Research and Application of Variable Frequency Remote Monitoring and Control System in Oilfields

Abstract: Currently, vector control frequency converters are widely used in many electric drive applications both domestically and internationally for stepless speed regulation of AC motors in industries such as general machinery, textiles, printing and dyeing, papermaking, steel rolling, chemicals, and petroleum. This has yielded significant energy-saving effects and meets process and automatic speed regulation requirements. Energy conservation and consumption reduction in oilfields, and the construction of digital oilfields, have become development goals for China's petroleum industry. Variable frequency speed regulation and wireless communication systems enable real-time monitoring and remote adjustment of the operating status of oil wells, thereby reducing the labor intensity of workers and ensuring that pumping units operate at their optimal state. This article introduces the working principle and application of the PL-ZNCY series variable frequency remote monitoring and control system in oilfields.

one,   Foreword
Most of my country's oil fields are low-energy, low-yield fields, unlike foreign oil fields which have strong self-flowing capabilities. Most oil fields rely on water injection to force oil into wells and on pumping units (nodding donkeys) to lift the oil from the formation. Replacing oil with water or electricity is currently the reality of my country's oil fields. Electricity costs account for a significant proportion of my country's oil extraction costs; therefore, the oil industry attaches great importance to saving energy, increasing production, and reducing equipment maintenance.
Currently, statistics show that my country has over 100,000 oil pumping units, with a total installed capacity of 3500MW and annual power consumption exceeding 10 billion kWh. The operating efficiency of these pumping units is particularly low, averaging only 25.96% in my country, compared to an international average of 30.05%, indicating an annual energy-saving potential of several billion kWh. Besides pumping units, oilfields also have numerous water injection pumps, oil transfer pumps, and submersible pumps, which account for over 80% of the total electricity consumption of oilfields. Therefore, the petroleum industry is a key sector for promoting energy-saving motor systems.
For energy saving in oil pumping units, the preferred solution is to retrofit the motor drive system with a frequency converter. Replacing the oil pumping unit with a frequency converter offers several advantages:
1) It significantly improves the power factor (to over 0.9), greatly reduces the apparent current, thereby alleviating the burden on the power grid and transformers, reducing line losses, and saving a significant amount of "capacity expansion" expenses;
2) The extraction speed can be dynamically adjusted according to the actual fluid supply capacity of the oil well, which can achieve energy saving and increase crude oil production at the same time;
3) Because true "soft start" is achieved, excessive mechanical impact is avoided on the motor, gearbox, and pumping unit, which greatly extends the service life of the equipment, reduces downtime, and improves production efficiency.
However, there are several issues that need to be addressed when using frequency converters for oil pumping unit motors.
(1) Inrush current problem
As shown in Figure 1, a beam pumping unit is a modified four-bar linkage mechanism. Its overall structure resembles a balance scale, with the pumping load at one end and a counterweight load at the other. For the support, if the torque generated by the pumping load and the counterweight load is equal or changes in the same direction, then the pumping unit can operate continuously with very little power. In other words, the energy-saving technology of the pumping unit depends on the quality of the balance. When the balance rate is 100%, the power provided by the motor is only used to lift half the weight of the liquid column and overcome friction. The lower the balance rate, the greater the power required from the motor. Because the pumping load changes constantly, and the counterweight cannot change in exactly the same direction as the pumping load, the energy-saving technology of the beam pumping unit becomes extremely complex. Therefore, it can be said that the energy-saving technology of the beam pumping unit is essentially the technology of balancing.

(2) Electromagnetic interference (EMI) problem, that is, the interference of the frequency converter to the microcomputer controller, sensor (transmitter) and communication equipment. Because the frequency converter is a strong source of electromagnetic interference, the switching devices in the frequency converter and the generated SPWM voltage waveform will cause great interference to the control and communication system. In addition to induction and radiation, the interference also includes conducted interference.

(3) Harmonic interference problem: The inverter generates harmonics on the input side. As long as there is a rectifier circuit, harmonics caused by nonlinearity will be generated in the inverter. According to Fourier series, the fundamental wave and each harmonic can be decomposed, which usually contains 6m+1 (m=1,2,3,...). In the inverter output circuit, both the output voltage and output current have harmonics. Since the inverter controls the three-phase 6-phase power element conduction/switching by the CPU generating 6 sets of pulse width adjustable SPWM waves, it forms a three-phase output voltage with adjustable voltage and frequency. Its output voltage and output current are generated by the intersection of SPWM wave and triangular wave, which is not a standard sine wave. The voltage square wave and current sine sawtooth wave can be analyzed by Fourier series decomposition and it can be found that they contain strong high-order harmonic components.

Therefore, the harmonic generation from frequency converters cannot be ignored. While the impact on large-capacity power systems may not be very noticeable, the harmonic generation in small-capacity power systems is significant. It pollutes the public power grid, deteriorates the environment in which electrical equipment operates, and harms surrounding communication systems and equipment outside the public grid. The main manifestations are as follows: 1. Harmonics cause harmonic losses in electrical components within the power grid, reducing the efficiency of transmission, transformation, and power consumption equipment. 2. Harmonics can cause localized series or parallel resonances in the power grid. 3. Current harmonics increase transformer copper losses. 4. Harmonics cause additional heating in motors, leading to extra temperature rise and affecting motor insulation.
(4) Reliability and environmental adaptability issues
Because oil pumping units operate in harsh outdoor environments, and many wells are unattended, high demands are placed on the reliability and environmental adaptability of frequency converters. This requires selecting frequency converter brands with high reliability ratings and creating the necessary conditions for their operation in harsh outdoor environments. For example, this necessitates designing a high-protection-level double-layer sealed (insulated) control cabinet; for areas with large diurnal temperature variations, a forced-air cooling system is incorporated within the cabinet to expel heat; and a cold air inlet is designed at the bottom of the cabinet to make it suitable for use in the high-temperature desert environment during summer. Furthermore, the control cabinet must be protected from direct sunlight and rain.

II . PL-ZNCY Series Variable Frequency Monitoring and Control System

The PL-ZNCY series variable frequency monitoring and control system is a high-tech system designed by Poly Deep Energy (Beijing) New Energy Technology Co., Ltd., leveraging its own R&D and technical strength to create a humanized, intelligent, and digital oilfield. The company integrates the R&D, design, production, and service of high-tech products, with green energy technology products and industrial automation control products as its core. Products include high-performance vector frequency converters, industrial automation control systems, pumping unit (screw pump) variable frequency intelligent monitoring and control cabinets, solar, wind, and complementary systems, portable power supplies, and field lighting systems. Adhering to the corporate culture of "creating benefits for society, creating value for users, and creating opportunities for employees" and the corporate spirit of "integrity, pragmatism, and professional service," the company has attracted a large number of professional talents with its "innovation, design, and development" philosophy. Strong technical strength, professional solution design, advanced testing equipment, rapid information exchange, and a modern management system have enabled the company to quickly become a high-tech enterprise integrating R&D, manufacturing, marketing, engineering design, installation, and after-sales maintenance services.

The PL-ZNCY series variable frequency monitoring and control system represents a revolutionary breakthrough in energy-saving control technology for oil pumping units. It aims to match the operating mode of the oil pumping unit with the actual load and environmental conditions of the oil well, improving the pumping unit's fill rate, thereby increasing the efficiency and power factor of the motor and achieving energy savings and increased production.

1. The PL-ZNCY series pumping unit frequency conversion monitoring and control system is a high-tech product that integrates advanced technologies from home and abroad and is specially designed for the special load conditions of pumping units. It features frequency conversion control, remote monitoring and other functions.

This system retains many advantages of beam pumping units and combines advanced oil production technology with computer technology, automatic control technology, and power electronics to achieve intelligent control of the pumping unit: stepless adjustment of the pumping unit's stroke rate and upstroke/downstroke speed ratio optimizes pumping parameters, achieving the goals of increased production, energy saving, and automatic control, ultimately improving the system efficiency of the pumping system. The system's human-machine interface is extremely simple, greatly facilitating use and maintenance by on-site personnel. It is particularly suitable for oil wells with extremely harsh working environments, oil wells requiring intermittent operation, oil wells prone to rod breakage, oil wells prone to wax buildup, and oil wells with frequently changing well conditions.

1.1 Product Design Principles:

The key equipment used in this system, the frequency converter, complies with European CE low-voltage electrical appliance standards, international UL508C power conversion equipment standards, and international EN50081/2 electromagnetic compatibility (EMC) standards.

It also features functions specifically designed for oil pumping units, including DC reactors, input reactors, and output reactors, reducing harmonic interference to the power grid and improving grid quality. Harmonic emissions comply with international standard IEC61000-2-2, with each harmonic voltage containing less than 8% THDV. It includes an RFI (Radio Frequency Interference) filter, significantly improving anti-interference and lightning strike protection. An integrated braking unit effectively releases braking energy, forming an integrated control system suitable for the specific applications of oil pumping units. The oil pumping unit's variable frequency monitoring and control system is designed with a "human-centered" philosophy, making the product more user-friendly. It can perform remote status monitoring. Based on the detection of various functional modules and corresponding sensors, the relevant data and parameters measured at the oil well are transmitted wirelessly to the CNC center for calculation and comparison. Vector variable frequency speed regulation technology is used to dynamically monitor and control the oil pumping unit, achieving efficient, energy-saving, automated, and digital control. The user-friendly design allows for remote parameter setting, operation data, status display, dynamometer cards, and daily oil production parameters via HMI (Human-Machine Interface).

1.2 . The frequency converter unit, as the implementation unit of variable frequency speed control technology, is one of the most important components of the entire system. This system is equipped with a vector-type PL-900 series high-performance frequency converter manufactured by Poly Shenyuan (Beijing) New Energy Technology Co., Ltd. The PL-900 series high-performance frequency converter is a continuation of the general-purpose frequency converter, and is equipped with vector control as standard in V/F control. It can monitor motor power and output torque in real time. The frequency converter has a rated voltage of 380-440V, a rated power range of 5.5-75KW, and an overload capacity of 200%*60 seconds.  

Switching loop flux vector control and open/closed loop V/F control can be freely selected.

Suitable for mechanical loads; for fluid loads and actual working conditions, the setting can be reduced accordingly.

Ultra-low speed high starting torque, low speed torque can reach 150% of rated torque (100~300 adjustable).

The speed control range can reach 1:1000 (in closed-loop control mode).

Speed ​​control accuracy can reach 0.02%, frequency accuracy can reach 0.01%, and output frequency resolution can reach 0.01Hz.

The acceleration time can be set to 0.01 seconds, with good dynamic response and extremely strong load adaptability.

It features slip compensation, torque compensation, automatic speed control (ASR), keyboard data copying, and self-learning.

It features a variety of advanced functions such as PID control, timing, energy saving, and anti-chasing.

It features multi-functional input and output terminals, multi-functional analog input and output functions, and a serial communication interface.

It has online monitoring functions for 7 parameters.

It features motor protection, instantaneous power outage handling and stall prevention, frequency detection, abnormal reset, over-torque output, torque limit, hardware protection functions, and diagnostic functions, which will prevent accidental damage to the machine.

The unique and advanced main circuit design and alarm function will fully guarantee the high reliability of the whole system.

It supports seven languages ​​including Chinese, English, and German, and has a user-friendly interface.

1.3 : The motor monitoring unit is a specialized motor monitoring unit developed by our company for oilfield motors. Users can monitor electrical parameters such as line voltage, phase voltage, active power, power factor, and current. It provides alarm and protection functions for the motor, including starting locked rotor protection, overload protection, undervoltage protection, phase loss protection, short circuit protection, and underload protection.  

1.4 : The wireless load displacement module is used to collect dynamometer diagrams, displacement, stroke, and is powered by a solar-powered lithium battery. It operates at a frequency of 426-441MHz, with low-power transmission, high-sensitivity reception, and strong anti-interference capabilities. It complies with European ETSI (EN300-220-1 and EN301-439) and (15.247 and 15.249) certification specifications. It wirelessly transmits oil pressure and temperature data from oil wells, enabling real-time monitoring of the well's operating status.

1.5 Temperature Control Unit: The electrical components in the oil pumping unit control cabinet have strict temperature requirements; therefore, the temperature inside the control cabinet must be monitored and controlled. The main components of this unit include a temperature controller, fan, temperature sensor, and heating system. It has a wide operating range of -40℃ to 75℃ and is particularly suitable for high-temperature deserts and cold northern regions.

1.6 Outdoor standard cabinet, patented design, double-layer insulation, wind and sand resistant, high protection level.

2. Product characteristics:

Designed with the specific conditions of major oilfields in China in mind, this product features a leading-edge, user-friendly design with multiple operating modes including power frequency and variable frequency, manual and automatic, remote and local operation. Remote frequency setting and stroke adjustment significantly reduce the labor intensity of oilfield workers.

This design has been widely used in Daqing Oilfield, Jilin Oilfield, and Karamay Oilfield in China, and has received unanimous praise for its significant energy-saving effect and simple operation.

III. Application Analysis: Energy Saving Analysis   Comparative Test Data Table of Intelligent Variable Frequency Monitoring and Control Cabinet for Oil Pumping Unit

Dyson Graph Analysis:

  IV. Conclusion

1. In terms of reducing labor intensity, the use of frequency converters allows for stepless speed regulation of existing oil pumping units within a certain range. This eliminates the need for manual adjustment of the motor pulley to regulate the stroke rate, thus reducing the labor intensity of workers.

2. The use of soft-start function for motors in the intelligent control system reduces mechanical impact on surface oil extraction equipment, extending the service life of the equipment and lowering oil extraction costs.

3. Due to the use of an intelligent self-learning control system, the system change control from underground to surface in oilfield production wells was completed for the first time in China, greatly improving the efficiency of the entire production well extraction system.

4. This technology integrates new achievements from multiple disciplines such as wireless communication and artificial intelligence to create a digital oilfield and remotely monitor the operating electrical parameters of the pumping unit, including power diagrams, current diagrams, dynamometer diagrams, oil temperature, and oil pressure. It allows for real-time monitoring of the pumping unit's operating status.

5. Experiments suggest that this technology will play a significant role in improving system efficiency, extending pump inspection cycles, saving energy, and reducing the labor intensity of workers in low-permeability oilfields.