A model of lightning-channel progression towards the Earth is given based on the latest knowledge on the physics of discharge in long air gaps. The physical assessment of the electric field is done with the charge simulation method, which is adopted to input into a computer program all parameters involved in the progression of the negative downward channels towards the Earth and those involved in the inception and propagation of upward positive channels from earthed structures. Starting from the geometrical configuration of the earthed object and taking into account the orographic conditions, the lateral distance may be evaluated as a function of the severity of the lightning stroke in terms of impulse charge and thus of current amplitude. Provided the statistical distribution of the currents is given, the model therefore allows the number of lightning strokes to an object to be estimated, yielding an assumed ground flash density. Reference is made to the critical radius concept acquired with laboratory tests with long front surges. A check of the model with field data relevant to free-standing structures is presented and a reasonable agreement is found. >

Lightning stroke simulation by means of the leader progression model. I. Description of the model and evaluation of exposure of free-standing structures

The paper presents the statistical data of the significant parameters of lightning flash, collected by many researchers over many years around the world. The significant parameters of a lightning flash are: peak current, waveshape and velocity of the return stroke, the total flash charge and /spl int/I/sup 2/dt. Negative first strokes have traditionally been considered to produce the worst stress on the system insulation. The subsequent negative strokes have significantly lower peak current but shorter wavefronts. This may stress the system insulation more. The positive strokes have about the same median current value as the negative first strokes and longer fronts, thus producing less stress. However, their duration is longer than that of the negative strokes. Therefore, the system insulation may be damaged because of the lower volt-time characteristic for long-duration waves. The positive strokes may also cause more thermal damage because of their significantly higher charge and /spl int/I/sup 2/dt. The relationship between the return-stroke velocity and the current peak is a significant parameter in estimating lightning-induced voltages and also in estimating the peak current from the radiated electromagnetic fields of the lightning channel. For better accuracy, the current and the velocity should be measured simultaneously. Better methods to measure the stroke current need to be developed. Correlation coefficient between various lightning parameters is another important parameter which will affect the analysis significantly. Lightning characteristics should be classified according to geographical regions and seasons instead of assuming these characteristics to be globally uniform.

Parameters of lightning strokes: a review

A new model for assessing the exposure of free-standing structures and horizontal conductors above flat ground to direct lightning strokes is introduced. A recently developed criterion for positive leader inception modified to account for positive leaders initiated under the influence of a negative descending lightning stroke, is discussed. Subsequent propagation of the positive leader is analyzed to define the point of encounter of the two leaders which determines the attractive radius of a structure or the attractive lateral distance of a conductor. These parameters are investigated for a wide range of heights and return-stroke currents. A method for analyzing shielding failure and determining the critical shielding angle is also described. The predictions of the model are compared with field observations and previously developed models. >

Modeling of transmission line exposure to direct lightning strokes

Methodology for large-scale systems

PREFACE. ACKNOWLEDGMENTS. 1. INTRODUCTION. 1.1. Scope and Objectives. 1.2. Power System Reliability. 1.3. The Insulation Coordination Process: What Is Involved? 1.4. Organization of the Book. 1.5. Precis. 2. INSULATORS FOR ELECTRIC POWER SYSTEMS. 2.1. Terminology for Insulators. 2.2. Classification of Insulators. 2.3. Insulator Construction. 2.4. Electrical Stresses on Insulators. 2.5. Environmental Stresses on Insulators. 2.6. Mechanical Stresses. 3. ENVIRONMENTAL EXPOSURE OF INSULATORS. 3.1. Pollution: What It Is. 3.2. Pollution Deposits on Power System Insulators. 3.3. Nonsoluble Electrically Inert Deposits. 3.4. Soluble Electrically Conductive Pollution. 3.5. Effects of Temperature on Electrical Conductivity. 3.6. Conversion to Equivalent Salt Deposit Density. 3.7. Self-Wetting of Contaminated Surfaces. 3.8. Surface Wetting by Fog Accretion. 3.9. Surface Wetting by Natural Precipitation. 3.10. Surface Wetting by Artificial Precipitation. 4. INSULATOR ELECTRICAL PERFORMANCE IN POLLUTION CONDITIONS. 4.1. Terminology for Electrical Performance in Pollution Conditions. 4.2. Air Gap Breakdown. 4.3. Breakdown of Polluted Insulators. 4.4. Outdoor Exposure Test Methods. 4.5. Indoor Test Methods for Pollution Flashovers. 4.6. Salt-Fog Test. 4.7. Clean-Fog Test Method. 4.8. Other Test Procedures. 4.9. Salt-Fog Test Results. 4.10. Clean-Fog Test Results. 4.11. Effects of Insulator Parameters. 4.12. Effects of Nonsoluble Deposit Density. 4.13. Pressure Effects on Contamination Tests. 4.14. Temperature Effects on Pollution Flashover. 5. CONTAMINATION FLASHOVER MODELS. 5.1. General Classifi cation of Partial Discharges. 5.2. Dry-Band Arcing on Contaminated Surfaces. 5.3. Electrical Arcing on Wet, Contaminated Surfaces. 5.4. Residual Resistance of Polluted Layer. 5.5. dc Pollution Flashover Modeling. 5.6. ac Pollution Flashover Modeling. 5.7. Theoretical Modeling for Cold-Fog Flashover. 5.8. Future Directions for Pollution Flashover Modeling. 6. MITIGATION OPTIONS FOR IMPROVED PERFORMANCE IN POLLUTION CONDITIONS. 6.1. Monitoring for Maintenance. 6.2. Cleaning of Insulators. 6.3. Coating of Insulators. 6.4. Adding Accessories. 6.5. Adding More Insulators. 6.6. Changing to Improved Designs. 6.7. Changing to Semiconducting Glaze. 6.8. Changing to Polymer Insulators. 7. ICING FLASHOVERS. 7.1. Terminology for Ice. 7.2. Ice Morphology. 7.3. Electrical Characteristics of Ice. 7.4. Ice Flashover Experience. 7.5. Ice Flashover Processes. 7.6. Icing Test Methods. 7.7. Ice Flashover Test Results. 7.8. Empirical Models for Icing Flashovers. 7.9. Mathematical Modeling of Flashover Process on Ice-Covered Insulators. 7.10. Environmental Corrections for Ice Surfaces. 7.11. Future Directions for Icing Flashover Modeling. 8. SNOW FLASHOVERS. 8.1. Terminology for Snow. 8.2. Snow Morphology. 8.3. Snow Electrical Characteristics. 8.4. Snow Flashover Experience. 8.5. Snow Flashover Process and Test Methods. 8.6. Snow Flashover Test Results. 8.7 Empirical Model for Snow Flashover. 8.8. Mathematical Modeling of Flashover Process on Snow-Covered Insulators. 8.9. Environmental Corrections for Snow Flashover. 8.10. Case Studies of Snow Flashover. 9. MITIGATION OPTIONS FOR IMPROVED PERFORMANCE IN ICE AND SNOW CONDITIONS. 9.1. Options for Mitigating Very Light and Light Icing. 9.2. Options for Mitigating Moderate Icing. 9.3. Options for Mitigating Heavy Icing. 9.4. Options for Mitigating Snow and Rime. 9.5. Alternatives for Mitigating Any Icing. 10. INSULATION COORDINATION FOR ICING AND POLLUTED ENVIRONMENTS. 10.1. The Insulation Coordination Process. 10.2. Deterministic and Probabilistic Methods. 10.3. IEEE 1313.2 Design Approach for Contamination. 10.4. IEC 60815 Design Approach for Contamination. 10.5. CIGRE Design Approach for Contamination. 10.6. Characteristics of Winter Pollution. 10.7. Winter Fog Events. 10.8. Freezing Rain and Freezing Drizzle Events. 10.9. Snow Climatology. 10.10. Deterministic Coordination for Leakage Distance. 10.11. Probabilistic Coordination for Leakage Distance. 10.12. Deterministic Coordination for Dry Arc Distance. 10.13. Probabilistic Coordination for Dry Arc Distance. 10.14. Case Studies. APPENDIX A: MEASUREMENT OF INSULATOR CONTAMINATION LEVEL. APPENDIX B: STANDARD CORRECTIONS FOR HUMIDITY, TEMPERATURE, AND PRESSURE. APPENDIX C: TERMS RELATED TO ELECTRICAL IMPULSES. INDEX.

Insulators for Icing and Polluted Environments

FRA tests in the frequency range up to 10 MHz were investigated using simulations and measurements. It was found that the FRA test was more sensitive to changes in the transformer at frequencies above 1 to 3 MHz than at lower frequencies. However, the effects of the conventional test circuits mask the changes at the higher frequencies. A new test method is proposed that is effective up to at least 10 MHz.

Improved detection of power transformer winding movement by extending the FRA high frequency range

Online transformer condition monitoring techniques based on transfer function methods, such as transmission-line diagnostics and swept frequency-response analysis, require the injection of a known test signal into the transformer On larger power transformers, a practical method is to use the available bushing tap connection In this paper, we will discuss a custom high-power signal generator that injects high-frequency signals on the bushing tap of the transformer under investigation, as well as a circuit to replace the bushing tap short and allow online operation of the system Finally, the system is demonstrated on a 650 kV transformer

Apparatus for Online Power Transformer Winding Monitoring Using Bushing Tap Injection

Electrical insulation performance of compressed gas insulated switchgear (GIS) and gas insulated transmission line (GITL) systems is adversely affected by metallic particle contaminants. Dielectric coatings applied to the inside surface of the outer enclosure of a coaxial GIS/GITL system improve the insulation performance in several ways. Coating has the effect of smoothing the electrode surface and reducing the prebreakdown current in the gas gap. Also, the electrostatic charge acquired by a particle is reduced and hence the range of its motion under an applied power frequency field is inhibited. The movement of such particles is complex and dependent on several parameters. In this paper the dynamics of wire particles in a coaxial system under AC voltage is studied when the inside surface of the outer enclosure of a coaxial GIS/GITL system is coated with a high resistance material. Suggestions for reducing the particle excursion in GIS/GITL systems are discussed.

Dynamics of metallic particle contaminants in GIS with dielectric-coated electrodes

M. Sargent (1972) concluded that the median current of the lightning strokes collected by a structure increases when the height of the structure is increased. In the present work, it is shown that Sargent's conclusion is applicable only to the case of equal striking distances, and that Sargent's use of too low a median current to flat ground has inadvertently exaggerated the effect of height on median current. For the realistic case of unequal striking distances, the median current versus height relation is a U-curve which is relatively flat. It is found that the electrogeometric model is not incompatible with the median findings of R.B. Anderson and A.J. Eriksson (1980), and that the median amplitude of the strokes collected by power lines is practically constant over the typical height range. >

The implications of the electrogeometric model regarding effect of height of structure on the median amplitude of collected lightning strokes

The effect of trees is analyzed using a three-dimensional electrogeometric model, with emphasis on the case of 500 kV lines. The effects of the height of the trees, the width of the right-of-way, and the voltage class of the power line on the number of strokes collected by the line and their median amplitude are discussed. The number of outages experienced on transmission lines running through forests is known to be less than the values predicted using the methods published in the literature. The present analysis indicate that the phenomenon is due to large reductions in the number of strokes to the line and the median amplitude of the collected strokes. The shielding effect of the forest is significantly reduced for higher voltage lines. Thus the low outage rate of unshielded 500 kV lines does not necessarily mean that unshielded higher voltage lines (765 kV or 1000 kV). >

K.D. Srivastava

Papers

Improved detection of power transformer winding movement by extending the FRA high frequency range

Apparatus for Online Power Transformer Winding Monitoring Using Bushing Tap Injection

Dynamics of metallic particle contaminants in GIS with dielectric-coated electrodes

The implications of the electrogeometric model regarding effect of height of structure on the median amplitude of collected lightning strokes

Effect of shielding by trees on the frequency of lightning strokes to power lines