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 a very important parameter for the determining of 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)?

Surge impedance is nothing but the characteristic impedance (Zc) of the lossless transmission line. It is also known as the Natural impedance of the line.

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 line (As shown in below figure).

Let us assume that the line has shunt admittance (y)

*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).**
Where,

**z = R + jwL**and**y = G + jwC.**
If we putt 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.**

Hence
according to definition Characteristic Impedance (Zc) is calculated as:

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

On
simplifying it we got a result as:

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

The above quantity has a dimension of resistance is known as 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 ohms.**

## Significance of Surge impedance

The significance of surge impedance is that if a pure resistance load that is equal to the surge impedance is connected to the end of line with no resistance, a voltage surge introduced by the shunt capacitor to the sending end of the line would be completely absorbed by the series inductance at the receiving end of the transmission line. In this case, the voltage at receiving end would have the same magnitude as the sending end voltage and also have 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 which is used mostly in electrical science in connection with the

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

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

If a
lossless transmission line terminates in its surge impedance (i.e. if the load is a pure resistance of value equal to the characteristic impedance of the line), then that transmission line is known as the

**infinite line or****flat line**. So, in that case, many interesting phenomena happen in such a line:- There will not any reflection of forward traveling wave 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 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 wire with zero resistance.

Now
come to our main topic Surge impedance loading (SIL) and its significance.

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

In our
power system there are some limitations of loading on the transmission line
network. Generally, loading of any transmission line depends on some factors
like:

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

So in context to these limitations 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.

**Also Read: 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 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). Its unit is MW.

Mathematically SIL is expressed as:

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

Hence the formula for SIL will be:

The
above expression gives the maximum power limit that can be delivered by any
transmission line which is very useful in designing the transmission line. SIL
can be used for the comparison of loads that can be transmitted through the overhead
transmission lines at different voltages.

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

As we
know that long transmission lines (length > 250 km) are represented by the distributed parameter model. In this model, the capacitance and inductance are
distributed uniformly along the line. When the line is charged then the shunt
capacitance generates reactive power and feeds to the line while the series
inductance absorbed the reactive power. Hence voltage drop occurs in line due
to series inductance is compensated by the shunt capacitance of line.

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

On
simplifying we got as:

Here
the quantity having a dimension of resistance is surge impedance denoted by the
symbol Zs. It is considered as a purely resistive load which when connected at
the receiving end of the transmission line, then the reactive power generated by
shunt capacitance will be completely absorbed by the series inductance of line.

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

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

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

From the above expression of SIL we observed that SIL
depends on the line voltage at the receiving end. Normally a line is loaded above SIL
for better utilization of conductor. In other words, we can say that SIL should always less than the maximum loading capacity of the line.

**Must Read: Electrical Bonding**

When the line is loaded less than its SIL, then it acts
like a shunt capacitor which means it will supply MVar to the system. In this
case, receiving end voltage will be greater than sending end voltage. In such a case line has to be compensated with an inductor to bring down the voltage at a normal level.

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.

The below figure contains a graphic of the effect of SIL. For a particular line of
SIL value 450 MW. So if the line is loaded to 450 MW, then MVAR produced by the line will exactly balance the MVAR absorbed by the line. Hence
there will no flow of reactive power in the line.

##
**How to
improve surge impedance loading?**

From
the above expression of SIL, we observe that the transmitted

**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}). Voltage transmission capability is increased 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 power network.
Other option
is by reducing the value of surge impedance. Since surge impedance is directly
proportional to inductance and inversely proportional to the capacitance. So the
value of surge impedance can be reduced either by increasing capacitor (C) of
line or by decreasing inductance (L) of line. But the inductance of the line cannot reduce
easily.

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.

Also by the use of shunt capacitor surge
impedance is reduced but the phase shift of 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.

I hope it will clear your concept. If you have any doubt then please ask in comment.

Thanks....

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