Abstract:
An embodiment of the invention is a DC power disconnect having a first mechanical switch for coupling a DC power module to a first rail of a DC bus and a second mechanical switch for coupling the DC power module to a second rail of the DC bus. A first solid-state switch couples the DC power module to the first rail of the DC bus and is positioned in parallel with the first mechanical switch. A second solid-state switch couples the DC power module to the second rail of the DC bus and is positioned in parallel with the second mechanical switch. A controller initiates closing the first solid-state switch and the second solid-state switch prior to changing state of the first mechanical switch and the second mechanical switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional application No. 60/385,685, filed Jun. 4, 2002, the entire contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    DC buses are used for a variety of power distribution systems. An exemplary system is disclosed in U.S. Pat. No. 6,559,559, the entire contents of which are incorporated herein by reference. Such systems may employ disconnect devices to interrupt power to a DC power module such as a DC load or a DC/DC converter, DC/AC converter, etc.  
           [0003]    Mechanical switches for this purpose are large and complicated mechanical devices to handle the large current and to deal with the large arcs that result from interrupting direct currents. Arc suppression mechanisms exist, but are typically complicated. Thus, there is a need for a DC power disconnect system for handling large currents with simple components.  
         SUMMARY OF THE INVENTION  
         [0004]    An embodiment of the invention is a DC power disconnect having a first mechanical switch for coupling a DC power module to a first rail of a DC bus and a second mechanical switch for coupling the DC power module to a second rail of the DC bus. A first solid-state switch also couples the DC power module to the first rail of the DC bus and is positioned in parallel with the first mechanical switch. A second solid-state switch also couples the DC power module to the second rail of the DC bus and is positioned in parallel with the second mechanical switch. A controller initiates closing the first solid-state switch and the second solid-state switch prior to changing state of the first mechanical switch and the second mechanical switch. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a block diagram of an exemplary DC power system.  
         [0006]    [0006]FIG. 2 is a schematic diagram of an exemplary DC power disconnect.  
         [0007]    [0007]FIG. 3 is a schematic diagram of an exemplary DC power disconnect in an alternate embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0008]    [0008]FIG. 1 is a block diagram of an exemplary DC power system  10 . The power system includes a DC bus having a negative rail  12  and a positive rail  14 . A number of DC power modules  16  are connected to the DC bus through a DC power disconnect  100 . DC power modules  16  may be a variety of devices including DC loads or DC power conditioning devices such as DC/DC converters, DC/AC converters, etc. The DC power disconnects  100  are used to disconnect a DC power module  16  from the DC bus for service, upgrade, etc. and then re-connect the DC power module  16  to the DC bus.  
         [0009]    In an exemplary embodiment, the DC power modules are part of a power generation system. Thus, it is desirable for the DC power disconnects  100  to operate under load conditions in order to service the DC power modules  16  without interrupting operation. The DC power disconnects  100  may be located physically in the DC main bus, which itself is located in the top section of each DC power module  16 . When service and/or maintenance is needed for a particular DC power module  16 , the operator will do the following to disconnect the particular module from the main system.  
         [0010]    1. Manually operate the DC power disconnect to remove power from the DC power module.  
         [0011]    2. Verify visually that the DC power disconnect is open.  
         [0012]    3. Perform any required “lock-out/tag-out” procedures.  
         [0013]    Reconnecting the DC power module back into the main system will simply require reversing the operation; i.e., arming and closing.  
         [0014]    [0014]FIG. 2 is a schematic diagram of an exemplary DC power disconnect  100 . A DC power module  16  is connected to the positive rail  14  through a first switch  102 . DC power module  16  is connected to the negative rail  12  through a second switch  104 . The first and second switches  102  and  104  are preferably mechanical switches actuated from controller  106 . Switches  102  and  104  may include a cam for activating electrical contact(s), a low contact resistance, and ready-made bus-bar connection points. The cam is coupled to mechanical linkage (e.g., a pneumatic drive mechanism) shown at line  103  that will open and close the switches  102 / 104  on command from an operator.  
         [0015]    In an exemplary embodiment, the first and second switches  102  and  104  have a minimum voltage rating of 600 VDC and a minimum current rating of 6000 ADC. The switch contacts have a resistance of less than 50 μohms. Preferably, switches  102  and  104  include a visible disconnect point and have a contact separation of at least one inch when open. The switches  102  and  104  also provide for the installation of a lock and tag to lock the switching device in the open or closed positions.  
         [0016]    An operator interfaces with an operator actuator  108  coupled to the controller  106 . An operator issues commands through the operator actuator  108  which are implemented by controller  106 . The controller  106  opens and closes switches  102  and  104  as long as certain safety conditions are met. If an unsafe condition is detected, controller  106  prevents operation that will either open or close switches  102  and  104 . Prevention of operation is accomplished by means of a positive locking device that will allow operation of the switch only if all conditions are satisfied as described in further detail herein.  
         [0017]    Switches  102  and  104  are shunted by a parallel-connected solid-state switches  112  and  114 , respectively. In one embodiment, the solid-state switches are insulated gate bipolar transistors (IGBT). The solid-state switches  112  and  114  handle the transition of switches  102  and  104  from open-to-closed contact or from closed-to-open contact. Auxiliary switches  132  and  134  are provided in the shunt path and are controlled by controller  106  as described herein. As with switches  102  and  104 , switches  132  and  134  may be opened or closed through a mechanical linkage (e.g., pneumatic drive) shown at line  133  actuated by controller  106 .  
         [0018]    The solid-state switches  112  and  114  can handle load conduction for a short period of time during the switching transition. However, the solid-state switches  112  and  114  cannot handle sustained loads because of heat build-up in the solid-state element. Fuses  122  and  124  (e.g., thermal fuses) protect solid-state switches  112  and  114 , respectively as described herein.  
         [0019]    Conversely, the switches  102  and  104  can handle sustained power without requiring any special cooling. However, switches  102  and  104  cannot handle switching transitions because of destructive arcing of the mechanical contacts. Therefore, the combination of mechanical and solid-state switches provides both long-term and low-resistance connection with arc-free switching under load.  
         [0020]    Operation of the DC power disconnect  100  will now be described. The connection operation is initiated by an operator at operator actuator  108 . Controller  106  receives the command from operator actuator  108  and closes auxiliary switches  132  and  134  to connect the solid-state switches  112  and  114  to the DC bus. Controller  106  confirms that prescribed operational safety conditions are satisfied to continue. Such safety conditions include detecting failures such as a shorted solid-state switch  112 / 114 , a blown fuse  122 / 124 , or a malfunctioning mechanical switch  102 / 104 .  
         [0021]    If the safety conditions are met, the controller  106  drives both solid-state switches  112  and  114  into conduction. The controller  106  also starts the closure of switches  102  and  104  to change state from open to closed. When switches  102  and  104  are closed, controller  106  turns off solid-state switches  112  and  114 . This allows the transition current from open to closed to be passed through the solid state switches  112  and  114  until switches  102  and  104  are closed.  
         [0022]    To disconnect the DC power module  16  from the DC bus, the operator selects the appropriate controls though operator actuator  108 . The controller  106  controls the actual switching sequence an initially closes auxiliary switches  132  and  134 . Controller  106  verifies that prescribed safety conditions are satisfied and then drives solid-state switches  112  and  114  into conduction. Controller  106  also starts the opening of switches  102  and  104  to change state from closed to open. When the switches  102  and  104  have opened, controller  106  turns off the solid-state switches  112  and  114 .  
         [0023]    The DC power disconnect  100  provides a number of safety benefits. Main DC bus connections are severed by visible switches  102 / 104  that are first in line before any of the DC power modules. The thermal fuses  122  and  124  protect the solid state switches  112  and  114  from excessive power dissipation. Thermal fuses  122  and  124  also protect the DC power module  16  in the event that one or both of solid-state switches  112  and  114  is shorted. Auxiliary switches  132  and  134  protect the DC power module  16  from accidental power from the main DC bus, should the solid-state switches  112  and  114  be shorted.  
         [0024]    Safety of personnel and equipment is provided by the DC power disconnect. The DC power disconnect has hardware features that allow for the DC power disconnect to be locked in either open or closed positions with proper visibility of the contacts along with any indicators or flags. In addition, the design of the DC power disconnect includes detection features and mechanisms that will prevent manual operation should certain unsafe electrical conditions exist. These conditions would include a shorted solid-state switch, a blown fuse, or a malfunctioning mechanical switch element.  
         [0025]    [0025]FIG. 3 is a schematic diagram of an exemplary DC power disconnect in an alternate embodiment. The system of FIG. 3 includes current sensors  152  and  154  coupled to controller  106  through signal conditioning devices (e.g., amplifier). In one embodiment, current sensors  152  and  154  are non-contact Hall-effect current sensors. In this embodiment, the DC disconnect operates as a DC circuit breaker by monitoring the current flowing in each switch  102  and  104 .  
         [0026]    The controller  106  receives a current signal from current sensors  152  and  154 . If a fault is detected (e.g., sensed current out of a predefined range), controller  106  closes auxiliary switches  132  and  134 . If prescribed safety conditions are satisfied, the controller  106  drives the solid-state switches  112  and  114  into conduction, opens the mechanical switches  102  and  104  and then turns off solid-state switches  112  and  114 .  
         [0027]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.