Asymmetrical Fault Conditions Research Articles

A smooth switching strategy for steady-state operation control and fault transient control of doubly fed induction generator

Abstract As the proportion of wind power generation systems in power systems continues to rise, their dynamic response during system malfunctions has gained significant importance. As the grid short-circuit ratio continues to decrease, traditional fault ride-through algorithms for wind power generators may no longer be applicable. This is evidenced by voltage oscillations under significant disturbances and overvoltage issues during low-voltage ride-through, which are further aggravated during asymmetric fault conditions. Therefore, this paper focuses on the low-voltage ride-through issues of doubly fed induction wind power generators in weak power grids. It investigates the steady-state operation and transient control schemes and proposes a smooth transition strategy for steady-state and fault transient operations of doubly fed wind power generators. This approach guarantees the synchronization of active and reactive power, mitigates steady-state reactive power imbalance, and prevents voltage fluctuations triggered by recurring low-voltage ride-through occurrences. Furthermore, during fault recovery, a ramped active power restoration approach is employed to mitigate the coupling of active and reactive power introduced by the phase-locked loop, enabling rapid and stable restoration of active and reactive power outputs. As a result, superior dynamic performance is achieved during both symmetric and asymmetric fault processes. Finally, the superiority of the proposed smooth transition strategy under different fault scenarios is validated through simulations with the RTLAB platform.

A fault ride-through strategy for grid-forming converters under symmetrical and asymmetrical grid faults

In order to maintain grid-forming converter (GFM) voltage source behaviour under current limiting mode, a threshold virtual impedance (TVI) current limiting control is proposed, which is controlled in the positive and negative sequence synchronous reference frames for symmetrical and asymmetrical fault conditions. The converter current is strictly limited within the maximum limit without the need for current saturation limiters. Since the TVI control is based on 3-phase sinusoidal currents, it is shown that using the measured current instead of the existing current reference for the TVI control may cause oscillatory behaviour when a large switching delay is considered. Since sequence extraction control is necessary, the paper also compares GFM dynamic stability under the three well-known sequence extraction methods (i.e. delay cancellation, dual second order generalized integrator, and decoupled double synchronous reference frame). It is shown that the differences are small when GFM is not in current limiting mode, but they are large when GFM is in current limiting mode or switching delays are considered.

Transient Synchronous Stability Analysis of Grid-Forming Photovoltaic Grid-Connected Inverters during Asymmetrical Grid Faults

Compared with the traditional grid-following photovoltaic grid-connected converter (GFL-PGC), the grid-forming photovoltaic grid-connected converter (GFM-PGC) can provide voltage and frequency support for power systems, which can effectively enhance the stability of power electronic power systems. Consequently, GFM-PGCs have attracted great attention in recent years. When an asymmetrical short-circuit fault occurs in the power grid, GFM-PGC systems may experience transient instability, which has been less studied so far. In this paper, a GFM-PGC system is investigated under asymmetrical short-circuit fault conditions. A novel Q-V droop control structure is proposed by improving the traditional droop control. The proposed control structure enables the system to accurately control the positive- and negative-sequence reactive current without switching the control strategy during the low-voltage ride-through (LVRT) period so that it can meet the requirements of the renewable energy grid code. In addition, a dual-loop control structure model of positive- and negative-sequence voltage and current is established for the GFM-PGC system under asymmetrical short-circuit fault conditions. Based on the symmetrical component method, the composite sequence network of the system is obtained under asymmetrical short-circuit fault conditions, and positive- and negative-sequence power-angle characteristic curves are analyzed. The influence law of system parameters on the transient synchronous stability of positive- and negative-sequence systems is quantitatively analyzed through the equal area criterion. Finally, the correctness of the theoretical analysis is verified by simulation and hardware-in-the-loop experiments.

Fault Current Multi-Stages Calculation Method for DFIG-Based Wind Farms with Whole Fault Process Attributes under Asymmetrical Grid Fault Conditions

Fault Current Multi-Stages Calculation Method for DFIG-Based Wind Farms with Whole Fault Process Attributes under Asymmetrical Grid Fault Conditions

Simulation-based post-fault analysis in WECS using DFIG systems under an unbalanced fault scenario

Fault analysis in an integrated Wind Energy Conversion System (WECS) is the highest priority factor to be taken into account for distributed generation (DG). The contribution of renewable energies (like solar and wind) to the power system is increasing continuously. New challenges are being faced while integrating such systems because of the no inertial response provided by renewable resources. So, for a reliable integration further exploration, development, and even new strategies are needed to improve the reliability, power quality, control and protection of renewable energy generation. Disintegration in distributed generation is still a huge question mark on the reliability of modern power systems. Disintegration has many causes and faults are one of them, that cause severe voltage profile distortions, which makes WECS a complex system in terms of control and protection of power system. In this work, focus has been paid to the analysis of asymmetrical faults in wind farms and their impacts are evaluated on the overall generation system. A 30 MW wind power plant has been developed in MATLAB to study the faults. Since wind farms are Low Voltage (LV) networks, therefore, fault impedance method (IEC 60909) and the Fort-cue Method have been used for the fault analysis. The fault current response and behaviour of the DG system under asymmetrical fault conditions at different locations in and around the wind farm have been obtained. It is found that fault loop impendence, transformer grounding type, and fault location largely affect the dynamic behaviour of the co-generation system under asymmetrical faults. In this work dynamic behaviour of the wind generation system has been analysed for the different asymmetrical faults induced at different locations of the system. The response of the wind generator has been evaluated based on the different fault simulations considering the sequence component method.

Enhanced DC-link voltage control of PMSG-based wind turbine generators by machine-side converter during asymmetrical grid faults

Under asymmetrical grid fault conditions, oscillating active power can occur on the grid-side converter (GSC) of permanent magnet synchronous generator (PMSG)-based wind turbine generators. The oscillating active power flows into the DC-link, causing voltage oscillations that can lead to a shortened lifetime of DC-link capacitors and additional harmonics injected into the grid. While conventional methods can mitigate the oscillating active power through negative sequence current injection by the GSC, this hinders the grid support of the GSC regarding negative sequence reactive current during grid fault. To address this issue, a novel control scheme is proposed that uses the machine-side converter (MSC) to transfer the oscillating power to the motor. Firstly, the active power capacity of the MSC is analyzed and compared with that of GSC, and the results show that MSC has enough capacity to accommodate the oscillating active power. On this basis, a novel MSC control scheme is proposed to transfer the oscillating active power on the DC-link to the motor, which has a much larger inertia to buffer the power oscillation. Compared with conventional methods, the proposed control scheme relieves the GSC from mitigating the power oscillation to support the grid. Simulation studies using DIgSILENT PowerFactory are presented to verify the effectiveness of the proposed control scheme for PMSG-based wind turbine generators.

DEVELOPMENT OF INTELLIGENT CONTROL STRATEGY FOR POWER QUALITY IMPROVEMENT OF HYBRID RES

The major problem in renewable energy system is that the variation in power generation from time to time because of the intermittent nature of the renewable sources. In this paper presents the Artificial Neural Network (ANN) based intelligent control strategy for hybrid standalone microgrid system for varying wind, varying solar irradiation, varying load and symmetrical and asymmetrical fault conditions is presented. A dynamic model of hybrid standalone microgrid consisting wind and solar PV renewable sources with maximum power point tracking control algorithm is developed in MATLAB/Simulink software. A solar PV panel, wind energy conversion system and dynamic model of hybrid energy storage consisting Lithium-ion batteries (Li-Ion) and supercapacitor is connected with the DC Bus. The DC link is connected through the power electronic interfacing circuit and converters connected to a source for a diverse electricity generation. The main objective of this thesis is to improve the power quality of the hybrid microgrid system by solving the voltage sag and swell problem and reduce total harmonics distortion occurring due to various symmetrical and asymmetrical condition. The simulation studies have been carried out to determine system performance with different scenarios of the sources such as typical solar radiation, temperature, wind, battery and supercapacitor charge or discharge conditions The proposed strategy gives better performance characteristics and reduces the harmonics of the system compared to the conventional solution.

Performance Evaluation of Circuit Breakers under Asymmetrical Fault Condition (Test Duty T100a)

Among several switchgear equipment largely used in high voltage Transmission systems, which change the gridconfiguration, isolates faulty parts from the grid, etc. circuit breaker is a remarkable one. For the reason to analyse theinterrupting capabilities, a circuit breaker has to undergo various test duties according to IEC 62271-100 among whichTest duty T100a is considered as the most onerous one. During this test the breaker has to prove its Interrupting capabilityduring maximum arc energy condition. This article focuses on the Performance evaluation of circuit breaker underasymmetric condition Test duty T100a.

Investigation of the Effect of Different Facts Devices on Stability in Case of Asymmetric and Symmetrical Fault Condition in Two-Machine System

Many methods have been developed for the control and stability of electrical energy systems. As a result of the researches, more complex systems are produced for electricity generation, transmission and distribution. New modern systems are developed which are capable of changing power flow parameters such as voltage, impedance, power angle, frequency, and power transmission capacity in transmission systems. In this context, the effects of static elements to be connected in parallel or in series to the stability of power system stability studies is one of the most important fields of study of electrical science. In this study, the effects of STATCOM and SVC, one of these shunt flexible AC devices, on the stable operation of the system in case of symmetrical or asymmetric fault situations that may occur in the transmission system are examined in matlab / simulink environment. This study showed that STATCOM provides superior control than SVC and maintains the stability of the system in case of any failure.

Enhancement of Line-to-Line Voltage Support During Asymmetrical Microgrid Faults Using a Four-Leg Three-Level Inverter

Recent research is focused not only to keep a three-phase inverter connected to the grid but also to support the voltage at the point of common coupling (PCC) during grid faults. The line-to-line PCC voltage can be enhanced by increasing the amplitude of positive and negative sequence current supplied to the grid for a given inverter rating. 3-leg inverters can supply only positive and negative sequence current to the grid. However, 4-leg inverters have the ability to supply zero-sequence current along with positive and negative sequence currents. This extra degree of freedom of control remains un-utilized during low voltage ride through operation. This paper presents a novel method to select the amplitude and phase angle of the zero-sequence current in such a way that the peak current amplitude of the 4-leg inverter remains minimum. Hence, it is possible to supply more positive and negative sequence currents to the grid. The proposed strategy can be applied to any asymmetrical grid fault condition. Experimental verification for two specific fault conditions has been carried out and results are presented.

Low-Voltage Ride Through Enhancement of Both DFIG and SEIG-Based Wind Power Plants by RC-Type Solid-State Fault Current Limiter

The low-voltage ride-through (LVRT) is the main requirement of grid codes for integration of wind power plants (WPPs) to the power system. This paper proposes a RC-type solid state fault current limiter (SSFCL) to enhance the LVRT requirement of WPPs. It is based on the RL-type SSFCL in which its configuration is changed to satisfy the LVRT requirements. To evaluate the proposed RC-type SSFCL performance, extended time domain simulations are executed in PSCAD/EMTDC software environment. Also, the performance of the RC-type and RL-type SSFCLs are compared under both symmetrical and asymmetrical fault conditions to show the efficiency of the proposed RC-type SSFCL. Both doubly-fed induction generator (DFIG) and self-excited induction generator (SEIG) are used to examine the RC-type SSFCL performance. Based on simulation results, the RC-type SSFCL provides an effective hardware scheme to augment the LVRT requirements of both DFIG and SEIG based WPPs and outperforms the RL-type SSFCL.

Submodule fault-tolerant control based adaptive carrier-PDPWM for modular multilevel converters

The modular multilevel converter (MMC) is considered a very promising candidate for medium-voltage and/or high-power grids. One of the major concerns in half-bridge MMCs, especially with large number of levels, is the fault occurs at the converter submodule (SM). Hence, an appropriate control with an SM fault tolerance of the MMC is required to enhance its reliability. This paper addresses the issue and proposes a fault tolerant control-based adaptive carrier phase disposition pulse width modulation (AC-PDPWM) technique for MMCs with superior symmetrical and asymmetrical fault-tolerant capabilities. The proposed control-based AC-PDPWM acts to regulate the flow of energy between arms, rebalance the SM capacitors, and redistribute the pulses to the SM switches. In this study, a single-phase grid connected MMC having both symmetrical and asymmetrical fault conditions is investigated. Compared with other control methods, the proposed control can tolerate the SM fault conditions without requiring redundant SMs or additional computational burden, which decreases the cost and volume of the MMC design and simplifies its control. Simulation and experimental results are included to verify the fault-tolerant capability of the proposed control.

Protection of DFIG during asymmetrical faults using FRT control technique

ABSTRACT Fault ride-through (FRT) capability of grid connected doubly fed induction generator (DFIG) for the wind energy system is an important feature. In this paper, FRT control technique of DFIG against the asymmetrical fault conditions is proposed. In which, the grid voltage unbalance is controlled with the help of sequence decomposition of rotor current, stator voltage and grid side current. The mathematical model of DFIG is presented in rotating synchronous reference frames using positive and negative sequence components of voltage and flux linkage. The main objectives of the proposed scheme are minimisation of electromagnetic torque oscillations, speed oscillations and DC link voltage oscillations. These objectives are achieved by using bandstop (Notch) filters. This technique also controls the rotor voltage of the converter by providing appropriate rotor current reference. The effectiveness of the proposed technique is proved by considering 2.6 MW grid-connected DFIG system in MATLAB Simulink environment.

A Novel Analog Circuit Soft Fault Diagnosis Method Based on Convolutional Neural Network and Backward Difference

This paper develops a novel soft fault diagnosis approach for analog circuits. The proposed method employs the backward difference strategy to process the data, and a novel variant of convolutional neural network, i.e., convolutional neural network with global average pooling (CNN-GAP) is taken for feature extraction and fault classification. Specifically, the measured raw domain response signals are firstly processed by the backward difference strategy and the first-order and the second-order backward difference sequences are generated, which contain the signal variation and the rate of variation characteristics. Then, based on the one-dimensional convolutional neural network, the CNN-GAP is developed by introducing the global average pooling technical. Since global average pooling calculates each input vector’s mean value, the designed CNN-GAP could deal with different lengths of input signals and be applied to diagnose different circuits. Additionally, the first-order and the second-order backward difference sequences along with the raw domain response signals are directly fed into the CNN-GAP, in which the convolutional layers automatically extract and fuse multi-scale features. Finally, fault classification is performed by the fully connected layer of the CNN-GAP. The effectiveness of our proposal is verified by two benchmark circuits under symmetric and asymmetric fault conditions. Experimental results prove that the proposed method outperforms the existing methods in terms of diagnosis accuracy and reliability.

Machine Learning-Based Intrusion Detection for Achieving Cybersecurity in Smart Grids Using IEC 61850 GOOSE Messages

Increased connectivity is required to implement novel coordination and control schemes. IEC 61850-based communication solutions have become popular due to many reasons—object-oriented modeling capability, interoperable connectivity and strong communication protocols, to name a few. However, communication infrastructure is not well-equipped with cybersecurity mechanisms for secure operation. Unlike online banking systems that have been running such security systems for decades, smart grid cybersecurity is an emerging field. To achieve security at all levels, operational technology-based security is also needed. To address this need, this paper develops an intrusion detection system for smart grids utilizing IEC 61850’s Generic Object-Oriented Substation Event (GOOSE) messages. The system is developed with machine learning and is able to monitor the communication traffic of a given power system and distinguish normal events from abnormal ones, i.e., attacks. The designed system is implemented and tested with a realistic IEC 61850 GOOSE message dataset under symmetric and asymmetric fault conditions in the power system. The results show that the proposed system can successfully distinguish normal power system events from cyberattacks with high accuracy. This ensures that smart grids have intrusion detection in addition to cybersecurity features attached to exchanged messages.

Current limiting strategy for grid-connected inverters under asymmetrical short circuit faults

Grid-connected inverter plays an essential role as an interface between energy resources and the power grid. The performance of the inverters is adversely affected by the grid disturbances such as imbalances and asymmetrical short circuit faults. Then, it is necessary to enhance the functionality of the inverter under such conditions. This paper proposes an unbalance current limiting strategy for grid-connected inverters under asymmetrical short circuit fault conditions. In the proposed current limiting strategy, two main features are included: (i) second-order harmonic elimination from instantaneous active power injected into the grid, and (ii) reactive current injection according to the grid code demand. In the proposed current limiting strategy, proper positive and negative sequence current components are calculated, by which the aforementioned goals are achieved. Furthermore, to be compatible with the well-known control structure of the power converters, the current references are obtained in the form that is applicable with the commonly-used double-synchronous-references-frame control structure. To verify the effectiveness of the proposed method, the behavior of the current limiting strategy is examined by simulation of an asymmetrical short circuit fault condition in a test system. In the simulation, both the transient response upon fault occurrence and steady-state of fault condition are considered and analyzed. Furthermore, the simulation results are supported by experimental test results, verifying the effectiveness of the proposed strategy.

Research on control strategy for AC side asymmetric fault of MMC-HVDC transmission system

When the Modular Multilevel Converter (MMC) fails on the AC side, the operating characteristics of MMC will be damaged. If the system wants to retain stable for a long time in the process of operation, it is obliged to design the control strategy in case of asymmetric fault. In this article, the control strategy based on feedback linearization principle is adopted to diminish the negative sequence current caused by breakdown, and a DC voltage controller is devised to suppress the fluctuation of the double frequency component under asymmetric fault conditions. The validity of the control strategy requires to be verified, the running results on PSCAD platform indicate this method is effective and feasible.

Power system stability enhancement in two machine system by using fuel cell as STATCOM (static synchronous compensator)

In this paper describes fuel cell functioning is a STATCOM to enhance the power system stability during various symmetrical and asymmetrical fault conditions. Nowadays our power utility confronting plenty of issues in retaining the stability of the system. Install the flexible alternating current transmission system (FACTS) compensator gives to be better results for this. But the expenses of compensating system extra payload to the consumers. For this reason a novel approach using fuel cell (FC) is a supply of compensation system was presented in this paper. Fuel cell (FC) was designed as a STATCOM – as a FACTS device. The potentiality of the FC STATCOM to improving transient stability of two machine system after a fault occurrence. In this paper fuzzy logic controller is a main controller was modeled to enhance the power system stability of two machine systems. Fuzzy logic controllers give the trigger pulse to improve the stability after the various fault occurrences. Proposed compensators are carried out in MATLAB simulink tool boxes. The design of with and without compensators was integrated in two machine power system and compared the results with traditional FCSTATCOM device. Various types of asymmetrical and symmetrical faults created in the proposed transmission line system. During the fault more oscillations occurred. After connecting the FC STATCOM oscillation level was reduced. Finally concluded that the results are found to be satisfactory in this proposed fuzzy logic based FC STATCOM compared with conventional FCSTATCOM.

New phase‐changing soft open point and impacts on optimising unbalanced power distribution networks

Three-phase unbalanced conditions in distribution networks are conventionally caused by load imbalance, asymmetrical fault conditions of transformers and impedances of three phases. The uneven integration of single-phase distributed generation (DG) worsens the imbalance situation. These unbalanced conditions result in financial losses, inefficient utilisation of assets and security risks to the network infrastructure. In this study, a phase-changing soft open point (PC-SOP) is proposed as a new way of connecting soft open points (SOPs) to balance the power flows among three phases by controlling active power and reactive power. Then an operational strategy based on PC-SOPs is presented for three-phase four-wire unbalanced systems. By optimising the regulation of SOPs, optimal energy storage systems dispatch and DG curtailment, the proposed strategy can reduce power losses and three-phase imbalance. Second-order cone programming (SOCP) relaxation is utilised to convert the original non-convex and non-linear model into an SOCP model which can be solved efficiently by commercial solvers. Case studies are conducted on a modified IEEE 34-node three-phase four-wire system and the IEEE 123-node test feeder to verify the effectiveness, efficiency and scalability of the proposed PC-SOP concept and its operational strategy.

Sequence-Impedance Modeling of Voltage Source Converter Interconnection Under Asymmetrical Grid Fault Conditions

Nonlinear behaviors of voltage source converter (VSC) and the interconnection under grid fault conditions are discussed in the article. Based on the discussion, a sequence-impedance model is developed from the complex space vector analysis. Lumped sequence admittances of the converter interconnection and perturbation propagation considering the coupling between ac and dc circuits are presented in this article. The influence of the phase-locked loop design for improving low-voltage ride-through performance on the sequence-admittance model is also introduced. The sequence admittances are calculated, compared, and verified with point-by-point simulations and experiment. Obtained results indicate that the sequence-admittance model is effective for describing and predicting the behavior of VSC interconnection under asymmetrical grid fault conditions.