Line modeling-and-performance
- 1. Line Modeling and Performance Line Presented by, Nafis Shahriar Munir Student ID: 170021026 Tamanna Tajin Student ID: 170021033
- 2. Why do we need line modeling ? Transmits power from one end to another over a distance with high efficiency and low voltage regulation.
- 3. Parameters of transmission line Performance of transmission lines
- 6. • Z: per-phase impedance of the short line • r: per-phase resistance per unit length • L: per-phase inductance per unit length • l: length of the line • R: per-phase resistance of the line • X: per-phase inductive reactance of the line
- 7. Two-port network model of short transmission line The ABCD parameters are given as in Short line transmission line can be translated into a two port network-
- 8. Voltage Regulation Voltage Regulation: Voltage regulation of the line may be defined as the percentage change in voltage at the receiving-end of the line (expressed as percent of full- load voltage) in going from no-load to full-load. Voltage regulation is a measure of voltage drop of the line and depends on the power factor of the load
- 10. Nominal π-Model • Half of the shunt capacitance may be considered to be lumped at each end of the line. • g represents the leakage current over the insulators and corona effects • g is generally ignored in normal operating conditions Y: total shunt admittance of the line g: shunt conductance per-unit length C: Line-to-neutral capacitance per unit length l: length of the line Z: Total series impedance of the lineNominal π-model for medium length line
- 12. Long line with distributed parameters z: series impedance per phase per unit length y: shunt admittance per phase per unit length z = r + jwL y = g + jwC Δx = small segment Using KVL we get :
- 13. Voltage and Current Waves for a Long Transmission Line Placing gamma’s value and transferring voltage Equation in time domain we get: First term second term At any point along the line, the voltage is the sum of these two components • As x increases (moving away from receiving-end), the first term becomes larger because of e^ax. • The first term is called ‘incident wave’. • As x increases (moving away from receiving-end), the second term becomes smaller because of e^- αx • The second term is called ‘reflected wave’.
- 14. Surge impedance • In summary, when line losses are neglected (g = r = 0); • Attenuation constant α becomes zero • Phase constant β becomes w*sqrt(LC) • Characteristic impedance ZC becomes sqrt(L/C) which is a real number and purely resistive • Characteristic impedance ZC is also known as “surge impedance”
- 15. Since, • In a lossless line under surge impedance loading, the voltage and current at any point is constant in magnitude • Since ZC is real number (no reactive part), there is no reactive power in the line (QS = QR = 0) • For SIL condition, the reactive power losses due to line inductance are exactly offset by reactive power supplied by the shunt capacitance • SIL is a useful measure of the transmission line loading capacity • For heavy loads (LOAD > SIL), shunt capacitors can be required to minimize voltage drop along the line • Generally transmission line full load is much higher • than SIL, so shunt capacitors should be used Surge Impedance Loading
- 16. Real and Reactive Transmission Line Losses
- 17. Power Transmission Capability The power carrying capacity of an AC transmission line is limited by three factors: • Thermal loading limit • Voltage stability limit • (Angle) stability limit
- 18. Thermal Loading Limit The excess amount of current flowing on the line produces heat leading to undesirable results such as • Increase sag due to stretching the conductors • Decreased clearance to ground due to conductor expansion at higher temperatures • Stretching the conductors may be irreversible • Gradual loss of mechanical strength of the conductor caused by temperature extremes
- 20. Transmission Line Compensation Two types of compensation: 1.Series Compensation 2.Shunt Compensation
- 21. Shunt Reactors • Shunt reactors are conventional solutions to compensate for the undesirable voltage effects associated with line capacitance. • Shunt reactors are used to control voltage during low- load period. • Shunt reactors are usually unswitched
- 22. Shunt Capacitive Compensation • Compensating reactive power of lagging power factor load • Supplying reactive power to maintain the receiving-end voltage at satisfactory level (around ~ 1.0 pu) • Capacitors are connected either directly to the bus to the tertiary winding of the main transformer
- 23. Series Capacitive Compensation Series capacitors are used for- • Reducing the series reactance between the load and the supply point. • Reducing the series reactance between the load and the supply point. • Yielding more economical loading.
- 24. Thankyou