Method for improving transient stability of converter under non-ideal power grid

Abstract

The invention discloses a method for improving the transient stability of a converter under a non-ideal power grid, the method for improving the transient stability of the converter under the non-ideal power grid is applied to a grid-connected converter based on a VSG technology, and the method for improving the transient stability of the converter under the non-ideal power grid is applied to the grid-connected converter based on the VSG technology. Comprising the steps of performing current-limiting control on a grid-connected converter based on a preset virtual impedance current-limiting control strategy; detecting the voltage amplitude of the power grid; and when the voltage amplitude of the power grid is smaller than the voltage threshold, adjusting the resistance proportionality coefficient so as to update the virtual impedance current-limiting control strategy and limit the current-limiting voltage drop within the voltage drop threshold. According to the method, the resistance proportionality coefficient can be adaptively adjusted according to the amplitude of the power grid voltage while the virtual impedance is utilized to start over-current limitation, so that the adjustment of the virtual impedance current-limiting control strategy is realized, the excessive increase of the current-limiting voltage drop is inhibited, the transient instability of the grid-connected converter caused by the excessive current-limiting voltage drop is prevented, and the stability of the grid-connected converter is improved. And the transient stability of the grid-connected converter is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of converters, and in particular to a method for improving the transient stability of a converter under a non-ideal power grid. Background Art

[0002] In recent years, a series of renewable energy sources, mainly wind, solar and electricity, have been vigorously developed. Since the DC power generated by renewable energy needs to be converted into AC power by grid-connected converters before being transmitted to the large power grid, the power system dominated by traditional synchronous machines is gradually transforming into a new power system dominated by power electronic converters. As an important grid-connected interface for renewable energy, converters undertake the important task of power conversion and transmission.

[0003] At present, grid-connected converters for phase locked loops (PLLs) still dominate grid-connected converters. However, since they do not have inertia and damping, their widespread application will seriously affect the safe and stable operation of the power grid, and the inertia and damping of the power grid will be further weakened. Therefore, in order to overcome the defects of low inertia and weak damping of grid-connected converters based on PLL phase locked loops, virtual synchronous generator (VSG) technology with self-synchronous grid connection capability has gradually become a feasible alternative. Nevertheless, while inheriting the external characteristics of traditional synchronous machines, grid-connected converters inevitably have the risk of transient instability. Especially when a short circuit fault occurs in the power grid and causes a sudden drop in transient voltage, the probability of transient instability of the grid-connected converter will be greatly increased. In addition, the small equivalent output impedance of the grid-connected converter will easily lead to a large overcurrent output during the grid fault. Since the overcurrent value of the power device inside the converter is low, a deeper grid fault will further increase the probability of burning of the grid-connected converter and the risk of transient instability.

[0004] Existing technologies have studied the transient instability phenomenon and output overcurrent problems of grid-connected converters that may be caused by non-ideal grid conditions such as transient voltage sags. Some technical studies have studied grid-connected converters under non-ideal grids from a single perspective, that is, only from the perspective of transient stability improvement or output current limiting control. However, transient instability and output overcurrent problems of converters under non-ideal grids often occur at the same time. Therefore, current limiting control and transient stability improvement of grid-connected converters under non-ideal grids are crucial. Summary of the invention

[0005] The present invention provides a method for improving the transient stability of a converter under a non-ideal power grid to solve the above technical problems and effectively improve the transient stability of the converter.

[0006] In order to solve the above technical problems, the present invention provides a method for improving the transient stability of a converter under a non-ideal power grid, which is applied to a grid-connected converter based on VSG technology. The method for improving the transient stability of a converter under a non-ideal power grid comprises:

[0007] Performing current limiting control on the grid-connected converter based on a preset virtual impedance current limiting control strategy;

[0008] Detect the voltage amplitude of the power grid;

[0009] When the grid voltage amplitude is less than the voltage threshold, the resistance proportional coefficient is adjusted to update the virtual impedance current limiting control strategy and limit the current limiting voltage drop within the voltage drop threshold.

[0010] The beneficial effects of the present invention are:

[0011] The present invention performs current limiting control on the grid-connected converter through a virtual impedance current limiting control strategy, and adjusts the resistance proportional coefficient when the grid voltage amplitude is less than the voltage threshold value. Thus, while using the virtual impedance to start overcurrent limitation, the resistance proportional coefficient can be adaptively adjusted according to the grid voltage amplitude, thereby realizing the adjustment of the virtual impedance current limiting control strategy, suppressing the excessive growth of the current limiting voltage drop, preventing the transient instability of the grid-connected converter caused by the excessive current limiting voltage drop, and thus improving the transient stability of the grid-connected converter.

[0012] As a preferred solution, the formula of the VSG technology is as follows:

[0013]

[0014] In the above formula, P and Q are the output power of the grid-connected converter; P ref is the active power command input to the grid-connected converter, Q ref is the reactive power command input to the grid-connected converter; ω and ω0 are the grid-connected converter output angular frequency and angular frequency input reference command respectively; J is the virtual inertia added by the grid-connected converter; D p and D q is the damping coefficient in VSG technology; K v is the gain coefficient of reactive voltage regulation; E0 is the reference instruction of the input internal potential of the grid-connected converter.

[0015] In this preferred solution, VSG technology adds inertia and damping on the basis of droop control, ensuring its own good voltage and frequency regulation capabilities while having the ability to self-synchronize and connect to the grid and cope with grid fluctuations.

[0016] As a preferred solution, the detecting of the grid voltage amplitude includes:

[0017] Collect three-phase grid voltage;

[0018] Perform dq transformation on the three-phase grid voltage to obtain the grid voltage in dq coordinate system;

[0019] The grid voltage amplitude is obtained based on the grid voltage in the dq coordinate system.

[0020] In this preferred scheme, the three-phase grid voltage is transformed into a dq coordinate system grid voltage to facilitate processing of the three-phase output signal of the grid-connected converter, simplify the control algorithm, and improve the control accuracy; in addition, due to the presence of the filtering device on the output side of the grid-connected converter and the equivalent resistance and inductance on the grid side, compared with the prior art of detecting the capacitor voltage amplitude, this preferred scheme directly detects the grid voltage amplitude and can make a judgment on transient faults more quickly.

[0021] As a preferred solution, the three-phase grid voltage is subjected to dq transformation by a dq transformation formula; the dq transformation formula is shown as follows:

[0022]

[0023] In the above formula, x0 is the zero-sequence voltage or current; x a 、x b and x c is a three-phase voltage signal or a three-phase current signal; x d is the voltage signal or current signal corresponding to the d-axis, x q is the voltage signal or current signal corresponding to the q-axis; θ is the phase angle of the internal output of the grid-connected converter.

[0024] In this preferred embodiment, the three-phase grid voltage is transformed into a dq coordinate system grid voltage to facilitate processing of the three-phase output signal of the grid-connected converter and simplify the control algorithm. By converting the three-phase signal into a dq axis DC signal, the active power and reactive power transmitted by the grid-connected converter can be more accurately controlled while simplifying the control design implementation.

[0025] As a preferred solution, the virtual impedance current limiting control strategy includes:

[0026]

[0027] In the above formula, I lim is the set current limiting threshold; k r is the resistance proportionality coefficient of the virtual impedance, k x is the reactance proportionality coefficient of the virtual impedance; R VIR is a virtual resistor, X VIR is the virtual reactance; i d is the d-axis current signal, i q is the q-axis current signal; R H is the high frequency suppression resistor, X His the high frequency suppression reactance; ω c is the cutoff frequency of the high-pass filter; V d,VIR is the d-axis equivalent voltage drop caused by the virtual impedance, V q,VIR is the q-axis equivalent voltage drop caused by the virtual impedance; E dref Enter the d-axis voltage reference value for the voltage loop, E qref Input the q-axis voltage reference value for the voltage loop; E is the internal potential amplitude output in the VSG technology.

[0028] When the grid voltage drops sharply, overcurrent is very likely to occur on the grid-connected converter line. At this time, it is necessary to increase the current limiting control strategy to suppress the impact current. However, the addition of the actual current limiting resistor usually introduces a certain current limiting resistor voltage drop, which will increase the converter's requirements for additional voltage compensation. The method based on virtual impedance is a better current limiting measure. In this preferred scheme, the current limiting control of the grid-connected converter is based on the preset virtual impedance current limiting control strategy, which can improve the dynamic response characteristics and transient stability of the grid-connected converter; and, the virtual impedance in this preferred scheme will be activated when the output current of the grid-connected converter exceeds the set current limiting threshold, not only during transient faults, which can effectively avoid the problem of compensating the current limiting impedance voltage drop when the grid-connected converter is running stably.

[0029] As a preferred solution, the resistance proportionality coefficient is adjusted by the following formula:

[0030]

[0031] In the above formula, k0 is the first adjustment coefficient of the resistance proportionality coefficient, k1 is the second adjustment coefficient of the resistance proportionality coefficient; v gamp is the grid voltage amplitude.

[0032] Since the virtual impedance presents adaptive changes, a larger output current will produce a larger virtual impedance value, corresponding to a larger virtual impedance voltage drop. During the transient voltage drop, the output equivalent internal potential of the inverter is relatively low. A larger virtual impedance voltage drop will further weaken the output internal potential amplitude of the inverter, causing it to be unable to maintain normal power output and unable to provide sufficient voltage support for the power grid, thereby causing integral saturation within the current loop control, and ultimately causing transient instability in the grid-connected converter. In addition, a larger virtual impedance voltage drop will also affect the dynamic response speed of the control system to a certain extent, causing the control system to delay response and be unable to quickly adjust the output to cope with the increase in transient voltage faults in the power grid, increasing the risk of transient instability of the grid-connected converter, and thus causing the system to be unable to operate stably. Therefore, in this preferred solution, when a transient voltage fault occurs on the grid side of the grid-connected converter, the current limiting voltage drop is dynamically adjusted according to the magnitude of the grid voltage amplitude to ensure that the grid-connected converter is suppressed during the transient period while improving its transient stability performance. Furthermore, in the preferred embodiment, by adjusting the resistance proportionality coefficient in a nonlinear manner, the transient instability phenomenon that may be caused by excessive growth of the virtual impedance during the overcurrent period can be greatly limited, thereby effectively improving the transient stability performance of the grid-connected converter.

[0033] As a preferred solution, the method for improving transient stability of a converter under a non-ideal power grid further includes:

[0034] When the grid voltage amplitude is greater than or equal to the voltage threshold, the resistance proportional coefficient is maintained at the initial proportional coefficient.

[0035] In this preferred solution, the grid voltage amplitude is greater than or equal to the voltage threshold, indicating that the transient voltage fault of the grid is relatively shallow, and the current limiting voltage drop is relatively small. The grid-connected converter can provide a certain voltage support for the grid. Therefore, this preferred solution keeps the resistance proportional coefficient unchanged. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 : A flow chart of an embodiment of a method for improving transient stability of a converter under a non-ideal power grid provided by the present invention;

[0037] Figure 2 : A schematic diagram of a control structure of a grid-connected converter based on VSG technology provided in an embodiment of the present invention;

[0038] Figure 3 : A schematic diagram of a grid-connected converter control structure including a virtual impedance current limiting control strategy provided by an embodiment of the present invention;

[0039] Figure 4 : A schematic diagram of a control structure of a grid-connected converter under the method for improving the transient stability of a converter under a non-ideal power grid provided by an embodiment of the present invention;

[0040] Figure 5 : A schematic diagram of the output current of the converter under the traditional virtual impedance current limiting control strategy provided by an embodiment of the present invention;

[0041] Figure 6 : A schematic diagram of the output power angle of the converter under the traditional virtual impedance current limiting control strategy provided by an embodiment of the present invention;

[0042] Figure 7 : A schematic diagram of the output current of the grid-connected converter under the method for improving the transient stability of the converter under the non-ideal power grid provided by an embodiment of the present invention;

[0043] Figure 8 : A schematic diagram of the output power angle of the grid-connected converter under the method for improving the transient stability of the converter under the non-ideal power grid provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] Embodiment 1

[0046] In order to solve the above technical problems, the present invention provides a method for improving the transient stability of a converter under a non-ideal power grid, which effectively improves the transient stability of the grid-connected converter.

[0047] Please refer to Figure 1 , which is a flow chart of an embodiment of a method for improving transient stability of a converter under a non-ideal power grid provided by an embodiment of the present invention.

[0048] The method for improving the transient stability of a converter under a non-ideal power grid is applied to a grid-connected converter based on VSG technology, such as Figure 1 As shown, the method for improving the transient stability of the converter under the non-ideal power grid includes steps 101 to 103, which are specifically:

[0049] Step 101: performing current limiting control on a grid-connected converter based on a preset virtual impedance current limiting control strategy;

[0050] Step 102: Detecting the voltage amplitude of the power grid;

[0051] Step 103: When the grid voltage amplitude is less than the voltage threshold, the resistance proportionality coefficient is adjusted to update the virtual impedance current limiting control strategy and limit the current limiting voltage drop within the voltage drop threshold.

[0052] The present invention performs current limiting control on the grid-connected converter through a virtual impedance current limiting control strategy, and adjusts the resistance proportional coefficient when the grid voltage amplitude is less than the voltage threshold value. Thus, while using the virtual impedance to start overcurrent limitation, the resistance proportional coefficient can be adaptively adjusted according to the grid voltage amplitude, thereby realizing the adjustment of the virtual impedance current limiting control strategy, suppressing the excessive growth of the current limiting voltage drop, preventing the transient instability of the grid-connected converter caused by the excessive current limiting voltage drop, and thus improving the transient stability of the grid-connected converter.

[0053] Furthermore, the formula of the VSG technique is as follows:

[0054]

[0055] In the above formula, P and Q are the output power of the grid-connected converter; P ref is the active power command input to the grid-connected converter, Q ref is the reactive power command input to the grid-connected converter; ω and ω0 are the grid-connected converter output angular frequency and angular frequency input reference command respectively; J is the virtual inertia added by the grid-connected converter; D p and D q is the damping coefficient in VSG technology; K v is the gain coefficient of reactive voltage regulation; E0 is the reference instruction of the input internal potential of the grid-connected converter.

[0056] Please refer to the following Figure 2 , Figure 2 A schematic diagram of a control structure of a grid-connected converter based on VSG technology provided in an embodiment of the present invention.

[0057] like Figure 2 As shown in the control structure of the grid-connected converter based on VSG technology, L f and C f are the inductance and capacitance of the filter device, R g and L g is the equivalent resistance and inductance of the line connecting the grid-connected converter and the grid, v abc and i abc is the voltage and current output by the grid-connected converter in the three-phase coordinate system, v dq and i dq is the output voltage and current of the converter in the dq coordinate system, E and θ are the internal potential amplitude and phase angle of the output in the VSG technology, I dqref and V dqref are the reference values ​​of the output voltage and current of the grid-connected converter respectively.

[0058] In this embodiment, the phase angle of the internal output of the grid-connected converter can be expressed as follows:

[0059] θ = ∫ωdt;

[0060] In the above formula, θ is the phase angle of the internal output of the grid-connected converter; ω is the output angular frequency of the grid-connected converter.

[0061] In this embodiment, VSG technology adds inertia and damping on the basis of droop control, ensuring its own good voltage and frequency regulation capabilities while having the ability to self-synchronize and connect to the grid and cope with grid fluctuations. In addition, compared with the existing VSG technology, the input quantity of its control loop is torque, which needs to be combined with angular frequency to be further connected with power, while the VSG technology in the present invention omits this step and can be applied more directly.

[0062] Further, in step 101, the virtual impedance current limiting control strategy includes:

[0063]

[0064] In the above formula, I lim is the set current limiting threshold; k r is the resistance proportionality coefficient of the virtual impedance, k x is the reactance proportionality coefficient of the virtual impedance; R VIR is a virtual resistor, X VIR is the virtual reactance; i d is the d-axis current signal, i q is the q-axis current signal; R H is the high frequency suppression resistor, X H is the high frequency suppression reactance; ω c is the cutoff frequency of the high-pass filter; V d,VIR is the d-axis equivalent voltage drop caused by the virtual impedance, V q,VIR is the q-axis equivalent voltage drop caused by the virtual impedance; E dref Enter the d-axis voltage reference value for the voltage loop, E qref Input the q-axis voltage reference value for the voltage loop; E is the internal potential amplitude output in the VSG technology.

[0065] Please refer to the following Figure 3 , Figure 3 A schematic diagram of a grid-connected converter control structure including a virtual impedance current limiting control strategy provided by an embodiment of the present invention. Figure 3 The expressions of the virtual impedance current limiting control strategy and the voltage loop input voltage reference value in the present invention can be obtained.

[0066] When the grid voltage drops sharply, overcurrent is very likely to occur on the grid-connected converter line. At this time, it is necessary to increase the current limiting control strategy to suppress the impact current. However, the addition of actual current limiting resistors usually introduces a certain current limiting resistor voltage drop, which will increase the converter's requirements for additional voltage compensation. The virtual impedance-based approach is a better current limiting measure, which can avoid the occurrence of transient instability in the converter to a certain extent.

[0067] In this embodiment, the current limiting control of the grid-connected converter is performed based on the preset virtual impedance current limiting control strategy, which can improve the dynamic response characteristics and transient stability of the grid-connected converter; in the virtual impedance strategy of the prior art, the virtual impedance is a virtual reactance, and changes with a simple linear change of the current. Different from the conventional virtual impedance strategy, in the current limiting control strategy based on the preset virtual impedance in this embodiment, the virtual impedance includes two parts, a virtual resistance and a virtual reactance. The relationship between them and the current is not a simple linear change, which includes corresponding specific judgment conditions and proportional coefficients, and a high-frequency suppression impedance part is added; and the virtual impedance in this embodiment is not only started during transient faults, but it will be started as long as there is an overcurrent phenomenon in the converter, that is, when the output current of the grid-connected converter exceeds the set current limiting threshold, thereby effectively avoiding the problem of needing to compensate for the current limiting impedance voltage drop when the grid-connected converter is running stably.

[0068] Further, in step 102, the detecting the grid voltage amplitude includes:

[0069] Collect three-phase grid voltage;

[0070] Perform dq transformation on the three-phase grid voltage to obtain the grid voltage in dq coordinate system;

[0071] The grid voltage amplitude is obtained based on the grid voltage in the dq coordinate system.

[0072] The grid-connected converter grid side is in different degrees of grid fault, and the corresponding grid voltage amplitude is different. At the moment of grid fault, the equivalent internal potential of the output of the grid-connected converter will not change instantly, and the equivalent impedance of the grid-connected line is small. A deeper grid fault will easily generate a larger output current, greatly increasing the probability of converter burning. Since the virtual impedance shows adaptive changes, a larger output current will produce a larger virtual impedance value, corresponding to a larger virtual impedance voltage drop. During the transient voltage drop, the output equivalent internal potential of the converter is relatively low. A larger virtual impedance voltage drop will further weaken the output internal potential amplitude of the inverter, causing it to be unable to maintain normal power output and unable to provide sufficient voltage support for the grid, thereby causing integral saturation inside the current loop control, and ultimately causing transient instability in the converter. In addition, a larger virtual impedance voltage drop will also affect the dynamic response speed of the control system to a certain extent, resulting in a response delay of the control system and an inability to quickly adjust the output to cope with transient voltage faults in the grid, resulting in the system being unable to operate stably. Therefore, when a transient voltage fault occurs on the grid side of the grid-connected converter, it is necessary to adjust the current limiting voltage drop to a certain extent according to the grid voltage amplitude to ensure that the converter can suppress overcurrent during the transient period while improving its transient stability performance. In this regard, the present invention detects the grid voltage amplitude, and when the grid voltage amplitude is less than the voltage threshold, adjusts the resistance proportional coefficient to update the virtual impedance current limiting control strategy and limit the current limiting voltage drop within the voltage drop threshold, which can ensure that the converter can suppress overcurrent during the transient period while improving the transient stability performance of the grid-connected converter.

[0073] In this embodiment, the three-phase grid voltage is subjected to dq transformation to obtain the grid voltage in the dq coordinate system, so as to process the three-phase output signal of the grid-connected converter, simplify the control algorithm, and improve the control accuracy; in addition, due to the existence of the filtering device on the output side of the grid-connected converter and the existence of the equivalent resistance and inductance on the grid side, the grid voltage amplitude and the capacitor voltage amplitude are not completely consistent in size and change. Compared with the prior art of detecting the capacitor voltage amplitude, the direct detection of the grid voltage amplitude in this embodiment can make a judgment on transient faults more quickly.

[0074] Furthermore, the three-phase grid voltage is subjected to dq transformation by a dq transformation formula; the dq transformation formula is shown as follows:

[0075]

[0076] In the above formula, x0 is the zero-sequence voltage or current; x a 、x b and x c is a three-phase voltage signal or a three-phase current signal; x d is the voltage signal or current signal corresponding to the d-axis, x qis the voltage signal or current signal corresponding to the q-axis; θ is the phase angle of the internal output of the grid-connected converter.

[0077] In this embodiment, the three-phase grid voltage is subjected to dq transformation to obtain the grid voltage in the dq coordinate system, so as to facilitate the processing of the three-phase output signal of the grid-connected converter and simplify the control algorithm. Moreover, by converting the three-phase AC signal into d-axis and q-axis DC components, the active power and reactive power transmitted by the grid-connected converter can be more accurately controlled while simplifying the control design implementation.

[0078] Further, after performing dq transformation on the three-phase grid voltage to obtain the dq coordinate system grid voltage, this embodiment obtains the grid voltage amplitude based on the dq coordinate system grid voltage, and specifically obtains the grid voltage amplitude through the following formula:

[0079]

[0080] In the above formula, v gamp is the grid voltage amplitude, v gd and v gq is the grid voltage in the dq coordinate system. Further, in step 103, the resistance proportionality coefficient is adjusted by the following formula:

[0081]

[0082] In the above formula, k0 is the first adjustment coefficient of the resistance proportionality coefficient, k1 is the second adjustment coefficient of the resistance proportionality coefficient; v gamp is the grid voltage amplitude.

[0083] Since the virtual impedance presents adaptive changes, a larger output current will produce a larger virtual impedance value, corresponding to a larger virtual impedance voltage drop. During the transient voltage drop, the output equivalent internal potential of the inverter is relatively low. A larger virtual impedance voltage drop will further weaken the output internal potential amplitude of the inverter, causing it to be unable to maintain normal power output and unable to provide sufficient voltage support for the power grid, thereby causing integral saturation within the current loop control, and ultimately causing transient instability in the grid-connected converter. In addition, a larger virtual impedance voltage drop will also affect the dynamic response speed of the control system to a certain extent, causing the control system to delay response and be unable to quickly adjust the output to cope with the increase in transient voltage faults in the power grid, increasing the risk of transient instability of the grid-connected converter, thereby causing the system to be unable to operate stably. Therefore, in this embodiment, when a transient voltage fault occurs on the grid side of the grid-connected converter, the current limiting voltage drop is dynamically adjusted according to the magnitude of the grid voltage amplitude to ensure that the grid-connected converter is suppressed during the transient period while improving its transient stability performance. Furthermore, in this embodiment, by adjusting the resistance proportionality coefficient in a nonlinear manner, the transient instability phenomenon that may be caused by excessive growth of the virtual impedance during the overcurrent period can be greatly limited, thereby effectively improving the transient stability performance of the grid-connected converter.

[0084] Furthermore, the method for improving transient stability of a converter under a non-ideal power grid further includes:

[0085] When the grid voltage amplitude is greater than or equal to the voltage threshold, the resistance proportional coefficient is maintained at the initial proportional coefficient.

[0086] In this embodiment, the grid voltage amplitude is greater than or equal to the voltage threshold, indicating that the transient voltage fault of the grid is relatively shallow. At this time, the current limiting voltage drop is small, and the grid-connected converter can provide a certain voltage support for the grid. Therefore, this embodiment keeps the resistance proportional coefficient unchanged.

[0087] Through the above scheme, this embodiment adjusts the resistance proportional coefficient when the grid voltage amplitude is less than the voltage threshold to update the virtual impedance current limiting control strategy and limit the current limiting voltage drop within the voltage drop threshold, and when the grid voltage amplitude is greater than or equal to the voltage threshold, maintains the resistance proportional coefficient as the initial proportional coefficient.

[0088] In this embodiment, the voltage threshold is set to 0.7v gn , where v gn is the rated voltage amplitude of the power grid.

[0089] Therefore, the present invention adjusts the resistance proportionality coefficient by the following formula:

[0090]

[0091] In the above formula, k0 is the first adjustment coefficient of the resistance proportionality coefficient, k1 is the second adjustment coefficient of the resistance proportionality coefficient; k r0 is the initial proportional coefficient; v gn is the rated voltage amplitude of the power grid; 0.7v gn is the voltage threshold; v gamp is the grid voltage amplitude.

[0092] Please refer to the following Figure 4 , Figure 4 A schematic diagram of a control structure of a grid-connected converter under the method for improving the transient stability of a converter under a non-ideal power grid provided in an embodiment of the present invention.

[0093] During a transient voltage fault, a larger virtual impedance voltage drop will greatly increase the risk of transient instability of the grid-connected converter. Therefore, in order to avoid the risk of transient instability of the converter while ensuring overcurrent suppression during a transient voltage fault, the transient stability performance can be improved by limiting the excessive growth of the current limiting voltage drop. The deeper the transient voltage fault depth of the power grid, the greater the output current of the converter will be. According to the virtual impedance current limiting control strategy described in the present invention, the grid-connected converter will generate a larger impedance value to achieve overcurrent suppression. Therefore, the excessive growth of the current limiting voltage drop can be limited by flexibly adjusting the resistance proportionality coefficient.

[0094] Moreover, compared with the prior art, the resistance proportional coefficient of the virtual impedance in the present invention is more flexible to adjust. The virtual impedance of the prior art only sets two virtual impedance values ​​according to the voltage drop depth, while the present invention can present different changes according to different voltage drop degrees, rather than simply two values; and in the case of a deep voltage drop fault, the resistance proportional coefficient will be adjusted nonlinearly to avoid the transient instability phenomenon caused by the large current limiting voltage drop of the converter caused by the virtual impedance as much as possible.

[0095] The present invention adjusts the resistance proportional coefficient according to the grid voltage amplitude, wherein when the grid transient voltage fault degree is relatively shallow (v gamp ≥0.7v gn ), at this time, the current limiting voltage drop is small, the grid-connected converter can provide a certain voltage support for the grid, and the resistance proportional coefficient remains unchanged; when a deep fault occurs in the grid (v gamp <0.7v gn ), the output current of the grid-connected converter is relatively large. In order to prevent the transient instability of the grid-connected converter caused by excessive current limiting voltage drop, the resistance proportional coefficient will be appropriately reduced at this time, so as to ensure that the virtual impedance starts to limit the overcurrent while suppressing the excessive increase of the current limiting voltage drop, thereby improving the transient stability performance of the grid-connected converter.

[0096] In this embodiment, according to the established grid-connected converter model based on VSG technology, the three-phase output voltage and current of the converter are collected in real time, and the dq axis voltage and current are obtained after dq transformation, and sent to the subsequent control program for processing. Then the dq axis voltage and current are calculated for power, and participate in the Pf loop and QV loop in the VSG technology, and the output internal potential and phase angle are obtained. Subsequently, the grid voltage amplitude is detected in real time to execute the transient stability improvement method of the grid-connected converter proposed in the present invention.

[0097] The following embodiment uses a simulation test example to illustrate the effectiveness of the method for improving the transient stability of a converter under a non-ideal power grid described in the present invention.

[0098] This embodiment sets Figure 4 The main variable parameters in the control structure of the grid-connected converter under the method for improving the transient stability of the converter under the non-ideal power grid are shown in the following table:

[0099] Variable Name illustrate Numeric <![CDATA[I lim ]]> Converter current limiting threshold 30A <![CDATA[k r0 ]]> Initial scale factor 1 <![CDATA[k x ]]> Reactance proportionality factor 0.3 <![CDATA[k0]]> First adjustment factor 1 0.11 <![CDATA[k1]]> Second adjustment factor 2 0.01 <![CDATA[R H ]]> High frequency suppression resistor 0.03 <![CDATA[X H ]]> High frequency suppression reactance 0.03 <![CDATA[P ref ]]> Active power command 10kW <![CDATA[Q ref ]]> Reactive power command 0var <![CDATA[E0]]> Reference command for inputting internal potential 320V <![CDATA[ω0]]> Angular frequency input reference command 314.15926rad / s <![CDATA[v gn ]]> Rated voltage amplitude of power grid 311V

[0100] This embodiment performs simulation tests on a grid-connected converter and obtains the output current and power angle of the grid-connected converter. The grid-connected converter is connected to the grid at 0s and reaches a stable state at 0.5s. A transient voltage drop fault occurs in the grid at 4s and is cleared at 4.5s.

[0101] Figure 5 A schematic diagram of the output current of the converter under the traditional virtual impedance current limiting control strategy provided by an embodiment of the present invention; Figure 6 A schematic diagram of the output power angle of the converter under the traditional virtual impedance current limiting control strategy provided by an embodiment of the present invention.

[0102] from Figure 5 and Figure 6 It can be seen that under the traditional virtual impedance current limiting control strategy, after the grid-side transient voltage fault of the grid-connected converter is cleared, the current still needs a long time to recover to the rated level.

[0103] As for the output power angle of the grid-connected converter, the output power angle of the converter was stable at 16.67° before the fault and remained stable; during the transient voltage fault, the maximum power angle of the converter output reached 39.59°.

[0104] When the method for improving the transient stability of the converter under the non-ideal power grid proposed by the present invention is used, the current and power angle curves of the grid-connected converter output are as follows: Figure 7 and Figure 8 As shown. Among them, Figure 7 A schematic diagram of the output current of a grid-connected converter under the method for improving the transient stability of a converter under a non-ideal power grid provided by an embodiment of the present invention; Figure 8A schematic diagram of the output power angle of a grid-connected converter under the method for improving the transient stability of a converter under a non-ideal power grid provided in an embodiment of the present invention.

[0105] from Figure 7 and Figure 8 It can be seen that when the method for improving transient stability of a grid-connected converter proposed in the present invention is used, the current of the grid-connected converter can quickly recover to the rated level after the grid-side transient voltage fault is cleared.

[0106] As for the output power angle of the grid-connected converter, during the transient voltage fault, the maximum output power angle of the converter is reduced to 35.72°.

[0107] Obviously, the method for improving the transient stability of the grid-connected converter proposed in the present invention can ensure that the output current of the converter is restored as soon as possible during a transient voltage fault, and the maximum power angle fluctuation is also reduced. While ensuring overcurrent suppression during the fault to a certain extent, it improves the transient stability performance of the grid-connected converter.

[0108] Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

[0109] The present invention proposes a method for improving the transient stability of a grid-connected converter under a non-ideal power grid, which improves the transient stability performance of the grid-connected converter while ensuring overcurrent suppression during a transient voltage drop fault. Different from the design concept of conventional virtual impedance, the present invention is not limited to relying solely on the grid voltage amplitude and the converter output current to adapt the virtual impedance value. At the same time, the resistance proportionality coefficient of the virtual impedance is flexibly adjusted according to the degree of grid voltage drop during the transient fault, ensuring overcurrent suppression during the transient fault of the grid-connected converter. In addition, the adjustment method of the resistance proportionality coefficient in the present invention is nonlinear, which effectively guarantees overcurrent suppression during the transient fault, and avoids the transient instability phenomenon that may be caused by excessive growth of virtual impedance, effectively improving the transient stability performance of the converter.

[0110] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for improving transient stability of a converter under a non-ideal power grid, characterized in that: Applied to a grid-connected converter based on VSG technology, the method for improving the transient stability of the grid-connected converter includes: Performing current limiting control on the grid-connected converter based on a preset virtual impedance current limiting control strategy; Detect the voltage amplitude of the power grid; When the grid voltage amplitude is less than the voltage threshold, the resistance proportional coefficient is adjusted to update the virtual impedance current limiting control strategy and limit the current limiting voltage drop within the voltage drop threshold.

2. The method for improving transient stability of a converter under a non-ideal power grid according to claim 1, characterized in that: The formula for the VSG technique is as follows: In the above formula, P and Q are the output power of the grid-connected converter; P ref is the active power command input to the grid-connected converter, Q ref is the reactive power command input to the grid-connected converter; ω and ω0 are the grid-connected converter output angular frequency and angular frequency input reference command respectively; J is the virtual inertia added by the grid-connected converter; D p and D q is the damping coefficient in VSG technology; K v is the gain coefficient of reactive voltage regulation; E0 is the reference instruction of the input internal potential of the grid-connected converter.

3. The method for improving transient stability of a converter in a non-ideal power grid according to claim 1, characterized in that: The detecting of the grid voltage amplitude comprises: Collect three-phase grid voltage; Perform dq transformation on the three-phase grid voltage to obtain the grid voltage in dq coordinate system; The grid voltage amplitude is obtained based on the grid voltage in the dq coordinate system.

4. The method for improving transient stability of a converter in a non-ideal power grid according to claim 3, characterized in that: The three-phase grid voltage is subjected to dq transformation by a dq transformation formula; the dq transformation formula is shown as follows: In the above formula, x0 is the zero-sequence voltage or current; x a 、x b and x c is a three-phase voltage signal or a three-phase current signal; x d is the voltage signal or current signal corresponding to the d-axis, x q is the voltage signal or current signal corresponding to the q-axis; θ is the phase angle of the internal output of the grid-connected converter.

5. The method for improving transient stability of a converter in a non-ideal power grid according to claim 1, characterized in that: The virtual impedance current limiting control strategy includes: In the above formula, I lim is the set current limiting threshold; k r is the resistance proportionality coefficient of the virtual impedance, k x is the reactance proportionality coefficient of the virtual impedance; R VIR is the virtual resistance, X VIR is the virtual reactance; i d is the d-axis current signal, i q is the q-axis current signal; R H is the high frequency suppression resistor, X H is the high frequency suppression reactance; ω c is the cutoff frequency of the high-pass filter; V d,VIR is the d-axis equivalent voltage drop caused by the virtual impedance, V q,VIR is the q-axis equivalent voltage drop caused by the virtual impedance; E dref Enter the d-axis voltage reference value for the voltage loop, E qref Input the q-axis voltage reference value for the voltage loop; E is the internal potential amplitude output in the VSG technology.

6. The method for improving transient stability of a converter in a non-ideal power grid according to claim 5, characterized in that: Adjust the resistance ratio by the following formula: In the above formula, k0 is the first adjustment coefficient of the resistance proportionality coefficient, k1 is the second adjustment coefficient of the resistance proportionality coefficient; v gamp is the grid voltage amplitude.

7. The method for improving transient stability of a converter in a non-ideal power grid according to claim 1, characterized in that: Also includes: When the grid voltage amplitude is greater than or equal to the voltage threshold, the resistance proportional coefficient is maintained at the initial proportional coefficient.

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