Fault Current Contribution Research Articles

Transient stability enhancement in multiple‐microgrid networks by cloud energy storage system alongside considering protection system limitations

AbstractThe gas turbine synchronous generators (GTSGs) are widely deployed as distributed generations (DGs) in countries with massive natural gas production owing to their low prices. However, due to the low inertia time constants of these synchronous‐based DGs, they are more susceptible to power grid faults which stands as a transient stability issue in networks with multiple microgrids (MGs). On the other hand, the cloud energy storage system (CESS) is a new concept that centralizes the individual distributed energy storage of one or more MGs. Here, the employment of CESS with synchronverter grid connections creates a suitable opportunity for improving the transient stability of the network by providing higher inertia. This is while the fault current contribution of the synchronverter‐based CESS imperils protection constraints in these networks. Therefore, a proper protection coordination index (PCI) is considered in the conducted study to identify the optimal size of the synchronverter‐based CESS through a two‐stage optimization algorithm that preserves the protection constraints among protective devices. Finally, the transient stability of the network with synchronverter‐based CESS is assayed by calculation of the critical clearing time (CCT) for faults. Numerical studies are carried out on the IEEE 33‐bus test system. Results are discussed in depth.

Improvement of Fault Current Calculation and Static Security Risk for Droop Control of the Inverter-Interfaced DG of Grid-Connected and Isolated Microgrids

The contribution current of an inverter-interfaced distributed generator unit during a fault is one of the significant challenges for two modes: grid-connected and isolated AC microgrid. For this challenge, this article is aimed to study two methods of fault current calculation for two modes: grid-connected and isolated microgrids. These methods include a virtual equivalent impedance and a proposed method. The proposed method is a new technique for calculating the fault current contribution depending on the droop control of inverter-interfaced DG. The proposed method can control the contribution short-circuit current of DG within its limit (2 p.u.) where it is dependent on the voltage value of the DG bus to calculate the short circuit current of DG by using the control criterion. Static security risk and load shedding are calculated after fault clearance using an operation scenario in which the distribution system will be divided into small subsystems and is then grid-connected and isolated due to the removal of the faulted bus by protection devices. The proposed technique is applied to a standard IEEE 33-bus distribution network with five DGs. The results show that the contribution current of inverter-interfaced DG during the fault has more effects than the fault current of the nearest faulted bus to the DG bus. The proposed technique improves the calculated fault current value by about 30% for the grid-connected microgrid and by about 50% for the isolated microgrid from its value of the virtual impedance method. The static security risk is improved after load shedding. The static security risk improved by about 0.025%.

Fast DC Fault Current Suppression and Fault Ride-Through in Full-Bridge MMCs via Reverse SM Capacitor Discharge

In the event of a DC side fault in modular multilevel converters (MMCs), the fault current contributions are initially made by submodule (SM) capacitor discharge, which occurs before the fault is detected, followed by the AC side contribution to the DC side fault. While the AC side currents can be regulated using fault blocking SMs, the initial discharge of the SM capacitors results in high DC fault currents, which can take several milliseconds to be brought under control. This paper presents a method to actively control the rate of rise of the DC fault current by regulating the discharge of SM capacitors and accelerating the suppression of fault current oscillations during fault ride-through (FRT) in a full-bridge (FB)-MMC system. In the proposed method, the discharge direction of the FBSM capacitors is reversed following the detection of a DC side fault, which leads to a reversal in the fault current direction and a fast drop-off towards the zero-crossing. Immediately after the zero-crossing of the DC fault current, the DC fault is cleared by adjusting the arm voltage references and operating the MMC as a static synchronous compensator (STATCOM) to provide voltage support to the AC grid. The proposed control scheme provides faster fault current suppression, more effective SM capacitor voltage regulation, low AC side and MMC arm current transient peaks, and an overall superior DC-FRT performance compared to methods in which the conventional fault ride-through operation is initiated immediately upon DC fault detection.

Quantitative criteria of considering AC infeed in DC fault assessment of modular multilevel converters

An accurate and efficient DC fault current assessment of modular multilevel converters (MMCs) is critical for designs and protection. The state-of-the-art often ignores the impact of the AC infeed in the DC fault assessment without MMC blocking. However, whether the AC infeed needs to be considered or not is a controversial problem. This paper proposes quantitative criteria to determine the necessity of considering the AC infeed in the fault current assessment. The method is based on the premise that the MMC blocking has not occurred. Based on the average model of the MMC, the fault current contribution from the AC side is quantified under different control schemes. The criteria are established by a contributing ratio of the AC infeed over the total fault current. The obtained criteria only rely on the system and control parameters, which enable designers to identify the impact of ignoring the AC infeed in the DC fault assessment. The effectiveness of the proposed criteria is validated by two case studies and two different control schemes. The results reveal that the submodule (SM) capacitance and the number of SMs have a large impact on the AC infeed contribution.

Grid Connection Studies for Large-Scale Offshore Wind Farms Considering High Penetration of Regional Renewables

There is a global focus on adding renewable energy sources to the mix of energy supplies. In this study, the grid connections for large-scale offshore wind farms in areas that have high penetration of renewable energy sources were examined. System strength evaluation considering the interaction of wind farms and inverter-based resources (IBRs) was performed; the fault current was then analyzed to determine their contribution to the total fault current at a bus level. These studies revealed that the interaction between offshore wind farms and IBRs may make the power system weaker, and it is possible that fault current contributions from offshore wind farms can violate the capacity limit of existing circuit breakers. The results of steady-state analysis were verified through case studies focused on the southwest area of the Korea Electric Power Corporation (KEPCO) system where large-scale offshore wind farms are planned to be established and connected. Power system planners will benefit from the results of this study with a better understanding of the factors to consider when integrating large-scale wind farms in areas with high penetration of renewables.

A Review of Current-Limiting Control of Grid-Forming Inverters Under Symmetrical Disturbances

Grid-forming (GFM) inverters are recognized as a viable solution to increase the penetration of renewable energy in bulk power systems. However, they are physically different from synchronous generators in terms of overcurrent capability. To protect the power semiconductor devices and support the power grid under severe symmetrical disturbances, the GFM control systems should be able to achieve the following requirements: current magnitude limitation, fault current contribution, and fault recovery capability. Various current-limiting control methods are reported in the literature to fulfill these goals, including current limiters, virtual impedance, and voltage limiters. This paper presents an overview of those methods. Emerging challenges that need to be addressed, including temporary overcurrent, unspecified output current vector angle, undesired current saturation, and transient overvoltage, are pointed out. Comparative simulations are conducted to demonstrate the performance of different methods under grid voltage drops and phase jumps. Finally, open issues of current-limiting control methods for GFM inverters, including transient stability assessment, voltage source behavior under overcurrent conditions, and windup of voltage controllers, are shared.

Equivalent Model of Photovoltaic Power Station Considering Different Generation Units’ Fault Current Contributions

The fault current calculation model of photovoltaic (PV) power stations is usually treated as a capacity weighted equivalent model of a single PV generation unit (PVGU). However, in the same PV power station, different PVGUs have various fault current characteristics. As a result, there are significant differences in fault current characteristics between a PVGU and a PV power station. It means that the existing capacity weighted equivalent model cannot be used for accurately describing the fault current contributions from a practical PV power station. In this paper, the fault behaviors of the PVGUs located at different access points of a PV power station are firstly analyzed. The difference in PVGUs’ fault current contributions is identified and reflected with the activation states of current limiters that are employed for PV inverters. The activation states are represented by a theoretical expression so as to distinguish the PVGUs’ fault contributions. Further, based on the proposed theoretical expression, a novel algorithm is developed for sorting all PVGUs included in a PV power station. The multi-machine calculation model is deduced in order to exactly express the fault current contribution from a PV station. Finally, some simulation tests are conducted. The tested results verify the effectiveness of the proposed calculation model. It can provide support for calculating the protection setting of power grid connected with large-scale PV stations.

Short-Circuit Analysis of DER-Based Microgrids in Connected and Islanded Modes of Operation

Since microgrids should be able to smoothly operate in two distinct modes—grid-connected and islanded, their fault currents can widely fluctuate depending on the operational mode. When the microgrid is connected to the grid, the highest fault current, by far, is supplied by the utility grid. In this mode, the fault current contribution from distributed energy resources (DERs) is less than 20%. However, when the microgrid switches to the islanded mode, the fault current contribution from the utility grid is lost and DERs are the sole fault current sources. Thus, the overall fault current in the islanded mode is multiple times lower when compared to the grid connected mode. Moreover, most of the DERs are inverter-based, with limited fault currents, which further reduces the overall fault current in the islanded mode. With the rapid rise of the microgrid penetration around the globe, this phenomenon can adversely influence the relay protection, and thus the microgrid fault current needs to be precisely analyzed. Therefore, the main purpose of this paper is to thoroughly analyze the fault current differences in two distinct operation modes of a microgrid, and to consequently derive conclusions regarding the required improvements in fault calculations and relay protection analysis in emerging microgrids. A representative microgrid test bed is developed and modelled using the in-house developed software as well as in a state-of-the-art hardware-in-the-loop environment. Several different short-circuit faults were simulated and analyzed in both grid-connected and islanded modes. The results show that the fault currents significantly differ depending on the operating mode, and thus highly influence the protection system. Moreover, test results show that the fault calculation algorithms aimed at radial distribution grids, mostly used for microgrid fault calculations in the available literature, need to be further improved to provide precise and time-efficient results when the emerging microgrids are considered. These results provide a valuable insight into the current state of the microgrids’ fault calculation and protection and reveal several important directions for future research.

A linearized MOV model-based method for fault location on off-terminal series capacitor bank-compensated transmission line using one-end current

This paper proposes a method for fault location on single and double-circuits series capacitor bank-compensated transmission line (SCTL). A common situation for installing the series capacitor bank (SCB) is to place it some point away from the terminal substations. One-end measurement-based fault locators are favorable for many utilities due to simplicity and less cost especially if they are accurate enough. For impedance-based one-end fault locators on SCTL, the current passing in the SCB is unknown for about 50% of faults. This prevents the prediction of the challenging metal oxide varistor (MOV) behavior that is inevitable for a one-end method. Herein, the nonlinear characteristics of the MOV protecting the SCB are simplified by a linearized simple yet precise model. This makes it possible to express the SCB-MOV characteristics as a linearized function of the far-end current for faults upstream the SCB keeping the nonlinear characteristics of the MOV. This eventually enables to estimate the fault current contribution of the far-end. Hence, an analytical fault distance estimation algorithm is derived for both single and parallel transmission lines using the sequence-networks modeling of the system. Beside simplicity and robustness, the proposed method is found to be accurate enough in locating a big number of faults under a variety of conditions.

Impact of converter‐interfaced renewable generations on breaker failure protection of the emanant transmission lines

The presence of Converter Interfaced Generations (CIGs) poses significant challenges to traditional protection schemes. This paper studies the impact of CIGs on the Breaker Failure Protection (BFP) of transmission lines emanating from CIGs. The instantaneous over-current (50BF) setting and reset time of BFP have been investigated. Fault current contribution of CIGs is usually comparable with load currents. Low fault current contribution by CIGs may lead to significant loss of security of BFP because of the existing practice of using lower setting for 50BF. This paper proposes a voltage-dependent adaptive setting of 50BF element to enhance the security of BFP schemes while maintaining dependability. Use of voltage helps in differentiating loads from fault situations. In traditional power systems, CT subsidence current is known to delay the reset of BFP schemes. The impact of low fault contribution by CIGs on the reset time of BFP has been studied. Mathematical expression for CT subsidence current, which influences the reset time, has been derived. It is observed that the BFP reset may not be delayed if the fault current seen by the breaker is low due to the presence of CIG. Discussions on the dependability and security of proposed 50BF setting are also presented.

Waveform Difference Feature-Based Protection Scheme for Islanded Microgrids

Unlike conventional synchronous generators, the fault characteristics of inverter-based distributed generators (IBDGs) are mainly determined by the inverter control strategies. Their limited fault current contributions make the protection design of islanded microgrids (MGs) become a major challenge. This article focuses on the fault characteristics and protection scheme design of islanded MGs. First, the phase differences between the positive-sequence current fault components (PSC-FCs) at both terminals of fault line are analyzed under different network topologies and fault severities. It points out the phase characteristic differences between internal and external faults. Then, on basis of this to overcome the problems of the phase measurement error and protection setting, a waveform difference feature-based protection scheme is proposed. The protection criteria are formulated using the first peak sign and time information of PSC-FCs between both terminals of line, which are extracted by the mathematical morphology (MM) technology. Only relatively low bandwidth communication is required to achieve accurate fault identification. Finally, the simulation results on a 10-kV islanded MG model demonstrate that the proposed scheme can identify internal faults reliably in both looped and radial islanded MGs under different fault resistances and measurement noises.

Integrated Outage Management With Feeder Restoration for Distribution Systems With DERs

The increasing deployment of distributed energy resources (DERs) and microgrids benefits power grids by improving system resilience. In a resilience mode without the utility system, the distribution grid relies on DERs to serve critical load. In such a severe event with multiple faults on the distribution feeders, actuation of various protective devices (PDs) divides the distribution system into electrical islands. The undetected actuated PDs due to fault current contributions from DERs can delay the restoration process, thereby reducing the system resilience. In this paper, algorithms are proposed for outage management and feeder restoration for distribution systems with multiple DERs. The Advanced Outage Management (AOM) identifies the faulted sections and actuated PDs in a distribution system with DERs by incorporating smart meter data. The Advanced Feeder Restoration (AFR) is proposed to restore a distribution system with available energy resources taking into consideration the availability of utility sources and DERs as well as the feeder configuration. By partitioning the system into islands, critical load will be served with the available generation resources within islands. When the utility systems become available, the optimal path will be determined to reconnect these islands back to substations and restore the remaining load. The proposed method has been validated with modified IEEE 123-Bus and 8500-Node Test Feeders. Simulation results demonstrate the capability of the integrated AOM and AFR to enhance distribution system resilience.

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.

Impact of DFIG‐based wind turbine's fault current on distance relay during symmetrical faults

As a new highly penetrated power source of modern power systems, doubly fed induction generator (DFIG)-based wind turbine (WT) has a different fault current contribution when compared with a synchronous generator. DFIG-based WT's fault current has significant impacts on distance relay's performances and may result in maloperation. In this study, the impacts of DFIG-based WT's fault current on the dynamic performances of two typical distance protection elements, mho and polarisation, are studied in terms of EMTP simulations and fault current expressions in symmetrical fault scenarios. It is discovered that the zone-1 distance relays of the lines connecting to DFIG-based wind power plants tend to overreach. Also, the following analysis indicates that the transient components of DFIG-based WT's fault current and the phasor estimation unit are the underlying reasons. With those understanding, this study is expected to help design new setting methods and for distance protection in power systems integrating wind power plants to prevent this kind of possible overreach.

Integration of a Heuristic Multi-Agent Protection System into a Distribution Network Interconnected with Distributed Energy Resources

In conventional method protection schemes are devised for certain operation conditions with fixed settings throughout their operation life, however, protection systems in interconnected distribution networks rely on a detailed analysis of fault current contribution and variation in operation conditions under real-world scenarios. In fact, the concept of Adaptive Protection System (APS) and possibility of the adjustment for protection settings within modern Intelligent Electronic Devices (IEDs) have introduced desired functionalities for protection systems to cope with complex operation scenarios envisioned within future power system networks. In this paper, a novel protection system based on Multi-Agent System (MAS) and heuristic decision-making is proposed to update protection settings of the protection IEDs with respect to prevailing operation conditions in an interconnected distribution network. The adopted methodology is reliant on real-time simulation of a distribution network interconnected with Doubly-fed Induction Generator (DFIG) wind farms where different fault scenarios are applied to evaluate the functionality and performance of the proposed Multi-Agent Protection System (MAPS). In addition to that, combination of knowledge sharing between different protection IEDs and logic reasoning are integrated to fulfill protection task under real-world operation scenarios. The significance of the proposed MAPS is the improvement in protection system of the interconnected distribution network.

Interharmonic Differential Relay With a Soft Current Limiter for the Protection of Inverter-Based Islanded Microgrids

Inverter-interfaced distributed generators (IIDGs) introduce a major challenge for the protection of inverter-based islanded microgrids (IBIMs) due to their limited fault current contribution. This paper proposes a new strategy for the protection of IBIMs, founded on two novel aspects including a soft current limiter and an interharmonic differential relay. In addition to limiting the fault current, the proposed current limiter performs an amplitude modulation of the output fault current from the IIDG which occurs only during fault conditions. This modulation produces interharmonic currents with specific signatures that flow towards the fault. The process of injecting interharmonics is spontaneously initiated by a subset of the IIDGs upon the fault occurrence without a need for a fault detection scheme. Thereafter, based on the locally detected interharmonics, an interharmonic differential relay requiring low bandwidth communication is proposed to identify and isolate the fault. The proposed strategy has been validated using PSCAD/EMTDC. The results confirm the ability of the proposed protection strategy to protect IBIMs considering different fault types, locations, and values of the fault resistance without the need for additional fault detection components or high-bandwidth communication which may constraint the applicability and reliability of the protection schemes.

New Approach of Calculation the Steady State Symmetrical Fault Current for Type 4 Wind Power Plants

With the increasing penetration of the distributed generation driven by renewable energy in distribution systems, there is a need to analysis fault currents for the next generation utility and smart grid systems. The steady state fault current contribution from Type 4 wind turbines is not calculated accurately by traditional short circuit analysis techniques. A new formulation for calculating the short circuit current sharing from voltage source converter wind turbines is presented, and taken into consideration the real behaviour of the type 4 WT during three phase short circuit in the electric system. The results are compared with that obtained from the dynamic simulation results using Power World Simulator program, which has the generation of Western Electricity Coordinating Council (WECC) generic model. The percentage difference between the two results is about one percent that verify the effectiveness of the proposed formulation

Protection of Inverter-Based Islanded Microgrids via Synthetic Harmonic Current Pattern Injection

Faults in inverter-based islanded microgrids can be a formidable protection challenge due to the limited fault current contribution of inverters. This paper designs a selective and sensitive protection scheme by injecting a pattern of up to three synthetic harmonics utilizing the flexibility of existing inverter-based distributed generation (IBDG) controllers (called injection pattern [IP]). The measured pattern (MP) by each relay is the result of the combination of IPs from IBDGs at different locations in a microgrid. To ensure that relays measure different MPs for forward and reverse faults, an optimization model is formulated to set and minimize the total number of IPs by IBDGs. A pattern-based directional element is proposed based on the cosine similarity measure between the MP and the predefined characteristic pattern (CP) of each relay. CPs are set offline as the normalized MPs of forward faults. Moreover, a new direction-comparison relaying scheme is devised, which comprises of permissive overreaching transfer trip (POTT) and direction zone-interlocking (DZI) for primary and remote backup protection, respectively. Transient studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults and different microgrid topologies.

Harmonic Directional Overcurrent Relay For Islanded Microgrids With Inverter-Based DGs

Conventional directional overcurrent relays (DOCRs) may fail to protect microgrids formed using inverter-based distributed generation (IBDG) units, as IBDGs have limited fault current contribution. Maloperation of DOCRs can be due to: 1) inability to distinguish load and fault currents; 2) incorrect fault direction identification; or 3) miscoordination of primary and backup DOCRs. This article proposes employing harmonic current injection capability of IBDGs to design a novel harmonic DOCR (HDOCR). In the proposed method, during fault conditions, each IBDG injects a harmonic current with an order different from the ones injected by its neighboring IBDGs. The magnitude of this current is modulated by an equivalent impedance that is locally computed at the terminals of each IBDG. New time-harmonic-current characteristic, directional element, and protection coordination optimization model are devised for the proposed HDOCRs. The proposed protection scheme is applicable to radial and ring microgrids with multiple IBDGs and does not require communication. Simulation studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults.

Protection Coordination of Radial Distribution Networks Connected with Distributed Generation Considering Auto-reclosing Schemes by Resistive Superconducting Fault Current Limiters

AbstractRadial distribution networks are normally protected against fault currents by fuses and reclosers. This protection is coordinated in a way that reclosers clear temporary faults before the melting of fuses and fuses melt to clear permanent faults. Distribution networks, which are recently connected with distributed generations (DGs), incur a variety of issues that are certainly worthy of closer considerations. One of these issues is the operation of protection devices. Recloser-fuse coordination may be lost when DGs are integrated with distribution networks due to the effect of current flowing from DGs during fault period and the breaking of the radiality nature of these networks. The impact of DGs on recloser-fuse coordination depends on penetration level, location, and type of DG. Recloser-fuse coordination of radial distribution networks connected with DGs can be kept by connecting resistive superconducting fault current limiter (R-SFCL) in series with the DG to limit the fault current contribution of DG during the fault period. However, when considering the auto-reclosing scheme of the recloser, recloser-fuse coordination may be lost even if R-SFCL is connected. This paper shows the effectiveness of R-SFCL to restore recloser-fuse coordination without considering auto-reclosing scheme. It also proposes solution methods based on three different configurations of R-SFCL to maintain recloser-fuse coordination when auto-reclosing scheme is considered. Simulation results illustrate the effectiveness of the proposed configurations of R-SFCL to keep recloser-fuse coordination. All simulations are performed using MATLAB/Simulink package.