The origin of electrical submersible pump adopting frequency variation technology.

From ten years, AC frequency variation device from small volume to big volume, from simple open-loop system to complex close-loop system, people try to use it  continually and get great benefit from it. As the speed  regulation device, it is used on AC motor that easy to use and not easy to regulate speed, whether from technology demand or energy saving, it is changing the state that old AC motor not easy to regulate speed.

Also, electrical submersible pump as momentum of oil well pumping unit, whether from technology demand or energy saving, using frequency variation and speed regulation technique can get benefit. But it is not widely used because the old technique project is not ideal. The technique project that our company adopt is the most advanced technique in the world.

1.1 The frequency variation technique that electrical submersible pump adopted is the best  drive project

Electrical submersible pump is the multiple stage centrifugal pump that worked down the oil well, it is one of the new machinery oil extraction device. It works in the poor environment of high temperature, strong erosive 2-3km down the oil well, because it goes deep into reservoirs, the work efficiency is high. recently, the oil wells are used widely, the power is between 55kw to 75kw, because the oil well is deeper, in order to reduce circuitry pressure drop, the drag motor doesn't adopt power frequency but 1000 level. The voltage belong to  medium-voltage, it is not high and low. If reduce voltage, the pressure drop of circuitry is too big, then the motor can't work normally. If increase voltage, the insulation problem of  power supply circuitry cable and motor is obvious, the malfunction rate is high and the reliability is low. So, the most  oil wells in China adopt electrical submersible pump of 1140 level.

By investigation, the main problem as following:

A. Impulse current is big when electricity start, distributing inductance heighten the anti-press in system, so it often damages most insulation of system.

B, Because the geology condition of oil well change greatly, the design margin of pump is often big, especially when the liquid in the well is not enough, oil-pressure of pump is too high,  this can shorten the using lifetime of pump, the rate of its repair and change will be increased.

To solve the above problem, speed regulation control for the pump motor must be taken. Adopting AC frequency variation and speed regulation is the ideal measure at present. The reasons are as following:

A, Inverter has the soft start function, during start,  motor rotate speed with the frequency variation, it will rise speed while it approaches synchronization state, so anti-electricity and impulse current  is small, it doesn't often has the problem that insulation is destroyed.

B, whether over load or light load, the power factor of system is higher, especially in the state of small load, reactive power reduce greatly, it has obvious effect of  energy-saving.

C. It can modulate oil quantity according to the current state of oil well, making the oil well work in the best state. To increase work efficiency while reducing malfunction rate.

D. It can also form of system of pressure、 temperature close-loop, to enhance automation degree and realize best control. After adopting frequency variation and speed regulation, it can increase the output for the rich oil well; it can produce continuously and reduce the times of stop well and has the aim of power saving for the poor oil well; it can reduce times of blocking pump and can reverse draining sand for the oil well that contains sand, so prolong the lifetime of pump; for the oil well that contain gas, it can increase rotate speed and reduce the air-lock phenomena because of no good separation of oil gas; for the oil well contain wax, it can reduce wax deposit, scaling and reduce the time of pipeline jam; for the oil well of thick oil, it can operate by low speed and high-power,  so it reduce the times of stop well and get considerable energy saving effect.

1.2 Compare with several frequency variation and speed regulation project that adopted in the past

1.2.1 Adopting general inverter of power frequency 

This project adopts transformer and universal inverter under power frequency to compose high-low-high mode for getting the aim of pump motor frequency variation.  It is easy and simple, but the system of it is too heavy, especially the output side-transformer must be designed by lowest work rotate speed, it is not universal and the design is not reasonable. The efficiency is lower because of often change of system.

1.2.2 Adopting versatility low voltage frequency variation (two level) technology to realize medium voltage frequency variation, the diagram of system is as following:

The project adopts low voltage rectifier and low voltage capacitance in series, high voltage IGBT invert directly.  The control mode also adopts traditional sine wave PWM. The system composing is simple, but the voltage wave of output is high voltage rectangle pulse, so it has the difficult dv/dt problem, this is not suitable for the state of long distance transmission line, and it also has the insulation problem for motor, the output current wave is also not ideal.

The technology project of medium voltage inverter of our company

1.3  We adopt technique of diode clamping three level, the system diagram is as following:

The technique was applied and promoted by foreign factories since 80s, the company of Toshiba 、ABB 、Mitsubishi、 Siemens etc. all have this product, it adopts medium-voltage inverter.  Compare with two level, the voltage wave of output  also compose of  rectangle pulse, but its pulse amplitude reduce one time, it solved the insulation and harmonic etc. problem of dv/dt.  there is little problem that the current commutation of invert components are also ensured by clamping diode, but this deficiency isn't the limiting factor for the inverter, the current wave is also ideal.

1.4 Application brief introduction of medium-voltage inverter of Jonweb technology

The medium-voltage inverter of our company is for AC asynchronous motor under domestic 1140V electrical net to design. But the medium-voltage products of foreign country are inverter of input voltage: 2300V、3300V、4160. For the medium-voltage inverter (6-10kv) of higher voltage, it takes multiple level inverter of invert unit in series mode as leading, now it is during developing. From the point of technology development, 1-2KV inverter first choose three level project, because the circuit frame is simple and there is suitable component(1700V-3300V IGBT) to use, it is also cheap. So this inverter has good future, it is widely used in the drive system of mine etc.