Power Systems Computer Aided Design/electromagnetic Transients Including DC Research Articles

Current-Limiting Reactors Based Time-Domain Fault Location for High-Voltage DC Systems With Hybrid Transmission Corridors

Accurately locating the fault distance helps in the rapid restoration of the isolated line back into the system. This article proposes a novel time-domain-based algorithm to determine accurate fault location in high-voltage direct current (HVdc) systems with hybrid dc transmission corridors i.e., a combination of underground cables (UGCs) and overhead lines (OHLs). The work gives a fault location method for a two-segment and three-segment hybrid transmission corridor (HTC) and then generalizes the analysis for an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -segment HTC. The algorithm offers flexibility to locate faults ranging from the homogeneous transmission line to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -segment HTC. The algorithm uses a simplified unit resistance–inductance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$RL$ </tex-math></inline-formula> ) representation of transmission lines along with time-domain-based measurements i.e., terminal voltage, current, and voltage across current-limiting reactors (CLRs). Power systems computer-aided design/electromagnetic transients including dc (PSCAD/EMTDC)-based simulations are used to validate robust performance against variation of key implementation parameters like type of faults, fault resistance, fault location, sampling frequency, and white Gaussian noise (WGN) in measurement. Further, the fault location calculation is analyzed under parameter variation i.e., change in the true value of unit resistance and unit inductance of line or cable and the true value of dc link capacitance.

Analysis and Design of a Nonisolated DC Transformer With Fault Current Limiting Capability

The dc&#x2013;dc converter, also termed as dc transformer, plays an important role in the future dc grids. It is used to interconnect network segments or lines with different voltage levels. A nonisolated modular multilevel dc&#x2013;dc converter is presented in this article, which can realize bidirectional power regulation and dc fault current limiting. The control strategy is simplified and the current stresses are decreased due to the employment of autotransformers. The nonisolated structure can reduce the requirements of autotransformers, because only part of the transmitted power flowing though the autotransformers. The operation principle and control strategy of this converter are illustrated in detail. The analyzes of power transmission, submodule (SM) capacitor voltage ripple, the optimal configuration, and the efficiency of this converter are also provided. The optimal design methods of the dc transformer are proposed to reduce the costs and power losses according to the analysis results. Simulations performed in power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) and experiments based on a down-scaled prototype verify the operation principle, control strategy, and theoretical analysis. The simulation results and experiment results show the performances of the converter under steady state, power regulation, and fault conditions.

Sub-Synchronous Oscillation Suppression Strategy Based on Impedance Modeling by Attaching Virtual Resistance Controllers for Doubly-Fed Induction Generator

A sub-synchronous oscillation (SSO) suppression strategy of attaching virtual resistance controllers to the rotor-side converter (RSC) of the doubly-fed induction generator (DFIG) is proposed in this study to suppress sub-synchronous oscillation (SSO) caused by series compensation and grid connection of DFIG. A DFIG-based frequency domain impedance model considering RSC control under small signal perturbations is developed in a three-phase stationary coordinate system. Subsequently, the factors and mechanisms of SSO in the system with different phase sequences are analyzed in combination with the equivalent RLC resonant circuit of a DFIG-based series-compensated grid-connected system (SCGCS). SSO occurs when RSC and rotor winding generate a large equivalent negative resistance at the SSO frequency, resulting in a negative total system resistance. Additionally, the influences of series compensation degree (SCD) of line and inner loop parameters (ILPs) of RSC related to the total impedance of the system on the SSO characteristics are analyzed to optimize the parameters and improve the system stability. Based on the causes of SSO, virtual resistance controllers are attached to RSC to provide positive resistance to the system and to offset the equivalent negative resistance of RSC and rotor winding at the SSO frequency, thereby avoiding SSO of the system. Finally, time-domain simulations using power system computer aided design/electromagnetic transients including dc (PSCAD/EMTDC) show that the SSO of the system is effectively suppressed.

Transient Stability Enhancement of a Grid-Connected Large-Scale PV System Using Fuzzy Logic Controller

This paper presents a new intelligent control strategy to augment the low-voltage ride-through (LVRT) potential of photovoltaic (PV) plants, and the transient stability of a complete grid system. Modern grid codes demand that a PV plant should be connected to the main power system during network disturbance, providing voltage support. Therefore, in this paper, a novel fuzzy logic controller (FLC) using the controlled cascaded strategy is proposed for the grid side converter (GSC) of a PV plant to guarantee voltage recovery. The proposed FLC offers variable gains based upon the system requirements, which can inject a useful amount of reactive power after a severe network disturbance. Therefore, the terminal voltage dip will be low, restoring its pre-fault value and resuming its operation quickly. To make it realistic, the PV system is linked to the well-known IEEE nine bus system. Comparative analysis is shown—using power system computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC) software—between the conventional proportional–integral (PI) controller-based cascaded strategy and the proposed control strategy to authenticate the usefulness of the proposed strategy. The comparative simulation results indicate that the transient stability and the LVRT capability of a grid-tied PV system can be augmented against severe fault using the proposed FLC-based cascaded GSC controller.

A Novel Voltage Clamping-Based Overvoltage Protection Strategy to Avoid Spurious Trip of Inverter-Based Resources and Eliminate the Risk of Wildfire Following the REFCL Operation in Compensated Networks

This article investigates the operational performance of the rapid earth fault current limiter (REFCL) that has been employed in Australia to reduce the risk of wildfires and shows that after the REFCL activation, an overvoltage is imposed on the grid which can lead to wildfire by creating a cross-country fault. Moreover, by increasing the penetration level of the inverter-based resources (IBRs), the potential inverter trip on transient overvoltage can become a vital issue. Since the protection functions of the IBRs are very sensitive to any transient overvoltage, their overvoltage or dc reverse current protection functions will trip the IBRs immediately following any overvoltage condition such as those caused during the REFCL operation. To tackle the mentioned problems, a novel voltage clamping-based overvoltage protection strategy is proposed and developed through a smart power electronic topology. The article presents the analytical analysis and the mode of operation of the proposed overvoltage protection strategy. The effectiveness of the proposed overvoltage protection strategy is tested through a comprehensive set of simulation studies in Power System Computer-Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC). The simulation results show that the proposed strategy can immediately limit the magnitude of the overvoltage, thereby it can not only reduce the risk of cross-country faults and wildfires but it can also mitigate the spurious trips of the IBRs.

Open-Circuit Fault Detection and Classification of Modular Multilevel Converters in High Voltage Direct Current Systems (MMC-HVDC) with Long Short-Term Memory (LSTM) Method.

Fault detection and classification are two of the challenging tasks in Modular Multilevel Converters in High Voltage Direct Current (MMC-HVDC) systems. To directly classify the raw sensor data without certain feature extraction and classifier design, a long short-term memory (LSTM) neural network is proposed and used for seven states of the MMC-HVDC transmission power system simulated by Power Systems Computer Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC). It is observed that the LSTM method can detect faults with 100% accuracy and classify different faults as well as provide promising fault classification performance. Compared with a bidirectional LSTM (BiLSTM), the LSTM can get similar classification accuracy, requiring less training time and testing time. Compared with Convolutional Neural Networks (CNN) and AutoEncoder-based deep neural networks (AE-based DNN), the LSTM method can get better classification accuracy around the middle of the testing data proportion, but it needs more training time.

Miniaturization of an Offshore Platform with Medium-Frequency Offshore Wind Farm and MMC-HVDC Technology

Offshore wind power has great development potential, for which the key factors are reliable and economical wind farms and integration systems. This paper proposes a medium-frequency wind farm and MMC-HVDC integration system. In the proposed scheme, the operating frequency of the offshore wind farm and its power collection system is increased from the conventional 50/60 Hz rate to the medium-frequency range, i.e., 100–400 Hz; the offshore wind power is transmitted to the onshore grid via the modular multilevel converter-based high-voltage direct current transmission (MMC-HVDC). First, this paper explains the principles of the proposed scheme in terms of the system topology and control strategy aspects. Then, the impacts of increasing the offshore system operating frequency on the main parameters of the offshore station are discussed. As the frequency increases, it is shown that the actual value of the electrical equipment, such as the transformers, the arm inductors, and the SM capacitors of the rectifier MMC, can be reduced, which means smaller platforms are required for the step-up transformer station and the converter station. Then, the system operation characteristics are analyzed, with the results showing that the power losses in the system increase slightly with the increase of the offshore AC system frequency. Based on time domain simulation results from power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC), it is noted that the dynamic behavior of the system is not significantly affected with the increase of the offshore AC system frequency in most scenarios. In this way, the technical feasibility of the proposed offshore platform miniaturization technology is proven.

Setting-Less Nonunit Protection Method for DC Line Faults in VSC-MTdc Systems

Existing nonunit protection schemes inevitably require setting, which is a serious problem in practical engineering. Faults occurred at different fault zones will result in different equivalent models, therefore, the fault zone can be determined by recognizing which equivalent model the fault fits well with. In this article, this “model recognition” idea is introduced in fault identification and a “setting-less” protection method is proposed. First, the Peterson equivalent circuits when faults occur at backward external zone, internal zone, and forward external zone are presented, respectively. Accordingly, the corresponding three fault voltage expressions are derived, which are defined as three fault modes. Then, the three fault modes are used to approximate the measured fault voltage using Levenberg-Marquardt optimal approximation method. The fault mode that best fits the measured fault voltage is recognized as the final determined fault mode, which is used for fault identification without setting threshold value. Numerous test studies carried out in Power Systems Computer Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) and real-time digital simulator have demonstrated that the proposed method can be utilized for different multiterminal dc systems and is effective under different fault locations, different fault types, and high fault impedances. The proposed method does not require high sampling frequency and has good robustness against measuring noise.

A Double-Terminal Traveling-Wave-Based Method Using Novel Noncontact Sensors for Fault Location in Transmission Cable Lines

The burgeoning demand for enhanced power-system safety means that there is increased attention on technologies for the detection and location of cable faults. The double-terminal fault location method, which senses a fault current using a current transformer (CT), is the most effective and widely applied method in industrial practice. However, CT sensors may suffer from magnetic saturation when exposed to a high fault current, resulting in significant location errors, and their sensitivity cannot be adjusted. To address this problem, this paper describes an improved method for the acquisition of fault signal and the identification of fault-wave arrival time in power cables. First, a novel noncontact sensor (NCS) is developed for the detection of electric fields, and its sensitivity-adjustment curves are obtained by theoretical calculation and simulation. Then, based on feedback variational mode decomposition (FVMD) and the Wigner-Ville distribution (WVD), an improved method (the FVMD + WVD method) for the identification of fault-wave arrival time is devised, and its efficacy and accuracy are verified by simulations in Power Systems Computer-Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) software. Finally, the fault-wave arrival-time detection performance of the NCS is examined in a series of on-site DC and AC experiments based on damped AC technology. The results show that the NCS exhibits feasible and effective performance, and that its response can be appropriately tuned based on its sensitivity-adjustment curves. The FVMD + WVD method is more accurate than several other current methods, as its error is only 0.48%, and thus this method enables the practical location of cable faults. In sum, these findings demonstrate that the NCS and the FVMD + WVD method comprise an improved system for the detection and location of cable faults, which will enhance cable safety in power systems.

Design and implementation of an improved power‐electronic system for feeding loads of smart homes in remote areas using renewable energy sources

This paper suggests an improved step-up step-down DC-DC system along with three-input and four-output for smart home applications. In this configuration, two unidirectional power ports have been identified as an Input power supply and one bidirectional power port for the power-saving element, which can be used as a bidirectional converter for the hybrid vehicle to discharge in a dependent structure. This system can be used to combine renewable energy sources like photovoltaic (PV), fuel cell, battery and hybrid vehicle (HV) to prepare power for remote smart homes. By using this system, serving different loads with different voltage range from high voltage to ultra-low voltage is possible, also battery charge and discharge with the energy-saving method can be achieved. In this system, the condition of all possible low-voltage load and high-voltage load conditions has been assumed. In this structure, nine power switches have been used, in which all of these switches are in control with independent and dependent duty cycles. By using these cycles, maximum power can be earned from PV sources, bus-bar voltage regulation, and battery power control is possible too. In this topology, depending on environmental conditions, five scenarios have been identified. To prove the capability of the system before the build, some valid simulations are needed. In this study, the suggested system has been simulated with power system computer aided design/electromagnetic transients including DC (PSCAD/EMTDC).

Line fault location for multi‐terminal MMC‐HVDC system based on SWT and SVD

This study presents a travelling wave (TW)-based method for locating DC line faults in a modular multilevel converter (MMC)-based high-voltage direct current (HVDC) system by using local information. Pole voltage signals are adopted and denoised via stationary wavelet transform (SWT) with improved threshold functions. Hankel matrix-based singular value decomposition (SVD) is utilised to detect TW arrivals. The arrival times of incidental and reflected wave heads are observed in SVD result. The reflected wave heads from the fault point and the opposite end can be discriminated by comparing surge polarities in SVD result. The proposed method relies on the TW principle but is independent of TW propagation velocity. The feasibility of the proposed algorithm is evaluated considering potential factors, such as fault resistance, close-in fault, remote fault, sampling rate and noise. The superiority of this method is validated by comparing it with other signal-processing techniques and TW-based fault location principles. Electromagnetic transient simulation of the multi-terminal HVDC system on Power Systems Computer Aided Design / Electromagnetic Transients including DC (PSCAD/EMTDC) is conducted to provide fault TW signals, which are analysed in MATLAB. A corresponding equivalent test model developed in a real-time digital simulator is also provided for conducting a supplementary study to verify and further research this fault location method.

Improved sliding‐mode control for MMC in DC power system

The modular multi-level converter based direct current (MMC-DC) power system is a dynamic equilibrium system, and the control of MMC is quite important to realise instantaneous balance of active and reactive power. Until now, the control architecture with outer-loop, inner-loop and circulating current controllers is widely used for MMCs in the DC power system, and these controllers are required to have excellent dynamic response and robustness. However, controllers based on existing proportional–integral (PI) control or conventional sliding-mode control (SMC) have unsatisfactory performances. Therefore, an improved SMC method is proposed in this study for inner-loop and circulating current controllers. Compared with PI and conventional SMC, the improved SMC has better dynamic response and robustness performance and can suppress chattering phenomenon substantially. The simulation model of a four-terminal MMC-DC system is established in power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC), based on which, comparison between PI control, conventional SMC and the improved SMC under typical working conditions, e.g. steady state, large disturbances of power step, is conducted, and the feasibility and superiorities of the improved SMC are verified.

Double Half-Bridge Submodule-Based Modular Multilevel Converters With Reduced Voltage Sensors

The half-bridge (HB) submodule (SM) based MMC requires a large number of sensors to measure capacitor voltages. In this letter, a double half-bridge (DHB) SM-based MMC is proposed, where each SM consists of two HBs. But only one voltage sensor is equipped in each DHB-SM for the capacitor voltage estimation, which is connected between the positive poles of two capacitors in the DHB-SM. A capacitor voltage estimation method is also proposed, which can estimate capacitor voltages in the proposed MMC with only a half number of voltage sensors in comparison with HB-SM-based MMCs. A DHB-SM-based three-phase MMC is simulated with power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) and a DHB-SM-based single-phase MMC prototype is built in the laboratory. Both the simulation and experimental results confirm the effectiveness of the proposed method.

Fault Detection and Classification in MMC-HVDC Systems Using Learning Methods.

In this paper, we explore learning methods to improve the performance of the open-circuit fault diagnosis of modular multilevel converters (MMCs). Two deep learning methods, namely, convolutional neural networks (CNN) and auto encoder based deep neural networks (AE-based DNN), as well as stand-alone SoftMax classifier are explored for the detection and classification of faults of MMC-based high voltage direct current converter (MMC-HVDC). Only AC-side three-phase current and the upper and lower bridges’ currents of the MMCs are used directly in our proposed approaches without any explicit feature extraction or feature subset selection. The two-terminal MMC-HVDC system is implemented in Power Systems Computer-Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) to verify and compare our methods. The simulation results indicate CNN, AE-based DNN, and SoftMax classifier can detect and classify faults with high detection accuracy and classification accuracy. Compared with CNN and AE-based DNN, the SoftMax classifier performed better in detection and classification accuracy as well as testing speed. The detection accuracy of AE-based DNN is a little better than CNN, while CNN needs less training time than the AE-based DNN and SoftMax classifier.

Fault Model and Travelling Wave Matching Based Single Terminal Fault Location Algorithm for T-Connection Transmission Line: A Yunnan Power Grid Study

Due to the complex structure of the T-connection transmission lines, it is extremely difficult to identify the reflected travelling wave from the fault point and that from the connection point by the measurement from only one terminal. According to the characteristics of the structure of the T-connection transmission line, the reflection of the travelling wave within the line after the failure of different sections in T-connection transmission line are analyzed. Based on the lattice diagram of the travelling wave, the sequence of travelling waves detected at the measuring terminal varies with the fault distance and the faulty section. Moreover, the sequence of travelling waves detected in one terminal is unique at each faulty section. This article calculates the arrival time of travelling waves of fault points at different locations in different sections to form the collection of the travelling wave arrival time sequence. Then the sequence of travelling waves of the new added fault waveforms is extracted to compare with the sequences in the collection for the faulty section identification and fault location. This proposed method can accurately locate the fault with different fault types, fault resistances and system impedances by only single-terminal fault data. Both Power Systems Computer Aided Design/ Electromagnetic Transients including DC (PSCAD/EMTDC) and actual measurement data are implemented to verify the effectiveness of this method.

A High-Performance and Economical Multiport Hybrid Direct Current Circuit Breaker

Hybrid dc circuit breaker (HCB) is an effective device for interrupting dc fault currents in dc grids. Conventional two-port HCB requires too many insulated gate bipolar transistors (IGBTs) in series in its main breaker, resulting in considerable costs. Recently proposed multiport hybrid dc circuit breakers (multiport HCBs, and the number of ports is greater than or equal to 2) interrupt fault currents on multiple lines using only one shared main breaker, significantly reducing the implementation costs but losing some fault handling capabilities. To overcome these obstacles, we propose a novel high-performance economical multiport hybrid dc circuit breaker (MP-HCB) in this article. The topology and its control method are proposed. Due to the designed ring-connection structure, the proposed MP-HCB can avoid dc bus fault, and thus is more reliable. The electrical stresses of the breaker are theoretically analyzed, and the parameters are determined to make the MP-HCB capable of multiple-line faults. Compared with the typical two-port HCB and the existing multiport HCBs, the proposed MP-HCB can achieve full fault current interruption capability with lower costs. To protect a dc node connecting with three lines, the cost of the semiconductors of the proposed MP-HCB is only 1/3 of the typical two-port HCB and 1/2 of the existing multiport HCB. Finally, the performances of the proposed MP-HCB are verified via a four-terminal HVdc system in power systems computer aided design/ electromagnetic transients including DC (PSCAD/EMTDC).

Novel Fault Detection and Localization Algorithm for Low-Voltage DC Microgrid

Accurate estimation of fault location is highly crucial for swift maintenance and early power restoration. Since faults in dc networks are time critical, this article proposes a new fault detection and localization scheme for a low-voltage direct current (LVdc) microgrid network. Low-resistance faults are detected by observing voltage across the inductor whereas high-resistance ground faults are identified by measuring ground current at the relay location. Thereafter, based on iterative method, fault location is estimated by comparing analytically derived fault current with the measured value of fault current. Genuineness of the suggested technique has been assessed by simulating various internal, external, and simultaneous faults on a typical LVdc microgrid network modeled in power system computer aided design/electromagnetic transients including dc (PSCAD/EMTDC) environment. The proposed method is capable of detecting and locating both low- and high-resistance dc faults without utilizing remote-end quantities. Subsequently, initial guess has minimal impact on the convergence and accuracy of the proposed algorithm. Comparative evaluation of the proposed technique with other techniques clearly proves its superiority in terms of better discrimination against external faults, rapid detection during internal faults, independency on the network topology, and higher accuracy for fault distance estimation against all types of internal faults.

Application of a Superconducting Fault Current Limiter to Enhance the Low-Voltage Ride-Through Capability of Wind Turbine Generators

The penetration of wind turbine generators onto the grid has grown worldwide at unprecedented rates in recent years. This raises the concern that the tripping of wind turbine generators could potentially cause system collapses. To alleviate these concerns, wind turbine generators need to maintain connection with the grid when a grid fault occurs. This has provoked many countries to adopt low-voltage ride-through (LVRT) for wind turbine generators. The LVRT is the capability of wind turbine generators to maintain connectivity during certain periods of voltage sag. The wind turbine generators should be connected to the grid to support fault recovery. Also, wind turbine generators must provide reactive power according to the grid voltage sag. Therefore, much research has been focused on enhancing LVRT capability. To enhance LVRT capability, this paper proposes the application of a superconducting fault current limiter (SFCL) in the system. The fault current was suppressed and the voltage sag was improved through the application of the SFCL. By improving the voltage sag, the wind turbine generator and the grid were able to maintain a connection. However, suppression of the fault current can cause a problem in the overcurrent relay (OCR) trip time delay. The trip time delay was solved by OCR resetting. Through a power system computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC), the enhancement of LVRT capability and improvement of the trip delay was confirmed.

A Novel Fault-Location Algorithm for AC Parallel Autotransformer Feeding System

In general, an autotransformer (AT) is used to improve the voltage sag and inductive interference in power lines and communication lines in the Korean ac electric railway feeding system. In particular, the Korean AT feeding system has a different configuration compared to those of the other countries, such as Japan or France; therefore, it has certain distinctive characteristics. However, a fault locator imported from the other countries has been used in Korea, and the settings of these equipment should be modified and reflect the distinct characteristics of the Korean AT feeding system. Thus, the development of a novel fault-location scheme for the Korean railway system is required. Herein, we analyzed the fault current and developed a specific relationship for use in our new algorithm. We presented the current magnitude of each branch, expressed by the boosting current of the ATs; further, the novel fault-location scheme using this relationship is developed. The novel scheme performance is verified through power system computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) and the field measured data.

Sub‐synchronous resonance mitigation by a STATCOM in doubly fed induction generator‐based wind farm connected to a series‐compensated transmission network

With the fast development of wind power generation, large-scale wind farms are connected to an electrical network which is series compensated to increase transmission capacity. However, a series-compensated power system may encounter with a sub-synchronous resonance (SSR) problem. Flexible AC transmission systems controllers are widely applied to mitigate SSR and improve the transient stability of the power system. A static synchronous compensator (STATCOM) with an SSR damping controller is proposed to mitigate the potential of SSR in a series-compensated doubly fed induction generator-based wind farm. The local signal is used as the input of the damping controller, which is feasible to be implemented in a practical project. Taking account of different series compensation levels and operating conditions, extensive transient simulations have been carried out using power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) to demonstrate the capability of STATCOM in damping SSR. It is shown that the SSR is successfully alleviated by STATCOM, which also verifies the effectiveness of the proposed STATCOM controller in improving the stability of the transmission system.