Abstract:
A dc energy store system includes a dc energy store, an AC/DC power converter having ac terminals connected to an ac power supply and dc terminals connected to the dc energy store, and at least one auxiliary unit associated with the dc energy store. The dc energy store system is adapted to be operated in a number of different operating modes including: (i) a first mode to supply power from the ac power supply to the dc energy store; (ii) a second mode to supply power from the dc energy store to the ac power supply; (iii) a third mode to supply power from the ac power supply to the auxiliary unit(s); and (iv) a fault mode where there is a fault in the ac power supply, and power is supplied from the dc energy store to the auxiliary unit(s).

Description:
FIELD OF THE INVENTION 
     The present invention relates to dc energy store systems, and in particular to methods of using the dc energy store to supply power to one or more auxiliary systems of the dc energy store. 
     BACKGROUND OF THE INVENTION 
     DC energy stores can be implemented from many different technologies such as batteries (e.g. lithium and sodium sulphur type), capacitors including supercapactitors and ultracapacitors, and flow cells (e.g. vanadium redox flow cells). In a typical dc energy store system an AC/DC power converter is interposed between the dc energy store and an ac power supply. More particularly, the AC/DC power converter has ac terminals connected to the ac power supply and dc terminals connected to the dc energy store by means of a dc link. The AC/DC power converter can be of any suitable type with a conventional two- or three-level topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation strategy. 
     AC power is supplied from the ac power supply and stored in the dc energy store. In this mode of operation the AC/DC power converter acts as an active rectifier and converts the ac power to dc power that is compatible with the dc energy store. When the stored energy is needed then the AC/DC power converter can act as an inverter to convert the dc power to ac power that is compatible with the ac power supply. The AC/DC power converter is therefore capable of bidirectional power flow. 
     This ability to store and release energy is useful in a number of different applications including storing excess wind energy and releasing that energy at a later time when less wind energy is available. A different application is to store energy in case an electrical generator stops operating. The energy stored in the dc energy store can then be used to keep essential systems and services operating until a standby or back-up generator is started and brought into operation. In some circumstances it can also be helpful to store energy slowly and allow the energy stored in the dc energy store to be released very quickly for applications that need high power pulses of energy like the linear electrical motors that are used to start roller coasters. 
     The dc energy store system will normally include one or more auxiliary units such as pumps, fans, control units or other devices that are connected to the ac power supply. A control unit can be used to control the operation of the AC/DC power converter and may be associated with its own uninterruptible power supply (UPS) so that it can continue to operate during short duration transients of the ac power supply. However, the UPS can be expensive if it is designed to supply power to the control unit for long periods of time such as 10 minutes or more. If the ac power supply is interrupted or experiences voltage or frequency transients then the other auxiliary units will not receive power and this can cause problems with the practical operation of the dc energy store. For example, if the fans that are used to cool the dc energy store stop working then this can result in unacceptable temperature increases. Certain types of dc energy store also require pumps to continue running at their optimum efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of operating a dc energy store system comprising a dc energy store, an AC/DC power converter having ac terminals connected to an ac power supply and dc terminals connected to the dc energy store (optionally by means of an interposing DC/DC power converter), and at least one auxiliary unit associated with the dc energy store, the at least one auxiliary unit optionally being a pump, a fan, an uninterruptible power supply (UPS), control unit or other device, the method comprising the steps of: (i) in a first mode supplying power from the ac power supply to the dc energy store; (ii) in a second mode supplying power from the dc energy store to the ac power supply; (iii) in a third mode supplying power from the ac power supply to the at least one auxiliary unit associated with the dc energy store; and (iv) in the event of a fault in the ac power supply, supplying power from the dc energy store to the at least one auxiliary unit associated with the dc energy store. 
     The present invention further provides a dc energy store system operated according to the method described above. 
     The present invention further provides a dc energy store system comprising: a dc energy store; an AC/DC power converter having ac terminals connected to an ac power supply and dc terminals connected to the dc energy store; and at least one auxiliary unit associated with the dc energy store, the at least one auxiliary unit optionally being a pump, a fan, a UPS, control unit or other device; wherein the dc energy store system is adapted to be operated: (i) in a first mode to supply power from the ac power supply to the dc energy store; (ii) in a second mode to supply power from the dc energy store to the ac power supply; (iii) in a third mode to supply power from the ac power supply to the at least one auxiliary unit associated with the dc energy store; and (iv) in the event of a fault in the ac power supply, to supply power from the dc energy store to the at least one auxiliary unit associated with the dc energy store. 
     During the first mode, energy is stored in the dc energy store. During the second mode, energy stored in the dc energy store is returned to the ac power supply. This return of stored energy can be carried out as part of the normal operation of the dc energy store system (e.g. for the reasons described in more detail above) or during a fault condition where power is supplied to the fault in the ac power supply. 
     Certain modes may run concurrently. For example, during normal operation of the dc energy store system, ac power can be supplied from the ac power supply to the auxiliary unit(s) (third mode) at the same time as power is being supplied from the ac power supply to the dc energy store (first mode) or from the dc energy store to the ac power supply (second mode). 
     During the first mode the AC/DC power converter acts as an active rectifier and converts the ac power to dc power that is compatible with the dc voltage of the dc energy store. An interposing DC/DC power converter can optionally be provided between the dc terminals of the AC/DC power converter and the dc energy store in situations where the dc voltage provided by the AC/DC power converter is not compatible with the dc energy store. During the second mode when the energy that is stored in the dc energy store needs to be returned to the ac power supply then the AC/DC power converter can act as an inverter to convert the dc power to ac power that is compatible with the ac power supply. The AC/DC power converter is therefore capable of bidirectional power flow. 
     If there is a fault in the ac power supply (e.g. the ac power supply is interrupted or experiences voltage or frequency transients) then ac power will no longer be supplied from the ac power supply to the auxiliary unit(s). The dc energy store system can therefore be operated during a fault condition to supply power from the dc energy store to the auxiliary unit(s) (fault mode). The AC/DC power converter will act as an inverter to convert the dc power to ac power that is compatible with the auxiliary unit(s). The present invention is therefore able to keep the auxiliary unit(s) operating for extended periods of time during a fault condition at minimal cost by using the energy that is stored in the dc energy store. Depending on the auxiliary unit(s) that are supplied with energy during the fault, this can ensure that the dc energy store remains cooled or that any pumps associated with the dc energy store continue to operate properly. In some fault conditions it is specified that power can also be supplied from the dc energy store to the ac power supply at the same time as being supplied to the auxiliary unit(s), i.e. the second mode and fault mode can be carried out at the same time. 
     The dc energy store can be a battery (e.g. lithium and sodium sulphur type), capacitor, flow cell or any other similar device that is capable of storing energy. 
     The dc energy store system can include any suitable number of auxiliary units, but at least two might be typical. For example, one of the auxiliary units might be a pump, fan, a control unit or any other similar device that forms an integral operational part of the dc energy store. Another of the auxiliary units can be a UPS that is used to supply power to a control unit for a short time during a fault condition. 
     The AC/DC power converter has to vary its dc voltage to supply power to the dc energy store or receive power from the dc energy store. The AC/DC power converter often has limitations on the range of variation in its dc voltage that is not compatible with the dc energy store. If this limitation applies then the interposing DC/DC power converter can be provided. The DC/DC power converter can include a first pair of dc terminals connected to the dc terminals of the AC/DC power converter and a second pair of dc terminals connected to the dc energy store. In this arrangement, the AC/DC power converter will normally maintain a nominally constant dc link voltage and the DC/DC power converter will provide a varying dc voltage. In other words, the first pair of dc terminals of the DC/DC power converter may be regulated to be at substantially constant dc voltage while the dc voltage of the dc energy store may be subject to a significant variation in dc terminal voltage during its various operating modes. 
     During normal operation, the frequency of the ac power supplied by the ac power supply (i.e. the supply frequency) is set by the external ac generators or other devices that typically supply ac power to the ac power supply. The AC/DC power converter is therefore synchronised by its control firmware to follow the supply frequency and by varying its ac voltage to import or export power from the ac power supply in line with the power being imported or exported by the dc energy store. However, during a fault condition the AC/DC power converter has to rapidly change to acting as an independent source of ac power and the AC/DC power converter then acts to deliver a fixed supply frequency. This change of operating mode is made by observing the voltage of the ac power supply. The ability of the AC/DC power converter to operate in the different operating modes described above is not a normal requirement and requires specific firmware to be provided. 
     The AC/DC power converter and the optional DC/DC power converter can have any suitable construction. 
     The ac terminals of the AC/DC power converter can be connected to the ac power supply by means of a switched connection. The switched connection can include a first switch and a second switch. 
     The auxiliary unit(s) can be connected to the switched connection at a first junction between the first and second switches. 
     The switched connection can include a transformer of any suitable type which can be connected between the first switch and the first junction. The switched connection can also include a harmonic filter of any type which can be connected between the second switch and the ac terminals of the AC/DC power converter. 
     In a first arrangement the first and second switches are closed during normal operation so that power can be supplied from the ac power supply to the dc energy store and the auxiliary unit(s), or from the dc energy store to the ac power supply. In the event of a fault in the ac power supply then the first switch is opened to disconnect the auxiliary unit(s) from the ac power supply and the second switch remains closed so that power can be supplied from the dc energy store to the auxiliary unit(s) so that they can continue to operate normally. 
     In a second arrangement the auxiliary unit(s) can be connected to the first junction of the switched connection by a third switch. The auxiliary unit(s) can be further connected to the switched connection at a second junction between the second switch and the ac terminals of the AC/DC power converter (optionally between the second switch and the harmonic filter) by a fourth switch. In other words, the auxiliary unit(s) can be connected to each side of the second switch by parallel switched connections. The first, second and third switches are closed during normal operation so that power can be supplied from the ac power supply to the dc energy store and the auxiliary unit(s) or from the dc energy store to the ac power supply. In the event of a fault in the ac power supply then the second and third switches are opened to disconnect the auxiliary unit(s) from the ac power supply. The fourth switch is closed so that power can be supplied from the dc energy store to the auxiliary unit(s) so that they can continue to operate normally. 
     In a third arrangement the auxiliary unit(s) can be further connected to the dc terminals of the AC/DC power converter by a DC/AC power converter and a fifth switch. (The fourth switch mentioned above in connection with the second arrangement is not required.) In other words, the dc terminals of the DC/AC power converter are connected to the dc link between the AC/DC power converter and the dc energy store. If the dc energy store system includes a DC/DC power converter having a first pair of dc terminals connected to the dc terminals of the AC/DC power converter by means of a dc link, and a second pair of dc terminals connected to the dc energy store, then the auxiliary units(s) can be further connected to the dc link by the DC/AC power converter and the fifth switch. Such an arrangement will typically be used when it is necessary to keep the auxiliary unit(s) operating during a fault condition while at the same time supplying power from the dc energy store to the ac power supply. The first, second and third switches are closed and the fifth switch is open during normal operation so that power can be supplied from the ac power supply to the dc energy store and the auxiliary unit(s) or from the dc energy store to the ac power supply. In the event of a fault in the ac power supply then the third switch is opened to disconnect the auxiliary unit(s) from the ac power supply. The fifth switch is closed so that power can be supplied from the dc energy store to the auxiliary unit(s) through the DC/AC power converter so that they can continue to operate normally. 
     A sixth switch can be provided in the dc link and this will be closed whenever power is to be supplied to or from the dc energy store. The sixth switch is either used for maintenance conditions or if a fault occurs in the dc energy store. It is not an essential part of the system. The various switches can have any suitable construction and can be made to open and close by any suitable control unit. 
     The present invention provides a method of operating a dc energy store system comprising an ac power supply, a switched connection having a first switch and a second switch (and optionally a transformer and a harmonic filter), an AC/DC power converter having ac terminals connected to the ac power supply by means of the switched connection and dc terminals connected to a dc energy store (optionally by means of an interposing DC/DC power converter), and at least two auxiliary units connected to the switched connection at a first junction between the first and second switches, the method comprising the steps of: in a first mode supplying power from the ac power supply to the dc energy store; in a second mode supplying power from the dc energy store to the ac power supply; in a third mode supplying power from the ac power supply to the auxiliary units; and in the event of a fault in the ac power supply, supplying power from the dc energy store to the auxiliary units. 
     The present invention further provides a dc energy store system comprising: an ac power supply, a switched connection having a first switch and a second switch (and optionally a transformer and a harmonic filter), an AC/DC power converter having ac terminals connected to the ac power supply by means of the switched connection and dc terminals connected to a dc energy store (optionally by means of an interposing DC/DC power converter), and at least two auxiliary units connected to the switched connection at a first junction between the first and second switches; wherein the dc energy store system is adapted to be operated: in a first mode to supply power from the ac power supply to the dc energy store; in a second mode to supply power from the dc energy store to the ac power supply; in a third mode to supply power from the ac power supply to the auxiliary units; and in the event of a fault in the ac power supply, to supply power from the dc energy store to the auxiliary units. 
     Further details of the method and the dc energy store system are as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will now be described, with reference to the accompanying drawings in which; 
         FIG. 1  is a schematic drawing of a dc energy store system in accordance with a first embodiment of the present invention; 
         FIG. 2  is a schematic drawing of a dc energy store system in accordance with a second embodiment of the present invention; and 
         FIG. 3  is a schematic drawing of a dc energy store system in accordance with a third embodiment of the present invention. 
     
    
    
     A first embodiment of a dc energy store system will now be described with reference to  FIG. 1 . 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A dc energy store  2  in the form of a battery (or any other suitable dc energy store) is connected to a DC/DC power converter  4  by means of a dc link  6  that includes a switch  8 . The switch  8  is closed whenever power is to be supplied to or from the dc energy store  2  and can be opened for safety and isolation purposes. The DC/DC power converter  4  is connected to the dc terminals of an AC/DC power converter  10  by means of a dc link  12 . In some embodiments, the DC/DC power converter  4  can be omitted so that the dc energy store  2  is connected directly to the dc terminals of the AC/DC power converter  10 . 
     The ac terminals of the AC/DC power converter  10  are connected to an ac power supply  14  by means of a switched connection  16 . The switched connection  16  includes a first switch  18 , a transformer  20  which is used to convert the ac supply voltage to a different ac voltage that is suitable for the AC/DC power converter  10 , a second switch  22  and a harmonic filter  24  which is used to reduce the harmonic voltages produced by the AC/DC power converter to a level that is acceptable to the ac power supply  14 . 
     A pump  28  and an uninterruptible power supply (UPS)  30  are connected to the switched connection  16  at a junction  26  between the transformer  20  and the second switch  22 . The UPS  30  supplies power to the control unit  32  during normal operation or for short duration transients in the ac power supply. The control unit  32  is used to control the operation of the AC/DC power converter  10  and the DC/DC power converter  4  to provide the correct power flow and control of the dc energy store  2 . It is therefore important that the control unit  32  continues to operate normally during a fault condition. It is for this reason that the UPS  30  is provided as part of the dc energy store system. The pump  28 , UPS  30  and control unit  32  are auxiliary units of the dc energy store system. 
     When no power is being supplied to or from the dc energy store  2  (Mode A) then the first and second switches  18 ,  22  are open. 
     During an initial starting of the dc energy store system (Mode B) the first switch  18  is closed and the second switch  22  remains open so that the UPS  30  can be charged. When the UPS  30  and the control unit  32  are ready for operation then the second switch  22  is closed to start the AC/DC power converter  10  and the DC/DC power converter  4 . 
     During normal operation, the first and second switches  18 ,  22  are closed so that power can be supplied from the ac power supply  14  to the dc energy store  2 , the pump  28  and the UPS  30  (Mode C). More particularly, ac power is supplied from the ac power supply  14  to the ac terminals of the AC/DC power converter  10  by means of the switched connection  16 . The AC/DC power converter  10  rectifies the ac power and supplies dc power to the dc energy store  2  by means of the DC/DC power converter  4 . AC power is also supplied directly from the ac power supply  14  to the pump  28  and the UPS  30  by means of a connection  34 . During normal operation, power can also be supplied from the dc energy store  2  to the ac power supply  14  (Mode D). More particularly, dc power is supplied from the dc energy store  2  to the dc terminals of the AC/DC power converter  10  by means of the DC/DC power converter  4 . The AC/DC power converter  10  inverts the dc power and supplies ac power to the ac power supply  14  by means of the switched connection  16 . Normal operation of the dc energy store system therefore provides for bidirectional power flow between the ac power supply  14  and the dc energy store  2  depending on whether energy is being stored in the dc energy store (Mode C) or returned to the ac power supply (Mode D). 
     In the event of a fault or transient in the ac power supply  14  then power can be supplied in to the fault by the system but this can result in the voltage at the junction  26  being very low, especially if the fault in the ac power supply is a short circuit. For this condition the low voltage at junction  26  can result in the pump  28  and the UPS  30  not receiving any significant power. This is acceptable for a short period (e.g. a few seconds) because the UPS  30  will keep the control unit  32  operating normally. For most faults in the ac power supply  14  the additional protection systems (not shown) will act to remove the fault and the dc energy store system can return to normal operation. 
     If the fault in the ac power supply  14  persists in giving a low voltage at junction  26  then the dc energy store system must adopt a shutdown condition to conserve stored energy (Mode E). To do this the first switch  18  must be opened to disconnect the pump  28 , the UPS  30  and the AC/DC power converter  10  from the fault in the ac power supply  14 . The second switch  22  remains closed so that power can be supplied from the dc energy store  2  to the pump  28  and the UPS  30  by means of the connection  34  that connects the auxiliary units to the junction  26  of the switched connection  16 . The UPS  30  can supply power to the control unit  32  for a short period but the power from the dc energy store  2  can be used to keep the auxiliary units operating for an extended period of time. This enables the correct shutdown procedure to be carried out. 
     If the fault in the ac power supply  14  is the loss of the external ac generator or other device that supplies power to the ac power supply then the first and second switches  18 ,  22  can remain closed so that power can be supplied from the dc energy store  2  to the ac power supply  14  (Mode F). Power can also be supplied from the dc energy store  2  to the pump  28  and UPS  30  by means of the connection  34  that connects the auxiliary units to the junction  26  of the switched connection  16 . The UPS  30  can supply power to the control unit  32  for a short period but the power from the dc energy store  2  can be used to keep the auxiliary units operating for an extended period of time. This enables the ac power supply  14  to receive power until the dc energy store  2  is exhausted. 
     The switch conditions for Modes A to F of the first embodiment are shown in Table 1 below. 
     A second embodiment of a dc energy store system will now be described with reference to  FIG. 2 . Like parts have been given the same reference numeral. The second embodiment is used when it is not possible for the dc energy store system to open the first switch  18  in the event of a fault condition. 
     The connection  34  that connects the pump  28  and the UPS  30  to the junction  26  includes a third switch  36 . The pump  28  and the UPS  30  are also connected in parallel to the switched connection  16  at a junction  38  by a connection  40  that includes a fourth switch  42 . The junction  38  is between the second switch  22  and the harmonic filter  24 . 
     When no power is being supplied to or from the dc energy store  2  (Mode A) then the first, second, third and fourth switches  18 ,  22 ,  36  and  42  are open. 
     During an initial starting of the dc energy store system (Mode B) the first switch  18  and the third switch  36  are closed and the second switch  22  remains open so that the UPS  30  can be charged. When the UPS  30  and the control unit  32  are ready for operation then the second switch  22  is closed to start the AC/DC power converter  10  and the DC/DC power converter  4 . The fourth switch  42  remains open during the initial starting mode. 
     During normal operation, the first, second and third switches  18 ,  22 ,  36  are closed so that power can be supplied from the ac power supply  14  to the dc energy store  2 , the pump  28  and the UPS  30  (Mode C). More particularly, ac power is supplied from the ac power supply  14  to the ac terminals of the AC/DC power converter  10  by means of the switched connection  16 . The AC/DC power converter  10  rectifies the ac power and supplies dc power to the dc energy store  2  by means of the DC/DC power converter  4 . AC power is also supplied directly from the ac power supply  14  to the pump  28  and the UPS  30  by means of the connection  34 . During normal operation, power can also be supplied from the dc energy store  2  to the ac power supply  14  (Mode D). More particularly, dc power is supplied from the dc energy store  2  to the dc terminals of the AC/DC power converter  10  by means of the DC/DC power converter  4 . The AC/DC power converter  10  inverts the dc power and supplies ac power to the ac power supply  14  by means of the switched connection  16 . 
     In the event of a fault or transient in the ac power supply  14  then power can be supplied in to the fault by the system but this can result in the voltage at the junction  26  being very low, especially if the fault in the ac power supply is a short circuit. For this condition the low voltage at junction  26  can result in the pump  28  and the UPS  30  not receiving any significant power. This is acceptable for a short period (e.g. a few seconds) because the UPS  30  will keep the control unit  32  operating normally. For most faults in the ac power supply  14  the additional protection systems (not shown) will act to remove the fault and the dc energy store system can return to normal operation. 
     If the fault in the ac power supply  14  persists in giving a low voltage at junction  26  then the dc energy store system must adopt a shutdown condition to conserve stored energy (Mode E). To do this the second and third switches  22 ,  36  must be opened to disconnect the pump  28 , the UPS  30  and the AC/DC power converter  10  from the ac power supply  14 . The fourth switch  42  can then be closed so that power can be supplied from the dc energy store  2  to the pump  28  and the UPS  30  by means of the connection  40  that connects the auxiliary units to the junction  38  of the switched connection  16 . The UPS  30  can supply power to the control unit  32  for a short period but the power from the dc energy store  2  can be used to keep the auxiliary units operating for an extended period of time. This enables the correct shutdown procedure to be carried out. 
     If the fault in the ac power supply  14  is the loss of the external ac generator or other device that supplies power to the ac power supply then the second and third switches  22 ,  36  can remain closed so that power can be supplied from the dc energy store  2  to the ac power supply  14  (Mode F). Power can also be supplied from the dc energy store  2  to the pump  28  and the UPS  30  by means of the connection  34  that connects the auxiliary units to the junction  26  of the switched connection  16 . The UPS  30  can supply power to the control unit  32  for a short period but the power from the dc energy store  2  can be used to keep the auxiliary units operating for an extended period of time. This enables the ac power supply  14  to receive power until the dc energy store  2  is exhausted. 
     The switch conditions for Modes A to F of the second embodiment are shown in Table 2 below. 
     A third embodiment of a dc energy store system will now be described with reference to  FIG. 3 . Like parts have been given the same reference numeral. The third embodiment is used when it is required that the dc energy store system supplies power in to a continuous fault in the ac power supply  14 . 
     The connection  34  that connects the auxiliary units  28 ,  30  to the junction  26  includes a third switch  36 . An additional DC/AC power converter  44  has its dc terminals connected to the dc link  12  and its ac terminals connected to the connection  34  by means of the fifth switch  46 . (The fourth switch  42  and the connection  40  of the second embodiment are not used.) 
     When no power is being supplied to or from the dc energy store  2  (Mode A) then the first, second, third and fifth switches  18 ,  22 ,  36  and  46  are open. 
     During an initial starting of the dc energy store system (Mode B) the first switch  18  and the third switch  36  are closed and the second switch  22  remains open so that the UPS  30  can be charged. When the UPS  30  and the control unit  32  are ready for operation then the second switch  22  is closed to start the AC/DC power converter  10  and the DC/DC power converter  4 . 
     During normal operation, the first, second and third switches  18 ,  22 ,  36  are closed so that power can be supplied from the ac power supply  14  to the dc energy store  2  and the pump  28  and the UPS  30  (Mode C). More particularly, ac power is supplied from the ac power supply  14  to the ac terminals of the AC/DC power converter  10  by means of the switched connection  16 . The AC/DC power converter  10  rectifies the ac power and supplies dc power to the dc energy store  2  by means of the DC/DC power converter  4 . AC power is also supplied directly from the ac power supply  14  to the pump  28  and the UPS  30  by means of the connection  34 . During normal operation, power can also be supplied from the dc energy store  2  to the ac power supply  14  (Mode D). More particularly, dc power is supplied from the dc energy store  2  to the dc terminals of the AC/DC power converter  10  by means of the DC/DC power converter  4 . The AC/DC power converter  10  inverts the dc power and supplies ac power to the ac power supply  14  by means of the switched connection  16 . 
     In the event of a fault or transient in the ac power supply  14  then power can be supplied in to the fault by the system but this can result in the voltage at the junction  26  being very low, especially if the fault in the ac power supply is a short circuit. For this condition the low voltage at junction  26  can result in the pump  28  and the UPS  30  not receiving any significant power. This is acceptable for a short period (e.g. a few seconds) because the UPS  30  will keep the control unit  32  operating normally. For most faults in the ac power supply  14  the additional protection systems (not shown) will act to remove the fault and the dc energy store system can return to normal operation. 
     If the fault in the ac power supply  14  persists in giving a low voltage at junction  26  then the third switch  36  is opened to disconnect the pump  28  and the UPS  30  from the ac power supply (Mode E). Power can continue to be supplied into the fault in the ac power supply  14  from the dc energy store  2  in line with its specification. The fifth switch  46  is also closed so that power can be supplied from the dc energy store  2  to the pump  28  and the UPS  30  by means of the dc link  12  and the additional DC/AC power converter  44 . The UPS  30  can supply power to the control unit  32  for a short period but the power from the dc energy store  2  can be used to keep the auxiliary units operating for an extended period of time. This enables the ac power supply  14  to receive power until the dc energy store  2  is exhausted. 
     If the fault in the ac power supply  14  is the loss of the external ac generator or other unit that supplies power to the ac power supply then the same operation can be carried out (Mode F). The UPS  30  can supply power to the control unit  32  for a short period but the power from the dc energy store  2  can be used to keep the auxiliary units operating for an extended period of time. This enables the ac power supply  14  to receive power until the dc energy store  2  is exhausted. 
     The switch conditions for Modes A to F of the third embodiment are shown in Table 3 below. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 First embodiment (FIG. 1) 
               
             
          
           
               
                   
                 Switch condition 
                   
               
               
                   
                 (open or closed) 
               
             
          
           
               
                   
                 Mode 
                 First switch 18 
                 Second switch 22 
               
               
                   
                   
               
               
                   
                 A 
                 Open 
                 Open 
               
               
                   
                 B 
                 Closed 
                 Open 
               
               
                   
                 C 
                 Closed 
                 Closed 
               
               
                   
                 D 
                 Closed 
                 Closed 
               
               
                   
                 E 
                 Open 
                 Closed 
               
               
                   
                 F 
                 Closed 
                 Closed 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Second embodiment (FIG. 2) 
               
             
          
           
               
                   
                 Switch condition (open or closed) 
               
             
          
           
               
                   
                   
                   
                   
                 Fourth 
               
               
                 Mode 
                 First switch 18 
                 Second switch 22 
                 Third Switch 36 
                 switch 42 
               
               
                   
               
               
                 A 
                 Open 
                 Open 
                 Open 
                 Open 
               
               
                 B 
                 Closed 
                 Open 
                 Closed 
                 Open 
               
               
                 C 
                 Closed 
                 Closed 
                 Closed 
                 Open 
               
               
                 D 
                 Closed 
                 Closed 
                 Closed 
                 Open 
               
               
                 E 
                 Closed 
                 Open 
                 Open 
                 Closed 
               
               
                 F 
                 Closed 
                 Closed 
                 Closed 
                 Open 
               
               
                   
               
             
          
         
       
     
                                                     TABLE 3                   Third embodiment (FIG. 3)                Switch condition (open or closed)                            Fifth       Mode   First switch 18   Second switch 22   Third Switch 36   switch 46               A   Open   Open   Open   Open       B   Closed   Open   Closed   Open       C   Closed   Closed   Closed   Open       D   Closed   Closed   Closed   Open       E   Closed   Closed   Open   Closed       F   Closed   Closed   Open   Closed                    
where for each of Tables 1 to 3:
     Mode A is where no power is being supplied to or from the dc energy store  2 ;   Mode B is an initial starting mode;   Mode C is where power is supplied from the ac power supply  14  to the dc energy store  2 , the pump  28  and the UPS  30 ;   Mode D is where power is supplied from the dc energy store  2  to the ac power supply  14 , the pump  28  and UPS  30  receiving power from the ac power supply  14 ;   Mode E is where power is being supplied from the dc energy store  2  to the pump  28  and the UPS  30  during a fault condition; and   Mode F is where power is being supplied from the dc energy store  2  to the ac power supply  14 , the pump  28  and the UPS  30  when the ac power supply suffers a loss of ac input power.   

     In all three embodiments described above the switch  8  is open during Modes A and B and closed during Modes C to F.