Short-circuit Calculation Research Articles

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.

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, ...

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.

Impacts of Photovoltaic Distributed Generation Location and Size on Distribution Power System Network

As the rapid development of photovoltaic (PV) technology in recent years with the growth of electricity demand, integration of photovoltaic distributed generation (PVDG) to the distribution system is emerging to fulfil the demand. There are benefits and drawbacks to the distribution system due to the penetration of PVDG. This paper discussed and investigated the impacts of PVDG location and size on distribution power systems. The medium voltage distribution network is connected to the grid with the load being supplied by PVDG. Load flow and short circuit calculation are analyzed by using DigSILENT Power Factory Software. Comparisons have been made between the typical distribution system and the distribution system with the penetration of PVDG. Impacts in which PVDG location and size integrates with distribution system are investigated with the results given from the load flow and short circuit analysis. The results indicate positive impacts on the system interconnected with PVDG such as improving voltage profile, reducing power losses, releasing transmission and distribution grid capacity. It also shows that optimal locations and sizes of DGs are needed to minimize the system’s power losses. On the other hand, it shows that PVDG interconnection to the system can cause reverse power flow at improper DG size and location and increases short circuit level.

Adaptability of phase selectors based on phase‐to‐phase superimposed currents when confronting cross‐voltage faults in mixed‐voltage quadruple‐circuit line system

For the mixed-voltage quadruple-circuit transmission lines on the same tower, its special construction form makes the cross-voltage faults, i.e. multi-circuit faults that occur between different voltage level systems, easy to occur. Although the phase selector based on phase-to-phase superimposed currents (PTPSC) shows excellent performance in the single transmission line, it cannot guarantee to identify faulted phases when confronting cross-voltage faults. In this study, the adaptability of phase selectors based on PTPSC is analysed and a new method of phase selection is put forward. First, the PTPSC measured by the relay can be obtained on the basis of cross-voltage short-circuit calculation. Then, the influencing factors and possible selection results are comprehensively analysed, and the corresponding measure has been proposed. Finally, a fault model of mixed-voltage quadruple-circuit transmission lines is constructed in PSCAD and simulation results agree well with the theoretical analysis, verifying the validity of this method.

Pandapower—An Open-Source Python Tool for Convenient Modeling, Analysis, and Optimization of Electric Power Systems

pandapower is a Python based, BSD-licensed power system analysis tool aimed at automation of static and quasi-static analysis and optimization of balanced power systems. It provides power flow, optimal power flow, state estimation, topological graph searches and short circuit calculations according to IEC 60909. pandapower includes a Newton-Raphson power flow solver formerly based on PYPOWER, which has been accelerated with just-in-time compilation. Additional enhancements to the solver include the capability to model constant current loads, grids with multiple reference nodes and a connectivity check. The pandapower network model is based on electric elements, such as lines, two and three-winding transformers or ideal switches. All elements can be defined with nameplate parameters and are internally processed with equivalent circuit models, which have been validated against industry standard software tools. The tabular data structure used to define networks is based on the Python library pandas, which allows comfortable handling of input and output parameters. The implementation in Python makes pandapower easy to use and allows comfortable extension with third-party libraries. pandapower has been successfully applied in several grid studies as well as for educational purposes. A comprehensive, publicly available case-study demonstrates a possible application of pandapower in an automated time series calculation.

Three-Phase Short-Circuit Current Calculation of Power Systems with High Penetration of VSC-Based Renewable Energy

Short-circuit current level of power grid will be increased with high penetration of VSC-based renewable energy, and a strong coupling between transient fault process and control strategy will change the fault features. The full current expression of VSC-based renewable energy was obtained according to transient characteristics of short-circuit current. Furtherly, by analyzing the closed-loop transfer function model of controller and current source characteristics presented in steady state during a fault, equivalent circuits of VSC-based renewable energy of fault transient state and steady state were proposed, respectively. Then the correctness of the theory was verified by experimental tests. In addition, for power grid with VSC-based renewable energy, superposition theorem was used to calculate AC component and DC component of short-circuit current, respectively, then the peak value of short-circuit current was evaluated effectively. The calculated results could be used for grid planning and design, short-circuit current management as well as adjustment of relay protection. Based on comparing calculation and simulation results of 6-node 500 kV Huainan power grid and 35-node 220 kV Huaisu power grid, the effectiveness of the proposed method was verified.

Sequence Domain Calculation of Active Unbalanced Distribution Systems Affected by Complex Short Circuits

In this paper, a calculation method for quantifying the impact of complex short circuits on active unbalanced distribution systems is proposed. Unlike previous distribution system short-circuit calculations, the proposed method is capable of solving complex short circuits with an arbitrary number of faulted busses and phases, without predefining the boundary (fault) conditions. Different short circuits are defined by simple inspection of these faults. Additionally, all distribution system elements are appropriately modeled and integrated in the proposed method, including electronically coupled distributed energy resources. The method can solve unbalanced systems, including mutually coupled sections, in unsymmetrical prefault states, and it is performed in the sequence domain. This makes the proposed method particularly fast and suitable for online distribution system analysis. The computation results show the advantages of the proposed method relative to previous short-circuit calculation methods, in regards to both the required calculation time and the accuracy.

Short-Circuit Current Analysis for DFIG Wind Farm Considering the Action of a Crowbar

With the increasing capacity of wind farms integrated into the power grid, the short-circuit current analysis for wind farms becomes more and more important. Since the wind turbine is usually integrated into the power grid via power electronic devices, the “crowbar” is installed in the wind turbine to protect the power electronic devices and to improve the fault ride through capability. The impact of the crowbar has to be considered during the short-circuit current analysis for the wind farm. In order to fully analyze the short-circuit current characteristics of a wind farm, the short-circuit currents for a doubly-fed induction generator (DFIG) wind turbine under symmetrical and asymmetrical faults considering the crowbar action characteristic are derived firstly. Then the action situation of the crowbar of a DFIG wind turbine is studied and the action area curve is obtained. Taking the crowbar action, or not, as the grouping criterion, wind turbines in the wind farm are divided into two groups, and the wind farm is aggregated into two equivalent wind turbines. Using the equivalent model, the short-circuit current of a wind farm can be calculated accurately. Finally, simulations are performed in MATLAB/Simulink which is the commercial math software produced by the MathWorks company in Natick, Massachusetts, the United States to verify the proposed short-circuit current calculation method for the DFIG wind farm.

Three-phase short circuit calculation method based on pre-computed surface for doubly fed induction generator

This paper presents an improved short circuit calculation method, based on pre-computed surface to determine the short circuit current of a distribution system with multiple doubly fed induction generators (DFIGs). The short circuit current, injected into power grid by DFIG, is determined by low voltage ride through (LVRT) control and protection under grid fault. However, the existing methods are difficult to calculate the short circuit current of DFIG in engineering practice due to its complexity. A short circuit calculation method, based on pre-computed surface, was proposed by developing the surface of short circuit current changing with the calculating impedance and the open circuit voltage. And the short circuit currents were derived by taking into account the rotor excitation and crowbar activation time. Finally, the pre-computed surfaces of short circuit current at different time were established, and the procedure of DFIG short circuit calculation considering its LVRT was designed. The correctness of proposed method was verified by simulation.

Currents in the overhead transmission line lighting wire in case of single-phase short circuit

The algorithms for currents induced in a grounded lightning wire (LW) with single-phase short circuit (SC) calculation are considered. It is shown that the method of short-circuit current and current in LW simultaneous determination gives more correct results for the assessment of LW thermal resistance

Adaptive Short-Circuit Current Calculation Model based on Colored Petri Net

Adaptive Short-Circuit Current Calculation Model based on Colored Petri Net

Inductive influence of 25 kV, 50 Hz electrified single rail traction system

The paper presents the methodology for calculation and measurement of induced voltage caused by railway system operation on the sensitive metallic structures. The arrangement and characteristics of railway system 25 kV, 50 Hz are provided. The procedures of short-circuit current calculation and supply current measurement in the railway system are described. The basic theoretic background of inductive interference is explained. The reduction factors of the communication cable, railway system and environment are mentioned and clarified. On a case study, the results of the induced voltage calculation and measurement is compared. It is proved that induced voltage directly depends of supply current or shortcircuit value.

A Short-Circuit Calculation Method for DFIG-Based Wind Farms

Short-circuit calculations for wind farms are crucial for determining equipment ratings and providing basic information for protection coordination. In this paper, a practical calculation method of short-circuit currents (SCCs) is proposed for doubly fed induction generator (DFIG)-based wind farms. The improved calculation method of the SCCs in single DFIG is developed based on electromagnetic transient analysis. Combined with the traditional method of calculating the SCCs of the power system, an equivalent model of the DFIG-based wind farm used for SCCs is studied. The equivalent model and expressions for SCCs are later validated through PSCAD/EMTDC-based simulation results, and a comparison with the national standard calculation method is analyzed. The results show that the calculation method proposed in this paper has better accuracy and practicability, which are important elements in the electrical design of DFIG-based wind farms.

Ground-Fault Characteristic Analysis of Grid-Connected Photovoltaic Stations with Neutral Grounding Resistance

A centralized grid-connected photovoltaic (PV) station is a widely adopted method of neutral grounding using resistance, which can potentially make pre-existing protection systems invalid and threaten the safety of power grids. Therefore, studying the fault characteristics of grid-connected PV systems and their impact on power-grid protection is of great importance. Based on an analysis of the grid structure of a grid-connected PV system and of the low-voltage ride-through control characteristics of a photovoltaic power supply, this paper proposes a short-circuit calculation model and a fault-calculation method for this kind of system. With respect to the change of system parameters, particularly the resistance connected to the neutral point, and the possible impact on protective actions, this paper achieves the general rule of short-circuit current characteristics through a simulation, which provides a reference for devising protection configurations.

Novel Strategy for Accurate Locating of Voltage Sag Sources in Smart Distribution Networks with Inverter-Interfaced Distributed Generators

With the aid of power quality monitoring systems (PQMSs), accurate locating of voltage sag sources, which has important significance for guiding maintenance personnel in finding and repairing faults as well as improving power supply reliability, has been becoming a new research hotspot. However, existing methods have unsatisfactory locating accuracy due to the integration of distributed generators (DGs) and fault resistance. In this paper, a novel strategy for accurately locating voltage sag sources in smart distribution networks is proposed. Based on inverse theory, which is well applied in geophysics, the accurate location issue is treated as a two-step optimization model. It aims at making the distribution of voltage phasors and current phasors obtained by theoretical short-circuit calculation match those actually observed as closely as possible. To guarantee the feasibility of the strategy, the effect of inverter-interfaced DGs (IIDGs) which are the main form of DG is considered in the short-circuit calculations. To guarantee the location accuracy of the strategy, fault resistance is treated as an optimization variable in the two-step optimization model to eliminate estimation error of fault resistance. Via two modified IEEE benchmarks with different scales, the validity and the superiorities in applicability and accuracy of the proposed strategy are verified.

DECISION ANALYSIS ON THE BASIS OF EXPERT DATA AND REFLEXIVE PROCEDURES OF THE CONCLUSION

Relevance. At decision-making the situation is rather widespread when difficult objects of management function in the conditions of uncertainty and are characterized by existence of several criteria of a choice, and also by existence the parameters which are badly giving in to measurement or having qualitative character. In such situations it is reasonable to apply expert approaches to receiving estimates and decision-making. It in turn demands the development of special methods of formalization of the considered objects and control systems, and also the corresponding procedures of numerical processing of expert data. Purpose. The purpose of work is development of procedures of calculation of estimates for consequences of interaction of factors of management on the basis of expert data and a method of a choice of rational decisions in the presence of several criteria. Method. A subject of consideration is the method of the expert analysis using reflexive procedures of a multiple-valued logical conclusion for receiving transitive short circuit of estimates of mutual influence of factors and a decision-making method on the basis of the balanced accounting of several criteria. Results. The offered method of decision-making includes two components: procedure of reflections for calculation of transitive short circuit of primary estimates and procedure of formation of the weighed criterion for the balanced accounting of several criteria considering various aspects of a choice. The flexible criterion which formation is based on balancing (the accounting of degree of importance) of maximin and minimax values of initial criteria and on use of a relevant subset of the considered factors is entered into double criteria situations. Calculation of a rational choice of structure of intangible assets of the enterprise is given as an example. Conclusions. Specifics of the considered approach possessing basic novelty consist in the accounting of the uncertainty concluded as in properties of expert data, and available several criteria of a choice. Application of algebra of multiple-valued logic in procedure of reflections allows considering most adequately expert data at estimation of options of adoption of administrative decisions. The flexible criterion is attribute of self-organization of the operated system. The developed method of the expert analysis of factors of management considering consequences of their interaction allows estimating and allocating the most significant factors from the point of view of the set criteria, to make rational decisions as for rapid response to current situation, and at long-term and strategic planning. The numerical example represents informative demonstration of efficiency of the offered approach.

Application of the Sherman–Morrison formula to short-circuit analysis of transmission networks with phase-shifting transformers

For transmission network with in-phase transformers only, the nodal admittance matrix is symmetric. For such networks the short-circuit calculations can be done using sparsity-oriented factorisation of large complex symmetric matrices. In some transmission networks the phase-shifting transformers are used in order to control the real power flows. The series branch of the π-port network modelling such transformer has directional property. When such anisotropic branches are included in the network model the admittance matrix becomes asymmetric. Typically the number of phase-shifting transformers installed in transmission network is very small when compared to the whole number of transformers. It means that very few asymmetric element of the matrix imposes a larger computational effort especially for factorisation, which is the most time consuming and memory using process. In this article it is shown that (in spite of the asymmetric elements) the short-circuit calculations can be done by factorisation performed by procedure applicable to symmetric matrices. A new calculation method is proposed where the admittance model of the phase-shifting transformer is divided into the isotropic π-port network and a series anisotropic branch. The network admittance matrix is formed only from the isotropic branches and is factorised by procedure for symmetric matrices. The anisotropic branches are taken into account by modification of the impedance matrix elements using the formulas derived in this paper on the basis of the Sherman–Morrison formula. Proposed method can be easily implemented in already existing computer programs originally developed for transmission networks without the phase-shifting transformers.

A Pole-to-Pole Short-Circuit Fault Current Calculation Method for DC Grids

This paper proposes a generic pole-to-pole short-circuit fault current calculation method for dc grids. The calculation procedure begins from the simplified RLC equivalent model of a single modular multilevel converter, and then the prefault matrices and faulted matrices are established and modified to calculate the dc fault currents of all the branches. The proposed approaches are validated by comparing with the electromagnetic transient (EMT) simulation results on PSCAD/EMTDC. Besides, two case studies showed that the calculation method can be easily used to evaluate the severity of a dc fault. Moreover, the calculation can be applied to select the parameters of a fault current limiter (to match the circuit breaker capacity. The main contributions of the proposed numerical calculation method are: 1) The proposed method is accurate and much more time efficient than the EMT simulations; 2) the proposed method can handle all kinds of dc grid networks including the ring, radial, and meshed topologies; and 3) the proposed method is applicable to dc grid with multiple dc voltage level areas connected with dc/dc converters.