Short-circuit Current Calculation Research Articles

Efficient Estimation and Characteristic Analysis of Short-Circuit Currents for MMC-MTDC Grids

Short-circuit current calculation and its characteristic analysis are the foundation of selecting circuit breakers and designing protection systems. First, a companion circuit-based method is presented to fast calculate the short-circuit current of asymmetric bipolar modular multilevel converter-based multiterminal direct current (MMC-MTDC) grids. With the reduction of the whole MTDC grid into an equivalent RLC circuit, the short-circuit current of the grid is efficiently and accurately solved by companion circuit method instead of the numerical integration. In addition to the efficiency, the proposed method can readily adapt to various types of MTDC grids and all kinds of dc faults. Second, unique short-circuit current characteristics of a bipolar MTDC grid following different types of dc faults are theoretically analyzed. A fault type discrimination method is then proposed for bipolar MMC-MTDC grids with dedicated metallic return. Finally, intensive numerical results of two MTDC grids validate the accuracy, adaptability, and efficiency of the presented dc grid short-circuit current calculation method as well as the correctness of the short-circuit current characteristic analyses.

Practical Calculation Method for the Short-Circuit Current of Power Grids with High Temperature Superconducting Fault Current Limiters

With the expansion of the power grid scale and the enhancement of network structure interconnection, the short-circuit current of power grids is increasingly close to the upper capacity limit of circuit breakers. The fault current limiter is effective in suppressing the short-circuit current of power grids, and with the mature development of superconducting technology, high-temperature superconducting fault current limiters (HTS-FCLs) have been widely used in power grids. Due to the nonlinear impedance–current characteristic of HTS-FCLs, the traditional short-circuit current calculation method is inapplicable to power grids with HTS-FCLs. First, based on the establishment of the electromechanical transient model and the short-circuit calculation model, the short-circuit current calculation procedure for power grids with HTS-FCLs is proposed. Second, the calculation methods for three-phase and single-phase short-circuit currents at the substation bus in power grids with HTS-FCLs are analyzed. Finally, three-phase and single-phase short-circuit currents at the 500 kV Xijiang substation bus in the Guangdong power grid are calculated as the study case. Simulation results show that compared with the situation where there is no HTS-FCL installed, the short-circuit current on the 500 kV bus decreases from 65.34 to 58.31 kA in terms of the three-phase short-circuit fault, and decreases from 54.56 to 46.62 kA in terms of the single-phase short-circuit fault, thus verifying the effectiveness and practicability of HTS-FCLs in suppressing the short-circuit current, which is significant for the safe, stable and reliable operation of the power grid.

Practical calculation of asymmetric short circuit current of DFIG connected to distribution network

In view of the existing complex analytic calculations of DFIG short-circuit current are hardly applied in engineering projects, a practical calculation of asymmetric short-circuit current of DFIG is proposed. According to the complex sequence and their Thevenin equivalent model of DFIG network, the composition of each sequence component of DFIG short-circuit current is analysed. Moreover, considering the low-voltage ride through strategy of DFIG, the negative sequence periodic components of short circuit current are well analysed during the crowbar activation and deactivation, and the formula of the negative sequence periodic components of short circuit current are derived. On the basis of positive and negative sequence open circuit voltage, calculating impedance and rotor current, the judgement of crowbar activation is established. The pre-calculated surfaces of negative sequence periodic components of short circuit current are proposed, and the procedure for calculating the asymmetric short-circuit current of DFIG is designed. Finally, the proposed method is verified by simulation.

Accurate and Efficient Estimation of Short-Circuit Current for MTDC Grids Considering MMC Control

DC short-circuit current calculation for the modular multilevel converter based multi-terminal direct current (MMC-MTDC) grid is critical and fundamental in equipment selection and protection setting. The existing MTDC grid short-circuit current calculation methods suffer from low efficiency, poor adaptability, and inflexibility. To address these issues, this paper proposes an efficient and general companion circuit-based method to estimate the short-circuit currents of asymmetric bipolar MTDC grids. First, an MMC is equivalent to an RLC series circuit in parallel with a time-dependent controlled current source. The equivalent model considers not only the discharge currents of the sub-module capacitors but also the ac in-feed current determined by MMC controllers. Then, the equivalent model for a bipolar MTDC grid with the dedicated metallic return is established by combing the equivalent MMC model with the lumped model of the dc transmission lines. Next, the companion circuit method is employed to efficiently and readily solve the equivalent model of the dc grid to obtain post-fault branch currents and node voltages. Both theoretical analyses and intensive tests on the CIGRE benchmark MTDC grid and Zhangbei bipolar MTDC grid verify the improved accuracy, efficiency, adaptability, and flexibility of the presented method.

A Novel Machine Learning-Based Short-Circuit Current Prediction Method for Active Distribution Networks

The traditional mechanism models used in short-circuit current calculations have shortcomings in terms of accuracy and speed for distribution systems with inverter-interfaced distributed generators (IIDGs). Faced with this issue, this paper proposes a novel data-driven short-circuit current prediction method for active distribution systems. This method can be used to accurately predict the short-circuit current flowing through a specified measurement point when a fault occurs at any position in the distribution network. By analyzing the features related to the short-circuit current in active distribution networks, feature combination is introduced to reflect the short-circuit current. Specifically, the short-circuit current where IIDGs are not connected into the system is treated as the key feature. The accuracy and efficiency of the proposed method are verified using the IEEE 34-node test system. The requirement of the sample sizes for distribution systems of different scale is further analyzed by using the additional IEEE 13-node and 69-node test systems. The applicability of the proposed method in large-scale distribution network with high penetration of IIDGs is verified as well.

Short-circuit Current Calculation Method for Distributed Power Distribution Network with Control Strategies

In recent years, large-scale distributed generation (DG) with control strategies, such as photo-voltaic and wind power generation, is connected to the distribution network, making the traditional short-circuit current calculation method for distribution network no longer applicable. This paper establishes the mathematical model of DG piece-wise function with control strategies based on the control objectives of new DG grid connection. On this basis, this paper proposes an iterative calculation method for short-circuit current with DG distribution network with control strategies by taking the piece-wise function equation as the modified equation. This method can accurately calculate the short-circuit current value of the DG distribution network with control strategies, which provides a basis for the study of the influence of DG access on relay protection and the corresponding countermeasures.

A Novel Overcurrent Suppression Strategy during Reclosing Process of MMC-HVDC

A modular multilevel converter based high-voltage DC (MMC-HVDC) system has been the most promising topology for HVDC. A reclosing scheme is usually configured because temporary faults often occur on transmission lines especially when overhead lines are used, which often brings about an overcurrent problem. In this paper, a new fault current limiter (FCL) based on reclosing current limiting resistance (RCLR) is proposed to solve the overcurrent problem during the reclosing process. Firstly, a mesh current method (MCM) based short-circuit current calculation method is newly proposed to solve the fault current calculation of a loop MMC-HVDC grid. Then the method to calculate the RCLR is proposed based on the arm current to limit the arm currents to a specified value during the reclosing process. Finally, a three-terminal loop MMC-HVDC test grid is constructed in the widely used electromagnetic transient simulation software PSCAD/EMTDC and the simulations prove the effectiveness of the proposed strategy.

Loop Analysis Method for Short Circuit Current Calculation of Distribution Network with Inverter-Interfaced Distributed Generators

To effectively calculate the short-circuit current of distribution network with IIDGs and meet the engineering demand, a novel short-circuit current calculation algorithm based on loop-analysis method is proposed. Equivalent fault models of current-connected IIDGs as well as voltage-controlled IIDGs are derived considering protection and control strategies. Moreover, the loop voltage function of distribution network with IIDGs during a fault is built, and an iterative correction method is proposed to solve the function. Test results show that the proposed method is effective and can satisfy the engineering requirement. It is also shown that the voltage improvement is more remarkable when IIDG adopt LVRT strategy than current limiting strategy. The degree of improvement is determined by many factors such as network structure, fault position, output current of IIDG, etc.

Modelling of design parameters of intrinsically safe instruments for the safety of oil, gas and coal industries

Intrinsic safety is the safest technique to prevent explosions that might occur in underground coal mines and process industries in the presence of explosive atmosphere. Designing of intrinsically safe equipment require keeping its energy below the minimum ignition energy of the explosive gases. In this work, three empirical relations based on standard IS/IEC 60079-11 have been proposed for the first time and used in the calculation of short circuit current (I), permitted capacitance (C) and inductance (L) of intrinsically safe circuits designed for gas groups IIA, IIB, IIC and I. The calculated values are compared with the available experimental and reported values. A fairly good agreement has been obtained between them. The designers of intrinsically safe circuits can use these relations as a very handy tool especially for the explosive gas atmosphere of underground coal mines and process industries. This research will contribute to the reduction of fatal accidents due to explosive gases. [Received: August 20, 2017; Accepted: March 11, 2018]

Modelling of design parameters of intrinsically safe instruments for the safety of oil, gas and coal industries

Intrinsic safety is the safest technique to prevent explosions that might occur in underground coal mines and process industries in the presence of explosive atmosphere. Designing of intrinsically safe equipment require keeping its energy below the minimum ignition energy of the explosive gases. In this work, three empirical relations based on standard IS/IEC 60079-11 have been proposed for the first time and used in the calculation of short circuit current (I), permitted capacitance (C) and inductance (L) of intrinsically safe circuits designed for gas groups IIA, IIB, IIC and I. The calculated values are compared with the available experimental and reported values. A fairly good agreement has been obtained between them. The designers of intrinsically safe circuits can use these relations as a very handy tool especially for the explosive gas atmosphere of underground coal mines and process industries. This research will contribute to the reduction of fatal accidents due to explosive gases. [Received: August 20, 2017; Accepted: March 11, 2018]

IEC 60909 and ANSI standards comparison with ASCC based fault calculations of Iraqi power system

The calculation of short circuit (SC) current has to be accurate because of their importance in power protection system’s design and to determine the capacity of the protective devices. Therefore, ...

Расчет токов короткого замыкания в тяговой сети с двухсторонним питанием

Расчет токов короткого замыкания в тяговой сети с двухсторонним питанием

Учет несинфазности генераторов при расчетах токов в начальной момент короткого замыкания

At present, short-circuit fault currents in electric power systems are calculated in accordance with national standards, which use a number of assumptions. One of these assumptions is neglecting possible out-of-phase operation of synchronous generators in the mode preceding the short-circuit fault. At present, the constantly growing level of short-circuit currents is a factor aggravating the problem of ensuring consistency between the equipment installed in the electric power systems and these currents. Accordingly, more stringent requirements are posed to the accuracy of short-circuit fault analysis methods. The aim of this work is to estimate the effect the out-of-phase operating conditions of synchronous generators have on the short-circuit current and to decide whether it is advisable to introduce changes in the short-circuit current calculation procedure stipulated by the relevant standards. To this end, the effect the out-of-phase operating conditions of synchronous generators have on the current at the initial moment of a short-circuit fault is studied taking into account the parameters of the mode preceding the short-circuit fault. To analyze the influence of out-of-phase operating conditions of the generators on the short-circuit fault currents, a mathematical model was developed, and case calculations were carried out using the Mathcad software package. The study was carried out for a synchronous generator operating in parallel with the electric power system; factors reflecting the design features of the synchronous generators, their power in the mode preceding the short-circuit fault, and the electrical remoteness of the synchronous generator from the grid were varied. The parameters of the electrical equipment installed in the electric power system of the Republic of Tajikistan were used as initial data. The effect the static out-of-phase operating conditions of synchronous generators have on the short-circuit current periodic component value has been evaluated. It is shown that calculations of short-circuit currents carried out according to the methods stipulated by the standards that are currently in force, in which out-of-phase operation of synchronous generators is not taken into account, yield overestimated short-circuit current values, and the calculation error can be as much as 14%. It has been recommended to take into account the static out-of-phase operating conditions of synchronous generators in solving matters concerned with coordination of short-circuit current levels, and to improve the methods for calculating short-circuit currents in power systems. The out-of-phase operation of synchronous generators should be taken into account in substantiating the need of replacing the electrical equipment of power plants and substations in view of growing levels of short-circuit currents.

Short-Circuit Current Estimation of Modular Multilevel Converter Using Discrete-Time Modeling

Short-circuit faults are hazardous for high-power modular multilevel converters (MMCs). Hence, an accurate current calculation will be helpful for the design of the system protection. In this letter, a new method for the short-circuit current calculation based on discrete-time modeling is proposed. With commonly adopted nearest level modulation, the operating state during one period can be divided into several stages according to the number of submodules inserted into a bridge arm. Then, a piecewise analysis allows the construction of a new discrete-time model by recursion of the state variables stage-by-stage. From this model, the current evolution during a short-circuit fault can be accurately calculated when given an initial state. The effects of different fault occurrence times in one period are also calculated and discussed. Finally, PSCAD/EMTDC simulations and a circuit experiment with a single-phase five-level MMC verify the effectiveness of the proposed calculation method.

Calculation of two-phase ground fault current with consideration of equipment grounding specifications

The article describes ways of calculating two-phase ground fault current in electrical networks with isolated neutral for needs of calculating thermal resistance in equipment's grounding elements. Grounding resistance and metal resistance of high-voltage equipment are added to the existing methods of short circuit currents calculation.

Closed-Form Determination of the Impedance Locus Plot of Fault Current Limiters: A Rigorous Approach With Graphical Representation

In this paper, a mathematically rigorous method is presented to calculate the impedance of FCLs based on a desirable change of short circuit currents. The method is found on the principle of superposition and can be easily incorporated into standard short circuit current calculations of power systems. The changes of the short circuit currents for each FCL installed are easily calculated without having to build a new impedance matrix for each new case. In addition, the method gives a clear graphical representation of the impact of the FCL on the power system taking advantage of the FCL impedance locus in the complex plane. The resultant plots provide valuable insight and information to utilities, system planners, and FCL manufacturers. FCL manufacturers can use these plots to identify the FCL impedance value and angle subject to the manufacturing and system constraints. The plots also allow utilities and system planners to compute the changes of fault current distribution caused by the FCL quickly and accurately. Results show that FCLs can increase short circuit currents in other branches. High-limiting FCLs may cause larger increase in short circuit currents of other branches and therefore a compromise may have to be made between fault current limitation and the maximum additional currents that are acceptable in other branches. Examples on a three-bus system and an eleven-bus system are presented for illustration and validation.

On Short-Circuit Current Calculations Including Superconducting Fault Current Limiters (ScFCLs)

This paper introduces a novel modeling approach for short-circuit studies incorporating superconducting fault current limiters (ScFCLs). The conventional formulation for short circuit studies, based on symmetrical components, has been extended to include the nonlinear limiting features of the ScFCL. Indeed, this is a novel iterative modeling method that has been derived from the classical calculations of short circuits. The operating point of the ScFCL is computed in the abc reference frame, while the solution of the whole network, including all of its components, is achieved using symmetrical components. The ScFCL model, which includes various parallel-connected superconducting tapes, is derived for the positive, negative, and zero sequences. Given that the resistance of the ScFCL is nonlinear and depends on the current flow, an iterative process is used to carry out short-circuit calculations. To validate the proposed method, radial and meshed distribution networks were studied. The solutions derived from the proposed approach and the ones obtained from the electromagnetic transient software Alternative Transients Program-Electromagnetic Transients Program were compared with both solutions agreeing very well with each other. The standard IEEE 14-bus test system, slightly modified to include two ScFCLs, was additionally simulated to demonstrate the practicality of the new approach in larger networks.

Short-Circuit Current Calculation and Harmonic Characteristic Analysis for a Doubly-Fed Induction Generator Wind Turbine under Converter Control

An accurate calculation of short-circuit current (SCC) is very important for relay protection setting and optimization design of electrical equipment. The short-circuit current for a doubly-fed induction generator wind turbine (DFIG-WT) under excitation regulation of a converter contains the stator current and grid-side converter (GSC) current. The transient characteristics of GSC current are controlled by double closed-loops of the converter and influenced by fluctuations of direct current (DC) bus voltage, which is characterized as high order, multiple variables, and strong coupling, resulting in great difficulty with analysis. Existing studies are mainly focused on the stator current, neglecting or only considering the steady-state short-circuit current of GSC, resulting in errors in the short-circuit calculation of DFIG-WT. This paper constructs a DFIG-WT total current analytical model involving GSC current. Based on Fourier decomposition of switch functions and the frequency domain analytical method, the fluctuation of DC bus voltage is considered and described in detail. With the proposed DFIG-WT short-circuit current analytical model, the generation mechanism and evolution law of harmonic components are revealed quantitatively, especially the second harmonic component, which has a great influence on transformer protection. The accuracies of the theoretical analysis and mathematical model are verified by comparing calculation results with simulation results and low-voltage ride-through (LVRT) field test data of a real DFIG.

Study and identification of short-circuit currents at the Complex Distribution Network SIDER-EL Hadjar (ex ArceloMittal-Algerie)

Because of an upcoming the modernization of the electricity distribution network in the SIDER EL-HADJAR steel complex, a new calculation of the short-circuit current must be taken into account. The objectives pursued in this article, is to determine the precise values of the currents of maximum and minimum fault by each start 225kV, 63kV and 15kV of the distribution network to ensure the reliability of the existing system, on the other hand is to validate the sizing of the 63kV bus bar follows the increase of the load of the transformer station P4 by replacing the 70MVA transformer with a 120MVA transformer and comparing the results to the old computation. The calculation of short-circuit currents is a key step in qualifying the equipment to withstand the thermal and electromagnetic effects. So it was question of appreciating the reliability of numerical computation of the currents of short-circuits which requires modeling and simulations with software NEPLAN V5.

Study and identification of short-circuit currents at the complex distribution network SIDER EL Hadjar (ex ArceloMittal-Algerie)

Because of an upcoming modernisation of the electricity distribution network in the SIDER EL HADJAR steel complex, a new calculation of the short-circuit current must be taken into account. The objectives are to determine the precise values of the currents of maximum and minimum fault by each start 225 kV, 63 kV and 15 kV of the distribution network to ensure the reliability of the existing system, and to validate the sizing of the 63 kV bus bar follows the increase of the load of the transformer station P4 by replacing the 70 MVA transformer with a 120 MVA transformer and comparing the results to the old computation. The calculation of short-circuit currents is a key step in qualifying the equipment to withstand the thermal and electromagnetic effects. So, to appreciate the reliability of numerical computation of the currents of short circuits requires modelling and simulations with software NEPLAN V5.
 Keywords: Initial short-circuit current, power supply system (SIDER EL HADJAR), IEC 60909-0, peak value of short-circuit current, NEPLA software.