Abstract : This paper describes the design and commissioning process of frequency conversion control for a water injection pump at a medium-pressure injection station in Qinghai Oilfield. It points out the significance of using frequency conversion control and demonstrates the future prospects of this type of equipment. Keywords : Medium-pressure water injection pump, frequency conversion control 1 Introduction Water injection into oil wells is a crucial aspect of oilfield production. In some wells with insufficient fluid supply, a certain amount of water needs to be injected first to dilute the oil and raise the fluid level to a certain height for easier extraction. To save energy and reduce consumption, several wells are often combined and injected using a single pump; this is the function of a medium-pressure injection station. Station 23 in the Yuexi Oil Production Area of ​​Qinghai Oilfield's Second Oil Production Plant is a centralized water injection station that injects water into wells YX-52, YX-47, and YX-35. It uses piston-type water injection pumps with a displacement of 23 m³/h and a pressure of 12 MPa, powered by medium-voltage 1140V/132kW motors. To achieve constant pressure water injection and effectively protect the pipelines, the oilfield company, after comprehensively considering various factors, decided to adopt variable frequency control. This system automatically adjusts the motor speed based on pressure sensor readings, increasing the speed when the pressure is low and decreasing it when the pressure is high, thus maintaining constant pipeline pressure. The system also includes pressure alarms: low-pressure alarm at the inlet with shutdown for ultra-low pressure; and high-pressure alarm at the outlet with shutdown for ultra-high pressure, meeting the operational requirements of the water injection pumps. 2 Requirements for Variable Frequency Control Cabinets Based on the above, the oilfield company put forward the following requirements for variable frequency control cabinets: 2.1 Technical tender for 1.14kV constant pressure water injection variable frequency cabinet (1) Variable frequency power of the variable frequency cabinet: 160kW (2 units); (2) Input voltage: 1140V±15%, 50Hz; (3) Water injection working pressure: 0-25Mpa; (4) The variable frequency cabinet has manual frequency adjustment and PID automatic adjustment modes. 2.2 Functions of the frequency converter cabinet: (1) Overpressure alarm and ultra-high pressure shutdown function for water injection pump (the value can be set); (2) Low pressure alarm and ultra-low pressure shutdown function for inlet (the value can be set); (3) High temperature alarm shutdown for lubricating oil (the value can be set); (4) Fault shutdown alarm (the sound has a time setting, and the light has a reset button); (5) Data storage function: voltage, current, output power, fault record; (6) Complete electrical protection function: overvoltage, undervoltage, overcurrent, underload, input and output phase loss, protection value and time limit can be set; (7) RS-485 data communication interface; (8) Cabinet cooling fan temperature control setting function; (9) Frequency converter cabinet panel display: voltage, current, operating frequency, water injection pressure; (10) On-site main control operation conversion switch; (11) Equipped with 4 pressure transmitters (with numerical display) 0-25MPa: 2 units; 0-2.5MPa: 2 units; (12) Set power frequency start; (13) Set on-site control box, display frequency, water injection pressure, with: start, stop, frequency adjustment button. 3 Principle of medium voltage frequency converter The key technology of this system is medium voltage frequency converter. After oilfield investigation, the medium voltage frequency converter technology and application produced by our company are very mature. It is widely used in both submersible electric pumps and oil pumping units, and is very stable. Therefore, the author's technology is adopted and the control cabinet is made. 3.1 Principle of frequency converter The speed formula of a three-phase asynchronous motor is: n=60f(1-s)/p Where: n——motor speed; f——power supply frequency; s——motor slip; p——number of pole pairs of the motor. In this formula, if p (number of pole pairs) and S (slip) are kept constant, and the power supply frequency of the motor is smoothly changed, then the motor speed n=Kf (k is the proportional coefficient) can be smoothly changed. This is the principle of frequency conversion speed regulation. 3.2 Medium-Voltage Three-Level Circuit Due to the relatively high operating voltage of 1140V, using a regular frequency converter is clearly unsuitable, while using a high-voltage module is costly. Furthermore, for high-voltage motors, a good waveform is essential for insulation and withstand voltage, which a regular two-level circuit cannot meet. To adapt to the voltage level and load requirements, a three-level circuit (also known as a midpoint clamping circuit) is used, as shown in Figure 1. [align=center] Figure 1 1140V Main Circuit Diagram[/align] In the main circuit, each phase is connected to four IGBTs, connected in series to bear the entire voltage. The three-phase AC power is rectified into DC power by a full-wave rectifier. The microcontroller controls the conduction and cutoff of the four IGBTs, forming an SPWM alternating output. To suppress voltage surges during IGBT alternation, a midpoint clamping circuit is used. Two fast recovery diodes are connected in series to form a clamping diode, whose center point is connected to the center point of the filter capacitor, forming the center point of the circuit (the center level of the three-level circuit). Its output waveform is shown in Figure 2. [align=center] Figure 2 Three-level waveform diagram[/align] As can be seen from Figure 2, the phase voltage is three-level and the line voltage is five-level. Therefore, it can not only output a higher voltage, but also reduce the output harmonics and voltage change rate (dv/dt). The good waveform is the feature of this circuit. 3.3 Control cabinet construction According to the requirements of the oilfield company and the special conditions of the site, the control cabinet was carefully constructed. (1) System control overview: To achieve automatic control, a constant pressure closed-loop system is formed by pressure transmitters and PID controllers with frequency converters. The system structure is shown in Figure 3: [align=center] Figure 3 System control diagram[/align] In Figure 3, P is the pressure transmitter. The working process is as follows: Two pressure transmitters, one at the inlet and one at the outlet, are used in the system to detect the pressure at the inlet and outlet respectively. The inlet pressure transmitter sends the detected value to the inlet PID1 for pressure display, and performs low pressure alarm and ultra-low pressure shutdown control through parameter setting. The outlet pressure transmitter sends the detected value to the outlet PID2 for outlet pressure display. Through parameter settings, it controls the frequency of the frequency converter to adjust the speed of the water pump, achieving automatic control. It also performs high-pressure detection to realize high-pressure alarm and over-pressure shutdown control functions. In the lubrication oil circuit system of the water injection pump, in order to control the oil temperature, a thermocouple is used to detect the temperature of the lubricating oil and sends the detected value to PID3 for oil temperature alarm control. (2) Control cabinet design The control cabinet block diagram is shown in Figure 4. [align=center] Figure 4 Main circuit diagram[/align] The switching method between power frequency and frequency converter is adopted to ensure that the power frequency can be temporarily put into operation when the frequency converter control has problems, thus ensuring the continuity of production. A motor protection device is also connected to the output end, which can display the voltage and current values ​​at any time and can effectively protect the load through on-site settings, such as overvoltage, undervoltage, overload and current imbalance. Through the alarm contacts and parameter settings of the PID, the system can perform high and low pressure alarm and overpressure shutdown control. The control block diagram is shown in Figure 5. [align=center] Figure 5 Control Schematic[/align] The alarm output of the three PIDs, AL2, is connected to the alarm device to realize the alarm. The output of AL1 is connected to an intermediate relay, and its normally closed contact is connected in series in the control circuit of the control contactors KM1 and KM2 of the frequency converter. When the system is over-pressured, AL1 outputs, KM1 and KM2 are de-energized, thereby stopping the frequency converter. (3) Remote control setting In order to facilitate the operators, the system is also equipped with a remote control device. The remote control device is also equipped with a PID, which is connected in series with the PID on site, so as to facilitate the operators to monitor and operate. (4) Iron sheet house maintenance Since it is an outdoor work, the wind and sand are relatively strong and the environment is relatively harsh, a special iron sheet house is built for protection. The iron sheet house is divided into two layers, with insulation material in the middle, which provides heat insulation in summer and heat insulation in winter, and has a special sand-proof net to prevent wind and sand invasion. 4 System debugging The key to system debugging is the setting of various parameters. The main one is the selection of PID parameters. To properly set the three parameters P, I, and D to ensure stable system operation, the PID can be self-tuned first, and then adjusted appropriately based on the tuned values, so that the frequency change of the inverter can track the change of system pressure in real time until the system is stable. For the setting of various alarm parameters, the PID controller manual can be used for setting. For the output PID, AL2 can be set as the upper limit alarm and the output control alarm can be set, and AL1 can be set as the ultra-high voltage output using the positive deviation alarm. For the inlet PID, AL2 can be set as the lower limit alarm and the negative deviation alarm can be set as the ultra-low voltage output, so that various control functions can be realized. The motor protection device should be set according to the actual operation of the site. Generally, the overload current is set to 1.2 times the rated current, the overvoltage is set to 1.2 times the rated voltage, and the undervoltage is set to 0.8 times, etc. 5 System operation status After installation and debugging, the system has been running normally for more than half a year without any faults. (1) Constant pressure automatic tracking speed regulation has been realized. During operation, the frequency converter can automatically track pressure changes and adjust the operating frequency in real time through the adjustment of the pressure transmitter, so that the pipeline pressure is constant and there are no large fluctuations. (2) When the inlet and outlet pressure fluctuates in a wide range, it can implement effective protection. According to the site requirements, the inlet pressure is set at 0.8MPa, the low pressure alarm is set at 0.5MPa, and the machine stops when it is below 0.2MPa. The outlet pressure is set at 8MPa, the high pressure alarm is set at 10MPa, and the machine stops when it is 12MPa. It effectively protects the pipeline and water pump, preventing them from operating under overpressure or overload, and the effect is good. (3) The cabinet has good protection performance. Due to the double protection, it can effectively isolate sand and dust and keep clean. (4) The frequency converter and motor protection device have good motor protection performance. Overvoltage, undervoltage, overload and short circuit can be effectively protected, which is beneficial to protect the motor and load. (5) The energy saving effect is obvious. For pump loads, frequency conversion control can achieve a lot of energy saving. According to fluid mechanics, power is proportional to the cube of the rotational speed, and rotational speed is proportional to the frequency. The current operating frequency of this system is approximately P1/P2 = (n1/n2)³ = (f1/f2)³ = (50/44)³. Therefore, P2 = 68%P1. The theoretical energy saving rate is: ΔP = (P1-P2)/P1 = (1-68%) = 32% . 6. Conclusion Through the modification of this system, significant features have been achieved in automatic control, energy-saving operation, and protection functions. Oilfields have numerous water injection pump systems, some medium-pressure, but most low-pressure. Modifying these systems with this approach yields the same results. Therefore, it can be widely promoted to achieve energy saving and efficiency improvement.