Network construction type control method based on weak power grid wind power cascade hybrid converter

Abstract

The invention discloses a grid construction type control method based on a weak grid wind power cascade hybrid converter, and the method comprises the steps: obtaining a cascade hybrid converter circuit and a cascade hybrid DC system circuit, and building a wind power generation cascade hybrid DC power transmission system based on the cascade hybrid converter circuit and the cascade hybrid DC system circuit; obtaining a cascade hybrid converter equivalent circuit and a power grid side converter equivalent circuit; obtaining a main circuit model based on the cascade hybrid converter equivalent circuit and the power grid side converter equivalent circuit; acquiring a control system model, and establishing a network construction type control small signal model based on the main circuit model and the control system model; and controlling the wind power generation cascade hybrid direct-current power transmission system by using the network construction type control small signal model. According to the invention, the wind power plant and the high-voltage direct-current system can operate normally under the weak power grid condition, and the system investment and operation cost can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy and new power systems, and in particular relates to a grid-connected control method based on a weak-grid wind power cascade hybrid converter. Background Art

[0002] With the rapid development of renewable energy, wind power, as an important clean energy, has been widely used around the world. However, wind power grid-connected power generation faces many challenges in practical applications. Wind power grid-connected power generation systems are usually composed of multiple wind turbines, converters, power grids, etc. The interaction and influence between these components make the stability of the system particularly prominent. The instability and unpredictability of wind power, as well as the weak characteristics of the power grid, may lead to voltage fluctuations, frequency instability and other problems, affecting the stable operation of the power grid. Therefore, how to improve the stability and compatibility of wind power grid-connected power generation systems has become an urgent problem to be solved in the wind power field.

[0003] Traditional wind power grid-connected power generation systems usually use a constant speed and frequency control method, which has many problems. First, due to the randomness and unpredictability of wind speed, the output power of wind turbines will also fluctuate, which will cause voltage and frequency fluctuations in the power grid. Second, traditional wind power grid-connected power generation systems usually do not take into account the weak characteristics of the power grid. When the power grid fails, the wind power grid-connected power generation system may be seriously affected.

[0004] There are still some problems with the existing hybrid converter control methods. First, traditional control methods usually only focus on the performance optimization of a single wind farm or a single wind turbine, while ignoring the optimization and control at the level of the entire wind power grid-connected power generation system. In fact, the performance of a wind power grid-connected power generation system depends not only on the performance of a single wind turbine, but also on the coordination and control of the entire system. Secondly, traditional control methods usually do not take into account the impact of the weak characteristics of the power grid on the wind power grid-connected power generation system. In fact, the power grids in many regions are relatively weak and cannot withstand the impact of large-scale wind power grid connection. In addition, with the large-scale grid connection of renewable energy, the complexity and uncertainty of the power grid are also increasing. The wind power grid-connected power generation system needs to be able to adapt to this complexity and uncertainty to ensure the stable operation of the power grid. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes a grid-connected control method based on a weak-grid wind power cascade hybrid converter, which can achieve the normal operation of wind farms and high-voltage DC systems under weak-grid conditions while reducing system investment and operating costs.

[0006] The present invention provides a grid-type control method based on a weak grid wind power cascade hybrid converter, comprising:

[0007] Acquire a cascade hybrid converter circuit and a cascade hybrid DC system circuit, and establish a wind power generation cascade hybrid DC transmission system based on the cascade hybrid converter and the cascade hybrid DC system circuit;

[0008] Obtaining an equivalent circuit of a cascade hybrid converter and an equivalent circuit of a grid-side converter;

[0009] Based on the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit, a main circuit model is obtained;

[0010] Acquire a control system model, and establish a networked control small signal model based on the main circuit model and the control system model;

[0011] The grid-type control small signal model is used to control the wind power generation cascade hybrid direct current transmission system.

[0012] Optionally, the cascade hybrid converter circuit is composed of a thyristor converter and a multi-level converter connected in series.

[0013] Optionally, establishing a wind power generation cascade hybrid DC transmission system includes:

[0014] Based on the cascade hybrid converter and the cascade hybrid DC system circuit, combined with constraint conditions, the wind power generation cascade hybrid DC transmission system is established;

[0015] The constraints are:

[0016]

[0017] Among them, P S To convert the grid output power, P W is the output power of the wind farm, P L is the power of LCC, P M is the power of MMC, V dcL is the DC voltage of LCC, V dcM is the DC voltage of MMC.

[0018] Optionally, obtaining the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit includes:

[0019] On the AC side, the thyristor converter is equivalent to a first current source i L On the DC side, the thyristor converter is equivalent to a first voltage source e dcL ;

[0020] The first current source i L for:

[0021] The first voltage source edcL for:

[0022] Among them, i Lx is the equivalent current source on the x side of the LCC in the xy rotating reference frame, i Ly is the equivalent current source of LCC on the y side, N b is the number of six-pulse units, i dc1 is the DC current of the rectifier, μ is the commutation angle, α is the actual ignition delay angle of the LCC, e dcL is the equivalent voltage source of LCC, v p is the voltage of PCC.

[0023] On the AC side, the multilevel converter is equivalent to a second voltage source e M On the DC side, the multilevel converter is equivalent to a third voltage source e dcM ;

[0024] The second voltage source e M for:

[0025]

[0026] The third voltage source e dcM for:

[0027]

[0028] Among them, e Mx is the voltage of the equivalent voltage source of the MMC on the x side in the xy rotating reference system, e My is the voltage of the equivalent voltage source of the MMC on the y side; is the differential mode modulation ratio of the MMC on the x side in the xy rotating reference frame, is the differential mode modulation ratio of MMC on the y side; V Cux2 is the dual-frequency capacitor voltage on the x side of the MMC in the xy rotating reference frame, V Cuy2 is the dual-frequency capacitor voltage of MMC on the y side; is the dual-frequency common-mode modulation ratio of the MMC on the x side in the xy rotating reference frame, is the dual-frequency common-mode modulation ratio of MMC on the y side; V Cux is the total capacitance voltage of the MMC on the x side in the xy rotating reference frame, V Cuy is the total capacitance voltage of the MMC on the y side; V Cua is the total voltage of the capacitor; R ram is the resistance of the MMC arm, L arm is the inductance of the MMC arm, L TM is the transformer leakage inductance of the MMC;

[0029] On the AC side, the grid-side converter is equivalent to a fourth voltage source e w On the DC side, the thyristor converter is equivalent to a second current source i dc ;

[0030] The fourth voltage source e w for:

[0031] The second current source i dc for:

[0032] Among them, m wx is the modulation ratio of the wind turbine (WT) on the x side in the xy rotating reference frame, m wy is the modulation ratio of WT on the y side; i wx is the GSC current of WT on the x side in the xy rotating reference frame, i wy is the GSC current of WT on the y side; v dc is the GSC DC voltage of WT; e wx is the equivalent voltage source voltage of GSC on the x side in the xy rotating reference frame, e wy is the equivalent voltage source voltage of GSC on the y side.

[0033] Optionally, based on the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit, obtaining the main circuit model includes:

[0034] The main circuit model is obtained by replacing the converter based on the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit.

[0035] Optionally, the control system model includes: a power synchronization loop, a vector current control module and a circulating current suppression control module;

[0036] The power synchronization loop is used to adjust the active power;

[0037] The vector current control module includes an outer loop and an inner loop, wherein the outer loop is used to adjust the active power and support the voltage of the common coupling point, and the inner loop is used to control the grid-connected current;

[0038] The circulating current suppression control module is used to eliminate the internal second harmonic circulating current.

[0039] Optionally, based on the main circuit model and the control system model, establishing a networked control small signal model includes:

[0040] The coordinates of the main circuit model and the control system model are unified to obtain a meshed control small signal model.

[0041] Using the grid-type control small signal model, controlling the wind power generation cascade hybrid DC transmission system includes:

[0042] The wind power generation cascade hybrid direct current transmission system is controlled by controlling the multi-level converter.

[0043] Compared with the prior art, the present invention has the following advantages and technical effects:

[0044] The present invention proposes a grid-type control method based on a weak grid wind power cascade hybrid converter, establishes a detailed small signal model of wind power generation combined with a weak grid and transmitted through a cascade hybrid high-voltage direct current, and studies the stability problem of the grid-type control scheme by using the eigenvalue analysis method, so as to achieve the normal operation of the wind farm and the high-voltage direct current system under weak grid conditions while reducing the system investment and operating costs. The PSL in the control method can obtain the reference phase by simulating the rotor of the synchronous generator, and can adjust the active power; the grid-connected current is controlled by the vector current inner loop to protect the safety of the grid and equipment. This control scheme is not only applicable to active grids, but also to renewable energy stations operating in passive networks or island modes, and has strong adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0046] Figure 1 This is a flow chart of a grid-connected control method based on a weak-grid wind power cascade hybrid converter according to an embodiment of the present invention;

[0047] Figure 2 is a topological diagram of a wind power transmission cascade hybrid DC system according to an embodiment of the present invention;

[0048] Figure 3 is a grid-type control structure diagram of a multi-level converter according to an embodiment of the present invention;

[0049] Figure 4 is an equivalent circuit diagram of a wind power cascade hybrid DC system according to an embodiment of the present invention;

[0050] Figure 5 is an equivalent circuit diagram of an application conversion interface according to an embodiment of the present invention;

[0051] Figure 6 is a time domain response comparison diagram of the networked control method according to an embodiment of the present invention;

[0052] Figure 7 It is a comparison diagram of the stability zones of the system under the network-building type and network-following type control schemes of the embodiment of the present invention. DETAILED DESCRIPTION

[0053] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0054] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0055] Figure 2 The rectifier station of the medium and high voltage DC system adopts a cascade hybrid converter, which consists of a 12-pulse thyristor converter (LCC) and a multilevel converter (MMC) based on a half-bridge submodule in series. The inverter station consists of two 12-pulse LCCs in series. The wind farm integrated with the sending-end grid is connected to the rectifier station of the cascade hybrid HVDC system at the common coupling point (PCC). When the receiving-end grid is strong enough and the control parameters of the DC regulator of the inverter station are reasonably configured, the inverter side of the HVDC system can be equivalent to a DC voltage source. The sending-end grid is modeled as a series equivalent voltage source with equivalent impedance, which reflects the strength of the grid. The wind farm is represented as an aggregated type 4 wind turbine (WT). Similarly, when the DC capacitor of the wavelet transform and the control loop bandwidth of the machine-side converter (MSC) of the wavelet transform are large enough, the generator side of the wavelet transform has little effect on the grid side. Therefore, the generator side of the WT can be equivalent to the power supply IMSC. The DC line is represented by a lumped π-type equivalent circuit. Figure 3 The overall structure of the grid-type control of multilevel converters in cascade hybrid converters is described. The grid-type control system includes three parts: power synchronization loop (PSL), vector current control (VCC), and circulating current suppression control (CCSC). The power synchronization loop plays the role of simulating the rotor of the synchronous generator in the entire grid-type control scheme and has the ability to regulate active power. The vector current control includes two closed loops, in which the outer loop regulates the active power and supports the voltage at the common coupling point, and the inner loop controls the grid current. The circulating current suppression control is used to eliminate the internal second harmonic circulating current.

[0056] The present invention proposes a grid-connected control method based on a weak grid wind power cascade hybrid converter. Figure 1 As shown, the specific steps include:

[0057] Acquire a cascade hybrid converter circuit and a cascade hybrid DC system circuit, and establish a wind power generation cascade hybrid DC transmission system based on the cascade hybrid converter and the cascade hybrid DC system circuit;

[0058] Obtaining an equivalent circuit of a cascade hybrid converter and an equivalent circuit of a grid-side converter;

[0059] Based on the equivalent circuit of the cascade hybrid converter and the equivalent circuit of the grid-side converter, a main circuit model is obtained;

[0060] Obtain the control system model, and establish a networked control small signal model based on the main circuit model and the control system model;

[0061] The grid-type control small signal model is used to control the wind power generation cascade hybrid DC transmission system.

[0062] Specifically, the present invention establishes a detailed small signal model of wind power generation combined with a weak power grid and transmitted through a cascaded hybrid high-voltage direct current (HVDC) transmission, and studies the stability problem of the grid-type control scheme by adopting the eigenvalue analysis method, so as to achieve the purpose of normal operation of wind farms and high-voltage direct current systems under weak power grid conditions while reducing system investment and operating costs.

[0063] Furthermore, the cascade hybrid converter circuit is composed of a thyristor converter and a multi-level converter connected in series.

[0064] Furthermore, the establishment of a wind power generation cascade hybrid DC transmission system includes:

[0065] Based on the cascade hybrid converter and cascade hybrid DC system circuit, combined with the constraint conditions, a wind power generation cascade hybrid DC transmission system is established;

[0066] The constraints are:

[0067]

[0068] Among them, P S To convert the grid output power, P W is the output power of the wind farm, P L is the power of LCC, P M is the power of MMC, V dcL is the DC voltage of LCC, V dcM is the DC voltage of MMC.

[0069] Specifically, the cascade hybrid converter is composed of a thyristor converter (LCC) and a multilevel converter (MMC) in series. The cascade hybrid DC transmission system can solve the problem of the impact of wind power grid-connected power generation system on the power grid, optimize the control strategy of wind power grid-connected power generation system, reduce dependence on the power grid and the impact on the power grid, and improve the operation efficiency and management level of wind farms and power grids. Figure 4 As shown, the equivalent circuit diagram of the wind power transmission cascade hybrid DC system is designed according to the above-mentioned system.

[0070] Furthermore, obtaining the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit includes:

[0071] From the AC side, LCC is an equivalent current source i L , from the DC side, LCC is the equivalent resistance R dcL and inductor L dcL Equivalent voltage source connected in series dcL ;

[0072] The first current source i L for:

[0073] The first voltage source dcL for:

[0074] μ can be determined as follows:

[0075] Among them, i Lx is the equivalent current source on the x side of the LCC in the xy rotating reference frame, i Ly is the equivalent current source of LCC on the y side, N b is the number of six-pulse units, i dc1 is the DC current of the rectifier, μ is the commutation angle, α is the actual ignition delay angle of the LCC, e dcL is the equivalent voltage source of LCC, v p is the voltage of PCC, X TL is the leakage inductance of the LCC transformer on the X side in the xy rotating reference frame, L TL is the leakage inductance of the LCC transformer.

[0076] Replace each bridge cascade submodule in the MMC with a controlled voltage source equal to the bridge modulation ratio m ua The total capacitor voltage V Cua The product of .

[0077] MMC can be equivalent to a voltage source M Connect in series, see the resistance R from the AC side eqM and inductor L eqM ;MMC can be regarded as the equivalent resistance R on the DC side dcM and inductor L dcM Equivalent voltage source in series dcM . These equivalent variables can be expressed as:

[0078] The second voltage source M for:

[0079]

[0080] The third voltage source e dcM for:

[0081]

[0082] Among them, e Mx is the voltage of the equivalent voltage source of the MMC on the x side in the xy rotating reference system, e My is the voltage of the equivalent voltage source of the MMC on the y side; is the differential mode modulation ratio of the MMC on the x side in the xy rotating reference frame, is the differential mode modulation ratio of MMC on the y side; V Cux2 is the dual-frequency capacitor voltage on the x side of the MMC in the xy rotating reference frame, V Cuy2 is the dual-frequency capacitor voltage of MMC on the y side; is the dual-frequency common-mode modulation ratio of the MMC on the x side in the xy rotating reference frame, is the dual-frequency common-mode modulation ratio of MMC on the y side; V Cux is the total capacitance voltage of the MMC on the x side in the xy rotating reference frame, V Cuy is the total capacitance voltage of the MMC on the y side; V Cua is the total voltage of the capacitor; R arm is the resistance of the MMC arm, L arm is the inductance of the MMC arm, L TM is the transformer leakage inductance of the MMC;

[0083] The grid-side converter (GSC) is essentially a two-level voltage source converter (TL-VSC). From the DC side, the GSC can be regarded as an equivalent current source i dc , from the AC side, GSC can be regarded as an equivalent resistor R eqw and inductor L eqw Equivalent voltage source in series w These equivalent variables can be expressed by the formula:

[0084] The fourth voltage source e w for: R eqw =R f ,L eqw =L f +L Tw ;

[0085] The second current source i dc for:

[0086] Among them, m wx is the modulation ratio of the wind turbine (WT) on the x side in the xy rotating reference frame, m wy is the modulation ratio of WT on the y side; i wx is the GSC current of WT on the x side in the xy rotating reference frame, iwy is the GSC current of WT on the y side; v dc is the GSC DC voltage of WT; e wx is the equivalent voltage source voltage of GSC on the x side in the xy rotating reference frame, e wy is the equivalent voltage source voltage of GSC on the y side, R f is the resistance of the filter; L f is the inductance of the filter; L Tw is the transformer leakage inductance of GSC.

[0087] Furthermore, based on the equivalent circuit of the cascade hybrid converter and the equivalent circuit of the grid-side converter, the main circuit model is obtained including:

[0088] The main circuit model is obtained based on the equivalent circuit of the cascade hybrid converter and the equivalent circuit of the grid-side converter instead of the converter.

[0089] Specifically, the above interface equivalent circuit is used to replace the converter to obtain the equivalent circuit of the designed system, as shown in Figure 5 The dynamics of the power grid can be expressed as:

[0090]

[0091] Where L g is the inductance of the power grid sending end; i gx is the current on the x side of the power grid sending end in the xy rotating reference system, i gy is the current at the y side of the power grid sending end; V gx is the voltage on the x side of the grid transmitting end in the xy rotating reference frame, V gy is the voltage of the power grid sending end on the y side; v px is the voltage on the x side of the PCC in the xy rotating reference frame, v py is the voltage of PCC on the y side; R g is the resistance of the power grid sending end; w0 is the rated angular frequency of the power grid; L g is the inductance of the sending end of the power grid.

[0092] Differential equations of grid-connected current of GSC and MMC w and i M It can be expressed as:

[0093]

[0094] Where L eqw is the equivalent inductance of GSC; i wx is the grid-connected current of the GSC on the x side in the xy rotating reference system, i wy is the grid-connected current of GSC on the y side; e wx is the voltage on the x side of the GSC in the xy rotating reference frame, ewy is the voltage of GSC on the y side; R eqw is the equivalent resistance of GSC; L eqM is the equivalent inductance of MMC; i Mx is the grid-connected current of the MMC on the x side in the xy rotating reference system, i My is the grid-connected current of MMC on the y side; e Mx is the voltage source on the x side of the MMC in the xy rotating reference frame, e My is the voltage source of MMC on the y side; R eqM is the equivalent resistance of MMC.

[0095] PCC voltage v p , and the fixed capacitor C in the LCC filter f The voltage expression can be expressed as:

[0096]

[0097] Where i fx is the current on the x side of the filter in the xy rotating reference frame, i fy is the current of the filter on the y side; i lhpx is the current on the x side of the low-pass filter in the xy rotating reference frame, i lhpy is the current of the low-pass filter on the y side; v chpx is the voltage on the x side of the high-pass filter capacitor in the xy rotating reference frame, v chpy is the voltage of the high-pass filter capacitor on the y side; R hp is the resistance of the high-pass filter; i l1x is the current on the x side of the inductor L1 in the filter in the xy rotating reference system, i l1y is the current of the inductor L1 on the y side of the filter; C f is the fixed capacitor in the filter.

[0098] where i f is the current flowing into the filter, i g is the current flowing out of the transmission grid, i w is the current flowing out of the grid-side converter (GSC), i L is the current flowing into the thyristor converter (LCC), i M is the current flowing into the multilevel converter (MMC), i f =i g +i w -i L -i M .

[0099] Further, the control system model includes: a power synchronization loop, a vector current control module and a circulating current suppression control module;

[0100] Power synchronization loop, used to regulate active power;

[0101] The vector current control module includes an outer loop and an inner loop, wherein the outer loop is used to adjust the active power and support the voltage of the common coupling point, and the inner loop is used to control the grid-connected current;

[0102] Circulating current suppression control module is used to eliminate internal second harmonic circulating current.

[0103] The power synchronization loop in the control system is established based on the PSL dynamic equation. The vector current control and circulating current suppression control are modeled based on their structure and function using PI controllers, feedback links, etc. The main difference of the networked control system is reflected in the synchronization unit of the MMC, that is, the power synchronization loop. Therefore, the main basis for control system modeling is the PSL dynamic equation.

[0104] Specifically, the difference in the grid-type control scheme of the cascade hybrid converter is mainly reflected in the synchronization unit of the MMC, that is, the power synchronization loop. The dynamic equation of the power synchronization loop can be expressed as:

[0105]

[0106] Where H psl is the inertia time constant, is the reference angular frequency in PSL, is the measured value of the unit value of MMC active power, The reference value of the unit value of MMC active power, D psl is the damping coefficient, θ psl is the reference phase in PSL, and w0 is the rated angular frequency of the power grid.

[0107] It can be seen that the differential equation of the power synchronization loop has a similar mathematical form to the differential equation of the synchronous generator rotor, which reflects that the power synchronization loop is essentially a simulation of the latter at the control level.

[0108] Furthermore, based on the main circuit model and the control system model, a networked control small signal model is established, including:

[0109] The coordinates of the main circuit model and the control system model are unified to obtain a small signal model of network control.

[0110] Furthermore, the grid-type control small signal model is used to control the wind power generation cascade hybrid DC transmission system including:

[0111] The small signal model is essentially a nonlinear model of the wind power cascade hybrid DC system obtained based on the main circuit and control system models. Since the voltage support function of the cascade hybrid converter is completed by the MMC, the control system can control the wind power cascade hybrid DC transmission system and the small signal model by controlling the multilevel converter.

[0112] Specifically, considering that the main circuit and control system of the system are modeled on different rotating reference frames, coordinate transformation formulas are required to combine them into a complete small signal model.

[0113] The coordinate transformation matrix between the xy and dq reference frames can be expressed as:

[0114]

[0115] Where θ represents the angle between the d-axis and the x-axis.

[0116] Combining the main circuit and control system models, the general form of nonlinear dynamics of the wind power cascade hybrid DC system is obtained, which can be expressed as:

[0117]

[0118] Among them, x and u are the state vector and input vector of the system; f represents the nonlinear function vector.

[0119]

[0120] Where A and B are the state matrix and input matrix respectively; Δx and Δu are small increments of the state and input vectors.

[0121] An electromagnetic transient (EMT) model of a cascaded hybrid HVDC wind power system is established, and its time domain response is compared with that of the above small signal model.

[0122] The definition of the silicon controlled rectifier (SCR) of the sending end grid can be expressed as:

[0123]

[0124] In the formula, Z g is the equivalent impedance of the power grid at the sending end, v g is the voltage at the transmission end of the power grid, V b is the grid voltage base value, P dcN is the rated power of the DC system.

[0125] Figure 6 The LCC DC voltage v is described when the cascade hybrid converter uses the GFM control scheme. dcL 、MMC active power P M 、PSL phase angle θpsl and the MMC total capacitance voltage V Cua Comparison of the time domain responses of the EMT and small signal models.

[0126] In order to evaluate the stability of the grid-forming control scheme under weak grid conditions, Table 1 shows the cascaded hybrid converter using the grid-following (GFL) and grid-forming (GFM) control schemes, and tests their tolerance under weak grid and ground-wind power output conditions through their stable region graphical visualization, using the mode with a larger real part (because a smaller real part means less impact on system stability). The operating conditions are SCR = 2, (SCR is the thyristor of the sending end power grid, is the unit value of wind farm output power).

[0127] Specifically, the eigenvalues ​​of the system in the five grid-following control modes and the five grid-forming control modes are first solved by numerical calculation methods. Secondly, the damping ratio and oscillation frequency of the system corresponding to the control mode are calculated according to the real and imaginary parts of the eigenvalues ​​to evaluate the stability and oscillation characteristics of the system. Finally, the stable boundary diagram is drawn to display the stable area of ​​the system under different control schemes, and the stability performance of the grid-following and grid-forming control schemes is compared.

[0128] Table 1

[0129]

[0130]

[0131] By seeking difference The corresponding critical SCR (or the critical value corresponding to different SCRs) ), the stable region of the system can be determined when the cascaded hybrid converter adopts the grid-building type and grid-following type control schemes, such as Figure 7 shown.

[0132] exist Figure 7 In the equation, the area above the stability boundary is the stable area of ​​the system, and the area below the stability boundary is the unstable area or the infeasible area (no equilibrium point).

[0133] Depend on Figure 7 It can be seen that the system stable region under the GFL control scheme is smaller than the system stable region under the GFM control scheme; the networked control system covers the entire region with equilibrium points. The results show that the GFM control scheme can maximize the system stable region.

[0134] Under the grid-based control scheme, the stability boundary of the system is linear, and the active power P of the grid is g It can be expressed as:

[0135]

[0136] Where α g is the complementary angle of the grid impedance angle, δ g is the power angle of the grid, v p is the voltage of PCC.

[0137] Divide both sides of the above formula by the power base value S of the power grid b =P dcN ,have to:

[0138]

[0139] in, is the unit value of the voltage at the transmission end of the power grid, is the unit value of PCC voltage, is the definition of the controllable oil ratio of the transmission end grid. Therefore, the above formula can be rewritten as:

[0140]

[0141] Combining the constraints and the above formula, we can get:

[0142]

[0143] In the formula is the unit value of wind farm output power, is the unit value of LCC power, is the unit value of MMC power, and SCR is the thyristor of the sending-end power grid.

[0144] The system stability boundary corresponding to the GFM control scheme means that the power grid reaches the power limit, that is, δ g =π / 2-α g This means that under the GFM control scheme, the satisfy:

[0145]

[0146] Where b is the intercept of the stable boundary of the network-building system, and k is the slope of the stable boundary of the network-building system.

[0147] The above equation represents the linearity of the system stability boundary under the GFM control scheme. This is because the GFM control scheme can push the power grid to its power limit, that is, the GFM control scheme can maximize the stability area of ​​the system.

[0148] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A grid-type control method based on a weak grid wind power cascade hybrid converter, characterized in that: include: Acquire a cascade hybrid converter circuit and a cascade hybrid DC system circuit, and establish a wind power generation cascade hybrid DC transmission system based on the cascade hybrid converter and the cascade hybrid DC system circuit; Obtaining an equivalent circuit of a cascade hybrid converter and an equivalent circuit of a grid-side converter; Based on the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit, a main circuit model is obtained; Acquire a control system model, and establish a networked control small signal model based on the main circuit model and the control system model; The grid-type control small signal model is used to control the wind power generation cascade hybrid direct current transmission system.

2. A grid-type control method based on a weak grid wind power cascade hybrid converter according to claim 1, characterized in that: The cascade hybrid converter circuit is composed of a thyristor converter and a multi-level converter connected in series.

3. A grid-type control method based on a weak grid wind power cascade hybrid converter according to claim 1, characterized in that: The establishment of a cascaded hybrid DC transmission system for wind power generation includes: Based on the cascade hybrid converter and the cascade hybrid DC system circuit, combined with constraint conditions, the wind power generation cascade hybrid DC transmission system is established; The constraints are: Among them, P S To convert the grid output power, P W is the output power of the wind farm, P L is the power of LCC, P M is the power of MMC, V dcL is the DC voltage of LCC, V dcM is the DC voltage of MMC.

4. A grid-type control method based on a weak grid wind power cascade hybrid converter according to claim 2, characterized in that: Obtaining the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit includes: On the AC side, the thyristor converter is equivalent to a first current source i L On the DC side, the thyristor converter is equivalent to a first voltage source e dcL ; The first current source i L for: The first voltage source e dcL for: Among them, i Lx is the equivalent current source on the x side of the LCC in the xy rotating reference frame, i Ly is the equivalent current source of LCC on the y side, N b is the number of six-pulse units, i dc1 is the DC current of the rectifier, μ is the commutation angle, α is the actual ignition delay angle of the LCC, e dcL is the equivalent voltage source of LCC, v p is the voltage of PCC; On the AC side, the multilevel converter is equivalent to a second voltage source e M On the DC side, the multilevel converter is equivalent to a third voltage source e dcM ; The second voltage source e M for: The third voltage source e dcM for: Among them, e Mx is the voltage of the equivalent voltage source of the MMC on the x side in the xy rotating reference system, e My is the voltage of the equivalent voltage source of the MMC on the y side; is the differential mode modulation ratio of the MMC on the x side in the xy rotating reference frame, is the differential mode modulation ratio of MMC on the y side; V Cux2 is the dual-frequency capacitor voltage on the x side of the MMC in the xy rotating reference frame, V Cuy2 is the dual-frequency capacitor voltage of MMC on the y side; is the dual-frequency common-mode modulation ratio of the MMC on the x side in the xy rotating reference frame, is the dual-frequency common-mode modulation ratio of MMC on the y side; V Cux is the total capacitance voltage of the MMC on the x side in the xy rotating reference frame, V Cuy is the total capacitance voltage of the MMC on the y side; V Cua is the total voltage of the capacitor; R arm is the resistance of the MMC arm, L arm is the inductance of the MMC arm, L TM is the transformer leakage inductance of the MMC; On the AC side, the grid-side converter is equivalent to a fourth voltage source e w On the DC side, the thyristor converter is equivalent to a second current source i dc ; The fourth voltage source e w for: The second current source i dc for: Among them, m wx is the modulation ratio of the wind turbine (WT) on the x side in the xy rotating reference frame, m wy is the modulation ratio of WT on the y side; i wx is the GSC current of WT on the x side in the xy rotating reference frame, i wy is the GSC current of WT on the y side; v dc is the GSC DC voltage of WT; e wx is the equivalent voltage source voltage of GSC on the x side in the xy rotating reference frame, e wy is the equivalent voltage source voltage of GSC on the y side.

5. The grid-type control method based on a weak grid wind power cascade hybrid converter according to claim 1 is characterized in that: Based on the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit, obtaining the main circuit model includes: The main circuit model is obtained by replacing the converter based on the cascade hybrid converter equivalent circuit and the grid-side converter equivalent circuit.

6. A grid-type control method based on a weak grid wind power cascade hybrid converter according to claim 1, characterized in that: The control system model includes: a power synchronization loop, a vector current control module and a circulating current suppression control module; The power synchronization loop is used to adjust the active power; The vector current control module includes an outer loop and an inner loop, wherein the outer loop is used to adjust the active power and support the voltage of the common coupling point, and the inner loop is used to control the grid-connected current; The circulating current suppression control module is used to eliminate the internal second harmonic circulating current.

7. A grid-type control method based on a weak grid wind power cascade hybrid converter according to claim 1, characterized in that: Based on the main circuit model and the control system model, a networked control small signal model is established, including: The coordinates of the main circuit model and the control system model are unified to obtain a meshed control small signal model.

8. A grid-type control method based on a weak grid wind power cascade hybrid converter according to claim 2, characterized in that: Using the grid-type control small signal model, controlling the wind power generation cascade hybrid DC transmission system includes: The wind power generation cascade hybrid direct current transmission system is controlled by controlling the multi-level converter.

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