Influences of ±800 kV Yunnan-Guangdong HVDC System on Security and Stability of China Southern Power Grid

The interaction mechanism between AC and DC systems in a hybrid AC-DC transmission grid is discussed with PSS/E software. Analysis shows that receiving-end AC faults may cause much more damage on the HVDC system operation than the sending-end AC faults in a multi-infeed HVDC system, and the damage severity depends on the power recovering rate of the HVDC systems. For HVDC systems with slow power recovering rate, the receiving-end AC faults may probably be a critical factor to constrain power transfer limits. Larger capacity of HVDC system means not only higher power transfer-limit of the parallel connected AC-DC transmission grid, but also more expensive stabilizing cost.

Frequency Control at the Power Sending Side for HVDC Asynchronous Interconnections Between Yunnan Power Grid and the Rest of CSG

For a large-scale high voltage direct current (HVDC)asynchronous interconnected power grid, the frequency issue atthe power sending side under DC faults is a crucial problem.To solve this problem, based on rotor motion equations, theeffect of unbalanced power on the system frequency under DCfaults is analyzed. The characteristics and dynamic developmentprocess of frequencies after the injection of disturbances areanalyzed. In addition, the actions and coordinated strategies ofvarious frequency control measures are also investigated. Basedon the testing projects of an asynchronous interconnection in theChina Southern Power Grid (CSG), the frequency features arestudied according to the measured PMU data. The outcome showsthat the frequency problem of the Yunnan Power Grid after anasynchronous interconnection can be solved and controlled. Italso shows that the frequency limit control (FLC) is importantfor the frequency regulation of large scale HVDC asynchronousinterconnected DC power grids. As demonstrated, DC FLC caneffectively suppress the deviation of the transient frequency.However, reasonable frequency regulation parameters shouldbe set and other area frequency control measures should becoordinated to maintain the frequency stability of the system.

Effects of Macroparticle Sizes on Two-phase Mixture Discharge Under DC Voltage

随着放电等离子体在各个领域应用的不断扩大，两相体放电问题越来越重要，需要解决的基础问题之一即是两相体中放电发生、发展的特点和规律及影响因素。采用21种不同的介质材料研究直流电压下两相体放电的效应，由实验结果可知介质的介电常数对放电路径基本无影响，而介质颗粒的粒径大小对两相体放电路径的选择起决定作用，即粒径效应。此外，对颗粒介电常数、电场畸变、光电离作用和颗粒荷电等几个方面进行研究，可知两相体放电的粒径效应与颗粒粒径大小紧密相关。

Application of Risk-based Flexibility Assessment Methods to Evaluate System Expansion Plans

This paper proposed a flexibility assessment approach based on and risk assessment methodology. System planners prioritize each planning scheme with consideration of three aspects: reliability, economics, and flexibility. In the past, there is lack of quantitative indices to measure flexibility of a power system. This paper proposes applying probabilistic risk assessment method to quantify system flexibility. The proposed approach is demonstrated to compare two transmission planning schemes during Guangdong expansion planning process.

Operation Cost Optimization Method of Regional Integrated Energy System in Electricity Market Environment Considering Uncertainty

In the electricity market environment,the regional integrated energy system(RIES)can reduce the total operation cost by participating in electricity market transactions.However,the RIES will face the risk of load and electricity price uncertainties,which may make its operation cost higher than expected.This paper proposes a method to optimize the operation cost of the RIES in the electricity market environment considering uncertainty.Firstly,based on the operation cost structure of the RIES in the electricity market environment,the energy flow relationship of the RIES is analyzed,and the operation cost model of the RIES is built.Then,the electricity purchase costs of the RIES in the medium-and long-term electricity markets,the spot electricity market,and the retail electricity market are analyzed.Finally,considering the risk of load and electricity price uncertainties,the operation cost optimization model of the RIES is established based on conditional value-at-risk.Then it is solved to obtain the operation cost optimization strategy of the RIES.Verification results show that the proposed operation cost optimization method can reduce the operation cost of high electricity price scenario by optimizing the energy purchase and distribution strategy,constrain the risk of load and electricity price uncertainties,and help balance the risks and benefits.

Knowledge-Based Efficient N-1 Analysis Calculation Method for Urban Distribution Networks with CIM File Data

The N-1 criterion is a critical factor for ensuring the reliable and resilient operation of electric power distribution networks.However,the increasing complexity of distribution networks and the associated growth in data size have created a significant challenge for distribution network planners.To address this issue,we propose a fast N-1 verification procedure for urban distribution networks that combines CIM file data analysis with MILP-based mathematical modeling.Our proposed method leverages the principles of CIM file analysis for distribution network N-1 analysis.We develop a mathematical model of distribution networks based on CIM data and transfer it into MILP.We also take into account the characteristics of medium voltage distribution networks after a line failure and select the feeder section at the exit of each substation with a high load rate to improve the efficiency of N-1 analysis.We validate our approach through a series of case studies and demonstrate its scalability and superiority over traditional N-1 analysis and heuristic optimization algorithms.By enabling online N-1 analysis,our approach significantly improves the work efficiency of distribution network planners.In summary,our proposed method provides a valuable tool for distribution network planners to enhance the accuracy and efficiency of their N-1 analyses.By leveraging the advantages of CIM file data analysis and MILP-based mathematical modeling,our approach contributes to the development of more resilient and reliable electric power distribution networks.

Effect of different coupling agents on the interfacial properties of tube-internally insulated foam materials interface in a composite cross-arm under water environment

In recent years,electrical breakdown accidents for composite cross-arms in rain-impacted environments have occurred which might be ascribed to the degradation of the tube-internally insulated foam materials interface.This research has focused on the different coupling agent treatments(KH-550,KH-560,KH-792,and IPTS)of the tube-internally insulated foam materials interface.The effect of different coupling agents on the inter-facial electrical and physical properties in a water environment have been studied.The leakage current and compressive shear strength of the interface were investigated after water diffusion pre-stress.Three-dimensional computed tomography(μCT)was used to study the interface cross-section tomogram,interface three-dimensional volume rendering,and relative interface porosity.The interfacial hydrolysis resistance ability and the interfacial strength modified by various coupling agents were effectively reflected by the compressive shear strength and relatively defect ratio.The results revealed the ranking of the comprehensive coupling effect was KH-560>KH-550>KH-792>IPTS,which provides significant guidance for modifying internal insulation interface performance for composite cross-arms.

Synthesis of high-purity mesoporous nanosilica microspheres from retired composite insulators based on orthogonal experiment

Silicone rubber is an indispensable insulation material for power equipment and is difficult to degrade naturally,posing a serious threat to the environment.It remains challenging to dispose this alarming amount of waste.In our previous research,a green pyrolysis recycling method was proposed for waste composite insulators.In this article,the experimental device was further improved,and the recycling process was optimised by orthogonal experiments to obtain nanosilica microspheres(NSMs)with higher purity and controllable particle size for the state-of-the-art engineering applications.Firstly,the effect of three factors(calcination temperature,constant-temperature duration,and heating rate)on the particle size of NSMs were investigated by single-factor analysis.ANOVA and response analyses were then used to examine the goodness of fit of the multi-parameter fitting equation and the interaction impact of multiple factors on the particle size.Finally,homogeneous mesoporous NSMs with higher purity(increased from 97%to 99%)and smaller particle size(decreased from 160 to 103 nm were prepared under the guidance of model.The SiO_(2)-recovery rate of this method was 74.7%,revealing superior economic value.

MCTSR:A super-resolution method for defects in gas-insulated switchgear X-ray images based on multi-scale contextual transformers

As the core equipment of transmission and distribution hubs,the operational status of gas-insulated switchgear(GIS)is closely linked to the safety of the power system.Recently,X-ray digital imaging technology has been extensively used in GIS equipment fault detection.However,the X-ray image of GIS is blurred,which is not conducive to the detection of tiny defects.Thus,a super-resolution method for GIS X-ray images based on multi-scale context transformers is proposed in this study,namely MCTSR.Firstly,a second-order image degradation model is introduced to generate GIS X-ray lowresolution images that more closely resemble the real world.Secondly,a contextual transformer gate module is constructed to improve attention to tiny defects in GIS X-ray images.Thirdly,a U-Net discriminator network based on multi-scale contextual transformers is intended to enrich the information of the generated images.Finally,the proposed discriminator is combined with the existing generator to compose a superresolution method applicable to GIS X-ray images.The experimental results demonstrate that the method outperforms other methods in peak signal-to-noise ratio and structural similarity on the constructed GIS X-ray image dataset.In addition,the output image of the proposed method facilitates the subsequent defect detection.

Dividing and setting strategy of improving calculation efficiency for needle electrode corona discharge with a large-scale space

Needle electrodes are widely used in the research of corona discharge.Aiming at the problem of needle electrode discharge in a large-scale space(tens or even hundreds of centimetres),a hybrid model was proposed in the previous work.However,its indis-pensable multiple iterations result in huge computation and further limit its wide appli-cations.Therefore,a strategy to improve the computing efficiency by setting initial values in different computational domains is put forward in this work.Three types of setting initial values(global constant,partition constant and partition exponent)are simulated and compared in detail.The calculation results show that the calculation efficiency can be increased by 1.4 times simply through setting initial values of charged particles as different constants for the divided subdomains,and further be improved by 4.3 times by setting those in the ionization region as an exponential distribution.The extension of the pro-posed strategy has also been discussed under various voltages,which shows that the results under other voltages can be quickly obtained based on the fitted coefficients under a specific voltage,and the improvement can reach up to 10 times.The accuracy of calculated results has been demonstrated by a needle-plate electrode device with a vertical distance of 25 cm.This research provides an effective strategy for improving the computing efficiency of corona discharge in a large-scale space,and implies the potential in optimizing the analyses of complex electrodes.

Robust State Estimation of Active Distribution Networks with Multi-source Measurements

The volatile and intermittent nature of distributed generators(DGs) in active distribution networks(ADNs) increases the uncertainty of operating states. The introduction of distribution phasor measurement units(D-PMUs) enhances the monitoring level. The trade-offs of computational performance and robustness of state estimation in monitoring the network states are of great significance for ADNs with D-PMUs and DGs. This paper proposes a second-order cone programming(SOCP) based robust state estimation(RSE) method considering multi-source measurements. Firstly, a linearized state estimation model related to the SOCP state variables is formulated. The phase angle measurements of D-PMUs are converted to equivalent power measurements. Then, a revised SOCP-based RSE method with the weighted least absolute value estimator is proposed to enhance the convergence and bad data identification. Multi-time slots of D-PMU measurements are utilized to improve the estimation accuracy of RSE. Finally, the effectiveness of the proposed method is illustrated in the modified IEEE 33-node and IEEE 123-node systems.

Day-ahead Network-constrained Unit Commitment Considering Distributional Robustness and Intraday Discreteness:A Sparse Solution Approach

Quick-start generation units are critical devices and flexible resources to ensure a high penetration level of renewable energy in power systems.By considering the wind uncertainty and both binary and continuous decisions of quickstart generation units within the intraday dispatch,we develop a Wasserstein-metric-based distributionally robust optimization model for the day-ahead network-constrained unit commitment(NCUC)problem with mixed-integer recourse.We propose two feasible frameworks for solving the optimization problem.One approximates the continuous support of random wind power with a finite number of events,and the other leverages the extremal distributions instead.Both solution frameworks rely on the classic nested column-and-constraint generation(C&CG)method.It is shown that due to the sparsity of L_(1)-norm Wasserstein metric,the continuous support of wind power generation could be represented by a discrete one with a small number of events,and the rendered extremal distributions are sparse as well.With this reduction,the distributionally robust NCUC model with complicated mixed-integer recourse problems can be efficiently handled by both solution frameworks.Numerical studies are carried out,demonstrating that the model considering quick-start generation units ensures unit commitment(UC)schedules to be more robust and cost-effective,and the distributionally robust optimization method captures the wind uncertainty well in terms of out-of-sample tests.

Key Technologies of Ultra-high Voltage Hybrid LCC-VSC MTDC Systems

The key technologies of ultra-high voltage hybrid LCC-VSC MTDC systems are investigated,focusing on the design of system configurations,converter topologies and the control and protection system.A double converter per pole of VSC connection is proposed along with the design of a 5000 MW VSC valve to develop a±800 kV/5000 MW large-capacity power transmission.The hybrid MMC topology capable of clearing the DC faults and the control strategy are developed to effectively improve the reliability in case of overhead line faults.The control and protection system of the LCC-VSC MTDC system is introduced to offer flexible operations under both normal and abnormal conditions,which includes voltage/current margin-based coordination,converter switch-in and switch-out,re-connection and drop-off of a third station.Simulations of an LCC-VSC MTDC system based on the LCC-VSC MTDC project are performed.

Design Aspects of Hybrid HVDC System

Hybrid high voltage direct current transmission(Hybrid HVDC)is a new type of HVDC technology developed in recent years.It combines the characteristics of large-capacity and low cost of the line commutated converters(LCC)and non-commutation failure and dynamic reactive power support of the voltage sourced converters(VSCs)in one HVDC system.It has technical advantages in the fields of unidirectional power transmission and in the application of improving the stability of multi-infeed HVDCs,giving it broad application prospects.This paper defines the basic form of hybrid HVDC first and discusses the characteristics and challenges of different hybrid HVDCs.Then,it analyzes the matching characteristics between LCC and VSC in the station-station hybrid HVDC,studies the key technologies,such as VSC topologies and control strategies for clearing the DC overhead line faults,and introduces the key technologies for analyzing and suppressing the high-frequency resonances between VSC and the power grid.In this paper,using the Wudongde UHV multi-terminal HVDC as an example,the proposed key technologies are analyzed and verified.