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Conventionally, the 2-parameter Weibull model, Arrhenius-Weibull model, has been used vastly for transformer aging-dependent unavailability modeling. However, this model only uses the lifetime feature to describe the transformer's degradation process and to calibrate the Weibull parameters, which harms the accuracy of aging-dependent unavailability forecasting. In response, this paper develops a 3-calibratable-parameter Weibull model for evaluating the transformer aging-dependent unavailability. In the proposed model, both the individualized aging threshold and lifetime are taken into the calibration of the Weibull parameters to accurately characterize the heterogeneity in transformer populations. First, a degree of polymerization (DP) analysis and Monte Carlo Simulation (MCS) based approach is proposed for estimating the transformers’ uncertain aging thresholds and lifetimes. Then, the Maximum Likelihood Estimate and Particle Swarm Optimization are jointly adopted to model the relationship among the calibratable Weibull parameters, aging threshold, and lifetime. Finally, the analytical formula of aging-dependent unavailability is derived from the established 3-calibratable-parameter Weibull model using an integral-discretization method. A real utility application example in China's Chongqing power system has been presented to validate and demonstrate the practicality and usefulness of this method. [ABSTRACT FROM AUTHOR]

Voltage sag insurance (VSI) is an effective measure to reduce the voltage sag loss and risk for enterprises with insufficient mitigation confidence or economic ability. However, due to the lack of understanding of voltage sag risk (VSR), it is difficult for power utilities to identify the enterprises with VSI demands, and it is also unable to realize differentiated premium according to different enterprises' situations. To solve the above problems, this paper proposes an improved voltage sag premium model based on VSR. Firstly, the VSR factors are analyzed, such as the hazard, the vulnerability, the exposure. The process immune time is introduced to describe the vulnerability. Based on the traditional consideration of only the ability of users to resist voltage sag, the ability of users to recover from voltage sag is further considered. Then, the risk evaluation model of voltage sag is established. The evaluation results can help power utilities to judge whether enterprises have the demand of VSI. Finally, the improved premium model of VSI based on VSR is constructed to ensure the differentiation and rationalization of the premium. The correctness and rationality of this method are proved by ten manufacturing enterprises in China by analyzing the VSR and comparing the premium before and after the improvement. [ABSTRACT FROM AUTHOR]

This paper proposes a load transfer optimization (LTO) model involving hot-spot temperature (HST) and top-oil temperature (TOT) limits of transformers. This work aims to address the overload problem and transformer HST and TOT excursions via LTO. First, the equivalent thermal model for the HST and TOT of transformers under external cooling conditions are discussed, and then the corresponding linearized HST and TOT constraints with varied thermal resistances are derived using the discretization method. Moreover, the proposed LTO model is formulated on the reconfigurable topology of high voltage distribution networks (HVDNs) in China. This proposed LTO model can achieve the optimal network reconfiguration scheme subject to temperature limits and system operation constraints, with the least switching operation cost and the maximum margins of reactive power exchange between HVDNs and transmission systems. Finally, the effectiveness of the proposed LTO model is validated by the modified IEEE-14 node system and the practical HVDNs in China. [ABSTRACT FROM AUTHOR]

The hybrid LCC/VSC cascaded HVDC (HC-HVDC) scheme, in which the LCC is adopted as rectifier, and one LCC in series with multiple paralleled VSC based MMCs is adopted as inverter, combines the advantages of both LCC and VSC-MMC technologies. Presently, a HC-HVDC project is planned to deliver the hydro power from Baihetan to Jiangsu in China. One of the major challenges is to suppress the DC overcurrent issue of VSC-MMCs especially under commutation failure condition of LCC at inverter side. Based on Baihetan-Jiangsu project under construction, the model of HC-HVDC system is developed in PSCAD/EMTDC. In the system, the rectifier employs a dual 12-pulse LCC, while the inverter adopts a hybrid scheme, i.e., one LCC converter in series with three paralleled VSC-MMC converters. This paper addresses the main cause and feature of the DC overcurrent, then proposes an overcurrent suppression control method for HC-HVDC system based on fuzzy clustering and identification approach. And the system transient performances with the proposed control approach are evaluated under AC fault conditions. The results show that the proposed approach can not only suppress the DC overcurrent following the commutation failure to a certain extent, but also improve the post-fault recovery performance of the overall system. [ABSTRACT FROM AUTHOR]

Calculation of geomagnetically induced current (GIC) flowing through power system during the geomagnetic storm has attracted more attention recently. However, for high voltage power systems with transmission lines over hundreds or even thousands of kilometers, the earth model and geomagnetical field generally vary significantly. So, it is essential to take them into consideration using limited earth survey sites and geomagnetic observatories. To address this problem, a Kriging method is introduced in this paper to make earth model and geomagnetical field interpolations. It has the characteristic of spatial autocorrelation by considering not only the distances between predicted points and training points but also the distances between training points themselves. Finally, a case study of the Central China 1000 kV ultra-high voltage (UHV) grid is carried out to illustrate the applicability and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The multi-terminal hybrid LCC/MMC HVDC has been applied to overcome the simultaneous commutation failure problem caused by the multi-infeed LCC-HVDCs in the East China. The high-speed and selectivity line protection method is required for the multi-terminal hybrid HVDC system. In this paper, the reflection and refraction of the fault traveling waves at different kinds of line terminals are characterized by transfer functions. Then the fault voltage as well as its first peak time at different line terminals when different internal and external faults occur are derived respectively. The conclusion proves that the first peak time of line-mode fault component voltage (FPTV) for internal faults is smaller than that for external faults, which is independent of fault impedance and fault type as well. Therefore, a FPTV-based non-unit ultra-high-speed dc line protection method is proposed. The algorithm is implemented in the developed protection prototype and validated in the Kun-Liu-Long three-terminal hybrid HVDC RTDS test system. The test results show that the proposed method can fast and correctly identify line faults in the multi-terminal hybrid HVDC system, even with 500 Ω fault impedance. Comparing with the traveling wave protection method of ABB, the proposed method improves the sensitivity when high-impedance faults occur. [ABSTRACT FROM AUTHOR]

The insulation paper loss of life (LOL) of a distribution transformer (DT) is largely determined by its winding hot-spot temperature. It is mainly estimated via the transformer load, ambient temperature, and related physical parameters. Fortunately, advanced metering infrastructure (AMI) can provide load profiles of DTs, allowing a cost-effective LOL estimation solution. However, the AMI dataset contains measurement errors that significantly reduce LOL estimation accuracy. A forecasting method is needed to recover the erroneous AMI data. This is a challenging problem, as the load profiles of DTs are nonstationary and volatile. We propose an ensemble of stacked long short-term memory (ES-LSTM) networks to simultaneously capture load consumption patterns of DTs and their correlations. The ES-LSTM consists of two independent LSTM networks and a feed-forward neural network (FFNN) stacked on top of them. A two-stage training procedure is applied. In the first stage, two LSTM networks are trained to capture the daily or weekly load patterns. In the second stage, the FFNN is trained to optimally combine the last hidden outputs of the two LSTM networks. Field verification is conducted using real AMI data obtained from an urban distribution utility in China. The test results confirmed the superiority of the proposed method. [ABSTRACT FROM AUTHOR]

Faults on the alternating current (AC) transmission lines could cause commutation failures of nearby line-commutated converter-based high-voltage direct-current (LCC-HVDC) systems, which in turn, may challenge the fault detection of AC systems. To address this issue, this paper proposes a fast fault detection method based on instantaneous active power and fault variation component to alleviate the impacts of commutation failures and reactive power compensation while mitigating the dead zone problem. Upon that, an integral-based differential protection algorithm is presented to enhance reliability. Simulations are carried out on a real hybrid AC/DC system in North China with different fault locations and resistances to verify the effectiveness of the proposed method. Finally, we further validate the proposed method by leveraging on-site recorded fault data. [ABSTRACT FROM AUTHOR]

Consumers’ transformers play an important role in power systems, and they are essential for operation reliability and commercial benefits. In the past, maintenance personnel had to spend plenty of time on examining consumers’ transformers one by one. Nowadays, with the wide deployment of power user electric energy data acquire system (PUEEDAS), informative metering data are becoming available, which can be utilized for further condition monitoring. Thus, this paper proposes a data-driven abnormal condition monitoring algorithm of data acquisition for consumers’ transformers, which could timely send abnormal condition alerts to operators and maintenance personnel. In the proposed algorithm, Spearman's rank correlation coefficient is utilized to show the degree of correlation among phase currents, and its t-Statistics is used to determine whether abnormal condition of data acquisition exists based on the hypothesis testing. Finally, actual acquisition data from Zhejiang power system in China are employed to validate the effectiveness of the proposed algorithm, and to analyze the characteristics of normal and abnormal conditions, respectively. Sensitive analyses on different significant levels and sampling rates are performed for considering its impact on monitoring results; the application in real power systems is also given to demonstrate the practicality of the proposed algorithm. [ABSTRACT FROM AUTHOR]

In China, two modular multilevel converter (MMC) projects were recently commissioned, and another two are currently under construction. The real-time hardware-in-the-loop (HIL) test bench played an important role in validating the manufacturer's controllers. The full detail MMC model does not fit with the HIL test bench, which has to solve the circuit containing numerous switches and handles a large amount of inputs and outputs at a small time step for real-time simulation. The main challenge is to find a method to model and simulate the MMC systems with sufficient detail, accuracy, and speed. This paper presents an equivalent circuit method for the HIL test bench. The circuit inside the submodule is represented by mathematical equations, implemented in CPU or a field-programmable gate array, and solved in parallel to achieve real-time performance. This method-based test bench is used in those MMC projects in China and connected to the manufacturer's controllers for HIL tests. The model accuracy and simulation speed achieved by this method met the requirements of the HIL tests. In this paper, various scenarios are tested in an MMC HVDC study system. The results achieved by the proposed method have high agreement with those of a reference model in EMTP-RV. [ABSTRACT FROM PUBLISHER]

Frequency response analysis (FRA) is an effective approach for detecting mechanical damage in the windings of a transformer. However, the application of FRA to estimate the winding condition of autotransformers (ATs) employed in the China High-Speed Railway is problematic. Because the split windings of the ATs are connected inside the tank, it is impossible to measure the FRA curves for each split winding outside the tank directly. Therefore, it is difficult to identify fault winding and effectively diagnose the overall condition of the windings. This paper explores the features of FRA curves under different fault conditions to help distinguish the fault winding. An equivalent circuit model is built for the AT with split windings, and the electrical parameters of the windings are determined using the finite-element method. The model is then employed to simulate typical axial displacement faults. The results show that the first antiresonance and resonance of the frequency response for connections involving series windings or series and common windings may be visible features for diagnosing winding faults and the fault level. Specifically, the first and second resonances of the frequency response for the common winding connection may be regarded as the primary feature for identifying fault windings. [ABSTRACT FROM PUBLISHER]

With the rapid development of China's economy, smart grid has been built in an all-round way. As an important basic node of smart grid, the technical complexity of smart substation automation system is getting higher and higher. Once a fault occurs, if not handled in time, it will seriously affect the safety and stability of the power grid, which poses a great loss to economic production activities. The traditional mode of local maintenance has the drawbacks of high time consuming, low efficiency, and heavy workload. To solve these problems, a remote maintenance technology for substation automation equipment based on general service protocol (GSP) is first proposed in this paper, and the brand new system architecture is designed which involves the service management center, the maintenance center, and substations. The standardized interaction mechanism, integrated deep security protection methods, and flow control strategy are adopted to ensure the standardization, security, and reliability of the remote maintenance system. Finally, the effectiveness of the proposed methods is successfully demonstrated, and the comprehensive performance of the remote maintenance system is evaluated by comparing with the traditional local maintenance mode. [ABSTRACT FROM PUBLISHER]

In order to solve the frequently occurring lightning flashover problems on ±500 kV dc transmission lines in China, the authors have conducted the research on the dc line arrester since 2011. In this paper, the structure and installation, technical parameters, and protection performances, as well as their deterministic processes of the arrester are presented and analyzed in detail. In 2013, the authors successfully developed the arrester products, and the type tests show that they have excellent performance parameters and can act effectively against the lightning impulse and cutoff the dc follow current. By now more than 500 dc line arresters have been installed and used on seven ±500 kV dc transmission lines in eight provinces of China. About four years of operating experience shows that the arresters work safely and reliably over a long period of time, and have been effectively reducing the lightning flashover rate of the ±500 kV dc lines. [ABSTRACT FROM PUBLISHER]

The development of new technologies in power generation and application, together with the demand for the reliability and capability of power supply has drawn increasing attention to dc distribution and microgrid technology. This paper introduces the renovation project for a single-circuit XLPE cable line being converted into dc operation after its second failure under 35 kV ac. This is the first time ever in China for this type of system conversion, in which the existing XLPE cable line was put into operation in a bipolar voltage-source converter (VSC) dc system of 10 kV voltage and 150 A current to maintain its 3 MW capacity. The test methods for cable performance that were checked under the designed dc scheme were put forward and conducted on sectioned cable samples. After the cable test and equipment installation, the line was put into operation under the VSC dc topology in 2011. It has now been running for four years without any interruption. Compared with other alternatives for ac operation, this proposal shows advantages in aspects of economy, reliability and engineering. Besides, the theoretical evaluation suggests that the system capacity can be further increased by four times, and the first step is underway for that goal. [ABSTRACT FROM PUBLISHER]

The smart substation, revolutionarily changing every aspect of the modern substation, is developing fast in the world and being massively deployed in China quickly. A smart substation is typically implemented with a sophisticated combination of smart primary high-voltage equipment and hierarchically networked secondary devices. Based on the IEC61850 communication protocol, the functionalities, such as the information sharing and interoperability among smart electric equipment, are realized in smart substations. It is regarded as the basis for the development of smart grid and represents the future development trend of substation technologies. This paper reviews the fundamental vision of the smart substation. The state of the art and the challenges encountered in the practice of engineering implementation are presented. Future developments to solve the present challenges and promote the development of smart substations are also described. [ABSTRACT FROM PUBLISHER]

A multi-purpose balanced (MPB) transformer is proposed in this paper for electric railway applications. The MPB transformer is developed from impedance-matching (IM) transformers widely used in China's railway systems. The proposed MPB transformer has three main functions: 1) it provides a symmetrical two-phase voltage of 27.5 kV for railway overhead lines to power electric trains; 2) it supplies a three-phase balanced voltage of 10.5 kV for railway substations; and 3) it can accommodate a three-phase harmonic filter with improved power factor and reduced manufacturing cost. In this paper, the operating principles of the MPB transformer are elaborated, the design of the harmonic filters is discussed, and the results of computer simulations are provided. A 16-MVA MPB transformer was designed and commissioned, and real-time field measurements are provided. [ABSTRACT FROM AUTHOR]

A few problems in the present calculation methods in the aspects of insulation flashover criteria, influence of operating voltages, induced overvoltage component, and shielding failure in China are briefly analyzed and discussed. Lightning performance of 500-ky double-circuit line schemes (nine tower types and thirteen schemes of insulation disposition) for the Three-Gorge Project is computed and compared with the existing operation experience of double-circuit lines. Different schemes are compared with each other. The cause of the remarkable increase of the total lightning outage rate of the usual unbalanced double-circuit lines, with one circuit insulation reduced, in the operation is analyzed. Different from the usual unbalanced insulation lines, a one-circuit insulation enhanced scheme is proposed in this paper. It is emphasized in the paper that the key of the lightning protection of double-circuit transmission lines is to reduce the double-circuit simultaneous trip-out rate and to keep a reasonable total trip-out rate at the same time. It can be achieved by choosing the proper insulation disposition scheme referring to the order of the weighted trip-out rate values. The viewpoints and methods in the paper would be useful to others working in this field. [ABSTRACT FROM AUTHOR]

This paper presents an investigation into cloud-to-ground lightning activity data collected by a lightning detection system in the Guang-Dong Province, the southernmost in China. Annual lightning days are acquired and ground flash density maps are constructed from the database, which contains the information of more than one million flashes. The lightning days are compared with annual thunderstorm days collected by the Guang-Dong Meteorological Agency, and a weak correlation is noted. The correlation between ground flash density and thunderstorm day is studied, and an empirical formula is presented. The relationship between transmission line faults and ground flash density is discussed in this paper too. It is noted that the correlation with the faults caused by lightning is relatively high. The correlation coefficient does not significantly vary with the threshold of discharge current set for constructing the map of revised ground flash density. [ABSTRACT FROM AUTHOR]

Earlier calculations of geomagnetically induced currents (GICs) in Chinese power grids have mainly concentrated on the highest voltage-level system whose geomagnetic risk is considered to be the largest, thus ignoring secondary voltage-level systems. With the 1000-kV system being newly added to China's power system, it is significant to figure out the interaction between GIC in the 500 kV [extremely high voltage (EHV)] system and GIC in the 1000 kV [ultra-high voltage (UHV)] system. Based on the North China-Central China-East China Power Grids, this paper establishes a single-voltage grid by only considering the 1000-kV system and a dual-voltage grid by considering the 1000- and 500-kV systems and investigates GIC in these two grids by developing their GIC “Full-node models.” The impact of the 500-kV system on GIC in the 1000-kV system is analyzed. GIC risks in the UHV and EHV systems are assessed by comparing calculated GIC data with monitored values of GIC. The results show that the impact of the 500-kV system on GIC in the 1000-kV system is obvious, both the EHV and the UHV grid have a high GIC risk. So calculating GIC in the UHV system and in the EHV system should utilize GIC modeling methods for multivoltage power grids. [ABSTRACT FROM PUBLISHER]

High-frequency harmonic resonance has occurred frequently in the traction power-supply system (TPSS) when high-speed trains that are equipped with pulsewidth-modulation converters were serviced in the China High-Speed Railway (HSR). This phenomenon can result in serious voltage distortion, overheating of electrical components, and electromagnetic interference (EMI) of communication circuits. In order to address this issue, a complete mathematical model of the 2\,\times\,27.5-kV ac railway system, by means of the nodal admittance matrix, is presented to attain equal modeling of TPSS and modern train for harmonic studies in this paper. The impact of the train on resonance behavior of TPSS by employing resonance-mode assessment and modal sensitivity analysis are investigated. Finally, numerical simulation results using the presented model are compared to measurements results of the China HSR to verify the accuracy of the TPSS model and Norton model of the train, and proved that the method of resonance frequency shifts described is doable. The comparison results illustrate that the proposed model and harmonic resonance assessment can be useful for determining and mitigating the harmonic resonance problems in TPSS as well as the power system. [ABSTRACT FROM AUTHOR]

The standard waveform of switching impulse is important for insulation tests of air gaps as well as external insulation and inner insulation of power apparatus in the power system, which should be consistent with the waveforms in actual power systems. Different definitions of waveform parameters of standard switching impulse are discussed in this paper, and a definition based on breakdown characteristics of air gaps is suggested to obtain the front times. Waveform parameters of switching overvoltages in two 1000-kV ultra-high voltage systems in China are studied and their statistical regularities under different operation conditions are analyzed. Based on the breakdown characteristics of air gaps and the concept of equivalent insulation performance, the typical front times in the system are obtained by a weighted average method. [ABSTRACT FROM PUBLISHER]

With the decision to construct the smart grids around China, the power line communications (PLC) on the low-voltage (LV) distribution networks have become one of the potential technologies to commute the information between the end users and the power provider. In order to provide communications services with different priorities under the smart grids environment, it is a must to design a completely new PLC system with variable information rate, which means understanding of the LV PLC channel characteristics become vital. This paper presents the measurement results of channel properties of LV PLC systems after giving a general overview of the topologies for the typical LV distribution networks in China. The testing results show that the main reason influencing the reliable communication of high-speed data of power line is the attenuation of the high-frequency signal, which exhibits more obviously in the branch of power line. It is almost impossible to use the frequency range from 10 to 20 MHz for the reliable communications from distribution transformer to end user, so it must be solved with the aid of such means as the repeater and the modulation schemes. Considering the signal's attenuation, the lower frequency range from 2 to 10 MHz is more suitable for the high-speed data access system. Based on the general topologies of the LV distribution networks, a new method with the help of the reflection matrix based on the multiconductor transmission line theory is proposed to model the channel transfer functions, taking into account of the source inner impedance matrix. The simulation results fit the measured data quite well and verify the feasibility of the proposed modeling scheme. [ABSTRACT FROM AUTHOR]

Ice melting with dc is one of the key technologies to prevent the Chinese power grid from ice storms. Especially after the severe ice storm in the southern part of China in early 2008, dc ice-melting technology has drawn more attention than ever before. However, there are few satisfactory methods to select correct parameters so the ice-melting project rarely achieves desired effects in some cases when applied in Hunan, Guizhou, and other provinces in China. Therefore, it is of great significance to develop a method to estimate parameters which is applicable in the practical situation for dc ice melting. To handle this, the factors, such as wind velocity, ambient temperature, current density and ice-layer thickness were analyzed and then a dc ice-melting model is put forward in this paper. Both the results of simulations and experiments show that the ice-melting process can be divided into three stages composed of temperature rising, ice melting, and ice shedding, among which the ice melting is the key stage consuming most of the ice-melting time. In this stage, an elliptic airgap is formed and widened gradually with an influence on the ice-melting time and the temperature of the conductor. The experiments in the artificial climate chamber demonstrate that the results of the presented model are consistent with those of the experiments generally, so it can be employed to estimate the parameters of ice melting in practical engineering as reference. [ABSTRACT FROM AUTHOR]

The breakage of overhead covered conductors on distribution lines resulting from the short-circuit arcs caused by lightning flash becomes a challenge as the application of covered conductors increases on distribution lines. Based on various methodologies and devices developed to avoid the covered conductor breakage in the world, novel arc-guided devices, including arcing protection hardware with barbs, clamping post composite insulator, and barb electrode clamping post porcelain/composite insulator are presented in this paper. Experimental results are also presented to validate these new developed devices. Devices based on the developed technologies have been widely applied on the power grids in China successfully. [ABSTRACT FROM AUTHOR]

With the operation of long transmission lines, unknown severe vibration and noise in transformers have sometimes been observed at the Shanghe substation and at the Ling'ao nuclear power plant in China. This paper gives a comparison of magnetic-storm recordings and measured transformer neutral current data and confirms that the disturbances were caused by geomagnetically induced currents (GICs). On November 7 and 9, 2004, the maximum GIC at the Ling'ao nuclear power plant was 47.2 A and 75.5 A, respectively, which are both larger than the neutral dc current caused by the HVdc monopole operation. The analysis of GIC data indicates that the GIC level is mainly determined by the geomagnetic-field variation rate, but the earth conductivity and power grid parameters are also important factors affecting GIC values. The data show that the GIC level is higher at the Ling'ao nuclear power plant than at the Shanghe substation. The reason is considered to be related to the grid structure and to the coast effect. Furthermore, because of smaller resistances, longer transmission lines, a larger scale, and the adoption of single-phase transformers, the influences of GIC on the ultra-high voltage power grid being constructed in China need to be studied more urgently. [ABSTRACT FROM AUTHOR]

This paper describes the principle of a new current differential relay developed for a 1000-kV UHV transmission line that is being constructed in China. The distributed capacitive current along the relatively long overhead line will have a significant effect on the relay performance and should be taken into account in the relay principle. The study results in a new current differential relay based on the steady state transmission line equations, in which the distributed capacitive current is inherently represented. Analysis is carried out for different practical situations where shunt reactors or series capacitors are present in the system for compensation. Laboratory tests show that the relay principle developed in this study can be used as a main protection scheme for the 1000-kV UHV transmission line. [ABSTRACT FROM AUTHOR]

The low-voltage powerline communication (PLC net- work) is a technical and attractive innovation in the field of communication. PLC utilities can support energy distribution and provide a pipe for high-speed reliable communication traffic. Wide business opportunities can then be expected. Rut the question that re- mains is how to make the PLC network a commercial reality in the competitive broadband market. The low-voltage electrical network is an unfriendly environment and several factors inherent to the PLC concept itself present technical challenges to using it for data communication. Besides interference and low signaling impedance, PLC transmitters also require a highly linear operating environment. In this paper, a coupling unit topology and design methodology, which could provide gain equalization and wideband mitigation of the effects of low-impedance loads on PLC at the frequency ranging from 0.1 to 30 MHz are presented. Maximization of the network signaling and implementation of a high-speed PLC-oriented system are main objectives for the present work. Some experimental measurements have been performed at residential and industrial environments in China. Motivations to conduct this experiment are given; some compromise was obtained and results are presented. [ABSTRACT FROM AUTHOR]