Abstract : This paper introduces the various drawbacks of traditional power supply methods for submersible electric pumps (SAPs), elaborates on the advantages of variable frequency power supply, and then provides several special requirements and solutions for SSP VEP inverters. Following the successful development and acceptance of its 1140V SSP VEP inverter by a wind and solar power company, a 2300V dedicated SSP VEP inverter has now been successfully developed. Keywords : inverter, latent oil electro-pump, low-pass filter, compensating voltage [align=center][b]2300V inverter for latent oil electro-pump The Shandong Phoenix Electronics Limited Company Li Luilai He Hongchen Han Wenzhao[/b][/align] Abstract : This paper presents several disadvantages of traditional power supply methods for latent oil electro-pumps and then describes some advantages of inverting frequency power supply methods. This paper describes the proper characteristics and solutions of this type of inverter. The company released a 1140V inverter for latent oil electro-pumps, which was approved by oilfields. After further research and manufacturing, they successfully produced a 2300V inverter for latent oil electro-pumps. Key words : inverter, latent oil electro-pump, low-pass filter, compensating voltage I. Introduction Using modern high technology to upgrade existing oilfield production equipment is an inevitable trend. Utilizing modern automatic control technology and variable frequency speed regulation technology to provide an ideal power source for submersible electric pumps (hereinafter referred to as submersible pumps) in oilfields is an important component of this technological transformation 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 and power frequency—lead to frequent malfunctions and significantly increased operating costs. Bringing a damaged submersible pump to the surface for repair costs alone of 50,000 yuan. Cables worth 100,000 yuan need replacement after an average of five hauls. Submersible pumps require maintenance on average every 10 months, with maintenance costs around 80,000 yuan. Traditional power supply methods pose numerous risks. For example: * When the submersible pump operates at full speed, it is prone to emptying the well when the downhole fluid volume is insufficient, potentially leading to a dead well and severe losses. * Full voltage and power frequency operation results in high starting current and impact torque, wasting electricity and significantly impacting motor lifespan. * Fluctuations in oilfield power supply voltage often lead to under-excitation or over-excitation of the motor, frequently resulting in motor burnout. * The several kilometers of downhole cable introduce approximately 150V of line loss, which, since uncompensated, affects the normal operation of the motor. Therefore, the traditional power supply method for submersible pumps must be modified. An ideal power supply device should possess the following characteristics: * Soft start * Convenient speed adjustment, i.e., variable frequency operation. Start-up time and running speed can be arbitrarily set according to working conditions. * Unaffected by power supply voltage fluctuations and able to compensate for cable line losses. * The transmitted signal on the cable must be a sine wave; otherwise, voltage pulses will be superimposed after cable reflection, easily burning out the motor. * Comprehensive protection functions. * Convenient control, simple operation, and clear display. Clearly, only a frequency converter can meet these requirements. However, readily available frequency converters designed for fans and pumps are unsuitable because of incompatible voltage levels, non-sine output waveforms, and the inability to compensate for cable voltage losses. Our company, commissioned by an oilfield, has successfully developed a series of 1140V, 30-100KW dedicated frequency converters for submersible pumps. We are now undertaking the development of a 2300V dedicated frequency converter for submersible electric pumps. II. Development of Dedicated Frequency Converters Although submersible pumps have 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 shortcomings. It is 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 electric pumps are already operating normally in several oilfields with excellent results. This article mainly introduces the performance and development status of the 2300V dedicated frequency converter for submersible electric 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: 2Hz～50Hz continuously adjustable; Voltage loss on the cable can be appropriately compensated; Output waveform: Sine wave control function and protection function are the same as ordinary frequency converters. This article only briefly describes the technical characteristics of this 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 frequency converters, a two-level circuit is generally used. If a two-level circuit is used to achieve a 2300V output, expensive high-voltage transistors must be used. In order to reduce the voltage withstand requirements of power devices and reduce the harmonic components of the output voltage, this design adopts a three-level circuit. The schematic diagram of the main circuit is shown in Figure 1. The main circuit diagram of the whole machine[/align] shows that the main circuit adopts a three-level circuit, also known as a netural point clamped (Npc) method. It can not only output a higher voltage, but also reduce output harmonics and voltage change rate (dv/dt). Good waveform is one of the goals of this design. The power switching device in the figure is a Siemens dual-unit IGBT module (1700V, 200A). After rectification, it is composed of two sets of large capacitors connected in series to form a filter. The connection point of the two sets of capacitors is the center point of this circuit (the middle level of the three levels). The three-level circuit structure and the 3300V IGBT module can achieve a 2.3KV inverter output. However, our familiar supplier of 3300V IGBT modules is not in stock and can only be ordered. Due to the tight schedule, we had to use a 1700V dual-unit module connected in series as a unit, which will reduce the cost. We took this opportunity to study the dynamic voltage equalization problem of device series connection. The IGBT symbol in Figure 1 is a simplified representation of dual-unit series connection. Direct series connection of IGBT power devices mainly addresses the voltage equalization problem. Steady-state voltage equalization is relatively easy, as the two IGBTs connected in series are devices within the same module, with similar manufacturing processes and ambient temperatures. Therefore, excessive measures are unnecessary, and the main focus should be on dynamic voltage equalization. After experimental screening, the voltage equalization circuit used in this design is shown in Figure 2. [align=center] Figure 2. Voltage Equalization Circuit[/align] The voltage equalization circuit consists of resistors R1 and R2, capacitor C, and diode D. Resistor R1 serves as a static voltage equalization component. R2, C, and D are in the same form as a typical buffer circuit, but their main purpose here is to provide dynamic voltage equalization. The voltage equalization process is primarily 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 impact 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 two IGBT switches' performance, 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 used. 2. Selection of carrier frequency Increasing the carrier frequency is very beneficial for improving the waveform and reducing noise. However, increasing the carrier frequency will increase the switching loss. Therefore, the advantages and disadvantages must be weighed when selecting. In this device, the carrier frequency is selected as 3.4KHZ. This value was chosen considering the weight factor of the inductor core of the LC low-pass filter at the output end. 3. The stable input voltage, after rectification and filtering, yields the DC bus voltage, denoted as 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 actual operation 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. Sine wave acquisition at the output: The voltage-type frequency converter 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 sent to the motor. Since the motor is an inductive load, it can obtain an approximately sinusoidal drive current. There are several kilometers of cable between the frequency converter 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 higher than the rated value due to the long-line effect, which may 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 about 1/3 of the carrier frequency. [align=center] Figure 3. Output low-pass filter[/align] 5. Compensation for cable loss The submersible pump does not have special requirements for the V/F curve. Oil production stops 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 starting performance of the motor. If Vb is too large, the starting current will be too large, causing an increase in losses. If Vb is too small, the motor will not start. As Vb gradually increases, the output current will inevitably change accordingly, resulting in a noticeable difference in current when the motor starts running. Based on this idea, we developed a software program called the Compensated Voltage Adaptive Program. If the software is successful, manual adjustment will not be necessary each time it starts. Currently, the software still needs further improvement and optimization, so the prototype still retains the manually adjustable potentiometer. [align=center] Figure 4. V/F Curve[/align] III. Operating Status The 2300V submersible pump-specific frequency converter has been successfully developed, and all technical indicators meet the design values. It operates well under rated load. The steady-state voltage imbalance of the series tubes is within 10%, and the dynamic voltage imbalance is within 15% as observed on an oscilloscope. The voltage after the output filter can be arbitrarily adjusted within the entire speed range of the submersible pump, and its waveform is always a sine wave (with very little distortion). Even at low speed and light load, the motor runs very smoothly and evenly without any pulsation. The motor speed adjustment is flexible and convenient throughout the entire frequency range.