Rockwell Automation’s Artificial Lift Solution Helps Operators Thrive in Challenging Times

By Tim Shea

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Summary
Most oil & gas industry headlines over the last several years have focused on the impact of lower oil prices and how oil and gas companies have been struggling to operate profitably based on this reality. In this challenging environment, ARC Advisory Group believes that owner-operators, independent E&P firms, and other stakeholders could benefit ts1.JPGfrom partnering with automation suppliers that can provide enhanced automation solutions to help lower costs, improve energy efficiency of critical equipment, and increase production.

Artificial lift is one of the solutions that can benefit from enhanced automation. Artificial lift provides a means to overcome bottom-hole pressure so a well can produce at a desired flow rate. It works by either injecting gas into the producing fluid column to reduce its hydrostatic pressure, or using a downhole pump to provide additional lift pressure downhole. Most oil and gas wells operated around the world require some form of artificial lift during their lifecycle.

ARC's research indicates that one of the more effective ways to optimize artificial lift is to optimize the associated drives and motors.

Electric submersible pumps (ESP) are some of the most sophisticated and common artificial lift systems deployed in the oilfield today. However, ESPs also have number of challenges that create problems impacting energy usage and uptime. Most ESPs today rely on traditional induction motors and variable speed drives. However, induction motors are not always ideal for every lift application. An alternative is permanent magnet synchronous motors (PMSM), which when combined with the right AC drive, can provide a viable option for certain applications.

Advantages of PMSMs for Electric Submersible Pumps
Like an induction motor, the PMSM has a classic three-phase stator. However, unlike an induction motor, the rotor in the PMSM has permanent magnets, so no heat is created when the rotor magnetic field is produced. The use of a permanent magnet to generate a substantial air gap magnetic flux makes it possible to design highly efficient PMSMs. These typically have advantages over induction motors relative to efficiency, size, and full torque when running at low speeds. However, PMSMs do require AC drives to operate.

Rockwell Automation is a strong advocate for using PMSMs within ESPs. As we learned in a recent briefing with company executives, Rockwell Automation’s PowerFlex 755 AC drive is designed to maximize PMSM performance within ESPs, especially in deep well pumping applications. Most oilfield players try to match PMSMs with certain drives, but the PowerFlex 755 is completely motor-agnostic and was tested successfully with different motors – two-pole, four-pole, as well as an electric submersible progressing cavity pump (ESPCP). Test with ESPs were conducted at 120 Hz and 40 percent motor load, at 120 Hz and 100 percent load, and also at 150 Hz and 100 percent load. Test results demonstrated smooth waveforms and energy savings.

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According to Rockwell Automation, a key differentiator in the PowerFlex 755 AC drive in artificial lift applications is that it is built to operate in traditional open loop when induction motors are used. However, when used with PMSMs, the drive switches to closed-loop control at between 5 to 15 Hz. In this manner, it operates much like a stepper motor or servo drive to ensure very smooth wave forms, save energy, reduce harmonic distortions, and enable higher load operating conditions, which likely helps improve uptime of the ESP.

Rockwell Automation claims that users will realize the following control advantages when combining the PowerFlex 755 AC drive with a PMSM in ESP applications:

  • Provide a true vector algorithm that tracks rotor position/flux angle on the PMSM

  • Offers a torque/amps maximum algorithm that takes reluctance torque into account

  • The combination of these two features maximizes energy efficiency for deep well pumps and keeps unity power factor for all load levels

  • Field tests of the technology demonstrate a 20-50 percent efficiency gain with a PMSM/ESP system versus an induction motor/ESP system based on high load conditions

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The PowerFlex 755 AC drive operates in three modes including starting, transition, and closed loop.

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Key product features of the PowerFlex 755 AC drive include:

  • PowerFlex 755 10HP to 400HP (frames 2 to 8); 14A to 518A output; 4kHz fixed carrier frequency

  • 480V AC input, 3PH, 60Hz

  • Permanent magnet motor control for ESPs

  • Sensorless Flux Vector Mode

  • Frequency operation up to 150Hz

  • Tested with flat cable and round cable up to 20,000 feet

  • Open transformer test mode to test drive integrity and transformer taps prior to connecting to the down-hole pump

  • Supports electronic data sheets (EDS) for configuration through RSlink and RSNetworks

  • Supports add-on profile compatibility with RSLogix 5000 software between the 17.xx and 21.xx versions and future RSLogix 5000 releases

Users can convert a PowerFlex 755 AC drive to PMSM control for ESPs in three steps:

  • Replace the standard control board with the PMM firmware control board (frame dependent)

  • Install three standard analog I/O cards into the PowerFlex 755 AC drive option slots

  • Install a voltage sense module kit external to the drive and wire each phase to one of the analog inputs in the drive

According to Rockwell Automation, other key points include:

  • 15 patents on algorithms around PMSM control and seven patents are pending. Some key patent areas include:

    • Method for maximum starting torque

    • Stable open loop starting control, with transition scheme to sensor-less position control

    • Steady state and instantaneous SWF capacitor current compensation

    • Method for speed reversal control

    • Method of estimating rotor position

    • Method of vector control based on equivalent Back-EMF approach

  • Torque pulses assist with clearing debris in the pump during a normal drive start

  • The carrier frequency of the drive is fixed at 4 KHz and tuned sine-wave filters can be sourced from partners, such as MTE

  • Users can replace control board with drive with special algorithm board

     

Rockwell Automation has already shipped more than 80 drives, of which 50 are installed on ESPs with the PMSM algorithm in South America for a joint venture of two public companies.

Conclusion
Companies operating artificial lift systems in their oilfields, especially deep pumping wells, today face constant price and margin pressures, increasing focus on capital and operating costs, the “Great Crew Change,” growing global demand for energy, and increasingly stringent regulatory oversight. It appears that, with its PowerFlex 755 AC drive operating with PMSMs and ESPs and the company’s significant domain knowledge, Rockwell Automation is well-positioned to help its customers in the challenging oil and gas business increase energy efficiencies, improve production and pump availability, reduce TCO, and realize operational excellence.

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Keywords: Rockwell Automation, PowerFlex 755, Artificial Lift, Permanent Magnet Synchronous Motor (PMSM), Electric Submersible Pump (ESP), AC Drives, ARC Advisory Group.

 

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