Under normal circumstances, the submersible pump system works at a constant speed, and the displacement of the submersible pump is manually adjusted through the nozzle, which inevitably increases the pipeline loss, and the adjustment range is not large. However, under certain special circumstances, such as the water well parameters are not clear, or the water well in the water production process, the displacement drops quickly, etc., the displacement of the submersible pump is required, and the head is adjustable. In the submersible pump system, adding a frequency conversion device can meet such requirements.
The frequency of the power supply is changed by the frequency converter. The speed of the motor changes proportionally with frequency. The displacement of the submersible pump is proportional to the speed of the motor, and the amount of the lift is inversely proportional to the motor.
The key part of the inverter is the inverter. For ICBT and other devices, there is a Z-lOKHz switching frequency, which can significantly improve the performance of the PWM inverter. However, the pulse output from the PWM inverter rises rapidly in a short period of time, which in turn affects the motor, causing problems such as high frequency leakage current, shaft voltage, bearing current, and bearing damage. In the submersible pump system, a long cable is required between the ground and the downhole motor. The output pulse of the PWM inverter is transmitted to the motor through the long-term cable, which causes a voltage reflection phenomenon, which approximately doubles the voltage at the motor end, thereby causing the motor to Insulation damage.
Second, the analysis of voltage reflection phenomenon When using long-term cable, the PWM pulse transmitted between the inverter and the motor is similar to the uplink wave of the transmission line. The PWM pulse acts as a forward traveling wave (incident wave), which is transmitted from the inverter to the motor. After the motor end reflects, the reverse traveling wave (reflected wave) is transmitted to the inverter, and the reflection is transmitted to the inverter output. The wave produces a second person wave and so on. In order to better understand the traveling wave repeated reflection phenomenon of the long-distance cable between the inverter and the motor, firstly, a reflection process of the PWM pulse is discussed. It is assumed here that the dv/dt of the PWM pulse is infinite.
Since the motor has a large impedance when the frequency is high, it can be considered as an open circuit. When the switching device is turned on, the voltage and current of the human wave are simultaneously transmitted to the motor, and the waveforms of the two are the same and the amplitudes are different. Reflection occurs when the incident wave reaches the end of the transmission line. The current when the transmission line is open is zero at any time, so the amplitude of the reflected wave current should be the same as that of the human wave, but the sign is opposite and the incident voltage forms a positive voltage reflected wave, which is transmitted to the inverter. The reflected wave is the same as the human wave. Double the motor terminal voltage. Before the reflected wave reaches the source. The voltage of the transmission line is 2U. However, if the output voltage of the inverter at the source is U, there should be a negative reflection of the voltage U, which is transmitted by the inverter to the motor. The voltage traveling wave must also be transmitted with a current. Since the transmission direction is positive, the sign of the current is the same as the voltage. That is -I, this first person wave quickly reaches the terminal and is also reflected. The second person's jet current is negative, since the current is zero when the current is open. Then the current of the second reflected wave is positive, that is, I. The positive current must be transmitted to the inverter along with a negative voltage. The situation of the third person's wave is the same as that of the first person, and there is no need to continue to explore.
2 PWM pulse rise time influence, according to the traveling wave transmission, the filtering technique used to eliminate the influence of the PWM inverter driving the long-distance cable of the AC motor.
In order to eliminate the effects of long-distance cable transmission of AC motors driven by PWM inverters, such as overvoltage, voltage reflection, high-frequency damping oscillations and insulation damage caused. Passive filtering techniques are generally used to solve these problems. Such filters are sometimes connected to the motor end, called motor end filtering, and some are connected to the output of the inverter, called inverter output filtering.
In the submersible pump system. The terminals of the motor are not accessible and cannot be filtered on the motor side. At this point, the output of the inverter is filtered. It is more convenient to filter at the output of the inverter at this time. There are two ways to design such a filter.
One is to use the reflected wave theory, but to connect the filter to the output of the inverter with a concentrated inductor and a concentrated capacitor such as a long cable. In series with the cable, this class is LR parallel type filtering.
Another idea is to increase the rise time of the inverter output PWM pulse beyond the critical value, so that the overvoltage caused by the reflection phenomenon can be significantly reduced. According to this principle, the structure of the second-order low-pass inverter output filter is obtained. The filter is connected to the output of the inverter, which can effectively reduce the dv/dt of the inverter output pulse, thereby reducing the overvoltage and damping oscillation of the motor end.
This filter can reduce the dv eight It of the inverter output line voltage (ie, differential mode dvdt) to reduce the voltage reflection phenomenon and reduce the insulation pressure of the motor. At present, the inverter output filter is such that the line voltage is close to a sine wave, and the values of Lf and Cf are too large, and the common mode dv/dt cannot be lowered, that is, the motor bearing leakage current and the shaft voltage are not inhibited.
Based on the second-order LRC low-pass filter, the neutral point of the star-connected resistor and capacitor is connected to the midpoint of the inverter DC bus voltage. This inverter output filter structure has many advantages. It can symmetrically reduce the inverter output dv/dt in the sixth switching instant, and its parameter determination method and second-order low-pass inverter output filter. the same. With reasonable parameter design, the filter can be mounted in the same housing as the inverter and allows long cable to be used between the inverter and the motor. With this filter, the overvoltage of the motor terminal can significantly reduce the ground leakage current and the induced shaft voltage. Since one filter can solve the common mode and differential mode dV/dt problems at the same time, the size loss and cost of the filter are low. It is only slightly more lossy than the previous filter. Of course, we can design the inductor to be adjustable so that the frequency of the filter can automatically follow the switching frequency of the inverter. Due to its variable inductance, it automatically tracks the switching frequency, making it more versatile. But this filter adapts to the frequency measurement.
Fast switching by IcBT, etc.篼 Switching frequency power electronics consists of a PWM inverter driven AC motor. When using long-distance cable transmission, a series of problems will occur, such as the common mode dV/dt of the 篼, the shaft voltage induced by the ground leakage current and the bearing current, and the voltage reflection caused by the motor terminal overvoltage 篼 frequency damped oscillation, etc. The filtering technique can effectively eliminate these phenomena.