TRANSMISSION LINE LOADABILITY INTRODUCTION
Load ability of a transmission line is defined as the optimum power transfer capability of a transmission line under a specified set of operating criteria. The load ability of short transmission lines is limited by the thermal rating of the conductors, medium line voltages regulation and long lines by stability consideration which is lower than the thermal rating. Compensation can be used to increase load ability of long lines toward their thermal limit. OBJECTIVES
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To underst understand and the use of series compensat compensation ion for increasing increasing transmis transmission sion line loadabil loadability ity.. To compare compare effect effect of compensa compensation tion at receivi receiving ng end, sending sending end end and at both ends. ends. THEORY:- There are some physical properties associated to the transmission system that limit power transfer in spite of the capability of the generator or the requirement of the load. Transmission systems are designed to operate according to specific voltage levels. Depending on the characteristic of the transferred power, the voltage at the transmission line ends, for instance, can be either below below or above certain certain limits, modifying modifying the system system capacity to transfer power. power. ctions ctions are frequently frequently ta!en to recover the assigned voltage levels, allowing the system to attend to the power demand at adequate operating condition. The physical parameters of transmission lines, which depend upon the line length and voltage level, strongly restrain power transfer. transfer. s s stated before, before, the load ability ability of short transmission lines is limited limited by the thermal thermal rating of the conductors. This is the magnitude of the current, continuing over time and increasingly heating the conductor that limits the loading. s the conductor heats up, the resistance of the conductor increases with temperature, it stretches, and the line sags "can be irreversible#. $eries and shunt compensations have been traditionally used to modify the natural parameters of transmission lines. Compensation generally describes the intentional insertion of reactive power devices "inductive or capacitive# into a power networ! to achieve a desired desired effect. $eries capacitors are sometimes used in long lines to increase the load ability. Capacitor ban!s are installed in series with each phase conductor at selected points along the line. They reduce the net series impedance of the line in series with the capacitor ban! thereby reducing line voltage drops and increasing the steady state stability limit. disadvantage disadvantage of series capacitor ban!s is that automatic protective devices have to be installed to bypass high currents during faults and to re%instate the capacitor ban!s after the fault has passed. They can also e&cite low frequency oscillations "sub%synchronous resonance# which may damage turbine%generator shafts. 'owever, there are techniques to counteract this effect such as use of static filters. The characteristic of power transfer "(%) characteristic# characteristic# relates the voltage at the receiving%end bus bar to the active power reaching it, it, for a given given sending%end voltage, voltage, power factor factor and impedance of transference. *t is affected affected by changes either in the sending%end voltage magnitude or in the impedance of transference between sending and receiving ends, or even in the transfer power factor. The graph below depicts a (%) characteristic where curves 1, 2 and + depict reactance 1, 2 and + respectively where 1- 2- +.The line )$ shows the point with ma&imum power transfer.
DISCUSSION
/or the )Lvs. (L curve, it is observed that an increase in the line loadability leads to a decrease in the receiving end voltage until the point where ma&imum amount of power transfer is attained.0eyond this point the s ystem is unstable thus a blac!out occurs. This corresponding real power at this point is highest for the system compensated at both ends "22 #, followed by those compensated at either the receiving of sending end "13 # and lastly is the system that has no compensation "14 #. The effect of transmission line reactance on the curve is to increase the ma&imum real power that can be transferred at a given receiving end voltage as is clearly depicted by the curves of the graph.The line that is compensated on both ends allows for the ma&imum transfer of power as compared to the rest. /or the 5$vs. (L curve, it is observed that as you increase the line loading the reactive power increases in an e&ponential mannerfrom negative values to positive ones.5 $ is negative for ( L6(7"where (7 is the surge impedance loading#and the system absorbs reactive power. This is when we have light loading. hen (L-(7, 5$ is positive and reactive power is supplied to the line. This is !nown as heavy loading. The effect of series compensation on the 5 $vs. (Lcurve is to increase the ma&imum power that can be transmitted for a specific value of 5$. *t is also noted that when there is compensation at both ends, the highest value of (Lis attained. CONCLUSION
$eries compensation by using capacitors reduces the net series impedance of the line in series with the capacitor ban! thus increasing the line loadability "power transfer can be increased#. /rom the graph of5$ vs. (L, we observe that when series compensation is introduced, we have an increase in the ma&imum power that can be transmitted for a specific value of 5$. The lines compensated at either the receiving end or the sending end have almost similar power transfer "(%) characteristic#. 'owever, the line compensated at both ends leads to the most reduction in series impedance, and as a result, to the largest increase in line loadability.
