Showing papers in "IEEE Transactions on Power Delivery in 2020"

IN-YOLO: Real-Time Detection of Outdoor High Voltage Insulators Using UAV Imaging

Abstract: The high voltage insulator requires continuous monitoring and inspection to prevent failures and emergencies. Manual inspections are costly as it requires covering a large geographical area where insulators are often subjected to harsh weather conditions. Automatic detection of insulators from aerial images is the first step towards performing real-time classification of insulator conditions using Unmanned Aerial Vehicle (UAV). In this paper, we provide a cost-effective solution for detecting insulators under the conditions of an uncluttered background, varied object resolution and illumination conditions using You Only Look Once (YOLO) deep learning neural network model from aerial images. We apply data augmentation to avoid overfitting with a training set size of 56000 image samples. It is demonstrated experimentally that this method can accurately locate insulator on UAV based real-time image data. The detected insulator images are then successfully subjected to insulator surface condition assessment for the presence of ice, snow and water using different classifiers.

Flexible Converters for Meshed HVDC Grids: From Flexible AC Transmission Systems (FACTS) to Flexible DC Grids

Abstract: Flexible Alternating Current Transmission Systems (FACTS) have achieved to enhance the flexibility of modern AC power systems, by providing fast, reliable and controllable solutions to steer the power flows and voltages in the network. The proliferation of High Voltage Direct Current (HVDC) transmission systems is leading to the opportunity of interconnecting several HVDC systems forming HVDC Supergrids. Such grids can eventually evolve to meshed systems which interconnect a number of different AC power systems and large scale offshore wind (or other renewable sources) power plants and clusters. While such heavily meshed systems can be considered futuristic and will not certainly happen in the near future, the sector is witnessing initial steps in this direction. In order to ensure the flexibility and controllability of meshed DC grids, the shunt connected AC-DC converters can be combined with additional simple and flexible DC-DC converters which can directly control current and power through the lines. The proposed DC-DC converters can provide a range of services to the HVDC grid, including power flow control capability, ancillary services for the HVDC grid or adjacent grids, stability improvement, oscillation damping, pole balancing and voltage control. The present paper presents relevant developments from industry and academia in the direction of the development of these converters, considering technical concepts, converter functionalities and possible integration with other existing systems. The paper explores a possible vision on the development of future meshed HVDC grids and discusses the role of the proposed converters in such grids.

A Traveling Wave-Based Fault Location Method Employing VMD-TEO for Distribution Network

Abstract: In this paper, a traveling wave based fault location method is proposed for distribution networks. Before fault occurs, an intrinsic distance difference matrix (IDDM) is established based on the characteristics of traveling wave propagation and the topology of distribution network. After the fault, the initial fault traveling wave signals are decomposed into several intrinsic mode function (IMF) signals by variational mode decomposition (VMD), and the first IMF signal is analyzed by teager energy operator (TEO) to determine the arriving time of initial fault traveling waves. According to the double-ended traveling wave method and the arrival time of traveling wave at all recorders, a fault distance difference matrix (FDDM) is built. By comparing the difference between IDDM and FDDM, a fault branch determination matrix (FBDM) and the lateral index Q are used to locate the fault. The effectiveness of the proposed method is verified via EMTP/ATP simulations. Results show that the proposed method has a high fault location accuracy, and does not need that all terminals of lines are installed traveling wave recorders.

Review of Wildfire Management Techniques—Part I: Causes, Prevention, Detection, Suppression, and Data Analytics

Abstract: This two-part paper is intended to inform power system engineers, electrical engineering academicians, and suppliers of electrical apparatus of the threat of wildfires initiated from mal-operation of electrical grids and the unexploited opportunity to develop proper solutions and preventive means to such lethal events. This part (Part I) reviews and categorizes research in different fields of science and industrial projects that attempt to address wildfire issues. The topics include prediction and prevention means, detection methods, monitoring and surveillance techniques, suppression methods, allocation and mapping algorithms, and a summary of research and educational efforts. Subsequently, this paper highlights the damages and negative effects that a wildfire can cause to the electric grid and the interruptions to its continuous operation. Finally, this paper analyzes and categorizes the various scenarios of faulty electrical networks that may lead to wildfires. Part I of this paper provides the ground work and information for the solutions and discussions presented in Part II.

A Control-Based Solution for Distance Protection of Lines Connected to Converter-Interfaced Sources During Asymmetrical Faults

Abstract: Full-scale converter-interfaced renewable energy sources (CIRESs) can cause misoperation of distance relays installed in their vicinity. Such failure stems from the different fault behavior of CIRESs and synchronous generators (SGs), based on which available relays have been developed. Several measures have been devised to secure the performance of distance protection by modifying existing relays. This paper proposes a new approach to address misoperation of distance relays during asymmetrical faults by regulating the fault currents of CIRESs such that available distance relays function properly without requiring modification. The prime objective of this method is to mimic certain features of the symmetrical components of SGs’ fault currents that affect distance relays. In the meantime, the proposed method satisfies the constraints of a converter, such as its limited phase current magnitude. As a result, correct operation of the distance relays close to CIRESs is ensured regardless of the fault conditions, including its type, resistance, and location. Some salient features of the proposed method are its simplicity, compatibility with off-the-shelf relays, and independence from the voltage and power rating of the CIRES. Moreover, the proposed method uses only local measurements, and so is cost-effective. PSCAD/EMTDC simulation studies verify the performance of this new method.

Impedance Modeling and Analysis of MMC-HVDC for Offshore Wind Farm Integration

Abstract: The offshore ac side impedance model of the modular multilevel converter (MMC) based high voltage direct current (HVdc) system is essential for analyzing the interaction stability between MMC-HVdc and the offshore wind power plants. This paper develops the offshore ac side impedance model of an MMC-HVdc system for wind power integration taking the effects of offshore station, dc cable and onshore station into consideration. The dc impedance model of the onshore station is first derived which includes the effects of dual closed loop dc bus voltage control and circulating current control. Then, the dc impedance of the onshore station and dc cable impedance are used to derive the ac side impedance of the offshore station under open loop control. In addition, the influence of the dual closed loop based ac bus voltage and frequency (VF) control on the offshore ac impedance is analytically derived. As a result, the proposed model can be used to reveal the coupling between offshore offshore ac system and dc system as well as to investigate the influence of the dc system and VF controller in the offshore ac impedance of the MMC-HVdc system. Furthermore, the proposed model overcomes the deficiencies in harmonic resonance analysis of MMC-HVdc based offshore wind power integration system. The results of the simulation in PSCAD/EMTDC validate the proposed models and analyses.

PMU Assisted Integrated Impedance Angle-Based Microgrid Protection Scheme

Abstract: Development of protection scheme for microgrid is a challenging issue due to the inclusion of distributed generations (DGs). The research work for developing the primary protection scheme for microgrid is progressing continuously. This paper proposes an integrated impedance angle (IIA) based protection scheme using wide-area positive sequence components of voltages and currents. The proposed scheme uses data retrieved from the phasor measurement units (PMUs) of IEEE C37.118.1 complied standards. Furthermore, the IIA for the line is computed considering both ends PMU information, which is the key indicator for identifying the faults in the microgrid. The proposed protection scheme is extensively tested considering the different operating conditions of microgrid, variation in DG penetration and variation in fault parameters such as fault resistance, fault types, and fault location. Furthermore, cross validation on no-fault situations including section cutoff, disconnection of high penetration DG, grid-islanding resynchronization, motor starting, capacitor switching, and load encroachment are carried out. The proposed scheme is also tested for the insensitivity towards the external faults. The test results on both 15-bus and 34-bus systems clearly indicate that the proposed IIA-based wide-area scheme can be a potential protection measure for microgrid under varied operating circumstances.

An Adaptive Reclosing Strategy for MMC-HVDC Systems With Hybrid DC Circuit Breakers

Abstract: Modular multilevel converter (MMC) based high voltage direct current transmission (HVDC) is an effective solution for large-scale renewable power integration over long-distance. In the overhead MMC-HVDC systems, the high voltage DC circuit breakers (DCCB) are implemented to interrupt the DC fault current. Subsequent to fault isolation, the DCCBs are required to automatically re-close to restore power transmission quickly. However, when the DCCBs are re-closed to permanent faults, they will be tripped again, resulting in a high requirement of interruption capacity for DCCBs and second overcurrent strikes on the HVDC systems. To overcome the drawbacks of the conventional auto-reclosing scheme, this paper proposes an adaptive reclosing scheme based on the active pulse injection from the converter associated with the coordination control of hybrid DCCBs. The single-end adaptive reclosing scheme as well as the two ends adaptive reclosing scheme dedicated to two-terminal HVDC systems and meshed DC grids are presented respectively. By applying this method, the location of faults can also be achieved in the case of permanent faults. In order to verify the effectiveness of the proposed adaptive reclosing schemes, extensive simulations have been conducted under PSCAD/EMTDC.

Two-Level Distributed Volt/Var Control Using Aggregated PV Inverters in Distribution Networks

Abstract: The penetration level of photovoltaic (PV) keeps increasing in modern distribution networks, which leads to various severe voltage limits violation problems. This article aims to aggregate and utilize the PV inverters for voltage regulation by a fully distributed two-level Volt/VAr control (VVC) scheme. In the lower-level VVC (real-time scale), the rooftop PV inverters are aggregated via consensus algorithms and then governed by droop controllers in medium-voltage networks. The droop controller adjusts the reactive power output of each PV aggregator in real-time from its dispatched value depending on the bus voltage variations. In the upper-level VVC (15-min timescale), the reactive power of PV aggregators is dispatched for power loss minimization, where control signals are set as base values for PV aggregators. This problem is formulated as second-order cone programming and solved by the alternating direction method of multipliers. The simulation results demonstrate the effectiveness of the proposed method in both short-term and long-term scenarios with different system scales.

Estimating Power Transformer High Frequency Model Parameters Using Frequency Response Analysis

Abstract: Frequency response analysis (FRA) has become a widely accepted technique by worldwide utilities to detect winding and core deformations within power transformers. The main drawback of this technique is its reliance on the personnel level of expertise more than standard or automated codes. To establish reliable FRA interpretation codes, accurate high frequency transformer model that can emulate the frequency characteristics of real transformers in a wide frequency range is essential. The model can be used to investigate the impact of various winding and core deformations on the transformer FRA signature. The transformer equivalent high frequency electric circuit parameters can be calculated based on design data, which are rarely available, especially for old transformers. As such, this paper presents an artificial intelligence technique to estimate these parameters from the transformer FRA signature. The robustness of the proposed technique is assessed through its application on three, 3-phase power transformers of different ratings, sizes, and winding structures to estimate their high frequency electric circuit parameters during normal and fault conditions. Results show that the proposed technique can estimate equivalent circuit parameters with high accuracy and helps interpret the FRA signature based on the numerical changes of these parameters. The main advantage of this approach is the physical meaning of the model parameters facilitates reliable identification of various faults and hence aids in establishing reliable interpretation codes for transformer FRA signatures.

Replicating Real-World Wind Farm SSR Events

Abstract: In 2017, three sub-synchronous resonance (SSR) events were reported in the transmission system of Electric Reliability Council of Texas (ERCOT). These three events with different consequences are due to the same cause, i.e., Type-3 wind farms radially connected to a series compensated transmission line. The objectives of this paper are: 1) to build a testbed to replicate these real-world events and investigate what causes the different consequences; 2) to find out the impact of wind penetration level and wind speed on SSR; and 3) to investigate how to mitigate SSR. The replication testbed is built in MATLAB/Simpowersystems with each wind farm represented by a single aggregated wind turbine with full dynamics. The challenge of replication is the limited information about those three events, the ERCOT system, and the wind farms. Nevertheless, relying on simplification and assumptions, we are able to replicate the three events. Our contributions include the configured testbed for Type-3 wind farm SSR studies, diagnosis of impacting factors on SSR, and the confirmation of an SSR mitigation strategy through grid-side converter ac voltage command signal modulation.

A Model-Based DC Fault Location Scheme for Multi-Terminal MMC-HVDC Systems Using a Simplified Transmission Line Representation

Abstract: Accurately determining the location of DC pole-to-pole short-circuit faults in modular multilevel converter (MMC) based multi-terminal HVDC (MTDC) systems is key issue in ensuring fast power recovery. This paper proposes an effective DC fault location scheme for the MMC-MTDC that uses an estimated R-L representation of the transmission lines. By using the measured voltage and current data from both ends of the faulted DC line, the proposed fault location formulas can calculate the location of the fault with high accuracy. The simplified R-L representation greatly reduces the computation burden of the fault detection algorithm. Electromagnetic transient (EMT) simulations of a four-terminal MMC-MTDC system on PSCAD/EMTDC are used to confirm the effectiveness of the proposed approach. The results verify that the proposed scheme is robust and almost not affected by the transmitted power or the fault resistance.

Probabilistic Analysis of Commutation Failure in LCC-HVDC System Considering the CFPREV and the Initial Fault Voltage Angle

Abstract: This paper investigates the in-depth mechanism of commutation failure for a line-commuted converter-based high-voltage direct current (LCC-HVDC) system. The commutation failure prevention control (CFPREV) and the initial fault voltage angle (IFVA) are considered from the view of the voltage-time area (VTA) in the analysis. It is revealed that the IFVA is among the dominant factors for commutation failures when the voltage drop of the inverter bus is relatively small, and CFPREV further intensifies the impact of the IFVA on commutation failures, while the fluctuation of the direct current plays a dominant role in commutation failures under a greater voltage reduction at the inverter bus. A quantitative division of the severity of AC faults is proposed to determine dominant factors for commutation failures. The relationship between the chance of commutation failures to occur and the IFVA is built, and the method used for computing probability of commutation failures is proposed. The influence of the dynamic of CFPREV output on our research is studied. Simulations based on a typical monopole LCC-HVDC system using PSCAD/EMTDC software are conducted to verify the correctness of the theoretic analysis and the effectiveness of the proposed computing methods.

A Novel DCCB Reclosing Strategy for the Flexible HVDC Grid

Abstract: The overhead line will be widely applied in the future flexible high-voltage dc (HVDC) grid. Correspondingly, effective reclosing strategy which can identify the fault property must be configured for the dc circuit breaker (DCCB) to improve the system power supply reliability. However, if the DCCB is reclosed directly, large second overcurrent may occur to damage the vulnerable devices in the system. Therefore, the inherent voltage characteristic difference between different dc faults (nonpermanent or permanent) is analyzed in the paper. And then a novel DCCB reclosing strategy for the flexible HVDC grid is proposed. Compared with the method which recloses DCCB directly, the proposed strategy can avoid the second damage under permanent fault condition, because it can identify the fault property without completely reclosing the DCCB (only the residual dc current breaker is reclosed before fault property identification). Moreover, compared with the typical sequential auto-reclosing strategy, the proposed strategy significantly reduces the additional control and design requirements on the DCCB. Finally, simulation cases based on the PSCAD/EMTDC are carried out to verify the correctness of the theoretical analysis and the superiorities of the proposed reclosing strategy.

A Novel Two-Stage Multi-Layer Constrained Spectral Clustering Strategy for Intentional Islanding of Power Grids

Abstract: Intentional islanding can be considered as the last action to prevent power grids from severe blackouts. In this strategy, the endangered network is deliberately decomposed into self-sustained islands to improve the power grid resilience, reliability, and security. In this way, this paper develops a novel optimal intentional islanding solution to deal with deliberate physical attacks on the power system. The proposed solution employs a modified multi-layer constrained clustering method based on multi-layer graphs via subspace analysis on Grassmann manifolds clustering. The main objectives are to minimize the active and reactive powers disruptions crossways for decomposed islands, as well as grouping the generators with high-frequency change similarity so that stable operation of each self-supplied grid is ensured. This technique guarantees that each island is only comprised of generators that are synchronized with each other. The proposed multi-layer technique increases the stability of the island by implementing different criteria such as frequency, active power, and reactive power. To better display the improvement, the method is compared with the single layer constrained clustering that can only handle one criterion at a time. The proposed intentional islanding technique is applied to IEEE-9 bus, IEEE-39 bus, and IEEE-118 bus as small, medium, and large-scale networks, respectively.

Fault Location Algorithm for Non-Homogeneous Transmission Lines Considering Line Asymmetry

Abstract: This article presents a novel phasor domain approach to determine the location of faults in non-homogeneous transmission lines. Best implementation requires synchronized voltage and current phasor measurements at all terminals of the transmission line. The overall non-homogeneous transmission line model with fault is established by systematically combining the generalized compact models of all homogeneous line sections. The location of the fault is introduced as an additional state of the overall model. The derivation procedure of the model is without further assumptions, resulting in full consideration of three phase line asymmetry as well as distributed parameters of non-homogeneous transmission lines. Afterwards, the location of the fault is identified using the state estimation algorithm. Extensive numerical experiments in a two-terminal and a multi-terminal non-homogeneous transmission line demonstrate that the method has higher accuracy than existing fault location methods, independent of fault types, locations and impedances.

Modeling, Experimental Validation, and Application of VARC HVDC Circuit Breakers

Abstract: This paper deals with the modeling, hardware results and model validation by measurements of a VSC assisted resonant current (VARC) dc circuit breaker (CB) and the application within a future network by simulation. The newly emerging VARC dc CB can be used as a solution for the protection of offshore multi-terminal HVDC (MTDC) grids. In this paper, the proposed VARC dc CB is modeled in detail in a PSCAD environment, by taking into account dielectric strength of the vacuum gap, high-frequency current quenching ability and parasitic components. The PSCAD-model is then verified by data from the testing of a 27 kV VARC dc CB prototype with maximum current interruption capability of 10 kA. Additionally, the initial transient interruption voltage and current slope at zero-crossing during the interruption are analyzed. With respect to scaling to a higher voltage level, three types of series connected modules are presented and the performances are compared. The performance of the series connected modules is simulated in a model of a 4-terminal HVDC grid. The obtained results validate the VARC dc CB as a promising solution for the dc fault isolation in MTDC grids.

A Novel Fault Impedance Calculation Method for Distance Protection Against Fault Resistance

Abstract: Performance of distance protection based on the traditional fault-line impedance calculation method is adversely affected by fault resistance, which may cause distance protection to operate incorrectly, threatening the safe operation of power system. To improve immunity to the fault resistance, this paper presents a novel fault-line-impedance calculation method. The fault-line impedance, supplementary impedance, and measured impedance are rotated simultaneously in the complex plane until the supplementary impedance coincides with the positive direction of the real axis. Following the geometrical relationship between the rotated fault-line impedance and the rotated measured impedance in the complex plane, the fault distance and fault-line impedance are solved. The proposed method is immune to the fault resistance and insensitive to power angle and fault location variations. Furthermore, the proposed method can work well under various types of faults, e.g., AG, BC, BCG, and ABC. The simulation results show that the proposed method can calculate the actual fault distance accurately and identify in-zone and out-of-zone faults correctly.

Systematic Approach to HVDC Circuit Breaker Sizing

Abstract: High-voltage direct current (HVDC) grids can be protected by HVDC circuit breakers (DCCB) to selectively clear DC side faults and assure continuous operation of the healthy parts in the grid. Line inductors in series with DCCBs are essential to reduce the rate of rise of the fault current and to slow down the voltage decline in the healthy parts of the grid. The line inductor value and DCCB requirements, i.e., operating time, current interruption capability, and absorbed energy, are dependent on the expected functional requirements of the grid. In this paper, three different categories of functional requirements, termed as HVDC grid fault-ride-through scenarios, have been defined. Based on these definitions, DCCB and line inductor parameters are determined in a systematic manner. Furthermore, a visual approach to DCCB operational characteristics, represented by DCCB operational graphs, is proposed to determine the suitable line inductor value, DCCB operating time, and interruption capability requirements given grid limitations, i.e., maximum current and energy, and/or required fault-ride-through scenario. The proposed method illustrates graphically the interdependence between the DCCB parameters and the line inductor value, and shows high dependence of the main DCCB parameters and the line inductor value on the fault-ride-through requirements of the converters during DC faults.

A Pilot Protection for HVDC Transmission Lines Based on Transient Energy Ratio of DC Filter Link

Abstract: To overcome shortages in the traditional current differential protection for HVDC transmission lines, a pilot protection based on transient energy ratio is proposed. The fault identification criterion is put forward based on the impedance-frequency characteristic of the line boundary, and the fault analysis which shows that under internal faults, with regard to either terminal of the dc line, the transient energy within a specific frequency band on the line side of the boundary is much greater than that on the valve side of the boundary, whereas after rectifier-terminal (inverter-terminal) external faults, the transient energy on the line side of the rectifier-terminal (inverter-terminal) boundary is much less than that on the valve side of the boundary. Moreover, the lightning disturbance identification criterion is implemented based on the magnitude ratio of fault components of high- and low-frequency band currents. Fault-pole selection is constructed by using the fault components of pole voltages. Simulation results demonstrate that the proposed scheme, with low computational complexity, can distinguish internal faults from external ones and protect the entire line reliably. It can also identify high-impedance faults and select the faulted pole correctly. Besides, it is not subject to lightning interferences and the dc line distributed capacitor.

Fault Characteristics of Distributed Solar Generation

Abstract: Inverter-based distributed energy resources (DERs) are characterized with low fault current and negligible amount of negative and zero sequence currents. Understanding DER's fault characteristics is critical for fault analysis and protective relay setting. Despite the abundant work on DER modeling, few research studies have been done to analyze DER's fault behaviors during actual fault events. This paper explores recorded fault events collected by Dominion Energy. Fault magnitude, angle, and sequence components are analyzed to show that actual DER fault response may differ from previous understandings.

A Novel Method to Determine Droop Coefficients of DC Voltage Control for VSC-MTDC System

Abstract: For droop control in voltage source converter based multi-terminal HVDC systems, the determination of droop coefficients is a key issue, which directly affects both power distribution and DC control performances. This paper proposes a novel design of droop coefficients considering the requirements of power distribution, DC voltage control and system stability. Considering the power margins of different converters, the ratio relationship among droop coefficients is established. Converters with larger power margins take bigger portion of power mismatch to avoid overload problem. Furthermore, the integral square error of converters DC voltage is adopted as the DC voltage control performance index, and optimization of droop coefficients to achieve coordinated DC voltage control of steady-state deviation and transient variation, is derived. Finally, the constraint of droop coefficients is established to guarantee the DC system stability after power disturbance. Case studies are conducted on the Nordic 32 system with an embedded 4-terminal DC grid to demonstrate the feasibility and effectiveness of the proposed droop control scheme.

Health Index, Risk and Remaining Lifetime Estimation of Power Transformers

Abstract: A new method for systematically estimating health index, probability of breakdown, and remaining life for power transformers is presented in this article. The method combines three basic models; a physical winding degradation model, a health index model based on condition monitoring data combined with expert judgement, and a statistics-based end-of-life model. The statistics-based model uses data from a database of scrapped transformers under development in Norway. Combining the first two models with the statistics-based model, an individual and condition-dependent probability of breakdown is obtained. From this, the expected remaining life is calculated. Finally, the stochasticity of the method is utilized for optimization of maintenance and replacement. The method hence provides key decision support for transformer managers, enabling them to identify transformers in poor condition, and to follow-up and prioritize transformers for maintenance and replacement. The proposed method has been implemented in a transformer asset management tool for Norwegian utilities. The usefulness of the method is illustrated by applying it to selected transformers from one of these utilities. Finally, important limitations, uncertainties, and further improvements are discussed.

A Novel Integrated Protection for VSC-HVDC Transmission Line Based on Current Limiting Reactor Power

Abstract: In this paper, a novel integrated protection for VSC-HVDC transmission line is proposed. The protection scheme includes a main protection and a backup protection. The main protection is based on current limiting reactor power (CLRP), while the backup protection is based on variation tendency of current. The changing characteristics of CLRP and current are theoretically analyzed under different fault conditions. Based on the characteristics, internal and external faults of DC transmission line can be identified. Then criteria of the novel integrate protection can be built. The main protection can detect the faults quickly without communication, while the backup protection is used to identify the high resistance faults and provide a backup if main protection fails. The scheme not only protect the whole transmission line without dead zone, but also improve the speed of protection with simplified algorithm. A typical VSC-HVDC system is modeled using PSCAD/EMTDC. Comprehensive simulations is done to verify the performance of proposed scheme. Furthermore, the protection does not require a high sample frequency and does not require data synchronization. These make it valuable for application in practical VSC-HVDC transmission system.

DC Fault Current Clearance at the Source Side of HVDC Grid Using Hybrid MMC

Abstract: This paper proposes a source-side DC fault current clearance scheme for HVDC grids under DC short-circuit faults. The scheme uses half-bridge submodules (HBSM) and full-bridge submodules (FBSMs)-based hybrid modular multilevel converter (MMC). The DC fault current behaviors of the HVDC grid is revealed as a primary criterion for the proposed approach. The additional degrees-of-freedom for control of hybrid MMC are incorporated into the HVDC grid to interrupt the fault current feeding at the source side. During the fault ride-through process, the reactive power compensation is maintained. The proposed method does not need the DC circuit breakers since the current in the fault line can be controlled to zero. The primary and backup protection schemes for hybrid MMC-HVDC grid are proposed. The effectiveness of the method is validated by electromagnetic transient simulations on PSCAD/EMTDC.

Time-Varying Interharmonics in Different Types of Grid-Tied PV Inverter Systems

Abstract: Widely existing circuit topologies and inverter control strategies for photovoltaic (PV) systems allow customer flexibility but also introduce different kinds of interharmonics into the grid. A complete understanding of interharmonics from PV systems, with reasons behind their origin, remains needed. In addition, the time-varying nature of interharmonics and the potential impacts on other equipment are yet to be understood. In this paper, laboratory and field measurements of seven different inverter types at multiple locations are presented. A comprehensive analysis is performed to understand the existence, persistence, and propagation of interharmonics in PV systems on the dc side as well as grid side for different power levels. The origins of the interharmonics are established with experimental evidence and through a comparative analysis. A rural low voltage six customer network, with two different impedance profiles caused by the installation of PV, is considered to show the potential impact on customer voltage. To address the time-varying nature of interharmonics, a sliding window ESPRIT method is preferred over fast fourier transform (FFT)-based methods.

An Improved Non-Unit Traveling Wave Protection Method With Adaptive Threshold Value and Its Application in HVDC Grids

Abstract: The performance of the traditional derivative-based non-unit traveling wave protection (TWP) in high voltage dc (HVDC) grids shows the problem of low-sensitivity when high-impedance faults occur. In this article, the characteristics of the fault initial traveling wave are analyzed first, two conclusions can be drawn: i) the distortion degree of the fault initial traveling wave is related to the fault location, ii) the amplitude of the fault initial traveling wave is related to the fault impedance. Then a zero-sequence current fitting method is proposed to obtain parameters respectively reflecting fault impedance and fault distance. When a fault occurs, a fault information extraction method based on the fitting parameters will give the estimation interval of the fault impedance, which provide reference for the adaptive threshold value selection of TWP. In this way, the influence of the fault impedance on TWP can be eliminated. A ±400 kV modular multilevel converter (MMC) HVDC grid is built in PSCAD/EMTDC. The verification of the proposed method considers different installation positions of the current limit inductor. The simulation results show that the proposed method can fast and correctly identify the dc line faults and effectively improves the protection sensitivity in detecting high-impedance faults.

A Novel Traveling Wave Protection Method for DC Transmission Lines Using Current Fitting

Abstract: Hybrid AC/DC transmission grids require high-speed operation and high reliability for relay protection. Traditional derivative-based traveling wave protection methods have exposed the problems of low sensitivity and low reliability. In this paper, the fault traveling wave is analyzed considering the attenuation, distortion, and boundary effect of the DC transmission line as well as three converter types. The expressions of the fault initial traveling wave are derived and the conclusions show that 1) the expressions of the fault initial traveling wave at the end of the transmission line have the formation of a (1 − exp (− bt )); 2) the index of the exponential function in the fault initial traveling wave expression is related to the fault location, and is not affected by the fault impedance and line attenuation. Then a Levenberg–Marquart fitting method is proposed to fit the measured zero-mode fault current initial traveling wave and extract the index coefficients. Finally, a novel traveling wave protection method for DC transmission lines is proposed using the obtained index coefficients. The simulation results show that 1) the proposed protection method can complete fault identification within 1 ms, even with high fault impedance; 2) the proposed protection method has high robustness to the influence of the noise.

High Impedance Arc Fault Detection Based on the Harmonic Randomness and Waveform Distortion in the Distribution System

Abstract: High impedance arc faults (HIAFs) happening in the medium-voltage distribution system may result in damages to devices and human security. However, great difficulties exist in identifying these faults due to the much weaker features and the varieties when grounded with different surfaces. This paper presents an integrated algorithm to detect the HIAFs with high-resolution waveform data provided by distribution-level PMUs deployed in the system. An improved arc model is proposed, which can continuously imitate the randomness and intermittence during the “unstable arcing period” of arc faults. The integrated algorithm consists of two branches. First, the variations of HIAFs during unstable arcing period are identified with the unified harmonic energy and global randomness index, which can unify the scales of harmonic content in different fault situations and enlarge the disparities from non-fault conditions. Then, the waveform distortions of HIAFs during the stable arcing period are identified with discrete wavelet transform to extract the detailed distribution characteristics. The reliability and security of the proposed algorithm are verified with numerical simulations and field tests in a 10-kV distribution system.

Stability Analysis of Systems With High VSC Penetration: Where Is the Limit?

Abstract: The paper addresses the stability of modern voltage-source-converter-dominated power systems, which are experiencing a progressive phase-out of conventional generation. An essential system model is used to capture the fundamental system dynamics, to conduct a thorough mathematical analysis and understand the principles governing network stability in converter-dominated power systems. A detailed analysis is developed to identify the stability limits of this system when voltage source converters operate in grid-following mode. A complete mathematical linear model including the dynamics of the network elements is used to identify these stability limits. Based on this model, a detailed assessment of the influence that the voltage source converter controllers have on the system stability is performed to identify potential instabilities and reveal the main mechanisms of interaction. Then, a study to identify the minimum synchronous generation to ensure system stability is developed, including analytical expressions of some critical system poles. The mathematical results obtained are validated with a complete non-linear simulation model of the system.