What is Ferranti Effect 

  A long transmission line draws a substantial quantity of charging current. If such a

line is open circuited or very lightly loaded at the receiving end, Receiving end

voltage being greater than sending end voltage in a transmission line is known as

Ferranti effect. All electrical loads are inductive in nature and hence they consume

lot of reactive power from the transmission lines. 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.

 

As the inductive load increases more of the capacitors are connected parallel via

electronic switching. Thus reactive power consumed by inductive loads is supplied by

the capacitors thereby reducing the consumption of reactive power from transmission

line. However when the inductive loads are switched off the capacitors may still be in

ON condition. The reactive power supplied by the capacitors adds on to the

transmission lines due to the absence of inductance. As a result voltage at the

receiving end or consumer end increases and is more than the voltage at the supply

end. This is known as Ferranti effect.

Why does voltage rise on a long, unloaded transmission

line? 

 

The Ferranti Effect occurs when current drawn by the distributed capacitance of the

transmission line itself is greater than the current associated with the load at the

receiving end of the line. Therefore, the Ferranti effect tends to be a bigger problem

on lightly loaded lines, and especially on underground cable circuits where the shunt

capacitance is greater than with a corresponding overhead line. This effect is due to

the voltage drop across the line inductance (due to charging current) being in phase

with the sending end voltages. As this voltage drop affects the sending end voltage,

the receiving end voltage becomes greater. The Ferranti Effect will be more

 pronounced the longer the line and the higher the voltage applied.

 

The Ferranti Effect is not a problem with lines that are loaded because line capacitive

effect is constant independent of load, while inductance will vary with load. As

inductive load is added, the VAR generated by the line capacitance is consumed by

the load.

How to Reduce Ferranti Effect: 

Shunt Reactors and Series Capacitors: 

  The need for large shunt reactors appeared when long power transmission lines for

system voltage 220 kV & higher were built. The characteristic parameters of a line are

the series inductance (due to the magnetic field around the conductors) & the shunt

capacitance (due to the electrostatic field to earth).

 

 

  Both the inductance & the capacitance are distributed along the length of the line. So

are the series resistance and the admittance to earth. When the line is loaded, there is a

voltage drop along the line due to the series inductance and the series resistance.

When the line is energized but not loaded or only loaded with a small current, there is

a voltage rise along the line (the Ferranti-effect)

  In this situation, the capacitance to earth draws a current through the line, which may

 be capacitive. When a capacitive current flows through the line inductance there will

 be a voltage rise along the line.

  To stabilize the line voltage the line inductance can be compensated by means of

series capacitors and the line capacitance to earth by shunt reactors. Series capacitors

are placed at different places along the line while shunt reactors are often installed in

the stations at the ends of line. In this way, the voltage difference between the ends of

the line is reduced both in amplitude and in phase angle.

 

Shunt reactors may also be connected to the power system at junctures where several

lines meet or to tertiary windings of transformers.

 

Transmission cables have much higher capacitance to earth than overhead lines. Long

submarine cables for system voltages of 100 KV and more need shunt reactors. The

same goes for large urban networks to prevent excessive voltage rise when a high load

suddenly falls out due to a failure.

  Shunt reactors contain the same components as power transformers, like windings,

core, tank, bushings and insulating oil and are suitable for manufacturing in

transformer factories. The main difference is the reactor core limbs, which have non-

magnetic gaps inserted between packets of core steel.

  3-phase reactors can also be made. These may have 3- or -5-limbed cores. In a 3-

limbed core there is strong magnetic coupling between the three phases, while in a 5-

limbed core the phases are magnetically independent due to the enclosing magnetic

frame formed by the two yokes and the two unwound side-limbs.

  The neutral of shunt reactor may be directly earthed, earthed through an Earthing-

reactor or unearthed.

  When the reactor neutral is directly earthed, the winding are normally designed with

graded insulation in the earthed end. The main terminal is at the middle of the limb

height, & the winding consists of two parallel-connected halves, one below & one

above the main terminal. The insulation distance to the yokes can then be made

relatively small. Sometimes a small extra winding for local electricity supply is

inserted between the main winding & yoke.

  When energized the gaps are exposed to large pulsation compressive forced with a

frequency of twice the frequency of the system voltage. The peak value of these

forces may easily amount to 106 N/m2 (100 ton /m2). For this reason the design of

the core must be very solid, & the modulus of elasticity of the non-magnetic (& non-

metallic) material used in gaps must be high (small compression) in order to avoid

 

large vibration amplitudes with high sound level consequently. The material in the

gaps must also be stable to avoid escalating vibration amplitudes in the end.

 

Testing of reactors requires capacitive power in the test field equal to the nominal

 power of the reactor while a transformer can be tested with a reactive power equal to

10 –  20% of the transformer power rating by feeding the transformer with nominal

current in short – circuit condition.

 

The loss in the various parts of the reactor (12R, iron loss & additional loss) cannot be

separated by measurement. It is thus preferable, in order to avoid corrections to

reference temperature, to perform the loss measurement when the average

temperature of the winding is practically equal to the reference temperature.

How does a phase shifting transformer help operators load and unload transmission

lines? 

  Power flow between two buses can be expressed as:

  Power Flow = (Vs*Vr / X) * Sine of the Power Angle.

  In other words: power flow (in watts) between two buses will be equal to the voltage

on the sending bus multiplied by the voltage on the receiving bus divided by the line

reactance, multiplied by the sine of the power angle between the two buses.

  This leaves grid operators with at least two options for making a path more conducive

to power flow, or if desired, making a path look less conducive to power flow. The

two options are to (1) adjust line reactance and (2) adjust power angle. The Phase

Shifting Transformer (PST) affects the second option, i.e. adjusting power angle.

  The physical appearance of the PST device is noteworthy, being one of the few

transformer types where the physical height and construction of the primary bushings

is the same as the secondary bushings. This makes sense since both bushing sets are at

the same potential. Internally, the primary voltage of a PST is bussed directly to the

secondary bushings, with one important addition. The primary voltage is applied to a

delta-wound transformer primary that has adjustable taps that inject “opposing phase”

signals. For instance the A-B primary winding has a C phase injection, the B-C

winding is injected with A, and the C-A winding is injected with B. These injection

 points are simultaneously adjustable taps that result in an adjustable shift of power

angle.

 

Since power angle is a direct contributor to the Power Flow formula provided above

(in the numerator, not the denominator), changing the PST tap settings can increase

 power angle making the path more conducive to power flow. The PST tap settings can

also decrease power angle making the path less conducive to power flow. (Remember

that “power flows downhill on angle”.) 

  Why is this important? Many transmission paths naturally have less impedance by

virtue of their construction and length, and these paths can carry scheduled flow as

well as unscheduled flow from parallel (but higher impedance) paths. In some cases

these low impedance paths become congested and PST devices and other devices and

techniques may be used to relieve the congestion. This is particularly the case in

regions where transmission paths are less densely developed.