Search

The purpose of this paper is to propose and analyze the high-performance control strategies for a novel current source converter (CSC)-based wind turbine-superconducting magnetic energy storage system (SMES) hybrid system, which will, in turn, improve the operating performance for the wind farm not only in the grid-connected operation, but also in the islanding mode. The key is to design the effective active power controller and the ac bus voltage controller for the CSC-based hybrid system in grid-connected operation. Thus, better power smoothing and robust operation can be achieved. For the islanding mode operation, the capacitor voltage control scheme is proposed for the CSC hybrid system to offer efficient operation when the wind farm is disconnected from the grid. Based on simulation, the performance of the proposed control for the CSC-type wind turbine-SMES hybrid system in the wind farm has been verified. [ABSTRACT FROM PUBLISHER]

A high-voltage direct current system using modular multilevel converter (MMC-HVDC) is a potential candidate for grid integration of renewable energy over long distances. The dc-link fault is an issue MMC-HVDC must deal with, especially for the nonpermanent faults when using overhead lines. This paper proposed a protection scheme to implement fast fault clearance and automatic recovery for nonpermanent faults on dc lines. By employing double thyristor switches, the freewheeling effect of diodes is eliminated and the dc-link fault current is allowed to freely decay to zero. Then, the dc arc can be naturally extinguished and the insulation on the short-circuit point can be restored. The thyristor switches convert the dc-link fault into an ac short circuit of the ac grid through MMC arms. The ac short-circuit current can be cleared simply by turning off all thyristor switches. Since circuit breakers are not tripped during fault clearance, MMC can immediately and automatically rebuild the dc-link voltage and restart power transmission. Simulation results using PSCAD/EMTDC have verified the validity of the proposed protection scheme. [ABSTRACT FROM AUTHOR]

In this paper, a new noncommunication-based scheme is presented for the protection of three-terminal transmission lines. Using this scheme, simultaneous operation of circuit breakers (CBs) at all terminals is provided for all internal faults. This scheme can discriminate between external and internal faults based on initiated transients by faults, considering the high-stop filtering property of busbars through modeling their stray capacitances and employing a newly defined mathematical morphology (MM) filter to analyze the captured transients. Moreover, the transients generated by lightning or switching are identified and prohibited to enter the algorithm. The presented scheme has been applied on one of three-terminal lines in Iran's Transmission Grid. The effects of fault resistance, fault inception angle, (internal) fault location, and fault type on the performance of the proposed scheme are evaluated, where the high dependability, accuracy, and speed are verified. [ABSTRACT FROM AUTHOR]

This paper proposes a new reconfiguration heuristic in order to reduce the total power loss and the maximum current of electrical radial networks. It is based on the branch-and-bound strategy, which is an implicit enumeration method that uses a tree structure and bounds to organize the searching process. The search tree in this paper is constructed by subdividing the feasible set using the branch-exchange technique in the networks. The constraints and the Pareto dominance are responsible for pruning the search tree. The heuristic also returns a feasible switching plan for each solution. The algorithm was successfully applied to medium- and large-scale problems. [ABSTRACT FROM AUTHOR]

This paper designs and implements an expert system adopting a newly proposed adaptive load transfer strategy and inference methodology as an integrated problem-solving solution of determining an appropriate load transfer strategy when diverse events occur in a distribution system under heavy load or severe overload conditions. In particular, to enhance the efficiency of search, the proposed inference engine is designed so that a solution for the load balancing and a solution for the minimization of the outage load can be inferred at the same time by searching the goal nodes and the fail nodes based on the local minimum tree search method. [ABSTRACT FROM AUTHOR]

Proper operation of a multilevel flying capacitor converter (FCC) necessitates a closed-loop voltage balancing strategy. This paper proposes a space vector modulation (SVM)-based approach that benefits from the switching state redundancy of an n-level FCC to implicitly carry out the voltage balancing task within the switching strategy. Based on the voltage deviations of the capacitor voltages, a cost function is defined and minimized to choose the proper redundant switching states among the available switching states. The performance of a grid-connected four-level FCC under the proposed SVM strategy for various operating conditions is studied and evaluated based on time-domain simulations in the PSCAD/EMTDC environment, and experimentally verified. The studies demonstrate the capability of the proposed SVM strategy to regulate the capacitor voltages at their nominal reference values. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is one of the most potential converter topologies for high-power/voltage systems, specifically for high-voltage direct current (HVDC). One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents of converter arms while the capacitor voltages are maintained balanced. This paper proposes a model predictive control (MPC) strategy that takes the advantage of a cost function minimization technique to eliminate the circulating currents and carry out the voltage balancing task of an MMC-based back-to-back HVDC system. A discrete-time mathematical model of the system is derived and a predictive model corresponding to the discrete-time model is developed. The predictive model is used to select the best switching states of each MMC unit based on evaluation and minimization a defined cost function associated with the control objectives of MMC units and the overall HVDC system. The proposed predictive control strategy: 1) enables control of real and reactive power of the HVDC system; 2) achieves capacitor voltage balancing of the MMC units; and 3) mitigates the circulating currents of the MMC units. Performance of the proposed MPC-based strategy for a five-level back-to-back MMC-HVDC is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate a satisfactory response of the MMC-HVDC station operating based on the proposed MPC strategy, under various conditions. [ABSTRACT FROM AUTHOR]

Voltage-source-converter (VSC) technologies present a bright opportunity in a variety of fields within the power system industry. New modular multilevel converters (MMCs) are expected to supersede two- and three-level VSC-based technologies for HVDC applications due to their recognized advantages in terms of scalability, performance, and efficiency. The computational burden introduced by detailed modeling of MMC-HVDC systems in electromagnetic-transients (EMT)-type programs complicates the study of transients especially when these systems are integrated into a large network. This paper presents a novel average-value model (AVM) for efficient and accurate representation of a detailed MMC–HVDC system. It also develops a detailed 401-level MMC-HVDC model for validating the AVM and studies the performance of both models when integrated into a large 400-kV transmission system in Europe. The results show that the AVM is significantly more efficient while maintaining its accuracy for the dynamic response of the overall system. [ABSTRACT FROM AUTHOR]

Automation is acknowledged by distribution utilities as a successful investment strategy to enhance reliability and operation efficiency. However, practical approaches that can handle the complex decision-making process faced by decision makers to justify the long-term financial effects of distribution automation have remained scarce. An automated and remote-controlled sectionalizing switch play a fundamental role in an automated distribution network. This paper introduces a new optimization approach for distribution automation in terms of automated and remotely controlled sectionalizing switch placement. Mixed-integer linear programming (MILP) is utilized to model the problem. The proposed model can be solved with large-scale commercial solvers in a computationally efficient manner. The proposed sectionalizing switch placement problem considers customer outage costs in conjunction with sectionalizing switch capital investment, installation, as well as annual operation and maintenance costs. The effectiveness of the proposed approach is tested on a reliability test system and a typical real size system. The presented results indicate the accuracy and efficiency of the proposed method. [ABSTRACT FROM PUBLISHER]

Electric power distribution systems of many commercial and industrial sites often employ variable frequency drives and other loads that internally utilize dc. Such loads are often based on front-end line-commutated rectifiers. The detailed switch-level models of such rectifier systems can be readily implemented using a number of widely available digital programs and transient simulation tools, including the Electromagnetic Transient (EMT)-based programs and Matlab/Simulink. To improve the simulation efficiency for the system-level transient studies with a large number of such subsystems, the so-called dynamic average models have been utilized. This paper presents the average-value modeling methodologies for the conventional three-phase (six-pulse) front-end rectifier loads. We demonstrate the system operation and the dynamic performance of the developed average models in discontinuous and continuous modes, as well as under balanced and unbalanced operation. [ABSTRACT FROM PUBLISHER]

Capacitor switching transients are harmful to the capacitor and the switching device. For reducing these transient phenomena, this paper proposes a symmetrical structure transient limiter (SSTL). The proposed SSTL can automatically provide high impedance at the instant of capacitor energization, and, thus, the switching transients can be effectively suppressed. After the restriction, owing to the compensation of all power losses caused by the diodes and reactors, the SSTL acts as a short circuit during the steady state and, thus, has no effect in the circuit. As a result, the capacitor voltage and current waveforms will not be distorted. Moreover, due to the freewheeling effect in the SSTL, no transient overvoltage will appear across the switch contacts at the instant of capacitor de-energization even though the reactors are inserted into the circuit. Theoretical analyses of the proposed SSTL for the energization state, steady state, and de-energization state have been completely carried out. Also, the selections of the related components are depicted. Finally, the experimental results obtained using different methods of capacitor switching verify the feasibility and performance of the proposed SSTL. [ABSTRACT FROM AUTHOR]

Numerical overcurrent relays used to protect capacitor banks can better discriminate between faults and switching inrush compared to their electromechanical counterparts. However, there is still scope and use for faster and reliable operation. This paper describes the implementation of a filter based on mathematical morphology that creates distinct signatures for capacitor inrush and faults. These signatures are then used to propose a rule-based classification method that can be easily implemented in microprocessor-based relays. Extensive simulations are carried out to test the classification method using waveforms generated from PSCAD/EMTDC simulation of a standardized test system. The classification method is shown to be very reliable, general, and significantly faster than methods currently used or previously proposed. [ABSTRACT FROM AUTHOR]

Automation can significantly enhance substation reliability by speeding up service restoration. Although much research works has been done in reliability assessment of substation control systems, there are still several aspects that need to be considered in automated substation reliability evaluation. The impacts of automation system on substation reliability indices have not been comprehensively covered in the literature so far. This paper aims at presenting a comprehensive technique to quantitatively assess the reliability of the automated substations. The technique is based on the event tree methodology and the concepts of conditional probability technique. A step by step evaluation procedure is presented using a typical substation automation architecture known as simple star. The proposed technique is applied to some conventional substation configurations to examine the effectiveness of the technique and also to examine the level of reliability improvement achieved by automation. Furthermore, different sensitivity analyses have been carried out for both automatic switching action and a specific automated substation. The proposed strategy is easy to implement and suitable to analyze for an automated substation reliability with arbitrary automation architectures. Also, automated substation reliability assessment tools can be developed in detail, and can be catered to the specific needs of substation design. [ABSTRACT FROM AUTHOR]

In this paper, a new method is proposed that can be used to discriminate faults from switching transients. The method is primarily intended for use in systems where fast fault detection and fast fault clearing before the first peak of the fault current are required. An industrial system, in which high short-circuit power is desired but in which high short-circuit currents cannot be tolerated is an example of such a system. A phase-locked loop (PLL) is used to perform the discrimination. Computer simulations have been performed and it has been demonstrated that the output of the PLL is completely different for a fault compared to a switching transient. This difference can be used for discrimination between a fault and a switching transient. [ABSTRACT FROM AUTHOR]

The dynamic voltage restorer (DVR) is one of the modern devices used in distribution systems to protect consumers against sudden changes in voltage amplitude. In this paper, emergency control in distribution systems is discussed by using the proposed multifunctional DVR control strategy. Also, the multiloop controller using the Posicast and P+Resonant controllers is proposed in order to improve the transient response and eliminate the steady-state error in DVR response, respectively. The proposed algorithm is applied to some disturbances in load voltage caused by induction motors starting, and a three-phase short circuit fault. Also, the capability of the proposed DVR has been tested to limit the downstream fault current. The current limitation will restore the point of common coupling (PCC) (the bus to which all feeders under study are connected) voltage and protect the DVR itself. The innovation here is that the DVR acts as a virtual impedance with the main aim of protecting the PCC voltage during downstream fault without any problem in real power injection into the DVR. Simulation results show the capability of the DVR to control the emergency conditions of the distribution systems. [ABSTRACT FROM AUTHOR]

This paper deals with the modeling and analysis of a five-legged rectifier transformer that is used for parallel operation of two three-pulse rectifiers, specifically, for low-voltage high-current applications. The five-legged system is compared with the conventional rectifier system consisting of a three-legged transformer and interphase transformer (IPT). Ensuring equal current sharing between the rectifiers and obtaining output voltage with a higher pulse number are the two major requirements which are satisfied in the conventional system. A mathematical explanation for the five-legged system satisfying the mentioned requirements is given. The system is treated as a strongly coupled circuit field one, where a nonlinear finite-element model of the five-legged transformer is coupled with nonlinear circuit equations. IPT-like behavior of the five-legged rectifier transformer is also investigated. The stated theory and analytical expressions are validated experimentally on a 12-V, 5000-A rectifier system. [ABSTRACT FROM AUTHOR]

This paper deals with the current state-of-the-art in interfacing issues related to real-time digital simulators employed in the simulation of power systems and power-electronic systems. This paper provides an overview of technical challenges encountered and their solutions as the real-time digital simulators evolved. Hardware-in-the-loop interfacing for controller hardware and power apparatus hardware are also presented. [ABSTRACT FROM AUTHOR]

This paper presents a neutral point voltage control strategy for the three-level active neutral point clamped (ANPC) converter using selective harmonic elimination pulsewidth modulation (SHE-PWM). The control strategy introduces a small change in the switching angles, which varies the duty cycle of the zero-level switching state, controls the neutral point current and, therefore, the neutral point voltage. The decision to vary a given switching angle takes into account the polarity of the output currents and the voltage across the lower dc-link capacitor. The effectiveness of the proposed strategy is verified through extensive simulation studies and experimentally validated using a low power laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper describes the development of a line-interactive single-phase dynamic voltage restorer (DVR) with a novel sag detection algorithm. The developed detection algorithm has a hybrid structure composed of an instantaneous detection part and the root-mean-square (rms) variation detection part. The source voltage passes through the instantaneous sag detection part. If instantaneous sag is detected, the rms variation detector-1 is selected to calculate the rms variation. The rms variation detector-2 is selected when the instantaneous sag occurs under the operation of the rms variation detector-1. The feasibility of proposed algorithm was verified through computer simulations and experimental works with a 3-kVA-rating prototype of line-interactive DVR. The line-interactive DVR with the proposed algorithm can compensate the input voltage sag or interruption within 2.0-ms delay. The developed DVR can be effectively utilized for the sensitive loads, such as a computer, communication device, and automation device. [ABSTRACT FROM PUBLISHER]

Due to different degradation mechanisms, onload tap changers (OLTCs) may contribute to power transformer failures. If offline diagnosis is undertaken during functional checks performed during the service life of OLTC devices, some of the degradation effects can be assessed. This paper addresses an OLTC diagnostic method based on the measurement of dynamic resistance. An experimental investigation of the degraded OLTC contacts was used to identify a number of defects revealed by dynamic resistance measurements. [ABSTRACT FROM PUBLISHER]

A mechanical switch can be used instead of a power electronic load commutation switch in orderto reduce the losses during normal power flow in standard hybrid DC circuit breakers. The success of current commutation in such hybrid breakers depends on the arc voltage formed across the contacts in the mechanical switch. The behaviour of rapidly elongated, wall constricted arcsgenerated during the opening of an ultra-fast mechanical switch in the air is studied here. The voltage-current characteristics of the generated arcs are obtained for steady state contact opening velocities of 6, 11 and 21 m/s. It is found that the arc voltage at a given current and contact separation increases with the contact opening velocity. It is shown that stationary,zero-contact-velocity characteristics can not be used to accurately quantify the voltage build-up in fast elongating arcs in hybrid breakers. A best-fitting black-box equation for the dynamic arc characteristics is presented for the tested velocities. The obtained voltage-current characteristics are used as input to simulate the current commutation process in a 12 kV hybridDC circuit breaker case-study. Different scenarios of current commutation failure related to the arc voltage build up are identified. It is shown that these failures scenarios can be avoided by increasing the contact opening velocity. [ABSTRACT FROM AUTHOR]

In this paper, the commutation and switching mechanisms between the operating low-voltage (LV) switch and parallel external branches are presented. Experiments for the limitation of electric arc energy during inductive current interruption were performed for three scenarios: 1) for standalone operated switch (base case), 2) for passive method of arc energy limitation (widely applied RC branch connected in parallel to operated switch), and 3) active method of electric arc energy limitation (current commutation into the external semiconductor branch controlled by the microprocessor system). A typical three-phase electromechanical LV switch with double arc quenching chamber was selected as the tested object. The following parameters were investigated: electric arc resistance, electric arc power, electric arc energy, as well as power and energy generated at the external branch connected in parallel to the operating switch. Measurements were performed for current interruption in a single-phase circuit. In addition, observations of a burning electric arc in the quenching arc chamber were registered by an ultraspeed camera (for two cases: base case and hybrid switching). [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC) based high voltage direct current (HVdc) transmission draws more attention in recent years for the grid connection of large-scale offshore wind farms (WF). This paper proposes nine switch converter (NSC) based configuration and control strategy for the VSC-HVdc connected offshore WFs to enhance the fault ride through operation of the system. The configuration allows having both shunt and series interfaces to the onshore grid, which allows the isolation of the faulty part of the network, continuous power delivery to the healthy portion, and significant reduction of the fault current. Reduced switch count NSC is utilized at the onshore station to provide simultaneous shunt and series compensation to the electric grid. The proposed control strategy ensures full power evacuation and the dc-link voltage regulation during network disturbances. The comparative analysis between the proposed and conventional VSC-HVdc systems carried out in MATLAB/ Simulink environment revealed the former's enhanced transient performance. [ABSTRACT FROM PUBLISHER]

A control technique is proposed in this paper for control of modular multilevel converters (MMC) in high-voltage direct current (HVDC) transmission systems. Six independent dynamical state variables are considered in the proposed control technique, including two ac currents, three circulating currents, and the dc-link voltage, for effectively attaining the switching state functions of MMCs, as well as for an accurate control of the circulating currents. Several analytical expressions are derived based on the reference values of the state variables for obtaining the MMC switching functions under steady state operating conditions. In addition, dynamic parts of the switching functions are accomplished by the direct Lyapunov method to guarantee stable operation of the proposed technique for control of MMCs in HVDC systems. Moreover, the capability curve of MMC is developed to validate maximum power injection from MMCs into the power grid and/or loads. The impacts of the variations of MMC output and dc-link currents on the stability of dc-link voltage are also evaluated in detail by small-signal analysis. [ABSTRACT FROM PUBLISHER]

This paper presents a concise frequency-domain interharmonic model for the current-source inverter-fed (CSI-fed) variable frequency drives (VFDs). This model is in the form of a Thévenin equivalent circuit. The equivalent voltage source is determined only by the VFD operating point, while the equivalent impedance is influenced by the system impedance, the dc-link impedance, and motor parameters. This model, verified by simulations and measurements, can be used to predict the interharmonic propagation in a system supplying CSI-fed VFDs. The model also reveals the interharmonic generation characteristics of the VFDs, which will help the proper selection of mitigation options. Based on the model, the analysis of the nature of the interharmonic source of CSI-fed VFD has been conducted. Some applications are demonstrated as well. [ABSTRACT FROM PUBLISHER]

Without additional circuitry, the half-bridge modular multilevel converter (HB-MMC) is endangered under dc-side faults. Typically, a bypass thyristor is augmented to each HB cell to take up fault current until ac circuit breakers interrupt the dc fault. This paper proposes a dc fault protection concept for HB-MMC stations that requires insignificant extra silicon area relative to the thyristor bypass concept. Herein, bypass thyristors of typical HB cells are rearranged such that an independent modular shadow rectifier bridge (SRB) is formed. A low-loss switch assembly is utilized to immediately isolate the MMC following fault detection and the SRB suppresses the fault current by injecting a reverse dc voltage. Among several advantages, the proposed arrangement incurs insignificant losses in steady state, and in some arrangements the MMC is capable of operating in STATCOM mode briefly after fault inception to support ac grid voltage. The proposed concept may be suitable for clearing temporary faults on overhead HVdc lines. Several structural variations will be viewed and discussed. Applicability for a two-level voltage source converter will be addressed. The concept is validated by detailed numerical simulations of a ±200-kV HB-MMC station under dc fault. [ABSTRACT FROM PUBLISHER]

Utilities continuously observe cable failures on aged cables that have an unknown degraded basic insulation level (BIL). One of the root causes is the transient overvoltage (TOV) associated with circuit breaker reclosing. To solve this problem, researchers propose a series of controlled switching methods, most of which belong to deterministic control. However, in power systems, especially in distribution networks, the switching transient is buffeted by stochasticity. Since it is hard to model transient overvoltage due to its complexity, we propose a model-free stochastic control method for reclosers under the existence of uncertainty and noise. Concretely, to capture high-dimensional dynamics patterns, we formulate the recloser control problem by incorporating the temporal sequence reward mechanism into a deep Q-network (DQN). Meanwhile, we embed our physical understanding of the problem into the action probability allocation and develop an infeasible-action-space-elimination algorithm. Through PSCAD simulation, we first reveal the impact of load types on cables’ TOVs. Then, to reduce the training burden for the proposed reinforcement learning (RL) control method in different applications, we establish a post-learning knowledge transfer method. After the validation with our industrial partner, we exhibit several learning curves to show the enhanced performance. The learning efficiency is proved to be outstanding due to the proposed time sequence reward mechanism and infeasible action elimination method. Moreover, the results on knowledge transfer demonstrate the capability of method generalization. Finally, a comparison with conventional methods is conducted. It illustrates the proposed method is most effective in mitigating the TOV phenomenon among three methods. [ABSTRACT FROM AUTHOR]

A challenge raised in today's power system is assessing the system protection security and dependability after the topology has been changed due to relay operation upon the occurrence of cascading faults or intentional operator switching action. This paper proposes an automated setting calculation module which could be used to review the adequacy of the distance relay settings following network topology changes. The calculation procedure is broken down into blocks which could be processed in parallel in order to improve the computation speed. A new concept called distance of impact is proposed to reduce the computational burden by conducting the calculations on a limited portion of the network affected by the topology change. The module performance is tested on a synthetic IEEE 118-bus and real-life Alberta transmission operator systems. A sensitivity analysis in the form of N-2 contingency impact on the network relay settings coordination is conducted on the test systems. [ABSTRACT FROM PUBLISHER]

A simple gas-blow switch, consisting of an axisymmetric tulip/pin contact and a cylindrical polytetrafluoroethylene (PTFE) nozzle, has been used for empirical studies of medium-voltage load current interruption in air. Only the thermal phase of the interruption is considered. Based on the results of more than 3000 interruption tests, a logistic regression model has been developed to describe the relationships between the switch design and test circuit parameters, and the current interruption capability. The most important design recommendation is that the inner diameter of the nozzle should be only slightly larger than the pin contact. The upstream overpressure needed for successful interruption increases as a polynomial function of degree 0.5–1 with the rate of rise of recovery voltage and of degree 2-3 with increasing current. [ABSTRACT FROM PUBLISHER]

To open electromechanical switches within the range of tens of microseconds, we exploit repulsive electromagnetic forces between current-carrying conductor loops. For this we study the power conversion efficiency from electromagnetic to kinetic form. These studies suggest connecting moving coils in series rather than employing inductive coupling. These techniques are put into practice in two prototype switches: the first one is based on induction and the other on series connected coils. Measurements with a high-speed camera verify that both switches achieve accelerations in the magnitude of 140000 g and breaking distances of 1 mm within 50 \mus. The switch with series connected coils obtains 44% greater force than the one based on induction. [ABSTRACT FROM AUTHOR]

Air is an environmentally benign alternative to SF_6 for use in medium-voltage load-break switches. A simple, axisymmetric test switch has been used for empirical studies of the thermal phase of current interruption in atmospheric air. The purpose is to quantify how the pressure drop across the nozzle influences the interrupting capability at a different rate of rise of the recovery voltages (RRRVs) and with different current amplitudes. Tests with pressure drops in the range 0.1–1.1 bar, RRRVs of 40, 80, and 160 V/\mus, and currents of 300, 600, and 900 A were carried out. In general, the current that can be successfully interrupted is proportional with the pressure drop. Likewise, a steeper RRRV requires a proportionally higher pressure drop for the interruption to be successful. For compact air load-break switches for the important 24 kV/630-A class, it seems sufficient to provide a pressure drop of around 0.25 bar lasting for at least 20 ms to comply with the “mainly active load” test-type requirements. [ABSTRACT FROM PUBLISHER]

A new controlled switching method for purely series compensated transmission lines is proposed here. The main idea is based on the estimation of trapped charges using a recursive digital filter for correction of the secondary voltage of the capacitive voltage transformer. Several operating conditions of series capacitor banks are taken into account to define the controlled switching strategy for transmission line reclosing. It is also shown that one can take advantage of the existing techniques for uncompensated and shunt compensated lines. The challenges and potentialities of the proposed method are highlighted through alternative transient program (ATP) of a 500-kV power system where switching transient overvoltages are limited to 1.85 p.u. [ABSTRACT FROM AUTHOR]

This paper proposes an LCL dc-dc converter concept which is capable of achieving very high stepping ratios with megawatt-level power transfers. The converter can find potential application in connecting high-power dc sources to high-voltage dc transmission including future dc transmission networks. This converter is based on two ac/dc insulated-gate bipolar transistor-based converters and an internal passive LCL circuit without internal ac transformers. The LCL circuit is designed to enable voltage stepping without any reactive power circulation and with potentially soft switching operation which minimizes the switching losses. The designed converter has the ability to achieve current regulation even under extreme external dc faults and, therefore, the converter can operate through dc faults. The switch utilization is better than similar topologies and losses are reasonably low. A dual-active-bridge transformer-based converter design is presented to compare with the proposed LCL converter. A detailed PSCAD model confirms conclusions using a 100-MW 20-kV/300 kV test system. An LCL 200-W 20-V/100-V dc/dc prototype converter is built to validate the proposed topology. [ABSTRACT FROM AUTHOR]

This paper introduces a computationally efficient surrogate model-based approach for tolerance analysis of power systems. Surrogate models are heuristic, simple representations of complex systems that are obtained through an automated sensitivity analysis of electromagnetic transient simulations results. These simpler models are shown in this paper to be significantly faster than full-detail simulation models to obtain accurate statistical tolerance information about complex power networks. Usefulness of the proposed approach is demonstrated by two application examples. In the first example, surrogate models are used for determining the statistical distribution of undesired remnant harmonics produced by a voltage-source converter, given the uncertainty in the firing angles. In the second example, the impact of variations in the system parameters around the nominal values on the transient behavior of a static compensator is analyzed. [ABSTRACT FROM PUBLISHER]

This paper presents a new hybrid diverter design for onload tap changers. The design uses “active-shunt” current diversion principles. At its core, the design employs a low-voltage high-current switch-mode amplifier to divert current out of the mechanical contacts and into a pair of anti-parallel thyristors. Commutation between transformer taps may then be performed by the thyristors. The amplifier and thyristors are placed outside the normal load current path and only conduct during a tap change, producing efficiency savings and improving robustness when compared to previous hybrid onload tap changer implementations. An amplifier control loop that autonomously produces zero-current conditions at switch opening and zero-voltage conditions at switch closure is demonstrated. Experimental results investigating the wear characteristics of contacts operated under the new hybrid diverter are presented, along with comparison results from a passive-type switching scheme. Contact lifetime of more than 25 million operations is demonstrated under the new scheme. [ABSTRACT FROM PUBLISHER]

This paper addresses the capacitor bank switching back to back to underground cables. The high currents recorded during the capacitor bank switching occur due to the traveling waves in the underground cables. The detailed analytical study of the capacitive inrush currents is done. The closed-form expressions of the inrush current waveforms due to the initial current surges as well as due to the superposition of the initial and reflected current waves are derived. The formulas for the peak inrush current and its maximum rate of rise are obtained. The case study of the 24-kV capacitor bank switching back to back to the underground cables is described. The results of the field measurements are compared with the results of Electromagnetic Transients Program simulation. The dependence of the inrush current peak on the number of the cables, and the equivalent inductance between the capacitor bank and the substation busbar as well as on the cable length are analyzed. The inrush currents caused failures of the capacitor bank circuit breakers. Since the described capacitor inrush current significantly exceeds the “inrush current of the isolated capacitor bank,” the selection of the capacitor bank switchgear should be taken into account. [ABSTRACT FROM PUBLISHER]

This paper describes a new approach for the maintenance of high-voltage devices in switching substations that is based upon upgraded reliability-centered maintenance (RCM). In addition to periodical maintenance, the Slovenian Transmission System Operator ELES also uses an application that enables RCM. The strategic maintenance concept was developed on the basis of ongoing monitoring maintenance. It takes into consideration not only the well-known indices regarding technical conditions and importance, but also the indices for risk and environment. This new approach is demonstrated using the example of a Podlog 400-kV switching substation before and after its overhaul. [ABSTRACT FROM AUTHOR]

This paper presents a harmonic state-space (HSS) model of a three-phase thyristor-controlled reactor (TCR) in a voltage-control application. In using the HSS description, the importance of incorporating the effect of harmonic interaction on the dynamics of a power system containing a TCR or other flexible ac transmission system controllers is illustrated. Although nonlinear in nature, the significant effect of switching instant variation is included in the linearized model. The model size is minimized with the selection of the minimum necessary set of harmonic frequencies. To validate the HSS models, the steady-state harmonics and the step response were validated with a PSCAD/EMTDC simulation. [ABSTRACT FROM AUTHOR]

Considering six common types of substation automation systems (SASs), this paper not only performs a quantitative reliability evaluation of different automated industrial substations, but also assesses the impacts of various automated substations on the distribution system reliability. The technique is based on the event-tree methodology, reliability block diagram approach, and the concept-of-expectation method. The proposed approach is applied to the eight basic industrial substation configurations, and quantitative assessment of reliability benefits from the utilization of substation automation is conducted through a range of studies. The results show the value of using a systematic quantitative approach for investigating the effects of automation systems on substation reliability. The main contributions of this paper are: 1) to give explicit formulas for reliability evaluations of the automated industrial substation with arbitrary time duration of the automatic switching operation; 2) to prove the SAS with a faster automatic switching operation has a higher degree of improvement in the substation reliability indices; 3) to provide an effective tool for ranking the most reliable industrial substation configurations in the presence of various SASs; and 4) to quantitatively evaluate the effects of the automated substation with various SASs on distribution reliability indices. [ABSTRACT FROM AUTHOR]

This paper presents an innovative generalized systematic approach and related analytical formulation to evaluate distribution system reliability in smart grids where islanded operation of microgrids helps to improve local and overall reliability. In order to do this, the analytical formulation involves the adequacy calculation of conventional and renewable distributed generators supplying microgrids by using probabilistic models. Adequacy is computed by means of a new general analytical expression which takes into account load shedding (user load disconnection) and curtailment (user load reduction) policies. [ABSTRACT FROM PUBLISHER]

In order to solve the problem of improper close/open of high-voltage isolating switches, a novel method was proposed based on the direction gradient histogram (HOG) and the support vector machine (SVM) in this paper. The concept of “switch state angle” was proposed first time to judge the state of high-voltage isolating switches. This method was experimental verification in the substation. Finally, we selected four types of high-voltage isolating switches to verify the validity and universality of this method. The experiment results showed that the algorithm can accurately identify the isolating switches improper close/open. This research provides an effective method to improve the operation reliability of high-voltage isolating switches. [ABSTRACT FROM AUTHOR]

The no-load curve of the dielectric recovery strength of the ultra-fast mechanical vacuum switch (MVS) changing with time is the theoretical foundation to optimize the turn-off strategy of the hybrid direct current circuit breaker with load commutation switch. To avoid the shortcomings of the classic synthetic circuit, a novel test circuit is proposed to measure the no-load dielectric recovery curve of the MVS in this paper. In the test circuit, a succession of pulse voltages is automatically applied on the MVS, and multiple breakdowns of the MVS occur in one test. Therefore, the dielectric strength of the MVS at multiple time points is obtained, and the envelope of the applied voltage on the MVS during one test can describe the dielectric recovery curve of the MVS; as a result, the test workload could be reduced dramatically and the measurement is more authentic. By changing test conditions, including the rate of rise of the test voltage and the operating speed of the MVS, no-load dielectric recovery characteristics are studied. The research has shown that the dielectric recovery speed of the MVS is irrelative with the rate of rise of the test voltage and it is directly proportional to the opening speed of the MVS. [ABSTRACT FROM AUTHOR]

This paper presents an accurate steady-state initialization method for electromagnetic transient (EMT) models of modular multilevel converters (MMCs). The proposed method uses steady-state arm voltages and currents to initialize internal electrical variables and control systems of the converter, including capacitor voltage and gating signals of each sub-module. In addition to typical voltage source converter upper level control, MMC specific controls are also considered, including circulating current control, dc ripple control, and capacitor voltage balancing algorithm. EMT simulations on 101-level and 401-level MMC-HVDC systems confirm the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

This paper discusses the sensitivity of programmable logic controllers (PLCs) to voltage sags based on mechanistic analysis and numerous tests. First, the sag sensitivity caused by different values of rated working voltage is shown through an analysis of the power supply and discrete input/output modules of PLCs. Second, a detailed sag-tolerance experimental scheme is proposed, and PLC sensitivity curves that differ from early curves are established through several experimental tests. Experiments are conducted with nine typical PLCs. The sensitivity of the power supply and discrete input modules are researched first, and then, the effect of the key characteristics of voltage sag, namely, magnitude, duration, points-on-wave of sag initiation, odd harmonics, multiple sags, and continuous sags, is tested. The uncertain area of the PLC sag tolerance is identified after intensively processing the extensive experimental data and curve fitting. A more accurate sag influence assessment for PLCs can be made with the given curves compared to the early curves. Furthermore, experiments have revealed that certain PLCs may work in an uncertain status during voltage sags. The reason and the method of elimination with the proper choice of dc bus capacitance for PLCs are also analyzed. [ABSTRACT FROM AUTHOR]

This paper addresses the subject of reducing outage durations and autorestoration applications within the distribution substation. Employment of intelligent electronic devices (IEDs) can aid in the reduction of the fault duration time, and also it enhances the protection, control, and monitoring at the distribution level. Further, communication among these IEDs can provide faster autorestoration, remote data analysis, event recording, etc. Many recent issues related to distribution substation protection are discussed and techniques to address these issues using current technologies are proposed. [ABSTRACT FROM AUTHOR]

This paper demonstrates the feasibility of radiometric on-line circuit breaker timing. The timing test is crucial and can reveal a wide range of faults or defects. The work presented concentrates on the timing of gas-insulated substation (GIS) circuit breakers, which due to their enclosed nature pose a variety of challenges not encountered in air insulated substations (AIS). It is demonstrated how key events in the arcing process such as arc ignition and extinctions can be identified from the emitted electromagnetic pulses and how this information can be used to assess the integrity of the circuit breaker being monitored. The results are backed by site measurements of a high voltage reactor switching SF6 double pole puffer breaker. The proposed technique can readily be extended to monitor closing operations as well as being adapted to monitor other types of circuit breakers. [ABSTRACT FROM AUTHOR]

This paper presents a new solution method that combines state-space and nodal analysis for the simulation of electrical systems. The presented flexible clustering of state-space-described electrical subsystems into a nodal method offers several advantages for the efficient solution of switched networks, nonlinear functions, and for interfacing with nodal model equations. This paper extends the concept of discrete companion branch equivalent of the nodal approach to state-space described systems and enables natural coupling between them. The presented solution method is simultaneous and enables benefitting from the advantages of two different modeling approaches normally exclusive from one another. [ABSTRACT FROM AUTHOR]

This paper proposes a new control method to reduce overvoltages due to three-phase reclosing for shunt compensated transmission lines (TL). The method detects the first minimum voltage region of the beating across the circuit breakers after protection dead time and works independently of voltage zero crossing, reducing significantly the reclosure time. The controller was tested on a Real Time Digital Simulator (RTDS) and its performance was compared to the traditional preinsertion resistor approach and another existing control method. [ABSTRACT FROM AUTHOR]

Applications of solid-state converters and inverters for adjustable speed drives (ASDs) are popular in industrial systems and lead to a significant amount of harmonic and interharmonic currents being injected into the connected system. This paper presents an accurate analytical approach for the assessment of the harmonic and interharmonic currents generated by the voltage-source inverter-fed ASDs operated under unbalanced supply voltages, where the commutaton effect of the ASD converter and the detailed motor circuit model with various load levels are included. To validate the accuracy of the proposed approach, simulation results are performed by comparing those obtained by a time-domain simulation tool and by laboratory tests. It is shown that the proposed method can effectively calculate the harmonic and interharmonic currents produced by the ASD, either under balanced or unbalanced supply-voltage conditions. [ABSTRACT FROM AUTHOR]

This paper applies nearest level control (NLC) to the modular multilevel converter (MMC). Since there are a number of submodules (SMs) in high-voltage applications of MMC and all SM voltages are required to be measured and sorted, if the uniform sampling frequency is not high enough, the SM will not be triggered as quickly as possible. Thus, the converter-output voltage levels and voltage harmonics will be affected. A method to systematically analyze the voltage harmonics is presented, considering the uniform sampling frequency. In order to select a proper sampling frequency, two critical values, which significantly influence the output voltage levels and voltage total harmonic distortions, are calculated. Simulation results based on PSCAD/EMTDC proved the validity of the proposed modulation scheme and the importance of a properly selected sampling frequency. [ABSTRACT FROM AUTHOR]