Practical Evaluation Method and Device for Transient Overvoltage of New Energy Low-Frequency Aggregation System

Abstract

The present application provides a practical evaluation method and device for transient overvoltage of a new energy low-frequency collection system. The purpose of the present application is to ensure the safe and stable operation of the system by calculating accurate transient overvoltages. Among them, the practical evaluation method for transient overvoltage of a new energy low-frequency collection system includes using the method of equivalent voltage source to determine the modular multilevel matrix converter in the new energy low-frequency collection system; obtaining the voltage, current, power, and short-circuit ratio of the new energy low-frequency collection system and the modular multilevel matrix converter, and calculating the voltage drop component caused by resistance and the voltage drop component caused by reactance based on the obtained data; determining the relationship between the voltage of the low-frequency side AC bus and the voltage of the sending-end unit; and using the voltage drop component caused by resistance, the voltage drop component caused by reactance, the relationship, and the short-circuit ratio to calculate the transient overvoltage.

Description

Technical Field

[0001] The present application relates to the field of power technologies, and particularly to a practical evaluation method and device for transient overvoltage of a new energy low-frequency aggregation system. Background Art

[0002] The new energy low-frequency aggregation system is an aggregation and transmission system for stable transmission of new energy power on a large scale and over a long distance. After collecting new energy power by the sending-end unit, it is transmitted through the transmission line circuit to the low-frequency converter, and the low-frequency converter performs frequency conversion to convert it into power frequency power suitable for accessing the existing power grid. However, during the actual operation process, it is easily affected by factors such as changes in equipment parameters and fluctuations in line impedance, thereby triggering overvoltage phenomena. Once a transient overvoltage occurs, the excessive voltage will cause serious damage to the equipment in the system and even lead to power outage faults of the equipment.

[0003] To address the problem of transient overvoltage, surge protectors are usually used for protection at present. However, since the value of the transient overvoltage cannot be determined in the existing technology, a larger specification surge protector is selected to cope with the potential overvoltage risk. This method not only causes waste of resources but also may not effectively suppress the transient overvoltage due to improper selection of the surge protector specification, resulting in the system equipment not being reliably protected.

[0004] Therefore, there is an urgent need for a method to scientifically and reasonably determine the accurate transient overvoltage of the new energy low-frequency aggregation system to ensure the safe and stable operation of the new energy low-frequency aggregation system. Summary of the Invention

[0005] The embodiments of the present application provide a practical evaluation method and device for transient overvoltage of a new energy low-frequency aggregation system, and the purpose is to ensure the safe and stable operation of the new energy low-frequency aggregation system by calculating the accurate transient overvoltage.

[0006] To solve the above technical problems, the embodiments of the present application provide the following technical solutions:

[0007] In a first aspect, the present application provides a practical evaluation method for transient overvoltage of a new energy low-frequency aggregation system, and the method includes:

[0008] Using the method of equivalent voltage source to determine the modular multilevel matrix converter in the new energy low-frequency aggregation system;

[0009] Obtain the resistance of the sending-end unit, the reactance of the sending-end unit, the rated power of the sending-end unit, the short-circuit capacity of the sending-end unit, the low-frequency side AC bus voltage of the modular multilevel matrix converter, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter in the new energy low-frequency aggregation system;

[0010] Determine the first component expression and the second component expression of the voltage drop in the sending-end unit according to the resistance of the sending-end unit, the reactance of the sending-end unit, the active power and the reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter;

[0011] According to Kirchhoff's voltage law, combine the low-frequency side AC bus voltage, the first component expression of the voltage drop and the second component expression of the voltage drop to determine the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency aggregation system;

[0012] Obtain the short-circuit ratio of the new energy low-frequency aggregation system, where the short-circuit ratio is the ratio of the short-circuit capacity of the sending-end unit to the rated power of the sending-end unit, and the short-circuit capacity of the sending-end unit is the ratio of the square of the voltage of the sending-end unit to the reactance of the sending-end unit;

[0013] Use the first component expression of the voltage drop, the second component expression of the voltage drop, the relationship and the short-circuit ratio to calculate the maximum voltage value of the sending-end unit and determine the maximum voltage value of the sending-end unit as the transient overvoltage.

[0014] In a second aspect, the present application provides a practical evaluation device for transient overvoltage of a new energy low-frequency aggregation system, and the device includes:

[0015] An acquisition unit, configured to determine a modular multilevel matrix converter in the new energy low-frequency aggregation system by using the method of an equivalent voltage source;

[0016] The acquisition unit is configured to acquire the resistance of the sending-end unit in the new energy low-frequency aggregation system, the reactance of the sending-end unit, the short-circuit capacity of the sending-end unit, the rated power of the sending-end unit, the low-frequency side AC bus voltage of the modular multilevel matrix converter, the active power and the reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter;

[0017] A determination unit, configured to calculate the first component expression and the second component expression of the voltage drop in the sending-end unit according to the resistance of the sending-end unit, the reactance of the sending-end unit, and the active power and the reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter in the acquisition unit;

[0018] The determination unit is configured to determine the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency aggregation system according to Kirchhoff's voltage law, in combination with the low-frequency side AC bus voltage, the first component expression of the voltage drop and the second component expression of the voltage drop;

[0019] A short-circuit ratio acquisition unit is used to acquire the short-circuit ratio of a new energy low-frequency aggregation system. The short-circuit ratio is the ratio of the short-circuit capacity of a sending-end unit to the rated power of the sending-end unit, where the short-circuit capacity of the sending-end unit is the ratio of the square of the voltage of the sending-end unit to the reactance of the sending-end unit.

[0020] A voltage determination unit is used to calculate the maximum voltage value of the sending-end unit by using the relationship, the first component expression of the voltage drop of the determination unit, the second component expression of the voltage drop, and the short-circuit ratio in the short-circuit ratio acquisition unit, and determine the maximum voltage value of the sending-end unit as the transient overvoltage.

[0021] In a third aspect, the present application provides a storage medium for storing a computer program. When the computer program runs, it controls the device where the storage medium is located to execute the above-mentioned practical evaluation method for transient overvoltage of a new energy low-frequency aggregation system.

[0022] In a fourth aspect, the present application provides an electronic device, which includes a processor and a memory. The processor is used to call the program instructions in the memory to execute the above-mentioned practical evaluation method for transient overvoltage of a new energy low-frequency aggregation system.

[0023] By means of the above technical solutions, the present application provides a practical evaluation method and device for transient overvoltage of a new energy low-frequency aggregation system. First, by introducing the equivalent voltage source method, the electrical characteristics of the converter can be accurately reflected according to the steady-state characteristics of the converter, providing reliable basic data for subsequent overvoltage calculation. Second, according to Kirchhoff's voltage law, combined with the low-frequency side AC bus voltage, the voltage drop component caused by resistance, and the voltage drop component caused by reactance, the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency aggregation system is determined, which can effectively reflect the internal voltage state and operating conditions of the system and provide accurate input data for overvoltage calculation. In addition, as a core index to measure the strength of a power system, the short-circuit ratio comprehensively reflects the relationship between the system structure, power supply, and load, further improving the calculation accuracy. That is, the present application combines Kirchhoff's voltage law, the low-frequency side AC bus voltage, and the short-circuit ratio, fully considering the operating characteristics of the new energy low-frequency aggregation system when overvoltage occurs, accurately calculating the overvoltage, and thus ensuring the safe operation of the system. Description of the Drawings

[0024] By reading the following detailed description with reference to the drawings, the above and other objects, features, and advantages of the exemplary embodiments of the present application will become readily understood. In the drawings, several embodiments of the present application are shown in an exemplary and non-limiting manner, and the same or corresponding reference numerals represent the same or corresponding parts, where:

[0025] Figure 1A practical evaluation method flowchart for transient overvoltage of a new - energy low - frequency collection system is proposed;

[0026] Figure 2 A schematic diagram of the equivalent process of the M3C sub - module is proposed;

[0027] Figure 3 Another practical evaluation method flowchart for transient overvoltage of a new - energy low - frequency collection system is proposed;

[0028] Figure 4 An equivalent circuit diagram of a low - frequency power transmission system is proposed;

[0029] Figure 5 A structural diagram of a practical evaluation device for transient overvoltage of a new - energy low - frequency collection system is proposed;

[0030] Figure 6 Another structural diagram of a practical evaluation device for transient overvoltage of a new - energy low - frequency collection system is proposed. Detailed implementation manners

[0031] The exemplary embodiments of the present application will be described in more detail with reference to the accompanying drawings. Although the exemplary embodiments of the present application are shown in the drawings, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present application can be more thoroughly understood and the scope of the present application can be completely conveyed to those skilled in the art. It should be noted that unless otherwise specified, the technical terms or scientific terms used in the present application should have the ordinary meaning understood by those skilled in the art to which the present application belongs.

[0032] To address the problem of transient overvoltage, surge protectors are usually used for protection at present. However, the existing methods mainly deal with potential overvoltage risks by selecting surge protectors with larger specifications. This method not only causes waste of resources but also may not effectively suppress transient overvoltage due to improper selection of surge protector specifications, resulting in the system equipment not being reliably protected.

[0033] Therefore, the inventors of the present application have proposed a practical evaluation method for transient overvoltage of a new - energy low - frequency collection system. After accurately calculating the transient overvoltage, a surge protector is selected according to the transient overvoltage. It can be seen that the present application can fully consider the characteristics of the new - energy low - frequency collection system, can calculate the overvoltage more accurately, and perform subsequent analysis based on the transient overvoltage and select equipment to suppress the transient overvoltage. The specific steps of the practical evaluation method for transient overvoltage of the new - energy low - frequency collection system implemented in the present application are as Figure 1 shown and include:

[0034] Step 101: Determine the modular multilevel matrix converter in the new energy low-frequency aggregation system by using the equivalent voltage source method.

[0035] Before introducing Step 101, this embodiment also introduces the new energy low-frequency aggregation system as follows: The new energy low-frequency aggregation system is a power system that integrates new energy power generation. Due to the dispersion and intermittency of new energy power generation, the generated electric energy needs to be centrally processed through the aggregation system. It operates at a relatively low frequency, which can reduce transmission losses. The sending-end unit is the source device that generates electric energy. The modular multilevel matrix converter (M3C) is a new type of power electronic converter topology that combines the characteristics of modular multilevel technology and matrix converters. M3C is usually composed of multiple sub-modules, and each sub-module contains one or more power electronic switches and capacitors and other components, which can realize the electric energy conversion between multiple AC ports and has good output waveform quality and high efficiency. In this embodiment, the main circuit structure of M3C is constructed based on a three-phase system. From the perspective of circuit topology, each phase is associated with the other two phases respectively. Under this three-phase interaction architecture, a specific arm connection method is formed. Specifically, as Figure 2 shown in the figure, the entire main circuit has a total of 9 arms, and each arm is cascaded by an inductor L, an equivalent resistance R, and N full-bridge sub-modules FBSM. Each sub-module contains a capacitor C0, 4 IGBTs, and 4 diodes. M3C connects two three-phase AC systems with different frequencies and amplitudes. O is the neutral point (zero potential) of the input voltage, and N is the neutral point of the output voltage. s represents the low-frequency input side, and r represents the power-frequency output side, as well as the voltage of each arm, the current in each arm, and the current direction. In the modular multilevel matrix converter, there are a low-frequency side and a power-frequency side. Among them, the low-frequency side AC bus is a key part connecting the converter and the AC system. The low-frequency side AC bus voltage is the AC voltage value on this bus, which reflects the electrical connection state and power exchange situation between the converter and the AC system.

[0036] Determine the modular multilevel matrix converter by determining the capacitance of the modular multilevel matrix converter in the new energy low-frequency aggregation system by using the equivalent voltage source method, and select the model of the modular multilevel matrix converter as the modular multilevel matrix converter average value model.

[0037] Step 102: Obtain the resistance of the sending-end unit, the reactance of the sending-end unit, the short-circuit capacity of the sending-end unit, the rated power of the sending-end unit, the low-frequency side AC bus voltage of the modular multilevel matrix converter, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter.

[0038] In this step, for the low-frequency side AC bus voltage of the modular multilevel matrix converter, high-precision voltage sensors can be installed at the low-frequency side AC bus of the modular multilevel matrix converter to collect voltage data in real time. Meanwhile, using the system's monitoring and analysis software, combined with the operating characteristics and historical data of the modular multilevel matrix converter, the collected voltage data is processed and analyzed to calculate the low-frequency side AC bus voltage of the modular multilevel matrix converter. The reactance and resistance of the sending-end unit can be obtained by direct measurement or by referring to the design data manual. The rated power of the sending-end unit can be calculated using the nameplate parameters of the sending-end unit. The active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter can be measured using power measuring instruments. The short-circuit capacity of the sending-end unit can be obtained from the power supply department. If there is no short-circuit capacity value of the sending-end unit, but there is a given short-circuit capacity value of the upper-level bus, the parameters of each power equipment after the given value can be obtained in detail, such as the resistance and reactance of the power line, the transformation ratio and short-circuit impedance of the transformer, the reactance value of the reactor, etc., and then the short-circuit capacity of this level of bus can be calculated by the nominal value method or the per-unit value method, and further the short-circuit capacity of the sending-end unit can be obtained.

[0039] Step 103: Calculate the first component expression of the voltage drop and the second component expression of the voltage drop in the sending-end unit according to the resistance of the sending-end unit, the reactance of the sending-end unit, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter.

[0040] In this step, according to the equipment in the above system (sending-end unit, transmission line, modular multilevel matrix converter, etc.), determine the voltage difference between the output end of the sending-end unit and the low-frequency side end of the modular multilevel matrix converter, that is, the first component of the voltage drop and the second component of the voltage drop. At this time, the calculation of the first component expression of the voltage drop and the second component expression of the voltage drop is as shown in Expression (1):

[0041] → (1)

[0042] Since in a system with a voltage level of 35 kV and above, the line resistance is small and can be ignored, therefore, the above expression can be simplified to the right-side expression. In the above expression, represents the low-frequency side AC bus voltage, represents the first component of the voltage drop, represents the second component of the voltage drop, P1 and Q1 are the active and reactive powers transmitted from the sending-end unit to the low-frequency side of the M3C respectively; R1 and X1 are the equivalent resistance and reactance of the sending-end system respectively.

[0043] Step 104: According to Kirchhoff's voltage law, in combination with the low-frequency side AC bus voltage, the voltage drop first component expression and the voltage drop second component expression, determine the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency collection system.

[0044] In this step, according to Kirchhoff's voltage law, at any time in the circuit, along the specified loop direction (clockwise or counterclockwise), the algebraic sum of the voltages across each component is zero, and the mathematical expression is ∑U = 0. From the perspective of potential, if you start from any point in the loop and go around the loop for one circle, the sum of the potential drops is equal to the sum of the potential rises. This is because the instantaneous potential at any point has a single value. At this time, the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency collection system, where the voltage of the sending-end unit can be the equivalent voltage of the sending-end unit. The expression of the relationship is: , where j represents the imaginary unit.

[0045] Step 105: Obtain the short-circuit ratio of the new energy low-frequency collection system.

[0046] In this step, the short-circuit ratio is the ratio of the short-circuit capacity of the sending-end unit to the rated power of the sending-end unit, wherein the short-circuit capacity of the sending-end unit is the ratio of the square of the voltage of the sending-end unit to the reactance of the sending-end unit. The short-circuit ratio (SCR) is an important indicator to measure the strength of the power system. It is defined as the ratio of the system short-circuit capacity to the rated capacity of the converter station. The short-circuit capacity reflects the short-circuit current that the system can provide in the event of a short-circuit fault. The larger the short-circuit ratio, the stronger the system and the greater its ability to withstand voltage changes; conversely, the weaker the system, the more easily the voltage stability is affected. In this step, the short-circuit ratio is expressed as follows:

[0047]

[0048]

[0049] In the above formula, represents the voltage of the sending-end unit, X1 is the equivalent reactance of the sending-end system, I is the short-circuit current, Expressed as the short-circuit ratio, Indicates the short-circuit capacity of the fan at the delivery end. Rated power of the sending end unit.

[0050] In addition, according to the simplified formula of the first component expression of the voltage drop in the sending-end unit and the second component expression of the voltage drop in step 103 and the short-circuit ratio formula in this step, a rewritten formula can be obtained:

[0051]

[0052] Step 106: Calculate the maximum voltage value of the sending-end unit by using the first component expression of the voltage drop, the second component expression of the voltage drop, the relationship, and the short-circuit ratio, and determine the maximum voltage value of the sending-end unit as the transient overvoltage.

[0053] In this step, the first component expression of the voltage drop and the second component expression of the voltage drop are used to reflect the difference between the performance origin and the power grid, that is, the voltage difference between the voltage of the sending-end unit and the voltage of the AC bus on the low-frequency side. Among them, the first component expression of the voltage drop and the second component expression of the voltage drop are substituted into the relationship between the voltage of the AC bus on the low-frequency side and the voltage of the sending-end unit in the new energy low-frequency aggregation system. The voltage of the sending-end unit in the relationship is expressed by the short-circuit ratio and the rated power of the sending-end unit, and the maximum value of the impedance modulus is calculated which is the transient overvoltage. The final expression of the transient overvoltage is as follows, where I is the short-circuit current:

[0054] (2)

[0055] In addition, it is worth noting that after obtaining the accurate transient overvoltage according to the practical evaluation method of the transient overvoltage of the new energy low-frequency aggregation system, the specification of the overvoltage protection device can be selected according to the transient overvoltage.

[0056] Based on the above Figure 1 implementation method, it can be seen that the present application provides a practical evaluation method for the transient overvoltage of the new energy low-frequency aggregation system. By combining Kirchhoff's voltage law, the voltage of the AC bus on the low-frequency side, and the short-circuit ratio, and fully considering the operating characteristics of the new energy low-frequency aggregation system when an overvoltage phenomenon occurs, the overvoltage can be accurately calculated, which can avoid waste of resources caused by blindly selecting equipment with a larger specification, and thus ensure the safe operation of the system.

[0057] Furthermore, according to the embodiment of the present application shown above, the embodiment of the present application further elaborates on how to determine the modular multilevel matrix converter by using the method of equivalent voltage source for the new energy low-frequency aggregation system, specifically as Figure 1 shown, including: Figure 3 shown, including:

[0058] 301. The establishment process of the average value model of the modular multilevel matrix converter bridge arm.

[0059] In this step, to better study the external equivalent characteristics and output characteristics of the M3C, the high-frequency effects of the switching devices and the differences between sub-modules are ignored. Therefore, an average value model of the M3C arm is established. The process of determining the capacitance of the M3C arm average value model is as follows: According to the main circuit structure of the modular multilevel matrix converter, obtain the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter. The first quantity is the number of sub-modules in a charging or discharging state in any arm, and the capacitor voltage of the sub-modules is the capacitor voltage of the sub-modules in the main circuit structure of the modular multilevel matrix converter; According to the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter, determine the equivalent capacitance of any arm; Set the equivalent capacitance as the arm capacitance of the modular multilevel matrix converter arm average value model. Specifically, according to the main circuit structure of the modular multilevel matrix converter, find the total number of sub-modules of the modular multilevel matrix converter. According to the working state of the sub-modules, obtain the number of sub-modules in a charging or discharging state in the arm, that is, the first quantity. After the acquisition is completed, first determine the capacitance voltage of the arm, and then determine the equivalent voltage of the arm and the current of the arm according to the capacitance voltage of the arm, further determine any phase voltage, and finally use the power balance principle and the energy equalization principle, combined with the following formula of any phase voltage, to determine the equivalent capacitance of any arm.

[0060] It should be noted that this embodiment gives a more specific implementation method for the step of determining the equivalent capacitance of any arm according to the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter, which is as follows: According to the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter, calculate the capacitance voltage of the arm. The calculation formula (3) of the capacitance voltage of the arm is:

[0061] (3)

[0062] In the above formula, is the capacitance voltage of the arm, is the capacitor voltage of the sub-module, n is the first quantity, N is the total number of sub-modules of the modular multilevel matrix converter, x = u, v, w, x is the phase of the low-frequency side AC voltage source, y = a, b, c, y is the phase of the sending-end AC voltage source; According to the capacitance voltage of the arm, determine the equivalent voltage of the arm and the current of the arm; According to Kirchhoff's voltage law, combined with the equivalent voltage of the arm and the current of the arm, determine any phase voltage of the low-frequency side AC voltage source. The following formula (4) is followed by any phase voltage:

[0063] (4)

[0064] Using the power balance principle and the equipartition principle of energy, combined with the formula followed by any one of the phase voltages, determine the equivalent capacitance of any bridge arm. In the above formula, is any phase voltage, is the equivalent voltage representing the series part of the sub-modules of bridge arm xy, is the current of bridge arm xy, R is the equivalent resistance of the bridge arm, and L is the inductance of the bridge arm, represents the derivative of the bridge arm current with respect to time, is the neutral point voltage measured at power frequency everywhere, is the equivalent voltage of the bridge arm. In addition, in the above steps, the specific steps for determining the equivalent voltage of the bridge arm according to the capacitance voltage of the bridge arm are as follows: According to the capacitance voltage balance between sub-modules, calculate the equivalent voltage of the bridge arm using the capacitance voltage of the bridge arm. The calculation formula (5) for the equivalent voltage of the bridge arm is:

[0065] (5)

[0066] According to formula (5), there can be the following formula (6), where is the modulation signal of the bridge arm:

[0067] (6)

[0068] In the above steps, the specific steps for determining the current of the bridge arm according to the capacitance voltage of the bridge arm are as follows: According to the capacitance voltage of the bridge arm, determine the current passing through the capacitor. The calculation formula (7) for the current of the capacitor is:

[0069] (7)

[0070] Based on the fact that the voltage of the bypassed sub-module is zero and the calculation formula of the capacitor current, determine the rewritten formula (8) for the capacitor current as:

[0071] (8)

[0072] Based on the rewritten formula of the capacitor current, determine the calculation formula (9) for the current of the bridge arm as:

[0073] (9)

[0074] In the above formula, represents the capacitance value of the capacitor in the full-bridge sub-module, is the sub-module capacitance voltage, represents the current of a single sub-module in the bridge arm, is the equivalent capacitive current of the sub-module, and the equivalent capacitance of the arm can be expressed as , represents the modulation signal of the arm.

[0075] Now take Figure 2 the schematic diagram of the equivalent process of the M3C sub-module shown as an example to illustrate: It is now known that the phase voltage and phase current of the AC voltage source on the low-frequency side are respectively represented by and i x , where x = u, v, w, and the phase voltage and phase current of the receiving-end AC voltage source are respectively represented by u y and i y , where y = a, b, c, represents the arm current, represents the circulating current of the arm xy, is the neutral point voltage on the power frequency output side, is the capacitor voltage of the sub-module. u xy represents the equivalent voltage source of the series part of the sub-modules of the arm. To simulate the oscillation characteristics of the capacitor voltage, the sub-module capacitor is equivalent to a controlled current source and the charging and discharging circuit of the equivalent capacitance of the sub-module in series. For a certain arm, there are a total of N sub-modules, among which n are in the charging and discharging state and the other (N - n) are in the bypass state. Solve for the equivalent capacitance of the nine arms of the M3C.

[0076] According to the above known conditions, the first quantity is n, the total number of sub-modules of the modular multilevel matrix converter is N, and the number of sub-modules in the bypass state is N - n. At this time, the capacitor voltage of the arm can be determined according to formula (3). Assuming that the capacitor voltages between the sub-modules are balanced at this time, the equivalent voltage of the arm can be determined according to formula (5), and there is formula (6). In addition, according to formula (3), the current passing through the capacitor can be determined. At this time, the current passing through the capacitor can be expressed by the mathematical formula as formula (7). Also, because the voltage of the (N - n) bypassed sub-modules is 0, the current formula of the capacitor is rewritten as formula (8) at this time. Transform formula (8), that is, divide both sides by N, and determine the calculation formula (9) of the arm current. According to Kirchhoff's voltage law, the arm xy satisfies formula (4) at this time.

[0077] Based on the above, since the increase or decrease of the capacitor voltage in the power module depends on the active power absorbed or released by the arm, the control of the DC component in the capacitor voltage only needs to analyze the active power. Assume that each arm of the M3C has N sub-modules, then the nine arms contain 9N sub-modules, that is, there are 9N capacitors. And assume that the energy between the nine arms is completely balanced. The DC circuit represents the average voltage of all full-bridge sub-modules on the arm. Use the power balance principle and the idea of energy equalization to deduce the average value model of the M3C arm, and simplify the detailed model to improve the simulation speed. Ignoring the internal losses of the M3C, it can be obtained that the power on the input side of the system must be equal to the power on the output side plus the power flowing through the M3C, that is:

[0078] (10)

[0079] Among them, P in is the low-frequency input-side power, P out is the power-frequency output-side power, and △P M3C is the power related to the current flowing through the M3C arm, which reflects the power situation generated due to the presence of current in the M3C arm. Similar to the average value model of the MMC, the DC circuit of the M3C can be composed of a controlled current source and a capacitor in parallel. At this time, the current of the controlled current source is , the capacitor is , and the voltage of the arm is , then the power flowing through the M3C arm can be expressed as:

[0080] (11)

[0081] Using the DC circuit to represent the average voltage of all full-bridge sub-modules on the arm, it is easy to obtain the total energy of the M3C = the number of capacitors × the energy stored in each capacitor according to the idea of energy equalization, that is:

[0082] (12)

[0083] Therefore, the equivalent capacitance of the nine arms of the M3C can be expressed as:

[0084] (13)

[0085] Among them, represents the current value of the controlled current source, represents the capacitance value of a single sub-module capacitor, represents the total energy of the M3C, is the sub-module capacitor voltage, represents the equivalent capacitor voltage of all full-bridge sub-modules of the M3C arm,

[0086] In this embodiment, in step 301, the average value model of the modular multilevel matrix converter arm with the equivalent capacitance of the nine arms of M3C is used to avoid the complex switching conditions during switching in the actual operation of M3C that connects two three-phase AC systems with different frequencies and amplitudes, enabling each sub-module of M3C to operate in a more ideal state, reducing the energy loss and uneven equipment stress caused by voltage imbalance, improving the overall performance and efficiency of the system, and thus ensuring the accuracy of subsequent calculations.

[0087] Step 302: Steady-state operation analysis of the M3C mathematical model in the new energy low-frequency aggregation system.

[0088] After determining the equivalent capacitance of the M3C arm in the new energy low-frequency aggregation system in step 301, this step analyzes the steady-state operation of the modular multilevel matrix converter in this system. The specific steps are as follows: According to the formula followed by any phase voltage, determine the dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system; Under the dynamic mathematical model of the modular multilevel matrix converter system, obtain the three-phase voltages and three-phase currents on the power frequency output side and the low-frequency input side within a preset time; According to the three-phase voltages and three-phase currents, obtain waveform diagrams, which include the three-phase voltage waveform diagram on the power frequency output side, the three-phase current waveform diagram on the power frequency output side, the three-phase current waveform diagram on the low-frequency input side, and the three-phase current waveform diagram on the low-frequency input side; Judge whether the waveform diagrams are three-phase sine waves; If so, determine the correctness of using the equivalent voltage source method to determine the modular multilevel matrix converter in the new energy low-frequency aggregation system.

[0089] For the specific implementation of determining the dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system according to the formula followed by any phase voltage in the step: According to the formula followed by any phase voltage, determine the AC side mathematical model of the new energy low-frequency aggregation system in the abc rectangular coordinate system; Perform coordinate transformation on the AC side mathematical model to determine the analytical expressions of the voltage and current components of the modular multilevel matrix converter; Based on the analytical expressions of the voltage and current components, determine the dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system. In this regard, this embodiment gives a more detailed implementation method, specifically as follows: First, according to any phase voltage that determines the low-frequency side AC voltage source in the modular multilevel matrix converter, establish an AC side mathematical model. The specific steps are to expand formula (4) to obtain the AC side mathematical model in the abc rectangular coordinate system:

[0090] (14)

[0091] The Clarke transformation matrix for equal power transformation is:

[0092] (15)

[0093] Among them, and and respectively represent the voltages of the u-phase, v-phase, and w-phase on the AC side. and and and and and and and and represent the currents on the corresponding bridge arms. and and and and and and and and respectively represent the voltage components on different bridge arms. L is the inductor in the bridge arm, and R is the equivalent resistance in the bridge arm. The Clarke transformation matrix is a linear transformation matrix that linearly combines the abc three-phase variables through specific coefficients to obtain aβ0 components.

[0094] To achieve the decoupling of the input and output frequency components of the bridge arm voltage and current, the formula (14) is aβ0 coordinate-transformed, and left-multiplied by the formula (5) to obtain the following equation:

[0095] (16)

[0096] According to the above formula (16), and and represent the components of the AC side phase voltage in the aβ0 coordinate axes ， and and and and and and and and represent the values on the corresponding bridge arms in the axis, axis, and the zero-sequence component value. and and and and and and , , respectively represent the voltage values of the sub-converter bridge arm voltage under different phases and different coordinate components. , , respectively represent the voltages of phases a, b, and c on the power frequency side.

[0097] According to the above formula (16), it can be seen that the zero-sequence components of the voltages and currents of the three sub-converter bridge arms are related to the 0 components of the input-side voltage, the voltage between the output-side voltage and the neutral point, and are independent of the aβ components of the input voltage. When the system is a three-phase three-wire system, this zero-sequence component is only related to the output-side voltage, that is, the 0 components of the bridge arm voltage and current are of the same frequency as the output-side system and can be analyzed independently. At this time, the analytical expressions of the aβ components of the voltages and currents of the three sub-converters and the analytical expressions of the 0 components of the voltages and currents of the three sub-converters can be obtained:

[0098] (17)

[0099] (18)

[0100] Among them, is the zero-sequence component of the sub-converter bridge arm voltage, , , represent the zero-sequence components of the sub-converter bridge arm current, , , represent the different phase voltage values of the sub-converter bridge arm voltage under the zero-sequence component. Since the 0 components of the bridge arm voltage and current are of the same frequency as the output-side system, at this time, perform a aβ0 coordinate transformation on formula (18), and the three-phase voltage equation on the output side can be transformed into the aβ0 coordinate system, that is, the mathematical model of the output frequency component in the aβ0 coordinate system is as follows:

[0101] (19)

[0102] At this time, formula (17) and formula (19) together constitute the mathematical model of the M3C system in the aβ0 coordinate system.

[0103] It should be noted that if the input and output systems are three-phase symmetric, the total zero-sequence voltage of the M3C is controlled to zero. At this time, u s0 = 0, u 00= 0, then there must be i 00 = 0, u N = 0 hold simultaneously, then the formula (18) determined according to formula (16) can be further reduced to a second-order model, as shown in the formula:

[0104] (20)

[0105] Through the order reduction of formula (18), formula (17) and formula (20) constitute the 8th-order mathematical model of the system during symmetric operation in aβ coordinate system. At this time, the arm voltage and current components are in the same frequency as the input-side system and are aβ related to the input-side voltage and have nothing to do with the output side; the zero-sequence components of the arm voltage and current are related to the output-side voltage and are in the same frequency as the output-side system.

[0106] Since, in aβ the stationary coordinate system, there are coupling relationships among physical quantities such as arm voltage and current in the system, making the mathematical model relatively complex and the analysis and control difficult. And transforming the system from aβ coordinate system to the dq rotating coordinate system can convert the original alternating quantities into direct current quantities according to the synchronous relationship between the dq axes and the rotating magnetic field, effectively decoupling the relevant variables, thereby simplifying the mathematical model. Transforming the mathematical model of the system in aβ coordinate system to the dq coordinate system, at this time, the two-phase stationary to two-phase rotating coordinate transformation matrix (2s / 2r transformation matrix) is:

[0107] (21)

[0108] (22)

[0109] Among them, and are the low-frequency side frequency and the power frequency side frequency respectively. At this time, perform coordinate transformation on formula (17) and formula (20) to convert them into the dynamic mathematical model of the M3C system in the dq coordinate system. The specific method is to left-multiply formula (17) by formula (21) to get formula (23), left-multiply formula (20) by formula (22) to get formula (24), and formula (23) and formula (24) form the dynamic mathematical model of the M3C system in the dq coordinate system, which is specifically as follows:

[0110] (23)

[0111] (24)

[0112] For the above formula (23), the dynamic relationships between the voltages and currents of the three sub-converters in the dq coordinate system are listed in a separate form, and they can be transformed into the mathematical model of the M3C system in the dq coordinate system represented in matrix form, as shown in formula (25):

[0113] (25)

[0114] Among them, the first term on the right side of the equation in formula (25) is the relevant term of the inductor's derivative of the current components of each sub-converter, the second term is the coupling term generated by rotation in the dq coordinate system, and the third term is the voltage source term of each sub-converter.

[0115] Through the above coordinate transformation, formula (23) and formula (24) together constitute the dynamic mathematical model of the M3C system in the dq coordinate system. Of course, formula (25) and formula (24) also together constitute the dynamic mathematical model of the M3C system in the dq coordinate system.

[0116] Furthermore, in this embodiment, for obtaining the three-phase voltages and three-phase currents on the power frequency output side and the low-frequency input side within a preset time under the dynamic mathematical model of the modular multilevel matrix converter system, specific implementation methods for obtaining the three-phase voltages and three-phase currents are given as follows:

[0117] According to formula (23) in the dynamic mathematical model of the M3C system in the dq coordinate system, the mathematical models of the inner-loop current controllers can be determined as shown in formula (26) and formula (27):

[0118] (26)

[0119] (27)

[0120] After determining the mathematical models of the inner-loop current controllers and the dynamic mathematical model of the M3C system in the dq coordinate system according to the above formulas, when the system adopts the synchronous dq coordinate system with the input-side grid voltage orientation, the q-axis voltage component at steady state is 0, and the input power of the system at this time can be determined as:

[0121] (28)

[0122] Among them, in the above formula, represents the active power input to the system, which reflects the part of the input energy of the system that can actually be used for doing work, represents the reactive power input to the system. The reactive power characterizes the scale of the energy exchange between the electric field and the magnetic field in the circuit and does not directly consume energy. In addition, since the q-axis voltage component at steady state is 0, that is, , then in formula (28) , it can be obtained that ida and i qa are respectively proportional to the active and reactive power on the input side. Therefore, a proportional-integral (PI) controller can be used for the outer power loop control.

[0123] For the M3C on both sides (low-frequency side and power-frequency side) of the new energy low-frequency aggregation system, the principle of active power conservation is followed. This means that in an ideal situation, the total active power input on the low-frequency side of the M3C must be equal to the total active power output on the power-frequency side. At this time, the active powers on both sides (low-frequency side and power-frequency side) of the M3C cannot be adjusted independently at will, but are interrelated and interact with each other. However, the reactive power on both sides can be independently controlled. That is, a change in the active power on one side will inevitably cause a corresponding change in the active power on the other side, and it is impossible to independently control the active power on one side unilaterally without affecting the other side. In addition, the outer loop control is divided into active power control and reactive power control. The low-frequency input side adopts constant active power control and constant AC voltage amplitude control, and the power-frequency output side adopts constant DC capacitor voltage control and constant reactive power control.

[0124] Adopt constant active power control to control the active current component on the low-frequency input side; adopt constant AC voltage amplitude control to control the reactive current component on the low-frequency input side, as shown in formula (29):

[0125] (29)

[0126] Adopt constant DC capacitor voltage control to control the active current component on the power-frequency output side; adopt constant reactive power control to control the reactive current on the power-frequency output side, as shown in formula (30):

[0127] (30)

[0128] According to the above formula, let the instantaneous values of the voltage and current on the low-frequency side of the M3C be , , and and i v , , and the instantaneous values of the voltage and current on the power-frequency side be , , and , , . Taking the phase of the u-phase voltage as the reference, the instantaneous values of the voltage and current on the low-frequency side under steady-state operation can be expressed as:

[0129] (31)

[0130] The instantaneous values of the voltage and current on the power-frequency side can be expressed as:

[0131] (32)

[0132] In the above formula, , , , are the voltage amplitudes and current amplitudes on the low - frequency side and the power - frequency side respectively, , are the phase differences between the voltage and current on the low - frequency side and the power - frequency side, is the initial phase difference of the power - frequency side compared with the low - frequency side.

[0133] Based on formula (31) and formula (32), the active power and reactive power on the input side and the output side can be obtained. The active power on the output side is given by formula (33) and formula (34):

[0134] (33)

[0135] (34)

[0136] In this embodiment, according to Figure 2 the nine - arm structure of the M3C shown, due to the symmetry of the M3C converter or the zero - sequence characteristics of the input and output currents, the expression of the arm current can be obtained as:

[0137] (35)

[0138] Based on formula (35), assuming that the capacitor voltages of the sub - modules on all arms of the M3C are balanced, that is, ignoring the influence of the AC voltage fluctuation on the DC - side capacitor of the arm, so the arm circulating current is ignored in this model, that is , and at this time the arm current can be expressed as:

[0139] (36)

[0140] At this time, the arm current connected to the a - phase in the M3C is:

[0141] (37)

[0142] Similarly, the arm currents connected to the b - phase and c - phase can be obtained as:

[0143]

[0144] (38)

[0145] In the embodiment of the present application, the correctness of the above - derived average - value model of the M3C is also verified as follows: Based on the PSCAD / EMTDC simulation platform, a structure as Figure 5The new energy low-frequency transmission system model shown in the figure, after completing the model construction and parameter setting, simulates the system in the dual dq coordinate system according to the converter control strategy proposed in the technical solution. During the operation, the voltage and current data of the input side u-phase converter are obtained from the set measurement points using the oscilloscope and other detection tools provided by the simulation platform. The selected measurement points are not limited here. They can be reasonably set according to the electrical connection position of the u-phase converter in the model. The voltage and current data obtained, when the system is in a steady state, the symmetry of the three-phase bridge arm voltage of each sub-converter reaches the first preset state, the symmetry of the three-phase bridge arm current of each sub-converter reaches the second preset state, and the superimposed current waveform envelopes a sinusoidal fluctuation to determine the correctness of the above theoretical analysis and modeling. The first preset state is the ideal state of waveform symmetry in the waveform diagram of the three-phase bridge arm voltage of each sub-converter changing with time, that is, it presents a well-symmetrical three-phase sine wave, and the second preset state is the ideal state of waveform symmetry in the waveform diagram of the three-phase bridge arm current of each sub-converter changing with time, that is, it presents a well-symmetrical three-phase sine wave.

[0146] Step 303: Calculate the overvoltage.

[0147] After determining the correctness of the modular multi-level matrix converter in the new energy low-frequency collection system in step 302, a new energy low-frequency collection and transmission system based on the M3C average value model is built based on the M3C average value model. The topology of the system is the low-frequency side, where there are wind farms, transformers and transmission lines on the low-frequency side. The above devices are connected in series in the system in sequence, and the modular multi-level matrix converter M3C is used to connect the low-frequency side and the power frequency side. Figure 5 As shown, P1 and Q1 are respectively the active and reactive power transmitted from the sending-end unit to the low-frequency side of M3C; R1 and X1 are respectively the equivalent resistance and reactance of the sending-end system; is the equivalent voltage of the AC system at the sending end; U M3C is the AC bus voltage at the low-frequency side of the M3C converter station. The voltage difference between the equivalent voltage of the sending-end unit and the AC bus voltage at the low-frequency side is given by formula (39), namely: with U M3C The relationship is:

[0148] (39)

[0149] in: (40)

[0150] In this embodiment, the voltage level of the system is 35 kV or above, and the line resistance is small and can be ignored. Therefore, formula (40) is usually simplified to:

[0151] (41)

[0152] Equation (41) gives the relationship between the transmission power of the sending - end system, the equivalent reactance, and the voltage change amount, ignoring the line resistance. This provides convenience for evaluating the operating state of the system. In power system analysis, the stability of the system is a key factor, and usually, the short - circuit ratio is used to measure it. The short - circuit ratio of a system is defined as , and the specific formula is as follows:

[0153] (42)

[0154] Combining Equation (41) and Equation (42) can obtain Equation (43):

[0155] (43)

[0156] By calculating Equation (43), the maximum value of the new - energy machine - terminal voltage during over - voltage occurrence can be obtained, that is, the over - voltage in the new - energy low - frequency aggregation system. The specific formula (44) is as follows:

[0157]

[0158] In the above - mentioned Equations (42), (43), and (44), represents the short - circuit capacity of the sending - end fan system, P dN represents the rated power of the HVDC transmission system, X1 are respectively the equivalent reactance of the sending - end system, is the short - circuit current, R1 is the equivalent resistance of the sending - end system, represents the equivalent voltage of the sending - end unit, represents the low - frequency - side AC bus voltage, Z represents the equivalent impedance of the sending - end system, and |Z| is equal to the equivalent reactance X1 of the sending - end system, is the short - circuit ratio, represents the line resistance, and I represents the short - circuit current.

[0159] After determining the over - voltage, select the specification of the surge protector according to the value of the transient over - voltage. This application also analyzes the voltage at the fan outlet during the M3C fault (i.e., the over - voltage in the new - energy low - frequency aggregation system) and its influencing factors, as follows: Based on the PSCAD / EMTDC simulation platform, build a new - energy low - frequency power - transmission system model as Figure 5 shown. The simulation parameters in this model are shown in Table 1. At this time, according to Equations (31) to (34) and the simulation parameter data in Table 1, the following data can be obtained: According to the data in Table 1, under the built simulation model, the voltage amplitudes on the power - frequency side and the low - frequency side can be calculated 220× = 179.629 kV, the current amplitudes on the power frequency side and the low frequency side are = 2 / 3 × 800 / 179.629 = 2969 A. According to the waveform diagram, the voltage and current amplitudes on the power frequency side and the low frequency side are the same, and the corresponding three-phase voltage and current phases differ by 120° each. Since the frequency ratio is 5:2, the period ratio is 2:5. They all maintain a good sinusoidal form at their respective frequencies. At this time, when the voltage and current of phase u of the low frequency side converter are input, under steady-state operating conditions, the dual-frequency coupling phenomenon of M3C only exists within the bridge arm. At steady state, the symmetry of the three-phase bridge arm voltage and current of each sub-converter is very good, and the superimposed current waveform envelopes into a sinusoidal fluctuation, verifying the correctness of the theoretical analysis and modeling.

[0160] After knowing the data in Table 1, at 1.5 s when the device is turned on, a three-phase short-circuit fault is artificially set at the machine-side collecting bus of the M3C system, and the fault is removed after maintaining the fault duration for 0.4 s. The voltage change at the fan port is as follows:

[0161] Before the fault occurs, the voltage data of the fan port is obtained from the electrical measurement device of the system. At this time, starting from t = 0 s, the voltage gradually increases from 0 until it maintains at a relatively stable level, and the completion time of this process is within 1 second, characterizing the voltage state of the fan port during normal system operation, and its value can reflect the electrical performance of the system under stable operating conditions.

[0162] When the fault occurs (set at 1.5 s, a three-phase short-circuit fault occurs at the machine-side collecting bus of the M3C system, and the fault duration is 0.4 s), a transient overvoltage phenomenon appears, and the voltage drop at the fan port can be observed through the same electrical measurement device. Due to the change in the system impedance caused by the short-circuit fault, after the voltage at the fan port drops rapidly, the voltage maintains at a stable low voltage level within a time interval of 0.4 s. This low voltage state in this stage reflects the operating characteristics of the system during the fault duration, and the continuous low voltage may affect the normal operation of the fan and related equipment.

[0163] After the fault is removed, a sudden increase in voltage occurs because after the fault is removed, the operating state of the system changes and the electrical parameters are readjusted, causing the voltage at the fan port to rise rapidly. After the voltage reaches the peak value, it starts to drop, which is the process of the system gradually returning to the normal stable operating state. By monitoring the voltage change, it can be observed that the voltage finally returns to the level under the normal stable operating state, indicating that it can gradually return to normal operation after the fault is removed, and the voltage at the fan port also returns to the normal range.

[0164] Table 1

[0165]

[0166] Influencing factor 1. Based on the voltage change data at the outlet of the fan sending end when the above-mentioned fault occurs and formula (44), it can be known that / represents the ratio of the equivalent reactance to the equivalent resistance of the sending-end system. At this time, according to formula (44), taking the partial derivative of / gives formula (45), from which it can be understood that when this parameter changes, the maximum value of the voltage at the outlet of the fan sending end has a changing trend.

[0167] (45)

[0168] Based on the simulation data on capacity for the current system, according to the simulation data, the voltage waveforms at the fan outlet under different transmission line lengths can be obtained. The horizontal axis of this figure represents time, with the unit of seconds (s), ranging from 0 to 3; the vertical axis represents voltage, with the unit of per-unit value (p.u.), ranging approximately from 0 to 1.6. There are four different curves in this figure, corresponding to the voltage waveforms at the fan outlet for transmission line lengths of 50 km, 75 km, 100 km, and 125 km respectively. In the interval of 0 - 0.9 s, each curve rises rapidly from 0 and the rising speed slows down when approaching 0.6 s. In the interval of 0.9 s - 1.5 s, the voltage basically remains stable at about 1.15 p.u. When a fault occurs at 1.5 s, between 1.5 s and 1.9 s, the curves start to decline and gradually approach the stable value. The lowest voltage values are 0.421 p.u., 0.404 p.u., 0.39 p.u., and 0.377 p.u. (these data are the lowest voltage values for 50, 75, 100, and 125 respectively). During this process, among them, the voltage of the curve corresponding to the 50 km length is slightly higher than that of the curve corresponding to the 75 km length, the voltage of the curve corresponding to the 100 km length is slightly higher than that of the curve corresponding to the 75 km length, and the voltage of the curve corresponding to the 125 km length is relatively lower among the four curves, and the changing trends of the four curves are the same. When the fault is removed, around 1.9 s - 2.3 s, the voltages of each curve rise significantly to the maximum values of 1.172 p.u., 1.229 p.u., 1.246 p.u., and 1.291 p.u. (these data are the peak overvoltages at the machine terminals for 50, 75, 100, and 125 respectively), and then they rise back to the stable state, that is, around 1.15 p.u. During this process, the voltage of the curve corresponding to the 125 km length is slightly higher than that of the curve corresponding to the 100 km length, the voltage of the curve corresponding to the 75 km length is slightly higher than that of the curve corresponding to the 50 km length, and the voltage of the curve corresponding to the 50 km length is relatively lower among the four curves, and the changing trends of the four curves are the same.

[0169] It should be noted that in this embodiment, to verify the influence of the ratio of the equivalent reactance to the equivalent resistance of the sending-end system on the maximum value of the outlet voltage of the wind turbine at the sending end, simulation tests are carried out using low-frequency cables of different lengths to verify the above conclusion, and the data shown in Table 2 are obtained:

[0170] Table 2

[0171]

[0172] In formula (45), if formula (45) is always greater than 0, it indicates that when a three-phase short circuit occurs in the wind turbine, as the length of the low-frequency transmission line increases, the line reactance gradually increases, and the voltage drop value also gradually increases. After the fault is removed, there is reactive power redundancy at the outlet of the wind turbine, and a transient voltage rise appears at the sending-end AC bus. The longer the length of the low-frequency transmission line, the greater the peak value of the transient overvoltage, indicating that there is a maximum value for the transmission distance under the same system capacity. This theoretical analysis that there is a maximum value for the transmission distance under the same system capacity is accurate.

[0173] Influence factor 2. Based on the change data of the voltage at the outlet of the wind turbine at the sending end during the above-mentioned fault and formula (44), the partial derivative of the parameter is obtained to get formula (46) as follows:

[0174] (46)

[0175] In formula (46),

[0176] Based on the current system, simulation data on capacity is carried out. According to the simulation data, the voltage waveform diagrams of the fan outlet under different capacities can be obtained. The horizontal axis of this diagram represents time, with the unit of seconds (s), ranging from 0 to 3; the vertical axis represents voltage, with the unit of per-unit value (p.u.), ranging approximately from 0 to 1.6. There are four different curves in this diagram, corresponding to the voltage waveforms of the fan outlet with capacities of 200MW, 400MW, 600MW, and 800MW respectively. In the interval of 0 - 1s, each curve rises rapidly from 0, and the rising speed slows down when approaching 0.5s. In the interval of 1 s - 1.5s, the voltage basically remains stable at 1.1 p.u. When a fault occurs at 1.5s, between 1.5 s – 1.9s, the curves start to decline and gradually approach the steady values of 0.354p.u., 0.376p.u., 0.392p.u., 0.408p.u. (these data are the minimum voltage values of 200, 400, 600, and 800 in sequence). During this process, the voltage of the curve corresponding to the 200MW capacity is slightly higher than that of the curve corresponding to the 400MW capacity, the voltage of the curve corresponding to the 600MW capacity is slightly higher than that of the curve corresponding to the 200MW capacity, and the voltage of the curve corresponding to the 800MW capacity is relatively in the lower position among the four curves, and the changing trends of the four curves are the same. When the fault is removed, around 1.9 s - 2.3s, the voltages of each curve rise significantly to the maximum values of 1.195p.u., 1.204p.u., 1.217p.u., 1.229p.u. (these data are the peak over-voltage values at the generator terminals of 200, 400, 600, and 800 in sequence), and then they rise back to the steady state, that is, 1.1p.u. During this process, the voltage of the curve corresponding to the 800MW capacity is slightly higher than that of the curve corresponding to the 600MW capacity, the voltage of the curve corresponding to the 400MW capacity is slightly higher than that of the curve corresponding to the 200MW capacity, and the voltage of the curve corresponding to the 200MW capacity is relatively in the lower position among the four curves, and the changing trends of the four curves are the same. Among them, the peak over-voltage data and the minimum voltage data in the simulation data are shown in Table 3.

[0177] Table 3

[0178]

[0179] According to the simulation data, it can be known that when the capacity of the system is larger, the short-circuit ratio of the system is smaller, the maximum value of the outlet voltage of the new energy generator terminal increases, that is, the system is weaker, and the maximum value of the new energy generator terminal voltage is larger after the three-phase short-circuit fault is cleared. That is, when a three-phase short circuit occurs and the transmission distance remains unchanged, with the increase of the transmission capacity, the minimum voltage value gradually increases, and the peak value of the transient over-voltage after the fault is removed also gradually increases, indicating that there is a maximum value of the electric energy that can be transmitted under the same transmission distance.

[0180] Further, as an implementation of the method shown in the above Figure 1 and Figure 4 The embodiment of the present application provides a practical evaluation device for transient overvoltage of a new energy low-frequency aggregation system. The purpose of this device is to accurately calculate the overvoltage when the overvoltage phenomenon occurs, in combination with the operating changes of each module in the new energy low-frequency aggregation system. For the convenience of reading, the details in the foregoing method embodiment will not be elaborated one by one in the device embodiment of the present application. However, it should be clear that the device in this embodiment can correspondingly implement all the contents in the foregoing method embodiment. The device is as shown in Figure 5 and specifically includes:

[0181] An acquisition unit 51, configured to determine a modular multilevel matrix converter in a new energy low-frequency aggregation system by using the method of an equivalent voltage source;

[0182] The acquisition unit 51 is configured to acquire the resistance of the sending-end unit, the reactance of the sending-end unit, the rated power of the sending-end unit, the low-frequency side AC bus voltage of the modular multilevel matrix converter, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter in the new energy low-frequency aggregation system;

[0183] A determination unit 52, configured to calculate a first component expression and a second component expression of the voltage drop in the sending-end unit according to the resistance of the sending-end unit, the reactance of the sending-end unit, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter in the acquisition unit 51;

[0184] The determination unit 52 is configured to determine the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency aggregation system according to Kirchhoff's voltage law, in combination with the low-frequency side AC bus voltage, the first component expression of the voltage drop, and the second component expression of the voltage drop;

[0185] A short-circuit ratio acquisition unit 53, configured to acquire the short-circuit ratio of the new energy low-frequency aggregation system, where the short-circuit ratio is the ratio of the short-circuit capacity of the sending-end unit to the rated power of the sending-end unit, and the short-circuit capacity of the sending-end unit is the ratio of the square of the voltage of the sending-end unit to the reactance of the sending-end unit;

[0186] A voltage determination unit 54, configured to calculate the maximum voltage value of the sending-end unit by using the relationship in the determination unit 52, the first component expression of the voltage drop, the second component expression of the voltage drop, and the short-circuit ratio in the short-circuit ratio acquisition unit 53, and determine the maximum voltage value of the sending-end unit as the transient overvoltage.

[0187] Further, as shown in Figure 6As shown, in the modular multilevel matrix converter, the average value model of the modular multilevel matrix converter bridge arm is applied, and the acquisition unit 51 includes:

[0188] The first acquisition module 511 is configured to obtain the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltages of the sub-modules of the modular multilevel matrix converter according to the main circuit structure of the modular multilevel matrix converter, where the first quantity is the number of sub-modules in a charging or discharging state in any bridge arm;

[0189] The bridge arm equivalent capacitance determination module 512 is configured to determine the equivalent capacitance of any bridge arm according to the first quantity obtained by the first acquisition module 511, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltages of the sub-modules of the modular multilevel matrix converter;

[0190] The bridge arm equivalent capacitance determination module 512 is configured to set the equivalent capacitance as the bridge arm capacitance of the average value model of the modular multilevel matrix converter bridge arm.

[0191] Furthermore, as Figure 6 shown, the bridge arm equivalent capacitance determination module 512 further includes:

[0192] The sub-module calculation module 5121 is configured to calculate the capacitor voltage of the bridge arm according to the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltages of the sub-modules of the modular multilevel matrix converter;

[0193] The first determination sub-module 5122 is configured to determine the equivalent voltage of the bridge arm and determine the current of the bridge arm according to the capacitor voltage of the bridge arm calculated by the sub-module calculation module 5121;

[0194] The second determination sub-module 5123 is configured to determine any phase voltage of the low-frequency side AC voltage source according to Kirchhoff's voltage law in combination with the equivalent voltage of the bridge arm determined by the first determination sub-module 5122 and the current of the bridge arm, and the formula followed by any phase voltage is the same as the formula in the above embodiment;

[0195] The third determination sub-module 5124 is configured to determine the equivalent capacitance of any bridge arm by using the power balance principle and the energy equalization principle in combination with the formula followed by any phase voltage determined by the second determination sub-module 5123.

[0196] Furthermore, as Figure 6 shown, the first determination sub-module 5122 includes:

[0197] According to the capacitance voltage balance between sub-modules, the equivalent voltage of the bridge arm is calculated by using the capacitor voltage of the bridge arm, and the calculation formula of the equivalent voltage of the bridge arm is the same as the formula in the above embodiment.

[0198] Further, the first determination sub-module 5122 includes:

[0199] Determine the current passing through the capacitor according to the capacitor voltage of the arm, and the calculation formula of the current of the capacitor is the same as the formula in the above embodiment;

[0200] Based on the fact that the voltage of the bypassed sub-module is zero and the calculation formula of the current of the capacitor, determine that the rewritten formula of the current of the capacitor is the same as the formula in the above embodiment;

[0201] Based on the rewritten formula of the current of the capacitor, determine that the calculation formula of the current of the arm is the same as the formula in the above embodiment.

[0202] Further, as Figure 6 shown, the device further includes a verification unit 55, and the verification unit 55 includes:

[0203] A dynamic model determination module 551, configured to determine a dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system according to the formula followed by any phase voltage;

[0204] A phase voltage and phase current acquisition module 552, configured to acquire three-phase voltages and three-phase currents of the power frequency output side and the low-frequency input side within a preset time under the dynamic mathematical model of the modular multilevel matrix converter system of the dynamic model determination module 551;

[0205] A waveform acquisition module 553, configured to acquire a waveform diagram according to the three-phase voltages and three-phase currents of the phase voltage and phase current acquisition module 552, where the waveform diagram includes a three-phase voltage waveform diagram of the power frequency output side, a three-phase current waveform diagram of the power frequency output side, a three-phase current waveform diagram of the low-frequency input side, and a three-phase current waveform diagram of the low-frequency input side;

[0206] A judgment module 554, configured to judge whether the waveform diagram of the waveform acquisition module 553 is a three-phase sine wave;

[0207] If the judgment module 554 judges yes, determine the correctness of using the equivalent voltage source method to determine the modular multilevel matrix converter in the new energy low-frequency aggregation system.

[0208] Further, as Figure 6 shown, the dynamic model determination module 551 includes:

[0209] An AC side mathematical model determination sub-module 5511, configured to determine the AC side mathematical model of the new energy low-frequency aggregation system in the abc rectangular coordinate system according to the formula followed by any phase voltage;

[0210] A determination expression sub-module 5512 is configured to perform coordinate transformation on the AC-side mathematical model of the determination AC-side mathematical model sub-module 5511 to determine an analytical expression of voltage and current components of the modular multilevel matrix converter.

[0211] A determination dynamic mathematical model sub-module 5513 is configured to determine a dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system based on the analytical expression of voltage and current components of the determination expression sub-module 5512.

[0212] This application also provides a processor, which is configured to run a program. When the program runs, it executes the Figure 1 and Figure 3 practical evaluation method for transient overvoltage of the new energy low-frequency aggregation system as described above.

[0213] In addition, this application also provides an electronic device, which includes a processor and a memory. The memory is configured to store a program, and the processor is coupled to the memory and configured to run the program to execute the Figure 1 and Figure 3 practical evaluation method for transient overvoltage of the new energy low-frequency aggregation system as described above.

[0214] In the above embodiments, the descriptions of the respective embodiments have their own emphases. For parts not detailed in a certain embodiment, reference may be made to the relevant descriptions of other embodiments.

[0215] It can be understood that the relevant features in the above methods and devices can be referred to each other. In addition, the "first", "second", etc. in the above embodiments are used to distinguish the respective embodiments, and do not represent the advantages and disadvantages of the respective embodiments. Those skilled in the art can clearly understand that for the convenience and simplicity of description, the specific working processes of the systems, devices, and units described above can refer to the corresponding processes in the foregoing method embodiments, and will not be elaborated herein.

[0216] The algorithms and displays provided herein are not inherently related to any particular computer, virtual system, or other device. Various general-purpose systems can also be used in conjunction with the teachings herein. The structures required to construct such systems are obvious from the above description. In addition, this application is not directed to any specific programming language. It should be understood that the content of this application described herein can be implemented using various programming languages, and the descriptions of specific languages above are for disclosing the best implementation modes of this application.

[0217] In addition, the memory may include non-permanent memory in a computer-readable medium, forms such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash memory (flash RAM), and the memory includes at least one storage chip.

[0218] Those skilled in the art will understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0219] The present application is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram can be implemented by computer program instructions, and the combination of the flows and / or blocks in the flowchart and / or block diagram can also be implemented. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate means for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0220] These computer program instructions can also be loaded onto a computer or other programmable data processing devices, so that a series of operation steps are executed on the computer or other programmable devices to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable devices provide steps for implementing the functions specified in Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0221] A computer-readable medium includes permanent and non-permanent, removable and non-removable media that can implement information storage by any method or technology. The information can be computer-readable instructions, data structures, program modules, or other data. Examples of computer storage media include, but are not limited to, phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory, or other memory technologies, compact disc read-only memory (CD-ROM), digital versatile disc (DVD), or other optical storage, magnetic cassette tapes, magnetic tape magnetic disk storage, or other magnetic storage devices, or any other non-transitory media that can be used to store information that can be accessed by a computing device. As defined herein, a computer-readable medium does not include transitory computer-readable media, such as modulated data signals and carrier waves.

[0222] It should also be noted that the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a series of elements includes not only those elements but also other elements not expressly listed, or elements that are inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0223] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0224] The above are only embodiments of the present application and are not used to limit the present application. For those skilled in the art, various changes and modifications can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the scope of the claims of the present application.

Claims

1. A practical evaluation method for transient overvoltage in a new energy low-frequency collection system, characterized in that The method includes: Determining a modular multilevel matrix converter in a new energy low-frequency collection system by using the method of equivalent voltage source; Obtaining the resistance of the sending-end unit, the reactance of the sending-end unit, the rated power of the sending-end unit, the short-circuit capacity of the sending-end unit, the low-frequency side AC bus voltage of the modular multilevel matrix converter, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter in the new energy low-frequency collection system; Determining the first component expression of the voltage drop and the second component expression of the voltage drop in the sending-end unit according to the resistance of the sending-end unit, the reactance of the sending-end unit, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter; Determining the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency collection system according to Kirchhoff's voltage law, combining the low-frequency side AC bus voltage, the first component expression of the voltage drop and the second component expression of the voltage drop; Obtaining the short-circuit ratio of the new energy low-frequency collection system, where the short-circuit ratio is the ratio of the short-circuit capacity of the sending-end unit to the rated power of the sending-end unit, and the short-circuit capacity of the sending-end unit is the ratio of the square of the voltage of the sending-end unit to the reactance of the sending-end unit; Calculating the maximum voltage value of the sending-end unit by using the first component expression of the voltage drop, the second component expression of the voltage drop, the relationship and the short-circuit ratio, and determining the maximum voltage value of the sending-end unit as the transient overvoltage.

2. The method according to claim 1, wherein The method of determining a modular multilevel matrix converter in a new energy low-frequency collection system by using the equivalent voltage source includes: Obtaining the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter according to the main circuit structure of the modular multilevel matrix converter, where the first quantity is the number of sub-modules in a charging or discharging state in any bridge arm; Determining the equivalent capacitance of any bridge arm according to the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter; Selecting the modular multilevel matrix converter by using the equivalent capacitance.

3. The method according to claim 2, wherein The method of determining the equivalent capacitance of any bridge arm according to the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter includes: Calculating the capacitor voltage of the bridge arm according to the first quantity, the total number of sub-modules of the modular multilevel matrix converter, and the capacitor voltage of the sub-modules of the modular multilevel matrix converter, and the calculation formula of the capacitor voltage of the bridge arm is: Determining the equivalent voltage of the bridge arm and determining the current of the bridge arm according to the capacitor voltage of the bridge arm; Determining any phase voltage of the low-frequency side AC voltage source according to Kirchhoff's voltage law, combining the equivalent voltage of the bridge arm and the determined current of the bridge arm, and any phase voltage follows the formula: Determining the equivalent capacitance of any bridge arm by using the power balance principle and the energy equalization principle, combining the formula followed by any phase voltage. In the above formula, is the capacitor voltage of the bridge arm, is the capacitor voltage of the sub-module, n is the first quantity, N is the total number of sub-modules of the modular multilevel matrix converter, x = u, v, w, x is the phase of the low-frequency side AC voltage source, y = a, b, c, y is the phase of the sending-end AC voltage source, is any phase voltage, is the equivalent voltage representing the series part of the sub-modules of the bridge arm xy, is the current of the bridge arm xy, R is the equivalent resistance of the bridge arm, L is the inductance of the bridge arm, represents the derivative of the bridge arm current with respect to time, is the neutral point voltage measured at power frequency everywhere.

4. The method according to claim 3, characterized in that, Determining the equivalent voltage of the bridge arm and determining the current of the bridge arm according to the capacitor voltage of the bridge arm, the method comprising: Calculating the equivalent voltage of the bridge arm by using the capacitor voltage of the bridge arm according to the capacitor voltage balance between sub-modules, the calculation formula of the equivalent voltage of the bridge arm being: In the above formula, is the capacitor voltage of the bridge arm, is the equivalent voltage of the bridge arm, n is the first quantity, and N is the total number of sub-modules of the modular multilevel matrix converter.

5. The method according to claim 3, characterized in that, Determining the equivalent voltage of the bridge arm and determining the current of the bridge arm according to the capacitor voltage of the bridge arm, the method comprising: Determining the current passing through the capacitor according to the capacitor voltage of the bridge arm, the calculation formula of the current of the capacitor being: Based on the calculation formula of the voltage of the bypassed sub-module being zero and the current of the capacitor, the rewritten formula for determining the current of the capacitor is: Rewriting the formula based on the current of the capacitor to determine the calculation formula of the current of the bridge arm: In the above formula, represents the capacitance value of the capacitor in the full-bridge sub-module, is the sub-module capacitor voltage, n is the first quantity, N is the total number of sub-modules of the modular multilevel matrix converter, x = u, v, w, x is the phase of the low-frequency side AC voltage source, y = a, b, c, y is the phase of the sending-end AC voltage source, represents the current of a single sub-module in the arm, is the equivalent capacitance current of the sub-module, and the equivalent capacitance of the arm can be expressed as , represents the modulation signal of the arm, is the current of arm xy.

6. The method according to claim 3, characterized in that, After determining any phase voltage of the low-frequency side AC voltage source, the method further comprises: Determining the dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system according to the formula followed by the any phase voltage; Under the dynamic mathematical model of the modular multilevel matrix converter system, obtaining the three-phase voltages and three-phase currents of the power frequency output side and the low-frequency input side within a preset time; Obtaining a waveform diagram according to the three-phase voltages and three-phase currents, the waveform diagram including the three-phase voltage waveform diagram of the power frequency output side, the three-phase current waveform diagram of the power frequency output side, the three-phase current waveform diagram of the low-frequency input side, and the three-phase current waveform diagram of the low-frequency input side; Judging whether the waveform diagram is a three-phase sine wave; If so, determining the correctness of the modular multilevel matrix converter in the new energy low-frequency aggregation system by using the method of equivalent voltage source.

7. The method according to claim 6, characterized in that, Determining the dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system according to the formula followed by the any phase voltage, comprising: Determining the AC side mathematical model of the new energy low-frequency aggregation system in the abc rectangular coordinate system according to the formula followed by the any phase voltage; Performing coordinate transformation on the AC side mathematical model to determine the analytical expressions of the voltage and current components of the modular multilevel matrix converter; Based on the analytical expressions of the voltage and current components, determining the dynamic mathematical model of the modular multilevel matrix converter system in the dq coordinate system.

8. Practical evaluation device for transient overvoltage of new energy low-frequency collection system, characterized in that, The device comprises: An obtaining unit, configured to determine a modular multilevel matrix converter in a new energy low-frequency aggregation system by using the method of equivalent voltage source; The obtaining unit is configured to obtain the resistance of the sending-end unit, the reactance of the sending-end unit, the short-circuit capacity of the sending-end unit, the rated power of the sending-end unit, the low-frequency side AC bus voltage of the modular multilevel matrix converter, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter; A determining unit, configured to calculate a first component expression of the voltage drop and a second component expression of the voltage drop in the sending-end unit according to the resistance of the sending-end unit, the reactance of the sending-end unit, the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multilevel matrix converter in the obtaining unit; The determining unit is configured to determine the relationship between the low-frequency side AC bus voltage and the voltage of the sending-end unit in the new energy low-frequency aggregation system according to Kirchhoff's voltage law, in combination with the low-frequency side AC bus voltage, the first component expression of the voltage drop, and the second component expression of the voltage drop. A short-circuit ratio acquisition unit is configured to acquire the short-circuit ratio of a new energy low-frequency aggregation system, where the short-circuit ratio is the ratio of the short-circuit capacity of a sending-end unit to the rated power of the sending-end unit, and the short-circuit capacity of the sending-end unit is the ratio of the square of the voltage of the sending-end unit to the reactance of the sending-end unit; A voltage determination unit is configured to calculate the maximum voltage value of the sending-end unit by using the relationship, the first voltage drop component expression and the second voltage drop component expression in the determination unit, and the short-circuit ratio in the short-circuit ratio acquisition unit, and determine the maximum voltage value of the sending-end unit as the transient overvoltage.

9. A storage medium, characterized in that, The storage medium is used to store a computer program, wherein when the computer program runs, it controls the device where the storage medium is located to execute the practical evaluation method for the transient overvoltage of the new energy low-frequency aggregation system according to any one of claims 1-7.

10. An electronic device, characterized in that, The electronic device includes a processor and a memory. The processor is configured to call the program instructions in the memory to execute the practical evaluation method for the transient overvoltage of the new energy low-frequency aggregation system according to any one of claims 1-7.

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