Research on hybrid power load matching control method of new energy pumping machine

In order to improve the energy saving effect of new energy pumping machine, the power matching energy saving control of wind energy, electric energy and pumping machine load should be carried out when the actual wind speed is lower than the rated wind speed of the system. In this paper, according to the wind power hybrid transmission system, we study the load matching energy saving control strategy and method that takes into account the simultaneous change of wind speed and load, and propose the hybrid power load matching adaptive energy saving control strategy based on the cascade controller. In the strategy, the motor torque control method is used to eliminate the pressure deviation and realize the system pressure tracking load matching adaptive control, while keeping the motor output torque stable.


Introduction
Frontier areas of China are rich in oil resources, while China's oil recovery equipment has long had problems of high energy consumption and low efficiency, and the cost of oil recovery is high [1].Due to the abundant wind energy resources in China's frontier areas, many oil fields in China have tried to use wind power generation to achieve energy saving.However, the actual operation results on site show that the energy saving effect of wind power is not obvious [2][3][4].
Therefore, this paper proposes a new model of oil pumping machine, the new energy pumping machine.This model eliminates the power generation system and realizes the wind-powered pumping machine by adopting a mechanical-hydraulic hybrid power transmission system and realizes the power mixing with the motor through the torque-coupled gear set.In this new wind energy utilization model, the wind energy utilization rate can reach 80%.However, the frequent change of wind speed in nature makes the wind power obtained by the system variable, and the load size of the pumping unit also varies during the pumping process, so the system faces the situation that its power input and load are both variable and unpredictable.This situation makes the system's load matching and following control difficult to achieve.
As the impact of wind speed on the pumping machine drive control is very obvious, when wind conditions change above the rated wind speed, wind energy can meet the independent drive load requirements, the motor can stop running at this time, as long as to ensure that the pumping machine can run at a steady speed to achieve oil recovery, this situation is relatively easy to achieve the control objectives [5][6].
When the wind conditions in the rated wind speed below, the wind machine and motor hybrid drive control should be carried out.Because wind speed and load both have time-varying characteristics, making the control process more complex and difficult to achieve the control objectives.The power matching and energy saving control of wind energy, electric energy and pumping machine load becomes a key research problem of hybrid power control system, which will directly affect the power saving of pumping machine.In order to solve the problems caused by such a situation, this paper proposes an adaptive load matching energy saving control strategy based on a cascade controller to obtain a more satisfactory control result by a two-stage PID control in the face of wind speed and load, the time-varying system input and output [7].

New energy pumping machine model
Right side of Figure 1 shows a sketch of the structure of wind power hybrid pumping unit.Left side of Figure 1 shows a sketch of the structure of the wind turbine-motor hybrid hydraulic transmission system.As can be seen from left side of Figure 1, the variable pump 3 can convert wind energy into hydraulic energy, and the speed control of the wind turbine can be achieved by adjusting the displacement of the variable pump, thus realizing the absorption and utilization of wind energy.The hydraulic system plays the role of energy storage and absorption, which can realize the capture and absorption of the most wind energy.The hybrid variable speed gearbox enables energy splitting and convergence.Through the torque coupling transmission, wind energy and electric energy can be power mixed to drive the load together, and the recovery and utilization of gravitational potential energy of the oil recovery system can be realized through the dosing pump 6.The variable motor 7 can play the role of controlling the energy output to realize the power matching with the load.

Direct torque load matching energy saving control method
By analyzing the characteristics of the hybrid drive train and combining the mathematical model of the hybrid drive train, it is known that the drive train power mixing and load matching control can be achieved by adjusting the hydraulic pump or motor displacement.Through simulation and experiment, it is known that the corresponding optimal motor torque target value can always be found at different wind conditions, which makes the hydraulic system pressure vary steadily around 15MPa.When the wind power varies randomly, the hydraulic system plays the role of energy transfer as long as the motor target torque value is set reasonably, and the hydraulic system neither stores nor discharges energy during the working cycle of the pumping unit.This paper proposes direct motor target torque tracking control load matching method, when the wind speed changes, according to the wind speed change range fixed variable pump displacement, by dynamically adjusting the motor displacement, change the motor output torque, to achieve the motor output power with the wind power changes.The direct torque load matching energy saving control flow chart is shown in Figure 2  Based on the wind speed estimation and prediction model, when the simulated wind speed varies randomly between 5-8 m/s, the output power P w-p variation curve of the wind turbine energy supply system can be derived from the randomly varying simulated wind speed as shown in Figure 3.When the wind speed is constant, the output power P w-p of the hydraulic energy supply system of the wind turbine is constant.By adjusting the motor tracking target torque in real time, the optimal torque target value Te* that can make the hydraulic system pressure stable can be obtained.By varying the power output of the pump in the hydraulic energy supply system of the wind turbine under constant load, multiple sets of corresponding optimal values of motor torque can be obtained.In this paper, the motor target torque data is obtained through simulation, and the fitting equation with the wind turbine-pump system output power P w-p as input and the motor target torque as output is established using Matlab simulation environment as follows: * 0.01277 85.43 The motor torque tracking signal at the change of P w-p can be calculated by Equation (1), and the motor torque target value tracking simulation is performed by PI controller, and the motor torque target tracking curve is obtained as shown in Figure 4.It can be seen from the figure that the motor torque target value follows the variation of the input power of the wind turbine hydraulic system, and the torque target can be well tracked by using PID control, so that the variation range of the motor torque fluctuates in the range of 35-75 N m  .Figure 5 shows the hydraulic system pressure variation pattern during the motor torque target tracking process.From the figure, it can be seen that the system pressure is always stable in the initial set pressure variation range during the motor torque tracking process.This indicates that the motor target torque value obtained by fitting the equation can achieve the stable pressure output well.The pressure variation follows the load in a cyclic trend without being affected by the variation of the hydraulic system input power.The effectiveness of the direct torque tracking control method is verified by Figure 4 and Figure 5, which realizes the control process that the input wind energy of the system varies randomly while the motor output power follows its variation.
Through the motor torque tracking control, it can simply and effectively realize the wind and electric energy mixing and power matching control, which can realize the electric power changing with the wind power while meeting the load matching requirements.This paper establishes the transmission system energy conservation Equation ( 2), when the wind power enters the hybrid system, there must exist an optimal motor torque output value T e * corresponding to it, making Equation ( 2 In Equation ( 2), e n is the motor speed, H P is the hydraulic system power, loss P is the system power loss, L T is the load torque, and L n is the load speed.
The motor target torque can be calculated from the load average powerT L , as shown in Equation (3).However, the system loss power is difficult to calculate accurately.Also, due to the special working conditions such as full load and half load when the actual pumping machine works, it leads to the average power of the load changing in one cycle.
Therefore, the target torque Equation ( 3) is difficult to be applied to practical production, and the main problem of motor torque tracking control is that the tracking target value is difficult to be accurately calculated.

Pressure tracking load matching energy saving control method
By changing the displacement of the pump or hydraulic motor in the hybrid drive system, not only the motor output power but also the hydraulic pipe pressure can be changed.According to the simulation calculation, as long as the wind energy and electric energy meet the working needs of the load, the pipeline pressure curve will remain unchanged.In the process of wind speed dynamic change by constantly adjusting the displacement value, so that the pipeline pressure is stable within the design value, the electric power will change with the wind power, which can realize the adaptive load matching control process.The system working pressure and pumping crank torque are not one-to-one correspondence, but the crank load-turn angle curve can be used to establish the correspondence between crank angle and system working pressure, and the pressure control can be carried out by collecting the crank angle signal.Since the pressure change follows the load change, it is not affected by the wind energy change.Therefore, the pressure target tracking signal can be designed based on the steady-state pressure variation curve in windless condition.Therefore, this paper proposes the pressure tracking adaptive load matching energy saving control method, and the principle of pressure tracking adaptive control system is shown in Figure 6.
In order to verify the feasibility of the pressure tracking adaptive load matching energy saving method, the controller still uses PI controller.Based on the output power of the wind turbine system, the output power of the first 100 s is taken as the wind energy input power P w-p of the hybrid system, and the power curve is shown in Figure 7.The output power of the energy supply system varies randomly between 0.8 and 2.6 KW. Figure 8 shows the hydraulic system line pressure change curve.As can be seen from the graph, the line pressure can be well stabilized at 14-15 MPa within 100 s of the numerical simulation time.when the system starts, the pressure tracking fluctuates at the initial stage because the initial value of the line pressure is set at 15 MPa, but the tracking error is small after 1.8 s.In the subsequent time, the hydraulic system output pressure follows the target pressure synchronously.Figure 9 shows the motor output torque variation curve during the pressure tracking process.From the figure, it can be seen that the motor output torque fluctuates drastically, especially at the start-up phase of the system, and the torque fluctuation even reaches -400 N m  .During the 300 s pressure tracking process, the motor torque fluctuation is cyclic, with a variation range of 10-150 N m  .As can be seen from the accompanying figure in Figure 8, the pressure tracking at the beginning stage is not stable, resulting in drastic changes in the motor torque output.The motor torque oscillates sharply during the pumping cycle because of the small fluctuation state of the target pressure curve.Motor output torque is sensitive to pressure changes, and unstable changes in line pressure will lead to unstable motor output torque.Compared with motor torque tracking control, pressure tracking control target curve can be easily obtained by cranking angle and cranking drive torque with more accuracy.With accurate pressure target tracking signal, adaptive dynamic matching control of wind energy, electric energy and pumping load can be realized.The pressure target tracking control can stabilize the system pressure near the design value during the random change of wind speed and the fluctuation change of pumping unit load, thus ensuring the normal working condition of the pumping unit, but there are drastic changes of the motor during the pressure target tracking control.
Therefore, pressure tracking control can achieve adaptive dynamic matching of hybrid loads, but the control system using traditional PID controller has the phenomenon of motor output power oscillation during the target tracking process, and even generates negative power.In the actual production of oilfield, the controller performance can be improved from improving the fast tracking performance of the controller to achieve a more stable output of motor power.

Adaptive load matching energy saving control strategy based on cascade controller
Direct motor torque control can achieve stable motor output torque while keeping the pressure stable, but the motor target torque value is difficult to obtain accurately.And the pressure tracking adaptive load matching control can accurately obtain the pressure tracking signal, but it is difficult to achieve the stable change of motor output torque.Since the direct motor torque control can quickly eliminate the torque deviation △T, the motor torque output is smooth in the target torque tracking process.So if the motor torque control process is used to realize the system pressure control and eliminate the pressure deviation △P h , then it can realize the system pressure tracking on the basis of load adaptive matching control while keeping the motor torque output smooth.For this reason, this paper proposes a hybrid load matching adaptive energy saving control strategy based on the idea of series control, and the principle of the series control system is shown in Figure 10.The control system selects a PID controller to use the crank angle-pressure calibration curve for pressure target value output, and the output value of the PID-I controller is used as the target value for motor torque tracking.The hybrid load matching tandem control system utilizes the motor torque tracking control with stable torque output for torque output control, thus realizing the system pressure tracking control and motor torque tracking control simultaneously.The control system uses the power output signal of the wind turbine-variable pump system shown in Figure 7 as the wind energy input signal of the hybrid powertrain system for the control method verification study.The pressure tracking curve and torque following curve are obtained as shown in Figure 11 and Figure 12, respectively.From Figure 11, it can be seen that the pressure output value of the hydraulic system basically changes synchronously with the pressure target value after 10 s of simulation time, which indicates that the pressure tracking target is achieved by using the cascaded PID control system.Since the steady state pressure variation is basically stable within the set value range, it meets the actual production requirements.Figure 12.Motor output torque variation curve.

MEIE-2023
As can be seen in Figure 12, compared to the direct pressure tracking control, the motor output torque is only -15 N m  at system start-up and is stable between 48.8 -77.7 N m  at system steady state.
The motor output torque does not oscillate due to small fluctuations in the pressure tracking target, and the output is very stable.The hybrid load-matching control method using a cascade control system solves the problem of violent fluctuations in motor output torque using direct pressure tracking, solves the problem of difficulty in precise tracking targets in direct motor torque control, and achieves adaptive control of power mixing and load matching of the hybrid power system with wind speed changes.

Conclusions
Two control methods, direct motor torque control and pressure tracking adaptive matching control, are proposed for wind power hybrid drive pumping conditions, which can realize the combined wind power and electric power to drive the load.However, the motor torque tracking control has the problem that the tracking target value is difficult to calculate accurately; compared with the motor torque tracking control, the pressure tracking control target curve can be easily obtained by crank angle and crank drive torque, and it is more accurate, but there are drastic changes of motor torque during the pressure target tracking control, which will damage the motor life.By comparing and analyzing the advantages and disadvantages of the two control methods, the hybrid power load matching adaptive energy-saving control strategy based on a series-level PID controller is proposed, which solves the problem of violent fluctuations of motor output torque in direct pressure tracking and the problem of difficulty in accurate tracking target in direct motor torque control, and realizes the adaptive control of power mixing and load matching of the hybrid power system with the change of wind speed.The adaptive hybrid power mixing and load matching control based on the cascade PID controller can adapt to the change of the suspension point load of the pumping machine and realize the positive torque working state of the motor.

Figure 1 .
Figure 1.Structural diagram of wind power hybrid pumping unit.

Figure 2 .
Figure 2. Direct motor torque control flow chart.

Figure 3 .
Figure 3. Wind turbine hydraulic system output power P w-p variation curve.

Figure 5 .
Figure 5. Hydraulic system line pressure change curve.

Figure 6 .
Figure 6.Pressure tracking adaptive load matching energy saving control.

Figure 7 .
Figure 7. Wind turbine energy supply system power curve.

Figure 10 .
Figure 10.Principle of hybrid load matching tandem control system.