Test: Ferranti Effect & Voltage Control Methods - Electrical Engineering (EE) MCQ

Ferranti effect is happens when line to earth capacitance becomes more than the inductance parameter. This effect is more predominant in medium and long transmission lines but is not a problem for short transmission lines. This is because line to earth capacitance for short transmission line is negligible.

During Ferranti effect the leading charging current is more than lagging component of current. Net line current leads the sending end and reciving end voltage. Hence the iR drop leads these voltages.

For high voltage and extra high voltage lines shunt reactors are provided to absorb part of charging current. As leading charging current is reduced. Ferranti effect is also reduced.

The voltage fluctuations at consumers end may cause malfunction or event damage of consumer’s equipments. So, the voltage at the consumers end must be maintained within prescribed limit upto ± 6% of declared supply voltage so that the consumers apparatus may operate satisfactorily.

Induction generators are not used for voltage control. Tap changing transformer is used to control distribution end voltage for upto limited voltage variations. Series compensators and Synchronous phase modifiers controls line voltage by supplying required leading or lagging VAR.

In tap changing transformer there is always a typing on the high voltage winding which when connected to the rated voltage gives rated voltage on the low voltage side. This tapping is called principal typing.

In a medium or long transmission line when open circuited or lightly loaded the receiving end voltage is found more than the standing in voltage. This phenomenon of rise in voltage at receiving end of open circuited or lightly loaded line is called Ferranti effect.

 Percentage rise in voltage = (ω2 l2 10-8)/18
Solving this % V_rise = {(2 × Ω × 50) × 2002 × 10-8}/18
= 2.19%.

All electrical equipments are designed to work at a certain voltage mentioned as voltage rating of that equipment. Change of supply voltage beyond a certain limit may cause malfunction or even damage to the equipment. So, the consumer end voltage must be maintained to a prescribed limit for satisfactory operation of consumer’s equipments.

AVR stands for Automatic Voltage Regulator. It operates on the principle of detection of error and is used to control the line voltage.