A harmonic injection based method and apparatus for fault protection of a photovoltaic grid-connected system

Abstract

The application relates to the technical field of low-inertia system protection and control, and discloses a photovoltaic grid-connected system fault protection method and device based on harmonic injection. When a fault occurs in a photovoltaic grid-connected system, a fault current limiting control strategy is adopted to limit the size of the fault current by adjusting the output voltage; then a harmonic injection control strategy is adopted to inject a harmonic signal into the photovoltaic grid-connected system on the basis of three-phase voltage; the fault equivalent circuit of the photovoltaic grid-connected system before and after the harmonic signal injection is acquired respectively, and the fault distance from the protection measuring point to the fault point is solved; and the exit time of the protection measuring point is determined according to the fault distance, so that the fault of the photovoltaic grid-connected system is processed. The application can inject a harmonic signal with characteristics after the system fault, can strengthen the fault characteristics, increase the fault information, improve the sensitivity and reliability of protection, and is helpful to solve the problem of protection failure caused by the weak feedback characteristics of the converter.

Description

Technical Field

[0001] This invention relates to the field of low inertia system protection and control technology, and in particular to a fault protection method and device for a photovoltaic grid-connected system based on harmonic injection. Background Technology

[0002] In a solar photovoltaic grid-connected system, when a fault occurs in the AC grid-connected line, the grid-connected converter exhibits weak feedback characteristics due to the vulnerability and high controllability of its own power semiconductor devices. It can only provide a short-circuit current of one to three times the rated current, which may cause the protection device on the photovoltaic side of the line to fail to operate because it cannot reach the protection threshold, thus making it impossible to isolate and clear the fault.

[0003] To address the issue of protection failure due to the weak feed characteristics of the converter, the current approach involves the overcurrent protection valve of the photovoltaic converter activating after a certain fault duration, causing the converter to lock out and thus severing the connection between the photovoltaic side and the fault. However, this lockout strategy means the entire photovoltaic system is taken offline, potentially leading to prolonged power outages and compromising the reliable power supply from the grid. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a fault protection method and device for photovoltaic grid-connected systems based on harmonic injection, which can enhance fault characteristics, improve the sensitivity and reliability of protection, make faults easier to identify, and prevent protection failure.

[0005] This invention provides a fault protection method for a photovoltaic grid-connected system based on harmonic injection, comprising:

[0006] When a fault occurs in the photovoltaic grid-connected system, the magnitude of the fault current is limited by adjusting the output voltage;

[0007] Based on the three-phase voltage, harmonic signals are injected into the photovoltaic grid-connected system;

[0008] Before and after the injection of harmonic signals, the first circuit data and the second circuit data of the photovoltaic grid-connected system are acquired respectively, and the fault distance from the protection measuring point to the fault point is determined based on the first circuit data and the second circuit data.

[0009] The exit time of the protection measuring point is set according to the fault distance, and the fault of the photovoltaic grid-connected system is handled according to the exit time of the protection measuring point.

[0010] Furthermore, when a fault occurs in the photovoltaic grid-connected system, the magnitude of the fault current is limited by adjusting the output voltage, specifically as follows:

[0011] When a fault occurs in the photovoltaic grid-connected system, the output voltage value at the previous moment is obtained;

[0012] According to a preset ratio, the current output voltage value is reduced to a preset ratio of the output voltage value at the previous moment.

[0013] Furthermore, the injection of harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage specifically involves:

[0014] Based on the preset voltage detection requirements, determine the amplitude, angular frequency, and initial phase of the harmonic signal;

[0015] Calculate the voltage magnitudes of the harmonic signals injected into the three-phase circuits of the photovoltaic grid-connected system, specifically:

[0016]

[0017] Among them, U det ω represents the amplitude of the harmonic signal. det The angular frequency of the harmonic signal; This represents the initial phase of the harmonic signal; and These are the voltage magnitudes of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system.

[0018] Furthermore, the determination of the amplitude, angular frequency, and initial phase of the harmonic signal according to the preset voltage detection requirements specifically involves:

[0019] Obtain the rated voltage of the photovoltaic grid-connected system;

[0020] The amplitude of the harmonic signal is determined based on the rated voltage of the photovoltaic grid-connected system and the preset amplitude detection requirements.

[0021] Based on the preset voltage detection requirements, determine the angular frequency and initial phase of the harmonic signal.

[0022] Furthermore, the step of acquiring the first circuit data and the second circuit data of the photovoltaic grid-connected system before and after the injection of the harmonic signal, and determining the fault distance from the protection measuring point to the fault point based on the first circuit data and the second circuit data, specifically involves:

[0023] Before and after the injection of harmonic signals, the first fault equivalent circuit and the second fault equivalent circuit of the photovoltaic grid-connected system are obtained respectively.

[0024] The fault distance from the protection measuring point to the fault point is solved based on the circuit state equations of the first fault equivalent circuit and the second fault equivalent circuit, specifically as follows:

[0025]

[0026] in, and Z represents the power frequency phasor of the protection measuring point before the injection of harmonic signals; n Z represents the equivalent impedance on the back side of the protection measuring point before the injected harmonic signal; L This represents the impedance per unit length of the line before the injection of harmonic signals. and Represent the power frequency fault component phasor of the line protection measuring point before the injection of harmonic signals; d represents the fault distance from the protection measuring point to the fault point; R f The value represents the transition resistance, and l represents the total length of the line. and This represents the power frequency phasor of the protection measuring point after the harmonic signal is injected; This represents the back-side equivalent impedance of the protection measuring point after the injection of harmonic signals. This represents the impedance per unit length of the line after the injection of harmonic signals.

[0027] Furthermore, the step of setting the exit time of the protection measuring point based on the fault distance specifically involves:

[0028] The inverse-time operating characteristic of the protection measuring point is calculated based on the fault distance to determine the exit time of the protection measuring point. Specifically:

[0029]

[0030] Among them, t act Indicates the protection segment I action time; t II Δt represents the protection's second-stage action time; Δt represents the coordination delay.

[0031] This invention provides a fault protection method for photovoltaic grid-connected systems based on harmonic injection. When a fault occurs in the photovoltaic grid-connected system, a fault current-limiting control strategy is adopted to limit the magnitude of the fault current by adjusting the output voltage. Then, a harmonic injection control strategy is employed to inject harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage. The fault equivalent circuit of the photovoltaic grid-connected system is obtained before and after the harmonic signal injection, and the fault distance from the protection measuring point to the fault point is calculated. The exit time of the protection measuring point is determined based on the fault distance to handle the fault in the photovoltaic grid-connected system. This invention injects characteristic harmonic signals after a system fault occurs, which can enhance fault characteristics, increase fault information, improve the sensitivity and reliability of protection, and help solve the problem of protection failure caused by weak feeder characteristics of the converter.

[0032] Accordingly, the present invention provides a fault protection device for a photovoltaic grid-connected system based on harmonic injection, comprising: an adjustment module, an injection module, a calculation module, and a setting module;

[0033] The adjustment module is used to limit the magnitude of the fault current by adjusting the output voltage when a fault occurs in the photovoltaic grid-connected system.

[0034] The injection module is used to inject harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage;

[0035] The calculation module is used to acquire the first circuit data and the second circuit data of the photovoltaic grid-connected system before and after the injection of harmonic signals, respectively, and determine the fault distance from the protection measuring point to the fault point based on the first circuit data and the second circuit data.

[0036] The setting module is used to set the exit time of the protection measuring point according to the fault distance, and to handle the fault of the photovoltaic grid-connected system according to the exit time of the protection measuring point.

[0037] Furthermore, the adjustment module includes: a voltage acquisition unit and a voltage setting unit;

[0038] The voltage acquisition unit is used to acquire the output voltage value at the previous moment when a fault occurs in the photovoltaic grid-connected system.

[0039] The voltage setting unit is used to reduce the current output voltage value to a preset ratio of the output voltage value at the previous moment, according to a preset ratio.

[0040] Furthermore, the injection module includes: a determination unit and a voltage calculation unit;

[0041] The determining unit is used to determine the amplitude, angular frequency and initial phase of the harmonic signal according to the preset voltage detection requirements;

[0042] The voltage calculation unit is used to calculate the voltage magnitude of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system, specifically:

[0043]

[0044] Among them, U det ω represents the amplitude of the harmonic signal. det The angular frequency of the harmonic signal; This represents the initial phase of the harmonic signal; and These are the voltage magnitudes of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system.

[0045] Furthermore, the calculation module includes: a circuit acquisition unit and a distance calculation unit;

[0046] The circuit acquisition unit is used to acquire the first fault equivalent circuit and the second fault equivalent circuit of the photovoltaic grid-connected system before and after the injection of harmonic signals, respectively.

[0047] The distance calculation unit is used to solve the fault distance from the protection measuring point to the fault point based on the circuit state equations of the first fault equivalent circuit and the second fault equivalent circuit. Specifically:

[0048]

[0049] in, and Z represents the power frequency phasor of the protection measuring point before the injection of harmonic signals; n Z represents the equivalent impedance on the back side of the protection measuring point before the injected harmonic signal; L This represents the impedance per unit length of the line before the injection of harmonic signals. and Represent the power frequency fault component phasor of the line protection measuring point before the injection of harmonic signals; d represents the fault distance from the protection measuring point to the fault point; R f The value represents the transition resistance, and l represents the total length of the line. and This represents the power frequency phasor of the protection measuring point after the harmonic signal is injected; This represents the back-side equivalent impedance of the protection measuring point after the injection of harmonic signals. This represents the impedance per unit length of the line after the injection of harmonic signals.

[0050] This invention provides a fault protection device for photovoltaic grid-connected systems based on harmonic injection. Based on the organic integration of modules, when a fault occurs in the photovoltaic grid-connected system, a fault current-limiting control strategy is employed to limit the magnitude of the fault current by adjusting the output voltage. Then, a harmonic injection control strategy is used to inject harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage. The fault equivalent circuit of the photovoltaic grid-connected system is obtained before and after the harmonic signal injection, and the fault distance from the protection measuring point to the fault point is calculated. The exit time of the protection measuring point is determined based on the fault distance to handle the fault in the photovoltaic grid-connected system. This invention injects characteristic harmonic signals after a system fault occurs, which can enhance fault characteristics, increase fault information, improve the sensitivity and reliability of the protection, and help solve the problem of protection failure caused by the weak feed characteristics of the converter. Attached Figure Description

[0051] Figure 1 This is a flowchart illustrating an embodiment of the fault protection method for photovoltaic grid-connected systems based on harmonic injection provided by the present invention.

[0052] Figure 2 A topology diagram of a photovoltaic grid-connected system provided by the present invention;

[0053] Figure 3 A schematic diagram of the fault current limiting control strategy provided by the present invention;

[0054] Figure 4 A schematic diagram of the harmonic injection control strategy provided by the present invention;

[0055] Figure 5 A schematic diagram of the full-scale equivalent circuit for power frequency provided by the present invention;

[0056] Figure 6 A schematic diagram of the power frequency fault component equivalent circuit provided by the present invention;

[0057] Figure 7 A schematic diagram of a fault equivalent circuit at harmonic frequencies provided by the present invention;

[0058] Figure 8 This is a schematic diagram of an embodiment of the fault protection device for a photovoltaic grid-connected system based on harmonic injection provided by the present invention. Detailed Implementation

[0059] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] Example 1

[0061] See Figure 1 This is a flowchart illustrating an embodiment of the fault protection method for photovoltaic grid-connected systems based on harmonic injection provided by the present invention. The method includes steps 101 to 104, each step being as follows:

[0062] Step 101: When a fault occurs in the photovoltaic grid-connected system, the magnitude of the fault current is limited by adjusting the output voltage.

[0063] Furthermore, in the first embodiment of the present invention, when a fault occurs in the photovoltaic grid-connected system, the magnitude of the fault current is limited by adjusting the output voltage, specifically as follows:

[0064] When a fault occurs in the photovoltaic grid-connected system, the output voltage value at the previous moment is obtained;

[0065] According to a preset ratio, the current output voltage value is reduced to a preset ratio of the output voltage value at the previous moment.

[0066] In the first embodiment of the present invention, considering that the number and capacity of grid-connected photovoltaic systems are increasing, the "local connection, automatic disconnection after fault" blocking strategy may cause grid collapse in systems with a high proportion of distributed energy. Therefore, it is necessary to modify the fault blocking of the grid-connected photovoltaic inverter system to a current limiting strategy to avoid the converter from shutting down for a short period of time, thereby providing the possibility for harmonic signal injection and protection actions.

[0067] As an example of the first embodiment of the present invention, after the fault current limiting control is started, the photovoltaic system is switched from MPPT control to constant output voltage control, and the photovoltaic cell terminal voltage is limited to 50% of the reference value at the moment before the current limiting start, so as to limit the output power and thus limit the fault current.

[0068] Step 102: Based on the three-phase voltage, inject harmonic signals into the photovoltaic grid-connected system.

[0069] Furthermore, in the first embodiment of the present invention, harmonic signals are injected into the photovoltaic grid-connected system based on the three-phase voltage, specifically as follows:

[0070] Based on the preset voltage detection requirements, determine the amplitude, angular frequency, and initial phase of the harmonic signal;

[0071] Calculate the voltage magnitudes of the harmonic signals injected into the three-phase circuits of the photovoltaic grid-connected system, specifically:

[0072]

[0073] Among them, U det ω represents the amplitude of the harmonic signal. det The angular frequency of the harmonic signal; This represents the initial phase of the harmonic signal; and These are the voltage magnitudes of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system.

[0074] Furthermore, in the first embodiment of the present invention, the amplitude, angular frequency, and initial phase of the harmonic signal are determined according to preset voltage detection requirements, specifically as follows:

[0075] Obtain the rated voltage of the photovoltaic grid-connected system;

[0076] The amplitude of the harmonic signal is determined based on the rated voltage of the photovoltaic grid-connected system and the preset amplitude detection requirements.

[0077] Based on the preset voltage detection requirements, determine the angular frequency and initial phase of the harmonic signal.

[0078] As an example of the first embodiment of the present invention, setting the amplitude of the harmonic signal mainly requires consideration of its impact on the power grid and the limitations of the instrument transformer's accuracy. Therefore, to ensure that the harmonic signal and its response signal are effectively detected, the amplitude of the injected harmonic signal should not be less than 1% of the rated voltage. Simultaneously, the amplitude of the harmonic signal should not be too large to avoid causing secondary impacts on equipment or affecting the normal operation of the power grid in the event of a fault outside the designated area. Therefore, after comprehensive consideration, the signal amplitude can be set to 5% of the rated voltage. Since synchronization with the AC system is not required, the initial phase of the harmonic signal can be arbitrarily chosen, and the angular frequency of the harmonic signal can be set to 200Hz. This invention, by designing a harmonic signal injection strategy, injects harmonic signals of characteristic frequencies into the power grid, thereby increasing fault information.

[0079] Step 103: Before and after the injection of harmonic signals, acquire the first circuit data and the second circuit data of the photovoltaic grid-connected system, respectively, and determine the fault distance from the protection measuring point to the fault point based on the first circuit data and the second circuit data.

[0080] Furthermore, in the first embodiment of the present invention, before and after the injection of harmonic signals, the first circuit data and the second circuit data of the photovoltaic grid-connected system are acquired respectively, and the fault distance from the protection measuring point to the fault point is determined based on the first circuit data and the second circuit data, specifically as follows:

[0081] Before and after the injection of harmonic signals, the first fault equivalent circuit and the second fault equivalent circuit of the photovoltaic grid-connected system are obtained respectively.

[0082] The fault distance from the protection measuring point to the fault point is solved based on the circuit state equations of the first fault equivalent circuit and the second fault equivalent circuit, specifically as follows:

[0083]

[0084] in, and Z represents the power frequency phasor of the protection measuring point before the injection of harmonic signals; n Z represents the equivalent impedance on the back side of the protection measuring point before the injected harmonic signal; L This represents the impedance per unit length of the line before the injection of harmonic signals. and Represent the power frequency fault component phasor of the line protection measuring point before the injection of harmonic signals; d represents the fault distance from the protection measuring point to the fault point; R f The value represents the transition resistance, and l represents the total length of the line. and This represents the power frequency phasor of the protection measuring point after the harmonic signal is injected; This represents the back-side equivalent impedance of the protection measuring point after the injection of harmonic signals. This represents the impedance per unit length of the line after the injection of harmonic signals.

[0085] In the first embodiment of the present invention, after harmonic injection is completed, based on the power frequency fault component network and the equivalent circuit at the corresponding harmonic frequency, a set of equations for solving the fault distance can be written, thereby calculating the fault distance from the protection measuring point to the fault point. In the above set of equations, Z L The impedance per unit length of the line can be expressed by Z. L =r L +jωl L The calculation shows that, r L l L These represent the resistance and inductance per unit length of the circuit, respectively.

[0086] Step 104: Set the exit time of the protection measuring point according to the fault distance, and handle the fault of the photovoltaic grid-connected system according to the exit time of the protection measuring point.

[0087] Furthermore, in the first embodiment of the present invention, the exit time of the protection measuring point is set according to the fault distance, specifically as follows:

[0088] The inverse-time operating characteristic of the protection measuring point is calculated based on the fault distance to determine the exit time of the protection measuring point. Specifically:

[0089]

[0090] Among them, t act Indicates the protection segment I action time; t II Δt represents the protection's second-stage action time; Δt represents the coordination delay.

[0091] In the first embodiment of the present invention, based on the solved fault distance and according to the inverse-time operating characteristics, the exit time of the protection measuring point can be set to achieve coordination between the upper-level line protection and the lower-level line protection, thereby completing the identification and isolation of faults across the entire line. According to the expression for the inverse-time operating characteristics, it is known that a faster operating speed is achieved for near-end faults (d=0), with the operating time equal to the basic coordination delay; for line-end faults (d=1), the operating time is equal to the operating time of the second-stage protection plus the coordination delay, thus forming coordination with the lower-level line protection for faults outside the protection zone.

[0092] In summary, the first embodiment of this invention provides a fault protection method for photovoltaic grid-connected systems based on harmonic injection. When a fault occurs in the photovoltaic grid-connected system, a fault current-limiting control strategy is adopted to limit the magnitude of the fault current by adjusting the output voltage. Then, a harmonic injection control strategy is adopted to inject harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage. Before and after the harmonic signal injection, the fault equivalent circuit of the photovoltaic grid-connected system is obtained, and the fault distance from the protection measuring point to the fault point is calculated. The exit time of the protection measuring point is determined based on the fault distance to process the fault in the photovoltaic grid-connected system. This invention injects characteristic harmonic signals after a system fault occurs, which can enhance the fault characteristics, increase fault information, improve the sensitivity and reliability of protection, and help solve the problem of protection failure caused by the weak feed characteristics of the converter.

[0093] Example 2

[0094] See Figure 2 This is a topology diagram of a photovoltaic grid-connected system provided by the present invention. The photovoltaic array is boosted by a Boost DC / DC converter, then inverted into AC by a two-level VSC inverter, and connected to the grid via a single output line L1. The DC / DC converter employs an MPPT control strategy, and the VSC employs a dual closed-loop control strategy with constant AC reactive power and constant DC voltage. In the diagram, K... 1m and K 1n The symbols represent protection measuring points installed on both sides of the dedicated outgoing line L1 of the photovoltaic grid-connected system. L2, L3, and L4 represent other feeders in the distribution network. K2, K3, and K4 represent protection measuring points installed on the busbar side of other corresponding feeders. Figure 2 In a grid-connected photovoltaic system, for the protection installation point K 1m Fault f1 occurring on line L1 is an in-zone fault, while faults f2, f3, f4 occurring on other feeders and bus fault f5 are out-of-zone faults. When an in-zone fault occurs, K... 1n The protection at the location will trip the L1 system-side circuit breaker based on the power frequency overcurrent criterion, while VSC will immediately lock out after the fault, causing K to... 1m The power frequency fault current at the location is relatively small, K 1m The protection system may fail to operate, and the circuit breaker on the photovoltaic side of line L1 may be difficult to disconnect. Therefore, it is necessary to design an additional control strategy to replace the blocking strategy with a current limiting strategy after a fault occurs on the AC grid-connected line, and then inject harmonic signals of characteristic frequencies to achieve line protection.

[0095] See Figure 3 This is a schematic diagram of a fault current limiting control strategy provided by the present invention. T1 U represents the start signal for fault current limiting control. * LV (0 -This represents the reference value of the photovoltaic cell terminal voltage calculated by the MPPT control strategy before the start-up fault current limiting control time.

[0096] During normal operation of the photovoltaic grid-connected system, the DC inter-electrode voltage is kept constant by the two-level VSC control of the downstream stage, that is, the high-voltage side voltage UHV of the DC / DC converter is constant. According to the operating characteristics of photovoltaic cells, under the same environmental factors, the output power of photovoltaic cells is a function of the terminal voltage ULV. The DC / DC converter uses the MPPT control strategy to ensure that the photovoltaic cells always operate at the terminal voltage corresponding to the maximum output power, thereby obtaining the maximum solar energy conversion efficiency.

[0097] However, in the event of a fault, it is necessary to limit the power generation of the photovoltaic system to limit the fault current, activate the fault current limiting control, switch the photovoltaic system from MPPT control to constant output voltage control, and limit the photovoltaic cell terminal voltage to 50% of the reference value at the moment before the current limiting is activated, so as to achieve the purpose of limiting the output power.

[0098] See Figure 4 This is a schematic diagram of a harmonic injection control strategy provided by the present invention. T2 U represents the start signal for additional control via harmonic signal injection. detabc This represents the reference value for the injected harmonic signal. After a fault occurs in the photovoltaic grid-connected system and the fault current-limiting control strategy is activated, additional harmonic signal injection control is initiated. The reference value u for the three-phase voltages abc is used. * abc Add a harmonic signal reference value U on top of that. * det,abc This enables the injection of harmonic signals.

[0099] See Figures 5 to 7 , is the fault equivalent circuit of a photovoltaic grid-connected system, in which, Figure 5 and Figure 6 These represent the full-scale equivalent circuit of the power frequency and the equivalent circuit of the power frequency fault component before harmonic injection, respectively. Figure 7 This represents the fault equivalent circuit after harmonic injection.

[0100] in, and Z represents the power frequency phasor of the protection measuring points at the beginning and end of the line before the injection of harmonic signals; m and Z n Z represents the equivalent impedance on the back side of the protection measuring points at both ends before the injected harmonic signal; L d represents the impedance per unit length of the line before the injection of harmonic signals; d represents the protection measuring point K. 1m Distance to the fault point; R f The value represents the transition resistance, and l represents the total length of the line. and These represent the power frequency fault component phasors at the protection measuring points at the beginning and end of the line before the injection of harmonic signals; and This represents the power frequency phasor of the protection measuring point after the harmonic signal is injected; This represents the back-side equivalent impedance of the protection measuring point after the injection of harmonic signals. This represents the impedance per unit length of the line after the injection of harmonic signals.

[0101] according to Figure 5 and Figure 6 From the full-frequency equivalent circuit and the power-frequency fault component equivalent circuit, the following equation can be derived:

[0102]

[0103] according to Figure 7 The fault equivalent circuit at the harmonic frequency can be used to derive the following equation:

[0104]

[0105] By solving the two equations above, the fault distance from the protection measuring point to the fault point can be obtained.

[0106] After determining the fault distance from the protection measuring point to the fault point, the output time of the protection measuring point can be set according to the inverse-time operating characteristics. This enables coordination between the upstream and downstream line protection systems, thereby completing the identification and isolation of faults across the entire line. The inverse-time operating characteristics of the protection measuring point output are as follows:

[0107]

[0108] Among them, t act Indicates the protection segment I action time; t II Δt represents the protection's second-stage action time; Δt represents the coordination delay.

[0109] Example 3

[0110] See Figure 8 This is a schematic diagram of an embodiment of the fault protection device for photovoltaic grid-connected systems based on harmonic injection provided by the present invention. The device includes an adjustment module 201, an injection module 202, a calculation module 203, and a setting module 204.

[0111] The adjustment module 201 is used to limit the magnitude of the fault current by adjusting the output voltage when a fault occurs in the photovoltaic grid-connected system.

[0112] The injection module 202 is used to inject harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage;

[0113] The calculation module 203 is used to acquire the first circuit data and the second circuit data of the photovoltaic grid-connected system before and after the injection of the harmonic signal, respectively, and to determine the fault distance from the protection measuring point to the fault point based on the first circuit data and the second circuit data.

[0114] The setting module 204 is used to set the exit time of the protection measuring point according to the fault distance, and to handle the fault of the photovoltaic grid-connected system according to the exit time of the protection measuring point.

[0115] Furthermore, in the second embodiment of the present invention, the adjustment module 201 includes: a voltage acquisition unit and a voltage setting unit;

[0116] The voltage acquisition unit is used to acquire the output voltage value at the previous moment when a fault occurs in the photovoltaic grid-connected system;

[0117] The voltage setting unit is used to reduce the current output voltage value to a preset ratio of the output voltage value at the previous moment, according to a preset ratio.

[0118] Furthermore, in the second embodiment of the present invention, the injection module 202 includes: a determination unit and a voltage calculation unit;

[0119] The determining unit is used to determine the amplitude, angular frequency, and initial phase of the harmonic signal according to the preset voltage detection requirements;

[0120] The voltage calculation unit is used to calculate the voltage magnitude of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system, specifically:

[0121]

[0122] Among them, U det ω represents the amplitude of the harmonic signal. det The angular frequency of the harmonic signal; This represents the initial phase of the harmonic signal; and These are the voltage magnitudes of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system.

[0123] Furthermore, in the second embodiment of the present invention, the amplitude, angular frequency, and initial phase of the harmonic signal are determined according to preset voltage detection requirements, specifically as follows:

[0124] Obtain the rated voltage of the photovoltaic grid-connected system;

[0125] The amplitude of the harmonic signal is determined based on the rated voltage of the photovoltaic grid-connected system and the preset amplitude detection requirements.

[0126] Based on the preset voltage detection requirements, determine the angular frequency and initial phase of the harmonic signal.

[0127] Furthermore, in the second embodiment of the present invention, the calculation module 203 includes: a circuit acquisition unit and a distance calculation unit;

[0128] The circuit acquisition unit is used to acquire the first fault equivalent circuit and the second fault equivalent circuit of the photovoltaic grid-connected system before and after the injection of the harmonic signal, respectively.

[0129] The distance calculation unit is used to solve the fault distance from the protection measuring point to the fault point based on the circuit state equations of the first fault equivalent circuit and the second fault equivalent circuit, specifically:

[0130]

[0131] in, and Z represents the power frequency phasor of the protection measuring point before the injection of harmonic signals; n Z represents the equivalent impedance on the back side of the protection measuring point before the injected harmonic signal; L This represents the impedance per unit length of the line before the injection of harmonic signals. and Represent the power frequency fault component phasor of the line protection measuring point before the injection of harmonic signals; d represents the fault distance from the protection measuring point to the fault point; R f The value represents the transition resistance, and l represents the total length of the line. and This represents the power frequency phasor of the protection measuring point after the harmonic signal is injected; This represents the back-side equivalent impedance of the protection measuring point after the injection of harmonic signals. This represents the impedance per unit length of the line after the injection of harmonic signals.

[0132] Furthermore, in the second embodiment of the present invention, the setting module 204 includes: a time calculation unit;

[0133] The time calculation unit is used to calculate the inverse time-delay action characteristic of the protection measuring point outlet based on the fault distance, so as to determine the outlet time of the protection measuring point, specifically:

[0134]

[0135] Among them, t act Indicates the protection segment I action time; t II Δt represents the protection's second-stage action time; Δt represents the coordination delay.

[0136] In summary, the second embodiment of this invention provides a fault protection device for a photovoltaic grid-connected system based on harmonic injection. Based on the organic integration of modules, when a fault occurs in the photovoltaic grid-connected system, a fault current-limiting control strategy is adopted to limit the magnitude of the fault current by adjusting the output voltage. Then, a harmonic injection control strategy is employed to inject harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage. Before and after the harmonic signal injection, the fault equivalent circuit of the photovoltaic grid-connected system is obtained, and the fault distance from the protection measuring point to the fault point is calculated. The exit time of the protection measuring point is determined based on the fault distance to handle the fault in the photovoltaic grid-connected system. This invention injects characteristic harmonic signals after a system fault occurs, which can enhance fault characteristics, increase fault information, improve the sensitivity and reliability of the protection, and help solve the problem of protection failure caused by the weak feed characteristics of the converter.

[0137] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention for those skilled in the art.

Claims

1. A fault protection method for a photovoltaic grid-connected system based on harmonic injection, characterized in that, include: When a fault occurs in the photovoltaic grid-connected system, the magnitude of the fault current is limited by adjusting the output voltage; Based on the three-phase voltage, harmonic signals are injected into the photovoltaic grid-connected system; wherein, according to preset voltage detection requirements, the amplitude, angular frequency, and initial phase of the harmonic signals are determined; the voltage magnitude of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system is calculated respectively, specifically as follows: in, U det The amplitude of the harmonic signal; ω det Angular frequency of the harmonic signal; φ det This represents the initial phase of the harmonic signal; , and These are the voltage magnitudes of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system; Before and after the injection of harmonic signals, the first circuit data and the second circuit data of the photovoltaic grid-connected system are obtained respectively, and the fault distance from the protection measuring point to the fault point is determined based on the first circuit data and the second circuit data; wherein, before and after the injection of harmonic signals, the first fault equivalent circuit and the second fault equivalent circuit of the photovoltaic grid-connected system are obtained respectively. The fault distance from the protection measuring point to the fault point is solved based on the circuit state equations of the first fault equivalent circuit and the second fault equivalent circuit, specifically as follows: in, and This represents the power frequency phasor of the protection measurement point before the injection of harmonic signals; Z n This represents the back-side equivalent impedance of the protection measuring point before the injection of harmonic signals. Z L This represents the impedance per unit length of the line before the injection of harmonic signals. and These represent the power frequency fault component phasors of the line protection measuring points before the injection of harmonic signals; d This indicates the fault distance from the protected measuring point to the fault point; R f Indicates the transition resistance. l Indicates the total length of the line; and This represents the power frequency phasor of the protection measuring point after the harmonic signal is injected; This represents the back-side equivalent impedance of the protection measuring point after the injection of harmonic signals. This represents the impedance per unit length of the line after the injection of harmonic signals. The exit time of the protection measuring point is set according to the fault distance, and the fault of the photovoltaic grid-connected system is handled according to the exit time of the protection measuring point.

2. The fault protection method for photovoltaic grid-connected systems based on harmonic injection according to claim 1, characterized in that, When a fault occurs in the photovoltaic grid-connected system, the magnitude of the fault current is limited by adjusting the output voltage, specifically as follows: When a fault occurs in the photovoltaic grid-connected system, the output voltage value at the previous moment is obtained; According to a preset ratio, the current output voltage value is reduced to a preset ratio of the output voltage value at the previous moment.

3. The fault protection method for photovoltaic grid-connected systems based on harmonic injection according to claim 1, characterized in that, The determination of the amplitude, angular frequency, and initial phase of the harmonic signal according to preset voltage detection requirements is specifically as follows: Obtain the rated voltage of the photovoltaic grid-connected system; The amplitude of the harmonic signal is determined based on the rated voltage of the photovoltaic grid-connected system and the preset amplitude detection requirements. Based on the preset voltage detection requirements, determine the angular frequency and initial phase of the harmonic signal.

4. The fault protection method for photovoltaic grid-connected systems based on harmonic injection according to claim 1, characterized in that, The step of setting the exit time of the protection measuring point according to the fault distance is specifically as follows: The inverse-time operating characteristic of the protection measuring point is calculated based on the fault distance to determine the exit time of the protection measuring point. Specifically: in, t act Indicates the duration of the protection segment I action; t II Indicates the duration of the second stage of protection action; Δ t Indicates a delay in coordination; the aforementioned d This indicates the fault distance from the protected measuring point to the fault point; the aforementioned l This indicates the total length of the line.

5. A fault protection device for a photovoltaic grid-connected system based on harmonic injection, characterized in that, include: Adjustment module, injection module, calculation module, and settings module; The adjustment module is used to limit the magnitude of the fault current by adjusting the output voltage when a fault occurs in the photovoltaic grid-connected system. The injection module is used to inject harmonic signals into the photovoltaic grid-connected system based on the three-phase voltage; wherein, the injection module includes: a determination unit and a voltage calculation unit; the determination unit is used to determine the amplitude, angular frequency, and initial phase of the harmonic signal according to preset voltage detection requirements; the voltage calculation unit is used to calculate the voltage magnitude of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system, specifically: in, U det The amplitude of the harmonic signal; ω det Angular frequency of the harmonic signal; φ det This represents the initial phase of the harmonic signal; , and These are the voltage magnitudes of the harmonic signals injected into the three-phase circuit of the photovoltaic grid-connected system; The calculation module is used to acquire first circuit data and second circuit data of the photovoltaic grid-connected system before and after the injection of harmonic signals, respectively, and to determine the fault distance from the protection measuring point to the fault point based on the first circuit data and the second circuit data. The calculation module includes a circuit acquisition unit and a distance calculation unit. The circuit acquisition unit is used to acquire the first fault equivalent circuit and the second fault equivalent circuit of the photovoltaic grid-connected system before and after the injection of harmonic signals, respectively. The distance calculation unit is used to solve for the fault distance from the protection measuring point to the fault point based on the circuit state equations of the first fault equivalent circuit and the second fault equivalent circuit, specifically: in, and This represents the power frequency phasor of the protection measurement point before the injection of harmonic signals; Z n This represents the back-side equivalent impedance of the protection measuring point before the injection of harmonic signals. Z L This represents the impedance per unit length of the line before the injection of harmonic signals. and These represent the power frequency fault component phasors of the line protection measuring points before the injection of harmonic signals; d This indicates the fault distance from the protected measuring point to the fault point; R f Indicates the transition resistance. l Indicates the total length of the line; and This represents the power frequency phasor of the protection measuring point after the harmonic signal is injected; This represents the back-side equivalent impedance of the protection measuring point after the injection of harmonic signals. This represents the impedance per unit length of the line after the injection of harmonic signals. The setting module is used to set the exit time of the protection measuring point according to the fault distance, and to handle the fault of the photovoltaic grid-connected system according to the exit time of the protection measuring point.

6. The fault protection device for photovoltaic grid-connected systems based on harmonic injection according to claim 5, characterized in that, The adjustment module includes: a voltage acquisition unit and a voltage setting unit; The voltage acquisition unit is used to acquire the output voltage value at the previous moment when a fault occurs in the photovoltaic grid-connected system. The voltage setting unit is used to reduce the current output voltage value to a preset ratio of the output voltage value at the previous moment, according to a preset ratio.

Images