Stable adjusting method for low-short-circuit-ratio network-building grid-connected system

Abstract

The invention discloses a stable adjustment method for a low-short-circuit-ratio networking grid-connected system, and relates to the technical field of power system grid connection, and the stable adjustment method comprises the steps: carrying out the low-pass filtering of an input voltage signal through a low-pass filter, and obtaining a stable input voltage signal; carrying out phase adjustment on the stable input voltage signal by utilizing an improved double-second-order generalized integrator phase-locked loop technology and combining an all-pass phase-shifting filter, so as to enable a voltage phase output by the grid-connected equipment to be synchronous with a voltage phase of a power grid system; and inputting the stable input signal after phase adjustment into an impedance simulation module for low-short-circuit-ratio working condition simulation, and optimizing the electric energy quality of the grid-connected equipment and the power grid system in the grid-connected process according to a simulation result. The multi-filtering technology is combined with the precise phase-locked loop, burrs and harmonic interference in the input voltage are effectively filtered out, meanwhile, advanced phase shift adjustment and steady-state adjustment of the phase are carried out, and harmonic interference is thoroughly removed.

Description

Technical Field

[0001] The present invention relates to the technical field of power system grid connection, in particular to a stable regulation method for a low short-circuit ratio grid connection system. Background Art

[0002] With the large-scale development of renewable energy (such as wind power, photovoltaics, etc.), more and more distributed power sources need to be connected to the grid. However, since the output characteristics of these new energy grid-connected devices are different from traditional synchronous generators, they often face the problem of low short circuit ratio (SCR) during the grid connection process.

[0003] A low short-circuit ratio means that the coupling between the generator and the grid is weak. When the system encounters grid disturbances or load changes, it is easy to experience voltage instability, power quality degradation, and even system instability. Traditional low short-circuit ratio systems usually rely on reactive power compensation equipment (such as static VAR compensators (SVCs) or static synchronous compensators (STATCOMs)) to improve voltage support capabilities and enhance system stability. However, these compensation devices have the problems of high cost, high system complexity, and delayed response in some scenarios, and cannot completely solve the system stability problems caused by low SCR.

[0004] In order to cope with the challenges of grid-connected control under low short-circuit ratio conditions, modern power systems have introduced regulation technologies based on control algorithms, such as virtual synchronous generator (VSG) control strategies. These control strategies effectively improve the grid-connected stability of the system by introducing the dynamic characteristics of simulated synchronous generators in the inverter. However, these strategies have high requirements on hardware performance and also increase the complexity and implementation cost of the system. In addition, grid-connected control under low short-circuit ratio conditions still faces the following major problems:

[0005] Insufficient voltage support capability: Under low short-circuit ratio conditions, the coupling between the generator and the grid is weak, which makes the system prone to voltage instability when encountering grid disturbances or load changes. Although existing reactive power compensation equipment can improve voltage support to a certain extent, it is costly, complex, and may not respond quickly enough.

[0006] Poor system stability: Since the dynamic characteristics of new energy power generation equipment are different from those of traditional synchronous generators, the existing grid-connected systems often exhibit voltage fluctuations and frequency oscillations under low SCR conditions, and may even cause system instability.

[0007] Reliance on expensive reactive power compensation equipment: Current technical solutions mostly rely on expensive reactive power compensation equipment such as SVC and STATCOM to improve the stability of low SCR systems. These devices are not only expensive, but also require complex control and maintenance, increasing the construction and operation costs of the overall system.

[0008] The control strategy is complex and has high hardware requirements: Although some modern control strategies such as virtual synchronous machine (VSG) can improve the stability of the system, their implementation requires high-performance hardware equipment, which increases the cost of system implementation and makes it difficult to promote and apply on a large scale.

[0009] Currently, no effective solution has been proposed for the problems in the related technologies. Summary of the invention

[0010] 1. Technical issues to be solved

[0011] In view of the deficiencies in the prior art, the present invention provides a stable regulation method for a low short-circuit ratio grid-connected system, which has the advantages of filtering out harmonics and high-frequency interference in the input voltage and adjusting the phase synchronization between the generator and the grid through phase shifting, thereby improving the voltage support capacity and power quality in the grid-connected process, and is suitable for new energy grid-connected applications with low short-circuit ratio, thereby solving the problem that under low short-circuit ratio conditions, the coupling between the generator and the grid is weak, resulting in the system being prone to voltage instability when encountering grid disturbances or load changes.

[0012] (II) Technical solution

[0013] In order to achieve the above advantages of filtering out harmonics and high-frequency interference in the input voltage and adjusting the phase synchronization between the generator and the power grid by phase shifting, thereby improving the voltage support capacity and power quality during the grid connection process, and being suitable for new energy grid connection applications with low short-circuit ratio, the specific technical solutions adopted by the present invention are as follows:

[0014] A stable regulation method for a low short-circuit ratio grid-connected system, the stable regulation method comprising:

[0015] S1, collecting the input voltage signal of the power grid system, and using a low-pass filter to perform low-pass filtering on the input voltage signal to obtain a stable input voltage signal;

[0016] S2. Use the improved dual second-order generalized integrator phase-locked loop technology and combine it with an all-pass phase-shift filter to adjust the phase of the stable input voltage signal so that the voltage phase output by the grid-connected equipment is synchronized with the voltage phase of the grid system;

[0017] S3. Input the phase-adjusted stable input signal into the impedance simulation module to simulate the low short-circuit ratio working condition, and optimize the power quality of the grid-connected equipment and the grid system during the grid-connected process according to the simulation results.

[0018] Preferably, collecting the input voltage signal of the power grid system and performing low-pass filtering on the input voltage signal using a low-pass filter to obtain a stable input voltage signal includes:

[0019] S11, taking the second-order discrete filter structure as the structural feature of the low-pass filter, calculating the filter coefficient of the low-pass filter by normalizing the angular frequency, and constructing the time transfer function of the low-pass filter according to the filter coefficient;

[0020] S12, converting the time transfer function into a time domain difference equation, and inputting the pre-collected input voltage signal of the power grid system into the time domain difference equation for filtering, filtering out the noise signal in the input voltage signal, and obtaining a stable input voltage signal.

[0021] Preferably, the time transfer function of the low-pass filter is:

[0022]

[0023] Where H(s) represents the continuous-time transfer function of the low-pass filter; ω c represents the cutoff angular frequency; s represents the variable in the complex frequency domain.

[0024] Preferably, taking a second-order discrete filter structure as a structural feature of a low-pass filter, calculating the filter coefficient of the low-pass filter by normalizing the angular frequency, and constructing a time transfer function of the low-pass filter according to the filter coefficient includes:

[0025] S111, obtaining a low-pass filter frequency under a second-order discrete filter structure, and performing frequency pre-distortion processing and normalization processing on the low-pass filter frequency to compensate for the frequency distortion of the low-pass filter;

[0026] S112, constructing a continuous-time transfer function of a low-pass filter, and converting the continuous-time transfer function of the low-pass filter into a discretized expression based on a predefined sampling period;

[0027] S113. Introduce the numerator coefficient, denominator coefficient and normalization coefficient of the discretization expression into the discretization expression to obtain the discrete-time transfer function of the low-pass filter.

[0028] Preferably, the phase of the steady input voltage signal is adjusted by using an improved biquad generalized integrator phase-locked loop technology in combination with an all-pass phase-shift filter so that the voltage phase output by the grid-connected device is synchronized with the voltage phase of the grid system, including:

[0029] S21, converting the steady input voltage signal into a two-phase orthogonal signal, and using the orthogonal detuning technology to eliminate the harmonic interference in the steady input voltage signal;

[0030] S22, using a second-order generalized integrator to decompose the steady input voltage signal, respectively extracting the sine component and the cosine component in the steady input voltage signal, and obtaining two-phase orthogonal components;

[0031] S23. Phase-adjust the steady input voltage signal through an all-pass phase-shift filter, and use the phase-adjusted steady input voltage signal as a control reference to align the voltage phase of the grid-connected device output with the voltage phase of the grid system.

[0032] Preferably, converting the steady input voltage signal into a two-phase orthogonal signal and eliminating harmonic interference in the steady input voltage signal by using an orthogonal detuning technique comprises:

[0033] S211, calculating the two-phase voltage based on the positive sequence component, negative sequence component and harmonic component output by the second-order generalized integrator, and obtaining orthogonal signals of the two-phase voltage;

[0034] S212. Generate an orthogonal voltage based on phase shifting of the orthogonal signal, and perform harmonic voltage elimination on the orthogonal voltage to eliminate harmonic interference in the stable input voltage signal.

[0035] Preferably, the phase adjustment of the steady input voltage signal by the all-pass phase shift filter comprises:

[0036] Obtaining a steady input signal frequency, and inputting the steady input signal frequency into a continuous-time phase response formula of an all-pass phase-shift filter, and calculating a characteristic frequency of the all-pass phase-shift filter;

[0037] The resistance value and the capacitance value of the circuit are calculated according to the characteristic frequency of the all-pass phase-shift filter, and the voltage transfer function of the all-pass phase-shift filter in a continuous time period is constructed based on the resistance value and the capacitance value;

[0038] The voltage transfer function is converted into a discrete time transfer function by using a linear transformation technique, the discrete time transfer function is simplified, and the simplified discrete time transfer function is converted into an all-pass phase shift difference equation of an all-pass phase shift filter.

[0039] Preferably, the expression of the two-phase voltage is:

[0040]

[0041] The expression of the orthogonal signal of the two-phase voltage is:

[0042]

[0043] In the formula, U′ α and U′ β Indicates SOGI output voltage; and Represents the fundamental component in the SOGI output voltage; and It represents the negative sequence component in the SOGI output voltage; i represents the harmonic order; and represents the i-th harmonic voltage in the SOGI output voltage; U′αq and U′ βq Indicates SOGI output voltage U′ α and U′ β orthogonal signals.

[0044] Preferably, the impedance simulation module represents a differential transfer function module of the RL parallel circuit, and the differential transfer function module of the RL parallel circuit is used to simulate the impedance characteristics of an actual low short circuit ratio device.

[0045] Preferably, the differential transfer function module of the RL parallel circuit includes:

[0046] Obtaining the RL circuit transfer function of the RL parallel circuit and calculating the time domain relationship of the RL circuit transfer function using the inverse Laplace transform;

[0047] Discretize the continuous time, set the sampling period interval to obtain the time discrete points, and obtain the input and output signals corresponding to the time discrete points;

[0048] The derivative terms are approximated by the backward difference method based on the input and output signals, and the discretized derivative terms are input into the time domain differential equation to obtain the differential equation of the RL parallel circuit.

[0049] (III) Beneficial effects

[0050] Compared with the prior art, the present invention provides a stable regulation method for a low short-circuit ratio grid-connected system, which has the following beneficial effects:

[0051] (1) The present invention uses multiple filtering technologies combined with a precise phase-locked loop to effectively filter out glitches and harmonic interference in the input voltage, and simultaneously performs phase advance shift adjustment and steady-state adjustment to completely remove harmonic interference. In addition, by combining the device resistive simulation module to simulate and analyze the grid-connected system, the stability and power quality of the system during the grid-connected process are greatly improved, and it is particularly suitable for stable control of the power grid system under low short-circuit ratio scenarios.

[0052] (2) The stable regulation method for a low short-circuit ratio grid-connected system provided by the present invention aims to improve the system stability by improving the input signal quality and accurately controlling the phase. This method filters out harmonics and high-frequency interference in the input voltage and adjusts the phase synchronization between the generator and the grid by phase shifting, thereby improving the voltage support capacity and power quality during the grid-connected process. It is particularly suitable for new energy grid-connected applications with a low short-circuit ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0054] Figure 1 is a flow chart of a stable regulation method for a low short-circuit ratio grid-connected system according to an embodiment of the present invention;

[0055] Figure 2 is a schematic diagram of a stable regulation method for a low short-circuit ratio grid-connected system according to an embodiment of the present invention;

[0056] Figure 3 It is a principle diagram of the orthogonal detuning method in the stable regulation method of the low short-circuit ratio grid-connected system according to an embodiment of the present invention;

[0057] Figure 4 It is a principle diagram of an all-pass phase-shift filter in a stable regulation method of a low short-circuit ratio grid-connected system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0058] To further illustrate each embodiment, the present invention provides drawings, which are part of the disclosure of the present invention and are mainly used to illustrate the embodiments and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these contents, ordinary technicians in the field should be able to understand other possible implementations and advantages of the present invention. The components in the figures are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0059] According to an embodiment of the present invention, a stable regulation method for a low short-circuit ratio grid-connected system is provided.

[0060] The present invention is further described with reference to the accompanying drawings and specific embodiments. Figure 1 As shown, according to a stable regulation method of a low short-circuit ratio grid-connected system according to an embodiment of the present invention, the stable regulation method includes:

[0061] S1. Collect the input voltage signal of the power grid system, and use a low-pass filter to perform low-pass filtering on the input voltage signal to obtain a stable input voltage signal.

[0062] The input voltage signal of the power grid system is collected, and the input voltage signal is low-pass filtered by a low-pass filter to obtain a stable input voltage signal, including:

[0063] S11. Taking the second-order discrete filter structure as the structural feature of the low-pass filter, the filter coefficient of the low-pass filter is calculated by normalizing the angular frequency, and the time transfer function of the low-pass filter is constructed according to the filter coefficient.

[0064] Among them, the second-order discrete filter structure is used as the structural feature of the low-pass filter, the filter coefficient of the low-pass filter is calculated by normalizing the angular frequency, and the time transfer function of the low-pass filter is constructed according to the filter coefficient, including:

[0065] S111, obtaining a low-pass filter frequency under a second-order discrete filter structure, and performing frequency pre-distortion processing and normalization processing on the low-pass filter frequency to compensate for the frequency distortion of the low-pass filter;

[0066] S112, constructing a continuous-time transfer function of a low-pass filter, and converting the continuous-time transfer function of the low-pass filter into a discretized expression based on a predefined sampling period;

[0067] S113. Introduce the numerator coefficient, denominator coefficient and normalization coefficient of the discretization expression into the discretization expression to obtain the discrete-time transfer function of the low-pass filter.

[0068] S12, converting the time transfer function into a time domain difference equation, and inputting the pre-collected input voltage signal of the power grid system into the time domain difference equation for filtering, filtering out the noise signal in the input voltage signal, and obtaining a stable input voltage signal.

[0069] In order to facilitate understanding of the above technical solution of the present invention, the present invention will be described in detail below in the actual process of collecting the input voltage signal of the power grid system, and using a low-pass filter to low-pass filter the input voltage signal to obtain a stable input voltage signal.

[0070] The low-pass filter is mainly used to filter out high-frequency noise and glitches in the input voltage to ensure the stability of the input signal. The noise and glitches in the input voltage may cause false triggering of the power device, resulting in voltage fluctuations and frequency oscillations in the system, thereby affecting the stability of the system. Therefore, the present invention ensures that the input signal is smoother and reduces disturbances to the grid-connected system by adding a digital low-pass filter in the control link.

[0071] The implementation of the low-pass filter adopts a second-order discrete filter structure, and its filter coefficients are normalized to make the data scale consistent. The digital filtering of the input signal is realized in the form of a differential equation and combined with the filter coefficients b0, b1, b2, a1, a2. The algorithm can be implemented in DSP28335 (Digital Signal Processor, DSP28335 is a high-performance digital signal processor designed for embedded control). Thanks to the fast floating-point computing capability of DSP, the real-time and high efficiency of the filtering process can be ensured.

[0072] In order to implement the low-pass filter in algorithmic form, it is necessary to convert the continuous transfer function into a difference equation. The continuous-time transfer function of the low-pass filter is known to be:

[0073]

[0074] Among them, ω c =2πf c represents the cut-off angular frequency; f c represents the cut-off frequency; H(s) represents the continuous-time transfer function of the low-pass filter; s represents a variable in the complex frequency domain.

[0075] Frequency Predistortion:

[0076] Before applying the bilinear transform, frequency predistortion is required to compensate for frequency distortion:

[0077]

[0078] because So the predistortion frequency is:

[0079]

[0080] Where, ω′ represents the predistortion frequency; f s represents the sampling frequency; T represents the sampling period.

[0081] Bilinear transformation:

[0082] Convert the continuous-time transfer function H(s) to the discrete-time transfer function H(z):

[0083]

[0084] in, represents the sampling period; z -1 Represents a z-domain variable delayed by one sampling period. Substitute s into H(s) to obtain the discretized expression of H(z):

[0085]

[0086] In order to simplify the calculation, the pre-distortion frequency ω′ and normalization processing are introduced.

[0087] Calculate the filter coefficients:

[0088] After algebraic manipulation, the coefficients of the discrete transfer function can be expressed as:

[0089] Numerator coefficient (b coefficient):

[0090]

[0091] Denominator coefficient (a coefficient):

[0092]

[0093] Among them, the normalization coefficient c is:

[0094]

[0095] After algebraic operations and sorting, the discrete-time transfer function is obtained:

[0096]

[0097] Express H(z) as a polynomial in the numerator and denominator:

[0098] Numerator:

[0099] B(z)=b0+b1z -1 +b2z -2

[0100] Denominator:

[0101] A(z)=1+a1z -1 +a2z -2

[0102] The inverse Z transform gives the difference equation according to get:

[0103] Y(z)·A(z)=X(z)·B(z)

[0104] Perform an inverse Z transform and obtain the time domain difference equation:

[0105] y[n]=-a1y[n-1]-a2y[n-2]+b0x[n]+b1x[n-1]+b2x[n-2]

[0106] Among them, ω dRepresents digital angular frequency; y[n] represents the current output; x[n] represents the current input; y[n-1], y[n-2] represent the historical output; x[n-1], x[n-2] represent the historical input; b0, b1, b2 represent the numerator coefficients; a1, a2 represent the denominator coefficients.

[0107] The input signal is processed by the differential equation of this second-order filter, which can filter out high-frequency glitches and interference signals in the input voltage through software. The filtered signal is more stable, which helps to improve the stability of the system, especially in the case of load changes or grid disturbances, and can effectively reduce transient fluctuations in the system. However, this will cause signal lag, and phase shifting is also required.

[0108] S2. Use the improved dual second-order generalized integrator phase-locked loop technology in combination with an all-pass phase-shift filter to adjust the phase of the stable input voltage signal so that the voltage phase output by the grid-connected equipment is synchronized with the voltage phase of the power grid system.

[0109] Among them, the phase of the steady input voltage signal is adjusted by using the improved dual second-order generalized integrator phase-locked loop technology and combining with the all-pass phase-shift filter, so that the voltage phase output by the grid-connected device is synchronized with the voltage phase of the power grid system, including:

[0110] S21, converting the steady input voltage signal into a two-phase orthogonal signal, and using the orthogonal detuning technology to eliminate the harmonic interference in the steady input voltage signal.

[0111] The method of converting the steady input voltage signal into a two-phase orthogonal signal and eliminating the harmonic interference in the steady input voltage signal by using the orthogonal detuning technology includes:

[0112] S211, calculating the two-phase voltage based on the positive sequence component, negative sequence component and harmonic component output by the second-order generalized integrator, and obtaining orthogonal signals of the two-phase voltage;

[0113] S212. Generate an orthogonal voltage based on phase shifting of the orthogonal signal, and perform harmonic voltage elimination on the orthogonal voltage to eliminate harmonic interference in the stable input voltage signal.

[0114] S22, using a second-order generalized integrator to decompose the steady input voltage signal, respectively extracting the sine component and the cosine component in the steady input voltage signal, and obtaining two-phase orthogonal components;

[0115] S23. Phase-adjust the steady input voltage signal through an all-pass phase-shift filter, and use the phase-adjusted steady input voltage signal as a control reference to align the voltage phase of the grid-connected device output with the voltage phase of the grid system.

[0116] The phase adjustment of the steady input voltage signal by the all-pass phase shift filter includes:

[0117] Obtaining a steady input signal frequency, and inputting the steady input signal frequency into a continuous-time phase response formula of an all-pass phase-shift filter, and calculating a characteristic frequency of the all-pass phase-shift filter;

[0118] The resistance value and the capacitance value of the circuit are calculated according to the characteristic frequency of the all-pass phase-shift filter, and the voltage transfer function of the all-pass phase-shift filter in a continuous time period is constructed based on the resistance value and the capacitance value;

[0119] The voltage transfer function is converted into a discrete time transfer function by using a linear transformation technique, the discrete time transfer function is simplified, and the simplified discrete time transfer function is converted into an all-pass phase shift difference equation of an all-pass phase shift filter.

[0120] In order to facilitate understanding of the above technical scheme of the present invention, the following is a detailed description of the present invention in the actual process of using the improved dual second-order generalized integrator phase-locked loop technology and combining it with a full-pass phase-shift filter to adjust the phase of the stable input voltage signal so that the voltage phase output by the grid-connected device is synchronized with the voltage phase of the power grid system.

[0121] Based on the dual second-order generalized integrator PLL (DSOGI-PLL), the AC signal is extracted by using a quarter-cycle delay and has the ability to filter out harmonics. It can lock the phase under the condition that the grid voltage is distorted. However, when the harmonic content of the grid is large, the baseband signal extracted by the DSOGI (Dual Second-Order Generalized Integrator) will cause directional voltage pulsation. The present invention proposes to use the orthogonal detuning method to eliminate the interference of harmonic voltage on the output voltage of the traditional DSOGI phase-locked loop, so as to achieve accurate tracking of the baseband signal.

[0122] DSOGI-PLL uses a second-order generalized integrator as its core. By decomposing the input signal (such as grid voltage or current), it extracts the sine component and cosine component respectively, thereby accurately estimating the phase information of the signal. It is particularly suitable for processing frequency fluctuations, harmonics or grid disturbances.

[0123] The principle of orthogonal detuning is as follows: Figure 3 As shown, assuming that the SOGI output voltage U′ α and U′β contain positive and negative sequence and harmonic components respectively, and the fundamental frequency is The two-phase voltage can be expressed as:

[0124]

[0125] In the formula, U′ α and U′ β Indicates SOGI output voltage; and Represents the fundamental component in the SOGI output voltage; and It represents the negative sequence component in the SOGI output voltage; i represents the harmonic order; and represents the i-th harmonic voltage in the SOGI output voltage; U′αq and U′ βq Indicates SOGI output voltage U′ α and U′ β The orthogonal signal U′ a and U′ β The orthogonal signals U′αq and U′ βq The expression is:

[0126]

[0127] It can be seen that the use of orthogonal signals can eliminate the negative sequence voltage in the voltage output by DSOGI, but the i-th harmonic voltage has not disappeared. Similarly, the harmonic signal is phase-shifted by 90° to generate an orthogonal voltage, and then the harmonic voltage is eliminated to eliminate the harmonic voltage. This method is called the orthogonal detuning method. and The harmonic components are eliminated by using the orthogonal detuning method. and Differentiation realizes 90° phase shift operation, and the orthogonal signal is obtained as follows: and The i-th harmonic elimination is shown below:

[0128]

[0129] Voltage after i-th harmonic elimination and Only the fundamental frequency component remains in the system. When the system contains multiple harmonics, they can be calculated in sequence to achieve successive elimination of harmonics.

[0130] The function of the all-pass phase-shift filter is to adjust the phase of the input signal. Combined with a precise phase-locked loop, the control system output can be better aligned with the grid voltage phase. In the grid-connected control process, precise phase alignment is the key to ensuring seamless grid switching and reducing impact current. The all-pass filter in the present invention is used to achieve advance phase shifting of a specific angle of the signal in order to achieve higher synchronization when connected to the grid.

[0131] In the project, the phase shifting operation is used to adjust the phase of the input signal before entering the control link to ensure that the output of the equipment is consistent with the power grid. This can not only reduce the current impact caused by phase mismatch during the grid connection process, but also effectively improve the voltage support capacity of the low short-circuit ratio system. The all-pass filter is implemented on the DSP28335 platform. After low-pass filtering, the phase is shifted with a precise phase-locked loop, making full use of the DSP's support for real-time phase adjustment, so that the system can respond quickly in a dynamic environment.

[0132] The specific derivation and implementation of the differential equation of the all-pass phase-shifting filter include: Since software is used for implementation, the hardware filter should be replaced with a differential equation to implement the algorithm, as shown below: Figure 4 As shown, the input signal U(s)in is subjected to voltage division processing after passing through the resistor R and the RC parallel circuit, and the RC parallel part filters the frequency component of the signal to play an impedance matching and filtering role.

[0133] First, based on the hardware structure of the all-pass filter, it can be seen that the voltage transfer function of the lead all-pass filter in continuous time is:

[0134]

[0135] Among them, R and C represent the resistance and capacitance values ​​of the circuit; s represents the complex frequency variable, which is used to describe the frequency domain characteristics of the system; the minus sign is used to adjust the overall phase so that the filter can achieve the phase advance characteristic.

[0136] The continuous-time phase response formula of the filter is:

[0137]

[0138] Where f represents the frequency of the input signal; represents the characteristic frequency of the filter; Indicates the amount of advance that the filter needs to achieve.

[0139] Find the characteristic frequency using the phase formula:

[0140] A phase advance is achieved at a frequency of f = 50 Hz Taking the discretization of the all-pass filter as an example, the characteristic frequency f0 can be calculated by substituting it into the phase response formula:

[0141] Substituting the known phase and frequency into the phase formula:

[0142]

[0143] Organizing formula:

[0144]

[0145] Convert an angle of 80° to radians:

[0146]

[0147] Solved:

[0148]

[0149] therefore:

[0150]

[0151] Solve RC according to f0, characteristic frequency formula:

[0152] Solve for RC:

[0153]

[0154] In order to convert the continuous time transfer function into discrete time, a bilinear transformation is used, which is expressed as:

[0155]

[0156] Where T is the sampling period (for example, if the sampling frequency is f s ,So ).

[0157] Substitute s for the above expression:

[0158]

[0159] set up:

[0160]

[0161] therefore:

[0162]

[0163] Substitute s into the transfer function Substitute s into the original transfer function A u (s):

[0164]

[0165] Simplifying the Discrete-Time Transfer Function To simplify this transfer function, first reduce the numerator and denominator to the same:

[0166]

[0167] Further simplification yields:

[0168]

[0169] Derive the difference equation:

[0170] Transfer function A u (z) can be expressed as the relationship between input X(z) and output Y(z):

[0171]

[0172] Multiply both sides of the equation by the denominator:

[0173] Y(z)((1+α)+(1-α)z -1 ) = X(z)(-(1-α)-(1+α)z -1 )

[0174] Convert to time domain difference equation:

[0175] Using the convolution property, z -1 Translating to the delay operator in the time domain, we obtain the difference equation:

[0176] (1+α)y[n]+(1-α)y[n-1]=-(1-α)x[n]-(1+α)x[n-1]

[0177] Listing y[n] separately gives the full-pass phase-advanced difference equation:

[0178]

[0179] Among them, x[n] represents the input signal at the current moment; x[n-1] represents the input signal at the previous moment; y[n-1] represents the output signal at the previous moment; by selecting an appropriate sampling frequency, the specific α value can be determined and used to achieve the phase compensation effect of the filter. The phase-shifted signal is used as a control signal input to ensure the phase synchronization of the device voltage during the grid-connected process. Using this differential equation can save a lot of computing resources to implement the corresponding software phase shift in the device.

[0180] S3. Input the phase-adjusted stable input signal into the impedance simulation module to simulate the low short-circuit ratio working condition, and optimize the power quality of the grid-connected equipment and the grid system during the grid-connected process according to the simulation results.

[0181] The impedance simulation module represents a differential transfer function module of the RL parallel circuit, and the differential transfer function module of the RL parallel circuit is used to simulate the impedance characteristics of an actual low short-circuit ratio device.

[0182] Among them, the differential transfer function module of the RL parallel circuit includes:

[0183] The RL circuit transfer function of the RL parallel circuit is obtained, and the time domain relationship of the RL circuit transfer function is calculated using the inverse Laplace transform; the continuous time is discretized, and the sampling period interval is set to obtain the time discrete points, and the input and output signals corresponding to the time discrete points are obtained; the derivative terms are approximated based on the input and output signals using the backward difference method, and the discretized derivative terms are input into the time domain differential equation to obtain the differential equation of the RL parallel circuit.

[0184] In order to facilitate understanding of the above technical scheme of the present invention, the following is a detailed description of the present invention in the actual process of inputting the phase-adjusted stable input signal into the impedance simulation module to simulate the low short-circuit ratio working condition, and optimizing the power quality of the grid-connected equipment and the grid system during the grid-connected process according to the simulation results.

[0185] The present invention provides a differential transfer function module of RL parallel circuit, which is used to simulate the impedance characteristics of actual low short-circuit ratio equipment. The module accurately reproduces the coupling characteristics between the generator and the power grid under low SCR conditions by modeling the RL parallel circuit, so as to test the filtering effect and the stability of the overall system.

[0186] Modeling principles of RL parallel circuits:

[0187] The RL parallel circuit is composed of resistance (R) and inductance (L). Its transfer function can better simulate the impedance characteristics in the low SCR system. Under low SCR conditions, the coupling between the generator and the grid is weak, which is manifested as voltage fluctuation and response time delay. Through the digital modeling of the RL parallel circuit, these system characteristics can be more accurately reflected, thus providing a more reliable and convenient test environment for filter design and optimization.

[0188] Digital model differential transfer function implementation:

[0189] The differential transfer function module of the RL parallel circuit is obtained based on the discretization of its continuous time transfer function. The transfer function in the continuous time domain is converted to the discrete time domain using the bilinear transformation method to facilitate implementation on the DSP28335 platform. The implementation of the differential transfer function includes the following steps:

[0190] Given the RL circuit transfer function:

[0191] H(s)=R+Ls

[0192] Among them, this is the representation of the RL system in the frequency domain, and it also needs to be converted into the time domain for discrete digital filtering processing.

[0193] Convert to a differential equation:

[0194] Using the inverse Laplace transform, the time domain relationship corresponding to the transfer function is:

[0195]

[0196] Among them, U out (t) represents the time domain relationship corresponding to the transfer function; U in (t) represents the voltage value of the input signal in the time domain; dt represents a small increment of time; d represents the differential operator; and t represents a discrete time point.

[0197] Time discretization:

[0198] Discretize the continuous time t, and set the sampling period to T, then the discrete time points are:

[0199] t=nTwhere n=0,1,2,…

[0200] The corresponding input and output signals are:

[0201] U in (t)→U in [n],U out (t)→U out [n]

[0202] Approximate derivative terms:

[0203] Use backward differences to approximate the derivative terms:

[0204]

[0205] The derivative term is expressed as the difference between the current and previous input signals.

[0206] Substituting into the differential equation:

[0207] Substitute the discretized derivative terms into the time domain differential equation:

[0208]

[0209] Expand and Arrange Expand the right side:

[0210]

[0211] Combine like terms:

[0212]

[0213] Introducing symbolic simplification:

[0214] U last =U in [n-1]

[0215] Then the difference equation can be written as:

[0216]

[0217] The dynamic response of the RL circuit is realized by using the differential equation to simulate the impedance characteristics of the actual equipment during the grid connection process.

[0218] It should be noted that by introducing the differential transfer function module of the RL parallel circuit, the effect of the filter can be evaluated under the condition of simulating the impedance characteristics of the low SCR system. This module can effectively test the actual effect of the dual filter (low-pass filter and full-pass phase-shift filter), including signal smoothness, phase synchronization effect and the ability to suppress transient disturbances.

[0219] Low-pass filter effect test: The input signal simulated by the RL parallel circuit is used to verify the effectiveness of the low-pass filter in filtering out high-frequency glitches and noise, ensuring that the filtered signal is suitable for grid-connected control.

[0220] Phase synchronization effect test: Utilize the RL parallel circuit characteristics to test the accuracy of the all-pass phase-shift filter in signal phase adjustment to ensure the synchronization of the generator output and grid voltage phase.

[0221] In summary, the present invention improves the quality of the input signal by filtering out the burrs and harmonic interference in the input voltage, and synchronizes the device voltage with the grid phase by phase shifting, thereby effectively improving the voltage support capability and stability of the system, reducing the dependence on expensive reactive power compensation equipment, and reducing the system complexity and implementation cost. Specifically, the present invention solves the problem in the following ways under the condition of low short-circuit ratio:

[0222] Low-pass filtering technology is used to remove high-frequency interference in the input signal, ensure the stability of the input signal, and improve the system's anti-disturbance ability. A dual second-order generalized integrator (DSOGI) phase-locked loop is used to remove harmonics and accurate phase. Combined with an all-pass filter to achieve phase adjustment, the voltage phase output by the grid-connected equipment is synchronized with the grid, reducing the current shock during the grid connection process. It can be connected to a digital analog RL circuit, and the entire software simulation test scr grid connection can be achieved, reducing the cost of experimental testing and achieving the purpose of quickly adjusting the grid-connected performance of the equipment. The synergy of multiple digital filters is used to optimize the input signal at a lower hardware cost, reducing dependence on complex compensation equipment. This solution is particularly suitable for grid-connected scenarios of new energy equipment with low short-circuit ratios, and has significant advantages in improving power quality, system stability, and reducing implementation costs.

[0223] The algorithm of the present invention is implemented on the DSP28335 platform. By utilizing its powerful real-time processing capability, it can efficiently execute filtering and control algorithms, thereby meeting the real-time requirements of the grid-connected system. In the real-time control system, stability and phase shift processing are performed for signals with large fluctuations and high low-latency requirements. Figure 2 As shown, before the low SCR device is connected to the grid, the voltage is first subjected to a low-pass filter LPF, and then phase-aligned by a phase-locked loop PLL combined with an all-pass phase-shift filter APF, and then the RL simulated impedance characteristics are output.

[0224] It should be noted that the combination of low-pass filtering, full-pass phase-shift filtering and RL simulation module (equivalent to the impedance simulation module in the present invention, and the impedance simulation module represents the differential transfer function module of the RL parallel circuit) makes the present invention have significant advantages in improving system stability and power quality. The low-pass filter is used to ensure the smoothness of the input signal and eliminate high-frequency burrs and noise; the full-pass phase-shift filter is used to ensure the synchronization of the signal phase with the power grid; and the RL parallel circuit simulation module is used to simulate the impedance characteristics of the actual low SCR device to test the filtering effect and ensure the robustness of the system. The combination of the three can effectively solve the voltage instability and system instability problems in the low short-circuit ratio system.

[0225] After the low-pass filter filters out the noise, the signal becomes more stable, providing a good foundation for phase-shifting operation; the phase-shifted signal keeps the output of the device synchronized with the grid phase, reducing the current shock and voltage fluctuation that may occur during the grid connection process; the RL simulation module further ensures that the filtered signal adapts to the characteristics of the low SCR system and ensures the overall stability of the system.

[0226] Through the application of filtering, phase shifting and testing modules, the present invention can significantly improve the stability and power quality of the grid-connected system of low short-circuit ratio equipment through software algorithm control without relying on expensive reactive compensation equipment. It has high practicality and economy, so that the control method can achieve efficient real-time control at low cost, which is very suitable for practical engineering applications.

[0227] To sum up, with the help of the above-mentioned technical scheme of the present invention, the present invention realizes effective filtering of glitches and harmonic interference in the input voltage through multiple filtering technology combined with a precise phase-locked loop, and simultaneously performs phase advance phase shift adjustment and steady-state adjustment to completely remove harmonic interference. In addition, by combining the equipment resistive simulation module to simulate and analyze the grid-connected system, the stability and power quality of the system during the grid-connected process are greatly improved, which is particularly suitable for stable control of power grid systems under low short-circuit ratio scenarios; the stable adjustment method of the low short-circuit ratio grid-connected system provided by the present invention aims to improve the system stability by improving the input signal quality and precisely controlling the phase. This method filters out harmonics and high-frequency interference in the input voltage, and adjusts the phase synchronization of the generator and the power grid through phase shifting, thereby improving the voltage support capacity and power quality during the grid-connected process, which is particularly suitable for new energy grid-connected applications with low short-circuit ratio.

[0228] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A stable regulation method for a low short-circuit ratio grid-connected system, characterized in that: The stable regulation method includes: S1, collecting the input voltage signal of the power grid system, and using a low-pass filter to perform low-pass filtering on the input voltage signal to obtain a stable input voltage signal; S2. Use the improved dual second-order generalized integrator phase-locked loop technology and combine it with an all-pass phase-shift filter to adjust the phase of the stable input voltage signal so that the voltage phase output by the grid-connected equipment is synchronized with the voltage phase of the grid system; S3. Input the phase-adjusted stable input signal into the impedance simulation module to simulate the low short-circuit ratio working condition, and optimize the power quality of the grid-connected equipment and the grid system during the grid-connected process according to the simulation results.

2. The method for stabilizing and regulating a low short-circuit ratio grid-connected system according to claim 1, characterized in that: The collecting of the input voltage signal of the power grid system and the low-pass filtering of the input voltage signal by using a low-pass filter to obtain a stable input voltage signal include: S11, taking the second-order discrete filter structure as the structural feature of the low-pass filter, calculating the filter coefficient of the low-pass filter by normalizing the angular frequency, and constructing the time transfer function of the low-pass filter according to the filter coefficient; S12, converting the time transfer function into a time domain difference equation, and inputting the pre-collected input voltage signal of the power grid system into the time domain difference equation for filtering, filtering out the noise signal in the input voltage signal, and obtaining a stable input voltage signal.

3. The method for stabilizing and regulating a low short-circuit ratio grid-connected system according to claim 2, characterized in that: The time transfer function of the low-pass filter is: Where H(s) represents the continuous-time transfer function of the low-pass filter; ω c represents the cutoff angular frequency; s represents the variable in the complex frequency domain.

4. The method for stabilizing and regulating a low short-circuit ratio grid-connected system according to claim 3, characterized in that: The method of using the second-order discrete filter structure as the structural feature of the low-pass filter, calculating the filter coefficient of the low-pass filter by normalizing the angular frequency, and constructing the time transfer function of the low-pass filter according to the filter coefficient includes: S111, obtaining a low-pass filter frequency under a second-order discrete filter structure, and performing frequency pre-distortion processing and normalization processing on the low-pass filter frequency to compensate for the frequency distortion of the low-pass filter; S112, constructing a continuous-time transfer function of a low-pass filter, and converting the continuous-time transfer function of the low-pass filter into a discretized expression based on a predefined sampling period; S113. Introduce the numerator coefficient, denominator coefficient and normalization coefficient of the discretization expression into the discretization expression to obtain the discrete-time transfer function of the low-pass filter.

5. A stable regulation method for a low short-circuit ratio grid-connected system according to claim 4, characterized in that: The method of using the improved biquad generalized integrator phase-locked loop technology in combination with an all-pass phase-shift filter to adjust the phase of the steady input voltage signal so that the voltage phase output by the grid-connected device is synchronized with the voltage phase of the grid system includes: S21, converting the steady input voltage signal into a two-phase orthogonal signal, and using the orthogonal detuning technology to eliminate the harmonic interference in the steady input voltage signal; S22, using a second-order generalized integrator to decompose the steady input voltage signal, respectively extracting the sine component and the cosine component in the steady input voltage signal, and obtaining two-phase orthogonal components; S23, phase-adjusting the steady input voltage signal through an all-pass phase-shifting filter, and using the phase-adjusted steady input voltage signal as a control reference, so that the voltage phase output by the grid-connected device is aligned with the voltage phase of the grid system.

6. A stable regulation method for a low short-circuit ratio grid-connected system according to claim 5, characterized in that: The method of converting the steady input voltage signal into a two-phase orthogonal signal and eliminating harmonic interference in the steady input voltage signal by using an orthogonal detuning technique comprises: S211, calculating the two-phase voltage based on the positive sequence component, negative sequence component and harmonic component output by the second-order generalized integrator, and obtaining orthogonal signals of the two-phase voltage; S212. Generate an orthogonal voltage based on phase shifting of the orthogonal signal, and perform harmonic voltage elimination on the orthogonal voltage to eliminate harmonic interference in the stable input voltage signal.

7. A stable regulation method for a low short-circuit ratio grid-connected system according to claim 6, characterized in that: The phase adjustment of the steady input voltage signal by the all-pass phase-shift filter comprises: Obtaining a steady input signal frequency, and inputting the steady input signal frequency into a continuous-time phase response formula of an all-pass phase-shift filter, and calculating a characteristic frequency of the all-pass phase-shift filter; The resistance value and the capacitance value of the circuit are calculated according to the characteristic frequency of the all-pass phase-shift filter, and the voltage transfer function of the all-pass phase-shift filter in a continuous time period is constructed based on the resistance value and the capacitance value; The voltage transfer function is converted into a discrete time transfer function by using a linear transformation technique, the discrete time transfer function is simplified, and the simplified discrete time transfer function is converted into an all-pass phase shift difference equation of an all-pass phase shift filter.

8. The method for stabilizing and regulating a low short-circuit ratio grid-connected system according to claim 7, characterized in that: The expression of the two-phase voltage is: The expression of the orthogonal signal of the two-phase voltage is: In the formula, U′ α and U′ β Indicates SOGI output voltage; and Represents the fundamental component in the SOGI output voltage; and It represents the negative sequence component in the SOGI output voltage; i represents the harmonic order; and represents the i-th harmonic voltage in the SOGI output voltage; U′αq and U′ βq Indicates SOGI output voltage U′ α and U′ β orthogonal signals.

9. A stable regulation method for a low short-circuit ratio grid-connected system according to claim 8, characterized in that: The impedance simulation module represents a differential transfer function module of the RL parallel circuit, and the differential transfer function module of the RL parallel circuit is used to simulate the impedance characteristics of an actual low short-circuit ratio device.

10. A stable regulation method for a low short-circuit ratio grid-connected system according to claim 9, characterized in that: The differential transfer function module of the RL parallel circuit includes: Obtaining the RL circuit transfer function of the RL parallel circuit and calculating the time domain relationship of the RL circuit transfer function using the inverse Laplace transform; Discretize the continuous time, set the sampling period interval to obtain the time discrete points, and obtain the input and output signals corresponding to the time discrete points; The derivative terms are approximated by the backward difference method based on the input and output signals, and the discretized derivative terms are input into the time domain differential equation to obtain the differential equation of the RL parallel circuit.

Images