CHAPTER -2
EXPERMANT (2)
Studying the effect of voltage changes on
transmission lines under different loads, the
effect of the Ferranti phenomenon, and
methods of compensation for voltage
prepared by :
We'am Al-Hemadi
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Abstract
This experiment uses equipment that emulates common power system
components, such as transmission lines and transformers. At a small scale, you will
explore voltage regulation and efficiency of a transmission line with deferent types of load.
These are the experiences that can be performed:
• Line losses and efficiency
• Reactive power and power factor improving
• Benefits of transporting energy at high voltage
• Increase of voltage owing to a capacitive load
To observe the Ferranti effect in a model of transmission line
To determine shunt compensation to counteract the voltage rise on no load
and zero regulation at different loads in a transmission line.
Introduction
A transmission line which delivers electric power dissipates heat owing to the resistance of
its conductors. It acts, therefore, as a resistance which in some cases is many miles long.
The transmission line also behaves like an inductance, because each conductor is surrounded
by a magnetic _eld which also stretches the full length of the line. Finally transmission line
behaves like a capacitor, the conductor acting as its more or less widely-separated plates.
The resistance, inductance and capacitance of a transmission line are uniformly dis-
tributed over its length, the magnetic _eld around the conductors existing side by side with
the electric _eld created by the potential di_erence between them. We can picture trans-
mission line as being made of thousands of elementary resistors, inductors and capacitors as
shown in Figure 1.
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*-the voltage regulation of a transmission line,
Resistive and inductive loads produce unacceptably large voltage drop. Therefore, some
mechanism for maintaining voltage is required. A typical approach is to add capacitors at
the receiving end to provide the reactive power needed by the load and the line.
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Ferranti Effect:
Long transmission line/cables draws a substantial quantity of charging current. If such a line/cable is
open circuited or very lightly loaded at the receiving end, the voltage at receiving end may become
greater than voltage at sending end due to capacitive reactance. This is known as Ferranti Effect.
Both capacitance and inductance is responsible to produce this effect. The capacitance (which is
responsible for charging current) is negligible in short line but significant in medium line and
appreciable in long line. Hence, this phenomenon occurs in medium and long lines. The figure shown
below is representing a transmission line by an equivalent pi (π)-model. The voltage rise is
proportional to the square of the line length.
In general practice we know, that for all electrical systems current flows from the region of higher
potential to the region of lower potential, to compensate for the electrical potential difference that
exists in the system. In all practical cases the sending end voltage is higher than the receiving end, so
current flows from the source or the supply end to the load. But Sir S.Z.Ferranti, in the year 1890,
came up with an astonishing theory about medium and long distance transmission line suggesting
that in case of light loading or no load operation of transmission system, the receiving end voltage
often increases beyond the sending end voltage, leading to a phenomena known as Ferranti Effect in
Power System.
A long transmission line can be considered to compose a considerably high amount of capacitance
and inductor distributed across the entire length of the line. Ferranti Effect occurs when current
drawn by the distributed capacitance of the line itself is greater than the current associated with the
load at the receiving end of the line (during light or no load). This capacitor charging current leads to
voltage drop across the line inductor of the transmission system which is in phase with the sending
end voltages. This voltage drop keeps on increasing additively as we move towards the load end of
the line and subsequently the receiving end voltage tends to get larger than applied voltage leading to
the phenomena called Ferranti Effect.
EFFECT OF SHUNT COMPENSATION
In shunt compensation, power system is connected in shunt (parallel) with the FACTS. It works as a
controllable current source. Shunt compensation is of two types:
Shunt capacitive compensation
This method is used to improve the power factor. Whenever an inductive load is connected to the
transmission line, power factor lags because of lagging load current. To compensate, a shunt capacitor is
connected which draws current leading the source voltage. The net result is improvement in power factor.
Shunt inductive compensation
This method is used either when charging the transmission line, or, when there is very low load at the
receiving end. Due to very low, or no load – very low current flows through the transmission line. Shunt
capacitance in the transmission line causes voltage amplification (Ferranti effect). The receiving end
voltage may become double the sending end voltage (generally in case of very long transmission lines).
To compensate, shunt inductors are connected across the transmission line.
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Procedures:
Make the same connections as described in section 1 but now add the internal
load. Refer to Schematic 2) Use the load switched on 120 kV.
In this case, the voltage drop is excessive, the efficiency of transmission
is very low. The line losses are the difference between the total power
at starting point and the one at arrival point (the table indicates the
total power with parameter W ∑ )
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Section 1 R=120KV: – Load line ( 120 V )
POINT V L1-2 A L1 W L1 Var L1 WΣ Var Σ PF Σ V L1
B
G
Section 1 -A– Load line ( 120 V ) WITH PHASING
R=120KV:
POINT V L1-2 A L1 W L1 Var L1 WΣ Var Σ PF Σ V L1
B
G
Proceed like section 2 but connect also the 3 capacitors on the the LOAD section (
triangle connection among them and then in parallel to the load)
You can observe the decrease of voltage drop ( now the arrival
voltage is 87 V). In this experience it is not possible to estimate the effect of phasing,
since when the output voltage increases, the current supplied by the load increases too.
In practice, the current flowing in line is no more the same.
If you want to estimate the the effect of phasing you have to adjust input voltage in order to
have the same voltage at arrival (with or without phasing)
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this experience it is not possible to estimate the effect of phasing, since when the
output voltage increases, the current supplied by the load increases too. In
practice, the current flowing in line is no more the same. If you want to estimate
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Section 2 – Load line ( 220 V ) WITHOUT PHASING
The scheme is the same of section 2 but with load
switched on 220 kV. Refer to Schematic 2. Set also the
input feeding at 220 V
POINT V L1-2 A L1 W L1 Var L1 WΣ Var Σ PF Σ V L1
B
G
The voltage drop is now of
The efficiency is
this value is definitely improved. As you can note, the same line at
higher voltage is able to transport the energy with lower voltage drop
and better efficiency.
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Section 2 -A – Load line ( 220 V ) WITH PHASING
Act as previous point, with the load switched on 220 kV and the capacitors connected among
them with star connection and in parallel with load.
Refer to Schematic 4
Feeding input at about 220 V
POINT V L1-2 A L1 W L1 Var L1 WΣ Var Σ PF Σ V L1
B
G
The voltage drop is
The efficiency is
R=120KV with Y capacitors connection:
POINT V L1-2 A L1 W L1 Var L1 WΣ Var Σ PF Σ V L1
B
G
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R=220KV with Y capacitors connection:
POINT VL12 AL12 W12 QL1 ∑W ∑Q P.F VL1
B
G
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Calculations and Question
1-Find the percent voltage regulation. What does a positive/negative
voltage
regulation indicate?
2-If the transmission line were purely resistive, would it be possible to
raise the receiving
end voltage by using static capacitors? Explain.
3-What is shunt compensation?
4-. Give methods of shunt compensation?
5. What is a Ferranti Effect?
6. How to Reduce Ferranti Effect in Transmission Line?
7. Why Ferranti Effect occurs in a Transmission Line?
8. What happens during the Ferranti effect?
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