Abstract:
Presented is a system and method for providing electrical isolation in vehicle power systems. The method comprises placing linear optimized isolation transformers in structures of a vehicle at positions that minimize the propagation of energy spikes into internal electronic systems, for example in the wing root of an aircraft where electrical cables from a generator associated with an engine enter the fuselage. The system includes a linear optimized isolation transformer with a core that has primary side winding isolated from a secondary side winding by an isolation dielectric. The isolation dielectric maintains a high value isolation independent of pressure differences due to operation at different altitudes. In embodiments, linear optimized isolation transformers for each phase of a power distribution system couple power from a generator through a structure of a vehicle thereby increasing electrical isolation of electrical components inside the structure from electrical surges originating outside the structure.

Description:
FIELD 
     Embodiments of the subject matter described herein relate generally to a system and method for providing electrical isolation for vehicle power systems. 
     BACKGROUND 
     Aerospace vehicles such as aircraft are susceptible to lightning strikes and other high intensity radiated fields (HIRF), or collectively voltage spikes or energy spikes. Voltage spikes and induced surges have the potential of interrupting the operation of electrical and control systems within the vehicles. In low-impedance systems, for example in power wiring, induced surges become high-current surges which can trip circuit breakers off-line and disrupt airplane services. In high-impedance systems, for example electronics, induced high-voltage spikes can trip logic, and damage semiconductor avionics. Current generations of aircraft use multiple low-voltage microprocessors, semiconductor devices, and high-frequency data busses, all of which are sensitive to voltage spikes. To mitigate these effects, protection in the form of shielding is used. 
     For example, in present airplanes with metal fuselages, and especially those produced in last 20 years, at least 90% of the protection required is achieved through the use of metallic shields on critical wiring and cable bundles. The demonstrated best-practice for such shielding (see e.g., “Lightning Protection of Aircraft”, Lightning Technologies Inc., Fisher, 2004 (LTI), Ch. 15, FIG. 15.1) is a copper-braid tube wrap on the entire bundle, terminated at each end by a bonded-ring to the connector back-shell, or other grounding methods depending on each individual case (see e.g., LTI, Ch. 15, FIG. 15.23.) While shielding has been proven to work quite well in metal airplanes by reducing the external effects by about 6 dB, it still leaves equipment exposed to 1500V spikes and 3000 Amp current surges (see Standards defined in “Environmental Conditions and Test Procedures for Airborne Equipment”, RTCA-DO-160E, RTCA Incorporated, 2007 (RTCA-DO-160E), Section 22, 23.) Because of these exposures, Line Replaceable Units (LRU&#39;s) typically include levels of internal protection to prevent damage, at extra cost and weight. Skilled workmanship is necessary to design and install copper-braided bundle-shields, and during their lifetime end-terminations are exposed to temperature-stress, current surges, and work-hardening breakages due to cable flexing. Special certification procedures are required for cable-shielding to demonstrate effectiveness to the FAA. Also, life expectancy has to be proven to the FAA, as shields are prone to coming loose and breakages are common. 
     Transformers used for Transformer-Rectifier 28 Vdc Units (TRU&#39;s) do provide some isolation, due in part because the secondary is not connected to the primary, but the isolation is nominal and provides only about −6 dB for the 400 Hz due to the 4:1 turns ratio. This protection is deemed acceptable for metal airplanes under RTCA-DO-160E design rules. Other traditional terrestrial solutions such as metal-oxide varistors (MOV&#39;s), diodes etc, have not been used mainly because they are not fault-tolerant, and a single latent-failure renders them useless for airplane purposes. 
     These solutions serve to mitigate the damage to electronics once a voltage spike is present in the vehicle, but do not prevent the voltage spike from entering the vehicle itself. Many fuselages of aircraft are constructed of metal, which provides some protection to the internal wiring and systems by inhibiting the flow of charge from outside into the enclosed metal fuselage. An enclosed metal structure is sometimes referred to as a “Faraday Cage.” In some vehicles, an additional enclosed metal compartment is created within the fuselage to further house and protect flight essential electronics and electrical systems from voltage spikes. However, a recent trend in modern aircraft is to use composite and other non-metal materials, in lieu of metal, in the construction of the vehicle. While these composite materials offer significant reductions in weight, and permit the use of advanced molding methods to achieve perfect aerodynamic forms not previously possible with metal-forming, they also significantly increase risk of damage from electromagnetic fields such as airport radars, high-power radio and TV transmitters Composite materials reduce the beneficial “Faraday Cage” effect of the fuselage, increasing the importance of using other means to prevent voltage spikes from harming the internal systems. 
     In terrestrial applications, electrical isolation is achieved through transorbs, spark gaps, gas tubes, and transformer isolation. For example, transformers having large volumes of dielectric liquid, or large air gaps, can be used as isolation transformers because there are generally no significant space or weight restrictions. Further, transorbs or components that deteriorate over a number of uses can be easily replaced in terrestrial environments. However, in an aerospace vehicle, there are significant space and weight considerations, and components whose performance deteriorates after every use must be periodically inspected and/or replaced, increasing maintenance time and costs. 
     SUMMARY 
     Presented is a system and method that mitigates voltages spikes and other high voltage radiated fields or HIRF. The aircraft power system protection uses optimized isolation transformer modules in the aircraft power feeder circuits to provide isolation between the generators coupled to external wiring and the electronics systems inside the fuselage of the vehicle. In an embodiment, the optimized isolation transformer modules reduce voltage spikes in the electrical system from lightning and HIRF by approximately 30 db, or reducing the induced effects by approximately 1/1000 Volts and 1/10,000 Joules of the original Voltage or energy. This reduction in the coupling of energy to system inside the vehicle reduces the need to require special treatment in every electronic unit to handle voltage spikes. 
     The method comprises inserting a linear optimized isolation transformer between a generator and a portion of a power distribution system; directing each phase of the power distribution system into a separate linear optimized isolation transformer; and, positioning the linear optimized isolation transformers relative to structures of the vehicle to increase the electrical isolation of electrical components within the structures. In embodiments, the structures are the fuselage, the wing root where electrical cables from the generator enter a fuselage, the aft bulkhead where the auxiliary power unit (APU) is located, the electronics bay, or Faraday Cage structures in the vehicle. In embodiments, the linear isolation transformers are positioned so that the primary and secondary sides are on opposite sides of the structure. 
     The system comprises a linear optimized isolation transformer having a magnetic core with a primary side winding that is isolated from a secondary side winding by an isolation dielectric that maintains a high value isolation independent of pressure differences due to operation at different altitudes. In embodiments, linear optimized isolation transformers associated with each phase of a power distribution system electrically couple power from a generator through a structure of a vehicle to increase electrical isolation of electrical components inside the structure from electrical surges originating outside the structure. 
     The features, functions, and advantages discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures depict various embodiments of the system and method for providing isolation for vehicle power systems. A brief description of each figure is provided below. Elements with the same reference number in each figure indicated identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number indicate the drawing in which the reference number first appears. 
         FIG. 1  is a diagram of a conventional isolation transformer; 
         FIG. 2  is a diagram of an optimal isolation transformer in one embodiment of the system and method for providing isolation for vehicle power systems; 
         FIG. 3  is a diagram of a linear optimized isolation transformer in one embodiment of the system and method for providing isolation for vehicle power systems; 
         FIG. 4  is a diagram of placement of linear optimized isolation transformers in an aerospace vehicle in one embodiment of the system and method for providing isolation for vehicle power systems; 
         FIG. 5  is a diagram of placement of linear optimized isolation transformers through a structure of a vehicle in one embodiment of the system and method for providing isolation for vehicle power systems; and 
         FIG. 6  is a flowchart of a process of placing linear optimized isolation transformers in a vehicle in one embodiment of the system and method for providing isolation for vehicle power systems. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the invention or the application and uses of such embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     There is a need to provide electrical isolation between the power generators in an aerospace vehicle and the internal electronics systems inside the vehicle that use the power from the power generators. Lightning strikes or high intensity radiated fields (HIRF) can create or induce voltage spikes that travel through the power lines leading from the power generators to the internal electronics systems inside the vehicle. The system and method of the present disclosure present a linear optimized isolation transformer for providing isolation for vehicle power systems. 
     Referring now to  FIG. 1 , an electrical diagram of a conventional isolation transformer  100  is presented. Although the conventional isolation transformer  100  is shown for a single phase system, multiple conventional isolation transformers  100  can be used to provide isolation for three phase power systems as would be understood in the art. The conventional isolation transformer  100  has a primary side  102  and a secondary side  104 . In the conventional isolation transformer  100 , the wires of the primary side  102  are wound over the core  106  of the conventional isolation transformer  100 , and the wires of the secondary side  104  are wound over the top of the wires of the primary side  102 . The wires are electrically insulated from each other, and the wires of the primary side  102  and secondary side  104  are electrically isolated from each other by a non-conductive electrostatic shield. Energy transfer from the primary side  102  to the secondary side  104  is effected only by magnetic coupling between the primary side  102  and secondary side  104 . By using equal numbers of windings in the primary side  102  and secondary side  104 , the conventional isolation transformer  100  provides the same voltage on the secondary side  104  as the voltage presented to the primary  102 . The conventional isolation transformer  100  is therefore said to be a 1:1 transformer. By including a center tap  108 , a reduced amount of voltage can be obtained on the secondary side  110 . For high power applications, the conventional isolation transformer  100  is sometimes placed in a dielectric container filled with a dielectric oil, and the terminals of the primary side  102  and secondary side  104  are physically distanced from one another to prevent arcing between the terminals. 
     Although the conventional isolation transformer  100  provides good electrostatic isolation between the primary side  102  and the secondary side  104 , there is little electromagnetic protection. Because the windings are directly on top of one another, surges on the primary side  102  can be electromagnetically coupled to the secondary side  104 . The core  106  acts as a reactive choke to some degree, but the proximity of the wires of the primary side  102  and secondary side  104  enable substantial energy to couple between the wires. 
     Isolation transformers, are seldom used in aircraft because the 115 Vac 400 Hz systems do not have transformers, and the extra weight of two isolation transformers does not trade-off well against bundle-shields, on the basis of protection from surges. However, one aspect of this disclosure is the design and placement of isolation transformers that prevent surges from occurring, rather than protection from surges that have already entered the vehicle. 
     Referring now to  FIG. 2 , an optimal isolation transformer  200  that provides both electrostatic and electromagnetic isolation is presented. The optimal isolation transformer  200  has a primary side  102  and a secondary side  104 . In the optimal isolation transformer  200 , the wires of the primary side  102  are wound over one part of the core  106  of the optimal isolation transformer  200 , and the wires of the secondary side  104  are wound over a different part of the core  106  of the optimal isolation transformer  200 . The primary side  102  and secondary side  104  are separated by an air gap  202 . The air gap  202  prevents the primary side  102  and secondary side  104  from directly coupling energy, and instead forces all electromagnetic coupling to be performed though the core  106 . The core  106  acts as a reactive electromagnetic choke, preventing large amounts of energy at high slew rates, such as those energies induced by a lightning strike, from being coupled from the primary side  102  to the secondary side  104 . 
     However, although the use of an air gap  202  is satisfactory for terrestrial applications, it is not acceptable for use in an aerospace vehicle where operation of the optimal isolation transformer  200  would also occur at high altitudes. This is because voltage breakdown flashover between terminals changes with altitude, in accordance with the Paschen curve. 
     Referring now to  FIG. 3 , the solution is to use a permanent high-Q material isolation dielectric  306  between the primary side  102  and the secondary side  104  of a linear optimized isolation transformer  300 . The isolation dielectric  306  provides similar electromagnetic isolation as the air gap  202  of the optimal isolation transformer  200  of  FIG. 2 , but with two additional advantages. First, because the isolation dielectric  306  is not a gas, the isolation dielectric is not affected by changes in altitude as is the air gap  202  of the optimal isolation transformer  200 . This feature allows the linear optimized isolation transformer  300  to be used in a wide range of aerospace applications. Second, because the isolation dielectric  306  can be a higher Q than air, the isolation dielectric permits the primary side  102  and secondary side  104  of the linear optimized isolation transformer  300  to be in closer proximity compared to the primary side  102  and the secondary side  104  of an optimal isolation transformer  200  that employs an air gap  202 . This reduces the necessary size or length of the linear optimized transformer  300  compared to the optimal isolation transformer  200 . Further, unlike the air gap  202 , the isolation dielectric  306  can be configured to extend beyond the core  106 , providing further suppression of potential arcing. 
     In an embodiment of the linear optimized transformer  300 , wires of a primary side  102  are wound around one portion of a center core member  310  of a squared-off figure-eight shaped core  308 . In an embodiment the core is an iron core. Wires of a secondary side  104  are wound around a second portion of a center core structure of the figure-eight shaped core  308 . The figure-eight shaped core  308  comprises a set of laminated layers configured to reduce eddy currents and associated losses due to eddy currents in the figure-eight shaped core  308 . The figure-eight shaped core  308  extends from the primary side  102  to the secondary side  104 . Between the primary side  102  and secondary side  104 , an isolation dielectric  306  separates the primary side  102  from the secondary side  104 . 
     The isolation dielectric  306  is comprised of a set of laminated members having a shape that fills all of the space between the primary side  102  and the secondary side that is not occupied by the figure-eight shaped core  308 . In an embodiment, the isolation dielectric  306  is an H-shape having two crossbar members as illustrated in  FIG. 3 . In an embodiment, the isolation dielectric  306  comprises layer members that interlock to facilitate assembly of the isolation dielectric  306  onto an existing figure-eight shaped core  308 . In an embodiment, the isolation dielectric  306  extends beyond the figure-eight shaped core  308  on at least one side, for example by having an additional top crossbar. In an embodiment, the isolation dielectric  306  extends beyond the figure-eight shaped core  308  on all sides. 
     In an embodiment, the primary side terminals  302  and secondary side terminals  304  are provided on opposite sides of the linear optimized transformer  300 . This separation of the primary side terminals  304  and secondary side terminals  306  provides superior electrostatic isolation. 
     In an embodiment, the linear optimized transformer  300  is a 1:1 isolation transformer. In embodiments the linear optimized transformer  300  is a 1:x or x:1 isolation transformer, where x is a real number greater than 1. For example, if the generator provides 230V power, and the system to be powered requires 115V power, then the linear optimized transformer  300  can be adapted to be a 2:1 transformer. In an embodiment, the linear optimized transformer  300  has one or more taps for 1:x or x:1 power coupling. For example, if two 115V power systems on the secondary side are to be powered using a single 230V power source fed to the primary side, then a center tap in the linear optimized transformer  300  can provide power to each 115V power system, each of which has a 2:1 power coupling ratio. In an embodiment, the linear optimized transformer  300  provides a 1:x step down voltage appropriate for providing power for 28 Vdc avionic systems. In embodiments, the linear optimized transformer  300  further comprises one or more transorbs, gas-discharge tubes, or other semiconductor or equivalent electronics to perform, for example, further R.F. choke or surge protection functionality. 
     Many aerospace vehicles use generators that are part of, or integrated into, the engines or jet turbines of an aircraft  400 . Power from the engines or jet turbines is typically generated as three-phase power. In an embodiment, three linear optimized transformers  300  are used to provide power isolation for each phase of a three-phase power generator. 
     Referring now to  FIG. 4 , an aircraft  400  comprises one or more linear optimized transformers  300 . Each of the linear optimized transformers  300  is used to isolate power from a generator coupled to a source such as a jet turbine engine  408  or auxiliary power unit or APU  404 . In one embodiment, one or more linear optimized transformers  300  is positioned within the wing root  402  where long electrical cables  412  come from the generator associated with the engine  408  into the fuselage  410 . In an embodiment, the primary side terminals  302  reside outside the fuselage  410  in the wing root  402 , whereas the secondary side terminals  306  reside inside the fuselage  410 . In this embodiment, the linear optimized transformers  300  help to ensure that charge does not enter the “Faraday Cage” environment of the fuselage  410  through the electrical cables in the wing root  402 . In another embodiment, linear optimized transformers  300  are placed near the aft pressure bulkhead near the APU  404  to isolate the long electrical cables  412  leading from the APU  404  to the avionics bay  406  in the front of the aircraft  400 . Electric cables  412  leading from the APU  404  to the avionics bay  406  are typically the longest cables and can be 200 ft or more. Collectively the electric cables  412  and power systems inside the avionics bay  406  comprise a power distribution system. Generally, the longer the aircraft  400  and the longer the electric cables  412 , the worse the induction effects become from lightning strikes and other HIRF. 
     Referring now to  FIG. 5 , a diagram of three linear optimized transformers  300  are illustrated passing through a structure  502 , for example a structure  502  associated with an aircraft fuselage  410  or wing root  402 . Each phase,  504 ,  506 , and  508  of the electrical cable attaches to a different linear optimized transformer  300 . The neutral wire  510  from each of the electrical cable  412  connects to the neutral terminals of each of the three linear optimized transformers  300 . The linear optimized transformers  300  help to ensure that charge does not pass through the structure  502 . 
     In an embodiment, linear optimized transformers  300  are used to isolate the components and systems inside the avionics bay  406  from the electric cables  412  delivering power from the generator associated with the engine  408  or APU  404 . In some aircraft  400 , the avionics bay  406  is isolated from the rest of the fuselage  410  by a cage that functions as a Faraday Cage to protect the components and systems inside of the avionics bay  406 . The cage serves to protect critical avionics flight control systems and navigation equipment from induced power surges. Passenger entertainment systems and other systems may similarly reside in the cage or in their own cage. In an embodiment, one or more linear optimized transformers  300  are positioned in proximity to the avionics bay  406  to provide power isolation. In a non-limiting example, the primary side terminals  302  reside outside the avionics bay, while the secondary side terminals  306  reside inside the avionics bay  406 . 
     Referring now to  FIG. 6 , a simplified process  600  of implementing a linear optimized transformers  300  in a vehicle such as an aircraft  400  is presented. In a first step, a linear optimized transformer  300  is inserted  602  between the outputs of the generator and the power distribution system. For example, the linear optimized transformer  300  is placed inline with one or more of the electrical cables  412 . In embodiments, the generator is on the engine  408  or APU  404 . Because most vehicle generators provide 3-phase power, in a second step, each phase of the power distribution system is directed  604  into separate linear optimized transformers  300 . In a third step, the linear optimized transformers  300  are positioned  606  relative to a structure of the vehicle in order to electrically isolate that structure. In embodiments, the linear optimized transformers  300  are positioned  606  in the wing root  402  in proximity to the avionics bay  406  and in proximity to the APU  404 , or placed between electrical cables  412  included in the power distribution system. In embodiments, the linear optimized transformers  300  are co-located, packaged together, or individually positioned independently from one another depending on available space in the vehicle or isolation design parameters. For example, in one embodiment the linear optimized transformers  300  can be separated from one another to prevent a localized lightning strike from affecting all of the linear optimized transformers  300 . In another embodiment, the linear optimized transformers  300  are positioned together so that a lightning strike will affect all of the linear optimized transformers  300  in approximately the same temporal frame, and thus any small amount of voltage surge that passes through the linear optimized transformers  300  will be common mode. In embodiments, in a fourth step, the linear optimized transformers  300  are equipped  608  with a device that provides a return path to divert energy spikes away from the power distribution system. For example, one or more transorbs, gas-discharge tubes, or other semiconductor or equivalent electronics will perform additional RF choke or surge protection functionality. 
     The embodiments of the invention shown in the drawings and described above are exemplary of numerous embodiments that may be made within the scope of the appended claims. It is contemplated that numerous other configurations of the system and method for providing electrical isolation for vehicle power systems may be created taking advantage of the disclosed approach. It is the applicant&#39;s intention that the scope of the patent issuing herefrom will be limited only by the scope of the appended claims.