Control method for improving grid-connected electric energy quality of natural gas differential pressure power generation system

Abstract

The invention discloses a control method for improving the grid-connected electric energy quality of a natural gas differential pressure power generation system, and the method comprises the following steps: S1, firstly, building a mathematical model of the natural gas differential pressure power generation system, and selecting a back-to-back converter based on NPC three-level topology as the power topology of natural gas differential pressure power generation energy transmission, modeling is carried out on a turbo expander-generator system, a machine-side converter and a grid-side converter; s2, then, respectively designing control systems of a machine-side converter and a grid-side converter of the system according to the requirements of the natural gas differential pressure power generation system; and S3, then, due to the fact that the output power of the differential pressure power generation system can change along with changes of the flow of the natural gas, the pressure of the inlet and the pressure of the outlet and the temperature, the output power of the system can have certain randomness and uncertainty, and it is considered that an energy storage battery is adopted to achieve power stabilization. According to the invention, the stability of the differential pressure power generation system is improved, and the grid-connected electric energy quality is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas pressure difference power generation, and in particular to a control method for improving the grid-connected power quality of a natural gas pressure difference power generation system. Background Art

[0002] With the continuous development of the new energy field, domestic and foreign scholars have conducted further research on the field of natural gas pressure difference power generation technology. First, a structural form in which a volumetric expander is directly connected to the rotating shaft of a generator is proposed. This method has a simple structure and is easy to maintain, but the operating efficiency is relatively low. Then, a structural form in which a turbine expander is connected to a generator through a gearbox reduction is adopted. Although the gearbox solves the problem of the high rated speed of the turbine expander, it also brings additional power loss and reduces the reliability of the system. In addition, the turbine expander can be directly connected to the generator. This method is not only conducive to reducing transmission losses and failure rates, but also facilitates the realization of a fully enclosed integrated design. However, due to the high speed characteristics of the motor, higher requirements are also placed on the inverter and control algorithm.

[0003] Considering that the output power of the pressure difference power generation system will change with the flow rate of natural gas, the pressure of the inlet and outlet, and the temperature, the output power of the system will have a certain degree of randomness and uncertainty. At present, the research on the power fluctuation of the pressure difference power generation system is still in its infancy. Considering that it uses a permanent magnet synchronous motor with a large inertia link as a generator to generate electrical energy, the power is directly transmitted to the energy storage battery, and the power fluctuation is suppressed by controlling the energy storage system. This method is simple to control, but the operation efficiency is low, and the frequent charging and discharging leads to a low battery life. The energy storage battery can also be connected to the DC bus through a DC-DC converter, and the power fluctuation in the wind power generation system can be smoothed by the fast response of the battery. This method also points out the direction for the pressure difference power generation system to suppress power fluctuations. However, in wind power generation, the role of the energy storage battery is usually to smooth the high-frequency fluctuating power, while the pressure difference power generation system needs to use the energy storage battery to make the total power fluctuation transmitted to the power grid less than 10%. Therefore, in the actual operation process, when considering the pressure difference power generation system working under different working conditions, it is of great research significance to study a control method that can ensure the smooth operation of the system and ensure that the system transmission power meets the design requirements. Summary of the invention

[0004] Based on the technical problems existing in the background technology, the present invention proposes a control method for improving the grid-connected power quality of a natural gas pressure difference power generation system.

[0005] The present invention proposes a control method for improving the grid-connected power quality of a natural gas pressure difference power generation system, wherein the natural gas pressure difference power generation system comprises a back-to-back NPC three-level topology, a natural gas pipeline, a turbine expander, a permanent magnet synchronous motor, an energy storage battery, a converter and a large power grid, and the control system comprises the following steps:

[0006] S1: Establish a mathematical model of natural gas pressure difference power generation system;

[0007] S2: Design control system;

[0008] S3: Smoothing out the output power of the pressure difference power generation system;

[0009] S4: Change the operating conditions of the system and verify the DC bus voltage;

[0010] The natural gas pressure difference power generation system adopts a structural form in which the generator is connected to the power grid after passing through the converter, and in order to maintain the stability of the system output power, the energy storage battery is connected to the DC bus through the DC-DC converter.

[0011] Preferably, the natural gas pressure difference power generation system adopts a permanent magnet synchronous motor. Since the peak-to-valley difference of the flow fluctuation in the natural gas pressure difference power generation system is large, the motor is required to work under wide load conditions. Therefore, compared with the asynchronous generator, the power generation efficiency of the permanent magnet synchronous motor has obvious advantages. For the permanent magnet synchronous motor, its motion equation and torque equation are respectively:

[0012]

[0013] The permanent magnet synchronous motor usually adopts field oriented control, that is, controlling the d-axis current:

[0014] i d =0

[0015] Motor torque equation:

[0016] Rated power of the permanent magnet synchronous motor: P m =T m ω m

[0017] Substituting the torque equation of the permanent magnet synchronous motor into the motion equation, the motion equation of the expander-generator system is:

[0018]

[0019] Preferably, the back-to-back NPC three-level converter consists of two parts, a machine-side converter and a grid-side converter, which are connected via a DC bus. The machine-side converter performs the functions of rectifying and controlling the motor speed, converting the AC power generated by the generator into DC power and storing it on the DC bus. The grid-side converter performs the functions of inverting and stabilizing the DC bus voltage, converting the DC power on the DC bus into AC power of industrial frequency, and transmitting the energy to the power grid.

[0020] Preferably, the control target of the machine-side converter in step S2 is to control the motor speed so that the generator operates at a high power factor and transmits the generated active power to the DC bus.

[0021] Preferably, the control target of the grid-side converter in step S2 is to maintain the balance between the total voltage of the DC bus and the upper and lower voltages, and to deliver active power to the grid, and the grid-side converter is connected to the DC bus of the generator-side converter.

[0022] Preferably, an expert control system is established to determine whether the system is operating under the same operating conditions. When the system power fluctuation is less than 2%, it is considered that the system operating conditions have not changed and the battery does not operate; when the generator power exceeds the system power by 2%, the battery enters the charging mode; conversely, if the generator power is less than 2% of the system power, the battery will operate in the discharging mode.

[0023] The beneficial effects of the present invention are:

[0024] 1. The control system of the natural gas differential pressure power generation system for stable operation is analyzed to improve the power quality of the system. First, a back-to-back NPC three-level converter is used to control the motor speed and DC bus voltage to transmit the active power of the generator to the main grid. On this basis, according to the working principle of the turboexpander, a DC-DC converter is designed to control the energy storage battery to reduce the power fluctuation of the system. The experimental results show that with the help of the proposed control system, the system can maintain a stable DC bus voltage under various working conditions, and the output power fluctuation is less than 10%, which improves the stability of the differential pressure power generation system and improves the power quality of the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the structural diagram of the grid-connected natural gas pressure difference power generation system;

[0026] Figure 2 This is the topology diagram of the back-to-back NPC three-level converter;

[0027] Figure 3 It is the control block diagram of the machine-side converter;

[0028] Figure 4 It is the control block diagram of the grid-side converter;

[0029] Figure 5 It is the DC-DC converter control block diagram;

[0030] Figure 6 It is a battery charging and discharging flow chart;

[0031] Figure 7 is the turboexpander output pressure diagram;

[0032] Figure 8 is the generator output power diagram;

[0033] Fig. 9 is the battery output power graph;

[0034] Fig.10 It is the system output power diagram. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0036] Reference Figure 1-10 A control method for improving the grid-connected power quality of a natural gas pressure difference power generation system, the natural gas pressure difference power generation system includes a back-to-back NPC three-level topology, a natural gas pipeline, a turbine expander, a permanent magnet synchronous motor, an energy storage battery, a converter and a large power grid, and the control system includes the following steps:

[0037] S1: First, a mathematical model of the natural gas pressure difference power generation system is established. The back-to-back converter based on the NPC three-level topology is selected as the power topology for the natural gas pressure difference power generation energy transmission. The turbine expander-generator system, the machine-side converter and the grid-side converter are modeled separately.

[0038] S2: Then, according to the requirements of the natural gas pressure difference power generation system, the control systems of the system machine-side converter and the grid-side converter are designed respectively;

[0039] S3: Next, since the output power of the pressure difference power generation system will change with the flow rate of natural gas, the pressure of the inlet and outlet, and the temperature, the output power of the system will have a certain degree of randomness and uncertainty, it is considered to use energy storage batteries to achieve power stabilization;

[0040] S4: Finally, by changing the operating conditions of the system, verify that the DC bus voltage always remains within a stable range and that the power fluctuations of the system are smoothed by the energy storage battery.

[0041] The natural gas pressure difference power generation system adopts a structural form in which the generator is connected to the power grid after passing through the converter, and in order to maintain the stability of the system output power, the energy storage battery is connected to the DC bus through the DC-DC converter.

[0042] Specifically, the NPC three-level topology back-to-back converter is an AC-AC power conversion device with the advantages of controllable DC bus voltage, independent control of active and reactive power, and bidirectional power flow. It has been widely used in renewable energy generation because the three-level converter can output positive, zero and negative voltages, thereby producing a waveform that is closer to a sine wave. In addition, since the three-level converter can directly output zero-level voltage, the current ripple near the zero point is almost eliminated. Near the peak value of the inverter output voltage, since the voltage change output by the three-level converter in one cycle is only half of that of the two-level converter, the ripple can be effectively suppressed.

[0043] Specifically, the essence of natural gas pressure difference power generation is to convert the internal energy of high-pressure natural gas into mechanical energy on the expander shaft through the expander. Since the expansion process in the expander is relatively short, it is usually approximated as an isentropic process, that is, the heat exchange with the external environment can be ignored, and it can be considered as a reversible adiabatic expansion process. When the expander operates under stable conditions, it can be regarded as an open system with steady state and steady flow. Therefore, according to the first law of thermodynamics of open systems, q = Δh + W t = 0, where q is the heat exchanged between the system and the outside world, Δh is the enthalpy drop, and W t It is technical work, also known as axis work.

[0044] Specifically, for an ideal gas, according to the constant specific heat capacity calculation, since the expansion of the expander is an isentropic process, the shaft work W t The calculation formula is

[0045]

[0046] Among them, κ is the isentropic index, R g is the molar gas constant, T1 is the temperature at the inlet of the expander, and p1 and p2 are the inlet and outlet pressures.

[0047] Considering the actual operating flow of the expander and the various losses, the mechanical power P of the expander can be obtained: m for

[0048]

[0049] Where, Q is the flow rate of the expander, η t is the efficiency of the expander.

[0050] In the present invention, the natural gas pressure difference power generation system adopts a permanent magnet synchronous motor. Since the peak-to-valley difference of the flow fluctuation in the natural gas pressure difference power generation system is large, the motor is required to work under a wide load condition. Therefore, compared with the asynchronous generator, the power generation efficiency of the permanent magnet synchronous motor has obvious advantages. For the permanent magnet synchronous motor, its motion equation and torque equation are respectively

[0051]

[0052] Permanent magnet synchronous motors usually use field oriented control, that is, controlling the d-axis current i d =0, which can not only improve the dynamic characteristics of the motor and realize high-performance closed-loop control of the motor, but also simplify the control algorithm so that the motor can obtain the maximum torque output. Therefore, the motor torque equation can be simplified to

[0053]

[0054] Rated power P of permanent magnet synchronous motor m =T m ω m Therefore, the mechanical power of the expander and the torque equation of the permanent magnet synchronous motor are substituted into the equation of motion to obtain the equation of motion of the expander-generator system:

[0055]

[0056] It can be seen that the mechanical characteristics of the system are affected by the operating conditions of the expander and the current of the generator. Since the operating conditions of the expander in the natural gas pipeline network are highly volatile, a good control system must be designed to maintain stable operation of the system.

[0057] In the present invention, the back-to-back NPC three-level converter consists of two parts, a machine-side converter and a grid-side converter, which are connected via a DC bus. The machine-side converter performs the functions of rectifying and controlling the motor speed, converting the AC power generated by the generator into DC power and storing it on the DC bus. The grid-side converter performs the functions of inverting and stabilizing the DC bus voltage, converting the DC power on the DC bus into AC power of industrial frequency, and transmitting the energy to the power grid.

[0058] The output voltage of the machine-side converter to point O has three output states: u po ,0,u on . Define its switch function as:

[0059]

[0060] According to the loop voltage equation of the machine-side converter, the motor is considered as a three-phase balanced system, and the current model in matrix form can be obtained as follows:

[0061]

[0062] After coordinate transformation, the current model in the two-phase rotating coordinate system is:

[0063]

[0064] Among them, esd 、e sq is the dq axis component of the induced electromotive force, S md , S mq is the dq axis component of the switching function of the machine-side converter. According to the machine-side current model in the two-phase rotating coordinate system, by changing the switching function of the machine-side converter, the dq axis current of the motor can be controlled, thereby controlling the torque and speed of the motor.

[0065] The power topology of the grid-side converter is similar to that of the machine-side converter. The switching function of the grid-side converter is defined as:

[0066]

[0067] According to the loop voltage equation of the grid-side converter, considering the grid as a three-phase balanced system, the current model in matrix form can be obtained as follows:

[0068]

[0069] The grid-side converter has many variables that are not independent of each other, which is not conducive to the design of the controller. Through the coordinate transformation based on the grid voltage orientation, the current model in the two-phase rotating coordinate system is obtained as follows:

[0070]

[0071] Among them, ω g is the grid voltage angular frequency, e gd ,e gq is the dq axis component of the grid voltage, i gd ,i gq is the dq axis component of the grid current, u gd ,u gq is the dq axis component of the inverter output voltage, S gd ,S gq is the dq-axis component of the switching function of the grid-side converter. According to the grid-side current model in the two-phase rotating coordinate system, the grid current can be controlled by changing the switching function of the grid-side converter.

[0072] According to the instantaneous power calculation principle, in the two-phase rotating coordinate system, the power equation exchanged between the power grid and the converter is:

[0073]

[0074] Among them, P g is the active power, Q g is the reactive power. When the grid voltage vector is used as the orientation, e gd The amplitude is the grid voltage amplitude, e gq The magnitude of is 0, so the power equation can be rewritten as:

[0075]

[0076] i.e. control gd The active power exchanged between the converter and the grid can be controlled to change i gd The amplitude and sign of can control the size and direction of active power, and control i gq The reactive power exchanged between the converter and the grid can be controlled to change i gq The magnitude and sign of the reactive power can control the size and direction of the reactive power. gq When =0, the reactive power is zero and the converter operates at unity power factor.

[0077] In the present invention, the control target of the machine-side converter in step S2 is to control the motor speed, so that the generator operates at a high power factor and transmits the generated active power to the DC bus. The control block diagram is as follows: Figure 3 As shown, due to the influence of factors such as magnetic circuit nonlinearity, the accurate values ​​of motor parameters are often difficult to know, and different parameter estimation deviations have different effects on the stability of the current loop. As the speed increases, the stability of motor control will decrease, and instability will occur in severe cases. In order to increase the speed range of stable operation of the motor, the present invention adopts a vector control system based on dq axis current feedforward, increases the equivalent stator resistance of the motor dq axis, and effectively increases the critical speed when the current loop becomes unstable.

[0078] In the present invention, the control target of the grid-side converter in step S2 is to maintain the balance between the total voltage of the DC bus and the upper and lower voltages, deliver active power to the grid, and minimize the flow of reactive power. The control block diagram is as follows: Figure 4 As shown, due to the different effects of different switch states on the midpoint voltage during the operation of the three-level converter, there is a midpoint voltage offset problem. Generally, a midpoint balance control algorithm is used to balance it. The grid-side converter is connected to the DC bus of the machine-side converter, so it is only necessary to balance the upper and lower capacitors on one side. The present invention adopts a zero-sequence voltage injection method based on a PI controller in the grid-side converter, superimposes the output of the controller on the modulation signal of the dq axis and then modulates it to achieve the balance of the upper and lower capacitor voltages.

[0079] In the present invention, during the actual operation of the pressure difference power generation system in step S3, the output power varies with the changes in the natural gas flow rate and the inlet and outlet pressures and temperatures. In order to ensure the stable operation of the system, it is considered to connect the battery in parallel to the DC bus through a DC-DC converter. The control block diagram is as follows: Figure 5 As shown, when the system operates under different working conditions, the current of the energy storage battery is controlled to smooth the system power fluctuation and improve the power quality.

[0080] In the present invention, it is considered that in the actual system, there may be slight fluctuations in the inlet pressure and flow rate, which may cause frequent charging and discharging of the battery, thereby reducing its life. Therefore, it is necessary to establish a simple expert control system, such as Figure 6 As shown, to determine whether the system is operating under the same working condition, when the system power fluctuation is less than 2%, it is considered that the system working condition has not changed and the battery is not running. When the generator power exceeds the system power by 2%, the battery enters the charging mode. On the contrary, if the generator power is less than 2% of the system power, the battery will operate in the discharging mode.

[0081] Through the operation analysis of a certain pressure regulating station, it is found that the natural gas inlet pressure of the pressure regulating station fluctuates greatly, while the outlet pressure remains relatively stable. Therefore, the present invention mainly analyzes how the existence of battery energy storage can improve the power quality while smoothing the power fluctuation of the system under various operating conditions. The output pressure condition is as follows: Figure 7 As shown, according to different working conditions, the curve is divided into five parts. When the system does not add energy storage, the power waveform is as follows Figure 8 As shown, the output power changes with the inlet pressure. After adding the energy storage battery, the output power waveform of the energy storage system is as follows Fig. 9 As shown, the system power waveform is as follows Fig.10 As shown, after zooming in, it can be seen that the system power fluctuation is less than 10%, which meets the design requirements while improving the system power quality.

[0082] In summary, this example first uses a back-to-back NPC three-level converter to control the motor speed and DC bus voltage to transmit the active power of the generator to the main grid. On this basis, according to the working principle of the turboexpander, a DC-DC converter is designed to control the energy storage battery, thereby reducing the system power fluctuation. The experimental results show that with the help of the proposed control system, the system can maintain a stable DC bus voltage under various working conditions, and the output power fluctuation is less than 10%, which improves the stability of the pressure difference power generation system and improves the power quality of the grid.

[0083] Those skilled in the art will appreciate that all or part of the steps for implementing the above embodiments may be accomplished by hardware, or may be accomplished by instructing related hardware through a program, and the program may be stored in a computer-readable storage medium, wherein the storage medium may be a magnetic disk, an optical disk, a read-only memory (ROM) or a random access memory (RAM), etc.

[0084] The above are only preferred specific implementation modes of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical solutions and inventive concepts of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A control method for improving the grid-connected power quality of a natural gas pressure difference power generation system, characterized in that: The natural gas pressure difference power generation system includes a back-to-back NPC three-level topology, a natural gas pipeline, a turbine expander, a permanent magnet synchronous motor, an energy storage battery, a converter and a large power grid. The control system The following steps are involved: S1: Establish a mathematical model of natural gas pressure difference power generation system; S2: Design control system; S3: Smoothing out the output power of the pressure difference power generation system; S4: Change the operating conditions of the system and verify the DC bus voltage; The natural gas pressure difference power generation system adopts a structural form in which the generator is connected to the power grid after passing through the converter, and in order to maintain the stability of the system output power, the energy storage battery is connected to the DC bus through the DC-DC converter.

2. A control method for improving the grid-connected power quality of a natural gas pressure difference power generation system according to claim 1, characterized in that: The natural gas pressure difference power generation system adopts a permanent magnet synchronous motor. Since the peak and valley differences of the flow fluctuation in the natural gas pressure difference power generation system are large, the motor is required to work under wide load conditions. Therefore, compared with the asynchronous generator, the power generation efficiency of the permanent magnet synchronous motor has obvious advantages. For the permanent magnet synchronous motor, its motion equation and torque equation are: The permanent magnet synchronous motor usually adopts field oriented control, that is, controlling the d-axis current: i d ＝0 Motor torque equation: Rated power of the permanent magnet synchronous motor: P m =T m ω m Substituting the torque equation of the permanent magnet synchronous motor into the motion equation, the motion equation of the expander-generator system is:

3. A control method for improving the grid-connected power quality of a natural gas pressure difference power generation system according to claim 2, characterized in that: The back-to-back NPC three-level converter consists of two parts, a machine-side converter and a grid-side converter, which are connected via a DC bus. The machine-side converter performs the functions of rectifying and controlling the motor speed, converting the AC power generated by the generator into DC power and storing it on the DC bus. The grid-side converter performs the functions of inverting and stabilizing the DC bus voltage, converting the DC power on the DC bus into AC power of industrial frequency, and transmitting the energy to the grid.

4. A control method for improving the grid-connected power quality of a natural gas pressure difference power generation system according to claim 3, characterized in that: The control target of the machine-side converter in step S2 is to control the motor speed so that the generator operates at a high power factor and transmits the generated active power to the DC bus.

5. A control method for improving the grid-connected power quality of a natural gas pressure difference power generation system according to claim 4, characterized in that: The control target of the grid-side converter in step S2 is to maintain the balance between the total voltage of the DC bus and the upper and lower voltages, and to deliver active power to the grid. The grid-side converter is connected to the DC bus of the machine-side converter.

6. A control method for improving the grid-connected power quality of a natural gas pressure difference power generation system according to claim 5, characterized in that: An expert control system is established to determine whether the system is operating under the same working condition. When the system power fluctuation is less than 2%, it is considered that the system working condition has not changed and the battery does not operate; when the generator power exceeds the system power by 2%, the battery enters the charging mode; On the contrary, if the generator power is less than 2% of the system power, the battery will operate in discharge mode.

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