A transmission line is generally used to transfer electricity which is generated in a substation to the numerous distribution lines. This function is carried out by transmitting a wave of current and voltage from one end of the line to the other.
The line is made up of a conductor that has a uniform cross-section all throughout its path. The air around the line acts as an insulation medium between the conductors. Generators that produce power, transmission lines that transmit this power from generators to loads, and loads that are consuming power are what make up the bulk electricity network system. These electrical networks are extensive and intermingled like a web and are under the control of a single network operator.
Optimal Transmission Switching
An innovative form of electricity grid management that takes advantage of this already existing power system is known as optimal transmission switching. There is a limited amount of power which each transmission line can handle at a time. The network of cables and generators is thus operated in a manner which respects the power flow limitations on each line. The power that flows from the generators respects the laws of physics which state that the power flow along all lines is in accordance with the electrical characteristics of those lines.
This means that the power that flows through the lines is influenced by the properties of all the lines in a network. Therefore, if the network consists of a line at any point which can not conduct a certain amount of power, the whole network will be affected by that particular line. This also means that by removing a particular line, the network of these lines can be improved.
However, when optimal transmission switching is in play, transmission lines are easily switched in and out of the network to get the most economically efficient use of the power generation dispatch and maximize the effectiveness of bulk electricity networks.
For safety reasons the transmission lines are kept high above the ground and an electrical tower is used to support the conductors for those lines. The lines utilize a high voltage direct current to transmit high voltage over a long distance. Steel is used to construct the towers which provide high strength to the conductors.
Parameters of transmission lines
The transmission lines have certain parameters and limitations according to those parameters. The four main parameters include resistance, shunt conductance, capacitance and inductance. Said parameters are distributed uniformly along the transmission line, and this action is known as the distributed parameter of the transmission line.
The capacitance and conductance make up the shunt admittance and the inductance and resistance make up the series impedance. Some important parameters that the transmission lines function is mentioned in detail below.
A magnetic flux is induced by the current flow in the transmission lines. Emf in the circuit is induced due to the variance of the magnetic flux after a change in current in the transmission line. The intensity of the induced emf relies upon the change of magnetic flux. A parameter known as the inductance of the line is caused due to the resistance of the flow of current in the conductor caused by the production of emf in the transmission line.
Air acts as a dialectical medium in the transition lines. This means that the air acts as an insulator and allows the transition of the current without conducting it. This dielectric medium makes up the capacitor between the conductors and enables an increase in the capacity of the lines, or stores the electrical energy. This capacity of the conductor and the ability to store energy is known as the present charge per unit of prospective difference.
The capacitance is one of the most important parameters in long transmission; however, in short transmission, it is insignificant. In long transmission, capacitance has an effect on voltage regulation, efficiency, the stability of the system and the power factor.
The air between the conductor acts as an insulating medium; however, when the voltage applied in the conductor alternates, some current can flow through the dielectric medium because of imperfections. This current is known as leakage current, and it depends on a few atmospheric factors such as pollution and moisture in the air and surface deposits.
This leakage in the current between conductors is known as shunt conductance. The measuring unit for shunt conductance is Siemens, and it is represented by the symbol Y in current flow equations.
Performance of Transmission Lines
The calculation of the performance of transmission lines consists of many factors. These factors are then applied into a specific formula, and the percentage voltage regulation of the transmission lines is calculated. The factors include:
- The sending end voltage
- The sending end power factor
- The sending end current
- The loss of power in lines during transmission
- The efficiency of transmission
- The regulation
- The limitations of power flow during the steady-state
- The limitation of power flows in transient conditions
All these factors are calculated and applied in the below-mentioned formula to calculate the performance of the transition lines:
This formula is used when calculating the voltage regulation of the transmission lines. The voltage regulation is known as the fluctuation in the magnitude of voltage during their journey through the sending and receiving ends of the transmission lines.
The above-mentioned formula is used to calculate the efficiency of the transmission lines, which is determined as the ratio of the input and output power.
Some Important Points to note:
- The capability of the electrical circuit is measured by admittance. In other words, we can say that the efficiency of the transmission lines is measured by admittance.
- The unit that measures admittance is known as Siemens and is denoted by the symbol Y
- Impedance is the opposite of admittance. It is used to measure the difficulty that occurs in the transmission line when the AC flows. The unit that measures impedance in ohms and it is denoted by the symbol z.
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