Post-fault Scenarios Research Articles

Fault Recovery Analysis of Grid-Forming Inverters With Priority-Based Current Limiters

Grid-forming (GFM) inverters are required to operate robustly against grid faults. However, due to the limited over-current capability of inverters, current-limiting controls are usually applied to protect these semiconductor devices, which may prevent GFM inverters from a successful fault recovery. To understand this phenomenon, this study analyzes the fault recovery process of a GFM inverter with a priority-based current limiter. According to whether the GFM inverter can ensure transient stability and exit the current-limiting mode after fault clearance, three post-fault scenarios are identified, including <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">normal operation, current limitation, and oscillations</i> . Further, the impacts of the short-circuit ratio and control parameters on the post-fault behavior of GFM inverters are demonstrated. To illustrate the implications of these theoretical results, typical numerical examples are presented. Finally, the theoretical findings are validated through experimental tests.

Dc-MMC for the Interconnection of HVDC Grids With Different Line Topologies

DC-DC converters are needed for the future development of high voltage direct current (HVDC) grids, as they allow to interconnect lines with different voltages and topologies. The dc-dc converters can increase the grid controllability adding power flow control, voltage regulation and/or fault blocking capability. The dc modular multilevel converter (dc-MMC) is a non-isolated solution proposed to interconnect HVDC systems with the same line topology. This paper proposes a new dc-MMC with a control strategy, which allows the converter to interconnect different line topologies (e.g., rigid bipole connected to a symmetric monopole). The paper presents the different line topologies in HVDC installations. Then, a mathematical model with a variable transformation is proposed for the new dc-dc converter. A control structure is proposed and implemented in Matlab/Simulink using an average arm model and simplified dc grids. The results validate the control in normal operation, fault blocking capability and post-fault scenario (degraded mode).

Improvement of Transient Stability of the Power Networks by an Intelligent Autoreclosing Scheme in the Presence of Synchronous-Based DGs

This article presents an adaptive autoreclosing scheme to preserve the network stability in postfault scenarios for the power grids equipped with synchronous-based distributed generations (SBDGs). Based on the precise fault location information, the proposed adaptive reclosing strategy will classify all the fault events in either reclosing or block reclosing zones. Furthermore, the proposed intelligent autoreclosing scheme will ensure safe and stable network operation by preventing hazardous futile reclosing attempts against persistent faults in forestry or densely populated areas due to serious safety concerns. The swift fault removal by the associated protective devices (PDs) will assist in ensuring the stable operation of the power grid after the fault clearance. If the fault is in the reclosing zones, the adaption of the proposed scheme will permit the reclosing at an appropriate instant to preserve the network stability against momentary faults. However, it will also ensure quick fault clearance by permitting the associated PD to directly operate against the permanent faults by disabling the fast operation of the autorecloser. The validation of the proposed protection strategy has been carried out using the IEEE standardized test system with multiple SBDG units to demonstrate its effectiveness in comparison with the conventional reclosing practices. A comparative analysis has been presented to illustrate the efficacy of the proposed scheme to reduce substantially the postfault swings for different fault events.

BESS Assisted Frequency Management for Black Start Process of Microgrid

This paper proposes a method for restoring the nominal frequency and improving the system recovery time using battery energy storage system (BESS) for an islanded microgrid (MG) which is operated by a black start unit (BSU). The frequency stability is controlled by varying the apparent power (MVA) rating of the BESS after simultaneously connecting with the BSU during the post fault scenarios. Simulations are performed on the IEEE Std. 399-1997 test MG using DIgSILENT PowerFactory. Results show that the nominal frequency of the system can be retained by connecting the BESS during the transient period and increasing the MVA rating up to a maximum value. Simulation results also show that with a higher distributed connection of the BESS units over the system, the frequency recovery time can be reduced.

A single phase five-level inverter with single and multiple switch fault tolerance capabilities

The reliability of multilevel inverters (MLIs) is of great importance, when they are employed for applications such as aircrafts, electric vehicles, standalone, and grid connected photovoltaic (PV) system. To achieve required output voltage during post-fault scenarios, several fault tolerant topologies of multilevel inverter have been proposed. However, the primary constraints which add on a limitation to their implementation are increased device count and inability to tolerate single and multiple switch faults with the conduction of lesser or equal switches as in pre-fault case. The inability to tolerate various types of faults, such as open circuit or short circuit, cause severe impact on the system and further increase the cost. In this regard, a fault tolerant (FT) multilevel inverter topology which tolerates aforementioned faults is proposed. The proposed topology is able to withstand both single switch and multiple switch faults with equal or lesser blocking voltage. The proposed topology employs only unidirectional switches, thus increasing the reliability and reducing the cost of inverter. The proposed work is validated experimentally.

Switch Fault Tolerant Model-Based Predictive Control of a VSC Connected to the Grid

During IGBT breakdowns, the grid side converter (GSC) of a renewable power plant must be disconnected from the grid. In the recent past, topological modifications propose the faulty phase to be connected to the midpoint of the dc bus as a solution. In this configuration, the converter can generate only four vectors. Moreover, since the capacitor voltages carry a voltage ripple at the grid frequency, the magnitude and angle of the vectors become variable during post-fault scenarios. The recent model predictive control (MPC) approaches select the vectors to be used in the cost function based on the location of the reference vector in a post-fault unsymmetrical vector diagram or by checking the effect of all of them, which is time consuming. This article proposes a simplified approach, which reduces the computing time by avoiding successive iterations to obtain the correct vector pairs. Furthermore, an approach to rebuild a regular hexagon from the four post-fault vectors is proposed. Hence, the proposed controller retains the advantages of the conventional space vector modulation (SVM) approach with minimal additions to handle post-fault scenarios. Finally, this article studies the limitations in the VSC output voltage and power magnitudes during post-fault scenarios. The approaches presented in this article have been verified using simulation and experimental results.

Investigating Variable Speed Wind Turbine Transient Performance Considering Different Inverter Schemes and SDBR

In wind energy applications, voltage source converters are employed to achieve energy conservation. Recently, multilevel converters have been showing promising advantages compared to the traditional 2-level converter scheme, due to the fact that they can overcome certain limitations during transient conditions. This paper investigates the transient performance of variable DFIG-based speed wind turbines taking into account different scheme configurations of the power converter system. The schemes investigated are a 2-level six step IGBT inverter, a parallel interleaved 2-level six step IGBT inverter, and a 3-level IGBT inverter. All schemes were compared during severe three-phase to ground fault at the terminal of the DFIG wind turbine using the conventional Phase Lock Loop (PLL) and a DC-chopper protection. A coordinated approach of improving the performance of all the converter schemes with series dynamic braking resistor (SDBR) was analyzed. Investigation of the best location for the SDBR in the DFIG architecture considering the best switching signal was also carried out. Furthermore, a new control strategy of PLL for the DFIG system was proposed in conjunction with the SDBR scheme for the converter systems. Simulations were carried out in Power System Computer Aided Design and Electromagnetic Transient Including DC (PSCAD/EMTDC). The results show that the proposed PLL and SDBR hybrid scheme in the various inverter topologies considered in the study can enhance the performance of the wind generator variables during severe three-phase to ground fault. This is because the proposed hybrid scheme could help to boost the capability of the current and recovery of the wind generator after post-fault scenarios. Also, the voltage source converter leg switched output voltage would be enhanced to maximum change in common mode voltage by the inverter schemes’ modulation of the space vector using the proposed strategy.

Optimization and Analysis of a High Power Density and Fault Tolerant Starter–Generator for Aircraft Application

Permanent magnet synchronous machines provide many dramatic electromagnetic performances such as high efficiency and high power density, which make them more competitive in aircraft electrification, whereas, designing a permanent magnet starter–generator (PMSG), with given consideration to fault tolerance (FT), is a significant challenge and requires great effort. In this paper, a comprehensive FT PMSG design process is proposed which is applied to power systems of turboprops. Firstly, potential slot/pole combinations were selected based on winding factor, harmonic losses and manufacture issues. Then, pursuing high power density, a multiple objective optimization process was carried out to comprehensively rank performances. To meet a fault tolerance target, electrical, magnetic and thermal isolation topologies were investigated and compared, among which 18 slot/12 pole with dual three-phase was selected as the optimal one, with a power density of 7.9 kW/kg. Finally, a finite element analysis verified the performance in normal and post-fault scenarios. The candidate machine has merits concerning high power density and post-fault performance.

A Single Machine Equivalent-Based Approach for Online Tracking of Power System Transient Stability

This paper presents a systematic single machine equivalent system based scheme for fast and accurate prediction of impending angular stability using measurements from generator terminals only. In a post-fault scenario, Kalman filter is designed to estimate rotor positions in real-time, based on damper winding observer with measurements gathered from generator terminals. To identify critical machines, short time-ahead prediction of rotor position is made by designing another Kalman filter. A general method is suggested to differentiate between critical and non-critical machines. To further enhance the accuracy of predicted time to instability, an extrapolation algorithm is developed, based on the knowledge of estimated rotor angles and short time-ahead predicted rotor angles. The proposed methods are successfully implemented on WSCC 9-bus, IEEE 39-bus, and NRPG 246-bus test systems.

Research on optimization of distribution network communication network structure based on invulnerability optimization model

In order to improve the power supply reliability and economy of the distribution network, from the perspective of optimizing its network structure and connectivity, an invulnerability optimization model based on the shortest path multi-scenario and multi-objective is constructed. For the normal operation and various single post-fault scenarios, aiming at the strong destructive and unloading or the minimum load shedding respectively, through the branch exchange method, find the best anti-destructive power of the completed distribution network in each scenario. The network topology provides a new research idea for the optimization of the network structure of medium voltage distribution network. Invulnerability measure index S(G)=1, the strongest invulnerability, the invulnerability measure index of any non-fully connected network is 0<s(G)<1, the larger S(G) is, the more compact the network structure is and the more resistant it is. The optimization results based on the approximate network example of the IEEE33 node distribution network test system show the rationality and practicability of the research method.

Power System Transient Stability Study by Involving Higher Order Corrections in Improved Quadratic Lyapunov Function for Singular Perturbed Model of Synchronous Generator

The assessment of transient stability of multi-machine power system, in a post-fault scenario, is a very challenging task due to highly non-linear nature of the system equations. In this work, the physical concept of maximum relative acceleration of a machine at a fault scenario is related with the boundary of stability region. This approach is extended to develop a quadratic Lyapunov function using singular perturbed modelling. Unlike the conventional energy function methods, this Lyapunov function candidate is a more general energy function and has a decaying nature around an equilibrium point. Moreover, with higher order corrections, an improved Lyapunov function candidate is presented with simulation results of power system examples. The decaying nature of Lyapunov function candidate remains similar even without higher order corrections. However, the relevance of inclusion of higher order corrections is brought out in the cases of faults occurring at other than generator buses. Also, the upper bound of singular perturbation parameter in the model is within allowable limits and ensures the accuracy of results.

Postfault Control of Scalar (V/f) Controlled Asymmetrical Six-Phase Induction Machines

With the accelerated development in semiconductor power devices along with the dictated rigorous reliability standards in some industrial sectors, the application of a medium-voltage high-power multiphase induction machine with multiple three-phase windings is now considered as a leading technology in high-power safety-critical applications. This paper proposes a parameter-independent postfault control scheme for an asymmetrical six-phase induction machine based on simple scalar V/f control, which can successfully ensure the most common postfault scenarios used in this respect, namely, equal stator copper loss and minimum copper loss modes. Moreover, the proposed controller can effectively be used in either open-loop or closed-loop speed control modes. The proposed controller is experimentally validated using a 1.5-kW prototype induction machine. The effect of the neutral arrangement on the dynamic performance is also explored.

Novel elastic protection against DDF failures in an enhanced software-defined SIEPON

Abstract Ever-increasing bandwidth demands on passive optical networks (PONs) are pushing the utilization of every fiber strand to its limit. This is mandating comprehensive protection until the end of the distribution drop fiber (DDF). Hence, it is important to provide refined protection with an advanced fault-protection architecture and recovery mechanism that is able to cope with various DDF failures. We propose a novel elastic protection against DDF failures that incorporates a software-defined networking (SDN) capability and a bus protection line to enhance the resiliency of the existing Service Interoperability in Ethernet Passive Optical Networks (SIEPON) system. We propose the addition of an integrated SDN controller and flow tables to the optical line terminal and optical network units (ONUs) in order to deliver various DDF protection scenarios. The proposed architecture enables flexible assignment of backup ONU(s) in pre/post-fault conditions depending on the PON traffic load. A transient backup ONU and multiple backup ONUs can be deployed in the pre-fault and post-fault scenarios, respectively. Our extensively discussed simulation results show that our proposed architecture provides better overall throughput and drop probability compared to the architecture with a fixed DDF protection mechanism. It does so while still maintaining overall QoS performance in terms of packet delay, mean jitter, packet loss, and throughput under various fault conditions.