BACKGROUND OF THE INVENTION 
       [0001]    The current invention generally relates to a generator control unit (GCU), and more particularly, to a generator control unit (GCU) that is capable of tolerating a ground fault in the generator, be that in the permanent magnet generator (PMG) wiring or field wiring This generator control unit (GCU) can tolerate a ground fault that commonly results from the incidental contact of the permanent magnet generator (PMG) wire or a field coil wire with the chassis of the generator. 
         [0002]    It is generally known that power generators require a generator control unit (GCU) to control their output. In order to control the generator output, the field current is adjusted to compensate for the load and speed variations. The field current is commonly derived from a permanent magnet generator (PMG), which is incorporated into the generator itself. 
         [0003]    A common problem that occurs in these generator control unit (GCU) is a ground shortage that occurs when the electrical wires touch the chassis as a result of insulation breakdown. This ground shortage is undesirable as such shortage causes the system to stop operation completely at the same time potentially damaging expensive electrical components within the generator control unit (GCU). 
         [0004]    Typically, the ground shortage issues can occur in permanent magnet generator (PMG) wire or in a field coil wire. Although deriving from two separate components, the shortage caused when either of these wires touches the chassis could be significantly detrimental to the continuous operation of an aircraft power generation system by damaging its generator and its generator control unit (GCU). 
         [0005]      FIG. 1  shows a prior art embodiment wherein the electrical components are connected directly to the ground referenced control domain, hence making it difficult to tolerate an electrical shortage to the chassis. 
         [0006]      FIG. 1  shows a direct connection from inverted buck regulator  106  to ground referenced control domain  112 . During a ground fault when a metal wire touches the chassis at a second location other than the intended ground connection  111 , a short circuit will occur, creating an undesirable over-current. The sudden spike in current could be detrimental to the operation of the generator control unit (GCU), often causing the entire system to shut down. 
         [0007]    As it can be seen from  FIG. 1 , the traditional prior art generator control unit (GCU) is not set up to withstand the ground fault caused when a permanent magnet generator (PMG)  100  wire or a field coil  108  wire come in touch with the chassis. 
         [0008]    In order to address the ground shortage issues deriving from the permanent magnet generator (PMG) wire shorting with the chassis, the current art sometimes attaches a heavy, bulky, and burdensome component called a permanent magnet generator transformer (PMG transformer) to isolate the input. These permanent magnet generator transformers (PMG transformers) have to be rated for the full power of the permanent magnet generators (PMG) that they are attached to making them expensive in addition to being heavy, bulky, and burdensome. 
         [0009]    In order to address the ground shortage issues deriving from the field coil wire shorting with the chassis, the current art lacks a suitable solution other than to add an additional isolator that can withstand the sudden spike in voltage. Consequently, without the additional isolator, the entire controlled frequency generator (CFG) will stop and shut down when the field coil wire touches the chassis. 
         [0010]    Hence, it can be seen that there is a need for an innovative system and method for providing continuous operation of a generator control unit (GCU) in the event of a ground short to the chassis from either a permanent magnet generator (PMG) wire or from a field coil wire without the need of heavy and burdensome isolators that are unnecessarily expensive. Moreover, there is also a need for an innovative system and method to be able to detect such a ground short to the chassis despite the continuous operation for future maintenance. 
       SUMMARY OF THE INVENTION 
       [0011]    In one aspect of the present invention, a generator control unit comprises a ground referenced control domain connected to a ground chassis, an isolated power pass domain controlled by the ground referenced control domain, a permanent magnet generator connected to the isolated power pass domain to provide a power, and a generator field coil connected the isolated power pass domain; wherein the connection between the isolated power pass domain and the ground referenced control domain is floating with respect to the ground referenced control domain. 
         [0012]    In another aspect of the invention a method of controlling a ground fault within a generator control unit comprises connecting a ground referenced domain to a ground chassis, connecting an isolated power pass domain to the ground referenced domain, connecting a permanent magnet generator to the isolated power pass domain, connecting a generator field coil to the isolated power pass domain, and floating the isolated power pass domain with respect to the ground referenced control domain. 
         [0013]    In a further aspect of the invention, a generator control unit capable of tolerating a ground fault comprises a ground referenced control domain connected to a ground chassis, a high ohmic resistor connected to the ground referenced control domain, and an isolated power pass domain connected to the high ohmic resistor; wherein the isolated power pass domain is floating with respect to the ground referenced control domain. 
         [0014]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a functional block diagram of the prior art generator control unit (GCU); 
           [0016]      FIG. 2  is a functional block diagram of one embodiment of the current invention with an isolated generator control unit (GCU); and 
           [0017]      FIG. 3  shows a flow chart of the current invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0019]    Various inventive features are described below that can each be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above or may only address one of the problems discussed above. Further, one or more of the problems discussed above may not be fully addressed by any of the features described below. The current invention of an isolated generator control unit (GCU) is suitable to be placed in an aircraft context wherein the variable speed of the electrical power generator is dependent on the operational speed of the main aircraft engine. However, many numerous other contexts where a generator control unit is used can be contained as embodiments of the current invention without departing from the scope of the present invention. 
         [0020]    The present invention generally provides an isolated power pass domain within a generator control unit (GCU) that is connected in a completely floating manner with respect to the ground referenced control domain. The completely floating connection allows the isolated power pass domain to continue operation in the event of an electrical shortage with the chassis. This is unlike the prior art generator control unit (GCU) where the electrical components are connected directly to the ground referenced control domain, causing the entire system to shut down in the event of an electrical shortage with the chassis. 
         [0021]      FIG. 2  shows a functional schematic block diagram of the current exemplary generator control unit (GCU) for an aircraft. It should be understood that the current invention, although shown in the exemplary embodiment of an aircraft generator control unit, could also be applicable to aerospace systems, industrial environments, electromechanical vehicles, or any other electrical system without departing from the scope of the present invention. 
         [0022]      FIG. 2  of the current exemplary embodiment may contain a permanent magnet generator (PMG)  200  as well as a generator field coil  220  connected to an isolated power pass domain  202  that may floating with respect to the ground referenced control domain  204 . Within the isolated power pass domain  202  may be a permanent magnet generator rectifier (PMG Rectifier)  206 , a buck converter unit  205 , a pair of opto-isolators  224  and  226 , as well as a high ohmic resistor  208 . Buck converter unit  205  may further contain additional components such as the DC/DC converter  208 , a buck regulator  210 , a fly-wheeling diode  212 , a buck inductor  214 , a gate driver  216 , an over-voltage protection switch  218 , and a transorb  222 . Isolated power pass domain  202  may also contain two opto-isolators  224  and  226 , as well as a high ohmic resistor  228 . Finally, the ground referenced control domain  204  may contain two essential elements named ground fault detection  230  and a ground connection  232 . 
         [0023]    First and foremost, it is worth noting that in this current exemplary embodiment the isolated power pass domain  202  may be completely floating by not being directly connection to the ground referenced control domain  204 . The ground connection  232  may be located only in the ground referenced control domain  204 , wherein the high ohmic resistor  228  serves as the only ground connection with the isolated power pass domain  208 . The high ohmic resistor  208  could provide sufficient resistance to the voltage spike in the event of an electrical short to allow the generator control unit (GCU) to continue to operate. 
         [0024]    Permanent magnet generator (PMG)  200 , in this current exemplary embodiment could be the source of the control and excitation power within the generator. This permanent magnet generator (PMG)  200  may be used to provide the excitation voltage to regulate the point of regulation voltage by adjusting the field current to compensate for load and speed variations. The permanent magnet generator (PMG)  200  in this exemplary embodiment may also contain electrical wires internally that may be protected from the chassis by a layer of insulation. When the insulation fail, the electrical wires within the permanent magnet generator (PMG)  200  may touch the chassis, which in turn creates a ground fault. Although a permanent magnet generator (PMG)  200  may be used in this current exemplary embodiment, other variations of power generators that are capable of producing electrical energy can also be used without departing from the scope of the present invention. 
         [0025]    The isolation of a voltage spike may be achieved by keeping the isolated power pass domain  202  completely floating with respect to the ground referenced control domain  204 . This floating arrangement may allow the isolated power pass domain  202  to continue operation even in the event of a ground fault without the need for a heavy, bulky and burdensome permanent magnet generator transformer (PMG Transformer). 
         [0026]    Isolated power pass domain  202  in this current exemplary embodiment may be connected between the permanent magnet generator (PMG)  200  and the generator field coil  220  to function as essentially a step down or buck DC/DC converter for the generator control unit (GCU). The isolated power pass domain  202  may operate entirely in a floating state with respect to the ground connection  232  within the ground referenced control domain  204  to provide the isolation. 
         [0027]    Ground referenced control domain  204  in this current exemplary embodiment may be connected to isolated power pass domain  202  via the high ohmic resistor  228 . Ground referenced control domain  204  may also derive its power from the DC/DC converter  208  as the power may be generated from the permanent magnet generator (PMG)  200  to power the main power controller section  240 . 
         [0028]    The buck converter unit  205  may perform the function of a step down or buck DC/DC converter in reducing the voltage of the permanent magnet generator (PMG)  200  under the control of the ground referenced control domain  204 , which in turn may be connected to the chassis and provides controlling capabilities of the generator. 
         [0029]    Permanent magnet generator rectifier (PMG Rectifier)  206  in this current exemplary embodiment may be connected to the permanent magnet generator  200  and the permanent magnet rectifier (PMG Rectifier)  206  may be used to convert alternating current (AC) to a direct current (DC) containing an average value proportional to the input speed. It is worth noting that the permanent magnet generator rectifier (PMG Rectifier)  206  may create noises and ripples that are byproducts of the AC to DC conversion. In order to address the noise and ripple issues, the current invention may utilize a control loop and a buck converter unit  205  to counter the effect of the ripple in the rectified voltage, eliminating the need of storage cap. The removal of the storage cap in turn would allow the PMG rectifier currents to become continuous. i.e. will reduce the emissions from the PMG wires. The common mode emissions would further be reduced by the floating nature of the isolated power pass domain  202 . 
         [0030]    DC/DC converter  208  in this current exemplary embodiment may be connected to the permanent magnet generator rectifier  206  and may utilize a fraction of the electrical energy generated by the permanent magnet generator (PMG)  200  to power the ground referenced control domain  204 . 
         [0031]    Buck regulator  210  in this current exemplary embodiment may be used to compare the actual field current to an internal fixed reference voltage, offset by a current command, coming through the opto isolator  224  from the ground referenced control domain  204 . The filed current command is generated in the ground referenced domain, based on comparison of Point of Regulation (POR) voltage feedback with internal reference. The buck regulator  210  may have a feedback voltage and an enabling voltage input coming from the ground referenced control domain  204  via opto-isolators  224  and  226  respectively. The feedback input may be used for duty cycle control of the buck regulator  210  as to enforce a field current, that is proportional to the current command from the isolated power pass domain  202 . The enable input may be used to enable/disable regulation and to enforce over-voltage protection by shutting the buck regulator  210  and the over voltage protection switch  218 . 
         [0032]    Fly-wheeling diode  212  in this current exemplary embodiment may be connected to the input of buck regulator  210 , and may be used to direct magnetic energy, stored in the buck inductor  214  during on time and to the field coil during off time The fly-wheeling diode  212  may allow for the field current to be continuous despite the On/Off operation of the buck regulator  210 . 
         [0033]    Buck inductor  214  in this current exemplary embodiment may be part of buck converter unit  205 , and may serve the purpose storing energy to be passed into the buck converter unit  205  depending on the buck regulator  210 . Buck inductor  214  may be used to allow the amount of power to and from the permanent magnet generator to be controlled. 
         [0034]    Gate driver  216  in this current exemplary embodiment may be connected to the over voltage protection switch  218 . Gate driver  216  may be connected to the output of the buck regulator  210  and the enabling opto-isolator  226  to control the electrical energy into the over voltage protection switch  218 . 
         [0035]    Over voltage protection switch  218  may be connected to the output of gate driver  216  and may serve the purpose of breaking the current path through the fly-wheeling diode  212  upon over-voltage detection. Opening of the flywheel path may force the generator field coil  220  to dissipate energy at a higher voltage, limited by the transorb  222 , causing the generator field coil  220  current to decay at a faster rate, limiting the over-shoot at the POR voltage during load off transients. 
         [0036]    Generator field coil  220 , in this current exemplary embodiment may be a magnetic field component containing field wires being wound together at either the rotor or the stator. Generator field coil  220  may be attached to buck converter unit  205  to allow the continuous operation of the generator control unit (GCU). The field wires within generator field coil  220  may be another source of a ground shortage when the wires of the filed coil  108  come in contact with the chassis. 
         [0037]    Transorb  222  in the current exemplary embodiment refers to a transient voltage suppression diode and may be connected in parallel to generator field coil  220  to protect the sensitive electric components within the generator control unit (GCU) from damages in the event of a over-voltage protection intervention by the switch  218 . 
         [0038]    Opto isolator  224  in this current exemplary embodiment may connect the feedback of the buck regulator  210  to the ground referenced control domain  204  to provide a current command to the isolated power pass domain  202 . Opto isolator  224  may utilize a short optical transmission path to transfer a signal while keeping the isolated power pass domain  202  isolated from the ground referenced control domain  204  to prevent damage occurring from an electrical short. 
         [0039]    Opto isolator  226  may function in the same way as opto isolator  224  in keeping the isolated power pass domain  202  from the ground referenced control domain  204 , but is directed towards providing the enabling command to the buck regulator  210 . 
         [0040]    High ohmic resistor  228  in this current exemplary embodiment is an essential component of the current invention in creating the only connection between isolated power pass domain  202  and ground referenced control domain  204 . Because only the ground referenced control domain  204  contains a ground connection  232 , by utilizing the high ohmic resistor  228  as the only connection to the isolated power pass domain  202 , the high ohmic resistor  228  keeps the entire power pass domain  202  floating with respect to the ground referenced control domain  204 . This floating configuration may allow the generator control unit (GCU) to continue operating even if there is an electrical short that stems from the permanent magnet generator wire or the field coil wire touching the chassis. When such an electrical short occurs, a ground loop is formed through the high ohmic resistor  228  allowing its current into the ground connection  232  to be detected as an indication of the ground fault. The high ohmic resistor  228  also limits the current in the ground loop as to allow continuous operation of generator control unit (GCU) despite the ground fault in the generator. 
         [0041]    Under normal operating conditions, the high ohmic resistor may carry no current because the isolated power pass domain  202  could be completely floating and isolated with respect to the ground referenced control domain  204 . However, because the isolated power pass domain  202  could be completely floating, static charging occurs as a result of a fully floating architecture, and the high ohmic resistor  228  may serve the additional purpose of absorbing the static charging during normal operating conditions without departing from the scope of the present invention. 
         [0042]    Ground fault detection  230  in this current exemplary embodiment may be located in the ground referenced control domain  204  and serves as the only connection to the high ohmic resistor  228  and the isolated power pass domain  202 . Ground fault detection  230  may serve the purpose of detecting a current that flows through the high ohmic resistor  228  as a result of an electrical short. This allows the detection of the occurrence of an electrical short despite continuous operation of the generator control unit (GCU). 
         [0043]    Finally, ground connection  232  here in the current exemplary embodiment refers to the general ground voltage of zero. This ground connection  232  may refer to an intentional grounding point within the ground referenced control domain  204  as indicated in  FIG. 2 , but could refer to any location in the chassis where a ground could occur without departing from the scope of the present invention. The ground connection  232  in this current exemplary embodiment may connected to the ground fault detection  230  to allow detection of the electrical short coming from the isolated power pass domain  202 . 
         [0044]      FIG. 3  shows a flow chart according to an embodiment of the present invention. 
         [0045]    The generator control unit (GCU) of the current exemplary embodiment functions by connecting a ground referenced control domain to ground chassis at step  302 . Subsequent to step  302 , an isolated power pass domain may be connected to the ground referenced control domain at step  304 . It should be noted that the connection between isolated power pass domain and ground referenced control domain at step  304  may be in a floating arrangement in accordance with the exemplary embodiment of the present invention. This floating arrangement allows the generator control unit (GCU) to continue operating in the event of a ground fault. 
         [0046]    At step  304 , a high ohmic resistor may be connected between the isolated power pass domain and the ground reference control domain to limit the current into the ground loop, formed between the ground fault point and the intentional ground connection  232  during ground fault condition. 
         [0047]    At step  308 , the generator control unit may determine whether a ground fault has occurred. If no ground fault has occurred, the generator control unit (GCU) may go through steps  310  and  312 . At step  310 , a voltage may not pass through high ohmic resistor when there is no ground fault, and at step  312 , the high ohmic resistor also may serve the purpose of absorbing static charge. 
         [0048]    If there is a ground fault at step  308 , then the generator control unit (GCU) may pass through steps  314  and  316 . At step  314 , the high ohmic resistor absorbs a voltage spike that may generally associated with a ground fault, allowing the continuous operation of generator control unit (GCU). Additionally, during the existence of a ground fault, at step  316 , the generator control unit (GCU) may also be capable of detecting a ground fault for subsequent maintenance purposes. 
         [0049]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.
An apparatus and method for allowing continuous operation of a generator control unit (GCU) despite a ground fault short circuit is disclosed. More specifically, a generator control unit (GCU) that maintains an isolated power pass domain in a completely floating set up with respect to the ground referenced control domain is used to allow continuous operation of a generator control unit (GCU) even when an internal wiring such as a permanent magnet generator wire or a generator field coil wire comes in contact with the chassis due to failure of the insulation. In addition, the apparatus and method also utilizes a high ohmic resistor as the only connection between the isolated power pass domain and the ground referenced control domain to control the voltage spike that occurs during a ground fault short circuit. Finally, the apparatus and method is also capable of detecting the occurrence of a ground fault short circuit by utilizing a ground fault detection to detect a disbalance current passing through a high ohmic resistor.