Abstract:
A single conversion stage bidirectional soft-switched AC/AC power converter system is capable of converting power in both directions between high- and low-voltage sources. The system has substantially loss-less switching and regulated output in both directions of power transfer. The semiconductor and electro-magnetic components of the system provide both output regulation and soft switching in both the step-up and the step-down directions of power conversion. The commonality of components between the two directions of power transfer reduces total component count, cost and volume, and enhances power conversion efficiency. An associated method of power transfer employs structural symmetry in a resonant circuit of the system to ensure high efficiency line power transfer in both directions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119 of U.S. Application No. 62/055,458 filed 25 Sep. 2014, and entitled SINGLE CONVERSION STAGE BIDIRECTIONAL SOFT-SWITCHED AC-TO-AC POWER CONVERTER which is hereby incorporated herein by reference for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a bidirectional and isolated AC-to-AC power converters. 
       BACKGROUND OF THE INVENTION 
       [0003]    Today&#39;s switched mode power converters are typically required to provide insulation between the primary and secondary sides and to have high power density, high efficiency and low cost. In addition, many applications including uninterruptible power supplies (UPS), power supplies utilizing renewable energy sources (e.g. solar, wind, fuel cells), as well as aerospace power supplies require bidirectional (step-up and step-down) power conversion with isolated and regulated output. Examples of isolated and pulse width modulation (PWM) regulated bidirectional DC to DC (DC/DC) converters are described in U.S. Pat. No. 5,140,509, U.S. Pat. No. 5,255,174, U.S. Pat. No. 7,433,207, U.S. patent Ser. No. 63/700,501 and U.S. Pat. No. 6,205,035. The pulse width modulation control techniques employed in these converters typically feature so called “hard-switching” which can lead to significant switching losses and adversely impact the ability to achieve high power densities and high power conversion efficiencies. 
         [0004]    Zero-voltage switching (ZVS) and zero-current switching (ZCS) are well-established switching techniques. These techniques reduce switching losses, which in turn allows for higher switching frequencies, reduced size of magnetic components, increased power density and reduced cost. U.S. Pat. No. 5,539,630, U.S. Pat. No. 6,370,050 and U.S. Pat. No. 6,330,170 describe bidirectional converters that feature ZVS. These converters provide only one direction of power conversion. 
         [0005]    Line frequency AC to AC (AC/AC) converters based on line frequency power transformers are very common and simple to build. However, these converters are bulky and heavy and their prices are rising due to the rising cost of the raw materials involved, including copper, aluminum and silicon steel. 
         [0006]    Line frequency AC/AC converters based on switched mode technology, on the other hand, can be relatively very small, light and efficient. They are based on high frequency power conversion, which dramatically reduces the size and the price of the magnetic components involved. In addition, the price of switched mode AC/AC power converters is dropping because of the steadily reducing price of components. 
         [0007]    A major challenge in building line frequency switched mode AC/AC converters resides in handling reactive loads. The phase-lagging line current, for example, stores energy in the load reactance at the zero crossings of the line voltage. This load reactance energy has either to be temporarily stored, or recovered to the source in a controlled manner. The latter is needed to retain a sinusoidal or other desirable line current waveform. Failure of the energy storage/recovery process described above results in load overvoltage and component failures. 
         [0008]    One prior art method for dealing with stored energy in the load reactance is to store it temporarily in an energy storage component, such as, for example, a bulk capacitor. This principle is embodied in double-conversion line frequency switched mode AC/AC converters. Double conversion switched mode converters have two power stages connected in series. The first power stage is an AC/DC stage, which is followed by a second DC/AC stage. Such double conversion units have an intermediate DC bus with a large storage capacitor or a battery connected to that bus in order to deal with reactive line frequency loads. The main drawbacks of double conversion line frequency switched mode AC/AC converters are reduced power efficiency, increased complexity and cost. Examples of double conversion converters are provided in U.S. Pat. No. 8,664,037 B2, U.S. Pat. No. 7,679,941 B2, U.S. Pat. No. 6,879,062, U.S. Pat. No. 5,943,229, and U.S. Pat. No. 4,894,763. 
         [0009]    Prior art multi-resonant converters are typically series type frequency controlled resonant converters having three resonant components: a resonant capacitor, a resonant inductor and a magnetizing inductor. The resonant components of such multi-resonant converters can be selected in relation to the operating frequency such that the converter will provide zero voltage switching (ZVS) for the switching devices connected to the power source and zero current switching (ZCS) for the switching devices connected to the load. In addition, the resonant components can be selected so that the ZVS and ZCS can be maintained when operating from no-load to full-load conditions. In such prior art multi-resonant converters the output voltage in the “reverse” direction cannot be controlled and such systems can therefore be employed only for power conversion in one direction. A multiresonant converter design procedure for meeting the above criteria for conversion in one direction is outlined in R. Petkov, “Analysis and Optimisation of a Multi-Resonant Converter Employed in a Telecom Rectifier”, 21st International Telecommunication Energy Conference Intelec′99, Copenhagen, Denmark, June 1999, poster 41; and Diambo Fu et al., “1 MHz High Efficiency LLC Resonant Converters with Synchronous Rectifier” 38-th Annual Power Electronics Specialists Conference PESC′07, Orlando, Fla., USA, June 2007, pp. 2404-2410 which are hereby incorporated herein by reference. 
         [0010]    While several power converters known in the art are configured for bidirectional power conversion and allow controllable output in both directions, the so-called “Green Revolution” has tightened the conversion efficiency requirements to a point where efficiencies above 95% in both directions is demanded. 
         [0011]    U.S. Pat. No. 8,363,427 B2 by Anguelov et al. describes a DC/DC power converter that implements bidirectional soft switching. This reference is directed toward DC/DC converters for a single polarity of input voltage and is therefore based on rectifying circuitry that make it inapplicable to AC/AC systems. 
         [0012]    Against the above background, there remains a need for a high efficiency and low cost bidirectional line frequency AC/AC converter with a wide range of output voltage controllability in both directions of power transfer. 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly, the present invention relates to improved bidirectional AC/AC converters. Some embodiments feature soft, substantially loss-less switching operation and output voltage controllability in both directions of power transfer. In addition, certain embodiments of the present invention can maintain soft-switching operation and output voltage controllability within the entire load operating range, from zero load to full load. In particular, an embodiment of the present invention provides an improved series-type frequency controlled bidirectional AC/AC resonant converter that not only allows for a full control of the output voltage in both directions of power transfer, but, when components are properly dimensioned, can provide ZVS for the input section devices (i.e. the ones connected to the power source) and ZCS for the output section devices (i.e. the ones connected to the load) in both directions of power transfer and for all load conditions. The combination of ZVS and ZCS for all devices enhances the power conversion efficiency. The use of the same components for bidirectional power conversion is a major contributor of achieving very high power density. The substantially loss-less switching provided by embodiments described in the present specification allows for further increase in the power density by operating at higher switching frequencies, also described herein as “chopping frequencies”. Increasing the chopping frequency allows the size of all magnetic and filter components to be reduced. This is a distinct advantage of certain embodiments of the present invention compared with Pulse Width Modulation-controlled bidirectional converters that feature hard switching in at least one of the directions of power conversion. 
         [0014]    Various embodiments of the present invention can employ input, or primary section devices that are connected in full-bridge or half-bridge switcher (“chopper”) configurations that chop the input power source AC signal at the chopping frequency. The resulting modulated input power signal is then applied to a resonant network circuit, while the output or secondary section devices are connected in full-bridge or half-bridge configurations and are controlled via a control signal to restore the shape of the output signal to that of the input signal. When the direction of power transfer reverses, the control functions of the primary section devices and the secondary section devices are effectively swapped. That is the devices that have performed the signal restoration now perform the “chopping” function while the former chopper devices perform the signal restoration function. The resonant circuit of various embodiments of the present invention is arranged in such a way that, when power transfer reverses, both substantially lossless switching (i.e. ZVS and ZCS operation) and the output voltage controllability of the circuitry are maintained. 
         [0015]    This invention has several aspects. These include methods for AC/AC power conversion, AC/AC power converters, and systems which provide bidirectional AC/AC power converters between a source and a load. In some embodiments, the load is a reactive load. 
         [0016]    In a first aspect a method is provided for transferring electrical line power along opposing first and second paths through a closed loop series reactance network comprising first, second, and third phase-retarding elements and a phase-advancing element. The method comprises: providing to a first switcher circuit a first input bipolar AC electrical line voltage signal having a first input signal shape; first modulating the first input bipolar voltage signal at a first chopping frequency in the first switcher circuit; providing across the first phase-retarding element a first modulated input voltage signal from the first switcher circuit; extracting across the second phase-retarding element a first modulated resonator output voltage signal; and first restoring in a second switcher circuit the first input signal shape to the first modulated resonator output voltage signal to create a first restored output voltage signal. The first restoring may comprise second modulating the first output voltage signal at the first chopping frequency. Extracting the first modulated resonator output voltage signal may comprise extracting the first modulated resonator output voltage signal through a transformer. The method may further comprise reversing power transfer through the closed loop series reactance network. 
         [0017]    Reversing the power transfer may comprise: providing to the second switcher circuit a second input bipolar AC electrical line voltage signal having a second alternating input voltage amplitude and a second input voltage signal shape; third modulating the second input bipolar voltage signal at a second chopping frequency in the second switcher circuit; providing across the second phase-retarding element a second modulated input voltage signal from the second switcher circuit; extracting across the first phase-retarding element a second modulated resonator output voltage signal; and second restoring in the first switcher circuit the second input voltage signal shape to the second modulated resonator output signal to create a second restored output voltage signal. The second restoring may comprise fourth modulating the second output voltage signal at the second chopping frequency. The second and fourth modulating may comprise square wave modulating. Providing the second modulated input power signal may comprise providing the second modulated input voltage signal through a transformer. 
         [0018]    The first and second chopping frequencies may be greater than frequencies of the first and second line voltage signals. Preferably the first and second chopping frequencies are at least twenty times the frequencies of the first and second line voltage signals. More preferably the first and second chopping frequencies are at least 8 kHz. Most preferably, the first and second chopping frequencies are at least 16 kHz. The modulating may be square-wave modulating. 
         [0019]    In another aspect, an AC to AC line frequency bipolar power converter is provided. The converter comprises: a closed loop series reactance network comprising a phase-advancing element and first, second, and third phase-retarding elements all connected in series; a first power transfer tank circuit comprising the phase-advancing element, the first phase-retarding element, and the second phase-retarding element, a second power transfer tank circuit comprising the phase-advancing element, the first phase-retarding element, and the third phase-retarding element; a first switcher circuit connected in parallel with the third phase-retarding element and in series with the first power transfer tank circuit; and a first load circuit connected in parallel with the second phase-retarding element and in series with the second power transfer tank circuit. 
         [0020]    The first switcher circuit may comprise a set of first switcher input terminals disposed for selectably connecting to one of a first electrical load and a first electrical power source providing a first input bipolar AC electrical line voltage signal having a first input signal shape. The first load circuit may comprise a second switcher circuit, the second switcher circuit comprising a set of second switcher input terminals and a set of second switcher output terminals disposed and configured to connect selectably to one of a second electrical load and a second electrical power source providing a second input bipolar AC electrical line voltage signal having a second input signal shape. 
         [0021]    The first switcher circuit may be configured for modulating at a first chopping frequency the first bipolar input line voltage signal to provide to the first power transfer tank circuit a first modulated input power signal when the second electrical load is connected to the set of second switcher output terminals and the first switcher input terminals are connected to the first electrical power source. The second switcher circuit may be configured for restoring the first input voltage signal shape to a first transmitted voltage signal obtained from the first power transfer tank circuit by modulating the first transmitted voltage signal at the first chopping frequency. The modulating may be square-wave modulating. 
         [0022]    The second switcher circuit may be configured for modulating the second line voltage signal at a second chopping frequency to provide to the second power transfer tank circuit a second modulated input voltage signal when the first electrical load is connected to the set of first switcher input terminals and the second switcher input terminals are connected to the second electrical power source. The first switcher circuit may be configured for restoring the second input voltage signal shape to a second transmitted voltage signal obtained from the second power transfer tank circuit by modulating the second transmitted voltage signal at the second chopping frequency. The modulating may be square-wave modulating. 
         [0023]    The first load circuit may further comprise a transformer electrically connected between the set of second switcher input terminals and the second phase-retarding element. The first and second switcher circuits may comprise discrete semiconductor power switching devices for carrying and modulating the first and second input voltage signals. Each such power-switching device may comprise at least three device terminals. For example, each such device may comprise a power input terminal, a power output terminal, and a control terminal. The first and second switcher circuits may be half-bridge switcher circuits or full-bridge switcher units. The phase-advancing element may be a capacitor. At least one of the first, second, and third phase-retarding elements may comprise an inductor. 
         [0024]    According to another aspect, a line frequency bipolar power converter presented for converting AC power in opposing first and second directions through the power converter comprises: a closed loop series reactance network comprising a phase-advancing element and first, second, and third phase-retarding elements all connected in series, a first switcher circuit connected in parallel with the third phase-retarding element and disposed to be selectably connected to one of a first electrical load and a first AC electrical power source providing a first bipolar AC input voltage signal; and a first load circuit connected in parallel with the second phase-retarding element and comprising a second switcher network disposed to be selectably connected to one of a second electrical load and a second AC electrical power source providing a second bipolar AC input voltage signal. When the first switcher circuit is selectably connected to the first power source the second switcher circuit is connected to the second load for power transmission in the first direction; and when the second switcher circuit is selectably connected to the second power source the first switcher circuit is connected to the first load for power transmission in the second direction. 
         [0025]    The first and second switcher circuits may be configured for modulating respectively the first input voltage signal and signals derived from the first input voltage signal at a first chopping frequency when the first switcher circuit is selectably connected to the first power source; and the second and first switcher circuits are configured for modulating respectively the second input voltage signal and signals derived from the second input voltage signal at a second chopping frequency when the second switcher circuit is selectably connected to the second power source. The first and second switcher circuits may be configured for modulating at differing phases. 
         [0026]    In another embodiment, a line frequency bipolar power converter is provided for converting AC power in opposing first and second directions through the power converter comprising: a closed loop series reactance network comprising a capacitor and first, second, and third phase-retarding elements all connected in series, a first switcher circuit arranged to induce a signal in the third inductor and disposed to be selectably connected to one of a first electrical load and a first AC electrical power source providing a first bipolar AC input voltage signal; and a first load circuit connected across the second phase-retarding element and comprising a second switcher network disposed to be selectably connected to one of a second electrical load and a second AC electrical power source providing a second bipolar input AC voltage signal; wherein: when the first switcher circuit is selectably connected to the first power source the second switcher circuit is connected to the second load for power transmission in the first direction; and when the second switcher circuit is selectably connected to the second power source the first switcher circuit is connected to the first load for power transmission in the second direction. 
         [0027]    The power converter may further comprise a common conductor disposed to connect the first AC electrical power source to the second electrical load and the second AC electrical power source to the first electrical load. The first switcher circuit may be arranged to electromagnetically induce a signal in the third inductor via a first 1:1 transformer comprising the third inductor and an inductor connected to the first switcher, the first transformer arranged for a primary of the first transformer to electromagnetically induce in a secondary of the first transformer an equal and opposite voltage. The second switcher circuit may be arranged to electromagnetically induce a signal in the first inductor via a second 1:1 transformer comprising the first inductor and an inductor connected to the first switcher, the second transformer arranged for a primary of the second transformer to electromagnetically induce in a secondary of the second transformer an equal and opposite voltage. 
         [0028]    Another aspect provides an AC/AC power converter comprising first and second line terminals for connecting to an AC power line; a closed-loop resonant circuit, first and second switcher circuits and a controller. The closed-loop resonant circuit comprises an input phase-retarding leg and an output phase-retarding leg. A first end of the input phase-retarding leg is connected to a first end of the output phase-retarding leg by a first connecting leg. A second end of the input phase-retarding leg is connected to a second end of the output phase-retarding leg by a second connecting leg. The first and second connecting legs each comprise at least one phase-shifting component. The first switcher circuit is connected between the line terminals and the input leg of the closed loop resonant circuit. The first switcher circuit comprises a plurality of switches controllable between: a first configuration in which a line AC waveform alternating at a line frequency presented between the first and second line terminals is applied across the input leg of the closed loop resonant circuit with a first line polarity; and a second configuration in which the line AC waveform is applied across the input leg of the closed loop resonant circuit with a second line polarity opposite to the first line polarity. The second switcher circuit is connected between the output leg of the closed loop resonant circuit and a load. The second switcher circuit comprising a plurality of switches controllable between: a first configuration in which a chopped AC waveform presented across the output leg of the closed loop resonant circuit is applied across the load with a first chopped waveform polarity; and a second configuration in which the chopped AC waveform is applied across the load with a second chopped waveform polarity opposite to the first chopped waveform polarity. The controller is connected to drive each of the first and second switcher circuits to alternate between their respective first and second configurations at a chopping frequency. 
         [0029]    Further aspects of the invention and features of various example embodiments are described below and/or illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Embodiments and applications of the invention are illustrated by the attached non-limiting drawings. The attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. 
           [0031]      FIG. 1  is a schematic circuit showing a single conversion stage bidirectional soft-switched AC/AC power converter employing a full-bridge primary or chopping section and a full-bridge secondary or restoration section according to an example embodiment. 
           [0032]      FIG. 2  is a schematic circuit showing a single conversion stage bidirectional soft-switched AC/AC power converter employing a half-bridge primary or chopping section and a half-bridge secondary or restoration section according to an example embodiment. 
           [0033]      FIG. 3A  shows an equivalent circuit of the example embodiment of  FIG. 1  during power transfer from the primary section to the secondary section. 
           [0034]      FIG. 3B  shows an equivalent circuit of the example embodiment of  FIG. 1  during power conversion from the secondary section to the primary section. 
           [0035]      FIGS. 4A ,  4 B,  4 C, and  4 D show four different example implementations of switcher sub-circuits that may be applied as switches in the single conversion stage bidirectional soft-switched AC/AC power converters of  FIG. 1  and  FIG. 2 . 
           [0036]      FIG. 5A  is a schematic circuit showing a single conversion stage bidirectional soft-switched AC/AC power converter employing a full-bridge primary or chopping section and a full-bridge secondary or restoration section without a transformer according to an example embodiment. 
           [0037]      FIG. 5B  is a schematic circuit showing a single conversion stage bidirectional soft-switched AC/AC power converter having no step-up/step-down transformer and having a common conductor connecting an AC power source to an electrical load according to an example embodiment. 
           [0038]      FIG. 6  is a flow diagram illustrating an example method for bidirectional power transfer through a single conversion stage bidirectional soft-switched AC/AC resonant power converter according to an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. 
         [0040]    The vast majority of AC loads are reactive. A power converter for supplying AC power to such loads should have bidirectional capabilities to permit interchange of reactive energy between the load and the source. Against the description in the “Background” section of double conversion power converters, it is desirable to have a power converter that improves on the efficiency and/or cost effectiveness of double conversion power converters. As described herein, a line frequency switched mode AC/AC converter built on a single power conversion stage without an intermediate energy storage component can operate so that energy stored in the load reactance can be recovered to the source. That is, the single power conversion stage can provide bidirectional AC power transfer capabilities. A failure to provide a path for interchange of reactive energy from the load to the source may result in load overvoltage in the case of an inductive load or load overcurrent in the case of a capacitive load, with possibly destructive consequences. To address the changing polarity of the AC source, a converter should also be bipolar. 
         [0041]    Another challenge in bidirectional power transfer is that of obtaining high efficiency in both directions of conversion. Soft switching of the switching devices is a very effective technique to reduce the switching losses and increase power conversion efficiency, and is discussed in more detail below. 
         [0042]      FIG. 1  is a schematic showing a single conversion stage bidirectional soft-switched AC/AC power converter  100  according to an example embodiment. In the case of power transfer from AC source  101  to load  102 , a full-bridge switcher circuit  107  comprising controlled switching subcircuits  103 ,  104 ,  105 , and  106  is connected to AC voltage source  101 . The terms “chopper circuit” and “switcher circuit” are employed interchangeably in the present specification. By way of example, AC source  101  may be providing an AC voltage at line frequency, which is usually nominally 50 or 60 Hz, depending on the territory. A full-bridge switcher circuit  127  comprising controlled switching subcircuits  123 ,  124 ,  125 , and  126  is connected to load  102 . Suitable controlled switching devices for use within switching subcircuits  103 ,  104 ,  105 ,  106 ,  123 ,  124 ,  125 , and  126  may include by way of non-limiting example MOSFETs, IGBTs, GTOs, and BJTs.  FIGS. 4A ,  4 B,  4 C, and  4 D show suitable non-limiting example embodiments of switcher subcircuits  103 ,  104 ,  105 ,  106 ,  123 ,  124 ,  125 , and  126  in more detail. Such switcher circuits may be built, for example, using standard electronic components as listed at the end of this specification.  FIG. 4A , when considered with  FIG. 1  and  FIG. 2 , shows that suitable full-bridge or half-bridge switcher circuits may be employed that have no discrete semiconductor diode devices. In such embodiments, all line power semiconductor switching devices carrying and modulating input power signals in the switcher subcircuits have three or more device terminals. 
         [0043]    Switching circuits  103 ,  104 ,  105 , and  106 , are turned on and off with an approximately 50% duty cycle and their switching frequency is controlled to allow full-bridge switcher circuit  107  to produce a square-wave voltage waveform with 50% duty cycle over source circuit terminals  108  and  109 . In the present specification, we refer to the frequency at which the input AC voltage is chopped (e.g. by full-bridge switcher circuit  107 ) as the “chopping frequency”. The chopping frequency may be adjustable. The frequency of the input voltage supplied by AC source  101  is referred to as the “line frequency” in the present specification. Electrical power supplied to the converter for conversion is herein referred to as “electrical line power” and its voltage and current alternate at the line frequency. The chopping frequency is preferably at least 20 times the line frequency, and is more preferably greater than 8 kHz, and most preferably greater than 16 kHz. 
         [0044]    In operation, switching circuit  103  and its diagonal partner switching circuit  106  are switched mutually in phase. Switching circuit  104  and its diagonal partner switching circuit  105  are switched mutually in phase. However, the two sets of partner switching circuits are switched 180 degrees out of phase with each other, so that, when switching circuits  103  and  106  are open and non-conducting, then switching circuits  104  and  105  are closed and conducting. This switching arrangement of full-bridge switcher circuit  107  also applies to full-bridge switcher circuit  127 , in that switching circuits  123  and  126  are operated mutually in phase and switching circuits  124  and  125  are operated mutually in phase, but the two sets of switcher circuits are operated 180 degrees out of phase. The phase of switching circuit  103  is substantially the same as the phase of switching circuit  123 . The duration of the control pulses of all switching devices is substantially half of the switching frequency period. That is, they operate with substantially 50% duty cycle. 
         [0045]    A closed loop series reactance network is connected across source circuit terminals  108  and  109  to be driven by the output voltage from full-bridge switcher/chopper circuit  107 . The series reactance network comprises three phase-retarding elements, symbolically represented by inductor symbols, and one phase-advancing element, symbolically represented by a capacitor symbol. The mix of phase-retarding and phase-advancing elements renders the series reactance network a resonant network. The term “phase-retarding” is employed in the present specification to describe retarding the phase of the current through the element with respect to the phase of the voltage across the element. By contrast, the term “phase-advancing” is used in the present specification to describe advancing the phase of the current through the element with respect to the phase of the voltage across the element. 
         [0046]    A first phase-retarding element  113  is connected across the source circuit terminals  108  and  109  of full-bridge switcher or chopper circuit  107 . In one embodiment, phase-retarding element  113  is an inductor, being in this specific case the first inductor of interest. In general, phase-retarding element  113  may be any device or circuit providing suitable phase retardation. 
         [0047]    The series reactance network comprises a second phase-retarding element  111  connected across primary  118  of transformer  116 , and thereby across load circuit terminals  115  and  114 . In general phase-retarding element  111  may be any device or circuit providing suitable phase retardation. In one specific embodiment, second phase-retarding element  111  is an inductor, being in this case the second inductor of interest. Inductor  111  may optionally be embedded in the magnetic structure of transformer  116 . The inductance of phase-retarding element  111 , often called the “magnetizing inductor”, can be controlled by providing an air gap in the magnetic core and adjusting its length. 
         [0048]    The series reactance network further comprises a third phase-retarding element  110  and a phase-advancing element  112 . In one embodiment, phase-advancing element  112  is a capacitor. In general phase-advancing element  112  may be any device or circuit providing suitable phase advancement. The third phase-retarding element  110  is connected between the second phase-retarding element  111  and one of source circuit terminals  108  and  109 , and the phase-advancing element  112  is connected between the second phase-retarding element  111  and the other of source circuit terminals  108  and  109 . In general phase-retarding element  110  may be any device or circuit providing suitable phase retardation. In one embodiment, phase-retarding element  110  is an inductor. 
         [0049]    Full-bridge switcher circuit  127  is connected across the secondary  117  of transformer  116 , and thereby across load circuit terminals  115  and  114 . Switcher circuit  127  is driven at the same chopping frequency with respect to the signal driving switcher circuit  107  to thereby restore the signal produced over load  102  to substantially the same form as that of the signal received by switcher circuit  107  from AC source  101 . For this reason, we refer in the present specification to switcher circuit  127 , when operated in this configuration, as a “restoration circuit”. In an idealized system that signal shape might very well be sinusoidal, but in practical power systems the signal shape may be distinctly different from sinusoidal. The user, or a suitable controller employed by the user, may control the chopping frequency of the driver control signals for switcher circuits  107  and  127 . 
         [0050]    In the case of power transfer from the right hand side to left hand side of the circuitry in  FIG. 1 , the power source and the load effectively exchange their places, i.e. load impedance  102  becomes an AC voltage source, while AC voltage source  101  becomes a load impedance. In addition, the switcher circuit  127  becomes a controlled switcher with controlled switching frequency and approximately 50% duty cycle that produces square-wave voltage with adjustable frequency across the load circuit terminals  115  and  114  of the primary  118  of transformer  116 . Furthermore, the full-bridge switcher circuit  107  is now in the role of a restoration circuit and can now be driven at the same frequency as the signal driving switcher circuit  127 . This allows the signal produced over element  101 , now the load impedance, to be restored to the same form as that of the voltage signal received by switcher circuit  127  from element  102 , which is now the AC source. 
         [0051]    The above switching arrangement provides a path for any reactive currents to flow from the load  102  to the source  101 . These currents are controlled by the controller  150  in a manner ensuring that the voltage produced by the stored energy of the inductive load  102  and reflected to the source side is higher than the voltage across source  101 . These currents therefore flow back to the source  101 , system  100  thereby fulfilling the requirement of restoring any load reactance energy to the source  101  in a controlled manner. 
         [0052]    The series reactance network comprising elements  110 ,  111 ,  112 , and  113  is employed as a resonant network that may be excited with equal effect across terminals  108  and  109  while loaded across terminals  114  and  115 , on the one hand, as when excited across terminals  114  and  115  while loaded across terminals  108  and  109 , on the other hand. The difference between these two scenarios is exactly that which pertains when the direction of conversion, as explained above, reverses. 
         [0053]    Controller  150  may monitor the operation of a power converter as described herein. In the example embodiments of  FIGS. 1 and 2 , controller  150  is configured by means of a suitable control algorithm to determine the instantaneous current through source  101  by means of ampere meter  132 , the instantaneous current through load  102  by means of ampere meter  142 , the instantaneous voltage over source  101  by means of voltmeter  131 , and the instantaneous voltage over load  102  by means of voltmeter  131 . Controller  150  is further configured by the control algorithm to supply switcher circuits  107  and  127  with respectively first and second chopping signals at the same frequency via respectively chopper control lines  133  and  143 . The first and second chopping signals control the gates of the various three-terminal devices in switcher subcircuits  103 ,  104 ,  105 ,  106 ,  123 ,  124 ,  125 , and  126 , shown in more detail in  FIG. 4   a ,  FIG. 4   b ,  FIG. 4   c , and  FIG. 4   d.    
         [0054]    Controller  150  may have, stored within a suitable memory, reference values for the source and load currents and for the source and load voltages. The controller  150  may also maintain internal sinusoids, or any other desired waveforms, synchronized with the zero crossings in the load current and load voltage, and it may use these waveforms as reference waveforms to be followed by the voltage and/or current of the power being transferred. Since the chopping frequency is much higher than the AC line frequency, the line current and voltage signal shapes may be adjusted very rapidly within one AC line voltage cycle. 
         [0055]    In some embodiments, a controller  150  monitors a voltage being delivered across the load and compares the monitored voltage to a reference value. The reference value may be time-varying. For example, the reference value may vary according to a desired output waveform. Controller  150  may select the reference value to compare to the monitored voltage at the current time by tracking a phase of the output AC waveform being applied across the load. In response to the comparison, controller  150  may control the chopper frequency. If the monitored voltage exceeds the reference value controller  150  may increase the chopper frequency. Conversely, if the monitored voltage is below the reference value controller  150  may decrease the chopper frequency. In some embodiments, the amount of increase or decrease of the chopper frequency is variable and is selected based on a magnitude of the difference between the monitored voltage and the current reference value. Such controllers  150  may be applied to drive any of the embodiments disclosed herein. 
         [0056]    With a controller  150  operating as described above, electrical power may flow from source to load. However, where the load is a reactive load, energy can also flow in the reverse direction, from load to source, during portions of a cycle of the AC line voltage. This reverse flow of energy can be regulated by controller  150  which varies the chopper frequency in real time to maintain the voltage delivered to the load according to a desired waveform. 
         [0057]    In the embodiment illustrated in  FIG. 1 , during energy transfer from left to right, controller  150  measures the instantaneous current through load  102  by means of ampere meter  142  and the instantaneous voltage over load  102  by means of voltmeter  141  as well as the instantaneous current through source  101  as measured by ampere meter  132  and the instantaneous voltage over source  101  as measured by voltmeter  131 . It then adjusts the frequency of the first and second chopping signals, ensuring that the current through the switches lags the voltage across the switches. It also ensures that the load voltage follows the reference waveform stored in the controller, and that the load current does not exceed the reference value stored in the controller. 
         [0058]    The phase of the current signal through switching pairs  103 ,  106  and  104 ,  105  of the switcher  107  lags the phase of the voltage produced across these switching pairs at frequencies above a certain minimum chopping frequency. This is the condition to be satisfied in order to provide substantially lossless soft-switching. This minimum chopping frequency is determined by the detailed choice of component values of elements  111 ,  110 , and  112 , and is distinctly higher than the resonance frequency of the resonant circuit. At or near the minimum chopping frequency, the voltage across the load  102  is at a maximum. 
         [0059]    At chopping frequencies greater than this minimum chopping frequency, the converter is in the soft-switching range where substantially lossless power conversion may be maintained over a wide chopping frequency range. As the chopping frequency is increased above the minimum chopping frequency, the phase lag of the current through the switching pairs  103 ,  106  and  104 ,  105  with respect the voltage across the switching pairs  103 ,  106  and  104 ,  105  monotonically increases, while the voltage across the load  102  monotonically decreases. 
         [0060]    When power is transferred in the reverse direction through the system  100 , the minimum chopping frequency is determined by the detailed choice of component values of elements  113 ,  110 , and  112 . Since, for reverse transfer of power, element  113  replaces element  111  in the determination of the minimum chopping frequency, the corresponding minimum chopping frequency is different from the value of the minimum chopping frequency for the forward power transfer configuration. As a result, the chopper frequencies will almost always differ for forward and reverse power transfer, but may under some circumstances be the same. During this reverse transfer, controller  150  adjusts the chopping frequency on the basis of the instantaneous current through source  101  as measured by ampere meter  132 , the instantaneous current through load  102  as measured by means of ampere meter  142  and the instantaneous voltage over load  102  as measured by means of voltmeter  141 . More specifically, it adjusts the frequency of the first and second chopping signals, ensuring that the current through the switches lags the voltage across the switches. It also ensures that the load voltage follows the reference waveform stored in the controller. 
         [0061]      FIG. 2  shows an alternative embodiment of a single conversion stage bidirectional soft-switched AC/AC power converter in the form of converter  200 . Elements numbered the same as in  FIG. 1  are similar types of elements, though their precise values may differ from the identically numbered elements in  FIG. 1 . Converter  200  differs from converter  100  of  FIG. 1 . Switching circuits  103 ,  104 ,  123  and  124  have all been replaced by capacitors, thereby making switcher/chopper circuit  207  and switcher/restoration circuit  227  half-bridge switchers. Other detailed arrangements of electronic switching circuits for switchers  107 ,  127 ,  207 , and  227 , are contemplated by the inventors, all such switching circuits ensuring that the chopper circuit and the restoration circuit in a given converter operate as synchronous phase controlled switches. 
         [0062]    In other embodiments employing the same elements as in  FIG. 1 , switcher circuit  107  may be connected over terminals  108  and  115  instead of terminals  108  and  109  of the closed loop series reactance network. Similarly, yet further embodiments employing the same elements as  FIG. 2  allow switcher circuit  207  to be connected over terminals  108  and  115  instead of terminals  108  and  109 . The distinction is merely in which of phase-retarding elements  110  and  113  spans the input terminals to the series reactance network. 
         [0063]    Returning to  FIG. 1  as example, the interchanging of the functions of the switcher circuits  107  and  127  when the source and the load exchange places is schematically illustrated in  FIG. 3A  and  FIG. 3B . These two figures are simplified versions of converters as illustrated in  FIG. 1  and  FIG. 2  during respectively power transfer from left to right ( FIG. 3A ), and from the right to left ( FIG. 3B ) through the bidirectional converter. In selecting components for such a design, it is found that for high efficiency power transfer the inductance of phase-retarding element  111  is typically larger than the inductance of phase-retarding element  110 . 
         [0064]      FIG. 3A  and  FIG. 3B  represent equivalent circuits of AC/AC power converter  100  when operated in two different modes, representing opposing power transfer directions. In converter  310  of  FIG. 3A  the power transfer is from the left to right along a first direction  311  in a first power transfer mode through bidirectional converter  100  configured as left-to-right power converter  310 , while in converter  320  of  FIG. 3B  the power transfer is from the right to the left along an opposing second direction  321  in a second power transfer mode through bidirectional converter  100  configured as right-to-left power converter  320 .  FIGS. 3A and 3B  may be applied exactly the same way to converter  200  of  FIG. 2 . All elements in  FIG. 3A  and  FIG. 3B  numbered the same as in  FIG. 1  and  FIG. 2  are the same types of elements, but may have different values. 
         [0065]    As shown in each of  FIGS. 3A and 3B  the electronic circuit includes terminals  114  and  115  and the series reactance network which comprises reactance elements  110 ,  111 ,  112 ,  113 . Phase-retarding element  111  is connected across a first set of resonant circuit terminals  115  and  114 , while phase-retarding element  113  is connected across a second set of resonant circuit terminals  108  and  109 . Phase-advancing element  112  and third phase-retarding element  110  are connected in series with the primary side of transformer  116  while second phase-retarding element  111  is connected in parallel with the primary side of transformer  116 . 
         [0066]    In the first power transfer mode shown in  FIG. 3A  and being along direction  311 , a first load circuit restoration circuit  327  and transformer  116  are connected across terminals  115  and  114 . In  FIG. 3 , restoration circuit  327  can be either restoration circuit  127  of  FIG. 1  or restoration circuit  227  of  FIG. 2 , or any other restoration/switcher circuit that conforms to the requirements described herein. In the first power transfer mode, phase-advancing element  112  and third phase-retarding element  110  are connected in series with the first load circuit, while the second phase-retarding element  111  is connected in parallel with the first load circuit. The shapes of voltage signals at the various stages of the converter are shown above the circuit in  FIG. 3A . 
         [0067]    In a second transfer mode, shown in  FIG. 3B  and being along direction  321 , a second load circuit comprising switcher circuit  307  is connected across terminals  108  and  109 . In  FIG. 3B , switcher circuit  307  can be either switcher circuit  107  of  FIG. 1  or switcher circuit  207  of  FIG. 2 , or any other switcher circuit that conforms to the restoration circuit requirements described herein. In the second power transfer mode, phase-advancing element  112  and third phase-retarding element  110  are connected in series with the second load circuit, while the first phase-retarding element  113  is connected in parallel with the second load circuit. The shapes of signals at the various stages of the converter are shown above the circuit in  FIG. 3B . 
         [0068]    The resonant circuit in  FIG. 3A  and  FIG. 3B  is of the same type for both directions of power transfer, and it performs the same role for both directions of power transfer. For example, with the load section connected across phase-retarding element  111  in  FIG. 3A , the resonant components involved in the power transfer and which therefore determine the voltage gain of the converter  310  (i.e. the ratio between the output voltage and the input voltage) are phase-retarding elements  110  and  111  together with phase-advancing element  112 . Phase-retarding element  113  is connected directly across the output terminals of the chopper circuit  307  and therefore it does not take part in power transfer. That is, element  113  does not affect the voltage gain characteristics of the resonant circuit. Accordingly, a first power transfer tank circuit  314 , comprising phase-retarding elements  110  and  111  together with phase-advancing element  112 , is provided by the electronic circuit of  FIG. 3A . 
         [0069]    With the load section connected across phase-retarding element  113  in  FIG. 3B , the resonant components involved in the power transfer and which therefore determine the voltage gain of the converter  310  (i.e. the ratio between the output voltage and the input voltage) are phase-retarding elements  110  and  113  together with phase-advancing element  112 . Phase-retarding element  111  is effectively connected across the output terminals of the chopper circuit  327  via the transformer  116  and therefore it does not take part in power transfer. That is, element  111  does not affect the voltage gain characteristics of the resonant circuit. Accordingly, a second power transfer tank circuit  324 , comprising phase-retarding elements  110  and  113  together with phase-advancing element  112 , is provided by the electronic circuit of  FIG. 3B . 
         [0070]    This very desirable equality of the resonant configurations in both directions of the power transfer is due to phase-retarding element  113 . In this example embodiment, phase-retarding element  113  is implemented as an external component. First power transfer tank circuit  314  has the same structural resonant configuration as second power transfer tank circuit  324 , phase-retarding element  111  of first power transfer tank circuit  314  being replaced by phase-retarding element  113  of second power transfer tank circuit  324 . That is, the combination of reactance elements employed by the first power transfer tank circuit  314  has the same structural resonant configuration as the combination of reactance elements employed by the second power transfer tank circuit  324 . 
         [0071]    To maintain the desirable characteristics of converter  310  in the opposing direction of power conversion, circuits such as those shown in  FIGS. 1 ,  2 ,  3 A, and  3 B provide the same resonant configuration in both directions of power conversion. Referring back to  FIG. 3A  and  FIG. 3B , which represent a simplified version of the circuits of  FIG. 1  and  FIG. 2 , during both directions of power transfer, the phase-retarding element  113  advantageously provides desired symmetry of both of the resonant configurations. 
         [0072]    Phase-retarding element  113  in the example embodiment of  FIG. 1  makes the resonant configurations symmetrical in both directions of power transfer resulting in step-down/step-up voltage conversion that can be accompanied by substantially loss-less ZVS/ZCS operation in both directions of power conversion. The exact values of resonant characteristics in both directions of power transfer are governed by the ratios of the inductances of phase-retarding elements  111  and  113  to the inductance value of phase-retarding element  110 . In an example case the turns ratio of transformer  116  is 1:1, phase-retarding elements  111  and  113  are equal and the input/output terminals of the circuit are equally loaded (during the bidirectional transfer). Under these circumstances bidirectional converter  100  will exhibit exactly the same DC-voltage gain and ZVS/ZSC characteristics in both directions of power transfer. 
         [0073]    It is to be noted that in some example embodiments, various ones of the corresponding resonant components employed to establish symmetrical resonant configurations in both directions of power transfer have different values. In some example embodiments, a value of a resonant component employed in a first resonant circuit is different from a value of a corresponding resonant component employed by a second resonant circuit that has the same resonant configuration as the first resonant circuit. In other embodiments, resonant circuits having different resonant configurations may be employed in each direction of power transfer. However, the frequency of chopper/switcher circuits is adjustable and fully under the control of the user, or a suitable controller provided by the user. It is therefore possible to program such a controller to adjust the frequency to ensure substantially lossless conversion in both directions through the circuits of  FIG. 1  and  FIG. 2 , based on the principles explained with reference to  FIG. 3A  and  FIG. 3B . 
         [0074]      FIG. 5A  shows a single conversion stage bidirectional soft-switched resonant AC/AC power converter according to another example embodiment. The converter is based on components and elements identical to those of  FIG. 1 , with the difference that transformer  116  of  FIG. 1  is absent. The first load circuit in this case comprises switcher circuit  127 . To the extent that the resonant circuit comprising reactance elements  110 ,  111 ,  112 , and  113  can have greater than unity voltage gain, as measured between the voltage across element  111  relative to the voltage across element  113 , the circuit of  FIG. 5  may be employed as a bidirectional soft switching resonant voltage converter in situations where a transformer is not desired or not appropriate. The converter may be operated the same way as that in  FIG. 1 , except that the chopping frequencies required to achieve desired voltages required for suitable signal restoration in the switcher/chopper circuits will be different from those in  FIG. 1 . The voltage converter of  FIG. 1  may in fact be viewed as the converter of  FIG. 5A  with an additional transformer  116  to achieve larger step-up voltages or smaller step-down voltages. The matter of non-unity voltage gain in resonant circuits of this general type is described in more detail in U.S. Pat. No. 8,363,427 B2 by Petkov et al, the specification of which is hereby incorporated by reference in full in the present specification for all purposes. 
         [0075]      FIG. 5B  shows a single conversion stage bidirectional soft-switched resonant AC/AC power converter  550  according to another example embodiment that does not employ a step-up/step-down transformer. Elements identically numbered to elements in  FIG. 5A  are of the same type as in  FIG. 5A , but have their own operational specifications and values, including, for example, their reactances. First center tap switcher circuit  507 , comprising switching subcircuits  103  and  104 , drives phase-retarding element  513   b , either directly via switching subcircuit  104  or indirectly via switching subcircuit  103 . Switching subcircuit  104  modulates the signal from power source  101  to a chopping frequency. The resulting signal is connected to the closed loop series reactance network comprising phase-advancing element  112  and phase-retarding elements  513   b ,  110 , and  511   a.    
         [0076]    Switching subcircuit  103 , in its turn, is connected to phase-retarding element  513   a . Phase-retarding elements  513   a  and  513   b , in the form of two inductors, may together form a 1:1 transformer  513  in which inductors  513   a  and  513   b  are mutually disposed and arranged to electromagnetically induce mutually opposed voltages as shown in  FIG. 5   b . This arrangement allows inductor  513   a  to induce a voltage equal and opposite to its own voltage in inductor  513   b.    
         [0077]    In this embodiment, at least one of switching subcircuits  103  and  104  is at any moment in time driving the power transfer tank circuit formed by reactance elements  112 ,  110  and  511   a , while all the stages of the system as a whole maintain a single unbroken common line  515 , shown at the bottom of  FIG. 5B . This allows the system to conform to various safety standards. 
         [0078]    At the output side of system  550  of  FIG. 5B , second center tap switcher circuit  527  either takes its input directly across reactance element  511   a  under the action of switcher subcircuit  124 , or by means of induction from reactance element  511   a  via reactance element  511   b  under the action of switcher subcircuit  123 . Phase-retarding elements  511   a  and  511   b , in the form of two inductors, may together form a 1:1 transformer  511  in which inductors  511   a  and  511   b  are mutually disposed and arranged to electromagnetically induce mutually opposed voltages as shown in  FIG. 5   b . This arrangement allows inductor  511   a  to induce a voltage equal and opposite to its own voltage in inductor  511   b.    
         [0079]    As in the foregoing embodiments, the power signal from source  101  is modulated at a first chopping frequency by switcher circuit  507  and supplied to the first power transfer tank circuit comprising elements  112 ,  113  and  511   a . From there the signal tapped over element  511   a  is transferred to switcher circuit  527 , directly or indirectly, where the signal shape of the signal from source  101  is restored to the output signal of the power transfer tank by suitable modulation at the first chopping frequency. This restored signal is then supplied to load  102 . 
         [0080]    For power transfer in the reverse direction, system  550  is electronically symmetrical in its structure, the order of elements  112  and  110  being immaterial to the working of the system  550 . In this case, load  102  is replaced by a source that provides a second input power signal, switcher circuit  527  does the modulation of this second input power signal at a second chopping frequency. The second chopping frequency will usually be different from the first chopping frequency, but may under some circumstances be the same as the first chopping frequency. The power is transmitted via a second power transfer tank circuit defined by elements  110 ,  112 , and  513   b . Switcher circuit  507  in this case then restores the shape of the second input signal to the power signal transmitted over the second power transfer tank circuit. In this reverse power transfer, element  511   b  induces an equal and opposite voltage in element  511   a  and element  513   b  induces an equal and opposite voltage in element  513   a.    
         [0081]    In a further aspect, illustrated by the flow chart of  FIG. 6 , a method  600  is provided for transferring electrical line power along opposing first and second paths through a closed loop series reactance network comprising first, second, and third phase-retarding elements and a phase-advancing element. The method comprises: providing  610  to a first switcher circuit (e.g. switcher circuit  107  of  FIG. 1 ) a first input AC electrical line voltage signal having a first input voltage signal shape; first modulating  620  the first input voltage signal at a first chopping frequency in the first switcher circuit  107 ; providing  630  to a first set of terminals (e.g. terminals  108  and  109  across the first phase-retarding element  113  of the series reactance network) a first modulated input voltage signal from the first switcher circuit  107 ; extracting  640  from a second set of terminals (e.g. terminals  114  and  115  across the second phase-retarding element  111  of the series reactance network) a first modulated resonator output voltage signal; first restoring  650  in a second switcher circuit (e.g. switcher circuit  127 ) the first input voltage signal shape to the first modulated resonator output voltage signal to create a first restored output voltage signal; and reversing  660  power transfer through the series reactance network. In the present specification the phrase “first modulated resonator output voltage signal” is used to describe the voltage signal taken directly from terminals  114  and  115  on the closed loop reactance network or the signal indirectly taken from the closed loop reactance network through transformer  116 . 
         [0082]    Reversing  660  the power transfer through the closed loop series reactance network comprises providing  662  to the second switcher circuit  127  a second input AC electrical line voltage signal having a second input voltage signal shape; second modulating  664  the second input voltage signal at a second chopping frequency in the second switcher circuit  127 ; providing  666  to the second set of terminals  114  and  115  across the second phase-retarding element  111  of the resonant circuit a second modulated input voltage signal from the second switcher circuit  127 ; extracting  668  from the first set of terminals  108  and  109  across the first phase-retarding element  113  of the series reactance network a second modulated resonator output voltage signal; second restoring  669  in the first switcher circuit  107  the second input voltage signal shape to the second modulated resonator output signal to create a second restored output voltage signal. The providing  666  a second modulated input voltage signal from the second switcher circuit may be directly from the second switcher circuit  127  or may be indirectly via the transformer  116 . 
         [0083]    Changes in the chopping frequency may alter the direction of the net flow of power through the closed loop reactance network. In some embodiments of method  600  the chopping frequency is varied continuously or nearly continuously. The chopping frequency may be set based on parameters (e.g. monitored voltages and/or currents) of the closed loop reactance network. The chopping frequency may be changed a plurality of times in a cycle or half-cycle of the line frequency. In some embodiments the chopping frequency is set to provide power transfer in a forward direction (from a source to a reactive load) during a first part of a half-cycle at the line frequency and is set to provide transfer of reactive power from the reactive load back to the source in a later part of the half-cycle while maintaining a net power flow in the forward direction over a full cycle at the line frequency. 
         [0084]    The first and second frequencies are preferably at least twenty times as high as a frequency of the electrical line voltage, preferably at least 8 kHz, and most preferably at least 16 kHz. The first restoring  650  comprises third modulating the first output voltage signal at the first chopping frequency. The second restoring [ 669 ] comprises fourth modulating the second output voltage signal at the second chopping frequency. The first, second, third, and fourth modulating may comprise square wave modulating or chopping. 
         [0085]    The first and second restoring further respectively comprise transferring the electrical power in opposing directions  311  and  321  of  FIG. 3A  and  FIG. 3B  respectively along a segment of the series reactance network comprising the third phase-retarding element and the phase-advancing element separated from the third phase-retarding element by the first and second phase-retarding elements. As already described, in some embodiments, the various phase-retarding elements may be inductors and the phase-advancing element may be a capacitor. 
         [0086]    The method may perform step-up-transformerless power conversion (using apparatus as shown, for example, in  FIGS. 5A and 5B ). The step-up-transformerless power conversion may comprise maintaining a single common voltage line between source  101  and load  102 . The maintaining a single common voltage line between source and load may comprise modulating an input voltage signal in a center tap switcher circuit  507 ,  527 . The maintaining a single common voltage line between source  101  and load  102  may comprise transferring power into a first and second power transfer tank circuits, comprising elements  112 ,  110 , and  511   a  on the one hand and elements  110 ,  112 , and  513   b  on the other, by 1:1 reverse polarity inducing of a voltage in a reactance element of the closed loop series reactance network consisting of elements  513   b ,  112 ,  110 , and  511   a . For example, for the forward transfer of power when switcher subcircuit  103  is conductive, the inducing is into element  513   b  and  511   b . In the reverse transfer direction, when switcher subcircuit  123  is conductive, the inducing is into elements  511   a  and  513   a.    
         [0087]    Referring back to  FIGS. 3A and 3B , it is to be noted that costs are advantageously reduced in this example embodiment since the first power transfer tank circuit  314  shares at least two common resonant components with the second power transfer tank circuit  324 . These are third phase-retarding element  110  and phase-advancing element  112 . Each of the first and second power transfer tank circuits  314  and  324  includes only a single different component. Specifically, the first power transfer tank circuit  314  includes a first resonant component, being phase-retarding element  111  that is different from a second resonant component, being phase-retarding element  113  employed by the second power transfer tank circuit  324 . While different example embodiments are contemplated for the circuitry surrounding the series reactance network, the first power transfer tank circuit  314  in forward transfer and the second power transfer tank circuit  324  in reverse transfer through the same converter always have at least one phase-advancing element and one phase-retarding element in common. 
         [0088]    Series type bidirectional line frequency AC/AC resonant converters as described in the present specification may be designed to provide a wide range of output voltage controllability in both directions of power transfer. Such circuits can provide, when needed, galvanic isolation between the power source and the load. By employing the same components for power conversion in both directions of power transfer, some embodiments can be very cost effective. The resonant converter of the present specification also provides for substantially loss-less switching operation in both directions of power transfer over the whole range of load conditions, from no load to full load, and substantially loss-less switching operation for all semiconductor devices in the circuitry. 
         [0089]    Example applications of bidirectional line frequency AC/AC resonant converter  100 ,  200 ,  500 ,  550  described in this specification include switched mode distribution transformers that step down, for example, the medium transmission voltage (from 2 kV to 35 kV) from suburban power distribution substations to 120V/208/240V required by ordinary households. Present transformers operate at mains frequency and are bulky, heavy and uncontrollable. They are also becoming more and more expensive due to the constantly increasing price of the raw materials used. The bidirectional line frequency AC/AC resonant converter described in this specification is much smaller and lighter, and has comparable power efficiency. The main advantage of the power converter described here is its controllability. Power converters as described herein may be incorporated into so-called “smart” grids and may be individually controlled/monitored from a remote location. 
         [0090]    Bidirectional line frequency AC/AC resonant converters as described in the present specification may also be employed to replace step-down power supplied for electronic equipment such as, for example, current cable television power supplies. Present cable television power supplies are essentially ferro-resonant step-down transformers powered from the 120V/208/240V mains supply. They produce trapezoidal output voltage to power cable TV equipment. These operate at relatively low power efficiencies of approximately 85%. In addition they are big and heavy and are becoming ever more expensive. Bidirectional line frequency AC/AC resonant converters as described in the present specification can operate with 98% efficiency and produce the trapezoidal output voltage needed in much smaller/lighter package. It may also be produced at lower unit cost. 
         [0091]    The various embodiments described herein can be combined or modified to provide other example embodiments. The scope of the invention is to be construed in accordance with the substance defined by the following claims. As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications to the above-described embodiments are possible. For example, certain modifications, permutations, additions and sub-combinations of the features described herein will be apparent to those skilled in the art. It is intended that the following appended claims and the claims hereafter introduced should be interpreted broadly so as to encompass all such modifications, permutations, additions and sub-combinations as are consistent with the language of the claims, broadly construed. 
       INTERPRETATION OF TERMS 
       [0092]    Unless the context clearly requires otherwise, throughout the description and the
       “comprise”, “comprising”, and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”;   “connected”, “coupled”, or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof;   “herein”, “above”, “below”, and words of similar import, when used to describe this specification, shall refer to this specification as a whole, and not to any particular portions of this specification;   “or”, in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list;   the singular forms “a”, “an”, and “the” also include the meaning of any appropriate plural forms.       
 
         [0098]    Words that indicate directions such as “vertical”, “transverse”, “horizontal”, “upward”, “downward”, “forward”, “backward”, “inward”, “outward”, “vertical”, “transverse”, “left”, “right”, “front”, “back”, “top”, “bottom”, “below”, “above”, “under”, and the like, used in this description and any accompanying claims (where present), depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly. 
         [0099]    Controllers for converters as described herein may be implemented using, as a controller, specifically designed hardware, configurable hardware, programmable data processors configured by the provision of software (which may optionally comprise “firmware”) capable of executing on the data processors, special purpose computers and/or data processors that are specifically programmed, configured, or constructed to perform one or more steps in a method as explained in detail herein and/or combinations of two or more of these. Examples of specifically designed hardware are: logic circuits, application-specific integrated circuits (“ASICs”), large scale integrated circuits (“LSIs”), very large scale integrated circuits (“VLSIs”), and the like. Examples of configurable hardware are: one or more programmable logic devices such as programmable array logic (“PALs”), programmable logic arrays (“PLAs”), and field programmable gate arrays (“FPGAs”)). Examples of programmable data processors are: microprocessors, digital signal processors (“DSPs”), embedded processors, graphics processors, math co-processors, general purpose computers, and the like. For example, one or more data processors in a controller for a device may implement methods as described herein by executing software instructions in a program memory accessible to the processors. 
         [0100]    In examples where processes or blocks are presented in a given order, alternative examples may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times. 
         [0101]    The invention may also be provided in the form of a program product. The program product may comprise any non-transitory medium which carries a set of computer-readable instructions which, when executed by a data processor, cause the data processor to execute or control a method of the invention. Program products according to the invention may be in any of a wide variety of forms. The program product may comprise, for example, non-transitory media such as magnetic data storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, EPROMs, hardwired or preprogrammed chips (e.g., EEPROM semiconductor chips), nanotechnology memory, or the like. The computer-readable signals on the program product may optionally be compressed or encrypted. 
         [0102]    Where a component (e.g. a circuit, component, software module, processor, assembly, device, switch, transformer, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention. 
         [0103]    Specific examples of systems, methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions, and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and/or acts with equivalent features, elements and/or acts; mixing and matching of features, elements and/or acts from different embodiments; combining features, elements and/or acts from embodiments as described herein with features, elements and/or acts of other technology; and/or omitting combining features, elements and/or acts from described embodiments. 
         [0104]    It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions, and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole. 
       LIST OF REFERENCES 
       [0000]    
       
           100  single conversion stage bidirectional soft-switched AC/AC power converter 
           101  alternating current (AC) source 
           102  load 
           103  switching subcircuit 
           104  switching subcircuit 
           105  switching subcircuit 
           106  switching subcircuit 
           107  full-bridge switcher circuit 
           108  terminal 
           109  terminal 
           110  phase-retarding element 
           111  phase-retarding element 
           112  phase-advancing element 
           113  phase-retarding element 
           114  terminal 
           115  terminal 
           116  transformer 
           117  transformer secondary 
           118  transformer primary 
           123  switching subcircuit 
           124  switching subcircuit 
           125  switching subcircuit 
           126  switching subcircuit 
           127  full-bridge switcher circuit 
           131  voltmeter 
           132  ampere meter 
           133  chopper control line 
           141  voltmeter 
           142  ampere meter 
           143  chopper control line 
           150  controller 
           200  single conversion stage bidirectional soft-switched AC/AC power converter 
           203  capacitor 
           204  capacitor 
           207  half-bridge switcher circuit 
           223  capacitor 
           224  capacitor 
           227  half-bridge switcher circuit 
           301  alternating current (AC) source 
           302  load 
           303  alternating current (AC) source 
           304  load 
           307  switcher circuit 
           310  single conversion stage bidirectional soft-switched AC/AC power converter 
         operated in forward direction 
           311  first direction of power transfer 
           314  power transfer tank circuit 
           320  single conversion stage bidirectional soft-switched AC/AC power converter operated in reverse direction 
           321  second direction of power transfer 
           324  power transfer tank circuit 
           327  switcher circuit 
           411   a  MOSFET enhanced mode (Metal Oxide Field Effect Transistor) 
           411   b  MOSFET enhanced mode (Metal Oxide Field Effect Transistor) 
           412  rectifying diode 
           413   a  IGBT (Insulated Gate Bipolar Transistor) 
           413   b  IGBT (Insulated Gate Bipolar Transistor) 
           500  single conversion stage bidirectional soft-switched AC/AC power converter without transformer 
           507  switcher circuit 
           511  1:1 transformer with primary and secondary coils arranged for opposing voltage induction 
           511   a  Phase-retarding element and inductor 
           511   b  Phase-retarding element and inductor magnetically connected to  511   a    
           513  1:1 transformer with primary and secondary coils arranged for opposing voltage induction 
           513   a  Phase-retarding element and inductor 
           513   b  Phase-retarding element and inductor magnetically connected to  513   a    
           515  Common line shared by the source and the load 
           527  switcher circuit 
           550  single conversion stage bidirectional soft-switched AC/AC power converter without step-up transformer and having a common conductor between source and load 
           600  Method for transferring electrical line power along opposing first and second paths through a closed loop series reactance network 
           610  Providing to a first switcher circuit a first input electrical line voltage signal 
           620  First modulating the first input voltage signal at a first frequency in the first switcher circuit 
           630  Providing the modulated first input voltage signal across a first phase-retarding element of a series resonant circuit that comprises a phase-advancing element and second and third phase-retarding elements 
           640  Extracting a first output voltage signal across the second phase-retarding element of the series resonant circuit 
           650  First restoring the shape of the first input voltage signal to the first output voltage signal in a second switcher circuit 
           660  The reversing power transfer through the closed loop series reactance network 
           662  Providing to the second switcher circuit a second input electrical line voltage signal 
           664  Second modulating the second input voltage signal at a second frequency in the second switcher circuit 
           666  Providing the modulated second input voltage signal across the second phase-retarding element of the series resonant circuit 
           668  Extracting a second output voltage signal across the first phase-retarding element of the series resonant circuit 
           669  Second restoring the shape of the second input voltage signal to the second output power signal in the first switcher circuit