Abstract: After introducing many shortcomings of traditional power supply mode of submersible electric pump, this article expounds the superiority of variable frequency power supply mode, and then gives some special requirements and solutions of submersible pump inverter. After the launch of the 1140V submersible pump inverter and recognition by the oil field, Scene Electronics has now developed the 2300V submersible pump-specific inverter.
Keywords: inverter, submersible pump, low pass filter, offset voltage
I. Introduction
It is the trend of the times to transform the existing oilfield production equipment with modern high technology. With modern self-control technology and frequency control technology to the oil field submersible pump (hereinafter referred to as submersible pump) to provide the ideal power supply is an important part of this technological transformation process. Submersible pump voltage levels are mostly 1140V and 2300V. Submersible pumps are placed 1000 to 3000 meters below the ground level, the working environment is extremely harsh (high temperature, strong corrosion, etc.), the traditional power supply - full pressure, frequency makes it frequent failures, operating costs greatly increased. Submersible pump damage mentioned on the ground to repair, only one project costs up to 50,000 yuan, the value of 100,000 yuan on average put on the cable to be placed on the 5 to be replaced, the submersible pump on average every 10 months to be repaired once, maintenance Cost about 80,000 yuan. Traditional power supply is very harmful.
E.g:
* The submersible pump runs at full speed. When the amount of liquid in the well is not enough, it is easy to take the time to evacuate or even cause the well. Once the well is dead, the loss will be heavy.
* Full pressure, work frequency starting current, impact torque, not only a waste of electricity, but also a great impact on motor life.
* Oilfield power supply voltage fluctuations, the motor under excitation or over excitation, the motor has been burned from time to time.
* A few kilometers of underground cable brings about 150V line loss, because this part of the loss can not be compensated, thus affecting the normal operation of the motor.
It can be seen from the above, the traditional submersible pump power supply must be transformed, the more ideal power supply equipment should have the following characteristics:
* Soft start
* Convenient speed control, that is, variable frequency operation. Start-up time and operating speed can be arbitrarily set according to working conditions.
* Unaffected by power supply voltage fluctuations, and to compensate for cable line losses.
* Cable must be transmitted on the sine wave, or by cable reflection, voltage pulse superimposed, easy to burn the motor.
* Various protection features complete.
* Convenient control, simple operation, clear display.
Obviously, to meet these requirements is non-inverter, but the market for fans, pumps are not suitable for those drives, because the voltage level does not match, the output waveform is not sinusoidal, cable loss voltage can not be compensated. My company commissioned by the oil field successfully developed a 1140V, 30 ~ 100KW submersible pump inverter series. Now also assume the 2300V submersible pump dedicated inverter development tasks.
Second, the development of special inverter
Although the submersible pump voltage levels are different, but mostly online operation of 1140V and 2300V. Useful 380V inverter industry with a special step-up transformer reports, this paper believes that this one high and one low-high program has inherent deficiencies, so that the work of the step-down transformer at low frequencies is very difficult, the addition of the transformer has increased the cost of the product Now IGBT devices have higher withstand voltage, 3000V below the inverter is not required to help the transformer, the company 1140V submersible pump has been in several normal operation of the oil field, the effect is very good.
This article introduces 2300V submersible pump dedicated inverter performance and development.
The inverter technical indicators:
Three-phase input: 2300V, 50Hz
Three-phase output: rated voltage of 2300V, capacity 110kW
Frequency range: 2HZ ~ 50HZ continuously adjustable
The voltage loss on the cable can be properly compensated.
Output waveform: sine
Control functions, protection functions with ordinary inverter.
This article only on the technical characteristics of the inverter system is as follows: (with 380V-class general-purpose inverter not repeat the same part).
1, the main circuit and power device selection
In the PWM voltage 380V-class inverter, the general use of two-level circuit. If the two-level circuit to achieve the output of 2300V is bound to require expensive high-pressure pipe, in order to reduce the power device voltage requirements and reduce the output voltage of the harmonic components, the design uses three-level circuit. The main circuit diagram shown in Figure 1. As shown
The main circuit part of the three-level circuit or the center point of the clamp (NeturalPointClamped-Npc) way, it not only can output higher voltage, but also reduce the output harmonic and voltage change rate (dv / dt), a good positive waveform It is one of the goals of this design. The power switching device in the figure uses the Siemens dual-cell IGBT module (1700V, 200A). After rectification, two large capacitors are connected in series to form a filter. The connection point of the two capacitors is the center point of the circuit Level). With a three-level circuit structure, 3300V IGBT module can achieve 2.3KV inverter output, but we are familiar with the supplier 3300V IGBT module is not available, only scheduled, because the task is tight, had to use 1700V dual unit module series When a unit to use, so the cost will be lower, just take this opportunity to study the device in series dynamic pressure equalization problems, Figure 1. The IGBT symbol in is a simplified representation of a dual cell series connection. The direct series connection of IGBT power devices mainly solves the voltage equalization problem. The steady-state voltage equalization is relatively easy. The two tubes in series are devices in the same module. The manufacturing process and the ambient temperature are basically the same. Therefore, it is unnecessary to take excessive measures. Put the main focus on dynamic pressure. After screening experiments, the design of the voltage equalizer circuit shown in Figure 2. Shown
Figure 2. Equalizing circuit
Equal voltage circuit by the resistor R1, R2, capacitor C, diode D composition. Resistor R1 plays the role of static voltage equalization, R2, C, D and ordinary buffer circuit in the same form, the main purpose here is to start the role of pressure equalization.
Equalization process is mainly done by the capacitor C. Two series IGBT, switching speed will not be exactly the same, but will be slightly different. The voltage on the capacitor C is the same under the static condition. During the switching process, the voltage drop on the two IGBTs will not be tripped because the voltage on the capacitor can not be abruptly changed.
Due to the switching process in the two IGBT current inconsistency caused by the charge and discharge capacitor C compensation.
The dynamic pressure equalization shows that the better the switch performance of the two IGBTs is, the better the voltage equalization is. The larger the value of the capacitor C, the better the voltage equalization. However, an excessively large C value will cause excessive power dissipation on R2. P = 1 / 2CV2f, V is the transition voltage of single IGBT. In order to limit the power dissipation on R2, the capacitor C value should be as small as possible and a lower modulation frequency f should be adopted.
2, the choice of carrier frequency
Increase the carrier frequency to improve the waveform, greatly reducing noise, but the carrier frequency increases, will increase the switching loss, so the choice must weigh the pros and cons, the device selected carrier frequency 3.4KHZ, select this value to take into account the output LC Low-pass filter inductor core weight factor.
3, the stability of the input voltage
The input voltage is rectified and filtered DC bus voltage to Uo said that there is a voltage sensor, the output voltage Ut is proportional to the bus voltage Uo, the Ut value sent to the microcontroller processing, so that Uo corresponding to the rated Ut value 1, Ut & gt; 1, Ut & lt; 1 when the grid voltage fluctuates downward, and the CPU multiplies the factor 1 / Ut when calculating the pulse width of the PWM wave. This achieves the goal of stabilizing the input voltage. Equipment in the actual operation of the oil field, when the grid side voltage fluctuations of + 10%, the motor side can not measure the voltage fluctuations, indicating Ut compensation effect is obvious.
4, the output of the sine wave acquisition
Voltage-frequency inverter output is three-phase SPWM wave, that is, the width of the sinusoidal distribution of rectangular pulse wave. This wave is sent directly to the motor, because the motor is inductive load, so you can get an approximate sinusoidal drive current. There is a few kilometers of cable from the drive to the submersible pump. If the PWM wave is applied directly to the cable input, the motor will be subjected to several times the peak value of the rated voltage due to the long-term effect, and the motor is likely to be Burn out. Therefore, three-phase low-pass LC filter is necessary, the filter circuit shown in Figure 3. In this design, the cut-off frequency of about 1/3 of the carrier frequency.
5, cable loss compensation
Submersible pump V / F curve and no special requirements, the frequency down to 30HZ the following has not oil, in order to achieve soft start, the device set the start frequency 2HZ, 50HZ corresponding to 2300V rated output, cable compensation voltage Vb according to each The specific conditions of oil well adjustment. V / F curve shown in Figure 4.
Vb determines the size of the motor starting performance is good or bad, Vb is big, will make the starting current is too large. Caused by the loss increase, Vb is too small, can not get up. Vb gradually increased by the small, the output current will inevitably change accordingly, when the motor starts running, the current will be significantly different. According to this idea we have written a software called compensation voltage adaptive process. If the software is very successful, each time you start it, you do not have to manually adjust it. At present, the software needs to be further improved and optimized. Therefore, the prototype developed by this time still retains the potentiometer that is manually adjusted.
Third, the operation
2300V submersible pump inverter has been successfully developed, the technical indicators are to meet the design value, with rated load is running well. The tandem tube's steady-state pressure unbalance is within 10%, and the dynamic pressure unbalance is within 15% on the oscilloscope. After the output filter voltage in the submersible pump within the entire range of arbitrary modulation, the waveform is a sine wave (distortion is very small), the motor is at low speed, light load, the operation is very smooth, uniform, without any Pulse phenomenon, the motor speed and flexibility in the entire frequency range.