Abstract:
This disclosure is concerned with devices and methods for voltage source transfer switching that reduces or eliminates transformer saturation due to DC flux built up during a transfer event. First and second voltage sources (primary and alternate) are connectable to a load via corresponding switches. A transformer is connected downstream of the switches. A controller operates the switches according to various transfer methods wherein a switching time is determined to minimize downstream saturation current.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/521,045, filed Feb. 10, 2004, which is incorporated by reference. This application is related to U.S. Pat. No. ______, “STATIC TRANSFER SWITCH DEVICE AND METHOD,” filed on the same day as the present application and incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     The present invention relates generally to voltage transfer switches, and more particularly, to AC voltage source transfer methods for switch systems having a transformer downstream of the transfer switch.  
         [0003]     Voltage transfer switches are commonly used to switch between a primary and one or more alternate power sources in the event of failure or instability of the primary source. Such transfer switches are commonly used in applications that require continuity of power, such as in hospitals and critical processes in both industrial and commercial settings. For example, in a power system having a primary voltage source and one alternate voltage source, fist and second switches are associated with the primary and alternate voltage sources, respectively. The switches are activated by a controller, such that upon a failure of the primary source, the first switch is opened to remove the primary voltage source from a load and the second switch is subsequently closed to connect the alternate source to the load, hence maintaining power to the load.  
         [0004]     Generally, the second switch is turned on as soon as possible after the load is disconnected from the primary source in an attempt to minimize the voltage disruption at the load side. However, in systems having a transformer connected downstream of the switches, this can cause a problem when the two sources are not initially synchronized, since the transformer would saturate due to the dc flux built up during the transfer event. The transformer saturations are highly undesirable since they can cause large saturation currents to flow, which in turn can cause system failure due to source overloading or upstream protective breakers tripping.  
         [0005]     The present application addresses these shortcomings associated with the prior art.  
       SUMMARY  
       [0006]     This disclosure is concerned with devices and methods for voltage source transfer switching that reduces or eliminates transformer saturation due to DC flux built up during a transfer event. First and second voltage sources (primary and alternate) are connectable to a load via corresponding switches. A transformer is connected downstream of the switches. A controller operates the switches according to various transfer methods to minimize downstream saturation current. One method includes computing the volt-seconds area of the load and alternate voltages in real time. Prior to and during the primary source failure, the controller continuously computes the target volt-seconds value, i.e., the area under the load voltage curve, and it computes a compensating area on the alternate source voltage curve. When a transfer is initiated, the controller waits until the target volt-second area is approximately equal to or complements the compensating volt-second area before it turns on the appropriate switch to connect the alternate source to the load.  
         [0007]     In another transfer method, two flux quantities are computed continuously prior to a transfer event. The first flux corresponds to the flux that is generated by the load voltage when it drives the transformer, and the second corresponds to an imaginary flux that would be generated by the alternate source if it were driving the transformer. Upon a primary source failure, the controller transfers the load to the alternate source when the two flux quantities are equal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0009]      FIG. 1  is a block diagram of an AC voltage transfer system.  
         [0010]      FIGS. 2A and 2B  illustrate two sets of voltage waveforms showing balanced voltage conditions.  
         [0011]      FIG. 3  is a flow diagram of a transfer method disclosed herein.  
         [0012]      FIGS. 4A and 4B  illustrate two voltage waveforms and corresponding flux waveforms.  
         [0013]      FIG. 5  is a flow diagram of another transfer method disclosed herein. 
     
    
       [0014]     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0015]     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0016]      FIG. 1  illustrates an uninterruptible power supply system including an AC voltage transfer switch system  100 . The transfer switch system  100  includes a first, or primary, voltage source  110  and a second, or alternate voltage source  111 . The first and second voltage sources  110 ,  111  are connected to a load  120  via a transformer  122 . First and second switches  130 ,  131  are connected to the first and second voltage sources  110 ,  111 , respectively, and a controller  134  activates the switches  130 ,  131 . Devices suitable for the switches  130 ,  131  include SCRs, IGBTs, Triacs, etc. The controller  134  may comprise, for example, a digital signal processor (DSP) or any suitable programmable logic device. The controller  134  receives the voltage levels of the first and second voltage sources V 1  and V 2 , and the load voltage Vload as inputs. The load voltage Vload here is defined as the output voltage of the transfer switch applied to the transformer primary.  
         [0017]     Under normal conditions, the first switch  130  is closed as shown in  FIG. 1 , connecting the first source  110  (primary source) to the load  120 . In the event that the first source  110  fails while the second source  111  (alternate source) is available, the controller  134  detects the condition, and turns off the first switch  130  and subsequently turns on the second switch  131 , maintaining power to the load  120 .  
         [0018]     In known transfer switch systems, the second switch  131  is typically turned on as soon as possible after the load  120  is disconnected from the first voltage source  110  in an attempt to minimize the voltage disruption at the load  120 . If the two sources  110 ,  111  are not initially synchronized, the transformer  122  will saturate due to the DC flux built up during the transfer event. The transformer saturations are highly undesirable since they can cause large saturation currents to flow, which in turn can cause system failure due to source overloading or upstream protective breakers tripping.  
         [0019]     To avoid transformer saturation, the optimum time to transfer from the first source  110  to the second source  111  is determined. In one embodiment, the transfer time is determined by computing the volt-seconds area of the load and alternate voltages.  FIGS. 2A and 2B  show waveforms for the load voltage  210  and alternate voltage  212 . When the primary source  110  fails, the controller  134  keeps track of the amount of volt-seconds, i.e., the area under the load voltage curve  210  (At) from the time of the last zero-cross until the first switch  130  is turned off, and it computes a compensating area for the alternate source curve  212  (Ac). Polarity is assigned to each volt-seconds area based on the polarity of the corresponding voltage, for example a positive polarity is assigned to the area under the positive voltage and a negative polarity is assigned to the area under the negative voltage. When a transfer is initiated, the controller  134  waits until the target volt-sec area is approximately equal to (as in  FIG. 2A ) or complements (as in  FIG. 2B ) the compensating volt-sec area before it turns on the second switch  131 . This results in little or no dc flux built up in the transformer  122  during transfer.  
         [0020]      FIGS. 2A and 2B  illustrate two different balanced conditions for transferring from the first to the second voltage source. In  FIG. 2A , At and Ac have different signs (At·Ac&lt;0). In  FIG. 2B , At and Ac have the same sign (At·Ac&gt;0). As noted above, At is the target volt-second area of the load voltage, and Ac is the compensating volt-second area of the alternate voltage.  FIG. 2B  also includes Af, which represents the full half-cycle volt-second area of the alternate voltage. The quantity Ac involves an event that occurs in the future (after the transfer occurs), and therefore can not be exactly determined. However, assuming that the alternate voltage  212  does not change considerably from the previous cycle before the transfer, the quantity Ac at any given time t can be approximated from Ac(t)=Af−Ar(t), where Af is the previous full half-cycle volt-second area, and Ar is the running integral of the volt-sec area from zero-cross to time t.  
         [0021]     Thus, to avoid transfer saturations, in the situation shown in  FIG. 2A , the controller  134  operates the switches  110 ,  111  such that the absolute values of At and Ac are equal (|At|=|Ac|). Defining quantity S 1  as S 1 =At+Ac, the controller  134  turns on the second switch  111  so that S 1 =0 when (At·Ac&lt;0).  
         [0022]     Referring now to  FIG. 2B , the controller  134  operates the switches  110 ,  111  such that the quantities At and Ac added together equal a full half-cycle volt-second area of the alternate voltage (At+Ac=Af). Defining quantity  52  as S 2 =At+Ac−Af, the controller  134  turns on the second switch  111  so that S 2 =0 when (At·Ac&gt;0).  
         [0023]      FIG. 3  is a flow diagram illustrating a specific implementation of the volt-second area method for transferring from the first voltage source  110  to the second voltage source  111 . In block  310 , the load voltage Vload(k) and the alternate voltage V 2 ( k ) are sampled at a predetermined sample rate, for example, 15 kHz. The target volt-sec area At is calculated in block  312  by integrating the load voltage Vload At(k+1)=At(k)+Vload(k). At each zero-cross of the load voltage Vload, the target volt-second area At(K) is reset, except after a command to transfer occurs. In block  314 , the running alternate volt-sec area Ar(k) is calculated by integrating the alternate voltage V 2 : Ar(k+1)=Ar(k)+Valt(k). The running volt-sec area Ar(k) is also reset to zero at every zero-cross of the alternate voltage V 2 , except after a command to transfer occurs.  
         [0024]     In block  316 , the maximum half-cycle volt-second area is calculated by latching the value of Ar(k) at every zero-cross, before resetting Ar(k). The compensating volt-sec area Ac(k) is calculated in block  318  based on the difference between the previous half-cycle volt-second area Af and the the running integral of the volt-sec area Ar Ac(k)=Af−Ar(k). In block  320 , the S 1  and S 2  values are calculated: S 1 ( k )=At(k)+Ac(k) and S 2 ( k )=At(k)+Ac(k)−Af. In block  322 , the controller  134  performs the transfer when S 1 ( k )=0 (At·Ac&lt;0) or when S 2 ( k )=0 (At·Ac&gt;0).  
         [0025]     In another embodiment, two flux quantities are computed continuously prior to a transfer event. The first flux corresponds to the flux that is generated by the load voltage Vload when it drives the transformer  122 , and the second flux corresponds to an imaginary flux that would be generated by the second voltage source  111 , if it were driving the transformer  122 . It can be shown that the optimum transfer point is achieved when these two flux quantities are equal.  
         [0026]      FIG. 4  shows two sets waveforms for the load voltage Vload and alternate voltage V 2 . The top waveforms are voltage curves for load voltage  210  and alternate voltage  212 , and the bottom waveforms are the corresponding flux curves for the load voltage  220  and alternate voltage  222 . The controller  134  performs the transfer when the fluxes are equal—shown by the broken line  230 .  
         [0027]     Ignoring the effect of leakage impedance of the transformer  122 , the fluxes built up on the transformer  122  due to the application of the first voltage source  110  and the second voltage source  111  satisfy the following differential equations:  
           ⅆ       ϕ   1     ⁡     (   t   )           ⅆ   t       ≈       V   1     ⁡     (   t   )           
             ⅆ     ϕ   2         ⅆ   t       ⁢     (   t   )       ≈       V   2     ⁡     (   t   )           
        where V 1 (t) and V 2 (t) are the first and second source voltages and φ 1 (t), φ 2 (t) are the fluxes corresponding to each voltage.        
 
         [0029]     The fluxes φ 1 (t), φ 2 (t) can be computed by solving the above differential equations at any given time: 
 
φ 1 ( t )∫= V   1 ( t ) dt  
 
φ 2 ( t )∫ V   2 ( t ) dt  
 
         [0030]     In the generalized flux method, the optimum transfer is achieved when the two flux quantities defined above are equal in values: φ 1 (t)=φ 2 (t).  
         [0031]      FIG. 5  is a flow diagram illustrating a specific implementation of the generalized flux method for transferring from the first voltage source  110  to the second voltage source  111 . In block  350 , the output voltage Vload(k) and the alternate voltage Valt(k) are sampled at a predetermined sample rate, for example 15 kHz. In blocks  352  and  354 , the load and alternate fluxes φ 1 , φ 2  are determined by integrating the load and alternate voltages, respectively: φ load (k+1)=φ load (k)+V load (k), φ alt (k+1)=φ alt (k)+V alt (k)  
         [0032]     The DC component is removed from both flux quantities periodically prior to transfer using any known techniques. In block  356 , the transfer is performed when the flux quantities are equal: φ load (k)=φ alt (k).  
         [0033]     Rather than balancing the volt-second area or fluxes to be exactly equal, as discussed above, some error tolerance can be used in the balanced condition to provide a trade off between reduced transfer time and some amount of transformer saturation current. Denoting the tolerance as zcth, the balanced conditions above can be rewritten as follows:  
         [0034]     Volt-second area method where At·Ac&lt;0: |S 1 |=|At+Ac|≦zcth; and where At·Ac&gt;0: |S 2 |=|At+Ac−Af|≦zcth.  
         [0035]     For the generalized flux method: |φ 1 −φ 2 ≦zcth.  
         [0036]     The methods described above use volt-second area and flux information that are computed continuously online. In these methods, the optimum switching times for transferring to the alternate source are not known until conditions that guarantee the minimization or elimination of the transformer saturation occur in real time. Known approaches determine optimum switching time based on offline voltage waveform analysis. Such methods, for example, may include determination of optimum switching time delay based on the last known measured phase angle difference between the two sources. In this case, the relationship between the optimum switching delay and the sources phase difference is first derived offline by imposing certain assumptions on the voltage waveforms during the transfer event. Clearly, this approach limits the effectiveness of the method when the actual voltage waveform during transfer deviates from the assumed shape. The methods disclosed herein, on the other hand, do not posses this drawback since the volt-second area and fluxes are continuously computed online based on actual real time voltage waveforms during transfer events.  
         [0037]     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. It should also be evident that the exemplary embodiments disclosed above may be readily applied to other similar or known power systems where transferring from one source of A/C power to another is necessary, such as a standard UPS system where the first source of power is an A/C voltage inverter and the second source of power is the utility or an A/C voltage generator. Consequently, all such similar applications are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.