Abstract:
The present invention relates to a multilevel inverter comprising: a converter unit converting an inputted AC power source to a direct current (DC) power source; a film capacitor rectifying the DC power source converted by the converter unit; an inverter unit converting the rectified DC power source to a three-phase current in response to a pulse width modulation (PWM) control signal and outputting the current; a current detector detecting a current outputted from the inverter unit; a power cell main controller generating a voltage instruction and a voltage instruction using the detected current; and a PWM controller generating the pulse width modulation (PWM) control signal using the voltage instruction and frequency instruction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on, and claims priority from, Korean Application Number 10-2008-0025967, filed Mar. 20, 2008, the disclosure of which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     The present disclosure relates to a multilevel inverter capable of replacing a chemical capacitor with a film capacitor. 
     In general, a large high voltage induction motor is variably designed with voltages ranging from 2,400v to 7,200v, while a high voltage inverter which is a variable speed motor device suffers from drawbacks such as increased cost, need of wide area for installation and decreased system efficiency due to application to various motors using separate step-up and step-down transformers and also due to lack of variable voltages. The high voltage inverter also suffers from drawbacks such as harmonic influence on bus, motor burnt-out and vibration caused by pulse width modulation voltage. 
     Various kinds of multilevel inverters have been developed to overcome the drawbacks, and one of the power topologies showing the most excellent characteristics in terms of input/output quality is an H-bridge multilevel inverter. The H-bridge multilevel inverter generally a multilevel inverter using a cascade configuration which is a multilevel topology of a high voltage and large capacity inverter, in which several single phase inverters (hereinafter, referred to as power cells or cells) are connected in series for each phase of a three-phase current and accordingly a high voltage can be obtained by using low voltage power semiconductor switches within the power cells. Thus, the H-bridge multilevel inverter is called a cascade inverter. 
     The H-bridge multilevel inverter using a cascade configuration has a feature of a pulse width modulation/phase shift wherein a phase difference is sequentially generated between power cells which are serially connected to one another. Accordingly, the H-bridge multilevel inverter can have a low rate of output voltage change (dv/dt). In addition, the multilevel inverter using the cascade configuration can obtain reduced total harmonic distortion (THD) due to an output voltage with multi levels, namely, many steps. 
     Further, the H-bridge multilevel inverter using the cascade configuration rarely incurs a voltage reflection. Accordingly, in spite of a long distance between the multilevel inverter using the cascade configuration and a motor, there is no need for a separate device to prevent the voltage reflection phenomenon. 
     Unlike other multilevel inverters, the H-bridge multilevel inverter has advantages such as no issues of voltage imbalance between DC-link capacitors and easy extension to a desired output voltage by modularization. 
       FIG. 1  is a circuit diagram illustrating a configuration of a conventional H-bridge multilevel inverter system. 
     Referring to  FIG. 1 , the H-bridge multilevel inverter system consists of a plurality of power cells  2  each connected in series, where the plurality of single phase power cells  2  are connected in series for each phase of a three-phase current and each power cell  2  has an independent single phase inverter structure. An input unit connected to a power system is a transformer  6  having several tabs of extended delta connection method at a secondary wiring side. 
       FIG. 2  is a circuit diagram illustrating configuration of power cell of the conventional H-bridge inverter. 
     Referring to  FIG. 2 , the H-bridge inverter comprises: an input alternating current (AC) power source  10 ; a converter unit  11  converting the inputted AC power source to a direct current (DC) power source; an initial charging resistor  12  preventing an inflow of a rush current during input of the inputted AC power source; an electronic contactor  13  separating the initial charging resistor  12  from the circuit following the prevention of the rush current; a chemical capacitor  14  rectifying a DC voltage; an inverter unit  15  converting the inputted DC power source in response to a pulse width modulation (PWM) control signal; a current detector  16  detecting a current outputted from the inverter unit  15 ; a power cell main controller  17  collecting various information including a three-phase current and the DC voltage of the inverter unit  15  and exchanging various instructions and information with a master controller ( 17 . not shown); and a PWM controller  18  receiving voltage instruction and frequency instruction from the power cell main controller  17  to generate a pulse width modulation (PWM) control signal. 
     Each power cell of the convention H-bridge inverter is equipped with the chemical capacitor  14 . The chemical capacitor  14  is an essential constituent element having a direct influence on the life of the inverter, such that the chemical capacitor  14  should be carefully chosen as it affects greatly on a current ripple rate and ambient temperature. 
     In general, the chemical capacitor  14  functions in the following manner. That is, the chemical capacitor  14  serves to compensate an instantaneous difference between an input power and an output power for each power cell unit, to compensate an output using an energy of the chemical capacitor  14  for a predetermined time period during instantaneous black-out, and functions to store a regenerative energy when the regenerative energy is generated. 
     However, the high-voltage inverter system which is a single phase output inverter system suffers from a drawback of being installed with more numbers of capacitors than those of the three phase output inverter, thereby increasing the overall size of the entire system. 
     The chemical capacitor  14  has a high volume capacitance, where a current inputted into a DC terminal generally shows a rabbit ear-shaped discontinuous current, which is a cause of generating harmonics. 
     Still another drawback of the chemical capacitor  14  is that circuits associated with initial charging, for example, circuits such as initial charging resistor  12  and electronic contactor  13 , are additionally required. 
     SUMMARY 
     Therefore, it is an object of the present invention to provide a multilevel inverter, the inverter comprising: a converter unit converting an inputted AC power source to a direct current (DC) power source; a film capacitor rectifying the DC power source converted by the converter unit; an inverter unit converting the rectified DC power source to a three-phase current in response to a pulse width modulation (PWM) control signal and outputting the current; a current detector detecting a current outputted from the inverter unit; a power cell main controller generating a voltage instruction and a voltage instruction using the detected current; and a PWM controller generating the pulse width modulation (PWM) control signal using the voltage instruction and frequency instruction. 
     The multilevel inverter may further comprise an input transformer, where the input transformer has several tabs at a secondary wiring side by way of zigzag connection or an extended delta connection, and an output power source of the input transformer is inputted into the input AC power source. 
     The inverter unit may include one of the elements out of metal oxide field effect transistors (MOSFET), insulated gate bipolar transistors (IGBT) and gate turn-off (GTO) thyristors. The inverter unit may further include a phase shift transformer, where an output power source of the phase shift transformer is inputted to the input AC power source. The phase shift transformer may include an inductor having a predetermined inductance value, where relation between the predetermined inductance value and the capacitance value of film capacitor is set at 1:3. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a configuration of a conventional H-bridge multilevel inverter system. 
         FIG. 2  is a circuit diagram illustrating configuration of a power cell of the conventional H-bridge inverter. 
         FIG. 3  is a circuit diagram illustrating a configuration of each power cell in a multilevel inverter according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The multilevel inverter according to the present invention will be described in detail with reference to  FIG. 3 . 
     In describing the present disclosure, detailed descriptions of constructions or processes known in the art may be omitted to avoid obscuring appreciation of the invention by a person of ordinary skill in the art with unnecessary detail regarding such known constructions and functions. Particular terms may be defined to describe the invention in the best mode as known by the inventors. Accordingly, the meaning of specific terms or words used in the specification and claims should not be limited to the literal or commonly employed sense, but should be construed or may be different in accordance with the intention of a user or an operator and customary usages. Therefore, the definition of the specific terms or words should be based on the contents across the specification. 
       FIG. 3  is a circuit diagram illustrating a configuration of each power cell in a multilevel inverter according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , the multilevel inverter may include an input AC power source  100 , a converter unit  110 , a film capacitor  120 , an inverter unit  130 , a current detector  140 , a power cell main controller  150 , and a PWM controller. 
     The input AC power source  100  is inputted by a secondary side output of an input transformer (not shown). The input transformer may be employed for two usages. First, the input transformer serves to supply an independent power source to each power cell of an H-bridge multilevel inverter. Second, the input transformer generates a phase difference between secondary side tabs to form a rectifier type converter of multi-pulse method, whereby a low total harmonic distortion (THD) can be obtained at an input terminal. The input transformer may be formed at the secondary wiring side with several tabs using extended delta connection or zigzag connection. 
     An input transformer connectible to the input AC power source  100  may use a phase shift transformer. The phase shift transformer may further include an inductor having a predetermined inductance value. Inductance value of the inductance and capacitance value of the film capacitor may be adjusted to improve a total harmonious distortion (THD) of an input terminal. For example, relation between the inductance value and the capacitance value of the film capacitor may be set up at 1:3. 
     The converter unit  110  serves to convert to a DC power source the AC power source inputted from the input AC power source  100 . The film capacitor  120  functions to rectify the DC power source converted by the converter unit  110 . 
     The inverter unit  130  functions to convert the inputted DC power source to a three-phase AC in response to a PWM control signal and output the three-phase AC. The inverter unit  130  may be comprised of the most common switching elements for output voltage control such as metal oxide field effect transistors (MOSFET), insulated gate bipolar transistors (IGBT) and gate turn-off (GTO) thyristors. A harmonic cut-off frequency by switching at inverter terminal may be determined within a scope of ⅕ to ½ of the switching frequency employed by the inverter. That is, the switching frequency may be designed based on 1 kHz, and the harmonic cut-off frequency by the switching at the inverter terminal may be 200 Hz to 500 Hz when the aforementioned switching is employed. The cut-off frequency (wc) may be set up as per the following equation, and the capacitance value may be determined in consideration of inductance value of input transformer according to cut-off frequency value. 
     
       
         
           
             
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     The current detector  140  serves to detect the current of each phase outputted from the inverter unit  130  and transmit the current to the power cell main controller  150 . The power cell main controller  150  functions to receive various information including the DC voltage inputted from the inverter unit  130  and the phase current outputted from the inverter unit  130  and exchange various instructions and information for various operation control with a master controller (not shown). The PWM controller  160  serves to receive voltage instruction and frequency instruction from the power cell main controller  150  to generate a pulse width modulation (PWM) waveform corresponding thereto and output a PWM control signal to the inverter unit  130 . The multilevel inverter according to the present invention is characterized in that it may use a film capacitor instead of the chemical capacitor as a capacitor of each power cell. 
     In a case of using the film capacitor, a characteristic equation useable by the power cell main controller  150  and the PWM controller  160  for power stabilization at the DC power source may be given as under: 
     
       
         
           
             
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     Use of film capacitor may be so designed as to allow the power detection and current control method to be robust to dynamic characteristic, and to be sensitive to input power change. 
     Now, the advantage of using the film capacitor instead of chemical capacitor as a capacitor of each power cell will be described. 
     First, the volume of the multilevel inverter may be reduced. The film capacitor has an approximate size ⅓ that of the chemical capacitor to thereby reduce the volume occupied by a capacitor in the multilevel inverter. As a result, the volume of the multilevel inverter can be reduced. Furthermore, the film capacitor is less expensive and has a longer life over that of the chemical capacitor. 
     Second, a charging circuit of an input terminal may be omitted. The film capacitor having a small capacity can be charged within a short time period, whereby there is no requirement of installing an initial charging resistor or an electronic contactor at the input terminal. 
     Third, reliability of multilevel inverter system can be enhanced. The film capacitor makes it possible to dispense with an input terminal charging circuit, such that the multilevel inverter is not affected by the input terminal charging circuit to enhance the reliability of the multilevel inverter system. 
     Fourth, a rush current can be reduced during charging. The film capacitor having a small capacity can greatly reduce the rush current during the charging compared with a case where the chemical capacitor is utilized. Thus, a safety of the multilevel inverter system can be increased during input of an initial power source to obtain an excellent harmonic characteristic. 
     If a system applied with a high voltage inverter is a light load system taking no stock of an overload rate, the system is preferred to use a film capacitor instead of a chemical capacitor. The light load system uses a voltage and frequency constant control method as a control method, and is hardly affected by sudden changes of load. The light load system is capable of implementing an automatic re-start even if an instantaneous black-out occurs or regenerative braking is not operated. 
     There is an advantage in the multilevel inverter according to the present disclosure in that an entire volume of the multilevel inverter can be reduced by using a film capacitor having a smaller size than that of a chemical capacitor. Other advantages are that use of a film capacitor having a small capacity can remove an input terminal charging circuit, and greatly reduce the rush current during charging to enhance the reliability and safety of the multilevel inverter. 
     Still furthermore, the multilevel inverter system may be dispensed with an initial charging circuit and a separate rectification circuit inductor by using the phase shift transformer. At the same time, the total harmonious distortion (THD) of input current at the rectifier terminal may be improved. 
     As the present invention may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within the scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.