The present invention relates to an electrical power system for a motor within a wellbore for use in a downhole power system, as well as to a method of powering a downhole motor for use in such a borehole electrical system. More specifically, the present invention relates to boosting the output of variable frequency drives employed to power electrical submersible pumps within wellbores.

US 4 928 771 discloses a centrifugal pump which is lowered into a wellbore having a downhole three-phase electrical motor, wherein a single-phase AC power is supplied from the surface down a conductor, with the return being the casing. A phase converter converts the single-phase AC power to three-phase AC power downhole for driving the pump motor.

US 4 541 041 discloses a control for a voltage-fed series resonant inverter regulating the output voltage of the inverter from no-load to full-load and reducing circulating continuous current in the resonant elements which can result in excessive power dissipation during no-load and light-load operation.

Electrical power is frequently transmitted to subterranean locations within boreholes to power downhole equipment, such as electrical submersible pumps (ESPs). Normally three phase electrical power is transmitted from the surface over cables running between the well casing and the production tubing.

In some downhole applications, high voltage electrical power is required. For example, electrical motors for ESPs may require voltages of 1,000 to 5,000 volts at the surface. However, electrical drives capable of providing output voltages at the required level may not be available, or may not be economical even when available. When lower output voltage drives are employed in such situations, typically step-up transformers at the output of the drive are utilized to boost the voltage of power transmitted downhole. Step-up transformers add to the expense of the system, however, and add additional sources of failure or disturbance to the electrical system.

There is, therefore, a need in the art for a system allowing an electric drive having a maximum output voltage lower than required to be utilized to power downhole equipment while eliminating the need for step-up transformers. It would further be advantageous to smooth the output of a pulse width modulated variable frequency drive while boosting the output voltage.

To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in powering downhole equipment, a sine wave filter including an inductor for each phase (three inductors) and three delta- or Y-connected capacitors. The sine wave filter is coupled within a three phase power system at the surface, between the output of a variable frequency drive and a three phase power cable transmitting power to a borehole location to boost the output voltage of the drive. The sine wave filter is designed to have a resonant frequency higher than the maximum operational frequency of the drive, and a Q such that, at the maximum operational frequency of the drive, the filter provides a voltage gain equal to the ratio of the desired voltage to the drive's maximum output power at the maximum operational frequency. The sine wave filter also smooths the voltage waveform of a pulse width modulated variable frequency drive.

The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention.

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which:

• FIGURE 1 depicts a three phase electrical power system employed to power downhole equipment according to one embodiment of the present invention;
• FIGURES 2A-2B illustrate in greater detail circuit diagrams for sine wave filters employed within a three phase electrical power system for downhole equipment according to one embodiment of the present invention; and
• FIGURE 3 depicts a plot of gain versus frequency for a sine wave filter employed within a three phase electrical power system according to one embodiment of the present invention.

FIGURES 1 through 3, discussed below, and the various embodiment used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device.

FIGURE 1 depicts a three phase electrical power system employed to power downhole equipment according to one embodiment of the present invention. The electrical power system 102 located at the surface of a borehole is coupled to a motor and pump 104 adapted for use within a borehole and disposed within the borehole by connection to tubing lowered within the well casing. Motor and pump assembly 104 includes an electrical submersible pump (ESP) in the exemplary embodiment, which may be of the type disclosed in U.S. Patent No. 5,845,709, coupled to an induction motor. The induction motor drives the ESP and is powered by three phase power transmitted over three phase transmission cable 106 electrically coupling motor and pump assembly 104 to a surface power system including generator 108 and drive 110.

Three phase transmission cable 106 include separate conductors for each electrical power phase and transmits power from the surface power system including generator 108, which produces three phase power, coupled to variable frequency drive (VFD) 110, designed to provide the appropriate voltage waveform at a selected frequency within a defined operating frequency range for powering motor and pump assembly 104. In the exemplary embodiment variable frequency drive 110 is a pulse width modulated (PWM) drive operationally regulated by a controller 112. Controller 112 for drive 110 changes the output frequency of drive 11o by altering the width of pulses forming the output voltage in accordance with the known art. Other suitable existing power electronics inverters may be employed for drive 110.

In the present invention, drive 110 may have a maximum output voltage (anywhere within the operating frequency range) which is lower than a voltage required for powering motor and pump assembly 104 disposed within the borehole. Drive 110 may be a low voltage drive having a maximum output voltage of only 480 volts (V), for example, while motor and pump assembly 104 may include a medium voltage motor requiring 1,000 V to 4,000 V at the surface. (Surface voltages are referenced since the cable 106, which may be thousands of feet long, will cause significant attenuation between the surface voltage and the voltage at the motor downhole.) Alternatively, drive 110 may have a maximum output voltage of 4,160 V, while a surface voltage of 5,000 V is requires to power motor and pump assembly 104. To boost the output voltage of drive 110, a sine wave filter 114 is coupled within the three phase power system 102 between the output of drive 110 and three phase cable 106 carrying power into the borehole.

While the sine wave filter 114 is preferably located at the surface, alternatively the sine wave filter may located downhole proximate to the motor, in which case the parameters of interest are the received input voltage at the input of the sine wave filter 114 received from the surface and the required motor voltage.

FIGURES 2A and 2B illustrate in greater detail circuit diagrams for sine wave filters employed within a three phase electrical power system for downhole equipment according to one embodiment of the present invention. Sine wave filter 114a depicted in FIGURE 2A includes three inductors L_A, L_B, and L_C each serially connected within a phase A, B and C, respectively, of the three phase power system between the output of the variable frequency drive and the three phase power cable 106 transmitting the power downhole. Sine wave filter 114a also includes three delta-connected capacitors C_AB, C_BC, and C_AC between phases A and B, between phases B and C, and between phases A and C, respectively, of the three phase power system.

Sine wave filter 114a depicted in FIGURE 2B also includes three inductors L_A, L_B, and L_C each serially connected within a phase A, B and C, respectively, of the three phase power system, but contains three Y-connected capacitors C_A, C_B, and C_C connected within phases A, B and C of the three phase power system, between the respectively phase and a common or neutral point.

In either implementation (114a in FIGURE 2A or 114b in FIGURE 2B), inductors L_A, L_B, and L_C each have the same inductance L, and either capacitors C_AB, C_BC, and C_AC or capacitors C_A, C_B, and C_C each have the same capacitance C (although the capacitance C of, for example, C_A is not necessarily the same as capacitance C of C_AB). The inductance L and capacitance C are selected to provide a filter voltage gain for three phase power at a maximum operational frequency of the variable frequency drive which is preferably equal to the ratio of the desired voltage for powering downhole equipment to the maximum output voltage of the drive.

FIGURE 3 depicts a plot of gain versus frequency for a sine wave filter employed within a three phase electrical power system according to one embodiment of the present invention. The sine wave filter 114a or 114b is tuned to have a resonant frequency f₀ which is offset from (higher than) the maximum operational frequency f_max of the variable frequency drive. The resonant frequency of the filter may be determined from: $$f_0=\frac{1}{2\pi \sqrt{3\mathit{LC}}}\mathrm{.}$$
The sine wave filter is also designed to have a quality factor Q, when excited by three phase power, which is greater than one. The quality factor Q may be determined from: $$Q=\frac{3\left(2\pi \right)f_{0}L}{R},$$
where R is the resistance of the sine wave filter components. The sine wave filter quality Q represents the gain of the filter at resonance, and thus the sine wave filter is capable of boosting the output voltage of the variable frequency drive by a factor equal to--or nearly equal to--the filter Q at the resonant frequency.

Because the drive frequency changes, however, it is not desirable to match the resonant frequency of the sine wave filter to the maximum operational frequency of the variable frequency drive. The high Q required to minimize filter losses under such circumstances would provide too much gain at the maximum operating frequency. Also, operating very close to the peak of the filter's resonance frequency would place operations on a very steep part of the filter's gain curve (gain plotted as a function of frequency, illustrated in FIGURE 3), making voltage regulation difficult.

Therefore, the sine wave filter is designed to have a resonant frequency offset from (and preferably higher than) maximum operating frequency of the variable frequency drive, on a portion of the frequency-dependent gain curve for the filter which is sufficiently gradual to permit voltage regulation (i.e., preferably within the range of voltage variances supported by the drive).

For example, if the maximum operational frequency of the variable frequency drive is 80 Hertz (Hz), the sine wave filter may be tuned to have a resonant frequency within the range of 90 Hz to 200 Hz, or more likely within the range of 90 Hz to 120 Hz. The filter is preferably always tuned for a resonant frequency higher than the drive's maximum operating frequency due to the need for a positive volts-per-Hertz ratio.

Since the gain G will vary with the frequency of the three phase power exciting the sine wave filter, the filter is preferably designed to provide a maximum gain G_max at the maximum operating frequency f_max of the drive. The maximum gain G_max is preferably equal to the ratio of the desired or required (surface) voltage to the maximum output voltage of the drive. In one of the examples described above, the sine wave filter would be designed to have a gain at the maximum operational frequency of the drive (e.g., 80 Hz) equal to 5,000/4,160, or about 1.2. In embodiments in which the filter resonant frequency is higher than the maximum operating frequency of the sine wave filter, the sine wave filter 114 will also have a minimum gain G_min at the minimum operational frequency f_min of the drive. It would be desirable, but is not necessary, for the minimum gain G_min to be greater than one.

The inductances and capacitances required to obtain a desired resonant frequency fo, and/or maximum gain G_max at the maximum operating frequency f_max of a particular generator/drive configuration, for the sine wave filter 114, may be determined utilizing existing electrical simulation programs.

Referring back to FIGURE 1, when excited by the output of drive 110 (utilizing power received from generator 108) filter 114 will (at least partially) resonate at the output frequency of drive 110, thus increasing the output voltage of filter 114 over the output voltage of drive 114 by a factor equal to the gain G of the filter 114 at the output frequency of drive 110. By tuning filter 114 to a resonant frequency above the maximum output frequency f_max of drive 110, the voltage boost provided by filter 114 will follow the output frequency of drive 110. In operation of electrical power system 102, the output voltage of filter 114 is connected by feedback loop 116 to controller 112. Controller 112 may thus monitor and regulate the output voltage of filter 114, altering the output voltage of filter 114 by controlling the output voltage and/or the output frequency of drive 110.

For a pulse width modulated variable frequency drive, sine wave filter 114 has the additional benefit of smoothing the voltage output of drive 110 into a very sinusoidal signal. For electrical submersible pumps, such smoothing of the power signal prevent problems from resonant frequencies and reflected waves, in addition to boosting the output voltage of the drive 110.