This paper proposes a multi-source multi-product framework for coupled multi-carrier energy supplies with a biogas-solar-wind hybrid renewable system. In this framework, the biogas-solar-wind complementarities are fully exploited based on digesting thermodynamic effects for the synergetic interactions of electricity, gas and heating energy flows, and a coupling matrix is formulated for the modeling of production, conversion, storage, and consumption of different energy carriers. The multi-energy complementarity of biogas-solar-wind renewable portfolio can be utilized to facilitate the mitigation of renewable intermittency and the efficient utilization of batteries, and a multi-carrier generation scheduling scheme is further presented to dynamically optimize dispatch factors in the coupling matrix for energy-efficient con-version and storage, while different energy demands of end-users are satisfied. The proposed methodology has been fully tested and benchmarked on a stand-alone Microgrid over a 24-hour scheduling horizon. Comparative results demonstrate that the proposed scheme can lower the battery charging/discharging actions as well as the degradation cost, and also confirm its capability to accommodate high penetration of variable renewables.

This paper proposes an adaptive droop control (ADC) strategy for the voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) system, which enables the VSC-station to provide frequency regulation for onshore ac grids. A V-I-f characteristic is derived to establish the relationship between the frequency deviation and dc voltage. A VSC-station working with the V-I-f characteristic is able to adjust its dc voltage reference autonomously according to the grid frequency deviation. Thus, the power flow of the VSC-MTDC system will be redistributed and more power is going to be injected into the adjacent grid of which the frequency deviation exceeds the threshold value. The ADC strategy inherits the advantage of the traditional droop control (TDC) that there is no need for communication system. An ac/dc hybrid system that includes VSC-MTDC system, onshore grids, and offshore wind farms is built in DIgSILENT/Powerfactory. Modal analysis and nonlinear simulations are performed to obtain the optimal control parameters and demonstrate the performance of the ADC strategy.

With the rapid increase of renewable energy sources and power electronic converters in the power system, the inertia of the power system is declined and the stability problem is more serious than ever before. In this paper, a virtual synchronous generator (VSG) control strategy is proposed to enable the voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) system to damp the low-frequency oscillation modes. Both rotational inertia part and governor part are modeled in the VSG controller. The V ² -P-ω characteristic is developed based on the V ² -P droop control to emulate the action of the speed governor when the frequency of ac grid deviates from the nominal value. The virtual inertia part is used to emulate the inertia response of the synchronous generator. The communication between VSC stations is unnecessary with the autonomous power allocation ability of the V ² -P-ω characteristic. Modal analysis is utilized to test the system stability and optimize the controller parameters. Two test systems which include offshore wind farms, VSC-MTDC systems, and onshore power grids are built in the DIgSIENT/PowerFactory. The simulation results verify the effectiveness of the proposed VSG strategy.

In this paper, a traveling wave based fault location method is proposed for distribution networks. Before fault occurs, an intrinsic distance difference matrix (IDDM) is established based on the characteristics of traveling wave propagation and the topology of distribution network. After the fault, the initial fault traveling wave signals are decomposed into several intrinsic mode function (IMF) signals by variational mode decomposition (VMD), and the first IMF signal is analyzed by teager energy operator (TEO) to determine the arriving time of initial fault traveling waves. According to the double-ended traveling wave method and the arrival time of traveling wave at all recorders, a fault distance difference matrix (FDDM) is built. By comparing the difference between IDDM and FDDM, a fault branch determination matrix (FBDM) and the lateral index Q are used to locate the fault. The effectiveness of the proposed method is verified via EMTP/ATP simulations. Results show that the proposed method has a high fault location accuracy, and does not need that all terminals of lines are installed traveling wave recorders.

In this paper, a flexible voltage control strategy, which takes good use of the distributed energy storage (DES) units, is proposed to enhance the voltage stability and robustness of dc distribution network. The characteristics of ac/dc interface in network are analyzed, and the virtual inertia and capacitance are given to demonstrate the interactive influence of the ac and the dc systems. The control strategy for DES which is located at the ac microgrid or at the network terminal bus is designed based on the interactive characteristics, enabling the DES to respond to both voltage variation of dc network and frequency change of utility ac grid. A cascading droop control strategy is proposed for DES in dc microgrid to relieve the pressure of voltage deterioration of dc network buses which connect the dc microgrid. The proposed comprehensive flexible control strategy for DESs at different interfaces features independence of communication as well as enhancement of system robustness, and reduces the impact of dc distribution network on utility ac grid. The performance of the proposed control strategy is validated under different operating conditions of the dc distribution network.