and voltage phase angle under each iteration; Step S2-2: Using equation (28), obtain the values ​​of all internal nodes except the stator node s in the first step. x The correction matrix for the positive sequence voltage amplitude in the next iteration Correction matrix for positive sequence voltage phase angle Virtual node m in the first... x Correction amount of negative sequence voltage amplitude in the next iteration Correction amount for negative sequence voltage phase angle : (28) In equation (28), For DFIG in the x The positive-sequence power imbalance matrix under the next iteration, and ;T indicates transpose; For DFIG in the x The negative-order power imbalance matrix under the next iteration, and ; For DFIG in the x The total power imbalance matrix under the next iteration, and The superscript -1 indicates that the matrix is ​​inverted. For the first x The Jacobian iteration matrix under the nth iteration, and we have: (29) Step S2-3: Using equation (30), obtain the values ​​of all internal nodes except the stator node s in the first step. x The new value matrix of the positive sequence voltage amplitude under the next iteration New value matrix of positive sequence voltage phase angle Virtual node m in the first... x New value of negative sequence voltage amplitude in the next iteration New value of negative sequence voltage phase angle : (30) Step S2-4: Construct the first using equation (31) x The convergence condition for the internal power flow calculation of DFIG in the next iteration is as follows: if equation (11) is satisfied, proceed to step S2-5; otherwise, proceed to step S2-5. Assign to ,Will Assign to ,Will Assign to ,Will Assign to Then, proceed to step S2-2; (31) In equation (31), e The set convergence precision; Step S2-5: Use equation (32) to obtain the DFIG in the first step. x Optimal output power of stator phase a in the next iteration Optimal output power of phase b optimal output power of phase C : (32) In equation (32), and The first x The optimal positive-sequence current and optimal negative-sequence current flowing from stator node s to virtual node m in the next iteration; and The first x The optimal positive-sequence current and optimal negative-sequence current flowing from stator node s to converter node g in the next iteration; , For each virtual node m in the th order, the virtual node m is in the th order. x New positive and negative sequence voltage values ​​under the next iteration; For converter node g at the th x The new positive-sequence voltage value of converter node g in the next iteration; R s , X s These are the equivalent resistance and equivalent reactance of stator node s, respectively; R k , X k These are the equivalent resistance and equivalent reactance values ​​of the transformer, respectively. For operators, The superscript * indicates the imaginary unit; the superscript * indicates conjugate.

[0008] Furthermore, step S3 is performed as follows: Step S3-1: Use equation (33) to convert DFIG into an equivalent PQ power supply: (33) In equation (33), , , For DFIG in the x The optimal output active power of stator phase a, phase b, and phase c under the next iteration; , , For DFIG in the x The optimal output reactive power of stator phase a, phase b, and phase c under each iteration; Re and Im represent the real and imaginary parts, respectively; Step S3-2: Construct any PQ load node in the medium and low voltage distribution network using equation (34). z In the x Three-phase load equivalent admittance under the next iteration This is then added to the diagonal elements of the admittance matrix of the three-phase nodes in the medium- and low-voltage distribution network, thereby establishing the three-phase nodes of the medium- and low-voltage distribution network with equivalent load admittance at the 1st... x Admittance matrix under the next iteration Y (x) ; (34) In equation (34), , , For any PQ load node in a medium- or low-voltage distribution network z In the x Equivalent load admittances for phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x The load power of phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x Voltage values ​​of phases a, b, and c under the next iteration; For the corresponding PQ load nodes in medium and low voltage distribution networks z The neutral point at the 1st x Voltage value under the next iteration; Step S3-3: Assume that the medium and low voltage distribution network grounded by the arc suppression coil has a total ofN Each of the three phases of a node in the medium- and low-voltage distribution network is treated as an independent node, denoted as a phase node, and the total number of such nodes is 3. N Among them, any PQ load node other than the balancing node will be included. z The nodes of phase a, phase b, and phase c are respectively denoted as 3 z +1, 3 z +2, 3 z +3; The PQ load node of the DFIG grid connection node p The grid-connected phase nodes of phases a, b, and c are respectively denoted as 3. p +1, 3 p +2, 3 p +3, thus using equation (35) to obtain the grid-connected phase node 3 of phase a. p +1 in the x Power injection active power in the next iteration and reactive power injected by the power source Phase b, grid-connected phase node 3 p +2 in the x In the next iteration, the power injected by the power source is active. and reactive power injected by the power source c-phase grid-connected node 3 p +3 in the x Power injection active power in the next iteration and reactive power injected by the power source : (35) Step S3-4: Using equation (38), obtain any phase node of the PQ load node at the non-grid-connected point in the medium and low voltage distribution network, taking into account the influence of the neutral point voltage. i In the x Active power imbalance in the next iteration and reactive power imbalance : (38) In equation (38), and These are the corresponding phase nodes. i In the x Injected active power and injected reactive power in the next iteration; G it and B it These are the corresponding phase nodes. i Phase nodes in medium and low voltage distribution networks t The real and imaginary parts of the mutual admittance; In the first x The corresponding phase node in the next iterationi Phase nodes in medium and low voltage distribution networks t The difference in voltage phase angle between them; and The first x The corresponding phase node in the next iteration i With the corresponding neutral point The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration i With the corresponding neutral point The difference in voltage phase angle between them; Indicates rounding up; For phase nodes in medium and low voltage distribution networks t In the x Voltage amplitude in the next iteration; For the corresponding phase node i In the x Voltage amplitude in the next iteration; For phase nodes in medium and low voltage distribution networks i Corresponding neutral point In the x Voltage amplitude in the next iteration; Step S3-5: Use equation (39) to obtain any phase node of the PQ load node of the DFIG grid-connected node in the medium and low voltage distribution network. e In the x Active power imbalance in the next iteration and reactive power imbalance : (39) In equation (39), , For the corresponding phase node e In the x Power injection active power and power injection reactive power under the next iteration; and These are the corresponding phase nodes. e In the x Injected active power and injected reactive power in the next iteration; and These are the corresponding phase nodes. e Phase nodes in medium and low voltage distribution networks t The real and imaginary parts of the mutual admittance; In the first x The corresponding phase node in the next iteration e Phase nodes in medium and low voltage distribution networks t The difference in voltage phase angle between them; and They were respectively in the secondx The corresponding phase node in the next iteration e With the corresponding neutral point The real and imaginary parts of the mutual admittance; In the first x The corresponding phase node in the next iteration e With the corresponding neutral point The difference in voltage phase angle between them; For the corresponding phase node e In the x Voltage amplitude in the next iteration; For the corresponding phase node in the medium and low voltage distribution network e Corresponding neutral point In the x Voltage amplitude in the next iteration; e =3 p + k , k Represents the number of phases, and k =1,2,3; Steps 3-6: Construct the first using equation (42) x Iterative equations for three-phase asymmetrical power flow in medium and low voltage distribution networks under the next iteration: (42) Equation (42), and The first x The corresponding phase node in the next iteration i The voltage phase angle correction matrix and the voltage amplitude correction matrix; and The first x The corresponding phase node in the next iteration e The voltage phase angle correction matrix and the voltage amplitude correction matrix; , , , Indicates the first x The four extended Jacobian matrices under the next iteration; Steps 3-7: Based on equation (42), obtain the corresponding phase nodes using equation (43). i In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude Corresponding phase nodes e In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude : (43) In equation (43), and These are the corresponding phase nodes. i In the x The initial value matrices of the voltage phase angle and voltage amplitude under each iteration; and These are the corresponding phase nodes. e In the x The initial value matrices of the voltage phase angle and voltage amplitude under each iteration; Steps 3-8: Construct the first using equation (44) x The convergence condition for power flow calculation of medium and low voltage distribution networks under the next iteration is as follows: if equation (20) is satisfied, proceed to step S4-1; otherwise, proceed to step S4-1. Assign to , Assign to , Assign to , Assign to Proceed to step S3-9; (44) Step 3-9: Use equation (21) to obtain any PQ load node in the medium and low voltage distribution network z In the x New value of neutral point voltage in the next iteration : (45) In equation (45), , , For any PQ load node in a medium- or low-voltage distribution network z In the x New voltage values ​​for phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x Mutual admittances between phase a, phase b, phase c and corresponding neutral points in each iteration; Y zn0 For the corresponding PQ load nodes in medium and low voltage distribution networks z Earth-to-ground admittance at the neutral point; Steps 3-10: [The text abruptly ends here, likely due to an incomplete sentence or a formatting error.] Assign to , used for calculation , Return to step S3-2.

[0009] Furthermore, step S4 is performed as follows: Step S4-1: Use equation (46) to obtain the PQ load node of the DFIG grid-connected node. p In the x New value of positive sequence voltage in the next iteration New value of negative sequence voltage New value of zero-sequence voltage : (46) In equation (46), , , Phase 3 of the grid connection for phase a p +1, Phase b grid connection node 3 p +2, c-phase grid connection node 3 p +3 in the x New voltage value in the next iteration; Step S4-2: Construct the first using equation (47) x The overall convergence criterion under the next iteration is, if it satisfies equation (47), then... , , , , , , , , , , If the result is the result of the three-phase unbalanced power flow calculation for a medium- and low-voltage distribution network containing doubly fed wind turbines, then proceed to step S4-3. (47) Step S4-3: ... Assign to ,Will Assign to ,Will Assign to and will x +1 is assigned to x Then return to steps S2-3 and execute them sequentially.

[0010] The present invention provides an electronic device, including a memory and a processor, characterized in that the memory is used to store a program that supports the processor in executing the three-phase asymmetrical power flow calculation method for medium and low voltage distribution networks, and the processor is configured to execute the program stored in the memory.

[0011] The present invention discloses a computer-readable storage medium storing a computer program, characterized in that the computer program, when executed by a processor, performs the steps of the method for calculating the three-phase unbalanced power flow of a medium- and low-voltage distribution network.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Based on the steady-state constraints of back-to-back converters, this invention constructs the sequence component equivalent circuit of DFIG, and based on this equivalent circuit, derives the internal node power balance equation of DFIG when the stator voltage is unbalanced, and then calculates the internal sequence component power flow of DFIG under the stator voltage unbalanced scenario to obtain the stator three-phase output power, laying a solid theoretical foundation for DFIG to be connected to the distribution network with node voltage unbalanced.

[0013] 2. This invention addresses the calculation of three-phase unbalanced power flow in medium and low voltage distribution networks grounded by arc suppression coils. It equates DFIG to PQ nodes and establishes node power balance equations that take into account the influence of neutral point voltage. The neutral point voltage is used as a key parameter in the iterative calculation of power flow in the distribution network, thereby improving the fit between the calculation results of three-phase unbalanced power flow in medium and low voltage distribution networks grounded by arc suppression coils and actual operating conditions.

[0014] 3. This invention combines the proposed DFIG internal sequence component power flow calculation under stator voltage asymmetry with the three-phase asymmetric power flow calculation of medium and low voltage distribution networks that takes into account the influence of neutral point voltage, and performs alternating iterative calculations to effectively improve the accuracy of power flow calculations for medium and low voltage distribution networks containing doubly fed wind turbine units. Attached Figure Description

[0015] Figure 1 The flowchart of the method for calculating three-phase unbalanced power flow in medium and low voltage distribution networks containing doubly fed wind turbines is shown in the present invention. Figure 2 This is the equivalent circuit diagram of the sequence components of the doubly-fed wind turbine of the present invention; Figure 3 This is a structural diagram of a medium- and low-voltage power distribution network containing a doubly-fed wind turbine generator, as presented in this invention. Detailed Implementation

[0016] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0017] In this example, the medium- and low-voltage distribution network structure diagram containing doubly-fed wind turbines is as follows: Figure 3As shown, a method for calculating three-phase unbalanced power flow in a medium- and low-voltage distribution network containing a doubly-fed induction generator (DFIG) wind turbine is presented. This method fully considers the impact of voltage asymmetry caused by three-phase unbalanced loads on the internal power flow of the DFIG. By constructing the sequence component equivalent circuit of the DFIG, the power balance equation of the internal nodes of the DFIG is derived when the stator voltage is unbalanced. The internal power flow of the DFIG under the stator voltage asymmetry scenario is calculated to obtain the stator three-phase output power. At the same time, in the power flow calculation of the distribution network, the DFIG is equivalent to a PQ power source, and a node power balance equation considering the influence of the neutral point voltage is established to calculate the three-phase unbalanced power flow of the medium- and low-voltage distribution network considering the influence of the neutral point voltage. By calculating the power flow of medium- and low-voltage distribution networks, the three-phase voltage of the DFIG grid-connected system is obtained. The convergence of the alternating iteration is judged based on the difference between the DFIG grid-connection point and the DFIG stator sequence voltage in the medium- and low-voltage distribution network. If convergence fails, the internal power flow of the unit is recalculated and iterated repeatedly, thus obtaining the three-phase asymmetrical power flow of the medium- and low-voltage distribution network containing doubly-fed induction generator (DFIG) wind turbines. This ultimately improves the accuracy of power flow calculations for medium- and low-voltage distribution networks containing DFIG wind turbines. Specifically, for example... Figure 1 As shown, the method includes the following steps: Step S1: There are a large number of unbalanced loads in the medium and low voltage distribution network, which leads to the asymmetry of the three-phase voltage at the nodes. When connecting the DFIG, the impact of this asymmetry on its steady-state power flow needs to be considered. Based on the steady-state constraints of the back-to-back converter, the sequence component equivalent circuit of the DFIG of the doubly fed wind turbine is constructed, and based on the relationship between the sequence component power and the total power, the power balance equation of the internal nodes of the DFIG when the stator voltage is unbalanced is established. Step S1-1: Construct the equivalent circuit of the sequence components of the DFIG based on the steady-state constraints of the back-to-back converter, including: stator node s, virtual node m, rotor node r, converter node g, and transformer k; since the DFIG adopts a neutral point ungrounded grounding method, there is no zero-sequence path; the existing positive-sequence equivalent circuit is as follows: Figure 2 As shown in (a); the rotation direction of the negative sequence component is opposite to that of the positive sequence component. The slip of the positive sequence equivalent circuit is s, therefore the equivalent slip of the negative sequence equivalent circuit is 2-s. Thus, the negative sequence equivalent circuit is constructed as follows: Figure 2 As shown in (b) of the diagram. Due to the control characteristics of the back-to-back converter, the negative sequence voltage of the converter and rotor node is zero.

[0018] Step S1-2: Power constraints exist in DFIGs, and the total power is usually given. Therefore, the three-phase total power needs to be expressed by sequence power. Since DFIGs use an ungrounded neutral point, there is no zero-sequence path and no zero-sequence power. Equation (1) is used to construct the relationship between the total power of any internal node of the DFIG and the positive-sequence power and negative-sequence power: (1) In equation (1), Ptotal , Q total These represent the total active power and total reactive power of any internal node in the DFIG, respectively. P (1) , Q (1) These represent the positive-sequence active power and positive-sequence reactive power of any node in the DFIG, respectively. P (2) , Q (2) These represent the negative-sequence active power and negative-sequence reactive power of any node in the DFIG.

[0019] Step S1-3: For the DFIG positive sequence equivalent circuit, the positive sequence power on both sides of the virtual node m satisfies the power balance equation, which is the same as the existing power balance form.

[0020] Step S1-3-1: Construct the positive-sequence power balance equation for the virtual node m in the positive-sequence equivalent circuit of DFIG using equation (2): (2) In equation (2), Δ P m(1) and Δ Q m(1) These are the positive-sequence active power imbalance and positive-sequence reactive power imbalance of virtual node m, respectively. P ms(1) and Q ms(1) These are the positive-sequence active power and positive-sequence reactive power flowing from virtual node m to stator node s, respectively. P mr(1) and Q mr(1) These are the positive-sequence active power and positive-sequence reactive power flowing from virtual node m to rotor node s, respectively. Q mm(1) This represents the positive-sequence reactive power of the excitation circuit in the DFIG.

[0021] Step S1-3-2: Based on the positive sequence voltage of each node, the equivalent circuit parameters, and equation (2), the specific expression of the positive sequence power balance equation of the virtual node m is obtained using equation (3): (3) In equation (3), Let be the positive sequence voltage value of virtual node m; This represents the positive sequence voltage value at stator node s; This represents the positive sequence voltage value at rotor node r; R s This is the stator resistance value; R r This is the rotor resistance value;X s This is the stator reactance value; X r This is the rotor reactance value; X m This is the value of the magnetizing reactance; s is the slip ratio; Re and Im represent the real and imaginary parts, respectively.

[0022] Step S1-4: For the DFIG negative sequence equivalent circuit, the negative sequence power on both sides of the virtual node m in the negative sequence network satisfies the power balance equation.

[0023] Step S1-4-1: Construct the negative-sequence power balance equation of the virtual node m in the DFIG negative-sequence equivalent circuit using equation (4): (4) In equation (4), Δ P m(2) and Δ Q m(2) These are the negative sequence active power imbalance and negative sequence reactive power imbalance of virtual node m, respectively. P ms(2) and Q ms(2) These are the negative-sequence active power and negative-sequence reactive power flowing from virtual node m to stator node s, respectively. P mr(2) and Q mr(2) These are the negative-sequence active power and negative-sequence reactive power flowing from virtual node m to rotor node s, respectively. Q mm(2) This represents the negative sequence reactive power of the excitation circuit in the DFIG.

[0024] Step S1-4-2: Based on the negative sequence voltage of each node, the equivalent circuit parameters, and equation (4), the specific expression of the negative sequence power balance equation of the virtual node m is obtained using equation (5): (5) In equation (5), The negative sequence voltage value of virtual node m; This represents the negative sequence voltage value of stator node s.

[0025] Step S1-5: For the total power constraint inside the DFIG, the positive-sequence equivalent circuit and the negative-sequence equivalent circuit of the DFIG need to be combined to construct the total power balance equation: Step S1-5-1: For the power balance equations on both sides of converter node g, the original method only considers the positive sequence network. Now, the positive sequence equivalent circuit of DFIG is combined with the negative sequence equivalent circuit to construct the total power balance equation. The active power on both sides of the back-to-back converters (RSC, GSC) is kept balanced. Based on equation (1), the total power calculated by positive sequence power and negative sequence power is balanced with the set value. The total power balance equation of converter node g is constructed using equation (6): (6) In equation (6), Δ P g and Δ Q g These represent the total active power imbalance and the total reactive power imbalance at converter node g, respectively. P rm(1) and P rm(2) These are the positive-sequence active power and negative-sequence active power flowing from rotor node r to virtual node m, respectively. P gs(1) and P gs(2) These represent the positive-sequence active power and negative-sequence active power flowing from converter node g to stator node s, respectively. Q gs(1) and Q gs(2) These represent the positive-sequence reactive power and negative-sequence reactive power flowing from converter node g to stator node s, respectively. Q g,set This is the reactive power setting value for converter node g.

[0026] Step S1-5-2: Based on the positive and negative sequence voltages of each node, the equivalent circuit parameters, and equation (6), the specific expression of the total power balance equation of converter node g is obtained using equation (7): (7) In equation (7), is the positive sequence voltage value of rotor node g; R k This is the equivalent resistance value of the transformer; X k This is the equivalent reactance value of the transformer; Step S1-5-3: For DFIG stator node s, the active power cannot be determined, and the reactive power is balanced with the set value. The total reactive power balance equation of stator node s is constructed using equation (8): (8) In equation (8), Δ Q s The stator reactive power imbalance of the DFIG; QDFIG,set The stator reactive power setting value for DFIG; Q sm(1) and Q sm(2) These are the positive-sequence reactive power and negative-sequence reactive power flowing from stator node s to virtual node m, respectively. Q sg(1) and Q sg(2) These represent the positive-sequence reactive power and negative-sequence reactive power flowing from stator node s to converter node g, respectively.

[0027] Step S1-5-4: Based on the positive and negative sequence voltages of each node, the equivalent circuit parameters, and equation (8), the specific expression of the total reactive power balance equation of stator node s is obtained using equation (9): (9) Steps S1-6: For the torque balance equation, when the stator voltage is asymmetrical, the existing method only considers the effect of positive-sequence electromagnetic power. Now, we will calculate based on the scenario of the combined effect of positive-sequence and negative-sequence electromagnetic power. After deducting the rotor active power loss in the positive and negative sequence networks, we will combine it with the power captured by the wind turbine. P wt Phase equilibrium.

[0028] Step S1-6-1: Use equation (10) to construct the relationship between the positive-sequence active power and negative-sequence active power consumed by the rotor and the electromagnetic power: (10) In equation (10), P r(1) and P r(2) These represent the positive-sequence active power and negative-sequence active power consumed by the rotor, respectively. P em(1) and P em(2) These are the positive-sequence electromagnetic power and the negative-sequence electromagnetic power, respectively.

[0029] Step S1-6-2: Based on (10), construct the relationship between DFIG electromagnetic power and wind turbine capture power using equation (11): (11) Step S1-6-3: Based on equation (11), construct the torque balance equation of DFIG using equation (12): (12) In equation (12), Δ T This represents the torque imbalance.

[0030] Step S1-6-4: Based on the positive and negative sequence voltages of each node, the equivalent circuit parameters, and equation (12), the specific expression of the torque balance equation is obtained using equation (13). (13) Step S2: Based on the internal node power balance equation of DFIG, the internal power flow of DFIG under stator voltage asymmetry is solved by the Newton-Raphson iterative method, thereby calculating the optimal three-phase output power of DFIG stator; Step S2-1: Define the current iteration number for calculating the three-phase unbalanced power flow in a medium- and low-voltage distribution network containing doubly-fed wind turbines as... x and initialize x= 1; Initialize the stator node s in DFIG at the 1st x The positive, negative, and zero-sequence voltage values ​​in the next iteration are respectively , , ; In the initialization of DFIG, all internal nodes except the stator node s are initialized in the 1st... x The initial value matrix of the positive sequence voltage amplitude in the next iteration is: The initial value matrix of the positive sequence voltage phase angle is: ; Initialize virtual node m in the first... x The initial value of the negative sequence voltage amplitude in the next iteration is... The initial value of the negative sequence voltage phase angle is ; The three-phase nodes and neutral point of the medium- and low-voltage distribution network are set at the [number]th [number]. x Voltage amplitude and voltage phase angle under each iteration.

[0031] Step S2-2: Construct the internal power flow equation of the DFIG when the stator voltage is unbalanced using equation (14): (14) In equation (14), C DFIG(1) Let be the positive-sequence power imbalance matrix of DFIG, and C DFIG(1) =[Δ P m(1) ,Δ Q m(1) ] T ;T indicates transpose; C DFIG(2) Let represent the negative-sequence power imbalance matrix of DFIG, and C DFIG(2) =[Δ P m(2) ,Δ Qm(2) ] T ; C DFIG Let be the power imbalance matrix of DFIG, and C DFIG =[Δ Q s ,Δ P g ,Δ Q g ] T ;Δ U DFIG(1) This represents the matrix of positive-sequence voltage magnitude corrections at internal nodes of the DFIG, and Δ U DFIG(1) =[Δ U m(1) ,Δ U r(1) ,Δ U g(1) ] T , where Δ U m(1) Δ U r(1) Δ U g(1) Δ represents the positive sequence voltage magnitude correction for virtual node m, rotor node r, and converter node g. U m(2) Δ is the negative sequence voltage magnitude correction for virtual node m. i DFIG(1) This represents the matrix of positive sequence voltage phase angle corrections at internal nodes of the DFIG, and Δ i DFIG(1) =[Δ i m1 ,Δ i r1 ,Δ i g1 ] T , where Δ i m(1) Δ i r(1) Δ i g(1) The positive sequence voltage phase angle correction for virtual node m, rotor node r, and converter node g; Δ i m(2) The negative sequence voltage phase angle correction for virtual node m; T represents transpose.

[0032] Step S2-3: Further calculate the Jacobian matrix in the internal node power flow iteration when the DFIG stator voltage is asymmetrical. A Element expression Step S2-3-1: The partial derivative of the power imbalance with respect to the negative-sequence voltage of the node in the positive-sequence network is 0, and the partial derivative of the power imbalance with respect to the positive-sequence voltage of the node in the negative-sequence network is 0. The Jacobian iteration matrix is ​​constructed using equation (15). A Simplified form: (15) Step S2-3-2: Construct the partial derivative matrix of the positive sequence power imbalance of DFIG with respect to the positive sequence voltage magnitude of the internal nodes using equation (16). Element expression: (16) Step S2-3-3: Construct the partial derivative matrix of the positive sequence power imbalance of DFIG with respect to the phase angle of the positive sequence voltage of the internal nodes using equation (17). Element expression: (17) Step S2-3-4: Construct the partial derivative matrix of the DFIG negative sequence power imbalance with respect to the magnitude of the negative sequence voltage at the internal nodes using equation (18). Element expression: (18) Step S2-3-5: Construct the partial derivative matrix of the DFIG negative sequence power imbalance with respect to the phase angle of the negative sequence voltage at the internal nodes using equation (19). Element expression: (19) Step S2-3-6: Construct the partial derivative matrix of the DFIG power imbalance with respect to the positive sequence voltage magnitude of the internal nodes using equation (20). Element expression: (20) Step S2-3-7: Construct the partial derivative matrix of the DFIG power imbalance with respect to the phase angle of the positive sequence voltage at the internal nodes using equation (21). Element expression: (twenty one) Step S2-3-8: Construct the partial derivative matrix of the DFIG power imbalance with respect to the magnitude of the negative sequence voltage at the internal nodes using equation (22). Element expression: (twenty two) Step S2-3-9: Construct the partial derivative matrix of the DFIG power imbalance with respect to the phase angle of the negative sequence voltage at the internal nodes using equation (23). Element expression: (twenty three) Step S2-3-10: Construct the partial derivative matrix of the torque imbalance with respect to the magnitude of the positive sequence voltage at the internal nodes using equation (24). Element expression: (twenty four) Step S2-3-11: Construct the partial derivative matrix of the torque imbalance with respect to the phase angle of the positive sequence voltage at the internal nodes using equation (25). Element expression: (25) Step S2-3-12: Construct the partial derivative of the torque imbalance with respect to the magnitude of the negative sequence voltage at the internal nodes using equation (26). : (26) Step S2-3-13: Construct the partial derivative of the torque imbalance with respect to the magnitude of the negative sequence voltage at the internal nodes using equation (27). : (27) Step S2-4: Calculate the... x The internal node power imbalance and Jacobian matrix in the nth iteration are calculated. x The internal node voltage magnitude and phase angle correction in the nth iteration are calculated, and the nth iteration is also calculated. x The new values ​​of the internal node voltage amplitude and phase angle under each iteration are determined, and convergence is judged to obtain the internal power flow calculation results of DFIG when the stator voltage is unbalanced.

[0033] Step S2-4-1: Based on step 2-3 and equation (14), use equation (28) to obtain the values ​​of all internal nodes except the stator node s in the first step. x The correction matrix for the positive sequence voltage amplitude in the next iteration is: The correction matrix for the positive sequence voltage phase angle is as follows: Virtual node m in the th... x The correction amount for the negative sequence voltage amplitude in the next iteration is The correction amount for the negative sequence voltage phase angle is : (28) In equation (28), For DFIG in the x The positive-sequence power imbalance matrix under the next iteration, and ; For DFIG in the x The negative-order power imbalance matrix under the next iteration, and ; For DFIG in the xThe total power imbalance matrix under the next iteration, and The superscript -1 indicates that the matrix is ​​inverted. For the first x The Jacobian iteration matrix under the nth iteration, and we have: (29) Step S2-4-2: Using equation (30), obtain the values ​​of all internal nodes except the stator node s in the first step. x The new value matrix of the positive sequence voltage amplitude under the next iteration is: The new value matrix of the positive sequence voltage phase angle is as follows: Virtual node m in the th... x The new value of the negative sequence voltage amplitude in the next iteration is The new value of the negative sequence voltage phase angle is : (30) Step S2-4-3: Construct the first using equation (31) x The convergence condition for the internal power flow calculation of DFIG in the next iteration is as follows: if equation (31) is satisfied, proceed to step S2-6; otherwise, proceed to step S2-6. Assign to ,Will Assign to ,Will Assign to ,Will Assign to Then, proceed to step S2-4; (31) In equation (31), e The set convergence precision.

[0034] Step S2-6: Use equation (32) to obtain the DFIG in the first step. x Optimal output power of stator phase a in the next iteration Optimal output power of phase b optimal output power of phase C : (32) In equation (32), and The first x The optimal positive-sequence current and optimal negative-sequence current flowing from stator node s to virtual node m in the next iteration; and The first x The optimal positive-sequence current and optimal negative-sequence current flowing from stator node s to converter node g in the next iteration; , For each virtual node m in the th order, the virtual node m is in the th order. x New positive and negative sequence voltage values ​​under the next iteration; For converter node g at the th x The new positive-sequence voltage value of converter node g in the next iteration; R s , X s These are the equivalent resistance and equivalent reactance of stator node s, respectively; R k , X k These are the equivalent resistance and equivalent reactance values ​​of the transformer, respectively. For operators, The superscript * indicates the imaginary unit; the superscript * indicates conjugate.

[0035] Step S3: The DFIG is equivalent to a PQ-type power source by using the optimal three-phase output power of the stator, and then substituted into the power flow calculation of the distribution network. Since there are many unbalanced loads in the distribution network, and the distribution network often uses a neutral point grounding method via an arc suppression coil, the neutral point voltage will shift. Existing three-phase power flow calculations do not effectively consider the impact of the neutral point voltage on the three-phase power flow calculation of the distribution network. Therefore, the calculation of the three-phase unbalanced power flow of low-voltage distribution networks with neutral point grounded via an arc suppression coil, taking into account the influence of the neutral point voltage, is used to obtain the three-phase voltage of the DFIG grid-connected node.

[0036] Step S3: Based on the optimal three-phase output power of the DFIG stator, the DFIG is equivalent to a PQ power supply, and the three-phase loads in the medium and low voltage distribution network are processed with equivalent admittance. Since there are many unbalanced loads in the distribution network, and the distribution network often uses a neutral point grounding method via an arc suppression coil, the neutral point voltage will shift. Existing three-phase power flow calculations fail to effectively consider the impact of the neutral point voltage on the three-phase power flow calculation of the distribution network. Therefore, this step calculates the three-phase unbalanced power flow of a medium and low voltage distribution network with a neutral point grounded via an arc suppression coil, taking into account the influence of the neutral point voltage, thereby obtaining the three-phase voltage of the DFIG grid-connected node. Step S3-1: Based on the three-phase output power of the DFIG stator, the DFIG is equivalent to a PQ power supply using equation (33): (33) In equation (33), , , For DFIG in the x The optimal output active power of stator phase a, phase b, and phase c under the next iteration; , , For DFIG in the xThe optimal output reactive power of stator phase a, phase b, and phase c under the next iteration.

[0037] Step S3-2: Treat the power flowing from a single-phase node to the neutral point as the load of that phase, and use this method as the equivalent load admittance. Construct any PQ load node in the medium and low voltage distribution network using equation (34). z In the x Three-phase load equivalent admittance under the next iteration This is then added to the diagonal elements of the admittance matrix of the three-phase nodes in the medium- and low-voltage distribution network, thereby establishing the three-phase nodes of the medium- and low-voltage distribution network with equivalent load admittance at the 1st... x Admittance matrix under the next iteration Y (x) ; (34) In equation (34), , , For any PQ load node in a medium- or low-voltage distribution network z In the x Equivalent load admittances for phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x The load power of phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x Voltage values ​​of phases a, b, and c under the next iteration; For the corresponding PQ load nodes in medium and low voltage distribution networks z The neutral point at the 1st x Voltage value in the next iteration.

[0038] Step S3-3: Medium and low voltage distribution networks containing doubly-fed wind turbines, such as Figure 3 As shown. The distribution network contains only two node types: balancing nodes and PQ load nodes. Assume the medium- and low-voltage distribution network grounded by arc suppression coils has a total of... N Each of the three phases of a node in the medium- and low-voltage distribution network is treated as an independent node, denoted as a phase node, and the total number of such nodes is 3. N Among them, any PQ load node other than the balancing node will be included. z The nodes of phase a, phase b, and phase c are respectively denoted as 3 z +1, 3 z +2, 3 z +3; The PQ load node of the DFIG grid connection node p The grid-connected phase nodes of phases a, b, and c are respectively denoted as 3. p +1, 3 p +2, 3 p +3, thus using equation (35) to obtain the grid-connected phase node 3 of phase a. p +1 in the x Power injection active power in the next iteration and reactive power injected by the power source Phase b, grid-connected phase node 3 p +2 in the x In the next iteration, the power injected by the power source is active. and reactive power injected by the power source c-phase grid-connected node 3 p +3 in the x Power injection active power in the next iteration and reactive power injected by the power source : (35) Step S3-4: Establish the power balance equations for phase nodes of the medium and low voltage distribution network, taking into account the influence of the neutral point voltage.

[0039] Step S3-4-1: Treat the neutral point as an independent node, taking into account any phase node of the neutral point relative to the PQ load node of the grid connection point. i Any phase node of the PQ load node of the DFIG grid-connected node e The influence of the node injection current is considered, and the value of any phase node of the PQ load node at the grid connection point is obtained using equation (36). i Any phase node of the PQ load node of the DFIG grid-connected node e Node injection current values: (36) In equation (35), , These are the corresponding phase nodes. i Corresponding phase nodes e Injecting current into the nodes; , These are the corresponding phase nodes. i With the corresponding neutral point Corresponding phase nodes e With the corresponding neutral point Inter-admittance; , These are the corresponding phase nodes. i Corresponding phase nodes e The corresponding neutral point voltage; , These are the corresponding phase nodes. i Corresponding phase nodes e With phase node t Mutual admittance; For phase node t Node voltage; This indicates rounding up to the nearest integer.

[0040] Step S3-4-2: Use equation (37) to obtain any phase node of the PQ load node at the grid connection point. i Any phase node of the PQ load node of the DFIG grid-connected node e Node injection power: (37) In equation (37), , These are the corresponding phase nodes. i Corresponding phase nodes e Node injection power; , For the corresponding phase node i Corresponding phase nodes e The node voltage.

[0041] Step S3-4-3: Based on equation (37), use equation (38) to obtain any phase node of the PQ load node at the non-grid-connected point in the medium and low voltage distribution network, taking into account the influence of the neutral point voltage. i In the x Active power imbalance in the next iteration and reactive power imbalance : (38) In equation (38), and These are the corresponding phase nodes. i In the x Injected active power and injected reactive power in the next iteration; G it and B it These are the corresponding phase nodes. i Phase nodes in medium and low voltage distribution networks t The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration i Phase nodes in medium and low voltage distribution networks t The difference in voltage phase angle between them; and The first x The corresponding phase node in the next iteration iWith the corresponding neutral point The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration i With the corresponding neutral point The difference in voltage phase angle between them; Indicates rounding up; For phase nodes in medium and low voltage distribution networks t In the x Voltage amplitude in the next iteration; For the corresponding phase node i In the x Voltage amplitude in the next iteration; For the corresponding phase node i Corresponding neutral point In the x Voltage amplitude in the next iteration; Step S3-4-4: Use equation (39) to obtain any phase node of the PQ load node of the DFIG grid-connected node in the medium and low voltage distribution network. e In the x Active power imbalance in the next iteration and reactive power imbalance : (39) In equation (39), , For the corresponding phase node e In the x Power injection active power and power injection reactive power under the next iteration; and These are the corresponding phase nodes. e In the x Injected active power and injected reactive power in the next iteration; and These are the corresponding phase nodes. e Phase nodes in medium and low voltage distribution networks t The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration e Phase nodes in medium and low voltage distribution networks t The difference in voltage phase angle between them; and The first x The corresponding phase node in the next iteration e With the corresponding neutral point The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration eWith the corresponding neutral point The difference in voltage phase angle between them; For the corresponding phase node e In the x Voltage amplitude in the next iteration; For the corresponding phase node in the medium and low voltage distribution network e Corresponding neutral point In the x Voltage amplitude in the next iteration; e =3 p + k , k Represents the number of phases, and k =1,2,3; Steps 3-5: For the three-phase unbalanced power flow of a medium- and low-voltage distribution network with the neutral point grounded by an arc suppression coil, the Newton-Raphson method is used for calculation. This step will calculate the Jacobian matrix elements required for the iterative calculation.

[0042] Step 3-5-1: Construct the first equation using equation (40) x The expanded Jacobian matrix in the next iteration Mid-phase node i The partial derivatives of the power imbalance with respect to its own voltage magnitude and phase angle are: (40) In equation (40), and Phase nodes i In the x Active power imbalance at phase nodes in the next iteration i Partial derivatives of voltage phase angle and amplitude; and Phase nodes i In the x The reactive power imbalance at the phase node in the next iteration i Partial derivatives of voltage phase angle and amplitude.

[0043] Step 3-5-2: Construct the first using equation (41) x The expanded Jacobian matrix in the next iteration Mid-phase node i Power imbalance at other PQ load phase nodes α Partial derivative elements of voltage amplitude and phase angle: (41) In equation (41), and Phase nodes i In the x Active power imbalance at phase nodes in the next iteration αPartial derivatives of voltage phase angle and amplitude; and Phase nodes i In the x The reactive power imbalance at the phase node in the next iteration α Partial derivatives of voltage phase angle and amplitude; , The first x Next iteration phase node α Voltage amplitude and phase angle; For the first x Next iteration phase node i With phase node α The difference in voltage phase angle between them; , Phase nodes i With phase node α The real and imaginary parts of the mutual admittance.

[0044] Step 3-5-3: The partial derivative expression in the middle and Same format , The partial derivative expression in equation (41) is the same; Steps 3-6: Based on equation (42), obtain the corresponding phase nodes using equation (43). i In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude Corresponding phase nodes e In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude : (43) In equation (43), and These are the corresponding phase nodes. i In the x The initial value matrices of the voltage phase angle and voltage amplitude under each iteration; and These are the corresponding phase nodes. e In the x The initial value matrix of voltage phase angle and voltage amplitude under the next iteration.

[0045] Steps 3-7: Based on equation (42), obtain the corresponding phase nodes using equation (43). i In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude Corresponding phase nodes e In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude : (43) In equation (43), and These are the corresponding phase nodes. i In the x The initial value matrices of the voltage phase angle and voltage amplitude under each iteration; and These are the corresponding phase nodes. e In the x The initial value matrix of voltage phase angle and voltage amplitude under the next iteration.

[0046] Steps 3-8: Construct the first using equation (44) x The convergence condition for power flow calculation of medium and low voltage distribution networks under the next iteration is as follows: if equation (20) is satisfied, proceed to step S4-1; otherwise, proceed to step S4-1. Assign to , Assign to , Assign to , Assign to Proceed to step S3-9; (44) Step 3-9: Use equation (45) to obtain any PQ load node in the medium and low voltage distribution network z In the x New value of neutral point voltage in the next iteration : (45) In equation (45), , , For any PQ load node in a medium- or low-voltage distribution network z In the x New voltage values ​​for phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x Mutual admittances between phase a, phase b, phase c and corresponding neutral points in each iteration; Y zn0 For the corresponding PQ load nodes in medium and low voltage distribution networksz The ground admittance at the neutral point.

[0047] Steps 3-10: [The text abruptly ends here, likely due to an incomplete sentence or a formatting error.] Assign to , used for calculation , The neutral point voltage of the nodes in the medium and low voltage distribution network is corrected, and the process returns to step S3-2.

[0048] Step S4: Convert the three-phase voltage of the DFIG grid-connected node into positive, negative, and zero-sequence voltages, and determine whether the overall convergence condition is met based on the difference between the three-phase voltage of the DFIG stator node and the positive, negative, and zero-sequence voltage of the DFIG. If the condition is met, it means that the calculation result of the three-phase unbalanced power flow of the medium and low voltage distribution network containing the doubly fed wind turbine is obtained; otherwise, return to step S2 to recalculate.

[0049] Step S4-1: Use equation (46) to obtain the PQ load node of the DFIG grid-connected node. p In the x New value of positive sequence voltage in the next iteration New value of negative sequence voltage New value of zero-sequence voltage : (46) In equation (46), , , Phase 3 of the grid connection for phase a p +1, Phase b grid connection node 3 p +2, c-phase grid connection node 3 p +3 in the x The new voltage value in the next iteration.

[0050] Step S4-2: Construct the first using equation (47) x The overall convergence criterion under the next iteration is, if it satisfies equation (47), then... , , , , , , , , , , If the result is the result of the three-phase unbalanced power flow calculation for a medium- and low-voltage distribution network containing doubly fed wind turbines, then proceed to step S4-3. (47) Step S4-3: ... Assign to ,Will Assign to ,Will Assign to and will x +1 is assigned to x Then return to steps S2-3 and execute them sequentially.

[0051] In this embodiment, an electronic device includes a memory and a processor. The memory stores a program that supports the processor in executing the above-described method, and the processor is configured to execute the program stored in the memory.

[0052] In this embodiment, a computer-readable storage medium stores a computer program, which is executed by a processor to perform the steps of the above method.

Claims

1. A method for calculating three-phase unbalanced power flow in a medium- and low-voltage distribution network containing doubly-fed wind turbine generators, characterized in that... Includes the following steps: Step S1: Based on the steady-state constraints of the back-to-back converter, construct the sequence component equivalent circuit of the doubly fed wind turbine DFIG, and establish the internal node power balance equation of the DFIG when the stator voltage is unbalanced based on the relationship between the sequence component power and the total power. Step S2: Based on the internal node power balance equation of DFIG, the internal power flow of DFIG under stator voltage asymmetry is solved by the Newton-Raphson iterative method, thereby calculating the optimal three-phase output power of DFIG stator; Step S3: Based on the optimal three-phase output power of the DFIG stator, the DFIG is equivalent to a PQ power supply, and the three-phase load in the medium and low voltage distribution network is processed with equivalent admittance to calculate the three-phase unbalanced power flow of the medium and low voltage distribution network with the neutral point grounded through the arc suppression coil, taking into account the influence of the neutral point voltage, so as to obtain the three-phase voltage of the DFIG grid connection node. Step S4: Convert the three-phase voltage of the DFIG grid-connected node into positive, negative, and zero-sequence voltages, and determine whether the overall convergence condition is met based on the difference between the voltage and the positive, negative, and zero-sequence voltages of the DFIG stator. If the condition is met, it means that the calculation result of the three-phase unbalanced power flow of the medium and low voltage distribution network containing the doubly fed wind turbine is obtained; otherwise, return to step S2 to recalculate.

2. The method for calculating three-phase unbalanced power flow in a medium- and low-voltage distribution network containing doubly-fed wind turbines according to claim 1, characterized in that, Step S1 is performed as follows: Step S1-1: Construct the equivalent circuit of the negative sequence component of DFIG, including: stator node s, virtual node m, rotor node r, converter node g, and transformer k; Step S1-2: Construct the relationship between the total power of any internal node of the DFIG and the positive-sequence power and negative-sequence power using equation (1): (1) In equation (1), P total , Q total These represent the total active power and total reactive power of any internal node in the DFIG, respectively. P (1) , Q (1) These are the positive-sequence active power and positive-sequence reactive power of the corresponding internal nodes in the DFIG, respectively. P (2) , Q (2) These are the negative-sequence active power and negative-sequence reactive power of the corresponding internal nodes in the DFIG, respectively. Step S1-3: Construct the positive-sequence power balance equation for the virtual node m in the positive-sequence equivalent circuit of DFIG using equation (2): (2) In equation (2), Δ P m(1) and Δ Q m(1) These are the positive-sequence active power imbalance and positive-sequence reactive power imbalance of virtual node m, respectively. P ms(1) and Q ms(1) These are the positive-sequence active power and positive-sequence reactive power flowing from virtual node m to stator node s, respectively. P mr(1) and Q mr(1) These are the positive-sequence active power and positive-sequence reactive power flowing from virtual node m to rotor node r, respectively. Q mm(1) This represents the positive-sequence reactive power of the excitation circuit in the DFIG. Step S1-4: Construct the negative-sequence power balance equation of the virtual node m in the DFIG negative-sequence equivalent circuit using equation (4): (4) In equation (4), Δ P m(2) and Δ Q m(2) These are the negative sequence active power imbalance and negative sequence reactive power imbalance of virtual node m, respectively. P ms(2) and Q ms(2) These are the negative-sequence active power and negative-sequence reactive power flowing from virtual node m to stator node s, respectively. P mr(2) and Q mr(2) These are the negative-sequence active power and negative-sequence reactive power flowing from virtual node m to rotor node r, respectively. Q mm(2) This refers to the negative sequence reactive power of the excitation circuit in the DFIG. Step S1-5: Construct the total power balance equation for converter node g using equation (6): (6) In equation (6), Δ P g and Δ Q g These represent the total active power imbalance and the total reactive power imbalance at converter node g, respectively. P rm(1) and P rm(2) These are the positive-sequence active power and negative-sequence active power flowing from rotor node r to virtual node m, respectively. P gs(1) and P gs(2) These represent the positive-sequence active power and negative-sequence active power flowing from converter node g to stator node s, respectively. Q gs(1) and Q gs(2) These represent the positive-sequence reactive power and negative-sequence reactive power flowing from converter node g to stator node s, respectively. Q g,set The reactive power setpoint for converter node g; Step S1-6: Construct the total reactive power balance equation of stator node s using equation (8): (8) In equation (8), Δ Q s The stator reactive power imbalance of the DFIG; Q DFIG,set The stator reactive power setting value for DFIG; Q sm(1) and Q sm(2) These are the positive-sequence reactive power and negative-sequence reactive power flowing from stator node s to virtual node m, respectively. Q sg(1) and Q sg(2) These are the positive-sequence reactive power and negative-sequence reactive power flowing from stator node s to converter node g, respectively. Step S1-7: Construct the torque balance equation of DFIG using equation (12): (12) In equation (12), Δ T This refers to the torque imbalance. P em(1) and P em(2) These are positive-sequence electromagnetic power and negative-sequence electromagnetic power, respectively. P wt Capture power for the wind turbine; s This refers to the slippage rate.

3. The method for calculating three-phase unbalanced power flow in a medium- and low-voltage distribution network containing doubly-fed wind turbines according to claim 2, characterized in that, Step S2 is performed as follows: Step S2-1: Define the current iteration number for calculating the three-phase unbalanced power flow in a medium- and low-voltage distribution network containing doubly-fed wind turbines as... x and initialize x= 1; Initialize the stator node s in DFIG at the 1st x The positive, negative, and zero-sequence voltage values ​​in the next iteration are respectively , , ; In the initialization of DFIG, all internal nodes except the stator node s are initialized in the 1st... x The initial value matrix of the positive sequence voltage amplitude in the next iteration is: The initial value matrix of the positive sequence voltage phase angle is: ; Initialize virtual node m in the first... x The initial value of the negative sequence voltage amplitude in the next iteration is... The initial value of the negative sequence voltage phase angle is ; The three-phase nodes and neutral point of the medium- and low-voltage distribution network are set at the [number]th [number]. x Voltage amplitude and voltage phase angle under each iteration; Step S2-2: Using equation (28), obtain the values ​​of all internal nodes except the stator node s in the first step. x The correction matrix for the positive sequence voltage amplitude in the next iteration Correction matrix for positive sequence voltage phase angle Virtual node m in the first... x Correction amount of negative sequence voltage amplitude in the next iteration Correction amount for negative sequence voltage phase angle : (28) In equation (28), For DFIG in the x The positive-sequence power imbalance matrix under the next iteration, and T represents transpose; For DFIG in the x The negative-order power imbalance matrix under the next iteration, and ; For DFIG in the x The total power imbalance matrix under the next iteration, and The superscript -1 indicates that the matrix is ​​inverted. For the first x The Jacobian iteration matrix under the next iteration is: (29) Step S2-3: Using equation (30), obtain the values ​​of all internal nodes except the stator node s in the first step. x The new value matrix of the positive sequence voltage amplitude under the next iteration New value matrix of positive sequence voltage phase angle Virtual node m in the first... x New value of negative sequence voltage amplitude in the next iteration New value of negative sequence voltage phase angle : (30) Step S2-4: Construct the first using equation (31) x The convergence condition for the internal power flow calculation of DFIG in the next iteration is as follows: if equation (11) is satisfied, proceed to step S2-5; otherwise, proceed to step S2-5. Assign to ,Will Assign to ,Will Assign to ,Will Assign to Then, proceed to step S2-2; (31) In equation (31), ε The set convergence precision; Step S2-5: Use equation (32) to obtain the DFIG in the first step. x Optimal output power of stator phase a in the next iteration Optimal output power of phase b optimal output power of phase C : (32) In equation (32), and The first x The optimal positive-sequence current and optimal negative-sequence current flowing from stator node s to virtual node m in the next iteration; and The first x The optimal positive-sequence current and optimal negative-sequence current flowing from stator node s to converter node g in the next iteration; , For virtual node m in the th... x New positive and negative sequence voltage values ​​under the next iteration; For converter node g at the th x The new positive-sequence voltage value of converter node g in the next iteration; R s , X s These are the equivalent resistance and equivalent reactance of stator node s, respectively; R k , X k These are the equivalent resistance and equivalent reactance values ​​of the transformer, respectively. For operators, The superscript * indicates the imaginary unit; the superscript * indicates conjugate.

4. The method for calculating three-phase unbalanced power flow in a medium- and low-voltage distribution network containing doubly-fed wind turbines according to claim 3, characterized in that, Step S3 is performed as follows: Step S3-1: Use equation (33) to convert DFIG into a PQ power supply: (33) In equation (33), , , For DFIG in the x The optimal output active power of stator phase a, phase b, and phase c under the next iteration; , , For DFIG in the x The optimal output reactive power of stator phase a, phase b, and phase c under each iteration; Re and Im represent the real and imaginary parts, respectively; Step S3-2: Construct any PQ load node in the medium and low voltage distribution network using equation (34). z In the x Three-phase load equivalent admittance under the next iteration This is then added to the diagonal elements of the admittance matrix of the three-phase nodes in the medium- and low-voltage distribution network, thereby establishing the three-phase nodes of the medium- and low-voltage distribution network with equivalent load admittance at the 1st... x Admittance matrix under the next iteration Y (x) ; (34) In equation (34), , , For any PQ load node in a medium- or low-voltage distribution network z In the x Equivalent load admittances for phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x The load power of phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x Voltage values ​​of phases a, b, and c under the next iteration; For the corresponding PQ load nodes in medium and low voltage distribution networks z The neutral point at the 1st x Voltage value under the next iteration; Step S3-3: Assume that the medium and low voltage distribution network grounded by the arc suppression coil has a total of N Each of the three phases of a node in the medium- and low-voltage distribution network is treated as an independent node, denoted as a phase node, and the total number of such nodes is 3. N Among them, any PQ load node other than the balancing node will be included. z The nodes of phase a, phase b, and phase c are respectively denoted as 3 z +1, 3 z +2, 3 z +3; The PQ load node of the DFIG grid connection node p The grid-connected phase nodes of phases a, b, and c are respectively denoted as 3. p +1, 3 p +2, 3 p +3, thus using equation (35) to obtain the grid-connected phase node 3 of phase a. p +1 in the x Power injection active power in the next iteration and reactive power injected by the power source Phase b, grid-connected phase node 3 p +2 in the x In the next iteration, the power injected by the power source is active. and reactive power injected by the power source c-phase grid-connected node 3 p +3 in the x Power injection active power in the next iteration and reactive power injected by the power source : (35) Step S3-4: Using equation (38), obtain any phase node of the PQ load node at the non-grid-connected point in the medium and low voltage distribution network, taking into account the influence of the neutral point voltage. i In the x Active power imbalance in the next iteration and reactive power imbalance : (38) In equation (38), and These are the corresponding phase nodes. i In the x Injected active power and injected reactive power in the next iteration; G it and B it These are the corresponding phase nodes. i Phase nodes in medium and low voltage distribution networks t The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration i Phase nodes in medium and low voltage distribution networks t The difference in voltage phase angle between them; and The first x The corresponding phase node in the next iteration i With the corresponding neutral point The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration i With the corresponding neutral point The difference in voltage phase angle between them; Indicates rounding up; For phase nodes in medium and low voltage distribution networks t In the x Voltage amplitude in the next iteration; For the corresponding phase node i In the x Voltage amplitude in the next iteration; For the corresponding phase node i Corresponding neutral point In the x Voltage amplitude in the next iteration; Step S3-5: Use equation (39) to obtain any phase node of the PQ load node of the DFIG grid-connected node in the medium and low voltage distribution network. e In the x Active power imbalance in the next iteration and reactive power imbalance : (39) In equation (39), , For the corresponding phase node e In the x Power injection active power and power injection reactive power under the next iteration; and These are the corresponding phase nodes. e In the x Injected active power and injected reactive power in the next iteration; and These are the corresponding phase nodes. e Phase nodes in medium and low voltage distribution networks t The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration e Phase nodes in medium and low voltage distribution networks t The difference in voltage phase angle between them; and The first x The corresponding phase node in the next iteration e With the corresponding neutral point The real and imaginary parts of the mutual admittance; For the first x The corresponding phase node in the next iteration e With the corresponding neutral point The difference in voltage phase angle between them; For the corresponding phase node e In the x Voltage amplitude in the next iteration; For the corresponding phase node in the medium and low voltage distribution network e Corresponding neutral point In the x Voltage amplitude in the next iteration; e =3 p + k , k Represents the number of phases, and k =1,2,3; Steps 3-6: Construct the first step using equation (42) x Iterative equations for three-phase asymmetrical power flow in medium and low voltage distribution networks under the next iteration: (42) Equation (42), and The first x The corresponding phase node in the next iteration i The voltage phase angle correction matrix and the voltage amplitude correction matrix; and The first x The corresponding phase node in the next iteration e The voltage phase angle correction matrix and the voltage amplitude correction matrix; , , , Indicates the first x The four expanded Jacobian matrices under the next iteration; Steps 3-7: Based on equation (42), obtain the corresponding phase nodes using equation (43). i In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude Corresponding phase nodes e In the x The new value matrix of voltage phase angle under the next iteration New value matrix of voltage amplitude : (43) In equation (43), and These are the corresponding phase nodes. i In the x The initial value matrices of the voltage phase angle and voltage amplitude under each iteration; and These are the corresponding phase nodes. e In the x The initial value matrices of the voltage phase angle and voltage amplitude under each iteration; Steps 3-8: Construct the first using equation (44) x The convergence condition for power flow calculation of medium and low voltage distribution networks under the next iteration is as follows: if equation (20) is satisfied, proceed to step S4-1; otherwise, proceed to step S4-1. Assign to , Assign to , Assign to , Assign to Proceed to step S3-9; (44) Step 3-9: Use equation (21) to obtain any PQ load node in the medium and low voltage distribution network z In the x New value of neutral point voltage in the next iteration : (45) In equation (45), , , For any PQ load node in a medium- or low-voltage distribution network z In the x New voltage values ​​for phases a, b, and c under the next iteration; , , For the corresponding PQ load nodes in medium and low voltage distribution networks z In the x Mutual admittances between phase a, phase b, phase c and corresponding neutral points in each iteration; Y zn0 For the corresponding PQ load nodes in medium and low voltage distribution networks z Earth-to-ground admittance at the neutral point; Steps 3-10: [The text abruptly ends here, likely due to an incomplete sentence or a formatting error Assign to Used for calculation , Return to step S3-2.

5. The method for calculating three-phase unbalanced power flow in a medium- and low-voltage distribution network containing a doubly-fed wind turbine, as described in claim 4, is characterized in that... Step S4 is performed as follows: Step S4-1: Use equation (46) to obtain the PQ load node of the DFIG grid-connected node. p In the x New value of positive sequence voltage in the next iteration New value of negative sequence voltage New value of zero-sequence voltage : (46) In equation (46), , , Phase 3 of the grid connection for phase a p +1, Phase b grid connection node 3 p +2, c-phase grid connection node 3 p +3 in the x New voltage value in the next iteration; Step S4-2: Construct the first using equation (47) x The overall convergence criterion under the next iteration is, if it satisfies equation (47), then... , , , , , , , , , , If the result is the result of the three-phase unbalanced power flow calculation for a medium- and low-voltage distribution network containing doubly fed wind turbines, then proceed to step S4-3. (47) Step S4-3: ... Assign to ,Will Assign to ,Will Assign to and will x +1 is assigned to x Then return to steps S2-3 and execute them sequentially.

6. An electronic device, comprising a memory and a processor, characterized in that, The memory is used to store a program that supports the processor in executing the three-phase asymmetrical power flow calculation method for medium and low voltage distribution networks according to any one of claims 1-5, and the processor is configured to execute the program stored in the memory.

7. A computer-readable storage medium storing a computer program thereon, characterized in that, When the computer program is run by the processor, it executes the steps of the three-phase asymmetrical power flow calculation method for medium and low voltage distribution networks as described in any of claims 1-6.

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