Injection control method and system of dual-frequency characteristic signal of centralized photovoltaic inverter

Abstract

The present invention discloses a method and system for controlling the injection of dual-frequency characteristic signals of a centralized photovoltaic inverter. The method includes: after a fault occurs in the power system, switching from a normal control strategy to a low voltage ride-through control strategy and an additional signal injection control strategy according to a starting criterion; performing frequency separation on AC electrical quantities; adopting a negative sequence suppression strategy for the negative sequence component of the power frequency, and a reactive power priority strategy for the positive sequence component of the power frequency, to obtain a first voltage modulation wave signal; obtaining a second voltage modulation wave signal for the dual-frequency characteristic electrical quantity under non-power frequency; superimposing the first voltage modulation wave signal and the second voltage modulation wave signal to realize the injection of dual-frequency signals into the transmission line. The present invention can provide a control scheme for generating two different non-power frequency signals while using an inverter to realize a low voltage ride-through strategy and current limiting, relying on its own regulation characteristics, to realize the stable injection of dual-frequency signals into the transmission line.

Description

Technical Field

[0001] The present invention belongs to the technical field of injection methods of non-industrial frequency electrical quantities, and in particular relates to an injection control method and system for a dual-frequency characteristic signal of a centralized photovoltaic inverter. Background Art

[0002] The new power system based on new energy has been widely promoted and applied. In order to ensure the flexible interconnection of various new energy power sources such as wind power, photovoltaics, and pumped storage, large-scale highly controllable power electronic equipment with inverters as the core has been developed and applied on a large scale.

[0003] The use of highly controllable power electronic equipment has brought challenges to traditional relay protection. Compared with the traditional power system dominated by synchronous machines, the large number of power electronic devices connected in the new power system have nonlinear time-varying characteristics when responding to disturbances, which further complicates the characteristics of transient faults in the power grid. Under the condition of vulnerability constraints, the highly controllable self-protection strategy of the converter makes the fault characteristics not only less obvious, but also shortens the duration. These characteristics are significantly different from those of synchronous generator power supplies, which brings challenges to traditional fault location and fault identification.

[0004] At the same time, the high controllability of power electronic equipment also brings the idea of ​​active detection to protection. That is, under the requirement of safe operation of power system equipment, by adding control strategies, a detection signal of a certain frequency and amplitude is injected within a certain period of time after the fault, so as to improve the weak feedback characteristics of power electronic equipment under traditional control strategies and improve the ability of fault location and fault identification. Compared with protection ranging schemes such as "S injection method" that rely on signal transmitting devices, the method of realizing active detection by relying on power electronic equipment in the power grid does not require the addition of other devices.

[0005] In summary, based on the idea of ​​coordinated control and protection, the regulation characteristics of power electronic converters are fully utilized, the control strategy after the inverter fault is optimized, and the additional harmonic injection strategy is designed after the centralized photovoltaic grid-connected transmission line fault, which is of great value for improving the sensitivity of line fault feature identification and positioning. Therefore, in summary, in the current scenario of centralized photovoltaic transmission lines, after the transmission line fault occurs, the inverter cannot be effectively used to implement the low voltage ride-through strategy and current limiting, and relies on its own regulation characteristics to give a control scheme that generates two different non-industrial frequency signals, resulting in insufficient stability of the dual-frequency signal injected into the transmission line. Summary of the invention

[0006] The present invention provides a method and system for controlling the injection of dual-frequency characteristic signals of a centralized photovoltaic inverter, with the aim of solving the problem that in the current scenario of centralized photovoltaic transmission lines, after a transmission line fault occurs, the inverter cannot be effectively utilized to implement a low voltage ride-through strategy and current limiting, and a control scheme for generating two different non-industrial frequency signals is provided by relying on its own regulation characteristics, resulting in insufficient stability of the dual-frequency signal injected into the transmission line.

[0007] In order to achieve the above object, the present invention adopts the following technical scheme:

[0008] The injection control method of the dual-frequency characteristic signal of the centralized photovoltaic inverter of the present invention comprises the following steps:

[0009] S1. After a power system fault occurs, the normal control strategy is switched to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criteria, and the AC electrical quantity is collected at the same time;

[0010] S2, the frequency separation link of the cascade structure of IIR type time domain digital notch filter based on triangular transformation, performs frequency separation on the AC electrical quantity, and extracts the power frequency electrical quantity after frequency separation and the dual-frequency characteristic electrical quantity under non-power frequency;

[0011] S3, performing positive-negative sequence separation on the power frequency electrical quantity after frequency separation to obtain a power frequency positive sequence component and a power frequency negative sequence component, adopting a negative sequence suppression strategy in a low voltage ride through control strategy for the power frequency negative sequence component, and adopting a reactive power priority strategy in a low voltage ride through control strategy for the power frequency positive sequence component, to obtain a first voltage modulation wave signal;

[0012] S4. For the dual-frequency characteristic electrical quantity under non-industrial frequency, the effective value of the current in the actual dual-frequency characteristic electrical quantity is calculated by fast Fourier transform, and the fixed characteristic frequency current control link output in the additional signal injection control strategy is adopted to obtain the second voltage modulation wave signal;

[0013] S5. Superimpose the first voltage modulated wave signal and the second voltage modulated wave signal, and output different pulses through a pulse width modulation link, so as to control the on and off of the inverter and realize the injection of the dual-frequency signal of the inverter into the output line.

[0014] In some embodiments, in S1, the starting criterion is the following formula (1):

[0015] (1);

[0016] in, is the power frequency voltage collected at the inverter outlet, is the time after voltage abnormality is detected, is the rated voltage of the power system.

[0017] In some implementations, in S2, the frequency separation link adopts a cascade structure of an IIR type time domain digital notch filter based on triangular transformation, and the transfer function of the frequency separation link is:

[0018] (2);

[0019] in, is the number of data intervals, is the zero pole distance, is a discrete complex frequency domain variable, is the center angular frequency, is the sampling interval, is the transfer function amplitude correction coefficient.

[0020] Furthermore, in S2, the expression of the transfer function amplitude correction coefficient is:

[0021] (3).

[0022] In some implementations, in S3, the reactive power priority strategy is expressed as follows:

[0023] (4);

[0024] in, , are the reference values ​​of the positive sequence d-axis and q-axis inner loop currents, is the d-axis current during normal operation, is the rated current of the power system, is the maximum withstand current of the converter, is the per unit value, is the power frequency voltage amplitude;

[0025] The negative sequence suppression strategy includes the negative sequence current suppression strategy and the negative sequence voltage suppression strategy. The negative sequence current suppression strategy sets the reference values ​​of the negative sequence current on the d-axis and q-axis to 0 respectively. The expressions are as follows:

[0026] (5);

[0027] in, , are the negative sequence inner loop currents on the d-axis and q-axis respectively, is the angular velocity at the power frequency, is the equivalent inductance of the converter, , are the negative sequence voltages on the d-axis and q-axis respectively, is a complex frequency domain variable, and is the adjustment parameter of proportional-integral control, , are the negative sequence components of the d-axis and q-axis grid-side voltages respectively;

[0028] The expression of the reference value of the q-axis inner ring current is as follows:

[0029] (6);

[0030] in, is the q-axis negative sequence current reference value, is the dynamic negative sequence reactive current proportional coefficient of the photovoltaic power station, It is the per unit value of the negative sequence component of the voltage at the grid connection point of the photovoltaic power station.

[0031] Furthermore, in S3, the maximum withstand current of the converter is in the range of 1.2 ~2 .

[0032] In some implementations, in S4, the output of the fixed characteristic frequency current control link in the additional signal injection control strategy includes: calculating the sum of the effective values ​​of the injected dual-frequency characteristic currents, setting a negative feedback link based on proportional integral control, and controlling the effective value of the dual-frequency injected current to be equal to a preset reference value; setting a voltage limiting link to limit the injected voltage amplitude when a high-resistance fault occurs, so that the sum of the injected voltage amplitude and the power frequency voltage amplitude does not exceed the rated voltage of the power system; and using the output result of the fixed characteristic frequency current control link as the amplitude of the second voltage modulation wave;

[0033] The expression of the output of the current control link with a fixed characteristic frequency is as follows:

[0034] (7);

[0035] in, I 1 and I 2 are the amplitudes at the dual-frequency characteristic frequencies, I det is the actual value of the input current of the control link, I * det is the reference value of the input current of the control link, s is a complex frequency domain variable, k p and k i is the adjustment parameter of proportional-integral control, U * det is the voltage amplitude of the characteristic signal, U0 is the power frequency voltage amplitude, U N is the voltage amplitude of the power system.

[0036] In some implementations, in S5, the superimposed voltage modulated wave signal is as follows:

[0037] (8);

[0038] in, U * 0 is the amplitude of the power frequency voltage modulation wave, ω 0 is the power frequency angular frequency, ω det , φ det The characteristic frequencies are f 1 and f 2 is the angular frequency and initial phase of the injected signal, t is the time, U * det is the voltage amplitude of the characteristic signal.

[0039] The present invention also provides an injection control system for a dual-frequency characteristic signal of a centralized photovoltaic inverter, which is used to implement the injection control method for the dual-frequency characteristic signal of the centralized photovoltaic inverter, including a fault detection and strategy switching module, a frequency separation and electrical quantity extraction module, a control and electrical quantity processing module, and a signal superposition and modulation output module, wherein:

[0040] The fault detection and strategy switching module is used to switch from the normal control strategy to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criteria after a fault occurs in the power system, and collect AC electrical quantities at the same time;

[0041] The frequency separation and electrical quantity extraction module is used for the frequency separation link of the cascade structure of the IIR type time domain digital notch filter based on triangular transformation, performs frequency separation on the AC electrical quantity, and extracts the power frequency electrical quantity after frequency separation and the dual-frequency characteristic electrical quantity under non-power frequency;

[0042] The control and electrical quantity processing module is used to separate the positive and negative sequences of the power frequency electrical quantity after frequency separation, and obtain the power frequency positive sequence component and the power frequency negative sequence component. The negative sequence suppression strategy in the low voltage ride-through control strategy is adopted for the power frequency negative sequence component, and the reactive power priority strategy in the low voltage ride-through control strategy is adopted for the power frequency positive sequence component to obtain the first voltage modulation wave signal; for the dual-frequency characteristic electrical quantity under non-power frequency, the effective value of the current in the actual dual-frequency characteristic electrical quantity is calculated by fast Fourier transform, and the fixed characteristic frequency current control link output in the additional signal injection control strategy is adopted to obtain the second voltage modulation wave signal;

[0043] The signal superposition and modulation output module is used to superimpose the first voltage modulation wave signal and the second voltage modulation wave signal, and output different pulses through the pulse width modulation link, so as to control the on and off of the inverter and realize the injection of the dual-frequency signal of the inverter into the transmission line.

[0044] Compared with the prior art, the injection control method and system of the dual-frequency characteristic signal of the centralized photovoltaic inverter of the present invention has the following beneficial effects:

[0045] The injection control method of the dual-frequency characteristic signal of the centralized photovoltaic inverter of the present invention can quickly switch from the normal control strategy to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criterion, so as to ensure the stable operation of the photovoltaic inverter during the fault and reduce the impact of the fault on the power grid. The present invention can effectively suppress the voltage and current fluctuations caused by the fault by switching the normal control strategy to a more robust control strategy. Based on the idea of ​​active detection, in the scenario of the centralized photovoltaic transmission line, after the transmission line fault occurs, the inverter is used to implement the low voltage ride-through strategy and current limiting, and the control scheme for generating two different non-industrial frequency signals is given by relying on its own regulation characteristics, thereby realizing the stable injection of the dual-frequency signal in the transmission line. The present invention is suitable for the voltage source converter (Voltage Source Converter, VSC) that realizes rectification and inversion by relying on PWM (Pulse Width Modulation, pulse width modulation) technology, and has better applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The drawings in the specification are used to provide further understanding of the present invention and constitute a part of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0047] Figure 1 It is a flow chart of a method for controlling injection of dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention;

[0048] Figure 2 It is a schematic diagram of a frequency separation link in a method for injecting and controlling dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention;

[0049] Figure 3 It is a negative sequence control block diagram of an inverter in a method for injecting control of dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention;

[0050] Figure 4 It is a schematic diagram of a detection signal constant current injection link in a method for injecting a dual-frequency characteristic signal of a centralized photovoltaic inverter according to the present invention;

[0051] Figure 5 It is a schematic diagram of realizing a dual-frequency signal injection strategy in a dual-frequency characteristic signal injection control method of a centralized photovoltaic inverter according to the present invention;

[0052] Figure 6It is a schematic diagram of a 110kV centralized photovoltaic transmission line model in a method for injecting a dual-frequency characteristic signal of a centralized photovoltaic inverter according to the present invention;

[0053] Figure 7 Schematic diagram of the waveform of electrical quantities at the protection measuring point under three-phase short circuit in a method for injecting dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention; wherein (a) is the measuring point voltage, and (b) is the measuring point current;

[0054] Figure 8 Schematic diagram of the voltage and current frequency components of the protection measuring point under three-phase short circuit in a method for injecting dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention; wherein (a) is the voltage amplitude-frequency characteristic, and (b) is the current amplitude-frequency characteristic;

[0055] Fig. 9 It is a curve schematic diagram of the dynamic response of characteristic current under three-phase short circuit in a method for injecting dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention;

[0056] Fig.10 Schematic diagram of the waveform of electrical quantities at the protection measuring point under two-phase short circuit in the injection control method of the dual-frequency characteristic signal of a centralized photovoltaic inverter of the present invention; wherein (a) is the measuring point voltage, and (b) is the measuring point current;

[0057] Fig.11 Schematic diagram of the voltage and current frequency components of the protection measuring point under two-phase short circuit in a method for injecting dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention; wherein (a) is the voltage amplitude-frequency characteristic, and (b) is the current amplitude-frequency characteristic;

[0058] Fig.12 The diagram is a curve diagram of the dynamic response of characteristic current under two-phase short circuit in a method for injecting dual-frequency characteristic signals of a centralized photovoltaic inverter according to the present invention. DETAILED DESCRIPTION

[0059] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0060] The following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0061] In the scenario of centralized photovoltaic transmission lines, after a transmission line fault occurs, how to use the inverter to implement low voltage ride-through strategy and current limiting, while relying on its own regulation characteristics to provide a control scheme to generate two different non-industrial frequency signals, thereby achieving stable injection of dual-frequency signals into the transmission line.

[0062] like Figure 1 As shown, the present invention provides a method for controlling the injection of dual-frequency characteristic signals of a centralized photovoltaic inverter, comprising the following steps:

[0063] S1. After a power system fault occurs, the normal control strategy is switched to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criteria, and the AC electrical quantity is collected at the same time;

[0064] S2, the frequency separation link of the cascade structure of IIR type time domain digital notch filter based on triangular transformation, performs frequency separation on the AC electrical quantity, and extracts the power frequency electrical quantity after frequency separation and the dual-frequency characteristic electrical quantity under non-power frequency;

[0065] S3, performing positive-negative sequence separation on the power frequency electrical quantity after frequency separation to obtain a power frequency positive sequence component and a power frequency negative sequence component, adopting a negative sequence suppression strategy in a low voltage ride through control strategy for the power frequency negative sequence component, and adopting a reactive power priority strategy in a low voltage ride through control strategy for the power frequency positive sequence component, to obtain a first voltage modulation wave signal;

[0066] S4. For the dual-frequency characteristic electrical quantity under non-industrial frequency, the effective value of the current in the actual dual-frequency characteristic electrical quantity is calculated by fast Fourier transform, and the fixed characteristic frequency current control link output in the additional signal injection control strategy is adopted to obtain the second voltage modulation wave signal;

[0067] S5. Superimpose the first voltage modulated wave signal and the second voltage modulated wave signal, and output different pulses through a pulse width modulation link, so as to control the on and off of the inverter and realize the injection of the dual-frequency signal of the inverter into the output line.

[0068] The injection control method of the present invention can quickly switch from the normal control strategy to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criteria. The cascade structure of the IIR type time domain digital notch filter based on the triangular transformation is used for frequency separation, and the first voltage modulation wave signal under the industrial frequency electrical quantity and the second voltage modulation wave signal under the non-industrial frequency can be accurately extracted, which is helpful for the implementation of the control strategy and fault analysis. After the positive and negative sequences of the industrial frequency electrical quantity are separated, the negative sequence suppression strategy and the reactive power priority strategy are respectively adopted, which helps to reduce the impact of the negative sequence current on the power grid and improve the operation ability of the photovoltaic inverter under low voltage conditions. The fixed characteristic frequency current control is performed on the extracted dual-frequency characteristic electrical quantity under the non-industrial frequency, which can accurately control the effective value of the dual-frequency injection current to make it equal to the preset reference value, and optimize the output characteristics of the photovoltaic inverter.

[0069] The injection control method of the present invention sets a voltage limiting link to limit the injected voltage amplitude when injecting into a high-resistance fault, ensuring that the sum of the injected voltage amplitude and the power frequency voltage amplitude does not exceed the rated voltage of the power system, thereby preventing the photovoltaic inverter from being damaged by overvoltage during the fault. The first voltage modulation wave signal and the second voltage modulation wave signal are superimposed to optimize the output waveform of the photovoltaic inverter.

[0070] The injection control method of the dual-frequency characteristic signal of the centralized photovoltaic inverter of the present invention is applicable to a voltage source converter (VSC) that realizes rectification and inversion by relying on PWM technology, and the specific steps are as follows:

[0071] Step 1: After a fault occurs, the control strategy is switched from the normal control strategy to the low voltage ride through (LVRT) control strategy and the additional signal injection control strategy according to the starting criteria. The specific criteria are starting after the power frequency voltage drops by more than 10% and lasts for 3ms.

[0072] The starting criteria are as follows:

[0073] (1);

[0074] in, is the power frequency voltage collected at the inverter outlet, is the time after voltage abnormality is detected, is the rated voltage of the power system.

[0075] Step 2: After the control strategy is switched, the collected AC electrical quantity is subjected to frequency separation. Considering that the delay of the frequency separation link should be as low as possible and implemented in the time domain, the cascade structure of the IIR time domain digital notch filter based on triangular transformation is used to construct the frequency separation link. The transfer function of the frequency separation link is:

[0076] (2);

[0077] in, is the number of data intervals, is the zero pole distance, is a discrete complex frequency domain variable, is the center angular frequency, is the sampling interval, is the transfer function amplitude correction coefficient;

[0078] The expression of the transfer function amplitude correction coefficient is:

[0079] (3).

[0080] like Figure 2 The figure shows the specific frequency separation process. n =2, α =0.99, ω The power frequency and two injection characteristic frequencies are set respectively. The time domain filters corresponding to the center frequencies are cascaded to achieve frequency separation.

[0081] The frequency separation link is only put into use when the additional control strategy is started after a fault, and theoretically there is only a delay of eight sampling points, so its impact on the existing power frequency control link can be ignored.

[0082] like Figure 3 As shown, in step 3, the positive and negative sequences of the power frequency electrical quantity after the frequency separation in step 2 are separated, and at the same time, the negative sequence suppression strategy in the low voltage ride through control strategy is adopted for the power frequency negative sequence component. Figure 3 middle, i * df , i * qf are the negative sequence d-axis and q-axis current reference values ​​respectively, i df , i qf are the actual values ​​of negative sequence d-axis and q-axis currents, respectively. L g is the AC equivalent inductance of the converter bridge arm, ω is the angular frequency of the power frequency, u gdf , u gqf The negative sequence voltage at the converter outlet is on the d-axis and q-axis components, respectively. u * df , u * qf They are the d-axis and q-axis components of the negative-sequence modulation signal of the converter valve respectively.

[0083] Step 4: For the power frequency positive sequence component extracted in step 3, the reactive power priority strategy in the low voltage ride through control strategy is adopted. The specific expression is as follows:

[0084] (4);

[0085] in, , are the reference values ​​of the positive sequence d-axis and q-axis inner loop currents, is the d-axis current during normal operation, is the rated current of the power system, is the maximum withstand current of the converter, is the per unit value, is the power frequency voltage amplitude. In actual working conditions, the maximum withstand current range of the converter is 1.2 ~2 .

[0086] The negative sequence suppression strategy includes the negative sequence current suppression strategy and the negative sequence voltage suppression strategy. The negative sequence current suppression strategy sets the reference values ​​of the negative sequence current on the d-axis and q-axis to 0 respectively. The expressions are as follows:

[0087] (5);

[0088] in, , are the negative sequence inner loop currents on the d-axis and q-axis respectively, is the angular velocity at the power frequency, is the equivalent inductance of the converter, , are the negative sequence voltages on the d-axis and q-axis respectively, is a complex frequency domain variable, and is the adjustment parameter of proportional-integral control, , are the negative sequence components of the d-axis and q-axis grid-side voltages respectively;

[0089] The reference value of the q-axis current of the negative sequence voltage suppression strategy is different, and the rest is consistent with the negative sequence current. The expression of the reference value of the q-axis inner loop current is as follows:

[0090] (6);

[0091] in, is the q-axis negative sequence current reference value, is the dynamic negative sequence reactive current proportional coefficient of the photovoltaic power station, It is the per unit value of the negative sequence component of the voltage at the grid connection point of the photovoltaic power station.

[0092] like Figure 4 As shown, step 5: for the dual-frequency characteristic electrical quantity under non-power frequency extracted in step 2, the effective value of the current under the actual characteristic frequency is calculated by fast Fourier transform (FFT), and then the fixed characteristic frequency current control link output in the additional signal injection control strategy is used to control I det track I * det , that is, the effective value of the dual-frequency injection current is equal to the preset reference value.

[0093] The output of the fixed characteristic frequency current control link in the control strategy of injecting an additional signal includes: calculating the sum of the effective values ​​of the injected dual-frequency characteristic currents, setting a negative feedback link based on proportional integral control, and controlling the effective value of the dual-frequency injected current to be equal to a preset reference value; setting a voltage limiting link to limit the injected voltage amplitude when a high-resistance fault occurs, so that the sum of the injected voltage amplitude and the power frequency voltage amplitude does not exceed the rated voltage of the power system; and using the output result of the fixed characteristic frequency current control link as the amplitude of the second voltage modulation wave.

[0094] The expression of the output of the current control link with a fixed characteristic frequency is as follows:

[0095] (7);

[0096] in, I 1 and I 2 are the amplitudes at the dual-frequency characteristic frequencies, I det is the actual value of the input current of the control link, I * det is the reference value of the input current of the control link, s is a complex frequency domain variable, k p and k i is the adjustment parameter of proportional-integral control, U * det is the voltage amplitude of the characteristic signal, U0 is the power frequency voltage amplitude, U N is the voltage amplitude of the power system.

[0097] exist Figure 4 Specifically, the sum of the injected dual-frequency positive-sequence harmonic current effective values ​​I det , set up a negative feedback link based on proportional integral control to achieve constant current injection and control I det Tracking I * det, that is, the effective value of the dual-frequency injection current is equal to the preset reference value. At the same time, a voltage limiting link is set to limit the injection voltage amplitude when injecting into a high-resistance fault, so that the sum of the injection voltage amplitude and the power frequency voltage amplitude does not exceed the rated voltage U of the power system. N , thus avoiding the overvoltage of the power system caused by the injection of signals during high-resistance faults. U * det As the amplitude of the dual-frequency characteristic signal.

[0098] like Figure 5 As shown, step 6: superimpose the voltage modulation wave signal obtained by the LVRT control and negative sequence suppression control link under the power frequency with the voltage modulation wave signal obtained by the fixed characteristic frequency current control link under the non-power frequency, and inject the dual-frequency signal of the transmission line after the modulation link. At this time, the superimposed voltage modulation wave signal is as follows:

[0099] (8);

[0100] in, U * 0 is the amplitude of the power frequency voltage modulation wave, ω 0 is the power frequency angular frequency, ω det , φ det The characteristic frequencies are f 1 and f 2 is the angular frequency and initial phase of the injected signal, t is the time, U * det is the voltage amplitude of the characteristic signal.

[0101] The injected dual-frequency signals are both positive sequence signals with a phase difference of 120°.

[0102] The present invention also provides an injection control system for dual-frequency characteristic signals of a centralized photovoltaic inverter, comprising a fault detection and strategy switching module, a frequency separation and electrical quantity extraction module, a control and electrical quantity processing module, and a signal superposition and modulation output module, wherein:

[0103] The fault detection and strategy switching module is used to switch from the normal control strategy to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criteria after a fault occurs in the power system, and collect AC electrical quantities at the same time;

[0104] The frequency separation and electrical quantity extraction module is used for the frequency separation link of the cascade structure of the IIR type time domain digital notch filter based on triangular transformation, performs frequency separation on the AC electrical quantity, and extracts the power frequency electrical quantity after frequency separation and the dual-frequency characteristic electrical quantity under non-power frequency;

[0105] The control and electrical quantity processing module is used to separate the positive and negative sequences of the power frequency electrical quantity after frequency separation, and obtain the power frequency positive sequence component and the power frequency negative sequence component. The negative sequence suppression strategy in the low voltage ride-through control strategy is adopted for the power frequency negative sequence component, and the reactive power priority strategy in the low voltage ride-through control strategy is adopted for the power frequency positive sequence component to obtain the first voltage modulation wave signal; for the dual-frequency characteristic electrical quantity under non-power frequency, the effective value of the current in the actual dual-frequency characteristic electrical quantity is calculated by fast Fourier transform, and the fixed characteristic frequency current control link output in the additional signal injection control strategy is adopted to obtain the second voltage modulation wave signal;

[0106] The signal superposition and modulation output module is used to superimpose the first voltage modulation wave signal and the second voltage modulation wave signal, and output different pulses through the pulse width modulation link, so as to control the on and off of the inverter and realize the injection of the dual-frequency signal of the inverter into the transmission line.

[0107] The present invention implements an injection control method for a dual-frequency characteristic signal of a centralized photovoltaic inverter through the above injection control system, thereby improving the operating performance of the photovoltaic inverter and ensuring the stability of the power system.

[0108] The following simulation experiments are conducted to verify the effectiveness of the injection control method and system of the dual-frequency characteristic signal of the centralized photovoltaic inverter of the present invention.

[0109] Simulation verification;

[0110] like Figure 6 As shown in the figure, in order to verify the effectiveness of the proposed dual-frequency characteristic signal injection, taking the 110kV transmission network transmission line as an example, the Berylon model based on the electromagnetic transient simulation software PSCAD is established as shown in the figure. Figure 6 The main parameters of the photovoltaic grid-connected system model are shown in Table 1. Among them, section L1 is the transmission line where the fault occurs, and the fault occurs at the local measuring point K. 1m Collect single-ended electrical quantities. The length of section L1 is divided into: =60km.

[0111] Table 1 Main parameters of photovoltaic grid-connected system model

[0112]

[0113] For Figure 6 In the transmission line model shown, different types of faults are set in section L1. After the fault occurs, the inverter is used to control the generation of 200Hz and 300Hz harmonics to verify the effectiveness of the dual-frequency signal injection control strategy.

[0114] Suppose a three-phase short circuit occurs in the transmission line area, the fault is 50km away from the head end, and the electrical quantity waveform of the protection measuring point is as follows Figure 7As shown, the frequency spectrum of electrical quantity collected by the local measuring point is as follows Figure 8 As shown, the response tracking curve of the current control with a fixed characteristic frequency is as follows Fig. 9 shown.

[0115] Suppose a two-phase short circuit occurs in the transmission line area, the fault is 10km away from the head end, and the electrical quantity waveform of the protection measuring point is as follows Fig.10 As shown, the frequency spectrum of electrical quantity collected by the local measuring point is as follows Fig.11 As shown, the response tracking curve of the current control with a fixed characteristic frequency is as follows Fig.12 shown.

[0116] The injection control method and system of the dual-frequency characteristic signal of the centralized photovoltaic inverter of the present invention can quickly adjust the control strategy when a fault occurs in the power system, ensure the stable operation of the photovoltaic inverter, and reduce the impact of the fault on the power grid. The cascade structure of the IIR type time-domain digital notch filter based on triangular transformation is used for frequency separation, and the voltage modulation wave signals under different control strategies are superimposed. After the modulation link, the dual-frequency signal of the transmission line is injected, which enhances the adaptability of the photovoltaic inverter to power grid faults.

[0117] Finally, it should be noted that the above is only a preferred embodiment of the present invention and does not impose any form of limitation on the present invention; slight changes, modifications and evolutions of equivalent changes made using the technical content disclosed above are all equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. A method for controlling the injection of dual-frequency characteristic signals of a centralized photovoltaic inverter, characterized in that: The steps include: S1. After a power system fault occurs, the normal control strategy is switched to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criteria, and the AC electrical quantity is collected at the same time; S2, the frequency separation link of the cascade structure of IIR type time domain digital notch filter based on triangular transformation, performs frequency separation on the AC electrical quantity, and extracts the power frequency electrical quantity after frequency separation and the dual-frequency characteristic electrical quantity under non-power frequency; S3, performing positive-negative sequence separation on the power frequency electrical quantity after frequency separation to obtain a power frequency positive sequence component and a power frequency negative sequence component, adopting a negative sequence suppression strategy in a low voltage ride through control strategy for the power frequency negative sequence component, and adopting a reactive power priority strategy in a low voltage ride through control strategy for the power frequency positive sequence component, to obtain a first voltage modulation wave signal; S4. For the dual-frequency characteristic electrical quantity under non-industrial frequency, the effective value of the current in the actual dual-frequency characteristic electrical quantity is calculated by fast Fourier transform, and the fixed characteristic frequency current control link output in the additional signal injection control strategy is adopted to obtain the second voltage modulation wave signal; S5. Superimpose the first voltage modulated wave signal and the second voltage modulated wave signal, and output different pulses through a pulse width modulation link, so as to control the on and off of the inverter and realize the injection of the dual-frequency signal of the inverter into the output line.

2. The injection control method of the dual-frequency characteristic signal of the centralized photovoltaic inverter according to claim 1 is characterized in that: In S1, the starting criterion is the following formula (1): （1）； in, is the power frequency voltage collected at the inverter outlet, is the time after voltage abnormality is detected, is the rated voltage of the power system.

3. The injection control method of the dual-frequency characteristic signal of the centralized photovoltaic inverter according to claim 1 is characterized in that: In S2, the frequency separation link adopts a cascade structure of an IIR type time domain digital notch filter based on triangular transformation, and the transfer function of the frequency separation link is: （2）； in, is the number of data intervals, is the zero pole distance, is a discrete complex frequency domain variable, is the center angular frequency, is the sampling interval, is the transfer function amplitude correction coefficient.

4. The injection control method of the dual-frequency characteristic signal of the centralized photovoltaic inverter according to claim 3 is characterized in that: In S2, the expression of the transfer function amplitude correction coefficient is: （3）。 5. The injection control method of dual-frequency characteristic signal of centralized photovoltaic inverter according to claim 1, characterized in that: In S3, the expression of reactive power priority strategy is as follows: （4）； in, , are the reference values ​​of the positive sequence d-axis and q-axis inner loop currents, is the d-axis current during normal operation, is the rated current of the power system, is the maximum withstand current of the converter, is the per unit value, is the power frequency voltage amplitude; The negative sequence suppression strategy includes the negative sequence current suppression strategy and the negative sequence voltage suppression strategy. The negative sequence current suppression strategy sets the reference values ​​of the negative sequence current on the d-axis and q-axis to 0 respectively. The expressions are as follows: （5）； in, , are the negative sequence inner loop currents on the d-axis and q-axis respectively, is the angular velocity at the power frequency, is the equivalent inductance of the converter, , are the negative sequence voltages on the d-axis and q-axis respectively, is a complex frequency domain variable, and is the adjustment parameter of proportional-integral control, , are the negative sequence components of the d-axis and q-axis grid-side voltages respectively; The expression of the reference value of the q-axis inner ring current is as follows: （6）； in, is the q-axis negative sequence current reference value, is the dynamic negative sequence reactive current proportional coefficient of the photovoltaic power station, It is the per unit value of the negative sequence component of the voltage at the grid connection point of the photovoltaic power station.

6. The injection control method of dual-frequency characteristic signal of centralized photovoltaic inverter according to claim 5, characterized in that: In S3, the maximum withstand current of the converter is in the range of 1.2 ~2 .

7. The injection control method of dual-frequency characteristic signal of centralized photovoltaic inverter according to claim 1, characterized in that: In the S4, the output of the fixed characteristic frequency current control link in the additional signal injection control strategy includes: calculating the sum of the effective values ​​of the injected dual-frequency characteristic currents, setting a negative feedback link based on proportional integral control, and controlling the effective value of the dual-frequency injected current to be equal to a preset reference value; setting a voltage limiting link to limit the injected voltage amplitude when a high-resistance fault occurs, so that the sum of the injected voltage amplitude and the power frequency voltage amplitude does not exceed the rated voltage of the power system; and using the output result of the fixed characteristic frequency current control link as the amplitude of the second voltage modulation wave; The expression of the output of the current control link with a fixed characteristic frequency is as follows: （7）； in, I 1 and I 2 are the amplitudes at the dual-frequency characteristic frequencies, I det is the actual value of the input current of the control link, I * det is the reference value of the input current of the control link, s is a complex frequency domain variable, k p and k i is the adjustment parameter of proportional-integral control, U * det is the voltage amplitude of the characteristic signal, U0 is the power frequency voltage amplitude, U N is the voltage amplitude of the power system.

8. The injection control method of dual-frequency characteristic signal of centralized photovoltaic inverter according to claim 1, characterized in that: In S5, the superimposed voltage modulation wave signal is as follows: （8）； in, U * 0 is the amplitude of the power frequency voltage modulation wave, ω 0 is the angular frequency of the power frequency, ω det , φ det The characteristic frequencies are f 1 and f 2 is the angular frequency and initial phase of the injected signal, t is the time, U * det is the voltage amplitude of the characteristic signal.

9. A control system for injection of dual-frequency characteristic signals of a centralized photovoltaic inverter, used to implement the injection control method for dual-frequency characteristic signals of a centralized photovoltaic inverter according to any one of claims 1 to 8, characterized in that: It includes fault detection and strategy switching module, frequency separation and electrical quantity extraction module, control and electrical quantity processing module and signal superposition and modulation output module, among which: The fault detection and strategy switching module is used to switch from the normal control strategy to the low voltage ride-through control strategy and the additional signal injection control strategy according to the starting criteria after a fault occurs in the power system, and collect AC electrical quantities at the same time; The frequency separation and electrical quantity extraction module is used for the frequency separation link of the cascade structure of the IIR type time domain digital notch filter based on triangular transformation, performs frequency separation on the AC electrical quantity, and extracts the power frequency electrical quantity after frequency separation and the dual-frequency characteristic electrical quantity under non-power frequency; The control and electrical quantity processing module is used to separate the positive and negative sequences of the power frequency electrical quantity after frequency separation, and obtain the power frequency positive sequence component and the power frequency negative sequence component. The negative sequence suppression strategy in the low voltage ride-through control strategy is adopted for the power frequency negative sequence component, and the reactive power priority strategy in the low voltage ride-through control strategy is adopted for the power frequency positive sequence component to obtain the first voltage modulation wave signal; for the dual-frequency characteristic electrical quantity under non-power frequency, the effective value of the current in the actual dual-frequency characteristic electrical quantity is calculated by fast Fourier transform, and the fixed characteristic frequency current control link output in the additional signal injection control strategy is adopted to obtain the second voltage modulation wave signal; The signal superposition and modulation output module is used to superimpose the first voltage modulation wave signal and the second voltage modulation wave signal, and output different pulses through the pulse width modulation link, so as to control the on and off of the inverter and realize the injection of the dual-frequency signal of the inverter into the transmission line.

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