Prototype Model for High Speed Railway Power Supply System Suitable for Indian Traction Sub Stations using Multi Modular Converter

As on 2020, India does not have any globally proven railway lines running with high-speed standards as above 250 km/h. Indian Railways looking at the prospect of high speed rail networks also known as bullet rail network in India. With the present Indian traction supply system which is based on the phase splitting or phase shifting method is not suitable for the high speed railway network unless we eliminate neutral sections as well as mitigating the power quality issues such as voltage unbalance, harmonics and high reactive power drawn from the utility grid. In this paper, Co-phase traction power supply system with zero neutral sections is proposed. Modular multilevel converter based three phase to single phase catenary system through a DC Link has been designed, simulated and laboratory prototype is made. A MATLAB simulation of the suggested method and droop characteristics based on load sharing between the two single phase parallel inverters had executed. The obtained results were satisfactory and shows that the proposed system is very much suitable for high speed railway.


INTRODUCTION
Nowadays speed, reliability and standard power quality are the essential parameters in case of high speed railway [1].Mainly the power supply system for locomotives has to be designed such a way that the system will satisfy the fore mentioned parameters.Many power transmission systems which are erected in various countries for traction network are presented in Figure 1.Indian Railway Traction substations receives an high voltage Three Phase 220/132/110 kV, 50 Hz power from local power generating stations and then the voltage is step down through a single-phase transformer [2].To attain this, duplicate feeders containing only two phases are served from the supply authorities nearby grid to the traction substation as mentioned in Figure 1(b).This method is known as phase shifting or phase splitting method [3].As the name itself indicates that, the supply phases are changes when the locomotive moves from one substation to another.Traction power supply stability depends on the sustaining traction voltage at feeding posts between 25kV and 27.5kV and a frequency between 48.5 and 51.5 Hz [4].Locomotives working on AC supply will incorporate voltage unbalance, harmonic and reactive power issues in the traction network.The problems concerning power quality upswings when the load on traction network increases [5].Moreover, neutral sections are necessary between substations especially in phase splitting network to avoid short circuit problems.The neutral section is described as a short isolated dead overhead device that usually separate contact wire fed by two adjacent traction substations [6].During meeting on the neutral section, the locomotive is turned off to prevent flickering while it is leaving and re-entering the live area that leads to reduction of locomotive speed [7].For an effective high-speed rail traction power supply the above mentioned problems must be solved.
There are many approaches to reduce the voltage unbalance Problems with the usage of balanced transformers such as Scott, YNvd, V/V connected transformers.But these solutions cannot adequately compensate for the issue of imbalance due to differences in traction load [8], [9].Passive components such as condensers and filters can compensate for reactive power and harmonics [10].Active compensators can have more complex and detailed coverage compared to passive compensators.Static VAR (SVC) and static synchronous compensator (STATCOM) are the most widely used traction compensators [11].In [12] the performance of SVC output for 25-kV voltage control is explained.However, the results are not adequate when the load is dynamic in nature [13].Hereby the Active Power compensators (APC) offered quick and vibrant response [14] but the system rating is too high [15].A half bridge multilevel RPC converter is proposed in [16].Co-phase traction power supply system is the newly developed traction power networks that will overcome the installation of neutral sections [17].Removal of neutral parts will Prototype Model for High Speed Railway Power Supply System Suitable for Indian Traction Sub Stations using Multi Modular Converter minimize the speed loss of locomotives [18], [19].The use of advanced co-phase supply was then observed as a possible system that can solve the above problems [20].

THREE PHASE TO SINGLE PHASE MMC STRUCTURE
The simplified configuration of the traction substation based on MMC circuit is represented in the Figure 2. The DC-system of the MMC is sometimes referred to it as the DClink, connected to the converter legs.The three-phase MMC rectifier is attached to the middle point of each leg.Each leg of the MMC shall be split into two arms [21].The arms which are attached to the positive strip are refers as the upper arms (u), and the arms attached to the negative strip are known to as the lower arms (l).Each and every arm has a submodules (capacitor & switches) and an arm inductor (L) as shown in Figure .Arm inductor which is connected in series helps to limit circulating current caused due to the difference of voltage between each arm [22].The design of MMC based on half-bridge submodule connection is shown in Figure 2. It consists of two switches with antiparallel diodes (S1 and S2) and a single DC capacitor (C) [23].The two IGBT devices are run in a complementary way in order to control voltage (Vc) across the capacitor and this voltage is specified as described in fallowing equation [24]. =

MODELLING OF MULTI MODULAR CONVERTER
The scientific modelling of the MMC having three-phase rectifier (grid side) and single phase inverter (locomotive side) is described below.The Figure 2  Inverter is operated and controlled independently by using droop control strategy as shown in the Figure 4.The single phase inverter output will be transferred to refuse the High Frequency (HF) switching with an LCL filter.The output of the LCL filter like voltage and current magnitudes are changed into d-q reference frame and these values are useful while calculating output power.The attained power is then passed through a low pass filter (LPF) and finally via droop controller components.

SIMULATION AND EXPERIMENTAL RESULTS
To validate the operational characteristics of the system mentioned above, a five-level modular multilevel converter based on three-phase to single-phase simulation is carried out and laboratory experimental setup is developed.The important simulation and control parameters are given in Table 1.The reference voltage for locomotive is taken as 25kv and the simulation is run for 4s to understand the voltage and current profiles effectively.

𝑛
, where 'n' indicates the no of modules per leg.Phase shifted PWM the most appropriate technique with minimal Total Harmonic Distortion (THD) for the MMC half-bridge network.Significant benefit of this technique compared to the phase-disposal techniques is that it leads to controlling the capacitors voltage since there is a movement of the modules activation embedded in the circuit which does not allow the overload or the high discharge of the individual modules.In the other hand, this dilemma can be seen in the methods of other PWM Methods.
. Three phase grid currents are mentioned in Figure 6.As the load is modelled by resistor, the power factor of the load is presumed nearly 1.The single phase traction voltage is displayed in Figure 7 and the wave form is of five level staircase under load condition.The single module voltage and DC link voltage of the MMC converter is shown in Figure 8, it is marked that the module capacitor voltage reaches to stable condition after 0.3s.
It can be found that the voltage wave, with decent sinusoidal wave shape, is a multilevel staircase.The further converters in the cascade structure, the narrower the filter circuit design spe cifications would be.When the cascade number is too high, thefilter circuit may be skipped to directly link the traction network.We can see that, to guarantee stable functioning, the load obtains a stable RMS voltage with a low THD (Total Harmonic Distortion) .When the current of the load is increased, the load voltage will decrease slightly, but it can be swiftly return to the reference value.

EXPERIMENTAL RESULTS
In order to verify simulation results, a low voltage experimental model has been developed as shown in Figure 9.To add the control device to an inverter built around an MOSET switch board, a Digital Signal Processor (DSP) was used.As a dc connection, a dc source was directly attached and the three phase inputs were connected to the rectifier.With a frequency of 5 kHz, space vector modulation was used as the switching scheme.The experimental findings gathered relate to the individual values that were logged in real time in the DSP.This was done by transmitting the necessary values over host system serial links.A schematic was given for the experimental setup in Figure 9.

Figure 1 .
Figure 1.Several Traction Power Supply Systems with (a) Balanced Transformer (b) Phase Splitting Method (c) Railway Active power compensator (d) Co-phase Traction MethodTraction power supply stability depends on the sustaining traction voltage at feeding posts between 25kV and 27.5kV and a frequency between 48.5 and 51.5 Hz[4].Locomotives working on AC supply will incorporate voltage unbalance, harmonic and reactive power issues in the traction network.The problems concerning power quality upswings when the load on traction network increases[5].Moreover, neutral sections are necessary between substations especially in phase splitting network to avoid short circuit problems.The neutral section is described as a short isolated dead overhead device that usually separate contact wire fed by two adjacent traction substations[6].During meeting on the neutral section, the locomotive is turned off to prevent flickering while it is leaving and re-entering the live area that leads to reduction of locomotive speed[7].For an effective high-speed rail traction power supply the above mentioned problems must be solved.There are many approaches to reduce the voltage unbalance Problems with the usage of balanced transformers such as Scott, YNvd, V/V connected transformers.But these solutions cannot adequately compensate for the issue of imbalance due to differences in traction load[8],[9].Passive components such as condensers and filters can compensate for reactive power and harmonics[10].Active compensators can have more complex and detailed coverage compared to passive compensators.Static VAR (SVC) and static synchronous compensator (STATCOM) are the most widely used traction compensators[11].In[12] the performance of SVC output for 25-kV voltage control is explained.However, the results are not adequate when the load is dynamic in nature[13].Hereby the Active Power compensators (APC) offered quick and vibrant response[14] but the system rating is too high[15].A half bridge multilevel RPC converter is proposed in[16].Co-phase traction power supply system is the newly developed traction power networks that will overcome the installation of neutral sections[17].Removal of neutral parts will

Figure 2 .
Figure 2. Three Phase to Single Phase MMC Converter with DC Link

Figure 4 .
Different switching operations of Singe Phase Inverter operation using half bridge MMC

Figure 6 .
Figure 6.The Three phase grid voltage and current wave forms

Figure 8 .
Figure 8. Single Module Voltage and DC Link Voltage of the MMC Converter

Figure 9 .
Figure 9. Low voltage experimental setup picture with two parallel inverters.

Figure 10 .
experimental results are shown in Figure10.Figure10(a) illustrate the module voltage of single phase inverter and the filtered output voltage of the catenary system is shown in Figure10(d).All the experimental results are done in this paper for minimum voltage value.The operational performance of the system for higher voltages has to be done in further research Laboratory prototype results (a) Single phase inverter side Module Volatge (b) Synchronized output voltage (c) individual single phase inverter voltage (d) Filtered output voltage of single phase output voltage 5. CONCLUSIONS Traction supply system by means of multi modular converters with load balancing is discussed in this paper.Mathematical model and control method of MMC for rectifier and inverter are analysed.In addition, the droop characteristics of single phase parallel inverters related to two successive traction substations is discussed.Transformer can be removed at inverter side with the suggested MMC inverter.The variable traction load can be balanced with single phase drooping characteristics.The laboratory prototype results indicates that the basic operation and load sharing characteristics of proposed method is valid and reliable.The obtained results were satisfactory.