第5-6次课 transmission.ppt

Lossless lines Equivalent π circuit Z` Y`/2 Y`/2 + _ Ir UR + _ Is Us Z’ and Y’ are both pure imaginary, Z’ is pure inductive and Y’ is Pure capacitive. Equivalent πcircuit is also lossless. Lossless lines Wavelength for a Lossless lines: Wavelength: For overhead lines the wavelength is 6000km. Surge impedance loading Surge impedance loading (SIL)(自然功率) is the power delivered by a lossless line to a load resistance equal to the surge impedance Zc 。 无损线路上任意点电压幅度是常量 。 Surge impedance loading The real power flow alone a lossless line at SIL remains constant from the sending end to the receiving end, the reactive power flow is zero. At rated line voltage, the real power delivered is： The power flow: 潮流 Voltage profile（电压特征曲线） In practice, power lines are not terminated by the surge impedance, instead, loadings can vary from a small fraction of SIL during light load conditions up to heavy load conditions. There are four loading conditions: 1, no-load, 2, SIL, 3, short circuit, 4, full load x=0 Receiving end U(x) US No-load SIL Full load Short circuit x=l Sendind end URSC=0 VRFL URSIL=Vs Voltage profile（电压特征曲线） 1, no-load, IRNL=0 2, SIL（自然功率）, 3, short circuit, URSC=0 4, full load 满载电压取决于满载电流，位于短路电压特征曲线上方。 Steady-state stability limit The equivalent πcircuit can be used to obtain an equation for the real power delivered by a lossless line. Steady-state stability limit Pmax represents the theoretical steady-state stability limit of a lossless line. If an attempt were made to exceed this steady-state stability limit , then synchronous machines at the sending end would lose synchronism with those at the receiving end. Steady-state stability limit It is convenient to express the steady-state stability limit in terms of Surge impedance loading (SIL)(自然功率): Steady-state stability limit Uspu=URpu=1,λ=5000km, and line lengths up to 1100km. The theoretical steady-state stability limit decreases from 4(SIL) for a 200-km line to about 2(SIL) for a 400-km lin