Search

Fault analysis is an essential prerequisite for fault detection, location and isolation. A detailed fault analysis method on the steady state of the pole‐to‐pole fault for voltage source converter (VSC) based DC systems is presented in this paper. According to fault characteristics under different transition resistances (TRs), the steady‐state response is divided into three scenarios: low transition resistance with blocked converter (LTRBC), medium transition resistance with blocked converter (MTRBC) and high transition resistance with unblocked converter (HTRUC). When TR is relatively low, fault currents through insulated gate bipolar transistors (IGBTs) are large and the converter is blocked. After an in‐depth study of freewheel diodes' conduction states, the iterative method is utilized to solve the steady state under blocked converter. When TR is relatively high, fault currents through IGBTs are not large enough to block the converter. With the unblocked converter, control strategy and pulse width modulation (PWM) are investigated to obtain steady‐state fault currents. Finally, a reasonable simulation model was built in PSCAD/EMTDC software. The simulation results verified the fault analysis accuracy, with much less calculation error than that of the conventional method. Besides, the proposed analysis method was proved applicable to fault scenarios under different TRs. [ABSTRACT FROM AUTHOR]

Voltage source converter‐based DC systems face severe overcurrent problem under DC line faults, which causes significant influence on security, reliability and stability of the power system. As the theoretical basis for relay coordination and protection, the transient fault analysis of voltage source converter‐based DC systems needs an in‐depth study. Based on the conventional fault analysis, an accurate transient fault analysis method for the DC pole‐to‐ground fault and pole‐to‐pole fault through freewheeling diode switch state signals is proposed. In the capacitor discharge stage, fault current calculation error is reduced by taking into consideration the current fed from the grid side. In addition, through detailed analysis of the conduction condition and state of the freewheeling diode, clear calculation equations of transient fault currents are obtained. With the prior knowledge of fault response characteristics, definitions of freewheeling diode switch state signals assist in unifying calculation processes of different fault stages and simplifying transient fault calculation. Finally, a typical simulation model of the voltage source converter‐based DC system was built in PSCAD/EMTDC software. The simulation results verified the conciseness and correctness of the proposed fault analysis method compared with the conventional fault analysis. [ABSTRACT FROM AUTHOR]

Fault location plays an important role in accelerating fault clearance and grid recovery, which greatly improves the reliability and robustness of low‐voltage direct current distribution network. A fault location method based on auxiliary injection unit (AIU) is proposed in this study with respect to its basic structure and principle. With single terminal information, the proposed method allows low requirement of sampling frequency and enhances the tolerance to fault resistance by utilising the characteristic of AIU. Furthermore, design criteria of components in AIU are discussed in detail. Optimised parameters contribute to better performance in estimating fault distance. Numerous simulations demonstrate that the proposed method can realise accurate fault location with fault distance and fault resistance varying in a wide range. [ABSTRACT FROM AUTHOR]

Multi-battery energy storage systems (MBESSs) are widely adopted to overcome the uncertainty problem of renewable energy sources. However, state-of-charge (SOC) imbalance issue of MBESSs brings great challenges to the optimal dispatching strategy and the lifetime management strategy for the MBESS. This study proposes a distributed equalisation strategy for the MBESSs to solve the stated problem. An equitable operation optimisation model is proposed in the distributed equalisation strategy. The equitable optimisation model can be simplified as a convex quadratic optimisation problem. An equitable approximation method is utilised to solve the convex quadratic optimisation problem according to the Karush–Kuhn–Tucker optimality conditions. Based on the equitable approximation method, the SOC balance process is distributed into different iteration steps. The iteration of the SOC balance process is implemented by the communication system connecting the distributed management systems (DMSs). The distributed SOC balance step is implemented by the DMSs of the batteries. The proposed strategy is validated by a hardware-in-the-loop simulation system based on the RT-LAB. Test results show that the proposed strategy ensure the SOC balance and improve the robustness of the MBESS. [ABSTRACT FROM AUTHOR]

With the advantage of saving resources and high utilisation, DC transmission becomes the vital part of the sustainable development. However, due to the control strategies of grid-side inverter, the fault characteristics of the grid will change dramatically, which influences the reliability of grid protection. By establishing the model in PSCAD, the fault characteristics of the inverter was obtained and the effects on current protection was analysed here. The results reveal that the magnitude of fault current was directly influenced because of being connected with the inverter. The grid inverter can make a difference in selectivity and reliability of current protection. In order to solve this problem, the paper constructs the equivalent model of the inverter and attains the unique feature that the fault component impedance measured in the location of protection decreases with the increasing distance of fault location. According to the characteristics, a targeted fault component impedance protection is designed. The feasibility and effectiveness of this protection is verified by simulation in PSCAD model. [ABSTRACT FROM AUTHOR]

With the considerable demand for distributed renewable energy and the significant increase of generalised DC loads, such as IT equipment, DC air-conditioner, electric vehicle, the traditional AC distribution network is no longer efficient. Driven by the rapid development of power electronics technology, the AC/ DC hybrid system is an effective way for large capacity AC/DC sources and loads efficient integrated. To ensure the stable operation of AC/DC hybrid system, the grounding mode analysis is necessary, which is the precondition for overvoltage analysis, lightning arrester configuration, and protection design. To select grounding mode of AC/DC hybrid system, the detailed system structure scheme is first determined considering the factors such as integrated object, reliability requirement, capacity of sources and loads, and converter type. Then the main connection scheme is selected considering the principle of line insulation, economy, and reliability. Based on the system structure and main connection scheme, a suitable grounding mode is selected for the AC/DC hybrid system with various distributed renewable energy, by weighing various factors such as power supply reliability, protection sensitivity, insulation level, and personal safety. [ABSTRACT FROM AUTHOR]

Power electronic transformer (PET) composed of modular dual active bridge (DAB) blocks has a promising future in ac–dc hybrid distribution network because of its energy routing ability. The key element that restrains the development of PET is the vulnerability of power electronic devices during faults, especially dc faults. DC faults (mostly line faults) have very short transient process and no obvious fundamental frequency, which means fault location is difficult to be realized for dc lines in distribution network. Considering that fault location plays an important role in realizing fast fault clearance and recovery, this paper analyzed fault characteristics of dc line faults with respect to the basic structure of modular DAB block and proposed a two-terminal fault location method based on transient voltages measured in dc line boundaries. To evaluate the validity of proposed method, numerical simulations with different fault conditions were implemented in PSCAD/EMTDC. The simulation results indicate that the proposed fault location method located the faulted point quickly and correctly. [ABSTRACT FROM AUTHOR]

The line protection methods for high‐voltage direct current (HVDC) transmission system in engineering applications are poor in sensitivity and reliability. In this paper, we introduce a new notion of energy, namely a particular AC energy, based on which a particular AC component protection scheme is proposed. The scheme is comprised of start‐up criterion, fault identification criterion, and faulty pole identification criterion. The start‐up criterion is based on the fact that particular AC component measured at both terminals increases rapidly when a fault occurs. The particular AC energy calculated by particular AC component at both terminals behaves differently in the face of an internal or external fault, and this characteristic is applied to construct the fault identification criterion. The particular AC energy on the non‐fault line is smaller than that on the faulty line, which can be used to establish the faulty pole identification criterion. Compared with other energy‐based protections, the proposed method does not require data synchronization and is not affected by the distributed capacitive current of the transmission line. Comprehensive simulation experiments including real‐time digital simulator (RTDS) tests indicate that this method is able to operate reliably under various fault conditions with satisfactory accuracy and high sensitivity. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Direct‐current (DC) line faults are a major issue in ultra‐high‐voltage direct‐current systems due to the complex and wicked environment. Thus, reliably detecting and isolating the DC line faults are not a trivial engineering task. This study proposes a DC line protection scheme based on resonance frequency. The scheme mainly consists of a fault discrimination criterion and a faulty pole selection criterion. The fault discrimination criterion is based on the phase of the resonance frequency impedance, and it is deployed to differentiate the faults on the protected DC line from the external faults. The faulty pole selection criterion is based on the ratio of the resonance frequency currents, and it is employed to select the faulty pole. The sensitivity analysis demonstrates that the performance of the proposed protection is promising. The simulations carried out in PSCAD/EMTDC under various conditions verify the feasibility and validity of the proposed protection scheme. [ABSTRACT FROM AUTHOR]

The high fault current of dc line is a major threat to multiterminal voltage-source-converter-based HVDC (VSC-HVDC) system. However, dc circuit breaker (DCCB) with large capacity and fast breaking speed is still under development. Therefore, fault current limitation is vital for the multiterminal VSC-HV DC system. This paper proposes a simple and easily applied hybrid current-limiting circuit (HCLC) at dc side, which consists of a current-limiting inductor (CLI) and an energy dissipation circuit (EDC) in parallel with the CLI. The CLI is designed to reduce the requirement for the DCCB's capacity and breaking speed. The EDC, which consists of thyristor-controlled resistors, is proposed to reduce the stress on energy absorption element (metal oxide arrester) in DCCB and to accelerate the fault current interruption. The design and discussion about the HCLC parameters are performed in detail. By employing the proposed HCLC, dc line fault in the multiterminal system can be isolated effectively with existing DCCB technology, and fast system restoration without power interruption of healthy part can be achieved. Numerous simulations with real-time digital simulator and comparisons with traditional schemes have demonstrated the promising performance of the proposed HCLC. The effectiveness of the HCLC's topology has also been verified by a simplified and scaled test circuit. [ABSTRACT FROM PUBLISHER]

DC line faults are major issues for a multiterminal high-voltage direct current (HVDC) system based on voltage-source converter (VSC). The fault current increases quickly along with a large peak, and complete isolation of the faulted system is not a viable option. Therefore, protection with high selectivity and accuracy is essential. In this paper, a new protection scheme for dc line in multiterminal VSC-HVDC system is proposed, which consists of a main protection and a backup protection. Both the protection principles are based on the supplemental inductor placed at each end of the dc line. Fault identification can be achieved by calculating the ratio of the transient voltages (ROTV) at both sides of the inductor. The main protection is able to detect the fault quickly without communication, while the backup protection is a pilot method based on the ROTVs at both ends of dc line, which is employed to identify the high-resistance faults and offer a backup in case the former fails. Comparison with some previous protection methods shows that the performance of the proposed protection scheme is promising. Numerous simulation studies carried out in PSCAD/EMTDC and real-time digital simulator (RTDS) under various conditions have demonstrated that fault identification with high selectivity and strong robustness against fault resistance and disturbance can be achieved by employing the proposed protection scheme. [ABSTRACT FROM PUBLISHER]

Crowbar circuit is a preferred solution to maintaining wind generators connected to the grid during a disturbance. However, the activation of crowbar protection will lead to an obvious difference between the positive sequence impedance of wind generators and the negative sequence one. This study analyses the influence of this unique fault characteristic of doubly‐fed induction generators (DFIGs) on the three zones of distance relays. When the transmission line emanates from a DFIG‐based wind farm, there exists a high probability that the conventional protective zones of distance relays fail to operate. Hence, an adaptive branch coefficient is proposed to calculate the zone 2 setting according to the fault type and equivalent sequence impedances of wind generators. The proposed adaptive setting principle realises a better coverage of zone 2 and enhances the reliability and sensitivity of the relays. Simulation results verify the improvement of the protective zone of distance relays. [ABSTRACT FROM AUTHOR]

The six-sequence component method is widely used in short-circuit calculation of double-circuit lines. However, this method can be used to decouple only two parallel transmission lines whose parameters are exactly the same. In this paper, a new sequence component method is introduced to deal with the lines whose parameters are not exactly the same. Mutual impedances between phases of the same line are decoupled first, and then the zero-sequence mutual impedances between different lines are decoupled. Six new independent components are obtained after decoupling. System impedances are modified based on the relationship between voltages and currents of the new sequence components and the traditional sequence components. According to the boundary conditions of the faults, the characteristics of new sequence components are studied, and the corresponding sequence networks are discussed. According to the combined sequence network, new sequence currents can be calculated, and the phase currents can be obtained easily from the new sequence currents by a transformation matrix. PSCAD simulation results demonstrate that the short-circuit calculation method proposed in this paper is appropriate and practical to deal with the two parallel transmission lines with different parameters and partially coupled transmission lines. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

The energy consumption statistics of buildings shows that cooling takes up a considerable portion of the energy demand. The peak-valley characteristics of cooling demand coincides with that of the electrical load in the microgrid. Thus the management of the cooling demand side can regulate the peak-valley demand and stabilize power fluctuations. This paper proposes a new energy management strategy that reduces the investment and loss of the battery energy storage system ( BESS) by applying ice storage air-conditioning ( ISAC) to the microgrid. Based on the load characteristics and BESS investment, the capacities of the chillers and the ice tank are analyzed. Penalty functions of ISAC and BESS are designed, according to which the stabilization command is dispatched. Finally, an actual case is analyzed by the proposed strategy, and the results verify the feasibility and effectiveness of the energy management strategy. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

DC line fault is a major threat to the security of ultra high‐voltage direct‐current system due to complex terrain and terrible weather conditions over long distances. This study proposes a DC line backup protection method on the basis of transient measured impedance (TMI), which is defined as the ratio of transient voltage and transient current at both sides of DC line. The characteristics of TMIs are theoretically analysed for DC line faults and external faults, and these characteristics are used to construct a protection scheme. Since the TMIs only depend on the system structure, the proposed method is able to identify the faults correctly, unaffected by the fault resistance and DC line attenuation. A selection method for the faulted line is also proposed based on the ratio of transient component on the bipolar lines. Comparison with the transient‐component‐based protections and conventional protections demonstrates the promising performance of the proposed protection. Numerous simulations on PSCAD/EMTDC platform and the tests with field fault‐recording data have validated the effectiveness of the proposed protection scheme. [ABSTRACT FROM AUTHOR]

Quadruple-circuit lines with asymmetric parameters have been widely built to increase power transmission capacity. The asymmetric parameters of the lines lead to different current characteristics compared with traditional lines. A cross-differential protection scheme based on negative sequence fault current is proposed to overcome the deficiency of traditional protection scheme. The fault superimposed networks of quadruple-circuit lines are discussed to analyze the fault sequence currents of each line. Differential currents are deduced to form the relay criterion. The threshold values of protection are calculated according to characteristics of superimposed fault sequence currents. Corresponding operational logic is also introduced to ensure the stability and reliability of protection. Finally, the reliability of the proposed protection scheme is verified. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Quadruple-circuit lines with different self-impedances have been applied in power system with the development of power industry. However, the different self-impedances in quadruple-circuit lines will cause problems in fault analysis and related relay protection in the lines. The impedance matrix of these lines is different from that of symmetrical lines, and existing methods cannot completely eliminate all mutual impedance. A new method is proposed to decouple these asymmetrical quadruple-circuits lines in this paper. The phase-to-phase mutual impedance is eliminated first, and then the line-to-line mutual impedance is eliminated according to the circulating current method. The mathematical deduction is formulated to ensure the complete decoupling of the impedance matrix. The fault conditions can be used to obtain the relationship between the independent sequence networks, which can be used to calculate the fault current. Power Systems Computer Aided Design (PSCAD) simulations indicate that the decoupling method is suitable for fault analysis of the quadruple-circuit lines with different self-impedances. The related fault analysis method is quite accurate and easy to be applied. The fault analysis accuracy is not affected by the fault resistance, fault type, fault location, or length of lines. [ABSTRACT FROM AUTHOR]

This study presents a new protection scheme for high‐voltage direct‐current (HVDC) transmission lines which only uses a specific frequency AC current (SFAC) at one of the line terminals during fault transients. The fluctuation characteristic of the root mean square (RMS) of SFAC is analysed under different fault conditions. The fluctuation in the RMS of SFAC during the transient period of internal and external faults is quite different. A fluctuation coefficient (FC) is proposed to measure the fluctuation. Fault types and the faulted line can be distinguished by calculating FC. A HVDC test system modelled in PSCAD/EMTDC is used to validate the proposed scheme considering different fault types, operation modes, sampling frequency and interference. Comprehensive test studies show that the performance of the proposed protection scheme is inspiring, and it is simpler and is of higher reliability compared with some traditional protections based on harmonic currents. [ABSTRACT FROM AUTHOR]

Distribution networks (DNs) gradually become more active and flexible with the massive integration of renewable resources. The conventional protection methods are unsuitable for the promising active DNs. This paper proposes a protection scheme on the basis of measured impedance, which includes an impedance differential method and an inverse-time low-impedance method. The former approach identifies fault instant and provides time reference for the exchanged data. As primary protection, it is able to clear different types of faults as soon as possible. The latter method operates in an automatic coordination way with inverse-time characteristic. It can be used as backup protection or provide protection for single-end lines alone. Both of the methods are independent of operation modes and are able to clear faults with high sensitivity. The efficiency of the protection scheme is validated by PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Jointed 12-phase-transmission lines are frequently used in power systems. However, it is subjected to a variety of technical problems on fault analysis and protection engineering. These problems are mainly due to the mutual coupling effects between adjacent circuits of the line. A method to decouple the mutual impedance completely was proposed in this paper. When fault occurs in the 12-phase-transmission lines, the faulted phase electrical measurements can be decomposed into twelve-sequence components which are completely independent from each other. The twelve-sequence components consist of positive sequence currents and inverted sequence components and the inverted sequence components will occurs under any internal faulted condition. When fault occurs in the jointed 12-phase-transmission lines, the inverted sequence component voltages calculated from two Bus sides are equal at the faulted point, which can be used to calculate the faulted location. As the twelve-sequence-component method has decoupled the mutual electromagnetic effect between the lines, the accuracy of the fault location is high. A large number of EMTP simulations indicate that the algorithm is completely suitable for the jointed 12-phase-transmission lines and the location accuracy is not affected by such factors as faulted resistance, generation mode and power angle, etc. [ABSTRACT FROM AUTHOR]