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    Reactive power compensation in railways using active impedance concept

                                             Sai Karteek Vyakaranam

                                                          PG Scholar

                                                Dr. K. Kiran Kumar

                                   Professor and HEAD OF EEE Department   

                                         VITS College of engineering, Visakhapatnam

Abstract ' This paper deals in analyzing the behaviour of linear, non-linear loads and the power   Conversion in electric traction system, which results in harmonics and changes the power conditions of  attraction system. Hence we have developed a p-q method that not only reduces the harmonics but also controls the power by using Scott transformer and the instantaneous complex power is constant for both the loads. The simulation results show the balancing effect on power system current and the reduction of total harmonic distortion.

Key words- Electric Traction, Active power, Reactive power, P-Q method.

1. Introduction

  The reactive power describes the back ground energy movement in ac system arising from the production of electric and magnetic field .it is required maintain the voltage deliver active power through transmission lines 'indexing of active power consumption is called reactive Power'. So reactive power deficiency cause the voltage to sag down and unbalanced load. Now coming to railway traction system 25kV/50Hz AC single phase supply is used. It is a longest transportation system in the world.  Railway electric load is a highly and continuous variable load. it will vary in kw  demand from train to train depending how many passengers and how many coaches are carrying .the daily load curves are vary as per the position of train .a railway train accelerating from station will draw a very high current and cause a voltage dip. So load will be changed station to station and cause load imbalance .the load imbalance cause harmonics distortion so the railway is paying a more amount of traction energy Charge's .now almost 70% of traction system electrified.  Electric system contains more reactive elements and consumed a large electric power. In order to balance the load between stations to station. Reactive power management most important concept. Nowadays in India for reactive power management capacitor banks that is fixed Filters will be used but nowadays technology will be developed and a vast amount of compensation techniques will be there so in order mange the reactive power a new topology will be introduced. Nowadays, the reactive power compensation is carried out by using fixed LC shunt filters.

       Figure 1: Revest Substation Equivalent Circuit

2. Proposed compensation system

    On three-phase systems the active and reactive powers are given by

p (t) = 3VI cos ('')

                q (t) = 3VI sin('')

 These two are same for balanced and Unbalanced Condition's

  In general the railway loads are single phase loads and having unbalance between the load and three phases so that the unbalance creates the negative sequence currents these currents flow into the system and increases the losses in generator and motor. These losses creates the disturbances in transmission line

    In the power system the power divided in to three types

1. Active power

2. Reactive power

3. Apparent power

      In order to deliver the active power into transmission lines the reactive power is required.it has

      To be balance in the grid to prevent the voltage problem and maintained secure and stable

       In case railway system the locomotive will draw 25kv/50Hz and it contains more reactive

       Elements and produces higher harmonic currents    

      The main sources of harmonics in traction system are

1. in the case of single phase supply  it produces 3rd harmonics

2. in the case of rectification of ac into dc

3. the rail having complex impedances so the impedance change with the current  

So by the above points we concluded that load is imbalance and here we want to reduce the harmonic currents from load to the supply

      So the proposed controller is that to balance the load and reduce the harmonic currents the shunt active filters are directly connected to power voltage step up transformer by using Scott or v-v transformers with pi controller for calculate the instantaneous active and reactive power taken by the combination of traction

The main purpose the transformer connections are

1. In the case Scott connected transformer loads on two transformer are equal the unbalance factor is zero. And it depends on the scheduling of trains

2. In the case of v-v connection lowest unbalance is obtained

    

   

                                           

  

    

 The proposed compensation scheme is show in below figure.1 and figure.2 shows that v-v and Scott connection

                            

Figure.1

                     

                               V-V Connection                                                          Scott connection

                                                                              Figure.2

2.1 Instantaneous Active and Reactive Power and Current Strategies

      In general the instantaneous reactive power also known as reactive power or P-Q  

      Theory and it is mainly based on set of instantaneous power and it can be applicable to

      Three phase system with and without nutralwire.it is not only in steady state but also

       In transient state. By this method is to calculate real power and reactive power this

       Method is applied to calculate reference current of active shunt filters. This theory can

       Be used in active control filters

       This theory have some interesting features

1. it is a three phase system theory

2. it is based on in instantaneous values and fast response

3. it can be applied for any three phase system

        4.  It allows two control strategies: constant instantaneous supply

             Power and sinusoidal supply current

Figure. Calculation for the constant instantaneous power

                              

                     Where     p0 = instantaneous zero-sequence power

                                     p = instantaneous real power

                        q = instantaneous imaginary power

Simulation Results

The scheme shown in Fig. 1 is modelled using the space vector representation of the state variables, at a 10 kHz sampling rate. Both, the V-V and Scott transformers are included in these simulations. The railroad system is represented using the measured harmonic currents distribution, injected to the power system in the secondary side of each transformer. The three-phase power system is modelled using a space vector The venin equivalent. Also, space vector representations of the Power transformer (V-V or Scott), three-p                                                             

                                         

     

            P & Q Method                                             Proposed Method

                                                                                                        

   

                                  

                                           Table .1

                               Parameters of the filter

    Vth    Lth       Rtrx    ltrx C(1,2) Vdc(1,2)

208v   0.1 mh     1m ohm    10 mh 1100 200~600v

                                             Table.2

                         Simulated THD and unbalance

  uncompensated        compensated

Simulated cases   THD I2/I1    THD     I2/I1

v-v rectifier case   0.4022 0.9494   0.0067 0.0024

v-v rail road case   0.2074 0.9239   0.0068 0.0018

Scott rectifier case   0.4061 0.9236   0.0070 0.0014

Scott rail road case   0.1226 0.8754   0.0051 0.0014

                                              Table.3

               Experimental results THD and unbalance

Un compensated

Compensated two  level

Compensated dual

 THD  I2/I1  THD   I2/I1  THD   I2/I1

  v-v 0.469 0.988  0.108 0.267 0.092 0.0336

 Scott 0.456 0.986  0.096 0.157 0.087 0.0178

CONCLUSION

In this paper, a low losses reactive power compensator based on Active Impedance concept has been proposed. A real case of study has been considered. Simulation results validate the correct working of the novel topology even when real waveforms are considered. Moreover, a 1.2 MVAR prototype of the compensator has been realized and tested at the test platform of SNCF. Experimental results confirm the analytical study and the good behaviour of the system.

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