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

Abstract

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

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of network-constructing new energy station low-voltage fault ride-through control strategies, and particularly relates to a GFM converter low-voltage ride-through method based on positive and negative current optimization. BACKGROUND

[0002] With the extensive access of renewable energy sources (RES) in global power systems, the global power generation pattern has undergone profound changes. This transition 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. Recent studies have shown that certain power systems can achieve nearly 100% renewable energy penetration under certain conditions, indicating that grid-connected converters are gradually taking a dominant 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 new energy power generation and traditional synchronous generator power generation methods, and this transition process also brings many challenges. Due to the lack of inherent ability to provide grid inertia and support grid strength, grid-connected converters make power systems more vulnerable when facing disturbances. In this context, the Grid-Following Converter (GFL) control strategy based on current loops and phase-locked loops to achieve synchronous control has gradually become the mainstream control method for grid-connected converters. However, the stability problem of low-voltage systems and the limitations of GFL converters in the case of power failure, which cannot operate independently or assist in grid recovery, highlight the need for new control strategies, which also promote the development of GFM converters.

[0004] As a voltage source converter, GFM maintains the phase angle and amplitude of the internal voltage during the transient process, thereby simulating the behavior of a synchronous generator, and possesses important characteristics such as inertia response, power oscillation damping, and adaptability to island mode and low system strength conditions. Another important feature is its current limiting strategy, which aims to prevent overcurrent situations during grid disturbances. Due to the low overcurrent bearing capacity of electronic converters (1.1p.u~1.3p.u), GFM converters effectively avoid current overruns through precise current limiting control when facing grid disturbances, ensuring safe operation of the converter under abnormal conditions. Recently, a new GFM control method based on virtual flux orientation has been published, which allows active and reactive currents to be limited in a simpler way to maintain system stability.

[0005] On the other hand, with the high penetration of renewable energy sources, the stability of the power system faces more complex challenges, especially when the grid fails. At this time, although the output current of the grid-connected converter is usually limited, some standards require the converter to quickly inject current in a short time to help maintain voltage stability on the premise of ensuring that the output current does not exceed the rated value. This requirement highlights the important role of grid-connected converters in the process of grid fault recovery, especially in improving the system's ability to resist disturbances and ensuring power quality. It highlights the higher requirements for the dynamic response capability of grid-connected converters in high-penetration renewable energy systems, especially when dealing with grid disturbances and faults. This rapid current injection capability is crucial in the event of grid failure or transient conditions, as it can effectively support the grid to restore normal voltage levels, ensuring the stability and reliability of the system, especially in weak grid or high renewable energy fluctuation environments.

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

[0007] Given the potential of GFM as an alternative to overcome the limitations of GFL converters, there is an urgent need 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 should be able to effectively deal with complex situations such as grid faults and unbalanced loads, ensuring that the injection of current complies with national standards under various operating conditions, ensuring the stability and safety of the system. SUMMARY

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

[0009] To achieve the above-mentioned purpose, the present application provides the following technical solution: A GFM converter low voltage ride through method based on positive and negative current optimization, comprising the following steps:

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

[0011] Step S2: Injecting positive and negative sequence reactive current into the converter under balanced and unbalanced fault conditions, introducing a proportional parameter for the injected positive and negative sequence reactive current, and obtaining the maximum value of the sum of the positive and negative sequence reactive current of the converter;

[0012] Step S3: Making the maximum value of the sum of the positive and negative sequence reactive current of the converter meet the set requirements under balanced and unbalanced fault conditions of the converter, synchronously connecting the converter with the power grid, aligning the virtual flux vector of the converter with the flux vector of the power grid, generating the same three-phase voltage as the power grid, and extracting the positive and negative sequence voltage components in the grid voltage, the converter terminal voltage and the three-phase voltage through the delay signal cancellation method;

[0013] Step S4: Designing a reactive power controller RPC based on the positive and negative sequence voltage components in step S3 + , a virtual flux measurement module VFM, a virtual flux orientation control module VFOV and an active power controller APC + , through the reactive power controller RPC + , the virtual flux measurement module VFM, the virtual flux orientation control module VFOV and the active power controller APC + , detecting and processing the converter under balanced and unbalanced fault conditions, coordinating and cooperating the output after detection and processing, obtaining a positive sequence voltage reference value, and the positive sequence voltage reference value being output by the virtual flux orientation control module VFOC after setting parameters, to obtain an accurate positive sequence voltage reference value;

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

[0015] Step S6: Processing the accurate positive sequence voltage reference value in step S4 and the negative sequence voltage reference value in step S5 by inverse Park transformation and space vector pulse width modulation technology 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 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 positive sequence reactive current proportional to the positive sequence voltage deviation, while providing positive sequence active current until the rated current is reached;

[0018] Under balanced fault condition, 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 between 2 and 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 definition of the positive sequence voltage proportional constant;

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

[0020] Under unbalanced fault condition, the converter must also inject / absorb a negative sequence current ΔI2, which is proportional to the negative sequence voltage deviation ΔU2; the adjustable constant K2 ranges between 2 and 6; as in the balanced fault condition, when the sum of the positive and negative sequence reactive currents is insufficient to reach the rated current value of the converter, a positive sequence active current must be injected until the rated value is reached; K2 represents the standard definition of the negative sequence voltage proportional constant;

[0021] Step S13: The total positive sequence reactive current injected is set to a maximum value of 1;

[0022] Step S14: The maximum time process for the converter to inject a positive sequence reactive current under balanced / unbalanced fault condition is:

[0023] When the grid-side voltage of the converter drops, the response delay time t i of the converter should not exceed 20 ms, and the response time t r of the converter needs to be within the time from the start of the injection of the positive sequence reactive current until the required positive sequence reactive current requirement is met; the delay time and the response time should satisfy t i + t r ≤ 50 ms.

[0024] Further, the specific process for the converter to inject positive and negative sequence reactive currents under balanced fault condition and unbalanced fault condition in step S2 is as follows:

[0025] Step S21: Under balanced fault condition, the converter needs to inject a positive sequence reactive current proportional to the positive sequence voltage deviation based on step S11, denoted as:

[0026]

[0027] In the formula, represents the standard positive sequence reactive current reference value; Δv + represents the positive sequence voltage change amount;

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

[0029]

[0030] wherein, denotes active negative sequence current reference value; denotes negative sequence reactive current reference value under flagging; - denotes negative sequence voltage increment;

[0031] When the standard positive sequence reactive current reference value and the negative sequence reactive current reference value under flagging do not reach the rated current value of the converter, positive sequence active current value must be injected until the rated current value is reached, denoted as

[0032]

[0033] wherein, and denote the modulus of positive sequence and negative sequence active current respectively;

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

[0035]

[0036] wherein, denotes active positive sequence current reference value; denotes positive sequence reactive current reference value under flagging;

[0037] Subtracting from the equation, we get:

[0038]

[0039] wherein, denotes active positive sequence current reference value;

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

[0041] A proportional parameter R is introduced, which proportionally reduces the positive sequence reactive current reference value under flagging and the negative sequence reactive current reference value under flagging , to obtain the maximum value of the sum of positive and negative sequence reactive current of the converter, denoted as:

[0042]

[0043] wherein, represents the maximum value of the sum of positive and negative sequence voltage components. and .

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

[0045] The maximum value of the sum of positive and negative sequence reactive currents of the converter is made to meet the set requirements under balanced fault conditions and unbalanced fault conditions of the converter, the converter is synchronously connected with the power grid, and the same three-phase voltage as the power grid is generated by aligning the virtual flux vector of the converter with the flux vector of the power grid. The positive and negative sequence components in the grid voltage, the converter terminal voltage and the three-phase current are extracted by using the delay 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 represented as:

[0046]

[0047] wherein, and represent the positive sequence components of the converter output voltage in the αβ coordinate axis, respectively; and represent the negative sequence components of the converter output voltage in the αβ coordinate axis, respectively. gfm-α and gfm-β represent the same components, respectively. gfm-α and gfm-β represent the same components, respectively.

[0048] The above formulae (8)-(11) are repeatedly calculated to extract the grid voltage and the three-phase current, and obtain the positive and negative sequence voltage components of the grid voltage and the three-phase current.

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

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

[0051] The reactive power controller RPC + contains two sub-rings. According to the determination of the reactive power controller RPC + whether a high-level signal under balanced fault conditions is detected, it is determined whether to work on the right side or the left side of the reactive power controller RPC + .

[0052] When detecting the presence of a high level signal in the balanced fault condition, the reactive power controller RPC + works on the left side, the reactive power controller RPC + left side of a proportional control loop, the proportional control loop sets the reference voltage of the photovoltaic PV controller, the reference voltage of the photovoltaic PV controller is set to 1, by comparing the reference voltage and the actual converter output voltage v gfm , an error signal is generated; the error signal is adjusted by a gain factor n q and the rated flux and the output of the reactive current limiting module RCL + summed to obtain the reference flux The output of the reactive current limiting module RCL + is obtained by summing the standard lower positive sequence reactive current reference value , the lower negative sequence reactive current reference value and the actual converter output voltage v gfm is expressed as:

[0053]

[0054] In the formula, represents the output of the reactive current limiting module RCL+ after adding the maximum current limiting value;

[0055] On the other hand, when detecting the presence of a high level signal in the unbalanced fault condition, the reactive power controller RPC + works on the right side, in the control loop on the right side, the positive sequence reactive current reference current during voltage fault is calculated and compared with the actual current positive sequence component , the generated deviation signal is processed by a PI regulator to obtain the positive sequence component of the grid flux to obtain the reference flux under the voltage sag condition is expressed as:

[0056]

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

[0058] The calculation method of the virtual flux oriented control module VFOV to be solved reference flux ψ *+ is as follows, expressed as:

[0059] ​

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

[0061] Through the active power controller APC + The control angle Θ is obtained as follows:

[0062]

[0063] In the formula, ω + Indicates the control angular frequency; This represents the loop output simulating the SG oscillation equation; Indicates Power System Stabilizer (PSS) 1,2 The output; This indicates the output of the active current limiter ACL+ module; the output of the active current limiter ACL+ module is determined by the active positive sequence current reference value. Active negative sequence current reference value and the actual converter output voltage v gfm Summing yields the result; dt represents the time-domain integral;

[0064] Active power controller APC + From the actual value of active power P + With active power reference value P *+ The injection enables the output of the control angle Θ, and the active power reference value P. *+ The expression is:

[0065]

[0066] The Virtual Flux Measurement Module (VFM) is responsible for acquiring the converter's virtual flux. Positive sequence component; positive sequence voltage under the αβ coordinate axis of the virtual flux measurement module (VFM). Integrating with the time constant 2πf, the integral value is obtained, where f is the rated frequency of 50Hz. This value is then passed through a high-pass filter with a cutoff frequency of 5Hz; the current output by the filter... Multiply by the filter impedance L f The value is obtained by multiplying the filter by its impedance. This output is then added to the integrated value to calculate the αβ-axis component flux of the virtual flux measurement module (VFM), thus obtaining the converter's virtual flux.

[0067] The active power controller (APC) is used in the virtual flux orientation control module (VFOV). + The calculated control angle Θ is used to calculate the virtual flux of the converter obtained in the Virtual Flux Measurement Module (VFM). The positive-sequence components are converted to αβ / dq axis components. The d-axis component is compared with the reference flux ψ calculated in the virtual flux oriented control module VFOV *+ The q-axis flux component is compared with 0, the comparison error signal is input into two PI regulators of the virtual flux oriented control module VFOV control loop, and the output is added to the feedforward signal compensation cross-coupling term The positive sequence voltage reference value is obtained ω represents the actual frequency.

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

[0069] The positive sequence voltage reference value The virtual flux oriented control module VFOV is taken as the output, the two PI regulators in the virtual flux oriented control module VFOV have the same control mode and consistent parameter selection method; and the compensation cross-coupling term transfer function G P (s) is represented as:

[0070]

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

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

[0076]

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

[0078]

[0079] The bandwidth value τ i is represented as:

[0080]

[0081] By setting the proportional constant K PI to 1 and the virtual flux oriented control module VFOV time constant T PIThe filter constant is set so that the bandwidth value equals the rated value ω; when the rated frequency is 50 Hz, the bandwidth value is 314.16 Hz; by setting the parameters of the virtual flux orientation control module VFOV, the virtual flux orientation control module VFOC with the set parameters is used as the positive sequence voltage reference value. The output is used to obtain an accurate positive sequence voltage reference value.

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

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

[0084] When a balance fault occurs, the reactive current controller RCC - By maintaining the negative sequence voltage d-axis component of the converter Calculate using zero When an unbalanced fault condition is detected, the reactive current controller RCC... - By comparing negative sequence reactive current references With actual negative sequence current The error value between them is obtained by correcting the error value through a PI controller. Item; Negative sequence voltage d-axis reference value Represented as:

[0085]

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

[0087] When a balance fault occurs, the active current controller ACC - By maintaining the q-axis component of the converter's positive sequence voltage Calculation based on PI for zero 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 value Addition based on PI output e q2 q-axis negative sequence voltage reference value The calculation is expressed as:

[0088]

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

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

[0091] By aligning the accurate positive sequence voltage reference value and the negative sequence voltage reference value Perform inverse Park transformation on the accurate positive sequence voltage reference value Use the angle as the control angle Θ, and it is represented as:

[0092]

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

[0094] The negative sequence voltage reference value The inverse Park transformation of the negative sequence voltage reference value Then, 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 prior art, the present application has the following beneficial effects:

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

[0097] Figure 1 The method flowchart of the present application.

[0098] Figure 2 The positive sequence voltage reference value control circuit diagram of the present application.

[0099] Figure 3 The negative sequence voltage reference value control circuit diagram of the present application. DETAILED DESCRIPTION

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

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

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

[0103] Step S3: making the maximum value of the sum of the positive and negative sequence reactive currents of the converter meet the set requirements under balanced fault conditions and unbalanced fault conditions of the converter, synchronously connecting the converter with the power grid, aligning the virtual flux vector of the converter with the flux vector of the power grid, generating the same three-phase voltage as the power grid, and extracting the positive and negative sequence voltage components in the grid voltage, the converter terminal voltage and the three-phase voltage through the delay signal cancellation method;

[0104] Step S4: based on the positive and negative sequence voltage components in step S3, designing a reactive power controller RPC + , a virtual flux measurement module VFM, a virtual flux orientation control module VFOV and an active power controller APC + , through the reactive power controller RPC + , the virtual flux measurement module VFM, the virtual flux orientation control module VFOV and the active power controller APC + detecting and processing the converter under balanced fault conditions and unbalanced fault conditions, coordinating and cooperating the output after the detection and processing, obtaining a positive sequence voltage reference value, and the positive sequence voltage reference value being output by the virtual flux orientation 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, designing a reactive current controller RCC - and an active current controller ACC - , through the reactive current controller RCC - and the active current controller ACC - detecting and processing the converter under balanced fault conditions and unbalanced fault conditions, 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, using inverse Park transformation and space vector pulse width modulation technology for processing, to obtain a converter switching pulse signal.

[0107] Wherein, the step S1 sets the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault condition and unbalanced fault condition, and the specific process is:

[0108] Step S11: Under balanced fault condition, the converter needs to inject positive sequence reactive current proportional to positive sequence voltage deviation, and provide positive sequence active current until reaching rated current;

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

[0110] Step S12: Under unbalanced fault condition, the converter needs to inject positive sequence active current until reaching rated current value, in addition to injecting negative sequence current proportional to negative sequence voltage deviation;

[0111] Under unbalanced fault condition, in addition to the positive sequence reactive current ΔI1 specified above, the converter must also inject / absorb negative sequence current ΔI2, which is proportional to negative sequence voltage deviation ΔU2; similar to K1, adjustable constant K2 ranges between 2 and 6; similar to balanced fault condition, when the sum of positive and negative sequence reactive current is insufficient to reach the rated current value of the converter, positive sequence active current must be injected until reaching rated value; K2 represents the standard definition of negative sequence voltage proportional constant;

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

[0113] Step S14: The maximum time process of positive sequence reactive current injection of the converter under balanced / unbalanced fault condition is:

[0114] When the grid-side voltage of the converter drops, the response delay time t i of the converter should not exceed 20 ms, and the response time t r of the converter needs to start injecting positive sequence reactive current until reaching the required positive sequence reactive current requirement; the delay time and the response time should satisfy t i +t r ≤ 50 ms.

[0115] Wherein, the step S2 sets the injection of positive and negative sequence reactive current of the converter under balanced fault condition and unbalanced fault condition, and the specific process is:

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

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

[0118]

[0119] wherein, denotes the reference value of positive sequence reactive current under standard; Δv + denotes the positive sequence voltage variation, the maximum value is 1 p.u.;

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

[0121]

[0122] wherein, denotes the reference value of active negative sequence current; denotes the reference value of negative sequence reactive current under standard; Δv - denotes the negative sequence voltage increment;

[0123] When the reference value of positive sequence reactive current under standard and the reference value of negative sequence reactive current under standard do 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, denoted as:

[0124]

[0125] wherein, and denote the modulus of positive sequence and negative sequence active current respectively, the unit is 1;

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

[0127]

[0128] wherein, denotes the reference value of active positive sequence current; denotes the reference value of positive sequence reactive current under standard;

[0129] Subtracting from the equation, we get:

[0130]

[0131] wherein, Iqref

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

[0133] When the voltage fault drop is deep, the sum of the positive sequence reactive current reference value and the negative sequence reactive current reference value may exceed the rated current value of the converter; in order to avoid this situation, a proportional parameter R is introduced, which proportionally reduces the positive sequence reactive current reference value and the negative sequence reactive current reference value , so that the maximum value of the sum of the positive and negative sequence reactive currents of the converter is obtained, 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 be satisfied; it is represented as:

[0134]

[0135] In the formula, represents the maximum value of the sum of and after introducing the proportional parameter;

[0136] The research proposes an input-output conversion strategy based on voltage drop; the core of the input-output conversion strategy is that the current limiter used is different from the traditional method, which 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 an overcurrent phenomenon during the transition phase of the fault; in order to prevent equipment damage, the power electronic blocking strategy is started, i.e., the switch is closed for 20 ms, when overcurrent is detected; during this period, it will quickly recover and inject a smooth current response according to the standard; for the output under voltage sag, if overcurrent is detected, the virtual flux direction converter will also stop the switch operation; subsequently, among the three reference current values, the active positive sequence current reference value , the active negative sequence current reference value , and the negative sequence reactive current reference value under the standard will be set to zero, and the reaction time during the fault is set to 30 ms; finally, when the fault is cleared, the virtual flux direction converter returns to the working mode before the fault.

[0137] In step S3, the positive and negative sequence voltage components are obtained, and the specific process is as follows:

[0138] The sum of the positive and negative sequence reactive currents of the converter is maximized to meet the set requirements under balanced fault conditions and unbalanced fault conditions, the converter is synchronously connected with the power grid, and the virtual flux vector of the converter is generated to generate the same three-phase voltage as the power grid; the positive and negative sequence components in the grid voltage, the converter terminal voltage and the three-phase current are extracted by using the delay 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 represented as:

[0139]

[0140] wherein, and respectively represent the positive sequence components of the converter output voltage in the αβ coordinate axis; and respectively represent the negative sequence components of the converter output voltage in the αβ coordinate axis;v gfm-α and gfm-β respectively represent the αβ coordinate axis components;v′ gfm-α and gfm-β respectively represent the same components;

[0141] The above formula (8)-(11) calculation steps are repeated to extract the grid voltage and the three-phase current, and obtain the positive and negative sequence voltage components of the grid voltage and the three-phase current.

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

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

[0144] The reactive power controller RPC + includes two sub-rings, and whether a high-level signal under balanced fault conditions is detected is determined according to the reactive power controller RPC + , to determine whether the reactive power controller RPC + works on the right side or the left side.

[0145] When no high-level signal under balanced fault conditions is detected, the reactive power controller RPC + works on the left side, and the reactive power controller RPC + left side has a proportional control loop, and the photovoltaic PV controller is set in the proportional control loop, and the reference voltage of the photovoltaic PV controller ​is set to 1 by comparing the reference voltage with the actual converter output voltage v gfm , an error signal is generated; the error signal is regulated by a gain factor n q and the rated flux The output of the reactive current limiting module RCL + is summed up to obtain the reference flux The output of the reactive current limiting module RCL + is obtained by summing up the standard positive sequence reactive current reference value the negative sequence reactive current reference value and the actual converter output voltage v gfm is expressed as

[0146]

[0147] In the formula, represents the output of the reactive current limiting module RCL+ after adding the maximum current limiting value;

[0148] On the other hand, in the presence of a high-level signal detected under the unbalanced fault condition, the right side of the reactive power controller RPC + works, in the control loop on the right side, the positive sequence reactive current reference current during voltage fault is calculated by comparing the actual current positive sequence component The deviation signal generated is processed by a PI regulator to obtain the positive sequence component of the grid flux to obtain the reference flux under the voltage sag condition is expressed as

[0149]

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

[0151] Figure 2 The S + of the reactive power controller RPC set in the middle is a high-level logic signal with a value of 1;

[0152] The main innovative feature of the RPC + control strategy is that during the fault period, instead of using the traditional proportional control loop, the grid flux is used as the reference, replacing the rated flux; this can provide more robust and rapid response to the instantaneous changes caused by the grid fault; therefore, the calculation method of the virtual flux oriented control module VFOV to be solved reference flux ψ *+ is as follows, expressed as ​

[0153]

[0154] wherein, represents a high level logic non-signal, taking value 0; S set represents a high level logic signal, taking value 1;

[0155] through the active power controller APC + obtaining the control angle Θ, expressed as:

[0156]

[0157] wherein ω + represents the control angle frequency; represents the loop output of the analog SG swing equation; represents the output of the power system stabilizer PSS 1,2 , which is helpful to compensate the damping of active response when the inertia constant is large and the damping coefficient is low; represents the output of the active current limiter ACL+ module; the output of the active current limiter ACL+ module is obtained by summing the active positive sequence current reference value the 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 similar to the reactive current limiter module RCL + , which cooperates to complete the corresponding current limiting control through the input of the converter terminal voltage and the maximum value of the active current; dt represents the time domain integral;

[0158] The main innovation of the active control loop APC+ lies in the implementation of its method and the operation in the voltage fault; the implementation of the function of the APC+ module depends on the injection of the active power actual value P + and the active power reference value P *+ , and then the output of the control angle Θ, wherein the active power reference value is expressed as:

[0159]

[0160] wherein, Figure 2 D + of the active power controller APC 1,2 represents the damping constant, H 1,2 represents the inertia constant; ω0represents the rated frequency, taking value 50 Hz; s represents the complex frequency domain, which is a form of expression of filter transfer function and has no actual meaning;

[0161] The virtual flux measurement module VFM is responsible for obtaining the virtual flux of the converter Positive sequence component; positive sequence voltage under the αβ coordinate axis of the virtual flux measurement module (VFM). Integrating with the time constant 2πf, the integral value is obtained, where f is the rated frequency of 50Hz. This value is then passed through a high-pass filter with a cutoff frequency of 5Hz; the current output by the filter... Multiply by the filter impedance L f The value is obtained by multiplying the filter by its impedance. This output is then added to the integrated value to calculate the αβ-axis component flux of the virtual flux measurement module (VFM), thus obtaining the converter's virtual flux.

[0162] Figure 2 The virtual flux measurement module VFM's ω c This indicates that the filter cutoff frequency is 5Hz.

[0163] The active power controller (APC) is used in the virtual flux orientation control module (VFOV). + The calculated control angle Θ is used to calculate the virtual flux of the converter obtained in the Virtual Flux Measurement Module (VFM). The positive-sequence components are converted to 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 flux component is compared to 0. The comparison error signal is input to the two PI regulators in the VFOV control loop of the virtual flux orientation control module. The output is then fed forward to compensate for the cross-coupling term. Obtain positive sequence voltage reference value ω represents the actual frequency.

[0164] The specific process for obtaining an accurate positive-sequence voltage reference value is as follows:

[0165] Positive sequence voltage reference value The output is provided by the Virtual Flux Orientation Control (VFOV) module. The two PI controllers within the VFOV module operate in the same manner, with identical parameter selection methods. The transfer function G for the compensation cross-coupling term introduced by the feedforward signal... P (s) is represented as:

[0166]

[0167] In the formula, T f Indicates the time constant of the feedforward signal;

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

[0169]

[0170] where K PI is the proportional constant; T PI is 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] The bandwidth value τ i is expressed as:

[0176]

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

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

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

[0180] When the balanced fault condition occurs, the reactive current controller RCC - calculates by keeping the negative sequence voltage d-axis component of the transformer to be zero When the unbalanced fault condition is detected, the reactive current controller RCC - calculates by comparing the error value between the negative sequence reactive current reference and the actual negative sequence current , and correcting the error value by the PI regulator, to obtain the item; the negative sequence voltage d-axis reference value can be expressed as:

[0181]

[0182] wherein,​ Reactive current controller RCC - The output value of the PI regulator during a balanced fault condition; The output value of the PI controller during the unbalanced fault condition; This indicates a high-level logic NOT signal, with a value of 0.

[0183] Active current controller ACC - Its working method is similar to RCC - Very similar, when a balance fault occurs, the active current controller ACC - By maintaining the q-axis component of the converter's positive sequence voltage Calculation based on PI for zero 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 value 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 These represent the active current controller ACC. - Reference values ​​for negative sequence voltage output during balanced and unbalanced faults.

[0186] In step S6, the converter switching pulse signal is obtained, and the specific process is as follows:

[0187] By using accurate positive sequence voltage reference values With negative sequence voltage reference value Perform the 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; The positive sequence voltage reference value is given under the three-phase coordinate axis, and the positive sequence three-phase voltage is given.

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

[0191] While embodiments of the application have been shown and described, it is to be understood that the application is not limited to these embodiments. Rather, it is the intention that modifications, changes, substitutions and variations be made to the embodiments without departing from the spirit and scope of the application, which is defined by the appended claims and their equivalents.

Claims

1. A GFM converter low voltage ride through method based on positive and negative current optimization, characterized in that, Comprising the following steps: Step S1: setting the injection or absorption of positive and negative sequence reactive current of the converter under balanced fault condition and unbalanced fault condition; Step S2: injecting positive and negative sequence reactive current of the converter under balanced fault condition and unbalanced fault condition, introducing a proportional parameter for the injected positive and negative sequence reactive current, and obtaining the maximum value of the sum of positive and negative sequence reactive current of the converter; Step S3: making the maximum value of the sum of positive and negative sequence reactive current of the converter meet the set requirements under balanced fault condition and unbalanced fault condition, synchronously connecting the converter with the power grid, aligning the virtual flux vector of the converter with the flux vector of the power grid, generating the same three-phase voltage as the power grid, and extracting the positive and negative sequence voltage components in the grid voltage, the converter terminal voltage and the three-phase voltage through the delay signal cancellation method; Step S4: Designing the reactive power controller RPC based on the positive and negative sequence voltage components in step S3 + , the virtual flux measurement module VFM, the virtual flux oriented control module VFOV and the active power controller APC + , through the reactive power controller RPC + , the virtual flux measurement module VFM, the virtual flux oriented control module VFOV and the active power controller APC + The converter is detected and processed under balanced fault conditions and unbalanced fault conditions, the output after the detection and processing is coordinated and matched, the positive sequence voltage reference value is obtained, the positive sequence voltage reference value is output by the virtual flux oriented control module VFOC after the setting parameters, and an accurate positive sequence voltage reference value is obtained. Step S5: Designing the reactive current controller RCC based on the positive and negative sequence voltage components in step S3 - with the active current controller ACC - by the reactive current controller RCC - with the active current controller ACC - Detecting and processing the converter under balanced fault conditions and unbalanced fault conditions, coordinating the output after detection and processing, and obtaining the 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, using inverse Park transformation and space vector pulse width modulation technology for processing, and obtaining the switch pulse signal of the converter.

2. The GFM converter low voltage ride through method 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 condition and unbalanced fault condition is set, and the specific process is as follows: Step S11: under balanced fault condition, the converter needs to inject positive sequence reactive current proportional to the positive sequence voltage deviation, and at the same time provide positive sequence active current until the rated current is reached; Under balanced fault condition, 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, in addition to the positive sequence reactive current, the converter must also provide 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 standard definition of positive sequence voltage proportional constant; Step S12: under unbalanced fault condition, 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 rated current value is reached; Under unbalanced fault condition, 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; the same as the balanced fault condition, when the sum of positive and negative sequence reactive current is not enough to reach the rated current value of the converter, the positive sequence active current must be injected until the rated value is reached; K2 represents the standard definition of negative sequence voltage proportional constant; Step S13: setting the maximum value of the total positive sequence reactive current injection to 1; Step S14: the maximum time process of the positive sequence reactive current injection of the converter under balanced / unbalanced fault condition is as follows: When the grid side voltage of the converter drops, the response delay time t i of the converter should not exceed 20 ms, the response time t r should be within the delay time and the response time should satisfy t i +t r ≤ 50 ms.

3. The GFM converter low voltage ride through method based on positive and negative current optimization according to claim 2, characterized in that: In step S2, the specific process of injecting positive and negative sequence reactive current of the converter under balanced fault condition and unbalanced fault condition is as follows: Step S21: based on step S11, under balanced fault condition, the converter needs to inject positive sequence reactive current proportional to the positive sequence voltage deviation, denoted as: In the formula, denotes the positive sequence reactive current reference value under the standard; Δv + represents the positive sequence voltage variation; Step S22: based on step S12, under unbalanced fault condition, the converter needs to inject negative sequence current proportional to the negative sequence voltage, denoted as: In the formula, denotes the active negative sequence current reference value; denotes the labeled negative sequence reactive current reference value; Δv - represents the negative sequence voltage increment; When the sum of the positive sequence active current reference value and the negative sequence active current reference value under the standard 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, denoted as wherein and | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | When the maximum value of the transformer total current module appears at the time of the vector and line, only the negative sequence reactive current will be absorbed, and the above formula (4) is rewritten as: In the formula, denotes the active positive sequence current reference value; denotes the marked down positive sequence reactive current reference value; Subtracting from the equation gives: In the formula, represents the active positive sequence current reference value.

4. The GFM converter low voltage ride through method based on positive and negative current optimization according to claim 3, characterized in that: The maximum value of the sum of the positive and negative sequence reactive currents of the converter is obtained in step S2, and the specific process is as follows: A proportion parameter R is introduced, which proportionally reduces the positive sequence reactive current reference value under the label and the negative sequence reactive current reference value under the label The two reference currents, the maximum sum of the positive and negative sequence reactive currents of the converter is obtained, which is expressed as: wherein represents the maximum value of the sum of and and 5. The GFM converter low voltage ride through method based on positive and negative current optimization according to claim 4, characterized in that: The positive and negative sequence voltage components are obtained in step S3, and the specific process is as follows: To ensure that the sum of the positive and negative sequence reactive currents of the converter meets the set requirements under both balanced and unbalanced fault conditions, the converter is synchronously connected to the power grid by means of the converter's virtual flux vector. Alignment of 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 of 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 represented as: wherein and denote the positive sequence components of the inverter output voltage in the αβ coordinate axes; and denote the negative sequence components of the inverter output voltage in the αβ coordinate axes;v gfm-α andv gfm-β denote the αβ coordinate components;v′ gfm-α andv′ gfm-β denote the same components; The above formula (8)-(11) is repeatedly calculated to obtain the positive and negative sequence voltage components of the grid voltage and three-phase current.

6. The GFM converter low voltage ride through method based on positive and negative current optimization according to claim 5, characterized in that: The positive sequence voltage reference value is obtained in step S4, and the specific process is as follows: Based on the positive and negative sequence voltage components in step S3, a reactive power controller RPC is designed + , a virtual flux measurement module VFM, a virtual flux oriented control module VFOV and an active power controller APC + ; Reactive power controller RPC + comprises two subrings, according to the reactive power controller RPC + determines whether a high level signal in a balanced fault condition is detected, determining whether the reactive power controller RPC + works on the right or on the left; When detecting that there is no high level signal under the balanced fault condition, the reactive power controller RPC + works on the left side, and the reactive power controller RPC + left side exists a proportional control loop, the proportional control loop sets the photovoltaic PV controller, the reference voltage of the photovoltaic PV controller is set to 1, and an error signal is generated by comparing the reference voltage with the actual converter output voltage v gfm ; The error signal is multiplied by a gain factor n q and the rated flux The regulating and reactive current limiting module RCL + The output terms are summed to obtain the reference flux The reactive current limiting module RCL + The output is given by the standard positive sequence reactive current reference value The negative sequence reactive current reference value is denoted by and the actual converter output voltage v g f m The sum is obtained is denoted by In the formula, represents the output item of the reactive current limiting module RCL+ after adding the maximum current limiting. On the other hand, in the presence of high level signals, indicative of unbalanced fault conditions, the reactive power controller RPC + works on the right side, in the control loop on the right, the positive sequence reactive current reference during voltage fault is calculated and compared with the actual positive sequence component of the current The resulting deviation signal is processed through a PI regulator, obtaining the positive sequence component of the magnetic flux injected into the grid obtaining the reference magnetic flux in the presence of voltage sag is expressed as: In the formula, represents the output value of the PI regulation of the current deviation signal; Virtual flux oriented control module VFOV reference flux ψ to be determined *+ is calculated as follows, expressed as: wherein represents a high level logic non-signal; S set is a high level logic signal; By means of an active power controller APC + The control angle Θ is obtained, expressed as: where ω + denotes the control angular frequency; ω1′ + denotes the loop output simulating the SG swing equation; denotes the output of the power system stabilizer PSS 1,2 ; denotes the output of the active current limiter ACL+ module; the output of the active current limiter ACL+ module is derived from the active positive-sequence current reference value the active negative-sequence current reference value and the actual converter output voltage v g f m ; dt denotes the time-domain integration; Active power controller APC + by the active power actual value P + with the injection of the active power reference value P *+ the output of the control angle Θ, the active power reference value P *+ is given by the equation The Virtual Flux Measurement Module (VFM) is responsible for acquiring the converter's virtual flux. Positive sequence component; positive sequence voltage under the αβ coordinate axis of the virtual flux measurement module (VFM). Integrating with the time constant 2πf, the integral value is obtained, where f is the rated frequency of 50Hz. This value is then passed through a high-pass filter with a cutoff frequency of 5Hz; the current output by the filter... Multiply by the filter impedance L f The value is obtained by multiplying the filter by its impedance. This output is then added to the integrated value to calculate the αβ-axis component flux of the virtual flux measurement module (VFM), thus obtaining the converter's virtual flux. Active power controller APC is used in virtual flux oriented control module VFOV + The control angle Θ is calculated, and the calculated virtual flux of the transformer in the virtual flux measurement module VFM is converted into αβ / dq-axis components The positive sequence components are converted into αβ / dq-axis components The d-axis components are compared with the calculated reference flux ψ in the virtual flux oriented control module VFOV, and the q-axis flux components are compared with 0 *+ The comparison error signals are input into two PI regulators of the control loop of the virtual flux oriented control module VFOV, and the output quantities are added to the feedforward signal compensation cross-coupling terms The positive sequence voltage reference value is obtained ω represents the actual frequency.

7. The GFM converter low voltage ride through method based on positive and negative current optimization according to claim 6, characterized in that: The accurate positive sequence voltage reference value is obtained, and the specific process is as follows: Positive sequence voltage reference value The virtual flux oriented control module VFOV outputs two PI regulators with the same control mode and consistent parameter selection method. The compensation cross-coupling term transfer function G P (s) is represented as: In the formula, T f denotes the feedforward signal time constant; s denotes the complex frequency; On the other hand, the transfer function G of the PI regulator PI (s) is represented as: where K PI represents a proportionality constant; T PI represents a PI regulator time constant; The open-loop transfer function G(s) can be represented as: The closed-loop transfer function G'(s) is represented as: Bandwidth value τ i is represented as: By setting the proportional constant K PI to 1 and the virtual flux oriented control module VFOV time constant T PI to the filter constant, the bandwidth value is 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 taken as the output of the positive sequence voltage reference value to obtain the accurate positive sequence voltage reference value.

8. The GFM converter low voltage ride through method based on positive and negative current optimization according to claim 7, characterized in that: The negative sequence voltage reference value is obtained in step S5, and the specific process is as follows: Based on the positive and negative sequence voltage components in step S3, a reactive current controller RCC is designed - with the active current controller ACC - ; When an unbalanced fault condition occurs, the reactive current controller RCC - by keeping the negative sequence voltage d-axis component of the converter to zero When an unbalanced fault condition is detected, the reactive current controller RCC - by comparing the error value between the negative sequence reactive current reference and the actual negative sequence current and correcting the error value by a PI regulator, obtaining e d2 term; the negative sequence voltage d-axis reference value is expressed as: wherein RCC is a reactive current controller - PI regulator output value during a balanced fault condition; PI regulator output value during an unbalanced fault condition; represents a high level logic NOT signal; When an unbalanced fault condition occurs, the active current controller ACC - by keeping the q-axis component of the converter positive sequence voltage to zero based on a PI calculation When a balanced fault condition occurs, the active current controller ACC - by comparing the active current with the actual negative sequence current q-axis component i q with the active negative sequence current reference value plus a PI output e q2 ; the q-axis negative sequence voltage reference value is calculated as In the formula, with respectively represent the active current controller ACC - Balancing and unbalancing fault output negative sequence voltage reference value.

9. The GFM converter low voltage ride through method based on positive and negative current optimization according to claim 8, characterized in that: The converter switching pulse signal is obtained in step S6, and the specific process is as follows: The accurate positive sequence voltage reference value is aligned with the negative sequence voltage reference value by performing an inverse Park's transformation on the accurate positive sequence voltage reference value The accurate positive sequence voltage reference value is aligned with the negative sequence voltage reference value by performing an inverse Park's transformation on the accurate positive sequence voltage reference value The accurate positive sequence voltage reference value is aligned with the negative sequence voltage reference value by performing an inverse Park's transformation on the accurate positive sequence voltage reference value Using the angle as the control angle Θ, is expressed as: In the formula, cos represents the cosine value of an angle; -sin represents the negative value of a sine function; respectively, are positive sequence voltage reference values under three-phase coordinate axes, and V1, V2, and V3 are positive sequence three-phase voltages. Negative sequence voltage reference value The inverse Park transformation uses an angle Θ' where Θ' = -Θ to obtain the negative sequence three-phase voltage Next, the positive sequence voltage and the negative sequence voltage obtained three-phase voltage are summed, and the converter switching pulse signal is obtained by the space vector pulse width modulation technology.

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