CONTROLLING A CONVERTER IN RESPONSE TO GRID FAULT EVENTS

BACKGROUND OF THE INVENTION

[0001] The subject matter disclosed herein relates generally to systems and methods for use in interfacing a power generator to an electric grid through a power module.

[0002] Electric grids are known for distribution of electric power. A utility power generator is generally known to provide a substantial amount of power to the electric grid, while independent sources are connected to the electric grid to provide a local grid power and reduced dependence on the utility power generator.

[0003] Each of the independent sources is connected to the electric grid through a power conditioner and/or a converter to provide consistent and efficient coupling of the independent source to the electric grid. Under certain conditions, the electric grid may experience one or more grid fault events, such as low voltage, high voltage, zero voltage, phase jumping, etc. Often, electric grid operators require that independent sources connected to the electric grid be sufficiently robust to ride through grid fault events. Under such conditions, power conditioners and/or converters may be required to protect the power generator from one or more over-voltage conditions, while providing the ride through functionality. Several known power conditioners and/or converters, for example, include braking resistors to absorb excessive energy in order to prevent over-voltage conditions. Other known methods instantaneously turn OFF switching devices within power conditioners and/or converters during a grid fault event, intending to preempt one or more over-voltage conditions. BRIEF DESCRIPTION OF THE INVENTION

[0004] In one aspect, a power module for use in interfacing a power generator to an electric grid is provided. The power module includes a converter configured to couple to the electric grid. The converter has an input configured to receive an input voltage from the power generator. The example power module further includes an energy storage device coupled across the input of the converter and a controller coupled to the converter. The controller is configured to establish at least one current path through the converter, in response to a grid fault event, to inhibit power flow from the electric grid to the energy storage device.

[0005] In another aspect, a power system is provided. The power system includes a power generator configured to generate a DC voltage and a power module coupled to the power generator. The power module includes a converter, which is coupled to the power generator and includes multiple phase outputs. The converter is configured to provide a multi-phase output voltage to an electric grid. The power module further includes an energy storage device coupled in parallel with the converter, and a controller coupled to the converter. The controller is configured to establish, in response to a grid fault event, a current path through said converter to inhibit power flow from the electric grid to said energy storage device.

[0006] In yet another aspect, a method for use in interfacing a power generator to an electric grid through a power module is provided. The method includes providing an output voltage from an output of a converter that is coupled to an electric grid. The method further includes detecting a grid fault event associated with the electric grid and, in response to the grid fault event, inhibiting power flow from the electric grid into an energy storage device coupled in parallel with an input of the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Fig. 1 is a block diagram of an exemplary power system. [0008] Fig. 2 is a circuit diagram of an exemplary power module that may be used with the power system of Fig. 1.

[0009] Fig. 3 is a block diagram of an exemplary method for use in interfacing a power generator to an electric grid through a power module.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The embodiments described herein relate to power systems and methods for use in interfacing a power generator to an electric grid through a power module. More particularly, the embodiments described herein relate to inhibiting reverse current flow into power modules from the electric grid during one or more grid fault events.

[0011] According to one or more embodiments, technical effects of the methods, systems, and modules described herein include at least one of: (a) providing an output voltage from an output of a converter, the converter coupled to an electric grid, (b) detecting a grid fault event associated with the electric grid, and (c) in response to the grid fault event, inhibiting power flow from the electric grid into an energy storage device coupled in parallel with an input of the converter.

[0012] Fig. 1 illustrates an exemplary power system 100. In the exemplary embodiment, power system 100 includes an electric grid 102, multiple power generators 104 coupled to electric grid 102, and a major power generator 106 coupled to electric grid 102. Major power generator 106 is configured to provide a relatively major portion of power to electric grid 102, as compared to each of the power generators 104. In various embodiments, each power generator 104 may include, without limitation, one or more photovoltaic (PV) cells, wind turbines, hydroelectric generators, fuel generators, and/or other power generator devices, etc. Further, major power generator 106 may include, for example, a nuclear, coal, or natural gas power generator. It should be appreciated that power system 100 may include a different number and/or configuration of generators in other embodiments.

[0013] Power system 100 includes a power module 108 coupled between each of power generators 104 and electric grid 102. Fig. 2 illustrates an exemplary power module 108. In the exemplary embodiment, power module 108 includes a DC- AC converter 110 and a controller 112 coupled to converter 110 to provide control signals to converter 110. Converter 110 includes an input 114 coupled to power generator 104 and an output 116 coupled to electric grid 102. While converter 110 includes a DC-AC converter in the exemplary embodiment, it should be appreciated that other converters may be used in other embodiments. For example, converter 110 may include a DC-DC converter, an AC-DC converter, and/or an AC-AC converter, etc. Further, as shown in Fig. 2, power module 108 includes an energy storage device 118 coupled in parallel with input 114 of converter 110. While energy storage device 118 is illustrated as a single capacitor, it should be understood that a different number and/or type of energy storage device may be used in other embodiments.

[0014] In the exemplary embodiment, converter 110 includes a three phase DC-AC converter, which is configured to provide a three-phase AC voltage to output 116. Output 116 includes three phase outputs A, B, and C to provide the three phase AC voltage to electric grid 102. As should be apparent, one or more converters having a different number of phases may be used in other power module embodiments.

[0015] Referring to Fig. 2, converter 110 includes a first branch 120 of switching devices and a second branch 122 of switching devices, which are coupled between energy storage device 118 and output 116. Specifically, in the exemplary embodiment, first branch 120 includes three first switching devices 124, 126 and 128, and second branch includes three second switching devices 130, 132, and 134. In the exemplary embodiment, converter 110 is configured such that each phase includes one of first switching devices 124, 126 and 128 and one of second switching devices 130, 132, and 134. As shown in Fig. 2, each switching device 124, 126, 128, 130, 132 and 134 includes a body diode D. Further, each switching device 124, 126, 128, 130, 132 and 134 is coupled to controller 112 to receive a control signal therefrom. As should be apparent, a different number of switching devices arranged in various configurations may be used in other power module embodiments. Further, as illustrated each switching device 124, 126, 128, 130, 132 and 134 includes an insulated gate bipolar junction transistor (IGBT). In other embodiments, one or more different switching devices or combinations thereof may be used. For example, each switching device 124, 126, 128, 130, 132 and 134 may include a field effect transistor (FET), a silicon controlled rectifier (SCR), a bipolar junction transistor (BJT), a thyristor, and/or another device suitable to perform as described herein.

[0016] In the exemplary embodiment, power module 108 includes three inductors 136, 138 and 140, which filter and/or condition the three-phase AC voltage provided from converter 110. Each inductor 136, 138 and 140 is coupled in series between one of phase outputs A, B, and C of output 116 and electric grid 102. Moreover, power module 108 includes a feedback unit 142 coupled to controller 112. Feedback unit 142 is positioned and/or structured to provide one or more signals indicative of a polarity and amplitude of an inductor current through one or more of inductors 136, 138 and 140. In one example, feedback unit 142 includes a rotating coordinate transformation module configured to provide output signals indicative of the inductor current to controller 112.

[0017] In the exemplary embodiment, controller 112 is implemented in one or more processing devices, such as a microcontroller, a microprocessor, a programmable gate array, a reduced instruction set circuit (RISC), an application specific integrated circuit (ASIC), etc. Accordingly, in the exemplary embodiment, one or more aspects of controller 112 as described herein are implemented through software and/or firmware embedded in the one or more processing device. In this manner, controller 112 is programmable, such that one or more predetermined thresholds and/or intervals may be programmed for a particular power generator 104 and/or operator of power generator 104. Further, in at least one embodiment, feedback unit 142 may be incorporated into one or more processing devices which embody controller 112.

[0018] Referring to Fig. 2, power module 108 includes a DC-DC boost converter 144 configured to adjust the voltage provided from power generator 104. More specifically, boost converter 144 is configured to step-up the DC output voltage from power generator 104 and provide the stepped-up DC voltage to converter 110. It should be appreciated that a different number and/or type of converters may be used in other power module embodiments. For example, a power module may include a buck converter to step-down the DC voltage from power generator 104, a buck-boost converter to step-up the DC voltage from power generator 104, or another type of converter, potentially depending on converter 110, power generator 104, and/or electric grid 102. In still other embodiment, converter 144 includes an AC- AC converter or an AC-DC converter to receive and convert an AC voltage from particular type of power generator 104, such as, for example, a wind turbine. In further embodiments, one or more converters 144 may be omitted.

[0019] During operation, power generator 104 provides a DC voltage to boost converter 144, which, in turn, provides a DC voltage to energy storage device 118. Energy storage device 118 is charged from the DC voltage to provide a voltage to converter 110. In response to control signals from controller 112, switching devices 124, 126, 128, 130, 132 and 134 are selectively turned ON and OFF to provide the three phase AC voltage to output 116. Inductors 136, 138 and 140 filter the AC voltage from converter 110 and provide the filtered AC voltage to electric grid 102. It should be appreciated that controller 112 monitors a voltage at electric grid 102 to ensure switching devices 124, 126, 128, 130, 132 and 134 provide the three-phase AC voltage substantially consistent with the voltage at electric grid 102, thereby providing a substantially efficient transfer of power from power generator 104 to electric grid 102. [0020] In the exemplary embodiment, electric grid 102 may experience one or more grid fault events during operation. A grid fault event may include, without limitation, a low voltage condition, a high voltage condition, a zero voltage condition, a phase jumping condition, and/or a transient voltage/current condition. Depending on the type of grid fault event, a voltage condition may exist that provide current flow from electric grid 102 into power module 108. Known power conditioner and/or converter often turn OFF switching devices included in a converter in response to reverse current flow into power module. The present disclosure recognizes that, when the switching devices are turned OFF, body diodes associated with such switching devices may conduct current from the electric grid into the energy storage device. Current conducted through the body diodes may be sufficient in some converters to cause an over-voltage condition, potentially resulting in stress, damage and/or shutdown of a power module. Further, as shown in Fig. 2, when switching devices 124, 126, 128, 130, 132 and 134 are turned OFF, body diode may be oriented such that current is only permitted to flow into energy storage device 118, but not be discharged from energy storage device 118.

[0021] In the exemplary embodiment, controller 112 is configured to create a current path through converter 110, in response to a grid fault event, to inhibit power flow from electric grid 102 into energy storage device 118 of power module 108. In one example, Accordingly, controller 112 controls first and second branches 120 and 122 to avoid one or more over- voltage conditions at energy storage device 118, which may reduce stress on power modules 108, and particularly, on energy storage device 118, to extend the useful life of power module 108.

[0022] In the exemplary embodiment, controller 112 is configured to provide a substantially zero voltage at output 116 (e.g., between two or more phases of output 116) to inhibit power flow from the electric grid to said energy storage device. . In this manner, power is permitted to flow through and/or across output 116, without flowing into energy storage device 118 of power module 108. Switching devices of converter 110 may be switched ON or OFF according to a variety of different schemes to provide such a voltage condition at output 116, potentially depending on the topology of converter 110 and the types of switching devices included therein. One exemplary switching scheme is described below with reference to controller 112 and converter 110. As used herein, the term "substantially zero voltage" refers to a voltage equal to zero or approximately equal to zero. The voltage across output 116 may be greater than zero, yet approximately equal tot zero, based on voltage across one or more switching devices included in converter 110.

[0023] In the exemplary embodiment, controller 112 is configured to selectively turn ON either switching devices 124, 126, andl28 or switching devices 130, 132, and 134. In this manner, phase outputs A, B, and C are electrically connected together to provide current paths into converter 110 through one of phase outputs A, B, and C and out of converter 110 through another phase output. Accordingly, output current flows into and out of converter 110 through its phase outputs A, B, and C. As shown in Fig. 2, such current paths, which include switching devices 124, 126, 128, 130, 132 and 134, exclude energy storage device 118, which inhibits current flow into energy storage device 118 through body diodes associated with switching devices 124, 126, 128, 130, 132 and 134. Further, the inclusion of switching devices 124, 126, 128, 130, 132 and 134 as described herein permits excess power from electric grid 102 to be absorbed through switching loss associated with alternately turning ON switching devices 124, 126, 128, 130, 132 and 134.

[0024] Moreover, in the exemplary embodiment, feedback unit 142 provides to controller 112 one or more feedback indicative of a polarity and/or amplitude of the inductor current flowing through one or more of inductors 136, 138 and 140 and/or voltage at output 116. In one example, the feedback is indicative of a current through one or more of inductors 136, 138 and 140, either directly or indirectly. In turn, controller 112 turns ON switching devices 124, 126, and 128 or switching devices 130, 132, and 134, based on the feedback. Additionally, in order to reduce the thermal stress on the ON switching devices of first and second branches 120 and 122, controller 122 alternately turns ON first branch 120 and second branch 122 during the grid fault event. More specifically, when first branch 120 is turned on, thermal stress on switching device of first branch 120 due to conducting current therethrough, After a predetermined interval (e.g., one half duty cycle, about 10 milliseconds, about 100 milliseconds) or a current condition at inductors 136, 138 and/or 140), controller alternate to turn on second branch 122, thereby permitting the thermal stress on first branch 120 to dissipate. As used herein, the term "thermal stress" refers a thermal condition of a switching device, such as, for example, heat generated by passing current through a switching device.

[0025] In the exemplary embodiment, controller 112 is configured to alternately turn ON switching devices 124, 126, and 128 and switching devices 130, 132, and 134 when an inductor current is less than a predetermined threshold. The predetermined threshold may be based on various factors including, without limitation, the inductance of inductors 136, 138, and 140. For example, when smaller inductors 136, 138 and 140 are included in power module 108, controller 112 is configured to selectively turn ON individual switching devices 124, 126, 128, 130, 132 and 134 to permit energy storage device to discharge electric grid 102 through individual phases of converter 110. More generally, switching devices 124, 126, 128, 130, 132 and 134 may be selectively turned ON to inhibit current flow from electric grid 102 to power module 108, but permit current flow from energy storage device to electric grid 102 during a grid fault event. As such, controller 112 is configured in the exemplary embodiment to control switching devices 124, 126, 128, 130, 132 and 134 individually and/or together as branches 120 and 122 to inhibit high current stress on power modules 108.

[0026] Additionally, or alternatively, controller 112 may be configured to alternately turn ON switching devices 124, 126, and 128 and switching devices 130, 132, and 134 for a predetermined interval, such as, for example, 100 milliseconds, 500 milliseconds, or 1 second, etc. The predetermined interval is selected to permit power module 108 to ride through one or more grid fault events, but not to indefinitely respond to the grid fault event when the grid fault event has ended or shutdown of power module 108 is warranted. In at least one embodiment, a phase-lock-loop (PLL) circuit of feedback unit 142 is configured to indicate to controller 112 when the PLL circuit has locked the phase of the voltage associated with electric grid 102. If the indication is received within the predetermined time, controller 112 may return to normal control of converter 110. Conversely, if the predetermined interval runs without an end to the grid fault event, controller 112 may shutdown power module 108.

[0027] In the exemplary embodiment, controller 112 monitors the voltage associated with electric grid 102 and/or voltages at converter 110. As such, controller 112 is able to detect the grid fault event and perform as described herein, within a predetermined time period, such as, for example, about 5 milliseconds, about 10, about 100 milliseconds, or another suitable time period. In this manner, controller 112 mitigates a potential over-voltage condition, before a known converter would be able to even detect the over-voltage condition at energy storage device 118. Therefore, as compared to known converters, controller 112 may permit one or more over- voltage conditions to be avoided, which may reduce stress on power modules 108 and extend its useful life. Further, by responding to a grid fault event before a over- voltage condition exists, controller 112 is able to maintain power module 108 and/or ride through the grid fault event, without requiring a shutdown or restart of power module 108 during or after the grid fault event.

[0028] Fig. 3 illustrates an exemplary method 200 for use in interfacing power generator 104 to an electric grid 102 through a power module 108. While method 200 is described herein with reference to power system 100, it should be appreciated that method 200 should not be understood to be limited to power system 100 and that method 200 may be used with a variety of different power systems. Further, power system 100 should not be understood to be limited to method 200.

[0029] Method 200 includes providing 202 an output voltage from an output 116 of converter 110. Converter 110 is coupled to electric grid 102. Method 200 further includes detecting 204 a grid fault event associated with electric grid 102 and in response to the grid fault event, inhibiting 206 power flow from electric grid 102 into energy storage device 118 coupled in parallel with input 114 of converter 110.

[0030] In one or more further exemplary methods, inhibiting power flow from electric grid 102 into energy storage device 118 may include electrically connecting at least two phases of the output of converter 110 to establish said at least one current path. Additionally, or alternatively, in some methods, electrically connecting the at least two phases of the output may include selectiving turning ON one of first branch 120 and second branch 122 of converter 110. Further, in some examples, method 200 may include turning ON at least one of switching device 130, 132, and 134 to discharge energy storage device 118 into electric grid 102, during a grid fault event, when a current associated with output 116 of converter 110 exceeds a predetermined threshold, selectively turning ON one of first branch 120 and second branch 122 of converter 110 includes alternately turning ON one of first branch 120 and second branch 122 to balance a thermal stress between first and second branches 120 and 122. Further, in some example methods, alternately turning ON one of first branch 120 and second branch 122 of converter 110 comprises alternately turning ON one of the first branch 120 and the second branch 122 based on a current induced in at least one of inductors 136, 138, and 140.

[0031] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. SYSTEMS AND METHODS FOR USE IN CONTROLLING A CONVERTER IN RESPONSE TO GRID FAULT EVENTS

PARTS LIST

Power system

Electric grid

Power generator

Major power generator

Power module

Converter

Controller

Input

Output

Energy storage device

First Branch

Second Branch

Switching Device

Switching Device

Switching Device

Switching Device

Switching Device

Switching Device

Inductor

Inductor

Inductor

Feedback Unit

Boost Converter

Method Providing

Detecting

Inhibiting