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This paper addresses the integrated volt-var control for distribution network operation via multiobjective optimization. This paper seeks to explore the problem of energy savings and peak demand relief through the voltage reduction procedure. Currently, due to the emergence of the distribution smart grids, these procedures are gaining renewed interest and attention. The proposal presented here is for the operation phase, with a strategy based on an hourly load forecast for the next day/week, taking into account the active power intake reduction, and the voltage deviation. Therefore, the result is a set of nondominated optimal solutions, and then one may decide when, where, and how to apply them to meet different goals. The obtained solutions, for two typical distribution networks, describe relevant economic and technical benefits.

Power system loads are one of the crucial elements of modern power systems and, as such, must be properly modelled in stability studies. However, the static and dynamic characteristics of a load are commonly unknown, extremely nonlinear, and are usually time varying. Consequently, a measurement-based approach for determining the load characteristics would offer a significant advantage since it could update the parameters of load models directly from the available system measurements. For this purpose and in order to accurately determine load model parameters, a suitable parameter estimation method must be applied. The conventional approach to this problem favors the use of standard nonlinear estimators or artificial intelligence (AI)-based methods. In this paper, a new solution for determining the unknown load model parameters is proposed—an improved particle swarm optimization (IPSO) method. The proposed method is an AI-type technique similar to the commonly used genetic algorithms (GAs) and is shown to provide a promising alternative. This paper presents a performance comparison of IPSO and GA using computer simulations and measured data obtained from realistic laboratory experiments.

Stray voltages have always been a concern to utility companies and customers. But methods to troubleshoot and monitor nonroutine stray voltage problems are very limited. This paper presents a passive, measurement-based method to identify the contributors to the common cause of stray voltages, the neutral-to-earth voltage (NEV) rise at the service entrance point. The paper shows that the NEV can be decoupled into two components: those related to the customer under investigation and those outside the customer facility. A method to quantify the relative contributions of the two components is proposed. A new concept called current return ratio is introduced to facilitate the decoupling and to quantify the grounding conditions of the system. This paper presents the background, circuit model, concept, and theory of the proposed method.

Hopf bifurcation phenomenon of a power system results in oscillatory dynamics which can lead to instabilities in the system. Therefore, it is desirable to operate the system such that a sufficient margin to Hopf bifurcation is ensured. This paper presents a methodology based on the adjustment of the setpoint values of the HVDC link controllers, to prevent instability or increase the stability margin of the system subject to Hopf bifurcation. In this paper, the first-order sensitivities of the Hopf stability margin to the setpoint values of the HVDC links are presented. These sensitivities identify the optimum direction to change the HVDC setpoints to steer the system away from instability, increase the stability margin, and improve the damping of oscillatory modes. The proposed method is evaluated on various system configurations subject to Hopf bifurcation phenomena caused by a variety of events, such as load and line impedance variations. Simulation results show that at the optimum operating point, for a variety of Hopf bifurcations, the stability margin and damping of the oscillatory modes improve.

This paper presents an economic study to find the switches that should participate in a switch upgrade plan, based on the results of reconfiguration. The traditional reconfiguration problem trend considers only one loading condition (mostly maximum demand), in order to find the optimum reconfiguration. However, as the distribution loading condition has hourly and daily variations, the optimum configuration continuously changes. Consequently, considering only one loading condition might lead to inefficiency of the results. In this paper, daily load curves of different types of distribution consumers, during various types of days (weekdays and holidays) and seasons (summer and winter), are used to obtain the best reconfiguration hours during a day. Then, genetic algorithm (GA) is used to obtain the optimum configuration during each time interval. The objective function applied to GA consists of loss and energy not supplied. The switches that contribute to reconfiguration should be remotely controlled in order to have the capability of immediate mode alteration. In order to evaluate the feasibility of automated switch installation, the benefit-to-cost ratio is calculated. The entire procedure is applied to a test distribution system, and the results are discussed in this paper.

A new wind turbine transformers protection method against high du/dt switching transients taking place during a wind turbine operation is described. High-frequency transients, characterized by a high rate of voltage rise (du/dt) and overvoltage at the wind turbine transformer high-voltage terminals may be a result of operations of the turbine switchgear typically comprising a vacuum circuit breaker. The protection concept described in this paper is a follow up of research activities on a transients suppressing method dedicated for distribution transformers. The principle of operation of the protective device proposed and parameters optimization procedures for windmill transformers are provided in this paper. The performance of the device was verified by means of Alternative Transients Program–Electromagnetic Transients Program simulations. The physical device was built and experimentally verified by full-scale functional tests reflecting a real wind turbine power network.

A parallel hybrid modular multilevel converter (PHMMC) belongs to the class of modular multilevel converters, which have become potential candidates for high-voltage direct-current (HVDC) transmission systems. Due to the circuit topology of a PHMMC, the dc bus voltage contains low-order harmonics and cannot be fully regulated at a constant dc voltage. The dc bus voltage, if not properly controlled, leads to improper power transfer and increases the magnitude of dc current ripple on the dc transmission line. This paper proposes a zero-sequence voltage injection (ZSVI)-based model predictive control (MPC) strategy to control the dc current/power flow and simultaneously minimize the dc current ripple. The proposed strategy takes advantage of a cost function minimization technique to determine and inject the optimal zero-sequence voltage components into the dc- bus voltage of a PHMMC system. This paper derives a discrete-time dynamic model of the dc transmission-line current and, correspondingly, develops a predictive model. The predictive model is used to inject the appropriate amount of zero-sequence voltage components to the dc bus reference voltage waveform. Compared with the existing triplen harmonics injection method, the proposed ZSVI-MPC strategy improves the performance of a PHMMC system in terms of minimization of the dc current/voltage ripple. Performance of the proposed strategy for a 21-level PHMMC-based HVDC station system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported results demonstrate superior performance of the PHMMC-HVDC station operating based on the proposed ZSVI-MPC strategy, under various operating conditions, as opposed to the existing triplen harmonics injection method.

Due to the electromagnetic induction between transmission lines and the associated grounding wires, power frequency current causes flow into the ground network via the metallic tower, which may interfere with grounding resistance measurements. This paper analyzes the mechanisms of this interference, and shows that power frequency interference becomes stronger at phase-wire transposition towers. Grounding resistance measurement by the variable frequency (VF) method has been proven to be effective in dealing with power frequency interference. However, the test equipment based on the VF method is always deactivated when the external interference voltage exceeds a preset level in order to avoid false readings. This paper aims at tackling the problem by introducing a reverse phase cancellation (RPC) method, which acquires and processes the interference signal before generating a cancelling signal with the same amplitude and frequency and a phase which is exactly opposite in polarity. This paper proves that the RPC method represents an excellent improvement over the VF method since it retains the advantages of VF and avoids the trip-out of test equipment due to excessive interference.

The method to identify the sources of the neutral-to- earth voltage (NEV) rise as presented in a companion paper is verified in this paper through extensive simulation studies and field measurements. Sensitivity studies are conducted to determine factors that affect the performance of the proposed method. The results show that the NEV contribution factor index is effective in identifying load unbalance caused NEV and its contributors. The current return ratio index is effective in diagnosing poor grounding caused NEV and its contributors. An NEV trouble shooting strategy and a system-wide NEV condition monitoring strategy are proposed based on the findings.

This paper presents the load-flow modeling and steady-state capabilities of the bundle-controlled line impedance modulator (LIM). This distributed flexible ac transmission system device is under development for the management of power flows. In particular, this paper presents the back-to-back interconnection of two bundle-controlled line segments. The LIM configuration is very interesting from an economic and operational point of view: it enables continuous control of the transmission-line impedance from its standard value up to 1.64 p.u. and, as a result, the power flow can be reduced smoothly.

This paper proposes a model predictive control (MPC) method for modular-multilevel-converter (MMC) high-voltage direct current (HVDC). To control the MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are taken into consideration. The existing MPC methods for the MMC-HVDC system utilize weighting factors to configure the cost function in combinations of the SM capacitor voltage balancing algorithm, ac current control, and circulating current control. Because all combinations of the switch states are considered in order to minimize the cost function, their possible combinations increase geometrically according to the increase of the level of the MMC, which is a significant disadvantage. This paper proposes a new MPC method with a reduced number of states for ac current control, circulating current control, and the SM capacitor voltage-balancing algorithm. The proposed cost functions are divided into three types according to their control purposes. Each cost function determines the minimum number of states for controlling the ac current, circulating current, and SM capacitor voltage. The efficacy of the proposed controlling method is verified through simulation results using PSCAD/EMTDC.

A novel approach for the calculation of transient fault current contributions from capacitive network components in HVDC cable networks during pole-to-ground faults is presented in this paper. Analytic approximations considering the skin effect and the resulting distortion of the waveshape of the fault surge are proposed. Only fault current contributions from capacitive components, such as dc capacitors and adjacent cable feeders, are taken into account in this paper, since they are dominant during the first few milliseconds and yield the highest rates of rise of fault current. The results of the proposed expressions are compared with a benchmark model implemented in PSCAD and exhibit an accurate representation of the time development of the fault current. The derived approximations may serve as a starting point for a short-circuit calculation standard for HVDC networks and the specification of HVDC circuit-breaker requirements.

A 240-MVA, 165-GRDY/20-kV generator step-up transformer was insulated with Envirotemp FR3 fluid. The transformer was energized during a cold temperature period in mid-December 2010. This is believed to be the largest generator step-up transformer filled with a natural ester fluid that was energized at low temperatures. The lack of cold start experience of natural ester-filled power transformers created questions and at least some uncertainty as to the outcome. This paper describes the instrumentation used and data collected during the cold startup procedure. In addition, a computational fluid dynamics analysis of this transformer design was performed that provided calculated temperatures and elapsed times until steady-state temperatures were achieved. Comparisons were made between the actual and calculated values. The results indicate that the cold startup procedures for a power transformer are the same whether filled with mineral oil or natural ester fluid of similar characteristics as used for this paper.

This paper deals with the impact of a gas discharge arrester's electrodes' shapes on its performance. When after extinguishing the electric arc between the electrodes of the gas discharge arrester and the electric-field strength between the electrodes exceeds the critical value, reignition occurs. Computation of the electric-field strength between the electrodes of the existing gas discharge arresters using the finite elements method shows that electric-field strength reaches its highest values at the edges of the electrodes. It therefore makes sense to reduce the electric-field strength $E$ at the edges of the electrodes by changing the electrodes' shapes. This also reduces the risk of reigniting the electric arc between the electrodes and improves the arrester's ability to self-extinguish. The electrodes can be geometrically shaped in such a way that ensures much uniform distribution of the electric-field strength $E$ as possible. This paper describes the model of a gas discharge arrester and the use of a differential evolution optimization algorithm for computation of the more adequate shapes of electrodes. Thus, uniform distribution of the electric-field strength is ensured between the gas discharge arrester electrodes.

This paper describes the application of the finite-elements method for determining the distribution of losses over windings and a detailed thermal-hydraulic network model for determining the value of the hot-spot factor and the hot-spot temperature. The influence of nonuniform losses in the conductor of the windings and the influence of nonuniform cooling are clearly separated. A detailed discussion of the components included in the hot-spot factor is given in this paper, enabling proper understanding of this complex and important quantity. The case study transformer was a 100-MVA oil power transformer with oil-directed water-forced cooling. The method was applied within the scope of the project of increasing the power of a hydro power station by 12%. The task was to calculate the temperatures and to recommend whether the reconstruction of the transformer is necessary. The developed calculation method and software were hitherto used for transformer design (in the production or the reconstruction stage of transformers) and it is the first application to give an answer as to whether it is possible to increase the power of an existing transformer. In this specific application, necessary onsite measurements for determining the characteristics of the coolers were performed.

Shunt reactors are applied to long extra high-voltage (HV) transmission lines to compensate for their natural capacitance which otherwise could cause overvoltages under light load conditions. However, when the level of compensation approaches around 65% or higher, these reactors themselves can, due to contingency, become the cause of hazardous overvoltages during unbalanced open-phase conditions on the healthy line. This paper reports on an overvoltage incident that occurred on a 500-kV 72% shunt-compensated line in the BC Hydro system. The overvoltage occurred when one of the breaker poles failed to interrupt the current following an inadvertent three-phase trip of the healthy line. The uninterrupted current kept one phase energized for an extended time. Temporary overvoltages during this condition and restrike transients eventually led to surge arrester failures at both terminals. Analysis of voltage and current recordings provided useful insight into the mechanism responsible for the damaging overvoltages as well as the protection and control responses during the disturbance. The intent of this paper is to share the knowledge gained from analyses of the recorded waveforms with other utilities either having or planning for highly compensated lines.

The phase-angle-jump is defined in this paper as another single-event characteristic of voltage dips, besides residual or characteristic voltage and duration. This paper proposes and compares six methods to calculate the phase-angle-jump for measured voltage dips in a three-phase system. This paper also studies the details of the calculation: correcting frequency and transition segments. The six proposed methods have been applied to 235 sets of measured voltage dips and a statistical comparison has been made. It is recommended that two of the methods be included in IEEE Standard 1564 and IEC 61000-4-30.

Over the years, there have been a number of cases where users experienced higher noise levels of transformers on-site versus what was measured in the factory acceptance tests. In some of these cases, measurements have shown that a power transformer can have significantly different noise characteristics (both noise level and frequency spectrum) on-site compared to what was measured in the factory. This difference can range from a few decibels to more than 10 dB in some cases. Some of these differences are found to be caused by local environmental factors that influenced the on-site measurements. Others were caused by operating conditions on-site that are different from those of the factory tests. This paper presents results of investigations and measurements performed, over the past several years, on the main contributors to this difference. For each of these contributors, data are presented that demonstrate the impact of this contributor as well as quantify typical magnitudes of this impact. It is hoped that the material presented in this paper will assist users in: 1) understanding some of the pitfalls of on-site measurement of transformer noise; 2) how operating conditions contribute to the noise performance of transformers on-site; and 3) how significantly higher on-site noise levels can, in some cases, indicate an issue with the transformer or any of its components/accessories. It is also hoped that the material in this paper will contribute to industry standards/application guides on noise determination and abatement.

in the companion paper (Part I), a new tool for lightning-induced voltage calculations, called CiLIV, has been presented. In this paper (Part II), predictions made with the new tool are compared to experimental data and to outputs of analytical procedures/numerical codes found in the literature.

Multiterminal dc (MTDC) systems are drawing a lot of interest lately in applications related to distributed generation, especially in those that integrate wind or photovoltaic (PV) generation with energy storage (ES). Several approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures are mainly application specific and, thus, can be still improved in order to provide a more generic approach. This paper proposes an improved primary control layer for an MTDC system. The concept is based on the combination of a droop control method and dc bus signaling in order to provide a more generic and flexible solution. In this paper, different droop characteristics are proposed for the various elements connected to the dc bus. All of them are specifically tailored around five operation bands, which depend on the dc bus voltage level. Special attention is paid to the integration of ES: the state of charge (SoC) is considered at the primary control level, yielding a surface characteristic that depends on the SoC and the dc bus voltage. The scaling of the system has been analyzed together with the proposed control strategy and the overall operation has been validated through simulations by considering a 100 kW PV system with energy storage. Experimental results were obtained on a scaled laboratory prototype rated at 10 kW.

Unbalanced power of three-phase stationary linear loads with a neutral conductor, supplied with sinusoidal symmetrical voltages and its reactive compensation, is the subject of this paper. A novel power equation of such loads is developed. The suggested power equation is based on the decomposition of the load current into Current Physical Components (CPC). This paper shows that all powers can be expressed in terms of the load parameters and this creates fundamentals for design of balancing reactive compensators capable of improving the power factor to unity. A reactive balancing compensator is composed, in general, of two compensators in the $\Delta$ and Y configuration. This paper presents a method of calculation of LC parameters of such a compensator using the CPC-based power theory.

This paper shows that a previously proposed linear-system identification method based on frequency partitioning and adaptive weighting can be successfully applied to the phase-domain frequency-dependent modeling of overhead transmission lines for electromagnetic transient (EMT) simulations. As the framework of the phase-domain modeling, the universal line model (ULM) is used, and the frequency responses of the characteristic-admittance and the propagation-function matrix are realized by linear equivalents obtained by the identification method mentioned above, instead of the well-known Vector Fitting (VF) method. In this paper, numerical techniques to enhance the identification method for this phase-domain line modeling application are also presented. For validation, the proposed approach is applied to the modeling of an existing 500-kV double-circuit transmission line. The effectiveness of the numerical techniques for enhancements are shown through the modeling process, and transient waveforms obtained by the proposed approach are compared with those by the rigorous Laplace transform method and also with a field-test result.

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.

Ice accretion on the overhead contact lines of the electric railways may cause severe inconveniences to train traffic. In order to avoid such problems, methods are presently applied that are characterized by high cost and/or some risks as described in this paper. This paper presents new simple techniques for the deicing of the overhead contact lines in the ac and in the dc electric railways that avoid any inconvenience. They consist in rearranging the system scheme in special configurations and inject a proper current into the circuits obtained. Under these configurations, the line continues to be protected against the faults. Experimental results have proved the viability of these techniques. The limits of applications of these techniques are discussed in this paper.

Active front-end (AFE) rectifiers are becoming widely used in medium-to-high-power adjustable speed drives (ASDs) to achieve regenerative operation and meet the energy efficiency and harmonic requirements. The typical control methods used with AFE rectifiers include voltage-oriented control (VOC), direct power control (DPC) and virtual-flux-based methods. This paper presents a dynamic average-value model (AVM) of the AFE rectifier system which is based on the voltage-source converter (VSC) operated with DPC. The developed AVM and the modeling methodology presented in this paper highlight the specifics of the hysteresis bang-bang type DPC that is used to track the real and reactive power references. The proposed AVM of the AFE rectifier system is demonstrated to significantly reduce the computing effort while accurately capturing the fast transient performance of the system.

This paper demonstrates the need for and provides an improved fitting of the line parameters when a wide range of frequencies with emphasis on the dc response is the objective. The improved fitting is based on a low-order fitting of a function error for each of the line parameters, that is, the characteristic admittance and propagation function, in the low-frequency range. The function error represents the difference between the analytical evaluation of the line parameters, that is, obtained from a frequency-dependent distributed parameters line model, and their initial rational approximation, for example, given by an electromagnetic transients software tool. The approximated function error, added to the initial rational approximation of the line parameters, provides the enhanced fitted line model. The enhanced fitting improves the dc response of the line and ensures numerical stability in time-domain simulations. This paper applies the proposed fitting approach to the universal line model for time-domain simulations. The simulation results by the enhanced fitting are validated by comparing the results with those obtained from a frequency-domain line model (FDLM). Two case studies are presented to illustrate the proposed fitting approach.

This paper presents the work carried out by the IEEE Working Grou[ on field-measured overvoltages and their analysis for validating power system component models to be used in switching transients studies. The work uses measurement data obtained from field tests performed by Bonneville Power Administration in June 1995 where switching overvoltages were measured on one of its 230 kV lines. This paper includes a description of the switching tests and the main results derived from field measurements. Details of the switching procedures that were followed to calibrate component models used to match field-recorded waveforms can be found in the Part II paper.

Very fast transient overvoltages (VFTO) are a well-known phenomenon in gas-insulated switchgear. They are mainly generated due to several pre- or restrikes during disconnector switching operations. Since VFTOs may cause complications in substations, a reliable method for damping these transient voltages is of great interest, especially in ultra-high voltage applications. Two new approaches are presented in this paper and compared with a matched radio-frequency resonator, which is an already existing theory. The new methods are an inductive arrangement with surge arrestors and rings of nanocrystalline tape-wound cores. All methods were investigated inside a high-voltage experimental test setup. Therefore, VFTOs with amplitudes of 600 kV and more were generated. The first VFTO amplitude was damped by up to 20%. Even higher damping is possible depending on the damping method and the test conditions. In addition to high-voltage tests and their results, this paper illustrates the theoretical background of all damping methods.

A typical maloperation case of the differential protection utilized in the ultra-high voltage (UHV) voltage-regulating transformer (VRT) under inrush conditions is investigated in this paper. It indicates that the second harmonic restraint principle cannot identify the inrush current reliably under such conditions. This paper proposes a normalized equivalent instantaneous inductance (NEII)-based inrush blocking method, which improves the equivalent instantaneous inductance (EII)-based method in the UHV VRT protection. Although the EII-based method is capable of discriminating inrush currents and internal faults, some factors constrain its application, such as the field limitation of measuring the terminal voltage of the transformer and the difficulty of determining a reasonable threshold due to the diversity of transformer magnetizing characteristics. The proposed NEII-based method overcomes limitations of the EII-based method and can be adopted in UHVVRT protection. A number of simulation tests have verified the effectiveness of the proposed method.

The testing of adaptive protection schemes is a problem that remains largely unaddressed. These schemes can be characterized by uncertainty in behavior due to the dynamic changes in their configuration to suit prevailing network conditions. This paper proposes a novel approach to formalizing this behavior using hybrid systems modeling. This unlocks the ability to verify the safety performance of the schemes using reachability analysis. In this paper, an adaptive setting selection logic for distance protection is verified for its safety, using reachability analysis, during changes in network conditions.

This paper proposes a technique for preventing saturation in series transformers from dynamic voltage restorer (DVR) systems. The method consists in correcting the voltages which are injected through the transformers into the power system to compensate voltage sags. It restricts the compensating voltages during the sag whenever it predicts that a maximum limit for the flux linkage is about to be exceeded. The prediction is carried out at the beginning of a stabilized voltage sag. Moreover, the technique allows a certain level of sag compensation even when the estimated flux is expected to exceed the saturation limit. The voltage sag level and phase are computed through an adaptive recursive least squares (RLS). The RLS estimation incorporates a transient period before it achieves a stable state whenever there is a sag event. The DVR is not supposed to operate in this period. Therefore, this paper also outlines a simple procedure for detecting the RLS estimation stable level. Simulation of different scenarios of voltage sags and the results from the experimental implementation of a DVR show the effectiveness of the method.

This paper presents a practical assessment and validation for the new proposed approach based on the continuous wavelet transform (CWT) to obtain the frequency response from online transient signals for an actual transformer. Apart from the mathematical procedure, an electronic system was also designed and implemented in order to inject superimposed controlled pulses to the power system (50/60 Hz) wave. The results show improved performance of the wavelet transform compared to the Fourier transform, for transient signal analysis to be applied on a nonintrusive transformer monitoring approach. Particularly, this new approach enables overcoming some problems related to the signal filtering and the signal processing in an online frequency-response analysis transformer diagnosis. This paper is presented in three parts. Part I showed the main theoretical basis of this approach. The current Part II shows a practical assessment based on tests performed in a three-phase transformer of 1150/345 V-5 kVA and in a single-phase transformer of 13200/240 V-15 kVA. Part III shows the performance of this new approach on transformers being diagnosed under real conditions.

Inrush currents in transformers can have very disruptive effects, such as voltage sags, false tripping of the protective devices, and mechanical stresses in the transformer windings. This paper shows that there are operating situations that may cause a transformer to draw abnormally high inrush currents. Examples include the normal operation of offline uninterruptible power-supply (UPS) systems, interruptions, voltage sags, and notching. These conditions may produce inrush-like currents of more than twice the value of the “normal” maximum inrush caused by energizing at voltage zero-crossing. For this condition, the term “phase-hop” is used in this paper. Laboratory experiments were performed on four different transformers (1 kVA) with varied characteristics and show the impact of phase-hop in the magnitude of inrush currents. The experiments are also used to validate the Electromagnetic Transients Program model used for the analysis of multiple cases. In addition, the behavior of the magnetic flux in a transformer under phase-hop is investigated and compared with different operating conditions using finite elements. The results of this paper have implications in transformer design and in the operation and design of UPS systems to prevent the damaging effects of phase-hop.

This paper presents a new technique for wideband characterization and modeling of multiport large grounding systems in the form of an accurate Electromagnetic Transients Program-compatible model. This helps to consider the exact behavior of grounding systems in the electromagnetic transient analysis of power systems. The methodology involves three stages. First, a general electromagnetic approach based on the method of moments solution to Maxwell's equations is utilized to obtain the grounding system admittance matrix over the frequency range of interest. In the next stage, a rational approximation of the grounding system admittance matrix is inferred by making use of a modified matrix pencil method (Modified-MPM) developed in this paper. The Modified-MPM is well suited to the matrix fitting applications with a common set of poles selected for all matrix elements. The obtained rational approximation is finally employed to construct a multiport time-domain model of the grounding system expressed in the form of state-space equations. The proposed modeling approach is applied to the lightning transient analysis of a real subtransmission (63/20 kV) substation considering its grounding system as an integrated part of the electrical system. It is shown that the traditional method of using a simple resistive model for representing the grounding system results in a noticeable underestimation in the calculation of lightning-generated overvoltages in electrical systems.

This paper presents a comprehensive analysis of the limitations and the key dynamics of closed-loop active power control systems for voltage-source converter (VSC) HVDC, regarding stability, performance, and robustness. Detailed dynamic models are derived and the controllability and robustness issues for VSC active power control are identified. Limitations imposed by ac system strength, converter operating point, and current control design on the stability and performance of the two leading active power control principles are addressed, using frequency-response analysis and time-domain simulations. The dynamic interactions between the active power control design and the dc voltage droop control are examined. The simulations are performed using average-value VSC models and a high-fidelity modular multilevel converter model. Impacts of the active power control design on dynamic behaviors of multiterminal dc (MTDC) systems are investigated using a four-terminal model. This paper provides a systematic study on the key stability and performance issues associated with the active power control. Furthermore, the methodology offers a framework for the analysis of more complex active power and dc voltage droop controllers for future MTDC systems.

This paper is divided into two main parts. The first one describes the computation of the number of lightning strokes that directly hit transmission lines as well as those strokes that hit the earth's surface near a line and can provoke very high overvoltages. The second one describes a process for computing the consequent short circuits in a line caused by both types of stroke incidence. In the companion paper, some application examples are presented.

Traditional distance relays often mis-operate for high transient resistance in phase-ground faults by measuring apparent impedance. To solve this problem, a novel distance protection algorithm is proposed in this paper. It is deduced in the RL lumped transmission line model and calculates the fault distance from the measuring point to the fault point by solving a linear differential equation. In addition, a protection criteria based on the comparison result between the calculated fault distance and the protective setting margin rather than the protective zone measured by apparent impedance is set up in this paper. As a result, the proposed method can avoid overreach for the given characteristics that the calculated fault distance is greater than the actual fault distance when the fault occurs next to the opposite terminal. Meanwhile, both the fundamental component and one decaying DC component are needed to solve the differential equation for the proposed distance algorithm, therefore it can trip the breaker fast during the fault transient period. Finally, EMTP simulation results verify the validity of the proposed distance algorithm on RL transmission line model. However, to put the proposed method into field use, the problem caused by the distributed capacitance of the transmission line on the proposed method will be further studied and solved.

An unbalance voltage between a positive pole and negative pole will exist for a long time in an ungrounded modular multilevel converter (MMC) HVDC transmission system. A zig-zag transformer has been proposed to construct an artificial neutral point and keep voltage balance between the positive pole and the negative pole. An efficient grounding system consists of the zig-zag transformer and the neutral grounding resistor. In this paper, the different impedances, including positive-, negative-, zero-sequence impedance, and dc current impedance, have been analyzed, respectively. Then, the computer simulations have been performed to evaluate the characteristic of the zig-zag transformer under normal voltage, zero-sequence voltage and unbalanced dc voltage, respectively. The simulation results show that 1) the zig-zag transformer can effectively balance the unbalanced voltage in dc side of the MMC under different work conditions; 2) the zero-sequence impedance is low and zero-sequence current is convenient for the detection of a single-phase fault; 3) the power consumption of the neutral grounding resistor is of a light rating, and the cooling of the resistor is unnecessary. The method presented in this paper proves to be an efficient grounding method for the MMC-HVDC transmission system.

This paper describes the establishment of a large-area outdoor experimental facility at the Dinorwig pumped-storage power station in North Wales for investigating the performance of practical grounding systems. The facility itself and arrangements of test electrodes and measurement equipment are described, providing a background for subsequent test methodologies. In addition, tests to determine the variation in the water and reservoir bed resistivities have been undertaken, resulting in a survey data set that better informs experimental work and simulations of vertical ground rod and grid electrodes. It is proposed that this facility may also be used to calibrate grounding meters and for validating computational modelling techniques. In a complementary paper, results of low-voltage dc, ac, and impulse tests of varying magnitudes are described, and experimental results are compared with computed values obtained from numerical models.

This paper presents tests carried out at an outdoor experimental facility, described in the companion Part 1 paper, to investigate the performance of practical grounding systems. Here, the results of low-voltage dc, ac, and impulse tests performed on rod and grid electrodes are described, and the measured quantities are compared with computed values obtained from numerical models. Measured ground resistance and impedance at low frequency (including power frequency) showed reasonable agreement with simple standard formulae and computational models, but revealed a significant falloff with current magnitude in the range often used for practical testing of high-voltage grounding systems. This may have implications for the specifications of grounding test equipment and extrapolation of measured ground resistance/impedance at low-voltage/current to values representative of realistic fault currents. A frequency dependence of ground impedance was also measured. Specifically, a fall in impedance over a frequency range up to 100 kHz is not generally accounted for in grounding models.

This paper presents the concept of complementary measurements with the use of two methods that allow for a more detailed assessment of the mechanical condition of a transformer's active part. These methods are frequency-response analysis (FRA) and vibroacoustic method (VM). The VM was interpreted separately in steady and transient conditions, thus allowing, together with the FRA measurements, the determination of possible source of vibrations. This paper presents a theoretical introduction, experimental measurements performed on a real transformer with introduced controlled deformations and, finally, an example of a practical application.

The regional energy management and optimized operating strategies of electric vehicles (EVs) and battery swapping station (BSS) are proposed in this paper based on smart microgrid according to the effects of the utility grid caused by uncoordinated charging of EVs and BSS. A price-incentive model is utilized to generate the management strategy to coordinate the charging of EVs and BSS to minimize the total cost of EVs and maximize the profit of BSS in grid-connected mode. In islanded mode, based on the power balance between renewable electric sources and loads, the fuzzy control method is applied to produce the service price of EV according to its state of charge. Combined with the interruptible-load scheduling, the energy management and dispatch of EVs and BSS are optimized to minimize the operational cost and maximize the benefit of islanded microgrid. The main optimization problems are formulated as a cost minimizing problem and a profit maximizing one, which are implemented in A Modeling Language for Mathematical Programming. The effectiveness of the proposed strategies for the optimized operation of EVs and BSS is validated by case studies. The approach given in this paper also provides ideas about the optimal operation of a future smart grid with the booming of EVs.

Plug-in electric vehicle (PEV) charging equipment incorporates protection that ensures that grid disturbances do not damage the charger or vehicle. When the grid voltage sags below 80% of nominal, undervoltage protection is likely to disconnect the charging load from the grid. Most PEV charging will occur overnight, when non-PEV load is at a minimum. This paper argues that PEV voltage-sag response, when synchronized across large numbers of PEVs, could result in the loss of a significant proportion of the total load. It is shown that this load loss can lead to unacceptably high voltages once the initiating event has been cleared. This paper explores the nature of this voltage-rise phenomenon. Analysis tools are developed to assist in determining PEV loading conditions that demarcate acceptable postdisturbance voltage response from unacceptable outcomes. Two examples, based on standard distribution test systems, are used to illustrate PEV-induced overvoltage behavior, and demonstrate applications of the analysis tools.

This paper presents a new approach to optimal exploitation and reactive power control of long inhomogeneous high-voltage ac cable lines, such as submarine interconnectors including significant land cable stretches. Voltage and reactive power profiles are discussed, highlighting the requirements for full exploitation of power transmission capacity, and giving formulae for optimal voltage control. Capability curves for the lossy inhomogeneous cable line are discussed; this paper also proposes a simple algorithm for controlling the reactive power flow at one end of the line independently from active power, using variable shunt reactors and tap-changing transformers. The application of the presented treatment to a real inhomogeneous line, that is, the 118-km-long 245-kV–50-Hz Malta–Sicily interconnector currently under construction, yields maximum power and optimal voltage drop values in close agreement with detailed simulation results. Power-flow simulations also confirm the effectiveness of the proposed reactive power control algorithm. Finally, total transmission losses associated with the proposed optimal voltage control are evaluated and shown to be competitive with those of state-of-the-art HVDC–VSC systems.

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

This paper presents a fully automatic leak detection system for slow oscillation high-pressure fluid-filled (HPFF) circuits. It also presents the concept of detecting leaks in such circuits, utilizing only a limited number of sensors. It will describe algorithms built upon this concept, including two different approaches to the construction of the decision module, crucial for detection capabilities. The proposed solution utilizes adaptive digital filters for modeling the circuit behavior and a Bayesian network coupled with a software leak simulator in the decision stage. These tests were performed using data obtained from a real-life circuit, including real-leak data. This paper may enable practical application of high-precision low-cost leak detection solutions for a wide range of HPFF circuits.

This paper proposes a new method of risk assessment for lightning shielding flashover in transmission lines. This is used to improve the performance of lightning shielding failure which is a frequently serious problem. The method can be efficiently implemented in the optimization of a 3-D model. Three-dimensional laser scanning is used to derive the analytical 3-D model to high precision. This includes the data of the whole span line's structure and terrain parameters. In this study, the evaluation of lightning shielding flashover risk of each tower is calculated using the whole span calculation method associated with the laser scanning data and the given parameters of lightning activities. The evaluation results for the Gechen line in the zone near the Three Gorges area are presented based on the proposed method. Using a comparison of theoretical values and actual statistical data, this paper concludes that the proposed new method has better performance than the traditional method.

The geomagnetically induced currents (GICs) produced in power systems during magnetic storms are a function of the electric-field amplitude and direction, and the characteristics of the power system. This paper examines the influence of a number of power system characteristics, which include the resistances and structures of the conductors; the length of the transmission lines; the number, type, and resistances of transformers, the substation grounding resistances, and the topology of the network. It is shown that GIC grows with increasing line length but approaches an asymptotic constant value, and a more relevant parameter than the individual line length is the length of the entire system. This paper also derives the effective GIC for a conventional transformer and an autotransformer, and analyzes the behavior of GIC when the network topology changes illustrated with the GIC-Benchmark Model. The results of these studies provide a guide to estimating GIC impacts on a power network.

In this paper, we present an improved and extended approximate formula for estimating the peak value of lightning-induced voltages in an overhead line, considering the horizontally stratified conducting ground. The approximate formula proposed in this paper is based on the return stroke transmission-line model (TL) and a trapezoidal lightning return stroke current waveform with the typical representative front time of 3.8 μs and the return stroke velocity of 120 m/ μs according to CIGRE and the IEEE Standard 1410 Guide. The extended approximate formula is validated by using the 2-D finite-different time-domain method and Agrawal field-line coupling model. The results show that the proposed approximate formula in this paper is simple and suitable for estimating the lightning-induced voltage peak value considering the horizontally stratified ground with satisfied accuracy.

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.