EFFECTIVE UTILIZATION OF PHOTOVOLTAIC ENERGY USING MULTIPHASE BOOST CONVERTER IN COMPARISON WITH SINGLE PHASE BOOST CONVERTER EFFECTIVE UTILIZATION OF PHOTOVOLTAIC ENERGY USING MULTIPHASE BOOST CONVERTER IN COMPARISON WITH SINGLE PHASE BOOST CONVERTER

The paper presents a novel concept of multiphase boost converter (MPBC) with high efficiency of energy conversion. The new topology of MPBC is compared with conventional single-phase boost converter (SPBC). It is shown that almost whole input energy from photovoltaic module entering to the proposed MPBC is utilized more effectively in comparison with conventional SPBC. This effective energy utilization is ensured by suitable algorithm of switches control. Modeling, simulation and experimental results are given. Subsequently the laboratory models of SPBC and MPBC were built and experimental results were obtained to confirm the simulation results. Also the control module of MPBC was designed, simulated and built to ensure the correct operation of proposed converter.


Introduction
Photovoltaic is the direct conversion of light into electricity in form of direct current electricity. Usually the DC/DC converters are used to convert this direct electrical power from one level to another.
There are many types of materials which are used to make of the PV modules. The main problem of these materials is low conversion efficiency which is usually moving from 5% (a-Si) to 25% -30% (GaAs) [1]. Nowadays, the efficiency of the soft switching DC/DC converters is very high and it is moving around the 97%. But, on the other hand, the efficiency of energy conversion is not good in comparison with abovementioned converter efficiency. This paper presents the novel concept of MPBC with high efficiency of energy conversion. The high efficiency of energy conversion is ensured by adding five more parallel legs to the conventional SPBC. The suitable algorithm of switches control in particular legs ensures that the almost whole PV output energy from the PV module is effective utilized. The impinging sun energy P IN_sun is converted by PV module direct to the electric energy. According to the material from which PV module is built this conversion efficiency is moving from 5% (a-Si) to 30% (GaAs) [1] and [2]. The output PV energy P OUT_PV is equal to the input energy to the converter P IN_con .

Efficiency of energy conversion
Let us assume that SPBC is used to adjustment of input energy P IN_con from one level to another.
The function principle of conventional SPBC is well known. When the switch S is turned on the input energy starts to accumulate in form of magnetic field in the inductor L. This accumulated inductor energy with input energy (source energy) are delivered to the output Z after switch S is turned off. (1) where duty cycle "z" is ratio between time when the switch S is turned on and the period T, z ϭ t on(s) /T. The theoretical waveforms and topology of conventional SPBC are shown in Fig. 2.
It can be seen that there exists a time interval within the period T when the energy delivered to the load Z is equal to zero. This is the main problem of conventional SPBC -effective utilization of input energy. We have to ensure that the input energy will be delivered to the load Z over the whole period T. We have to remove the time interval within the period T where the energy delivered to the load Z is zero. The another important fact is, that in case of unfavorable operating conditions of PV module, the SPBC is unable to obtain ist maximum power point (MPP) It stands to reason that the efficiency of energy conversion is very low because we are unable to utilize the whole potential of input energy. This is done by using the proposed MPBC, [3] and [4]. The three different efficiencies can be defined as: The conversion efficiency of PV module η PV : The converter efficiency η con : The efficiency of energy conversion η E : where P OUT_PV(MPP) is the instantaneous maximal output power of PV module for its certain operating conditions.

The proposed concept of MPBC
The proposed topology of MPBC is in Fig. 3a. The MPBC has, in comparison with the conventional SPBC, five more parallel legs with five inductors (L 2 -L 6 ), five rectifier diodes (D 2 -D 6 ) and five switches (S 2 -S 6 ).
This MPBC allows the effective utilization of energy delivered form the PV module. Appropriate control algorithm of switches allows converter to take the PV output energy by cooperation of all six parallel legs in every moment.

Principle of operation
The principle of operation of SPBC was mentioned above. This described process can be repeated six times because six parallel legs are presented in proposed MPBC which allows effective utilization of delivered energy from PV module.
The proposed MPBC has 6 operating cycles within each period. The corresponding operation waveforms are shown in Fig. 3b.

Mode 1 (t 0 -t 1 ):
The switch S 1 (leg A) is turned on at the time t 0 . The energy in form of magnetic field begins to accumulate in inductor L 1 . The input current is closed in loop ϩU IN Ϫ L 1 Ϫ S 1 Ϫ ϪU IN . In this mode the switches S 5 (leg E) and S 6 (leg F) are in on-state. The input energy is delivered to the inductor L 5 and L 6 in particular legs, too. The switches S 2 and S 3 (leg C) are in off-state. The inductor energy W L2 and W L3 is delivered through diodes D 2 and D 3 to the load Z. The equivalent equations are: The inductor voltages u L1 (t), u L5 (t) and u L6 (t) are: The inductor voltage u L2 (t), u L3 (t) and u L4 (t) are: The currents flow through inductors L 1 , L 5 , L 6 and switches

Fig. 2 Topology and theoretical waveforms of SPBC
The currents flow through inductors L 2 , L 3 , L 4 and diodes D 2 , D 3 , D 4 are: The inductor current i L1 (t) exponentially increases from the initial value I L1 to the maximum value I L1max (reached at the time t 3 ) with time constant τ 1 ϭ L 1 /R.

Mode 2 (t 1 -t 2 ) and mode 3 (t 2 -t 3 ) are the same as mode 1.
Only other switches S 2 (leg B, mode 2) and S 3 (leg C, mode 3) are turned on, on-state S 1 , S 6 (leg A and leg F, mode 2) and S 1 , S 2 (legs A and B, mode 3) and off-state S 3 , S 4 (leg D and leg C, mode 2) and S 4 , S 5 (legs D and E, mode 3). The corresponding equations are the same. Only subscripts are changed.

Mode 4 (t 3 -t 4 ):
The switch S 1 is turned off and S 4 is turned in the beginning of this mode at the time t 3 . The inductor energy W L1 begins to deliver through diode D 1 to the load Z. The output current i Z (t) is enclosed in the loop L 1 - The switches S 2 (leg B) and S 3 (leg C) are on-state and the input energy is delivered to the inductor L 2 and L 3 . The switches S 5 (leg E) and S 6 (leg F) are in off-state. The inductor energy W L5 and W L6 is delivered through diodes D 5 and D 6 to the load Z. The equivalent equations are: The inductor voltages u L4 (t), u L2 (t) and u L3 (t) are (9) The inductor voltages u L1 (t), u L5 (t) and u L6 (t) are   The inductor current i L4 (t) exponentially increases with time constant τ 4 ϭ L 4 /R.

Mode 5 (t 4 -t 5 )
and mode 6 (t 5 -t 6 ) are the same as mode 4. Only other switches S 5 (leg E, mode 5) and S 6 (leg F, mode 6) are turned on, on-state S 3 , S 4 (leg C and leg D, mode 5) and S 4 , S 5 (legs D and E, mode 6) and off-state S 1 , S 2 (legs A and B, mode 6) and S 1 , S 6 (leg A and leg F, mode 5). The corresponding equations are the same. Only subscripts are changed.  Fig. 3 The proposed topology and theoretical waveforms of MPBC

Simulation Results
The simulation models of MPBC and SPBC shown in Fig. 4 were created in simulation environment OrCAD Capture CSI to verify its theoretical properties. The power MOSFET transistors were used as switches. The two DC voltage sources were used to simulate output photovoltaic voltage UPV and battery voltage U bat . Parameters: Three different levels of input voltage U PV were set to compare the properties of simulation models of MPBC and SPBC. The tested values of input voltage were U PV ϭ {2 V, 6 V, 12 V}.

Experimental results
The laboratory models of SPBC and proposed MPBC were built and tested to verify the theoretical assumptions and simulation results. The laboratory model of control structure of MPBC was built and connected to the converter to generate corresponding gate signals for transistors in particular legs. Figure 9 shows the overall view connection of converter with designed control.
The DC regulated voltage source was used to simulate different operating conditions and thus different levels of output PV voltage U PV . The battery was used as a load Z. The laboratory model of proposed MPBC works in discontinuous conduction mode at the U PV = 4V in comparison with simulation model. This is because the real components were used in comparison with the simulation model where many of parasitic and physical properties cannot be taken into account.

Comparison of efficiency of energy conversion
Efficiency of energy conversion η E can be defined as a ratio of output converter energy P OUTcon to the maximum output energy P OUT_PV(MPP) , of PV module which it is able to deliver under the given operating conditions. It is evident, that the efficiency of energy conversion η E of MPBC is several times higher in comparison with SPBC, Fig. 12, for the whole range of operating conditions. The low efficiency of energy conversion η E of SPBC results from its principle of operation. The SPBC operates in pulse mode so only a portion of its input energy is delivered to output. The PV module operates

Conclusion
The most popular material used to make the solar cell is silicium (Si). The Si is also one of major factors in long-term energy recovery and high cost of PV modules. One way to reduce the long -term energy and financial recovery is to use the proposed MPBC. The MPBC works with high efficiency of energy conversion in comparison with SPBC. The MPBC continually delivers the input source energy to the load Z using six parallel phases. In this case, the time interval when the output energy is equal to zero is removed. The MPBC ensures that the PV module is operating in the maximum power point for whole range of its operating conditions, so we can effectively utilize the output PV energy.