- All electrical loads are inductive in nature and hence they consume lot of reactive power from the transmission lines or bus-bar. Hence there is voltage drop in the lines.
- Capacitors which supply reactive power are connected parallel to the transmission lines at the receiving end so as to compensate the reactive power consumed by the inductive loads.
- A long transmission line has a large capacitance. Ferranti effect is observed by closing the switch at load side. In this case output side is open circuited or light loaded to be assumed, hence resulting voltage at this remote terminals in more than sending end voltage.
- The magnitude of the voltage at the receiving end becomes higher than the voltage at the sending end. This phenomenon is called Ferranti effect.
- Capacitance and inductance are the main parameters of the lines having a length 240km or above.
- under-ground cable compensation may be needed at intervals of every 15 to 20 km.
MODEL OF TRANSMISSION LINE
Generally, transmission line is modelled by a few parameters like R (series resistance), L (series inductance), C (shunt capacitance), all are measured per unit length of line and Zo and γ (propagation constant). R, L, C and G are named as primary constant while Zo and γ are named as secondary constant.
π-section transmission line consists of series impedance Z, shunt admittance Y are lumped to get proper concept involved in it.
Z = R+ jwL,
Y = G+ jwC
Phasor diagram of Ferranti effect in long transmission line
OM = receiving end voltage Vr
OC = Current drawn by capacitance = Ic
MN = Resistance drop
NP = Inductive reactance drop
resistance is small compared to reactance; resistance can be neglected in calculating Ferranti effect.
For open circuit line,
The quantity 1/√(LC) is constant
from above expression is < 1,
Vs < Vr or Vr > Vs
Reduce Ferranti effect
- Ferranti effect on long transmission lines at low load or no load increases the receiving end voltage.This voltage can be controlled by placing the shunt reactors at the receiving end of the lines.
- Shunt reactor is an inductive current element connected between line and neutral to compensate the capacitive current from transmission lines.
- When this effect occurs in long transmission lines, shunt reactors compensate the capacitive VAr of the lines and therefore the voltage is regulated within the prescribed limits.
- Voltage rise is directly proportional to the square of the length of a line.
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