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Under no load conditions the current in a transmission line is because of
Capacitance effect
Corona effect
Proximity effect
Back flow from earth
Capacitance effect
Under no-load conditions, a transmission line behaves like a large capacitor due to the potential difference between conductors separated by air (dielectric). This distributed shunt capacitance draws a leading charging current from the source, even when no active power is being delivered to a load.
Under no-load conditions, a transmission line behaves like a large capacitor due to the potential difference between conductors separated by air (dielectric). This distributed shunt capacitance draws a leading charging current from the source, even when no active power is being delivered to a load.
IchтАЛ=VphтАЛтЛЕ2╧АfC тАФ Charging current in a transmission line
C=ln(D/r)2╧А╧╡0тАЛтАЛ тАФ Capacitance per unit length of a two-wire line
The conductors of a transmission line form the plates of a capacitor, and the air between them acts as the dielectric medium. Since the line is energized by an AC voltage, the charging current flows through these shunt capacitors. This current is given by IchтАЛ=VphтАЛтЛЕ╧ЙC, leading to the Ferranti effect, where the receiving end voltage can become higher than the sending end voltage at no-load.
Charging current is capacitive and leads the voltage by 90 degrees.
This effect is more pronounced in long transmission lines at high voltages.
The Ferranti effect is a direct consequence of this no-load capacitive charging current.
Shunt reactors are often used to compensate for this effect at light loads.
Helps in maintaining voltage profile in long lines
Provides reactive power support
Causes rise in receiving end voltage (Ferranti effect)
Can lead to insulation stress during overvoltage conditions
EHV (Extra High Voltage) transmission line planning
Reactive power compensation studies
Corona effect occurs only when the electric field intensity exceeds the dielectric strength of air (approx 30 kV/cm).
Proximity effect is due to the non-uniform distribution of current in conductors due to magnetic fields of nearby conductors, significant only under load.
Back flow from earth is not a standard term for no-load current mechanisms.
A is correct тАФ The no-load current is primarily the charging current required to charge the shunt capacitance of the transmission line.
Always remember that charging current is proportional to the line length and voltage; hence, it becomes critical in EHV/UHV lines (400kV and above).