Network construction type unbalanced fault low-voltage ride-through method and network construction type unbalanced fault low-voltage ride-through device

Abstract

The invention provides a network construction type unbalanced fault low-voltage ride-through method and device, and the method comprises the steps: collecting unbalanced three-phase voltage and three-phase current at a grid connection point of a power grid; the grid-connected point is connected in parallel with a grid-following type converter and a grid-constructing type converter; converting the three-phase voltage and the three-phase current into three groups of balance components by using a positive and negative sequence separation method; eliminating harmonic waves in the three-phase current based on the three-phase balance component; based on the three groups of balance components, generating a reference voltage by using a network-forming converter; performing virtual impedance on the reference voltage by using a virtual impedance current limiting algorithm to obtain an actual expected output voltage; based on the actual expected output voltage, overcurrent caused by a short-circuit fault is suppressed; based on the grid following type converter and the grid constructing type converter, when low-voltage ride-through occurs in the power grid, the grid following type converter is used for reactive compensation, and low-voltage ride-through suppression of the power grid is completed. A grid-connected system can be controlled to generate overcurrent and low-voltage ride-through caused by current harmonics and short-circuit faults.

Description

Technical Field

[0001] The present invention belongs to the field of electric power technology, and specifically relates to a method and device for low-voltage crossing of a grid-type unbalanced fault. Background Art

[0002] At present, in order to promote the upgrading and optimization of the energy industry, the penetration rate of new energy power generation such as photovoltaic power generation and wind power generation is increasing year by year at a very fast rate. These emerging energy generation methods have advantages such as green and environmental protection compared to traditional energy generation methods, but at the same time, their power sources are more uncertain and unstable than traditional energy, making their output power have greater fluctuation characteristics. In the process of connecting these power generation equipment to the power grid in a suitable manner, the grid-connected converter plays a vital role in the transmission and conversion of current and voltage between the single machine and the power grid. Grid-connected converters are mainly divided into the following two types:

[0003] (1) Grid-following converter: It uses a phase-locked loop to synchronize the grid phase and uses a current source to output current, thereby controlling the active or reactive power fed into the grid.

[0004] (2) Grid-type converter: Drawing on the mechanism of synchronous generator, it achieves synchronization by controlling its own output power and controls the converter into voltage source mode.

[0005] At present, most grid-connected transformer systems adopt the grid-following grid-connected mode. However, with the continuous increase in the installed capacity of new energy, the control capability of the power grid system is weak, and the traditional low-voltage ride-through strategy often fails to obtain satisfactory ride-through performance. Summary of the invention

[0006] In order to overcome the above-mentioned deficiencies of the prior art, in a first aspect, the present invention proposes a method for low voltage ride-through of a grid-type unbalanced fault, comprising:

[0007] Collecting unbalanced three-phase voltage and three-phase current at the grid connection point; the grid connection point is connected in parallel with a grid-following converter and a grid-forming converter;

[0008] Using a positive-negative sequence separation method, the three-phase voltage and the three-phase current are converted into three groups of balanced components; based on the three-phase balanced components, harmonics in the three-phase current are eliminated;

[0009] Based on the three groups of balanced components, the grid-type converter is used to generate a reference voltage; using a virtual impedance current limiting algorithm, the reference voltage is subjected to virtual impedance to obtain an actual desired output voltage; based on the actual desired output voltage, overcurrent caused by a short circuit fault is suppressed;

[0010] Based on the grid-following converter and the grid-forming converter, when low voltage ride-through occurs in the power grid, the grid-following converter is used to perform reactive power compensation to complete the low voltage ride-through suppression of the power grid.

[0011] Preferably, the positive-negative sequence separation method is used to convert the three-phase voltage and the three-phase current into three groups of balanced components, including:

[0012] Separating the three-phase voltage by using a positive-negative sequence separation method to obtain three groups of balanced components of the three-phase voltage;

[0013] Separating the three-phase current by using a positive-negative sequence separation method to obtain three groups of balanced components of the three-phase current;

[0014] The three groups of balanced components of the three-phase voltage satisfy the following formula:

[0015]

[0016] In the above formula, E a ,E b ,E c Represents the three balanced components of the three-phase voltage, E p is the amplitude of the positive sequence component of the three-phase voltage after the positive and negative sequence are separated, E n It is the amplitude of the negative sequence component obtained after the positive and negative sequences of the three-phase voltage are separated;

[0017] The three groups of balanced components of the three-phase current satisfy the following formula:

[0018]

[0019] In the above formula, I a ,I b ,I c Represents three sets of balanced components of three-phase current, I p It is the amplitude of the positive sequence component obtained after the positive and negative sequence separation of the three-phase current, I n It is the amplitude of the negative sequence component of the three-phase current obtained after the positive and negative sequence separation of the three-phase current, ω represents the angular frequency, t represents the operation time of the grid connection, θ1 represents the angle of the positive sequence component, and θ2 represents the angle of the negative sequence component.

[0020] Preferably, eliminating harmonics in the three-phase current based on the three-phase balanced components includes:

[0021] The positive sequence components and the negative sequence components in the three groups of balanced components of the three-phase current are respectively converted into the d and q two-phase components of the three-phase current through Clarke-Pike transformation;

[0022] The d and q two-phase components of the three-phase current are processed by a notch filter to obtain positive sequence components of the d and q two-phase currents;

[0023] The positive sequence components of the currents of the two phases d and q are used in the closed-loop control of the grid-following converter or the grid-forming converter to eliminate the current harmonics in the three-phase current.

[0024] Preferably, the generating a reference voltage based on the three groups of balanced components using the grid-type converter comprises:

[0025] After the coordinates of the positive sequence components and the negative sequence components in the three groups of balanced components of the three-phase voltage are changed, they are converted into the d and q two-phase components of the three-phase voltage;

[0026] The d and q two-phase components of the three-phase voltage are processed by a notch filter to obtain positive sequence components of the d and q two-phase voltages;

[0027] Based on the positive sequence components of the voltages of the two phases d and q, the positive sequence components of the voltages of the two phases d and q are used as reference voltage values ​​generated by the grid-connected converter;

[0028] According to the reference voltage value, a reference voltage is generated by using the grid-connected converter.

[0029] Preferably, before using the virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain the actual expected output voltage, the method includes:

[0030] Based on the reference voltage, the positive sequence components of the voltages of the d and q phases are input into a first-order inertia link to form a first-order voltage, and the first-order voltage is introduced into a proportional differential link for calculation to obtain a virtual resistance and a virtual inductive reactance.

[0031] Preferably, the method of using a virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain an actual expected output voltage includes:

[0032] The actual expected output voltage is determined based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter.

[0033] Preferably, the process of calculating the actual expected output voltage based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter satisfies the following formula:

[0034]

[0035] In the above formula: represents the actual expected output voltage of phases d and q, and is the positive sequence component of the voltage of phases d and q, I gdrepresents the current in phase d, I gq represents the current in phase q, R v is the virtual resistance, L v is the virtual inductive reactance, and ω is the angular frequency of the grid-type converter.

[0036] Preferably, suppressing overcurrent caused by a short circuit fault based on the actual expected output voltage includes:

[0037] The actual expected output voltage is used as a reference input of the voltage loop of the grid-type converter, the output of the voltage loop is obtained based on the grid-type converter, the reference current of the current loop of the grid-type converter is calculated based on the output of the voltage loop, and the reference current is used to suppress overcurrent caused by short-circuit faults.

[0038] Preferably, based on the grid-following converter and the grid-forming converter, when low voltage ride-through occurs in the power grid, the grid-following converter is used to perform reactive power compensation to complete low voltage ride-through suppression of the power grid, including:

[0039] Controlling the grid-forming converter and the grid-following converter to share phase information;

[0040] When low voltage ride-through occurs in the power grid, the voltage at the grid connection point is reduced, and the grid-following converter is used to output reactive power for reactive compensation based on shared phase information to increase the voltage at the grid connection point;

[0041] If the grid-type converter can only output reactive power that can suppress part of the low voltage ride through, the grid-type converter is used to perform reactive power compensation for the part of the low voltage ride through that cannot be suppressed by the grid-type converter.

[0042] In a second aspect, a grid-type unbalanced fault low-voltage ride-through device applied for by the present invention comprises:

[0043] A collection module is used to collect unbalanced three-phase voltage and three-phase current at a grid connection point; a grid-following converter and a grid-building converter are connected in parallel at the grid connection point;

[0044] A harmonic elimination module: used to convert the three-phase voltage and the three-phase current into three groups of balanced components by using a positive-negative sequence separation method; and eliminate harmonics in the three-phase current based on the three-phase balanced components;

[0045] An overcurrent suppression module: used to generate a reference voltage based on the three groups of balanced components using the grid-type converter; use a virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain an actual expected output voltage; based on the actual expected output voltage, suppress overcurrent caused by a short circuit fault;

[0046] The grid-following converter is used to perform reactive power compensation when low voltage ride-through occurs in the power grid, thereby suppressing the low voltage ride-through of the power grid.

[0047] In a third aspect, the present invention application further proposes an electronic device, comprising: at least one processor and a memory; the memory and the processor are connected via a bus;

[0048] The memory is used to store one or more programs;

[0049] When the one or more programs are executed by the at least one processor, a grid-type unbalanced fault low voltage ride-through method as described is implemented.

[0050] In a fourth aspect, the present invention application further proposes a readable storage medium having an execution program stored thereon, and when the execution program is executed, a method for low-voltage ride-through of a grid-type unbalanced fault as described above is implemented.

[0051] Compared with the closest prior art, the present invention has the following beneficial effects:

[0052] The present invention discloses a method for low-voltage crossing of a grid-type unbalanced fault. In the method, unbalanced three-phase voltage and three-phase current are collected at a grid connection point. A grid-type converter and a grid-type converter are connected in parallel at the grid connection point. A positive-negative sequence separation method is used to convert the three-phase voltage and the three-phase current into three groups of balanced components. Harmonics in the three-phase current are eliminated based on the three-phase balanced components. A reference voltage is generated based on the three groups of balanced components using a grid-type converter. A virtual impedance current limiting algorithm is used to perform virtual impedance on the reference voltage to obtain an actual expected output voltage. Based on the actual expected output voltage, overcurrent caused by a short-circuit fault is suppressed. Based on the grid-type converter and the grid-type converter, when low-voltage crossing occurs in the grid, reactive compensation is performed using the grid-type converter. The low-voltage crossing suppression of the grid is completed through reactive compensation, and the generation of current harmonics in the grid-connected system, overcurrent caused by short-circuit faults, and low-voltage crossing can be controlled. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 A schematic flow chart of a method for low-voltage ride-through of a network-type unbalanced fault provided by the present invention;

[0054] Figure 2 An architecture diagram of a grid-type unbalanced fault low-voltage ride-through system provided by the present invention;

[0055] Figure 3 The structure diagram of the three-phase voltage component positive and negative sequence separation method provided by the present invention;

[0056] Figure 4 A flowchart of adaptive generation of virtual impedance in the d and q coordinate systems provided by the present invention;

[0057] Figure 5 A control structure diagram of a grid-type converter provided in the present invention;

[0058] Figure 6 The structural diagram after the grid-type converter is introduced provided by the present invention;

[0059] Figure 7 Selecting parameters and simulation structure diagram for simulation verification of the method of the present invention;

[0060] Figure 8 The active power and reactive power response curves of the grid-type converter structure under different voltage drop depths of the method of the present invention are shown;

[0061] Fig. 9 The active power and reactive power response curves of the grid-type converter structure under different voltage drop depths of the method of the present invention are shown;

[0062] Fig.10 It is a graph of the response curves of each component of the grid-type converter of the method of the present invention;

[0063] Fig.11 A schematic diagram of the operation of an electronic device provided by the present invention. DETAILED DESCRIPTION

[0064] The specific implementation methods of the present invention are further described in detail below with reference to the accompanying drawings.

[0065] Embodiment 1:

[0066] like Figure 1 As shown: A method for low voltage ride-through of a grid-type unbalanced fault, comprising:

[0067] Step 1: collecting unbalanced three-phase voltage and three-phase current at a grid connection point; a grid-following converter and a grid-forming converter are connected in parallel at the grid connection point;

[0068] Step 2: using a positive-negative sequence separation method, converting the three-phase voltage and the three-phase current into three groups of balanced components; eliminating harmonics in the three-phase current based on the three-phase balanced components;

[0069] Step 3: Based on the three groups of balanced components, the grid-type converter is used to generate a reference voltage; using a virtual impedance current limiting algorithm, the reference voltage is subjected to virtual impedance to obtain an actual desired output voltage; based on the actual desired output voltage, overcurrent caused by a short circuit fault is suppressed;

[0070] Step 4: Based on the grid-following converter and the grid-forming converter, when low voltage ride-through occurs in the power grid, the grid-following converter is used to perform reactive power compensation to complete low voltage ride-through suppression of the power grid.

[0071] For example, in the above step 1, a grid-following converter and a grid-forming converter can be connected in parallel at the grid connection point of the grid-connected system; after collecting the unbalanced three-phase voltage and three-phase current at the grid connection point, proceed to step 2;

[0072] For example, in the above step 2, the use of the positive and negative sequence separation method to convert the three-phase voltage and the three-phase current in step 1 into three groups of balanced components may include:

[0073] like Figure 2 As shown, the three-phase voltage is separated by the positive and negative sequence separation method, and three groups of balanced components of the three-phase voltage can be obtained;

[0074] The three-phase current is separated by a positive-negative sequence separation method, so that three groups of balanced components of the three-phase current can be obtained;

[0075] The three groups of balanced components of the three-phase voltage obtained satisfy the following formula:

[0076]

[0077] In the above formula, E a ,E b ,E c Represents the three balanced components of the three-phase voltage, E p is the amplitude of the positive sequence component of the three-phase voltage after the positive and negative sequence are separated, E n It is the amplitude of the negative sequence component obtained after the positive and negative sequences of the three-phase voltage are separated;

[0078] The three groups of balanced components of the three-phase current obtained satisfy the following formula:

[0079]

[0080] In the above formula, I a ,I b ,I c Represents three sets of balanced components of three-phase current, I p It is the amplitude of the positive sequence component obtained after the positive and negative sequence separation of the three-phase current, I n It is the amplitude of the negative sequence component of the three-phase current obtained after the positive and negative sequence separation of the three-phase current, ω represents the angular frequency, t represents the operation time of the grid connection, θ1 represents the angle of the positive sequence component, and θ2 represents the angle of the negative sequence component.

[0081] It can be concluded from the formula of three groups of balanced components of three-phase voltage and three groups of balanced components of three-phase current that the three groups of balanced components of three-phase voltage and three-phase current include: positive sequence component, negative sequence component and zero sequence component; the present invention uses the separated positive sequence component and negative sequence component for the subsequent control of the grid-following converter or the grid-forming converter;

[0082] In a possible implementation, the eliminating harmonics in the three-phase current based on the three-phase balanced components includes:

[0083] The positive sequence component and the negative sequence component in the three groups of balanced components of the three-phase current are respectively converted into the d and q two-phase components of the three-phase current through Clarke-Pike transformation;

[0084] The d and q two-phase components of the three-phase current are processed by a notch filter to obtain positive sequence components of the d and q two-phase currents;

[0085] The positive sequence components of the currents of the two phases d and q are used in the closed-loop control of the grid-following converter or the grid-forming converter to eliminate the current harmonics in the three-phase current.

[0086] For example, in the above step 3, based on the three groups of balanced components of the three-phase voltage, generating a reference voltage using the grid-type converter may include:

[0087] After the coordinates of the positive sequence components and the negative sequence components in the three groups of balanced components of the three-phase voltage are changed, they are converted into the d and q two-phase components of the three-phase voltage;

[0088] The d and q two-phase components of the three-phase voltage are processed by a notch filter to obtain positive sequence components of the d and q two-phase voltages;

[0089] Based on the positive sequence components of the voltages of the two phases d and q, the positive sequence components of the voltages of the two phases d and q are used as reference voltage values ​​generated by the grid-connected converter;

[0090] According to the reference voltage value, a reference voltage is generated by using the grid-connected converter.

[0091] According to the above, it can be seen that the three sets of balanced zero-sequence components of the three-phase voltage and three-phase current are expressed as zero vectors in the d, q coordinate system, so the zero-sequence components are ignored;

[0092] The above process of generating the reference voltage using the grid-type converter satisfies the following expression:

[0093]

[0094] E=U n +n(Q ref -Q)

[0095] In the above formula: represents the positive sequence component of the voltage of phase d; E represents the reference voltage generated by the grid-type converter; U n Indicates the rated output voltage of the grid-connected converter, Q ref represents the reference reactive power value, Q represents the reactive power output by the grid-type converter, and n represents the reactive droop coefficient.

[0096] In the above, before using the virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain the actual expected output voltage, the following steps are included:

[0097] Based on the reference voltage, the positive sequence components of the voltages of the d and q phases are input into a first-order inertia link to form a first-order voltage, and the first-order voltage is introduced into a proportional differential link for calculation to obtain a virtual resistance and a virtual inductive reactance.

[0098] In the above, the virtual impedance current limiting algorithm is used to perform virtual impedance on the reference voltage to obtain the actual expected output voltage, including:

[0099] The actual expected output voltage is determined based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter.

[0100] In the above, the process of calculating the actual expected output voltage based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter satisfies the following formula:

[0101]

[0102] In the above formula: represents the actual expected output voltage of phases d and q, and is the positive sequence component of the voltage of phases d and q, I gd represents the current in phase d, I gq represents the current in phase q, R v is the virtual resistance, L v is the virtual inductive reactance, and ω is the angular frequency of the grid-type converter.

[0103] In view of the harmonic interference caused by the negative sequence component under unbalanced fault during the grid-connected low voltage ride-through process, the present invention obtains the actual expected output voltage by adopting virtual impedance, and adopts the actual expected output voltage for subsequent overcurrent suppression.

[0104] An adaptive method is used in the process of generating virtual impedance, so as to effectively use the reference voltage generated by the grid configuration to obtain the actual expected output voltage through the virtual impedance, and the size of the virtual impedance is adaptively generated according to the voltage drop depth. When used for subsequent control, it can improve the applicability of the grid-connected system to different working conditions;

[0105] In the above, suppressing the overcurrent caused by the short circuit fault based on the actual expected output voltage may include:

[0106] The actual expected output voltage is used as a reference input of the voltage loop of the grid-type converter, the output of the voltage loop is obtained based on the grid-type converter, the reference current of the current loop of the grid-type converter is calculated based on the output of the voltage loop, and a dual closed-loop control method of the voltage loop and the current loop is adopted to suppress the overcurrent caused by the reference current and the short-circuit fault.

[0107] Using the actual expected output voltage to suppress overcurrent can ensure steady-state fault current and effectively improve the reliability and safety of the grid-connected system.

[0108] The phase angle and voltage of the grid-forming converter are used to generate a reference voltage, and a power command requirement is set. The reference voltage realizes the command requirement of active power, and the reference current realizes the command requirement of reactive power. The grid-forming converters share phase information with the grid-following converter, thereby allocating all reactive power to the grid-following converter, allowing the grid-following converter to participate in power distribution.

[0109] For example, in the above step 4, based on the grid-following converter and the grid-forming converter, when low voltage ride-through occurs in the power grid, reactive power compensation is performed by using the grid-following converter to complete low voltage ride-through suppression of the power grid, and the method may also include:

[0110] Controlling the grid-forming converter and the grid-following converter to share phase information;

[0111] When low voltage ride-through occurs in the power grid, the voltage at the grid connection point is reduced, and the grid-following converter is used to output reactive power for reactive compensation based on shared phase information to increase the voltage at the grid connection point;

[0112] If the grid-type converter can only output reactive power that can suppress part of the low voltage ride through, the grid-type converter is used to perform reactive power compensation for the part of the low voltage ride through that cannot be suppressed by the grid-type converter.

[0113] When the grid-connected system voltage decreases, the grid-following converter outputs reactive power to increase the grid-connected system voltage;

[0114] When the grid-connected system voltage increases, the grid-following converter absorbs reactive power to reduce the grid-connected system voltage;

[0115] When the grid-following converter outputs reactive power, the grid-following converter may be unable to output sufficient reactive power. At this time, the grid-forming converter compensates for the reactive power that the grid-following converter cannot provide.

[0116] The present invention introduces a grid-following converter at the grid-connected point and connects it in parallel with the grid-connected converter, thereby achieving effective reactive power compensation during the reactive power compensation period and improving the stability of the grid-connected system.

[0117] In summary, the present invention selects three-phase current for closed-loop control of a grid-following converter or a grid-forming converter to eliminate current harmonics in the grid-connected system; the grid-forming converter shares phase information with the grid-following converter, and distributes the reference voltage generated by the grid-forming converter to the grid-following converter to stabilize the voltage in the grid-connected system; the reference voltage is transformed to generate an actual expected output voltage, and the actual expected output voltage is used as a reference input for the voltage loop of the grid-forming converter, and the output of the voltage loop of the grid-forming converter is used to calculate the reference current of the current loop of the grid-following converter, so as to suppress overcurrent caused by short-circuit faults in the grid-connected system; the grid-connected system can be controlled to generate current harmonics, voltage instability, and overcurrent caused by short-circuit faults.

[0118] At the same time, the positive and negative sequence separation-virtual impedance combined with low voltage ride-through design concept proposed in the present invention has good universality, can be applied to the output control design of other types of grid-connected power grids, and can be used in complex and varied scenarios with good flexibility.

[0119] Embodiment 2:

[0120] like Figure 3 As shown: A grid-type unbalanced fault low-voltage ride-through device applied by the present invention comprises:

[0121] A collection module is used to collect unbalanced three-phase voltage and three-phase current at a grid connection point; a grid-following converter and a grid-building converter are connected in parallel at the grid connection point;

[0122] A harmonic elimination module: used to convert the three-phase voltage and the three-phase current into three groups of balanced components by using a positive-negative sequence separation method; and eliminate harmonics in the three-phase current based on the three-phase balanced components;

[0123] An overcurrent suppression module: used to generate a reference voltage based on the three groups of balanced components using the grid-type converter; use a virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain an actual expected output voltage; based on the actual expected output voltage, suppress overcurrent caused by a short circuit fault;

[0124] The grid-following converter is used to perform reactive power compensation when low voltage ride-through occurs in the power grid, thereby suppressing the low voltage ride-through of the power grid.

[0125] Furthermore, the harmonic elimination module includes:

[0126] A three-phase voltage separation submodule, used for separating the three-phase voltage by using a positive-negative sequence separation method to obtain three groups of balanced components of the three-phase voltage;

[0127] A three-phase current separation submodule, used for separating the three-phase current by using a positive-negative sequence separation method to obtain three groups of balanced components of the three-phase current;

[0128] The three groups of balanced components of the three-phase voltage satisfy the following formula:

[0129]

[0130] In the above formula, E a ,E b ,E c Represents the three balanced components of the three-phase voltage, E p is the amplitude of the positive sequence component of the three-phase voltage after the positive and negative sequence are separated, E n It is the amplitude of the negative sequence component obtained after the positive and negative sequences of the three-phase voltage are separated;

[0131] The three groups of balanced components of the three-phase current satisfy the following formula:

[0132]

[0133] In the above formula, I a ,I b ,I c Represents three sets of balanced components of three-phase current, I p It is the amplitude of the positive sequence component obtained after the positive and negative sequence separation of the three-phase current, I n It is the amplitude of the negative sequence component of the three-phase current obtained after the positive and negative sequence separation of the three-phase current, ω represents the angular frequency, t represents the operation time of the grid connection, θ1 represents the angle of the positive sequence component, and θ2 represents the angle of the negative sequence component.

[0134] Furthermore, the harmonic elimination module further includes:

[0135] A current two-phase component conversion submodule, used for converting the positive sequence component and the negative sequence component in the three groups of balanced components of the three-phase current into the d and q two-phase components of the three-phase current through Clarke-Pike transformation respectively;

[0136] The d and q two-phase components of the three-phase current are processed by a notch filter to obtain positive sequence components of the d and q two-phase currents;

[0137] The current harmonic elimination submodule is used to use the positive sequence components of the currents of the d and q phases in the closed-loop control of the grid-following converter or the grid-forming converter to eliminate the current harmonics in the three-phase current.

[0138] Furthermore, the overcurrent suppression module includes:

[0139] A voltage two-phase component conversion submodule is used to convert the positive sequence component and the negative sequence component of the three groups of balanced components of the three-phase voltage into the d and q two-phase components of the three-phase voltage after coordinate change;

[0140] The d and q two-phase components of the three-phase voltage are processed by a notch filter to obtain positive sequence components of the d and q two-phase voltages;

[0141] A reference voltage value generating submodule, for using the positive sequence components of the voltages of the two phases d and q as the reference voltage value generated by the grid-connected converter based on the positive sequence components of the voltages of the two phases d and q;

[0142] The reference voltage generation submodule is used to generate a reference voltage using the grid-type converter according to the reference voltage value.

[0143] Furthermore, the system further comprises:

[0144] The virtual resistance and virtual inductive reactance determination module inputs the positive sequence component of the voltage of the d and q phases into the first-order inertia link based on the reference voltage to form a first-order voltage, and introduces the first-order voltage into the proportional differential link for calculation to obtain the virtual resistance and virtual inductive reactance.

[0145] Furthermore, the overcurrent suppression module further includes:

[0146] The actual expected output voltage determination submodule is used to determine the actual expected output voltage based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter.

[0147] Furthermore, the process of calculating the actual expected output voltage based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter satisfies the following formula:

[0148]

[0149] In the above formula: represents the actual expected output voltage of phases d and q, and is the positive sequence component of the voltage of phases d and q, I gd represents the current in phase d, Igq represents the current in phase q, R v is the virtual resistance, L v is the virtual inductive reactance, and ω is the angular frequency of the grid-type converter.

[0150] Furthermore, the overcurrent suppression module further includes:

[0151] An overcurrent suppression submodule is used to use the actual expected output voltage as a reference input of the voltage loop of the grid-type converter, obtain the output of the voltage loop based on the grid-type converter, calculate the reference current of the current loop of the grid-type converter based on the output of the voltage loop, and use the reference current to suppress overcurrent caused by short-circuit faults.

[0152] Furthermore, the system further comprises:

[0153] A phase information control module, used for controlling the grid-forming converter and the grid-following converter to share phase information;

[0154] A voltage boosting module, used for, when low voltage ride-through occurs in the power grid, the voltage at the grid connection point is reduced, and the grid-following converter is used to output reactive power for reactive compensation based on the shared phase information to increase the voltage at the grid connection point;

[0155] The reactive power compensation module is used to use the grid-forming converter to perform reactive power compensation for the part of low voltage ride-through that the grid-forming converter cannot suppress if the grid-forming converter can only output reactive power that suppresses part of low voltage ride-through.

[0156] Embodiment 3:

[0157] The embodiment of the present invention provides a specific embodiment, a method for low voltage ride-through of a network-type unbalanced fault, comprising:

[0158] Step 1: Determine the positive and negative sequence components of the three-phase voltage according to the three-phase voltage; determine the positive and negative sequence components of the three-phase current according to the three-phase current;

[0159] Step 2: Connecting the grid-following converter and the grid-forming converter in parallel in the grid-connected system, and the grid-forming converter and the grid-following converter share phase information;

[0160] Step 3: Select three-phase current for closed-loop control of a grid-following converter or a grid-forming converter to eliminate current harmonics in the grid-connected system;

[0161] Step 4: Based on the shared phase information between the grid-forming converter and the grid-following converter, the reference voltage generated by the grid-forming converter is distributed to the grid-following converter to stabilize the voltage in the grid-connected system;

[0162] Step 5: Determine the reference voltage according to the positive and negative sequence components of the three-phase voltage, transform the reference voltage to generate the actual expected output voltage, use the actual expected output voltage as the reference input of the voltage loop of the grid-type converter, and use the output of the voltage loop of the grid-type converter as the reference current of the current loop of the grid-type converter to suppress overcurrent caused by short-circuit faults in the grid-connected system.

[0163] In step 1, the positive and negative sequence components of the three-phase voltage are determined according to the three-phase voltage; the positive and negative sequence components of the three-phase current are determined according to the three-phase current, including:

[0164] Based on the three-phase voltage, the positive and negative sequence separation method is used to separate the positive and negative sequence components of the three-phase voltage;

[0165] Based on the three-phase current, the positive and negative sequence separation method is adopted to separate the positive and negative sequence components of the three-phase current.

[0166] The positive and negative sequence separation method is used to separate the positive and negative sequence components of the three-phase voltage, satisfying the following formula:

[0167]

[0168] In formula (1), E a ,E b ,E c Represents the components of the three-phase voltage, E p is the amplitude of the positive sequence component of the three-phase voltage after the positive and negative sequence are separated, E n It is the amplitude of the negative sequence component obtained after the three-phase voltage is separated into positive and negative sequences. θ represents the phase angle of the three-phase voltage, ω represents the angular frequency of the grid-connected converter, t represents the operating time of the grid-connected system, θ1 represents the angle of the positive sequence component, and θ2 represents the angle of the negative sequence component.

[0169] The conversion process of the positive sequence component and the negative sequence component of the d and q phases is as follows:

[0170]

[0171] In formula (2): On the left side of the equation, E d and E q Represent the d and q axis components of the voltage respectively, and t represents time;

[0172] The positive and negative sequence separation method is used to separate the positive and negative sequence components of the three-phase current, satisfying the following formula:

[0173]

[0174] In formula (3), I a ,I b ,I cThe inductor currents of the three-phase currents a, b, and c of the grid-following converter and the grid-forming converter in the three-phase stationary coordinate system, I p It is the amplitude of the positive sequence component obtained after the positive and negative sequence separation of the three-phase current, I n It is the amplitude of the negative sequence component of the three-phase current obtained after the positive and negative sequence of the three-phase current are separated.

[0175] In step 3, if Figure 3 As shown, three-phase current is used in the closed-loop control of the grid-following converter or the grid-forming converter to eliminate the current harmonics in the grid-connected system, including:

[0176] The positive and negative sequence components of the three-phase current are converted into d and q two-phase components V through Clarke-Pike transformation. d 、V q , after the d and q two-phase components are processed by the notch filter, the positive sequence components and negative sequence components of the d and q two-phase currents corresponding to the positive and negative sequence components of the corresponding three-phase currents are obtained;

[0177] like Figure 3 As shown in the figure, the positive and negative sequence components of the three-phase voltage are transformed into d and q two-phase components E through Clarke-Pike transformation. d 、E q After the two-phase components of d and q are processed by the notch filter, the positive and negative sequence components of the corresponding three-phase voltage are obtained. The positive and negative sequence components of the current of the two-phase d and q are u dp 、u qp 、u dn 、u qn ;

[0178] A notch filter is used to separate the three-phase voltage and the three-phase current to obtain the positive and negative sequence components. The function of the notch filter is to filter out the components of the three-phase voltage and the three-phase current in special frequency bands. The negative sequence component of the three-phase current is a harmonic of the second frequency, so it can be filtered out and extracted. Among them, the transfer function of the notch filter is as follows:

[0179]

[0180] In the above formula, F(s) represents the transfer function of the notch filter, ω c Indicates the notch frequency, Q p represents the quality factor, s represents the Laplace differential operator;

[0181] After the d-axis and q-axis components are separated into positive and negative sequences by the notch filter, the d-axis positive and negative sequence components and the q-axis positive and negative sequence components can be obtained respectively. According to the transient power expression in the d-axis and q-axis coordinate systems, after the positive and negative sequences are separated, the expression is as follows:

[0182]

[0183] In the above formula, P a Indicates active power, Q a represents reactive power, the subscript a represents the average value, sin2 represents the double frequency component of sine, and cos2 represents the double frequency component of cosine. dp 、u qp 、u dn 、u qn Respectively represent the positive sequence components of the d and q axis voltages and the negative sequence components of the d and q axis voltages. dp ,i qp ,i dn ,i qn Represent the positive sequence components of the d and q axis currents and the negative sequence components of the d and q axis currents respectively. To eliminate the grid-connected current harmonics, the expected value of the negative sequence component of the grid-connected current is set to 0;

[0184] So we have:

[0185]

[0186] in and They respectively represent the expected values ​​of the negative-sequence components of the d-axis and q-axis inductor currents of the grid-connected converter.

[0187] The three-phase current is selected for transformation to generate the corresponding positive-sequence components and negative-sequence components of the currents of the d and q phases, and the positive-sequence components of the currents of the d and q phases are used in the closed-loop control of the grid-following converter or the grid-forming converter to eliminate the current harmonics in the grid-connected system.

[0188] In step 4:

[0189] The grid-connected converter and the grid-following converter share phase information, so controlling the reactive current of the grid-connected system can control the reactive power of the grid-connected system. The grid-connected converter mainly plays the role of reactive power compensation, so the reference current of the grid-connected converter is designed to be the positive sequence component of the grid-connected reactive current. For the active current control loop, the reactive compensation of the grid-connected converter may cause fluctuations in the DC capacitor voltage. In order to ensure the stability of the grid-connected converter, the DC capacitor voltage error is generated as a reference value through the active current loop. The design process is as follows:

[0190]

[0191] in, and Indicates the positive sequence component reference value of the d and q axis inductor current. K Pdc represents the proportional coefficient in the DC capacitor voltage PI controller, Indicates the reference value of the DC capacitor voltage of the grid-following converter, V dc2 Indicates the true value of the DC capacitor voltage of the grid-following converter, K Idc represents the integral, I gqp The q-axis component of the grid-connected current. The negative sequence loop of the grid-connected converter and the grid-connected converter have the same goal of eliminating grid-connected current harmonics. Their control objectives are as follows:

[0192]

[0193] in and It represents the expected value of the negative sequence component of the d-axis and q-axis inductor current of the grid-following converter.

[0194] In step 5, the reference voltage generation process includes:

[0195] The positive-sequence component amplitude and the negative-sequence component amplitude of the three-phase voltage are transformed into d and q two-phase components after coordinate transformation. The d and q two-phase components are processed by a notch filter to obtain the positive-sequence component and negative-sequence component of the voltage of the d and q two phases corresponding to the positive-sequence component of the three-phase voltage; based on the positive-sequence component and negative-sequence component of the voltage of the d and q two phases, the positive-sequence component of the voltage of the d and q two phases is combined with the phase angle of the grid-type converter to calculate the reference voltage.

[0196] For the positive sequence loop, the reference voltage value generation process is as follows:

[0197]

[0198] In the above formula, θ represents the phase angle, t represents the grid-connected operation time, ω represents the virtual synchronous machine exciter angular frequency, and ω n represents the rotor angular velocity, P m Represents the grid-connected converter output power reference value, P e represents the active power output by the grid-type converter, J represents the rotational angular momentum, D represents the attenuation coefficient, E represents the voltage reference value generated by the grid-type converter, and U n represents the rated output voltage generated by the grid-connected converter, Q ref represents the reference reactive power value, Q represents the reactive power output by the grid-type converter, and n represents the reactive droop coefficient.

[0199] Thus:

[0200]

[0201] in, and Represents the positive and negative sequence components of the d and q axis grid connection point voltages. Formulas (7) and (8) constitute the reference voltage of the positive sequence voltage and current loop.

[0202] In step 5, the reference voltage is transformed to generate the actual desired output voltage, which specifically includes:

[0203] The grid-type converter inputs the positive sequence components of the voltages of the d and q phases corresponding to the positive and negative sequence components of the three-phase voltage into the first-order inertia link to form a first-order voltage. The first-order voltage is introduced into the proportional differential link for calculation to obtain the virtual resistance R in the virtual impedance. v With virtual inductive reactance L v , according to the virtual resistance R v 、Virtual inductive reactance L v and the angular frequency ω of the grid-connected converter, determine Represents the actual expected output voltage of the d and q phases.

[0204] like Figure 4 As shown, the virtual impedance size is adaptively generated according to the voltage drop depth, and a first-order inertia link is designed to prevent overcurrent caused by the sudden withdrawal of the virtual impedance;

[0205] After obtaining the three-phase voltage components, the adaptive virtual impedance is calculated according to the three-phase voltage components. The virtual impedance is used to maintain the stability of the grid-connected system. The adaptive generation method of the virtual impedance in the d and q coordinate systems is as follows: the voltage drop depth is input and after the first-order inertia link is acted on, the proportional-differential link is introduced for calculation to obtain the size of the virtual resistance and virtual inductive reactance in the adaptive virtual impedance. The process is as follows:

[0206]

[0207] In the above formula, T(s) represents the transfer function of the low-pass filter, τ represents the time constant of the low-pass filter, s represents the Laplace differential operator, and R v represents the virtual resistance, L v Indicates the size of the virtual inductive reactance, P rv represents the proportional link coefficient used by the virtual resistor, P lv Denotes the proportional link coefficient used in the virtual inductive reactance, D rv Denotes the differential link coefficient used by the virtual resistor, D lv represents the differential link coefficient used in the virtual inductive reactance, V n Indicates the rated voltage of the power grid, V g represents the real voltage of the power grid, t represents the operation time of the power grid connection, and d is the differential operator.

[0208] After the voltage reference value E and the phase angle value are generated by the virtual synchronous machine, after d and q transformation, the two-phase reference output voltage in the d and q coordinate systems is generated. The two-phase reference output voltage is respectively and Indicates that I gd Indicates the grid current in phase d, I gq Indicates the grid current in phase q. I gd ,I gq After four variables, R v , L v , ω are combined into coefficients to amplify and superimpose the variables respectively. and Generate the expected output voltage corresponding to the output under the adaptive virtual impedance in the d, q coordinate system.

[0209] According to the virtual resistance R v With the virtual inductive reactance L v , and the angular frequency ω of the grid-connected converter determine the desired output voltages corresponding to d and q, satisfying the following formula:

[0210]

[0211] In the above formula: represents the expected output voltage of phases d and q, and is the positive sequence component of the voltage of phases d and q, I gd represents the current in phase d, I gq represents the current in phase q, R v is the virtual resistance, L v is the virtual inductive reactance, and ω is the angular frequency of the grid-type converter.

[0212] The method of the present invention further comprises:

[0213] When the voltage of the grid-connected system decreases, the grid-following converter outputs reactive power to increase the voltage of the grid-connected system;

[0214] When the voltage of the grid-connected system increases, the grid-following converter absorbs reactive power to reduce the voltage of the grid-connected system;

[0215] When the grid-following converter cannot output sufficient reactive power during the process of outputting reactive power, the grid-building converter compensates for the reactive power that cannot be provided by the grid-following converter.

[0216] During the low voltage ride-through period of unbalanced fault in the grid-connected system, such as Figure 5 In the figure, PCC is the grid connection point. The voltage at the PCC grid connection point shown will produce obvious fluctuations. gIndicates the impedance of the power grid system;

[0217] like Figure 6 As shown in the figure, a grid-following converter is introduced to compensate for reactive power. The control strategy of the grid-following converter mainly acts on the reactive power compensation period of low voltage ride-through. The following two power loops are designed for control:

[0218] (1) Active current loop. Control the DC side capacitor voltage within its normal range and basically do not output active power. The active power of the system is mainly output by the grid-type converter structure designed in step 3;

[0219] (2) Reactive current loop. When the grid voltage value is basically constant, the control of reactive current is also the control of reactive power. In the reactive power loop, the reactive current is controlled with the grid-connected reactive current as the reference value.

[0220] In order to verify the effectiveness of the grid-type unbalanced fault low-voltage ride-through method and device proposed in the present invention, numerical simulation is introduced into the grid-type unbalanced fault low-voltage ride-through device and the grid-connected system with load. The parameters selected for the simulation are shown in Table 1 below:

[0221]

[0222] Table 1 Parameters selected for simulation

[0223] V PCCabc Represents the voltage of the grid-connected point. The simulation structure is as follows Figure 7 As shown. Take the expected reference active power as P ref =40kW, the expected reactive power is Q ref =40Var. The grid-connected system suddenly experiences an unbalanced fault (Phase A is grounded) at 0.5s. Where L1 is the inductance in the grid-connected converter, L2 is the inductance in the grid-following converter, and C1 is the capacitor in the grid-connected converter.

[0224] According to a device for running simulation of a grid-type unbalanced fault low-voltage ride-through method of the present invention, Figure 8 , Fig. 9 They are respectively the active power and reactive power response curves of the grid-forming converter and the grid-following converter when the grid-connected system has different drop depths (20%, 50%, 80%) under the action of the device designed by the present invention. Figure 8The power curve of the grid-connected converter output of the method of the present invention. Wherein, (a) is the active power output of the grid-connected system, the horizontal axis represents the response time (unit: second), the vertical axis represents the active power output of the grid-connected system (unit: watt), and pu represents the voltage drop depth; (b) is the reactive power output of the grid-connected converter, the horizontal axis represents the response time (unit: second), the vertical axis represents the reactive power output of the grid-connected system (unit: watt), and pu represents the voltage drop depth. It can be seen that the control structure based on the grid-connected converter effectively suppresses the impact of the voltage drop and improves the stability and anti-interference performance of the grid-connected system. At the same time, when the voltage drop depth increases, the interference to the grid-connected system also increases, and the difficulty of controlling the grid-connected system increases. Fig. 9 The power curve output by the grid-type converter structure of the method of the present invention, wherein (a) is the active power output by the grid-type converter, the horizontal axis represents the response time (unit: seconds), the vertical axis represents the active power output by the grid-connected system (unit: watt), and pu represents the voltage drop depth; (b) is the reactive power output by the grid-type converter, the horizontal axis represents the response time (unit: seconds), the vertical axis represents the reactive power output by the grid-connected system (unit: watt), and pu represents the voltage drop depth. It can be seen that the introduction of the grid-type converter structure effectively compensates for reactive power and effectively achieves the control target. At the same time, when the voltage drop depth increases, the reactive power compensation value required by the grid-connected system increases, and improving the performance of the grid-connected system requires higher control costs.

[0225] Fig.10 Figure 3 is a graph of the response curves of each component of the grid-type converter, where (a) is the frequency response curve of the grid-type converter, the horizontal axis represents the response time (unit: second), the vertical axis represents the angular velocity of the system output (unit: radians per second), and pu represents the voltage drop depth; (b) is the output voltage amplitude response curve of the grid-type converter under different drop depths, the horizontal axis represents the response time (unit: second), the vertical axis represents the amplitude of the system output voltage (unit: volt), and pu represents the voltage drop depth; (c) is the influence curve of the grid-type converter on the output voltage amplitude under 80% of the grid voltage drop depth, the horizontal axis represents the response time (unit: second), the vertical axis represents the amplitude of the system output voltage (unit: volt), and pu represents the voltage drop depth. It can be seen that under the condition of the same voltage drop depth, the introduction of the parallel grid-connected converter can, on the one hand, reduce the absolute value of the output voltage amplitude reduction and optimize the control range of the device of the present invention; on the other hand, it can effectively improve the fluctuation of the output voltage amplitude during the drop and enhance the stability of the grid-connected system.

[0226] When the voltage at the grid connection point drops due to a fault, the converter is required to inject reactive power into the grid system to help the grid restore voltage. The low voltage ride-through capability of the grid-connected converter can support the grid voltage and at the same time ensure the safety and stability of the power system operation to a large extent.

[0227] Based on the above, during the low voltage ride-through of the grid-type converter, the device of the present invention mainly handles the following two goals:

[0228] (1) Voltage drop (reactive power compensation). One of the main features of the grid-connected converter that distinguishes it from the grid-following converter is that it can provide voltage and frequency support, compensate the grid through reactive power, and at the same time ensure the voltage at the grid connection point, thus reducing the impact on the load in the grid-connected system during a fault.

[0229] (2) Current suppression: The moment a fault occurs and the moment it is restored will both bring impact current to the grid-connected system, affecting the safety of the grid-connected system. Therefore, the device of the present invention needs to focus on performance in this aspect.

[0230] The present invention is based on a control method for separating positive and negative sequences and a virtual impedance current limiting technology. First, the positive and negative sequence separation method is used to convert the unbalanced three-phase voltage and current into three groups of balanced components for processing, so that the separated positive and negative sequence components are controlled separately. Secondly, at the same time, a grid-type converter is introduced and connected in parallel with the grid-forming converter to compensate for reactive power during the reactive compensation period of low-voltage crossing to ensure the stability of the grid-connected voltage. Thirdly, the virtual impedance current limiting technology is introduced to use the reference voltage generated by the grid-forming converter to obtain the actual expected output voltage through the virtual impedance to suppress overcurrent caused by faults in the grid-connected system. The present invention is simple in form, can realize grid-forming low-voltage crossing under unbalanced faults, effectively suppress overcurrent and harmonics in the grid-connected system, and effectively improve the stability and safety of the grid-connected system.

[0231] Embodiment 4:

[0232] like Fig.11 As shown, the present invention also provides an electronic device, which may be a computer device, a single-chip device, an intelligent mobile device, etc. The electronic device in this embodiment may include a processor, a memory, a transceiver component, etc. The memory, the processor, and the transceiver component are connected via a bus; the memory may be used to store an execution program, and an exemplary execution program may include instructions; the processor is used to execute the instructions stored in the memory. The memory may also be used to store data, which may be called and / or modified when the instructions are executed.

[0233] The processor may be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components, etc. It is the computing core and control core of the terminal, which is suitable for implementing one or more instructions, specifically suitable for loading and executing one or more instructions in a storage medium to implement the corresponding method flow or corresponding functions, so as to implement the steps of a network-type unbalanced fault low-voltage ride-through method in the above-mentioned embodiment.

[0234] Embodiment 5:

[0235] Based on the same inventive concept, the present invention also provides a readable storage medium, specifically an electronic device readable storage medium (Memory), which is a memory device in the electronic device for storing programs and data. It can be understood that the storage medium here can include both the built-in storage medium in the electronic device and the extended storage medium supported by the electronic device. The storage medium provides a storage space, which stores the operating system of the terminal. In addition, one or more instructions suitable for being loaded and executed by the processor are also stored in the storage space, and these instructions can be one or more execution programs (including program codes). It should be noted that the storage medium here can be a high-speed RAM memory or a non-volatile memory, such as at least one disk memory. The processor loads and executes one or more instructions stored in the storage medium, which can implement the steps of a low-voltage crossing method for a network-type unbalanced fault in the above embodiment.

[0236] Those skilled in the art will appreciate that the embodiments of the present invention application can be provided as methods, systems, or computer program products. Therefore, the present invention application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present invention application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0237] The present invention application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present invention application. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of the processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0238] These computer program instructions may also be stored in a computer readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture including an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0239] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0240] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention application rather than to limit its protection scope. Although the present invention application is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that after reading the present invention application, those skilled in the art can still make various changes, modifications or equivalent substitutions to the specific implementation methods of the application, but these changes, modifications or equivalent substitutions are all within the protection scope of the claims to be approved.

Claims

1. A method for low voltage ride through of grid-type unbalanced fault, characterized in that: include: Collecting unbalanced three-phase voltage and three-phase current at the grid connection point; the grid connection point is connected in parallel with a grid-following converter and a grid-forming converter; Using a positive-negative sequence separation method, the three-phase voltage and the three-phase current are converted into three groups of balanced components; Eliminate harmonics in the three-phase current based on the three-phase balanced components; Based on the three groups of balanced components, the grid-type converter is used to generate a reference voltage; using a virtual impedance current limiting algorithm, the reference voltage is subjected to virtual impedance to obtain an actual desired output voltage; based on the actual desired output voltage, overcurrent caused by a short circuit fault is suppressed; Based on the grid-following converter and the grid-forming converter, when low voltage ride-through occurs in the power grid, the grid-following converter is used to perform reactive power compensation to complete the low voltage ride-through suppression of the power grid.

2. The method according to claim 1, characterized in that The positive-negative sequence separation method is used to convert the three-phase voltage and the three-phase current into three groups of balanced components, including: Separating the three-phase voltage by using a positive-negative sequence separation method to obtain three groups of balanced components of the three-phase voltage; Separating the three-phase current by using a positive-negative sequence separation method to obtain three groups of balanced components of the three-phase current; The three groups of balanced components of the three-phase voltage satisfy the following formula: In the above formula, E a ,E b ,E c Represents the three balanced components of the three-phase voltage, E p is the amplitude of the positive sequence component of the three-phase voltage after the positive and negative sequence are separated, E n It is the amplitude of the negative sequence component obtained after the positive and negative sequences of the three-phase voltage are separated; The three groups of balanced components of the three-phase current satisfy the following formula: In the above formula, I a ,I b ,I c Represents three sets of balanced components of three-phase current, I p It is the amplitude of the positive sequence component obtained after the positive and negative sequence separation of the three-phase current, I n It is the amplitude of the negative sequence component of the three-phase current obtained after the positive and negative sequence separation of the three-phase current, ω represents the angular frequency, t represents the operation time of the grid connection, θ1 represents the angle of the positive sequence component, and θ2 represents the angle of the negative sequence component.

3. The method according to claim 1 or 2, characterized in that: Eliminating harmonics in the three-phase current based on the three-phase balanced components comprises: The positive sequence component and the negative sequence component in the three groups of balanced components of the three-phase current are respectively converted into the d and q two-phase components of the three-phase current through Clarke-Pike transformation; The d and q two-phase components of the three-phase current are processed by a notch filter to obtain positive sequence components of the d and q two-phase currents; The positive sequence components of the currents of the two phases d and q are used in the closed-loop control of the grid-following converter or the grid-forming converter to eliminate the current harmonics in the three-phase current.

4. The method according to claim 1 or 2, characterized in that: The method of generating a reference voltage based on the three groups of balanced components by using the grid-type converter comprises: After the coordinates of the positive sequence components and the negative sequence components in the three groups of balanced components of the three-phase voltage are changed, they are converted into the d and q two-phase components of the three-phase voltage; The d and q two-phase components of the three-phase voltage are processed by a notch filter to obtain positive sequence components of the d and q two-phase voltages; Based on the positive sequence components of the voltages of the two phases d and q, the positive sequence components of the voltages of the two phases d and q are used as reference voltage values ​​generated by the grid-connected converter; According to the reference voltage value, a reference voltage is generated by using the grid-connected converter.

5. The method according to claim 4, characterized in that Before using the virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain the actual expected output voltage, the method includes: Based on the reference voltage, the positive sequence components of the voltages of the d and q phases are input into a first-order inertia link to form a first-order voltage, and the first-order voltage is introduced into a proportional differential link for calculation to obtain a virtual resistance and a virtual inductive reactance.

6. The method according to claim 5, characterized in that The method of using a virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain an actual expected output voltage includes: The actual expected output voltage is determined based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter.

7. The method according to claim 6, characterized in that The process of calculating the actual expected output voltage based on the virtual resistance and the virtual inductive reactance, the positive sequence components of the voltages of the d and q phases, and the angular frequency of the grid-connected converter satisfies the following formula: In the above formula: represents the actual expected output voltage of phases d and q, and is the positive sequence component of the voltage of phases d and q, I gd represents the current in phase d, I gq represents the current in phase q, R v is the virtual resistance, L v is the virtual inductive reactance, and ω is the angular frequency of the grid-type converter.

8. The method according to claim 1, characterized in that The step of suppressing an overcurrent caused by a short circuit fault based on the actual expected output voltage includes: The actual expected output voltage is used as a reference input of the voltage loop of the grid-type converter, the output of the voltage loop is obtained based on the grid-type converter, the reference current of the current loop of the grid-type converter is calculated based on the output of the voltage loop, and the reference current is used to suppress overcurrent caused by short-circuit faults.

9. The method according to claim 1, characterized in that: The method based on the grid-following converter and the grid-forming converter, when low voltage ride-through occurs in the power grid, uses the grid-following converter to perform reactive power compensation to complete low voltage ride-through suppression of the power grid, including: Controlling the grid-forming converter and the grid-following converter to share phase information; When low voltage ride-through occurs in the power grid, the voltage at the grid connection point is reduced, and the grid-following converter is used to output reactive power for reactive compensation based on shared phase information to increase the voltage at the grid connection point; If the grid-type converter can only output reactive power that can suppress part of the low voltage ride through, the grid-type converter is used to perform reactive power compensation for the part of the low voltage ride through that cannot be suppressed by the grid-type converter.

10. A grid-type unbalanced fault low-voltage ride-through device, characterized in that: include: A collection module is used to collect unbalanced three-phase voltage and three-phase current at a grid connection point; a grid-following converter and a grid-building converter are connected in parallel at the grid connection point; Harmonic elimination module: used for converting the three-phase voltage and the three-phase current into three groups of balanced components by using a positive and negative sequence separation method; Eliminate harmonics in the three-phase current based on the three-phase balanced components; Overcurrent suppression module: used to generate a reference voltage based on the three groups of balanced components using the grid-type converter; use a virtual impedance current limiting algorithm to perform virtual impedance on the reference voltage to obtain an actual desired output voltage; Based on the actual expected output voltage, suppressing overcurrent caused by a short circuit fault; The grid-following converter is used to perform reactive power compensation when low voltage ride-through occurs in the power grid, thereby suppressing the low voltage ride-through of the power grid.

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