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Which method of voltage control is applied for long line AC transmissions?
Switching by shunt capacitors
Tap changing transformers
Switching by shunt reactors
Static Var sources
Switching by shunt reactors
Long AC transmission lines exhibit the Ferranti effect, where the receiving end voltage exceeds the sending end voltage under light load or no-load conditions due to line capacitance. Shunt reactors are connected in parallel to compensate for this charging current, thereby regulating and controlling the voltage profile along the line.
Long AC transmission lines exhibit the Ferranti effect, where the receiving end voltage exceeds the sending end voltage under light load or no-load conditions due to line capacitance. Shunt reactors are connected in parallel to compensate for this charging current, thereby regulating and controlling the voltage profile along the line.
VrтАЛ=VsтАЛcos(╬▓l)тИТIsтАЛZcтАЛsin(╬▓l) тАФ General voltage relation for a long transmission line
QcompтАЛ=V2╧ЙClineтАЛ тАФ Reactive power compensation required to neutralize line charging
Transmission lines possess distributed shunt capacitance. Under light load, the charging current IcтАЛ=V╧ЙC creates a voltage rise at the receiving end. Shunt reactors provide inductive reactive power (VAR absorption) to neutralize the leading capacitive VARs, bringing the voltage back to the nominal level.
Long lines are typically defined as those greater than 250 km.
The Ferranti effect is primarily caused by line shunt capacitance C.
Shunt reactors are switched in during low-load conditions to maintain voltage stability.
Inductive compensation is required to prevent overvoltage at the receiving end.
Prevents insulation failure due to overvoltage.
Improves voltage regulation profile.
Reduces stress on terminal equipment.
Increases the cost of transmission substations.
Introduces harmonics if not properly filtered.
Not required during peak load when the line is naturally loaded.
Extra High Voltage (EHV) and Ultra High Voltage (UHV) AC lines.
Underground cables (which have very high capacitance).
Long distance overhead power lines with low loading.
Tap changing transformers are used for voltage control, but shunt reactors are specifically required for long-line charging current compensation.
Shunt capacitors are used for lagging power factor correction (to raise voltage), which would exacerbate the Ferranti effect on long lines.
C is correct тАФ Switching by shunt reactors is the standard method for absorbing excess capacitive reactive power in long lines to prevent the Ferranti effect.
Always remember: Shunt capacitors for light loads (raising voltage) and Shunt reactors for no/light loads on long lines (lowering voltage to prevent Ferranti effect).