A method for calculating large-scale renewable energy transmission line faults based on time-domain and phasor-domain alternation

Abstract

The present invention discloses a method for calculating large-scale renewable energy transmission line faults based on alternating time domain and phasor domain, comprising the following steps: 1. establishing a controlled current source mathematical model of a renewable energy inverter; 2. establishing a composite sequence network model of a renewable energy grid-connected system; 3. solving the renewable energy grid-connected point voltage during the fault period in the composite sequence network; 4. converting the renewable energy grid-connected point voltage from the phasor domain to the time domain; 5. solving the mathematical model of the renewable energy grid-connected inverter using a numerical integration method to obtain the output current of the renewable energy grid-connected inverter; 6. converting the output current of the renewable energy from the time domain to the phasor domain; 7. selecting a corresponding step size according to the dynamic change of the current and updating the calculation time; 8. repeating steps 3-7 until the calculation is completed. The present invention can be applied to nonlinear scenarios involving control switching and controller saturation, has a simple calculation process, is highly applicable, and has significant calculation efficiency when processing large-scale and complex system-level fault calculations.

Description

Technical Field

[0001] The present invention belongs to the field of control and protection of new power systems, involving fields such as grid-connected technology of new energy and control technology of power electronic equipment, and specifically relates to a method for calculating large-scale new energy transmission line faults based on alternating time domain and phasor domain. Background Art

[0002] In recent years, numerous large-scale renewable energy bases have been built, using multi-stage boosting to transmit power via high-voltage AC or high-voltage DC transmission. These grid-connected renewable energy bases contain numerous power electronic converters, whose response characteristics are highly controlled, exhibiting complex coupling and nonlinearity, making traditional fault calculation methods inapplicable. Furthermore, the current limiting and low-voltage ride-through characteristics of inverter power supplies raise concerns about the adaptability of existing protection systems. There is an urgent need to research new power supply equivalence methods and fault calculation methods that integrate the various fault control strategies for power electronic equipment.

[0003] Current research on analytical calculations for grid-connected inverter power supplies can only accurately model certain control links and cannot account for all nonlinear aspects. Furthermore, analytical calculations are primarily targeted at single-unit grid-connected scenarios. For multi-unit systems at the network level, analytical analysis is difficult due to the coupling between units.

[0004] For systems containing multiple renewable energy generators, the contradiction between the network and the individual generators must be resolved, taking into account the complexity and diversity of the network topology, as well as the impact of node voltage on fault current. The sequence component method can decouple the network, and combined with the inverter's positive and negative sequence decoupling control algorithm, it can reduce the scale and difficulty of calculation. Traditional fault calculation methods can obtain steady-state fault current solutions through iteration, but they do not take into account the nonlinear characteristics of the fault and the transient process of fault changes. Therefore, existing fault calculation methods have problems such as high analytical difficulty, large computational workload, and difficulty in nonlinear characterization, and their applicability is not high. Summary of the Invention

[0005] In order to overcome the problems existing in the above-mentioned prior art, the purpose of the present invention is to propose a method for calculating large-scale renewable energy transmission line faults based on alternating time domain and phasor domain. The method can be applied to nonlinear scenarios involving control switching and controller saturation. The calculation process is simple and the applicability is strong. In addition, the calculation efficiency of the present invention is significant when processing large-scale and highly complex system-level fault calculations.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for calculating large-scale renewable energy transmission line faults based on alternating time and phasor domains is proposed. Based on the asymmetric fault calculation method, a time-domain solution process of the mathematical model of the renewable energy controlled current source is added using a numerical integration method. This method can take into account the dynamic process of the output current of the renewable energy grid-connected inverter during the fault period. At the same time, the renewable energy grid-connected point voltage calculated by formula (6) in the phasor domain takes into account the mutual coupling between renewable energy units. The method comprises the following steps:

[0008] Step 1: Establish a mathematical model of the controlled current source of the new energy source;

[0009] Based on the control principle of the renewable energy grid-connected inverter during fault period, a mathematical model of the controlled current source of renewable energy during fault period is established;

[0010] Step 2: Establish a composite sequence network model for the new energy grid-connected system;

[0011] Based on the mathematical model of the controlled current source of the new energy, the structure of the new energy grid-connected system and the fault conditions established in step 1, a composite sequence network model of the new energy grid-connected system is established in the phasor domain;

[0012] Step 3: Based on the composite sequence network model of the renewable energy grid-connected system, solve the voltage at the renewable energy grid-connected point during the fault period;

[0013] The fault calculation starts at the moment of fault occurrence and ends at the moment of fault completion. Based on the superposition theorem, the voltage at the renewable energy grid connection point during the fault period, including both positive and negative sequence voltages, is calculated in the phasor domain using the composite sequence network model of the renewable energy grid connection system.

[0014] In the composite sequence network model, considering that the new energy grid-connected inverter adopts the negative sequence current suppression strategy, the positive sequence voltage of the kth new energy grid-connected point and negative sequence voltage for:

[0015]

[0016] The superscript (n) indicates the short circuit type, where n = 3 indicates a three-phase short circuit, n = 1 indicates a single-phase ground short circuit, n = 2 indicates a two-phase short circuit, and n = 1 indicates a two-phase ground short circuit; m is the total number of new energy power stations; is the additional impedance; is the negative sequence voltage divider impedance; Z G+ is the positive sequence impedance from the fault point to the AC grid; Z M+ is the positive sequence impedance from the collection point to the fault point; is the grid voltage; is the positive sequence current output by the kth new energy power station; is the positive sequence current of the i-th feeder; ZLk+ is the positive sequence impedance of the AC feeder of the kth renewable energy power station.

[0017] Step 4: Time domain conversion;

[0018] Convert the voltage of the new energy grid connection point solved in step 3 from the phasor domain to the time domain;

[0019] Step 5: Calculate the output current of the new energy grid-connected inverter;

[0020] Combined with the time domain conversion result of the voltage at the grid-connected point of the new energy in step 4, the numerical integration method is used to solve the mathematical model of the controlled current source of the new energy in step 1 to obtain the output current of the new energy grid-connected inverter;

[0021] Step 6: Phase conversion;

[0022] Convert the output current of the new energy grid-connected inverter solved in step 5 from the time domain to the phasor domain and use it as a known quantity to solve the voltage at the new energy grid-connected point in step 3 in the next step;

[0023] Step 7: Select the corresponding step size according to the dynamic change of current and update the fault calculation time;

[0024] When the output current of the new energy grid-connected inverter remains stable within several step sizes in step 5, a larger step size is selected for calculation in subsequent steps and the fault calculation time is updated;

[0025] Step 8: Repeat steps 3-7 until the calculation is completed, and the fault calculation ends when the fault ends.

[0026] Since the output current of the renewable energy grid-connected inverter is solved in the time domain, the calculation does not depend on the form of control, which makes it applicable also in nonlinear scenarios involving control switching and saturation.

[0027] Since the calculation of the voltage at the renewable energy grid connection point during a fault is performed in the phasor domain, and the corresponding step size can be selected according to the dynamic changes in the output current of the renewable energy grid-connected inverter, large step size calculation can save calculation time. Therefore, it can significantly improve the computational efficiency when dealing with fault calculations of large-scale renewable energy systems.

[0028] Step 1: Establish a mathematical model of a controlled current source of new energy, including the output power, DC side voltage, current reference value, port voltage reference value, port voltage and output current of the new energy grid-connected inverter.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The method of the present invention can be used in large-scale renewable energy AC transmission systems. Because the present invention establishes a mathematical model of a controlled current source for renewable energy, this model is solved in the time domain using a numerical integration method. This model is applicable to nonlinear scenarios involving control switching and controller saturation, and features a simple calculation process and strong applicability. Furthermore, because the composite sequence network of renewable energy grid-connected systems is solved in the phasor domain and a variable-step solution method is used, the present invention achieves significant computational efficiency when handling large-scale, highly complex system-level fault calculations. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flow chart of the method of the present invention;

[0032] Figure 2 It is a topological diagram of the large-scale new energy transmission system in the present invention;

[0033] Figure 3a It is a composite sequence network model for single-phase ground short circuit fault;

[0034] Figure 3b It is a composite sequence network model of phase-to-phase short-circuit fault;

[0035] Figure 3c It is a composite sequence network model of two-phase ground short circuit fault;

[0036] Figure 4 This is the topology diagram of the new energy transmission system of the example;

[0037] Figure 5a It is the waveform of the single-phase ground short circuit fault simulation and calculation results.

[0038] Figure 5b It is the waveform of phase-to-phase short circuit fault simulation and calculation results.

[0039] Figure 5c It is the waveform of the two-phase ground short circuit fault simulation and calculation results. DETAILED DESCRIPTION

[0040] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0041] Step 1: Establish a mathematical model of the controlled current source of the new energy source;

[0042] Based on the control principle of the new energy grid-connected inverter during a fault, a mathematical model of the controlled current source of the new energy during a fault is established. The model includes the output power, DC side voltage, current reference value, port voltage reference value, port voltage, and output current of the new energy grid-connected inverter. The steps to establish the mathematical model of the controlled current source of the new energy are as follows:

[0043] Step 1.1 The output power P of the new energy grid-connected inverter is calculated based on the voltage at the new energy grid-connected point and the output current of the new energy grid-connected inverter.

[0044] Step 1.2 Calculate the DC side voltage u of the new energy grid-connected inverter dc

[0045] According to the law of conservation of energy, the energy input to the DC side of the new energy grid-connected inverter is equal to the sum of the inverter output energy and the energy consumed by the DC capacitor. Since the DC side input power P0 of the new energy grid-connected inverter remains unchanged during the fault period, the DC side voltage u of the new energy grid-connected inverter can be calculated. dc .

[0046] Step 1.3 Calculate the d-axis component i of the current reference value of the new energy grid-connected inverter dref and the q-axis component i of the current reference value of the new energy grid-connected inverter qref

[0047] During normal operation or when the fault depth of the power grid is shallow, the per-unit value of the positive sequence voltage amplitude of the renewable energy grid connection point after the fault is U + Greater than 0.9pu, at this time, the current reference value of the new energy grid-connected inverter is obtained through the DC voltage outer loop. The d-axis component i of the DC voltage outer loop output current reference value of the new energy grid-connected inverter dref0 and the q-axis component i qref0 Expressed as:

[0048]

[0049] Among them, K uI K is the integral coefficient of the DC voltage outer loop, uP is the proportional coefficient of the DC voltage outer loop, u dcref It is the DC voltage reference value of the new energy grid-connected inverter.

[0050] When a fault occurs in the power grid, the per-unit value of the positive sequence voltage amplitude at the renewable energy grid connection point after the fault is U + When the voltage drop is lower than 0.9 pu, the current reference value of the new energy grid-connected inverter is obtained by low voltage ride-through control combined with the DC voltage outer loop output current reference value of the new energy grid-connected inverter in formula (2) as follows:

[0051]

[0052] Among them, i dref is the d-axis component of the current reference value of the new energy grid-connected inverter, i qref is the q-axis component of the current reference value of the new energy grid-connected inverter, K is the reactive current compensation coefficient, which is generally set to 1.5; I N is the rated current of the new energy grid-connected inverter; Imax The maximum short-circuit current that the new energy grid-connected inverter can withstand; U + It is the per-unit value of the positive sequence voltage amplitude at the renewable energy grid connection point after the fault.

[0053] Step 1.4 Calculate the d-axis component m of the voltage reference value of the new energy grid-connected inverter port d and the q-axis component m of the voltage reference value of the inverter port connected to the new energy grid q

[0054] The current inner loop equation of the new energy grid-connected inverter is expressed as:

[0055]

[0056] Among them, i d is the d-axis component of the output current of the new energy grid-connected inverter; i q is the q-axis component of the output current of the new energy grid-connected inverter; K iI is the integral coefficient of the inner current loop, K iP is the proportional coefficient of the inner current loop, m d is the d-axis component of the voltage reference value of the inverter port connected to the new energy grid, m q is the q-axis component of the voltage reference value of the inverter port connected to the new energy grid; ω N is the rated angular frequency, L f is the filter inductor of the new energy grid-connected inverter, u d is the d-axis component of the voltage at the new energy grid connection point, u q is the q-axis component of the voltage at the renewable energy grid connection point.

[0057] The d-axis component i of the current reference value of the new energy grid-connected inverter in step 1.3 is dref and the q-axis component i of the current reference value of the new energy grid-connected inverter qref Substitute into formula (3). Use the anti-Pike transformation to convert the d-axis component m of the port voltage reference value of the new energy grid-connected inverter in step 1.4 into d and the q-axis component m of the voltage reference value of the inverter port connected to the new energy grid q Converted to the abc coordinate system, the voltage after coordinate conversion is expressed as m s .

[0058] Step 1.5 Calculate the port voltage u of the new energy grid-connected inverter s

[0059] New energy grid-connected inverter port voltage u s Expressed as:

[0060]

[0061] Among them, V cris the carrier amplitude, which is a constant. dc and the new energy grid-connected inverter port voltage reference value m after coordinate conversion in step 1.4 s Substituting into formula (4), we can get the new energy grid-connected inverter port voltage u s .

[0062] Step 1.6 Calculate the output current i of the new energy grid-connected inverter

[0063] The filter equation of the new energy grid-connected inverter is:

[0064]

[0065] Among them, u is the grid-connected point voltage of the new energy; i is the output current of the new energy grid-connected inverter

[0066] The port voltage u of the new energy grid-connected inverter in step 1.5 s Substitute into equation (5) to solve the output current i of the new energy grid-connected inverter.

[0067] Step 2: Based on the mathematical model of the controlled current source of the new energy, the structure of the new energy grid-connected system and the fault conditions established in step 1, a composite sequence network model of the new energy grid-connected system is established in the phasor domain.

[0068] Large-scale renewable energy power stations are connected to the power grid through multi-voltage transmission lines, forming a large-scale renewable energy sending-end power grid. Figure 2 The figure shows the system architecture of the sending-end power grid connected to the grid via the high-voltage transmission line. When a fault occurs at point F of the transmission line, the composite sequence network models of single-phase grounding fault, two-phase short circuit fault and two-phase grounding short circuit fault are as follows: Figure 3a 、 Figure 3b and Figure 3c shown.

[0069] Figure 3a 、 Figure 3b and Figure 3c middle, I Lk+ is the positive sequence current output by the kth new energy power station, I Lk- is the negative sequence current of the kth feeder, Z Lk+ is the positive sequence impedance of the AC feeder of the kth renewable energy power station, Z Lk- is the negative sequence impedance of the AC feeder of the kth new energy power station, Z G+ and Z G- are the positive sequence impedance and negative sequence impedance from the fault point to the AC grid respectively. M+ and Z M- are the positive sequence impedance and negative sequence impedance from the collection point to the fault point respectively. Σ0 is the equivalent zero-sequence impedance of the system. Fis the fault resistance. U G is the grid voltage. F+ , I F- and I F0 are respectively the positive sequence component, negative sequence component and zero sequence component of the fault current. F+ 、U F- and U F0 They are the positive sequence component, negative sequence component and zero sequence component of the fault point voltage respectively.

[0070] Step 3: Based on the composite sequence network model of the renewable energy grid-connected system, solve the voltage at the renewable energy grid-connected point during the fault period.

[0071] The current fault calculation time is t, and the fault calculation step is ΔT. Based on the superposition theorem, the composite sequence network model of the renewable energy grid-connected system in step 2 is solved to obtain the voltage at the renewable energy grid-connected point during the fault period.

[0072] In the composite sequence network model, considering that the new energy grid-connected inverter adopts the negative sequence current suppression strategy, the positive sequence voltage of the kth new energy grid-connected point and negative sequence voltage for:

[0073]

[0074] The superscript (n) indicates the short circuit type, where n is 3 for a three-phase short circuit, n is 1 for a single-phase ground short circuit, n is 2 for a two-phase short circuit, and n is 1 for a two-phase ground short circuit. m is the total number of new energy power stations; Z G+ is the positive sequence impedance from the fault point to the AC grid; Z M+ is the positive sequence impedance from the collection point to the fault point; is the grid voltage; is the positive sequence current output by the kth new energy power station; is the positive sequence current of the i-th feeder; Z Lk+ is the positive sequence impedance of the AC feeder of the kth renewable energy power station; For additional impedance, is the negative sequence voltage divider impedance, see Table 1 for details.

[0075] Table 1: Additional impedance and negative sequence divider impedance for various short-circuit conditions

[0076]

[0077] Step 4: Time Domain Conversion

[0078] The voltage of the new energy grid connection point solved in step 3 is and Convert from the phasor domain to the time domain.

[0079] Step 5: Combined with the time domain conversion result of the voltage at the new energy grid connection point in step 4, the numerical integration method is used to solve the mathematical model of the controlled current source of the new energy in step 1 to obtain the output current of the new energy grid-connected inverter.

[0080] Establish a mathematical model of the controlled current source of new energy by step 1, and use the numerical integration method to solve the output power, DC side voltage, current reference value, port voltage reference value, port voltage and output current of the new energy grid-connected inverter in sequence.

[0081] Step 6: Phase Conversion

[0082] Convert the output current of the new energy grid-connected inverter solved in step 5 from the time domain to the phasor domain and use it as a known quantity to solve the voltage at the new energy grid-connected point in step 3 in the next step;

[0083] Step 7: Select the corresponding step size ΔT according to the dynamic change of current and update the fault calculation time t;

[0084] Since the output current of a renewable energy grid-connected inverter enters a steady-state phase after the transient fault process, the output current remains stable. Therefore, if the output current is detected to be stable within a few steps, a larger step size can be selected for subsequent calculations. At the same time, the fault calculation time is updated based on the new step size ΔT.

[0085] Step 8: Repeat steps 3-7 until the calculation is completed, and the fault calculation ends when the fault ends.

[0086] The effect of the present invention is described below by a specific embodiment:

[0087] The structure of the new energy transmission system of the embodiment is as follows Figure 4 As shown, it contains two new energy units. The maximum current allowed to pass through the equipment is 1.5 times the rated current. The new energy grid-connected inverter adopts negative sequence current suppression control.

[0088] The line parameters are shown in Table 2.

[0089] Table 2: Line parameters of new energy transmission system

[0090]

[0091] When the system is operating normally until the second second, an asymmetric fault (including a single-phase ground fault, a two-phase short circuit, and a two-phase ground fault) occurs on the transmission line. The fault calculation results using the method of the present invention and the PSCAD simulation results are shown in Figure 5. In the figure, the simulation results are represented by dotted lines, and the calculation results are represented by solid lines.

[0092] The example results show that the present invention can obtain the steady-state calculation results of the output current of the new energy grid-connected inverter before and after the fault, the calculation process is simple, and the calculation results are accurate.

[0093] In summary, the present invention proposes a method for calculating large-scale renewable energy transmission line faults, based on alternating time and phasor domains. This method effectively handles nonlinear control in the time domain and performs system-level solutions in the phasor domain. Calculation and simulation results demonstrate that the proposed method can describe the dynamic response of the output current of renewable energy grid-connected inverters during faults, demonstrating its applicability to complex grid fault calculations involving multiple renewable energy sources.

[0094] The above disclosure is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for calculating large-scale renewable energy transmission line faults based on time-domain and phasor-domain alternation, characterized by: On the basis of the asymmetric fault calculation method, the time domain solution process of the mathematical model of the renewable energy controlled current source is added with the numerical integration method, which can take into account the dynamic process of the output current of the renewable energy grid-connected inverter during the fault period. At the same time, the voltage at the grid connection point of the renewable energy source calculated by formula (6) in the phasor domain takes into account the mutual coupling between the renewable energy units; it includes the following steps: Step 1: Establish a mathematical model of the controlled current source of the new energy source; Based on the control principle of the renewable energy grid-connected inverter during fault period, a mathematical model of the controlled current source of renewable energy during fault period is established; Step 2: Establish a composite sequence network model for the new energy grid-connected system; Based on the mathematical model of the controlled current source of the new energy, the structure of the new energy grid-connected system and the fault conditions established in step 1, a composite sequence network model of the new energy grid-connected system is established in the phasor domain; Step 3: Based on the composite sequence network model of the renewable energy grid-connected system, solve the voltage at the renewable energy grid-connected point during the fault period; The fault calculation starts at the moment of fault occurrence and ends at the moment of fault completion. Based on the superposition theorem, the voltage at the renewable energy grid connection point during the fault period, including both positive and negative sequence voltages, is calculated in the phasor domain using the composite sequence network model of the renewable energy grid connection system. In the composite sequence network model, considering that the new energy grid-connected inverter adopts the negative sequence current suppression strategy, the positive sequence voltage of the kth new energy grid-connected point and negative sequence voltage for: The superscript (n) indicates the short circuit type, where n = 3 indicates a three-phase short circuit, n = 1 indicates a single-phase ground short circuit, n = 2 indicates a two-phase short circuit, and n = 1 indicates a two-phase ground short circuit; m is the total number of new energy power stations; is the additional impedance; is the negative sequence voltage divider impedance; Z G+ is the positive sequence impedance from the fault point to the AC grid; Z M+ is the positive sequence impedance from the collection point to the fault point; is the grid voltage; is the positive sequence current output by the kth new energy power station; is the positive sequence current of the i-th feeder; Z Lk+ is the positive sequence impedance of the AC feeder of the kth renewable energy power station; Step 4: Time domain conversion; Convert the voltage of the new energy grid connection point solved in step 3 from the phasor domain to the time domain; Step 5: Calculate the output current of the new energy grid-connected inverter; Combined with the time domain conversion result of the voltage at the grid-connected point of the new energy in step 4, the numerical integration method is used to solve the mathematical model of the controlled current source of the new energy in step 1 to obtain the output current of the new energy grid-connected inverter; Step 6: Phase conversion; Convert the output current of the new energy grid-connected inverter solved in step 5 from the time domain to the phasor domain and use it as a known quantity to solve the voltage at the new energy grid-connected point in step 3 in the next step; Step 7: Select the corresponding step size according to the dynamic change of current and update the fault calculation time; When the output current of the new energy grid-connected inverter remains stable within several step sizes in step 5, a larger step size is selected for calculation in subsequent steps, and the fault calculation time is updated; Step 8: Repeat steps 3-7 until the calculation is completed, and the fault calculation ends when the fault ends.

2. The method for calculating large-scale renewable energy transmission line faults based on time-domain and phasor-domain alternation according to claim 1, characterized in that: Since the output current of the renewable energy grid-connected inverter is solved in the time domain, the calculation does not depend on the form of control, which makes it applicable also in nonlinear scenarios involving control switching and saturation.

3. The method for calculating large-scale renewable energy transmission line faults based on time-domain and phasor-domain alternation according to claim 1, characterized in that: Since the calculation of the voltage at the renewable energy grid connection point during a fault is performed in the phasor domain, and the corresponding step size can be selected according to the dynamic changes in the output current of the renewable energy grid-connected inverter, large step size calculation can save calculation time. Therefore, it can significantly improve the computational efficiency when dealing with fault calculations of large-scale renewable energy systems.

4. The method for calculating large-scale renewable energy transmission line faults based on time-domain and phasor-domain alternation according to claim 1, characterized in that: Step 1: Establish a mathematical model of a controlled current source of new energy, including the output power, DC side voltage, current reference value, port voltage reference value, port voltage and output current of the new energy grid-connected inverter.

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