TRANSMISSION LINE LOADABILITY

TRANSMISSION LINE LOADABILITY INTRODUCTION Loadability of a transmission line is defined as the optimum power transfer capability of a transmission line under a specified set of operating criteria. The loadability 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 loadability of long lines toward their thermal limit. OBJECTIVES To understand the use of series compensation for increasing transmission line loadability. To compare effect of compensation at receiving end, sending end and at both 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 therequirement of the load.Transmission systems are designed to operate according to specific voltagelevels. Depending on the characteristic of the transferred power, the voltage at thetransmission line ends, for instance, can be either below or above certain limits,modifying the system capacity to transfer power. Actions are frequently taken to recover the assigned voltagelevels, allowing the system to attend to the power demand at adequate operating condition. The physical parameters of transmission lines, which depend upon the linelength and voltage level, strongly restrain power transfer. As stated before,the loadability of short transmission lines is limited by the thermal rating of the conductors. This is the magnitude of the current, continuingover time and increasingly heating the conductor that limits the loading.As the conductor heats up, the resistance of theconductor increases with temperature, it stretches, and the line sags (can be irreversible). Series and shuntcompensations have been traditionally used to modify the natural parameters oftransmission lines. Compensation generally describes the intentional insertion of reactive power devices (inductive or capacitive) into a power network to achieve a desired effect. Series capacitors are sometimes used in long lines to increase the loadability. Capacitor banks 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 bank thereby reducing line voltage drops and increasing the steady state stability limit. A disadvantage of series capacitor banks is that automatic protective devices have to be installed to bypass high currents during faults and to re-instate the capacitor banks after the fault has passed. They can also excite low frequency oscillations (sub-synchronous resonance) which may damage turbine-generator shafts. However, there are techniques to counteract this effect such as use of static filters. The characteristic of power transfer (P-V characteristic) relates the voltage at the receiving-end bus bar to the active power reaching it, for a given sending-end voltage, power factor and impedance of transference. It is 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 P-V characteristic where curves 1, 2 and 3 depict reactance X1, X2 and X3 respectively where X1> X2> X3.The line VSM shows the point with maximum power transfer.  CASE STUDY The power system shown below was studied. The generator connected at bus 1 represents a large power system with an equivalent rating of 3750 MVA and the nominal voltage of both the generator and the load buses is 765 kV. PROCEDURE The PowerWorld Simulator was started and case example 5_10 with series capacitive compensation on both ends of the line in service was opened. The load was set such that PL = 0 MW and QL = 0 MVAr. After running the simulation, the corresponding values of sending end reactive power, QS, load bus voltage, VL and load angle, δ (measured with VL as the reference) were tabulated. The power factor was maintained as a constant (0.8 lagging) and PL was increased in steps of 200 MW until the system became unstable while corresponding values of QS, VL and δ were tabulated. Steps 2 to 4 were repeated with series compensation at the receiving-end in service and that of the sending-end out of service. Steps 2 to 4 were repeated with series compensation at the receiving-end out of service and that of the sending-end in service. Steps 2 to 4 were repeated with series compensation at both ends out of service.  RESULTS The load power factor is kept constant at 0.8 lagging. PL (MW) QL (MVAr) Compensation at both ends VL (kV) δ (deg) QS (MVAr) 0 0 804.3 0 -842.9 200 150 790.2 -1.3 -685.6 400 300 775.1 -2.7 -512.2 600 450 758.8 -4.1 -321.4 800 600 741.2 -5.6 -109.7 1000 750 721.9 -7.2 126.5 1200 900 700.5 -9.0 392.6 1400 1050 676.3 -10.9 696.9 1600 1200 648.4 -13.0 1053.1 1800 1350 614.5 -15.5 1487.8 2000 1500 569.5 -18.6 2066.7 2200 1650 477.4 -24.8 3234.6 BLACKOUT PL (MW) QL (MVAr) Compensation at receiving end VL (kV) δ (deg) QS (MVAr) 0 0 821.6 0 -861.0 200 150 804.6 -1.6 -702.4 400 300 786.1 3.2 -524.8 600 450 765.9 -4.9 -325.1 800 600 743.6 -6.7 -98.8 1000 750 718.5 -8.7 160.4 1200 900 689.6 -10.9 463.2 1400 1050 655.3 -13.5 828.5 1600 1200 611.4 -16.6 1298.5 1800 1350 543.0 -21.2 2028.8 BLACKOUT  PL (MW) QL (MVAr) Compensation at sending end VL (kV) δ (deg) QS (MVAr) 0 0 804.3 0 -842.9 200 150 787.3 -1.6 -684.0 400 300 768.7 -3.3 -505.8 600 450 748.4 -5.0 -304.7 800 600 725.9 -6.9 -76.3 1000 750 700.5 -9.0 186.6 1200 900 671.1 -11.3 495.6 1400 1050 635.7 -13.9 872.3 1600 1200 589.5 -17.2 1219.3 1800 1350 509.5 -22.7 2041.4 BLACKOUT PL (MW) QL (MVAr) No compensation VL (kV) δ (deg) QS (MVAr) 0 0 821.6 0 -861.0 200 150 801.7 -1.8 -700.9 400 300 779.7 -3.7 -518.2 600 450 755.3 -5.8 -308.2 800 600 727.5 -8.0 -64.0 1000 750 695.3 -10.5 226.1 1200 900 656.1 -13.4 584.2 1400 1050 603.9 -17.1 1064.7 1600 1200 502.0 -23.9 1994.3 BLACKOUT  CALCULATIONS Determination of the maximum amount of power that can be transferred to the load 0.8 p.u. is the minimum value allowed for the load voltage. This means that the corresponding power on the VL vs. PL plot will be the maximum amount of power that can be transferred to the load since after this point the system would be unstable. Transforming this to actual values, we have: Taking the Vbase as 765 kV; Thus reading from the graph, the values can be approximated as: Compensation at both ends: Pmax≈ 1820 kW Compensation at receiving end: Pmax≈1610 kW Compensation at sending end: Pmax≈1520 kW No compensation: Pmax≈1390 kW DISCUSSION For the VLvs. PL curve, it is observed that an increase in the line loadability leads to a decrease in the receiving end voltage until the point where maximum amount of power transfer is attained.Beyond this point the system is unstable thus a blackout occurs. This corresponding real power at this point is highest for the system compensated at both ends (2200 MW), followed by those compensated at either the receiving of sending end (1800 MW) and lastly is the system that has no compensation (1600 MW). The effect of transmission line reactance on the curve is to increase the maximum 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 maximum transfer of power as compared to the rest. For the QSvs. PL curve, it is observed that as you increase the line loading the reactive power increases in an exponential mannerfrom negative values to positive ones.QS is negative for PL<PO(where PO is the surge impedance loading)and the system absorbs reactive power. This is when we have light loading. When PL>PO, QS is positive and reactive power is supplied to the line. This is known as heavy loading. The effect of series compensation on the QSvs. PLcurve is to increase the maximum power that can be transmitted for a specific value of QS. It is also noted that when there is compensation at both ends, the highest value of PLis attained. CONCLUSION Series compensation by using capacitors reduces the net series impedance of the line in series with the capacitor bank thus increasing the line loadability (power transfer can be increased). From the graph ofQS vs. PL, we observe that when series compensation is introduced, we have an increase in the maximum power that can be transmitted for a specific value of QS. The lines compensated at either the receiving end or the sending end have almost similar power transfer (P-V characteristic). However, 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. REFERENCES Power Systems Analysis and Design, J. Glover, P. Sarma, T. Overbye, pgs. 261 – 270. Electrical Power Systems, Alexandra von Meier, pgs. 182 – 183. Lecture Notes on Power Systems I, K. K. Kaberere.  COLLEGE OF ENGINEERING AND TECHNOLOGY SCHOOL OF ELECTRICAL, ELECTRONICS AND INFORMATION ENGINEERING DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING COURSE: ELECTRICAL AND ELECTRONICS ENGINEERING EEE 2413: POWER SYSTEMS I EXPERIMENT: TRANSMISSION LINE LOADABILITY GROUP MEMBERS EN271-0294/2008 PETER W. GICHUKI EN271-2181/2010 LAWRENCE T. KAGWAINI EN271-1245/2008 JEAN CLAIRE OTSYULA LECTURER: DR. K. K. KABERERE