Power factor correction circuit, power factor correction assembly and on-line uninterruptible power supply comprising same

The present invention provides a power factor correction circuit (21, 22), a power factor correction assembly (2) and an on-line uninterruptible power supply including the same. The power factor correction circuit (21) comprises a pulse width modulated rectifier (211, 221) and an isolated DC-DC converter (212, 222), wherein an output of the pulse width modulated rectifier (211, 221) is connected to an input of the isolated DC-DC converter (212, 222). The power factor correction assembly (2) comprises a plurality of power factor correction circuits (21, 22) described above, wherein inputs of pulse width modulated rectifiers (211, 221) in the plurality of power factor correction circuits (21, 22) are connected in series, and outputs of isolated DC-DC converters (212, 222) in the plurality of power factor correction circuits (21, 22) are connected in parallel. The power factor correction assembly (2) of the present invention needs no line-frequency transformer and has the advantages of small size, low cost and improved operation reliability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 371 National Stage Entry of International Application No. PCT/EP2019/025432 having an international filing date of Dec. 3, 2019, which claims the benefit of Chinese Patent Application No. 201811473139.2 entitled POWER FACTOR CORRECTION CIRCUIT, POWER FACTOR CORRECTION ASSEMBLY AND ON-LINE UNINTERRUPTIBLE POWER SUPPLY COMPRISING SAME, filed Dec. 4, 2018, the disclosures of each of which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to the field of electronic circuits, and in particular, to a power factor correction circuit, a power factor correction assembly and an on-line uninterruptible power supply comprising the same.

BACKGROUND

The operating principle of an on-line uninterruptible power supply is as follows: no matter whether the grid voltage is normal, an alternating current voltage used by a load needs to pass through an inverter so that the inverter is always in an operating state. Given that on-line uninterruptible power supplies can continuously supply power to the load, they have been widely applied in various fields.

FIG.1is a circuit block diagram of an on-line uninterruptible power supply in the prior art. The on-line uninterruptible power supply1includes a power factor correction circuit11, an inverter12, and a bi-directional DC-DC converter13.

When a low-voltage alternating current Vi′ (for example, mains supply) is normal, a control device (not shown inFIG.1) controls the power factor correction circuit11to convert the low-voltage alternating current Vi′ at the input thereof into a direct current and then transmit the direct current to positive and negative direct-current buses10,10′, and controls the bi-directional DC-DC converter13to transmit electric energy on the positive and negative direct-current buses10,10′ to a rechargeable battery B, so as to charge the rechargeable battery B. When a failure occurs in the low-voltage alternating current Vi′, the control device controls the bi-directional DC-DC converter13to transmit the direct current in the rechargeable battery B to the positive and negative direct-current buses10,10′. No matter whether the low-voltage alternating current Vi′ is normal, the inverter12is controlled to convert the low-voltage direct current on the positive and negative direct-current buses10,10′ into a low-voltage alternating current and then supply power to the load.

However, considering performance parameters of a power switching tube in the on-line uninterruptible power supply1, the existing on-line uninterruptible power supply cannot be directly connected to an alternating-current power supply of a medium-voltage distribution network (1 kilovolt to 35 kilovolts). Accordingly, a bulky line-frequency transformer must be added to first convert a medium-voltage alternating current into a low-voltage alternating current and then transmit the low-voltage alternating current to an alternating current input of the on-line uninterruptible power supply. Under a high-power load, transmission lines for the low-voltage alternating current generate large costs and loss, thus lowering the profitability and efficiency of the entire power distribution system. In addition, an incoming cabinet and an outgoing cabinet matching the line-frequency transformer must be added, which increases equipment cost and floor area of the entire equipment, thus causing low power density.

SUMMARY

In view of the aforementioned technical problem in the prior art, an embodiment of the present invention provides a power factor correction circuit, comprising a pulse width modulated rectifier and an isolated DC-DC converter, wherein an output of the pulse width modulated rectifier is connected to an input of the isolated DC-DC converter.

Preferably, the pulse width modulated rectifier is a full-bridge pulse width modulated rectifier or a half-bridge pulse width modulated rectifier.

Preferably, the isolated DC-DC converter comprises:an inverter, wherein an input of the inverter is connected to the output of the pulse width modulated rectifier;a transformer, wherein a primary side of the transformer is connected to an output of the inverter; anda rectifying device, wherein an input of the rectifying device is connected to a secondary side of the transformer, and an output of the rectifying device serves as an output of the isolated DC-DC converter.

Preferably, the inverter is a full-bridge inverter or a half-bridge inverter, the transformer is a high-frequency transformer having a working frequency greater than 10 kHz, and the rectifying device is a full-bridge pulse width modulated rectifier, a half-bridge pulse width modulated rectifier, or a bridge rectifying circuit.

Preferably, the power factor correction circuit further comprises a first fuse and a second fuse connected to an input of the pulse width modulated rectifier, and a third fuse and a fourth fuse connected to the output of the isolated DC-DC converter.

Preferably, the power factor correction circuit further comprises a first switch, a second switch, a third switch, and a fourth switch respectively connected in series to the first fuse, the second fuse, the third fuse, and the fourth fuse.

The present invention further provides a power factor correction assembly, comprising a plurality of power factor correction circuits described above, wherein inputs of pulse width modulated rectifiers in the plurality of power factor correction circuits are connected in series, and outputs of isolated DC-DC converters in the plurality of power factor correction circuits are connected in parallel.

Preferably, each of the power factor correction circuits further comprises a first fuse and a second fuse connected to the input of the pulse width modulated rectifier, a third fuse, and a fourth fuse connected to the output of the isolated DC-DC converter, and a first switch, a second switch, a third switch, and a fourth switch respectively connected in series to the first fuse, the second fuse, the third fuse, and the fourth fuse; and the power factor correction assembly further comprises fifth switches each connected between inputs of the power factor correction circuits.

Preferably, the power factor correction assembly further comprises a first control device, whereinwhen the plurality of power factor correction circuits are all normal, the first control device controls the first switch, the second switch, the third switch, and the fourth switch in each of the power factor correction circuits to all turn ON, and controls the fifth switches in the power factor correction assembly to all turn OFF; andwhen the plurality of power factor correction circuits include a failed power factor correction circuit, the first control device controls a fifth switch connected to an input of the failed power factor correction circuit to turn ON, and controls a first switch, a second switch, a third switch, and a fourth switch in the failed power factor correction circuit to all turn OFF, and meanwhile controls first switches, second switches, third switches, and fourth switches in other power factor correction circuits to all turn ON, and controls other fifth switches in the power factor correction assembly to turn OFF.

The present invention further provides an on-line uninterruptible power supply, comprising:the power factor correction assembly described above, wherein an output of the power factor correction assembly is connected to positive and negative direct-current buses;a bi-directional DC-DC converter assembly, connected between a rechargeable battery and the positive and negative direct-current buses; and an inverter assembly, wherein an input of the inverter assembly is connected to the positive and negative direct-current buses, and an output of the inverter assembly is used for providing an alternating current.

Preferably, the bi-directional DC-DC converter assembly comprises a plurality of bi-directional DC-DC converters connected in parallel.

Preferably, each of the plurality of bi-directional DC-DC converters comprises:a bi-directional DC-DC conversion circuit, having a first connection end and a second connection end;a sixth fuse and a seventh fuse respectively connected to a positive terminal and a negative terminal of the first connection end of the bi-directional DC-DC conversion circuit, and a sixth switch and a seventh switch respectively connected in series to the sixth fuse and the seventh fuse; andan eighth switch and a ninth switch respectively connected to a positive terminal and a negative terminal of the second connection end of the bi-directional DC-DC conversion circuit.

Preferably, the on-line uninterruptible power supply further comprises a second control device, whereinwhen the plurality of bi-directional DC-DC converters are all normal, the second control device controls the sixth switch, the seventh switch, the eighth switch, and the ninth switch in each of the plurality of bi-directional DC-DC converters to all turn ON; andwhen the plurality of bi-directional DC-DC converters include a failed bi-directional DC-DC converter, the second control device controls a sixth switch, a seventh switch, an eighth switch, and a ninth switch in the failed bi-directional DC-DC converter to all turn OFF, and controls sixth switches, seventh switches, eighth switches, and ninth switches in other bi-directional DC-DC converters to all turn ON.

Preferably, the inverter assembly comprises a plurality of inverters connected in parallel.

Preferably, each of the plurality of inverters comprises:an inversion circuit;a tenth fuse and an eleventh fuse respectively connected to a positive input terminal and a negative input terminal of the inversion circuit;a tenth switch and an eleventh switch respectively connected in series to the tenth fuse and the eleventh fuse; andan output switch connected to an output terminal of the inversion circuit.

Preferably, the on-line uninterruptible power supply further comprises a third control device, whereinwhen the plurality of inverters are all normal, the third control device controls the tenth switch, the eleventh switch, and the output switch in each of the plurality of inverters to all turn ON; andwhen the plurality of inverters include a failed inverter, the third control device controls a tenth switch, an eleventh switch, and an output switch in the failed inverter to all turn OFF, and controls tenth switches, eleventh switches and output switches in other inverters to all turn ON.

The power factor correction circuit and power factor correction assembly of the present invention both do not require a line-frequency transformer and have the advantages of small size and low cost. An input of the power factor correction assembly can receive a medium-voltage alternating current, so as to improve system efficiency and operation reliability and implement the on-line service and hot-swap function.

DETAILED DESCRIPTION

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below through specific embodiments with reference to the accompanying drawings.

FIG.2is a circuit diagram of a power factor correction assembly according to a first embodiment of the present invention. As shown inFIG.2, the power factor correction assembly2includes a power factor correction circuit21and a power factor correction circuit22. Inputs of the power factor correction circuits21,22are connected in series and then connected to a medium-voltage alternating current Vi (for example, 10 kilovolts). Outputs are connected in parallel between a positive direct-current bus20and a negative direct-current bus20′.

The power factor correction circuit21includes a full-bridge pulse width modulated rectifier211and an isolated DC-DC converter212that are cascaded. The full-bridge pulse width modulated rectifier211includes four metal-oxide-semiconductor field effect transistors and a capacitor C21. The four metal-oxide-semiconductor field effect transistors are provided with a pulse-width modulated signal so that the current and voltage of an alternating current at an input of the full-bridge pulse width modulated rectifier211have the same phase, a power factor of the alternating current approximates 1, and meanwhile a required direct current is obtained on the capacitor C21.

The isolated DC-DC converter212includes a half-bridge inverter2121, a transformer Tr2and a half-bridge pulse width modulated rectifier2122. An input of the half-bridge inverter2121is connected to an output of the full-bridge pulse width modulated rectifier211, and an output of the half-bridge inverter2121is connected to a primary side of the transformer Tr2. An input of the half-bridge pulse width modulated rectifier2122is connected to a secondary side of the transformer Tr2, and an output of the half-bridge pulse width modulated rectifier2122is connected to (or serves as) the positive direct-current bus and the negative direct-current bus20′.

The half-bridge inverter2121is controlled to convert the direct current on the capacitor C21into a high-frequency alternating current and output the high-frequency alternating current to the primary side of the transformer Tr2, so as to obtain a high-frequency alternating current on the secondary side of the transformer Tr2. The half-bridge pulse width modulated rectifier2122is controlled to convert the alternating current on the secondary side of the transformer Tr2into a direct current and transmit the direct current to the positive and negative direct-current buses20,20′.

The power factor correction circuit22has the same circuit structure as that of the power factor correction circuit21, and also includes a full-bridge pulse width modulated rectifier221and an isolated DC-DC converter222that are cascaded. An input of the full-bridge pulse width modulated rectifier221is connected in series to the input of the full-bridge pulse width modulated rectifier211. That is, one input terminal of the full-bridge pulse width modulated rectifier221is connected to one input terminal of the full-bridge pulse width modulated rectifier211, and the other input terminal of the full-bridge pulse width modulated rectifier221and the other input terminal of the full-bridge pulse width modulated rectifier211are connected to the alternating current Vi. An output of the isolated DC-DC converter222and an output of the isolated DC-DC converter212are connected in parallel between the positive direct-current bus20and the negative direct-current bus20′.

A control device (not shown inFIG.2) controls the full-bridge pulse width modulated rectifiers211,221to each operate, and the metal-oxide-semiconductor field effect transistors in each full-bridge pulse width modulated rectifier withstand a decreased voltage, so as to reduce the risk of the power factor correction assembly2failing and so that the input of the power factor correction assembly2can receive a medium-voltage alternating current having a large voltage value. Since the transmission efficiency of the medium-voltage alternating current is higher than the transmission efficiency of a low-voltage alternating current, the efficiency is improved.

In addition to achieving the function of reducing voltage, the transformer Tr2further isolates a medium-voltage grid connected to the primary side from the low-voltage alternating current connected to the secondary side, thereby improving safety.

Meanwhile, since the magnetic core of the transformer Tr2provides a magnetic loop for an alternating magnetic field produced by the high-frequency alternating current, magnetic saturation is not caused even with a magnetic core of very small size. Thus a high-frequency transformer of small size can be used, for example, a high-frequency transformer having a working frequency greater than 10 kHz is used. Compared with a line-frequency transformer of large size, the total size of all high-frequency transformers in the power factor correction assembly2in this embodiment is far smaller than the size of one line-frequency transformer; at the same time, a cabinet of small size can be used, and the plurality of power factor correction circuits in the power factor correction assembly2in this embodiment can have a small spacing between circuit modules during connection and assembly, so that the entire cabinet can be designed more compactly.

In other embodiments of the present invention, the power factor correction assembly2includes more than two power factor correction circuits. In practical applications, an appropriate number of power factor correction circuits are selected according to the voltage value of the alternating current Vi and the withstand voltage value of the metal-oxide-semiconductor field effect transistors. For example, when the alternating current Vi is a medium-voltage alternating current, far more than two power factor correction circuits are selected; inputs of the power factor correction circuits are connected in series and then connected to the medium-voltage alternating current; and outputs of the power factor correction circuits are connected in parallel between the positive direct-current bus and the negative direct-current bus. When the number of the power factor correction circuits increases, the input of the power factor correction assembly2can receive an alternating current having a larger voltage value.

FIG.3is a circuit diagram of a power factor correction assembly according to a second embodiment of the present invention. As shown inFIG.3, the power factor correction assembly3is basically the same as the power factor correction assembly2inFIG.2. The difference lies in that the isolated DC-DC converter312includes a full-bridge inverter3121connected to a primary side of a transformer Tr3and a full-bridge pulse width modulated rectifier3122connected to a secondary side of the transformer Tr3. The isolated DC-DC converter322is the same as the isolated DC-DC converter312and will not be described herein again.

FIG.4is a circuit diagram of a power factor correction assembly according to a third embodiment of the present invention. As shown inFIG.4, the power factor correction assembly4is basically the same as the power factor correction assembly2inFIG.2. The difference lies in that the isolated DC-DC converter412includes a bridge rectifying circuit4122connected to a secondary side of a transformer Tr4. The isolated DC-DC converter422is the same as the isolated DC-DC converter412and will not be described herein again.

FIG.5is a circuit diagram of a power factor correction assembly according to a fourth embodiment of the present invention. As shown inFIG.5, the power factor correction assembly5is basically the same as the power factor correction assembly2inFIG.2. The difference lies in that the power factor correction circuit51includes a half-bridge pulse width modulated rectifier511and an isolated DC-DC converter512that are cascaded, where the isolated DC-DC converter512includes a full-bridge inverter5121connected to a primary side of the transformer Tr5. The power factor correction circuit52is the same as the power factor correction circuit51and will not be described herein again.

FIG.6is a circuit diagram of a power factor correction assembly according to a fifth embodiment of the present invention in a normal working state. As shown inFIG.6, the power factor correction assembly6includes two identical power factor correction circuits61,62. Only the power factor correction circuit61is used as an example for description below.

The power factor correction circuit61is basically the same as the power factor correction circuit21inFIG.2. The difference lies in that the power factor correction circuit61further includes a switch S61and a fuse F61connected in series to one input terminal of a full-bridge pulse width modulated rectifier611, and a switch S62and a fuse F62connected in series to the other input terminal of the full-bridge pulse width modulated rectifier611; and a fuse F63and a switch S63connected in series to one output terminal of an isolated DC-DC converter612, and a fuse F64and a switch S64connected in series to the other output terminal of the isolated DC-DC converter612.

The power factor correction assembly6further includes a switch S65and a switch S65′. The switch S65is connected between inputs of the power factor correction circuit61, and the switch S65′ is connected between inputs of the power factor correction circuit62.

As shown inFIG.6, in the normal operating state, a control device (not shown inFIG.6) controls the switch S65and the switch S65′ to be in an OFF state; controls the switches S61, S62, S63, and S64in the power factor correction circuit61to be all in an ON state; and controls the switches S61′, S62′, S63′, and S64′ in the power factor correction circuit62to be all in an ON state. At this time, the inputs of the power factor correction circuit61and the power factor correction circuit62are connected in series, and the outputs are connected in parallel between a positive direct-current bus60and a negative direct-current bus60′. The power factor correction assembly6has the same equivalent circuit as that of the power factor correction assembly2shown inFIG.2. The working mode thereof will not be described herein again.

FIG.7is a circuit diagram of the power factor correction assembly shown inFIG.6with one power factor correction circuit failed. As shown inFIG.7, when a failure occurs in the circuit on a primary side of a transformer Tr6in the power factor correction circuit61, the fuse F61and the fuse F62will blow; or when a failure occurs in the circuit on a secondary side of the transformer Tr6, the fuse F63and the fuse F64will blow. Then, the switch S65is controlled to turn ON to short-circuit the input of the power factor correction circuit61. Afterwards, the switches S61, S62, S63and S64are controlled to turn OFF. At this time, since the full-bridge pulse width modulated rectifier611is short-circuited by the turned ON switch S65, an alternating current Vi is connected to the input of the power factor correction circuit62through the turned ON switch S65, thereby not affecting operation of the power factor correction circuit62(in other words, not affecting the normal operation of a power factor correction circuit that does not have a failure in the power factor correction assembly).

After the switches S61, S62, S63and S64are controlled to turn OFF, maintenance personnel can remove the failed power factor correction circuit61and then reconnect a new power factor correction circuit61to the power factor correction assembly6. Next, the switches S61, S62, S63and S64are controlled to turn ON, and then the switch S65is controlled to turn OFF. At this time, the input of the power factor correction circuit61and the input of the power factor correction circuit62are connected in series, whereas the output of the power factor correction circuit61and the output of the power factor correction circuit62are connected in parallel between the positive and negative direct-current buses60,60′.

Thus, even if other power factor correction circuits in the power factor correction assembly6fail, the maintenance personnel can replace the failed power factor correction circuit while the power factor correction assembly6is still in operation. Therefore, the power factor correction assembly6implements the on-line service and hot-swap function.

In other embodiments of the present invention, the switches S61to S65and the fuses F61to F64shown inFIG.6are connected to each power factor correction circuit in the power factor correction assembly3,4or5.

In other embodiments of the present invention, other rectifier devices may further be used in place of the half-bridge pulse width modulated rectifier2122, the full-bridge pulse width modulated rectifier3122, or the bridge rectifying circuit4122in the aforementioned embodiment.

In other embodiments of the present invention, the power factor correction assembly includes more than two power factor correction circuits. When a failure occurs in one or a plurality of power factor correction circuits in the power factor correction assembly, the power factor correction assembly can still operate, thereby greatly improving operation reliability. Meanwhile, the one or plurality of failed power factor correction circuits can be replaced without interrupting operation of the power factor correction assembly.

The power factor correction assemblies2,3,4,5and6in the aforementioned embodiments of the present invention all include the same power factor correction circuits. Hence mass production can be carried out and replacing a failed power factor correction circuit is convenient, thereby effectively avoiding connection failure and mis-assembly. Moreover, the control process of the control device over the same power factor correction circuits is simple.

FIG.8is a circuit diagram of an on-line uninterruptible power supply according to a first embodiment of the present invention. As shown inFIG.8, the on-line uninterruptible power supply includes a power factor correction assembly7, a bi-directional DC-DC converter71, and an inverter assembly72.

The power factor correction assembly7is the same as the power factor correction assembly6and will not be described herein again. The bi-directional DC-DC converter assembly71is connected between a rechargeable battery B and positive and negative direct-current buses70,70′. An input of the inverter assembly72is connected to the positive and negative direct-current buses70,70′, and an output of the inverter assembly72is used for providing an alternating current Vo to a load.

FIG.9is a schematic enlarged view of the bi-directional DC-DC converter assembly inFIG.8. As shown inFIG.9, the bi-directional DC-DC converter assembly71includes bi-directional DC-DC converters711,712connected in parallel. That is, the bi-directional DC-DC converters711,712are both connected between the rechargeable battery B and the positive and negative direct-current buses70,70′.

The bi-directional DC-DC converters711,712have the same circuit structure. Only the bi-directional DC-DC converter711is used as an example for description below. The bi-directional DC-DC converter711includes a bi-directional DC-DC conversion circuit7111; a switch S711and a fuse F711connected in series to a positive terminal of a first connection end of the bi-directional DC-DC conversion circuit7111; a switch S712and a fuse F712connected in series to a negative terminal of the first connection end of the bi-directional DC-DC conversion circuit7111; and a switch S713and a fuse F714respectively connected to a positive terminal and a negative terminal of a second connection end of the bi-directional DC-DC conversion circuit7111.

The bi-directional DC-DC conversion circuit7111includes a metal-oxide-semiconductor field effect transistor T711having an anti-parallel diode D711; a metal-oxide-semiconductor field effect transistor T712having an anti-parallel diode D712; and an inductor L71and a capacitor C71. The inductor L71, the metal-oxide-semiconductor field effect transistor T711and the anti-parallel diode D712are connected to form a Boost circuit; at the same time, the metal-oxide-semiconductor field effect transistor T711, the anti-parallel diode D711, and the inductor L71are connected to form a Buck circuit. The capacitor C71is connected between one end of the inductor L71and an anode of the anti-parallel diode D711and is used for filtering a high-frequency alternating current to effectively protect the rechargeable battery B.

If the bi-directional DC-DC converters711,712are both normal (namely, having no failure), in an on-line working mode (that is, the mains supply is normal), a control device (not shown inFIG.9) controls the bi-directional DC-DC converters711,712to achieve buck conversion, so as to store electric energy on the positive and negative direct-current buses70,70′ into the rechargeable battery B; in a battery mode (that is, the mains supply fails), the bi-directional DC-DC converters711,712are controlled to achieve boost conversion, so as to store electric energy in the rechargeable battery B into the positive and negative direct-current buses70,70′.

If a failure occurs in the bi-directional DC-DC conversion circuit7111, the fuses F711, F712will blow, and the control device controls the switches S711, S712, S713and S714to be in an OFF state. Since the bi-directional DC-DC converter712connected in parallel to the bi-directional DC-DC converter711is still in the working state, discharging or charging of the rechargeable battery B is not affected. At this time, the maintenance personnel can remove the failed bi-directional DC-DC converter711from the bi-directional DC-DC converter assembly71and replace it with a new bi-directional DC-DC converter711, and finally control the switches S711, S712, S713, and S714to turn ON, so as to reconnect the bi-directional DC-DC converter711to the bi-directional DC-DC converter assembly71.

In other embodiments of the present invention, the fuse F711and the fuse F712are respectively connected in series to the switches S713and S714.

In other embodiments of the present invention, the bi-directional DC-DC converter assembly71includes more than two bi-directional DC-DC converters, and the plurality of bi-directional DC-DC converters are connected in parallel and then connected between the rechargeable battery B and the positive and negative direct-current buses. The plurality of bi-directional DC-DC converters operate simultaneously, which not only provides high output power but also effectively protects components in the bi-directional DC-DC converter assembly71from damage, thereby reducing the risk of failure.

In other embodiments of the present invention, a DC-DC converter and a charger that are separate are used in place of the bi-directional DC-DC conversion circuit7111in the aforementioned embodiment, where an input of the charger is connected to an output of the DC-DC converter, and an output of the charger is connected to an input of the DC-DC converter.

FIG.10is a schematic enlarged view of the inverter assembly inFIG.8. As shown inFIG.10, the inverter assembly72includes two inverters721,722connected in parallel. That is, inputs of the inverters721,722are both connected to the positive and negative direct-current buses70,70′, and outputs of the inverters721,722are connected together and provide the required alternating current Vo to the load.

The inverters721,722have the same circuit structure. Only the inverter721is used as an example for description below. The inverter721includes a full-bridge inversion circuit7211formed by connecting four metal-oxide-semiconductor field effect transistors and an inductor L72; a switch S721and a fuse F721connected in series between a positive input terminal of the full-bridge inversion circuit7211and the positive direct-current bus70; a switch S722and a fuse F722connected in series between a negative input terminal of the full-bridge inversion circuit7211and the negative direct-current bus70′; and output switches S723, S724connected to two output terminals of the full-bridge inversion circuit7211. Under normal operating conditions, the switches S721, S722and the output switches S723, S724are all in an ON state. The control mode of the full-bridge inversion circuit7211is the same as that in the prior art and will not be described herein again.

If a failure occurs in a full-bridge inversion circuit (for example, the full-bridge inversion circuit7211) of the inverter assembly72, the fuses F721and F722will blow, and a control device (not shown inFIG.10) controls the switches S721, S722and the output switches S723, S724to be in an OFF state. Since other inverters (for example, the inverter722) in the inverter assembly72are still in an operating state, supply of the alternating current Vo to the load is not affected. At this time, the maintenance personnel can remove the failed inverter721and replace it with a new inverter721, and then the control device controls the switches S721, S722and the output switches S723, S724to be in an ON state, and the inverter721will be reconnected to the inverter assembly72.

The inverter assembly72in the present invention is not limited to being formed by two inverters connected in parallel, and may also be formed by more than two inverters connected in parallel in other embodiments of the present invention. When the positive and negative direct-current buses70,70′ can provide high power, the inverter assembly72in this embodiment not only can increase output power but also can reduce the risk of damage to each inverter.

After a failure occurs in one or a plurality of inverters in the inverter assembly of the present invention, the maintenance personnel can also replace the failed inverter while the inverter assembly continues working and provides an alternating current, thereby improving operation reliability.

In other embodiments of the present invention, a half-bridge inversion circuit is used in place of the full-bridge inversion circuit in the inverter assembly72.

FIG.11is a circuit diagram of an inverter assembly in an on-line uninterruptible power supply according to a second embodiment of the present invention. As shown inFIG.11, the inverter assembly72′ is basically the same as the inverter assembly72shown inFIG.10. The difference lies in that T-type three-level inverters7211′,7221′ are used respectively in place of the two full-bridge inversion circuits inFIG.10. The replacement mode of a failed T-type three-level inverter in the inverter assembly72′ is the same as the replacement mode of the full-bridge inversion circuit inFIG.10and will not be described herein again.

In other embodiments of the present invention, other inverters such as I-type three-level inverters are used respectively in place of the T-type three-level inverters7211′,7221′ inFIG.11.

FIG.12is a circuit diagram of an on-line uninterruptible power supply according to a third embodiment of the present invention. As shown inFIG.12, the on-line uninterruptible power supply8includes three identical power factor correction assemblies83, a bi-directional DC-DC converter81and a three-phase inverter assembly82. Inputs of the three power factor correction assemblies83are respectively connected to medium-voltage alternating currents of three phases A, B and C whereas outputs are all connected in parallel to positive and negative direct-current buses80,80′. The bi-directional DC-DC converter81is connected between a rechargeable battery and the positive and negative direct-current buses80,80′. An input of the three-phase inverter assembly82is connected to the positive and negative direct-current buses80,80′, and an output of the three-phase inverter assembly82is used for providing low-voltage alternating currents of three phases U, V and W.

The power factor correction assemblies83have the same circuit structure as that of the power factor correction assembly6shown inFIG.6, and the bi-directional DC-DC converter81has the same circuit structure as that of the bi-directional DC-DC converter71shown inFIG.9, which will not be described herein again.

FIG.13is a schematic enlarged view of a three-phase inverter assembly inFIG.12. As shown inFIG.13, the three-phase inverter assembly82includes two three-phase inverters821,822connected in parallel. That is, an input of the three-phase inverter821and an input of the three-phase inverter822are connected in parallel to the positive and negative direct-current buses80,80′; one output terminal of the three-phase inverters821,822is connected to a neutral line N; and the remaining three output terminals are connected in parallel and used for providing the alternating currents of three phases U, V and W.

The three-phase inverters821,822have the same circuit structure. Only the three-phase inverter821is used as an example for description below. The three-phase inverter821includes a three-phase four-leg inversion circuit8211; a switch S821and a fuse F821connected in series between the positive direct-current bus80and a positive input terminal of the three-phase four-leg inversion circuit8211; a switch S822and a fuse F822connected in series between the negative direct-current bus80′ and a negative input terminal of the three-phase four-leg inversion circuit8211; and an output switch S823, an output switch S824, an output switch S825and an output switch S826respectively connected to four output terminals of the three-phase four-leg inversion circuit8211.

In a normal operating state, a control device (not shown inFIG.13) controls all switches in the three-phase inverters821,822to turn ON and controls three-phase four-leg inversion circuits to work to invert a direct current between the positive and negative direct-current buses80,80′ into a three-phase alternating current, so as to obtain low-voltage (for example, 400 volts, 480 volts or 600 volts) alternating currents of three phases U, V and W at the output thereof.

If a failure occurs in the three-phase four-leg inversion circuit8211in the three-phase inverter821, the fuse F821and the fuse F822blow, and the control device controls the switches S821, S822and the output switches S823, S824, S825, S826in the three-phase inverter821to be in an OFF state. Since the three-phase inverter822connected in parallel to the three-phase inverter821is still in the operating state, the maintenance personnel can remove the failed three-phase inverter821and replace it with a new three-phase inverter821without affecting the three-phase inverter822providing low-voltage alternating currents of three phases U, V and W. Finally, the control device controls the switches S821, S822and the output switches S823, S824, S825, S826to be in an ON state.

In the on-line uninterruptible power supply8of the present invention, the three power factor correction assemblies83are the same, and the three-phase inverters821,822are the same. Hence mass production can be carried out, replacement and maintenance are convenient, and the operator can conveniently conduct assembly and connection, thereby avoiding misassembly.

In other embodiments of the present invention, the three-phase inverter assembly82includes more than two three-phase inverters connected in parallel, thereby reducing the risk of failure of each of the three-phase inverters connected in parallel, and providing higher output power to increase power density.

In other embodiments of the present invention, a three-phase inversion circuit such as a three-phase three-leg inversion circuit, a T-type three-level three-phase inversion circuit or an I-type three-level three-phase inversion circuit may further be used in place of the three-phase four-leg inversion circuit in the aforementioned embodiment. Accordingly, three output switches are connected to output terminals of such three-phase inversion circuits.

In another embodiment of the present invention, an input of the on-line uninterruptible power supply is connected to a single-phase alternating current, and an output of the on-line uninterruptible power supply is used for providing a three-phase alternating current.

In still another embodiment of the present invention, the input of the on-line uninterruptible power supply is connected to a three-phase alternating current, and the output of the on-line uninterruptible power supply is used for providing a single-phase alternating current.

In other embodiments of the present invention, a switching tube such as an insulated gate bipolar transistor having an anti-parallel diode may further be used in place of the metal-oxide-semiconductor field effect transistor in the aforementioned embodiment.

Although the present invention has been described through preferred embodiments, the present invention is not limited to the embodiments described herein, but includes various changes and variations made without departing from the scope of the present invention.