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This paper introduces adaptive decision-making logic to improve the performance of distance protection of transmission lines. The proposed approach incorporates the trajectory of the measured impedance into the relay final decision-making logic through a weighting procedure. Two criteria are used to weigh the impedance plane appropriately, where higher weights are assigned to the faults occurring at the interior areas of the relay protective zone and low resistance faults. The weights assigned to impedance samples are added together considering the amount of smoothness of the measured impedance trajectory. An adaptive neurofuzzy inference system (ANFIS) is used to implement the proposed decision logic. Meanwhile, the fault current amplitude is considered as the other input of the ANFIS network to accelerate the relay operation for close-in faults and to incorporate an index for the source-to-line impedance ratio into the final decision-making logic. Extensive simulation studies using PSCAD/EMTDC software indicate that the proposed approach provides a very reliable technique to make the distance protective scheme faster and smarter. It helps to achieve the optimal compromise between the protective relay operation time and security.

This paper describes ensuring safe working conditions for workers performing electrical installation work and maintenance on overhead lines within the vicinity of high voltage. Due to harsh operating conditions and the necessity of ensuring uninterrupted electricity supply, it is sometimes necessary to replace a conductor on a double-circuit overhead line with one circuit for normal operations. In such cases, there is a high-risk of conductor swing that may cause contact with the phase conductor of the adjacent energized circuit. The presented special “sliding grounding clamp” has been designed and constructed in order to carry the fault current to ground. It enables a good flow of the current from the drawn conductor and, thus, protects the workers from excessive “step and touch” voltage on the worksite. The sliding grounding clamp has been constructed, and tested in practice. It has also been granted a Slovenian national patent.

This paper analyzes long duration overvoltages due to backfeeding currents from the low-voltage network to the medium-voltage network through network protectors in heavily meshed underground distribution networks. Overvoltages above 3 p.u. may be developed as a result of a simultaneous occurrence of three phenomena: neutral displacement, Ferranti effect, and current chopping. A detailed model of a typical distribution network is utilized for the study. Time-domain simulations are performed using an Electromagnetic Transients Program-type program. The results demonstrate that overvoltages due to backfeeding last longer during peak load operating conditions, and the largest peak overvoltages occur at no load.

The aim of this paper is to present an analytical, state-space model of an indirect, voltage-controlled cascaded-type multilevel static synchronous compensator (STATCOM) with ldquosquare wave control.rdquo The multilevel converter model is segmented into a dynamic and static part in order to accurately represent all internal feedback connections. Each voltage component is analyzed in detail and described mathematically by an averaged expression with an equivalent capacitance. The STATCOM model is linearized and linked with a DQ frame AC system model and the controller model, and implemented in MATLAB. The controller gains are selected by analyzing the root locus of the analytical model to give optimum responses. The validity and accuracy of the proposed model are verified against non-linear digital simulation PSCAD/EMTDC in the time and frequency domain. The model is very accurate in the subsynchronous range, and it is adequate for most control design applications and practical stability issues below 100 Hz. Furthermore, the developed model can be used for multilevel cascaded converters which exchange real power.

Multi-inverter microgrid systems, particularly those with loop topology, offer higher power supply reliability and robustness compared to conventional radial distribution systems. In meshed systems, communication-less protection schemes have proved to be ineffective for multi-inverter microgrids due to bidirectional power flow, and limited and controlled fault currents generated by the voltage source converters interfacing the energy source to the network. This makes communicationassisted line protection schemes preferable for such systems despite the necessity for communication means. While these protection schemes are effective, their reliability depends much on the communication availability. This requires a back-up communication path in case the main one fails bringing up the cost issue that hinders their uptake. This paper proposes a novel and low-cost line protection based on directional blocking strategy that can operate as a main as well as a back-up protection to any protection scheme using communication means between the line terminals. As the main, it requires low-bandwidth communication system. As a back-up, it would share the same communication means with the main one and use those of the healthy lines when the faulted lines fails. Thus saving the cost of back-up communication system.

The Neher/McGrath paper and the IEC Standard 60287-2-1 contain the formulae for the calculation of the external thermal resistance of cables installed in backfills or duct banks. These models are based on several assumptions, which often are not valid for practical installations. This paper examines the applicability of the Neher/McGrath model and proposes a new solution for the calculation of the external thermal resistance for such installations. Numerical examples illustrate the difference between the two methods.

Because traditional electric power distribution systems have been designed assuming the primary substation is the sole source of power and short-circuit capacity, DR interconnection results in operating situations that do not occur in a conventional system. This paper discusses several system issues which may be encountered as DR penetrates into distribution systems. The voltage issues covered are the DR impact on system voltage, interaction of DR and capacitor operations, and interaction of DR and voltage regulator and LTC operations. Protection issues include fuse coordination, feeding faults after utility protection opens, impact of DR on interrupting rating of devices, faults on adjacent feeders, fault detection, ground source impacts, single phase interruption on three phase line, recloser coordination and conductor burndown. Loss of power grid is also discussed, including vulnerability and overvoltages due to islanding and coordination with reclosing. Also covered separately are system restoration and network issues.

This paper presents a three-phase unified power-quality conditioner based on current source converters (CSC-UPQC), including the design guidelines of the key components, an appropriate control scheme, and a selection procedure of the dc current level. Particularly, the ride through capability criterion is used to define a minimum dc current level so that the CSC-UPQC achieves the same characteristics as a UPQC based on voltage-source converters in terms of voltage disturbance compensation in the point of common coupling (PCC) and load power factor compensation. A 1.17 MVA load fed from a 3.3 kV system is used to show the proposed design procedure, and a laboratory prototype is implemented to show the system compensating sags and swells using low switching frequency in the CSC and maintaining a unitary displacement power factor in the PCC.

Room Temperature Vulcanized (RTV) silicone rubber is widely used on the surface of insulator of power transmission and transformation equipment because of its excellent hydrophobicity. With the increase of operating life, RTV coating is subject to the influence of high temperature, high humidity and complex outdoor environmental factors, which tends to fall off, seriously affecting the working state of the insulator. In this paper, based on the realization of insulator identification, the defect in RTV coating is identified, and the damage area percentage of RTV coating in the insulator shed is calculated. Experiments show that this method can effectively characterize the severity of defects in RTV coating, which provides a technical basis for the online detection and evaluation of RTV coating status.

This paper presents the results of an EPRI study on development of a new method for coupling capacitor voltage transformer (CCVT) frequency response measurements from the secondary side. The method is especially suitable for field measurements since it does not require any internal CCVT disassembly or access to its individual components. It has been verified by performing the field measurements on actual CCVTs installed in a substation. The results were compared with the results obtained by carrying out the CCVT frequency response measurements on the same type of CCVTs in a laboratory and using the method of frequency response measurement from the primary side. The proposed method is easy to use and gives accurate results. The method may be used for the EMTP-based CCVT model development and the CCVT performance analysis. >

This paper proposes a technique to approximate a multi-layer earth structure by a homogeneous conductivity soil in ground return impedance calculations. An equivalent real-valued conductivity parameter is obtained, which can be used with reasonable accuracy in a simpler expression or in commonly available EMTP software, such as the Line Constants routine of the Alternative Transients Program (ATP). Several actual soil models are evaluated at frequencies ranging from 1 Hz to 2 MHz. Results show that the proposed method is accurate for modeling most power system problems, from steady-state conditions to transients commonly verified in electrical systems. The proposed expression is easy to use and introduces a considerable performance gain in terms of floating-point operations, compared to the analytical formulation of the general multi-layer soil structure.

The modular multilevel converter is the state-of-the-art topology for voltage source converter HVDC. Despite its advantages, this converter handles large internal low-frequency energy ripples, and the capacitance that supports these dynamics is a key design parameter that affects the operating region of the converter. Different strategies can be found in the literature to increase the feasible region of operation of the converter. Nevertheless, they are typically open loop in nature and use precalculated control references. This paper presents an alternative based on model predictive control that steers the system through optimal control trajectories that are calculated online. This provides feedback and corrective control action in real time. The predictive controller used for this purpose is presented and a linear time-varying approximation is used to reduce the computational burden of the algorithm. The feasible boundaries of the converter are sought and the final performance of the control algorithm is evaluated through detailed simulations using a switching model of the converter.

The ruling span method is used by line engineers to calculate the approximate tension in a multi-span dead-ended line section. Providing satisfactory results for level spans, it may, however, result in an error of over 30% if it is used to calculate tensions in significantly long and steep spans e.g., in mountainous terrain. Following the ruling span concept in general, this paper presents equations to calculate tensions for inclined single spans or multi-span line sections based on calculation of conductor length in three-dimensional space. These equations include the equivalent parameters such as equivalent inclined span, equivalent slope, equivalent load, equivalent effective tension-all needed to take into account the effect on tension of the changing geometry and length of conductors in significantly inclined spans. The numerical examples show the effect of span inclination on conductor tension calculated using these equations. A simple and accurate equation (without hyperbolic functions) is derived to calculate conductor length in an inclined span. This paper also addresses calculation of sags in nonlevel spans. The presented method integrates the practicality of the ruling span concept with the accuracy of the three-dimensional vector method for analysis of inclined spans. This method is most efficient for overhead lines in mountainous terrain.

This paper proposes a novel approach to achieve monitoring and control of modern substations using advanced computer communication and control technology. The proposed approach is based on a code division multiple access (CDMA) and interleave division multiple access (IDMA) system joining with optical-fiber communication technology to realize the utility infrastructure of Smart Grid. Recently, owing to the rapid growth on optical-fiber communication, it can offer standardized communication technology for wide-area, metropolitan-area, and local-area networks. Moreover, optical-fiber communication technologies offer not only significant benefits on connection among different CDMA-IDMA systems such as rapid deployment, higher baud rate (equal or higher than 14 Tb/s), enhanced bandwidth, less bit error rate (equal or less than 10-9), less optical-amplifier quantities, reduced interference among them, lower signal attenuation, etc. but also the advantages of being more suitable for remote-end applications in Smart Grid environment. With spreading spectrum modulation and demodulation combing encoding, it can be used to transmit data and images over power lines to accomplish opportunities and challenges in the effort on comparisons and assure safety, security, and reliability in order to achieve reliability and demand efficiency for modern substations of the utility system as a supervisory control and data acquisition (SCADA) system.

This paper shows one way to calculate grounding potentials in high voltage substation fences coupled by soil to a grounding grid in a two-layer soil. This problem is of special concern due to the presence of passers-by near metallic fences, whose potentials usually have not been evaluated, not directly connected to the grounding grid. In this context, the paper presents a general analytical formulation aiming to estimate the possible risk to human beings close to the substation. In order to do that the generated potentials originating from a short-circuit in the electric power system are calculated considering the coupling by soil between the main grounding system and the adjacent electrode (the metallic fence). Finally, the proposed methodology is tested and validated using different grounding topologies.

Reliable protection and communication are the key features in a digital substation from protection, control and automation perspective. Implementation of digital protection in high voltage (HV) substations could provide increased reliability, optimized maintenance cost and improved communication capabilities. The application of IEC 61850-9-2 process bus technology in a utility substation has reduced the fault diagnosis downtime due to decreased number of nodes likely to be tested for investigation. Additionally, manufacturers are offering new smart primary plants that have intelligent features built within to makes it a fit case to replace Conventional Instrument Transformers (CITs). These Non-Conventional Instrument Transformers (NCITs) coupled with Merging Units and Intelligent Electronic Devices (IEDs) provide better accuracy, transient response and wider bandwidth when implemented in a process bus system guided by IEC 61850-9-2 standard. It is seen that use of NCIT could mitigate issues arising out of CIT failures, provide better safety, reliability, and increased asset life. At present, majority of the CITs operating in the air insulated and gas insulated high voltage (HV) substations are either oil filled or SF6 type which has few environmental issues in the event of a catastrophic failure or leakage. NCITs have better performance as compared to CITs. However, before these high voltage NCITs could find their way in a mass implementation in smart grids and digital substations, there is a need for modelling and simulation at the design stage for its application as per IEC61850-9-2 process bus and its successful implementation. Optimized Network Engineering Tool (OPNET) or Riverbed Modeler is a popular simulation software for such design and modeling of Ethernet networks. This paper presents a model of OPNET software for the simulation of a NCIT based on digital substation in a process bus environment.

This paper proposes a new modulated model predictive control method for the control of modular multilevel converters (MMCs) in voltage source converter-high voltage dc systems. The proposed method retains the advantages of the conventional finite control set-model predictive control methods by programing the nonlinear properties of the MMC into the design calculations while minimizing the ac line and circulating current ripples and steady-state error by generating modulated switching signals with a fixed switching frequency. In this paper, first, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to an MMC system is detailed. Finally, experimental results from an MMC system connected to a three-phase grid are provided to validate the theoretical outcomes.

Gas-insulated substation (GIS) equipment served in the traction power-supply system (TPSS) of the high-speed railway (HSR) suffers from extremely severe traction load conditions during its lifetime. In this paper, three major issues related to the actual field maintenance operations of GIS equipment in TPSS are addressed: 1) $s$ -dependent degradation and shock process; 2) both spatial and temporal failure thresholds of shocks; and 3) incomplete maintenance (i.e., nonignorable maintenance time). To model the deterioration and failure process of GIS equipment, the hard failure model and the soft failure model are proposed. The characteristic function is utilized to bridge the s -dependent degradation and shock process in the soft failure model, while the extreme shock model (with spatial threshold) and $\delta $ -shock model (with temporal threshold) are introduced in the hard failure model. Furthermore, the index of availability, which combines both reliability and maintainability, is derived under the incomplete maintenance consideration. Based on the balance of availability and maintenance cost, the maintenance strategy with periodic inspections for GIS equipment in the long-run time span is developed. Optimizations of three decision parameters (i.e., inspection period, extreme shock threshold, and $\delta $ -shock threshold) are implemented to achieve the best performance (with high availability and low maintenance cost) of the maintenance strategy.

When designing utility substations, it is necessary to consider all directional components of the electromagnetic reaction forces during short-circuit faults to properly account for their effects on substation equipment. IEEE 605 assumes straight long parallel conductors in its two-dimensional (2-D) force calculations while ignoring any bends and 3-D placement of conductors. CIGRE's Brochure 105 provides practical information on the mechanical effects of short-circuit currents in HV and EHV open air, conventional substations. Although these computational methods are easy to implement, they have several limitations and are sometimes too conservative. Previously published papers address such conservatism using dynamic analysis approaches that provide more accurate results when compared to experimental data. This paper provides two extensions to IEEE 605 and CIGRE 105 for calculating fault current reaction forces. A closed-form equation defining the simplified 2-D short-circuit force time history during a fault based on CIGRE 105 and a computationally inexpensive numerical 3-D force calculation technique are presented.

Submitted to wind-induced vibrations, overhead conductors are vulnerable to fatigue damage, especially at restraining fixtures such as the suspension clamp. This paper proposes an efficient finite-element modeling approach providing a full 3-D representation of both the conductor and suspension clamp. Validation based on experimental data shows the precision of the approach. An in-depth model response analysis also demonstrates its ability to describe inter-wire and conductor–clamp contact interactions. Finally, a study of conductor stress distributions reveals that in critical regions, conductor wires mostly sustain alternating bending loads.

This paper proposes a methodology for transient state estimation in power systems. The proposed methodology is formulated using approximation methods for derivatives to relate the state variables to measurements. It does not require knowledge of the steady state to establish the pre-disturbance operation conditions. The method uses an optimal monitoring system based on topological analysis to obtain full observability. A saving index is introduced to analyze the effectiveness of the instrumentation used. The adverse effect of noisy measurements in the estimation process is mitigated using an Infinite Impulse Response (IIR) filter. A transient index is introduced to estimate the fault location. The transient state estimation is assessed using two test systems. The results are validated through direct comparison against those obtained by simulation using SimPowerSystems toolbox of Simulink®. With the proposed methodology, the transient state estimation can be obtained with an important saving in the implementation of the measuring system and with considerably less computational effort.

Transmission lines are vital components in power systems. Tripping of transmission lines caused by over-temperature is a major threat to the security of system operations, so it is necessary to efficiently simulate line temperature under both normal operation conditions and foreseen fault conditions. Existing methods based on thermal-steady-state analyses cannot reflect transient temperature evolution, and thus cannot provide timing information needed for taking remedial actions. Moreover, conventional numerical method requires huge computational efforts and barricades system-wide analysis. In this regard, this paper derives an approximate analytical solution of transmission-line temperature evolution enabling efficient analysis on multiple operation states. Considering the uncertainties in environmental parameters, the region of over-temperature is constructed in the environmental parameter space to realize the over-temperature risk assessment in both the planning stage and real-time operations. A test on a typical conductor model verifies the accuracy of the approximate analytical solution. Based on the analytical solution and numerical weather prediction (NWP) data, an efficient simulation method for temperature evolution of transmission systems under multiple operation states is proposed. As demonstrated on an NPCC 140-bus system, it achieves over 1000 times of efficiency enhancement, verifying its potentials in online risk assessment and decision support., Accepted by IEEE Transactions on Power Delivery

The protective effect of a shield wire against direct lightning flashes to a buried power cable has been clarified in this paper. The finite-difference time-domain (FDTD) method is used in the investigation. The FDTD model considers a 1-km-long insulated cable protected by a shield wire, and a lightning strike right above them. The model has been validated by comparing its results with experimental results available in the literature. The computed results show that the presence of the shield wire can effectively prevent the cable from being directly struck by the lightning flash. The voltage across the cable insulation at the striking point depends on the ground resistivity (the higher the resistivity, the higher the voltage), but this dependence is not linear. It is also shown that the soil surface ionization is relevant to the current delivered to the shield wire and to the voltage developed across the cable insulation. On the other hand, the soil ionization around the shield wire has negligible effects. The intrinsic resistance of the shield wire does not influence significantly the peak voltage across the cable insulation, but it influences its duration (the higher the intrinsic resistance, the longer the voltage duration).

Oil flow distribution in the winding has a direct impact on the cooling performance. In addition, static pressure drop over the winding determines oil split among windings connected hydraulically in parallel. In this paper, experimental verifications are provided to support computational fluid dynamics (CFD) simulations for disc-type windings in oil forced and directed (OD) cooling modes. Oil flow distribution in and pressure drop over disc-type winding models are measured using a particle image velocimetry (PIV) system and a differential pressure instrument, respectively. Dimensional analysis is adopted to analyze the relationship between flow distribution, or pressure drop, and the controlling parameters. CFD parametric sweeps of the dimensionless parameters obtained from the dimensional analysis are conducted and the CFD results are then correlated with the dimensionless parameters. The comparisons between measured results and the corresponding results obtained from the correlations demonstrate constant consistency, proving the validity of both the method of dimensional analysis and the correlations. Finally, comparisons of experimental results from isothermal and nonisothermal conditions in OD cooling modes are executed, which show that the isothermal conclusions can be extended to nonisothermal cases because the effects of buoyancy force and hot-streak dynamics are proved to be negligible.

Vibro-acoustic measurement on a transformer's on-load tap changer (OLTC) can provide indications on its mechanical condition. Recently, a joint vibro-acoustic and arcing measurement system has been proposed, which can correlate the vibro-acoustic signal to mechanical events of OLTC's operation. However, there are still considerable difficulties in extracting useful information from both vibro-acoustic signal and arcing signal in a synchronized manner without any distortions in the extracted signals. In this paper, Savitzky–Golay filter is introduced to process the signals acquired from a joint vibro-acoustic and arcing measurement system installed on in-service OLTCs. It proves that the Savitzky–Golay filter can process both vibro-acoustic and arcing signals induced by OLTC, extract essential information without any time delay from both types of signals, and retrieve voltage phase information from the arcing signal. The methodologies developed in this paper can improve the visibility of OLTC's mechanical operation for an effective online condition monitoring.

An electric arc furnace exhibits highly nonlinear characteristics and can cause power quality issues. Its performance is a vital issue for electric utilities. In this paper, a novel multiobjective optimization technique has been proposed for the estimation of the electric arc furnace parameters to predict the voltage and current waveforms and evaluate the performance of the furnace under various conditions. The multiobjective optimization technique minimizes the extinction voltage error and arc resistance error simultaneously to enhance the performance of the electric arc furnace model. The proposed optimization model employs a genetic algorithm and the stochastic variation of the arc length to estimate the parameters of a nonlinear time-variant resistance model of the electric arc furnace by using actual current and voltage data. These data have been obtained from a steel plant in Saudi Arabia. Optimization has been carried out in MATLAB/SIMULINK environment and the model of the electric arc furnace with the estimated parameters has been developed in PSCAD/EMTDC. The results obtained from the PSCAD model have been validated with actual data from the steel industry. The validation shows that the proposed method is efficient and accurate in the estimation of the electric arc furnace parameters.

With the rapid development of China's economy, smart grid has been built in an all-round way. As an important basic node of smart grid, the technical complexity of smart substation automation system is getting higher and higher. Once a fault occurs, if not handled in time, it will seriously affect the safety and stability of the power grid, which poses a great loss to economic production activities. The traditional mode of local maintenance has the drawbacks of high time consuming, low efficiency, and heavy workload. To solve these problems, a remote maintenance technology for substation automation equipment based on general service protocol (GSP) is first proposed in this paper, and the brand new system architecture is designed which involves the service management center, the maintenance center, and substations. The standardized interaction mechanism, integrated deep security protection methods, and flow control strategy are adopted to ensure the standardization, security, and reliability of the remote maintenance system. Finally, the effectiveness of the proposed methods is successfully demonstrated, and the comprehensive performance of the remote maintenance system is evaluated by comparing with the traditional local maintenance mode.

A power transformer is one of the most crucial items of equipment in the electricity supply chain. The reliability of this valuable asset is strongly dependent on the condition of its subsystems such as insulation, core, windings, bushings and tap changer. Integration of various measured parameters of these subsystems makes it possible to evaluate the overall health condition of an in-service transformer. This paper develops an artificially intelligent algorithm based on multiple general regression neural networks to combine the operating condition of various subsystems of a transformer to form a quantitative health index. The model is developed using a training set derived from four conditional boundaries based on IEEE standards, the literature and the knowledge of transformer experts. Performance of the proposed method is compared with expert classifications using a database of 345 power transformers. This shows that the proposed method is reliable and effective for condition assessment and is sensitive to poor condition of any single subsystem.

Cables intended to interconnect offshore wind farms with the mainland are currently treated as separate lead-type cables by IEC 60287. A single core equivalent thermal model (SCETM) is implied, that is, a thermal model accounting for 1-D (radial) heat transfer only. While this approach is expected to be reasonable when smaller cables are rated, the 1-D representation is less valid in larger cable sizes. This paper uses 2-D finite-element models to reveal certain weak points of the representation of the thermal resistances in the existing SCETM. For constant losses, the IEC SCETM seems to underestimate the temperature by up to 8 °C. Suitable replacement formulae are proposed that could improve the accuracy of the SCETM. Finally, the issue of nonsolid fillers is considered and modelling guidelines are given.

A BC Hydro regional system experienced a blackout after multiple 500 kV lines, sharing the same transmission corridor, were subjected to lightning strikes in the time span of about three minutes. These lines connect two large generating stations, regional loads, and five nonutility generators to the rest of the BC Hydro system. The protection scheme on the first line tripped three-phase and initiated autoreclose as designed. However immediately after three-phase opening, high magnitudes of low-frequency (about 9 Hz) currents appeared in the line shunt reactor, causing a protection trip, and aborting the autoreclose. Subsequent lightning strikes coupled with a pre-existing line outage disconnected both generating stations from each other as well as from the integrated system. The larger of two generating stations and almost all regional load formed an islanded subsystem, which eventually collapsed and blacked out. During the restoration of the subsystem, the system operator inadvertently picked up the entire regional system without load, causing steady-state resonant overvoltages higher than 145% and damaging a transmission customer. This paper shares lessons learned from forensic analysis of disturbances leading to regional blackout and describes the risk of unsafe overvoltages from near power frequency resonant condition during the restoration process.

This paper presents a mathematical model for the selection of an optimal power cable conductor cross section and the dimensions of a corrective backfill. To this end, a detailed model for the calculation of the life-cycle cost of cable ownership is presented. The formula considers the material and labor costs in the production of a power cable as well as the cost of losses during its operation. Simultaneously, the procedure seeks the optimal size of the corrective backfill, taking ampacity constraints into account. Since the formulation is complex, a genetic algorithm is proposed to solve the optimization problem. A real-life numerical example is presented.

With the integration of the renewable energy-based distributed small power plants, whose generated energies are uncertain, in addition to the increasing usage of the charging stations for electric vehicles, the determination of the dynamic loading of distribution transformers appears to be crucial. Therefore, the issues of the model-based online monitoring of distribution transformers may achieve acceptance, if a cost–benefit compromise is reached. In this paper, a new thermal model is proposed to calculate the top-oil temperature of indoor distribution transformers using calculated tank temperature which accounts for convection, radiation, conduction, and ventilation heat transfer phenomena. The model is validated using measurements during the in-service operation of two indoor distribution transformers. Moreover, a method for calculating the loading capability of indoor distribution transformers without sacrificing the transformer's life is presented which can be easily adjusted for other distribution transformers. Furthermore, the results of the hardware implementing the proposed model and method for a model-based online assessment of the loading capability during the in-service operation of an indoor distribution transformer are presented.

In hydroelectric power plants, which are characterized by particularly high short-circuit current levels and high time constants, conventional current transformer (CT) specifications can result in increased equipment dimensions, thus complicating their installation, especially during refurbishment projects. This paper presents a study to assess the potential of various low power instrument transformers (LPITs) for generator protection applications. Two types of LPITs were evaluated: the optical current transformer (OCT) and the stand alone merging unit (SAMU). To test the OCT, a 65-kA-peak test source was developed using a real-time simulator. This source was used to inject symmetrical and asymmetrical currents into LPITs for the assessment of protection accuracy class. Low-current tests were also performed to assess their metering classes. This paper presents the design of a high-current test bench, measurement uncertainty analysis and analysis algorithms used for device evaluation. Analysis results are then presented and discussed in terms of their applicability to generator differential protection using SAMUs and OCTs.

Current differential protection typically requires symmetrical communications channels—with equal latency in each direction—for correct operation. Conventionally, this has been delivered using protocols such as IEEE C37.94 over a Time-Division Multiplexing (TDM) wide-area network (WAN). Modern packet-based WANs offer improvements in efficiency, flexibility, and cost-effectiveness for utility applications. However, jitter is unavoidable in packet-based networks and, in extreme cases, jitter inevitably results in substantial asymmetrical latency in communications paths. This paper clearly defines how a new source of asymmetry arises due to the use of "de-jitter" buffers, which can jeopardize critical protection services. This is demonstrated using an analytical modelling approach, which precisely quantifies the degree of risk, and through real-time demonstration with actual devices, involving current differential protection over an IP/MPLS WAN. Using a novel method of real-time manipulation of Ethernet traffic to emulate large WANs, the modelling approach has been validated. It is shown how the sensitivity of relays to asymmetry depends on the protection settings and the magnitude of the measured load current. To address the risk of protection maloperation, a new approach for compensating for asymmetrical latency has been comprehensively validated. These developments will be of immediate interest to utilities operating, or migrating to, a packet-based infrastructure.

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.

The inductor-capacitor-inductor ( LCL ) dc/dc converter has been extensively studied for high power and stepping ratio because of elimination of internal transformer, lower footprint/weight, higher efficiency, and most importantly providing dc fault isolation from both dc sides. This paper presents a two-channel, two-layer controller including two inner current loops, which is symmetrical for each bridge of LCL dc/dc. The real-time implementation of the control scheme and its performance under normal conditions and during transient dc faults at both sides are studied on a 30 kW 200 V/900 V 1.7 kHz prototype. The prototype development is presented in some depth. The experimental results show that the converter with closed-loop control operates well at full power and under fast power reversal. Further dc fault testing concludes that there is no need for blocking since the internal voltage and current variables are within the rated values. Detailed study of converter losses is performed and results show that full power efficiency is around 93.4%.

In order to improve features, such as fault current blocking and capacitor voltage balancing, modular multilevel converter (MMC) topologies incorporating multiport submodules (SMs) are being considered as candidates for HVdc transmission applications. This paper presents high-speed and accurate electromagnetic transient (EMT) models for MMCs composed of such multiport SMs. The approach uses the Schur's complement technique to recursively eliminate internal nodes of the converter structure to create a multiport Norton equivalent that connects to the external network. Thus, the final admittance matrix seen by the EMT solver has a dimension orders of magnitude smaller than that of the unreduced structure. As with previously developed approaches for MMCs with single-port SMs, all internal information, such as individual SM capacitor voltages, is preserved and can be output by the program if needed. This increases the bookkeeping effort, but the overall reduction in matrix size more than compensates for any resulting time penalty. Approximately two to three orders of magnitude speedup over a straightforward implementation in an EMT program is achieved.

This paper presents a comprehensive analysis and a novel solution for the discharge problem on the half-bridge submodules’ (HBSMs’) capacitors in the hybrid modular multilevel converter (MMC)-based high-voltage direct current transmission systems under a long-term (e.g., several minutes or longer) pole-to-ground fault condition. The mechanism of this issue is analyzed, and then a solution based on fundamental-frequency reactive circulating current (FFRCC) injection is proposed. The influence of FFRCC injection is analyzed in terms of the fluctuated arm capacitor energy and the current stress of semiconductors. Moreover, an enhanced arm voltage modulation method based on HBSMs and full-bridge submodules coordinate control is proposed to accelerate the capacitor voltage balance of HBSMs. Finally, the performance of the proposed control strategy is verified by the simulation results.

A new type of overvoltage phenomenon in half-wave length transmission line (HWLL) is caused by power fluctuation (PF). This paper defines this phenomenon as PF overvoltage (PFO), which belongs to the temporary overvoltage category. The amplitude of PFO is generally high, the duration time is long, and the coverage on HWLLs is wide in the case of single-phase transient fault, when the transmission power is large. This will pose a new problem for ultrahigh voltage (UHV) half-wave length ac transmission (HWACT). First, we analyze the unique transmission characteristics of HWLLs with incident and reflected wave theory. Next, according to the application scenario of HWACT in China, we construct a plant-to-grid UHV half-wave length transmission system. Then, based on a large number of electromagnetic transient simulations on the system using electromagnetic program, we obtain the PF characteristics of the UHV HWLL. Furthermore, the mechanism and characteristics of PFO are described by the characteristics of transmission and PF. Finally, we propose the mitigation measures of PFO for UHV HWLL, which include configuring metal oxide arresters differentially along HWLLs, closing high-speed grounding switches during the fault period, generator tripping after a fault, and shortening the reclosing time.

This paper presents a new method based on the relay agent for single-phase earth fault protection in radial distribution systems with distributed generators. In order to get the most informative fault features to discriminate fault for this system, traditional protection schemes based on different fault features are analyzed. The selection of fault features is discussed. Moreover, the cluster center of historical fault data is calculated to analyze the space distribution of fault data for each feeder by applying a fuzzy clustering algorithm. The space relative distance between the online sample data and the cluster center and the minimum value of the space relative distance among all of the relays at the relay agent are obtained. Finally, the coordination strategy of the relay agent is proposed to discriminate the faulty feeder. The proposed protection scheme is evaluated in a radial distribution network with distributed generations using PSCAD simulation.

Low-voltage (LV) feeder voltage magnitude and unbalance are often the constraining factors on a feeder's capacity to absorb rooftop photovoltaic (PV) generation. This paper presents a new probabilistic method for voltage management in distribution networks through the placement of distribution static compensators (dSTATCOMs) and on-load tap changers (OLTCs) considering the reactive capability of PV inverters in multiple LV and medium-voltage distribution networks. The method uses a modified particle swarm optimization. In this paper, several scenarios for the placements of multiple dSTATCOMs with and without embedded energy storage systems using both reactive and real power compensation are investigated in combination with an OLTC equipped with independent per-phase tap-changing control. The voltage constraints in the proposed method are statistically defined using three duration curves. These are the voltage unbalance, maximum voltage, and minimum voltage duration curves. The method is comprehensively tested for varying load and PV generation based on data from a real Australian distribution network with considerable unbalance and distributed PV generation. The results show that PV hosting capacity increases where the proposed approach is applied.

This paper presents a novel online diagnostics method capable of detecting winding deformations in two-winding single-phase transformers. The main idea is to identify changes in the short-circuit impedance. The combination of 3-D Lissajous curve methods with a Butterworth low-pass filter allows for the accurate determination of winding deformation of large power transformers in real time. The method is very robust and capable of detecting deformations at the early stage even when the measurements are noisy. Only information already available to the differential protection relay is needed. The proposed diagnostics method has been validated with circuit and finite-element simulations plus a lab experiment. The results show that the proposed online diagnostics technique has the ability to identify winding deformation problems under severe conditions, such as nonsinusoidal input, nonlinear loading, and measurement noise. Under ideal conditions (no signal noise), the inductive identification error of the proposed online diagnostics method identifies the parameters with less than 0.09% error. When accepting a measurement noise of 1%, the error on the identification of inductance is less than 0.13%.

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.

Typically, in cable system modeling for power system transients studies, the ground return effect is taken into account in the per unit length impedance, its effect on the admittance being neglected. In this paper, the per unit length impedance and admittance matrices for a cable system are established, based on the approach previously presented by the authors for the case of a single insulated cable in a dispersive medium. The presented formulation is derived from a full-wave approach, assuming quasi-TEM propagation which was proved suitable for the frequency range of interest. A comparison of the propagation modes indicates that the inclusion of the ground return admittance affects significantly the ground and intersheath modes in the high frequency range, i.e., above a few kilohertz. Test cases where the different modes are excited are used to analyze the impact of including the ground return admittance in computing the transient response of underground cable systems. The results indicate that this impact increases for high resistivity soils and shorter cables.

Subsynchronous control interaction (SSCI) is the interaction between the power-electronics control and the series-compensated transmission system that occurs at frequencies below the system nominal frequency. SSCI may occur between the doubly-fed induction generator (DFIG) control system and the series-compensated transmission line, to which the wind park (WP) is connected. Not only do the DFIG control system parameters, but also the WP operating conditions have a significant impact on SSCI. In this paper, the impact of WP operating conditions and DFIG control system parameters on SSCI is analyzed in detail. Guidelines are presented for modifying the DFIG control system parameters to ensure safe operation and acceptable transient responses due to faults. This paper also examines the accuracies of various analytical tools used for SSCI problem identification and proposes a new frequency scan analysis approach for accurate prediction of potential SSCI problems.

With an increased focus on renewable power generation, dynamic thermal line rating is being investigated as a way to connect the new intermittent generation to the grid without needing to build additional infrastructure. Traditionally, dynamic thermal line rating is only applied to thermally limited lines. This paper expands this notion by taking advantage of the relationship between temperature, conductor resistance, and voltage drop across a line; to increase the rating of non-thermally limited lines. This investigation is carried out with changes to the thermal limit to demonstrate how the intersection between thermal and voltage limits changes when resistance is considered. The sensitivity of the rating to different environmental parameters and the impact of changing the voltage limit are also investigated.

This paper proposes a novel damping controller for subsynchronous resonance (SSR) mitigation using existing 11-kV variable frequency drive interfaced auxiliary power plant loads. The input of the proposed auxiliary damping controller is the turbine output power $(P_{t})$ . This is a standard signal that is monitored in power plant control rooms and is available locally without the need for additional measurement and/or communication infrastructure. The auxiliary damping controller (ADC) adds an auxiliary speed signal to the existing speed reference in the closed-loop motor drive control in response to any torsional range oscillations seen in $P_{t}$ , via a feedback compensator. The ADC extracts damping by exploiting SSR load interactions and is tuned using a residue-based pole placement technique. The ADC performance is evaluated for both torsional interaction and torque amplification types of SSR in the IEEE First Benchmark and IEEE 68-bus networks. The results show that the ADC is effective in providing positive damping to mitigate unstable SSR oscillations under a range of operating conditions. The proposed solution is a simple yet effective means of providing local control of SSR with minimal additional cost.