Open-loop injection control method and system of ac signal on dc side of mmc

Abstract

The application discloses a kind of open-loop injection control method and system of MMC direct current side alternating current signal, and the modulation wave generating unit generates alternating current multi-frequency component modulation wave, and each phase multi-frequency component modulation wave in modulation wave generating unit is synthesized by the power frequency modulation wave component, lower sideband modulation wave component and upper sideband modulation wave component of its corresponding phase;Each modulation wave component in each phase multi-frequency component modulation wave is kept at the fixed phase difference between three-phase alternating voltage, so that the alternating current multi-frequency component modulation wave after synthesis forms the fluctuation characteristics directly related to low-frequency detection signal frequency;Modulation wave generating unit realizes low-frequency detection signal open-loop injection to MMC converter by gate signal generating unit, overcomes the problem that weak alternating current signal is difficult to accurately extract from direct current voltage detection signal in prior art by adopting closed-loop injection, with the advantages of high injection accuracy, simple implementation, not affected by working condition etc.

Description

Technical Field

[0001] This invention relates to the field of high-voltage DC cable insulation monitoring, and specifically to an open-loop injection control method and system for AC signals on the DC side of MMC. Background Technology

[0002] High-voltage direct current (HVDC) cables are crucial physical carriers for modular multilevel converter (MMC) flexible DC transmission systems. The cross-linked polyethylene (XLPE) insulating dielectric material is susceptible to aging and dielectric breakdown under the combined effects of thermo-electrical-mechanical stress, posing a serious threat to the safe and stable operation of the power system. Injecting specific AC signals into the cable can effectively improve the accuracy of online insulation condition monitoring, which is of great significance for ensuring the safe and stable power supply from the cable.

[0003] Patent application CN120490704A discloses a "Method and System for Online Monitoring of High-Voltage DC Cable Condition Based on MMC Low-Frequency Injection," proposing to inject low-frequency AC signals of specific amplitude and frequency into DC cables by controlling the MMC converter valve, thereby monitoring the dielectric loss angle of the DC cable under low-frequency conditions online and realizing online assessment of the insulation condition of the DC cable; (See attached specification of this patent application). Figure 1 As shown, the proposed MMC DC-side AC signal injection method employs a closed-loop control strategy. Specifically, the low-frequency injection target voltage generation module first generates a target voltage Ugoal signal output by the MMC to the DC cable based on DC and low-frequency AC voltage commands. Then, a dual closed-loop control method consisting of an outer voltage loop and an inner current loop controls the DC voltage to track the target voltage Ugoal signal. However, in practical system control applications, although the closed-loop controller can control the MMC DC-side voltage to track the target voltage signal, its control accuracy is often affected by multiple factors such as voltage and current sensors, controller bandwidth, and electromagnetic interference. To ensure that cable condition monitoring does not affect the normal operation of the MMC system, the amplitude of the AC injection voltage signal is usually very weak (e.g., no more than 1% of the rated DC voltage). This requires the DC bus voltage sensor to be able to accurately measure both large-amplitude DC voltage and small-amplitude AC voltage simultaneously, which is difficult to implement in engineering.

[0004] Patent application CN114244165A discloses a hybrid multilevel converter control method based on open-loop common-mode voltage injection. This method suppresses capacitor voltage fluctuations in the multilevel converter by injecting a third-harmonic common-mode voltage with specific amplitude and phase onto a reference wave. Although this method uses open-loop control and eliminates the need for capacitor voltage feedback signals, thus reducing system complexity and cost, it is only applicable to hybrid multilevel converters. The injected third-harmonic common-mode voltage (i.e., a three-phase AC signal with completely equal amplitude, frequency, and phase) cannot generate an AC voltage for online monitoring of DC cable insulation status on the direct side of the MMC system, failing to meet the requirements of high-voltage DC cable insulation status online monitoring applications that demand extremely high signal accuracy. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide an open-loop injection control method and system for AC signals on the DC side of an MMC converter. The modulation wave generation unit realizes open-loop injection of low-frequency detection signals into the MMC converter through the gate signal generation unit, which overcomes the problem that it is difficult to accurately extract weak AC signals from DC voltage detection signals in the prior art using closed-loop injection. It has the advantages of high injection accuracy, simple implementation, and no influence from operating conditions.

[0006] The technical solution adopted in this invention is as follows:

[0007] An open-loop injection control method for AC signal on the DC side of an MMC (Multi-Channel Converter), wherein the MMC DC side includes an MMC converter as the DC output, which outputs DC voltage through a DC cable connected to the power grid; the control method includes:

[0008] An AC multi-frequency component modulated wave is generated by a modulation wave generation unit. Each phase multi-frequency component modulated wave in the modulation wave generation unit is synthesized from the power frequency modulated wave component, the lower sideband modulated wave component, and the upper sideband modulated wave component of its corresponding phase. The power frequency modulated wave is generated according to the DC voltage and power command and through closed-loop control. The lower sideband modulated wave is set to the power frequency minus the low-frequency detection signal frequency, and the upper sideband modulated wave is the power frequency plus the low-frequency detection signal frequency. Each modulated wave component in each phase multi-frequency component modulated wave maintains a fixed phase difference between the three-phase AC voltages, so that the synthesized AC multi-frequency component modulated wave forms a fluctuation characteristic directly related to the low-frequency detection signal frequency.

[0009] A signal transmission link is formed between the gate signal generation unit and the MMC converter to realize the open-loop injection of low-frequency detection signals into the MMC converter.

[0010] Preferably, in the ABC three-phase stationary coordinate system, the power frequency modulation wave components in the multi-frequency component modulation waves of each phase constitute a set of symmetrical vectors with constant amplitude and phase difference of 120°; at the same time, the side-frequency modulation wave components in the multi-frequency component modulation waves of each phase are superimposed on the low-frequency component modulation waves on their corresponding phase power frequency modulation wave components, wherein the low-frequency component modulation waves of each phase are manifested as three voltage fluctuation signals with completely consistent phase and equal amplitude.

[0011] Preferably, through the Clarke transform, the three-phase power frequency modulated wave components with a spatial phase difference of 120° are projected onto mutually perpendicular α and β axes, and combined into a time-varying complex vector U. αβ The synthesis formula is as follows:

[0012] ;

[0013] In the formula, U is the resultant vector on the α-β plane; α U is a vector on the α-axis; β U is a vector on the β-axis; m ω1 is the vector magnitude; j is the imaginary unit; ω1 is the angular velocity of the power frequency component, which is the power frequency; t is time; θ is the initial phase of the power frequency; the synthesized vector rotates counterclockwise around the origin at a constant speed with the angular velocity of the power frequency component ω1, its magnitude remains constant, and the trajectory of the vertex forms a standard circle on the α-β plane;

[0014] Simultaneously, the low-frequency component modulation wave superimposed on the three-phase power frequency modulation wave component constitutes the zero-sequence voltage vector. The voltage vector corresponding to the projection of this zero-sequence voltage vector on the α-β plane is 0, which will not affect the rotation trajectory of the composite vector. It only manifests as fluctuations with the same phase and equal amplitude in the ABC three-phase stationary coordinate system.

[0015] Preferably, in the closed-loop control of the power frequency modulation wave, the DC-side output voltage of the MMC is measured in real time, and the DC voltage and its output power are subjected to directional dual closed-loop vector control. The angular frequency ω1 (as the angular velocity of the power frequency component) and the initial phase θ1 of the grid-side voltage are extracted through a phase-locked loop (PLL). These parameters are substituted into the generation control process of the power frequency modulation wave. It should be noted that the phase-locked loop PLL involved in this application is common knowledge in the field and is not considered as the innovative content of this application. Therefore, this application will not elaborate on it.

[0016] Preferably, during modulation by the modulation wave generation unit, the DC-side output amplitude corresponding to the three-phase power frequency modulation wave component is the DC-side output voltage amplitude U. DC The low-frequency component modulated wave corresponds to the DC-side output amplitude of the DC voltage fluctuation ΔU. DC The DC voltage fluctuation amplitude ΔUDC DC-side output voltage amplitude U not exceeding 1% DC .

[0017] Preferably, the AC multi-frequency component modulation wave includes an A-phase multi-frequency component modulation wave, a B-phase multi-frequency component modulation wave, and a C-phase multi-frequency component modulation wave, wherein,

[0018] A-phase multi-frequency component modulation wave includes A-phase power frequency modulation wave and A-phase low-frequency component modulation wave superimposed on A-phase power frequency modulation wave;

[0019] The B-phase multi-frequency component modulation wave includes the B-phase power frequency modulation wave and the B-phase low-frequency component modulation wave superimposed on the B-phase power frequency modulation wave.

[0020] The C-phase multi-frequency component modulation wave includes the C-phase power frequency modulation wave and the C-phase low-frequency component modulation wave superimposed on the C-phase power frequency modulation wave.

[0021] The modulation generation process of the three-phase multi-frequency component modulated wave is the same and carried out synchronously. At the same time, the initial phase of the B-phase power frequency modulated wave lags behind the A-phase power frequency modulated wave by 120°, and the initial phase of the C-phase power frequency modulated wave lags behind the A-phase power frequency modulated wave by 240°, so that the three maintain a symmetrical relationship with a phase difference of 120°.

[0022] Preferably, the A-phase multi-frequency component modulation is generated and controlled according to the following formula:

[0023] In the formula, U a This is the A-phase multi-frequency component modulated wave voltage. The voltage of the power frequency modulation wave component of phase A. The voltage of the sideband modulation wave component under phase A. U is the voltage of the sideband modulation wave component on phase A. DC_A Let ω1 be the voltage amplitude of the A-phase power frequency modulated wave, t be the angular velocity of the power frequency component, θ1 be the initial phase of the A-phase power frequency, ω2 be the angular velocity of the low-frequency component, and ΔU be the voltage amplitude of the A-phase power frequency modulated wave. DC The DC-side output amplitude corresponding to the low-frequency component modulation wave is the DC voltage fluctuation amplitude.

[0024] Preferably, the power frequency is set to 45-55Hz, and the low-frequency detection signal frequency is 1-10Hz.

[0025] Preferably, an open-loop injection control system for DC-side AC signals of an MMC includes:

[0026] Modulation wave generation unit: responsible for generating AC multi-frequency component modulated wave signals;

[0027] Gate signal generation unit: responsible for converting the AC multi-frequency component modulated wave signal generated by the modulation wave generation unit into the gate drive signal of the power device in the MMC converter;

[0028] MMC converter: As the mainstream topology of the power grid, it consists of an upper arm and a lower arm, and each arm includes multiple fully controlled power devices;

[0029] An open-loop injection control method based on the DC-side AC signal of the MMC described above is adopted.

[0030] Preferably, the modulation wave generation unit comprises a power frequency modulation module, a side-frequency modulation module, and an adder. The power frequency modulation module generates the power frequency modulation wave component based on a closed-loop modulation strategy, and the side-frequency modulation module generates the lower side-frequency modulation wave component and the upper side-frequency modulation wave component based on an open-loop modulation strategy. The three components are superimposed by the adder to form the AC multi-frequency component modulation wave signal.

[0031] This application, based on the closed-loop modulation strategy (generated according to DC voltage and power commands and through closed-loop control) for generating the DC-side power frequency modulation wave of the MMC, specifically introduces a particular open-loop modulation strategy to generate the lower and upper sideband modulation waves respectively. Specifically, the lower sideband modulation wave is set to the power frequency minus the low-frequency detection signal frequency, and the upper sideband modulation wave is the power frequency plus the low-frequency detection signal frequency. This results in multi-frequency component modulation waves synthesized from the power frequency modulation wave component, the lower sideband modulation wave component, and the upper sideband modulation wave component, ensuring that each modulation wave component in each phase of the multi-frequency component modulation wave maintains a fixed phase difference between the three-phase AC voltages. This makes the synthesized AC multi-frequency component modulation wave form a fluctuation characteristic directly related to the low-frequency detection signal frequency. The modulation wave generation unit realizes open-loop injection of the low-frequency detection signal into the MMC converter through the gate signal generation unit, overcoming the problem in the prior art that it is difficult to accurately extract weak AC signals from the DC voltage detection signal using closed-loop injection. It has the advantages of high injection accuracy, simple implementation, and no influence from operating conditions.

[0032] It should also be noted that the synergistic relationship between the modulation wave generation unit, the gate signal generation unit, and the MMC converter proposed in this application transforms the complex DC-side signal injection task into a multi-frequency component superposition problem at the modulation wave level. Through the inherent multi-level modulation characteristics of the MMC converter in the system, the detection signal and the power transmission function are organically integrated, avoiding the dependence on DC-side voltage feedback in traditional closed-loop control, and significantly improving the accuracy and reliability of signal injection. Attached Figure Description

[0033] Figure 1 This is a module connection diagram of the open-loop injection control system for the DC-side AC signal of the MMC according to a specific embodiment of this application;

[0034] Figure 2 This is a schematic diagram illustrating the process of the modulation wave generation unit generating an AC multi-frequency component modulation wave signal in a specific embodiment of this application;

[0035] Figure 3 This is a schematic diagram of the synthesis of A, B, and C phase AC multi-frequency component modulated wave signals under a specific embodiment of this application;

[0036] Figure 4 This is a complex plane schematic diagram of the open-loop injection of low-frequency detection signal on the DC side of the MMC according to a specific embodiment of this application;

[0037] Figure 5 This is a structural diagram of the simulation model of the ±30 kV / 18 MW MMC flexible DC transmission system in a specific embodiment of this application;

[0038] Figure 6 This is a schematic diagram of the process of generating an AC multi-frequency component modulated wave signal by the modulation wave generation unit in Embodiment 1 of this application (its power frequency is 50Hz, and the low-frequency detection signal frequency is 5Hz).

[0039] Figure 7 This is a global waveform diagram of the AC multi-frequency component modulated wave signal generated in Embodiment 1 of this application;

[0040] Figure 8 yes Figure 7 A magnified view of a portion of the waveform;

[0041] Figure 9 This is a Fourier analysis diagram of the MMC output voltage after injecting a low-frequency detection signal into the MMC converter in Embodiment 1 of this application;

[0042] Figure 10 This is a partially enlarged view of the voltage monitoring waveform output by the MMC converter in Embodiment 2 of this application;

[0043] Figure 11 This is a partially enlarged view of the voltage monitoring waveform output by the MMC converter in Embodiment 3 of this application;

[0044] Figure 12 This is a partial enlarged view of the voltage monitoring waveform output by the MMC converter in Embodiment 4 of this application (its power frequency is 50Hz, and the low-frequency detection signal frequency is 2Hz).

[0045] Figure 13 This is a partial enlarged view of the voltage monitoring waveform output by the MMC converter in Embodiment 5 of this application (its power frequency is 50Hz, and the low-frequency detection signal frequency is 10Hz). Detailed Implementation

[0046] Please refer to the above. Figure 1As shown, this embodiment proposes an open-loop injection control system for the AC signal on the DC side of an MMC. The MMC DC side includes an MMC converter as the DC output, which outputs DC voltage through a DC cable connected to the power grid. The open-loop injection control system includes:

[0047] Modulation wave generation unit: responsible for generating AC multi-frequency component modulated wave signals (i.e., Figure 1 (The marked "modulated wave signal"); preferably, in this embodiment, please refer to further details. Figure 2 As shown, the modulation wave generation unit consists of a power frequency modulation module, a side frequency modulation module, and an adder. The power frequency modulation module generates power frequency modulation wave components based on a closed-loop modulation strategy, and the side frequency modulation module generates lower side frequency modulation wave components and upper side frequency modulation wave components based on an open-loop modulation strategy. The three components are superimposed by the adder to form an AC multi-frequency component modulation wave signal.

[0048] Gate signal generation unit: Responsible for converting the AC multi-frequency component modulated wave signal generated by the modulation wave generation unit into the gate drive signal of the power devices in the MMC converter (i.e., the gate drive signal). Figure 1 The specific working process is as follows: when the modulation wave generated by the modulation wave generation unit contains power frequency components and side frequency components, the gate signal generation unit will generate a PWM pulse sequence with a specific duty cycle and timing relationship (based on the known carrier phase shift modulation technology) and use it as the gate drive signal of each power device in the MMC converter. This enables the modulation wave generation unit to accurately control the output characteristics of the MMC converter, so that the MMC converter can not only complete the conventional AC / DC power conversion function, but also actively respond to the cable insulation detection requirements and inject a low-frequency detection signal with a specific frequency and amplitude on the DC side.

[0049] MMC converter (i.e.) Figure 1 The marked "MMC converter valve" is a mainstream topology of the power grid, consisting of an upper arm and a lower arm. Each arm includes multiple fully controlled power devices, preferably IGBTs, which have advantages such as high voltage, large capacity, low harmonics, and modularity. It is widely used in offshore wind power transmission, long-distance power transmission, asynchronous grid interconnection and other scenarios.

[0050] This embodiment employs an open-loop injection control method based on the following MMC DC-side AC signal:

[0051] Please refer to the above. Figure 2 and Figure 3 As shown, this embodiment proposes an open-loop injection control method for the DC-side AC signal of an MMC, including:

[0052] An AC multi-frequency component modulated wave is generated by a modulation wave generation unit. Each phase multi-frequency component modulated wave in the modulation wave generation unit is synthesized from the power frequency modulated wave component, the lower sideband modulated wave component, and the upper sideband modulated wave component of its corresponding phase. The power frequency modulated wave is generated according to the DC voltage and power command and through closed-loop control. The lower sideband modulated wave is set to the power frequency minus the low-frequency detection signal frequency, and the upper sideband modulated wave is the power frequency plus the low-frequency detection signal frequency. Each modulated wave component in each phase multi-frequency component modulated wave maintains a fixed phase difference between the three-phase AC voltages, so that the synthesized AC multi-frequency component modulated wave forms a fluctuation characteristic directly related to the low-frequency detection signal frequency.

[0053] A signal transmission link is formed between the gate signal generation unit and the MMC converter to realize the open-loop injection of low-frequency detection signal into the MMC converter; preferably, in this embodiment, the power frequency is set to 45-55Hz and the low-frequency detection signal frequency is 1-10Hz.

[0054] Preferably, in this embodiment, in the closed-loop control of the power frequency modulation wave, the DC side output voltage of the MMC is measured in real time, the DC voltage and its output power are subjected to directional dual closed-loop vector control, and the angular frequency ω1 (as the angular velocity of the power frequency component) and the initial phase θ1 of the DC voltage are extracted by the phase-locked loop (PLL), and these parameters are substituted into the generation control process of the power frequency modulation wave.

[0055] Preferably, in this embodiment, when the modulation wave generation unit modulates, the DC-side output amplitude corresponding to the three-phase power frequency modulation wave component is the DC-side output voltage amplitude U. DC The low-frequency component modulated wave corresponds to the DC-side output amplitude of the DC voltage fluctuation ΔU. DC The DC voltage fluctuation amplitude ΔU DC DC-side output voltage amplitude U not exceeding 1% DC .

[0056] Preferably, in this embodiment, the AC multi-frequency component modulation wave includes the A-phase multi-frequency component modulation wave (i.e., ... Figure 3 The marked "A-phase synthesized modulated wave" and B-phase multi-frequency component modulated wave (i.e., Figure 3 The marked "B-phase synthesized modulated wave" and "C-phase multi-frequency component modulated wave" (i.e., Figure 3 The marked "C-phase synthesized modulated wave"), where,

[0057] Phase A multi-frequency component modulation wave includes Phase A power frequency modulation wave (i.e. Figure 2 The marked "A-phase power frequency modulation signal" and the A-phase low-frequency component modulation wave superimposed on the A-phase power frequency modulation wave (i.e., the A-phase low-frequency component modulation wave) Figure 2 (The marked "A-phase sideband modulation signal"); please also refer to [the relevant source]. Figure 3As shown, the spectrum components of each phase modulation wave correspond to the synthesized modulation wave of each phase. In the spectrum component diagram of each phase modulation wave, the sideband modulation wave 1 corresponds to the lower sideband modulation wave, and the sideband modulation wave 2 corresponds to the upper sideband modulation wave.

[0058] Phase B multi-frequency component modulation wave includes phase B power frequency modulation wave (i.e. Figure 2 The marked "B-phase power frequency modulation signal" and the B-phase low-frequency component modulation wave superimposed on the B-phase power frequency modulation wave (i.e., Figure 2 The marked "B-phase sideband modulation signal");

[0059] The C-phase multi-frequency component modulation wave includes the C-phase power frequency modulation wave (i.e., ... Figure 2 The marked "C-phase power frequency modulation signal" and the C-phase low-frequency component modulation wave superimposed on the C-phase power frequency modulation wave (i.e., Figure 2 The marked "C-phase sideband modulation signal");

[0060] The modulation generation process of the three-phase multi-frequency component modulated wave is the same and occurs synchronously.

[0061] In a preferred embodiment, phase A multi-frequency component modulation is generated and controlled according to the following formula:

[0062] In the formula, U a This is the A-phase multi-frequency component modulated wave voltage. The voltage of the power frequency modulation wave component of phase A. The voltage of the sideband modulation wave component under phase A. U is the voltage of the sideband modulation wave component on phase A. DC_A Let ω1 be the voltage amplitude of the A-phase power frequency modulated wave, t be the angular velocity of the power frequency component, θ1 be the initial phase of the A-phase power frequency, ω2 be the angular velocity of the low-frequency component, and ΔU be the voltage amplitude of the A-phase power frequency modulated wave. DC The DC-side output amplitude corresponding to the low-frequency component modulation wave is the DC voltage fluctuation amplitude.

[0063] The modulation generation process of phase B and phase C power frequency modulation waves is the same as and synchronous with the modulation generation process of phase A multi-frequency component modulation as described above. The only difference is that the initial phase of phase B power frequency modulation wave lags behind phase A power frequency modulation wave by 120°, and the initial phase of phase C power frequency modulation wave lags behind phase A power frequency modulation wave by 240°, so that the three maintain a symmetrical relationship with a phase difference of 120°.

[0064] Preferably, please refer to [reference needed]. Figure 4As shown, in this embodiment, in the ABC three-phase stationary coordinate system, the power frequency modulation wave components in the multi-frequency component modulation waves of each phase constitute a set of symmetrical vectors with constant amplitude and phase difference of 120°; at the same time, the side frequency modulation wave components in the multi-frequency component modulation waves of each phase are superimposed on the low frequency component modulation waves on their corresponding phase power frequency modulation wave components, wherein the low frequency component modulation waves of each phase are manifested as three voltage fluctuation signals with completely consistent phase and equal amplitude.

[0065] More preferably, in this embodiment, the three-phase power frequency modulation wave components with a spatial phase difference of 120° are projected onto mutually perpendicular α and β axes through Clarke transform, and combined into a complex vector U that varies with time. αβ The synthesis formula is as follows:

[0066] ;

[0067] In the formula, U is the resultant vector on the α-β plane; α U is a vector on the α-axis; β U is a vector on the β-axis; m ω1 is the vector magnitude; j is the imaginary unit; ω1 is the angular velocity of the power frequency component, i.e., the power frequency; t is time; θ is the initial phase of the power frequency; the synthesized vector rotates counterclockwise around the origin at a constant speed with the power frequency component angular velocity ω1, its magnitude remains constant, and the trajectory of its vertex forms a standard circle on the α-β plane; please refer to further details. Figure 4 As shown, U α0 Let U be the initial vector on the α-axis, with a value of 0, i.e.: U α0 =0;U β0 The vector on the β-axis has a value of 0, which is: U β0 =0;

[0068] Simultaneously, the low-frequency modulated wave superimposed on the three-phase power frequency modulated wave constitutes the zero-sequence voltage vector. The voltage vector corresponding to the projection of this zero-sequence voltage vector onto the α-β plane is 0, which does not affect the rotation trajectory of the composite vector. It only manifests as in-phase and equal-amplitude fluctuations in the ABC three-phase stationary coordinate system, which can be expressed by the formula:

[0069] In the formula, U a0 This represents the voltage fluctuation amplitude of phase A. U b0 This refers to the voltage fluctuation amplitude of phase B. U c0 This represents the voltage fluctuation amplitude of phase C.

[0070] To enable those skilled in the art to better understand the technical solutions of this invention, based on the above embodiments, the following specific embodiments will be proposed in conjunction with the accompanying drawings. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort should fall within the scope of protection of this invention.

[0071] Simulation analysis is conducted using a ±30 kV / 18 MW MMC flexible DC transmission system as an example. The Matlab simulation model can be found in [link to Matlab simulation model]. Figure 5 As shown, where, Figure 5 The “MMC sending-end converter valve” marked is the “MMC converter” described in the entire application. During the startup phase, the system adopts a closed-loop modulation strategy, and the power frequency modulation module in the modulation wave generation unit outputs the power frequency modulation wave. After the MMC runs stably, the side frequency modulation wave component is generated by the side frequency modulation module in the modulation wave generation unit, and the PWM control signal (as the gate drive signal) is generated by the gate signal generation unit, so that the MMC converter located on the DC side of the MMC has a low-frequency AC injection signal superimposed on the rated output voltage.

[0072] In Adoption Figure 5 Based on the simulation model shown, this application performs open-loop injection of low-frequency detection signals in the following specific embodiments:

[0073] Example 1: Please refer to [link / reference] Figure 6 , Figure 7 , Figure 8 and Figure 9 As shown, in this embodiment, the power frequency is set to 50Hz, the low-frequency detection signal frequency is 5Hz, and the DC voltage fluctuation amplitude U DC Under the condition of 1%, the modulation wave generation unit generates A-phase multi-frequency component modulation waves (i.e., ... Figure 6 The marked "A-phase modulated wave") and B-phase multi-frequency component modulated wave (i.e. Figure 6 The marked "B-phase modulated wave" and the C-phase multi-frequency component modulated wave (i.e., Figure 6 The marked "C-phase modulation wave" is used to generate a 5Hz low-frequency detection signal; the waveform of the AC multi-frequency component modulation wave signal generated in this embodiment 1 can be found in [link to example]. Figure 7 and Figure 8 As shown; please see further. Figure 9 The figure shows the Fourier analysis diagram of the MMC output voltage after injecting a low-frequency detection signal into the MMC converter (using an FFT analyzer). Figure 9 The left side of the image shows the parameter settings for the FFT analyzer. Figure 9 The meanings of the relevant parameters shown are as follows: 1. U_0408 represents the name of the data sample of the time structure being tested. It has no specific meaning and is just a test data sample name (also known as "the structure containing time"). 2. unnamed (input 1) means that the signal has not been named and is taken from the time-domain waveform acquired by the software's first input channel. 3. Dimension (dimension = 1) means: one-dimensional time-domain scalar signal, which refers to single-channel time-series data, not a signal in two-dimensional matrix or three-dimensional array format; 4. Start time=14.2s means: the time domain start point of the FFT analysis window is captured, and the waveform is taken for spectral decomposition starting from time 14.2 seconds. 5. Number of cycles=29 means that the captured window contains a total of 29 complete cycles of the 5Hz fundamental frequency; the more cycles, the higher the frequency resolution of the FFT spectrum and the smaller the spectral leakage error. 6. Zoom on: Signal, which means that when zooming the canvas, the view zooms on the original time domain signal waveform above. 7. Fundamental frequency = 5Hz, which means that the fundamental frequency of the signal being measured is artificially calibrated to be 5Hz, and this fundamental frequency will be used as the reference for subsequent THD calculations and amplitude percentage conversions. 8. Max frequency=10Hz means that the maximum scale of the horizontal axis of the spectrum plot is limited to 10Hz, and spectrum data above 10Hz will not be plotted and displayed on the bar chart. 9. THD (Total Harmonic Distortion) means that the total effective value of harmonic components is divided by the effective value of the fundamental frequency. The smaller the percentage value, the better the sinusoidal nature of the waveform and the lower the degree of distortion. 10. Nyquist frequency: The theoretically highest analyzable frequency at which a signal can be reconstructed without aliasing in a sampling system. If the signal frequency exceeds the Nyquist frequency, spectral aliasing will occur, and the harmonic results calculated by FFT will be distorted and unreliable. When the software calculates the total harmonic distortion (THD), the statistical cutoff upper limit of the harmonics is automatically set to the Nyquist frequency corresponding to the current sampling. Harmonic components exceeding this frequency are not included in the THD value calculation, thus avoiding interference from aliasing frequencies on the distortion rate statistics. 11. Bar (relative to fundamental) axis (relative to the fundamental frequency): The amplitude is normalized, and the amplitude of each harmonic is converted into a percentage of the fundamental frequency amplitude using the fundamental frequency amplitude as a 100% reference. This display style can intuitively compare the strength of the harmonics relative to the fundamental frequency. 12. Base value = 1.0, which means that the normalized calculation base coefficient is set to 1. Combined with the bar chart style, the percentage amplitude base of the fundamental frequency is fixed at 100%, that is: base multiple 1.0; 13. Hertz (Hz) is the international standard unit of frequency, representing the number of periodic oscillations of a signal per second. After completing the above parameter settings, click the "Compute FFT" button. The FFT analyzer will then extract the time-domain waveform according to the above parameter settings, perform fast Fourier transform analysis, and output the spectral amplitude (pu) and THD data, as shown in Table 1 below.

[0074] It should also be noted that these parameter settings are standard settings in the field of Fourier FFT analysis, and will not be elaborated on in this embodiment; as can be seen from the Fourier analysis graph, the energy of the signal is almost entirely concentrated at the 5Hz fundamental frequency (i.e., the... Figure 9 The amplitude of the marked "Fundamental" component is normalized to 100%; and no significant harmonic peaks appear at other frequency points, indicating that the signal is a very pure sine wave, and its time-domain waveform is highly consistent with the ideal sine curve, which fully meets the requirements of high-precision signal analysis.

[0075] Example 2: The remaining technical solutions of Example 2 are the same as those of Example 1, except that: in this example, the power frequency is set to 50Hz, the low-frequency detection signal frequency is 5Hz, and the DC voltage fluctuation amplitude U DC The condition of 0.25% is used to generate a 5Hz low-frequency detection signal; the voltage monitoring waveform output by the MMC converter in this embodiment 2 can be found in [reference needed]. Figure 10 As shown.

[0076] Example 3: The remaining technical solutions of Example 3 are the same as those of Example 1, except that: in this example, the power frequency is set to 50Hz, the low-frequency detection signal frequency is 5Hz, and the DC voltage fluctuation amplitude U DC The condition of 0.5% is used to generate a 5Hz low-frequency detection signal; the voltage monitoring waveform output by the MMC converter in this embodiment 3 can be found in [reference needed]. Figure 11 As shown.

[0077] Example 4: The remaining technical solutions of Example 4 are the same as those of Example 1, except that: in this example, the power frequency is set to 50Hz, the low-frequency detection signal frequency is 2Hz, and the DC voltage fluctuation amplitude U DC The condition of 1% is used to generate a 2Hz low-frequency detection signal; the voltage monitoring waveform output by the MMC converter in Example 4 can be found in [reference needed]. Figure 12 As shown.

[0078] Example 5: The remaining technical solutions of Example 5 are the same as those of Example 1, except that: in this example, the power frequency is set to 50Hz, the low-frequency detection signal frequency is 10Hz, and the DC voltage fluctuation amplitude U DC The condition of 1% is used to generate a 2Hz low-frequency detection signal; the voltage monitoring waveform output by the MMC converter in Example 5 can be found in [reference needed]. Figure 13 As shown.

[0079] This application also performs Fourier analysis on the waveforms obtained in Examples 1-5 above, and the test results are shown in Table 1 below:

[0080] Table 1

[0081] Example 1 5 0.25 1.75 Example 2 5 0.5 1.63 Example 3 5 1 0.73 Example 4 2 1 0.91 Example 5 10 1 0.9

[0082] As shown in Table 1 above, the embodiments of this application all exhibit excellent harmonic suppression capabilities. The total harmonic distortion (THD) under all test conditions is controlled below 2%, which is far lower than the conventional requirements for signal purity of power electronic equipment (typically requiring THD < 5%). In particular, from an engineering practicality perspective, the system maintains a THD level of < 1% in both the low-frequency (2Hz) and high-frequency (10Hz) bands, indicating its strong robustness and ability to cover common low-frequency detection signal requirements. At the same time, the inverse correlation between the injected amplitude (i.e., the DC voltage fluctuation amplitude) and THD provides flexibility for the system design of this embodiment: when the detection sensitivity requirement is high, the harmonic interference can be significantly reduced by appropriately increasing the injected amplitude (e.g., from 0.25 pu to 1 pu).

[0083] Furthermore, this result demonstrates that the specific open-loop control strategy provided in this application embodiment achieves high-precision signal injection, avoiding the stability risks and computational burdens caused by complex closed-loop control. This characteristic enables the system to have higher reliability and lower debugging difficulty in actual deployment, making it particularly suitable for DC power grid online monitoring scenarios with high real-time requirements.

[0084] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0085] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. An open-loop injection control method for AC signals on the DC side of an MMC, characterized in that, The MMC DC side includes an MMC converter as the DC output, which outputs DC voltage through a DC cable connected to the power grid; the control method includes: An AC multi-frequency component modulated wave is generated by a modulation wave generation unit. Each phase multi-frequency component modulated wave in the modulation wave generation unit is synthesized from the power frequency modulated wave component, the lower sideband modulated wave component, and the upper sideband modulated wave component of its corresponding phase. The power frequency modulated wave is generated according to the DC voltage and power command and through closed-loop control. The lower sideband modulated wave is set to the power frequency minus the low-frequency detection signal frequency, and the upper sideband modulated wave is the power frequency plus the low-frequency detection signal frequency. Each modulated wave component in each phase multi-frequency component modulated wave maintains a fixed phase difference between the three-phase AC voltages, so that the synthesized AC multi-frequency component modulated wave forms a fluctuation characteristic directly related to the low-frequency detection signal frequency. A signal transmission link is formed between the gate signal generation unit and the MMC converter to realize the open-loop injection of low-frequency detection signals into the MMC converter.

2. The open-loop injection control method for the DC-side AC signal of the MMC according to claim 1, characterized in that, In the ABC three-phase stationary coordinate system, the power frequency modulation wave components in the multi-frequency component modulation waves of each phase constitute a set of symmetrical vectors with constant amplitude and phase difference of 120°; at the same time, the side frequency modulation wave components in the multi-frequency component modulation waves of each phase are superimposed on the low frequency component modulation waves on their corresponding phase power frequency modulation wave components. Among them, the low frequency component modulation waves of each phase are manifested as three voltage fluctuation signals with completely consistent phase and equal amplitude.

3. The open-loop injection control method for the DC-side AC signal of the MMC according to claim 2, characterized in that, Through the Clarke transform, the three-phase power frequency modulated wave components, which are 120° out of phase in space, are projected onto mutually perpendicular α and β axes, and combined into a complex vector U that varies with time. αβ The synthesis formula is as follows: ； In the formula, U is the composite vector on the α-β plane; α U is a vector on the α-axis; β U is a vector on the β-axis; m ω1 is the vector magnitude; j is the imaginary unit; ω1 is the angular velocity of the power frequency component, i.e., the power frequency; t is time. θ is the initial phase of the power frequency; the composite vector rotates counterclockwise around the origin at a constant angular velocity ω1 of the power frequency component, its magnitude remains constant, and the trajectory of the vertex forms a standard circle on the α-β plane; Simultaneously, the low-frequency component modulation wave superimposed on the three-phase power frequency modulation wave component constitutes the zero-sequence voltage vector. The voltage vector corresponding to the projection of this zero-sequence voltage vector on the α-β plane is 0, which will not affect the rotation trajectory of the composite vector. It only manifests as fluctuations with the same phase and equal amplitude in the ABC three-phase stationary coordinate system.

4. The open-loop injection control method for the DC-side AC signal of the MMC according to claim 1 or 3, characterized in that, In the closed-loop control of the power frequency modulation wave, the DC side output voltage of the MMC is measured in real time, and the DC voltage and its output power are subjected to directional dual closed-loop vector control. The angular frequency ω1 and initial phase θ1 of the grid side voltage are extracted through the phase-locked loop (PLL). The angular frequency ω1 of the grid side voltage is used as the angular velocity of the power frequency component. These parameters are substituted into the generation control process of the power frequency modulation wave.

5. The open-loop injection control method for the DC-side AC signal of the MMC according to claim 3, characterized in that, When the modulation wave generation unit modulates, the DC-side output amplitude corresponding to the three-phase power frequency modulation wave component is the DC-side output voltage amplitude U. DC The low-frequency component modulated wave corresponds to the DC-side output amplitude of the DC voltage fluctuation ΔU. DC The DC voltage fluctuation amplitude ΔU DC DC-side output voltage amplitude U not exceeding 1% DC .

6. The open-loop injection control method for the DC-side AC signal of the MMC according to claim 3, characterized in that, The AC multi-frequency component modulation wave includes a phase A multi-frequency component modulation wave, a phase B multi-frequency component modulation wave, and a phase C multi-frequency component modulation wave, wherein... A-phase multi-frequency component modulation wave includes A-phase power frequency modulation wave and A-phase low-frequency component modulation wave superimposed on A-phase power frequency modulation wave; The B-phase multi-frequency component modulation wave includes the B-phase power frequency modulation wave and the B-phase low-frequency component modulation wave superimposed on the B-phase power frequency modulation wave. The C-phase multi-frequency component modulation wave includes the C-phase power frequency modulation wave and the C-phase low-frequency component modulation wave superimposed on the C-phase power frequency modulation wave. The modulation generation process of the three-phase multi-frequency component modulated wave is the same and carried out synchronously. At the same time, the initial phase of the B-phase power frequency modulated wave lags behind the A-phase power frequency modulated wave by 120°, and the initial phase of the C-phase power frequency modulated wave lags behind the A-phase power frequency modulated wave by 240°, so that the three maintain a symmetrical relationship with a phase difference of 120°.

7. The open-loop injection control method for the DC-side AC signal of the MMC according to claim 6, characterized in that, Phase A multi-frequency component modulation is generated and controlled according to the following formula: In the formula, U a This is the A-phase multi-frequency component modulated wave voltage. The voltage of the power frequency modulation wave component of phase A. The voltage of the sideband modulation wave component under phase A. U is the voltage of the sideband modulation wave component on phase A. DC_A Let ω1 be the voltage amplitude of the A-phase power frequency modulated wave, ω2 be the angular velocity of the power frequency component, t be time, θ1 be the initial phase of the A-phase power frequency, ω2 be the angular velocity of the low-frequency component, and ΔU be the voltage amplitude of the A-phase power frequency modulated wave. DC The DC-side output amplitude corresponding to the low-frequency component modulation wave is the DC voltage fluctuation amplitude.

8. The open-loop injection control method for the DC-side AC signal of the MMC according to claim 1, characterized in that, The power frequency is set to 45-55Hz, and the low-frequency detection signal frequency is 1-10Hz.

9. An open-loop injection control system for AC signals on the DC side of an MMC, characterized in that, include: Modulation wave generation unit: responsible for generating AC multi-frequency component modulated wave signals; Gate signal generation unit: responsible for converting the AC multi-frequency component modulated wave signal generated by the modulation wave generation unit into the gate drive signal of the power device in the MMC converter; MMC converter: As the mainstream topology of the power grid, it consists of an upper arm and a lower arm, and each arm includes multiple fully controlled power devices; The open-loop injection control method for the DC-side AC signal of the MMC is adopted according to any one of claims 1-8.

10. The open-loop injection control system for the DC-side AC signal of the MMC according to claim 9, characterized in that, The modulation wave generation unit comprises a power frequency modulation module, a side-frequency modulation module, and an adder. The power frequency modulation module generates the power frequency modulation wave component based on a closed-loop modulation strategy, and the side-frequency modulation module generates the lower side-frequency modulation wave component and the upper side-frequency modulation wave component based on an open-loop modulation strategy. The three components are superimposed by the adder to form the AC multi-frequency component modulation wave signal.

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