09-03-2011, 02:22 PM
Submitted by
Ashish Kumar
Keerti Kr. Mahawar
Nishant Bansal
Nitesh Garg
Palak Kumar Jain
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SIMULATION OF EXTRA HIGH VOLTAGE LONG TRANSMISSION LINES
ABSTRACT
The electrical power system mainly consists of three principle divisions the generating stations, y he transmission system and the distribution system. The transmission lines are the connecting links between generating station and the distribution system and lead to other power system interconnections.
Now a day, we are using Extra High Voltage (EHV) transmission lines for transmission of power between the generating station and distribution system in India.
The main reasons behind it are:
1. The construction of super power stations of very large capacities necessities the transmission at high voltage for this we use EHV lines.
2. At high voltages power loss is also reduced because losses are directly proportional to the square of current.
The simulation of transmission line using MATLAB helps us to analyze the behaviors and parameters of transmission line under actual conditions. We are simulating a long transmission line and analyze the waveforms at sending and receiving end.
The results obtained after simulation are used in the designing of Extra High Voltage Long Transmission Line Model.
INTRODUCTION
Electrical energy is generated in large hydro electric, thermal and nuclear super and super critical power stations these stations are generally situated far away from the load centers. This necessitates an extensive power supply network between the generating station and consumer load. This network may be divided into two parts transmission and distribution the main part of this transmission system. Transmission line transmits bulk electrical power from sending end to receiving end stations without supplying any consumer en route and it can be divided into two parts primary and secondary. The transmission voltage is in India re 66kV, 110kV, 132kV, 220kV, 400kV and 765kV.
The more the voltages of transmission line the better the performance and efficiency of the system. For this we use high voltage and extra high voltage transmission lines to transmit electrical power from the sending end substations to the receiving end substations. At the receiving end substations the voltage is stepped down to a lower value of 66kV, 33kv or 11kV. The secondary transmission system forms the page link between the main receiving end substations and secondary substations. In the transmission line the voltage can vary as much as 10% or even 15% DUE TO variation in loads
The transmission line is the main energy corridor in a power system. The performance of a power system is mainly dependent on the performance of the transmission lines in the system. It is necessary to calculate the voltage current and power at any point on the transmission line provided the values at one point are known. We are aware that in 3 phase circuit problem it is sufficient to compute results in one phase and subsequently predict results in the other 2 phases by exploiting the three phase symmetry. Although the lines are not spaces equilaterally and not transposed the resulting asymmetry is slight and the phases are considered to be balanced as such transmission line calculations are also carried out on per phase basis.
The transmission line performance is governed by its four parameters
1. Series resistance
2. Series inductance
3. Shunt capacitance
4. Shunt conductance
All these parameters are distributed over the length of the line. The insulation of a line us seldom perfect and leakage currents flow over the surface of insulators especially during bad weather this leakage is simulated by shunt conductance. The shunt conductance is in parallel with the system capacitance. Generally the leakage currents are small and the shunt conductance is ignored in calculations.
The transmission line may be classified as short, medium and long. When the length of the line is less than about 80km the effect of shunt capacitance can be ignored and the line is designated as a short line.
When the length is between 80 and 250km the shunt capacitance can be considered as lumped and the line is termed as medium length line.
Lines more than 250km long require calculation in terms of distributed parameters are knows as ling lines.
TWO PORT NETWORKS
A pair of terminals at which a signal (voltage or current) may enter or leave is called a port. A network having only one such pair of terminals is called a one port network.
A two-port network (or four-terminal network, or quadripole) is an electrical circuit or device with two pairs of terminals. Examples include transistors, filters and matching networks. The analysis of two-port networks was pioneered in the 1920s by Franz Breisig, a German mathematician.
A two-port network basically consists in isolating either a complete circuit or part of it and finding its characteristic parameters. Once this is done, the isolated part of the circuit becomes a "black box" with a set of distinctive properties, enabling us to abstract away its specific physical buildup, thus simplifying analysis. Any circuit can be transformed into a two-port network provided that it does not contain an independent source.
A two-port network is represented by four external variables: voltage and current at the input port, and voltage and current at the output port, so that the two-port network can be treated as a black box modeled by the relationships between the four variables Vs, Is, Vr and Ir. There exist six different ways to describe the relationships between these variables, depending on which two of the four variables are given, while the other two can always be derived.
Note: All voltages and currents below are complex variables and represented by phasors containing both magnitude and phase angle.
The parameters used in order to describe a two-port network are the following:
Z, Y, A , h and g. They are usually expressed in matrix notation and they establish relations between the following parameters:
(1)Input voltage V1
(2) Output voltage V2
(3) Input current I1
(4) Output current I2
