Raj Emtp Model
Short Description
Download Raj Emtp Model...
Description
ii
INSULATION COORDINATION OF QUADRUPLE CIRCUIT HIGH VOLTAGE TRANSMISSION LINES USING ATP-EMTP
SITI RUGAYAH BTE DUGEL
A thesis submitted in partial fulfillment of the requirements for the award of the degree of Master of Engineering (Electrical-Power)
Faculty of Electrical Engineering Universiti Teknologi Malaysia
MEI 2007
iv
To my beloved husband and dear children, who are always giving their support and understanding. They are always with me when I need support and advice and without their understanding, I will not be able to complete my master study
v
ACKNOWLEDGEMENT
I would like to express my sincere appreciation and special thanks to my project supervisor, Prof. Dr. Zulkurnain Abdul Malek, for his support, advices, encouragement, guidance and friendship. I wish to thank the grateful individuals from TNB research and TNB Generation. I am grateful for their cooperation and willingness to assist me in this matter. I am also would like to thank all my friends especially Adzhar Bin Khalid for their assistance towards the successful completion of this project. I am also indebted to Universiti Teknologi Malaysia (UTM) for their assistance in supplying the relevant literatures. Last but not least, I wish to thank my beloved husband, Surmazalan B. Ngarif who give me his undivided attention and support throughout this research.
vi
ABSTRACT
A significant number of faults in overhead transmission lines are due to lightning strikes which cause back flashovers and hence single or double circuit outages. The continuity and quality of the power supply is therefore can be severely affected by the outages, especially in Malaysia where the isokeraunic level is rather high. The lightning performance of transmission lines is also influenced by the transmission line configuration itself. In Malaysia, the TNB's transmission lines consist of 500 kV or 275 kV double circuits, and 275/132 kV quadruple circuits. It is known that the lower portion of the 132 kV line apparently has the lowest lightning performance.
The application of transmission line arresters is also known to be the best method in improving the lightning performance of transmission lines in service. However, its usage requires proper coordination and placement strategy to ensure optimum improvement in lightning performance.
In this work, the ATP-EMTP simulation program was used to study the lightning performance of the quadruple circuit transmission line behaviour towards lightning activities. The models used include those for the surge arresters, overhead lines, towers and insulators. All models were based on the data supplied by the utility. Initial results show that the configuration 6 gives the best protection or lowest flashover rate.
vii
ABSTRAK
Kebanyakkan gangguan bekalan pada talian atas penghantaraan adalah disebabkan oleh panahan petir yang mana telah mengakibatkan kerosakkan dan gangguan bekalan pada litar sediada dan litar berkembar. Gangguan bekalan ini telah mengakibatkan keterusan dan kualiti bekalan elektrik terganggu teruk. Tahap panahan petir di talian atas penghantaran adalah juga dipengaruhi oleh configurasi talian atas itu sendiri. Di Malaysia, talian penghantaran TNB adalah terdiri dari 500kV atau 275kV litar berkembar dan 275/132kV litar berkembar empat(quadruple circuits). Telah dikenalpasti bahawa pada bahagian bawah talian 132kV adalah merupakan tahap panahan petir yang terendah.
Penggunaan penangkap kilat untuk talian atas adalah merupakan cara terbaik dalam memperbaiki tahap panahan petir di talian atas yang sedang beroperasi. Walau bagaimanapun, penggunaanya memerlukan koordinasi yang tepat dan lokasi yang strategik bagi mendapatkan kesan yang optimum.
Untuk kajian ini, aturcara simulasi ATP-EMTP telah digunakan bagi mengkaji tahap dan aktiviti panahan petir terhadap litar berkembar empat. Model yang digunakan adalah termasuk penangkap kilat, talian atas penghantaraan, menara dan penebat. Semua data yang digunakan untuk dimodelkan adalah diperolehi dari pembekal elektrik
Keputusan dari simulasi yang dibuat menunjukkan configurasi 6 telah
menghasilkan perlindungan yang terbaik dan kadar gangguan bekalan yang terendah
viii
UTM(PS)-1/02
School of Graduate Studies Universiti Teknologi Malaysia
VALIDATION OF E-THESIS PREPARATION
Title of the thesis:
INSULATION COORDINATION OF QUADRUPLE CIRCUIT HIGH VOLTAGE TRANSMISSION LINES USING ATP-EMTP
Degree:
MASTER OF ENGINEERING (ELECTRICAL POWER)
Faculty:
KEJURUTERAAN ELEKTRIK
Year:
2007
I SITI RUGAYAH BINTI DUGEL hereby declare and verify that the copy of e-thesis submitted is in accordance to the Electronic Thesis and Dissertation’s Manual, School of Graduate Studies, UTM
_____________________
______________________
(Signature of the student)
(Signature of supervisor as a witness)
Permanent address:
Name of Supervisor: Prof Madya Dr Zulkurnain
No 17, Lorong Gurney off Jalan Semarak, 54100 Kuala Lumpur, Wilayah Persekutuan.
Bin Abdul Malek Faculty: KEJURUTERAAN ELEKTRIK
TABLE OF CONTENTS
CHAPTER
TITLE TITLE PAGE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
xiii
LIST OF FIGURES
xv
LIST OF ABBREVIATIONS
1
2
CHAPTER
PAGE
xviii
LIST OF SYMBOLS
xix
INTRODUCTION
1
1.1 Background
1
1.2 The Objectives of the Research
2
1.3 Scope of Study
3
LITERATURE REVIEW
3
2.1 Case Study by Kerk Lee Yen(TNBT Network SB)
3
2.1.1 Objective
3
2.1.2 Methodology
4
2.1.2.1 Line Section
4
2.1.2.2 Basic input data
4
TITLE
PAGE
10
2.1.3 Configuration of TLA installation
5
2.1.4 Result
5
2.1.5 Various installation of TLA
6
2.1.6 Simulation Result
7
2.1.6.1 Application of 3 TLA per tower
7
2.1.6.2 Application of 2 TLA per tower
7
2.2 Case Study by S.J Shelemy and D.R.Swatek 2.2.1 Objective
7
2.2.2 Introduction
8
2.2.3 Model Overview
8
2.2.4 Methodology
9
2.2.4.1 Tower Model
9
2.2.4.2 Line Termination
10
2.2.4.3 Insulator String
10
2.2.4.4 Tower Ground Resistance
10
2.2.4.5 Point of Contact
11
2.2.4.6 Lightning stroke
12
2.2.5 Results
12
2.2.6 Conclusion
13
2.3 Case Study by Y.A.Wahab, Z.Z.Abidin and S.Sadovic
CHAPTER
7
13
2.3.1 Objective
13
2.3.2 Introduction
14
2.3.3 Model Overview
14
2.3.4 Methodology
14
2.3.4.1 Electromagnetic model
15
2.3.4.2 Tower footing resistance model
16
2.3.4.3 Line insulation flashover model
16
2.3.4.4 Tower Model
17
2.3.4.5 Transmission Line Surge Arrester
17
TITLE
PAGE
11
2.3.4.6 Corona model
3
18
2.3.5 Result of Lightning Performance
18
2.3.6 Conclusions
20
TRANSMISSION SYSTEM
21
3.1 Transmission Line and Ground Wire
21
3.2 Insulator
22
3.3 Insulation Coordination
23
3.1.1 Definitions of Insulation Coordination
23
3.3.2 Insulation Coordination
24
3.3.3 Insulation Coordination Involves
24
3.3.4 Selection of Insulation Levels
24
3.3.5 Basic Principles of Insulation Coordination
25
3.3.6 Insulation Withstand Characteristics
26
3.3.7 Standard Basic Insulation Levels
26
3.4 Arching Horn
27
3.5 Earthing
28
3.6 Tower Types
28
3.6.1 Tower with wooden cross arm
29
3.7 Design Span
30
3.8 System Over voltages
30
3.9 Fast Front Over voltages
31
3.10 Fast Front Over voltages
31
3.11 Metal-Oxide Arresters
33
3.12 Gapped TLA and Gapless TLA
33
3.13 Surge Lightning Arrester placement (TLA)
34
3.14 Comparison of Available Surge Arresters (Gapless Type)
35
12 CHAPTER 4
TITLE
TRANSMISSION SYSTEM
5
PAGE 37
4.1
System Modelling
37
4.2
EMTP Simulation
37
4.3
Selected model and Validation
38
4.4
Transmission Line
38
4.5 Line exposure to lightning
39
4.6
Shielding Failure
40
4.7
Overhead Transmission Lines
41
4.8
Line length and Termination
41
4.9 Tower Model
42
4.10 Tower footing resistance model
45
4.11 Insulators
46
4.12 Backflashover
46
4.13 Corona
47
4.14 Line surge arrester
47
4.15 Selection of Lightning Configuration
51
AVAILABLE METHOD FOR LIGHTNING
52
PERFORMANCE IMPOVEMENT 5.1
Additional Shielding Wire
53
5.2 Tower Footing Resistance
53
5.3 Increase the Tower Insulation
54
5.4
Unbalance Insulation
54
5.5 Transmission Line Arrester
55
5.6
Installation of TLA based on TFR
55
5.6.1
57
Additional of TLA at low TFR Section
5.6.2 Installation of TLA on one circuit
58
5.6.3
59
Coordination of Gap Spacing fot Transmission
13 CHAPTER
6
TITLE
PAGE
5.6 Extended Station Protection
61
SIMULATION METHOD
61
6.1 ATP-EMTP Simulation
61
6.2
Selected Model and Validation
62
6.2.1
62
Tower Model
6.3 Model And Parameters Used In The Simulation
7
66
6.3.1
Tower Model
66
6.3.2
Transmission Line model
67
6.4 Selection of Lightning Parameter
70
6.5
Lightning Amplitude
70
6.6
Time of Rising
71
6.7
Time of Falling
71
6.8
Limitation of Simulation
74
6.9
Statistical Approach
74
SIMULATION: 275 kV DOUBLE CIRCUIT
75
AND 275/132kV QUADRUPLE CIRCUIT LINE 7.1
275/132kV Quadruple Circuit and
75
Specification used in this Simulation 7.1.1
Model Used In The Simulation
78
7.2 Lightning Surge Arrester Configuration
79
7.3 Results Of Simulation For Lightning
80
Current of 17kA And Strike at Tower 2 7.3.1
Response without transmission line arrester
80
7.3.2
Response with transmission line arrester
81
7.3.2.1 TLA with configuration 1
82
7.3.2.2 TLA with configuration 2
83
14 7.3.2.3 TLA with configuration 3
83
7.3.2.4 TLA with configuration 4
84
7.3.2.5 TLA with configuration 5
85
7.3.2.6 TLA with configuration 6
85
7.3.2.7 TLA with configuration 7
86
7.3.2.8 TLA with configuration 8
87
7.4 Results Of Simulation For Lightning
88
Current of 120kA And Strike at Tower 2
8
7.4.1
Response without transmission line arrester
88
7.4.2
Response with transmission line arrester
89
7.4.2.1 TLA with configuration 1
89
7.4.2.2 TLA with configuration 2
90
7.4.2.3 TLA with configuration 3
90
7.4.2.4 TLA with configuration 4
91
7.4.2.5 TLA with configuration 5
92
7.4.2.6 TLA with configuration 6
92
7.4.2.7 TLA with configuration 7
93
7.4.2.8 TLA with configuration 8
94
7.4.2.9 TLA with configuration 9
94
7.5 Summary of the Simulation
95
7.4
98
Limitation of simulation
RECOMMENDATION AND CONCLUSION
REFERENCES
99 101
15
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Section of BBTG-RSID 132 kV
4
2.2
Flashover rate for individual section of line
5
2.3
Various installation of TLA
6
2.4
Strike distances for the Nelson River HVDC
11
transmission line 2.5
Critical peak lightning current amplitudes for
11
the Nelson River HVDC transmission line towers 2.6
Back flashover rates and shielding failure rates
12
per 10,000 lightning strikes 2.7
Two-line stroke distribution to flat ground
15
2.8
Flashover rate for different circuits without line
18
surge arresters( flashover rate/100km/year) 2.9
Line total and multi circuit flashover rate without
19
line surge arresters( flashover rate/100km/year) 2.10
Line Total Flashover Rate Different Arrester
19
Installation Configurations( flashover rate/100km/year) 2.11
Line Double Total Flashover Rate Different Arrester
20
Installation Configurations( flashover rate/100km/year) 3.1
Conductors Type and Their Specification
22
3.2
Number of insulator set required based on voltage
23
and type of insulator set
16 TABLE NO.
TITLE
PAGE
3.3
Standard Basic Insulation Levels(BIL)
27
3.4
Arching distance and BIL for various circuit
27
and towers 3.5
Tower types and deviation angle
29
3.6
The major differences between gapped SLA
32
and gapless SLA 3.7
SLA placement and energy consideration
34
3.8
TLA Placement and Energy Consideration
35
3.9
Data on Gapless Transmission line arrester
36
manufactured by several company 4.1
Balakong to Serdang 132kV line information
39
4.2
Value for A0 and A1 based on 8/20 us residual
48
voltage supplied by manufacturer for the application of Pinceti’s arrester model. 5.1
Arrester installation strategy to eliminate double
56
circuit flashover 7.1
Parameter of the 275kV double circuit tower model
77
7.2
Value for A0 and A1 based on 8/20 us residual voltage
75
supplied for the application of Pinceti’s arrester model with 120kV rated Siemens 3EQ4-2/LD3 7.3
Line Performance For Different TLA Configuration
95
For Lightning Current of 17kA 7.4
Line Performance For Different TLA Configuration For Lightning Current of 120kA
97
17
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
4.1
Model of Transimission Line
40
4.2
Overhead Transmission Line, Tower and Insulator
42
model 4.3
Tower Representation for Quadruple Circuit
43
Transmission Line 4.4
M. Ishii’s tower model for a double circuit line tower
44
4.5
Pinceti’s arrester model used for representing
49
surge arrester 4.6
Relative error of residual voltage for representing
49
Siemens 120kV rated 3EQ4-2/LD3 SA with Picenti’s model compared to manufacturer performance data 4.7
Example of Gapless-type Surge Arrester installed
50
at 132kV BLKG-SRDG 4.8
Different arrester Installation Configurations
51
5.1
Available Method for Lightning Improvement
52
5.2
Unbalance tower insulation for double circuit line
55
5.3
Circuit location and TLA placement for a double
56
circuit line 5.4
Additional TLA at Low TFR section along the high
57
TFR section 5.5
TLA added only at one circuit of a double circuit line tower
58
18 FIGURE NO.
TITLE
PAGE
5.6
Extended station protection
60
6.1
M.Ishii’s tower model for a double circuit line tower
64
6.2
Tower equivalent radius
64
6.3
Modified M.Ishii’s tower model for a quadruple
66
circuit line tower modeling 6.4
Voltage Amplitude for Time of Falling 20µs
72
6.5
Voltage Amplitude for Time of Falling 50µs
72
6.6
Voltage Amplitude for Time of Falling 100µs
73
6.7
Voltage Amplitude for Time of Falling 200µs
73
6.8
Voltage Amplitude for Time of Falling 500µs
73
7.1
Simulated 275/132kV quadruple circuit line
76
7.2
Conductor identification for 275/132kV double
76
circuit line used in simulation 7.3
Modified M.Ishii’s tower model for a quadruple
78
circuit line tower modeling 7.4
Current injected at top tower 2
80
7.5
Lightning strike has caused voltage rise at top tower 2
80
7.6
Voltage measured at tower 2 which are connected to
81
275kV Line 7.7
Flashover Voltages when TLA are equipped at
82
conductor RBT and RBT1 7.8
Flashover Voltages when TLA are equipped at
83
conductor RBT132, RBT131 and BBT131 7.9
Flashover Voltages when TLA are equipped at
83
conductor RBT131, YBT 131 and BBT131 7.10
Flashover Voltages when TLA are equipped at
84
conductor RBT, RBT1 and RBT131 7.11
Flashover Voltages when TLA are equipped at conductor RBT132, RBT131, YBT132 and YBT131
85
19 FIGURE NO. 7.12
TITLE Flashover Voltages when TLA are equipped at
PAGE 85
conductor RBT, RBT1, RBT132 and RBT131 7.13
Flashover Voltages when TLA are equipped at
86
conductor RBT, RBT1, BBT, RBT132 and RBT131 7.14
Flashover Voltage when TLA are equipped at
87
conductor RBT, RBT1, YBT, RBT132 and RBT131 7.15
Current injected at top tower 2
88
7.16
Lightning strike has caused voltage rise at top tower 2
88
7.17
Flashover Voltage across insulators when TLA are
89
equipped at conductor RBT and RBT1 7.18
Flashover Voltage across insulators when TLA are
90
equipped at conductor RBT132, RBT131 and BBT1 7.19
Flashover Voltage across insulators when TLA are
90
equipped at conductor RBT132, RBT131 and BBT1 7.20
Flashover Voltage across insulators when TLA are
91
equipped at conductor RBT, RBT1 and RBT131 7.21
Flashover Voltage across insulators when TLA are
92
equipped at conductor RBT132, RBT131, YBT132 and YBT131 7.22
Flashover Voltage across insulators when TLA are
92
equipped at conductor RBT, RBT1, RBT132 and RBT131 7.23
Flashover Voltage across insulators when TLA are
93
equipped at conductor RBT, RBT1, BBT, RBT132 and RBT131 7.24
Flashover Voltage across insulators when TLA are
94
equipped at conductor RBT, RBT1, BBT, RBT132 and RBT131 7.25
Flashover Voltage across insulators when TLA are equipped at all conductors of 275kV and 132kV lines
94
20
LIST OF ABBREVIATIONS
ac
-
Alternating Current
ACSR
-
Aluminium Conductor Steel Reinforced
AIS
-
Air Insulated Substation
ATP
-
Alternative Transient Program
BFR
-
Back Flashover Rate
BIL
-
Basic Lightning Insulation Level
CB
-
Circuit Breaker
CBPS
-
Connaught Bridge Power Station
CFO
-
Critical Flashover
EMTP
-
Electro Magnetic Transient Program
FDQ
-
Frequency Dependent Q Matrix
GIS
-
Gas Insulated Substation
GPS
-
Global Positioning System
IEE
-
The Institution of Electrical Engineers
IEEE
-
Institute of Electrical and Electronic Engineers
IVAT
-
High Voltage and Current Institute
LOC
-
Leader Onset Conditions
MO
-
Metal Oxide
MOV
-
Metal Oxide Varistor
OPGW
-
Optical Fibre Composite Ground Wire
SA
-
Surge Arresters
SiC
-
Silicon Carbide
S/S
-
Substation
21 TFR
-
Tower Footing Resistance
TLA
-
Transmission Line Arresters
TNB
-
Tenaga Nasional Berhad
ZnO
-
Zinc Oxide
22
LIST OF PRINCIPLE SYMBOLS
µF
-
micro-Farad
µH
-
micro-Hendry
µs
-
nicro-second
A
-
Ampere
C
-
Capacitive
Ng
-
Ground Flash Density per Kilometer2 per year
kA
-
kilo-Ampere
kJ
-
kilo-Joule
kV
-
kilo-Volt
L
-
Inductive
MV
-
Mega-Volt
R
-
Resistance
Uc
-
Maximum Continuous Operating Voltage
Ur
-
Rated Surge Arrester Voltage
Km
-
kilometer
V
-
Volt
Z
-
Impedance
Zt
-
Surge Impedance
CHAPTER 1
INTRODUCTION
1.1
Background
Transmission system in services can be divided into two which are overhead transmission system and cable type transmission system. The main focus here is the overhead transmission systems, which are directly subjected to lightning over voltage. A significant number of the faults on overhead transmission lines are due to lightning. Lightning Faults may be single or multiple, and their elimination causes voltage dips and outages. Therefore, the outage rate of a line and the quality of the delivered voltage depend on the lightning performance of the line.
Many procedures have been presented over the years with the aim of predicting the lightning performance of transmission lines. Modern understanding about lightning phenomena and lightning attraction mechanisms allowed developing methods for estimating the lightning performance of overhead lines which avoid such empiricism.
2 For this purpose, the performance of transmission lines is estimated using ATP-EMTP simulation programs
1.2
The Objectives of the Research
The main objective of the project is to improve the lightning performance of transmission lines by the application of line surge arresters on the quadruple circuit transmission line and to analyze different line surge arresters application configurations in order to optimize application of this technology to the existing and to the future quadruple transmission lines.
1.3
Scope of Study
The main scope of this project is to study the applications of surge arresters on transmission line to improve the lightning and transient performance of the transmission line which is includes: ■
Arrangement of line arresters for optimum technical and economic
■
Performance which include where or which tower along the line arresters to be installed
■
The rating and withstand energy of the surge arresters
■
The arresters configurations
3
CHAPTER 2
LITERATURE REVIEW
The purposes of these Studies are to measure the performance to eliminate circuit trippings on Overhead Transmission Line Protection for double and quadruple circuit transmission line. The performance of the system are justified by installation of transmission line arrester (TLA) and analyzing different line surge arresters application configurations also to determine best location and configuration of TLA installation.
2.1
Case Study by Kerk Lee Yen(TNB Transmission Network SB): Studies on Optimum Installation of TLA for BBTG-RSID 132kV
2.1.1
Objective
The prime focus of this study is on elimination double circuit tripping by installation of transmission line arrester (TLA) and also to determine best location and configuration of TLA installation.
4 2.1.2
Methodology
The study using Sigma SLP software developed by Sodovic Consultant. Simulations are performed assuming no shielding failure as the software could generate only vertical stokes.
2.1.2.1 Line Sectioning
Based on tower footing resistance (TFR) and soil resistivity measurement collected, BBTG-RSID is sectionalized to 3 sections as follow to simply the analysis:
Table 2.1: Section of BBTG-RSID 132 kV Section Tower
Average TFR Soil Resistivity Land Profile Average
No.
(ohm)
1
1-5
8.86
(ohm-meter) 200
Length(km) Bushes/Flat
1.5
Land 2
6 – 29
56
1000
Hilly
7.2
3
30 - 39
5.04
30
Flat land
3.0
2.1.2.2 Basic input data
i ) Tower structure – standard drawings ii ) Total Thunderstorm day –200 thunderstorm days (Worst case scenario) iii) Type of lightning arrester – NGK gapless
5 2.1.3
Configuration of TLA installation 1) 1-3 arrangement 2) Double bottom 3) L-arrangement 4) I – arrangement
2.1.4 RESULT
1.
Without TLA installation
Result of flashover rate for the transmission line without TLA installation as table below:
Table 2.2: Flashover rate for individual section of line Section
Total Flashover
Single
Double Circuit
(Flash/km-year)
Circuit
Flashover
Flashover 1 (T1 – T5)
0
0
0
2 (T6 – T29)
55.71
35.05
20.65
0
0
0
3 (T30 – T39)
6 Above result shows that total flashover rate for both sections 1 and 3 are essentially zero. Hence, no installation of SLA is required for these two sections. The main factor that minimizes the flashover rate is their good tower footing resistance (TFR) values. TFR of these two sections are reasonably well maintained with values below requirement – 10 ohm
However, section 2 records high total flashover rate from simulation performed. Double circuit flashover constitutes about 37% of total flashover. As section 2 is hilly area, TFR readings are excessively high with an average of 56 ohms. High TFR hence contributes to higher back flashover.
2.1.5
Various installation of TLA
Table 2.3: Various installation of TSLA No
Scenario
Total Flashover
Single Circuit Double Circuit
(flashover/km/year)
Flashover
Flashover
1
1-3 arrangement
24.97
24.49
0.48
2
Double bottom
9.6
8.64
0.96
3
L-arrangement
3.36
3.36
0
4
I – arrangement
19.21
19.21
0
7 2.1.6
Simulation Result
2.1.6.1 Application of 3 TLA per tower
Configuration 3 and 4 are capable of eliminating double circuit flashover. However configuration “L” is the most effective solution as it helps to reduce single circuit flashover to the lowest compared to configuration “I”
2.1.6.2 Application of 2 TLA per tower
Configuration 2 “Double Bottom” records lowest total flashover rate. However, configuration 1 “1-3 arrangement” is more effective in reducing double circuit tripping
2.2
Case Study by S.J Shelemy and D. R. Swatek from System Planning Department, Manitobe Hydro, Manitoba, Canada
2.2.1
Objective
Study on Lightning Performance of Manitoba Hydro’s Nelson River HVDC Transmission Lines using Monte Carlo Model
8 2.2.2
Introduction
A Monte Carlo model of the lightning performance of Manitoba Hydro’s Nelson River HVDC Transmission Lines has been constructed in PSCAD/EMTDC Version 3. The value of key parameters is randomly drawn from user specified probability density functions (pdf’s). Most significant of these are the pdf’s of the positive and negative lightning stroke amplitudes which have been derived from actual data measured within 1km radius buffer of the lines. Estimates of the back flashover rate and shielding failure rate were calculated using various “zone-of-attraction” models.
2.2.3
Model Overview
A hierarchical multi-layered graphical representation of the conductor-towerinsulator-ground system was implemented in PSCAD/EMTDC Version 3. For each run(1 run = 1 lightning stroke), random number generators select values for the preionization tower footing resistance and for the amplitude and rise time of the lightning current pulse based on user defined probability density functions(pdf’s). Transmission tower geometry, stroke amplitude, and initial location are fed into zone-of-attraction model in order to determine the most likely point of contact between the stroke and the conductor-ground system
Each tower is represented by the detail traveling wave model. False reflections from the artificial truncation are eliminated by a multi conductor surge impedance termination. The non-linear time dependent characteristics of the insulator strings are
9 represented by the “Leader Progression Model” (LPM). The outcome of each run stored in a “Monte Carlo Accumulator”. Which compare the number and nature of insulator flashover to the total number of lightning strokes in order to obtain the rates of back flashover and shielding failure.
2.2.4
Methodology
Lightning outage statistics are estimated by way of a monte carlo simulation, by which we mean a multi-run case in which the key model parameters (pre-ionization footing resistance, lightning stroke rise time, lightning stroke peak current amplitude, and lateral position of stroke), for each separate run, are randomly drawn from a predefined pool of values. The Monte Carlo simulation was run for a total of 20,000 lightning strokes (10,000 strokes for both positive and negative lightning)
2.2.4.1 Tower Model
The tower was divided into five equivalent transmission line sections including the upper member, two cross arms, tower base, and a single equivalent of four parallel guy wires. The propagation time along the tower member is taken to be 3.92 x 10-9 sec/m. A 5 nsec simulation time step is used.
10
2.2.4.2 Line termination
To prevent false reflections from the truncations, the line model is terminated into its multi-conductor surge impedance.
2.2.4.3 Insulator String
The insulator string was modeled as a stray capacitance (0.476pF) in parallel with a volt-time controlled circuit breaker. The recursive equations selected by CIGRE for leader velocity v (t) and unabridged gap length are used. V(t) = kLe(t)( e(t) - E50 ) m/sec x
2.2.4.4 Tower Ground Resistance
High Magnitudes of lightning current flowing through ground decrease the ground resistance significantly below the measured low current values. Rt =
Ro/ √( 1 + Ir ) Ig
Ig =
1
ρEo
2π
Ro ²
11 A PSCAD/EMTDC Version 3 component was developed to calculate each individual tower grounding resistance.
2.2.4.5 Point of Contact
Electromagnetic models that treat the zone-of-attraction as a strike distance include the strike distance to ground Rg, The strike distance to the shield wire, Rs and the strike distance to the pole conductor, Rc. Five electromagnetic models were studied as table below: Table 2.4: Strike distances for the Nelson River HVDC transmission line Model
rg
rs
rc
Young
27 Im0.32
1.07I rg
1.046 rg
Love
10 Im0.65
rg
rg
IEEE 1992 T&D
9 Im0.65
1.256 rg
1.256 rg
1.274 rg
1.180 rg
6.8 Im0.74
5.9 Im0.74
Brown & Whitehead Eriksson
6.4
Im0.75
N.A
List of the critical peak current amplitudes predicted for the Nelson River HVDC transmission line tower as table below
Table 2.5: Critical peak lightning current amplitudes for the Nelson River HVDC transmission line towers Model Young Love IEEE 1992 T&D Brown & Whitehead Eriksson
Critical Current(kA) 25 30 70 20 15
12
2.2.4.6 Lightning Stroke
The lightning stroke was modeled as a current impulse, idealized as a triangular wave. Because insulation occurs shortly after the lightning strike, the fall time was fixed at 100 micro-sec.
2.2.5
Results
Using fault data for the Nelson River HVDC transmission lines collected between 1998 to 2000, faults were correlated to lightning strikes occurring at the same time and location. Over this period of time the FALLS program found 5066 negative lightning strokes and 530 positive lightning strokes within 1 km radius buffer of the transmission line. Out of these lightning strikes, only 6 are found to have caused lightning failure and due to shielding failures and back flashovers. The result of the simulation of lightning strike to the Nelson River HVDC transmission line as listed in table below:
Table 2.6: Back flashover rates and shielding failure rates per 10,000 lightning strikes Model Young Love IEEE 1992 T&D Brown & Whitehead Eriksson Measured
Back Flashover 2 3.3 6 7.5 2.1 3.6
Shielding Failure 11.2 58.5 75.5 3 6.2 7.1
13 From the result above, model of Young’s, Love’s and the IEEE 1992 T&D greatly over predicted the number of shielding failures. On the other hand the two remaining models, Brown & Whitehead and Eriksson’s model predicted failure rates closer to those actually observed, however the Brown & Whitehead model predicted a disproportionately high ratio of back flashovers to shielding failures. Of the models tested, Eriksson’s model yielded failure rates most consistent with the recorded data.
2.2.6 Conclusion
Through this analysis, the estimation of the back flashover rate and shielding failure rate were calculated using various zone-of-attraction models and Eriksson’s model yielded failure rate most consistent with lightning correlated fault data measured.
2.3
Line Surge Arrester Application on Quadruple Circuit Transmission Line by Y. A. Wahab, Z. Z. Abidin and S.Sadovic
2.3.1
Objective This paper is dealing with the application of line surge arresters on the
quadruple circuit transmission line and to analyze different line surge arresters application configurations in order to optimize application of this technology to the existing and to the future quadruple transmission lines.
14
2.3.2
Introduction
Line surge arresters are normally installed on all phase conductors of one circuit of the double circuit line. Arresters are installed on all towers of the considered 132kV line. With this arrester configuration, double circuit outages are eliminated but there exists possibility to have flashovers on the circuit without arresters.
Based on the positive experience with the surge line arresters on 132kV double circuit lines, it was decided to extend line surge arrester application to the quadruple circuit lines: 2 x 275kV and 2 x 132kV. By the application of the line arresters on 132kV circuits only, line overall lightning performance is improved since the majority of the flashovers will happen to 132kV circuit.
2.3.3
Model Overview
The circuit needs to be modeled is a quadruple circuit transmission line Balakong-Bandar Tun Razak, being commissioned in 1992, consists of two 275kV circuit and two 132kV circuit. Route length is 10.6km and number of towers is 37. Average line span is 300m. Line is operating with an average ground flash density of 10-20 strokes/km squared/year
2.3.4
Methodology
15 All computer simulations are performed using sigma slp simulation software tool. 2.3.4.1 Electromagnetic model
Line span is divided into short sections (10-15m each), in order to accept lightning stroke to the ground wires or to the phase conduction along the span. A total number of 20 to 30 thousand strokes are used in the electromagnetic simulations. Following striking distances are used: The Striking Distance to the conductor (CIGRE) rc = 10I 0.65 The striking distance to earth (IEEE) re = 5.5I 0.65 The striking distance to tower top rT = 1.05 rc I(kA) – lightning stroke current Two line CIGRE stroke distributions are modified to represent stroke distribution to flat ground as table below:
Table 2.7: Two-line stroke distribution to flat ground Parameter
Shielding
Failure Backflashover
range ( I < 15.9 kA)
Range( I > 15.9 kA)
Im (kA)
48.4
26.4
σ
1.33
0.605
16
2.3.4.2 Tower footing resistance model
The tower footing impulse resistance by the following equation: Ri =
Rt / √ ( 1 + I ) Ig
Ig =
1
ρEg
2π
Rlc²
where: Rt - Tower footing resistance at low current and low frequency, (ohm) (Rt = 10 – 40 ohm) Ri - Tower footing resistance, (ohm) Ig
- The limiting current to initiate sufficient soil ionization,(A)
I
- The lightning current through the footing impedance, (kA)
ρ
- Soil resistivity ( 100 ohm-m)
Eg - soil ionization critical electric field (kV/m), (Eg = 400 kV/m) Rlc - tower low current resistance ρ
- 50
Rlc
2.3.4.3 Line insulation flashover model
The leader propagation model is used to represent line insulation flashovers: Vl
=
17 0d {(u(t) ) - Eo }e0.0015{u(t)/d }
17 D–lL Where : V1
=
Leader velocity, m/s
d
=
Gap distance, m
lL
=
Leader length, m
u(t)
=
Applied Voltage, kV
E0
=
520, kV/m
2.3.4.4 Tower Model
Section of the tower from the bottom cross arm to the ground is represented as propagation element, which is defined by the surge impedance Zt and the propagation length Iprop. Wave propagation speed on the tower was taken to be equal to the velocity of light. Section on the tower top(between tower top and top cross arm) are modeled as inductance branches parallel with damping resistors
2.3.4.5 Transmission Line Surge Arrester
Polymer housed line surge arrester with an external gap is used with the following characteristic: Rated Voltage:
120kV
Series gap spacing:
650mm
IEC Line discharge class:
II
Critical flashover voltage:
620kV
18
2.3.4.6 Corona model
The influence of the corona is modeled by the capacitance branches which are connected between conductors and ground
2.3.5
Result of Lightning Performance
Line lightning performance is first determined for the line without arresters and then several arrester installation configurations are studied. Lightning performance of a line without line surge arresters is presented in table below:
Table 2.8: Flashover rate for different circuits without line surge arresters ( flashover rate/100km/year) Rt(Ω) 10 15 20 25 30 35 40
C1(275) 0 0 0 0 0.19 0.19 0.19
C2(275) 0 0 0 0.19 0.39 0.58 0.19
C3(132) 0 0.78 5.66 12.69 20.69 29.67 42.55
C4(132) 0 2.14 4.88 10.92 20.69 33.58 46.85
From the above table has shown the majority of the flashover happened on 132kV circuits. For the tower footing resistance less than 10Ω, zero flashover rates is obtained. Table below is line total, single, double and triple line flashover rates presented.
19
Table 2.9: Line total and multi circuit flashover rate without line surge arresters ( flashover rate/100km/year) Rt(Ω) 10 15 20 25 30 35 40
Total 0 2.93 8.39 18.35 32.6 49.26 65.64
Single 0 2.93 6.24 13.08 23.81 32.41 41.98
Double 0 0 2.14 5.07 8.19 14.64 22.84
Triple 0 0 0 0.19 0.58 0.78 0.78
Number of double circuit flashovers depends on the tower footing resistance, and may reach value of 35% of the line total flashover rate, for the tower footing resistance of 40 Ω. Results of the simulation for the different line arrester installation configuration are presented in tables below: Table 2.10: Line Total Flashover Rate Different Arrester Installation Configurations ( flashover rate/100km/year) o : o o : o o : o o : o o : o Rt(Ω) o : o o : o o : o o : o o : o 10 0 0 15 2.93 0 20 8.39 0.78 25 18.35 2.53 30 32.6 5.66 35 49.26 8.78 40 65.64 13.08 o - Without Lightning Surge Arrester
o : o o : o o : o o : o o : o 0 0.19 2.14 6.24 9.37 15.62 23.62
o : o o : o o : o o : o o : o 0 0 0 0.58 0.97 3.12 3.89
o - With Lightning Surge Arrester The substantial improvement in the line total flashover rate is obtained by the installation of line arresters on the two bottom conductors of 132kV circuits than the
20 three arresters installed on the all phase conductors of one 132kV circuit. The best improvement in the line total flashover rate is obtained by the installation of the arrester on the bottom conductor of one 132kV circuit and on the one top conductor of one 132kV circuit.
Table 2.11: Line Double Total Flashover Rate Different Arrester Installation Configurations ( flashover rate/100km/year)
Rt(Ω) 10 15 20 25 30 35 40
o o o o o
: o : o : o : o : o 0 0 2.14 5.07 8.19 14.64 22.84
o o o o o
: o : o : o : o : o 0 0 0 0.19 0 0.58 1.17
o o o o o
: : : : : 0 0 0 0 0 0 0
o o o o o
o o o o o
: o : o : o : o : o 0 0 0 0 0 0.19 0.39
When line surge arresters are installed on all phases conductors of one 132kV circuit, double circuit flashover are completely eliminated. But it is to note that with this arrester installation configuration line total flashover rate remains high. Arrester installation configuration with the arresters on the bottom conductors of both 132kV circuits and on the one top conductor of one 132kV be able to reduce line total flashover rate and at the same time reducing double circuit flashover rate.
2.3.6
Conclusions
21 Lightning Performance with different voltage levels can be improved by the installation of the LSA on the lower voltage level circuits only and the ‘L’ arrester configuration will give the best improvement in the line total flashover rate
CHAPTER 3
TRANSMISSION SYSTEM
3.1
Transmission Line and Ground Wire
Three type of transmission tower are being used in the transmission system, whish sre the old single circuit, double circuit and the quadruple circuit. The current practiced is to build double circuit or quadruple circuit due to the needs to transfer large quantity of power. The double circuit transmission lines are used for voltage from 132kV, 275kV to 500kV. The quadruple circuits are used either for 132kV/132kV circuit or for 275kV/132kV circuit.
The conductor used in transmission lines is called ACSR (Aluminium Conductor Steel Reinforced). The ACSR conductor consists of aluminium and steel stranding. Two conductors are used per phase and are kept apart at a distance of
22 400mm by the use of spacers. For quadruple circuit line design the 275kV lines used a configuration of 2x400mm squared bundle namely zebra and 132kV lines uses a configuration of 2x300mm squared bundle namely batang. The earth wire used is ACSR 60mm squared namely skunk. Two earth wires are used per tower, one on each side Table 3.1: Conductors Type and Their Specification Conductor Code Name ACSR Curlew ACSR Zebra ACSR Curlew ACSR Curlew
3.2
Nominal Cross Sectional Area (mmⁿ) 500
Actual Cross Sectional Area (mmⁿ) n/a
Maximum Resistance At 20C (Ω) n/a
Diameter (mm)
Voltage (kV)
n/a
500
400
428.9
0.0674
28.62
275
300
338.5
0.0892
24.16
275 & 132
150
n/a
n/a
n/a
132
Insulator
Insulators are defined as non-conductive materials that cover separate or support a conductive material to prevent a passage of electricity to ground. From the point of transmission system, the insulator are being used to separate the conductors that carries large amount of current and the tower body that are directly connected to ground. The insulators are very important to the operating performance of the transmission system itself. The insulator provide mechanical support to the conductors and all the current carrying parts and subjected to normal operating and transient voltage
23
Standard materials used on transmission tower insulator are usually glass and porcelain because it has high dielectric strength and easily spotted if break. The type used are pin and cap types. Table 3.2: Number of insulator set required based on voltage and type of insulator set Type of Insulator set
Installation location
Upright Light Duty Tension Set Inverted Light Duty Tension Set Jumper suspension Set
Upper end of the slack spans between terminal tower Lower end of Slack end with line end and earth end arching horns Heavy angle towers to maintain the electrical clearances between the jumper loops to the tower body
3.3
132kV insulator units 10
275kV insulator units 20
10
16
13
20
Insulation Coordination
3.3.1 Definitions of Insulation Coordination
IEC: The selection of the dielectric strength of equipment in relation to the voltages which can appear on the system for which the equipment is intended and taking into account the service environment and characteristics of the available protective devices
IEEE: The selection of insulation strength consistent with expected over voltages to obtain an acceptable risk of failure
General: The protection of electrical systems and apparatus from harmful overvoltages by the correlation of the characteristics of protective devices and the equipment being protected
24
3.3.2. Insulation Coordination
Insulation coordination is an optimization process where the attempt is made to keep the overall cost of insulation, protection devices and service interruption to a minimum.
For self-restoring insulation some failures have to be tolerated. However, insulation failures should be confined to areas where they cause minimum damage and least interruption of supply; and they should not compromise the safety of operating personnel.
3.3.3. Insulation Coordination Involves
Estimating of credible over voltages that may appear in the network, their peak values, wave shapes and frequency of occurrence.
Exploring means of reducing and/or diverting the over voltages.
Selection of insulation levels to achieve the performance criteria.
3.3.4. Selection of Insulation Levels
25
For Voltages up to 300 kV ♦ Insulation is designed to withstand lightning and power frequency overvoltages. ♦ Sufficient margin is kept between the maximum overvoltage and the minimum withstand strength. For Voltages Higher than 300 kV ♦ Choice of insulation and tower dimensions to withstand switching overvoltages. ♦ Check the number of failures due to atmospheric overvoltages. ♦ Check the ability of the design to withstand power frequency overvoltages under different operating conditions.
3.3.5. Basic Principles of Insulation Coordination
The process of correlating the insulation strengths of electric equipment with expected overvoltages and with the characteristics of surge protective devices. Main Issues: ♦ System overvoltages, their wave shapes, peak values and probabilities of occurrence. ♦ Withstand characteristics of different types of insulation to different types of overvoltages. ♦ Measures used to reduce system overvoltages and protective devices to divert them
26
3.3.6. Insulation Withstand Characteristics
Voltage/Clearance Characteristics ♦ Withstand voltage as a function of gap spacing for lightning and switching surges. Voltage/Time Characteristics ♦ Withstand voltage as a function of time to crest of the voltage surge. Observations ♦ Lightning overvoltage is important for high voltage systems. ♦ Switching overvoltages are more important in extra- and ultra-high voltage systems.
3.3.7. Standard Basic Insulation Levels
Standard BILS developed for various system voltages based on experience. Test voltage levels for other types of surges and tests are usually associated with the equipment BIL. Standard BILS originally intended irrespective of how system was grounded.
27 It became recognized that lower voltage arresters could be used on solidly grounded systems thus providing lower protective levels (better protection) for equipment insulation. Resulted in reduced insulation levels (one or more steps lower). At EHV, much greater economic incentive to use lower insulation levels through better arrester protection. Table 3.3: Standard Basic Insulation Levels(BIL) System Voltage
Standard BIL
Class (kV)
Reduced BIL (kV)
115
550
450
138
650
550
161
750
650
198
900
230
1050
287
1300
345
1550
900
*For effectively grounded system using 80% arrester
3.4
Arching Horn Arching horn put at the live end of the conductor string to create a preferred
path for lightning impulse to prevent flashing over at the conductors and insulators, which might damage it. Table 3.3 shows at arching distance and BIL at various circuit and tower design Table 3.4: Arching distance and BIL for various circuit and towers Circuit & Towers 275kV High Insulation Suspension
No.of Insulator Disc 16
Arching Distance (m) 2.16
BIL (kV) 1819
28 275kV Low Insulation Suspension 275kV High Insulation Tension 275kV Low Insulation Tension 132kV Suspension 132kV Tension 3.5
16
1.78
1411
2x20
2.62
1440
2x20
1.83
1100
10 2x14
1.40 1.87
1160 1000
Earthing
The basic lightning protection consists of atroke interceptor (earth wire), a down lead (tower) and an earth connection, whose primary function is to dissipate the lightning current safely into the ground. When the lightning struck the earth wire, a large amount of current flows through the tower and into the ground through the grounding system. The potential of the tower will be raised above the earth potential by an amount equal to or at least to the product of current and impedance of the earth path. If the potential rise minus and conductor voltage are much higher than the withstand voltage of the arching horn, a back flashover would occur from the tower to the conductor.
It is found that the BFR of a shielded line are very sensitive to tower footing resistance of the tower and that the BFR decrease with the decrease of the tower footing resistance. In addition, the discharge of the lightning currents into the ground will raised the potential around the earth point and those potential are related to the earth resistance and soil resistivity. Good earthing will reduce the BFR as well as the spread of dangerous voltages around the earth point.
3.6
Tower Types
29
The tower family are usually selected based on their distribution of line angle. Line angle could be grouped such that angle would follow a pattern of light medium and heavy suspension and medium and heavy tension types. Tower types are divided into four categories: 1. 23 Series tower : 2x132kV – twin(duplex) 2x300mm sq. “Batang” 2. 24 Series tower : 2x275kV – twin(duplex) 2x400mm sq. “Zebra” 3. 2423 Series tower : 2x275kV + 2x132kV – twin(duplex) 2x400mm sq. “Zebra” + twin(duplex) 2x400mm sq. “Zebra” 3. 2323 Series tower : 2x132kV – twin(duplex) 2x300mm sq. “Batang” + 2x132kV – twin(duplex) 2x300mm sq. “Batang”
Table 3.5: Tower types and deviation angle Tower Type
Angle of Dviation
Suspension Tower (Line-tower)
0-2
Tension Tower (Section Tower)
2-10
Tension Tower (Medium Tower)
10-30
Tension Tower (Heavy Tower)
30-60
Tension Tower (Right Tower)
60-90
Tension Tower (Terminal Tower)
0-10
3.6.1 Tower with wooden cross arm
The advantages of wooden cross arm is its higher impulse level and its good arch quenching properties, which result in a better lightning performance of the line. The
30 275kV cross arm is made from 4 pieces of chengal timber , 2 strut and 2 tie members and the 132kV design is made from 3 pieces of chengal timber, 2 strut and 1 tie. Due to its structursl strength limitation, wooden cross arm are used only in light suspension tower while the heavy suspension and tension towers are fitted with steel cross arm and longer tension strings. 3.7
Design Span
Span is defined as a distance between a tower top to the next adjacent tower with both tower are in the same tower family. The three terms used for span are:
1.
Basic Span defined as horizontal distance between centres of adjacent supports on level ground from which the height of standard supports is derived with the specified conductor’s clearances to ground in still air at maximum temperature
2.
Wind Span defined as half the sum of adjacent horizontal span length supported on any on tower
3.
Weight Span defined as equivalent length of the weight of conductor supported at any one tower at minimum temperature in still air
3.8 System Over voltages
Characteristic: ♦ They appear during abnormal operating conditions or during transitions between steady states.
31 ♦ They can have values much higher than system operating voltage. ♦ They form a threat to the integrity of the system and the safety of personnel. Classification: ♦ By origin:
internal overvoltages
external overvoltages
♦ By waveshape:
3.9
temporary overvoltages
slow front overvoltages
fast front overvoltages
Fast Front Over voltages
Due to lightning strokes hitting towers, ground wires, or phase conductors. Can also be induced by coupling with structures hit by lightning. Frequency of occurrence depends on the thunderstorm activity in the area which is measured by the number of thunderstorm days per year. The amplitude of the overvoltage surge generated by atmospheric discharge depends on the discharge current, line surge impedance and tower footing resistance.
3.10
Surge Arresters
32
The application of surge arresters on distribution lines started around 1975 in Japan. Field trial of surge arresters 66kV, 77kV and 138kV lines were carried out in and around 1980 in Japan and the United States. An external air gap in series with the surge arrester was introduced in 1985 to electrically isolate the surge arrester from the system. No international Standards specific on TLA available. Table below summarizes the basic requirements of TLA
Table 3.6: Fundamental requirements for surge arresters on line(TLA) Functional
Practical
Suppress over voltages and prevent flashover Light and compact at the instant of lightning Is able to cut off follow current before Easy to install on towers operation of the circuit breaker Not operate under the switching over voltages
Minimum maintenance and easy to inspect
Not cause permanent fault and impedance Long life and adverse weather proof circuit re closing in the event the arrester fails
Safe and explosion-proof
The idea of using metal oxide-surge arresters to prevent lightning fault on lines has existed quite a long time. However, there was a practical concern on “stresses” that these surge arresters may be subjected when installed on towers and the line tripping that the disconnecting device fail to operate to isolate the faulty arresters. Four general classes of devices that have been used to limit over voltages and permit lower (more economical) insulation levels of equipment: ♦ spark gaps ♦ expulsion-type arresters ♦ gapped valve-type arresters
33 ♦ (gapless) metal-oxide arresters Devices do not provide the same degree of protection Spark gaps have been used up to 245 kV in locations with modern lightning activity. 3.11
Metal-Oxide Arresters
Advantages using Metal-Oxide arresters as below:
Use metal-oxide (zinc oxide) for non-linear resistor element. Metal-oxide has a much more non-linear characteristic than silicon carbide. Characteristic is so flat that current at normal (1 per unit) voltage is in milliamps range that element can conduct continuously without overheating. No series gap required Some early versions used a shunt gap across some of the elements to reduce discharge voltage at high currents. No longer used in current designs. Present metal-oxide arresters have better protective characteristics than gapped silicon-carbide arresters and also other advantages. Metal-oxide arresters are very suitable for HVDC applications due to possibility of using parallel columns to share duty.
3.12 Gapped TLA and Gapless TLA
34 Both type of TLA are being used in many utilities worldwide to prevent lightning faults on transmission lines. However some utilities may prefer one to the other. Both types have advantages and disadvantages owing primary to the fact that the gapless TLA is connected directly to the system while gapped TLA is only connected directly to the system temporarily during the gap (spark over) operation. Table below shows the major differences between gapped TLA and gapless TLA. This table is used as basis for selecting of gapless TLA in this modeling Table 3.7: The major differences between gapped TLA and gapless TLA Subject Basic Construction Operation Principle
TLA Reliability
Faulty arrester isolation
Electrical Stresses
Gapped TLA Metal-oxide surge arrester unit in series with external air gap connected between live phase conductor and earth When lightning strikes the tower or shield wire, there will be a rise of voltage on the tower. The voltage across the insulator may reach sufficiently high value and cause the gap to spark over. A large lightning surge arrester is discharged through surge arrester and overvoltages are suppressed. As lightning surge current is discharged, resistance of ZnO block increases and current becomes small. Faulty arresters may reduce the breakdown strength of the SLA and upset the overall insulation coordination. Gap distance critical. Air gap operation can be affected by conditions during service.
Provided by external series air gap
Not subjected to temporary over voltages. Lower arrester rating and smaller arrester feasible
Gapless TLA Metal-oxide surge arrester directly connected live conductor and earth Metal oxide surge arrester to suppress the overvoltages across the insulator. Disconnecting device will operate to isolate the arrester in case the arrester fails during service Very critical because faulty arrester would cause the line to trip Arrester is usually equipped with a disconnecting device to isolate faulty arrester. The disconnecting devices operation must not affect line clearance or impedance line reclosing subjected to temporary over voltages on the system. Higher electrical stresses on the arresters
35
Installation
Maintenance Subject Arrester size and weight
More hardware needed (external spark gap rods). Gap distance critical
Periodic inspection and maintenance of surge arresters and spark gap rods recommended Gapped TLA Smaller arrester unit possible
Energy sharing Energy less assured and can be affected by the “non-ideal” air gap spark-over Operational Failure of air gap to operate correctly and Risk consistently. Gap operation may be affected by conditions in service
Strokes
TLA Placement
MOV block and polymeric housing Less hardware needed. Spaces on the strength of (existing) towers may be the limiting factors for large and heavy SLA Periodic inspection and maintenance of surge arresters recommended Gapless TLA Connected continuously to the system and subjected to over voltages in the system. Bigger and larger arrester may be required Better energy sharing between arresters Failure of the disconnecting device to operate to isolate faulty arresters
Energy Consideration
3.13 Surge Lightning Arrester placement (TLA) The placement of arrester for black-flashover, direct stroke and induced surges are summarized below :
Table 3.8: TLA placement and energy consideration High energy direct strokes Fir arresters on the other phases and Arrester must withstand to line without shield wire adjacent structures if high TFR discharge energy and causes back flashover from struck high current amplitude phase and structure to another phase in lightning impulse. Low energy direct strokes Fit arresters to phases for which Shielding angle arising from shielding shielding failure is expected failure also occurring on unshielded lines Backflashover – low Fix arresters on exposed structures on Low energy rating energy injected into the those phases most likely to suffer adequate because most
36 phase conductor from the structure Induced surges greater than line insulation
back flashover
energy is discharged into the shield wire earthing Low energy rating adequate because discharge is shared by many arresters and structures
Install arresterson all phases on all structures exposed to high induced surges
3.14 Comparison of Available Surge Arresters (Gapless Type)
There are number of high voltage surge arrester manufactured by several company which can be discussed here as mean of reference in designing the transmission line arrester. Below is the data for SLA which is PROTECTA*LITE by Ohio Brass, PEXLIM R120-YH145H by ABB and SLA.2.120.030 by Sediver
Table 3.9: Data on Gapless Transmission line arrester manufactured by several company PROTECTA*LITE Manufacturer Rated Voltage (kV) Uc (kV) MCOV (kV) Nominal discharge current (kA) Line discharge class Energy capability (kJ/KVur) Insulation material Creepage distance (mm) Weight (kg) Length (mm)
98 10
PEXLIM R120YH145H ABB 120 92 98 10
96 10
2
2
2
5.1
5.2
5
ESP 4,694
Silicon 3,726
Silicon 3,736
18 2,140
25 1,216
32 1,470
Ohio Brass 120
SLA.2.120.030 Sediver 120
37
CHAPTER 4
METHODOLOGY
4.1
System Modelling
The main emphasis is to identify the models of power system components to be used in the lightning studies. For each component, the important model parameters will be described and typical values will be provided.
38 4.2
EMTP Simulation
EMTP is a computer simulation program specially designed to study a transient pheno mena in the power system It contains a large variety of detailed power equipment models or builds in setups that simplify the tedious work of creating a system representation. Generally, this simulation software can be used in design of an electrical system or in detecting or predicting an operating problem of a power system. ATP-EMTP is used in this simulation process of observing the electrical response of the transmission system. To represent the electrical response of the transmission system, electrical model of the transmission system apparatus have to be selected and validated to gain high accuracy result.
4.3
Selected model and Validation
Models are circuit or mathematical or electrical representation of a physical apparatus so that its characteristic by the means of an output when applied with certain input. In EMTP simulation, the input and output that are usually observed are current, voltage, power and energy. A complete set of representation of a transmission system are combination of every model of the transmission line apparatus itself.
4.4
Transmission Line
39 The line which is Quadruple circuit 2 x 275kV and 2 x 132kV from Balakong to Bandar Tun Razak was commissioned in January 1995. The 12.07 km long line span across the urban areas of Balakong, Seri kembangan and Serdang with a number of spans cut across plantation, jungle and hills. The line comprises of 37nos. of 2423 series towers with steel cross-arms. The detail of the overhead transmission line system as below:
Table 4.1: Balakong to Serdang 132kV line information Line length (km)
12.07
Number of Tower
37 (No. T49A – No. T85A) Suspension: 11
Tower
2423 series
Cross arms
Steel
Span Length (m)
Min; 173
Max: 530
Avg: 335
TFR (Ohm)
Min: 1.1
Max: 11.0
Avg: 3.7
Altitude from sea level(m)
Min: 37
Max: 142
Avg: 77
Terrain
4.5
Tension: 16
16 towers on flat land and 21 towers on hills
Line exposure to lightning
How often an overhead transmission line is likely to be struck by lightning must be known to assess its lightning performance. For this purpose, the first step is to characterize the lightning activity in the region crossed by the line. Number of lightning activity can be calculated as :
N
g
= 0 . 04 T D
1 . 25
40
N
=
s
N
g
10
(4 h
1 . 09
+ b
)
Ng
=
Number of flashes to ground per square kilometer per year
h
=
Average height of the line
b
=
Width of the line
Ns
=
Total hit to line
The Lightning performance of an overhead line depends on the ground flash density of the region and on the incidence of lightning strikes to a line.
Lightning
Transition
Transition OHLine 1000 MVA
PMU Bdr Tun
PMU Balakong
Razak S/S Switching
Switching
Underground Cable 1000 MVA
Underground Cable 1000 MVA
Fig. 4.1: Model of Transimission Line
4.6
Shielding Failure
41 The phase conductor exposure to lightning is evaluated using an electrogeometric model. Line span is divided into short sections (10m to 15m), in order to accept lightning stroke to the ground wires or to the phase conductors along the span.By using IEEE 1992, the following striking distances are used : The striking distance to a conductor : rc = 1.256 rg The striking distance to earth : rg = 9Im 0.65 The striking distance to top tower : rt = 1.05 rc Im(kA) = 70kA (Lightning Stroke Current)
4.7
Overhead Transmission Lines
The overhead lines are represented by multi-phase models considering the distributed nature of the line parameters due to the range of frequencies involved Phase conductors and shield wires are explicitly modeled between towers and only a few spans are considered. The line parameters can be determined by a line constants, using the tower structure geometry and conductor data as input.
4.8
Line length and Termination
42
Since peak voltage at the struck tower is influnced by reflections from the adjacent tower sufficient number of adjacent towers at both sides of the struck tower should be modeled to determine the overvoltages accurately. Number of line span need to modeled in such that the travel time between the struck tower and the fartest tower is more than one-half of the lightning surge front time. Fig 2, shows the model of transimmision line and tower used for lightning studies
Phase Conductors and Shield Wires
Insulators
Towers
Fig. 4.2: Overhead Transmission Line, Tower and Insulator model
4.9
Tower Model
Transmission line tower model used in all simulation is represented as in Fig 3. Section of the top tower (between tower top ad top cross arm and between cross arm) are modeled as inductance branches which is determined according to section length, tower surge impedance and the propagation velocity.
43
Section of the tower from the bottom cross arm to a ground is represented by the surge impedance Zt and the propagation length Iprop as in Fig 4 . Wave propagation speed on the tower was taken to be equal to the velocity of light. Detail parameters as shown in Fig.4 This tower model is based on the work of M.Ishii, which is recognized as a detail tower model widely used in Japan for EMTP simulation purpose. This tower model does not consider the tower wrm, which is available with new tower model develop by other author. There is six-tower model parameter, which is important in selecting its parameter ♦ Zt = Tower Surge Impedance ♦ VL = Surge Propagation Velocity ♦
γ = Attenuation coefficient
♦
α
= Damping coefficient
♦ R
= Damping resistance
♦ L
= Damping Inductance
44
275 kV circuit U50% = 1120 kV
Parallel ResistanceInductance branch
132 kV circuit U50% = 880 kV
Propagation element Zt, Iprop
Fig 4.3: Tower Representation for Quadruple Circuit Transmission Line
Fig 4.4: M. Ishii’s tower model for a double circuit line tower
45 This double circuit tower model could be extended into quadruple circuit tower model with little modification. To produce high accuracy result, current impulse test should be conducted to the quadruple tower to validate its voltage and current response so that the selected parameter of Zt, R and L could be modified to increase the accuracy of the simulation. Formula used for double circuit tower could be modified to be used as quadruple tower model as follows: H
= h1 + h2 + h3 + h4 + h5 + h6 + h7
Ri =
-2Zt1 x ln√γ
x h1
h1 + h2 + h3 + h4 + h5 + h6 R7 = -2Zt2 x ln√γ Li
= α x Ri x 2H Vt
4.10
Tower footing resistance model
Steel towers are represented as a single conductor distributed parameter line terminated by resistance representing the tower footing impedance. By using a soil ionization model, the tower footing impulse resistance is described by the following equations : Rt =
Ro √(1 + I/Ig)
Ig =
ρEg 2π Rlc ²
46 where: Ro - Tower footing resistance at low current and low frequency, (ohm) (Ro = 10 – 40 ohm) Rt - Tower footing resistance, (ohm) Ig
- The limiting current to initiate sufficient soil ionization,(A)
I
- The lightning current through the footing impedance, (kA) - Soil resistivity ( 100 ohm-m)
Eg - soil ionization critical electric field (kV/m), (Eg = 400 kV/m) Rlc - tower low current resistance
The tower footing low current resistance was varied between 10 – 40 ohm and the ratio between the soil resitivity and the tower low current resistence was kept constant at 50 and typical tower grounding resistance is between 10 – 100 ohms. 4.11
Insulators
The insulators are represented by voltage-dependent flashover switches in parallel with capacitors connected between the respective phases and the tower. Refer to Fig.4.2 Typical capacitance = 80pF/unit
4.12
Back flashover
47 The back flashover of the insulators can be represented by volt-time curves. CIGRE suggested that the leader propagation model is used to represent line insulation flashovers and can be calculated using the equation as below :
V1 = 170d
u(t)
- Eo
e0.0015(u(t)/d)
d – l1 Where : V1
=
Leader velocity, m/s
d
=
Gap distance, m
l1
=
Leader length, m
u(t)
=
Applied Voltage, kV
Eo
=
520 ( kV/m )
Critical flashover voltage ( U50%) of 275 kV circuits was 1120kV and value for 132kV was 880kV.
4.13
Corona
The influence of the corona is modeled by the capacitance branches, which are connected between conductors and ground. Although corona effects may reduce the peak of lightning related over voltages by 5 – 20%, in this study corona is neglected in order to be on the pessimistic side and take the worst condition of the lightning struck. From the evaluation procedures implemented in PLASH and DESCARGA, the corona effect does not significantly affect the computation result
48
4.14
Line surge arrester
For representing the non-linear characteristic of ZnO surge arrester, Pinceti’s model has been planned to be used in simulation. Pinceti’s model was introduced in year 1999 and are the easiest model to be used. The model is from the IEEE working group with some minor difference. The model can be realized directly rom performance data rom residual voltage with various current impulses supplied by the manufacturer
The inductance value can be obtained directly from residual voltage in kV of ½, ¼ or 1/20 us impulse and 8/20 us impulse of the same current impulse. The formula used to obtained the inductances is as below. All value obtained are in Uh
L0 =
1 (V 1 / 20 − V 8 / 20) (Ur) V 8 / 20 12
L1 =
1 (V 1 / 20 − V 8 / 20) (Ur) V 8 / 20 4
For the non-linear element A0 and A1, the value can be obtained from 8/20 us impulse data as supplied by the manufacturer. The table bellows shows the value of A0 and A1 based on the recommendation from the author
49 Table 4.2: Value for A0 and A1 based on 8/20 us residual voltage supplied by
manufacturer for the application of Pinceti’s arrester model. I [kA]
A0[p.u]
A0[V]
A1[p.u]
A1[V]
2x10-6
0.810
-
0.623
-
0.1
0.974
-
0.788
-
1
1.052
109829
0.866
90410
2.5
1.096
121875
0.910
101192
3
1.108
-
0.922
-
5
1.147
134199
0.961
124437
10
1.195
153797
1.009
129858
20
1.277
-
1.091
-
Fig. 4.5: Pinceti’s arrester model used for representing surge arrester
Polymer housed line surge arrester with gapless type is choosen to be used for the lightning performance improvement. The example of SLA installation as in Fig 6.
50
14.00 5 kA 1/10 us 1 kA 8/20 us 2.5 kA 8/20 us 5 kA 8/20 us 10 kA 8/20 us 100 A 30/60 us 250 A 30/60 us 500 A 30/60 us 1 kA 30/60 us
12.00
Relative Error [%]
10.00 8.00 6.00 4.00 2.00 0.00 1
-2.00 -4.00 -6.00
Figure 4.6: Relative error of residual voltage for representing Siemens 120kV rated
3EQ4-2/LD3 SA with Picenti’s model compared to manufacturer performance data
The selection of gapless type compare to gapped type is as per appendix 1. Both types have advantages and disadvantages owing primarily to the fact that the gapless type is connected directly to the system (and continuously exposed to the system voltage and to any over voltages that may appear on the system) while gapped type is only “connected” directly to the system temporarily during the gap (spark over) operation. Proposed Surge arrester (manufactured by ABB) has the following characteristics: Rated Voltage
120 kV
Uc(kV)
92kV
MCOV(kV)
96kV
Nominal discharge current
10kA
IEC Line discharge class
2
Energy capability(kJ/KVur) 5.2 (kJ/kVmcov) Insulation Material
Silicon
Critical Flashover voltage
620 kV
Creepage distance(mm)
3,736mm
Weight(kg)
25kg
Length(mm)
1,216mm
51
Fig 4.7: Example of Gapless-type Surge Arrester installed at 132kV BLKG-SRDG
The arresters can be modeled as nonlinear resistors with 8 x 20us maximum voltage-current characteristics. SiC surge arresters will be installed at 132kV circuits only because the lower voltage (132kV) circuits of the quadruple line have lower line insulation critical flashover voltages, which means that the majority of the backflashover will happen on the 132kV circuits
4.15
Selection of Lightning Configuration
Several arrester installation configuration will be studied with maximum number of the arrester to be used is less or equal to three. The example installation configuration are as Fig 7.
52
1) 1-3 arrangement
3) L-arrangement
2) Double bottom
4) I - arrangement
Fig 4.8: Different arrester Installation Configurations
- With `Line Surge Arrester Installed -
Without Line Surge Arrester Installed
CHAPTER 5
AVAILABLE METHOD FOR LIGHTNING PERFORMANCE IMPOVEMENT
53
There are several methods available for improving the lightning performance of a transmission line in services. This method can be applied for improving the lightning performance of a transmission line already in services.
Additional Shiels Increasing
Under built Ground
Line Surge
Foot resistance
Fig. 5.1: Available Method for Lightning Improvement 5.1 Additional Shielding Wire
Shield wire could be added or modified to a tower design. The shielding angle could be decrease or in the case of quadruple circuit, additional of an under running and over running ground wire to the 132kV circuit could be done. Improvement in the performance of the 132kV circuit are expected due to coupling of the lower phases and upper phases with over running ground wire are comparable with the normal double circuit line. However, this solution would translate into high cost especially if the provisions are not in the original tower design. It would eliminate a large number of interruptions but not enough to obtain a new demanded degree reliability.
54
5.2 Tower Footing Resistance
Resistance value for transmission line with high Footing Resistance can be improved by method of counterpoise that could lead to improve the performance of the line. However this method is often difficult and expensive especially in hilly terrain. In the case of quadruple circuit line with good grounding and low lightning performance, this method ie useless because the low performance of the lower portion of the quadruple circuit line are caused low coupling with the ground wire, lower insulation and sacrificial nature of lower circuit due to the tower height.
5.3 Increase the Tower Insulation
This method can be used to increase the lightning performance of the line but also would require a large modification of the clearance and mechanical strength of the tower structure, which would lead to high cost. The insulation of the station equipment would also have to be increased to cope up with the modification, which is not a very good choice for a line in services.
5.4 Unbalance Insulation
55
This method is only applicable to double and quadruple circuit line. Double circuit outages could also be reduced by use of unbalanced insulation, which the basic principle of unbalanced insulation is to install one circuit at a higher insulation level than the other circuit. The decrease in double circuit outage rate depends on the insulation differential and on the tower footing resistance. Unbalanced insulation does not reduce total line outage rate.
Double circuit flashovers and flashovers on the higher insulation circuit are reduced, but with an inceased number of flashovers on the lower insulation circuit which is not a very reliable and good solutions for improving the lightning performance of a line. Rhis method maybe cheap considering the fact that the only thing that should be done is to decrease the arching distance of one circuit and to maintain the next circuit arching distance but this method will affect realibility and quality of power.
5.5 Transmission Line Arrester
Double circuit outages can be eliminated by installation of line surge arresters on all conductors of all circuit, which is uneconomical considering the total amount of money compared to the improvement gain. Lower TLA configuration could result in performance improvement. There are several methods available for optimizing the application of TLA for improving the lightning performance of a transmission line in services.
56
Circuit with lower Insulation level
Circuit with higher Insulation level
Fig 5.2: Unbalance tower insulation for double circuit line 5.6 Installation of TLA based on Tower Footing Resistance(TFR)
This method is used for improving a lightning performance of a double circuit line with high TFR on certain section of a line. This method is based on a strategy of arrester installment as in Figure 5.1. This strategy is based on an assumption that the higher TFR the higher the current have to be diverted by the surge arrester. Table 5.1: Arrester installation strategy to eliminate double circuit flashover
No
TFR(Ω)
TLA location
1
TFR < 10
No TLA
2
10 < TFR < 20
3
3
20 < TFR < 40
2&3
4
40 < TFR
1,2 and 3
57
1
2 3
1 2
3
Fig. 5.3: Circuit location and TLA placement for a double circuit line
Base on the strategy as in Table 5.1, an improvement of a lightning performance can be conducted based in this procedure: ♦ Divide the line into several sections and apply arrester installation configuration related to the section’s tower footing resistance. ♦ Apply same arrester installation configuration throughout the particular line section. ♦ Perform statistical study in determining the arrester energy duties ♦ The most energy sensitive installation is a configuration with arrester installed on all conductors. Lightning current have to be shared by TFR, ground wire and surge arresters.
5.6.1 Additional of TLA at low TFR Section
58
These methods are proposed by ABB for increasing the availability of the line. This procedure specially designs for protecting the line against the abnormal lightning surges (frequent or high amplitude) and reduce the outages caused by such lightning surges. Two steps needed to perform this procedur are: ♦ Place TLA at all tower section with high TFR ♦ Add additional TLA at one tower at both of the low TFR section along the section of high TFR that have been equipped with TLA
4 3
5 6
7
2
1
Fig. 5.4: Additional TLA at Low TFR section along the high TFR section
5.6.2
Installation of TLA on one circuit
Double circuit flashover can be reduced using arrester installation configuration, which are three and more arresters on one circuit installation scheme. An important difference between the application of line surge arresters on one circuit and unbalanced insulation is in the fact that line surge arresters substantially reduce the number of flashovers on the other circuit, hence improving total line lightning performance
59
TLA added only at one circuit
Fig. 5.5: TLA added only at one circuit of a double circuit line tower
5.6.3
Coordination of Gap Spacing fot Transmission Line Arrester with External Gap
The purpose of adding an external gap to the TLA are to protect the TLA from acceleration aging due to leakage current and to protect the TLA from being stressed from switching over voltage that could be in duration of 2 second which could also caused accelerated ageing of TLA. Gap distance of TLA external gap are coordinated based on two main references, the maximum switching over voltage predicted to the
60 system and the lightning impulse withstand voltage of the system insulation with TLA added. This is based on the gap distance that wide enough for not permitting operation of TLA when switching over voltage occurs but low enough to ensure operation of TLA when lightning over voltage occur.
5.7 Extended Station Protection
By locating TLA on towers near a substation, the risk of back flashover near the station is eliminated.
This result in reduction of steepness and amplitude of coming
travelling wave, thus improving the protection performance of station arresters and eliminating the need for additional expensive metal-enclosed arresters even for large GIS.
Usually a gapped TLA with higher rating are applicable to cope with the probability of being stressed by switching over voltage however higher rated compare to the substation surge arrester are advisable so that the TLA are less stressed by the switching over voltage. TLA with external gap are also applicable but the gap distance must be kept as minimum as possible so that the surge can be reduce effectively
61
Substation
Fig. 5.6: Extended station protection
CHAPTER 6
62
SIMULATION METHOD
6.1
ATP-EMTP Simulation
The study is performed with the aid of ATP-EMTP software. This software program is a computer simulation program specially design to study a transient phenomena in the power system. It contains a large variety of detailed power equipment models or builds in setups that simplify the tedious work of creating a system representation.
Generally this simulation software can be used in design of an electrical system or in detecting or predicting an operating problem of a power system. To represent the electrical response of the transmission system, electrical model of transmission system apparatus have to be selected and validated to gain high accuracy result
6.2
Selected Model and Validation
In this simulation, the parameters as input and output and need to be observed are current, voltage, power and energy. The models are circuit or electrical
63 representation of a physical apparatus and the characteristic can be observed through the output when the input applied. A complete set of representation of a transmission system are combination of every model of the transmission line apparatus itself
6.2.1
Tower Model
M.Ishii multi-storey tower model is selected as tower model for EMTP simulation purpose. This tower model does not consider the tower arm, which is available in other model. The parameters need to be modeled using M.Ishii model are : Zt = Tower Surge Impedance VL = Surge Propagation Velocity R = Damping Resistance L = Damping Inductance Hi = Height of Tower
Where: H = h1 + h2 + h3 + h4 + h5 + h6 + h7 The value of surge impedance for each level of the tower can be based on IEEE and CIGRE formula for inverted cone Z = 60 x Loge cos{0.5x tan (R/H)}
64 If R
View more...
Comments
