RAJ KUMAR SINGH Dy. Chief Design Engineer (Engg-TL) Power Grid Corporation of India limited New Delhi Email: firstname.lastname@example.org Mobile: 9910379468
Transmission Line Categorized by • Voltage Class - 132 kV / 220 kV / 400 kV / 500 kV / 800 kV • Number of Circuits - Single / Double / Multi Circuit • AC or DC - EHV AC / UHV AC / HVDC
MAJOR COMPONENTS OF A TRANSMISSION LINE • Conductor • Towers (and Foundations) • Earthwire • Insulators ] Insulator • Hardware Fittings ] strings • Accessories
TRANSMISSION LINE DESIGN OPTIMIZATION Tower Design Study Bundle Conductor studies Review of Existing systems & Practices Tower Config. Analysis Selection of clearances Line Cost & Optimization Insulator string design Tower Fdn. Study Economic Eval. Of Line Results
BASIC DESIGN ASPECTS • Electrical Design Aspects - Power Flow / Line Loadability - Electrical Clearances (Operational, safety) - Corona & Interference - Insulation Requirements • Mechanical Design Aspects - External (Dynamic) loads due to wind, ice etc. - Self Weight of components - Temperature conditions, Climatological factors - Vibrations
REVIEW OF EXISTING SYSTEM AND PRACTICES • Review of practice adopted in different countries as well as India w.r.t following - Clearances adopted for different insulation levels - Swing angles adopted and clearances thereof - Configuration & Rating of insulator string, no of discs per string - Bundle conductor configuration, diameter of conductor - Surface gradient, Electric field, AN,TVI, RIV limitations
LOADABILITY OF TRANSMISSION LINES • Stability limit: Determined by system configuration. • Thermal limit: Determined by conductor size & its permissible temp. • Indian practices for max. conductor temp for ACSR: - 65deg C in 1970’s . - Increased to 75 degrees in 1980’s. • Line Loadability generally restricted by stability limit. Thermal limits are not fully exploited for longer lines. • FACTs, Series compensation etc.,improve stability limits & enable loading close to thermal limits. • Maxm. Permissible temperature limits increased to 85 deg C in general by POWERGRID and upto 95 deg C on case to case basis
SELECTION OF CLEARANCES • Tower Clearance (Strike Distance) for different swing angles • Phase to Phase Spacing (Vertical, Horizontal) • Ground Clearance • Mid Span Clearance and Shielding Angle
TYPICAL 400KV S/C TOWER: CLEARANCES MID SPAN CLEARANCE = 9.0 M (MIN) A B PHASE TO PHASE CLEARANCE = 8.0M (MIN) MAXM. SAG=12.87 M A= CLEARNCE AT 0 DEG SWING (FOR SWITCHING / LIGHTNIG OVERVOLTAGE) B= CLEARNCE AT MAX SWING (FOR POWER FREQ.OVERVOLTAGE) GROUND CLEARANCE = 8.84 M
SELECTION OF CLEARANCES: TYPES OF OVER VOLTAGES • Power Frequency Over voltage - Line to ground faults (Typically 1.4p.u to 1.7p.u) • Switching Over voltages -Energizing or High speed Reclosings • Lightning Over voltages
SELECTION OF CLEARANCES • Strike distance (Live metal clearances): Clearance requirements are to be based on two assumptions; - In still air or under very moderate winds, the clearance should be wide enough to withstand the lightning or switching impulse voltages. - Under high wind the clearance may be related to the power frequency voltage.
SELECTION OF CLEARANCES(CONTD.) • Phase to Phase Clearances: Dictated by live metal clearances for standard tower configurations adopted in India • Ground Clearances: Min clearance Based on I.E rules and interference criteria (Electric field, surface gradient, AN, RIV) • Mid Span Clearance: Between earthwire and conductor: Based on voltage level, span etc.
LINE INTERFERENCE • Electric field at ground • Magnetic field • Audible Noise • Radio Interference • TV Interference
BUNDLE CONDUCTOR SELECTION AND OPTIMISATION • Size, Type and Configuration of Conductor influences - Tower and its geometry - Foundations - Optimum spans - Rating and configuration of Insulator string - Insulator swings - Ground clearance - Line interferences like electric field at ground, corona, radio & TV interference, audible noise etc
CONDUCTOR TYPES • ACSR • AAAC • ACAR • AAC • New Technology Conductors - Trapezoidal - ACSS - INVAR - Self Damping - Vibration Resistant
DESIGN OF TOWERS Transmission Line Towers are designed as per IS:802:1995 considering wind zones as per IS:875:1987 SALIENT DESIGN CONDITIONS CLIMATIC LOADS RELIABILITY REQUIREMENTS UNDER NORMAL CONDITION SECURITY REQUIREMENTS FAILURE CONTAINMENT LOADS UNDER BROKEN WIRE CONDITION SAFETY REQUIREMENTS LOADS DURING CONSTRUC- TION AND MAINTENANCE LOAD.
DESIGN OF TOWERS • The reliability of transmission line towers depends on the appropriate selection of design criteria/parameters. • Climatic conditions play an important role in determining the reliability of transmission line tower. • A significant number of transmission line failures can be the result of wind speed exceeding design limits due to deficiencies in selection of design parameters/criteria.
TOWER DESIGN 1. TOWER TYPES 2. CLASSIFICATION OF TOWER 3. TOWER CONFIGURATION 4. LOADING OF TOWER 5. ANALYSIS AND DESIGN
Type of Towers a). Tangent towers with suspension string (0° to 2 °) b). Small angle towers with tension strings (2° to 15 °) c). Medium angle towers with tension strings (15 ° to 30 °) d). Large angle (30 ° to 60 °) and dead end towers with tension strings.
CLASSIFICATION OF TOWERS • ACCORDING TO CONSTRUCTIONAL FEATURE: - Self Supporting Towers. - Conventional Guyed Towers. - Chainette Guyed Towers. • ACCORDING TO NO OF CIRCUITS THEY CARRY: - Single Circuit Towers. - Double Circuit Towers. - Multi Circuit Towers. • ACCORDING TO TOWER SHAPES: - Horizontal Towers. - Vertical towers.
TOWER CONFIGURATION • A TOWER IS CONSTITUTED OF FOLLOWING: - PEAK - CROSS ARM - BOOM - CAGE - TOWER BODY - BODY EXTENSION - LEG EXTENSION - STUB/ANCHOR BOLT & BASE PLATE ASSEMBLY.
Wind Zones and Basic Wind Speeds WindBasic Wind Speed Zone(Vb) m/sec 1 33 2 39 3 44 4 47 5 50 6 55
Reliability Levels RELIABILITYRETURNSUGGESTED FOR LEVELPERIOD 1 50 FOR EHV TRANS LINES UPTO 400KV CLASS 2 150 FOR TRANS LINES ABOVE 400KV CLASS AND TRIPLE & QUAD CIRCUIT TRANS LINE UPTO 400KV. 3 500 FOR TALL RIVER CROSSING TOWERS AND SPECIAL TOWERS.
Loads Due To Conductor & Earthwire i). Transverse Load a). Due to Conductor & Earthwire. Pd . Cdc. L . Gc. d b). Due to insulator string. Where, Cdi. Pd. Ai . Gi Pd = Design wind pressure c). Deviation loads Cdc, Cdi = Drag co-officients 2T. Sin(D/2) L = Wind span Gc, Gi = Gust response factors ii). Vertical Load d = Dia of cable T = Design tension iii). Longitudinal Load D = Deviation angle
Tower Loads i). Transverse Load Pd. Cdt. Ae. Gt ii). Self Weight Load Combinations Reliability Conditions Security Conditions Safety Conditions
Analysis And Design • ANALYSIS i). GRAPHICAL METHOD ii). ANALYTICAL METHOD iii). COMPUTER AIDED ANALYSIS • DESIGN AS COMPRESSION AND TENSION MEMBERS. • CODAL PROViSION FOR LIMITING SLENDERNESS RATIO FOR COMPRESSION MEMBER DESIGN i). LEG MEMBERS - 120 ii). BRACINGS - 200 iii). REDUNDANTS - 250 iv). TENSION MEMBERS - 400
NAME, VOLTAGE, CLASS, WIND ZONE & BASIC DESIGN PARAMETERS ( FROM APPROVED FR OR SEF GROUP) GEOLOGICAL CONSTRAINTS DETAILS OF ROUTE & BILL OF QUANTITIES (FROM SITE) DESIGN PHILOSPHY (FROM IS / IEC/ STANDARDISATION COMMITTEE REPORTS) INPUTS REVIEW CONFOIGURATION & TYPE OF TOWERS REVIEW TOWER LOADINGS & CONDITIONS REVIEW DESIGN STAGES • STURUCTIRAL ANALYSIS • BY COMPUTER • BY MANUAL VERIFICATION REVIEW FINAL DESIGN (THEORITICAL) STRUCTURAL DRAWINGS PROTO MANUFACTURE/ FABRICATION TESTING & FINALISATION MODIFY DESIGN SUCCESSFUL FAILED PROTO TESTING (FULL SCALE) DESIGN FINALISED FLOW CHART FOR TOWER DESIGN
START MAXIMUM/ CRITICAL TOWER LOADINGS FROM TOWER DESIGN/ PREVIOUS SIMILAR FDN DESIGN , TOWER DIMENSIONS & SLOPE FROM TOWER DESIGN REVIEW INPUTS FOUNDATION LOADINGS TYPE OF FDN FROM BOQ (SITE INPUT), SOIL PROPERTIES FROM SPECN/ SOIL INV. REPORT & CONCRETE PROPERTIES FROM SPECN DESIGN PHILOSPHY (FROM IS/ CBIP / (STANDARDISATION COMMITTEE REPORTS) REVIEW FOUNDATION DESIGN BY COMPUTER /MANUALLY REVIEW REVIEW LARGE VARIATION WRT PREVIOUS SIMILAR DESIGN/ NIT ESTIMATE ? YES YES FOUNDATION DRAWINGS FINALISATION REVIEW DESIGN FINALISED END FLOW CHART FOR FOUNDATION DESIGN
INSULATION CO-ORDINATION • Insulation co-ordination aims at selecting proper insulation level for various voltage stresses in a rational manner. The objective is to assure that insulation has enough strength to meet the stress on it. How many Flashovers? Strength Stress Over Voltage Probability Density Insulation Flashover Probability Voltage-kV
Pollution Level Equiv. Salt Deposit Density (mg/cm2) Minm nominal specific creepage dist (mm/Kv) Light 0.03 to 0.06 16 Medium 0.10 to 0.20 20 Heavy 0.20 to 0.60 25 Very Heavy >0.60 31 INSULATOR AND INSULATOR STRING DESIGN Electrical design considerations • Insulation design depends on - Pollution withstand Capability Min. nominal creepage dist. = Min nominal specific creepage dist X highest system voltage phase to phase of the system Creepage Distance of insulator string required for different pollution levels - Switching/ Lightning Over voltage
INSULATOR AND INSULATOR STRING DESIGN Mechanical design considerations • a) Everyday Loading Condition Everyday load 20 to 25% of insulator rated strength. • b) Ultimate Loading Condition Ultimate load on insulator to not exceed 70% of its rating. This limit corresponds roughly to pseudo-elastic limit. • c) In addition, capacity of tension insulator strings at least 10 % more than rated tensile strength of the line conductors.
Earthwire • Function • To protect conductor against lightning flashovers • To provide a path for fault current
LIGHTNING FLASHOVERS • Direct Flashover • Occurs due to shielding failure with lightning on the conductor , flashover taking place across the insulator string from conductor to ground. • Back Flashover • Occurs due to high tower footing resistance with a high voltage at the grounded tower cross arm compared to conductor, resulting in a flashover across the insulator string from ground to conductor.
HARDWARE FITTINGS • For attachment of insulator string to tower • D-Shackles,Ball clevis, Yoke plate, Chain link • For attachment of insulator string to the conductor • Suspension & tension assembly • Fittings like D-Shackles, Socket clevis, chain link • For protection of insulator string from power follow current • Arcing Horn • For making electric field uniform and to limit the electric field at the live end • Corona Control Ring/ Grading Ring • For fine adjustment of conductor sag • Sag Adjustment Plate, Turn Buckle
HARDWARE FITTINGS • Arcing Horn • The air gap is maintained for satisfactory performance under actual field conditions. • For power follow current • Yoke Plate • To withstand mechanical loads- Thickness & shear edge maintained • To maintain sub conductor spacing • Corona Control Ring/ Grading Ring • To cover atleast one live end insulator disc • To cover hardware fittings susceptible for Corona/RIV
HARDWAREFITTINGS • Suspension Assembly • Shaped to prevent hammering between clamp & conductor • To minimize static & dynamic stress in conductor under various loading conditions • Minimum level of corona/RIV performance • For slipping of conductor under prescribed unbalanced conditions between adjacent conductor spans • Tension Assembly • To withstand loads of atleast 95% of conductor UTS • To have conductivity more than that of conductor • Sag Adjustment Plate/ Turn Buckle • To adjust sag upto 150mm in steps of 6mm
ACCESSORIES FOR CONDUCTOR & EARTHWIRE • For joining two lengths of conductor/earthwire • Mid Span Compression joint for Conductor/ earthwire • For repairing damaged conductor • Repair Sleeve • For damping out Aeolian vibrations • Vibration Damper for conductor & earthwire • For maintaining sub conductor spacing along the span • Spacers • For damping out Aeolian vibrations, sub span oscillation and to maintain sub conductor spacing • Spacer Damper
ACCESSORIES FOR CONDUCTOR & EARTHWIRE • Mid Span Compression joint for Conductor/ earthwire & Repair Sleeve • To withstand at least loads equivalent to 95% of the conductor UTS • To have conductivity better than equivalent length of conductor (99.5% Aluminium)
TYPE OF WIND INDUCED VIBRATIONS • AEOLIAN VIBRATIONS High frequency, low amplitude vibrations induced by low, steady & laminar wind • WAKE INDUCED VIBRATIONS Low frequency, medium amplitude vibrations induced by high velocity steady winds on bundle conductors • GALLOPING Very low frequency, high amplitude vibrations induced by high velocity steady winds on conductors with asymmetrical ice deposit
FACTORS INFLUENCING VIBRATION PERFORMANCE • TYPE , STRANDING & DIA OF CONDUCTOR, EARTHWIRE • CONDUCTOR/EARTHWIRE TENSION • SUB-CONDUCTOR SPACING IN BUNDLE CONDUCTORS • BUNDLE CONFIGURATION