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  Photovoltaic Inverter Structures  13 2.2.3 HERIC Inverter (Sunways) In 2006, Sunways patented a new topology also derived from the classical H-bridge calledHERIC (highly efficient and reliable inverter concept) by adding a bypass leg in the AC sideusing two back-to-back IGBTs (insulated gate bipolar transistors), as shown in Figure 2.7 [6].The AC bypass provides the same two vital functions as the fifth switch in case of the H5topology:     Prevents the reactive power exchange between  L 1(2)  and  C  PV   during the zero voltage state,thus increasing efficiency.     Isolates the PV module from the grid during the zero voltage state, thus eliminating thehigh-frequency content of   V  PE  .The switching states for positive and negative generated AC currents are depicted inFigure 2.8.The main features of this converter are:     S  1– S  4 and  S  2– S  3 are switched at high frequency and  S  + ( S  − ) at grid frequency.     Two zero output voltage states are possible:  S  += on and  S  −= on (providing the bridge isswitched off).Advantages:     Voltage across the filter is unipolar (0 →+ V  PV   → 0 →− V  PV   → 0), yielding lower corelosses.     Higher efficiency of up to 97 % is due to no reactive power exchange between  L 1(2)  and  C  PV  during zero voltage and to lower frequency switching in one leg.     V  PE   has only a grid frequency component and no switching frequency components, yieldingvery low leakage current and EMI. Grid   N  L Filter FilterHERIC FB inverter PV Array 1 S  1  D 3 S  3  D 2 S  2  D 4 S  4  D  PV  V   PV  C   PE  V  1  L S   +  D − S   −  D + 2  L  g  V  Figure 2.7  HERIC topology (Sunways)  14  Grid Converters for Photovoltaic and Wind Power Systems Grid   N  L Filter FilterHERIC FB inverter PV Array V   g  > 0. S  1 and S  4 = ON, S  + = ON S  1 and S  4 are switched at high frequency. S  + is switched at line frequency S  1  D 1 S  3  D 3 S  2  D 2 S  4  D 4 S  +  D +  D − S −  A B  AB PV  V V  =+ 1  L 2  L  g  V   PV  V   PV  C   PE  V  Grid   N  L Filter FilterHERIC FB inverter PV Array V   g  > . 0  S  1 and   S  4 =OFF. S  + and  D - = ON S  5 and S  4 are switched at high frequency. S  + is switched at line frequency S  1  D 1 S  3  D 3 S  2  D 2 S  4  D 4 S  +  D + S  −  D − 1  L 2  L  g  V  0  AB V  =  A B  PV  V   PV  C   PE  V  Grid   N  L Filter FilterHERIC FB inverter PV Array V   g  < 0. S  2 and S  3 = ON. S   – = ON S  2 and S  3 are switched at high frequency. S  -is switched at line frequency S  1  D 1 S  3  D 3 S  2  D 2 S  4  D 4 S +  D +  D − S  − 1  L 2  L  g  V   AB PV  V V  = −  PV  C   PV  V   PE  V   A  B Grid   N  L Filter FilterHERIC FB inverter PV Array V   g  < 0. S  2 and S  3 =OFF. S  -and  D + = ON S  2 and S  3 are switched at high frequency. S  -is switched at line frequency S  1  D 1 S  3  D 3 S  2  D 2 S  4  D 4 S  +  D +  D − S  − 2  L 1  L  g  V  0  AB V  =  B  A  PV  C   PV  V   PE  V  (a)(b) Figure 2.8  The switching states of the HERIC inverter in the case of generating: (a) positive currentand (b) negative current Drawbacks:     Two extra switches.Remark: The HERIC improves the performance of FB with BP modulation by adding the zero voltage obtained with the AC bypass, thus increasing the efficiency. Thistopology is therefore very suitable for use in transformerless PV applications dueto high efficiency and low leakage current and EMI. It is currently commercialized by Sunways in the AT series (2.7–5 kW) with reported European efficiency of 95 %and maximum efficiency of 95.6 % (Photon International, July 2008). The behavior of HERIC and H5 are quite similar as both realize the decoupling of the PVgenerator from the grid during the zero voltage state on the AC side and DC side respectively.They both use two switches switched at high frequency and one switched at grid frequency,and H5 has three switches conducting at the same time, while HERIC has only two.
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