Circuit for transferring power between a direct current line and an alternating-current line

A system, method, and apparatus is disclosed for interfacing and transferring power unidirectionally or bidirectionally between a direct current (DC) line and a single or multiphase alternating-current (AC) line for only half of any given phase and only a single phase at a time when polarity is matched between the DC and the AC system. A circuit with simplified, robust, and reduced-cost components perform the power conditioning and the synchronization as a system that simulates a half-wave rectifier/inverter.

FIELD OF TECHNOLOGY

This disclosure relates generally to the technical fields of power electronics, and in one example embodiment, this disclosure relates to a method, apparatus and system of providing an interface to transfer power between a direct-current (DC) line and an alternating current (AC) line.

BACKGROUND

Alternative energy sources such as photovoltaic (PV) and wind power generation are becoming more commonplace in the global effort to conserve energy, move away from fossil fuels, become more self-reliant, and to reduce carbon footprints. Most alternative energy sources are intrinsically DC current sources. However, the modern commercial, industrial, and residential world runs almost entirely on AC current, including the power grid that distributes the power to consumers. Thus a grid-interactive inverter, or grid-tie inverter (GTI), is used to invert the DC current to an AC current. A significant cost element of the alternative energy solution is the GTI that converts the DC current supplied by the PV panels or wind turbine to AC current used by the utility power grid. As the price of PV panels falls, the inverter(s) will become a more significant fraction of the total lifecycle cost of the alternative energy system. Additionally, many PV systems operate at high voltages that subsequently require trained installers and maintenance workers familiar with the hazards of high voltage, thereby adding to the cost of the alternative energy system.

Some inverter designs use a transformer while others use no transformer (transformerless), and have a corresponding lower weight. High-frequency transformers follow an unconventional pattern of converting the DC into a high frequency AC, then back to DC, then finally to the desired line frequency AC. Transformerless inverters are less favored because of the possibility of transmitting DC faults directly into the AC grid, which could cause subsequent problems to the substation and the system at large.

Most inverters use maximum power point tracking (MPPT) that aligns the voltage and the current such that the product of the two, equal to the power, is maximized. Misalignment between the two could otherwise result in a high level of current being multiplied by a low voltage level, and vice versa, with the overall product resulting in a substantially reduced power output.

Residential electricity users in North America utilize a split single-phase AC line comprising a neutral line and two lines called phase and antiphase, which are sometimes inaccurately referred to as a two-phase line. The split single AC phase has a line voltage of 120 volts AC (VAC) root mean square (RMS) with an actual peak voltage of 170 volts (120 volts * √2) relative to neutral and a “peak-to-peak” voltage of 340 V. Traditional methodology involves converting the DC input source to drive the phase and antiphase lines via power semiconductor electronics that should be capable of withstanding the peak system voltage

SUMMARY

An apparatus, method, and system for interfacing, or transferring power between, a direct current line to an alternating-current utility grid is disclosed. Significant cost reduction and improved robustness is realized by driving only one polarity of an AC system at a time. With this method, components can be optimized for a polarity matching circuit (PMC), used to select an appropriate phase for the DC line, and a power conditioning circuit (PCC) coupled thereto, for controlling shape of the current on the DC line. Semiconductor devices for the power conditioning circuit, which operate at a high kHz frequency, can be rated for a low voltage rating of the AC peak voltage as they are only exposed to one polarity of an AC phase, rather than the full peak-to-peak AC voltage, e.g., only the half of the phase waveform with the same polarity rather than the full sinusoid waveform having both polarities. In comparison, semiconductor devices with more robust voltage ratings yet slower speeds can be used for the phase selector circuit that operate at a frequency not greater than an AC line, e.g., operates at the slower rate of 60 Hz. In one embodiment, the power conditioning circuit performs an inverter function to a split single-phase or to a multi-phase (2 or more phases) AC line to a utility grid as described. In another embodiment, the power conditioning circuit performs a rectifier function for either a split single-phase or a multi-phase AC line. The power conditioning can operate as a boost DC-DC converter to step up a DC voltage to an AC voltage level, or as a buck DC-DC converters to step down a DC voltage to an AC voltage level.

The PMC includes one or more solid state switches (SSSs) coupled to the DC line and a phase-selector. Each of the one or more switches is individually coupled to one respective phase of the one or more AC phases on the AC line. The phase-selector is configured to control the one or more switches such that no more than ONE SWITCH IS CLOSED when power is being transferred between the AC line and the DC line. The PMC functions to selectively couple via solid-state switches (SSSs) the DC line to one phase of the AC line that has a polarity that matches the polarity of the DC line. Only one phase is coupled to the DC line, e.g., no more than one switch coupling a phase to the DC line is closed when power is being transferred between the AC line and the DC line, to prevent a potential short-circuit. Thus, the PMC functions as a half-wave rectifier mode when the AC line powers the DC line and a half-wave inverter mode when the DC line powers the AC line. The phase selector is configured to couple each of the one or more switches selectively to the AC line such that no more than half of each phase, for all the one or more phases on the AC line, is communicated to the DC line. Regardless, current flows only in one direction through each of the one or more switches in the PMC for a given mode. The SSSs are chosen from a group of switches consisting of: a silicon-controlled rectifier (thyristor), a triac, a power field-effect transistor (FET), and an insulated gate bipolar transistor (IGBT). In general, N number of SSS is used in the PMC for N quantity of phases on the AC line. A phase selector is coupled to each of the one or more switches for controlling the switching function, and is coupled to the output of each of the one or more switches to monitor for an overvoltage condition that would cause the phase-selector to shut down, e.g., open, all of the one or more switches to prevent an over-voltage condition from failing the conditioning stage, and alternatively or additionally deactivating the PWM-controller to cease current generation in the PCC. The phase-selector opens a given switch of the one or more switches before a polarity of the AC phase to which the given switch is coupled changes to a polarity that is opposite of the polarity of the DC line. If the switch is an SCR or triac, then it is opened by either: turning off the PWM controller to stop driving current; shorting current to neutral via a shunt; by closing a second switch to another phase thereby reverse biasing and turning off a first switch that was previously closed; or by skipping a phase in a multi-phase system. Alternatively, if the switch is a power field-effect transistor (FET) then it can communicate current for both polarities of the AC current and can open while conducting current. For bidirectional current flow between the DC line and the AC line, the switch can be a FET or a pair of oppositely coupled SCRs coupled in parallel.

The PCC includes an input from a DC power source and a switching block coupled to the DC power source. The switching block includes: a pulse-width modulated (PWM) transistor coupled to, and controlled by, a pulse width modulated (PWM)-controller to obtain a desired current. The PCC can be configured as a buck converter if a voltage on the DC line exceeds a peak voltage of the AC line, or a boost converter if the peak voltage of the AC line exceeds a voltage on the DC line. The PWM transistor and controller operate at, below the peak AC voltage, or below the maximum DC voltage. A current generated by the PCC can either be shaped for delivering an optimum power factor (PF) to the AC system or can be shaped for delivering constant power (CP) to the AC system. The constant power mode is configured by drawing only a constant current from a DC source by measuring the current drawn via a current sensor on the DC line into the PCC. A constant voltage on the DC line is maintained by selectably coupling or shedding loads on the DC line. Based on conservation of energy, by outputting all the received power, the PCC provides a current profile that is inherently the inverse of the voltage level of the AC line to which the PCC is driving, thereby resulting in a product of the two, which is constant. In the alternative, the optimum power factor mode is obtained by shaping the current output from the PCC, as measured by a current sensor on the output of the PCC and fed back to the PWM controller for modifying the current as required to obtain the desired PF.

The system includes a DC power source coupled to the DC line, wherein the DC power source is chosen from a group consisting of: a photovoltaic (PV) DC power source, a wind-powered DC power source, and an alternative energy DC power source.

DETAILED DESCRIPTION

A method, apparatus and system for transferring power between a near constant, or approximately constant, voltage direct current (DC) power source to an alternating-current utility grid is disclosed. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. It will be evident, however to one skilled in the art that various embodiments may be practiced without these specific details.

Functional Block Diagram

Referring now toFIG. 1A, a functional diagram of a bidirectional system100-A for transferring power between a direct current (DC) line and an alternating current (AC) line is shown, according to one or more embodiments. A DC voltage (VDC) function102can be supplied by a power source102-A, such as an alternative energy source. The power source102-A can power components tied to it, e.g., power sinking function102-B, and/or to power an AC line, e.g., a AC voltage power sinking function124-B, such as a utility grid, for compensation such as energy credit. The voltage level of VDC102provided by an alternative energy source can be a fixed or variable level that provides power to an AC line and that powers other devices that can operate in a fixed and/or variable voltage level environment. Alternatively, VDC function102can be supplied primarily by power from the AC line124, e.g., when the alternative energy source function102-A is not available to provide power to the power sinking function102-B. An optional isolating function block108processes the DC sourced power to prevent DC faults from propagating into the AC line, that otherwise might disturb the sensitive phase timing and/or the demand-response balance of the AC system. Power conditioning function112drives current unidirectionally, or optionally bidirectionally depending upon a system design, at a voltage level that is either boosted or bucked, depending upon the application and design specification of the VDC function102vis-à-vis the AC voltage (VAC) function124. Polarity-matching function122only couples the VAC function124and the VDC function102when a polarity of both the VAC function124and the VDC function102is the same. In one embodiment, the polarity is a fixed polarity, e.g., either a positive polarity or negative polarity, but not both. Thus, system100-A appears from the outside as a half-wave rectifier and/or half-wave inverter. The VAC124can be a power source124-A, such as a functional utility grid, or fixed VAC124can be a power sink124-B, such as a functional utility grid able to accept power, or a browned-out or blacked-out utility grid that needs power. Some of the many benefits of bidirectional system100-A is that simpler, less expensive, and more robust electronic power components may be used to implement the power conditioning function112and the polarity-matching function122because power is only transferred between VAC function124and VDC function102when polarity of both functions is the same. The functions illustrated inFIG. 1Aare implemented in structure, processes, systems, and their equivalents, as described in subsequent figures.

Referring now toFIG. 1Ba functional diagram of a power-conditioning system100-B having a constant power input from a direct current line and a resultant constant power output to an alternating current line, or a resultant output current shaping, is shown, according to one or more embodiments. In particular, a constant voltage supply132-A is provided, e.g., by power management of an alternative energy power source via selectable load coupling and shedding to maintain the constant voltage. One embodiment of such a circuit is provided in provisional patent application Ser. No. 61/489,263, filed May 24, 2011, entitled “System And Method For Integrating And Managing Demand/Response Between Alternative Energy Sources, Grid Power, And Loads,” with common inventorship to the present disclosure, which application is hereby incorporated by reference in its entirety. Power conditioning block142has a function that draws a constant current132-B in one embodiment, e.g., via current sensing, feedback and control capabilities therein. The resultant power input to power conditioning block142is the product of a constant input voltage supply132-A times a constant input current draw132-B, which results in a power input132-C that is constant. On the output side, power condition block142provides a current shaping function144-C as output144-A that shapes current to match the shape of the AC voltage level so as to obtain a beneficial power factor (PF) or alternatively as output144-B to shape current as an inverse of the AC voltage level for constant power. In the first case, an optimal power factor is typically required by an electrical utility grid to provide for cost-effective power transfer. The first case does not require a constant power into power conditioning block142. In the latter case, the principle of conservation of energy dictates that if power-conditioning block142has a constant power input, with no shunting and negligible losses, then the function of supplying the maximum amount of current at any voltage level will result in a constant power output from the power-conditioning block142. A constant power supply is a desirable property in a power source, especially if the recipient is a utility grid, which typically needs to maintain a balance between the demand and supply thereon. Moreover, drawing constant power from the DC line minimizes the ripple on the DC line and reduces the need for short-term storage normally performed by electrolytic capacitors, which tend to be failure prone.

System Schematic

Referring toFIG. 2, a block diagram of a system200for bidirectionally transferring power between a DC line and an AC line is shown, according to one or more embodiments. DC system202, also referred to as a DC/AC interface, can be a DC alternative energy source that is uncontrolled, e.g., solar, wind, wave, etc. that provides DC power via a DC line210within system200. The DC power source in this embodiment is specifically controlled to produce a near-constant voltage level, e.g., less than 2 volts in one embodiment and less than 5 volts variance in another embodiment, with an associated cut out voltage levels for both over-voltage and under-voltage conditions beyond a given narrow operating voltage band dictated by system performance needs and equipment and load sensitivities. Optional isolation block203, as provided per wiring codes, receives power from the DC system and provides safety functions to prevent any potential DC faults from propagating to the AC line207, and subsequently disrupting AC system250, e.g., a utility grid. One embodiment of isolation block203is an H-bridge switched-mode DC-DC converter.

Power conditioning block212, which couples optional isolation block203and polarity-matching block230, drives current via a switching block214coupled to an inductor216and a subsequent filter218, at a rate that creates a voltage level matching to that of the selected phase of the AC system250. Switching block214is coupled to receive a sensed voltage level on the AC phases via voltage input215, on the high side204and low side205of DC line210, and on neutral206for feedback and control purposes. As known by those skilled in the art, power-conditioning circuit212can be either a boost or a buck circuit, or combination thereof. Inductor216can be configured to provide isolation functions as well, with a dual winding operating in a “flyback” mode, e.g., energy is transferred in on one winding and extracted on a second winding when the first is undriven. If PCC212and PMC230have circuit components and control logic for buck and boost conversion, and have switches that are bidirectional, then the system can be configured to provide a bidirectional current213via functioning as either a half-wave inverter or half-wave rectifier.

Polarity-matching circuit (PMC), or phase selector circuit,230has one or more switches, e.g.,231-1through231-P, where P≧2 when multiple switches are used, that are each controlled by control lines coupled thereto from control block234. The quantity of switches is not greater than a quantity of phases supplied by the AC line in the present embodiment. Specifically, the control provided by control block234separately and selectively couples DC system202, via DC line210and power conditioning block212, to only one respective phase of one or more AC phases on AC system250, via AC line207, at a time when a polarity of both the DC line210and the AC line207are the same and such that no more than one of switches231-1to-231-P is closed at any given time, e.g., no more than one switch is fully closed when power is being transferred between the AC line and the DC line. That is, control block234will selectively close switch231to AC Phase I line241-1when a polarity of the AC phase I matches a polarity of DC line210, and selectively close switch231-P to AC phase P line241-P when a polarity of AC phase P matches a polarity of DC line210, so long as no more than one of switches231-1through231-P are closed at any given time. Thus, switching speed for switches231-1through231-P is that of the AC system, e.g., 60 Hz. Because the polarity of DC line210is constant, it is only when a polarity of a given phase matches the DC line that no more than one switch in the polarity-matching block230will fully close. Switches231-1through231-P can alternatively be a triac, e.g.,231-1′, or an insulated gate bipolar transistor (IGBT), e.g.,231-1″.

Control block234is a phase selector that receives inputs from DC high side voltage level204; neutral voltage206; conditioned voltage level209; and voltages from AC phase I241-1through AC phase P241-P, where P≧2 for a multi-phase system. By having polarity-matching block230match the polarity between DC system202and AC system250the compatibility of power transfer between the two systems is ensured. By permitting no more than one switch in polarity matching block230to be fully closed at any given time, non-conflicting power transfer is ensured in system200. Control block234senses the output voltage from switches231-1to231-P, and if the output voltage exceeds a threshold, then control block234shuts down all of the one or more switches to protect the AC system250.

In an alternative embodiment, more than one of switches231-1to-231-P can be closed at a time, providing that no more than one switch is forward biased at a time, e.g., all but one switch is either open or is reverse biased. This embodiment is accomplished by switching a following phase's SCR from an open to a closed state while a first SCR is already in a closed state for a first AC phase. The following phase's SCR steals current from the first SCR because the following phase has a lower potential than the phase to which the first SCR is coupled at the time it is closed. The polarity of the AC phases, respectively coupled to the first SCR and the second SCR, is the same as the phase of the DC line. The process of stealing current at a lower potential thus reverse biases the first SCR and effectively opens it, thus maintaining the condition that only one switch be fully closed to only transfer power between one phase and the DC line. The switching occurs prior to the voltage of the two phases crossing each other.

AC loads tapping into the AC line will alter the power factor of the AC power. By feeding back the current level248,249sensed on the phases of the AC lines to a power factor (PF) correction circuit that provides a feedback221to the switch function block214, the current output from PCC212can be adjusted to provide an optimal PF of the current supplied to AC system250. In North American applications, the interface to AC line207, e.g. the utility grid, is typically a local transformer that serves a modest quantity of houses. Such a transformer will effectively integrate current on AC phase I (1) line241-1and on AC phase P line241-P, e.g., phase II (2) on line242, to present power symmetrically to the AC grid. The topology ofFIG. 2can be simplified for use with a single-phase application by eliminating switch231-P and AC Phase P. This configuration is referred to as a “single half-wave inverter.” In a unidirectional system, e.g., one that transfers power only from DC line210to AC line207, control block234determines whether conditioned voltage209falls below the peak phase-to-neutral grid voltage, in which case it will open switches231-1through231-P and not attempt to transmit power to AC system250.

Referring now toFIG. 3A, a schematic is shown of a split single-phase AC system selectively coupled to a buck DC-DC converter for unidirectionally transferring power from a DC line to an AC line via solid-state switches with turnoff capability only at zero current, according to one or more embodiments. System300-A includes power conditioning circuit (PCC)312-A having a pulse-width modulated (PWM) controller314coupled to T1switching transistor315to control its duty cycle, and to flyback diode309, of together implement switching block function block214ofFIG. 2. PWM controller314receives current measurement input from DC line210, to which is coupled, via current (I) sensing device303to determine whether DC line210is being pulled down by excessive current draw from power conditioning block312-A. PWM controller314also receives current measurement input from current (I) sensing device302for sensing actual duty cycle performance of switching transistor T1315as indicated by unidirectional current flow313to inductor316. Current sensor302and303may be implemented using any type of conventional current sensor, such as a resistive drop measurement of current, e.g., across an access transistor, or as an inductive measurement of current, or any device or method that provides reasonably accurate current measurements. A DC input308from low side304(−190 V DC) of DC line210is communicated to PWM controller314in order to detect performance of DC system. For example, a slight degradation of voltage sensed on line308can indicate a degradation of power-generating capability of DC system202, and thus, current draw can be reduced by a predetermined performance curve, as embodied in a look up table describing the relationship between DC system202voltage versus current output.

Inductor316drives current because of its inherent property of maintaining a given current flow state, e.g., either current flowing or not flowing. Thus, when a closed and conducting switching transistor T1315is opened by controller314per the desired duty cycle, current flow to inductor316is interrupted, resulting in a collapse of the magnetic field in inductor windings316. The collapsing magnetic field is naturally converted into current to help maintain the existing state of current flow through inductor316, as known by those skilled in the art. When the magnetic field collapses in inductor316, the voltage is reversed across inductor to maintain current flow, with diode309providing clamping against neutral line306. Switching transistor T1315switches at a sufficient speed to operate the DC-DC converter function, e.g., a silicon power-FET working at a rate of 100 KHz or more in one embodiment. Alternative device materials could be GaN or SiC. However, power conditioning block312-A is not exposed to the full range of AC voltage, as polarity-matching circuit330-A only opens and exposes power conditioning block to half the voltage swing of a single AC phase at a time, and thus, electronic components in power conditioning block312-A can be rated at a lower voltage, with potential cost savings and higher performance in other areas and more robustness.

In the present embodiment, power conditioning circuit312-A is configured as a buck DC-DC converter to reduce voltage from DC line210of −190 VDC to AC line207peak voltage of 170 VAC (120root mean square (RMS) voltage * √2). Filter block218ofFIG. 2is implemented in capacitor318that couples inductor316to neutral306, in order to filter out high frequency noise arising from inductor316.

Polarity-matching circuit330-A, coupling power conditioning circuit (PCC)312-A and AC line207-2, implements polarity matching block230ofFIG. 2. In particular, switches231-1through231-P ofFIG. 2are implemented in the present embodiment using silicon-controlled rectifiers (SCR)331and332type of solid-state switches coupled to inductor316on the reverse biased side and individually coupled to respective AC phase I line341and AC phase II line342on the forward biased side. Switch controller334, coupled similarly to control block234ofFIG. 2, provides an input to each of switches331and332to individually and selectively open and close them. SCR switch331and332turn off, or open, only when current through the switch ceases. Thereafter SCR switch331or332must receive a control signal from switch controller334to turn back on, or close. Control block334provides an input319to PWM controller314to sense voltage on AC lines341and342, e.g., for sensing overvoltage, and for sensing voltage of AC line for shaping of current313via PWM T1315.

SCR switches331-332direct the filtered output of inductor316to the appropriate phase line341or342of the AC line207-2, and turn off automatically at the end of the respective half cycle, that is, when the current through the switch falls to zero. Transistor T2307acts as a shunt circuit to draw current away from polarity-matching circuit330-A in order to create a zero-current condition through the SCR and thereby forces SCR switches331-332to open at a at a time period near phase crossover, e.g., from Phase I to Phase II and vice versa. The amount of current conducted through T2307shunt is normally small because the sinusoid is near zero current and zero voltage, yet provides a safety factor to prevent multiple phase from conducting through system300-A at one time since it may work independently of the PWM controller314management of T1315. In another embodiment configured for constant-power output for a multiphase version, e.g., in subsequentFIG. 4A, the shunt current may be higher, but the duration will be short, thereby resulting in only an acceptable amount of power loss.

Referring now toFIG. 3B, a schematic is shown of a split single-phase AC system selectively coupled to a boost DC-DC converter for unidirectionally transferring power from a DC line to an AC line via solid-state switches with turnoff capability only at zero current, according to one or more embodiments. System300-B is configured similarly to system300-A ofFIG. 3Ain terms of basic components and functions, e.g., capacitor318, inductor316, PWM controller314, polarity-matching block330-B, etc. However, because system300-B utilizes a higher AC voltage split single-phase line341-H and342-H with a peak AC voltage that exceeds the voltage of 190 V on DC line210, the power conditioning block312-B now acts as a DC-DC boost circuit for unidirectional current313supplied from DC line210to AC line207-2, rather than a buck circuit as used inFIG. 3A. That is, the PCC will boost the DC voltage from −190 V to a peak AC voltage of −360 V (−240 RMS *√2). In particular, flyback diode309is now coupled between polarity-matching circuit330-B and both T1switching transistor315, coupled to the high side305of the DC line210, and inductor316, coupled to low side of the DC line210. Capacitor318is coupled on the forward biased side of diode309down to the low voltage side of the DC line210. Switch controller334and switching transistor T1315, are coupled sensing inputs similar to system300-A ofFIG. 3A, but switch controller314now switches T1to drive current at a boosted voltage level from DC line210to AC line207-2.

In one embodiment, for either system300-A or300-B, diode309and/or solid-state switches331and332are replaced with transistors coupled to control block334to achieve greater power efficiency, albeit at slightly higher component cost. Additionally, while bothFIGS. 3A and 3Billustrate two phases, e.g., a typical residential or commercial application, the present disclosure is well suited to running a single phase or multiple phases (e.g., 3 to 6 phases) of an AC line, for a very simple, low-cost, and robust circuit. In this embodiment, either one of switch331or332is not operated, or are not designed in the circuit, assuming the utility grid has sufficient robustness to accept an imbalanced feed of a single polarity of a single phase.

Referring now toFIG. 3C, a timing diagram is shown of current (I) versus time (t) for the power supplied from a DC line to a split single-phase AC line via solid-state switches with turnoff capability only at zero current, according to one or more embodiments. Inductor current313is the composite non-continuous current from both AC phase I current (IP1)341-C and AC phase II current (IP2)342-C as the secondary current on the load side of the transformer to the AC system250. All current flowing through inductor316, and subsequently through polarity-matching block330-A and330-B will be only one polarity, e.g., traveling in a single direction. Hence, all current is shown on the top of the axis and none below, simulating a half-wave output. Power conditioning block312-A or312-B fromFIGS. 3A or 3Bshapes the current drawn from DC system202into a sinusoid shape as shown, to match the current profile of half of the full-wave AC system250. The PMC312-A and312-B ofFIG. 3AandFIG. 3Bis configured to couple each of the one or more switches selectively to the AC line such that no more than half of each phase, or only one polarity of each phase, for all the one or more phases on the AC line, is communicated to the DC line. The phase-selector334ofFIGS. 3A and 3Bis configured to open a given switch of the one or more switches331to332before: a polarity of an AC phase to which the given switch is coupled changes to a polarity that is opposite of a polarity of the DC line; or before a zero voltage level is reached by the AC phase to which the given switch is coupled, as shown by current lobes351to354ofFIG. 3C.

A switch cutoff gap222exists between the composite current flow through polarity-matching block330-A and330-B to prevent a condition where more than one phase will be conducting with power conditioning block312-A or312-B. A phase is only coupled to the DC line210after the phase voltage has reached an active voltage level, e.g., one in which the utility grid is safely established to be operating, such as 10-20 volts. This safety feature is to prevent powering a utility grid intended to be turned off for servicing by a technician. If transistors are used for switch331,332, then the switch cutoff gap222is accomplished by control block334turning off transistors331,332and the cutoff gap222can be much narrower because a transistor can switch to an open state more accurately and with current flowing through the switch. However, if switches331and332are SCR switches, then current must be eliminated through the SCR in order for the SCR to open. Current can be eliminated through the SCR in numerous ways, such as: 1) eliminating current generation from the PCC by turning off the switching T315; 2) by shunting current away from the SCR; 3) by reverse biasing the SCR by closing a second SCR at a lower potential, etc. such that the SCR will open automatically at zero current. Shunting can occur via a switch-controlled link, from T2307to neutral306, as shown inFIG. 3A. Because only one phase is conducted at a time, voltage remains at a level commensurate with a single phase AC system, e.g., 110 V RMS, or 155 VAC peak, thereby allowing lower voltage-rated components to be used for power conditioning circuits, e.g.,312-A,312-B.

Polarity-matching circuit330-B, in turn, is responsible for directing the current to the appropriate polarity matched AC line207-2. PWM controller314can sense voltage level of AC line207-2, e.g., shown as exemplary voltage profile356and level355, via voltage feed310from control block334. In turn, PWM controller314senses current drawn from DC line210via current sensor303or from current pumped through inductor316via current sensor302that is accumulated over a longer period of time. For example, the sensed current can be accumulated over a period of time greater than the periodicity of a transistor T1315switching rate, e.g., 1/100 KHz, and less than the period of the current waveform itself, e.g., less than half of the AC period of 1/60 Hz. Thus, AC current lobe351is conducted through switch331because its polarity matches that of the DC line210at the noted time, while AC current lobe352is conducted through switch332because its polarity matches that of the DC line210at the subsequent noted time. A resultant AC current on the primary winding of a transformer, will receive the phase and antiphase of the split single phase as a sine wave, as shown with the appropriate polarity for the AC system. Gaps222can be smoothed out with inductive filtering devices on the AC line feed.

Referring now toFIG. 3D, a transformer is shown illustrating how the half-wave inverter for a split single-phase produces a full wave on the utility grid, according to one or more embodiments. Primary winding on the left coupled to phase A, which is a single phase of a typical three-phase utility grid, is transformed to secondary winding on the right with a split single phase having a center tap to ground, and V1on the top line and V2on the bottom line. In illustration (A) a top diode only allows current for IP1that drives a current IAC317in the polarity shown on phase A of the utility grid as current lobe351. In illustration (B) by comparison, bottom diode only allows current for IP2that drives a current IAC317in the polarity shown on phase A of the utility grid as current lobe352. Consequently, a full wave is driven onto AC utility grid.

Buck Mode 2-Phase Half-Wave Inverter Circuit with FETs

Referring now toFIG. 4A, a schematic is shown of a three-phase AC system selectively coupled to a buck DC-DC converter for unidirectionally transferring power from a DC line to an AC line via solid-state switches with turnoff capability while carrying current, according to one or more embodiments. System400-A is configured similarly to system300-A ofFIG. 3Bin terms of basic component properties and functions, e.g., capacitor318, inductor316, PWM controller314, etc. Moreover, likeFIG. 3B, presentFIG. 4Aoperates a DC line210voltage at minus 350 volts, which is greater than the 155 VAC peak voltage. Thus to condition the current from the DC line210to the AC line207-3, power conditioning block412-A is configured and operated as a “buck” type DC-DC converter for unidirectional current413.

Because system400-A accommodates three phases, it uses three solid-state switches SSS-I431, SSS-II432, and SSS-III433, in the polarity-matching circuit430-A, to couple same-polarity current with the DC line210for each of the three phases to which they are independently and respectively coupled, AC phase I on AC phase line441, AC phase II on AC phase line442, and AC phase III on AC phase line443. System400-A implements solid state switches431-433as transistors, only needing a slow switching speed compatible with AC frequency, e.g., 60 Hz, such as power FET transistors or insulated gate bipolar transistors (IGBT), which achieve greater power efficiency than SCR type switches. The power FET transistors provide turn-off capability, as controlled by control block334, to allow a sustained non-zero current flow across the three phases, as described in a subsequent timing diagram. Thus, system400-A does not need a shunt circuit to redirect current away from polarity-matching circuit430-A in order to turn-off current to SCR switches, which require zero current to open. While system400-A is configured as a three-phase system, it may be selectively operated as a two-phase or a single-phase system, as programmed and controlled by control circuit334operating only the needed quantity of SSS in polarity-matching block430-A.

Referring now toFIG. 4B, a timing diagram is shown of current versus time for the power supplied from a DC line to each of three phases on an AC line via solid-state switches in constant power mode with turnoff capability while driving carrying current, according to one or more embodiments. Inductor current413is the composite current through inductor316from consecutive and contiguous AC phases of: phase I current (I)441-C conducted through switch SSS-I431, AC phase II current442-C conducted through switch SSS-II432, and AC phase III current443-C conducted through SSS-III433ofFIG. 4A, as controlled by control block334of polarity-matching block430-A. All current flowing through inductor316, and subsequently through polarity-matching block330-A and330-B will be only one polarity. Hence, all current is on the top of the axes and none is below, simulating a half-wave output across three-phases.

Power conditioning block412-A fromFIG. 4Ashapes current drawn from DC system202into a current profile451-453that is an inverted sinusoid shape as shown, e.g., to approximately match an inverse shape of the voltage profile460-463, respectively, of half of the AC sinusoid. In particular, current profile451-453times a sinusoid AC voltage460, that exists on AC lines for AC Phase I441through AC Phase III443provides an approximately constant power output from power conditioning circuit412-A ofFIG. 4A, over time. Shaping of current profile451-453to achieve constant power output is provided by power conditioning circuit412-A drawing only constant power from DC system202via DC line210. Constant power draw from DC system202is ensured by PWM controller314that modulates switching transistor T1315to draw a constant current. Resultantly, the sensed DC line voltage input308times DC current draw input from current sensor303, is maintained at a near constant value, e.g., a constant current drawn time its near-constant voltage equals a near constant power draw.

The DC power supply system202in the present embodiment is configured with power supply electronics that maintain its output voltage at a near constant level, regardless of its output current, in addition to a cutout for overvoltage and undervoltage conditions on its output. The constant voltage configuration is described inFIG. 1B. Other configurations of DC power supply systems202can be utilized with the present disclosure. Thus, the resulting output current from power-conditioning block430-A ofFIG. 2, naturally results in the current profile shown, considering conservation of power in vs. power out from the power-conditioning block412-A. A cutoff gap422for switches exists between current flow from different phases,451,452,453into composite current flow413is implemented via control block334of polarity-matching block430-A to prevent a condition where more than one phase will be conducting with power conditioning block412-A. Because transistors432-433are used, the switch cutoff gap422is accomplished by control block334, and the spacing of cutoff gap422can be minimized to reduce power loss and ensure a smoother power transfer to AC system across all phases.

Buck Mode 3-Phase Half-Wave Inverter Circuit with SCRs

Referring now toFIG. 4C, a schematic is shown of a three-phase AC system selectively coupled to a buck DC-DC converter for unidirectionally transferring power from a DC line to an AC line via solid-state switches with turnoff capability only at zero current, according to one or more embodiments.FIG. 4Cutilizes SCR type solid-state switches461-463for polarity-matching circuit430-C that require zero current to switch off. Thus, power conditioning circuit412-C provides a sufficient gap between adjacent phases to allow switches461-463to shut off. Alternatively, system400-C can use a shunt, as exemplified by T2transistor307ofFIG. 3A, to redirect current from polarity-matching block430-C at times where phases would have otherwise overlapped (not shown). Alternatively, the phases coupled to DC line210can be staggered such that there is a gap between phases conducting to the DC line210, as illustrated in the subsequent timing diagram ofFIG. 4D.

A mechanical energy storage device or generator450, such as a rotary flywheel, is coupled in parallel to system400-C and the AC loads and AC system250, the utility grid. The inertial storage solves many potentially undesirable properties of the present half-wave inverter/rectifier architecture to provide a smoothing of the current and a damping of any voltage spikes and other noise caused in the system400-C conditioning and switching power between the DC system202and the AC system250. In the present embodiment, the storage device450is a synchronous motor/generator (SMG), which offers many advantages over costly and failure-prone alternative smoothing devices such as static electrolytic, batteries, and/or supercapacitors. An SMG450can operate reliably and maintenance free for decades as a high-speed mechanical flywheel with both motor and generator windings in a vacuum chamber.

SMG450may be connected in parallel with straight feed through from phase switches to the AC system to act as a UPS. Alternatively, SMG450can function as an isolation stage with extra functionality as a step-up or step-down voltage transformer. DC system400-C drives current via lines471,472, and473into motor windings of SMG450while generator windings in SMG450are coupled out to AC system250. Any changes in current and voltage are absorbed by the spinning inertia of the rotor in SMG450, thereby providing the smoothing capacitive and inductive functions.

When the SMG450is combined with a DC system, e.g., the DC/AC interface,400-C, the result is a balanced high power-factor feed from AC lines270-3to AC loads and to AC system250. The SMG450has sufficient inertia to accept any type of power generated by DC/AC interface400-C, such as constant power output configuration, PF corrected current shaping, skipped phases, etc. Multiple phases431to433and471-473can be driven by the DC/AC interfaces400-A and400-C, respectively, with either all phases (shown) or only one phase can be fed out from SMG450into AC loads or AC system250, depending on the winding of the electrical generator portion of the SMG450. The AC loads can be a local residential or commercial split single-phase or multiphase system that benefits from a clean AC cycle generated from the generator portion of the SMG450without power dead spots. Applications needing very reliable power and/or that have heavy AC loads, such as medical institutions, server farms, facilities with rotating equipment, etc., that can't easily be run off the DC system, benefit from the flywheel which can provide smooth power and provide high peak currents for motor starts. SMG450can be utilized in any of the architectures described herein.

If the AC system250is disconnected from the DC/AC interface400-C, the power level generated from the DC/AC interface400-C will be adjusted to maintain the AC voltage and the AC frequency output from the SMG450. The output voltage from SMG450is fed back to PWM controller314for this control purpose.

Referring now toFIG. 4D, a timing diagram400-D is shown of current versus time for the power supplied from a DC line to three phases on an AC line in a staggered fashion while skipping some of the phases via solid-state switches with turnoff capability only at zero current, according to one or more embodiments.FIG. 4Dresembles timing diagram inFIG. 3C, because both timing diagrams use SCR type solid-state switches for polarity matching. Inductor current (IL)493is the composite current from consecutive and non-contiguous AC phases of: AC phase I current (I)471-I conducted through switch SSS-I461, AC phase III current473-I conducted through switch SSS-III463, and AC phase II current472-I conducted through switch SSS-II462, in that order, as controlled by control block334of polarity-matching block430-C, as shown inFIG. 4C.

Current flow493through inductor316, and subsequently through polarity-matching block430-C ofFIG. 4Cis only one polarity. Hence, all inductor current493is above the top of the axis and none is below the axis, thereby simulating a half-wave output across three-phases. Unlike inductor current413in timing diagram400-B, Inductor current493in present diagram400-D is not continuous. This is because adjacent phases in a three-phase system have an overlap423, where the current and voltage are too high to efficiently shunt current, or the resulting current discontinuities are not tolerable to the AC line. Instead, polarity-matching block430-C of system400-C only communicates power between DC line210and AC line207-3at every other phase, thus allowing a sufficient non-conducting time gap424between phases, e.g., current lobe481from Phase I current471-C and current lobe483from Phase III current473-C, where switches in polarity-matching block430-C can turn off. Consequently, current lobe482for Phase II current472-C is not communicated to AC phase II line472, and thus system400-C does not communicate as much power as does system400-A. However, components and operation of system400-C are simpler than that of400-A, providing a reasonable tradeoff to be chosen by a given application's needs.

The phases communicated between the DC system and the AC system, and the phases skipped, alternate in a round robin manner. Thus, for a three-phase system, the phases that receive power from the DC system (no brackets) and the phases skipped (in brackets) are: 1, [2], 3, [1], 2, [3], 1, [2], 3, [1], 2 . . . Consequently, over time, the power received by each phase in a split-single phase, or a multi-phase AC system is equal. More than one phase can be skipped as well, providing that it is not equal to one minus a total quantity of all the AC phases and is not a divisor to the total quantity of AC phases that would result in a whole number quotient. Thus, for a three-phase system, N=3, and skipping N−1 phases (2 phases) is not recommended, as only phase1will ever be powered, and phase2and3will always be skipped. Likewise, for a four-phase system, N=4, and skipping 2 phases (a divisor that result in a whole number quotient from the total number of phases) is not recommended, as only phase1and4will ever be powered, and phase2and3will always be skipped.

Power conditioning block412-C ofFIG. 4Ccan be configured to shape current drawn from DC system202into current profile481-486ofFIG. 4D, similar to that described and shown for of timing diagram300-C ofFIG. 3C. Because three phases are used in timing diagram400-D, the separation between every other phase, e.g., phase I and phase III, provides ample gap in current flow such that no clipping is necessary of any given phase being conducted through polarity-matching circuit430-C, and thus no shunt circuit is required for the operation of the circuit inFIG. 4C, but it may be included for safety.

Referring now toFIG. 5A, a schematic is shown of a split single-phase AC system coupled via a selectable buck or boost DC-DC converter for bidirectionally transferring power between a DC line and an AC line via solid-state switches with bidirectional current carrying capability, according to one or more embodiments. System500-A differs from all prior systems described in that power can be transferred bidirectionally, rather than just unidirectionally, as shown by bidirectional current flow513through inductor316. Thus, system500-A provides both an alternative energy DC power source to power an electrical, or utility, grid, as well as a DC system to receive power from the utility grid when the DC power source is down. Resultantly, a comprehensive power management system is provided for residential, commercial, and industrial applications.

System500-A is configured with basic components whose properties and functions have been previously described, e.g., capacitor318, inductor316, PWM controller314, etc. Furthermore, system500-A utilizes block portions from prior systems, such as solid state switches with turn-off capability531and532in polarity-matching block530, and DC voltage level of −190 VDC, both as previously described in system400-A ofFIG. 4A. However, power-conditioning block512is uniquely configured with multiple switching transistors to selectively accommodate a buck or boost DC-DC converter configuration, depending on the direction of current transfer. For example, a “buck” type DC-DC converter configuration bucks the −190 DCV to a VAC anywhere from 0 to −155 V peak when the alternative energy source DC power source generates sufficient power for the AC utility grid system. Complementarily, a “boost” type DC-DC converter configuration boosts the −155 VAC peak voltage from the utility AC system to the −190 V DC when the DC system demand exceeds the DC power source supply.

For example, when DC system202operates as a power sourcing function via DC line210, then power conditioning circuit512is configured to operate as a “buck” type DC-DC converter, by: 1) controlling transistor T3515-B, via PWM controller514, to act as an synchronous rectifier, providing clamping similar to diode309ofFIG. 4A; and 2) controlling transistor T1515-A, via PWM controller514, to act as a switching transistor for generating current through inductor316similar to T1315transistor ofFIG. 4A. In contrast, when DC system202operates as a power sinking function, receiving power from AC line207-2via DC line210, then power conditioning circuit512is configured to operate as a “boost” type DC-DC converter, by: 1) controlling transistor T3515-B, via PWM controller514, to act as a switching transistor for generating current through inductor316similar to T1315transistor of system400-B ofFIG. 3B; and 2) controlling transistor T1515-A, via PWM controller514, to act as an synchronous rectifier, e.g., a power FET, providing clamping similar to diode309ofFIG. 4A. System500-A can be configured for any quantity of phases, e.g., from a single phase up to three-phases.

A timing diagram for system500-A would perform similarly to timing diagram300-C ofFIG. 3Cwhen DC system acts as a power source, and would perform similarly when DC system acts as a power sink except that current profile would be moved below the axis to reflect the changed direction of current flow.

Referring now toFIG. 5B, a bi-directional solid-state switch500-B with turnoff capability only at zero current for bidirectionally transferring power between a DC line and an AC line is shown, according to one or more embodiments. Switch500-B includes oppositely biased SCRs coupled in parallel to each other such that one SCR531-A conducts current when DC circuit acts as a power source and another SCR531-B conducts current when DC circuit acts as a power sink. Switch500-B can be substituted for each of solid-state switches531and532in polarity-matching circuit530ofFIG. 5A. However, a switching transistor circuit similar to307ofFIG. 3Bwould be used in a two-phase AC system with overlapping phases in order to shunt current from SCR500-B when changing the phase line conducting current such that a zero current condition through switch500-B would allow the SCR to turn off.

Referring now toFIG. 5Ca timing diagram is shown that illustrates current stealing on a three-phase AC line using switches with turnoff capability only at zero current, according to one or more embodiments. The curves represent the phase voltages, with the dashed line above the axis being the voltage on the AC line for each respective phase, and the solid line below the axis being the voltage level on the common node440between inductor316and phase switches461to463. While a given first switch is closed, e.g., solid state SCR switch461with turnoff capability only at zero current ofFIG. 4Cto communicate current IP1from the DC line210to phase1of the AC line471, and at a voltage AC (VAC) point550before reaching a crossover point552with another subsequent and adjacent phase, a second switch is closed, e.g., solid state SCR switch462with turnoff capability only at zero current to communicate current IP2from the DC line210to phase2of the AC line472. When second switch462closes it reduces the voltage at the common node440ofFIG. 4Cas shown by voltage reduction554inFIG. 5C, thereby reverse biasing and opening first switch461, and thereby stealing its current and preventing more than one phase from communicating to the DC line304. For a multi-phase system greater than 3 phases, a phase may be skipped and the next phase might still overlap, and thereby allow use of current stealing for switching between phases. While the foregoing figures illustrated specific quantity of phases for a given topology, the architecture in each figure can be adapted to a different quantity of phases.

Referring now toFIG. 6A, a flowchart600-A is shown of a process for isolating, power conditioning, and switching power between a DC line and an AC line, according to one or more embodiments. Flowchart600-A begins with operation652of isolating DC line from AC line, as provided by isolation block203ofFIG. 2to prevent DC faults from interfering with the AC system. The isolation block may also include an operation of level shifting e.g. from the 190V DC used for easy 120VAC compatibility to 48V DC to avoid regulatory issues with wiring high voltage DC in buildings.

Power-condition operation654conditions the voltage and current received from either the DC system202to AC system250as a half-wave inverter, or from the AC system250to the DC system202as a half-wave rectifier, depending upon operating conditions and on the circuit configuration provided in priorFIG. 3AthroughFIG. 5A. Power-condition operation654conditions the voltage and/or current from the DC system202to AC system250if a sensed DC operating voltage input654-A is above a threshold level, e.g., indicating that the DC power source has surplus power after meeting the existing DC loads. If, conversely, a sensed DC voltage level input654-A is below a threshold level, then power-condition operation654is programmed to power-condition a voltage and current from AC system250to DC system202, as DC power source is unable to meet existing DC loads. Power conditioning operation654utilizes a suboperation of boosting voltage654-C using a boost DC-DC converter or a suboperation of bucking voltage654-D using a buck DC-DC converter, depending on the voltage levels of a DC or AC system between which power is transferred. The DC-DC converters in PCC512,412-C,412-A, and312-B,312-A, and212can be utilized to implement operation654. Output current level654-D is sensed to determine duty cycle of DC-DC converter, and to condition the current to one of multiple current profiles. A first current profile matches a shape of a voltage waveform of the AC line250with adjustments to compensate for clipping the current in order to open SSS that require zero current to open and to compensate for any power factor offsetting from DC loads, e.g., dimmer switch bias to the back half of a phase. A second current profile is an inverse of the voltage waveform of the AC line250, to provide a constant power output to the AC line250.

Referring now toFIG. 6B, a flowchart600-B is shown of a process for selectively coupling a DC line to an AC line when their respective polarities are the same, according to one or more embodiments. Operation602senses a polarity for each phase of the plurality of phases of the AC line. When a polarity of one AC phase line matches the polarity of the DC line it is a candidate for being coupled to the DC line. Operation604controls each of the plurality of switches that individually and selectively couple the DC line to one respective phase of the one or more phases on the AC line. Operation606selects a phase of the plurality of phases that has a polarity that matches a polarity of the DC line. Operation608closes a given switch that is respectively coupled to the phase with the matching polarity. Operation610opens a given switch transferring current before the polarity of the phase changes to a polarity opposite that of the DC line. If, during operations606through610, two phases each have a polarity that match the polarity of the DC line at the same time, then a controller is configured to ensure that no more than one phase is fully coupled to the DC line at a time, e.g., no more than one switch is fully closed, and in one embodiment, both phase lines with matching polarity are switched off so that no phase is communicating with the DC line. A phase line with a rising voltage level from neutral will have priority for coupling to the DC line versus another phase line with a decreasing voltage level. In operation612an inquiry determines whether excessive voltage exists in the AC line, per sensed voltage input612-A. An affirmative response will proceed to operation614that opens all switches to prevent a shorting failure between AC and DC systems, while a negative response returns to operation602for repeating the overall process herein.

Representative circuit300utilizes a negative (with respect to neutral) DC supply “−190 VDC” as shown such that an n-channel field effect transistor (FET) may be conveniently utilized for transistors, e.g., T1315. It is appreciated that p-channel FETs can be similarly utilized with a positive DC supply. Additional devices such as snubbers, ancillary power supplies, fuses, etc. may be utilized in the present disclosure to provide smoother and more regulated power transfer. Such devices are well known in the art and are not shown for clarity. Furthermore, one skilled in the art” could combine the positive and negative versions of the circuit to get better utilization of the utility transformer. For example, one circuit handles positive polarity on any phase while the other circuit handles negative polarity on any phase, such that for a 2-phase system both phase and anti-phase are driven continuously. The DC supply can be common or separate—i.e. the DC power supply can be 380V.

Methods and operations described herein can be in different sequences than the exemplary ones described herein, e.g., in a different order. Thus, one or more additional new operations may be inserted within the existing operations or one or more operations may be abbreviated or eliminated, according to a given application.

Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description. In addition, it will be appreciated that the various operations, processes, and methods disclosed herein may be carried out, at least in part, by processors and electrical user interface controls under the control of computer readable and computer executable instructions stored on a computer-usable storage medium. The computer readable and computer executable instructions reside, for example, in data storage features such as computer usable volatile and non-volatile memory and are non-transitory. However, the non-transitory computer readable and computer executable instructions may reside in any type of computer-usable storage medium.

The foregoing descriptions of specific embodiments of the present disclosure have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching without departing from the broader spirit and scope of the various embodiments. The embodiments were chosen and described in order to explain the principles of the invention and its practical application best and thereby to enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It should be appreciated that embodiments, as described herein, can be utilized or implemented alone or in combination with one another. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the claims appended hereto and their equivalents.