Abstract : During the operation of the Luning pipeline, the difference in parameter characteristics between the oil pumps in Phase I and Phase II resulted in a large pressure difference in the pump manifold before and after the pump outlet valve, leading to significant energy consumption and waste. A high-pressure variable frequency speed control system was implemented to eliminate the throttling losses caused by the pump-pipe pressure difference, reducing the unit consumption of oil transportation and saving electricity. Keywords : Oil pump; High-pressure variable frequency speed control; Energy saving 1 Losses of the Oil Pump The Luning pipeline mainly transports Shengli oil and imported oil from Linyi, Shandong to Yizheng, Jiangsu. The pipeline has a diameter of 720mm and a designed annual transport capacity of 2000×10⁴t. It has 12 oil transfer stations and 43 oil pump units, with three pumps operating in series. The models of oil pumps selected for Phase I and Phase II renovations are different. The main technical parameters of the oil pump units are shown in Table 1. Due to the differences between the two types of oil pumps, the characteristics of the upstream and downstream oil pipelines are mismatched. Under essentially the same throughput across the entire line, especially in the case of a closed-loop system, the flow rate needs to be adjusted by regulating the outlet valves of the oil pumps (three pumps operate in series; the outlet valves of the first two pumps are fully open, and the outlet pressure is adjusted using the outlet valve of the third pump). The pressure difference across the outlet valve of the third oil pump is typically 1.29 MPa, with a throttling effect of 1.48 MPa within the station. This results in a significant pump-pipe pressure difference across the outlet valves. The throttling at the pump outlet valves leads to a substantial energy loss, and the oil pumps perform a significant amount of wasted work, shortening the maintenance cycle and service life of the pump units. Table 2 shows the statistical data on the pump-pipe pressure differences across the outlet valves of the oil pumps under different operating conditions. As shown in Table 2, under the various matching operation modes of the three pumps connected in series at Ningyang and Sihong stations, the average throttling losses before and after the pump outlet valves are 1.29 MPa, 1.21 MPa, and 1.54 MPa, respectively, while the average throttling losses within the stations are 1.49 MPa, 1.39 MPa, and 1.69 MPa. The throttling losses due to the pump outlet valves under the three operating conditions (three pumps connected in series) are: N_lossi = 0.278P_lossiQ_i Where: N_lossi—power loss due to valve throttling under different operating conditions, kW; P_lossi—pressure loss due to valve throttling under different operating conditions, MPa; Q_i—displacement of a single pump under different operating conditions, m³/h. N_lossi = 0.278 × 1.29 × 2099 = 752kW 752/1800 × 100% = 41.8% N_loss2 = 0.278 × 1.21 × 1987 = 665kW 665/1800 × 100% = 36.9% N_loss3 = 0.278 × 1.54 × 2018 = 863kW 863/1800 × 100% = 47.9% From the above calculations, it can be seen that under the three pumps connected in series and operating conditions, the throttling loss of the oil pump outlet valve accounts for 41.8%, 36.9%, and 47.9% of the rated power of one pump, respectively. It is evident that the energy waste is staggering. Therefore, it is necessary to apply variable frequency speed control technology to oil pump units to achieve the same operating conditions. Variable frequency operation meets the operating requirements, allowing the pump outlet valve to be fully open and the outlet pressure to be controlled by the outlet regulator, avoiding throttling losses at the pump outlet valve. 2. Variable Frequency Speed ​​Control System for Oil Pump Units Based on the characteristics of centrifugal pumps, their operating condition regulation mainly involves flow rate adjustment. The two most common methods for adjusting the flow rate of centrifugal pumps are adjusting the opening of the pump outlet valve and changing the pump speed. While the former is convenient, it results in significant energy waste. Changing the motor speed by using variable frequency drive (VFD) to adjust the oil pump's operating conditions is a feasible technical approach to meet process operating conditions. Currently, 6kV high-voltage VFD speed control systems are in the development stage. Their basic principle is to change the motor speed by altering the voltage frequency of the motor stator through an AC-DC-AC inversion process. After thorough research into the application of various 6kV variable frequency speed control systems both domestically and internationally, and after conducting technical and economic performance evaluations of various variable frequency speed control systems, the HARSVERT-A06/220 high-voltage variable frequency speed control system manufactured by Beijing Leadford Co., Ltd. was ultimately selected for application on the oil pump unit of the Luning power transmission line. The HARSVERT-A series high-voltage variable frequency speed control system adopts unit series multi-level technology and is a high-high voltage source type frequency converter. The frequency converter mainly consists of a phase-shifting transformer, power modules, and a controller. The power modules are basic AC-DC-AC single-phase inverters, with a diode three-phase full-bridge rectifier. Single-phase AC output can be obtained by sinusoidal PWM control of the IGBT inverter bridge. Each power module is structurally and electrically identical and interchangeable. The input side is powered by a phase-shifting transformer, which has three secondary windings. Based on the voltage level and the number of series stages of the modules, a multi-stage superimposed rectification method is formed, which can greatly improve the current waveform on the grid side. This ensures the grid-side power factor under load is close to 1, eliminating the need for any power factor compensation or harmonic suppression devices. The output side supplies power to the motor via a star connection formed by interconnecting the U and V output terminals of each power module. By recombining the PWM waveforms of each unit, a stepped sinusoidal PWM waveform can be obtained. When a power module fails, its output terminal can be short-circuited, bypassing the unit and removing it from the system, allowing the inverter to operate at a reduced mechanical rating; this avoids losses caused by downtime in many situations. The controller consists of a high-speed microcontroller, an industrial PC, and a PLC. The microcontroller implements PWM control and also enables remote monitoring and networked control. Fiber optic communication technology is used between the controller and the power units, ensuring complete and reliable isolation between the low-voltage and high-voltage sections, providing extremely high system safety and excellent electromagnetic interference resistance, significantly improving reliability. Furthermore, in the event of a power failure in the control power supply, the controller can be powered by the equipped UPS, allowing the inverter to continue operating. 3 Technical Solution While the application of high-pressure variable frequency speed control system to oil pump units can generate good energy-saving benefits, the oil pump unit system is an important hub in oil production and operates continuously for a long time. In addition to requiring high reliability of the equipment itself, the technical solution must be combined with the process characteristics of the site and fully consider the safety, applicability and convenience of on-site operation, start-up, shutdown and adjustment. The following technical measures are adopted in the application of this system. (1) The system has the function of manual switching between power frequency and variable frequency. Once the variable frequency system has a fault, it can be manually switched to the power frequency setting to disconnect the variable frequency system. During the maintenance of the variable frequency system, the operation of the oil pump can be guaranteed normally to meet the production needs of the Lu-Ning line. (2) The operating frequency of the system is adjusted by open-loop/closed-loop adjustment. ① The outlet valve of the last pump in the series pump is fully opened, and the required outlet pressure is adjusted by the outlet regulating valve. ② Open-loop state is given speed: The set value of the frequency converter is the speed, and the actual operating speed is the same as the set speed. The actual oil pressure is not controlled. Closed-loop state is given oil pressure: The frequency converter will perform PID adjustment according to the feedback of the actual oil pressure to change the operating frequency (motor speed) in order to keep the actual oil pressure constant. ③ Press manual/automatic on the frequency converter operation interface of the station control machine. The computer keyboard for setting will appear. Enter the required value and press OK. (3) On-site setting, start, stop and emergency stop buttons. The host computer in the control room displays the operating parameters in real time, which greatly facilitates the operation of on-site operators and the monitoring of equipment operation status. (4) Optimize the protection parameters of the system to ensure the continuous and stable operation of the oil transportation system. When applied to the oil transportation system, it is necessary to carefully select and set some protection parameters according to actual needs. Avoid the oil pump stopping due to the overly sensitive protection of the frequency converter system, which will affect the safe and stable operation of the oil transportation system. (5) An alarm function suitable for the actual site conditions is set in the variable frequency speed control system, and a detailed recording function is provided for the operating parameters, operation status, and fault conditions. 4 Application Effect The variable frequency speed control system of the Ningyang and Sihong oil pump units of the Luning Line was put into normal operation after trial operation on May 19, 2005, and achieved significant energy-saving effect. The comparison data of the three pumps running in series under different operating conditions are shown in Tables 3, 4, and 5. According to the statistical data in Tables 3, 4, and 5, under different operating conditions, the variable frequency speed control operation of the No. 10 oil pump can generate significant power-saving benefits. When the No. 10 pump at Ningyang Station is matched with the No. 7 and No. 8 pumps in series, the power saving rate is 21.68% compared with the original operating conditions. When the No. 10 pump is matched with the No. 7 and No. 9 pumps in series, the power saving rate is 28.57%. When the No. 10 pump is matched with the No. 7 and No. 9 pumps in series at Sihong Station, the power saving rate is 30.05%. According to operational data from recent years, the series operation time of the three pumps on the Luning line accounts for 99% of the total operating time. Based on this, it can be calculated that if the No. 10 oil pump unit operates throughout the year, compared with before the implementation of variable frequency speed control, the comprehensive power saving rate is 26.77% (conservative data). If calculated based on 350 days of operation per year, the annual power saving is approximately 1909.2500 × 10⁴ kWh. Based on the 6kV industrial electricity price of 0.58 yuan/kWh, the annual power saving benefit is 11.07365 million yuan, and the equipment investment can be basically recovered in one year. Furthermore, the use of a frequency converter system for soft start and stop of the oil pump unit reduces the impact during startup and extends the maintenance cycle of the oil pump. Because the pump operates at a lower speed after frequency conversion, the wear on the pump shaft and bearings is reduced. For example, before frequency conversion, the bearing temperature of pump #10 during normal operation reached 82.6–90℃, while during frequency conversion, the bearing temperature is only 61.1–68.3℃. This will significantly extend the life of vulnerable parts such as bearings and mechanical seals of the oil pump, while also reducing noise during operation. Therefore, in addition to significant direct economic benefits, it also has good indirect economic benefits. 5. Conclusion High-pressure variable frequency speed control technology for oil pump units is an effective technical approach to achieve energy conservation in oil transportation systems. It replaces the valve throttling adjustment method with a centrifugal pump speed change adjustment method (open-loop/closed-loop), which is convenient to adjust and allows the pump outlet valve to be fully open. This effectively avoids throttling losses at the pump outlet valve, generating significant energy benefits. It also reduces mechanical shock, wear, and noise in the oil pump unit, extending its maintenance cycle and service life. However, since high-pressure variable frequency speed control systems are currently in the technological development stage and require high initial investment, it is essential to select high-reliability, cost-effective, and widely used high-pressure variable frequency devices. During application, it is crucial to closely integrate with the on-site process conditions and select an appropriate speed control method to ensure the safe and stable operation of the oil transportation system while achieving significant energy-saving benefits.