Active support method and device for grid-connected system under power grid fault and storage medium

Abstract

The invention relates to an active support method and device for a grid-connected system under a power grid fault and a storage medium, and belongs to the technical field of smart power grids. According to the invention, when the power grid generates the symmetric fault, the optimal short-circuit current control point is utilized to re-assign the d-axis current reference value and the q-axis current reference value, the short-circuit current response characteristic of the receiving-end converter station to the power grid fault is changed, and the problems that the sea wind-flexible direct current grid-connected system cannot provide voltage support for the power grid and cannot provide voltage support for the power grid are effectively solved. Active power can be continuously output to the power grid, and the problems of frequency stability and synchronous stability are caused; when the power grid generates an asymmetric fault, the converter station actively absorbs negative sequence current to provide active support for the power grid, the unbalance degree of a fault point is reduced, and meanwhile, the positive sequence voltage support capability of the power grid is reserved; the short-circuit current bearing capacity of the flexible DC converter station is improved, and the power grid supporting capacity of the sea wind-flexible DC grid-connected system is improved on the premise that the fault ride-through performance is guaranteed.

Description

Technical Field

[0001] The present application belongs to the field of smart grid technology, and specifically relates to a method, device and storage medium for active support of a grid-connected system under grid fault conditions. Background Art

[0002] With the high penetration of new energy sources such as wind power and photovoltaics, the overall inertia of the power grid has decreased, seriously affecting the frequency stability of the power grid operation. In order to improve the frequency stability of the power grid, the wind power-flexible direct current grid-connected system needs to be able to respond to changes in the grid frequency and provide active power support.

[0003] The wind power-flexible DC system can extract energy from the DC bus capacitor or wind turbine to provide frequency response for the grid. Since the DC bus capacitor is too small, its ability to damp grid frequency fluctuations is very weak. Some literature proposes to enhance the frequency response capability of the flexible DC system by increasing the DC bus capacitor, but this increases the construction cost. From a cost perspective, providing grid frequency response by wind turbines is the most economical and reliable strategy. Wind turbines can increase or release the kinetic energy in the blades and rotors through variable speed operation to achieve inertia response to the grid. When necessary, they can also achieve primary frequency regulation through active standby. However, when flexible DC transmission is used to send power out to the grid, Due to the isolation effect of the flexible DC system, wind turbines cannot sense the grid frequency, and the grid frequency needs to be transmitted to the wind turbines; some literature also proposes that the grid frequency is transmitted to the wind turbines through communication to assist the wind farm in grid frequency response; some literature also proposes using DC voltage amplitude instead of communication means to transmit grid frequency information to the sending-end converter, and the sending-end converter extracts the grid frequency information from the DC voltage and maps it to the wind farm AC frequency; however, the above strategies are all improved on the basis of current source control, relying on the phase-locked loop to detect the grid frequency, which is a non-autonomous response and is prone to induce stability problems under weak grid conditions.

[0004] There is a certain degree of conflict between the fault ride-through of the offshore wind-flexible DC grid-connected system and the grid support. If the stability of the fault ride-through is to be enhanced, the support capacity for the grid will be weakened. If effective support is to be provided to the grid, the performance of the fault ride-through will be affected. However, the overcurrent capacity of power electronic devices is far inferior to that of synchronous generators of the same capacity. How to improve the grid support capacity of the offshore wind-flexible DC grid-connected system while ensuring the fault ride-through performance is one of the important issues that need to be urgently addressed in the current "double-high" power system. Summary of the Invention

[0005] To this end, the present application provides a method, device and storage medium for active support of a grid-connected system under a power grid fault, so as to solve the problem in the prior art of the conflict between fault ride-through and grid support of the sea wind-flexible direct current grid-connected system to a certain extent, and how to improve the grid support capability of the sea wind-flexible direct current grid-connected system while ensuring the fault ride-through performance.

[0006] According to a first aspect of an embodiment of the present invention, a method for actively supporting a grid-connected system under a grid fault is provided, the method comprising:

[0007] Establishing an equivalent circuit of the receiving-end converter station and the AC power grid under a power grid fault, obtaining a phase diagram of the voltage and current at the fault point based on the equivalent circuit, and obtaining an optimal short-circuit current control point based on the phase diagram;

[0008] Determine the type of fault in the power grid;

[0009] If the fault type is a symmetrical fault, shielding the DC voltage and reactive power control loops, and reassigning the d-axis and q-axis current reference values of the current loop according to the optimal short-circuit current control point so that the output current meets the preset first standard;

[0010] If the fault type is an asymmetric fault, the positive-sequence and negative-sequence current of the receiving-end converter station are separated, and a positive-sequence current reference value and a negative-sequence current reference value are respectively obtained according to the separated positive-sequence component and negative-sequence component; the positive-sequence current reference value and the negative-sequence current reference value are reassigned according to the optimal short-circuit current control point, and the positive-sequence current reference value and the negative-sequence current reference value are reassigned at the optimal short-circuit current control point according to a preset second standard;

[0011] The reassigned current reference value is used as the initial given value of the current. The initial given value of the current passes through a symmetrical limiting link to obtain a final given value of the positive sequence current, and the final given value of the positive sequence current is used as the output current.

[0012] Preferably,

[0013] The obtaining of the optimal short-circuit current control point according to the phase diagram includes:

[0014] Obtaining the position where the fault point voltage is the highest according to the phase diagram, and taking the position where the fault point voltage is the highest as the optimal short-circuit current control point;

[0015] The obtaining of the position of the highest fault point voltage according to the phase diagram includes:

[0016] The point in the phase diagram where the short-circuit current output by the converter is in phase with the short-circuit current of the power grid is obtained. The point where the phase is the same is the position where the fault point voltage is the highest.

[0017] Preferably,

[0018] The output current meeting the preset first standard includes:

[0019] Obtaining the dynamic reactive current increment injected by the wind farm in the output current, setting the wind farm dynamic reactive current proportional coefficient, and obtaining the per-unit voltage of the wind farm grid connection point and the wind farm rated current;

[0020] The dynamic reactive current increment injected by the wind farm is equal to the difference between the dynamic reactive current proportional coefficient of the wind farm multiplied by a preset parameter minus the per-unit value of the voltage at the wind farm grid connection point, multiplied by the rated current of the wind farm.

[0021] Preferably,

[0022] The output current meeting the preset first standard also includes:

[0023] During a voltage drop in the grid-connected system, the reactive current output by the wind farm to the power system is the sum of the reactive current output by the system during normal operation before the voltage drop and the dynamic reactive current increment injected by the wind farm;

[0024] The maximum output capacity of the wind farm reactive current is not less than a preset multiple of the wind farm rated current.

[0025] Preferably,

[0026] The output current meeting the preset first standard also includes:

[0027] When the grid connection point voltage of the grid-connected system begins to drop, the rise time of the reactive current output by the wind farm to the power system is no longer than a preset first time period;

[0028] When the grid connection point voltage of the grid-connected system recovers to a preset percentage of the nominal voltage, the wind farm exits the dynamic reactive current increment within a preset second time period.

[0029] Preferably,

[0030] The preset second standard includes:

[0031] When the positive sequence component of the grid connection point voltage of the grid-connected system is within a preset percentage interval of the nominal voltage, the wind farm injects a positive sequence dynamic reactive current into the grid to support the recovery of the positive sequence voltage, and absorbs a negative sequence dynamic reactive current from the grid to suppress the increase of the negative sequence voltage;

[0032] When the positive sequence component of the grid connection point voltage of the grid-connected system is less than a preset percentage interval of the nominal voltage, the wind farm injects positive sequence dynamic reactive current into the grid and absorbs negative sequence dynamic reactive current from the grid;

[0033] The injected positive-sequence dynamic reactive current and the absorbed negative-sequence dynamic reactive current respectively satisfy a preset magnitude relationship.

[0034] Preferably,

[0035] The preset second standard also includes:

[0036] During a voltage sag in the grid-connected system, the positive-sequence reactive current output by the wind farm to the power system is the sum of the reactive current output before the voltage sag and the positive-sequence dynamic reactive current increment. The maximum output capacity of the wind farm's reactive current is not less than a preset multiple of the wind farm's rated current. The maximum output capacity of the wind farm's reactive current is controlled by the positive-sequence dynamic reactive current and the negative-sequence dynamic reactive current.

[0037] Preferably,

[0038] If the fault type is an asymmetric fault, performing positive and negative sequence separation on the short-circuit current of the receiving-end converter station includes:

[0039] The two output closed-loop transfer functions of the second-order generalized integrator are obtained, and the two outputs of the second-order generalized integrator are subjected to algebraic operation to obtain the positive-sequence component and the negative-sequence component of the AC side voltage of the receiving converter station.

[0040] Preferably,

[0041] The step of respectively obtaining a positive-sequence current reference value and a negative-sequence current reference value according to the separated positive-sequence component and negative-sequence component comprises:

[0042] Constructing an MMC average value model under an asymmetric fault AC power grid, and obtaining expressions for AC side voltage and current according to the MMC average value model;

[0043] The dynamic equation of the AC side current is constructed using Kirchhoff's voltage law and superposition theorem;

[0044] Transforming the dynamic equation of the AC side current into a two-phase static αβ coordinate system, and obtaining the positive and negative sequence αβ axis components of the AC side according to the positive sequence component and the negative sequence component;

[0045] Transforming the dynamic equation of the AC side current into a dual synchronous rotating dq coordinate system to obtain the positive and negative sequence dq axis components of the AC side respectively;

[0046] Obtaining space vector expressions of the voltage and current on the AC side according to the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, and the positive and negative sequence dq axis components;

[0047] Obtain the expressions of instantaneous active and reactive power of asymmetric AC grid according to instantaneous power theory;

[0048] Substituting the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, the positive and negative sequence dq axis components, and the space vector expressions of the voltage and current on the AC side into the expressions of the instantaneous active and reactive power of the asymmetric AC power grid to obtain the instantaneous power expression of the AC power grid;

[0049] A positive sequence current reference value and a negative sequence current reference value are obtained according to the instantaneous power expression of the AC power grid.

[0050] Preferably,

[0051] If the final given value of the positive sequence current exceeds the tolerance value of the device, the receiving-end converter proportionally reduces the final given value of the positive sequence current so that the final given value of the positive sequence current reaches the tolerance value of the device.

[0052] According to a second aspect of an embodiment of the present invention, there is provided an active support device for a grid-connected system under a grid fault, the device comprising:

[0053] Optimal short-circuit current control point acquisition module: used to establish an equivalent circuit of the receiving-end converter station and the AC power grid under a power grid fault, obtain a phase diagram of the voltage and current at the fault point based on the equivalent circuit, and obtain the optimal short-circuit current control point based on the phase diagram;

[0054] Fault type determination module: used to determine the fault type of the power grid;

[0055] Symmetrical fault assignment module: used for shielding the DC voltage and reactive power control loops if the fault type is a symmetrical fault, and reassigning the d-axis and q-axis current reference values of the current loop according to the optimal short-circuit current control point so that the output current meets the preset first standard;

[0056] an asymmetric fault assignment module configured to, if the fault type is an asymmetric fault, separate the positive-sequence and negative-sequence current of the receiving-end converter station, obtain a positive-sequence current reference value and a negative-sequence current reference value according to the separated positive-sequence component and negative-sequence component, respectively; reassign the positive-sequence current reference value and the negative-sequence current reference value according to the optimal short-circuit current control point, and reassign the positive-sequence current reference value and the negative-sequence current reference value at the optimal short-circuit current control point according to a preset second standard;

[0057] Final given value acquisition module: used to use the reassigned current reference value as the initial given value of the current, the initial given value of the current passes through a symmetrical limiting link, obtain the final given value of the positive sequence current, and use the final given value of the positive sequence current as the output current.

[0058] According to a third aspect of an embodiment of the present invention, a storage medium is provided, wherein the storage medium stores a computer program, and when the computer program is executed by a host controller, each step in the above method is implemented.

[0059] The technical solutions provided by the embodiments of the present invention may have the following beneficial effects:

[0060] When a symmetrical fault occurs in the power grid, the present application uses the optimal short-circuit current control point to reassign the d-axis and q-axis current reference values, thereby changing the short-circuit current response characteristics of the receiving-end converter station to the power grid fault, outputting more q-axis current to support the power grid voltage, and at the same time will not reduce the active output capacity of the receiving-end converter station, increase the pressure on the flexible DC system unloading device and the wind turbine unloading device, and can effectively solve the problem that the sea wind-flexible DC grid-connected system is not only unable to provide voltage support for the power grid, but also continues to output active power to the power grid, which easily causes the synchronous generator rotor to accelerate, causing frequency stability and synchronization stability problems; when an asymmetrical fault occurs in the power grid, the converter station actively absorbs the negative sequence current to provide active support for the power grid, reduce the imbalance of the fault point, and retain the positive sequence voltage support capability of the power grid; the present application improves the short-circuit current carrying capacity of the flexible DC converter station, and on the premise of ensuring fault ride-through performance, improves the power grid support capability of the sea wind-flexible DC grid-connected system, and also has good power grid voltage support performance under weak power grids.

[0061] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0063] Figure 1 This is a flow chart showing a method for actively supporting a grid-connected system under a power grid fault according to an exemplary embodiment;

[0064] Figure 2 is a schematic diagram of an equivalent circuit of a receiving-end converter station and an AC power grid under a power grid fault according to an exemplary embodiment;

[0065] Figure 3 is a phase diagram of voltage and current at a fault point according to an exemplary embodiment;

[0066] Figure 4is a structural block diagram of a second-order generalized integrator according to an exemplary embodiment;

[0067] Figure 5 is a schematic diagram of an MMC average value model under an asymmetric AC power grid according to an exemplary embodiment;

[0068] Figure 6 is a system schematic diagram showing an active support device for a grid-connected system under a power grid fault according to an exemplary embodiment;

[0069] In the attached figure: 1-optimal short-circuit current control point acquisition module, 2-fault type determination module, 3-symmetrical fault assignment module, 4-asymmetrical fault assignment module, 5-final given value acquisition module. DETAILED DESCRIPTION

[0070] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions of this application will be described in detail below. Obviously, the embodiments described are only some of the embodiments of this application, rather than all of them. Based on the embodiments in this application, all other implementation methods obtained by ordinary technicians in this field without making any creative work are within the scope of protection of this application.

[0071] Example 1

[0072] Figure 1 FIG. 1 is a flow chart showing a method for actively supporting a grid-connected system under a grid fault according to an exemplary embodiment. Figure 1 As shown, the method includes:

[0073] S1, establishing an equivalent circuit of the receiving-end converter station and the AC power grid under a power grid fault, obtaining a phase diagram of the voltage and current at the fault point based on the equivalent circuit, and obtaining an optimal short-circuit current control point based on the phase diagram;

[0074] S2, determine the fault type of the power grid;

[0075] S3, if the fault type is a symmetrical fault, shielding the DC voltage and reactive power control loops, and reassigning the d-axis and q-axis current reference values of the current loop according to the optimal short-circuit current control point so that the output current meets the preset first standard;

[0076] S4, if the fault type is an asymmetric fault, performing positive-sequence and negative-sequence separation on the short-circuit current of the receiving-end converter station, obtaining a positive-sequence current reference value and a negative-sequence current reference value according to the separated positive-sequence component and negative-sequence component, respectively; reassigning the positive-sequence current reference value and the negative-sequence current reference value according to the optimal short-circuit current control point, and reassigning the positive-sequence current reference value and the negative-sequence current reference value at the optimal short-circuit current control point according to a preset second standard;

[0077] S5, using the reassigned current reference value as an initial given current value, passing the initial given current value through a symmetrical amplitude limiting link to obtain a final given positive sequence current value, and using the final given positive sequence current value as the output current;

[0078] It can be understood that, first, according to the equivalent circuit of the receiving-end converter station and the AC power grid under the power grid fault, as shown in the attached figure, Figure 2 As shown in the figure, draw the phase diagram of the voltage and current at the fault point according to the equivalent circuit, as shown in the attached figure. Figure 3 As shown in the figure, the optimal short-circuit current control point is obtained according to the phase diagram of the voltage and current at the fault point. The optimal short-circuit current control point is the position where the voltage at the fault point is the highest, that is, Figure 3 It can be seen that when the short-circuit current output by the converter is in phase with the short-circuit current of the grid, the voltage at the fault point is the highest;

[0079] Since flexible DC systems are usually connected to high-voltage AC grids, the grid impedance is generally inductive, so the output of pure reactive current by the receiving-end converter is the best way to support the voltage at the fault point.

[0080] The fault types of the power grid include symmetrical faults and asymmetrical faults:

[0081] Symmetrical fault:

[0082] When a symmetrical fault occurs in the power grid, its DC voltage and reactive power control loops are shielded, and the d-axis and q-axis current reference values are reassigned according to the optimal short-circuit current control point so that the system output current meets the preset first standard;

[0083] Under conventional grid-following control, when a three-phase short-circuit fault (symmetrical fault) occurs in the power grid, if no control is applied, the DC voltage control loop of the receiving-end converter station will be saturated. Since the reactive loop generally controls the output reactive power to 0 under normal circumstances, the reactive loop will generally not be saturated. At this time, in the short-circuit current of the receiving-end converter station, the d-axis output current is the maximum current limiting amplitude, and the output current on the q-axis is 0. The advantage of this feature is that it can retain the active power output capacity of the converter station to the greatest extent, reduce surplus power, and thus reduce the pressure on the flexible DC system unloading device and the wind turbine unloading device, which is beneficial to the fault crossing of the sea wind-flexible DC grid-connected system itself. However, in this case, the sea wind-flexible DC grid-connected system not only cannot provide voltage support for the grid, but also continues to output active power to the grid, which is not conducive to the protection of the power grid. It is easy to cause the synchronous generator rotor to accelerate, causing frequency stability and synchronization stability problems. By using the optimal short-circuit current control point to re-assign the d-axis and q-axis current reference values, the short-circuit current response characteristics of the receiving-end converter station to the grid fault are changed, and more q-axis current is output to support the grid voltage. However, due to the overcurrent capacity limitation of the converter, the vector sum of the d-axis and q-axis currents cannot exceed the maximum output current limit of the converter. Therefore, increasing the q-axis current will also reduce the maximum output amplitude of the d-axis current, reduce the active output capacity of the receiving-end converter station, and increase the pressure on the flexible DC system unloading device and the wind turbine unloading device, which is not conducive to the fault ride-through of the sea wind-flexible DC grid-connected system itself. Therefore, the distribution of the d-axis and q-axis current reference values needs to be assigned according to the set first standard requirements;

[0084] The first preset standard: When a three-phase short circuit occurs in the power system and the positive sequence component of the grid connection point voltage is lower than 80% of the nominal voltage, the wind farm should have dynamic reactive power support capability:

[0085] The dynamic reactive current increment of the wind farm should respond to the voltage change at the grid connection point and meet the following requirements:

[0086] ΔI t =K1×(0.9-U t )×I N (4-1)

[0087] Where:

[0088] ΔI t The dynamic reactive current increment injected by the wind farm, in ampere (A);

[0089] K1 is the dynamic reactive current proportional coefficient of the wind farm. The value range of K1 should be no less than 1.5 and preferably no more than 3;

[0090] U t is the per-unit voltage of the wind farm grid connection point, in per-unit value (pu);

[0091] IN is the rated current of the wind farm, in A;

[0092] During voltage sag, the reactive current output by the wind farm to the power system should be the output reactive current I0 during normal operation before the voltage sag plus the dynamic reactive current increment ΔI t The maximum output capacity of the wind farm’s reactive current should not be less than 1.05 times the rated current of the wind farm;

[0093] From the moment the grid connection point voltage drops, the wind farm's dynamic reactive current rise time shall not exceed 60ms. From the moment the grid connection point voltage recovers to more than 90% of the nominal voltage, the wind farm shall exit the dynamic reactive current increase within 40ms.

[0094] Asymmetric fault:

[0095] When an asymmetric fault occurs in the power grid, the positive-sequence current reference value in the short-circuit current is reassigned according to the optimal short-circuit current control point. For the negative-sequence current component, the converter station is required to actively absorb the negative-sequence current to provide active support to the power grid and reduce the imbalance of the fault point. However, in order to avoid single-phase overcurrent in the output current of the converter station, the converter station absorbs the negative-sequence current at the expense of a portion of the positive-sequence current output, which will reduce the active power output capacity of the converter station and the positive-sequence voltage support capacity of the power grid. Therefore, the negative-sequence current reference value needs to be reassigned according to the preset second standard.

[0096] The second preset standard includes: when an asymmetric short-circuit fault occurs in the power system, the wind farm should have dynamic reactive power support capability during the low voltage ride-through process;

[0097] When the positive-sequence component of the grid-connected point voltage is between 60% and 80% of the nominal voltage, the wind farm should be able to inject positive-sequence dynamic reactive current into the grid to support positive-sequence voltage recovery and absorb negative-sequence dynamic reactive current from the grid to suppress the increase in negative-sequence voltage. The wind farm's dynamic reactive current increment should respond to changes in grid-connected point voltage and satisfy the following formula:

[0098]

[0099] Where:

[0100] ——The positive sequence dynamic reactive current increment injected by the wind farm, in ampere (A);

[0101] ——Negative sequence dynamic reactive current increment absorbed by the wind farm, in ampere (A);

[0102] -- Dynamic positive sequence reactive current proportional coefficient of wind farm, K: the value range should be no less than 1.0;

[0103] --The wind farm dynamic negative sequence reactive current proportional coefficient, K, should be no less than 1.0;

[0104] ——The per-unit value of the positive sequence component of the voltage at the wind farm grid connection point, in per-unit value (pu);

[0105] ——The per-unit value of the negative sequence component of the voltage at the wind farm grid connection point, in per-unit value (pu);

[0106] I N ——Rated current of the wind farm, in ampere (A);

[0107] If the positive sequence component of the grid connection point voltage is less than 60% of the nominal voltage, the wind farm should, based on the actual control capability of the wind turbines and the actual conditions of the wind farm's access to the grid, inject appropriate positive sequence dynamic reactive current into the grid and absorb appropriate negative sequence dynamic reactive current from the grid without increasing the voltage imbalance at the grid connection point;

[0108] During voltage sag, the positive sequence reactive current output by the wind farm to the power system should be the output reactive current I0 before the voltage sag plus the positive sequence dynamic reactive current increment. The maximum output capacity of the wind farm reactive current should not be less than 1.05 times the rated current of the wind farm, which should be reduced by and To meet the limitation of maximum output capacity of reactive current;

[0109] The specific steps of separating the positive and negative sequence of the short-circuit current at the receiving-end converter station based on the second-order generalized integrator (SOGI) are as follows:

[0110] As attached Figure 4 As shown, attached Figure 4 The block diagram of the second-order generalized integrator (SOGI) is shown in Figure 1. The two output closed-loop transfer functions of the second-order generalized integrator are obtained as follows:

[0111]

[0112] From formula (2-38), we can see that H d (s) shows the characteristics of a frequency selector, which can track signals with frequencies of ±ω; H q (s) shows the characteristics of a low-pass filter, which can output the signal with a frequency of +ω after π / 2 lag and output the signal with a frequency of -ω before π / 2 lag. k determines the bandwidth of the closed-loop system and is usually taken as

[0113] From formula (2-38), we can see that the output du of SOGI has exactly the same amplitude and phase angle as the input signal u, while the output qu has the same amplitude as the input signal u, with a phase lag of π / 2. qu and u always maintain an orthogonal relationship and are independent of the parameter k.

[0114] Perform algebraic operations on the outputs du and qu of the second-order generalized integrator to obtain the positive sequence component of the AC side voltage of the receiving converter station: and Negative sequence component and

[0115] Then, according to the positive sequence component of the AC side voltage of the receiving converter station and Negative sequence component and The calculation process of the positive sequence current reference value and the negative sequence current reference value is as follows:

[0116] Construct the MMC average value model under asymmetric fault AC power grid, as shown in the attached Figure 5 As shown in the figure, since there is no zero-sequence voltage and zero-sequence current on the valve side, the voltage and current on the AC side of the MMC can be expressed as:

[0117]

[0118] Where: h = 1, 2, 3, and are the positive and negative sequence voltage amplitudes on the AC side, θ + and θ - are the initial phases of the positive and negative sequence voltages on the AC side, and are the positive and negative sequence current amplitudes on the AC side, φ + and φ - They are the initial phases of positive and negative sequence currents on the AC side respectively;

[0119] According to Kirchhoff's voltage law and superposition theorem, the dynamic equation of the AC side current is:

[0120]

[0121] Transforming Equation (2-27) into the two-phase stationary αβ coordinate system, we can obtain:

[0122]

[0123] Where: and and and are the α and β axis components of the MMC positive sequence AC internal potential, AC side voltage, and AC side current, respectively. and and and They are the α and β axis components of the negative sequence AC internal potential, AC side voltage and AC side current of the MMC respectively;

[0124] Transforming Equation (2-27) into the dual synchronous rotating dq coordinate system, we can obtain:

[0125]

[0126] Where: and and and are the dq axis components of the MMC AC internal potential, AC side voltage, and AC side current in the positive sequence rotating coordinate system, respectively. and and and are the dq axis components of the MMC AC internal potential, AC side voltage and AC side current in the negative sequence rotating coordinate system respectively;

[0127] Combining Equations (2-27) and (2-31), the space vectors of the AC side voltage and current can be expressed as:

[0128]

[0129] According to the instantaneous power theory, the instantaneous active and reactive power of an asymmetric AC grid can be expressed as:

[0130] p(t)=P0+P c2 cos(2ωt)+P s2 sin(2ωt) (2-34)

[0131] q(t)=Q0+Q c2 cos(2ωt)+Q s2 sin(2ωt) (2-35)

[0132] Where: p(t) and q(t) are the instantaneous active and reactive power of the asymmetric AC grid respectively; P0 and Q0 are the average values of the instantaneous active and reactive power respectively; P c2 and P s2 are the cosine and sine amplitudes of the instantaneous active power respectively; Q c2 and Q s2 are the cosine and sine amplitudes of the instantaneous reactive power, respectively;

[0133] Substituting equations (2-27) to (2-33) into equations (2-34) and (2-35), we can obtain the instantaneous power expression of the AC grid:

[0134]

[0135] The converter in an asymmetric AC grid usually has three control objectives: (1) suppressing negative sequence current control; (2) suppressing active power double frequency fluctuation control; (3) suppressing reactive power double frequency fluctuation control. According to the AC grid instantaneous power expression shown in formula (2-36), to achieve the above control objectives, the AC side current reference value should be set as:

[0136]

[0137] Where r = 0 corresponds to target (1), r = -1 corresponds to target (2), and r = 1 corresponds to target (3). D1 and D2 can be expressed as:

[0138]

[0139] Under the asymmetric AC grid, if the MMC adopts any of the three control strategies mentioned above, its DC side will not have double frequency fluctuations. However, as a voltage source converter, the MMC only outputs positive sequence fundamental voltage during normal operation. When there are negative sequence voltage components and zero sequence voltage components in the AC bus of the converter station, if the MMC does not provide the corresponding back electromotive force, a large negative sequence current component will be generated on the valve side of the MMC. After superposition with the positive sequence current component and the DC current component, it may greatly exceed the current tolerance range of the power device, thus seriously threatening the safe operation of the MMC. Therefore, in order to make the MMC transmit as much active power as possible under the asymmetric grid and meet the application scenarios with special requirements for grid-connected negative sequence current, the control target of the MMC can be set to target (1), that is, suppressing negative sequence current control:

[0140] Therefore, in order to achieve the control goal of suppressing negative sequence current, according to formula (2-36), the reference value of the AC side current is:

[0141]

[0142] The reassigned current reference value is used as the initial given value i of the current * d and i * q , the initial given value of current i * d and i * q After a symmetrical limiting link, the final given value of the positive sequence current i is obtained. dref and i qref :

[0143]

[0144] When a grid fault occurs, if the given current value exceeds the tolerance of the device, the receiving end converter will reduce the current proportionally. dref and i qref , making Its essence is to increase L v With R v , that is, an adaptive virtual impedance is used to avoid saturation of the AC voltage outer loop, so that the synchronization link will not fail.

[0145] Example 2

[0146] Figure 6 1 is a system schematic diagram showing an active support device for a grid-connected system under a grid fault according to an exemplary embodiment, the device comprising:

[0147] Optimal short-circuit current control point acquisition module 1: used to establish an equivalent circuit of the receiving-end converter station and the AC power grid under a power grid fault, obtain a phase diagram of the voltage and current at the fault point based on the equivalent circuit, and obtain the optimal short-circuit current control point based on the phase diagram;

[0148] Fault type determination module 2: used to determine the fault type of the power grid;

[0149] Symmetrical fault assignment module 3: configured to, if the fault type is a symmetrical fault, shield the DC voltage and reactive power control loops, and reassign the d-axis and q-axis current reference values of the current loop according to the optimal short-circuit current control point so that the output current meets a preset first standard;

[0150] Asymmetric fault assignment module 4: configured to, if the fault type is an asymmetric fault, perform positive-sequence and negative-sequence separation on the short-circuit current of the receiving-end converter station, obtain a positive-sequence current reference value and a negative-sequence current reference value based on the separated positive-sequence component and negative-sequence component, respectively; reassign the positive-sequence current reference value and the negative-sequence current reference value based on the optimal short-circuit current control point, and reassign the positive-sequence current reference value and the negative-sequence current reference value at the optimal short-circuit current control point according to a preset second standard;

[0151] Final given value acquisition module 5: used to use the reassigned current reference value as the initial given value of the current, the initial given value of the current passes through a symmetrical limiting link, obtain the final given value of the positive sequence current, and use the final given value of the positive sequence current as the output current.

[0152] Example 3:

[0153] This embodiment provides a storage medium, wherein the storage medium stores a computer program, and when the computer program is executed by a host controller, each step in the above method is implemented;

[0154] It is understandable that the storage medium mentioned above can be a read-only memory, a magnetic disk or an optical disk, etc.

[0155] It can be understood that the same or similar parts of the above embodiments can be referenced to each other, and the contents not described in detail in some embodiments can refer to the same or similar contents in other embodiments.

[0156] It should be noted that, in the description of the present invention, the terms "first", "second", etc. are used for descriptive purposes only and should not be understood as indicating or implying relative importance. In addition, in the description of the present invention, unless otherwise specified, the meaning of "plurality" is at least two.

[0157] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment or portion of code comprising one or more executable instructions for implementing the steps of a specific logical function or process, and the scope of the preferred embodiments of the present invention includes alternative implementations in which functions may be performed out of the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order depending on the functions involved, which should be understood by those skilled in the art to which the embodiments of the present invention pertain.

[0158] It should be understood that various parts of the present invention can be implemented using hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application-specific integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0159] Those skilled in the art will understand that all or part of the steps in the method of the above embodiment can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium. When the program is executed, it includes one or a combination of the steps of the method embodiment.

[0160] In addition, the functional units in the various embodiments of the present invention may be integrated into a single processing module, or each unit may exist physically separately, or two or more units may be integrated into a single module. The aforementioned integrated modules may be implemented in the form of hardware or in the form of software functional modules. If the integrated modules are implemented in the form of software functional modules and sold or used as independent products, they may also be stored in a computer-readable storage medium.

[0161] The storage medium mentioned above can be a read-only memory, a magnetic disk or an optical disk, etc.

[0162] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0163] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A method for actively supporting a grid-connected system under a power grid fault, characterized in that: The method comprises: Establishing an equivalent circuit of the receiving-end converter station and the AC power grid under a power grid fault, obtaining a phase diagram of the voltage and current at the fault point based on the equivalent circuit, and obtaining an optimal short-circuit current control point based on the phase diagram; Determine the type of fault in the power grid; If the fault type is a symmetrical fault, shielding the DC voltage and reactive power control loops, and reassigning the d-axis and q-axis current reference values of the current loop according to the optimal short-circuit current control point so that the output current meets the preset first standard; If the fault type is an asymmetric fault, the positive-sequence and negative-sequence current of the receiving-end converter station are separated, and a positive-sequence current reference value and a negative-sequence current reference value are respectively obtained according to the separated positive-sequence component and negative-sequence component; the positive-sequence current reference value and the negative-sequence current reference value are reassigned according to the optimal short-circuit current control point, and the positive-sequence current reference value and the negative-sequence current reference value are reassigned at the optimal short-circuit current control point according to a preset second standard; The reassigned current reference value is used as the initial given value of the current. The initial given value of the current passes through a symmetrical limiting link to obtain a final given value of the positive sequence current, and the final given value of the positive sequence current is used as the output current.

2. The method according to claim 1, characterized in that The obtaining of the optimal short-circuit current control point according to the phase diagram includes: Obtaining the position where the fault point voltage is the highest according to the phase diagram, and taking the position where the fault point voltage is the highest as the optimal short-circuit current control point; The obtaining of the position of the highest fault point voltage according to the phase diagram includes: The point in the phase diagram where the short-circuit current output by the converter is in phase with the short-circuit current of the power grid is obtained. The point where the phase is the same is the position where the fault point voltage is the highest.

3. The method according to claim 2, characterized in that The output current meeting the preset first standard includes: Obtaining the dynamic reactive current increment injected by the wind farm in the output current, setting the wind farm dynamic reactive current proportional coefficient, and obtaining the per-unit voltage of the wind farm grid connection point and the wind farm rated current; The dynamic reactive current increment injected by the wind farm is equal to the difference between the dynamic reactive current proportional coefficient of the wind farm multiplied by a preset parameter minus the per-unit value of the voltage at the wind farm grid connection point, multiplied by the rated current of the wind farm.

4. The method according to claim 3, characterized in that The output current meeting the preset first standard also includes: During a voltage drop in the grid-connected system, the reactive current output by the wind farm to the power system is the sum of the reactive current output by the system during normal operation before the voltage drop and the dynamic reactive current increment injected by the wind farm; The maximum output capacity of the wind farm reactive current is not less than a preset multiple of the wind farm rated current.

5. The method according to claim 4, characterized in that The output current meeting the preset first standard also includes: When the grid connection point voltage of the grid-connected system begins to drop, the rise time of the reactive current output by the wind farm to the power system is no longer than a preset first time period; When the grid connection point voltage of the grid-connected system recovers to a preset percentage of the nominal voltage, the wind farm exits the dynamic reactive current increment within a preset second time period.

6. The method according to claim 1, wherein The preset second standard includes: When the positive sequence component of the grid connection point voltage of the grid-connected system is within a preset percentage interval of the nominal voltage, the wind farm injects a positive sequence dynamic reactive current into the grid to support the recovery of the positive sequence voltage, and absorbs a negative sequence dynamic reactive current from the grid to suppress the increase of the negative sequence voltage; When the positive sequence component of the grid connection point voltage of the grid-connected system is less than a preset percentage interval of the nominal voltage, the wind farm injects positive sequence dynamic reactive current into the grid and absorbs negative sequence dynamic reactive current from the grid; The injected positive-sequence dynamic reactive current and the absorbed negative-sequence dynamic reactive current respectively satisfy a preset magnitude relationship.

7. The method according to claim 6, characterized in that The preset second standard also includes: During a voltage sag in the grid-connected system, the positive-sequence reactive current output by the wind farm to the power system is the sum of the reactive current output before the voltage sag and the positive-sequence dynamic reactive current increment. The maximum output capacity of the wind farm's reactive current is not less than a preset multiple of the wind farm's rated current. The maximum output capacity of the wind farm's reactive current is controlled by the positive-sequence dynamic reactive current and the negative-sequence dynamic reactive current.

8. The method according to any one of claims 1 to 7, characterized in that If the fault type is an asymmetric fault, performing positive and negative sequence separation on the short-circuit current of the receiving-end converter station includes: The two output closed-loop transfer functions of the second-order generalized integrator are obtained, and the two outputs of the second-order generalized integrator are subjected to algebraic operation to obtain the positive-sequence component and the negative-sequence component of the AC side voltage of the receiving converter station.

9. The method according to claim 8, characterized in that The step of respectively obtaining a positive-sequence current reference value and a negative-sequence current reference value according to the separated positive-sequence component and negative-sequence component comprises: Constructing an MMC average value model under an asymmetric fault AC power grid, and obtaining expressions for AC side voltage and current according to the MMC average value model; The dynamic equation of the AC side current is constructed using Kirchhoff's voltage law and superposition theorem; Transforming the dynamic equation of the AC side current into a two-phase static αβ coordinate system, and obtaining the positive and negative sequence αβ axis components of the AC side according to the positive sequence component and the negative sequence component; The dynamic equation of the AC side current is transformed into the dual synchronous rotating dq coordinate system to obtain the positive and negative sequence of the AC side respectively. d q Axis component; Obtaining space vector expressions of the voltage and current on the AC side according to the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, and the positive and negative sequence dq axis components; Obtain the expressions of instantaneous active and reactive power of an asymmetric AC grid based on instantaneous power theory; Substituting the dynamic equation of the AC side current, the positive and negative sequence αβ axis components, the positive and negative sequence dq axis components, and the space vector expressions of the voltage and current on the AC side into the expressions of the instantaneous active and reactive power of the asymmetric AC power grid to obtain the instantaneous power expression of the AC power grid; A positive sequence current reference value and a negative sequence current reference value are obtained according to the instantaneous power expression of the AC power grid.

10. The method according to claim 1, characterized in that If the final given value of the positive sequence current exceeds the tolerance value of the device, the receiving-end converter proportionally reduces the final given value of the positive sequence current so that the final given value of the positive sequence current reaches the tolerance value of the device.

11. An active support device for a grid-connected system under a power grid fault, characterized in that: The device comprises: Optimal short-circuit current control point acquisition module: used to establish an equivalent circuit of the receiving-end converter station and the AC power grid under a power grid fault, obtain a phase diagram of the voltage and current at the fault point based on the equivalent circuit, and obtain the optimal short-circuit current control point based on the phase diagram; Fault type determination module: used to determine the fault type of the power grid; Symmetrical fault assignment module: used for shielding the DC voltage and reactive power control loops if the fault type is a symmetrical fault, and reassigning the d-axis and q-axis current reference values of the current loop according to the optimal short-circuit current control point so that the output current meets the preset first standard; an asymmetric fault assignment module configured to, if the fault type is an asymmetric fault, separate the positive-sequence and negative-sequence current of the receiving-end converter station, obtain a positive-sequence current reference value and a negative-sequence current reference value according to the separated positive-sequence component and negative-sequence component, respectively; reassign the positive-sequence current reference value and the negative-sequence current reference value according to the optimal short-circuit current control point, and reassign the positive-sequence current reference value and the negative-sequence current reference value at the optimal short-circuit current control point according to a preset second standard; Final given value acquisition module: used to use the reassigned current reference value as the initial given value of the current, the initial given value of the current passes through a symmetrical limiting link, obtain the final given value of the positive sequence current, and use the final given value of the positive sequence current as the output current.

12. A storage medium, characterized in that: The storage medium stores a computer program, and when the computer program is executed by the processor, the steps of the method for active support of a grid-connected system under a grid fault according to any one of claims 1 to 10 are implemented.

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