Transmission and Distribution Lines

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Registration / Licence Examination — Study Material Electrical Engineering (AElE) CHAPTER 7 Transmission and Distribution Lines Sub-code Topic AElE0701 Transmission Lines:

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Overhead & Underground Cabling AElE0702 Transmission Line Circuit Selection AElE0703 Mechanical Design of Overhead Line AElE0704 Electrical Loads AElE0705 Distribution Systems AElE0706 Voltage Regulation & Power Factor Correction This detailed edition contains expanded revision notes, comparison tables, exam-style single-correct MCQs with explanations, and a dedicated “Things to Remember” section after every topic plus a master revision sheet at the end — designed specifically for MCQ-based licensing examinations.

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7.1 Transmission Lines:

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Overhead Transmission Line and Underground Cabling (AElE0701) This section introduces the two basic means of transporting bulk electrical power — overhead transmission lines and underground cables — comparing their construction, components, and relative merits.

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Overhead Transmission Line An overhead line consists of bare (uninsulated) conductors suspended in open air on supports (towers or poles), relying on air as the insulating medium between conductors and between conductor and support.

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Main components: conductors (carry current), supports (towers/poles, hold conductors at safe height), insulators (isolate live conductors from the earthed support), cross-arms (mount insulators on the support), and an overhead earth (shield) wire (lightning protection).

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Underground Cabling An underground cable is a fully insulated conductor assembly buried in the ground, used where overhead construction is impractical (dense urban areas, river crossings, aesthetic/right-of-way constraints).

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Basic cable construction (working outward): conductor (stranded copper/aluminium core) → conductor insulation (e.g. impregnated paper, or XLPE — cross-linked polyethylene) → metallic sheath (lead or aluminium, moisture barrier) → bedding/armouring (mechanical protection, steel tape/wire) → serving (outer protective/anti-corrosive covering).

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PILC (Paper Insulated Lead Covered) — traditional, oil-impregnated paper insulation;

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XLPE cables — modern, higher thermal rating, lighter, no oil leakage risk, now the dominant technology for new installations.

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Underground Comparison Factor Overhead Line Underground Cable Initial cost Much lower Much higher (several times overhead cost) Fault location & repair Faster, visible faults, easier access Slower, faults hard to locate, excavation needed Exposure to weather/lightning Exposed (wind, ice, lightning, pollution) Well protected, unaffected by weather Appearance / right-of-way Visually intrusive, needs wide corridor Aesthetically unobtrusive, saves surface space Typical use Long-distance transmission, rural/suburban areas Dense urban areas, short high-value runs, special crossings

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This section examines why high voltage is used for bulk power transmission, the factors governing voltage level and conductor spacing choices, the classification of insulating materials, and voltage regulation/efficiency for short and medium transmission lines.

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Necessity of High Voltage Transmission For a given power P transmitted at voltage V and power factor cosφ, line current I = P / (√3 × V × cosφ) for a three-phase system — so raising the transmission voltage reduces the line current for the same power delivered.

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Since line (I²R) power loss and the required conductor cross-sectional area both depend on current, a lower current at higher voltage means lower I²R losses and less conductor material (smaller, cheaper conductors) for the same power transfer — the fundamental economic justification for high-voltage transmission.

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Higher voltage also improves voltage regulation and allows greater power to be transmitted over long distances economically, though it requires costlier insulation, larger clearances, and more expensive terminal equipment (transformers, switchgear) — an economic optimum voltage exists (related to Kelvin's Law) balancing conductor cost against insulation/equipment cost.

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Choice of Voltage Level & Conductor Spacing The choice of transmission voltage level depends on the quantity of power to be transmitted, the transmission distance, standard/available equipment voltage ratings, and overall economic optimization (conductor cost vs. insulation/equipment cost).

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Conductor spacing must satisfy minimum electrical clearance (to prevent flashover between phases and to limit corona discharge at higher voltages) as well as mechanical clearance (to accommodate conductor sag/swing under wind and temperature variation).

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Corona — a discharge/ionization effect around a conductor when the surface voltage gradient exceeds the breakdown strength of air — increases with higher voltage and closer/smaller conductors, so spacing and conductor diameter are chosen partly to keep corona losses within acceptable limits.

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Insulating Materials & Classification Insulator Material Characteristics Porcelain Traditional, widely used, good mechanical strength and weather resistance, moderate cost Glass (toughened) Good dielectric strength, self-indicating failure (shatters visibly when damaged), used for high voltage strings Polymer / composite (silicone rubber) Lightweight, excellent hydrophobic (water-repellent) surface reducing flashover in pollution/wet conditions, increasingly preferred for HV/EHV lines Voltage Regulation & Efficiency of Short and Medium Lines Voltage regulation (%) = [(VS − VR) / VR] × 100, where VS is sending-end voltage (at no-load-equivalent, i.e. with the same VR magnitude reference) and VR is the receiving-end (load) voltage — it measures how much the receiving-end voltage would rise if load were removed, expressed relative to full-load voltage.

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Transmission efficiency (%) = (Power delivered at receiving end / Power sent from sending end) × 100 = (PR / PS) × 100 = [PR / (PR + line losses)] × 100.

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A short transmission line (typically up to ~80 km) is modeled with only series resistance and inductive reactance (shunt capacitance neglected); a medium transmission line (roughly 80–250 km) additionally accounts for shunt capacitance, commonly using the nominal-π or nominal-T equivalent circuit.

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This section covers the mechanical aspects of overhead line design — sag and tension, supports, conductor materials, insulator types and string efficiency, and jumpers/vibration dampers.

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Sag, Tension & Clearances Sag is the vertical difference between the level of supports and the lowest point of the conductor, arising because the conductor cannot be perfectly straight/taut; sag increases with conductor weight/span and decreases with increased tension.

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Tension and sag are inversely related for a given span and conductor weight — higher tension gives lower sag, but excessive tension risks conductor breakage, especially under added ice/wind loading and temperature contraction, so an optimum working tension (with adequate safety factor) is chosen.

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Adequate ground clearance must be maintained at maximum sag condition (highest temperature/heaviest loading) to ensure safety beneath the line, per applicable clearance standards.

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Supports & Cross Arms Supports (poles for lower voltage/shorter spans, lattice steel towers for higher voltage/longer spans) hold the conductors at the required height and withstand conductor weight, wind, and ice loading, plus unbalanced tension (e.g. at angle or dead-end towers).

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Cross-arms mounted on the support carry and space the insulators, and their design/strength must accommodate the mechanical loads transferred from the conductors and insulators.

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Conducting Materials Conductor Material Characteristics ACSR (Aluminium Conductor Steel Reinforced) Aluminium strands (conductivity) around a steel core (mechanical strength); the most widely used overhead transmission conductor, offering a good strength-to-weight and cost balance AAAC (All Aluminium Alloy Conductor) Aluminium alloy throughout, better corrosion resistance than ACSR (no steel core to corrode), lighter, used where corrosion is a concern AAC (All Aluminium Conductor) Pure aluminium, high conductivity but lower mechanical strength, used mainly for short spans/low voltage or in areas with limited mechanical stress Insulators, String Efficiency & Vibration Control Insulator Type Application Pin insulator Mounted on a pin fixed to the cross-arm, conductor tied on top; used for lower voltages and straight-line/tangent supports Suspension insulator A string of insulator discs suspended vertically from the cross-arm, conductor hangs at the bottom; used for higher voltages, easily extended by adding discs Strain (tension) insulator Used at dead-ends, angle towers, or river crossings where the insulator string must withstand the full conductor tension Shackle insulator Used for low-voltage lines, typically at angle points or terminations, directly bolted/strapped to the support String efficiency = (Voltage across the whole string) / (n × voltage across the disc nearest the line conductor), where n = number of discs — ideally 100% if voltage divided equally across all discs, but in practice the disc nearest the conductor experiences the highest voltage (due to shunt capacitance to the earthed tower), so string efficiency is always less than 100% and decreases as more discs are added.

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Jumpers are short flexible conductor loops used to electrically connect the two conductor sections at a strain/dead-end tower (where the main conductor is not continuous), allowing current to bypass the insulator string.

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Vibration dampers (commonly Stockbridge dampers) are weighted devices clamped onto the conductor near the support to absorb the energy of wind-induced (aeolian) vibrations, preventing fatigue damage/breakage of the conductor strands at the support point.

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This section covers the classification and characteristics of electrical loads encountered in power systems, and the basic principles of load forecasting.

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Types of Electrical Loads Load Type Characteristics Domestic (residential) Lighting, fans, small appliances; relatively low power factor, peaks in morning/evening hours Commercial Shops, offices; lighting, air-conditioning, computing loads, peaks during business hours Industrial Motors, process equipment; typically the largest, steadiest load with relatively flat demand across the day Agricultural Irrigation pumping loads; often seasonal, may have poor power factor Municipal / street lighting & public utility Public lighting, water supply pumping; largely predictable, fixed-schedule loads Load Characteristics Term Definition Demand factor = Maximum demand / Total connected load (≤ 1); indicates how much of the total connected load is actually used simultaneously at peak Load factor = Average load / Maximum demand (≤ 1), over a given period; a higher load factor indicates more uniform loading and better utilization of generation/transmission capacity Diversity factor = Sum of individual maximum demands / Coincident (system) maximum demand (≥ 1); accounts for the fact that individual peak demands typically do not occur simultaneously Plant capacity factor = Average load on plant / Rated plant capacity; reflects how much of the installed generation capacity is utilized on average A higher diversity factor (individual peaks more spread out in time) allows a smaller generation/distribution capacity to serve a given set of connected loads economically, since the system need not be sized for the (rare) coincidence of all individual peaks at once.

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A higher load factor is generally desirable for utilities, since it means generating capacity is used more uniformly (fewer idle hours), improving the economics of generation.

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Load Forecasting Load forecasting predicts future electrical demand (short-term: hours to days ahead, for operational scheduling; medium/long-term: months to years ahead, for capacity planning and investment decisions).

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Common approaches include trend/extrapolation methods (projecting historical load growth patterns), correlation methods (relating load to factors such as population growth, GDP, weather/temperature), and increasingly, statistical/computational forecasting techniques.

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Accurate load forecasting is essential for planning generation capacity additions, transmission/distribution reinforcement, and scheduling maintenance, avoiding both costly over-investment and risky under-capacity.

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This section covers primary and secondary distribution system configurations, the selection of supports and conductors for secondary distribution, and the role of the stay wire.

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Primary Distribution System Configuration Characteristics Radial system Power flows in a single direction from substation to consumers along a tree-like network; simplest and cheapest, but a fault anywhere upstream interrupts supply to all downstream consumers (poor reliability) Ring main system Feeder forms a closed loop starting and ending at the substation, with consumers tapped along the ring; each point can be fed from two directions, so a single fault can be isolated without interrupting the whole ring (improved reliability over radial) Interconnected network system Multiple feeders/substations are interconnected, allowing power to flow from more than one source to any point; offers the highest reliability and best voltage regulation but is the most complex and costly Secondary Distribution System Three-phase, four-wire distribution (three phases plus neutral) is used where both three-phase power (for motors/industrial loads) and single-phase power (for lighting/domestic loads, taken between one phase and neutral) are required from the same network.

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Single-phase, two-wire distribution (one phase plus neutral/return) is used for purely single-phase loads such as small residential or rural consumers where three-phase supply is not needed.

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Selection of Supports & Conductors in Secondary Distribution Support selection (wood, concrete, or steel poles) and spacing depend on span length, conductor loading (weight, wind, ice), soil conditions, and cost, with concrete/steel generally preferred for durability in permanent installations.

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Conductor selection for secondary distribution considers current-carrying capacity (ampacity) for the expected load, permissible voltage drop over the run, mechanical strength for the span, and economic factors (material cost vs. losses).

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Stay Wire A stay wire (guy wire) is a tensioned wire anchored diagonally from a pole to the ground (or to another structure), used to counteract unbalanced mechanical pull on the pole — for example at angle poles, dead- end poles, or poles carrying an unequal number of conductors on each side — preventing the pole from bending or toppling.

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Stay wires are typically fitted with a strain insulator in the wire itself, isolating the upper (pole-side) section from the lower (ground-anchored) section, so that if the main conductor were to touch the stay wire, the ground-level section (and anyone near it) remains at a safe potential.

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This final section of the chapter covers methods of voltage regulation in distribution systems and the principles and methods of power factor correction.

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Voltage Regulation in Distribution Systems Voltage at the consumer end must be maintained within a permissible band (per standards/regulation) despite variation in load current along the feeder, which causes voltage drop (I × Z) that varies with loading level.

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Common voltage regulation methods include: tap-changing transformers (on-load or off-load, adjusting turns ratio at the substation or distribution transformer), voltage/feeder regulators (auto-transformer-based devices inserted along a feeder to boost/buck voltage locally), capacitor banks (shunt capacitors reduce reactive current drawn from the source, reducing the I × X component of voltage drop and raising voltage), and booster transformers (series-connected, injecting a small additional voltage).

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Proper conductor sizing (to limit I × R drop) and balanced loading across phases also help maintain acceptable voltage regulation in distribution networks.

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Power Factor Correction Most practical loads (induction motors, transformers, fluorescent lighting ballasts) are inductive, drawing a lagging power factor, meaning the current drawn is larger than the minimum needed to deliver the real (active) power — this increases I²R losses, requires larger conductors/equipment, and typically incurs utility penalty charges for low power factor.

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Power factor correction means adding a source of leading reactive power (capacitive) to partially or fully offset the lagging reactive power drawn by inductive loads, bringing the net power factor closer to unity.

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Power Factor Correction Methods Method Description Static (shunt) capacitor banks Most common and economical method; capacitors connected in parallel with the load supply leading reactive power (VARs), reducing the reactive power drawn from the source; can be fixed or automatically switched in steps to match varying load Synchronous condenser An over-excited synchronous motor running unloaded, supplying leading reactive power; provides smoothly variable compensation but has higher cost, losses, and maintenance than static capacitors (rotating machine) Static compensators (SVC / STATCOM) Power-electronic-based devices offering fast, continuously variable reactive power compensation, used especially where rapid response to changing reactive demand is needed Benefits of power factor correction: reduced I²R losses, reduced current (freeing up capacity in conductors/transformers), improved voltage regulation, and reduced/avoided low-power-factor utility penalty charges.