Patent Publication Number: US-2022224246-A1

Title: Current control device and power conversion system employing same

Description:
RELATED APPLICATION 
     This application claims priority to China Patent Application No. 202110030390.7, filed on Jan. 11, 2021, the entire contents of which are incorporated herein by reference for all purposes. 
     TECHNICAL FIELD 
     The present disclosure relates to a current control device and a power conversion system employing the current control device with high current control accuracy. 
     BACKGROUND 
     Nowadays, with the rapid development of electronic technology, the requirement for current control accuracy becomes higher and higher. Conventionally, in order to achieve high current control accuracy, an additional power circuit is required to generate a compensation current, and the compensation current must be synthesized with the original DC current to eliminate the error. 
     However, because the additional power circuit is an extra hardware, the overall circuit becomes more complex. Further, the additional power circuit cannot be installed and disassembled flexibly. Therefore, the power circuit can only be inherently disposed in a certain apparatus, and the power circuit is unable to be disposed in the existing product or system externally. 
     Therefore, there is a need to provide a current control device and a power conversion system employing the same in order to overcome the drawbacks of conventional technologies. 
     SUMMARY 
     The present disclosure provides a current control device and a power conversion system employing the current control device. A current sensor and an error compensator can be utilized to realize the error compensation control for the total current of one or more power conversion units. Consequently, high current control accuracy can be achieved with a low cost solution that takes up only a very small space. 
     In one aspect, the present disclosure provides a current control device configured to control N power conversion unit(s), where N is an integer greater than or equal to 1. The N power conversion units are connected in parallel when N is greater than 1. Each power conversion unit includes a signal input terminal and a current-controlled output terminal electrically connected to an external circuit. The external circuit can be a load or a power source. The current control device includes a first current sensor and an error compensator. The first current sensor is electrically connected between the current-controlled output terminal and the external circuit. The first current sensor is configured to sample a current flowing through the external circuit and acquire a current sampling value. The error compensator is electrically connected to the N power conversion unit(s) and the first current sensor. The error compensator receives the current sampling value and a reference current value and generates a compensation value accordingly. The error compensator outputs N current command(s) to the N power conversion unit(s) respectively according to the reference current value and the compensation value. 
     In another aspect, the present disclosure provides a power conversion system including a power grid, an inverter, N power conversion unit(s) connected to each other, and a current control device. N is an integer greater than or equal to 1. The N power conversion units are connected in parallel when N is greater than 1. Each power conversion unit includes a signal input terminal and a current-controlled output terminal electrically connected to an external circuit. The external circuit can be a load or a power source. The current control device is configured to provide current command(s) to the N power conversion unit(s) for controlling current(s) flowing through the N power conversion unit(s). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  and  FIG. 2  are schematic circuit diagrams illustrating a current control device and a power conversion unit according to an embodiment of the present disclosure. 
         FIG. 3  and  FIG. 4  are schematic circuit diagrams illustrating a current control device and a power conversion unit according to an embodiment of the present disclosure. 
         FIG. 5  is a schematic circuit diagram illustrating a current control device and power conversion units according to an embodiment of the present disclosure. 
         FIG. 6  and  FIG. 7  are schematic block diagrams illustrating a power conversion system according to various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described in more detail with reference to the drawings. It is to be noted that the following detailed descriptions are presented for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
       FIG. 1  and  FIG. 2  are schematic circuit diagrams illustrating a current control device and a power conversion unit according to an embodiment of the present disclosure. As shown in  FIG. 1  and  FIG. 2 , a current control device  1  is configured to control N power conversion unit(s)  2 , where N is an integer greater than or equal to 1. The power conversion unit  2  can be, for example, but not limited to, a DC/DC converter, an AC/AC converter, or a DC/AC converter.  FIG. 1  shows the implementation of N being equal to 1, and  FIG. 2  shows the implementation of N being greater than 2. When N is greater than 1, the N power conversion units  2  can be connected in parallel. Each power conversion unit  2  includes a signal input terminal  21  and a current-controlled output terminal  22 . The current-controlled output terminal  22  is electrically connected to an external circuit  3 , which may be a load or a power source. The current flowing direction between the current-controlled output terminal  22  and the external circuit  3  is not limited. That is, the current-controlled output terminal  22  may output current to the external circuit  3  (when the external circuit  3  as a load), or the current-controlled output terminal  22  may receive current from the external circuit  3  (when the external circuit  3  as a power source). 
     The current control device  1  includes a first current sensor  11  and an error compensator error compensator  12 . The first current sensor  11  is coupled (electrically or magnetically) between the current-controlled output terminal  22  and the external circuit  3 . The first current sensor  11  is configured to sample the current flowing through the external circuit  3  and acquire a current sampling value Is. The first current sensor  11  has a high sampling accuracy, and the measurement error of the first current sensor  11  is preferably less than 0.5%, but not so limited. The error compensator  12  is electrically connected to all the power conversion units  2  and the first current sensor  11 . The error compensator  12  receives the current sampling value Is and a current reference value Iref and generates a compensation value Ip according to the current sampling value Is and the current reference value Iref. Based on the current reference value Iref and the compensation value Ip, the error compensator  12  outputs N current command(s) Ic to the signal input terminal(s)  21  of the N power conversion unit(s)  2 , respectively. 
     In the current control device  1  of the present disclosure, the error compensator  12  may be implemented as a software or firmware module, the operation of which corresponds to a computer algorithm. The error compensator  12  can be disposed or otherwise installed in a processing unit of the power conversion unit  2  or a separate controller, and the processing unit may execute the algorithm of the error compensator  12 . Therefore, there is no need to additionally dispose hardware compensation circuits, and the occupied space volume and cost can be reduced. Accordingly, through the first current sensor  11  and the error compensator  12 , the error compensation control for the total current of one or more power conversion unit(s)  2  can be realized. Consequently, the high current control accuracy can be achieved with low cost, and the occupied space volume is small. 
     In addition, when N is greater than 1, as shown in  FIG. 2 , the sum of the N current commands Ic is equal to the sum of the current reference value Iref and the compensation value Ip. In one embodiment, the N current commands Ic are equal. In another embodiment, the N current commands Ic are unequal, and each current command Ic can be adjusted individually according to the actual requirements. 
     In some embodiments, as shown in  FIG. 1  and  FIG. 2 , the error compensator  12  includes a compensation unit  121  and a calculation unit  122 . The compensation unit  121  generates the compensation value Ip according to the current sampling value Is and the current reference value Iref. The calculation unit  122  outputs the N current command(s) Ic to the N power conversion unit(s)  2  according to the current reference value Iref and the compensation value Ip. Specifically, the compensation unit  121  performs subtraction operation to the current sampling value Is and the current reference value Iref so as to acquire the current error (e.g., Ierror=Is−Iref). Then, the compensation unit  121  generates the compensation value Ip by regulating the current error with proportional, PI (proportion integration) or PID (proportion integration differentiation) algorithm and limiting the compensation amplitude to be a small part such as 10% of the rated value of the current through the external circuit. Afterwards, the calculation unit  122  adds the compensation value Ip to the current reference value Iref for acquiring a sum (e.g., Isum=Ip+Iref). The sum equals the current command Ic when N equals 1, and the calculation unit  122  divides the sum into the N current commands Ic when N is greater than 1. Preferably but not exclusively, the compensation value is not greater than 20% of the rated value of the current through the external circuit. 
     In some embodiments, as shown in  FIG. 3  and  FIG. 4 , the error compensator  12  further includes a first low-pass filter  123  and a second low-pass filter  124 . The first low-pass filter  123  is coupled between the first current sensor  11  and the compensation unit  121 . The first low-pass filter  123  receives and filters the current sampling value Is, and outputs the filtered current sampling value Is to the compensation unit  121 . The second low-pass filter  124  is coupled to the compensation unit  121 . The second low-pass filter  124  receives and filters the current reference value Iref, and outputs the filtered current reference value Iref to the compensation unit  121 . When the current sampling value Is fluctuates rapidly, the current sampling value Is received by the compensation unit  121  becomes more stable through the filtering of the first low-pass filter  123 . In addition, the power conversion unit  2 , the first current sensor  11  and the first low-pass filter  123  all have a time delay. As such, the second low-pass filter  124  is disposed, and the time delay of the second low-pass filter  124  is set to be equal to the sum of the time delays of the power conversion unit  2 , the first current sensor  11 , and the first low-pass filter  123 . Consequently, the effect of time delay can be eliminated. Therefore, the current control becomes more accurate. 
     In some embodiments, as shown in  FIG. 5 , each power conversion unit  2  includes a second current sensor  23 . Each second current sensor  23  is configured to detect the current flowing through its corresponding power conversion unit  2  (i.e., the power conversion unit  2  including this second current sensor  23 ). The sampling accuracy of the second current sensor  23  is lower than the sampling accuracy of the first current sensor  11 . Of course, the power conversion unit  2  in other embodiments disclosed herein can include the second current sensor  23  shown in  FIG. 5 . 
     In some embodiments, the current control device may be employed in a power conversion system. For example, the power conversion system may include a power grid, an inverter, N power conversion unit(s)  2 , a current control device, and an external circuit. The composition and operation of the current control device of the power conversion system are the same as that of the above-mentioned current control device, and thus the detailed descriptions thereof are omitted herein. The power grid, the inverter, and the N power conversion unit(s)  2  are connected with each other. The current control device  1  is configured to provide the current command for the N power conversion unit(s)  2  so as to control the current flowing through the N power conversion unit(s)  2 . 
       FIG. 6  and  FIG. 7  schematically illustrate a power conversion system according to various embodiments of the present disclosure. 
     In one embodiment, as shown in  FIG. 6 , the power grid, the inverter, the power conversion unit and the external circuit of the power conversion system  101  are an AC power grid  61 , a grid-tie inverter  63 , DC/DC converters  65 , and a SOFC (solid oxide fuel cell) system  67 . The error compensator  12  of the current control device  1  (as shown in  FIGS. 1-5 ) can be disposed in a controller  69  configured to control the DC/DC converter  65 . As shown in  FIG. 6 , in this embodiment, three DC/DC converters  65  are connected in parallel to boost the low-voltage DC power outputted by the SOFC system, and the grid-tie inverter  63  converts the boosted DC power into an AC power and outputs the AC power to the AC power grid  61 . The first current sensor  11  of the current control device  1  can be coupled (electrically or magnetically) to the parallelly-connected input terminal of the DC/DC converters for sampling current. According to the sampled current, the error compensator  12  in the controller  69  compensates the current error and provides the current commands Ic to the DC/DC converters  65 . Consequently, high current control accuracy can be achieved. 
     In one embodiment, as shown in  FIG. 7 , the power conversion system  102  includes a power grid, an inverter, N power conversion unit(s)  2 , N current control device(s), and N external circuit(s). The N external circuit(s) is/are respectively electrically connected to the N power conversion unit(s)  2 . The N current control device(s) is/are respectively corresponding to the N power conversion unit(s)  2 . The error compensator of each current control device can be disposed in the corresponding power conversion unit  2 . The first current sensor  11  of each current control device is configured to detect the current flowing through the corresponding external circuit. The external circuit may be a power generator or a battery like lithium battery or SOFC. 
     In summary, the present disclosure provides a current control device and a power conversion system employing the same. The current sensor and the error compensator are utilized to realize the error compensation control for the total current of one or more power conversion unit(s). Consequently, the high current control accuracy can be achieved with low cost, and the occupied space is small. In addition, through the introduction of low-pass filters, the fluctuation of the current value sampled by the current sensor can be reduced, and the effect of time delay can be eliminated. 
     While embodiments of the present disclosure has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that other embodiments may be apparent to one of ordinary skill in the art upon review of the present disclosure. Accordingly, it is intended that the present disclosure covers any modifications and/or alterations so long as such modifications and/or alterations fall within the spirit and scope of the appended claims.