Abstract:

Frequency disturbances caused by power system faults propagate in the form of electromechanical waves. This propagation process affects the frequency response analysis and decision-making of safety control measures such as frequency stability maintenance and under-frequency load shedding. The conventionally used offline simulation method offers high calculation accuracy, but it has limitations such as poor adaptability to complex and variable new power systems and insufficient online computing capability. An online calculation method is proposed for frequency propagation based on approximate analytical solutions and physics-informed neural network (PINN). Firstly, a partial differential equation model for the propagation of electromechanical disturbance frequencies is established. Through engineering simplifications, an analytical expression of frequency disturbance with respect to time and position is derived, which meets the requirement for fast calculation of frequency disturbances but suffers from reduced calculation accuracy due to the simplifications. Subsequently, PINN is introduced to improve calculation accuracy. A small amount of high-precision simulation data and a large amount of low-precision data quickly obtained from analytical solutions are used as samples, which solves the problem of excessive sample generation time. Furthermore, the electromechanical disturbance frequency propagation model before simplification is incorporated into the network layer as a physical constraint. This not only ensures fast calculation but also enhances the calculation accuracy and generalization performance across different operating scenarios. Finally, a case study system is built using RT-Lab and PyTorch. The feasibility and superiority of the proposed method are verified through simulation and comparative analysis.

Abstract:

There is uncertainty in the output of wind farms, and there are also certain correlation in time and space. The random variables brought by these wind farms are introduced into the system, which may bring certain deviations to the calculation of the transient stability constrained optimal power flow (TSCOPF) if their temporal and spatial correlations are not adequately taken into account. Therefore, a TSCOPF model and calculation method considering the spatio-temporal correlation of wind power output are proposed. Firstly, a wind power output model containing spatio-temporal correlation is constructed to consider the correlation between wind farm power output in time and space dimensions. Secondly, a probability constraint is constructed based on the joint chance constraint (JCC) theory and a TSCOPF model based on JCC theory is established on this basis. Then a mix sample average approximation (MSAA) is used to process JCC, and JCC problem is transformed into a linear programming (LP) problem, which is solved by the CPLEX solver. Finally, simulation analysis is carried out at the improved IEEE 39-bus system, and the simulation results show that the proposed method can obtain the optimal operation scheme while ensuring system safety and stability.

Abstract:

With the large-scale integration of new energy into the power grid, the problem of uneven distribution of power flow is prominent, seriously affecting the reliability and flexibility of the power grid. Therefore, there is an urgent need for means to improve the power grid dynamic control capability of power flow. Firstly, a new hybrid power flow controller (HPFC) based on a three-level neutral point clamp (NPC) inverter is proposed in this paper, combined with a phase-shifting transformer (PST) large discrete adjustment and three-level NPC small-range flexible adjustment to achieve precise control of large-scale power flow. Secondly, the working principles of PST and three-level NPC are theoretically analyzed, a control strategy for synergy between the two is proposed, and the overall operation mode and working scope of HPFC are determined. Finally, taking the 220 kV double-circuit line as the application scenario, the effectiveness of the proposed new HPFC control strategy and its ability to control the power flow were verified through simulation on the PSCAD/EMTDC platform. The results show that the proposed new HPFC effectively optimizes the shortcomings of the traditional two-level voltage source converter (VSC) output with high harmonic content, it can achieve large-scale flexible control of power flow, and can also quickly and accurately respond to dynamic scenarios where the target power flow changes frequently.

Abstract:

Multi-source self-adaption STATCOM and line commutated converter (SLCC) technology can overcome the inherent shortcomings of conventional line commutated converter (LCC) based high-voltage direct current (HVDC) transmission technology. In the "embedded" scenario, the topology and control strategy of a symmetrical unipolar SLCC-HVDC is proposed, which possesses significant technical and economic advantages. Then, the operating principles of SLCC are analyzed. During the non-commutation process, the SLCC can directly control the grid current through the static synchronous compensator (STATCOM) supplementing current. During the commutation process, the SLCC need to passively absorb the commutation current from the grid, while the grid current can still be indirectly controlled through changing the commutation angle. Finally, the operation characteristics of the traditional LCC and the SLCC under the symmetrical unipolar topology are compared. Both the commutation angle and the commutation voltage drop of the SLCC-HVDC are smaller than the LCC-HVDC and the reactive power of the SLCC-HVDC can be adjusted smoothly. Under transient condition, the fluctuation of the commutation angle is smaller, taking less risk of commutation failure. For the harmonic characteristics, SLCC-HVDC obviously decreases the harmonics compared with the LCC-HVDC during the unsymmetrical operation, thus is supposed to possess more flexible operation modes.

Abstract:

To limit the overvoltages of the flexible DC converter valves and absorb surplus energy, a DC controllable energy dissipation device based on metal oxide varistors has been applied for the first time in the Baihetan-Jiangsu ±800 kV hybrid cascaded ultra-high voltage direct current (UHVDC) transmission project. The control switches of the device consist of three types of switches connected in parallel, so the transient current transfer and distribution among the three control switches are critical to the safe operation of the device. Firstly, the operating conditions and current transfer characteristics of the control switches are analyzed. Theoretical calculations and electromagnetic transient simulations are performed on current transfer characteristics under a sinusoidal half-wave current waveform. The results show that after the fast switch is closed, the current in the thyristor switch branch can be rapidly transferred to the fast switch branch, and the thyristor turns off reliably. Furthermore, the current transfer characteristics among the control switches are validated via a synthetic test circuit, and the test results verify the accuracy of theoretical calculations and simulations. In the tests, the thyristor turn-off time is delayed by approximately 5.7 ms compared with the simulation results, mainly due to discrepancies in the nonlinear characteristics between the simulated and actual saturated reactors. This reduces the current transfer rate of the thyristor branch. After adjusting the saturated reactor parameters in the simulation, the thyristor turn-off time in the simulation lags the test result by about 1.8 ms. Finally, the current transfer characteristics between the control switches under engineering operating conditions are verified using the electromagnetic transient model of the Baihetan-Jiangsu hybrid cascaded UHVDC system. Under extreme operating conditions, the current through the fast switch branch accounts for approximately 30% of the total current.

Abstract:

To address the issue of harmonic source localization in low-observability power distribution systems, a hierarchical localization method based on physical network structure information is proposed. Firstly, according to the physical characteristics of distribution networks, the distribution rules of harmonic voltages and currents under radial topology are analyzed. Then, measurement points are reasonably placed to partition the network and construct a new topology. Based on harmonic current path identification, the number of harmonic sources and their located sections are determined. Finally, precise estimation is carried out in the determined sections to obtain the specific locations of harmonic sources. The model is established and solved based on mixed integer quadratic programming (MIQP), and simulations are implemented in MATLAB. Taking the IEEE 33-bus system as an example, the effectiveness and accuracy of the proposed method are verified by comparison with existing methods. The results demonstrate that the hierarchical harmonic source localization method presented in this paper can solve harmonic source localization problems that are closely aligned with real-world scenarios, achieving localization of harmonic sources in low-observability distribution systems where both the number and location of harmonic sources are unknown.

Abstract:

The generalized load refers to the sum of all electrical equipment within the power supply scope of a substation, including traditional loads, new type loads, distributed power sources, distributed energy storage, etc. Its flexible components can be divided into three categories, pure load type, pure power source type, and energy storage type. The generalized load is gradually becoming a research hotspot at home and abroad. Power modeling of it is an essential part of the research on the new power system. In view of the problem that most current reviews are oriented towards the electrical models of loads, the research status of the power models of typical flexible components of the generalized load is reviewed and prospected in this paper. Firstly, the modeling methods of the flexible components of the generalized load are summarized, and the similarities and differences among white box, black box, and grey box models are elaborated from the modeling principles. Then, in the order of pure load type, pure power source type, and energy storage type, the power models of the most typical, widely used and numerous flexible components in each category are classified and reviewed as white box, black box, and grey box models, and then the differences among various models are comprehensively compared. Finally, the future research directions of the modeling of the flexible components of the generalized load in the new power system are prospected.

Abstract:

To achieve the power sharing and improve misalignment tolerance of dual-channel wireless power transfer (WPT) systems, a dual-channel magnetic coupler is proposed based on the traditional double-layer quadrature double-D coil structure. This coupler exhibits excellent misalignment tolerance, namely tolerance for misalignment in any direction within the plane composed of X and Y axis. Its transmitting and receiving sides respectively include two decoupled coils, and mutual inductance between the two pairs of coils is symmetry within a certain misalignment range, which can achieve the power sharing of dual-channel WPT systems. Firstly, for dual-channel WPT systems using resonant compensation, detuned compensation and hybrid topology, the common requirements of magnetic couplers are discussed. Then, a design method for the proposed magnetic coupler is given and a case design is carried out. Furthermore, the conformity of the designed case with various performance indicators is analyzed by finite element simulation. Finally, a magnetic coupler prototype is developed for experimental verification. The results show that, within 100 mm misalignment in X and Y axis direction except for the extreme misalignment position, the magnetic coupler efficiency remains above 91.12% and the ratio of channel output power is greater than 0.87, demonstrating a good power sharing effect.

Abstract:

Thermally induced defects in the encapsulated insulation layer of dry-type transformers are prominent. To detect abnormal heating of winding insulation materials in a timely manner, this paper proposes an inorganic thermochromic coating-based temperature measurement method for the encapsulated insulation layer of dry-type transformer windings and investigates its performance. The inorganic temperature-indicating material is synthesized via the liquid-phase method, and its structure and micromorphology are characterized by Fourier transform infrared spectroscopy, thermogravimetry-differential thermogravimetry (TG-DTG), and other techniques, revealing the discoloration mechanism of the material. Considering the discoloration temperature range and sensitivity, the optimal mass ratio of the components (oxalic acid, potassium oxalate, and cobalt carbonate) is determined. Temperature-indicating coatings are prepared using two different basecoats, and their composition ratios are optimized according to discoloration, adhesion, hydrophobicity, and electrical properties. The results show that coatings using RTV-Ⅱ as the basecoat have a discoloration temperature of 103~120 ℃ and a grade 1 adhesion level. The static contact angle is greater than 100° both before and after discoloration, and the surface flashover voltage exceeds 9 kV/cm. The coating achieves the best comprehensive performance when the ratio of the basecoat to thermochromic material is 10∶3. After thermal aging at 60~80 ℃ for 168 h, no obvious degradation is observed in its discoloration and electrical insulation performance. The results can provide technical support for the temperature detection and overheating warning of thermally induced defects in the encapsulated insulation layer of dry-type transformer windings.

Abstract:

To enhance the application of ultra-low frequency dielectric loss testing technology in evaluating cable insulation conditions and to improve the testing efficiency of ultra-low frequency dielectric spectroscopy, the dielectric properties of cross-linked polyethylene (XLPE) cable insulation before and after thermal aging are investigated using a improved frequency domain spectroscopy (iFDS). A novel method for characterizing the thermal aging state of cable insulation is proposed. Firstly, XLPE cable insulation samples with varying aging levels are prepared through thermal aging experiments. The dielectric properties of these aged XLPE samples are measured in the ultra-low frequency range from 10^-3 Hz to 10³ Hz. The results demonstrate that the ultra-low frequency dielectric spectrum is highly sensitive to changes in the XLPE state. As the test frequency decreases, the dielectric loss gradually increases, with more severely aged samples showing a more pronounced rise in dielectric loss. This indicates that ultra-low frequency dielectric loss exhibits strong anti-interference characteristics and can effectively detect XLPE aging conditions. Based on dielectric physics, the sharp increase in dielectric loss in the ultra-low frequency region is primarily attributed to the combined effects of DC conductance and interface polarization. Finally, a correlation between the ultra-low frequency dielectric spectrum and volt-ampere characteristics is established. A volt-ampere hysteresis loop, incorporating amplitude and phase information, is proposed to evaluate cable insulation aging. Relevant characteristic parameters, such as the rate of deflection angle change (RDA) and the rate of curve deformation (RCD), are extracted to characterize the aging status of cable insulation.

Abstract:

To more effectively cooperation response of adjustable resources within the integrated energy system, a two-layer optimization method based on chance constraints for integrated demand response (IDR) in industrial parks is proposed. Firstly, based on the integrated energy system framework of the industrial park, an incentive-based integrated demand response strategy is developed, along with an interactive response framework involving the industrial park operator, load aggregator and the main grid. Then, to address the uncertainty in load response, the chance constraint method is introduced,and coordinated response strategy between air-conditioning and aluminum electrolysis load is proposed to improve response reliability. Finally, a two-layer optimization method is established, with the industrial park operator as the upper layer and the load aggregator as the lower layer. The model is solved using a hybrid approach that embeds the Gurobi solver within a particle swarm optimization algorithm. The simulation results show that the proposed two-layer optimization method can achieve a coordinated response of energy conversion equipment and multiple loads. It reduces the response cost of the industrial park and improves the reliability and economic efficiency of integrated demand response.

Abstract:

The construction scale of communication base stations is increasing, and the reliability of power supply in distribution networks is enhancing. Therefore, backup energy storage resources of base stations are commonly experiencing idle conditions. Dispersed and low-capacity backup energy resources of base stations can be aggregated and utilized through the form of a virtual power plant (VPP). This paper presents a methodology for calculating the dynamic minimum backup time of communication base stations based on the availability index. The method aims to accurately evaluate the dispatchable capacity of an individual energy storage base station. This paper employs VPP technology to efficiently aggregate the dispatchable capacity of distributed base station backup energy storage. Thereby a base station backup energy storage VPP is formed and participates in the power market as a qualified entity. An economic dispatch model is proposed for a backup energy storage VPP to participate in both price arbitrage and frequency regulation ancillary services to maximize operational revenue. Case studies demonstrate that dynamic evaluations of minimum backup time enhance the accuracy of dispatchable capacity assessments in comparison to fixed-time methodologies. The energy storage VPP of base stations optimizes energy storage utilization and maximizes operational revenue through collaborative participation in price arbitrage and frequency regulation services.

Abstract:

Microgrids controlled by virtual synchronous generators (VSG) experience inrush currents and voltage offsets during transitions between grid-connected and islanded modes. These phenomena cause fluctuations in active power and frequency, thereby affecting the safe and stable operation of the system. To address this issue and achieve seamless mode switching in microgrids, the fundamental principles of VSG control and the linear active disturbance rejection control (LADRC) algorithm are investigated in this paper. The second-order LADRC model is simplified and incorporated into the active power-frequency control loop of the VSG. This replaces the traditional proportional-integral (PI) controller in the phase pre-synchronization unit. The output frequency of the VSG is taken as the output of the LADRC, and the rated frequency is taken as the input of the LADRC, while regulating both the frequency and phase simultaneously. It also reduces the impact of inrush currents and voltage offsets during grid connection. This prevents excessive inrush currents that could destabilize grid integration and enables seamless switching between grid-connected and islanded modes. MATLAB/Simulink simulation models are constructed to compare the proposed control strategy with direct grid connection without pre-synchronization and with traditional pre-synchronization control strategies. The results validate that the proposed control strategy suppresses inrush currents and voltage fluctuations more effectively, which provides more accurate power tracking, and achieves better performance in mode switching.

Abstract:

With the development of new-type power systems, source-load uncertainty and their mutual coupling have become increasingly prominent. Power system analysis must therefore fully account for the impact of uncertain factors on probabilistic power flow. To address the issue that traditional stochastic response surface method (SRSM) rely heavily on a large number of samples to achieve high modeling accuracy in probabilistic power flow calculations, a sample selection strategy based on the stochastic reduced order method (SROM) is proposed to extract representative samples from the full ensemble for constructing the surrogate model, thereby ensuring high computational accuracy. Furthermore, to better capture the spatial correlation and nonlinear relationships among input variables, the Kriging method is integrated with SRSM to develop an enhanced probabilistic power flow model. In addition, the global sensitivity analysis method is used to establish the sensitivity indices, calculate the sensitivity of the output variable of the probabilistic power flow to the input random variables, and quantify the impact on the operation state variables of the distribution network. Finally, numerical simulations demonstrate the feasibility and effectiveness of the proposed SROM-Kriging enhanced SRSM framework for accurate and efficient probabilistic power flow modeling.

Abstract:

Power level improvement is an important trend of wireless charging system (WCS) for electric vehicles. To improve the power level considering the misalignment tolerance of WCSs, a high-power WCS prototype with S-S topology is proposed based on the comprehensive optimal load configuration method. Firstly, the power and efficiency characteristics of S-S and LCC-LCC topology WCSs are compared in the unified circuit by equivalent conversion of the compensation topology, and the advantages of the traditional S-S topology for high-power WCS are evaluated by considering the cost, power density and reliability, etc. Meanwhile, a comprehensive optimal load configuration method that accounts for the statistical characteristics of the parking position is proposed, and the finite element method is used to calculate the efficiency characteristics under different load conditions. The results show that the proposed method achieves the highest expected efficiency, modestly improves the average efficiency, and mitigates the efficiency degradation resulting from the misalignment. Finally, a 22 kW S-S topology WCS prototype is designed and manufactured for experimental validation. The results show that, with the proposed comprehensive optimal load configuration method, the magnetic coupler efficiency of the WCS prototype is around 95% under different misalignment conditions. With 200 mm lateral misalignment, the efficiency of the prototype is 94.688%, only 0.464% lower than that under alignment condition. These results verify the effectiveness of the proposed method in enhancing misalignment tolerance. The achievement of this study can provide a theoretical basis for the misalignment tolerance improvement of high-power WCSs.