Classification of Overhead Transmission Lines

1.1.1 Tower

Transmission tower are the most visible component of the power transmission system. Their function is to keep the high-voltage conductors (power lines) separated from their surroundings and from each other.

The overhead transmission line consists of a group of conductors running parallel to each other and carried on supports which provide insulation between the different conductors and between each conductor and earth. A variety of tower designs exist that generally employ an open lattice work or a monopole, but generally they are very tall metal structures. For example, a 500 kV tower with cross-arms is higher than a 230 kV tower. Figure 1.2 shows various types of transmission towers.

Fig. 1.2: Types of Transmission Towers

3

1.1.2 Conductor

Conductors are the power lines that carry the electricity to and through the grid to consumers. Generally, several conductors are strung on a tower for each electrical circuit. In general, the types of conductors are solid, stranded and hollow conductors. Conductors are constructed primarily of twisted metalstrands, but newer conductors may incorporate ceramic fibers in a matrix of aluminum for added strength with lighter weight. An overhead transmission line usually consists of three conductors or bundles of conductors containing the three phases of the power system. The bare conductors are suspended from a tower via insulators, usually made of Aluminium Conductor Steel Reinforced (ACSR), which are steel core (for strength) and aluminium wires (having low resistance) wrapped around the core. Other types of conductors are All Aluminium Conductor (AAC) and All Aluminium Alloy Conductor (AAAC).

Fig. 1.3: Cross-sectional view of ACSR conductor (7 steel strands and 24 aluminium strands)

4

1.1.3 Insulators

Generally, insulators for overhead lines provide insulation to the power conductor from ground. The insulators are connected to the cross arm of the tower and the power conductor passes through the clamp of the insulator.

Fig. 1.4: Insulator on Transmission Tower

1.2 Transmission of Electrical Power

Fundamentally there are two systems by which electrical energy can be transmitted;

1.2.1 High Voltage DC (HVDC) Transmission System 1.2.2 High voltage AC (HVAC) Transmission System

Generally electrical power mainly transmitted by Three Phase High Voltage Alternating Current (HVAC) system because of certain reasons. Since power is the product of voltage and current, for a given load, the higher the supply voltage, the lower the load current. Reducing the current ensures (a) minimum voltage drop along the line, (b) minimum line losses, and (c) conductors of practical crosssectional areas. This is necessary to minimize the voltage drop along the transmission line, and to reduce energy losses which are proportional to current squared.

5

1.3 Advantages of using HVAC

1.3.1 Reduce the line losses (P) Electrical power is directly proportional to the product of electrical current and voltage of system. For a given amount of power, transmitting certain electrical power from one place to another, if the voltage of the power is increased then associated current of this power is reduced. The series resistance causes a real power loss (P= I2R) in the conductor. Reduced current means less energy losses, I2R loss in the system. Decreased current causes improvement in voltage regulation of power transmission system and improved voltage regulation indicates quality power

1.3.2 Smaller conductors The advantage of transmitting electrical power at higher voltages is that smaller conductors can be used, since the current flow would be less for the same amount of power (Watts). Less cross sectional area of the conductor means less capital involvement.

1.3.3 The alternating voltages can be easily stepped up and down using transformers, which is not possible in DC transmission system.

1.3.4 Maintenance of AC substation is quite easy and economical compared to DC system.

1.3.5 The transforming in AC electrical substation is much easier than motor-generator sets in DC system.

6