For decades the pump jack has been a common sight in oil fields, especially across western North America. Historically, their most common driver has been a low-voltage, three-phase induction motor. The beam pump, or sucker rod pump, is driven with a cam action that has the effect of loading, then unloading the driver (motor). This loading and unloading occurs on every rotation of the cam.
Needing a motor that can go from unloaded to loaded has led the industry to rely on Design D motors. These motors operate with high slip, or the inherent characteristic of producing high torque, as they “catch”
the load. Another common feature of the oil field motor is having it wound so that it is connectable on multiple voltages, usually 230/460/796 volts.
Traditionally, Design D motors have been controlled by pump-off control (POC). Variable frequency drives (VFDs), with or without POC, are more common. An initial benefit of using VFDs is that users can choose a more traditional and common Design B motor. Operating via the VFD, these motors are more efficient than the uncontrolled Design D motor.
By connecting a VFD to a supervisory control and data acquisition (SCADA) system, the operator can remotely monitor the system. A VFD with software specifically designed for artificial lifting further enables this capability. While today’s VFD technology follows the same laws of physics as those designed a decade ago, advancement in the design and manufacturing of hardware and software enable them to have a higher power density and improved reliability.
The VFD’s output section, also referred to as an inverter, is typically configured with insulated gate bipolar transistors (IGBT) that are controlled by software using some type of pulse-width modulation (PWM) algorithm. An IGBT-based inverter always creates common mode voltages (CMV) to the output circuit. CMV can cause bearing currents on the motor and can migrate throughout the rod pumping station, potentially causing disturbances in measurement and programmable logic controller (PLC) systems. The most effective way to control CMV effects is through proper VFD cabling practices—using VFD rated output cables, having a cable gland with a 360-degree shield, and connecting the VFD, motor and cabling to ground.
The VFD’s input, also called a rectifier, can be configured as a traditional six-pulse diode rectifier, a multi-pulse rectifier with matching transformer or an active front end (AFE), where rectifier diodes have been replaced with IGBTs. Diode-based rectifier units draw blocks of current from the supply side, creating harmonics.
Harmonic current will create additional heating and needs to be added to supply dimension. Harmonic current also causes harmonics in the voltage, distorting sinusoidal supply voltages. A stiff (high short-circuit capacity) supply network can withstand voltage distortion to higher load currents, so the relationship between short circuit current (Isc) and demand load current (Iload) is important.
Rod pump units can be fed from a local generator plant or utility via high line. Utilities typically require IEEE 519 compliance at the service entrance. IEEE 519 harmonic limits are set at the point of common coupling (PCC) to be met at continuous maximum load. At a small rod pump station, the application motor’s VFD load may be the main load. To comply with the IEEE 519 standard, 18-pulse VFDs typically require a sizeable additional load. However, a VFD with an AFE complies on its own. An AFE also can feed energy back to the supply network, which reduces operational costs and negates the need for a braking chopper and matching resistor bank.
CAPEX vs. OPEX
When power is fed from a generator plant, the operator can decide whether compliance with IEEE 519 is necessary. Oil fields, for example, have many direct current drives still in operation. The thyristors or silicon controlled rectifiers (SCR) create disturbance to the supply network due to their commutation notches. The harmonic limits on this network typically are not clean enough to meet IEEE 519 requirements at PCC, and these systems operate just fine. The user should calculate capital expenditure (CAPEX) and operational expenditure (OPEX) to determine which solution is the most beneficial.
For CAPEX, a VFD with AFE is typically slightly more expensive than a six-pulse VFD with chopper and resistor bank, but the lack of harmonics allows optimal size generator plant. For OPEX, a VFD with AFE always has a power factor of 1 by default, does not require operation of an oversized power plant and does not waste produced energy to the resistor bank via the chopper.
VFDs are applied to a system to provide better control of a process. VFD software provides control because it is enabled by more powerful processors and increased memory inside the VFDs. The control technology in today’s drives usually uses vector control based on flux or direct torque control (DTC). The improved motor model inside the software, together with better and more accurate torque control, empowers the VFD to consume the minimum current from the supply and maintain the desired rotational speed. Modern VFD software can have separate speed controllers for upstroke and downstroke.
Today, a VFD also can “learn.” For example, it can adjust the pump’s speed based on varying flow rate, minimizing wear of the rods. Having fewer sensors and inputs than traditional POC should be a significant benefit because the cost of unexpected shutdowns resulting from faulty sensors or inputs can be high.
The Internet of Things (IoT) and pumping stations on oil fields are no longer strangers. SCADA and other similar remote-monitoring systems are becoming more common and more extensive. The VFD can be a great source of information for operators and maintenance professionals. By trending the speed, current and temperature, the operator gains valuable information regarding the condition of the station.
Additionally, VFDs typically have extra input/output (I/O) points, which might be suitable for other monitoring devices or operations, providing savings over external relay logic or PLC I/O units. VFD systems today have either built-in communication capabilities or communications that may be added as optional cards to enable seamless communication to any supplier’s SCADA system. Modern VFDs can provide a status package to be emailed—or email it directly if connected to the internet—to the supplier’s support line for analysis.