Patent Publication Number: US-2003227785-A1

Title: On-line uninterruptible power supplies with two-relay bypass circuit and methods of operation thereof

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to power supply apparatus and methods of operation thereof, and more particular, to uninterruptible power supply (UPS) apparatus and methods.  
       [0002] Many conventional uninterruptible power supplies (UPSs) use an on-line topology. As shown in FIG. 1, a typical on-line series train UPS  10  includes an AC/AC converter circuit  12  that produces an AC output voltage at a load from an AC input voltage provided by an AC power source  20 , such as a utility. As shown, the AC/AC converter circuit  12  may include a combination of a rectifier and an inverter, connected by a DC bus that can isolate the load  30  from disturbance and other degradation of the AC power source  20 . Typically, the DC bus is also coupled to an auxiliary source of power, such as a battery, which maintains the DC voltage on the DC bus in the event the AC power source fails. Some on-line UPSs use circuit topologies other than a series train, including more complex topologies, such as delta converters, or other techniques.  
       [0003] Under normal operating conditions, on-line UPS&#39;s supply power to a load through a rectifier/inverter chain or similar regulating circuitry, providing relatively clean and regulated power at the output of the UPS. When the AC power source  20  fails, the UPS  10  may achieve an uninterrupted transition to battery power, as there typically is no need to change the state of a transfer switch. As illustrated in FIG. 1, the UPS  10  may also include a bypass feature such that if, for example, the AC/AC converter circuit  12  fails or becomes overloaded, the load  30  may be decoupled from the AC/AC converter circuit  12  (e.g., the output inverter) and coupled directly to the AC source  20  by a form C (double-pole double-throw) relay  14 . Such a feature may also be used to provide an “economy” mode of operation, as power dissipation associated with the operation of the rectifier/inverter chain may be reduced when the load is transferred to the bypass path.  
       [0004] Many conventional low-cost UPSs use a “break before make” technique to transfer between normal and bypass modes, i.e., they produce a disruption in the output voltage as the form C relay  14  transitions between states. However, some units, as shown in FIG. 1, use a solid state switch  16 , e.g., anti-parallel connected silicon controlled rectifiers (SCRs), to smooth transfer of the load  30  to and from the AC source  20  until the form C relay  14  has transitioned. In particular, when transfer of the load  30  from the AC/AC converter circuit  12  to the AC source  20  is initiated, the solid state switch  16  may be turned on immediately preceding triggering of the relay  14 . Because the solid state switch  16  can begin conducting nearly instantaneously, the AC source  20  can be connected to the load  30  via the solid state switch  16  while the contacts of the relay  14  move between positions. Similarly, when a transfer of the load  30  back to the AC/AC converter circuit  12  is initiated, the solid state switch  16  may be turned on immediately preceding triggering of the relay  14 , and may be maintained in an “on” state until the contacts of the relay  14  have switched over to the AC/AC converter circuit  12 . The solid state switch  16  may then be turned off to break the direct connection between the AC power source  20  and the load  30 .  
       SUMMARY OF THE INVENTION  
       [0005] According to some embodiments of the invention, an on-line uninterruptible power supply (UPS) comprises an AC input configured to be coupled to an AC power source and an output configured to be coupled to a load. The UPS includes an AC/AC converter circuit coupled to the AC input and operative to generate an AC voltage from the AC power source. A first mechanical relay (e.g., a single-pole single-throw (SPST) relay) is coupled between the AC/AC converter circuit and the output. A second mechanical relay (e.g., another SPST relay) is coupled between the AC input and the output. The UPS further includes a control circuit operative to control the first and second mechanical relays to selectively place the UPS in an on-line mode in which the first mechanical relay couples the AC/AC converter circuit to the output and the second mechanical relay decouples the AC input from the output or a bypass mode in which the first mechanical relay decouples the AC/AC converter circuit from the output and the second mechanical relay couples the AC input to the output.  
       [0006] According further embodiments of the invention, the control circuit is operative to place the UPS in the bypass mode by closing the second mechanical relay while maintaining the first mechanical relay in a closed state and then opening the first mechanical relay a predetermined time thereafter. The control circuit may be further operative to place the UPS in the on-line mode by closing the first mechanical relay while maintaining the second mechanical relay in a closed state and then opening the second mechanical relay a predetermined time thereafter. According to further aspects, the control circuit is operative to operate at least one of the first and second mechanical relays as a hypervelocity mechanical relay, for example, by applying a relatively high coil voltage to speed operation of the relay, followed by a reduced coil voltage that maintains the relay in its new state. In this manner, performance approaching that provided by bypass circuits employing solid state switches can be achieved.  
       [0007] According to various embodiments of the invention, relatively smooth transitions between on-line and bypass modes in an on-line UPS can be provided without requiring the use of solid state switches. Accordingly, the need for back-feed and/or fail-safe circuitry can be obviated, potentially reducing the complexity and/or cost of the UPS. The invention may be embodied as apparatus and methods. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008]FIG. 1 is a schematic diagram illustrating a conventional uninterruptible power supply (UPS).  
     [0009]FIG. 2 is a schematic diagram illustrating a UPS according to some embodiments of the invention.  
     [0010]FIG. 3 is a schematic diagram illustrating a UPS according to further embodiments of the invention.  
     [0011]FIG. 4 is a waveform diagram illustrating exemplary operations of the UPS of FIG. 3. 
    
    
     DETAILED DESCRIPTION  
     [0012] The present invention now will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.  
     [0013]FIG. 2 illustrates an uninterruptible power supply (UPS)  200  according to some embodiments of the invention. The UPS  200  includes an AC input  201  configured to be coupled to an AC power source  20  (e.g., an AC utility line) and an output  202  configured to be coupled to a load  30 . An AC/AC converter circuit  210  is coupled to the input  201 . A first mechanical relay, here shown as a single-pole single-throw (SPST) relay  220 , is operative to couple and decouple an output of the AC/AC converter circuit  210  to and from the output  202 . A second mechanical relay  230  is operative to couple and decouple the AC input  201  to and from the output  202 . A control circuit  240 , e.g., a relay driver circuit, controls the first and second mechanical relays  220 ,  230 .  
     [0014] Several advantages can be provided by the configuration of FIG. 2. In particular, the use of mechanical relays  220 ,  230  instead of a combination of a form C relay and a solid state switch can obviate the need to provide additional circuitry to prevent back-feed and provide fail-safe operation in compliance with regulatory requirements. Because the two relays  220 ,  230  can be operated in a “make before break” fashion, smooth transition between on-line and bypass modes can be achieved.  
     [0015] It will be appreciated that the components of the UPS  200 , such as the control circuit  240 , may, in general, be implemented using discrete circuitry, integrated circuits, data processing circuits configured to execute software and/or firmware, and combinations thereof. For example, the control circuit  240  may include a microprocessor, microcontroller or other data processing circuit in combination with discrete relay driving circuitry, such as transistors and logic circuit elements. It will be further understood that all or some of such circuitry may be integrated in one or more devices, such as application specific integrated circuits (ASICs) or hybrid circuits.  
     [0016] As shown in FIG. 3, a UPS  300  according to further embodiments of the invention includes an AC/AC converter circuit  310 , first and second mechanical SPST relays K1, K2, and a control circuit  340 . The AC/AC converter circuit  310  includes an input rectifier  312  that is coupled to an AC input  301  of the UPS  300  at which an AC power source  20  is connected. The AC/AC converter circuit  310  also includes an output inverter  314  coupled to the rectifier  312  by an intermediate DC bus  315 . The AC/AC converter circuit  310  further includes an auxiliary DC power source  316  (e.g., a combination of a battery and a DC/DC converter) connected to the DC bus  315 . The first mechanical SPST relay K1 is operative to couple and decouple the inverter  314  to and from an output  302  of the UPS  300  at which a load  30  is connected. The second mechanical SPST relay K2 is operative to couple and decouple the AC input  301  to and from the output  302 . The control circuit  340  controls the mechanical SPST relays K1, K2, and the operation of the AC/AC converter circuit  310 .  
     [0017] Exemplary operations for the UPS  300  of FIG. 3 are illustrated in FIG. 4. Initially, the UPS is in a bypass mode, i.e., a zero voltage is applied to the coil of the first relay K1 to maintain its contacts in an open state O, while a voltage V1 is applied to the coil of the second relay K2 to maintain its contacts in a closed state C. In order to transfer the load  30  from the AC power source  20  to the inverter  314 , such that the UPS  300  operated in an “on-line” mode, the control circuit  340  first drives the coil of the first relay K1 with a voltage V3 sufficient to cause the contacts of the first relay K1 to begin a transition from an open state O to a closed state C. Following transition of the contacts of the first relay K1 to the closed state C, the control circuit  340  then applies a zero voltage to the coil of the second relay K2 to transition the contacts of the second relay K2 from a closed state C to an open state O.  
     [0018] In order to transfer the load  30  back to the AC power source  20  to place the UPS  300  in a “bypass” mode, the control circuit  340  applies a voltage V2 to the coil of the second relay K2 to transition its contacts from an open state O to a closed state C. After transition of the contacts of the second relay K2 to the closed state C, the control circuit  340  applies a zero voltage to the coil of the first relay K1 to transition its contacts from a closed state C to an open state O. As shown, the second relay K2 may be a “hypervelocity” relay, e.g., the control circuit  340  may apply a relatively high voltage V2 to transition the second relay K2 to the closed state C, followed by a lower voltage V1 that maintains the relay K2 in the closed state. In this manner, performance approaching conventional designs using solid state switches may be achieved in many fault scenarios. It is believed that such operation can result in at least 50% faster operation over use of a nominal coil voltage.  
     [0019] The coil current in a typical DC relay builds up slowly. In particular, after a voltage is applied to the coil, the shape of the coil current is similar to current that builds up in an inductance, except that motion of the relay&#39;s armature typically changes the inductance of the relay and, thus, affects the usual time-current relationship. When the armature “seats,” the coil current typically exhibits a dip, after which the coil current increases based on the inductance of the closed relay until it reaches a steady state value dominated by the resistance of the coil.  
     [0020] The application of a higher than rated coil voltage generally changes the way in which current builds up in the coil. Because of the higher voltage applied to the coil, coil current sufficient to start the armature moving typically occurs earlier than with a normal coil voltage applied. The more rapidly increasing coil current can increase the force applied to the armature, which can reduce the time to first strike (and can increase contact bounce, which may not be a problem in the bypass mode operation of a UPS). Upon closure of the contacts, the applied voltage can be reduced to a level that maintains the contacts in the closed position.  
     [0021] Circuits for providing such multi-level drive are known to those skilled in the art, and will not be discussed in greater detail herein. For example, relay manufacturers have used this principal on some large DC contactors. Such contactors typically have a coil that has a low inductance and DC resistance and that is capable of moving the armature rapidly, but that cannot continuously sustain the magnitude of coil current required to close the contacts. After the contacts have switched state, such contactors may reduce the current in the coil using an external series resistor that is switched in to lower the coil current to a level that will maintain the contacts in the closed position by effectively reducing voltage across the coil. It will be appreciated that these and other techniques for producing hypervelocity relay operation may be used with the present invention.  
     [0022] It will also be appreciated that the first relay K1 may also be a hypervelocity relay, and may be operated in a manner similar to the second relay K2 in transitioning the UPS  300  to the “on-line” mode. However, rapid operation of the second relay K2 may be more desirable. Accordingly, the first relay K1 may be operated as a “standard” relay, e.g., without such a two-level drive, which may entail simpler, lower cost drive circuitry.  
     [0023] Referring back to FIG. 3, the control circuit  340  may be operatively associated with the AC/AC converter circuit  310 . For example, the control circuit  340  may be operative to sense an operating state of the AC/AC converter circuit  310 , e.g., loading, output voltage or the like, and may operate the relays K1, K2 responsively thereto. It will be further appreciated that during times that the relays K1, K2 are both in a closed state, such that the inverter  314  and the AC power source  20  are both connected to the load  30 , the control circuit  340  may be operative to synchronize operation of the AC/AC converter circuit  310  to the AC power source  20  such that backflow into the inverter  314  and other undesirable phenomena arising from lack of synchronization can be reduced.  
     [0024] For example, as shown in FIG. 3, the control circuit  340  may be responsive to an output voltage Va produced by the AC power source  20  and an output voltage Vb produced by the inverter  314 . The control circuit  340  may, for example, adjust the frequency of operation of the inverter  314  responsive to a detected phase difference between the output voltage Va of the AC power source  20  and the output voltage Vb of the inverter  314  and adjust the amplitude of the output voltage Vb produced by the inverter  314  responsive to a detected amplitude difference between the output voltage Va of the AC power source  20  and the output voltage Vb of the inverter  314 . For example, if the output voltage Vb of the inverter  314  is lagging the output voltage Va of the AC power source  20 , the control circuit  340  may increase the frequency of operation of the inverter  314  to reduce phase error before transferring the load  30  to the inverter  314 . Conversely, if the output voltage Vb of the inverter  314  is leading the output voltage Va of the AC power source  20 , the control circuit  340  may responsively reduce the frequency of operation of the inverter  314  to reduce phase error before the transfer. Similarly, the amplitude of the output voltage Vb of the inverter  314  can be adjusted, e.g., by adjusting the DC voltage on the DC link  315 , to substantially match the amplitude of the output voltage Va of the AC power source before transfer of the load  30  from the AC power source  20  to the inverter  314 . Following transition of the load to the inverter  314 , the frequency and/or amplitude of the output voltage Vb produced by the inverter  314  can be controlled independently of the output voltage Va produced by the AC power source  20 .  
     [0025] In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.