Search

The power calculation is an indispensable element in droop-controlled inverters because the bandwidth of the measured power has a direct impact on the controller performance. This paper proposes a fast and accurate power calculation algorithm based on the combined Second Order Generalized Integrator (SOGI) filters in stationary coordinates for a three-phase system, which takes into consideration the use of nonlinear loads. The power calculation scheme is formed by the two-stage SOGI filters that are employed for obtaining the active and reactive powers required to perform a droop-based inverter operation, respectively. From the two-stage structure, the first SOGI is used as a band-pass filter (BPF) for filtering harmonics and obtaining the fundamental current of the nonlinear load; The second SOGI is used as a low-pass filter (LPF) for extracting the DC-component, which corresponds with the average power. A small-signal model of a two droop-controlled inverters system is built to obtain the dynamical response and stability margin of the system. And compared it with the dynamical behaviour of a standard droop-control method. Next, the proposed power calculation system is designed in order to achieve the same ripple amplitude voltage as that obtained with the standard droop-control method by adjusting the bandwidth gains. Through simulation and hardware in the loop (HIL) validation, the proposed approach presents a faster and more accurate performance when sharing nonlinear loads, and also drives the inverters’ output voltage with lower distortion.

In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current (MTDC) grids, this paper proposes a modular multi-terminal DC power flow controller (MM-DCPFC) with fault current limiting function. The topology structure, operation principle, and equivalent circuit of MM-DCPFC are introduced, and such a structure has the advantages of modularity and scalability. The power balance mechanism is studied and a hierarchical power balance control strategy is proposed. The results show that MM-DCPFC can achieve internal power exchange, which avoids the use of external power supply. The fault characteristics of MM-DCPFC are analyzed, fault current limiting and self-protection methods are proposed, and the factors affecting the current limiting capability are studied. The simulation models are established in PLECS, and the simulation results verify the effectiveness of MM-DCPFC in power flow control, fault current limiting, and scalability. In addition, a prototype is developed to validate the function and control method of MM-DCPFC.

In order to eliminate the disadvantages of the keyhole in conventional friction stir spot welding joint and attain the high-strength lap joint of Al/Cu dissimilar metals, a novel welding technique, named as friction stir spot riveting (FSSR), was proposed. A pinless tool and an extra filling stud were employed. The Al/Cu spot joints without keyhole defect were achieved by the FSSR. A Cu anchor-like structure was formed, which greatly increased the mechanical interlocking between the upper Al sheet and lower Cu sheet. The thin intermetallic compounds containing CuAl2 and CuAl at the Al/Cu interface strengthened the joining interface between the Al sheet and the Cu stud. Increasing rotating velocity increased frictional heat and plastic deformation and then eliminated the interfacial joining defects. The FSSR joint with the maximum tensile shear load of 3.50 kN was achieved at a rotating velocity of 1800 rpm and a dwell time of 20 s, whose fracture path passed through the softened region of upper Al sheet. In summary, the novel FSSR technique has the advantages of strong mechanical interlocking and metallurgical bonding between dissimilar materials, thereby attaining the high-strength spot joint.

Nonlinear dynamical systems such as weakly coupled oscillators are an interesting approach to be adopted for the regulation of power inverters inside microgrids. Aiming at the synchronization and load sharing in islanded mirogrid, this paper is inspired by oscillator synchronization property to propose a Hopf oscillator controller for the single-phase inverters. The Hopf oscillator dynamic equations are used for providing the inverter's frequency and amplitude voltage references which lead to a robust nonlinear droop behavior for driving the system without using communications. The Hopf oscillator provides better sharing of the load between inverters with higher robustness, less harmonic distortion, and faster time response of the associated limit cycle than the achieved by the other approach made with a Van der Pol oscillator. In addition, global asymptotic synchronization of system is proven by Lyapunov approach. Simulation results of a system composed by paralleled inverters are provided and compared with a Van der Pol oscillator approach reported in literature. Experimental results are also provided to prove the Hopf oscillator based controller under different circumstances.

A new method of hybrid technology for controlling welding residual stress and distortion is proposed, which contains the transient thermal tensioning with two symmetrical heat sources ahead and the...

The dynamic performance of the local control of single-phase voltage source inverters (VSIs) can be degraded when supplying to nonlinear loads (NLLs) in microgrids. When this control is based on the droop principles, a proper calculation of the active and reactive averaged powers (P&ndash, Q) is essential for a proficient dynamic response against abrupt NLL changes. In this work, a VSI supplying to an NLL was studied, focusing the attention on the P&ndash, Q calculation stage. This stage first generated the direct and in-quadrature signals from the measured load current through a second-order generalized integrator (SOGI). Then, the instantaneous power quantities were obtained by multiplying each filtered current by the output voltage, and filtered later by utilizing a SOGI to acquire the averaged P&ndash, Q parameters. The proposed algorithm was compared with previous proposals, while keeping the active power steady-state ripple constant, which resulted in a faster calculation of the averaged active power. In this case, the steady-state averaged reactive power presented less ripple than the best proposal to which it was compared. When reducing the velocity of the proposed algorithm for the active power, it also showed a reduction in its steady-state ripple. Simulations, hardware-in-the-loop, and experimental tests were carried out to verify the effectiveness of the proposal.

A novel passivity-based coordinated control strategy is proposed for an islanded AC microgrid including renewable energy source and energy storage system units. The main advantage is that the proposed coordinated control strategy manages the microgrid without using a phase-locked loop system. In the microgrid, the energy storage system supports the voltage of the microgrid, and the renewable energy sources inject their maximum power to the microgrid in the normal operation. For the energy storage system, we use a proportional-resonant controller, and for the renewable energy sources, we use a voltage modulated direct power control method, which has not only a good tracking performance but also a good steady-state behavior. Another advantage of the proposed method is that the asymptotical stability of the whole microgrid can be guaranteed by using the passivity principle when the heterogeneous renewable energy sources are integrated into the microgrid. To validate the proposed coordinated control law, we use a microgrid consisting of one energy storage system, one wind turbine, one photovoltaic and two controllable loads. Simulation results show that the plug-and-play capability of the wind turbine and photovoltaic in the microgrid is enhanced when comparing with the conventional vector current control method with a phase-locked loop system. Moreover, the voltage and frequency of the microgrid are recovered to its nominal value by the energy storage system with the proposed method as well. Finally, experimental verification of the proposed coordinated control algorithm is performed on a 10 kW microgrid system. The experimental results match the simulation ones as well.

To enhance the high-temperature oxidation resistance of Ti6Al4V alloy, an anti-oxidation coating is fabricated on Ti6Al4V by simple surface aluminization. The interdiffusion of Al/Ti leads to the TiAl3 formation. With the increasing diffusion temperature or holding time, Al is depleted, and the generated TiAl3 reacts with Ti subsequently, forming the Ti-Al IMC layers. A smooth, dense, and uniform coating (Al + TiAl3) is obtained at 700 ℃/20 min. The cyclic oxidation test exhibits that the coating remarkably enhances the oxidation resistance of Ti6Al4V, and the surface aluminized Ti6Al4V possesses long-term stability (≥ 205 h) at 800 ℃ in air.

The joining of metals and polymer-matrix composites (PMCs) is a highly pressing and challenging task in the manufacturing industries. To achieve the reliable hybrid joint, a solid-state joining technique of top-thermic friction stir lap welding (TT-FSLW) was proposed and performed in the joining of 7075 aluminum alloy and SGF/PEEK materials. Through the TT-FSLW with low rotational speed, the excessive overflow of polymer material in conventional FSLW was suppressed, so the satisfactory joint with smooth surface was formed. Thanks to the low heating and cooling rates and the reduced interfacial stress, common FSLW defects of void and crack were eliminated in the TT-FSLW joint. Characterized by the widened effective interfacial joining area and the enhanced interfacial joining with micro and macro mechanical interlockings, the maximum shear strength of joint was enhanced to 59.9 MPa under TT-FSLW, which was increased by 52.4% compared to that under the conventional FSLW. The TT-FSLW technique has been preliminarily proved to be superior to other conventional welding methods in joining metals and PMCs.

To meet the requirements of high-power compensating occasion such as electrochemical industries, a multifunctional hybrid active power converter (HAPC) with LC series filter is studied to effectively integrate functions of rectification and power compensation. HAPC can be used as a high-power pulse width modulation rectifier to supply its dc load, meanwhile, it can also compensate large amount of reactive and harmonic current without increasing the voltage rating of itself. A compound current control method for HAPC is introduced so as to improve the dynamic tracking performance. The current and voltage dual closed-loop system of HAPC is built in s domain, and the detailed control parameters are designed to tune control performance further. Finally, a set of 800 kVA HAPC system for electrolytic copper-foil is developed. The results show that the HAPC system can achieve expected operation performance and largely improves power quality of power grid.

The active and reactive powers, P and Q, are crucial variables in the parallel operation of single-phase inverters using the droop method, introducing proportional droops in the inverter output frequency and voltage amplitude references. P and Q, or P-Q, are calculated as the product of the inverter output voltage and its orthogonal version with the output current, respectively. However, when sharing nonlinear loads these powers, Pav and Qav, should be averaged by low-pass filters (LPFs) with a very low cut-off frequency to avoid the high distortion induced by these loads. This forces the droop method to operate at a very low dynamic velocity and degrades the system stability. Then, different solutions have been proposed in literature to increase the system velocity, but only considering linear loads. Therefore, this work presents a method to calculate Pav and Qav using second-order generalized integrators (SOGI) to face this problem with nonlinear loads. A double SOGI (DSOGI) approach is applied to filter the nonlinear load current and provide its fundamental component to the inverter, leading to a faster dynamic velocity of the droop-based load sharing capability and improving the stability. The proposed method is shown to be faster than others in the literature when considering nonlinear loads, while smoothly driving the system with low distortion levels. Simulations, hardware-in-loop (HIL) and experimental results are provided to validate this proposal.

Although virtual synchronous generator (VSG) is a promising solution to low inertia and damping of the power electronics-based power system, improper parameter design will lead to instability of such a system. In this paper, a VSG parameter design strategy is proposed by considering both AC and DC restraints for a paralleled inverters system, where the active power output of VSG is restricted by the maximum power limit of the converters in AC side, with the energy supply bounded by the energy margin of power source in DC side. Detailed stable operation boundaries are investigated using small-signal mode by considering the dual physical constraints and the stable constraints. Therefore, the proposed parameter tuning method can not only provide enough virtual inertia for stable operation of the system, but also satisfy the power and energy restraints in practice. Detailed simulation results verified the effectiveness of the proposed strategy.

This paper presents a method which mimics coupled nonlinear Poincaré–Bendixson oscillators synchronization to realize current sharing in islanded three-phase Microgrid. We propose that each inverter be controlled to imitate the dynamics of coupled oscillators. When they are controlled to interact strongly enough, they will all start synchronization. We also introduce the comparison between Van der Pol oscillator corresponding to their physical characteristics. The dual-loop based on the oscillator is designed in αβ frame, and the phase dynamics are derived to observe system initial state performance. The behavior and performance of the proposed method were verified experimentally regarding load connection, inverter removal and current ratios changes.

In the rapid development process of a high-speed electrified railway, power quality problems in traction power grid have become increasingly deteriorative. In order to ensure a three-phase balanced traction power grid, a modular multilevel converter based railway traction power conditioner (RTPC) is presented. The RTPC consists of four H-Bridge clusters and filter inductors, and it can be directly connected to traction feeders in a co-phase traction system without insulation transformers. According to the equivalent circuit analysis of RTPC system, it can be considered as four single-phase inversion systems. Based on the equivalent control model of each single-phase cluster, the relationship between the multilevel output voltage and current slopes in a control period is analyzed, and an improved model predictive control is proposed. Moreover, a linear combination of two different output levels is proposed to improve tracking performance of cluster current control, and enhance the waveform quality of ac current. Finally, both simulation and experiment results are presented to verify the effectiveness of the structure and its control method.

Conventional autonomous current-sharing controller (ACSC) will induce inevitable frequency and voltage magnitude deviations due to the output currents and virtual impedances. Thereby secondary control has been proposed to restore the frequency and voltage magnitude deviations by using communication links. In this paper, an autonomous deviation eliminative control method is proposed for the parallel-connected inverters in AC islanded microgrids in order to simplify the control scheme and to improve power quality and control reliability. Two high-pass filters are included in synchronous-reference-frame virtual impedance loop and are activated in steady-state operation to attenuate frequency and voltage magnitude deviations, meanwhile maintaining the autonomous current-sharing performance among the parallel inverters. Simulation results based on two parallel-connected voltage controlled inverters from Matlab/Simulink verify the effectiveness of the proposed approach in different scenarios.

In this paper, a voltage modulated direct power control for three-phase pulse-width modulated rectifier is proposed. With the suggested method, the differential equations describing the rectifier dynamics are changing from a linear time-varying system into a linear time-invariant one. In this way, the conventional feedback and feedforward controllers are applicable for the independent control of active and reactive powers. The proposed method is guaranteed that the closed system is globally exponentially stable. A feedback linearization method is also employed for generating the active power reference of inner loops. Finally, some experimental tests are conducted to verify its effectiveness.

In this paper, an alternative realization method of droop control and virtual impedance for paralleled converters in DC microgrids is proposed as complement to the existing method. The major feature of the proposed realization is that the virtual impedance behavior is realized by using current reference together with a properly designed low-pass filter. Therefore, it will inherently get rid of the noise problem in real-world measurement, which is critical when realizing inductive virtual components. In addition to that, the impact of low-pass filter used in the proposed method on the dynamic performance is also comparatively studied. To validate the proposed method, simulations and experiments are carried out. The results show that the proposed method can be fully compatible with existing methods. Moreover, the low-pass filter is proven to be optional, however, with a specially designed low-pass filter, the transient response of the system can be modified significantly.

Aluminum (Al) and copper (Cu) have been widely used in many industrial fields thanks to their good plasticity, high thermal conductivity and excellent electrical conductivity. An effective joining of dissimilar Al and Cu materials can make full use of the special characteristics of these two metals. Friction stir spot welding (FSSW), as an efficient solid-state welding method suitable for joining of dissimilar metal materials, has great prospects in future industrial applications. In this paper, the FSSW studies on Al-Cu dissimilar materials are reviewed. The research progress and current status of Al-Cu FSSW are reviewed with respect to tool features, macroscopic characteristics of welded joints, microstructures, defects in welds and mechanical properties of joints. In addition, some suggestions on further study are put forward in order to promote the development and progress of Al-Cu FSSW studies in several respects: material flow, thermal history, addition of intermediate layer, auxiliary methods and functionalization of Al-Cu FSSW joint.

This paper presents an adaptive synchronization for current-controlled grid-connected inverter based on a time-domain virtual oscillator controller (VOC). Inspired by the phenomenon of dynamics of adaptive oscillator under the perturbation effect. Firstly, the fast learning rule of the oscillator frequency is derived in order to make the oscillator evolve to the grid voltage. Thus realizing a phase-locking behavior, but without requiring the use of a dedicated phase-locked loop (PLL). Then, the unit orthogonal components are obtained to generate the current reference, and a current loop is designed by using a PR controller in the αβ frame. Experimental results were obtained with comparison to PI current controller, showing the salient performances of the proposed method in front of grid frequency variations, voltage sags and swells.

In this paper, the multi-mode operation of the on-line UPS system is investigated and corresponding control strategies are proposed. The proposed control strategies are able to achieve the seamless transition in traditional normal mode, PV-aided normal mode, enhanced eco-mode and burn-in test mode. At the same time, the uninterruptible load voltage is guaranteed during the transition between these modes. Furthermore, the stability of enhanced eco-mode and burn-in test mode is studied by small signal analysis. Finally, extensive experimental results are provided to validate the effectiveness of the proposed methods.

Harmonic current caused by nonlinear loads and parametric variations of output filter of inverters make popular proportional–integral–derivative (PID) voltage controller far beyond excellent performance in case of microgrid operating in islanded mode. Motivated by this limitation, this paper proposes an adaptive sliding-mode controller (ASMC) to enhance disturbance-rejection performance of control system of islanded parallel inverters. And adaptive algorithms are designed to observe external disturbances and internal perturbation so as to guarantee the globe robustness of control system of inverter. The switching gain of control input is designed to be a time-varying value which effectively reduces undesirable chattering of control input signal. Simulating and experimental results are presented that the total harmonic distortion, chattering, and steady-state error of output voltage of islanded parallel inverters are effectively reduced and the dynamic performances and the capability of perturbation rejection of control system of inverter are effectively enhanced.

BaTiO3 (BT) thin films were prepared on Pt/Ti/SiO2/Si and Ru/Ti/SiO2/Si substrates by a modified sol-gel technique. The microstructure of the films was characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy. The results showed that BT thin films crystallized with perovskite structure. Compared to BT film on Pt/Ti/SiO2/Si substrate, BT thin film deposited on Ru electrode has similar dielectric constant, while it has higher dielectric loss. C–E curve for BT film on Pt/Ti/SiO2/Si was more symmetrical around zero-bias field than C–E curve for BT film on Ru/Ti/SiO2/Si substrate. The tunability was 52.02% for BT film on Pt electrode, which was 33.42% on Ru electrode, at 275 kV/cm and room temperature. The leakage current density of BT on Pt electrode was about an order of magnitude lower than BT film on Ru electrode at the applied electrical field below 150 kV/cm. The leakage conduction mechanism was investigated.

The resonance problem in multi-paralleled grid-connected inverters in microgrid is more complex than single one. The Norton's equivalent circuit associated with grid-connected inverter with LCL filter is figured out in this paper. And in terms of different reasons of causing resonance, resonance problems associated with multi-paralleled grid-connected inverters are categorized into three types: individual resonance, parallel resonance, series resonance. In order to attenuate resonance peak, an active damping compensation term associated with filter capacitor voltage feedback is embedded into current control forward path. The proposed active damping approach is based on the concept of virtual damping resistor which acts as an additional resistor paralleled with filter capacitor. The virtual resistance is deliberately designed to guarantee the requirement of effective resonance damping. Comparative study is conducted to verify the feasibility and effectiveness of resonance damping.