| Dielectric Limits |
Exceeding dielectric limits (maximum electric field strength) results in failure of insulation, causing faults. See a photograph of dielectric breakdown in an insulator here. Electric fields may be excessive (due to overvoltage) under low loading conditions on long ac transmission lines (Ferranti Effect) or during abnormal conditions like lightning strokes. |
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Deviation of voltages beyond certain limits can also be considered to be an unacceptable compromise on the quality of power being supplied to consumers. Low or high voltages can also damage electrical equipments. |
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| Voltages and reactive power demand of transmission lines are affected by: |
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Line parameters |
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Length of line |
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Power transfer |
Given a nominal voltage rating, a steady state overvoltage of about 10% is often permissible. Shunt reactors (inductors) are often connected in shunt on transmission lines to prevent overvoltages under low loading. If permanently connected to a line, however, they may excessively reduce the voltages during heavy load conditions.
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Line Parameters: Line parameters are dependent on the conductor dimensions and relative placement. The surge impedance of most overhead lines is around 250-350 ohms whereas it is 30-50 ohms for cables.
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Typical positive sequence inductance and capacitance parameters for a 400 kV overhead line: L = 1.044mH/km, C = 12 nF/km. For EHV lines , X/R ratio is large. The resistance per unit length of this line = 0.0296 ohm / km
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For a 400 kV paper-insulated lead-covered(PILC) cable, typical positive sequence parameters are: L = 0.78 mH/km, C = 0.95 uF/km. |
Note the high capacitance as compared to an overhead line.
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What is the Surge Impedance Loading (SIL) of the overhead line and cable?
To analyse the effect of various parameters, we now have a quick look at the basic equations of a lossless transmission line. |
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