Wind power dc collection and transmission system grid-connected port impedance modeling and oscillation analysis method

Abstract

The present disclosure relates to a wind power DC collection and sending system grid-connected port impedance modeling and oscillation analysis method, and relates to the technical field of electric power, and comprises the following steps: obtaining a small signal equation of a system according to a wind power DC collection and sending system topology and control strategies of each link; further obtaining an admittance model of each DC wind turbine port in a wind farm; and finally obtaining an admittance model of an MMC AC port on the grid side according to a series-parallel circuit relationship, in combination with a collection line and a wind farm topology. In this way, the coupling influence of the wind power DC collection and sending system on the grid-connected port can be reflected through the impedance model, and the system oscillation risk can be reduced.

Description

Technical Field

[0001] The present disclosure relates to the field of electric power technology, and in particular to a method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system. Background Art

[0002] Driven by the "dual carbon" goals, my country is vigorously developing wind power in deep-sea and desert regions, with planned base capacity reaching 10GW. Flexible DC transmission technology is suitable for long-distance transmission of renewable energy power without synchronous power sources, but its construction costs are high. Its AC aggregation method results in significant reactive power losses and overvoltage issues, making it difficult to improve aggregation efficiency. Using DC lines to aggregate and transmit power from DC wind turbines can improve aggregation efficiency while leveraging the technical advantages of flexible DC transmission.

[0003] Wind power DC transmission systems consist of DC wind turbines, DC transformers (DCTs), and grid-side modular multilevel converters (MMCs), among other power electronics. These systems exhibit complex dynamic characteristics and present oscillation risks. The impedance method, with its clear physical implications, is widely used to analyze and resolve oscillation issues in wind power grid-connected systems. Current research focuses primarily on impedance modeling and oscillation analysis of wind power transmitted via flexible DC transmission. However, modeling and oscillation analysis of wind power DC transmission systems remains limited, and an impedance model for the grid-connected ports of wind power DC transmission systems is lacking. Summary of the Invention

[0004] In order to solve the above technical problems, the present disclosure provides a wind power DC collection and transmission system grid-connected port impedance modeling and oscillation analysis method, which is used to construct a wind power DC collection and transmission system grid-connected port impedance model, reflect the coupling effect of the collection and transmission system on the grid-connected port, and reduce the oscillation risk.

[0005] The present disclosure provides a method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC transmission system, including:

[0006] According to the wind power DC transmission system topology and the control strategy of each link, the small signal equations of the system are obtained, including the small signal equations of the DC wind turbine electrical circuit, the small signal equations of the DC wind turbine control circuit, the small signal equations of the DC transformer electrical circuit, the small signal equations of the DC transformer control circuit, the small signal equations of the grid-side MMC electrical circuit, and the small signal equations of the grid-side MMC control circuit;

[0007] According to the obtained DC wind turbine electrical circuit small signal equation and the DC wind turbine control circuit small signal equation, a DC port admittance model of a single DC wind turbine is obtained; repeating the above steps to obtain the port admittance model of each DC wind turbine in the wind farm;

[0008] According to the port admittance model of each DC wind turbine, combined with the series-parallel circuit relationship of the collection line and the wind farm topology, the collection wind farm admittance is obtained, and the collection wind farm admittance is the DCT low-voltage port input admittance;

[0009] Wherein, the wind farm topology is n s After the DC wind turbines are connected in series, they are connected in parallel to the DCT low-voltage port through the busbar. The number of parallel branches is n. p ;

[0010] The DCT high-voltage port admittance is obtained based on the DCT low-voltage port input admittance, combined with the DC transformer electrical circuit small-signal equation and the DC transformer control circuit small-signal equation; the DC bus outlet admittance is obtained in combination with the high-voltage DC bus, and the DC bus outlet admittance is the grid-side MMC DC port input admittance;

[0011] According to the grid-side MMC DC port input admittance, combined with the grid-side MMC electrical circuit small signal equation and the grid-side MMC control circuit small signal equation, a grid-side MMC AC port admittance model is obtained.

[0012] Optionally, the small signal equation of the DC wind turbine electrical circuit is:

[0013]

[0014] in, is the small signal vector of the MMC DC port on the machine side, and are the small signal vectors of the voltage, current and modulation signal of the bridge arm of phase a respectively; I aurT 、M aurT and V cauT are the steady-state vector matrices of the a-phase bridge arm current, modulation signal, and capacitor voltage respectively; Z pLr and Y pCr They are the impedance and admittance matrices of the bridge arm inductance and submodule capacitance, respectively, and their inverse matrices Y pLr and Z pCr is the admittance and impedance matrix of the bridge arm inductance and submodule capacitance, and is the AC port voltage and current small signal vector, is the neutral point component, A p0 is the zero-sequence matrix of the bridge arm small signal vector, G var_ i ar The voltage of the MMC AC port on the machine side and current The transfer function relationship.

[0015] Optionally, the small-signal equation of the direct-current wind turbine generator control loop is:

[0016]

[0017] wherein, is a small-signal vector of the a-phase bridge arm current, a modulation signal, G pr is a matrix of active power control related transfer functions, G cirr is a matrix of circulating current control related transfer functions.

[0018] Optionally, the small-signal equation of the direct-current transformer electrical circuit is:

[0019]

[0020] wherein, is a small-signal vector of the direct-current transformer high-voltage port voltage, is a small-signal vector of the direct-current transformer low-voltage port voltage, is a small-signal vector of the direct-current transformer high-voltage port current, is a small-signal vector of the direct-current transformer low-voltage port current, and are small-signal vectors of the W, H, and L bridge arm port voltages and currents, respectively.

[0021] Optionally, the small-signal equation of the direct-current transformer control loop is:

[0022]

[0023] wherein, are small-signal vectors of the direct-current transformer W, L, and H bridge arm modulation signals, G iW and G vW are matrices of W bridge arm current and alternating-current voltage control related transfer functions; G iL , G vcL , G vWL , and G vdcL are matrices of L bridge arm current, global voltage, alternating-current voltage feedforward, and direct-current voltage control related transfer functions; G iH , G vcH , and G vWH are matrices of H bridge arm current, global voltage, and alternating-current voltage feedforward control related transfer functions; are a-phase currents of the direct-current transformer W, L, and H bridge arms; is a small-signal vector of the direct-current transformer low-voltage port voltage, is a sum of direct-current transformer L bridge arm sub-module capacitor voltages, is a sum of direct-current transformer H bridge arm sub-module capacitor voltages, is the voltage of phase a at the common connection point.

[0024] Optionally, the grid-side MMC electrical circuit small signal equation is:

[0025]

[0026] in, and is the voltage and current small signal vector of the grid-side MMC AC port, Z g is the grid impedance matrix, is the small signal vector of the grid-side MMC DC port, and are the small signal vectors of the voltage, current and modulation signal of the bridge arm of phase a respectively, is the shore MMC AC voltage phase a, is the shore MMC AC voltage phase a, I augT 、M augT and V caugT are the steady-state vector matrices of the a-phase bridge arm current, modulation signal, and capacitor voltage, respectively. pLg and Y pCg They are the grid-side MMC bridge arm inductance impedance matrix and the submodule capacitance admittance matrix, the grid-side MMC bridge arm inductance admittance matrix Y pLg and submodule capacitance impedance matrix Z pCg Z pLg and Y pCg The inverse matrix of .

[0027] Optionally, the DC port admittance of the x-th series branch and the y-th DC wind turbine generator set is:

[0028]

[0029] Among them, A pd is the differential mode matrix of the bridge arm small signal vector, (x=1,...,n p ) represents the parallel branch number, (y=1,...,n s ) indicates the serial number of the series units.

[0030] Optionally, the aggregated wind farm admittance, that is, the DCT low-pressure port admittance is:

[0031]

[0032] Among them, Y dcrx is the impedance of the x-th series branch port, Y ClineLx is the equivalent capacitance admittance of the collection line, Z ClineH is the equivalent inductance impedance of the collection line.

[0033] Optionally, the grid-side MMC DC port input admittance is:

[0034] Y dcg =1 / (1 / (Y dcH +Y ClineH )+2Z LlineH )+Y ClineH

[0035] Among them, Y dcH is the DCT high-voltage port admittance, Y ClineH is the equivalent capacitance admittance of the high-voltage DC line, Z ClineH is the equivalent inductive impedance of the high-voltage DC line.

[0036] Optionally, the grid-side MMC AC port admittance is:

[0037] Y acC =2(3Γ4Z dcg A p0 -Γ1-Γ2) -1 Γ5

[0038] Among them, Z dcg Y dcg The inverse matrix of Γ1~Γ5 is the transfer function of the analytical derivation process of the onshore MMC impedance.

[0039] The technical solution provided by the disclosed embodiments offers the following advantages over the prior art: By constructing an impedance model for the grid-connected port of a wind power DC transmission system, this approach addresses the lack of such a model in the prior art. By analyzing the impedance of the DC wind farm, transmission lines, DCT, and HVDC lines, the coupling effect of the entire transmission system on the grid-connected port can be reflected. Comparing this impedance model with the measured impedance values ​​calculated from the voltage and current components at the disturbance frequency reveals high accuracy within the 1-1000Hz frequency range. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0041] In order to more clearly illustrate the embodiments of the present disclosure or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0042] Figure 1FIG2 is a schematic diagram of a topological structure of an offshore wind power DC collection and transmission system provided by an embodiment of the present disclosure;

[0043] Figure 2 FIG2 is a schematic diagram of a topological structure of a DC wind turbine provided by an embodiment of the present disclosure;

[0044] Figure 3 FIG2 is a schematic diagram of a network-side MMC topology structure provided by an embodiment of the present disclosure;

[0045] Figure 4 The figure shows a block diagram of a DC wind turbine side MMC control provided by an embodiment of the present disclosure;

[0046] Figure 5 FIG2 is a block diagram of a network-side MMC control provided by an embodiment of the present disclosure;

[0047] Figure 6 Shown is a circuit diagram of a collection line and a high-voltage DC bus provided by an embodiment of the present disclosure;

[0048] Figure 7 FIG2 is a block diagram of a DCT inner loop current control provided by an embodiment of the present disclosure;

[0049] Figure 8 FIG2 is a block diagram of a DCT outer loop voltage control provided by an embodiment of the present disclosure;

[0050] Figure 9 The figure shows a schematic diagram comparing the measured impedance value and the impedance analytical model result in a simulation model provided by an embodiment of the present disclosure. DETAILED DESCRIPTION

[0051] In order to more clearly understand the above-mentioned objectives, features and advantages of the present disclosure, the scheme of the present disclosure will be further described below. It should be noted that the embodiments of the present disclosure and the features therein can be combined with each other in the absence of conflict.

[0052] In the following description, many specific details are set forth to facilitate a full understanding of the present disclosure, but the present disclosure may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present disclosure, rather than all of the embodiments.

[0053] Figure 1 The figure shows a topological structure diagram of an offshore wind power DC collection and transmission system provided by an embodiment of the present disclosure. Please refer to Figure 1The present disclosure provides a method for modeling and oscillation analysis of the grid-connected port impedance of a wind power DC transmission system. This method is applied to an offshore wind power DC transmission system. In an optional embodiment provided by the present disclosure, the offshore wind power DC transmission system comprises a DC wind turbine, a transmission line, a DC transformer (DCT), a high-voltage DC line, and a grid-side multi-layered capacitor (MMC). The DCT has a T-type multi-layered capacitor (MMC) topology, in which the L and W arms form the low-voltage port, the H and W arms form the high-voltage port, and the W arm is a reused arm. dcL is the DC voltage of the low voltage port, v dc H is the DC voltage at the high voltage port; i xZ (x=a,b,c,Z=L,H,W) is the bridge arm current, L armZ 、R armZ are the bridge arm inductance and resistance, C armZ is the submodule capacitance, n Z is the number of bridge arm submodules.

[0054] The present disclosure provides a method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system, including:

[0055] According to the wind power DC transmission system topology and the control strategy of each link, the small signal equations of the system are obtained, including the small signal equations of the DC wind turbine electrical circuit, the small signal equations of the DC wind turbine control circuit, the small signal equations of the DC transformer electrical circuit, the small signal equations of the DC transformer control circuit, the small signal equations of the grid-side MMC electrical circuit, and the small signal equations of the grid-side MMC control circuit;

[0056] Based on the obtained DC wind turbine electrical circuit small signal equation and DC wind turbine control circuit small signal equation, a DC port admittance model of a single DC wind turbine is obtained; the above steps are repeated to obtain the port admittance model of each DC wind turbine in the wind farm;

[0057] According to the port admittance model of each DC wind turbine, combined with the series-parallel circuit relationship of the collection line and wind farm topology, the collection wind farm admittance is obtained. The collection wind farm admittance is the DCT low-voltage port input admittance.

[0058] Among them, the wind farm topology is n s After the DC wind turbines are connected in series, they are connected in parallel to the DCT low-voltage port through the busbar. The number of parallel branches is n. p ;

[0059] Based on the DCT low-voltage port input admittance, combined with the DC transformer electrical circuit small-signal equation and the DC transformer control circuit small-signal equation, the DCT high-voltage port admittance is obtained; combined with the high-voltage DC bus, the DC bus outlet admittance is obtained. The DC bus outlet admittance is the grid-side MMC DC port input admittance.

[0060] According to the grid-side MMC DC port input admittance, combined with the grid-side MMC electrical circuit small signal equation and the grid-side MMC control loop small signal equation, the grid-side MMC AC port admittance model is obtained. In this way, through the port admittance model of each DC wind turbine in the wind farm, combined with the series-parallel circuit relationship between the collection line and the wind farm topology, the collection wind farm admittance, DTC low-voltage port admittance, DTC high-voltage port admittance, grid-side MMC DC port input admittance can be gradually obtained, and finally the grid-side MMC AC port admittance can be obtained. That is, through the admittance model of each link and each port of the wind power DC collection and transmission system, the coupling influence of the wind power DC collection and transmission system on the grid-connected port can be intuitively reflected, which can be further used to analyze the stability of the system and reduce the risk of system oscillation.

[0061] It should be noted that the present disclosure adopts the frequency domain linearization method to establish the port impedance model, and the small signal vector of each physical quantity is represented by “^”, which is the perturbation frequency f p A 2g+1 column vector centered at .

[0062] Figure 2 The figure shows a schematic diagram of a DC wind turbine topology structure provided by an embodiment of the present disclosure. Figure 1 and Figure 2 In an optional embodiment provided by the present disclosure, a DC wind turbine is composed of a permanent magnet synchronous motor, an AC transformer and a machine-side MMC, wherein the SM shown in the figure is an MMC submodule, v xm 、i xm They are the three-phase AC voltage and current on the machine side, v xr 、i xr (x=a,b,c) are the three-phase AC voltage and current on the valve side, and the transformer ratio is n1 / n2. The DC voltage and current of MMC are v dcr 、i dcr , the three-phase upper and lower bridge arm currents are i xyr , y=u, l represent the upper and lower bridge arms respectively. L armr 、R armr are the bridge arm inductance and resistance, C armr is the submodule capacitance, n M is the number of bridge arm submodules.

[0063] Establish the small signal equation of the DC wind turbine electrical circuit:

[0064]

[0065] in, is the small signal vector of the MMC DC port on the machine side, and are the small signal vectors of the voltage, current and modulation signal of the bridge arm of phase a respectively; I aurT 、MaurT and V cauT are the steady-state vector matrices of the a-phase bridge arm current, modulation signal, and capacitor voltage, respectively; and are the steady-state values ​​of each frequency component in the form of a Toeplitz matrix; Z pLr and Y pCr They are the impedance and admittance matrices of the bridge arm inductance and submodule capacitance, respectively, and their inverse matrices Y pLr and Z pCr is the admittance and impedance matrix of the bridge arm inductance and submodule capacitance, and is the AC port voltage and current small signal vector, is the neutral point component, A p0 is the zero-sequence matrix of the bridge arm small signal vector, G var_iar The voltage of the MMC AC port on the machine side and current The transfer function relationship of , taking into account the generator dynamics and transformer ratio.

[0066] Furthermore, G var_iar The expansion of non-zero elements is

[0067]

[0068] Among them, L d and L q is the AC and DC axis reactance of the generator, and n1 and n2 are the transformer ratios.

[0069] In this way, under the conditions of considering the dynamics of the generator and the transformation ratio of the transformer during actual operation, the small signal equation of the electrical circuit of the DC wind turbine is established by combining the three-phase AC voltages on the machine side and valve side, as well as the bridge arm current, inductance, resistance, etc., which is conducive to the subsequent establishment of the DC port admittance model of a single DC wind turbine.

[0070] Please refer to Figure 1 and Figure 2 In an optional embodiment provided by the present disclosure, the small signal equation of the DC wind turbine control loop is:

[0071]

[0072] in, are the small signal vectors of the a-phase bridge arm current and modulation signal, G pr is the active power control related transfer function matrix, G cirr is the transfer function matrix related to circulation control.

[0073]

[0074] Among them, G ad , G aq , Ga2d and G a2 q is the transformation matrix from dq coordinate system to abc coordinate system at fundamental frequency and double fundamental frequency respectively, V qr0 , V dr0 and I qr0 are steady-state vector matrix of active and reactive voltage, current respectively.H pr is active power control transfer function matrix, H ir is inner loop current control transfer function matrix, H icr is circulating current control transfer function matrix.

[0075] In this way, under the fundamental frequency and double fundamental frequency respectively, combined with the active, reactive voltage, active power, inner loop current and circulating current of the DC wind turbine, the control loop small signal equation of the DC wind turbine in the actual operation state is established, which is beneficial to reflect the coupling influence of the control loop of the DC wind turbine on the system, and is convenient for further analysis or reducing the influence of oscillation or signal disturbance on the grid-connected port.

[0076] Please continue to refer to Figure 1 and Figure 2 In an optional embodiment provided in the present disclosure, the DC transformer electrical loop small signal equation is:

[0077]

[0078] wherein, is a small signal vector of the high-voltage port voltage of the DC transformer, is a small signal vector of the low-voltage port voltage of the DC transformer, is a small signal vector of the high-voltage port current of the DC transformer, is a small signal vector of the low-voltage port current of the DC transformer, and are small signal vectors of the W, H, and L bridge arm port voltage and current respectively.

[0079] In this way, according to the small signal vectors of the current and voltage of the high-voltage port and the low-voltage port of the DC transformer, and the small signal vectors of the bridge arm port voltage and current, the DC transformer electrical loop small signal equation is obtained, which is beneficial to reflect the oscillation or signal disturbance of the DC transformer electrical loop, and provides a calculation basis for further reducing the oscillation risk of the system.

[0080] Further, for the three groups of bridge arms, the port voltage and current are three-phase bridge arm voltage and current zero sequence components:

[0081]

[0082] wherein, is a small signal vector of the three groups of bridge arm voltage and current.

[0083] Please continue to refer to Figure 1 and Figure 2 In an optional embodiment provided in the present disclosure, the small signal equation of the DC transformer control loop is:

[0084]

[0085] wherein, are the small signal vectors of the DC transformer W, L and H bridge arm modulation signals, G iW and G vW are the W bridge arm current and AC voltage control related transfer function matrices; G iL , G vcL , G vWL and G vdcL are the L bridge arm current, global voltage, AC voltage feedforward and DC voltage control related transfer function matrices; G iH , G vcH and G vWH are the H bridge arm current, global voltage and AC voltage feedforward control related transfer function matrices; are the a-phase currents of the DC transformer W, L and H bridge arms, is the small signal vector of the DC transformer low voltage port voltage, is the sum of the L bridge arm submodule capacitor voltages of the DC transformer, is the sum of the H bridge arm submodule capacitor voltages of the DC transformer, is the a-phase voltage of the common connection point.

[0086]

[0087] wherein, G iL , G vcL , G vWL and G vdcL have the expressions

[0088]

[0089] wherein, G iH , G vcH and G vWH have the expressions

[0090]

[0091] In this way, according to the related conditions of the DC transformer bridge arm current, bridge arm modulation signal, low voltage port voltage, and each bridge arm submodule capacitor voltage, the small signal equation of the DC transformer control loop is obtained, which is conducive to reflecting the oscillation or signal disturbance of the DC transformer control loop, and provides a calculation basis for further reducing the oscillation risk of the system.

[0092] Figure 3 FIG. 1 is a schematic diagram of a network-side MMC topology structure provided by an embodiment of the present disclosure. Figure 4 The figure shows a DC wind turbine side MMC control block diagram provided by an embodiment of the present disclosure. Figure 5 The figure shows a network side MMC control block diagram provided by the embodiment of the present disclosure. Figures 1 to 5 In an optional embodiment provided by the present disclosure, the grid-side MMC DC voltage and current are v dcg 、i dcg , the AC voltage and current are v xg 、i xg , the three-phase upper and lower bridge arm currents are i xyg , L armg and R armg are the bridge arm inductance and resistance, C armg is the submodule capacitance, n N is the number of submodules; the DC collection line and high-voltage DC bus adopt π-type equivalent, R lineL / H , L lineL / H and C lineL / H The equivalent resistance, inductance and capacitance of the collection line and high-voltage DC bus are shown in Table 1.

[0093] Table 1:

[0094]

[0095] The network-side MMC parameters are shown in Table 2:

[0096] Table 2:

[0097]

[0098] Based on the above known conditions, the small signal equation of the grid-side MMC electrical circuit is established:

[0099]

[0100] in, and is the voltage and current small signal vector of the grid-side MMC AC port, Z g is the grid impedance matrix, is the small signal vector of the grid-side MMC DC port, and are the small signal vectors of the voltage, current and modulation signal of the bridge arm of phase a respectively, is the shore MMC AC voltage phase a, is the shore MMC AC voltage phase a, I augT 、M augT and V caugTare the steady-state vector matrices of the a-phase bridge arm current, modulation signal, and capacitor voltage, respectively. pLg and Y pCg They are the grid-side MMC bridge arm inductance impedance matrix and the submodule capacitance admittance matrix, the grid-side MMC bridge arm inductance admittance matrix Y pLg and submodule capacitance impedance matrix Z pCg Z pLg and Y pCg The inverse matrix of .

[0101] Please continue to refer to Figures 1 to 5 , and Table 1 and Table 2, the machine side MMC inner loop AC voltage control, H ir (s) is the AC current controller; K dr is the AC current decoupling coefficient, the outer loop active power control, H pr (s) is the active power controller, with additional circulating current control, H icr (s) is the circulation controller, K 2dr is the circulating decoupling coefficient; θ r It is the synchronous rotation reference angle obtained by the position sensor.

[0102] Grid-side MMC inner loop AC current control, H ig (s) is the AC current controller; K dg is the AC current decoupling coefficient, the outer loop determines the DC voltage control, H vdcg (s) is a constant DC voltage controller with additional circulating current control, H icg (s) is the circulation controller, K 2dg is the circulating decoupling coefficient; θ g is the synchronous rotation reference angle, obtained by the phase-locked loop, H PLL (s) Phase-locked loop controller.

[0103] Establish the small signal equation of the grid-side MMC control loop:

[0104]

[0105] Among them, G ig , G cirg , G vg and G vdcg is the transfer function matrix related to the grid-side MMC AC current, circulating current, voltage feedforward, phase-locked loop, and DC voltage control, and its expression is as follows:

[0106]

[0107] In this way, based on the grid-side MMC AC port current and voltage, the grid-side MMC DC port small signal vector, grid impedance, bridge arm current, voltage, modulation signal small signal vector, bridge arm inductance impedance and sub-module capacitance admittance and other related conditions, the grid-side MMC electrical circuit small signal equation is obtained, which is beneficial to reflecting the oscillation or signal disturbance of the grid-side MMC electrical circuit, and is beneficial to intuitively reflect the coupling effect of the DC wind turbine group collection and transmission system on the grid-connected port, thereby reducing the risk of system oscillation.

[0108] Please continue to refer to Figures 1 to 5 In an optional embodiment provided by the present disclosure, the DC port admittance of the x-th series branch and the y-th DC wind turbine is:

[0109]

[0110] Among them, A pd is the differential mode matrix of the bridge arm small signal vector, (x=1,...,n p ) represents the parallel branch number, (y=1,...,n s ) indicates the serial number of the series units.

[0111] Furthermore, the impedance of the x-th series branch port is:

[0112]

[0113] In this way, the DC port admittance model of a single DC wind turbine can be obtained based on the series-parallel circuit relationship of the wind farm topology, as well as the obtained small-signal equation of the DC wind turbine electrical circuit and the small-signal equation of the DC wind turbine control circuit. By repeating the above steps, the port admittance model of each DC wind turbine in the wind farm can be obtained, and then the port impedance of the series branch can be obtained, reflecting the oscillation or signal disturbance of the series branch of the DC wind turbine, which is conducive to intuitively reflecting the coupling effect of the series branch of the DC wind turbine on the grid-connected port and reducing the risk of system oscillation.

[0114] Figure 6 FIG. 1 is a circuit diagram of a collection line and a high-voltage DC bus provided by an embodiment of the present disclosure. Figure 7 FIG. 1 is a block diagram of a DCT inner loop current control provided by an embodiment of the present disclosure. Figure 8 The figure shows a DCT outer loop voltage control block diagram provided by the embodiment of the present disclosure. Figures 1 to 8 , Table 1 and Table 2, in an optional embodiment provided by the present disclosure, each bridge arm of the DCT adopts the current inner loop control, θ s is the synchronous rotation reference angle, H iZ (s) is the current controller of the W and H bridge arms, which controls the AC circulating current, v dsW 、v qs W is the dq axis voltage at the common connection point, K fZis the voltage feedforward coefficient, K dZ is the current decoupling coefficient. The L bridge arm controls the 0 axis current, and H i0L (s) is the DC current controller. The outer loop of the W bridge arm of DCT adopts constant AC voltage control, H vW (s) is the AC voltage controller; the L and H bridge arms control the global voltage, H vcL (s) and H vcH (s) is the global voltage controller. The DCT parameters are shown in Table 3:

[0115] Table 3:

[0116]

[0117] Please combine Figures 1 to 8 , Table 1 to Table 3, in an optional embodiment provided by the present disclosure, the aggregated wind farm admittance, that is, the DCT low-pressure port admittance is:

[0118]

[0119] Among them, Y dcrx is the impedance of the x-th series branch port, Y ClineLx is the equivalent capacitance admittance of the collection line, Z ClineH is the equivalent inductance impedance of the collection line.

[0120] Furthermore, considering the coupling effect of the aggregated wind farm and the simultaneous DCT small signal equations, we obtain:

[0121]

[0122] Among them, G 11 ~G 43 The transfer function matrix of the derivation process is expressed as:

[0123]

[0124] Among them, P1~P4, Q1~Q4 and G maW_iaW The expanded form is as follows:

[0125]

[0126]

[0127] G maW_iaW =(EG vW (M aWT Z pCW I aWT +V caWT )) -1

[0128] (G iW+G vW (Z pLW +M aW0T Z pCW M aWT ))(31)

[0129] Please continue to refer to Figures 1 to 8 , Table 1 to Table 3, considering the coupling effect of the wind farm, the DCT high-voltage port admittance is:

[0130] Y dcH =3A p0 (G 11 +G 12 G iaL_iaH +P3G maH_iaH ) -1 (32)

[0131] Among them, G iaL_iaH and G maH_iaH The expanded form is as follows:

[0132]

[0133] Among them, Z WFL is the impedance of the wind farm, that is, Y WFL The inverse matrix reflects the dynamics of the aggregation system.

[0134] In this way, the DCT low-voltage port admittance can be obtained based on the circuit connection relationship, equivalent capacitive admittance, equivalent inductive impedance, and the DC wind turbine series branch port impedance of the collection line, reflecting the oscillation or signal disturbance of the DCT low-voltage port, which is conducive to intuitively reflecting the coupling impact on the grid-connected port and reducing the risk of system oscillation.

[0135] Please continue to combine Figures 1 to 8 In an optional embodiment provided by the present disclosure, the line parameters are as shown in Table 4:

[0136] Table 4:

[0137]

[0138] Establish the grid-side MMC DC port input admittance:

[0139] Y dcg =1 / (1 / (Y dcH +Y ClineH )+2Z LlineH )+Y ClineH (34)

[0140] Among them, Y dcH is the DCT high-voltage port admittance, Y ClineH is the equivalent capacitance admittance of the high-voltage DC line, Z ClineHis the equivalent inductive impedance of the high-voltage DC line.

[0141] Please continue to refer to Figures 1 to 8 , Table 1 to Table 4, in an optional embodiment provided by the present disclosure, the grid-side MMC AC port admittance is:

[0142] Y acC =2(3Γ4Z dcg A p0 -Γ1-Γ2) -1 Γ5 (35)

[0143] Among them, Z dcg Y dcg The inverse matrix of Γ1~Γ5 is the transfer matrix, and the expression of Γ1~Γ5 is:

[0144]

[0145] Furthermore, the positive and negative sequence impedances of the grid-side MMC AC port are:

[0146]

[0147] Among them, Y acC is the admittance of the grid-side MMC AC port, j represents the imaginary unit, ω1 represents the angular frequency (50 Hz), s is the Laplace operator, and Z acCnn * Z acCnn The conjugate matrix of .

[0148] In this way, the input admittance of the MMC DC port can be obtained based on the equivalent inductive impedance and equivalent capacitive admittance of the high-voltage DC line, as well as the admittance of the DCH high-voltage port, and the grid-side MMC AC port admittance can be further obtained. Thus, through a step-by-step process, the oscillation or signal disturbance of the grid-side MMC AC port can be finally reflected, which is beneficial for reflecting the coupling influence of the wind power DC collection and transmission system on the grid-connected port from a holistic perspective under actual operating conditions, facilitating system stability analysis and reducing system oscillation risks.

[0149] Figure 9 The figure shows a comparison diagram of the impedance measured value and the impedance analytical model result in a simulation model provided by the embodiment of the present disclosure. Figures 1 to 9, Parameters in Tables 1 to 4, in an optional embodiment provided by the present disclosure, the impedance analytical model provided by the present disclosure is obtained according to the impedance modeling and oscillation analysis method of the grid-connected port of the wind power DC collection and transmission system provided by the present disclosure, and based on the electromagnetic transient simulation model of the offshore wind power DC collection and transmission system, a disturbance sinusoidal voltage of 0.03 pu of AC voltage is injected into the AC port of the grid-side MMC, and its frequency range is 1 Hz-1000 Hz. After each disturbance voltage injection and simulation, the voltage and current components at the disturbance frequency are extracted at the AC port by FFT, and the measured impedance value is calculated and compared with the analytical model. The comparison results are as follows Figure 9 As shown by Figure 9 It can be seen that the impedance analysis results obtained according to the impedance modeling and oscillation analysis method for the grid-connected port of a wind power DC transmission system provided by the present disclosure have high accuracy in the frequency range of 1-1000 Hz.

[0150] In summary, the present disclosure provides a method for modeling and oscillation analysis of the impedance of the grid-connected port of a wind power DC transmission system. This method constructs an impedance model for the grid-connected port of a wind power DC transmission system, thus addressing the shortcomings of this model. This method models the impedance of DC wind farms, transmission lines, DCTs, and high-voltage DC lines, reflecting the coupling effect of the entire transmission system on the grid-connected port. The model is highly accurate, and when compared with the measured impedance values ​​obtained by extracting the voltage and current components at the disturbance frequency through FFT at the AC port, it is verified that the impedance model provided by the present disclosure has high accuracy within the frequency range of 1-1000Hz.

[0151] The foregoing description is intended only to provide specific embodiments of the present disclosure, intended to enable those skilled in the art to understand and implement the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure is not intended to be limited to the embodiments described herein, but rather to be construed in the broadest manner consistent with the principles and novel features disclosed herein.

Claims

1. A method for modeling and oscillation analysis of the grid-connected port impedance of a wind power DC transmission system, characterized in that: include: According to the wind power DC transmission system topology and the control strategy of each link, the small signal equations of the system are obtained, including the small signal equations of the DC wind turbine electrical circuit, the small signal equations of the DC wind turbine control circuit, the small signal equations of the DC transformer electrical circuit, the small signal equations of the DC transformer control circuit, the small signal equations of the grid-side MMC electrical circuit, and the small signal equations of the grid-side MMC control circuit; According to the obtained DC wind turbine electrical circuit small signal equation and the DC wind turbine control circuit small signal equation, a DC port admittance model of a single DC wind turbine is obtained; repeating the above steps to obtain the port admittance model of each DC wind turbine in the wind farm; According to the port admittance model of each DC wind turbine, combined with the series-parallel circuit relationship of the collection line and the wind farm topology, the collection wind farm admittance is obtained, and the collection wind farm admittance is the DCT low-voltage port input admittance; Wherein, the wind farm topology is n s After the DC wind turbines are connected in series, they are connected in parallel to the DCT low-voltage port through the busbar. The number of parallel branches is n. p ; The DCT high-voltage port admittance is obtained based on the DCT low-voltage port input admittance, in combination with the DC transformer electrical circuit small-signal equation and the DC transformer control circuit small-signal equation. The DC bus outlet admittance is obtained in combination with the high-voltage DC bus, and the DC bus outlet admittance is the grid-side MMC DC port input admittance. The grid-side MMC AC port admittance model is obtained based on the grid-side MMC DC port input admittance, in combination with the grid-side MMC electrical circuit small-signal equation and the grid-side MMC control circuit small-signal equation.

2. The method for modeling and oscillation analysis of the grid-connected port impedance of a wind power DC collection and transmission system according to claim 1, characterized in that: The small signal equation of the DC wind turbine electrical circuit is: in, is the small signal vector of the MMC DC port on the machine side, and are the small signal vectors of the voltage, current and modulation signal of the bridge arm of phase a respectively; I aurT 、M aurT and V cauT are the steady-state vector matrices of the a-phase bridge arm current, modulation signal, and capacitor voltage respectively; Z pLr and Y pCr They are the impedance and admittance matrices of the bridge arm inductance and submodule capacitance, respectively, and their inverse matrices Y pLr and Z pCr is the admittance and impedance matrix of the bridge arm inductance and submodule capacitance, and is the AC port voltage and current small signal vector, is the neutral point component, A p0 is the zero-sequence matrix of the bridge arm small signal vector, G var_ i ar The voltage of the MMC AC port on the machine side and current The transfer function relationship.

3. The method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system according to claim 1, characterized in that: The small signal equation of the DC wind turbine control loop is: in, are the small signal vectors of the a-phase bridge arm current and modulation signal, G pr is the active power control related transfer function matrix, G cirr is the transfer function matrix related to circulation control.

4. The method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system according to claim 1, characterized in that: The small signal equation of the DC transformer electrical circuit is: in, is the small signal vector of the voltage at the high voltage port of the DC transformer, is the small signal vector of the DC transformer low voltage port voltage, is the small signal vector of the current at the high voltage port of the DC transformer, is the small signal vector of the current at the low voltage port of the DC transformer, and are the small signal vectors of voltage and current at the W, H, and L bridge arm ports, respectively.

5. The method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system according to claim 1, characterized in that: The small signal equation of the DC transformer control loop is: in, are the small signal vectors of the modulation signals of the W, L, and H bridge arms of the DC transformer, G iW and G vW is the transfer function matrix related to the control of the W bridge arm current and AC voltage; G iL , G vcL , G vWL and G vdcL is the transfer function matrix related to the L bridge arm current, global voltage, AC voltage feedforward and DC voltage control; G iH , G vcH and G vWH is the transfer function matrix related to the H-bridge arm current, global voltage and AC voltage feedforward control; is the a-phase current of the W, L, and H bridge arms of the DC transformer; is the small signal vector of the DC transformer low voltage port voltage, is the sum of the capacitor voltages of the L arm submodules of the DC transformer, is the sum of the capacitor voltages of the H-bridge arm submodules of the DC transformer, is the voltage of phase a at the common connection point.

6. The method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system according to claim 1, characterized in that: The small signal equation of the grid-side MMC electrical circuit is: in, and is the voltage and current small signal vector of the grid-side MMC AC port, Z g is the grid impedance matrix, is the small signal vector of the grid-side MMC DC port, and are the small signal vectors of the voltage, current and modulation signal of the bridge arm of phase a respectively, is the shore MMC AC voltage phase a, is the shore MMC AC voltage phase a, I augT 、M augT and V caugT are the steady-state vector matrices of the a-phase bridge arm current, modulation signal, and capacitor voltage, respectively. pLg and Y pCg They are the grid-side MMC bridge arm inductance impedance matrix and the submodule capacitance admittance matrix, the grid-side MMC bridge arm inductance admittance matrix Y pLg and submodule capacitance impedance matrix Z pCg Z pLg and Y pCg The inverse matrix of .

7. The method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system according to claim 2, characterized in that: The DC port admittance of the xth series branch and the yth DC wind turbine is: Among them, A pd is the differential mode matrix of the bridge arm small signal vector, (x=1,...,n p ) represents the parallel branch number, (y=1,...,n s ) indicates the serial number of the series units.

8. The method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system according to claim 7, characterized in that: The aggregated wind farm admittance, i.e., the DCT low-voltage port admittance is: Among them, Y dcrx is the impedance of the x-th series branch port, Y ClineLx is the equivalent capacitance admittance of the collection line, Z ClineH is the equivalent inductance impedance of the collection line.

9. The method for modeling and oscillation analysis of grid-connected port impedance of a wind power DC collection and transmission system according to claim 8, characterized in that: The grid-side MMC DC port input admittance is: AND dcg =1 / (1 / (And dcH +Y ClineH )+2Z LlineH )+Y ClineH Among them, Y dcH is the DCT high-voltage port admittance, Y ClineH is the equivalent capacitance admittance of the high-voltage DC line, Z ClineH is the equivalent inductive impedance of the high-voltage DC line.

10. The method for grid-connected port impedance modeling and oscillation analysis of a wind power DC collection and transmission system according to claim 9, characterized in that: The grid-side MMC AC port admittance is: Y acC =2(3Γ4Z dcg A p0 -C1-C2) -1 C5 Among them, Z dcg Y dcg The inverse matrix of Γ1~Γ5 is the transfer function of the analytical derivation process of the onshore MMC impedance.

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