EP0157855A1 - Uninterruptible power supply and line conditioner - Google Patents

Abstract

Conventional uninterruptible power supplies require different stages of energy conversion, and other systems, during normal equilibrium operation, have minimum reverse currents under zero load conditions. An uninterruptible power supply and line conditioner use a local accumulator (40) and the public high voltage lines (96, 98) as power sources for an alternating current load where an inverter (46) modulating pulse width (PWM) is used to convert the DC current from the accumulator to AC charging current and to charge the accumulator from a high voltage line. The public high voltage lines (96, 98) are efficiently connected to the load by means of a series inductor (78) and the phase angle between the output voltage of the inverter (46) and the voltage d. effective input to the inductor (78) from the high voltage lines controls the distribution of current to the load and to the accumulator (40) from the high voltage lines, as well as from the inverter to the load . The required reversing current is reduced and the flow efficiency improved by providing a predetermined ratio (R) between the voltage applied to the input side of the inductor and the voltage at the output of the inverter; in a preferred embodiment, the effective input voltage is approximately 1.1 times the voltage of the inverter and is produced by the use of magnetic shunts (114, 116) in a transformer (78) coupling the high voltage line to the load.

Description

UNINTERRUPTIBLE POWER SUPPLY AND LINE CONDITIONER

Background of the Invention

There are many applications in which it is becoming increasingly important to assure that equipment will be supplied with an uninterrupted AC supply voltage, and that this voltage will be a substantially pure and substantially noise-free sinewave of predetermined fixed frequency. The usual utility power lines are intended to provide such a supply voltage, but are subject to complete power outages, to reductions in voltage level, to surges which cause the voltage to rise above the normal level and to various types of interfering noise picked up by the power lines.

For many purposes such inadequacies of the power lines are relatively harmless, or at most inconvenient. However, with other more critical loads, for example computer apparatus, any of the foregoing departures of the power line from a constant, fixed-frequency, substantially noise-free sinewave can cause loss of stored information and/or improper handling bϊ information by the load apparatus, either of which can have very serious adverse results. There are a variety of types of equipment which are known in the prior art and which will operate. In one degree or another, to mitigate one or more of the foregoing defects in the line voltage supply. One of these consists of so-called uninterruptible power supply (UPS) equipment and employs a battery charger, a battery and an inverter in tandera with each other, the charger being supplied from the AC power line and the inverter supplying AC to the computer or other critical load. The power line keeps the battery adequately charged despite small over-voltages, under-voltages or interfering noise on the utility line, and the inverter utilizes the stored energy of the battery to produce substantially pure single-frequency sinewaves of constant amplitude for supply to the critical load. In the event of longterm power line failure, the battery and inverter will maintain the desired AC current and voltage at the load for a substantial period of time, after which discharge of the battery can be detected and the equipment appropriately shut down, its use discontinued, or other protective measures taken, such as shifting to other standby power.

While quite effective for its purposes, this type of uninterruptible power supply equipment requires two distinct stages of power conversion, one for conversion from the AC power of the power line to the DC power needed to charge the battery, and the second for subsequent conversion from the DC of the battery to the AC supplied to the critical load, accomplished by the inverter. Such process and apparatus are often unduly inefficient and expensive.

It is one object of the present invention to provide an efficient uninterruptible power supply and line conditioner which provides the desired alternating voltage for the critical load, yet does not require a separate charger and inverter and therefore is more efficient and less expensive than those systems which require such apparatus.

More recently there have been developed uninterruptible power supply and line conditioner apparatus in which the utility line is coupled to the load and to the inverter output by way of a series inductance, and in which the inverter comprises a bidirectional pulsewidth modulation (PWM) sinewave inverter connected between the battery and the load. In such systems the phase of the sinewave generated by the inverter can be varied as desired with respect to the phase of the utility line voltage, thereby varying the magnitude and phase of the contribution of the inverter current to the load current. In a typical operation this phase angle may be set, and preferably automatically maintained, at a value sufficient to supply the power demanded by the load, plus any losses in the system, plus any amount of power which it is desired to supply to the battery to maintain it charged or to recharge it.

in such systems, the magnitude of the alternating voltage supplied to the load from the utility line, and hence also supplied to the output terminals of the inverter, is substantially equal to the utility line voltage itself. Thus where the coupling between the inverter output, the load terminals and the utility line terminals is by way of three corresponding windings of a transformer, the ratio of the turns of the winding connected to the utility line is equal to the number of turns coupled to the load terminals; that is, the ratio of the turns is 1:1 based on the concept that the load equipment is to be supplied with the same alternating voltage as is present on the power lines. While this arrangement will operate, it has been found that, for reasons set forth hereinafter, such a system, during normal "break even" operation, has its minimum inverter current under zero load conditions, and with a load present has a substantially greater-than-minimum inverter current; with a lagging load power factor, the required inverter current (and consequent inverter size) can become very great, and the system is therefore unduly expensive. Furthermore, its through-put efficiency is, in general, not the maximum obtainable.

Accordingly, it is also an object of the present invention to provide an uninterruptible power supply and line conditioner of the type which employs an inductance through which the utility line voltage is fed to the inverter output and the load terminals, and in which the inverter is of the four-quadrant PWM sinewave type, but in which the inverter current required for normal operation near the "break even" operating point is minimized and the through-put efficiency of the system maximized.

It is a further object to provide such a system in which isolation is maintained between and among the load, the inverter output terminals and the line voltage terminals.

Summary of the Invention

These and other objects of the invention are achieved by the provision of a system of the above-described general type, but in which the voltage applied to the series inductance differs from the utility line voltage in a ratio and in a sense substantially to minimize the inverter current required during normal operation and substantially to maximize the throughput efficiency of the system. The departure from unity of the ratio R between the voltage supplied to the series inductance and the utility line voltage is in an amount and in a direction which depends upon the power factor of the load; for example, in one typical application of the invention in which the power factor of the load was unity, the ratio R of the voltage applied to the series inductance to the utility line voltage was 1.1:1, with the improvements in operation set forth fully hereinafter.

Brief Description of Figures

These and other objects and features of the invention will be more readily understood from a consideration of the following detailed description, taken with the accompanying drawings, in which:

Figure 1 is a schematic representation of a UPS system of the prior art;

Figure 2 is a schematic representation of a class of system to which the present invention is applicable;

Figure 3 is an equivalent-circuit diagram for the system of Fig. 2; Figures 4 and 5 are phasor diagrams to which reference is made in explaining the operation of the system of Fig. 3;

Figures 6 and 8 are graphical representations of the relationship between certain parameters in the system of Fig. 2;

Figure 7 is an equivalent-circuit diagram of a system according to the present invention;

Figure 9 is a graphical representation showing the variations of certain parameters in a system according to the present invention;

Figure 10 is a circuit diagram of apparatus in accordance with a preferred embodiment of the invention;

Figure 11 is a schematic view of a form of transformer used in the system of Fig. 10;

Figure 12(a) is a phasor diagram for a system not using a principal feature of the present invention, and Figure 12(b) is a similar type of diagram for a system using the present invention; Figure 13 is a block diagram illustrating one type of complete microprocessor-controlled system in which the apparatus of the present invention may be used; and

Figures 14A and 14B are graphical plots of voltages and currents, respectively, in a preferred system according to the invention, illustrating respectively the substantial immunity of load voltage to electrical noise on the power lines and the absence of line-current distortion in the power-line current when the load current is substantially distorted.

Description of Specific Embodiments

Without thereby in any way limiting the scope of the invention, in the interest of clarity it will be described with specific reference to the embodiment shown in the accompanying figures. In these figures, Figure 1 illustrates in broad form a system previously known in the prior art in which the utility line 10 is connected to a rectifier/charger 12, which converts the alternating utility voltage to direct voltage so as to charge a battery 16. The voltage across the latter battery is then utilized to operate an inverter 18, which converts the DC voltage of the battery to alternating current and supplies it over output line 22 to the critical load. With this system, the utility line can be disconnected for substantial periods while the inverter continues to supply the desired alternating voltage, while at the same time substantial protection is provided against interfering noise, current surges, momentary voltage drops and irregularities in the waveform of the utility line voltage.

Figure 2 is a diagram similar to Figure 1 but illustrating a class of equipment to which the invention is particularly applicable. In this case the utility line 24 supplies alternating voltage to the four-quadrant PWM sinewave inverter 26 by way of a series inductance, and the output of the inverter is connected over line 28 to the critical load; the battery 30 is connected to the inverter, and the inverter determines how much of the critical load current is supplied from the utility line and how much from the battery, and how much of the utility line current is supplied to charge the battery.

Arrangements generally like that of Figure 2 are known in the prior art, but as described previously, are subject to the unnecessarily high inverter current and less-than-optimum through-put efficiencies described above. The present invention represents an improvement in the general type of system illustrated in Figure 2, and accordingly the construction and operation of a system in accordance with Fig. 2 will now be explained and described, after which the improvement thereon according to the present invention will be set forth in more detail.

Figure 3 is a simplified equivalent circuit for the general arrangement of Fig . 2 , depicting the utility line voltage E_u, the series inductance L_s through which the current I_L flows, the inverter with a voltage E. across it and a current I- through it, and the critical load Z_O to which the load current I_O is supplied. The inverter and critical load are effectively in parallel with each other, and supplied with voltage from the utility line by way of the series inductor L_s.

The generalized phaser diagram for such a circuit is shown in Fig. 4 for the case in which the angle between the utility line voltage E_u and the inverter output voltage E_i is β , with the inverter voltage lagging. The voltage E_L across the inductance is the vector difference between the vectors E_u and E_i, and hence is a vector joining the heads of the vectors of the latter two quantites, as shown. I_L, for a substantially lossless inductance, is at right angles to E_L and the load current I_O is assumed to lag the inverter voltage by a load power factor angle θ . The inverter current I_i is equal to the vector difference between the load current I_O and the inductance current I_L, as shown in the drawing. Also shown is the angle by which I_L lags E_u.

Figure 5 illustrates the effect of changing the angle β between the utility line voltage E_u and the inverter output voltage E_i; the magnitudes of E_u and E_i are equal, and for simplicity the case is shown wherein the load power factor is unity.

As shown, wheny β is small (e.g. β = β_I) , the input or inductor current I_LI is also small; the inverter current I_il is nearly in phase with the inverter voltage E_i and therefore the inverter is delivering real power to meet the power requirements of the load not supplied by the utility line; thus, in this case the inverter battery is discharging.

When β is somewhat larger ( β = β ₂ ) I_L2 is considerably larger and, in fact, its real part (i.e. its projection along the horizontal axis) is equal to

I_O, and the utility supplies all of the load power. Since I_i2 is at 90° to the inverter voltage E_i, no real power flows into or out of the inverter and therefore the battery current is zero (ignoring losses). However, there is a substantial reactive current in the inverter, as depicted by the vector I_i2. This condition in which sub stantially no real power flows in or out of the inverter we designate as the "break even" case.

For a still larger input voltage displacement angle ( β - β 3) , I_L3 is substantially larger, as is the inverter current I_i3; however, the direction of the vector I_i3 indicates that real power is flowing into the inverter, while the inverter battery is being charged during such operation at the angle β₃.

As is seen from Figure 5, varying the input displacement angle β significantly changes the magnitude of the inverter current I_i. Table I hereof summarizes the variation of β and I_i for different full load power factors and values of input inductance (L_s). This table was computed assuming an 83% efficient inverter for both th'e "break even" (battery charging current and real inverter power = 0) and "battery charging" (charge current or real inverter power = 0.2 P.U.) cases, wherein P.U. indicates per unit, i.e. all parameters including inductance have been normalized to the load voltage and current.

The input voltage displacement angle β for any given load, input voltage, and charge current condition increases with increasing value of the inductance L_S. However, at -15% utility voltage (E_u = 0.85), inverter current is minimal when L_S equals 0.4 P.U. At this optimal inductance value, the inverter still must be sized to handle 130% full load current (at "break even") when the load power factor is 0.8 lagging. Moreover, to charge the battery at 0.2 P.U. requires an inverter rated at 150%.

Figure 6 illustrates the variation of inverter current as a function of input voltage (normalized), at the break-even operation condition. It is noted that for unity power factor, inverter current is minimum when the input or utility voltage E is equal to 1.1 P.U.

In accordance with the present invention the system performance is improved by scaling or transforming the input utility voltage upward by a factor of 1.1, as shown in Figure 7.

Thus Figure 7 shows a system according to the invention in equivalent circuit form, with a step-up of

1 to 1.1 in voltage between the line voltage terminals and the input to the inductance Ls. (in this equivalent circuit , shown as if it were provided by an autotransformer connection). At this ratio of 1.1, inverter current at no load is actually higher than at full load, as indicated by the vertical arrow in Fig. 6. Table II shows the reduction in the required inverter current when the input voltage has been transformed by the 1.1 ratio. Included in this Table is the input power factor angle between E_u and I_L. As indicated, the input power factor actually improves at -15% line voltage, viz, when E_u = 0.935 (1.1 X 0.85).

The effect of transforming the input voltage in this manner is further illustrated in Figure 8, wherein the through-put efficiency is plotted as ordinate and the normalized utility voltage E_u is plotted as abscissa, for an 83% efficient inverter. Through-put efficiency is maximum approximately when the inverter current is minimum, at a transformed input voltage of about 1.1. The values indicated were calculated for a 120-volt, 3 kilovolt-ampere system. From this it will be seen that, for a unity power-factor load, the 1.1 ratio gives substantially maximum through-put efficiency, and gives reasonable efficiencies for both 0.8 lag power factor and 0.9 lead power factor. For other loads having different power factors, maximum through-put and minimum "break-even" inverter current may be obtained by using other suitable values of R. In Figure 9, load current I_O is plotted as abscissae and two variables are plotted as ordinates, namely input voltage displacement angle β and input currents (I_u, I_L). The graphs contained therein illustrate the effects on load current I_O of varying I_u, I_L and β for a 1.1 transformation ratio.

Turning now to Figure 10, there is shown a preferred embodiment of the invention for the typical case of a utility voltage of 120 volts AC, a load voltage of 120 volts AC, a load power requirement of three KVA at 60hz, and a load power factor of unity. A battery 40, in this example providing 120 volts DC, is connected through an appropriate fuse 42 to a shunt capacitor 44, typically having a value of about 15,000 microfarad. Also connected across the battery is the four-quadrant PWM sinewave inverter 46 made up of the PWM filter 48 and the four transistor-diode sections A, B, C and D arranged in a bridge configuration, where the battery is connected between the top and bottom junctions 50, 52 of the bridge and the opposed side junctions 54 and 56 of the bridge are connected to the respective input lines 58 and 60 of the PWM filter. Each of the bridge sections A, B, C and D is made up of a high current NPN switching transistor having a high-current semiconductor diode in parallel therewith. In each of the upper sections A and C of the bridge, the collectors of the two transistors are connected to the positive side of the battery and their emitters are connected to bridge output lines 58 and 60 respectively; the two diodes in the upper sections A and C are poled so that their cathodes are connected to the positive end of the battery. The transistors and diodes in the lower bridge sections B and D are poled oppositely from those in sections A and C. Such circuits and their operation are well known in the art for use as PWM inverters. In such operation, the bases of the four switching transistors are turned ON and OFF in pairs in a predetermined sequence at predetermined times and for predetermined intervals, in this example 26 times per sinewave cycle, so that the output leads 58 and 60 of the bridge circuit are provided with a pulse-width modulated pulse signal having energies representing a sinewave, which signal after passage through the low-pass PWM filter 48 therefore produces a sinewave in response to energy from the battery. In a typical case, each of the capacitors C_T and C_F of the filter may have a value of about 200 microfarad, the inductance of each of the two coils L_F may be about 400 microhenries and the inductance of the coil L_T may be about 13 microhenries, producing a low-pass filter having an upper band limit at about 400 Hz and a rejection trap at the carrier frequency of the PWM pulses. The output termimls 70,72 of the inverter are connected across the inverter winding 76 of a transformer 78. In a typical case, the transformer winding 76 may have a number of turns equal to about 1/2 the number of turns of the load winding 80 thereon which supplies power to the load, that is, if the numbar of turns of winding 80 is N₂ then the number of turns of inverter output winding 76 may equal 1/2 N₂.

Transformer windings 76 and 80 are tightly coupled to each other, e.g. may be wound one on top of the other on a common iron core 84 so that the inverter output voltage is the load voltage. Transformer winding 80 is connected directly to the load input terminals 88 and 90, in this example by way of a normally-closed manual switch 92. A bypass switch-contact 94 is provided so that switch 92 may be placed in an alternate position wherein the high side of the transformer winding 80 is replaced by the high side 96 of the AC utility line, the neutral side 98 of the utility line being permanently connected to the lower end of transformer winding 80, thus enabling an operator to mechanically bypass the entire UPS system and connect the critical load directly to the utility line vhen conditions warrant r.uch action. For more rapid switching from the UPS init to the utility line, a static bypass circuit 102 may be employed, made up of a pair of parallel, oppositely-pol ed silicon-controllod-rectifiers each of which can be triggered on by signals applied to its gate electrode, the pair thus serving as a bidi rectional electronic switch, actuatable in response to electrical signals indicative of any selected malfunction, such as a large change in load voltage due to a load disturbance.

The portion of the Figure 10 thus far described in detail represents, in its general form, a known type of inverter system for operating a critical AC load from a battery and for charging the battery from an AC source, and hence need not be described in even further detail.

The AC utility line, made up of the high line

96 and the neutral line 98 is connected, via utility line input terminals 108 and 110, to transformer winding 112, which is located on the same core as the windings 76 and 80 but is loosely coupled thereto by virtue of the intervening magnetic shunts 114 and 116, which typically comprise bodies of ferro-magnetic material positioned to shunt or bypass a portion of the magnetic flux which otherwise would extend between coil 112 and the coils 76 and 80; each magnetic shunt is designed to provide at least, a small air gap on each side of the shunt so that complete shunting does not occur. Such constructions an d procedures are well known in the art and need not be described herein in detail, and a physical arrangement of such a transformer is illustrated schematically in Fig. 11, wherein the transformer windings are designated by the same numerals as previously and the magnetic shunts are designated as 140 and 142. This decoupling by the inductance permits the vectors representing the voltages at winding 112 and at winding 76 to be independently adjusted.

Such types of Systems, their backgrounds and the theory of their operation are described, for example in G. J. Smollinger and W. J. Raddi, "Reverse Energy Through an A.C. Line Synchronized Pulse Width Modulated Sine-Wave Inverter", Intelec 81, pp. 126-131; R. Rando, "AC Triport - A New Uninterruptible AC Power Supply", Intelec 78, pp. 50-58; H. E. Menks, "A Stored-Program Controlled Triport UPS", Intelec 81, pp 210-215; and Z. Noworolski and K. Goszyk, "High Efficiency Uninterruptible Power Supply", 4th International PCI Conference on Power Conversion, March, 1982, pp. 521-529.

In addition, in the present embodiment the connection between the high utility line 96 and winding 112 may include a series fuse 150 and an AC disconnect switch 152, similar in form to the static bypass switch 102 and similarly operable, when desired, by electrical signals applied to the gate electrodes of the SCR's; for example, when the utility line fails, switch 152, is automatically opened and the load is supplied with AC power entirely from the battery and inverter. In this embodiment of the invention, a primary inventive feature is that the ratio R of the number of turns N₂ of transformer winding 80 to the number of turns N₁ of transformer winding 112 is other than unity, e.g. in this example N₂ may be 46 turns and N^ may be 42 turns, i.e. N₂/N₁ = 1.1. The significance of this will now be described with respect to Figs. 7, 11 and 12 especially.

The simplified equivalent circuit illustrated in Fig. 7 is applicable to the system of Fig. 10, the the ratio N₂/N₁ of the turns of windings 80 and 112 being represented by the tap position on an autotransformer which, in effect, increases the line voltage applied to the input end of inductance L_s from E_u to a 10% higher value Eu'. The series inductance L_s is effectively provided, in the example of Fig. 10, by the transformer and the magnetic shunts built into it. As described previously with respect to Figs. 6 and 8, this step-up ratio of 1.1 minimizes the break-even inverter current required by the system during normal operation and maximizes the through-put efficiency.

The transformer 78 in this example is of El constrti tion, with the magnetic shunts described previously serving to attenuate the magnetic path between winding 112 and windings 76 and 80, in this example giving an effective value for L_s of about 5 millihenries. Figures 12A and 12B illustrate from a different viewpoint the operation and effect of the line voltage step-up employed according to the present invention. Figure 12A illustrates the phasor relationships in a typical prior-art apparatus in which E_u=E_I and E_i lags E_u by, for example, 23° in a typical operating condition. The difference vector E_L again represents the voltage across the series inductance L_S, and the current through that inductance is represented by the vector I_L at right angles thereto. The output current in this example is assumed to be in phase with the inverter output voltage, i.e. the load is unity power factor, so that the I_O vector lies along the same direction as the E_i vector as shown. The difference vector I_i then represents the substantial circulating current in the inverter, which always exists under these conditions even though no real power is then being delivered to or from the inverter.

Figure 12B shows conditions existing in a comparable system modified according to the present invention so that the line voltage E_u is in effect, transformed upwardly by a factor 1.1 to a new value E_u', this increased value of E_u' being sufficient so that the E_L vector is vertical and the I_L vector, being at right angles to E_L, lies directly along the direction of the inverter current I_O and is equal thereto. It therefore supplies all of the load current, leaving no current, reactive or real, in or out of the inverter, as is desired to produce the previously-described improvements with regard to minimizing inverter current and improved through-put efficiency.

It can be seen from Figures 12A and 12B that varying the length of E_u' exerts an action as if the E_L and I_L vectors were fixed at right angles to each other but rotatable together about the end of the E_i vector so that, by appropriate selection of the length of E_u', I_L can be turned into alignment with I_O regardless of the direction of I_O, which direction may vary depending upon the load power factor, for example.

Figure 13 illustrates by way of example one type of system in which the UPS of the invention may be included. In this case a microprocessor 300, such as a Z80 microprocessor chip, controls the frequency and phase β of the inver ter output sinewave and is supplied with appropriate program memory information from memory 302; with system personality information indicative of the particular application parameters from system personality 304; with digital information with respect to line voltage, load voltage, line current, load current, battery current and battery voltage from A/D device 308; with a variety of monitoring information with respect to conditions of the line switch, the bypass switch, overtemperature, over-voltage or any other parameters which it is desired to monitor, by way of I/O port 310; and with a mutual interchange of information with an appropriate display device 312. The microprocessor also preferably receives information from an interrupt control 360 with respect to such parameters as line voltage, inverter voltage, time and any other parameters found desirable. In this example, the microprocessor controls a counter timer chip (CTC) carrier generator 400 and a CTC 60Hz generator 402, which operate to produce on line 404 a carrier frequency equal to the repetition rate of the pulse-width modulated pulses (typically at 26 times the 60 Hertz line frequency) and to produce from sine generator 410 a substantially pure sinewave function at utility-line frequency and of the desired 120-volt magnitude. The PWM control 420 controls the inverter 422, which in this case is assumed to be the entire circuit of Figure 10, so as to determine the phase and the widths of the pulses which turn on the transistors in the PWM bridge circuit. An inverter feedback connection extends from the load 450 to a comparison or error amplifier circuit 452 which detects and amplifies any differences between the voltage fed beck from the inverter and the idealized sinewave from sine generator 410, this difference: then being fed to PWM control 420 in a polarity and amount to correct any deficiencies in the sinewave appearing at the load. Normally the sinewave is locked to the utility line sinewave. However, to provide a suitable sinewave to the comparison circuit upon a utility line outage, the microprocessor includes a stable crystal-controlled reference oscillator, powered by the battery, from which the desired ideal sinewave at the desired line frequency is derived.

The microprocessor maintains frequency lock between the sine-wave reference and the utility as well as manipulating the displacement phase angle between them. It also examines all system parameters and compares then against preset software limits. The user can access these system parameters through a front graphics display panel.

Figures 14A and 14B illustrate the bidirectional line conditioning obtained with applicants' isolated system. As shown in Fig. 14A, if the line voltage E_u consists of a sinewave with the noise spikes shown thereon, the inverter voltage E_i supplied to the load has the substantially pure sinewave appearance shown in the latter figure; Figure 14B shows that even if the load current

I_O were distorted as shown, this would not reflect back into, or substantially distort, the utility supply line current I_u, which remains a substantially pure sinewave. The preferred embodiment of the invention has been shown as utilizing a transformer in which magnetic shunts provide the effective series inductance L_S, and the voltage step-up R is provided by the ratio N₂/N₁ in the number of turns of entirely separate and isolated transformer windings. However, many of the advantages of the invention with regard to minimizing inverter current and maximizing through-put can be obtained where the series inductance L_S is in fact a real lumped-circuit series inductor connected between the high side of the line and the inverter output, much as represented schematically in the simplified equivalent circuit of Fig. 7, and it is in fact possible to utilize an autotransformer as suggested by the equivalent circuit of Fig. 7 rather than the completely isolated transformer winding arrangement of the preferred embodiment.

Also, although the ratio of E_u'/E_u of 1.1 has been found preferable for many practical purposes, the minimum inverter current may in some instances occur at a different value than 1.1, in which case R may be differently chosen to minimize inverter current during breakeven operation. That is, in some cases the load power factor may not be centered about unity, but may have a known average fixed value departing substantially from unity, in which case the value of R may be chosen to be substantially different from 1.1, so as to minimize the required inverter current during normal operation. Thus while the invention has been described with respect to certain specific embodiments in the interest of complete definiteness, it will be understood that it may be embodied in a variety of forms diverse from those specifically shown and described without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. In a system for supplying AC electrical power at a predetermined voltage to a load apparatus, comprising: a rechargeable DC source of power; a bidirectional four-quadrant sinewave inverter having its DC input terminals connected to said DC source for developing AC power at its AC output terminals in response to energy from said DC source, said inverter being responsive to AC power externally supplied to said AC output terminals to charge said DC source for certain phase angles between said externally applied AC power and the AC power developed by said inverter; load terminals connectable to said load apparatus; means for maintaining the AC voltage at said load terminals at a substantially constant value; utility line terminals connectable to an AC utility line; first coupling means tightly coupling said inverter output terminals to said load terminals; and series inductive means loosely coupling said utility line terminals to said load terminals and to said inverter output terminals; the improvement which comprises means responsive to voltages applied to said utility line terminals from said utility lines for changing the effective voltage applied to said inductive element by said utility line to a value differing from said substantially constant value at said load terminals, in a direction to reduce the inverter current during normal "break even" operation wherein no real power flows into or out of said inverter.

2. The system of Claim 1, wherein said first coupling means comprises transformer means having a ferromagnetic core and a first winding connected with said inverter output terminals and a second winding tightly coupled with said first winding and connected with said load terminal; and further wherein said series inductor means comprises a third winding on said transformer means, spaced from said first and second windings, and magnetic shunt means on said transformer means between said third winding and said first and second windings.

3. The system of Claim 1, wherein said inverter is of the pulse-width-modulation type and comprises a bridge circuit including at least four switching transistors and a diode rectifier in parallel with each transistor, and means for switching said transistors ON and OFF at times and for intervals of time such as to produce a sequence of width-modulated pulses representing a sinewave, and low-pass filter means supplied with said pulses for deriving said sinewave from said pulses and for applying said sinewave to said load terminals.

4. The system of Claim 3, and further comprising means for shifting the phase angle between said effective voltage applied to said inductor and said inverter output voltage thereby to provide sufficient power to said inverter output terminals to satisfy said load, to maintain said battery in a charged condition and to provide for losses in the circuit.

5. A power supplying system comprising: load apparatus to be supplied with AC electrical power at a predetermined AC voltage level: battery means providing a source of DC power; utility lines providing a source of AC power: a bidirectional four-quadrant pulse-width-modulation inverter having a pair of DC input terminals connected to said battery means, and having a pair of AC output terminals; transformer means having a first winding connected to said AC output terminals of said inverter, a second winding tightly coupled to said first winding and connected across said load apparatus, a third winding spaced from said first and second winding and connected with with utility lines, and magnetic shunt means between said third winding and said first and second windings for producing the effect of a series inductance between said utility lines and said first and second windings; the improvement wherein the ratio R of the number of turns of said second winding to the number of turns of said third winding differs substantially from unity in the direction to rotate the phasor of the current through said inductance more nearly into coincidence with the average direction of the phasor of the current in said first winding during normal operation of said power supplying system.

6. The system of Claim 5, wherein the value of said ratio R is substantially 1.1.

7. The system of Claim 5, wherein the value of said ratio R is substantially that required to rotate the phasor of the current in said inductance into the same orientation as the phasor of the inverter current in said first winding when the power factor of said load apparatus is substantially unity.

8. The system of Claim 5, wherein said inverter comprises a four-section bridge circuit each section of which comprises a switching transistor in parallel with a diode rectifier, and a low-pass filter, whereby power from said battery is supplied by said inverter to said first winding in AC form, and AC power supplied by said third winding to said first winding is supplied in part to said battery means in DC form, to charge it.

9. The system of Claim 8, and further comprising signal comparison means, feedback means for feeding a sign representative of the voltage across said loan apparatus back to an input of said comparison means, a sinewave generator having the desired frequency and form for the voltaqe waveform across said load apparatus, means for applying said sinewave from said generator to another input of said comparison means to produce from said comparison means an error siqnal representative of departures of said voltage across said load apparatus from its desired sinusoidal form, and PWM control means responsive to said error siqnal for controlling conduction of said transistors in said bridge to cause said voltaqe across said load apparatus to conform with said sinewave from said generator,