High Frequency Transient Sparse Measurement-Based Fault Location for Complex DC Distribution Networks

With mature of the direct current (dc) distribution grid technology, the distributed energy is embraced as much as possible, and reducing network loss becomes an effective way to increase energy efficiency and system stability. The conventional reducing loss approaches include complex algorithms, loss modeling in power devices and adding hardware, which lead to costs and system response time increase and system stability reduce. Considering the Intermittent characteristics of distributed energy, for reducing network loss, the effective option is to control error and response speed directly through the upper control, when distributed energy accesses on dc distribution network, which is no need to change grid topologies, run cumbersome power flow algorithms, nor add additional equipment. A reducing network loss method is proposed based on this idea. The details are as follows: Firstly, the network loss formula is derived based on power flow calculation, and the network loss change rules are analyzed. Secondly, one optimal power flow mathematical model of dc distribution network is established, which takes the minimum network loss as the objective functions and conforms to constraints of system security and components operating limit. The tide optimization is solved by using the artificial bees (ABC) algorithm. Thirdly, the network loss reduction method is proposed in the dc distribution network by using master–slave control through real-time control instruction optimization. The node voltage, branch current and main voltage source converter power are precisely regulated to control the power flow; thus, the network loss of multiple distributed energy access dc distribution network can be reduced. Fourthly, when wind and solar energy is connected to the dc distribution network, a typical IEEE16 node case is demonstrated to verify the feasibility of the proposed method using software MATLAB/SIMULINK. Fifthly, evaluation and prospect are made for the research.

The multi-terminal flexible DC distribution network can realize the power supply of distributed power supply and the flexible operation of multiple DC loads, which is an important development direction of the future DC distribution network. The coordinated control and communication coordination of each converter station and multi-converter is the key to the safety and stability of DC distribution network. Therefore, in view of the physical information coupling characteristics of multi-terminal flexible DC systems, considering the coordinated control stability characteristics and information interaction capabilities between multi-terminal flexible converter stations, a master-slave control strategy based on multi-site cooperative communication is proposed. Then, combined with the real-time power simulation system RT-LAB and the network simulation tool OPNET, a power information hybrid real-time simulation platform for multi-terminal flexible DC distribution network is constructed to analyze the power information interaction process in different communication environments and its application to DC distribution network. Finally, with the Suzhou Four-Terminal Flexible DC Network Project as an example, the effectiveness of the proposed coordinated control strategy and power information hybrid simulation platform is verified by setting network communication congestion and delay in typical scenarios such as fault-locking and DC load disturbance of the master-slave converter station.

One recurring problem in reinforced concrete structures is hidden corrosion. Detection of hidden corrosion is difficult due to incomplete information that can be measured. To overcome this problem, inverse analysis can be employed. Inverse analysis uses incomplete information from field measurement, and employs optimization to infer further information. Previous research has demonstrated the inference capability of inverse analysis for hidden corrosion detection in reinforced concrete. However, question remains on the limit of incompleteness of measurement data. The aim of this study is to simulate and examine the limit of incomplete information that can be allowed for an effective inverse analysis. The inverse analysis of hidden corrosion in reinforced concrete is formulated as a PDE-constrained optimization. The objective function of the optimization is the distance function between measurement data and FEM simulation. The search space is the PDE boundary conditions configuration. A FEM simulation is used to simulate measurement data by half-cell potential mapping on a concrete block of 2 by 1 meters. To simulate incomplete information, the number of measurement points is varied from 2 to 20 points on the surface of the concrete block. Results show that 20 measurement points are needed to successfully infer hidden corrosion profile. However, when the number of measurement points are below this, the effectiveness of inverse analysis varies and does not seem to correlate to the amount of information (i.e., number of measurement points). Further study is needed if this is due to a fluke or choice of measurement point’s location.

The estimation of the localization of targets in wireless sensor network is addressed within the Bayesian compressive sensing (BCS) framework. BCS can estimate not only target locations but also noise variance of the environment. Furthermore, we provide adaptive iteration BCS localization (AIBCSL) algorithm, which is based on BCS and will choose measurement sensors according to the environment adaptively with only an initial value, while other frameworks require prior knowledge such as target numbers to choose measurements. AIBCSL suppose that environment noise variance is identical in interested area in a short period of time and change measurement numbers until terminal condition is reached. To suppress noise, we optimize estimation result by energy threshold strategy (ETS), which takes that transmit energy of noise focused on single grid is much lower than signal into consideration. And multisnapshot BCS (MT-BCS) will be explained and lead to a good result in low SNR level situation.

Voltage source converters (VSCs) are highly vulnerable to DC fault current; thus, protection is one of the most important concerns associated with the implementation of multi‐terminal VSC‐based DC networks. This study proposes a protection strategy for medium voltage DC distribution systems. The strategy consists of a communication‐assisted fault location method and a fault isolation scheme that provides an economic, fast and selective protection by means of using the minimum number of DC circuit breakers. This study also introduces a backup protection which is activated if communication network fails. The effectiveness of the proposed protection strategy is analysed through real‐time simulation studies by use of the hardware‐in‐the‐loop approach. Furthermore, the effects of fault isolation process on the connected loads are also investigated. The results show that the proposed strategy can effectively protect multi‐terminal DC distribution networks and VSC stations against different types of faults.

Compared to the AC distribution network, DC distribution network has the advantage on reliability, transmission capacity and power flow control. This article firstly introduces the current development of DC power distribution technology and the power distribution demonstration projects at home and abroad, and sorts out the main technical standards of DC power distribution system voltage levels. On this basis, the five typical application scenarios of DC power distribution technology are clarified and the main characteristics of various scenarios are further analyzed. Study the structural differences and configuration requirements of various typical medium-voltage DC distribution networks. By comparing and analyzing the technical characteristics of various AC/DC converters, DC transformers and DC circuit breakers, the technical requirements of various DC power distribution equipment in terms of topology, control strategy, and magnetic integration technology are put forward. Furthermore, in view of the advantages and disadvantages of different control methods, research requirements for multi-operation state conversion, circulation suppression, and energy optimization are proposed. Finally, the protection requirements of AC side faults, converter faults, DC side faults and other types of faults are analyzed, and the basic requirements for DC distribution protection configuration and technical requirements for fault identification, fault location, coordination and cooperation of multiple voltage levels are put forward.

To improve the fault location ability of the DC distribution network and ensure its power supply reliability, a fault location method based on the beetle swarm optimization algorithm is proposed. Firstly, the fault characteristics of the DC side after short-circuit fault are analyzed, and the mathematical model for fault location is established; then, the problem of fault location is transformed into the problem of parameter identification by discretization; finally, to verify the effectiveness of the method, a flexible DC distribution network model is built in MATLAB. The simulation results show that the proposed fault location method is accurate, not affected by the transition resistance, and has good robustness.

The DC distribution network based on the hybrid modular multilevel converter (MMC) has the ability of active current-limiting control, which plays an active role in fault clearing of distribution network. However, the fault characteristics of the DC distribution network become complicated because of the active current-limiting control. It brings new problems to the fault detection and judgment of relay protection. According to the structural features of the hybrid MMC, the start-up strategy and control objectives of the active current-limiting control are analyzed in this paper. The improvement suggestions are put forward to maintain the operational safety of the power grid, as well as meet the application requirements of the relay protection. Moreover, The characteristics of the short-circuit current of the DC distribution network are analyzed based on the active current-limiting control. Meanwhile, the influences on the relay protection are estimated. Finally, the effectiveness of the proposed current-limiting control strategy and the correctness of the fault characteristic analysis results are verified by simulation tests.

This paper deals with an inverse problem for estimating the magnitude of the residual stress in a butt-welded plate from measured data. The welding residual stresses in the butt-welded plate are caused by inherent strain. Thus, the welding residual stress in the butt-welded plate can be calculated if the inherent strains are estimated from measurement data. In previous works, a method was proposed to estimate the inherent strains in the butt-welded plate using thermo-elastic boundary element inverse analysis. It was found in the paper that the evaluated value of the inherent strains in the butt-welded plate depended on the location or the number of measurement points in the butt-welded plate. The accuracy of the evaluated inherent strains was low when the location of the measurement point was far from the welding line and the number of measurement points was small. Thus, this paper uses Tikhonov's regularization to improve the accuracy of the evaluated values of the inherent strains in the butt-welded plate. Numerical simulations were conducted for an analysis model which was the same with that used in the previous paper. In the simulations, measurement errors were included in stresses at measurement points. The maximum value of the error was 4.9 (MPa). The results of the calculations showed that the inherent strains in the butt-welded plate were evaluated in high accuracy from a few measured data by using Tikhonov's regularization. The discrepancy principle was applied for estimating a proper value of the regularization parameter. It was found that Tikhonov's method with the discrepancy principle was useful in evaluating the residual stress in the butt-welded plate.

In this paper, a heating process control law with steam supply is designed for a fluid in a heat exchanger. The process is described by a linear hyperbolic equation of the first order with a nonlocal boundary condition with a time-delayed argument. The temperature of the supplied steam is found as a linear dependence on fluid temperature values at measurement points in the heat exchanger. Explicit formulas are obtained for the gradient of the objective functional of the control problem in the space of the feedback coefficients (parameters) of this dependence. A numerical scheme is developed for determining the feedback parameters based on these formulas. Finally, an algorithm is proposed for determining the rational (optimal) number of measurement points.

Fault location and distance protection in transmission lines are essential smart grid technologies ensuring reliability of the power system and achieve the continuity of service. The objective of this chapter is to presents an accurate algorithm for estimating fault location in Extra High Voltage (EHV) transmission lines using Artificial Neural Networks (ANNs) for high speed protection. The development of this algorithm is based on disturbed transmission line models. The proposed fault protection (fault detection/classification and location) uses only the three phase currents signals at the one end of the line. The proposed technique uses five ANNs networks and consists of two steps, including fault detection/classification and fault location. For fault detection/classification, one ANN network is used in order to identify the fault type; the fault detection/classification procedure uses the fundamental components of pre-fault and post-fault sequence samples of three phase currents and zero sequence current. For fault location, four ANNs networks are used in order to estimate the exact fault location in transmission line. Magnitudes of pre-fault and post-fault of three phase currents are used. The ANNs are trained with data under a wide variety of fault conditions and used for the fault classification and fault location on the transmission line. The proposed fault detection/classification and location approaches are tested under different fault conditions such as different fault locations, different fault resistances and different fault inception angles via digital simulation using MATLAB software in order to verify the performances of the proposed methods. The ANN-based fault classifier and locator gives high accuracy for all tests under different fault conditions. The simulations results show that the proposed scheme based on ANNs can be used for on-line fault protection in transmission line.

This article provides a detailed analysis of the dynamic characteristics of DC pole-to-pole faults in modular multilevel converters and proposes a protection and control strategy based on the current rate of change. By detecting the current rate of change, a resistance current limiter is put into operation to achieve DC bipolar short circuit non-blocking fault crossing. The fault current is cut off by the DC circuit breaker. This method is an active current limiting control method that adds current limiting devices before fault current identification, leading to fault identification. Finally, a single-ended flexible DC distribution network model was built by using a real-time simulation system. The simulation results verified the effectiveness and accuracy of the protection and control strategy in the case of pole-to-pole faults.

A fault detection and location algorithm on three-terminal transmission line is proposed. Vast literature on fault location shows that to simplify the analysis, line charging currents are often ignored. To improve the accuracy of fault location, synchronized current and voltage measurements from three ends of the three-terminal line are utilized to detect and locate distance to fault while considering the line charging currents. The method first detects the zone at which the fault has occurred and then locates it from the respective line ends. Since three-terminal lines provide solutions for connecting existing system to the grid, facilitating bulk transfer of power, thus the fault detection and location on the same is important to maintain reliability and security of the system. Performance of the proposed algorithm is tested for various fault types, fault locations, fault resistances and fault inception angles. The 200-kV system is simulated in ATP/EMTP; however, the algorithm is developed in MATLAB. The extensive simulation studies demonstrate accuracy of the proposed scheme.

In dc distribution networks, the parallel H-bridge dc/dc converters (HBDCs) are widely adopted to convert voltage levels with the higher power rating and reliability, in which the parallel HBDCs are naturally connected in input-parallel output-parallel (IPOP) form. However among IPOP HBDCs, there are complicated circulating currents that will influence the safe and steady operation of dc distribution networks. This paper focuses on the suppression of these circulating currents. First, the detailed mathematic models of circulating currents among IPOP HBDCs are derived. Through the model, it is found that various types of circulating currents exist in the system, including the circulating currents within the single HBDC and the circulating currents among the multiple HBDCs. Hence, the suppression of circulating currents among IPOP HBDCs is a multiobjective control problem. In this paper, it is proven that the conventional one degree of freedom control based on the bipolar modulation cannot eliminate all the circulating currents. Second, a novel two degrees of freedom control method is proposed to suppress all kinds of circulating currents based on the improved modulation way of HBDCs, which consists of two parts. The droop based control is used to suppress the circulating currents among the multiple HBDCs, whereas the common mode control is used to control the circulating currents within the single HBDC. All the theoretical analyses are verified by the real-time hardware-in-loop tests.

In order to transmit massive amounts of power generated by remotely located power plants, especially offshore wind farms, and to balance the intermittent nature of renewable energy sources, the need for a reliable high voltage transmission grid is anticipated. Due to power transfer limitations by AC transmission lines and its cost, the most attractive choice for such a power transfer is a high voltage DC (HVDC) lines [1]. The need to detect the fault location in the transmission line as quickly and accurately as possible has increasingly been demanded by utilities, and the use of traveling wave-based fault location technology has been implemented in order to improve the efficiency and to minimize the electrical system downtime and thus to avoid or minimize penalties [2]. The location method consists from measuring accurate time, when the traveling waves (generated by wave fronts caused by transients during line fault) pass through known measurement points, usually substations located at the ends of the transmission line. Different from fault locators using impedance methods, the location methods using traveling waves can achieve much higher accuracy regardless of fault type and line characteristics. The Travelling Wave Fault Locators (TWFL) currently available on the market rely on measurements from inductive CTs and inductive/capacitive VTs, which are not applicable to DC systems. This paper presents a means to acquire the readings of traveling waves in a HVDC transmission system. In addition, results of the field deployment of a TWFL system on a HVDC transmission line are presented. The described system was implemented on the longest in the world IE Madeira HVDC overhead line over a distance of 2375 kilometers, connecting Porto Velho to Araraquara II substations from Northwest to Southeast of Brazil and tested for stage faults during commissioning.