Parallel power converter topology

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Patent Application Ser. No. 61/291,624 titled Parallel Power Converter Topology filed Dec. 31, 2009 and U.S. Provisional Patent Application Ser. No. 61/370,089 titled Inverters With Combined Converters and Current Source filed Aug. 2, 2010.

BACKGROUND

Power converters are used to convert electric power from one form to another, for example, to convert direct current (DC) power to alternating current (AC) power. One important application for power converters is in transferring power from energy sources such as solar panels, batteries, fuel cells, etc., to electric power distribution systems such as local and regional power grids. Most power grids operate on AC current at a line (or mains) frequency of 50 or 60 cycles per second (Hertz or Hz). Power in a single phase AC grid flows in a pulsating manner with power peaks occurring at twice the line frequency, i.e., 100 Hz or 120 Hz. In contrast, many energy sources supply DC power in a steady manner. Therefore, a power conversion system for transferring power from a DC source to an AC grid typically includes some form of energy storage to balance the steady input power with the pulsating output power.

This can be better understood with reference to FIG. 1 which illustrates the mismatch between a DC power source and a 60 Hz AC load. As shown, the amount of power available from the DC source is constant (or could be varying slowly). However, the amount of power transferred to the load is of the form sine-squared which is the product of the sinusoidal load voltage, and the corresponding sinusoidal load current. As shown, the sine-squared load-power waveform fluctuates from the zero power level at the minimum of the sin-squared waveform to a maximum value and back down to minimum twice every line cycle. For a system with a grid frequency, f_grid, a cycle-time corresponding to twice the line frequency is given by 1/(2*f_grid), which is 10 millisecond (ms) for 50 Hz systems, and 8.33 ms for 60 Hz system. During time T1, the power available from the DC source exceeds the instantaneous power required by the AC load. During time T2, however, the maximum power available from the DC source is less than that required by the AC load.

FIG. 2 illustrates a conventional system for converting DC power from a photovoltaic (PV) panel to AC power. The PV panel 10 generates a DC output current I_PV at a typical voltage V_PV of about 35 volts, but panels having other output voltages may be used. A DC/DC converter 12 boosts V_PV to a link voltage V_DC of a few hundred volts. A DC/AC inverter 14 converts the DC link voltage to an AC output voltage V_GRID. In this example, the output is assumed to be 120VAC at 60 Hz to facilitate connection to a local power grid, but other voltages and frequencies may be used.

The system of FIG. 2 also includes a DC link capacitor C_DC and a decoupling capacitor C₁. Either or both of these capacitors may perform an energy storage function to balance the nominally steady power flow from the PV panel with the fluctuating power requirements of the grid. Power ripple within the system originates at the DC/AC inverter 14, which must necessarily transfer power to the grid in the form of 120 Hz ripple. In the absence of a substantial energy storage device, this current ripple would be transferred all the way back to the PV panel where they would show up as fluctuations (or “ripple”) in the panel voltage V_PV and/or current I_PV. Therefore, the DC link capacitor C_DC or the decoupling capacitor C₁, is used to store enough energy on a cycle-by-cycle basis to reduce the ripple at the PV panel to an acceptable level.

In a relatively recent development, the ripple at the PV panel has been reduced to essentially zero through the use of one or more control loops that cause the DC/DC converter 12 to draw constant power from the PV panel while allowing the voltage on the link capacitor C_Dc to vary over a relatively wide range. See, e.g., U.S. Patent Application Publication Nos. 2010/0157638 and 2010/0157632 which are incorporated by reference.

A problem with prior art approaches, however, is that the power may be processed sequentially through multiple power stages. But each stage introduces various inefficiencies, so the overall system efficiency is reduced. Also, since each stage must be designed to carry the full system power, the components in each stage must be sized accordingly, which may increase the cost and reduce the reliability of the components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the mismatch between a DC power source and an AC load.

FIG. 2 illustrates a prior art system for converting DC power from a photovoltaic panel to AC power.

FIG. 3 illustrates an embodiment of an energy conversion system according to some inventive principles of this patent disclosure.

FIG. 4 illustrates an example embodiment of an energy conversion system according to some inventive principles of this patent disclosure.

FIG. 5 illustrates waveforms within the embodiment of FIG. 4.

FIG. 6 illustrates another example embodiment of an energy conversion system according to some inventive principles of this patent disclosure.

FIGS. 7 and 8 illustrate waveforms within the embodiment of FIG. 6.

DETAILED DESCRIPTION

FIG. 3 illustrates an embodiment of a power conversion system according to some inventive principles of this patent disclosure. The embodiment of FIG. 3 includes a power source 22 having has a source waveform 24 that is generally required for optimum performance of the power source. For example, if the power source is a photovoltaic (PV) panel, the source waveform may be a flat quasi-constant power waveform. That is, the power available from the PV panel may appear to be constant on the scale of a few line cycles, but it may vary slowly over time in response to shading and/or changes in temperature, solar inclination, etc. As used herein, waveform with a constant value may refer to a truly constant value, or a quasi-constant value. If the power source is an AC generator, however, the source waveform may be a sine wave.

The embodiment of FIG. 3 also includes a load 26 having a load waveform 28 that is generally required for optimum performance of the load. As with the power source, the load waveform depends on the nature of a load. If the load includes an electrochemical battery, the load waveform may be a flat constant power waveform, whereas, if the load is a utility grid, the load waveform may be a sine wave.

Power is transferred from the power source to the load through a first power path 30 that has a first input waveform 32 and a first output waveform 34.

Power is also transferred from the power source to the load through a second power path 36 that has a second input waveform 38 and a second output waveform 40.

The first and second input waveforms 32 and 38 are complementary with respect to the source waveform 24, e.g., the source waveform 24 may split as shown by arrow 23 to form the first and second input waveforms 32 and 38. Thus, with the first and second power paths operating in parallel, the first and second input waveforms to the power paths combine to create a composite waveform that matches the source waveform of the power source.

Likewise, the first and second output waveforms 34 and 40 are complementary with respect to the load waveform 28, e.g., the first and second output waveforms 34 and 40 may combine (as shown by arrow 27) to form the load waveform 28. Thus, with the first and second power paths operating in parallel, the first and second output waveforms from the power paths combined to create a composite waveform that matches the load waveform of the load.

The first and second power paths 30 and 36 may include any suitable power conversion apparatus, typically one or more power stages such as rectifiers, inverters, commutators, push-pull stages, buck converters, flyback converters, etc.

The two paths may have different characteristics that enable the implementation of additional features according to some inventive principles of this patent disclosure. For example, in some embodiments, one path may have better efficiency than the other path, thereby enabling some power to be routed through the more efficient path and improving the overall efficiency of the system. As another example, one path may include energy storage capacity 42 while the other path does not include any substantial energy storage, or both paths may have equal energy storage, or both paths may have unequal but substantial energy storage. As yet another example, the two paths may have different numbers and/or types of power stages that enable the implementation of additional power conversion functionality according to some inventive principles of this patent disclosure.

FIG. 4 illustrates an example embodiment of a power conversion system according to some inventive principles of this patent disclosure. The embodiment of FIG. 4 is described in the context of a panel-mounted microinverter system for converting DC power from a solar panel to AC power that can be fed into a utility grid, but the inventive principles are not limited to these implementation details. Referring to FIG. 4, the power source is a PV panel 44 and the load is a utility grid 46. The first power path is essentially a flyback converter 48, while the second power path includes a flyback converter 50, an energy storage capacitor C_S, and a buck converter 52 configured as a current source. An H-bridge 54 is arranged as a simple commutator between the parallel connected power paths and the grid. A controller 56 includes monitoring and control circuitry to control the overall operation of the system.

To operate at peak efficiency, power should be drawn from a PV panel as a pure DC waveform at current and voltage levels corresponding to the maximum power point (MPP). Thus, the source waveform is simply a flat DC value I₅ as shown in FIG. 5. At the downstream end of the system, the load is a conventional power grid, so the load waveform is a pure sinusoidal power waveform, preferably with the voltage component and current component I₇ in phase to achieve a power factor of one (unity).

To accommodate these input and output constraints while striving for a high level of efficiency, half of the power flows through the first power path which only includes a single power stage, in this example, a flyback converter 48. Because the first power path includes only a single power stage, it has a relatively high efficiency. However, because it does not include any substantial energy storage, the instantaneous power at its output must always match the instantaneous power at its input.

The output current I₂ from the first power path is a rectified sine wave which, when multiplied by the sinusoidal grid voltage, provides a sinusoidal power output equal to half of the total system output. The output power from the PV panel, however, is a constant DC value. Therefore, to match the instantaneous sinusoidal power output, the first power path draws power from the PV panel in the form of a sine-squared (sine²) current waveform I₁ as shown in FIG. 5.

The other half of the power flows through the second power path which includes the second flyback converter 50. To maintain the output power from the PV panel at a constant value, the input current I₃ to the second power path must necessarily have a sine-squared waveform that is 180 degrees out of phase with I₁ so that I₁ and I₃ combine to provide a constant input current I₅ as shown in FIG. 5. Being out of phase with I₁ however, means that the second power path needs energy storage to shift the energy taken in at the input peaks in I₃ to the output peaks in I₄. This energy storage is provided by capacitor C_S which receives the output from the second flyback converter. The second power path also includes a second power stage, in this example the buck converter 52 configured as a current source, following the energy storage capacitor to modulate the output current I₄ which is combined with the output current I₂ from the first power path to provide the combined output current I₆.

Thus, each power path provides one of the two currents I₂ and I₄ which are rectified sinusoidal currents at half the grid frequency. The two currents I₂ and I₄ are summed to provide the rectified sinusoidal current I₆ which is switched synchronously with the grid voltage. The H-bridge 54 then commutates the current I₆ so the resulting output current I₇ is sinusoidal and in phase with the grid voltage. The capacitor voltage V₂ and input current I₈ to the buck converter 52 are also shown in FIG. 5.

A potential advantage of the embodiment of FIG. 4 is that only half of the power delivered to the grid needs to be processed twice through the second power path which includes two power stages and the energy storage capacitor. The other half of the power flows directly through the first power path which only includes a single power stage, i.e., the first flyback converter 48, thus providing an opportunity for higher overall efficiency. Moreover, since only half of the power flows through the second power path, the components in this path need only be sized to handle half of the system power, thereby reducing the cost and improving the reliability of these components.

The embodiment of FIG. 4 may be modified in countless ways in accordance with the inventive principles of this patent disclosure. For example, the embodiment of FIG. 4 is shown as a single-phase implementation, but the inventive principles may be applied to three-phase and other types of systems as well. As another example, the flyback converters may be replaced with boost converters, in which case a switch may be added in series with the output of the first converter 48 and opened during times when the first boost converter is not providing power.

This is because a boost topology can typically only control the input current when the output voltage is greater than the input voltage. Non-isolated boost inverters may potentially provide higher efficiency operation, whereas flyback converters may make it easier to provide galvanic isolation.

FIG. 6 illustrates another example embodiment of a power conversion system according to some inventive principles of this patent disclosure. The embodiment of FIG. 6 is described in the context of a panel-mounted microinverter system for converting DC power from a solar panel to AC power that can be fed into a utility grid, but the inventive principles are not limited to these implementation details.

The embodiment of FIG. 6 is similar in structure to the embodiment of FIG. 4, however, the controller 58 is configured to operate the system in a manner that maximizes the amount of power transferred through the first power path. Since the first power path only includes a single power stage, this may optimize the system efficiency by transferring as much power as possible through the more efficient path.

The operation of the embodiment of FIG. 6 is illustrated with the waveforms shown in FIGS. 7 and 8. Rather than splitting the input current equally between the two boost converters, the controller 58 causes the first boost converter 48 to transfer as much power as possible at each point in the line cycle of the grid load. Referring to FIG. 7, between time t₀ and time t₁, the input current I₁ to the first boost converter accounts for the entire input current I₅ to the system, and the input current I₃ to the second boost converter is zero. The output of the PV panel is essentially a constant voltage, and the current I₁ drawn from the panel during this time period is constant, and thus, a constant amount of power is drawn from the panel.

Since the first boost converter does not have any significant energy storage, this constant amount of power must be transferred to the output at each instant during the time between t₀ and time t₁. The voltage at the output of the first boost converter follows the rectified value of the grid voltage which is sinusoidal. Therefore, to maintain a constant product, and thus constant power at the output of the first boost converter, the output current I₂ from the first boost converter takes on the waveform shown in FIG. 8. The second power path outputs the current I₄ to make up the difference between I₂ and the (rectified) sinusoidal current waveform I₆ demanded by the grid. However, as shown in FIG. 7, the input current I₃ to the second power path is zero during the time between t₀ and time t₁, and therefore, all of the current I₄ is provided by charge stored on capacitor C_S.

The current I₄ is synthesized by the buck converter 52 in response to the PWM signal from the controller 58 so that when I₁ is added to I₂, the resulting combined current I6 is a rectified sinusoidal current that is switched synchronously with the grid voltage. After passing through the commutator 54, the resulting grid current is sinusoidal and in phase with the grid voltage.

Referring again to FIG. 7, at time t₁, the input currents I₁ and I₃ to the first and second boost converters begin a transition in which I_I decreases while I₃ increases until time t₃ at which point the entire input current I₅ is attributed to I₃. The rate at which I₁ decreases is coordinated with the rate at which I₃ increases to maintain a constant sum between I₁ and I₃, and therefore, a constant power draw from the PV panel.

During the time period between t₃ and t₄, the first boost converter does not draw any power from the input, so the entire current I₆ is provided by the output current I₄ from the second boost converter. The second boost converter also charges the storage capacitor C_S when the power it is drawing from the input exceeds the power it is providing to the output. For example, toward the midpoint of the time period between t₃ and t₄, the output current transitions through a V-shaped valley which provides the second boost converter with an opportunity to transfer a relatively large amount of charge to C_S.

In the transition period between times t₁ and t₃, the two power paths provide varying proportions of the total output current depending on the instantaneous amount of power the first boost converter can provide. The second power path then makes up the difference in the output current, and also takes advantage of additional opportunities to charge the storage capacitor C_S during moments when the power it is drawing from the input exceeds the power it is providing to the output.

A switch 60 may be included in series with the output of the first boost converter. This switch may be opened during times when the first boost converter is not providing power to the output.

A potential advantage of the embodiment illustrated with respect to FIGS. 6-8 is that the average power may be weighted in favor of the first boost converter which has fewer power stages and higher efficiency and therefore may increase the overall efficiency of the system. For example, in some embodiments, the first power path may be able to handle a theoretical maximum of 67 percent of the total power. Depending on the implementation details, a practical implementation may be able to handle from 50-67 percent of the total power, thus providing an opportunity for higher overall efficiency.

The inventive principles of this patent disclosure have been described above with reference to some specific example embodiments, but these embodiments can be modified in arrangement and detail without departing from the inventive concepts. Such changes and modifications are considered to fall within the scope of the following claims.