Abstract:
The invention presents a structure for the conversion of direct current electric power into alternating current electric power, characterised in that it is simple, highly efficient and minimises the problem of electromagnetic compatibility. The circuit includes, in its first preferred embodiment, six switching elements governed by a command unit, four switches forming an H-bridge (T 1,  T 2,  T 3,  T 4 ) and two auxiliary switches (T 5 D, T 6 D), and two auxiliary diodes (Daux 1  and Daux 2 ). The elements of the H-bridge switch at grid frequency, whereas T 5 D and T 6 D switch at high frequency by means of pulse width modulation (PWM), or other appropriate modulation techniques. The voltage of these auxiliary switching elements (T 5 D, T 6 D) is limited topologically to half the direct current input voltage (Vin), thereby reducing switching losses and resulting in a high performance converter.

Description:
OBJECT OF THE INVENTION 
       [0001]    The present invention has its main field of application in the industry dedicated to the design of electronic devices and, more particularly, to those conceived within the sector of photovoltaic solar energy power systems. 
         [0002]    The object of the invention is to provide a dc/ac conversion structure specially designed for photovoltaic systems connected to the electric grid without a transformer, characterized in being simple, highly efficient and minimizing the problems of electromagnetic compatibility. 
       BACKGROUND OF THE INVENTION 
       [0003]    Photovoltaic systems connected to the grid are today enjoying wide acceptance in our society, and are now being used with increasing frequency. Commonly, they involve installations formed by a group of solar panels and an electronic converter, called an inverter which conditions the energy produced by the panels and injects it into the electric grid. 
         [0004]    In most cases, the converters for these installations are private low-power single-phase circuits, wherein the objective sought is to maximize the economic return obtained by selling the energy produced to electricity companies. For this reason, the inverters required must be cheap, reliable and highly efficient. 
         [0005]    Usually a low-frequency transformer is included in the conversion stage in photovoltaic installations connected to the grid. This transformer guarantees galvanic isolation between the installation and the grid and reduces electromagnetic emissions. The main drawback of the low-frequency output transformer is its considerable size and weight, as well as substantially increasing the price of the conversion stage and, therefore, making the photovoltaic installation as a whole more expensive. 
         [0006]    The evolution of technology means that today it is possible to dispense with this transformer without reducing the properties of the system as regards personal safety and integration in the grid, thereby questioning the need for its use. In the near future, the obligation to use an output transformer is expected to gradually disappear. 
         [0007]    If the transformer is removed, the galvanic isolation between the photovoltaic system and the grid is lost, which impairs system behaviour regarding Electromagnetic Compatibility (EMC). In this event, it is desirable to use conversion topologies that minimize EMC problems. 
         [0008]    The use of a transformer in photovoltaic systems has allowed an H-bridge with unipolar modulation to be employed as the conversion structure, which structure has demonstrated the best efficiency versus complexity relationship. However, this structure behaves poorly from the point of view of EMC.  FIG. 1  shows an H-bridge. This structure comprises two parallel branches, each with its own pair of switches or switching elements in series (T 1 , T 2  and T 3 , T 4 ), usually transistors, with diodes in anti-parallel (D 1 , D 2 , D 3 , D 4 ). 
         [0009]    An option to improve inverter behaviour regarding EMC, is to use bipolar modulation. In this modulation, the T 1 -T 4  and T 2 -T 3  switch pairs switch alternately, obtaining voltages at the output points of the H-bridge having the value of the input voltage with positive and negative sign (+Vin or −Vin). However, bipolar modulation has two disadvantages with respect to unipolar modulation. On one hand, the current ripple in the coil with bipolar modulation is greater. On the other hand, to obtain the same current ripple frequency in the coil in bipolar modulation, it is necessary to switch at twice the frequency, which means it has twice the switching loss. This, together with the fact that the semiconductors have to withstand all the input voltage, implies a decrease in the efficiency obtained with this structure. 
         [0010]    In the resolution of the deficiencies in the structures of the aforementioned inverters, it is necessary to cite the proposal of European patent application EP1369985. The inverter circuit described in the aforementioned document consists of an H-bridge switching at high frequency with bipolar modulation, to which a third branch is added on the alternating side, between the output points (A, B) of the full bridge inverter which is switching at the grid frequency, as shown schematically in  FIG. 2 . This structure, which includes six transistors, improves the behaviour and global efficiency of the photovoltaic converter with respect to the H-bridge with bipolar modulation, according to the operating mode explained in EP1369985. 
         [0011]    This converter described in EP1369985 has two advantages with respect to the H-bridge with bipolar modulation: one, the switching of the transistors of the H-bridge is carried out with half the input voltage, which reduces the switching losses of the converter; two, the maximum current ripple in the coil is half that in the bipolar H-bridge, which allows a smaller coil to be used. However, although the transistors switch with half the input voltage, in the cutoff state they support all the input voltage (Vin) whereby all of them, those in the H-bridge plus those of the additional branch on the alternating side (T 5 A-T 6 A), have to be sized for said voltage. Since switching losses increase with the voltage capacity of the transistor, this feature restricts improvements in performance. 
       DESCRIPTION OF THE INVENTION 
       [0012]    The invention described herein corresponds to a dc/ac inverter circuit specially applicable as a conversion stage in photovoltaic installations connected to the grid, as shown in  FIG. 3 . 
         [0013]    Said circuit minimizes EMC problems, and has a higher efficiency than those previously proposed. 
         [0014]    The circuit of the invention is a single-phase inverter that is connected to a direct current energy source and transforms it into alternating current energy to be fed into an electric grid. The topology of the inverter circuit essentially comprises:
       a temporary energy accumulator that can consist of one or more accumulators connected in series in one or more branches in parallel with the energy source, across the direct current connections of the circuit;   an inverter that is configured as a full bridge or H-bridge comprising at least two parallel branches each with a pair of switching elements in series, consisting of MOSFET, IGBT or another type of transistor that adapts to this configuration with or without their corresponding diodes in anti-parallel;   two auxiliary switching elements (T 5 D, T 6 D), with or without their respective diodes in anti-parallel (D 5 D and D 6 D), connected across the direct current connections and the input of the H-bridge;   a branch with at least one auxiliary diode connected in anti-parallel to the H-bridge across the respective connections of the auxiliary switching elements and said H-bridge; and   at least two output points, which correspond with the centre-taps of the branches of the H-bridge, which constitute alternating current connections, to which inductances are connected, it being possible to connect the electric grid between the inductances.       
 
         [0020]    The switching elements of the H-bridge, which comprise a first pair of transistors (T 1 , T 4 ) and a second pair (T 2 , T 3 ), work as an inverter switching at grid frequency and in synchronism therewith. During the positive half-cycle T 1  and T 4  are on, while in the negative half-cycle T 2  and T 3  will be on. 
         [0021]    The pair of auxiliary switching elements, T 5 D and T 6 D, are capable of switching synchronously by means of a given trigger signal or with independent signals for each switch. 
         [0022]    The operation of the converter can be explained during a switching period of the positive grid half-cycle. T 1  and T 4  are on during the entire positive half-cycle. When T 5 D and T 6 D are on, the input voltage is applied across points A and B. The current flows through T 5 D, T 1 , T 4  and T 6 D. 
         [0023]    When T 5 D and T 6 D are off, the current in the coils is closed through Daux 1 , T 1 , T 4  and Daux 2 . During this period of time a decoupling takes place between the direct current side and the alternating current side. 
         [0024]    The control signals are defined in a command unit that has at least one computation unit and software to implement the control strategy. The computation module comprises at least one programmable electronic device, which can be a general-purpose microprocessor, a micro-controller, a digital signal microprocessor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or any combination of the foregoing, serving to establish the updated values of the energy source working point. 
         [0025]    The main advantages of the invention are:
       The maximum ripple is half that in the H-bridge with PWM bipolar modulation, which allows smaller output inductances to be used.   The problems associated with EMC are minimized.   Transistors T 1 , T 2 , T 3 , T 4 , which switch at low frequency, have to be sized to withstand the input voltage, while the auxiliary switching elements (T 5 D, T 6 D) which switch at high frequency, have to be sized to support half the input voltage. This implies an improvement when compared with the conversion structure that EP1369985 proposes, wherein six transistors are also used but all have to be sized to withstand the entire input voltage.   A further improvement with regard to the precedent EP1369985 is that the T 1 -T 4  and T 2 -T 3  pairs switch at grid frequency, whereby there are hardly any switching losses in them, it being possible to use low saturation voltage semiconductors.       
 
         [0030]    The number of semiconductors that conduct at all times in this converter is higher than in some of the conversion topologies of the current state of the art, for which reason there will be more conduction losses. However, the switching losses in the circuit object of the invention are smaller than in the other structures. This is because the semiconductors that switch at high frequency, T 5 D and T 6 D, in addition to switching at half the input voltage, in contrast with the topologies already cited, this is also the maximum voltage that they have to withstand, therefore this is the voltage for which they should be sized. Thus, a better efficiency is achieved than in state of the art converters. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0031]    To supplement the description that is being made and with the object of assisting in a better understanding of the characteristics of the invention, in accordance with a preferred example of a practical embodiment thereof, attached as an integral part of said description is a set of drawings wherein by way of illustration and not restrictively, the following has been represented: 
           [0032]    FIG.  1 .—Shows a configuration for a photovoltaic converter known in the state of the art as an H-bridge. 
           [0033]    FIG.  2 —Shows another possible configuration for a direct voltage to alternating current or voltage converter, also applicable in photovoltaic systems, according to an embodiment proposed in European patent application EP1369985 pertaining to the state of the art. 
           [0034]    FIG.  3 .—Shows a diagram of the structure of the circuit of the invention according to a preferred embodiment. 
           [0035]    FIG.  4 .—Shows a diagram of the structure of the circuit of the invention according to another preferred embodiment. 
       
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       [0036]    In the light of  FIG. 3 , a first practical embodiment of the invention can be described as a single-phase inverter circuit to condition an energy source ( 8 ), formed by a photovoltaic array or another energy source adapted to supply a continuous input voltage (Vin) that can be transformed into an alternating grid voltage capable of delivering an alternating current to an electric grid ( 9 ), comprising:
       two direct current connections ( 6 ,  7 ) across which at least one branch is connected with one or several temporary energy accumulators;   an inverter with in an H-bridge configuration formed by at least two parallel branches, a first pair of switching elements in series (T 1 , T 2 ) being connected to one branch and a second pair of switching elements in series (T 3 , T 4 ) to the other branch, and it being possible for their respective diodes in anti-parallel (D 1 , D 2 , D 3 , D 4 ) to be connected in anti-parallel to each switching element (T 1 , T 2 , T 3 . T 4 );   at least two alternating current connections (A, B) corresponding to the centre-taps of the H-bridge branches, their corresponding inductances (L 1 , L 2 ) being connected to each one of them, preferably with the same value L 1 =L 2 =L/2;   characterised in adding the following elements in its structure:   two branches connected in series with the H-bridge in the direct current connections ( 6 ,  7 ) with two auxiliary switching elements (T 5 D, T 6 D) with or without their respective protection diodes (D 5 D, D 6 D) in anti-parallel; and   at least one of the branches with temporary energy accumulator elements, connected across the points ( 6 ,  7 ) has a centre-tap ( 12 ); and   a branch with two auxiliary diodes in series (Daux 1 , Daux 2 ) which are connected in anti-parallel to the input of the H-bridge in some points ( 10 ) and ( 11 ). The centre-tap of this branch is joined to the centre-tap of the temporary energy accumulators ( 12 ). These branches signify a topological limit to the voltage that can be applied to T 5 D and T 6 D, at the voltage of C 1  and C 2  respectively.       
 
         [0044]    A second alternative implementation of the invention is that presented in  FIG. 4  and which relates to a single-phase inverter circuit with a substantially similar structure but which requires a smaller number of semiconductor elements. The distinguishing characteristics of this second configuration are:
       the temporary energy accumulator (C) connected across the two direct current connections ( 6 ,  7 ) does not need a centre-tap;   there is a single auxiliary diode (Daux) in the branch connected across points ( 10 ) and ( 11 ).   In this case, the maximum voltage applied to the auxiliary switches (T 5 D, T 6 D) is not limited by topology, but instead it is conditioned by the switching characteristics of these elements.       
 
         [0048]    It is possible to add a command unit to either of these two structures with a function similar to that of any of the control strategies possible for known converters, which basically consists in governing the switching of all the switching elements by means of a series of on signals produced at the output of said command unit. 
         [0049]    The terms in which this specification has been written are to be taken always in the broadest sense and not restrictively.