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Test methods for evaluating flashover voltage of ceramic and nonceramic insulators under ice, snow, and cold-fog conditions are recommended. The paper describes the procedure to be followed at various stages of an evaluation test including preparation of the test object, insulator precontamination, ice, snow, and cold-fog deposits as well as voltage applications and flashover voltage evaluation. Some relevant parameters for ice accretion, flashover voltage evaluation, artificial pollution, and power source requirements are taken from IEC 60129, IEC 60507, and IEEE Std.-4-1995 on standard techniques for high-voltage testing.

Many utilities still have paper-insulated pipe type cables in service. In the near future they may have to replace some of these systems due to local increases in electric power demand or aged cable insulation. In many cases the steel pipes are well protected against corrosion and thus suitable for further use. For retrofitting in such installations, XLPE-insulated cables offer several advantages. Felten & Guilleaume Energietechnik AG has developed a new three-core XLPE-insulated 64/110 kV cable with reduced insulation thickness (10 mm), that can be pulled into a pipe. In co-operation with VEW ENERGIE AG, the cable performance is being investigated in a field test, that started in October 1996. This long-term loading cycle test will last for three years. The test voltage, applied between conductor and earth, amounts to 127 kV. The temperature is measured with the help of an optical-fiber measurement system. This device is computer controlled and works as a remote monitoring system.

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

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

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

The Nelson River HVDC system currently consists of two thyristor-based bipole schemes: Bipoles I and II. Pole 1 in Bipole I was found to have suffered from a serious thyristor leakage problem (TLP) after 20 years in service. In order to understand, evaluate, and resolve this problem, a comprehensive study was conducted in 2013 at Manitoba Hydro, which involved thyristor voltage monitoring, leakage current measurements, and simulations. The study revealed that the majority of thyristors in two of the three valve groups at both ends were leaky (i.e., exhibiting increased leakage currents) to various degrees. This paper provides an overview of the TLP and related investigation. The effects and symptoms/behaviors of the leaky thyristors are illustrated with the study results for one of the worst valves at each end. The failure mechanism of the leaky thyristors is also discussed. This paper should provide a clear understanding of the TLP from the user's perspective. A question, however, remains with regard to the root-cause of the TLP. Further research is required to answer this question.

In this paper, the finite-elements method was used to analyze the approximation being typically used in the international standards for the calculation of the touch voltage, appearing at earth faults in power networks. The calculations in this paper were performed on the series of earthing systems applied in practice and the results are compared with the results of the widely accepted approximate method from standards. The origin of discrepancy in the results is analyzed and evaluated. The outcome of this paper is the justification of the application of Thevenin's theorem and the development of the higher accuracy equations for the resistance between feet and remote earth in case of the application of high resistivity additional surface material.

Several methods have been proposed and investigated so far on mitigation of very fast transient overvoltages (VFTO) in gas-insulated switchgear (GIS). The state-of-the-art methods are primarily based on dissipation of the energy associated with electromagnetic waves that the VFTO originate from and are composed of. Present paper reports on an alternative concept of VFTO mitigation based on the principle of controlling voltage conditions preceding voltage breakdown in SF6 gas that leads to VFTO generation. The paper introduces different control algorithms and shows how the algorithms can limit VFTO maximum value and total number of voltage breakdowns during operation of the GIS disconnector. The concept is applied for mitigation of VFTO in ultra-high voltage GIS. As the study case, an 1100 kV test set-up is used as recently reported for Wuhan (China) GIS station, with the disconnector characteristics obtained from 1100 kV development tests.

This paper proposes a flexible neutral grounding method based on electromagnetic hybrid Petersen coil (EHPC), which consists of a magnetically controlled reactor and an active power compensator (APC). Under normal conditions, the system's neutral point grounds through a reactor of high reactance. When a single line-to-earth fault occurs, the EHPC can be controlled flexibly to suppress the recovery voltage of faulty phase and to reduce the ground-fault current to almost zero. This paper expounds the two operating modes and corresponding characteristics, the measuring method of insulation parameters of the system (mainly including capacitance to earth and leakage resistance), and the full compensation principle for arc suppression based on the equivalent negative impedance of EHPC. This paper also proposes a compound control method of neutral-to-earth impedance based on neutral-point displacement voltage and a compensating method for the harmonic component of the ground-fault current. Compared to traditional grounding methods, this method can be fast and reliably make the residual current approach almost zero without arc reignition while greatly reducing the output capacity of APC and, thus, decreasing the cost of this device. Simulations and experiments verify the correctness of this measuring method and feasibility of this flexible grounding control strategy.

In this paper, a new adaptive single-phase reclosing method for high-voltage transmission lines is proposed. This method employs local voltage magnitude and angle of the faulted phase. It is shown in this paper that the proposed method is able to quickly identify the fault type and the arc extinction if the fault is transient. This method is effective for various system configurations, including ideally and partially transposed transmission lines, untransposed lines, as well as transmission lines with or without a shunt reactor. Superior performance of the proposed method has been verified using 550 case studies simulated in PSCAD and Matlab and a real case related to a 765-kV transmission line.

This paper describes a systematic method for determining an area of vulnerability (AOV) to fault induced voltage sags. The concept of an AOV is very useful for evaluating vulnerability and the relationship between sensitive loads and system voltage sag performance. In general, a system performance assessment pertaining to voltage sags requires a determination of AOVs for system buses. Because voltage sags can be caused by remote faults in a transmission system, to completely identify an AOV, a large part of the power system including adjacent circuits and remote supply systems should be considered. Therefore, accurately calculating critical points and identifying AOVs are challenging tasks. In this paper, we present an effective numerical method to determine an accurate AOV in a large scale power system and to overcome limitations of other well-known methods.

This paper proposes a frequency-based synchronization control (FSC) approach for the voltage-sourced converter in rectification and inversion modes of an HVDC station that is connected to a weak ac grid. Similar to the conventional vector current control (VCC) approach, FSC includes a conventional phase-locked loop (PLL), and its outer loop controller generates the current reference for the inner loop. Different from VCC, the inner loop of FSC utilizes a supplementary frequency-based control loop to provide damping for the oscillations due to the PLL and generates the angle/magnitude for the converter ac-side voltage. The inner loop of FSC switches to VCC to limit over-current during fault scenario. This paper develops a linearized dynamic model of VSC-HVDC station, analytically evaluates behavior of the FSC dynamics, and validates the results based on time-domain simulation studies in the PSCAD platform. The eigenanalysis and simulation results show that FSC can effectively mitigate the instability issues of the PLL even under very weak ac grid condition.

This paper proposes a probabilistic method aimed at estimating the voltage sag frequency of nonmonitored sites given the number of sags recorded by a limited set of voltage sag monitors. The proposed method is based on simulating a representative database of feasible pseudomeasurements by using probabilistic values of fault coefficient, fault impedance, fault type, and fault location. This information is processed through Bayesian filtering based on the Bayes' probabilistic theorem. This enables obtaining the most probable voltage sag index at nonmonitored busbars given the index measured at monitored busbars. This statistical estimation of sag indices at locations where there are no monitors constitutes the main contribution of this paper. This study is developed in three power systems: the IEEE 24-busbar reliability test system, the IEEE 118-busbar test system, and the transmission network of Ecuador with 357 busbars.

A method of calculating LC parameters of a reactive balancing compensator for AC arc furnaces at the supply voltage asymmetry is the subject of this paper. The developed method is illustrated numerically with the calculation of LC parameters of a balancing compensator for an AC arc furnace approximated by a linear model with fixed parameters. The presented method can be regarded as an initial step toward developing balancing compensation for ultra-high-power AC arc furnaces. The method of calculation of LC parameters of balancing compensators is based in this paper on the Currents' Physical Component (CPC) power theory.

Circulating current can be decomposed into double line-frequency positive-, negative- and zero-sequence components under unbalanced voltage conditions. First, in order to decrease peak/rms value of arm current, double line-frequency circulating current should be eliminated. Second, ac-side current that consists of positive- and negative-sequence components under unbalanced voltage conditions should be regulated to track the reference signal. Furthermore, fundamental frequency circulating current caused by asymmetry of the upper and lower arm should be also removed. To effectively control ac-side current and eliminate all circulating currents, an arm current control strategy based on a decoupler and proportional-integral controller plus the vector proportional integral (VPI) controller is proposed in this paper. The proposed control strategy works well under balanced and unbalanced voltage conditions as well as asymmetry of the upper and lower arm. In addition, this paper proposes a design procedure for the VPI controller to reduce the uncertainty of this process. A three-phase modular multilevel converter simulation model is set up on PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.

This paper presents challenges and potential impacts of photovoltaic (PV) integration in the low-voltage downtown secondary networks (downtown networks). In the conventional secondary networks, substation feeders are the sole source of electric power and establish unidirectional power to the downtown network. The network protectors prevent the flow of power from inside the network to the upstream feeder by disconnecting the circuit to protect the feeder transformer against upstream faults. The assumption of unidirectional power flow can be violated by PV generation due to the possibility of excess power inside the network. It is shown in this paper that a large number of network protector trips can occur and, thus, voltage collapse may follow even in low PV penetration levels. In addition, it is demonstrated that the reclose action of the network protector relays is adversely affected by the PV power. Other adverse effects of such distributed-generation units, such as voltage profile, line overloads, and flicker, are also briefly discussed. Finally, a solution is proposed, based on differential currents, to upgrade the network protector relays in order to avoid false trips due to excessive PV power. Part of the New Orleans downtown network is modeled and the study is performed through simulations.

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

This paper proposes and evaluates an alternative statistical methodology to analyze a large number of voltage dips. For a given voltage dip, a set of lengths is first identified to characterize the root mean square (rms) voltage evolution along the disturbance, deduced from partial linearized time intervals and trajectories. Principal component analysis and K-means clustering processes are then applied to identify rms-voltage patterns and propose a reduced number of representative rms-voltage profiles from the linearized trajectories. This reduced group of averaged rms-voltage profiles enables the representation of a large amount of disturbances, which offers a visual and graphical representation of their evolution along the events, aspects that were not previously considered in other contributions. The complete process is evaluated on real voltage dips collected in intense field-measurement campaigns carried out in a wind farm in Spain among different years. The results are included in this paper.

This paper investigates the voltage profiles and harmonic impacts of high-speed trains on the traction power-supply system (TPSS) and primary utility system. Power-quality (PQ) assessment has become not only a computing tool for TPSS design and planning, but also an indispensable technique for utilities to estimate the accurate PQ impacts from the railway systems. Therefore, to achieve comprehensive PQ assessment in TPSS, a dynamic fundamental/harmonic power-flow (DF/HPF) method is developed in a companion paper, while further application of the technique is described in this paper. The fundamental and harmonic results calculated in a 24-h period, such as loading levels, voltage profiles, unbalance, power loss, and harmonic distortions have been computed. In addition, the statistical measured background harmonics of the utility system are represented by Weibull function and considered in the harmonic evaluation. The unbalance and harmonic impacts are investigated and checked with national standards in this paper. The proposed method can be effective for excavating and predicting the potential serious PQ problems existing in the TPSS by using a train timetable.

IEEE Standard 1547 defines grid connection standards for distributed resources (DRs). This standard sets requirements for harmonic performance, which are particularly relevant when converter-interfaced DRs are considered. Specifically, the standard limits the allowable harmonic current injected by a DR, assuming grid voltage at the point of common coupling (PCC) is harmonic free. In this paper, it is demonstrated, both mathematically and experimentally, that the harmonic current injection of a converter-interfaced DR depends highly on the level of PCC background voltage. This paper presents a mathematical formulation that enables the impact of PCC voltage harmonics on the harmonic current injection of a converter-interfaced DR to be determined via experimental identification of the converters frequency coupling matrix. Experimental tests show that an IEEE 1547-compliant solar inverter with measured total demand distortion (TDD) of 0.66% for an undistorted PCC can produce TDD levels in excess of 20% in the presence of 3% voltage distortion. It is concluded that TDD metrics, as employed in IEEE Standard 1547, provide little insight into the field performance of converter-interfaced DRs when background voltage harmonics are present. Improved FCM-based metrics for assessing the harmonic performance of converter-interfaced DRs in the presence of voltage harmonics are provided.

This paper presents a new multilevel cascaded-type dynamic voltage restorer (MCDVR) with a fault current-limiting function. This topology can operate in two operational modes: 1) a compensation mode for voltage fluctuations and unbalances and 2) short-circuit current-limiting mode. The current-limiting function of the MCDVR is performed by activating antiparallel thyristors during the short-circuit fault, and deactivating them during normal operation. The mathematical model of the MCDVR system is also established in this paper. The control scheme design and optimal parameter selection are outlined based on the detailed theoretical analysis of the converter. The transient states of the MCDVR in the compensation mode and current-limiting mode are also analyzed. Simulation results based on the PSCAD/EMTDC software, and experimental results on a laboratory setup help to validate the proposed topology and the theoretical analysis.

In this paper, a static synchronous compensator (STATCOM) operation scheme of the modular multilevel converter adopting clamp double submodules (CDSM-MMC) during a pole-to-pole dc fault is proposed. By analyzing the current paths of a CDSM, this paper proposes three new operation states of the CDSM, which allow the CDSM to provide bipolar voltages when the arm current that flows through it is negative. Through control of the CDSMs to operate in these new states along with the blocked state, the arms of the CDSM-MMC can operate alternately in two different modes, that is, the conducting mode and blocked mode even when the dc-link voltage drops to zero. Thus, the CDSM-MMC is capable of working as a STATCOM during a dc fault. Moreover, a control strategy of the dc-fault STATCOM operation mode is designed to control the reactive power provided by the CDSM-MMC, to limit the dc fault current, and to reserve and balance the energies stored in capacitors. Simulation results conducted in PSCAD/EMTDC demonstrate the validity of the proposed STATCOM operation scheme and control strategy of the CDSM-MMC during a pole-to-pole dc fault.

Powers of three-phase aggregates of single-phase static linear loads supplied with a nonsinusoidal voltage are the subject of this paper. It is demonstrated in this paper that the supply current of such systems with a nonsinusoidal voltage can be decomposed into six mutually orthogonal components associated with distinctive physical phenomena in the load, two of which do not exist at a sinusoidal voltage. Such decomposition enables evaluation of contribution of these phenomena to the supply-current rms value increase and the power factor decline. A power equation and definitions of powers in such systems in the presence of voltage harmonics are proposed. This paper shows that all powers can be expressed in terms of load parameters, which can be identified by a measurement at the load terminals.

This paper proposes a robust frequency-domain method to tune the $d$ – $q$ decoupled control system used in Modular Multilevel Converter-type Voltage Source Converter (MMC-VSC) systems. A linearized state-space model of the MMC-VSC system is developed and used to calculate the stability-related frequency-domain attributes. The controller design problem is formulated as an optimization problem. In this paper, the simulated annealing optimization technique is applied to find the proportional-integral (PI) controller parameters that give desired damping for the oscillatory modes and desired values for decaying exponential modes. The efficacy of this method is tested on the electromagnetic transient model of a two-terminal MMC-VSC system on the real-time digital simulators, and the results are provided in this paper. Finally, tuned controller parameters for different ac system strengths are discussed and it is shown that this mathematical model is suitable to tune the PI-controller parameters for MMC-VSC systems connected to strong as well as weak ac networks.

Series impedance and shunt admittance parameters of transmission lines are fundamental data for various online and offline power system studies. This paper shows that the active power, reactive power, and voltage magnitude ( $P$ , $Q$ , $V$ ) data measured at the two ends of a transmission line are sufficient to determine the positive-sequence line parameters. The phase-angle information is not essential. Since the $P$ , $Q$ , and $V$ data are readily available from the supervisory control and data-acquisition systems, the proposed method can be easily implemented in existing control centers. Algorithms, characteristics, performances, and potential applications of the proposed method are presented in this paper. Simulation study and field test results show that the proposed technique can be a useful addition to the energy-management systems of utility companies.

The objective of this paper is to present a 126-kV/40-kA–2500-A single-break vacuum circuit breaker (VCB) and its type test results based on the IEC 62271-100: 2006 and GB1984-2003 standards. A single-break 126-kV vacuum interrupter (VI), with a pair of 2/3 coil-type contacts, is used in the 126-kV VCB. The external insulation of the VI is provided by ${\rm SF}_{6}$ at 0.1 MPa. A spring-type operating mechanism is used to provide a closing velocity of 1.15 m/s, which is an average velocity for 4 mm before the moving contact mates with the stationary contact. The opening velocity within 20 mm, which is 1/3 of a full contact stroke of 60 mm, is 3.5 m/s. Two kinds of test circuits are used in the type tests to verify the making and breaking capabilities of this 126-kV VCB prototype. One is a direct test circuit for test duty cycles, such as out-of-phase making and breaking test duties, that is, OP1 and OP2; capacitive current switching test of class C1, that is, CC1/LC1 and CC2/LC2. The other is a synthetic circuit, used for tests, such as the terminal fault test, the short-line fault test, the double earth fault test, and the electrical endurance test of class E2. The dielectric tests of a 230-kV/1 min power frequency voltage withstand test and a 550-kV lightning impulse voltage withstand test were used to verify the dielectric withstand performance of the VCB. Experiment results presented in this paper showed that the 126-kV/40 kA–2500 A single-break VCB passed the type tests according to IEC 62271-100 and GB1984-2003.

This paper investigates on a new concept for very-fast transient overvoltages (VFTOs) attenuation in gas-insulated switchgear (GIS). The concept, first introduced by the authors in this paper, involves modification of the GIS disconnector contact system in order to dissipate the VFTO energy in damping elements placed inside the GIS busbar conductor. The concept is an alternative to the state-of-the-art method where magnetic rings are placed in the space between the GIS conductor and the enclosure. The proposed arrangement has no impact on the damping element on the dielectric design of the GIS busbar, and it highly reduces the impact on the GIS thermal design. This paper presents analyses which show feasibility of the concept, through 1) principles which govern voltage conditions inside the GIS busbar and 2) full-Maxwell finite-element method electromagnetic simulations of VFTO attenuated with the use of the proposed concept.

This paper proposes a novel method for detecting parasitic dc components in ac grids. The method uses a parallel-connected sensing reactor and a low-cost digital signal processor. From the reactor current, the algorithm derives a sum of even harmonics, considering at the same time their phase shift with respect to the fundamental. The method is insensitive to the measurement noise and offset, and it does not require additional passive filters. Precision better than 100 μV in 0.4-kV grids is maintained over a wide range of ac voltages. This paper details the detection method, provides the guidelines for the reactor design, and includes the key implementation details related to the signal acquisition-sampling and digital processing. High precision of the new sensor is thoroughly verified on an experimental setup connected to a 0.4-kV grid.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

HVDC cable systems are growing in number. This is a result of globally defined targets leading to an increase of investments in renewables. The HVDC cable systems enable us to integrate these new energy sources in a manner that reduces losses and stabilizes operations. In this paper, the focus is on extruded HVDC cables. Starting from a general description of the forces that drive us toward more cables, this paper quantifies the push for higher voltages. The extruded cable system on a product level is described followed by design aspects. Particularly, the effect of thermal runaway is highlighted. The accessories, joints, and terminations are described. The advantages of the concept nonlinear field grading are described. This paper ends with some words on realized commercial projects and future trends.

The increase of harmonic current circulation in electric system affects the shunt capacitor banks protection arrangement, causing false alarms and trips and leading to a reduction of the relay sensitivity. This paper analyzes the behavior of the protection system on a reduced-scale model, which reproduces the main features of grounded wye shunt capacitor banks. In such experiments, high levels of current harmonic distortion, voltage unbalance, and bank inherent unbalance were considered. This paper brings experimental analyses for protection arrangements considering robustness and sensitivity.

Single-pole tripping (SPT) operation imposes problem to many power network relaying schemes. In this paper the directional relaying issue for faults during SPT condition is addressed. Algorithms applying negative sequence components of fault voltage and current for directional relaying find limitation during SPT as the prefault signals also contain negative sequence components. This paper proposes a directional relaying algorithm during SPT using phase-change in negative sequence current. The proposed technique is an attractive option as it does not use the fault voltage signal in the decision process. The technique is evaluated using data simulated with EMTDC/PSCAD for single and double circuit lines and the performance of the algorithm is also evaluated.

This paper presents a technique for simulation of transient skin effect of the dc railway system based on MATLAB/Simulink. The classic calculation of skin effect based on frequency domain is not suitable for time-domain simulation. The aim of this paper is to develop a time-domain calculation method which can be realized in MATLAB/Simulink. Through the analysis of the step series algorithm, results are applied to the calculation of frequency voltage signal and compared with skin effect impedance of frequency. Accurate simulation of the transient skin effect produces accurate modeling of the remote fault current. This is important for the dc railway protection researchers to develop techniques for discriminating remote short-circuit currents from locomotive starting currents.

This paper proposes a short-circuit diagnosis tool for electrical low-voltage (LV) distribution networks. The tool relies on analyzing Park's vector representation of the LV phase voltage in the sag produced by a short circuit. Park's vector representation is considered to be an identification mark for the type of short circuit that the installation is exposed to and the phases that it affects. In addition, it shows whether the fault is located in the medium voltage or LV network. The characteristics of the vector representation corresponding to the different types of short circuits are analyzed in this paper. Finally, the relationship between Park's vector representation and the location of the defect is also studied. The results can be used as part of an expert system for the automatic classification of disturbances.

Thyristor-based onload tap-changing ac voltage stabilizers can replace mechanical tap changers associated with transformers feeding medium-voltage (MV) lines (typically 69 kV primary, 34.5 kV line, 6.3-50 MVA), thus avoiding frequent contacts revision. As shown in a previous paper, the topology using a compensating transformer in the main circuit and a multiwinding coil in the commutating circuit give good technical and economical results independent of the coil operation mode (shared, nonshared, and mixed load). This second paper focuses on this topology and defines the criteria for an optimum design and the synthesis formulae that give the definition parameters of the components as a function of the equipment characteristics. Numerical results are added for a typical 10-MVA stabilizer. One of the best circuit solutions (single switch comb, mixed load, auxiliary short-circuit winding), which does not need a series association of thyristors, is developed in detail. Final synthesis formulae, effective power of the transformers and coil, and summatory of thyristors I·U products for all three operation modes are contrasted in a table as the final step of the iterative analysis and synthesis cost-comparative method followed along the research.

The conventional interphase power controller (IPC) can control the power flow of the transmission line. In this paper, a novel topology for IPC, called the unified interphase power controller (UIPC), is introduced. In this topology, the phase-shifting transformers (PSTs) are replaced by voltage-source converters. In this paper, the transient behavior of UIPC is modeled and investigated. The results are compared with the simulation results of IPC and UPFC. It is shown that UIPC has all of the capabilities of IPC and UPFC, such as power-flow control, fault current limitation, voltage isolation, and local bus voltage regulation simultaneously. The results also reveal that UIPC increases the variations range of control variables of IPC and, as a result, the range of variations of active and reactive powers.