20-09-2014, 11:19 AM
Integration of low frequency alternator to utility grid Via Fractional FrequencyTransmission System Project Report
Integration of low.docx (Size: 345.25 KB / Downloads: 23)
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.In this paper two studies of FFTS are presented to investigate its feasibility. Firstly, a phase-controlled cycloconverter as the frequency changer, stepping up 50/3 Hz electricity to 50 Hz electricity and supplying it to the utility grid by synchronizing process of 50/3 Hz transmission system with 50 Hz utility system. Finally, the simulation model of FFTS and conventional 50 Hz system is developed in Simulink platform of MATLAB and stability is assessed under different contingencies. The results show that, rotor swings of FFTS are compatible with conventional 50 Hz system. Thus it is concluded that, FFTS is feasible and stable and there is no essential difficulty for practicing it in engineering
INTRODUCTION
Increasingtransmission distance and capacity is alwaysthe 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 multi terminal HVDC system.
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 extent.
The Fractional Frequency Transmission System (FFTS) proposedin the literature [3] is anotherpromising approach toincrease transmission line capacity. In this system, generationand transmission is done at fractional frequency of 50/3 Hz.Frequency Tripler is used at grid side to step the frequency from50/3 Hz to 50 Hz and power is fed into the grid. Due to reductionin frequency for transmission, line reactance
is decreasedand hence the transmission capability is increased and voltageprofile is improved. A laboratory
STUDY-1
A. Problem formulation
We know that, all the systems connected in an interconnectedpower system have to maintain synchronism with one anotherunder contingencies. In this section stability study of FFTS iscarried out under different contingencies and the rotor swingsare compared with the conventional system to investigate thefeasibility of FFTS
Methodology
1. Structure of FFTS and conventional system
The structure of FFTS and conventional 50 Hz system consideredfor the study are as shown in Fig 6 and 7 [10]. In theFFTS, power is generated at 50/3 Hz and transmitted. At the grid side frequency Tripler step up the frequency from 50/3Hzto 50 Hz and power is fed into the grid. In the conventionalSystem since the power is generated at 50 Hz, there is no needof frequency Tripler
FFTS: Sudden change in mechanical input
Sudden change in mechanical input is simulated by step inputblock, which change the state from 1.068 p. u to 1.4 p. u. in zerotime at 2 sec from the simulation run. Due to sudden change inthe state of the system it drives into transient condition androtor swing obtained is as shown in Fig. 11. Excitation is keptconstant. Insimilar manner the transient stability of conventional 50Hz system for sudden change in mechanical input isperformed and the result is shown in Fig. 11.
The result shows that, the FFTS is stable for sudden changein mechanical input and is compatible with the conventional 50Hz system. The rotor swing has a peak overshoot of 1. 004p. u. on the base of 500 rpm and the settling time is 1.688sec from the time of contingency. Hence it is concluded that, theFFTS can maintain synchronism in an interconnected powersystem and can be a part of it as conventional system.
FFTS: Three-phase fault
Three-phase fault is simulated by a 3-phase fault block. It is introduced at the sending-end of the line at 2 sec from the simulation run and open at 2.05 sec. Due to change in the state of the system it drives into oscillation and the rotor swing obtainedfrom the simulation model is as shown in the Fig. 12.In similar manner stability study of conventional system is performed and the result is shown in Fig. 12. The result shows that, the FFTS is stable and is compatible with conventional system. The peak overshoot is high, because of more excitation and more reactive power injection from the generator. The settling time is found 1.942sec from the time of contingency. So it is concluded that, the FFTS can maintain synchronism in an interconnected power system and can be a part of it as conventional system.
CONCLUSIONS
A new transmission system-Fractional Frequency TransmissionSystem (FFTS) proposed in the literature provides an alternativeapproach to overcoming the inherent difficulties ofconventional AC transmission system. Three studies of FFTSpresented in this paper, demonstrates the stability and feasibilityof this new transmission system. A phase controlled Cycloconverteris employed as frequency Tripler in performing stabilitystudies. The result of various studies shows that, the FFTS cantransmit 3 times more power than conventional system. FFTScan also maintain stability with the interconnected power systemunder contingencies. In the FFTS, frequency Tripler is the key component whichinjects harmonics into the system and hence a complete studyon restrain of harmonics is essential. Short circuit studies, financialfeasibility study can also be done in future.