Photovoltaic grid system fault current limiting control method and device

Abstract

The present invention discloses a fault current limiting control method and device for a photovoltaic grid-connected system, and relates to the field of photovoltaic grid-connected technology. The present invention generates a switch tube control signal based on the voltage of the DC voltage-stabilizing capacitor on the input side of the inverter; obtains the grid connection point voltage amplitude and the voltage signal and current signal in a stationary synchronous coordinate system based on the voltage signal and current signal at the grid connection point of the converter; determines the fault depth based on the grid connection point voltage amplitude to determine the active output rating of the inverter; generates the voltage reference value and phase reference value of the inverter based on the voltage signal and current signal in the stationary synchronous coordinate system and the active output rating of the inverter, and converts them into a current reference value through a fixed virtual impedance; obtains an inverter modulation wave based on the current reference value, and obtains the control signal of each switch tube of the inverter after the inverter modulation wave is modulated by SPWM. The present invention has good dynamic performance and can meet the control requirements under fault conditions of the photovoltaic grid-connected system.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic grid-connected technology, and in particular to a method and device for controlling fault current limiting in a photovoltaic grid-connected system. Background Art

[0002] To address energy and environmental challenges, renewable energy generation is gaining increasing attention. Photovoltaic power generation, as a key form of renewable energy generation, is seeing increasing installed capacity. However, with the reduction in synchronous generators and the increase in grid-connected inverters, the overall grid inertia and stability are significantly impacted. Therefore, control measures are needed to enhance the stability of inverter grid-connected systems, ensuring that the inverters have a sufficient level of grid support. These inverters are often referred to as grid-connected or networking inverters.

[0003] Since converters are usually designed according to their energy transmission requirements, their overcurrent capacity is usually weak. When unexpected faults occur in the system (such as low voltage faults on the grid side), these faults may cause a sharp increase in current, damaging capacitors, switching tubes, etc.

[0004] Therefore, how to ensure that the converter supports the power grid as much as possible without exceeding the rated output current limit is of great practical significance for alleviating the energy shortage crisis, large-scale grid connection of photovoltaic power generation systems, and improving their operating performance.

[0005] Currently, many fault current limiting solutions exist for three-phase and single-phase grid-connected photovoltaic systems. However, most of these solutions are implemented under the premise of grid-following control. Fault current limiting solutions for grid-connected systems are relatively rare. Furthermore, existing fault current limiting solutions for grid-connected systems involve a switching process between operating modes, resulting in poor dynamic performance and failing to meet the control requirements of photovoltaic grid-connected systems under fault conditions. Summary of the Invention

[0006] In order to solve the above problems existing in the prior art, the present invention provides a method and device for controlling fault current limiting in a photovoltaic grid system.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] A photovoltaic grid system fault current limiting control method, the method comprising:

[0009] Obtain the voltage of the DC stabilizing capacitor on the input side of the inverter in the photovoltaic grid system;

[0010] Generate a switch control signal based on the voltage of the DC stabilizing capacitor at the input side of the inverter;

[0011] Obtain voltage and current signals at the grid-connected points of the converters in the photovoltaic grid system;

[0012] Based on the voltage signal and current signal of the grid-connected point of the converter, the voltage amplitude of the grid-connected point and the voltage signal and current signal in the stationary synchronous coordinate system are obtained;

[0013] Determining a fault depth based on the grid connection point voltage amplitude, and determining an active output rating of the inverter based on the fault depth;

[0014] generating a voltage reference value and a phase reference value of the inverter based on the voltage signal and the current signal in the stationary synchronous coordinate system and the active output rated value of the inverter;

[0015] Converting the voltage reference value and phase reference value of the inverter into a current reference value through a fixed virtual impedance;

[0016] An inverter modulation wave is obtained based on the current reference value, and the inverter modulation wave is modulated by SPWM to obtain control signals of each switch tube of the inverter.

[0017] Optionally, generating a switch control signal based on the voltage of a DC voltage stabilizing capacitor at the input side of the inverter specifically includes:

[0018] Based on the voltage of the DC stabilizing capacitor on the input side of the inverter, a flexible power point tracking controller is used to control the working voltage of the photovoltaic array to obtain the difference between the photovoltaic working voltage reference value and the actual working voltage value;

[0019] Based on the difference between the photovoltaic working voltage reference value and the actual working voltage value, the switch tube control signal is generated through the PI controller and the modulation link.

[0020] Optionally, obtaining the grid connection point voltage amplitude and the voltage signal and current signal in a stationary synchronous coordinate system based on the voltage signal and current signal of the grid connection point of the converter specifically includes:

[0021] Separate the positive and negative sequences of the voltage signal at the grid-connected point of the converter to obtain the voltage amplitude at the grid-connected point;

[0022] Clarke transformation is performed on the voltage signal of the converter grid-connected point and the current signal of the converter grid-connected point to obtain the voltage signal and the current signal in the stationary synchronous coordinate system.

[0023] A photovoltaic grid system fault current limiting control device, the device is used to implement the photovoltaic grid system fault current limiting control method provided above; the device comprises: a flexible power point tracking controller, a fault current limiting controller, a grid type controller, a short-time current limiting controller and a PWM modulator;

[0024] The converter in the photovoltaic grid system is controlled by the FPPT controller; the inverter in the photovoltaic grid system is controlled by the fault current limiting controller, the grid type controller, the short-time current limiting controller and the PWM modulator connected in sequence;

[0025] The flexible power point tracking controller is used to generate a switch control signal based on the voltage of the DC stabilizing capacitor at the input side of the inverter;

[0026] The fault current limiting controller is used to obtain a grid connection point voltage amplitude and a voltage signal and a current signal in a stationary synchronous coordinate system based on a voltage signal and a current signal at a grid connection point of the converter, and is used to determine a fault depth based on the grid connection point voltage amplitude, and determine an active output rated value of the inverter based on the fault depth;

[0027] The grid-type controller is used to generate a voltage reference value and a phase reference value of the inverter based on the voltage signal and the current signal in the stationary synchronous coordinate system and the active output rated value of the inverter;

[0028] The short-time current limiting controller is used to convert the voltage reference value and the phase reference value of the inverter into a current reference value through a fixed virtual impedance;

[0029] The PWM modulator is used to obtain an inverter modulation wave based on a current reference value, and is used to modulate the inverter modulation wave to obtain control signals for each switch tube of the inverter.

[0030] Optionally, the meshed controller includes an active loop and a reactive loop; the active loop obtains a voltage phase reference value through the active output rated value and the current frequency difference; the reactive loop obtains a voltage amplitude reference value through the current voltage difference.

[0031] Optionally, a PR controller is further included;

[0032] The PR controller is used to obtain a current reference value after passing through a current limiting link, and obtain an inverter modulation wave based on the current reference value.

[0033] Optionally, the inverter modulation wave is modulated by SPWM to obtain the control signal of each switch tube of the inverter.

[0034] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0035] The present invention uses a fault current limiting controller, a grid-type controller and a short-time current limiting controller to control the converter, so that the converter can support the voltage or frequency of the grid when a fault occurs in the grid, while ensuring that the output current of the converter is within the range of its maximum allowable current. A flexible power point tracking (FPPT) controller is used to control the photovoltaic system so that it can autonomously adjust the operating point according to the energy demand of the subsequent converter to match the energy of the previous and subsequent stages. When a fault occurs, the present invention can limit the output current and output as much active or reactive power as possible within the allowed range to support the grid. Since the present invention does not have the process of switching operating modes, it has good dynamic performance and high economy, and can meet the control requirements in the event of a fault in the photovoltaic grid-type system. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0037] Figure 1 This is a flowchart of the implementation of the photovoltaic grid system fault current limiting control method provided by the present invention;

[0038] Figure 2 A control flow chart of the FPPT controller provided by the present invention;

[0039] Figure 3 A control flow chart of the fault current limiting controller provided by the present invention;

[0040] Figure 4 This is a control block diagram of the active loop in the network-type controller provided by the present invention;

[0041] Figure 5 This is a control block diagram of the reactive loop in the grid-type controller provided by the present invention;

[0042] Figure 6 A flow chart of the short-time current limiting controller provided by the present invention;

[0043] Figure 7 This is a schematic diagram of the structure of the photovoltaic grid system fault current limiting control device provided by the present invention;

[0044] Figure 8 A diagram showing the current changes at the grid connection point in the event of a grid voltage fault provided by the present invention;

[0045] Figure 9A diagram showing the voltage change at the grid connection point in the event of a grid voltage fault provided by the present invention;

[0046] Figure 10 A diagram showing changes in the active power output of the inverter in the event of a grid voltage failure provided by the present invention;

[0047] Figure 11 This is a diagram of the changes in photovoltaic array output power under the condition of grid voltage failure provided by the present invention. DETAILED DESCRIPTION

[0048] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0049] The purpose of the present invention is to provide a method and device for controlling current limiting in the event of a photovoltaic grid system fault, which can limit the output current and output as much active or reactive power as possible within the allowed range to support the grid. In addition, the method and device have good dynamic performance and high economy, and can meet the control requirements in the event of a photovoltaic grid system fault.

[0050] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0051] like Figure 1 As shown, the photovoltaic grid system fault current limiting control method provided by the present invention includes:

[0052] Step 1: Measure the DC stabilizing capacitor voltage v on the inverter input side dc . Use FPPT controller to adjust the working voltage of photovoltaic array v pv The FPPT controller first maintains the photovoltaic operating voltage v pv Lower than its maximum power point voltage to ensure the power P of the photovoltaic array pv With v pv The power of the photovoltaic array P is monotonically related to pv With the working voltage of the photovoltaic array v pv Then the desired DC bus voltage v pvref As a reference value, the actual DC bus voltage v dc The power reference value P of the FPPT controller is output through the PI controller. pvref Since the power of the photovoltaic array P pv The working voltage of the photovoltaic array is v pvThere is a monotonic relationship, so when the actual power of the photovoltaic array (abbreviated as photovoltaic power) is greater than the power reference value P pvref When the working voltage of the photovoltaic array is controlled pv Reduce (i.e. reduce the PV array voltage reference value), when the PV power is less than or equal to the power reference value P pvref When the working voltage of the photovoltaic array is controlled pv Increase (i.e. increase the PV array voltage reference value), and eventually control the P pv , so that the DC stabilizing capacitor voltage v on the inverter input side dc Stable. The DC stabilizing capacitor voltage v on the inverter input side dc The stability of the inverter grid-connected output can be said to be equal to the active power generated by the photovoltaic array. The difference between the photovoltaic operating voltage reference value given by the flexible power point tracking controller and the actual operating voltage is used to obtain the boost circuit duty cycle through the PI controller. The switching tube signal is modulated by the modulation link to control the converter. Among them, the control process of the FPPT controller is as follows: Figure 2 shown.

[0053] Step 2: Use voltage sensor and current sensor to measure the voltage signal v at the grid connection point of the converter cABC and the current signal i cABC , by the voltage signal v cABC Perform positive and negative sequence separation to obtain the grid voltage amplitude V pu , by the voltage signal v cABC and the current signal i cABC Clarke transform obtains the voltage signal and current signal in the stationary synchronous coordinate system, thereby calculating the current output power P of the inverter inv ;

[0054] Step 3: Control the active output reference value according to the fault depth through the fault current limiting controller. The fault depth is determined by the grid voltage amplitude V obtained in step 2. pu Sure.

[0055] Since the inverter needs to output additional power to the grid or absorb reactive power to support the grid during a voltage fault, this will cause the inverter output current to increase, which may exceed the maximum capacity limit of the inverter. Therefore, it is necessary to reduce the active power output accordingly after determining the fault depth so that the inverter output current remains within the current limit range. The fault current limiting controller can determine the active power output rating P of the inverter in the current situation. ref , passed to the subsequent virtual synchronous machine controller, since the reactive power output is more prioritized, the active power output rated value P refThat is, the smaller value between the original active setting value and the maximum active value that the converter can output after giving priority to reactive power output; Based on this, the control flow of the fault current limiting controller is as follows: Figure 3 shown.

[0056] Step 4: Generate the voltage reference value and phase reference value of the inverter through the grid-type controller.

[0057] The grid-type controller is mainly divided into two parts: active loop and reactive loop. The active loop outputs rated value P ref The voltage phase reference value is obtained by the difference between the current frequency and the current frequency. The specific control structure is as follows: Figure 4 As shown. The reactive loop obtains the voltage amplitude reference value through the current voltage difference. The specific control structure is as follows Figure 5 shown. Figure 4 and Figure 5 In, P ref 、P inv 、D p are active power given, active power output and damping coefficient, J, ω, ω N , θ are the corresponding moment of inertia, angular frequency, angular frequency setting value and voltage phase reference value of the modulation signal respectively; Q ref , Q inv 、D q They are reactive power setting, reactive power output, QV loop coefficient, U g 、U N , K, E N 、E m They are the grid connection point voltage amplitude, grid voltage amplitude setting value, integral coefficient of modulation signal and output voltage setting value, and voltage amplitude reference value.

[0058] Specifically, the active loop compares the current output angular frequency ω with the angular frequency setting value ω N By taking the difference, we can get the damping coefficient D p The gain is then fed back to the active output rated value P ref and actual power P inv The difference is divided by the angular frequency setting value and then divided by the set moment of inertia J. After passing through an integrator, the angular frequency ω is obtained, and then the current phase θ is obtained through another integrator.

[0059] The reactive loop will connect the grid voltage amplitude U g and the grid voltage amplitude setting value U N Make the difference, through the QV loop coefficient D q The gain is then fed back to the reactive reference value Q ref and actual reactive power Q inv The difference is divided by the integral coefficient K, and then the output voltage setting value E is obtained through an integrator. Nand the output voltage reference value E m The difference is finally passed through the output voltage setting value E N The output voltage reference value E is obtained by feedforward m .

[0060] Step 5: The short-time current limiting controller converts the voltage reference value and voltage phase reference value obtained by the network controller into a current reference value through a fixed virtual impedance, and passes it to the PR controller after the current limiting link, and finally obtains the inverter modulation wave. After SPWM modulation, the control signal of each switch tube of the inverter is obtained. The specific control structure is as follows Figure 6 shown.

[0061] In order to implement the above-mentioned photovoltaic grid system fault current limiting control method, a photovoltaic grid system fault current limiting control device is set up by taking the fault current limiting control of a three-phase two-level grid-connected photovoltaic system with an LCL filter, which is mainly composed of a photovoltaic array, a boost converter, a three-phase two-level inverter and an output filter as an example. The structure of the device obtained by the setting is as follows: Figure 7 As shown, where v pv 、i pv Represent the voltage and current of the photovoltaic array respectively, d b and v dc Represent the duty cycle and output voltage of the boost converter, v invA 、v invB 、v invC They represent the midpoint voltages of the bridge arms of phases a, b, and c of the three-phase two-level inverter, respectively, and v cA 、v cB 、v cC with i cA 、i cB 、i cC Represent the voltage of phase a, phase b and phase c at the grid connection point and the inverter output current, P ref Represents the active power setting value in the grid-type controller, E m and θ represent the amplitude reference value and phase reference value of the converter output voltage, v m The modulated waveform of the three-phase two-level inverter is shown in Figure 1. The converter is controlled by the FPPT controller, and the three-phase two-level inverter is controlled by the fault current limiting controller, the meshing controller, the short-time current limiting controller, and the PWM modulator.

[0062] Based on this device structure, the fault current limiting control of the grid-connected photovoltaic system is carried out, and the changes in various parameters of the three-phase two-stage grid-connected photovoltaic system with LCL filter under the condition of grid voltage fault are shown as follows: Figures 8 to 11 shown.

[0063] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0064] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A photovoltaic grid system fault current limiting control method, characterized in that: The method comprises: Obtain the voltage of the DC stabilizing capacitor on the input side of the inverter in the photovoltaic grid system; Generate a switch control signal based on the voltage of the DC stabilizing capacitor at the input side of the inverter; Obtain voltage and current signals at the inverter grid connection point in the photovoltaic grid system; Based on the voltage signal and current signal of the inverter grid connection point, the voltage amplitude of the grid connection point and the voltage signal and current signal in the stationary synchronous coordinate system are obtained; Determining a fault depth based on the grid connection point voltage amplitude, and determining an active output rating of the inverter based on the fault depth; generating a voltage reference value and a phase reference value of the inverter based on the voltage signal and the current signal in the stationary synchronous coordinate system and the active output rated value of the inverter; Converting the voltage reference value and phase reference value of the inverter into a current reference value through a fixed virtual impedance; An inverter modulation wave is obtained based on the current reference value, and the inverter modulation wave is modulated by SPWM to obtain a control signal for each switch tube of the inverter; The switch control signal is generated based on the voltage of the DC stabilizing capacitor on the inverter input side, specifically including: The flexible power point tracking controller is used to control the operating voltage of the photovoltaic array based on the voltage of the DC voltage-stabilizing capacitor on the input side of the inverter. When the actual power of the photovoltaic array is greater than the power reference value, the photovoltaic operating voltage reference value is reduced. When the photovoltaic power is less than or equal to the power reference value, the photovoltaic operating voltage reference value is increased. The flexible power point tracking controller gives the photovoltaic operating voltage reference value and the actual operating voltage, and then uses the PI controller to obtain the boost circuit duty cycle. The modulation link modulates the switch tube control signal to control the converter.

2. The photovoltaic grid system fault current limiting control method according to claim 1, characterized in that: Based on the voltage signal and current signal of the inverter grid connection point, the voltage amplitude of the grid connection point and the voltage signal and current signal in the stationary synchronous coordinate system are obtained, specifically including: Separate the positive and negative sequences of the voltage signal at the inverter grid connection point to obtain the grid connection point voltage amplitude; Clarke transformation is performed on the voltage signal of the inverter grid-connected point and the current signal of the inverter grid-connected point to obtain the voltage signal and current signal in the stationary synchronous coordinate system.

3. A photovoltaic grid system fault current limiting control device, characterized in that: The device is used to implement the photovoltaic grid system fault current limiting control method according to any one of claims 1-2; the device comprises: a flexible power point tracking controller, a fault current limiting controller, a grid type controller, a short-time current limiting controller and a PWM modulator; The converter in the photovoltaic grid system is controlled by the flexible power point tracking controller; the inverter in the photovoltaic grid system is controlled by the fault current limiting controller, the grid type controller, the short-time current limiting controller and the PWM modulator connected in sequence; The flexible power point tracking controller is used to generate a switch control signal based on the voltage of the DC stabilizing capacitor at the input side of the inverter; The fault current limiting controller is used to obtain a grid connection point voltage amplitude and a voltage signal and a current signal in a stationary synchronous coordinate system based on a voltage signal and a current signal at a grid connection point of the inverter, and is used to determine a fault depth based on the grid connection point voltage amplitude, and determine an active output rated value of the inverter based on the fault depth; The grid-type controller is used to generate a voltage reference value and a phase reference value of the inverter based on the voltage signal and the current signal in the stationary synchronous coordinate system and the active output rated value of the inverter; The short-time current limiting controller is used to convert the voltage reference value and the phase reference value of the inverter into a current reference value through a fixed virtual impedance; The PWM modulator is used to obtain an inverter modulation wave based on a current reference value, and is used to modulate the inverter modulation wave to obtain control signals for each switch tube of the inverter.

4. The photovoltaic grid system fault current limiting control device according to claim 3, characterized in that: The grid-type controller includes an active loop and a reactive loop; the active loop obtains a voltage phase reference value through the active output rated value and the current frequency difference; the reactive loop obtains a voltage amplitude reference value through the current voltage difference.

5. The photovoltaic grid system fault current limiting control device according to claim 3, characterized in that: Also includes PR controller; The PR controller is used to obtain a current reference value after passing through a current limiting link, and obtain an inverter modulation wave based on the current reference value.

6. The photovoltaic grid system fault current limiting control device according to claim 3, characterized in that: The inverter modulation wave is modulated by SPWM to obtain the control signal of each switch tube of the inverter.

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