I. Introduction
Upgrading existing oilfield production equipment with modern high technology is an inevitable trend. Using modern automatic control technology and variable frequency speed regulation technology to provide an ideal power supply for submersible electric pumps (hereinafter referred to as submersible pumps) is an important part of this technological upgrade process. Submersible pumps typically operate at voltage levels of 1140V and 2300V. Located 1000-3000 meters below ground level, submersible pumps operate in extremely harsh environments (high temperatures, strong corrosion, etc.). Traditional power supply methods—full voltage, mains frequency—lead to frequent failures and significantly increased operating costs. Bringing a damaged submersible pump to the surface for repair costs as much as 50,000 yuan in engineering fees alone. Cables worth 100,000 yuan need to be replaced after an average of five hauls. Submersible pumps require maintenance on average every 10 months, with maintenance costs of approximately 80,000 yuan. Traditional power supply methods pose numerous disadvantages.
For example:
When a submersible pump is running at full speed, it can easily run dry when the fluid level in the well is low, potentially leading to a dead well. A dead well can result in significant losses.
Full-voltage, power frequency operation results in a large starting current and a large impact torque, which not only wastes electricity but also has a significant impact on the lifespan of the motor.
The power supply voltage in oil fields often fluctuates, causing motors to be under-excited or over-excited, and motors sometimes burn out.
The underground cable, which is several kilometers long, resulted in a line loss of about 150 volts. Since this loss could not be compensated, it affected the normal operation of the motor.
As can be seen from the above, the traditional power supply method for submersible pumps must be modified. An ideal power supply device should have the following characteristics:
soft start
Speed adjustment is convenient, i.e., variable frequency operation. Start-up time and running speed can be set arbitrarily according to working conditions.
It is unaffected by fluctuations in power supply voltage and can compensate for cable line losses.
The signal transmitted over the cable must be a sine wave; otherwise, the voltage pulses will be superimposed due to reflection from the cable, which can easily burn out the motor.
It has a full range of protective functions.
It is easy to control, simple to operate, and has a clear display.
Obviously, only frequency converters can meet these requirements. However, the frequency converters readily available on the market that serve fans and water pumps are unsuitable because of incompatible voltage levels, non-sinusoidal output waveforms, and inability to compensate for cable losses. Our company, commissioned by the oilfield, has successfully developed a series of 1140V, 30-100KW dedicated frequency converters for submersible pumps. We are now undertaking the development of a dedicated frequency converter for 2300V submersible oil pumps.
II. Development of Dedicated Frequency Converters
While submersible pumps come in different voltage levels, most operate online at 1140V and 2300V . There are reports in the industry of using 380V frequency converters with specially designed step-up transformers. This article argues that this high-low-high scheme has inherent limitations. It's difficult to make the step-up and step-down transformers operate at low frequencies, and the addition of transformers increases product costs . Modern IGBT devices have high voltage withstand capabilities , and frequency converters below 3000V do not need to rely on transformers . Our company's 1140V submersible oil pumps are already operating normally in several oil fields with excellent results. This article mainly introduces the performance and development of a dedicated frequency converter for 2300V submersible oil pumps.
The technical specifications of this frequency converter are as follows:
Three-phase input: 2300V, 50Hz
Three-phase output: rated voltage 2300V, capacity 110kW
Frequency range continuously adjustable from 2Hz to 50Hz
Voltage loss on the cable can be adequately compensated.
Output waveform: sine wave
The control and protection functions are the same as those of ordinary frequency converters.
This article only briefly describes the technical features of the frequency converter system as follows: (the parts that are the same as those of the 380V general-purpose frequency converter will not be repeated).
1. Selection of main circuit and power devices
In PWM voltage-type 380V inverters, a two-level circuit is generally used. However, achieving a 2300V output using a two-level circuit would necessitate expensive high-voltage transistors. To reduce the voltage withstand requirements of power devices and lower the harmonic content of the output voltage, this design employs a three-level circuit. The main circuit schematic is shown in Figure 1.
The main circuit adopts a three-level circuit, also known as a netural point clamped (NPC) method . This not only outputs a higher voltage but also reduces output harmonics and voltage change rate (dv/dt), resulting in a favorable waveform – one of the design goals. The power switching device in the diagram is a Siemens dual-unit IGBT module (1700V, 200A). After rectification, a filter is formed by two sets of large capacitors connected in series. The connection point of the two sets of capacitors is the center point of the circuit (the middle level of the three-level circuit). A three-level circuit structure and a 3300V IGBT module could achieve a 2.3KV inverter output. However, our familiar supplier of 3300V IGBT modules was unavailable and could only be pre-ordered. Due to the tight schedule, we had to use a 1700V dual-unit module connected in series as a single unit, which would reduce costs. This provided an opportunity to study the dynamic voltage equalization problem of connected devices. The IGBT symbol in Figure 1 is a simplified representation of a dual-unit connected in series. The main challenge in directly connecting IGBT power devices in series is voltage equalization. Steady-state voltage equalization is relatively easy, as the two IGBTs connected in series are from the same module, with similar manufacturing processes and ambient temperatures. Therefore, excessive measures are unnecessary; the focus should be on dynamic voltage equalization. After experimental selection, the voltage equalization circuit used in this design is shown in Figure 2 .
The voltage equalization circuit consists of resistors R1 and R2, capacitor C, and diode D. Resistor R1 serves as a static voltage equalizer, while R2, C, and D are configured similarly to a typical buffer circuit, but their primary purpose here is to provide dynamic voltage equalization.
The voltage equalization process is mainly accomplished by capacitor C. When two IGBTs are connected in series, their switching speeds will not be completely identical, but will differ slightly. The voltage across capacitor C is the same under static conditions. During switching, because the voltage across the capacitor cannot change abruptly, the voltage drop across the two IGBTs is forced to remain constant. The effect caused by the inconsistent current in the two IGBTs during switching is compensated by the charging and discharging of capacitor C.
As can be seen from the dynamic voltage equalization process, the better the consistency of the performance of the two IGBT switches, the better the voltage equalization effect; the larger the value of capacitor C, the better the voltage equalization effect. However, an excessively large value of C will result in excessive power consumption on R2. P = 1/2CV²f , where V is the switching voltage on a single IGBT. To limit the power consumption on R2, the capacitor C value should be as small as possible and a lower modulation frequency f should be adopted.
2. Selection of carrier frequency
Increasing the carrier frequency is beneficial for improving waveform and reducing noise. However, increasing the carrier frequency will increase switching losses. Therefore, the advantages and disadvantages must be weighed when making the selection. In this device, the carrier frequency is selected as 3.4KHZ . The weight of the inductor core of the LC low-pass filter at the output end was taken into account when selecting this value.
3. Stability of input voltage
The input voltage is rectified and filtered to obtain the DC bus voltage, denoted by Uo. A voltage sensor is installed here, and its output voltage Ut is proportional to the bus voltage Uo. The Ut value is sent to the microcontroller for processing. The rated value of Uo corresponds to a Ut value of 1. When the grid voltage fluctuates upward, Ut > 1; when the grid voltage fluctuates downward, Ut < 1. The CPU multiplies the PWM pulse width by a factor of 1/Ut. This achieves the purpose of stabilizing the input voltage. In the actual operation of the equipment in the oil field, when the grid voltage fluctuates by +10%, no voltage fluctuation can be detected on the motor side, indicating that the Ut compensation effect is significant.
4. Obtaining the sine wave at the output terminal
The voltage-source inverter outputs a three-phase SPWM wave, which is a rectangular pulse wave with a width distributed according to a sinusoidal law. This wave is directly fed to the motor, and since the motor is an inductive load, it can obtain an approximately sinusoidal drive current. There are several kilometers of cable between the inverter and the submersible pump. If the PWM wave is directly applied to the cable input, the motor side will be subjected to a voltage spike several times the rated value due to the long-line effect, which could potentially burn out the motor. Therefore, a three-phase low-pass LC filter is necessary. The filter circuit is shown in Figure 3. In this design , its cutoff frequency is approximately 1/3 of the carrier frequency.
5. Compensation for cable loss
Submersible pumps do not have special requirements for the V/F curve. Oil production ceases when the frequency drops below 30Hz. To achieve soft start, the starting frequency of this equipment is set at 2Hz. 50Hz corresponds to a rated output of 2300V. The cable compensation voltage Vb is adjusted according to the specific conditions of each oil well. The V/F curve is shown in Figure 4 .
The value of Vb determines the motor's starting performance . A large Vb will result in excessive starting current , increasing losses ; a small Vb will prevent the motor from starting . As Vb gradually increases , the output current will inevitably change accordingly, showing a significant difference when the motor starts running. Based on this idea, we developed software called a compensated voltage adaptive program. If the software is successful, manual adjustment will be unnecessary for each start-up. Currently, the software still needs further improvement and optimization; therefore, the prototype developed in this project still retains the manually adjustable potentiometer.
III. Operational Status
A dedicated 2300V submersible pump frequency converter has been successfully developed, with all technical specifications meeting design values and operating well under rated load. The steady-state voltage imbalance of the series tubes is within 10%, and the dynamic voltage imbalance, as observed on an oscilloscope, is within 15%. The voltage after the output filter can be arbitrarily adjusted across the entire RPM range of the submersible pump, with a consistently sinusoidal waveform (minimal distortion). Even at low speeds and light loads, the motor operates smoothly and evenly without any pulsation, and speed adjustment is flexible and convenient across the entire frequency range.
