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Introduction: This paper proposes a sigmoid-based adaptive inertia control strategy for grid-forming (GFM) inverter to enhance frequency stability.Methods: Firstly, the frequency response characteristics under different disturbances are analyzed theoretically. Then, to solve the problem that fixed inertia leads to a contradiction between the maximum frequency deviation and the setting time, a non-linear inertia regulator is investigated. This non-linear inertia regulator, which is based on an improved sigmoid function, is applied to achieve real-time inertia by the frequency deviation. Moreover, the full-order small-signal model of the system containing the nonlinear inertia regulator is established and thus the stability of the proposed strategy is analyzed.Result and Discussion: The proposed control strategy is implemented without derivative action, which may suffer from high-frequency noises. Finally, the hardware-inthe-loop (HIL) results verify the remarkable performance of the proposed control strategy.

Combined with Empirical Mode Decomposition method(EMD) and travelling-wave protection, a strategy is put forward to precisely locate the fault point by calculating the distance between converters and it. After the fault occurred, signals of line current were collected and separated into different functions each with single frequency component through EMD method. Then, functions were transformed into time spectrum which clearly exhibited the relationship among time, frequency and the signal. The time when travelling waves arrived the monitoring point was thus confirmed and the fault distance was obtained afterwards. Performances of the proposed criterion show a high degree of accuracy through numerical simulations. The strategy effectively realizes the full-length protection and the tolerance under different transition resistances in fault locating problem avoiding complex calculation or additional modules.

Due to the increasing penetration of the uncertain single-phase distributed generations (DGs), the current distribution networks are becoming more uncertain, unbalanced, and complicated than ever before, which brings great challenges for distribution network operators. This study proposes an adaptive robust optimal reactive power dispatch approach for the unbalanced distribution networks (U-DNs) considering uncertainty caused by DGs. Leveraging the reactive power compensation from inverters of DGs, the purpose of the proposed method is to minimise power losses and maintain the voltage within regulatory limits. The feasible region of DGs is estimated and considered as a new constraint, aiming to guarantee the reliable operation of U-DNs. The optimal reactive power dispatch problem is then formulated as an adaptive robust optimisation problem based on semidefinite programming. The adaptive function, which derives the relationship between reactive power and active power outputs of DGs, is utilised to make the method more flexible and less conservative. The cutting plane algorithm is introduced to solve the proposed adaptive robust reactive power dispatch model efficiently. Moreover, case studies are separately conducted on the modified IEEE 13-bus and 123-bus test systems to demonstrate the effectiveness of the proposed method.

Converter blocking in the case of pole-to-pole short circuit of the direct current (DC) distribution network can cause large-scale power outage and slow system recovery. Given this issue, a scheme of short circuit protection between poles of the DC distribution network based on the cooperation of fault current limiting module and DC circuit breaker is proposed in this paper. According to the inductive current limiting topology, the transient characteristics of the bridge arm current and the fault current on the DC side of the converter are analyzed, and the theoretical calculation method and selection principle of the current limiting inductor parameters are given. The proposed protection scheme can realize the fault ride-through of the converter during the pole-to-pole short circuit, reduce the scope of power outage and quickly restore the system. The simulation results under the PSCAD/EMTDC environment verify the correctness and feasibility of the protection scheme.

This letter proposes a new design of frequency-locked loop (FLL) which is based on synchronous (dq) reference frame instead of stationary ({\alpha}\b{eta}) reference frame. First, a synchronous reference frame FLL (briefly called SRF-FLL0) equivalent to the conventional FLL is proposed. Then the SRF-FLL0 is improved by utilizing the phase error to acquire a better performance. The small-signal modeling and parameter tuning of the improved synchronous reference frame FLL (SRF-FLL) are presented. Finally, the theoretical analysis and experiment results verify the superiority and effectiveness of proposed SRF-FLL., Comment: 4 pages, 6 figures

Virtual synchronous generator (VSG), an emerging technique that has been proposed for its possible grid-friendly features in these years. Since VSGs mimic synchronous generators with power electronic devices, they obtain both the high frequency converter’s features and low frequency characteristics of the synchronous generator. Full-band output impedance model of the VSG should be investigated for further stability analysis before large scale VSG applications. This paper provides an in-depth investigation of the VSG, whose full-band output impedance model in dq framework is proposed, derived and verified, leading to the extraction of information for system stability.

The modular multilevel converters (MMCs) are attractive for high-voltage direct current (HVDC) transmission system. However, due to the complicated structure of MMC, which normally consist of thousands of sub-modules, how to accurately and efficiently model the MMCs is one of the key technical challenges. In this paper, based on sub-module modelling method, a circuit-oriented average-value model of MMC involving switching-cycle averaging is proposed. Different from traditional MMC model, which is based on the modelling of converter bridge and the dynamic performance of the sub-modules in which is ignored, for the proposed model in this paper the chopper-cells are replaced by controlled voltage and current sources. As a result, the proposed model can not only accelerate simulation speed but also describe the characteristics of sub-modules, such as the dynamic voltage-balancing process of sub-module capacitors. Then static synchronous compensator based on the modular multilevel converters (MMC-STATCOM) is simulated in PSIM simulations. Both the circuit-oriented average-value model and switching model are established to simulate the steady state of the system, as well as the procedures of one sub-module failure and transient of AC current command. The validity, rapidity, accuracy, intuitiveness and compatible reconfiguration of the presented model are well verified by simulations.

Established a set of common practical and strong operational post-project evaluation system the method for distribution network project, and also the post-project evaluation index system; According to the Analytic Hierarchy Process (AHP) method was improved, the average value method fixed judgment matrix, get the index weight at all levels; comprehensive evaluation for distribution network project is made by level 2 three layers of fuzzy comprehensive evaluation model. Applied to a distribution network project, the paper give the objective scientific post-project evaluation process, and completed the comprehensive evaluation of this project. the system and method of distribution network post-project evaluation is validated scientific and practical, and play an improving role for distribution network project decision-making and management level.

With a large number of nonlinear and asymmetric loads being connected to the power system, power quality problem has been increasingly severe. Therefore, sufficient attention shall be paid to power quality monitoring. Based on the Power Quality Management section of the latest IEC 61850 2nd editions, this paper established information model of power quality monitoring device. On this basis, hardware and software platforms are designed and developed to realize the mapping of Abstract Communication Service Interface (ACSI) and Manufacturing Message Specification (MMS). The development platform of power quality monitoring device, inside which VxWorks operating system and MMS-EASE Lite software have been put, and the platform uses AT91RM9200 as the core chip to parse the information model, and restore the tree structure of the information model by creating the communication links between power quality monitoring device and IEDScout client software.