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- Designed a system for power transmission at 1/3rd reduced frequency over long distances to improve efficiency. - Cycloconverters were designed using IGBT in MATLAB for conversion of frequency at both the ends of the system for transmission.

•1 recomendación•644 vistas

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- Designed a system for power transmission at 1/3rd reduced frequency over long distances to improve efficiency. - Cycloconverters were designed using IGBT in MATLAB for conversion of frequency at both the ends of the system for transmission.

- 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 [1] P.P. BIRINGER, J.D. LAVERS, RECENT ADVANCES IN THE DESIGN OF LARGE MAGNETIC FREQUENCY CHANGERS. [2] FREQUENCY CHANGERS, IEEE TRANS. ON MAGNETICS VOL. MAG-12, NO. 6, NOVEMBER 1976. [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/