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Surge Impedance Loading (SIL) of Transmission Line


The surge impedance loading (SIL) of a line is the power load at which the net reactive power is zero. So, if your transmission line wants to “absorb” reactive power, the SIL is the amount of reactive power you would have to produce to balance it out to zero. You can calculate it by dividing the square of the line-to-line voltage by the line’s characteristic impedance.

Transmission lines can be considered as, a small inductance in series and a small capacitance to earth, – a very large number of this combinations, in series. Whatever voltage drop occurs due to inductance gets compensated by capacitance. If this compensation is exact, you have surge impedance loading and no voltage drop occurs for an infinite length or, a finite length terminated by impedance of this value (SIL load). (Loss-less line assumed!). Impedance of this line can be proved to be sqrt (L/C). If capacitive compensation is more than required, which may happen on an unloaded EHV line, then you have voltage rise at the other end, the ferranti effect. Although given in many books, it continues to remain an interesting discussion always.

The capacitive reactive power associated with a transmission line increases directly as the square of the voltage and is proportional to line capacitance and length.

Capacitance has two effects:

1 Ferranti effect
2 rise in the voltage resulting from capacitive current of the line flowing through the source impedances at the terminations of the line.

SIL is Surge Impedance Loading and is calculated as (KV x KV) / Zs their units are megawatts.

Where Zs is the surge impedance….be aware…one thing is the surge impedance and other very different is the surge impedance loading.

Loading of any transmission line depends on,
  • Thermal limitation (I2R limitation)
  • Voltage regulation
  • Stability Limitation
This is defined as the load (of unity power factor) that can be delivered by the line of negligible resistance.
Where VLL is the receiving end voltage in kV and Zo is the surge impedance in ohms, and SIL is the surge impedance loading or natural loading of the line
The above  expression gives a limit of the maximum power that can be delivered by a line and is useful in designing the transmission line. This can be used for the comparison of loads that can be carried on the transmission lines at different voltages
From the above expression power transmitted through a long transmission lines can be either increased by increasing the value of the receiving end line voltage (VLL) or by reducing the surge impedance (Zo). Voltage transmission capability is increased day by day, this is the most commonly adopted method for increasing the power limit of the heavily loaded transmission line. But there is a limit beyond which is neither economical nor practical to increase the receiving end line voltage
By applying some methods such as introducing series capacitors (capacitors in series with the transmission line) or shunt capacitors (capacitors in parallel with transmission lines) can be used to reduce the value of surge impedance (Zo).
Surge Impedance Loading (SIL) can be increased by reducing the Surge impedance of the line. From the above expression Zo can be decreased by either increasing the capacitance (C) of the line or by reducing the inductance (L) of the line. Inductance (L) of the transmission line cannot be reduced easily
By use of the series capacitors surge impedance (Zo) and the phase shift get reduced due to decrease in the line inductance (L). This improves the system stability limit. These capacitors also helps in reducing the line drops and so voltage variations. But this method causes difficulty under short circuit conditions of system as capacitors will get damage.
By use of shunt capacitors though the surge impedance (Zo) is reduced but the phase shift of the system increases this affects the poor stability in the system specially when synchronous machines are under the load. This method is not employed in long transmission lines specially when stability limits are present

Surge Impedance loading- download

Pumped Storage


Smart Grid Technology

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Phasor measurement unit (PMU)

A phasor measurement unit (PMU) or synchrophasor is a device which measures the electrical waves on an electricity grid, using a common time source for synchronization. Time synchronization allows synchronized real-time measurements of multiple remote measurement points on the grid. In power engineering, these are also commonly referred to as synchrophasors and are considered one of the most important measuring devices in the future of power systems.

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