Abstract : Surface direct-drive screw pumps are oil pumping machines developed from screw pumps. They utilize permanent magnet synchronous motors to drive the polished rod directly, reducing the need for mechanical reducers and belt reducers, resulting in high efficiency. This paper introduces the application of INVT's GOODRIVE300 open-loop vector inverter in surface direct-drive screw pumps, demonstrating reliable control performance and high energy efficiency. This paper analyzes a 37kW GOODRIVE300 inverter as an example and provides application solutions for energy-saving retrofitting of surface direct-drive screw pumps.
Keywords : frequency converter, ground direct drive screw pump, Invt
1 Introduction
Surface direct-drive screw pumps are a type of oil extraction machinery that has developed rapidly in recent years. They are particularly suitable for crude oil extraction from high-viscosity, high-gas-content, high-sand-content, high-water-content, and low-production wells. They effectively overcome phenomena such as gas lock and sand blockage, and have a small footprint and high efficiency. Their advantages in increasing production and saving energy are becoming increasingly apparent. Surface direct-drive screw pumps are driven by permanent magnet synchronous motors, eliminating the need for belt drives and two-stage gear reduction. They are equipped with INVT's new generation open-loop vector inverter GOODRIVE300 and use electromagnetic braking to slowly release the elastic deformation of the pump rod. The entire system is simple and compact, easy to operate, and runs efficiently and smoothly.
2. Introduction to Ground Direct Drive Screw Pumps
The permanent magnet synchronous motor direct-drive screw pump consists of a mechanical transmission system and an electrical control system. It eliminates traditional belt drives, gearboxes, and anti-reverse mechanisms. A low-speed permanent magnet synchronous motor directly drives the polished rod to power the downhole screw pump, as shown in Figure 1. The power transmission method is as follows: the motor torque is transmitted from the motor hollow shaft to the sealed connecting sleeve, then to the square clamp, and finally to the polished rod. The polished rod extends through the motor hollow shaft and connects to it via the square clamp, transmitting the motor torque to the pump rod. The axial load-bearing and mechanical seal devices at the wellhead are designed within the lower end face of the motor and the hollow shaft, thus achieving an integrated design of axial load-bearing, mechanical seal, and power motor. The sealing assembly combines dynamic and static seals to ensure no leakage of produced fluid. The lower end of the motor connects to the load-bearing section, which bears the weight of the entire pump unit. The load-bearing bearings in the load-bearing section are lubricated with lubricating oil for easy maintenance and oil changes. In addition, an overflow channel was added to the upper flange of the load-bearing device housing to drain the tiny leaks of liquid generated during normal operation of the mechanical seal, preventing oil emulsification and reducing lubrication.
Figure 1. External view and structural diagram of the permanent magnet synchronous motor direct drive screw pump.
Direct drive eliminates the need for a gearbox, which not only improves system efficiency but also avoids problems that often occur in traditional screw pump oil extraction systems, such as the inconvenience of replacing pulleys, adding lubricating oil, changing production parameters, and management and maintenance.
3. Technical Features of the Direct-Drive Screw Pump Drive Control System Based on GOODRIVE300
3.1 Advantages of using permanent magnet synchronous motor drive
The mechanical transmission efficiency of a conventional screw pump is only around 80%, while the transmission efficiency can reach 98% when the polished rod is directly driven by a motor. The drive motor of a direct-drive screw pump is a permanent magnet synchronous motor. Although permanent magnet synchronous motors cannot be directly started, they have the following characteristics:
(1) Small size, high power density and compact structure.
(2) High operating efficiency. The rotor speed is completely synchronized with the stator rotating magnetic field, resulting in no rotor loss compared to asynchronous motors; compared to ordinary electrically excited synchronous motors, the rotor does not require an external excitation power supply, eliminating excitation losses. Therefore, the rated efficiency of permanent magnet synchronous motors can reach over 94%. Permanent magnet synchronous motors have high efficiency under light loads, reaching up to about 96% of the rated value, while the efficiency of asynchronous motors under light loads is far lower than their rated value. Therefore, under conditions where most of the operating time is under light load, the energy-saving effect of permanent magnet synchronous motors is more ideal. Overall, the efficiency of permanent magnet synchronous motors is 6%~10% higher than that of induction motors of the same specifications.
(3) High power factor. Since the rotor is a permanent magnet, the stator side of the permanent magnet synchronous motor does not need to provide excitation current, and the inductor voltage drop is relatively small. The power factor is relatively high in both light and heavy load ranges, which ensures that a high power factor is maintained throughout the entire power range. Since the power factor of the asynchronous drive motor is low when running under light load, the reactive power saving effect of using the permanent magnet synchronous motor is quite significant.
(4) It has a large starting torque, strong overload capacity, and is suitable for multiple pole pairs. It can drive the load directly without a transmission box.
3.2 GOODRIVE300 Permanent Magnet Synchronous Motor System
Ground direct-drive screw pump oil pumping units have high starting torque and require high-performance drive controllers. The INVT new generation GOODRIVE300 frequency converter uses the latest 32-bit DSP chip from TI, which has vector control specifically for permanent magnet synchronous motors and has high motor control performance and accuracy. Its main technical features [1] are as follows:
●The starting torque of the permanent magnet synchronous motor with open-loop vector control is 2Hz/200%;
●Overload capacity: 150% rated current for 60 seconds; 180% rated current for 10 seconds;
● Open-loop control accuracy can reach 0.2% of the maximum speed;
●Supports switching control of 2 sets of synchronous motors;
●Supports Modbus, Profibus, Ethernet, and CAN bus communication protocols;
●Features speed tracking function, easily enabling shock-free motor start-up at any time;
●0-second acceleration time, allowing for direct startup without overload;
● Comes with a digital potentiometer as standard and supports multiple flexible frequency adjustment modes;
● Supports parameter copying and downloading;
● Up to 36 types of fault codes are recorded, including the first 6 fault codes and detailed fault information for the first 3.
● Flange installation is available, saving customers installation space;
The GOODRIVE300 uses current injection to detect the initial magnetic pole position within 30ms, ensuring high starting torque and preventing the motor from losing steps. The control system is simple, and its starting performance, speed accuracy, and stability fully meet the requirements of the operating conditions.
Because the shutdown process of a screw pump oil extraction unit system is relatively complex, there are safety hazards during reversal. If the reversing torque is too large and the speed becomes uncontrollable, it can cause serious consequences such as the sucker rod breaking. The GOODRIVE300 frequency converter has a speed tracking function. When the screw pump speed becomes uncontrollable, it can smoothly and shocklessly start the frequency converter (motor), allowing its counter-torque to be released slowly under control.
The GOODRIVE300 supports communication functions such as Modbus and Profibus, providing standard communication interfaces for building oilfield automation and digitalization. Furthermore, preventing reverse rotation of screw pump pumping units has always been the biggest control issue for this type of pumping method. If other controllers (such as PLCs) are used to control the frequency converter, anti-reverse rotation programs can be programmed to ensure safe and reliable shutdown to the greatest extent possible.
The GOODRIVE300 variable frequency drive permanent magnet synchronous motor exhibits high overall efficiency (variable frequency converter + motor) of up to 93% under different frequencies and loads (average data from tests conducted at a machinery plant of Jianghan Petroleum). The screw pump pumping unit operates under constant torque load, exhibiting different optimal operating conditions under varying well conditions. By controlling the screw pump speed to the minimum while ensuring maximum fluid production per well, the motor's operating power can be minimized. Combined with the high efficiency of the entire system, the single-well pumping system will achieve optimal operation. From a macro perspective, this can also save costs for the entire oilfield, contributing positively to economic development.
4 Electrical Control Scheme for Ground Direct Drive Screw Pumps Based on GOODRIVE300
4.1 Control scheme for preventing reverse rotation using shutdown procedures in digital oilfields
In response to national development strategies, to enhance core competitiveness, and to achieve energy conservation and emission reduction goals, domestic oil companies are accelerating the upgrading and transformation of their technological equipment, with digitalization becoming a development trend in oilfield construction. A typical digital well site solution is shown in Figure 2.
Figure 2. Typical well site digitalization scheme
In a digital solution for a direct-drive screw pump, the frequency converter can be controlled via communication. If the start-up, shutdown, and frequency setting of the frequency converter are entirely controlled by communication, a shutdown program can be programmed into the controller to slowly release the counter-torque when automatic control (communication control) is used. When a shutdown command is issued, the GOODRIVE300 switches the frequency converter control mode to torque mode and then sends a braking torque signal from the controller. After a certain delay, the frequency converter stops.
In actual oilfield operation, to improve the reliability of pumping unit control, a backup terminal-connected control mode is typically provided. This creates a manual/automatic control system for the entire direct-drive screw pump. Manual control uses a terminal-controlled frequency converter, while automatic control uses a communication-controlled frequency converter. The frequency converter is directly connected to the main circuit, and control mode switching is achieved through terminal switching.
The scheme of slowly releasing the counter-torque using a frequency converter requires the frequency converter to be equipped with a braking unit and a braking resistor. The elastic deformation energy stored in the sucker rod can be dissipated by the braking resistor. Its main circuit and control circuit are shown in Figures 3 and 4.
Figure 3 System main circuit
Figure 4 System control circuit
4.2 Energy Consumption Braking Anti-Reverse Scheme
If the digitalization level of an oilfield is relatively backward, a local manual control + energy-consumption braking anti-reverse method can be adopted. Taking a well site in Dongying, Shengli Oilfield as an example, this control scheme is illustrated. The requirement is that the motor speed reaches 200 rpm and the starting torque reaches 1600 N·m under full load driven by the frequency converter. The motor parameters are shown in Table 1.
Table 1 Motor Parameters
Motor rated power | 31kW |
Rated current | 64A |
Rated frequency | 40Hz |
Extreme logarithm | 12 |
Rated voltage | 380V |
Rated output torque | 1500 N·m |
The permanent magnet synchronous motor of the ground-driven screw pump has a power of 31kW and a rated current of 64A. Considering its high starting torque load characteristics, a GOODRIVE300-037G/045P-4 frequency converter is selected for drive. Because of the elastic counter-torque present when the screw pump stops, the sucker rod will reverse at high speed. If not controlled, this will inevitably cause the sucker rod to disengage. Therefore, an electrical energy-consumption braking method can be adopted to prevent reverse reversal. This is achieved using a bleed resistor to release the electrical energy generated by the motor shaft rotation due to the elastic deformation energy stored in the sucker rod. The circuit uses an AC contactor group with normally open/normally closed contacts. During normal operation, the main contacts of the AC contactor are closed, connecting the driver to the torque motor windings, and the motor operates normally. When power is off, the main contacts of the AC contactor open, disconnecting the motor windings from the driver. Simultaneously, the auxiliary contacts of the AC contactor close, short-circuiting the motor windings to the three energy-consumption resistors. If the screw pump reverses direction, the motor becomes a generator, and the generated electricity is consumed by the resistor. This means the mechanical energy generated by the deformation of the drive rod is converted into heat energy through the resistor. At this point, the motor rotor will slowly reverse direction until it stops, achieving the protection purpose. The electrical control principle diagram of a ground-driven screw pump is shown in Figure 5.
Figure 5. Electrical control schematic diagram of a ground direct-drive screw pump.
The surface direct-drive screw pump oil pumping unit is operated from the inverter cabinet door panel. The inverter control cabinet door panel has start, stop, and fault reset buttons. The corresponding inverter control scheme is a terminal control method. If the well site is to be digitized in the future, the Modbus communication protocol standardly configured in the GOODRIVE300 will provide a digitization solution for the well site. The inverter control cabinet layout and the standard wiring diagram of the GOODRIVE300 inverter control I/O are shown in Figure 6.
Figure 6. Layout of the frequency converter control cabinet and standard wiring diagram of the GOODRIVE300 frequency converter control I/O.
The GOODRIVE300 frequency converter, through its own wiring terminals and supplemented by electrical control circuits, enables local control of the screw pump. The entire system is installed in a frequency converter cabinet, and the proper design of the frequency converter cabinet is the key to ensuring its long-term stable operation under harsh conditions (such as dusty areas).
The system was connected according to the electrical control scheme wiring diagram. After on-site testing, the starting torque reached more than 1800 N·m. Under the condition of not exceeding 420 volts, the frequency converter could drive a full-load motor at a speed of 230 rpm, and the current was always maintained at about 64.5A. The output current waveform was also very good, exceeding the user's requirements.
5. Conclusion
Through long-term operational observation in oilfields and feedback from frontline staff, the surface direct-drive screw pump pumping unit system driven by GOODRIVE300 has performed well, achieving energy savings of over 30% compared to traditional asynchronous motor-driven screw pumps with reduction gears. Currently, after nearly a year of practical operation, INVT's GOODRIVE300 has received high praise from oilfield staff in both Shengli and Daqing oilfields. Practice has proven that GOODRIVE300 has reliable performance in driving permanent magnet motors for direct-drive screw pumps. The frequency converter itself also boasts numerous technical features; as a general-purpose frequency converter with functions specifically designed for synchronous motors, GOODRIVE300 will provide a fundamental solution for energy-saving renovations and digital infrastructure development in oilfields.
