A Seminar Report
On
FRACTIONAL FREQUENCY TRANSMISSION SYSTEM
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ABSTRACT
Present day Power System uses the frequency of 50Hz and facing some
Voltage and Power stability problems. In order to limit the problem a new method is
proposed. 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. The proposed approach can multiply increase transmission
capacity and remarkably improve operating performance. The structure and
implementation scheme of the FFTS is discussed here along with the feasibility study
and experiment conducted on FFTS. The performance of FFTS against conventional
AC transmission system is compared here. The proposed method gives better result
when compared to the conventional method.

CHAPTER 1
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. 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. The FACTS exploits power electronic
techniques to regulate the parameters of the ac transmission, which can raise
transmission capacity to some degree.
China is very rich in hydro-power resources. They are mostly concentrated in
the West China. But the large loads are mainly located in the East and the South coast
area where industry and agriculture are well developed. These conditions determine a
basic structure of electric transmission from the West to East. The distance of
transmission ranges from 1000 to 2500 km. The long distance transmission of
electricity becomes an obstacle to hydro power development in China. At present, the
DC transmission system is too expensive for China to widely use. The highest voltage
level of AC transmission in China is 550 kV. This voltage level does not satisfy
power transmission over 1000 km. It is said that at least 20 years are necessary to
develop the transmission of higher voltage class and to manufacture the respective
equipment.
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 reactanceâ„¢s. 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.


Submitted in partial fulfilment for the award of the Degree of
Master of Technology in Electrical Engineering
(Computer Controlled Industrial Power)
Submitted by
Nihas K
Roll No. M080394EE
M.Tech in Electrical Engineering
(Computer Controlled Industrial Power)
Department of Electrical Engineering
NATIONAL INSTITUTE OF TECHNOLOGY CALICUT
Calicut, Kerala - 673 601.
April 2009


REFERENCE
[1] X. Wang, The fractional frequency transmission system, in Proc. Inst.
Elect. Eng. Jap. Power Energy, Tokyo, Japan, Jul. 1994, pp. 53“58.
[2] X. Wang and X. Wang, Feasibility study of fractional frequency transmission
system, IEEE Trans. Power Syst., vol. 11, no. 2, pp. 962“967, May 1996.
[3] O. I. Elgerd, Electric Energy Systems Theory. New York: McGraw-Hill, 1985.
[4] P.P. Biringer, J.D. Lavers, Recent Advances in the Design of Large Magnetic
Frequency Changers, IEEE Trans. on Magnetics Vol. MAG-12, No. 6,
November 1976
[5] Wang Xifan, Experiment on Fractional Frequency Transmission System,
IEEE Trans. Power Syst., vol. 21, no. 1, February 2006
[6] V.K Mehta, Rohith Mehta, Principles of Power System, S Chand 2004
[7] B.L.Theraja, A.K. Theraja, A textbook of electrical technology, vol II, S
Chand, 2003

Why the fractional frequency that they have used in their experiment is specifically (50/3) and not (50/2) or (50/4) or any similiar fraction.inteaashkhan[at]gmail.com" class="mycode_email

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CHAPTER-1

INTRODUCTION

1.1 History

Increasing 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. This thesis 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 and supplying it to the utility grid.

Thus, a new FACTS device is successfully established in this thesis and also illustrates that there is no essential difficulty to realize FFTS in engineering practice.
1.2 Literature Survey

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. 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. 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 [3], [4].

1.3 Objective

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. The simulation 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 established in this simulation. The synchronizing process of 50/3 Hz transmission system with 50 Hz utility system is used.

1.4 Organization of the Thesis

Design of Three phase to three phase (50/3 Hz to 50 Hz) step up cycloconverter based on FFTS to increase electricity transmission capacity is the central subject of this thesis. The thesis starts with an introduction in chapter 1 about history of FACTS advantages and a brief survey of long distance power transmission system in literature.

The theory of power system transmission, FACTS and its various controls are presented in chapter 2.

Chapter 3 presents the principle and structure of FFTS, generation and synchronization of 50/3 Hz. Chapter 4 describes the phase controlled cycloconverter. The simulation for the three phase to three phase cycloconverter and its waveforms are presented in Chapter 5, and finally Chapter 6 gives the contribution of this thesis and future research in this area.

CHAPTER-2
POWER TRANSMISSION SYSTEM
2.1 Long Distance Power Transmission System

As Power resources near the country's major load centers become exhausted, the transmission system will enlarge its interconnection as part of the long range expansion strategy. The energy resources to be exploited in the near future are located, where there is great hydroelectric potential. Major power markets on the other hand are expected to be in the near by region. The distances between the hydro plants and the main load centers ranging from 2000 to 2800 km. Therefore a great challenge will have to be faced in future, namely the transmission of large amounts f energy from very remote sources.

The power transfer capacity of the interties will be determined as a function of the optimum generation / transmission expansion. It was decided to create several working groups to develop studies in the following areas: power market, generation expansion, transmission planning, technological development, environmental aspects and industrial development. The studies have been divided in 4 stages as follows:

1. Pre-selection of Technology “ With the objective of starting-up the analysis of the technologies and to dismiss the less promising ones.

2. Selection of Technology “ Which aim is the selection of technologies that will be used in the selected transmission region.

3. Selection of Configuration “ With the objective of defining the configuration of the transmission trunks and to adequate the sending and receiving end networks to these trunks.

4. Detailing of selected Configuration “ Which objective is to establish main technical characteristics of the system and its components.
The results studied from Pre-selection of technology for the transmission systems were analyzed:

AC Transmission

1. Ultra High voltage (1050V to 1200V).
2. Extra High voltage (800 KV) with compact lines.
3. Half wavelength (800KV to 1200KV).
4. Six phase (317KV to 577KV)

DC Transmission

1. Ultra High voltage (above + / - 600KV).
2. Extra High voltage (+ / - 600KV).
3. Unit connection.
4. Multi terminal systems.

2.2 Introduction to FACTS (Flexible AC Transmission System).

Flexible Alternating Current Transmission System (FACTS) is static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability. It is generally a power electronics-based device.

FACTS is defined by the IEEE as "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability."

2.2.1 FACTS Controllers.

A power electronic-based system and other static equipment that provide control of one or more AC transmission system parameters.
2.2.2 Types of Controllers

I. Shunt Connected Controllers
1. BESS (Battery Energy Storage System)
2. STATCOM or SSC (Static Synchronous Compensator)
3. SSG(Static Synchronous Generator)
4. SVC(Static Var Compensator)
5. SVS(Static Var System)
6. TCBR(Thyristor Controlled Braking Resistor)
7. TCR(Thyristor Controlled Reactor)
8. TSC(Thyristor Switched Capacitor)
9. TSR(Thyristor Switched Reactor)

II.Series Controlled Controllers

1. SSSC (Static Synchronous Series Compensator)
2. TCSC (Thyristor Controlled Series Capacitor)
3. TCSR (Thyristor Controlled Series Reactor)
4. TSSC (Thyristor Switched Series Capacitor)
5. TSSR (Thyristor Switched Series Reactor)

III.Combined Shunt and Series Connected Controllers

1. TCPST (Thyristor Controlled Phase Shifting Transformer)
2. UPFC (Unified Power Flow Controller)
3. IPFC (Interline Power Flow Controllers)


CHAPTER-3

FRACTIONAL FREQUENCY TRANSMISSION SYSTEM
3.1 Principle of FFTS

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. The stability limit of an ac transmission line can be approximately evaluated by

Where V is the normal voltage, and is the X 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 % can be evaluated by

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

Where L is the total inductance of the transmission line. Hence, decreasing the electricity frequency f 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 50/3 Hz, the theoretically transmission capability can be raised three times.

We can also look through the principle of FFTS from another perspective. It is well known that the velocity of electricity transmission is approximately equal to the light velocity, 300 000 km/s. When electricity frequency is 50 Hz, the wave length is 6000 km; for 50/3 Hz, the wave length enlarges to 18 000 km. Thus, when frequency is 50 Hz, a transmission line of 1200 km corresponds to one fifth of the wave length; for the 50/3 Hz case, this transmission line only corresponds to one fifteenth of the wave length. Therefore, the electrical length decreases to one third. This is the essential reason why the FFTS can increase transmission capability several fold and remarkably improves its performance.

3.2 Basic Structure of FFTS

The basic structure of FFTS is illustrated in Fig. 3.1. The hydro power 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 hydro power 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 hydro power 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 electronic 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.


Fig. 3.1. Basic structure of FFTS

Fig. 3.2. Simulation Configuration of FFTS

3.3. Input Source 50/3 Hz Generation

The control system of the cycloconverter consists of three parts: the fraction frequency generator, synchronizing circuit, and cycloconverter control circuit. The turbine which will gives the input to the hydro generator(generates 50/3 Frequency)The turbine and the generator parameters are shown in Fig. 3.3.The Generated voltage is fed to the three phase transformer and to the three phase transmission line.
Fig.3.3. 50/3Hz Source Generation

3.4. Operation of Frequency Synchronizer
The IGBT based (frequency synchronizer) step up cycloconverter is shown in Fig.3.4. A fixed frequency (50/3 Hz) AC voltage source is applied to the primary of the transformer. Using mid point transformer the input AC voltage is splitted into two voltages. These voltages are converted in to variable output frequency (50 Hz) using pair of IGBT switches. Each pair of IGBT switch consists of two anti parallel connected IGBTâ„¢s. Using two pulse generators the triggering pulses given to switches. Now we are getting the required output voltage with frequency 50 Hz, across Ro. The time period can be calculated from the output frequency. The output voltage is shown in Fig.3.5.

Fig.3.4.IGBT based(frequency synchronizer) step up cycloconverter
Fig.3.5. Output voltage of frequency synchronizer

3.5. IGBT (Insulated-Gate Bipolar Transistors) Device
IGBT is a hybrid MOS-gated turn-on / turn-off bipolar transistor that combines that attributes of MOSFET, BJT and thyristors. It has a high impedance gate, low on-state voltage drop and bipolar voltage blocking capability. These attempts have led to the development of the IGBT, which is becoming the device of choice in most new applications. The performance of the IGBT is thus midway between that of a MOSFET and a BJT. It is faster than a comparable BJT but slower than a MOSFET. Its on-state losses are much smaller than those of a MOSFET, and are comparable with those of a BJT. The turn-on speed of the IGBT can be controlled by the rate of change of the gate source voltage. The symbolic representation of IGBT is shown in Fig.3.6.

Fig: 3.6 Symbolic Representation for IGBT

CHAPTER-4

CYCLOCONVERTER

4.1Configuration of Cycloconverter in FFTS

A Cycloconverter is a Frequency changer that converts AC power at one input frequency to output power at a different frequency with a one “ stage conversion process. The block diagram of cycloconverter is shown in Fig. 4.1
Fig.4.1. Block diagram of a cycloconverter

The three phase to three phase cycloconverter configuration is shown in Fig.4.2.It consists of two three phase bridge converters connected in anti-parallel and is built up by 36 thyristors (K200A/1600 V).
Fig. 4.2. Configuration of Cycloconverter in FFTS

4.2. Phase-Controlled Cycloconverters

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 page link converters or 2- in one stage (ac-ac) cycloconverters (Fig. 1). Cycloconverters are used in high power applications driving induction and synchronous motors. They are usually phase-controlled and they traditionally use thyristors due to their ease of phase commutation.

4.2.1. Single-phase to Single-phase (1?-1?) Cycloconverter
To understand the operation principles of cycloconverters, the single-phase to single-phase cycloconverter (Fig.4.3) should be studied first. This converter consists of back-to-back connection of two full-wave rectifier circuits. Fig 4.4 shows the operating waveforms for this converter with a resistive load.

The input voltage, vs is an ac voltage at a frequency, fi as shown in Fig. 4.4a. For easy understanding assume that all the thyristors are fired at a=0° firing angle, i.e. thyristors act like diodes. Note that the firing angles are named as a P for the positive converter and a N for the negative converter.

Consider the operation of the cycloconverter to get one-fourth of the input frequency at the output. For the first two cycles of vs, the positive converter operates supplying current to the load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen in Fig.4.4b. In the next two cycles, the negative converter operates supplying current to the load in the reverse direction. The current waveforms are not shown in the figures because the resistive load current will have the same waveform as the voltage but only scaled by the resistance. Note that when one of the converters operates the other one is disabled, so that there is no current circulating between the two rectifiers.

The frequency of the output voltage, vo in Fig.4.4b is 4 times less than that of vs, the input voltage, i.e. fo/fi=1/4. Thus, this is a step-down cycloconverter. On the other hand, cycloconverters that have fo/fi>1 frequency relation are called step-up cycloconverters. Note that step-down cycloconverters are more widely used than the step-up ones. The frequency of vo can be changed by varying the number of cycles the positive and the negative converters work. It can only change as integer multiples of fi in 1f-1f cycloconverters.

Fig. 4.3 Single-phase to single-phase cycloconverter

Fig. 4.4. Single-phase to single-phase cycloconverter waveforms
a) input voltage
b) output voltage for zero firing angle
c) output voltage with firing angle p/3 rad.
d) output voltage with varying firing angle
4.2.2. Three phase to Three Phase Cycloconverter
The configuration of three phase to three phase cycloconverter in FFTS is shown in Fig.4.2.The block diagram of three phase to three phase cycloconverter used in FFTS and the corresponding output for A,B,C phases are shown in Fig.4.5.The simulation diagram is shown in Fig.4.6.

Fig. 4.5. Block Diagram of 3 phase to3 phase cycloconverter


Fig.4.6 Simulation diagram of 3 phase to 3 phase cycloconverter
CHAPTER-5

RESULTS AND ANALYSIS

The three phase to three phase cycloconverter is used to convert the input source frequency 50/3 Hz to the required output frequency 50 Hz. The operation of phase A is given below. The simulated results of phase A, B, C are shown in Fig.5.1 to 5.7 respectively.

Phase A operation: The output frequency is three times higher than that of the input frequency. The model input and output wave forms are given below.. From figure the time axis is separated by electrical degrees with respect to input voltage.

i. 0<?t<30º : The most positive phase is C and the most negative phase is B. Hence the entering switch is 3 and leaving switch is 5.

ii. 30º<?t<60º: The most positive phase is A and the most negative phase is B. Hence the entering switch is 1 and leaving switch is 5.

iii. 60º<?t<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 8.

iv. 90º<?t<120º: The most positive phase is A and the most negative phase is C. Hence the entering switch is 10 and leaving switch is 9.

v. 120º<?t<150º: During the period the output voltage is in again 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 leaving switch is 6.
vi. 150º<?t<180º: The most positive phase is B and the most negative phase is C. Hence the entering switch is 2 and leaving switch is 6.

vii. 180º<?t<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.

viii. 210º<?t<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.

ix. 240º<?t<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.

x. 270º<?t<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.

xi. 300º<?t<330º: The most positive phase is C and the most negative phase is A. Hence the entering switch is 12 and leaving switch is 7.

xii. 330º<?t<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 / down ward corresponding to the positive and negative cycles.
The phase A, B and C (three phase input ) / Output voltage wave forms are given below.
Fig 5.1 Three phase input Voltage Model wave form for phase - A

Fig.5.2 A - phase Output Voltage
Fig 5.3 Three phase input Voltage Model wave form for phase - B
Fig.5.4 B - phase Output Voltage

Fig .5.5 Three phase input Voltage Model wave form for phase - C
Fig.5.6 B - phase Output Voltage
Fig.5.7 Combined output voltages of Phases A, B, C


CHAPTER-6

CONCLUSION AND FUTURE WORK

The thesis discusses the simulation that employs the cycloconverter as the frequency changer to step up 50/3 Hz power to 50 Hz power. The step up frequency of desired voltage is to be supplied to the utility grid with the 1200 km/500 kV transmission line. The 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.

Further the scope of the works 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.



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