07-09-2016, 12:01 PM
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ABSTRACT
Railway traction system which is now very important part of daily life of people mostly living in the city areas. Old systems offers much drawbacks mainly due to use of the DC motor. Day by day previous old systems are implementing. So in this advanced power conversion drive system overcoming the previous drawbacks by using three phase induction motor. Also other new power electronics circuits are implemented in this system which are compact in size & also having negligible power losses. Also it offers smooth speed control and braking system. Transmission and conversion of AC supply is more easier & effective than that of in case of DC supply. Also the additive benefit is getting in case of use of AC motor which is called Regenerative Braking. Thus by seeing this much benefits and flexibility due to use of power electronics circuits new system is encouraging to bring in application all over the world.
INTRODUCTION
As the railway travelling is the most preferred transportation system mostly in the city areas, traction system has necessity to being better. The old system which was in application and now is in application has so many drawbacks so that system needs to redevelop. Old system which in use consists DC motor which is connected to the wheels of the train and with the aid of motor by giving supply to it train has to be run. Generally in old days where DC overhead system was used in the traction system has huge power loss because DC supply cannot step up or step down, so because of this current in the line is more which was causing huge I2R loss in the system. After looking at this drawback the AC overhead system is came in application which is proved effective. In this case as we using DC series motor to run, it has to be convert in DC only and then by using arc rectifier or vacuum tube rectifier this AC to DC conversion had been done. But these rectifiers had very large size, then compact size diode rectifiers came in use instead of arc & vacuum tube rectifier . But as DC motors used it requires rheostat for the speed control purpose. In past AC overhead line started to use instead of DC line to reduce I2R loss; but because of rheostat used for speed control, I2R loss taking place. This again tends to voltage drop, power loss & heating In motor. Also speed control is not so smooth. So in recent a system came which is called “Advance Power Conversion Drives In Traction System”, which overcoming these drawbacks causing due to use of Dc motor. In this system, three phase induction motor is using to run the train. Firstly supply coming of the overhead line is of 25KV which is bring by pantograph and then feeding to power conversion drives, output of power drive system is 3 phase,750V AC which is given to the motors which are actual cause to run the train. In this system as AC motor is used and new compact size power electronics circuits are available so that speed control is very smooth & losses are very less. Thus, this system found so much effective it is necessary to bring this all over the world.
LITERATURE SURVEY
In old systems as DC motors was used so there were so many disadvantages offered and speed control was not smooth. Also because of bulkiness of the system these systems were so costly and difficult to handle. Also by seeing the demerits of the DC motor over AC motor the preference to use of AC motor was more. Also old system not giving smooth control and braking system.
If any fault occurred in the power drive system of old traction then that was very difficult to repair it, The cost of maintenance and repairing was much higher as compared to recent system. Also it was taking too long time for total remedial action. The size of old power drive system was too large as compared to recent system.
In old system as DC motor were using so that at the time of down gradient when speed increasing then train wheels started to float in the air. These DC series motors are connected in series to each other so that overall load decreases and speed of motor goes to higher extent which can cause slippage from the track. Thus old systems offering many demerits.
Use of VFD:-
Train in the traction system require the quickly pick up. It is generally desirable at every station. But as the load of train is much higher so, that is very difficult to achieve. Dc series motor is only which is giving that much starting torque so that pick up is quicker. But in recent system we are using three phase induction motor and the starting torque of this motor is less as compared to the DC series motor. So that it cannot give desirable high starting torque. So we require to maintain high starting torque of the motor. At starting we have maximum starting torque but that is only for small period, it is not giving up to good pick of the train. We cannot do anything with any other parameter to maintain it at maximum but only the frequency is the parameter by which we can do. So to do this VFD(Variable Frequency Drive) use is essential.
Role of VFD:-
For maximum torque, slip should be 1. That means rotor resistance and rotor reactance should be equal then only slip will be maximum which is cause to maximum torque. So if we maintain the high starting torque then train will take pick up quickly.
Following is equation of rotor resistance, rotor reactance & slip-
Where,
R2= Rotor Resistance
X2= Rotor Reactance
F = Frequency
L = Inductance
Thus at starting there is maximum slip i.e. 1 and high starting torque in standstill condition. But as motor started to run then torque gets reduced as slip getting reduce. So there is only one way to keep torque maximum i.e. slip should be maximum.
Now, in above equation R2 is always constant at standstill & running condition also. Only slip & X2 get changed at standstill to running condition. So we have to vary X2 to keep slip at maximum.
As per equation shows we can vary reactance by varying frequency. Now, this is done by the VFD.
We can also vary the speed by changing R2 in case of slip ring induction motor.
REGENERATIVE BRAKING
When down gradient comes then because of the sliding train is started to run fastly. At this time speed of the motor i.e. rotor goes to higher extent which is more than the synchronous speed of the motor.
In normal condition rotor is cause to drive the wheels but at this condition wheels are cause to rotate the rotor then speed of rotor increases and rotor started to acting as generator. Then this generated supply is fed back to the system line again. But then lense law is act on the rotor and speed of rotor started reducing that means regenerative braking takes place. The advantage of this braking is that, when train started to run faster, then chances of slippage may be there which can cause accident but this braking maintains the speed of train in limit and avoids the slippage. Also because of the regenerative braking generation takes place which is extra benefit.
Regenerative braking also takes place at the time of stopping at the station.
ADVANTAGES & DISADVANTAGES
9.1 ADVANTAGES:-
Better accuracy in system operation.
Less maintenance required as AC motors are used where brushes are absent.
Accuracy and flexibility in the speed control.
Regenerative braking takes place so power fed back to the supply.
Slippage from the track is reduces due to regenerative braking.
9.2 DISADVANTAGES:-
Initial cost of this system is more as more power electronic circuits are used.
Complexity of the system increases to some extent.
CONCLUSION
The recent developments outlined briefly in this paper bear testimony to the major progress that has been accomplished during the past few years in applying new power electronics technology to industrial and traction drives. Although the improvements sometimes seem painfully slow and labored to technical experts working in the field every day, the rate of technical progress is actually very impressive when one takes a step back to see how far the technology has progressed during the past 25 years. Where do we go from here? The future of both industrial and traction drives depends not only on advances in the underlying technologies, but the economic and regulatory climate in which they are developing.
Despite the risks of predicting future trends, there are many reasons to expect that increasing global concerns about efficient electrical energy utilization, transportation fuel economy, pollutant emissions levels, and electrical power quality will increase during coming years. In light of these pressing concerns, the desire for further major improvements in industrial and traction drives will almost certainly continue to place a high premium on new advances in power electronics technology.