1 Introduction
Currently, frequency converters are widely used in polymer injection pump stations in oilfields. Taking the Gudong Oil Production Plant of Shengli Oilfield as an example, statistics show that more than 270 low-voltage frequency converters have been used in polymer injection pump stations, accounting for about 58% of the total number of low-voltage frequency converters in the oilfield. In the tertiary oil recovery technology of oilfields, frequency converters are widely used in the control of single-well injection pump motors, as well as in the mother liquor preparation system, transfer system, and feed and delivery system in polymer field tests. They have the advantages of energy saving, convenient flow regulation, good communication performance with PLC and host computer, and stepless speed regulation capability. When used in conjunction with flow meters, regulating valves, and automatic mixing controllers, they can freely change the amount of powder (polymer) fed, maintain constant pressure, track level adjustment, ensure relatively stable liquid level in the preparation tank, and the standby pump system can universally replace other single pumps. However, with the increasing number of frequency converters in use and the increasing service life, frequency converter failures are becoming more and more apparent. How to ensure the normal operation of frequency converters in use and reduce their downtime has become an important research topic for oilfields.
2. Inverter working principle and terminal wiring
2.1 Working Principle
A device that converts AC power with a fixed voltage and frequency into AC power with a variable voltage or frequency is generally called a "frequency converter". To generate variable voltage and frequency, this device first converts the AC power from the power supply to DC power (DC) through a bridge rectifier, and then converts the DC power (DC) back to AC power (AC). The electrical term for this is "inverter". Since the main device in a frequency converter that generates changing voltage or frequency is called an "inverter", the product itself is named "inverter". Frequency converters used for motor control can change both voltage and frequency, but frequency converters used for fluorescent lamps are mainly used to adjust (increase) the frequency of the power supply to reduce the eye glare caused by low-frequency flicker. The working principle of frequency converters is widely used in various fields, such as computer power supplies (UPS). In this application, frequency converters are used to suppress reverse voltage and frequency fluctuations, as well as momentary power outages. During a momentary power outage, the UPS can convert the DC power stored in the battery into AC power with the same voltage and frequency as the mains power, ensuring that computer users have enough time to save their work. It is also suitable for backup power systems in hospitals and other industrial and military systems where power outages are unavoidable.
The internal structure of a frequency converter can be described by the schematic diagram in Figure 1. Taking a three-phase industrial power supply as an example, the internal structure and working principle of the frequency converter are briefly explained.
Figure 1 Schematic diagram of the internal structure of the frequency converter
Figure 1 illustrates the basic working principle of a frequency converter. Three-phase AC power is rectified by a bridge rectifier circuit into pulsating DC power, then filtered by a large-capacity capacitor to become more stable DC power. Finally, an oscillating inverter system converts this DC power into AC power with variable voltage and frequency to drive the load motor. This process involves closed-loop control with the participation of a central control system. This includes monitoring the input power voltage and frequency, the rectified DC voltage, the output voltage, frequency, and current, the frequency converter's temperature, and external input faults. It also responds to external manual input parameters, adjusts internal feedback data, and handles fault alarms and shutdowns. The human-machine interface (HMI) allows users to input various parameters, view internal operating status, fault alarm content (sometimes in codes), and input pre-programmed procedures, enabling the frequency converter to perform specific operations according to user intentions. Visually, the HMI typically appears as an LED display or LCD screen with waterproof soft-touch keys, functioning similarly to a computer keyboard and monitor. Therefore, it is both an input and output device for the frequency converter.
2.2 Terminal wiring of frequency converter
Figure 2 provides a simple introduction to some commonly used wiring terminals of frequency converters in polymer injection pumping stations. This helps to understand the basics of frequency converters and external connections, and also facilitates the inspection and handling of frequency converter faults.
Generally speaking, the terminals of a frequency converter can be simply divided into main circuit terminals and control circuit terminals:
(1) Main circuit terminals
The input three-phase power terminals are marked with the letters r, s, and t, and their input voltage is AC 380～450V ±10% (50, 60Hz ±5%). Foreign frequency converters also have an input voltage range of 200～240V. Output (load) terminals are generally marked with u, v, and w. The grounding terminal is marked with g.
Other optional DC reactor terminals, external braking resistor terminals, and external braking unit connection terminals are not shown in the figure. They are not commonly used in injection molding and will not be described in detail here.
(2) Control circuit wiring terminals
The inverter has two input terminals: a common terminal (l) for analog input and a frequency input terminal (h) for frequency input. A +10V DC current can be supplied between h and l. The frequency command terminal has both voltage and current input options, typically 0-10V for voltage and 4-20mA for current. The analog monitor can monitor the inverter's output frequency, voltage, current, and input power. The internal alarm output terminal transmits internal inverter faults to connected external automation equipment for联动 (interlocking/interaction). The external alarm input terminal transmits faults from peripheral devices to the inverter, causing it to shut down, such as in cases of load failure or automatic control system malfunction. The external thermistor input terminal allows the inverter to detect temperature; overheating triggers a trip and stops the output. The forward/reverse input terminal, when connected, prevents the inverter from being controlled by keyboard input for direction of rotation; therefore, it is commonly used to input operation and stop commands from the outside.
Common problems and solutions for frequency converters in 3-pump-station pumping stations
Based on the problems encountered during actual field application, the following are the troubleshooting methods and precautions for handling these issues:
3.1 The inverter draws excessive current at both low and high speeds.
Excessive current during low-speed and high-speed operation of the frequency converter leads to overheating of both the converter itself and the load, resulting in excessive noise and vibration from the load (motor). In a 1992 test of a three-dimensional composite drive at Gudong, five Toshiba 30kW frequency converters were used. Due to design and production discrepancies, the actual operating frequency required the converters to run at 8-10Hz, a common low-frequency condition. Consequently, the motors connected to each converter experienced widespread excessive vibration and emitted a "squeaking" noise. Motor surface temperatures exceeded 80°C, reaching as high as 90°C, and the converter output current exceeded 35A with significant fluctuations. After debugging by on-site electrical engineers in the United States, it was concluded that the frequency converter should not operate continuously at low frequencies. Furthermore, the slow motor fan speed resulted in poor heat dissipation, making overheating inevitable. The author carefully analyzed the frequency converter's instruction manual at the time, discovering the meaning and setting methods of parameters such as V/F and torque. It was speculated that the excessive current at low frequencies was caused by insufficient output torque at those frequencies. According to the motor power formula p=uicosφ, when the output power is constant, appropriately increasing the output voltage can reduce the output current. Therefore, the author tested two methods: The first method is to change the V/F curve, changing the inverter's fundamental frequency from 50Hz to around 40Hz. This means that the inverter outputs approximately 380V at around 40Hz, resulting in a roughly 20% increase in output voltage between 8 and 10Hz. After adjusting the fundamental frequencies of the five inverters to between 38 and 42Hz, the output current during operation was about 50% of the original. The temperatures of both the inverter and the motor were significantly reduced, with the motor temperature between 34 and 39℃. Noise and vibration were also minimized, and the motor ran more powerfully and smoothly than at the mains frequency. The second method is to set a torque boost, which involves increasing the output voltage by 10 to 50V while keeping the V/F ratio constant. This method also achieves the same effect.
Frequency converters can operate normally at low frequencies, but the output voltage gain, i.e., the output torque, should be adjusted appropriately according to the load conditions. However, care must be taken not to increase the voltage too much, as this could cause the motor stator to exceed magnetic saturation, resulting in excessive current and increased power consumption. Proper adjustment can improve equipment performance and achieve energy savings by increasing the power factor. Simultaneously, attention should be paid to whether the pump being driven has adequate lubrication at low speeds. Generally, piston pumps rely on the crankshaft rotation to propel the lubricating oil. At low speeds, this propulsion is less effective, so the lubricating oil level can be appropriately increased. This will allow the piston pump to achieve better lubrication and generate less heat.
3.2 Handling of overcurrent protection during inverter startup and shutdown
The following are some of the reasons why an inverter may experience overcurrent protection during startup and shutdown:
(1) Improper acceleration and deceleration times can cause problems. Excessive acceleration time can lead to a prolonged period of low torque output from the inverter, resulting in excessive starting current. Conversely, insufficient acceleration time can cause the motor to fail to keep up with the inverter's voltage and frequency output values in a short time, similar to the effect of direct starting at the power frequency. Acceleration time is generally set according to load changes. For single-well injection pump inverters in polymer and chemical injection systems, a setting of 5-10 seconds is typically used. The input ammeter should be observed to ensure that the current during acceleration is not greater than 1.5 times the actual current. Deceleration time is also crucial, exhibiting similar characteristics to acceleration time. It is generally set between 5-15 seconds, adjusted according to load conditions. Braking is prohibited during deceleration; for example, the pump outlet gate should not be closed while the motor is still running.
(2) The starting torque is too small, resulting in an excessive starting current. This is generally caused by an unsuitable v/f curve. Since the injection pump is a constant pressure output pump, a linear v/f curve should be selected. The corresponding values of v and f should be adjusted appropriately according to the load to achieve the best starting torque. For example, when the load is small, the v/f value can be smaller, and vice versa.
3.3 Troubleshooting a frequency converter that inexplicably shuts down automatically during operation and can be restarted after a reset.
This phenomenon typically occurs when other large motors in the same pumping station start simultaneously (at power frequency). When a motor starts, a starting current corresponding to its capacity flows, causing a voltage drop in the transformer on the motor stator side. This voltage drop has a greater impact when the motor capacity is large. The frequency converter connected to the same transformer will then detect undervoltage or momentary shutdown, sometimes triggering the protection function (IPE) and causing the operation to stop. This can be resolved by adding an inductive filter coil to the frequency converter input.
3.4 A centrifugal pump driven by a frequency converter at 50 Hz cannot achieve the flow rate and head required at the power frequency.
On-site inspection revealed that the inverter's basic frequency was set to 60Hz, meaning that it only achieved an output voltage of 380V at 60Hz. This was clearly due to insufficient torque at 50Hz, causing the centrifugal pump to run at low speed. Changing the basic frequency to 50Hz increased the maximum output voltage to 400V (410V from the grid input), which met the flow and head requirements. The operating current was also reduced, and the motor heating was also less.
3.5 Troubleshooting and Repair of Alarm Outputs Caused by Damage to Internal Components of the Frequency Converter
The following are some of the reasons for damage to internal components of a frequency converter:
(1) Mainboard damage. Generally, at this time, the inverter's HMI displays nothing and the inverter does not respond. The mainboard needs to be replaced. The manufacturer's provided mainboard can be directly replaced. When replacing, ensure the inverter is completely discharged, and do not touch the circuit board to prevent static electricity from burning out the internal CMOS circuit. I once repaired a Hitachi inverter with no HMI display. Inspection revealed that the mainboard power circuit was burnt out. After replacing the mainboard, it returned to normal. I also repaired a Fuji inverter with both HMI and fault codes. The problem was found to be a fault in the charging/discharging circuit. Using a multimeter, I discovered that a 3A fuse in the charging/discharging circuit on the mainboard was blown. Replacing it restored normal operation.
(2) Output module failure. This usually causes the inverter to shut down due to overload, and no amount of adjustment can fix it. I have replaced the output modules of Toshiba and ABB inverters before. Of course, some inverters directly display fault codes. There are various reasons for output module failure. Generally, the problems are nothing more than no output on the U, V, and W phases, unbalanced output, balanced output but jitter at low frequencies, and startup alarms. When the fast fuse after the large capacitor of an inverter is open, or the IGBT inverter module is damaged, the output module will basically not be intact. Do not replace it with a good fast fuse or IGBT inverter module, as this can easily cause the newly replaced good device to be damaged again. At this point, you should focus on checking for arcing marks on the output circuit. You can first disconnect the output pins of the IGBT inverter module and use a multimeter in resistance mode to measure whether the resistance values of the output circuit are the same. If they are basically the same, it cannot completely prove that the output circuit is intact. Next, you need to use a digital electronic multimeter to measure the DC voltage of the output circuit. Generally speaking, the DC voltage on each output circuit is about 10V when not started and about 2-3V after starting. If the measurement results are normal, you can confirm that the output circuit of the inverter is good.
(3) Input phase loss. All three phases have fuses inside. If the fuse burns out, a phase will be lost. Most frequency converters will give a prompt in this case. You can find the problem by checking according to the prompt.
(4) The rectifier module is damaged. If this module is damaged, the AC-DC conversion cannot be completed and the frequency converter will not work. This is not difficult to find through inspection. Sometimes it is a diode in the three-phase bridge that is damaged, which will cause the main DC voltage to be too low alarm. Damage to the filter electrolytic capacitor will also cause the main DC voltage to be too low alarm.
(5) Temperature sensor failure. I once encountered a frequency converter that frequently issued a mainboard overheating alarm. After inspection, it was found that the sensor was damaged. After replacement, it worked normally.
(6) Monitor failure. I have encountered many inverters that do not display a fault. Most of the faults are caused by open circuits in the monitor circuit due to long-term operation. After resoldering, they are restored to normal.
(7) Cooling fan failure. The inverter will automatically shut down when encountering this fault, and the fault code will be displayed as inverter overheating fault. This type of fault is also common in injection pump stations. After replacing the cooling fan, the system will function normally. The normal service life of the cooling fan is 20,000 hours in a dust-free indoor environment with moderate temperature and humidity. Using it in high temperature, high humidity and dusty environments will seriously affect its service life.
(8) Alarm output circuit failure. This type of failure is also quite common. It is mainly caused by a fault in the internal alarm relay of the frequency converter, resulting in false alarms, automatic shutdown, or even failure of the control system, making the system unable to operate normally. The method to check the frequency converter fault is to use a multimeter to measure whether the voltage of the output terminal of the alarm relay is normal and whether the normally closed and normally open contacts are correct. If a relay fault is found, the relay can be replaced.
3.6 Potentiometer Failure
When the frequency converter is remotely controlled by a potentiometer, the frequency is always 0 or at the maximum (e.g., 50Hz) and cannot be adjusted. This is generally because an open circuit at the upper and adjustment terminals of the potentiometer causes a 0Hz reading, while an open circuit at the lower terminal causes a maximum reading. Of course, it has also been seen that employees have incorrectly adjusted the internal minimum and maximum frequencies, setting them to the same value, resulting in the inability to adjust. There are also cases where the remote control has been modified to a panel control, making it impossible to adjust. These issues are not difficult to correct based on the symptoms and internal settings. Additionally, if there is an internal fault in the potentiometer, the output at the adjustment terminal will remain unchanged, causing the frequency converter to be unable to be adjusted normally. In this case, replacing it with a potentiometer of the same resistance value will solve the problem.
4. Precautions for practical application
(1) If you don’t know which parameter is wrong, or if the parameter has been messed up by someone else, you should first change it back to the factory setting.
(2) The output terminal of the inverter must never be connected to the power frequency power supply. To prevent such errors, when switching between frequency conversion and power frequency, circuit interlocking is generally not used, but mechanical interlocking should be used. Multi-position switches or auxiliary contacts are also used for switching, but overcurrent protection fuses must be added to prevent the contacts from sticking together.
(3) The power capacity of the selected frequency converter should ideally be 120% of the actual load. In practical applications, the problem of using a small power frequency converter with a large power motor should be avoided. If the motor load rate increases beyond the maximum power of the frequency converter, the frequency converter will easily burn out.
(4) The frequency converter can operate at more than 50 Hz. First, ensure that there is no overload, and second, ensure that the motor and pump it is connected to are allowed to operate at overspeed.
(5) The frequency converter can also operate below 10 Hz. The key is to adjust the torque compensation to ensure that there is appropriate torque at low speed. It is also necessary to ensure the motor cooling at low speed, because the cooling fan of most motors is synchronized with the motor shaft. At the same time, ensure that the pump has sufficient lubrication, because the oil of the plunger pump for polymer injection is also driven by the rotation of the pump itself to lubricate the system. Only a few large displacement plunger pumps are provided with independent lubrication pumps and are not affected by the pump speed.
(6) If a high-power frequency converter is used to drive a small load device, be sure to set its internal protection value to a value close to the load to avoid damage to the motor and to truly monitor whether the equipment is operating normally.
(7) The size of the external potentiometer resistor of the frequency converter should be selected according to the requirements of the frequency converter instruction manual. Too large or too small is not suitable. If the potentiometer is too small, the frequency cannot be adjusted. If the potentiometer is too large, it is not easy to control.
(8) The problem of incorrect wiring of the external potentiometer of the frequency converter should be avoided. If the wiring is incorrect, although the frequency converter can still run normally, the change of the potentiometer resistance value is exactly opposite to the change of frequency, which makes it inconvenient to adjust on-site. The solution is to readjust the wiring.
5. Conclusion
In summary, the commissioning, maintenance, and troubleshooting of frequency converters in oilfield polymer injection pump stations require not only familiarity with the converter's working principles and oilfield production processes, but more importantly, continuous accumulation of practical experience. Only through this accumulation can on-site maintenance personnel truly improve their troubleshooting capabilities and maintenance skills, thereby better mastering frequency converter repair techniques to meet production needs. Therefore, this analysis of common faults in oilfield polymer injection pump stations is intended to be helpful to colleagues in the frequency converter repair industry.
