Bidirectional power flow in storage inverters

Abstract

A bidirectional storage inverter includes a bidirectional power inverter circuit and a controller. The bidirectional power inverter circuit includes alternating current (AC) terminals to be coupled to an AC power system, direct current (DC) terminals that are electrically isolated from the AC terminals and are to be coupled to an electrical energy storage device, and a bidirectional circuit path between the AC terminals and the DC terminals. Power flow at the AC terminals is determined based on current and voltage at the AC terminals. The bidirectional power inverter circuit is controlled based on the determined power flow and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from a further bidirectional storage inverter coupled to the AC power system, and from the bidirectional storage inverter through the AC terminals to the further bidirectional storage inverter.

Description

92557216BIDIRECTIONAL POWER FLOW IN STORAGE INVERTERSFIELD

[0001] The present application relates generally to electrical power inverters that are capable of both supplying power to an alternating current (AC) electrical load from a direct current (DC) electrical energy source and receiving power from an AC electrical energy source and supplying that power to a DC electrical load. This includes power inverters that are connected in parallel with each other and / or in parallel with a utility grid.BACKGROUND

[0002] For a power inverter to be capable of supplying both real power and reactive power, the inverter topology must be capable of operating in four quadrants of a cartesian plane formed by plotting voltage versus current. The four quadrants include quadrants where: voltage and current are both positive; voltage is positive, and current is negative; voltage and current are both negative; and voltage is negative, and current is positive. This will enable bidirectional power flow between AC and DC terminals in some non-isolated power inverters. However, this is not sufficient to allow bidirectional power flow where the power may flow in either direction for periods that are much greater than the period of an AC cycle and when electrical isolation is required between the AC terminals and the DC terminals. In this case, a path or paths must be provided to supply power to / from the DC terminals from / to the AC terminals.

[0003] In the case of power inverters that provide electrical isolation between their DC terminals and AC terminals, each path must provide electrical isolation. It is common to provide two isolated, unidirectional DC to DC conversion paths between DC terminals and a bidirectional AC inverter. One path provides power conversion from the AC inverter to the DC terminals to, for example, charge a battery from the AC terminals of the AC inverter. The second path provides power conversion from the DC terminals to the AC inverter to, for example, supply power to an AC electrical grid through the AC terminals of the AC inverter. In a second common configuration, two unidirectional DC / AC conversion paths are provided between the DC and AC terminals, with each converting and carrying power in opposite directions.92557216

[0004] In both configurations, control must be provided to prevent power from circulating through the two paths. Switching between these two paths causes a discontinuity in the control function, leading to a delay in the transition between power flow directions, for example from charging to discharging batteries or from discharging to charging batteries. This delay may also cause a momentary change in impedance that the power inverter presents at its AC terminals, to an AC electrical grid for example, which may result in a transient loss of regulation of AC potential.

[0005] Bidirectional power flow is, by itself, insufficient to maintain regulation when large load or generation transients occur on the AC electrical grid. It is desirable to provide a control and transfer function of a power inverter that also has the necessary response and stability to operate under transient conditions.

[0006] More generally, improved approaches to bidirectional power flow in power inverters are desirable.SUMMARY

[0007] The present disclosure encompasses various embodiments, including embodiments in which an apparatus utilizes a single bidirectional circuit that provides isolation and allows power to flow in either direction. Such a single bidirectional circuit may be formed, for example, by a bidirectional inverter stage and by a bidirectional isolated DC to DC converter.

[0008] An apparatus with a single bidirectional circuit may be useful, for example, in implementing or supporting one continuous transfer function in a control system, which is continuous in that it extends from maximum negative power (charging batteries, for example) to zero power to maximum positive power (supplying AC loads). A continuous transfer function can provide very rapid response to transitions in the direction of power flow and to transient changes in the power supplied or received. This rapid response can help maintain regulation of AC electrical grid potential and provide continuous power flow to or from the grid.

[0009] Examples of features or functions that may be provided in embodiments, alone or in any of various combinations, include the following:92557216bidirectional power flow, which can be useful in realizing a smaller and less expensive inverter with smaller, lighter magnetic components relative to implementations with separate unidirectional paths, for example;a bidirectional circuit that provides isolation, which can be useful in realizing enhanced safety isolation by eliminating the need to treat all electrical nodes as high voltage from a safety point of view - for example, there could be no user serviceable parts or access that would otherwise require suitable insulation such that the user would not be able to access anything hazardous, thereby potentially reducing required safety service level of technicians, reducing difficulty to pass safety certification, and / orreducing expense and size of the circuit by not requiring a higher level of insulation of related components such as a battery enclosure;control to avoid power flow from one bidirectional storage inverter to charge another bidirectional storage inverter;one otherwise continuous transfer function, to provide fast response to load and / or generation transients;fault tolerance enabled by such fast transient response and sharing of power between inverters.

[0010] According to one aspect of the present disclosure, bidirectional storage inverter includes a bidirectional power inverter circuit and a controller. The bidirectional power inverter circuit includes AC terminals to be coupled to an AC power system, DC terminals that are electrically isolated from the AC terminals and are to be coupled to an electrical energy storage device, and a bidirectional circuit path between the AC terminals and the DC terminals. The controller is coupled to the bidirectional power inverter circuit, to determine power flow at the AC terminals based on current and voltage at the AC terminals, and to control the bidirectional power inverter circuit. The controller is to control the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from a further bidirectional storage inverter coupled to92557216the AC power system and from the bidirectional storage inverter through the AC terminals to the further bidirectional storage inverter.

[0011] According to another aspect of the present disclosure, a method involves determining power flow at AC terminals of a bidirectional power inverter circuit of a bidirectional storage inverter based on current and voltage at the AC terminals, and controlling the bidirectional power inverter circuit. The bidirectional power inverter circuit includes the AC terminals coupled to an AC power system, DC terminals that are electrically isolated from the AC terminals and coupled to an electrical energy storage device, and a bidirectional circuit path between the AC terminals and the DC terminals. Controlling the bidirectional power inverter circuit is based on the determined power flow, and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from a further bidirectional storage inverter coupled to the AC power system and from the bidirectional storage inverter through the AC terminals to the further bidirectional storage inverter.

[0012] Other aspects and features of embodiments of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments will be described herein, by way of example only, with reference to the accompanying drawings.

[0014] Fig. 1 illustrates an example of a non-isolated inverter.

[0015] Fig. 2 illustrates an example of an isolated inverter.

[0016] Fig. 3 illustrates an example of an isolated bidirectional inverter according to an embodiment.

[0017] Fig. 4 illustrates an example of an isolated bidirectional DC to DC converter.

[0018] Fig. 5 illustrates an example isolated bidirectional inverter with a power control system according to an embodiment.

[0019] Fig. 6 illustrates an example of a system including a bidirectional storage inverter and a constant power inverter.92557216

[0020] Fig. 7 illustrates an example of a system that includes two bidirectional storage inverters and a constant power inverter.

[0021] Fig. 8 illustrates example power relationships in a system.

[0022] Fig. 9 illustrates power versus voltage examples of various loads and generation.

[0023] Fig. 10 illustrates inverter sharing according to an embodiment.

[0024] Fig. 11 illustrates power versus voltage examples of various loads and storage inverters.

[0025] Fig. 12 illustrates a zero-power region according to an embodiment.

[0026] Figs. 13 A and 13B illustrate a second embodiment of zero-power region.

[0027] Fig. 14 illustrates a third embodiment of zero-power region.

[0028] Fig. 15 illustrates an example method according to an embodiment.DETAILED DESCRIPTION

[0029] Fig. 1 shows an example of a non-isolated DC to AC inverter 100. Non-isolated DC to AC inverter 100 includes DC terminals 104, first capacitance 108, power switches 112, filter inductances 116, second capacitance 120, and AC terminals 124. DC power is received at DC terminals 104 and is provided to first capacitance 108 and power switches 112. First capacitance 108 provides a path for AC currents created by switching of power switches 112. Power switches 112 switch the DC potential at DC terminals 104 and first capacitance 108 to produce pulse width modulated waveforms at filter inductances 116. Filter inductances 116 and second capacitance 120 filter the pulse width modulated waveforms to produce low frequency AC potential at AC terminals 124. The low frequency potential may have a frequency of 60 Hz, for example. DC terminals 104 are connected to a DC energy source (not shown). AC terminals 124 are connected to an AC power grid (not shown) such as a utility grid or a grid formed by other inverters. Power may flow from DC terminals 104 to AC terminals 124 or from AC terminals 124 to DC terminals 104. This inverter example does not provide electrical isolation between DC terminals 104 and AC terminals 124.92557216

[0030] Fig. 2 shows an example of an isolated DC to AC inverter 200. Isolated DC to AC inverter 200 includes DC terminals 204, first capacitance 208, power switches 212, filter inductances 216, second capacitance 220, isolation transformer 224, and AC terminals 228. DC power is received at DC terminals 204 and is provided to first capacitance 208 and power switches 212. First capacitance 208 provides a path for AC currents created by switching of power switches 212. Power switches 212 switch the DC potential at DC terminals 204 and first capacitance 208 to produce pulse width modulated waveforms at filter inductances 216. Filter inductances 216 and second capacitance 220 filter the pulse width modulated waveforms to produce low frequency AC potential at isolation transformer 224. Isolation transformer 224 provides AC potential at AC terminals 228. Isolation transformer 224 provides electrical isolation between DC terminals 204 and AC terminals 228. By way of example, the low frequency potential may have a frequency of 60 Hz. DC terminals 204 are connected to a DC energy source (not shown). AC terminals 228 are connected to an AC power grid (not shown) such as a utility grid or a grid formed by other inverters. Power may flow from DC terminals 204 to AC terminals 228 or from AC terminals 228 to DC terminals 204.

[0031] Fig. 3 shows an example of an isolated bidirectional inverter 300 according to an embodiment. Isolated bidirectional inverter 300 includes DC terminals 304, isolated bidirectional DC to DC converter 308, bidirectional DC to AC inverter 312, and AC terminals 316. DC terminals 304 are coupled to isolated bidirectional DC to DC converter 308. Isolated bidirectional DC to DC converter 308 is coupled to bidirectional DC to AC inverter 312.Bidirectional DC to AC inverter 312 is coupled to AC terminals 316. DC potential from / to DC terminals 304 is isolated from bidirectional DC to AC inverter 312 by isolated bidirectional DC to DC converter 308, and isolated bidirectional DC to DC converter 308 presents DC potential to / is presented with DC potential from bidirectional DC to AC inverter 312. Isolated bidirectional DC to DC converter 308 may, for example, be isolated bidirectional DC to DC converter 400 of Fig. 4, which is described below. Bidirectional DC to AC inverter 312 may, for example, be non-isolated DC to AC inverter 100 of Fig. 1. Energy from / to isolated bidirectional DC to DC converter 308 is converted from / to DC to / from AC by bidirectional DC to AC inverter 312. Bidirectional DC to AC inverter 312 provides AC potential to / is provided with AC potential from AC terminals 316. AC terminals 316 are connected to an AC power grid such as a utility92557216grid or a grid formed by other inverters. Power may flow from DC terminals 304 to AC terminals 316 or from AC terminals 316 to DC terminals 304. Bidirectional DC to AC inverter 312 is controlled such that the output at AC terminals 316 in the case of power flow from DC terminals 304 to AC terminals 316 is a sinusoidal voltage waveform as shown at F(x), where the amplitude is defined as u and the frequency is defined as co. The relationship between u and co is shown by the equation for F(x) in Fig. 3. The regulation of u, to control the amplitude of the sinusoidal voltage waveform to a reference value, is not shown in Fig. 3. An example control method related to example isolated bidirectional inverter 300 is described below with reference to Fig. 5.

[0032] Fig. 4 shows isolated bidirectional DC to DC converter 400, which is an example of isolated bidirectional DC to DC converter 308 in Fig. 3. Isolated bidirectional DC to DC converter 400 includes first DC terminals 404, first capacitance 408, first quadra of power switches 412, isolation transformer 416, second quadra of power switches 420, second capacitance 424, and second DC terminals 428.

[0033] First DC terminals 404 are coupled to first capacitance 408 and first quadra of power switches 412. First quadra of power switches 412 is coupled to isolation transformer 416. Isolation transformer 416 is coupled to second quadra of power switches 420. Second quadra of power switches 420 is coupled to second capacitance 424 and second DC terminals 428.

[0034] First DC power is supplied to / from first DC terminals 404 from / to a first DC source / load (not shown). First DC power supplied from / to DC terminals 404 is present at first capacitance 408 and first quadra of power switches 412. First quadra of power switches 412 switch to convert first DC power to / from first AC power. First AC power is present at isolation transformer 416. First capacitance 408 provides a path for first AC currents generated by switching of first quadra of power switches 412.

[0035] Isolation transformer 416 transforms first AC power to / from second AC power.

[0036] Second AC power is present at second quadra of power switches 420. Second quadra of power switches 420 switch to convert second AC power to / from second DC power. Second DC power is present at second capacitance 424 and second DC terminals 428. Second capacitance 424 provides a path for second AC currents generated by switching of second quadra of power switches 420. Second DC power is supplied from / to second DC terminals 428 to / from a92557216second DC load / source (not shown). Power can flow in either direction between first DC terminals 404 and second DC terminals 428. Electrical isolation between first DC terminals 404 and second DC terminals 428 is provided by isolation transformer 416. The turns ratio of isolation transformer 416 may, for example, be 1:1. Other turns ratios of isolation transformer 416 may provide ratios of first DC voltage at first DC terminals 404 to second DC voltage at second DC terminals 428 that is other than 1:1. First quadra of power switches 412 and second quadra of power switches 420 may be, for example, semiconductor switches such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).

[0037] Fig. 5 illustrates an example isolated bidirectional inverter with power control system, generally denoted 500, according to an embodiment. The illustrated example 500 includes DC terminals 504, inverter power circuit 508, current sense element 520, AC terminals 524, voltage sense element 528, power calculator 532, compensated error generator 536, difference block 540, and power reference generator 544. Inverter power circuit 508 is a bidirectional DC to AC inverter power circuit. Inverter power circuit 508 may, for example, be non-isolated DC to AC inverter 100 of Fig. 1 and isolated bidirectional DC to DC converter 400 of Fig. 4. AC terminals 524 are connected to an AC power grid (not shown) such as a utility grid or a grid formed by other inverters. Power may flow from DC terminals 504 through inverter power circuit 508 to AC terminals 524 or from AC terminals 524 through inverter power circuit 508 to DC terminals 504. Current sense element 520 senses current flowing through AC terminals 524 to or from the grid. For example, the sensed current may be in phase with the voltage and supplying power to the grid. DC terminals 504 are coupled to the DC terminals of inverter power circuit 508. The AC terminals of inverter power circuit 508 are coupled to AC terminals 524 and to sense input terminals of voltage sense element 528. Voltage sense element 528 is coupled to power calculator 532 and to power reference generator 544. Current sense element 520 senses the current flowing between inverter power circuit 508 and AC terminals 524. The output of current sense element 520 is coupled to power calculator 532. The output of power calculator 532 is coupled to the inverting input of difference block 540. The non-inverting input of difference block 540 is coupled to power reference generator 544. The output of difference block 540 is coupled to compensated error generator 536. Compensated error generator 536 is coupled to inverter power circuit 508.92557216

[0038] Current sense element 520 may be or include a current sensor or device such as an ammeter or other current sensor. Voltage sense element 528 may be or include a voltage sensor or device such as a voltmeter or other voltage sensor. Other types of current and / or voltage sensors or devices will be apparent to those familiar with power electronics and control.Embodiments are not restricted to any particular type of current sense element or voltage sense element. Compensated error generator 536 may include, for example, a gain block, a low pass filter block, or a combination of these blocks. Embodiments are not restricted to these two types of blocks. Various implementation options for difference block 540 and power reference generator 544 will be readily apparent to those familiar with power conversion, power electronics, and power electronics control.

[0039] Fig. 6 illustrates an example first system 600, including a bidirectional storage inverter and a constant power inverter. Example first system 600 includes electrical energy storage device 604, bidirectional storage inverter 608, photovoltaic (PV) panel 612, constant power inverter 616, and load 620. The AC terminals of constant power inverter 616 are coupled to the AC terminals of bidirectional storage inverter 608 and to load 620. PV panel 612 is coupled to the DC terminals of constant power inverter 616. Electrical energy storage device 604 is coupled to the DC terminals of bidirectional storage inverter 608. Power may flow from PV panel 612 to constant power inverter 616 and from constant power inverter 616 to load 620 and to bidirectional storage inverter 608; and from bidirectional storage inverter 608 to electrical energy storage device 604. When both PV panel 612 and electrical energy storage device 604 are supplying power, power may flow from PV panel 612 to constant power inverter 616 and to load 620 and power may flow from electrical energy storage device 604 to bidirectional storage inverter 608 and to load 620. PV panel 612 may not always supply power, such as when it is dark, in which case power may flow from electrical energy storage device 604 to bidirectional storage inverter 608 and to load 620.

[0040] Although batteries are commonly used to store electrical energy in operation with storage inverters, the present disclosure is not in any way limited to batteries or any other type of electrical energy storage elements or devices at 604. A battery is shown in Fig. 6 at 604 as an example of an electrical energy storage device, and may be of any type. Embodiments herein are not in any way restricted to any particular type of battery, or to an electrical energy storage92557216device that includes a battery. For example, the electrical energy storage device 604 may include several cells or batteries connected in series and / or several cells or batteries connected in parallel. Electrical energy storage devices may include one or more electrical energy storage elements, such as one or more batteries, other electrochemical storage elements, storage capacitors, inductors or inductive elements, and / or other types of electrical energy storage elements. More than one type of electrical energy storage element may be provided in some embodiments.

[0041] Similarly, PV panel 612 is shown in Fig. 6 as an example of a DC power source. A DC power source may include multiple PV panels, and / or one or more other types of devices or systems that produce or otherwise provide DC power.

[0042] Fig. 7 illustrates an example second system 700 with two bidirectional storage inverters and a constant power inverter. Example second system 700 includes first electrical energy storage device 704, first bidirectional storage inverter 708, second electrical energy storage device 712, second bidirectional storage inverter 716, PV panel 720, constant power inverter 724, and load 728. The AC terminals of constant power inverter 724 are coupled to the AC terminals of first bidirectional storage inverter 708 and to the AC terminals of second bidirectional storage inverter 716, and to load 728. PV panel 720 is coupled to the DC terminals of constant power inverter 724. First electrical energy storage device 704 is coupled to the DC terminals of first bidirectional storage inverter 708. Second electrical energy storage device 712 is coupled to the DC terminals of second bidirectional storage inverter 716. Power may flow from PV panel 720 to constant power inverter 724, and from constant power inverter 724 to load 728; and from constant power inverter 724 to first bidirectional storage inverter 708, and from first bidirectional storage inverter 708 to first electrical energy storage device 704; and from constant power inverter 724 to second bidirectional storage inverter 716, and from second bidirectional storage inverter 716 to second electrical energy storage device 712. Power may flow from PV panel 720 to constant power inverter 724 and to load 728; power may flow from first electrical energy storage device 704 to first bidirectional storage inverter 708 and to load 728; power may flow from second electrical energy storage device 712 to second bidirectional storage inverter 716 and to load 728.92557216

[0043] Implementation options provided above in the context of Fig. 6 also apply to similarly labelled components in Fig. 7.

[0044] In some embodiments, inverter control is distributed, and more inverters may be added as needed and still have distributed control. For example, systems 600, 700 are similar, apart from system 700 having one more bidirectional storage inverter than system 600.Distributed control, with a respective controller that is coupled to or potentially integrated with each bidirectional storage inverter, avoids the need to rely on a potentially unreliable communication system or a shared control cable that introduces a single point of failure, and may be advantageous for providing a more robust and less complex system. Such features may be especially useful to facilitate customer implementation of an inverter-based system.

[0045] Fig. 8 illustrates example power relationships, in a graph 800 of power versus time for the example first system 600 in Fig. 6. Graph 800 depicts constant power inverter power plot 804, bidirectional storage inverter power plot 808, zero-reference line 812, and load power plot 816. On load power plot 816, segments 820, 824, 828, 832, and 840 illustrate a first, second, third, fourth, and fifth load power level, respectively. On bidirectional storage inverter power plot 808, segments 876, 864, 860, 852, 848, 844, and 836 illustrate a first, second, third, fourth, fifth, sixth, and seventh bidirectional storage inverter power level, respectively. On constant power inverter power plot 804, segments 872, 868, and 856 illustrate a first, second, and third constant power inverter power level, respectively.

[0046] In graph 800, power flowing to load 620 in Fig. 6 is shown in the upper positive region of the plot. Power flowing from constant power inverter 616 in Fig. 6 is shown in the lower negative region of the plot. Power flowing from bidirectional storage inverter 608 in Fig. 6 is shown in the lower negative region of the plot in segments 876, 864, 860, 844 and power flowing to bidirectional storage inverter 608 in Fig. 6 is shown in the upper positive region of the plot in segments 852, 848, and 836. The upper positive region of the plot is the area above the zero-reference line 812. The lower negative region of the plot is the area below the zeroreference line 812. Power flowing from bidirectional storage inverter 608 and from constant power inverter 616 in Fig. 6 shown in segments 876 and 872 respectively, sum to be equal and opposite to power shown in segment 820 flowing to load 620 in Fig. 6. Power flowing from bidirectional storage inverter 608 and from constant power inverter 616 in Fig. 6 shown in92557216segments 864 and 868 respectively, sum to be equal and opposite to power shown in segment 820 flowing to load 620 in Fig. 6. Power flowing to bidirectional storage inverter 608 and from constant power inverter 616 in Fig. 6 shown in segments 852 and 856 respectively, sum to be equal and opposite to power shown in segment 824 flowing to load 620 in Fig. 6. This segment-by-segment sum of bidirectional inverter power plot 808 and of constant power inverter power plot 804 result in a power equal and opposite to the power shown in load power plot 816 for all segments in Fig. 8. Bidirectional storage inverter 608 in Fig. 6 is controlled to produce additional power required by load 620 in Fig. 6 that is not produced by constant power inverter 616 in Fig.6 and is controlled to absorb additional power produced by constant power inverter 616 in Fig. 6 that is not required by load 620 in Fig. 6. This action by bidirectional storage inverter 608 to produce or absorb power ensures that the power required by load 620 in Fig. 6 is provided by the combination of constant power inverter 616 and bidirectional storage inverter 608 in Fig. 6.

[0047] The example plots in Fig. 8, and others herein, are provided solely for illustrative purposes. Observations may be similar, or different, during testing and / or operation.

[0048] Fig. 9 shows graph 900, which is an example of power versus voltage for the example first system 600 in Fig. 6 and the example second system 700 in Fig. 7. Graph 900 depicts power generation curves 904, 908, 912, 916, and 920 and load power curves 924, 928, 932, 936, and 944, maximum operating voltage 940, and minimum operating voltage 948. The horizontal line showing 240 volts is the nominal-voltage line in this example. The vertical line labeled zero is a zero-power axis. Power generation curves 904, 908, 912, 916, and 920 show the sum of power generation of one or more bidirectional storage inverters and 0%, 25%, 50%, 75%, and 100% constant power generated by one or more constant power inverters, respectively. Load power curves 924, 928, 932, 936, and 944 show 20%, 40%, 60%, 80%, and 100% load power, respectively. In graph 900, power is expressed in percent of rated power, which is the sum of power of all bidirectional storage inverters and all constant power inverters in a system.

[0049] Power generation curve 904 shows an example of power generation of bidirectional storage inverter 608 and constant power inverter 616 in Fig. 6 with no power generated by the constant power inverter. Power generation curves 908, 912, 916, and 920 show examples of power generation of bidirectional storage inverter 608 and constant power inverter 616 in Fig. 6 with 25%, 50%, 75%, and 100% power generated by the constant power inverter.92557216Power generation curve 904 also shows an example of the sum of power generation from first bidirectional storage inverter 708, second bidirectional storage inverter 716, and constant power inverter 724 in Fig. 7 with no power generated by the constant power inverter. Power generation curves 908, 912, 916, and 920 also show examples of power generation of first bidirectional storage inverter 708, second bidirectional storage inverter 716, and constant power inverter 724 in Fig. 7 with 25%, 50%, 75%, and 100% power generated by the constant power inverter.

[0050] Load power curves 924, 928, 932, 936, and 944 each intersect with each of power generation curves 904, 908, 912, 916, and 920. The coordinates of each of these intersection points show the voltage across the respective load, and the power flowing in that load. All of these intersection points are in the region of graph 900 between maximum operating voltage 940 and minimum operating voltage 948. The mechanisms described herein provide a means to regulate the grid voltage within a specified range as available power generation varies from zero power to maximum power and as the load power varies from zero power to maximum power.

[0051] Fig. 10 illustrates an example of inverter sharing. Graph 1000 is a region of graph 900 in Fig. 9 around an intersection point of a load power curve and a power generation curve. Graph 1000 includes first power generation curve 1004, second power generation curve 1008, load power curve 1012, and power difference 1016. First power generation curve 1004 may, for example, show the power generation from first bidirectional storage inverter 708 in Fig. 7.Second power generation curve 1008 may, for example, show the power generation from second bidirectional storage inverter 716 in Fig. 7. Load power curve 1012 may be an example of one of load power curves 924, 928, 932, 936, and 944 in Fig. 9. The vertical separation of power generation curve 1004 and power generation curve 1008 illustrates an example error in the grid voltage sensing between first bidirectional storage inverter 708 and second bidirectional storage inverter 716 in Fig. 7, resulting in a difference in the power delivered. This error is identified by the Error band in graph 1000 (labelled “Error” in Fig. 10). Power difference 1016 shows the difference in power generation between power generation curves 1008 and 1004 that results from the example error in the grid voltage sensing.

[0052] The magnitude of power difference 1016 is dependent upon the magnitude of the error in the grid voltage sensing and the slope of power generation curves 1008 and 1004. These magnitudes constrain the difference in the power generated by first bidirectional storage inverter92557216708 and second bidirectional storage inverter 716 in Fig. 7. This constraint results in sharing of power between two parallel coupled bidirectional storage inverters as shown in the example second system in Fig. 7. The horizontal axis in Figs. 9 and 10 is expressed as a percentage of the rated power of the system or the sum of the rated powers of the generation elements. The rated power of the generation elements need not be equal; however, each element will generate the same percentage of its rated power based on the slope. This enables bidirectional power inverters with different power ratings to share power proportionate to their rated power. It will be appreciated by those skilled in the art that multiple inverters may be connected in parallel and can be controlled in this way to share power. Bidirectional storage inverter power plot 808 in Fig. 8 may represent the power from a single bidirectional storage inverter or may represent the sum of power from multiple bidirectional storage inverters that are sharing power.

[0053] Power generation curves 904, 908, 912, 916, and 920 in Fig. 9 may be representative of the sum of one or more bidirectional storage inverters sharing power and one or more constant power inverters.

[0054] Fig. 11 shows graph 1100 which is an example of power versus voltage for the example first system 600 in Fig. 6 and the example second system 700 in Fig. 7. Graph 1100 depicts power generation curve 1104 and load power curves 1108, 1112, 1116, 1120, and 1128, maximum operating voltage 1124, and minimum operating voltage 1132. The horizontal line showing 240 volts is the nominal-voltage line in this example. The vertical line labeled zero is a zero-power axis. Power generation curve 1104 shows the sum of power generation of one or more bidirectional storage inverters. Load power curves 1108, 1112, 1116, 1120, and 1128 show 20%, 40%, 60%, 80%, and 100% load power respectively. In graph 1100, power is expressed in percent of rated power, which is the sum of power of all bidirectional storage inverters in a system.

[0055] Power generation curve 1104 shows an example of power generation of bidirectional storage inverter 608 and constant power inverter 616 in Fig. 6 with no power generated by the constant power inverter. Power generation curve 1104 also shows an example of the sum of power generation from first bidirectional storage inverter 708, second bidirectional storage inverter 716, and constant power inverter 724 in Fig. 7 with no power generated by the92557216constant power inverter. Power generation curve 1104 may represent the sum of power generation from a plurality of bidirectional storage inverters.

[0056] Load power curves 1108, 1112, 1116, 1120, and 1128 each intersect with power generation curve 1104. The coordinates of each of these intersection points show the voltage across the respective load, and the power flowing in that load. Power generation curve 1104 shows a step in voltage at zero power. This step in voltage, which is not a feature of the power generation curves in Fig. 9, is an example mechanism to prevent power from flowing between bidirectional storage inverters when the grid voltage is near nominal and power is near zero. For example, if bidirectional storage inverters measure slightly different grid voltages, there is a region close to zero power where one (or more) bidirectional storage inverter(s) may absorb power and another (one or more) bidirectional storage inverter(s) may supply power. Under this operating condition, there is power flow from the one (or more) bidirectional storage inverter(s) to the other bidirectional storage inverter(s). The step in voltage at zero power prevents or at least reduces this flow of power between bidirectional storage inverters.

[0057] Fig. 12 illustrates a zero-power region according to an embodiment. Graph 1200 is a region of graph 1100 in Fig. 11 around the zero-power axis and nominal -voltage line intersection point. Graph 1200 includes first power generation curve 1204, second power generation curve 1220, extended power generation curves 1208, nominal -voltage line 1212, and zero-power axis 1216. First power generation curve 1204 shows the power generation from a first bidirectional storage inverter which may be, for example, bidirectional storage inverter 708 in Fig. 7. Second power generation curve 1220 shows the power generation from a second bidirectional storage inverter which may be, for example, bidirectional storage inverter 716 in Fig. 7. The vertical separation of power generation curve 1204 and power generation curve 1220 illustrates an example error in the grid voltage sensing and the resulting difference in the power delivered by first bidirectional storage inverter 708, for example, and second bidirectional storage inverter 716 in Fig. 7, for example.

[0058] This example error in the grid voltage sensing is identified by the Error band in graph 1200 (labelled “Error” in Fig. 12). If these curves were to continue as per the extended power generation curves 1208, there would be a range around zero-power axis 1216 in which first bidirectional storage inverter 708 in Fig. 7, for example, would set its output power to a92557216small negative value and second bidirectional storage inverter 716 in Fig. 7, for example, would set its output power to a small positive value. In Fig. 12, the horizontal distance between power generation curve 1204 and power generation curve 1220 illustrates the power difference that results from the voltage sensing error, and it is this power difference that can cause one bidirectional storage inverter (such as 708 in Fig. 7, for example) to set its output power to a negative value and another bidirectional storage inverter (such as 716 in Fig. 7, for example) to set its output power to a positive value in the range around zero-power axis 1216. With reference to the extended power generation curves 1208, the extension of power generation curve 1204 at the bottom crosses zero-power axis 1216 at 1230, when power generation curve 1220 at the bottom is still in the positive-power region at 1232. Without the step and zero-power region in Fig. 12, this condition of different output power polarities (one bidirectional storage inverter with negative output power and absorbing power from another bidirectional storage inverter with positive output power and supplying the power that is being absorbed) would persist, because of the power difference, until the extension of power generation curve 1220 at the bottom also crosses zero-power axis 1216 at 1234. At this point, without the step and zero-power region in Fig. 12, the extension of power generation curve 1204 at the bottom would still be in the negative power region as shown at 1236, but the power that is being absorbed is not being provided by another bidirectional storage inverter. A line is shown between 1234 and 1236 in Fig. 12 to again illustrate the power difference that results from the voltage sensing error.

[0059] In the case of the power generation curves 1204 and 1220 extending as shown at 1208, power would flow from second bidirectional storage inverter 716 to first bidirectional storage inverter 708, for example. This power would provide no useful function and a fraction of it would be lost by the inverters. Introduction of a zero-power region in the power generation curves of the bidirectional storage inverters, labeled as “Zero Power” in graph 1200, prevents or at least reduces power flow from one bidirectional storage inverter to another bidirectional storage inverter. Provided the voltage error between the bidirectional storage inverters is less than the voltage range of the zero-power region, flow of power from one bidirectional storage inverter to another bidirectional storage inverter is at least reduced and may be prevented. The step in the power generation curves at zero power, which creates the zero-power region, may be92557216a static feature of power generation curve 1104 of Fig. 11, or may be a step that is formed by changing the zero-power intercept of power generation curve 904 of Fig. 9.

[0060] The introduction of this step may occur when power generation is close to zero for a period of time. This period of time refers to a time duration, and may be a few seconds, for example, such as 3 seconds. The step may be introduced, for example, by incrementing the zeropower intercept. The step may also be removed, by decrementing the zero-power intercept when power is no longer close to zero for example. . Removal of the step, like introduction of the step, may be time-dependent, and may occur when power generation is no longer close to zero for a period of time. The period of time for introduction and removal may be the same, or different time periods may be used.

[0061] Power close to zero, may, for example, be within a few percent of the rated power of a bidirectional storage inverter, such as 3% in an embodiment, but not so close to zero power that tolerances of voltage and power measurements could reduce this range of “power close to zero”, or the voltage range of a zero-power region, to zero or a negative value.

[0062] Figs. 13 A and 13B illustrate a zero-power region according to another embodiment. Graph 1300 is a region of graph 900 in Fig. 9 around the zero-power axis and nominal -voltage line intersection point. This region is shown twice, illustrating in Fig. 13 A the power voltage relationship of the two inverters, and a mechanism in Fig. 13B of correcting for differences in sensed voltage between the two inverters. Graph 1300 includes first power generation curve 1304, second power generation curve 1308, nominal-voltage line 1312, zeropower axis 1316, and regions of change of slope 1320 in Fig. 13B. First power generation curve 1304 shows the power generation from, for example, first bidirectional storage inverter 708 in Fig. 7. Second power generation curve 1308 shows the power generation from, for example, second bidirectional storage inverter 716 in Fig. 7. The vertical separation between power generation curve 1304 and power generation curve 1308 illustrates an example error in the grid voltage sensing, and the horizontal separation between the power generation curves illustrates the resulting difference in the power delivered by first bidirectional storage inverter 708 and second bidirectional storage inverter 716 in Fig. 7. This example error in the grid voltage sensing is identified as the Voltage Error between two storage inverters shown in graph 1300.92557216

[0063] Fig. 13A shows an area within the voltage error where first power generation curve 1304 shows the power generation from first bidirectional storage inverter 708 in Fig. 7 absorbing power and second power generation curve 1308 shows the power generation from second bidirectional storage inverter 716 in Fig. 7 supplying power. The horizontal lines bordering the cross-hatched areas in Fig. 13A are similar to those between 1230 / 1232 and 1234 / 1236 in Fig. 12, if power generation curves 1204 and 1220 in Fig. 12 were to extend as shown at 1208.

[0064] Fig. 13B illustrates regions of change of slope 1320, which serve to eliminate or at least reduce the voltage overlap of the regions of different bidirectional storage inverters absorbing power and supplying power.

[0065] The change of slope in regions 1320 may be formed, for example, by changing the slope of power generation curve 904 of Fig. 9. These regions of change of slope 1320 may be formed when power generation is close to zero and may be static or may be formed when power generation is close to zero for a period of time. This time may be a few seconds, for example. The slope change may be introduced by incrementing the slope of power generation curve 904 and may be removed by decrementing the slope of power generation curve 904 when power is no longer close to zero, for example. Illustrative examples of “power close to zero” are provided above.

[0066] Introduction of features including the step introduced in Fig. 11 and discussed in Fig. 12 and the regions of change of slope in Fig. 13B that appear following a period of time provide a continuous power voltage relationship with no points of inflection for rapid changes in load or generation, during which power generation by different bidirectional storage inverters would not remain close to zero for that period of time. The elimination of points of inflection reduces the response time to transient changes and deviation of voltage regulation of the system. Introduction of these features after a period of time prevents or at least reduces power from flowing from one bidirectional storage inverter to another for extended periods of time. It should be apparent to those skilled in the art that mechanisms such as a step or change in slope to prevent or at least reduce unwanted power flow due to voltage errors can be implemented in multiple ways, such as those illustrated by way of example herein.92557216

[0067] Fig. 14 illustrates a zero-power region according to another embodiment. Graph 1400 is a region of graph 900 in Fig. 9 around the zero-power axis and nominal-voltage line intersection point. Graph 1400 includes a first quantized power generation curve 1404, a second quantized power generation curve 1408, nominal-voltage line 1412, and zero-power axis 1416. First quantized power generation curve 1404 shows the power generation from a first bidirectional storage inverter which may be, for example, bidirectional storage inverter 708 in Fig. 7. Second quantized power generation curve 1408 shows the power generation from a second bidirectional storage inverter which may be, for example, bidirectional storage inverter 716 in Fig. 7. The vertical separation of first quantized power generation curve 1404 and second quantized power generation curve 1408 illustrates an example error in the grid voltage sensing, and the horizontal separation illustrates the resulting difference in the power delivered by first bidirectional storage inverter 708 and second bidirectional storage inverter 716 in Fig. 7. This example error in the grid voltage sensing is identified by the Voltage Error between two storage inverters shown in graph 1400. Flow of power from one bidirectional storage inverter to another bidirectional storage inverter is prevented or at least reduced, provided the voltage error between the bidirectional storage inverters is less than the voltage step of the voltage quantization. The voltage region that is the Difference in Error and Quantization shown in graph 1400 creates a region where both inverters are near zero power, and in some embodiments the inverters may both be at zero power in this region.

[0068] Even if both inverters in this example are not quite at zero power at the same time, at least one is near or at zero power when the inverters are operating with different output power polarities (one with its output power set to a negative value and the other with its output power set to a positive value). When power generation curve 1404 crosses zero-power axis 1416, first bidirectional storage inverter 708 (Fig. 7) is absorbing power, but at a low power level, while second bidirectional storage inverter 716 is generating power. Second bidirectional storage inverter 716 may transition to a low power generation level at the lower limit of the “Difference in Error and Quantization” zone shown in Fig. 14, and both inverters are in a low power state. When power generation curve 1404 transitions to a higher power absorbing state as voltage continues to rise, power generation curve 1408 is still at or near zero power, so second92557216bidirectional storage inverter 716 is not generating a significant amount of the power that is being absorbed by first bidirectional storage inverter 708.

[0069] With a larger voltage error, both inverters may be in higher-power states with opposite polarities, and thus there would be higher power flow between the inverters. In that scenario, a zero-power region as disclosed elsewhere herein may be applied to reduce or prevent such power flow. In other words, the features disclosed herein are not limited to analog embodiments, and may also or instead be implemented in conjunction with digital sensing and / or digital control.

[0070] Referring to Fig. 5 and the example 500, the voltage at AC terminals 524 is measured by voltage sense element 528 and presented to power calculator 532 and power reference generator 544. The current flowing between inverter power circuit 508 and AC terminals 524 is sensed by current sense element 520 and presented to power calculator 532. The outputs of voltage sense element 528 and current sense element 520 may, for example, be digital signals or may be analog signals. Power calculator 532 calculates the power flowing to / from AC terminals 524 and from / to inverter power circuit 508 and presents this power to the inverting input of difference block 540. Difference block 540 receives a reference at its non-inverting input from power reference generator 544. The difference between the reference and the calculated power is presented to compensated error generator 536. Compensated error generator 536 adjusts the gain relative to frequency to produce a compensated error signal. Compensated error generator 536 may, by way of example, be formed by a Proportional-Integral-Differential (PID) controller. The compensated error signal is presented to inverter power circuit 508 to control the voltage at AC terminals 524.

[0071] A servo mechanism is formed by the feedback path that includes current sense element 520, voltage sense element 528, power calculator 532, difference block 540, compensated error generator 536, and inverter power circuit 508 to control the power to / from inverter power circuit 508 from / to AC terminals 524. Voltage sense element 528 presents the voltage information to power reference generator 544. Power reference generator 544 calculates a target output power reference from the voltage information. The relationship between the voltage and the target output power reference produces a voltage-power relationship that enables power sharing between isolated bidirectional DC to AC inverters which have their AC terminals92557216coupled. An example of the voltage-power relationship is shown in power generation curves 904, 908, 912, 916, and 920 of graph 900 of Fig. 9. The control of power in isolated bidirectional inverter with power control system 500 in Fig. 5 is a monotonic relationship of grid voltage and power that encompasses the range from maximum negative power to maximum positive power of isolated bidirectional inverter with power control system 500.

[0072] Changes in power are responsive to the voltage at AC terminals 524, which is in turn responsive to AC load power as illustrated in Fig. 9. Changes in load power or generation which may result in a change in direction of power flow are illustrated in Fig. 8. These changes in power flow represent different points on the power voltage relationship defined in Fig. 5. Responses to step changes are a function of the bandwidth determined by compensated error generator 536 of the servo control loop defined in Fig. 5. The gain and bandwidth of this servo loop provide a mechanism to maintain regulation within a specified range while changing power levels or reversing power flow direction as is illustrated in Fig. 8. In Fig. 5, a first (inner) loop has a gain that is set by H(s) and the transfer function of DC to AC inverter 508. The signal path from voltage sensing element 528 to power reference generator 544 forms a second (outer) loop in which the power reference is responsive to the sensed AC voltage. This second loop gain is the k term in the example shown in Fig. 5 for power reference generator 544, and determines the slope of the droop relationship (power curve). The Po term in the example shown in Fig. 5 for power reference generator 544 determines the zero-power intercept of the power curve.

[0073] In some prior implementations, a change in power direction requires switching from one power path to another and establishing control of regulation following the switching event. This switching action causes loss of output power regulation for a period of time.According to embodiments disclosed herein, however, such path switching and loss of power regulation can be avoided by incorporating a single bidirectional path for power and providing a control mechanism with a single monotonic transfer function.

[0074] Isolated DC to AC inverter 200 in Fig. 2 may be an example of inverter power circuit 508. However, isolated DC to AC inverter 200 does not provide a power voltage control function as is shown in Fig. 5. Isolated DC to AC inverter 200 in Fig. 2 requires isolation transformer 224. Isolation transformer 224 must be suitable to transform power at the grid frequency, resulting in a very large, heavy transformer. Isolation transformer 416 in Fig. 4 may,92557216for example, be operated at frequencies of 200 kHz. Isolation transformer 416 in Fig. 4 will be a very small fraction of the size and weight of isolation transformer 224 in Fig. 2 when both are designed for the same power level.

[0075] In the event of failure of one inverter in the case where the inverter suddenly stops supplying or absorbing power and where more than one bidirectional storage inverter are coupled and sharing power, the fast transient response will help ensure that voltage remains within the range between minimum and maximum, provided that the total power available from generation is greater than or equal to the load power and power absorbed by bidirectional storage inverters.

[0076] Various features related to bidirectional storage inverters and control are described in detail above and shown in the drawings. The disclosed features are intended as illustrative examples, any of which may be provided, individually or in any of a number of combinations, in embodiments. Fig. 5, for example, is illustrative of an embodiment that includes several elements or components, whereas other embodiments may include additional, fewer, and / or different elements or components, interconnected in the same way as shown in Fig. 5, in a similar way, or in a different way. Similarly, Fig. 7 illustrates an example that includes two bidirectional storage inverters 716 and 708, and more than two bidirectional storage inverters may be provided in other embodiments. Further variations are also possible.

[0077] According to an embodiment, a bidirectional storage inverter includes a bidirectional power inverter circuit and a controller. The bidirectional power inverter circuit provides a single path for bidirectional power flow, instead of a separate path for each direction. The circuit includes AC terminals to be coupled to an AC power system, DC terminals that are to be coupled to an electrical energy storage device and are electrically isolated from the AC terminals, and a bidirectional circuit path between the AC terminals and the DC terminals. Such a circuit is shown by way of example in Fig. 5, with AC terminals 524, DC terminals 504, and inverter power circuit 508. Each bidirectional storage inverter 708 and 716 in Fig. 7 similarly includes AC terminals at the right, DC terminals at the left, and a bidirectional circuit path between the AC terminals and the DC terminals.92557216

[0078] As disclosed herein, inverter control may be distributed. A controller may be coupled to a bidirectional power inverter circuit, and potentially integrated with a bidirectional storage inverter, to determine power flow at the AC terminals based on current and voltage at the AC terminals, and to control the bidirectional power inverter circuit. In the example shown in Fig. 5, a bidirectional storage inverter further includes current sense element 520 and voltage sense element 528 to measure the current and voltage, respectively, at the AC terminals, and power calculator 532 is coupled to current sense element 520 and voltage sense element 528, to receive the measured current and voltage and to determine the power flow at the AC terminals based on the measured current and voltage. These elements, and / or others that are involved in storage inverter control, may be described as being part of a bidirectional storage inverter or part of a bidirectional storage inverter controller. The present disclosure is not limited to any particular implementation or division of functions.

[0079] The controller may control the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from a further bidirectional storage inverter that is coupled to the AC power system, and from the bidirectional storage inverter through the AC terminals to the further bidirectional storage inverter. Although power flows are described in these terms below and elsewhere herein, such power flows may also be described or characterized in other ways. For example, power flows may also be described as being to or from the bidirectional power inverter circuit, to or from the electrical energy storage device, or to or from the DC terminals.

[0080] Control based on limiting power flow may help avoid or prevent, or at least reduce, power flow between storage inverters. For example, a controller may be configured to control a bidirectional power inverter circuit based on limiting the power flow to (or from) a bidirectional storage inverter from (or to) a further bidirectional storage inverter by time. Power flow between bidirectional storage inverters may be permitted for up to a maximum period of time, such as 3 seconds in an example provided above. Time-based limiting prevents or at least reduces power from flowing from one bidirectional storage inverter to another for extended periods of time, but can help avoid introducing a step or slope change, for example, for more transient power flows.92557216

[0081] A controller may be configured to control a bidirectional power inverter circuit based on limiting an amount of power flow to (or from) a bidirectional storage inverter from (or to) another bidirectional storage inverter. As disclosed herein, a zero-power region may be provided so that no power, or relatively little power, flows between bidirectional storage inverters.

[0082] For example, a controller may be configured to control a bidirectional power inverter circuit based on limiting the amount of the power flow where a target power for the bidirectional storage inverter would fall within a threshold of zero power. Examples that are provided above for power being close to zero are examples of power falling within a threshold of zero power. Actual power output (generating or absorbing power) may or may not necessarily fall within the threshold. For example, with reference to Figs. 9 and 11, under normal operating conditions, target power for bidirectional storage inverter may fall within a threshold of zero power based on a continuous control function illustrated by power generation curve 904.However, actual power output may not fall within that threshold before a step is introduced into the power generation curve 1104 as shown in Fig. 11. Thus, control may be based on limiting the amount of power flow where a target power “would” fall within a threshold of zero power.

[0083] Time-based and amount-based limiting of power flow may potentially be applied separately or in combination. For example, a controller may be configured to control a bidirectional power inverter circuit based on limiting the power flow to (or from) the bidirectional storage inverter from (or to) a further bidirectional storage inverter by time, for up to a maximum period of time, and limiting the amount of the power flow where a target power for the bidirectional storage inverter falls within a threshold of zero power for more than the maximum period of time.

[0084] In some embodiments, a controller may be configured to determine target power for a bidirectional storage inverter based on a control function that implements a monotonic relationship between the voltage at the AC terminals and the target power, and to control a bidirectional power inverter circuit based on limiting the amount of the power flow by determining the target power as zero power for a range of voltages for which the target power would fall within the threshold of zero power according to the monotonic relationship. Examples of such a range of voltages are shown by the vertical height of the step in power generation curve925572161104 in Fig. 11, the vertical height of the step in each of power generation curves 1204 and 1220 in Fig. 12, and the vertical height of the step in each of power generation curves 1304 and 1308 in Figs. 13A and 13B.

[0085] A monotonic relationship between voltage and target power may be consistent with a power curve, which is also referred to herein as a power generation curve. A controller may be configured to adjust a zero-power intercept of a power curve to implement the range of voltages within which the target power is determined as zero power. A controller may also or instead be configured to adjust a slope of a power curve to implement the range of voltages within which the target power is determined as zero power. Adjustment of an intercept is described by way of example at least above as incrementing and / or decrementing the zero-power intercept, and adjusting an intercept is one way to introduce and / or remove a step in a power curve. Slope adjustments are illustrated by way of example in Fig. 13B.

[0086] With reference again to Fig. 5, the k term in the example shown for power reference generator 544 determines the slope of a droop relationship (power curve), and the Po term in the example shown determines the zero-power intercept of the power curve. The example in Fig. 5 illustrates that adjusting the zero-power intercept and / or slope may involve adjusting one or more parameters of a relationship between the voltage at the AC terminals and a power reference that sets or controls the target power for the bidirectional storage inverter.

[0087] The range of voltages within which the target power is determined as zero power may be based on a voltage error between voltage sensing at the AC terminals of a bidirectional storage inverter and voltage sensing for another bidirectional storage inverter, and a resultant power difference between the bidirectional storage inverter and the other bidirectional storage inverter that results from the voltage error. Voltage errors and power differences are shown by way of example in Figs. 10, 12, 13A, 13B, and 14.

[0088] In some embodiments, a controller is configured to control a bidirectional power inverter circuit further based on a continuous control function. A control function is also referenced above, and that control function may be a continuous control function in some embodiments. As used herein, a continuous control function refers to a function that extends from a maximum power flow from the AC power system to the AC terminals and through the92557216bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, to zero power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, and to a maximum power flow to the AC power system from the AC terminals and through the bidirectional power inverter circuit and the DC terminals from the electrical energy storage device. Such a continuous control function may help maintain regulation of voltage at the AC terminals within a voltage range, and / or may help provide continuous power flow to or from the AC power system.

[0089] As shown by way of example in Fig. 5, a controller (or an element thereof such as power reference generator 544 in the example shown) may be configured to determine a target power reference based on the voltage at the AC terminals (as measured by voltage sense element 528 in the example shown), and to control the bidirectional power inverter circuit 508 based on a difference (determined by difference block 540 in the example shown) between the determined power flow (determined by power calculator 532 in the example shown) and the target power reference. As also described at least above, a relationship between the voltage at the AC terminals and the target power reference, an example of which is illustrated at 544 in Fig. 5, may provide a voltage-power relationship that enables power sharing between a bidirectional storage inverter and another bidirectional storage inverter. With respective power generation curves for different bidirectional storage inverters based on a percentage or proportion of respective rated power, bidirectional storage inverters can share power (generation and / or absorption) based on their respective rated power.

[0090] Fig. 7 and other examples herein may refer to two bidirectional storage inverters, such as 708 and 716 in Fig. 7. However, the features herein are not in any way limited to systems that include only two bidirectional storage inverters. More generally, systems in which power flow between bidirectional storage inverters may include two, or more than two, bidirectional storage inverters. Thus, a controller of a bidirectional storage inverter may be configured to control a bidirectional power inverter circuit based on determined power flow and further based on limiting power flow to (or from) the bidirectional storage inverter through the AC terminals from (or to) any of one or multiple other bidirectional storage inverters that are coupled to the same AC power system.92557216

[0091] In some system embodiments, a system may include a bidirectional storage inverter with a bidirectional power inverter circuit and a controller, and a further bidirectional storage inverter with a further bidirectional power inverter circuit and a further controller. The further bidirectional power inverter circuit may include elements that are described by way of example herein, such as: further AC terminals coupled to the AC terminals of the bidirectional storage inverter and to the AC power system; further DC terminals, electrically isolated from the further AC terminals, to be coupled to a further electrical energy storage device; and a further bidirectional circuit path between the further AC terminals and the further DC terminals. The further controller may be coupled to the further bidirectional power inverter circuit, to determine power flow at the further AC terminals based on current and voltage at the further AC terminals, and to control the further bidirectional power inverter circuit based on the determined power flow and limiting power flow to (or from) the bidirectional storage inverter through the AC terminals from (or to) the further bidirectional storage inverter.

[0092] Expanding on this system example, the further bidirectional storage inverter may be one of multiple further bidirectional storage inverters, each including a respective bidirectional power inverter circuit of the same or similar structure and a respective controller to control each further bidirectional power inverter circuit based on determined power flow and limiting power flow to or from other bidirectional storage inverters of the system. In such a system, in the context of previous examples there is a bidirectional storage inverter and multiple further bidirectional storage inverters. The controller of the bidirectional storage inverter may be configured to control the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to (or from) the bidirectional storage inverter through the AC terminals from (or to) any of the multiple further bidirectional storage inverters.

[0093] Other combinations of components or elements are also possible, and the present disclosure is not limited to any particular combination of disclosed components or elements, or to the specific example implementations of such components or elements that are shown in the drawings or otherwise disclosed by way of example herein.

[0094] Embodiments are described above primarily in the context of power circuits or components thereof. Method embodiments are also possible.92557216

[0095] Fig. 15 illustrates an example method according to an embodiment. Method embodiments, like other embodiments herein, may include disclosed features individually or in any of a number of combinations. The example method 1500 in Fig. 15 is illustrative of an embodiment that is consistent with features that may be provided in, or in conjunction with, a bidirectional storage inverter or a system that includes multiple bidirectional storage inverters. Other method embodiments may include additional, fewer, and / or different features.

[0096] The example method 1500 involves determining, as shown at 1502, power flow at AC terminals of a bidirectional power inverter circuit of a bidirectional storage inverter. The determining at 1502 may be based on current and voltage at the AC terminals, and for completeness measuring the voltage and the current are also shown, at 1510 and 1512, respectively. As in other embodiments herein, the bidirectional power inverter circuit includes AC terminals coupled to an AC power system, DC terminals that are electrically isolated from the AC terminals and coupled to an electrical energy storage device, and a bidirectional circuit path between the AC terminals and the DC terminals.

[0097] The method 1500 also involves controlling the bidirectional power inverter circuit, as shown at 1504. The controlling at 1504 is based on the determined power flow and further based on limiting power flow to (or from) the bidirectional storage inverter through the AC terminals from (or to) a further bidirectional storage inverter coupled to the AC power system.

[0098] The controlling at 1504 may be based on limiting the power flow to (or from) the bidirectional storage inverter from (or to) the further bidirectional storage inverter by time, for up to a maximum period of time, and / or based on limiting an amount of the power flow. In one amount-based limiting embodiment, the controlling at 1504 may involve controlling the bidirectional power inverter circuit based on limiting the amount of the power flow where a target power for the bidirectional storage inverter would fall within a threshold of zero power. A combined embodiment may involve controlling at 1504 further based on limiting the power flow to the bidirectional storage inverter by time, for up to a maximum period of time, and further based on limiting the amount of the power flow where a target power for the bidirectional storage inverter falls within a threshold of zero power for more than the maximum period of time, for example. Time and / or amount monitoring, to determine when the maximum period of92557216time is exceeded and / or an amount of power flow (including whether target power is within a threshold of zero power) is shown in Fig. 15 at 1520. In response to detecting the condition(s), a control function may be adjusted as shown at 1522.

[0099] Some embodiments may involve determining, at 1520 for example, the target power for the bidirectional storage inverter based on a control function that implements a monotonic relationship between the voltage at the AC terminals and the target power. The controlling at 1504 may then be based on limiting the amount of the power flow by determining the target power as zero power for a range of voltages for which the target power would fall within the threshold of zero power according to the monotonic relationship. Such a monotonic relationship may be consistent with a power curve, for example, and a method may involve adjusting a zero-power intercept of the power curve, and / or adjusting a slope of the power curve, to implement the range of voltages. Adjusting an intercept and / or a slope are illustrative of adjusting a control function at 1522.

[0100] As in other embodiments, the range of voltages within which target power is determined as zero power may be based on a voltage error (between voltage sensing at the AC terminals and voltage sensing for the further bidirectional storage inverter) and a power difference between the bidirectional storage inverter and the further bidirectional storage inverter that results from the voltage error.

[0101] The controlling at 1504 may be further based on a continuous control function, as described in further detail elsewhere herein. A continuous control function may help maintain regulation of the voltage at the AC terminals within a voltage range, and / or may help provide continuous power flow to or from the AC power system.

[0102] A method may involve determining a target power reference based on the voltage at the AC terminals, in which case the controlling at 1504 based on the determined power flow may involve controlling the bidirectional power inverter circuit further based on a difference between the determined power flow and the target power reference.

[0103] A relationship between the voltage at the AC terminals and the target power reference may provide a voltage-power relationship that enables power sharing between the bidirectional storage inverter and the further bidirectional storage inverter. Such power sharing92557216may thus be implemented or provided by controlling the bidirectional power circuit at 1504 based on the target power reference that is determined in accordance with the voltage and target power reference relationship.

[0104] A method may be applied in conjunction with two or more bidirectional storage inverters. Method examples herein may refer to a bidirectional storage inverter and a further bidirectional storage inverter, but there may be more than two bidirectional storage inverters in a system. For example, the further bidirectional storage inverter referenced herein may be one of multiple further bidirectional storage inverters. The controlling at 1504 may then involve controlling the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to or from any of the multiple further bidirectional storage inverters.

[0105] Not all the features referenced in the method examples herein are explicitly shown in Fig. 15, in an effort to avoid further congestion in the drawing. Many of these features may be within the context of the controlling at 1504, the monitoring at 1520, and / or the adjusting at 1522, for example.

[0106] Fig. 15 illustrates an example method. The illustrated operations may be provided or supported in any of various ways, and other embodiments may include fewer, additional, and / or different operations or features, performed in a similar or different order. At least some variations to the example method shown in Fig. 15 may be or become apparent, for example, from features that are disclosed above, with reference to any of Figs. 1-14 and apparatus embodiments.

[0107] What has been described is merely illustrative of the application of principles of embodiments of the present disclosure. Other arrangements and methods can be implemented by those skilled in the art.

[0108] For example, embodiments need not include all elements or components that are shown in the drawings or described herein. Embodiments may include additional, fewer, and / or different components or elements.92557216

[0109] It should also be appreciated that features disclosed herein in the context of a particular embodiment, such as an apparatus embodiment, are not limited only to that embodiment. Features may also or instead be implemented in other embodiments, such as a method embodiment. Similarly, method features may also or instead be implemented, supported, or otherwise provided in apparatus embodiments.

[0110] In addition, although described primarily in the context of methods and apparatus such as power circuits, other implementations are also contemplated, as instructions stored on a non-transitory computer-readable medium, for example.

Claims

92557216CLAIMS:

1. A bidirectional storage inverter comprising:a bidirectional power inverter circuit that comprises: alternating current (AC) terminals to be coupled to an AC power system; direct current (DC) terminals, electrically isolated from the AC terminals, to be coupled to an electrical energy storage device; and a bidirectional circuit path between the AC terminals and the DC terminals;a controller, coupled to the bidirectional power inverter circuit, to determine power flow at the AC terminals based on current and voltage at the AC terminals, and to control the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from a further bidirectional storage inverter coupled to the AC power system and from the bidirectional storage inverter through the AC terminals to the further bidirectional storage inverter.

2. The bidirectional storage inverter of claim 1, wherein the controller is configured to control the bidirectional power inverter circuit based on limiting the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter by time, for up to a maximum period of time.

3. The bidirectional storage inverter of claim 1, wherein the controller is configured to control the bidirectional power inverter circuit based on limiting an amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter.

4. The bidirectional storage inverter of claim 3, wherein the controller is configured to control the bidirectional power inverter circuit based on limiting the amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter where a target power for the bidirectional storage inverter would fall within a threshold of zero power.925572165. The bidirectional storage inverter of claim 3, wherein the controller is configured to control the bidirectional power inverter circuit further based on limiting the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter by time, for up to a maximum period of time, and limiting the amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter where a target power for the bidirectional storage inverter falls within a threshold of zero power for more than the maximum period of time.

6. The bidirectional storage inverter of claim 4 or claim 5,wherein the controller is configured to determine the target power for the bidirectional storage inverter based on a control function that implements a monotonic relationship between the voltage at the AC terminals and the target power,wherein the controller is configured to control the bidirectional power inverter circuit based on limiting the amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter by determining the target power as zero power for a range of voltages for which the target power would fall within the threshold of zero power according to the monotonic relationship.

7. The bidirectional storage inverter of claim 6,wherein the monotonic relationship is consistent with a power curve,wherein the controller is configured to adjust a zero-power intercept of the power curve to implement the range of voltages.

8. The bidirectional storage inverter of claim 6,wherein the monotonic relationship is consistent with a power curve,wherein the controller is configured to adjust a slope of the power curve to implement the range of voltages.925572169. The bidirectional storage inverter of claim 7, wherein the controller is further configured to adjust a slope of the power curve to implement the range of voltages.

10. The bidirectional storage inverter of any one of claims 6 to 9, wherein the range of voltages is based on a voltage error between voltage sensing at the AC terminals and voltage sensing for the further bidirectional storage inverter and a power difference between the bidirectional storage inverter and the further bidirectional storage inverter that results from the voltage error.

11. The bidirectional storage inverter of any one of claims 1 to 5, wherein the controller is configured to control the bidirectional power inverter circuit further based on a continuous control function that extends from a maximum power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, to zero power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, and to a maximum power flow to the AC power system from the AC terminals and through the bidirectional power inverter circuit and the DC terminals from the electrical energy storage device.

12. The bidirectional storage inverter of any one of claims 6 to 10, wherein the control function comprises a continuous control function that extends from a maximum power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, to zero power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, and to a maximum power flow to the AC power system from the AC terminals and through the bidirectional power inverter circuit and the DC terminals from the electrical energy storage device.

13. The bidirectional storage inverter of claim 11 or claim 12, wherein the continuous control function maintains regulation of the voltage at the AC terminals within a voltage range.

14. The bidirectional storage inverter of any one of claims 11 to 13, wherein the continuous control function provides continuous power flow to or from the AC power system.9255721615. The bidirectional storage inverter of any one of claims 1 to 14, wherein the controller is configured to determine a target power reference based on the voltage at the AC terminals, and to control the bidirectional power inverter circuit based on a difference between the determined power flow and the target power reference.

16. The bidirectional storage inverter of claim 15, wherein a relationship between the voltage at the AC terminals and the target power reference provides a voltage-power relationship that enables power sharing between the bidirectional storage inverter and the further bidirectional storage inverter.

17. The bidirectional storage inverter of any one of claims 1 to 16, wherein the controller is configured to control the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from any of a plurality of further bidirectional storage inverters coupled to the AC power system and from the bidirectional storage inverter through the AC terminals to any of the plurality of further bidirectional storage inverters.

18. A system comprising:the bidirectional storage inverter of any one of claims 1 to 16; andthe further bidirectional storage inverter,the further bidirectional storage inverter comprising:a further bidirectional power inverter circuit that comprises: further AC terminals coupled to the AC terminals of the bidirectional storage inverter and to the AC power system; further DC terminals, electrically isolated from the further AC terminals, to be coupled to a further electrical energy storage device; and a further bidirectional circuit path between the further AC terminals and the further DC terminals;a further controller, coupled to the further bidirectional power inverter circuit, to determine power flow at the further AC terminals based on current and voltage at the further AC terminals, and to control the further bidirectional power inverter circuit based on the determined power flow and limiting power flow to the bidirectional storage inverter through the AC92557216terminals from the further bidirectional storage inverter and from the bidirectional storage inverter through the AC terminals to the further bidirectional storage inverter.

19. The system of claim 18,wherein the further bidirectional storage inverter is one of a plurality of further bidirectional storage inverters, each comprising a respective bidirectional power inverter circuit and a respective controller to control each further bidirectional power inverter circuit based on determined power flow and limiting power flow to or from other bidirectional storage inverters of the system,wherein the controller is configured to control the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from any of the plurality of further bidirectional storage inverters and from the bidirectional storage inverter through the AC terminals to any of the plurality of further bidirectional storage inverters.

20. A method comprising:determining power flow at alternating current (AC) terminals of a bidirectional power inverter circuit of a bidirectional storage inverter based on current and voltage at the AC terminals, the bidirectional power inverter circuit comprising: the AC terminals coupled to an AC power system; direct current (DC) terminals, electrically isolated from the AC terminals, coupled to an electrical energy storage device; and a bidirectional circuit path between the AC terminals and the DC terminals;controlling the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to the bidirectional storage inverter through the AC terminals from a further bidirectional storage inverter coupled to the AC power system and from the bidirectional storage inverter through the AC terminals to the further bidirectional storage inverter.

21. The method of claim 20, wherein the controlling is based on limiting the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the92557216bidirectional storage inverter to the further bidirectional storage inverter by time, for up to a maximum period of time.

22. The method of claim 20, wherein the controlling is based on limiting an amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter.

23. The method of claim 22, wherein the controlling comprises controlling the bidirectional power inverter circuit based on limiting the amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter where a target power for the bidirectional storage inverter would fall within a threshold of zero power.

24. The method of claim 22, wherein the controlling comprises controlling the bidirectional power inverter circuit further based on limiting the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter by time, for up to a maximum period of time, and further based on limiting the amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter where a target power for the bidirectional storage inverter falls within a threshold of zero power for more than the maximum period of time.

25. The method of claim 23 or claim 24, further comprising:determining the target power for the bidirectional storage inverter based on a control function that implements a monotonic relationship between the voltage at the AC terminals and the target power,controlling the bidirectional power inverter circuit based on limiting the amount of the power flow to the bidirectional storage inverter from the further bidirectional storage inverter or from the bidirectional storage inverter to the further bidirectional storage inverter by determining the target power as zero power for a range of voltages for which the target power would fall within the threshold of zero power according to the monotonic relationship.9255721626. The method of claim 25, wherein the monotonic relationship is consistent with a power curve, wherein the method further comprises:adjusting a zero-power intercept of the power curve to implement the range of voltages.

27. The method of claim 25, wherein the monotonic relationship is consistent with a power curve, wherein the method further comprises:adjusting a slope of the power curve to implement the range of voltages.

28. The method of claim 26, further comprising:adjusting a slope of the power curve to implement the range of voltages.

29. The method of any one of claims 25 to 28, wherein the range of voltages is based on a voltage error between voltage sensing at the AC terminals and voltage sensing for the further bidirectional storage inverter and a power difference between the bidirectional storage inverter and the further bidirectional storage inverter that results from the voltage error.

30. The method of any one of claims 20 to 24, wherein the controlling is further based on a continuous control function that extends from a maximum power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, to zero power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, and to a maximum power flow to the AC power system from the AC terminals and through the bidirectional power inverter circuit and the DC terminals from the electrical energy storage device.

31. The method of any one of claims 25 to 29, wherein the control function comprises a continuous control function that extends from a maximum power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, to zero power flow from the AC power system to the AC terminals and through the bidirectional power inverter circuit and the DC terminals to the electrical energy storage device, and to a maximum power flow to the AC power system92557216from the AC terminals and through the bidirectional power inverter circuit and the DC terminals from the electrical energy storage device.

32. The method of claim 30 or claim 31, wherein the continuous control function maintains regulation of the voltage at the AC terminals within a voltage range.

33. The method of any one of claims 30 to 32, wherein the continuous control function provides continuous power flow to or from the AC power system.

34. The method of any one of claims 20 to 33, further comprising:determining a target power reference based on the voltage at the AC terminals,wherein the controlling based on the determined power flow comprises controlling the bidirectional power inverter circuit further based on a difference between the determined power flow and the target power reference.

35. The method of claim 34, wherein a relationship between the voltage at the AC terminals and the target power reference provides a voltage-power relationship that enables power sharing between the bidirectional storage inverter and the further bidirectional storage inverter.

36. The method of any one of claims 20 to 35, wherein the further bidirectional storage inverter is one of a plurality of further bidirectional storage inverters, wherein the controlling comprises controlling the bidirectional power inverter circuit based on the determined power flow and further based on limiting power flow to or from any of the plurality of further bidirectional storage inverters.

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