Solar array inverter with maximum power tracking

• US 20060171182A1
• Filed: 01/28/2005
• Published: 08/03/2006
• Est. Priority Date: 01/28/2005
• Status: Active Grant

• Current Assignee: Aerospace Corporation
• Original Assignee: Issa Batarseh, Kasemsan Siri
• Inventors: Siri, Kasemsan, Batarseh, Issa
• Granted Patent: US 7,193,872 B2
• Time in Patent Office: Days
• US Class Current: 363/131
• CPC Class Codes: H02M 1/0012 Control circuits using digital or numerical techniques in DC/DC converters H02M3/157, H02M3/33515; in DC-AC converters H02M7/53873
• CPC Class Codes: H02M 1/007 Plural converter units in cascade push-pull DC/DC converters with pre-regulator H02M3/3374; DC-AC converters following a DC-DC stage including a high frequency transformer H02M7/4807; DC-AC converters following a DC-DC conversion stage generating periodically varying voltages H02M7/4826
• CPC Class Codes: H02M 3/33592 having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
• CPC Class Codes: H02M 7/53803 with automatic control of output voltage or current
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

An inverter for use in connecting a DC power source to the utility grid includes a single DC-DC conversion stage, maximum (source) power tracking, and current control based on feed-forward compensation as a function of an input power commanding voltage, rectified utility line voltage, and either a scaled and squared inverse or a scaled inverse of RMS utility line voltage. Various embodiments also include over-voltage, over-current, under-voltage, and over-temperature protection, and a stand-by battery with its battery management control, while others adapt a multiple-channel front-end distributed power system with distributed maximum power tracking to serve as a single DC power source input to the inverter system downstream with controllers, emergency or auxiliary loads, and alternative current feedback control systems for providing an in-phase output current with respect to the utility grid voltage while absorbing maximum power available from the DC power source and minimally utilizing the stand-by battery for maximal system reliability.

Claims

1. A power supply adapted to supply AC power to a utility grid, comprising:

a DC power source having an output voltage;

a power converter that provides uni-polar output power through a single power stage of DC-DC conversion;

an unfolding and output filtering grid interface that commutates and filters the uni-polar output power into AC current that is supplied to the utility grid;

a system controller for

regulating the output voltage of the DC power source at its maximum power point,

controlling the waveform of the converter output current to be rectified sinusoidal in phase with the utility grid, and

reducing the output power when the utility grid voltage exceeds a pre-determined threshold;

wherein the power converter is a cascade connection of an input bulk capacitor, a line-filter for providing sufficient filtering of the converter input current, and the single power stage.

2. The power supply of claim 1, further comprising

an input-output isolation means for electrically isolating input of the power converter from its output while electrical power is transferred from the converter input to the converter output through the isolation means operating at a switching frequency, which has an input and an output being electrically isolated from each other,

a set of one or more controllable input semiconductor switches that are parts of the converter input circuit but are electrically isolated from the power converter output and controlled by the system controller at the switching frequency to regulate the converter output current to have a rectified sinusoidal waveform,

a set of one or more controllable output semiconductor switches that are electrically isolated from the power converter input circuit and controlled by the system controller to convert the rectified sinusoidal output current into an AC output current of sinusoidal waveform that is in phase with the utility grid voltage,

an output filtering means for

absorbing AC switching ripple current from the AC sinusoidal output current and transferring the filtered AC sinusoidal output current into the utility grid and

attenuating common-mode switching noise that radiate from the output semiconductor switches through the utility grid transmission line,

wherein the input-output isolation means and the input semiconductor switches are parts of the power converter and operate together at the same switching frequency which is at least about 20 kHz,

wherein the converter output has its power components internally interconnected from the output of the input-output isolation means to the converter output that is externally terminated by the unfolding and output filtering grid interface which comprises the controllable output semiconductor switches and the output filtering means, and

wherein the converter input circuit includes the DC power source, the input bulk capacitor, the line filter, and all interconnected components within the converter input section that is connected to the input of the input-output isolation means.

3. The power supply of claim 1, further comprising:

a control-to-output isolation means for electrically isolating the output of the power converter from the system controller whereas controlling signals, that are delivered from the controller to the converter output circuit, and the feedback signals, that are sensed from the power converter output circuit to the controller, are transferred through the control-to-output isolation means that have their inputs and outputs electrically isolated from one another,

wherein the system controller, while being isolated from the converter output, can be designed to share a common reference node located at the input-power return terminal that is part of the converter input circuit and the common reference node serves as the signal ground for the controller and the power ground for the converter input circuit.

4. The power supply of claim 1, further comprising:

a transient energy absorption means for absorbing excess or residual transient energy within the power converter and external transient voltage surges produced by or coupled into the utility grid and for protecting components within the power converter output circuit from being over-stressed by excessive transient voltages occurring as a result of the converter residual transient energy and the utility grid voltage surges,

wherein the transient energy absorption means comprises a set of interconnected active and passive semiconductor devices that are parts of the unfolding and output filtering grid interface for dissipating the converter transient energy and the utility grid voltage surges,

wherein the transient energy absorption means always functions to protect the power converter output circuit during any controlled mode in which the unfolding and output filtering grid interface is commanded to operate,

wherein the transient energy absorption means primarily provides energy dissipation and voltage clamping across the converter output for protecting the converter output circuit from being over-stressed when the unfolding and output filtering grid interface is controlled to disconnect the converter output from supplying its output current to the utility grid, and

wherein the converter output circuit includes all interconnected components within the converter output section starting from the output of the input-output isolation means to the unfolding and output filtering grid interface.

5. The power supply of claim 1, further comprising:

a relay switching network for directing the filtered AC current to either the utility grid or an emergency stand-alone load; and

a utility voltage dropout detector circuit for detecting a grid under-voltage or over-voltage condition, providing a mode transfer control signal for changing the state of the relay switching network from being connected to the utility grid to being connected to the stand-alone load due to the grid under-voltage or over-voltage condition, or vice versa a pre-determined duration after the grid under-voltage or over-voltage condition disappears for at least a pre-determined duration.

6. The power supply of claim 1, further comprising a free-running sinusoidal waveform generator that provides a rectified sinusoidal reference waveform for the system controller and a utility switching signal for controlling the unfolding and filtering utility grid interface.

7. The power supply of claim 1, further comprising an over-current protection circuit for

limiting the AC output current to a pre-determined RMS value and

preserving the sinusoidal waveform of the AC output current.

8. The power supply of claim 1, further comprising:

one or more semiconductors in a component selected from the component group consisting of

the DC-DC converter and

the unfolding and output filter grid interface; and

an over-temperature protection circuit for sensing the temperature of the one or more semiconductors and regulating the sensed temperature not to exceed a pre-determined temperature by reducing the output power transferred to the utility grid or a stand-alone load.

9. The power supply of claim 1, wherein the system controller comprises a three-input multiplier that produces a current reference signal for commanding the power converter output current to be rectified sinusoidal.

10. The power supply of claim 1, further comprising:

a stand-by battery for providing additional transient power to support high transient power demand from a stand-alone load to bring the load to operate properly during and after its start-up or a transient load condition;

a first semiconductor battery switch and a second semiconductor battery switch, connected in series, for managing power transfer to and from the stand-by battery; and

a battery management control circuit for determining proper mode of the power transfer to and from the said stand-by battery;

wherein each of the first and second semiconductor battery switches has a built-in body diode, and the body diodes within the battery switches are connected in series and with opposite polarity effectively to block current flow when both of the battery switches are turned off.

11. The power supply of claim 10, wherein:

the battery management control circuit controls the first and second semiconductor battery switches to operate the stand-by battery in an isolation mode, a stand-by mode, a charging mode, an input voltage clamping mode, and a battery-dominated input mode;

the stand-by battery is in the isolation mode when the first and second semiconductor battery switches are turned off;

the stand-by battery is in the stand-by mode when the first semiconductor battery switch is turned on, the second semiconductor battery switch is turned off, and the power converter and unfolding and output filtering grid interface provide transient power to the stand-alone load;

the stand-by battery is in the charging mode when the first semiconductor battery switch is turned off and the second semiconductor battery switch is linearly regulated to maintain a healthy state of charge in the stand-by battery at low power, while the system controller regulates the DC power source output voltage at a level that is typically above the battery voltage;

the stand-by battery is in the input voltage clamping mode when the first semiconductor battery switch is turned off, the second semiconductor switch is turned on continuously or linearly regulated with a pulse-width modulation control, and the power converter and unfolding and output filtering grid interface are shut down, to restore a healthy state of charge to the stand-by battery without power transfer to the power converter; and

the stand-by battery is in the battery-dominated input mode, when the first semiconductor battery switch is turned on and the second semiconductor switch is turned on continuously or linearly regulated with a pulse-width modulation control, for providing power transfer either

to the stand-alone load when there is no power available from the DC power source, or

for fast battery charging while the power converter is shut down.

12. The power supply of claim 1, further comprising:

a back-up battery;

an input relay switching network that is controlled to electrically connect the input of the power converter alternately to the DC power source or the back-up battery;

a house keeping supply means for providing a DC voltage bias for each control circuit that controls power transfer from the DC power source and the back-up battery to the power converter;

a backup charging means for maintaining a healthy state of charge of the backup battery using power from the utility grid; and

a mode controller means for controlling the backup charging means and the input relay switching network for reliable utilization of the backup battery.

13. The power supply of claim 1, wherein the system controller comprises a means for producing a current reference commanding signal I_REF as a function of the inverse RMS value of the converter output voltage, but not as a function of the square of the inverse RMS value of the converter output voltage.

14. The power supply of claim 1, wherein the system controller comprises a means for producing a current reference commanding signal I_REF as a function of a rectified sinusoidal reference signal that is synchronized and in-phase with, but produced independently of the magnitude of, the utility grid voltage.

15. The power supply of claim 1, wherein the system controller comprises a means for producing a current reference commanding signal I_REF as a function of V_ERR, the scaled difference between a maximum-power set point voltage and the sensed output voltage of the DC power source.

16. The power supply of claim 15, wherein V_ERR serves as the commanding power input that is proportional to the average output power being transferred from the power converter to the utility grid and independent from the utility grid voltage.

17. The power supply of claim 15, further comprising:

a slow voltage-limiting error amplifier for providing active pull-down, through a first diode and an PNP transistor, of V_ERR when the RMS voltage across the stand-alone load or the RMS of the utility grid voltage exceeds a pre-determined RMS value, while preserving the sinusoidal waveform of the AC inverter output current;

a fast over-voltage shut-down circuit for providing a quick pull-down, through a second diode and the PNP transistor, of V_ERR to shut down the power converter stage when the voltage across the stand-alone load or the utility grid exceeds a predetermined instantaneous value; and

an isolated voltage sense and scaling circuit that senses the rectified inverter output voltage with scaling factor K_V with electrical isolation from the inverter output for feeding the input of both the slow voltage-limiting amplifier and the fast over-voltage shut-down circuit.

18. The power supply of claim 15, further comprising:

a current sense circuit that senses the power converter output current I_OUT, the current sense circuit being electrically isolated from the uni-polar output, and generating a sensed current signal that represents I_OUT;

a voltage sense circuit that senses the power converter output voltage V_UT, the voltage sense circuit being electrically isolated from the uni-polar output, and generating a sensed voltage signal that represents V_UT;

a slow current-limiting error amplifier for providing active pull down, through a first diode and a PNP transistor, of V_ERR when the RMS of the sensed current signal exceeds an RMS threshold, where the RMS threshold is a first function of the voltage supplied to the utility grid, and where the waveform of the AC inverter output current remains sinusoidal;

a fast over-current shut-down circuit for providing a quick pull-down, through a second diode and the PNP transistor, of V_ERR to shut down the power converter stage when the instantaneous value of the sensed current signal exceeds an instantaneous threshold, where the instantaneous threshold is a second function of the voltage supplied to the utility grid;

a negative voltage detection circuit means for

detecting that the sensed voltage signal represents a negative voltage at the power converter output; and

producing an enable signal, responsive to the detecting, that enables or shuts down the unfolding and output filtering grid interface;

wherein the enable signal controls a switching bridge driver circuit that enables and disables a square-wave signal generator that drives the unfolding and output filtering grid interface.

19. The power supply of claim 1, further comprising:

a waveform generator that produces a sinusoidal waveform that;

is synchronized and in-phase with, but produced independently of the voltage waveform of the utility grid power,

has total harmonic distortion of less than 2% and has no substantial discontinuity in its waveshape, and

has a fixed amplitude that is independent from the amplitude of the utility grid voltage; and

a current level controller that controls the output current of the single power stage as a function of the high-fidelity sinusoidal waveform.

20. A power supply adapted to supply AC power to a utility grid, comprising:

a DC power source comprising a plurality of distributed DC sources that are individually connected in cascade with respective upstream DC-DC converters, where each upstream DC-DC converter has an output, and the outputs are connected in parallel;

a power converter that provides uni-polar output power through a single power stage of DC-DC conversion;

an unfolding and output filtering grid interface that commutates and filters the uni-polar output power into AC current that is supplied to the utility grid;

a system controller for

controlling the waveform of the converter output current to be rectified sinusoidal in phase with the utility grid, and

reducing the output power when the utility grid voltage exceeds a pre-determined threshold;

wherein the power converter is a cascade connection of an input bulk capacitor, a line-filter for providing sufficient filtering of the converter input current, and the single power stage;

each of the distributed DC sources includes a dedicated maximum power tracking control through the respective upstream DC-DC converter;

the parallel-connected outputs of the upstream DC-DC converters provide DC power to the single power stage; and

the system controller regulates the power source voltage at a fixed DC voltage.

21. A method of converting power from a DC power source into AC power, comprising:

detecting the utility grid power ripple;

sensing a current proceeding from the DC power source into a power conversion stage, wherein the output current of the power conversion stage is controlled by a control signal that possesses a rectified sinusoidal waveform of a proper amplitude;

sensing a voltage across the DC power source;

generating a power signal as a function of the product of the sensed current and the sensed voltage;

detecting an AC power ripple from the power signal and an AC voltage ripple from the sensed voltage, each ripple appearing as an inherent result of operation of the power conversion stage;

extracting a first dither signal from the AC power ripple, and a second dither signal from the AC voltage ripple;

deriving a dither signal inherently available from the utility grid power ripple; and

converting power from the DC power source into AC power, including;

determining the proper tracking direction toward the maximum power point of the DC source, and

providing inherent dithering as a result of regulating the sensed output current to track the rectified sinusoidal control signal, and

deriving the proper amplitude of the rectified sinusoidal within the control signal through a three-input multiplier.

22. A power supply adapted to supply AC power, comprising:

a DC power source having an output voltage V_SA and an output current I_SA;

a first low-pass filter taking V_SA as an input and providing a filtered voltage level V_LP as an output;

a second low-pass filter taking I_SA as an input and providing a filtered current level I_LP as an output;

an analog multiplier taking filtered voltage level V_LP and filtered current level I_LP as inputs, and providing a scaled power output level P_SA=k·

V_LP·

I_LP as an output, for a predetermined value k;

a first analog differentiator taking filtered voltage level V_LP as an input, and providing voltage derivative signal Vac_SA as an output;

a second analog differentiator taking scaled power output level P_SA as an input, and providing power derivative signal Pac_SA as an output;

a set point searching means for searching for a set point voltage V_SP as a function of the voltage derivative signal Vac_SA and power derivative signal Pac_SA; and

a feedback means for controlling the DC power source output voltage V_SA to track the voltage, being proportional to the set point voltage V_SP, that eventually reaches and swings back and forth around the maximum power voltage of the DC power source.

23. The power supply of claim 22, wherein at least one of the first and second analog differentiators is a two-stage active filter, comprising:

a first-order high-pass active filter having a high-pass-filter input, the first-order high-pass active filter being connected in cascade with

a first-order inverted low-pass active filter that includes an inductor;

wherein the first-order high-pass active filter has a transfer function of the form

$\frac{k_1·<br><br>s}{\frac{s}{\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>o}+1},$

including a network comprising a feedback resistor, a series R-C two-terminal network, and an operational amplifier;

the first-order inverted low-pass active filter has a transfer function of the form

$\left(\frac{s}{\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>o}+1\right)/k_2,$

, and includes a network comprising an input resistor, a series L-R two-terminal network, and an operational amplifier;

the series R-C two-terminal network comprises a capacitor and a resistor connected in series, and is terminated between the input of the high-pass active filter and the inverting input of the operational amplifier within the high-pass active filter;

the series L-R two-terminal network comprises an inductor and a resistor connected in series, and is terminated between the output of the inverted low-pass active filter and the inverting input of the operational amplifier within the inverted low-pass active filter;

the feedback resistor is terminated across the inverting input and the output of the operational amplifier within the high-pass active filter;

the input resistor is terminated across the input of the inverted low-pass filter and the inverting input of the operational amplifier within the inverted low-pass active filter; and

the overall transfer function of the two-stage filter has the form (k₁/k₂)·

s.

24. The power supply of claim 22, wherein at least one of the first and second analog differentiators is a two-stage active filter, comprising:

a first-order high-pass active filter having a high-pass-filter input, the first-order high-pass active filter being connected in cascade with

a first-order inverted low-pass active filter having no inductor;

wherein the first-order high-pass active filter has a transfer function of the form

$\frac{k_1·<br><br>s}{\frac{s}{\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>o}+1},$

including a network comprising a first feedback resistor, a series R-C two-terminal network, and a first operational amplifier;

the first-order inverted low-pass active filter has a transfer function of the form

$\left(\frac{s}{\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>o}+1\right)/k_2,$

including a network comprising a second feedback resistor, a parallel R-C two-terminal network, and a second operational amplifier;

the parallel R-C two-terminal network comprises a capacitor and a resistor connected in parallel, and is terminated between the input of the inverted low-pass active filter and the inverting input of the operational amplifier within the inverted low-pass filter;

the series R-C two-terminal network comprises a capacitor and a resistor connected in series, and is terminated between the input of the high-pass active filter and the inverting input of the operational amplifier within the high-pass active filter;

each of the feedback resistors is terminated across the inverting input and the output of the respective operational amplifier; and

the overall transfer function of the two-stage filter has the form (k₁/k₂)·

s.

25. A DC-to-AC power conversion system, comprising:

a utility grid interface having two input terminals and two output terminals;

a uni-directional switching bridge, comprising two upper uni-directional semiconductor switches and two lower uni-directional semiconductor switches, that takes the rectified converter output current I_OUTas an input and provides AC output as inverter output current (I_AC) feeding into the two input terminals of the utility grid interface, wherein each uni-directional semiconductor switch has an upper terminal and a lower terminal;

a DC-DC converter output having positive and return terminals, where the positive terminal is connected to the upper terminals of the two upper uni-directional semiconductor switches within the switching bridge, and the return output terminal is connected to the lower terminals of the two lower uni-directional semiconductor switches within the switching bridge;

a full bridge rectifier taking the signal between the two input terminals of the utility grid interface as input, and providing a positive output and a negative output; and

an energy absorption device (730) electrically connected between the positive output and the negative output of the rectifier;

wherein the DC-DC converter positive output terminal is connected to the positive output of the rectifier (720) through a forward-biased diode (D5);

wherein each of the upper and lower uni-directional semiconductor switches comprises a diode and a semiconductor switch connected in series and has an upper terminal and a lower terminal; and

wherein each semiconductor switch is a transistor selected from the group consisting of;

a bipolar junction transistor (BJT),

a metal-oxide semiconductor field effect transistor (MOSFET), and

an insulated-gate bipolar junction transistor (IGBT).

26. The apparatus of claim 25 wherein the output current of the apparatus is controlled to make its output current have a sinusoidal waveform that

in a first mode, is in-phase with the utility grid voltage that possesses insignificant inductive characteristics, or

in a second mode, is delivered into a stand-alone resistive load that has insignificant inductive characteristics;

wherein the first mode and the second mode are accomplished by controlling a relay switching network for directing the filtered AC current to either the utility grid or an emergency stand-alone load.

27. The system of claim 25, wherein:

the lower terminal of each upper uni-directional semiconductor switch is connected in series with the upper terminal of the respective lower uni-directional semiconductor switch;

further comprising a control signal that controls the upper semiconductor switches to disconnect the output terminal of the DC-DC converter from supplying AC power to the utility grid as a result of loss of synchronization with the utility grid voltage.

28. A power supply adapted to provide AC power from a DC power source to a utility grid having a utility grid transmission line, comprising:

a DC-DC power conversion stage

having a converter output inductor and

providing a rectified converter output current I_OUT between a first conductor and a second conductor, where the voltage potential of the first conductor is at least about as great as the potential of the second conductor;

a uni-directional switching bridge, comprising four or more uni-directional switches, that takes the rectified converter output current I_OUT as an input and provides AC output as inverter output current (I_AC) to the utility grid;

a full-bridge rectifier that takes the AC output as its input and provides a fully rectified output between a positive terminal and a negative terminal;

a transient energy absorption device (730, 930, 931) electrically connected between the positive terminal and the negative terminal; and

a bleeding diode that provides a current path for a portion of

excess energy stored in the converter output inductor and

external transient energy coupled to or residual energy stored in the utility grid transmission line to be absorbed by the transient energy absorption device;

wherein the excess inductive energy in the rectified converter output current I_OUT is directed from the first conductor into the transient energy absorption network, and excess energy in forms of voltage surges produced from the utility grid is directed through the following devices;

the bleeding diode,

one or more of the semiconductor switches in the uni-directional switching bridge,

the full-bridge rectifier, and

the transient energy absorption device;

wherein the external transient energy in forms of voltage surges produced from the utility grid is directed through the following devices;

one of the semiconductor switches in the uni-directional switching bridge,

the full-bridge rectifier, and

the transient energy absorption device; and

wherein the uni-directional switches each comprise a series connection between a diode and a semiconductor switch selected from the switch group consisting of a MOSFET, a BJT, and an IGBT.

29. A power supply adapted to provide AC power from a DC power source to a utility grid with energy transient absorption capability, comprising:

a DC-DC uni-directional converter power stage having an AC converter output current I_AC between a first conductor and a second conductor, where the voltage potential of the first conductor relative to the second conductor is sinusoidal and in-phase with the utility grid voltage, and comprising an output-coupled inductor having a first winding and a second winding;

a first and a second uni-directional transient energy absorption network, each comprising a diode and a uni-directional transient absorber connected in series, the uni-directional transient energy absorption devices being connected in series with opposite polarity to provide bi-directional transient energy absorption; and

wherein excess energy in the output-coupled inductor is directed through either

a first diode, the first winding of the output-coupled inductor, a second diode, and the first uni-directional transient energy absorption network, or

a third diode, the second winding of the output-coupled inductor, a fourth diode, and the second uni-directional transient energy absorption network; and

excess energy from the utility grid is directed through either

a first semiconductor switch and the first transient energy absorption network,

a second semiconductor switch and the second transient energy absorption network; and

wherein the first and second semiconductor switches have their respective paralleled or built-in body diodes for providing current paths for the excess energy from the utility grid to be absorbed by the first and the second transient energy absorption networks.