Application of IGBT in petroleum induction heating power supply

1 Introduction

In order to exploit the heavy oil containing " three highs " such as asphalt, wax and high freezing point , oil fields usually adopt steam stimulation, chemical viscosity reduction and hot water jet accompaniment. These methods have the disadvantages of high investment and inconvenient management. At present, Jiangsu Oilfield adopts steel hollow sucker rod induction heating technology in combination with its own characteristics when exploiting heavy oil. This technology has the following characteristics: simple equipment, convenient downhole operation and maintenance; adjustable input power; easy to realize automatic control; uniform heating along the way without local overheating; high thermal efficiency (heating in the center of the oil pipe); no fear of power outage and pumping; extended operation cycle; improved production efficiency; no pollution to the environment; safe to use and easy to manage; small one-time investment and fast capital recovery [ 1 ].

According to the current domestic power supply mode, the hollow sucker rod induction heating system adopts the industrial frequency induction heating method. In order to balance the three-phase power consumption, the industrial frequency heating power supply shifts one phase of the industrial frequency three-phase AC power to the other two phases through the reactor and capacitor series phase shift, and then directly converts it into a single-phase industrial frequency AC power suitable for various heating requirements through the transformer and then connects the heating conductor. Therefore, the industrial frequency heating power supply has high cost, large size, bulky weight and low efficiency.

The petroleum medium-frequency induction heating power supply uses IGBT as the inverter switch device. Compared with the commonly used industrial frequency heating power supply, its volume is reduced by 40 % and its weight is reduced by 50 %.

2 Medium frequency induction heating power supply

The circuit structure of the medium frequency induction heating power supply is shown in Figure 1. The three-phase rectifier rectifies the industrial frequency three-phase AC into DC. After filtering, it is converted into a single-phase medium frequency AC with continuously adjustable frequency and duty cycle within a certain range by the full-bridge inverter, and then output to the heating load through the isolation transformer. The full-bridge inverter adopts a pulse width modulation ( PWM ) zero voltage switching circuit, which has the advantages of low switching loss and low electromagnetic interference [ 2 ]. The control circuit adopts the SG3524 integrated block to adjust the voltage of the 9th pin to ensure the dead time of the output signal. The pulse width of the output signal is modulated by the oil temperature detector. The temperature of the oil is controlled at 45 ~ 70℃ . Too high temperature can easily change the chemical properties of the oil. Too low temperature will reduce the fluidity of the oil.

3 IGBT gate drive circuit

3.1 Selection of IGBT gate drive module

IGBT gate drive modules EXB841 and M57962L can be used to drive IGBT modules within 400A of the 1200V series , and have overcurrent detection and protection functions. The output waveforms of these two drive modules under short-circuit protection are shown in Figure 2. EXB841 generates a 5V negative bias internally and is not adjustable; M57962L uses a voltage-stabilizing diode to generate a 9V negative bias externally , and the gate-off reliability is higher than that of EXB841 . In addition, the protection action time of M57962L (from overcurrent to gate voltage dropping to 0V) is 6.3μs ( see Figure 2a ) ; the protection action time of EXB841 is 16μs and the gate-off voltage cannot drop below -2V (see Figure 2b ), which leads to a greater risk of IGBT explosion than M57962L . Therefore, M57962L should be selected .

3.2 Improvement of peripheral circuit of driver module

When the IGBT is turned off, the voltage rise rate between the collector and emitter is as high as 30000V / μs . If it is too high, it will produce a large displacement current. It will also lead to a large collector pulse surge current, which can easily cause the IGBT to be dynamically locked. In order to avoid this malfunction of the IGBT , a negative bias must be added to the IGBT gate [ 3 ]. However, in actual applications, the following two situations will cause the negative bias of the drive circuit to disappear: ① The voltage regulator diode breaks down and shorts; ② The drive circuit loses the +24V power supply.

In view of the above two situations, the author improved the peripheral circuit of the IGBT driver module M57962L based on the manufacturer's recommended application circuit , as shown in Figure 3. Under normal circumstances, VZ4 is turned on, M57962L⑧ foot is high potential, VD1 is turned off, VT2 is turned on, and the output end of VO1 is in a low impedance state. If the voltage-stabilizing diode VZ1 or VZ3 breaks down and shorts, VZ4 is turned off, VT1 is turned off, and the output end of VO1 is in a high impedance state. If the driving circuit loses the +24V voltage, no current flows through the input end of VO1 , and the output end of VO1 is in a high impedance state.

4 Connection between IGBT module and filter capacitor

The input characteristics of IGBT are similar to those of MOSFET . The input impedance is high. If the drive circuit loses voltage, the gate of the IGBT loses negative bias. It is in a high impedance state to the emitter. At this time, once interference reaches the gate of the IGBT , the upper and lower tubes of the IGBT module are likely to be turned on at the same time. If the IGBT module is directly connected to a filter capacitor of several thousand microfarads, the energy stored in the filter capacitor will be directly released through the upper and lower tubes of the IGBT module, which may easily cause damage to the IGBT module. Therefore, when the induction heating power supply is turned on, the power supply of the control and drive circuits is first connected, and then the IGBT module is connected to the filter capacitor. When shutting down, the IGBT module is disconnected from the filter capacitor first , and then the power supply of the control and drive circuits is turned off.

5 Test results and conclusions

The induction heating power supply of PetroChina has been tested in the field of the second trial production plant since June 1997 and is still working normally. The power supply power is 3588kw . The induction heating power supply of PetroChina can not only be used in the hollow sucker rod induction heating system. It can also be used in the medium frequency induction electric heating system of the gathering and transportation pipelines in complex small fault block oil fields, which is of great significance to reducing the infrastructure investment and operation costs of the gathering and transportation pipelines.