Showing papers in "IEEE Transactions on Power Delivery in 2018"

Fault Analysis of Inverter-Interfaced Distributed Generators With Different Control Schemes

Abstract: Diversification of control schemes adopted by inverter-interfaced distributed generators (IIDGs) leads to difficulties in fault current estimation in a microgrid, which might make preexisting protection systems invalid and threaten the safety of power electronic devices. It is therefore important to study fault characteristics of IIDGs. This paper investigates characteristics of fault current of IIDGs caused by both symmetrical and asymmetrical faults. Two kinds of widely used control modes, current control (constant current control and PQ control) and voltage control (V/F control and droop control), are under investigation to provide an intuitive comparison on fault current. In particular, a novel algorithm is proposed to calculate fault current of droop-controlled IIDGs. It is found that different limiters have great impacts on fault response of IIDGs and detailed research works are carried out to identify the effects in this paper. Simulation results based on PSCAD/EMTDC and calculation results based on MATLAB/Simulink verify the correctness of the proposed fault models.

A Comprehensive Assessment of PV Hosting Capacity on Low-Voltage Distribution Systems

Abstract: Rooftop photovoltaic (PV) hosting capacity has become a concern for utilities in scenarios of high penetration due to impacts on voltage quality, such as over/undervoltage and voltage unbalance, and on equipment loading (conductors and transformers). This paper uses a simplified Monte Carlo-based method to analyze this issue, which is applied to 50 000 real low-voltage (LV) systems. Results show that it is possible to perform a risk-based analysis of hosting capacity by means of a lognormal distribution. Furthermore, overvoltage is found to be the most restrictive impact of PV integration; such information can help to guide utility actions to avoid technical violations. Extensive sensitivity studies are also presented to quantify the effects of several factors on the PV hosting capacity. The effects of number of customers with PV generators, PV power factor, voltage magnitude on the medium-voltage system, load level, and conductor impedances are investigated. It is also shown that the hosting capacity for the entire utility can be estimated by performing simulations only on 1% of the circuits randomly selected. In addition to providing a comprehensive overview of PV hosting capacity in real systems, the method can be used by utilities to improve the management of LV systems with high PV penetration.

Unstable Operation of Photovoltaic Inverter From Field Experiences

Abstract: This letter presents records of unstable operations in grid-connected photovoltaic generation plants. The instabilities involve a wide range of frequencies from tens to thousands of Hertz. Possible causes of the instabilities are discussed based on the literature survey. This letter suggests new industry standards or grid codes for photovoltaic generation integration should consider such practical challenges.

Transient Characteristics Under Ground and Short-Circuit Faults in a ${\pm \text{500}\,\text{kV}}$ MMC-Based HVDC System With Hybrid DC Circuit Breakers

Abstract: The development of voltage-source-converter (VSC)-based high-voltage direct current (HVDC) systems has increased significantly. However, the implementation of VSC-HVDC systems is limited for lacking fast, low-loss, and reliable circuit breakers. Hybrid dc circuit breakers (DCCBs) may help to improve this situation. In this paper, an efficient modular multilevel converter (MMC) HVDC system model based on the Zhangbei–Beijing ${\pm \text{500}\,\text{kV}}$ bipolar MMC-HVDC transmission (overhead line) project is first established. Then, the system transient characteristics are investigated involving permanent line-to-ground faults (LGFs), permanent short-circuit faults (SCFs), temporary LGFs, and temporary SCFs with either ac circuit breakers (ACCBs) or DCCBs in service. Finally, the performances, including the fault clearance and recovery, comparison of DCCBs with two different breaking methods, and comparison of ACCBs and DCCBs, in the MMC-HVDC system are presented. This paper provides a deep understanding of LGF and SCF in the MMC-HVDC systems, operation of DCCB, and comparison of faults clearance and recovery under these faults conditions using ACCB and DCCB, respectively. Results also benefit the application of the DCCB and the operation strategies in high-voltage and high-power MMC-HVDC systems with overhead lines.

Assessing Gaussian Assumption of PMU Measurement Error Using Field Data

Abstract: Gaussian phasor measurement unit (PMU) measurement error has been assumed for many power system applications, such as state estimation, oscillatory modes monitoring, voltage stability analysis, to cite a few. This letter proposes a simple yet effective approach to assess this assumption by using the stability property of a probability distribution and the concept of redundant measurement. Extensive results using field PMU data from WECC system reveal that the Gaussian assumption is questionable.

A Review on Grid-Connected Converter Control for Short-Circuit Power Provision Under Grid Unbalanced Faults

Abstract: As an increasing amount of converter-based generation on power electronics is connected to power systems, transmission system operators are revising the grid-connection requirements to streamline the connectivity of the devices to maintain security of supply. Converter-based generation can behave significantly different from the traditional alternators under grid faults. In order to evaluate the potential impact of the future converter-based power systems on protective relays, it is necessary to consider diverse current control strategies of voltage-source converters (VSCs) under unbalanced faults as the performance of converters primarily depends on their control objectives. In this paper, current control strategies of VSCs under unbalanced faults for short-circuit power provision are reviewed in two groups, namely, power-characteristic-oriented and voltage-support-oriented control strategy, respectively. As the fault current provided by converters should be restricted within secure operation limits considering semiconductor capabilities, converter current limit issue is also discussed.

A Fast DC Fault Detection Method Using DC Reactor Voltages in HVdc Grids

Abstract: This paper proposes a fast dc short-circuit fault detection method for HVdc grids, which relies on the voltage across the dc fault-current-limiting reactor. In the proposed approach, primary detection uses only the single-terminal dc reactor voltage information and is shown to be able to identify the faulted line as well as the faulted pole within 1 ms. The backup detection is primarily required for high resistance faults and uses communication between the two sides of the line. Its speed is dependent on the travel time of signals between the two ends, but will operate, as shown in the paper, typically in 2 ms or less. The approach is justified using analytical calculations and exhaustively validated via electromagnetic transient simulation for pole-to-pole and pole-to-ground faults, different fault resistance, fault locations, and dc reactor values. The validation shows that the method is able to identify all genuine dc line faults, but is not prone to misoperation with ac line faults, different loading levels, and so on.

Accurate Two-Terminal Transmission Line Fault Location Using Traveling Waves

Abstract: This paper presents an innovative two-terminal traveling wave (TW)-based fault location formulation. It depends only on the time difference between the first incident TW and the successive reflection from the fault point, at both line ends. Thereby, the proposed formulation requires neither data synchronization nor line parameters, which are sources of error that usually affect TW-based fault location schemes. Several faults on a typical 500 kV line were simulated to compare the proposed formulation performance with that of a classical two-end approach. The obtained results attest that the proposed formulation is able to accurately locate faults on transmission lines, even when data synchronism errors and uncertainties in the monitored line parameters exist.

Analysis of Faults in Multiterminal HVDC Grid for Definition of Test Requirements of HVDC Circuit Breakers

Abstract: This paper provides a detailed analysis of the temporal development of fault currents in a multiterminal high-voltage direct-current (MT-HVDC) grid composed of a bipolar converter configuration. The sequence of events following the occurrence of a pole-to-ground fault is identified, divided into three distinct periods; namely, submodule capacitor discharge, arm current decay, and ac in-feed periods. The critical parameters that have a significant impact on the fault current in each period are discussed. The impacts of various parameters of the HVDC grid such as the size of the current limiting reactor, ac grid strength, as well as the location of the fault within the grid are studied through PSCAD/EMTDC simulation. Then, a fault current interruption process using models of various HVDC circuit breaker technologies and the resulting stresses are studied. Both serve as important inputs to define test procedures. It is found that the HVDC circuit breakers are subjected to not only dc current and voltage stresses but also energy stress. These stresses are translated into test requirements.

Fault Location in Ungrounded Photovoltaic System Using Wavelets and ANN

Abstract: Identifying ground faults is a significant problem in ungrounded photovoltaic (PV) systems because such earth faults do not provide sufficient fault currents for their detection and location during system operation. If such ground faults are not cleared quickly, a subsequent ground fault on the healthy phase will create a complete short circuit in the system. This paper proposes a novel fault-location scheme in which high frequency noise patterns are used to identify the fault location. The high-frequency noise is generated due to the switching transients of converters combined with the parasitic capacitance of PV panels and cables. Discrete wavelet transform is used for the decomposition of the monitored signal (midpoint voltage of the converters) and features are extracted. Norm values of the measured waveform at different frequency bands give unique features at different fault locations and are used as the feature vectors for pattern recognition. Then, a three-layer feedforward artificial neural networks classifier, which can automatically classify the fault locations according to the extracted features, is investigated. The proposed fault-location scheme has been primarily developed for fault location in the PV farm (PV panels and dc cables). The method is tested for ground faults as well as line–line faults. These faults are simulated with a real-time digital simulator and the data are then analyzed with wavelets. Finally, the effectiveness of the designed fault locator is tested with varying system parameters. The results demonstrate that the proposed approach has accurate and robust performance even with noisy measurements and changes in operating conditions.

A Novel GIS Partial Discharge Detection Sensor With Integrated Optical and UHF Methods

Abstract: A novel gas insulated switchgear (GIS) partial discharge (PD) sensor with integrated optical fiber and ultrahigh frequency (UHF) method is developed in this paper. The integrated sensor is formed by placing an optical fiber on the surface of the UHF sensor. The novel sensor can be mounted inside the GIS and measure the PD through optical and UHF signals at the same time. The validity and sensitivity of the sensor are proved by experiments on a physical 110 kV GIS. This PD detection sensor solves the problem that the optical fiber is difficult to be mounted inside GIS and provides a novel method for PD detection in GIS.

A Decentralized Fault Detection Technique for Detecting Single Phase to Ground Faults in Power Distribution Systems With Resonant Grounding

Abstract: This paper presents a decentralized fault detection technique for power distribution systems with resonant grounding. The aim of this paper is to detect single phase to ground faults and identify the faulty feeder within three cycles of the fault occurrence. In the proposed technique, faults are first detected based on the neutral voltage displacement. The pre-fault and post-fault voltages (phase to ground) are then used to identify the faulty phase. Finally, the faulty feeder (as well as the faulty area of a long feeder) is identified from the relationship between the initial transient of the zero-sequence current and the faulty phase voltage just after the occurrence of faults. A signal processing tool called mathematical morphology is utilized to identify the faulty feeder. To identify the faulty feeder, it is also required to know the fault occurrence time that is estimated using the slope of the neutral voltage. The main feature of the proposed scheme is that this technique only uses voltage and current signals from the corresponding voltage transformers and current transformers. Therefore, it does not require communication among protection devices in the same or different feeders to identify the faulty feeder. The proposed technique also has the ability to distinguish the nature of faults, i.e., whether the faults are permanent or temporary. The effectiveness of the proposed scheme is tested, on an IEEE test system as well as on a practical test system, using MATLAB/SimPowerSystems . Simulation results show that the proposed technique can quickly detect the single phase to ground faults in a resonant grounding power distribution system and identify the faulty feeder. It is also capable of distinguishing faults from other disturbances. Moreover, it works under different compensation levels as well as for different fault inception angles.

A Wavelet-Based Transformer Differential Protection With Differential Current Transformer Saturation and Cross-Country Fault Detection

Abstract: The current transformer (CT) saturation phenomenon has been one of the main problems for the power transformer differential protection, leading to incorrect current measurements and relay misoperation. This paper proposes a fast and efficient transformer differential protection scheme with additional differential CT saturation and cross-country fault detection modules after the external fault detection, all of them based on the differential wavelet coefficient energy with border distortions in order to stabilize the relay during external faults and distinguish accurately CT saturation from cross-country internal faults. The proposed method was assessed by using representative simulations of internal faults, transformer energizations, and external faults with CT saturation followed by cross-country internal faults, and good results were achieved.

Commutation Failure Elimination of LCC HVDC Systems Using Thyristor-Based Controllable Capacitors

Abstract: The adverse impacts of commutation failure (CF) of a line-commutated converter (LCC)-based high-voltage direct current (HVdc) system on the connected ac system are becoming more serious for high-power ratings, for example, the development of ultra-HVdc systems. Aiming to solve the problem of CF particularly for higher power/current LCC HVdc systems, this paper proposes a new method, which utilizes a thyristor-based controllable capacitor (TBCC), to eliminate CFs. The topology of the proposed TBCC LCC HVdc and its operating principles are presented. Then, mathematical analysis is carried out for the selection of component parameters. To validate the performance of the proposed method, modified LCC-HVdc and capacitor-commutated converter (CCC)-based HVdc systems based on the modified CIGRE HVdc system are modeled in a real-time digital simulator. Simulation studies for zero impedance single-phase and three-phase faults are carried out, and comparisons are made with both LCC-HVdc and CCC-HVdc systems. Furthermore, voltage and current stress of the TBCC are investigated and power-loss calculations are presented. The results show that the proposed method is able to achieve CF elimination under the most serious faults while the increase of power losses due to the TBCC is small.

Use of Clustering to Reduce the Number of Different Setting Groups for Adaptive Coordination of Overcurrent Relays

Abstract: The facility to save multiple setting groups (SGs) within digital overcurrent relays (OCRs) is to adapt the active setting of the relays to the current topology of the power network. However, the number of available SGs is much lower than the possible topologies of the network, so this facility is usually applied to a few local changes in the network. This paper aims to extend the application to a complete set of topologies. A proper index is introduced and applied with a k -means clustering technique to classify the topologies into some clusters, whose number is equal to the number of SGs. Then, for every topology cluster, the optimal settings are calculated and saved in the OCRs as a distinct SG to be activated by changing the topology to any member of the cluster. Applying this technique to OCRs of the IEEE 14-bus test network and a 63/20kV substation verifies its effectiveness on reserving coordination for all topologies and reducing the relays’ time dial setting. Using available communication links, this technique can be implemented by making a computer network between substations of the power network without needing to interact with the control center.

Modular Multilevel Converter DC Fault Protection

Abstract: High-voltage direct current (HVDC) grids will require the development of dc protections that provide fast fault isolation and minimize the disturbance caused to the existing ac power networks. This paper investigates how the dc fault recovery performance of a half-bridge modular multilevel converter (HB-MMC) is impacted by different dc protection design choices. An HB-MMC point-to-point HVDC system that is protected with dc circuit breakers (CBs) is simulated on a real-time digital simulator using detailed switch models of the converters and switch gear. A dc CB controller has been developed and implemented in a software-in-the-loop fashion, and has been made available free for download. A novel blocking scheme for the HB-MMC is proposed, which limits the prospective dc-side fault current, benefiting dc switch gear. A comparison of circulating current controllers shows that the standard d — q controller is likely to be unsuitable for fault studies. Finally, benchmarking shows that a 48% reduction in power-flow recovery time and a 90% reduction in the energy dissipated in the circuit breaker can be achieved, along with other benefits, depending on the protection design.

Detecting Single-Phase-to-Ground Fault Event and Identifying Faulty Feeder in Neutral Ineffectively Grounded Distribution System

Abstract: In neutral ineffectively grounded distribution systems, a conventional zero-sequence voltage criterion to detect a single-phase-to-ground (SPG) fault event suffers from some drawbacks, and existing fault feeder selection techniques demonstrate low reliability in the field. For improvement, a new protection scheme is proposed in this paper. It is composed of a fault event detection unit and a fault feeder identification unit. Based on data distribution kurtosis of transient zero-sequence current, the former unit is employed to detect whether a SPG fault event has occurred and capture a fault starting moment. According to the behavior of average data distribution skewness, the latter unit is employed to identify which feeder is faulty. Superior to the zero-sequence voltage criterion, the proposed scheme can reliably detect the SPG fault event under a three-phase voltage imbalance condition and intermittent arc grounding fault. Besides, it is also found that the proposed scheme can validly identify a faulty feeder even under high background noises. Case test results indicate that the proposed scheme is able to work well with not only simulation data but also recorded field data, which demonstrates the high value in engineering practice.

A New Fault Location Technique in Smart Distribution Networks Using Synchronized/Nonsynchronized Measurements

Abstract: This paper proposes a new impedance-based technique to locate all fault types in distribution networks with/without distributed generators. A new procedure to form an impedance matrix using only series impedances of the distribution lines is introduced. The impedance matrix along with the prefault and during-fault voltage phasors at few buses is used to estimate the injection fault current via the least-squares technique. Linear least-squares estimator is utilized if microphasor measurement units ${({\mu} \rm{PMUs)}}$ are installed along the network. However, a nonlinear least-squares problem solved by the trust-region-reflective algorithm is used when only the voltage magnitudes are provided by smart meters. The operation of the standard protective devices in the distribution networks is used to reduce the computational burden of the proposed method. Also, a generalized measurement placement algorithm is studied using the discovered features of the impedance matrix. In addition, the Sobol's sensitivity analysis is conducted to quantify the importance of different input factors on the fault location accuracy. The effectiveness of the proposed method is validated on a real 134-bus, 13.8 kV distribution network under several fault scenarios and noisy measurements.

Distributed PLL-Based Control of Offshore Wind Turbines Connected With Diode-Rectifier-Based HVDC Systems

Abstract: Distributed phase-locked loop-based frequency control is proposed in this paper for offshore wind turbine converters connected with diode-rectifier-based high-voltage-direct-current systems. The proposed control enables a large number of wind turbines to work autonomously to contribute to the offshore ac frequency and voltage regulation. The proposed control also provides automatic synchronization of the offline wind turbines to the offshore ac grid. Stability of the proposed frequency control is analyzed using the root locus method. Moreover, active dc voltage control of the onshore modular multilevel converter (MMC) is proposed to ride-through an onshore ac fault, where the onshore MMC converter quickly increases the dc voltage by adding additional submodules in each phase, in order to rapidly reduce wind farm active power generation so as to achieve quick active power rebalance between the offshore and onshore sides. Thus, the overvoltage of the submodule capacitor is alleviated during the onshore fault, reducing the possibility of system disconnection. Simulation results in PSCAD verify the proposed control strategy during startup, synchronization, and under onshore and offshore fault conditions.

High-Sensitivity Vegetation High-Impedance Fault Detection Based on Signal's High-Frequency Contents

Abstract: High-impedance faults (HIFs) are linked to enduring unaddressed knowledge gaps due to their diverse and complex behavior, despite being extensively researched disturbances. Vegetation HIFs, for instance, are a particular type of fault that can lead to great fire hazards and life risks. They have unique fault signatures and should receive special attention if fire risk mitigation is desired. This paper focuses on the detection of these distinct, very small current faults. As the main correlational features, the proposed methodology uses the vegetation fault signatures’ high-frequency content. Different from many previous works that rely on HIF models, the approach validation is performed using a real dataset comprising a large number of experiments, sampled in a functioning network in the presence of noise. The classification is performed by boosted decision trees, which showed high dependability and security in the classification of small phase-to-earth and phase-to-phase HIFs.

Improving Small-Signal Stability of an MMC With CCSC by Control of the Internally Stored Energy

Abstract: The dc-side dynamics of modular multilevel converters (MMCs) can be prone to poorly damped oscillations or stability problems when the second-harmonic components of the arm currents are mitigated by a circulating current suppression controller (CCSC). This paper demonstrates that the source of these oscillations is the uncontrolled interaction of the dc-side current and the internally stored energy of the MMC, as resulting from the CCSC. Stable operation and improved performance of the MMC control system can be ensured by introducing the closed-loop control of the energy and the dc-side current. The presented analysis relies on a detailed state-space model of the MMC, which is formulated to obtain constant variables in steady state. The resulting state-space equations can be linearized to achieve a linear time invariant model, allowing for eigenvalue analysis of the small-signal dynamics of the MMC. Participation factor analysis is utilized to identify the source of the poorly damped dc-side oscillations, and indicates the suitability of introducing control of the internal capacitor voltage or the corresponding stored energy. An MMC connected to a dc power source with an equivalent capacitance, and operated with dc voltage droop in the active power flow control, is used as an example for the presented analysis. The developed small-signal models and the improvement in small-signal dynamics achieved by introducing control of the internally stored energy are verified by time-domain simulations in comparison to an electro-magnetic transient (EMT) simulation model of an MMC with 400 submodules per arm.

Operating DC Circuit Breakers With MMC

Abstract: High voltage direct current (HVDC) grids may be protected from dc faults through the application of HVDC circuit breakers. Recent advances in dc circuit breaker technologies may allow faults in the dc grid to be cleared without a permanent loss of power to the connected ac grids. The requirements for the protection have yet to be fully defined; especially where half-bridge modular multilevel converter (MMC) controls are concerned. This paper investigates integrating dc circuit breakers with half-bridge MMC converters, specifically looking to at how to recover from a pole-to-pole fault. The fault response of the converter to a fault is analyzed in depth. This analysis highlights key stages in the converter response to a dc fault, allowing the MMC fault currents to be predicted. This analysis is then verified in PSCAD simulations and the power flow recovery is shown. The converter controls are investigated, improvements made to the power flow recovery, and the need for arm current controllers highlighted.

An Improved Measure of AC System Strength for Performance Analysis of Multi-Infeed HVdc Systems Including VSC and LCC Converters

Abstract: A small signal model of a multi-infeed high-voltage direct current (HVdc) transmission system containing a line commutated converter (LCC) and a voltage source converter (VSC) is developed. This model represents the LCC and VSC converters as operational impedances as seen from the converter ac busbar. This permits the converters to be included in the effective short-circuit ratio (ESCR) calculations. The resulting ESCR is referred to in this paper as the “Impedance based Effective Short Circuit Ratio” (IESCR). It is shown that the maximum power transfer limit (referred to as the maximum available power or MAP) of the converters is better predicted by this index compared to the conventional ESCR which ignores the operational impedances of the converters. The question also arises as to how close to this theoretical maximum power transfer limit can the HVdc system operate. Using the small-signal model, it is shown that with commonly used control strategies, the predicted MAP can only be achieved by reducing the controller gains. The results are validated using detailed electromagnetic transients simulation of the multi-infeed VSC-LCC system.

Direct Current Gas-Insulated Transmission Lines

Abstract: The technology behind dc power transmissions shows an ever-increasing usage worldwide. Predominantly installed in remote areas, the use of overhead transmission lines is reasonable. Today, dc transmission technology is entering a new era, where rather densely populated areas are being firmly looked at with the desire of nonvisibility transmission lines. DC gas-insulated transmission lines (DC GIL) provide a powerful solution for underground installations. The acclaimed reliability of ac gas-insulated technology has, in turn, further initiated changes in the development of DC GIL. Driven by the German Energy Policy (“Energiewende”), additional north-south power transmission corridors are essential for power transmission of wind energy from the north and photovoltaic from the south, based on the required availability of energy. The DC GIL uses an aluminum conductor and enclosure pipes of high cross sections for rated currents of up to 5000 A and a voltage rating of ±550 kV. The assembly and installation technique of DC GIL is highly automated with the mobile factory and the friction stir welding process for fast and safe jointing of aluminum pipes. Development results and the state of the art are explained in this contribution.

A Robust Transform-Domain Deep Convolutional Network for Voltage Dip Classification

Abstract: This paper proposes a novel method for voltage dip classification using deep convolutional neural networks. The main contributions of this paper include: 1) to propose a new effective deep convolutional neural network architecture for automatically learning voltage dip features, rather than extracting hand-crafted features; 2) to employ the deep learning in an effective two-dimensional (2-D) transform domain, under space-phasor model (SPM), for efficient learning of dip features; 3) to characterize voltage dips by 2-D SPM-based deep learning, which leads to voltage dip features independent of the duration and sampling frequency of dip recordings; and 4) to develop robust automatically-extracted features that are insensitive to training and test datasets measured from different countries/regions. Experiments were conducted on datasets containing about 6000 measured voltage dips spread over seven classes measured from several different countries. Results have shown good performance of the proposed method: average classification rate is about 97% and false alarm rate is about 0.50%. The test results from the proposed method are compared with the results from two existing dip classification methods. The proposed method is shown to outperform these existing methods.

Enhanced Independent Pole Control of Hybrid MMC-HVdc System

Abstract: This paper presents an enhanced independent pole control scheme for hybrid modular multilevel converter based on a full-bridge submodule and half-bridge submodule. A detailed analysis of the power distribution between upper and lower arms under asymmetrical dc pole voltages is presented. It is found that the fundamental ac currents in the upper and lower arms are asymmetrical. To enable operation under asymmetrical dc pole voltages, an enhanced independent pole control scheme is proposed. The controller is composed of two dc control loops, two ac control loops, and circulating current suppression control based on current injection. Six modulation indices are presented to independently control the upper and lower arms. With this controller, the dc voltage operating region is significantly extended. To ride through a pole-to-ground dc fault without bringing dc bias at the neutral point of the interface transformer, a pole-to-ground dc fault ride through strategy is proposed. The feasibility and effectiveness of the proposed control scheme are verified by simulation results using PSCAD/EMTDC.

Improvement of Transmission Line Ampacity Utilization by Weather-Based Dynamic Line Rating

Abstract: Most of the existing overhead transmission lines (TLs) are assigned a static rating by considering the conservative environmental conditions (e.g., high ambient temperature and low wind speed). Such a conservative approach often results in underutilization of line ampacity because the worst conditions prevail only for a short period of time during the year. Dynamic line rating (DLR) utilizes local meteorological conditions and grid loadings to adaptively compute additional line ampacity headroom that may be available due to favorable local environmental conditions. This paper details Idaho National Laboratory-developed weather-based DLR, which utilizes a state-of-the-art general line ampacity state solver for real-time computation of thermal ratings of TLs. Performance of the proposed DLR solution is demonstrated in existing TL segments at AltaLink, Canada, and the potential benefits of the proposed DLR for enhanced transmission ampacity utilization are quantified. Moreover, we investigated a hypothetical case for emulating the impact of an additional wind plant near the test grid. The results for the given system and data configurations demonstrated that real-time ratings were above the seasonal static ratings for at least 76.6% of the time, with a mean increase of 22% over the static rating, thereby demonstrating huge potential for improvement on ampacity utilization.

Fault Analysis and Traveling-Wave-Based Protection Scheme for Double-Circuit LCC-HVDC Transmission Lines With Shared Towers

Abstract: Double-circuit transmission lines, with bipolar transmission lines of two different line-commutated-converter-based high-voltage direct-current (HVDC) systems constructed on the same towers, are a new technology emerging in China to increase the power transmission capacity. Double-circuit HVDC transmission lines sharing the same towers will lead to an electromagnetic coupling effect between the two circuits. According to the geometry configuration of same-tower double-circuit transmission lines in HVDC transmission projects, double-circuit transmission lines are unbalanced without transposition. Therefore, the coupling effects on the healthy circuit cannot be completely eliminated. These coupling components will affect the sensitivity of conventional traveling-wave-based protection criteria. This work first develops a fault analysis for double-circuit HVDC transmission lines with various internal fault conditions, and then, the linear combinations of four independent modal components based only on each single circuit are obtained and investigated. Finally, an integrated traveling-wave-based protection scheme for double-circuit HVDC transmission lines is presented. Numerous simulation results verify that the proposed protection scheme can operate correctly in double-circuit HVDC transmission lines even for a fault with high resistance up to the maximum of 300 Ω.

Small-Signal Stability Analysis of Offshore AC Network Having Multiple VSC-HVDC Systems

Abstract: This paper presents a methodology to perform small-signal analysis of an offshore ac network, which is formed by interconnecting several offshore wind power plants. The offshore ac network is connected with different onshore ac grids using point-to-point voltage-source-converter high-voltage direct current (VSC-HVDC) transmission links. In such a network, each offshore VSC-HVDC converter operates in grid-forming mode. In this paper, the offshore VSC grid-forming control is enhanced by using a frequency and voltage droop scheme in order to establish a coordinated grid control among the offshore converters. A small-signal model of the offshore ac network is developed that includes the high-voltage alternating current cables' model, the converters' current, and voltage-control model, the frequency droop scheme, and the voltage droop scheme. Based on this model, an eigenvalue analysis is performed in order to study the influence of the frequency and voltage droop gains on overall offshore ac network stability. Finally, theoretical analysis is validated by performing nonlinear dynamic simulation.

Performance of Interconnection Protection Based on Distance Relaying for Wind Power Distributed Generation

Abstract: This paper proposes a method to analyze and improve the performance of interconnection protection based on distance relaying for wind power distributed generation (DG) in distribution systems. Of particular importance is distance protection that uses the concept of prefault voltages as reference quantities found to have issues with intermittent behavior of wind power DG. This concept is normally used in different distance protective relaying applications in order to increase the fault resistance coverage capability of the distance relays as well as to ensure selectivity, dependability, and security under extreme undervoltages. The main contributions of this paper are to analyze this issue and propose a method to enhance the performance of distance protection to mitigate this issue. In this methodology, several case studies are investigated with different penetration levels, weather patterns, and configuration topology of DGs under both normal operating conditions as well as fault conditions. Results for a case study are given.