Abstract:
An electrical switch apparatus for use in connecting and disconnecting a DC power source and a load includes first and second pairs of controllable electromechanical contacts coupled to the DC power source and the load for connecting the power source to the load when the contacts are closed, and disconnecting the power source from the load when the contacts are open. A controller is coupled to the electromechanical contacts and programmed to produce control signals for opening and closing the contacts. A diode is coupled to the electromechanical contacts to prevent electrical current from flowing from the load to the power source, and a controllable semiconductor switch is coupled to the controller and across the power source for momentarily short circuiting the source in response to a control signal indicating a transition of either or both of the first and second pairs of electromechanical contacts from a closed condition to an open condition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a hybrid electrical switch having a closed, conducting state for connecting a DC power source to a load, and an open, non-conducting state for disconnecting the DC power source from the load. 
     BACKGROUND OF THE INVENTION 
     Breaking high DC currents at relatively high voltages has typically been accomplished with high-cost equipment. For example, a large number of electromechanical contacts in series have been used to achieve DC load break capability. Magnetic arc blowouts or arc chutes have also been used in conjunction with electromagnetic contactors, and contacts have been put in vacuum-encased glass “bottles” to reduce arc potential under load break. There is a need for a lower-cost way of breaking high DC currents at relatively high voltages. 
     SUMMARY 
     In accordance with one embodiment, an electrical switch apparatus for use in connecting and disconnecting a DC power source and a load includes first and second pairs of controllable electromechanical contacts coupled to the DC power source and the load for connecting the power source to the load when the contacts are closed, and disconnecting the power source from the load when the contacts are open. A diode is coupled to the electromechanical contacts to prevent electrical current from flowing from the load to the power source, and a controllable semiconductor switch is coupled to the controller and across the power source. A controller coupled to the electromechanical contacts and the controllable semiconductor switch is programmed to produce a control signal for turning the semiconductor switch on and off, and to produce a control signal for turning the semiconductor switch on to momentarily short circuit the DC power source when at least one of the first and second pairs of electromechanical contacts transitions from a closed condition to an open condition. 
     In one implementation, the controller is programmed to control the semiconductor switch to momentarily short the DC power source, and to open at least one of the pairs of electromechanical contacts while the DC power source is short circuited by the semiconductor switch. 
     In another implementation, the controller is programmed to open at least one of the first and second pairs of electromechanical contacts, and to control the semiconductor switch to momentarily short the DC power source immediately after the opening of the at least one of the first and second pairs of electromechanical contacts. 
     A further implementation includes a third pair of controllable electromechanical contacts connected in parallel with the diode, and the controller is programmed to close the third pair of electromechanical contacts in response to a command to open at least one of the first and second pairs of contacts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the present disclosure will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
         FIG. 1  is an electrical schematic diagram of a hybrid electrical switch coupling a DC source and resistive and capacitive loads. 
         FIG. 2  is an electrical schematic diagram of a modified version of the hybrid electrical switch of  FIG. 1 . 
         FIG. 3  is an electrical schematic diagram of another modified version of the hybrid electrical switch of  FIG. 1 . 
         FIG. 4  is an electrical schematic diagram of a further modified version of the hybrid electrical switch of  FIG. 1 . 
         FIG. 5  is an electrical schematic diagram of yet another modified version of the hybrid electrical switch of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       FIG. 1  illustrates a hybrid electrical switch  10  that couples a DC power source  20 , such as a photovoltaic source, with a load  30  that is illustrated as having a resistive component  30   a  and a capacitive component  30   b . The illustrative switch  10  is shown in  FIG. 1  as a two-port device having the source  20  connected to the switch  10  at + and − input terminals  21  and  22 , respectively, and having the load  30  connected to the switch  10  at + and − output terminals  31  and  32 , respectively. The switch  10  has an open, non-conducting state in which the source  20  and the load  30  are disconnected, and a closed, conducting state in which the source  20  and the load  30  are connected. In the conducting state, current flows from the + input terminal  21  through a diode D 1  and a pair of closed contacts C 1   a  to the + terminal  31  of the load  30 . Current returns from the − load terminal  32  through a pair of contacts C 1   b  to the − terminal  22  of the source  20 . 
     The source  20  is shown as a non-ideal current source, but other types of DC power sources may be used. For example, the switch  10  may be used with a voltage source having limited current capability, and may also have an associated complex distributed LRC impedance. 
     The switch  10  includes a programmable controller  11 , such as a microprocessor, that provides coil power to a contactor coil C 1  that controls the opening and closing of the two pairs of contacts C 1   a  and C 1   b , which in turn determine whether the switch  10  is in its open or closed state. The controller  11  also provides power to a contactor coil C 2  that controls when a pair of contacts C 2   a  are closed to shunt current around the diode D 1 , during steady state conditions when the switch is in its closed, conducting state. The shunt formed by closing the contacts C 2   a  avoids conduction losses in the diode D 1  when the diode is not needed. 
     The controller  11  also provides a gate drive signal to a transistor Q 1  connected across the input terminals  21  and  22 . The controller  11  can receive inputs such as external commands to open or close the hybrid switch and/or can generate commands internally in response to inputs from one or more sensors. The controller  11  provides specific timing sequences when transitioning the switch  10  between its closed and open states. 
     When the switch  10  is in the open, non-conducting steady state, the contacts C 1   a  and C 1   b  are open, and the transistor Q 1  is off. When the switch  10  is in the closed, conducting steady state, the contacts C 1   a  and C 1   b  are closed, and the transistor Q 1  is off. When the switch  10  transitions between its open and closed states, there are two primary “make” sequences and two primary “break” sequences that can be executed by the controller  11 , as follows: 
     Load Make Sequence # 1 
         (i) Contactor coil C 2  is energized to close contacts C 2   a.      (ii) After the worst case close and bounce time for contacts C 2   a  has expired, contactor coil C 1  is energized to close contacts C 1   a  and C 1   b.          

     Load Make Sequence # 2 
         (i) Transistor Q 1  is driven “on.”   (ii) Contactor coils C 2  and C 1  are energized to close contacts C 2   a , C 1   a  and C 1   b.      (iii) After the worst case close and bounce time for contacts C 2   a , C 1   a  and C 1   b  has expired, transistor Q 1  is driven “off.”       

     Load Break Sequence # 1 
         (i) Contactor coil C 2  is de-energized to open contacts C 2   a.      (ii) After the worst case time for contacts C 2   a  to open, transistor Q 1  is driven on and conducts all the current from source  20  plus the transient diode D 1  recovery current.   (iii) After diode D 1  has recovered, the current path from load capacitance  33  through transistor Q 1  is blocked.   (iv) Coil C 1  is de-energized to open contacts C 1   a  and C 1   b.      (v) After a delay to ensure contacts C 1   a  and C 1   b  are fully open, transistor Q 1  is driven off.       

     Load Break Sequence # 2 
         (i) Contactor coil C 2  is de-energized to open contacts C 2   a.      (ii) After the worst case time for contacts C 2   a  to open, coil C 1  is de-energized to open contacts C 1   a  and C 1   b , after a sub-second delay time. Contacts C 1   a  and C 1   b  may (by design) sustain an arc.   (iii) After a delay to ensure that contacts C 1   a  and C 1   b  are fully open, transistor Q 1  is driven on and conducts all of the current from source  20  plus transient diode D 1  recovery current as a function of the available arc current conducted pole-to-pole across contacts C 1   a  and C 1   b.      (iv) After the worst cased diode recovery time, the arc is quenched and transistor Q 1  is driven off.       

     The controller can be programmed to execute any combination of the above sequences. In both Load Break Sequences # 1  and # 2 , the contacts C 1   a  and C 1   b  need only be AC rated because the contacts are not required to break a sustained DC arc. The potential arc energy is removed from the conduction paths that include the contacts C 1   a  and C 1   b  by shorting the source  20  with the transistor Q 1 . In Load Break Sequence # 1 , the recovery current of the diode D 1  is much greater than that in Load Break Sequence # 2 , and therefore the stress on the diode D 1  is greater. In Load Break Sequence # 2 , the arcing time of the contacts C 1   a  is much longer than that in Load Break Sequence # 1 . The best sequence is determined as a function of the application and the type of components used in a given hybrid switch design. The contacts C 2   a  are only used to remove diode D 1  conduction losses by shunting diode D 1  current through contacts C 2   a  during steady state conditions when the hybrid switch is in the closed, conducting state. As part of any state transition sequence, i.e., in either a making or breaking sequence, the contacts C 2   a  are always fully open before the transistor Q 1  is driven on. 
       FIG. 2  illustrates a modified hybrid switch  40  that includes a manually operated disconnect switch having a power pole  41  and a ganged auxiliary switch contact  42  connected to the control circuit  11  to enable the control circuit to detect opening and closing of the power pole  41 . This disconnect switch may be integral to the hybrid switch as shown or may be external and logically interlocked by any number of methods. When the disconnect switch is opened under load, one of the following Load Break Sequences is executed by the control circuit  11 : 
     Load Break Sequence # 1 
         (i) Transistor Q 1  is driven on and conducts all the current from source  20  plus the transient diode D 1  recovery current.   (ii) After diode D 1  has recovered, the current path from load capacitance  33  through transistor Q 1  is blocked.   (iii) Coil C 1  is de-energized to open contacts C 1   a  and C 1   b.      (iv) After a delay to ensure contacts C 1   a  and C 1   b  are fully open, transistor Q 1  is driven off.       

     Load Break Sequence # 2 
         (i) Coil C 1  is de-energized to open contacts C 1   a  and C 1   b , after a sub-second delay time. Contacts C 1   a  and C 1   b  may (by design) sustain an arc.   (ii) After a delay to insure that contacts C 1   a  and C 1   b  are fully open, transistor Q 1  is driven on and conducts all of the current from source  20  plus transient diode D 1  recovery current as a function of the available arc current conducted pole-to-pole across contacts C 1   a  and C 1   b.      (iii) After the worst cased diode recovery time, the arc is quenched and transistor Q 1  is driven off.       

     The disconnect switch power pole  41  need not be rated for DC load break because the transistor Q 1  automatically “steals” the potential arc energy from the contacts C 1   a  and the power pole  41  after an open disconnect switch condition is indicated by the auxiliary switch contact  42 . 
       FIG. 3  illustrates another modified hybrid switch  50  that includes additional components to protect the semiconductor components from switching- or lightning-induced voltage transients. A transient voltage suppressor such as a varistor  51  connected across the input terminals  21  and  22 , and thus across the transistor Q 1 , ensures that the breakdown voltage of the transistor Q 1  is not exceeded. A diode D 2  is also connected across the transistor Q 1  to provide reverse polarity protection for the transistor Q 1  and to clamp any reverse polarity differential voltage transients across the input terminals  21  and  22 . A clamp network formed by a diode  52 , a capacitor  53  and resistor  54  slows the voltage rise time across the input terminals  21  and  22  when the transistor Q 1  turns off and serves to clamp and damp ringing from parasitic inductances. This clamp network also reduces the stress on the varistor  51 . A resistor  55  and a capacitor  56  damp the ringing across the diode D 1  during diode recovery, and a transient voltage suppressor such as a varistor  57  ensures that the breakdown voltage of the diode D 1  is not exceeded. 
       FIG. 4  illustrates another modified hybrid switch  60  that includes additional components and control functions to protect the hybrid switch under fault conditions. As part of any sequence where the transistor Q 1  is turned on, a number of steps are taken to ensure that the semiconductor ratings will not be exceeded. First, the open circuit input voltage across the terminals  21  and  22  is read, via divider resistors  62  and  63 , and is recorded by the programmable controller  11 . Next, a second transistor Q 2 , connected across the terminals  21  and  22  in series with a resistor  64 , is momentarily pulsed on, and the input terminal voltage is again read and recorded while the source  20  is loaded by the resistor  64 . The ratio of (a) the open circuit input terminal voltage to (b) the input terminal voltage when the source  20  is momentarily loaded by the resistor  64 , is used by the controller  11  to calculate the available short circuit current from source  20 . If this calculated value is not within the capabilities of the transistor Q 1 , a fault is indicated, and the hybrid switch  60  will not close. Additionally, whenever the transistor Q 1  is driven on, the terminal voltage is again read to look for a desaturated condition in the transistor Q 1 . If detected, the transistor Q 1  is turned off, a fault is indicated, and the hybrid switch will not close. 
     The transistor Q 2  and the resistor  64  may also be used to discharge any differential capacitance associated with the source  20  before the transistor Q 1  is driven on. A current sensor  61  is coupled to the controller  11  to permit the controller to identify reverse current, overcurrent and leakage fault conditions. Under steady state conditions, when the transistors Q 1  and Q 2  are without drive and the coil C 1  is not energized, if current is detected by the sensor  61 , then a Load Break Sequence is re-initiated and a fault is logged by the controller  11 . The signal from the current sensor  61  can also be used to compare the load current to a preprogrammed reference value stored in the controller  11  so that the hybrid switch can function as a circuit breaker. 
     If the programmable controller  11  detects an internal component failure such as welded contacts C 1   a  or a failed transistor Q 1 , a fault is annunciated, and a non-load-break-rated latching contactor C 3  is used as a failsafe device to indefinitely short circuit the source  20  via closed contacts  63   a  until the hybrid switch  60  can be serviced. In solar photovoltaic applications, additional latching contactor contacts (not shown) may be used in series with the current sensor  61  to break the circuit created by the latching contactor C 3  after sunset to isolate the failed hybrid switch. Ideally, the hybrid switch should be single-fault-tolerant so that any of the power components can fail without presenting a safety or fire hazard. 
       FIG. 5  illustrates a hybrid switch  70  that is part of a solar photovoltaic (PV) power conversion system. A pair of solar photovoltaic arrays  20   a  and  20   b  are connected across respective terminal pairs  21   a ,  22   a  and  21   b ,  22   b , respectively. The negative pole of the array  20   a  and the positive terminal of the array  20   b  are connected to earth ground  71  via terminal  72  through ground fault protection fuses  73  and  74 , respectively, having respective blown-fuse indicating switches  75  and  76  connected to the controller  11 . This photovoltaic array configuration is typically referred to as bipolar. The function of the hybrid switch  70  is basically the same as that of  FIG. 2 , but the controller  11  is logically integrated with the overall control of the power converter system. An additional contactor having a coil C 3  and contacts C 3   a  permits direct connection of the negative terminal  22   a  of the source  20   a  with the positive terminal  21   b  of the source  20   b . In a grid-interactive PV power converter, the load resistor  30  is proportional to the power delivered to the electrical grid. The “value” of the load resistor  30  can be controlled by the power converter under normal operating conditions. As such, when no faults are present, the power into the grid, and therefore the current through the hybrid switch  70 , can be reduced to zero before the contacts C 1   a , C 1   b , C 2   a  and C 3   a  are commanded to open, and thus the transistor Q 1  need not be brought into conduction. The load capacitor  33  is the DC buss capacitance of the PV power converter and is essentially constant. The primary function of the hybrid switch  70  in PV applications is to interrupt full short circuit PV array current and to interrupt and isolate PV array ground faults. A secondary function is to provide protection from catastrophic PV power converter faults where the load resistance  30  becomes shorted or cannot be controlled. The hybrid switch works well with photovoltaic sources because the short circuit current of a PV source is typically only 125% that of the PV current at maximum power transfer. 
     As an operational example of the circuit topology shown in  FIG. 5 , assume that the PV power converter is operational and is transferring nominal power to the electric grid with contactors C 1   a , C 1   b , C 2   a  and C 3   a  closed when a ground fault (a short) from terminal  22   b  to earth  40  is established, as illustrated in  FIG. 5 . The following sequence will occur:
         (i) Current from the fault is the available short circuit current from the PV array  20   b  and flows through the fuses  73  and  74 .   (ii) The fuses  73  and  74  clear and blown-fuse indicators  75  and  76  signal a fault condition to the controller  11 .   (iii) The contact coils C 1  and C 2  are energized by the controller  11  to open the contacts C 1   a , C 1   b  and C 2   a.      (iv) After a delay to ensure that contacts C 1   a , C 1   b  and C 2   a  are fully open, the transistor Q 1  is pulsed “on” to momentarily short circuit the series combination of the PV sources  20   a  and  20   b . The conduction time of the transistor Q 1  is just long enough to ensure that the diode D 1  has been recovered and that arcing in the contacts C 1   a  and C 1   b  has been quenched.   (v) After the transistor Q 1  has turned off, the coil C 3  is de-energized and contacts C 3   a  open.       

     This entire sequence takes place in less than 1 second. The PV array monopole  20   a  now floats with respect to ground, the PV power converter and the array monopole  20   b . The PV array monopole  20   b  is grounded at the negative pole, terminal  22   b , through the fault, but no fault current flows because the fault current return path has been eliminated. 
     The application illustrated in  FIG. 5  can be configured from two of the circuits illustrated in  FIG. 2 , so that each photovoltaic monopole  20   a  and  20   b  is individually shorted while the electromechanical contacts open. 
     The controller  11  in most practical applications will be microprocessor-based and may have a number of current, voltage and temperature inputs, a number of transistor and contactor coil drive outputs, isolated external command input and outputs, isolated serial communications, an external or internal power supply, data and fault logging capability and self-diagnostic capabilities. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations will be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.