Abstract:
A system, method and device for power switching and power control, particularly for switching the source of power between two or more power sources are provided. Power control elements testing the availability and stability of alternate power sources and switch loads between these power sources in short periods of time and with advantageous switching characteristics. In a fuel-cell system, the embodiments of the invention may be advantageously deployed to power up balance of plant loads using opto-electronic couplers and electronic relays.

Description:
BACKGROUND OF THE INVENTION 
     Embodiments of the present invention relate generally to the field of power supply control systems. Particular embodiments relate to the application of fuel cells for power generation. More particularly, embodiments of the present invention relate to the control of power supply to and from fuel cell systems used for primary or backup power supply. 
     Fuel cell systems can be employed as a reliable and efficient primary or backup power source for critical applications that normally use power drawn from the utility grid. A typical fuel cell system for electrical power includes a fuel cell stack which is the power generating component of the system, an inverter to convert the direct current produced by the fuel cell stack to an alternating current, a fuel source, for example in a solid oxide fuel cell system (SOFC) typically a hydrogen or hydrocarbon fuel, and a number of components, such as heat exchangers, valves, blowers, etc., designed to ensure the proper functioning of the power generation components, often referred to as the “balance of plant”. The balance of plant components can be powered by utility grid power rather than by the fuel cell stack. 
     A SOFC backup power system, for example, can be regeneratively coupled to the utility grid. This means that the fuel cell can provide power to the primary application if the grid goes offline or, when the grid is functioning normally, the fuel cell can reverse cycles and regenerate fuel for operating the system, or function as a current source for the grid. 
     In grid interconnected systems, it is important that a power source be applied to the balance of plant loads in a short period of time in the event of grid failure, otherwise there may be damage to the fuel cell stack. Unfortunately, it has been seen that mechanical contactors are often too slow to provide backup power to the balance of plant loads in the event of grid loss. In the past, this problem has been alleviated by means of an uninterruptible power supply (UPS), having batteries and other storage devices such as capacitors. Uninterruptible power supplies are, however, expensive and cumbersome to implement. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate generally to methods, systems and devices for accomplishing a switch to and from fuel cell or grid power in a fast and electrically optimal manner, while lessening the need for an uninterruptible power supply. 
     One aspect of the invention relates to a circuit, comprising: a first power source connected to a first opto-electric coupler; a first line configured to provide a control signal related to a second power source, the first line being connected to a second opto-electric coupler; and logic for emitting a second control signal for switching between the first power source and a second power source upon a change in state of at least one of the first or second opto-electric couplers. Of course, as will be clear to a person of skill in the art, the term “connected to” should not be interpreted to mean “directly connected to”. 
     Another aspect of the invention relates to a method for providing power in a fuel cell system, comprising: determining in a first step, using a first opto-electric coupler, whether a first power source is present; determining in a second step, using a second opto-electric coupler, a presence of a control signal indicative of a second power source; and responsive to the determining in the first and second steps, powering a fuel cell balance of plant with either the first or second power sources. 
     Yet another aspect of the invention relates to a system for power control, comprising: a load to be supplied with power; a first power source connected with the load over at least a first electronic relay; a second power source connected with the load over at least a second electronic relay; and a control circuit for controlling the first and second relays, such that the relays are in opposite states except during moments of transition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a system overview of an exemplary embodiment of the invention. 
         FIG. 2  shows a control circuit that can be used with embodiments of the invention. 
         FIG. 3  is a plot of voltage versus time for various voltages in an exemplary embodiment of the invention. 
         FIG. 4  is a plot of voltage versus time for various voltages in an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a fuel cell power generation system  100  according to an embodiment of the present invention having connections  102  and  104  to the utility power grid, an inverter  106  for converting fuel cell generated direct current (DC) to an alternating current, a stand alone balance of plant load  108 , as well as contactors  110 ,  112  and  114 .  FIG. 1  further shows a battery  116 , a control circuit  200  with a grid input, and control outputs  250  and  252  from circuit  200  which are connected to relays  112  and  114  respectively. 
     During normal grid operation, contactor  110  is closed to allow reverse grid interconnection. Contactor  110  can be a standard mechanical contactor such as, for example, mechanical contactors driven by a pilot relay. Contactors  112  and  114  are electronic relays, preferably solid state relays such as Silicon Controlled Rectifiers (SCRs), and in a preferred embodiment capable of changing states in 8.33 milliseconds or less. In a de-energized state, or in the event of grid failure, it can be necessary to power the balance of plant loads from an alternate source. If there is no or limited power in the system, however, it is difficult to control electronic relays  112  and  114 . 
     Embodiments of the present invention can therefore be advantageously used with a control circuit that is powered from a fuel cell battery. A fuel cell battery  116  can either be a battery present in the fuel cell system as a supplemental DC voltage source, or a battery present in the system for an auxiliary purpose, such as the control of transients or the absorption of power ripples produced by the fuel cell inverter. 
     Turning to  FIG. 2 , an exemplary circuit  200  is shown for effecting embodiments of the present invention. The circuit  200  has a grid connection  202 , battery or other DC source connections  204  and  206 , which can be connected to a fuel cell battery  116  as described above, for example, ground or reference potential connections  208 ,  210  and  212 , resistors  214 ,  216 ,  218  and  220 , and a capacitor  222 . Circuit  200  also has an NAND gate  224  that can be, for example, from a 4012 dual 4-input NAND gate, an inverter  228 , and a pass-through gate  230 . Both inverter  228  and gate  230  can be constructed from 4069 Hex Inverters. The use of multiple inputs for the NAND gate and inverters assists in developing a three-phase circuit, which will react to changes in any phase of the system in an economical manner. The circuit  200  is shown for a single phase, but it will be clear to people of skill in the art that a three-phase variant can be constructed. 
     Circuit  200  further comprises opto-electric couplers  232  and  234  (i.e. opto-couplers), which can be, for example, 2271 opto-couplers sold by NEC, Inc., each comprising a respective light-emitting diode (LED)  238 ,  240  and a corresponding phototransistor  242 ,  244 . As is known in the art, in an opto-coupler, the radiation emitted by a light-emitting diode is received by a photo transistor or other suitable photodetector. The phototransistor or other suitable photodetector is turned on in response to receiving the LED radiation. Circuit  200  further comprises a diode  236 , a switch  246 , a DC voltage source  248  and outputs  250  and  252 . Outputs  250  and  252  are connected to contactors  114  and  112 . These contactors are shown in  FIG. 1 . 
     If the fuel cell ignition key is in the “off” position, reflecting an inactive state of the fuel cell system, switch  246  will be open. Once the ignition key is turned on, the balance of plant loads will need to be powered up quickly. Assuming the grid is powered on at connection  202 , current will flow through resistor  214 , which serves to limit current flow to a manageable amount according to the circuit implementation, and which in exemplary embodiments can be dimensioned at approximately 50 kilo Ohms, resulting in an AC current of approximately 1.2 milliamperes. 
     Diode  236  allows current to pass through only during the positive cycle of the current. This excites light-emitting diode  238  and simultaneously charges capacitor  222 , which in exemplary embodiments can be approximately 22 microFarads for a 120 Volt alternating current grid connection. On the negative half cycle, capacitor  222  discharges to hold light-emitting diode  238  in an excited state. 
     The light emitted by light-emitting diode  238  acts as a gate of transistor  242 , providing radiation to the channel of transistor  242  and serves to allow current to flow from source to drain of transistor  242 , thus allowing the flow of direct current from battery or other direct current connection  204 . Resistor  216  is so dimensioned as to hold one of the inputs to NAND gate  224  at high potential (HI) when opto-electric coupler  232  is closed (active). Resistor  216  is also so dimensioned so that when opto-electric coupler  232  is open (inactive), the respective input to NAND gate  224  is held at low potential (LO), that is, that the resistance of the opto-electric coupler  232  is much larger than that of the resistor when opto-electric coupler  232  is open. The transistor  242  thus functions in this manner as a pull-up transistor. Resistor  216  is further dimensioned to take into account the desired shape of any voltage tail, as described hereinafter. In exemplary embodiments, the resistor  216  can be approximately 27 kilo Ohms. 
     When switch  246  is open, LED  240  is de-energized and phototransistor  244  is open, causing the top of  220  to be LO. This causes NAND gate  224  to have a HI output, causing the output of  228  to be LO and correspondingly turning on relay  112  and turning off relay  114 , which isolates the inactive inverter from the utility grid  104  (shown in  FIG. 1 ). 
     Thus, the normal functioning of the grid with switch  246  open will cause contact  114  to be open, and require the balance of plant to be driven from the inverter. Since the fuel cell ignition key is in the “off” position, the balance of plant will be driven by the inverter drawing on battery power. Contact  114  will also be open, isolating the grid from the balance of plant. 
     If the grid is operating normally, but switch  246  is closed indicating that the fuel cell ignition key has been placed in the “on” position, the circuit changes output states. As the grid is still active, the functioning of opto-electric coupler  232  remains the same, and the top input to NAND gate  224  is still HI. The bottom input to NAND gate  224  will also be HI, as opto-electric coupler  234  will be closed, causing transistor  244  to pull up the voltage at the second input of NAND gate  224 . If all three grid phases are present in a three-phase system, the output of NAND gate  224  will be LO. In this situation, output  250  will be HI and output  252  will be LO, which will apply power from the grid to the balance of plant and isolate the balance of plant from the inverter, by opening contactor  112  and closing contactor  114  (shown in  FIG. 1 ). 
     Similarly, if the grid should suffer from an interruption, regardless of the state of switch  246 , opto-electric coupler  232  will open, causing the upper input to NAND gate  224  to be LO. In this situation, output  250  will be LO and output  252  will be HI, which will also apply power from the inverter to the balance of plant and isolate the balance of plant from the grid, by closing contactor  112  and opening contactor  114 . 
     The circuit  200  generally provides for the contactors  112  and  114  (shown in  FIG. 1 ) to be in opposite states. Thus, for example, if contactor  112  is open, contactor  114  will be closed, and vice versa. This will hold true with the present exemplary embodiments except in moments of transition. Of course, it is possible to design circuit  200  such that any number of logical states of different contactors may be encompassed. 
       FIG. 3  shows plots  302 ,  304  and  306  of various voltages with time for the embodiment shown in  FIG. 2 . Plot  302  is a plot of grid voltage between contact  202  and ground contact  208  with time. Plot  304  is a plot of voltage over resistor  216  with time. Plot  306  is a plot of voltage over the LED  238  in opto-coupler  232 . 
     As can be seen from  FIG. 3 , on the positive half cycle of the grid voltage in plot  302 , capacitor  222  is charged to its full voltage, and the opto-electric coupler  232  is closed, allowing the full voltage drop across resistor  216 . In the negative half cycle, capacitor  222  begins to discharge, but is so dimensioned in conjunction with resistor  216 , that insufficient discharge current can flow to allow the voltage over light-emitting diode  238  (provided by the capacitor) to drop fully below the emission threshold. Thus, the potential at the input of NAND gate  224  does not drop below the switching threshold. 
     There can be, however, a voltage tail  308  at the input to NAND gate  224 , as shown in  FIG. 3 . The voltage tail can be caused by an increase in channel resistance of transistor as the capacitor  222  discharges and as the voltage over light-emitting diode  238  drops. Before the channel resistance of transistor  242  can become so high that the potential at the input of NAND gate  224  drops below the switching threshold, however, grid voltage enters the positive half cycle again. 
     The sharpness of voltage tail  308  can be regulated by optimizing the resistance of resistor  216 . An increase in resistance lessens drop in potential at the input of NAND gate  224  caused by increases in channel resistance of transistor  242 . Since channel resistance increases exponentially with decreasing voltage near the emission threshold of light-emitting diode  238 , however, a slight voltage drop may be observable toward the end of the negative half-cycle. An increase in the resistance of resistor  216  postpones the observability of voltage tail  308 . 
     Voltage tail  308  can be used in a number of applications. For example, since the voltage tail occurs primarily toward the end of the negative half-cycle, the voltage tail can be used in combination with a comparator as a positive/negative half-cycle indicator. Since the voltage tail  308  occurs once per cycle, it can be communicated to a synch circuit, processor, microcontroller or similar circuit to perform a frequency monitoring function. 
       FIG. 4  provides an illustration of the switching quality of embodiments of the present invention. Plot  402  shows grid voltage during normal operation. At time  404 , a control signal  406 , for example from the output of opto-coupler  232  in  FIG. 2 , is received indicating loss of grid power. According to  FIG. 1 , this means the closure of coupler  114  and the opening of coupler  112 . As can be seen from  FIG. 4 , the use of solid state electronic relays causes the control signals at outputs  250  and  252  to activate (open or close) couplers  114  and  112  only at the zero point of a cycle, as shown by the deactivation of voltage in plot  402  and the activation of voltage in plot  408 . This reduces current spikes that would otherwise be caused by non-smooth transitions. 
     Embodiments of the present invention provide numerous advantages over standard systems employing UPS-based solutions. First and foremost, the requirement of a UPS is obviated, thus saving cost. Furthermore, the electronic relays and control circuit provide a method of switching that is as much as 80% faster than standard mechanical contactors. 
     Embodiments of the present invention are also useful in analyzing and handling grid disturbances. For example, a circuit of the type  200  can respond within a single half-cycle to power loss, and can respond to the loss of a single phase in a three-phase system. The circuit can also be used to detect a high or low grid condition and disconnect based on an undesirable state. Furthermore, circuits of type  200  can function as frequency detectors for frequency monitoring. 
     Moreover, embodiments of the present invention obviate the need for more expensive solutions employing digital signal processors (DSPs). If desired, however, embodiments of the present invention can be designed to cooperate with a DSP, for example, by providing an interrupt signal to a DSP upon grid loss. 
     Embodiments of the present invention are also believed to be advantageous in their modularity. That is, the system can be designed without reference to the particular fuel cell or inverter being used, which allows wide application with minimal integration costs. 
     The invention has been presented with reference to certain specific and exemplary embodiments. It will be recognized by persons of skill in the art, however, that the invention is not so limited, and may be modified in numerous ways within the scope of this disclosure.