Field of the Invention

This invention relates to an electrical inverter assembly used to provide alternating electrical current when connected to a direct current source.

Background of the Invention

Electrical DC-AC converters or inverters are circuits used in applications where a direct current source is employed to power electrical loads designed to work with alternating currents. Electrical inverter technology is

particularly important in applications which employ direct current electrical energy generation or supply systems - such as photovoltaic solar cells, or when energy is supplied from energy storage devices such as fuel cells or battery systems. Battery based technologies are also increasingly being deployed in electrically powered vehicles which require inverter circuitry. Battery systems provided within long duration uninterruptable power supplies (UPS) also normally require inverter technology capable of delivering high quality alternating current.

Presently the operation of existing inverter circuits consumes electrical energy in the inverter components as direct current is converted to alternating current. It is preferable to have an inverter circuit or assembly which is as efficient as possible, lowering energy wastage within the system and mitigating heat dissipation issues. A common design approach used within prior art inverters is to provide a single inverter circuit capable of receiving the entire input power of a direct current source and suppling an alternating current output with desired output characteristics. However, the higher the input DC current received by such circuits, the more significant the ripple current issues within the operation of the inverter. This in turn leads to the need for larger smoothing capacitors to be included within the input stages of such designs, increasing space requirement and heat dissipation problems in the resulting inverter circuit. In portable devices, or within electric vehicles, space is at a premium and it is preferable where possible to minimize the weight of any components used to construct an electrical inverter, while increasing the efficiency of the overall inverter circuit.

It would therefore be of advantage to have an improved electrical inverter assembly which mitigated any of the above issues, or at least provided the public with an alternative choice. In particular, it would be of advantage to have an improved inverter assembly which is more energy efficient and smaller in size than prior art inverters.

Disclosure of the Invention

According to one aspect of the present invention there is provided an electrical inverter assembly arranged to connect to a direct current electrical supply and configured to supply alternating current to at least one electrical load, the inverter assembly including

at least one pair of inverters, being a primary inverter and a secondary inverter, each inverter having a set of input terminals and output terminals, and

at least two banks of energy storage capacitors provided in association with each pair of primary and secondary inverters, and

a capacitor switching structure associated with each pair of inverters, said capacitor switching structure being arranged to cyclically connect a discharged energy storage capacitor bank in series with the input terminals of the primary inverter and the direct current electrical supply to charge said capacitor bank, and to concurrently connect a charged energy storage capacitor bank with the input terminals of the secondary inverter to discharge said capacitor bank.

According to a further aspect of the present invention there is provided a method of operating an electrical inverter assembly using a capacitor switching structure characterised by the steps of;

i. connecting a first bank of capacitors in series with a direct current source and a set of input terminals of a primary inverter while connecting a second bank of capacitors across a set of input terminals of a secondary inverter, and ii. allowing the first bank of capacitors to become at least partially charged while the second bank of capacitors becomes at least partially discharged, and

iii. connecting the second bank of capacitors in series with the direct

current source and the input terminals of the primary inverter while connecting the first bank of capacitors across the input terminals of the secondary inverter, and

iv. repeating the above steps starting from step (i) after the second bank of capacitors has become at least partially charged while the first bank of capacitors has become at least partially discharged.

The present invention provides an electrical inverter assembly. This assembly is arranged to connect to a direct current electrical energy supply and to output an alternating current to at least one electrical load. The invention may be connected to a variety of direct current sources - such as, for example, photovoltaic solar cells, battery systems, fuel cells or any equivalent or effective source of direct current electrical energy. For example, those skilled in the art will appreciate that an electrical inverter assembly provided by the invention may be connected to any one or combination of photovoltaic solar cells, battery systems and/or fuel cells.

The inverter assembly provided by the invention incorporates a combination or collection of existing prior art standalone inverter circuits. These inverter circuits can be assembled together in conjunction with the invention to provide a novel and innovative arrangement. Each individual inverter incorporated within the invention defines a set of input terminals used to connect to a DC source and a set of output terminals provided to deliver output alternating current. The individual inverters selected for the invention's assembly can be drawn from any known inverter circuit arrangement depending on the particular application in which the invention is used, or the electrical loads to be supplied by the invention. Those skilled in the art will appreciate that the invention may incorporate various forms of square wave, modified sine wave or true sine wave type inverters in a range of embodiments.

The assembly provided by the invention is designed or specified with the input voltage and current of the direct current source in mind in combination with the output power characteristics required for the various alternating current loads to be supplied. The invention provides an alternative to the prior art which employs a single inverter circuit design to receive the entire output power of a direct current source. Conversely with the invention, each inverter of the assembly receives a portion or a fraction of the output power received from the direct current source or to be delivered to an AC load. Furthermore the invention operates to supply a fraction of the initial input direct current to each inverter to reduce ripple current magnitudes. The output of each of the inventions "fractional" inverters can then be recombined to deliver alternating current with the desired power

characteristics required for a particular application.

The invention therefore provides a significant degree of flexibility in terms of how these fractional inverter outputs can be combined. Those skilled in the art will appreciate that these inverter outputs can be connected in series in some embodiments, in parallel in others, or in any combination of serial and parallel connections if required. Furthermore in some embodiments the invention may provide one distinct output, whereas in other cases two or more separate output AC power supplies may be implemented as required.

For example in various embodiments inverter output connections may include the following possibilities:

a) Series connected outputs where inverters have individual high frequency filters within the inverter itself.

b) Parallel connected outputs where inverters have individual high frequency filters within the inverter itself.

c) Series connected high-frequency unfiltered outputs where inverters do not have individual high frequency filters within the inverter itself, but a single high frequency filter is provided to extract the low frequency AC output as a whole.

d) Parallel connected high-frequency unfiltered outputs where inverters do not have individual high frequency filters within the inverter itself, but a single high frequency filter will extract the low frequency AC output as a whole.

e) Any combination of the above options. An inverter assembly provided by the invention includes at least one pair of inverters, each member of the pair defined as either the primary inverter or alternatively the secondary inverter. Those skilled in the art will also appreciate that in various embodiments the assembly provided by the invention may include a single pair of primary and secondary inverters, or alternatively may include multiple pairs of inverters. For example, in some potential embodiments this assembly may be formed from two inverters only defining a single pair, four inverters defining two pairs or potentially six or eight inverters defining three and four pairs respectively. Those skilled in the art will appreciate that the physical size of the assembly, the cost of individual components, and the performance required of the assembly may dictate the number of primary and secondary inverter pairs provided.

Reference in general throughout this specification will however be made to the invention including a single pair of primary and secondary inverters, or alternatively two pairs of primary and secondary inverters in various preferred embodiments. Again however those skilled in the art will appreciate that alternative arrangements are also envisioned

Reference throughout this specification will also be made to the output terminals of all inverters incorporated into the assembly being connected together in series, thereby providing output terminals which deliver the entire alternating current output of the assembly to a single load or collection of loads. Again however, those skilled in the art will also

appreciate that in various alternative embodiments the outputs of each of the invention's standalone or fractional inverters may be combined in various ways to deliver alternating current independently to several different loads.

Those skilled in the art will also appreciate that references made throughout this specification to various components being connected Ίη series' encompasses both the direct physical connection of these components to each other, in addition to a connection made through an intervening component In addition, the invention's use of at least two banks of energy storage capacitors provides connections for these components so that they act as ripple current smoothing capacitors. Furthermore, the serial connection of a capacitor being charged with the input terminals of a primary inverter also provides the resistance of the primary inverter as a useful resistance in the capacitor charging path.

The invention employs at least two energy storage capacitor banks for each pair of primary and secondary inverters incorporated into the invention. A bank of such energy storage capacitors may be formed in some

embodiments by a single capacitor whereas in other embodiments a plurality of capacitors may be connected together to provide the various properties required of a particular bank.

Reference throughout this specification will also be made to either a capacitor or a capacitor bank being employed by the invention, and those skilled in the art will appreciate the equivalency of these terms. Reference will also be made throughout this specification to the invention providing two energy storage capacitor banks only in association with each pair of primary and secondary inverters. Again however, those skilled in the art will appreciate that other arrangements are envisioned and may be implemented depending on the particular application with which the invention is used. In a preferred embodiment the energy storage capacitors used by the invention are provided by super capacitors or ultra-capacitors.

In a further preferred embodiment the invention may employ large energy storage capacitors provided by electrical double layer capacitors. These EDL or electrical double layer capacitors are also known as super capacitors or ultra-capacitors. EDL capacitors have a high capacitance giving these components high relative time constants and relatively long charging periods. However, those skilled in the art will appreciated that other types of capacitors - such as for example hybrid capacitors - may also be used as energy storage capacitors in conjunction with the present invention. In various embodiments storage capacitors with large capacitance values may be employed within the invention. For example, in a number of preferred embodiments storage capacitors with capacitance values greater than or equal to at least 0.2 Farad may be used.

The present invention also incorporates at least one capacitor switching structure arranged to cyclically connect and disconnect the capacitor banks of the assembly in various configurations.

For example in one potential embodiment a microprocessor may be provided as a capacitor switching structure. In yet another embodiment a capacitor switching structure may be formed by a low frequency oscillator circuit. Those skilled in the art will appreciate that various types of switching technologies may be used to implement such a capacitor switching structure in different embodiments.

Furthermore in some implementations of the invention a single capacitor switching structure may be provided for each pair of primary and secondary inverters integrated into the assembly, or alternatively a single switching structure may be provided to service all pairs of inverters incorporated into the assembly.

As referenced above, the capacitor switching structure is configured to periodically or cyclically swap the connections of two banks of capacitors provided in association with an inverter pair.

In the first part of this operational switching cycle a first bank of capacitors - which is normally discharged - is connected in series with the direct current source and the input terminals of one of the primary inverters. These connections result in the recharging of the first discharged capacitor bank while the primary inverter is supplied with direct current from the direct current source. Concurrently with making the above connections the switching structure also connects a second bank of capacitors - normally being charged energy storage capacitors - across the input terminals of the secondary inverter of the pa ir. This connection results in the dischargi ng of the second capacitor ba nk to supply di rect current energy to the secondary i nverter.

In the second and final part of this switching cycle the connections of the first and second capacitor ban ks are reversed, al lowing the origina l newly charged first capacitor bank to be connected across the i nput terminals of the seconda ry i nverter while the origi nal second capacitor bank is recharged by the DC source with the pri mary inverter in series with the discharged capacitor.

The capacitor switching structure can therefore drive the operation of the inverter assembly provided by the invention, continuously swapping the connections of two banks of energy storage capacitors to supply direct current to a secondary inverter from one bank whi le the other ban k is being recharged in series with the pri mary i nverter.

The present i nvention may therefore provide many potential advantages over the prior art, or i n the least provide the public with an alternative choice .

The i nvention's use of charging and discharging capacitor paths ca n mitigate the problem of high i nput currents. The multiple fractiona l inverters employed by the i nvention initially drop the input DC current suppl ied to each separate i nverter circuit, leading to a reduction in ripple cu rrent magnitudes. Furthermore as the invention preferably employs super capacitors in the construction of an inverter assembly, the large capacitance of these components acts to smooth a nd attenuation the effects of such currents. These characteristics reduce the need for dedicated ripple current smoothing capacitors and ca n result in a size reduction for the resulti ng assembly.

The serial connection of a ca pacitor bei ng cha rged with the input stage of a primary inverter can also i mprove the overall efficiency in the resulting inverter assembly design . The pri mary i nverter provides a useful resistance RL in the ca pacitor charging path havi ng an initial resistance R, where it is assumed RL> > R - reducing energy losses by a factor of RL / R + RL by circumventing the losses in the capacitor charging loop. The invention improves energy efficiencies by recovering resistive losses in the capacitor charging loop which are supplied to the paired secondary inverter when the capacitor is discharged.

Brief description of the drawings Additional and further aspects of the present invention will be apparent to the reader from the following description of one embodiment, given by way of example only, with reference to the accompanying drawings in which:

• Figure 1 shows a basic schematic view of an improved electrical

inverter assembly provided in accordance with one embodiment of the invention,

• Figure 2a shows a plot of voltage against time for the primary and secondary capacitor bank pair CI, C3 of Figure 1,

• Figure 2b shows a plot of voltage against time for the primary and secondary capacitor bank pair C2, C4 of Figure 1,

· Figure 2c shows a plot of high frequency filtered voltage against time at the output terminals of Inverter 1 of Figure 1, and

• Figure 2d shows a plot of high frequency filtered voltage against time at the output of the inverter assembly of Figure 1. Further aspects of the invention will become apparent from the following description of the invention which is given by way of example only.

Best modes for carrying out the invention Figure 1 shows an electrical inverter assembly as provided in accordance with one embodiment of the invention with the example of 480V DC input supply and a 240V AC output inverter capable of 10 A root mean square current. This assembly is connected to a direct current electrical supply which delivers 5 Amps at 480 Volts, as shown by the connection terminals illustrated to the left side of Figure 1. The assembly incorporates four prior art inverters - being inverters 1 through 4. The four inverters are arranged in two pairs, with inverter 1 being a primary inverter and inverter 3 being a secondary inverter of the first of these pairs. The second pair is composed of inverter 2 as the primary inverter and inverter 4 as the secondary inverter.

Each inverter has a set of input terminals as shown to the left of figure 1, and a set of output terminals as shown to the right of figure 1. The output terminals of all four inverters are connected in series to deliver 10 Amps of alternating current at 240 Volts to the connection terminals illustrated to the right side of Figure 1. As will be appreciated by those skilled in the art other connection combinations of these inverter outputs may also be implemented in other embodiments. Each pair of primary and secondary inverters are associated with two energy storage capacitor banks. The paired combination of primary inverter 1 and secondary inverter 3 is associated with capacitor banks CI and C3. The paired combination of primary inverter 2 and secondary inverter 4 is associated with capacitor banks C2 and C4. In the embodiment shown each capacitor bank is made up of a collection of EDL capacitors or

supercapacitors.

This assembly also includes two capacitor switching structures, each associated with one of the pairs of inverters. These structures are

represented by each of the double ended switching arrows adjacent to the combinations of capacitor banks 1 and 3, and 2 and 4 respectively.

The capacitor switching structures work to cyclically connect a discharged energy storage capacitor bank in series with the input terminals of the primary inverter and the direct current electrical supply to charge said capacitor bank. At the same time they connect a charged energy storage capacitor bank with the input terminals of the secondary inverter to discharge this capacitor. Looking at the first part of this cycle of operations for the connection arrangement currently illustrated by Figure 1 : • For the first inverter pair, CI represents a discharged capacitor bank which is being recharged in series with the DC supply and inverter 1, while C3 represents a charged capacitor bank which is being

discharged into the input terminals of inverter 3.

· For the remaining inverter pair, C2 represents a discharged capacitor bank which is being recharged in series with the DC supply and inverter 2, while C4 represents a charged capacitor bank which is being discharged into the input terminals of inverter 4. After sufficient time has elapsed to allow for the discharge and recharging of each capacitor bank the capacitor switching structures swap the respective connections of each associated bank to implement the following:

• For the first inverter pair, C3 provides a discharged capacitor bank which is being recharged in series with the DC supply and inverter 1, while CI provides a charged capacitor bank which is being discharged into the input terminals of inverter 3.

• For the remaining inverter pair, C4 provides a discharged capacitor bank which is being recharged in series with the DC supply and inverter 2, while C2 provides a charged capacitor bank which is being discharged into the input terminals of inverter 4.

The operation of the capacitor switching structures of Figure 1 are also illustrated in the voltage plots provided as Figures 2a and 2b. These show how the periodic switching of the connections between banks CI and C3, and C2 and C4 respectively allow for the concurrent charging and

discharging of each bank.

Figure 2c shows a plot of high frequency filtered voltage against time at the output terminals of Inverter 1 of Figure 1. As can be seen from this figure the output terminals of Inverter 1 provide a 60 V RMS 50/60 Hz alternating current output. An equivalent 60 V RMS 50/60 Hz alternating current is also provided at the output terminals of Inverters 2, 3 and 4. Figure 2d shows a plot of high frequency filtered voltage against time at the output of the inverter assembly of Figure 1. This figure shows how the outputs of each of Inverters 1 , 2, 3 and 4 are summed together to provide a 240 V RMS 50/60 Hz alternating current.

It is to be understood that the present invention is not limited to the embodiment described herein and further and additional embodiments within the spirit and scope of the invention will be apparent to the skilled reader from the examples illustrated with reference to the drawings. In particular, the invention may reside in any combination of features described herein, or may reside in alternative embodiments or combinations of these features with known equivalents to given features. Modifications and variations of the example embodiments of the invention discussed above will be apparent to those skilled in the art and may be made without departure of the scope of the invention as defined in the appended claims.