Abstract:
A method and apparatus comprises a filter receiving an input voltage signal from a drive circuit, and a filter producing an output voltage signal with reduced resonance and transients. The amplitude of the output voltage signal is boosted using the filter.

Description:
BACKGROUND  
       [0001]     Variable speed voltage drive systems are used to vary the speed of motors, such as submersible motors used in submersible pumping systems deployed in wells. A typical submersible pumping system includes a pump and a motor, with the motor being electrically connected to a variable speed drive system over a cable that extends from the downhole location of the motor to an earth surface location of the variable speed drive system. The motor powers downhole components, such as the pump, to perform downhole tasks, such as to pump fluids from the downhole location to the earth surface. An example submersible motor is a three-phase induction-type motor. In the three-phase configuration, the variable speed drive system provides a three-phase input to the three-phase induction-type motor.  
         [0002]     The load impedance of the cable and the downhole motor may cause resonance in signals from the variable speed drive system to the motor. The resonance is caused by undesirable harmonic components generated by the output of the drive system, which can cause voltage distortion and/or transients, zero-crossing noise, and other issues. To reduce resonance, a filter can be used to filter out harmonic components of each input signal from the variable speed drive system.  
         [0003]     In some applications, the cable from the variable speed drive system to the downhole motor can be quite long, some as long as 25 kilometers or more. The long cable is associated with a large resistance that can cause a substantial voltage drop of each signal from the motor drive system along the cable. As a result, a separate step-up transformer (separate from the filter) typically has to be used to boost the voltage amplitude of an input signal from the variable speed drive system to compensate for the voltage drop along the cable. Use of separate units (a filtering unit and a voltage boost unit) to perform the filtering and amplitude boosting tasks may result in greater complexity and costs associated with deployment into a well of a submersible pump system, or other type of downhole system that includes a motor.  
       SUMMARY  
       [0004]     In general, according to an embodiment, a method comprises receiving, at a filter, an input signal from a drive circuit. The filter produces an output signal with reduced resonance, and the filter also boosts the amplitude of the output signal.  
         [0005]     Other or alternative embodiments will become apparent from the following description, from the drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates an example production string that includes a motor that is driven by an output provided by an autotransformer filter according to an embodiment, where the autotransformer filter filters and boosts the output voltage of a variable speed drive circuit, and where the autotransformer filter and variable speed drive circuit are contained in the same enclosure.  
         [0007]      FIG. 2  is a more detailed diagram of components of the autotransformer filter according to an embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0008]     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.  
         [0009]      FIG. 1  shows a downhole string deployed in a wellbore  100  that is lined by casing  102 . The casing  102  has perforations  106  that enable communication of fluids, such as hydrocarbon fluids, between the wellbore  100  and a reservoir surrounding the wellbore  100 . The downhole string, according to an example embodiment, includes a submersible motor  104 , such as an induction-type motor. Other types of submersible motors can be used in other embodiments. The submersible motor  104  is part of a submersible pump system that also includes a pump  108 . The downhole string is deployed into the wellbore  100  by a tubing  110 . In other embodiments, the downhole string can be deployed by other deployment mechanisms, such as cables, slicklines, and so forth.  
         [0010]     The submersible motor  104  is connected by a cable  112  to equipment located at an earth surface  122 . The cable  112  extends along the length of the wellbore  100  between the earth surface  122  and the motor  104 . The tubing  110  also extends to the earth surface from the submersible pump system. When activated by input signals transmitted over the cable  112 , the motor  104  powers the pump  108  to pump fluids from the surrounding reservoir up the tubing  110  to the earth surface.  
         [0011]     Although described in the context of a variable speed drive system for driving a submersible motor in a downhole environment, it is contemplated that other types of drive systems for driving other types of motors (whether used in downhole applications or otherwise) can be used in other embodiments.  
         [0012]     The surface equipment that provides output signaling for communication over the cable  112  to the motor  104  includes a variable speed drive system  121 , which includes variable speed drive power circuits  120  and an autotransformer filter  116 , according to an embodiment. The variable speed drive circuits  120  and autotransformer filter  116  can be contained in the same enclosure. The autotransformer filter  116  receives an input  118  from the variable speed drive power circuits  120 . The autotransformer filter  116  then provides an output  114  for communication over the cable  112  to the motor  104 .  
         [0013]     According to one embodiment, the input  118  is a three-phase input to the autotransformer filter  116 , and the output  114  from the autotransformer filter is a three-phase output, which powers the three-phase induction-type motor  104 . However, according to another embodiment, a single-phase input and output can be used. The three-phase input includes three input signals that are out of phase with respect to each other by 120°, and the three-phase output includes three output signals that are out of phase with respect to each other by 120°.  
         [0014]     The autotransformer filter  116  filters out undesirable harmonic components from the input  118 . Also, in accordance with some embodiments of the invention, the autotransformer filter  116  also boosts an amplitude of the output  114  such that the amplitude of the output  114  is greater than (stepped up from or boosted from) the amplitude of the input  118 . According to an embodiment, the autotransformer filter  116  steps up the voltage of each input signal to a higher voltage at the output  114 . Boosting the output voltage from the autotransformer filter  116  allows compensation for voltage loss caused by resistance of the cable  112 . The voltage drop along a relatively long cable (such as 25 kilometers or greater) can be substantial.  
         [0015]     In addition, by filtering out undesirable harmonic components in each input signal from output signal, resonance due to the load impedance provided by the cable  112  and motor  104  is reduced or eliminated. The ability of the autotransformer filter  116  to both perform filtering and amplitude boosting tasks reduces complexity in the equipment used for providing signals down the cable  112  to the motor  104 , since use of separate filter and transformer units can be avoided.  
         [0016]      FIG. 2  illustrates components of the autotransformer filter  116  in greater detail. As depicted in  FIG. 2 , a three-phase input  200  is provided to the variable speed drive power circuits  120 , which produces the three-phase input  118  to the autotransformer  116 . The three-phase input  118  from the variable speed drive system includes three signals  118 A,  118 B,  118 C that are out of phase with respect to each other. Each signal  118 A,  118 B,  118 C from the variable speed drive power circuits  120  is a pulsed DC voltage signal that switches between positive and negative voltages.  
         [0017]     The signals  118 A,  118 B,  118 C from the variable speed drive power circuits  120  are provided to the autotransformer filter  116 . Each signal  118 A,  118 B,  118 C is provided to a tap point of a respective transformer  202 A,  202 B, and  202 C. Each transformer  202 A,  202 B, and  202 C includes a primary coil and secondary coil. A node of the primary coil of each of the transformers  202 A,  202 B, and  202 C is connected to a common node N 1 . A node of the secondary coil of each of the transformers  202 A,  202 B, and  202 C is connected to a respective output signal  114 A,  114 B, and  114 C (which are part of the three-phase output  114  from the autotransformer filter  116 ).  
         [0018]     Also, the output signals  114 A,  114 B, and  114 C are connected to respective capacitors  204 A,  204 B, and  204 C. The inductance of a respective transformer  202 A,  202 B, and  202 C and capacitance of a respective capacitor  204 A,  204 B, and  204 C cooperate to provide a filter to filter out certain harmonic components in a respective input signal  118 A,  118 B,  118 C. In other words, the inductance of the transformer  202 A cooperates with the capacitance of the capacitor  204 A to provide a filter for input signal  118 A; the inductance of the transformer  202 B cooperates with the capacitance of the capacitor  204 B to provide a filter for input signal  118 B; and the inductance of the transformer  202 C cooperates with the capacitance of the capacitor  204 C to provide a filter for input signal  118 C.  
         [0019]     According to one embodiment, the harmonic components that are filtered out by the filters include high frequency components of each pulsed DC voltage input signal  118 A,  118 B, or  118 C. Filtering the high-frequency harmonic components in each input signal  118 A,  118 B,  118 C produces a sine wave at a respective output signal  114 A,  114 B,  114 C. The term “sine wave” refers to a waveform of a signal that can be exactly a sine wave or approximately or generally a sine wave. Approximately or “generally” a sine wave means that a signal has a waveform shape resembling a sine wave. Each sine wave signal at the output  118  of the autotransformer  116  has reduced resonance (or no resonance) when communicated to the load impedance represented by the cable  122  and motor  104 . Resonance can cause vibrations that may produce harmful results in the electrical system that includes the variable speed drive power circuits  120  and motor  104 .  
         [0020]     The tap point  203 A,  203 B, and  203 C of the respective transformer  202 A,  202 B, and  202 C that connect to input signal  118 A,  118 B,  118 C enables selection of the amount of boosting for the voltage amplitude of the input signal to the voltage amplitude of the output signal. Varying the tap point  203 A,  203 B, and  203 C of the transformers  202 A,  202 B, and  202 C allows variation of the amount of boosting or stepping up of the amplitude of the output signal. Boosting or stepping up of the amplitude of an output signal of the autotransformer filter  116  refers to receiving an input signal at the autotransformer filter  116  having a first amplitude, and increasing the amplitude to a second, greater amplitude that defines the amplitude of the output signal from the autotransformer  116 .  
         [0021]     Varying of the tap point  203 A,  203 B, and  203 C also allows the inductance of the transformer  202 A,  202 B, and  202 C seen by the input signal  118 A,  118 B, and  118 C to be varied, such that the filters provided by the autotransformer filter  116  can be adjusted.  
         [0022]     As depicted in  FIG. 2 , the autotransformer  116  includes both an amplitude boosting portion and a filtering portion. The amplitude boosting portion includes the transformers  203 A,  203 B, and  203 C. The filtering portion includes the inductance provided by the transformers  203 A,  203 B, and  203 C, and respective capacitors  204 A,  204 B, and  204 C. Use of the autotransformer  116  results in more simplified implementation of equipment associated with the variable speed drive power circuits  120 , which reduces costs and likelihood of equipment failure. Also, the autotransformer filter  116  provides variable tap points (at the transformers  203 A,  203 B, and  203 C) that enable adjustment of the amplitude boosting and filtering provided by the autotransformer  116 .  
         [0023]     In the foregoing description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details. While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.