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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
“Traditional” versus “Arrester” Approach
· 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
“Traditional” versus “Arrester” Approach
· 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
“Traditional” versus “Arrester” Approach
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
345 kV Transmission Line Lightning Performance Mitigation
· 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
345 kV Transmission Line Lightning Performance Mitigation
· 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
345 kV Transmission Line Lightning Performance Mitigation
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
345 kV Transmission Line Lightning Performance Mitigation
No jumpers around bolted connections
Steel foundation is insulated by protective coating
Rust from weathering steel dripping onto
top insulators
Considerations Included:
345 kV Transmission Line Lightning Performance Mitigation
· 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
345 kV Transmission Line Lightning Performance Mitigation
The Vaisala data along with the structure locations were displayed on Google Earth
345 kV Transmission Line Lightning Performance Mitigation
· Distance from strike to tower determined
345 kV Transmission Line Lightning Performance Mitigation
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
345 kV Transmission Line Lightning Performance Mitigation
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
345 kV Transmission Line Lightning Performance Mitigation
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
345 kV Transmission Line Lightning Performance Mitigation
Foundation · Steel culverts
installed first· Foundation steel
had a Corrocote coating
345 kV Transmission Line Lightning Performance Mitigation
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
345 kV Transmission Line Lightning Performance Mitigation
· Top sensor measures shield wire impedance
· Bottom sensor measures structure impedance
345 kV Transmission Line Lightning Performance Mitigation
· Two sensors on each pole
· Two turns in each sensor
345 kV Transmission Line Lightning Performance Mitigation
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
345 kV Transmission Line Lightning Performance Mitigation
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
Line Arrester Problems – Early Years
· Improper Installation
Line Arrester Problems – Early Years
Manufacturing Defect
Defective End Cap Moisture Seal
Line Arrester Problems – Early Years
Manufacturing Defect
Defective End Cap Causing Internal Tracking from Moisture Ingress
Line Arrester Problems – Early Years
Manufacturing Defect
Inferior Lugs Broke in Service
Line Arrester Problems – Early Years
Lead & Chain Worn Out After 6 Years of Service
Line Arrester Problems – Early Years
· Close-up of Chain (6 years service)
Line Arrester Problems – Early Years
Lead Broken Away From Lug
Line Arrester Problems – Early Years
Summary of Defective Arrester Quantities
Supplier 69 kVInstalled/Defective
138 kVInstalled/Defective
A 627/120 794/220
B 516/3
C 30/30
Line Arrester Problems – Early Years
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
J Williamson Engineering Inc
J Williamson Engineering Inc
Transmission Line Lightning 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
J Williamson Engineering Inc
Transmission Line 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
J Williamson Engineering Inc
Lightning Performance AnalysisSigma SLP Software
J Williamson Engineering Inc
Transmission Line Lightning Performance Example # 1 with GFD = 0.2
J Williamson Engineering Inc
Transmission Line Lightning Performance Example # 1 with GFD = 1.0
J Williamson Engineering Inc
Transmission Line Lightning Performance Example # 1 with GFD = 8
J Williamson Engineering Inc
Transmission Line Lightning Performance
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
J Williamson Engineering Inc
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
J Williamson Engineering Inc
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