Method for alternately calculating large-scale new energy transmission line fault based on time domain and phasor domain

Abstract

The invention discloses a method for alternately calculating a large-scale new energy transmission line fault based on a time domain and a phasor domain. The method comprises the following steps: 1, establishing a controlled current source mathematical model of a new energy inverter; 2, establishing a composite sequence network model of the new energy grid-connected system; 3, solving the new energy grid-connected point voltage in the fault period in the composite sequence network; 4, converting the new energy grid-connected point voltage from a phasor domain to a time domain; 5, solving the mathematical model of the new energy grid-connected inverter by using a numerical integration method to obtain the output current of the new energy grid-connected inverter; 6, converting the output current of the new energy from the time domain to a phasor domain; 7, selecting a corresponding step length according to the dynamic change of the current, and updating the calculation time; and 8, repeating the steps 3-7 until the calculation is finished. The method can be suitable for nonlinear scenes involving control switching and controller saturation, the calculation process is simple, the applicability is high, and when system-level fault calculation with large processing scale and high complexity is carried out, the calculation efficiency is remarkable.

Description

Technical Field

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

[0002] In recent years, a large number of large-scale new energy bases have been built, and they are transmitted through multi-stage boosting via high-voltage AC or high-voltage DC transmission. Large-scale new energy bases connected to the grid contain a large number of power electronic converters, whose response characteristics are highly controlled, showing complex coupling and nonlinearity, and traditional fault calculation methods are difficult to apply. In addition, the current limiting characteristics and low voltage ride-through characteristics of the inverter power supply have attracted attention to the adaptability of the original protection. It is urgent to study new power supply equivalence methods and new fault calculation methods in combination with different fault control strategies of power electronic equipment.

[0003] At present, in the research on analytical calculation of grid-connected inverter power supply, only some control links can be accurately modeled, and all nonlinear links cannot be considered. At the same time, analytical calculation is mainly aimed at single-machine grid-connected scenarios. For multi-machine systems at the network level, due to the coupling between units, it is difficult to solve their fault characteristics through analytical analysis.

[0004] For systems containing multiple renewable energy units, what needs to be resolved is the contradiction between the network and the individual, taking into account the complexity and diversity of the network topology, as well as the impact of the node voltage on the fault current. The sequence component method can decouple the network, and combined with the positive and negative sequence decoupling control algorithm of the inverter, it can reduce the scale of calculation and reduce the difficulty of calculation. The traditional fault calculation method can obtain a steady-state fault current solution through iteration, but does not take into account the nonlinear characteristics of the fault and the transient process of the fault change. Therefore, the existing fault calculation method has problems such as high difficulty in analytical calculation, large amount of calculation, and difficulty in nonlinear characterization, and its 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 calculations in the 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 solution:

[0007] A method for calculating large-scale renewable energy transmission line faults based on time domain and phasor domain alternation is provided. On the basis of the asymmetric fault calculation method, a time domain solution process of a new energy controlled current source mathematical model is added by a 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 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] According to the control principle of the grid-connected inverter of renewable energy 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 controlled current source mathematical model 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: According to the composite sequence network model of the renewable energy grid-connected system, solve the voltage of the renewable energy grid-connected point during the fault period;

[0013] The fault occurrence time is taken as the starting time of the fault calculation, and the fault end time is taken as the termination time of the fault calculation; at one step, based on the superposition theorem, in the phasor domain, according to the composite sequence network model of the renewable energy grid-connected system, the voltage at the renewable energy grid-connected point during the fault period is solved, including the positive sequence voltage and the negative sequence voltage;

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

[0015]

[0016] The superscript (n) indicates the short circuit type, where n is 3 for three-phase short circuit, n is 1 for single-phase ground short circuit, n is 2 for two-phase short circuit, and n is 1 for 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 ith 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 mathematical model of the controlled current source of the new energy in step 1 is solved by using the numerical integration method to obtain the output current of the grid-connected inverter of the new energy;

[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 of 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 in step 5 remains stable within several step sizes, a larger step size is selected to perform calculations 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 end time is the end time of the fault calculation.

[0026] Since the solution of the output current of the renewable energy grid-connected inverter is performed 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-connected point during the fault period is performed in the phasor domain, and the corresponding step size can be selected according to the dynamic change of the output current of the renewable energy grid-connected inverter, the calculation time can be saved under large step size calculation. Therefore, the calculation efficiency can be significantly improved when dealing with fault calculations of large-scale renewable energy systems.

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

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] The method of the present invention can be used in large-scale renewable energy AC power transmission systems. Since the present invention establishes a mathematical model of a controlled current source of renewable energy, the model is solved in the time domain using a numerical integration method, and can be applied to nonlinear scenarios involving control switching and controller saturation, with a simple calculation process and strong applicability. At the same time, since the solution of the composite sequence network of the renewable energy grid-connected system is carried out in the phasor domain, and a variable step-size solution method is used, the computational efficiency of the present invention is significant when processing large-scale and highly complex system-level fault calculations. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 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 delivery 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 It is the topology diagram of the new energy delivery system of the example;

[0037] Figure 5a It is the waveform of 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 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] According to the control principle of the new energy grid-connected inverter during faults, a mathematical model of the controlled current source of new energy during faults 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 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 new energy grid-connected inverter current reference value qref

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

[0048]

[0049] Among them, K uI is the integral coefficient of the DC voltage outer loop, K 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 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 combining the DC voltage outer loop output current reference value of the new energy grid-connected inverter in formula (2) through low voltage ride-through control and is expressed as:

[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 It is 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 a fault.

[0053] Step 1.4 Calculate the d-axis component m of the voltage reference value of the inverter port connected to the new energy grid 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 renewable energy grid connection point, u q It 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 dref and the q-axis component i of the new energy grid-connected inverter current reference value qref Substitute into formula (3). Through the reverse Pike transformation, the d-axis component m of the voltage reference value of the inverter port of the new energy grid-connected inverter in step 1.4 is converted 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 It is expressed as:

[0060]

[0061] Among them, V cris the carrier amplitude, which is a constant. Substitute the DC side voltage u of the new energy grid-connected inverter in step 1.2 dc and the reference value m of the new energy grid-connected inverter port voltage after coordinate conversion in step 1.4 s Substituting into formula (4), we can get the port voltage u of the new energy grid-connected inverter: 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 grid-connected inverter of the new energy

[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 a 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 renewable 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 renewable energy power station, Z G+ and Z G- are the positive sequence impedance and negative sequence impedance from the fault point to the AC power 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. G is the grid voltage. F+ ,I F- and I F0 They are the positive sequence component, negative sequence component and zero sequence component of the fault current respectively. 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 grid-connected inverter of renewable energy adopts the negative sequence current suppression strategy, the positive sequence voltage of the kth renewable energy grid-connected point and negative sequence voltage for:

[0073]

[0074] The superscript (n) indicates the short circuit type, where n is 3 for three-phase short circuit, n is 1 for single-phase ground short circuit, n is 2 for two-phase short circuit, and n is 1 for 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 ith 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 voltage 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-connected point in step 4, the mathematical model of the controlled current source of the new energy in step 1 is solved by the numerical integration method to obtain the output current of the new energy grid-connected inverter.

[0080] The mathematical model of the controlled current source of the new energy is established by step 1, and 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 are solved in sequence by numerical integration method.

[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 of the new energy grid-connected point in step 3 in the next step;

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

[0084] Since the output current of the new energy grid-connected inverter will enter the fault steady-state stage after the fault transient process ends, the output current of the new energy grid-connected inverter will remain stable. Therefore, when it is detected that the output current remains stable within a few step sizes, a larger step size can be selected for subsequent calculations. At the same time, the fault calculation time is updated according to the new step size ΔT.

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

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

[0087] The structure of the new energy delivery 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, and 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 running normally until the second second, an asymmetric fault (including single-phase ground short circuit, two-phase short circuit and two-phase ground short circuit) occurs on the transmission line. The fault calculation results of the method of the present invention and the simulation results of PSCAD 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 is based on a method for calculating large-scale renewable energy transmission line faults alternately in the time domain and the phasor domain. This method effectively handles nonlinear control in the time domain and performs system-level solutions in the phasor domain. The calculation and simulation results show that the method of the present invention can describe the dynamic response of the output current of the renewable energy grid-connected inverter during the fault process, and has good applicability to the calculation of complex power grid faults containing multiple renewable energy sources.

[0094] The above disclosure is only the preferred embodiment of the present invention, which certainly cannot be used 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, phasor domain and alternating calculations, 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 by 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 of the new energy grid connection point calculated by formula (6) in the phasor domain takes into account the mutual coupling between the new energy units; it includes the following steps: Step 1: Establish a mathematical model of the controlled current source of the new energy source; According to the control principle of the grid-connected inverter of renewable energy 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 controlled current source mathematical model 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: According to the composite sequence network model of the renewable energy grid-connected system, solve the voltage of the renewable energy grid-connected point during the fault period; The fault occurrence time is taken as the starting time of the fault calculation, and the fault end time is taken as the termination time of the fault calculation; at one step, based on the superposition theorem, in the phasor domain, according to the composite sequence network model of the renewable energy grid-connected system, the voltage at the renewable energy grid-connected point during the fault period is solved, including the positive sequence voltage and the negative sequence voltage; In the composite sequence network model, considering that the grid-connected inverter of renewable energy adopts the negative sequence current suppression strategy, the positive sequence voltage of the kth renewable energy grid-connected point and negative sequence voltage for: The superscript (n) indicates the short circuit type, where n is 3 for three-phase short circuit, n is 1 for single-phase ground short circuit, n is 2 for two-phase short circuit, and n is 1 for 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 ith 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 mathematical model of the controlled current source of the new energy in step 1 is solved by using the numerical integration method to obtain the output current of the grid-connected inverter of the new energy; 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 of 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 in step 5 remains stable within several step sizes, a larger step size is selected to perform calculations in subsequent steps and the fault calculation time is updated; Step 8: Repeat steps 3-7 until the calculation is completed, and the fault end time is the end time of the fault calculation.

2. A method for calculating large-scale renewable energy transmission line faults based on time domain, phasor domain alternation as claimed in claim 1, characterized in that: Since the solution of the output current of the renewable energy grid-connected inverter is performed 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. A method for calculating large-scale renewable energy transmission line faults based on time domain, phasor domain alternation as claimed in claim 1, characterized in that: Since the calculation of the voltage at the renewable energy grid-connected point during the fault period is performed in the phasor domain, and the corresponding step size can be selected according to the dynamic change of the output current of the renewable energy grid-connected inverter, the calculation time can be saved under large step size calculation. Therefore, the calculation efficiency can be significantly improved 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, phasor domain alternation as claimed in claim 1, characterized in that: Step 1: Establish a mathematical model of a controlled current source of a new energy source, including the output power, DC side voltage, current reference value, port voltage reference value, port voltage and output current of a new energy grid-connected inverter.

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