Transmission Line Surge Arrester Application and Overhead … · 2018-08-23 · Transmission Line Surge Arrester Application and Overhead Shield Wire Grounding Issues [North American

Transmission Line Surge Arrester Application and Overhead Shield Wire Grounding Issues [North American experience]

Presented at the CIGRE Committee C4 Colloquium on

“Lightning and Power Systems”

In Kuala Lumpur, MalaysiaMay 17, 2010

Location of New Brunswick

NB Power Transmission System

· Includes 6800 km of transmission lines

·Voltage classes include 69, 138, 230 and 345 kV

Lightning Flash Density Map

Transmission Line Lightning Protection“Traditional” Approach

· In the traditional approach shield wires are used to capture the lightning strike

· Lightning performance using shield wires is a calculated prediction based on probabilistic methodologies

· Good lightning performance with shield wires is achieved with good grounding

“Traditional” versus “Arrester” Approach

· The “Traditional” approach using shield wires is to conduct the lightning strike into the ground before a flashover occurs

· The “Arrester” approach is to conduct the lightning strike around the insulator string without a flashover in the air


· In the “Traditional” approach, a lightning strike directly to a phase or a high energy lightning strike can still cause a flashover

· In the “Arrester” approach, a high energy lightning strike or a defective arrester can cause an arrester failure and a system line to ground fault


· In the case of shield wires:· Low resistance grounding is necessary for good

performance

· In the case of arresters applied to all insulator strings:· Good performance is not dependent on the ground

resistance


Shield Wire Grounding· It is not always possible to achieve low resistance grounding at every structure

Arrester Energy Sharing· The ability of transmission line surge arresters to share in the discharge of lightning current can be an important consideration

345 kV Transmission Line Lightning Performance Mitigation

· In 2007, NB Power constructed a 95 km 345 kV transmission line


· In 2008, this new 345 kV transmission line experienced 4 lightning outages

· The design target for lightning outages is typically 1 outage per year

· 4 outages is a significant deviation from typical lightning performance on existing 345 kV lines on the NB Power system


· Existing NB Power 345 kV lines use a guyed portal design

· The new line uses a tubular H-Frame design· The shielding angle on the new line is identical to existing portal structure design

· Clearances are slightly reduced on the middle phase for the new H-Frame design


Mitigation Strategy

· Determination of location of the four lightning strikes· Fault Locating Relays· Vaisala Lightning Detection System· Field inspections

· Determination of the cause of the lightning outages· Grounding System analysis· Field testing· Helicopter aerial inspection


No jumpers around bolted connections

Steel foundation is insulated by protective coating

Rust from weathering steel dripping onto

top insulators

Considerations Included:


· The Vaisala Lightning Detection system provided a Strikenet report for each of the 4 lightning strikes

· Within the Strikenet report a lightning strike was identified with the exact time as the recorded fault time on the transmission line

· From this report the strike time, location and intensity were determined


The Vaisala data along with the structure locations were displayed on Google Earth


· Distance from strike to tower determined


Conclusions· July 24 fault Flashover

· -68 kA strike, 175 m to structure 184, measured 120 ohm impedance at structure, long span both sides

· July 29 fault Flashover or shielding failure· -45 kA strike, 320 m to mid span

· Aug 3 fault Flashover· -29 kA strike, 66 m to structure 109, flash mark on

tower peak

· Oct 2 fault Shielding failure· -11 kA strike, good grounding, long span


Grounding system analysis was carried out :

· Soil resistivity measurements near structures (Wenner 4 point out to 8 meters)

Megger Earth Tester Model - DET4TD· Computer modeling grounding design software

CDEGS RESAP and MALTZ modules· Field measurements to verify structure grounding

impedance AEMC 6472\6474 Ground Tester


Structure Ground Impedance Measurement

· Testing of a single structure while it is interconnected to a multi-grounded system is difficult to do accurately

· A new test set became available in 2008 which addresses this problem using Rogowski coils to measure electrode currents

AEMC Pylon Ground Tester Model 6472 and Model 6474

AEMC® InstrumentsTower Ground Resistance Testing

System

GroundFlex Coil Sensors(Rogowski Coils)

6472/74GroundFlex® Adaptor

· Capable of testing ground resistance of towers without disconnecting the Overhead ground wire

· Tests both ground resistance of tower legs (individually and total) and Overhead ground wires

· Test at frequencies up to 5kHz to profile impedance, important to characterize for lightning strikes

GroundFlex® Method


Foundation · Steel culverts

installed first· Foundation steel

had a Corrocote coating


Structure Grounding· Steel poles, culverts and counterpoise connected together

345 kV Transmission Line Lightning Performance

Mitigation

· Structure Grounding Electrical Circuit

· The AEMC Pylon Ground Tester permits a more detailed analysis of structure impedances


· Top sensor measures shield wire impedance

· Bottom sensor measures structure impedance


· Two sensors on each pole

· Two turns in each sensor


Structure 243· Calculated 7.9 ohms· Measured 6.7 ohms

Structure 248· Calculated 13.3 ohms· Measured 12.9 ohms

Structure 184· Calculated 110.8 ohms· Measured 120 ohms

Structure 144· Calculated 155 ohms· Measured 40 ohms

Important NoteResistivity and Resistance tests were done on the same day

Structure Grounding Results


Summary

•CDEGS Grounding Software and the AEMC Pylon Ground Test Set can achieve a very close comparative result for determination of structure ground impedance

•Lightning performance improvement can be best achieved by improving the structure ground impedance

•A simple soil resistivity measurement along with the CDEGS model can easily predict which structures will require grounding improvement

Lightning Improvements69 and 138 kV Transmission Lines

· Improvements were first considered in 1999 on 69 and 138 kV lines supplying the City of Saint John

· These lines use double circuit steel lattice towers with shield wires as part of the design

· Built in 1950’s these lines are installed over very rocky terrain

Typical 138 kV Double Circuit Structure

Saint John Area Lines69 and 138 kV Transmission Lines

·Salt and industrial contamination has resulted in failure of the shield wires over the years without replacement

·With no shield wires and with towers sitting on rock (poor grounding) lightning outages were a problem

Saint John Area 69 and 138 kV Transmission Lines

Problems with re-installing shield wires:

·Line outages were difficult to obtain· Improving the ground resistance in this urban environment was a huge challenge

·With towers sitting on rock flashovers would still be expected

Saint John Area Line Arrester Application

·Towers prone to lightning strikes were identified using historical information from fault locating relays

·Arresters were applied to all insulator strings on the selected double circuit towers

·No improvements were made to existing tower grounding systems

Typical 138 kV Tower with Arresters

Arrester Installation69 and 138 kV Transmission Lines

·1500 Gapless Arresters were installed under de-energized conditions

· Installation included:· drilling one hole in the steel crossarm· hanging the arrester· connecting the hotline clamp to the conductor

Arrester Kit

Saint John Area Lightning Performance Summary

Structures Before Arresters Structures With Arresters

Year 69 kV Faults 138 kV Faults 69 kV Faults 138 kV Faults

1996 0 0

1997 1 7

1998 4 2

1999 2 1

2000 1 4 0 0

2001 4 0 0 0

2002 0 2 0 0

2003 0 0

2004 1 0

2005 0 0

2006 1 2

2007 0 1

2008 0 0

2009 0 0

69 and 138 kV Transmission LinesConclusions

Line arresters installed on all insulator strings on a steel tower can provide zero lightning outages for lightning within the arrester energy rating.

This was demonstrated in July 2002 when the City of Saint John was struck by a severe lightning storm which lasted 3 days. Numerous outages were experienced but none at structures where arresters were installed.

Line Arrester Problems – Early Years

·NB Power’s experience spans 10 years with line arresters

·Numerous manufacturing defects as well as installation problems have been experienced

·Some components are showing prematured signs of wear

Manufacturing Defect


· Improper Installation



Defective End Cap Moisture Seal



Defective End Cap Causing Internal Tracking from Moisture Ingress



Inferior Lugs Broke in Service


Lead & Chain Worn Out After 6 Years of Service


· Close-up of Chain (6 years service)


Lead Broken Away From Lug


Summary of Defective Arrester Quantities

Supplier 69 kVInstalled/Defective

138 kVInstalled/Defective

A 627/120 794/220

B 516/3

C 30/30


Transmission Line Lightning Performance

Mitigation•Lightning performance of transmission lines with shield wires can be improved by lowering the structure ground impedance

•Transmission line lightning performance can also be improved by installing transmission line surge arresters (TLSA)

J Williamson Engineering Inc

Shield Wire Performance




New Transmission LinesLightning Performance Determination

· Identify the Lightning Ground Flash Density for the location

· Select structure type and voltage· Model the structure and line in Sigma SLP Lightning Performance software

· Calculate the Lightning Flashover Rates for:· Different values of ground impedance· With and without shield wires· Partial and full application of arresters

· Select the appropriate design to match the desired lightning performance



Example # 1· 138 kV Transmission Line· Wood Pole H-Frame Construction· Two Shield Wires with 30 degree Shield Angle· 7 Suspension Porcelain Insulators· 695 kV Critical Impulse (+)· Structure Ground Resistances 25 to 125 ohms


Lightning Performance AnalysisSigma SLP Software


Transmission Line Lightning Performance Example # 1 with GFD = 0.2


Transmission Line Lightning Performance Example # 1 with GFD = 1.0


Transmission Line Lightning Performance Example # 1 with GFD = 8



Example # 2

· 138 kV Transmission Line· Single Pole Wood Construction· No Shield Wire· Two Side and One Vertical Post Polymer Insulator· 675 kV Critical Impulse (+)· Structure Ground Resistances - 25 to 125 ohms


Arrester on Top Phase

UnderbuiltShield Wires 5 ohms

25 ohms

50 ohms

75 ohms

100 ohms

125 ohms

No No 86.06 83.77 85.89 85.27 85.76 85.06No Yes 84.55 87.67 83.91 84.74 86.95 85.89Yes No 0 0 0 6.76 18.3 21.51Yes Yes 0 0 0 0.85 3.46 7.82

Transmission Line Lightning Performance Example # 2 with GFD = 8

Flashover Rates

•Top phase arresters will significantly improve lightning performance•For lines with top phase arresters and where ground impedance is high, lowering ground impedance and/or installing under built shield wire will improve lightning performance


Presentation Summary

· NB Power Transmission System· Traditional Shield Wire versus Arrester Approach· Shielded 345 kV Transmission Line Lightning

Performance Mitigation· 69 and 138 kV Arrester Application on Steel Towers

· Performance Results over 10 Years· Defects in Early Years

· Lightning Performance Determination on New Lines

THE END

This has been a joint presentation by

J Williamson Engineering IncTransport Transmission

Documents

Transmission Line Surge Arrester Application and Overhead … · 2018-08-23 · Transmission Line Surge Arrester Application and Overhead Shield Wire Grounding Issues [North American