Hybrid regenerative braking energy absorption system and method of controlling the same

Abstract

The application discloses a hybrid regenerative braking energy absorption system and a control method thereof. The system has three different branches, namely a branch one feeding back to a 400V power distribution system, a branch two feeding back to a secondary side 1180V loop of a rectifier transformer and a branch three storing regenerative braking energy by using an energy storage unit. The regenerative braking energy is firstly supplied to a low-voltage power and lighting system through the branch one. If the braking energy is greater than the rated capacity of the branch one, the excess energy is fed back to a high-voltage AC power grid through the branch two and the rectifier transformer for a traction substation high-voltage. If the regenerative braking energy is greater than the capacity of the entire feedback part, resulting in continuous increase of the traction grid voltage, the excess energy is stored by the energy storage unit. Any branch in the system can be operated independently, and the required floor area is small, so that the system is convenient to access the already operated urban rail transit traction substation.

Description

Technical Field

[0001] This invention belongs to the field of regenerative braking energy absorption, specifically relating to a hybrid regenerative braking energy absorption system and its control method. Background Technology

[0002] There are currently several ways to utilize regenerative braking energy in urban rail transit, including regenerative braking, energy storage, and resistive consumption. Resistive consumption involves directly consuming braking energy through resistors, which wastes energy and is not economical or environmentally friendly. Regenerative braking typically involves feeding the energy back to AC400V, AC1180V, AC10kV, or AC35kV AC power grids. Energy storage involves storing the energy through mediums such as supercapacitors or flywheels, and then using the stored energy for vehicle traction when the vehicle is in motion.

[0003] The most common application of existing regenerative braking energy utilization systems is to invert the current to AC via a converter and then feed it back to the high-voltage AC grid through an energy feeder transformer, primarily for new line construction. However, with the increasing national awareness of energy conservation and environmental protection, driven by the "dual carbon" target, incorporating more energy conservation and emission reduction into the upgrading of existing urban rail transit lines is a major future trend. Due to the limited space in existing line substations, this invention addresses this issue by constructing a novel isolated energy feeder and energy storage device. On the one hand, the isolated energy feeder eliminates the need for the energy feeder transformer in traditional feedback-type energy feeders; on the other hand, the energy storage device stores the excess energy fed back by the isolated energy feeder. During traction, the stored energy can be released to smooth grid voltage. This solution meets the needs of both new line construction and existing line renovation projects, satisfying the diverse needs of different subway owners. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a hybrid regenerative braking energy absorption system and its control method. The system feeds regenerative braking energy back to a 400V power distribution system or the 1180V side of a rectifier transformer, while simultaneously storing the regenerative braking energy through an energy storage unit. When the vehicle is towed, the stored energy is released to the DC traction network for secondary use by the vehicle.

[0005] A hybrid regenerative braking energy absorption system consists of branch line one, branch line two, and branch line three.

[0006] One end of the branch is connected to the DC traction network, and the other end is connected to the first AC power grid. The DC power from the DC traction network is converted into first isolated DC power through a first isolated DC / DC converter. Then, the first isolated DC power is converted into first AC power through a first DC / AC converter and a first filter inductor in sequence and fed back to the first AC power grid.

[0007] One end of the second branch is connected to the DC traction network, and the other end is connected to the second AC power grid. The DC power from the DC traction network is converted into second isolated DC power through the second isolated DC / DC converter. Then, the second isolated DC power is converted into second AC power through the second DC / AC converter and the second filter inductor in sequence, and then fed back to the second AC power grid through the rectifier transformer.

[0008] One end of the third branch is connected to the DC traction network, and the other end is connected to the energy storage unit. The DC power from the DC traction network charges the energy storage unit through a bidirectional DC / DC chopper.

[0009] Once the voltage of the DC traction network meets the feedback conditions, branch one is first activated to provide energy feedback, feeding regenerative braking energy back to the first AC grid to stabilize the DC traction network voltage at the set threshold value. If the DC traction network voltage continues to rise and exceeds the activation threshold voltage of branch two, branch two is activated to feed regenerative braking energy back to the second AC grid. If the DC traction network voltage continues to rise and the energy storage unit voltage is less than the set value, a bidirectional DC / DC chopper is activated to charge the energy storage unit. If the DC traction network voltage is lower than the energy storage unit discharge threshold value and the energy storage unit meets the discharge conditions, the energy storage unit releases energy to the DC traction network.

[0010] Furthermore, both the first isolated DC / DC converter and the second isolated DC / DC converter use high-frequency isolation transformers for isolation.

[0011] Furthermore, in order to obtain a higher switching frequency and reduce switching losses, the high-frequency isolation transformers all adopt LLC type resonance, which is composed of two inductors and one capacitor.

[0012] Furthermore, both the first and second DC / AC converters include a three-level power module and supporting capacitors, as well as a fan. The switching frequency of the three-level power module can typically reach 3kHz to 4kHz, which can reduce the inductance value of the filter inductor while meeting grid harmonic requirements, thereby reducing its size and weight.

[0013] Furthermore, the first AC power grid is a 400V distribution network, supplying power and lighting systems, while the second AC power grid is an 1180V grid, which is fed back to the high-voltage AC power grid through a rectifier transformer to supply high voltage to the traction substation.

[0014] Furthermore, the energy storage unit is a supercapacitor bank or a flywheel energy storage unit.

[0015] Furthermore, the feedback condition is that the voltage of the DC traction network is greater than the feedback start threshold value.

[0016] Furthermore, the bidirectional DC / DC chopper operates in either Buck mode or Boost mode. In Buck mode, the DC traction network charges the energy storage unit, thereby reducing the voltage of the DC traction network. In Boost mode, the energy storage unit discharges to the DC traction network, thereby increasing the grid voltage of the DC traction network.

[0017] Furthermore, the system's operating states are divided into standby state, running state, and fault state. The standby state refers to the state in which the system is waiting for the braking condition to arrive; the running state is the state in which the system provides feedback or stores energy; the fault state is a state in which the system enters a self-protection state when the monitored parameters exceed the required range, the monitored equipment fails to act as required, or other equipment malfunctions during standby or running.

[0018] This invention also proposes a hybrid regenerative braking energy absorption control method, which is applied to a hybrid regenerative braking energy absorption system. When the system is in operation, it first detects whether the system meets the power supply conditions. If the power supply conditions are met, it enters the feedback process. If the power supply conditions are not met, it detects whether the system meets the discharge conditions. If the system meets the discharge conditions, it enters the discharge process.

[0019] The feedback process is as follows: execute the feedback control program and detect whether the system meets the charging conditions. If the system does not meet the charging conditions, execute the feedback control program until the system meets the feedback stop conditions, and then exit the control. If the system meets the charging conditions, execute the bidirectional DC / DC chopper charging control program to charge the energy storage unit until the system meets the charging stop conditions, and then exit the control.

[0020] The discharge process is as follows: execute the energy storage unit discharge program until the system meets the discharge stop conditions, and then exit control.

[0021] The control method of the system in standby mode is as follows: when the system receives the input command, it checks the status of each switch and whether the current DC traction grid voltage is normal. If it is normal, it enters the pre-charging process until the pre-charging is completed.

[0022] The control method of the system under fault conditions is as follows: when the system detects one or more of the following in standby and running states: external fault signal, exit command, emergency stop signal and system fault signal, the circuit breaker is opened and fault information is sent.

[0023] The power supply conditions are as follows: the DC traction grid voltage is greater than the start-up threshold value of the first DC / AC converter and / or the second DC / AC converter; the charging conditions are as follows: the DC traction grid voltage is greater than the charging threshold value of the bidirectional DC / DC chopper and the voltage across the energy storage unit is less than the rated value; the charging stop conditions are as follows: the DC traction grid voltage is less than a fixed value and the DC traction grid current is less than a set value; the feedback stop conditions are as follows: the DC traction grid current is less than a set value; the discharging conditions are as follows: the DC traction grid voltage is less than the discharge threshold value of the bidirectional DC / DC chopper and the voltage across the energy storage unit is greater than its minimum discharge depth voltage value; the discharging stop conditions are as follows: the voltage across the energy storage unit is less than its minimum discharge depth voltage value and / or the DC traction grid current is less than a set value.

[0024] The beneficial effects of this invention are that any two branches in the system can operate independently, or any one branch can operate alone. Branches one and two are relatively small in size and require less floor space, facilitating connection to existing urban rail transit traction substations. This allows for application in both new line construction and old line renovation, meeting the diverse needs of different subway owners. Furthermore, this invention utilizes an isolated DC / DC converter, incorporating a high-frequency isolation transformer into the isolated DC / DC circuit, thus eliminating the need for a bulky transformer.

[0025] The inverter circuits of branch one and branch two adopt a three-level inverter scheme, which increases the switching frequency of the system and reduces the value of the filter inductor; compared with the two-level inverter topology, all performance indicators of the system are improved.

[0026] Considering the high manufacturing process and difficulty of the high-power high-frequency isolation transformers in Branch Lines 1 and 2, the current capacity of high-frequency isolation transformers is mainly concentrated in the range of several kilowatts to several hundred kilowatts. However, the regenerative braking energy of urban rail transit trains can typically reach the megawatt level. This scheme prioritizes feeding the regenerative braking energy back to Branch Lines 1 and 2. If there is any excess energy, it will be stored using energy storage units. In this way, the regenerative braking energy can be supplied to locomotives, rolling stock, and other loads within the substation in real time. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0028] Figure 1 This is a topology diagram of a hybrid regenerative energy absorption system.

[0029] Figure 2 This is a topology diagram of an LLC structure.

[0030] Figure 3 This is a topology diagram of a three-level power module.

[0031] Figure 4 This is a schematic diagram of a bidirectional DC / DC chopper circuit.

[0032] Figure 5 This is a flywheel energy storage unit topology.

[0033] Figure 6 This is the control flowchart for standby mode.

[0034] Figure 7 This is the control flow diagram in the running state.

[0035] Figure 8 This is the control flowchart under fault conditions. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0037] Figure 1 The diagram shows the topology of a hybrid regenerative braking energy absorption system. The system consists of branch one, branch two, and branch three. One end of branch one is connected to a DC traction network, and the other end is connected to a first AC power grid. The DC power from the DC traction network is converted into first isolated DC power through a first isolated DC / DC converter. Then, the first isolated DC power is converted into first AC power through a first DC / AC converter and a first filter inductor, and fed back to the first AC power grid.

[0038] One end of the second branch is connected to the DC traction network, and the other end is connected to the second AC power grid. The DC power from the DC traction network is converted into second isolated DC power through the second isolated DC / DC converter. Then, the second isolated DC power is converted into second AC power through the second DC / AC converter and the second filter inductor in sequence, and then fed back to the second AC power grid through the rectifier transformer.

[0039] One end of the third branch is connected to the DC traction network, and the other end is connected to the energy storage unit. The DC power from the DC traction network charges the energy storage unit through a bidirectional DC / DC chopper.

[0040] In the system described in this invention, any two branches can operate independently, or any one branch can operate alone. Branches one and two are relatively small in size and require minimal floor space, facilitating connection to existing urban rail transit traction substations. This allows for application in both new line construction and the renovation of existing lines, meeting the diverse needs of different subway owners. Both ends of the first isolated DC / DC converter utilize high-frequency isolation transformers for isolation, achieving voltage conversion and energy transmission while ensuring electrical isolation and safety.

[0041] To achieve higher switching frequencies and reduce switching losses, high-frequency isolation transformers all employ LLC-type resonants. An LLC-type resonant is composed of two inductors and one capacitor, and its topology is as follows: Figure 2 As shown, it possesses the advantages of both series resonance and parallel resonance, and can easily achieve wide-range soft-switching characteristics.

[0042] The first and second DC / AC converters typically employ a three-level scheme, consisting of a three-level power module, supporting capacitors, and a fan. The switching frequency of the three-level power module can usually reach 3kHz to 4kHz, which, while meeting grid harmonic requirements, reduces the inductance value of the filter inductor, thus lowering its size and weight. A topology diagram of the three-level power module is shown below. Figure 3 As shown, it adopts a parallel connection of devices, with the parallel devices sharing a single drive circuit, so that the AC output is completed in parallel within the module.

[0043] Energy storage units can employ supercapacitor banks or flywheel energy storage units. Supercapacitor banks include voltage regulators and supercapacitor banks. The relationship between the voltage regulators, supercapacitor banks, and the bidirectional DC / DC chopper is as follows: the voltage regulators and the bidirectional DC / DC chopper are connected in parallel to a reactor, which is then connected to the supercapacitor. A series branch consisting of a contactor and a resistor is connected in parallel across the supercapacitor to discharge it during equipment maintenance. The bidirectional DC / DC chopper enables bidirectional energy flow, operating in two modes: Buck mode and Boost mode. In Buck mode, energy flows from the high-voltage side to the low-voltage side, and the energy storage device absorbs energy, thus reducing the traction network voltage. Conversely, in Boost mode, energy flows from the low-voltage side to the high-voltage side, and the energy storage device releases energy, increasing the traction network voltage. The topology of the bidirectional DC / DC chopper is shown in Figure 4.

[0044] The topology of an energy storage unit using flywheel energy storage is as follows: Figure 5As shown, the charging and discharging mode of the flywheel energy storage unit is basically the same as that of the supercapacitor energy storage. When the regenerative braking of the train causes the DC traction grid voltage to exceed the starting threshold, the flywheel energy storage unit charges and stores energy. When the train traction causes the DC traction grid voltage to fall below the discharge threshold, the flywheel energy storage unit discharges to smooth the DC traction grid voltage.

[0045] Because the hybrid regenerative braking energy absorption system has three different branches—branch one feeding back to the 400V power distribution system, branch two feeding back to the secondary 1180V circuit of the rectifier transformer, and branch three storing regenerative braking energy using supercapacitors—there is mutual coordination and cooperation between various operating conditions. When the voltage of the DC traction network meets the feedback conditions, branch one is first activated to feed back energy, supplying regenerative braking energy to the first AC grid for use by the low-voltage power and lighting systems, thus stabilizing the DC traction network voltage at the set threshold value. If the DC traction network voltage continues to rise and exceeds the activation threshold voltage of branch two, branch two is activated to feed back regenerative braking energy to the second AC grid for use by the traction substation. If the DC traction network voltage continues to rise and the energy storage unit voltage is lower than the set value, the bidirectional DC / DC chopper is activated to charge the energy storage unit. When the DC / DC chopper's activation value is reached, the switch Q1 turns on, and the anti-parallel diode D2 (Q2) freewheels, charging the energy storage unit. The DC / DC chopper operates in Buck mode, storing excess energy. When the train starts, the DC traction network voltage drops. When it drops to the DC / DC chopper's discharge activation value, the switch Q2 turns on, and the anti-parallel diode D1 (Q1) freewheels, putting the converter in boost mode. The energy storage unit then begins releasing energy to the DC traction network to increase its voltage. Additionally, in certain situations such as maintenance or repairs, when the energy storage unit needs to discharge rapidly, the contactor KM1 closes via the discharge resistor R1.

[0046] The working states of the hybrid regenerative braking energy absorption system are divided into standby state, running state, and fault state. The standby state refers to the state in which the system is waiting for the braking condition to arrive. At this time, after the pre-charging is completed, the corresponding DC switch cabinet and AC switch are closed. The running state refers to the state in which the system regenerates and stores energy. The fault state refers to the state in which the system enters a state to protect itself and other related equipment in the traction substation when the monitored parameters exceed the required range, the monitored equipment does not act as required, or other equipment fails during the standby or running process.

[0047] After the hybrid regenerative braking energy absorption system enters standby mode, when it detects that the DC traction grid voltage is higher than the feedback threshold, it first activates the DC / AC converter pulse signal of branch one to perform energy feedback, feeding regenerative braking energy back to the 400V distribution network to stabilize the DC bus (DC traction grid) voltage at the set threshold. If the DC traction grid voltage continues to rise and exceeds the activation threshold voltage of branch two, branch two starts working, feeding regenerative braking energy back to branch two and then back to the high-voltage AC grid through a rectifier transformer. If the grid voltage continues to rise and the voltage across the supercapacitor is lower than the set value, the buck control signal of the DC / DC chopper is activated to operate in step-down mode, charging the energy storage unit to stabilize the DC traction grid voltage. When it detects that the DC traction grid voltage is lower than the energy storage unit's discharge threshold and the voltage across the energy storage unit is higher than the voltage corresponding to the maximum discharge depth of the energy storage unit, the energy storage unit begins to discharge, smoothing the DC traction grid voltage.

[0048] The corresponding control flow diagrams of the system in each state are as follows: Figure 6 , 7 As shown in Figure 8.

[0049] Figure 6 The control method for the system in standby mode is as follows:

[0050] 1) The system receives the input command, checks the status of each switch and whether the current DC traction network voltage is normal. If it is normal, it executes 2); if it is not normal, it executes 5).

[0051] 2) Close the pre-charging contactor to enter the charging process and determine whether the system is operating normally. If it is normal, proceed to 3); otherwise, proceed to 5.

[0052] 3) Determine if charging is complete. If complete, disconnect the charging contactor, close the DC traction network switch, and then proceed to step 4).

[0053] 4) Determine if the status of each switch is normal. If it is normal, enter standby mode. If it is not normal, proceed to step 5).

[0054] 5) Send system fault signals.

[0055] Figure 7 The control method for the system in the running state is as follows:

[0056] a) Determine if the system meets the feedback conditions. If it does, execute b); otherwise, execute f.

[0057] b) Execute the feedback control procedure and determine whether the system is normal. If it is normal, execute c); if the system has a fault, execute g).

[0058] c) Determine if the system meets the charging conditions. If it does, proceed to d). If it does not, proceed to e).

[0059] d) Execute the charging control program and determine whether the energy storage unit is normal. If it is normal, execute d1); otherwise, execute g).

[0060] d1) Determine if the system meets the charging stop condition. If it does, exit control and execute a).

[0061] e) Determine if the system meets the feedback stop condition. If it does, block the DC / AC converter pulse, exit control, and execute a). If it does not meet the condition, continue the feedback.

[0062] f) Determine if the system meets the discharge conditions. If it does, execute f1); otherwise, execute a).

[0063] f1) Determine if the energy storage unit is in normal condition. If it is, execute f2). If there is a fault, execute g).

[0064] f2) Determine whether the charging stop condition is met. If it is met, exit control and execute a); otherwise, continue charging.

[0065] g) Send a system fault signal.

[0066] Figure 7 In this system, the conditions for determining whether the system is feeding back are: the DC traction grid voltage is greater than the start-up threshold of the first DC / AC converter and / or the second DC / AC converter; the conditions for stopping the feedback are: the DC traction grid current is less than a set value; the conditions for determining whether the system is charging are: the DC traction grid voltage is greater than the charging threshold of the bidirectional DC / DC chopper and the voltage across the energy storage unit is less than the rated value; the conditions for stopping charging are: the DC traction grid voltage is less than a fixed value and the DC traction grid current is less than a set value; the conditions for determining whether the system is discharging are: the DC traction grid voltage is less than the discharge threshold of the bidirectional DC / DC chopper and the voltage across the energy storage unit is greater than its minimum discharge depth voltage; and the conditions for stopping the discharge are: the voltage across the energy storage unit is less than its minimum discharge depth voltage and / or the DC traction grid current is less than a set value.

[0067] Figure 8 The control method for the system under fault conditions is as follows: when one or more of the following are detected: external fault signal, exit command, emergency stop signal and system fault signal, the circuit breakers on the DC traction grid side and AC grid side are disconnected, the fault-oriented strategy is executed, and fault information is sent.

[0068] As described above, these embodiments of the present invention do not exhaustively cover all details, nor do they limit the invention to the specific embodiments described. Many modifications and variations can be made based on the above description. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to effectively utilize the invention and its modifications. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A hybrid regenerative braking energy absorption system, characterized in that, The system consists of branch one, branch two, and branch three. One end of the branch is connected to the DC traction network, and the other end is connected to the first AC power grid. The DC power from the DC traction network is converted into first isolated DC power through a first isolated DC / DC converter. Then, the first isolated DC power is converted into first AC power through a first DC / AC converter and a first filter inductor in sequence and fed back to the first AC power grid. One end of the second branch is connected to the DC traction network, and the other end is connected to the second AC power grid. The DC power from the DC traction network is converted into second isolated DC power through the second isolated DC / DC converter. Then, the second isolated DC power is converted into second AC power through the second DC / AC converter and the second filter inductor in sequence, and then fed back to the second AC power grid through the rectifier transformer. One end of the third branch is connected to the DC traction network, and the other end is connected to the energy storage unit. The DC power from the DC traction network charges the energy storage unit through a bidirectional DC / DC chopper. Once the voltage of the DC traction network meets the feedback conditions, branch one is first activated to provide energy feedback, feeding regenerative braking energy back to the first AC grid to stabilize the DC traction network voltage at the set threshold value. If the DC traction network voltage continues to rise and exceeds the activation threshold voltage of branch two, branch two is activated to feed regenerative braking energy back to the second AC grid. If the DC traction network voltage continues to rise and the energy storage unit voltage is less than the set value, a bidirectional DC / DC chopper is activated to charge the energy storage unit. If the DC traction network voltage is lower than the energy storage unit discharge threshold value and the energy storage unit meets the discharge conditions, the energy storage unit releases energy to the DC traction network.

2. The system according to claim 1, characterized in that, Both the first isolated DC / DC converter and the second isolated DC / DC converter use high-frequency isolation transformers for isolation.

3. The system according to claim 2, characterized in that, The high-frequency isolation transformer adopts LLC type resonance, which is composed of two inductors and one capacitor.

4. The system according to claim 1, characterized in that, Both the first DC / AC converter and the second DC / AC converter include a three-level power module and a supporting capacitor.

5. The system according to claim 1, characterized in that, The first AC power grid is a 400V distribution network, and the second AC power grid is an 1180V power grid.

6. The system according to claim 1, characterized in that, The energy storage unit is a supercapacitor bank or a flywheel energy storage unit.

7. The system according to claim 1, characterized in that, The feedback condition is that the voltage of the DC traction network is greater than the feedback start threshold value.

8. The system according to claim 1, characterized in that, The bidirectional DC / DC chopper operates in either Buck mode or Boost mode. In Buck mode, the DC traction network charges the energy storage unit, thereby reducing the voltage of the DC traction network. In Boost mode, the energy storage unit discharges to the DC traction network, thereby increasing the grid voltage of the DC traction network.

9. The system according to any one of claims 1-8, characterized in that, The system operates in three states: standby, running, and fault. The standby state refers to the state in which the system is waiting for the braking condition to arrive. The running state is the state in which the system is feeding back or storing energy. The fault state is a state in which the system protects itself when the monitored parameters exceed the required range, the monitored equipment fails to act as required, or other equipment malfunctions during standby or operation.

10. A hybrid regenerative braking energy absorption control method, characterized in that, The system applied to any one of claims 1-9, when the system is in operation, first detects whether the system meets the power supply conditions. If the power supply conditions are met, the system enters the feedback process. If the power supply conditions are not met, the system detects whether the system meets the discharge conditions. If the system meets the discharge conditions, the system enters the discharge process. The feedback process is as follows: execute the feedback control program and detect whether the system meets the charging conditions. If the system does not meet the charging conditions, execute the feedback control program until the system meets the feedback stop conditions, and then exit the control. If the system meets the charging conditions, execute the bidirectional DC / DC chopper charging control program to charge the energy storage unit until the system meets the charging stop conditions, and then exit the control. The discharge process is as follows: execute the energy storage unit discharge program until the system meets the discharge stop conditions, and then exit control.

11. The control method according to claim 10, characterized in that, The power supply condition is: the DC traction grid voltage is greater than the start-up threshold value of the first DC / AC converter and / or the second DC / AC converter; the charging condition is: the DC traction grid voltage is greater than the charging threshold value of the bidirectional DC / DC chopper and the voltage across the energy storage unit is less than the rated value; the charging stop condition is: the DC traction grid voltage is less than a fixed value and the current of the DC traction grid is less than a set value; the feedback stop condition is: the current of the DC traction grid is less than a set value; the discharging condition is: the DC traction grid voltage is less than the discharge threshold value of the bidirectional DC / DC chopper and the voltage across the energy storage unit is greater than its minimum discharge depth voltage value. The discharge cessation conditions are: the voltage across the energy storage unit is less than its minimum discharge depth voltage and / or the current of the DC traction network is less than a set value.

12. The control method according to claim 10, characterized in that, The control method of the system in standby mode is as follows: when the system receives the input command, it checks the status of each switch and whether the current DC traction grid voltage is normal. If it is normal, it enters the pre-charging process until the pre-charging is completed. The control method of the system under fault conditions is as follows: when the system detects one or more of the following in standby and running states: external fault signal, exit command, emergency stop signal and system fault signal, the circuit breaker is opened and fault information is sent.

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