Abstract:
A bidirectional inverter is provided for a renewable energy storage system which may simplify the circuitry and lower manufacturing cost by reducing the numbers of switches and control signals. The bidirectional inverter includes a pulse-width-modulation (PWM) signal provider for providing a PWM signal, a push-pull inverter coupled to the PWM signal provider and a direct current (DC) link, and an offset voltage provider coupled to the push-pull inverter and the electric power system. Accordingly, the bidirectional inverter converts DC power from the DC link into AC power or AC power from the electric power system into DC power.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0125896, filed Dec. 17, 2009 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Aspects of embodiments of the present invention relate to a bidirectional inverter for an energy storage system. 
         [0004]    2. Description of Related Art 
         [0005]    In general, a renewable energy storage system, such as a solar cell based system or a wind power generator based system, includes a plurality of converters and a plurality of inverters for storing generated energy in various levels of alternating current (AC) or direct current (DC) power. That is to say, the renewable energy storage system uses a DC-to-AC inverter to convert DC power generated by a solar cell to AC power that is provided to an electric power system. Further, since the power generated by a solar cell has a different power level (e.g., voltage level) from that of a battery, a DC-to-DC converter is used to change the power generated by the solar cell to the power having a voltage level suitably provided to the battery. 
       SUMMARY 
       [0006]    Aspects of embodiments of the present invention provide a bidirectional inverter for a renewable energy storage system with reduced numbers of switches and control signals. 
         [0007]    According to an embodiment of the present invention, a bidirectional inverter is provided for a renewable energy storage system capable of providing power from a direct current (DC) link to an electric power system or providing power from the electric power system to the DC link. The bidirectional inverter includes a pulse-width-modulation (PWM) signal provider for providing a PWM signal, a push-pull inverter coupled to the PWM signal provider and the DC link, and an offset voltage provider coupled to the push-pull inverter and the electric power system. 
         [0008]    The push-pull inverter may be configured to convert DC power from the DC link into alternating current (AC) power to then provide the AC power to the offset voltage provider. 
         [0009]    The PWM signal provider may be configured to provide a PWM signal having a same phase as that of the electric power system. 
         [0010]    The push-pull inverter may be configured to convert AC power from the offset voltage provider into DC power to then provide the DC power to the DC link. 
         [0011]    The PWM signal provider may be configured to provide a PWM signal having a same phase as that of the electric power system. 
         [0012]    The offset voltage provider may be configured to provide an offset voltage to the electric power system, the offset voltage being a sum of a negative offset voltage and an AC voltage from the push-pull inverter. 
         [0013]    The negative offset voltage may level-shift the AC voltage from the push-pull inverter in a negative direction by one half of the AC voltage. 
         [0014]    The offset voltage provider may be configured to provide an offset voltage to the push-pull inverter, the offset voltage being a sum of a positive offset voltage and an AC voltage from the electric power system. 
         [0015]    The positive offset voltage may level-shift the AC voltage from the electric power system in a positive direction by one half of the AC voltage. 
         [0016]    In an embodiment of the present invention, the bidirectional inverter may further include a voltage sensor for sensing a voltage of the electric power system and a phase information calculator for sensing a phase information using a voltage obtained from the voltage sensor and providing the phase information to the PWM signal provider. 
         [0017]    In an embodiment of the present invention, the push-pull inverter may include a first switch having a first electrode coupled to the DC link, a second electrode coupled to the PWM signal provider, and a third electrode, a second switch having a first electrode coupled to the third electrode of the first switch, a second electrode coupled to the PWM signal provider, and a third electrode coupled to a ground terminal, an inductor having a first electrode coupled to the third electrode of the first switch and the first electrode of the second switch, and a second electrode coupled to the offset voltage provider, and a capacitor coupled between the third electrode of the second switch and the second electrode of the inductor. 
         [0018]    Here, the first switch may include an N-channel field-effect transistor (FET), and the second switch may include a P-channel FET. 
         [0019]    The offset voltage provider may include a first offset voltage provider for supplying power from the push-pull inverter to the electric power system as a first offset voltage, and a second offset voltage provider for supplying power from the electric power system to the push-pull inverter as a second offset voltage. 
         [0020]    In an embodiment of the present invention, the first offset voltage provider may include a first offset switch coupled to the push-pull inverter, and a first adder coupled between the first offset switch and the electric power system, the first adder for providing the first offset voltage to the electric power system by adding a negative offset voltage to a voltage supplied from the push-pull inverter. 
         [0021]    In an embodiment of the present invention, the second offset voltage provider may include a second offset switch coupled to the electric power system, and a second adder coupled between the second offset switch and the push-pull inverter, the second adder being for providing the second offset voltage to the push-pull inverter by adding a positive offset voltage to a voltage supplied from the electric power system. 
         [0022]    A bidirectional inverter for a renewable energy storage system according to the above described embodiments of the present invention has reduced numbers of switches and control signals, thereby lowering the manufacturing cost and simplifying the circuitry. 
         [0023]    Additional aspects and/or features of the invention will be set forth in the description which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    These and/or other aspects and features of the present invention will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
           [0025]      FIG. 1  is a schematic block diagram of a new renewable energy storage system according to an embodiment of the present invention; 
           [0026]      FIG. 2  is a block diagram of a bidirectional inverter for a new renewable energy storage system according to an embodiment of the present invention; 
           [0027]      FIG. 3  is a block diagram illustrating a first mode operation of the bidirectional inverter according to an embodiment of the present invention; 
           [0028]      FIGS. 4A and 4B  are graphs illustrating variations in the voltage across nodes A and B shown in  FIG. 3 ; 
           [0029]      FIG. 5  is a block diagram illustrating a second mode operation of the bidirectional inverter according to an embodiment of the present invention; and 
           [0030]      FIGS. 6A and 6B  are graphs illustrating variations in the voltage across nodes C and D shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey embodiments of the invention to those skilled in the art. 
         [0032]    Throughout the specification, like numerals refer to like elements. It will be understood that when an element is referred to as being “electrically connected to” another element, it may be directly connected to the other element, or intervening elements may also be present therebetween. 
         [0033]    Referring to  FIG. 1 , the renewable energy storage system  100  according to one embodiment includes a renewable energy  110 , a Maximum Power Point Tracking (MPPT) converter  120 , a Direct Current (DC) link, a bidirectional inverter  140 , a load  150 , a grid connector  160 , an electric power system  170 , a battery  180 , a battery monitoring system  190 , a bidirectional converter  200 , and an integrated controller  210 . 
         [0034]    The renewable energy  110  refers to energy generated from various renewable energy sources of power such as, without limitation, sunlight, wind, water, and geothermal heat. In some embodiments, the renewable energy  110  is an electrical energy produced by a wind generator, a photovoltaic (PV) generator, a wind power generator, or equivalents thereof. In the following, the renewable energy  110  is described with regard to a solar cell as an example. 
         [0035]    The MPPT converter  120  extracts the maximum power from the renewable energy  110  and converts it into a different level of an output DC power. The output of the solar cell varies nonlinearly with respect to the amount of solar radiation and surface temperature, which is the main cause of degradation in power generation efficiency of the solar cell. The MPPT converter  120  makes the solar cell operate at a maximum power point. The maximum power point of the solar cell varies nonlinearly with respect to the amount of solar radiation and surface temperature. DC power extracted at the maximum power point is converted into a different level of DC power and provided to the DC link  130 . 
         [0036]    The DC link  130  temporarily stores a DC voltage supplied from the MPPT converter  120 . The DC link  130  may be a substantial high capacity capacitor or other suitable devices. Thus, the DC link  130  removes an alternating current (AC) component from the DC power output from the MPPT converter  120  and stores stable DC power. The DC link  130  also stabilizes and temporarily stores a DC voltage supplied from the bidirectional inverter  140  or the bidirectional converter  200 , which will be described in detail later. 
         [0037]    The bidirectional inverter  140  converts the DC power provided by the DC link  130  into AC power (e.g., commercial AC power) and outputs the AC power. More specifically, the bidirectional inverter  140  converts a DC voltage from the renewable energy  110  or the battery  180  into AC power that is suitable for home use. The bidirectional inverter  140  also converts AC power (e.g., commercial AC power) provided by the electric power system  170  into DC power and feeds the DC power into the DC link  130 . The power stored in the DC link  130  is provided to the battery  180  through the bidirectional converter  200 . 
         [0038]    The load  150  may be residential or industrial facility using AC voltage (e.g., commercial AC voltage). The load  150  receives AC power sourced from the renewable energy  110 , the battery  180 , or the electric power system  170 . 
         [0039]    The grid connector  160  connects the bidirectional inverter  140  to the electric power system  170 . More specifically, the grid connector  160  adjusts the range of voltage variations and suppresses harmonic frequencies. The grid connector  160  also provides AC power from which a DC component has been removed to the electric power system  170 , or AC power output from the electric power system  170  to the bidirectional inverter  140 . 
         [0040]    The electric power system  170  may be an electric company or an AC power system provided by an electricity generating company. For example, the electric power system  170  may include power plants, substations, and transmission lines electrically interconnected over a wide area. The electric power system  170  is commonly referred to as a “grid.” 
         [0041]    The battery  180  may be a secondary battery capable of charging and discharging. The battery  180  may be, for example, a lithium-ion (Li-ion) battery, a lithium polymer (Li-poly) battery or equivalents thereof, but aspects of the present invention are not limited thereto. 
         [0042]    The battery monitoring system  190  maintains and manages the battery  180  to be in an optimal state. More specifically, the battery monitoring system  190  monitors the voltage, current and temperature of the battery  180  and warns a user upon detection of a failure. Further, the battery monitoring system  190  calculates the State of Charge (SOC) and State of Health (SOH) of the battery  180 , performs cell balancing to equalize voltages or capacities of battery cells constituting the battery  180 , and controls a cooling fan to prevent overheating of the battery  180 . 
         [0043]    The bidirectional converter  200  converts DC power from the DC link  130  into a different level of DC power suitable for charging the battery  180 . On the other hand, the bidirectional converter  200  converts DC power from the battery  180  into a different level of DC power suitable for use in the DC link  130 . The bidirectional converter  200  may have a unitary structure. In addition, the bidirectional converter  200  may be an insulation-type or a non-insulation type. 
         [0044]    The integrated controller  210  monitors and controls the MPPT converter  120 , the bidirectional inverter  140 , the grid connector  160 , and the bidirectional converter  200 . The integrated controller  210  also communicates with the battery monitoring system  190  to monitor the battery monitoring system  190 . The integrated controller  210  controls the MPPT converter  120 , the bidirectional inverter  140 , the grid connector  160 , and the bidirectional converter  200  by sensing their voltages, currents, and temperatures. Further, the integrated controller  210  controls an interceptor  155  located between the load  150  and the grid connector  160  to cut off the connection in the event of an emergency. 
         [0045]      FIG. 2  is a block diagram of a bidirectional inverter  140  for a renewable energy storage system according to an embodiment of the present invention. 
         [0046]    Referring to  FIG. 2 , the bidirectional inverter  140  for a renewable energy storage system according to one embodiment includes a pulse-width-modulation (PWM) signal provider  141 , a push-pull inverter  142 , and an offset voltage provider  143 . The bidirectional inverter  140  further includes a power grid voltage sensor  146  and a phase information calculator  147 . The bidirectional inverter  140  converts DC power from a DC link  130  into AC power to provide to an electric power system  170  or converts AC power from the electric power system  170  into DC power to provide to the DC link  130 . Here, the DC link  130  and the electric power system  170  are substantially the same as those shown in  FIG. 1 . 
         [0047]    The PWM signal provider  141  provides a PWM signal to the push-pull inverter  142 . The PWM signal provider  141  includes at least one waveform generator  141   a,  a comparator  141   b,  and an amplifier having a power supply  141   c.  The PWM signal provider  141  is provided for illustration only, and aspects of the present invention are not limited to the embodiment illustrated in  FIG. 2 . 
         [0048]    The PWM signal provider  141  provides a PWM signal having the same phase as that of AC power from the electric power system  170 . To achieve this, the power grid voltage sensor  146  senses a voltage of the electric power system  170 . 
         [0049]    The phase information calculator  147  then calculates phase information using a voltage obtained from the power grid voltage sensor  146  and outputs the phase information to the PWM signal provider  141 . The PWM signal provider  141  thereafter provides a PWM signal having the same phase as that of AC power from the electric power system  170  based on the phase information output by the phase information calculator  147 . In both first and second modes, which will be described below, the PWM signal provider  141  provides a PWM signal having the same phase as that of AC power from the electric power system  170 . 
         [0050]    The push-pull inverter  142  is electrically connected to the DC link  130 , the PWM signal provider  141 , and the offset voltage provider  143 . In the first mode, for example, the push-pull inverter  142  converts DC power from the DC link  130  into AC power and provides the AC power to the offset voltage provider  143 . In the second mode, for example, the push-pull inverter  142  converts AC power from the offset voltage provider  143  into DC power and provides the DC power to the DC link  130 . 
         [0051]    To accomplish the above described function, the push-pull inverter  142  includes a first switch Q 1 , a second switch Q 2 , an inductor L, and a capacitor C. 
         [0052]    The first switch Q 1  has a first electrode (e.g., drain), a second electrode (e.g., gate), and a third electrode (e.g., source). The first and second electrodes are connected to the DC link  130  and the PWM signal provider  141 , respectively. The third electrode is connected to the second switch Q 2  and the inductor L. In this case, the first switch Q 1  may include a parasitic diode that is forward biased from the third electrode towards the first electrode. The first switch Q 1  may be one selected from an N-channel field-effect transistor (FET), an Insulated gate bipolar transistor (IGBT), an NPN-type bipolar transistor, and the equivalents thereof, but aspects of the present invention are not limited thereto. 
         [0053]    The second switch Q 2  has a first electrode (e.g., drain), a second electrode (e.g., gate), and a third electrode (e.g., source). The first and second electrodes are connected to the third electrode of the first switch Q 1  and the PWM signal provider  141 , respectively. The third electrode is connected to a ground terminal. In this case, the second switch Q 2  may include a parasitic diode that is forward biased from the first electrode towards the third electrode. The second switch Q 2  is one selected from a P-channel FET, an IGBT, a PNP-type bipolar transistor, and the equivalents thereof, but aspects of the present invention are not limited thereto. 
         [0054]    The push-pull inverter  142  further includes a buffer  142   a  connected to the second electrode of the first switch Q 1 , the second electrode of the second switch Q 2 , and the PWM signal provider  141 . 
         [0055]    The inductor L has a first electrode connected between the third electrode of the first switch Q 1  and the first electrode of the second switch Q 2  and a second electrode connected to the offset voltage provider  143 . Due to the above configuration, the first and second switches Q 1  and Q 2  are turned on or off in response to a single common control signal. Thus, the number of control signals can be reduced, and the circuitry can be simplified. 
         [0056]    The capacitor C has a first electrode connected between the third electrode of the second switch Q 2  and the ground terminal and a second electrode connected between the second electrode of the inductor L and the offset voltage provider  143 . 
         [0057]    The offset voltage provider  143  is connected between the push-pull inverter  142  and the electric power system  170 . In the first mode, for example, the offset voltage provider  143  adds a negative offset voltage to an AC voltage provided by the push-pull inverter  142  and provides the resultant voltage to the electric power system  170 . As a result of adding the negative offset voltage, the AC voltage from the push-pull inverter  142  is level-shifted in a negative direction, e.g., by about a half of the AC voltage, and provided to the electric power system  170 . In the second mode, for example, the offset voltage provider  143  adds a positive offset voltage to an AC voltage provided by the electric power system  170  and provides the resultant voltage to the push-pull inverter  142 . As a result of adding the positive offset voltage, the AC voltage from the electric power system  170  is level-shifted in a positive direction, e.g., by about a half of the voltage, and provided to the push-pull inverter  142 . 
         [0058]    To accomplish the above described functions, the offset voltage provider  143  includes a first offset voltage provider  144  for supplying power from the push-pull inverter  142  to the electric power system  170  and a second offset voltage provider  145  for supplying power from the electric power system  170  to the push-pull inverter  142 . 
         [0059]    The first offset voltage provider  144  includes a first offset switch  144   a  and a first adder  144   c.  The first offset switch  144   a  is connected between the push-pull inverter  142  and the first adder  144   c.  The first adder  144   c  is connected between the first offset switch  144   a  and the electric power system  170 . The first offset voltage provider  144  further includes a first switch controller  144   b  for controlling the first offset switch  144   a  to turn on or off and a first power supply  144   d  connected to the first adder  144   c  to provide a negative offset voltage to the first adder  144   c.  For example, by adding the negative offset voltage, a voltage Vlink of the DC link  130  may be level-shifted in the negative direction, e.g., by one half of the voltage Vlink. 
         [0060]    The second offset voltage provider  145  includes a second offset switch  145   a  and a second adder  145   c.  The second offset switch  145   a  is connected between the electric power system  170  and the second adder  145   c.  The second adder  145   c  is connected between the second offset switch  145   a  and the push-pull inverter  142 . The second offset voltage provider  145  further includes a second switch controller  145   b  for controlling the second offset switch  145   a  to turn on or off and a second power supply  145   d  connected to the second adder  145   c  to provide a positive offset voltage to the second adder  145   c.  For example, by adding the positive offset voltage, a voltage Vgrid of the electric power system  170  may be level-shifted in the positive direction, e.g., by one half of the voltage Vgrid. 
         [0061]    The electric power system  170  is electrically connected to the offset voltage provider  143 . Although not shown in  FIG. 2 , a load, a cut-off switch, and a grid connector are connected between the offset voltage provider  143  and the electric power system  170 . Further, a resistor and a capacitor may be additionally connected to the electric power system  170 , but aspects of the present invention are not limited thereto. 
         [0062]      FIG. 3  is a block diagram illustrating a first mode operation of the bidirectional inverter  140  according to an embodiment of the present invention.  FIGS. 4A and 4B  are graphs illustrating variations in the voltage across nodes A and B shown in  FIG. 3 . 
         [0063]    According to one embodiment, in the first mode, the bidirectional inverter  140  converts DC power from the DC link  130  into AC power and provides the AC power to the electric power system  170 . The first mode is also referred to as an inverter mode. In this case, the DC link  130  may be kept charged by power generated by a solar cell or battery. 
         [0064]    The power grid voltage sensor  146  senses a voltage of the electric power system  170  and outputs the voltage to the phase information calculator  147 . The phase information calculator  147  then calculates phase information using the voltage and provides the phase information to the PWM signal provider  141 . 
         [0065]    Based on the phase information about the electric power system  170 , the PWM signal provider  141  thereafter provides a PWM signal having the same phase as that of the power from the electric power system  170  to the push-pull inverter  142 . 
         [0066]    The push-pull inverter  142  alternately turns on and off the first and second switches Q 1  and Q 2  in response to the PWM signal input from the PWM signal provider  141 . Then, DC power from the DC link  130  is transferred to a LC filter consisting of the inductor L and the capacitor C and converted into AC power. In this case, the first and second switches Q 1  and Q 2  may be N and P-channel FETs, respectively. Thus, the first and second switches Q 1  and Q 2  turn on and off alternately, and not simultaneously. 
         [0067]    According to the operation of the push-pull inverter  142 , AC power is output to node A of the push-pull inverter  142 , as illustrated in  FIG. 4A  where the y axis (e.g., ordinate) and x axis (e.g., abscissa) represent voltage and time, respectively. Referring to  FIG. 4A , AC power of about 0 V to about 400 V is output through the node A. 
         [0068]    Subsequently or simultaneously, the offset voltage provider  143 , more specifically, the first offset voltage provider  144  starts to operate. First, the first switch controller  144   b  controls the first offset switch  144   a  to turn on. Here, the second offset switch  145   a  remains turned off. The first switch controller  144   b  may be controlled by a control signal output from the integrated controller  210  (shown in  FIG. 1 ). For example, if the integrated controller  210  recognizes the current mode as the first mode, the integrated controller  210  may issue a command to the first switch controller  144   b  to turn on the first offset switch  144   a.    
         [0069]    Turning on the first offset switch  144   a  causes the nodes A and B to be electrically connected to each other. That is, the push-pull inverter  142  is electrically connected to the electric power system  170 . 
         [0070]    Here, the first power supply  144   d  supplies a negative offset voltage to the first adder  144   c.  Thus, an AC voltage provided by the push-pull inverter  142  is level-shifted in the negative direction by some extent, for example, about one half of the AC voltage, and provided to the electric power system  170 . In this way, as illustrated in  FIG. 4B , an AC voltage of about −200 V to about 200V can be provided to the electric power system  170  through the node B of the offset voltage provider  143 . 
         [0071]      FIG. 5  is a block diagram illustrating a second mode operation of the bidirectional inverter  140  according to an embodiment of the present invention.  FIGS. 6A and 6B  are graphs illustrating variations in the voltage across nodes C and D shown in  FIG. 5 . 
         [0072]    According to the embodiment of  FIG. 5 , in the second mode, the bidirectional inverter  140  converts AC power from the electric power system  170  into DC power and provides the DC power to the DC link  130 . The second mode is also referred to as a power factor correction (PFC) mode. 
         [0073]    First, the offset voltage provider  143 , more specifically, the second offset voltage provider  145  starts to operate. The second switch controller  145   b  controls the second offset switch  145   a  to turn on. Here, the first offset switch  144   a  remains turned off. The second switch controller  145   b  may be controlled by a control signal output from the integrated controller  210  (shown in  FIG. 1 ). For example, if the integrated controller  210  recognizes the current mode as the second mode, the integrated controller  210  may issue a command to the second switch controller  145   b  to turn on the second offset switch  145   a.  Turning on the second offset switch  145   a  causes the push-pull inverter  142  to be electrically connected to the electric power system  170 . 
         [0074]    The second power supply  145   d  supplies a positive offset voltage to the second adder  145   c.  Thus, an AC voltage provided by the electric power system  170  is level-shifted in the positive direction by some extent, for example, about one half of the AC voltage, and provided to the push-pull inverter  142 . In this way, as illustrated in  FIG. 6A  where the y axis (e.g., ordinate) and x axis (e.g., abscissa) represent voltage and time, respectively, an AC voltage of about 0 V to about 400V may be provided to the push-pull inverter  142  through node C of the offset voltage provider  143 . 
         [0075]    Subsequently or simultaneously, the power grid voltage sensor  146  senses a voltage of the electric power system  170  and outputs the voltage to the phase information calculator  147 . The phase information calculator  147  then calculates phase information using the voltage and provides the phase information to the PWM signal provider  141 . 
         [0076]    Based on the phase information about the electric power system  170 , the PWM signal provider  141  thereafter provides a PWM signal having the same phase as that of power from the electric power system  170  to the push-pull inverter  142 . 
         [0077]    The push-pull inverter  142  alternately turns on and off the first and second switches Q 1  and Q 2  in response to the PWM signal input from the PWM signal provider  141 . Then, power stored in the LC filter consisting of the inductor L and the capacitor C is provided to the DC link  130 . In this case, the first and second switches Q 1  and Q 2  may be N and P-channel FETs, respectively. Thus, the first and second switches Q 1  and Q 2  turn on and off alternately, and not simultaneously. 
         [0078]    According to the operation of the push-pull inverter  142 , stable DC power as illustrated in  FIG. 6B  may be provided. The voltage illustrated in  FIG. 6B  is substantially equal to a voltage charged in the DC link  130 . In  FIG. 6B , the y axis (ordinate) and x axis (abscissa) represent voltage and time, respectively. Referring to  FIG. 6B , a DC voltage greater than 400V may be provided through node D. 
         [0079]    Although a few exemplary embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.