High-frequency oscillation suppression method and system for network-forming type converter

Abstract

The invention discloses a high-frequency oscillation suppression method and system for a network-constructing type converter, and the method comprises the steps: obtaining a voltage parameter and a current parameter of the converter, so as to construct a wind field high-frequency impedance model and a converter station high-frequency impedance model; constructing a high-frequency impedance model corresponding to the converter according to the wind field high-frequency impedance model and the converter station high-frequency impedance model; performing equivalent conversion on the high-frequency impedance model to obtain an equivalent circuit where the current converter is located; determining influence factors causing the high-frequency oscillation of the current converter according to the equivalent circuit, and formulating a suppression strategy corresponding to the current converter according to the influence factors; and performing high-frequency oscillation suppression on the converter according to the suppression strategy to improve the suppression effect of high-frequency oscillation.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power grids, and in particular to a method and system for suppressing high-frequency oscillation of a grid-type converter. Background Art

[0002] With the rapid development of offshore wind power, offshore wind power has rich engineering practice experience in flexible direct current transmission. In this scenario, the offshore wind farm is connected to the grid in an isolated way, and the offshore flexible direct current converter station operates in grid construction control (GFM-MMC). Since both the wind farm and the flexible direct current converter station have significant power electronic characteristics, the interactive coupling of power electronic control occurs in the system, introducing high-frequency oscillation problems. Once oscillation occurs, it may cause equipment damage at the least, or even cause the wind turbine to be disconnected from the grid and the system to stop operating, threatening the stable operation of the AC system.

[0003] The suppression strategy of high-frequency oscillation usually adopts three ideas: reducing the total delay of the control link, installing passive filtering devices and active damping control. Since the control link delay is affected by many factors such as the control algorithm, hardware device, and operating environment, the cost of passive filtering devices is relatively high and the floor space is large. At present, most of them use additional damping control strategies to suppress various oscillation problems. However, the existing high-frequency oscillation suppression strategies for grid-type converters have the following two problems: 1) The description of the control link of the grid-type converter is not complete, and the high-frequency oscillation of the converter cannot be effectively suppressed; 2) Most strategies only reshape the impedance from a single control loop, which is difficult to apply to the scenario of multiple resonance peaks, and may introduce new oscillation modes, resulting in poor oscillation suppression effect. Summary of the invention

[0004] In order to solve the above technical problems, the present invention discloses a method and system for suppressing high-frequency oscillation of a grid-type converter, which are used to improve the effect of suppressing high-frequency oscillation of the converter.

[0005] In order to achieve the above object, the present invention discloses a method for suppressing high-frequency oscillation of a grid-type converter, comprising:

[0006] Obtaining the voltage parameters and current parameters of the converter to construct a high-frequency impedance model of the wind farm and a high-frequency impedance model of the converter station;

[0007] Constructing a high-frequency impedance model corresponding to the converter according to the high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station;

[0008] Performing equivalent transformation on the high-frequency impedance model of the interconnected system to obtain an equivalent circuit corresponding to the converter;

[0009] Determining the influencing factors causing the high-frequency oscillation of the converter according to the equivalent circuit of the interconnected system, and formulating a corresponding suppression strategy for the converter according to the influencing factors;

[0010] High-frequency oscillation of the converter is suppressed according to the suppression strategy.

[0011] The present invention discloses a method for suppressing high-frequency oscillation of a grid-type converter. First, the voltage parameters and current parameters of the converter are obtained to construct a high-frequency impedance model of a wind farm and a high-frequency impedance model of a converter station. The control structure of the converter is derived by the constructed high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station to construct an impedance model corresponding to the converter, thereby improving the accuracy of the derivation of the high-frequency impedance of the converter. Furthermore, after the high-frequency impedance model corresponding to the converter is derived, it is equivalent to a circuit, so as to obtain the influencing factors that cause the high-frequency oscillation of the converter by analyzing the circuit, and then the oscillation is suppressed according to the influencing factors to improve the suppression effect.

[0012] As a preferred example, the step of obtaining voltage parameters and current parameters of the converter includes:

[0013] Collecting the valve-side three-phase AC voltage and the valve-side three-phase AC current of the converter, and calculating the virtual synchronization phase angle according to the rated frequency of the converter connected to the power grid;

[0014] Performing Park changes on the valve-side three-phase AC voltage and the valve-side three-phase AC current according to the virtual synchronous phase angle to obtain the valve-side AC voltage and the valve-side AC current of the converter in a virtual synchronous speed rotating coordinate system;

[0015] The grid-side three-phase AC voltage and grid-side three-phase AC current of the converter and the intermediate DC bus voltage and intermediate DC current between the converter and the wind farm are collected.

[0016] According to the function of the converter between the wind farm and the converter station, the present invention collects the voltage and current of the converter on the valve side and the grid side, as well as the intermediate current and intermediate voltage flowing between the converter and the wind farm, so as to provide an accurate parameter basis for subsequent impedance calculation and ensure the effect of oscillation suppression.

[0017] As a preferred example, the step of calculating the virtual synchronization phase angle according to the rated frequency of the converter connected to the power grid includes:

[0018] The virtual synchronous phase angle is obtained according to the rated frequency and the preset phase angle calculation formula; wherein the expression of the phase angle calculation formula is:

[0019]

[0020] Wherein, the θ1 represents the virtual synchronous phase angle; and the ω1 represents the rated frequency of the power grid.

[0021] The present invention obtains the virtual synchronous phase angle to perform Park transformation on the obtained current parameters and voltage parameters, thereby realizing the transformation of the current parameters and voltage parameters on different coordinate systems, and then performing impedance calculation through unified data.

[0022] As a preferred example, the step of obtaining the valve-side AC voltage and the valve-side AC current of the converter in a virtual synchronous speed rotating coordinate system includes:

[0023] The Parker change of the valve-side three-phase AC voltage is performed according to the virtual synchronous phase angle to obtain the component voltages of the valve-side three-phase AC voltage in different axes in the virtual synchronous speed rotating coordinate system; wherein the expression of the Parker change corresponding to the valve-side three-phase AC voltage is:

[0024]

[0025] Among them, the v ga 、v gb 、v gc represents the three-phase AC voltage on the valve side; the v gd 、v gq represents the component voltages of the valve-side three-phase AC voltage on the d-axis and q-axis in the virtual synchronous speed rotating coordinate system; wherein, V g and I g Refers to the valve side AC voltage and threshold AC current, where a, b, and c are used to represent different phases;

[0026] The Park change of the valve-side three-phase AC current is performed according to the virtual synchronous phase angle to obtain component currents of the valve-side three-phase AC current in different axes in the virtual synchronous speed rotating coordinate system; wherein the expression of the Park change corresponding to the valve-side three-phase AC current is:

[0027]

[0028] Among them, the i ga 、i gb 、i gc represents the three-phase alternating current on the valve side; the i gd 、i gq It represents the component current of the three-phase alternating current on the valve side on the d-axis and the q-axis in the virtual synchronous speed rotating coordinate system.

[0029] The present invention simplifies data through the Park transformation, reduces the amount of data processing, and improves the efficiency of oscillation suppression.

[0030] As a preferred example, the construction of the wind farm high-frequency impedance model and the converter station high-frequency impedance model includes:

[0031] The wind farm is simplified to calculate the outlet voltage of the converter on the grid side of the wind turbine in the wind farm according to the component voltage and the component current; wherein the calculation formula of the outlet voltage of the converter on the grid side of the wind turbine is:

[0032]

[0033] Among them, the i Ldq The inductor current of the grid-side converter comprises the component currents on the d-axis and the q-axis in the virtual synchronous speed rotating coordinate system; the k wp ,k wi are the proportional coefficient and integral coefficient of the current controller respectively; the G dw is the digital control delay,; the L f is the filter inductance;

[0034] The grid-side converter is equivalent to a controlled current source and a parallel impedance, and the impedance in the virtual synchronous speed rotating coordinate system is transformed into a stationary coordinate system to construct the high-frequency impedance model of the converter station and the high-frequency impedance model of the wind farm; wherein the expression of the high-frequency impedance model of the converter station is:

[0035] Z gvsc =G wi (s-jω1)G dw (s-jω1)+sL f

[0036] Among them, the Z gvsc represents the high-frequency impedance model of the grid-side converter, the G wi represents the control parameters of the current controller; s and j represent the current model in the time domain;

[0037] The expression of the wind farm high frequency impedance model is:

[0038] Z w =(Z gvsc / / Z cf )+Z Lg

[0039] Among them, the Z cf represents the capacitor filter impedance; the Z Lg Represents the equivalent filter inductance impedance on the grid side; the Z w Represents the high-frequency impedance model of the wind farm.

[0040] The present invention constructs a high-frequency impedance model connected to the converter and a high-frequency impedance model of a converter station based on a control structure corresponding to the converter, so as to improve the accuracy of the impedance model corresponding to the converter, thereby improving the effect of suppressing high-frequency oscillation.

[0041] As a preferred example, the step of constructing a high-frequency impedance model corresponding to the converter includes:

[0042] According to the control structure corresponding to the converter and the circuit structure in which it is located, an impedance model of the converter in a synchronous rotating coordinate system is constructed; wherein the expression of the impedance model is:

[0043]

[0044] Among them, the L eq =L trf +L arm / 2, L trf is the transformer leakage reactance, the L arm is the bridge arm reactance, the G v , G i are the transfer functions of the controllers for the AC voltage outer loop and the current inner loop, respectively. gref is a phase modulated wave, the G vref For the modulation wave transfer function expression, the G d For digital control delay, the G fv For grid voltage feedforward control;

[0045] According to the impedance model, the wind farm high-frequency impedance model and the converter station high-frequency impedance model, a high-frequency impedance model of the converter in a stationary coordinate system is constructed, so as to calculate the high-frequency output impedance of the converter in a stationary coordinate system through the high-frequency impedance model; wherein the expression of the high-frequency impedance model of the interconnected system is:

[0046]

[0047] Among them, the Z MMC (s) represents the high frequency impedance of the converter, and Z MMC_dq (s) represents the high-frequency impedance of the converter in the dq coordinate system, and R eq Indicates the equivalent resistance of the converter.

[0048] The present invention is based on the fact that the high-frequency impedance is basically independent of the internal dynamics of the converter and is not affected by the system balance point, and thus only retains the structural parameters of the converter to build a corresponding impedance model, so as to improve the efficiency of high-frequency oscillation suppression through the impedance model.

[0049] As a preferred example, the obtaining of the equivalent circuit corresponding to the converter includes:

[0050] The high-frequency impedance model is subjected to Thevenin equivalent conversion processing to construct an equivalent circuit where the converter is located; wherein the expression of the equivalent circuit is:

[0051]

[0052] Among them, the I w represents the wind farm equivalent current, the U MMC (s) represents the converter equivalent voltage.

[0053] The present invention constructs the equivalent circuit to determine the dominant factors affecting high-frequency oscillation by analyzing the circuit, thereby improving the effect of suppressing high-frequency oscillation.

[0054] As a preferred example, determining the influencing factors causing the high-frequency oscillation of the converter according to the equivalent circuit includes:

[0055] A high-frequency output impedance model of the converter is constructed according to the equivalent circuit, and then the influencing factors causing high-frequency oscillation of the converter are determined according to the high-frequency output impedance model; wherein the expression of the high-frequency output impedance model is:

[0056]

[0057] Among them, the k vp represents the converter voltage outer loop proportionality coefficient, the k ip represents the converter current inner loop proportional coefficient, the L eq represents the equivalent resistance of the converter, and the M(s) and the D(s) are the corresponding numerator and denominator.

[0058] As a preferred example, formulating the suppression strategy corresponding to the converter according to the influencing factors includes:

[0059] Determine the parameter value corresponding to the influencing factor according to the voltage feedforward loop and the voltage control loop where the converter is located and in combination with the high-frequency output impedance model;

[0060] A suppression strategy corresponding to the converter is generated according to the parameter value.

[0061] The present invention designs high-frequency oscillation suppression measures through a voltage feedforward loop and a voltage control loop, realizes mutual compensation of disturbance components in the grid voltage feedforward and voltage control paths, and improves the suppression effect of high-frequency oscillation.

[0062] On the other hand, the present invention discloses a high-frequency oscillation suppression system for a grid-type converter, comprising: an impedance analysis module, a high-frequency impedance simulation module, an equivalent conversion module, an oscillation analysis module and an oscillation suppression module;

[0063] The impedance analysis module is used to obtain the voltage parameters and current parameters of the converter to construct a high-frequency impedance model of the wind farm and a high-frequency impedance model of the converter station;

[0064] The high-frequency impedance simulation module is used to construct a high-frequency impedance model corresponding to the converter according to the high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station;

[0065] The equivalent conversion module is used to perform equivalent conversion on the high-frequency impedance model of the interconnected system to obtain an equivalent circuit corresponding to the converter;

[0066] The oscillation analysis module is used to determine the influencing factors that cause the high-frequency oscillation of the converter according to the equivalent circuit of the interconnected system, and formulate a corresponding suppression strategy for the converter according to the influencing factors;

[0067] The oscillation suppression module is used to suppress high-frequency oscillation of the converter according to the suppression strategy.

[0068] The present invention discloses a method for suppressing high-frequency oscillation of a grid-type converter. First, the voltage parameters and current parameters of the converter are obtained to construct a high-frequency impedance model of a wind farm and a high-frequency impedance model of a converter station. The control structure of the converter is derived by the constructed high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station to construct an impedance model corresponding to the converter, thereby improving the accuracy of the derivation of the high-frequency impedance of the converter. Furthermore, after the high-frequency impedance model corresponding to the converter is derived, it is equivalent to a circuit, so as to obtain the influencing factors that cause the high-frequency oscillation of the converter by analyzing the circuit, and then the oscillation is suppressed according to the influencing factors to improve the suppression effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 : A schematic flow chart of a method for suppressing high-frequency oscillation of a grid-type converter disclosed in an embodiment of the present invention;

[0070] Figure 2 : It is a structural schematic diagram of a high-frequency oscillation suppression system of a grid-type converter disclosed in an embodiment of the present invention;

[0071] Figure 3 : A schematic flow chart of a method for suppressing high-frequency oscillation of a grid-type converter disclosed in another embodiment of the present invention;

[0072] Figure 4 : is a structural diagram of an interconnection system disclosed in another embodiment of the present invention;

[0073] Figure 5 : A schematic diagram of a control structure of a converter disclosed in another embodiment of the present invention;

[0074] Figure 6 : is a schematic diagram of an equivalent circuit disclosed in another embodiment of the present invention;

[0075] Figure 7 : A Bode diagram of a numerator / denominator in a high-frequency impedance of a converter disclosed in another embodiment of the present invention;

[0076] Figure 8 : A schematic diagram of PCC voltage and current when high-frequency oscillation suppression is not performed, disclosed in another embodiment of the present invention;

[0077] Fig. 9 : A schematic diagram of FFT of a PCC voltage when high-frequency oscillation suppression is not performed, disclosed in another embodiment of the present invention;

[0078] Fig.10 : A schematic diagram of PCC voltage and current after high-frequency oscillation suppression disclosed in another embodiment of the present invention;

[0079] Fig.11 : A schematic diagram of PCC voltage and current under different delays after high-frequency oscillation suppression disclosed in another embodiment of the present invention. DETAILED DESCRIPTION

[0080] 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.

[0081] Embodiment 1

[0082] This embodiment discloses a method for suppressing high-frequency oscillation of a grid-type converter. For the specific implementation process of the suppression method, please refer to Figure 1 , mainly including steps 101 to 105, wherein the steps are mainly:

[0083] Step 101: Obtain voltage parameters and current parameters of the converter to construct a high-frequency impedance model of the wind farm and a high-frequency impedance model of the converter station.

[0084] In this embodiment, this step mainly includes: collecting the valve-side three-phase AC voltage and the valve-side three-phase AC current of the converter, and calculating the virtual synchronous phase angle according to the rated frequency of the converter connected to the power grid; performing Parker transformation on the valve-side three-phase AC voltage and the valve-side three-phase AC current according to the virtual synchronous phase angle to obtain the valve-side AC voltage and the valve-side AC current of the converter in the virtual synchronous speed rotating coordinate system; collecting the grid-side three-phase AC voltage and the grid-side three-phase AC current of the converter and the intermediate DC bus voltage and the intermediate DC current between the converter and the wind farm.

[0085] Wherein, the virtual synchronization phase angle is obtained according to the rated frequency and the preset phase angle calculation formula; wherein the expression of the phase angle calculation formula is:

[0086]

[0087] Wherein, the θ1 represents the virtual synchronous phase angle; and the ω1 represents the rated frequency of the power grid.

[0088] The Parker change of the valve-side three-phase AC voltage is performed according to the virtual synchronous phase angle to obtain the component voltages of the valve-side three-phase AC voltage in different axes in the virtual synchronous speed rotating coordinate system; wherein the expression of the Parker change corresponding to the valve-side three-phase AC voltage is:

[0089]

[0090] Among them, the v ga 、v gb 、v gc represents the three-phase AC voltage on the valve side; the v gd 、v gq represents the component voltages of the valve-side three-phase AC voltage on the d-axis and q-axis in the virtual synchronous speed rotating coordinate system; wherein, V g and I g Refers to the valve side AC voltage and threshold AC current, where a, b, and c are used to represent different phases;

[0091] The Park change of the valve-side three-phase AC current is performed according to the virtual synchronous phase angle to obtain component currents of the valve-side three-phase AC current in different axes in the virtual synchronous speed rotating coordinate system; wherein the expression of the Park change corresponding to the valve-side three-phase AC current is:

[0092]

[0093] Among them, the i ga 、i gb 、i gc represents the three-phase alternating current on the valve side; the i gd 、i gq represents the component currents of the three-phase alternating current on the valve side on the d-axis and the q-axis in the virtual synchronous speed rotating coordinate system;

[0094] After obtaining the parameters, the wind farm is simplified to calculate the outlet voltage of the converter on the grid side of the wind turbine in the wind farm according to the component voltage and the component current; wherein the calculation formula of the outlet voltage of the converter on the grid side of the wind turbine is:

[0095]

[0096] Among them, the i Ldq The inductor current of the grid-side converter comprises the component currents on the d-axis and the q-axis in the virtual synchronous speed rotating coordinate system; the k wp ,k wi are the proportional coefficient and integral coefficient of the current controller respectively; the G dw is the digital control delay,; the L f is the filter inductance;

[0097] The grid-side converter is equivalent to a controlled current source and a parallel impedance, and the impedance in the virtual synchronous speed rotating coordinate system is transformed into a stationary coordinate system to construct the high-frequency impedance model of the converter station and the high-frequency impedance model of the wind farm; wherein the expression of the high-frequency impedance model of the converter station is:

[0098] Z gvsc =G wi (s-jω1)G dw (s-jω1)+sL f

[0099] Among them, the Z gvsc represents the high-frequency impedance model of the grid-side converter, the G wi represents the control parameters of the current controller; s and j represent the current model in the time domain. The expression of the wind farm high-frequency impedance model is:

[0100] Z w =(Z gvsc / / Z cf )+Z Lg

[0101] Among them, the Z cf represents the capacitor filter impedance; the Z Lg Represents the equivalent filter inductance impedance on the grid side; the Z w express.

[0102] In this embodiment, this step collects the voltage and current of the converter on the valve side and the grid side, as well as the intermediate current and intermediate voltage flowing between the converter and the wind farm, based on the functional role of the converter between the wind farm and the converter station, to provide accurate parameter basis for subsequent impedance calculation and ensure the effect of oscillation suppression.

[0103] Furthermore, by acquiring the virtual synchronous phase angle, the obtained current parameters and voltage parameters are subjected to Park transformation, thereby realizing the transformation of current parameters and voltage parameters in different coordinate systems, and then impedance calculation is performed through unified data, and the data is simplified through the Park transformation, thereby reducing the amount of data processing and improving the efficiency of oscillation suppression.

[0104] Step 102: construct a high-frequency impedance model corresponding to the converter according to the high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station.

[0105] In this embodiment, this step mainly includes: constructing an impedance model of the converter in a synchronous rotating coordinate system according to the control structure corresponding to the converter and the circuit structure in which the converter is located; wherein the expression of the impedance model is:

[0106]

[0107] Among them, the L eq =L trf +L arm / 2, L trf is the transformer leakage reactance, the L arm is the bridge arm reactance, the G v , G i are the transfer functions of the controllers for the AC voltage outer loop and the current inner loop, respectively. gref is a phase modulated wave, the G vref For the modulation wave transfer function expression, the G d For digital control delay, the G fv For grid voltage feedforward control;

[0108] According to the impedance model, the wind farm high-frequency impedance model and the converter station high-frequency impedance model, a high-frequency impedance model of the converter in a stationary coordinate system is constructed, so as to calculate the high-frequency output impedance of the converter in a stationary coordinate system through the high-frequency impedance model; wherein the expression of the high-frequency impedance model of the interconnected system is:

[0109]

[0110] Among them, the Z MMC (s) represents the high frequency impedance of the converter, and Z MMC_dq (s) represents the high-frequency impedance of the converter in the dq coordinate system, and R eq Indicates the equivalent resistance of the converter.

[0111] In this embodiment, this step constructs an impedance model of the converter based on the high-frequency impedance model connected to the converter and the high-frequency impedance model of the converter station to improve the accuracy of the impedance model corresponding to the converter, thereby improving the effect of high-frequency oscillation suppression. At the same time, based on the fact that the high-frequency band impedance is basically irrelevant to the internal dynamics of the converter and is not affected by the system balance point, only the structural parameters of the converter are retained to construct the corresponding impedance model, so as to improve the efficiency of high-frequency oscillation suppression through the impedance model.

[0112] Step 103: performing equivalent transformation on the high-frequency impedance model to obtain an equivalent circuit where the converter is located.

[0113] In this embodiment, this step mainly includes: performing Thevenin equivalent conversion processing on the high-frequency impedance model to construct an equivalent circuit where the converter is located; wherein the expression of the equivalent circuit is:

[0114]

[0115] Among them, the I w represents the wind farm equivalent current, the U MMC (s) represents the converter equivalent voltage.

[0116] In this embodiment, this step constructs the equivalent circuit to determine the dominant factors affecting high-frequency oscillation by analyzing the circuit, thereby improving the effect of suppressing high-frequency oscillation.

[0117] Step 104: determining the influencing factors causing the high-frequency oscillation of the converter according to the equivalent circuit of the interconnected system, and formulating a corresponding suppression strategy for the converter according to the influencing factors.

[0118] In this embodiment, this step mainly includes: constructing a high-frequency output impedance model of the converter according to the equivalent circuit, and then determining the influencing factors causing the high-frequency oscillation of the converter according to the high-frequency output impedance model; wherein the expression of the high-frequency output impedance model is:

[0119]

[0120] Among them, the k vp represents the converter voltage outer loop proportionality coefficient, the k ip represents the converter current inner loop proportional coefficient, the L eq represents the equivalent resistance of the converter, and the M(s) and the D(s) are the corresponding numerator and denominator.

[0121] Furthermore, according to the voltage feedforward loop and the voltage control loop in which the converter is located, the parameter values ​​corresponding to the influencing factors are determined in combination with the high-frequency output impedance model; and according to the parameter values, the suppression strategy corresponding to the converter is generated.

[0122] In this embodiment, this step designs high-frequency oscillation suppression measures through a voltage feedforward loop and a voltage control loop, realizes mutual compensation of disturbance components in the grid voltage feedforward and voltage control paths, and improves the suppression effect of high-frequency oscillation.

[0123] Step 105: Suppress high-frequency oscillation of the converter according to the suppression strategy.

[0124] On the other hand, this embodiment also discloses a high-frequency oscillation suppression system for a grid-type converter. For the specific structural composition of the suppression system, please refer to Figure 2 , including: an impedance analysis module 201, a high-frequency impedance simulation module 202, an equivalent conversion module 203, an oscillation analysis module 204 and an oscillation suppression module 205.

[0125] The impedance analysis module 201 is used to obtain voltage parameters and current parameters of the converter to construct a high-frequency impedance model of the wind farm and a high-frequency impedance model of the converter station.

[0126] The high-frequency impedance simulation module 202 is used to construct a high-frequency impedance model corresponding to the converter according to the high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station.

[0127] The equivalent conversion module 203 is used to perform equivalent conversion on the high-frequency impedance model of the interconnected system to obtain an equivalent circuit where the converter is located.

[0128] The oscillation analysis module 204 is used to determine the influencing factors that cause the high-frequency oscillation of the converter according to the equivalent circuit, and formulate a corresponding suppression strategy for the converter according to the influencing factors.

[0129] The oscillation suppression module 205 is used to suppress high-frequency oscillation of the converter according to the suppression strategy.

[0130] A method and system for suppressing high-frequency oscillation of a grid-type converter disclosed in this embodiment first obtains the voltage parameters and current parameters of the converter to construct a high-frequency impedance model of a wind farm and a high-frequency impedance model of a converter station, and derives the control structure of the converter through the constructed high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station to construct an impedance model corresponding to the converter, thereby improving the accuracy of the derivation of the high-frequency impedance of the converter. Furthermore, after the high-frequency impedance model corresponding to the converter is derived, it is equivalent to a circuit, so as to obtain the influencing factors that cause the high-frequency oscillation of the converter by analyzing the circuit, and then suppress the oscillation according to the influencing factors to improve the suppression effect.

[0131] Embodiment 2

[0132] In the prior art, the method for suppressing high-frequency oscillations in a grid-type converter does not take into account the complete control structure corresponding to the converter, and often reshapes the impedance of the converter according to a single control loop, resulting in a suppression frequency formulated according to the impedance reshaping that is not only poor in effect and difficult to adapt to the scenario of multiple resonance peaks, but is also likely to cause a new oscillation mode in the converter.

[0133] In order to solve the above technical problems, this embodiment derives the high-frequency impedance of the converter output according to the complete control structure corresponding to the converter, and implements full feedforward control of the grid voltage and formulates suppression measures according to the proportional coefficient of voltage and current to improve the effect of oscillation suppression. In this regard, in certain implementations of this embodiment, a high-frequency oscillation suppression method for a grid-type converter is disclosed. For the specific implementation process of the suppression method, please refer to Figure 3 , including steps 301 to 305, wherein the steps are mainly:

[0134] Step 301: Obtain voltage parameters and current parameters for performing impedance analysis on the converter.

[0135] In this embodiment, the steps are mainly as follows: first, a virtual synchronous phase angle θ1 is obtained according to the rated frequency ω1 of the power grid; wherein the calculation expression of θ1 is:

[0136]

[0137] Next, collect the AC three-phase voltage v on the converter valve side gabc and three-phase current i gabc , for v gabc and i gabc Perform Park transformation to obtain the converter station AC voltage v in the virtual synchronous speed rotating coordinate system gdq and the converter station AC current i gdq The process of the Pike change is:

[0138]

[0139] Among them, the v ga 、v gb 、v gc Indicates the v gabc The voltages corresponding to the three phases a, b, and c respectively; the v gd 、v gq Indicates the v gabc The component voltages on the d-axis and q-axis in the virtual synchronous speed rotating coordinate system; the i ga 、i gb 、i gc Indicates the i gabc The currents corresponding to the three phases a, b, and c respectively; the i gd 、i gq Indicates the i gabc The component currents on the d-axis and q-axis in the virtual synchronous speed rotating coordinate system; the V g and I g Refers to the valve side AC voltage and threshold AC current.

[0140] Collect the three-phase AC voltage v on the grid side of the converter sj and the alternating current i sj (j = a, b, c), and the intermediate DC bus voltage V between the converter and the wind farm bus dc and the DC current i dc .

[0141] Step 302: constructing a wind farm high-frequency impedance model and a converter station high-frequency impedance model corresponding to the converter according to the voltage parameters and the current parameters.

[0142] In this embodiment, the step mainly includes: simplifying the wind turbines, generators, and machine-side converters in the wind farm connected to the converter as controlled current sources, and only considering the grid-side converter and the intermediate DC link. Then the grid-side converter outlet voltage connected to the wind turbine group can be expressed by the following formula:

[0143]

[0144] Among them, the i Ldq The inductor current of the grid-side converter comprises the component currents on the d-axis and the q-axis in the virtual synchronous speed rotating coordinate system; the k wp ,k wi are respectively the proportional coefficient and integral coefficient of the current controller; wherein the current controller is a part of the control link of the grid-side converter in the wind farm; the G dw is the digital control delay,; the L f is the filter inductance;

[0145] Next, the grid-side converter is equivalent to a controlled current source and a parallel impedance, and the impedance in the dq coordinate system, i.e., the virtual synchronous speed rotating coordinate system, is transformed into the stationary coordinate system. Then, the converter side impedance, i.e., the converter station high-frequency impedance model, can be expressed by the following formula:

[0146] Z gvsc =G wi (s-jω1)G dw (s-jω1)+sL f

[0147] Among them, the Z gvsc represents the high-frequency impedance model of the grid-side converter, the G wi represents the control parameters of the current controller; and s and j represent the current model in the time domain.

[0148] Therefore, the high-frequency impedance of the wind turbine at the grid common connection point, that is, the constructed high-frequency impedance model of the wind farm, can be expressed by the following formula:

[0149] Z w =(Z gvsc / / Z cf )+Z Lg

[0150] Among them, the Z cf represents the capacitor filter impedance; the Z Lg Represents the equivalent filter inductance impedance on the grid side; the Z w Represents the high-frequency impedance model of the wind farm.

[0151] Step 303: construct a high-frequency impedance model corresponding to the converter according to the high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station.

[0152] In this embodiment, this step is mainly based on: the high-frequency impedance of the converter is basically independent of the internal dynamics of the converter, and is not affected by the balance point of the interconnected system consisting of the wind farm and the converter station including the converter to be oscillated. The specific structure of the interconnected system can be referred to Figure 4 ,Depend on Figure 4 It can be seen that the interconnected system includes wind turbines and converter stations. Therefore, only the structural parameters of the converter are retained. Figure 5 The control block diagram and circuit structure corresponding to the converter shown in the figure are used to establish an impedance model taking into account the control link, that is, the impedance model of the converter in the synchronous rotating coordinate system can be expressed by the following formula:

[0153]

[0154] Among them, the L eq =L trf +L arm / 2, L trf is the transformer leakage reactance, the L arm is the bridge arm reactance, the G v , G i are the transfer functions of the controllers for the AC voltage outer loop and the current inner loop, respectively. vref is a phase modulated wave, the G vref For the modulation wave transfer function expression, the G d For digital control delay, the G fv It is the grid voltage feedforward control.

[0155] Next, the coordinate system of the impedance model is transformed, and the MMC high-frequency output impedance in the stationary coordinate system can be expressed by the following formula:

[0156]

[0157] Among them, the Z MMC (s) represents the high frequency impedance of the converter, and Z MMC_dq (s) represents the high-frequency impedance of the converter in the dq coordinate system, and Req Indicates the equivalent resistance of the converter.

[0158] Step 304: construct an equivalent circuit of the converter according to the high-frequency impedance model, so as to determine key influencing factors causing high-frequency oscillation of the converter according to the equivalent circuit.

[0159] In this embodiment, the step mainly includes: adopting grid control based on the grid-type converter, and the grid control is embodied as a voltage source characteristic, then the impedance model corresponding to the converter adopts the Thevenin equivalent, that is, the equivalent circuit where the converter is located is constructed in the form of a voltage source series output impedance.

[0160] In this embodiment, the rectifier station where the converter is located in the interconnected system controls the AC bus voltage, and the wind turbine converter controls the output current. At this time, the equivalent circuit of the converter is as follows: Figure 6 As shown. The rectifier station is an MMC converter station. The wind farm is subjected to Norton equivalent and the converter station is subjected to Thevenin equivalent. The two are connected in series to obtain an equivalent circuit where the converter is located.

[0161] After obtaining the equivalent circuit, the feedback control system of the converter can be determined according to the equivalent circuit. According to the equivalent circuit, it can be known that the interconnection system is stable when ZMMC(s) / Zw(s) satisfies the Nyquist criterion. The calculation expression of the equivalent circuit is:

[0162]

[0163] Among them, the I w represents the wind farm equivalent current, the U MMC (s) represents the converter equivalent voltage.

[0164] Furthermore, since the integral terms of the voltage controller and the current controller are mainly used to track the steady-state quantity without error, and the integral terms of the controller can be ignored when analyzing the high-frequency impedance, the high-frequency output impedance of the converter can be simplified to the following formula:

[0165]

[0166] Among them, the k vp represents the converter voltage outer loop proportionality coefficient, the k ip represents the converter current inner loop proportional coefficient, the L eq represents the equivalent resistance of the converter, and the M(s) and the D(s) are the corresponding numerator and denominator.

[0167] Referring to the above impedance formula, we can see that the frequency characteristics of the numerator and denominator of the high-frequency impedance of the converter are as follows: Figure 7 See Figure 7, the numerator M(s) is determined by the passive impedance part and current control. In the high frequency band, the passive component inductance is dominant. From the phase characteristics, it can be seen that the delay term in the current control leads to the existence of the negative damping area. For the denominator 1 / D(s), the existence of the delay term causes the phase to show periodic changes. It can be concluded from the figure that the existence of the delay term in the denominator is the root cause of the periodic change of the MMC impedance.

[0168] Step 305: Formulate a suppression strategy according to the key influencing factors, and suppress high-frequency oscillation of the converter according to the suppression strategy.

[0169] In this embodiment, the steps are mainly as follows: in a converter station adopting grid-type fixed Vf control, both the voltage feedforward loop and the voltage control loop will introduce voltage disturbances, and the high-frequency impedance characteristics are jointly determined by the two loops. Therefore, when designing active damping based on grid voltage feedforward and feedback, both control loops should be taken into account at the same time.

[0170] In order to coordinate the design of grid voltage feedforward link and voltage control link parameters, the case of full grid voltage feedforward is considered, that is, G fv =1, and the high-frequency equivalent inductance impedance is much larger than the base frequency equivalent inductance impedance. At this time, the high-frequency impedance of the converter can be expressed by the following formula:

[0171]

[0172] When k vp and k ip When the product of is 1, Z MMC (s)≈sL eq +R eq , at this time, the converter station is characterized by filter inductance, indicating full voltage feedforward, and the high-frequency components in the voltage control loop cancel each other out, thereby avoiding the negative damping characteristics introduced by the voltage disturbance component. Parameters are selected based on the principle of current loop bandwidth limitation, voltage loop bandwidth limitation, and high-frequency band proportional coefficient dominance. There exists K = k vp k ip =1 parameter selection. Among them, the suppression strategy formulated is the k vp , k ip The parameters are set to ensure that the k vp , k ip The product of is 1.

[0173] In some implementations of this embodiment, in order to verify the inhibitory effect of the inhibitory method, the k vp =0,5,k vi =100, k ip =1, k ii= 20, the simulation step is 2us, then the wind farm is started within 0 to 1s, and when t = 1.2s, the control system delay is increased from 300μs to 500μs. Figure 8 It can be seen that after the delay increases, the voltage at the common connection point of the interconnected system immediately appears high-order harmonics and diverges and becomes unstable. FFT analysis of the voltage at the common connection point shows that Fig. 9 It can be seen that the oscillation frequency is mainly 1400Hz, and the high-order harmonic THD reaches 319.90%.

[0174] Furthermore, in the same simulation timing, according to the oscillation suppression strategy provided in this embodiment, the voltage loop and current loop proportional coefficients are modified at t=1.26s, and k vp =2, k ip =0.5, by Fig.10 It can be seen that after the control delay increases, the oscillation gradually converges, verifying that the high-frequency characteristics of the system are significantly improved under this control parameter. Under this control parameter, the delay is increased by 100μs every 1s. Fig.11 It can be seen that during the delay change process, the voltage and current of the common connection point do not diverge, proving that under this control parameter combination, the negative damping area of ​​the flexible DC interconnection system is significantly reduced when the delay changes are relatively wide. In summary, this example verifies the effectiveness of the high-frequency oscillation suppression strategy of the present invention.

[0175] This embodiment discloses a method for suppressing high-frequency oscillation of a grid-type converter. The impedance model of the converter is derived by considering the control structure of the converter, and the equivalent circuit of the interconnected system composed of the wind farm and the converter station is analyzed to determine the dominant factors affecting the high-frequency oscillation. By implementing full feedforward control of the grid voltage and designing high-frequency oscillation suppression measures based on the proportional coefficient of voltage and current, mutual compensation of disturbance components in the grid voltage feedforward and voltage control path is achieved, which not only ensures that the output impedance is approximately passive in the high-frequency range, thereby optimizing the damping performance in the high-frequency region, and effectively suppressing the high-frequency oscillation of the converter.

[0176] 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 suppressing high-frequency oscillation of a grid-type converter, characterized in that: include: Obtaining the voltage parameters and current parameters of the converter to construct a high-frequency impedance model of the wind farm and a high-frequency impedance model of the converter station; Constructing a high-frequency impedance model corresponding to the converter according to the high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station; Performing equivalent transformation on the high-frequency impedance model to obtain an equivalent circuit corresponding to the converter; Determining the influencing factors causing the high-frequency oscillation of the converter according to the equivalent circuit, and formulating a corresponding suppression strategy for the converter according to the influencing factors; High-frequency oscillation of the converter is suppressed according to the suppression strategy.

2. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 1, characterized in that: The step of obtaining the voltage parameters and current parameters of the converter includes: Collecting the valve-side three-phase AC voltage and the valve-side three-phase AC current of the converter, and calculating the virtual synchronization phase angle according to the rated frequency of the converter connected to the power grid; Performing Park changes on the valve-side three-phase AC voltage and the valve-side three-phase AC current according to the virtual synchronous phase angle to obtain the valve-side AC voltage and the valve-side AC current of the converter in a virtual synchronous speed rotating coordinate system; The grid-side three-phase AC voltage and grid-side three-phase AC current of the converter and the intermediate DC bus voltage and intermediate DC current between the converter and the wind farm are collected.

3. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 2, characterized in that: The step of calculating the virtual synchronization phase angle according to the rated frequency of the converter connected to the power grid comprises: The virtual synchronous phase angle is obtained according to the rated frequency and the preset phase angle calculation formula; wherein the expression of the phase angle calculation formula is: Wherein, the θ1 represents the virtual synchronous phase angle; and the ω1 represents the rated frequency of the power grid.

4. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 3, characterized in that: The obtaining of the valve-side AC voltage and the valve-side AC current of the converter in a virtual synchronous speed rotating coordinate system comprises: The Parker change of the valve-side three-phase AC voltage is performed according to the virtual synchronous phase angle to obtain the component voltages of the valve-side three-phase AC voltage in different axes in the virtual synchronous speed rotating coordinate system; wherein the expression of the Parker change corresponding to the valve-side three-phase AC voltage is: Among them, the v ga 、v gb 、v gc represents the three-phase AC voltage on the valve side; the v gd 、v gq represents the component voltages of the valve-side three-phase AC voltage on the d-axis and q-axis in the virtual synchronous speed rotating coordinate system; wherein, V g and I g Refers to the valve side AC voltage and threshold AC current, where a, b, and c are used to represent different phases; The Park change of the valve-side three-phase AC current is performed according to the virtual synchronous phase angle to obtain component currents of the valve-side three-phase AC current in different axes in the virtual synchronous speed rotating coordinate system; wherein the expression of the Park change corresponding to the valve-side three-phase AC current is: Among them, the i ga 、i gb 、i gc represents the three-phase alternating current on the valve side; the i gd 、i gq It represents the component current of the three-phase alternating current on the valve side on the d-axis and the q-axis in the virtual synchronous speed rotating coordinate system.

5. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 4, characterized in that: The constructing of the wind farm high frequency impedance model comprises: The wind farm is simplified to calculate the outlet voltage of the converter on the grid side of the wind turbine in the wind farm according to the component voltage and the component current; wherein the calculation formula of the outlet voltage of the converter on the grid side of the wind turbine is: Among them, the i Ldq The inductor current of the grid-side converter comprises the component currents on the d-axis and the q-axis in the virtual synchronous speed rotating coordinate system; the k wp ,k wi are the proportional coefficient and integral coefficient of the current controller respectively; the G dw is the digital control delay,; the L f is the filter inductance; The grid-side converter is equivalent to a controlled current source and a parallel impedance, and the impedance in the virtual synchronous speed rotating coordinate system is transformed into a stationary coordinate system to construct a high-frequency impedance model of the grid-side converter and a high-frequency impedance model of the wind farm; wherein the expression of the high-frequency impedance model of the grid-side converter is: Z gvsc =G wi (s-jω1)G dw (s-jω1)+sL f Among them, the Z gvsc represents the high-frequency impedance model of the grid-side converter, the G wi represents the control parameters of the current controller; s and j represent the current model in the time domain; The expression of the wind farm high frequency impedance model is: WITH w =(From gvsc / / WITH cf )+Z Lg Among them, the Z cf represents the capacitor filter impedance; the Z Lg Represents the equivalent filter inductor impedance; the Z w Represents the high-frequency impedance model of the wind farm.

6. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 5, characterized in that: The constructing of the high-frequency impedance model corresponding to the converter includes: According to the control structure corresponding to the converter and the circuit structure in which it is located, an impedance model of the converter in a synchronous rotating coordinate system is constructed; wherein the expression of the impedance model is: Among them, the L eq =L trf +L arm / 2, L trf is the transformer leakage reactance, the L arm is the bridge arm reactance, the G v , G i are the transfer functions of the controllers for the AC voltage outer loop and the current inner loop, respectively. gref is a phase modulated wave, the G vref For the modulation wave transfer function expression, the G d For digital control delay, the G fv For grid voltage feedforward control; According to the impedance model, the wind farm high-frequency impedance model and the converter station high-frequency impedance model, a high-frequency impedance model of the converter in a stationary coordinate system is constructed, so as to calculate the high-frequency output impedance of the converter in a stationary coordinate system through the high-frequency impedance model; wherein the expression of the high-frequency impedance model is: Among them, the Z MMC (s) represents the high frequency impedance of the converter, and Z MMC_dq (s) represents the high-frequency impedance of the converter in the dq coordinate system, and R eq Indicates the equivalent resistance of the converter.

7. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 6, characterized in that: The obtaining of an equivalent circuit corresponding to the converter comprises: The high-frequency impedance model is subjected to Thevenin equivalent conversion processing to construct an equivalent circuit where the converter is located; wherein the expression of the equivalent circuit is: Among them, the I w represents the wind farm equivalent current, the U MMC (s) represents the converter equivalent voltage.

8. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 7, characterized in that: The determining, according to the interconnected system equivalent circuit, the influencing factors causing the high-frequency oscillation of the converter comprises: A high-frequency output impedance model of the converter is constructed according to the equivalent circuit, and then the influencing factors causing high-frequency oscillation of the converter are determined according to the high-frequency output impedance model; wherein the expression of the high-frequency output impedance model is: Among them, the k vp represents the converter voltage outer loop proportionality coefficient, the k ip represents the converter current inner loop proportional coefficient, the L eq represents the equivalent resistance of the converter, and the M(s) and the D(s) are the corresponding numerator and denominator.

9. A method for suppressing high-frequency oscillation of a grid-type converter according to claim 1, characterized in that: The formulating a suppression strategy corresponding to the converter according to the influencing factors includes: Determine the parameter value corresponding to the influencing factor according to the voltage feedforward loop and the voltage control loop where the converter is located and in combination with the high-frequency output impedance model; A suppression strategy corresponding to the converter is generated according to the parameter value.

10. A high-frequency oscillation suppression system for a grid-type converter, characterized in that: include: Impedance analysis module, high-frequency impedance simulation module, equivalent conversion module, oscillation analysis module and oscillation suppression module; The impedance analysis module is used to obtain the voltage parameters and current parameters of the converter to construct a high-frequency impedance model of the wind farm and a high-frequency impedance model of the converter station; The high-frequency impedance simulation module is used to construct a high-frequency impedance model corresponding to the converter according to the high-frequency impedance model of the wind farm and the high-frequency impedance model of the converter station; The equivalent conversion module is used to perform equivalent conversion on the high-frequency impedance model to obtain an equivalent circuit corresponding to the converter; The oscillation analysis module is used to determine the influencing factors that cause the high-frequency oscillation of the converter according to the equivalent circuit, and formulate a corresponding suppression strategy for the converter according to the influencing factors; The oscillation suppression module is used to suppress high-frequency oscillation of the converter according to the suppression strategy.

Images