Application of Sunwoele VFD (Variable-frequency Drive) in Oilfield Kowtow Machine

I. Introduction

In the 21st century, frequency conversion speed regulation technology has been widely used in China's oil fields thanks to its excellent energy-saving and speed regulation characteristics. China's output value energy consumption is one of the highest in the world. To solve the problem of product energy consumption, in addition to other related technical issues that need to be improved, frequency conversion speed regulation technology has become an effective measure for energy saving and product quality improvement.

As a special industry, the oil field has its own unique background. The application of VFD (Variable-frequency Drive) in the oil field mainly focuses on beam pumping unit control, electric submersible pump control, water injection well control and oil and gas gathering and transportation control. occasion. The beam pumping unit, commonly known as "kowtow machine", is the pumping unit commonly used in various oil fields at present. However, the current pumping unit system generally has obvious problems such as low efficiency, high energy consumption, and inconvenient adjustment of stroke and stroke times. Shortcomings. This article mainly introduces the application of HSF600 (Variable-frequency Drive) on beam pumping units.

1. The working principle of kowtow machine

As shown in the physical picture of the beam pumping unit, when the kowtow machine is working, the load acting on the suspension point of the donkey head changes. During the upstroke, the suspension point of the donkey head needs to lift the sucker rod column and the liquid column. When the pumping unit is not balanced, the motor will spend a lot of energy. During the downstroke, the rod column of the pumping unit turns to do work on the motor, so that the motor is in the running state of the generator. When the pumping unit is not balanced, the load on the up and down strokes is extremely uneven, which will seriously affect the efficiency and life of the four-bar linkage mechanism, reduction box and motor of the pumping unit, worsen the working conditions of the sucker rod, and increase the number of times it breaks. In order to eliminate these shortcomings, a balance weight is generally added to the end of the beam or the crank or both of the pumping units, as shown in Figure 1. In this way, during the downstroke of the suspension point, the balance weight should be lifted from a low place to a high place to increase the potential energy of the balance weight. In order to raise the balance counterweight, in addition to relying on the potential energy released by the drop of the sucker rod string, the electric motor must pay part of the energy. During the upstroke, the balance weight falls from a high place, releasing the potential energy stored during the downstroke, helping the motor to lift the sucker rod and liquid column, and reducing the energy required by the motor during the upstroke. The more beam pumping units used at present all adopt the working mode of adding balance counterweight, so in one working cycle of the pumping unit, there are two motor operating states and two generator operating states. When the balance counterweight is adjusted well, the time of its generator running state and the energy produced are all small.

2. VFD (Variable-frequency Drive) control problem in pumping unit

At present, among the pumping equipment used in Shengli Oilfield, the beam pumping unit is the most common and the number is the largest. Its number reaches more than 100,000 units. The power consumption of pumping units accounts for about 40% of the total power consumption in the oil field, and the operating efficiency is very low, with an average operating efficiency of only 25%. The power factor is low and the waste of electric energy is large. Therefore, the energy-saving potential of pumping units is very huge, and the petroleum industry is also a key industry to promote "energy-saving of motor systems".

2.1 The control problems of VFD (Variable-frequency Drive) in the pumping unit are mainly reflected in the following aspects

On the one hand, there is the problem of regenerative energy processing. As shown in Figure 2, the movement of the beam pumping unit is to lift up and down repeatedly, one stroke per stroke, and its power comes from two heavy steel sliders driven by the motor. When the slider is lifted, the rod of the oil production unit is sent into the well similar to the action of leverage; If it cannot be attracted by the load, it will inevitably look for energy consumption channels, causing the motor to enter the state of regenerative power generation, and feed back excess energy to the grid, causing the voltage of the main circuit bus to rise, which will inevitably have an impact on the entire grid, resulting in a decline in the quality of grid power supply and power consumption. The risk of factor reduction; frequent high-voltage shocks will damage the motor, resulting in reduced production efficiency and increased maintenance, which is extremely unfavorable for energy saving and consumption reduction of oil pumping equipment, causing large economic losses to the enterprise.

On the other hand, there is the problem of inrush current. As shown in Figure 2, the beam pumping unit is a deformed four-bar linkage mechanism. The structure of the whole machine is like a balance. heavy load. For the support, if the torque formed by the pumping load and the balance load is equal or consistent, the pumping unit can work continuously and uninterruptedly with a small power. That is to say, the energy-saving technology of the pumping unit depends on the quality of the balance. When the balance rate is 100%, the power provided by the motor is only used to lift 1/2 of the liquid column weight and overcome friction, etc. The lower the balance rate, the greater the power provided by the motor. Because the pumping load is changing all the time, and the balance counterweight can not be completely consistent with the pumping load, which makes the energy-saving technology of the beam pumping unit very complicated. Therefore, it can be said that the energy-saving technology of the beam pumping unit is a balancing technology.

The survey of dozens of oil wells in Changqing Oilfield shows that only 1 or 2 wells have a good counterweight balance, and most of the pumping units have serious unbalanced counterweights, and more than half of the wells have small counterweights. The counterweight of several other wells is too large, resulting in excessive inrush current. The ratio of inrush current to working current can exceed 5 times at most, or even exceed 3 times of rated current. Not only is a large amount of electric energy wasted needlessly, but also seriously threatens the safety of equipment. At the same time, it also causes great difficulties to adopt VFD (Variable-frequency Drive) speed control: Generally, the capacity of VFD (Variable-frequency Drive) is selected according to the rated power of the motor, and excessive inrush current will cause VFD ( Variable-frequency Drive) overload protection action and cannot work normally.

In addition to the above two problems, the special geographical environment of oil production in the oil field determines that the oil production equipment has its own unique operating characteristics: in the early stage of oil well production, the oil storage capacity is large and the liquid supply is sufficient. In order to improve the efficiency, power frequency operation can be used to ensure higher oil production. In the middle and late stages, due to the reduction of oil reserves, it is easy to cause insufficient liquid supply. If the motor is still running at power frequency, it will waste electric energy and cause unnecessary loss. At this time, the actual working situation must be considered, and the motor speed should be appropriately reduced. Improve filling rate.

2.2 The frequency conversion transformation of the beam pumping unit mainly includes the following three aspects

(1) The power factor is greatly improved (it can be increased from 0.25 to 0.5 to above 0.9), and the power supply (apparent) current is greatly reduced, thereby reducing the burden on the power grid and transformers, reducing line losses, and saving a lot of The "capacity increase" expenditure. This is mainly concentrated in the occasions where the power supply enterprises have high requirements on the quality of the power grid. In order to avoid the decline of the quality of the power grid, frequency conversion control needs to be introduced. The main purpose is to reduce the impact of the pumping unit's working process on the power grid.

(2) Frequency conversion transformation with energy saving as the first goal. This is quite common. On the one hand, in order to overcome the large starting torque, the electric motor used by the oilfield pumping unit is far greater than the actual required power. The utilization rate of the electric motor is generally 20% to 30% during operation, and the maximum will not exceed 50%. , The motor is often in a light load state, resulting in a waste of resources. On the other hand, the continuous change of the working condition of the pumping unit depends on the state of the ground. If it is always in power frequency operation, it will also cause waste of electric energy. In order to save energy and improve the working efficiency of the motor, frequency conversion transformation is required.

(3) Due to the real "soft start", excessive mechanical impact on the motor, gearbox and pumping unit is avoided, which greatly prolongs the service life of the equipment, reduces downtime and improves production efficiency. Frequency conversion transformation for the purpose of improving power grid quality and energy saving. This situation combines the advantages of the above two transformations and is an important development direction in the application.

3. Technical development of pumping units

The first generation: the main motor of the first pumping unit was mainly started by a three-phase asynchronous motor