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Maintenance of transmission lines

Power Transmission Line

This article covers information about the health diagnostic methods, as well as about preventive and predictive maintenance techniques in cold line and in hot line conditions
Maintenance of transmission linesThe transmission lines play a very important role in transmitting the huge quantum of electrical power generated at various generating stations to the whole area of the country from one end to other end over a distance of several hundreds of kilometers. By such transmission it is possible to distribute the power to the various types of customers through substations, at lowest possible line losses.
For uninterrupted power supply it is essential to maintain the transmission lines in trim and healthy condition.
For preventive and predictive maintenance of transmission lines following methods are applied.

Regular (monkey) thorough line patrolling with digital photography of the line components is carried out.
Thermo-graphic scanning for detecting Hot spots.
Punctured insulator detection by electric field measurement.
Punctured insulator detection by voltage measurement method.
Corona-Detection by Decor camera.
Resistance measurement of electric joint in live line condition (CRM).
Tower footing earth resistance measurement.
Various techniques for EHV line maintenance

Cold line maintenance
Deteriorated tower stub strengthening
Live Line Maintenance techniques  Live line maintenance by hot       stick method.  Bare hand technique for       system maintenance.  Hot line washing.
Ground patrolling is carried out by Inspection of the line, by walk down survey, and thorough patrolling by climbing on the tower through internal body of the tower and observing the line components keeping safe body clearance. While visual inspection thermo- scanning, digital photograph of the components and corona detection by Daycor camera is carried out.

Tower foundation
Tower earthing
Nut-bolts
Tower members
Anti climbing devices
Number plates, phase plate, danger boards
Clearances, as required by IE Rules
1956 for Ground / river /canal /dam/ structure/ Buildings / Road/ Tree/ Railway/ HT- LT lines/ telephone line crossing, etc. are checked
Broken disc insulator
Flashover disc insulator
Contaminated disc insulator
Split pins / cotter pins/ W pin/ hangers
Conductor and earth wire hardware (tension and suspension hardware, mid span joint)
Conductor condition
Jumper condition
Repair sleeve
Sag and ground clearance
Tack welding to the nuts and bolts of tower
Tower members missing
Armouring, suspension clamp, split pin
Conductor spacer
Vibration dampers for conductor and earth wire
Earth wire fittings flexible earth bond, jumper
Mid span joint and repair sleeve
Bird guard
Conductor and conductor fittings
Armour rod, suspension hardware
Insulator and insulator fittings, hanger, eye bolt, yoke plate, nut- bolts
Corona measurement
Thermo-vision scanning.
Modern health diagnostic methods for EHV linesThermo-vision scanning: By Thermo-vision scanning hot spots on the line live components can be detected.Electromagnetic spectrum having wave length bands of x-rays, ultra violate rays. Visible light, infra red rays and microwaves
Measurement of resistance of electric joint in live line condition (CRM): The sophisticated instrument (live line micro ohm meter) is designed to be attached directly to an energised EHV line through a hot stick and directly reads the resistance in micro ohms. Live line Micro ohm meter is designed to store up to 9 sets of readings.
It measures the voltage drop, the line current, and the calculated resistance is displayed. The ratio of the resistance of the joint (Rj) and the reference resistance of equal length of the conductor (Rc) indicates the condition of the joint. The value Rj / Rc should be < 1 for healthy joint. The ratio > 3 is serious, needs urgent replacement.
Punctured insulator detection using positron instrument by electric field measurement (PID): The positron instrument detects punctured insulator in the string by measuring the electric field across the insulator. The electric field across the punctured insulator is less as compared to healthy insulator.
Punctured insulator detection by voltage measurement method using Ritz’s instrument:
The Ritz’s instrument detects punctured insulator in the string by measuring the voltage across the insulator. The voltage across the punctured insulator is zero.
Coronagraphy by Daycor camera: When voltage on an electrical conductor exceeds threshold value, the air around it begins to ionise to form a blue or purple glow and produces hissing noise.Corona is the polarisation of air molecules due to energy on high voltage.
Electromagnetic spectrum having wave length bands of x-rays, ultra violate rays. Visible light, infra red rays and Micro waves
Tower footing earth resistance: Tower footing earth resistance is measured by the instrument earth tester. The limit for the tower footing earth resistance is 10 ohms. After seeing the modern health diagnostic methods for EHV lines let us see the various techniques for EHV line maintenance as below.
Cold line maintenanceIn the cold line method the maintenance is carried out by availing outage on the line. The works of replacement of defective insulator strings, tightening loose nuts and bolts of the joints, cold line Insulator washing etc. and defects noticed in the thorough line patrolling are rectified, however since there is interruption in the line, there is huge revenue loss and customer’s dissatisfaction, which are the main disadvantages of cold line maintenance. Therefore hot line (live line) maintenance is highly preferable and adopted where ever possible.
Live line maintenance

Advantages of hot line technique
No outage, so no interruption to consumers
No revenue loss
Problem is attended immediately, loose nut-bolts can be tightened by hot line technique and breakdown is avoided. Hence no interruptionEmergency outage is avoided
Contaminated disc insulators can be cleaned by hot line washing, so flashover of disc insulators resulting in to break-down can be avoided
Punctured insulator can be detected with hot stick attached to punctured insulator detecting instrument either by Electric field measurement method or by Voltage measurement method. The punctured insulator can be replaced in live line
Hot spots noticed during thermo graphic scanning can be attended by hot stick method or by bare hand technique as preventive maintenance
Defective jumper can be relieved from the danger of breakdown by providing parallel jumper and emergency is avoided
Very less man-power and very less time is required as compared to cold line method.
Live line maintenance by hot stick methodIn hot stick method the works are carried out by using hot line tools.
Bare hand technique for system maintenance It is based on faraday’s principle. Therefore, when a lineman is shielded in Faraday’s cage wearing Conductive Suit and it gets bonded to an energised conductor, he can work on the conductor and associated hard ware without shock or discomfort.
Hot line tools The tools required for hot line work are made as per IEC 60832, 60895 and 61478.

Tool head required to operate (heat treated aluminium alloy which is used in aero plane)Hot sticks on which the tool is fitted were previously made of wooden handles, but they were heavy. Now epoxy glass handles are used
Engineers and the line men should be trained and authorised for hot line work. Training centres in India are at, Bengaluru (Karnataka) and Gangawal (Punjab)
Conductive suit, hand gloves, socks (woven from thin stainless steel wires and special type of fibers), boots and static belts are also manufactured on above principle
Hot line ladder and swivel sticks (manufactured from epoxy-glass) and adjustable ladder hook. Strain sticks, insulating cradle, tie sticks, roller link sticks, suspension, gin poles, cum along clamps, strain carrier sticks, double string dead end insulator toll, yoke plates, nylon and poly propylene rope, etc.
Ladder monitor kit: To detect micro ampere leakage through the ladder, this monitor kit is connected before it is placed into use and periodical readings are taken during work progress to ensure safe working condition.
Hot line washingThe insulators of lines and equipment are contaminated due to dust, bird droppings, chemical pollution, industrial pollution, saline weather, smoke due to vehicle exhaust, agricultural burning, volcanic ash etc.
The contaminated insulators are susceptible for flashover during humid atmosphere, Drizzling, dew or foggy weather etc. hence need to be cleaned.
The contaminated insulators can be cleaned by hot line washing method for lines including 400 Kv level.

When de-mineralised water jet at very high pressure (300 to 800 psi) is released in the air, the water particles split due to air resistance and so the water path is not continuous, offering the sufficient electrical resistivity more than 30,000 ohm – cm. For better cleaning high pressure compact stream is maintained.
Probe system can be used for continuous monitoring of water conductivity. A light or audible warning and pump throttle control can be integrated in to monitor circuit.For hot line washing water used should have conductivity less than 200 micro mhos per CC to avoid leakage current.
The high pressure water jet is created by electric motor pump or diesel engine operated pump.
Stub strengtheningSince, the transmission line towers are working in agricultural fields, industrial areas, sea shore areas etc. and the tower foundation is embedded in ground, the tower stubs get heavily rusted in life span of about 10-15 years.
The major causes for deterioration are wet, marshy soil around tower stubs on account of rain and drain water, watering of crops by cultivators, chemical reaction due to fertilisers and chemicals, in the farms as well as in industrial areas.
It is necessary to inspect the tower stubs condition if those are deteriorated and such towers are likely to fall in stormy weather. Hence, the deteriorated tower stubs need to be repaired and strengthened.
Tower Stub strengthening process

The area around the tower leg is cleaned
Area of one meter wide and one meter deep is excavated around the leg so as to make a room for the process
Chimney concrete of the tower stub is broken by vibro hammer
The rusted portion of the stub is thoroughly cleaned using power tools
Phospating is done to avoid corrosion. This helps to get perfect etching
Two nos M.S. angle of the same size as the stub, which are pre-treated in the factory for anti-rusting and notching is done for perfect fitting
The angles are welded with specialised welding procedure
Then the welded surfaces are thoroughly cleaned by MRF cleaning agent and protected from rusting by applying special MRF anticorrosive solution
Anti-corrosive, low viscosity epoxy resin is filled up in the cavity in between old and new stub angles
Then after the whole joint is sealed by specialized MRF putty to make the surface plain and leak proof and MRF special paint is applied
Joint is wrapped with glass wool, fibre mesh clothing giving extra protection to the stub.In re-concreting process homogeneous jointing of old and new concrete, special bonding agent which is a specific epoxy adhesive is applied over the surface of the old concrete before final concreting is started
Advanced machines like needle vibrator for re-concreting process are usedIn the concrete special fibre mesh fibre is mixed which gets uniformly dispersed in the concrete as millions of fibres in every cubic metre
For achieving perfect waterproofing of newly cast sub concrete, special 100 per cent MRF PU paint is applied
The concrete is cured by using a curing catalyst for avoiding excess wateringThe concrete portion and the tower leg are painted
And finally the remaining pit portion around the concrete is backfilled with soil by compacting the layers
Thus finally a strong waterproof and anti-rust structure is developed for the tower stub which avoids possibility of tower collapsing due to corroded stub and ensures long and healthy life for the tower lines.
Authored by__
 
Sudhir S. Kulkarni, Technical Director, RPL.
The author is Technical Director in Ramelex Pvt. Ltd, Pune since last 11 years. He retired as Superintending Engineer (EHV), MSEB-Pune.  Ramelex Pvt. Ltd. is a company founded by Mr. Ram B Jogdand, Chairman and Managing Director in the year 1990.
The company provides services in the field of line and substation, construction and maintenance, health diagnostic methods and preventive and predictive maintenance activities in cold line as well as in Hot line techniques.
It also manufactures electric power connectors and conductor accessories and insulator hardware up to 400 Kv level for ACSR, AAAC as well as for HTLS conductors.

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