In a medium or long transmission line when open-circuited or lightly loaded the receiving end voltage is found more than the sending end voltage this phenomenon of rise in voltage at the receiving end of the open circuit or lightly load line is called Ferranti effect
The effect is firstly observed on the Depford man laid down by S.Z de Ferranti in 1890 .This effect is due to voltage drop across the line inductance ,due to charging current ,being in phase with applied voltage at the end of the line. Thus both capacitance and inductance are necessary to cause this phenomenon. The capacitance and therefore the charging current is negligible in short transmission line but significant in medium and long transmission line. On a 300 kilometers line operating on 40Hz supply, the receiving-end voltage on open circuit is usually found to be roughly 5% higher than the sending end voltage .
For long high voltage and extra High Voltage lines shunt reactors are provided to absorb part of charging current or shunt capacitive VA, of the line on no- load and light load ,in order to prevent over voltage on the line.
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