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Abstract

Urban rail vehicle power supply short interruptions or prolonged paralysis situation sometimes occurs, which will cause the entire line outage. If the train through the battery to achieve emergency traction, it will be able to effectively solve the problem of train interval stop due to external power supply reasons. This article proposes an emergency traction system, using lithium-ion batteries as traction power, carrying out the design and research on the function of lithium-ion emergency traction system for rail transit vehicles. Through the design of the power supply system, the design of the emergency traction power supply, the design of the conversion control and safety protection circuit, the design of the traction system, and the design of the auxiliary system, the train can be run to the next station under the emergency power supply, and the effectiveness of the scheme is verified by simulation. Moreover, the transformation on the basis of the existing Shanghai 952 train illustrates the feasibility of the existing train to achieve the emergency traction function and provides guarantee for the safe operation of the train.

Keywords

Emergency traction lithium-ion battery auxiliary power battery management system charging method

Introduction

With the development of urbanization, the urban population is increasing. In order to alleviate the increasing traffic pressure, many countries have vigorously developed rail transit.¹ The energy source of rail transit is electricity. After the power plant generates electricity, it is sent to the substation through the transmission line, and then the power is transmitted to the train through the contact network.² However, accidents of blackouts in metropolitan areas around the world have occurred from time to time. On 28 August 2003, a major blackout occurred in parts of London and southeast England. Nearly two-thirds of London’s rail traffic was shut down, and about 250,000 people were trapped in London’s rail transit. On 16 December 2011, a serious malfunction occurred in Singapore’s rail transit. Thousands of passengers were trapped in the compartment for 1 h and several passengers were taken to hospital for treatment. To this end, a number of vehicle manufacturers are conducting research on emergency traction of vehicle batteries, and there are practical applications.³ The Hitachi Research Institute cooperated with the Tokyo Metro Co., Ltd. to carry out the driving test of the train during emergency power supply. The test train was a 6-knot train on the Ginza line; when the driver switched the button to the emergency mode, he can continue driving for 1.2 km and take passengers to the nearest station. In November 2007, French NICE opened a city tram with a contact network and vehicle power supply. The NiMH battery can maintain the train driving 1 km under the emergency power supply. However, these emergency power traction systems have a limited ability to pull the train through the battery. For example, the running distance, running speed, and climbing ability cannot meet the emergency requirements.

Lead–acid batteries are not suitable for emergency power supply for rail transit because of low energy density and serious environmental pollution.⁴ Currently, conventional rail transit energy storage components are supercapacitors, NiMH, and lithium-ion batteries.⁵ Lithium-ion batteries are widely used because of their superior performance.

The performance of lithium-ion batteries is susceptible to thermal management capabilities. Many scholars have studied the performance of lithium-ion batteries under different thermal management forms.^6–9 HY Hwang et al. studied the effects of the ventilation locations of the inlets and outlets and the gaps among battery cells on the rate of heat dissipation and temperature distribution in the pack. By establishing a computational fluid dynamics model to analyze the temperature distribution, it is found that the location and shape of the inlet and outlet have a significant effect on the heat dissipation of the battery.¹⁰ Y Liu et al. proposed a new design of thermal management system for lithium-ion battery pack with thermoelectric coolers. The results show that the thermoelectric coolers (TEC) battery thermal management system has good heat dissipation and temperature balance.¹¹ Giuliano et al.¹² found that as the air flow rate decreases, the load power consumed by air cooling also decreases, but there is a significant increase in the air temperature difference between the outlet and inlet, which significantly reduces the heat dissipation performance of the battery pack. Li et al.¹³ established a two-dimensional model to simulate the thermal management problems in air-cooled battery packs in detail; he found that as the air flow rate decreases, the maximum temperature-rising and maximum temperature difference in the module increase. Yu et al.¹⁴ developed a battery thermal management system; it has the function of inputting two airflows of independent air channel and fan at the same time. Their study found that the air flow from the bottom air duct rises vertically, resulting in a more uniform battery temperature distribution.

The state of charge (SOC) has a great influence on the performance of the lithium-ion battery, so it is important to accurately evaluate the SOC of the lithium-ion battery.¹⁵ ZC Gao et al. presented an integrated SOC estimation model and active cell balancing method of a battery power system. This strong tracking cubature extended Kalman filter gave a more accurate SOC prediction, and the proposed active cell balancing method exhibited a higher balancing speed and lower balancing loss.¹⁶ J Li et al.¹⁷ combined particle filter with sample entropy feature of discharge voltage and proposed a method of remaining capacity estimation for lithium-ion battery; the results indicate that the algorithm enhances the accuracy. SC Cheng et al.¹⁸ proposed an adaptive online sequential extreme learning machine to predict the SOC of the battery cells at different ambient temperatures; it is found that the SOC prediction has a small root mean square error and reasonable training time in the test. X Li et al.¹⁹ proposed an improved gray prediction model to address low-accuracy prediction issue, the results show that the effect of gray support vector machine model is optimal, and the corresponding root mean square error is only 3.18%, which can predict the remaining life of lithium-ion battery accurately.

In order to improve the ability of rail vehicles to cope with faults, many scholars have studied the control strategies and control methods of rail vehicles under emergency conditions. K Hu et al.²⁰ designed a digital three-phase emergency inverter power supply in order to supply power for the ventilation equipment of the confined compartment when urban rail vehicles power supply fails. F Ciccarelli et al. suggested an energy management control strategy of wayside lithium-ion capacitor–based energy storage for light railway vehicles. The strategy can maximize the recovery of kinetic energy during braking operations of the running vehicles, and the validity was verified.²¹ M Meinert et al. focused on assessing energy storage systems and the design of hybrid system architectures to determine their potential use in specific diesel-driven rail duty cycles. The results show that double-layer capacitors and lithium-ion batteries have the highest potential to be successfully integrated into the system architecture of diesel-driven rail vehicles.²² VI Herrera et al.²³ presented an optimal energy management strategy for a light rail vehicle with an onboard energy storage system combining batteries and supercapacitors to minimize the daily operating cost of the tramway. Z Li et al.²⁴ presented a bilateral direct current (DC)/DC converter rated at 1500 V with a peak power of 500 kW for battery energy storage systems; it can support railway DC feeder systems.

Through the analysis of the above documents, it is found that the current research on the control of vehicles under emergency conditions of rail vehicles is mainly the design of some specific components. However, the lack of an effective overall solution does not fundamentally solve the problem that the vehicle cannot operate when it fails. In order to solve the problem of intermittent stoppage caused by the external power supply of the train, this article designs the lithium-ion battery emergency traction system for rail transit. The main work of this article includes determining the power battery structure adopted in this scheme and establishing a mathematical model, then designing the configuration form of emergency power supply, then determining the traction power supply scheme, and simulating the verification scheme is feasible, and formulating the corresponding control strategy. Finally, combined with the Shanghai 952 train, the reconstruction plan of the traction system is proposed. Compared with the existing train reconstruction scheme, the scheme has the advantages of high power density, flexible configuration, large traction capacity, and small transformation cost. This article can provide some reference for the design and modification of train traction system.

Mechanical system and mathematical model

Mechanical system

The lithium-ion battery emergency power supply for rail transit is made up of a plurality of battery packs connected in series. The smallest component of the battery pack is a cell, a plurality of cells constitutes a module in a certain manner, and a plurality of modules is further assembled into a battery pack.

1. Battery cell

The battery cell is designed with a stainless steel metal casing, and the upper cover plate is laser welded with the battery casing to ensure structural strength. Metal shell adopts integral stretching process design, which effectively ensures structural sealing. At the same time, the upper cover is designed with an explosion-proof valve. When the pressure inside the battery exceeds the design upper limit, pressure will be released to ensure the safety of the battery. In the internal mechanism and material selection design, fully consider the battery safety performance, to ensure that the battery cannot ignite or explode under the extreme conditions of vibration, acupuncture, extrusion, external short circuit, and so on.

2. Battery module

High-voltage busbars are connected between battery modules. The high-voltage busbars are tightly fixed with the cable ties and structural components to prevent busbar damage or connector failure during train operation. The overall structure of the battery module is produced by a full mold. The structural parts of the module are made of engineering plastics, featuring high strength, flame retardant, and recyclable characteristics. The housing of the battery module is composed of a plurality of components, and two stainless steel rings are externally arranged for fastening, which can facilitate the assembly of the module and improve the structural strength of the module. The upper cover of the module is fixed by nuts, which is easy to assemble and maintain. In terms of insulation design, the materials used in the battery module can meet the electrical strength requirements such as insulation. In the electrical connection of the module, the nickel-plated copper jumper is used to form the battery module, so that it has good electrical conductivity without causing pressure damage to the battery pole. Epoxy boards are used between the batteries to isolate the battery, preventing the battery from being abnormal due to a cell failure. The overall structure of the module is shown in Figure 1.

Figure 1.

Battery module: (a) module overall structure, (b) module external dimensions, and (c) module exploded view.

3. Battery pack

The battery pack adopts a fully sealed design, and each of the three modules in the interior is designed to be isolated to prevent the expansion of the influence of a single module. The housing of the battery pack adopts 2 mm cold-rolled steel plate to ensure the mechanical strength of the box. At the same time, three explosion-proof valves are arranged on the side of the box to relieve pressure when the battery pack is abnormal.

Heat generation model of battery

In the process of rail transit emergency power supply, the heat generation of the battery pack is a problem that cannot be ignored. When the battery temperature is too high, battery management system (BMS) control or effective thermal management technology can be used to prevent the battery from overheating to prevent the performance degradation and the risk of thermal runaway.²⁵ The heat generating power per unit volume of a lithium-ion battery is referred to as a heat generating rate.^26–28 The heat generation power of the battery cell can be calculated based on the general model of battery heating established by Bernardi et al.²⁹ As shown in equation (1)

$Φ = (\sum_{i} I_{i} E_{i, avg} - IE) - \sum_{i} I_{i} T \frac{d E_{i, avg}}{dT}$ (1)

where Φ is the heat generating power of the battery cell; I is charge and discharge current; I_i is the ith layer electrode corresponding charging and discharging current; E is the battery cell load voltage; E_i,avg is the open-circuit voltage of the ith layer electrode; T is the battery temperature.

Since I and I_i are equal in size, the above equation can be further simplified as

$Φ = I [(E_{0} - E) - T \frac{d E_{0}}{dT}]$ (2)

where E₀ is the cell open-circuit voltage; E is the cell load voltage.

In addition, (E₀ – E) can be expressed by the product of ohmic resistance and current

$E_{0} - E = I R_{s}$ (3)

$Φ = I^{2} R_{s} - IT \frac{d E_{0}}{dT}$ (4)

where $d E_{0} / dT$ is the coefficient of variation of battery voltage with temperature; it is thought as a constant for a certain type of battery.

Emergency traction system design

Power system design

Power system is the core of the train emergency traction function. If the train relies on its own battery capacity to pull the nearest station to clear the passengers, choosing the right power battery is the key.

Different from the traditional power system, lithium-ion power system contains BMS in addition to battery pack systems.³⁰ For the sake of safety, in addition to the BMS that comes with the power system itself, a safety monitoring unit is also designed. The basic composition of the power system is shown in Figure 2.

Figure 2.

Power system composition.

Design of emergency power system

At present, the emergency power supply system applied to rail vehicles is included emergency traction batteries and backup batteries. The emergency power system has two functions: the first is to provide power to the emergency traction system, and the second is to provide emergency ventilation and lighting power to the train. According to the configuration of the cell, the emergency power supply system currently applied to the rail vehicle mainly has two configurations. The first is the combination of emergency traction power supply and backup power supply. The change of working conditions needs to be realized by electrical conversion. The second is that the emergency traction power supply and the backup power supply are independent of each other, and the change of working conditions does not need to be realized by electrical conversion.

1. Emergency traction power supply combined with backup power supply

When the emergency traction power supply is combined with the backup power supply, in the face of different working conditions, the connection relationship between the 110 V battery cells is different, and it is necessary to set a certain series-parallel conversion device inside the battery pack. The working principle of this battery pack is shown in Figure 3.

Figure 3.

Emergency traction power supply combined with backup power supply.

The battery of this solution uses 60 Ah single cells, and the battery system includes a total of 10 units; each unit voltage is 115 V. (1) In the charging condition, 10 units in the battery system are connected in parallel, and the 124 V charger is used for charging. A high-power diode is added to the charging input of each unit to prevent short-circuiting in series, and a fuse is added to the K1, K3, and K4 switching lines to prevent short circuits due to relay failure. (2) Under emergency conditions, the battery pack system needs to supply power to the emergency load. At this time, K1, K3, and K4 are closed, K2 is disconnected, and 10 115 V units are discharged in parallel. (3) In the emergency traction state, the emergency traction, ventilation, and lighting need to work at the same time. At this time, K1 and K3 are disconnected, K2 and K4 are closed, 9 115 V units are connected in series to supply traction, and one 115 V unit is separately supplied for emergency ventilation and lighting. (4) In the case of emergency ventilation and lighting, 10 115 V units are connected in parallel to output for emergency load. (5) In the floating state, the battery system is in the online floating state. At this time, K1, K2, and K4 are disconnected, K3 is pulled in, and 10 115 V units are charged in parallel.

2. Emergency traction power supply and backup power supply are independent

When the emergency traction power supply and the backup power supply are independent of each other, the working states of the respective power supplies are different in different working conditions. The working principle of this battery pack is shown in Figure 4.

Figure 4.

Emergency traction power supply and backup power supply are independent.

The emergency traction power supply uses 8 Ah single cells, and the battery cells are composed of 3-parallel and 300-series to form a 960 V–24 Ah battery system. The backup power supply uses 50 Ah single cells, and the battery cells are composed of 3-parallel and 36-series to form a 115.2 V–150 Ah battery system. (1) Under normal conditions, the emergency traction power supply and the backup power supply are independent of each other, K2 is disconnected, the emergency traction power supply is in a self-discharge state, and the backup power supply is floated online using a 124 V charger. (2) Under emergency conditions, K2 is disconnected, emergency traction power is discharged, and 110 V power is supplied for emergency load. (3) In the emergency traction state, the K2 pull-in and emergency traction power supply are in a discharge state, providing 900 V power for the traction system and a part of the auxiliary system equipment, and the backup power supply is also in a discharge state, providing 110 V power for the emergency load. (4) After returning to the garage, charge the emergency traction power supply with a special charger as appropriate.

3. Battery configuration of this program

In scheme 1, more relays need to be set up, and the battery is always in a floating state during the running of the train, which has potential safety hazards, and the emergency traction power supply time is short. Scheme 2 uses offline charging, which has higher safety performance and can meet emergency traction requirements. Taking into account the economic and safety factors, it is recommended to use the scheme that emergency traction power supply and backup power supply are independent of each other. The technical indicators of the scheme are shown in Table 1.

Table 1.

Battery pack technical indicators of this scheme.

Technical indicators	Emergency traction: 24 Ah cell (3-parallel and 300-series)	Backup: 150 Ah cell (3-parallel and 36-series)
Cell voltage range	2.0–3.65 V	2.5–3.65 V
Total electricity	46.08 kWh	34.56 kWh
Available electricity at the end of the cycle	33.18 kWh	23.50 kWh
Rated capacity (1C)	24 Ah × 2	150 Ah × 2
Nominal voltage	960 V	115.2 V
Maximum working voltage	1020 V	126 V
Minimum working voltage	840 V	111.6 V
Maximum discharge current (SOC > 10%)	720 A × 2	450 A × 2
Maximum output power	561.6 kW	50.22 kW

SOC: state of charge.

BMS system design

The role of BMS is to monitor the status of all batteries and communicate externally, while actively balancing the differences between the batteries. The main functions include the following:

Protection: over-discharge, over-charge, over-voltage, over-current, over-temperature, and insulation failure.

Performance improvement: SOC estimation and system status monitoring.

Control: discharge, charge, high-voltage power-on, and power-on self-test.

Detection: battery voltage, bus voltage, bus current, insulation resistance, cell temperature, and cell voltage.

For this scheme, the power system consists of two sets of BMS, one for emergency traction power and one for backup power. The BMS for emergency traction power supply consists of battery management unit (BMU), local electronic control unit (LECU), and high-voltage box. The BMS for backup power supply consists of BMU, battery protection device (BPD), and high-voltage box. Each battery module is equipped with a LECU/BPD that collects the voltage and temperature of the cell and performs controller area network (CAN) communication with the BMU. The BMU analyzes the collected data and protects cells according to the control strategy. The BMU communicates with external devices through the CAN bus at the same time and reports the battery pack information to the display screen of the driver’s cab. The high-voltage box is mainly an internal and external interface device of the battery system, including a housing, a high-voltage device, a connecting bus, a communication interface, an electrical interface, and so on.

1. BMS of traction battery system

BMS of traction battery system mainly realizes the power management, state management, and balance management of the battery system. One BMS consists of 30 LECUs, 1 BMU, and 1 high-voltage box. Traction battery pack system is divided into two charging methods; when the capacity of SOC is lower than 80%, the battery pack system will be charged after the train back into the garage. When the traction power supply is not required, the battery pack is independent of the power supply system and remains in a stationary state. When the train is unable to receive power normally, such as pantograph failure, catenary failure, and train entering the no-electric zone, the train enters the emergency traction mode. At this point, the vehicle emergency load device is power supplied by the backup battery system and the traction system is power supplied by the traction battery system. Traction battery system external input and output share a system total positive and total negative, output total positive and total negative busbars are connected from the bellows to the traction system, as shown in Figure 5.

Figure 5.

Traction battery system connection.

2. BMS of backup battery system

When the electricity of backup battery system is not full, the charger charges the backup battery pack system. The BMS collects the cell voltage and the battery module temperature, and performs equalization control on the charging circuit. One BMS consists of nine BPDs, one BMU, and one high-voltage box. When the train is unable to receive power normally, such as pantograph failure, catenary failure, and train entering the no-electric zone, the train enters the emergency traction mode. At this time, the vehicle emergency load device is power supplied by the emergency battery system, and the emergency battery system is in a discharged state. The BPD collects the cell voltage and battery module temperature and passes the data to the BMU for battery status control, protection, monitoring, and alarming. System charging and discharging share a high-voltage circuit, and the external input and output share one system total positive and total negative. The output total positive and negative busbars are connected from the bellows to the auxiliary electrical control box, as shown in Figure 6.

Figure 6.

Backup battery system connection.

Safety monitoring unit design

The urban rail train battery pack safety monitoring system consists of a collecting board, a communication unit, and a monitoring host. The security monitoring unit works in conjunction with the BMS to create redundancy.³¹ It collects the environmental, voltage, and current information of the battery pack through sensors. And the data are transmitted to the communication unit through the acquisition board and transmitted to the monitoring host through the CAN bus for information display and fault warning. The communication unit collects the status information of the BMU of the battery system and transmits it to the GPRS module in the driver’s cab monitoring host, sends it to the WEB server, and saves it into a database. The battery pack can be queried in real time through the network. At the same time, the security monitoring unit has a short message alarm function, which can send the alarm information of the battery pack system to the mobile phone of the relevant person for reminding, and its working principle is shown in Figure 7.

Figure 7.

Working principle of safety monitoring unit.

The collector is installed near the battery pack; its main function is to collect battery temperature and case temperature and collect battery pack smoke information. The sensor for collecting battery temperature and ambient temperature is a pre-embedded thermistor with a range of −40°C to +125°C and an accuracy of ±1.5°C. The point-type photo-electricity smoke sensor is used to collect smoke information. The communication unit is located in the high-voltage box and provides four CAN bus interfaces. CAN1 connects three acquisition boards to complete the collection and summary of the body temperature, case temperature, and smoke of three battery packs. CAN2 connects the BMS of high-voltage box and receives the battery pack condition data sent by the BMS at regular intervals. CAN3 connects the Isabel IVT sensor to measure the total voltage and total current of the battery pack. CAN4 connects the driver’s cab monitoring host. The vehicle monitoring host is installed in the train cab on both sides. The monitoring host communicates with the communication unit through the CAN bus to obtain the battery pack operating condition data and display it. The panel uses an LED screen, and an alarm indicator and an alarm confirmation button (self-resetting) are provided below the screen. The installation mode of the monitoring host and the back interface is shown in Figure 8.

Figure 8.

Display installation method and back interface.

The monitoring host displays the current battery pack voltage, charge and discharge current, maximum temperature, and remaining battery capacity. When the parameter exceeds the limit or the internal fault occurs, the text and icon will appear on the display interface. At the same time, the alarm indicator flashes and the buzzer sounds to remind the driver to pay attention and take measures, as shown in Figure 9.

Figure 9.

Failure corresponding interface.

Emergency traction power supply scheme

Traction power supply scheme

So far, for the conventional A-type train with a total of 6 knots, the power configuration uses a way of 4-moving and 2-traction (4M2T). The traction power supply of the 4 knots is divided into two units; the units are independent of each other, and the traction power supply is not connected to each other. The traction of a knot train is not sufficient to provide traction in emergency traction mode, so the number of connected trains is determined to be 2 knots. In terms of the traction power supply circuit, the traction power supply of the 6-knot train is divided into two independent units. A group of B and C knots are one unit, which are respectively powered by one pantograph. The traction power supply of the two units is not connected to each other. The train’s battery box is located at the A train at both ends. According to the principle of proximity, a group of batteries supplies power to the nearest B train traction system. The emergency traction power supply scheme is applicable to all 6-knot trains, and Figure 10 is a simplified diagram of the traction power supply layout of the 6-knot trains. Since each unit only needs to access one moving train, the moving train access point is easy to be after the high-speed switch of the moving train B. In the emergency traction mode, the high-speed switch is disconnected, so as to achieve the function of isolating the emergency traction main circuit and the pantograph to prevent the pantograph from receiving power in the emergency traction mode.

Figure 10.

Layout diagram of emergency traction train.

Auxiliary power supply scheme

For the 952 train, the traction system equipment is self-ventilated and cooled, and there is no ventilation equipment. The auxiliary reverse system does not need to supply power for ventilating the equipment of the traction system. However, considering the braking requirements in the emergency traction mode, the train air supply system must work normally, so the auxiliary inverter must provide AC 380 V power to drive the air compressor motor. For other kinds of trains, the auxiliary inverter must provide ventilation for the working electrical equipment in addition to the AC 380 V power supply for the air compressor. Figure 11 is a simplified diagram of the power supply arrangement of the auxiliary system in the emergency traction mode. As can be seen from Figure 11, each group of trains is equipped with two sets of compressor units, which are installed in two A trains, which are respectively powered by the auxiliary inverter of the A train. Considering that the brake air cylinder of the train is not supplied by the wind supply unit, it is necessary to ensure that the train completes the five-time full air brake-relief process. Only one auxiliary inverter of the A train needs to work normally to provide AC 380 V. Since the output voltage of the backup power supply is DC 110 V and only supplies power for the emergency load, and the power supply for the air compressor needs to have AC power for the auxiliary system of one train, so the emergency traction power supply needs to supply power for the auxiliary system of one train. Based on the traction power supply mode in the above emergency traction mode, the auxiliary power supply of the train must be additionally equipped with a high-voltage access point. Because the high-speed switch is disconnected in the emergency traction mode, the traction power supply cannot be shared with the auxiliary power supply. In addition, to ensure that the auxiliary power supply enters the pantograph, the auxiliary power supply access point should be behind the diode.

Figure 11.

Layout diagram of the auxiliary working auxiliary inverters in emergency traction mode.

Main circuit access scheme

From the analysis above, for the TC1 traction train, the emergency traction power needs to output power for the traction system and the auxiliary system at the same time; for the TC2 traction train, the emergency traction power only needs to supply power to the traction system, so the circuits from the emergency traction power to both ends of the two systems should be increased; Figure 12 is a schematic diagram of the emergency traction main circuit access. The contactor K2 is normally in the off state. Emergency traction batteries are installed on the train; it is not connected to the traction circuit and auxiliary circuit under normal operating conditions.

Figure 12.

Emergency traction main circuit access schematic.

When the train does not need to carry out emergency battery traction by high voltage, first the driver must stop the train and turn the driver controller main control handle and direction handle to zero and disconnect the relevant load. Then the train reduces the bow, the drive confirms that the train is not in a high-voltage state through the display and then presses the “main break” button to break the four high-speed moving train breakers. Figure 13 shows the battery emergency traction and pantograph interlock circuit.

Figure 13.

Battery emergency traction and pantograph interlock circuit.

A “battery emergency traction” switch is set in the driver’s cab; the switch signal is collected by the network module digital input/output module (DXM) and sent to the traction control unit via the multifunction vehicle bus (MVB). The K2 contactor is controlled by the network. After the network receives the traction state feedback of the B-train traction inverter and the A-train auxiliary inverter battery, high level is outputted to drive K2 to close. The driver switches the “battery emergency traction” switch from the “OFF” position to the “ON” position in the driver’s cab. At this time, K2 is closed, and the traction power is lead into the traction circuit and the auxiliary circuit. The traction control unit and the auxiliary control unit receive the battery emergency traction signal sent by the network, and the traction control unit simultaneously determines whether the network voltage is greater than DC 750 V and whether the high-speed circuit breaker state is disconnected, thereby converting the traction control to the battery power supply mode. The auxiliary control unit determines whether the network voltage is greater than DC 750 V and converts the working mode into an emergency traction battery module. The network feedbacks state through the traction control unit and the auxiliary control unit, and the traction inverter and the auxiliary inverter in the emergency traction battery power supply mode are shown in the monitor. At this time, the two B trains are in the traction preparation state, and one auxiliary inverter works in the whole train. The driver can control the traction and braking by operating the driver controller direction handle and the main control handle; the train speed limit is 20 km/h in this mode. After the train is back to the garage, the driver turns the controller direction handle and the main control handle back to zero and stops the train; after closing the six static inverters (SIVs), the “battery emergency traction” switch can be turned from the “ON” bit to “OFF” bit; at this time K2 is disconnected, the train will drop out the emergency traction battery mode.

Conversion control and safety protection circuit

The circuit conversion contactor is controlled by a cab control switch. When the train is not in a high-voltage state and the train itself does not affect driving and driving safety, if the train needs emergency traction, first the driver must stop the train and turn the driver controller main control handle and direction handle to zero, and disconnect the relevant load (such as air conditioning refrigeration). Then the train reduces the bow, the drive confirms that the train is not in a high-voltage state through the display and then presses the “main break” button to break the four high-speed moving train breakers. A “battery emergency traction” switch is set in the driver’s cab, the switch signal is sent to the traction control unit, the auxiliary inverter control unit, the DC/DC charger and the BMS via the MVB network, the corresponding contactors’ actions are driven through hardwire at the same time. The driver operates the “battery emergency traction” switch from the “normal” bit to “emergency traction” bit in the cab, the contactors are powered, normal open contacts are closed, and the emergency traction main circuit is established. The B train traction control unit receives the battery’s emergency traction signal from the network and determines whether the network voltage is greater than DC 600 V and whether the state of the high-speed circuit breaker and pantograph is off, thereby converts the traction control to the emergency traction operation mode, and the message that B train is in the battery power supply mode will be sent to the monitor through the network. The driver can conduct the traction and brake control by operating the driver controller direction handle and the main control handle. In the emergency traction mode, the train low-voltage power supply busbar is powered by a group of batteries, train emergency lighting and emergency ventilation can provide the lowest comfort protection for the passenger room. After the train brakes to stop and the traction power supply system returns to normal, the controller direction handle and the main control handle will be turned back to zero, the “battery emergency traction” switch will be converted from the “emergency traction” position to “normal” bit, at this point the corresponding contactors will take actions, and the main circuit will return to normal. For existing trains, an emergency traction command can be provided for the BMS by adding K51 and train line contactors. Figure 14 is a schematic diagram of mode switching control.

Figure 14.

Emergency traction mode conversion control schematic of existing trains.

For new trains, the emergency traction commands can be provided for the BMS via the train control and management system (TCMS) MVB network or RS485; the circuit to be added to the circuit schematic diagram is shown in Figure 15.

Figure 15.

Emergency traction mode conversion control schematic of new trains.

Traction system design

Simulation method

1. Basic train parameters

Under the AW3 load, the basic parameters of the train are shown in Table 2.

Table 2.

Basic parameters of the train.

Wheel diameter	840 mm
Gear ratio	6.6875
Train formation	-A-B-C-C-B-A(4M2T)
Train weight	229.655 t
Passenger weight	25.01 t/knot
Train total weight	379.715 t
Train conversion weight	398.987 t
Train starting resistance	18.606 kN
Additional resistance on the maximum slope (i = 35‰)	130.375 kN

Basic resistance formula

$F_{rr} = a + bv + c v^{2}$ (5)

where F_rr is resistance, N; under AW3 load, a is 9987, N; b is 0, N/(km/h); c is 1.334, N/(km/h)².

2. Train dynamic performance requirements

Under the AW3 load, the train can start on 35‰ ramp; on the flat, the train can run at a maximum speed of 20 km/h.

3. Line conditions

(a) 500 m (i = 30‰) + 800 m (i = 0‰) + 200 m (i = 2‰)

(b) 800 m (i = 0‰) + 500 m (i = 30‰) + 200 m (i = 2‰)

(d) 800 m (i = 0‰) + 400 m (i = 35‰) + 200 m (i = 2‰)

4. Other conditions

The capacity of each air compressor is calculated according to 11.82 kVA (power factor is 0.85, auxiliary inverter efficiency is 0.9); one train is calculated by one air compressor; braking resistor fan is not activated in emergency traction power supply mode. The voltage at the power supply terminal is a changing process. In order to make the simulation result closer to the actual situation, the simulation check is performed according to the voltage of 900 and 840 V, respectively.

Simulation results

Table 3 shows the simulation results of the train using the emergency power supply for traction.

Table 3.

Simulation results of the train using the emergency power supply for traction.

Line conditions	Traction voltage (V)	Maximum speed (km/h)	Mileage (m)	Running time (s)	Technical speed (km/h)	DC side equivalent current (A)	Motor equivalent current (A)	Traction energy consumption (kWh)	Auxiliary energy consumption (kWh)	Total energy consumption (kWh)
1	840	15	1500	443.98	12.16	178.7699	111.8599	29.27	1.38	30.65
	840	20	1500	391.28	13.8	194.5458	114.8589	30.09	1.22	31.31
	900	15	1500	430.16	12.55	173.7125	111.7682	29.04	1.33	30.37
	900	20	1500	377	14.32	189.753	114.7308	29.84	1.17	31.01
2	840	15	1500	420.98	12.83	171.2629	105.7417	26.34	1.31	27.65
	840	20	1500	360.06	15	176.8649	107.7882	26.97	1.12	28.09
	900	15	1500	409.27	13.19	166.1216	105.6337	26.15	1.27	27.42
	900	20	1500	347.88	15.52	184.6079	107.1864	26.78	1.08	27.86
3	840	15	1400	434.92	11.59	171.58	120.3897	29.16	1.35	30.51
	840	20	1400	382.64	13.17	199.9495	124.3268	29.9	1.19	31.09
	900	15	1400	421.96	11.94	178.0655	120.0722	28.87	1.31	30.18
	900	20	1400	369.25	13.65	194.4453	124.0045	29.68	1.14	30.82
4	840	15	1400	404.34	12.46	174.4572	111.5919	25.4	1.25	26.65
	840	20	1400	344.33	14.64	192.9302	114.0811	25.99	1.07	27.06
	900	15	1400	393.4	12.81	168.8276	111.1992	25.17	1.22	26.39
	900	20	1400	333.02	15.13	187.4703	113.2292	25.75	1.03	26.78

It can be seen from the simulation results that the power and energy consumption of the 952 train when starting on the 30‰ ramp and the 35‰ ramp are relatively large, and the energy consumption is sufficient to support the train to travel 5000 m on the flat ground.

It can be seen from Table 3 that the emergency traction power source consumes the most energy of 31.31 kWh when the voltage is 840 V and the maximum speed is 20 km/h under the line condition 1. The traction simulation results under this condition are shown in Figure 16(a) shows time–mileage curve, (b) shows time–speed and energy consumption curve, (c) shows time–DC current and motor current curve, and (d) shows time–DC side power curve.

Figure 16.

Curve of different parameters over time when the voltage is 840 V and the maximum speed is 20 km/h under the line condition 1: (a) time–mileage curve; (b) time–speed, energy consumption curve; (c) time–DC current, motor current curve; (d) time–DC side power curve.

Based on the results of comprehensive traction simulation, this scheme can meet the emergency traction power capability requirements of the 952 train.

Traction and braking control strategy

Design of traction enveloping line for battery traction mode

According to the motor speed–torque characteristic, the maximum torque envelope of the traction and braking conditions is designed. The given traction and braking torque cannot exceed the envelope.

In the 840–1020 V network voltage, the traction enveloping line consists of the following two sections

$T = {\begin{matrix} 1460, 0 r / min \leq n \leq 280 r / min, Constant torque stage \\ \frac{1460 \times 280}{n} = \frac{408800}{n}, 280 r / min \leq n \leq 800 r / min, Constant power stage \end{matrix}$ (6)

Calculation of given traction/braking torque under battery traction mode

In the emergency traction mode, according to the given traction level (0–4096) of the vehicle, it is converted into the traction torque that each motor should exert. Under normal operating conditions, the traction torque is calculated as follows

$T = \frac{10 \times M \times a \times D \times 1.5}{1000 \times 2 \times W \times U \times 4} = \frac{MaD}{533.33 WU}$ (7)

where M is the load, 0.01 t; a is the ratio of traction level to 4096; D is the wheel diameter, mm; W is transmission ratio; U is transmission efficiency, 98%.

Generally, the torque provided by the system in the constant torque phase of the envelope is under the condition of half wear, AW3 load, and full level, so it can be used for load compensation, level compensation, and wheel diameter compensation

$T = 1460 \times a \times \frac{D}{805} \times \frac{M}{M_{aw 3}} = \frac{1.81 aDM}{M_{aw 3}}$ (8)

where M is the current load of the train; M_AW3 is the load of the train under AW3.

Battery traction mode excitation setting

After the inverter start command is issued for 1 s; the torque rises from 0 to the target value at the torque change rate, as shown in Figure 17.

Figure 17.

Excitation time setting.

Limits of the traction/electric braking torque under battery traction mode

1. Limits of the traction/electric braking torque from the network

If the net voltage is less than 750–840 V, the torque drops linearly to zero; if the net voltage is higher than 1020–1100 V, the torque drops linearly to 0, as shown in Figure 18.

Figure 18.

Limits of the traction/electric braking torque from the network.

2. Limits of the traction/electric braking torque under other conditions

Any of the following conditions are met, and the traction torque is reduced to 0 according to a given slope: the start command is revoked; the traction command is revoked; the braking command is effective; the train is not in traction conditions (the traction command sent to the motor control unit [MCU] is 0).

Battery traction mode traction and braking conversion

When the traction turns to the braking, it is necessary to judge whether the traction force is greater than 0, the working mode can be converted to the braking condition only when the traction drops to 0 according to the slope; otherwise, it is necessary to reduce the traction force. The purpose of this is to ensure that the traction/braking force changes smoothly and continuously during the entire operation of the train, avoiding sudden changes in force. Figure 19 is a graph showing the torque variation of the traction condition to braking condition.

Figure 19.

Torque changes from traction condition to braking condition.

When the working mode is from the braking condition to the traction condition, as shown in Figure 20.

Figure 20.

Torque changes from braking condition to traction condition.

Protection

1. Battery traction undervoltage and over-voltage protection

The undervoltage protection value is set as 750 V in battery traction mode, it needs to block the pulse and disconnect the line contactor when the voltage is less than 750 V for more than 50 ms, and the recovery threshold is 800 V; the over-voltage protection value is set as 1200 V in battery traction mode; it needs to block the pulse and disconnect the line contactor when the voltage is more than 1200 V for more than 50 ms, and the recovery threshold is 1150 V.

2. Battery traction DC over-current protection and output over-current protection

The DC over-current protection value is set to 450 A in the battery traction mode, and the output current protection value is set to 350 A.

3. Battery traction high-speed circuit breaker control

The high-speed circuit breaker remains disconnected in battery traction mode.

4. Battery traction charging threshold change

In battery traction mode, charging is started when a network voltage is detected more than 750 V.

Rail vehicle self-traction system transformation plan

Different types of trains have different self-traction transformation schemes. The following takes the 952 train as an example to introduce the transformation plan.

Emergency traction power configuration

From the perspective of system security, a battery pack configuration in which the emergency traction power source and the backup power source are independent of each other is adopted.

Emergency traction power supply parameters

The emergency traction power supply is used to provide power for the traction system and the auxiliary system under the emergency traction state of the train. The main technical parameters of the system are shown in Table 4.

Table 4.

Main technical parameters of the emergency traction power supply system.

Technical parameters	24 Ah monomer scheme (3-parallel and 300-series)
Traction electricity	46.08 kWh
Available electricity at the end of the cycle	33.18 kWh
Rated capacity (1C)	24 × 2 Ah
Nominal voltage	960 V
Maximum working voltage	1020 V
Minimum working voltage	840 V
Maximum discharge current	720 × 2 A
SOC working range	10% to 100%
Cycle life	1500 times
Calendar life	10 years
Single cell self-discharge rate	<6%/month
System storage temperature	−30°C to +65°C
System working temperature	−20°C to +65°C
Charging method	Charged by new kind of charger in garage

SOC: state of charge.

The voltage–capacity curve of the emergency power system is shown in Figure 21.

Figure 21.

Voltage–capacity curve of 960 V–24 Ah system.

Backup power supply parameters

The backup power supply is used to provide emergency load power in the emergency traction state of the train and in the emergency state, replacing the existing train nickel–cadmium battery. After the configuration is complete, the main technical parameters of the system are shown in Table 5.

Table 5.

Main technical parameters of the backup power supply system.

Technical parameters	50 Ah monomer scheme (3-parallel and 36-series)
System total electricity	34.56 kWh
Available electricity	29.38 kWh
Available electricity at the end of the cycle	23.50 kWh
Rated capacity	150 × 2 Ah
Nominal voltage	115.2 V
Maximum working voltage	126 V
Minimum working voltage	111.6 V
Floating charge voltage/charge constant voltage value	126 V
SOC working range	10% to 95%
Maximum discharge current	450 × 2 A
Maximum charge current	26 × 2 A
Calendar life	10 years
Single cell self-discharge rate	<6%/month
System storage temperature	−30°C to +70°C
System working temperature	−20°C to +65°C

SOC: state of charge.

Battery pack structure

Figure 22 is a photograph of the original nickel–chromium battery pack system of the 952 train. The battery pack can be pulled out of the box. Since the modification of the existing vehicle requires replacement of the original vehicle battery case, the cabinet is redesigned and the fixing method is unchanged.

Figure 22.

Original nickel–cadmium battery system.

A total of two sets of traction backup battery pack systems are required for each train, which are fixed in the original battery pack box position. Each traction backup battery system consists of two chassis, divided into an A chassis and a B chassis, with different internal layouts. The A chassis contains a backup battery pack and a partial traction battery pack system, and the B chassis contains a portion of the traction battery pack system. The structural design of the traction and backup battery pack system is shown in Figure 23.

Figure 23.

Two battery pack internal structures: (a) internal structure of the chassis A and (b) internal structure of the chassis B.

Auxiliary power system design

The Shanghai 952 train SIV is a static inverter device with a 12-pulse main circuit. The input is DC 1500 V (1000–1800 V). The DC 1500 V is stream inputted by the pantograph and is sent to two parallel IGBT inverters (A5, A6) through the DC Filter Reactor (FL), capacitor charging circuits (V1, KM2, RR, KM1), and DC filter capacitors (FC1, FC2). The control unit (A100) controls the two insulated gate bipolar transistor (IGBT) inverters and sends the pulse width modulation (PWM) output voltages of the two inverter modules staggered at a certain angle to the Dy, Dz three-phase transformers (TR1, TR2), respectively. A 12-step wave close to sinusoidal is obtained by transformer coupling, and finally, a three-phase quasi-sinusoidal voltage with low harmonic content is obtained by three-phase alternating current filter (ACC) filtering. The output voltage is finally output through the electromagnetic immunity (EMI) filter (S1) and the load contactor KMA. The schematic diagram is shown in Figure 24.

Figure 24.

952 train SIV circuit schematic.

The existing auxiliary reverse design input high-voltage 1000 V or more can provide stable 380 V AC power supply, and automatically lock below 1000 V. However, the existing battery power supply solution cannot guarantee the supply of more than 1000 V in the entire emergency traction process, so the auxiliary system of the train needs to be modified.

Add boost chopper circuit

The input voltage of the SIV is increased by increasing the boost chopper circuit. The schematic diagram is shown in Figure 25. If the chopper circuit needs to be added, it is necessary to add the components and control unit. According to the calculation, the existing cabinet cannot be installed, and the existing cabinet of the 952 train cannot be used. Therefore, the use of this program requires the development of a new cabinet, replacing the original SIV cabinet, which has certain limitations in practical use.

Figure 25.

Adding the auxiliary inverse system circuit of the boost chopper circuit.

Add boost chopper cabinet

Increase the input voltage of the SIV by adding a boost chopper cabinet. The schematic is shown in Figure 26. If the chopper cabinet needs to be added, the SIV cabinet of the original 952 train can keep constant. However, it is necessary to add the equipment and wiring of the train body and make more modifications. For this scheme, the power supply input voltage of the SIV can be raised by adding the boost chopper circuit or adding the boost chopper cabinet. The output voltage of the scheme is between 840 and 1020 V. Considering that the load on the vehicle does not require a large power in an emergency situation, the output voltage can be appropriately reduced, and only the software needs to be modified on the original equipment. Therefore, this scheme can use the boost method, but it is not mandatory.

Figure 26.

Boost chopper cabinet circuit principle.

The above-mentioned auxiliary reverse power supply boosting scheme is a general-purpose scheme, which is applicable to various types of vehicles. When implementing existing vehicle modification or new vehicle procurement, it is possible to select whether to adopt the boosting scheme or which boosting scheme is adopted according to the project situation.

Management of auxiliary load

The 952 train has an auxiliary inverter for each train. Under normal conditions, each inverter supplies power to the AC load of the vehicle. When an inverter fails to supply power, the AC power of the adjacent vehicle is connected to the faulty vehicle through the expansion controller. Some kinds of trains do not have an expansion circuit or cannot be removed separately. At this time, the auxiliary load has relatively large energy consumption. It is necessary to recalculate the energy consumption of this part and manage the auxiliary load according to the actual situation. The following is a detailed description of the auxiliary load management using the 952 train as an example; the auxiliary power system box is shown in Figure 27.

Figure 27.

Auxiliary power system box.

Each vehicle is equipped with one SIV, which normally supplies power to the auxiliary AC load of the respective vehicle. In the case of one SIV failure, the extended power supply device automatically expands and supplies power in the adjacent two workshops. When two SIV faults occur in one unit, one vehicle is ventilated by the emergency inverter, and the other vehicle is powered by its adjacent SIV through the expansion circuit. The DC/DC power supply unit is equipped with one unit per unit, that is, one for each A vehicle, which supplies power to the DC load of the unit. In the case of another unit power failure, it can supply power for the entire train DC load. For some reason, when the vehicle cannot provide normal working power for the air conditioning unit, the emergency inverter starts, and the air conditioning unit operates in the emergency ventilation mode. The extended power supply unit is placed in the same cabinet as the emergency inverter of the same vehicle, and the B vehicle is equipped with a separate emergency inverter box.

The scheme is used for accounting. The emergency power supply range is 840–1020 V, and the auxiliary inverter is calculated according to the minimum battery voltage of 840 V, which ensures that the auxiliary inverter can output AC 380 V normally under the condition of 85% rated load.

Due to the variety of auxiliary power systems, the management form of the auxiliary load can also take many forms. For most trains, the removal of the auxiliary load can be achieved by modifying the train control system software. For some cases where the auxiliary system cannot be turned off by software, it can be turned off by the expansion circuit or the driver’s manual operation.

Charging scheme

This solution adopts the form that the emergency traction power supply and the backup power supply are independent of each other. The backup power supply is an energy type battery, and the charging current is smaller than it of emergency power supply, so two types of chargers are used.

For emergency traction power supply, it needs to be considered separately in two cases. When the train is powered by the catenary, the train usually only needs a period of time to carry out the floating charge of the emergency traction battery. Once the emergency traction battery is used, it needs to be charged with high power. Therefore, it is considered to be charged by the ground charger to reduce the safety risk when charging the lithium-ion batteries. The voltage value of the emergency battery should be connected to the train network control system through the sensor for real-time monitoring. When the train adopts three-track power supply, the train emergency traction battery needs to be charged every time the train returns to the warehouse. For safety reasons, it is recommended to install a three-track charging device on each parking line to charge the train.

For the backup power supply, the interface design of the battery charger does not change due to the use of the original charger. The charging characteristics of the battery need to be modified to the constant current and constant voltage charging mode according to the requirements of the emergency battery. The constant voltage value is set to 124 V.

Discussion

With the increase of urbanization and economic development, urban rail transit has become more and more common. However, problems with city power supply or other failures may result in loss of power during train operation, resulting in casualties and property damage. Therefore, it is very important to propose an emergency power system scheme for urban rail transit.

Throughout the research status at domestic and abroad, it is found that the current research under the train emergency conditions mainly focuses on the vehicle control strategy and control methods, such as the design of three-phase emergency inverter power supply, research on energy management control strategy. However, there is still a lack of systematic research on how to solve the problem of stopping the train in the middle.^20–23

In order to solve the problem that the train is forced to stop in the middle, this article proposes a lithium-ion battery emergency traction system for rail transit. The battery configuration of this solution includes emergency traction power supply and backup power supply. And the mechanical structure of the battery module is described and the heat generation model of the battery is established. The detailed design of the power system was proposed, including the design of the BMS and the design of the safety monitoring unit. The power supply scheme of the traction system was studied and the effectiveness of this scheme was verified by simulation. Finally, combined with Shanghai’s 952 train, the existing trains were retrofitted with emergency traction systems, including new battery packs and increased chopper circuits. Compared with other methods, the method proposed in this article has the advantages of high power density, flexible configuration, small transformation range, and so on and has certain innovation and practicability.

Conclusion

Through the comprehensive design of power supply system, emergency traction power supply scheme, conversion control and safety protection circuit, traction system scheme and auxiliary system transformation scheme, the function of lithium-ion batteries emergency traction system can be realized effectively to provide reference for the existing trains and to ensure the safe operation of the train.

Footnotes

Handling Editor: Crinela Pislaru

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This study was funded by National Natural Science Foundation of China (51505196,51705208).

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