Short-circuit Current Calculation Method Research Articles

Calculation method of external fault short-circuit current for variable-speed pumped storage units based on time-domain dynamic circuit

The calculation method of the external fault short-circuit current of large units with clear mechanisms and practicality is of great value in engineering practice. Due to the special AC excitation structure and control mode, the external fault short-circuit characteristics of variable-speed pumped storage units (VSPSUs) are very different from those of conventional units. Therefore, this paper proposes a calculation method for external fault short-circuit current based on a time-domain dynamic circuit. According to the excitation and control characteristics of VSPSUs under external fault, the VSPSU under external fault is divided into two cases: continuous excitation and jumper action. Based on space vector analysis, the time-domain dynamic circuit models are established, respectively. On this basis, the analytical formula of the external fault short-circuit current of VSPSUs is derived by combining the transition boundary conditions of the two cases and the dynamic characteristics of the stator flux linkage. Finally, the comparison with PSCAD/EMTDC simulation results shows that the proposed VSPSUs’ external fault short-circuit current calculation method has high accuracy and practical value, which can meet the needs of primary equipment selection and relay protection of variable-speed pumped storage power plants.

A New Short-Circuit Current Calculation and Fault Analysis Method Suitable for Doubly Fed Wind Farm Groups.

The transient characteristics of wind farms in groups are quite different; in addition, there is a strong coupling between the wind farms and the grid, and these factors make the fault analysis of the grid with wind farm groups complicated. In order to solve this problem, a mathematical model of the converter is established based on the input-output external characteristics of the converter, and a transient model of a doubly fed wind turbine (DFIG) is presented considering the influence of the low-voltage ride-through control (LVRT) of the converter, and the effect mechanism of the LVRT strategy on the short-circuit current is analyzed. Finally, a short-circuit current calculation model of a doubly fed wind turbine with low-voltage crossing control is established. The interaction mechanism between wind farms during the fault is analyzed, and a short-circuit current calculation method of doubly fed wind farm groups is proposed. RTDS is used to verify the accuracy of the proposed short-circuit current calculation method for doubly fed field groups. On this basis, a method of power grid fault analysis after doubly fed field group access is discussed and analyzed.

Short-circuit Current Engineering Calculation Method Considering Crowbar Operation Priority Degree of DFIG

When fault occurs in high wind power proportion grid, the control strategy and external characteristics of the wind power plant are affected by voltage drop of grid, so the traditional short-circuit current engineering calculation is difficult to converge. A short-circuit current engineering calculation method for high wind power proportion grid, considering grid node importance evaluation and crowbar protection of DFIG(double-fed induction generator), is proposed. This paper proposes the external equivalent model of the DFIG when fault occurs in grid, considers voltage drop degree and node importance indexes, and determines the weight of each index through Delphi method. The DFIG crowbar operation priority is evaluated to ensure the successful convergence of calculation and the maximum voltage support of wind power plants. The study case shows that proposed method is superior to the traditional method.

Short Circuit Current Algorithm and Software Design Based on IEC60909 Standard

With the continuous growth of load and the enhancement of grid structure, the problem of excessive short circuit current has become a key factor affecting the safe operation of the China Southern Power Grid. The comparison of measured data shows that the short circuit current calculation results of China Southern Power Grid are optimistic based on the existing short circuit current calculation method. It brings difficulties to the arrangement of the power grid operation mode. This paper first briefly introduces the calculation principle of the IEC60909 Standard (short for IEC60909 Short Circuit Current Calculation Standard). And based on this standard, an improved algorithm for short-circuit current calculation is proposed. FCAP (short for Fault Current Analysis Program), a newly developed software based on the improved algorithm, is compared with other similar software being popular in domestic and foreign to prove its engineering practical value.

GCN-Based Short-Circuit Current Calculation Method for Active Distribution Networks

GCN-Based Short-Circuit Current Calculation Method for Active Distribution Networks

Impact of System Inherent Characteristics on Initial-Stage Short-Circuit Current of MMC-Based MTDC Transmission Systems

This paper studies the impact of the inherent characteristics on the initial-stage short-circuit current of modular multilevel converter based multi-terminal direct current (MMC-based MTDC) transmission system. A short-circuit current calculation method based on input-output characteristics is proposed. The MMC’s input-output characteristics are factorized into a series combination of a linear and a nonlinear part through the operator approach. The inherent characteristics of the system represent the inherent properties of the MMC-based MTDC system, which are unrelated to the short-circuit fault. Therefore, short-circuit currents are decoupled and quantitative indices are proposed to measure the impact of the system inherent characteristics on the initial-stage short-circuit current. The proposed decoupled short-circuit current calculation method and the quantitative indices are verified in a typical MMC-based MTDC transmission system under different system parameters and structures. The proposed quantitative indices can effectively evaluate the short-circuit current of the MMC-based MTDC system without calculating the short-circuit current, which indicates that they can provide guidance for the planning of DC system structure, determination of operational modes, and selection of system parameters from the perspective of short-circuit current.

Calculation method of short-circuit fault current in flexible DC grid

The flexible DC grid has the capability to transport and distribute electric energy in multiterminal power system, and avoid the adverse impact on AC power grid in reactive power consumption and current harmonic. Unfortunately, because of the discharge of submodule capacitor facing the short-circuit fault of the overhead line, The overcurrent problem caused by the fault restricts the further development of the DC grid. At present, the research on short-circuit fault of flexible DC grid is mainly based on computer software simulation without mathematical expression to describe failure process quantitatively. This paper proposes a short-circuit fault current calculation method describing flexible DC grid fault process. Firstly, the linearized equivalent model of the MMC converter station is obtained by considering the discharge characteristics of the submodule capacitors. Secondly, considering the DC grid space topology, the parsed expression of overcurrent in short circuit fault is derived. Finally, the analysis results of the paper are compared with the software simulation results of the PSCAD/EMTDC platform to verify the validity and accuracy of the overcurrent algorithm.

Study on Calculation Method for Steady-State Short-Circuit Current of MMC During a DC Pole-to-Pole Fault

The modular multilevel converter based high-voltage direct current (MMC-HVDC) has drawn great attention in the field of HVDC transmission. Research on short-circuit fault current calculation method is of great significance for equipment protection and key MMC components selection. In common cases, IGBT will be blocked within a short period of time after the occurrence of a bipolar fault, and the short-circuit current will be suppressed. However, there are also cases where the fault progresses to stable stage. Therefore, research on the steady-state short-circuit current calculation method is of high importance for protection design and failure analysis. Taking AC-side reactance and DC-side short-circuit impedance into consideration, a steady-state short-circuit current calculation method based on solving the circuit initial value is proposed. Current calculation is discussed in three situations divided based on different fault parameters after defining the commutation overlap angle are respectively analyzed. The short-circuit current can be solved by formulating the equations based on the conduction state of the diode in the converter, and then solving the electrical quantities through the current boundary conditions of each state. Finally, a fault circuit model is built in PSCAD/EMTDC for simulation, and the accuracy of the calculation method is verified under different fault parameters.

Analytical Calculation of Transient Short-Circuit Currents for MMC-Based MTDC Grids

DC short-circuit current calculation is necessary to design and protect modular multilevel converter-based (MMC-based) multiterminal dc (MTDC) grids. In this article, a general and analytical short-circuit current calculation method is proposed for bipolar MTDC grids. For pole-to-pole (P2P) faults, the superposed fault equivalent network of the MTDC grid is first developed in the frequency-domain. Then, a decoupling method based on the network inductance ratio is proposed, which simplifies the high-order network into the superposition of several independent two-order <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> circuits. As a result, the short-circuit current can be expressed as the sum of the currents of these <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> circuits. For pole-to-ground (P2G) faults, the phase-domain dc line impedance considering the coupling effects is determined based on its modal-domain parameters, so that the P2G fault current can be calculated using the method of the P2P fault. The proposed method’s effectiveness is verified through simulations and experiments, and the calculation error is classified and analyzed. Compared with the existing methods, the proposed method is more accessible, more accurate, and available for hand-solution.

The adaptability of IEC 60909: 0-2016 to power grid with voltage levels above 400kV

The equivalent voltage source method is recommended in IEC 60909 for short-circuit current calculation, and the accuracy is verified by 400 kV and below voltage level power grids. There are 500 kV, 750 kV or 1000 kV power grids in some countries where IEC standards are applied, but there is no relevant research to verify the accuracy of the calculation method. In this paper, verification methods for IEC 60909 calculation accuracy for the three voltage level power grids are adopted. Calculation examples based on the operation and equipment parameters of the 500 kV, 750 kV and 1000 kV power grids are established. The results show that IEC 60909 short-circuit current calculation method is conservative, and the calculation accuracy meets the requirements. The research expands the application scope of this standard to all AC power grids, which is conducive to the engineering application of IEC 60909.

Influence of reactive power support control strategy on short-circuit current calculation and fault analysis method of DFIG

More and more Doubly-fed Induction Generators (DFIG) have a low voltage ride through (LVRT) capability, which needs to provide reactive power for the grid during the low voltage ride-through period. Traditional short-circuit current calculation of DFIG ignore the influence of reactive power support on short-circuit current; it is assumed that the rotor current is zero when the Crowbar is input after the fault. This inaccurate short-circuit current will indirectly affect the protection’s operating characteristics and produce certain errors. Specially, in China’s new grid-connected wind power standards, DFIG need to output reactive current during fault to provide reactive power support for the system. At this time, the rotor-side converter should no longer be locked, and provide continuous reactive current during the fault. In view of the above issues, according to the input–output characteristic of rotor converter, the equivalent model of rotor converter and the transient model of DFIG were provided. The influence mechanism of reactive power support control strategy was analyzed. The short circuit current calculation model of DFIG was established. Based on RTDS the experiment platform with physical controller of converter was founded, the proposed short-circuit current calculation model considering the reactive power support control strategy has been verified. The fault analysis method for the grid with penetration of DFIG is analyzed. Result of this research is available for the study of DFIG influence on the protection characteristics.

Interaction between damping characteristics and the initial‐stage short‐circuit current of MMC‐HVDC transmission systems

This paper studies the interaction between the damping characteristics and the initial-stage short-circuit current of modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system. First, the MMCs' input–output characteristics are obtained through the operator approach, and the influences of both linear part and nonlinear part are analysed. Second, an input–output characteristics based short-circuit current calculation method is proposed. Third, an eigenvalue-based comprehensive index is proposed to evaluate the damping characteristics of the MMC-HVDC transmission system. Finally, the interaction between the damping characteristics and the initial-stage DC short-circuit current is analysed under different system parameters and DC system structures. The analysis results show that there is a negative correlation between the system damping characteristics and the average rising rate of the short-circuit current. The proposed index G can effectively evaluate the short-circuit current of MMC-HVDC and can provide guidance for the planning of DC system structure, the selection of operation mode and system parameters.

Research on Short-Circuit Current Calculation Method of Doubly-Fed Wind Turbines Considering Rotor Dynamic Process

With the enlarging scale of doubly-fed induction generators (DFIGs) connected to power systems, it is important to analyze the influence of a short-circuit current to system relay protection. Due to the correct evaluation of the protection operation characteristics, the DFIG short-circuit current needs to be calculated accurately. But the current research on the short-circuit current of DFIG is based on the following assumption: the rotor excitation current is zero after the rotor crowbar is put, and the influence of its dynamic process is ignored. This will bring errors to the calculation results. This paper takes into account the influence of rotor current dynamics by studying the mechanism of the potential transient change of DFIG. The stator rotor flux linkage of DFIG in the event of a three-phase short-circuit is accurately calculated, and an improved RMS calculation method of doubly-fed wind turbine short circuit current is proposed. A physical experiment platform with an actual controller of a doubly-fed fan is established, based on RTDS. It can be seen from the experiment that the short-circuit current calculation method proposed in this paper is more accurate than those methods that ignore the rotor dynamic process. This study lays a foundation for further study of the influence of DFIG on the protection operation characteristics.

Improved Short-Circuit Current Calculation of Doubly Fed Wind Turbines With Uninterrupted Excitation

With the enlarging scale of a doubly fed induction generator (DFIG) connected to a power system, the influence of short-circuit current on the system relay protection could not be ignored. Setting and configuring relay protection would be affected by an imprecise short-circuit current calculation. However, some existing studies only consider the condition that the input is the Crowbar and the rotor excitation is blocked. China's new network standard requires the output reactive support current of a DFIG and will change the characteristics of short-circuit current. To solve this problem, on the basis of analyzing the characters of the transient equivalent potential of a DFIG, the transient model of a DFIG with uninterrupted excitation is provided. Based on the characteristics of a non-abrupt change of flux linkage and the requirement of a new grid standard reactive support current, the short-circuit current calculation method of a DFIG with uninterrupted excitation is put forward. Based on the real-time digital simulator (RTDS), a digital-analog experimental platform containing the actual control unit of the DFIG converter is founded, the proposed short-circuit current root mean square (RMS) value calculating method is validated.

A Harmonic Time-Current-Voltage Directional Relay for Optimal Protection Coordination of Inverter-Based Islanded Microgrids

Inverter-interfaced distributed generators (IIDGs) are characterized by their low fault current contributions that impose a major challenge for microgrid protection. This article proposes a new overcurrent protection strategy that utilizes a third harmonic voltage generated by the IIDG controller during short-circuit faults. By generating harmonic voltages, a new current flow layer in the harmonic domain is created during faults and is independent of the fundamental fault current, which is limited by IIDGs. A harmonic time-current-voltage directional relay is developed based on the measured harmonic currents and voltages at the relay location to ensure optimal protection coordination (OPC) for isolated microgrids. The OPC with the proposed relay characteristics is formulated as a nonlinear program and equipped with a harmonic short-circuit current calculation method to determine the optimal relay settings. The 9-bus Canadian test system and the IEEE 33-bus system are adopted to validate the operation of the proposed OPC. Simulation results demonstrate that the proposed harmonic time-current-voltage directional relay is capable of detecting the direction of the fault current and maintaining precise protection coordination without the need for communication.

A new short-circuit calculation method of power system with photovoltaic power sources

A large number of photovoltaic power sources connected to the grid will increase short-circuit current level of power system, and its fault transient process will change the fault characteristics of power grid. In this paper, the equivalent models of transient and steady state faults are obtained, by analysing the full current expression of photovoltaic power sources and its current characteristics in different fault stages. Based on the superposition theorem, the short-circuit current calculation method of power system with photovoltaic power sources is proposed. Finally, the effectiveness of the proposed method is verified by the analysis of actual power grid calculation examples, and the results can be used for power grid planning and relay protection setting.

Short-Circuit Fault Current Calculation Method for the Multi-Terminal DC Grid Considering the DC Circuit Breaker

The analysis and calculation of the short-circuit fault current in the DC grid is of great significance to the design and configuration of the converter station and DC circuit breaker parameters. The existing flexible DC system not only includes the modular multilevel converter (MMC) converter, but also needs power equipment such as the fault current limiter and DC circuit breaker. Therefore, the system modeling and short circuit calculation of the multi-terminal DC system after adding the DC circuit breaker are also of great significance for the design of DC power system parameters and the grid troubleshooting ability. In this paper, firstly, the parameters of the four-terminal DC system of the modular multilevel converter (MMC) are simplified, and the analytical solution of the short circuit fault current of the multi-terminal DC system is given. Then, the external characteristics of the cascaded hybrid DC circuit breaker are modeled. Based on the equivalent circuit of the fault current in different stages, the short circuit calculation method of four-terminal MMC system with DC circuit breaker is obtained. This method can effectively describe the overall trend of fault current and provide the basis for the configuration of DC line protection settings and DC circuit breaker related parameters.

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.

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.