2. 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,
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.
3. 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 a long transmission line (length > 250 km) is represented by a
distributed parameter model. In the 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).
4. 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 put the value of z and y in the definition of (Zc), then we find
that Characteristic Impedance is a complex quantity. However, for lossless
transmission lines (R=0 and G= 0)
z = jwL and y = jwC.
5. According to the definition, Characteristic Impedance (Zc) is calculated as:
Characteristic Impedance (Zc) = square root of (jwL/jwC).
On simplifying it we got the result:
Zs= Zc = square root of (L/C).
The above quantity has a dimension of resistance known as the 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.
6. 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 the 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 the receiving end would have the same magnitude as
the sending end voltage and also have a phase angle lagging with respect to the
sending end by an amount equal to the time required to travel across the line from
the sending end to the 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.
7. 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 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, the 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.
Now come to our main topic Surge impedance loading (SIL) and its significance.
8. What is Surge impedance loading (SIL)?
In our power system, there are some limitations of loading on the transmission
line network. Generally, the loading of any transmission line depends on some
factors like:
Thermal limitation (I2R 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.
9. 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 the transmission line for which total reactive
power generated (Capacitive VAR) is equal to total reactive power consumed
(Inductive VAR). So as 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 the transmission line will be zero
and hence transmission line is assumed to be loaded as purely resistive load.
10. 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)
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 line voltages.
11. Calculation of Surge impedance loading (SIL):-
As we know 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 absorbs the reactive power. Hence voltage drop occurs in line due to
series, inductance is compensated by the shunt capacitance of the line.
If we take a balance of reactive powers due to inductance and capacitance then we
get an expression as:
12. To simplify we got the following:
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 the line.
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)
13. 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 the conductor. In other words, we can say that SIL should always be less than the
maximum loading capacity of the line.
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, the 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 to 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 voltage drop occurs in the line, due to this receiving end
voltage will be smaller than the sending end voltage. Hence a compensator is
required to maintain the voltage level.
14. 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 be no
flow of reactive power in the line.
15. Also when we observed the line voltage vs. length curve of the transmission line
( as shown in the below figure), we concluded different voltage profiles for loading
the line in different conditions.
If the loading is equal to SIL, then the voltage profile of the line is Flat.
If the loading is greater than SIL, then the line has an inductive nature.
If the loading is less than SIL, then the line has a capacitive nature.
16. 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 (VLL) or by reducing the value of surge impedance (Zs).
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. However, there is a limit beyond
which it is neither economical nor practical to increase the receiving end line voltage
of the power network.
Another option is by reducing the value of surge impedance (Zs) or characteristics
impedance of the 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 the capacitor (C) of the line or by decreasing the
inductance (L) of the line. However, the inductance of the line cannot be reduced
easily.
17. Further, the capacitance value can be increased in two ways either by using a series
capacitor or by using a shunt capacitor. Hence there are two methods to improve the
surge impedance loading of transmission lines:
Using series capacitor: By the use of a series capacitor surge impedance and
phase shift get 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 with this method is It causes difficulty under the short circuit condition as
a series capacitor will get damaged.
Using a 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.
Hence surge impedance loading of line is increased by using either series capacitor or
shunt capacitor in the transmission line. But practically series capacitor is a more
feasible and effective method for improving the SIL of a line.
20. Introduction
Transmission lines involve heavy voltages during the transmission of power from
the sending end to the receiving end.
The power transferred consists of real power and the reactive power. The
reactive power may be higher or less due to the reactance parameter of the
transmission lines and the load impedance at the receiving end.
Reactive power generated by the a.c power source is stored in a capacitor or
reactor during a quarter of a cycle and in the next quarter of the cycle it is sent
back to the power source. Therefore the reactive power oscillates between the
a.c source and the capacitor or reactor at a frequency of twice the value(50 or60).
So to avoid the circulation between the load and source, the reactive power
needs to be compensated.
Therefore, series compensation is used to modify the reactance parameter of the
lines or power system while shunt compensation is to change the equivalent load
impedance.
In both cases, the line reactive power can be effectively controlled thereby
improving the performance of overall electric power system.
21. Series Compensation
• Series Compensation is basically used to improve performance of extra high
voltage transmission lines.
• It consists of a device connected in series with the line at suitable locations.
• There are two modes of operation: - capacitive mode of operation and the
inductive mode of operation.
• A simplified model of transmission system with series compensation is shown
below, the magnitude of the two buses is assumed to equal as V and δ is the
phase angle between the two buses.
22. Series Compensation (cont.…..)
• By inserting reactive power in series with transmission line, the line
impedance is reduced which improves the power transfer capability
of the line.
• Series compensation can be achieved by using fixed or switched
capacitors or by using a thyristor-controlled reactor.
• Fixed capacitors provide constant level of compensation while
switched capacitors can be turned on or off to adjust the level of
compensation as needed.
• TCRs can provide continuous and adjustable series compensation by
using thyristors to control the flow of current through the reactor.
23. Advantages of series compensation
1. Increases power transfer capacity: by reducing the impedance
of the transmission line, series compensation can increase the
power transfer capacity of the line, allowing it to transmit more
power.
Power transfer capacity of the line is given by;
P =
𝐸.𝑉
𝑋
sinδ, where
E= sending voltage
V= receiving voltage
X= reactance of the line
δ= phase angle between E and V
24. Advantages…..
• Power transfer without and with compensation
P1 =
𝐸.𝑉
𝑋𝑙
sinδ
P2 =
𝐸.𝑉
𝑋𝑙−𝑋𝑐
sinδ
P1
P2
=
𝑋𝑙
(𝑋𝑙−𝑋𝑐)
=
1
(1−𝑋𝑙 𝑋𝑐)
=
1
1−𝑘
Where k is the degree of compensation
The economic degree of compensation lies in the range of 40-70%
25. Advantages….
2. Improvement of system stability: series compensation can
help to improve the stability of transmission system by reducing
voltage drop along the line and providing a stable source of reactive
power.
For the same amount of power transfer and same value of E and V, the δ in
the case of compensated line is less than that of uncompensated line.
P=
𝐸.𝑉
𝑋𝑙
sinδ1 , P== 𝐸.𝑉
𝑋𝑙−𝑋𝑐
sinδ2 ,
sinδ2
sinδ1
=
𝑋𝑙−𝑋𝑐
𝑋𝑙
A lower δ means better system stability.
Series compensation offers most economic solution for the system stability as
compared to other methods (reducing generator, x-mer reactance, bundled
conductors, increase no. of parallel circuits).
26. Advantages…..
3. Less installation time: the installation time of the series
capacitor is smaller( 2 years approx.) as compared to the installation
time of the parallel circuit line(5 years approx.)
• This reduces risk factor.
• Hence used to hit the current thermal limit.
• The life of transmission line and capacitor is generally 20-25years.
4. Load division between parallel circuits: When a system is to
be strengthened by the addition of a new line or when one of the
existing circuit is to be adjusted for parallel operation in order to
achieve maximum transfer of or minimise losses, series
compensation can be used.
27. 5) Improves voltage transmission: By injecting voltage in to the
transmission line, series compensation helps to improve the voltage
profile at the load end and reduces the voltage drop along the line.
This can help to improve the performance of equipment and reduce
power loss.
6) Reduced transmission losses: Series compensation can help to
reduce transmission losses by improving the transmission of power
along the distances
7) Easy to maintain: Series compensation devices, such as fixed
capacitors, are generally self-regulating and require little or no
control equipment, which makes them easy to maintain.
28. Disadvantages of Series Compensation
Increase in fault calculation.
Mal operation of distance relay- if the degree of compensation and
location is not proper.
High recovery voltage of lines- across the circuit breaker contacts and
is harmful.
Limited effectiveness: Series compensation is most effective during
heavy load conditions, when the voltage drop along the transmission
line is significant. During light load conditions, shunt compensation
may be more effective in improving the power factor of the system.
29. Outage issues: When an outage occurs on a transmission line with series
compensation, the series compensation must be removed to prevent overloading
of the other parallel lines. This can be a complex process and may require
additional protection and control measures.
Parallel line issues: If series compensation is added to an existing transmission
system, it may be necessary to have it on all lines in parallel to ensure that the
system is balanced. This can be a complex and expensive process.
High voltage issues: During system outages, the series capacitors in the
transmission line may be subjected to high voltage, which can lead to damage or
failure.
Sub-synchronous resonance: Series compensation can cause sub-synchronous
resonance (SSR) in some systems, which can lead to instability and damage to
equipment. Additional expenses may be needed to address this issue.
Problem of ferro-resonance; occurring between the iron created inductance and
the compensated line, leading to flow of high current.
Disadvantages of series compensation….
30. Location of series compensating element
The choice of the location of the series compensating element depends
on many technical and economical considerations.
In each case, a special system study concerning load flow, stability,
transient overvoltage, protection requirement, system voltage profile,
etc is necessary before the optimal location is chosen.
1. Location along the line; capacitor bank located at middle of the
line(if 1 bank) and at 1/3th distance along the line(if 2 banks).
This has advantage of better voltage profile along the line, lesser SC current
the capacitor in even of fault and simpler protection of capacitor
The capacitor stations are generally unattended.
31. Location of series compensating element….
2. Location at one or both ends of the line section on the line side in
the switching station
The main advantage of this location is that the capacitor installation is near
the manned sub stations.
However, requires more advanced line protection.
For the same degree of compensation, more MVAr capacity is needed as
compared method 1
32. Degree of series compensation
• We know that the surge impedance
• Zc=
𝐿
𝐶
=
𝑗ω𝐿
𝑗ω𝐶
= xxL
• Suppose Cse is the series capacitance per unit length for series
compensation,
• Then the total series reactance will be
• jω𝐿′ = 𝑗ωL −
𝑗
ωCse
= jωL−
𝑗
ωCse
.
𝑗ω𝐿
𝑗ω𝐿
• Jω𝐿(1 −
1
ω2LCse
)= 𝑗ω𝐿(1 −
𝑋𝑐𝑠𝑒
𝑋𝐿
)= 𝑗ω𝐿 1 − γse
• Where is known as the degree of series compensation. Therefore,
virtual surge impedance 𝑍𝑐 =
𝑗ω𝐿 1−γse
𝑗ω𝐶
= Zc 1 − γse
33. Shunt compensation
• Shunt compensation involves the use of a capacitor or reactor in
parallel with a transmission line to improve its reactive power
transmission characteristics.
• Shunt compensation is used to improve the power factor of the
transmission system by providing a source of reactive power to the
transmission line.
• For high voltage transmission line the capacitance is high and plays a
significant role in voltage conditions of the receiving end.
• When line is loaded then the reactive power demand of the load is
partially met by the reactive power generated by the line capacitance
and the remaining reactive power flow through the line from the
sending end to the receiving end.
34. Shunt compensation….
• When load is high (more than SIL) then a large reactive power flows
from the sending end to the receiving end resulting in large voltage
drop in the line.
• To improve the voltage at the receiving end, shunt capacitors may be
connected at the receiving end to generate and feed the reactive
power to the load so that reactive power flow through the line and
consequently the voltage drop in the line is reduced.
• To control the receiving end voltage, a bank of capacitors (large
number of capacitors connected in parallel) is installed at the
receiving end and a suitable number of capacitors are switched in
during the high load condition depending upon the load demand.
35. Shunt compensation……
• Thus the capacitors provide leading VAr to partially meet reactive
power demand of the load to control the voltage.
If 𝑋𝑐 = 1 ω𝐶 be the reactance of the shunt capacitor then the reactive power
generated of leading VAr supplied by the capacitor.
𝑄𝑐 =
|𝑉2
|2
𝑋𝑐
= |𝑉2|2ω𝐶, where |𝑉2|is the magnitude of receiving end voltage.
36. Shunt compensation……
• When load is small(less than SIL) then the load reactive power
demand may even be lesser than the reactive power generated by the
line capacitor. Under these conditions the reactive power flow
through the line becomes negative, i.e. the reactive power flows from
receiving end to sending end, and the receiving end is higher than the
sending end voltage (Ferranti effect).
• To control the voltage at the receiving end it is necessary to absorb or
sink reactive power. This is achieved by connecting shunt reactors at
the receiving end.
37. Shunt compensation…..
• If 𝑋𝐿 = ωL be the reactance of the shunt reactor (inductor) then the
reactive VAr absorbed by the shunt reactor.
• 𝑄𝐿 =
|𝑉2
|2
𝑋𝐿
= 𝑉2
2ωL, where|𝑉2|is the magnitude of receiving end
voltage.
• To control the receiving end voltage generally one shunt reactor is
installed and switched in during the light load condition.
• To meet the variable reactive power demands requisite number of
shunt capacitor are switched in, in addition to the shunt reactor,
which results in adjustable reactive power absorption by the
combination.
38. Advantages of shunt compensation
Improves voltage profile: this done by providing a source of reactive power to
the transmission line. This can help to reduce voltage drop and improve the
performance of equipment.
Effective at all load levels: Unlike series compensation, shunt compensation is
effective at all load levels. This makes it a useful tool for improving the power
factor of a transmission system regardless of the load on the system.
Fast control of over voltages: Shunt compensation can provide fast control of
temporary overvoltage that may occur in a transmission system. This can help to
protect equipment and improve the stability of the system.
Cost-effective: Shunt compensation is generally less expensive than series
compensation, making it a cost-effective option for improving the transmission of
power and correcting the power factor of a system.
Easy to maintain: Shunt compensation devices, such as fixed capacitors, are
generally self-regulating and require little or no control equipment, which makes
them easy to maintain.
39. Disadvantages of shunt compensation
• Higher cost: Shunt compensation is generally more expensive than series
compensation, particularly when large amounts of reactive power are required.
• Limited overload capability: The overload capability of shunt compensation is
limited, as the capacitors can only provide a certain amount of reactive power
before they become overloaded.
• Complex control: Shunt compensation requires complex control systems to
ensure that the correct amount of reactive power is provided to the transmission
line. This can increase the cost and maintenance requirements of the system.
• Limited power transfer capability: Shunt compensation does not directly
improve the power transfer capability of a transmission line. To increase the
power transfer capacity of the line, other measures, such as series compensation
or upgrading the line, may be needed.
• Risk of overvoltage: If the shunt compensation is not properly coordinated with
the rest of the transmission system, it can lead to overvoltage and instability in
the system.
40. Differences between series and shunt
compensation
Series compensation
• Lower cost
• They used to control loading of
parallel lines
• They are self regulating and increases
the system stability
• Series reactor used to limit SC current.
• Series capacitor used to increase
transmission capacity and improve
stability
Shunt compensation
• Higher cost compared to series
compensation
• They are used to regulate the grid
voltage
• It has greater voltage control and
stability
• Shunt reactor used to avoid
Ferranti effect
• Shunt capacitor used to improve
power factor
41. Conclusion
Following the above discussion, we can conclude that the series
compensation is used in long transmission lines to improve the
voltage profile at the receiving end, while shunt compensation is used
to provide reactive power compensation and improve power factor of
the transmission system.
The capacity of the shunt compensation device depends on the
reactive power requirement of the transmission system and the size of
the lagging load.
