Abstract:
A power supply arrangement ( 1 ) has a first input for connecting to a first power source, at least one second input for connecting to at least one second power source and an output for connecting to a load. The arrangement ( 1 ) comprises at least two paralleled inverter modules ( 11, 12, 13 ) supplied by said at least one second input and a static switch ( 30 ) for switching a connection from said first power source to said load. Each of said inverter modules ( 11, 12, 13 ) comprises an internal switch ( 11   a   , 12   a   , 13   a ) for switching a connection from the respective inverter module ( 11, 12, 13 ) to said load. This architecture allows for directly connecting the critical load to the inverter group ( 10 ), thereby eliminating the susceptibility for single point failures at a common static switch for all inverter modules. Redundancy of the whole system may be easily established or increased by providing additional inverter modules. The proposed architecture guarantees a fail safe maintenance procedure as there is no manual bypass needed for maintenance of the static switch.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a power supply arrangement having a first input for connecting to a first power source, at least one second input for connecting to at least one second power source and an output for connecting to a load, the arrangement comprising at least two paralleled inverter modules supplied by said at least one second input and a static switch for switching a connection from said first power source to said load. The invention further relates to an uninterruptible modular power supply system comprising such a power supply arrangement as well as to an inverter module for such a power supply arrangement. 
   2. Description of the Related Art 
   Uninterruptible power supplies (UPS) are widely used. They provide an interface between a standard power source (such as AC mains) and sensitive loads (computer systems, security equipment, instrumentation etc.). The uninterruptible power supply comprises an alternate power source which is usually a DC power source (e.g. rectifiers with backup batteries). Inverters are employed for generating an AC output current from the DC input current by recomposing a regulated and continuous sine-wave output. Usual inverters comprise a DC/AC-converter, a regulation system and an output filter. 
   Often, at least two paralleled inverters are employed for converting the DC input to the AC output utilizable by the critical load in the case of a failure of the standard power source. Providing a plurality of inverters not only allows for higher loads but a certain redundancy may be provided such that in the event an inverter module fails other units may pick up its share and guarantee for uninterrupted supply. 
   It is further known that the inverters are comprised in inverter modules, that may be easily replaced in the case of defects, or further modules may be easily added if the load is increased or if a higher redundancy level is desired. 
   In a so-called off line topology, the primary (standard) power source (e.g. AC mains) as well as the inverter group (consisting of the plurality of inverter modules) are connected to inputs of a static switch. The output of the switch is connected to the critical load to be supplied by the power supply arrangement. The static switch (or static bypass switch) is a power-electronics device that allows for ultra-fast switching by employing suitable electronic switching devices (instead of slow mechanical parts). In normal use the static switch is in a position where the load is directly connected to the primary power source (off line mode). As soon as a power failure is detected (i.e. the standard power source fails) the static switch switches to the alternate power source (on line mode). Due to the ultra-fast switching device this change of power source is possible without interruption of the power supply for the critical load. 
   In the context of these existing modular inverter architectures the critical load is always connected to the energy source via the static switch, in off line as well as in on line mode. Therefore, if the static switch fails the power supply for the critical load is interrupted. This susceptibility to single point failures of the static switch constitutes a major drawback or the known architecture. Such single point failures may be caused by a number of reasons such as microcontroller resets, defects of the quartz clock circuit for the microcontroller or the microcontroller itself, defects in the circuit, which may short circuit the auxiliary supply, a defect of the auxiliary supply or many other reasons. Additionally, a cooling system for the static switch is required, either based on convection cooling or employing redundant fans. 
   In principle, these drawbacks may be avoided by building a static switch with full redundancy. However, this leads to a major impact on the cost of the arrangement. 
   Furthermore, even with a switch with full redundancy there is the additional drawback that the static switch has to be replaced by using a manual bypass if maintenance procedures are to be carried out. The manual bypass switch allows for supplying the load via direct connection to the standard power source. If this replacement is not carried out correctly by the maintenance staff the power supply for the critical load will be interrupted, i.e. the reliability of the system depends on the maintenance instructions and human skills and attention. This calls for complicated and lengthy prescribed maintenance procedures in order to minimize the risk of failures. However, human errors can never be completely prevented and are therefore a permanent threat to the reliability of the known systems. 
   SUMMARY OF THE INVENTION 
   It is the object of the invention to create a power supply arrangement pertaining to the technical field initially mentioned, that is reliable and cost-effective. 
   According to one aspect of the invention each of said inverter modules comprises an internal switch for switching a connection from the respective inverter module to said load. 
   Therefore, each inverter module has an input for connecting the second power source and an output for connecting the load and comprises an inverter circuit and an internal switch for switching a connection from said input to said output. 
   Thereby, according to the invention the two switching devices that normally reside inside the static switch for switching the connection to the first and to the second power source, respectively, are separated into different locations. The switch connected to the first (standard) energy source remains within the static switch. The switch connected to the inverter group is moved and distributed into the inverter modules. In the static switch this switching device is not required anymore. 
   This architecture allows for directly connecting the critical load to the inverter system, in particular to a redundant inverter system, which is a very reliable energy source. In redundant systems at least n+1 inverter modules are provided if n modules are required for supplying the load. There is no critical element anymore between the reliable energy source and the critical load. Due to this fact, single point failures at the static switch are no longer critical events. Therefore, there is no need for a costly static switch with full redundancy. In contrast, all system relevant redundancy is integrated in the modular inverter modules. The redundancy of the whole system may be easily increased by providing an additional inverter module. 
   Furthermore, the proposed architecture guarantees a fail safe maintenance procedure as there is no manual bypass needed for maintenance of the static switch. However, if desired, a manual bypass for the static switch may be still employed with this proposed architecture. 
   The plurality of inverter modules may be supplied by a single alternate power source or there may be a plurality of alternate power sources, each of them connected to a single inverter module or to a group of modules. 
   Preferably, an output of each of said inverter modules is permanently connected to said output for connecting to the load. Thereby, additional elements in between the inverter group and the critical load such as further switching devices may be avoided. Via the permanent connection each of the inverter modules may always obtain information about whether the load is presently supplied by power. 
   The inverter modules are controlled such that the internal switch of each of the modules is open as long as the load is supplied by said first power source. As soon as a power failure is detected at the load the internal switches of the modules are closed in order to supply the load by said second (alternate) power source. The operation of a plurality of independent fast switches arranged in the inverter modules guarantees redundancy and therefore reliable operation of the power supply arrangement. 
   Preferably, each of said inverter modules comprises a detection circuit connected to an output of the inverter module for detecting power failures at the load. If the outputs of the modules are permanently connected to the load as mentioned above they may always detect power failures at the load and ensure that the respective module immediately switches to on line mode. If at least one redundant inverter module is provided, failure of an inverter module does not lead to failure of the system because this additional paralleled inverter module picks up the share of the failed module and ensures further operation of the arrangement. Providing detection circuits in each of the modules ensures redundancy of this crucial element of the arrangement in the same way as it is done for the inverters and the switching devices. 
   Alternatively, a detection circuit or several detection circuits are arranged outside the inverter modules, e.g. within the static switch, and all the modules are controlled by this circuit or these circuits, respectively. For example, if there is a large number of inverter modules they may be separated into groups where each group is connected to a separate, preferably modular, detection device. The detection circuits are connected to the output of the arrangement for connecting to the load, to the first input of the arrangement for connecting to the standard power source or directly to the power input of the load. 
   Advantageously, the internal switch of the inverter module is connected between the inverter circuit and the output of the inverter module. This guarantees a short reaction time and allows for a simpler architecture if the inverter module comprises a detection circuit which is as well connected to the output of the module. 
   Usually, the first input of the arrangement will be adapted to be connected to an AC power source (e.g. the usual 230 V 50 Hz, 120 V 60 Hz or other standard mains systems), whereas the at least one second input is adapted to be connected to a DC power source. There are a lot of possible DC power sources: They may include storage elements such as batteries, capacitors, flywheels etc. that are charged by the AC power source or by independent power generators such as solar converters or wind generators. Alternatively the DC power sources may include power generators that allow for continuous production of power such as fuel cells. If necessary, rectifiers are employed to generate a DC output current from AC power sources. 
   The reliability of the system may be increased by additional redundancy if a plurality of DC power sources are provided that are connected to single inverter modules or groups of inverter modules. The modularity of the arrangement allows for easy adaptation to different needs relating to reliability, stability of the output current, required maximum load and power sources available. 
   Together with the DC power source the power supply arrangement constitutes a modular uninterruptable power supply system of high reliability. The modular architecture ensures easy maintenance and upgrading. The power supply system usually comprises further components, e.g. for surge protection, monitoring of the device, providing error messages etc. 
   Other advantageous embodiments and combinations of features come out from the detailed description below and the totality of the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings used to explain the embodiments show: 
       FIG. 1A , B Schematic representations of a prior art power supply arrangement in off line and on line mode, respectively; 
       FIG. 2A , B schematic representations of a power supply arrangement according to the invention in off line and on line mode, respectively; and 
       FIG. 3  a schematic representation of an uninterrupted power supply employing the power supply architecture according to the invention. 
   

   In the figures, the same components are given the same reference symbols. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The  FIGS. 1A , B are schematic representations of a prior art power supply arrangement in off line and on line mode, respectively. The arrangement  101  features an inverter group  110  comprising a plurality of inverter modules  111 ,  112 ,  113  each of them connected to an input  120  for connecting a DC power source such as a backup battery. The arrangement  101  further comprises a static switch  130  providing two switching devices  131 ,  132  for switching a connection from AC mains input  140  to an output  150  for connecting a critical load and a connection between the inverter modules  111 ,  112 ,  113  and the output  150 , respectively. 
   In off line mode ( FIG. 1A ), power for the load output  150  is directly supplied from the AC mains input  140  through the closed first switching device  131 . The inverter group  110  is not connected to the load output  150  as the other switching device  132  is open. 
   As soon as a power failure is detected, the static switch  130  switches, i.e. the switching device  132  closes, such that the inverter group  110  is connected to the load output  150  and the other switching device  131  opens to separate the load output  150  from the AC mains  140 . Closing of the switching device  132  connected between the inverter group  110  and the load output  150  happens very rapidly such that the load output  150  is permanently supplied by power. 
   Because the critical load is always connected to the energy source via the static switch  130 , in off line as well as in on line mode, the existing modular inverter architecture has major drawbacks related to single point failure of the static switch  130 . The power supply for the critical load output  150  is interrupted if the static switch  130  fails, e.g. due to microcontroller resets or defects of the microcontroller or other circuits within the static switch  130 . 
   To avoid these drawbacks, a new modular inverter architecture is proposed. A corresponding power supply arrangement  1  is schematically represented in  FIGS. 2A ,  2 B in off line and on line mode, respectively. Again, the arrangement  1  features an inverter group  10  comprising a plurality of inverter modules  11 ,  12 ,  13  each of them connected to an input  20  for a DC power source such as a backup battery. The arrangement  1  further comprises a switch module  30  providing a switching device  31  for switching a connection from an AC mains input  40  to the output  50  for connecting the critical load. 
   In contrast to the prior art architecture described above each of the inverter modules  11 ,  12 ,  13  features a switching device  11   a ,  12   a ,  13   a  in the connection from the respective inverter module to the output  50  for connecting the critical load. Thereby, the switching devices  11   a ,  12   a ,  13   a  integrated in the inverter modules  11 ,  12 ,  13  take over the function of the second switching device of the prior art static switch. 
   In off line mode ( FIG. 2A ), power for the load output  50  is again directly supplied from the AC mains through the closed switching device  31 . There is no connection from the inverter group  10  to the load output  50  as the switching devices  11   a ,  12   a ,  13   a  of the inverter modules  11 ,  12 ,  13  are open. 
   As soon as a power failure is detected the switching devices  11   a ,  12   a ,  13   a  of the inverter modules close, such that the inverter group  10  is connected to the load output  50  and the other switching device  31  opens to separate the load output  50  from the AC mains input  40 . Again, closing of the switching devices  11   a ,  12   a ,  13   a  connected between the inverters of the inverter modules  11 ,  12 ,  13  and the load  50  output happens very rapidly such that the load output  50  is permanently supplied by power. 
   This happens not only in cases where the AC mains power source fails but also in cases where the switch module  30  fails or is unplugged for maintenance purposes. Therefore, it is not anymore required to install a manual bypass for maintenance operations of the switch module  30 . 
   Preferably, the number of inverter modules  11 ,  12 ,  13  is chosen such that reliable power supply for the load output  50  is ensured even if one or more of the inverter modules fail, i.e. a certain redundancy is provided. In the context of the proposed power supply architecture this scheme ensures automatically that the switching devices  11   a ,  12   a ,  13   a  making sure that power is supplied without interruption in case of a failure of the AC mains, are as well provided redundantly. Therefore, failure of one of these switching devices  11   a ,  12   a ,  13   a  does not anymore lead to a general failure of the arrangement. 
   The  FIG. 3  is a schematic representation of a modular uninterrupted power supply system employing the power supply architecture according to the invention. The UPS 2comprises the power supply arrangement  1  that has been briefly discussed above in connection with  FIGS. 2A ,  2 B. It further comprises batteries  3 ,  4  and a rectifier/charger module  5  that mainly consists of a rectifier  5   a  that is connected to the AC mains input  2   a  of the UPS and a charging circuit  5   b  that is connected between the output of the rectifier  5   a  and the batteries  3 ,  4 . The rectifier/charger module  5  ensures that the batteries  3 ,  4  are charged when there is AC mains power such that they are ready to backup the power supply in cases of AC mains power failures. 
   In  FIG. 3  further details of the inverter modules  11 ,  12 ,  13  are schematically displayed. Apart of the switching device  11   a ,  12   a ,  13   a  each of the inverter modules  11 ,  12 ,  13  features an inverter circuit  11   b ,  12   b ,  13   b  and a detection circuit  11   c ,  12   c ,  13   c . The input of the inverter circuit  11   b ,  12   b ,  13   b  is connected to the batteries  3 ,  4 ; the output of the inverter circuit  11   b ,  12   b ,  13   b  is connected to the switching device  11   a ,  12   a ,  13   a . The inverter circuit generates a sinusoidal AC output voltage with a specified frequency (such as 50 Hz or any other desired frequency) and amplitude (such as 230 V or any other desired amplitude). Such circuits are known as such and e.g. comprise a DC/AC converter circuit and an output filter (which may be passive or controlled, e.g. by a DSP). The detection circuit  11   c ,  12   c ,  13   c  is connected to the output of the respective inverter module  11 ,  12 ,  13  for sensing the state of the power supply of the load. This is possible because the outputs of the inverter modules  11 ,  12 ,  13  are always directly connected to the output  2   b  of the modular uninterrupted power supply system  2  for connecting the critical load. In off line mode, the load is supplied from the AC mains through the closed switching device  31 . As soon as a power failure is detected the detection circuit  11   c ,  12   c ,  13   c  instructs the switching device  11   a ,  12   a ,  13   a  to close its switch in order to immediately supply the load with power from the batteries  3 ,  4 . 
   In on line mode, the load is supplied from the batteries  3 ,  4 . In the case of a power failure in the DC power source the switching device  31  of the static switch  30  closes and the switching devices  11   a ,  12   a ,  13   a  open in order to supply the load directly from the AC mains. For detecting the power failure the static switch  30  features as well two detection circuits (not displayed), one at the input to monitor the AC mains and the second at the output to monitor the load supply. 
   In the case of a short circuit of the load in on line mode the power supply system  2  must be able to blow the load fuse in a short time. In such a case, the detection circuits  11   c ,  12   c ,  13   c  in the inverter modules  11 ,  12 ,  13  detect that the output voltage is not present anymore. Based on additional internal information (e.g. a current limit condition) the inverter modules  11 ,  12 ,  13  know that this is a short circuit condition. The inverters  11 ,  12 ,  13  therefore open the switching devices  11   a ,  12   a ,  13   a . The static switch  30  also detects the output voltage fail by means of its detection circuits, and turns on its switching device  31  to blow the load fuse with high current from the AC mains. 
   In case of an overload (to much load connected), the inverters  11 ,  12 ,  13  limit their output currents which results in a decrease of the load voltage. This is detected by the detection circuits  11   c ,  12   c ,  13   c  of the inverter modules  11 ,  12 ,  13 . As soon as the voltage drop is outside a given threshold the inverters open their switching devices  11   a ,  12   a ,  13   a . Again, also the static switch  30  will detect the output voltage error, and may thus turn on its switching device  31 , to provide power from the AC mains. 
   The modular uninterrupted power supply system  2  further comprises a controller that has a variety of monitoring and controlling tasks, e.g. controlling the charging process, cooling equipment, regulating the function of the inverters, supervision of all internal processes, providing error messages, etc. For simplicity this controller and related devices, which as such are known from the prior art, are not shown in  FIG. 3 . Some of the tasks of the controller may be performed by subordinate controllers integrated into the components of the power supply system  2  such as the inverter modules  11 ,  12 ,  13 , the rectifier/charger module  5  or the switch module  30 . 
   The number of inverter modules may be freely varied depending on the required maximum power and the desired redundancy. Similarly, the number of backup power sources may be decreased to just one or increased to three or more; instead of batteries other power sources may be employed such as power capacitors etc. As mentioned above, the detection circuits for detecting failure of the standard power source may be arranged or distributed in a different way. The same is true for the arrangement of the switching devices within the inverter modules. 
   In a modular uninterrupted power supply system according to the invention the batteries and the rectifier/charger module may be replaced by any other kind of suitable DC power source (such as solar converters, fuel cells etc.). 
   In summary, it is to be noted that the invention creates a power supply arrangement that is reliable and cost-effective.