SIL is defined as the maximum load (at unity power factor) that can delivered by the transmission line when the loads terminates with the value equal to surge impedance (Zs) of line. Simply if any line terminates with surge impedance then the corresponding loading in MW is known as Surge Impedance loading (SIL).

**Surge Impedance Loading (SIL)**is the most important parameter for determining the maximum loading capacity (MW loading) of transmission lines. Before understanding SIL in detail, at first we have to understand the concept of Surge and

**Surge impedance (Zs)**and its physical significance. So let’s discuss the topic in detail.

## What is Surge Impedance (Zs)?

As we all know that a long transmission line (length > 250 km) is represented by a distributed parameter model. In Distributed parameter model of the long transmission line, resistance (R), inductance (L), capacitance (C), and conductance (G) are uniformly distributed over the whole length of the line (As shown in the below figure).

*per unit length*

*series impedance (z) per unit length. Then the*

**Characteristic impedance (Zc) of any lossless transmission line is defined as the square root of**

**(z/y).**

**z = R + jwL**and

**y = G + jwC.**

If we put the value of **z** and **y** in the definition of (**Zc**), then we found that Characteristic Impedance is a complex quantity. However, for lossless transmission line (R=0 and G= 0)

**z = jwL**and

**y = jwC.**

**Characteristic Impedance (Zc) = square root of (jwL/jwC).**

**Zs= Zc = square root of (L/C).**

**Surge impedance**of the line. When a purely resistive load of value equal to surge impedance is connected at the receiving end of the line, then the reactive power generated by the shunt capacitor will be completely absorbed by the series inductor of the transmission line.

The value of Surge impedance for overhead transmission line is around 400 ohm, whereas surge impedance value for underground cable is around 40 ohm.

## Significance of Surge impedance

In this case, the voltage at receiving end would have the same magnitude as the sending end voltage and also have a phase angle lagging with respect to sending end by an amount equal to the time required to travel across the line from sending end to receiving end.

**Surge impedance (Zs)**is a technical term that is used mostly in electrical science in connection with the

**Surges on transmission lines**which may appear due to switching or lightning operation in our Electrical power system.

##
**What happens if the line terminates in surge impedance?**

**infinite line or**

**flat line**.

So, in that case, many interesting phenomena happen in such a line:

- There will not be any reflection of forwarding traveling waves and hence there will be no standing wave in the line. Therefore, the voltage will be the same throughout the line. Hence in this case, receiving end and sending end voltage will be the same.
- The line will compensate itself. That is, the reactive power demanded by the series inductance of the line will be supplied by the shunt capacitance. That's why there will be no voltage drop (due to series inductance) and also no voltage boost (due to shunt capacitance).
- The load, as seen by the generator, is a pure resistance that will be equal to characteristic impedance. Hence the line is observed as equivalent to a pair of wires with zero resistance.

**Surge impedance loading (SIL) and its significance**.

##
**What is Surge impedance loading (SIL)?**

- Thermal
limitation (I
^{2}R Limitation) - Voltage regulation
- Stability limitation

**Surge impedance loading (SIL)**is an important parameter in electrical science to predict the maximum loading capacity of any transmission line. It is the maximum MW loading of the transmission line at which reactive power balance occurs.

**[ ##eye## Fundamentals of LVDT]**

**SIL is defined as the maximum load (at unity power factor) that can be delivered by the transmission line when the loads terminate with a value equal to surge impedance (Zs) of the line.**Simply if any line terminates with surge impedance then the corresponding loading in MW is known as Surge Impedance Loading (SIL).

In other words we can define surge impedance loading (SIL) as: **SIL is the maximum load connected in transmission line for which total reactive power generated (Capacitive VAR) is equal to total reactive power consumed (Inductive VAR)**. So that to maintain an exact balance of reactive power consumption (by series inductance of line) and generation (by shunt capacitance of line). That's why the net flow of reactive power in transmission line will be zero and hence transmission line is assumed to be loaded as purely resistive load.

**SI unit of surge impedance loading (SIL) is Mega-Watt (MW).**

Mathematically SIL is expressed as:

**SIL (in
MW) = (Square of line voltage in kV)/(Surge impedance in ohm)**

##
**Calculation of Surge impedance loading
(SIL)**

If we take a balance of reactive powers due to inductance and capacitance then we got an expression as:

Now the exact value of SIL can be calculated by putting the **surge impedance** (Zs) value in
the above mathematical formula of SIL is expressed as:

SIL (in MW) = (Square of line voltage in kV)/(Surge impedance in ohm)

##
**Effect of
Surge impedance loading (SIL)**

**Have a Look:**

However when the line is loaded above its SIL, then it acts like a shunt reactor that will absorb MVAR from the system. In such a case a voltage drop occurs in the line, due to this receiving end voltage will be smaller than sending end voltage. Hence a compensator is required to maintain voltage level.

**If the loading is equal to SIL, then voltage profile of the line is Flat.****If the loading is greater than SIL, then the line has inductive nature.****If the loading is less than SIL, then the line has capacitive nature**.

**How to
improve surge impedance loading?**

**Electrical power**through a transmission line can be either increased by increasing the value of the receiving end line voltage (V

_{LL}) or by reducing the value of surge impedance (Z

_{s}).

Since Voltage transmission capability is increasing day by day. So the most commonly adopted method for increasing the power limit of the heavily loaded transmission line is by increasing the voltage level. But there is a limit beyond which it is neither economical nor practical to increase the receiving end line voltage of the power network.

**surge impedance**

**(Z**

_{s}**)**or charestristics impedance of transmission line, we can easily improve its surge impedance loading (SIL).

Since surge impedance is directly
proportional to inductance and inversely proportional to the capacitance. Hence the
value of surge impedance can be reduced * either* by increasing capacitor (C) of
line

**by decreasing inductance (L) of line. But the inductance of the line cannot reduce easily.**

*or***[ ##eye## Construction and Working of RVDT]**

**Using series capacitor:**By the use of series capacitor surge impedance and also phase shift gets reduced due to a decrease in inductance value (L). It also improves system stability. This capacitor also helps in reducing the line voltage drop. But the main problem in this method is It causes difficulty under the short circuit condition as a series capacitor will get damaged.

**Using shunt capacitor:**Also by the use of a shunt capacitor surge impedance is reduced but the phase shift of the system increases. This affects the poor stability of the system especially when the synchronous machines are present in the load. So this method is not feasible where the stability limit is the main concern in the power system.

**Read More Article**

**Theory and working of Star-Delta Starter****Power Factor Correction techniques****Construction and Working of RVDT****Notch filter - Theory, circuit design and Application****Different Parts of transformer and their functions****What is power factor and why it is important**

Very good ...

ReplyDeleteðŸ˜„ðŸ˜„

ReplyDeleteBest Explaination so far

ReplyDeletea thyrister lA has a characterise R1 0.72 =7200 what is ratio of voltage appearing at the end of the line having a surge impedance of 500 ohm due to a 500 kv surge when 1) the line is open circuited 2) the line is terminated by arrester 3) the value of R omh

ReplyDeleteNicely explained...

ReplyDeleteWhat is surge impedance value for transmission line

ReplyDeleteWhat is surge impedance value for cable

ReplyDeleteSurge impedance value for cable is around 40-50 ohm

ReplyDeleteThis comment has been removed by a blog administrator.

ReplyDeleteWhat can I use to draw the SIL curves? Is it possible to generate the SIL curve from a simulink model in matlab

ReplyDeleteCable impedance is low then the surge voltege should pass easly through cable .how cable reduce the stifness of surge voltage

ReplyDeleteThank you!

ReplyDeleteThanks

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