Energy‐saving system of secondary balance for the hybrid power of the pumping unit

The pumping unit is the main equipment for oil production in the oilfield. However, its serious energy consumption has greatly increased the cost of oil extraction. The reason of this phenomenon is the undesirable primary and secondary balance cannot make the motor run steadily in the high efficiency load rate interval, leading to low efficiency and great loss of the motor. In this paper, the secondary balance system for hydraulic hybrid power of pumping unit is developed, which can store the potential energy of the lower stroke in the accumulator and release it in the up stroke. It can effectively fill and reduce the wave valley and peak of the load rate curve, so that the motor can run in the high efficiency for a long time. The controller of the secondary balance system has a self-learning method based on the multiple populations' genetic algorithms. It can reflect the real load by detecting the motor operating parameters, improving the response speed. At last, use AMEsim to simulate the hydraulic system, and compare the load fluctuation and motor energy consumption before and after setting energy-saving system. It is proved that the energy saving effect of the system is extremely remarkable.


Introduction
Energy saving of pumping unit is an important part of equipment energy-saving technology in China's oil field, but the energy consumption of the mechanical extraction system accounts for about 40-50% of the energy consumption of the oil field. At present, there are about 140,000 pumping wells on land in China, and the annual electricity consumption is around 140 × 108 kW h, but the average efficiency is only 20-25%, which wastes a lot of energy.
Conventional beam pumping units are widely used in oil field of our country. Due to the inherent load imbalance of the beam pumping unit, as well as excessive load caused by cold start and tubing waxing, the overload condition must be considered when selecting the motor of the pumping unit. Therefore, the installed power of the motor is inevitably increased, resulting in low efficiency and low power factor, which leads to high energy consumption of the system.
In order to improve the balance rate of pumping units under cyclic conditions effectively, the researchers improved the traditional mechanical structure and balance method. To realise the whole cycle positive torque operation of the pumping unit, Xu Jinchao added speed-increasing box on the original crank shaft to carry out secondary balance for the opposite direction crank pumping unit. Although the secondary equilibrium structures have some energy-saving effects, the field adjustment of the abnormal crank structure is difficult and inconvenient to operate. If the adjustment is not in place, there will be energy consumption [1]. Using the energy-saving principle of torque converter, the effective length of the rear arm of the double donkey pumps can be changed with the rotation of the crank, so that the net torque curve can be slowed down, but the vulnerability rate of the pumping unit, the poor bearing capacity of the support, and the fracture of the rear donkey pin shaft and the driving rope still need to be solved [2].
Another way to save energy is to use energy-efficient drive equipment or to add energy-saving devices. The motor runs at light load for most of the stroke of pumping unit, resulting in large energy loss. With the exploitation of some oil wells, the degree of immersion also varies. To save the unnecessary power consumption, Yu Xiaoming and He Guanzhong focused on the study of the time-matching relationship between pumping wells [3], but for some special conditions of the non-stop machine, it will lead to the wellhead wax or oil, so it cannot be widely used.
In this article, a set of secondary balance system is designed without changing the drive mode and weight distribution structure of the original pumping unit. The system can eliminate structural imbalance and reduce the need for large torque when the well starts after the motor is installed.

Primary balance analysis of traditional pumping unit
The pumping unit is composed of motor, reducer, rack, connecting rod mechanism etc. The high-speed rotation of the motor is converted to the low-speed rotation of the crank of the four-bar mechanism through the reducer; the low-speed rotation movement of the crank shaft is changed from the connecting rod to the vertical reciprocating movement of the donkey head suspension; the rod string is attached below the polished rod eye, and the sucker rod string drives the plunger or the piston in the pump cylinder to move up and down in the pump cylinder. The crude oil in the well is lifted to the ground to achieve the purpose of pumping.
Taking the beam pumping unit as an example, the geometric relationship can be seen as follows (see Fig. 1): where θ is the angle of the crank, and the symbol ± of the expression ϕ depends on the direction of the crank rotation. The distance between the crank pin centre and the beam support is as follows: where ψ is the angle between BO′ and OO′. α is the angle between the crank and the connecting rod.
As the kinematics and dynamics equation of pumping unit is much more tedious, the torque expression of crank shaft is given directly: where TF = [ARsin α/(Csin β)] is torque factor, M cmax is crank maximum balance torque, P is suspension point load, B is structure unbalanced weight, and τ is balance lag angle. The geometric parameters of the pumping unit are shown in Table 1. The pumping unit is modelled in the Pro/E environment and imported into Adams software. The simulation results are shown in Fig. 2.
The asynchronous motor has a higher efficiency when the load rate in the range of 60%-100%. During the operation of pumping unit, the suspension load of sucker rod, crank shaft torque and motor shaft torque change periodically. Although the vibration amplitude of the torque has been weakened by balancing crank and beam, the load rate is still only about 30%, which cannot improve the motor efficiency. The research and development of the secondary balance system of pumping units is imminent.

Principle of the secondary balance
The key to the design of the secondary balance system is to reduce the wave peak and trough of the load, which involves the storage and release of energy. Compared with the traditional secondary balance scheme of mechanical structure, the hydraulic energy storage method has the advantages of low-energy density, highpower density, convenient speed regulation, easy to realise overload protection, and more safety [4]. At present, the hydraulic hybrid power scheme is only applied to hydraulic pumping units, which cannot be extended to a large number of beam pumping units, and the scope of application is small [5]. Fig. 3 is the schematic diagram of the secondary balance system for pumping units.
The external clutch of the secondary balance system is added to enable the motor to store energy for the accumulator without dragging load when starting cold. The secondary balance system is mainly composed of secondary components, reversing valves, controllers, and sensors. Sensor captures hydraulic signal and gives feedback to the controller; the controller can adjust the secondary element displacement and reversing valve switch state; and secondary components according to the working condition of automatic switching working condition of hydraulic pump/motor can realise the storage and release. The motor and suspension load give gradual feedback and abrupt feedback to the controller, respectively, in order to realise closed-loop control and adaptive control.
In order to smooth the load rate curve of the motor to the maximum extent, it is necessary to reasonably match the load of the secondary element and the suspension point according to energy conservation: where T s (t) is the output shaft torque of secondary component, T(t) is the point load torque, k 1 is the energy loss coefficient, and the motor shaft end torque is T m (t): where p a (t) is the accumulator pressure, V s (t) is the secondary component displacement.

Simplest monitor model of motor
The acquisition cost of non-electrical parameters (especially the torque) of pumping unit transmission system is comparatively high.
As the separate collection of efficiency calculation parameters leads to the increase of data processing amount, collection channel number, and computation duration accordingly, it on one hand improves the costs of technology application and on the other hand influences the timeliness of real-time control negatively. Based on theoretical derivation and experimental analysis among the motor parameters, it explores the mechanism among the various state variables, simplifies the state variables in the motor efficiency function, and builds up the simplest monitor model of the motor. Although the energy loss rules of different motors tend to be consistent, some parameters are slightly different, and the unified law cannot be drawn. Therefore, solving the energy loss of motor is a regular non-unified solution problem.
According to the sources of loss [6], the energy losses of PMSM are mainly divided into the following parts: the stator core loss, the winding copper loss, the eddy current loss of rotor and permanent magnet, and the mechanical loss during motor movement [7,8]. The latter formula shows the motor input power as follows: P 1 = 3UIcos φ = P 2 + P Fe + P m + P Cu = P sF + P rF + P m + P Cu = k 1 T 2 + k 2 T + k 3 + k 4 I 2 + k 5 + mI 2 R Cu In order to enlarge the individuality of the simplest monitoring model of motor [9], a self-learning method of parameter identification based on multi-population genetic algorithm (MPGA) is established. MPGA is not easy to fall into local optimal, and it greatly improves the robustness and global search ability of the algorithm [10] (see Fig. 4).

AMEsim simulation analysis
Part of the system parameters is shown in Table 2. Fig. 5 is the simulation model diagram of the system. Figs. 6 and 7 are sub-models of the motor and controller, respectively.
As can be seen from Figs. 8 and 9, energy-saving system of secondary balance for the hybrid power of the pumping unit can well eliminate the negative torque of the motor of the traditional pumping unit and add energy to the upper stroke, effectively reducing the electrical energy consumption caused by the larger load fluctuation.

Conclusion
Using AMEsim simulation platform, compare the load fluctuation and motor energy consumption before and after setting up secondary balance energy-saving system, which proves that the system can effectively reduce the pumping unit energy consumption problem caused by the fluctuating load rate. A self-learning method based on MPGA is developed. The controller can obtain load change by monitoring the running state of the motor and improve the response of control strategy.