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` First proposed in 1994 by Wang Xifan and team.` A very promising long distance transmission approach.` Uses fractional frequency to reduce the electrical length of

the ac power line.` 50/3 Hz used in the study.` Reactance of the system is reduced.` Transmission capacity of the system increases by several

folds` Simulation study on magnetic frequency changer` Experimental installation carried out using 50/3 Hz

electricity using cycloconvertor.` Cycloconvertor is a new FACTS device.` Is specially useful for remote hydropower.

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` Has 2 basic parameters

1. Voltage Conventionally voltage levels changed to guarantee

efficiency for different segments of the power system. Withthe invention of transformers voltage levels could bechanged easily.

2. Frequency In the history of electricity transmission many frequencies

were used such as 25Hz,50/3 and 133Hz.

50-60 Hz was selected as the standard. Changing frequency considered as taboo since to transformfrequency is more difficult than to transform voltage.

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` With the recent advances in power electronictechniques different kinds of large frequencychangers are being developed rapidly. And thistrend may lead to use of different frequencies for

electricity transmission and utilization.

` Lower frequency electricity can be used totransmit larger power for longer distances.

` Higher frequency electricity can be used moreefficiently to drive the electric tools.

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` There are three main factors:

` For long distance ac transmission

` Thermal limit` Stability limit

` Voltage drop limit

` Thermal limitation is not a significantimpediment.

` Its load ability mainly depends on the -Stability Limit & Voltage Drop Limit.

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` Increasing transmission distance.

` Increasing transmission capacity.

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Depends mainly on Raising the voltage level of transmission lines.

The highest voltage level at present in operation is 750 KV.

Further upgrading of voltage level difficult due to material andenvironmental issues.


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` High Voltage Direct Current (HVDC) Transmission. Has no stability limit problem.

However the current converters at two ends of HVDC are veryexpensive.

Limited to the point to point transmission ,upto now.

Still difficult to operate a multiterminal HVDC.

` Flexible AC Transmission System(FACTS) Has been used to improve power system performance.

Has become a very hot research field.

Exploits power electronic techniques to regulate the parametersof AC transmission and raise the transmission capacity thereby.

FFTS (Fractional Frequency Transmission System) is apromising new FACTS device.

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` Stability limit of an ac transmission line can beapproximately evaluated by

` Voltage drop V % can be evaluated by

` where V is the normal voltage,Q is the reactive power,& X is the the reactance of transmission line.Decreasing reactance X will give rise to better

voltage regulation and improved system stability.

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In order to increase

the transmission

capability we caneitherincrease the

Voltage level or

decrease the

reactance of the

transmission line.


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` Reactance of a transmission line X is given byReactance of a transmission line X is given by

X=2X=2fLwhere f is the power frequency &

L is the inductance of the transmission line.` Decreasing the electricity frequency can proportionally increase theDecreasing the electricity frequency can proportionally increase the

transmission capabilitytransmission capability..

` In conventional ac transmission voltage level changed to improveIn conventional ac transmission voltage level changed to improvetransmissiontransmission..

` FFTS utilizes the principle of using lower power frequencyFFTS utilizes the principle of using lower power frequency toto

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Reduce the reactance of transmission

system ,thereby increasing the transmission

capability by several folds.


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` Velocity ofelectricity transmission light velocity i.e.

300,000 km/s

` Transmission capability Pmax

` X - reactance

` Electric length is the measurement in terms of theelectromagnetic wave length corresponding to length of the

transmission line

` Electric length frequency

` X frequency

electrical length

transmission capability 1/ electrical length

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` Hydropower generatorgenerates ac power offractional frequency.

` Stepped up by atransformer.

` Transmitted to thereceiving end oftransmission line.

` Fractional ac powerstepped up to industrial

frequency.

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HYDROPOWER GENERATOR

TRANSFORMER

TRANSMISSION LINE

FREQUENCY CHANGER

FFTSCONSISTSOF FOUR

BASICUNITS


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` Hydropower generatorLow rotational speed, hence suitable for generating low

frequency electric power .

Only change is to reduce the no. of poles.

Little influence on the cost and efficiency of the hydropower unit

` TransformerElectric power stepped up at lower frequency.

Core section area and coil turn nos. must be increased.

More expensive than 50 Hz transformer.

` Transmission lineConventional transmission line can be used in FFTS without any

change.

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` 1200 km, 500kV transmission line can transmission line can

transmit upto 1700 MW

` Reversible property of FFTS can be used to inter connect two

50 Hz power systems

` Efficiency of magnetic frequency changer can reach 95% andabove

` Simpler structure, lower cost, more reliable

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` Model generator unit

DC motor as prime mover (500 r/min)

AC generator generates 50/3 Hz 133v electric power

` Stepped up to 500Vby transformer unit

` Transmitted through 1200 km,500 KV transmission line

` 50/3 Hz converted to 50 Hz power by cycloconvertor.

` Injected to utility grid through isolating transformer bank

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` Key apparatus in FFTS` Two three phase bridge convertors

Connected in antiparallel

Built up by 36 thyristors

p & n group bridges connect the utility system through the

isolating transformer bank

Both the terminals of cyclconvertor connected with voltage

sources

p & n group operated in inversion mode

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CYCLOCONVERTOR CONTROL CIRCUIT

CONSISTS OF THREE PARTS

Fractional frequency generator

Synchronizing circuit

Cycloconvertor control circuit

1.Fraction frequency generator generates 50/3 Hz reference

signal

2. Synchronizing circuit measures and compares frequencies,

magnitudes and phases of voltages and sends starting

impulses to the control circuit of cycloconvetor

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3. Control circuit

Zero crossing of current at the end of each half cycle

detected

To regulate the control signal either to p or n group

50/3 Hz voltage synthesized by sinusoidal modulation of

a fire angle stored in a digital circuit in advance.

Successively used to control cycloconvertor operation

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` FFTS BASED ON CYCLOCONVERTOR

1.Only primary experiments conducted.

2.Experimental results achieved in power systems lab of Xian Jiatong

University for1200km/500V transmission line model

50 HZ Pmax800 MW

50/3 Hz- Pmax 2000 MW

3.Much work needs to be done such as

-economic feasibility

-analysis of transients

-dynamic stability

-optimal control

-improvement of transmission efficiency-restraint of harmonics

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` FFTS can provide good long distance power transmission solution.

` Especially suitable for developing hydro electricity resources

` Can improve power system performance remarkably

` Reversibility of power transmission.

` Comparing with HVDC, FFTS can save an electronic convertorterminal

` HVDC only point to point transmission.

` Network like conventional ac system

` Feasibility study for a project from west to east china underway

` Work like analysis of transient and dynamic stability, improvement ofefficiency restraint of harmonics still needs to be done

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Fractional n

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