The underwater wireless sensor networks (UWSNs) have been applied in lots of fields such as environment monitoring, military surveillance, data collection, etc. Deployment of sensor nodes in 3D UWSNs is a crucial issue, however, it is a challenging problem due to the complex underwater environment. This paper proposes a growth ring style uneven node depth-adjustment self-deployment optimization algorithm (GRSUNDSOA) to improve the coverage and reliability of UWSNs, meanwhile, and to solve the problem of energy holes. In detail, a growth ring style-based scheme is proposed for constructing the connective tree structure of sensor nodes and a global optimal depth-adjustment algorithm with the goal of comprehensive optimization of both maximizing coverage utilization and energy balance is proposed. Initially, the nodes are scattered to the water surface to form a connected network on this 2D plane. Then, starting from sink node, a growth ring style increment strategy is presented to organize the common nodes as tree structures and each root of subtree is determined. Meanwhile, with the goal of global maximizing coverage utilization and energy balance, all nodes depths are computed iteratively. Finally, all the nodes dive to the computed position once and a 3D underwater connected network with non-uniform distribution and balanced energy is constructed. A series of simulation experiments are performed. The simulation results show that the coverage and reliability of UWSN are improved greatly under the condition of full connectivity and energy balance, and the issue of energy hole can be avoided effectively. Therefore, GRSUNDSOA can prolong the lifetime of UWSN significantly.

Recently, virtual synchronous generators (VSGS) are a hot topic in the area of microgrid control. However, the traditional fixed-parameter-based VSG control methods have an obvious disadvantage. Namely, if the damping value is set to be small, the amplitude of frequency deviations under external power disturbances is large, meaning that the frequency suppression capacity is insufficient, but if the damping value is large, the dynamics of the system will be greatly sacrificed. To solve the problem, taking the dynamic characteristics and the maximum allowable frequency deviation (MAFD) into account, in this paper an improved fuzzy adaptive damping-based VSG control strategy is proposed to simultaneously attenuate the microgrid frequency fluctuations and guarantee the system dynamics. Firstly, in order to address the necessity of using an adaptive damping-based VSG, the structure of a fixed-parameter VSG method that incorporates the f-p/Q-V droop controllers is introduced, based on which a small signal model is established to discuss the impacts of the virtual damping on the frequency response characteristics concerning the different penetration levels of power disturbances. Then, considering the dynamics and MAFD, a fuzzy adaptive controller is constructed relying on the well-designed membership functions, control rules and output scaling factors. The main feature of the improved fuzzy controller is that two alternative output scaling factors are employed to allow the system to be overdamped when the frequency deviation is large and undamped when the frequency deviation is small, balancing the frequency response dynamics and stability characteristics. To verify the effectiveness of the proposed fuzzy adaptive damping-based VSG technique, a computer simulation is conducted on a microgrid system in MATLAB/Simulink, and the obtained results are compared with the conventional droop control and fixed-parameter based VSGs. By using the proposed fuzzy adaptive damping-based VSG control method, the peak frequency deviations under the large power disturbances would become at least 8% lower compared to the traditional droop control and fixed-parameter VSG control, and meanwhile, the frequency response speed is fast when the disturbance stands at a low position. Consequently, it is valuable to promote the proposed techniques in engineering.

In order to reduce the torque and flux ripples of interior permanent magnet synchronous motor (IPMSM) drives, a variable structure speed controller with integral-operation switching surface for direct torque controlled IPMSM has been proposed. The variable structure control law is derived from the proposed integral-operation switching surface and the system equations of direct torque control (DTC) IPMSM. Furthermore, integral-operation switching surface can possess the exponential stability for variable structure control (VSC) by using its integral component. Moreover, the dynamic response of the variable structure speed controller is studied. Then, the chattering which is a very significant problem in variable structure control is investigated. In this paper, we proposed a proper continuous function to substitute the traditional discontinuous sign function to obtain a continuous control signal which can reduce the chattering of the system. Finally, the validity of the proposed scheme is demonstrated through computer simulations. The results show that, in comparison with conventional DTC, variable structure DTC (VS-DTC) obviously reduces the torque and flux ripples of IPMSM and with this integral-operation switching surface, the stability is guaranteed and insensitivity to uncertainties and disturbance is obtained as well.

Multilevel inverter has been widely investigated for high-power applications in recent years. Various topologies and modulation strategies have been proposed for utility and drive applications in the literature. There are three main types of transformerless multilevel inverters proposed so far, diode-clamped inverter, flying-capacitor inverter and cascaded inverter. The cascaded inverter has drawn tremendous interest. The most attractive features of cascaded multilevel inverters are as follows: firstly, they require the least number of components to achieve the same number of voltage levels. Secondly, modularized circuit layout and packaging is possible and there are no extra clamping diodes or voltage balancing capacitors. Various multilevel PWM methods have been proposed during the past two decades. They can be classified into two catalogues, carrier-based PWM method and space vector PWM (SVPWM) method. Carrier-based PWM is superior to SVPWM for five-level converter and above. In order to overcome the disadvantages of low modulation index and fundamental component existing in subharmonic PWM (SHPWM) and phase-shifted PWM (PSPWM), a new triangular carrier-based phase-shifted suboptimal PWM (PS-SUB-PWM) suitable for cascaded multilevel converters is proposed in this paper. The triangular carriers in this method are same as that in the PSPWM method, but the modulation waveform is modified by adding the 3^rd , or the 6^th, or the 9^th harmonics and so on to it. Injecting harmonics into modulation waveform makes modulation index and fundamental component increase. Through analysis of PS-SUB-PWM strategy, the parameters of optimal wave and switching angles are derived. The simulation results indicate that the proposed method is valid

The infotaxis scheme is a search strategy for a diffusive source, where the sensor platform is driven to reduce the uncertainty about the source through climbing the information gradient. The infotaxis scheme has been successfully applied in many source searching tasks and has demonstrated fast and stable searching capabilities. However, the infotaxis scheme focuses on gathering information to reduce the uncertainty down to zero, rather than chasing the most probable estimated source when a reliable estimation is obtained. This leads the sensor to spend more time exploring the space and yields a longer search path. In this paper, from the context of exploration-exploitation balance, a novel search scheme based on minimizing free energy that combines the entropy and the potential energy is proposed. The term entropy is implemented as the exploration to gather more information. The term potential energy, leveraging the distance to the estimated sources, is implemented as the exploitation to reinforce the chasing behavior with the receding of the uncertainty. It results in a faster effective search strategy by which the sensor determines its actions by minimizing the free energy rather than only the entropy in traditional infotaxis. Simulations of the source search task based on the computational plume verify the efficiency of the proposed strategy, achieving a shorter mean search time.