New energy low-frequency collection system transient overvoltage practical evaluation method and device

Abstract

The invention provides a practical evaluation method and device for transient overvoltage of a new energy low-frequency collection system, and aims to guarantee safe and stable operation of the system by calculating accurate transient overvoltage. The practical evaluation method for the transient overvoltage of the new energy low-frequency collection system comprises the following steps: determining a modular multi-level matrix converter in the new energy low-frequency collection system by using an equivalent voltage source method; acquiring voltage, current, power and short-circuit ratio of the new energy low-frequency collection system and the modular multilevel matrix converter, and calculating a voltage drop component caused by resistance and a voltage drop component caused by reactance based on the acquired data; determining a relational expression between the low-frequency side AC bus voltage and the sending end unit voltage; and calculating the transient overvoltage by using the resistance-induced voltage drop component, the reactance-induced voltage drop component, the relational expression and the short-circuit ratio.

Description

Technical Field

[0001] The present application relates to the field of electric power technology, and in particular to a practical transient overvoltage assessment method and device for a new energy low-frequency collection system. Background Art

[0002] The new energy low-frequency collection system is a collection and transmission system for large-scale, long-distance and stable transmission of new energy electricity. After collecting new energy electricity through the sending-end unit, it is transmitted to the low-frequency converter through the transmission line circuit. The low-frequency converter performs frequency conversion and converts it into industrial frequency electricity suitable for access to the existing power grid. However, in the actual operation process, it is easily affected by factors such as changes in equipment parameters and line impedance fluctuations, which in turn cause overvoltage phenomena. Once a transient overvoltage occurs, the excessively high voltage will cause serious damage to the equipment in the system and even cause power outages.

[0003] In order to deal with transient overvoltage problems, surge protectors are usually used for protection. However, since the value of transient overvoltage cannot be determined in the existing technology, a larger-sized surge protector is selected to deal with potential overvoltage risks. This method not only wastes resources, but also may fail to effectively suppress transient overvoltage due to improper selection of surge protector specifications, resulting in the inability to reliably protect system equipment.

[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 collection system to ensure the safe and stable operation of the new energy low-frequency collection system. Summary of the invention

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

[0006] In order to solve the above technical problems, the embodiments of the present application provide the following technical solutions: In a first aspect, the present application provides a practical evaluation method for transient overvoltage of a new energy low-frequency collection system, the method comprising: Using the equivalent voltage source method, the modular multi-level matrix converter in the renewable energy low-frequency collection system is determined; 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 multi-level matrix converter, and the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multi-level matrix converter in the new energy low-frequency collection system; Determine a first component expression of a voltage drop in the sending-end unit and a second component expression of a voltage drop according to the resistance of the sending-end unit, the reactance of the sending-end unit, and the active power and reactive power transmitted by the sending-end unit to the low-frequency side of the modular multi-level matrix converter; 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; Obtaining the short-circuit ratio of the new energy low-frequency collection system, the short-circuit ratio being 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 maximum voltage value of the sending-end unit is calculated by using the voltage drop first component expression, the voltage drop second component expression, the relationship and the short-circuit ratio, and the maximum voltage value of the sending-end unit is determined as a transient overvoltage.

[0007] In a second aspect, the present application provides a practical evaluation device for transient overvoltage of a new energy low-frequency collection system, the device comprising: An acquisition unit is used to determine a modular multi-level matrix converter in a new energy low-frequency collection system by using an equivalent voltage source method; The acquisition unit is used to acquire 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 multi-level matrix converter, and the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multi-level matrix converter in the new energy low-frequency collection system; a determining unit, configured to calculate a first component expression of a voltage drop in the sending-end unit and a second component expression of a voltage drop according to the resistance of the sending-end unit in the acquiring unit, the reactance of the sending-end unit, and the active power and reactive power transmitted by the sending-end unit to the low-frequency side of the modular multi-level matrix converter; The determining unit is used 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 collection system 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; A short-circuit ratio acquisition unit is used to acquire the short-circuit ratio of the new energy low-frequency collection system, wherein 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; A voltage determination unit is used to calculate the maximum voltage value of the sending-end unit and determine the maximum voltage value of the sending-end unit as a transient overvoltage by using the relationship and the voltage drop first component expression of the determination unit, the voltage drop second component expression and the short-circuit ratio in the short-circuit ratio acquisition unit.

[0008] In a third aspect, the present application provides a storage medium, which is used to store a computer program, wherein when the computer program is running, it controls the device where the storage medium is located to execute the above-mentioned practical evaluation method for transient overvoltage of the new energy low-frequency collection system.

[0009] In a fourth aspect, the present application provides an electronic device, comprising a processor and a memory, wherein the processor is used to call program instructions in the memory to execute the above-mentioned practical evaluation method for transient overvoltage of a new energy low-frequency collection system.

[0010] By means of the above technical scheme, the present application provides a practical evaluation method and device for transient overvoltage of a new energy low-frequency collection 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. Secondly, according to Kirchhoff's voltage law, combined with the low-frequency side AC bus voltage, the voltage drop component caused by the resistance and the voltage drop component caused by the 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 collection system is determined, which can effectively reflect the voltage state and operating conditions inside the system, and provide accurate input data for overvoltage calculation. In addition, the short-circuit ratio, as a core indicator for measuring the strength of the power system, comprehensively reflects the relationship between the system structure, power supply and load, and further improves the calculation accuracy. That is, the present application fully considers the operating characteristics of the new energy low-frequency collection system when overvoltage occurs by combining Kirchhoff's voltage law, the low-frequency side AC bus voltage and the short-circuit ratio, accurately calculates the overvoltage, and thus ensures the safe operation of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] By reading the detailed description below with reference to the accompanying drawings, the above and other purposes, features and advantages of the exemplary embodiments of the present application will become easy to understand. In the accompanying 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, wherein: Figure 1 A flow chart of the practical evaluation method for transient overvoltage in the new energy low-frequency collection system is proposed; Figure 2 A schematic diagram of the equivalent process of M3C submodules is proposed; Figure 3 A flow chart of practical evaluation method for transient overvoltage of new energy low-frequency collection system is proposed; Figure 4 An equivalent circuit diagram of a low-frequency power transmission system is proposed; Figure 5 The structural diagram of the practical evaluation device for transient overvoltage of new energy low-frequency collection system is proposed; Figure 6 The structural diagram of another practical transient overvoltage assessment device for new energy low-frequency collection system is proposed. DETAILED DESCRIPTION

[0012] The exemplary embodiments of the present application will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present application are shown in the accompanying drawings, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present application and to be able to fully convey the scope of the present application to those skilled in the art. It should be noted that, unless otherwise stated, the technical terms or scientific terms used in this application should be the common meanings understood by those skilled in the art to which this application belongs.

[0013] In order to deal with transient overvoltage problems, surge protectors are currently commonly used for protection. However, the existing method mainly deals with potential overvoltage risks by selecting larger-sized surge protectors. This method not only wastes resources, but may also fail to effectively suppress transient overvoltages due to improper selection of surge protector specifications, resulting in the inability to reliably protect system equipment.

[0014] To this end, the inventor of the present application has proposed a practical evaluation method for transient overvoltage in a new energy low-frequency collection system. The method calculates an accurate transient overvoltage and selects a surge protector based on 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 practical evaluation method for transient overvoltage in a new energy low-frequency collection system implemented by the present application has the following specific steps: Figure 1 As shown, including: Step 101: Determine a modular multi-level matrix converter in a new energy low-frequency collection system by using an equivalent voltage source method.

[0015] Before introducing step 101, this embodiment also introduces the new energy low-frequency collection system, as follows: The new energy low-frequency collection system is an electric power system that integrates new energy power generation. Due to the dispersion and intermittency of new energy power generation, the electric energy generated by it needs to be centrally processed through the collection system. It operates at a lower frequency and can reduce transmission losses. The sending-end unit is the source equipment for generating electric energy. The modular multi-level matrix converter (M3C) is a new type of power electronic converter topology that combines the characteristics of modular multi-level technology and matrix converter. M3C is usually composed of multiple sub-modules, each of which contains one or more power electronic switches and capacitors and other components, which can realize power conversion between multiple AC ports and have 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 interactive architecture, a specific bridge arm connection method is formed. Specifically, as Figure 2 As shown in the figure, the entire main circuit has a total of 9 bridge arms, each of which is composed of an inductor L, an equivalent resistor R and N full-bridge sub-modules FBSM cascaded. 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 input voltage neutral point (zero potential), and N is the output voltage neutral point. s represents the low-frequency input side, r represents the industrial frequency output side, the voltage of each bridge arm, the current in each bridge arm and the current direction. In the modular multi-level matrix converter, there are low-frequency side and industrial frequency side, among which the low-frequency side AC bus is the key part connecting the converter and the AC system. The low-frequency side AC bus voltage is the AC voltage value on the bus, which reflects the electrical connection status and power exchange between the converter and the AC system.

[0016] Determining the modular multilevel matrix converter is to determine the capacitance of the modular multilevel matrix converter in the new energy low-frequency collection system through an equivalent voltage source method, and selecting the model of the modular multilevel matrix converter as the modular multilevel matrix converter average value model.

[0017] 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 multi-level matrix converter, and the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multi-level matrix converter in the new energy low-frequency collection system.

[0018] In this step, the low-frequency side AC bus voltage of the modular multi-level matrix converter can be measured by installing a high-precision voltage sensor at the low-frequency side AC bus of the modular multi-level matrix converter to collect voltage data in real time. At the same time, the system monitoring and analysis software is used to process and analyze the collected voltage data in combination with the operating characteristics and historical data of the modular multi-level matrix converter to calculate the low-frequency side AC bus voltage of the modular multi-level matrix converter. The reactance and resistance of the sending-end unit can be obtained by direct measurement, or by looking up 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 by the sending-end unit to the low-frequency side of the modular multi-level matrix converter can be measured using a power measuring instrument. 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 value of the short-circuit capacity 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 the bus at this level is calculated by the nominal value method or the per-unit value method, and then the short-circuit capacity of the sending-end unit is obtained.

[0019] Step 103: Calculate a first component expression of a voltage drop and a second component expression of a 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 reactive power transmitted by the sending-end unit to the low-frequency side of the modular multi-level matrix converter.

[0020] In this step, according to the equipment in the above system (sending-end unit, transmission line, modular multilevel matrix converter, etc.), the voltage difference between the output end of the sending-end unit and the low-frequency side end of the modular multilevel matrix converter, i.e., the first voltage drop component and the second voltage drop component, is determined. At this time, the calculation of the expression of the first voltage drop component and the expression of the second voltage drop component is as shown in Expression (1): → (1) Since the line resistance is small and can be ignored in systems with voltage levels of 35kV and above, the above expression can be simplified to the expression on the right. In the above expression, Indicates the AC bus voltage on the low-frequency side, represents the first component of voltage drop, Represents the second component of voltage drop, P 1 , Q 1 are the active and reactive power transmitted from the sending-end unit to the low-frequency side of M3C; R 1 , X 1 are the equivalent resistance and reactance of the sending end system respectively.

[0021] 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.

[0022] 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.

[0023] Step 105: Obtain the short-circuit ratio of the new energy low-frequency collection system.

[0024] 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: In the above formula, Indicates the voltage of the sending end unit, X 1 are 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.

[0025] 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: Step 106, using the voltage drop first component expression, the voltage drop second component expression, the relationship and the short-circuit ratio, calculate the maximum voltage value of the sending-end unit and determine the maximum voltage value of the sending-end unit as a transient overvoltage.

[0026] In this step, the voltage drop first component expression and the voltage drop second component expression 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 low-frequency side AC bus. The voltage drop first component expression and the voltage drop second component expression are introduced into 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, and 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 impedance modulus is calculated. The maximum value of is the transient overvoltage. The final expression of transient overvoltage is as follows, where I is the short-circuit current: (2) In addition, it is worth mentioning that after obtaining the accurate transient overvoltage according to the practical evaluation method of transient overvoltage in the new energy low-frequency collection system, the specifications of the overvoltage protection device can be selected according to the transient overvoltage.

[0027] Based on the above Figure 1 It can be seen from the implementation method that the present application provides a practical evaluation method for transient overvoltage in a new energy low-frequency collection system. By combining Kirchhoff's voltage law, the AC bus voltage on the low-frequency side and the short-circuit ratio, the operating characteristics of the new energy low-frequency collection system when overvoltage occurs are fully considered, and the overvoltage is accurately calculated, which can avoid the waste of resources caused by blindly selecting larger-sized equipment, thereby ensuring the safe operation of the system.

[0028] Furthermore, according to the above Figure 1 The embodiment of the present application shown in the embodiment of the present application describes in more detail how the new energy low-frequency collection system uses the equivalent voltage source method to determine the modular multi-level matrix converter, specifically as follows Figure 3 As shown, including: 301. The process of establishing the average value model of the bridge arm of the modular multi-level matrix converter.

[0029] In this step, in order to better study the external equivalent characteristics and output characteristics of M3C, the high-frequency effect of the switching device and the difference between the submodules are ignored, so the M3C bridge arm average value model is established, and the process of determining the M3C bridge arm average value model capacitance is as follows: according to the main road structure of the modular multi-level matrix converter, the first number, the total number of submodules of the modular multi-level matrix converter, and the submodule capacitance voltage of the modular multi-level matrix converter are obtained, the first number is the number of submodules in the charging and discharging state in any bridge arm, and the submodule capacitance voltage is the capacitance voltage of the submodule in the main road structure of the modular multi-level matrix converter; according to the first number, the total number of submodules of the modular multi-level matrix converter and the submodule capacitance voltage of the modular multi-level matrix converter, the equivalent capacitance of any bridge arm is determined; the equivalent capacitance is set to the bridge arm capacitance of the bridge arm average value model of the modular multi-level matrix converter. Specifically, according to the main road structure of the modular multi-level matrix converter, the total number of submodules of the modular multi-level matrix converter is found. According to the working state of the submodule, the number of submodules in the bridge arm in the charging and discharging state, that is, the first number, is obtained. After the acquisition is completed, the capacitor voltage of the bridge arm is first determined, and the equivalent voltage of the bridge arm and the current of the bridge arm are determined according to the capacitor voltage of the bridge arm, and any phase voltage is further determined, and finally the power balance principle and the energy sharing principle are used, combined with the compliance formula of any phase voltage, to determine the equivalent capacitance of any bridge arm.

[0030] It is worth noting that, in this embodiment, a more specific implementation method is given for determining the equivalent capacitance of any bridge arm according to the first number, the total number of submodules of the modular multilevel matrix converter, and the capacitance voltage of the submodules of the modular multilevel matrix converter. Specifically, the capacitance voltage of the bridge arm is calculated according to the first number, the total number of submodules of the modular multilevel matrix converter, and the capacitance voltage of the submodules of the modular multilevel matrix converter. The calculation formula (3) of the capacitance voltage of the bridge arm is: (3) In the above formula, is the capacitor voltage of the bridge arm, is the submodule capacitor voltage, n is the first number, N is the total number of submodules 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 capacitor voltage of the bridge arm, the equivalent voltage of the bridge arm and the current of the bridge arm are determined; according to Kirchhoff's voltage law, combined with the equivalent voltage of the bridge arm and the current of the bridge arm, any phase voltage of the low-frequency side AC voltage source is determined, and the voltage of any phase follows formula (4): (4) By using the power balance principle and energy sharing principle, combined with the formula followed by any phase voltage, the equivalent capacitance of any bridge arm is determined. In the above formula, is any phase voltage, To represent the equivalent voltage of the series connection of the bridge arm xy submodules, is the current of bridge arm xy, R is the equivalent resistance of bridge arm, L is the inductance of bridge arm, represents the time derivative of the bridge arm current, To measure the neutral point voltage at the power frequency, is the equivalent voltage of the bridge arm. In addition, in the above steps, the specific steps of determining the equivalent voltage of the bridge arm according to the capacitor voltage of the bridge arm are as follows: according to the capacitor voltage balance between the submodules, the equivalent voltage of the bridge arm is calculated using the capacitor voltage of the bridge arm, and the calculation formula (5) of the equivalent voltage of the bridge arm is: (5) According to formula (5), we can get the following formula (6), where is the modulation signal of the bridge arm: (6) In the above steps, the specific steps of determining the current of the bridge arm according to the capacitor voltage of the bridge arm are as follows: determining the current passing through the capacitor according to the capacitor voltage of the bridge arm, and the calculation formula (7) of the current of the capacitor is: (7) Based on the bypassed submodule voltage being zero and the calculation formula of the capacitor current, the capacitor current is rewritten as formula (8) as follows: (8) Based on the rewritten formula of the capacitor current, the calculation formula (9) for determining the current of the bridge arm is: (9) In the above formula, represents the capacitance value of the capacitor in the full-bridge submodule, is the submodule capacitor voltage, Represents the current of a single submodule in the bridge arm, is the equivalent capacitance current of the submodule, and the equivalent capacitance of the bridge arm can be expressed as , Represents the modulation signal of the bridge arm.

[0031] Now Figure 2 The equivalent process diagram of the M3C submodule shown in FIG. 1 is taken as an example to illustrate: It is known that the phase voltage and phase current of the low-frequency side AC voltage source are respectively and ix Indicates that x=u, v, w, and the phase voltage and phase current of the receiving end AC voltage source are respectively u y and i y It means, y=a,b,c, represents the bridge arm current, represents the circulation of bridge arm xy, is the neutral point voltage on the power frequency output side, is the capacitor voltage of the submodule. xy Represents the equivalent voltage source of the series part of the bridge arm submodule. To simulate the oscillation characteristics of the capacitor voltage, the submodule capacitor is equivalent to a controlled current source and the submodule series equivalent capacitor For a certain bridge arm, there are N sub-modules, n of which are in the charging and discharging state, and the other (Nn) are in the bypass state. Find the equivalent capacitance of the nine bridge arms of M3C.

[0032] According to the above known conditions, the first number is n, the total number of submodules of the modular multilevel matrix converter is N, and the number of submodules in the bypass state is Nn. At this time, the capacitor voltage of the bridge arm can be determined according to formula (3). At this time, assuming that the capacitor voltage between the submodules is balanced, the equivalent voltage of the bridge 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 a mathematical formula as formula (7). Because the voltage of the bypassed (Nn) submodules is 0, at this time, the current of the capacitor is rewritten as formula (8). Formula (8) is transformed, that is, both sides are divided by N at the same time, and the calculation formula (9) of the current of the bridge arm is determined. According to Kirchhoff's voltage law, the bridge arm xy at this time satisfies formula (4).

[0033] Based on the above, since the increase or decrease of the power module capacitor voltage depends on the active power absorbed or released by the bridge arm, the control of the DC component in the capacitor voltage only needs to analyze the active power. Assuming that there are N sub-modules in each bridge arm of M3C, then 9 bridge arms contain 9N sub-modules, that is, there are 9N capacitors, and assuming that the energy between the nine bridge arms is completely balanced, the DC circuit represents the average voltage of all full-bridge sub-modules on the bridge arm. The M3C bridge arm average value model is derived using the power balance principle and the idea of ​​energy equalization, and the detailed model is simplified to increase the simulation speed. Ignoring the internal loss of 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 M3C, that is: (10) Among them, P in is the low frequency input side power, P out is the power at the industrial frequency output side, △P M3Cis the power related to the current flowing in the M3C bridge arm, reflecting the power generated by the current in the M3C bridge arm. Similar to the average value model of MMC, the DC circuit of 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 capacitance is , the voltage of the bridge arm is , then the power flowing through the M3C bridge arm can be expressed as: (11) Using a DC circuit to represent the average voltage of all full-bridge sub-modules on the bridge arm, it is easy to obtain the total energy of M3C = number of capacitors × energy stored in each capacitor using the energy sharing idea, that is: (12) Therefore, the equivalent capacitance of the nine bridge arms of M3C can be expressed as: (13) in, represents the current value of the controlled current source, Indicates the capacitance value of a single submodule capacitor, represents the total energy of M3C, is the submodule capacitor voltage, Represents the equivalent capacitance voltage of all full-bridge submodules of the M3C bridge arm, In this embodiment, step 301 utilizes the modular multi-level matrix converter bridge arm average value model of the equivalent capacitance of the nine bridge arms of the M3C, so that in the actual operation of the M3C connecting two three-phase AC systems with different frequencies and amplitudes, the switching complexity during the switching period is avoided, and each submodule of the M3C operates 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.

[0034] Step 302: steady-state operation analysis of the M3C mathematical model in the new energy low-frequency collection system.

[0035] After determining the equivalent capacitance of the M3C bridge arm in the new energy low-frequency collection system in step 301, this step analyzes the steady-state operation of the modular multi-level matrix converter in the system. The specific steps are as follows: according to the compliance formula of any phase voltage, determine the dynamic mathematical model of the modular multi-level matrix converter system in the dq coordinate system; under the dynamic mathematical model of the modular multi-level matrix converter system, obtain the three-phase voltage and three-phase current on the power frequency output side and the low-frequency input side within a preset time; according to the three-phase voltage and three-phase current, obtain a waveform diagram, the waveform diagram includes a three-phase voltage waveform diagram on the power frequency output side, a three-phase current waveform diagram on the power frequency output side, a three-phase current waveform diagram on the low-frequency input side, and a three-phase current waveform diagram on the low-frequency input side; determine whether the waveform diagram is a three-phase sine wave; if so, determine the correctness of the modular multi-level matrix converter in the new energy low-frequency collection system by using the equivalent voltage source method.

[0036] The specific implementation method of the step of determining the dynamic mathematical model of the modular multi-level matrix converter system in the dq coordinate system according to the formula followed by any phase voltage is as follows: determining the AC side mathematical model of the new energy low-frequency collection system in the abc rectangular coordinate system according to the formula followed by any phase voltage; transforming the coordinates of the AC side mathematical model to determine the analytical expression of the voltage and current components of the modular multi-level matrix converter; based on the analytical expression of the voltage and current components, determining the dynamic mathematical model of the modular multi-level matrix converter system in the dq coordinate system. In this regard, this embodiment provides a more detailed implementation method, which is as follows: first, according to any phase voltage of the low-frequency side AC voltage source determined in the modular multi-level matrix converter, an AC side mathematical model is established, and the specific steps are to expand formula (4) to obtain the AC side mathematical model in the abc rectangular coordinate system: (14) The Clarke transformation matrix for equal power transformation is: (15) in, , , Respectively represent the voltage of the u-phase, v-phase, and w-phase on the AC side, , , , , , , , , Expressed as the current on the corresponding bridge arm, , , , , , , , , Respectively represent the voltage components on different bridge arms, L is the inductance 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 three-phase variables abc through specific coefficients to obtain aβ0 Quantity.

[0037] In order to achieve the decoupling of the input and output frequency components of the bridge arm voltage and current, formula (14) is modified aβ0 Coordinate transformation, multiplying formula (5) on the left, yields the following formula: (16) According to the above formula (16), , , Indicates the AC side phase voltage aβ0 Components in the coordinate axes ， , , , , , , , , It is represented by the corresponding bridge arm axis, axis and the value of the zero-sequence component, , , , , , , , , They represent the voltage values ​​of the sub-converter bridge arm voltage in different phases and different coordinate components, , , They respectively represent the voltages of phase a, phase b, and phase c on the power frequency side.

[0038] According to the above formula (16), the zero-sequence components of the voltage and current of the three sub-converter bridge arms and the input side voltage 0 The component, output voltage and neutral voltage are related to the input voltage. aβ When the system is a three-phase three-wire system, the zero-sequence component is only related to the output side voltage, that is, the bridge arm voltage and current. 0 The components have the same frequency as the output side system and can be analyzed independently. At this time, the voltage and current of the three sub-converters can be obtained.aβ Analytical expressions of components and obtaining voltage and current of three sub-converters 0 Analytical expressions for the components: (17) (18) in, is the zero-sequence component of the sub-converter bridge arm voltage, , , It is represented as the zero-sequence component of the sub-converter bridge arm current, , , It is represented by the different phase voltage values ​​of the sub-converter bridge arm voltage under the zero-sequence component. 0 The component has the same frequency as the output side system. In this case, formula (18) is aβ0 Coordinate transformation can transform the three-phase voltage equation on the output side into aβ0 Coordinate system, that is, the output frequency component is aβ0 The mathematical model in the coordinate system is as follows: (19) At this time, formula (17) and formula (19) together constitute the M3C system in aβ0 Mathematical model in coordinate system.

[0039] It is worth noting that if the input and output system is three-phase symmetrical, the total zero-sequence voltage of M3C is controlled to be zero. At this time, u in formula (16) is s0 =0,u 00 =0, then there must be i 00 =0,u N = 0, then the formula (18) determined according to formula (16) can be further reduced to a second-order model, as shown in the formula: (20) By reducing the order of formula (18), formula (17) and formula (20) form a symmetric runtime system. aβ The 8th-order mathematical model in the coordinate system. At this time, the voltage and current components of the bridge arm are in the same frequency as the input side system, and the input side voltage aβ The zero-sequence components of the bridge arm voltage and current are related to the output side voltage and have the same frequency as the output side system.

[0040] Because, in aβ In the stationary coordinate system, there is a coupling relationship between the bridge arm voltage, current and other physical quantities in the system, which makes the mathematical model relatively complex and difficult to analyze and control. aβBy transforming the coordinate system to the dq rotating coordinate system, the original AC quantity can be converted into DC quantity according to the synchronous relationship between the dq axis and the rotating magnetic field, effectively decoupling related variables and thus simplifying the mathematical model. aβ The mathematical model in the coordinate system is transformed into the dq coordinate system. At this time, the two-phase stationary to two-phase rotating coordinate transformation matrix (2s / 2r transformation matrix) is: (twenty one) (twenty two) in, , are the low-frequency side frequency and the power-frequency side frequency, respectively. At this time, coordinate transformation is performed 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 multiply formula (21) on the left to obtain formula (23), and multiply formula (20) on the left to obtain formula (22) to obtain formula (24). Formula (23) and formula (24) constitute the dynamic mathematical model of the M3C system in the dq coordinate system, which is as follows: (twenty three) (twenty four) The above formula (23) lists the dynamic relationship between the voltage and current of the three sub-converters in the dq coordinate system in a column form, which can be converted into a mathematical model of the M3C system represented by a matrix in the dq coordinate system, such as formula (25): (25) Among them, the first term on the right side of the formula (25) is the related term of the inductance to the derivative of the current component of each sub-converter, the second term is the coupling term generated by the rotation in the dq coordinate system, and the third term is the voltage source term of each sub-converter.

[0041] 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.

[0042] Furthermore, this embodiment provides a specific implementation method for obtaining the three-phase voltage and the three-phase current in obtaining the three-phase voltage and the three-phase current at the power frequency output side and the low-frequency input side within a preset time under the dynamic mathematical model of the modular multi-level matrix converter system, as follows: According to formula (23) in the dynamic mathematical model of the M3C system in the dq coordinate system, the mathematical model of the inner loop current controller can be determined as formula (26) and formula (27): (26) (27) After determining the mathematical model of the inner loop current controller and the dynamic mathematical model of the M3C system in the dq coordinate system according to the above formula, when the system adopts the synchronous dq coordinate system oriented by the input side grid voltage, the q-axis voltage component in the steady state is 0, and the system input power at this time can be determined as: (28) Among them, in the above formula, Indicates the active power input to the system, reflecting the part of the system input energy that can actually be used to do work. Represents the reactive power input to the system. Reactive power represents the scale of 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 is 0 in the steady state, that is, , then in formula (28) , we can get i da 、i qa They are proportional to the active and reactive power on the input side respectively, so a proportional-integral PI controller can be used for power outer loop control.

[0043] For the two sides of M3C (low-frequency side and power frequency side) in the new energy low-frequency collection system, the active power conservation principle is followed, which means that under ideal conditions, the sum of the active power input on the low-frequency side of M3C must be equal to the sum of the active power output on the power frequency side. At this time, the active power on both sides of M3C (low-frequency side and power frequency side) cannot be adjusted separately and arbitrarily, but are interrelated and mutually influential, but the reactive power on both sides can be controlled independently. That is, the change of active power on one side will inevitably cause the corresponding change of active power on the other side, and it is impossible to unilaterally control the active power on one side independently without affecting the other side. In addition, the outer loop control is divided into active control and reactive control. The low-frequency input side adopts fixed active power control and fixed AC voltage amplitude control, and the power frequency output side adopts fixed DC capacitor voltage control and fixed reactive power control.

[0044] The active power control is adopted to control the active current component on the low-frequency input side; the reactive current component on the low-frequency input side is controlled by adopting the constant AC voltage amplitude control, as shown in formula (29): (29) The constant DC capacitor voltage control is used to control the active current component on the power frequency output side; the constant reactive power control is used to control the reactive current on the power frequency output side, as shown in formula (30): (30) According to the above formula, the instantaneous voltage and current values ​​of the low-frequency side of M3C are , , and 、i v , , the instantaneous values ​​of voltage and current on the power frequency side are , , and , , , taking the phase of the u-phase voltage as the reference, the instantaneous value of the voltage and current on the low-frequency side during steady-state operation can be expressed as: (31) The instantaneous value of the voltage and current on the power frequency side can be expressed as: (32) In the above formula, , , , are the voltage amplitude and current amplitude on the low frequency side and the power frequency side respectively, , is the phase difference between the voltage and current on the low frequency side and the power frequency side, is the initial phase difference between the power frequency side and the low frequency side.

[0045] Based on formula (31) and formula (32), the active power and reactive power on the input side and output side can be obtained. The active power on the output side is formula (33) and formula (34): (33) (34) In this embodiment, according to Figure 2 The 9 bridge arm structures of the M3C are shown. According to the symmetry of the M3C converter or the zero-sequence characteristics of the input and output currents, the expression of the bridge arm current can be obtained as follows: (35) Based on formula (35), it is assumed that the voltage of the submodule capacitors on all bridge arms of M3C is balanced, that is, the influence of the DC side capacitor of the bridge arm on the AC voltage fluctuation is ignored, so the bridge arm circulating current is ignored in this model, that is, , at this time the bridge arm current can be expressed as: (36) At this time, the bridge arm current connected to phase a in M3C is: (37) Similarly, the current of the bridge arm connected between phases b and c is: (38) In the present application example, the correctness of the above-mentioned derivation of the M3C average value model is also verified, as follows: Based on the PSCAD / EMTDC simulation platform, the following Figure 5 The 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.

[0046] Step 303: Calculate the overvoltage.

[0047] 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, P 1 , Q 1 are the active and reactive power transmitted from the sending-end unit to the low-frequency side of M3C; R 1 , X 1 are the equivalent resistance and reactance of the sending end system respectively; 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: (39) in: (40) 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: (41) Formula (41) gives the relationship between the transmission power, equivalent reactance and voltage change of the sending end system when the line resistance is ignored, which provides convenience for evaluating the system operation status. In power system analysis, the stability of the system is a key factor, which is usually measured by the short-circuit ratio. The short-circuit ratio of a system is defined as , the specific formula is as follows: (42) Combining formula (41) and formula (42) we can get formula (43): (43) By calculating formula (43), the maximum voltage at the terminal of the new energy generator when overvoltage occurs can be obtained: , that is, the overvoltage in the new energy low-frequency collection system. The specific formula (44) is as follows: In the above formulas (42), (43), and (44), Indicates the short-circuit capacity of the fan system at the supply end, P dN Indicates the rated power of the power transmission system, X 1 are the equivalent reactance of the sending end system, is the short-circuit current, R 1 is the equivalent resistance of the sending end system, Indicates the equivalent voltage of the sending end unit, represents the AC bus voltage on the low-frequency side, Z represents the equivalent impedance of the sending-end system, and |Z| is the equivalent reactance X of the sending-end system. 1 equal, is the short circuit ratio, represents the line resistance and I represents the short-circuit current.

[0048] After determining the overvoltage, the specifications of the surge protector are selected according to the value of the transient overvoltage. This application also analyzes the voltage at the fan outlet under M3C failure conditions (i.e., the overvoltage in the new energy low-frequency collection system) and the influencing factors, as follows: Based on the PSCAD / EMTDC simulation platform, the following Figure 5 The new energy low-frequency transmission system model shown in the figure has simulation parameters as shown in Table 1. At this time, according to formula (31) to formula (34) and the simulation parameter data in Table 1, the following data can be obtained: According to the data in Table 1, under the constructed simulation model, the voltage amplitudes on the power frequency side and the low frequency side can be calculated. 220× =179.629kv, the current amplitude on the power frequency side and the low frequency side is =2 / 3×800 / 179.629=2969A. 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°. Since the frequency ratio is 5:2, the period ratio is 2:5. All maintain a good sinusoidal form at their respective frequencies. At this time, the voltage and current of the u-phase converter on the low frequency side are input. Under steady-state operating conditions, the dual-frequency coupling phenomenon of M3C only exists in the bridge arm. In steady state, the voltage and current of the three-phase bridge arm of each sub-converter are very symmetrical, and the superimposed current waveform envelopes a sinusoidal fluctuation, which confirms the correctness of theoretical analysis and modeling.

[0049] After knowing the data in Table 1, at 1.5s after the equipment is turned on, a three-phase short circuit fault is artificially set to occur at the machine-side collection bus of the M3C system, and the fault duration is maintained for 0.4s before the fault is removed. At this time, the voltage changes at the fan port are as follows: Before the fault occurs, the fan port voltage data is obtained from the system's electrical measuring device. At this time, starting from t=0s, the voltage gradually increases from 0 until it remains at a relatively stable level, and this process is completed within 1 second, which represents the voltage state of the fan port when the system is operating normally. Its value can reflect the electrical performance of the system under stable working conditions.

[0050] When a fault occurs (set at 1.5s, a three-phase short circuit fault occurs at the M3C system's machine-side collection busbar, and the fault lasts for 0.4s), a transient overvoltage phenomenon occurs, and the voltage drop at the fan port can be observed through the same electrical measurement device. Due to the change in system impedance caused by the short circuit fault, the voltage at the fan port drops rapidly, and then maintains a stable low voltage level within a time interval of 0.4s. The low voltage state at this stage reflects the operating characteristics of the system during the duration of the fault. The continuous low voltage may affect the normal operation of the fan and related equipment.

[0051] After the fault is removed, the voltage suddenly increases. This is because after the fault is removed, the system's operating state changes and the electrical parameters are readjusted, causing the fan port voltage to rise rapidly. The voltage begins to drop after reaching the peak value. This is the process of the system gradually returning to a normal and stable operating state. By monitoring the voltage changes, it can be observed that the voltage eventually returns to the level of a normal and stable operating state, indicating that normal operation can be gradually restored after the fault is removed, and the fan port voltage also returns to the normal range.

[0052] Table 1 Influencing factors 1. Based on the voltage change data at the outlet of the fan when the above fault occurs and formula (44), / 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, we get formula (45), which shows that when this parameter changes, the maximum voltage at the outlet of the fan is 's changing trend.

[0053] (45) Based on the simulation data of the capacity of the current system, the voltage waveform of the wind turbine outlet under different transmission line lengths can be obtained according to the simulation data. The horizontal axis of the figure represents time, in seconds (s), ranging from 0 to 3; the vertical axis represents voltage, in units of per unit (pu), ranging from 0 to 1.6. There are four different curves in the figure, corresponding to the voltage waveform of the wind turbine outlet for four transmission line lengths of 50km, 75km, 100km and 125km. In the 0-0.9s interval, each curve rises rapidly from 0, and the rising speed slows down when it approaches 0.6s. In the 0.9s-1.5s interval, the voltage basically remains stable at around 1.15pu. When the fault occurs at 1.5s, the curve starts to drop and gradually approaches a stable value from 1.5s to 1.9s, with the lowest voltage values ​​being 00.421pu, 0.404pu, 0.39pu, and 0.377pu (these data are the lowest voltage values ​​of 50, 75, 100, and 125, respectively). In this process, the voltage of the curve corresponding to the length of 50km is slightly higher than that of the curve of the length of 75km, and the voltage of the curve of the length of 100km is slightly higher than that of the curve of the length of 75km. The voltage of the curve of the length of 125km is relatively low among the four curves, and the change trends of the four curves are the same. After the fault is removed, at about 1.9 s-2.3 s, the voltage of each curve rises significantly to maximum values ​​of 1.172 pu, 1.229 pu, 1.246 pu, and 1.291 pu (these data are the machine-end overvoltage peaks of 50, 75, 100, and 125, respectively), and then recovers to a stable state, i.e., about 1.15 pu. During this process, the voltage of the curve corresponding to the length of 125 km is slightly higher than that of the curve of 100 km, and the voltage of the curve of 75 km is slightly higher than that of the curve of 50 km. The voltage of the curve of 50 km is relatively lower among the four curves, and the change trends of the four curves are the same.

[0054] It is worth noting that, in order 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 fan sending end, this embodiment uses low-frequency cables of different lengths to conduct simulation tests to verify the above conclusions, and obtains the data shown in Table 2: Table 2 In formula (45), if formula (45) is always greater than 0, it means 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, the wind turbine outlet has reactive power redundancy, and a transient voltage rise occurs on the AC bus at the sending end. The longer the low-frequency transmission line is, the greater the transient overvoltage peak is, indicating that the system has a maximum transmission distance under the same capacity. This shows that the theoretical analysis that the system has a maximum transmission distance under the same capacity is accurate.

[0055] Influencing factor 2: Based on the voltage change data at the outlet of the fan when the above fault occurs and formula (44), the parameters Taking partial derivatives, we get formula (46) as follows: (46) In formula (46), Based on the simulation data of the capacity of the current system, the waveform of the wind turbine outlet voltage under different capacities can be obtained according to the simulation data. The horizontal axis of the figure represents time, in seconds (s), ranging from 0 to 3; the vertical axis represents voltage, in units of per unit (pu), ranging from 0 to 1.6. There are four different curves in the figure, corresponding to the outlet voltage waveforms of the four capacities of 200MW, 400MW, 600MW and 800MW. In the 0-1s interval, each curve rises rapidly from 0, and the rising speed slows down when it approaches 0.5s. In the 1s-1.5s interval, the voltage basically remains stable at 1.1 pu. When the fault occurs at 1.5s, the curve begins to decline and gradually approaches the stable values ​​of 0.354pu, 0.376pu, 0.392pu, and 0.408pu (these data are the lowest voltage values ​​of 200, 400, 600, and 800, respectively). In this process, the curve voltage corresponding to the 200MW capacity is slightly higher than the curve of the 400MW capacity, and the curve voltage of the 600MW capacity is slightly higher than the curve of the 200MW capacity. The curve voltage of the 800MW capacity is relatively low among the four curves, and the change trends of the four curves are the same. After the fault is removed, at about 1.9s-2.3s, the voltage of each curve rises significantly to the maximum values ​​of 1.195pu, 1.204pu, 1.217pu, and 1.229pu (these data are the overvoltage peak values ​​of 200, 400, 600, and 800, respectively), and then rises to a stable state, that is, 1.1pu. In this process, the curve voltage corresponding to the 800MW capacity is slightly higher than the curve of the 600MW capacity, and the curve voltage of the 400MW capacity is slightly higher than the curve of the 200MW capacity. The curve voltage of the 200MW capacity is relatively low in the four curves, and the four curves have the same change trend. The overvoltage peak data and the voltage minimum value data in the simulation data are shown in Table 3.

[0056] Table 3 According to the simulation data, when the capacity of the system is larger, the short-circuit ratio of the system is smaller, and the maximum value of the voltage at the new energy machine end increases, that is, the weaker the system, the greater the maximum value of the voltage at the new energy machine end after the three-phase short-circuit fault is cleared. That is, when a three-phase short circuit occurs, the transmission distance remains unchanged. As the transmission capacity increases, the minimum voltage value gradually increases, and the transient overvoltage peak value after the fault is removed also gradually increases, indicating that under the same transmission distance, there is a maximum value for the electric energy that can be transmitted.

[0057] Furthermore, as a response to the above Figure 1 and Figure 4 The implementation of the method shown in the embodiment of the present application provides a practical transient overvoltage assessment device for a new energy low-frequency collection system. The device is mainly used to accurately calculate the overvoltage when an overvoltage phenomenon occurs, combined with the operating changes of each module in the new energy low-frequency collection system. For ease of reading, this device embodiment will no longer repeat the details of the aforementioned method embodiment, but it should be clear that the device in this embodiment can correspond to all the contents of the aforementioned method embodiment. The device is as follows Figure 5 As shown, specifically including: An acquisition unit 51 is used to determine a modular multi-level matrix converter in a new energy low-frequency collection system by using an equivalent voltage source method; The acquisition unit 51 is used 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 multi-level matrix converter, and the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multi-level matrix converter in the new energy low-frequency collection system; A determining unit 52 is used to calculate a first component expression of a voltage drop in the sending-end unit and a second component expression of a voltage drop according to the resistance of the sending-end unit, the reactance of the sending-end unit, and the active power and reactive power transmitted by the sending-end unit to the low-frequency side of the modular multi-level matrix converter in the acquiring unit 51; The determining unit 52 is used 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 collection system 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; The short-circuit ratio obtaining unit 53 is used to obtain the short-circuit ratio of the new energy low-frequency collection system, wherein 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 voltage determination unit 54 is used to calculate the maximum voltage value of the sending-end unit and determine the maximum voltage value of the sending-end unit as a transient overvoltage by using the relationship in the determination unit 52 and the expression of the first component of the voltage drop, the expression of the second component of the voltage drop and the short-circuit ratio in the short-circuit ratio acquisition unit 53.

[0058] Further, such as Figure 6 As shown, the modular multi-level matrix converter applies the modular multi-level matrix converter bridge arm average value model, and the acquisition unit 51 includes: A first acquisition module 511 is used to acquire a first quantity, a total number of sub-modules of the modular multi-level matrix converter, and a sub-module capacitance voltage of the modular multi-level matrix converter according to a main path structure of the modular multi-level matrix converter, wherein the first quantity is the number of sub-modules in a charging and discharging state in any bridge arm; A bridge arm equivalent capacitance determination module 512 is used to determine the equivalent capacitance of any bridge arm according to the first number of the first acquisition module 511, the total number of submodules of the modular multi-level matrix converter, and the capacitance voltage of the submodules of the modular multi-level matrix converter; The bridge arm equivalent capacitance determination module 512 is used to set the equivalent capacitance as the bridge arm capacitance of the bridge arm average value model of the modular multi-level matrix converter.

[0059] Further, such as Figure 6 As shown, the bridge arm equivalent capacitance determination module 512 further includes: A calculation submodule 5121, configured to calculate a capacitor voltage of a bridge arm according to the first quantity, a total number of submodules of the modular multi-level matrix converter, and a capacitor voltage of a submodule of the modular multi-level matrix converter; A first determining submodule 5122, configured to determine an equivalent voltage of the bridge arm and a current of the bridge arm according to the capacitor voltage of the bridge arm of the calculating submodule 5121; The second determination submodule 5123 is used 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 of the first determination submodule 5122 and the current of the determined bridge arm, wherein the formula followed by the any phase voltage is the same as the above-mentioned implemented formula; The third determination submodule 5124 is used to determine the equivalent capacitance of any bridge arm by utilizing the power balance principle and the energy equalization principle in combination with the compliance formula of any phase voltage of the second determination submodule 5123.

[0060] Further, such as Figure 6 As shown, the first determining submodule 5122 includes: According to the capacitor voltage balance between the submodules, the equivalent voltage of the bridge arm is calculated 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.

[0061] Further, the first determining submodule 5122 includes: Determine the current passing through the capacitor according to the capacitor voltage of the bridge arm, and the calculation formula of the current of the capacitor is the same as the formula in the above embodiment; Based on the bypassed submodule voltage being zero and the calculation formula of the capacitor current, the rewritten formula for determining the capacitor current is the same as the formula in the above embodiment; The calculation formula for determining the current of the bridge arm based on the current rewriting formula of the capacitor is the same as the formula in the above embodiment.

[0062] Further, such as Figure 6 As shown, the device further includes a verification unit 55, and the verification unit 55 includes: A dynamic model determination module 551 is used to determine a dynamic mathematical model of a modular multi-level matrix converter system in a dq coordinate system according to a compliance formula of any phase voltage; The module 552 for obtaining phase voltage and phase current is used to obtain the three-phase voltage and three-phase current at the power frequency output side and the low frequency input side within a preset time under the dynamic mathematical model of the modular multi-level matrix converter system of the dynamic model determination module 551; A waveform acquisition module 553 is used to acquire a waveform according to the three-phase voltage and three-phase current of the phase voltage and phase current acquisition module 552, wherein the waveform includes a three-phase voltage waveform at the power frequency output side, a three-phase current waveform at the power frequency output side, a three-phase current waveform at the low frequency input side, and a three-phase current waveform at the low frequency input side; A judging module 554, used for judging whether the waveform obtained by the waveform obtaining module 553 is a three-phase sine wave; If the judgment module 554 judges that it is yes, it determines whether the modular multi-level matrix converter in the new energy low-frequency collection system is correct by using an equivalent voltage source method.

[0063] Further, such as Figure 6 As shown, the dynamic model determination module 551 includes: Determine AC side mathematical model submodule 5511, used to determine the AC side mathematical model of the new energy low-frequency collection system in the abc rectangular coordinate system according to the compliance formula of any phase voltage; The determination expression submodule 5512 is used to transform the coordinates of the AC side mathematical model of the determination AC side mathematical model submodule 5511 to determine the analytical expressions of the voltage and current components of the modular multi-level matrix converter; The dynamic mathematical model determination submodule 5513 is used to determine the dynamic mathematical model of the modular multi-level matrix converter system in the dq coordinate system based on the voltage and current component analytical expressions of the expression determination submodule 5512.

[0064] The present application also provides a processor, which is used to run a program, wherein the program is executed as follows when it is run: Figure 1 and Figure 3 The practical evaluation method for transient overvoltage of the new energy low-frequency collection system is described.

[0065] In addition, the present application also provides an electronic device, which includes a processor and a memory, the memory is used to store a program, and the processor is coupled to the memory to run the program to perform the following steps: Figure 1 and Figure 3 The practical evaluation method for transient overvoltage of the new energy low-frequency collection system is described.

[0066] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0067] It is understandable that the relevant features in the above methods and devices can refer to each other. In addition, the "first", "second", etc. in the above embodiments are used to distinguish the various embodiments, and do not represent the advantages and disadvantages of the various embodiments. Those skilled in the art can clearly understand that for the convenience and simplicity of description, the specific working processes of the above-described systems, devices and units can refer to the corresponding processes in the aforementioned method embodiments, and will not be repeated here.

[0068] The algorithm and display provided herein are not inherently related to any particular computer, virtual system or other device. Various general purpose systems can also be used together with the teachings based on this. According to the above description, it is obvious that the structure required for constructing such systems. In addition, the application is not directed to any specific programming language either. It should be understood that various programming languages ​​can be utilized to realize the content of the application described herein, and the description of the specific language above is for the purpose of disclosing the best mode of implementation of the application.

[0069] In addition, the memory may include non-permanent memory in a computer-readable medium, random access memory (RAM) and / or non-volatile memory in the form of read-only memory (ROM) or flash RAM, and the memory includes at least one memory chip.

[0070] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt 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 codes.

[0071] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0072] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0073] Computer readable media include permanent and non-permanent, removable and non-removable media that can be implemented by any method or technology to store information. 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 technology, compact disk read-only memory (CD-ROM), digital versatile disk (DVD) or other optical storage, magnetic cassettes, magnetic tape magnetic disk storage or other magnetic storage devices or any other non-transmission media that can be used to store information that can be accessed by a computing device. As defined herein, computer readable media does not include temporary computer readable media (transitory media), such as modulated data signals and carrier waves.

[0074] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0075] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems or computer program products. Therefore, the present application may take the form of a complete hardware embodiment, a complete software embodiment or an embodiment combining software and hardware. Moreover, the present application may 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 codes.

[0076] The above are only embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included within the scope of the claims of the present application.

Claims

1. A practical evaluation method for transient overvoltage of a new energy low-frequency collection system, characterized in that: The method comprises: Using the equivalent voltage source method, the modular multi-level matrix converter in the renewable energy low-frequency collection system is determined; 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 multi-level matrix converter, and the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multi-level matrix converter in the new energy low-frequency collection system; Determine a first component expression of a voltage drop in the sending-end unit and a second component expression of a voltage drop according to the resistance of the sending-end unit, the reactance of the sending-end unit, and the active power and reactive power transmitted by the sending-end unit to the low-frequency side of the modular multi-level matrix converter; 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; Obtaining the short-circuit ratio of the new energy low-frequency collection system, the short-circuit ratio being 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 maximum voltage value of the sending-end unit is calculated by using the voltage drop first component expression, the voltage drop second component expression, the relationship and the short-circuit ratio, and the maximum voltage value of the sending-end unit is determined as a transient overvoltage.

2. The method according to claim 1, characterized in that The method of using an equivalent voltage source to determine a modular multi-level matrix converter in a new energy low-frequency collection system includes: According to the main path structure of the modular multi-level matrix converter, a first quantity, a total number of sub-modules of the modular multi-level matrix converter, and a sub-module capacitance voltage of the modular multi-level matrix converter are obtained, wherein the first quantity is the number of sub-modules in a charging and discharging state in any bridge arm; Determine an equivalent capacitance of any bridge arm according to the first number, the total number of submodules of the modular multilevel matrix converter and the capacitance voltage of the submodules of the modular multilevel matrix converter; As the equivalent capacitance, a modular multi-level matrix converter is selected.

3. The method according to claim 2, characterized in that The method of determining the equivalent capacitance of any bridge arm according to the first number, the total number of submodules of the modular multilevel matrix converter, and the capacitance voltage of the submodules of the modular multilevel matrix converter comprises: The capacitor voltage of the bridge arm is calculated according to the first number, the total number of submodules of the modular multilevel matrix converter and the capacitor voltage of the submodule of the modular multilevel matrix converter. The calculation formula of the capacitor voltage of the bridge arm is: Determining the equivalent voltage of the bridge arm and the current of the bridge arm according to the capacitor voltage of the bridge arm; According to Kirchhoff's voltage law, combined with the equivalent voltage of the bridge arm and the current of the determined bridge arm, any phase voltage of the low-frequency side AC voltage source is determined, and the formula followed by any phase voltage is: Using the power balance principle and the energy sharing principle, combined with the compliance formula of any phase voltage, the equivalent capacitance of any bridge arm is determined; In the above formula, is the capacitor voltage of the bridge arm, is the submodule capacitor voltage, n is the first number, N is the total number of submodules 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, To represent the equivalent voltage of the series connection of the bridge arm xy submodules, is the current of bridge arm xy, R is the equivalent resistance of bridge arm, L is the inductance of bridge arm, represents the time derivative of the bridge arm current, Measure the neutral point voltage at the industrial frequency.

4. The method according to claim 3, characterized in that The method of determining the equivalent voltage of the bridge arm and the current of the bridge arm according to the capacitor voltage of the bridge arm comprises: According to the capacitor voltage balance between the submodules, the equivalent voltage of the bridge arm is calculated using the capacitor voltage of the bridge arm. The calculation formula of the equivalent voltage of the bridge arm is: In the above formula, is the capacitor voltage of the bridge arm, is the equivalent voltage of the bridge arm, n is the first number, and N is the total number of sub-modules of the modular multi-level matrix converter.

5. The method according to claim 3, characterized in that: The method of determining the equivalent voltage of the bridge arm and the current of the bridge arm according to the capacitor voltage of the bridge arm comprises: The current passing through the capacitor is determined according to the capacitor voltage of the bridge arm, and the calculation formula of the current of the capacitor is: Based on the bypassed submodule voltage being zero and the calculation formula of the capacitor current, the capacitor current rewrite formula is determined as: Based on the current rewriting formula of the capacitor, the calculation formula for determining the current of the bridge arm is: In the above formula, represents the capacitance value of the capacitor in the full-bridge submodule, is the submodule capacitor voltage, n is the first number, N is the total number of submodules 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 submodule in the bridge arm, is the equivalent capacitance current of the submodule, and the equivalent capacitance of the bridge arm can be expressed as , represents the modulation signal of the bridge arm, is the current in bridge 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 includes: According to the compliance formula of any phase voltage, a dynamic mathematical model of a modular multi-level matrix converter system in a dq coordinate system is determined; Under the dynamic mathematical model of the modular multi-level matrix converter system, three-phase voltage and three-phase current at the power frequency output side and the low frequency input side within a preset time are obtained; According to the three-phase voltage and the three-phase current, a waveform diagram is obtained, wherein the waveform diagram includes a three-phase voltage waveform diagram on the power frequency output side, a three-phase current waveform diagram on the power frequency output side, a three-phase current waveform diagram on the low frequency input side, and a three-phase current waveform diagram on the low frequency input side; Determining whether the waveform diagram is a three-phase sine wave; If so, determine the correctness of the modular multi-level matrix converter in the renewable energy low-frequency collection system by using the equivalent voltage source method.

7. The method according to claim 6, characterized in that Determining the dynamic mathematical model of the modular multi-level matrix converter system in the dq coordinate system according to the compliance formula of any phase voltage includes: According to the compliance formula of any phase voltage, determine the AC side mathematical model of the new energy low-frequency collection system in the abc rectangular coordinate system; Transforming the coordinates of the AC side mathematical model to determine analytical expressions of voltage and current components of the modular multi-level matrix converter; Based on the analytical expressions of the voltage and current components, a dynamic mathematical model of the modular multi-level matrix converter system in a dq coordinate system is determined.

8. A practical evaluation device for transient overvoltage of a new energy low-frequency collection system, characterized in that: The device comprises: An acquisition unit is used to determine a modular multi-level matrix converter in a new energy low-frequency collection system by using an equivalent voltage source method; The acquisition unit is used to acquire 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 multi-level matrix converter, and the active power and reactive power transmitted from the sending-end unit to the low-frequency side of the modular multi-level matrix converter in the new energy low-frequency collection system; a determining unit, configured to calculate a first component expression of a voltage drop in the sending-end unit and a second component expression of a voltage drop according to the resistance of the sending-end unit in the acquiring unit, the reactance of the sending-end unit, and the active power and reactive power transmitted by the sending-end unit to the low-frequency side of the modular multi-level matrix converter; The determining unit is used 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 collection system 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; A short-circuit ratio acquisition unit is used to acquire the short-circuit ratio of the new energy low-frequency collection system, wherein 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; A voltage determination unit is used to calculate the maximum voltage value of the sending-end unit and determine the maximum voltage value of the sending-end unit as a transient overvoltage by using the relationship and the voltage drop first component expression of the determination unit, the voltage drop second component expression and the short-circuit ratio in the short-circuit ratio acquisition unit.

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

10. An electronic device, characterized in that: The electronic device includes a processor and a memory, and the processor is used to call program instructions in the memory to execute the practical transient overvoltage assessment method for a new energy low-frequency collection system as described in any one of claims 1-7.

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