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Multiple strokes are the main factors affecting the safe and stable operation of power equipment. As the core lightning protection equipment, arresters are also frequently thermally collapsed due to multiple strokes. This paper investigates thermal effects of polymer-housed metal-oxide surge arrester (PMOSA) under multiple strokes. The comparative experiments of PMOSA lightning impulse are carried out through multiple strokes of experiment platform, including continuous lightning stroke and multiple strokes. Furthermore, the temperature distribution of the housing surface is obtained by infrared imager for thermal effect analysis. Subsequently, some typical DC parameters are tested to explore the association of impulse aging and thermal effects of the arrester under multiple strokes. Finally, a thermal-electric coupling model (TECM) is constructed to investigate the influence of some factors on the temperature of PMOSA from the perspective of materials and structure. The research in this paper can provide a theory reference and brief data support for improving the heat dissipation performance of arresters under multiple strokes based on structural optimization design and material modification. [ABSTRACT FROM AUTHOR]

This paper provides a pragmatic solution to the challenge of testing fault current interruption of high-voltage direct current (HVDC) circuit breakers (CBs). The critical parameters in the design of a test circuit capable of supplying the necessary stresses: current, energy, and voltage (both during and after interruption) are discussed. In addition, a practical implementation of a test circuit based on ac short-circuit generators operated at low power frequency, which is capable of testing the current interruption performance of the proposed technologies of HVDC CBs, is discussed. Tests validating the proposed method and circuit have been conducted on a prototype of an HVDC CB and the test results are presented. Since the performance of some technologies of HVDC CBs can depend on the magnitude of the interrupted current, four test duties are defined and demonstrated in the paper. Moreover, testing of HVDC CBs using ac short-circuit generators poses new challenges, such as the application of dielectric dc stress after current interruption and the protection of both the test-object as well as the test-circuit components when the HVDC CB fails to interrupt. Methods to overcome these challenges are developed and practically demonstrated in a test laboratory. Finally, taking into account the available resources of the author's test laboratory, the capability to test multiple series-connected modules of different technologies of HVDC CBs is verified and example cases are demonstrated. Six short-circuit generators (13 500 MVA @ 50 Hz) and up to ten step-up transformers (up to 550 kV) were actually used. [ABSTRACT FROM AUTHOR]

The short-circuit response of inverter-interfaced distributed generators (IIDGs) is not adequately represented in many conventional protection studies. This paper presents an in-depth analysis of IIDG behavior during grid faults and proposes a more accurate reduced-order parameterized short-circuit current (RPSC) model of inverters. Typical inverter components are thoroughly investigated to identify those that play dominant roles during faults. The paper shows that the current limiter is the dominant factor for the steady-state fault-current of inverter; while the inverter filter along with the severity of the voltage disturbance largely determine the initial transient spike of inverter fault current. The proposed model is low-order and can be used in large scale simulations. The parameters of the proposed RPSC model can be extracted from laboratory experiments without requiring proprietary manufacturer information. The proposed fault-current model is analogous to the well-known synchronous machine model that segregates the inverter fault current into subtransient, transient, and steady-state fault currents. Experimental and simulation tests are presented to validate the model. [ABSTRACT FROM AUTHOR]

Autonomous power systems with a single power station and a high-voltage (HV) network may be susceptible to overvoltage instability, especially during light-load conditions. Moreover, in low inertia systems, considerable frequency excursions may be observed after disturbances. To deal with voltage and frequency stability problems in such isolated power systems, several measures should be taken, such as rules for adequate automatic voltage control of the synchronous generators and load-tap-changers (LTCs), and/or adequate design of actions applied during emergency system conditions such as reactors switching, and redesigning underfrequency load shedding taking into account overvoltage problems. The effectiveness of the proposed countermeasures against overvoltage instability in autonomous power systems is illustrated in this paper. In 2016, the autonomous power system of the Greek island of Rhodes experienced similar voltage/frequency problems leading to a blackout. This event has been accurately reconstructed in this paper. The application of the suggested countermeasures during the simulation of the 2016 blackout event in Rhodes, further highlighted the effectiveness of these measures, providing a guideline for similar cases. [ABSTRACT FROM AUTHOR]

Electrical equipment of the floating nuclear power plant operates in a long-term vibration environment. Ground faults occur frequently and are often accompanied by arc, a reliable arc suppression algorithm for the generator stator winding is necessary. In this paper, an active arc suppression method is proposed by connecting a controllable voltage source to the generator neutral point. Through regulating the neutral point voltage to be equal in amplitude and opposite in phase to the fault potential, the fault point voltage can be clamped to a low level to achieve arc suppression. Since the generator's phase potential contains certain harmonic components, the controllable voltage source includes fundamental and third-order harmonic components. To accurately calculate the output voltage, online calculation methods for the fault location, fault potential and grounding capacitance are proposed. The accuracy of the fault location method is verified by simulation and dynamic test. And simulation results based on PSCAD/EMTDC software show that the proposed method can achieve effective arc suppression at different ground fault conditions. [ABSTRACT FROM AUTHOR]

With the increase of photovoltaic penetration, the power-electronic converter (PEC)-interfaced generators with traditional control strategy may cause severe power oscillations after being disturbed due to lack of inertia and damping. The photovoltaic power plant controlled by virtual synchronous generator (VSG) technology, while introducing virtual inertia and virtual damping to improve the stability of the system, will also inherit its oscillation characteristics and cause a new oscillation mode. Aiming at this problem, this paper analyzes the difference between the system low-frequency oscillation (LFO) modes before and after the VSG is connected to the two-area system by the modal analysis. It is pointed out that the PEC-interfaced renewable energy generators based on VSG control technology could change the oscillation mode of the original system. The influence of VSG's main parameters and capacity on system dynamics is analyzed. Finally, a synchrophasor-based inter-area oscillation suppressed comprehensive control strategy is proposed to suppress the LFO of active power on the tie line, and the superiority of the strategy is verified by DIgSILENT /PowerFactory. [ABSTRACT FROM AUTHOR]

The risk of unstable subsynchronous oscillations (SSO) in DFIG-based wind parks radially connected to transmission lines with series capacitive compensation is typically assessed with numerous electromagnetic transient (EMT) simulations, which is a time-consuming process. However, not all operating scenarios present risk of SSO and require such a detailed investigation. This paper proposes two charts based on impedance equivalents of wind generators obtained through frequency scans to quickly map which combinations of parameters can create unstable SSOs and which combinations are risk-free. With the information, engineers can immediately filter out scenarios with no risk of instability and conduct detailed studies on fewer scenarios that are truly critical. These charts can be obtained with simple equations that use only information readily available in practice to engineers, without the need to run any computer simulation. The charts also reveal the key operating conditions that can increase the risk of an unstable SSO and provide real-time quantitative insights into how close a wind park is to an instability. Application examples are provided to illustrate how this approach can help engineers speed up studies to identify the risk of SSO. [ABSTRACT FROM AUTHOR]

Metal-oxide arresters (MOA) are widely used to protect equipment against transient electromagnetic disturbances (TED) like very fast transient overvoltage (VFTO), high-altitude electromagnetic pulse (HEMP), lightning pulse, et al. Researches on the performance of MOA under microsecond level rise-time TED are abundant, such as lightning pulse. However, when MOA is subjected to TED of rise-time at nanosecond level, the voltage/current responses of MOA are not fully studied due to the lack of a standardized and reliable test setup. In this paper, a test setup is proposed for acquiring the response behavior of MOA under nanosecond level pulse. The setup is comprised of a pulse generator, a two-branch transmission line system and a high voltage power divider. The generator can provide stable double exponential pulse with tunable rise-time ranging from 5 ns to 100 ns. The divider ensures an even distribution of voltage/current surge wave along the two-branch line. Through the numerical analysis, both impinging and residual surge waveforms could be retrieved from measurements. The proposed method has been applied to investigate response behavior of MOA subjected to nanosecond level rise-time pulses. [ABSTRACT FROM AUTHOR]

The penetration of inverter-based resources is growing rapidly around the globe. During a fault, the behavior of inverter-based resource is very different from a traditional synchronous machine based generation. As the penetration of inverter-based resources increases, there is an urgency for the industry to understand their behavior so that these resources can be accurately modeled and their impact on various protection schemes can be studied in detail. This path begins with standardizing the response of inverter-based resources to a fault. Additionally, this paper discusses issues with modeling inverter-based resources in a short circuit model which is developed for systems dominated by synchronous machines and finally a need for further research to understand impacts on traditional protection schemes as inverter-based resources continue to displace traditional generation. This paper does not offer solutions however, it is intended to encourage further study by industry professionals toward development of solutions. [ABSTRACT FROM AUTHOR]

As the weakest components of wind turbine converters, dc-link capacitor withstands high-frequency ripple currents from both rectifier and inverter. Traditional wind turbine control strategy does not consider the influence of the harmonic current generated by both side converters on the dc-link capacitor, and ignores the restriction on the reliability of direct-drive wind turbine converter. For the reason, this paper synthesized the wind turbine model and dc-link capacitor harmonic spectrum analytical approach to derive the connection between mission profile and harmonic spectrum. Regarding with the capacitor lifetime model, the optimal mapping from control strategy to mission profile, maximizing the service life of capacitor, was solved. With the proposed health-oriented control strategy, the core temperature of dc-link capacitor can be effectively reduced, which can increase the service life of capacitor, making the dc-link capacitor no longer the most fragile component in wind turbine converter and ultimately improving the reliability of converters. [ABSTRACT FROM AUTHOR]

This paper presents frequency domain (FD) Norton-type equivalents of single- and three-phase photovoltaic (PV) generators, aimed at transient calculations. The proposed equivalents can be interfaced with a general frequency-dependent network and/or nonlinear loads and include voltage/current sources representing initial conditions aimed at partitioned-time transient simulation. The transient solution is performed via the numerical Laplace transform (NLT) in a sequential scheme by using time sub-windows with corresponding initial conditions and using reduced number of samples. The proposed approach overcomes the issue of computationally heavy simulations as in classical implementations of NLT. Two case studies involving PV generator and nonlinear load are presented to illustrate the performance of the proposed FD PV models. [ABSTRACT FROM AUTHOR]

Determining the Marx generator configuration, including the values of series and parallel resistors as well as the number of series and parallel stages to obtain standard lightning impulse (LI) waveforms, is usually done in a trial and error process in the high voltage test fields of power transformer manufacturers. This practical process may take excessive time and is boring for test engineers. In this paper, a simple simulation method is proposed to model the LI test circuit of the transformer by computer, which is helpful in identifying the Marx generator set up. This method is based on modeling the LI test circuit as well as the transformer's high frequency impedance in discrete frequency domain. The measured frequency response of the transformer is used to model its high frequency characteristics. Digital signal processing methods are used to simulate the circuit and obtain waveforms. The accuracy of the proposed method is validated using experimental tests performed on three transformers with different ratings. [ABSTRACT FROM AUTHOR]

The unbalanced nature of distribution systems requires a multiphase load-flow solution capable of handling arbitrary network topologies and providing accurate results. The need for detailed analysis of secondary grid systems found in dense urban areas and the modeling of distribution networks including the subtransmission level requires using highly efficient and large-scale system-capable methods. In this paper, three different load-flow solution algorithms are presented using the modified-augmented-nodal-analysis formulation. The load-flow solution algorithms are compared for the IEEE 8500-node distribution test feeder using a proposed regulator tap control strategy. [ABSTRACT FROM PUBLISHER]

This paper presents original records of distributed-generation unintentional islanding events in three categories. The islands were formed by naturally occurred faults without the transfer trip initiated to generation facilities. Hydraulic, natural gas, and wind generators were involved in the events. The records exemplify important issues in DG anti-islanding planning, such as coordination with underfrequency (or undervoltage) load shedding, reactive power compensation, generation-to-load ratio, reclosing coordination, etc. The events reported in this paper can serve as first-hand references in applying IEEE Standard 1547 to distributed generation anti-islanding planning and operation. [ABSTRACT FROM PUBLISHER]

Voltage imbalances are one of the most severe challenges in electrical networks, which negatively affect their loads and other connected equipment. This paper proposes a voltage support control strategy to mitigate the voltage imbalance in inverter-based low voltage distribution networks. The control scheme is derived taking in mind the following control objectives: a) to increase the positive sequence voltage as much as possible, b) to decrease the negative sequence voltage as much as possible, c) to inject the power generated by the primary source, and d) to minimize the output current of the inverter. The innovative contribution of the proposed solution is based on the design of a control algorithm that meets the aforementioned objectives without resorting to communications with other grid components. The theoretical results are experimentally validated by selected tests on a laboratory setup with X/R ratio close to one. [ABSTRACT FROM AUTHOR]

The possibility of operating distributed generation (DG) in islanded mode brings several benefits but still faces some problems, especially related to the transition from a grid-connected to an islanded operating condition. When connected to the main grid, DG units operate in PQ mode. During this transition, their controls must be changed to regulate frequency and voltage. The delay for islanding detection, associated with abrupt changes in the control references, may result in large and detrimental transients. This paper proposes a new control structure for a distributed synchronous generator (DSG) to allow a smooth transition from the grid-connected to the islanded operation mode. The proposed control structure uses a compensation loop that was conceived based on the main ideas of the anti-windup bumpless transfer (AWBT) theory (although it is not, strictly speaking, an AWBT controller). The results show that the proposed structure ensures smaller excursions of the DSG variables during the transition and, therefore, the possibility of a successful formation of the islanded system is extended to a wider range of operating conditions. Moreover, with the proposed control, the success of this transition does not depend on the islanding detection time, which is the main contribution of this paper. [ABSTRACT FROM AUTHOR]

This paper presents a new approach in adaptive out-of-step (OOS) protection settings in power systems in real time. The proposed method uses extended equal area criterion (EEAC) to determine the critical clearing time and critical clearing angle of the system, which are vital information for the OOS protection setting calculation. The dynamic model parameters and the coherency groups of the system for EEAC analysis are determined in real time to ensure that the newly calculated settings suit with the prevalent system operating condition. The effectiveness of the method is demonstrated in simulated data from a 16-machine 68-bus system model. [ABSTRACT FROM PUBLISHER]

The integration of high bandwidth energy-storage systems (ESS) in compact dc electrical power systems can increase the operational capability and overall flexibility of the network. However, the impact of ESSs on the performance of existing dc protection systems is not well understood. This paper identifies the key characteristics of the ESS that determine the extent of the protection blinding effects on slower acting generator systems on the network. It shows that higher fault impedances beyond that of an evaluated critical level will dampen the response of slower-acting generator systems, decreasing the speed of corresponding overcurrent protection operation. The paper demonstrates the limitations of existing protection solutions and identifies more suitable protection approaches to remove/minimize the effects of protection blinding. [ABSTRACT FROM AUTHOR]

The power system planning and protection studies are becoming more challenging due to the rapid increase in penetration levels of converter-interfaced renewables. Type-IV wind turbine generators (WTGs) and photovoltaic panels are interfaced to the grid through a full-scale converter, and their short-circuit current contributions are mainly designated by the converter control and associated current limits. This paper proposes a new phasor domain modeling approach for the wind parks (WPs) with Type-IV WTGs, using the concept of control-based equivalent circuits. The proposed model precisely represents the detailed electromagnetic transient (EMT)-type model in steady state, and is able to account for the fault-ride-through function of the WTG control as well as its specific decoupled sequence control scheme in addition to the traditional coupled control scheme. Although the collector grid and WTGs inside the WP are represented with their aggregated models, the overall reactive power control structure of the WP is preserved by taking the central WP controller into account. The accuracy of the proposed model is validated through detailed EMT simulations. [ABSTRACT FROM AUTHOR]

The increased penetration of wind power has made its low voltage ride through (LVRT) capability more crucial than ever. Currently, the LVRT performance of wind generators is verified using experimental voltage sags with different magnitudes and durations. The actual fault, however, also causes the phase angle jump (PAJ) of the grid voltage. In this paper, the impact of PAJ on the LVRT capability of doubly fed inductor generator (DFIG) under three-phase fault is analyzed. To achieve this, detailed analytical calculations and experimental verifications are conducted. The results demonstrate that the PAJ imposes more challenges on the LVRT by intensifying the rotor overcurrent through the natural stator flux. As a result, DFIGs that have passed the existing LVRT test may still fail to comply with the grid code requirements in practice. To tackle this issue, an improved test procedure is recommended. The new procedure considers the impedance angle of the target system as an additional test condition, thus can reveal the LVRT performance of DFIG under the potential PAJ. [ABSTRACT FROM AUTHOR]

Utility engineers, professionals from regulatory agencies, manufacturers and researchers have discussed if high-frequency distortions produced by rooftop photovoltaic generators are a general power quality concern for distribution systems. In this context, this paper presents the results of an extensive research to investigate the emission and propagation characteristics of such high-frequency distortions. Analytical and theoretical analyses, computer simulations, laboratory experiments and field measurements revealed that high-frequency distortions emitted by rooftop photovoltaic inverters are produced with a voltage source characteristic. As a result, the equivalent grid impedance limits and attenuates significantly the high-frequency distortions injection and propagation into the distribution systems. Moreover, the front-end, low-pass filters found in many home appliances also contribute with such attenuation by sinking part of the high-frequency distortions. In summary, the results indicate that these distortions are unlikely to become a generalized power quality concern, so that the front-end filters commonly available for such devices, when properly designed, are enough to deal with this issue. [ABSTRACT FROM AUTHOR]

Power generators contain control systems that are able to regulate system frequency, but the frequency setpoint values are rarely modified from nominal values. This paper describes design considerations for a communication system from generators to frequency-sensitive distributed energy resources (FS-DER) using changes to frequency setpoint values of generators. Signaling discrete system states by generating off-nominal system frequency values can be used as a novel narrowband unidirectional broadcast communications channel. This paper describes two protocols for utilizing off-nominal frequencies to carry information: First, a protocol for dispatching blocks of FS-DER that is suitable for systems restricted to relatively slow rates of change of frequency (ROCOF). Second, for systems that allow higher ROCOF values, the feasibility of using power generation resources as a power-line communication transmitter is shown. Data from an operating islanded power system with diesel generators are analyzed to demonstrate the feasibility of the proposed communication system in systems fed by rotating machines. The feasibility of the proposed communication system in systems fed by voltage-source inverters is shown with laboratory tests of a 20-kVA inverter. The inverter was found to have a maximum ROCOF of 2.2 Hz/s, sufficient to enable its use as a power-line communication transmitter. [ABSTRACT FROM AUTHOR]

This paper applies a con-resistant trust mechanism to improve the performance of a communications-based special protection system to enhance its effectiveness and resiliency. Smart grids incorporate modern information technologies to increase reliability and efficiency through better situational awareness. However, with the benefits of this new technology come the added risks associated with threats and vulnerabilities to the technology and to the critical infrastructure it supports. The research in this paper uses con-resistant trust to quickly identify malicious or malfunctioning (untrustworthy) protection system nodes to mitigate instabilities. The con-resistant trust mechanism allows protection system nodes to make trust assessments based on the node's cooperative and defective behaviors. These behaviors are observed via frequency readings which are prediodically reported. The trust architecture is tested in experiments by comparing a simulated special protection system with a con-resistant trust mechanism to one without the mechanism via an analysis of the variance statistical model. Simulation results show promise for the proposed con-resistant trust mechanism. [ABSTRACT FROM PUBLISHER]

Timely identification of the size and location of loss of generation (LoG) events improves the effectiveness of remedial actions taken against this type of disturbances. This paper sets forth a novel method for LoG localization and size estimation by phasor measurements received in the control center within a reasonable wait time following the event inception. The bus impedance matrix is utilized to obtain a transfer function from the LoG location to variations of voltage and current phasors following the event. This results in an overdetermined system of linear equations whose closed-form solution provides the LoG location and size based upon the sum of squared residuals concept. The proposed method removes the need for the reception of specific measurements in the control center, and/or knowing the system inertia. This is in contrast with previous methods resorting to the swing equation of the center of inertia. The method lends itself to real-time applications for its robustness against partial communication network failures and losses of the time synchronization signal. Extensive simulations conducted on the IEEE 39-bus and 118-bus test systems verify the effectiveness and superiority of the proposed method over existing ones. [ABSTRACT FROM AUTHOR]

This paper proposes a transient stability constrained controlled islanding (TSC-CI) approach against cascading events in power systems. Using the wide area measurements, the proposed islanding algorithm is implemented regarding the actual dynamic conditions of the resulted islands. Since the main concern, right after the execution of controlled network splitting, is the transient stability of synchronous machines, a transient energy function is utilized for proper network partitioning. The proposed transient stability criterion is expressed as a function of the transfer impedance between each pair of generators in the resulted islands. Transfer impedance between each pair of coherent generators determines the direction of network splitting for transient stability improvement. A multi-objective function is then introduced to provide the transient stability with least possible power imbalance, as a resilience measure, over the created islands considering operational and structural constraints. Based on the wide-area measurements, a proper weighting procedure is utilized to prioritize the critical islands, avoiding the unnecessary power imbalance in favor of transient stability improvement. The proposed TSC-CI model is formulated as a mixed integer linear programming (MILP) multi-objective model and is solved using CPLEX in GAMS. The obtained results are compared with the conventional islanding scheme using the IEEE 118-bus system. [ABSTRACT FROM AUTHOR]

Natural calamities always have been a serious threat to energy systems. In this regard, this paper constitutes a stochastic mixed integer linear programming (SMILP) model to enhance the resilience of power distribution systems to deal with disastrous events. In particular, the proposed model is developed to enhance both survivability and restoration capability of distribution systems. In this regard, to increase the preparedness of the power distribution system, the system operator reconfigures the network by utilizing remote-control switches (RCSs), manual switches (MSs), and distributed generations (DGs) before the natural calamity hits. The proposed model contemplates the traveling time of crew teams to the MS sites in the transportation system in order to achieve the best switching sequence. In addition, it pre-positions the crews and mobile emergency generators (MEGs) in staging locations to hasten the likely post-disturbance operations. To do so, likely post-event operations are included in the model using the scenario generation technique. To validate the performance of the developed model a distribution system is employed. The results of simulations confirm the effectiveness of the proposed approach in declining the interruption of electric energy for customers. [ABSTRACT FROM AUTHOR]

Impulse corona characteristics are very important for the analysis of overvoltage along overhead transmission lines. For DC line, the space charge due to normal corona may affect the impulse corona characteristics. The paper develops a circuit for combined application of DC and impulse voltage to measure the effect of space charges on impulse corona characteristics along DC conductor. Based on the circuit, a test platform with a corona cage was established. By measuring the q-u curve, the impulse corona onset voltages under different DC pre-voltages were obtained. It was proposed that the space charges would affect the corona onset voltage. The space charges of the opposite polarity would weaken the corona and reduce the impulse corona loss, which is beneficial to the overvoltage wave propagation. While the space charges of the same polarity would enhance the corona and increase the impulse corona loss, which is not conducive to the overvoltage wave propagation. In addition, the influence of wave front time and conductor radius on the impulse corona characteristics was measured. The test results are important references for the transient overvoltage protection of overhead HVDC lines. [ABSTRACT FROM AUTHOR]

The stator ground fault location of large generators can guide rapid maintenance. In this paper, a location criterion without fault transition resistance is constructed by using the fundamental potential and the third harmonic potential of the faulty winding simultaneously. The above potentials of the faulty winding are corresponding to the fault position. To precisely locate the grounding fault, it is necessary to clarify the analytical relationship between the faulty winding potential and the fault position. However, the existing methods often take the coil potential as the unit to analyze the winding potential, which has theoretical errors for the generators with short-pitch winding. Aiming at this problem, a more precise analysis method for winding potential is established by taking the slot conductor potential as the unit. To solve the location criterion, the optimization model for each slot conductor of the faulty phase is constructed. The slot conductor with the smallest optimal objective function value is judged as the faulty slot conductor, and the corresponding decision variable is judged as the fault position. Simulation and experiments verify the effectiveness of the proposed method, which can still meet the needs of engineering applications under multiple adverse conditions. [ABSTRACT FROM AUTHOR]

Geomagneticallyinduced current (GIC) studies usually focus on transformer response to dc but seldom analyze generator excitation systems. A GIC is driven by an electric field which is induced by a very low-frequency geomagnetic field. It is widely acceptable to model GICs with dc, mostly suitable for static voltage stability analysis, but this does not adequately characterize a power system's dynamic response. Considering this, we studied a multi-machine power system on an OPAL-RT digital real-time simulator (DRTS)-MATLAB/Simulink system. We used real GIC measurement data from high and mid-latitude regions. Wavelet analysis related the frequency components of the GIC to the power system's response. A dynamic generator model with excitation voltage control modelled the source. Our main finding is that excitation system control contributes to reducing the voltage drop caused by increased var demand in saturated transformers with GICs. Measured GICs revealed non-negligible short-term dynamics affecting bus voltages that are not captured with dc-modelling assumptions, yet they are important for voltage stability analysis. This study implies that excitation control creates a buffer for high GICs to flow without violating the grid codes for minimum operating voltages and this paper demonstrates the optimization of this novel approach. [ABSTRACT FROM AUTHOR]

This paper presents a new method for measuring grounding resistances, which is based on the measured net charge drained by the grounding system when excited by an impulse generator. The circuit property used by the method is briefly described, alongside with the method itself. An advantage of this method with respect to the traditional ones is that it is not affected by the self and mutual reactances present in the grounding system and in the measuring wires. Moreover, the proposed method does not require the measurement of surface potentials, which are prone to interference due to soil non-homogeneities, buried conductive bodies, and stray currents. This paper presents experimental results from the method using lumped components and using actual grounding systems. The results show that the method provides consistent and accurate values of grounding resistances. [ABSTRACT FROM AUTHOR]

Third-harmonic based schemes are used to detect stator-ground faults in high-impedance grounded generators. These methods use third-harmonic voltage measures in the neutral and in terminals to detect faults. As third-harmonic voltage levels are different according to each generator and operational point, simulations, studies and tests need to be carried out to ensure that the chosen third-harmonic scheme is able to cover the portion closest to neutral while the conventional method (59GN) covers the portion closest to the stator winding terminals. This paper presents the solution of the third-harmonic generator model used for a stator-ground fault study in terms of the mesh currents and uses the derived equations to develop a third-harmonic based fault location method. The fault location method was tested over 1785 simulations and presented a maximum error of 0.004 p.u., determining the fault location accurately. [ABSTRACT FROM AUTHOR]

This paper proposes a systematic approach to simulating stochastic electromagnetic transients in emtp, through the reliable numerical solution of stochastic differential equations. Actually, numerical integration schemes for stochastic differential equations (sdes) are not available in emtp. It is shown how to set up the Semi-Implicit Backward Euler scheme specific for stochastic differential equations. The implemented interaction strategy between the engine for the solution of electrical circuits and the engine for the Transient Analysis of Control Systems of emtp is exploited. This is a bolt from the blue feature of this simulator that makes it the only one on the shelf able to handle stochastic differential equations. Exploiting this unveiled capability, some basic and tutorial case studies are presented. They pave the way for the simulation of more complex circuits and systems that go beyond the scope of this work. Simulation files are provided as supplementary material and they are available on GitHub. [ABSTRACT FROM AUTHOR]

The paper presents a new adaptive 100% stator ground protection for high-impedance-grounded (HIG) synchronous generators. This is done by first examining reliability, that is, dependability and security of the most prominent conventional 3rd-harmonic voltage schemes. Using field data from various capacity industrial generators, it is shown that the conventional 3rd-harmonic differential voltage scheme, while being dependable, lacks security. Using the same field data, it is shown that the new adaptive scheme is a highly secure and sensitive scheme. Moreover, the new adaptive scheme is a zero-setting scheme which does not require any setting calculations, 3rd-harmonic voltage testings, or additional hardware. [ABSTRACT FROM PUBLISHER]

This paper describes the advantages of using nonsynchronous microgrids in networked systems containing densely concentrated loads. The nonsynchronous bus arrangement, in addition to allowing for the integration of substantially larger distributed generation, completely isolates transient disturbances from and to the network and the microgrid. Significant is the fact that distributed generators installed in the microgrid do not contribute to the short-circuit current that needs to be interrupted by the substation breakers. The behavior of the grid and the microgrid is investigated by comparing: the occurrence of faults, voltage reduction, and losses, in the presence and absence of the microgrid. The benefits of the dc microgrid are made evident with steady-state and transient studies performed on a real distribution network in New York City. [ABSTRACT FROM PUBLISHER]

This paper presents a noninvasive and easy-to-implement technique using phasor measurement units for accurate estimation of subsynchronous torsional frequencies. This information is relevant for the optimal design of high-voltage direct-current (HVDC) subsynchronous damping controllers that enhance the effect of HVDC on subsynchronous damping. The method is rigorously justified using mathematical proofs as well as thorough electromagnetic-transients simulations. The method was implemented in the Finnish transmission network and proved to be effective. [ABSTRACT FROM PUBLISHER]

In hydroelectric power plants, which are characterized by particularly high short-circuit current levels and high time constants, conventional current transformer 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper 1) introduces a new approach for dependable and secure detection of loss-of-field (LOF) events based on estimating rotor signals, 2) implements digital algorithm of the corresponding LOF relay in an industrial hardware platform, and 3) evaluates performance of the LOF relay in a real-time-digital-simulator (RTDS) based control-hardware-in-the-loop (CHIL) environment. The main features of the proposed relay as compared with the conventional approaches include 1) capability to discriminate between system disturbances and LOF events, and 2) dependability and higher speed of the LOF detection mechanism. To evaluate the proposed approach performance for various types of LOF events, a detailed excitation system is modeled and implemented in the RTDS-based CHIL environment. This paper also verifies superior performance of the proposed LOF relay as compared with that of a conventional impedance-based LOF relay, using the RTDS-based CHIL platform. [ABSTRACT FROM PUBLISHER]

Power-electronic sources, such as Type 3 wind turbine generators and static synchronous compensators (STATCOM), interface to the grid through, partial- or full-scale, and power converters that have inherently fast switching capability to control their output current during short circuits. The short-circuit current is a function of the specific converter control algorithm and differs significantly from the conventional rotating machine sources without converter interfaces. Therefore, if a transmission-line protection scheme is designed for conventional sources, (not taking into account these differences in short-circuit current characteristics), reliability can be at risk. Using real-life short-circuit currents on lines supplied by sources having a power converter interface, this paper illustrates the reliability risk to conventional line protection schemes, in particular, to those which use negative-sequence quantities for the detection of unbalanced faults. This paper discusses the protection schemes, adopted by BC Hydro–a large Canadian Electric Utility, for transmission lines interconnecting Type 3 wind turbine and STATCOM sources. Their application for ground faults is independent of the converter control algorithm as long as the source is interconnected to the grid via a transformer which is a source of zero-sequence current. [ABSTRACT FROM PUBLISHER]

This paper proposes a new passive Islanding detection approach for the Inverter Interfaced Distributed Generators (IIDGs) operating under different control modes. First, a novel index is formulated using Root Mean Square (RMS) value of the voltage and the frequency information, acquired at the Point of Common Coupling (PCC) of the target DG. The index is then post-processed by estimating the number of peaks and valleys $(N_{p_{k}-v})$ in a cycle using the sliding window concept, which indicates the occurrence of an event. Finally, the nature of the event, i.e., Islanding or non-Islanding, is decided by exploiting the two statistical features viz., Square of RMS $(S_{RMS})$ of $N_{p_{k}-v}$ and Average Crest Factor $(ACF^{m})$ of the proposed index. The performance of the proposed method (P-IDM) is demonstrated through comprehensive studies in PSCAD/EMTDC for the single-bus and IEEE 34-bus Active Distribution System (ADS). Also, the real-time validation is performed for the IEEE 34-bus using Controller Hardware-in-the-Loop (CHIL) setup comprising of Real-Time Digital Simulator (RTDS) and dSPACE1104. The simulation and hardware results for different test systems indicate that the proposed scheme, being generic in nature, sustains its efficacy under various interface controls of IIDGs. Finally, a comparative assessment with the other existing Signal Processing (SP) based passive methodologies shows the supremacy of the proposed method in terms of accuracy and faster detection. [ABSTRACT FROM AUTHOR]

Cables short circuit withstand-capacity plays a crucial role in medium voltage distribution system design. For a given short circuit level at substation, fault current at any point in a radial distribution system depends on the location with respect to the substation, distributed generation as well as size and length of upstream cable sections. This paper addresses optimal routing, cable sizing, distributed generation allocation and sizing of radial medium voltage power distribution network considering cables short circuit withstand capacity. Both synchronous-based and inverter-based distributed generators are considered. Complete formulation of cables short circuit constraints (SCCs) is introduced in mixed-integer linear programming (MILP) form. A Benders-decomposition-based approach is used to solve the problem by considering short circuit at each system bus in a separate slave feasibility problem. Combinatorial cuts are proposed to exclude infeasible solutions in each iteration by investigating upstream routes, cable sizes and distributed generation. The proposed optimization framework and solution algorithm are applied on 54 bus, 84 bus and 138 bus systems. The results revealed potential saving in investment, maintenance and losses costs compared to upgrading method. Also the proposed solution algorithm showed great saving in the number of added constraints to the original problem. [ABSTRACT FROM AUTHOR]

Integration of wind-based renewable energy sources into long Extra High Voltage/Ultra High Voltage uncompensated/compensated power transmission network poses significant problems in terms of proper detection of power swings and effective discrimination between symmetrical faults and power swing situations. As the existing protection strategies are unable to detect the said circumstances, a novel technique, based on the difference between sending end and receiving end positive sequence currents angles of the transmission line, is proposed in this paper. The required data is collected with the help of Phasor Measurement Units placed on both sides of the line. The scheme results in effective discrimination between distinct faults and circumstances of asymmetrical/symmetrical power swing and achieves satisfactory outcome during current transformer saturation condition. The proposed algorithm is evaluated on the wind-integrated IEEE-9 bus system by producing power swings, various cases of faults, and faults during power swings. Validation of the suggested technique was carried out by the execution of hardware-in-loop simulation on a Real-Time Digital Simulator. The achieved outcomes disclose higher sensitivity and better discriminating ability of the presented technique in comparison with those of numerous prevailing methods. [ABSTRACT FROM AUTHOR]

Loss of excitation (LOE) is one of the most important issues of synchronous alternators, which may be a source of genuine harms and could lead to power system instability. The principle issue to recognize LOE and stable power swing (SPS) is the time delay, which is a primary segment of the recognition time in all ongoing promise strategies. This paper introduces an effective scheme based on the first derivative of equivalent power factor angle (EPFA) of 3-phase voltage and current quantities in the direct-quadrature axis, to detect the LOE. The new algorithm, based on EPFA, is developed to reduce the required time of LOE recognition scheme. Comprehensive simulation studies are accomplished using MATLAB/Simulink as a robust simulation tool for various generator conditions such as full load and sudden load changes in addition to 3-phase faults with different location and duration times. The relay setting is determined and the algorithm performance is evaluated depending on various scenarios in power system. The simulated study results confirm that the proposed method can discriminate SPS from LOE conditions under different modes of operations. [ABSTRACT FROM AUTHOR]

In this paper, a multiagent system-based wide-area protection and control scheme is proposed to deal with the long-term voltage instability-induced cascading trips. Based on sensitivity analysis between the relay operation margin and power system state variables, an optimal emergency control strategy is defined to adjust the emergency states timely and prevent the unexpected relay trips. In order to supervise the control process and further minimize the load loss, an agent-based process control is adopted to monitor the states of distributed controllers and adjust the emergency control strategy. A hybrid simulation platform based on LabVIEW and real-time digital simulator is set up to simulate a blackout case in the power system of Eastern Denmark and to demonstrate the effectiveness of the proposed MAS-based protection strategy. [ABSTRACT FROM AUTHOR]

Fractional Frequency Transmission is a competitive technology for offshore wind power transmission. Modular Multilevel Matrix Converter (M3C) plays a key role in a Fractional Frequency Transmission System (FFTS) as the frequency changer. M3C is broadly considered as the AC–AC converter for the future with its attractive advantages in high voltage and high power applications. Due to the lack of a DC link, electrical quantities at different frequencies from two AC systems couple in M3C, resulting in a complex harmonic condition. Harmonics can lead to stability issues and its analysis is of great importance. This paper focuses on the harmonic analysis of M3C. The arm capacitor voltage ripples and the harmonic currents are analyzed at various frequencies. Major factors influencing the harmonics magnitude are discussed. Analysis is conducted on sub-module capacitance and arm inductance selection. A zero-sequence current mitigation controller for M3C is implemented and tested. It is found that for a FFTS, some current harmonics flow into AC systems even though the system is balanced, while the others circulate within the converter. The theoretical harmonic analysis is verified by simulations in Real Time Digital Simulator (RTDS) of a M3C system where each arm consists of forty sub-modules. [ABSTRACT FROM AUTHOR]

Massive deployment of plug-in electric vehicles (PEVs) in the coming years will create more challenges for the power system including the aging rate of transformers. It will be an essential requirement to propose solutions to minimize the impacts related to the integration of PEVs. Special attention must be given to the residential electric grid where charging will mostly take place. In this paper, first we propose a rule-based (RB) algorithm which determines the minimum charging power levels of home-charged PEVs with/without a charging ban during peak hours. Second, we evaluate the consequences of supplying an RB algorithm on life duration of a low-voltage transformer supplying a residential area. [ABSTRACT FROM AUTHOR]

Synchronous-machine DGs are well known to cause miscoordination of distribution system protections because of the generator's ability to contribute large fault currents to the fault. This paper proposes a field discharge circuit to limit the generator's fault current, thus leading to a synchronous-machine DG with little impact on distribution system protection. In this paper, the operation of a solid-state switch-based field discharge circuit is studied, and its effects on the generator's output current during the fault are investigated. It is shown that the proposed scheme removes the steady-state component of the fault current and accelerates the decay of the transient ac component of the current. The results demonstrate that the proposed field discharge circuit is sufficient to prevent miscoordination of the feeder protections when short time-delay and/or inverse-time overcurrent relays are involved in the protection scheme. [ABSTRACT FROM PUBLISHER]

The traditional overcurrent protection is not suitable for the microgrid. One of the main reasons is that the fault response of the inverter-interfaced distributed generator (IIDG) is very different from that of the rotational generator. This paper develops fault models of IIDGs within a low-voltage microgrid, including active/reactive power (PQ)-controlled IIDGs and voltage-controlled IIDGs. Considering that the control strategy of the PQ-controlled IIDG varies, this paper introduces adjustable parameters into the model to reflect its fault characteristics as comprehensive as possible. Voltage-controlled IIDG is of great significance on maintaining a stable microgrid. The model of voltage-controlled IIDG proposed in this paper keeps its voltage-source nature while accomplishing current limiting. The performance of the proposed IIDG fault models has been tested by Matlab simulations. The achievements of this paper are expected to have a positive impact on the development of microgrid fault analysis and protection. [ABSTRACT FROM AUTHOR]

This paper highlights the importance of chronology on voltage protection and control apparatuses in studies of voltage security in active distribution networks with high generation capacity. This paper presents simulation results of a real-life example of a long rural distribution feeder having three voltage regulators in cascade, and a customer whose demand accounts for a large percentage of the total feeder load. The customer is also an independent power producer and its location at the far end of the feeder yields an intricate voltage protection coordination when the customer ramps up and down its own generation at the peak-demand period. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of modelling of different aspects of wind farms (WF) as a first step towards development of the equivalent harmonic model initially of WF, and later of other types of converter connected generation, for studies of harmonic propagation and mitigation in power electronics (PE) rich transmission networks. The aim of the study was to establish a set of scaling factors that can be used when modelling the entire WF as a single probabilistic harmonic generator in harmonic propagation studies, considering different WF configurations, weather conditions, harmonic cancellation effects, individual wind generator availability, cable disconnection, and the influence of the non-ideal external grid. The importance of modelling resonance and individual characteristic harmonics on the equivalent modelling of WF has also been established. The results facilitate the assessment of global contribution of PE connected RES to harmonic levels in transmission networks and more generally harmonic propagation studies in large PE rich transmission networks. [ABSTRACT FROM AUTHOR]