GFM converter low-voltage ride-through method based on positive and negative current optimization

Abstract

The invention discloses a GFM converter low-voltage ride-through method based on positive and negative current optimization, and the method comprises the steps: injecting positive and negative sequence reactive current into a converter, and obtaining positive and negative sequence voltage components, designing a reactive power controller RPC +, a virtual flux measurement module VFM, a virtual flux oriented control module VFOV and an active power controller APC + according to the obtained data to detect and process the converter to obtain an accurate positive sequence voltage reference value; and designing a reactive current controller RCC-and an active current controller ACC-to detect and process the converter to obtain a negative sequence voltage reference value, and processing the accurate positive sequence voltage reference value and the negative sequence voltage reference value to obtain a converter switching pulse signal. Through cooperation of advanced control strategies such as virtual magnetic flux directional control, reactive power control and active power control, stability and electric energy quality of a power system under a fault condition can be effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of low-voltage fault ride-through control strategies for grid-connected new energy stations, and in particular to a low-voltage ride-through method for a GFM converter based on positive and negative current optimization. Background Art

[0002] With the widespread access of renewable energy sources (RES) to the global power system, the global power generation pattern has undergone profound changes. This shift marks a significant adjustment in the global energy structure. Since 2013, the annual growth of renewable energy has exceeded the sum of fossil fuels and nuclear energy. The latest research shows that under certain conditions, some power systems can achieve a renewable energy penetration rate close to 100%, indicating that renewable energy with grid-connected converters as the interface is gradually taking the leading position in the power generation field. The relevant report of the International Renewable Energy Agency also confirms this trend, further strengthening the core position of renewable energy in the global energy transformation.

[0003] However, there are essential differences between converter-based renewable energy generation and traditional synchronous generator generation, and this transformation process also brings many challenges. Grid-following converters lack the inherent ability to provide grid inertia and support grid strength, making the power system more vulnerable to disturbances. In this context, the grid-following converter (GFL) control strategy based on current loop and phase-locked loop to achieve synchronous control has gradually become the mainstream control method for grid-connected converters. However, the stability problem of the low-voltage system and the limitation of the GFL converter that cannot operate independently or assist in grid recovery in the event of a power outage highlight the need for new control strategies, which has also prompted the proposal and development of the GFM converter.

[0004] As a voltage source converter, GFM simulates the behavior of a synchronous generator by maintaining the phase angle and amplitude of the internal voltage during transient processes. It has important characteristics such as inertial response, power oscillation damping, and adaptability to islanding mode and low system strength conditions. Another important feature is its current limiting strategy, which is designed to prevent overcurrent conditions during grid disturbances. Since electronic converters have a low overcurrent tolerance (1.1pu~1.3pu), when facing grid disturbances, GFM converters effectively avoid current overload through precise current limiting control, ensuring the safe operation of the converter under abnormal conditions. Recently, some scholars have published a new GFM control method based on virtual flux orientation, which allows a simpler way to limit active and reactive currents to maintain system stability.

[0005] On the other hand, with the access of a high proportion of renewable energy, the stability of the power system faces more complex challenges, especially when a grid fault occurs. At this time, although the output current of the grid-connected converter is usually limited, some standards require the converter to be able to quickly inject current in a short period of time to help maintain voltage stability while ensuring that the output current does not exceed the rated value. This requirement emphasizes the important role of the grid-connected converter in the grid fault recovery process, especially in improving the system's anti-disturbance capability and ensuring power quality. It highlights the higher requirements for the dynamic response capabilities of the grid-connected converter in high-penetration renewable energy systems, especially when dealing with grid disturbances and faults. This fast current injection capability is crucial in the event of a grid fault or transient condition. It can effectively support the grid to restore normal voltage levels and ensure the stability and reliability of the system, especially in an environment where the grid is weak or renewable energy fluctuates greatly.

[0006] Although positive- and negative-sequence current injection strategies for GFL converters during balanced faults have been extensively studied, most existing literature focuses on balanced fault scenarios. Although some studies have attempted to explore unbalanced fault scenarios, there is still limited research on the performance of GFM converters under fault conditions. In addition, many existing control strategies focus on limiting the total current, which may lead to uncontrollable negative-sequence current injection during faults. This control method fails to fully meet the precise requirements for current injection in grid standards, especially in terms of fine-tuning the current response during faults.

[0007] Given that GFM is a potential alternative to overcome the limitations of GFL converters, it is urgent to develop new control methods to ensure that the injection of current during low-voltage faults strictly complies with the technical requirements set by national standards. The new method needs to be able to effectively cope with complex situations such as grid faults and unbalanced loads, ensure that the injection of current complies with national standards under various operating conditions, and ensure the stability and safety of the system. Summary of the invention

[0008] In view of the deficiencies in the prior art, the present invention provides a low voltage ride-through method for a GFM converter based on positive and negative current optimization, which aims to solve the problems mentioned in the background technology.

[0009] To achieve the above object, the present invention provides the following technical solution: a low voltage ride-through method for a GFM converter based on positive and negative current optimization, comprising the following steps:

[0010] Step S1: setting the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault conditions and unbalanced fault conditions;

[0011] Step S2: injecting positive and negative sequence reactive currents into the converter under balanced fault conditions and unbalanced fault conditions, introducing proportional parameters into the injected positive and negative sequence reactive currents, and obtaining the maximum value of the sum of the positive and negative sequence reactive currents of the converter;

[0012] Step S3: Make the maximum value of the sum of the positive and negative sequence reactive currents of the converter meet the set requirements under the balanced fault condition and the unbalanced fault condition of the converter, synchronously connect the converter with the power grid, align the virtual magnetic flux vector of the converter with the magnetic flux vector of the power grid, generate the same three-phase voltage as the power grid, and extract the positive and negative sequence voltage components in the power grid voltage, the converter terminal voltage and the three-phase voltage by the delayed signal cancellation method;

[0013] Step S4: Based on the positive and negative sequence voltage components in step S3, design the reactive power controller RPC + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + , through the reactive power controller RPC + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + The converter is detected and processed under balanced fault conditions and unbalanced fault conditions, and the output after the detection and processing is coordinated and matched to obtain a positive sequence voltage reference value. The positive sequence voltage reference value is output by the virtual flux oriented control module VFOC after setting parameters to obtain an accurate positive sequence voltage reference value;

[0014] Step S5: Based on the positive and negative sequence voltage components in step S3, design the reactive current controller RCC - With active current controller ACC - , through the reactive current controller RCC - With active current controller ACC - Performing detection and processing under balanced fault conditions and unbalanced fault conditions of the converter, coordinating the output after the detection and processing, and obtaining a negative sequence voltage reference value;

[0015] Step S6: Based on the accurate positive sequence voltage reference value in step S4 and the negative sequence voltage reference value in step S5, inverse Park transform and space vector pulse width modulation technology are used to obtain a converter switching pulse signal.

[0016] Further, in step S1, the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault conditions and unbalanced fault conditions is set, and the specific process is as follows:

[0017] Step S11: Under balanced fault conditions, the converter needs to inject a positive-sequence reactive current proportional to the positive-sequence voltage deviation, and simultaneously provide a positive-sequence active current until the rated current is reached;

[0018] Under balanced fault conditions, the converter needs to inject / absorb a positive sequence reactive current, denoted as ΔI1; the positive sequence reactive current is proportional to the positive sequence voltage deviation ΔU1; the adjustable constant K1 ranges from 2 to 6; in addition to the positive sequence reactive current, the converter must also provide a positive sequence active current until the rated current value is reached; in addition to the positive sequence reactive current, the converter must also provide a positive sequence active current until the rated current value is reached; K1 represents the standard defined positive sequence voltage proportional constant;

[0019] Step S12: Under unbalanced fault conditions, the converter needs to inject positive-sequence active current in addition to the negative-sequence current proportional to the negative-sequence voltage deviation until the current rating is reached;

[0020] Under unbalanced fault conditions, the converter must also inject / absorb negative sequence current ΔI2, which is proportional to the negative sequence voltage deviation ΔU2; the adjustable constant K2 ranges from 2 to 6; similar to balanced fault conditions, when the sum of positive and negative sequence reactive currents is not enough to reach the rated current value of the converter, positive sequence active current must be injected until it reaches the rated value; K2 represents the negative sequence voltage proportional constant defined by the standard;

[0021] Step S13: setting the maximum value of the injected total positive sequence reactive current to 1;

[0022] Step S14: The maximum time course of the converter injecting positive sequence reactive current under balanced / unbalanced fault conditions is:

[0023] When the grid-side voltage of the converter drops, the response delay time of the converter is t i Not more than 20ms, response time t r It is necessary to inject positive sequence reactive current at the beginning until the required positive sequence reactive current requirement is reached. The delay time and response time should meet t i +t r ≤50ms.

[0024] Further, in step S2, positive and negative sequence reactive currents are injected into the converter under balanced fault conditions and unbalanced fault conditions. The specific process is as follows:

[0025] Step S21: Based on step S11, under the condition of balanced fault, the converter needs to inject a positive sequence reactive current proportional to the positive sequence voltage deviation, which is expressed as:

[0026]

[0027] In the formula, Indicates the positive sequence reactive current reference value under the standard; Δv + Indicates the change in positive sequence voltage;

[0028] Step S22: Based on step S12, under unbalanced fault conditions, the converter needs to inject a negative sequence current proportional to the negative sequence voltage, expressed as:

[0029]

[0030] In the formula, Indicates the active negative sequence current reference value; Indicates the negative sequence reactive current reference value under the mark; Δv - Indicates the negative sequence voltage increment;

[0031] When the positive sequence reactive current reference value under the standard The negative sequence reactive current reference value is marked If the sum of the positive sequence active current does not reach the rated current value of the converter, the positive sequence active current value must be injected until it reaches the rated current value, which is expressed as

[0032]

[0033] In the formula, and Represent the magnitude of the positive-sequence and negative-sequence active currents respectively;

[0034] When the maximum value of the total converter current magnitude appears in the vector and At the moment of the line, only the negative sequence reactive current will be absorbed, and the above formula (4) can be rewritten as:

[0035]

[0036] In the formula, Indicates the active positive sequence current reference value; Indicates the positive sequence reactive current reference value under marking;

[0037] Subtract from the equation get:

[0038]

[0039] In the formula, Indicates the active positive sequence current reference value;

[0040] Further, in step S2, the maximum value of the sum of the positive and negative sequence reactive currents of the converter is obtained, and the specific process is as follows:

[0041] A proportional parameter R is introduced, which reduces the positive sequence reactive current reference value proportionally. The negative sequence reactive current reference value is marked The two reference currents are used to obtain the maximum value of the sum of the positive and negative sequence reactive currents of the converter, expressed as:

[0042]

[0043] In the formula, Indicates that after introducing the scale parameter and The maximum value of the sum.

[0044] Further, in step S3, the positive and negative sequence voltage components are specifically processed as follows:

[0045] The maximum value of the sum of the positive and negative sequence reactive currents of the converter meets the set requirements under the balanced fault condition and the unbalanced fault condition of the converter, and the converter is synchronously connected to the grid. Aligning the grid flux vector To generate the same three-phase voltage as the power grid; the delayed signal cancellation method is used to extract the positive and negative sequence components in the grid voltage, converter terminal voltage and three-phase current; taking the converter terminal voltage component as an example, the positive and negative sequence voltage components of the converter terminal voltage are expressed as:

[0046]

[0047] In the formula, and They represent the positive sequence components of the converter output voltage under the αβ coordinate axis respectively; and They represent the negative sequence components of the converter output voltage on the αβ coordinate axis; v gfm-α and v gfm-β Respectively represent the αβ coordinate axis components; v′ gfm-α and v′ gfm-β Respectively represent the same quantity;

[0048] Repeat the calculation steps of formulas (8)-(11) above to extract the grid voltage and three-phase current to obtain the positive and negative sequence voltage components of the grid voltage and three-phase current.

[0049] Further, in step S4, the positive sequence voltage reference value is obtained, and the specific process is as follows:

[0050] Based on the positive and negative sequence voltage components in step S3, the reactive power controller RPC is designed. + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + ;

[0051] Reactive Power Controller RPC + It consists of two sub-rings, according to the reactive power controller RPC + Determine whether a high-level signal is detected under a balanced fault condition and determine whether it is in the reactive power controller RPC + Work on the right or left side;

[0052] When the balanced fault condition is detected and there is no high level signal, the reactive power controller RPC + The left side of the reactive power controller RPC + There is a proportional control loop on the left side, which sets the reference voltage of the photovoltaic PV controller. Set to 1, by comparing the reference voltage The actual converter output voltage v gfm , generating an error signal; the error signal is passed through the gain factor n q and rated flux Regulation and reactive current limiting module RCL + The output items are summed to obtain the reference flux Reactive current limiting module RCL + The output is the positive sequence reactive current reference value under the standard Mark the negative sequence reactive current reference value and the actual converter output voltage v gfm The sum is It is expressed as;

[0053]

[0054] In the formula, It indicates the output item of reactive current limiting module RCL+ after adding the maximum current limit;

[0055] On the other hand, when an unbalanced fault condition is detected, a high level signal is present in the reactive power controller RPC. + The right side of the control loop calculates the positive sequence reactive current reference during voltage fault. The actual current positive sequence component The resulting deviation signal is processed by the PI regulator to obtain Introducing the positive sequence component of the grid flux Get the reference flux under voltage sag It is expressed as:

[0056]

[0057] In the formula, Indicates the output value of the current deviation signal after PI adjustment;

[0058] Virtual flux orientation control module VFOV reference flux ψ to be sought *+ The calculation method of is as follows, expressed as:

[0059]

[0060] In the formula, Indicates a high-level logical NOT signal; S set It is a high level logic signal;

[0061] Active Power Controller APC + The control angle θ is obtained, which is expressed as:

[0062]

[0063] In the formula, ω + Indicates the control angular frequency; represents the output of the loop simulating the SG swing equation; PSS stands for Power System Stabilizer 1,2 Output: Represents the output of the active current limiter ACL+ module; the output of the active current limiter ACL+ module is composed of the active positive sequence current reference value Active negative sequence current reference value and the actual converter output voltage v gfm The sum is obtained; dt represents the time domain integral;

[0064] Active Power Controller APC + The actual value of active power P + and the active power reference value P *+ The injection of the control angle Θ is realized, and the active power reference value P *+ The expression is;

[0065]

[0066] The virtual flux measurement module VFM is responsible for obtaining the converter virtual flux Positive sequence component; positive sequence voltage under αβ coordinate axis of virtual flux measurement module VFM The integrated value is obtained by integrating with the time constant of 2πf, where f is the rated frequency of 50Hz, and passes through a high-pass filter with the cutoff frequency set to 5Hz; the current output by the filter Multiply by the filter impedance L f The output of the multiplication of the filter and the filter impedance is obtained, the output of the multiplication of the filter and the filter impedance is added to the integral processing value, the αβ axis component flux of the virtual flux measurement module VFM is calculated, and the converter virtual flux is obtained.

[0067] Using Active Power Controller (APC) in Virtual Flux Oriented Control (VFOV) + The calculated control angle θ is converted into the converter virtual flux calculated in the virtual flux measurement module VFM. The positive sequence component is converted into αβ / dq axis components The d-axis component is compared with the reference flux ψ calculated in the virtual flux orientation control module VFOV *+ The q-axis magnetic flux component is compared with 0, and the comparison error signal is input into the two PI regulators of the virtual flux orientation control module VFOV control loop, and the output is added with the feedforward signal to compensate the cross-coupling term. Get the positive sequence voltage reference value ω represents the actual frequency.

[0068] Furthermore, the accurate positive sequence voltage reference value is obtained, and the specific process is as follows:

[0069] Positive sequence voltage reference value The virtual flux oriented control module VFOV is used as the output. The two PI regulators in the virtual flux oriented control module VFOV have the same control mode and the same parameter selection method. The transfer function G of the compensation cross-coupling term introduced by the feedforward signal is P (s) is expressed as:

[0070]

[0071] Where, T f represents the time constant of the feedforward signal; s represents the complex frequency;

[0072] On the other hand, the transfer function of the PI regulator G PI (s) is expressed as:

[0073]

[0074] In the formula, K PI represents the proportional constant; T PI Represents the PI regulator time constant;

[0075] The open-loop transfer function G(s) can be expressed as:

[0076]

[0077] The closed-loop transfer function G′(s) is expressed as:

[0078]

[0079] Bandwidth value τ i It is expressed as:

[0080]

[0081] By changing the proportionality constant K PI Set to 1 and set the virtual flux orientation control module VFOV time constant T PISet as the filter constant to make the bandwidth value equal to the rated value ω; when the rated frequency is 50, the bandwidth value is 314.16; by setting the parameters of the virtual flux oriented control module VFOV, the virtual flux oriented control module VFOC after setting the parameters is used as the positive sequence voltage reference value The output of the positive sequence voltage reference is obtained.

[0082] Further, in step S5, a negative sequence voltage reference value is obtained, and the specific process is as follows:

[0083] Based on the positive and negative sequence voltage components in step S3, design the reactive current controller RCC - With active current controller ACC - ;

[0084] When a balanced fault condition occurs, the reactive current controller RCC - By maintaining the negative sequence voltage d-axis component of the converter Calculated as zero When an unbalanced fault condition is detected, the reactive current controller RCC - By comparing the negative sequence reactive current reference The actual negative sequence current The error value between the two is corrected by the PI regulator to obtain Item; Negative sequence voltage d-axis reference value It is expressed as:

[0085]

[0086] In the formula, Reactive current controller RCC - The output value of the PI regulator during the balanced fault condition; is the output value of the PI regulator during the unbalanced fault condition; Indicates a high-level logical NOT signal;

[0087] When a balanced fault condition occurs, the active current controller ACC - By maintaining the q-axis component of the converter positive sequence voltage Zero based on PI calculation Under unbalanced fault conditions, the active current controller ACC - By comparing the active current with the actual negative sequence current q-axis component i q With active negative sequence current reference Addition based on PI output e q2 ;q-axis negative sequence voltage reference value The calculation is expressed as:

[0088]

[0089] In the formula, and Respectively represent the active current controller ACC - Balanced and unbalanced fault output negative sequence voltage reference value.

[0090] Further, in step S6, a converter switch pulse signal is obtained, and the specific process is as follows:

[0091] By accurately determining the positive sequence voltage reference value With negative sequence voltage reference Perform inverse Park transformation to obtain the accurate positive sequence voltage reference value The angle used is the control angle θ, expressed as:

[0092]

[0093] In the formula, cos represents the cosine of the angle; -sin represents the negative value of the sine function; are the positive sequence voltage reference values ​​under the three-phase coordinate axis, and are the positive sequence three-phase voltage;

[0094] Negative sequence voltage reference value The inverse Park transform uses an angle of Θ′, where Θ′ = -Θ, to obtain the negative sequence three-phase voltage Next, the three-phase voltages obtained by the positive sequence voltage and the negative sequence voltage are summed, and the converter switching pulse signal is obtained through the space vector pulse width modulation technology.

[0095] Compared with the existing technology, the present invention has the following beneficial effects:

[0096] (1) The present invention can effectively improve the stability and power quality of the power system under fault conditions by combining advanced control strategies such as virtual flux oriented control, reactive power control and active power control. In addition, the present invention can also dynamically adjust the control strategy to ensure that the converter can operate reliably under various power grid fault conditions, and has strong adaptability and robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Figure 1 The figure is a flow chart of the method of the present invention.

[0098] Figure 2 This is a positive sequence voltage reference value control circuit diagram of the present invention.

[0099] Figure 3 This is a negative sequence voltage reference value control circuit diagram of the present invention. DETAILED DESCRIPTION

[0100] like Figure 1As shown, the present invention provides a technical solution: a low voltage ride-through method of a GFM converter based on positive and negative current optimization, comprising:

[0101] Step S1: setting the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault conditions and unbalanced fault conditions;

[0102] Step S2: injecting positive and negative sequence reactive currents into the converter under balanced fault conditions and unbalanced fault conditions, introducing proportional parameters into the injected positive and negative sequence reactive currents, and obtaining the maximum value of the sum of the positive and negative sequence reactive currents of the converter;

[0103] Step S3: Make the maximum value of the sum of the positive and negative sequence reactive currents of the converter meet the set requirements under the balanced fault condition and the unbalanced fault condition of the converter, synchronously connect the converter with the power grid, align the virtual magnetic flux vector of the converter with the magnetic flux vector of the power grid, generate the same three-phase voltage as the power grid, and extract the positive and negative sequence voltage components in the power grid voltage, the converter terminal voltage and the three-phase voltage by the delayed signal cancellation method;

[0104] Step S4: Based on the positive and negative sequence voltage components in step S3, design the reactive power controller RPC + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + , through the reactive power controller RPC + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + The converter is detected and processed under balanced fault conditions and unbalanced fault conditions, and the output after the detection and processing is coordinated and matched to obtain a positive sequence voltage reference value. The positive sequence voltage reference value is output by the virtual flux oriented control module VFOC after setting parameters to obtain an accurate positive sequence voltage reference value;

[0105] Step S5: Based on the positive and negative sequence voltage components in step S3, design the reactive current controller RCC - With active current controller ACC - , through the reactive current controller RCC - With active current controller ACC - Performing detection and processing under balanced fault conditions and unbalanced fault conditions of the converter, coordinating the output after the detection and processing, and obtaining a negative sequence voltage reference value;

[0106] Step S6: Based on the accurate positive sequence voltage reference value in step S4 and the negative sequence voltage reference value in step S5, inverse Park transform and space vector pulse width modulation technology are used to obtain a converter switching pulse signal.

[0107] Wherein, in step S1, the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault conditions and unbalanced fault conditions is set, and the specific process is as follows:

[0108] Step S11: Under balanced fault conditions, the converter needs to inject a positive-sequence reactive current proportional to the positive-sequence voltage deviation, and simultaneously provide a positive-sequence active current until the rated current is reached;

[0109] Under balanced fault conditions, the converter needs to inject / absorb a positive sequence reactive current, denoted as ΔI1; the positive sequence reactive current is proportional to the positive sequence voltage deviation ΔU1; the adjustable constant K1 ranges from 2 to 6, with a typical value of 3.5; in addition to the positive sequence reactive current, the converter must also provide a positive sequence active current until the rated current value is reached; K1 represents the standard defined positive sequence voltage proportional constant;

[0110] Step S12: Under unbalanced fault conditions, the converter needs to inject positive-sequence active current in addition to the negative-sequence current proportional to the negative-sequence voltage deviation until the current rating is reached;

[0111] Under unbalanced fault conditions, in addition to the positive sequence reactive current ΔI1 specified in the previous point, the converter must also inject / absorb negative sequence current ΔI2, which is proportional to the negative sequence voltage deviation ΔU2; similar to K1, the adjustable constant K2 ranges from 2 to 6; similar to balanced fault conditions, when the sum of the positive and negative sequence reactive currents is not enough to reach the rated current value of the converter, the positive sequence active current must be injected until it reaches the rated value; K2 represents the negative sequence voltage proportional constant defined by the standard;

[0112] Step S13: setting the maximum value of the injected total positive sequence reactive current to 1 p.u;

[0113] Step S14: The maximum time course of the converter injecting positive sequence reactive current under balanced / unbalanced fault conditions is:

[0114] When the grid-side voltage of the converter drops, the response delay time of the converter is t i Not more than 20ms, response time t r It is necessary to inject positive sequence reactive current at the beginning until the required positive sequence reactive current requirement is reached. The delay time and response time should meet t i +t r ≤50ms.

[0115] Wherein, in step S2, positive and negative sequence reactive current is injected into the converter under balanced fault conditions and unbalanced fault conditions, and the specific process is as follows:

[0116] The converter fault injection current strategy requires the implementation of positive and negative sequence reactive current injection or absorption according to the fault type and voltage fault drop depth. The process is:

[0117] Step S21: Based on step S11, under the condition of balanced fault, the converter needs to inject a positive sequence reactive current proportional to the positive sequence voltage deviation, which is expressed as:

[0118]

[0119] In the formula, Indicates the positive sequence reactive current reference value under the standard; Δv + Indicates the change in positive sequence voltage, with a maximum value of 1p.u.;

[0120] Step S22: Based on step S12, under unbalanced fault conditions, the converter needs to inject a negative sequence current proportional to the negative sequence voltage, expressed as:

[0121]

[0122] In the formula, Indicates the active negative sequence current reference value; Indicates the negative sequence reactive current reference value under the mark; Δv - Indicates the negative sequence voltage increment;

[0123] When the positive sequence reactive current reference value under the standard The negative sequence reactive current reference value is marked If the sum of the current and current does not reach the rated current value of the converter, the positive sequence active current value must be injected until the rated current value is reached, which is expressed as:

[0124]

[0125] In the formula, and Respectively represent the modulus of positive-sequence and negative-sequence active currents, with the unit being 1;

[0126] Since the maximum value of the total converter current magnitude appears in the vector and At the moment of the line, only the negative sequence reactive current will be absorbed, and the above formula (4) can be rewritten as:

[0127]

[0128] In the formula, Indicates the active positive sequence current reference value; Indicates the positive sequence reactive current reference value under marking;

[0129] Subtract from the equation get:

[0130]

[0131] In the formula, Indicates the active positive sequence current reference value.

[0132] Among them, in step S2, the maximum value of the sum of the positive and negative sequence reactive currents of the converter is obtained, and the specific process is:

[0133] When the voltage fault drops deeply, mark the positive sequence reactive current reference value. The negative sequence reactive current reference value is marked The sum may exceed the rated current value of the converter; in order to avoid this situation, a proportional parameter R is introduced, which reduces the positive sequence reactive current reference value proportionally. The negative sequence reactive current reference value is marked Two reference currents are used to obtain the maximum value of the sum of the positive and negative sequence reactive currents of the converter, thereby ensuring that the sum does not exceed the rated current value of the converter; even if this adjustment causes the constants K1 and K2 to be compressed outside the predetermined range, the converter can still maintain stable operation and will always meet; expressed as:

[0134]

[0135] In the formula, Indicates that after introducing the scale parameter and The maximum value of the sum;

[0136] This study proposed an input-output conversion strategy based on voltage drop; the core of the input and output conversion strategy is that the current limiter used is different from the traditional method. The latter usually ensures stability by limiting the current modulus, while the virtual flux direction converter starts to limit when the current exceeds the preset maximum value; this design may cause overcurrent in the transition stage of the fault; to prevent equipment damage, when overcurrent is detected, the power electronics blocking strategy is activated, that is, the switch is turned off for 20ms; during this period, it will quickly recover and inject a smooth current response according to the standard; for the output in the case of voltage sag, if overcurrent is detected, the virtual flux direction converter will also stop the switching operation; then, the active positive sequence current reference value among the three reference current values Active negative sequence current reference value And the negative sequence reactive current reference value under the mark will be set to zero, and The reaction time during the fault is set to 30ms; eventually, when the fault is cleared, the virtual flux direction converter returns to the pre-fault operating mode.

[0137] Among them, the positive and negative sequence voltage components in step S3 are specifically processed as follows:

[0138] The maximum value of the sum of the positive and negative sequence reactive currents of the converter meets the set requirements under the balanced fault condition and the unbalanced fault condition of the converter, and the converter is synchronously connected to the grid. Aligning the grid flux vector To generate the same three-phase voltage as the power grid; the delayed signal cancellation method is used to extract the positive and negative sequence components in the grid voltage, converter terminal voltage and three-phase current; taking the converter terminal voltage component as an example, the positive and negative sequence voltage components of the converter terminal voltage are expressed as:

[0139]

[0140] In the formula, and They represent the positive sequence components of the converter output voltage under the αβ coordinate axis respectively; and They represent the negative sequence components of the converter output voltage on the αβ coordinate axis; v gfm-α and v gfm-β Respectively represent the αβ coordinate axis components; v′ gfm-α and v′ gfm-β Respectively represent the same quantity;

[0141] Repeat the calculation steps of formulas (8)-(11) above to extract the grid voltage and three-phase current to obtain the positive and negative sequence voltage components of the grid voltage and three-phase current.

[0142] like Figure 2 As shown, in step S4, the positive sequence voltage reference value is obtained, and the specific process is as follows:

[0143] Based on the positive and negative sequence voltage components in step S3, the reactive power controller RPC is designed. + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + ;

[0144] Its reactive power controller RPC + It consists of two sub-rings, according to the reactive power controller RPC + Determine whether a high-level signal is detected under a balanced fault condition and determine whether it is in the reactive power controller RPC + Work on the right or left side;

[0145] When the balanced fault condition is detected and there is no high level signal, the reactive power controller RPC + The left side of the reactive power controller RPC + There is a proportional control loop on the left side, which sets the reference voltage of the photovoltaic PV controller. Set to 1, by comparing the reference voltage The actual converter output voltage v gfm , generating an error signal; the error signal is passed through the gain factor n q and rated flux Regulation and reactive current limiting module RCL + The output items are summed to obtain the reference flux Reactive current limiting module RCL + The output is the positive sequence reactive current reference value under the standard Mark the negative sequence reactive current reference value and the actual converter output voltage v gfm The sum is It is expressed as;

[0146]

[0147] In the formula, It indicates the output item of reactive current limiting module RCL+ after adding the maximum current limit;

[0148] On the other hand, when an unbalanced fault condition is detected, a high level signal is present in the reactive power controller RPC. + The right side of the control loop calculates the positive sequence reactive current reference during voltage fault. The actual current positive sequence component The resulting deviation signal is processed by the PI regulator to obtain Introducing the positive sequence component of the grid flux Get the reference flux under voltage sag It is expressed as:

[0149]

[0150] In the formula, Indicates the output value of the current deviation signal after PI adjustment;

[0151] Figure 2 Reactive Power Controller RPC + S set It is a high-level logic signal, and its value is 1;

[0152] Introducing RPC + The main innovative features of the control strategy are: During a fault, instead of using the traditional proportional control loop, the grid flux is used as a reference instead of the rated flux; this provides a more robust and rapid response to transient changes caused by grid faults; therefore, the virtual flux oriented control module VFOV requires a reference flux ψ *+ The calculation method of is as follows, expressed as:

[0153]

[0154] In the formula, Indicates a high-level logical NOT signal, with a value of 0; S set It is a high-level logic signal, and its value is 1;

[0155] Active Power Controller APC + The control angle θ is obtained, which is expressed as:

[0156]

[0157] In the formula, ω + Indicates the control angular frequency; represents the output of the loop simulating the SG swing equation; PSS stands for Power System Stabilizer 1,2 The output of helps to compensate for the damping of the active response when the inertia constant is large and the damping factor is low; Represents the output of the active current limiter ACL+ module; the output of the active current limiter ACL+ module is composed of the active positive sequence current reference value Active negative sequence current reference value and the actual converter output voltage v gfm The active current limiter ACL+ module is responsible for ensuring that the active current of the active power controller does not exceed its maximum value; its function is realized by the reactive current limiting module RCL + Similarly, the corresponding current limiting control is completed through the synergistic effect of the input of the converter terminal voltage and the maximum active current; dt represents the time domain integral;

[0158] The main innovation of the active control loop APC+ is that its method is implemented in the operation during voltage failure; the implementation of the APC+ module functions depends on the actual value of the active power P + and the active power reference value P *+ The injection of the control angle θ is then realized, where the active power reference value is expressed as follows:

[0159]

[0160] in, Figure 2 Active Power Controller APC + D 1,2 represents the damping constant, H 1,2 represents the inertia constant; ω0 represents the rated frequency, which is 50Hz; s represents the complex frequency domain, which is a form of expression of the filter transfer function and has no practical meaning;

[0161] The virtual flux measurement module VFM is responsible for obtaining the converter virtual flux Positive sequence component; positive sequence voltage under αβ coordinate axis of virtual flux measurement module VFM The integrated value is obtained by integrating with the time constant of 2πf, where f is the rated frequency of 50Hz, and passes through a high-pass filter with the cutoff frequency set to 5Hz; the current output by the filter Multiply by the filter impedance L f The output of the multiplication of the filter and the filter impedance is obtained, the output of the multiplication of the filter and the filter impedance is added to the integral processing value, the αβ axis component flux of the virtual flux measurement module VFM is calculated, and the converter virtual flux is obtained.

[0162] Figure 2 Virtual Flux Measurement Module VFM ω c Indicates that the filter cutoff frequency is 5Hz.

[0163] Using Active Power Controller (APC) in Virtual Flux Oriented Control (VFOV) + The calculated control angle θ is converted into the converter virtual flux calculated in the virtual flux measurement module VFM. The positive sequence component is converted into αβ / dq axis components The d-axis component is compared with the reference flux ψ calculated in the virtual flux orientation control module VFOV *+ The q-axis magnetic flux component is compared with 0, and the comparison error signal is input into the two PI regulators of the virtual flux orientation control module VFOV control loop, and the output is added with the feedforward signal to compensate the cross-coupling term. Get the positive sequence voltage reference value ω represents the actual frequency.

[0164] Among them, the accurate positive sequence voltage reference value is obtained, and the specific process is as follows:

[0165] Positive sequence voltage reference value The virtual flux oriented control module VFOV is used as the output. The two PI regulators in the virtual flux oriented control module VFOV have the same control mode and the same parameter selection method. The transfer function G of the compensation cross-coupling term introduced by the feedforward signal is P (s) is expressed as:

[0166]

[0167] Where, T f represents the time constant of the feedforward signal;

[0168] On the other hand, the transfer function of the PI regulator G PI (s) is expressed as:

[0169]

[0170] In the formula, K PI represents the proportional constant; T PI Represents the PI regulator time constant;

[0171] The open-loop transfer function G(s) can be expressed as:

[0172]

[0173] The closed-loop transfer function G′(s) is expressed as:

[0174]

[0175] Bandwidth value τ i It is expressed as:

[0176]

[0177] By changing the proportionality constant K PI Set to 1p.u. and set the virtual flux orientation control module VFOV time constant T PI Set as the filter constant to make the bandwidth value equal to the rated value ω; when the rated frequency is 50Hz, the bandwidth value is 314.16rad / s; by setting the parameters of the virtual flux orientation control module VFOV, the virtual flux orientation control module VFOC after setting the parameters is used as the positive sequence voltage reference value The output of the positive sequence voltage reference is obtained.

[0178] like Figure 3 As shown, in step S5, the negative sequence voltage reference value is obtained, and the specific process is:

[0179] Based on the positive and negative sequence voltage components in step S3, design the reactive current controller RCC- and the active current controller ACC-;

[0180] When a balanced fault condition occurs, the reactive current controller RCC - By maintaining the negative sequence voltage d-axis component of the converter Calculated as zero When an unbalanced fault condition is detected, the reactive current controller RCC - By comparing the negative sequence reactive current reference The actual negative sequence current The error value between the two is corrected by the PI regulator to obtain Item; Negative sequence voltage d-axis reference value It can be expressed as:

[0181]

[0182] In the formula, Reactive current controller RCC - The output value of the PI regulator during the balanced fault condition; is the output value of the PI regulator during the unbalanced fault condition; Indicates a high-level logical NOT signal, with a value of 0;

[0183] Active current controller ACC - It works in the same way as RCC - Very similarly, when a balanced fault condition occurs, the active current controller ACC - By maintaining the q-axis component of the converter positive sequence voltage Zero based on PI calculation Under unbalanced fault conditions, the active current controller ACC - By comparing the active current with the actual negative sequence current q-axis component i q With active negative sequence current reference Addition based on PI output e q2 ;q-axis negative sequence voltage reference value The calculation is expressed as:

[0184]

[0185] In the formula, and Respectively represent the active current controller ACC - Balanced and unbalanced fault output negative sequence voltage reference value.

[0186] Wherein, in step S6, the converter switch pulse signal is obtained, and the specific process is as follows:

[0187] By accurately determining the positive sequence voltage reference value With negative sequence voltage reference Perform inverse Park transform (dq-abc) to obtain the accurate positive sequence voltage reference value The angle used is the control angle θ, expressed as:

[0188]

[0189] In the formula, cos represents the cosine of the angle; -sin represents the negative value of the sine function; is the positive sequence voltage reference value under the three-phase coordinate axis, is the positive sequence three-phase voltage;

[0190] Negative sequence voltage reference value The inverse Park transform uses an angle of Θ′, where Θ′ = -Θ, to obtain the negative sequence three-phase voltage Next, the three-phase voltages obtained by the positive sequence voltage and the negative sequence voltage are summed, and the converter switching pulse signal is obtained through the space vector pulse width modulation technology.

[0191] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A low voltage ride-through method for a GFM converter based on positive and negative current optimization, characterized in that: The steps include: Step S1: setting the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault conditions and unbalanced fault conditions; Step S2: injecting positive and negative sequence reactive currents into the converter under balanced fault conditions and unbalanced fault conditions, introducing proportional parameters into the injected positive and negative sequence reactive currents, and obtaining the maximum value of the sum of the positive and negative sequence reactive currents of the converter; Step S3: Make the maximum value of the sum of the positive and negative sequence reactive currents of the converter meet the set requirements under the balanced fault condition and the unbalanced fault condition of the converter, synchronously connect the converter with the power grid, align the virtual magnetic flux vector of the converter with the magnetic flux vector of the power grid, generate the same three-phase voltage as the power grid, and extract the positive and negative sequence voltage components in the power grid voltage, the converter terminal voltage and the three-phase voltage by the delayed signal cancellation method; Step S4: Based on the positive and negative sequence voltage components in step S3, design the reactive power controller RPC + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + , through the reactive power controller RPC + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + The converter is detected and processed under balanced fault conditions and unbalanced fault conditions, and the output after the detection and processing is coordinated and matched to obtain a positive sequence voltage reference value. The positive sequence voltage reference value is output by the virtual flux oriented control module VFOC after setting parameters to obtain an accurate positive sequence voltage reference value; Step S5: Based on the positive and negative sequence voltage components in step S3, design the reactive current controller RCC - With active current controller ACC - , through the reactive current controller RCC - With active current controller ACC - Performing detection and processing under balanced fault conditions and unbalanced fault conditions of the converter, coordinating the output after the detection and processing, and obtaining a negative sequence voltage reference value; Step S6: Based on the accurate positive sequence voltage reference value in step S4 and the negative sequence voltage reference value in step S5, inverse Park transform and space vector pulse width modulation technology are used to obtain a converter switching pulse signal.

2. A low voltage ride-through method for a GFM converter based on positive and negative current optimization according to claim 1, characterized in that: In step S1, the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault conditions and unbalanced fault conditions is set. The specific process is as follows: Step S11: Under balanced fault conditions, the converter needs to inject a positive-sequence reactive current proportional to the positive-sequence voltage deviation, and simultaneously provide a positive-sequence active current until the rated current is reached; Under balanced fault conditions, the converter needs to inject / absorb a positive sequence reactive current, denoted as ΔI1; the positive sequence reactive current is proportional to the positive sequence voltage deviation ΔU1; the adjustable constant K1 ranges from 2 to 6; in addition to the positive sequence reactive current, the converter must also provide a positive sequence active current until the rated current value is reached; In addition to the positive sequence reactive current, the converter must also provide positive sequence active current until the rated current value is reached; K1 represents the positive sequence voltage proportional constant defined by the standard; Step S12: Under unbalanced fault conditions, the converter needs to inject positive-sequence active current in addition to the negative-sequence current proportional to the negative-sequence voltage deviation until the current rating is reached; Under unbalanced fault conditions, the converter must also inject / absorb negative sequence current ΔI2, which is proportional to the negative sequence voltage deviation ΔU2; the adjustable constant K2 ranges from 2 to 6; similar to balanced fault conditions, when the sum of positive and negative sequence reactive currents is not enough to reach the rated current value of the converter, positive sequence active current must be injected until it reaches the rated value; K2 represents the negative sequence voltage proportional constant defined by the standard; Step S13: setting the maximum value of the injected total positive sequence reactive current to 1; Step S14: The maximum time course of the converter injecting positive sequence reactive current under balanced / unbalanced fault conditions is: When the grid-side voltage of the converter drops, the response delay time of the converter is t i Not more than 20ms, response time t r It is necessary to inject positive sequence reactive current at the beginning until the required positive sequence reactive current requirement is reached. The delay time and response time should meet t i +t r ≤50ms.

3. The low voltage ride-through method of a GFM converter based on positive and negative current optimization according to claim 2, characterized in that: In step S2, positive and negative sequence reactive currents are injected into the converter under balanced fault conditions and unbalanced fault conditions. The specific process is as follows: Step S21: Based on step S11, under the condition of balanced fault, the converter needs to inject a positive sequence reactive current proportional to the positive sequence voltage deviation, which is expressed as: In the formula, Indicates the positive sequence reactive current reference value under the standard; Δv + Indicates the change in positive sequence voltage; Step S22: Based on step S12, under unbalanced fault conditions, the converter needs to inject a negative sequence current proportional to the negative sequence voltage, expressed as: In the formula, Indicates the active negative sequence current reference value; Indicates the negative sequence reactive current reference value under the mark; Δv - Indicates the negative sequence voltage increment; When the positive sequence reactive current reference value under the standard The negative sequence reactive current reference value is marked If the sum of the positive sequence active current does not reach the rated current value of the converter, the positive sequence active current value must be injected until it reaches the rated current value, which is expressed as In the formula, and Represent the magnitude of positive-sequence and negative-sequence active currents respectively; When the maximum value of the total converter current magnitude appears in the vector and At the moment of the line, only the negative sequence reactive current will be absorbed, and the above formula (4) can be rewritten as: In the formula, Indicates the active positive sequence current reference value; Indicates the positive sequence reactive current reference value under marking; Subtract from the equation get: In the formula, Indicates the active positive sequence current reference value.

4. The low voltage ride-through method of a GFM converter based on positive and negative current optimization according to claim 3, characterized in that: In step S2, the maximum value of the sum of the positive and negative sequence reactive currents of the converter is obtained. The specific process is: A proportional parameter R is introduced, which reduces the positive sequence reactive current reference value proportionally. The negative sequence reactive current reference value is marked The two reference currents are used to obtain the maximum value of the sum of the positive and negative sequence reactive currents of the converter, expressed as: In the formula, Indicates that after introducing the scale parameter and The maximum value of the sum.

5. The low voltage ride-through method of a GFM converter based on positive and negative current optimization according to claim 4, characterized in that: The positive and negative sequence voltage components in step S3 are specifically as follows: The maximum value of the sum of the positive and negative sequence reactive currents of the converter meets the set requirements under the balanced fault condition and the unbalanced fault condition of the converter, and the converter is synchronously connected to the grid. Aligning the grid flux vector To generate the same three-phase voltage as the power grid; to extract the positive and negative sequence components of the grid voltage, converter terminal voltage and three-phase current by using the delayed signal cancellation method; Taking the converter terminal voltage component as an example, the positive and negative sequence voltage components of the converter terminal voltage are expressed as: In the formula, and They represent the positive sequence components of the converter output voltage under the αβ coordinate axis respectively; and They represent the negative sequence components of the converter output voltage on the αβ coordinate axis; v gfm-α and v gfm-β Respectively represent the αβ coordinate axis components; v′ gfm-α and v′ gfm-β Respectively represent the same quantity; Repeat the calculation steps of formulas (8)-(11) above to extract the grid voltage and three-phase current to obtain the positive and negative sequence voltage components of the grid voltage and three-phase current.

6. A GFM converter low voltage ride-through method based on positive and negative current optimization according to claim 5, characterized in that: In step S4, the positive sequence voltage reference value is obtained, and the specific process is as follows: Based on the positive and negative sequence voltage components in step S3, the reactive power controller RPC is designed. + , virtual flux measurement module VFM, virtual flux orientation control module VFOV and active power controller APC + ; Reactive Power Controller RPC + It consists of two sub-rings, according to the reactive power controller RPC + Determine whether a high-level signal is detected under a balanced fault condition and determine whether it is in the reactive power controller RPC + Work on the right or left side; When the balanced fault condition is detected and there is no high level signal, the reactive power controller RPC + The left side of the reactive power controller RPC + There is a proportional control loop on the left side, which sets the reference voltage of the photovoltaic PV controller. Set to 1, by comparing the reference voltage The actual converter output voltage v gfm , generating an error signal; The error signal is passed through the gain factor n q and rated flux Regulation and reactive current limiting module RCL + The output items are summed to obtain the reference flux Reactive current limiting module RCL + The output is the positive sequence reactive current reference value under the standard Mark the negative sequence reactive current reference value and the actual converter output voltage v g f m The sum is It is expressed as; In the formula, It indicates the output item of reactive current limiting module RCL+ after adding the maximum current limit; On the other hand, when an unbalanced fault condition is detected, a high level signal is present in the reactive power controller RPC. + The right side of the control loop calculates the positive sequence reactive current reference during voltage fault. The actual current positive sequence component The resulting deviation signal is processed by the PI regulator to obtain Introducing the positive sequence component of the grid flux Get the reference flux under voltage sag It is expressed as: In the formula, Indicates the output value of the current deviation signal after PI adjustment; Virtual flux orientation control module VFOV reference flux ψ to be sought *+ The calculation method of is as follows, expressed as: In the formula, Indicates a high-level logical NOT signal; S set It is a high level logic signal; Active Power Controller APC + The control angle θ is obtained, which is expressed as: In the formula, ω + Indicates the control angular frequency; ω1′ + represents the output of the loop simulating the SG swing equation; PSS stands for Power System Stabilizer 1,2 Output: Represents the output of the active current limiter ACL+ module; the output of the active current limiter ACL+ module is composed of the active positive sequence current reference value Active negative sequence current reference value and the actual converter output voltage v g f m The sum is obtained; dt represents the time domain integral; Active Power Controller APC + The actual value of active power P + and the active power reference value P *+ The injection of the control angle Θ is realized, and the active power reference value P *+ The expression is; The virtual flux measurement module VFM is responsible for obtaining the converter virtual flux Positive sequence component; positive sequence voltage under αβ coordinate axis of virtual flux measurement module VFM The integrated value is obtained by integrating with the time constant of 2πf, where f is the rated frequency of 50Hz, and passes through a high-pass filter with the cutoff frequency set to 5Hz; the current output by the filter Multiply by the filter impedance L f The output of the multiplication of the filter and the filter impedance is obtained, the output of the multiplication of the filter and the filter impedance is added to the integral processing value, the αβ axis component flux of the virtual flux measurement module VFM is calculated, and the converter virtual flux is obtained. Using Active Power Controller (APC) in Virtual Flux Oriented Control (VFOV) + The calculated control angle Θ is converted into the converter virtual flux calculated in the virtual flux measurement module VFM. The positive sequence component is converted into αβ / dq axis components The d-axis component is compared with the reference flux ψ calculated in the virtual flux orientation control module VFOV *+ The q-axis magnetic flux component is compared with 0, and the comparison error signal is input into the two PI regulators of the virtual flux orientation control module VFOV control loop, and the output is added with the feedforward signal to compensate the cross-coupling term. Get the positive sequence voltage reference value ω represents the actual frequency.

7. A low voltage ride-through method for a GFM converter based on positive and negative current optimization according to claim 6, characterized in that: To obtain an accurate positive sequence voltage reference value, the specific process is as follows: Positive sequence voltage reference value The virtual flux oriented control module VFOV is used as the output. The two PI regulators in the virtual flux oriented control module VFOV have the same control mode and the same parameter selection method. The transfer function G of the compensation cross-coupling term introduced by the feedforward signal is P (s) is expressed as: Where, T f represents the time constant of the feedforward signal; s represents the complex frequency; On the other hand, the transfer function of the PI regulator G PI (s) is expressed as: In the formula, K PI represents the proportional constant; T PI Represents the PI regulator time constant; The open-loop transfer function G(s) can be expressed as: The closed-loop transfer function G′(s) is expressed as: Bandwidth value τ i It is expressed as: By changing the proportionality constant K PI Set to 1 and set the virtual flux orientation control module VFOV time constant T PI Set as the filter constant to make the bandwidth value equal to the rated value ω; when the rated frequency is 50, the bandwidth value is 314.16; by setting the parameters of the virtual flux oriented control module VFOV, the virtual flux oriented control module VFOC after setting the parameters is used as the positive sequence voltage reference value The output of the positive sequence voltage reference is obtained.

8. The low voltage ride-through method of a GFM converter based on positive and negative current optimization according to claim 7, characterized in that: In step S5, the negative sequence voltage reference value is obtained, and the specific process is as follows: Based on the positive and negative sequence voltage components in step S3, design the reactive current controller RCC - With active current controller ACC - ; When a balanced fault condition occurs, the reactive current controller RCC - By maintaining the negative sequence voltage d-axis component of the converter Calculated as zero When an unbalanced fault condition is detected, the reactive current controller RCC - By comparing the negative sequence reactive current reference The actual negative sequence current The error value between the two is corrected by the PI regulator to obtain e d2 Item; Negative sequence voltage d-axis reference value It is expressed as: In the formula, Reactive current controller RCC - The output value of the PI regulator during the balanced fault condition; is the output value of the PI regulator during the unbalanced fault condition; Indicates a high-level logical NOT signal; When a balanced fault condition occurs, the active current controller ACC - By maintaining the q-axis component of the converter positive sequence voltage Zero based on PI calculation Under unbalanced fault conditions, the active current controller ACC - By comparing the active current with the actual negative sequence current q-axis component i q With active negative sequence current reference Addition based on PI output e q2 ;q-axis negative sequence voltage reference value The calculation is expressed as: In the formula, and Respectively represent the active current controller ACC - Balanced and unbalanced fault output negative sequence voltage reference value.

9. A low voltage ride-through method for a GFM converter based on positive and negative current optimization according to claim 8, characterized in that: In step S6, the converter switch pulse signal is obtained, and the specific process is as follows: By accurately determining the positive sequence voltage reference value With negative sequence voltage reference Perform inverse Park transformation to obtain the accurate positive sequence voltage reference value The angle used is the control angle θ, expressed as: In the formula, cos represents the cosine of the angle; -sin represents the negative value of the sine function; are the positive sequence voltage reference values ​​under the three-phase coordinate axis, and are the positive sequence three-phase voltage; Negative sequence voltage reference value The inverse Park transform uses an angle of Θ′, where Θ′ = -Θ, to obtain the negative sequence three-phase voltage Next, the three-phase voltages obtained by the positive sequence voltage and the negative sequence voltage are summed, and the converter switching pulse signal is obtained through the space vector pulse width modulation technology.

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