The main merit of grid-connected adjustable-speed pumped storage unit (ASPSU) is the capability to control the pumping power flexibly, which contributes to active power balance in power system operation. Physics-based dynamic modeling of ASPSU in pumping mode under power regulation is the foundation of the quantitative analysis on its transient performance, which has been studied inadequately. This study aims to derive a dynamic model of ASPSU in pumping mode which embodies reversible pump-turbine characteristics, and to conduct eigen analysis on ASPSU in pumping mode which facilitates a better understanding of its intrinsic dynamics. Firstly, the dynamic model of reversible pump-turbine is revealed to depict its regulation properties that the pumping power is adjusted by rotational speed rather than guide vane opening. Then, the state-space representation of ASPSU in pumping mode is established to be utilized with small disturbances (including indicial and ramp tests) and validated by comparing simulated results with actual records of a commissioned ASPSU in Japan. It is also effective for compensating the wind power fluctuations based on a timescale of seconds in simulation. Finally, the eigenvalues, damping ratios, and participation factors of the state variables are investigated based on the small-signal-stability model of ASPSU in pumping mode under power regulation. The results of eigen analysis indicate that the small-signal behavior of the system is characterized by electromechanical mode and electromagnetic mode, and the latter is dominant. Moreover, the damping ratio of electromagnetic mode which is sensitive to system stability needs to be increased.

Collective oscillations of free electrons generate plasmons on the surface of a material. A whispering-gallery microcavity effectively confines the light field on its surface based on the total reflection from its internal wall. When these two kinds of electromagnetic waves meet each other, the stimulated emissions from an individual ZnO microrod were enhanced more than 50-fold and the threshold was reduced after the whispering-gallery microcavity was coated with a monolayer of graphene and Al nanoparticles. The improvement of the lasing performance was attributed to the synergistic energy coupling of the graphene/Al surface plasmons with ZnO excitons. The lasing characteristics and the coupling mechanism were investigated systematically. Open image in new window

Crowbar protection and chopper protection (CCP) are commonly used hardware protection measures in the process of low voltage ride through (LVRT) of doubly-fed induction generator (DFIG). The protections operating states during fault have an important influence on the electromagnetic transient (EMT) process of DFIG. Therefore, different from only considering the influence of single protection, the coordinated control of crowbar protection and chopper protection are further considered in this paper, and a novel analysis method of CCP operation criterion is proposed by analyzing the relationship between rotor current, DC-link voltage and post-fault terminal voltage (PFTV) of DFIG. On this basis, the states of CCP under different voltage drop depths are analyzed. And an EMT analysis model of DFIG considering CCP is studied. The proposed method and EMT analysis model for DFIG are later validated through PSCAD/EMTDC-based simulation results. The results show that the method and model proposed in this paper have good accuracy and practicability, which provides more references for further study of the transient characteristics of DFIG.

Having great performance on active power regulation, grid-connected adjustable-speed pumped storage unit (ASPSU) has attracted world-wide concern. This superior transient response is achieved by both its excitation system and speed governing system, but dynamic characteristics study of ASPSU taking into account the optimal control of speed governor has seldom been explored. This paper first presents small-signal-stability model of on-grid ASPSU. Furthermore, instead of the dynamic-performance-indexes rule, a stability-degree criteria is proposed to optimize parameters for the speed regulator of ASPSU, which aims at improving the dynamic behavior of rotational speed and guide vane opening. Firstly, a mathematical model of on-grid ASPSU with power priority control is derived and it is validated by comparing simulated performance with on-site measurements of a Japanese commissioned ASPSU. Secondly, the small-signal-stability model is showcased, and the influence of control parameters on the stability degree which builds the bridge between eigenvalues and transient performance is investigated in details. Thirdly, instead of improved integral of time-weighted-absolute-error criteria, a novel stability-degree criteria is utilised in particle swarm optimization algorithm to optimise the regulator’s parameters of ASPSU. Finally, with the optimised parameters, the indicial response of a 400MW on-grid ASPSU built with PSCAD/EMTDC is significantly enhanced.

The early fault of a cable is usually a incipient fault, which is difficult to find and can easily develop into an insulation breakdown fault. Thus, it is of great significance to quickly and accurately find an incipient cable fault. This paper presents an fault location method for cable insulation based on the electric quantities of both ends. Based on the equivalent circuit model of three-phase single-core cable distribution parameters, the calculation method of the voltage along the cable is derived. The equivalent model of cable insulation in resistive and capacitive parallel is proposed and applied to the early fault modeling of cable. On the basis of the calculation formula of the voltage along the cable and the electric quantity at both ends of the cable, the analytical expression of the cable fault location is obtained by solving the fault equation directly. Extensive simulations show that the algorithm, this method can locate the incipient fault accurately, and is not affected by the fault point, fault resistance and fault inception angle.

Sustainable development of combined electricity and heat system (CEHS) can effectively facilitate the energy transition from fossil energy consumption to comprehensive utilization of multiple energy. The coupled energy system, featured with the risk of components failure and the integrated uncertain renewable energy sources (RESs), can face a safety and reliability operation challenge. Therefore, achieving the tradeoff between the economy and risk aversion for the CEHS operation is an urgent issue to be settled. This paper proposes a contingencies-based distributionally robust co-risk operation model (DRCROM) for the CEHS, which provides the failure risk sample of components applied by the non-sequenced Monte Carlo simulation (MCS) method and the penalty of energy spillage (including wind spilling and load shedding) under contingencies. Moreover, the uncertain power output risk of RESs can be addressed by using the distributionally robust individual chance-constrained (DRICC) methodology with Wasserstein metric ambiguity set. As a result, incorporating the uncertain component state and power output of RESs into the risk operation can be considered in this paper. Case studies can be conducted to give risk analyses that the average violation probability $V_{p}$ and the total co-risk dispatch cost $C^{\text {Obj}}$ are mainly affected by $\varepsilon $ . Contingencies can also increase risk indices and influence the dispatch energy and energy spillage.