Abstract:
A DC/AC inverter is disclosed having two DC input terminals ( 1, 2 ), between which are connected an energy buffer capacitor (C), two output voltage terminals ( 3, 4 ) connected to filter section, switch configuration comprising the active switches (S 1 -S 6 ), and freewheeling diodes (D 1 -D 6 ) between the output voltage terminals ( 3, 4 ) and DC input terminals ( 1, 2 ) to provide real and/or reactive power to a public or islanded electric network. It is provided that, capacitive leakage currents occurring on the generator side be avoided while conserving high efficiency. This is achieved in that a freewheeling current path, is established for the line current (I N ) to freewheel through one of the freewheeling diodes (D 3,  D 6 ) in conjunction with one of their respective parallel semiconductor switches (S 3,  S 6 ) when the two output voltage terminals ( 3, 4 ) are decoupled from the DC input terminals ( 1, 2 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a DC/AC inverter to convert DC current/voltage into AC current/voltage as shown in  FIG. 1 . Such types of inverters are used to feed public electric network or to constitute an islanded electric network. These inverters are suitable for electric network connected DC voltage/current energy sourced applications such as fuel cell and photovoltaic systems. 
         [0003]    2. Description of the Prior Art 
         [0004]    The energy injection of DC/AC inverters to an existing AC electric network is based on generating sinusoidal current depending on the voltage of the electric network. Besides, the establishment of an islanded electric network with DC/AC inverters is achieved by providing preferably sinusoidal voltage with fixed frequency and amplitude. 
         [0005]    In addition to real power provision capability, reactive power provision capability of a DC/AC inverter is preferable to meet the reactive power requirements of local loads or to prevent voltage rise at the point of common coupling. 
         [0006]    DC/AC inverters connected to a public electric network or utilized in an islanded electric network should have high efficiency of energy conversion, low volume, lightness, and low cost. Besides, systems utilizing these inverters should maintain human and electric network safety by preventing capacitive leakage currents on the generator side causing electro-magnetic compatibility (EMC) problems especially in photovoltaic systems. 
         [0007]    The low voltage connection of sources or energy storage systems having DC current/voltage to an electric network can be realized via transformer-based or transformerless DC/AC inverters. 
         [0008]    Since transformer-based DC/AC inverters provide galvanic isolation, they have negligible capacitive leakage currents on the generator side. Thus, EMC problems due to leakage currents are minimized in transformer-based systems. However, transformer-based DC/AC inverters suffer from the core and copper losses of the transformer. Moreover, inclusion of a transformer increases the cost, size, and weight. The non-existence of transformer core and copper losses in transformerless inverters provide an increase in energy conversion efficiency, and decrease in the cost, size, and weight. Therefore, payback period of systems with transformerless inverters are shorter than systems with transformer-based inverters. Despite such advantages of transformerless DC/AC inverters, systems utilizing these inverters, especially photovoltaic energy sourced electric network connected systems, may encounter EMC problems caused by capacitive leakage currents. Utilized DC/AC inverter topology and related switching technique in such systems have primary importance to suppress capacitive leakage currents and related EMC problems. 
         [0009]    The conventional and widely utilized H-bridge DC/AC inverter topology shown in  FIG. 2  has two switching techniques. These are unipolar modulation and bipolar modulation. The unipolar modulation of the H-bridge DC/AC inverter topology has certain advantages that increase energy conversion efficiency such as three-level output voltage and no energy backflow to the DC bus capacitor (C). Although it is applicable in transformer-based DC/AC inverters due to these advantages, the H-bridge DC/AC inverter topology with unipolar modulation switching technique restricts its utilization as a transformerless DC/AC inverter because of the EMC problems caused by capacitive leakage currents especially in grid connected photovoltaic source applications. In bipolar modulation of the H-bridge topology, EMC problems are not dominant. However, line current ripple is increased due to two-level output voltage characteristic or such a characteristic yields a requirement of higher inductance value for the same peak line current ripple value. The increase in the line current ripple or in the value of the required filter inductance of bipolar modulation decrease the energy conversion efficiency and increase the cost as compared to unipolar modulation of the H-bridge topology. 
         [0010]    A prior art circuit configuration comprising an H-bridge and an additional switch S 5 ″ connected to positive DC bus rail as shown in FIG. 3 is described in U.S. Pat. No. 7,411,802 B2. This circuit avoids the high frequency capacitive leakage currents on the generator side of that are existent in the unipolar modulation of the H-bridge. This is achieved by the fact that at freewheeling states decoupling of AC and DC sides is provided. Moreover, the output voltage is three-level so that related core and copper losses are reduced on the filter inductors and there is no backflow of the energy to the DC bus capacitor (C) so that efficiency is increased. However, at each time interval that the output voltage terminals are connected to the DC bus terminals, said active state, the number of semiconductor on the line current path is three, resulting in energy conversion efficiency deterioration thereof. 
         [0011]    In the present invention, besides capacitive leakage currents are avoided by decoupling output voltage terminals ( 3 ,  4 ) from the DC input terminals ( 1 ,  2 ) (shown in  FIG. 1 ) at zero states, the number of semiconductors on the line current path at active states is reduced to two for half of the electric network period. This reduction of number of semiconductors on the line current path increases the energy conversion efficiency. Besides the low leakage current and high energy conversion efficiency characteristics, the present invention provides a DC/AC inverter with reactive provision capability which is nonexistent in the prior art shown in  FIG. 3 . In addition to real power, reactive power provision capability of a grid connected DC/AC inverter is preferable to meet the reactive power requirements of the local loads or to regulate the voltage at the point of common coupling. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The aim of the present invention is to provide a high efficiency transformerless DC/AC inverter with reactive power provision capability either in a public electric network connected mode or in an islanded mode while preventing high frequency leakage currents on the generator side. 
         [0013]    To achieve the objective of prevention of high leakage currents on the generator side, there is a DC/AC inverter circuit configuration in accordance with the present invention as shown in  FIG. 1 . The circuit consists of an input capacitor (C), configuration of the switches S 1 -S 6  and their respective anti parallel freewheeling diodes D 1 -D 6 , and filter section to suppress high frequency content of the line current. The prevention of leakage current on the generator side is achieved by decoupling alternating current voltage circuit from the direct current voltage circuit at freewheeling states. In accordance with the invention, the freewheeling current flows through a freewheeling current path determined by the polarity of the freewheeling current when the path is provided in the freewheeling states. 
         [0014]    With the prevention of capacitive leakage currents, safety is increased for direct current circuit components, for electric network, and for persons. Moreover, reliability and lifetime are increased as the generator side photovoltaic modules are not exposed to high leakage currents and additional losses due to leakage currents are nonexistent even the circuit does not include a transformer. 
         [0015]    Therefore, preferred embodiment of the circuit of the invention is a transformerless inverter with low cost and high efficiency as compared to transformer-based units. 
         [0016]    In the present invention, size of the filter chokes and related losses are reduced as compared to H-bridge bipolar modulation. This is achieved by means of pulse width modulation of the switches and the H-bridge unsymmetrical modulation voltage output characteristics of the invention. 
         [0017]    The circuit of the present invention can be designed and implemented to operate at unity power factor. In this configuration, the semiconductors can be optimized for high efficiency as S 1 , S 2 , S 4 , S 5  being MOSFETs and their respective freewheeling diodes D 1 , D 2 , D 4 , D 5  being their body diodes. The diodes D 1 , D 2 , D 4 , and D 5  can be also nonexistent at unity power factor operation design. The switches S 3 , S 6  can be chosen as IGBTs wherein their respective diodes (D 3 , D 6 ) correspond to fast built-in diodes or these freewheeling diodes can be fast external diodes. 
         [0018]    To increase efficiency in the design with reactive power provision capability, meaning design for non-unity power factor operation, the switches S 1 -S 6  can be selected as IGBTs and the respective freewheeling diodes D 1 -D 6  can be fast built-in diodes of the IGBTs or fast diodes can be mounted externally. Since all the diodes carries are intended to carry pulsed current in this kind of operation, such embodiments of fast diodes yield better commutation characteristics. 
         [0019]    The proposed circuit within the scope of this invention provides lower conduction losses as compared to the transformerless DC/AC inverter topology described in U.S. Pat. No. 7,411,802 B2 and shown in FIG. 3. This is achieved by means of less number of switches on the line current path at active states when the electric network voltage is positive. Thus, the energy conversion efficiency is increased in the present invention. 
         [0020]    The implementation of this circuit configuration can be performed for one-phase or multi-phase either in an electric network connected mode or in an island mode. 
         [0021]    In dependent claims, further advantageous features and the implementations are given. 
         [0022]    The invention and the advantages will be described in further detail with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The present invention is made more apparent using preferred embodiments with reference to the related drawings without the intention of limiting the scope of the invention. In the drawings: 
           [0024]      FIG. 1  shows an illustration of a circuit arrangement of a DC/AC inverter in accordance with the invention, 
           [0025]      FIG. 2  shows an illustration of a prior art DC/AC inverter circuit arrangement, 
           [0026]      FIG. 3  shows an illustration of a prior art DC/AC inverter circuit arrangement, 
           [0027]      FIG. 4  shows an illustration of a single-phase electric network voltage and injected current in accordance with the circuit arrangement in  FIG. 1 , 
           [0028]      FIG. 5  shows an illustration of the circuit arrangement shown in  FIG. 1  with active state current path during time region R 1  in  FIG. 4 , 
           [0029]      FIG. 6  shows an illustration of the circuit arrangement shown in  FIG. 1  with zero state current path during time region R 1  or R 4  in  FIG. 4 , 
           [0030]      FIG. 7  shows an illustration of the circuit arrangement shown in  FIG. 1  with active state current path during time region R 3  in  FIG. 4 , 
           [0031]      FIG. 8  shows an illustration of the circuit arrangement shown in  FIG. 1  with zero state current path during time region R 2  or R 3  in  FIG. 4 , 
           [0032]      FIG. 9  shows an illustration of the circuit arrangement shown in  FIG. 1  with active state current path during time region R 2  in  FIG. 4 , and 
           [0033]      FIG. 10  shows an illustration of the circuit arrangement shown in  FIG. 1  with active state current path during time region R 4  in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    In  FIG. 1 , a transformerless DC/AC inverter is shown in accordance with the present invention. This circuit allows for a method to convert DC current/voltage to AC current/voltage with reactive power provision capability, improved efficiency, and reduced leakage currents on the generator side. 
         [0035]    The DC/AC inverter shown in  FIG. 1  has a DC voltage input with positive and negative terminals  1 ,  2  respectively, high frequency voltage output terminals  3 ,  4 , and AC voltage output with terminals  6 ,  7 . Connected in parallel to the DC input terminals  1 ,  2 , there is an input energy buffer capacitor C and an energy source G. A half bridge configuration comprising switches  51  and S 2  is connected between the terminals  1  and  2 . Switch S 5  is connected between the nodal point  1  and  5  to allow current flow from the nodal point  1  to nodal point  5  when triggered. Between the nodal points  5  and  2 , another half bridge configuration comprising switches S 3  and S 4  is connected. A switch S 6  is connected between the nodal point  5  and the midpoint called nodal point  3  of the first half bridge that consists of S 1  and S 2  to allow current flow from the nodal point  5  to the nodal point  3 . All the switches S 1 -S 6  have anti-parallel diodes D 1 -D 6 . 
         [0036]    The high frequency voltage output terminals  3 ,  4 , also called filter section input terminals, are connected to the filter section residing between the so called the filter section input terminals  3 ,  4  and filter section output terminals  6 ,  7 . The filter circuit, which is the circuit residing between the filter input terminals  3 ,  4 , and filter output terminals  6 ,  7 , has two filter inductors L 1 , L 2  preferably with equal ratings. The filter inductor L 1  is connected in between the filter circuit input terminal  3  and the filter circuit output terminal  6 . The other filter inductor L 2  is connected in between the filter circuit input terminal  4  and filter circuit output terminal  7 . The filter circuit output terminals  6 ,  7  are connected to an electric network N to provide preferably sinusoidal current/voltage for example at 50 Hz or 60 Hz. 
         [0037]    Any known semiconductor switch such as MOSFETs, IGBTs or FETs can be used in principle for the realization of the switches S 1 -S 6  of the DC/AC inverter in accordance with the present invention. The freewheeling diodes D 1 -D 6  of the DC/AC inverter can be built-in diodes or they can be external diodes. However, to optimize the energy conversion efficiency the switches S 1 -S 6  and the freewheeling diodes D 1 -D 6  should be chosen accordingly. The switches S 1 -S 6  and freewheeling diodes D 1 -D 6  of the DC/AC inverter circuit can be selected according to reactive power provision capability intention in the design of the inverter. In other words, shown in  FIG. 4 , the value of the phase angle φ between the line current I N  and the electric network voltage V N  of the DC/AC inverter shown in  FIG. 1  can be used to determine the type of the semiconductor switches S 1 -S 6  and their anti-parallel diodes D 1 -D 6  for the purpose of optimization of energy conversion efficiency of the DC/AC inverter in accordance with the invention. 
         [0038]    When the reactive power provision capability is not desired in the implementation of the DC/AC inverter circuit of the present invention, in other words the inverter is designed to operate at phase angle φ ideally being zero, said unity power factor operation, MOSFETs are suitable for the realization of the semiconductor switches S 1 , S 2 , S 4 , and S 5 . This is due to the fact that, low on-state resistance of MOSFETs yields low losses. 
         [0039]    In the embodiment of the design of the DC/AC inverter of the present invention for the unity power factor operation, the semiconductor switches S 1 , S 2 , S 4 , and S 5  are clocked at high frequency (between 1 kHz and 1 MHz, for example 20 kHz) to modulate the output voltage (the voltage between the nodal points  3  and  4 ) of the inverter. At unity power factor operation, to provide zero output voltage states (in other words, the states when the voltage between the nodal points  3  and  4  is zero), said zero states, the switches S 3  and S 6  are clocked at the electric network frequency, for example at 50 Hz. In the case of unity power factor operation, since there is no backflow of electric energy from AC side to DC bus capacitor (C), the diodes D 3  and D 6  are sufficient to allow the flow of line current I N  at zero states. According to the sign of the line current I N , one of the freewheeling diodes D 3 , D 6  takes the line current I N  in conjunction with the one of the switches S 3 , S 6  to establish a freewheeling path at zero states. To increase the efficiency characteristics, the freewheeling diodes D 3  and D 6  can be selected as fast diodes. 
         [0040]    In unity power factor operation mode, the DC/AC inverter in accordance with the invention operates either in time region R 1  or R 3  shown in  FIG. 4 . In this mode, the phase angle φ is zero; therefore, the electric network voltage V N  and the inverter current I N  are aligned. While V N  and I N  are positive (corresponding to region R 1  in  FIG. 4  with phase angle φ being zero), the switches S 1  and S 4  are clocked and pulse-width modulated in synchronism at high frequency (between 1 kHz and 1 MHz) whereas they are switched-off in the other half time duration (when V N  and I N  are negative). In  FIG. 5 , the line current I N  path is illustrated for the condition of positive active state and positive line current values, in other words for the region R 1  in  FIG. 4 . In this condition, the current flows through the switches S 1  and S 4 . When commutated to zero state, the positive line current I N  flows from the path through the switch S 6  and the diode D 3 , as shown in  FIG. 6 . In this condition, the DC input terminals  1 ,  2  and inverter output voltage terminals  3 ,  4  are decoupled from each other in accordance with the present invention. While V N  is negative (corresponding to region R 3  in  FIG. 4  with phase angle φ being zero), the switches S 2  and S 5  are clocked and pulse-width modulated in synchronism at high frequency, whereas they are switched-off when V N  is positive. The line current I N  path when the inverter provides negative output voltage (negative state) and the line current I N  is negative is shown in  FIG. 7 . In this condition, the line current I N  flows through the switches S 2 , S 3 , and S 5 . Zero states in the unity power factor operation mode in region R 3  (when V N  and I N  are negative) are provided as shown in  FIG. 8 . 
         [0041]    When the DC/AC inverter of the present invention is designed to be able to provide reactive power, the semiconductor switches S 1 -S 6  should be preferably realized as IGBTs since MOSFETs have slow parasitic body diodes that introduce high switching losses. For reactive power provision capability, the DC/AC inverter in accordance with the invention should be able provide positive active states with negative I N  and negative active states with positive I N  that correspond to region R 2  and region R 4  respectively in  FIG. 4 . In  FIG. 9 , negative line current I N  path at positive active states is illustrated. On this path, the freewheeling diodes D 1 , D 4  become forward biased and they provide current for the backflow of the electric energy to the DC bus capacitor C. In  FIG. 10 , positive line current I N  path at negative active state is illustrated. On this path, the freewheeling diodes D 2 , D 3 , and D 5  become forward biased and they provide current for the backflow of the electric energy to the DC bus capacitor C. According to the sign of the line current I N , zero states of the DC/AC inverter can be provided through the pairs S 3 , D 6  or S 6 , D 3  as shown in  FIG. 6  and  FIG. 8  with decoupling of DC input terminals  1 ,  2  and the inverter voltage output terminals  3 ,  4  in accordance with the invention. The freewheeling diodes D 1 -D 6  of the DC/AC inverter should be fast diodes for low switching losses and high efficiency. Accordingly, these freewheeling diodes D 1 -D 6  can be built-in diodes or they can be connected externally. 
         [0042]    With the decoupling of DC and AC terminals at zero states, with the low number of semiconductors on the line current path both in active states and zero states, and with the three-level output voltage, the invention provides a low leakage current and high efficiency transformerless DC/AC inverter with reactive power provision capability. 
       LIST OF NUMERALS (LIST OF PART NUMBERS) 
       [0043]      1 ,  2  inverter DC input terminals 
         [0044]      3 ,  4  inverter voltage output terminals 
         [0045]      5  inverter intermediate nodal point 
         [0046]      6 ,  7  AC electric network connection terminals 
         [0047]    C input buffer capacitor 
         [0048]    V dc  DC bus voltage 
         [0049]    G generator 
         [0050]    S 1 -S 6  semiconductor switching elements 
         [0051]    D 1 -D 6  freewheeling elements 
         [0052]    L 1 , L 2  filter inductors 
         [0053]    N either a public electric network or an islanded electric network 
         [0054]    V N  electric network voltage 
         [0055]    I N  electric network current 
         [0056]    φ phase angle between electric network voltage and current 
         [0057]    R 1 -R 4  time regions based on the sign of electric network voltage and current