Hans Kristian Høidalen

Analytical formulation of lightning-induced voltages on multiconductor overhead lines above lossy ground

This paper presents analytical expressions for calculation of lightning-induced voltages (LIV) on multiconductor overhead lines above lossy ground. The transmission-line return-stroke model is used and the propagation effect on the electromagnetic field is approximated by the surface impedance method. Assuming a linear return-stroke current, the standard numerical integrations along the lightning channel and the overhead line are avoided, and the result is a fast and stable formulation that contains only a single convolution integral. The described method applies for distances between 100 m and 10 km and is valid the first few microseconds where the maximum LIV usually occurs. A simplified approach to how to include overhead line losses is also outlined. A model is proposed that can be included in the electromagnetic transients program ATP-EMTP for calculation of LIV in larger and practical systems.

Transformer Model for Inrush Current Calculations: Simulations, Measurements and Sensitivity Analysis

The modeling of inrush currents that occur upon energization of a transformer is a challenge for Electromagnetic Transients Programs due to limitations in available transformer models and the ability to determine and specify initial flux. The estimation of transformer model parameters is also an issue. This paper presents a transformer model for low- and mid-frequency transient studies with a focus on the behavior in saturation and the estimation of residual fluxes. The comparison of the simulation results with analytical calculations and measurements proves the capability of the model to accurately represent energization and de-energization transients of a three-legged-core distribution transformer. A novel property is the ability of auto initialization after disconnection, made possible by the implementation of a hysteretic core model which properly simulates and remembers residual flux from the previous de-energization. Special attention is paid to parameter estimation. Detailed core and winding design data are not used as they are seldom available from the manufacturer. Sensitivity analysis is performed to verify the influence of each parameter on the inrush current envelope curve. It is observed that the parameter that most influences the current amplitude is the slope of the magnetization curve at extreme saturation.

Continuous monitoring of circuit breakers using vibration analysis

This paper reports the results from three years of continuous condition monitoring of high-voltage circuit breakers (CBs) using vibration analysis. More than 1000 vibration patterns from the operating mechanism of three different spring-operated SF/sub 6/ breakers in normal service are analyzed. The patterns are recorded during each opening/closing operation and compared with references to identify timing and frequency deviations. Two of the three investigated CBs were operating well, while the third suffered from several severe problems. A developing failure that caused this breaker to not open on command was identified in advance. The reproducibility of the vibration patterns is high for a faultless CB and this makes it possible to separate faulty conditions from natural variations. The vibration analysis method is, however, dependent on a well-conditioned vibration pattern without noise and with distinct events. If the instrumentation is made sufficiently robust to withstand both the electrical disturbances in a substation environment and the mechanical shocks during CB operation, vibration analysis seems to be a promising technique that can be used on a continuous basis to detect malfunctions in the operating mechanism.

Novel Approach for Reducing Transformer Inrush Currents: Laboratory Measurements, Analytical Interpretation and Simulation Studies

Transformer inrush currents can lead to a reduction of transformer lifetime and inadvertent tripping of relays. This paper investigates a novel approach for minimizing the inrush current with a potential application in circuit-breaker (CB) control strategies without independent-pole operation and residual flux estimation. For the analyzed transformer, the worst-case inrush current is halved compared to the rapid-closing switching strategy. Measurements of inrush current transients are performed on an unloaded 11-kV distribution transformer varying disconnection and connection instants systematically. This reveals a characteristic pattern in the extremal value of the inrush current as a function of switching times. The pattern is reproduced with simulations and extended to alternative winding configurations. A condition for minimum inrush currents, consistent for all phases and winding configurations, is identified and explained physically. The impact of the current chopping capability of the CB is important and is discussed in this paper.

Effects of reduced pressure and additives on streamers in white oil in long point-plane gap

Recent experiments show that modern dielectric liquids behave differently from traditional mineral oil, particularly with respect to breakdown voltages for lightning impulse. This paper describes an experimental investigation addressing underlying reasons for this. The influences of reduced pressure and additives on streamers in white oil were investigated under both positive and negative polarities using an 8 cm long point-plane gap. Reduced pressure significantly accelerates streamers, thus increasing stopping length and reducing both breakdown and acceleration voltages. With increasing applied voltage, different typical propagation modes of streamers were recorded for both polarities. A low ionization potential additive strongly affects positive streamers. It significantly changes streamer velocity and reduces the breakdown voltage but increases the acceleration voltage where breakdown streamer velocity increases drastically. Adding an electron scavenger influences streamers of both polarities, but it mainly increases the velocity of negative streamers and results in a reduction of both the breakdown and the acceleration voltages. The propagation mechanisms of streamers are also discussed.

Core loss behavior in high frequency high power transformers—II: Arbitrary excitation

High frequency high power transformers used in power electronic converters are frequently subjected to non-sinusoidal excitations. The main purpose of this paper is to study the effects of some general arbitrary waveforms on magnetic core loss in these transformers. First, using well-known empirical equations, general expressions were derived based on the parameters of the waveforms. Second, the impacts of different orders of voltage harmonics were investigated. Finally, capabilities of nanocrystalline and amorphous magnetic materials were compared. It is shown that the loss inside the core is highly sensitive to the rise time and duty cycle of trapezoidal and rectangular waveforms, respectively. Furthermore, although amorphous materials have higher saturation flux density, the total core loss inside the transformer designed using nanocrystalline material is considerably lower than the similar transformer with amorphous materials.

Analysis of Fault Detection and Location in Medium Voltage Radial Networks with Distributed Generation

This paper focus on how distributed generation (DG) may impact on the fault detection and location in medium voltage (MV) distribution networks. Typical possible problems are protection blinding and sympathetic tripping. The purpose of the work is to gain an understanding of when problems are most likely to occur, and of possibilities for fault location in networks with distributed generation. The paper presents simulations (PSCAD/EMTDC) done on a MV radial distribution network with a DG-unit (synchronous generator). An analytical approach is used to explain simulation results.

Core loss behavior in high frequency high power transformers—I: Effect of core topology

This two-part paper presents an overview of core loss computations performed in both time and frequency domains in order to evaluate their behavior in single phase transformers with different core topologies. Moreover, the effects of non-sinusoidal waveforms on well-known core loss calculation methods are investigated with both analytically and finite element calculations. Three well-known configurations of transformers utilized in high frequency high power applications are investigated, namely, the core type, the shell type, and the matrix transformer. Based on the results obtained from a large number of FEM simulations for different operating conditions, the efficiencies of the transformers are compared in terms of distribution of magnetic flux density, loss density, total core loss, and weight. The analysis shows that for lower range of frequency and power, the shell type core could be the favorable option, and on the other hand, core type seems to be an appropriate solution for higher values of the operating frequency and nominal power.

Medium frequency high power transformers, state of art and challenges

The worldwide increasing interests towards medium frequency high power transformers (MFHPTs) as a key element of high power converters in offshore wind farms and traction systems makes it a promising area of research. Since the electrical environment and excitations are different in these applications, conventional transformers are not applicable and hence new design procedures should be utilized. Several prototypes and design procedures have been proposed in recent years to optimize the efficiency and maximize the power density of such transformers. Consequently, in this paper, a review on the status of art, trends and challenges of such transformers is presented in detail. In addition, several potential future works are proposed and addressed completely.

A Multiagent System-Based Protection and Control Scheme for Distribution System With Distributed-Generation Integration

In this paper, a multiagent system-based protection and control scheme is proposed to deal with diverse operation conditions in distribution system due to distributed-generation (DG) integration. Based on cooperation between DG controller and relays, an adaptive protection and control algorithm is designed on converter-based wind turbine DG to limit the influence of infeed fault current. With the consideration of DG control modes, an adaptive relay setting strategy is developed to help protective relays adapt suitable settings to different operation conditions caused by the variations of system topology and DG status. The proposed scheme is tested and validated on a test distribution system in a hardware-in-the-loop real-time testing platform.