Download USE OF SERIES AND SHUNT CAPACITORS IN TRANSMISSION

USE OF SERIES AND SHUNT CAPACITORS
IN TRANSMISSION LINE
NAME
:
D.K.PATHIRANA
INDEX NO
:
080332P
GROUP
:
EE09
DATE OF PER. :
29-11-2010
DATE OF SUB. :
13-12-2010
OBSERVATION SHEET
Name
:
Index no
:
Group
:
Date of per :
Instructed By:
D.K.Pathirana
080332P
EE09
29-11-2010
Ms. V Vijayarajah
1) With series capacitance
Capacitance (μF)
Power received (W)
Current (A)
0
50
0.59
6
6
12
17
18
31
0.38
24
51
0.6
30
47
0.55
2) With shunt capacitance
Capacitance (μF)
Power received (W)
Current (A)
6
75
0.8
12
95
1.1
18
110
24
120
1.4
30
120
1.4
Calculations
1) With series capacitance
Capacitive reactance
Inductive reactance
Here,
L=0.15H
Per unit compensation of line
⁄
Capacitance (μF)
Power received
(W)
Capacitive
reactance (Ω)
Per unit
compensation
0
50
6
6
530.52
11.26
12
17
265.26
5.63
18
31
176.84
3.75
24
51
132.63
2.81
30
47
106.10
2.25
Plot of power received vs. series capacitive reactance
70
60
Power received (W)
50
40
30
20
10
0
0
100
200
300
400
Series capacitive reactance (Ω)
500
600
700
Series Capacitive
Reactance (Ω)
Power received (W)
-
50
530.52
6
265.26
17
176.84
31
132.63
51
106.10
47
Plot of Power received vs. per unit compensation of line
70
60
Power received (W)
50
40
30
20
10
0
0
2
4
6
8
Per Unit Compensation
10
12
14
Per unit compensation
Power received (W)
50
11.26
6
5.63
17
3.75
31
2.81
51
2.25
47
Plot of Power Received vs. Shunt Capacitive Reactance
130
120
110
Power received (W)
100
90
80
70
60
50
0
100
200
300
400
Shunt capacitance reactance (Ω)
500
600
Shunt capacitive
reactance (Ω)
Power received (W)
530.52
75
265.26
95
176.84
110
132.63
120
106.10
120
Discussion
The power factor of an AC electric power system is defined as the ratio of the real power
to the apparent power, and is a number between 0 and 1. Real power is the capacity of the
circuit for performing work in a particular time. Apparent power is the product of the current
and voltage of the circuit. Due to energy stored in the load and returned to the source, or due
to a non-linear load that distorts the wave shape of the current drawn from the source, the
apparent power can be greater than the real power. Low-power-factor loads increase losses in
a power distribution system and result in increased energy costs. When power factor is equal
to 0, the energy flow is entirely reactive, and stored energy in the load returns to the source
on each cycle. When the power factor is 1, all the energy supplied by the source is consumed
by the load.
When a purely resistive load is connected to a power supply, current and voltage will
change polarity in step, the power factor will be unity, and the electrical energy flows in a
single direction across the network in each cycle. Inductive loads such as transformers and
motors (any type of wound coil) generate reactive power with current waveform lagging the
voltage. Capacitive loads such as capacitor banks or buried cable generate reactive power
with current phase leading the voltage. Both types of loads will absorb energy during part of
the AC cycle, which is stored in the device's magnetic or electric field, only to return this
energy back to the source during the rest of the cycle.
It is often possible to adjust the power factor of a system to very near unity. This
practice is known as power factor correction and is achieved by switching in or out banks of
inductors or capacitors. For example the inductive effect of motor loads may be offset by
locally connected capacitors.
Shunt capacitive compensation 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.
In series connection, due to the series connection due to the inductivity of the line there
can be a resonance occurring at a certain capacitive value. This will lead to very low
impedance and may cause very high currents to flow through the lines. In shunt connection
the capacitor is connected in parallel to the unit. The voltage rating of the capacitor is usually
the same as (or a little higher than) the system voltage.
In certain situations capacitors are not connected directly to the supply lines. The reason
for this is the presence of harmonics in the waveform caused by switched mode power supply
units. The simplest way to control the harmonic current is to use a filter. It is possible to
design a filter that passes current only at line frequency (e.g. 50 or 60 Hz). This filter kills the
harmonic current, which means that the non-linear device now looks like a linear load. At this
point the power factor can be brought to near unity, using capacitors or inductors as required.
This filter requires large-value high-current inductors which are bulky and expensive.
For power factor correction, instead of using a capacitor, an unloaded synchronous
motor can be used. This is referred to as a synchronous condenser. It is started and connected
to the electrical network. It operates at full leading power factor and puts VARs onto the
network as required to support a system’s voltage or to maintain the system power factor at a
specified level. The condenser’s installation and operation are identical to large electric
motors. The reactive power drawn by the synchronous motor is a function of its field
excitation. In this method the amount of correction can be adjusted; it behaves like an
electrically variable capacitor.