1. Fractional Frequency Transmission System
Ramya Bhat, Madhumita Bhattacharya, Sidharth Goyal, Sneha Abhang
Fr C Rodrigues Institute of Technology
Abstract— The fractional frequency transmission system
(FFTS) is a very promising long-distance transmission approach,
which uses lower frequency (50/3 Hz) to reduce the electrical
length of the ac power line, and thus, its transmission capacity
can be increased several fold. This technology uses the phase-controlled
cycloconverter as the frequency changer, stepping up
50/3 Hz electricity to 50 Hz electricity and supplying it to the
utility grid. Thus, a new flexible ac transmission system device is
successfully established. The experiment results show that a 1000
km transmission line can transmit more electric power when
employing the FFTS.
Keywords—FFTS,Cycloconverter
I. INTRODUCTION
Increasing transmission distance and capacity is always the
motivation to advance power industry technologies. In the
history of the ac transmission system, increasing distance and
capacity mainly depends on raising voltage level of
transmission lines. At present, the highest voltage level of the
ac power transmission line in operation is 750 kV. To further
upgrade, the voltage level encounters difficulties of material
and environment issues. The high-voltage direct current
(HVDC) transmission that has no stability limit problem once
became another approach to increasing electricity
transmission capacity. However, the current converters at two
ends of HVDC are very expensive. It is still difficult to
operate a multiterminal HVDC system. From 1982 to 2003,
the total HVDC transmission capacity in the world was only
70 GW. The flexible ac transmission system (FACTS) has
been used to improve power system performance and has
become a very hot research field. The FACTS exploits power
electronic techniques to regulate the parameters of the ac
transmission, which can raise transmission capacity to some
degree.
In 1994, X. Wang proposed a novel electricity transmission
approach, the fractional frequency transmission system
(FFTS), which provides an efficient approach to solving the
above problem. As it is well known, AC long distance
transmission is mainly subject to problems with respect to
steady and transient stability which are in turn restricted by its
reactances. The new transmission system uses fractional
frequency (i.e. 50/3 Hz in our study) to reduce the reactances
of the AC transmission system, hence can multiply increase
transmission capacity and remarkably improve its operating
performances.
Generally speaking, there are three factors limiting
transmission capability, i.e., the thermal limit, stability limit,
and voltage drop limit. For the long-distance ac transmission,
the thermal limitation is not a significant impediment. Its load
ability mainly depends on the stability limit and voltage drop
limit. The stability limit of an ac transmission line can be
approximately evaluated by
Pmax=V2/X
Where V is the normal voltage, and X is the Reactance of the
transmission line.
We can see from the above equation that transmission
capacity is proportional to the square of the normal voltage
and inversely proportional to the reactance of the transmission
line. The voltage drop Δv% can be evaluated by
Δv%=QX/V2 *100
Where Q is the reactive power flow of transmission line.
Thus, the voltage drop is inversely proportional to the square
of voltage and proportional to the reactance of the
transmission line. Therefore, in order to raise transmission
capability, we can either increase the voltage level or decrease
the reactance of the transmission line. The reactance is
proportional to power frequency f.
X=2πfl
Where L is the total inductance of the transmission line.
Hence, decreasing the electricity frequency can proportionally
increase transmission capability. The FFTS uses fractional
frequency to reduce the reactance of the transmission system;
thus, its transmission capacity can be increased several fold.
For instance, when frequency is 50/3Hz, the theoretically
transmission capability can be raised three times.
The principle of FFTS can also be viewed from another
perspective. It is well known that the velocity of electricity
transmission is approximately equal to the light velocity,
300000 km/s. When electricity frequency is 50Hz, the
wavelength is 6000km; for 50/3Hz, the wave length enlarges
to 18000km. Thus, when frequency is 50 Hz, a transmission

2. line of 1200km corresponds to one fifteenth of the wave
length. Therefore, the “ELECTRICAL LENGTH” decrease to
one third. This is the essential reason why the FFTS can
increase transmission capability several fold and remarkably
improve is performance.
II. Operation of cycloconverter
The three phase to three phase cycloconverter is used to
convert the input source frequency 50/3 to the required output
frequency 50 Hz. The operation of phase A is given below
Phase A operation
The output frequency is three time higher than that of input
frequency. The model input and output waveforms are given
below. From the figure time axis is separated by electrical
degrees with respect to input voltage.
Fig 1. Cycloconverter
Fig 2. Input waveforms
Fig 3. Output waveforms
1. 0o< ωt <30o: The most positive phase in C and most
negative phase in B. Hence enter switch is 3 and leaving
switch is 5.
2. 30o< ωt <60o: The most positive phase in A and most
negative phase in B. Hence the entering switch is 1 and
leaving switch is 5.
3. 60o< ωt <90o: In this period the output voltage is in negative
cycle. So the download switch is operated to get the negative
cycle. The most positive phase is A and the most negative
phase is B. Hence the entering switch is 10 and leaving switch
is 9.
4. 90o< ωt <120o: The most positive phase is A and most
negative phase is C. Hence the entering switch is 10 and
leaving switch is 9.
5. 120o< ωt <150o: During this period the output voltage is
again in positive cycle. So the upward switch is to be operated
to get the positive cycle at the load. The most positive phase is
A and the most negative phase is C .hence the entering switch
is 1 and the leaving switch is 6.
6.150o< ωt <180o: The most positive phase is B and the most
negative phase is C. Hence the entering switch is 2and leaving
switch is 6.
7.180o< ωt <210o: The most positive phase is B and the most
negative phase is C. Hence the entering switch is 11 and
leaving switch is 9.

3. 8. 210o< ωt <240o: The most positive phase is B and the most
negative phase is A. Hence the entering switch is 11 and
leaving switch is 7.
9. 240o< ωt <270o: The most positive phase is B and the most
negative phase is A. Hence the entering switch is 2 and
leaving switch is 4.
10. 270o< ωt <300o: The most positive phase is C and the most
negative phase is A. Hence the entering switch is 3 and
leaving switch is 4.
11. 300< ωt <330: The most positive phase is C and the most
negative phase is A. Hence the entering switch is 12 and
leaving switch is 7.
12. 330o< ωt <360o: The most positive phase is C and the most
negative phase is B. Hence the entering switch is 12 and
leaving switch is 8.
Similarly the other phases B, C are switched
upward/downward Corresponding to the positive and negative
cycles.
II. VERIFICATION MODEL OF FFTS
The verification model is used to check the frequency and
corresponding voltage levels, verification model also indicates
the phase system. Thus the real power flow through the
transmission line can be measured by knowing the voltage and
current parameters of the three phase system. The model is
designed for 1000km and the voltage of the system is varied
in steps.
The value of inductance, capacitance and resistance is chosen
from the default values of the distributed line parameters, the
3 phase supply is taken from standard AC source available in
the SIM POWER SYSTEM.
The verification model is just used to note the drop across the
transmission line of 1000kms and to note the voltage level at
different frequencies. The model employs voltmeters and
ammeters and output scope from the inbuilt tool box in
MATLAB7.0
The AC voltage source is kept at different voltage levels and
the phase sequence is established as R phase 0 degree, Y
phase 120 phase shift &B phase- 120 degree phase shift .it can
also be noted that the three phase source is common grounded.
Fig 4. Simulation cicuit diagram
Fig 5. Current and voltage waveforms for 50/3 Hz.
Fig 6. Current and voltage waveforms for 50 Hz.

4. VOLTAGE
LEVEL
Vph(V)
for 50/3
Hz
Iph(A) for
50/3 Hz
Vph(V)
for 50
Hz
Iph(A) for
50Hz
1.1 kv 989.35 9.82 968.32 9.68
3.3 kv 3149 31.49 2915 27.93
6.6 kv 6325.67 64.78 5888 58.9
11 kv 10571 105.71 9813..2 98.2
Table 1. Voltage and current values for different frequencies
IV. CONCLUSIONS
We proposed FFTS, the main idea of which is raising
transmission capacity by reducing power frequency. The study
employs the cycloconverter as the frequency changer to step
50/3 Hz power to 50Hz power and then supply it to the utility
grid. The result of the study demonstrates that a 1000km
transmission line can transmit more electrical power by using
FFTS. Comparing with 50 Hz AC transmission line, the
transmission capability increases by 1.25-1.5 times. It
demonstrates the great potential of applying this new FACTS
device.
Comparing with HVDC, the FFTS can save an electronic
converter terminal, thus reducing investment. In addition,
usually HVDC can be used only for point to point
transmission, but FFTS can easily form a network-like
conventional ac system. Therefore, FFTS on/under 750kv can
be completed without any special technical difficulty.
REFERENCES
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[3] V.K Mehta, Rohith Mehta, Principles of Power System, S Chand
2004
[4] B.L.Theraja, A.K. Theraja, A textbook of electrical technology,
Vol II,
S. Chand, 2003.
[5] The IEEE website. [Online]. Available: http://www.ieee.org/