Abstract:
A reverse current protection device for a fuel cell of a power circuit protects the fuel cell from damage caused by reverse current into the fuel cell caused by a faulty diode. The reverse current protection device includes a contactor for opening the power circuit to prevent current from flowing therethrough and for closing the power circuit to allow current to flow therethrough, means for measuring the voltage across the diode to determine if the diode is in a good condition or a fault condition, means for causing the contactor to be closed if the diode is in a good condition and open if the diode is in a fault condition so that current does not flow through the contactor, protecting the fuel cell from damage caused by reverse current through the diode and into the fuel cell.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of the invention relates, in general, to systems and methods for protecting direct current (DC) high power sources from reverse current damage, and, in particular, to systems and methods for protecting fuel cells in buses or other vehicles from reverse current damage.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various computers are usually employed in monitoring and control systems associated with high power sources. Conceptually, sensing and actuation devices interfaced to a computer as controlled by the computer software could provide reverse current protection. However, the response time of such a system is generally considered as not fast or reliable enough to provide the desired level of protection.  
           [0003]    It is common to use a diode to allow electric current to flow in one direction but not the other. Furthermore, it is common to use diodes to isolate and effectively “OR” DC power sources connected in parallel. High current power sources are often expensive and may be dangerous if damaged or destroyed. If a reverse current protection diode fails in a “shorted” condition, protection of the power source is lost.  
           [0004]    A bus fuel cell used to power an electric motor of a bus supplies high-current, high-voltage power to the electric motor through output power wiring. The output power wiring works in parallel with battery energy storage to supply power to an inverter that controls and powers the electric motor that propels the bus. If any condition arises that causes a reverse current into the fuel cell, significant and very expensive damage to the fuel cell may occur. To prevent this, a high current (e.g., 400 amp) diode may be used in series with the fuel cell power circuit. However, as mentioned above, if the diode fails, the expensive fuel cell is no longer protected from reverse-current damage.  
         SUMMARY OF THE INVENTION  
         [0005]    A reverse current protection device was developed by the present inventors to sense for a “shorted” diode condition and provide protection for the fuel cell in addition to the diode because of the potential consequences of reverse-current damage to a fuel cell. The protected fuel cell may be mounted in a bus and used in parallel with a battery energy storage system to supply power for the electric motor that propels the bus. The reverse current protection device may also be used to provide an additional layer of protection against accidental reverse-current damage for other mobile and fixed fuel cell applications, and other types of power sources susceptible to reverse-current damage.  
           [0006]    An aspect of the invention involves a reverse current protection device designed as, but not limited to, a fuel cell diode check circuit. The circuit senses a healthy diode from its forward current voltage drop. When a forward current voltage drop exists, a “good” condition is sensed, amplified, and used to maintain closure on a high current relay, called a “contactor”, that is connected in series with the high current power bus. Conversely, if no forward voltage exists, a “fault” condition is sensed and the contactor is opened to prevent current flow in the power circuit. For the purpose of sensing forward current flow, a resistor could function as well as a diode, but the nonlinear voltage-current characteristics of the diode together with the fuel cell diode check circuit provide redundant protection for the fuel cell.  
           [0007]    A “start up override delay” signal is part of the start up sequence until the power current starts flowing as a means to sense voltage drop. The start up override delay may be supplied by a simple RC time delay that times out before the sensing of the diode voltage drop. Prior to the start up override delay the start up sequence may include a preliminary sensing of other circuit voltage and/or impedance levels to protect against other possible faults caused by incorrect installation, accident, software bug, or other hardware component failure. The preliminary sensing may be implemented by hard wire circuit connections or by sensing devices connected to a computer and controlled by the computer software.  
           [0008]    Another aspect of the invention involves a method of protecting a fuel cell of a power circuit from damage caused by reverse current into the fuel cell caused by a faulty diode. The power circuit may power an electric motor of a bus for propelling the bus. The method includes providing a contactor in the power circuit that opens to prevent current from flowing therethrough and closes to allow current to flow therethrough; measuring the voltage across the diode to determine if the diode is in a good condition or a fault condition; causing the contactor to be closed if the diode is in a good condition so that current flows from the fuel cell through the diode and the contactor to the electric motor to propel the bus; and causing the contactor to be opened if the diode is in a fault condition so that current does not flow through the contactor, protecting the fuel cell from damage caused by reverse current through the diode and into the fuel cell.  
           [0009]    A further aspect of the invention involves a reverse current protection device for a fuel cell of a power circuit for protecting the fuel cell from damage caused by reverse current into the fuel cell caused by a faulty diode. The power circuit may power an electric motor of a bus for propelling the bus. The reverse current protection device includes means in the power circuit for opening the power circuit to prevent current from flowing therethrough and for closing the power circuit to allow current to flow therethrough; means for measuring the voltage across the diode to determine if the diode is in a good condition or a fault condition; means for causing the power circuit opening and closing means to be closed if the diode is in a good condition so that current flows from the fuel cell to the electric motor to propel the bus; and causing the power circuit opening and closing means to be opened if the diode is in a fault condition so that current does not flow through the power circuit opening and closing means, protecting the fuel cell from damage caused by reverse current through the diode and into the fuel cell.  
           [0010]    A still further aspect of the invention involves a reverse current protection device for a fuel cell of a power circuit for protecting the fuel cell from damage caused by reverse current into the fuel cell caused by a faulty diode. The power circuit may power an electric motor of a bus for propelling the bus. The reverse current protection device includes a contactor in the power circuit that opens to prevent current from flowing therethrough and closes to allow current to flow therethrough; an impedance buffer to invert the voltage across the diode; a threshold detector to compare the inverted voltage to a reference voltage to determine if the diode is in a good or fault condition; an opto-isolator actuatable by the threshold detector when at least one of a good diode condition is determined or a fault diode condition is determined; and a relay in communication with the opto-isolator to cause at least one of the contactor to close in a good diode condition so that power is supplied to the electric motor to propel the bus or open in a fault diode condition for protecting the fuel cell from damage caused by reverse current into the fuel cell caused by a faulty diode.  
           [0011]    It is therefore an object of the present invention to protect direct current (DC) high power sources from reverse current damage to associated electrical components. It is a further object of this invention to protect expensive fuel cell components form reverse current damage.  
           [0012]    An even further object of the invention is to protect expensive fuel cell components from inadvertent electrical power surges in the reverse direction due to incorrect installation, accident, software bug, or other hardware component failure.  
           [0013]    Lastly, it is an object of the present invention to advance and protect the technology of the fuel cell used in the automobile industry, especially the bus and mass transit industry.  
           [0014]    These together with other objects and advantages which become subsequently apparent reside in the details of the construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part thereof, wherein like numerals refer to like parts throughout. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of this invention.  
         [0016]    [0016]FIG. 1 is a block diagram of an embodiment of a reverse current protection device for a fuel cell diode.  
         [0017]    [0017]FIG. 2 is a circuit schematic of the reverse current protection device of FIG. 1.  
         [0018]    [0018]FIG. 3 is a block diagram of another embodiment of a reverse current protection device for a fuel cell diode where the reverse current protection device includes dual redundant circuits.  
         [0019]    [0019]FIG. 4 is a circuit schematic of the reverse current protection device of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    With reference to FIGS. 1 and 2, an embodiment of a reverse current protection device  10  for protecting a fuel cell  15  of a bus will now be described. Although the reverse current protection device  10  is described as protecting a fuel cell  15  of a bus, the reverse current protection device  10  may be used to protect fuel cells used in other mobile and fixed applications and may be used to protect power sources other than fuel cells.  
         [0021]    The fuel cell  15  is part of a power circuit  17  for supplying power in the form of high current at high voltage to a load  20 . In the embodiment shown, the fuel cell  15  is a bus fuel cell sold by UTC Fuel Cells of South Windsor, Conn. Although a single fuel cell  15  is shown, in an alternative embodiment, the power circuit  17  may include multiple fuel cells  15  with multiple protection devices  10 . The output power wiring for the fuel cell  15  may work in parallel with an active load such as a battery  25  to supply power to an inverter (not shown) that controls and powers the load  20 . In an alternative embodiment, additional or alternative active loads such as a dc/dc converter, a battery charger, or another battery may be part of the power circuit  17 . In the embodiment shown, the load  20  is an electric motor that propels the bus.  
         [0022]    If any condition arises, including but not limited to, incorrect installation, accident, software bug, or other hardware component failure, that causes a reverse current into the fuel cell  15 , significant and very expensive damage to the fuel cell  15  may occur. To prevent this, a high power diode  40  in a negative return line to the fuel cell  15  or anywhere in series with the fuel cell power circuit  17  may be used to protect the fuel cell  15  against reverse current. In the embodiment shown, the diode  40  is a high-current power diode that typically operates with a forward current rating of 400 amps. As used herein high-current power diodes are power diodes with a forward current rating of at least 30 amps. Reverse current could inadvertently occur, for example, because of some over-voltage of the battery  25 , some other power source connected in parallel, incorrect installation, accident, software bug, or other hardware component failure.  
         [0023]    The reverse current protection device  10  includes an impedance buffer  50  and a threshold detector  60  designed using a dual operational amplifier integrated circuit which requires the + and voltage for power as provided by a dc/dc converter  100 . The impedance buffer  50  acquires and inverts the voltage Vdiode across the diode to an inverted voltage V1. The threshold detector  60  compares the inverted voltage V1 to a reference voltage derived by a resistor divider pair. When Vdiode exceeds the reference voltage, the threshold detector  60  outputs a voltage that indicates that the diode  40  is good at a diode status indicator  102  (e.g., LED) and also drives a relay  80  through an opto-isolator  70  that, in turn, causes a contactor or contactor relay  30  to close the power circuit  17  and allow power from the fuel cell  15  to be delivered to the load  20 . The opto-isolator  70  isolates the high-voltage power ground from the low-voltage signal ground. The driver relay  80  provides sufficient current to operate the 400 amp contactor relay  30 .  
         [0024]    When first powering up the fuel cell  15 , there is no current across the diode  40  and, therefore, no voltage drop. This would indicate a bad diode and not allow the contactor relay  30  to close and turn on the load circuit  20 . A start up delay override circuit  90  and driver relay  92  may be used to close the contactor relay  30  at startup and allow current to flow. Once the power circuit  17  is in operation, the override signal is removed and the reverse current protection device  10  takes over to continuously monitor the diode condition. If a failed diode (e.g., short) fault occurs, the fuel cell  15  is at risk of damage from reverse current. The reverse current protection device  10  senses the fault within one millisecond and opens the contactor relay  30  within 30 milliseconds and removes the risk to the fuel cell  15  by turning off the power circuit  17 . In alternative embodiments, the reverse current protection device  10  opens the contactor relay  30  in less than 30 milliseconds.  
         [0025]    [0025]FIG. 2 is a circuit schematic of the reverse current protection device  10  of FIG. 1 and shows an embodiment of the integrated circuits, discrete resistors, and discrete capacitors that may make up the reverse current protection device  10 . The capacitors may be added to a feedback loop of the threshold detector  60  to provide a Schmidt-trigger effect to speed up and latch the threshold detector  60 .  
         [0026]    With reference to FIG. 3, another embodiment of a reverse current protection device  98  for protecting a fuel cell  15  of a bus will be described. The reverse current protection device  98  includes dual redundant first reverse current protection circuit  10 A and second reverse current protection circuit  10 B. Although dual redundant reverse current protection circuits  10 A,  10 B are shown, the reverse current protection device  98  may include two or more redundant reverse current protection circuits.  
         [0027]    The reverse current protection circuits  10 A,  10 B include impedance buffers  50 A, B and threshold detectors  60 A, B designed using a dual operational amplifier integrated circuit that requires the + and voltage for power as provided by the dc/dc converters  100 A, B. The impedance buffers  50 A, B acquire and invert the voltage Vdiode across the diode  40  to inverted voltage V1. The threshold detectors  60 A, B compare the inverted voltage V1 to a reference voltage derived by a resistor divider pair. When V1 exceeds the reference voltage, the threshold detectors  60 A, B output a voltage that indicates that the diode  40  is good and drive a driver relay  80  through opto-isolators  70 A, B that, in turn, causes the contactor relay  30  to close the power circuit  17  and allow the fuel cell  15  to power the load  20 . The opto-isolators  70 A, B isolate the high-voltage power ground from the low-voltage signal ground. The driver relay  80  provides sufficient current to operate the 400 amp contactor relay  30 . Second opto-isolators  72 A, B, which are activated by the threshold detectors  60 A, B when V1 exceeds the reference voltage, cause a diode status indicator  102  (e.g., LED) to indicate the diode is good. As indicated above, when first powering up the fuel cell  15 , there is no current across the diode  40  and therefore no voltage drop. This would indicate a bad diode and not allow the contactor relay  30  to close and turn on the load circuit  20 . Redundant detect and time delay circuits  140 A,  140 B and driver relay  92  are used to close the contactor relay  30  at startup and allow current to flow if a startup condition is detected by measuring VregA, VregB, the output voltages of voltage regulators  130 A,B. Once the power circuit  17  is in operation, the override signal is removed and the reverse current protection device  98  takes over to continuously monitor the diode condition. If a failed diode (e.g., short) fault occurs, the reverse current protection device  98  opens the contactor relay  30  and removes the risk to the fuel cell  15  by turning off the power circuit  17 .  
         [0028]    [0028]FIG. 4 shows a circuit schematic of the reverse current protection device  98  of FIG. 3 and shows an embodiment of all the components and connections to implement the functions of the reverse current protection device  98  of FIG. 3. The circuit of FIG. 4 may be installed on an operating fuel cell powered bus.  
         [0029]    Thus, the reverse current protection device provides a redundant safety check system and method for sensing a short circuit fault condition of a reverse-current protection diode for a fuel cell and turns off or “opens” the fuel cell main power circuit in the event of a protection diode short circuit fault condition.  
         [0030]    The reverse current protection device shown in the drawings and described in detail herein disclose arrangements of elements of particular construction and configuration for illustrating preferred embodiments of structure and method of operation of the present invention. For example, the circuit could be reduced to an integrated circuit that is enclosed within the contactor package. It is to be understood, however, that elements of different construction and configuration and other arrangements thereof, other than those illustrated and described may be employed for providing a reverse current protection device  10  in accordance with the spirit of this invention, and such changes, alterations and modifications as would occur to those skilled in the art are considered to be within the scope of this invention as broadly defined in the appended claims.