Multistage electric energy conversion system

Abstract

A power conversion system includes a direct current (DC) element, a source inverter interfacing an alternating current (AC) source and the DC element, a load inverter interfacing a load on the DC element, and a controller. A voltage of an output of the load inverter is controlled by a controller to reduce power consumption of the load according to a variable charge state of the DC element.

Description

Technical Field

[0001] The present invention generally relates to power converters and more particularly to a power conversion system that can be applied to the rapidly growing market of solar photovoltaic systems with power storage functions, which are usually interconnected with and dependent on an external large-scale alternating current (AC) power grid. Background Art

[0002] The interconnection of AC power electronic converters to the grid involves synchronization of voltage, phase and frequency. However, over time, the voltage stability and harmonic content of most grids around the world have become increasingly unstable, resulting in fluctuations in user loads.

[0003] For example, the Australian national grid often delivers excessive voltage levels during the day, where a 240VAC node can measure anywhere from 240V to 255V over the course of a day.

[0004] Since average power is proportional to supply voltage, overvoltage will cause the meter to read higher than required, and this overvoltage will be dissipated as harmful heat within the appliance with no benefit to the user. Summary of the invention

[0005] An AC circuit element consumes or generates power (P) according to P = I·V, where I is the current in the element and V is the voltage across the circuit. The instantaneous power p(t) = i(t)·v(t) is time dependent. For a resistive circuit, i(t) and v(t) are in phase and have the same sign (+ or -) at any instant in time.

[0006] Many loads are resistive in nature. However, some loads have capacitive or inductive components as reactance, where the relative signs of i(t) and v(t) change within a cycle due to the phase difference. The vector sum of these components is the load circuit impedance Zc.

[0007] Therefore, p(t) is sometimes positive and sometimes negative, indicating that the impedance also transmits or absorbs power under different circumstances.

[0008] It is also possible to consider the instantaneous power p(t) within a power line cycle, ie the time average of the instantaneous power (Pave=1 / T·∫p(t)·dt), where T=2·π / w is the oscillation period.

[0009] The average power consumed by the load impedance can be expressed as Pave = 1 / 2·Is·Vs·cosφ, where cosφ is the power factor. The power factor essentially quantifies the loss (the effective power delivered in the circuit is less than the theoretical maximum due to the out-of-phase components Is and Vs).

[0010] Therefore, the power demand on the power supply can be interpreted as follows: Pave = 1 / 2·Is·Vs·cosφ=Vs2·Rc / Zc, where Zc is proportional to the sum of the circuit Rc and the reactance, which is usually inductive in nature, and Xl is expressed in ohms.

[0011] However, the inductive reactance Xl (whose size causes losses) is determined by Xl = 2·π·f·L, where f is the system frequency and L is the component inductance (in Henrys).

[0012] Therefore, from the circuit analysis it can be seen that Pave can be reduced by reducing impedance with careful optimization of voltage and system frequency.

[0013] Therefore, this paper provides a multi-level back-to-back power conversion system, in which the impedance can be reduced by optimizing the control of the voltage and frequency of the DC element according to the charge level of the DC element to reduce the Pave.

[0014] The present system includes two independently controlled DC-AC inverters, wherein each DC-AC inverter preferably has an ultra-high efficiency of more than 99%.

[0015] These inverters include a source converter for an AC source (usually the grid) and a load inverter for a load (usually comprising one or more consumer appliances).

[0016] Each inverter is operatively coupled to a DC element (usually a suitable capacitor, but could also take the form of an electrochemical cell) that holds a certain amount of charge.

[0017] A source inverter operating between a DC element and an AC source has an output Vs which can be controlled by a bidirectional T-neutral point clamped (TNPC) multi-level converter, where grid power can be input into or output from the DC element.

[0018] Since the standard TNPC usually uses three half-bridge power stages to realize three levels, the present system can apply ultra-high frequency interleaved pulse width modulation to two H-bridge power stages to realize five-level control.

[0019] The load inverter interfaces the DC element and the load, and is a unidirectional converter having an output for delivering power from the DC element to the user load.

[0020] The system further includes a controller that independently controls the voltage and frequency of the inverter according to the amount of electricity of the DC element.

[0021] For example, the controller is used to set the voltage and / or frequency of the output of the load inverter according to the measured voltage and frequency of the AC source when the power supply is high.

[0022] Additionally, the controller is configured to reduce at least one of a voltage and a frequency of an output of the load inverter when the charge level is deemed low.

[0023] The control algorithm for inverters operating in parallel can take the form of an artificial neural network with multiple input layers feeding two output layers, whereby the largest weight of the input is the DC component power. Other inputs can include solar energy, ambient temperature, time of day, etc.

[0024] The grid-connected source inverter can effectively buffer the power level that excess solar energy may output to the grid. Any local power shortage in the solar shortage section can be optimally input from the grid to the DC element through a ramp rate current control link embedded in a proportional integral derivative loop (PID).

[0025] The local inverter is connected to the user load through an interface, and the controller performs key power optimization at the user load. The control of the load inverter can be based on improved linear and nonlinear hyperbolic control.

[0026] The back-to-back control of the inverter is based on continuous calculations performed by the controller (eg using a neural network) and can be controlled on a power line cycle by power line cycle basis, while the charge of the DC element will continuously fluctuate due to the varying current circuit conditions.

[0027] The typical Australian home consumes 20kWh of electricity per day (7.3MWh per year), resulting in fossil fuel emissions of around 7 tonnes of carbon dioxide (CO2) per year.

[0028] According to data estimates, a typical implementation of the system proposed by the present converter will save about 20% of electricity and reduce more than 1.4 tons of CO2 per house per year, which means a reduction of 14 tons in ten years (or 14 million tons if used for 1 million houses).

[0029] Assuming 27.5 cents / kW, the reduction in energy consumption could save the exemplary home $305 per year. Over 10 years, the net present value (NPV) of these direct cash flow savings would be $2,355 in 2021 dollars. If the system product were installed in 1 million homes in Australia, the NPV savings would be an estimated $2,355 billion over 10 years.

[0030] Furthermore, users can expect that the life of their appliances will be extended (an estimated 25% by reducing the heat dissipated by the connected home appliances).

[0031] Australia has 10 million homes, more than 3 million of which have rooftop solar, making them potential candidates for solar photovoltaic (PV) and energy storage retrofits. This conversion system can reduce the need for solar PV and energy storage for new smaller loads.

[0032] Other aspects of the invention are also disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Although there are other forms that fall within the scope of the present invention, preferred embodiments of the present invention are described herein by way of example only with reference to the accompanying drawings, in which:

[0034] Figure 1 shows a logical schematic diagram of a power conversion system;

[0035] Figure 2 and Figure 3 A simplified circuit schematic diagram showing a power conversion system; and

[0036] FIG. 4 shows an exemplary circuit of a power conversion system. DETAILED DESCRIPTION

[0037] Figure 2 A simplified schematic diagram of a power conversion system 100 is shown, which includes back-to-back inverters 101 that are interfaced with DC elements 102 controlled by a controller.

[0038] The controller may include a processor for processing digital data and computer program code instructions. The controller may be in operable communication with a storage device via a system bus. The storage device may be used to store digital data and computer program code instructions. In use, the processor obtains these computer program code instructions and related data to implement the control functions described herein.

[0039] For various purposes, the computer program code instructions may be logically divided into multiple computer program code instruction controllers.In an embodiment, the controller may take the form of a local microprocessor based controller.

[0040] Block A represents an intermittent DC source 103 (typically a solar photovoltaic array or an equivalent DC source such as a solar PV boost-buck converter).

[0041] Block B represents a DC element 102, which may take the form of a capacitor, a battery storage device, or the like.

[0042] Box C represents a bidirectional source AC / DC inverter 101A, which interfaces a DC element 102 and an AC source (typically a grid). The source inverter 101A has an output 104 (shown as Vs), wherein its voltage and / or frequency (preferably both) can be controlled by a controller. The source inverter 101A is bidirectional because when the state of charge of the DC element 102 is high, the source inverter 101A can output power to the grid, and when the state of charge of the DC element 102 is low, the source inverter 101A can import power from the grid.

[0043] Block D represents a unidirectional load AC / DC inverter 101B that interfaces with a load (typically one or more consumer appliances). The load inverter 101B has an output 105 (shown as Vo) whose voltage and / or frequency is controlled by a controller to optimize power delivery to the load to reduce energy consumption.

[0044] The voltage and frequency output of the inverter 101 are independently controlled by a controller according to the state of charge of the DC element 102 .

[0045] The controller may implement three levels of control based on the amount of power in the DC element 102 and the power factor of the local user load.

[0046] The controller may reduce the voltage of the output 105 of the load inverter 101B based on the amount of charge of the DC element 102. For example, the controller may reduce the voltage of the output 105 of the load inverter 101B to 10% to 15% (compared to the voltage of the AC source).

[0047] In an embodiment, the controller may be configured to have multiple operating modes. These operating modes may include an off-peak operating mode, wherein the controller may further reduce the voltage output of the output 105 of the load inverter 101B by up to 25% (compared to the voltage of the AC source) to substantially keep the appliance in a standby state during off-peak hours.

[0048] Another operating mode may include a critical operating mode or an operating interruption when power is low, wherein the voltage of the output 105 of the load inverter 101B may be further reduced to 30% to 35% (ie, approximately 180V).

[0049] Additionally, the controller may reduce the frequency of the output 105 of the load inverter 101B to reduce impedance and improve throughput efficiency.

[0050] Furthermore, the controller may control the voltage and frequency of the output 105 of the load inverter 101B in a non-linear manner based on the overall power factor determined by a real-time signature at the load impedance measured at the output 105 of the load inverter 101B.

[0051] This real-time operational control can reduce the energy consumption of the load by approximately 20% and provide other benefits (including appliance longevity).

[0052] Figure 1 A logical schematic diagram of the present power conversion system 101 is shown, showing a source inverter 101A (shown as a grid I / F power stage) interfacing with an AC source 106 (shown as a typical grid).

[0053] The schematic also shows a load inverter 101B (shown as an ESS-load power stage).

[0054] The schematic also shows a DC element 102 (shown as an ESS).

[0055] The schematic also shows an optional variable DC power source 103 which may include a solar PV input.

[0056] As shown, the RMS VAC of AC source 106 can vary between -15% and 10% of the set value, and its frequency can vary within a range of up to 5% of the set value. In addition, AC source 106 may have sustained brownouts, surges, harmonics, notches, etc.

[0057] As described above, the source inverter 101A is bidirectional, such that power can be output from the DC element 102 to the AC source 106 or input from the AC source 106 to the DC element 102 .

[0058] As described above, the load inverter 101B is unidirectional to supply power from the DC element to the load 107. As shown in the figure, the RMS VAC supplied to the load 107 can be reduced by 10% (e.g. Figure 1 ), but may be further reduced in the case described above. In addition, the frequency supplied to the load 107 may be reduced by 10%.

[0059] The controller optimizes the voltage and frequency of the output 105 of the load inverter 101B according to the state of charge of the DC element 102. Figure 1 As shown, the controller may take other inputs that are indicative of the power delivery capability of the DC element 102. These inputs may include nominal VDC, operating VDC, state of charge, and / or cell temperature, among others.

[0060] Figure 3 Yet another simplified schematic diagram of a power conversion system 101 is shown, where class A is the DC element 102, class B is the source inverter 101A, and class C is the load inverter 101B.

[0061] The output voltage and frequency of the B class (Vs, fs) and the C class (Vo, fo) can be set in the first order according to the coulomb charge (Aq) of the A class DC element 102 .

[0062] The output AC of class B can be a T-type quasi-multilevel bidirectional structure, in which the midpoint clamp controls the transmission (Vs, fs).

[0063] The coupled C-stage is independently controlled by the controller to deliver optimized (Vo, fo) and can take the form of an H-bridge.

[0064] At higher charge levels of the DC element 102, the B level (Vs, fs) can be set in second order by synchronization with an external AC source.

[0065] At lower charge levels of the DC element 102, the C class (Vo, fo) may be reduced to reduce the effective reactive power delivered to the local AC loads.

[0066] Class B and C can be realized with suitable power semiconductors at a control frequency of 100 kHz.

[0067] An exemplary circuit diagram of power conversion is shown in Fig. 4. As can be seen from the circuit, the load inverter is a T-type quasi-multilevel bidirectional circuit, in which the midpoint clamp controls the delivery (Vs, fs).

[0068] The coupled C-stage is independently controlled by the controller to deliver (Vo, fo) and comprises an H-bridge.

[0069] For the purpose of illustration, the above description uses specific terms to provide a comprehensive understanding of the present invention. However, it is obvious to those skilled in the art that the present invention can be practiced without specific details. Therefore, for the purpose of illustration and description, the above description of specific embodiments of the present invention is presented. These descriptions are not intended to be exhaustive or to limit the present invention to the exact form disclosed, because in view of the above teachings, there can obviously be many modifications and variations. These embodiments are selected and described in order to maximize the principles of the present invention and its practical application, so that other technical personnel in the art can maximize the use of the present invention and various embodiments with various modifications suitable for the intended specific use. The following claims and their equivalents are intended to define the scope of the present invention.

Claims

1. A power conversion system, comprising: Direct current (DC) components; A source inverter, which interfaces an alternating current (AC) source and a DC element; A load inverter, connecting the load and the DC element via an interface; as well as A controller, wherein: The voltage of the output of the load inverter is controlled by the controller to reduce the power consumption of the load according to the variable state of charge of the DC element.

2. The system of claim 1, wherein the controller reduces the voltage of the output of the load inverter when the state of charge is low.

3. The system of claim 1, wherein the controller reduces the voltage in proportion to the amount of charge of the DC element.

4. The system of claim 2, wherein the voltage is reduced by more than 5% relative to the voltage of the AC source.

5. The system of claim 2, wherein the voltage is reduced by more than 10% relative to the voltage of the AC source.

6. The system of claim 1, wherein a frequency of the output of the load inverter is controlled by the controller to reduce power consumption according to the variable state of charge of the DC element.

7. The system of claim 6, wherein the controller reduces the frequency of the output of the load inverter when the state of charge is low.

8. The system of claim 7, wherein the controller reduces the frequency in proportion to the amount of charge of the DC element.

9. The system of claim 7, wherein the frequency is reduced by more than 5% relative to the frequency of the AC source.

10. The system of claim 7, wherein the frequency is reduced by more than 10% relative to the frequency of the AC source.

11. The system of claim 1 , wherein the state of charge is determined based on at least one of a nominal VDC, an operating VDC, a state of charge, and a cell temperature data input of the DC element.

12. The system of claim 1, wherein the DC element is a capacitor.

13. The system of claim 1, wherein the DC element is a battery.

14. The system of claim 1, further comprising a variable DC source interfaced to the DC element.

15. The system of claim 14, wherein the DC source is a solar photovoltaic (PV) DC source.

16. The system of claim 1, wherein the controller is configurable to a plurality of operating modes, and wherein the controller is configured to control the voltage of the output of the load inverter according to the operating modes.

17. The system of claim 16, wherein in one operating mode, the controller is configured to reduce the voltage by more than 20%.

18. The system of claim 1, wherein the source inverter is bidirectional in that the source inverter can be controlled by the controller to: exporting power from the DC element to the grid; or Power is input to the DC element from the grid.

19. The system of claim 18, wherein the controller independently controls the voltages of the outputs of the load inverter and the source inverter.

20. The system of claim 18, wherein when the state of charge is higher, the controller sets at least one of a voltage and a frequency of the output of the source inverter based on at least one measured voltage and frequency of the AC source.

21. The system of claim 18, wherein the controller changes a phase of the output voltage of the source inverter according to the state of charge.

22. The system of claim 1, wherein the controller controls a voltage and a frequency of the output of the load inverter based on a load impedance power factor measured at the output of the load inverter.

23. The system of claim 1, wherein the load comprises at least one consumer electrical appliance.

24. The system of claim 1, wherein the AC source is a utility grid.

25. The system of claim 1, wherein the source inverter includes a midpoint clamp control circuit.

26. The system of claim 1, wherein the load inverter comprises an H-bridge circuit.

27. The system of claim 1, wherein the power semiconductors of the inverter are controlled at a control frequency of approximately 100 kHz.

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