Abstract:
An energy storage apparatus has developed for railway vehicles by adopting a bidirectional DC-DC converter to increase the efficiency of charge/discharge, comprising that; a power receiving unit, filter unit, charging unit storage unit having a plurality of super-capacitors, capacitor monitoring unit, a plurality of bidirectional DC-DC converters arranged in parallel, and voltage detector electrically connected to the filter unit, current detector for detecting the currents flowing. The controller further comprises; an analog interface board, signal identifying board, signal control board, digital output contact unit, communicating board, PWM control board, optical output board, external gate driver. The PWM control board includes; a sensor input circuit, A/D converter, calculation unit, calculation control processor, and power monitoring unit. It is easy to add a storage unit including supercapacitor in a form of box module for increasing capacity and efficient of charging/discharging of the storage unit through various sensors and signal checking.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an energy storage apparatus for railway vehicles by adopting a bidirectional DC-DC converter. More particularly, the energy storage device for railway vehicle is adopting the bidirectional DC-DC converter, which has increased the efficiencies of the charge/discharge. The design of controller is associated the economical operation to increase the efficiency and convenience through the innovation of the function and configuration, so that the capacity of the energy storage is increased. 
     2. Related Prior Art 
     The worldwide industrialization has rapidly increased the oil consumption. As a result, our environment has changed. Due to increase the pollutants in our environment, the world&#39;s climate has rapidly warmed. To make the matter worse, the natural resources, such as the fossil fuel energy in the world is becoming depleted. Thus, the depletion of the natural resources has caused to raise the oil price steeply. Because the escalation of oil consumption causes to increase the carbon-dioxide in our atmosphere, the Kyoto protocol was established to prevent the global warming. As an urgent project, the research has performed for recycling the energy to reduce the emission of carbon monoxide. 
     Under such circumstances, the natural energy, such as a wind, tide, solar, or water energy has investigated for utilizing in our life. The new technology has developed to utilize the natural energy for applying in our life. However, the new technology has intensively developed to increase the efficiency. The conventional energy generation device and the storage systems have innovated to improve the certain problems of energy loss for minimizing the loss rate. 
     In a modern subway, some of trains employ a regeneration brake system to save energy. When the subway train is decelerated, the regeneration brake system converts kinetic energy to the electrical energy and accumulates the converted electric energy. The regeneration brake system is not only reducing the power consumption of the entire system, but also preventing the noise generation by the frictions of brake and wearing of the brake shoes. Regeneration brake system has increasingly applied to the modern train because of such an advantage. 
     However, the conventional regeneration brake system has a problem of the efficiency of the charging/discharging through the energy storage components. Because the controller of the conventional energy storage apparatus is limited in the function, the conventional controller has merely performed to check the voltage or current, while it is monitoring supercapacitors as the major energy storage components without monitoring the operation of the device. 
     The regeneration brake system of the conventional energy storage apparatus has a problem of the DC/DC converter and a storage medium. They should be developed for increasing the capacity and efficiency of the energy storage apparatus. For example, when the demand is increased, the capacity of the energy storage apparatus associated with the DC/DC converter and the storage medium from 750V and 10 MJ to 750V and 15 MJ, it is not possible to produce the energy storage apparatus with capacity of 750V and 15 MJ by simply modifying the existing regeneration power storage system with capacity of 750V and 10 MJ. Thus, it is required research to develop and manufacture a new regeneration power storage system having 750V and 15 MJ. 
     When a 750V, 20 MJ storage medium is needed, a recently developed technology connects a plurality of 750V, 10 MJ storage media to achieve the 750V, 20 MJ storage medium, thereby increasing use efficiency of existing components. 
     However, an energy storage apparatus which employs such technology needs to include a plurality of units for almost all components (for example, charging units, filter units, current and voltage detectors, and controllers, DC/DC converters, and storage media) and also needs to include an additional current balancing controller to achieve current balancing. Therefore, the energy storage apparatus has problems in that it is necessary to include an additional component such as the current balancing controller without advantages such as a small number of parts and a small volume. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an energy storage apparatus for urban railways using a bidirectional DC-DC converter, which increases charge and discharge efficiency through design of a controller taking into consideration operation of associated devices and increases economic efficiency and use convenience through change or addition of a minimal configuration for energy storage capacity variation or increase. 
     In accordance with the present invention, the above and other objects can be accomplished by the provision of an energy storage apparatus for urban railways using a bidirectional DC-DC converter, the energy storage apparatus including a power receiving portion that is electrically connected to an overhead line to receive power and that includes a disconnector therein, a filter unit that is electrically connected to the overhead line to remove high frequency components, a charging unit that is electrically connected between the overhead line and the filter unit to prevent excess current from flowing into the filter unit during charging of a capacitor of the filter unit, the charging unit including a block switch that opens and closes the electrical connection between the charging unit and the overhead line, at least one storage unit including a plurality of supercapacitors to store electrical energy, at least one capacitor monitoring unit that monitors measurements of capacity, input/output current, and power of the storage unit and outputs a monitoring result to a controller, a plurality of bidirectional DC-DC converters provided in parallel, the bidirectional DC-DC converters being electrically connected between the filter unit and the storage unit and being controlled by turning on or off first to sixth transistors according to a control signal, a voltage detector electrically connected to the filter unit to detect voltage of the overhead line, a current detector that detects the amount of current flowing into the filter unit and outputs the same to the controller, and the controller that receives the current and voltage of the overhead line detected by the current detector and the voltage detector and outputs an on/off control signal to each of the first to sixth transistors of the bidirectional DC-DC converters. 
     Preferably, the number of the at least one capacitor monitoring unit is equal to the number of the bidirectional DC-DC converters. 
     Preferably, the number of the at least one storage unit is equal to the number of the bidirectional DC-DC converters. 
     Preferably, the controller includes an analog interface board that receives a detected DC link voltage and supercapacitor voltage in real time from a PT unit and receives a DC link current and a supercapacitor charging and discharging current from a CT unit, a signal identification board that is provided in parallel with the analog interface board and identifies a feedback signal of a contactor, a fuse contact, a temperature contact, and a switch contact of a front panel and performs drive sequence and guard operations of the controller, a signal control board that receives a signal from the analog interface board and the signal identification board and performs sequence and guard operations and communication operations of the apparatus, a digital output contact portion that receives a signal from the signal control board and outputs a contact signal, a communication board that receives a signal from the signal control board and performs communication with a GP monitor and also performs communication with a supercapacitor monitoring unit, a PWM control board that receives a signal from each of the analog interface board and the signal control board and provides a PWM pulse signal, an optical output board that receives the PWM pulse signal from the PWM control board and converts the PWM pulse signal into a PWM optical signal, and an external gate driver that receives the PWM optical signal from the optical output board and generates an external gate drive signal. 
     Preferably, the PWM control board includes a sensor input circuit that receives an analog signal, an A/D converter that converts an output signal from the sensor input circuit into a digital signal, a calculation unit that receives, as inputs, a reference malfunction value and an output signal of the sensor input circuit and outputs a malfunction detection signal and an interrupt request signal, a calculation control processor that receives an output signal of the calculation unit, creates a charge and discharge current instruction, generates a pulse pattern of a bidirectional chopper, and controls a duty ratio of the chopper, and a power monitoring unit that monitors power state. 
     Preferably, the signal control board performs communication with another board, a main circuit, and a TMS unit and stores general drive information and a malfunction record and also stores state information of each part in an NVSRAM when the energy storage apparatus malfunctions. 
     Preferably, the signal identification board includes an input circuit that receives a 100V input signal, a surge blocking unit that is connected to an output terminal of the input circuit to block surge in the input signal, a filter circuit that filters an output signal of the surge blocking unit, and an insulation circuit that insulates the signal identification board from external signals. 
     Preferably, the digital output contact portion includes an insulation circuit for separating output signals of a motherboard from external signals, and a relay driver for driving a contactor relay. 
     Preferably, the optical output board includes an optical signal converter that receives 12-channel gate PWM signals from the PWM control board and converts the same into optical signals to drive the gate driver to turn a corresponding IGBT on or off. 
     Preferably, the controller sequentially activates GCK, MCB, and HSCB units when 220V AC power is applied to the controller and each contactor is in normal state, turns a CHK unit on to enter an initial charging mode of the supercapacitors for charging and discharging when FC voltage exceeds a reference voltage, and identifies, when initial charging is completed, an operating mode of the supercapacitors according to a voltage state of the overhead line and alternately drives the bidirectional DC-DC converters in a buck mode and a booster mode. 
     The energy storage apparatus for urban railways using a bidirectional DC-DC converter according to the present invention has advantages in that it is possible to easily add a storage unit including supercapacitors in the form of a box module as capacity increases or changes and it is possible to achieve accurate and efficient charging and discharging of the storage unit through various sensors and signal checks (inspections). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of an energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 2  is a block diagram of a controller included in the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 3  is a detailed block diagram of a PWM control board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 4  is a detailed block diagram of a signal control board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 5  is a detailed block diagram of a signal identification board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 6  is a detailed block diagram of a digital output contact portion included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 7  is a detailed block diagram of an optical output board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 8  is a detailed block diagram of a power input board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 9  is a detailed block diagram of a power control board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
         FIG. 10  is an operation sequence diagram of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described in detail with reference to the drawings. 
       FIG. 1  is a circuit diagram of an energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the energy storage apparatus  2  for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention increases charge and discharge efficiency through design of a controller taking into consideration operation of associated devices and increases economic efficiency and use convenience through change or addition of a minimal configuration for energy storage capacity variation or increase. 
     More specifically, the energy storage apparatus  2  for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention includes a power receiving portion  4 , a filter unit  8 , and storage units  28   a ,  28   b , and  28   c . The power receiving portion  4  is electrically connected to an overhead line to receive power and includes a disconnector  6  therein. The filter unit  8  is electrically connected to the overhead line to remove high frequency components. The storage units  28   a ,  28   b , and  28   c  include a plurality of supercapacitors to store electrical energy. 
     The energy storage apparatus  2  for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention also includes a plurality of bidirectional DC-DC converters  20   a ,  20   b , and  20   c , a voltage detector  18 , a current detector  16 , and a controller  32 . The bidirectional DC-DC converters  20   a ,  20   b , and  20   c  are provided in parallel and are electrically connected between the filter unit  8  and the storage units  28   a ,  28   b,  and  28   c . The bidirectional DC-DC converters  20   a ,  20   b , and  20   c  are controlled by turning on or off first to sixth transistors  22   a  to  22   f  according to a control signal. The voltage detector  18  is electrically connected to the filter unit  8  to detect voltage of the overhead line. The current detector  16  detects the amount of current flowing into the filter unit  8  and outputs the same to the controller  32 . The controller  32  receives the current and voltage of the overhead line detected by the current detector  16  and the voltage detector  18  and outputs an on/off control signal to each of the first to sixth transistors  22   a  to  22   f  of the bidirectional DC-DC converters  20   a ,  20   b , and  20   c.    
     The energy storage apparatus  2  for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention may further include a plurality of capacitor monitoring units  30   a ,  30   b , and  30   c  and a charging unit  10 . The capacitor monitoring units  30   a ,  30   b , and  30   c  monitor measurements of capacity, input/output current, and power of the storage units  28   a ,  28   b , and  28   c  and output the monitoring result to the controller  32 . The charging unit  10  is electrically connected between the overhead line and the filter unit  8  to prevent excess current from flowing into the filter unit  8  during charging of a capacitor of the filter unit  8  and includes a block switch that opens and closes the electrical connection between the charging unit  10  and the overhead line. 
     Reference numerals  12  and  14  that are not described above denote a current sensor and a voltage sensor, respectively. 
     The controller  32  in each energy storage module configured as described above opens the block switch of the charging unit when excess current is detected by the current detector. In a charging mode, the controller  32  outputs a control signal for turning the 1st, 3rd, and 5th transistors off when the storage unit has been fully charged. In a power supply mode, the controller  32  outputs a control signal for turning the 2nd, 4th, and 6th transistors off to control charging and discharging of the storage unit when a voltage input from the capacitor monitoring unit is less than or equal to a reference voltage. 
     The disconnector  6  includes an electric disconnector for connecting or disconnecting the main circuit to or from the secondary side of a substation upon ESS maintenance and repair or inspection and also includes a 64P relay, which is the same ground relay as that of an existing substation facility, and an input smoothing filter reactor. 
     The disconnector  6  safely and reliably separates and reconnects an overhead line voltage from and to the energy storage apparatus when the energy storage apparatus is subjected to maintenance and repair and inspection of malfunction. The disconnector  6  can be operated to perform such separation and reconnection simply by pressing on and off buttons through an RS relay and a switch included in the disconnector. 
       FIG. 2  is a block diagram of a controller included in the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 2 , the controller  32  included in the energy storage apparatus  2  for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention includes an analog interface board (INF23-1, 2)  34 , a signal identification board (DIP13 &amp; DIP-AUX)  36 , a signal control board (PUZ33)  38 , a digital output contact portion (RYO45)  46 , a communication board (INF79)  48 , a PWM control board (PWM33)  40 , an optical output board (EOC09-1)  42 , and an external gate driver (IGBT Gate Driver)  44 . The analog interface board  34  receives a detected DC link voltage and supercapacitor voltage in real time from a PT unit and receives a DC link current and a supercapacitor charging and discharging current from a CT unit. The signal identification board  36  is provided in parallel with the analog interface board  34  and identifies a feedback signal of a contactor, a fuse contact, a temperature contact, and a switch contact of a front panel and performs drive sequence and guard operations of the controller  32 . The signal control board  38  receives a signal from the analog interface board  34  and the signal identification board  36  and performs sequence and guard operations and communication operations of the system. The digital output contact portion  46  receives a signal from the signal control board  38  and outputs a contact signal. The communication board  48  receives a signal from the signal control board  38  and performs communication with a GP monitor and also performs communication with the supercapacitor monitoring units  30   a ,  30   b , and  30   c . The PWM control board  40  receives a signal from each of the analog interface board  34  and the signal control board  38  and provides a PWM pulse signal. The optical output board  42  receives the PWM pulse signal from the PWM control board  40  and converts the PWM pulse signal into a PWM optical signal. The external gate driver  44  receives the PWM optical signal from the optical output board  42  and generates an external gate drive signal. 
     More specifically, the controller  32  is located in the main box to perform all sequence, control, guard operations and communication functions of the system. 
     A 32-bit, high-performance microprocessor (TMS320VC33) is used to perform such functions of the controller  32 . The controller  32  is designed such that boards are classified and arranged by functions to facilitate system extension. 
     First, the INF23-1/2 board is an analog interface board wherein an INF23-1 board receives a detected DC link voltage and a supercapacitor voltage in real time from the PT unit of the system and an INF23-2 board receives a DC link current and a supercapacitor charging and discharging current from the CT unit of the system. 
     The ISO26 board is designed to monitor control sequence and analog values of the controller using a DA converter. The DIP13 &amp; DIP-AUX board identifies a feedback signal of a contactor of the system, a fuse contact, a temperature contact, and a switch contact of the front panel and performs drive sequence and guard operations of the controller  32  using the identified results. A total of 32 contacts may be used as such digital contacts. 
     The RYO45 board may output a total of 8 digital output contact signals and perform functions such as a function to drive exciting coils of the relay and the contactor or a function to turn on a lamp of the front panel. 
     The EOC09 board is an optical output board that functions to convert a PWM pulse signal that is provided by the PWM33 board into an optical signal and transmits the optical signal to the external gate driver (IGDU). The purpose of optical conversion is to avoid the influence of externally generated electrical noise. 
     The INF79 board is a communication board that is designed such that the communication interface can be changed to RS-485/422 according to setting of jumper pins therein. The INF79 board may use up to 2 channels, channel A and channel B. In the current system, channel A uses an RS-422 communication scheme to perform communication with the GP monitor provided on the front side of the main device and channel B uses an RS-485 communication scheme to perform communication with the supercapacitor status monitoring module. 
     All signals of such external interface devices are transmitted to the PWM33 board and the PUZ33 board to perform control, sequence, and guard operations. The PUZ33 board manages sequence, guard, and communication operations of the apparatus and is controlled using 2 processors. The PWM33 board receives sequence-related information of the PUZ33 board through a DPRAM and performs control functions of a power conversion device. 
       FIG. 3  is a detailed block diagram of a PWM control board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 3 , the PWM control board (PWM33)  40  includes a sensor input circuit  62 , an A/D converter  64 , a calculation unit  68 , a calculation control processor  72 , and a power monitoring unit  80 . The sensor input circuit  62  receives an analog signal  60 . The A/D converter converts an output signal from the sensor input circuit  62  into a digital signal. The calculation unit  68  receives, as inputs, a reference malfunction value and an output signal of the sensor input circuit  62  and outputs a malfunction detection signal and an interrupt request signal. The calculation control processor  72  receives an output signal of the A/D converter  64  and an output signal of the calculation unit  68 , creates a charge and discharge current instruction, generates a pulse pattern of a bidirectional chopper, and controls the duty ratio of the chopper. The power monitoring unit  80  monitors power state. 
     Specifically, the PWM33-1 CARD uses a TMS320VC33 processor from Texas Instruments Inc., which is a high-performance 32-bit digital signal processor, as a main calculation unit (arithmetic unit) for executing a control algorithm in real time. In addition, a flash memory is used as the memory to allow a program to be directly downloaded to the ROM to facilitate maintenance and repair. 
     This board serves as a central part of the energy storage apparatus and thus implements various processes by software using a 32-bit DSP. This board receives various analog signals and creates a charge and discharge current instruction for the energy storage apparatus and generates a pulse pattern of the bidirectional chopper. The board has a function to control the duty ratio of the chopper in order to control a varying input overhead line voltage at a regular DC voltage 
     A PWM pulse waveform generated by this board is transmitted to the EOC09 board and is then converted into an optical signal which is used to drive each gate driver, thereby turning the IGBT on or off. 
       FIG. 4  is a detailed block diagram of a signal control board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 4 , the signal control board (PUZ33)  38  performs communication with another board, the main circuit and a TMS unit and stores general drive information and a malfunction record and also stores state information of each part in an NVSRAM when the energy storage apparatus malfunctions. 
     Specifically, the PUZ33A board is an interface card for communicating with the PWM33-1 card, the main circuit, and the TMS unit in order to control the energy storage apparatus. This card communicates with the PWM33-1 to receive various information and stores general information, a malfunction record, and the like and communicates with the TMS unit. 
     This card also regularly stores various state information in the NVSRAM when the energy storage apparatus malfunctions and stores a malfunction record and the like therein. The user may download the stored malfunction record to a PC through RS232 communication between the PC and the card to allow the user to be notified of the state of the energy storage apparatus when the energy storage apparatus malfunctions. 
       FIG. 5  is a detailed block diagram of a signal identification board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 5 , the signal identification board (DIP13 &amp; DIP-AUX)  36  includes an input circuit  100 , a surge blocking unit  102 , a filter circuit  104 , and an insulation circuit  106 . The input circuit  100  receives a 100V input signal. The surge blocking unit  102  is connected to an output terminal of the input circuit  100  to block surge in the input signal. The filter circuit  104  filters an output signal of the surge blocking unit  102 . The insulation circuit  106  insulates this board from external signals. 
     Specifically, the DIP13-1/DIP-AUX board includes a digital input circuit  100  for receiving a 100V input signal, a surge blocking circuit for blocking surge in the input signal, a filter circuit for performing filtering, and an insulation circuit for providing insulation from external signals. 
     The DIP13-1 board is designed to receive digital signals of a total of 24 channels and the DIP-AUX board is designed to receive digital signals of 8 channels. The received signals are transmitted to the PUZ33A board through a motherboard and are then used for control. 
       FIG. 6  is a detailed block diagram of a digital output contact portion included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 6 , the digital output contact portion (RYO45)  46  includes an insulation circuit  114  for separating output signals of the motherboard  110  from external signals and a relay driver  116  for driving a contactor relay. 
     The digital output (RYO45) board includes a power separation circuit for separating output signals from external power and a relay circuit for driving the contactor relay. 
     The controller  32  of the energy storage apparatus includes 8-channel digital output circuits to drive each contactor of the energy storage apparatus or to externally output digital signals. 
       FIG. 7  is a detailed block diagram of an optical output board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 7 , the optical output board (EOC09-1)  42  includes an optical signal converter  122  that receives 12-channel gate PWM signals from the PWM control board  40  and converts the same into optical signals to drive the gate driver  44 , thereby turning the IGBT on or off. 
     That is, the EOC09-1 board receives 12-channel gate PWM signals from the PWM33-1 unit and converts the same into optical signals to drive the gate driver, thereby turning the IGBT on or off. 
       FIG. 8  is a detailed block diagram of a power input board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention and  FIG. 9  is a detailed block diagram of a power control board included in the controller of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIGS. 8 and 9 , the power input board (PWS24A)  130  included in the controller  32  receives DC 100V and inputs the same to the motherboard through a filter circuit and a power stabilization circuit to provide the same to the PWS27 board which is a control power unit. 
     The PWS27 board receives DC 100V from the PWS24 board and generates and provides control power required for each controller component through a Switch Mode Power Supply (SMPS). 
       FIG. 10  is an operation sequence diagram of the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention. 
     Referring to  FIG. 10 , first, when 220V AC power is applied to the controller  32  included in the energy storage apparatus for urban railways using a bidirectional DC-DC converter according to an embodiment of the present invention, the controller  32  starts operation and sequentially activates GCK, MCB, and HSCB units at short intervals when each contactor is in normal state. 
     When FC voltage exceeds a reference voltage, a CHK unit is turned on to enter an initial charging mode of the supercapacitors for charging and discharging. 
     In the initial charging mode, an operation for protecting the supercapacitor from surge current is performed as described above. 
     When initial charging is completed, the controller identifies the operating mode of the supercapacitors according to a voltage state of the overhead line and performs a normal operation by alternately driving the bidirectional DC-DC converters in a buck mode and a booster mode. 
     As is apparent from the above description, an energy storage apparatus for urban railways using a bidirectional DC-DC converter according to the present invention has advantages in that it is possible to easily add a storage unit including supercapacitors in the form of a box module as capacity increases or changes and it is possible to achieve accurate and efficient charging and discharging of the storage unit through various sensors and signal checks (inspections). 
     The energy storage apparatus for urban railways using a bidirectional DC-DC converter according to the present invention is not limited to the above embodiments and may be modified into various forms without departing from the spirit of the present invention.