Test Research on the Knock of a Common-Rail Diesel Engine Fueled with Diesel-Methanol Dual-Fuel

Experiments were conducted on a diesel-methanol dual-fuel(DMDF)engine modified by a six-cylinder,turbocharged,inter-cooled diesel engine.According to the number of diesel injection,the experiments are divided to two parts:the single injectionmode and double injectionmode.The results show that,at the double injectionmode,themaximumof pressure rise rate is small and the engine runs smoothly,however,knock still occurswhen the cocombustion ratio(CCR)is big enough.Under knock status,the power density of the block vibration concentrating at some special frequencies rises dramatically,and the special frequency of single injection mode(about 4.1 kHz)is lower than that of double injection mode(7–9 kHz).The cylinder pressure oscillations of knock status are very different fromthe non-knock status.Under knock status,cylinder pressure oscillations become more concentrated and fiercer at some special frequencies,and the same as the block vibration.The special frequency of single injection mode(3–6 kHz)is lower than that of double injection mode(above 9 kHz).

An Improved Algorithm of Grounding Grids Corrosion Diagnosis Based on Total Least Square Method

A new model considering corrosion property for grounding grids diagnosis is proposed,which provides reference solutions of ambiguous branches.The constraint total least square method based on singular value decomposition is adopted to improve the effectiveness of grounding grids' diagnosis algorithm.The improvement can weaken the influence of the model's error,which results from the differences between design paper and actual grid.Its influence on touch and step voltages caused by the interior resistance of conductors is taken into account.Simulation results show the validity of this approach.

Properties of vacuum cast CuCr25 and CuCr25Te contact material

The CuCr25 and CuCr25Te contact materials were manufactured by vacuum casting process.The microstructures of two alloys were observed by metallographic microscope,the electrical conductivity,density and hardness were measured.The tensile test was done by universal testing machine while the fractography was observed by SEM,the breaking current tests were carried out in Weil Synthetic Circuit with arc-igniting branch in Xi’an Jiaotong University.The results show that the microstructures and physical parameters change after adding Te element.The tensile strength decreases and the toughness turns bad,which is propitious to improve the anti-welding property.But the breaking current capacity of CuCr25Te alloy is inferior to CuCr25 alloy.Thus,adding Te element has both advantages and disadvantages on main properties of CuCr contact material.

Preparation and Performance of Corona-proof Conductive Composite Coatings

Because of its merits,acrylic resin was chosen to improve the mechanical,conductive and hydrophobic properties.Carbon fiber powders (CF),carbon nanotubes (MWCNT),and nano-TiO_(2) were incorporated into the acrylic resin to prepare the corona-proof conductive composite coatings.The incorporation of CF and MWCNT may improve the conductivity and mechanical strength of the coatings.However,the addition of nano-TiO_(2) may increase the hydrophobicity of the coatings.Thus,the effects of different additives on the mechanical properties,conductivity,hydrophobicity and heat resistance of the conductive film were studied.The experimental results show that the incorporation of carbon fiber powders and multi walled carbon nanotubes can significantly improve both the conductivity and mechanical properties of the conductive coatings,and the addition of nano titanium dioxide can improve the hydrophobicity of the conductive film.

Wind power prediction based on variational mode decomposition multi-frequency combinations

Because of the uncertainty and randomness of wind speed, wind power has characteristics such as nonlinearity and multiple frequencies. Accurate prediction of wind power is one effective means of improving wind power integration. Because the traditional single model cannot fully characterize the fluctuating characteristics of wind power, scholars have attempted to build other prediction models based on empirical mode decomposition(EMD) or ensemble empirical mode decomposition(EEMD) to tackle this problem. However, the prediction accuracy of these models is affected by modal aliasing and illusive components. Aimed at these defects, this paper proposes a multi-frequency combination prediction model based on variational mode decomposition(VMD). We use a back propagation neural network(BPNN),autoregressive moving average(ARMA)model, and least square support vector machine(LS-SVM) to predict high, intermediate,and low frequency components,respectively. Based on the predicted values of each component, the BPNN is applied to combine them into a final wind power prediction value.Finally,the prediction performance of the single prediction models(ARMA,BPNN and LS-SVM)and the decomposition prediction models(EMD and EEMD) are used to compare with the proposed VMD model according to the evaluation indices such as average absolute error, mean square error,and root mean square error to validate its feasibility and accuracy. The results show that the prediction accuracy of the proposed VMD model is higher.

Multi-objective interval prediction of wind power based on conditional copula function

Interval prediction of wind power,which features the upper and lower limits of wind power at a given confidence level,plays a significant role in accurate prediction and stability of the power grid integrated with wind power.However,the conventional methods of interval prediction are commonly based on a hypothetic probability distribution function,which neglects the correlations among various variables,leading to the decrease of prediction accuracy.Therefore,we improve the multi-objective interval prediction based on the conditional copula function,through which we can fully utilize the correlations among variables to improve prediction accuracy without an assumed probability distribution function.We use the multi-objective optimization method of nondominated sorting genetic algorithm-II(NSGA-II)to obtain the optimal solution set.The particular best solution is weighted by the prediction interval average width(PIAW)and prediction interval coverage probability(PICP)to pick the optimized solution in practical examples.Finally,we apply the proposed method to three wind power plants in different cities in China as examples forvalidation and obtain higher prediction accuracy compared with other methods,i.e.,relevance vector machine(RVM),artificial neural network(ANN),and particle swarm optimization kernel extreme learning machine(PSO-KELM).These results demonstrate the superiority and practicability of this method in interval prediction of wind power.

Parallel computing of multi-contingency optimal power flow with transient stability constraints

To deal with the high dimensionality and computational density of the Optimal Power Flow model with Transient Stability Constraints(OTS),a credible criterion to determine transient stability is proposed based on swing curves of generator rotor and the characteristics of transient stability.With this method,the swing curves of all generator rotors will be independent one another.Therefore,when a parallel computing approach based on the MATLAB parallel toolbox is used to handle multi-contingency cases,the calculation speed is improved significantly.Finally,numerical simulations on three test systems including the NE-39 system,the IEEE 300-bus system,and 703-bus systems,show the effectiveness of the proposed method in reducing the computing time of OTS calculation.

Onset electric field of soil ionisation around grounding electrode under lightning

Onset electric field of soil ionisation is one of the basic parameters to analyse the lightning impulse performance of a grounding electrode,which has been fully investigated based on experiments on hemispherical electrode or rod-to-rod gap.Until now,there has been lack of onset electric field of rod-to-plane gap,which is the real situation for grounding electrode.In this study,the soil ionisation around the grounding electrode is studied by experiment.Method to judge the start of ionisation is discussed.Both the effect of the soil resistivity and that of the radius of the grounding electrode on the onset electric field of soil ionisation are investigated.It shows that the onset electric field decreases as the radius of the electrode increases,while increases as the soil resistivity increases.A formula to predict the onset field with respect to soil resistivity and the radius of the grounding electrode is proposed.

A matrix-perturbation-theory-based optimal strategy for small-signal stability analysis of large-scale power grid

In this paper,a sensitivity matrix based approach is proposed to improve the minimum damping ratio.The proposed method also avoids burdensome deviation calculations of damping ratio of large-scale power grids when compared to the Small-Signal-Stability Constrained Optimal Power Flow(SSSC-OPF)approach.This is achieved using the Matrix Perturbation Theory(MPT)to deal with the 2nd order sensitivity matrices,and the establishment of an optimal corrective control model to regulate the output power of generating units to improve the minimum damping ratio of power grids.Finally,simulation results on the IEEE 9-bus,IEEE 39-bus and a China 634-bus systems show that the proposed approach can significantly reduce the burden of deviation calculation,while enhancing power system stability and ensuring calculation accuracy.

Defect recognition of oil-pressboard insulation based on frequent arc reignition phenomenon under switching impulse voltage

In the past three years,a number of oil-immersed ultra-high-voltage shunt reactors have experienced discharge defects when being put into operation,resulting in an overload of acetylene.However,detecting and identifying these discharge defects caused by switching impulse voltage is challenging under steady-state conditions.This poses unpredictable and difficult-to-assess safety risks for the longterm operation of the equipment and subsequent transient processes.Hence,comprehending the discharge behavior of oilpressboard(PB)insulation under switching impulse voltage and devising a method to identify defects becomes crucial.This study focuses on investigating the frequent arc reignition(FAR)pattern exhibited by typical defects under both standard and oscillation switching impulse voltages.The objective is to uncover the mechanism behind FAR and propose a defect recognition strategy suitable for transient processes.The study reveals the FAR process will occur at least once during the breakdown process;the FAR phenomenon is the weakest in the surface defect with a weak vertical electric field.The average recovery voltage percentage and the average discharge interval of the FAR process decrease with increasing impulse amplitude or oscillation frequency.Additionally,the average number of discharges decreases with higher oscillation frequency,while it initially increases and then decreases with increaseing amplitude.Based on the analysis of the number of FAR processes and their variation in terms of amplitude or discharge interval,a method for recognizing oil-PB defects during switching transient processes is developed and successfully applied to a case study involving acetylene overload in a 1000 kV shunt reactor.

Investigation on corrosion of yttrium-doped magnesium-based sacrificial anode in ground grid protection

In the simulated DC and AC discharge circuit in ground grid protection of substation,corrosion and potential of magnesium anode with yttrium addition were studied by potential measurement and scanning electron microscopy.The results showed that potential of anode increased with increasing current density in DC simulation,potential increase of anode with 0.1% yttrium addition was the smallest,and the affordable current density of the anode reached 1.85 A/cm2,which was two times that of the anode without yttrium addition.In AC simulation,addition of yttrium had less effect on its potential.Grains of magnesium alloy sacrificial anode without yttrium addition were the equiaxed grains of magnesium grains coated with β-Mg17Al12,while the anode with yttrium was α-Mg phase with dispersed β phase.The corroded magnesium anode with 0.1% yttrium addition showed the most uniform and the smallest pit on its surface.

The effect of dynamic behaviours of the water droplet on DC/AC flashover performance on silicone rubber surface:Experiment,simulation and theoretical analysis

It is usually seen that DC flashover voltage of the water droplet on the surface of silicone rubber(SIR)composite insulator is lower than AC flashover voltage.It is necessary to study the mechanisms and influencing factors of the surface properties.In this article,experiments are performed to study the moving processes of the water droplet and the influences of its dynamic behaviours on flashover voltage under DC/AC electric field.Besides,an electrohydrodynamic coupling method is employed to establish the simulating model.Moreover,the theoretical physical model is established to reveal the mechanisms of dynamic behaviours of a droplet.It is indicated that the simulation results are in good consistency with the experimental data.The elongation behaviours of a droplet on the surface of a composite insulator can influence the homogeneity of the electric field distribution along the sample's surface.It is shown that the different dynamic behaviours and flashover performances for a single water droplet on the SIR surface are both affected by the volume of the droplet and the type of voltage applied.

Influence of yttrium addition on properties of Mg-based sacrificial anode

To improve lower current efficiency of Mg-based sacrificial anode,a Mg-based sacrificial anode material with yttrium addition was investigated.Its electrochemical properties were evaluated by the galvanostatic method,and its microstructure and constitution were characterized by metallurgical microscope and X-ray diffraction.The results showed that,the addition of yttrium refined dentrite grains,maximized current efficiency,and minimized potential of magnesium sacrificial anode.When 0.1% yttrium was doped,the anode showed the highest current efficiency,62.5%,which improved its current efficiency by 14%.

Insulation Properties and Degradation Mechanism for XLPE Subjected to Different Aging Periods

Due to its excellent insulation and mechanical properties,cross-linked polyethylene(XLPE)is widely used as the insulation candidate of power cables in power transmission and distribution systems.However,its performance can be affected due to various stresses,which endangers safe operation of the power system in service.In this paper,dielectric and insulation properties of XLPE materials of 110 kV cables with the service lives of 0,15,and 30 years were tested and evaluated.Results show that with increase of service life,dielectric loss caused by the polarization process increases significantly,and dielectric strength decreases as well.Power frequency breakdown field strength and crystallinity decrease significantly,and carbonyl index increases.The degradation process of XLPE can be divided into two stages:the structure of the crystalline region of the original sample is relatively complete,and loss can be almost ignored.After long-term service,the crystallization region is destroyed,and thermal oxygen reaction occurs.Intermolecular force is weakened,which makes breakdown easier to occur.However,there is no direct relation between the carbonyl index and macroscopic characteristics.This paper reveals the deterioration law of the XLPE cable insulation,which provides a theoretical basis for evaluating cable aging condition and is of great significance to improve cable operation safety and maintenance quality.

Fuzzy Logic-based Management of Hybrid Distribution Transformers Using LoRa Technology

Hybrid distribution transformers(HDTs)have better performance than traditional distribution transformers in improving power quality through reducing harmonics,unbalance,voltage fluctuations and low power factors in future smart power distribution systems.In order to increase the service life and reliability of hybrid distribution transformers,this paper proposes a remote management system using LoRa technology based on fuzzy logic.HDT based on a fuzzy logic judgment system(FLJS)replaces the Boolean logic with fuzzy logic and several power quality problems including power factors,load-side current harmonics and voltage unbalance are considered,as well as grid-side voltage deviation and unbalance.This management system can dynamically adjust the working states of HDT according to the output results of the FLJS to reduce the use time of power electronic devices.Due to the application of LoRa,this management system can remotely adjust the parameters of the FLJS in real time for different distribution network nodes to avoid frequent switching of HDT working states.In addition,it is able to remotely monitor the real-time working states and fault states of HDT to reduce recovery time and maintenance costs in case of HDT failure.Finally,simulation and experimental results are presented to verify the effectiveness of the proposed management system for HDT.