I. Introduction
Water injection is one of the important measures for stabilizing and increasing oil production in oilfield development. It effectively replenishes the energy of the formation, maintains the formation pressure, and plays a positive role in improving the oil production rate and crude oil recovery rate, ensuring high and stable oilfield production.
For a long time, the high-voltage, high-power frequency converters used in the high-power water injection motors of Daqing Oilfield were monopolized by foreign brands. In August 2007, Jiuzhou Electric's self-developed heat pipe cooling system perfectly solved the heat dissipation technical bottleneck of high-power high-voltage frequency converters (>200A). The first 2500KVA/6KV high-voltage, high-power frequency converter was successfully put into operation at Shandong Kangda Cement and ran continuously for a year without failure. In September 2008, Jiuzhou Electric won the bid for four skid-mounted high-voltage, high-power frequency converters for energy-saving retrofitting of high-power water injection motors in Daqing Oilfield. These were for the water injection pumps at Xing V-2, Xing V-1, and Junan 24 water injection stations, and the water injection pump at Junan 25 water injection station, totaling 6KV-2500KW.
This breakthrough broke the monopoly of foreign brands in high-voltage, high-power frequency converters used in high-power water injection motors in the Daqing Oilfield, and pioneered the application of domestic high-voltage, high-power frequency converters in high-power water injection motors in the Daqing Oilfield.
II. Current Status and Problems of the Water Injection System in Xingnan District of Oil Production Plant No. 5
1. Current Situation
By the end of June 2008, the Xingnan Development Zone had completed the construction of 1,822 water injection wells, of which 1,511 were operational, along with 9 water injection stations and 25 water injection pumps, with a total installed power of 49,140 kW. The designed water injection capacity was 11.60 × 10⁴ m³/d, the actual load was 8.24 × 10⁴ m³/d, and the load rate was 71%. The pure oilfield area adopted a single-pipeline, single-well water distribution process, while the transition zone adopted a single-pipeline, multi-well water distribution process.
2. Existing Problems
Currently, water injection systems are categorized by water quality into ordinary sewage injection systems, tertiary infill well injection systems, and polymer-containing sewage injection systems. Due to the lack of adjustment mechanisms, changes in the water injection plan and well shut-in operations can only be addressed by adjusting the number and model of the injection pumps in operation. This results in low adjustment efficiency and poor adaptability. The injection system often operates under high pressure, with an average drilling and shut-in water volume of approximately 5000 m³/d. The system's energy consumption per unit volume is more than 0.15 kWh/m³ higher than during normal operation, reaching a maximum increase of 0.4 kWh/m³. In severe cases, the injection pumps cannot start properly due to insufficient water volume.
III. Solutions
To address the issue of insufficient pump water consumption in the ordinary sewage injection system of Xingnan Oilfield due to the lack of comprehensive regulation measures, it was decided to adopt high-voltage variable frequency technology to achieve comprehensive regulation of water volume and pressure in the Xingnan Development Zone, ensuring that the injection pressure is within a reasonable range and reducing pump water consumption.
Based on the layout of the water injection stations and the actual consumption of each station, energy-saving technology was implemented at the XingV-2 water injection station (5.1.1 and 8.3.2), a dual-water-quality injection station. A skid-mounted high-voltage frequency converter with a capacity of 3150KVA/6KV, manufactured by Harbin Jiuzhou Electric Co., Ltd., was installed to achieve dual-system operation, enabling regulation of both the "8.3.2" and "5.1.1" water quality systems.
IV. Jiuzhou Electric Skid-Mounted High-Voltage Frequency Converter
(I) Design of the cooling system for high voltage frequency converter room. The high voltage frequency converter room adopts a color steel plate factory building. In order to ensure good heat insulation, the plate wall is required to be more than 100mm. Considering that the transformer cabinet in the high voltage frequency converter is the main heat source of the frequency converter, the transformer cabinet, power cabinet and control cabinet are placed in two separate rooms. This can fully ensure the reliability of the frequency converter operation.
Because Daqing Oilfield is located in a cold, high-altitude region in northern my country, with short spring, summer, and autumn seasons and relatively long winters, the inverters employ a combined air conditioning and duct cooling system. Air conditioning is used in spring, summer, and autumn, while duct cooling is used in winter. Additionally, electric heaters are installed in the prefabricated buildings to maintain the temperature of the inverter room during winter when the inverters are shut down for maintenance and repair.
The cross-sectional view of the prefabricated steel structure house is as follows:
(II) Design of the primary system scheme:
The schematic diagram is as follows:
illustrate:
1. QS1～QS5 are high-voltage isolating switches.
2. QS4 and QS5 mechanical interlocks.
3. QS4 is electrically interlocked with QS1 and QF1, and a program lock is installed.
4. QS5 is electrically interlocked with QS2 and QF2, and a program lock is installed.
5. The motor can operate at either the mains frequency or the variable frequency, but at any given time, a single motor is only allowed to operate at either the mains frequency or the variable frequency.
6. When overhauling and maintaining the frequency converter, disconnect all QS3, QS4, and QS5. There are obvious disconnection points, which protects the safety of maintenance personnel.
7. During inverter maintenance, the motors (M1, M2) can run at the power frequency, ensuring the continuity of production.
(III) Equipment Parameters:
1. Motor parameters:
Rated voltage: 6KV
Rated current: 265A
Rated power: 2500KW
2. Technical parameters of high-voltage frequency converter:
The high-voltage variable frequency speed control system adopts a high-high configuration, with the input side directly connected to a 6kV power supply and the output connected to the client's 6kV asynchronous motor.
| Design using standards | DL/T994-2006 |
| Installation location | indoor |
| Technical solution | Intersection-direction-intersection, high-high method |
| Requirements for electric motors | Ordinary squirrel-cage (or wound-rotor) asynchronous motor |
| Allowable variation range of rated input voltage | 6kV±10% |
| Rated capacity | 3150KVA |
| System output voltage | 0～6kV |
| System output current | 0～315A |
| Frequency output range | 0～50Hz |
| Rated input frequency / permissible variation range | 50Hz±10% |
| Inverter efficiency | ≥97% at full load |
| harmonic | Input voltage ≤ 4%, output current ≤ 4% |
| Input power factor | ≥0.95 (within the speed regulation range) |
| Control power supply | 380V±10%AC, 6kVA |
| UPS type, parameters and capacity | Online, 3000 VAh |
| High and low voltage isolation | Using optical fiber |
| Noise level | ≤75dB at 1 meter from the device |
| Cooling method | Forced air cooling (heat pipe) |
| Overload capacity | 120% overload time 1 minute (every 10 minutes) |
| Analog signal (input) specifications and quantity | 4-20mA, 2 channels |
| Analog signal (output) specifications and quantity | 4-20mA, 2 channels |
| Specifications and quantity of digital input signals | Dry contact point, Route 12 |
| Specifications and quantity of switch signal (output) | Dry contact point, Route 12 |
| Protection level | IP30 |
| Keyboard Operation | LCD Touch Screen |
| Interface language | Simplified Chinese |
| cabinet color | RAL7000 |
| Incoming line method | bottom line |
| Ambient temperature | 0～40°C |
| Altitude | No more than 1000 meters |
| air pollution | Only dry, non-conductive contamination |
| relative humidity | Up to 90% (20℃), no condensation. |
3. Brief introduction to the functions of Jiuzhou Electric's skid-mounted high-voltage high-power frequency converter:
The inverter room and inverter are integrated into one design, with comprehensive protection up to IP54, making it suitable for harsh working environments. The PowerSmart 6000 series high-voltage inverter is a voltage source type high-to-high voltage inverter.
• It adopts dual DSP control, eliminating the need for an industrial control computer, ensuring high reliability and achieving speeds up to nanosecond levels. This is 1000 times faster than the response speed of an industrial control computer, thus eliminating the problem of inverter crashes.
• Employing 36-pulse rectification and space vector multiplexing PWM technology, each phase consists of 6 units connected in series, directly driving the motor without the need for an output step-up transformer. It boasts a very high number of output levels, a very small dv/dt ratio, and an output waveform close to a sine wave, eliminating the need for a sine wave filter and ensuring smooth motor operation.
It features real-time verification of PWM control waveform and inverter output waveform, improving the accuracy of the output waveform and enhancing the system's fault-free operation capability. This verification function cannot be achieved by using data from similar manufacturers via packetized communication.
• The system has a high equivalent switching frequency. Each power unit has a switching frequency of 1.2kHz. Through series superposition, the actual phase-to-phase equivalent switching frequency of the inverter can reach 14.4kHz.
The inverter output torque pulse is narrow, the control precision is high, mechanical resonance is avoided, the wear of the transmission mechanism is reduced, the electrical stress intensity of the motor is similar to that when using power frequency power supply, there is no obvious additional impact, and the motor noise is similar to that when using power frequency power supply.
The input employs a multi-stage split transformer with H-class insulation. Grounding and shielding measures are implemented between the primary and secondary sides. It provides transformer overheat alarm and protection functions, with an alarm at 130℃ and a fault trip at 150℃. Alarm and fault points can be set. The transformer has strong overload capacity of 120% for 60 minutes and 200% for 10 seconds. The input impedance is as high as 8%, and its resistance to short-circuit current surges can reach 12.5 times the rated current.
• A comprehensive self-diagnostic and fault early warning mechanism, including power-on self-test and real-time monitoring during operation, with high detection speed. Through a dual DSP system, nanosecond-level calculations and comprehensive judgments are achieved, resulting in accurate analysis and reduced false alarms from the frequency converter. The self-diagnostic and protection functions are quite complete, providing over 400 diagnostic information entries, and exhibit strong adaptability to power grids and loads.
• It adopts patented real-time fiber optic transmission technology to control the power unit, avoiding the waveform delay caused by the data packet communication method of similar manufacturers.
It features reverse start and flyback start functions, allowing the inverter to achieve direct high-torque start regardless of whether the motor is rotating forward or in reverse.
The frequency converter has a soft start function, which can realize soft start for one-to-many connections.
• It provides GPRS remote monitoring functionality, allowing the high-voltage frequency converter's operating data to be transmitted to a local server for display, processing, and storage via GPRS. Remote fault diagnosis is also possible, and operators can activate remote control functionality when necessary. The system can promptly alarm or protect against situations such as short circuits, grounding faults, overcurrent, overload, overvoltage, undervoltage, and overheating.
• Built-in PID controller enables closed-loop control.
It can achieve multiple frequency setting methods such as touch screen, digital keypad, analog potentiometer, and remote DCS to meet various user needs.
It features user-isolated digital and analog input/output interfaces, ensuring reliable connection to existing user equipment. It is available with 32 digital input/output channels each, and 4 or 8 analog input/output channels, or can be configured according to user requirements.
The inverter is powered by two control power supplies: one from the secondary winding of the input transformer and the other from the user's site. It also has a built-in UPS power supply to ensure smooth, uninterrupted switching and prevent the inverter from shutting down in the event of a control power failure.
• Full parameterization capabilities, comprehensively supporting various user applications.
Meanwhile, to facilitate the previous operating habits of on-site operators, the frequency converter has added four speed regulation levels and jogging up and down functions, which greatly facilitates the operation of on-site operators.
Since its commissioning, it has received unanimous praise from on-site operators.
V. Effects of applying a high-voltage variable frequency speed control system:
(1) Improved process. After the frequency converter is put into operation, the water injection pump can output water smoothly, and the operators can adjust it freely. The operating parameters of the water pump are improved and the efficiency is increased.
(2) Extend the service life of motors and water pumps. When the water pump is running at the power frequency, the starting current is large (about 5 to 8 times the rated current) and the mechanical impact is very large. After adopting frequency conversion speed regulation, soft start and soft braking can be achieved, which will hardly produce impact on the motor and greatly extend the service life of the machinery.
(3) Reduce valve mechanical wear and pipeline impact. Extend the overhaul cycle of water injection pumps and save maintenance costs and time.
Compared with the traditional valve regulation method, this approach shows that, under the same flow rate, speed control avoids the energy loss caused by increased pressure head and pipe resistance under valve control, thus reducing manual valve adjustment. It completely eliminates the "water hammer" phenomenon during pump startup and shutdown.
VI. Economic Benefit Measurement and Evaluation
According to fluid mechanics, P = (Q*H)/102η.
In the formula: P is the shaft power of the water pump; Q is the flow rate; H is the pressure; η is the pump efficiency.
The formula for calculating the energy consumption of a water pump is: P = (K × H × Q) / η
K: Margin coefficient; η: Efficiency; Q1/Q2 = nl/n2; P1/P2 = (nl/n2)2; N1/N2 = (nl/n2)3.
That is, Q∝n; H∝n²; P∝n³ (where n is the rotational speed).
The rated power of the water injection pump is 2500kW, and the actual average shaft power during operation is 2200kW.
The average shaft power under actual operation with frequency conversion control was 1868kW, with an average energy saving rate of 15.1%. Operational results show that after frequency conversion speed control, the response and adjustment speed are fast, the electrical control system operates continuously, smoothly, and reliably, the pumps and motors achieved soft start, and the speed operated between 2503 and 2800 r/min, reducing pump and motor wear.
The above analysis shows that after the water injection pump is retrofitted with variable frequency speed control, it not only saves a significant amount of electrical energy but also achieves true soft starting for the motor. This greatly reduces the starting impact on the motor, pump, valves, various process components, high-voltage switches, and the power grid, eliminating the "water hammer" phenomenon during pump commissioning and decommissioning. Their service life is extended, and maintenance costs for these devices are significantly reduced.
Frequency converters are a new type of highly intelligent equipment that can fully meet the requirements of improving production efficiency and unit automation.
