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.