Method for upqc detection and dc voltage suppression for microgrid with pulse load

Abstract

The application discloses a UPQC detection and DC voltage suppression method for a microgrid containing a pulse load, and belongs to the technical field of microgrid power quality treatment. When the operation of the pulse load causes the voltage distortion of the microgrid, the series compensation module is used for voltage compensation to eliminate the harmonic voltage. For the current distortion problem of the microgrid caused by the pulse load access, the system detects the harmonic current for compensation to eliminate the nonlinear distortion of the microgrid side current. When the operation of the pulse load causes the power quality problem of the microgrid, the UPQC high-frequency switching operation causes the frequent and large amplitude charging and discharging of the DC capacitor, which causes the large amplitude fluctuation of the UPQC DC capacitor. The DC side voltage control based on the charging and discharging of the energy storage module is used for voltage stabilization. The voltage detection technology based on the synchronous coordinate transformation is used to realize the fast response of the series side, significantly improves the real-time performance of the harmonic extraction, realizes the steady-state maintenance of the DC bus voltage, and effectively suppresses the harmonic and voltage fluctuation.

Description

Technical Field

[0001] This invention belongs to the field of microgrid power quality management technology, specifically relating to a UPQC detection and DC voltage smoothing method for microgrids with pulse loads. Background Technology

[0002] With the rapid development of microgrids and power electronics technology, pulsed power loads are increasingly widely used in radar, electromagnetic launch, high-power weapons, mobile digital devices, and rectifier-related industrial scenarios. These loads are highly impulsive nonlinear loads, characterized by high peak power, frequent switching, rapid change rate, and high harmonic content, which cause severe instantaneous power surges to the power supply system.

[0003] In large public power grids, the system has high inertia and sufficient capacity, limiting the impact of pulse load disturbances. However, in small-capacity independent microgrids such as those on ships, islands, and vehicles, the system has low inertia and weak regulation capabilities. Power sources such as diesel generator sets have inherent response delays, making it impossible to quickly smooth out instantaneous power fluctuations caused by pulse loads. This directly leads to a series of power quality problems such as microgrid voltage distortion, frequency deviation, excessive harmonics, and three-phase imbalance, exacerbating equipment losses and even causing system instability, equipment failure, and power outages.

[0004] Currently, microgrid power quality management mostly uses single-function devices such as SVG, APF, and DVR, which can only achieve one of the following: reactive power compensation, harmonic filtering, or voltage sag control. When faced with the complex problems of voltage, current, harmonic, and DC fluctuations caused by pulse loads, multiple devices need to work together. This has drawbacks such as low integration, complex coordination and control, high installation and maintenance costs, and slow compensation response, making it difficult to match the high-frequency and high-power dynamic disturbance requirements of pulse loads.

[0005] While Unified Power Quality Conditioner (UPQC) can achieve comprehensive voltage and current compensation, the traditional UPQC detection and control methods are designed for conventional loads. In terms of harmonic detection, there are problems such as excessive delay and incomplete extraction of specific harmonics. The DC side relies solely on capacitor voltage regulation, which cannot suppress the severe voltage fluctuations caused by pulse loads. It is difficult to meet the high dynamic and high reliability management requirements of microgrids with pulse loads. Summary of the Invention

[0006] The purpose of this invention is to provide a UPQC detection and DC voltage stabilization method for microgrids with pulse loads, in order to solve the problem that traditional UPQC control methods cannot quickly compensate for the severe impact of pulse loads on the power grid.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for UPQC detection and DC voltage smoothing in microgrids with pulsed loads, comprising the following steps:

[0008] S1. When the pulse load causes the microgrid voltage to distort, the system activates the series compensation module for real-time correction.

[0009] The series compensation module first uses a detection algorithm based on synchronous coordinate transformation to lock the grid voltage phase through a phase-locked loop and perform dq0 transformation to accurately extract the distorted voltage component containing harmonics. Then, it calculates the difference between the extracted voltage component and the reference voltage to generate the corresponding voltage compensation command. By injecting a controlled compensation voltage into the grid, it achieves complete cancellation of the harmonic voltage, thereby restoring the waveform quality of the microgrid voltage.

[0010] S2. To address the microgrid current distortion problem caused by pulsed load access, the system detects harmonic current to compensate for and eliminate the nonlinear distortion of the current on the microgrid side.

[0011] S3. When the pulse load causes power quality problems in the microgrid, the UPQC operates at high frequency, and the DC capacitor is frequently charged and discharged in large quantities, causing the DC capacitor of the UPQC to fluctuate greatly. Voltage stabilization is achieved by using DC side voltage control based on the charging and discharging of the energy storage module.

[0012] Preferably, in step S1, the step of the series compensation module performing voltage compensation to eliminate harmonic voltage is as follows:

[0013] S11. The microgrid signal acquisition system acquires the output voltage of the series compensation module and the voltage on the diesel generator set side, and the grid phase-locked loop obtains the voltage phase.

[0014] S12, Three-phase voltage after transformation matrix After dq0 conversion, the fundamental signal is converted into DC components on the d and q axes, while the original harmonic signal remains an AC quantity after the change.

[0015] S13. The DC component is obtained by removing the AC component using a low-pass filter, and then the fundamental signal is obtained after inverse dq0 transformation.

[0016] S14. Subtract the extracted fundamental signal from the original signal to obtain the signal harmonic components;

[0017] S15. Utilize the dq0 matrix to output an ideal reference command voltage U that is equal to the amplitude of the grid's rated voltage. m ;

[0018] S16. The acquired error signal is input into the double closed-loop decoupling control loop in the dq rotating coordinate system on the series side. Through the coordinated adjustment of the inner loop current and the outer loop voltage, dynamic tracking and decoupling control of the system are realized. The modulated voltage signal output by the control loop is then sent to the series side inverter as the reference for the pulse width modulation (PWM) signal.

[0019] S17. The series-side inverter converts its DC-side energy into a compensation voltage according to the control command, and couples it to the power distribution system through the series transformer.

[0020] This process aims to counteract voltage deviations and waveform distortions in the system, thereby ensuring that the load-side voltage is rated and sinusoidal.

[0021] Preferably, the series compensation module is used to obtain compensation instructions based on synchronous coordinate transformation, lock the grid voltage phase using a phase-locked loop, extract the positive sequence voltage component through dq0 transformation and calculate the difference with the reference voltage to generate series side voltage compensation instructions.

[0022] Preferably, step S2 is as follows:

[0023] S21. The microgrid signal acquisition system monitors and acquires the output current of the parallel compensation module and the total current signal on the load side in real time, providing feedback input for subsequent harmonic extraction and closed-loop control.

[0024] S22. Based on the load current, the current signal is decomposed into fundamental and harmonic signals using the sliding discrete Fourier method.

[0025] S23. The fundamental component of the current is obtained from step S22, and its expression is:

[0026] ;

[0027] in, A1 and B1 are the real and imaginary coefficients of the fundamental component, respectively, and N cur For the current sampling point, N cur－N+1 The oldest sampling point, The amplitude of the discrete sampling sequence;

[0028] S24. Extract the complex coefficients A1 and B1 obtained by the sliding discrete Fourier transform (SDFT), which characterize the DC offset and fundamental wave characteristics in the current signal. By subtracting the real-time reconstructed fundamental wave and DC component from the original sampling sequence, the high-order harmonic components can be accurately extracted.

[0029] S25. Let the instantaneous values ​​of the three-phase grid currents be i a、 i b、 i c， Transform them to α-β coordinates using matrix operations;

[0030] S26. Assume a load current of 6K ± 1st odd harmonic. After i p -i q After the method is transformed, the distorted load current is obtained. , ;

[0031] S27. The parallel-side inverter generates controlled compensation based on modulation commands and injects it into the AC line. This process utilizes the interaction between the inverter output impedance and the grid impedance to generate a compensation current with the opposite phase to the distortion component at the physical level, ultimately achieving real-time correction of the line current waveform.

[0032] Preferably, the parallel compensation module employs a specified harmonic detection and compensation based on an improved discrete Fourier transform algorithm. By placing the transform in the pq coordinate system and combining it with generalized discrete Fourier detection, the acquisition time of the command signal is shortened to 1 / 3 of that of the traditional discrete Fourier transform method, thereby achieving accurate extraction and compensation of specific harmonics of the pulse load.

[0033] Preferably, in step S3, the step of voltage stabilization based on the DC-side voltage control of the energy storage module's charging and discharging is as follows:

[0034] S31, when the DC side voltage u on Higher than the set reference voltage value u ref When the upper bridge arm is turned on and the lower bridge arm is turned off, the DC-side capacitor discharges directly to the energy storage system. The energy storage system absorbs the excess energy of the DC-side capacitor, causing the DC-side capacitor voltage to drop rapidly to the reference voltage value. At the same time, the voltage of the energy storage system also rises slowly.

[0035] S32, When the DC side voltage is lower than u ref When the upper bridge arm is turned off, the lower bridge arm is controlled by the switch, and charges the DC capacitor through the inductor and the voltage of the energy storage system, causing the DC capacitor voltage to rise rapidly to the reference voltage value. During the charging process, the energy storage system charge decreases.

[0036] S33. Energy balance is achieved by monitoring the status of the energy storage module: If the output voltage drops to a preset lower limit as the power level decreases, a constant compensation value is added to the current command signal to start charging; until the energy storage voltage recovers to the target power level, the constant value charging command is canceled, and the system returns to normal operation.

[0037] Preferably, the energy storage system on the DC side adopts a DC voltage smoothing control strategy based on the charging and discharging of energy storage units instead of traditional PI regulation. It is connected to the DC side through a bidirectional DC / DC converter to achieve real-time energy buffering against pulse power surges, so as to maintain the stability of the DC bus voltage.

[0038] Compared with the prior art, the present invention has the following advantages:

[0039] (1) The present invention uses synchronous coordinate transformation, phase-locked loop and dq0 transformation on the series side to quickly extract positive sequence voltage components and generate compensation instructions, which can offset voltage distortion, spurious rise and spurious fall in real time, and ensure the stable sinusoidal voltage on the load side.

[0040] (2) The parallel side of this invention innovatively adopts the pq coordinate system, the sliding discrete Fourier transform (SDFT) combined with the generalized discrete Fourier transform (GDFT) to detect specified harmonics. The instruction acquisition time is only 1 / 3 of that of the traditional DFT (about 6.7ms), accurately extracting the 5th, 7th, 11th, 13th and 3rd and 9th key harmonics, and adapting to the high-frequency impact characteristics of pulse loads.

[0041] (3) The present invention introduces an external energy storage combined with a bidirectional DC / DC converter on the DC side, and uses energy storage charge and discharge control to replace the traditional PI regulation, quickly buffering pulse power impact, suppressing large fluctuations in DC bus voltage from the source, and improving the reliability of UPQC operation.

[0042] (4) The present invention adopts the UPQC-R series-parallel collaborative architecture, and the voltage compensation and current compensation do not interfere with each other. It takes into account the comprehensive management of voltage, current, harmonics and reactive power, and is suitable for small-capacity independent microgrids such as ships and islands and high-dynamic high-power pulse load scenarios. Attached Figure Description

[0043] Figure 1 This is a schematic flowchart of the UPQC detection and DC voltage stabilization method for microgrids with pulsed loads according to the present invention.

[0044] Figure 2 A diagram of the sliding window iterative algorithm model provided by this invention;

[0045] Figure 3 The following is a diagram illustrating the implementation of the sliding window iterative DFT algorithm provided by this invention;

[0046] Figure 4 The theoretical vector diagram of instantaneous reactive power provided for this invention;

[0047] Figure 5 A block diagram of the improved current detection method provided by the present invention;

[0048] Figure 6 The waveform diagram of the detection current signal provided by the present invention;

[0049] Figure 7 The DC voltage control circuit diagram provided for this invention;

[0050] Figure 8 The DC voltage control flowchart provided for this invention. Detailed Implementation

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] Please see Figure 1 This invention provides a UPQC detection and DC voltage stabilization method for microgrids with pulsed loads. The method includes: a series compensation module, used to obtain compensation commands based on synchronous coordinate transformation, lock the grid voltage phase using a phase-locked loop, extract the positive-sequence voltage component through dq0 transformation, calculate the difference with a reference voltage, and generate a series-side voltage compensation command; such as... Figure 2-6 As shown, the parallel compensation module employs a specified harmonic detection compensation based on an improved discrete Fourier transform algorithm. By placing the transform in the pq coordinate system and combining it with generalized discrete Fourier detection, the command signal acquisition time is reduced to 1 / 3 of that of the traditional discrete Fourier transform method, thus achieving accurate extraction of specific harmonics from the pulsed load. The DC-side energy storage system uses a DC voltage smoothing control strategy based on the charging and discharging of energy storage units to replace the traditional PI regulation. It is connected to the DC side through a bidirectional DC / DC converter to achieve real-time energy buffering against pulsed power surges, thereby maintaining the stability of the DC bus voltage. The DC voltage regulator circuit and its principle are as follows. Figure 7-8 As shown.

[0053] Specifically, it also includes the following steps:

[0054] S1. When the pulse load causes voltage distortion in the microgrid, the system activates the series compensation module for real-time management. This module first uses a detection algorithm based on synchronous coordinate transformation to lock the grid voltage phase through a phase-locked loop and perform dq0 transformation to accurately extract the distorted voltage component containing harmonics. Then, the difference between the extracted voltage component and the reference voltage is calculated to generate a corresponding voltage compensation command. By injecting a controlled compensation voltage into the grid, the harmonic voltage is completely canceled, thereby restoring the waveform quality of the microgrid voltage.

[0055] S11. The microgrid signal acquisition system acquires the output voltage of the series compensation module and the voltage on the diesel generator set side, and the grid phase-locked loop obtains the voltage phase.

[0056] The distorted voltage is expressed as:

[0057] ;

[0058] ;

[0059] ;

[0060] Among them, u sa u sb u sc For three AC voltages, U an U bn U cn These correspond to the magnitudes of each sequence component of the three-phase voltage (including positive, negative, and zero sequence components). Let be the angular frequency of the system voltage. The initial phase of each sequence component;

[0061] S12, Three-phase voltage after transformation matrix After dq0 conversion, the fundamental signal is converted into DC components on the d and q axes, the original harmonic signal is transformed into AC quantities, and the three-phase voltage is converted to u in the dq0 coordinate system. d u q The expression for u0 is:

[0062] ;

[0063] Among them, U n1 U n2 U n0 These correspond to the positive, negative, and zero sequence amplitudes of the nth harmonic, respectively. , , These are the initial phases of each sequence component.

[0064] S13. The DC component is obtained by removing the AC component using a low-pass filter, and then the fundamental signal is obtained after inverse dq0 transformation.

[0065] S14. Subtract the extracted fundamental signal from the original signal to obtain the signal harmonic components;

[0066] S15. In actual power grid operation, the grid voltage not only undergoes distortion but also experiences voltage spikes and dips, resulting in a deviation between the reference command voltage and the rated voltage. Since the amplitude and frequency of the desired voltage are constant in both simulation and actual power grid operation, this invention directly utilizes the dq0 matrix to output an ideal reference command voltage U equal to the rated voltage amplitude of the power grid. m At this time, the compensation voltage command value on the series side , , for:

[0067] ;

[0068] In the formula, This is the initial phase.

[0069] S16. The acquired error signal is input into the dual-closed-loop decoupling control loop in the dq rotating coordinate system on the series side. Dynamic tracking and decoupling control of the system are achieved through the coordinated adjustment of the inner loop current and the outer loop voltage. The modulated voltage signal output from the control loop is then sent to the series-side inverter as a reference for the pulse width modulation (PWM) signal.

[0070] S17. The series-side inverter converts its DC-side energy into a compensating voltage according to control commands and couples it to the power distribution system through a series transformer. This process aims to compensate for voltage deviations and waveform distortions in the system, thereby ensuring the rated value and sinusoidal nature of the load-side voltage.

[0071] In this embodiment of the invention, the series compensation module is used to obtain compensation instructions based on synchronous coordinate transformation, lock the grid voltage phase using a phase-locked loop, extract the positive sequence voltage component through dq0 transformation and calculate the difference with the reference voltage to generate the series side voltage compensation instruction.

[0072] S2. The overall principle block diagram of current detection is as follows: Figure 5 To address the microgrid current distortion problem caused by pulsed load access, the system detects harmonic current to compensate for and eliminate the nonlinear distortion of the current on the microgrid side.

[0073] S21. The microgrid signal acquisition system monitors and acquires the output current of the parallel compensation module and the total current signal on the load side in real time, providing feedback input for subsequent harmonic extraction and closed-loop control.

[0074] S22. Based on the load current, the current signal is decomposed into fundamental and harmonic signals using the sliding discrete Fourier method.

[0075] S23. The fundamental component of the current is obtained from step 22, and its expression is:

[0076] ;

[0077] in, A1 and B1 are the real and imaginary coefficients of the fundamental component, respectively, and N cur For the current sampling point, N cur－N+1 The oldest sampling point, The amplitude of the discrete sampling sequence;

[0078] S24. Extract the complex coefficients A1 and B1 calculated by the sliding discrete Fourier transform (SDFT), which characterize the DC offset and fundamental frequency characteristics in the current signal. Accurate extraction of higher harmonic components is achieved by subtracting the real-time reconstructed fundamental frequency and DC component from the original sampling sequence. The sliding window iterative algorithm model is as follows: Figure 2 , Figure 3 As shown;

[0079] S25. In practical engineering implementation, a portion of the lower-order harmonics with a relatively large content is usually extracted for compensation. Due to the inherent characteristics of DFT (Dynamic Power Flow), there is an inherent delay, the length of which is one power frequency cycle. When UPQC uses the DFT method to obtain the command signal in a three-phase stationary coordinate system to compensate for the impact of pulsed power load, the 20ms delay is obviously too long and does not meet the compensation speed requirement of this design; it must be improved. Let the instantaneous values ​​of the three-phase grid currents be i... a、 i b、 i c， By transforming them to α-β coordinates through matrix operations, we obtain:

[0080] ;

[0081] Among them, C 32 for:

[0082] ;

[0083] Instantaneous reactive power theory, such as Figure 4 As shown.

[0084] S26. In this scheme and in actual three-phase power grids, even-order harmonics cancel each other out, resulting in extremely low content in the power grid. Due to the influence of rectifiers, inverters, and nonlinear loads, odd-order harmonics (6K±1) account for a relatively large proportion in the power grid. Assume the load current containing such harmonics... :

[0085] ;

[0086] Where I1 is the effective value of the fundamental current; I represents the initial phase of the fundamental current. n This is the effective value of the nth harmonic current; and The initial phases of the corresponding order harmonics are respectively

[0087] S27, the parallel-side inverter generates controlled compensation based on modulation commands and injects it into the AC line, ultimately achieving real-time correction of the line current waveform. After i p -i q After the method is changed, the distorted load current , ;

[0088] ;

[0089] As shown in the equation, after Clark and Park transformations, the fundamental component of the distorted load current becomes a DC component. The 6k-1 negative sequence harmonic and the 6k+1 positive sequence harmonic both become the 6k harmonic, and the frequencies of the positive and negative 6k±1 harmonics are unified. Therefore, the component values ​​of two harmonics can be obtained in a single extraction. Furthermore, after the original signal is transformed using the instantaneous reactive power theory, ignoring even harmonics in the power grid, 300Hz can be used as the fundamental frequency for extraction. The extracted harmonic signals can be transformed back into the original harmonic signals through inverse coordinate transformation.

[0090] Using only the sliding window iteration method in the pq coordinate system cannot detect harmonic components such as the 3rd and 9th harmonics. In island-type independent microgrids, the 3rd harmonic content is the highest and therefore cannot be simply ignored. Here, we use the Generalized Discrete Fourier Transform (GDFT) to detect the specified 3K harmonics to compensate for this deficiency. The GDFT is currently widely used in the detection of specified harmonics, and its mathematical model for detecting the mr+l harmonic is described below. for:

[0091] ;

[0092] In the formula, m is the delay factor, r is the fundamental frequency multiple factor, and together with l, they determine the harmonic order of target detection; mr+l represents the specific number of detections locked by GDFT.

[0093] When detecting the 3Kth harmonic, l=0, and only the angular frequency of the resonator needs to be amplified and phase compensation performed based on the sliding discrete Fourier transform method described in the previous section. The 5th, 7th, 11th, and 13th harmonics are detected using the SDFT method based on pq coordinate transformation, while the 3rd and 9th harmonics are detected using the generalized discrete Fourier transform method. The SDFT detection method based on pq coordinate transformation has a delay of 1 / 6 of a cycle, and the generalized discrete Fourier transform method has a delay of 1 / 3 of a cycle. When this design is used in a system containing multiple harmonics, the overall delay is theoretically 1 / 3 of a cycle. The overall block diagram of the improved current detection method is shown below. Figure 5 As shown.

[0094] The parallel-side inverter generates controlled compensation based on modulation commands and injects it into the AC line. This process utilizes the interaction between the inverter's output impedance and the grid impedance to physically generate a compensation current that is out of phase with the distortion component, ultimately achieving real-time correction of the line current waveform. The current signal tracking effect is as follows: Figure 6 As shown;

[0095] S3. When the pulse load causes power quality problems in the microgrid, the UPQC operates at high frequency, and the DC capacitor is frequently charged and discharged in large quantities, causing the DC capacitor of the UPQC to fluctuate greatly. Voltage stabilization is achieved by using DC side voltage control based on the charging and discharging of the energy storage module.

[0096] S31, such as Figure 7 As shown, when the DC side voltage u on Higher than the set reference voltage value u ref At this time, the upper bridge arm is turned on and the lower bridge arm is turned off. The DC-side capacitor discharges directly to the energy storage system. The energy storage system absorbs the excess energy from the DC-side capacitor, causing the DC-side capacitor voltage to drop rapidly to the reference voltage value. At the same time, the voltage of the energy storage system also rises slowly.

[0097] S32, When the DC side voltage is lower than u ref When the upper bridge arm is turned off, the lower bridge arm is controlled by a switch, and the DC capacitor is charged through the inductor and the voltage of the energy storage system, causing the DC capacitor voltage to rise rapidly to the reference voltage value. During the charging process, the energy storage system charge decreases.

[0098] S33. When the energy storage module's power decreases, causing the output voltage to drop to a certain value, the control current command signal increases by a constant value to charge the energy storage module. When the energy storage voltage is charged and returns to the specified power level, the constant value charging is canceled. The DC voltage control process is as follows: Figure 8 As shown.

[0099] The UPQC method of this invention uses a UPQC-R structure, i.e., a series inverter on the left and a parallel inverter on the right. When the UPQC is running, the series side is responsible for compensating for grid voltage swells, drops, and distortions, while the parallel side compensates for harmonic and reactive currents on the load side. The advantage of UPQC is that after the series compensation module compensates for the power supply voltage, the operation of the parallel compensation module is unaffected by distorted voltage. And after the parallel compensation module compensates for the current, the series compensation module is unaffected by distorted current during operation.

[0100] The UPQC series-side compensator of this invention is used to compensate for grid voltage quality issues. It consists of a rectifier, a series transformer, etc., connected in series between the power source and the load. The parallel compensator is used to compensate for current quality issues. It consists of an inverter, etc., connected in parallel near the user side. The DC energy storage link is shared by the series and parallel units. The compensation amount detection circuit measures the voltage and current values ​​to be compensated and inputs them to the control circuit. Based on the input voltage and current compensation amounts and the set control strategy, the output compensation voltage and current values ​​are calculated and then converted into trigger signals, which are input to the inverter driver circuit to control the inverter switching. After the passive LC filter filters out high-order harmonics, the actual output compensation voltage and current are generated, thereby improving the voltage, current, and reactive power of the power system. The series-side APF is connected to the system side through a transformer to isolate harmonics between the system and the load and prevent resonance.

[0101] Furthermore, considering the characteristics of pulsed power loads reaching their maximum power in a very short time and with extremely high switching frequencies during operating mode switching, this invention incorporates an energy storage structure into the traditional UPQC-R structure to suppress DC voltage fluctuations in the capacitor. The DC voltage control module based on the energy storage system mainly consists of the energy storage system and a bidirectional DC / DC converter circuit. When the DC-side voltage is higher than the set reference voltage value, the DC-side capacitor directly discharges into the energy storage system. The energy storage system absorbs the excess energy from the DC-side capacitor, causing the DC-side capacitor voltage to rapidly decrease to the reference voltage value, while the voltage of the energy storage system slowly rises. When the DC-side voltage is lower than the reference value, the DC capacitor is charged through the inductor and the energy storage system voltage, causing the DC-side capacitor voltage to rapidly rise to the reference voltage value. During the charging process, the energy storage system's charge decreases.

[0102] The series module of this invention adopts a voltage command signal detection method based on synchronous coordinate transformation. It uses a phase-locked loop to lock the grid voltage phase, extracts the positive sequence voltage component through dq0 transformation, and calculates the difference with the reference voltage to generate a voltage compensation command for the series side. The parallel module adopts a specified harmonic detection and compensation method. Based on the improved Discrete Fourier Transform (DFT) algorithm, by placing the transformation in the pq coordinate system and combining it with generalized Discrete Fourier detection, the command signal acquisition time is shortened to 1 / 3 of the traditional DFT method, realizing accurate extraction and compensation of specific harmonics generated by pulse load.

[0103] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for UPQC detection and DC voltage smoothing in microgrids with pulsed loads, characterized in that, Includes the following steps: S1. When the pulse load causes voltage distortion in the microgrid, use a series compensation module to compensate for the voltage and eliminate harmonic voltage. S2. To address the microgrid current distortion problem caused by pulsed load access, the system detects harmonic current to compensate for and eliminate the nonlinear distortion of the current on the microgrid side. S3. When the pulse load causes power quality problems in the microgrid, the UPQC operates at high frequency, and the DC capacitor is frequently charged and discharged in large quantities, causing the DC capacitor of the UPQC to fluctuate greatly. Voltage stabilization is achieved by using DC side voltage control based on the charging and discharging of the energy storage module.

2. The UPQC detection and DC voltage smoothing method for microgrids with pulsed loads according to claim 1, characterized in that: In step S1, the step of the series compensation module performing voltage compensation to eliminate harmonic voltage is as follows: S11. The microgrid signal acquisition system acquires the output voltage of the series compensation module and the voltage on the diesel generator set side, and the grid phase-locked loop obtains the voltage phase. S12, Three-phase voltage after transformation matrix After dq0 conversion, the fundamental signal is converted into DC components on the d and q axes, and the original harmonic signal is converted into AC quantity after change; S13. The DC component is obtained by removing the AC component using a low-pass filter, and then the fundamental signal is obtained after inverse dq0 transformation. S14. Subtract the extracted fundamental signal from the original signal to obtain the signal harmonic components; S15. Utilize the dq0 matrix to output an ideal reference command voltage U that is equal to the amplitude of the grid's rated voltage. m ; S16. The acquired error signal is input into the double closed-loop decoupling control loop in the dq rotating coordinate system of the series side. Through the coordinated adjustment of the inner loop current and the outer loop voltage, the modulation voltage signal output by the control loop is then sent to the series side inverter as the reference for the pulse width modulation (PWM) signal. S17. The series-side inverter converts its DC-side energy into a compensation voltage according to the control command, and couples it to the power distribution system through the series transformer.

3. The UPQC detection and DC voltage smoothing method for microgrids with pulsed loads according to claim 1 or 2, characterized in that: The series compensation module is used to obtain compensation commands based on synchronous coordinate transformation, lock the grid voltage phase using a phase-locked loop, extract the positive sequence voltage component through dq0 transformation and calculate the difference with the reference voltage to generate series side voltage compensation commands.

4. The UPQC detection and DC voltage smoothing method for microgrids with pulsed loads according to claim 1, characterized in that: The steps of step S2 are as follows: S21. The microgrid signal acquisition system monitors and acquires the output current of the parallel compensation module and the total current signal on the load side in real time, providing feedback input for subsequent harmonic extraction and closed-loop control. S22. Based on the load current, the current signal is decomposed into fundamental and harmonic signals using the sliding discrete Fourier method. S23. The fundamental component of the current is obtained from step S22, and its expression is: ； in, A1 and B1 are the real and imaginary coefficients of the fundamental component, respectively, and N cur For the current sampling point, N cur－N+1 The oldest sampling point, The amplitude of the discrete sampling sequence; S24. Extract the complex coefficients A1 and B1 obtained by the sliding discrete Fourier transform (SDFT), which characterize the DC offset and fundamental characteristics in the current signal. Subtract the real-time reconstructed fundamental and DC components from the original sampling sequence. S25. Let the instantaneous values ​​of the three-phase grid currents be i a、 i b、 i c， Transform them to α-β coordinates using matrix operations; S26. Assume a load current of 6K ± 1st odd harmonic. After i p -i q After the method is transformed, the distorted load current is obtained. , ; S27. The parallel-side inverter generates controlled compensation based on modulation commands and injects it into the AC line, ultimately achieving real-time correction of the line current waveform.

5. The UPQC detection and DC voltage smoothing method for microgrids with pulsed loads according to claim 4, characterized in that: The parallel compensation module employs a specified harmonic detection and compensation based on an improved discrete Fourier transform algorithm. By placing the transform in the pq coordinate system and combining it with generalized discrete Fourier detection, the acquisition time of the command signal is reduced to 1 / 3 of that of the traditional discrete Fourier transform method, thereby achieving accurate extraction and compensation of specific harmonics of the pulse load.

6. The UPQC detection and DC voltage smoothing method for microgrids with pulsed loads according to claim 1, characterized in that: In step S3, the voltage stabilization step based on the DC-side voltage control of the energy storage module's charging and discharging is as follows: S31, when the DC side voltage u on Higher than the set reference voltage value u ref When the upper bridge arm is turned on and the lower bridge arm is turned off, the DC-side capacitor discharges directly to the energy storage system. The energy storage system absorbs the excess energy of the DC-side capacitor, causing the DC-side capacitor voltage to drop rapidly to the reference voltage value. At the same time, the voltage of the energy storage system also rises slowly. S32, When the DC side voltage is lower than u ref When the upper bridge arm is turned off, the lower bridge arm is controlled by the switch, and charges the DC capacitor through the inductor and the voltage of the energy storage system, causing the DC capacitor voltage to rise rapidly to the reference voltage value. During the charging process, the energy storage system charge decreases. S33. Energy balance is achieved by monitoring the status of the energy storage module: If the output voltage drops to a preset lower limit as the power level decreases, a constant compensation value is added to the current command signal to start charging; until the energy storage voltage recovers to the target power level, the constant value charging command is canceled, and the system returns to normal operation.

7. The UPQC detection and DC voltage smoothing method for microgrids with pulsed loads according to claim 6, characterized in that: The energy storage system on the DC side adopts a DC voltage smoothing control strategy based on the charging and discharging of energy storage units to replace the traditional PI regulation. It is connected to the DC side through a bidirectional DC / DC converter to achieve real-time energy buffering against pulse power surges in order to maintain the stability of the DC bus voltage.

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