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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 paper introduces the experimental installation
of FFTS and primary experiment results. The experiment
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 in this experiment. The synchronizing
process of 50/3 Hz transmission system with 50 Hz utility
system is introduced in this paper. The experiment results show
that a 1200 km/500 kV transmission line can transmit more than
2000 MW electric power when employing the FFTS. The experiment
also illustrates that there is no essential difficulty to realize
FFTS in engineering practice
INTRODUCTION
I NCREASING transmission distance and capacity is always
the motivation to advance power industry technologies [1].
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. In addition,
up to now, the HVDC practices have been limited to the
point-to-point transmission. 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 [2]. The FACTS exploits power electronic
techniques to regulate the parameters of the ac transmission,
which can raise transmission capacity to some degree.
In 1994, we proposed a novel electricity transmission approach,
the fractional frequency transmission system (FFTS),
which uses lower frequency (50/3 Hz) to reduce the electrical length of the ac transmission line, and thus, its transmission capacity
can be increased several fold [3], [4]. The system is especially
suitable to transmit remote hydropower for developing
countries, such as China.
This paper introduces the experimental installation of FFTS
and primary experiment results. The experiment uses the phasecontrolled
cycloconverter as the frequency changer, stepping up
50/3 Hz electricity to 50 Hz and supplying it to the utility grid.
Thus, a new FACTS device is successfully established in this experiment.
The experiment results show that a 1200 km/500 kV
transmission line can transmit more than 2000 MW electric
power when employing FFTS. The experiment also illustrates
that there is no essential difficulty to realize FFTS in engineering
practice.
The structure of this paper is as follows. The next section
briefly introduces the principle of FFTS. Section III discusses
the components of the experimental FFTS. Section IV gives a
special description of the key equipment, the cycloconverter.
Section V analyzes the experiment results, and finally in Section
VI, we give some conclusions.
II. PRINCIPLE AND STRUCTURE OF FFTS
The ac electricity supplied by utilities has two basic parameters:
voltage and frequency. After the transformer was invented,
different voltage levels could be used flexibly in generating,
transmitting, and consuming electricity to guarantee efficiency
for different segments of the power system. In the history of
electricity transmission, besides of 50–60 Hz, many frequencies
were used, such as 25, 50/3, and 133 Hz. In 1896, the first two
generators and the transmission line from Niagara to Buffalo,
NY were put into operation. A 25-Hz electric system had been
chosen as the winning design [5]. However, since 50–60 Hz
was selected as the standard, changing frequency apparently became
taboo. The reason for this might consist in that to transform
frequency is more difficult than to transform voltage. As
new materials and power electronic techniques continuously advance,
different kinds of large-frequency changers are developed
rapidly. This trend may possibly lead to more reasonably
selecting different frequencies for electricity transmission and
utilization. For instance, the lower frequency electricity can be
used to transmit larger power for longer distance, and the higher
frequency electricity can be used more efficiently to drive the
electric tools.
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
CONCLUSION
We proposed FFTS in 1994, the main idea of which is multiplying
raising transmission capacity by reducing the power frequency.
This paper introduces the main experiment results of
FFTS achieved in the Power System Lab of the Xi’an Jiaotong
University. The experiment employs the cycloconverter as the
frequency changer to step up 50/3 Hz power to 50 Hz power and
then supply it to the utility grid. Thus, a new FACTS device is
successfully established. The results of experiment demonstrate
that a 1200 km/500 kV transmission line can transmit electric
power to 2000 MW by using FFTS. Comparing with the 50-Hz
ac transmission line, the transmission capability increases 2.5
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.
Nowadays, it is mature to transform power frequency by the
electronic converter (e.g., the cycloconverter). Therefore, FFTS
on/under 750 kV can be completed without any special technical
difficulty. A feasibility study for an industrial project to transmit
hydropower from west to east China is underway.
At present, we have just done some primary experiments;
much work urgently needs to be done, such as a study on economic feasibility, analysis of transient and dynamic stability, optimal
control of the cycloconverter, improvement of transmission
efficiency, and restraint of harmonics.