000000000001012313

Outline of Guide for Application of Transmission Line Surge Arresters42 to 765 kV

Extended Outline

1012313

Outline of Guide for Application of Transmission Line Surge Arresters42 to 765 kV

Extended Outline

1012313

Technical Update, October 2006

EPRI Project Manager A. Phillips

ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California 94304-1338 . PO Box 10412, Palo Alto, California 94303-0813 . USA

800.313.3774 . 650.855.2121 . [email protected] . www.epri.com

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT Kinectrics North America, Inc.

NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e-mail [email protected].

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.

Copyright 2006 Electric Power Research Institute, Inc. All rights reserved.

CITATIONS

This report was prepared by

Kinectrics North America Inc. 800 Kipling Avenue Toronto, Ontario, Canada

Principal Investigator W.A. Chisholm

This report describes research sponsored by the Electric Power Research Institute (EPRI).

This publication is a corporate document that should be cited in the literature in the following manner:

Outline of Guide for Application of Transmission Line Surge Arresters42 to 765 kV: Extended Outline. EPRI, Palo Alto, CA: 2006. 1012313.

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PRODUCT DESCRIPTION

Lightning flashovers are the most frequent cause of transmission line outages. Transmission line surge arresters (TLSA) limit lightning overvoltages between phase conductors and towers, and thus eliminate most outages on protected structures. This guide provides a tutorial on the relevant lightning phenomena, with an in-depth look at the operation, application, and placement of TLSA to maximize flashover protection and minimize capital investment. The guide also describes ways to improve tower grounding for better performance of overhead groundwires.

Results and Findings The guide contains an in-depth description of the following areas:

The parameters that influence transmission line lightning performance / parameters. Lightning incidence scales performance in all regions. With overhead groundwires, lightning currents act against local soil resistivity to create insulator stress. Adding arresters reduces the influence of grounding. When OHGW are removed, leaving only arresters, the lightning charge replaces peak current as a dominant stress. The section also discusses other transmission line features that affect line lightning performance, including line, tower, insulator and arrester air gap geometries; tower impedance; and nonlinear corona effects.

TLSA selection/specification. Before selecting a TLSA, utility engineers should consider a number of design questions concerning arrester operating characteristics and rating, temporary overvoltages, arrester protective levels and insulation coordination, TLSA energy capability, arrester failures, TLSA housings, and TLSA installation and handling. Selecting an arrester system (possibly including a series gap or insulator) for a particular transmission line is the process of simultaneously satisfying these concerns with a single arrester type.

Placement of arresters for improved lightning performance. The efficient application of TLSA to improve line performance requires the investigation of all available mitigation options and weighing of the performance benefits against real cost. Estimating the effects of changes in tower structure and design, shielding, grounding, and arresters on the lightning performance of transmission lines is crucial to this process. This section discusses back-flashover protection, unshielded applications, and transmission lines over varying terrain.

Challenges and Objectives One difficulty in focusing this report is the wide range of technical backgrounds of the readers. Electrical engineers will be most interested in insulation coordination and risk management. Civil engineers will be more interested in what will be gained and lost if a new line is designed without overhead groundwires (OHGW) and with compact insulation, protected by TLSA. While

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these readers will find what interests them, the main focus of this report is a utility project manager facing a decision to replace existing overhead groundwires (OHGW), the fastest-decaying transmission line component, with a typical life of 25 to 55 years. What has changed in ten years is that the decision to put up TLSA in place of OHGW is commercially and technically viable in many areas. Having this new alternative, with its reduced visual impact and peak-load loss reduction, can help the utility bottom line, especially considering that the OHGW conductors represent 4% of the total line investment.

Applications, Values, and Use New lines with reduced visual impact are already taking advantage of TLSA to replace overhead groundwires. So far, these applications have been made in areas of difficult grounding and low lightning incidence. However, the alternative of buried transmission cable looms like the sword of Damocles, motivating overhead line engineers to deliver more reliability with fewer resources.

EPRI Perspective Lightning causes power outages that cost utilities more than $1 billion per year directly, in damaged or destroyed equipment. The indirect damage to customers from all power quality problems is estimated to exceed $100 billion per year, with more than half of these disturbances having lightning as a root cause. This guide presents TLSA theory and design information to enable utilities to minimize the number outages due to lightning. EPRI developed this guide with the understanding that users may not be familiar with either TLSA or the current standards that do, or should, apply to them. The guide is tutorial in nature and does not anticipate every situation or utility need. In general, however, experience has shown that properly designed, installed, inspected, and maintained hardware such as TLSA, counterpoise, and overhead groundwires can significantly improve system reliability and power quality.

Approach In 1997, EPRI delivered a TLSA application guide (TR-108913), consolidating literature with results of a survey of 31 EPRI-member utilities. EPRI also supported arrester energy and mechanical tests at the EPRI Power Delivery Center-Lenox.

The state of the art of TLSA has advanced considerably since the last EPRI guide was published. Line arresters have proved themselves as technically and economically feasible for improving performance of conventional lines with overhead groundwires. TLSA have also been used on 230-kV and 400-kV lines without overhead groundwires, where the extra surge duties raise new electrical reliability concerns. Most of the difficulties found in applying TLSA relate to the spotty reliability of mechanical components. This can be addressed by ensuring that TLSA components meet the same high reliability standards that apply to other line components.

This document is an extended outline that will be built upon and refined over the new few years to develop a completed guide

Keywords Reliability Lightning and weather impacts Power quality Transmission lines

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ABSTRACT

In most areas, flashovers from lightning are by far the most frequent cause of transmission line outages. Transmission line surge arresters (TLSA) limit the lightning over-voltages between phase conductors and the tower structure. This prevents flashovers and insulation damage on that structure. This guide provides a tutorial of the relevant lightning phenomena, in general, and offers an in-depth look at the operation, application, and placement of TLSA to maximize flashover protection and minimize capital investment.

The guide also considers other mitigation measures, including improved tower grounding and the application of overhead groundwires. EPRI developed this guide with the understanding that users may not be familiar with either TLSA or the current standards that do or should apply to them. The guide, therefore, is tutorial and does not anticipate every situation or utility need. Overall, however, experience has shown that properly designed, installed, inspected, and maintained hardware such as TLSA, counterpoise, and overhead groundwires can significantly improve system reliability and power quality.

This document is an extended outline that will be built upon and refined over the new few years to develop a completed guide

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ix

CONTENTS

1 PURPOSE ..............................................................................................................................1-1

2 DEFINITIONS .........................................................................................................................2-1

3 WHY PROTECT TRANSMISSION LINES FROM LIGHTNING.............................................3-1 Economic Impact of Power Quality Problems .......................................................................3-1 Classification of Power Quality Problems..............................................................................3-1 Lightning as a Root Cause of Short-Duration Faults.............................................................3-3 Typical Power Line Lightning Mitigation Options...................................................................3-4 Utility Investment in Lightning Protection using Overhead Groundwires...............................3-4 Utility Investment in Other Lightning Protection Methods......................................................3-5

4 TRANSMISSION LINE LIGHTNING PERFORMANCE PARAMETERS ...............................4-1 Introduction ...........................................................................................................................4-1 Lightning Incidence Parameters............................................................................................4-2

Ground Flash Density (GFD)............................................................................................4-2 Lightning Incidence to Lines .............................................................................................4-4

Lightning Current Parameters ...............................................................................................4-8 Stroke Current Peak Magnitudes .....................................................................................4-9 Stroke Current Rate of Rise ...........................................................................................4-11 Stroke Current Waveshapes ..........................................................................................4-12 Total Charge Delivered...................................................................................................4-13 Number of Strokes in a Flash .........................................................................................4-14

Transmission Line Parameters............................................................................................4-14 Line Conductor Geometries............................................................................................4-15 Tower Geometries ..........................................................................................................4-15 Insulator / Air Gap Geometries .......................................................................................4-16

Volt-Time Curve Penetration Method.........................................................................4-18

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Disruptive Index Method ............................................................................................4-18 Leader Progression Method.......................................................................................4-19

Tower Ground Characteristics........................................................................................4-19 Buried Tower Grillage ................................................................................................4-20 Driven Ground Rods ..................................................................................................4-20 Counterpoise .............................................................................................................4-20

Transmission Line Surge Arresters (TLSA)....................................................................4-21 Nonlinear Corona Effects ...............................................................................................4-21

5 TLSA SELECTION AND SPECIFICATION ...........................................................................5-1 Introduction ...........................................................................................................................5-1 Transmission Line Arresters..................................................................................................5-1 Arrester Operating Characteristics ........................................................................................5-5 Arrester Rating and MCOV ...................................................................................................5-7 Temporary Overvoltages.......................................................................................................5-7 Arrester Protective Levels and Insulation Coordination ........................................................5-8

Lightning Insulation Coordination .....................................................................................5-9 Switching Surge Insulation Coordination........................................................................5-11

Energy Capability of TLSA ..................................................................................................5-12 Lightning Energy.............................................................................................................5-12 Switching Energy............................................................................................................5-13

Arrester Failures..................................................................................................................5-15 Electrical Failure Modes .................................................................................................5-15 Arrester Disconnects ......................................................................................................5-16

TLSA Housings ...................................................................................................................5-18 TLSA Installation and Handling...........................................................................................5-19 Handling and Installation Recommendations ......................................................................5-20

Handling .........................................................................................................................5-20 Installation and Maintenance..........................................................................................5-20 Arrester Markings ...........................................................................................................5-20

6 PLACEMENT OF ARRESTERS FOR IMPROVED LIGHTNING PERFORMANCE..............6-1 Economics.............................................................................................................................6-1 Backflashover Protection.......................................................................................................6-1

Phase Location of TLSA...................................................................................................6-1

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Coupling to Overhead Groundwires.............................................................................6-1 Crossarm Voltage ........................................................................................................6-3

TLSA Location for High Tower Footing Resistance..........................................................6-4 Unshielded Applications........................................................................................................6-5

Arrester Energy ................................................................................................................6-6 Vertical Circuits.................................................................................................................6-6 Horizontal Circuits ............................................................................................................6-8

Transmission Lines over Unchanging Terrain.......................................................................6-9 Compact Transmission Lines ..............................................................................................6-11

Compact Transmission Lines with Overhead Groundwires and TLSA...........................6-11 Compact Unshielded Transmission Lines with TLSA.....................................................6-13

7 APPLICATION SOFTWARE ..................................................................................................7-1 Introduction ...........................................................................................................................7-1 Lightning Performance Design Workstation (LPDW) ............................................................7-1 TFLASH Overview.................................................................................................................7-1 Building a TFLASH Model .....................................................................................................7-2

TFLASH Capabilities ........................................................................................................7-2 General Procedure for Constructing a Line Model ...........................................................7-2

Analyzing a TFLASH Model ..................................................................................................7-5 The Classical Solution - The Average Performance of the Line.......................................7-5 Oscillographs - Line Behavior for a User-Specified Stroke ..............................................7-6 General Procedure / Sample Application .........................................................................7-7

8 CASE STUDIES .....................................................................................................................8-1 44 kV Case Studies...............................................................................................................8-1

Comparison of OHGW versus TLSA using Customer Momentary Disturbance Benchmark .......................................................................................................................8-1 Application Experience with 44-kV Arrester Application...................................................8-3

46 kV Case Studies...............................................................................................................8-4 66 kV Case Study .................................................................................................................8-4 69 kV Case Study .................................................................................................................8-4 115 kV Vertical Line Case Study...........................................................................................8-5 115 kV Horizontal H-Frame Line Case Study .......................................................................8-7 115 kV Horizontal Steel Lattice Line Case Study..................................................................8-8

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115-kV Horizontal Line: Predicted Outage Rate...............................................................8-8 115 kV Horizontal Line: Outage Rates with Partial TLSA Treatments .............................8-8 115 kV Horizontal Steel Lattice Line: Lessons Learned ...................................................8-9

138 kV Case Studies...........................................................................................................8-10 154 kV Case Study .............................................................................................................8-10 161 kV Case Studies...........................................................................................................8-10 230 kV Case Studies...........................................................................................................8-10

230 kV Horizontal Line: Application Experience.............................................................8-10 230 kV Circuit with 35-kV Underbuild .............................................................................8-11

275 kV Case Studies...........................................................................................................8-11 400 kV Case Study .............................................................................................................8-11 500 kV Case Studies...........................................................................................................8-12 765 kV Case Study .............................................................................................................8-13

9 REFERENCES .......................................................................................................................9-1

A TLSA MECHANICAL PERFORMANCE TESTS .................................................................. A-1 Line Disconnector Testing.................................................................................................... A-1 Arrester Body Testing........................................................................................................... A-2 Evaluation of Different Arrester Installation Configurations.................................................. A-2

B TLSA ENERGY WITHSTAND TEST DATA ......................................................................... B-1 Definition of Withstand Criteria............................................................................................. B-1 Tests of 63-mm TLSA to Destruction ................................................................................... B-1 Tests of 8.4-kV MCOV Samples to Thermal Runaway ........................................................ B-1

C TRANSMISSION LINE LIGHTNING PERFORMANCE CASE STUDIES ............................ C-1

D MECHANICAL FORCE ANALYSIS FOR GAPLESS TLSA INSTALLATIONS................... D-1

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LIST OF FIGURES

Figure 3-1 Power Quality Acceptability Curves. Left: Computer Business Equipment Manufacturers Association (CBEMA) 1996; Right: Information Technology Industry Council (2000)....................................................................................................................3-2

Figure 3-2 Origins of Power Quality Disturbances < Substitute EPRI figure here > [Plata 2002] ..................................................................................................................................3-2

Figure 3-3 Power System Areas where Transmission Faults will Cause 50% and 70% Sags .........................................................................................3-3

Figure 4-1 Lightning Ground Flash Density for Continental USA, 1989-1998 [Orville Huffines] .............................................................................................................................4-3

Figure 4-2 Lightning Ground Flash Density from NALDN, 2000 to 2003 (Vaisala, need permission).........................................................................................................................4-3

Figure 4-3 Optical Transient Density Map from (NASA 2006) and Estimate of Ground Flash Density .....................................................................................................................4-4

Figure 4-4: Relation between Lateral Attractive Distance Da of Horizontal Conductor and Average Conductor Height h . Curve 1: Eriksson; Curve 2: D=2h; Curve 3: Rizk.............4-5

Figure 4-5 Striking Distances from Ground and Conductor to a Downward Leader ..................4-6 Figure 4-6: Modeling of Lightning Shielding Failures using L2 Applet [Red 2005] for Peak

Stroke Currents of 5, 15 and 25 kA....................................................................................4-7 Figure 4-7 Relation between Lightning Leader Potential and Stroke Charge [Mazur 2001] ......4-8 Figure 4-8 Lightning to Instrumented Rods on Tokyo Electric Transmission Towers

[Takami 2005] ..................................................................................................................4-10 Figure 4-9 Relation between Maximum Rate of Rise and Peak Amplitude of Lightning to

Tokyo Electric Transmission Towers [Takami 2006]........................................................4-11 Figure 4-10 Relation between Virtual Front Time and First Peak Amplitude for Lightning

to Tokyo Electric Towers [Takami 2006] ..........................................................................4-12 Figure 4-11 Percentage of Positive Cloud-to-Ground Lightning Flashes (Left) and

Density of Large-Amplitude Positive Flashes (Right) in USA [Boccippio et al .................4-14 Figure 4-12 Flashover paths for a V-String Configuration .......................................................4-17 Figure 5-1 Internal Construction of Silicon Carbide Lightning Surge Arrester ...........................5-3 Figure 5-2 General Electric 138-kV Gapped MOV TLSA in Virginia [Koch 1985, Zed

2004] ..................................................................................................................................5-4 Figure 5-3 Classification of Arrester Design Features [Richter et al 2004] ................................5-5 Figure 5-4 TLSA Volt-Amp Curve < substitute a modern version >...........................................5-6 Figure 5-5 Typical Lightning Current Distribution on an Unshielded Transmission Line

with a Top-Phase Arrester having R=20 at 40 kA. .......................................................5-11

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Figure 5-6 Effect of Small Variation in Reference Voltage on Discharge Current....................5-14 Figure 5-7 Comparison of 63-mm MOV Block Charge Absorption at Destruction with

Firing Level for DLSA disconnect [Lat CEA Guide, CEATI 3312A, permission or substitute needed]............................................................................................................5-17

Figure 6-1 Plot of Voltage versus Time at Various Points on a Double-Circuit Tower using L-5 Applet and Step Waveshape (final to use CIGRE concave) ..............................6-3

Figure 6-2 Plot of Voltage versus Time at Various Points on Double-Circuit Tower, Taking Into Account Relative Coupling from Shield Wires (final to use CIGRE concave).............................................................................................................................6-3

Figure 6-3 Schematic of Traveling Waves Propagating Towards a Structure with Low Footing Resistance: If Tower 3 has no arrester, it may flashover. .....................................6-5

Figure 6-4 A Schematic showing the Shielding Angle on an Unshielded Transmission Line with the Top Phase protected by TLSA......................................................................6-7

Figure 6-5 Options for Improving Compact Line Lightning Performance.................................6-13 Figure 6-6 Typical 115-kV Compact Line Geometry from 1980, using Polymer Post and

Semiconductive Glaze Bell Insulators [Ontario Hydro 1980]............................................6-15 Figure 7-1 Tower Modeling Screen from EPRI TFLASH (dummy) ............................................7-3 Figure 7-2 Conductor Information Screen from TFLASH (dummy)............................................7-4 Figure 8-1 Probability of Flashover on 44-kV Line in Delta Configuration with Overhead

Groundwire.........................................................................................................................8-2 Figure 8-2 69-kV Line Configurations considered by TU Electric for Improved Lightning

Performance.......................................................................................................................8-4 Figure 8-3 Application of TLSA on TU Electric 69-kV Lines [Sanders and Newman 1992] .......8-5 Figure 8-4 Voltage across 115-kV or 138-kV class TLSA compared to Insulator

Flashover Levels ................................................................................................................8-5 Figure 8-5 Strategy for Mounting TLSA with Suitable Mechanical Rating to Restrain

Conductor...........................................................................................................................8-6 Figure 8-6 Mounting of TLSA with Insufficient Horizontal Clearance.........................................8-7 Figure 8-7 Single Circuit 115-kV Structure with Single OHGW Lightning Protection

[Tarasiewicz 2000] .............................................................................................................8-8 Figure 8-8 Typical 400-kV Line Geometries at Statnett (Norway)............................................8-12 Figure 8-9 Compact 400-kV Unshielded Design with Top-Phase TLSA..................................8-12 Figure 8-10 Traveling Waves near Open Terminal ..................................................................8-13 Figure 8-11 TLSA on AEP 765-kV Line for Switching Surge Control ......................................8-14 Figure A-1 Shear and Tension Tests on TLSA Disconnects .................................................... A-1 Figure A-2 Arrester Body Bending Test Setup.......................................................................... A-2 Figure D-1 Example of Typical Conductor to Pole Suspension ................................................ D-1 Figure D-2 Example of Typical Conductorto Tower Mast Suspension ..................................... D-2

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LIST OF TABLES

Table 3-1 Typical Power Line Lightning Mitigation Options .......................................................3-4 Table 4-1 Geometric and Contact Resistance for Typical Surface Electrodes .......................4-19 Table 5-1 External Gap or MCOV Requirements for TLSA .....................................................5-21 Table 6-1 Footing Resistance at Steel Lattice Towers along Hypothetical 138-kV

Transmission Line ..............................................................................................................6-4 Table 6-2 Flash Incidence for 161 km (100 miles) of a Horizontal Circuit, 18 m (60 feet)

above Flat Terrain with Ground Flash Density of 3.9 per km2 per year..............................6-8 Table 6-3 Flashover Data for 161 km (100 miles) of Unshielded Transmission Line for

Various TLSA Installations ...............................................................................................6-10 Table 6-4 Flashover Data for 161 km (100 miles) of Vertical Circuit, Steel Pole, Shielded

Transmission Line for Various TLSA Installations............................................................6-11 Table 6-5 Effects of Compact Line Insulation and Phase Spacing on Lightning

Performance (Outages per 100 km per year)...................................................................6-12 Table 6-6 Options for Improving Compact Line Lightning Performance (Outages per 100

km per year) .....................................................................................................................6-13 Table 8-1 Comparison of Costs for 44-kV Lightning Protection Options ...................................8-3 Table 8-2 Reduction in Total Lightning Outages for Nine Treatment Options ...........................8-9 Table A-1 Observed TLSA Failure Loads ................................................................................. A-2

1 PURPOSE

The purpose of this application guide is to help utilities use transmission line surge arresters (TLSA) for the reduction or prevention of outages caused by lightning on transmission lines that operate at system voltages up to and including 765 kV. Other mitigation measures, such as improved tower grounding and the application of overhead groundwires, are considered in this guide.

The guide was developed with the understanding that users may not be familiar with either TLSA or the current standards that do or should apply to them. While it has the penalty of making this a long document, considerable tutorial material is included. Even so, the guide cannot anticipate every situation or utility need. The user must consider the particular requirements of a given application and select those criteria that fit that application. In certain situations the user may want to develop additional criteria to address a particular use.

(Description of target audience I want to help an electrical engineer to ask the right questions and make the right decisions when she receives a report that the overhead groundwires on a line have reached their end-of-life. This help includes the technical vocabulary and basic understanding of mechanical issues such as vibration, wind load and conductor restraint force needed to work efficiently with mechanical engineers doing tower head layouts.)

1-1

2 DEFINITIONS

Arc A continuous luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of the electrodes

Backflashover A flashover of insulation resulting from a lightning stroke to part of a network or electric installation that is normally at ground potential

Basic Lightning Impulse Insulation Level (BIL)

For self-restoring insulation such as air. The crest value of a standard 1.2/50 s lightning impulse for which the insulation exhibits a 10% chance of flashover under standard conditions.

Basic Switching Impulse Insulation Level (BSL)

For self-restoring insulation such as air. The crest value of a standard 250/2500 s switching impulse for which the insulation exhibits a 10% chance of flashover under standard conditions.

Chopped-Wave Lightning Impulse

A prospective full lightning impulse during which any type of discharge causes a rapid collapse of the voltage.

Chopped-Wave Lightning Impulse Withstand

The insulation strength necessary to withstand a surge exceeding the BIL, but chopped at 2 or 3 s. Transformers and circuit breakers must have a chopped-wave withstand of at least 1.15 times the BIL.

CIGR Conference Internationale des Grands Reseaux Electriques a haute tension (International Conference on Large High Voltage Electric Systems). CIGRE is an international technical organization, similar to the IEEE Power Engineering Society, which focuses primarily on transmission voltage systems. CIGRE holds a general conference in Paris every two years. The various study committees meet more frequently at other locations. CIGRE'S official publication is "Electra."

Continuing Current A small-amplitude (100 A) long-duration (tens to hundreds of milliseconds) current that flows between strokes of a lightning flash, with moderate return-stroke channel luminosity and significant transfer of charge from cloud to ground.

2-1

Definitions

Core The internal insulating part of an arrester that carries the mechanical loads and restrains the metal oxide valve blocks.

Corona A luminous discharge due to ionization of the air surrounding a conductor caused by a voltage gradient exceeding a certain critical value

Critical Current The lightning current at a specific waveshape that will produce a 50% probability of flashover when applied to a particular conductor and location

Critical Flashover Voltage (CFO)

The amplitude of the voltage of a given waveshape that, under specified conditions, causes flashover through the surrounding medium on 50% of the voltage applications

Disconnector (Arrester) A device that disconnects a failed surge arrester to prevent a permanent fault on the circuit. It also provides a visual indication of a failed arrester

Electrogeometric Model A model for the reach of the final jump of a downward lightning leader to lines, objects or ground based on electrostatic estimates of relations among leader potential, charge and current. Striking distances in this model become functions of the peak magnitude of the first stroke current.

Electromagnetic Transients Program (EMTP)

A large "industry-standard" computer program that simulates transient overvoltages on power systems using pi-section circuit approximations to distributed lines.

Electrostatics The branch of science treating electrical phenomena associated with electric charges at rest (with no time variation) in the frame of reference

Erosion The loss of material by leakage current or corona discharge. Erosion is nonconductive and can be uniform, localized or tree-shaped. Shallow surface traces can occur on insulator surfaces after arcing.

Ethylene Propylene Diene Monomer (EPDM)

A base polymer previously used in rubber housings for insulators and arrester housings, now alloyed with silicone for hydrophobicity

External insulation The external insulating surfaces and the surrounding air. The dielectric strength of external insulation depends on atmospheric conditions.

2-2

Definitions

Flashover A disruptive discharge through air around or over the surface of solid or liquid insulation, between parts of different potential or polarity, produced by the application of voltage wherein the breakdown path becomes sufficiently ionized to maintain an electric arc.

Magnetic Flux A condition in a medium produced by magnetomotive force such that, when altered in magnitude, a voltage is induced in an electric circuit linked with the flux.

Front Time (Rise Time) The time-to-peak of an impulse that is estimated by drawing a straight line through two points on the front of the wave. One point is located at 90% of the crest value; the other point is either 30% or 10% of the crest value. The front time is the first number in the description of a wave shape, i.e., 8 in a wave shape described as 8/20 s.

Ground A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth.

Ground Flash Density (GFD)

The average number of lightning flashes to ground per square kilometer per year.

Housing The external covering of a TLSA that protects the core from the weather and may be equipped with weather sheds. In some designs the housing may also include insulating materials between the weather sheds ad the core.

Hydrolysis A chemical process involving the reaction of a material with water (in liquid or vapor form) that can lead to electrical and mechanical degradation.

Induction Field (Magnetostatic Field)

The electric and magnetic fields created by a constant current.

Insulator A device intended to give flexible or rigid support to electrical conductors or equipment and to insulate these conductors or equipment from ground or from other conductors or equipment. An insulator comprises one or more insulating parts to which connecting devices (metal fittings) are often permanently attached.

Internal Insulation Insulation inside a sealed container, often holding gas or oil media. Internal insulation is usually not self-restoring.

2-3

Definitions

Insulator Arcing Horn / Insulator Arcing Ring

A metal part, usually shaped like a (Horn / Ring), placed at one or both ends of an insulator or a string of insulators to establish an arc- over path, thereby reducing or eliminating damage by arc-over to the insulator or conductor or both.

Keraunic Level (TD) The number of days in a year during which thunder was heard, expressed in days per year (TD). Used in the past to estimate lightning ground flash density but no longer recommended for this purpose, as the relation between KL and GFD varies with location.

Lightning flash The complete lightning discharge, most often composed of leaders from a cloud followed by one or more return strokes.

Lightning impulse protective level

The maximum lightning impulse voltage expected at the terminals of a surge protective device under specified conditions of operation. The lightning impulse protective levels are given by: a) Front-of-wave impulse sparkover or discharge voltage, and b) the higher of either a 1.2/50 impulse sparkover voltage or the discharge voltage for a specified current magnitude and waveshape.

Lightning first stroke A lightning discharge to ground initiated when the tip of a downward stepped leader meets an upward leader from the earth.

Maximum Continuous Operating Voltage (MCOV)

The maximum line-to-ground power frequency voltage (RMS) that is specified by the manufacturer.

Metal Oxide Varistor (MOV)

Zinc Oxide, sintered with a number of other metal elements to give a highly non-linear semiconducting material used in modern surge arresters. In contrast to the older silicon carbide arresters, MOV arresters do not require spark gaps but can benefit from them.

North American Lightning Detection Network (NALDN)

A system of broadband receivers (20-400 kHz and Global Positioning System (GPS) time) that provides ground flash density, peak radiated field and rise time data for North America.

Overhead Groundwire Grounded wire or wires placed above phase conductors for the purpose of intercepting direct strokes in order to project the phase conductors from direct strokes. They may be grounded directly or indirectly through short gaps.

Reflection Coefficient, The portion of an incident wave that, on reaching a change in surge impedance, travels back down the line towards the source. The voltage reflection coefficient is v = (Zb-Za)/(Zb+Za) where Za is the initial impedance and Zb is the new impedance encountered.

2-4

Definitions

Refraction Coefficient The portion of an incident wave that, upon reaching a change in surge impedance, travels on in the new impedance. The refraction coefficient for voltage is (2Zb)/(Zb+Za) where Za is the initial impedance and Zb is the new impedance encountered.

Return Stroke The luminescent, high-current discharge that is initiated after the stepped leader and pilot streamer have established a highly-ionized path between charge centers. Lightning current flow removes the charge deposited by the stepped leader along the stroke channel.

Self-Restoring Insulation Insulation, such as porcelain or non-ceramic insulators, that is not damaged by flashovers and soon regains all or most of its insulation strength after a flashover event.

Spark Gap An air gap between one conducting electrode that is connected to the transmission line and another conducting electrode that is connected to ground, or to the high-voltage terminal of a line surge arrester. Spark gaps were the first lightning protection devices used on transmission lines and active spark gaps, with series nonlinear elements, are still an important surge proactive device technology.

Standard Impulse Voltage Wave

A voltage waveshape with a front time of 1.2s and a time to half value of 50 s that is used to test insulation in the laboratory. Testing standards such as IEEE Standard 4 describe the allowable tolerances on this waveshape.

Stepped Leader Static discharge that propagates from a cloud into the air. Current magnitudes that are associated with stepped leaders are small (on the order of 100 A) in comparison with the return stroke current. The stepped leaders progress in a random direction in discrete time steps of 10 to 80 m in length. Their most frequent velocity of propagation is 0.05% of the speed of light. It is not until the stepped leader is within the striking distance of the point to be struck that the stepped leader is positively directed toward this point.

Stroke A highly luminous discharge component of the lightning flash. Strokes typically last less than 100 s. Each component stroke of a flash is separated by several tens of milliseconds. In many cases, a small continuing current flows between strokes.

Surge Arrester A nonlinear device used to limit transient electrical overvoltages

2-5

Definitions

Surge Impedance The intrinsic ratio of voltage to current in a conductor. Neglecting losses, Z = (L/c)w, here L and C are the inductance and capacitance per unit length for a transmission line or cable. For lumped circuits, L and C are the total equivalent inductance and capacitance.

Temporary Fault A flashover of line insulation that will clear itself after the circuit is momentarily de-energized.

Temporary Overvoltage (TOV)

An oscillatory overvoltage, associated with switching or faults (for example load rejection, single-phase faults) and/or nonlinearities (ferroresonance effects, harmonics) of relatively long duration, which is undamped or slightly damped.

Tower or Pylon A structure that supports overhead transmission line conductors.

Transmission Line Surge Arrester (TLSA)

Arresters designed specifically for application on transmission lines to prevent line insulation flashovers. They may be gapped or gapless.

Upward Leader A stepped leader that travels up toward the cloud from a tall object, such as a transmission tower, skyscraper, or mountain-top. Most stepped leaders are downward traveling. Downward leaders tend to have higher potential and charge, both of which lead to higher first return stroke peak current magnitudes.

Volt-Time Curve The curve relating the disruptive discharge voltage of a test object to the time of chopping, which may occur on the front, at the crest or on the tail. The curve is obtained by applying impulse voltages of constant shape, but with different peak values.

Wave Velocity The speed at which voltage and current disturbances travel through a circuit. Neglecting losses, v = (1/LC), where L and C are the inductance and capacitance per unit length for a transmission line or cable.

Weathershed The part of a housing that increases the distance measured along insulating surfaces (leakage distance) between the conductive parts of an insulator. Weathersheds also provide protected bottom surfaces that tend to stay dry in wet weather, further improving electrical flashover performance.

2-6

3 WHY PROTECT TRANSMISSION LINES FROM LIGHTNING

Economic Impact of Power Quality Problems

Citation from 2001 EPRI report, noting power disturbance problems cost the US economy between $119B and $188B per year. [IEEE Spectrum Jan 2006]

Classification of Power Quality Problems

Standards have been developed by several interested parties to negotiate what constitutes acceptable power, through the use of time-duration curves of disturbances. Important standards have been recommended by:

Computer Business Equipment Manufacturing Association (CBEMA) Information Technology Information Council (ITIC) Semiconductor Equipment and Materials International (SEMI-47) American National Standards Institute (ANSI) C84.1 Customer computer equipment used to meet the CBEMA standard for power quality, shown in Figure 3-1. Ride-through for short-duration sags (70% voltage dip for 100 ms) in the 1970s was provided by rotating machines or other locally stored energy for mainframe computers. Linear power supplies with large electrolytic capacitors provided this function in electronic equipment of the period.

Revisions to the CBEMA curve were made by its replacement, the Information Technology Industry Council (ITIC), starting in 1996 and adopted in 2000. Figure 3-1 shows that the general nature is the same, but there are differences in detail. The ITIC:

Raised the tolerance level for short-duration overvoltages, because these are easy to eliminate with surge protective devices inside the equipment

Reduced the tolerance level for short-duration voltage sags, because providing additional energy storage in typical switching power supplies adds cost, consumes more power

3-1

Why Protect Transmission Lines from Lightning

0.0001 0.001 0.01 0.1 1 10 100 1000-100

-50

0

50

100

150

200

250

TIME IN SECONDS

PE

RC

EN

T C

HA

NG

E IN

BU

S V

OLT

AG

E

8.33

ms

OVERVOLTAGE CONDITIONS

UNDERVOLTAGE CONDITIONS

0.5

CYC

LE

RATEDVOLTAGE

ACCEPTABLEPOWER

10%+--

0.0001 0.001 0.01 0.1 1 10 100 1000

-100

-50

0

50

100

150

200

250

TIME IN SECONDS

PER

CEN

T C

HAN

GE

IN B

US

VOLT

AGE

8.33

ms

OVERVOLTAGE CONDITIONS

UNDERVOLTAGE CONDITIONS

0.5

CY

CLE

RATEDVOLTAGE

ACCEPTABLEPOWER

Figure 3-1 Power Quality Acceptability Curves. Left: Computer Business Equipment Manufacturers Association (CBEMA) 1996; Right: Information Technology Industry Council (2000)

Figure 3-2 classifies typical voltage dip duration curves for six different power quality disturbance root causes. Nearby Distribution Faults are classed as under-voltage conditions in both the CBEMA and ITIC curves. Fuse Operations sit right on the CBEMA curve of acceptable power, but are classed as unacceptable in the ITIC graph.

Transmission faults lead to short-duration voltage dips of three to ten ac cycles (50 to 200 ms) that affect a large number of customers at once. For the Transmission Faults in Figure 3-2:

15% are classified as under-voltage conditions in the CBEMA curve 35% are classified as under-voltage conditions in the ITIC curve

Figure 3-2 Origins of Power Quality Disturbances < Substitute EPRI figure here > [Plata 2002]

3-2


Lightning as a Root Cause of Short-Duration Faults

EPRI has made extensive simulations of power system disturbances, leading for example to the area map in Figure 3-3 where transmission faults will cause unacceptable 70% voltage dips.

Figure 3-3 Power System Areas where Transmission Faults will Cause 50% and 70% Sags

In the area of 70% vulnerability of Figure 3-3, there are about 190 km (120 miles) of trans-mission lines. In the USA, with its average level of lightning risk, about 120 flashes would strike these average transmission lines every year.

If the transmission lines are completely exposed to lightning, like distribution lines, then every flash would cause a flashover. Protective relaying would operate each time, leading to 120 unacceptable voltage dips at the customer load every year. These would not be spread evenly over the year, however about 80 of the 120 would occur in July and August.

Normal practice would be to protect the transmission lines with overhead groundwires (OHGW). These will steer lightning away from the phase conductors and lead it safely to ground. The combination of OHGW, high insulation strength and good grounding from wide-base towers is highly effective, especially in the central US where the soil resistivity is low.

With OHGW and good grounding, the lightning outage rate for a typical transmission line will be less than 1 tripout per 100 km per year. For the sensitive area in Figure 3-3, this works out to (190 km x 1 tripout per 100 km per year) or an average of 1.9 voltage dips every year. Put into other terms, 2 of the 120 flashes will cause faults on the protected line, an efficiency of 98.3%.

3-3


Typical Power Line Lightning Mitigation Options

The consequences of a lightning fault depend to a large extent on the operating voltage of the line. Figure 3-3 has shown that there will be a region around the fault where customers may record an unacceptable voltage dip. The number of customers in the affected area will scale somewhere between linearly and quadratically with the system voltage, as higher-voltage lines tend to use larger conductors or bundles.

The efficiency of various types of lightning protection also varies with operating voltage. High system voltage usually calls for larger electrical clearances, and lightning impulse strength scales linearly at about 500-540 kV per meter of clearance. As an additional factor, however, the insulation requirements for uniform lightning performance also vary with local soil type.

Table 3-1 Typical Power Line Lightning Mitigation Options

Flashover Type

Influence of Poor* Soil

System Voltage 42 to 100 kV

System Voltage of 100 to 230 kV

System Voltage of 275 to 765 kV

Typical Impulse Insulation Strength (CFO)

No effect on strength

150 to 550 kV

0.3 to 1 m

550 to 1100 kV

1 to 2 m

1100 to 2200 kV

2 to 4 m

Induced Overvoltages 30% extra stress

Add TLSA**, or 300-400 kV CFO Induced flashovers not likely

Shielding Failures No effect

Add TLSA to top and poorly protected phases

Add second overhead groundwire, and/or move them outboard

Add TLSA to poorly protected phases

Backflashover 9x more stress on

tower with poor soil

Add TLSA to top phase(s) Add TLSA to bottom phase(s)

Improve grounding

Add TLSA on bottom phases

Improve grounding

Add gapped TLSA

* Poor soil = 1000 m where 100 m is typical; ** TLSA = Transmission Line Surge Arrester

Utility Investment in Lightning Protection using Overhead Groundwires

Overhead groundwires (OHGW) in Table 3-1 are used for systems operating above 100 kV to provide effective lightning protection over a long service life. Simple visual cues such as broken strands or missing wire signal the end of OHGW service life. However, they do not carry any power in fact, they dissipate power at peak loads because nearby phase conductors induce circulating currents. One evaluation of the value of OHGW protection was carried out [Red 2006] using a benchmark of the cost per avoided customer momentary disturbance. Using the example in Figure 3-3, this benchmark is:

The cost of avoiding 118 of the 120 unacceptable voltage sags using OHGW (The number of affected customers in the area) times (118 avoided disturbances)

3-4


A rough guide is that the OHGW protection adds a total of 10% to the line cost. This breaks down as 4% for the extra conductors (there are usually two), 3% for stronger towers and 3% to make up generation capacity for the peak-power losses. Using typical transmission line cost of $US 100k per mile, the utility capital investment in OHGW in the sensitive area of Figure 3-3 works out to $1.2 million. With about 100,000 customers in the area, the benchmark works out to about 10 per avoided customer momentary disturbance.

Depending on the system voltage, this value was found to range from 1 to 14 in (Red 2006). The value is lower for areas of dense lightning and for EHV lines that take advantage of low-reactance phasing to limit induced-current power consumption at peak loads.

Utility Investment in Other Lightning Protection Methods

Utilities have other options to OHGW, and have used them with moderate success in areas of minimal lightning like California, British Columbia, New Brunswick, Newfoundland and Quebec. Some utilities reduce their sensitivity to single-phase to ground flashovers by providing single-pole reclosing. Others provide redundant transmission paths. However, high rates of multiple-pole flashover from lightning have forced some of these utilities to retrofit protective measures, such as transmission line surge arresters (TLSA).

This guide covers two important applications of TLSA as lightning protection:

1. To improve the efficiency of OHGW protection. If the OHGW is reaching 98.3%, there is not much more to be gained. However, if the ground resistivity is high, OHGW efficiency can fall off to 30-40%. Spot treatment of high-resistance towers (and their neighbors) is also a feasible option.

2. To replace the OHGW completely. The protection budget above $1.2 M would cover an investment of $2k per tower (at five towers per mile). Again, depending on ground resistivity and insulation level, this investment may be better spent on TLSA than on OHGW.

3-5

4 TRANSMISSION LINE LIGHTNING PERFORMANCE PARAMETERS

Introduction

The parameters that govern transmission line lightning performance fall into two broad categories:

1. Those that govern lightning incidence to a line

2. Those that govern the development of insulator and air gap voltages when lightning hits a line or hits the earth near a line

The designer can substantially improve line lightning performance by paying careful attention to the parameters in both categories.

It should be recognized that lightning flashovers are meteorological phenomena that can vary widely from year to year. As such, it is easy to understand that 100% accuracy in forecasting flashover rates is no more possible than 100% accuracy in long-range weather forecasting [r1], except for the situation where TLSA suppress nearly all flashovers that would otherwise normally occur. It should also be noted that the number of lightning flashovers can vary widely from one year to the next. One should not be surprised at a variation of 3:1, and sometimes substantially more than that.

Lightning performance calculations are useful for the comparison of line designs in the same meteorological environment. The designer can compare design options, understanding that while the absolute performance of the line may be uncertain, the design is the best option evaluated based on performance requirements, economic considerations, soil conditions and terrain, and the estimated average ground flash density (GFD) where the line is to be located.

It is possible to perform some limited lightning performance calculations manually, but the evaluation of multiple design options generally requires the use of a high-speed computer. Transmission line lightning computer programs forecast an average expected flashover rate by first assuming a prescribed average lightning incidence to a line that is based on average or median thunderstorm weather records. Some programs also provide estimates of return periods, i.e., probabilities that flashover rate X will be exceeded only once in Y years, based on statistical distributions of lightning currents and GFD statistics. Transmission line lightning programs

4-1

Transmission Line Lightning Performance Parameters

perform these calculations using concepts outlined in this chapter. Application software is discussed in more detail in Section 7.

Lightning Incidence Parameters

"Lightning incidence" is a generic term relating to the likelihood of lightning hitting a line over a designated period of time. To assess line performance, engineers usually refer to the number of flashes to a line per 100 miles, or 100 kilometers, per year. With this information they can use modern analytical procedures to determine the number of flashes to each overhead groundwire and phase conductor. These values are fixed by GFD and the "electrogeometric" twists and contortions of each lightning leader as it approaches the line or the earth.

Ground Flash Density (GFD) Every transmission line is located in a specific meteorological environment. Therefore, a key lightning incidence parameter is the average number of flashes to earth per square kilometer, per year along the line corridor. This parameter, called the ground flash density (GFD), is determined by averaging years of ground flash counts recorded by electronic locating systems. This technology has been operated for fifteen years or more, giving satisfactory average values of GFD even in areas of North America where thunderstorms vary substantially from year to year and the density is low. Figure 4-1 shows a ten-year average GFD for the USA.

4-2


Figure 4-1 Lightning Ground Flash Density for Continental USA, 1989-1998 [Orville Huffines]

Figure 4-2 Lightning Ground Flash Density from NALDN, 2000 to 2003 (Vaisala, need permission)

For those regions where the GFD is unknown or of dubious accuracy, engineers ten years ago used the local "keraunic level" as an alternative means of estimating the GFD. The keraunic level is defined as the average number of days per year at a specific location on the ground that someone can expect to hear thunder. Weather bureaus in many countries keep yearly records of the keraunic level at their meteorological observation points. From these records, national or international isokeraunic maps can be prepared to show contours of constant keraunic level.

A rough relationship between local GFD and keraunic level is given by:

1.125/1 054.004.0 hd TTGFD == Equation 4-1: Ground Flash Density from Annual Thunder-Days or Thunder-Hours

Where: GFD = average flashes to earth/km2/year, Td = average thunder days per year (keraunic level) and Th = average thunder hours per year (keraunic level).

This relationship, recommended by IEEE [2] and CIGRE [3] ten years ago, assumes that the ratio of the number of cloud-to-cloud flashes to the number of cloud-to-ground flashes is the same in both tropical and temperate zones. There is considerable evidence that this is not the

4-3


case, but Equation 4-1 should be roughly correct for temperate zones. Other possible relationships have been tabulated in [Red Book 1981].

In tropical areas with more than 100 thunder-days per year, estimates of GFD using Equation 4-1

alf

for moderate climates give unreasonably high results. Even for temperate climates, there is factor-of-two variation: the relation between GFD and TH in Canada [CEA 179 T 372] has hof the slope of the relation in Equation 4-1. Starting in 1995, this problem was addressed with measurements of optical transient density (OTD), carried out by satellite. Comparison of OTD with GFD values suggests that there are roughly three optical flashes for every ground flash. Anup-to-date map of OTD is given by [Bocc NASA]

Figure 4-3 nsient Density Map from (NASA 2006) and Estimate of Ground Flash Density

Lightning Incidence to Lines

From the regional value of GFD, the approximate number of flashes per year collected by a

Optical Tra

transmission line in the same region is given by Equation 4-2 [2,3]:

( )bhGFDN ts += 6.02810 Equation 4-2: Eriksson Expression for Number of Flashes to Transmission Line

Where Ns = number of flashes to a line 100 km/yr, GFD = ground flash density (flashes/km2/yr), ht = overhead groundwire height at the tower (m) and b = horizontal separation distance between overhead groundwires (m).

4-4


Erikssons expression in Equation 4-2 uses the overhead groundwire height at the tower, ht, which can be confusing. Most other expressions use the average height of the wire over ground h given by this height at the tower minus two-thirds of the sag, assuming that the terrain is flat. Other equations for h have been proposed for rolling and mountainous terrain. If no overhead

int

groundwires exist, h in Equation 4-2 becomes the average height of conductor attachment poat the tower, and b is the distance between the outmost phases. If only one shield wire exists, b isalso zero. A preferred expression for average flash incidence as a function of first peak return stroke current I and average conductor height h is:

( )bhIGFDNs += 45.069.014.310 Equation 4-3: Preferred Expression for Lightning Flash Incidence to Power Lines

This expression gives similar numerical estimates to the Equation 4-2, as shown in Figure 4-4, and has a much stronger basis in the physics of switching-surge gap flashover.

Figure 4-4: Relation between Lateral Attractive Distance Da of Horizontal Conductor and Average Conductor Height h . Curve 1: Eriksson; Curve 2: D=2h; Curve 3: Rizk

Flashes to a line terminate either on one, of the overhead groundwires (if any exist) or on one of the of the insu e line has very high insulation strength, and the first stroke is weak, one of the subsequent strokes is likely

phases. A strike to a phase, known as a "shielding failure, will usually cause flashover lation at one or more towers which may involve one or more phases. Even if th

4-5


to have enough energy to cause flashover. This means that every shielding failure can be considered to cause a shielding failure flashover.

The "electrogeometric theory" of shielding failures is reviewed in [Red Books, IEEE 1243]. The model is based on an assumed perfect correlation between leader charge and flash current. stepped downward leader of a flash approaches a li

As a ne, the last step has a choice of striking the

earth or jumping to a shield wire or phase wire (Figure 4-2). The "striking distance" to a shield

m Lines to Vertical Leader (IEEE 1243) Where: r = striking distance, leader tip to nearest conductor (m) and I = stroke current (kA). The

wire or phase wire is approximated by Equation 4.3

65.010Irc = Equation 4-4: Recommended Striking Distance fro

striking distance, rg from the same leader tip to the earth is given by:

[ ] 65.0)43ln(7.16.3 Ihrg += Equation 4-5: Recommended Striking Distance from Earth to Vertical Leader (IEEE 1243)

where: rg= striking distance to earth (m) and h = average conductor height, < 40 m.

Theconsidered to be the only one that will develop from streamers and leaders into a full return

shortest striking distance from the leader to overhead groundwire, phase or ground is

stroke.

Figure 4-5 Striking Distances from Ground and Conductor to a Downward Leader

4-6


Further research has adjusted some of the electrogeometric relationships, with a view towards dev ielding failure rate accurately. The expression in Equation 4-3 is one model that accomplishes this.

mines the

,

ell-.

eloping a unified model that predicts both the flash incidence and the sh

The flash termination point depends not only on the location of its tip as it approaches the line but also on the current, I, that will be delivered by the first stroke in the flash. This current is proportional to the leader charge near the leader tip, and it is this charge that deterstriking distances rc and rg The final breakdown of the air is assumed to occur over the shortest distance and this determines the anchoring point for the stroke current. Electrogeometric theoryas outlined in [Red Books] or [Rizk 1990], can be applied over the expected wide range of lighting amplitudes (from 3 to 200 kA), and the results integrated to estimate the number of expected hits to each shield or phase wire. Alternately, numerical simulations can be performed.An applet (LI2) provided with this guide can be used to explore the various models of shielding failure. Figure 4-6 illustrates the occurrence of a few shielding failures, even for a wshielded double circuit line, when some randomness is introduced into the leader propagation

P(I5 kA) = 1%

4-7


P(I15 kA) = 13%

P(I25 kA) = 36% Figure 4-6: Modeling of Lightning Shielding Failures using L2 Applet [Red 2005] for Peak Stroke Currents of 5, 15 and 25 kA

Lightning Current Parameters

For computational purposes, a lightning flash to a line is idealized as a vertical, infinite-impedance, surge current source. The actual surge impedance of a flash channel is still a matter of discussion after a century of lightning research. Values from 300 to infinity have been used. In principle, the flash acts like a single-conductor lossy transmission line that has been lowered from an overhead thundercloud into the vicinity of a line, charged to extremely high voltage and then suddenly connected to the line. Estimates of the leader potential V can be plotted against leader charge Q, both obtained from multiple-point synchronized measurements of electric fields at ground level, as shown in Figure 4-7. The relation CLeader=Q/V then gives an estimate that these vertical charged rods with their corona envelopes have capacitances of about (13.2 MV per coulomb) or 75 nF in the area where [Mazur 2001] carried out these studies.

4-8


Figure 4-7 Relation between Lightning Leader Potential and Stroke Charge [Mazur 2001]

The return stroke is a traveling wave of current that rushes upward, discharging the stored charge on the leader channel and the corona envelope around it. The return stroke current wave of zero potential moves upward at roughly one-third of the speed of light. Charge conservation forces a high current transient out the base of the channel, passing through the stricken point. When the flash discharges a charge pocket in the cloud overhead, it may upset the voltage distribution in the cloud and cause other charge pockets to flash to the channel, creating multiple current transients that can be measured on the ground as a series of high current peaks.

The return stroke current, traveling up the stroke channel, creates a powerful electromagnetic field that is well modeled by a simple transmission line model [Uman Maclain Krider 1975]. This creates a burst of static, familiar to people using AM radio in the summer, that can be detected with broadband receivers at distances of more than 600 km. Calibrated, remote measurements of EM fields can be inverted to estimate the strength of the return stroke current and, in some terrain, its rise and fall times. This physics is what makes lightning location networks practical and effective for studying lightning parameters.

The lightning parameters important in establishing line performance are:

3. Stroke current peak magnitude 4. Stroke current rate-of-rise 5. Stroke current waveshapes 6. Total charge delivered 7. Number of strokes in a flash

4-9


Stroke Current Peak Magnitudes

When a lightning flash terminates on a shield or phase wire, the likelihood of a line flashover depends to a great extent on the peak magnitudes of all the strokes in the flash and, for each stroke, on the time interval that each stroke current requires to rise from near zero to its crest value, i.e. the "front time." The IEEE Guide [2] and [I] suggest a simple probability equation (Equation 4.6) to describe expected peak currents in any first stroke in a flash:

6.2

311

1

+=

IPf

Equation 4-6: Probability Distribution for First Return Stroke Peak Current

where: Pf = probability that any first stroke in a flash will equal or exceed stroke peak current, I and I = first stroke peak current magnitude (kA).

Field observations have shown that subsequent strokes in a flash are often (but not always) weaker in peak amplitude than first strokes. A cumulative probability equation is given in [IEEE 1243] for subsequent stroke peak magnitudes as:

7.2

121

1

+=

ss

IP

Equation 4-7: Probability Distribution for Subsequent Return Stroke Peak Current

Where: Ps = probability that any subsequent stroke in a flash will equal or exceed peak current magnitude, Is and Is = subsequent stroke peak magnitude (kA).

Lightning location systems give good estimates of the product of (peak stroke current) times (return stroke velocity). If, using wishful thinking, the first return stroke velocity v is fixed (and v=0.3c is a common value) then the current can be established with an error of less than 10%. The log-normal statistical distribution of peak currents will then generate a log-normal distribution of remote peak radiated fields. With more than 107 remote measurements per year, the remote peak-field distributions in wide-area networks have proved to be log normal.

Using pessimistic thinking, the return stroke velocity and peak stroke current are perfectly correlated, a condition needed to satisfy charge conservation (Wagner 1963). If an empirical model for perfect correlation is adopted, as for example by (Chowdhuri et al 2004), then the log-normal distribution of peak stroke current should generate remote radiated field statistics that have high skew and kurtosis. They do not (Chisholm and Cummins 2005).

Data tend instead to support a hypothesis that charge and peak stroke current are only moderately correlated, like many other variables in lightning parameters. Berger found that the Pearson

4-10


correlation coefficient R between first-stroke peak current and impulse charge was moderate (R=0.77). The correlation observed by [Mazur 2001, Red 2005] recently between the stroke charge and the peak radiated field was fairly similar, at R=0.6, with the relation between stroke charge and leader potential in Figure 4-7 being much stronger (R2=0.61 or R=0.78).

This will be a difficult issue to sort out in future research. Measurements of stroke currents to towers can be helpful. The enhancement of the radiated field from speed-of-light propagation in tall towers is well established: For example 9-kA flashes to the CN Tower in Toronto should and do read out as 30-kA flashes in the NALDN lightning detection network, using the fixed return stroke velocity of v=0.3c. This unfortunately means that measurements on short towers will be more meaningful unfortunate because short towers receive few flashes per year, leading to a long-term research commitment.

One ten-year commitment was reported by [Takami, Okabe 2005] for Tokyo Electric Company. Sixty transmission towers were instrumented, leading to 120 flashes for analysis, including those shown in Figure 4-8. Rogowski coils were used to record impulse currents above 9 kA with good high-frequency response.

Figure 4-8 Lightning to Instrumented Rods on Tokyo Electric Transmission Towers [Takami 2005]

Stroke Current Rate of Rise

Line flashovers are determined not only by stroke current magnitudes but also by stroke current rate of rise (time derivative). The overall voltage rise with a series R-L circuit is given by V = RI + L dI/dt, where R is the resistance of the path and L is the inductance. For transmission lines, L is the tower or down-lead inductance and the R is the resistance of the grounding system.

The faster the current changes through the tower inductance, the greater the voltage across that inductance and, hence, the greater the tower top voltage and the voltage at the tower cross arms. A significant component of any insulator voltage created by a lightning flash to a tower or

4-11


overhead groundwire will be the inductive voltage drop created by stroke currents flowing in the towers. This component is described in [Red Books] in more detail. If a flash hits the earth near a line, the voltage coupled into the line is also dependent on the rise time. A cumulative probability equation for the maximum rate of rise of negative first strokes in a flash is given in [Chowdhuri IEEE, Red 2005] as:

4max

241

1

+=

SPSm

Equation 4-8: Probability Distribution for First Return Stroke Maximum Steepness Smax

Where: PSm = probability that the rate of rise in a first stroke will equal or exceed Smax, expressed in (kA/s). A median value of Smax =18.9 kA/s is given by [Takami 2006] for transmission towers as an alternative to Bergers median value of Smax=24 kA/s on standalone masts and chimneys.

Figure 4-9 Relation between Maximum Rate of Rise and Peak Amplitude of Lightning to Tokyo Electric Transmission Towers [Takami 2006]

Takami found that the maximum steepness of lightning flashes to transmission towers was highly correlated with peak current, with R=0.82 and poorly correlated with front time (R=0.26). This supports evaluation of the circuit response with a fixed 2-s virtual front time, as in the IEEE FLASH program, as shown in Figure 4-10.

4-12


2 s Ramp

Figure 4-10 Relation between Virtual Front Time and First Peak Amplitude for Lightning to Tokyo Electric Towers [Takami 2006]

Typical first return strokes have a concave shape that ensures the maximum rate of rise occurs nearly at the same time as the peak of maximum current. In contrast, laboratory test waves with 1.2/50 s waveshape have Smax at t=0 and zero slope (dI/dt=0) at the peak of current wave. This means that calculations of insulator voltages using double-exponential current waves in EMTP tend to understate the importance of component inductance.

The contribution of tower inductive voltages to insulator voltages depends very much on the stroke wave shape.

Stroke Current Waveshapes

No two stroke current wave shapes are exactly alike, and the variations in wave shape are substantial. Some examples- are given in [Red Books, Narita 2000 etc]. Computer programs calculate line lightning performance by assuming one of the following:

1. A linear-rising front (such as 2 s in the IEEE FLASH program [4]) 2. A double exponential wave shape such as the Heidler wave in the EPRI L4 applet 3. A concave front, such as in the CIGRE wave in the L4 applet provided with this guide For line performance where resistive terms are important, all models are acceptable, depending on what is done with it within the program. However, the linear-rising front and concave front both have Smax at the peak of current wave, and they are both superior to the double exponential waveshape in this important respect.

4-13


Total Charge Delivered

Most of the charge by a stroke current is delivered after crest current is reached. If one integrates the stroke current wave to obtain the charge delivered, it becomes obvious that the charge delivered is governed by the tail time.

It is the charge in a lightning flash that determines the energy fed into TLSA, and it is also the charge that causes pitting, burning, and ablation of overhead groundwires at contact points. It should be noted that, between some stroke current peaks and at the current decay at the end of lightning flashes, a low, continuing current of hundreds of amps can flow for hundreds of milliseconds. The product of 100 A and 100 ms is a charge of 10 coulomb, and this means that the charge delivered by continuing currents can greatly exceed the charge from the current peaks in a flash. These low currents, because of their longer duration, act somewhat like an arc welder and in fact plasma cutting torches make realistic test apparatus when simulating this damage on optical-fiber groundwires.

Berger [5] integrated the current records of downward flashes to Mount San Salvatore in Switzerland to determine the charges delivered. The results can be approximated by:

7.1

71

1

+=

QPC

Equation 4-9: Probability Distribution for Negative-Flash Charge

2

851

1

+=+

QPC

Equation 4-10: Probability Distribution for Positive-Flash Charge to 2 ms

Where: PC- is the probability that a negative flash will deliver a total charge equal or greater than Q and PC+ probability that a positive flash will deliver a total charge equal or greater than Q, with Q expressed in coulomb.

Berger reported that in one case a positive charge reached 300 coulombs. Note that positive flashes tend to deliver almost 10 times as much charge as negative flashes. Positive flashes are infrequent but there is some evidence in Figure 4-11 that the central US is an area where there can be a large number of high-amplitude positive flashes, associated with severe thunderstorms there.

4-14


Figure 4-11 Percentage of Positive Cloud-to-Ground Lightning Flashes (Left) and Density of Large-Amplitude Positive Flashes (Right) in USA [Boccippio et al

Number of Strokes in a Flash

The previous section provided two equations that describe the probabilities of observing magnitudes of first and subsequent strokes in a flash. More than half of all observed lightning flashes contain more than one stroke, and the average number of strokes in a flash is roughly three. The subsequent strokes in a flash tend to have a much faster rise time than first strokes, but they tend to be lower in amplitude. A number of studies [I] have concluded that, in calculating line lightning performance, one can assume that the first stroke in a flash is generally at least as severe as subsequent strokes, so the latter can be ignored. This certainly does not hold true in every case. When evaluating circuit breaker reliability, for instance, a subsequent transient can arrive just as breaker contacts are opening and cause a breaker restrike. Also, as mentioned previously, a weak first stroke causing a shielding failure to a phase conductor is likely to be followed up with one or more subsequent strokes, each with a median 12-kA current.

Transmission Line Parameters

There are many transmission line parameters that govern line lightning performance. The six fundamental categories are:

1. Line conductor geometries 2. Tower geometries 3. Insulator /air gap geometries 4. Tower ground characteristics 5. Transmission line surge arresters 6. Nonlinear corona effects

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Because these parameters can vary from one tower to the next, calculating line lightning performance in a rigorous way is very difficult, requiring a lot of program input data and computational effort.

Line Conductor Geometries

Line conductor geometry is a dominant factor in line lightning performance. The size and location in space of the line conductors determines which conductor is hit and how often. The spacing between conductors and their height above the ground determines how they couple electromagnetically to one another and to the lightning stroke. The height, sizes and bundling of conductors establishes the "surge impedance," an important variable that in turn determines the voltage on a conductor for a given stroke current flowing through it.

Conductor resistance is normally ignored in lightning surge calculations, since far more distortion of voltage and current wave shapes is created by corona than by the resistance of the metal. Because of the short distances of propagation (usually only a few spans), even the difference between steel in the overhead groundwires and aluminum in the phase conductors is usually ignored, unless induced power losses in overhead groundwires also have to be calculated. The high frequencies involved in lightning surge currents cause a substantial skin effect, with practically all currents flowing on the outer surfaces of the line conductors. Very little current flows inside, where the steel cores of aluminum conductor steel reinforced (ACSR) are located. In a similar fashion, it is assumed that the skin effect causes earth currents to flow on the surface of the earth, except around the ground electrodes at each tower.

Tower Geometries

Tower geometry is an important parameter in the insulator voltage development process. A typical transmission tower might have an equivalent inductance on the order of 20H, and a lightning surge current flowing down that tower, changing at a rate of 75 kA/s, would create a tower top voltage of 1500 kV with respect to ground. A substantial part of this voltage appears at the tower crossarms, and consequently, across the line insulators connected to the crossarm.

Tower shapes and heights vary widely, from simple wood poles with one ground wire to tall lattice river crossing structures. Some towers have guy wires stretching out to earth anchors that further complicate the analysis of their electromagnetic contributions to insulator voltage. Whatever the tower geometry, the following two fundamental parameters are needed for analysis:

1. Tower Height - The tower height determines the travel time of lightning transients from top to bottom. All other variables being equal, if the tower height is doubled, the inductive component of voltage across each insulator will double.

2. Tower Surge Impedance - A transmission tower can be regarded as a network of metallic elements, each with a finite travel time for any transient current moving along it. In effect, the tower becomes a network of short transmission lines carrying current from the tower top

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overhead groundwires to the earth below, where some of the current enters the earth resistance and some reflects back up the tower toward the top. Then the tower itself can be considered a short vertical transmission line. Like any transmission line its surge response (voltage per unit current) can be described by a scalar surge impedance and a travel time. The surge impedance and travel time are different for different tower geometries. A rough value for a conventional lattice tower might be 150 , but this can vary substantially. Simple towers, including pole bonds, have travel times given by the tower height divided by the speed of light, while towers with crossarms have multiple paths that add complications. The contributions of tower surge impedance to lightning voltages across insulators are discussed in some detai

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