Abstract:
A clustered energy-storing micro-grid system includes a renewable energy device, a clustered energy-storing device, an electrical power conversion device and a local controller. Before coordinating and allocating power to a plurality of loads, the clustered energy-storing device stores and releases the power in a centralized manner. This, coupled with the control exercised by the local controller over the electrical power conversion device, controls the micro-grid system in its entirety so that the micro-grid system operates in cost-efficient optimal conditions, under a predetermined system operation strategy, and in a system operation mode.

Description:
FIELD OF TECHNOLOGY 
       [0001]    The present invention relates to micro-grid systems and more particularly to a clustered energy-storing micro-grid system. 
       BACKGROUND 
       [0002]    A micro-grid evolves from a grid framework and usually involves various renewable energy sources, electrical power conversion devices, communication facilities, control devices, energy-storing components and client loads. Compared with traditional large-scale grids, micro-grids are close to power-consuming loads and therefore dispense with long-distance power transmission/distribution lines; hence, micro-grids reduce line loss, dispense with investments required for power transmission/distribution lines and cut operation expenses. Furthermore, micro-grids operate in multiple power management modes with respect to power generation, power transmission and power distribution; hence, micro-grids exhibit high energy utilization efficiency, high system reliability and high grid security performance while operating effectively, flexibly and independently. 
         [0003]    Renewable energy-derived power in a micro-grid system is intermittently unstable and dispersive; hence, it is necessary to strike a balance between the demand and supply of system power by carrying out power regulation with an energy-storing device in the system. The energy-storing device takes the power left over from renewable energy-derived power supplied to a load and allocates, when an energy-storing power level accumulates to a certain extent, the energy-storing power level to system auxiliary power use, for example, supplying emergency standby power, assisting the system in adjusting the frequency or voltage, reducing the use of conventional fossil fuel-derived power, and saving power clients&#39; electricity costs. Every conventional micro-grid system equipped with an energy-storing device has a “single-point” framework and therefore is available to a single specific power client only. When connected to multiple power clients, a conventional micro-grid system equipped with an energy-storing device is restrained by the capacity of the energy-storing device and therefore needs a multi-client control strategy, and in consequence the conventional micro-grid system fails to satisfy multiple power clients. 
       SUMMARY 
       [0004]    It is an objective of the present invention to provide a “clustered” energy-storing micro-grid system to thereby integrate clustered energy-storing devices in the micro-grid system and supply power in a clustered multiple-point manner to local power clients. 
         [0005]    Another objective of the present invention is to provide an operation control strategy suitable for a clustered energy-storing micro-grid system so that, by predicting the power level required for a power-consuming load, the micro-grid system operates in cost-efficient optimal conditions, for example, in a situation conducive to prevention of a waste of power which might otherwise occur if the energy-storing device power level reaches its rated level and therefore causes the micro-grid system to generate excessive power. 
         [0006]    In order to achieve the above and other objectives, the present invention provides a clustered energy-storing micro-grid system, having micro-grids coupled to an AC utility power end to form a clustered network and supply power to loads formed from power consumption levels of clients, respectively, the micro-grid each comprising: a renewable energy device for generating power from a renewable energy source; a clustered energy-storing device coupled to the renewable energy device to store power left over from power consumed by the load and supplied by the renewable energy device; an electrical power conversion device coupled to the AC utility power end, the renewable energy device and the clustered energy-storing device so that a power form of power received from the renewable energy device and power received from the clustered energy-storing device is converted into a power form required for the load; and a local controller coupled to the electrical power conversion device to determine a system operation mode of the local controller by detecting a current level of power required for the load, a current level of power generated from the renewable energy device, and a level of power stored in the clustered energy-storing device, and control a power form of the power supplied by the electrical power conversion device to the load in accordance with the determined system operation mode. 
         [0007]    In the micro-grid system, the electrical power conversion device comprises: a DC/DC converter coupled to the renewable energy device to convert DC power generated from the renewable energy device into DC power which is stable and capable of maximum power generation; a bidirectional DC converter coupled to the clustered energy-storing device and the DC/DC converter to thereby, when the clustered energy-storing device is supplying power, convert an output of the clustered energy-storing device into an output DC power or convert input power into an input DC power to be input to the clustered energy-storing device; and a DC/AC converter coupled to the local controller, the load, the AC utility power end, the DC/DC converter and the bidirectional DC converter to convert DC power into AC power and vice versa, wherein the output DC power provided by the clustered energy-storing device and converted is converted into AC power required for the load, or AC power provided by the AC utility power end is converted into power to be input to the bidirectional DC converter. 
         [0008]    In the micro-grid system, the local controller controls the bidirectional DC converter to output the output DC power from the clustered energy-storing device or input the input DC power to the clustered energy-storing device, according to the configured system operation mode. 
         [0009]    In the micro-grid system, the local controller controls AC power which the DC/AC converter outputs to the load in accordance with the configured system operation mode. 
         [0010]    In the micro-grid system, a plurality of system operation modes configured for the local controller includes a load following mode in which, when the power generated from the renewable energy device exceeds the power required for the load, the local controller controls the electrical power conversion device so that the renewable energy device solely supplies a power consumption level of each client of the load and stores in the clustered energy-storing device a residual portion of power supplied by the renewable energy device; and when the power generated from the renewable energy device is less than the power required for the load, the local controller controls the electrical power conversion device so that the clustered energy-storing device provides standby power to thereby charge the renewable energy device with residual power left over from power consumed by the load. 
         [0011]    In the micro-grid system, a plurality of system operation modes configured for the local controller includes a fixed power mode in which the local controller controls the electrical power conversion device so that the renewable energy device solely supplies the load with a fixed power level and stores in the clustered energy-storing device power left over from power consumed by the load and supplied by the renewable energy device when power generated from the renewable energy device exceeds power required for the load, and the local controller controls the electrical power conversion device so that the clustered energy-storing device serves as a source of standby power, and the power supplied by the clustered energy-storing device compensates for inadequacy of power supplied by the renewable energy device to the load when power generated from the renewable energy device is less than power required for the load. 
         [0012]    In the micro-grid system, the clustered energy-storing device has a predetermined stored power level so that, in the load following mode, when the power stored in the clustered energy-storing device has not reached the predetermined stored power level, the clustered energy-storing device does not provide standby power, and the AC utility power end serves as a source of standby power, thereby allow AC utility power to compensate for inadequacy of power supplied by the renewable energy device to the load. 
         [0013]    In the micro-grid system, the system operation modes further include an emergency power mode, wherein the local controller is configured to operate in the emergency power mode when the AC utility power end stops supplying power, wherein, in the emergency power mode, the local controller controls a direction of current and a strength of current in the electrical power conversion device to allow the power stored in the clustered energy-storing device to be output to function as emergency power or allow the clustered energy-storing device to store power. 
         [0014]    In the micro-grid system, the local controller controls the electrical power conversion device so that the clustered energy-storing device can only be charged at a specific time. 
         [0015]    Therefore, the present invention provides a clustered energy-storing micro-grid system which has a clustered energy-storing device to store and release power in a centralized manner, coordinate and allocate power to a plurality of loads timely. This, coupled with the control exercised by the local controller over the electrical power conversion device, controls the micro-grid system in its entirety so that the micro-grid system operates in cost-efficient optimal conditions, under a predetermined system operation strategy, and in a system operation mode. 
     
    
     
       BRIEF DESCRIPTION 
         [0016]    Objectives, features, and advantages of the present invention are hereunder illustrated with specific embodiments in conjunction with the accompanying drawings, in which: 
           [0017]      FIG. 1  is a schematic view of the framework of a clustered energy-storing micro-grid system according to an embodiment of the present invention; 
           [0018]      FIG. 2  is a schematic view of a clustered energy-storing micro-grid according to an embodiment of the present invention; 
           [0019]      FIG. 3  is a schematic view of another clustered energy-storing micro-grid according to an embodiment of the present invention; 
           [0020]      FIG. 4  is a schematic view of yet another clustered energy-storing micro-grid according to an embodiment of the present invention; and 
           [0021]      FIG. 5  is a schematic view of the process flow of operation of a system operation mode of the clustered energy-storing micro-grid system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Referring to  FIG. 1 , there is shown a schematic view of the framework of a clustered energy-storing micro-grid system  1  according to an embodiment of the present invention. 
         [0023]    The clustered energy-storing micro-grid system  1  comprises a plurality of micro-grids  4 ,  5 ,  6 . The micro-grids  4 ,  5 ,  6  are each connected to a load. In this embodiment, the micro-grids  4 ,  5 ,  6  are each connected to two loads  3  for exemplary purposes. Referring to  FIG. 1 , for example, each load matches a client to thereby allow each micro-grid matches a plurality of clients, and the loads  3  which must be dealt with by each micro-grid are collectively known as a load of the micro-grid. The micro-grids  4 ,  5 ,  6  are connected to the loads  3  by an AC wire  2  for coupling purposes. The micro-grids  4 ,  5 ,  6  and the loads  3  are coupled to an AC utility power end  7  through the AC wire  2 . Therefore, the micro-grids  4 ,  5 ,  6  together form a clustered micro-grid. 
         [0024]    In this embodiment, the micro-grids  4 ,  5 ,  6  and the loads  3  are in the number of three and six, respectively, for exemplary purposes, but the present invention is not limited thereto. 
         [0025]    In the aspect illustrated with  FIG. 1 , the clustered energy-storing micro-grid system  1  of the present invention has a stack framework and comprises at least one of the micro-grids  4 ,  5 ,  6  according to the quantity of power clients (that is, the loads  3 ). Users may expand the micro-grids  4 ,  5 ,  6  so as to increase the micro-grids  4 ,  5 ,  6  as needed. 
         [0026]    In the aspect illustrated with  FIG. 1 , the micro-grids  4 ,  5 ,  6  in the micro-grid system  1  of the present invention are each provided with only one energy-storing device (that is, a clustered energy-storing device described later). Therefore, unlike the conventional single-point framework which has a specific energy-storing unit capable of supplying power to only one specific power client (that is, load), an energy-storing device in each micro-grid  4 ,  5 ,  6  of the present invention controllably allocates power to two or more loads  3  according to the power level required for the loads  3  and a system operation mode in operation, thereby circumventing the conventional limitation of the scope of power supply to a single client. 
         [0027]    In the aspect illustrated with  FIG. 1 , the micro-grid system  1  of the present invention is coupled to the AC utility power end  7  so that the loads  3  can selectively use the micro-grids  4 ,  5 ,  6  or AC utility power as the sole power supply source, or the power supplied by the micro-grids  4 ,  5 ,  6  and an AC utility power end  140  is mixed so that the mixed power is supplied to the loads  3 . 
         [0028]    Referring to  FIG. 2  through  FIG. 4 , there are shown schematic views of the clustered energy-storing micro-grids  4 ,  5 ,  6 , respectively, according to an embodiment of the present invention. Parts and components of the micro-grids  4 ,  5 ,  6  are described below. 
         [0029]    The clustered energy-storing micro-grid  4  comprises a solar power generation device  41 , a fuel cell device  42 , a local controller  43 , a clustered energy-storing device  44  and an electrical power conversion device  45 . The electrical power conversion device  45  comprises a DC/DC converter  46 , a bidirectional DC converter  47 , a DC power bus  48  and a DC/AC converter  49 . The AC output end of the DC/AC converter  49  is coupled to the AC utility power ends  7  and the loads  3 . 
         [0030]    The clustered energy-storing micro-grid  5  comprises a solar power generation device  51 , a wind power generation device  52 , a local controller  53 , a clustered energy-storing device  54  and an electrical power conversion device  55 . The electrical power conversion device  55  comprises a DC/DC converter  56 , a bidirectional DC converter  57 , a DC power bus  58  and a DC/AC converter  59 . The AC output end of the DC/AC converter  59  is coupled to the AC utility power ends  7  and the loads  3 . 
         [0031]    The clustered energy-storing micro-grid  6  comprises a solar power generation device  61 , a local controller  63 , a clustered energy-storing device  64  and an electrical power conversion device  65 . The electrical power conversion device  65  comprises a DC/DC converter  66 , a bidirectional DC converter  67 , a DC power bus  68  and a DC/AC converter  69 . The AC output end of the DC/AC converter  69  is coupled to the AC utility power ends  7  and the loads  3 . 
         [0032]      FIG. 2 ,  FIG. 3  and  FIG. 4  show renewable energy devices and regard the solar power generation devices  41 ,  51 ,  61  as the first renewable energy device.  FIG. 2 ,  FIG. 3  and  FIG. 4  differ from each other in terms of the second renewable energy device. Referring to  FIG. 2 , the fuel cell device  42  serves as the second renewable energy device. Referring to  FIG. 3 , the wind power generation device  52  serves as the second renewable energy device. Referring to  FIG. 4 , no second renewable energy device is provided. The clustered energy-storing micro-grids of the present invention are hereunder described and illustrated with  FIG. 2 . Persons skilled in the art understand that the description of  FIG. 2  is applicable to related parts of  FIG. 3  and  FIG. 4 . 
         [0033]    The quantity of the renewable energy devices shown in diagrams illustrative of the embodiments of the present invention serves illustrative purposes; hence, the quantity of the renewable energy devices is not limited to one or two. The types of renewable energy sources are not restricted to sunlight, wind and fuel. Whatever device which generates power from a renewable energy source can function as a renewable energy device of the present invention to thereby provide the power consumption level required for a load. Referring to  FIG. 2 , renewable energy devices, such as the solar power generation device  41  and the fuel cell device  42 , are coupled to the clustered energy-storing device  44  and the electrical power conversion device  45 , respectively. Alternatively, renewable energy devices, such as the solar power generation device  41  and the fuel cell device  42 , are coupled to the clustered energy-storing device  44 , and then the clustered energy-storing device  44  is coupled to the electrical power conversion device  45 . 
         [0034]    The clustered energy-storing device  44  is coupled to the solar power generation device  41  to store the residual power left over from the power consumed by the loads  3  and supplied by the solar power generation device  41 . The clustered energy-storing device  44  comprises batteries of different types, such as a lead-acid battery, a lithium ferrous battery and a sodium-sulfur battery. The clustered energy-storing device  44  consists of a combination of energy-storing components of different types or different specifications. 
         [0035]    The electrical power conversion device  45  is coupled to the solar power generation device  41  and the clustered energy-storing device  44  to convert the DC power generated from the renewable energy device, such as the solar power generation device  41 , and the DC power stored in the clustered energy-storing device  44  into power of a power form required for a load so that the required power is supplied to the load. For example, when the loads  3  require AC power, the electrical power conversion device  45  converts the DC power into AC power. 
         [0036]    The local controller  43  is coupled to the electrical power conversion device  45  and adapted to provide multiple system operation modes (which are described later) so that one of the system operation modes is determined at the user&#39;s request or in accordance with specific system operation information, such as the current power level required for the loads  3 , current level of power generated from the solar power generation device  41 , and level of power stored in the clustered energy-storing device  44 . The local controller  43  communicates with, for example, an electric meter for detecting the current power level required for the loads  3 , a maximum power tracking circuit for detecting the current level of power generated from the solar power generation device  41 , and a battery management system for detecting the level of power stored in the clustered energy-storing device  44  separately, so as to gather system operation information. 
         [0037]    The local controller  43  controls the electrical power conversion device  45 . The electrical power conversion device  45  determines the level of power supplied by the solar power generation device  41  to the loads  3 , the level of power stored in the clustered energy-storing device  44 , and the level of power which is supplied by the clustered energy-storing device  44  to the loads  3  and must be consumed. Therefore, the local controller  43  substantially controls the operation of the micro-grid  4  in its entirety. 
         [0038]    The DC/DC converter  46  is coupled to the solar power generation device  41  to convert the DC power generated from the solar power generation device  41  into DC power which is stable and capable of maximum power generation. The DC/AC converter  49  is coupled to the local controller  43 , the loads  3 , the AC utility power ends  7 , the DC/DC converter  46  and the bidirectional DC converter  47  to convert DC power into AC power, wherein the output DC power provided by the clustered energy-storing device  44  and converted is converted into AC power required for the loads  3 , and AC power provided by the AC utility power ends  7  is converted into power to be input to the bidirectional DC converter  47 . 
         [0039]    The bidirectional DC converter  47  is coupled to the clustered energy-storing device  44  and the DC/DC converter  46  to thereby, when the clustered energy-storing device  44  is supplying power, convert the output of the clustered energy-storing device  44  into an output DC power (that is, releasing power) or convert input power into an input DC power to be input to the clustered energy-storing device  44  (that is, storing power). 
         [0040]    The DC power of the DC/DC converter  46  and the bidirectional DC converter  47  is collected by the DC power bus  48 . Then, the DC/AC converter  49  converts the collected DC power into AC power for use by the loads  3 . 
         [0041]    The local controller  43  is coupled to the bidirectional DC converter  47  and the DC/AC converter  49  by connection lines (not shown). By being coupled to the connection lines, the local controller  43  transmits control signal S Bi  in accordance with a system operation mode to control the bidirectional DC converter  47  to output the output DC power from the clustered energy-storing device  44  (that is, releasing power) or input the input DC power into the clustered energy-storing device  44  (that is, storing power) and transmit control signals S 11 , S 12  to thereby control the level of AC power which the DC/AC converter  49  outputs to each load  3 . Therefore, given the transmission of instructions, such as control signals S Bi , S 11 , S 12 , the local controller  43  not only controls the direction of current and the strength of current in the electrical power conversion device  45  but also controls the direction of current and the strength of current between devices (such as an AC grid, each load  3 , the solar power generation device  41 , and the clustered energy-storing device  44 ) coupled to the electrical power conversion device  45 , so as to substantially control the operation of the micro-grid  4  in its entirety. 
         [0042]    The system operation modes include a load following mode, a fixed power mode and an emergency power mode as described below. 
         [0043]    In the load following mode, the micro-grid system  1  provides the required power level to the loads  3  one by one. Referring to  FIG. 2 , in the situation where not only has the required power level of the loads  3  exceeded the level of power generated from the solar power generation device  41  but the clustered energy-storing device  44  has also reached a predetermined stored power level, the local controller  43  controls the electrical power conversion device  45  to thereby transmit the power generated from the solar power generation device  41  to the DC/DC converter  46 , and then the DC/AC converter  49  converts the DC power into AC power so that the AC power is supplied to meet a portion of the power consumption requirement of the loads  3 ; meanwhile, in case of insufficient renewable energy-derived power, the clustered energy-storing device  44  will release power, and then the bidirectional DC converter  47  transmits DC power to the DC/AC converter  49  for conversion into AC power to supplement the aforesaid insufficient other portion of the power consumption requirement while the solar power generation device  41  is supplying power to the loads  3 . In the situation where the level of power required for the loads  3  exceeds the level of power generated from the solar power generation device  41  and the clustered energy-storing device  44  has not reached the predetermined stored power level, the other portion of power required for the loads  3  is supplied by the AC utility power ends  7 , wherein the AC power is directly transmitted to each load  3  by the AC wire  2 . 
         [0044]    If the level of power required for the loads  3  is lower than the level of power generated from the solar power generation device  41 , the solar power generation device  41  will solely supply all the loads  3  with their respective required levels of power, regardless of the level of the power stored in the clustered energy-storing device  44 ; if residual power is available, it will flow to the clustered energy-storing device  44  for storage (that is, charging), or the AC utility power ends  7  will perform a power resale process (by feeding the residual power to the AC grid to thereby achieve the purpose of reselling power to an electric utility of the AC grid). 
         [0045]    In the fixed power mode, the micro-grid system  1  provides a fixed level of power to all the loads  3 . Referring to  FIG. 2 , in the situation where the clustered energy-storing device  44  has reached a predetermined stored power level and the level of power generated from the solar power generation device  41  is insufficient, the bidirectional DC converter  47  transmits DC power to the DC/AC converter  49  for conversion into AC power to meet the other portion of power requirement of the loads  3 , thereby compensating for the inadequacy of power supplied by the solar power generation device  41 . When the clustered energy-storing device  44  has not reached the predetermined stored power level, the local controller  43  controls the electrical power conversion device  45  to give priority to clients having low accumulative power consumption level in the loads  3 . When the power generated from the solar power generation device  41  is less than the power required for the loads  3 , the local controller  43  controls the electrical power conversion device  45  so that the clustered energy-storing device  44  serves as a source of standby power, thereby allowing the clustered energy-storing device  44  to provide power which compensates for the inadequacy of power supplied by the solar power generation device  41  to the loads  3 . 
         [0046]    Regarding the emergency power mode, the local controller  43  switches quickly to this mode as soon as a utility grid malfunctions (for example, as a result of a breakdown), so as to control the micro-grid system  1  to operate independently and maintain the level of power supplied to the loads  3 . For example, the DC/AC converter  49  is capable of performing island detection to detect whether the AC grid is malfunctioning. When the DC/AC converter  49  detects that the AC grid is malfunctioning, it is feasible to disconnect the AC grid from an AC utility power end  1  so that the micro-grid system  1  operates independently and therefore maintains the level of power required for the loads  3 ; meanwhile, the local controller  43  controls the clustered energy-storing device  44  to release power for use as emergency power. For example, when the power generated from a renewable energy device (such as the solar power generation device  41 ) is insufficient for use by the loads  3 , the local controller  43  uses control signal S Bi  to control the bidirectional DC converter  47  to transmit supplementary power to the DC/AC converter  49  to serve as emergency power and be converted into AC power for use by the loads  3 . For example, before the micro-grid system  1  begins to operate in the emergency power mode or after the micro-grid system  1  has operated in the emergency power mode, the clustered energy-storing device  44  can be charged according to the time configured by a system user. For example, in the situation where a renewable energy device (such as the solar power generation device  41 ) has supplied power required for the loads  3  and residual power is available, the local controller  43  uses control signal S Bi  to control the bidirectional DC converter  47  to store the residual power in the clustered energy-storing device  44  so that the power thus stored serves as emergency power subsequently. 
         [0047]    In an embodiment, the load following mode is denoted by mode  1 , the fixed power mode by mode  2 , and the emergency power mode by mode  3  to thereby match the system operation modes; after considerations have been given to the stored power level status and weather status (in the daytime and the nighttime) of the clustered energy-storing device  44 , the sources of electrical power which the loads  3  receive from the micro-grid  4  under different system operation modes in this embodiment are shown in Table 1 below. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 relation between energy-storing status and power source of load 
               
             
          
           
               
                 stored power level 
                 source of electrical power which a load receives from 
               
               
                 status of clustered 
                 the micro-grid under different system operation modes 
               
             
          
           
               
                 energy-storing device 
                 mode 1 
                 mode 2 
                 mode 3 
               
               
                   
               
             
          
           
               
                 daytime 
                 sufficient 
                 renewable 
                 renewable 
                 renewable energy + 
               
               
                   
                 stored 
                 energy + 
                 energy + 
                 energy-storing 
               
               
                   
                 power level 
                 energy-storing 
                 energy-storing 
               
               
                   
                 insufficient 
                 renewable 
                 renewable 
                 power generation 
               
               
                   
                 stored 
                 energy + 
                 energy + 
                 is unavailable or 
               
               
                   
                 power level 
                 AC utility 
                 AC utility 
                 trace power is 
               
               
                   
                   
                 power 
                 power 
                 provided by 
               
               
                   
                   
                   
                   
                 renewable energy 
               
               
                   
                   
                   
                   
                 only 
               
               
                 nighttime 
                 sufficient 
                 energy-storing + 
                 energy-storing + 
                 energy-storing 
               
               
                   
                 stored 
                 AC utility 
                 AC utility 
               
               
                   
                 power level 
                 power 
                 power 
               
               
                   
                 insufficient 
                 AC utility 
                 AC utility 
                 power generation 
               
               
                   
                 stored 
                 power 
                 power 
                 is unavailable or 
               
               
                   
                 power level 
                   
                   
                 trace power is 
               
               
                   
                   
                   
                   
                 provided by 
               
               
                   
                   
                   
                   
                 renewable energy 
               
               
                   
                   
                   
                   
                 only 
               
               
                   
               
             
          
         
       
     
         [0048]      FIG. 5  is a schematic view of the process flow of operation of a system operation mode of the clustered energy-storing micro-grid system  1  according to an embodiment of the present invention. 
         [0049]    When the clustered energy-storing micro-grid system  1  starts and begins to operate (S 101 ), one of the system operation modes (operating modes) is selected (S 102 ) so that the clustered energy-storing micro-grid system  1  operates in the selected system operation mode. The system operation modes include a load following mode (S 103 ), a fixed power mode (S 104 ) and an emergency power mode (S 105 ). 
         [0050]    The present invention is disclosed above by preferred embodiments. However, persons skilled in the art should understand that the preferred embodiments are illustrative of the present invention only, but should not be interpreted as restrictive of the scope of the present invention. Hence, all equivalent modifications and replacements made to the aforesaid embodiments should fall within the scope of the present invention. Accordingly, the legal protection for the present invention should be defined by the appended claims.