Abstract: This paper mainly introduces the application of INVT's Goodrive300 open-loop vector frequency converter in an oil pump at a branch of China National Petroleum Corporation. The Goodrive300 frequency converter plays an important role in its digital oil transportation system, demonstrating reliable control performance and high energy efficiency. The control of the oil pump includes manual/automatic switching control. The PLC control system and the Goodrive300 series frequency converter's manual-to-frequency switching system together constitute the core control system. This paper analyzes its application using a 15kW Goodrive300 frequency converter as an example and provides a promotion plan for the application of INVT's Goodrive300 open-loop vector general-purpose frequency converter in oil pumps.
Keywords: frequency converter, oil pump, remote control, PLC
I. Introduction
A branch of PetroChina is a regional oilfield company under PetroChina, operating in the Ordos Basin, China's second largest basin, spanning five provinces (regions): Shaanxi, Gansu, Ningxia, Inner Mongolia, and Shanxi, with a total exploration area of 370,000 square kilometers. This oilfield has implemented digital management, fully utilizing automatic control technology, computer network technology, reservoir management technology, data integration technology, and data sharing and exchange technology to improve the monitoring level of processes and establish a unified digital management platform for production management and comprehensive research across the entire oilfield, thereby saving human resources and improving efficiency.
The following diagram shows the digital management process of a booster station in one of the company's oilfield operating areas.
Currently, with the continuous increase in crude oil water content, changes in production, and increased resistance in external pipelines in most oilfields in my country, oilfield pumps generally suffer from inefficiency and high energy consumption. To address this issue, many oilfields have adopted variable frequency drive (VFD) technology for motors, replacing outlet regulating valves with adjustments to the pump motor speed, thereby reducing pressure loss at the pump outlet and achieving significant energy savings. In the digital construction process of oilfield booster stations, the requirement to adjust the start-up, shutdown, and speed of the oil pump based on the buffer tank level is a crucial aspect of achieving intelligent oil transportation. Since the oil pump is driven by an asynchronous motor, motor control is paramount. INVT's Goodrive300 VFD not only offers significant energy savings but also meets the intelligent speed control requirements of the oil transportation process.
II. Application Environment Analysis of Variable Frequency Oil Pumps
The well sites under the jurisdiction of one of the company's oil production areas are located in the mountainous area of Yan'an, Shaanxi Province. The well sites are not concentrated. The oil collected by the pumping units at the well sites is transported through oil pipelines to the booster station. After necessary simple treatments such as metering and degassing at the booster station, the oil is pumped out to the joint station and then to the oil distribution point for unified external distribution.
To achieve digital and intelligent oil transportation and save energy, the oilfield well station proposed a variable frequency control oil transportation upgrade scheme. The scheme requires the ability to automatically start and stop the oil pump and adjust the pump speed based on the buffer tank level. The booster station has two oil pumps, one for backup and one for use. Under normal conditions, the pumps are required to be controlled by variable frequency and intelligently adjusted. When the buffer tank level is too high and an alarm is triggered, the backup pump will be automatically activated. The entire oil transportation control system is required to be unattended, as shown in the figure below.
The well site provides 380V/50HZ AC input power. Depending on the well site's fluid production volume, the asynchronous motor power of the oil pumps at each booster station varies, ranging from 11KW to 45KW. This results in differences in cabinet size and inverter model required for each station's frequency converter. Based on on-site investigation, the INVT Goodrive300 series frequency converter meets these requirements. This article uses an 11KW oil pump as an example for analysis; the following diagram shows the structure of a centrifugal oil pump.
III. Energy Saving Analysis Using Variable Frequency Speed Control
When the oil pump motor is running, the flow rate is directly proportional to the speed. As the motor speed changes, the flow rate also changes. Therefore, by using a frequency converter, the motor speed, flow rate, and pressure can be adjusted to meet the requirements of the current production process. During frequency conversion operation, the motor speed changes, and the motor load current also changes accordingly. Naturally, a decrease in motor speed leads to a decrease in the frequency converter output voltage, and consequently, a decrease in the oil pump motor load current.
Once the pump and motor are determined, the system's energy efficiency is primarily related to the pump's remaining head, and secondarily to the pump's operating state (whether it operates in the high-efficiency zone). Variable frequency speed control improves energy efficiency from these two aspects, thereby achieving energy savings. Theoretically, with appropriate adjustments, the remaining head can be reduced to zero, allowing the pump to operate in its high-efficiency zone. For systems with a high remaining head, the energy-saving effect is particularly significant.
By using variable frequency speed control technology, the pump speed can be adjusted so that the pump characteristic curve shifts from 1 to 2 (as shown in the figure above) [1]. The new operating point of the pipeline system is point C. The pump outlet head is equal to the total pressure drop of the pipeline. There is no remaining head and no throttling loss, so the energy utilization rate is high. According to the similarity principle, the efficiency of the pump is approximately equal after speed adjustment, which can ensure that the pump works in the high-efficiency zone. Therefore, the energy saving benefit is very considerable.
IV. Variable Frequency Cabinet Design Scheme
Based on INVT's years of product experience and the existing equipment and requirements on site, a variable frequency drive (VFD) retrofit plan for the control cabinet was ultimately determined. The motor parameters of the on-site oil pump are shown in the table below:
The frequency converter and the industrial frequency conversion circuit are installed in the frequency converter cabinet. Local industrial frequency conversion switching is performed on the door of the frequency converter cabinet. Local start and stop of industrial frequency is also designed on the door. Local start and stop of frequency conversion are directly operated by the keyboard. Line interface for direct start and stop of pump room is reserved. The structure of the frequency converter cabinet after main installation is shown below.
The circuit design of the frequency converter cabinet is shown below.
The inverter I/O control wiring is not enabled; only a fault signal needs to be provided to the PLC. The inverter function code uses version V4.19. The inverter communicates with the Siemens PC via RS485, and the Siemens PLC is configured as the intelligent control master station throughout the booster station. The monitoring room can manually and remotely start and stop the oil pumps and control their speed through the PC's operating interface. The Siemens PLC module collects buffer tank liquid level data for automatic control of the oil pump's start and stop. When the liquid level is too high, the standby pump will automatically start, and the variable frequency oil pump will also run at a variable frequency, achieving unattended operation.
The main control method used by the frequency converter in this solution is RS485 remote communication control. In the power distribution control room of each booster point, there are not only frequency converter control cabinets for the oil pumps, but also, in some cases, frequency converter control cabinets for the water injection pumps, which often have higher power than the oil pumps. There are also PLC control cabinets in the power distribution control room. The entire power distribution control room is generally only about 8-12 square millimeters. Therefore, our frequency converter control cabinets operate in an environment with electromagnetic interference, and their EMC requirements must pay attention to two aspects: interference and immunity. Since there is only general radiated interference, shielded twisted-pair cable can be used for the RS485 communication line of the frequency converter. Other control lines with strict requirements (such as the water injection pump communication line) should also use shielded twisted-pair cable. Field operation has shown that this is reliable.
The use of variable frequency speed control for oil transportation has achieved two major breakthroughs in process and control technology: First, the buffer tank level is segmented, and the pump operating frequency is different in different level segments, thereby controlling the operation of the mixed pump more accurately and effectively; Second, the characteristics of the mixed pump operation in winter and the current oil volume are fully considered, and the pump start and stop levels are set so that the pump stop time does not exceed 1 hour, which protects the pump itself and prevents the oil-water mixture in the downstream pipeline from freezing due to prolonged stagnation.
V. Analysis of Operational Results
With the advancement of the oilfield's digital construction project, the oil production plant has basically achieved digital management of booster points, and all oil pumps have implemented variable frequency drive (VFD) operation. After a long period of on-site operation, the advantages of VFD speed regulation have become apparent:
(1) The power consumption of the booster station with variable frequency speed regulation is 40% lower than that of the booster station without variable frequency speed regulation, and the power saving effect is better.
(2) The worse the matching of the oil pump, that is, the lower the load rate of the booster station, the better the energy saving effect after frequency conversion modification. When the load rate of the external pump of the booster station is higher than 85%, the energy saving effect is significantly reduced.
(3) Under the condition of similar frictional resistance along the pipeline, the unit consumption of oil transportation during automatic operation of frequency conversion is lower than that during manual operation. Statistical results show that the unit consumption of oil transportation at booster stations with automatic frequency conversion operation is lower than that of booster stations with manual frequency conversion operation. Analysis suggests that during automatic operation of frequency conversion, the motor speed is automatically adjusted according to the liquid level, which can ensure that the transportation capacity matches the demand in real time; while during open-loop operation of frequency conversion, the motor speed needs to be manually adjusted by the operator according to the liquid level. If the amount of oil transportation at the booster station is unstable, the operator's adjustment is far less effective than the automatic adjustment by the equipment, which will inevitably lead to an increase in the unit consumption of oil transportation.
VI. Conclusion
Through long-term operational observation and feedback from frontline employees, the unmanned automatic variable frequency continuous oil transfer operation has proven reliable, fully meeting the production process requirements of the booster points. It eliminates accidents such as buffer tank overflows that are easily caused by manual operation, improving production safety, reducing labor intensity, and achieving overall energy savings of over 20%. This has been well-received by frontline operators and the oilfield. The variable frequency automatic control solution for the oil transfer pumps is now widely used at various booster points throughout the oilfield. The INVT Goodrive300 open-loop vector general-purpose frequency converter boasts high reliability and rich functionality, and has been operating extensively and for a long time at oilfield booster points.
