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1 Elements of Transmission and Distribution Systems BEX 44603/BEK 4213 – Electrical Power Transmission and Distribution
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Elements of Transmission and
Distribution Systems
BEX 44603/BEK 4213 – Electrical Power Transmission and Distribution
BEE 3243 – Electric Power Systems – Module 4
Module Outline
1.0 Introduction
1.1 Transmission System
1.2 Performance of Transmission Line
1.3 Design of Transmission Line
1.4 Different systems of Power
Transmission
1.6 Comparison on Volume of Copper Used
for Different Power Transmission Systems
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BEE 3243 – Electric Power Systems – Module 4
Module Outline
1.7 Distribution System
1.8 Design considerations in Distribution
System
1.9 Underground Cables vs. Overhead lines
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BEE 3243 – Electric Power Systems – Module 4
Introduction - Transmission System
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Figure 1: Concept of electric energy transmission
BEE 3243 – Electric Power Systems – Module 4
Introduction – Transmission System
• The transmission system is made up of the high voltages lines and bulk power subs that connect the generating stations with the distribution subs.
• Electrical power transmission lines can be ac, dc, underground or overhead lines.
• Overhead ac is the most used method of electrical power transmission.
• The transmission system can be divided into the transmission and subs transmission system.
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BEE 3243 – Electric Power Systems – Module 4
High voltage transmission Lines
• Interconnect power plants and loads, and form an electric network.
• This system contains 500kV, 275kV and 132kV.• The maximum length of high voltage lines is
around 200 miles.• The major components of an HV are:
– Tower : a lattice, steel tower– Insulator : V string hold four bundled conductors in
each phase.– Conductor: Each conductor is stranded, steel
reinforced aluminum cable.
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BEE 3243 – Electric Power Systems – Module 4
High voltage transmission Lines
Figure 2: Typical of HV transmission line
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BEE 3243 – Electric Power Systems – Module 4
High voltage transmission Lines
Figure 3: Foundation of transmission line
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BEE 3243 – Electric Power Systems – Module 4
Sub-transmission Lines
• Subs transmission system is defined as the lines with voltages/ capabilities between those used for transmission and those used for distribution.
• It used to carry power from power substations to the distribution substation and also large individual customers.
• The common voltage levels are 33kV and 66kV.
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BEE 3243 – Electric Power Systems – Module 4
Transmission system
Transmission System in Malaysia
• Transmission line in Malaysia mostly for HVAC transmission, with a short portion HVDC to Thailand.
• The transmission voltage at Malaysia are:
– HVAC 132 kV
– HVAC 275 kV
– HVAC 500 kV
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BEE 3243 – Electric Power Systems – Module 4
Efficiency of the line
Performance of Transmission Line• The transmission line performance is governed by
its four parameters:– Series resistance– Series inductance– Shunt capacitance– Shunt conductance
• The leakage current over the surface of the insulator is modeled as shunt conductance.
• Usually shunt conductance is ignored because the leakage current are small.
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BEE 3243 – Electric Power Systems – Module 4
Efficiency of the Line
• Performance of transmission lines is meant the determination of efficiency and regulation of lines.
100% X V
VVVR %
R(FL)
R(FL)R(NL) 100
P
P
)S(3
)R(3
Transmission efficiency Voltage Regulation
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• The design of a transmission lines involves a number of technical and economic aspects.
• The design details include:– Line voltage
– Size of conductors,
– Span , spacing and configuration of conductors
– Number and size of earth wires
– Number of insulators
– Clearances and sag
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• Line voltage– The cost and performance of the line depend on
the line voltage.
– An empirical formula for the optimum voltage is:
• Where V is line voltage in kV
L is the distance in km
P is the power in kW
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• Selection of conductor size– The cost of conductor is about 30 to 45 % of the
total cost of the line.
– The cost of tower, foundations and line losses also depend on the conductor size.
– The size of the conductor should be able to carry the rated current continuously without excessive rise in temperature.
– The temperature affects the sag and the loss of the tensile strength of the conductor.
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
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Conventional ACSR and modern ACCC conductors
BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• Choice of Span, number of circuit, conductor configuration
– For HV line the economical value of span lies in the range of 200 to 300 m. For 400kV lines span ranging from 350 to 400 m.
– A transmission line may be a single circuit line or a double circuit line. A double circuit line has a higher power transfer capability and greater reliability.
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
Single circuit line Double circuit line18
BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• Insulation design
– The insulation design has a greater impact on the performance of the line.
– Insulation should be sufficient to take care of switching, temporary overvoltage and lightning.
– Insulators are usually made of wet-process porcelain or toughened glass, with increasing use of glass-reinforced polymer insulators.
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• Insulation design
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• Selection of Earth/Ground wire– The primary function of ground wires is to shield the
conductors from the lightning strokes. (placed above the conductors and grounded at every tower)
– The selection of the number and configuration is important in transmission line protection against lightning.
– A shielding angle of 30º is considered to be adequate for high voltages lines.
– A ground wires should be able to carry max expected lightning current without heating and have sufficient mechanical strength.
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BEE 3243 – Electric Power Systems – Module 4
Design of Transmission Lines
• Selection of Earth/Ground wire
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BEE 3243 – Electric Power Systems – Module 4
Different Systems of Power Transmission
• D.C. systemi) D.C. two-wire ii) D.C. two wire with mid-point earthediii) D.C. three-wire
• Single-phase A.C. systemi) Single-phase two-wireii) Single-phase two-wire with mid-point
earthediii) Single-phase three-wire
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BEE 3243 – Electric Power Systems – Module 4
Different Systems of Power Transmission
• Two-phase A.C. system
i) Two-phase four-wire
ii) Two-phase three-wire
• Three-phase A.C. system
i) Three-phase three-wire
ii) Three-phase four-wire
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper• Overhead System1) 2-wire D.C. system with one conductor earthed
● The load is connected between two wires.Max. voltage between conductors = Vm
Figure 1
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper• Overhead System
2) 3-phase 3-wire A.C. system
Figure 2
● System that almost adopted for electric power transmission
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of Copper
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of CopperVolume of copper required
System Overhead Underground
D.C. 2-wire with 1 conductor earthed
K K
D.C. 2-wire with mid-point earthed
K
D.C. 3-wire 1.25K
A.C. single phase 2-wire with 1 conductor
A.C. single phase 2-wire with mid-point earthed
A.C. single phase 3-wire
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BEE 3243 – Electric Power Systems – Module 4
Comparison on Volume of CopperVolume of copper required
System Overhead Underground
A.C. 2 phase 4-wire
A.C. 2 phase 3-wire
A.C. 3 phase 3-wire
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BEE 3243 – Electric Power Systems – Module 4
Example 1
• A 50 km long transmission line supplies a load of 5 MVA at 0.8 p.f. Lagging at 33 kV. The efficiency of transmission is 90%. Calculate the volume of aluminium conductor required for the line when:
(i) Single phase, 2-wire system is used
(ii) 3-phase, 3-wire system is used.
The specific resistance of aluminium is
2.85 x 10-8 Ωm.
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BEE 3243 – Electric Power Systems – Module 4
Classification of Distribution System
• A distribution system may be classified according to:
1) Nature of current
(a) D.C. distribution system
(b) A.C. distribution system
2) Type of construction
(a) overhead system
(b) underground system
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BEE 3243 – Electric Power Systems – Module 4
Classification of Distribution System
3) Scheme of connection(a) radial system
Single line diagram for:
(i) D.C. distribution system(ii) A.C. distribution system
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BEE 3243 – Electric Power Systems – Module 4
Classification of Distribution System
(b) ring main system
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BEE 3243 – Electric Power Systems – Module 4
Classification of Distribution System
(c) inter-connected system
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BEE 3243 – Electric Power Systems – Module 4
Design Considerations in Distribution System
• Most important factor for delivering good service to consumers is good voltage regulation. For this purpose, 2 parts require careful consideration:
i) Feeders – designed from the point of view its current carrying capacity.
ii) Distributors – designed from the point of view of the voltage drop in it. The size and length of the distributor should be such that voltage at the consumer’s terminals is within permissible limits.
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BEE 3243 – Electric Power Systems – Module 4
Underground vs Overhead System
• The types of systems are used for the transmission and distribution:– Overhead system
– Underground system
• Overhead system– The transmission and distribution of electrical power
using overhead lines over long distance.
– The appropriate spacing between the conductors is provided which prevent an electric discharge to occur between the conductors.
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BEE 3243 – Electric Power Systems – Module 4
Underground vs Overhead System
• Overhead system
– Overhead lines are subjected to the faults occurring due to lightning, short circuits.
– It is easy to repair compare to underground system.
– However difficult to find exact point of fault as transmission lines are very long.
– The insulation must be provided between the conductors and also supporting structure and withstand both the normal operating voltage and surges due to switching and lightning.
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BEE 3243 – Electric Power Systems – Module 4
Underground vs Overhead System
• Underground system– Underground transmission lines are mostly used to
supply urban substation in high load density areas.
– All the conductors must be insulated from each other.
– The voltage level used in underground system is below 66kV due to difficulties in designing the appropriate insulation for higher voltage levels.
– Underground lines are more expensive due to underground cables must be insulated, installed in pipe, cooled with oil circulation system and difficulty in access to repair.
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BEE 3243 – Electric Power Systems – Module 4
Underground cables vs. Overhead lines
• Underground cables have the following advantages:– It ensures uninterrupted continuity of supply. The
possible supply interruptions due to lightning, storm or weather are eliminated due to underground cable.
Steel pipe
Paper/oil
insulator
Conductor
(Stranded
copper)
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BEE 3243 – Electric Power Systems – Module 4
Underground cables vs. Overhead lines
• Underground cables have the following advantages:
– It requires less maintenance.
– The accidents caused due to breakage of overhead lines conductors are eliminated.
– The life of underground cables is longer.
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BEE 3243 – Electric Power Systems – Module 4
Underground cables vs. Overhead lines
• Underground cables have the following advantages:
– The voltage drop in the underground cable is less.
– The visual impact can be minimized.
– More appropriate to use in populated areas due to safety issues in overhead lines.
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BEE 3243 – Electric Power Systems – Module 4
Underground cables vs. Overhead lines
• Disadvantages of the underground cables:
– The only drawbacks of underground cables are the extremely high initial cost and insulation problems at high voltages.
– The use of underground cables is mainly for distribution of an electrical power at low and medium voltages.
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BEE 3243 – Electric Power Systems – Module 4
Underground cables vs. Overhead lines
• Advantages of the overhead lines:
– Long distance transmission is possible by the overhead lines.
– The conductor in overhead lines is less expensive.
– The size of the conductor in overhead lines is smaller than underground cables due to good heat dissipation.
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BEE 3243 – Electric Power Systems – Module 4
Underground cables vs. Overhead lines
• Advantages of the overhead lines:
– The cost of insulation is less due to using the air as insulator between the conductors. The gas or oil is not required.
– The cost to erect the tower is less than laying the underground cable which is difficult and complicated.
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