Abstract:
A generator which is self-configurable to selectively bond the neutral lead to the ground lead of the generator. The generator includes a sensor configured to detect current flow on the ground lead of the generator. The sensor generates a signal corresponding to the current flow which is, in turn, provided to a controller present on the generator. A switch device, such as a relay selectively connects the neutral conductor to the ground conductor at the generator. The controller outputs a signal to control the switch device in response to the signal from the current sensor. If the controller detects current flowing on the ground lead, it opens the switch between the neutral and ground conductors; however, if the controller detects no current on the ground lead, it closes the switch between the neutral and ground conductors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to generators and, more particularly, to a generator that detects ground currents and selectively bonds the neutral lead of the generator to the ground lead of the generator responsive to a detected ground current. 
     Electrical panels, breaker boxes, or load centers frequently include a main contactor, switch, or breaker, which electrically isolates a series of individual circuit or load breakers from a utility power input. In a typical utility power input installation, the main contactor selectively connects or disconnects a first hot lead, L 1 , and a second hot lead, L 2 , with a respective bus bar, to which the individual circuit or load breakers are connected. A neutral lead, N, is connected to a neutral bar from which neutral connections are distributed to each of the loads. In addition, a ground connection is established in the load center, for example, by a ground bar connected to a conductive rod inserted into the ground or to a metal wire or pipe exiting the site into the ground. The ground bar provides connection points for ground conductors to be distributed to the loads as required. According to wiring standards, the neutral lead, N, from the utility power is connected to the ground connection at a single location within the electrical distribution system. Thus, in a standard installation a bonding wire may be connected between the neutral bar and the ground bar in the load center. 
     Occasionally, such load centers are configured to receive a secondary input power source, such as from a generator, to provide electrical power to certain of the individual loads or circuits in the event of a utility power failure. During interruption of utility power, the generator supplies power to the load center, which the load center distributes to selected circuits of the building. Depending on factors such as the size of the generator, the number of electrical loads, and whether a load is considered critical (i.e., must remain on during a utility power outage), such as a furnace, sump pump, etc., the secondary power source may power all of the electrical loads or only a portion of the loads. Similar to the utility power input, the generator includes a first hot lead, L 1 , a second hot lead, L 2 , and a neutral lead, N. The generator may also include a ground connection, such as a receptacle grounding terminal. The generator may be configured either with a bond between the neutral lead, N, and the ground connection (i.e., a bonded-neutral generator) or without a bond between the neutral lead, N, and the ground connection (i.e., a floating-neutral generator). 
     In a non-separately derived system, the neutral lead, N, from the generator is solidly connected to the neutral connection from the utility supply, and the neutral lead, N, of the generator must neither be directly connected to an earth ground nor to the ground connection of the generator (i.e., a floating-neutral generator). As previously indicated, a connection between the neutral and ground leads should be established at a single location in the electrical distribution system. Because the neutral connection in the generator is floating, the connection between the neutral and ground leads is established in the service entrance panel. In the same manner as a distribution system having no generator, this connection may be established via a bonding wire connected between the neutral bar and the ground bar in the load center. The neutral lead from the generator is bonded with the neutral lead from the utility, for example, at the neutral bar. A transfer switch is provided to selectively connect the hot leads, L 1  and L 2 , from either the utility or the generator to the electrical loads. 
     A generator that has its neutral lead, N, bonded to the ground connection of the generator (i.e., a bonded-neutral generator) must be installed as a separately derived system. The neutral conductor from the loads is switched between the neutral lead, N, of each of the power sources to maintain a single bonding point in the system. The neutral leads, N, of the utility power source and the secondary power source, such as the generator, are switched by the transfer switch in addition to the hot leads, L 1  and L 2 . Further, the connection between neutral and ground for the utility system must occur prior to the switched connection such that it is removed when the secondary power source is connected and the equipment grounds for the loads are separated from the neutral. Typically the neutral bar and the ground bar are not connected directly in a service entrance panel but rather they are combined into one dual-function terminal bar. This separation is needed in a service entrance transfer panel to keep the neutrals and ground wires separate when switched to the secondary power source since the connection between neutral and ground takes place further upstream toward the power source and cannot occur again in the panel. 
     A potential problem arises when a bonded-neutral generator, having a neutral connection connected to the equipment ground at the generator, is connected to a service panel having a transfer switch that is only configured to switch the hot leads, L 1  and L 2 . In such a configuration, multiple connection points can be established between the neutral leads and the ground leads, creating a second conduction path parallel to the neutral conduction path. As a result, a portion of the current that is supposed to be conducted via the neutral lead may be present on the ground lead, raising the potential for damage to the electrical loads connected to the system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a generator which is self-configurable to selectively bond the neutral lead to the ground lead of the generator. The generator includes a sensor configured to detect current flow on the ground lead of the generator. The sensor generates a signal corresponding to the current flow which is, in turn, provided to a controller present on the generator. A switch device, such as a relay, is provided to selectively connect the neutral conductor to the ground conductor at the generator. The controller outputs a signal to control the switch device in response to the signal from the current sensor. If the controller detects current flowing on the ground lead, it opens the switch between the neutral and ground conductors. By monitoring the current on the ground lead, the generator can detect whether the neutral leads between the utility and the generator are being switched by the transfer switch and select whether to bond the neutral lead at the generator accordingly. The switch device may have a normally open default position, and the controller may close the switch device at periodic intervals when the generator is running to check for current on the ground lead. Optionally, the switch device may have a normally closed position, and the controller may open the switch if the level of current and/or voltage detected exceeds a predefined level. The controller may then hold the switch device open until the generator is stopped. 
     According to one embodiment of the invention, a generator for providing electrical energy to an electrical distribution system includes an electrical energy generation device configured to generate the electrical energy, and a plurality of electrical conductors configured to transmit the electrical energy. The plurality of electrical conductors includes at least one hot conductor, a neutral conductor, and a ground conductor. The generator also includes a bonding jumper connected between the neutral conductor and the ground conductor, a sensor configured to generate a signal corresponding to an amplitude of current present on the bonding jumper, a switch connected in series with the bonding jumper and configured to selectively establish an electrical connection between the neutral conductor and the ground conductor in a first mode and break the electrical connection between the neutral conductor and the ground conductor in a second mode, and a controller configured to receive the signal from the sensor and configured to generate a control signal transmitted to the switch as a function of the signal from the sensor. The control signal places the switch in either the first mode or the second mode. The switch may be an electro-mechanical switch, such as a relay, or a solid state switch. The controller may be a logic circuit, including discrete electronic components, or a microcontroller, including an input configured to receive the signal from the sensor, an output configured to transmit the control signal, and a plurality of instructions configured to generate the control signal as a function of the signal from the sensor. 
     According to another embodiment of the invention, a method of selectively bonding a neutral connection in a generator is disclosed. A sensor is provided in a generator, and the generator is configured to provide electrical energy to an electrical distribution system via an electrical energy generation device and a plurality of electrical conductors, including at least one hot conductor, a neutral conductor, and a ground conductor. A bonding jumper and a switch are connected in series between the neutral conductor and the ground conductor. The amplitude of current present on the bonding jumper is measured with the sensor, and an electrical connection is selectively established between the neutral conductor and the ground conductor with the switch as a function of the amplitude of current present on the bonding jumper. In a first mode, the electrical connection is established, and in a second mode, the electrical connection is broken. 
     According to another aspect of the invention, the step of selectively establishing an electrical connection between the neutral conductor and the ground conductor further includes the steps of receiving a feedback signal at a controller, the signal corresponding to the amplitude of current present on the bonding jumper, and generating a control signal transmitted to the switch as a function of the feedback signal. The control signal places the switch in either the first mode or the second mode. 
     According to still another embodiment of the invention, a generator includes means for generating electrical energy, means for providing electrical energy to an electrical distribution system, including at least a neutral conductor and a ground conductor, means for sensing current present on a bonding jumper selectively connected between the neutral conductor and the ground conductor; and means for selectively connecting the bonding jumper as a function of the current sensed on the bonding jumper. 
     Various other features, objects and advantages of the present invention will be made apparent from the following detailed description of the drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
         FIG. 1  is a block diagram representation of a generator according to one embodiment of the invention connected to a non-neutral switching transfer switch; and 
         FIG. 2  is a block diagram representation of a generator according to one embodiment of the invention connected to a neutral switching transfer switch. 
     
    
    
     DETAILED DESCRIPTION 
     The various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non-limiting embodiments described in detail in the following description. 
     Referring first to  FIG. 1 , one embodiment of an electrical distribution system including a load center  10  having a transfer switch  20  configured to selectively provide electrical energy to a load from one of two power sources is disclosed. The load center  10  includes an enclosure  12  having an electrical panel  11  mounted within the enclosure  12 . The transfer switch  20  is mounted to the electrical panel  11  and receives a first set of electrical conductors  14  from a first power source, such as the utility supply, at a first set of terminals  22 . The transfer switch  20  receives a second set of electrical conductors  16  from a second power source, such as a backup generator  50 , at a second set of terminals  24 . The transfer switch  20  includes a switch arm  26  movable between a first position, which connects each of the hot leads, L 1 , L 2  from the utility power source to one of a pair of terminal bars  30 , and a second position, which connects each of the hot leads, L 1 , L 2  from the generator  50  to one of the terminal bars  30 . As illustrated, the switch arm  26  is manually transferred between the first and second positions. Optionally, a sensor may be included to monitor operation of the utility power source, and, if the utility power source fails, a solenoid or other actuator may be energized to automatically transfer the switch arm  26  between the first and second positions. Alternately, still other configurations of manual and or automatic transfer switches may be employed without deviating from the scope of the invention. 
     The load center  10  distributes the power from one of the two sources to an electrical load. Circuit breakers  32  are mounted to the terminal bars  30  such that a first terminal on the rear of each circuit breaker  32  establishes an electrical connection with the terminal bar  30  to which it is mounted. A second terminal on the side of each circuit breaker  32  is configured to receive a first electrical conductor providing power to the load. The neutral conductor, N, from the utility supply is connected to one of the terminals  38  on a neutral bar  36 . Other terminals  38  of the neutral bar  36  are configured to receive a second electrical conductor providing power to the load. The first and second electrical conductors define a forward conduction path and a return conduction path, establishing a circuit, between from the load center  10  and the load. One or more of the circuits may also include a ground conductor. A ground bar  40  includes multiple terminals  42  to which each of the ground conductors may be connected. An earth ground  44 , which may be, for example, a copper rod sunk into the ground next to the building in which the load center  10  is housed, is electrically connected to the ground bar  40 . Because the illustrated transfer switch only switches the two hot leads, L 1  and L 2 , of each power supply and does not switch neutral leads, N, a permanent bond between the neutral lead, N, and earth ground  44  is established. A bonding jumper  46  establishes this bond connecting the neutral bar  36  to the ground bar  40 . 
     A generator  50  is connected to the load center  10  to provide a secondary energy source. The generator  50  includes an energy generation device  52 , in a manner as is known. According to one embodiment of the invention, the energy generation device includes a combustion engine fueled by gasoline, diesel fuel, propane, or any other fuel. As the combustion engine operates, it spins a rotor within a stator of a generator. The rotor has permanent magnets mounted thereto which excite the windings of the stator, generating electricity. Optionally, various other configurations of an energy generation device  52  may be utilized without deviating from the scope of the invention. The windings on the stator are configured to provide at least one hot lead, L 1 , and a neutral lead, N. As illustrated, the windings of the energy generation device  52  provide a first hot lead, L 1 , and a second hot lead, L 2 , in addition to the neutral lead, N. A ground lead, G, may be connected, for example to the chassis of the generator  50 . A bonding jumper  53  is provided internal to the generator  50  to connect the neutral lead, N, to the ground lead, G. 
     A sensor  54  measures the amplitude of current present on the bonding jumper  53  and generates a feedback signal  56  corresponding to the amplitude of current. The feedback signal  56  is input to a controller  58 , and the controller  58  is configured to generate an output signal  60  as a function of the feedback signal  56 . The output signal  60  is transmitted to a switch  62  connected in series with the bonding jumper  53 , and the output signal  60  controls operation of the switch  62 . According to one embodiment of the invention, the controller  58  includes discrete logic devices, including, for example, a buffer and/or voltage conditioner to convert the feedback signal from a first level to a second level, a voltage regulator to generate reference voltages either from the generated voltage or from a battery, a comparator to compare the feedback signal to the reference voltages, and an isolation circuit or output buffer to supply the output signal  60  to the switch  62 . Optionally, the controller  58  may be a microcontroller including an input configured to receive the feedback signal and an output configured to supply the output signal  60  to the switch. The microcontroller may include integrated memory or may access external memory to retrieve a series of stored instructions. The microcontroller is configured to execute the instructions to generate the output signal  60  as a function of the feedback signal  56 . It is contemplated that the controller  58  may include still other combinations of devices without deviating from the scope of the invention. 
       FIG. 2  illustrates another embodiment of an electrical distribution system including a load center  10  having a transfer switch  20  with a neutral switch  21  configured to selectively provide electrical energy to a load from one of two power sources. The load center  10  includes an enclosure  12  having an electrical panel  11  mounted within the enclosure  12 . The transfer switch  20  and the neutral switch  21  are each mounted to the electrical panel  11  and together receive a first set of electrical conductors  14  from a first power source, such as the utility supply  14 , at a first set of terminals  22 ,  23 . The transfer switch  20  and the neutral switch  21  also receive a second set of electrical conductors  16  from a second power source, such as the backup generator  50 , at a second set of terminals  24 ,  25 . The transfer switch  20  includes a switch arm  26  movable between a first position, which connects each of the hot leads, L 1 , L 2  from the utility power source to one of a pair of terminal bars  30 , and a second position, which connects each of the hot leads, L 1 , L 2  from the generator  50  to one of the terminal bars  30 . The neutral switch  21  includes a switch arm  27  movable between a first position, which connects the neutral lead, N, from the utility power source to the neutral switch output  29 , and a second position, which connects the neutral lead, N, from the generator  50  to neutral switch output  29 . With the neutral switch  21 , the neutral lead, N, from the utility power source is first connected to a first terminal  42  of the ground bar  40  and another conductor runs from a second terminal  42  of the ground bar  40  to the first terminal  23  of the neutral switch  21 , establishing a ground connection with the utility neutral lead prior to switching the neutral connections. 
     As illustrated, each of the switch arms  26 ,  27  is manually transferred between the first and second position. A mechanical interlock device  28  is included such that both switch arms  26 ,  27  are moved in tandem. Optionally, the mechanical interlock may be configured to toggle the switch arms  26 ,  27  in a desired sequence. According to still another embodiment of the invention, a sensor may be included to monitor operation of the utility power source, and, if the utility power source fails, a solenoid or other actuator may be energized to automatically transfer each of the switch arms  26 ,  27  between the first and second positions. Alternately, still other configurations of manual and or automatic transfer switches may be employed without deviating from the scope of the invention. 
     The load center  10  distributes the power from one of the two sources to an electrical load. Circuit breakers  32  are mounted to the terminal bars  30  such that a first terminal on the rear of each circuit breaker  32  establishes an electrical connection with the terminal bar  30  to which it is mounted. A second terminal on the side of each circuit breaker  32  is configured to receive a first electrical conductor providing power to the load. The neutral switch output  29  is electrically connected to one of the terminals  38  on a neutral bar  36 . Other terminals  38  of the neutral bar  36  are configured to receive a second electrical conductor providing power to the load. The first and second electrical conductors define a forward conduction path and a return conduction path, establishing a circuit, between the load center  10  and the load. One or more of the circuits may also include a ground conductor. A ground bar  40  includes multiple terminals  42  to which each of the ground conductors may be connected. An earth ground  44 , which may be, for example, a copper rod sunk into the ground next to the building in which the load center  10  is housed, is electrically connected to the ground bar  40 . 
     The generator  50  is connected to the load center  10  to provide a secondary energy source. The generator  50  includes an energy generation device  52  as discussed above with respect to  FIG. 1 . The windings of the energy generation device  52  provide a first hot lead, L 1 , a second hot lead, L 2 , and a neutral lead, N. The generator  50  also includes a ground connection  64 . The generator  50  is normally connected to the earth ground  44  via a ground conductor  90  connected between the ground connection  64  and one of the terminals  42  on the ground bar  40 . A bonding jumper  53 , which may be internal to the generator  50 , connects the neutral lead, N, to the ground conductor  90 . A sensor  54  measures the amplitude of current present on the bonding jumper  53  and generates a feedback signal  56  corresponding to the amplitude of current. The feedback signal  56  is input to a controller  58 , and the controller  58  is configured to generate an output signal  60  as a function of the feedback signal  56 . The output signal  60  is transmitted to a switch  62  connected in series with the bonding jumper  53 , and the output signal  60  controls operation of the switch  62 . According to one embodiment of the invention, the controller  58  includes discrete logic devices. Optionally, the controller  58  may be a microcontroller. It is contemplated that the controller  58  may include still other combinations of devices without deviating from the scope of the invention. 
     In operation, the controller  58  in the generator  50  monitors the feedback signal  56  from the sensor  54  to control operation of the bonding switch  62 . According to one embodiment of the invention, the switch  62  is normally closed, such that the bonding jumper  53  and switch  62  normally establish an electrical connection between the neutral lead, N, and the ground lead, G, in the generator  50 . If the feedback signal  56  indicates the presence of current on the bonding jumper  53 , the controller  58  generates the output signal  60  to open the bonding switch  62  thereby interrupting the conduction path between the neutral lead, N, and the ground lead, G. To avoid spurious trips, the feedback signal  56  may be filtered and/or a minimum threshold may be defined which the current on the bonding jumper  53  must exceed prior to opening the switch  62 . 
     According to one embodiment of the invention, the controller  58  leaves the switch  62  open after detecting current on the bonding jumper  53 . The switch  62  may be configured to be positively retained in either the first or second position. For example, the switch  62  may be a rocker or toggle-style switch with one or more actuators to move the switch  62  between the first and second positions. As the switch moves between positions, it overcomes a mechanical resistance point in the center position and “snaps” over to the other position. In this configuration, the generator  50  detects, for example, when a bonded generator is connected to a load center  10  having a transfer switch  20  configured to switch only the hot leads, L 1 , L 2 . If a bonded neutral generator is connected to a non-neutral switching transfer switch  20 , a conduction path, parallel to the neutral conduction path, is established via the ground lead, G, from the generator and the bonding jumper  46  in the load center  10 . This parallel conduction path results in undesirable current being conducted via the ground lead, G. By opening the switch  62  and leaving the switch  62  in the open state, this undesired conduction path is broken. The controller  58  may further be configured to reset the switch  62  to a closed state if, for example, a neutral switching transfer switch  20  is installed in the load center  10 . 
     According to another embodiment of the invention, the controller  58  may allow the switch  62  to return to the normally closed position. The controller  58  may include, for example, a timer relay which opens for a period of time responsive to the control signal  60  from the controller  58  and automatically closes upon expiration of the timer. If the sensor  54  again detects current on the bonding jumper  53  after closing the switch  62 , the controller  58  generates another control signal  60  to re-open the switch  62 . This may continue at a periodic interval until the condition causing the current to flow on the ground lead, G, is resolved or, optionally, the controller  58  may include a counter which holds the switch  62  in an open state after the switch  62  is opened and closed a predefined number of times. The counter may be manually reset to allow the switch  62  to be closed again. Thus, the controller  58  in the generator  50  may differentiate between an intermittent fault condition and a continuous current being conducted on the ground lead, G, and determine whether to break the electrical connection between the neutral lead, N, and the ground lead, G, according to predetermined parameters. 
     It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention