Abstract:
An electrical system includes an alternating current (AC) source; a transformer rectifier unit (TRU) connected to the AC source, the TRU configured to receive AC power from the AC source, convert the AC power to direct current (DC) power, and output DC power; a DC bus configured to receive the DC power output by the TRU; at least one DC load powered by the DC bus; and a shunt regulator connected to the output of the TRU, the shunt regulator configured to overload the output of the TRU in the event of an overvoltage condition at the output of the TRU, such that a voltage available to the DC bus during the overvoltage condition does not exceed a reference voltage.

Description:
FIELD OF INVENTION 
     The subject matter disclosed herein relates generally to the field of overvoltage protection in an electrical generating system. 
     DESCRIPTION OF RELATED ART 
     Overvoltage conditions in an electrical generating system may cause damage to electrical loads, including alternating current (AC) or direct current (DC) loads, powered by the electrical generating system. For example, an aircraft electric power system may have various loads, including loads that include circuitry that is critical for flight operations, powered by a generator in the electrical generating system. Overvoltage conditions from the generator may result in a catastrophic event in the aircraft due to exposure of a critical load to a voltage outside the load&#39;s rated limits, which may cause a critical load to fail. 
     One way of preventing overvoltage in an electrical generating system is to provide one or more overvoltage protection modules in a generator control unit (GCU) that is connected to the generator. A constant frequency (CF) generator may magnetically saturate at a voltage below the maximum ceiling voltage tolerated by the overvoltage protection module circuitry. This gives time for the overvoltage protection module(s) to de-excite and disconnect the generator from the loads. However, if the generator is a variable frequency (VF) generator a maximum ceiling voltage may be produced that is far in excess of the maximum allowable voltage for the critical loads. Also, the voltage rise for a VF generator due to a failed on excitation may be too fast to protect with the traditional overvoltage protection circuitry; the overvoltage may be on the bus for a period of time of 10 s or 100 s of milliseconds (ms) before the overvoltage is detected by an overvoltage protection module and the generator is taken off line, allowing the overvoltage to be experienced by loads powered by the generator, which may including flight critical equipment in an aircraft electrical generating system that may act in an adverse way or suffer damage when exposed to the overvoltage. 
     BRIEF SUMMARY 
     According to one aspect of the invention, an electrical system includes an alternating current (AC) source; a transformer rectifier unit (TRU) connected to the AC source, the TRU configured to receive AC power from the AC source, convert the AC power to direct current (DC) power, and output DC power; a DC bus configured to receive the DC power output by the TRU; at least one DC load powered by the DC bus; and a shunt regulator connected to the output of the TRU, the shunt regulator configured to overload the output of the TRU in the event of an overvoltage condition at the output of the TRU, such that a voltage available to the DC bus during the overvoltage condition does not exceed a reference voltage. 
     According to another aspect of the invention, a method for limiting an overvoltage condition at an output of a transformer rectifier unit (TRU) of an electrical system includes activating a shunt regulator in response to the overvoltage condition, the shunt regulator being connected to the output of the TRU; overloading the output of the TRU by the shunt regulator such that a voltage available to a DC bus is less than a reference voltage; limiting an alternating current (AC) voltage received from an AC source at an input of the TRU in response to the overvoltage condition by an overvoltage protection module; and deactivating the shunt regulator. 
     According to another aspect of the invention, a shunt regulator for an electrical system, the electrical system comprising a transformer rectifier unit (TRU), includes an operational amplifier, the operational amplifier configured to receive a reference voltage and an output voltage of the TRU as inputs; and a transistor, the transistor configured to receive an output of the operational amplifier as a gate voltage, the output voltage of the TRU as a source voltage, and wherein a drain of the transistor is connected to ground. 
     Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  illustrates an embodiment of an electrical generating system having a shunt regulator for overvoltage protection at a transformer rectifier unit. 
         FIG. 2  illustrates an embodiment of a shunt regulator for overvoltage protection. 
         FIG. 3  illustrates an embodiment of a method of operating an electrical generating system having a shunt regulator for overvoltage protection at a transformer rectifier unit. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an electrical generating system having a shunt regulator for overvoltage protection at a transformer rectifier unit (TRU), and a method of operating an electrical generating system having a shunt regulator for overvoltage protection at a TRU, are provided, with exemplary embodiments being discussed below in detail. An overvoltage in an electrical generating system can be dealt with downstream of a TRU, as the output of the TRU may be relatively low power as compared to the power on an AC bus of the electrical generating system. Current output of the TRU may be limited due to impedance and saturation, and the output voltage of the TRU may also be relatively low. The shunt regulator may act to limit the TRU output voltage to less than a maximum ceiling voltage allowed for any loads powered by the electrical generating system by applying additional output load to the TRU, thereby overloading the TRU output. In an embodiment in which the electrical generating system is located in an aircraft, flight-critical aircraft loads, which are typically DC powered, may be protected by the shunt regulator at the TRU output. Any overvoltage protection modules in the GCU may then have time to react to the overvoltage and disconnect the generator without damage to the DC loads. The shunt regulator is an active clamp that may be tested pre-flight, and may only be applied as needed to TRU outputs that power flight critical equipment. 
       FIG. 1  illustrates an embodiment of an electrical generating system  100  having a shunt regulator  116  for overvoltage protection at a TRU  111 . Electrical generating system  100  includes a permanent magnet generator (PMG)  101  and a generator  102 . PMG  101  provides an excitation voltage to generator  102  via GCU  103 . PMG  101 , generator  102 , and GCU  103  are shown for illustrative purposes only; a shunt regulator for overvoltage protection at a TRU may be used to protect a DC bus in any electrical system that includes a TRU. Generator  102  powers AC bus  110  via line contactor  109 . AC bus  110  may power one or more AC loads  115  in some embodiments. AC power from AC bus  110  is converted to DC power by TRU  111 , and the DC power from TRU  111  powers DC bus  112 . Shunt regulator  116  is connected to the output of TRU  111 . DC loads  113   a - b  are powered by DC bus  112 . Regulation point  114 , located between generator  102  and line contactor  109 , is connected to points of regulation (PORs)  107   a - c ; voltage from regulation point  114  is provided to the GCU  103  at PORs  107   a - c . DC loads  113   a - b  and AC load  115  are shown for illustrative purposes only; an electrical generating system  100  may power any appropriate number and type of loads in various embodiments. Electrical generating system  100  may be located in an aircraft in some embodiments, and DC loads  113   a - b  may be flight-critical loads in the aircraft in some embodiments. 
     GCU  103  acts to regulate the excitation voltage from PMG  101  to generator  102 , thereby limiting the output of generator  102  and prevent overvoltage conditions in electrical generating system  100 . GCU  103  includes a generator control relay  108 , a voltage regulator  104 , overvoltage protection module  105 , and redundant overvoltage protection module  106 . The excitation voltage from PMG  101  is sent via generator control relay  108  to voltage regulator  104  en route to generator  102 . Voltage regulator  104  acts to keep the excitation voltage output by GCU  103  to generator  102  at a target voltage based on input from POR  107   a . Overvoltage protection module  105  and redundant overvoltage protection module  106  are also each connected to a respective POR  107   b - c , which receives voltage data from regulation point  114 . During normal operation, generator control relay  108  is closed, and PMG  101  excites generator  102  via generator control relay  108  and voltage regulator  104 . PORs  107   a - c  are shown for illustrative purposes only, the PORs may be configured in any appropriate manner that provides voltage data from a regulation point in the electrical generating system to the GCU. Overvoltage protection module  105  and redundant overvoltage protection module  106  are also shown for illustrative purposes only; a GCU may include any appropriate number or type of overvoltage protection modules. 
     Overvoltage conditions at regulation point  114  may be detected by either or both of overvoltage protection module  105  or redundant overvoltage protection module  106  based on input from their respective PORs  107   b - c . In response to overvoltage conditions, either overvoltage protection module  105  or redundant overvoltage protection module  106  may open generator control relay  108  and/or line contactor  109 , and/or may turn off voltage regulator  104 . However, due to possible delays in operation of overvoltage protection module  105  and redundant overvoltage protection module  106 , shunt regulator  116  is also triggered by overvoltage conditions at the output of TRU  111 . Shunt regulator  116  acts to draw power from the output of TRU  111  to protect loads  113   a - b  connected to DC bus  112 . 
       FIG. 2  illustrates an embodiment of a shunt regulator  200  for overvoltage protection. Shunt regulator  200  may comprise shunt regulator  116  of  FIG. 1  in some embodiments. Input  201  is connected to the output of TRU  111  of  FIG. 1 . Transistor  202  includes a gate, source, and drain; the source of transistor  202  is connected to input  201  and to an input of operational amplifier  203 ; the drain of transistor  202  is connected to ground connection  205 ; and the gate is connected to the output of operational amplifier  203 . Operational amplifier  203  receives reference voltage  204 . Reference voltage  204  is a voltage limit that is below a predetermined ceiling voltage at which loads  113   a - b  may be damaged. When the voltage output by TRU  111  to shunt regulator input  201  exceeds the reference voltage  204  (i.e., an overvoltage condition exists at the output of TRU  111 ), the operational amplifier  203  biases the transistor gate voltage to on, such that transistor  202  draws current from the output of the TRU  111  at input  201 . This causes the voltage on DC bus  112  from TRU  111  to drop to the level of reference voltage  204 . The shunt regulator  116  acts to protect DC loads  113   a - b  until overvoltage protection modules  105  or  106  in the GCU  103  have time to react to the overvoltage. Shunt regulator  200  is shown for illustrative purposes only; the shunt regulator  116  of  FIG. 1  may have any appropriate configuration for overloading the output of TRU  111 . 
       FIG. 3  illustrates an embodiment of a method  300  of operating a GCU having a shunt regulator for overvoltage protection at a TRU output. Method  300  is discussed with reference to  FIG. 1 . In block  301 , an overvoltage occurs in electrical generating system  100 , i.e., a voltage at the output of TRU  111  exceeds a reference voltage. In block  302 , the shunt regulator  116  is activated. In block  303 , the shunt regulator overloads output of TRU  111 , causing the voltage available to the DC bus  112  to drop to the level of the reference voltage. In block  304 , an overvoltage protection module (such as either of overvoltage protection modules  105  or  106 ) responds to the overvoltage, and the shunt regulator is deactivated. 
     The technical effects and benefits of exemplary embodiments include relatively fast response to overvoltage conditions in an electrical generating system. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while various embodiment of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.