Fault Current Contribution Research Articles

Comprehensive Study on Different Possible Operations of Multiple Grid Connected Microgrids

This paper proposes an efficient strategy for control of multiple grid connected microgrids (MGs) through converters, to improve network management during normal, islanding, and fault conditions. In normal condition, the converters control the bidirectional power flow between the MGs (downstream) and the main network (upstream). When the upstream is disconnected from downstream, if one of the MGs cannot handle its local loads, the deficit of power is provided by the other MGs. Transferred powers of the MGs are controlled regarding power quality (PQ) preserving criteria of the network. If PQ in a point of common coupling of an MG degrades, its converter goes into semi-isolated mode. Furthermore, in case of fault occurrence in the upstream, the converters limit fault current contribution of the MGs, in order to preserve the MGs PQ. However, in case of fault occurrence in the MG, the converters are bypassed by the relevant static switches. Therefore, short circuit current contribution of the upstream will be maximized to improve the MGs PQ. This strategy can also overcome the problems related to the system short circuit capacity increment in case of a newly added distributed generation unit, such as loss of the over current relays coordination.

An adaptive current limiting strategy to prevent fuse-recloser miscoordination in PV-dominated distribution feeders

The ever-increasing penetration of photovoltaic (PV) systems in distribution networks has resulted in proposing a new control strategy to mitigate the negative impacts of PVs on the protection coordination. However, disconnecting DGs, limiting penetration level, using Fault Current Limiter (FCL) and executing communication-based adaptive approaches are not such affordable solutions. In this paper, an applicable communication-free adaptive technique is proposed to prevent miscoordination problems in distribution networks. It is not only cost effective, but also immune to PV penetration-changes. Firstly, according to the amount of fault current variation in the beginning of the feeder, the fault location is estimated. Next, using a new-added auxiliary control mode, the fault current contributions of PV systems are decreased. Given the parameters are locally measured in each amount of penetration, the proposed methodology needs no communication link to be adapted against penetration variations. The effectiveness of the proposed method for different types of fault scenarios, variations in the photovoltaic penetration rates and different fault resistances is evaluated via detailed simulation case studies. The results clearly show that the proposed adaptive approach can successfully prevent miscoordination problems.

Fault current contribution and short circuit behaviour of a solar PV integrated distribution network

This paper investigates the short circuit behaviour and variation in fault level with solar PV integrated distribution network. In order to investigate the issue, a generic distribution network, which supplies power to the load of varying nature is designed and simulated using electrical transient analyser program (ETAP) software. The network is analysed according to IEC 60909 standard and the alteration in short circuit current and fault level due to various types of faults is reported. It is found that the cumulative contribution of upstream and downstream grid makes the three-phase fault most severe in comparison to other faults. However, if the transformer neutral is solidly grounded, then the severity of single line to ground fault will be highest. Furthermore the variation in grid strength considerably affects the fault current level of the distribution network. Such situation is the cause of major concern when solar PV is added to the network.

Fault current contribution and short circuit behaviour of a solar PV integrated distribution network

This paper investigates the short circuit behaviour and variation in fault level with solar PV integrated distribution network. In order to investigate the issue, a generic distribution network, which supplies power to the load of varying nature is designed and simulated using electrical transient analyser program (ETAP) software. The network is analysed according to IEC 60909 standard and the alteration in short circuit current and fault level due to various types of faults is reported. It is found that the cumulative contribution of upstream and downstream grid makes the three-phase fault most severe in comparison to other faults. However, if the transformer neutral is solidly grounded, then the severity of single line to ground fault will be highest. Furthermore the variation in grid strength considerably affects the fault current level of the distribution network. Such situation is the cause of major concern when solar PV is added to the network.

Theoretical Analysis on the Short-Circuit Current of Inverter-Interfaced Renewable Energy Generators with Fault-Ride-Through Capability

Renewable energy generators (REGs) usually employ power electronic devices for connecting with the grid, which makes their fault characteristics completely different from those of conventional synchronous generators. In the existing studies, the simulation methods are mainly adopted to analyze fault current contribution from REG. As a result, the explanations on the fault current show diversity and cannot reach a recognized standard. The REGs’ mathematical model in relay-setting calculations is unknown. Thus, this paper theoretically analyses the fault current characteristics of inverter-interfaced REGs (IIREGs) with fault-ride-through (FRT) ability. In order to understand the fault current characteristics, the FRT control strategy for IIREGs is firstly studied. Then the characteristics of high-frequency and fundamental-frequency fault currents from IIREGs are theoretically analyzed after and during the faults. The affecting factors and duration time of different frequency fault currents are, respectively, revealed. Further, the mathematical expression of fundamental fault currents from IIREGs are derived and verified based on the experimental test bench. The results can be used in estimating the IIREGs’ fault contributions and developing the fault calculation model.

Performance evaluation of distance protection of transmission lines connected with VSC-HVDC system using closed-loop test in RTDS

Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) can control voltage and power flow on ac transmission lines with the priority of reactive current support during the fault as suggested in the grid codes for HVDC connections. Furthermore, the fault current contribution from the converter is limited in magnitude and modified in the waveform in order to protect the power electronic devices during the fault, which on the other hand affects the apparent impedance seen by the distance relay. This paper addresses the challenges faced by the distance protection of transmission lines connected with VSC-HVDC system by performing a closed-loop test of a commercial distance relay using Real Time Digital Simulator (RTDS). The power system with VSC is modeled in detail under a more realistic scenario in RTDS so that the performance of the distance protection can be tested under near to actual field conditions. The responses of the commercial distance relay for balanced and unbalanced faults are analyzed. The oscillograph data of the voltage, current and protection bits recorded by the relay are extracted and compared with the simulation results.

Comprehensive Assessment of Fault Current Contribution in Smart Distribution Grid with Solar Photovoltaic

The aim of this paper is to analyze the short circuit (SC) behavior and variation in fault level due to solar PV inverters in a smart distribution network. In order to investigate the issue, a generic urban distribution feeder is modeled, which supplies power to the loads of varying nature. The network is analyzed according to IEC 60909 standard and the alteration in SC current and fault level by the inverters is reported. Paper also reports that the grid strength and neutral grounding techniques significantly affect the inverter SC current. It is found that, with solar PV integration, the major change in fault level occurs due to the three phase fault then followed by double line to ground and other faults. Moreover, the varying network conditions also alter the SC current and fault level of the system. Such situation has to be managed to ensure the protection coordination and reliability of the system. The integration of more and more renewable energy sources like solar PV is expected in future power grid, therefore a proper planning study is required at the time of integration to ensure the reliable operation of the system.

Use of superconducting fault current limiters for mitigation of distributed generation influences in radial distribution network fuse–recloser protection systems

In this study, extensive dynamic simulation studies are carried out to explore the impact of synchronous machine (SM)-based distributed generation (DG) integration on existing radial fuse–recloser protection infrastructure. Furthermore, dynamic simulation studies are also conducted to highlight the use of superconducting fault current limiters (SFCLs) to mitigate such an impact. These studies have included the effects of SM-based DG sources on fuse–recloser coordination and recloser sensitivity adequacy. In addition, a comparison between the performances of two different SFCL types has been also offered. The dynamic results of these investigations have shown that the presence of SFCLs has prevented any excessive fault current contribution from SM-based DG sources, as a result, it has restored the fuse–recloser coordination and recloser sensitivity adequacy. Within the frame of reference of the study is the dynamic simulations of a test benchmark that have been conducted using the PSCAD/EMTDC software.

Simplified and Automated Fault-Current Calculation for Fault Protection System of Grid-Connected Low-Voltage AC Microgrids

Abstract Fault currents inside a grid-connected AC microgrid are significantly varied because fault current contributions of the main grid and DG units are different from each other due to various fault locations, fault types, and high penetration of inverter-based distributed generators (IBDGs) and rotating-based distributed generators (RBDGs). A traditional fault-analysis method cannot be sufficiently applicable for AC microgrids with the presence of both rotating-based distributed generators and inverter-based distributed generators. From the above viewpoint, this paper proposes a simplified and automated fault-current calculation approach for grid-connected AC microgrids to quickly and accurately calculate fault-current contributions from IBDGs and RBDGs as well as the grid fault-current contribution to any faulted microgrid sections. The simplified and automated fault-current calculation approach is mainly focused on grid-connected and small-sized low-voltage AC microgrids with the support of communication system. Under the grid-connected microgrid operation mode, fault-tripping current-thresholds of adaptive overcurrent relays are properly adjusted thanks to the proposed fault analysis method. Relying on fault-current distribution-coefficients of IBDGs, RBDGs, and the utility grid, the setting values of adaptive overcurrent relays in a low-voltage AC microgrid are effectively self-adjusted according to various microgrid configurations and the operation status of DG units during the grid-connected mode.

Assessing the technical impact of integrating largescale photovoltaics to the electrical power network of Bahrain

Electricity demand of Bahrain is met almost entirely by gas turbine power stations at present. The peak demand growth rate of around 6.5% per year necessitates installing new generation in near future. Bahrain’s rich natural solar resource could be exploited to meet this need. However, so far, no detailed studies on the grid integration of large scale photovoltaics (PV) systems have been carried out. Potential impacts of integrating a 1MW PV plant to the power network of Bahrain is examined in this paper by means of a systematic modelling analysis using PVsyst and PSSE. Results demonstrate how PV systems can exploit the advantage provided by the weather of Bahrain to match the peak demand. Other concerns addressed in this work are impact of PV generation during times when electrical power network is more susceptible to voltage limit violations and impact of fault current contribution from the PV plant to the security of the network considered. The overarching view from the results of these grid integration studies is that the outlook for network integrated PV in the future In Bahrain is positive. It is hoped that the results of this work would inform Bahrain’s utility’s policies on PV systems.

Recloser time–current–voltage characteristic for fuse saving in distribution networks with DG

Utilities apply a fuse-saving strategy during auto-reclosing on transient faults. Integration of distribution generation (DG) into distribution networks (DNs) challenges this strategy as the fault current contribution from the DG may lead to the loss of recloser–fuse coordination. This study proposes a relaying scheme to be applied on microprocessor-based reclosers for fuse saving under transient fault conditions. A relay operating characteristic is defined which utilises voltage and current magnitudes at the recloser location. The voltage term in the relay characteristic compensates for the reclosing delay resulting from the DG fault contribution. The main features of the relaying scheme are use of local measurements, i.e. no communication link is required, and its independence of number and location of DG. The proposed relaying scheme is validated by simulation study on a practical 20 kV Iranian DN. It is shown that the new scheme maintains proper recloser–fuse coordination for different fault conditions and DG configurations. Moreover, the maximum DG penetration in different locations is obtained whilst the recloser–fuse coordination is upheld. It is shown in a comparative study that the proposed relaying scheme can maintain the coordination for higher DG penetrations than a recent adaptive method reported in the literature.

A Comprehensive Review of Protection Schemes for Distributed Generation

Due to the increasing demand of energy and the need for nonconventional energy sources, distributed generation (DG) has come into play. The trend of unidirectional power flow has been gradually shifting. With new technology comes new challenges, the introduction of DG into the conventional power system brings various challenges; one of the major challenges is system protection under DG sources. These sources pose a significant challenge due to bidirectional flows from DGs as well as lower fault current contribution from inverter interfaced DGs. This paper reviews existing protection schemes that have been suggested for active distribution networks. Most of these protection strategies apply only to smaller distribution systems implying that they may need to be extended to larger systems with a much higher penetration of distributed generation. In the end, a potential protection scheme has also been recommended as a future work.

Post‐retirement utilisation of synchronous generators to enhance security performances in a wind dominated power system

Power system security concerns due to significant wind generation could be twofold: first, the frequency response and second, the short-circuit performance. Continuous development of wind energy may cause economic replacement and retirement of existing synchronous generators. Modern wind turbine generators do not inherently offer inertia and governor response after a disturbance, which may result in declining frequency response behaviour. Besides, they have limited fault current contribution, which may cause unacceptable short-circuit ratio at the grid connection point of those wind power plants. Traditionally, frequency response and short-circuit performance are individually improved. However, both of them are concurrently affected by high wind penetration. Hence, a common approach to simultaneously enhance both indices is essential. Therefore, this study introduces an idea of operating some of the retired synchronous generators in the synchronous condenser mode, which is termed as ‘post-retirement scheme (PRS)’. Such a second use of the retired synchronous generators can jointly upgrade frequency response and short-circuit strength during high wind generation. This study also proposes a methodology to evaluate when and how much PRS should be deployed for ensuring an adequate security performance in a wind dominated power system.

Fault analysis of unbalanced radial and meshed distribution system with inverter based distributed generation (IBDG)

The fault current contribution of inverter based DGs (IBDGs) may affect the operation of protective devices present in the system. Hence, it is necessary to consider the presence of IBDGs in short-circuit analysis of distribution system. A short-circuit analysis approach for unbalanced distribution system with IBDG, incorporating different voltage dependent control modes, is proposed in this paper. Comparison of the results obtained by the proposed method with those obtained by the time domain simulation studies carried out using PSCAD/EMTDC software, shows the accuracy of the proposed technique.

Analysis of fault current contribution of Doubly-Fed Induction Generator Wind Turbines during unbalanced grid faults

The fault current contribution of the Doubly-Fed Induction Generator Wind Turbines (DFIG-WTs) under unbalanced grid faults did not draw researchers' attention due to the absence of any related requirements in the former grid codes. However, in 2015 new requirements were established dictating the DFIG-WT/generating unit to remain connected to the grid during phase-to-phase short-circuit and to perform reactive power provision. Thereupon, the knowledge of the fault current magnitude as well as the dynamic response of the DFIG-WT are crucial for the power system component design and for the validation of its commitment to the grid codes requirements. There are concrete control objectives regarding negative sequence control that cannot be fulfilled simultaneously but rather set according to the owner's preferences. Therefore, in this paper, a detailed analysis of the negative sequence current response under each control objective were carried out, considering all the controller parameters. The influence of each controller structure on the short-circuit quantities is investigated and based on it an approximated mathematical models of the short-circuit quantities and currents were proposed. Finally, a brief analysis, due to space limitation, of the response during the voltage limitation period is also performed where the influences of voltage limitation on the negative sequence current were shown.

Fault contribution from large photovoltaic systems in building power supply networks

This paper presents a detailed analysis for determining the impact of adding large three phase photovoltaic (PV) systems in secondary (building) power distribution networks. The analysis highlights the protection relay coordination problems arising due to the increase in network fault levels caused due to the contribution from PV generators. A typical distribution network for power supply to large buildings with multiple apartments in a housing complex has been modeled and used as a test network. Simulation result shows that the magnitude of fault current contribution from PV system depends on a number of different factors, not only on the size of the PV system. The analysis emphasizes the requirement to review protection settings of similar installations prior to connection of PV generators to the network and suggests a method of protection coordination to minimize the requirement of reviewing the protection setting every time a new PV system is connected to the distribution network. It is found that fault current contribution from PV systems, depending on the size, can cause significant relay coordination problems in terms of discrimination and thereby reduce system reliability.

Arc Flash Hazard in Distribution System with Distributed Generation

This work presents the arc flash hazard assessment in distribution system including Distributed Generations (DGs). The results demonstrate the impacts of DGs on the incident energy released from arc due to short circuits. The fault current contribution from DGs decreases the arcing time consequently decreases the incident energy. This situation causes the underestimated arc flash hazard. Finally, the circumstances and the parameters should be aware for arc flash assessments are discussed.

Enhanced Dynamic Voltage Control of Type 4 Wind Turbines During Unbalanced Grid Faults

The fully rated converter of type 4 wind turbines is capable of providing dynamic voltage control during grid faults by injecting controlled reactive currents. This paper describes three different dynamic voltage control options during unbalanced grid faults: 1) the positive sequence voltage control with only a positive sequence reactive current injection and suppression of the negative sequence current; 2) the positive sequence voltage control with limitation of the positive sequence reactive current injection and suppression of the negative sequence current; and 3) the positive and negative sequence voltage control with both a positive and a negative sequence reactive current injection. These different control options are compared in simulations of a wind power plant connected to a meshed power system, including synchronous generators. It is shown that both the positive sequence voltage control with limitation and the positive and negative sequence voltage control can overcome the voltage rise and voltage distortion that can occur with pure positive sequence voltage control without limitation. Both of these options have a distinct fault response, where the positive and negative sequence voltage control results in a fault response that resembles the fault response of a synchronous generator with higher fault current contributions in the faulted phases.

Optimal protection considering fault current characteristic of wind turbines in active distribution networks

Wind Turbines (WTs) make the traditional distribution networks act as active distribution networks and change the fault currents behavior and protection system operation. The fault current contribution of WTs depends on the type of WTs generators. This paper shows the impacts of WTs on Over Current Relays (OCRs) operation and proposes a new OCR with voltage dependent curve to tackle protection problems. The proposed relay changes its settings based on the voltage measured at the location of the relay. The capability of the relay is tested on two networks, including different types of WTs by Particle Swarm Optimization algorithm. Two distribution networks with different types of WTs are modeled and simulated in MATLAB\Simulink.

Management of Fault Current Contribution of Synchronous DGs Using Inverter-Based DGs

Of the numerous types of distributed generators (DGs), synchronous DGs represent the most substantial contribution to fault currents and, consequently, have the greatest effect on the operation of the protection system. On the other hand, many types of DGs require power electronic (PE) interfaces at the points where they connect to the grid, which are normally left idle during fault conditions. This paper presents a novel idea for employing the PE interfaces of inverter-based DGs (IBDGs) as a means of managing the fault current contribution of synchronous DGs through the modification of the IBDG current phase angle during fault conditions. This operation enables synchronous DGs and IBDGs to be kept connected to the system during fault conditions but with their contribution to the current having no effect on the magnitude of the fault current. Constraints related to DG locations and capacities are thus relieved. More interestingly, it is demonstrated that larger-capacity IBDGs are more effective for managing the fault current contribution of synchronous DGs, which means that for systems that include synchronous DGs, the availability of more numerous IBDGs would be beneficial.