ELNG 403: Substation and Transmission Line Design

• ELNG 403 covers Substation and Transmission Line Design
• This course is worth 3 credits
• The course was written by E. K. Anto in June 2015

Course Content

• Mechanical design for overhead lines involves determining the right of way, tower/pole spotting, and wood-pole steel tower configuration
• Sag, tension, equivalent span, and vibrations due to wind loading are also factors in mechanical design
• Electrical design considers voltage selection, line power ratings, and ground wire arrangement
• Line clearances, spacing, and tower footing resistance are determined, and arching rings are used
• Insulator size, number, and spacing are chosen
• Performance characteristics, corona, and corona loss are assessed
• Types of insulators, flashover characteristics, and electric stress are studied
• Cables are analyzed for types, construction, electric stress, thermal characteristics, charging currents, and sheath currents
• Cost factors of cables are compared to overhead lines
• Cable fault localization techniques are learned
• System earthing principles include component and substation earthing
• Earthing schemes are designed, and neutral-earthing devices are sized
• The time rating of neutral earthing devices is determined
• Earth resistance is determined
• Elements of a substation, layout, and operation are examined
• Busbar and switching arrangements are studied
• Substation location, appearance, and surroundings are considered
• Voltage regulation equipment is chosen
• Ratings are determined, and application curves are used
• Area load density and load diversity are analyzed from an economic viewpoint

Substation Types

• Electrical substations transform and switch power between powerhouses and consumers
• An electrical substation is an assembly of switchgear components used to direct the flow of electrical energy in a power system
• Switchgear includes switching, protective, regulating, and metering devices, controls, interconnections, and accessories
• Substations can be classified based on their purpose:
• Step-up substations are associated with generating stations to increase voltage for primary transmission (e.g. 13.8 kV to 161 kV)
• Primary grid substations are located at load centers, stepping down primary transmission voltage to secondary voltages (e.g. 33kV or 11kV)
• Secondary substations further step down voltage to sub-transmission and primary distribution voltage at load points
• Distribution substations step down voltage to supply consumers through distribution and service lines and include metering, relaying, and automatic controlling
• Bulk supply and industrial substations serve individual consumers who take a large or medium supply group
• Mining substations require special design considerations due to safety needs
• Rural substations are simple and designed for low cost, using disconnecting switches, HRC fuses, and pole-mounted transformers (5 to 315 kVA)
• The use of aerial bunched conductors is increasing in rural areas

Substation Components

• Substation components include sections of busbars, switching devices, instrument transformers, power transformers, protective devices, voltage regulating devices, telecontrol systems, and auxiliaries
• Switching devices include circuit breakers, fuses, disconnecting switches, or isolators
• Instrument transformers are current and voltage transformers
• Protective devices are lightning arresters, earthing grids, and surge diverters
• Voltage regulating devices are shunt and series capacitors, and voltage regulators
• Study notes focus on busbar arrangements, with some coverage of switching, instrument transformers, and protective devices

Voltage Regulation

• Voltage quality affects customers and can vary over time, change suddenly, or have rapid fluctuations, harmonics, frequency variations, or imbalance of 3-phase voltages
• Fluctuations and irregularities can cause equipment malfunction or damage
• Voltage control equipment failures or system faults can cause excessively high voltage
• Excessive voltage drops in the distribution network can cause low voltages
• Voltage tolerances are regulated to avoid harmful effects on equipment
• A common voltage tolerance for LV customers in Europe is ±5%, with -10% allowed in some rural areas
• Relaxation of the lower limit by a further 5% tolerance may be allowed under abnormal conditions
• The US standard for service voltage tolerance is ±5%
• The IEC recommends a standard voltage of 230/400 V ± 6%
• Voltage standardization will be difficult economically, technically, and politically
• Voltage fluctuations are largely due to voltage drops in conductors caused by changing loads and network conditions
• Larger conductors reduce voltage drop but are costly
• Available facilities for voltage control include automatic on-load tap-changers, voltage regulators, line-drop compensation (voltage compounding), and power factor correction equipment

Telecontrol Systems

• Telecontrol systems enable real-time information gathering for monitoring and remote control of switching equipment
• Modular telecontrol systems (microprocessor-based) improve fault clearance times and supply security
• Telecommunication networks are essential for telecontrol systems
• Public and utility-owned telephone networks radios, power line carrier and fiber optics lines can send data

Substation Layout

• Substation layout entails arranging switchgear components in an ordered pattern based on function and spatial rules, connected electrically via a busbar system
• Layout principles are not affected by voltage or current variations but influenced by considerations specific to the situation, like requirements from planning authorities
• Spatial relationships, maintenance zoning, electrical separations, site limits, indoor or outdoor type, and aesthetics are all considerations
• Four clearance distances are needed:
• Earth clearance between live parts and earthed structures
• Phase clearance between live parts of different phases
• Isolating distance between isolator terminals or connections
• Section clearance between live parts and maintenance zone limits with a minimum ground clearance of 2.44 m based on British Standard BS 162
• Clearance values are determined by maximum overvoltages the system is subjected to
• System insulation is subject to continuous power-frequency voltage and transient impulse voltages from lightning and switching surges
• Lightning is the determining factor for outdoor substations up to 300 kV
• Switching surges become the determining factor above 300 kV
• Insulation coordination uses shielding, surge diverters, or arc gaps to limit overvoltages to around 80% of the basic insulation level (BIL)

Zoning and Electrical Separation

• Substation layout involves zoning equipment for maintenance
• Isolation and physical separation from live gear helps safe access
• Electrical separation of substation components (isolating distances and phase clearances) are the main substations layout basis
• A minimum of three separation factors are needed per phase in a circuit, that are between the terminals of:
• Busbar isolators or their connections
• Circuit breakers or their connections
• Feeder isolators connections
• Additional separations may be needed where conductors of different phases cross

Substation Site

• Site limitations include limited ground area, shape, line entry position, and building integration
• Construction may occur underground or on terraces
• Unusual site conditions require special arrangements, aiming for standard layouts with compactness and flexibility

Indoor or Outdoor Substations

• Putting EHV open-type switchgear indoors or outdoors depends on atmospheric pollution, maintenance concerns, site area, and aesthetics
• Substations placed indoors may cost 10-25% more than outdoors
• Environmental or weather obstacles require closer looks at these factors
• Atmospheric pollution lowers insulation security and increases corrosion, maintenance frequency, and difficulty
• Creepage length increase, live washing and insulator coatings can lower pollution effects

Safety and Site Considerations

• Individual opinion decides how much importance safety and comfort receive
• Climactic conditions are the sole decisive factor
• Indoor substations are only sometimes smaller than outdoors
• Future pollution must be taken into account where accurately predictable
• Design outdoor substations for later conversion to indoor
• Space must be paid for to line-terminal towers, power transformers, and auxiliary-plant buildings
• Personal choice inevitably plays a part in controversial aesthetic judgements
• Most people find well-designed buildings preferable to outdoor substations
• Indoor arrangements combine the need for small ground area, aesthetics and pollution protection in urban areas
• Modern tools now accommodate trees, but this step may be costly
• Planning rules affect design through site constraints and appearance

Busbar Systems

• Busbars are used to join a number of circuits together
• Continuity of supply is helped by facilities to bypass parts for maintenance
• Because circuit breakers contain elements subject to deterioration, isolators are generally used for CB maintenance
• Reactors can be added to reduce fault level and accommodate switchgear
• Reactors are attached between busbar sections and short-circuited by a CB
• Sometimes reactors are used in substations that cannot expand
• Reactor placement should note consideration since their presence directly affects the arrangement of busbar systems

Busbar System Types

• Single Busbar or Radial System: Incoming feeders bussed together with local transformers; has a low level of security for power supply
• Ring system: Better security than single busbar through alternative routes, but immediate result of busbar fault is the same
• Mesh system: CBs are in the busbar, not tee offs. Operation of two circuit breakers required
• Interconnected system: Increased security through interconnected supply circuits; can accept loss of one feeder
• Transfer busbar system: Single busbar with facilities to bypass CB on load, and the circuits protection is retained
• Duplicate/Double Busbar System: Common and allows changeability for grouping circuits
• Some may have wraps around a central busbar

Single Busbar Disadvantages

• With a single-busbar, a circuit takes transformers of service when maintenance occurs
• Any busbar extension is a substation shutdown
• A fault trips connected CBs
• Additional circuits lost during busbar maintenance can be reduced through sectionalizing by adding a busbar-section

Buses, Faults and Outages

• In mesh systems, one circuit outage per bus fault
• A CB fault in a mesh bus loses two circuits at most
• Duplicate and Mesh substations circuits can be extended without losing another

Busbar Protection

• Differential protection is used to protect against busbar faults and ensure that all circuit breakers connected open in order to clear the fault
• Two sets of busbar protections are typically provided because of the importance of not isolating the busbars unnecessarily
• Mechanical, electrical, and electromechanical interlocking systems can ensure that disconnectors cannot be opened or energized

System Earthing

• Can be divided into equipment/general earthing or the system/neutral earthing
• Improves safety for personnel, property, and equipment
• Minimizes equipment frames, uses a ring main, and connects to buried electrodes
• Includes calculating electrode resistance, electrode types/sizes, structure earthing, fences, lightning devices, and carrier-current systems covered

System Earthing Impact

• Earthing has a particular impact on an electrical power system's performance, security, and reliability
• Consists of maintaining correct earthing systems in utility power and networks
• Earthing star-point reduces voltage stresses from switching/surges and controls fault currents to a value considered satisfactory
• A low earthing impedance leads to reduced earth-fault current, less transient overvoltages during overvoltage conditions

Safety and Grounding

• Hazard voltage (voltage gradient) can occur
• Transfer potential hazard exists
• Step voltage and touch voltage values are of concern
• Limiting the hazard voltages requires reducing the fault current or metallic frame resistance

Earthing Arrangements

• Neutral systems can be configured a number of ways
• Isolation of the neutral entirely
• Solid or direct earthing
• Impedance earthing
• Short arcing or Peterson-coil earthing
• Arrangement selection is difficult
• Arrangements depend on the transformers earthing set for systems in use
• Frames are often solidly grounded
• Earth is connected to star-windings
• Isolated networks have no connections between neutrals and leads to zero-sequence impedances for the line

Line Capacitance

• Conductor lines have capacitances between one another and to the ground
• The grounding has little effect on the capacitances
• Imbalanced operation of capacitances means earth capacitances voltages across lines lead to current voltages at 90°
• Vector sums lead to zero on most currents, however, a phase earth fault sees voltage leads to zero
• After line-ground fault on isolation system with system ground voltage on unfaulted lines increase to line-to-line value out of phase
• Capacitive charging current is three times the line current
• Faulty phase supplies equal current as capacitive lagging leads to 90°

Arcing System

• Current zero happens when voltage is high which can lead to the phenomenon of having arcing grounds
• Capacitors are trapped to ground and charges become DC which may lead to tree branch failure
• Maximum value in isolated systems depends on capacitance to the lines

Ungrounded systems

• Low earth-fault is equal to capacitance
• Voltage between earth and equipment is small improving relative acceptability

Ungrounded System Shortcomings

• Arc's unsafe build up leads to transient overvoltages
• Requires code authorities to frown on them
• Reducing insulation permissible makes equipment less costly
• High voltages cause saving on transformers

Solid or Direct Earthing

• The simplest method for earthing is solid, wye-connected transformer to earth
• The voltage on earth becomes zero if a ground fault appears
• Capacity current flows from earth over to two components that sources have to fault out and connect
• Symmetrical components are the result since the zone’s impedance is small

System and Earth Benefits

• A neutral system is simple with no extra equipment
• Neutral point is held for all equipment which does nullify effect of capacity’s current causing overvoltage conditions

System Advantages

• Low hazard voltages and the lowest phase-earth voltage
• Add additional savings with grading insulations at high kV
• Fault current magnitude simplified provides earth and special sensitivity

Earth Systems

• Solid earth is dangerous due to fault magnitude
• Increased ground interferes with communications when it occurs
• 3rd currents are problematic

Impedance Earthing

• Different MV systems use it
• A limiting device introduces the neutral and Earth
• Current and ohmic affect zero sequence performance
• Voltages run by ground cause phases increase to linear output

Overhead and Underground Transmission

• Electrical power is transmitted through overhead lines or underground cables based on economical issues
• Economic transmission is determined by balancing cost and conductors

Overhead v. Underground

• Overhead lines mar beauty
• Susceptible to weather and interference
• Underground transmission is less common since construction of cable costs and voltage is a factor

Line Construction Designs

• Conductors - Carry power
• Supports - Tower for stability
• Insulators - Insulation
• Cross arms - Support for insulators

Conductor Material

• The design and choice of conductor importance
• Over dimension limits voltage drops, the expansion of loads, and financial stress
• Selection infulenced by cost, environmental conditions, and mechanics

Conductors

• Steel has poor ratings to corrosion
• Copper has high tensile strength on overhead lines are drawn in stranded state
• High current density
• Smaller cross sections reduce cost.
• Homogenous qualities are important

Conductor Considerations

• Aluminium costs are increasingly high with the expansion of conductivity
• Lightweight concerns with aluminum has made it conducive to tensile weight, etc.
• Wire in a circle allows better weight control

Conductor types

• Conductors may be flat surfaced, stranded or tubing
• Terminals consist of flat palms and must be secured by clamping
• Flat or Solid lack mechanical strength

Cable Systems

• Cables are installed in large quantities to prevent a multitude of routes
• Cables are made to handle:
• Large range ratings in large suspension cables
• Armour helps reduce the amount of stress that will make its way to the consumer

Non Metallic

• PVC is the most common insulation since there are plenty options withstanding adverse effects

Under the Ground

• Most cable networks are installed to protect the wire end bolts

Diggin

• Dig to reach 1.5m of dirt with 45cm of room
• The trench should prevent moisture and decay
• Cement is used to prevent further decay to an extent
• Cabling must have bituminous covering material

Draw Ins

• Conduits help lay cabling underground with manholes
• Cables mus have servings for jutes

Sold

• CABLES can be put below oil, water ect. for insulation purposes

Cable currents

• Cable are separated by insulation and therefore conduct currents
• Potentials conduct electrical and electric static due to these wires

Sheaths and Cores

Cables are split to two equal potential conductors or the boundary line as a boundary of metal