[Abstract] Surface direct-drive screw pump oil production systems have become an effective mechanical oil production equipment due to their simple structure, low noise, low energy consumption, low investment, ease of use, and convenient maintenance. This paper uses the Delta C2000 frequency converter as the control center, detailing the application methods of the frequency converter in the operation and shutdown processes of the screw pump. Furthermore, combining electrical control principles, it rationally solves the problem of reverse rotation in direct-drive screw pumps, making the system operation more reliable and stable.
【Abstract】The oil production system of ground driving screw pump with simple structure,lower noise,low energy consumption and less investment,convenient to use,easy maintenance etc become a kind of effective mechanical oil production equipment.This paper sets up to C2000 inverter as the control center,introduces in detail application methods of the inverter in the process of screw pump operation and stop,combining with the electrical control principle,solve the problem of the reversal of direct driving screw pump, make the system more reliable and stable.
[Keywords] Direct-drive screw pump; C2000 frequency converter; permanent magnet synchronous motor; energy-consuming resistor; PLC function
【Keywords】Straight drive screw pump;C2000 inverter;Permanent-magnet synchronous motor;Energy resistance;PLC functions
1. Introduction
Currently, the most commonly used types of oil pumping units in China are beam pumping units, tower pumping units, and screw pumping units. Among them, screw pumping units have developed rapidly in recent years. They are mechanical oil extraction equipment suitable for extracting crude oil with high viscosity, high sand content, and high gas-oil ratio. As a simple, efficient, economical, and energy-saving artificial lifting method, screw pumping units have been widely used in the extraction of heavy oil and crude oil containing sand and water.
Screw pump oil production systems are further divided into surface-driven and downhole-driven types. Surface direct-drive screw pumps use ordinary screw pumps downhole, driven at the wellhead. They are simple in structure, technologically mature, and are currently the most common type of screw pump used in oilfields across China. Surface direct-drive screw pumps are driven by permanent magnet synchronous motors and equipped with Delta C2000 high-order magnetic flux vector universal frequency converters. During shutdown, a two-stage braking system using both the frequency converter and an external resistor is employed. The entire system is simple, compact, reliable in operation, and achieves excellent energy-saving results.
2. Composition and Working Principle of Direct-Drive Screw Pump System
As an excellent artificial lift method, the surface direct-drive screw pump system has shown more obvious advantages than other oil production methods in heavy oil wells and sand-carrying oil wells.
2.1 System Composition
The surface direct-drive screw pump system consists of a bottom-hole screw pump, a sucker rod string, a sucker rod centralizer, and a surface drive system. During operation, the surface power drives the sucker rod string to rotate, and the screw pump rotor connected to the bottom of the sucker rod rotates along with it. Well fluid is drawn in from the bottom of the screw pump, discharged from the top, flows out of the wellhead through the tubing, and is then transported to the metering station through surface pipelines.
2.2 Working Principle
Screw pumps discharge oil through cavities, which are isolated, closed chambers formed between the rotor and stator. As the rotor rotates, these closed cavities move axially from the suction end to the discharge end. At the discharge end, the closed cavities disappear, and the crude oil within them is uniformly squeezed from the suction end to the discharge end. Simultaneously, new low-pressure cavities are formed at the suction end to draw in crude oil. This continuous formation, movement, and disappearance of closed cavities constantly fills, squeezes, and discharges crude oil, thereby continuously drawing oil from the well and lifting it to the wellhead through the tubing.
Figure 1. Screw pump structure diagram
3. Reverse Reversal Characteristics of Direct-Drive Screw Pumps
Screw pumps can reverse when they lose external drive or stop. There are generally two reasons for this: First, when the screw pump is working, the torque of the drive motor is transmitted to the pump body through the nearly kilometer-long sucker rod. The sucker rod will inevitably undergo elastic deformation and a torsional angle, thus storing elastic energy. When the drive motor has no driving torque, the sucker rod will drive the mechanical system to twist in the opposite direction. Second, when the drive system stops, due to the liquid level difference in the oil casing, the nearly kilometer-long oil will fall back from the oil pipe under the action of gravity, and the pressure of the liquid surface will also cause the screw pump rotor to reverse.
Reverse rotation of a screw pump can cause the rod to disengage, the polished rod to bend, damage to components of the ground drive unit, and may also endanger the safety of operators. Therefore, it is particularly important to properly address the reverse rotation problem of direct-drive screw pumps.
4 System Design
Delta C2000 frequency converters use open-loop control to operate permanent magnet synchronous motors, directly driving downhole screw pumps via sucker rods. When the system stops, the frequency converter uses its built-in PLC programmable function to pre-control the screw pump to reverse, allowing the reverse torque of the screw pump to be released slowly under controlled conditions. At the same time, based on the generator characteristics of the permanent magnet motor, appropriate energy-consuming resistors are added to each phase winding of the motor rotor to make the screw pump system operate more reliably.
4.3 Control Principle of Direct Drive Screw Pump
The direct-drive screw pump oil pumping unit is operated on the door panel of the frequency converter cabinet. The door panel of the frequency converter control cabinet has start, stop, and fault reset buttons, as well as operation indicator lights and a potentiometer knob for adjusting the motor speed (as shown in Figure 3). The frequency converter cabinet can be operated in real time on site.
The working states of a direct-drive screw pump can be divided into forward rotation, inverter braking, and energy consumption braking.
4.1 Features of C2000 Frequency Inverter
The Delta C2000 high-order magnetic flux vector universal frequency inverter integrates drive (IM/PM), control (PLC), display (text), and multiple protocol fieldbus technologies, with built-in CANOpen for use as a master station. It features high performance, easy maintenance, and multi-language operation. Specifically, it integrates asynchronous and synchronous motor control, offering multiple control modes and enabling open-loop and closed-loop motor control; starting torque can reach over 150% at 0.5Hz, and it supports heavy-load mode; overload capacity is up to 200% of torque current; built-in PLC function with a program capacity of 10K steps; powerful panel editing function, equivalent to text-based operation, with text prompts for panel parameter settings, making it easy to use; modular design, with detachable control terminals and fans, and support for through-wall installation; rich network expansion functions, with built-in CANOpen, expandable with ProfiBUS, DeviceNet, MODBUS TCP, EtherNet-IP fieldbus and Ethernet cards; 50℃ operating ambient temperature, and global safety compliance with CE/UL/CUL.
4.2 Open-loop commissioning method for permanent magnet synchronous motors
In open-loop mode, the C2000 frequency converter drives a permanent magnet synchronous motor. When using the frequency converter for the first time with the motor, it needs to learn the motor parameters and perform function settings. The relevant setting steps are as follows: Set 00-02 to 9 to reset to factory defaults; then set 05-33=1 sequentially. PM motor, 05-35 motor power, 05-37 motor poles, 01-00 maximum frequency, 01-02 rated voltage, 05-34 motor current, 05-36 motor speed, 05-38 motor inertia, 01-01 rated frequency setting parameters; set 05-00 to 13, then press the RUN button on the digital panel. The motor will then automatically detect the motor. After several tens of seconds, the detection will be complete. If the detection is correct, continue to the next step. If the detection fails, please check if the parameter settings in step two are correct and re-detect; set 00-10=0 (speed control), 00-11=6 (control mode), and set 01-12 and 01-13 acceleration/deceleration times as required; set 10-39 (IM and PM motor control switching point), generally 0.2 times the maximum frequency, but not less than 5; set 11-00 to 1 (inertia estimation), and set 11-01 system inertia value;
Figure 2. Commissioning steps for permanent magnet synchronous motors
4.3 Control Principle of Direct Drive Screw Pump
The direct-drive screw pump oil pumping unit is operated on the door panel of the frequency converter cabinet. The door panel of the frequency converter control cabinet has start, stop, and fault reset buttons, as well as operation indicator lights and a potentiometer knob for adjusting the motor speed (as shown in Figure 3). The frequency converter cabinet can be operated in real time on site.
The working states of a direct-drive screw pump can be divided into forward rotation, inverter braking, and energy consumption braking.
Figure 3 Variable frequency control cabinet panel
4.3.1 Forward Rotation Status
During the oil extraction phase, the C2000 frequency converter drives the permanent magnet synchronous motor to maintain forward rotation through the built-in PLC function. The motor speed is adjusted by the potentiometer on the control cabinet. The frequency converter outputs the operating and fault status of the frequency converter through RA1 and RA2. The wiring diagram of the frequency converter control part is shown in Figure 4 (MI1 is the fault reset input).
Figure 4 Wiring diagram of the inverter control section
Before operating the C2000 frequency converter, the PLC function needs to be enabled through the operation panel, and the control program needs to be input into the frequency converter through the RS485 interface using Delta WPLSoft software.
The inverter's default communication format is 9600, 7, N, 2, and the communication station number is 2 (inverter PLC station number). This must be consistent with the communication settings in Delta WPLSoft for the computer to communicate correctly with the inverter. The forward rotation program is shown in Figure 5.
Figure 5. PLC program for forward rotation of the frequency converter
4.3.2 Inverter Braking State
When the control system issues a shutdown command, the C2000 frequency converter first uses the PLC function to brake the screw pump to reverse (using torque detection braking or constant speed timed braking) to prevent the elastic potential energy stored in the sucker rod and the effect of the hydrostatic column in the annulus of the tubing from causing the sucker rod to reverse at high speed and resulting in serious consequences.
Through the primary braking method of the Delta C2000 frequency converter, the reverse torque of the screw pump is released slowly under controlled conditions.
Relevant data of the frequency converter during reverse operation (such as running time, current change value, etc.) can be input through unused parameter items of the frequency converter, such as items 04-00, 04-01, etc.
The reverse part of the PLC program is shown in Figure 6.
Figure 6. PLC program for inverter reversal
4.3.3 Energy Consumption Braking State
After the inverter stops braking output, the power connection between the inverter and the permanent magnet synchronous motor is disconnected using the inverter's operating status output point. At the same time, the rotor winding of the permanent magnet synchronous motor is connected to the three-phase external energy-consuming resistor to start the secondary braking (the energy-consuming braking principle diagram is shown in Figure 7).
Figure 7 Schematic diagram of energy-saving braking principle
The braking process is as follows: the sucker rod, due to its stored reversing energy, drives the motor rotor in reverse, and the motor's angular velocity gradually increases. Because of the characteristics of a permanent magnet motor, a permanent magnet magnetic field exists on the rotor, thus inducing an electromotive force (EMF) in the motor windings. This induced EMF forms a circuit through the energy-dissipating resistor, generating a current. This current produces a braking torque in the opposite direction of rotation. The smaller the resistance, the greater the braking torque, the less heat energy is converted, and the longer the sucker rod remains stationary. Therefore, the optimal energy-dissipating resistor value must be selected based on the specific well conditions.
The energy-consuming resistor braking method not only achieves the effect of secondary braking, but also ensures that the energy of the screw pump is effectively controlled and released when the frequency converter or control cabinet fails, making the system control more reliable and safe.
5. System energy saving
During the oil extraction process, the oil conditions may change at any time. The C2000 frequency converter can automatically detect the motor output through its built-in PLC function and adjust the output frequency or torque reasonably through internal calculations. According to the oil plant's oil production targets, the oil extraction workers can change the output frequency manually or at regular intervals, which satisfies the dual function of manual and automatic control. It not only saves electricity but also saves manpower and material resources, and has been well received by users.
6. Conclusion
Since its commissioning in October 2011 at multiple surface direct-drive screw pump wellheads in a well-known oilfield in China, the Delta C2000 frequency converter has demonstrated stable operation, significant energy savings, and ease of operation, receiving positive feedback from customers.
Due to the simple, efficient, economical, and energy-saving manual lifting method of direct-drive screw pumps, their application in oil fields will become more widespread. Since 2007, nearly 2,000 direct-drive screw pump wells have been put into use annually in this well-known oil field. Although the open-loop synchronization function of the Delta C2000 frequency converter was introduced relatively late, its stable and efficient output function is in no way inferior to that of well-known foreign brands. It is hoped that this article will play a positive role in promoting the Delta C2000 frequency converter in the oilfield surface direct-drive screw pump industry.
About the author:
Zhao Chengliang, male, born in July 1984, graduated from Jiamusi University with a major in Automation. He currently works as an Application Engineer in the Northeast China Business Support Department of Delta Electronics, providing technical support and promotion for Delta's electromechanical products in the Heilongjiang region, and has extensive industry experience.
