A ground type lithium battery and super capacitor hybrid energy storage system for urban rail transit

Abstract

This invention discloses a ground-based hybrid energy storage system of lithium batteries and supercapacitors for urban rail transit, relating to the field of rail transit power supply and energy storage control technology. The system includes a traction substation, a train simulation device, and a hybrid energy storage device. The traction substation forms a DC bus, and the train simulation device simulates the operation of an urban rail train under acceleration, coasting, and deceleration conditions. The hybrid energy storage device includes a lithium battery energy storage unit, a supercapacitor energy storage unit, and corresponding bidirectional DC / DC converters, connected in parallel to the DC bus. The system is controlled based on the DC bus voltage threshold. Through the synergistic effect of the voltage outer loop, filtered power distribution, and current inner loop, it replenishes energy to the bus during train acceleration and absorbs regenerated energy during train braking, thereby suppressing bus voltage fluctuations. The system has a clear structure and well-defined control logic, making it suitable for verification and research of hybrid energy storage systems for urban rail transit in a laboratory environment.

Description

Technical Field

[0001] This invention relates to the field of power supply and energy storage control technology for rail transit, and more specifically, to a hybrid energy storage system of ground-mounted lithium batteries and supercapacitors for urban rail transit. Background Technology

[0002] With the continuous advancement of urbanization, urban rail transit has become an important component of urban public transportation systems due to its advantages such as large capacity, high punctuality, and low energy consumption. In recent years, the scale and operating mileage of urban rail transit lines have continued to grow, and the safety, stability, and energy efficiency of its power supply system have received increasing attention.

[0003] Urban rail transit systems typically use DC traction power supply. During operation, trains frequently experience switching between acceleration, coasting, and deceleration (braking). During acceleration, the traction motor absorbs a large amount of energy from the DC bus, which can easily lead to a drop in bus voltage. During deceleration or braking, the traction motor operates in generator mode, converting mechanical energy into electrical energy and feeding it back to the DC bus, which can easily cause a rise in bus voltage. If these energy fluctuations cannot be effectively regulated, they will not only affect the voltage stability of the traction power supply system but may also cause overvoltage, undervoltage, and other problems, thereby affecting train operation safety and the lifespan of power supply equipment.

[0004] To improve the utilization rate of regenerative braking energy and suppress DC bus voltage fluctuations, existing technologies have proposed configuring ground-based energy storage devices in traction power supply systems. These devices absorb energy during train braking and release energy during train acceleration, thereby achieving energy recovery and voltage stabilization. However, existing ground-based energy storage systems mostly use a single type of energy storage element, such as only supercapacitors or only lithium batteries. Single energy storage structures have significant limitations in practical applications: while supercapacitors have advantages such as high power density, fast charging and discharging speeds, and long cycle life, their energy density is relatively low, making it difficult to meet long-term energy support needs; while lithium batteries have advantages such as high energy density and large energy storage capacity, their power density and transient response capabilities are relatively insufficient, and frequent high-power charging and discharging can easily accelerate battery aging and reduce lifespan.

[0005] To address the aforementioned issues, existing research has attempted to combine supercapacitors with lithium batteries to construct hybrid energy storage systems, aiming to achieve a balance between high power density and high energy density. However, current hybrid energy storage solutions generally suffer from the following shortcomings: First, the system structure and control strategies are mostly engineering application-oriented, making it difficult to verify their operating mechanisms and control effects in a controllable and repeatable manner under laboratory conditions; second, some solutions do not accurately simulate train operating conditions, failing to truly reflect the dynamic changes in DC bus voltage during acceleration, coasting, and deceleration; and third, the description of the collaborative control and power distribution mechanisms among different energy storage units in the hybrid energy storage system is not clear enough, making it difficult to balance bus voltage stability and energy storage unit lifespan.

[0006] Therefore, there is an urgent need for a hybrid energy storage system combining ground-mounted lithium batteries and supercapacitors for urban rail transit to solve these problems. Summary of the Invention

[0007] The purpose of this invention is to solve the technical problems mentioned in the background section and to provide a hybrid energy storage system of lithium batteries and supercapacitors for urban rail transit, comprising:

[0008] Traction substation equipment, train simulation equipment, and hybrid energy storage equipment;

[0009] The traction substation is electrically connected to the train simulation device and the hybrid energy storage device respectively, and is used to form the DC bus of the urban rail transit system.

[0010] The hybrid energy storage device is used to absorb or release energy generated during train operation according to changes in the DC bus voltage, so as to suppress DC bus voltage fluctuations and realize energy recovery and utilization.

[0011] Furthermore, the traction substation device includes a DC source, voltage divider resistors, diodes, and bus capacitors, used to equivalently simulate the DC-side output characteristics of a multi-pulse uncontrolled rectifier circuit, wherein the voltage across the bus capacitor is the DC bus voltage.

[0012] Furthermore, the train simulation device includes a DC brushed motor, a permanent magnet DC brushless motor, and a load mechanism. The DC brushed motor and the permanent magnet DC brushless motor form a motor-to-drive platform, which is used to equivalently simulate the operating state of urban rail trains under acceleration, coasting, and deceleration conditions.

[0013] Furthermore, the hybrid energy storage device includes a lithium battery energy storage unit, a supercapacitor energy storage unit, and a bidirectional DC / DC converter connected to each of them. The high-voltage side of the bidirectional DC / DC converter is connected in parallel to the DC bus, and its low-voltage side is connected to the lithium battery energy storage unit and the supercapacitor energy storage unit respectively, so as to realize bidirectional energy flow.

[0014] Furthermore, the lithium battery energy storage unit includes a lithium battery pack and a battery management system (BMS) connected thereto, used to monitor and manage the charging and discharging status of the lithium battery pack.

[0015] Furthermore, the hybrid energy storage device also includes a control system, which adopts a threshold control method based on DC bus voltage, including a voltage outer loop, a filtered power distribution module, and a current inner loop, for generating charging and discharging control signals for the lithium battery energy storage unit and the supercapacitor energy storage unit.

[0016] Furthermore, when the DC bus voltage is detected to be lower than a preset discharge threshold, the control system controls the lithium battery energy storage unit and the supercapacitor energy storage unit to discharge to the DC bus through the corresponding bidirectional DC / DC converter, so as to replenish the bus energy and prevent the bus voltage from continuing to drop.

[0017] Furthermore, when the DC bus voltage is detected to be higher than a preset charging threshold, the control system controls the DC bus to charge the lithium battery energy storage unit and the supercapacitor energy storage unit through the bidirectional DC / DC converter in order to absorb the energy generated during the train's regenerative braking process.

[0018] Furthermore, when the DC bus voltage is higher than the preset maximum bus voltage or lower than the preset minimum bus voltage, the control system controls the hybrid energy storage device to disconnect from the DC bus, causing the hybrid energy storage device to enter a prohibited charging and discharging mode, thereby achieving system protection.

[0019] Furthermore, when the DC bus voltage is between the discharge threshold and the charging threshold, the control system controls the hybrid energy storage device to be in standby mode, and neither the lithium battery energy storage unit nor the supercapacitor energy storage unit participates in energy exchange.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The urban rail transit ground-based lithium battery and supercapacitor hybrid energy storage system provided by this invention, by setting up a ground-based hybrid energy storage device composed of lithium battery energy storage units and supercapacitor energy storage units in the traction power supply system, and using a bidirectional DC / DC converter to connect the two types of energy storage units in parallel to the DC bus, enables the system to simultaneously possess high power density and high energy density energy storage capabilities. Compared with existing ground-based energy storage systems that only use a single energy storage element, this invention can more effectively adapt to the rapid energy changes caused by frequent starts and stops of urban rail trains, achieving rapid energy release and absorption during train acceleration and braking, thereby significantly enhancing the adaptability of the DC traction power supply system to power fluctuations.

[0022] 2. This invention constructs a threshold control mechanism based on DC bus voltage, and combines it with a control structure of voltage outer loop, filtered power distribution, and current inner loop. This enables the hybrid energy storage system to automatically switch between discharge mode, charging mode, standby mode, and prohibited charging / discharging mode according to changes in bus voltage. When train acceleration causes a drop in bus voltage, the hybrid energy storage system can promptly replenish energy to the bus to prevent excessive voltage drop. When train deceleration or braking causes a rise in bus voltage, the hybrid energy storage system can actively absorb regenerative braking energy to suppress the rise in bus voltage. Through the above methods, this invention can stably control the DC bus voltage within a preset threshold range, improving the operational stability and power supply reliability of the traction power supply system under multi-condition switching conditions.

[0023] 3. This invention achieves collaborative operation between lithium battery energy storage units and supercapacitor energy storage units through a filtered power allocation method. This allows the supercapacitor to prioritize high-power response tasks corresponding to rapid fluctuations in bus voltage, while the lithium battery primarily undertakes relatively gentle energy support tasks. This effectively reduces the frequent high-current surges experienced by the lithium battery during operation, thus slowing down performance degradation and extending its lifespan. Furthermore, this invention employs an experimental platform structure combining a train simulation device and a traction substation device. This allows for realistic equivalent simulations of typical urban rail train operating conditions in a laboratory environment, providing a reliable basis for verifying control strategies and engineering applications of hybrid energy storage systems. It possesses significant practical value and potential for widespread adoption. Attached Figure Description

[0024] Figure 1 This is the system overview diagram of the present invention;

[0025] Figure 2 This is the main circuit diagram of the present invention;

[0026] Figure 3 This is the pattern determination diagram of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the following description is provided in conjunction with embodiments and appendices. Figures 1-3 The present invention will be further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0028] Example 1: As Figure 1As shown, this invention provides a ground-mounted hybrid energy storage system for urban rail transit, combining lithium batteries and supercapacitors. The system includes a traction substation 1, a train simulation device 2, and a hybrid energy storage device 3. The traction substation 1, train simulation device 2, and hybrid energy storage device 3 are connected in parallel to a DC bus to construct and maintain the DC bus voltage for urban rail traction power supply. The train simulation device 2 is used to simulate the absorption and feedback of bus energy by urban rail trains under acceleration, coasting, deceleration / braking conditions, thereby causing a decrease or increase in bus voltage. The hybrid energy storage device 3 is used for charge and discharge control based on bus voltage changes. It replenishes energy to the bus when train acceleration causes a decrease in bus voltage and absorbs energy when train braking causes an increase in bus voltage, thus suppressing bus voltage fluctuations and achieving regenerative braking energy recovery.

[0029] like Figure 2 As shown, the traction substation device 1 mainly consists of a DC source 6, voltage divider resistors 7, diodes 8, and bus capacitors 9, used to equivalently simulate the DC-side output characteristics of a multi-pulse uncontrolled rectifier circuit. The voltage across the bus capacitor 9 is the real-time DC bus voltage. (Unit: V) It serves as the input to the control system to determine the voltage state of the entire system.

[0030] The train simulation device 2 mainly consists of a DC active motor, a permanent magnet DC brushless motor 4, and a load 5 forming a motor-driven platform. In specific implementation, a controller (such as an STM32F407) is used to adjust the motor speed / torque commands, thereby equivalently forming train acceleration (absorbing bus energy, descent), coasting (less energy exchange), Approaching no-load), deceleration / braking (energy feedback to the busbar, Three typical operating conditions (rising)

[0031] The hybrid energy storage device 3 consists of a supercapacitor energy storage unit 11, a lithium battery energy storage unit 12, and two bidirectional DC / DC converters 10. The high-voltage sides of the two bidirectional DC / DC converters 10 are connected in parallel to the DC bus, while their low-voltage sides are connected to the supercapacitor energy storage unit 11 and the lithium battery energy storage unit 12 respectively, enabling bidirectional energy flow. The lithium battery energy storage unit 12 includes a lithium battery pack and a battery management system (BMS). The BMS collects data on battery terminal voltage, current, temperature, and SOC, and provides permissible charging and discharging conditions to ensure the battery operates within a safe range. The supercapacitor energy storage unit 11 provides high-power, fast response to support rapid charging and discharging when the bus voltage fluctuates rapidly.

[0032] Combination Figure 1 The control structure shown in this invention employs a control method of "bus voltage threshold criterion + voltage outer loop / current inner loop dual closed loop + filter power allocation". First, the real-time bus voltage is acquired. (Unit: V), and set the discharge threshold. (Unit: V), charging threshold (Unit: V) Maximum allowable voltage of busbar (Unit: V) and minimum allowable voltage of the busbar (Unit: V). Among them... Used to determine the boundary where a bus voltage drop requires discharge to replenish energy; Used to determine the boundary of energy absorption required for bus voltage rise; , Used to determine busbar abnormalities and trigger protection to prohibit charging and discharging.

[0033] Based on the above thresholds, the operating mode of this system is switched according to the following criteria:

[0034] when Enter discharge mode at this time;

[0035] when It enters charging mode at that time;

[0036] when Enter standby mode when; or It enters the mode that prohibits charging and discharging.

[0037] In the no-charge / discharge mode, the control system puts the two bidirectional DC / DC converters in the off state (PWM output is disabled or the duty cycle is set to zero), so that they no longer exchange energy with the bus side; in engineering implementation, electrical isolation can also be achieved in conjunction with the bus side contactor / circuit breaker (either one can be used), thereby protecting the hybrid energy storage device and the DC bus.

[0038] In discharge or charging mode, the control system performs an outer voltage loop calculation of the bus voltage deviation and generates a total current setpoint, specifically by setting the target bus voltage. (Unit: V, can be the no-load voltage or the desired stable value), calculate the voltage deviation.

[0039] ;

[0040] in Voltage deviation (unit: V). This is the reference value for bus voltage (unit: V). Real-time bus voltage (unit: V).

[0041] The voltage outer loop uses PI regulation to obtain the total current command on the bus side. (Unit: A):

[0042] ;

[0043] in The total current on the busbar side is given (unit: A). This is the voltage outer loop proportionality coefficient (unit: A / V). The voltage outer loop integral coefficient (unit: A / (Vs)). The integral time variable (unit: s).

[0044] In discharge mode, This corresponds to the demand for injecting current into the bus; in charging mode... To meet the requirement of absorbing current from the bus (consistency can be achieved through symbol conventions or current direction definitions).

[0045] Then on The signal is filtered and then fed into the power / current distribution module to achieve the distribution goal of "supercapacitors prioritizing rapidly changing components and lithium batteries handling relatively smoother components." This is in line with... Figure 1 As shown in the "Filter Power Allocation" example, this embodiment uses a first-order low-pass filter to obtain the battery-side current reference.

[0046] ;

[0047] in The current of the lithium battery branch is given (unit: A). The filtering time constant (in seconds) is used to characterize the extent to which the battery handles low-frequency / gradual components.

[0048] The supercapacitor branch current is given by the difference between the total current given and the battery current given:

[0049]

[0050] in The current of the supercapacitor branch is given (unit: A). Through the above allocation, the supercapacitor is used to quickly respond to transient fluctuations in the bus voltage, while the lithium battery is used to undertake relatively smooth energy exchange, thus balancing high power density and high energy density and helping to extend battery life.

[0051] In the inner current loop, the actual current of the supercapacitor branch is collected respectively. (Unit: A) and actual current of lithium battery branch (Unit: A), and this results in a current deviation:

[0052] ;

[0053] in , These represent the current deviations (in A) between the supercapacitor branch and the lithium battery branch. The inner loop of each branch's current is PI-regulated to obtain the converter control input (which can correspond to the duty cycle or equivalent modulation signal).

[0054] ;

[0055] ;

[0056] in , Modulation / duty cycle commands for two bidirectional DC / DC converters (dimensionless or defined by implementation). This is the proportionality coefficient for the inner current loop (unit: dimensionless / A or implementation-dependent). The integral coefficient of the inner current loop (unit: 1 / s or implementation-dependent).

[0057] The control system will , The signal is fed into the PWM module to generate a drive signal, which controls the switching devices of the bidirectional Buck / Boost converter to achieve precise tracking of the charging and discharging currents of the two energy storage branches. To ensure safety, the control system can superimpose amplitude limiting constraints when generating the current setpoint and duty cycle command, for example... ,in , These are the maximum allowable currents (in A) for batteries and supercapacitors, respectively. This limit can be determined by the BMS allowable value or the device rating.

[0058] like Figure 3 As shown, according to the above control process, when the train simulation device is in acceleration mode, the bus voltage... Decline, if The system enters discharge mode, and the hybrid energy storage device injects current into the bus to compensate for energy and suppress voltage drops.

[0059] When the train simulation device is in deceleration / braking mode, the bus voltage Rise, if The system enters charging mode, where the hybrid energy storage device absorbs feedback energy and suppresses voltage rise.

[0060] when When the system is in standby mode, it does not participate in energy exchange; when or The system enters a no-charge / discharge mode and shuts off the DC / DC converter to protect the system.

[0061] Example 2: In this example, the same urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system as in Example 1 is used. The overall system structure is as follows: Figure 1 and Figure 2As shown, it mainly includes a traction substation device 1, a train simulation device 2, and a hybrid energy storage device 3. The traction substation device 1 is electrically connected to the train simulation device 2 and the hybrid energy storage device 3, respectively, to form the DC bus of the urban rail transit system.

[0062] like Figure 2 As shown, the traction substation device 1 mainly consists of a DC source 6, voltage divider resistors 7, diodes 8, and capacitors 9. The voltage across capacitor 9 is the DC bus voltage, used to equivalently simulate the DC-side output characteristics of a 24-pulse uncontrolled rectifier circuit. The train simulation device 2 mainly consists of a DC brushed motor, a permanent magnet DC brushless motor 4, and a load 5. It uses a motor-driven drag system to equivalently simulate the acceleration, coasting, and deceleration conditions of an urban rail train during operation.

[0063] The hybrid energy storage device 3 includes a supercapacitor energy storage unit 11, a lithium battery energy storage unit 12, and two bidirectional DC / DC converters 10 connected to each unit. The high-voltage sides of the two biphase DC / DC converters 10 are connected in parallel to the DC bus formed by the traction substation unit 1, while their low-voltage sides are connected to the supercapacitor energy storage unit 11 and the lithium battery energy storage unit 12, respectively, thereby achieving bidirectional energy flow between the hybrid energy storage system and the DC bus. The lithium battery energy storage unit 12 includes a lithium battery pack and a connected BMS (Battery Management System) for managing the operating status of the lithium battery.

[0064] This embodiment focuses on verifying the operation of the hybrid energy storage system under frequent start-stop conditions of urban rail trains. At the start of the experiment, the traction substation device 1 supplies power to the DC bus, the train simulation device 2 is in an initial static state, the DC bus voltage is maintained at the no-load voltage level by capacitor 9, the hybrid energy storage device 3 is in standby mode, and the bidirectional DC / DC converter 10 does not participate in energy exchange.

[0065] Subsequently, the controller applies an acceleration command to the train simulation device 2, causing the motor-driven platform, composed of a brushed DC motor and a permanent magnet brushless DC motor 4, to enter acceleration mode. The train simulation device 2 absorbs energy from the DC bus, causing the DC bus voltage, represented by capacitor 9, to gradually decrease. When the control system detects that the DC bus voltage has dropped to a preset discharge threshold, the control core (such as...) Figure 1 The voltage outer loop, filter distribution and current inner loop modules 4, 5 and 6 marked in the figure issue control commands to drive the bidirectional DC / DC converter 10 to work, so that the supercapacitor energy storage unit 11 and the lithium battery energy storage unit 12 release energy to the DC bus, thereby supporting the bus voltage and preventing the bus voltage from dropping further.

[0066] After the train simulation device 2 completes the acceleration process and enters a short-term coasting condition, its energy demand on the DC bus decreases, and the DC bus voltage gradually recovers and stabilizes between the discharge threshold and the charging threshold. At this time, the control system determines that the bus voltage is in the normal stable range, controls the hybrid energy storage device 3 to enter standby mode, the bidirectional DC / DC converter 10 stops modulation, and neither the supercapacitor energy storage unit 11 nor the lithium battery energy storage unit 12 participates in energy exchange.

[0067] Subsequently, the control system sends a deceleration command to the train simulation device 2, causing the motor to engage in braking mode. The train simulation device 2 feeds energy back to the DC bus, causing the DC bus voltage to gradually increase. When the bus voltage rises to a preset charging threshold, the control system controls the bidirectional DC / DC converter 10 to enter a charging state, allowing the DC bus to replenish energy to the supercapacitor energy storage unit 11 and the lithium battery energy storage unit 12 via the bidirectional DC / DC converter 10. This absorbs the regenerative energy generated during train braking and prevents the bus voltage from continuing to rise.

[0068] During the repeated start-stop operation, if the DC bus voltage is detected to rise abnormally to the maximum value or drop abnormally to the minimum value, the control system will immediately control the bidirectional DC / DC converter 10 to shut down, disconnecting the hybrid energy storage device 3 from the DC bus. The system will then enter a no-charge / discharge mode to protect the hybrid energy storage device 3 and the traction substation device 1.

[0069] As can be seen from the above embodiments, in the typical operating scenario of frequent start-stop of urban rail trains, the urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system provided by the present invention can automatically switch between discharge mode, charging mode, standby mode and prohibited charging and discharging mode according to the change of DC bus voltage, effectively suppress bus voltage fluctuations, realize the recovery and utilization of regenerative braking energy, thereby verifying the stability and feasibility of the system under complex operating conditions.

[0070] Through the above methods, the present invention can stabilize the bus voltage within the threshold range, while simultaneously realizing the recovery and reuse of regenerative braking energy, reducing the pressure on traction substations and improving system economy and energy storage life. The present invention is not limited to the above embodiments. Equivalent substitutions or modifications made by those skilled in the art without departing from the spirit of the present invention should fall within the protection scope of the present invention.

[0071] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the present invention is not limited to these embodiments. Equivalent modifications made by those skilled in the art without departing from the principles of the present invention should fall within the protection scope of the present invention.

[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A hybrid energy storage system of lithium battery and supercapacitor for urban rail transit, characterized in that, include: Traction substation equipment, train simulation equipment, and hybrid energy storage equipment; The traction substation is electrically connected to the train simulation device and the hybrid energy storage device respectively, and is used to form the DC bus of the urban rail transit system. The hybrid energy storage device is used to absorb or release energy generated during train operation according to changes in the DC bus voltage, so as to suppress DC bus voltage fluctuations and realize energy recovery and utilization.

2. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 1, characterized in that: The traction substation device includes a DC source, voltage divider resistors, diodes, and bus capacitors, which are used to equivalently simulate the DC-side output characteristics of a multi-pulse uncontrolled rectifier circuit, wherein the voltage across the bus capacitor is the DC bus voltage.

3. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 1, characterized in that: The train simulation device includes a DC brushed motor, a permanent magnet DC brushless motor, and a load mechanism. The DC brushed motor and the permanent magnet DC brushless motor form a motor-to-drive platform, which is used to equivalently simulate the running state of urban rail trains under acceleration, coasting, and deceleration conditions.

4. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 1, characterized in that: The hybrid energy storage device includes a lithium battery energy storage unit, a supercapacitor energy storage unit, and a bidirectional DC / DC converter connected to each of them. The high-voltage side of the bidirectional DC / DC converter is connected in parallel to the DC bus, and its low-voltage side is connected to the lithium battery energy storage unit and the supercapacitor energy storage unit respectively, so as to realize bidirectional energy flow.

5. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 4, characterized in that: The lithium battery energy storage unit includes a lithium battery pack and a battery management system (BMS) connected thereto, which is used to monitor and manage the charging and discharging status of the lithium battery pack.

6. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 1, characterized in that: The hybrid energy storage device also includes a control system, which adopts a threshold control method based on DC bus voltage and includes a voltage outer loop, a filtered power distribution module, and a current inner loop, used to generate charging and discharging control signals for the lithium battery energy storage unit and the supercapacitor energy storage unit.

7. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 6, characterized in that: When the DC bus voltage is detected to be lower than the preset discharge threshold, the control system controls the lithium battery energy storage unit and the supercapacitor energy storage unit to discharge to the DC bus through the corresponding bidirectional DC / DC converter in order to replenish the bus energy and prevent the bus voltage from continuing to drop.

8. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 6, characterized in that: When the DC bus voltage is detected to be higher than the preset charging threshold, the control system controls the DC bus to charge the lithium battery energy storage unit and the supercapacitor energy storage unit through the bidirectional DC / DC converter in order to absorb the energy generated during the train's regenerative braking process.

9. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 6, characterized in that: When the DC bus voltage is higher than the preset maximum bus voltage or lower than the preset minimum bus voltage, the control system controls the hybrid energy storage device to disconnect from the DC bus, so that the hybrid energy storage device enters the prohibited charging and discharging mode to achieve system protection.

10. The urban rail transit ground-mounted lithium battery and supercapacitor hybrid energy storage system according to claim 6, characterized in that: When the DC bus voltage is between the discharge threshold and the charging threshold, the control system controls the hybrid energy storage device to be in standby mode, and neither the lithium battery energy storage unit nor the supercapacitor energy storage unit participates in energy exchange.

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