WO2013079067A1

WO2013079067A1 - Converter with storage device

Info

Publication number: WO2013079067A1
Application number: PCT/DK2012/000125
Authority: WO
Inventors: Franke WULF-TOKE
Original assignee: Danfoss Power Electronics A/S
Priority date: 2011-11-29
Filing date: 2012-11-27
Publication date: 2013-06-06

Abstract

The present invention relates to a converter comprising DC input and DC output, said DC input and output being interconnected by a first and a second power path, wherein the first power path comprises a voltage adjusting circuit, and wherein the second power path comprises a DC storage device being charged by energy from a first circuit point, and discharged to a second, and different, circuit point. Preferably, the first circuit point is at or near the DC input, and the second circuit point is at or near the DC output. Compared to prior art systems, conversion losses may be reduced by around 60%.

Description

CONVERTER WITH STORAGE DEVICE

FIELD OF THE INVENTION

The present invention relates to a converter for renewable energy sources. In particular, the present invention relates to a converter with reduced conversion losses.

BACKGROUND OF THE INVENTION

The storage of electrical energy that has been generated by renewable energy sources is liable to become a particular area of interest and importance in the future, due to the fact that the amount and the time of energy production often depends on weather, tidal, river flow, windspeed or other conditions and which therefore cannot be easily influenced.

Today, many manufacturers, especially of solar inverters, have concepts ready to store the captured energy in batteries, hydrogen or other storage devices. Most of these concepts require an additional inverter that is connected between the storage device and the grid. This additional inverter introduces additional losses since the DC-energy is, for example, first boosted and converted into AC and afterwards back-converted into DC and adjusted to the required voltage level for the storage device. If the stored energy is to be discharged from the storage device, the voltage is again boosted and converted into DC. Thus, in traditional systems, the energy has to pass three DC/ AC inverters and three DC/DC converters from production, via storage, to consumption. The large number of inverters and converters induce unavoidable conversion losses which, in relation to renewable-related applications, are undesirable.

It may be seen as an object of embodiments of the present invention to provide a more efficient conversion scheme for renewable-related applications.

DESCRIPTION OF THE INVENTION The above-mentioned object is achieved by providing, in a first aspect, a converter comprising DC input and DC output, said DC input and output being interconnected by a first and a second power path, wherein the first power path comprises a first voltage adjusting circuit, and wherein the second power path comprises a DC storage device being charged using energy from a first circuit point, and discharged to a second, and different, circuit point. Compared to prior art systems, the present invention requires only two DC/DC conversions and one DC/AC conversion in the storage/discharge path. The reduced number of inverters and converters reduce the conversion losses by approximately 60% compared to prior art solutions. It is a further advantage of the present invention that the second power path comprising the DC storage device may be implemented using low-component count circuitries, such as, for example, buck and boost converter circuitries. A reduction in the number of components used in such circuits is a distict advantage, since it leads to reduced costs of manufacture.

The DC input of the converter may, in principle, be connected to any type of power generating unit. However, the converter according to the present invention is particularly suitable in combination with photovoltaic arrays. A load, such as a power distributing grid, may be connected to the DC output of the converter.

The first circuit point may comprise a point at or near the DC input. Similarly, the second circuit point may comprise a point at or near the DC output. Thus, the DC storage device is charged by energy drawn from one point, but discharges energy back to another point.

The first voltage adjusting circuit may comprise a boost converter or any other type of voltage adjusting circuit, such as transformerless DC/DC converter, such as buck, buck- boost, SEPIC, Cuk converter. The DC storage device may comprise a battery and/or a capacitor, or a fuel cell. The converter may further comprise a second voltage adjusting circuit for charging the DC storage device.

The converter may further comprise a third voltage adjusting circuit for discharging the DC storage device.

A boost, buck, buck-boost, SEPIC, Cuk or other suitable converter may be applied as the second voltage adjusting circuit for charging the DC storage device from the first circuit point. Moreover, a boost, buck, buck-boost, SEPIC, Cuk or other suitable converter may be applied as the third voltage adjusting circuit for discharging the DC storage device to the second circuit point. The choice of converter type will depend upon the relative voltages of the first circuit point, the second circuit point, and the optimum voltage supplied or stored by the DC storage device. For example, if the voltage at the first circuit point is always higher than voltage of the DC storage device, then a buck converter may be an appropriate choice for the second voltage adjusting circuit. In a second aspect, the present invention relates to a method for operating a converter comprising DC input and DC output, said method comprising the steps of

- providing power directly from the DC input to a DC storage device during a charging process, and - providing stored power from the DC storage device directly to the DC output port during a discharging process when demands so require.

A second voltage adjusting circuit may be applied for charging the DC storage device and a third voltage adjusting circuit may be applied for discharging the DC storage device.

Similar to the first aspect, a boost, buck, buck-boost, SEPIC, Cuk or other suitable converter may be applied as the second voltage adjusting circuit for charging the DC storage device and a boost, buck, buck-boost, SEPIC, Cuk or other suitable converter may be applied as the third voltage adjusting circuit for discharging the DC storage device. In a preferred embodiment, a buck converter may be applied for charging the DC storage device, and a boost converter may be applied for discharging the DC storage device. In a third aspect, the present invention relates to a converter comprising DC input and DC output, the converter further comprising first converter means for providing power directly from the DC input to a DC storage device, and second converter means for providing stored power from the DC storage device directly to the DC output.

The first converter means may comprise a buck or a boost converter for charging the DC storage device. The second converter means may comprise a boost or a buck converter for discharging the DC storage device. Buck and boost converter circuitries are both considered low-component circuitries. Thus, the circuitries charging and discharging the DC storage device may be implemented using simple and low-component count circuitries.

In a fourth aspect, the present invention relates to a converter comprising DC input and DC output, the converter further comprising

- a first converter for providing power directly from the DC input to a DC storage device, and

- a second converter for providing stored power from the DC storage device directly to the DC output, wherein the first converter comprises an energy reservoir, a first switching device, and a first diode, and wherein the second converter comprises the energy reservoir, a second switching device, and a second diode.

The energy reservoir may comprise an inductor and/or a capacitor. The reduction of conversion losses by approximately 60% also apply to the second, third and fourth aspects of the present invention.

In a fifth aspect, the present invention relates to a power generating unit comprising a photovoltaic array, a solar inverter, and a frequency converter, where the solar inverter comprises a converter according to the first or third aspects. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in further details with reference to the accompanying figures, wherein

Fig. 1 shows the general concept of the present invention,

Fig. 2 shows a number of parallel connected converters according to the present invention, Fig. 3 shows a first embodiment of the present invention, Fig. 4 shows a second embodiment of the present invention Fig. 5 shows a third embodiment of the present invention, and Fig. 6 shows a fourth embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been disclosed by way of examples. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. DETAILED DESCRIPTION OF THE DRAWINGS

Generally the present invention relates to a new type of energy saving DC converters where a storage device is charged from the input, and discharged to the output of a DC converter. Thus, the storage device receives it energy for charging from a separate point of the converter than that to which it delivers the energy while discharging. By following this approach, the same components for charging and discharging are used, leading to a compact and cheap solution. In addition, the energy stored in the storage device does not have to pass the same converter circuit twice, and this leads to lower conversion losses.

Fig. 1 shows the basic inventive idea in a simplified version. As depicted in Fig. 1, DC energy is supplied to the input of a DC converter from, for example, a photovoltaic array. The converter supplies energy to a load connected to the output of the converter. A boost converter circuit is used between the input and the output. Moreover, an energy storage system is used for storing excess energy, under certain conditions, by charging it using DC energy from the input. The stored energy is discharged when that energy is needed at the output. Traditionally, charging and discharging of such energy storage systems happens through a single point in the circuit. The present invention deviates from this concept by charging directly from the input, and discharging directly to the output whereby conversion losses can be reduces with approximately 60%.

Referring now to Fig. 2, the lower branch of each string comprises devices that are required in order to store the produced energy, S1-S3. It is possible to use one storage device for each input or apply one single storage device for all inputs. In each power string of Fig. 2, a photovoltaic array feeds power to a boost converter B1-B3 via an input. The boost converters B1-B3 feed power to a common DC-link. A storage device S1-S3 is coupled in parallel with each boost converter B1-B3. Power from the common DC-link is fed to the grid via a DC/ AC inverter and a grid filter.

Before explaining the topology in more detail, the working principle of the system as a whole will be discussed. There are, in principle, six operation modes that have to be distinguished :

1. The total amount of energy produced is to be fed into the grid, and no additional power from the storage is fed into the grid : Therefore the storage device does not operate.

2. The total amount of energy produced should be stored, and no energy is to be fed into the grid : In this case all power flows to the storage device. The new topology adjusts the power level for the storage device and works as a maximum power point tracker (MPPT) for the photovoltaic array. The boost converter and the inverter are not operating.

3. A part of the produced energy should be stored, and the rest should be fed into the grid: In this case both the boost converter and the new topology share the power with the required ratio. The reference value of the MPPT is divided according the required ratio. In this way it is ensured that the photovoltaic array delivers the maximum power and the predefined amount of energy is stored. The inverter is operating, and feeds in the partial power.

4. The photovoltaic array is not delivering any power: Therefore, energy from the

storage device should be fed into the grid: Therefore the boost converter is not operating, but the new topology feeds energy from the storage device to the DC-link. The inverter forwards the energy into the grid.

5. The generated power from the photovoltaic array is low, and additional energy from the storage device should be fed into the grid: In this case the boost converter operates as usual, and additional power is fed from the storage device into the DC- link by the new topology.

6. Optional case: Power from the grid should be stored. The power flows from the grid through the inverter into the DC-link. From the DC-link the power is taken by the new topology and fed into the storage device. Referring now to Fig. 3, an embodiment of the present invention in which a boost converter circuit is placed in the positive line is illustrated. The working principle of this topology is as follows, for the two cases storing/charging energy and delivering/discharging energy according the operation states mentioned above:

Charging energy from the input: Involved components: L2, T2, D3 and D5 while T3 and D2 are blocking. The involved components form a buck converter that is able to transform the input voltage to a lower voltage for the storage device. Therefore the switch T2 is switching with a typical switching frequency (e.g. 17 kHz). While T2 is turned on, the current flows from the positive rail through the diode D5, the switch T2, the inductance L2 and the DC storage device to the negative rail and energy is stored in the inductance L2. The inductance L2 is acts in this case as a short term energy reservoir, temporarily storing some of the energy present in the system. In the current path described above there are two major voltage drops: One fixed voltage drop across the DC storage device and one adjustable voltage drop across the inductance. The sum of these voltage drops equals the input voltage level. In this way the voltage is reduced at the output of the buck converter. To keep the voltage drop across the inductance constant, the current through the inductance has to increase linearly. Since this is only possible for a short time, to prevent the device from over-current, the switch T2 has to be turned off. When this happens, the current continues to flow through the inductance but the current path is now through the diode D3 and the DC storage device. In this case the voltage across the inductor changes its orientation and has the same magnitude as the voltage across the DC storage device. In this state, the magnitude of the current drops steadily until T2 is turned on again.

Discharging energy to the output:

Involved components: L2, T3 and D2 while D5, D3 and T2 are blocking. The involved components form a boost converter that is able to transform the input voltage to a higher voltage for the output. Therefore the switch T3 is switched with a typical switching frequency (e.g. 17 kHz). While T3 is turned on the current flows from the DC storage device through L2 and T3 and energy is stored in the inductance L2. The inductance L2 is acts in this case as a short term energy reservoir, temporarily storing some of the energy present in the system. The voltage drop across L2 is the same as across the DC storage device. After T3 is turned off, the voltage across the inductance changes its orientation and is added to the voltage of the DC storage device. The sum of these two voltages is equivalent to the output. The current is now flowing from the negative rail of the DC link through the storage device, L2 and D2 and to the positive rail of the DC link.

Thus, DC energy is charged directly from the input and discharged directly to the output, i.e. without passing through the boost converter circuit. This direct charging/discharging scheme reduces conversion losses by approximately 60% compared to conventional schemes.

In the case that two or more inputs are available, cf. Fig. 2, it might be beneficial to wind all winding of the inductors L2 on the same core. Moreover, by applying interleaved switching the ripple of the current at the output and the DC storage device can be reduced. Another advantage is that the DC part of magnetic flux is cancelled out. This allows a much cheaper and smaller inductance.

It can clearly be seen from the above description of the storing/charging energy and delivery/discharging energy operation states that the inductance L2, which acts as an energy reservoir, is utilised in both the charging and the discharging operation states. That is to say, that the same component is used in two different operations. This is a distinct advantage over circuits where separate components are used for the charging and discharging operation states, since fewer components need to be used, and this in turn leads to greater reliability and to reduced costs of manufacture.

It should be noted that the principle of the present invention also applies to implementations where the boost converter is replaced by any kind of transformerless DC/DC converter, such as buck, buck-boost, SEPIC or Cuk converters, further described below

The above-mentioned embodiments all relates to implementations were the boost converter, or any kind of transformerless DC/DC converter, is positioned in the positive voltage rail. It should be noted however, that the concept underlying the present invention also applies to implementations where boost converters, or the like, are positioned in the negative voltage rail.

As previously mentioned a single and common DC storage device may be used in case of a plurality of inputs. In this situation T3 and D2 are removed and the connection remains open, while D5 has to be short-circuited in most cases.

Referring now to Figs. 4 and 5 two variants both involving galvanic isolation are disclosed. In both figures the galvanically isolated DC/DC converter for MPP-T racking without storage is illustrated by the box.

Regarding the DC storage circuit two variants are possible: one with the galvanic isolation on the charger side (Fig. 4), and one with the galvanic isolation on the discharger side (Fig. 5). For the galvanically isolated charger and discharger full bridge DC/DC inverter are shown. Obviously the full bridge DC/DC inverters can be replaced by any other galvanically isolated DC/DC inverter topology (e.g. Flyback, half bridge etc.). In terms of functioning the circuits shown in Figs. 4 and 5 are operated as disclosed in connection with Fig. 3.

Referring now to Fig. 6, a variant is shown where a capacitor C is used as the energy reservoir. In terms of functioning the circuit shown in Fig. 6 is operated as disclosed in connection with Fig. 3 taking into account the different components involved. The components involved in the storing/charging energy operation state are T10, L10, C, Lll, Dl l and the storage device. The switch Ti l remains in an open condition, and is not used in the operation state.

The components involved in the delivery/discharging energy operation state are Til, L10, C, Ll l, D10 and the storage device. The switch T12 remains in an open condition, and is not used in the operation state. To keep the implementation simple, the galvanically non-isolated converters are illustrated by simple boost and buck converters. However, these can also be replaced by any other DC/DC converters - galvanically isolated or not.

In the case that the voltage of the DC storage device is in the range of either the input voltage or the output voltage, or above these voltages, bidirectional buck-boost or boost converters are required.

Claims

1. A converter comprising DC input and DC output, said DC input and output being interconnected by a first and a second power path, wherein the first power path comprises a first voltage adjusting circuit, and wherein the second power path comprises a DC storage device being charged using energy from a first circuit point, and discharged to a second, and different, circuit point.

2. A converter according to claim 1, wherein the first circuit point comprises the DC input.

3. A converter according to claim 1 or 2, wherein the second circuit point comprises the DC output.

4. A converter according to any of claims 1-3, wherein the first voltage adjusting circuit comprises a boost or buck converter.

5. A converter according to any of claims 1-4, wherein the DC storage device comprises a battery and/or a capacitor.

6. A converter according to any of claims 1-5, further comprising a second voltage adjusting circuit for charging the DC storage device.

7. A converter according to any of claims 1-6, further comprising a third voltage adjusting circuit for discharging the DC storage device.

8. A method for operating a converter comprising DC input and DC output, said method comprising the steps of - providing power directly from the DC input to a DC storage device during a charging process, and

- providing stored power from the DC storage device directly to the DC output port during a discharging process when demands so require.

9. A method according to claim 8, wherein a second voltage adjusting circuit is applied for charging the DC storage device.

10. A method according to claim 8 or 9, wherein a third voltage adjusting circuit is applied for discharging the DC storage device.

11. A converter comprising DC input and DC output, the converter further comprising first converter means for providing power directly from the DC input to a DC storage device, and second converter means for providing stored power from the DC storage device directly to the DC output.

12. A converter according to claim 11, wherein the first converter means comprises a buck or a boost converter for charging the DC storage device.

13. A converter according to claim 11 or 12, wherein the second converter means comprises a boost or a buck converter for discharging the DC storage device.

14. A converter comprising DC input and DC output, the converter further comprising

- a first converter means for providing power directly from the DC input to a DC storage device, and

- a second converter means for providing stored power from the DC storage device directly to the DC output, wherein the first converter means comprises an energy reservoir, a first switching device, and a first diode, and wherein the second converter means comprises the energy reservoir, a second switching device, and a second diode.

15. A converter according to claim 14, wherein the energy reservoir comprises an inductor.

16. A converter according to claim 14, wherein the energy reservoir comprises a capacitor.

17. A power generating unit comprising a photovoltaic array, a solar inverter, and a frequency converter, where the solar inverter comprises a converter according to any of claims 1-7 or any of claims 11-16.