Regenerated Power Accumulator for Rod Lift Drive

Abstract

A variable speed drive for use in connection with a beam pumping unit has a pair of DC bus rails that include a positive DC bus rail and a negative DC bus rail. The variable speed drive further comprises a rectifier section, an inverter section connected between the DC bus rails and a primary capacitor bank. The primary capacitor bank includes a primary capacitor and an overvoltage switch. The variable speed drive further includes an auxiliary capacitor bank that has an auxiliary capacitor, a charge diode connected between the auxiliary capacitor and the overvoltage switch, and a discharge diode connected between the auxiliary capacitor and the positive DC bus rail.

Description

FIELD OF THE INVENTION

This invention relates generally to oilfield equipment, and in particular to a drive system for surface-mounted reciprocating-beam, rod-lift pumping units.

BACKGROUND

Hydrocarbons are often produced from wells with reciprocating downhole pumps that are driven from the surface by pumping units. A pumping unit is connected to its downhole pump by a rod string. Although several types of pumping units for reciprocating rod strings are known in the art, walking beam style pumps enjoy predominant use due to their simplicity and low maintenance requirements.

Beam pumping units and their upstream drive components are exposed to a wide range of loading conditions. These vary by well application, the type and proportions of the pumping unit's linkage mechanism and counterbalance matching. The primary function of the pumping unit is to convert rotating motion from the prime mover (engine or electric motor) into reciprocating motion above the wellhead. This motion is in turn used to drive a reciprocating down-hole pump via connection through a sucker rod string.

The “4-bar linkage” comprising the articulating beam, pitman, cranks, and connecting bearings processes the load from the polished rod into one component of the gear box torque (well torque). The other component, counterbalance torque, is adjusted on the pumping unit to yield the lowest net torque on the gearbox. Counterbalance torque can be adjusted in magnitude but typically not in phase (timing) with respect to the well load torque.

Counterbalance may be provided in a number of forms ranging from beam-mounted counterweights, to crank-mounted counterweights (as shown in FIG. 1), to compressed gas springs mounted between the walking beam and base structure to name only a few. The primary goal in incorporating counterbalance is to offset a portion of the well load approximately equal to the average of the peak and minimum polished rod loads encountered in the pumping cycle. This technique typically minimizes the torque and forces at work on upstream driveline components reducing their load capacity requirements and maximizing energy efficiency.

Although generally effective at offsetting a portion of the load produced by the downhole components of the reciprocating pumping system, the rotating mass of the crank-mounted counterweights are difficult to rapidly adjust under advanced control schemes. The elasticity of the sucker rod string may present an oscillatory response when exposed to variable loads. The motion profile of the driving pumping unit combined with the step function loading of the pump generally leaves little time for the oscillations to decay before the next perturbation is encountered. The flywheel effect produced by massive rotating components within the pumping unit resists rapid changes in speed. Attempts to substantially alter speed within the pumping cycle have generally consumed disproportionately more power which negatively affects operating cost.

In many cases, the beam pumping unit is driven by an electric motor (prime mover) that is controlled by a variable speed drive. The electric motor is connected to the gearbox to rotate the crankshaft arms according to a control scheme. During portions of the reciprocating cycle, the motor is energized by the drive to apply torque to the crankshaft through the gearbox. In other portions of the cycle, the weight of the rod string, counterbalance weights, and other portions of the beam pumping unit cause the electric motor to passively rotate without the application of drive current. During this portion of the pumping cycle, the electric motor acts as a generator and produces a current that may be fed back into the variable speed drive.

To accommodate the generated current, the variable speed drive may include a dynamic braking resistor (DBR) that is configured to release excess energy as heat. Prior art variable speed drives often include a rectifier, a capacitor bank, a dynamic braking section, and an inverter motor output section connected along common bus rails for connecting a three phase power input to a three phase output provided to the motor.

When the motor is drawing power, diodes in the charging section charge the capacitor bank and an insulated-gate bipolar transistor (IGBT) bridge arrangement in the inverter motor output section modulates capacitor voltage to control current in the motor windings. When the motor is regenerating power, due to braking action, as the rod string pulls the walking beam downward, for example, diodes in the inverter motor output section transfer power to the capacitor bank, causing the capacitor bank voltage to rise. If the capacitor bank voltage exceeds a threshold amount, a dynamic braking resistor can be temporarily switched on to rheostatically dissipate the energy from the variable speed drive.

Although generally accepted, the reliance on dynamic braking resistors to control overvoltage situations can be inefficient as a large portion of the regenerated energy is intentionally dissipated as heat. There is, therefore, a need for an improved variable speed drive that more efficiently manages electrical current generated by the motor of a beam pumping unit.

SUMMARY OF THE INVENTION

In one aspect, embodiments of the present invention a variable speed drive for use in connection with a beam pumping unit has a pair of DC bus rails that include a positive DC bus rail and a negative DC bus rail. The variable speed drive further comprises a rectifier section, an inverter section connected between the DC bus rails and a primary capacitor bank. The primary capacitor bank includes a primary capacitor and an overvoltage switch. The variable speed drive further includes an auxiliary capacitor bank that has an auxiliary capacitor, a charge diode connected between the auxiliary capacitor and the overvoltage switch, and a discharge diode connected between the auxiliary capacitor and the positive DC bus rail.

In another aspect, embodiments of the present invention include a motor drive assembly for controlling the operation of a plurality of beam pumping units. In these embodiments, the motor drive assembly includes a central auxiliary capacitor bank and a plurality of variable speed drives. Each of the plurality of variable speed drives is connected between the central auxiliary capacitor bank and a corresponding one of the plurality of beam pumping units.

In yet another aspect, embodiments of the present invention include an energy efficient drive system for a beam pumping unit. The energy efficient drive system includes a variable speed drive, an external switch connected to the DC bus rails of the variable speed drive, and an auxiliary capacitor bank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a beam pumping unit and well.

FIG. 2 is a circuit diagram for a variable speed drive constructed in accordance with an exemplary embodiment.

FIG. 3 is a depiction of multiple beam pumping units connected to a common auxiliary capacitor bank.

WRITTEN DESCRIPTION

FIG. 1 shows a beam pumping unit 100 constructed in accordance with an exemplary embodiment of the present invention. The beam pumping unit 100 is driven by a prime mover 102, typically an electric motor or internal combustion engine. The rotational power output from the prime mover 102 is transmitted by a drive belt 104 to a gearbox 106. The gearbox 106 provides low-speed, high-torque rotation of a crankshaft 108. Each end of the crankshaft 108 (only one is visible in FIG. 1) carries a crank arm 110 and a counterbalance weight 112. The reducer gearbox 106 sits atop a sub-base or pedestal 114, which provides clearance for the crank arms 110 and counterbalance weights 112 to rotate. The gearbox pedestal 114 is mounted atop a base 116. The base 116 also supports a Samson post 118. The top of the Samson post 118 acts as a fulcrum that pivotally supports a walking beam 120 via a center bearing assembly 122.

Each crank arm 110 is pivotally connected to a pitman arm 124 by a crank pin bearing assembly 126. The two pitman arms 124 are connected to an equalizer bar 128, and the equalizer bar 128 is pivotally connected to the rear end of the walking beam 120 by an equalizer bearing assembly 130, commonly referred to as a tail bearing assembly. A horse head 132 with an arcuate forward face 134 is mounted to the forward end of the walking beam 120. The face 134 of the horse head 132 interfaces with a flexible wire rope bridle 136. At its lower end, the bridle 136 terminates with a carrier bar 138, upon which a polish rod 140 is suspended.

The polish rod 140 extends through a packing gland or stuffing box 142 on a wellhead 144 above a well 200. A rod string 146 of sucker rods hangs from the polish rod 140 within a tubing string 148 located within the well casing 150. The rod string 146 is connected to a subsurface pump 202. In a reciprocating cycle of the beam pumping unit 100, well fluids are lifted within the tubing string 148 during the rod string 146 upstroke. In accordance with well-established rod lift pump design, a stationary standing valve and reciprocating traveling valve cooperate to lift fluids to the surface through the tubing string 148.

The motor 102 is driven by a variable speed drive 152. The variable speed drive 152 receives a source of electrical power from a power source such as an established electrical grid or a generator. The input current to the variable speed drive 152 may be processed through an input transformer to adjust the voltage of the current. Turning to FIG. 2, shown therein is a simplified depiction of the major circuits within, or connected to, the variable speed drive 152. In the exemplary embodiment depicted in FIG. 2, the variable speed drive 152 includes a rectifier (converter) section 154, an inverter section 156, a primary capacitor bank 158 and an auxiliary capacitor bank 160. Each of the sections of the variable speed drive 152 is interconnected by common DC bus rails (+, −) 162, 164.

In accordance with well-established motor drive technology, the rectifier section 154 converts the input voltage to the variable speed drive 152 to direct current coupled to the DC bus rails 162, 164. The inverter section 156 is utilized to convert the DC bus voltage to a variable frequency AC signal, in response to motor drive control commands in the variable speed drive 152. In some configurations, the output from the inverter section 156 is provided directly to the motor 102 or to an intermediate step-up transformer (not shown). The inverter section 156 of the variable speed drive 152 can be configured to produce a six-step commutation sequence that can be adjusted manually or automatically to adjust the operating parameters of the beam pumping unit 100. The inverter section 156 may include a plurality of insulated-gate bipolar transistors (IGBTs) arranged in a bridge configuration to selectively adjust the output of the variable speed drive 152.

The primary capacitor bank 158 includes a primary capacitor 166 and an overvoltage switch 168. The primary capacitor 166 is charged by the DC bus rails 162, 164. During a driving mode of operation, the voltage within the primary capacitor 166 is drained as current is applied to the motor 102 by the variable speed drive 152. During a braking mode of operation, the motor 102 acts as a generator to produce current and a corresponding retarding torque. The current generated by the motor 102 is fed back to the variable speed drive 152 and charges the primary capacitor 166. Although a single primary capacitor 166 is depicted within FIG. 2, it will be appreciated that the primary capacitor bank 158 may include a plurality of primary capacitors 166 linked together to provide an aggregated capacitance.

In exemplary embodiments, the overvoltage switch 168 is an insulated-gate bipolar transistor (IGBT). The overvoltage switch 168 can be switched on to connect the auxiliary capacitor bank 160 to the primary capacitor bank 158 through the DC bus rails 162, 164. The auxiliary capacitor bank 160 generally provides the variable speed drive 152 with sufficient capacity to minimize the risk of the voltage across the DC bus rails 162, 164 exceeding established thresholds. Unlike prior art systems in which an overvoltage switch would be used to route excess voltage for dissipation on a dynamic braking resistor (DBR), the overvoltage switch 168 of the variable speed drive 152 is configured to route excess voltage generated during a braking mode of operation to the auxiliary capacitor bank 160 for storage and subsequent use during a driving mode of operation.

The auxiliary capacitor bank 160 includes an auxiliary capacitor 170, a charge diode 172 and a discharge diode 174. Although a single auxiliary capacitor 170 is depicted, it will be appreciated that the auxiliary capacitor bank 160 may include a plurality of auxiliary capacitors 170 linked together to provide an aggregated capacitance. In exemplary embodiments, the combined capacitance of the primary capacitor 166 and the auxiliary capacitor 170 is designed to accommodate more than the maximum anticipated charge produced by the motor 102 during the regenerating (braking) phase of operation.

The charge diode 172 permits the unidirectional flow of current from the DC bus rail 162 to the anode side of the auxiliary capacitor 170. The discharge diode 174 permits the unidirectional flow of current from the anode side of the auxiliary capacitor 170 to the DC bus rail 162. In this way, the charge diode 172 and discharge diode 174 cooperate to automatically control the charge at the auxiliary capacitor 170 based on differences in voltage between the DC bus rail 162 and the anode side of the auxiliary capacitor 170. The auxiliary capacitor bank 160 optionally includes a resistor 176 between the charge diode 172 and the auxiliary capacitor 170. The resistor 176 can be configured to reduce the voltage at the anode side of the auxiliary capacitor 170 so that it remains less than the voltage on the DC bus rail 162.

During a braking mode of operation, current generated by the motor 102 is fed to the primary capacitor bank 158. As the voltage in the primary capacitor 166 begins to rise toward a predetermined limit, the overvoltage switch 168 is automatically closed to connect the auxiliary capacitor bank 160. If the voltage on the DC bus rail 162 is greater than the voltage at the anode side of the charge diode 172, current flows through the charge diode 172 to the anode side of the auxiliary capacitor 170. The voltage at the auxiliary capacitor 170 continues to rise during the regeneration (braking) phase of the pumping cycle, but current does not flow out of the auxiliary capacitor 170 until the voltage on the DC bus rail 162 is less than the voltage at the anode side of the auxiliary capacitor 170.

When the variable speed drive 152 switches to a driving mode of operation, the primary capacitor 166 begins to drain as current is fed through the inverter section 156 to the motor 102. As the voltage on the primary capacitor 166 and the DC bus rail 162 drops, current flows out of the auxiliary capacitor 170 through the discharge diode 174 to the DC bus rail. In this way, the charge in the auxiliary capacitor 170 is released during the driving mode of operation to offset a portion of the power that would otherwise be required to operate the motor 102. It will be appreciated that in well balanced pumping systems, the charge generated by the motor 102 during the downstroke portion of the pumping cycle may not exceed the limits of the primary capacitor 166. In that situation, the overvoltage switch 168 is not activated and the auxiliary capacitor bank 160 remains functionally disconnected from the primary capacitor bank 158.

The auxiliary capacitor bank 160 can be easily retrofitted onto existing variable speed drives, which often include overvoltage switches that direct current to a conventional dynamic braking resistor. In this way, the conventional dynamic braking resistor can be easily replaced with the more efficient auxiliary capacitor bank 160. The auxiliary capacitor bank 160 can be housed inside the variable speed drive 152 or in an appropriate cabinet that is connected to the variable speed drive 152. In other situations, an external switch connected to the DC bus rails 162, 164 can be easily installed to provide a mechanism for connecting the auxiliary capacitor bank 160 to existing variable speed drives 152 that do not include overvoltage switching mechanisms and dynamic braking resistors. In certain applications, it may be desirable to implement the auxiliary capacitor bank 160 in combination with a traditional dynamic braking resistor.

As depicted in FIG. 3, in some applications a common central auxiliary capacitor bank 160 is connected to multiple variable speed drives 152 that are each connected to a motor 102 of a different beam pumping unit 100. In this embodiment, the common auxiliary capacitor bank 160 must be appropriately sized to accommodate the aggregate charge anticipated by the simultaneous regeneration of power from the multiple beam pumping units 100. In this embodiment, the charge and discharge of the common auxiliary capacitor bank 160 remains an automatic function as the charge and discharge diodes 172, 174 control the flow of current to the auxiliary capacitor 170 without intelligent intervention.

It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functions of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.

Claims

1. A variable speed drive for use in connection with a beam pumping unit, the variable speed drive comprising: a pair of DC bus rails, wherein the pair of DC bus rails comprise a positive DC bus rail and a negative DC bus rail;a rectifier section connected between the DC bus rails;an inverter section connected between the DC bus rails;a primary capacitor bank connected between the DC bus rails, wherein the primary capacitor bank comprises: a primary capacitor; andan overvoltage switch; andan auxiliary capacitor bank, wherein the auxiliary capacitor bank comprises: an auxiliary capacitor;a charge diode connected between the auxiliary capacitor and the overvoltage switch; anda discharge diode connected between the auxiliary capacitor and the positive DC bus rail.

2. The variable speed drive of claim 1, wherein the overvoltage switch is selected from the group consisting of insulated-gate bipolar transistors, semiconductor switches and mechanical switches.

3. The variable speed drive of claim 2, wherein the overvoltage switch is an insulated-gate bipolar transistor.

4. The variable speed drive of claim 2, wherein the auxiliary capacitor bank further comprises a resistor between the charge diode and the auxiliary capacitor.

5. A motor drive assembly for controlling the operation of a plurality of beam pumping units, the motor drive assembly comprising: a central auxiliary capacitor bank; anda plurality of variable speed drives, wherein each of the plurality of variable speed drives is connected between the central auxiliary capacitor bank and a corresponding one of the plurality of beam pumping units.

6. The motor drive assembly of claim 5, wherein each of the plurality of variable speed drives is connected to the central auxiliary capacitor bank through an overvoltage switch that permits the central auxiliary capacitor bank to be automatically charged by the corresponding variable speed drive.

7. The motor drive assembly of claim 6, wherein each of the overvoltage switches is selected from the group consisting of insulated-gate bipolar transistors, semiconductor switches and mechanical switches.

8. The motor drive assembly of claim 7, wherein each of the overvoltage switches is an insulated-gate bipolar transistor.

9. The motor drive assembly of claim 5, wherein each of the plurality of variable speed drives comprises: a pair of DC bus rails, wherein the pair of DC bus rails comprise a positive DC bus rail and a negative DC bus rail;a rectifier section connected between the DC bus rails;an inverter section connected between the DC bus rails; anda primary capacitor bank connected between the DC bus rails, wherein the primary capacitor bank comprises a primary capacitor.

10. The motor drive assembly of claim 5, wherein the central auxiliary capacitor bank comprises: an auxiliary capacitor;a charge diode connected between the auxiliary capacitor and the overvoltage switches of each of the plurality of variable speed drives; anda discharge diode connected between the auxiliary capacitor and the positive DC bus rails of each of the plurality of variable speed drives.

11. The motor drive assembly of claim 10, wherein the auxiliary capacitor bank further comprises a resistor between the charge diode and the auxiliary capacitor.

12. An energy efficient drive system for a beam pumping unit, the energy efficient drive system comprising: a variable speed drive,an external switch connected to the DC bus rails of the variable speed drive; andan auxiliary capacitor bank.

13. The energy efficient drive system of claim 12, wherein the variable speed drive comprises: a pair of DC bus rails, wherein the pair of DC bus rails comprise a positive DC bus rail and a negative DC bus rail;a rectifier section connected between the DC bus rails; andan inverter section connected between the DC bus rails.

14. The energy efficient drive system of claim 13, wherein the variable speed drive further comprises a primary capacitor bank connected between the DC bus rails.

15. The energy efficient drive system of claim 14, wherein the primary capacitor bank further comprises an overvoltage switch.

16. The energy efficient drive system of claim 15, wherein the overvoltage switch is selected from the group consisting of insulated-gate bipolar transistors, semiconductor switches and mechanical switches.

17. The energy efficient drive system of claim 16, wherein the overvoltage switches is an insulated-gate bipolar transistor.

18. The energy efficient drive system of claim 12, wherein the auxiliary capacitor bank comprises: an auxiliary capacitor;a charge diode connected between the auxiliary capacitor and the external switch; anda discharge diode connected between the auxiliary capacitor and the positive DC bus rail.

19. The energy efficient drive system of claim 18, wherein the auxiliary capacitor bank further comprises a resistor between the charge diode and the auxiliary capacitor.