1. Introduction Most of China's oilfields are low-energy, low-yield oilfields, unlike foreign oilfields which have strong self-flowing capabilities. Most oil production relies on water injection to force oil into wells, and pumping units (nodding donkeys) to lift the oil from the formation. In China, replacing oil with water and electricity is currently the reality of oilfield operations, and electricity costs account for a significant proportion of China's oil extraction costs. Therefore, the oil industry attaches great importance to energy conservation, as saving electricity directly reduces oil extraction costs. Currently, China has over 100,000 pumping units, with a total installed capacity of 3500MW, consuming over 10 billion kWh annually. The operating efficiency of these pumping units is particularly low, averaging 25.96% in China, compared to an average of 30.05% abroad, indicating an annual energy-saving potential of several billion kWh. In addition to pumping units, oil fields also have a large number of water injection pumps, oil transfer pumps, submersible pumps, and other equipment, which account for more than 80% of the total electricity consumption of the oil field. It can be seen that the petroleum industry is also a key industry for promoting "energy saving of motor systems". The preferred solution for energy saving of pumping units is to use frequency converters to modify their motor drive system. The following are the benefits of using frequency converters to drive pumping units: (1) It greatly improves the power factor (from the original 0.25-0.5 to more than 0.9), greatly reduces the power supply current, thereby reducing the burden on the power grid and transformer, reducing line loss, and saving a lot of "capacity expansion" expenses. (2) The pumping 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) Since true "soft start" is achieved, excessive mechanical impact is avoided on the motor, gearbox, and pumping unit, greatly extending the service life of the equipment, reducing downtime, and improving production efficiency. However, when frequency converters are used in pumping unit motors, several problems need to be solved, mainly the inrush current problem and the regenerative energy handling problem, which will be analyzed below. 2 Inrush Current Problem [align=center] Figure 1 Conventional Crank-Balanced Pumping Unit[/align] The structural diagram of a conventional crank-balanced pumping unit is shown in Figure 1. In Figure 1, 1—base; 2—support; 3—suspension rope device; 4—donkey head; 5—walking beam; 6—crossbeam bearing seat; 7—crossbeam; 8—connecting rod; 9—crank pin device; 10—crank device; 11—reducer; 12—brake safety device; 13—brake device; 14—motor; 15—distribution box. The walking beam pumping unit is a modified four-bar linkage mechanism. Its overall structural characteristics resemble a balance scale, with the pumping load at one end and the balancing counterweight load at the other end. For the support, if the torque formed by the pumping load and the balancing load is equal or changes in the same direction, then a very small amount of power can make the pumping unit work continuously without interruption. In other words, the energy-saving technology of a pumping unit depends on the quality of its balance. When the balance is 100%, the power provided by the motor is only needed to lift half the weight of the liquid column and overcome friction. The lower the balance, the greater the power required from the motor. Because the pumping load changes constantly, and the counterweight cannot change exactly in sync with the pumping load, the energy-saving technology of a beam pumping unit becomes extremely complex. Therefore, it can be said that the energy-saving technology of a beam pumping unit is essentially a balancing technology. According to the author's investigation of 18 wells in a certain oilfield, only 1 or 2 wells had good counterweight balance; the vast majority of pumping units had severely unbalanced counterweights. Ten wells had counterweights that were too small, and six wells had counterweights that were too large, resulting in excessive inrush currents. The ratio of inrush current to operating current could exceed 5 times, and even exceed 3 times the rated current! This not only wastes a large amount of electrical energy but also seriously threatens the safety of the equipment. This also presents significant challenges for variable frequency drive (VFD) speed control: VFD capacity is typically selected based on the motor's rated power; excessive inrush current can trigger the VFD's overload protection, preventing normal operation. Adjusting the counterweight of the pumping unit's crankshaft can reduce the inrush current to within the motor's rated current, ensuring the ratio of inrush current to normal operating current is within 1.5 times. Therefore, using a VFD with the same capacity as the motor's rated power, or even slightly less (depending on the pumping unit's motor load rate), allows for long-term stable operation. Because pumping units often have high starting torque and inertia, the VFD's acceleration and deceleration times must be set sufficiently long, typically 30–50 seconds, to prevent overload protection during startup. 3. Problems in handling regenerative energy Since the pumping unit is a potential energy load, especially when the counterweight is unbalanced, the motor will be in a regenerative braking state (power generation state) during one stroke cycle of the pumping unit. Due to potential energy or inertia, the motor speed will exceed the synchronous speed. The regenerative energy is fed back to the DC bus through the rectification effect of the freewheeling diode connected in parallel with the inverter bridge switching device (IGBT) of the frequency converter. Since the DC bus of the AC-DC-AC frequency converter is powered by a common diode rectifier bridge, it cannot feed back electrical energy to the grid. Therefore, the regenerative energy fed back to the DC bus can only charge the filter capacitor and increase the DC bus voltage, which is called "pump voltage". When the DC bus voltage is too high, it will threaten the filter capacitor and the power switching device. In order to protect the safety of the capacitor and the power switching device, the frequency converter is equipped with "OUD" protection - DC bus voltage high protection shutdown function. (1) One way is to increase the capacity of the filter capacitor on the DC bus of the frequency converter to store the regenerative energy and release it to the motor to do work when in motor state. This method is beneficial for energy saving, but the energy storage capacity of capacitors is limited. For example, if the average power of a pumping unit motor is 10kW and the feedback power is 25% (2.5kW), and the power generation state lasts for 2-3 seconds in one stroke cycle, then the feedback energy Ad = 6000 joules. If a 15kW frequency converter is used, its DC bus filter capacitor has a capacitance of 2200μF. Under normal operation, the DC bus voltage is less than 600V (Us), and the "OUD" protection voltage (Usm) is 800V. Then, As = 1/2CUsm2 - 1/2CUs2 = 1/2 × 2200 × 10-6 × (640000 - 360000) = 308 joules, which is much smaller than the 6000 joules of feedback energy. Even if an additional 10,000μF filter capacitor is added, only 1400 joules of energy can be stored. Therefore, in systems with large capacity or high load inertia, it is impossible to limit the pump rise voltage solely by relying on filter capacitors. (2) The second method is to use the "release" method, which can be achieved by using a pump-up voltage limiting circuit composed of a shunt resistor Rp and a switching transistor VB, as shown in Figure 2. That is, the feedback energy is consumed on the resistor, which is an energy-consuming method and is not conducive to energy saving. Especially in large-capacity or large-inertia drive systems, the energy loss is large. Of course, ready-made frequency converter options: braking unit and braking resistor can also be used to achieve this. The principle is the same as in Figure 2, but the investment is larger and the energy consumption is also larger. [align=center] Figure 2 Pump-up voltage limiting circuit[/align] (3) For oil pumping units located in cold northern regions, in order to increase the fluidity of crude oil and prevent waxing in winter, the wellhead return pipe is electrically heated, such as by a medium-frequency electric heating device. In this case, the frequency converter and the medium-frequency electric heating device can share the rectifier circuit and DC bus. In this way, the regenerative energy fed back to the DC bus by the motor can be used for the medium-frequency heater, while preventing the DC bus voltage from pumping up. (4) For sites with multiple oil wells on the same well site, a shared DC bus system can be adopted: that is, the frequency converters of several pumping units can share a rectifier and connect their DC buses together. Utilizing the principle that the feedback energy of each frequency converter cannot occur simultaneously, the feedback energy of one frequency converter is used as the power for the other frequency converters. This saves energy and prevents the generation of pump boost voltage. As shown in Figure 3. [align=center] Figure 3 Main circuit of a multi-inverter system using a shared DC bus[/align] (5) For systems with higher power, in order to feed back regenerative energy and improve efficiency, an energy feedback device can be used to feed the regenerative energy back to the grid. Of course, this makes the system more complex and the investment higher. The so-called energy feedback device is actually an active inverter. According to the different power switching devices used, it can be divided into two types: thyristor (SCR) active inverters and insulated gate bipolar transistor (IGBT) inverters, each with its own characteristics and requirements. a) Thyristor Active Inverter [align=center] Figure 4 Main circuit of regenerative energy feedback system using thyristor active inverter[/align] As shown in Figure 4, when a three-phase bridge controllable rectifier circuit is used as an active inverter, it becomes a three-phase bridge active inverter circuit. The only difference is that the direction of energy flow within the circuit is reversed compared to rectification; the DC bus outputs power, while the power grid absorbs power. To prevent overcurrent, the condition UD≈Um should be satisfied. UD depends on the magnitude of the motor's feedback energy, while Um can be adjusted by the control angle α (or inverter angle β, β=π-α). Since Um is negative during inversion, α should be between π/2 and π (or β between π/2 and 0) during inversion. In fact, due to the influence of inductive load and transformer leakage reactance, the minimum inverter angle βmin ≥ π/6. From the above analysis, it can be seen that there are two conditions for inversion: First, there must be a DC voltage, the polarity of which must be consistent with the conduction direction of the thyristor, and its value should be slightly greater than the average voltage Um on the DC side of the converter; Second, the control angle of the thyristor α must be greater than π/2, so that Um is negative. Both conditions must be met simultaneously to achieve active inversion. The key to the thyristor active inverter is the voltage matching between the AC and DC sides, otherwise active inversion cannot be achieved. Since Um = -2.34U2cosβ (or = -1.35U2l cosβ) (1) If the AC side of the inverter is directly connected to a 380V AC power supply, and the minimum inversion angle is β = π/6, then UMmax = about 480V, while the DC bus voltage of the inverter is about 540V during normal operation. UD > Um, which will form an unnecessary circulation of energy between the inverter rectifier, inverter and grid, and will reduce the DC bus voltage, thus reducing the output power of the inverter. What we require is that when the feedback energy is small, the energy feedback device does not work, allowing the energy to be stored in the filter capacitor. When the DC bus voltage reaches a certain set value (e.g., UD > 670V), the energy feedback device will start working and feed the excess energy back to the grid. According to the reverse calculation of equation (1), the line voltage of the secondary side of the inverter transformer should be greater than 540V and the phase voltage should be greater than 300V to achieve voltage matching. b) Although the main circuit structure of the IGBT active inverter is basically the same as that of the passive inverter in the frequency converter, its function and control method are very different. The load of the passive inverter in the frequency converter is a three-phase AC motor, and its output frequency, voltage and phase can be arbitrarily controlled by the frequency converter; while the output of the IGBT active inverter is connected to the AC grid, which is an active load. Its output frequency, phase and voltage must be consistent with the grid, otherwise it will cause a short circuit and burn out the inverter. Therefore, frequency detectors, phase detectors and phase-locked loop control are added to the control of the IGBT active inverter. The voltage is controlled by PWM, which is easier to implement than thyristor active inverters. In addition, an AC reactor is connected to the output terminal to suppress overcurrent. (6) The control of the inverter with four-quadrant operation is more complicated and the investment is higher, as shown in Figure 5. [align=center] Figure 5 Main circuit of inverter with four-quadrant operation[/align] 4 Electromagnetic compatibility issues This section mainly discusses electromagnetic interference (EMI) issues, that is, the interference of the inverter on the microcomputer controller, sensor (transmitter) and communication equipment. Because the inverter is a very strong source of electromagnetic interference, the switching power supply in the inverter and the generated SPWM voltage waveform will cause great interference to the control and communication system. The interference path includes induction, radiation and conduction interference, that is, interference conducted through connecting wires. In the control system, the inverter is just an actuator. Its operating frequency (speed) command is sent to the inverter by the controller after controlling the signal such as the oil well production rate. The inverter is set to operate according to the external signal. The frequency converter caused significant interference to the microcomputer controller through this signal line, rendering the controller malfunction. Because the interference is conducted, shielding the cable is insufficient; the problem must be addressed by tackling both common-mode and differential-mode interference on the signal line, as shown in Figure 6. [align=center] Figure 6 Signal Line Anti-Interference Measures[/align] 5. Reliability and Environmental Adaptability Issues Since oil pumping units operate in harsh outdoor environments, and many wells are unattended, high demands are placed on the reliability and environmental adaptability of the frequency converter. On the one hand, high-reliability frequency converter brands must be selected; on the other hand, necessary conditions must be created for the frequency converter to operate in harsh outdoor environments. For example, a high-protection-level double-layer sealed (insulated) control cabinet should be designed, with a forced-air cooling system to dissipate heat, and a cold air inlet at the bottom of the cabinet, making it suitable for use in the high-temperature desert environment during summer. If possible, a control cabinet shelter can be built to protect the control cabinet from direct sunlight and rain. 6. IMOC-2000 Series Intelligent High-Efficiency Energy-Saving and Production-Enhancing Control Device for Pumping Units. The IMOC-2000 series intelligent high-efficiency energy-saving and production-enhancing control device for pumping units is a high-tech product specifically designed and manufactured for energy-saving and production-enhancing control of pumping units. It was developed through a collaboration between the Electrical Equipment Plant of Nanjing Electric Power Automation Equipment General Factory and senior technical personnel with extensive experience in AC motor energy-saving control at the State Power Corporation Thermal Engineering Research Institute. Based on extensive and in-depth investigations and research of domestic oil wells, and considering the functional shortcomings and deficiencies of similar products both domestically and internationally, this device is tailored to the specific conditions of different oil wells. This device can accurately sense the thickness of the downhole oil layer by measuring flow rate, pressure, and temperature. Measurement of the upper and lower dead center positions further improves the accuracy of control. This product integrates microcomputer intelligent control technology, advanced variable frequency speed regulation technology, and comprehensive energy-saving control technology for electric motors, transforming the traditional, coarse, and inefficient pumping of pumping units into intelligent and high-efficiency pumping. As a result, it effectively avoids pump cavitation, significantly reduces ineffective strokes, achieves energy savings of 30%–50%, and increases crude oil production by 20%–30%. The control device is also equipped with a remote control module, which can receive command signals from the host computer at any time, and can also transmit parameters such as well flow, pressure, temperature, motor temperature, and pumping unit status to the host control computer at any time, realizing remote operation and management of the entire oilfield and greatly improving the automation level of oil production. The successful development of the IMOC-2000 series pumping unit intelligent, efficient, energy-saving and production-enhancing control device represents a revolutionary breakthrough in pumping unit energy-saving control technology. The pumping unit intelligent, efficient, energy-saving and production-enhancing controller uses a variety of different methods to match the pumping unit's working mode with the actual load and environmental conditions of the oil well as much as possible, improves the pumping unit's fill rate, thereby improving the motor's efficiency and power factor, and achieving the goal of energy saving and production enhancement. There are three models of products according to different oil well conditions, which are described below. 6.1 Type A: Intermittent operation, Y/Δ conversion control energy-saving type. For old oil wells and lean oil wells that have been in operation for a long time, if the pumping unit operates continuously, the oil production will be low, or even dry pumping will occur, resulting in a waste of electrical energy. Faced with this problem, the traditional method is to use a timer to make the pumping unit work intermittently. However, this method still cannot solve the problem of making the pumping unit's working capacity dynamically respond to changes in the well load. At the same time, this approach also comes at the cost of losing the well's production. The pumping unit's intelligent energy-saving controller uses microcomputer technology to accurately sense the well load by detecting the well's output. When the well's output is less than the economic flow rate, pumping stops. The adaptive fuzzy control algorithm scientifically determines the start-up and shutdown times, ensuring the maximum pumping unit efficiency when starting up, avoiding the phenomenon of half-full pumping or empty pumping, and saving a lot of electricity. If controlled properly, it can also increase the oil production to a certain extent. However, most oil wells are not allowed to work intermittently. Otherwise, it will affect the oil production at best, and at worst, the oil well will be unable to be opened again. For example: (1) Oil wells with high wax content, high salt content, and high oil viscosity, and located in cold regions, will cause wax, salt, or oil to form at the wellhead if they work intermittently, making the oil well unable to be opened again. (2) For water-injected oil wells, stopping extraction will inevitably affect oil production, which would be a loss. For such oil wells, other energy-saving methods should be adopted. In order to solve the problem of inefficient extraction by the pumping unit, the excitation voltage of the pumping unit motor can be reduced to improve the power factor and efficiency of the motor, thereby achieving the purpose of energy saving. The IMOC-2000 controller uses two methods to reduce the voltage of the motor: one is Y/Δ conversion for energy saving. When the controller detects that the load rate of the pumping unit motor is <33%, the motor windings in the original delta connection are changed to star connection through the contactor. This reduces the voltage of the motor windings from 380V to 220V, thereby greatly improving the power factor and efficiency, and achieving the purpose of energy saving. When the controller detects that the load rate of the motor is >40%, the star connection is changed to delta connection to ensure the output of the pumping unit and prevent the motor from burning out due to excessive current. 6.2 Type B: Dynamic voltage regulation energy-saving type of power electronic devices. Another method for saving energy by reducing the voltage of the electric motor under light load is to utilize the phase-shifting voltage regulation function of the thyristor to dynamically adjust the terminal voltage of the motor, so that the working capacity of the pumping unit matches the actual load. The voltage can also be adjusted in a timely manner according to load changes during the up and down strokes, maximizing energy saving and consumption reduction. Y/Δ conversion control equipment is simple and inexpensive, but its energy-saving effect is slightly worse, and it cannot achieve energy saving based on load changes during the up and down strokes. Electronic automatic voltage regulation has good energy-saving effects, but requires a larger investment. The IMOC-2000 controller also adopts a local reactive power compensation method. Compensation capacitor banks are installed in both Type A and Type B control cabinets. Their capacity can be adjusted according to the power of the pumping unit motor to achieve a better compensation effect, greatly reducing reactive power loss and line loss, while also reducing transformer capacity and saving on capacity expansion costs. 6.3 Type C: Variable Frequency Speed ​​Regulation Energy-Saving Type. Utilizing modern variable frequency speed regulation technology, the pumping unit's stroke frequency and up and down stroke speed are dynamically adjusted to achieve both energy saving and increased production. (1) Dynamically adjusting the stroke frequency of the pumping unit to save energy. We know that the stroke frequency of the pumping unit can be adjusted mechanically. However, once it is adjusted, it is unlikely to be adjusted frequently, and the method of adjusting the frequency by the diameter of the pulley is also stepped and cannot dynamically adapt to the needs of the oil well load. Only by dynamically adjusting the speed of the pumping unit motor can the pump filling degree be adjusted, the extraction efficiency be improved, and the crude oil production be increased, achieving a double benefit. As the oil well is extracted from shallow to deep, the amount of oil downhole decreases. If the extraction is still performed at the original adjusted frequency, the pump filling degree will inevitably be insufficient and the pump efficiency will decrease. At this time, if the motor speed is reduced by using variable frequency speed regulation technology to reduce the extraction frequency, not only is the motor power reduced, achieving the purpose of energy saving, but the pump filling degree is also improved, ensuring that each extraction is full, greatly improving the pump efficiency and increasing the crude oil production. (2) Dynamically adjusting the speed of the pumping unit's up and down strokes to achieve the purpose of energy saving and production increase. Due to the use of microcomputer control and variable frequency speed regulation technology. In addition to dynamically changing the stroke frequency of the pumping unit, the speed of each stroke's up and down strokes can be adjusted separately according to actual needs, ensuring the pumping unit operates at its optimal state. Appropriately reducing the downstroke speed in each stroke increases the crude oil filling rate within the pump, while appropriately increasing the upstroke speed reduces leakage during lifting, effectively increasing crude oil production per unit time. Simultaneously, dynamic speed regulation significantly saves energy consumption. The IMOC-2000 series pumping unit intelligent, high-efficiency, energy-saving, and production-enhancing control device is designed according to the principles of safety, reliability, economic rationality, and field applicability. Considering the harsh environment of oilfields, large temperature differences between winter and summer, and frequent sandstorms and dust storms, a double-layer sealed, heat-insulated (thermal-insulated) control cabinet is designed with anti-theft functionality and an IP44 protection rating. The cabinet is equipped with a forced-air cooling system that dissipates heat from inside while preventing external heat from entering, making it suitable for use in high-temperature environments such as deserts during summer.