CA1317635C

CA1317635C - A.c. power supply apparatus

Info

Publication number: CA1317635C
Application number: CA000592977A
Authority: CA
Inventors: Nobuo Sashida; Takao Kawabata
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1988-04-27
Filing date: 1989-03-07
Publication date: 1993-05-11
Anticipated expiration: 2010-05-11
Also published as: JPH07108092B2; JPH01278266A; DE3912941A1; KR890016743A; KR910008549B1

Abstract

ABSTRACT OF THE DISCLOSURE

An a.c. power supply apparatus, categorized to be an uninterruptible power supply apparatus or a fuel cell power generation apparatus, has its first, second and third converting devices connected in star configuration with a common bus, whereby the number of power converting devices is reduced and a compact, light weighty and efficient power supply apparatus is realized.

Description

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A.C. POWER SUPPLY APPARATUS

BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to an a.c. power supply apparatus categorized to be an uninterruptible power supply (UPS) or a fuel cell power generation apparatus.
Prior art A.C. power supplies will be discussed hereinbelow in conjunction with the drawings.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an a.c.
power supply apparatus which is compact, light weight, efficient and economical by using a small number of converting devices.
In accordance with one aspect of the invention there is provided an a.c. power supply apparatus, comprising: a first controllable converting device which receives a.c. power of a first frequency from a power source, converts the frequency of said a.c. power into a second frequency higher than said first frequency, converts the voltage of said a.c. power to a value equal to a demanded voltage control signal, thereby providing converted a.c. power, and provides said converted a.c. power to a bus; a second voltage inverter type converting device which receives d.c. power and converts said d.c. power into a.c. power of said second frequency and provides said converted d.c. power to said bus, and converts a.c. power from said bus into d.c. power to charge a battery, said second 131 763~

~onverting device being responsive to a voltage control signal for controlling the voltage level of said converted a.c. and d.c. power; and a third converting device which is supplied with a.c. power of said second frequency from one of said first and second converting devices through said bus, converts the supplied power into a.c. power of a third frequency, and provides the a.c. power of said third fre~uency to a load.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram showing the arrangement of the conventional a.c. power supply apparatus;
Fig. 2 is a block diagram showing the arrangement of the conventional a.c. power supply apparatus based on the high-frequency intermediate link method;
Figs. 3 and 4 are block diagrams showing other arrangements of the conventional a.c. power supply apparatus;
Fig. 5 is a block diagram showing the inventive a.c.
power supply apparatus;
Fig. 6 is a circuit diagram of the apparatus shown in Fig. 5;
Fig. 7 is a circuit diagram of switching circuits used in the cycloconverter;
Fig. 8 is a waveform diagram used to explain the operation of the cycloconverter;
Figs. 9 and 10 are block diagrams showing other embodiments of this invention:
~ig. 11 is a circuit diagram showing another embodim~nt which converts commercial power into high-frequency power; and Fig. 12 is a block diagram showing the inventive ~ -?
",, . ~ ' , 1317~3~i control circuit for the a.c. power supply apparatus.
Descri~ion of the Prior Art Fig. 1 shows the arrangement of a typical conventional UPS. In the figure, a charger 3 converts a.c~ power of frequency f2, received through a transformer T1 from a commercial power source 5, into d.c. power, and supplies the d.~. power to a voltage-type inverter 1 while charging a battery 2.
The inverter 1 converts the d~c. power into a.c. power of frequency f3 including less amount of low-order harmonics. The aOc. power is made sinusoidal by being fed through a filter constructed of a reactor Ls and capacitor Cp, and, by being fed through a transformer T2 so as to meet a load voltage, it is supplied to a load 4.
Generally, the load of UPS, e.g., a computer, is isolated from the power source line for the protection against noises and provided with an exclusive ground, in most cases, and therefore the transformer T1 is required for the purpose of power line isolation as well as voltage matching.
Although, in some cases, the input power transformer T
is omitted, the inverter 1 has its d.c. output voltage ~etermined on the basis of economy of the inverter 1 and battery 2, and therefore the transformer T1 is used to ~-5 provide a pr-per voltagc for the lnv-rtcr and also to 13176~
isolate the load line from the power source line, in most cases.
Accordingly, a most orthodox-designed conventional UPS
necessitates two heavy and bulky transformers, and this hampers the reduction in the size and weight of the UPS.
A no~el apparatus which has been developed with the intention of overcoming the above problem is the high-frequency int~rmediate link method as shown in Fig. 2.
The UPS apparatus shown in Fig. 2, with the function n similar to that of Fig. 1, is based on the high-frequency intermediate link DC/AC inverter disclosed in an article entitled "Classification of Inverters and Their Characteristics", Electric Review, Fig. 14 in pp. 987 -992, Nov. 1981.
In the figure, an inverter 1 is a voltage-type inverter producing a single-phase rectangular waveform of f1 = 10 kHz for example, and it supplies the output to a cycloconverter 6 through a transformer T2 for isolation.
The cycloconverter 6 converts the frequency of a.c. power from f1 to f3 = 60 Hz for example, and the power is fed through a filter constructed of a reactor Ls and capacitor Cp, so that it becomes sinusoidal, and supplied to a load 4. This apparatus has its transformer T2 designed to operate at 10 kHz, and therefore it can be compact and 25~ light~weight. However, the apparatus necessitates a transformer T1 of the commercial power fre~uency f2 for a ~ 1317~3~

charger 3, as in the case of Fig. 1.
Fig. 3 shows a more advanced apparatus, in which the same high-frequency intermediate link method is further applied to the charger 3 in consideration that the DC/AC
inverter of Fig. 2 is reversible. Although this apparatus can have compact transformers, power is transmitted through two cycloconverters 6 and 8 and two inverters 1 and 7 between the commercial power input and power output, resulting in a degraded efficiency and increased cost of converters. Accordingly, the apparatus of Fig. 3 is less practical from the view point of economy and efficiency.
A more innovative apparatus intended to overcome the above deficiencies is offered in an article entitled "Small UPS Using Phase Control", INTELEC '87 Conference Proceedings, Session 12, Fig. 16(b) in pp. 516 - 520. The apparatus, which does not necessitate a charger, is shown in Fig. 4 in the same depictive manner as of Figs. 1 through 3.
In this apparatus~ when the commercial power source 5 is normal, a.c. power of frequency f2 from the commercial power source 5 is directly supplied through a switch SW to the load 4 and at the~same time fed to a cycloconverter 6 through a filter ~ormed of a reactor Ls and capacitor ep.
~The converter 6 converts the power to have a frequency f1, 25 ~ which is fed through a transformer Tz to an inverter 1, which then produces d.c. power to charge the battery 2.

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When the commercial power source 5 is out, the switch SW is opened so that power of the battery 2 is fed through the inverter 1, transformer T2, cycloconverter 6 and filter and supplied to the load 4.
This apparatus is highly practical because of its need of only two converting devices, however, power with the same voltage and frequency as those of the commercial power source is supplied to the load 4, and therefore it is not suited to applications which require precise constant frequency.
DETAILED DESCRIPTION OF THE PREFERRED ~MBODIMENTS
Preferred embodiments of this invention will be described in detail with reference to the accompanying drawings.
Fig. 5 shows in block diagram the inventive a.c. power supply apparatus, in which indicated by 10 is an inverter, 11 is a converting device, and 12 is a cycloconverter.
The remaining functional blocks identical to tho~e of Figs. 1 through 4 are referred to by the common symbols and explanation thereof will not ba repeated.
Fig. 6 shows a circuit diagram of the apparatus shown in Fig. 5. In Fig. 6, the inverter 10 is a rectangular-wave inverter made up of transistors Q1 - ~4 and diodes D1 - D4, and it supplies a rectangular-wave voltage derived from the voltage of the battery 2 to a bus B1. The inverter 10 does not have voltage control and has its frequency fixed, and it establishes the .

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voltage and frequency on the bus Bl as reference values for the whole apparatus.
The converting device 11 is constructed of a diode rectifier ll-B, a transistor inverter ll-A and a capacitor CD, and it operates to convert a.c. power of the commercial power source 5 into a.c. power with the same fre~uency as of the inverter 10. The inverter ll-A
has PWM control and ph~se control for its output voltage with respect to the bus Bl voltage, thereby controlling the output power fed to the bus Bl and also controlling power for charging the battery 2 through the inverter 10 and power delivered to the load through the cyc~loconverter 12.
The reactor LA has an impedance of several ~ to 30%
P.U., and the purpose thereof is:to suppress harmonic currents caused by the difference between the non-controlled rectangular wave on the bus Bl and the PWM wave produced by the inverter ll-A and also to ,~
~acilitate t~e control of the power fed from the ~ 20 invertex ll-A to the bus Bl.

: Since the voltage on the bus Bl is fixed to the , ; : ~ :virtually complete rectangular wave by the battery 2 and a smoothing capacitor~CB in paraIlel connection with it, the~cycloconver~ter lZ oper:ates~;~independently of the 25~ inverter ll-A without the mutual influence. With fl being set~to several~kHz or~above and f3 being set to : 60 Hzj the cycloconverter 12 can be any of the line :..
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commutation t~pe or self commutation type. Switches Sl - S6 may be of the sel~ commutation type as shown by (a) and (b) in Fig. 7.
The operation of the embodiment shown in Fig. 6, particularly -the operation of the cycloconverter 12, will be explained in more detail. By designing the transformer T2 to have a sufficiently small leakage inductance, it produces on the secondary winding the same rectangular wave as that on the bus Bl, as shown by (a) in Fig. 8. A surge absorbing capacitor CA is provided to facilitate the switching of the cycloconverter 12.
In the first positive half cycle of VRs in Fig. 8, closing the switch Sl provides a positive voltage at point X, or closing the switch S2 provides a negative voltage a-t point X. In a negative half cycle of VRs, the same voltages appear at point X by the switching S1 and S2 vice versa. Closing the Sl and S2 simultaneously results in a short-circuit on the transformer secondary winding and it must be avoided, while leaving both the Sl and S2 open results in the absence of current path : for the reactor LSu and it rnust be avoided.
: ~ In a~ha~f cycle of the secondary voltage VR5 of the transformer T2 shown in (a) of Fig.~8, a saw-tooth wave ; ~ 25: as shown in (b) of Fig. 8 is~ generated, so that the timing of switchir~g the Sl and S2 is determined at the : intersection of the saw-tooth wave and a control signal :: :

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level shown by the dashed line. As shown by (c) of Fig. 8, the voltage Vx of the point X relative to the virtual neutral point, which may be the center tap of the secondary windiny of T2, increases progressively as the control signal level rises. It will be understood from the figure that the timing of switching is determined from the relation of the levels oE the saw-tooth wave and control signal and from the polarity of the voltage V~S .
For a positive half cycle of T at R relative to S, where T= l/(2fl), when the switch Sl is closed in the former hal TA of that pexiod and the switch S2 is closed in the latter half TB = T - TA, the average voltage of the point X relative to the virtual neutral point N in the period ~ is evaluated as follows.

VX = VS(2T~/T - 1) where Vs i9 the voltage between R and S. Accordingly, by controlling the T~, the average voltage at point X
can be varied in a range from ~Vs to Vs.
~ These are the operation~oE the U-phase of the cycloconverter in Fig. 6. The V-phase and W~phase are ; also~provlded with comparators for comparing the saw tooth ~wave so as to control a switch pair S3 and 54 and another switch pair S5 and S6, respectively. By :
appIying the control signals corresponding to the intended 3-phase output voltage to the three ::
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comparators, the voltages at points X, Y and Z have their mean values varying in 3-phase sinusoidal waveforms, and, after being fed through the filters, the 3 phase sinusoidal voltages are delivered to the output terminals U, V and W.
The two transformers T1 and T2 used in the embodiment of Fig. 6 can be integrated to a single transEormer T4 as shown in FigO 9, in which the remaining portions identical to those of Fig. 6 are drawn as blocks. In the figure, the transformer T4 has three windings, and the output of the converting device 11 is mostly fed through the windings Wl and W2 to the cycloconverter 12. The output is partially fed through the winding W3 and rendered AC/DC conversion by the reverse operatio~ of the inverter lO thereby to charge the battery 2.
In the occurrence of power outage, the inverter 10 performs DC/AC conversion for the power of battery 2, and it is supplied through the windings W3 and W2 to the cycloconverter 12. Accordingly, the apparatus of Fig. 9 always transmits power through only one transformer T4, and i~t is superior in the efficiency and economy. It is also possible in Fig. 9 to connect~the output of the inverter lO to the winding Wl or W2, with the winding W3 b~eing omitted.
In the above explanation, the inverter 10 is operated uninterruptedly, whi~le an al~ternative apparatus - 10 -- `

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13176~5 is possible in which the inverter 10 is operated only during power outage. In this case, a larger capacitor CA of 40 - 100~ PU is used so as to stabilize the bus B
voltage to be sinusoidal during the operation of the inverter 10. In order to absorb the voltage difference between the sinusoidal wave of the bus Bl and the rectangular-wave of the inverter 10, it is recommended to provide a reactor of 20 - 30% PU in series to the output of the inverter 10. The cycloconverter 12 implements phase control on the basis of the varying single-phase sinusoidal wave on the bus Bl, thereby producing 3-phase sinusoidal waves.
It is also possible for this embodiment to charge the battery 2 as follows. The inverter ll-A has PWM
control to change the voltage of the bus Bl, which is rectified by the diodes Dl - D4 of the inverter 10 to charge the battery 2, In this operation, the transistors Ql - Q4 are kept cutoff. In the occurrence of power outage, the inverter 10 is activated immediately so that the bus voltage is retained. In this case, even though the battery voltage is varied by `
the PWM control of the inverter 10, the voltage on the bus Bl can be maintained constant.~;

Although the inverter 10 lS a~single-phase inverter ;25~ in the foregoing embodiment, it is known that the ; ~ cycloconverter 12 is operable on a 3-phase sinusoidal-; wave power source.~ Accordingly, the inverter 10 in : ~ , :: :

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Fig. 9 can be replaced with a 3-phase sinusoidal-wave inverter as shown in Fig. 10. In the figure, the 3-phase inverter consists of three single-phase bridge inverters 10-A, 10-B and 10-C operating under pulse width control of l-pulse PWM signal to maintain the voltage of capacitors CA irrespective of the voltage variation on the battery 2.
The converting device 11 controls the phase of its output voltage relative to the voltage of the capacitors CA, thereby regulating power introduced to the winding Wl through the reactors LA. The cycloconverter 12 produces stabiliæed 3-phase, 60 Hz power on the output te~minals U, V and W on the basis of the stabilized high-frequency 3-phase voltages established on the capacitors CA. The apparatus with a 3-phase intermediate link provides a satisfactory output waveform even with a relatively low intermediate link frequency, and it is suitable for a large-capacity power UPS .
Although the embodiment of Fig. 6 has the converting device 11 constructed in combination o~ the rectifier ll-B and inverter ll-A, it may be replaced with a 3-to-2 phase converting cycloconverter as shown in Fig. 11. Switches Sl through S6 in the figure may be of the type shown in Fig. 7O
Next, the control circuit of the inventive a.c.
power supply apparatus will be described with reference ~ ' ~317~3~
to Fig. 12. In this embodiment, the bus Bl has a single-phase sinusoidal waveform of high frequency fl under constant-voltage, constant-frequency control by the inverter 10, and the cycloconverter 12 and inverter 11 are controlled with reference to the bus voltage~
Transformers are omitted for simplification.
The inverter 10 is of the single-phase bridge type and operates to regulate the voltage of the bus Bl under control of l-pulse PWM signal. The inverter 10 has its operating frequency fixed by an oscillator OSC. A
voltage controller VC2 controls the output pulse width of PWM2 in accordance with a feedback signal provided by a voltage sensor VS2, thereby maintaining the bus voltage VB aonstant.
The cycloconverter 12 performs phase control for the sinusoidal single-phase power on the bus Bl to produce single-phase 60 Hz sinusoidal-wave power. With the hus Bl having a sufficiently high frequency of 600 Hz or above relative to the 60 Hz output frequency, a filter formed of a relatively small reactor Ls and capacitor Cp can remove harmonics enough to produce a sinusoidal wave with a distortion Eactor of 8 - 5~ or .

less, in general. ~ -The cycloconverter control circuit is provided with 25~ a minor loop for controlling the in=tantaneous value of the output current. By~providing a reference current of Ic* = Icm cos ~t = ~CpVcm cos ~t for the output filter oapacitor Cp current, the no-load~voltage is established.

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The load current IL is fed forward so that the apparatus is responsive to the load variation and operates as a low-impedance voltage source. Finally, a sinusoidal voltage reference generator REF produces a voltage reference Vc* = Vcm sin ~t, and the voltage controller VC3 operates to nullify the difference between the ac-tual voltage and the reference.
The sum of -these three signals is limited by a limiter LIM below the allowable current of the cycloconverter, and it is applied as a reference value to the current minor loop. Consequently, the cycloconverter 12 produces 60 Hz sinusoidal single-phase power from the single-phase high-frequency power established on the bus 1 .
Next, the control of the inverter 11 which supplies demanded power to the system will be described. The inverter 11 has its frequency and phase determined by a voltage-controlled oscillator VCO. The VC0 has a center ; frequency set to fO = mfl and, after it is divided to f by a m-blt counter CNTl, it is fed to a modulation circuit PWMl. The PWM circuit issues a l-pulse PWM
signal to the inverter ll, thereby controlling the :, ;output~voltage of the inverter 11. The voltage control is~intended to bring the voltage Vl in its mean value at 25~ the front of the reactor LA to the reference value Vl*, i.e.;,~the voltage controller VCl nullifies the signal Vl* - Vl based on the mean value of Vl provided by the voltage sensor VSl.

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The center phase of the generated voltage Vl of the inverter 11 is evaluated from CNTl, ana the lead angle ~ relative to the bus Bl voltage VB is detected by a phase detector PD. A PPL amplifier Al controls for the ~ in correspondence to the demanded power. Since the most of the demanded power of the system is the input to the cycloconverter 12, the power Pl is evaluated by a multiplier MLT and, after being smoothed by a filter FIL, applied to the PLL amplifier Al as the phase lQ difference command ~1*
For charging the battery 2, an amplifier A2 is operated so that the difference between the actual -~alue VD and voltage command VD* is nullified, and a phase signal ~2* corresponding to the charging power is applied to the PLL amplifier Al. Furthermore, a phase differential angle signal ~3* for compensating the no-load loss of the inverter lO is applied as a bias to the PLL amplifier Al. In this way, the PLL amplifier A
finely adjusts the frequency of the oscillator VCO, and the INVl supplies the system demand power to the bus B.

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Claims

1. An a.c. power supply apparatus, comprising:
a first controllable converting device which receives a.c. power of a first frequency from a power source, converts the frequency of said a.c. power into a second frequency higher than said first frequency, converts the voltage of said a.c. power to a value equal to a demanded voltage control signal, thereby providing converted a.c. power, and provides said converted a.c. power to a bus;
a second voltage inverter type converting device which receives d.c. power and converts said d.c. power into a.c.
power of said second frequency and provides said converted d.c. power to said bus, and converts a.c. power from said bus into d.c. power to charge a battery, said second converting device being responsive to a voltage control signal for controlling the voltage level of said converted a.c. and d.c.
power; and a third converting device which is supplied with a.c.
power of said second frequency from one of said first and second converting devices through said bus, converts the supplied power into a.c. power of a third frequency, and provides the a.c. power of said third frequency to a load.

2. An a.c. power supply apparatus according to claim 1, further comprising:
reactor means for eliminating harmonic frequencies of said source a.c. power connected between an output terminal of said first converting device and said bus; and capacitor means connected across input terminals of said third converting device for smoothing converted power from said second converting device.

3. An a.c. power supply apparatus according to claim 1, wherein said first converting device includes a diode rectifier, a voltage-type inverter and a capacitor.

4. An a.c. power supply apparatus according to claim 2, wherein said reactor means has an impedance ranging from several percentage P.U. to 30% P.U.

5. An a.c. power supply apparatus according to claim 1, wherein each of said first and second converting device comprises a 3-phase sinusoidal-wave inverter.

6. An a.c. power supply apparatus according to claim 1, wherein said third converting device comprises a cycloconverter of the line commutation type.

7. An a.c. power supply apparatus according to claim 2, wherein said second converting device controls the voltage and the frequency of a voltage across said capacitor means;
said first converting device controls the phase angle of output voltage according to a demand power of said third converting device and the d.c. side voltage of said second converting device; and said third converting device produces sinusoidal a.c.
power from high frequency power established in said capacitor means, and controls the voltage and the frequency of said sinusoidal a.c. power constantly.

8. An a.c. power supply apparatus according to claim 1, wherein said third converting device comprises a cycloconverter of the self-commutation type.