Abstract:
The present disclosure relates to electrical generators, more specifically to an apparatus and method for dehumidifying the windings and insulation system of a generator. The application of low-voltage, low-frequency, high-AC-current provides precise control of the rise of temperature in the winding. The controlled temperature increase occurs uniformly over the complete length of the winding, providing uniform dehumidification of the insulation system such that connection to the inverter and power production occurs without risk of damage to the insulation system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and benefit of U.S. Provisional Patent Application No. 61/917,730, filed Dec. 18, 2013, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the disclosure relate generally to electric motors and generators, and more particularly to systems and methods for dehumidifying the windings and insulation system of electric motors and generators. 
         [0003]    High-voltage systems designed to handle the flow of maximum rated power can undergo stress during initial start or during a start after a dormant period. Various situations can cause moisture to become deposited in the insulation proximal to the windings of an electrical machine. For example, during transportation from a manufacturing facility to an installation site, or during a dormant period in a humid environment, an electrical machine is subjected to ambient humidity. Protective covers are often employed; however, sufficient exposure time and/or humidity levels tend to cause humidity to penetrate an electrical machine enclosure and influence the winding insulation system. Humidity in the insulation surrounding the windings alters the properties of the insulation and can make the insulation conductive. Current flowing through insulation can destroy the insulation and hence the generator winding. 
         [0004]    Conventional heaters or dehumidifying devices placed proximal to the generator can only heat or dehumidify the air inside the generator and not the winding itself where the problematic humidity is situated. The process is unreliable as a heater might raise the temperature of one portion of the generator but not portions where the winding could still be humid. Such devices are commonly engaged for long periods of time in an attempt to compensate for the aforementioned shortcomings. 
       SUMMARY 
       [0005]    The present disclosure relates to electric motors and generators, more specifically to an apparatus and method for dehumidifying the windings and insulation system of a motor or generator. Some embodiments may be implemented in conjunction with wind, water or other fluid turbines. The process is commonly performed prior to commissioning the machinery or after a period of time during which the machinery has been shut down. 
         [0006]    In embodiments taught herein, a turbine converter is electrically coupled to a low voltage alternating current (AC) or direct current (DC) source, thereby providing systems and methods of generating low-voltage, low-frequency alternating current in the generator windings. Temperature rise of the winding system can be controlled by setting andr adjusting the amount of low-voltage, low-frequency current delivered. The appropriate amount of low-voltage, low frequency current to be delivered can vary. The application of AC low-voltage, low-frequency, high-current eliminates the effect of inductance and provides precise control of the energy losses that cause a rise of temperature in the winding. The controlled temperature increase occurs uniformly over the complete length of the winding, providing uniform dehumidification of the insulation system such that connection to the inverter and power production occurs without risk of damage to the insulation system. It will be apparent in view of this disclosure that additional embodiments may include the application of a low-voltage, high-frequency, high-current to said electrical machine windings for the aforementioned intended purpose. However, as explained in further detail below, the use of a semiconductor switching frequency lower than a semiconductor switching frequency used for normal converter operation results in a beneficial lowering of the risk of stressing of the insulation system during the heating cycle. 
         [0007]    Various embodiments are directed to methods and systems, the systems comprising a combination generator and converter; a method comprised of utilizing the converter to dehumidify the generator before commissioning or after a dormant period. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The following is a brief description of the drawings, which are presented for the purposes of illustrating the disclosure set forth herein and not for the purposes of limiting the same. 
           [0009]      FIG. 1  is a schematic diagram illustrating an example embodiment of a system for dehumidifying insulation in an electrical machine as taught herein. 
           [0010]      FIG. 2  is a schematic diagram illustrating another example embodiment of a system for dehumidifying insulation in an electrical machine as taught herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    A more complete understanding of the components, processes, and apparatuses disclosed herein can be obtained by reference to the accompanying figures. These figures are intended to demonstrate the present disclosure and are not intended to show relative sizes and dimensions or to limit the scope of the disclosed embodiment(s). 
         [0012]    Although specific terms are used in the following description, these terms are intended to refer only to particular structures in the drawings and are not intended to limit the scope of the present disclosure. It is to be understood that like numeric designations refer to components of like function. 
         [0013]    The term “about” when used with a quantity includes the stated value and also has the meaning dictated by the context. For example, it includes at least the degree of error associated with the measurement of the particular quantity. When used in the context of a range, the term “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the range “from about 2 to about 4” also discloses the range “from 2 to 4.” 
         [0014]    Electrical machinery operated by a converter incurs a high load on the winding insulation system due to high switching frequency, and high voltage, are also referred to as a high rate of voltage change over time (dv/dt levels). Engaging a converter as per its normal function is not an appropriate method for drying out a humid electrical machine as doing so incurs a risk of catastrophic damage to the insulation system. In particular, due to the reduced insulation capability of humid or wet insulation systems described above, the high switching frequency and voltage associated with operating load causes insulation breakdown which, in turn, exacerbates the reduced insulation capability, leading to destruction of the insulation and, ultimately, the windings. 
         [0015]      FIG. 1  is a schematic diagram of an exemplary system  100 .  FIG. 1  illustrates a electrical machine  118  such as, for example, a three-phase generator with star-connected windings  117  as shown. A converter  114  is electrically coupled to the three-phase electrical machine  118 . The converter  114  is electrically coupled to the grid voltage system  110  through a three-phase, high-voltage switch  112  during normal operation. A low voltage AC supply  122  is electrically coupled to a low-voltage switch  120 . Low-voltage, low-frequency, high alternating current is delivered to the converter when the low-voltage switch  120  is closed and the high-voltage switch  112  is open. Low-voltage, low-frequency, high alternating current  116  is delivered to the generator  118  prior to start-up of the generator. The low-voltage, low frequency, high alternating current  116  encounters ohmic resistance in the windings  117 , thereby dissipating energy as heat evenly throughout the generator windings  117  via resistive heating and thus providing a system for evaporating moisture in the generator insulation system  119 . 
         [0016]    In accordance with various embodiments, the converter  114  can be, for example, a three-phase inverter/voltage source converter and can include an AC/DC inverter  113   a  in electrical communication with a DC link  115 , which is also in electrical communication with a DC/AC inverter  113   b,  thereby allowing the system to control the voltage, frequency, and current level of the AC current delivered to the windings  117 . 
         [0017]    In accordance with various embodiments, the electrical machine  118  includes an insulation system  119  including an insulating material configured to create an electrical barrier from the current-carrying winding  117  to the magnetic system of the generator. Insulating materials of the insulating system can include, for example, mica, kapton epoxy, and/or any other suitable insulating material. Insulation  119  can insulate the windings  117  of the electrical machine  118 . 
         [0018]    In accordance with various embodiments, low voltage AC supply  122  can be any suitable supply, including for example, a low-voltage transformer and/or a secondary winding of a transformer. 
         [0019]    Delivering low-voltage, low-frequency, high alternating current  116  to the windings  117  is advantageous for multiple reasons. High alternating current advantageously increases the resistive heating effect as compared to a low alternating current and can be, for example, between approximately 50%-100% of the rated generator current for the generator. 
         [0020]    Additionally, for every switching of the voltage in the AC current, there is a capacitive charge current flowing in the insulation system  119  and a voltage overshoot. Each of these charges and overshoots stress the insulation system  119 . When dry, the insulation system  119  is designed to withstand the charges and overshoots resulting from operational voltages and frequencies. However, in the wet state, the insulation system  119  cannot withstand those same operational conditions. Therefore, reducing voltage and frequency can advantageously reduce both the amplitude of the charges and overshoots and the overall number of charges and overshoots delivered to the insulation system  119  during dehumidification. 
         [0021]    Low frequencies are further advantageous for reducing the impedance imposed on the current flow by the winding inductance. Accordingly, for example, in some embodiments, the frequency can be between approximately 0.01 Hz and 5 Hz (e.g., 0.2 Hz), although it will be understood in view of this disclosure that any suitable frequency can be used with various embodiments depending on the design capabilities of the insulation system. Furthermore, for example, in some embodiments, the voltage can advantageously be between approximately 1% and 10% (e.g., 2%) of a nominal voltage (e.g., line voltage, such as 120, 240 or 480 VAC), although it will be understood in view of this disclosure that any voltage low enough to avoid excessive stress on wet insulation but high enough to drive a desired current through the windings  117  can be used in accordance with various embodiments. 
         [0022]      FIG. 2  is a schematic diagram of an exemplary system  200 .  FIG. 2  illustrates an electrical machine  218  such as, for example, a three-phase generator with delta-connected windings  217  as shown. It will be apparent in view of this disclosure that various electrical machines  218  may be affected by the systems and methods of the present embodiment. A converter  214  is electrically coupled to the three-phase electrical machine  218 . The converter  214  is electrically coupled to the grid voltage system  210  through a three-phase, high-voltage switch  212  during normal operation. A low voltage DC supply  222  is electrically coupled to a low-voltage switch  220 . Low-voltage, high direct current is delivered to the converter  214  when the low-voltage switch  220  is closed and the high-voltage switch  212  is open. The converter  214  then converts the low-voltage, high direct current into low-voltage, low-frequency, high alternating current  216 , which is delivered to the generator  218  prior to start-up of the generator. The low-voltage, low frequency alternating current encounters ohmic resistance in the windings  217 , thereby dissipating energy as heat evenly throughout the generator windings  217  via resistive heating and thus providing a means of evaporating moisture in the generator insulation system  219 . 
         [0023]    In accordance with various embodiments, converter  214  can be, for example, a three-phase inverter/voltage source converter and can include an AC/DC inverter  213   a  in electrical communication with a DC link  215 , which is also in electrical communication with a DC/AC inverter  213   b,  thereby allowing the system to control the voltage, frequency, and current level of the AC current delivered to the windings  217  of the electrical machine  218 . It will be apparent in view of this disclosure that the low-voltage, high direct current delivered to the converter  214  can be treated as AC current having a frequency of zero, thereby allowing the AC/DC inverter to receive the supplied low-voltage, high direct current from the low voltage DC supply  222 . 
         [0024]    In accordance with various embodiments, the insulation  219  includes an insulating material configured to create an electrical barrier from the current-carrying winding  217  to the magnetic system of the generator. Insulating materials of the insulation  219  can include, for example, mica, kapton epoxy, and/or any other suitable insulating material. Insulation  219  can insulate the winding  217  of the electrical machine  218   
         [0025]    In accordance with various embodiments, low voltage DC supply  222  can be any suitable supply, including for example, a low-voltage transformer, a secondary winding of a transformer, and/or a switch mode DC power supply. 
         [0026]    Delivering low-voltage, low-frequency, high alternating current  216  to the windings  217  is advantageous for multiple reasons. High alternating current advantageously increases the resistive heating effect as compared to a low alternating current and can be, for example, between approximately 50%-100% of the rated generator current for the generator. 
         [0027]    Additionally, for every switching of the voltage in the AC current, there is a capacitive charge current flowing in the insulation system and a voltage overshoot. Each of these charges and overshoots stress the insulation system. When dry, the insulation system is designed to withstand the charges and overshoots resulting from operational voltages and frequencies. However, in the wet state, the insulation system cannot withstand those same operational conditions. Therefore, reducing voltage and frequency can advantageously reduce both the amplitude of the charges and overshoots and the overall number of charges and overshoots delivered to the insulation system during dehumidification. 
         [0028]    Low frequencies are further advantageous for reducing the impedance imposed on the current flow by the winding inductance. Accordingly, for example, in some embodiments, the frequency can be between approximately 0.01 Hz and 5 Hz (e.g., 0.2 Hz), although it will be understood in view of this disclosure that any suitable frequency can be used with various embodiments depending on the design capabilities of the insulation system. Furthermore, for example, in some embodiments, the voltage can advantageously be between approximately 1% and 10% (e.g., 2%) of a nominal voltage (e.g., line voltage, such as 120, 240 or 480 VAC), although it will be understood in view of this disclosure that any voltage low enough to avoid excessive stress on wet insulation but high enough to drive a desired current through the windings  217  can be used in accordance with various embodiments. 
         [0029]    The present disclosure has been described with reference to exemplary embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.