Abstract:
A method and system is provided for operating an electrical machine. The method includes the steps of, providing a brushless excitation system including a diode rectifier having at least one diode, sensing heat energy generated by at least one resistor connected in parallel with the diode, and detecting a deviation of the generated heat energy from the at least one resistor. Another step generates a signal indicating an error in a diode or a failed or faulty diode if the deviation in generated heat energy exceeds a predetermined threshold deviation level.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to brushless excitation systems for rotating electrical machines and, particularly, relates to a temperature sensing fault detector for a brushless excitation system. 
         [0002]    A brushless excitation system (or more simply a “brushless exciter”) applies a direct current (DC) to the field coils of a rotor in an electrical machine. The current in the rotor field coils generates an electromagnetic field that induces current in, for example, the coils of a stator surrounding the rotor and in a generator producing AC current. Alternatively, the electromagnetic field from the rotor field coils may be used to turn the rotor of a motor. 
         [0003]    Typically, a brushless excitation system is mounted on and rotates with the rotor of the electrical machine. The brushless excitation system includes a rotating armature and a diode rectifier, which may be configured as a diode wheel. Alternating current (AC) generated within the rotating armature is converted by the diode rectifier to direct current, which is applied to the field windings of the rotor. 
         [0004]    A fault in a diode of the rectifier can impair the conversion of AC to DC by the rectifier. A diode rectifier typically has two or more redundant diodes connected in series for each phase of the AC power applied to the input to the rectifier. It is generally difficult to reliably detect a fault in one diode, due to the presence of redundant diodes. The failure of a single diode may not substantially reduce the ability of the rectifier to convert AC to DC power. The failure of two or more diodes in series can impair the conversion of AC to DC, lead to a failure of the rectifier and result in an unscheduled shutdown of the electrical machine. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    According to one aspect of the present invention, a method is provided for operating an electrical machine, including the steps of, providing a brushless excitation system including a diode rectifier having at least one diode, sensing heat energy generated by at least one resistor connected in parallel with the diode, and detecting a deviation of the generated heat energy from the at least one resistor. Another step generates a signal indicating a failed or faulty diode, or an error in a diode, if the deviation in generated heat energy exceeds a predetermined threshold deviation level. 
         [0006]    According to another aspect of the present invention, a brushless excitation system is provided for an electrical machine. The system includes a diode rectifier electrically coupled to a source of alternating current and the rectifier produces a direct current which is applied to field windings of a rotor of the electrical machine. A plurality of resistors is configured so that each resistor is connected in parallel with a diode in the diode rectifier. A plurality of temperature sensors proximate to the plurality of resistors are each arranged to sense heat energy from one of the resistors and each temperature sensor generates a temperature signal indicative of the sensed heat energy of the resistor adjacent the sensor. A controller receives temperature data indicative of the temperature signals from the resistors, and detects whether one of the diodes has failed or is faulty based on the temperature data 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a circuit for a brushless excitation system. 
           [0008]      FIG. 2  is a plan view of a connector lead in a brushless excitation system, with a temperature sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]      FIG. 1  is a schematic view of an exemplary generator brushless excitation system  10  for providing excitation power to the field coils  12  of the rotor  13  of an alternating current (AC) generator  14 , such as a synchronous generator. The components of the brushless excitation system  10  are within the dotted line box with uniform dashes shown in  FIG. 1 . The components within the dot-dash line  11  rotate with the rotor  13  of the generator  14 . 
         [0010]    The AC generator  14  may be a three-phase synchronous generator providing electrical power for an electric power utility, such as by providing power at a frequency and current level suitable for an electric power grid serving homes, businesses and other facilities. As the rotor  13  turns, an electromagnetic field formed by the field coils  12  induces a current in the stator  15  of the generator. Alternatively, the brushless excitation system disclosed herein may be applied to an electrically-driven motor. 
         [0011]    An electric power source  16  provides DC power to the brushless exciter field  18 . The power source  16  may be a permanent magnet generator (PMG) generating electrical alternating current (AC) power or a transformer connected to an alternate source of AC power. The AC power from the power source  16  is rectified in the controller  20 , providing DC to the brushless exciter field winding  18 . The exciter field applies the magnetic flux to an armature  26  of the brushless excitation system  10 . The power source  16  may be controlled and monitored by a controller  20 , such as a programmable logic controller (PLC), microcontroller, excitation regulator or computer. The controller  20  monitors the condition of the brushless excitation system, analyzes data regarding the condition of the system and generates reports and alarms regarding the condition of the system  10 . 
         [0012]    The receiver  22  collects data from the rotating components of the brushless excitation system  10 , such as by a slip ring in contact with the rotor  13  or a wireless transmitter  24  attached to the rotor. The wireless transmitter may send infrared, radio frequency or other types of wireless signals with data regarding the condition of the brushless excitation system  10 . 
         [0013]    The exciter field coils  18  of the brushless excitation system  10  are electromagnetically coupled to coils of the armature  26  for the brushless excitation system  10 . The coils of the armature are mounted on a rotor  28 , which may be attached to an end of the rotor  13  in generator  14 . AC current is induced by the exciter field winding  18  in the exciter coils of the armature  26 . The AC power from the exciter field coils in armature  26  is applied to an electric current diode rectifier  30 . The AC power is converted to DC power by the diode rectifier  30 . The DC power from the diode rectifier  30  is applied to the rotor field coils  12  of the rotor  13  for the generator  14 . 
         [0014]    The diode rectifier  30  may include an array of diodes  32  for each phase of the AC current, e.g., three current phases, from the exciter rotor armature coils  26 . The diodes may be arranged on a diode wheel. The output terminals  34  of the diode rectifier  30  apply DC power to connector leads  35  that are coupled to the rotor field coils  12 . The input terminals  36  to the diode rectifier are connected to the coils of the armature to receive AC power. The diodes  32  in each array allow current to flow in one direction and thereby convert the alternating current to direct current. The diodes  32  are arranged in series. Alternating current at the input terminals  36  flows in a single direction through the diodes  32 . 
         [0015]    The diodes  32  ensure that direct current is applied to the rotor field coils  12 . Two or more diodes are preferably connected in series to provide redundancies in the rectifier. If one or more the diodes  32  fail in each array of diodes, the rectification of the alternating current may be fully performed by the redundant diode in the array. The failure of a single diode  32  may not substantially impair the conversion of AC to DC because the other diode in series with the failed diode can perform the rectification. If both diodes in a series fail, the conversion of that phase will fail. If two or more diodes in the array fail, alternating current may flow through the failed diodes and be applied to the rotor field coils  12 . Alternating current applied to the rotor field windings will interfere with the formation of the electromagnetic fields by the rotor, reduce the power generation efficiency of the generator  14  and typically causes the generator to shut down. 
         [0016]    The blocking or reverse voltage amplitude across each of the diodes  32  may be relatively large, typically between 40 and 500 volts. In some applications or conditions this voltage could be up to 1000 Volts. 
         [0017]    The addition of a high ohmic, high voltage resistor  33  in parallel to the diode  32 , adjacent and in proximity of a RTD or temperature sensing device  42  will generate a discernable temperature above ambient representing a normal operating condition. The addition of this parallel resistor  33  amplifies thermal characteristics of diode  32  operation or failure. While the diode  32  is in a forward operating condition, the voltage drop is small generating almost no heat. When the diodes  32  are blocking, the blocked potential will pass through the resistor  33  generating heat. With two diodes  32  in series and one of the diodes in a failed, shorted condition the blocking potential will generate little heat while the resistor  33  in parallel with the functional diode will produce nearly twice the heat then in a normal condition. Using a comparison algorithm on the diode-resistor array will determine an error in a diode, a diode failed short or a resistor failed open. Resistor  33 -RTD  42  pairs can be mounted or isolated in such a manner that the RTD  42  will sense a discernable temperature with the resistor to minimize the power dissipated. 
         [0018]    The temperature of each resistor  33  indicates whether the diode  32  has failed. A diode failure in a brushless excitation system almost always results in a short, or nearly short, circuit in the diode. The resistor connected in parallel with a failed diode will experience reduced current flow and reduced temperature, when compared to resistors connected across functional diodes. Likewise the companion resistor, across the functional diode in a diode module pair with one failed diode, will dissipate nearly twice the energy of a resistor in a normal operating state. 
         [0019]    A temperature sensor  42  is positioned near each resistor  33  and, preferably, is thermally isolated with the resistor. The temperature sensors  42 , such as resistance temperature detectors (RTDs), generate an output signal indicative of the operating temperature of the adjacent resistor(s)  33 . The temperature signals from the temperature sensors can be conducted by wire to the transmitter  24 . The transmitter sends the temperature signals to the receiver  22  and controller  20 . Alternatively, the temperature signals from the temperature sensors can be transmitted by wireless link (e.g., radio frequency, RFID tags, etc.) to the transmitter  24 . 
         [0020]    To detect a failed diode  32 , the controller  20  monitors the temperature signals from each of the temperature sensors  42 . The temperature signals are indicative of the temperature of the resistor adjacent to the sensor and the operating environment. When the controller detects that the temperature of a resistor has fallen or risen above a predetermined or comparison threshold, the controller determines that the diode has failed. The controller may issue an alarm or a report identifying the failed diode. The controller may also indicate which diode has failed and/or the temperature sensor issuing a low or high diode parallel resistor temperature signal. 
         [0021]    To determine whether a temperature signal from a sensor  42  indicates a failed diode, the controller  20  compares the signal to the temperature signals from the other temperature sensors  42 . The comparison may include calculating an average of all of the temperature signals from all sensors  42  in the rectifier, and checking for signals that are above or below the average by more than a threshold amount, such as by more than about 5 degrees Celsius below the average temperature signal. The average temperature signal may be a determined over a recent period of time, such as an average of all temperature signals over a period of the last minute. In addition, the controller may compare the temperatures of each resistor in a series of diodes for one of the AC phases. If one of the resistors in a series is at a substantially lower or higher temperature, e.g., higher or lower by about 5 degrees Celsius, the controller  20  determines that the appropriate diode has failed. 
         [0022]    As non-limiting examples only, if diode  32 A fails by shorting while diode  32 B remains functional, then resistor  33 A will experience a reduced current flow when compared to resistor  33 B. As a result, resistor  33 A will be “cooler” compared to resistor  33 B. A relatively “hotter” resistor  33 B could indicate a failed diode  32 A. Similarly, a relatively “cooler” resistor  33 A could indicate a failed diode  32 A. 
         [0023]    Further, the direct current and power generated by the brushless excitation system may be determined by a temperature sensor  44  and electrical contacts  46  mounted on each of the connector leads  35  extending between the diode rectifier  30  and the field windings  12  of the rotor. The temperature sensor  44 , e.g., a RTD, may be placed in the middle of the connector lead  35  and mid-way between two points to which electrical contacts  46  are bonded to the lead. The resistance of each of the connector leads is a function of the temperature of the lead. By measuring the temperature of the connector lead, the resistance of the connector lead can be reliably determined. 
         [0024]    The current in the connector lead can be determined by sensing the voltage potential across the lead connector  35  and calculating the resistance of the lead connector. The voltage potential at two points at far ends of the connector is measured by determining the difference of the voltage potential at the electrical contacts  46 . The output of an operational amplifier  48  indicates the voltage difference between the two points on the lead connector. The voltage difference signal from the operation amplifier and the temperature signal from sensor  44  are transmitted to the controller  20 . Using Ohm&#39;s law, it is known that the voltage equals the product of the current and resistance. The controller may determine the current in the lead controller by dividing the voltage difference across the connector by the resistance between the two points on the connector to which the electrical contacts  46  are connected. 
         [0025]      FIG. 2  is a front view of a lead connector  35  having a temperature sensor  44  and electrical leads  46  bonded to and spaced apart on the connector. The lead connector may be a conductive bar or strap extending between the brushless excitation system and the field coils of the rotor. The bar may be composed of 2 or more parallel leafs for stress relief. 
         [0026]    The distance (D) on the lead connector is known between the electrical leads  46 . The electrical resistance between the electrical leads is determined by the controller based on the distance (D) and the temperature of the connector lead. The controller may store a look-up table or formula, for example that identifies the resistance between the electrical leads  46  based on the temperature of the connector lead. 
         [0027]    The temperature sensors  42  are applied to detect faults in the diode rectifier. Detection of diode faults provides a technical effect of reporting when the diode rectifier in an brushless excitation system is in need of repair, before the system entirely fails to generate sufficient DC power for the rotor field windings. For example, the detection of a single diode failure in a diode array provides an indication of a needed repair. The failure of a single diode in a diode array may not cause the entire diode rectifier to fail. However, the failure of two or more diodes in series in a diode array may result in the failure of the diode rectifier. Having an indication that a single diode has failed, provides the operator of the brushless excitation system that a repair is needed, such as during the next scheduled shut down of the generator. The prompt repair of a single failed diode reduces the risk that the entire diode rectifier will fail and cause an unscheduled shut down of the generator. 
         [0028]    The temperature sensors  44  are applied to determine the direct current in each of the lead connectors. A real time reading of the direct current from the brushless excitation system provides an indication to the controller and the operator of the generator of the operating condition of the rotor field windings and of the generator. 
         [0029]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.