Abstract:
A drive system for driving large capacity motors includes a motor and a variable frequency drive which accepts input from a three-phase power source. The drive system includes a step-up transformer, preferably of a high-capacity three-phase type, positioned between and electrically connected to the motor and the variable frequency drive to thereby step-up voltage received from the variable frequency drive to be supplied to the motor. The transformer includes a transformer chamber formed in the transformer tank containing a cooling fluid for cooling transformer internal components. A plurality of inductors forming part of a harmonic filter are positioned within the transformer chamber such that they can be protected from the environment and simultaneously cooled with other transformer internal components by the dielectric fluid. The filter includes capacitors that are preferably mounted outside of the tank.

Description:
1. RELATED APPLICATIONS  
       [0001]    This application claims priority to provisional application serial No. 60/436,593 titled Systems and Methods for Driving Large Capacity AC Motors, filed on Mar. 17, 2003. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates in general to systems and methods for driving large capacity AC motors and, in particular, to such systems and methods including a transformer.  
           [0004]    2. Brief Description of Related Art  
           [0005]    The oil drilling industry uses large volume submersible pumps typically located thousands of feet into a well. The pump assembly includes a centrifugal pump which is driven by an AC motor. The motors may range from 15 to 2000 horsepower, and thus, require a large supply of power. Normally, 60-cycle, three-phase power is supplied with voltage phase-to-phase being 480 volts or more. Common rotational speeds of the motor are about 3500 revolutions per minute (rpm). Most of these types of pumps are generally single speed pumps. Because of different viscosities, densities, well flowing characteristics, and the like, it is desirable to vary the speed of the motor. One way in which to vary the speed is to vary the frequency of the power being supplied. Normally, however, the line power comes from a utility company and cannot be changed from the standard 60-cycle per second. There are circuits that will convert the standard frequency to different frequencies. These circuits can also change the amplitude in proportion to the frequency change for efficient operation of the motor.  
           [0006]    A technique for controlling the speed of a three-phase induction motor uses an electronic variable frequency drive (VFD). The electronic VFD has a rectifier circuit that requires multiple phases of alternating current. For example, a six-pulse rectifier needs three-phases of electric power to be input so that six pulses are provided by the full-wave rectification. One type of VFD uses pulse width modulation (PWM). Others use square waves, such as a six-step waveform.  
           [0007]    Although multi-phase PWM inverters are useful, they can cause high order harmonic voltages resulting in detrimental high peak voltages at the motor, particularly with transformers and long lengths of cable between the VFD and the motor.  
           [0008]    System filters consisting of an inductor and a shunt capacitor can be used to prevent or attenuate harmonic distortion. Applicant has recognized, however, that these filters have only been marginally successful due to problems related to environmental exposure. Applicant has also recognized that positioning the inductors in containers sufficient to prevent degradation due to the environment is problematic due to the excessive heat generated in the inductor coil of the filter arrangement.  
         SUMMARY OF THE INVENTION  
         [0009]    With the foregoing in mind, embodiments of the present invention advantageously modify the traditional system for driving motors such as large capacity motors and advantageously eliminate or substantially reduce the harmful effect caused by PWM inverters. For example, in an embodiment of the present invention, a drive system for driving large capacity motors includes a motor and a variable speed drive which accepts input from a three-phase power source. The variable speed drive is electrically connected to the motor and positioned to vary speeds of the motor. The drive system further includes a transformer, preferably of a high-capacity three-phase type, positioned between and electrically connected to the motor and the variable speed drive to thereby step-up voltage received from the variable speed drive to be supplied to the motor.  
           [0010]    In an embodiment of the present invention, the transformer includes a transformer housing defining a transformer tank and a transformer chamber formed in the transformer tank. A plurality of typically vertically oriented magnetic core elements and primary and secondary windings are positioned within the transformer chamber. The primary and secondary windings substantially surround at least portions of the core and provide the step-up voltage. A filter, generally configured as a low pass filter, includes a plurality of inductors and a plurality of capacitors and is provided to filter harmonics created by the variable speed drive. The plurality of inductors are uniquely also positioned within the transformer chamber. Each of the three-phases of the transformer includes at least one of the inductors preferably connected between a bushing connected to the transformer tank and the primary windings or windings associated with that individual phase of the three-phases.  
           [0011]    The transformer chamber of the transformer tank further includes or contains a dielectric fluid, such as an insulating dielectric oil, for cooling the transformer internal components, i.e., the core, the primary and secondary windings, and advantageously also cools the plurality of inductors. Each of these transformer internal components including the plurality of inductors is at least partially immersed in the dielectric fluid. Convection currents in the dielectric fluid can help promote or cause cooling of the inductors along with the other transformer internal components.  
           [0012]    The transformer can also include a recirculation line for cooling the dielectric fluid mounted exterior to the transformer tank. The recirculation line can be and preferably is connected between an outlet manifold adjacent the top of the transformer tank and an inlet manifold adjacent the bottom of the transformer tank. When so positioned, the heated dielectric fluid entering the recirculation line can be cooled by radiation cooling to the atmosphere, and can naturally, or with mechanical help, circulate downwardly to re-enter the tank adjacent the bottom.  
           [0013]    Advantageously, according to an embodiment of the present invention, the transformer can function both as a conventional transformer and as the above described transformer capable of filtering harmonic frequencies created by a variable speed drive unit and capable of dissipating heat associated with such filtering. To utilize the above described transformer as a conventional transformer, the transformer also can include a plurality of inductor bypasses positioned to bypass the plurality of inductors. In a preferred configuration, each inductor bypass has a first end and second end, the first end connected to a bushing, the second end connected between an inductor and the primary winding to thereby bypass the inductor to allow the transformer to be used without the inductor.  
           [0014]    The filter is generally configured in the form of a low pass filter comprising the plurality of inductors and a capacitor bank including a plurality of capacitors and positioned external to the transformer tank. Each capacitor within the capacitor bank is electrically connected to at least one of the inductors. In the preferred configuration, each capacitor is also connected preferably phase-to-phase at an input of the transformer. This configuration allows the capacitors to be easy disconnected in order to perfect bypassing the filter arrangement to allow the transformer to function as a conventional step-up transformer, or to electrically disconnect malfunctioning capacitors from the electrical circuit.  
           [0015]    Advantageously, embodiments of the present invention also include a method for transforming electricity. For example, a method of transforming electricity can include a user providing a transformer including a transformer tank having a chamber at least partially filled with a dielectric cooling fluid, and containing a core, and a plurality of primary and secondary windings. The user positions a plurality of inductors in the chamber immersed either individually or in a separate container in the dielectric cooling fluid. This positioning allows the inductors to be cooled with the same dielectric fluid that cools other transformer internal components. By connecting each of the inductors in series with and between a primary bushing connected to the transformer tank and one of the plurality of primary windings, the user can form part of a low pass filter. The user also mounts preferably a capacitor bank to an external surface of the transformer tank and can electrically connect each capacitor in the capacitor bank to at least one of the inductors to further form the low pass filter. The user can supply AC power to the primary windings, filter the AC power with the inductor and capacitor, and deliver power from the second winding, while cooling the inductors and the windings with the dielectric fluid in the tank.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]    [0016]FIG. 1 is a block diagram illustrating a system for driving a motor, according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 2 is a front elevational view of a transformer of a system for driving a motor, according to an embodiment of the present invention.  
         [0018]    [0018]FIG. 3 is a side elevational view of a transformer of a system for driving a motor, according to an embodiment of the present invention.  
         [0019]    [0019]FIG. 4 is a fragmentary perspective view of a transformer of a system for driving a motor having portions thereof broken away for clarity, according to an embodiment of the present invention.  
         [0020]    [0020]FIG. 5 is a schematic diagram of a transformer showing inductors and a magnetic core and windings immersed in a fluid coolant in a transformer tank with provisions for external cooling of the circulated coolant, according to an embodiment of the present invention.  
         [0021]    [0021]FIG. 6 is a schematic circuit diagram depicting three-phases of a three-phase transformer configured Delta-Delta, according to an embodiment of the present invention.  
         [0022]    [0022]FIG. 7 is a schematic circuit diagram depicting three-phases of a three-phase transformer configured Delta-Wye, according to an embodiment of the present invention.  
         [0023]    [0023]FIG. 8 is a schematic circuit diagram depicting a filter arrangement of a three-phase or three single-phase inductors within a transformer tank and capacitors mounted exterior of the transformer tank, according to an embodiment of the present invention.  
         [0024]    [0024]FIG. 9 is a schematic circuit diagram depicting capacitors and capacitor disconnect switches in a Delta and in a Wye configuration, according to an embodiment of the present invention.  
         [0025]    [0025]FIG. 10 is a schematic circuit diagram depicting a method of bypassing the inductors using a switch, according to an embodiment of the present invention.  
         [0026]    [0026]FIG. 11 is a schematic circuit diagram depicting a method of bypassing the inductors by using a separate set of primary bushings, according to an embodiment of the present invention.  
         [0027]    [0027]FIG. 12 is a schematic diagram of a transformer tap array utilized to select voltage showing tapped secondary winding and tap changer switches for a single output phase of a transformer, according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0028]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments.  
         [0029]    Referring to FIG. 1, illustrated is a drive system  12  capable of delivering a substantially sinusoidal waveform to a multi-phase alternating current (AC) motor such as three-phase high-power AC motor  19 . The power normally available in a well site is a three-phase sinusoidal waveform (not shown), having Phase A, Phase B, and Phase C. Each phase alternates between positive and negative in a sine wave. The individual phases of this AC waveform or all three-phases can be used to drive a high-power motor, such as AC motor  19 . In an embodiment of the present invention, the motor  19  typically operates between 15 to 2000 horsepower and is generally used in well-site operations. A variable speed drive unit, such as a variable frequency drive  15 , of a pulse width modulated type and generally having a 480 volt output, is provided to vary the speed of the motor  19 . Referring also to FIGS. 2-4, a high-capacity step-up transformer  13 , having a typical output of 1000 to 5000 volts, is positioned between and electrically connected to the motor  19  and the variable frequency drive  15 . The transformer  13  typically functions to step-up voltage received from the variable frequency drive  15 , to be supplied to the motor  19 . Note, the transformer  13  can also be implemented as a step-down transformer.  
         [0030]    Referring to FIGS. 1 and 8, a filter arrangement such as low pass filters  17  can be employed to filter harmonics created or translated by the variable frequency drive  15  so that a more sinusoidal wave form is provided to the motor  19 . Filter  17  preferably includes a set of inductors  21  and a set of capacitors  22 . The inductors  21  are positioned within the transformer  13 , and each are electrically connected in series between an output of variable frequency drive  15  and a corresponding input to transformer primary windings  31 , respectively. The capacitors  22  are electrically connected to the transformer primary binary  31  in either a delta or wye configuration (FIG. 9). In the exemplary embodiment, the filter  17  includes three inductors  21 , three single phases or one three-phase, each connected in series between the output of the variable frequency drive  15  and the primary windings  31  of one of the phases, and a three-phase capacitor bank  23  (FIG. 2) is preferably configured to connect phase-to-phase at the input of the transformer  13 . Capacitor bank  23  comprises three capacitors  22  mounted as a unit. Note, the filter  17  can be positioned on the secondary winding side of the transformer  13 , but additional benefits are realized by positioning the filter  17  on the primary winding side of the transformer  13 .  
         [0031]    Referring to FIG. 5, in an embodiment of the present invention, transformer  13  has a chamber  25  within a transformer housing or tank  20 . A suitable liquid coolant, such as insulating dielectric fluid  26 , typically having both insulating and cooling properties, can be positioned within chamber  25  of tank  20 . The dielectric fluid  26  is preferably oil or an equivalent, as understood by those skilled in the art, which can be used for cooling the various transformer components. Transformer  13  has at least one magnetic core element per phase,  28 A,  28 B,  28 C, located inside the transformer tank  20  to conduct magnetic flux.  
         [0032]    Referring to FIGS. 5-8, transformer  13  has at least one primary winding  31  per phase. Each primary winding  31  has a first end and a second end, each electrically connected to a different primary bushing  33  in order to form a Delta configuration or Wye configuration (not shown). Each primary winding  31  is positioned to substantially surround at least portions of at least one of the vertically oriented magnetic core elements  28 . The primary windings  31  are further at least partially immersed in dielectric solution  26  contained within the chamber  25  of the transformer tank  20 .  
         [0033]    Referring to FIGS. 2 and 3, the primary bushings  33  are mounted to the walls of the transformer tank  20 . The primary bushings  33  send the transformer input current to the primary windings  31  located within the transformer tank  20 , from external electric circuits, such as the variable frequency drive  15  (FIG. 1). The primary bushings  33  provide electrical insulation between the conductor (not shown) of each primary bushing  33  and the transformer tank  20 .  
         [0034]    Referring to FIGS. 5-8, transformer  13  has at least one secondary winding  32  per phase. Each secondary winding  32  has a first end and a second end, each electrically connected to a different secondary bushing  35  in order to form a Delta configuration (FIG. 6) or Wye configuration (FIG. 7). Each secondary winding  32  is positioned to substantially surround at least portions of at least one of the magnetic core elements  28 . The secondary windings  32  are further at least partially immersed in dielectric solution  26  contained within the chamber  25  of the transformer tank  20 .  
         [0035]    Referring to FIG. 4, although embodiments of the present invention have provisions for using a transformer tank  20  designed and equipped for forced cooling and have provisions for being either air-cooled or cooled by a non-fluid method, transformer  13  is preferably liquid-fluid cooled by natural circulation and convection. Where the cooling requirements are rather small, generally the surface area of the transformer tank  20  can provide for sufficient radiant cooling. Transformers requiring additional cooling can include fins which can increase the surface area available for cooling. Transformers requiring even more cooling can include cooling tubes or a radiator-type cooling system. For example, referring to FIG. 5, the transformer  13  can include at least one cooling and recirculation line  40  preferably mounted on an exterior portion of transformer tank  20  adjacent one side wall or completely around its periphery. Each cooling and recirculation line  40  is fluidly connected to the chamber  25  of transformer tank  20  via an outlet manifold  41  ideally located near the top of tank  20  and an inlet manifold  42  ideally located in the bottom of the tank. In a natural circulation and convection arrangement, heated dielectric fluid  26  rises to the top of the inner chamber  25  of tank  20 . Heated fluid  26  entering the cooling and recirculation lines  40  is cooled by radiation cooling to the atmosphere or by forced air cooling typically using cooling panels  45  and fans (not shown). The fluid  26  naturally circulates downwardly to re-enter the inner chamber  25  of tank  20  at the bottom of the chamber  25 . Channel members  46  (FIG. 2) fixed to the bottom surface of tank  20  also provide for additional cooling through air circulation, as can the general design of tank  20 . Note, the use of pumps (not shown) in order to improve the recirculation is also within the scope of the present invention.  
         [0036]    Referring to FIGS.  4 ,  8 - 11 , in an embodiment of the present invention, the transformer electrical circuit includes filter  17  (FIG. 1) formed via one or more inductors  21  and one or more capacitors  22 . The array of inductors  21  include at least one inductor  21  per electrical phase. Thus, in a three-phase arrangement, there are preferably three inductors  21  connected in series with the respective primary windings  31 . The inductors  21  are housed within the inner chamber  25  of transformer tank  20  either individually, grouped together, or in their own protective container (not shown) immersed in a dielectric fluid. In the illustrated embodiment, the inductors  21  are immersed in the same dielectric solution  26  that is located in the inner chamber  25  of tank  20  for cooling windings  31 ,  32 . Each inductor  21  is electrically connected between the respective primary bushings  33  and primary windings  31  associated with the individual phases, e.g., phase A, B, or C (FIG. 6).  
         [0037]    Referring to FIG. 10, in an embodiment of the present invention, the inductors  21  could also include a bypass switch  24  for each inductor  21 . At least one bypass switch  24  is probably electrically connected in parallel across inductors  21  for each phase A, B, C, forming means for bypassing inductors  21  to bypass the filter arrangement, and thus, a stage of the harmonic voltage filtering. The bypass switch  24  is typically a three-phase switch in the form of a rotary switch but can, however, be implemented using other various methods known and understood by those skilled in the art.  
         [0038]    Referring to FIG. 11, the inductor bypass can also be implemented without bypass switches  24 . For example, an embodiment of the present invention can include a conductor  36  for each phase electrically connected between each auxiliary primary bushing  34  and each primary winding  31 . By applying the transformer input power to the auxiliary primary bushings  34  rather than primary bushing  33 , the inductors  21  are electrically bypassed with current flowing directly to the primary windings  31 .  
         [0039]    Referring to FIGS. 8 and 9, the capacitors  22  include at least one capacitor or capacitor group inter connected to act as a single capacitor, for each phase. Each of the terminals for the capacitors  22  is either electrically connected between a pair of the phases A, B, C, in the circuit as a delta connection, or one of the phases A, B, C, and neutral as a wye connection. The capacitors  22  are positioned at a location on or close to the transformer. More particularly, as illustrated, the capacitors  22  can be electrically connected between the connection of inductor  21  and primary windings  31 . The capacitors  22  are preferably located exterior of tank  20  if the cooling fluid is a liquid. In a configuration, however, where the cooling fluid for the transformer internal components is not a liquid, capacitors  22  can instead be located inside transformer tank  20 .  
         [0040]    As shown in FIG. 2, the capacitors  22  can be in the form of a modular capacitor bank  23  mechanically attached to the exterior of tank  20 . In this configuration, the capacitors  22  are electrically connected to the auxiliary primary bushings  34 . Disconnecting capacitors  22  from electrical communication with the circuit is necessary for bypassing the filter arrangement. A switch  49  can perform such function. Switch  49  is preferably a three-phase switch that is electromechanically actuated, but can be implemented by other means known and understood by those skilled in the art.  
         [0041]    Referring to FIGS. 2 and 12, the transformer circuit can include a tap array  50  (FIG. 12) and tap changer  55  (FIG. 2) which provides for discrete voltage selection. For example, each of the secondary windings  32  (FIG. 12) can include a set of taps  51  and a tap switch  52 . The tap switches  52  can be selectively electrically connected to taps  51  on the secondary winding  32 . The tap switches  52  are each provided to allow corresponding secondary bushings  35  (FIG. 2) to be mechanically and electrically connected to the corresponding secondary winding&#39;s taps  51 , which together comprise tap array  50 . The tap switches  52  are typically located inside tank  20  and operated external to tank  20  via an actuator (not shown) connected to tap changer  55  and that extends through the wall of tank  20 . In the preferred configuration, the voltage between taps is approximately a maximum of seven percent per tap  51 .  
         [0042]    In operation, a suitable source, such as a power utility, provides three-phase electric power to the pulse-width modulated variable frequency power drive  15  (FIG. 1). The variable frequency drive  15  converts fixed frequency power into variable frequency, variable voltage power that contains higher order harmonic voltages. Within the transformer  13 , internal inductors  21  (FIG. 8) in electrical communication with the external or alternatively internal capacitors  22  filter the higher order harmonic voltages producing a substantially sinusoidal voltage waveform. The voltage is stepped up via the primary and secondary windings  31 ,  32 . The heat generated from the inductors  21  and windings  31 ,  32 , is dissipated from within the tank  20  through convection or other techniques. The substantially sinusoidal waveform with increased voltage is then available to the motor  19 .  
         [0043]    The invention has significant advantages. Application of a filter arrangement having inductors within the transformer tank not only can serve to eliminate the harmful effect caused by a non-sinusoidal voltage waveform but provide a methodology for cooling those inductors through use of the cooling system already associated with a transformer. Advantageously, when necessary, either the inductors or capacitors comprising a filter can be bypassed.  
         [0044]    In the drawings and specification, there have been disclosed a typical preferred embodiment of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The invention has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification. For example, the drive system may be in a form other than pulse width modulation.