Application analysis of VFD (Variable-frequency Drive) dedicated to electric submersible pumps

Abstract: After introducing the various shortcomings of the traditional power supply method of submersible electric pumps, this article explains the advantages of the variable frequency power supply method, and then gives several special requirements and solutions for VFD (Variable-frequency Drive) of submersible electric pumps. Method. After Sunwoele launched the 1140V submersible electric pump VFD (Variable-frequency Drive) and was recognized by the oil field, it has now successfully developed a dedicated VFD (Variable-frequency Drive) for the 2300V submersible electric pump.

I. Introduction

It is a general trend to use modern high and new technologies to transform existing oil field production equipment. Using modern automatic control technology and variable frequency speed regulation technology to provide ideal power for oilfield submersible electric pumps (hereinafter referred to as submersible pumps) is an important part of this technological transformation process. The voltage levels of submersible electric pumps are mostly 1140V and 2300V. The submersible pump is placed 1,000 to 3,000 meters below the ground level. The working environment is extremely harsh (high temperature, strong corrosion, etc.). The traditional power supply method - full voltage and power frequency causes frequent failures and greatly increases operating costs. After the submersible pump is damaged and raised to the ground for repair, the engineering fee alone reaches 50,000 yuan. The cable worth 100,000 yuan needs to be replaced after being raised and lowered an average of 5 times. The submersible pump needs to be repaired every 10 months on average. Maintenance The cost is about 80,000 yuan. Traditional power supply methods have many hazards. For example:

* The submersible pump operates at full speed. When the downhole fluid volume is not sufficient, it is easy to empty the well and even cause a dead well. Once the well is dead, the losses will be heavy.

* Full voltage, power frequency operation has large starting current and large impact torque, which not only wastes electricity, but also has a great impact on the life of the motor.

* The power supply voltage in oil fields often fluctuates, causing the motor to be under- or over-excited and the motor to be burned.

* Several kilometers of underground cables bring a line loss of about 150V. Since this loss cannot be compensated, it affects the normal operation of the motor.

It can be seen from the above that the traditional power supply method of submersible pumps must be transformed. The ideal power supply equipment should have the following characteristics:

＊Soft start

* Convenient speed adjustment, that is, variable frequency operation. The starting time and running speed can be set arbitrarily according to the working conditions.

* It is not affected by power supply voltage fluctuations and can compensate for cable line losses.

＊ What is transmitted on the cable must be a sine wave, otherwise it will be reflected by the cable and the voltage pulses will be superimposed, which will easily burn the motor.

* Various protection functions are complete.

＊Convenient control, simple operation and clear display.

Obviously, VFD (Variable-frequency Drive) must meet these requirements, but those VFDs (Variable-frequency Drive) that are easily available on the market for fans and water pumps are not suitable because the voltage level does not match and the output waveform is not sinusoidal. Cable voltage losses cannot be compensated. Our company was commissioned by the oil field and successfully developed a VFD (Variable-frequency Drive) series dedicated to 1140V, 30-100KW submersible pumps. Now it has undertaken the development task of VFD (Variable-frequency Drive) dedicated to 2300V electric submersible pump.

2. Development of special VFD (Variable-frequency Drive)

Although submersible pumps have different voltage levels, most of them running online are 1140V and 2300V. There are reports in the industry of using 380V-class VFD (Variable-frequency Drive) with a special step-up transformer. This article believes that this high-low-one-high scheme has inherent shortcomings. It is very difficult to make the step-up and step-down transformers work at low frequencies. The addition of transformers It also increases the product cost. The withstand voltage of current IGBT devices is already relatively high. There is no need to resort to a transformer for VFD (Variable-frequency Drive) below 3000V. Our company's 1140V submersible electric pump has been operating normally in several oil fields with great results. good. This article mainly introduces the performance and development of VFD (Variable-frequency Drive) dedicated to 2300V submersible electric pumps.

The technical indicators of the VFD (Variable-frequency Drive) are:

Three-phase input: 2300V, 50Hz

Three-phase output: rated voltage 2300V, capacity 110kW

Frequency range: 2HZ～50HZ continuously adjustable

Voltage losses on the cable can be appropriately compensated.

Output waveform: sine

The control functions and protection functions are the same as those of ordinary VFD (Variable-frequency Drive).

This article only briefly describes the technical characteristics of the frequency conversion system as follows: (The same parts as the 380V-class general-purpose VFD (Variable-frequency Drive) will not be repeated again).

1. Selection of main circuit and power devices

In PWM voltage type 380V class VFD (Variable-frequency Drive), a two-level circuit is generally used. If a two-level circuit is used to achieve an output of 2300V, expensive high-voltage tubes must be used. In order to reduce the withstand voltage requirements of the power devices and reduce the harmonic components of the output voltage, this design uses a three-level circuit.

The main circuit part adopts a three-level circuit or Netural Point Clamped-Npc method. It can not only output a higher voltage, but also reduce output harmonics and voltage change rate (dv/dt), and have good waveform positive This is one of the goals of this design. The power in the picture