An improved control method for low voltage ride-through of renewable energy power generation system

Abstract

The present application discloses an improved control method for low voltage ride-through of a renewable energy power generation system, which specifically includes: measuring the three-phase voltage signal at the fault point and the equivalent resistance of the transmission circuit between the grid connection point and the fault point of the renewable energy power generation system; calculating the total current command value I injected by the renewable energy power generation system into the power grid; * ; Collect the three-phase voltage signal U of the grid connection point of the renewable energy power generation system s , calculate the virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system v ; The virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system v After passing through the digital phase-locked loop, the electrical angle θ of the positive sequence voltage vector of the power grid is obtained. PLL and synchronous electrical angular velocity ω g , and the virtual terminal voltage d-axis orientation method is used for subsequent control. The present application improves the control by processing the three-phase voltage signal input signal of the phase-locked loop of the renewable energy power generation system to avoid the large interference instability problem of the renewable energy power generation system during the low voltage ride-through period.

Description

Technical Field

[0001] The present invention relates to the field of renewable energy power generation, and in particular to an improved control method for a renewable energy power generation system during low voltage ride-through. Background Art

[0002] At present, conventional wind power / photovoltaic and other renewable energy power generation systems mostly adopt a phase-locked synchronous control mode based on a phase-locked loop, and their operating characteristics are heavily dependent on the operating status of the power grid. However, with the large-scale centralized development of renewable energy such as wind power / photovoltaic, and cross-regional and long-distance transmission, the connection between the renewable energy power generation system and the main power grid is tending to weaken. The interaction between the output flow of the renewable energy power generation system and the grid impedance is becoming more and more obvious. Especially during the low voltage ride-through period, the power grid no longer has a strong voltage support and phase reference function for the renewable energy power generation system, which increases the risk of the renewable energy power generation system and the power grid being out of step and disconnected from the grid, seriously threatening the safe and stable operation of the power system. Therefore, improving the low voltage ride-through control method of the renewable energy power generation system and enhancing its synchronization stability have important practical value for improving the safety and reliability of the power system. At present, scholars at home and abroad have carried out relevant research, such as the following published literature:

[0003] [1] P.Sun, J.Yao, R.Liu, J.Pei, H.Zhang and Y.Liu, "Virtual capacitancecontrol for improving dynamic stability of the DFIG-based wind turbines during a symmetrical fault in a weak AC grid," IEEE Transactions on Industrial Electronics, vol.68, no.1, pp.333-346, Jan.2021.

[0004] [2] W.Tang, J.Hu, Y.Chang, F.Liu., "Modeling of DFIG-based WT for powersystem transient response analysis in rotor speed control timescale," IEEETransactions on Power System, vol.33, no.6, pp.6795-6805, Nov.2018.

[0005] Reference [1] proposed an impedance cancellation control scheme based on virtual capacitors for doubly-fed wind power systems. This scheme can enhance the connection strength between the system and the grid during low voltage ride-through by equivalently canceling the grid inductive reactance, thereby reducing the interaction between the wind power system and the grid impedance. However, this method can only improve the small signal stability of the system and cannot avoid the risk of system out-of-step. Reference [2] studied the large-interference instability phenomenon of doubly-fed wind power generation systems during low voltage ride-through of weak grids from the perspective of electromechanical time scales, and found that the large-interference instability phenomenon of doubly-fed wind power generation systems during low voltage ride-through caused out-of-step and grid disconnection. However, this document did not propose corresponding improved control measures. Therefore, how to avoid large-interference instability of renewable energy power generation systems during low voltage ride-through through improved control remains to be solved. Summary of the invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to propose an improved control method suitable for a renewable energy power generation system during low voltage ride-through, and to avoid the problem of large interference instability of the renewable energy power generation system during low voltage ride-through by improving the control of the processing of the three-phase voltage signal input signal of the phase-locked loop of the renewable energy power generation system.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] An improved control method for low voltage ride-through of a renewable energy power generation system, the method is to process a phase-locked loop three-phase voltage signal input signal of the renewable energy power generation system during low voltage ride-through, specifically comprising the following steps:

[0009] Step 1: When a fault occurs, measure the three-phase voltage signal U at the fault point f , and the equivalent resistance R of the transmission circuit between the grid connection point and the fault point of the renewable energy power generation system g ;

[0010] Step 2: During the fault period, the renewable energy generation system adopts the generator convention and sets the total reactive current instruction injected by the renewable energy generation system to the grid according to the grid regulations. And set the total active current command injected by the renewable energy generation system into the grid

[0011] Step 3: Calculate the total current command value I injected by the renewable energy power generation system into the power grid * ;

[0012] Step 4: Collect the three-phase voltage signal U of the grid connection point of the renewable energy power generation system s , calculate the virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system v ;

[0013] Step 5: convert the virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system obtained in step 4 into v After passing through the digital phase-locked loop, the electrical angle θ of the positive sequence voltage vector of the power grid is obtained. PLL and synchronous electrical angular velocity ω g , and the virtual terminal voltage d-axis orientation method is used for subsequent control.

[0014] Furthermore, the total current command value I of step 3 is * The calculation formula is shown in formula (1):

[0015]

[0016] Furthermore, in step 4, the virtual terminal voltage signal U v The calculation formula is shown in formula (2):

[0017]

[0018] In the formula, I s is the total current input to the grid by the renewable energy generation system during the fault period, R V is the virtual resistance value.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention is applicable to renewable energy power generation systems based on phase-locked loop synchronous grid-connected control. Without increasing the hardware of the equipment, it can avoid the risk of loss of balance point caused by the resistance effect of the transmission line during low voltage ride-through of the renewable energy power generation system, and solve the problem of transient loss of step of the renewable energy power generation base under power grid failure. The present invention can significantly improve the large interference stability of the renewable energy power generation system during low voltage ride-through, and greatly avoid the risk of grid disconnection accidents caused by short circuit faults in the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Shown is a flow chart of an improved control method for low voltage ride-through of a renewable energy power generation system according to the present application.

[0022] Figure 2 This is a schematic diagram of the structure of a photovoltaic power generation system connected to the power grid.

[0023] Figure 3 When the grid voltage drops to 20%, the photovoltaic power generation system adopts the traditional control strategy of the machine-side voltage effective value waveform and the phase-locked loop output frequency waveform.

[0024] Figure 4When the grid voltage drops to 20%, the photovoltaic power generation system adopts the terminal voltage effective value waveform and the phase-locked loop output frequency waveform of the method of the present application. DETAILED DESCRIPTION

[0025] The specific implementation scheme of the present invention is described in detail below with reference to the accompanying drawings.

[0026] The improved control method of the renewable energy power generation system during low voltage ride-through is used to improve the large disturbance stability of the renewable energy power generation system during low voltage ride-through. The method involves processing a three-phase voltage signal input signal of a phase-locked loop of the renewable energy power generation system.

[0027] like Figure 1 The specific steps shown are as follows:

[0028] S1: When a fault occurs, measure the three-phase voltage signal U at the fault point f , and the equivalent resistance R of the transmission circuit between the grid connection point and the fault point of the renewable energy power generation system g ;

[0029] S2: During the fault period, the renewable energy generation system adopts the generator convention and sets the total reactive current instruction injected by the renewable energy generation system into the grid according to the grid regulations. And set the total active current command injected by the renewable energy generation system into the grid

[0030] S3, calculate the total current command value I injected by the renewable energy power generation system into the power grid * , the calculation formula is shown in formula (1):

[0031]

[0032] S4, collecting the three-phase voltage signal U of the grid connection point of the renewable energy power generation system s , calculate the virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system v , the calculation formula is shown in formula (2):

[0033]

[0034] In the formula, I s is the total current input to the grid by the renewable energy generation system during the fault period, R V is the virtual resistance value;

[0035] S5, the virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system obtained in step S4 is converted into v After passing through the digital phase-locked loop, the electrical angle θ of the positive sequence voltage vector of the power grid is obtained. PLL and synchronous electrical angular velocity ωg , and the virtual terminal voltage d-axis orientation method is used for subsequent control.

[0036] The following specific implementations are used to compare and analyze the system responses during low voltage ride-through using a traditional control strategy and using the control method of the present invention.

[0037] Taking photovoltaic power generation system as an example, Figure 2 This is a schematic diagram of the structure of a photovoltaic power generation system connected to the power grid.

[0038] Figure 3 The figure shows that the grid voltage drops to 20%, and the traditional control strategy (directly using the real three-phase voltage signal U s The voltage waveform of the photovoltaic power generation system and the frequency waveform of the phase-locked loop output are used as the input signal of the phase-locked loop module. A three-phase short circuit fault occurs in the power grid at 0.4s, and the fault lasts for 0.5s to 1.3s. Figure 3 It can be seen that when the photovoltaic power generation system adopts the traditional control strategy and operates according to the simulation waveform of the traditional power grid guidelines, the photovoltaic power generation system will become unstable due to large interference during the low voltage ride-through period, and the terminal voltage of the photovoltaic power generation system will diverge rapidly and cannot be maintained at a stable level. In addition, the phase-locked loop output frequency of the photovoltaic power generation system will gradually oscillate and decrease, and quickly deviate from the rated frequency of the power grid. The photovoltaic power generation system will lose step with the power grid and there will be a risk of disconnection from the grid, which will pose a great threat to the safe and stable operation of the power grid.

[0039] Figure 4 When the grid voltage drops to 20%, the voltage waveform of the photovoltaic power generation system and the frequency waveform of the phase-locked loop output when the control method of the present invention is adopted. A three-phase short circuit fault occurs in the grid at 0.4s, and the fault lasts for 0.5s to 1.3s. Figure 4 It can be seen that when the photovoltaic power generation system adopts the control method proposed in the present invention, the terminal voltage of the photovoltaic power generation system is steadily maintained at a stable level, and the phase-locked loop output frequency of the photovoltaic power generation system is consistent with the rated frequency of the power grid after a dynamic adjustment process, and maintains synchronous operation with the power grid, so that the photovoltaic power generation system can safely and stably achieve low voltage ride through.

[0040] The embodiments of the present application are described in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and its core idea of ​​the present application. At the same time, changes or deformations made by those skilled in the art based on the ideas of the present application, the specific implementation methods and the scope of application of the present application, all belong to the scope of protection of the present application. In summary, the content of this specification should not be construed as a limitation on the present application.

Claims

1. An improved control method for low voltage ride-through of a renewable energy power generation system, characterized in that: The three-phase voltage signal input signal of the phase-locked loop of the renewable energy power generation system during the low voltage ride-through period is processed, specifically including the following steps: Step 1: When a fault occurs, measure the three-phase voltage signal U at the fault point f , and the equivalent resistance R of the transmission circuit between the grid connection point and the fault point of the renewable energy power generation system g ; Step 2: During the fault period, the renewable energy generation system adopts the generator convention and sets the total reactive current instruction injected by the renewable energy generation system to the grid according to the grid regulations. And set the total active current command injected by the renewable energy generation system into the grid Step 3: According to the total reactive current instruction and the total active current command Calculate the total current command value I injected by the renewable energy power generation system into the power grid * ; Step 4: Collect the three-phase voltage signal U of the grid connection point of the renewable energy power generation system s , calculate the virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system v ; The virtual terminal voltage signal U v The calculation formula is shown in formula (2): In the formula, I s is the total current input to the grid by the renewable energy generation system during the fault period, R V is the virtual resistance value; Step 5: convert the virtual terminal voltage signal U of the phase-locked loop module of the renewable energy power generation system obtained in step 4 into v After passing through the digital phase-locked loop, the electrical angle θ of the positive sequence voltage vector of the power grid is obtained. PLL and synchronous electrical angular velocity ω g , and the virtual terminal voltage d-axis orientation method is used for subsequent control.

2. The improved control method for low voltage ride through of a renewable energy power generation system according to claim 1, characterized in that: The total current command value I of step 3 * The calculation formula is shown in formula (1):

Images