FIELD OF INVENTION

The present invention relates to the field of electrical power inverting techniques and more particularly relates to power inverting techniques suitable for renewable energy sources.

BACKGROUND

An inverter is an electrical power converter that converts direct current (DC) into alternating current (AC). Inverters of three-leg topology are used to perform under different switching algorithms to generate AC voltage and current from a DC source. Inverters of three-leg topologies are commonly used to supply AC power from DC sources such as renewable energy sources. Renewable energy sources include photovoltaic cells, fuel cells etc.

Distribution Static Compensators (DSTATCOMs) are used to eliminate unbalanced and distorted currents from being drawn from the distribution bus.

In the known art, DSTATCOMs are used for power quality improvisation at the distribution end and a boost DC-DC converter along with an inverter is used for stepping up the DC voltage level from renewable energy sources, for integration into grids. The PV cells are used to share the real power of the load and the DSTATCOM were used to eliminate the unbalanced and nonlinearities in the source end currents. For two of these purposes three leg topology inverters were used with a DC link at one end and a three phase output at the other end.The three-leg topology of inverter known in the current art makes integration of the inverter with the renewable energy sources to the electrical grid difficult, since there are complex connections involved in integrating inverter with the renewable energy sources at a single point along the electrical grid. The current state of art also uses many power electronic devices in the process of integration of the converter with the renewable energy sources to the electrical grid.

The present invention caters to the above problem by designing an inverter which is fundamentally based on boost DC-DC converter unlike the three leg topology. This invention uses devices to achieve inversion of DC to AC, which are all encapsulated in a single converter. This single inverter topology can boost the renewable energy voltages, integrate them into the grid and also improvise the power quality simultaneously. So this gives the flexibility of connecting this inverter at the distribution end at a single point to integrate the PV cells, fuel cells and also improvise the power quality.

BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more clearly understood from the following description of the preferred embodiments of the invention read in conjunction with the attached drawings, in which:

Fig. 1 shows a circuit representing a system for converting a direct current (DC) from renewable energy sources into a three-phase alternating current (AC), improving power quality and integrating the said system with the renewable energy sources at a point before the load terminal in an electrical grid;
Fig. 2 depicts the representation of output voltage of a boost DC-DC converter in relation to a single phase of three-phase AC; and

Fig. 3 shows the location at which the system of invention is integrated at a point before the load terminals along an electrical grid.

The drawings are merely representative and are not intended to limit the scope of the appended claims.

DETAILED DESCRD?TION OF THE INVENTION

Fig. 1 shows the schematic representation of the system 1 for improving power quality of source DC from renewable energy sources and converting DC into three-phase AC. The system 1 is integrated with the renewable energy sources at a point before the load terminal in an electrical grid.

The system 1 comprises of a plurality of units of boost DC-DC converters as shown in Fig. 1. Each unit of boost DC-DC converters4, 5 and 6 of the plurality of units of boost DC-DC converters further comprises of two boost DC-DC converters. Each unit of boost DC-DC converters converts the source DC voltagesVdc1, VdC22, 3derived from the renewable energy sources into a phase constituting three-phase AC. Unit of boost DC-DC converters 4 converts the DC voltages Vdc1, VaC2 into phase 'a' of AC.

Similarly, unit of boost DC-DC converters 5, 6converts the DC voltages Vdcl, Vdc2 into phase 'b' and phase 'c' of AC respectively.Hence, the plurality of units of boost DC-DC converters together contribute in converting DC from renewable energy sources into a three-phase AC.

All the units of boost DC-DC converters 4, 5, 6in Fig. 1 operate in the same manner. The operation of the unit of boost DC-DC converters 4 of the system 1 is described as an exemplary part of the complete embodiment of the system lin Fig.l. The unit of boost DC-DC converters 4 comprises of two individual boost DC-DC converters 7, 8as shown in Fig.l. Each individual boost DC-DC converter comprises of a switch, an inductor, a diode and a capacitor.

Boost DC-DC converter 7 is described as an exemplary embodiment of the unit of boost DC-DC converters 4 as shown in Fig.l. The working of other boost DC-DC converters present in different units of boost DC-DC converters 4, 5 and 6 are similar. Each boost DC-DC converter includes a switch Swl for transferring charge from an inductor LI to a capacitor C1 through a diode. Each individual boost DC-DC converter is connected to filters. The boost DC-DC converter7is connected in series with filter9and the boost DC-DC converter 8 is connected in series with filter 10 as shown in Fig. 1. Each filter 9, lOinclude a resistance in series with an inductance constituting together as output impedance. The unit of boost DC-DC converters 4provides phase 'a' of the three-phase AC. The voltagevc1and current ifa+.as shown in Fig. 1 represents the reference output voltage and reference output current respectively of the boost DC-DC converter 7. Similarly, vc2and current ifa.shown in Fig. 1 represents the reference output voltage and reference output current respectively of the boost DC-DC converter 8. The output currents ifa+. and ifa.. shown in Fig. l,constitute phase 'a' of the three-phase AC.

Similarly, unit of boost DC-DC converters 5 provides output currents ifab and ift. that constitute phase 'b' of the three-phase AC. And unit of boost DC-DC converters 6 provides output currents ifc+.and ifc.that constitute phase 'c' of the three-phase AC as shown in Fig.l.

The conversion of the source DC voltage Vdc1, Vdc2 from the renewable energy sources into three-phase AC involves switching each boost DC-DC converter based on the input parameters to the system 1 from the user. The switching of each boost DC-DC converter is based on calculating an output voltage of each of the boost DC-DC converter and then determining a reference input current to the boost DC-DC converter. Output voltage of the boost DC-DC converter is calculated from the input parameters to the system 1 obtained from the user. The input parameters to the system 1 are voltage and current at the load terminals. The filter currents ifa, ifb. and ifc.shown in Fig.l are the reference input currents for the system 1 that are to be determinedby the application of the instantaneous symmetrical technique, the reference input currents are represented as below:

where are the output currents of boost DC-DC converters as shown in Fig.l. The output voltage of each boost DC-DC converter is determined from the input parameters to the system 1 obtained from the user.

The user chooses the minimum output voltage Vbmin shown in Fig. 2for each of the boost DC-DC converters. The output voltage and output current of each boost DC-DC converter is determined by voltage enveloping technique. The output voltages vc1,vc2 of the boost DC-DC converters 7, 8 respectively, shown in Fig. 2, are determined by using voltage enveloping technique. Fig. 3 shows the representation of the output voltages vc1,vc2 of the boost DC-DC converters 7, 8. The output voltages vc1,vC2 are as shown in Fig. 2 for the system 1 to convert DC voltage from renewable energy sources into three-phase AC voltage and operate as desired. In Fig.2, V1 depicts the voltage at the point of intersection of the system 1 with the renewable energy sources to the load terminal.

For the positive half cycle of Vj,
where Z = (R + jXLf)is the output impedance of the respective filter connected in series with each boost DC-DC converter and; Vbminin Fig. 2 is the minimum attainable voltage by boost DC-DC converter as chosen by the user to be the minimum output voltage across each boost DC-DC converter. So, the output voltages vc1,vc2 are calculated by solving the above equations.

Further, reference inputs current to each boost converters are determined from the calculated output voltage of the respective boost DC-DC converters, by average power balance technique. This technique
operates on the principle that the average output power and average input power in a boost DC-DC
converter remains constant. Hence, reference input currents to each boost DC-DC converter are
determined from the source DC voltage of the renewable energy sources and the output voltage of the
boost DC-DC converters.

Then further, switching of each boost DC-DC converters is controlled by hysteresis current control technique based on the determined reference input current to each of the boost DC-DC converters.
The system 1 with renewable energy sources such as Photovoltaic cells, fuels cells, is integrated at a single point before the load terminals of an unbalanced load 20 along an electric grid in the distribution end as shown in Fig. 3. Currents ifa*, ifb* and ifc*in Fig. 3 represent the determined reference input currents for the system 1 and ija, ij b and ijc represent the load currents respectively of each of the phases 'a', 'b' and 'c' of the three-phase AC.

WE CLAIMS:

1. A system for converting a direct current (DC) from renewable energy sources into a three-phase
alternating current (AC) and improving power quality, the system integrated with the renewable
energy sources at a point before the load terminal in an electrical grid, the system comprising of:

a plurality of units of boost DC-DC converters to convert DC from renewable energy sources into the
three-phase AC based on input parameters to the system from user; and

a filter connected in series with each boost DC-DC converter of the unit of boost DC-DC converters,
before the load terminals in the electrical grid.

2. The system as claimed in claim 1, wherein converting a DC from renewable energy sources into a three-phase AC and improving power quality involves switching each boost DC-DC converter based on the input parameters to the system from user.

3. The system as claimed in claim 2, wherein switching each boost DC-DC converter comprises steps of:

calculating an output voltage of the said boost DC-DC converter from the input parameters to the
system by user;

determining a reference input current to the said boost converter from the calculated output voltage of
the boost DC-DC converter by average power balance technique; and

switching the said boost DC-DC converter by hysteresis current control technique using the
determined reference input current.

4. The system as claimed in claim 1, wherein each unit of boost DC-DC converters of the plurality of units of boost DC-DC converters, is coupled together with two DC outputs derived from the renewable energy source to provide a single phase of AC.

5. The system as claimed in claim 1, wherein the said boost DC-DC converter includes a switch for transferring charge from an inductor to a capacitor through a diode.

6. The system as claimed in claim 1, wherein the input parameters to the system from user includes voltage and current levels to be maintained at load terminals.

7. The system as claimed in claim 1, wherein the said filter connected in series with each boost DC-DC converter comprises a resistance in series with an inductance.