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(1.Zhuhai Wanlida Electrical Automation Co.,Ltd.,Zhuhai 519085,China; 2.Guangdong Province Voltage Sag Engineering Technology Research Center of Enterprise Power Supply & Distribution System,Zhuhai 519085,China; 3.China Railway First Survey and Design Institute Group Co.,Ltd.,Xi’an 710043,China)

Abstract: Railway power system is an inseparable part of the power system,therefore,the intelligent architecture of the railway power system should also be focused on.The unique power supply characteristics of the railway power system are analyzed and integrated railway smart grid architecture based on energy routers is proposed.Importantly,three corresponding resilient mode control methods are suggested for the proposed architecture.In the fourth section,a simulation model corresponding to the resilient control mode is built and the simulation results prove the feasibility of the proposed control mode.Equally,for the novel network-connected backbone router (NCBR),a 1 000 kVA,27.5/10 kV NCBR engineering prototype is used to prove its effectiveness in practical applications.Finally,a differentiation analysis is given,followed by conclusions regarding the traditional power system and proposed system.

Keywords: Railway traction power system,railway electric distribution system,integrated railway smart grid architecture,energy migration router,network-connected backbone router,engineering prototype

1 Introduction

The outstanding flexibility,good intelligence,high efficiency,enhanced resilience,and excellent sustainability[1]are unparalleled advantages of a smart grid.From the perspectives of smart power generation,transmission[2],distribution[3],consumption,etc.,many studies have been conducted on smart grid.In addition,the smart grid has made great progress in the fields of sensors[4],big data[5],communications[6],security[7],and energy storage[8].The railway power system should not be excluded from the development of the smart grid,which is a very important part of the power system.

Generally,a railway power system (RPS)includes a railway traction power system (RTPS)and a railway electric distribution system (REDS),as shown in Fig.1.Conventionally,a traction power supply substation (TPSS)and electric distribution supply substation (EDSS)are independently constructed and operated and maintained for RTPSs and REDSs to avoid mutual interference.Because of the power quality problems caused by the locomotive,such as voltage harmonics and voltage fluctuations[9-13],the different phases of RTPSs and REDSs make it difficult to obtain the REDS power supply from RTPS via simple conversion.

Fig.1 Conventional and integrated railway power system

An active power filter (APF)[14],static var compensator (SVC)[15],thyristor control reactor (TCR)[16],static var generator (SVG)[17-18],dynamic voltage regulator (DVR)[19],railway power conditioner (RPC)[20-21],CO-phase power supply (CO-P PS)[22-26],and other solutions,are constantly being tried and applied in RPS.However,each only improves a certain function,from the perspective of a single device,or improves the performance of a specific aspect.

From the perspective of the system,this paper proposes the concept of an integrated railway smart grid architecture,based on an energy router (IRSG-ER).The “grid” not only refers to the network architecture but also includes the functions of controlling,detecting,and running the network,etc.In this paper,only the interconnected network architecture of the RPS is discussed.After interconnection,the concept of energy internet[27]will be further expanded to railways.

This paper summarizes the challenges in implementing the proposed IRSG-ER.

(1)System control mode challenges.Versatile network architecture creates flexible power flows,such as railway traction locomotives with different TPSS and EDSS loads,and the power flow between each.Therefore,the IRSG-ER system will contain multiple control modes according to the cooperative operation.

(2)Interconnected equipment technology challenges.Connecting different RTPS arms and connecting REDSs to RTPSs requires different energy routers.However,they all face complicated equipment technology challenges,such as extreme variations,rapidly changing speed of traction voltage caused by the locomotive,excellent output power quality for REDS,etc.Conversely,the routers must interface with the RTPS and REDS and the installation method is an issue that requires consideration.

(3)Challenges of coordinated control category with operational habits of REDS.In addition to the power quality standard,the special energy router needs to meet the operational habits of REDS.Particularly,when the single-phase grounding fault occurs,the router should meet the habit of continuous operation for more than 2 h on a single-phase grounding point when is the neutral ungrounded system.

During the process of solving the challenges,the contribution of the paper is summarized as follows.

(1)Some problems existing in railway power systems are analyzed and,specifically,the integrated railway smart grid architecture based on an energy migration router (EMR) and network-connected backbone router (NCBR)is proposed to reduce the cost of RPS construction,operation,and maintenance and solve the problem of a power supply shortage in particular environmental areas.Most importantly,the proposal enhances the resilience of RPS.

(2)To ensure reasonable use,a reasonable installation method for EMR and NCBR with fewer challenges is elaborated.

(3)Considering the different working conditions,this paper proposes a corresponding system control method for different operation modes.

(4)Architecture system simulations based on PSIM are conducted to prove achievability.

(5)A nonlinear load test and input/output voltage quality test,especially single-phase grounding fault tests are performed on a 1 000 kVA,27.5/10 kV NCBR engineering prototype,proving the feasibility of the IRSG-ER.

The paper is organized as follows.In Section 2,technologies regarding RPS are briefly reviewed and the integrated railway smart grid architecture is proposed.In Section 3,the railway smart grid architecture system control mode is divided into the traction state,braking state,and traction transformer offline.In Section 4,the system-level simulation,consistent with the proposed topology and control mode,is realized via PSIM.A series of tests prove the feasibility of NCBR.Finally,some conclusions and perspectives are provided in Section 5.

2 Integrated railway smart grid architecture

To detail the railway power system before and after integration,this section is divided into three parts.

2.1 Technologies review of railway power system

This section is reviewed from the perspective of RTPS and REDS,consistent with the schematic shown in Fig.2.

Fig.2 Conventional railway power supply system architecture

2.1.1 About railway traction power system

RTPS is the top priority of RPS.The current technological developments are mainly focused on the following innovations.

(1)Power quality compensation.Due to the impact and nonlinear characteristics of the locomotive,the harmonic current,reactive current,three-phase unbalance,negative sequence current,voltage fluctuation,and other problems in the RTPS are becoming increasingly concerning.Recently,harmonics and reactive power compensation have been optimized from traditional fixed capacitors and passive filters to active power filters[14]and dynamic var compensators[15-18].Currently,RPC[20-21]is proposed to balance the current of the RTPS from the perspective of energy movement and can optimize the three-phase unbalanced current and negative sequence current.

(2)Co-phase power supply.Due to the influence of neutral sections on the current balance and high-speed operation of the locomotive,the co-phase power supply technology has been proposed.A co-phased traction power system based on a balanced transformer[23]and impedance-matching balance transformer[26]is proposed.Ref]24]indicates a three-phase modular multilevel converter topology of co-phase power supply.Ref]25]describes the technical solution of 10 MVA co-phase power supply and its power balance control method.

(3)Regenerative braking energy recovery[28-29].Recently,many cities have begun operating high-speed railways and inter-city railways.According to the analysis of China Heavy-Duty Railway Technology Research Center,the significant overvoltage caused by the locomotive regenerative braking energy has become a serious concern for railway systems.

Improvements on traction transformers[30-32],power electronic traction transformers[33-35],and locomotive converters[36-37],etc.,are consistently being developed.

2.1.2 About railway electric distribution system

The REDS mainly supplies the railway station feeder (RSF),auto-blocking power transmission line (ABPTL),continuous power transmission line (CPTL),and other power lines,as illustrated in Fig.2.The main loads are the station lighting system,disaster prevention warning,signal equipment,pump equipment[38],etc.,on each line.

The 10 kV power supply of REDS is extremely difficult to obtain in some special environments,e.g.,China’s Qinghai,Xinjiang,Tibet,and other high altitude and sparsely populated areas where it is extremely difficult to find a corresponding 10 kV power supply.To solve this engineering problem,designers have proposed a variety of methods to power the REDS; power is often supplied via long-distance or ultra-long-distance power transmission or a 27.5/10 kV transformer.However,the long-distance power supply has a large power loss,which is prone to overvoltage and other problems,affecting the economics and safety of the REDS.The 27.5/10 kV transformer is easily affected by the traction locomotive,which causes power quality problems (such as voltage fluctuation,voltage harmonics,and three-phase voltage imbalance in the 10 kV voltage),reducing the reliability and quality of the power supply.At the same time,during the application process,the transformer is also limited by the traction transformer winding form and cannot be used freely.

2.2 Integrated railway smart grid architecture

According to the reviews and aforementioned problems regarding the RPS and current technical means,this paper proposes the concept of using an energy router to reconstruct the connection of the RPS.As shown in Fig.3,the energy migration router (EMR)connects the different traction arms of the RTPS.At the same time,the network-connected backbone router (NCBR)takes energy from the traction side and converts it to 10 kV power,which is suitable for the REDS.EMR and NCBR can greatly improve the flexibility and resilience of the RPS and move the RPS architecture toward a smart grid.The improved railway power grid architecture includes interconnected RTPS and interconnected traction-distribution supply systems.Together with the original rail network,it constitutes a truly interconnected railway system.

Fig.3 Integrated railway smart grid topology

As illustrated in Fig.3,for the specific integrated railway power grid topology,the integration is reflected as follows: ① in the sectioning post for electric traction (SP-ET),the EMR connects the power supply arms of different TPSSs.② In the same TPSS,the EMR connects different arms of the same traction transformer.③ Specifically,the NCBR connects the RTPS with REDS and supplies power to the load.In particular,this involves integrating the TPSS with the railway EDSS to form a joint construction substation (JCSS).That is,the new JCSS consists of an original TPSS,EDSS,NCBR,and EMR.

2.3 Optimal installation method for routers

The EMR and NCBR installation method is very important for reducing the application challenges and achieving rational use.As shown in Fig.4,the connection feeders of the EMR in the traction arms are N3 and N4 and the connection feeders of the NCBR are N1 and N2.Most importantly,the feeder N1 is farther away from the load side (the locomotive side)than N3.

Fig.4 Optimal installation method

And the connection method can have the following advantages.

(1)The voltage fluctuations and harmonics encountered by NCBR are reduced.The EMR optimizes the reactive and harmonics current generated by the locomotive while reducing the single-sided current of the traction arm.Therefore,the output voltage fluctuation of the traction transformer caused by the locomotive current and transformer impedance will be reduced.

(2)The input side overvoltage faced by the NCBR is optimized.The EMR transfers the energy of the locomotive regenerative braking process,reducing the resulting overvoltage.

(3)Power quality problems,such as harmonics and reactive power generated by NCBR itself,can be optimized by EMR,reducing the influence on the RTPS.

3 System operation mode

The optimal operation of the system needs to match the appropriate control mode and the operation objective functions are designed herein.

3.1 System operation objective functions

(1)Minimize unbalanced current of transformer

The traction power supply is very important in the operation of electrified railways.However,the single-phase current flow and reverse current during braking cause a current imbalance on the left and right arms of the traction transformer.

To improve the service life of the traction transformers,and reduce the impact of the railway power system on the public power grid while increasing the flexibility of the architecture,the first objective function is established

whereNis the total number of transformers in operation on line;iis thei-th traction transformer on the line;Lis the left arm of the traction transformer;Ris the right arm of the traction transformer.

(2)When the traction transformer of JCSS is offline,the energy transferred by EMRs to the two near SP-ET tends to be balanced.

wherejrepresents thej-th transformer of JCSS is offline;Lrepresents the energy transferred to JCSS on the left side;Rrepresents the energy transferred to JCSS on the right side.

(3)Establish a voltage source around the JCSS in which the transformer is offline.

When the transformer is offline,the EMRs in the SP-ET adjacent to JCSS switch from the on-grid control mode to the off-grid mode.In summary,many effects can be achieved with the objective functions.

(a)The imbalanced energy problem in JCSS can be reduced.

(b)The problem of impact on the public power grid when the energy flows in the reverse direction can be improved.

(c)Even if JCSS’s traction transformer is offline,the integrated system can operate flexibly.

3.2 Resilient control mode

To improve flexibility,the integrated power system operation can be divided into three working modes according to the energy transfer path.The losses of NCBR and EMR are ignored to simplify the analysis process.For the convenience of analysis,we only discuss the situation when the power factor is 1.However,the locomotive is a non-linear load,although its power factor is very high after PWM rectification and the harmonic content is very small.Considering the reactive component,it can also be distributed according to the proposed control mode.Taking the traction line (27.5 kV)as a reference,the flowing-out of the traction line direction is positive and the flowing-into the traction line direction is negative.

3.2.1 Traction state mode

As shown in Fig.5,inside the IRSG-ER,the following energy relationship is founded when the locomotive passes by

Fig.5 Resilient control in traction state

According to the objective function (1),we obtain

Therefore,through the proposal of IRSG-ER,it is possible to reduce the imbalance of the traction transformer,greatly improving flexibility.It is also possible to use EMR in the adjacent JCSS to transfer energy to the JCSS where the locomotive is located.The specific amount of energy transferred and the number of EMR put into operation should conform to the transfer principle of keeping the transformer in a balanced state.Here,the author only discusses the situation where the energy balance is performed by the EMR in a single JCSS.

3.2.2 Braking state mode

JCSS_A and JCSS_B are two of the JCSSs in the rail line,as illustrated in Fig.6.The locomotive at position A is braking and the locomotive at position B normally receives energy from the IRSG-ER.According to the conservation of energy,the equation can be obtained for JCSS_A and JCSS_B

Fig.6 Resilient control in braking state

When ignoring the losses of EMR,we obtain

In addition,when there are several JCSSs between JCSS_A and JCSS_B,a balance can be achieved by adding and subtracting the power requirements of NCBR and EMR in order.If there is only one locomotive and it is braking,the reverse braking power could be distributed to many NCBRs.

3.2.3 Traction transformer offline mode

Even if the reliability of the transformer is high,it may face unpredictable situations,such as inspection,maintenance,and damage.Therefore,when the traction transformer is offline,normal locomotive operation must be ensured.Therefore,the traction transformer offline mode is proposed.

As depicted in Fig.7,the traction transformer B of the JCSS_B is offline and,at this time,there is a train going to the power supply area from JCSS_B to JCSS_A.

Fig.7 Resilient control in traction transformer offline

According to the power flow,the equation of power can be obtained; for JCSS_A

In SP-ET_AB (SP-ET connecting JCSS_A and JCSS_B)，there is

In SP-ET_BC (SP-ET connecting JCSS_B and JCSS_C)，there is

At the same time,the voltage source is established according to the principle of proximity.The power supply area of JCSS_B is offline due to the traction transformer B.The EMR of SP-ET_AB and SP-ET_BC output voltage for the left and right arms of JCSS_B,respectively.

According to the above analysis process,the EMR needs to transfer power in JCSS_A is

In JCSS_B,the energy transferred is

In JCSS_C,the transferring energy is

In SP-ET_AB

In SP-ET_BC

Through the above process,IRSG-ER can continue to operate when the transformer is offline,greatly improving flexibility.

3.3 Single-phase grounding fault operation

The REDS is usually a 10 kV neutral ungrounded system; hence,in the IRSG-ER,the NCBR needs to meet the power system operation regulations.

(1) Once a single-phase grounding fault occurred,NCBR needs to run continuously for more than 2 h.

(2)The moment that single-phase grounding fault occurs,the NCBR should not switch the operating state and the line voltage remains unchanged,the phase voltage of the faulty phase relative to ground is zero,and the phase voltage of non-faulted phase relative ground rises to the line voltage.

Importantly,NCBR needs to strictly abide by the regulations to improve supply reliability.

4 Simulation and experiment

4.1 IRSG-ER simulation by PSIM

The low-voltage simulation models of the proposed control modes are established via PSIM software and the positions of the locomotive are consistent with those shown in Figs.5,6 and 7.The simulation parameters are listed in Tab.1 and the EMR and NCBR both adopt the back to back converter in the simulation.The results show that the resilient control mode can be effective.

Tab.1 Simulation parameters

4.1.1 Traction state

As illustrated in Fig.8,the locomotive obtains up to 56.9 kW power from the traction voltage and the NCBR_A converts the single-phase traction voltage into a three-phase to supply 14.5 kW of power for the ABPTL and RSFL.The left and right arms of the traction transformer of JCSS_ are unbalanced.In 0.5 s,the EMR_A in JCSS_A begin to transfer energy so that the power supply of JCSS_A was gradually balanced at 35.7 kW.In a short time,when the speed of EMR output energy is greater than the speed of energy absorption,part of the energy stored on the DC side will be released.This causes the observed short-term power mismatch.

Fig.8 Resilient control in traction state

4.1.2 Braking state

As shown in Fig.9,the locomotive in JCSS_A began to brake after 0.5 s and generate reverse active power of approximately 39 kW.To highlight the effect of EMR on power flow changes for better reading,the EMRs in IRSG-ER began to transfer energy to JCSS_B from JCSS_A at 0.6 s.The two NCBRs in JCSS_A and JCSS_B always provide power to ABCPTL and RSFL and cut down 15 kW in braking power.Since the braking energy is greater than the NCBR energy,the power transferred by EMRs from the left to the right arm of JCSS_A and from JCSS_A to JCSS_B is the difference between the two,i.e.,approximately 24.5 kW.In JCSS_B,the sum of the transferred power,locomotive power,and NCBR power consumption is obtained from the two arms.EMR tends to balance the power of the two arms at 23.5 kW.

Fig.9 Resilient control in locomotive braking

4.1.3 Traction transformer offline

According to Fig.10,in JCSS_A and JCSS_C,NCBR_A and NCBR_C supply power to REDS steadily,with a power of approximately 14.5 kW; At the same time,EMR_A transfers 10.6 kW from the left arm,EMR_C transfers 25 kW from the right arm,and finally the power of the left and right arms of JCSS_A and JCSS_C are balanced at approximately 25.1 kW.ERM_AB and EMR_BC output voltage while JCSS_A and JCSS_C transfer approximately 35.7 kW of power to JCSS_B respectively,ensuring the balance between the left and right arms of JCSS_B.Inside JCSS_B,the energy is transferred from EMR_B’s right arm and superimposed from JCSS_A to meet the running power requirements of the locomotive and NCBR_B,as shown in Fig.11.

Fig.10 Resilient control in traction transformer offline in JCSS_A

Fig.11 Resilient control in traction transformer offline in JCSS_B

4.2 Prototype experiment

The IRSG-ER architecture proposed in this paper mainly contains two key devices: EMR and NCBR.There are many specific implementation methods of EMR and NCBR.A typical study of EMR is the railway power conditioner (RPC).Much research on RPCs has been conducted[20-21,39-40]; hence,this paper does not re-prove the experiment data.Therefore,the experiment mainly focused on NCBR.

In this section,the 27.5/10 kV,1 000 kVA NCBR prototype is constructed in two containers as shown in Fig.12.The topology is shown in Fig.13.The purpose of the multi-winding transformer is to transfer the high voltage input 27.5 kV to a multi-low voltage output.Compared with a larger number of traditional,directly connected AC-DC-AC converter units,the multi-winding transformer significantly reduces the required number of AC-DC-AC converter units.

Fig.12 Prototype assembling of the NCBR

Fig.13 Topology of the RESPS

The purpose of the AC-DC-AC converter is to isolate voltage fluctuations and harmonics and also transfer the poor quality single-phase AC voltage to a stable three-phase AC voltage.

The LCR-T filter comprises two parts,namely an LCR filter and T transformer (Δ-Y output transformer).The purpose of the LCR filter is to reduce the high-order harmonic voltage produced by the converter.The Δ-Y output transformer achieves electrical isolation between the cascaded multilevel H-bridge inverter (Fig.13)and the loads.

Fig.14 illustrates the circuit structure of the AC-DC-AC converter (9 power units)in each phase.Each power unit has an uncontrolled rectifier,storage capacitors,inverter,and bypass switch.

Fig.14 Circuit structure of AC-DC-AC converter in each phase

In this section,the experimental results are provided to verify the characteristics and functions of the NCBR.The circuit network on the field and industrial platform are provided in Fig.15 for 27.5/10 kV,1 000 kVA NCBR in the TuKu-Xinjiang railway system.As shown in Fig.15,the purpose of the NCBR is to maintain the voltage quality of test point 2 (10 kV)to satisfy Chinese standards[41-42].

Fig.15 Industrial 27.5/10 kV,1 000 kVA NCBR in TuKu-Xinjian railway

The characteristics tests of the NCBR include a nonlinear load test.The function tests of the NCBR include input and output railway application tests and,specifically,a single-phase grounding fault test.

4.2.1 Nonlinear load

This part aims to prove that the output voltage (10 kV)can maintain a high power quality when the loads are unbalanced and nonlinear.The waveforms and harmonic spectrums of the NCBR output current and voltage are provided in Figs.16 and 17,respectively.The unbalanced rectifier loads and motor loads (400 kW)are used in this test.

Fig.17 The output current of the NCBR

According to Figs.17b and 17c,the total harmonic distortion of the output current (THDi)is as high as 8.9 % and the load current unbalance factor is 2.1%.As shown in Figs.16a and 16b,the NCBR output voltage deviation range is 10.25 kV±0.01 kV and the total harmonic distortion of the output voltage (THDu)is only approximately 0.9 %.Moreover,the unbalance factor of the NCBR output voltage is only 0.1 %,as shown in Fig.16c.Therefore,the NCBR can deliver high-quality voltage,even when the loading is unbalanced and nonlinear.

4.2.2 Input and output application test

This section aims to prove the practical functions of the NCBR.Figs.18 and 19 show the input traction voltage (test point 1)and output voltage (test point 2)of the NCBR.

As shown in Fig.18a,when a regular train passes through the traction arm,the input voltage waveform of the NCBR is distorted.The input voltage fluctuates from 26.85 kV to 28.74 kV,as reflected in the PT measurement (85.25 V to 91.25 V in Fig.18b).Moreover,the input voltage total harmonic distortion (THDu)is approximately 3.6 %,as shown in Fig.18c.

Fig.18 Input voltage of NCBR (Test point 1)

In contrast,as in Fig.19a,the output voltage waveform of the NCBR is very smooth.Over a 96 h observation period,the output voltage of the NCBR was consistently maintained within 10.25 kV±0.08 kV,as in Fig.19b.Moreover,the output voltageTHDucan be maintained at less than 1%,as in Fig.19c.

Fig.19 The output voltage of NCBR (Test point 2)

4.2.3 Single-phase grounding fault test

To simulate the actual situation,NCBR normally operates a supply power to REDS.At a certain moment,a single-phase grounding fault suddenly occurs.The fault is modeled as being directly connected to the ground through a circuit breaker on phase C.

Grounding faults occurred during operation.This matches the actual situation.As shown in Fig.20c,during normal operation,the A/B/C phase voltage is normal and the grounding fault in phase C suddenly occurs,the C-phase voltage drops to zero,and the A/B phase voltage turns to a line voltage of approximately 10.25 kV.Simultaneously,the voltage on the input side of NCBR is the traction voltage; a 75/5 current transformer and 100/5 A current probe are used for measurement and the input side current displayed by chance is very small.

Fig.20 Grounding fault occurred during operation

5 Conclusions

From the RPS and the proposed IRSG-ER,some comparisons are summarized in Tab.2.The following speculations can be made inferred from Tab.2.

Tab.2 Comparison between the conventional RPS and the proposal

(1)EMR and NCBR have fully improved the flexibility of JCSS and greatly improved the resilience of IRSG-ER.

(2)The typical representative of EMR is RPC and there have been many related studies.At present,NCBR research is particularly urgent.In many areas of China,the cost of construction,investment,operation,and maintenance of 10 kV power supplies is significant.According to Chinese railway standards,REDS generally requires at least two 10 kV power supplies.When NCBR is adopted along the railway,the railway-related costs can be greatly reduced and a lot of resources can be saved in the future.

(3)Through the simulation model and engineering prototype verification,the integrated concept proposed in this paper is very practical.In engineering,this concept is gradually becoming more valued by design experts; especially,NCBR in the Turku Railway,Geku Railway,Lalin Railway,etc.There will be more practical application requirements in the future.