Abstract:Weighted least-square support vector machine (WLS-SVM) is proposed in this research as a real-time transient stability evaluation method using the synchrophasor measurement received from phasor measurement units (PMUs). This method considers the directional overcurrent relays (DOCRs) for the transmission system, whereas in previous studies, the effect of protective mechanisms on the transient stability was largely ignored. When protective relays are activated in power system, the configuration of the power system is altered to mitigate the risk of the power system becoming unstable. The present study considers the operation of DOCRs in transmission lines for the transient stability so that the proposed method can respond to changes in the configuration of the case study system. In addition, WLS-SVM is employed for an online assessment of the transient stability. WLS-SVM not only is effective in response due to its faster speed, but also is resistant to noise and has excellent performance against the measurement errors of PMUs. To extract the characteristics of the vectors that are fed into the WLS-SVM algorithm, principal component analysis is used. The findings of the suggested technique reveal that it has higher accuracy and optimum performance, as compared to the extreme learning machine method, the adaptive neuro-fuzzy inference system method, and the back-propagation neural network method. The proposed technique is validated in the New England 39-bus system and the IEEE 118-bus system.

Abstract:Numerous renewable energy sources (RESs) are coupled with the power grid through power electronics to advance low-carbon objectives. These RESs predominantly connect to the AC collection network via inverters, with the electricity they produce either transmitted over long distances through high-voltage lines or utilized locally within the distribution system. The unique interfacing of RESs alters their fault response characteristics, typically resulting in limited fault currents, frequency deviations, and fluctuating sequence impedance angles. Therefore, existing protection principles based on fault signatures of synchronous generators will face significant challenges including distance relays, directional elements, differential relays, phase selectors, and overcurrent relays. To solve these issues, innovative protection technologies have been developed to bolster grid stability and security. Furthermore, the superior controllability of power converters presents an opportunity to devise effective control strategies that can adapt existing protection mechanisms to function correctly in this new energy landscape. Nevertheless, the complexity of fault behaviors exhibited by RESs necessitates further refinement of these schemes. Therefore, this paper aims to consolidate current research methodologies and explore prospective avenues for future investigation.

Abstract:The stability problem arising from the DC-link voltage outer loop and current inner loop control in weak-grid-tied interlinking voltage-source converters (VSC) system has been extensively investigated. However, the different impacts on system stability caused by these controller parameters have not been well solved. A three-port hybrid AC/DC admittance-based model for VSC taking into account AC- and DC-side dynamics is developed for revealing the mechanism intuitively on the formation of the positive and negative feedback effects in the DC-link voltage control loop. Subsequently, the stability characteristics of the voltage outer loop and current inner loop controller parameters on VSC are analyzed. Compared to adjusting the voltage outer loop controller parameters which has significant effort on the dynamic regulation characteristics, it is more sensitive on system stability by the parameter change of the current inner loop controller. Theoretical results, along with experimental verification, are provided to validate the analysis.

Abstract:Nonlinear loads and power electronics components inject harmonics into the connected power system, which affects the power quality of the system. Harmonic suppression, reactive power compensation, and load balancing can be accomplished with the use of shunt converters. This paper proposes a proportionate normalized least mean M-estimate controlled shunt converter to separate the distorted load current’s active and reactive components. The control algorithm introduces the M-estimate based cost function, which helps remove the impact of impulsive noise on weight update. The shunt converter is designed to balance load, compensate for reactive power, and reduce harmonics in the distribution system. To determine the proposed control algorithm’s robustness, the system is tested under a variety of linear and nonlinear load scenarios. The proposed control is compared to four existing control strategies to demonstrate its effectiveness in evaluating the components of load current. Experimental analysis is carried out through real-time simulation analysis using the dSPACE 1202 processor. The proposed system is validated to comply with the IEEE-519 standards.

Abstract:The rapid expansion of offshore wind power plays a crucial role in China’s pursuit of its ‘dual carbon goals’. Fractional frequency transmission, an emerging technology for delivering large-scale offshore wind power, currently lacks extensive research attention. This paper addresses this gap by proposing a reliability assessment model for fractional frequency systems, encompassing generation, boosting, transmission, and conversion processes. Additionally, the study conducts a quantitative analysis of severe weather impacts on offshore component maintenance. With a focus on China’s offshore wind power development, the research includes comparative analyses of various offshore regions, system topologies, and transmission methods to evaluate system reliability. This comprehensive analysis serves as a valuable reference for the strategic planning and large-scale deployment of grid-connected offshore wind power systems.

Abstract:In recent years, a significant number of distributed small-capacity energy storage (ES) systems have been integrated into power grids to support grid frequency regulation. However, the challenges associated with high-dimensional control and synergistic operation alongside conventional generators remain unsolved. In this paper, a partitioning-based control approach is developed for the participation of widespread distributed ES systems on frequency control in power systems. The approach comprises a network partitioning method and a two-layer frequency control scheme. The partitioning method utilizes a community detection algorithm in which the weights between the buses are calculated based on the electrical distances. After partitioning the buses into different groups, an optimization-based frequency control system with two layers is established to aggregate and dis-aggregate the inertia and droop coefficients so that frequency regulation and economical operation can be achieved. The effectiveness of the proposed method is demonstrated through numerical simulations on an IEEE 39-bus system. The results confirm the successful elimination of frequency deviations and low operating cost of the proposed approach.

Abstract:In this work, a consistency detection method is proposed, to overcome the inconsistencies in the use of large-scale lead-carbon energy storage batteries (LCESBs) and the difficulties of large-scale detection for LCESBs. Based on the chemical materials and physical mechanisms of LCESBs, the internal and external factors that affect the consistency and their characterization parameters are analyzed. The inconsistent characterization parameters, such as voltage, temperature, and resistance, are used to construct a high-dimensional random matrix and calculate the matrix eigenvalue. Single loop theorem and average spectral radius are then employed to carry out preliminary consistency detection. Next, short-term discharge experiments are conducted on individual batteries with inconsistent initial screening. The voltage and temperature data is collected, and sequential overlapping derivative (SOD) transformation is performed to extract the characteristics of voltage and temperature changes. The consistency of individual cells using the Wasserstein distance is quantitatively characterized. Finally, the reliability of the consistency detection method is evaluated by the confusion matrix. The large amounts of actual measurement data shows a false negative rate of the algorithm of 0 and an accuracy of 99.94%. This study shows that using random matrix theory for preliminary detection is suitable for processing high-dimensional data of large-scale energy storage power plants. Using SOD for precise detection can amplify the voltage, temperature, and resistance differences of inconsistent batteries, making the consistency detection more accurate.

Abstract:Modular multilevel converters (MMCs) have limited ability to withstand overcurrent. Additionally, the complex characteristics of faults make it extremely difficult to reliably identify the fault area within a short period. In this paper, the waveform of the transient current component is transformed into multiple intersecting curves in an alternative coordinate system, utilizing the principle of coordinate mapping. The internal and external faults are identified by determining the range of angles between the current waveforms and the time axis based on the intersecting regions of the curves. Subsequently, a unit protection scheme based on the transient current coordinate mapping interval is proposed. Finally, a modular multilevel converter-high voltage direct current (MMC-HVDC) system is built using PSCAD/EMTDC to validate the proposed scheme. The simulation results show that the proposed protection scheme is insensitive to factors such as current fluctuations caused by noise and distributed capacitive currents. In addition, it shows high robustness against fault resistance.

Abstract:Transmission blockage caused by structural defects of small hydropower distribution network and insufficient regulation flexibility, which has a certain coupling relationship in time series, is the main reason for water abandonment. Based on the characteristics of time-sharing multiplexing of energy storage system (ESS), utilizing the coupling relationship of water abandonment events and rationally configuring ESS can solve the problems of small hydropower transmission congestion and lack of regulatory flexibility. The causes of insuffi cient flexibility and transmission blockage resulting in water abandonment, and the coupling relationship with ESS are analyzed. A source-grid-storage collaborative planning model for small hydropower distribution net work is established. The upper-level objective is to max imize on-site consumption of small hydropower and the profits of regional small hydropower investors. The low er-level objective is to minimize average voltage offset, the input of VAR compensation system (VCS) and the num ber of transformer tap changer operation. The YALMIP solver is used to solve the optimization problems. Taking the 90 MW small hydropower unit in a certain area as an example, collaborative planning and operation analysis is conducted. The results prove that the proposed method can improve the on-site consumption of small hydropower, increase the income of regional small hydropower inves tors, and can also increase the voltage quality of the dis tribution network.

Abstract:A high-boost interleaved DC-DC converter that utilizes coupled inductors and voltage multiplier cells (VMC) is proposed in this paper. The input power supply connects to switches through the primary sides of two coupling inductors with an interleaved structure, which reduces the voltage stresses of the switches and lowers the input current ripple. Two capacitors and a diode are placed in series on the secondary side of the coupled in ductors to enhance the high boost capability. The imple mentation of maximum power point tracking (MPPT) is facilitated by the simplification of the control system through common ground. To verify the effectiveness of the proposed converter, an experimental platform and a prototype based on a turns ratio of 1 are presented. The test results show that the voltage stresses on the switches are only 1/8 of the output voltage. The operating principle and design guidelines of the proposed converter are de scribed in detail. The experimental results show that the converter is efficient and stable over a wide power range.

Abstract:Solving optimization problems plays a vital role in ensuring the secure and economic operation of distribution systems. To enhance computational efficiency, this paper proposes a general simplification and acceler ation method for distribution system optimization prob lems. Firstly, the capacity boundary and voltage bound ary model of distribution systems are established. The relative position between the two boundaries reflects the strength of capacity and voltage constraints, leading to the definition of two critical feeder lengths (CFLs) to quantify these strengths. Secondly, simplification criteria and an acceleration method are proposed. Given a distribution system, if the distance from the end load/DG node to the slack bus is less than the corresponding CFL, we can conclude that the capacity constraints are stricter than the voltage constraints. Then, the distribution system can be simplified by adopting DC power flow model or disre garding the voltage constraints. After that, the reference value tables of CFL are presented. Finally, the effective ness of the proposed method is verified by exemplifying the method in network reconfiguration and reactive power optimization problems. By implementing the pro posed acceleration method, a significant reduction in computation time is achieved while ensuring accuracy. This method applies to most urban distribution systems in optimization problems involving power flow equations or voltage constraints.