As the penetration of power electronic converters and renewable generation increases, voltage and current waveforms in power systems are increasingly distorted and nonstationary. Accurate estimation and fast tracking of electric power quantities are essential for energy metering and billing, online monitoring, and safe operation of power systems. However, most existing estimation methods assume that the waveform within an observation window is stationary and periodic, which often leads to a slow dynamic response under nonstationary conditions and introduces estimation errors when the fundamental frequency deviates from its nominal value. To address these challenges, this paper introduces a framework termed operating point fluctuations (OPF). In the OPF framework, a nonstationary waveform is reinterpreted by treating the time-varying fundamental component as a dynamic operating point and describing all distortions as fluctuations around it. Based on this framework, a set of single-phase and three-phase power quantity expressions is derived. Furthermore, an estimation method is proposed. Comprehensive simulation studies demonstrate that the proposed method achieves high accuracy under stationary conditions, fast dynamic tracking under nonstationary conditions, low computational burden, and robustness to noise and fundamental frequency deviation. Validation using field measurement data further confirms these results and demonstrates its suitability for practical applications.

With the increasing integration of power electronic converters and renewable energy resources, power system voltage and current waveforms have become increasingly distorted and nonstationary. Rapid tracking of power quality (PQ) indices is crucial for the safe operation of equipment. Conventional Fourier transform (FT)-based methods suffer from limited accuracy under nonstationary conditions because of spectral leakage, while existing time-frequency methods still suffer from limited estimation accuracy and high computational burden. To address these challenges, this paper introduces a conceptual framework termed operating point fluctuations (OPF), in which the time-varying fundamental component is regarded as a dynamic operating point and all distortions are modeled as fluctuations around it. Based on this framework, a set of instantaneous PQ indices is reformulated, and a real-time estimation method is proposed using recursive least squares (RLS) filters and the Hilbert transform (HT). Simulation studies demonstrate that the proposed method achieves high steady-state accuracy, fast dynamic tracking, robustness against a certain degree of frequency estimation error, interharmonics, and noise, and low computational burden. Hardware-in-the-loop (HIL) experiments and field measurement data verification further demonstrate its practical potential.

The harmonics and interharmonics in the power system is a critical issue of power quality, which not only undermine the economic efficiency of system operation but also pose a potential threat to system safety. In light of the increasingly complex characteristics in future power loads, it becomes imperative to establish a fair determination of dominant harmonic sources and harmonic responsibilities. By adopting the time domain analysis method that transcends specific frequency domains, harmonics and interharmonics can be examined with greater intuitiveness and interpretability. A method is proposed based on time domain power analysis to evaluate the harmonic responsibility in cases where voltage and current contain harmonics and interharmonics at the point of common coupling (PCC). The extensibility of the proposed method renders it suitable for application across theories that possess orthogonal decomposition properties. Simulation comparison results illustrate that the proposed method efficiently assesses the harmonic and interharmonic responsibilities of load branches, and possesses significant potential for practical application.

AbstractMost of the existing methods for harmonic analysis are from frequency‐domain perspective. In fact, the essential factor for generating harmonics is non‐linear characteristic or time‐varying property. Therefore, the harmonic generation mechanism and the harmonic source location method are investigated from the time domain perspective in this paper. Firstly, a general model of non‐harmonic sources is established. The non‐harmonic sources will match well with the proposed general model while the harmonic source will not. Then correlation coefficient that reflects compliance degree between each device and the general model is defined for distinguishing harmonic sources from non‐harmonic sources. On this basis, a novel time‐domain harmonic source location method is proposed. Numerous simulations and experiments have demonstrated that the proposed method has good performance under fluctuations, saturations, pre‐distortions, transient process, and resonances. The defined correlation coefficient can reflect non‐linearity/time‐varying degree of the harmonic source, which can be used for evaluating harmonic emission level of each harmonic source. On this basis, a simple harmonic responsibility division method is proposed, which is immune to possible fluctuations, pre‐distortions, saturations, resonances, and impedance variation.

High voltage dc grids are raised high-speed operation and high reliability requirement for relay protection. This article aims at proposing a protection scheme with high operation speed, high sensitivity, and high reliability. First, the symbolic expressions of the line-mode fault component traveling wave in case of the internal and external faults are derived, which consist of the traveling wave propagation term and the dc line boundary term. It is theoretically demonstrated that the index of the traveling wave propagation term in case of internal faults is greater than that for the external faults, and the index is independent of fault impedance. Second, the Levenberg-Marquardt algorithm is introduced to extract the index and an index-based nonunit protection method as well as the protection scheme is proposed. The proposed method is theoretically robust and independent of fault impedance and fault type. In the validation, the fitting results are indicated that the symbolic expressions are correct and the extracted indices are effective to characterize the location of the faults. Numerous fault cases of different fault distances, fault impedances, and fault types have demonstrated that the proposed protection scheme is of high sensitivity, high operation speed, and high reliability.

The power capture performance of an adaptive bistable point absorber wave energy converter (WEC) in irregular waves is investigated in this paper. Based on the linear potential flow theory and Cummins equations, the equation of motion for the WEC is established in time domain. Then a parametric study is performed on the power capture performance of the WEC by considering the influences of different wave and system parameters such as the spring combination parameter, the stiffness of auxiliary springs and main springs, the original length value of the main spring, damping coefficient of the PTO systems and the significant wave height. The results show that in an irregular wave, each parameter will have a certain impact on the performance of the device. Appropriate device parameters need to be selected according to actual environmental parameters to ensure that the adaptive bistable WEC has better power capture performance than its linear and conventional bistable counterparts.

DC micro-energy system is an effective pattern to integrate high-penetration distribution generators. It has flexible operation modes and complicated fault characteristics, which requires protection with higher selectivity and sensitivity. This paper proposes a DC feeder protection method using the transient high-frequency currents. The fault direction and fault feeder are identified by comparing the amplitude of high-frequency currents of all ends. The amplitude ratio coefficient of DC voltages is introduced to detect the fault pole. The transient high-frequency components will not be affected by the communication delay and fault impedance. The protection scheme realizes the fast detection and clearance of different faults on feeders before the failure of the inverter-interfaced generators and loads, which ensure the reliable and safe operation of the non-fault zone. The model of a DC micro-energy system is established in MATLAB/Simulink and the efficiency of this method is verified by detailed simulations.

Abstract The multi‐terminal hybrid line commutated converter (LCC)/modular multilevel converter (MMC)‐based high voltage dc (HVDC) has been applied to solve the simultaneous commutation failure problem in East China. Here, the propagation of the travelling wave is characterised by transfer functions and the characteristic of the fault current travelling wave in the multi‐terminal hybrid LCC/MMC HVDC system is analysed. Then a directional pilot protection for the multi‐terminal hybrid LCC/MMC HVDC system is proposed. The proposed method is implemented to the designed HCM5000 dc control and protection device and verified in the Kun–Liu–Long three‐terminal hybrid LCC/MMC HVDC real‐time digital simulator (RTDS) test system. The test results show that the proposed method meets the engineering requirement of multi‐terminal hybrid LCC/MMC HVDC system for line protection.

Due to the absence of the natural current zero-crossing point, the dc interruption has been a critical issue in the dc system. The interruption schemes currently used are based on power electronics or precharged capacitors, leading to the high cost and complicated control strategy. In this article, a passive oscillation dc load current breaking scheme based on a vacuum interrupter is proposed. The external transverse magnetic field (ETMF) is applied to the vacuum interrupter to excite the oscillation of the arc voltage. According to the spectrum analysis of the arc voltage, a passive LC path is designed to realize resonant oscillation with the interrupter, by which 2 kA/10 kV interruption is realized within 2 ms. The influence of the gap separation, ETMF intensity, and arcing energy on the resonant oscillation process is investigated. At last, the practicability of applying the proposed scheme in HVdc interruption is discussed.