Short-circuit Contribution Research Articles

Identifying optimal combinations of synchronous condensers for minimum grid strength compliance

This paper proposes a method for the automatic optimization of the size and location of synchronous condensers (SCs) to meet a minimum grid strength requirement that enables the high penetration of inverter-based resources (IBRs) in a weak network. The grid strength is quantified by the effective short-circuit ratio (ESCR), which is an heuristic index that accounts for the voltage interactions between IBRs. The basin hopping algorithm (BHA) is used in combination with the Nelder–Mead algorithm to minimize the total additional short-circuit contribution from the SCs, subject to a minimum ESCR constraint. It is shown that this optimization problem has several solutions with the minimum values of the objective function lying within a narrow range but different allocation of the SCs’ capacities to the candidate buses. Therefore, the BHA is iteratively applied to identify the area of the search space in which the optimum solutions are located, and additional criteria are proposed to rank the solutions based on cost and reliability attributes. The effectiveness of the proposed approach is shown using a modified 39-bus New England benchmark system in which conventional synchronous generators were replaced by IBRs, thus leading to weak network conditions.

Grid Forming Inverter With Increased Short-Circuit Contribution to Address Inverter-Based Microgrid Protection Challenges

Grid Forming Inverter With Increased Short-Circuit Contribution to Address Inverter-Based Microgrid Protection Challenges

Grain boundary self- and Mn impurity diffusion in equiatomic CoCrFeNi multi-principal element alloy

Grain boundary self-diffusion of Co, Cr and Fe and impurity diffusion of Mn are measured in a coarse-grained equiatomic CoCrFeNi multi-principal alloy. The tracer diffusivities are determined in a wide temperature range of 643 K to 1273 K, which encompasses both the C- and B-type kinetic regimes of grain boundary diffusion in polycrystalline materials after Harrison’s classification. At higher temperatures (T>800 K), only one short-circuit (grain boundary) contribution is observed, while the existence of two distinct contributions is elucidated by thorough analysis of the penetration profiles corresponding to the C-type kinetic regime (643–703 K). The latter observations are explained in terms of a grain boundary phase decomposition after prolonged annealing below 700 K. The product of the segregation factor and the grain boundary width is found to be about 0.5 nm for all constituting elements. The grain boundary diffusion data indicate that Mn does not reveal a strong (if any) segregation in the equiatomic CoCrFeNi alloy.

Short-circuit currents from wind turbines with full-scale inverters

An important aspect of any wind power plant integration analysis is the short-circuit contribution. Reliable results can only be obtained by considering every aspect of the wind turbine behaviour that can impact its short-circuit current injection. This information is required to accurately represent the wind plant in power system models used by the system operators, to properly size circuit breakers and for protection coordination within the wind power. This task is often challenging not only due to the inherent differences between the synchronous generating units and wind turbine generators (WTGs) but also due to the variability of the WTG types and settings. This paper presents results of detailed short circuit analysis performed on hundreds of types and configuration variants of Vestas WTGs with full-scale converters. The WTG current contribution during the 1st, 3rd and 5th cycles for faults varying between 0.9pu and 0.01pu residual voltage were obtained using detailed WTG models. Various statistical techniques were used to assess the variability in the short circuit contributions and generalise the most appropriate values for the minimum and maximum transient contributions for a given WTG type and configuration. Finally, the paper dwells on the techniques used to obtain the equivalent positive and negative sequence impedances for the WTG for various fault conditions and turbine configuration and provides recommendations on the appropriate use of these impedances for wind power plant integration analysis.

Implementation of an Alternative Frequency-Dependent Three-Phase Transmission Line Model Based on the Folded Line Equivalent Model in MatLab-Simulink

This paper proposes an alternative multiconductor transmission line model that combines the folded line equivalent with the modal transformation. The folded line equivalent decomposes the nodal admittance matrix of a transmission line into its open-circuit and short-circuit contributions. These contributions are fitted to rational functions, which are associated with Norton equivalent circuits based on their state space models. The proposed model uses an orthogonal matrix to transform voltages and currents from the phase domain to the folded line equivalent domain and vice versa. Because the transformation matrix is orthogonal, we represent it using ideal transformers in simulation software. First, we use a circuit representation of Clarke’s matrix to decompose a transmission line into its modes. Then, each mode is decomposed into its open-circuit and short-circuit contributions using a circuit implementation of the proposed matrix. The proposed approach can accurately represent short lines in simulations with time steps equal to or greater than the propagation time of the transmission line. We compare the results obtained with the proposed approach to those obtained with power systems computer-aided design/electromagnetic transients including the DC universal line model.

A modified implementation of the Folded Line Equivalent transmission line model in the Alternative Transient Program

This paper proposes a modified implementation of the Folded Line Equivalent transmission line model to represent three-phase transmission lines. The Folded line equivalent decomposes a line into its open circuit and short circuit contributions. We propose an alternative orthogonal matrix to transform single-phase transmission line parameters, voltages, and currents to the Folded Line Equivalent domain. Because the proposed matrix is orthogonal, it can directly represent bidirectional transformations using circuit components in simulation software, such as the ATP-EMTP. First, the circuit implementation of Clarke’s matrix decouples a three-phase transmission line into its modes. Then, the circuit implementation of the proposed matrix decouples each mode into its open circuit and short circuit contributions. This paper proves that the proposed approach outputs similar results as those obtained by the Universal Line Model and the JMarti model in the simulation of transmission lines under open-circuit, energization, and fault conditions. However, because the proposed model does not derive from the method of characteristics, it can run with simulation times greater than the propagation delay of the transmission line. It is shown that the proposed approach outputs accurate results for time steps that are 10%, 100%, 200%, and 400% of the propagation delay of the transmission line. Thus, one major advantage of the proposed approach is that it can be used to reduce runtimes in the representation of short transmission lines in large complex networks by increasing time steps without compromising accuracy.

Fault ride through capability of grid forming wind turbines: A comparison of three control schemes

The popularity of converter-based generations, such as wind and solar, suggests that future power systems will be dominated by power-electronic converters, reducing the share of conventional generation, and, inevitably decreasing inertia and short circuit contribution of the system. When this happens in power systems, they become more vulnerable to instabilities. A promising solution for counteracting this vulnerability could be equipping converter-based generation units with grid forming control schemes. A study of the fault-ride through capability of wind turbines with different grid forming control schemes is presented in this paper. In this context, Virtual Synchronous Machines, Power Synchronisation Control, and Distributed PLLs based Control are considered. The performance of these control schemes is evaluated and compared during three-phase symmetrical faults using a case study that consists of a wind turbine connected to an AC grid. The simulated results indicate the strengths and drawbacks of each control scheme during faults.

Reducing Microgrids Integration Complexity in Distribution Networks Considering Bidirectional Power Flow in SFCLs

In this paper, investigations are carried out to explore the application of the well-known unique bidirectional power flow nature of superconducting fault current limiters (SFCLs) to reduce the complexities of integrating microgrids into distribution networks. More specifically, their adverse impact on the coordination of existing protection of distribution networks, as well as the complexity of setting their internal protection to handle the change in fault current due different modes of operation. In this context, the capability of the proposed SFCL scheme is tested while demonstrating the tandem operation of the utility distribution network and microgrid protection systems and problems related to sensitivity and selectivity are addressed. In addition, different fault contingencies are considered, namely, faults located on the incoming main-lateral connecting the microgrid to the distribution network (where the fault is between the distribution substation protection and the microgrid) and on an adjacent main-lateral from the same main feeder (where the microgrid is located between the distribution substation protection and the fault). Moreover, other faults are located inside the microgrid on a local-line and at a local end-user. Results have shown the efficiency of the proposed SFCL scheme in reducing the short circuit contributions of both the microgrid and the utility network within the coordination limits depending on the direction of flow into the point of connection (PoC). Thus, it allowed a safe continuous integration of microgrids in distribution networks during faulted conditions, while, simultaneously permitted the microgrid to utilize a single protection setting to handle the changes in short circuit current levels when transitioning between grid-connected and islanded modes of operation. The time-domain simulation studies are conducted using PSCAD/EMTDC software.

Short-circuit modeling of three-phase 4-wire unbalanced networks in presence of single-phase photovoltaic systems

Due to the growing diffusion of single-phase Photovoltaic Systems (PVSs), the analysis of active unbalanced three-phase 4-wire distribution networks in short-circuit operating conditions is nowadays of great interest. However, this analysis is not trivial due to various factors (e.g., environmental conditions, inverter control system, limits of the inverter current, location and type of the fault, PVS self-protection systems) influencing the contribution of PVSs to the short-circuit conditions. Among these factors, the greatest challenge is to model the complex behavior of the inverter control system that must comply with actual and future grid requirements, or with national and international standards, in both normal and abnormal operating conditions.An analytical model based on the phase-coordinates approach is proposed in this paper to evaluate the short-circuit contributions of single-phase PVSs connected to unbalanced three-phase 4-wire LV distribution networks. The model of the PVSs takes into account the main factors that influence the PVS contributions, with particular attention to the model of the control system which injects reactive power to support grid voltage in normal operating conditions and to perform low-voltage ride-through (LVRT) dynamic grid support during the fault.In this paper, control systems for next-generation single-phase PVSs able to satisfy grid code requirements are explored; they include constant average active power control, constant active current control and constant peak current control. Extensive numerical experiments are presented with reference to the Cigrè European LV Distribution Network Benchmark, considering different fault types, effects of various control strategies and the evaluation of the impact of the PVSs in LVRT conditions w.r.t. the passive network, i.e., the network under fault with no installed PVSs.

Short-Circuit Current Contribution of Doubly-Fed Wind Turbines According to IEC and IEEE Standards

The calculation of short-circuit currents is critical for the design of protection systems and electrical equipment rating. The short-circuit current contribution of traditional synchronous generators, which depends on common machine parameters, has been validated in recent decades. In this sense, the increasing penetration of wind power generation in current power systems means that network operators need to evaluate the response of wind turbines when a fault occurs. The recent publication of Edition 2 of IEC 60909-0 in 2016 defined a formulation to represent the short-circuit contribution for modern wind turbines, such as doubly-fed and full-converter technologies. Nevertheless, both the IEC and the ANSI/IEEE guidelines are based on new parameters that must be provided by the wind turbine manufacturer. This paper develops a methodology to calculate the parameters that define the short-circuit contribution of doubly-fed wind turbines for both international guidelines, IEC 60909-0 Ed. 2 and ANSI/IEEE. Field measurements and simulations are used for the validation of the developed method. Furthermore, some of the shortcomings of the IEC short-circuit calculation method are identified and alternative approaches are proposed and evaluated.

Grain boundary diffusion and grain boundary structures of a Ni-Cr-Fe- alloy: Evidences for grain boundary phase transitions

Grain boundary structure-property relationship was studied in a Ni-base 602CA coarse-grained alloy using a novel correlative tracer diffusion-analytical microscopy approach. Co-existence of several short-circuit contributions to tracer diffusion was distinguished at higher temperatures. These contributions were related to different families of high-angle grain boundaries with distinct coverages by precipitates and segregation levels as revealed by HAADF-STEM combined with EDX measurements and a detailed atom probe tomography analysis. Annealing at such conditions resulted in Cr23C6-type carbides co-existing with an α-Cr-Mn-enriched phase in addition to sequential segregation layers of Al, Ni, and Fe around them. Curved and hackly grain boundaries showed a high density of plate-like carbides. In contrast, straight grain boundaries were composed of globular carbides with similar chemical composition variations and additionally with alternating layers of Cr and Ni in-between the carbides, similar to spinodal microstructures. At lower temperatures, hackly interfaces with Cr and Cr-carbide enrichment dominated and the alloy annealed at 403 K contained plate-like Cr23C6-type carbides surrounded by a Ni-rich layer around them. The Ni grain boundary diffusion rates at these relatively low temperatures showed an anomalous character being almost temperature independent. This specific diffusion behaviour corresponds to a metastable grain boundary transition which is explained by a concomitant relaxation of transformation-induced elastic strains occurring on a longer time scale in comparison to those for grain boundary diffusion. Thermodynamic insights into the probable mechanism of decomposition at grain boundaries are provided. The paper provides solid evidences towards the existence of grain boundary phase transitions in the Ni-base multi-component alloy. It underlines the capabilities of the correlated kinetic-structure measurements as a tool to probe such changes.

Grain boundary diffusion in CoCrFeMnNi high entropy alloy: Kinetic hints towards a phase decomposition

Grain boundary diffusion of the principal elements 57Co, 51Cr, 59Fe and 54Mn in a coarse-grained equiatomic CoCrFeMnNi high entropy alloy is measured in a wide temperature range of 643 to 1273 K in both C- and B-type kinetic regimes after Harrison’s classification. The results suggest that the product of the pertinent segregation factors, s, and the grain boundary width, δ, is about 0.5 nm for all elements at temperatures T > 800 K. Whereas one short-circuit contribution is observed at higher temperatures above 800 K, the penetration profiles in the C-type kinetic regime (643 – 703 K) reveal two distinct contributions that hint towards a phase decomposition at a fraction of high-angle grain boundaries at these temperatures and the existence of a structural multiplicity of high-angle grain boundaries. A correlative microscopy combining transmission Kikuchi diffraction and atom probe tomography manifests formation of neighboring Ni-Mn-rich and Cr-rich precipitates at high angle grain boundaries. Transmission electron microscopy revealed an increased dislocation density in the vicinity of such interfaces which is suggested to be a reason of the enhanced diffusion rates at low temperatures for such short circuits.

An Accurate Type III Wind Turbine Generator Short Circuit Model for Protection Applications

The integration of renewables into power systems introduces several technical challenges including the development of appropriate models for system protection studies and accurate evaluation of short circuit contributions. Converter-interfaced wind turbines (WTs) produce significantly different current waveform signatures compared to the traditional synchronous or asynchronous generators. This paper proposes a new phasor domain modeling approach for Type III WTs with doubly fed induction generators using the concept of control-based equivalent circuits. The proposed model accounts for the impact of negative sequence quantities on the WT control response to achieve accurate simulation of unbalanced faults.

Directional Overcurrent Relays Coordination Problems in Distributed Generation Systems

This paper proposes a new approach to the distributed generation system protection coordination based on directional overcurrent protections with inverse-time characteristics. The key question of protection coordination is the determination of correct values of all inverse-time characteristics coefficients. The coefficients must be correctly chosen considering the sufficiently short tripping times and the sufficiently long selectivity times. In the paper a new approach to protection coordination is designed, in which not only some, but all the required types of short-circuit contributions are taken into account. In radial systems, if the pickup currents are correctly chosen, protection coordination for maximum contributions is enough to ensure selectivity times for all the required short-circuit types. In distributed generation systems, due to different contributions flowing through the primary and selective protections, coordination for maximum contributions is not enough, but all the short-circuit types must be taken into account, and the protection coordination becomes a complex problem. A possible solution to the problem, based on an appropriately designed optimization, has been proposed in the paper. By repeating a simple optimization considering only one short-circuit type, the protection coordination considering all the required short-circuit types has been achieved. To show the importance of considering all the types of short-circuit contributions, setting optimizations with one (the highest) and all the types of short-circuit contributions have been performed. Finally, selectivity time values are explored throughout the entire protected section, and both the settings are compared.

An Improved Arc Flash Energy Calculation Method and Its Application

In this paper, we describe an improved algorithm for arc flash calculations. This method can be used in any software that performs arc flash calculations to determine the incident energy, arc flash boundaries, and other related parameters for arc flash risk assessment purposes. The method provides a more sophisticated and precise way of calculating short circuit contributions to the fault location. To illustrate the effectiveness of the method, a typical renewable power plant (wind energy) with common types of protection is analyzed. The arc flash risk assessment results are compared in both the conventional way and the presented method. The new approach is called the Integrated Method. This method is an accurate way of modelling the short circuit contribution of the devices, which can potentially change the total short circuit current at the fault location. This Integrated Method considers the decay of the contributing machines and the devices where currents are interrupted due to a protection device. It also accounts for the current decays over time.

Effects of Distributed Generation on Overcurrent Relay Coordination and an Adaptive Protection Scheme

Integration of distributed generation (DG) such as renewable energy sources to electrical network becomes more prevalent in recent years. Grid connection of DG has effects on load flow directions, voltage profile, short circuit power and especially protection selectivity. Applying traditional overcurrent protection scheme is inconvenient when system reliability and sustainability are considered. If a fault happens in DG connected network, short circuit contribution of DG, creates additional branch element feeding the fault current; compels to consider directional overcurrent (OC) protection scheme. Protection coordination might get lost for changing working conditions when DG sources are connected. Directional overcurrent relay parameters are determined for downstream and upstream relays when different combinations of DG connected singular or plural, on radial test system. With the help of proposed flow chart, relay parameters are updated and coordination between relays kept sustained for different working conditions in DigSILENT PowerFactory program.

Modeling the three-phase short-circuit contribution of photovoltaic systems in balanced power systems

A high penetration of Photovoltaic (PV) systems into power networks can alter the fault currents and negatively impacts on the operation of the protection systems. In this paper, a new analytical model of the fault contribution of a three-phase PV system equipped with a voltage source converter is proposed. Such a model extends the classical steady-state short-circuit analysis to balanced active power networks including PV systems, with the aim to analyze their impact on the breaking capacity of the interrupting devices. The proposed model: (i) takes into account environmental conditions and VSC current limits; (ii) includes the effects of reactive power injections by a detailed representation of different type of VSC control systems; and (iii) can be easily integrated in a software package for power system analysis. The proposed approach is validated by comparing analytical results with time-domain simulations of the IEEE benchmark PV system. Furthermore, it is used to evaluate the impact of a large penetration of PV systems on the fault currents.

Power-Flow Control and Short-Circuit Current Limitation of Wind Farms Using Unified Interphase Power Controller

This paper presents analytical and simulation studies of wind farms (WFs) connected to power system through a unified interphase power controller (UIPC), which should control power flow and the short-circuit contribution of WF. The UIPC is based on an interphase power controller (IPC) in which the phase-shifting transforms are replaced by two series converters (SECs). In this paper, the UIPC model is developed based on phase angles of injected SEC voltage and a control scheme is proposed to control power flow and limit the short-circuit contribution of WF. The WF model is based on an equivalent aggregated variable-speed double-fed induction generator. The PSCAD/EMTDC simulation results show that the UIPC is an effective and novel solution for power-flow control and limitation of short-circuit contribution of WF in power systems.

Dynamic Phasor Modeling of Type 3 DFIG Wind Generators (Including SSCI Phenomenon) for Short-Circuit Calculations

Short-circuit modeling of wind generators is crucial to determine protective relay and control settings, equipment ratings, and to provide data for protection coordination. The short-circuit contribution of a Type 3 wind farm connected to a series-compensated line is affected by subsynchronous interactions, making it essential to model such behavior. Fundamental frequency models are unable to represent the majority of critical wind generator fault characteristics. The complete electromagnetic transient (EMT) models, though accurate, demand high levels of computation and modeling expertise. This paper proposes a novel modeling technique for a Type 3 wind farm based on the generalized averaging theory, where system variables are represented using time-varying Fourier coefficients known as dynamic phasors. The novelty and advantage of the proposed modeling technique is that it does not just include 60-Hz frequencies but also other dominant frequencies, such as 36 Hz, that are present due to the SSCI in the system. Methods currently used by the industry mostly rely on fundamental frequency-based analysis. Only the appropriate dynamic phasors are selected for the required fault behavior to be represented, improving computational efficiency. Once the SSCI behavior (waveforms showing resonant frequency at the point of common coupling) of a series-compensated Type 3 wind farm from real-time field data is available, the developed model could be used to simulate the scenario without necessarily having to know the exact control blocks of the wind generator controls. A 450-MW Type 3 wind farm, consisting of 150 units, was modeled using the proposed approach. The method is shown to be accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for nonfundamental frequency components present in fault currents during subsynchronous interactions.

Short-circuit calculations in networks with distributed generation

Fault level considerations may be an inhibiting factor for the interconnection of distributed generation (DG) to the network, particularly at the medium voltage level. In this paper, the latest edition of the IEC 60909 Standard is applied for the calculation of the resulting fault level in medium and low voltage distribution networks with DG. First, an outline of the IEC calculation methodology is presented, including all relevant equations for DG cases studies. Then the short-circuit contribution of the various DG types is extensively discussed. The application of the methodology is demonstrated on a study case medium voltage distribution network, which includes all representative types of DG sources. Emphasis is placed on the contribution of the upstream system, which is the dominant source of short-circuit current. A discussion is also included on potential measures for fault level reduction.