Equivalent modeling method for photovoltaic power station

Abstract

The invention provides a photovoltaic power station equivalent modeling method, and the method comprises the steps: constructing a photovoltaic power station detailed model, inputting illumination intensity information to the photovoltaic power station detailed model, and obtaining the transient operation data of a single-machine model of each photovoltaic power generation unit when the photovoltaic power station detailed model breaks down; according to the illumination intensity information, the battery parameters of each photovoltaic power generation unit under the marked working condition and the voltage threshold value of the action of the chopping protection circuit, calculating to obtain chopping motion critical current, and according to the transient operation data, calculating to obtain end current of the photovoltaic power generation unit; and dividing the photovoltaic power generation units to a chopping protection action cluster or a chopping protection non-action cluster according to the chopping motion critical current and the end current, and respectively establishing a first photovoltaic power station equivalent model and a second photovoltaic power station equivalent model. The method ensures accurate fitting of the output external characteristics of the photovoltaic power station, reflects the operation characteristics of the photovoltaic power station during the fault ride-through Chopper protection action, and improves the intuition and high efficiency of unit grouping.

Description

Technical Field

[0001] The present application belongs to the field of photovoltaic power generation technology, and specifically relates to a photovoltaic power station equivalent modeling method. Background Art

[0002] As ecological and environmental problems become increasingly serious, new energy sources such as wind and light are gradually replacing fossil energy and are receiving more and more attention. The large-scale grid connection of photovoltaic power generation has brought challenges to the stable operation of the power grid. Grid failures can cause voltage drops at the grid connection point of photovoltaic power stations. In the early stages of photovoltaic development, due to its small proportion, photovoltaic power generation systems may not have the ability to ride through faults. When a grid fault occurs, it automatically disconnects from the grid to ensure its own safety and has little impact on the grid. However, as the proportion of photovoltaic power generation in the grid increases, if the photovoltaic power generation system still does not have a qualified ability to resist grid faults, it will automatically disconnect when a grid fault occurs, which will increase the difficulty of recovering from local grid faults, deteriorate grid stability, and even aggravate faults and cause system collapse. Therefore, it is required that photovoltaic power stations can withstand a certain voltage drop when a grid fault occurs, and quickly restore voltage after the fault, that is, low voltage ride-through of photovoltaic power stations. In order to improve the low voltage ride-through capability of photovoltaic power generation units, a chopper protection circuit is usually installed on the DC side, but its action has a certain impact on the operating characteristics of photovoltaic power stations.

[0003] When studying the characteristics of large-scale photovoltaic grid-connected, if a detailed model is used for simulation, it is necessary to perform multiple iterations of iterative calculations on all power generation units, main systems and control systems in the photovoltaic power station. When there are many photovoltaic power generation units, a large main system and a complex control system, the amount of data processing and calculation is very large, iterative calculations are difficult to solve, and the convergence of the model is reduced. Therefore, photovoltaic power stations generally use equivalent models to effectively fit the dynamic operating characteristics of photovoltaic power stations after they are connected to the grid while meeting the simulation speed and simulation accuracy, providing a theoretical basis and model support for the research on the stability of photovoltaic power station grid connection. The traditional single-machine equivalent method is to equate all photovoltaic power generation units to one unit, ignoring the differences in the operating characteristics of each unit within the photovoltaic power station, and the equivalent error is large. At present, the research on multi-machine equivalent relies on clustering algorithms for grouping, which is relatively complex to calculate, and does not consider the impact of Chopper protection on the equivalent modeling of photovoltaic power stations during low voltage ride-through. Summary of the invention

[0004] The technical problem solved by the present application is: how to provide a photovoltaic power station equivalent modeling method that can take into account the impact of chopping protection action during low voltage ride-through.

[0005] The present application provides a photovoltaic power station equivalent modeling method, the photovoltaic power station equivalent modeling method comprising:

[0006] Constructing a detailed model of a photovoltaic power station, wherein the detailed model of the photovoltaic power station includes single-machine models of multiple photovoltaic power generation units;

[0007] Inputting light intensity information into the photovoltaic power station detailed model to obtain transient operation data of the single-machine model of each photovoltaic power generation unit when a failure occurs in the photovoltaic power station detailed model;

[0008] The chopping action critical current is calculated based on the light intensity information, the battery parameters of each photovoltaic power generation unit under the marked working condition and the voltage threshold of the chopping protection circuit action, and the terminal current of the photovoltaic power generation unit is calculated based on the transient operation data;

[0009] Dividing the photovoltaic power generation unit into a chopper protection action group or a chopper protection non-action group according to the chopper action critical current and the terminal current;

[0010] A first photovoltaic power station equivalent model is established to characterize the group of machines with chopper protection in action, and a second photovoltaic power station equivalent model is established to characterize the group of machines with chopper protection inaction.

[0011] Optionally, the method of inputting light intensity information into the photovoltaic power station detailed model includes:

[0012] The maximum input light intensity S of multiple photovoltaic power generation units is calculated by using a random function. max and the minimum input light intensity S min The input light intensity of each photovoltaic power generation unit is randomly generated between T_i for:

[0013] S T_i =(S max -S min )×RAND()+S min

[0014] In the formula, RAND() represents any random number between 0 and 1.

[0015] Optionally, the method for obtaining transient operation data of a single-machine model of each photovoltaic power generation unit when a failure occurs in the detailed model of the photovoltaic power station comprises:

[0016] It is assumed that a three-phase short circuit fault occurs at the outlet of the detailed model of the photovoltaic power station at time t0, and the fault is removed at time t1, and the transient operation data of the photovoltaic power generation unit under the low voltage ride-through condition is collected. The transient operation data includes the active output of the photovoltaic power generation unit and the terminal voltage drop value at the short circuit moment.

[0017] Optionally, the method for calculating the chopping action critical current according to the light intensity information, the battery parameters of each photovoltaic power generation unit under the marked working condition and the voltage threshold of the chopping protection circuit action includes:

[0018] Using the light intensity information, the battery parameters of each battery of each photovoltaic power generation unit under the marked working condition are corrected;

[0019] The output volt-ampere characteristic value of the photovoltaic power generation unit is calculated based on the battery parameters of each battery after correction;

[0020] The chopping action critical current of the photovoltaic power generation unit is calculated according to the output volt-ampere characteristic value and the voltage threshold.

[0021] Optionally, the method of dividing the photovoltaic power generation unit into a chopper protection action group or a chopper protection non-action group according to the chopper action critical current and the terminal current includes:

[0022] When the terminal current is less than the chopping action critical current, the photovoltaic power generation unit is divided into a chopping protection non-action group;

[0023] When the terminal current is greater than or equal to the chopping action critical current, the photovoltaic power generation unit is divided into a chopping protection action group.

[0024] Optionally, the stand-alone model of the photovoltaic power generation unit includes an inverter controller, and the inverter controller includes:

[0025] A low voltage ride-through judgment module, which is used to detect whether the grid connection point voltage drops below the standard requirement value of 0.9pu. When it drops below 0.9pu, it switches to the low voltage ride-through control mode during the fault period;

[0026] A dynamic reactive power support module is used to calculate a reactive current reference value emitted by a photovoltaic inverter.

[0027] Optionally, the single-machine model of the photovoltaic power generation unit includes a DC side chopper protection circuit, and the DC side chopper protection circuit includes:

[0028] A protection action judgment module, the protection action judgment module is used to detect whether the DC bus voltage exceeds the voltage threshold of the protection circuit action;

[0029] The unloading protection circuit is used to input the unloading resistor to consume the excess energy on the DC side to prevent overvoltage and overcurrent when the voltage exceeds the voltage threshold; when the DC bus voltage drops below the voltage threshold, the unloading protection circuit is used to cut off the unloading resistor.

[0030] The present application provides a photovoltaic power station equivalent modeling method, which has the following technical effects:

[0031] This method ensures accurate fitting of the external characteristics of the photovoltaic power station output and reflects the operating characteristics of the photovoltaic power station when the fault-through Chopper protection is activated. At the same time, it is relatively simple and convenient to implement, which improves the intuitiveness and efficiency of unit grouping. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A flowchart of a photovoltaic power plant equivalent modeling method according to one or more embodiments;

[0033] Figure 2 A topological diagram of a photovoltaic power generation unit and a collector line according to one or more embodiments;

[0034] Figure 3 is a graph showing active power experimental results of various comparative examples according to one or more embodiments;

[0035] Figure 4 is a graph showing reactive power experimental results of various comparative examples according to one or more embodiments;

[0036] Figure 5 Graph showing positive sequence current experimental results of comparative examples according to one or more embodiments. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0038] Before describing in detail the various embodiments of the present application, the technical concept of the present application is first briefly described: the current photovoltaic power stations usually use an equivalent model, ignoring the impact of Chopper protection on the equivalent modeling of photovoltaic power stations during low voltage crossing, resulting in large errors. To this end, the photovoltaic power station equivalent modeling method provided by the present application divides the photovoltaic power generation units into two groups: Chopper protection action and Chopper protection inaction, and establishes a dual-machine equivalent model of the photovoltaic power station. Compared with the traditional photovoltaic power station grouping equivalent modeling method, the method proposed in the present application ensures accurate fitting of the external characteristics of the photovoltaic power station output and reflects the operating characteristics of the photovoltaic power station when the fault crosses the Chopper protection action, while being relatively simple and convenient to implement, thereby improving the intuitiveness and efficiency of the unit grouping. The specific principles of the photovoltaic power station equivalent modeling method of the present application are described in conjunction with more embodiments below.

[0039] Specifically, Figure 1 As shown, the photovoltaic power station equivalent modeling method provided in this embodiment includes:

[0040] Step S10: constructing a detailed model of a photovoltaic power station, wherein the detailed model of the photovoltaic power station includes single-machine models of multiple photovoltaic power generation units;

[0041] Step S20: inputting light intensity information into the detailed model of the photovoltaic power station, and obtaining transient operation data of the single-machine model of each photovoltaic power generation unit when a failure occurs in the detailed model of the photovoltaic power station;

[0042] Step S30, calculating the chopping action critical current according to the light intensity information, the battery parameters of each photovoltaic power generation unit under the marked working condition and the voltage threshold of the chopping protection circuit action, and calculating the terminal current of the photovoltaic power generation unit according to the transient operation data;

[0043] Step S40, dividing the photovoltaic power generation units into a chopper protection action group or a chopper protection non-action group according to the chopper action critical current and the terminal current;

[0044] Step S50: establishing a first photovoltaic power station equivalent model to characterize the chopper protection action group, and establishing a second photovoltaic power station equivalent model to characterize the chopper protection non-action group.

[0045] In one or more embodiments, a detailed model of a photovoltaic power station is built on the DIgSILENT / Power Factory simulation modeling software platform, and the detailed model of the photovoltaic power station includes: a stand-alone model of each photovoltaic power generation unit in the photovoltaic power station, a line topology model between each photovoltaic power generation unit, a main transformer model, a machine-end transformer model, and a power grid replaced by an ideal voltage source; wherein the stand-alone model of the photovoltaic power generation unit includes a photovoltaic array, maximum power tracking, an inverter, an inverter controller, and a DC side Chopper protection.

[0046] Exemplarily, the inverter controller constructed includes: a low voltage ride-through judgment module and a dynamic reactive power support module. The low voltage ride-through judgment module is used to detect whether the grid connection point voltage drops below the standard requirement value of 0.9pu. When it drops below 0.9pu, it switches to the low voltage ride-through control mode during the fault period. The dynamic reactive power support module calculates the reactive current reference value ΔI generated by the photovoltaic inverter according to formula (1): T ;

[0047] ΔI T =K×(U T -0.9)×I N (1)

[0048] In formula (1), K is the reactive current proportional coefficient of the photovoltaic inverter, I N is the rated current of the photovoltaic power generation unit, U T is the grid connection point voltage.

[0049] Exemplarily, the constructed DC side chopper protection circuit includes: a protection action judgment module and a unloading protection circuit; the protection action judgment module is used to detect whether the DC bus voltage exceeds the voltage threshold of the protection circuit action. When the voltage threshold is exceeded, the unloading resistor is put into use to consume excess energy on the DC side to prevent overvoltage and overcurrent; when the DC bus voltage drops below the voltage threshold, the unloading resistor is cut off.

[0050] In one or more embodiments, in step S20, considering that the actual weather is complex and changeable, cloud cover may cause different outputs of photovoltaic power generation units, and the method of inputting light intensity information into the photovoltaic power station detailed model includes:

[0051] The maximum input light intensity S of multiple photovoltaic power generation units is calculated by using a random function. max and the minimum input light intensity S min The input light intensity of each photovoltaic power generation unit is randomly generated between T_i for:

[0052] S T_i =(S max -S min )×RAND()+S min (2)

[0053] In the formula, RAND() represents any random number between 0 and 1.

[0054] In one or more embodiments, the method for obtaining the transient operation data of the single-machine model of each photovoltaic power generation unit when a photovoltaic power station detailed model fails includes: setting a three-phase short circuit fault at the exit of the photovoltaic power station detailed model at time t0, and removing the fault at time t1, collecting the transient operation data of the photovoltaic power generation unit under the low voltage ride-through condition, the transient operation data including the active output of the photovoltaic power generation unit and the terminal voltage drop value at the time of short circuit. Wherein, the terminal current is I i =P i / U i , where P i For meritorious contribution, U i It is the voltage drop value at the terminal at the moment of short circuit.

[0055] In one or more embodiments, a method for calculating a chopping action critical current based on light intensity information, battery parameters of each photovoltaic power generation unit under a marked working condition, and a voltage threshold of a chopping protection circuit includes:

[0056] First, the light intensity information is used to correct the battery parameters of each battery in each photovoltaic power generation unit under the marked working conditions. Among them, the battery parameters include the short-circuit current I sc , open circuit voltage U oc, Maximum power point current I m and the maximum power point voltage U m , when the light intensity and temperature are not under standard conditions and the corrected battery parameters include the short-circuit current I s ' c , open circuit voltage U' oc , Maximum power point current I' m and maximum power point voltage U' m , the revised calculation formula is as follows:

[0057]

[0058] In the formula, S ref 、T ref They represent the light intensity and battery module temperature under standard working conditions, S and T represent the light intensity and battery module temperature under actual working conditions, a, b, c are correction coefficients, which are 0.0025 / ℃, 0.5 / ℃, and 0.00288 / ℃ respectively.

[0059] Next, the output volt-ampere characteristic value of the photovoltaic power generation unit is calculated based on the battery parameters of each battery after correction. Specifically, the photovoltaic array model composed of several photovoltaic cells connected in series and parallel is analyzed and simplified, and the photovoltaic cell equivalent circuit is simplified, and its series equivalent resistance R s Considered as infinitesimal, the parallel equivalent resistance R sh Assume N s PV cells are connected in series to form a PV string. p The photovoltaic groups are connected in parallel to form a photovoltaic array, and the simplified photovoltaic array output volt-ampere characteristic is obtained:

[0060]

[0061] Where I and U represent the output current and output voltage of the photovoltaic cell respectively.

[0062] Finally, according to the output volt-ampere characteristic value and voltage threshold U ref Calculate the critical current I of the photovoltaic power generation unit ref , the calculation formula is as follows

[0063]

[0064] In one or more embodiments, the method of dividing the photovoltaic power generation unit into a chopper protection action group or a chopper protection non-action group according to the chopper action critical current and the terminal current includes: when the terminal current is less than the chopper action critical current, it means that under the short-circuit fault condition, although the photovoltaic power generation unit has generated a voltage drop, it has not triggered the Chopper protection control during low voltage crossing, and the photovoltaic power generation unit is divided into the chopper protection non-action group; when the terminal current is greater than or equal to the chopper action critical current, it indicates that the Chopper protection action mechanism of the photovoltaic power generation unit is activated, and the photovoltaic power generation unit is divided into the chopper protection action group.

[0065] Furthermore, the equivalent parameters of the first photovoltaic power station equivalent model and the second photovoltaic power station equivalent model are calculated according to the following formula (9), and the equivalent impedance of the collector line is directly solved using the equal power loss method to establish a dual-machine equivalent model of the photovoltaic power station.

[0066]

[0067] Where, λ represents the number of photovoltaic power generation units in the photovoltaic power station; N seq 、N peq , S eq , Z ceq , C eq and L eq Respectively represent the number of PV array series batteries, the number of parallel batteries, the inverter capacity, the inverter impedance, the filter capacitor and filter inductor on the AC side of the inverter in the equivalent model; N s 、N p , S, Z c , C, and L represent the number of series batteries in a single PV array, the number of parallel batteries, the capacity of a single PV inverter, the inverter impedance, and the AC side filter capacitor and filter inductor of a single PV inverter in the detailed model, respectively.

[0068] Experimental verification: In order to verify the effectiveness of the equivalent modeling method of the photovoltaic power station in this embodiment, a detailed model of the photovoltaic power station is built on the DIgSILENT / PowerFactory simulation modeling software platform, which consists of 35 photovoltaic power generation units and collector lines. The topology diagram is shown in the figure. Figure 2 As shown. The photovoltaic power generation unit of the photovoltaic power station has an outlet voltage of 0.315V, which is connected to the 35kV medium voltage busbar through the machine-side transformer and the overhead line. It includes 4 feeders, 10 photovoltaic power generation units on feeder 1, 10 photovoltaic power generation units on feeder 2, 8 photovoltaic power generation units on feeder 3, and 7 photovoltaic power generation units on feeder 4. Then the main transformer is connected to the 220kV power grid through the transmission line. The photovoltaic power station model parameters and collector line parameters are shown in Tables 1 and 2 below:

[0069] Table 1 Detailed parameters of photovoltaic power station model

[0070]

[0071] Table 2 Parameters of photovoltaic power station collection line

[0072]

[0073]

[0074] Considering the complex and changeable actual weather conditions, cloud cover may lead to different outputs of photovoltaic power generation units. A random function is used to generate the light intensity of each photovoltaic power generation unit, as shown in Table 3 below:

[0075] Table 3 Light intensity of each photovoltaic power generation unit

[0076]

[0077] The three-phase short circuit fault is set to occur at the midpoint of the transmission line of the photovoltaic power station grid connection point, starting at 2s and cleared after 0.5s. The transient operation data of each photovoltaic power generation unit under the low voltage ride-through condition is measured, and the photovoltaic power station equivalent modeling method proposed in this embodiment is used to divide the 35 photovoltaic power generation units into two groups: Chopper protection action and Chopper protection inaction. The grouping results are shown in Table 4.

[0078] Table 4 Photovoltaic power generation unit grouping results

[0079]

[0080] Then, the equivalent parameters are calculated based on the clustering results, and the equivalent model of the photovoltaic power station is established in the DIgSILENT / PowerFactory software.

[0081] The simulation results of the dual-machine equivalent model, the traditional single-machine equivalent model and the detailed model established by the PV power station using the equivalent method proposed in this embodiment are compared to verify the effectiveness of the equivalent method of the present invention. The power response characteristics at the outlet of the PV power station are compared. Figure 3 , Figure 4 and Figure 5 shown.

[0082] Quantitatively analyze the error between the equivalent model and the detailed model, and define the error index of the equivalent model:

[0083]

[0084] Where W i , W i *They represent the active output or reactive output of the detailed model and the equivalent model of the PV power station at the outlet of the PV power station respectively, and n represents the number of sampling points.

[0085] The error evaluation indexes of the single-machine equivalent model and the dual-machine equivalent model established by the method of the present invention during the fault ride-through period are calculated respectively, and the results are shown in Table 5. It can be seen that the method proposed by the present invention more accurately reflects the operating characteristics of the photovoltaic power station when the fault ride-through Chopper protection is activated, and improves the accuracy of the established equivalent model in fitting the external characteristics of the photovoltaic power station output.

[0086] Table 5 Error evaluation index of equivalent model

[0087] Equivalence Model Active Power Reactive power Positive sequence current Single machine equivalent model 0.03578 0.00762 0.09721 Dual-machine equivalent model 0.00051 0.00030 0.00073

[0088] It can be seen that this method more accurately reflects the operating characteristics of the photovoltaic power station when the fault-through Chopper protection is activated, and improves the accuracy of the constructed equivalent model in fitting the external characteristics of the photovoltaic power station output.

[0089] The specific implementation methods of the present application are described in detail above. Although some embodiments have been shown and described, those skilled in the art should understand that these embodiments can be modified and improved without departing from the principles and spirit of the present application whose scope is defined by the claims and their equivalents. These modifications and improvements should also be within the scope of protection of the present application.

Claims

1. A photovoltaic power station equivalent modeling method, characterized in that: The photovoltaic power station equivalent modeling method comprises: Constructing a detailed model of a photovoltaic power station, wherein the detailed model of the photovoltaic power station includes single-machine models of multiple photovoltaic power generation units; Inputting light intensity information into the photovoltaic power station detailed model to obtain transient operation data of the single-machine model of each photovoltaic power generation unit when a failure occurs in the photovoltaic power station detailed model; The chopping action critical current is calculated based on the light intensity information, the battery parameters of each photovoltaic power generation unit under the marked working condition and the voltage threshold of the chopping protection circuit action, and the terminal current of the photovoltaic power generation unit is calculated based on the transient operation data; Dividing the photovoltaic power generation unit into a chopper protection action group or a chopper protection non-action group according to the chopper action critical current and the terminal current; A first photovoltaic power station equivalent model is established to characterize the group of machines with chopper protection in action, and a second photovoltaic power station equivalent model is established to characterize the group of machines with chopper protection inaction.

2. The photovoltaic power station equivalent modeling method according to claim 1, characterized in that: The method of inputting light intensity information into the photovoltaic power station detailed model comprises: The maximum input light intensity S of multiple photovoltaic power generation units is calculated by using a random function. max and the minimum input light intensity S min The input light intensity of each photovoltaic power generation unit is randomly generated between T_i for: S T_i =(S max -S min )×RAND()+S min In the formula, RAND() represents any random number between 0 and 1.

3. The photovoltaic power station equivalent modeling method according to claim 1, characterized in that: The method for obtaining transient operation data of a single-machine model of each photovoltaic power generation unit when a failure occurs in the detailed model of the photovoltaic power station includes: It is assumed that a three-phase short circuit fault occurs at the outlet of the detailed model of the photovoltaic power station at time t0, and the fault is removed at time t1, and the transient operation data of the photovoltaic power generation unit under the low voltage ride-through condition is collected. The transient operation data includes the active output of the photovoltaic power generation unit and the terminal voltage drop value at the short circuit moment.

4. The photovoltaic power station equivalent modeling method according to claim 1, characterized in that: The method for calculating the chopping action critical current according to the light intensity information, the battery parameters of each photovoltaic power generation unit under the marked working condition and the voltage threshold of the chopping protection circuit action includes: Using the light intensity information, the battery parameters of each battery of each photovoltaic power generation unit under the marked working condition are corrected; The output volt-ampere characteristic value of the photovoltaic power generation unit is calculated based on the battery parameters of each battery after correction; The chopping action critical current of the photovoltaic power generation unit is calculated according to the output volt-ampere characteristic value and the voltage threshold.

5. The photovoltaic power station equivalent modeling method according to claim 1, characterized in that: The method for dividing the photovoltaic power generation unit into a chopper protection action group or a chopper protection non-action group according to the chopper action critical current and the terminal current comprises: When the terminal current is less than the chopping action critical current, the photovoltaic power generation unit is divided into a chopping protection non-action group; When the terminal current is greater than or equal to the chopping action critical current, the photovoltaic power generation unit is divided into a chopping protection action group.

6. The photovoltaic power station equivalent modeling method according to claim 1, characterized in that: The stand-alone model of the photovoltaic power generation unit includes an inverter controller, and the inverter controller includes: A low voltage ride-through judgment module, which is used to detect whether the grid connection point voltage drops below the standard requirement value of 0.9pu. When it drops below 0.9pu, it switches to the low voltage ride-through control mode during the fault period; A dynamic reactive power support module is used to calculate a reactive current reference value emitted by a photovoltaic inverter.

7. The photovoltaic power station equivalent modeling method according to claim 1, characterized in that: The single-machine model of the photovoltaic power generation unit includes a DC side chopper protection circuit, and the DC side chopper protection circuit includes: A protection action judgment module, the protection action judgment module is used to detect whether the DC bus voltage exceeds the voltage threshold of the protection circuit action; The unloading protection circuit is used to input the unloading resistor to consume the excess energy on the DC side to prevent overvoltage and overcurrent when the voltage exceeds the voltage threshold; when the DC bus voltage drops below the voltage threshold, the unloading protection circuit is used to cut off the unloading resistor.

Images