CA1236524A - Uninterruptible power supply and line conditioner - Google Patents

Abstract

Abstract of the Disclosure An uninterruptable power supply and line con-ditioner of the class using a local battery and the util-ity power lines as the sources of power for an AC load, and in which a pulse width modulation (PWM) inverter is used to convert the battery DC to the load AC and to charge the battery from the power lines. The utility power lines are effectively connected to the load by way of a series inductance, and the phase angle between the inverter out-put voltage and the effective input voltage to the induc-tance from the power lines controls the proportioning of current to the load and to the battery from the power lines, and from the inverter to the load. The necessary inverter current is reduced and the throughput efficiency improved by, in effect, providing a predetermined ratio R between the voltage applied to the input side of the inductor and the voltage at the inverter output; in a preferred embodiment the effective input voltage is about 1.1 times the inverter voltage, and is produced by use of magnetic shunts in a transformer which couples the utility line to the load.

Description

UNINTERRUPTIBLE POWER S~PPLY
AND LINE CONDITIONE~

Background of the Invention There are many applications in which it is be-coming 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 com-p~ete power outages, to reductions in voltage level, to surges which cause the voltage to rise above the normal level an~ to various types of interfering noise picked up by the power lines.

For many purposes such inadequacies o~ the power 'lines are relatively harmless, or at most inconvenient.
~owever, with other more critical loads, ~or example com-puter apparatus,"any of the foregoing departures of the power line from a constant, ixed-frequency, substantially no;se-free sinewaVe can cause loss of stored information and~or improper .handling o~ information by the load ap-paratus, either of which can have very serious adverse re~ults.

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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 ~pply (UPS) equipment and employs a battery charger, a battery and an inverter in tandem with each other, the charger being supplied from the AC power line and the inverter supplying AC to the computer or othe~ critical load.
The power line keeps the battery adequately charged des-pite 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 long-term power line failure, the batte~y 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 appro-priately shut down, its use discontinued, or other pro-tective measures taken, such as shifting to other stand-by power.

While quite effective for its purposes~ this type of uninterruptible power supply equipment requlres two distinct stages of power conversion, one for conver-sion from the ~C power o~ the p~wer line to the DC power `. ~L2;3~S~
needed to charge the battery, and the second for subse-quent conversion from the DC of the battery to the AC
supplied to the critical load, accomplished by the in-~erter. Such process and apparatus are often unduly in-efficient and expensive.

It is one object of the present invention to provide an efficient uninterruptible power supply and line conditioner which prov.ides the desired alternating voltage for the critical load, yet does not require a separate charger and inverter and therefore is more ef-ficient and less expensive than those systems which re-quire such appara.tus.

More recently there have been developed unin-terruptible power supply and line conditioner a~paratus in which ~he 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 pulse-width modulation ~PWM) sinewave inverter connected between the battery and the load. In such sy~ems 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 vàrying 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 automa~ically maintained, at a value suf~ic.ient .

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to supply the power demanded by the load, plus any losses in the system, plus any amount of power which it is de-sired to supply to the battery to maintain it charged or to recharge it~

In such systems, the magnitude of the alter-nating 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 volt-age 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 pre-sent on the power lines. While this arrangement will operate, it has been found that, for reasons set orth 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 substan-tially greater-than-minimum inverter current; with a lagg-ing load power factor, the required inverter current (and consequent inverter size) can become very great, and the , ~;36~
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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 supp~y and line conditioner of the type which employs an inductance through which the utility line voltage is fed to the in-verter 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 opera-tion 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 lo~d, the inverter output terminals and the line voltage terminals~

5urnmary of the Invention These and other objects of the invention are achieved by the provision of a system of the above-des-cribed general type, but in which the voltage appliedto the series inductance difers ~rom the utility line voltage in a ratio and in a sense substantially to mini-miæe the inverter current required during normal operation i52~

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 ~he 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 Fiqures These and other objects and features of the invention will be more readily understood from a considera-tion of the following detailed description, taken with the accompanying drawings, in which:

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

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

2~ Figure 3 is an equivalent-circuit diagram for the systcm of Fig. 2;

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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;
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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 wi~h a preferred embodiment of ~he invention;

Figure 11 is a schematic view o~ a form of trans-former used in the system o~ 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;

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~ igure 13 i5 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 ~cco~d-ing to the invention, illustrating respectively the sub-stantial immunity of load voltage to electrical noise on the power lines and the absence of line-current dis-tortion in the power-line current when the load current is substantially distorted.

D cri tion of S ecific Embodiments es p p ~ ithout thereby in any way limiting the scope of the invention, in the interest of clarity it will be described with speciic reerence to the embodiment shown in the accompanying ~igures. In these figures, Figure 1 illustrates in broad form a system pceviously known in the prior art in which the utility line 10 is connected to a re~tifier/charger 12, which converts the alternating utility voltage to direct voltage so as to charge a bat-tery 16. The voltage across the latter battery is thenutili~ed to operate an in~erter 18, which converts the DC voltage of the battery to alternating current and sup-plies it over output line 22 to the c~itical load. With this system, the utility line can be disconnected ~or ~L2~

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substantial periods while the inverter continues to supply the desired alternatir,g voltage, while at the same time substantial protection is provided against interfering noise, current s~rg~, momentary voltage drops and ir-regularities in the waveform of the utility line voltage.

Figure 2 is a diagram similar to Fig~re l 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-~uadrant PW~ 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 o~
the critical load current is supplied from the utility line and how much ~rom the battery, and how much o the utility line current i5 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, _g_ and accordingly the c~nstruction and operation of a system in accordance with Fig. 2 will now be explained and des-cribed, after which the imp~ovement thereon according to the p~esent inve~tion will be set forth in more de~ail.

Figure 3 is a simplified equivalent cir~uit for the general arrangement of Fig. 2, depicting the util-ity line voltage ~u~ the series inductance Ls through which the current IL flows, the inverter with a voltage Ei across it and a current Ii through it, and the critical load Z0 to which the load current Io is supplied. The inverter and critical load are effectively in parallel with each other, and supplied with voltage from the util-ity line by way of the series inductor Ls.

The generalized phaser diagram for such a cir-cuit is shown in Fig. 4 for the case in which the angle between the utility line voltage Eu and the inverter out-put voltage Ei is ~ , with the inverter voltage lagging.
The voltage EL across the inductance is the vector difference between the vectors Eu and Ei, and hence is a vector join-ing the heads of the vectors of the latter two quantites,as shown. I~, for a substantially lossless inductance, is at right angles to ~L and the load current Io is as-sumed to lag the inverter voltage by a load power factor angle ~ ~ The inverter current Ii is equal to the vector 3~

di~ference between the load current Io and the inductance current IL, as shown in ~he drawing. Also shown is the angle ~ by which I~ lags Eu.

Figure 5 illustrates the effect of changing the angle ~ be~ween the utility line voltage ~u and the inverter output voltage Ei; the magnitudes of Eu and Ei are equal, and f~r simplicity the case is shown where-in the load power factor is unity.

As shown, when ~ is small (e.g. ~ = ~ 1)' the input or inductor current ILl is also small; the in-verter current Iil is nearly in phase with the invertec voltage Ei and therefore the inverter is delivering real power to meet the power requirements of the load not sup-plied by the utility line; thus, in this case the inverter battery is discharging.

When ~ is somewhat larger (~ a ~ 2) I 2 i5 considerably larger and, in fact, its real part (i.e.
its projection along the horizontal axis) is equal to I~, and the utility supplies all o the load power. Since Ii2 is at 90 to the inverter voltage Ei, no real power flows into or out of the inverter and therefore the b~t-tery current is ~ero (ignoring losses). However, there is a substantia1 reactive current in the inverter, as depicted by the vector Ii2. This c(,ndition in which sub---11-- , ~L236~

stantially no real power flows in or out o the inverterwe designate as the ~break even" case.

For a still larger input voltage displacement angle ( ~ z ~ ~), IL3 is substantially larger, as is the inverter current Ii3; however, the direction o~ the vector Ii3 indicates that real power is flowing into the inverter, while the inverter battery is being charged during such operation at the angle ~ 3.

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

The input voltage displacement angle ~ for any given load, input voltage, and charge current condi-tion increases with increasing value of the inductance Ls~ However, at -15% utility voltage (Eu = 0.85), in-verter current is minimal when Ls equals 0.4 P.U. ~t ~6~2~
.
this optimal inductance value, the inverter still must be sixed to handle 130~ full load current (at "break even~) when the load power factor is 0.8 lagging. More-over, to charge the battery at 0.2 P.U. requires an in-verter rated at 150%.

Figure 6 illustrates the variation of inverter current as a function o~ input voltage (normalized), at the break-even operation condition. It i5 noted that for unity power factor, inverter current is minimum when the input or utility voltage Eu is equal to 1.1 P.U.
In accordance with the present invention the system per-formance 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 e~uivalent 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 i~ were provided by an autotrans-former connection). At this ratio of 1.1, inverter cur-rent at no load is actually higher than at full load, as indicated by the vertical arrow in Fig. 6.

T~ble II shows the re~ction in the required inverter current when the input voltage has been trans~
formed by the 1.1 ratio. Included in this Table is the input power factor angle ~f between EU and IL. AS in-di~ated, the input power factor actually improves at -15~ line voltage, viz, when Eu = 0 935 (1.1 X O.gS)~

The effect of transforming the lnput voltage in this manner is further illustrated in Figure B, where-in the through-put efficiency is plotted as ordinate and the normaliæed utility voltage Eu 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 substan-tially maximum through put efficiency, and gives reason-able efficiencies or both 0.8 lag power factor and 0.9 lead power factor. For other loads having different power factoes, maximum through-put and minimum ~break~even"
inverter current may be obtained by using other suitable values of R.

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In Figure 9, load current Io is plotted as ab-scissae and two variables are plotted as ordinates, namely input voltage displacement angle ~ and input currents tI~, IL). The graphs contained therein illustrate the ef~ects on load current Io of varying Iu, IL and ~ ~or a 1.1 transformation ratio.

Turning now to Figure 10, there is shown a pre-ferred embodiment of the invention for the typical case of a utility voltage of 120 volts AC, a load voltage o~
120 volts AC, a load power requirement of three XVA 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 P~M sinewave inverter 46 made up of the PWM filter 48 and the four transistor-diode sections A, B, C and D ar-ran4ed in a bridge configuration, where the battery is connected between the top and bottom junctions 50,52 of the bridge and the opposed ~ide 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 hi~h current NPN switching transistor having a high-current semiconductor diode in parallel therewith.

In each o~ the upper sections A and C o~ the bridge, the collectors of the two transistors ~Ire con-nected to the positive ~ide of the battery and their emitters are connected to bridge output lines 58 and 60 re6pectively; the two diodes in the upper sections A
and C are poled so that their cathodes are connected to the positive end o~ 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 ou~put leads 58 and 60 o~ the bridge circuit are provided with a pulse-width modu-lated pulse signal having energies representing a slne- ~ ~
wave, which signal after passage through the low-pass PWM filter 48 thereore produces a sinewave in response to energy from the battery. In a typical case, each of the capacitors CT and CF o~ the filter may have a value o~ about 200 microfarad, the inductance of each of the two coils LF may be about 400 microhenries and the in-doctance of the coil LT 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 fre-quency of the PWM pulses. The output terminals 70,72 /~ .

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of the inverter are connect~d across the inv~rter w;~ding 76 of a tr~nsfor~er 78. ~n a t~pical case, the t~ns-former winding 76 may have a number o turns equal to ~bout 1/2 the number of t~rns of the load winding 80 there-on w~ich supplies power to the load, that is, if the num-ber of turns o~ winding R~ is N2 then the nu~ber ~f turns of inverter output winding 7~ may equal 1/2 N~.

Transformer windincs 7fi and 80 aré tightly coupled to each other, e.g. may be wound one on top of the other on a common iron core 84 so t~at the inverter output volt-age is the load voltage. Transformer winding ~0 is con-nected direc~ly to the load inpu~ 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 g2 may be placed in an alterna~e position wherein the h~gh 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 e~ld o~ transformer winding 80, thus enabling an operator to mechanically bypa~s the entire UPS system and connect the critical load directly to the utility line when conditions warrant such a~tion. For more rapid switching from the UPS unit to the utility line, a static bypass circuit 102 may be employed, made up oE a ~air o parallel, oppositely-poled sllicon-controlled-recti-fiers each of which can be triggered on by signals applied to its gate electrode, the pair thus servinq as a bidi-k ,.
rectional electronic switch, actuatable in response toelectrical signals indicative of any selected malfunction, such as a large change in load voltage due to a load dis-turbance.

The portion of the ~igure lO thus far described in detail represents, in its general form, a known type of ~nverter 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 t~he in-t~rvening magnetic shunts 114 and 116, which typically comprise bodies of ferro-magnetic material positioned .
to shunt or bypass a po~tion of the magnetic flux whi~h otherwise would extend between coil 112 and the coils 76 and 80; each magnetic shunt is designed to provide 20 at least a small air gap on each side of the shunt so that complete shunting does not occur. Such construc-tions and procedures are well known in the art and need not be described herein in detail, and a physical arrange-ment of such a transformer is ill~strated schematically in Fig. 11, wherein the transformer windings are desig-nated by the same numerals as previously and the magnetic 3~i;2 9L
shunts a~e designated as 140 and 142. This decoupling by the inductance permits ~he vectors representing the voltages at winding 112 and at winding 76 to be independently adjusted.

Such types of systems, their background~ 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 Tri-port UPS", Intelec 81, pp 210-215; and Z. Noworolski and X. Goszyk, ~High Efficiency Uninterruptible Power Supplyn, 4th International PCI Con~ere ~ , March, 1982, pp. 521-529.

In additlon, in the present embodiment the con-nection between khe 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 ~C power entirely from the battery and inverter.

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~ n this embodiment o the invention, a primary inventive feature is that the ratio R of the number o turns N2 of tranc,former winding 80 to the number of turns Nl of transformer winding 112 is other than unity, e.g.
in this example N2 ~ay be 46 t~rns and Nl may be 42 turns, ieO N2/Nl = 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/Nl of the turns of windings 80 and 112 being eprcsented by the tap position on an autotransformer which, in effect, increa~es the line voltage applied to the input end of inductance Ls from Eu to a 10% higher value Eu'. The series inductance Ls is effectively pro-vided, in the example of Fig. 10, by the transformer and the magnetic shunts bùilt into it. As described previous-ly 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 eficiency.

The transformer 78 in this example is o~ ~I
construction, with the magnetic sh~nts described previ-ously servinq to attenuate the nla~netic path between wind-ing 112 and windings 76 and 80, in this example giving an effec~ive value fo~ Ls Of abo~t ~ millihcnrics.

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Fig~res 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 EU=EI and Ei lags Eu by, for example, 23 1n a typical operating condition.
The difÇerence vector EL again represents the volta~e across the series inductance Ls, and the current through that inductance is represented by the vector IL 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 Io vector lies along the same direction as ~he Ei vector as shown.
The difference vector Ii then represents the substantial circulating current in the lnverter, which always exists under these conditions even though no real power is then being delivered to or from the inverter.

Figu~e 12~ shows conditions existing in a com-parable system modified according to the present inven-tion so that the line voltage Eu is in effect, transformed upwardly by a factor 1. 1 to a new value Eu', this increased value of Eu' being sufficient so that the EL vector is vertical and the IL vec~or, being at right angles to EL, lies directly along the direction of the inverter current Io and is equal thereto. It therefore supplies all o ~:3~

., the load current, leaving no current, reactive or real, in or out of the inverter, as is de~ired to produce the previously-desc~ibed improvements with regard to mini-mizing inverter current and improved through-put effi-ciency.

It can be seen from Figures 12A and 12B that varying the length of Eu' exerts an action as if the EL
and IL vectors were ~ixed at right angles to each other but rotatable together about the end of the Ei vector so that, by appropriate selection of the length of Eu', I~ can be turned into alignment with Io regardless of the direction of Io~ 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 o the invention may bé
included. In this case a microprocessor 300, such as a %80 microprocessor chip, controls the frequency and phase ~ of the invexter output sinewave and is supplied with appropriate program memory information Erom memory 302; with system personality information indicative of the particular application parameters from system per~
sonality 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 ~3~
~.
tO conditions o the line switch, the bypass switch, over-temperature, ovet-voltage or any other parameters which it is desired to monitor, by way of I/O por~ 310; and with a mutual interchange of information with an appro-priate display device ~12. The microproces50r also pre-ferably receives information from an interrupt control 360 with respect to such parameters as line voltage, in~
verter voltage, time an~ any other parameters found de-sirable. In this example, the microprocessoL controls a counter time~ chip (CTC) carrier generator 400 and a CTC 60Hz genera~or 402, which operate to produce on line 404 a carrier frequency equal to the repetition rate of the pulse-width modulated pulses (typically at 25 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 magni-~de. The PWM control 420 controls the invertet 422, which in this case is assumed to be the entire circuit o Figute 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 cir-: cuit 452 which detects and amplifies any differences between the voltage fed back from the inverter and the idealized sinewave ~rom sine genera~or 410, ~his di~fetence then , .

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~eing 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 util-ity line sinewave. However, to provide a suitable sinewave to the comparison circui~ upon a utility line outage, the microprocessor includes a stable crystal-cont~olled reference oscillator, powered by the battery, from which the desired ideal sinewave at the desired line frequency is derivedO

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

Figures l~A and 14B illustrate the bidirectional line conditioning obtained with applicants' isolated sys-tem. As shown in Fig. 14~, if the line voltage Eu con-sists of a sinewave with the noise spikes shown thereon, the inverter voltage Ei supplied to the load has the sub-stantially pure sinewave appearance shown in the latter figu~e; Figure l~B shows that even if the load current were distorted as s~own, this would not reflect back into, or substantially distort, the utility supply line current Iu, which remains a substantially pure sinewave.

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The preferred embodiment of the invention has been shown as ~tili~ing a trans~ormer in which magnetic shunts provide the efective series inductance Ls, and the voltage step-up R is p~ovided by the ratio N2/~1 ln the number of turns of entirely separate and isolated transformer windings. ~owever, many of the advantages of the invention with regard to minimi~ing inverter cur-rent and maximizing 'hrough-put can be obtained where the series inductance Ls is in fact a real lumped-circuit series inductor connected between the high side of the line and the inverter output, much as represented sche-matically in the simplified equivalent circuit of Fig.
7, and it is in fact possible to utilize an autotrans-ormer as suggested by the equivalent circuit of Fig.
7 rather than the completely isolated transformer winding arrangement of the preerred embodiment.

Also, although the ratio o Eu'/Eu of 1.1 has been found pre~erable for many practical pur~oses, the minimum inverter current may in some instances occur at a different value than 1.1, in which case R may be dif-ferently chosen to minimize inverter current during break-even 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 frorn 1.1, so as to minimize the required inverter current during normal operation.

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Thus while the invention has been described with respect to certain specific embodiments in the in-terest of complete definiteness, it will be understood that it may be embodied in a variety of forms diverse fr;om tho~e speciically shown and described without de-parting from the spirit and scope of the inYentio~ as defined by the appended claims.

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., TABLE I
-Eu = 1.0Load PF = 1 Load PF - 0.8 lag Load PF - 0.9 lead ~5Breakeven Charge sreakeven Charge Breakeven Charge _ .
0.2 P.U.Ii 0.19 0.39 0.77 0.87 _ 0.34 0.49 ~P 13 15.5 12 14 12 15.5 -0.4 P.U.Ii 0.31 0.55 0.86 1.01 0.25 0.36 ~ 27 33 24 29 25 30 0.6 P.~.Ii 0 53 0.87 1.04 1.12 0.17 0.~1 ~ ~5 57 40 48 4~O 50O

Eu ~ 0.85 (-15%) 0.2 P.U.Ii 0 99 1.12 1.5S 1.66 1.51 0.67 ~ 17 20 15 18 15 18 _ 0.6 P.U.Ii 0-77 1.02 1.29 1.46 0.25 0.49 ~ 3~ ~2 30o 36 29 36 0.6 P.U.Ii 0 99 1.61 1.39 1.75 0~37 0.74 ~ 6~ 85 S0 65 500 65 _ , .

T~.r.E 2 Eu = 1.1 Load PF = 1 Load PF = 0.8 lag Load PF = 0.9 lead L5Breakeven Charge Breakeven Charge Breakeven Charge .
0.4 P.U. Ii 0.12 0~35 0.58 0.72 0.49 . 0.51 2~ 29 21 25 23 27 _ Eu = 0~935 (-15%) 0.4 P.U. Ii 0.52 0.72 1.03 1.21 0.12 0,35 y 7 11 4 8 4 8 -

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

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 in-verter 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 coupl-ing 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 transis-tors 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 volt-age 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-modula-tion 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 con-nected 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 sup-plying 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 re-presentative of the voltage across said load apparatus back to an input of said comparison means, a sinewave generator having the desired frequency and form for the voltage 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 signal representa-tive of departures of said voltage across said load apparatus from its desired sinusoidal form, and PWM control means respon-sive to said error signal for controlling conduction of said transistors in said bridge to cause said voltage across said load apparatus to conform with said sinewave from said generator.