Dependence of residual voltage ratio behavior of SnO_2-based varistors on Nb_2O_5 addition

SnO2-based varistor samples doped with Co2O3,Nb2O5 and Cr2O3 were prepared by ball-mixed oxide method.The microstructure,nonlinear I-V characteristic and surge current performances of these samples were investigated.This paper mainly focused on the dependence of the residual voltage ratio behavior of SnO2-based varistors on Nb2O5 addition,different factors influencing the residual voltage ratio in different concentration of Nb2O5 were analyzed.The Nb2O5 addition influences its residual voltage ratio by changing the grain size,forming defects(especially the free electrons) and cumulative effect.The measured results indicated that the optimally obtained sample with 0.07mol% Nb2O5 possesses the lowest residual voltage ratio of 1.86,the corresponding nonlinear coefficient and the threshold electric field are 42.6 and 364.6 V/mm,respectively.

An Active Triggered Surge Protective Gap

A triggered surge protective device is designed and its discharge characteristics axe studied. The experimental results show that the triggered surge protective device has excellent surge protective characteristics. When the gap distance is 5 mm, p. d is 90 Pa.mm and without an active energy trigger circuit, the DC breakdown voltage of the triggered surge protective device is 2.32 kV and the pulse breakdown voltage is 5.75 kV. Therefore, the pulse voltage ratio, which is defined as the specific value of pulse breakdown voltage and DC breakdown voltage, is 2.48. With a semiconductor ZnO flashover trigger device and an active energy coupling trigger circuit, the pulse breakdown voltage can be reduced to 3.32 kV, the pulse voltage ratio is 1.43 and the response time is less than 100 ns. These results are helpful in laying a theoretical foundation for further studies on triggered surge protective devices.

A SIZE-REDUCED LOG PERIODIC DIPOLE ANTENNA

A rectangle capacity patch was adopted as the resonance unit of the Log Periodic Dipole Antenna (LPDA) so as to realize the miniaturization of this aerial in this paper. Fifteen rectangle capacity patch units of different parameters were analyzed in this paper and three design laws of size-reduction were found. Accord-ing to these design laws, a 70% miniaturization ratio LPDA was designed and fabricated. The Voltage Standing Wave Ratio (VSWR) and pattern of the fabricated LPDA were measured. The results indicate that this size-reduction method do not deteriorate performance.

Evaluation of High Step-up Power Conversion Systems for Large-capacity Photovoltaic Generation Integrated into Medium Voltage DC Grids

With the increase of dc based renewable energy generation and dc loads,the medium voltage dc(MVDC)distribution network is becoming a promising option for more efficient system integration.In particular,large-capacity photovoltaic(PV)-based power generation is growing rapidly,and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid.Different from traditional ac grid-connected converters,the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios.This is an important issue,yet received limited research so far.In this paper,a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted.The required structural features for such a conversion system are first discussed.Based on these features,the conversion system is classified into four categories by series-parallel connection scheme of power modules.Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance,conversion efficiency,modulation method,control complexity,power density,reliability,and hardware cost.In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.

AN ULTRA-WIDEBAND ANTENNA FOR THE REINFORCED CONCRETE DETECTION

In this paper,an Ultra-Wideband(UWB) planar antenna is proposed for the reinforced concrete detection,which consists of a pair of planar waterdrop arms,a microstrip to coplanar parallel-strips transition and a shallow rectangular cavity.In order to overcome the disadvantages of the shallow cavity,some absorbing material is loaded to weaken the narrow-band effect of the cavity and the crosstalk interference.The simulated and measured results show that the proposed antenna has a large bandwidth from 0.48 GHz to 3.6 GHz with Voltage Standing Wave Ratio(VSWR) below 2 and a fractional bandwidth about 200% under the center frequency of 1.6 GHz,directional radiation characteristics and small late-time ringing in the time domain,which can be suitable for nondestructive detection of the reinforced concrete.

Defected Ground Structure Multiple Input-Output Antenna For Wireless Applications

In this paper,the investigation of a novel compact 2×2,2×1,and 1×1 Ultra-Wide Band(UWB)based Multiple-Input Multiple-Output(MIMO)antenna with Defected Ground Structure(DGS)is employed.The proposed Electromagnetic Radiation Structures(ERS)is composed of multiple radiating elements.These MIMO antennas are designed and analyzed with and without DGS.The feeding is introduced by a microstrip-fed line to significantly moderate the radiating structure’s overall size,which is 60×40×1 mm.The high directivity and divergence characteristics are attained by introducing the microstripfed lines perpendicular to each other.And the projected MIMO antenna structures are compared with others by using parameters like Return Loss(RL),Voltage Standing Wave Ratio(VSWR),Radiation Pattern(RP),radiation efficiency,and directivity.The same MIMO set-up is redesigned with DGS,and the resultant parameters are compared.Finally,the Multiple Input and Multiple Output Radiating Structures with and without DGS are compared for result considerations like RL,VSWR,RP,radiation efficiency,and directivity.This projected antenna displays an omnidirectional RP with moderate gain,which is highly recommended for human healthcare applications.By introducing the defected ground structure in bottom layer the lower cut-off frequencies of 2.3,4.5 and 6.0 GHz are achieved with few biological effects on radio propagation in human body communications.The proposed design covers numerous well-known wireless standards,along with dual-function DGS slots,and it can be easily integrated into Wireless Body Area Networks(WBAN)in medical applications.This WBAN links the autonomous nodes that may be situated either in the clothes,on-body or beneath the skin of a person.This system typically advances the complete human body and the inter-connected nodes through a wireless communication channel.

An Over Modulation Strategy and Simulation Study for Matrix Converter

The over modulation strategy of matrix converter(MC) is put forward on the basis of conventional AC/DC/AC MC control theory. Firstly,imaginary AC/DC converter is over modulated to get the greatest fictitious DC voltage,secondly,imaginary DC/AC converter is over modulated,which effectively improve the voltage transfer ratio. Two voltage transfer ratio is emphatically analyzed with Fourier transform. At last,different modulation method is simulated with MATLAB,the simulation results verified the theoretical analysis and validity of the over modulation strategy,which provide a theoretical basis for the further research.

Novel high-PSRR high-order curvature-compensated bandgap voltage reference

This paper proposes a novel high-power supply rejection ratio（high-PSRR） high-order curvature-compensated CMOS bandgap voltage reference（BGR） in SMIC 0.18 μm CMOS process. Three kinds of current are added to a conventional BGR in order to improve the temperature drift within wider temperature range, which include a piecewise-curvaturecorrected current in high temperature range, a piecewise-curvature-corrected current in low temperature range and a proportional-to-absolute-temperature T^（1.5） current. The high-PSRR characteristic of the proposed BGR is achieved by adopting the technique of pre-regulator. Simulation results shows that the temperature coefficient of the proposed BGR with pre-regulator is 8.42x10^（-6）′ /℃ from - 55 ℃ to 125 ℃ with a 1.8 V power supply voltage. The proposed BGR with pre-regulator achieves PSRR of - 123.51 dB, - 123.52 dB, - 88.5 dB and - 50.23 dB at 1 Hz, 100 Hz, 100 kHz and 1 MHz respectively.

Effects of manganese dioxide additives on the electrical characteristics of Al-doped ZnO varistors

The effects of manganese dioxide (MnO 2 ) additives on the electrical characteristics of the Al-doped ZnO varistors are investigated. The leakage current densities of the samples decrease first and then increase again with the increase of the amount of MnO 2 additives. Correspondingly, the nonlinear coefficients of the samples keep on increasing and then decrease. In addition, the donor densities increase monotonously with the amount of the doped MnO 2 increasing. The effects of MnO 2 additives on the electrical characteristics of the samples are different from the conclusion of previous literatures. The reasons of the distinct effects are supposed to be related with the donor and interface state densities of the samples.

Harmonic influence analysis of unified power flow controller based on modular multilevel converter

The unified power flow controller(UPFC)based on modular multilevel converter(MMC) is the most creative flexible ac transmission system(FACTS) device. In theory, the output voltage of the series MMC in MMCUPFC can be regulated from 0 to the rated value. However,there would be relatively large harmonics in the output voltage if the voltage modulation ratio is small. In order to analyze the influence of MMC-UPFC on the harmonics of the power grid, the theoretical calculation method and spectra of the output voltage harmonics of MMC are presented. Subsequently, the calculation formulas of the harmonics in the power grid with UPFC are proposed. Based on it, the influence of UPFC on the grid voltage harmonics is evaluated, when MMC-UPFC is operated with different submodular numbers and voltage modular ratios. Eventually, the proposed analysis method is validated using digital simulation. The study results would provide guideline for the design and operation of MMC-UPFC project.

Non-isolated stacked bidirectional soft-switching DC-DC converter with PWM plus phase-shift control scheme

In this paper, a non-isolated stacked bidirectional DC-DC converter with zero-voltage-switching(ZVS) is introduced for the high step-up/step-down conversion systems. The extremely narrow turn-on and/or turn-off duty cycle existing in the conventional bidirectional buck-boost converters can be extended due to the stacked module configuration for large voltage conversion ratio applications. Furthermore, the switch voltage stress is halved because of the series connection of half bridge modules. The PWM plus phase-shift control strategy is employed, where the duty cycle is adopted to regulate the voltages between the input and output sides and the phaseshift angle is applied to achieve the power flow regulation.This decoupled control scheme can not only realize seamless bidirectional transition operation, but also achieve adaptive voltage balance for the power switches. In addition, ZVS soft-switching operation for all active switches is realized to minimize the switching losses. Finally, a prototype of 1 kW operating at 100 kHz is built and tested to demonstrate the effectiveness of the proposed converter and the control strategy.