Voltage Control Scheme For MVDC-RES Containing Renewable Energies And Energy Storage Systems

Since last couple of decades,electric rail transportation system has been emerged as fast,reliable and environment friendly source of transportation.The long-distance high-speed railways mainly utilizes AC electrification system,where urban metro railway utilizes DC electrification system.Classically,the AC systems have attained easy voltage transformation mechanisms since late 19^th century and hence has been utilized throughout long-distance corridors.Although DC systems have higher efficiencies than AC systems,the unavailability of mature voltage transformation technology have restricted its utilization only to short-distance urban rail transportation system.Meanwhile,the revolutionary advancement in the field of power electronics and converter technology have enabled engineers to consider high voltage DC technology in conventional power systems.Several high voltage direct-current（HVDC）transmission line projects have already been operationalized in recent years.In this connection,scientists are also exploring possible ways to get the benefits of DC technology in long-distance high-speed railways as well.Application of DC technology in long-distance railways can have the advantage of less reactance;reduced harmonic issues;reduced line losses;more distance between traction substations;and easy integration with energy storage systems（ESSs）and renewable energies（REs）.Considering so many advantages of the DC technology in long-distance railways,there is a great need to analyze the possible integration scenarios and control scheme to get the actual benefits.In this work,a voltage control scheme for voltage source converter（VSC）-based MVDC traction system has been proposed through adaptive droop mechanism.The scheme is aimed to provide enhanced current sharing capabilities amongst traction substations with improved voltages as compared to conventional droop control scheme.The information of current generated by each substation is shared with all other substations;the current generated by the considered substation is compared with the average of current generated by all substations in the system.An exponential droop mechanism is used for output voltage control;the droop function has the advantage of generating lower adjustments for less concerning small current deviations and has larger adjustments for more concerning large current deviation from the average current value.Moreover,critical point voltage regulators are used to monitor the voltage at most critical location i.e.at mid-section along the traction line.The proposed scheme demonstrates promising results in terms of current sharing with improved voltages.The improved voltage also reduces current in the catenary system hence reducing losses as compared to conventional droop scheme.In DC railways,the return current from the trains flows back to traction substation through running rails.A part of the track current leaks into the earth due to rail potential and is known as stray current.In MVDC railways,the distance between the traction substations is many times higher as compared to the urban DC railways.On the other hand,the catenary voltage in urban DC railway is many folds lower as compared to MVDC railways.As the rail potential and stray current mainly depends upon distance between substations and voltage level used;therefore,it will be very interesting to investigate the profile of rail potential and stray current in MVDC railway electrification system.For this purpose,a detailed analysis has been performed for rail potential and stray current in MVDC railways through a proposed simulation model.Analysis has been performed for grounded as well as floating scheme for different traction substation lengths.The results provide essential information of maximum loading and substation lengths for safe operation i.e.within the limits of maximum allowed rail potential and stray current values.The key advantage of the MVDC system is less reactance which allows the placement of traction substations farther as compared to conventional AC long-distance high-speed railways.This results in increased traction line resistance which causes the issues of catenary voltage sag and increase in stray current,specifically,at mid-section between adjacent traction substations.Furthermore,the integration of energy storage systems and renewable energies to railway systems are performed in a fashion to solve the issues of railway electrification system.In this regard,an integration topology and control scheme of photovoltaic and energy storage system has been proposed to solve the issues of MVDC railways.The scheme proposes a secondary energy source consists of photovoltaic（PV）and energy storage systems at mid-section between primary grid-tied substations.The control scheme consists of different operation modes which can be operationalized on the basis of voltage at the point of connection between the catenary and secondary energy source.The scheme improves catenary voltage and stray current at mid-sections along the traction line.The scheme also improves overall catenary voltage which also reduces line losses;moreover,the system becomes more robust and can operate in case of loss of a primary traction substation as well.