Symmetrical Faults Research Articles

Synchrophasor Assisted Power Swing Detection Scheme for Wind Integrated Transmission Network

Integration of wind-based renewable energy sources into long Extra High Voltage/Ultra High Voltage uncompensated/compensated power transmission network poses significant problems in terms of proper detection of power swings and effective discrimination between symmetrical faults and power swing situations. As the existing protection strategies are unable to detect the said circumstances, a novel technique, based on the difference between sending end and receiving end positive sequence currents angles of the transmission line, is proposed in this paper. The required data is collected with the help of Phasor Measurement Units placed on both sides of the line. The scheme results in effective discrimination between distinct faults and circumstances of asymmetrical/symmetrical power swing and achieves satisfactory outcome during current transformer saturation condition. The proposed algorithm is evaluated on the wind-integrated IEEE-9 bus system by producing power swings, various cases of faults, and faults during power swings. Validation of the suggested technique was carried out by the execution of hardware-in-loop simulation on a Real-Time Digital Simulator. The achieved outcomes disclose higher sensitivity and better discriminating ability of the presented technique in comparison with those of numerous prevailing methods.

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.

On wind turbine's loss of synchronisation during severe faults

AbstractGrid codes mandate that wind turbines (WTs) remain connected even during severe faults. This requirement can lead to WTs' loss of synchronisation (LoS) with the AC grid under severe symmetrical faults. In this paper, the LoS is illustrated by the power balance principle. Then, a solution with improved performance for avoiding LoS during severe faults is proposed, implemented and verified by simulations. The results show that the solution eliminates the steady‐state frequency deviation during faults, resulting in a smoother WT transient during and post‐fault.

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.

A Two-Stage Simultaneous Control Scheme for the Transient Angle Stability of VSG Considering Current Limitation and Voltage Support

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed.

Reduction of gearbox loads of a DFIG wind turbine during grid faults with optimized converter configurations

Grid faults in wind turbines (WT) with doubly fed induction generator (DFIG) result in dynamic generator torque excitations, which can lead to dynamic load changes within the gearbox. Dynamic load changes in combination with changing rotational speeds can increase the risk of damage in the gearbox. WT gearbox damage occurs mainly on the high speed shaft (HSS) components. The torque excitations have the highest influence on the HSS since it is coupled to the generator. Therefore, an investigation of the correlation between grid faults and gearbox damage is necessary. The torque excitation in DFIG WTs due to grid faults is dependent on the converter and its fault ride through capabilities. The load analysis in this paper is done for a state of the art converter configuration and for one that is optimized in order to stabilize the performance during grid faults. It is shown via simulation with a WT drivetrain model that dynamic load changes of the HSS gear wheel are prevented for symmetrical grid faults with the optimized configuration. The analysis of a HSS bearing shows that the smearing damage risk can be significantly reduced (minus up to 56 percent) by using the optimized configuration. Therefore, the possibility to decrease the gearbox damage risk during grid faults via an optimization of the converter configuration is shown in this paper.

Transient Synchronization Stability Improvement Control Strategy for Grid-Connected VSC Under Symmetrical Grid Fault

Under grid fault, the grid-connected voltage source converters (VSCs) have the risk of transient synchronization instability during low voltage ride through. Based on the equivalent rotor swing equation of VSC, this letter proposes an improved transient synchronization stability control strategy, which improves the existence of equilibrium points and transient synchronization behaviors of the VSC system. Thus, the transient synchronization stability of grid-connected VSC can be significantly enhanced. The simulation and experimental results demonstrate the validity of the proposed strategy.

Double integral sliding mode controller based bridge-type flux-coupling non-superconducting fault current limiter to protect DFIG-based multi-machine power system under transient-state

This paper proposes a double-integral sliding mode controller (DISMC) based bridge-type flux-coupling non-superconducting fault current limiter (BFC-NSFCL) to enhance the fault ride-through (FRT) capability of a DFIG-based wind farm connected to a multi-machine power system. At first, a complete modeling of the BFC-NSFCL is derived to understand its behavior during the normal and fault period more accurately. Then, the DISMC is designed based on that dynamic model obeying the double-integral theory, along with the single-integral sliding mode controller (SISMC) and non-integral sliding mode controller (NISMC) for comparison purpose. Finally, the performance of the DISMC in controlling the BFC-NSFCL has been analyzed and verified by comparing with that of the SISMC and the NISMC. The DISMC removes the chattering problems induced by the traditional SMCs and improves the transient performance by reducing the overshoot and steady-state error by implementing double-integral action. All the graphical and mathematical analyzes favor the DISMC based BFC-NSFCL under symmetrical and unsymmetrical fault (both temporary and permanent) scenarios.

Stator winding fault detection of an Alternator using MATLAB

The paper represents the comprehensive analysis of stator winding faults of an alternator. An alternator is main component of power system which plays a big role in whole system for security and stability. The conventional system only detect symmetrical and unsymmetrical faults, they will not detect inter turn fault. The fault detection of stator winding is an effective and important measure to ensure the safe operation of generator. Early detection of stator winding fault would eliminate damage to stator core & adjacent coils, also reduce repairing cost and outage time of generator. As stator winding fault causes unbalance in phase voltage, this concept is discussed in this paper for the detection of stator winding fault. The negative sequence voltage of generator is used as fault indicator for fault detection. This new method is done using MATLAB software. Also, this method operate for external as well as internal fault and helps the generator winding keep healthy. Keywords— stator winding fault, internal negative sequence voltage, internal and external faults.

Comparative assessment of four low voltage fault ride through techniques (LVFRT) for wind energy conversion systems (WECSs)

This paper presents a comparative assessment of four low voltage fault ride through (LVFRT) techniques that alleviate the effects of power imbalance during AC network faults for voltage source converter (VSC)-based wind energy conversion systems (WECSs). The techniques appraised include a software-based coordinated control (CC) technique, commercially available dumping resistor (DR) technique, and two energy storage techniques, namely, superconductive magnetic energy storage (SMES) and supercapacitor energy storage (SCES). The techniques presented are technically viable solutions for alleviating impacts of AC network faults on the WECS. The techniques have previously been assessed by performance-based metrics, whereas this paper aims to form a more comprehensive assessment by expanding the scope from the performance-based comparison to include economic considerations and practical limitations. Based on qualitative review of technically viable designs, this study presents quantitative substantiation with time-domain simulations of symmetrical and asymmetrical AC network faults. The comparative assessment is formulated based on assessing key performance metrics including grid code compliance, electrical and mechanical stress reduction, LVFRT response speed, hardware implementation, system efficiency and investment reduction. The assessment findings suggest that a performance-based study fails to highlight the potential financial and practical barriers that may exist regarding the implementation of the LVFRT solutions especially with energy storage-based techniques. However, the flexibility, benefits and increasing feasibility regarding energy storage systems may make them a preferred option for LVFRT of critical WECSs.

A Current Limiting Method for Single-Loop Voltage-Magnitude Controlled Grid-Forming Converters During Symmetrical Faults

This article proposes a current limiting method for single-loop voltage-magnitude controlled grid-forming (GFM) converters to avoid overcurrent during symmetrical faults. The proposed method consists of two parts, which are the outer power reference adjustment to limit the steady-state fault current, and a transient virtual resistor, implemented by directly modifying the modulation voltage reference, to limit the transient fault current during fault-inception and -clearing periods. Furthermore, a nonlinear dynamic model for the GFM converter with the proposed control is developed, and the phase portrait is plotted to reveal the influences of power reference adjustment and of transient virtual resistor control on the transient stability. Finally, experimental tests confirm that the proposed method can effectively limit the fault current to an admissible value while simultaneously keeping the GFM mode during fault ride-through.

Improved MIMO Modeling and Enhanced Transient Performance of Phase-Locked Loop During Grid Fault

A phase-locked loop (PLL) is commonly used in grid-connected power converter for synchronization. A single input single output (SISO) linear model of PLL with the phase angle of the point of common coupling (PCC) voltage as input and the estimated phase angle as the output is generally used in the PLL analysis and design. However, an undesirable coupling between the magnitude and phase of the input voltage is present when a filtering stage is incorporated before the control loop, which a SISO model cannot capture, and could result in a significant disturbance in the PLL estimated phase during grid faults. This paper proposes a generalized multi-input multi-output (MIMO) linear model that captures the complete PLL dynamics during symmetrical faults for a PLL equipped with any prefilter. Furthermore, a compensation method that decouples the magnitude dynamics of the input voltage from the phase dynamics is proposed in this paper. The proposed compensation improves the PLL's phase tracking performance by ensuring that the prefiltered PLL acts only on the PCC voltage phase changes. A vital application of the proposed compensation method for PLL is during a grid fault case, wherein the converters are expected to contribute to the fault current, which requires an accurate measurement of the PCC voltage phase. The compensation method's effectiveness is demonstrated using power hardware in the loop simulation of a hardware voltage source converter interfaced to a distribution system simulated in real-time.

Experimental study on short-circuit current characteristics of a photovoltaic system with low voltage ride through capability under a symmetrical fault

Large number of photovoltaic (PV) power plants connected to a power grid can bring significant impacts to fault currents and the operation of protection systems. In this paper, short-circuit current characteristics of a PV system with low voltage ride through (LVRT) capability under a symmetrical fault is studied. PV system short-circuit experiments with different voltage dips at high and low output power levels are designed and conducted. The experiment results provide useful and valuable references for researches of PV system short-circuit current characteristics, modeling and PV system short-circuit current contribution to a power grid. Then, based on the experiment results, steady-state values, peak values and steady state short-circuit currents harmonic characteristics at different conditions are obtained and analyzed. It is found that, the DC component amplitude of a PV system short-circuit current is almost negligible compared with that of the power frequency component, which is different from conventional synchronous generator short-circuit current. Meanwhile, based on the LVRT control strategy and experimental results, the expression of steady-state short-circuit current RMS value is derived and verified, which provides an important basis for short-circuit current calculation of power system with large scale of PV plants.

Analysis of Grid-Forming IIDG’s Transient- and Steady-State Fault Model

Due to the inherent characteristics of inverterinterfaced distributed generators (IIDG), their fault responses are different from those of conventional synchronous generators. Investigating fault responses of IIDGs and establishing their fault models will benefit the fault calculation and protection scheme design for microgrids. This paper focuses on grid-forming IIDGs (GFGs) and hierarchically analyzes their fault characteristics under symmetrical and asymmetrical faults. A generic fault model of GFG, which covers both the transient-and steady-state, is proposed. Impacts of the transfer function of controllers, current limiting module, and fluctuation of DC-link voltage on GFG’s transient-and steady-state fault responses are respectively investigated. The influence of GFG’s topology is also considered, and both three-phase three-wire and three-phase four-wire GFGs are covered in the fault analysis. Model parameters calculation methods for all kinds of fault conditions are presente. It is found that the fault type, the fault severity, and the response speed of the DC power regulator all have significant impacts on GFG’s fault responses. The fault analysis and fault model are verified by simulation results in MATLAB /Simulink.

Synchronization Instability of Inverter-Based Generation During Asymmetrical Grid Faults

The transient stability of traditional power systems is concerned with the ability of generators to stay synchronized with the positive-sequence voltage of the network, whether for symmetrical or asymmetrical faults. In contrast, both positive- and negative-sequence synchronizations should be of concern for inverter-based generation (IBG) under asymmetrical faults. This is because the latest grid codes stipulate that IBG should inject dual-sequence current when riding through asymmetrical faults. Currently, much less is known about the synchronization stability during asymmetrical faults. This significantly differs from the positive-sequence synchronization alone because the coupled dual-sequence synchronization is involved. This paper aims to fill this gap. Considering the sequence coupling under asymmetrical faults, the dual-sequence synchronization model of IBG is developed. Based on the model, the conditions that steady-state equilibrium points should follow are identified. The conditions throw light on the possible types of synchronization instability, including the positive-sequence dominated instability and the negative-sequence dominated one. For different types of instability, the dominant factors are analyzed quantitatively, which are reflected by the limit on the current injection amplitude. Exceeding the limit will lead to the loss of both positive- and negative-sequence synchronizations. The model and the analysis are verified by simulations and hardware-in-the-loop experiments.

Analytical Calculation of Transient Current From an Inverter-Interfaced Renewable Energy

Short-circuit current analysis, especially transient current calculation, provides a basis for protection design. Because the control systems of inverter-interfaced renewable energy sources have a high degree of nonlinearity, it is difficult to analyze the transient current. To address this issue, in this work, a nonlinear differential equation of transient current of symmetrical fault is derived while considering the saturation characteristics of the current loop. Aiming at the solution of this nonlinear differential equation, an analysis method based on the phase plane is proposed, which makes use of the linear relationship between the current deviation and its derivative to simplify the direct determination of the nonlinear working state in the time domain. The analytical expression of the transient current can then be obtained by the inverse transform from the phase plane to the time domain. The proposed calculation method is verified by both simulations and field tests.

Event-Triggered, Adaptive, Exponentially Asymptotic Tracking Control of Stochastic Nonlinear Systems

This paper investigates the problem of event-triggered, adaptive, asymptotic tracking control for a class of non-strict feedback stochastic nonlinear systems with symmetrical structures and sensor faults. Based on the negative exponential function, the event-triggered adaptive tracking control strategy deals with the problem of exponentially asymptotic convergence for the first time. The radial basis function neural network (RBFNN) mechanism addresses uncertain factors and unknown external disturbances in the system. The developed strategy ensures that all the signals of the closed-loop system are semi-globally uniformly bounded in probability, and that the tracking error can exponentially converge to zero. Finally, a simulation example demonstrates the effectiveness of the proposed method.

Dynamic voltage restore based on switched-capacitor multilevel inverter with ability to compensate for voltage drop, harmonics, and unbalancing simultaneously

The voltage harmonics and voltage unbalance, as well as voltage drop due to the electrical faults, can damage the sensitive loads. Various voltage compensators, such as the series active power filter (SAPF) and dynamic voltage restore (DVR) have been developed to overcome these problems. In this paper, a new compensator is introduced, which can simultaneously operate as SAPF and DVR. To this end, an adaptive control method is utilized for generating the compensator reference signal to mitigate the voltage drop, harmonics, and unbalance. On the other hand, for reduction of complexity and cost, a switched-capacitor multilevel inverter has been employed in the proposed compensator. The effectiveness of the compensator based on the improved control method is studied in the presence of symmetrical and asymmetrical faults with distorted utility.

An intelligent differential protection of power transformer based on artificial neural network

This paper describes the application of artificial neural network (ANN) techniques for protecting small power transformer 2 kVA (Terco type). ANN network trained according to the primary and secondary currents data under NN tool, this network performs as a function of differential protection relay. Symmetrical and unsymmetrical faults are analyzed using Matlab environment. ANN network senses the difference in the internal current of both transformer sides and sending a trip to the circuit breaker (CB) at moment of fault occurrence. All the voltages and the currents waveforms affected with the fault and the response time increased according to this technique. Finally, the trip signal and the quick disconnect time were improved according to ANN technique.

Development of Symmetrical Fault Detection During Power Swing Based on Entropy

Development of Symmetrical Fault Detection During Power Swing Based on Entropy