Power Generation and Transmission Notes

CLO-3: Insulation and Protection of Transmission Lines

3.1 Ceramic Insulator Characteristics (Medium Voltage Suspension)
- Insulators prevent free flow of internal electric charge.
- They support overhead line conductors, preventing current flow to the earth.
- Materials: rubber, wood, plastic, mica, glass, ceramic, porcelain, PVC, polymers.
Types of Insulators
- Classified based on ratings for transmission & distribution systems
- Types:
  - Pin Insulator
  - Suspension Insulator
  - Strain Insulator
  - Shackle Insulator
Pin Insulator
- Used in distribution systems.
- Voltage capacity: < 33kV
- High mechanical strength
- Connected vertically or horizontally.
- Simple construction, less maintenance.
Suspension Insulator
- Also called disc insulator, made of ceramic, porcelain, or glass.
- Voltage capacity: $11 ewline kV$ to $765 ewline kV$
- Used in overhead transmission lines for flexibility.
- Uses multiple discs based on voltage level.
- Connected by steel towers, requiring more height.
- If one disc is damaged, the remaining discs continue to function.
Strain Insulator
- Similar to suspension insulators but with different specifications.
- Voltage capacity: $33kV$
- Placed at bends or arm places in TL.
- Number of discs vary based on rated system voltage:
  - $33KV$ : 3 discs (strain), 3 discs (suspension)
  - $66KV$ : 5 discs (strain), 4 discs (suspension)
  - $132KV$ : 9 discs (strain), 8 discs (suspension)
  - $220KV$ : 15 discs (strain), 14 discs (suspension)
Shackle Insulator
- Small size insulators used in overhead distribution systems.
- Connected using a metallic strip.
- Voltage capacity: < 33
 ewline kV
- Used in bend or circular turn positions.
- Vertical or horizontal positioning using bolts or cross arms.
Insulator Voltage Capacity and Use
- Pin insulator: < 33
 ewline kV, distribution system
- Post insulator: $11 ewline kV$ to $765 ewline kV$ , substation system
- Suspension insulator: > 11
 ewline kV (High), transmission system
- Shackle insulator: < 33
 ewline kV, distribution system
Ceramic Insulator Characteristics
- Major developments since WWI.
- Properties tailored to specific requirements.
- Important electrical properties:
  - Dielectric constant: $1.5$ (porous) to $10.5$ , common range $5.5$ to $6.5$
  - Power factor: $0.02$ (porcelain) to $0.000015$ , common range $0.004$ to $0.0005$
  - Resistivity: $10^{14}$ to $10^{16}$ ohms cm at room temperature.
  - Dielectric strength: $190$ to $350$ volts per mil.
  - Frequency effect: stable up to $10$ megacycles, higher frequencies increase loss factor.
Mechanical and Physical Characteristics of Ceramics
- Brittleness
- Low tensile strengths
- High compressive strengths
- High-temperature stability
- Chemical stability
- Low thermal conductivities
3.2 Wooden Support Poles
- Used in $400$ volt and $230$ volt low tension lines.
- Lower cost than other poles.
- Foundation cost comparable to wood pole cost.
- Long service life.
Voltage Distribution Over A Suspension Insulator String and String Efficiency
- Each disc between metal links forms a capacitor, $C$ (self-capacitance or mutual capacitance).
- Air capacitance (shunt capacitance) exists between metal links and earthed tower.
- Shunt capacitance = $k$ * self-capacitance.
Kirchoff's Current Law Application
- Node A: $I2 = I1 + i1$ => $V2ωC = V1ωC + V1ωkC$ => $V2 = V1 + V1k$ => $V2 = (1 + k)V_1$ …… eq.(i)
- Node B: $I3 = I2 + i2$ => $V3ωC = V2ωC + (V2 + V1)ωkC$ => $V3 = V2 + (V1 + V2)k$ => $V3 = kV1 + (1 + k) V2$ => $V3 = kV1 + (1 + k)^2 V1$ …… from eq.(i) => $V3 = V1 [k + (1 + k)^2]$ => $V3 = V1 [k + 1 + 2k + k^2]$ => $V3 = V_1 (1 + 3k + k^2)$ …… eq.(ii)
- Voltage between the conductor and the earth tower:
 - $V = V1 + V2 + V3$ => $V = V1 + (1 + k)V1 + V1 (1 + 3k + k^2)$ => $V = V_1 (3 + 4k + k^2)$ …… eq.(iii)
- Voltage across the top disc is minimum, and voltage across the disc nearest to the conductor is maximum ( $V3 = V1 (1 + 3k + k^2)$
- Non-uniform voltage distribution causes maximum electrical stress on the disc nearest to the conductor.
String Efficiency
- String efficiency = (Voltage across the string) / (number of discs X voltage across the disc nearest to the conductor).
- Greater string efficiency implies more uniform voltage distribution.
- 100% string efficiency is ideal but impossible in practice.
- For DC voltages, insulator capacitances are ineffective, and voltage across each unit is the same (100% string efficiency).
- Inequality in voltage distribution increases with the number of discs in a string.
- Shorter strings are more efficient than longer string insulators.
Problem Solving Question - 1
- An overhead 3-phase transmission line, which has phase voltage $30 ewline kV$ , is hanging from a three insulators suspension strain. The capacitance between the links and the earth is $0.2 ewline C$ , where $C$ is the capacitance of an insulator. Compute the:
  - a. Voltage across each disc.
  - b. String Efficiency
Solution – 1
- a. $V2 = (1 + k)V1=1.2V1$ $V3 = V1 (1 + 3k + k^2) =1.64 V1$
 $V = V1 + V2 + V3$ $30000 = 3.84 V1$ $V_1 =7812.5 V$
- $V2 = (1 + k)V1 =1.2V1 =9375 V$ $V3 = V1 (1 + 3k + k^2) =1.64 V1 =12812 V$
- b. String efficiency= $30000/(3 * V_3 ) = 0.7804=78.04\%$
Problem Solving Question – 2.
- Repeat the above problem when the capacitance between the links and the earth is:
 - $0.1 ewline C$
 - $0.0 ewline C$
 - $0.3 ewline C$
 - $0.4 ewline C$
 - Where C is the capacitance of an insulator.
3.3 Cleaning Insulators
- Cleaning prevents flashovers, blackouts, and equipment damage.
- Hot Line Washing: Cleaning insulators with water while energized.
- White vinegar: Removes water residue and weathering accumulation; soak insulators overnight, brush, and rinse.
- Oxalic acid: Used for cleaning due to availability, cheapness, safeness.
3.4 Transmission Line Towers: Spacing and Slack
- Conductors swing synchronously with wind, but non-synchronous swinging can occur.
- Minimum horizontal spacing should be 1% of the span length.
- Ground clearance requirements:
 - $66 ewline kV$ : $6.5 ewline m$ at +65°C conductor temperature
 - $132 ewline kV$ : $7.0 ewline m$ at +65°C conductor temperature
 - $220 ewline kV$ : $7.5 ewline m$ at +80°C conductor temperature
- Minimum clearance over non-navigable rivers: $3.05 ewline m$ above maximum flood level.
- Minimum clearance between power lines and telecommunication cables:
 - $132 ewline kV$ : $2.44 ewline m$
 - $220 ewline kV$ : $2.74 ewline m$
 - $400 ewline kV$ : $4.88 ewline m$
- Minimum spacing between power lines:
 - $132 ewline kV$ : $2.75 ewline m$
 - $220 ewline kV$ : $4.55 ewline m$
 - $400 ewline kV$ : $6.00 ewline m$
- Power lines can overheat in hot weather, causing expansion and increased slack.
- Tension exists in a string under opposing forces; tight cables are prone to cutting due to contraction or expansion.
3.5 Ground Wires and Tower Grounding
- Ground wires (earth wires): bare conductors at the top of transmission towers.
- Shield the line and intercept lightning strikes.
- Run parallel to main conductors, higher than main conductors, grounded at every tower.
- Horizontal arrangement: two ground wires for effective shielding.
- Vertical configuration: one ground wire.
- Requirements:
  - Mechanically strong and provide sufficient shielding.
  - Sufficient clearance between power conductors and tower structure.
  - Adequate clearance between line conductors and ground wires.
  - Low tower footing resistance.
  - Made of galvanized steel or ACSR conductors.
  - Protective angle: $20^o$ to $45^o$ .
Tower Earthing
- Interconnected system of conductors, rods, connectors, foundation, and local soil.
- Considers electrical performance of the line and individual tower.
- Varies from tower to tower due to varying parameters and conditions.
- Each tower should be earthed.
- Footing resistance measured in the dry season should not exceed $10 ewline </li></ul></li> </ul> Ω$ .
 * Pipe earthing is used.
 * River crossing and railway crossing towers are earthed at diagonally opposite legs.
 - 3.6 Medium/High Voltage Circuit Breaker Components
 - Circuit Breaker (CB) Function:
 - Controls a circuit manually or remotely under normal and fault conditions.
 - Automatically breaks a circuit under fault conditions (overcurrent, short circuit).
 - Working Principle of Circuit Breaker
 - Fixed and moving contacts (electrodes) in a closed chamber with a quenching medium (liquid or gas).
 - Contacts remain closed under normal operating conditions.
 - Trip coils energize when a fault occurs, separating the moving contacts and opening the circuit.
 - Arc Phenomenon
 - When contacts separate under fault conditions, an arc is struck.
 - The breaker must extinguish the arc quickly to prevent dangerous heat buildup.
 - Current depends on arc resistance during the arcing period.
 - Factors Affecting Arc Resistance
 - Degree of ionization: Resistance increases with decreased ionization.
 - Length of the arc: Resistance increases with increased length.
 - Cross-section of the arc: Resistance increases with decreased cross-section.
 - Methods of Arc Extinction
 - High resistance method
 - Low resistance or current zero method
 - Components of the Circuit Breaker
 - Actuator lever: ON/OFF switch; trip-free feature.
 - Actuator mechanism: Opens and closes the contact.
 - Contacts: Allow current flow when closed; break the current when open.
 - Terminals: Connect to the circuit.
 - Bimetallic strip: Separates contacts under high voltage or short circuit.
 - Calibration screw: Sets the trip current.
 - Solenoid: Separates contacts under overcurrent.
 - Arc extinguisher/divider: Separates an arc during interruption.
 - Types of Circuit Breakers
 - Low Voltage Circuit Breakers: For DC applications in domestic, commercial, and industrial fields. (MCB, MCCB)
 - Medium Voltage Circuit Breakers: For indoor applications in metal enclosed switchgear or outdoor applications in substations. (Vacuum, air, SF6 CBs)
 - High Voltage Circuit Breakers: Protect and control electrical power transmission networks; solenoid operated with current sensing protective relays and current transformers.
 - Medium & High Voltage CB
 - Medium Voltage: $1 ewline KV$ - $69 ewline KV$
 - High Voltage: $69 ewline KV$ - $230 ewline KV$
 - Vacuum Circuit Breaker (VCB): Used for medium voltage; arc quenching in vacuum bottles.
 - SF6 Circuit Breaker: Used in medium voltage applications; SF6 gas quenches arc; less preferred due to poisonous nature.
 - Oil Circuit Breaker: Used on high voltages; oil used as arc quenching medium.
 - 3.7 Types of Circuit Breakers
 - Oil Circuit Breakers
 - Designed for $11kv-765kv$ .
 - Types: BOCB (Bulk Oil Circuit Breaker), MOCB (Minimum Oil Circuit Breaker).
 - Contacts are immersed in oil.
 - Oil provides cooling and acts as a dielectric medium.
 - Advantages:
 Good dielectric strength.
 Low cost and easily available.
 Wide range of breaking capability.
 - Disadvantages:
 Slower operation (20 cycles for arc quenching).
 Highly inflammable, high risk of fire.
 High maintenance cost.
 - Vacuum Circuit Breakers
 - Designed for medium voltage range ( $3.3-33kv$ ).
 - Vacuum pressure ( $1*10^{-6}$ ) inside arc extinction chamber.
 - Arc burns in metal vapor when contacts disconnect.
 - High dielectric strength recovery.
 - Advantages:
 Free from arc and fire hazards.
 Low maintenance & simpler mechanism.
 Low arcing time & high contact life.
 Silent and less vibrational operation.
 Contacts remain free from corrosion due to vacuum.
 No byproducts formed.
 - Disadvantages:
 High initial cost due to vacuum creation.
 Surface of contacts are depleted due to metal vapors.
 High cost & size required for high voltage breakers.
 - Air Blast Circuit Breakers
 - Uses high-velocity air blast to quench the arc.
 - Consists of blast valve, blast tube & contacts.
 - No carbonization of surface.
 - Advantages:
 High-speed operation compared to OCB.
 Ability to withstand frequent switching.
 Facility for high-speed reclosure.
 Less maintenance compared to OCB.
 - Disadvantages:
 Moisture prolongs arcing time.
 Pressure should be checked frequently.
 Risk of fire hazards due to overvoltages.
 Can't be used for high voltage operation due to prolonged arc quenching.
 - SF6 Circuit Breakers
 - Contains an arc interruption chamber containing SF6 gas.
 - In closed position, contacts surrounded by SF6 gas at $2.8 ewline kg/cm^2$ .
 - During opening, high-pressure SF6 gas flows toward the chamber by valve mechanism.
 - SF6 rapidly absorbs free electrons to form immobile negative ions and build up high dielectric strength.
 - Cools the arc and extinguishes it.
 - Advantages:
 Very short arcing period due to arc quenching property of SF6.
 Can interrupt much larger currents.
 No risk of fire.
 Low maintenance, light foundation.
 No overvoltage problem.
 No carbon deposits.
 - Disadvantages:
 Costly due to the high cost of SF6.
 SF6 gas has to be reconditioned after every operation, which requires additional equipment.
 - LOW VOLTAGE CIRCUIT BREAKER- MINIATURE CIRCUIT BREAKER (MCB)
 - MCB stands for Miniature Circuit Breaker.
 - It automatically switches OFF electrical circuit during any abnormal conditions in the electrical network, such as overload and short circuit conditions.
 - However, fuse may sense these conditions but it has to be replaced though MCB can be reset.
 - The MCB is an electromechanical device, which guards the electric wires & electrical load from overcurrent to avoid any kind of fire or electrical hazards.
 - Handling MCB is quite safer and it quickly restores the supply.
 - When it comes to house applications, MCB is the most preferred choice for overload and short circuit protection.
 - MCB can be reset very fast and don’t have any maintenance cost.
 - LOW VOLTAGE CIRCUIT BREAKER- MCCB (MOULDED CASE CIRCUIT BREAKER)
 - MCCB stands for Molded Case Circuit Breaker.
 - It is another type of electrical protection device which is used when load current exceeds the limit of a miniature circuit breaker.
 - The MCCB provides protection against overload, short circuit faults and is also used for switching the circuits. It can be used for higher current rating and fault level even in domestic applications.
 - The wide current ratings and high breaking capacity in MCCB find their use in industrial applications.
 - MCCB can be used for protection of capacitor bank, generator protection, and main electric feeder distribution.
 - Difference between MCB and MCCB
 - MCB:
 - Miniature Circuit Breaker
 - Rated current not more than 125 Ampere.
 - Interrupting current rating is under 10KA
 - mainly used for low Breaking capacity requirement mainly domestic.
 - trip characteristics are normally not adjustable
 - MCCB
 - Molded Case Circuit Breaker
 - Rated Current up to 1600A
 - interrupting current ranges from around 10KA -85KA
 - mainly used for both low and high Breaking capacity requirements mainly industrial.
 - trip current may be fixed as well as adjustable for overload and magnetic setting.
 - CB Size Calculation
 - Single Phase Supply
 - Circuit breaker size should be $125\%$ of the cable and wire capacity.
 - Example: For a 20A lighting circuit, the CB size should be $1.25 ewline * ewline 20A = 25A$ .
 - Three Phase Supply
 - Considers load type (light, motor, inductive, capacitive).
 - Uses the formula $P = V ewline * ewline I ewline * ewline </li></ul></li></ul></li> </ul> √3$
 * Example: For a 6.5kW, 480V three-phase load:
 * $I = 6.5kW / (480V ewline * ewline 1.732) = 7.82A$
 * CB size = $1.25 ewline * ewline 7.82A = 9.77A$ , so use a 10A breaker.
 3.8 Electrical Installation Systems
 Principal components:
 Incoming supply
 Suppliers fuse
 Meter
 Consumer unit
 Main earth bar
 Conductors (copper, silver, aluminum)
 Insulators (rubber, plastic, glass)
 Cables:
 1.5mm twin and earth = lighting
 2.5mm twin and earth = ring main
 6mm twin and earth = smaller electric shower (7kW)
 10mm twin and earth = larger electric showers (11kW)
 Flex: used for extension cables and leads for appliances
 Electrical components
 Heat resistant flex: used on heat producing appliances; kettles, irons and immersion heaters.
 An immersion heater is usually rated at $2.5-3.0kW$ so the cable used would be $1.5mm$ Butyl heat resistant flex.
 This flex can be heated up to $85°C$ without being damaged or going brittle.
 Electrical components
 Two way switch: allows the live to be switched off in two different places (eg upstairs and downstairs).
 One way switch: allows the live to be switched off in one place (eg for a single room