POWER TRANSMISSION THROUGH FRACTIONAL FREQUENCY SYSTEM

ABSTRACT

The fractional frequency transmission system FFTS is a very promising long -distance transmission approach, which uses lower frequency (50/3) to reduce the electrical length of the AC power line, and thus its transmission capacity can be increased several fold . This paper introduces the primary experiments results of FFTS.

The experiment uses the phase-controlled cycloconverter as the frequency changer, stepping up 50/3Hz electricity to 50Hz 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 50/3Hz transmission system with 50Hz utility system is introduced in this paper. The experiment results show that a 1200km/500kv transmission line can transmit more than 2000MW electric power when employing the FFTS. The experiment also illustrates that there is no essential difficulty to realize FFTS in engineering practice

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 of transmission lines. At present, the highest voltage level of the AC power transmission line in operation is 750kv.To further up grade, the voltage level encounters difficulties of material and environment issues.
The high voltage direct current ( HVDC) transmission that has no stability limit program once become 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.

The flexible AC transmission system (FACTS) has been used to improve power system performance and has become a research field. The FACTS exploits power electronic techniques to regulate the parameters of the ac transmission, which can raise transmission capacity to some degree.

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 50/3Hz electricity to 50Hz and supplying it to the utility grid. Thus, a new FACTS device is successfully established I this experiment. The experiment results show that a 1200km/500kv transmission line can transmit more than 200MW 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 FACTS. In that section discusses the components of the experimental FFTS.

This approach can multiply increase transmission capacity and remarkably improve operating performance. The feasibility arid efficiency of the FFTS is investigated in this paper by the computer simulation method.

The fractional frequency transmission system (FFTS) is a very promising long distance transmission approach, which uses lower frequency(50/3Hz) 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 50/3Hz electricity to 50 Hz electricity.

Thus, a new flexible ac transmission system device is successfully established in this experiment. The synchronizing process of 50/3Hz transmission system with 50Hz utility system is introduced in this paper, the experiment also illustrates that there is no essential difficulty to realize FFTS in engineering practice.

2. Flexible AC Transmission systems (facts)

The need for more efficient electricity systems management has given rise to innovative technologies in power generation and transmission. The combined cycle power station is good example of a new development in power generation and flexible AC transmission system FACTS as they are generally known, are new devices that improve transmission systems. Worldwide transmission systems are undergoing continuous changes and restructuring .They are becoming more heavily loaded and are being operated in ways not originally envisioned .Transmission systems must be flexible to react to more diverse generation and load patterns. In addition the economical utilization of transmission system assets is of vital importance to enable utilities in industrialized countries to remain competitive and to survive.

In developing countries, the optimized use of transmission systems investments is also important to support industry, create employment and utilize efficiency scarce economics resources. Flexible AC transmission systems (FACTS) are a technology that responds to these needs. It significantly alters the way transmission systems are developed and controlled together with improvements in asset utilization, system flexibility and system performance


FRACTIONAL FREQUENCY TRANSMISSION SYSTEM (FFTS)

The fractional frequency transmission system FFTS is a very promising long distance transmission approach, which uses lower frequency (50/3) to reduce the electrical length of the AC power line, and thus its transmission capacity can be increased several fold

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 systems. In the history of electrical transmission, besides of 50_60 Hz, many frequencies were used, such as 25, 50/3and 133 Hz. In 1896, the first two generators and the transmission line from Niagara to Buffalo, NY were put into the operation A 25 Hz electric system had been chosen as the winning design

How ever, since 50_60Hz was selected as the standard, changing frequency apparently become taboo. The reason for this might consist in that o 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 low frequency electricity can be used to transmit large power for longer distance, and the high frequency electricity can be used more efficiently to drive 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 [6]. 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 ∆v% can be evaluated by

∆v%=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.
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The principle of FFTS can also view 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 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.

The basic structure of FFTS is illustrated in fig (). The hydropower generator in the figure generates ac power of fractional frequency (say 50/3 Hz), which is then stepped up by a transformer and transmitted to the receiving end of the transmission line where the fractional frequency ac power is stepped up to the industrial frequency.

The hydropower generator can easily generate low-frequency electric power because its rotating speed is usually very low. To generate low- frequency power, the only change for the generator is to reduce its pole number .this change has little influence on cost and efficiency of the hydropower unit. For the transformer, since the electric power that has to be stepped up is of low frequency, the core section area and the coil turn number must be increased.

Therefore, the cost of the transformer in FFTS is higher than that of the conventional transformer. The conventional transmission line can be used in FFTS Without any change.

The frequency changer is the key equipment in FFTS, which can be either the saturable transformer [7] or the power electronics ac-ac frequency changer, such as the cycloconverter [8]. The ferromagnetic frequency changer has advantages of simpler structure, lower cost, and more reliable operation, while the electronic type is superior in higher efficiency and more flexible in installation.

CYCLOCONVERTER

In industrial applications, two forms of electrical energy are used: direct current (dc) and alternating current (ac). Usually constant voltage constant frequency single phase or three-phase ac is readily available. However, for different applications, different forms, magnitude and/or frequencies are required. There are four different conversions between dc and ac power sources.

These conversions are done by circuits called power converters. The converters are classified as:

1-rectifiers: from single – phase or three-phase ac to variable voltage dc
2-choppers: from DC to variable voltage DC
3-inverters: from DC to variable magnitude and variable frequency, single phase or three phase AC.
4-cycloconverter: from single-phase or three-phase ac to variable magnitude and variable frequency, single-phase or three- phase

Traditionally, ac-ac conversion using semiconductor switches is done in two different ways: 1-in two stages (ac-dc and then dc-ac)as in dc link converters or 2-in one stage (ac-ac) cycloconverters . Cycloconverters are used in high power applications driving induction and synchronous motors. They are usually phase-controlled and they traditionally use to their ease of phase commutation.

There are other newer forms of cycloconversion such as ac-ac matrix converters and high frequency AC-AC (HFAC-AC) converters and these use self-controlled switches. These converters, however, are not popular yet.
Simulation and analysis

Operation of cycloconverter

The three phase to three phase cycloconverter is used to convert the input source frequency 50/3 to the required output frequency50 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

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. 0o< 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. 0o< 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. 0o< 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.

* 180< ωt<210: The most positive phase is B and the most negative phase isC.
Hence the entering switch is 11 and leaving switch is 9.

* 210< ωt<240: The most positive phase is B and the most negative phase isA.
Hence the entering switch is 11 and leaving switch is 7.

* 240< ωt<270: The most positive phase is B and the most negative phase is A. Hence the entering switch is 2 and leaving switch is 4.

* 270< ωt<300: The most positive phase is C and the most negative phase is A. Hence the entering switch is 3 and leaving switch is 4.

* 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.

* 330< ωt<360: 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.

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 1200km 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 value of neutral resistance is 100 ohm; 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 1200kms and to note the voltage level at different frequencies. The model employees voltmeters and multimeters and output scope from the inbuilt tool box in MATLAB6.0
The AC voltage source is kept at 500kv 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.

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 1000 10 968.32 9.68
3.3 kv 3149.28 31.49 2915 27.93
6.6 kv 6336.795 63.36 5888 58.9
11 kv 10571 105.71 9813.2 98.2



CONCLUSION

We proposed FFTS, in this he main idea of which is multiplying raising transmission capacity by reducing power frequency. The experiment employs the cycloconverter as the frequency changer to step 50/3 Hz power to 50Hz power and then supply it to the utility grid. Thus a new FACTS device is successfully established. The result of the experiment demonstrates that a 1200km/500kv transmission line can transmit electrical power to by using FFTS. Comparing with 50 Hz AC transmission line, the transmission capability increases by 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. Nowadays, it is mature to transform power frequency by the electronic converter(e.g., the cycloconverter). Therefore, FFTS on/ under 750kv can be completed without any special technical difficulty.
The fractional frequency transmission system (FFTS) is a very promising long – distance transmission approach, which uses lower frequency (50/3Hz)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 .as new materials and power electronic techniques continuously advance different kinds of large-frequency changes 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 can be used more efficiently to drive the electric tools.