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This paper proposes an optimal power flow model that takes into account the uncertainty in the probability distribution of wind power. The model can schedule controllable generators under any possible distribution of wind power to ensure the safe and economic operation of the system. Firstly, considering the incompleteness of historical wind power data, the paper models the uncertainty of wind power using second-order moments of probability distribution and their fluctuation intervals. Subsequently, a robust optimal power flow model based on probability distribution model and joint chance constraints is established. The Lagrangian duality theorem is then employed to eliminate random variables from the optimization model, transforming the uncertainty model into a deterministic linear matrix inequality problem. Finally, a convex optimization algorithm is used to solve the deterministic problem, and the results are compared with traditional chance-constrained optimal power flow model. The feasibility and effectiveness of the proposed method are validated through case study simulations.

To systematically analyze the impact of the DC frequency limit controller (FLC) configuration scheme on the frequency characteristics of asynchronous interconnected power grids with renewable energy integration, a comprehensive frequency analysis model for three-area asynchronous interconnected power grids including the FLC is proposed. The model is based on the SFR model considering renewable energy integration and includes a simplified model of the DC FLC. A rigorous validation of the rationality and stability of the model is achieved through detailed mathematical calculations and pole-zero analysis. Taking the local power grids as an example, the established model is used to study the difference between single- and double-sided configuration of the DC FLC, the relationship between the increase in the renewable energy penetration rate and the frequency characteristics of each regional power grid, and the influence of DC FLC configuration on the maximum acceptance ratio of renewable energy. Through data comparison and theoretical analysis, the influence of FLC configuration on the frequency characteristics of each region of the asynchronous power grid under renewable energy integration is obtained. The results show that the configuration of a bilateral FLC on the DC line has more advantages in alleviating the complementary power disturbance at both ends and obvious limitations on the non-complementary power disturbance at both ends. Compared with the configuration of the bilateral FLC, the configuration of a unilateral FLC on the weak grid has a larger proportion of renewable energy acceptance. When the DC FLC power transmission capacity is sufficient, the weak grid and the adjacent two asynchronous grid DC tie lines are configured with a single-sided FLC, and the weak grid is theoretically connected to the renewable energy penetration rate of up to 100%.

In the evolving landscape of power systems, the integration of various renewable energy resources (RERs) introduces complex challenges, particularly in maintaining power quality, which are paramount for system stability. To address this issue, an adaptive power quality disturbance (PQD) detection framework is implemented in this paper. First, the optimal mode decomposition (OMD) is developed to decompose the compound PQDs into sub-ingredients to make them more visible based on the optimal energy ratio. Subsequently, we propose an improved attention convolutional neural network (IACNN), an advanced neural network architecture that leverages an enhanced attention mechanism to expedite the identification of PQDs. Importantly, the sub-ingredients can be strengthened based on the established PQD detection framework. Finally, a series of experiments are conducted under different noise levels and various types of PQDs. The results demonstrate that the proposed framework has profound detection effectivity with about 99.2% accuracy under the simulation condition of 20 dB noise level. In addition, the experimental verification analysis proves a satisfactory real-time performance. This underscores the potential of the proposed framework as a significant advancement in the realm of power quality management, offering a robust solution to the challenges posed by the integration of RERs into modern power systems.

With the rapid development of photovoltaic and wind power, the penetration rate of renewable energy in power system is gradually increasing. This escalation poses a challenge as it results in a continuous reduction in the inertia and damping of the power grid, accentuating the issue of frequency stability in power systems. It is one of the effective measures to deal with this risk that renewable energy sources such as wind turbines actively provide frequency support for power grid. This paper carries out research into the examination of wind turbines’ capacity to contribute to system frequency support, considering two aspects: inertia support and primary frequency regulation capabilities. Subsequently, the frequency control method of the wind turbine support system is analyzed, emphasizing the roles of rotor kinetic energy control and power reserve control in facilitating frequency support. The evaluation of the transient frequency support capability of wind turbines is introduced, incorporating factors such as control methods, controller parameters, and the duration of transient frequency support. Key metrics, including accumulated energy and frequency change rate indices during the transient frequency support stage, are proposed to quantitatively assess the transient frequency support capability of wind turbines. These indices provide a comprehensive framework for the quantitative evaluation of wind turbine transient frequency support capabilities.

With the rapid development of offshore wind power and large-scale grid connection, the mechanical inertia and frequency regulation ability of the power system are greatly reduced, which seriously affects the frequency stability of the receiving end power grid. Aiming at the frequency stability analysis of large-scale offshore wind power connected to the receiving end power grid, this paper proposes a frequency response aggregation model which includes the frequency limiting controller (FLC) and the wind turbine generator (WTG) with frequency modulation (FM) capability. The rationality of model aggregation and the stability of transfer function are proved by theoretical analysis. Taking a southern province in China as an example, the effects of wind turbine generator access form and DC block capacity on different frequency response indexes are analyzed by using the proposed frequency response aggregation model. Through theoretical and simulation analysis, the correlation of system inertia, FM capacity and DC FLC capacity with frequency deviation nadir and quasi-steady state frequency index is obtained, and the relevant conclusions affecting frequency stability indexes are drawn. Finally, through the model simulation method, the improvement of the system frequency stability when the wind turbine provides inertia and primary FM support is analyzed. This paper concludes that when the wind power penetration is in the range of 10%∼40% and the wind power assisted frequency modulation capacity reaches 5% of the installed capacity, the minimum frequency of the system can be maintained above 49.5 Hz.

Power supply for IGBT drive circuit of HVDC circuit breaker is a great challenge in practical engineering. At present, most of the HVDC circuit breakers rely on external power sources. This paper proposes a self-powered hybrid HVDC circuit breaker. The HVDC circuit breaker consists of an ultra-fast mechanical disconnector (UFD), a load commutation switch (LCS) and a main circuit breaker (MB). The LCS and the MB are formed by series connection of enhanced half-bridge sub-modules (EHBSMs). By this design, the energy stored in the EHBSM capacitor can be used for IGBT triggering. Thus, the self-powered function of the HVDC circuit breaker is realized. Three operation modes of the HVDC circuit breaker, including the star-up mode, the steady-state operation mode and the DC fault handling mode are analysed in detail. Finally, a four-terminal HVDC model is constructed in PSCAD simulation software. The simulation results verify the feasibility and effectiveness of the self-powered HVDC circuit breaker.

Abstract Traditional permanent magnet synchronous generator (PMSG)‐based wind turbine (WT) normally utilises phase lock loop (PLL) for obtaining accurate phase angle of AC grid to make sure of maximum power point tracking (MPPT). However, a weak AC system with a low short circuit ratio (SCR) and high control bandwidth of PLL might have impaired effects on the stability of the system. To overcome these, a novel power synchronous control of PMSG‐based WT with grid forming ability of PMSG for integrating very weak AC system is first proposed in this study. Grid‐side converter (GSC) of PMSG emulates the inertia response of synchronous generator (SG) by artificially coupling the DC‐link voltage with the grid frequency based on the certain droop property, while the converter voltage magnitude is regulated according to DC‐link voltage deviation by controlling the modulation index of GSC. This novel scheme can realise the grid synchronisation without PLL by mimicking the natural characteristic of SG. Small‐signal analysis of the proposed scheme indicates the DC‐link capacitor energy can be exerted for system inertia support during system disturbances, while the system damping can be provided by spontaneously alternating converter voltage magnitude. More importantly, MPPT property of PMSG is not compromised while providing inertia and damping for the system, and the related inertia and damping coefficient of WT with the proposed control can be easily tuned for satisfying the system requirement. Non‐linear simulations of one PMSG connected with one large SG considering a sudden change of active power reference and variable wind speeds have been studied to verify the effectiveness of the proposed grid‐forming control.

To better utilize the diversity of renewable energies in the U.S., this paper proposes a cross-seam hybrid multi-ter-minal high-voltage direct current (MTDC) system for the integration of different types of renewable energies in the U. S. Based on a developed station-hybrid converter design, the proposed hybrid MTDC system further investigates the connection methods of renewable energies and develops novel flexible power flow control strategies for realizing uninterrupted integration of renewable energies. In addition, the frequency response control of the hybrid MTDC system is proposed by utilizing the coordination between the converters in the hybrid MTDC system. The feasibility of the hybrid MTDC system and the performance of its corresponding control strategies are conducted in the PSCAD/EMTDC simulation. The simulation results indicate that the proposed hybrid MTDC system could realize the uninterrupted integration of renewable energies and flexible power transmission to both coasts of U.S.

High-dynamic-range (HDR) image has a wide range of applications, but its access is limited. Multi-exposure image fusion techniques have been widely concerned because they can obtain images similar to HDR images. In order to solve the detail loss of multi-exposure image fusion (MEF) in image reconstruction process, exposure moderate evaluation and relative brightness are used as joint weight functions. On the basis of the existing Laplacian pyramid fusion algorithm, the improved weight function can capture the more accurate image details, thereby making the fused image more detailed. In 20 sets of multi-exposure image sequences, six multi-exposure image fusion methods are compared in both subjective and objective aspects. Both qualitative and quantitative performance analysis of experimental results confirm that the proposed multi-scale decomposition image fusion method can produce high-quality HDR images.

In this paper, a control method called synchronous generator imitation control (SGIC) as well as a dynamic power sharing for distributed power generation systems (DPGSs) are presented. In order to imitate the behavior of a synchronous generator (SG), the SGIC method for the voltage source converter (VSC) station is proposed. The SGIC includes three loops, namely active power and frequency loop (Pf loop), reactive power and voltage loop (QU loop) as well as inner current loop. The Pf loop is used to emulate the swing equation of the SG. The exciter of the SG is mimicked by the QU loop. The inner current loop is developed for fast current and voltage regulations as well as current limiting. The system stability performances are analyzed through small-signal model, and the effectiveness of the proposed control is validated by PSCAD/EMTDC simulations. The results show that the Pf loop of the VSC emulates the motion equation of the SG rotor very well, which makes the VSC station to have inertia just as a SG; the VSC station can maintain a stable output voltage and regulate the reactive power at the same time; through the dynamic power sharing, precise power control, frequency offset elimination and system stability improvement are achieved; the system has the merits of fault ride-through capability as well as good dynamic performance.

The urban power distribution system is one of the most complex artificial systems in the world [...]

This paper proposed a second harmonic capacitor voltage eliminating controller (SHCVEC) for capacitor voltage fluctuation suppression of modular multilevel converters (MMCs). The proposed controller consists of two PI controllers, namely the outer controller and the inner controller. The outer controller determines the reference value of the circulating current, and the inner controller generates the circulating current in each arm. The second harmonic capacitor voltage is controlled to zero by the outer PI controller, so there will be no static error between the generated second harmonic capacitor voltage and its reference value, which is the main improvement compared to other conventional methods. The PSCAD/EMTDC simulation verification of SHCVEC is carried out based on a 2-terminal MMC-HVDC system, and the effectiveness and feasibility of the proposed controller are proved. Furthermore, a comparison in terms of capacitor voltage fluctuation and rms arm current is made between the SHCVEC and the conventional circulating current suppressing controller, and the results demonstrate that the capacitor voltage fluctuation could be significantly reduced with only a small increase of rms arm current.

The commutation overshoot is an important reference index for the insulation coordination of the converter station as well as the thyristor voltage capacity. Although the dispersity of reverse recovery charges of series thyristors may affect commutation overshoots, especially those at the thyristor level, little literature has tried to analyse it in detail. Firstly, this study compares the turn-off models of independent thyristor and elaborates their applicability to series thyristors. Then, a novel model for series thyristors during the turn-off process is proposed, based on which the turn-off model of converter valve is also established. Subsequently, the validity and efficiency of the proposed model are discussed by simulations and it is applied to the analysis of commutation overshoots. The analysis shows that though the average model can be used to estimate the valve voltage stresses, it may lead to large errors in the thyristor voltage stress. And selecting the 1:n − 1 distribution as the distribution mode of reverse recovery charge in series thyristors will result in very conservative results.

As a key element of the DC grid, the technology of DC circuit breakers (DCCBs) still have difficulty to be widely used due to its large cost and volume. This paper studies the basic principle and its realisation methods for DCCBs. First, two basic ways of breaking the fault current are discussed, including the way by inserting a metal oxide varistor (MOV) to force the fault current to zero and the way by inserting capacitor to change the nature of the fault current. Second, a new approach to realise the DCCB has been proposed based on the inserting capacitor way. According to this new approach, three topologies of DCCB are proposed, including the single-branch DCCB with inserted capacitor (SB-DCCB-C), the double-branch DCCB with inserted capacitor (DB-DCCB-C) and the triple-branch DCCB with inserted capacitor (TB-DCCB-C). All these three DCCBs are proved to be able to deal with DC faults effectively according to the simulation results in PSCAD/EMTDC. Finally, the economy of four circuit breaker structures are compared.

This paper studies the electromechanical transient model and the control strategy of line commutated converter (LCC) and modular multilevel converter (MMC) based decentralized hybrid High Voltage Direct Current (HVDC) Transmission systems. The decentralized hybrid HVDC system is a new type of topology, and the related electromechanical transient model and control strategy have not been studied well. In this paper, the electromechanical transient model of a decentralized hybrid HVDC system is devloped through mathematical deduction. This model can be easily implemented in electromechanical transient simulation software and meet the time domain simulation requirements of large-scale systems. Then, in order to ensure the safe absorption of the DC power under various conditions, an optimal power flow model considering the decentralized hybrid HVDC system is proposed. Finally, the electromechanical transient model proposed in this paper is verified by the electromagnetic transient model, and the control strategy is validated in a modified New England 39-bus system.

A method for electromechanical modeling of line commutated converter (LCC)-modular multilevel converter (MMC)-based hybrid multi-terminal High Voltage Direct Current Transmission (HVDC) systems for large-scale power system transient stability study is proposed. Firstly, the general idea of modeling the LCC-MMC hybrid multi-terminal HVDC system is presented, then the AC-side and DC-side models of the LCC/MMC are established. Different from the conventional first-order DC-side model of the MMC, an improved second-order DC-side model of the MMC is established. Besides considering the firing angle limit of the LCC, a sequential power flow algorithm is proposed for the initialization of LCC-MMC hybrid multi-terminal HVDC system. Lastly, simulations of small scale and large scale power systems embedded with a three-terminal LCC-MMC hybrid HVDC system are performed on the electromechanical simulation platform PSS/E. It is demonstrated that if the firing angle limit is not considered, the accuracy of the power flow solutions will be greatly affected. Steady state calculation and dynamic simulation show that the developed LCC-MMC hybrid MTDC model is accurate enough for electromechanical transient stability studies of large-scale AC/DC system.

In this paper, the DC short-circuit fault and corresponding clearance solutions of modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems are analyzed in detail. Firstly, the analytical expressions of DC fault currents before and after blocking the MMC are derived based on the operation circuits. Before blocking the MMC, the sub-module (SM) capacitor discharge current is the dominant component of the DC fault current. It will reach the blocking threshold value in several milliseconds. After blocking the MMC, the SM capacitor is no longer discharged. Therefore, the fault current from the AC system becomes the dominant component. Meanwhile, three DC fault clearance solutions and the corresponding characteristics are discussed in detail, including tripping AC circuit breaker, adopting the full-bridge MMC and employing the DC circuit breaker. A simulation model of the MMC-HVDC is realized in PSCAD/EMTDC and the results of the proposed analytical expressions are compared with those of the simulation. The results show that the analytical DC fault currents coincide well with the simulation results.

With the increasing utilization of power electronic equipment in power systems, there has been an increase in the occurrence of oscillatory behavior from unknown sources in recent years. This paper puts forward the concept of electric network resonance stability (ENRS) analysis and tries to classify the above-mentioned oscillations into the category of ENRS. With this method, many complex power system oscillations can be analyzed with the linear network theory, which is mathematically mature. The objective of this paper is to establish a systematic approach to analyze ENRS. By introducing the s-domain nodal admittance matrix (NAM) of the electric network, this paper transforms the judgment of ENRS into the zero-point solution of the determinant of the s-domain NAM. First, the zero-points of the determinant of the s-domain NAM are proved to correspond to the eigenvalues of the system. Then, a systematic approach is proposed to analyze ENRS, which includes the identification of the dominant resonance region and the determination of the key components related to resonance modes. The effectiveness of the proposed approach for analyzing ENRS is illustrated through case studies.

As the penetration of the integrated intermittent and fluctuating new energy (e.g., wind and photovoltaic power) increases, the conventional grid- following voltage source converter (VSC)-based high voltage direct current (HVDC) transmission system faces the problem of interactive instability with the grid. A novel grid-forming control strategy is proposed to overcome these issues, which adopts the dynamics of a DC capacitor to realize the function of self- synchronization with the grid. Moreover, the per-unit DC voltage can automatically track the grid frequency, acting as a phase-locked loop. Next, the small-signal model of the grid-forming VSC-HVDC system is established, and the stability of the system is analyzed using the eigen-value analysis method and the complex power coefficient method. In addition, the stabilization controller is proposed for the grid-forming (GFM) control structure, which further enhances the grid-forming VSC-HVDC system's stability and helps it operate stably under both stiff and weak grid conditions. Research results show that the VSC-HVDC system under the proposed grid-forming control can work stably in both stiff and weak grids. The grid-forming VSC-HVDC system is robust and can maintain stable operations with a large range variation of the parameters in the current and voltage control loop. Simulations are carried out on the PSCAD/EMTDC platform to verify the proposed grid-forming control strategy. [ABSTRACT FROM AUTHOR]