Abstract:建设新型电力系统、加速新能源的利用消纳，是推进电力能源革命和构建清洁低碳、安全高效能源体系的重大举措，也是保障我国能源安全的必然选择。随着新型电力系统建设的日益推进，高电压、大容量的传统电力装备被广泛应用于新型电力系统源、网、荷、储等多个环节中；与此同时，以高功率密度电力电子装备、大容量直流气体绝缘设备等为代表的新型电力装备被大量投入使用，其可能出现的电、热失控给电网运行带来巨大的安全隐患。

Abstract:A fiber-optic Fabry-Perot (F-P) ultrasonic sensor based on grooved silicon diaphragm for partial discharge (PD) detection in power equipment has been designed and prepared. The parameters of the grooved diaphragm are optimized by finite element simulation. The static sensitivity of grooved diaphragm is 4.09 times higher than that of traditional circular diaphragm,while the resonant frequency is basically the same. The coupling efficiency is introduced to modify the traditional two-beam interference model,and the effect of the F-P cavity length on the contrast of the interference spectrum is studied to improve the acoustic pressure sensitivity of the sensor. A grooved silicon diaphragm with a grooved diameter of 829.44 μm and a thickness of 2.09 μm is prepared by the micro-electro-mechanical system (MEMS). The F-P cavity length of fabricated ultrasonic sensor is 163.600 μm. The acoustic pressure sensitivity of the sensor is up to 357.78 mV/Pa at the resonance frequency of 61.5 kHz,and the sensor performance is verified by combining with the gas insulated switchgear (GIS) partial discharge defect model. The experimental results show that the proposed sensor has the advantages of high sensitivity,good real-time performance, and strong capable of detecting ultrasonic signals.

Abstract:Electrical,thermal,mechanical,and other stresses acting together on dry-type transformers may lead to core loosenings,winding deformations,and other faults. The vibration-based analysis method can sensitively reflect the mechanical state of the core and windings. Thus,it is suitable for the mechanical fault detection of dry-type transformers. In this paper,a finite element model of the dry-type transformer is constructed,and its vibration mechanism and characteristics are investigated by simulation. The simulation results show that,at normal operation,the winding vibration is much smaller than the core vibration caused by the magnetostrictive effect. A dry-type transformer experiment platform is built,and then the vibration signals of the transformer are measured and analyzed at normal and mechanical fault states. Experimental results show that with the gradual magnetic saturation of the iron core,the vibration signal is no longer linearly related to the squared voltage. Moreover,the peak value of the total vibration signal grows rapidly and the amplitude of the fundamental frequency vibration signal increase slowly. The vibration signal spectrum of the dry-type transformer under different operating conditions is measured. Experimental results show that the ratio of fundamental frequency to total and high frequency to low frequency can be used to diagnose the mechanical state of dry-type transformers.

Abstract:In order to ensure the safety and reliability of the optical microphone in the internal application of electrical equipment,polyurethane,polytetrafluoroethylene,epoxy resin,polyetheretherketone and phenol-formaldehyde resin are selected as packaging materials to carry out environmental compatibility tests related to electrical equipment and test material properties before and after the tests by scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectrometer. The results show that the polyurethane surfaces are susceptible to infiltration of transformer oil and have a marked colour change. It can be used in the environment of hydrophone,air and ambient temperature,but not in the environment of oil-immersed transformer. The polytetrafluoroethylene is compatible with the transformer oil,and can be used in the environment of oil-immersed transformer,gas insulated switchgear (GIS) and environment with large temperature change. The compatibility of other materials in electrical equipment related environments is not good,and their applicability needs further research. A method for testing the environmental compatibility of packaging materials is presented in this paper. It can be used for reference in the selection of packaging materials for optical microphones and has certain engineering practical value.

Abstract:Photothermal interference (PTI) is a new optical method for gas detection. Due to its high sensitivity,high accuracy,and "zero background" advantages,it is expected to be promoted and applied in the field of dissolved gas analysis (DGA). However,the influence of the temperature and pressure of dissolved gases in oil on the photothermal phase is not yet clear. To improve the applicability of PTI technology in the DGA,an intensity-modulated-PTI-based acetylene sensing scheme using a Herriott cell is proposed. Experimental measurements are conducted on mixed gases containing acetylene and other characteristic gases to simulate fault conditions in transformer oil,and the influence of temperature and pressure on the detection results during the measurement process is focused. It is found that the photothermal phase increases with decreasing temperature or increasing pressure. Therefore,the accuracy and stability of acetylene detection depend on the reasonable setting of temperature and pressure during the detection process. The system demonstrates a strong linear relationship with acetylene concentration,with a detection sensitivity of 0.151 mV/(μL·L^-1) and a detection lower limit of 5.3 μL/L. The proposed solution provides insights and lays the foundation for the future development of a new type of DGA based on PTI technology.

Abstract:To overcome the influence of background noise interference on vibration detection efficiency,an anti-interference framework based on generation adversarial networks and convolutional neural networks (CNNs) is proposed to realize the contact defect detection for field gas insulated switchgear (GIS). Firstly,by current-carrying tests on prototype GIS platform,vibration waveforms of GIS with four artificial designed contact defects (missing finger,loosening bolt,with decomposed products and insufficient conductor insert depth) are acquired. Vibration waveforms on field GIS which contain background noise interference are also collected as a reference. Dataset for background noise interference removal and contact fault classification is built through spectrogram transform. Secondly,a cycle-consistent generative adversarial network (CycleGAN) with field vibration spectrogram as input is adopted to remove background noise interference on GIS. Then,two classical CNN architectures (AlexNet,ResNet18) are empirically designed to extract defeat features hidden in vibration spectrograms. Finally,the contact faults are identified via fully connected classifier. Influence of different time-frequency transformation algorithms on fault classification results are also compared. The results show that the proposed model can obtain maximum mean discrepancy (MMD) with 0.956 0 and Fréchet inception distance (FID) with 62.09 on field dataset,and the Mel-ResNet18 model could obtain 99.43% contact defect classification accuracy on test dataset. The proposed method in this paper can bring sound application value on improving the effectiveness of vibration detection and contact defect diagnosis results of field GIS.

Abstract:The vibration and noise generated during the operation of the transformer are directly related to its operating state and internal defects. The analysis of its voiceprint characteristics is helpful to further understand the operating conditions of the equipment,and ensure the safety and stability of the power system. Based on the analysis of voiceprint features,a transformer defect diagnosis method based on deep neural network and ensemble learning model is proposed. Taking the time-domain and frequency-domain features of transformer voiceprint data as multi-channel input,an integrated learning model is constructed based on a deep neural network model,and the effective recognition of transformer voiceprint features is realized. An ensemble learning model improves the accuracy of transformer defect diagnosis. Based on the transformer voiceprint sample library constructed in this paper,the recognition accuracy rate of the method for single transformer defects is 99.2%,and the recognition accuracy rate for transformer mixed defects is 99.7%. The research results show that the method can effectively identify the operating state of the transformer,and can provide technical reference for the operation and maintenance of the transformer.

Abstract:Lithium iron phosphate batteries have been widely used in energy storage system because of the long service life and environmental protection. However,in recent years,there are many safety accidents in the energy storage power station,which threaten the stable operation of the power grid. In the process of thermal runaway,lithium iron phosphate batteries will produce combustible gases such as C₂H₂ and C₂H₆,which is an important cause of combustion,explosion and other disasters. Therefore,real-time monitoring of combustible gases such as C₂H₂ and C₂H₆ in the prefabricated cabin can ensure the safe and stable operation of the battery. In this paper,a fiber loop ring-down spectroscopy (FLRDS) technology based on open optical paths is proposed,which can realize on-line monitoring of trace gas in open space. The optical loss of gradient index (GRIN) lens is analyzed theoretically and experimentally. An open optical path FLRDS gas detection system with an insertion loss of 0.95 dB is established. According to the infrared spectral characteristics of C₂H₂ and C₂H₆,the performance test of laser source is carried out. The gas concentration detection methods of C₂H₂ and C₂H₆ are studied by using the simulated open space. The results show that the system has good stability,and the standard deviation S of the measured signal under N₂ background is only 0.156% of the average value. There is a good linear relationship between ring-down time and concentration. R² of C₂H₂ is 0.998 32,R² of C₂H₆ is 0.994 72. The results of the inversion calculation show that the maximum relative errors of C₂H₂ and C₂H₆ are 3.215% and 4.72%. The maximum absolute errors are 16.86 μL/L and 12.74 μL/L. The research proves that the method has good measurement accuracy.

Abstract:Due to the complexity of the operation environment,partial discharge (PD) detection is accompanied by a large amount of noise interference,resulting in the phenomenon of missed and false alarms,which affects the subsequent operation and maintenance of power equipment. In this paper,an adaptive denoising algorithm for PD fluorescence signal based on spectral kurtosis and improved empirical wavelet transform (EWT) is proposed by fluorescence fiber PD detection method. Firstly,the fast kurtogram is used to determine the compact support boundary of the Fourier spectrum of the fluorescence signal,and then the EWT is used to decompose the noise-containing fluorescence signal to obtain the useful signal components where the fluorescence signal is located. Finally,the wavelet threshold method is used to remove the residual noise from the useful signal components to obtain the denoised PD fluorescence signal. This proposed method is used to carry out denoising analysis on simulated fluorescent signals. The de-noising results are compared with the empirical mode decomposition-wavelet transform (EMD-WT) and the EWT method,which show that the method in this paper improves the signal-to-noise ratio,root-mean-square error and normalized correlation coefficient,proving that the method has good denoising effect. In addition,the denoising results of the measured signals demonstrate that this method in this paper has a higher noise reduction rate than the EMD-WT method or the EWT method does,as well as superior noise suppression capacity.

Abstract:As a new distribution network structure,the honeycomb active distribution network uses the multi-port energy hub (MEH) to realize the interconnection of multiple microgrids/distribution network units. The MEH equipped with energy storage can improve the efficiency of renewable energy and flexibility of the system. In this paper,a MEH with energy storage and the corresponding hierarchical coordination control strategy are proposed. The upper-layer control coordinates the power distribution among the energy storage and grid-connected ports converters with considering the state of charge (SOC) of the energy storage and the operation states of the distribution system. Therefore,the energy management within the MEH can be realized in renewable energy fluctuations suppression and distribution network fault recovery modes. The lower layer control utilize the differential value of the output active power to compensation the control loop of the energy storage system for accelerating its response speed. The MEH system and its parameters are further designed. Finally, MATLAB/Simulink is used to simulate the application of the MEH in the distribution network. The effectiveness of the proposed hierarchical coordinated control strategy can be verified by the comparison of simulation results under different working conditions. By using this strategy, the working time of the energy storage system can be extended and the active power change rate of the energy storage system can be increased. Also,the voltage fluctuation of the DC bus can be reduced.

Abstract:Supraharmonics are a new power quality problem that can not be ignored in new energy power generation technology. As a typical supraharmonic source,the grid-connected inverter has received extensive attentions. To reveal the characteristics of its generation and propagation mechanism,the mechanism of supraharmonic generation in sinusoidal pulse width modulation (SPWM) inverters is analyzed firstly. It is found that the supraharmonics generated are mainly distributed around integer multiples of the switching frequency,and can be equivalent to a voltage source. At the same time,the influencing factors that cause changes in the voltage source are theoretically analyzed. Then,a supraharmonic circuit model including inverter,filter and grid side impedance is established,and the mathematical expression of supraharmonic emission current is derived. On this basis,the influence of capacity and number of inverters on supraharmonic propagation characteristics when multiple inverters are connected in parallel is explored through the control variates. Finally,the simulation experiment is carried out in Simulink. The results show that supraharmonics in the grid-connected inverter are mainly affected by the DC side voltage and modulation ratio,and the emission intensity of supraharmonics is proportional to the capacity when multiple inverters operate in parallel. The correctness of the theoretical analysis of supraharmonics in the grid-connected inverter is verified in this paper.

Abstract:In order to meet the requirements of high power density,high performance and low total harmonic distortion (THD) for photovoltaic microinverters, a multi-envelope resonant soft-switch inverter control strategy based on the critical current mode is proposed. Through time-sharing control of switching tubes,the rise and fall time of inductance current is balanced,so as to alleviate zero-crossing distortion. Then,the working mode of the control strategy of multi-envelope resonant soft-switch inverter is introduced in detail,and compared with the traditional critical current mode in terms of soft switch,switching frequency,switching tube loss and inverter efficiency,zero-crossing distortion degree. In order to verify the effectiveness of the control strategy,a 500 W soft-switched single-phase full-bridge inverter is built based on PSIM for open-loop simulation verification. The simulation results indicate that the multi-envelope mode,when compared to the traditional unipolar critical current mode,offers several advantages. These advantages include effective optimization of the zero crossing issue,harmonic filtration leading to reduced THD,and the achievement of zero current switch (ZCS) and zero voltage switch (ZVS) for all switches.

Abstract:In the hybrid direct current (DC) transmission system composed of modular multi-level converter (MMC) and line commutated converter (LCC),the commutation failure of LCC seriously affects the security and stability of the system. Firstly,the current characteristics and alternating current (AC)/DC voltage characteristics of MMC-LCC hybrid DC transmission system are analyzed when commutation failure occurs. Secondly,considering the influence of modulation ratio on half-bridge MMC,the feasible region of voltage modulation ratio is extended by using enhanced control strategy of MMC voltage. Then,the segmented control strategy of MMC voltage is proposed. According to the different degrees of AC voltage sag,the adjustment methods of DC voltage reference value are designed respectively,and the voltage control logic of hybrid HVDC transmission system is optimized to realize the effective switching of MMC voltage between normal operation and fault condition. Finally,the model of MMC-LCC hybrid DC transmission system is built in MATLAB/Simulink,and the simulation results show that the proposed control strategy can improve the DC voltage control accuracy and system stability while achieving fault ride through.

Abstract:When the oscillation suppression measures on the new energy side fail,the sub-synchronous oscillation induced by the interaction between the double-fed wind farm and the series compensation will result in grid disconnection of a large number of wind-driven generators,causing adverse effect on system stability. Since the existing measures cannot guarantee reliable oscillation suppression under any circumstances,it is necessary to take the initiative to conduct emergency control of the wind field at the network side. On the basis of frequency domain impedance,an emergency control strategy is formulated to address the problem of excessive real-time calculation during slicing of radial network,so as to reduce the amount of calculation and decision-making time. The relationship between the real part of the frequency domain impedance and the mode value,phase angle of branch impedance at the new energy side under the polar coordinate system is deduced,followed by a three-step judgement depending on the change of the real part. Firstly,the 'suspension of invalid units' is excluded. Secondly,the valid units with effective emergency control are selected from the rest units. Thirdly,the order of suspension is determined based on the size of projection of the complex impedance,so as to prevent excessive suspension. On the basis of the actual parameters of transmitted grid after series compensation at a domestic wind field,the effectiveness of the proposed strategy is subject to PSCAD simulation. The simulation result shows that emergency control with adoption of the proposed strategy herein can effectively improve the system damping during the sub-synchronous oscillation and prevent rapid divergence of oscillation.

Abstract:Line commutated converter-modular multi-level converter (LCC-MMC) parallel multi-terminal hybrid DC transmission system combines the technical advantages of line commutated converter-high voltage direct current (LCC-HVDC) and modular multi-level converter-HVDC (MMC-HVDC). However,due to its inconsistent boundary characteristics,special busbar structures,and different control strategies for converter stations,the existing line protection schemes are difficult to apply directly. Therefore,a protection scheme of multi-terminal hybrid DC line based on measuring wave impedance phase characteristics is proposed. Firstly,the expressions of wave impedance measured at different fault locations are deduced,and it is found that in the high-frequency band,the phase of wave impedance measured at the measurement point is significantly different under the condition of internal and external faults. And the phase characteristics have nothing to do with factors such as fault distance,type and transition resistance. Then,the S-transform is used to extract the impedance phase information of the measured wave,and combined with the high-low frequency energy ratio criterion of the traveling wave to realise the fault identification. Finally,the simulation verification in PSCAD shows that the proposed protection can distinguish the faults of the T-zone bus and the adjacent line end,and has a certain anti-transition resistance (500 Ω) and noise immunity (20 dB) while satisfying rapidity. The proposed peotection meets the main protection requirements of the multi-terminal hybrid DC system line.

Abstract:With the large-scale development of new elements such as distributed new energy,distributed generation and electric vehicles under the new power system,the distribution network voltage control is faced with challenges such as heterogeneous control object characteristics,complex models and variable scenarios. In this context,a distribution network voltage control strategy based on model-free adaptive control is proposed. The data from dominant node voltages,distributed generation,and energy storage device output are used by this strategy to establish a real-time dynamic linearized data rolling pool based on the distribution network voltage control partitioning and dominant node selection. Based on the characteristic model theory and data drive,the complex system voltage control characteristics are time-varyingly modified by the controller characteristic parameters,and the control model complexity is reduced without losing system model information. Finally,the effectiveness of the proposed control strategy is shown in the results by simulating and comparing in the modified IEEE 33-node distribution network system arithmetic. The response of the proposed control strategy is quicker than that of traditional model-based control method,thus reducing the voltage fluctuation caused by disturbances. The proposed control strategy has good application prospects.

Abstract:Lately,deep learning techniques have developed rapidly. To address the problem of the poor processing capability of discrete data and the problem of poor generalization in short-term load forecasting tasks,a load forecasting method based on a long short-term memory network with attention mechanism (Attention-LSTM) and Stacking multi-model integration is proposed,which can take into account their respective advantages. Firstly,the discrete features are processed by mean coding,and then Attention-LSTM is used to extract the features of the load data. Next,the processed data are input into the multi-model integrated prediction model based on Stacking. The input features are analyzed and processed through three basic learners. Finally,the prediction is completed through the meta-learner. Actual load data from two datasets are used for analysis in the case study,and the load data in the two datasets are predicted separately,and compared with gated recurrent units,light gradient boosting machine and support vector machine. The simulation results show that the prediction accuracy based on the proposed method in the two datasets can exceed 98% in terms of accuracy,which is higher than the other three methods.

Abstract:Hybrid modular multi-level converter (MMC) consisting can operate with a high modulation index,and it borrows the capacitor voltage balance method from half-bridge MMC. Thus,a strong coupling effect occurs in the charging behaviours of full bridge sub-modules (FBSMs) and half bridge sub-modules (HBSMs),which causes a low volatility of the ripple voltage of HBSMs' capacitors and challenges for its capacitance optimization. To address this issue,the ripple coefficients of HBSMs' and FBSMs' capacitor voltages are calculated for MMC operating with a high modulation index quantitatively. The optimize of HBSMs' capacitance can be clarified. Then,design principles of the optimal HBSMs' and FBSMs' average switching functions are explored. Hence,the optimization of HBSMs' capacitance is achieved with the constraint of the maximal value of sub-modules' instantaneous capacitor voltages. Further,under the conventional basic control framework of MMC,a control strategy based on module decoupling control which can regulate the dynamics of capacitor voltage accurately is proposed. Finally,hybrid MMC operating is modeled with MATLAB/Simulink. The proposed capacitor optimization method is verified. The simulation results show that the DC component and ripple component of capacitors' voltages can be regulated accurately,and the capacitance of HBSMs can be reduced effectively.

Abstract:Accurate multivariate load forecasting is a prerequisite for optimal scheduling and stable operation of integrated energy system (IES). Aiming at the complex coupling relationship between multiple loads and many factors affecting load forecasting in IES,a method for multivariate load prediction based on extreme gradient boosting (XGBoost) and multi task learning (MTL) is proposed. Firstly,the contribution of each influencing factor to the multivariate load is obtained by XGBoost importance ranking,and the key factors influencing the load prediction are selected as the input of the prediction model based on the contribution,which ensures the effective correction of the input features to the multivariate load prediction. Secondly,the gated recurrent unit (GRU) is used as a shared layer to build the MTL prediction model,and the sub-tasks share information with each other to effectively exploit the complex coupling relationship between the loads. Finally,the validity of the proposed model is verified by using the load data of an integrated energy station in Shanghai as an example,and the results show that the model can adapt to the changes of various types of loads in the actual integrated energy system,effectively improving the prediction accuracy and reducing the training time.

Abstract:To bring the improvement of the transient stability of the power grid by the operation mode control and generator tripping control into full play,a power system transient stability prevention-emergency coordinated control decision-making method embedded in the light gradient boosting machine (LightGBM) assessment model is proposed. In order to quickly evaluate the degree of improvement of the system stability margin by the control measures,firstly,the hybrid control sample generation method and the LightGBM algorithm is used to construct a assessment model of the prevention-emergency control on the system stability margin. Considering that the unreasonable generator tripping and load shedding control may deteriorate the system stability. The numerical sensitivity of the LightGBM surrogate model is used to identify effective control sites and reduce the decision space. The LightGBM model is further embedded in the transient stability two-layer optimal control model,which replaces the transient stability time-domain simulation,combines the improved non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ) to realize the rapid solution of the coordinated control strategy. Through the IEEE 39-node test example,it is verified that the proposed method can realize the coordination and cooperation between the preventive control before the occurrence of faults and the emergency control after the occurrence of faults of different severity,improve the security and stability of the power grid, reduce the cost of optimal dispatching.

Abstract:Locating the voltage sag source quickly and accurately is of great significance to improve the reliability of power supply and clarify the responsibility of both supply and demand. In this paper,a precise location method of voltage sag source in radial distribution network is proposed based on micro-synchronous phasor measurement unit (μPMU) and binary search method. Firstly,the sag source current is calculated by the layered circuit equipped with μPMU. In each layered circuit,it is assumed that the sag source current is injected into each bus node. The virtual voltage variation at the terminal bus is calculated,and the error is obtained by comparing it with the measured voltage variation at the terminal bus. According to the size of the error,the sag source adjacent bus is determined. Secondly,the virtual bus is set at the midpoint of the adjacent section of the sag source adjacent bus. The binary search method is used to quickly narrow the locating interval and achieve precise locating of the sag source. Finally,the IEEE 33-node model is used to verify the proposed method in MATLAB. The results show that the proposed method has good locating accuracy for different positions or different types of voltage sag sources in radial distribution networks. The proposed method has certain anti-interference ability.

Abstract:To solve the problem of power system outages caused by extreme weather,a reasonable strategy for restoring the power supply is needed to reduce losses. Therefore,a dynamic optimization method of black start recovery path considering the probability of device failure is proposed. Firstly,to accurately measure the probability of line fault under disaster conditions,a line fault model is founded by taking into account the spatiotemporal coupling of typhoons,rainfall,and landslides. Secondly,the black-start power evaluation model is established by combining the optimal weights of motor characteristics. Furthermore,the online recovery strategy is developed to determine the black-start recovery scheme,which can be solved by the improved Monte Carlo tree search algorithm and the Dijkstra algorithm. Finally,the IEEE 30-node distribution network system is used to verify the applicability of the dynamic decision system under different scenarios. The results show that the dynamic decision system can reduce the riskiness of the recovery scheme by 22.5% while keeping the power supply recovery time stable,effectively improving the rapidity and reliability of the power supply scheme.

Abstract:The hybrid multilevel inverter is widely used because it can output more levels and uses fewer switching devices. However,the traditional phase disposition pulse width modulation (PD-PWM) strategy is used for hybrid cascaded H-bridge 13-level inverters with a voltage ratio of 3:1:1,and the power of each unit is unbalanced. In order to achieve the goal of power equalization,an improved hybrid modulation method for the hybrid 13-level topology is proposed. Based on this method,it further reconstructs the triangular carrier under PD modulation and adjusts the output pulse of the high-voltage unit. Under laboratory conditions,taking the modulation degree of 0.9 as an example,the switching losses of the three low-voltage units L₁,L₂,and L₃ are 4.6 W,4.4 W,and 4.2 W respectively. The switching losses are basically equal. The output power of L₁,L₂,and L₃ is basically the same. The output power of high-voltage unit H₁ is 51.7 W, and the power of the three low-voltage units is 17.2 W. The output power ratio of high-voltage and low-voltage units is close to 3:1. The research shows that the switching losses of the three low-voltage H-bridge units are the same and the power of the four units is balanced. The power switches of the high-voltage and low-voltage parts work at the fundamental frequency and high frequency respectively,and the harmonic characteristics are good.

Abstract:The J-A hysteresis theory is widely used in the modeling of the magnetic properties of electromagnetic current transformers. However,under harmonic conditions,the model has significant errors. To solve this problem,the classical J-A model parameters are modified for each harmonic,and genetic-annealing algorithm is used for parameter identification. Then,the magnetic characteristics of each harmonic are linearly added to achieve the magnetic characteristics modeling of the current transformer under non sinusoidal excitation. The premise of adopting this correction method is that the nonlinear error of the measuring current transformer meets the specified level 0.2S,so the simulation analysis and error correction of harmonic hysteresis loops are approximately applicable to the linear superposition principle. Taking 0.2S level current transformers made of nanocrystalline materials as an example,the simulation analysis and experimental verification of angle errors and ratio errors show that the J-A model after parameter correction improves measurement accuracy under harmonic conditions,and the approximate linear superposition method of multiple harmonic magnetic characteristics has a negligible impact on measurement accuracy. This confirms that the J-A hysteresis model correction considering harmonics is reasonable and effective.

Abstract:In order to reduce the capacitor volume of the modular multi-level converter,a modular multi-level converter with the hybrid submodule based on capacitor voltage ripple compensation is proposed. Firstly,the system structure of the proposed hybrid modular multi-level converter and the mechanism of reducing the capacitor volume are introduced,and the switching principle of the hybrid submodule is analyzed in detail. Secondly,the energy fluctuation of capacitors is analyzed to size the capacitance of all capacitors,and the overall capacitor volume is evaluated and compared with the conventional half-bridge modular multi-level converter. Finally,the effectiveness of the proposed structure and control is verified by simulation and experimental results. The results show that the proposed structure in this paper can greatly enhance the energy utilization ratio of the capacitors in the modular multi-level converter with lower switching device cost,thus,reducing the capacitor volume and improving the system power density.

Abstract:High sensitivity in condition monitoring is a key link in digital electrical equipment operation and maintenance,besides,it is also an important premise of status analysis and life cycle management. The partial discharge (PD) transient earth voltage (TEV) detection is the primary mean to insulation state perception of high voltage switchgear. The detection sensitivity of TEV method is affected by the type and location of the defect,the location of TEV detection. However,there is no systematic research on this subject by now. A finite difference time domain (FDTD) electromagnetic wave simulation model based on the measured pulse current waveform is established. The stuexplores the effects of factors such as the rise-up time of PD pulse current waveform,the position of discharge source,and the direction of pulse current on the amplitude and attenuation degree of TEV. A series of PD tests and TEV measurements are conducted in a laboratory setting to identify the primary factors influencing TEV detection sensitivity. The results reveal that both the rise-up time and spatial position of PD significantly affect the amplitude of the TEV signal. Specifically,the amplitude of the TEV signal exhibits a decreasing trend as the pulse current's rise-up time increases. Notably,a tenfold increase in the rise-up time results in a decrease in TEV signal amplitude within the range of 3.17 to 3.37 times. Furthermore,the TEV signal's attenuation degree in the busbar room compartment is approximately 9.5 to 10.0 times greater than that observed in the cable compartment. This conclusion has reference value for testing the sensitivity of TEV detection.

Abstract:In order to explore the influence of the modulation circuit fault of the fiber optical current transformer (FOCT) on the output signal of the detector,and facilitate fault prediction and fault diagnosis,the mathematical model of sine wave modulation FOCT output signal which includes the phase delay between the optical circuit and the electrical circuit is firstly established in this paper. The mathematical expressions of modulation depth,driving voltage, second and fourth harmonic are given,and the basic methods of closed-loop modulation and demodulation of the target value modulator are analyzed. On this basis,a mathematical model is established to analyze the influence of modulation depth on the peak of the detector output light intensity,average light intensity,second and fourth harmonic. Therefore,it is proposed that the modulation depth decrease is the key factor leading to the abnormal output signal of the detector when the modulation loop fault occurs,and the increase of the average light intensity of the detector output,the decrease of the fourth harmonic and the increase of the second/fourth harmonic ratio should be the typical characteristics of the modulation loop fault. Finally,the practical FOCT simulation test is carried out to prove the validity of the theory.

Abstract:To ensure safe operation of instrumentation and control (I&C) cables in nuclear power plants (NPP),time-frequency domain reflectometry (TFDR) is widely applied to identify and locate typical defects in I&C cables. The advantages of Wigner-Ville distribution (WVD),pseudo Wigner-Ville distribution (PWVD),and smooth pseudo Wigner-Ville distribution (SPWVD)are compared. In addition,the energy range scaling method is proposed in the time-frequency cross-correlation (TFCC) processing. During the experiment,a 50-meter and a 148-meter multi-core cross-linked polyolefin I&C cable are prepared to simulate several failure cases,including short circuit,open circuit,metal shield damage,and local thermal agingdefect. Based on the TFDR method,three time-frequency distribution algorithms are adopted. Then,the energy range scaling method is applied to improve the problem of wide main lobe of TFCC localization peak. The experimental results show that the main lobe of TFCC is wider after SPWVD processing. PWVD has a better inhibitory effect on cross terms in normal cable detection. In terms of detecting a defect,SPWVD has better detection ability. By using the energy range scaling method,similar localization peaks can be separated to enhance the discrimination of weak reflection signals.