Wideband Harmonic Impedance Testing Equipment and Methods for Wind Power Grid

Abstract

This invention discloses a broadband harmonic impedance testing device and method for wind power grids. The device employs a hierarchical control strategy to transmit harmonic voltages to the wind power grid with a frequency range of 100Hz to 5000Hz and an amplitude range of 0 to 1000V. By injecting harmonics with controllable amplitude and frequency into the wind power grid using this technology, and collecting voltage and current responses at different frequencies at the point of common coupling (PCC) port, the harmonic impedance characteristics of the wind power grid are obtained based on a wind power grid impedance algorithm. This is a reliable technology for pre-assessment of wind power grid system construction and for the inspection and maintenance of power transmission lines.

Description

Technical Field

[0001] This invention is used for wind power grid impedance testing and belongs to the technical field of wind power grid systems. Specifically, this invention relates to a wideband harmonic impedance testing device and a harmonic impedance testing method suitable for wind power grids. Background Technology

[0002] Since the birth of power systems, harmonic resonance has been a crucial aspect of their dynamic and stability studies. In recent years, broadband oscillations occurring in power systems with a high proportion of power electronics have exhibited new and complex characteristics. Oscillations dominated by power electronic equipment include weakly connected system oscillations of VSC equipment (such as direct-drive wind power / STATCOM), oscillations of doubly-fed wind power transmitted via flexible direct current, and oscillations caused by VSC control parameter mismatches. The dynamic interaction between VSC equipment and the grid causes the control systems of direct-drive wind power / STATCOM to exhibit negative damping characteristics for oscillation modes. Furthermore, resonances caused by control parameter mismatches of equipment using VSC as the grid interface also fall under the category of control resonances, primarily resulting from inappropriate parameters in the control loops of equipment such as phase-locked loops (PLLs) or control parameters that are not adapted to the grid operating conditions.

[0003] Currently, research on harmonic impedance in power grid systems mainly focuses on modeling the harmonic transmission characteristics of power grids. Researchers establish simulation models of power grids using methods such as the state variable method and multi-conductor transmission line theory, analyzing the laws and characteristics of harmonic resonance from aspects such as resonant frequency, harmonic power flow calculation, resonant overvoltage, and overcurrent transmission characteristics. However, due to the complexity of the power grid structure, model parameters are difficult to obtain accurately; the impedance viewed from the high-voltage incoming line to the power system cannot be accurately given at harmonic frequencies, yet this portion of the harmonic impedance has a significant impact on the occurrence of resonance. In other words, it is difficult to accurately calculate the harmonic impedance of the power grid through simulation modeling. Therefore, it is necessary to obtain the harmonic impedance characteristics of the power grid through testing methods.

[0004] Harmonic currents generated by nonlinear load intervention methods in power systems, such as capacitor switching and thyristor switching, suffer from the drawback that the amplitude of harmonic current decreases with increasing frequency. Measurement methods based on voltage disturbance generators require an additional DC source. Non-interventional harmonic impedance estimation methods based on data processing algorithms, such as fluctuation methods, regression methods, and statistical methods, exist. Currently, strategies for broadband harmonic impedance testing in wind power grids are still in their early stages, lacking a systematic technical solution. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides a wideband harmonic impedance testing device for wind power grids and a testing method based on the control system of the device.

[0006] To achieve the above-mentioned objectives, the technical solution provided by this invention is: a wideband harmonic impedance testing device for wind power grids, comprising a step-down transformer TP, a harmonic source converter HG, and a converter reactor Lc. The primary winding of the step-down transformer TP is connected in series with the wind power grid system to extract energy. The secondary main winding of the step-down transformer TP is connected to the harmonic source converter HG through the converter reactor Lc, providing power supply voltage to the harmonic source converter HG. The harmonic source converter HG is composed of power electronic switching devices, consisting of N H-bridge modules cascaded in three phases, where 2≤N≤10, and N is an integer. Harmonic current generates harmonic voltage through the converter reactor Lc. The harmonic voltage emitted by the harmonic impedance testing device has a frequency range of 100Hz~500Hz and an amplitude range of 0~1000V.

[0007] The rated voltage of the primary winding of the step-down transformer TP is taken according to the nominal voltage of the wind power grid, and the rated voltage of the secondary winding of the step-down transformer TP is matched with that of the harmonic source converter HG, with a range of 200V to 3000V.

[0008] The power switching devices of the three-phase cascaded three-phase H-bridge harmonic source converter HG are IGBTs, IGCTs, or MOSFETs, which generate harmonic currents with variable amplitude and frequency through PWM carrier phase shift modulation.

[0009] This invention also provides a control system for the aforementioned broadband harmonic impedance testing equipment for wind power grids. The system employs a hierarchical control strategy, including top-level power control, intermediate-level harmonic control, and bottom-level voltage equalization control. The top-level power control absorbs active power from the grid and stores harmonic energy; the intermediate-level harmonic control generates three-phase symmetrical harmonic currents; and the bottom-level voltage equalization control balances the DC-side voltages between and within phases.

[0010] Top-level power control is used to balance its own losses and store energy in the capacitor as the energy source for emitting harmonics. The top-level power control enables the harmonic source converter of the harmonic impedance testing technology to absorb the fundamental active power from the grid, balance its own losses, and store energy in the capacitor as the energy source for emitting harmonics. In the top-level power control, the harmonic source converter operates in PWM rectification mode, the capacitor voltage of each module is stabilized near the given value, and the harmonic source converter does not emit harmonic current to the wind grid.

[0011] Intermediate layer harmonic control controls the generation of harmonic current. The intermediate layer harmonic control specifies the frequency and amplitude of the harmonic to be emitted. The harmonic modulation wave is superimposed on the total modulation wave and modulated by the harmonic source converter to generate a three-phase symmetrical harmonic current of the required frequency. This current is injected into the wind power grid system and applied to the system impedance to generate a harmonic voltage response.

[0012] The underlying voltage equalization control balances the capacitor voltages of each module. The underlying voltage equalization control adopts the modulation wave fine-tuning superposition method: a small interphase voltage equalization modulation wave and an intraphase voltage equalization modulation wave are superimposed on the main modulation wave of each module to adjust the phase or amplitude of the modulation wave of the sub-module, change the AC side output of the sub-module, adjust the active energy absorbed by the sub-module, and balance the capacitor voltage of the sub-module.

[0013] By employing a hierarchical control strategy, the top-level power control modulation wave, the middle-level harmonic modulation wave, and the bottom-level voltage equalization modulation wave are superimposed to form the total modulation wave output. This output is compared with a triangular carrier wave to obtain the pulse switching signal of the H-bridge IGBT, thereby realizing the various functions of the harmonic source converter.

[0014] The present invention also provides a method for testing the harmonic impedance of a wind power grid applicable to the above-mentioned broadband harmonic impedance testing equipment, comprising the following steps;

[0015] Step S1: The harmonic source converter of the harmonic impedance testing equipment is connected to the common connection point between the wind power generation terminal and the public power grid through a transformer.

[0016] Step S2: Based on the hierarchical control strategy of the control system of the test equipment, the harmonic source converter can output positive sequence harmonic current, negative sequence harmonic current and zero sequence harmonic current with controllable amplitude and frequency, and obtain the user-set sampling frequency, number of storage points, storage interval, lower limit of harmonic current and lower limit of harmonic frequency.

[0017] Step S3: Inject the positive-sequence harmonic current, negative-sequence harmonic current, and zero-sequence harmonic current generated by the harmonic source converter into the common coupling point.

[0018] Step 4: Collect the voltage and current responses at different frequencies at the common connection point port, convert the collected voltage and current responses into low-voltage analog signals, pass them through the data acquisition card, convert them into digital signals, and upload them to the industrial control computer.

[0019] Step 5: By calculating the ratio of voltage response to current response, the positive-sequence harmonic impedance, negative-sequence harmonic impedance, and zero-sequence harmonic impedance of the wind power grid are obtained. Then, the impedance of each phase of the three-phase wind power network is obtained through inverse transformation using the symmetrical component method. Beneficial effects of this invention:

[0020] Simulation modeling methods cannot accurately obtain the frequency characteristics and resonant frequency of wind power grid impedance. This invention provides a hierarchical control strategy for the control system of a broadband harmonic impedance testing device for wind power grids. The three-layer control strategy has clearly defined functions and works in concert, providing a high-capacity, high-voltage, wide-bandwidth, and amplitude-adjustable harmonic source control strategy for the actual measurement of wind power grid impedance frequency characteristics. Based on the hierarchical control strategy of this testing device, the wind power grid impedance algorithm establishes a positive / negative / zero-sequence model that can be decoupled under certain conditions, overcoming the influence of the dynamic process of different phase-to-phase voltages, which is beneficial to improving measurement accuracy and obtaining more accurate wind power grid harmonic impedance characteristics. Attached Figure Description

[0021] When considered in conjunction with the accompanying drawings, the invention will be more fully and better understood, and its many accompanying advantages will become readily apparent, by referring to the following detailed description. However, the accompanying drawings, which are provided to further illustrate the invention and form part of this invention, are used to explain the invention and do not constitute an undue limitation thereof, as shown in the figures:

[0022] Figure 1 A schematic diagram of the topology for impedance testing of cascaded H-bridge wind power grids (single-phase schematic).

[0023] Figure 2 Connection relationship of harmonic source converter module for impedance testing of cascaded H-bridge wind power grid (single-phase schematic diagram);

[0024] Figure 3 It is a top-level power control structure;

[0025] Figure 4 It is a current loop control structure;

[0026] Figure 5 To simplify the control structure for the current loop;

[0027] Figure 6 For intermediate layer harmonic control;

[0028] Figure 7 For bottom-level pressure equalization control;

[0029] Figure 8 For overall structural control;

[0030] Figure 9 For current clamping;

[0031] Figure 10 It is a high-voltage divider;

[0032] Figure 11 It is an HS4 data acquisition card. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0034] like Figure 1 As shown, a broadband harmonic impedance testing device for wind power grids according to the present invention includes: a step-down transformer TP, a harmonic source converter HG, and a converter reactor Lc. The primary winding of the step-down transformer TP is connected in series with the wind power grid system, and the secondary main winding of the step-down transformer TP is connected to the harmonic source converter HG through the converter reactor Lc, providing power supply voltage to the harmonic source converter HG. The harmonic source converter HG is composed of power electronic switching devices and consists of N cascaded H-bridge modules. The connection relationship between any two adjacent module units (Mm and Mm+1, respectively) is as follows: Figure 2 As shown. Each module consists of two "branch arms" L1 and L2, as follows. Figure 2 As shown in the red dashed box, the midpoint of the branch arm is the circuit connection point. The midpoint of the L2 branch arm of module Mm is connected to the midpoint of the L1 branch arm of the next adjacent module Mm+1. This process is repeated until all N modules are connected. Figure 1 , Figure 2 C1-C N Represents the DC-side capacitors of the module, R1-R N Lc represents the equivalent load resistance, Lc represents the converter branch inductance, Ze represents the equivalent impedance of the wind power grid, and C represents the equivalent load resistance. m C represents the DC-side capacitor of the m-th module. m+1 R represents the DC-side capacitance of the module adjacent to the m-th module. m R represents the equivalent load resistance of the m-th module. m+1 This represents the equivalent load resistance of the module next to the m-th module.

[0035] The control system of the harmonic impedance testing equipment adopts a hierarchical control strategy: the top layer is power control, which balances its own losses and provides energy for the harmonics emitted by the control system; the middle layer is harmonic control, which controls the generation of harmonic currents; and the bottom layer is voltage equalization control, which balances the capacitor voltages of each module. Specifically:

[0036] Top-level power control enables the harmonic source converter in the harmonic impedance testing equipment to absorb a certain amount of fundamental active power from the grid, balance its own losses, and store energy in the capacitor as the energy source for emitting harmonics. In this layer of control, the harmonic source converter operates in PWM rectification mode, the capacitor voltage of each module is stable near the given value, and the harmonic source converter does not emit harmonics to the wind grid;

[0037] like Figure 3 As shown, the input current I of the harmonic source converter abc After transformation to the dq coordinate system, the d-axis component I of the input current is obtained. d and q-axis component I qThe input voltage U of the harmonic source converter abc After transformation to the dq coordinate system, the d-axis component U of the input voltage is obtained. s d-axis and q-axis components U sq The total DC-side voltage U of a cluster of harmonic source converters. dcsum With a given value The error is used by the PI controller to generate the current loop d-axis current. I d and The error, as determined by the PI controller, is used as the input to the d-axis of the outer voltage loop and is related to U. sd The output U, which is the d-axis active voltage superimposed with the q-axis coupling voltage as a harmonic source, is... rd , where ω is the angular frequency of the grid voltage, obtained from the grid voltage through a phase-locked loop. q With q-axis given current The error, as determined by the PI controller, is used as the input to the q-axis of the outer voltage loop and is related to U. sq The voltage coupled to the d-axis is superimposed to form the output of the q-axis harmonic source. rd and U rq After inverse transformation to the dq coordinate system, and then through mean reduction, divided by... Then the top-level modulated wave is obtained.

[0038] Figure 4 The top-level power control current inner loop structure shown uses the error between the current setpoint and the current sampling value as the input to the current controller w(s). The controller output... The summation with the voltage feedforward compensation is then used as the output power control modulation wave. Voltage feedforward compensation can suppress power supply voltage disturbances for control. The current controller w(s) can be a proportional regulator, proportional-integral regulator, proportional-resonant regulator, quasi-proportional-resonant regulator, etc. Figure 4 Zhong K PWM For the bridge equivalent gain, u c This is the output voltage at the converter bridge port. Considering the sampling delay of the current inner loop signal and the small inertia characteristic of PWM, the simplified control structure of the current inner loop is as follows: Figure 5 As shown in the figure, T s To control the cycle time constant.

[0039] Intermediate layer harmonic control specifies the frequency and amplitude of the harmonics to be emitted. The harmonic modulation wave is superimposed on the total modulation wave and modulated by the harmonic source converter to generate a three-phase symmetrical harmonic current of the required frequency. This current is injected into the wind power system and applied to the system impedance to generate a harmonic voltage response.

[0040] The harmonic generation reference signal is generated by the h-th harmonic sinusoidal signal. The reference voltages for the three phases A, B, and C harmonics are 120° apart, and the reference signals are sin(hωt) respectively. sin(hωt±2π / 3). When a positive-sequence harmonic disturbance is issued, phases A, B, and C lag by 120° sequentially; when a negative-sequence harmonic disturbance command is issued, phases A, B, and C lead by 120° sequentially; when a zero-sequence harmonic disturbance command is issued, phases A, B, and C are in phase. The given signal is multiplied by the corresponding amplitude |U. af |、|U bf |、|U cf Then, after mean normalization and division by the DC-side reference voltage of the single-phase harmonic source converter... Then, a given modulated wave is generated as a harmonic through switch control. Superimposed on the total modulated wave. Intermediate layer harmonic control, as follows: Figure 6 As shown. By controlling the given frequency parameters of harmonic generation, the current signal injected at the point of common coupling can be dynamically adjusted, thereby indirectly regulating the harmonic voltage response of the system impedance. The intermediate layer harmonic control circuit can generate harmonic currents of 100–5000 Hz, which can inject harmonics and interharmonics into the point of common coupling port.

[0041] Bottom-level voltage equalization control. The harmonic source converter consists of a three-phase H-bridge cascaded system. Since the DC capacitors of each module are independent, voltage imbalances inevitably arise between the three phases and within each phase sub-module due to varying power losses. This layer of control employs a modulation wave fine-tuning superposition method: tiny inter-phase and intra-phase voltage equalization modulation waves are superimposed on the main modulation wave of each module. This adjusts the phase or amplitude of the sub-module's modulation wave, altering the sub-module's AC output, adjusting the sub-module's absorption of active power, and balancing the sub-module capacitor voltage.

[0042] The change in active power on the DC side of the harmonic source converter is:

[0043]

[0044] In the formula: P absorbk The active power absorbed by the k-th H-bridge module, W; P lossk The power loss of the k-th H-bridge module is expressed in W; P. ck W represents the change when the k-th H-bridge module is working.

[0045] P absorbk =U k I n cosθ k (2)

[0046] In the formula: U k Let V be the AC side voltage of the k-th H-bridge mode; I be the voltage across the bridge. n θ is the input current of the H-bridge, in A; k It is the angle between the AC side voltage and the input current.

[0047] When the input current of the H-bridge remains constant, the amplitude U of the DC-side capacitor voltage can be changed by altering the active power absorbed by the DC capacitor. k And the angle θ between voltage and current k This allows for adjustment of the DC-side voltage of the submodule to achieve phase-to-phase and phase-to-phase voltage equalization. The underlying voltage equalization control strategy is as follows: Figure 7 As shown. By comparing the total voltage U on the DC side of a certain phase. dcsum_k With a given value The error, after passing through the PI control loop, is then multiplied by the input phase current i. c The sign of the variable is then divided by the DC-side reference voltage of the single-phase harmonic source converter. Obtain interphase equal voltage modulation wave When the total DC voltage is lower than a given value, active power needs to be absorbed to compensate for the DC voltage. Conversely, when the total DC voltage is higher than a given value, active power needs to be consumed, resulting in a loss of DC voltage. The introduction of voltage equalization control ensures that the AC output voltage of the harmonic source converter is not constrained by the DC voltage, thus obtaining a clean harmonic current.

[0048] The top-level power control modulation wave Intermediate layer harmonic modulation wave Bottom-layer phase-to-phase voltage equalization modulation wave Superimposed, and then superimposed again with the voltage equalization modulation wave of the in-phase modules. As the total modulating wave u r The output is compared with a triangular carrier wave to obtain the pulse switching signal of the H-bridge IGBT. The overall structure diagram after the three-layer control is combined is shown below. Figure 8 As shown, the harmonic source converter achieves the following functions: top-level power control to absorb active power from the grid and store harmonic energy; middle-level harmonic control to generate three-phase symmetrical harmonic current; and bottom-level voltage equalization control to balance the DC-side voltage between and within phases.

[0049] This harmonic source converter control process has ended.

[0050] This invention also proposes a harmonic impedance testing method suitable for broadband harmonic impedance testing equipment for wind power grids, comprising the following steps:

[0051] When measuring the harmonic impedance of a wind power grid system, the harmonic source converter becomes a harmonic source. The converter injects positive-sequence, negative-sequence, and zero-sequence harmonic currents of different frequencies into the wind farm's common bus through the point of common coupling. By measuring the voltage and current data at the injection point, the positive-sequence, negative-sequence, and zero-sequence harmonic impedances of the wind farm port are obtained. Then, through inverse transformation using the symmetrical component method, the frequency characteristics of the impedance of each phase of the three-phase network of the wind power system are obtained. Based on the distribution characteristics of the wind power grid impedance frequency characteristic curve, its extreme points are selected to obtain the resonant frequency. The specific explanation is as follows:

[0052] Step S1: Connect the harmonic source converter of the harmonic impedance testing equipment to the common connection point between the wind power generation terminal and the public power grid through a transformer.

[0053] Step S2: Based on the hierarchical control strategy, configure the harmonic source converter to output positive-sequence harmonic current with controllable amplitude and frequency. Negative sequence harmonic current Zero-sequence harmonic current Obtain the user-defined sampling frequency, number of storage points, storage interval, lower limit of harmonic current, and lower limit of harmonic frequency;

[0054] Step S3, convert the positive sequence harmonic current generated by the harmonic source converter Negative sequence harmonic current Zero-sequence harmonic current Inject a public connection point;

[0055] Step S4: Using a current sensor (e.g., the ZH15XX series clamp-type current clamp shown in Figure 9), acquire the current response at the point of common coupling (PCC) port; using a voltage sensor (e.g., the high-voltage divider shown in Figure 10), acquire the voltage response at the PCC port. The voltage and current sensors convert the acquired voltage and current responses into low-voltage analog signals, which are then processed by the HS4 data acquisition card, such as... Figure 11 As shown, it is converted into a digital signal and uploaded to the industrial control computer.

[0056] Step S5: The measurement and control software installed on the industrial control computer performs wind power grid impedance algorithm processing on the signal.

[0057] The current response is then processed using a Fast Fourier Transform to obtain the three-phase current phasors at a specific frequency. The voltage response is then processed using a Fast Fourier Transform to obtain the voltage drop phasor of the system impedance at a specific frequency. The numerical relationship between the current phasor and the voltage drop phasor is as follows:

[0058]

[0059] In the formula: Z s Z represents the self-impedance of each phase of the system under test. m It represents mutual impedance.

[0060] It can be simply remembered as:

[0061] ΔV abcf =ZI abcf (4)

[0062] In a three-phase network, the symmetrical component method can be used to convert the impedance of the three-phase stationary coordinate system into positive and negative sequence impedances, which facilitates engineering measurement.

[0063] The positive-sequence harmonic impedance, negative-sequence harmonic impedance, and zero-sequence harmonic impedance of the wind power grid are obtained by calculating the ratio of voltage response and current response. The phase impedances of the three-phase wind power network are then obtained by inverse transformation using the symmetrical component method.

[0064] The symmetric component method can be used to decompose the port current vector at the point of common coupling into positive-sequence components. Negative order components Zero-order component

[0065]

[0066] In the formula: operator α = e j120° .

[0067] Equation (4) can be written as I 120 =SI abc , where the S matrix is ​​the symmetric component transformation matrix.

[0068] Applying the inverse symmetric component transform can transform the positive-order components Negative order components Zero-order component Synthetic three-phase current phasors:

[0069] I abcf =S -1 I 120 (6)

[0070] In the formula S -1 The matrix is ​​the inverse transformation matrix of the symmetric components.

[0071] The voltage drop across the system impedance is transformed into a symmetrical component in three phases:

[0072] ΔV 120 =SΔV abcf (7)

[0073] Substituting equation (4) into equation (7), and then applying equation (6), we can obtain...

[0074]

[0075] From (8), the order impedance matrix can be obtained:

[0076]

[0077] The sequence impedance of the port at the point of common coupling can be expressed as:

[0078]

[0079] The phase impedances of the three-phase wind power network are then obtained through inverse transformation using the symmetrical component method:

[0080] Z = S-1 Z sc S (11)

[0081] As described above, embodiments of the present invention have been explained in detail. However, many modifications are possible without departing substantially from the inventive points and effects of the present invention, which will be apparent to those skilled in the art. Therefore, all such modifications are also included within the scope of protection of the present invention.

Claims

1. A wind power grid wideband harmonic impedance test device, characterized in that, The harmonic source converter HG is composed of power electronic switching devices, and is a three-phase H-bridge harmonic source converter with N cascaded three-phase modules, 2≤N≤10, N being an integer; the harmonic current passes through the converter reactor Lc to generate a harmonic voltage, and the harmonic voltage frequency range emitted by the harmonic impedance test device is 100Hz-500Hz, and the amplitude range is 0-1000V; The rated voltage of the primary side winding of the step-down transformer TP is taken according to the nominal voltage of the wind power grid, and the rated voltage of the secondary side winding of the step-down transformer TP is matched with the harmonic source converter HG, and the range is 200V-3000V; The power switching devices of the three-phase H-bridge harmonic source converter HG with three-phase cascaded are IGBT or IGCT or MOSFET, which emits harmonic current with variable amplitude and frequency through PWM carrier phase modulation; the control system of the test device adopts a hierarchical control strategy, including top power control, intermediate harmonic control and bottom voltage balancing control, the top power control absorbs active power from the power grid and stores harmonic energy; the intermediate harmonic control emits three-phase symmetrical harmonic current; the bottom voltage balancing control balances the DC side voltage between phases and within phases, wherein the harmonic source converter control process is: The top power control is used to balance the self-loss and store energy in the capacitor as the energy source for emitting harmonics; The top power control enables the harmonic source converter of the harmonic impedance test technology to absorb the fundamental active power from the power grid, balance the self-loss, and store energy in the capacitor as the energy source for emitting harmonics; the harmonic source converter works in PWM rectification mode in the top power control, and the capacitor voltage of each module is stable around the given value, and the harmonic source converter does not emit harmonic current to the wind power grid; Specifically, Specifically, The input current I of the harmonic source converter abc After transformation to the dq coordinate system, the d-axis component I of the input current is obtained. d and q-axis component I q The input voltage U of the harmonic source converter abc After transformation to the dq coordinate system, the d-axis component U of the input voltage is obtained. sd and q-axis component U sq The total DC-side voltage U of a cluster of harmonic source converters dcsum With a given value The error is used by the PI controller to generate the current loop d-axis current. I d and The error, as determined by the PI controller, is used as the input to the d-axis of the outer voltage loop and is related to U. sd The output U, which is the d-axis active voltage superimposed with the q-axis coupling voltage as a harmonic source, is... rd Where ω is the angular frequency of the grid voltage, obtained from the grid voltage through a phase-locked loop; I q With q-axis given current The error, as determined by the PI controller, is used as the input to the q-axis of the outer voltage loop and is related to U. sq The voltage superimposed on the d-axis coupling voltage is used as the output of the harmonic source q-axis voltage; U rd and U rq After inverse transformation to the dq coordinate system, and then through mean reduction, divided by... Then the top-level modulated wave is obtained. Intermediate layer harmonic control controls the generation of harmonic current; the intermediate layer harmonic control specifies the frequency and amplitude of the harmonic to be emitted, the harmonic modulation wave is superimposed on the total modulation wave, and after being modulated by the harmonic source converter, a three-phase symmetrical harmonic current of the required frequency is generated and injected into the wind power grid system, applied to the system impedance, and a harmonic voltage response is generated. By controlling the given frequency parameters of the harmonic generation, the current signal injected at the point of common coupling can be dynamically adjusted, thereby indirectly adjusting the harmonic voltage response of the system impedance; the intermediate harmonic control link can generate harmonic current of 100-5000Hz, and can inject harmonics and interharmonics to the point of common coupling port; The harmonic generation given signal is generated by h harmonic sine signal, the ABC three-phase harmonic generation reference voltage is 120°, the given signal is sin(hωt), sin(hωt±2π / 3); when the positive sequence harmonic disturbance is issued, the ABC three-phase is 120° behind in turn; when the negative sequence harmonic disturbance instruction is issued, the ABC three-phase is 120° ahead in turn; when the zero sequence harmonic disturbance instruction is issued, the ABC three-phase is in the same phase; the given signal is multiplied by the corresponding amplitude |U af |、|U bf |、|U cf |, and then the mean value is calculated and divided by the single-phase harmonic source converter DC side reference voltage After that, the switch control is used as the harmonic generation given modulation wave Superimposed to the total modulation wave; The bottom voltage balancing control balances the capacitor voltage of each module; The bottom control strategy adopts a modulation wave fine adjustment method: a small phase-to-phase voltage balancing modulation wave and an intra-phase voltage balancing modulation wave are superimposed on each module main modulation wave, the phase or amplitude of the sub-module modulation wave is adjusted, the sub-module AC side output is changed, the sub-module active power absorption is adjusted, and the sub-module capacitor voltage is balanced; The change amount of the active power of the DC side of the harmonic source converter is: The harmonic source converter control process is ended. In the formula, P absorbk P is the active power absorbed by the kth H-bridge module, W; P lossk P is the power dissipated by the kth H-bridge module, W; P ck P is the change in the kth H-bridge module when it is working, W; P absorbk = U k I n cos θ k (2) where: U k is the AC side voltage of the kth H-bridge module, V; I n is the input current of the H-bridge, A; θ k is the angle between the AC side voltage and the input current; When the input current of the H-bridge is constant, the amplitude of the DC side capacitor voltage can be changed by changing the active power absorbed by the DC capacitor k And the angle θ between the voltage and the current k And further, the DC side voltage of the sub-module can be adjusted to achieve inter-phase and intra-phase balance. By comparing the total voltage U dcsum_k of a certain phase with a given value , the error is passed through a PI control link, multiplied by the sign of the input phase current i c , and divided by the single-phase harmonic source converter DC side reference voltage to obtain the inter-phase voltage balancing modulation wave When the total voltage of the DC side is lower than the given value, active power needs to be absorbed to compensate for the DC side voltage; conversely, when the total voltage of the DC side is higher than the given value, active power needs to be consumed to waste the DC side voltage; the introduction of the voltage balancing control makes the AC side output voltage of the harmonic source converter not be restricted by the DC side voltage, and a pure harmonic current is obtained; Top layer power control modulation wave Intermediate layer harmonic modulation wave Bottom layer interphase voltage sharing modulation wave Superimposed, superimposed phase internal module voltage sharing modulation wave As the total modulation wave u r Output, compared with the triangular carrier, get the pulse switching signal of H bridge IGBT; The harmonic impedance test device includes the following steps:

2. A wind power grid harmonic impedance test method of the wind power grid broadband harmonic impedance test device according to claim 1, characterized in that Step S1, the harmonic source converter of the harmonic impedance test device is connected to the point of common coupling between the wind power generation end and the public power grid through the transformer; ​ Step S2, based on the layered control strategy of the control system of the test device, the harmonic source converter can emit positive sequence harmonic current, negative sequence harmonic current and zero sequence harmonic current with controllable amplitude and frequency, and obtain the sampling frequency, storage point number, storage interval, harmonic current lower limit and harmonic frequency lower limit set by the user; Step S3, injecting the positive sequence harmonic current, negative sequence harmonic current and zero sequence harmonic current emitted by the harmonic source converter into the point of common coupling; Step S4, collecting the voltage response and current response of the point of common coupling under different frequencies, converting the collected voltage response and current response into low-voltage analog signals, passing through a data acquisition card to become digital signals, and uploading to an industrial computer; Step S5, obtaining the positive sequence harmonic impedance, negative sequence harmonic impedance and zero sequence harmonic impedance of the wind power grid by calculating the ratio of the voltage response and current response, and then obtaining the impedance of each phase of the wind power three-phase network through inverse transformation by the symmetrical component method.

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