Basic Structure of Power System
Electrical energy generated at generating stations by synchronous generator. The generating voltages are generally 11 KV and 33 KV. This voltage is then stepped up by step up transformer upto 132 KV, 220 KV, 400 KV for transmission over long distances. Again this high voltages are brought down to subtransmission level i.e. 66 KV to supply large consumer and further stepped down for primary distribution i.e. 33 KV, 11 KV. For secondary distribution level voltage is brought down to 400 V for 3-phase and 230 V for 1-phase for residential and commercial used.
Single Line Diagram of AC Power Transmission System
A single Line diagram that represents the flow of energy in a given power system is shown below:-
Single Line Diagram of AC Power Transmission System
Note :
- Generating Stations are interconnected by the lines.
- Transmission Lines, when interconnected with each other, becomes transmission networks.
- The combined transmission and distribution network is known as the "power grid".
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Electric power is commonly generated at 11KV in generating stations in India and Europe. While in some cases, generation voltage might be higher or lower. Generating machines, to be used in power stations, are available between 6KV to 25KV for some big manufacturers. This generating voltage is then stepped up to 132KV, 220KV, 400KV or 765 KV etc. Stepping up the voltage level depends upon the distance at which power is to be transmitted. Longer the distance, higher will be the voltage level. Stepping up of voltage is to reduce the I²R losses in transmitting the power (when voltage is stepped up, the current reduces by a relative amount so that the power remains constant and hence I²R loss also reduces). This stage is called as primary transmission.
The voltage is the stepped down at a receiving station to 33KV or 66KV. Secondary transmission lines emerge from this receiving station to connect substations located near load centers (cities etc.).
The voltage is stepped down again to 11KV at a substation. Large industrial consumers can be supplied at 11KV directly from these substations. Also, feeders emerge from these substations. This stage is called as primary distribution.
Feeders are either overhead lines or underground cables which carry power close to the load points (end consumers) up to a couple of kilometres. Finally, the voltage is stepped down to 415 Volts by a pole-mounted distribution transformer and delivered to the distributors. End consumers are supplied through a service mains line from distributors. The secondary distribution system consists of feeders, distributors and service mains.
Different types of Transmission Systems
1. Single phase AC System
- single phase, two wires
- single phase, two wires with mid-point earthed
- single phase, three wires
2. Two-phase AC System
- two-phase, three wires
- two-phase, four wires
3. Three-phase AC System
- three-phase, three wires
- three-phase, four wires
4. DC System
- DC two wires
- DC two wires with mid-point earthed
- DC three wires
Electric power transmission can also be carried out using underground cables. But, construction of an underground transmission line generally costs 4 to 10 times than an equivalent distance overhead line. However, it is to be noted that, the cost of constructing underground transmission lines highly depends upon the local environment. Also, the cost of conductor material required is one of the most considerable charges in a transmission system. Since conductor cost is a major part of the total cost, it has to be taken into consideration while designing. The choice of transmission system is made by keeping in mind various factors such as reliability, efficiency and economy. Usually, overhead transmission system is used.
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| Image Source - Google | Image By - Wikimedia Commons |
Effects of system voltage on transmission of power
- Power loss in the line is inversely proportional to the system voltage and power factor both.
- Percentage voltage drop in resistance decreases with the increase in the system voltage.
- Weight of the conductor material for the line will decreases with the increase in supply voltage and power factor.
- Efficiency of transmission, increases with the increase of supply voltge and power factor.
- Higher supply voltages also enhances the system stability.
- The problems encountered with high voltages are the insu;ation of the equipment, corona, radio and television interference.
- The voltage level of a system is therefore governed by the amount of power to be transmitted and the lenght of the line.
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| Overhead Transmission Line |
Voltage Level
(a) Low Voltage
→ 230 V (1-phase)
→ 400 V (3-phase)
(b) High Voltage
→ 11 KV
→ 33 KV
(c) Extra High Voltage: 66 KV, 132 KV, 220 KV.
(d) Modern Extra High Voltage: 400 KV
(e) Ultra High Voltage: 765 KV and above.
Conductor Used for Transmission Line
- Copper Conductor.
- ACSR (Aluminium conductor steel reinforced).
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| Image Source - Google | Image By - Wikimedia Commons |
- ACAR (Aluminium conductor alloy reinforced).
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| Image Source - Google | Image By - Wikimedia Commons |
- AAAR (All Aluminium alloy reinforced).
- Expanded ACSR conductor: Normally used for EHV lines.
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| Image Source - Google | Image By - Wikimedia Commons |
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