Frequency-locked loop and power grid state tracking method based on discrete three-phase second-order generalized integrator

Abstract

The present invention discloses a frequency-locked loop and a power grid state tracking method based on a discrete three-phase second-order generalized integrator, including a Clark converter, two discrete second-order generalized integrator prefilters in parallel rotating reference axis directions, a discrete frequency-locked loop system with normalization, a positive and negative sequence calculator, and a Clark inverse converter. The present invention first performs discretized prefiltering in two directions perpendicular to the three-phase voltage rotating reference system through a discrete three-phase second-order generalized integrator frequency-locked loop to obtain two pairs of orthogonal fundamental component sequence signals, obtains a frequency estimation signal after processing through a discrete frequency-locked loop, and then transmits it to the prefilter to form a closed-loop system to estimate the frequency. The present invention not only tracks the fundamental frequency of the three-phase power grid more accurately, but also can obtain more accurate fundamental positive and negative sequence fundamental voltages in a stationary reference system, thereby observing whether the power grid has a fault or imbalance, and allowing energy storage to make a coordinated response to power grid systems in different states.

Description

Technical Field

[0001] The present invention relates to a real-time frequency estimation method for a power grid, and in particular to a frequency-locked loop and a power grid state tracking method based on a discrete three-phase second-order generalized integrator. Background Art

[0002] With the advancement of the carbon neutrality strategy, for a long time, improving the consumption of new energy will remain the primary task of the Xinjiang power grid. Because energy storage has multiple functions of rapid response, power regulation and energy transfer, it can break through or improve the rigid constraints of existing consumption restrictions as a new means of regulation. However, since the direct output of energy storage is mostly in the form of direct current, it is necessary to invert the direct current into alternating current with the same real-time frequency of the power grid in order to connect to the grid normally without introducing too much voltage fluctuation. The premise of using power electronic equipment to invert direct current is to obtain the frequency of the alternating current after inversion, that is, the real-time frequency of the power grid, which cannot be directly measured. At present, the way to estimate the frequency of the power grid is usually to use a continuous state frequency-locked loop model. For the actual measurement, the continuous frequency-locked loop model will increase the error when used due to discrete sampling. In addition, the real-time voltage of the power grid is also an important parameter of the power grid state. By tracking and observing the measured voltage of the power grid, the voltage fluctuation can be timely known, but it is impossible to know whether there is a fault or imbalance in the power grid, so it cannot provide an effective reference for power grid regulation. Summary of the invention

[0003] Purpose of the invention: In view of the above problems, the present invention proposes a frequency-locked loop and a grid status tracking method based on a discrete three-phase second-order generalized integrator, which can improve the accuracy of tracking the positive and negative sequence components of the three-phase grid frequency and the voltage fundamental wave, and judge the real-time situation of the grid through the positive and negative sequence components of the fundamental wave, thereby solving the problem of how to track the status of the three-phase grid in real time and accurately during the energy storage inversion process.

[0004] Technical solution: The technical solution adopted by the present invention is a frequency-locked loop based on a discrete three-phase second-order generalized integrator, comprising a Clark converter, two subtractors, two discrete second-order generalized integrators, a discrete frequency-locked loop with normalization, a positive / negative sequence calculator, and a Clark inverse converter; the Clark converter is used to convert the three-phase sequence voltage from a stationary reference system to an αβ two-phase orthogonal voltage in a rotating reference system; the two discrete second-order generalized integrators are used as pre-filters for the αβ two-phase orthogonal voltage to obtain two pairs of mutually orthogonal sequence fundamental wave signals in two directions parallel to the reference axis of the rotating reference system; the subtractor is used to convert the three-phase sequence voltage from a stationary reference system to an αβ two-phase orthogonal voltage in a rotating reference system. The αβ two-phase orthogonal voltages are respectively subtracted from the filtered sequence fundamental signal to obtain two groups of error signals; the normalized discrete frequency locked loop is used to multiply the error signals in the αβ directions by the corresponding sequence fundamental signals in the orthogonal directions and then add them to obtain frequency error signals, and the frequency error signals are normalized and then passed through the accumulator of the negative gain coefficient to obtain the estimated frequency, and the estimated frequency is passed as the center frequency to the two discrete second-order generalized integrators to form a closed-loop estimation of the frequency signal; the positive / negative sequence calculator is used to calculate the positive and negative sequence components of the sequence fundamental signal, and then obtain the positive and negative sequence components of the three-phase voltage fundamental through the Clark inverse transformer.

[0005] Among them, the closed-loop state space expressions of the discrete second-order generalized integrator are:

[0006]

[0007] in It corresponds to the two orthogonal sequence fundamental signals observed by the second-order generalized integrator at the kth moment. The superscript k+1 represents the k+1th moment, ω represents the estimated frequency transmitted from the outside, and T s is the sampling period, k op is the open-loop gain coefficient, u k Represents the state space input sequence signal at the kth moment.

[0008] The positive / negative sequence calculator calculates the positive and negative sequence components of the sequence fundamental signal, and the expression is:

[0009]

[0010]

[0011] In the formula, and Represents the fundamental voltage of positive and negative sequence in the αβ reference frame, v α It represents the fundamental voltage sequence signal parallel to the α reference axis after being filtered by the discrete second-order generalized integrator on the α reference axis, qv αIt represents the fundamental voltage sequence signal orthogonal to the α reference axis after being filtered by the discrete second-order generalized integrator on the α reference axis, v β It represents the fundamental voltage sequence signal parallel to the β reference axis after being filtered by the discrete second-order generalized integrator on the β reference axis, qv β It represents the fundamental voltage sequence signal which is orthogonal to the β reference axis after being filtered by a discrete second-order generalized integrator on the β reference axis.

[0012] The present invention also proposes a power grid state tracking method applied to the above-mentioned discrete three-phase second-order generalized integrator-based frequency-locked loop, comprising the following steps:

[0013] Step 1: Clark converter transforms the three-phase electricity from the stationary reference frame to the αβ two-phase orthogonal voltage in the rotating reference frame;

[0014] Step 2: A discrete second-order generalized integrator performs filtering processing on the αβ two-phase orthogonal voltages to obtain two pairs of mutually orthogonal filtered sequence fundamental wave signals in two directions of the reference axis of the parallel rotating reference system, and subtracts the αβ two-phase orthogonal voltages from the filtered sequence fundamental wave signals at the input end of the second-order generalized integrator to obtain two sets of error signals;

[0015] Step 3, the normalized discrete frequency locked loop multiplies the error signals in the αβ directions by the corresponding sequence fundamental signals in the orthogonal directions and then adds them to obtain a frequency error signal, and normalizes the frequency error signal and obtains the estimated frequency through an accumulator with a negative gain coefficient, so that the estimated frequency follows the fundamental frequency of the three-phase power grid;

[0016] Step 4, the normalized discrete frequency-locked loop transmits the estimated frequency to the discrete second-order generalized integrator as the center frequency, thereby forming a closed-loop estimation of the frequency signal;

[0017] Step 5, the positive / negative sequence calculator calculates the positive and negative sequence components of the sequence fundamental signal obtained in step 2 to obtain the positive and negative sequence components of the fundamental wave in the rotating reference frame, and then transforms the positive and negative sequence components of the fundamental wave in the rotating reference frame to the stationary three-phase reference frame through the Clark inverse transformer to obtain the positive and negative sequence components of the fundamental wave of the three-phase voltage.

[0018] The state space expression of the discrete second-order generalized integrator and the calculation formula of the positive / negative sequence calculator are the same as above. The above method can accurately estimate the frequency and the positive and negative sequence classification of the three-phase voltage, thereby realizing the observation and tracking of the fundamental wave of the abc three-phase voltage.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following advantages: The present invention improves the conventional continuous second-order generalized integrator, and proposes a discrete three-phase second-order generalized integrator frequency-locked loop based on the zero-pole matching method. The discrete three-phase second-order generalized integrator frequency-locked loop is first used to perform discretized pre-filtering in two directions perpendicular to the three-phase voltage rotating reference system, filter out high-order harmonic components, and obtain two pairs of mutually orthogonal fundamental component sequence signals. Then, the above two pairs of orthogonal sequence signals are comprehensively processed, and the frequency estimation signal is obtained by accumulating the discrete frequency-locked loop, and then passed to the pre-filter to form a closed-loop system for frequency estimation. The present invention not only tracks the fundamental frequency of the three-phase power grid more accurately, but also can obtain more accurate fundamental positive and negative sequence fundamental voltages under the stationary reference system, and judge the real-time situation of the power grid according to the positive and negative sequence components of the grid voltage fundamental, so as to observe whether the power grid has faults or imbalances, and allow energy storage to make coordinated responses to power grid systems in different states. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of Clark transformation;

[0021] Figure 2 It is a continuous state adaptive prefilter based on a second-order generalized integrator;

[0022] Figure 3 The system state diagram of the discrete second-order generalized integrator closed-loop controller of the present invention is

[0023] Figure 4 It is a system structure block diagram of the discrete single-phase second-order generalized integrator frequency-locked loop described in the present invention;

[0024] Figure 5 It is a structural block diagram of the discrete three-phase second-order generalized integrator frequency-locked loop system of the present invention;

[0025] Figure 6 It is a flow chart of a method for tracking power grid status based on a frequency-locked loop of a discrete three-phase second-order generalized integrator according to the present invention;

[0026] Figure 7 It is an input waveform diagram of the discrete three-phase second-order generalized integrator frequency-locked loop described in the present invention;

[0027] Figure 8 It is the FFT analysis of phase A voltage after the fault;

[0028] Fig. 9 is the output three-phase positive and negative sequence voltage and estimated frequency of the discrete three-phase second-order generalized integrator frequency-locked loop;

[0029] Fig.10 It is the FFT analysis of the output positive sequence voltage of the discrete three-phase second-order generalized integrator frequency-locked loop after the fault;

[0030] Fig.11 It is the FFT analysis of the output negative sequence voltage of the discrete three-phase second-order generalized integrator frequency-locked loop after the fault; DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0032] The frequency locked loop based on discrete three-phase second-order generalized integrator of the present invention has a system structure diagram as shown in FIG. Figure 5 As shown, it includes a Clark converter, two discrete second-order generalized integrator pre-filters in the direction of parallel rotating reference axis, a discrete frequency locked loop system with normalization, positive and negative sequence calculators, and a Clark inverse converter.

[0033] The entire three-phase second-order generalized integrator frequency-locked loop discrete system projects the collected three-phase voltage of the power grid from the stationary reference frame to the rotating reference frame, and then filters it with a discrete pre-filter in its two vertical directions to filter out the high-order harmonic components, and obtains two pairs of mutually orthogonal fundamental wave component sequence signals in the two vertical directions parallel to the rotating reference axis. When the above two pairs of orthogonal sequence signals are comprehensively processed, the discrete frequency-locked loop is accumulated after normalization to obtain the frequency estimation signal, and then the pre-filter is sent to form a closed-loop system for frequency estimation. After further calculation, the two pairs of orthogonal fundamental wave signals can obtain the positive and negative sequence components of the fundamental wave in the three-phase stationary reference frame, thereby judging the real-time status of the power grid, so that the energy storage system can respond to the power grid under different conditions.

[0034] The grid state tracking method applied to the above frequency locked loop according to the present invention has a flow chart as shown in FIG. Figure 6 As shown, the following steps are included:

[0035] Step 1, in view of the property that the sampled three-phase sequence voltage has a phase difference of 120° between the three-phase electricity abc, the present invention transforms the three-phase electricity abc from a stationary reference system to an αβ two-phase orthogonal voltage in a rotating reference system based on the property of Clark transformation, which can reduce the number of pre-filters for processing the three-phase electricity in the filtering link. At the same time, the converted two-phase orthogonal voltage can still contain all the information of the original three-phase voltage, and does not affect the accuracy of the final frequency estimation.

[0036] For an ordinary single-phase second-order generalized integrator frequency-locked loop, it can only act on a single-phase circuit to estimate its frequency. However, for a three-phase circuit, that is, the voltage is composed of three sinusoidal voltages with a phase difference of 120°, the single-phase second-order generalized integrator frequency-locked loop cannot be directly used for frequency estimation. Therefore, it is necessary to use Clark transform to process the sampled three-phase voltage. Figure 1 , the three-phase voltage forms a space phasor In the abc reference system, the amplitude is used as the radius and the circular motion is performed at an angular frequency of 50 Hz. The α axis is parallel to the a axis, and the β axis is 90° ahead of the α axis. A rotating reference system is established that also rotates at 50 Hz. Decomposed on the a-axis and β-axis, we get two rotating voltage phasors v a and v β Then the pair of orthogonal rotating components contains all the information of the original orthogonal three-phase voltage, and only the components in the two rotating directions need to be pre-filtered, which reduces the use of filters and improves the accuracy of filtering compared to direct three-phase filtering. The basic expression of Clark transformation is:

[0037]

[0038] Where v abc represents the three-phase electricity in the abc reference system, v αβ represents the three-phase electricity in the αβ reference frame, and the matrix [T αβ ] is the Clark transformation matrix.

[0039] Step 2, the present invention performs filtering processing on the αβ two-phase orthogonal voltage transformed into the rotating reference system, and discretizes the continuous second-order generalized integral filter as a pre-filter for the αβ two-phase orthogonal voltage by means of the zero-pole matching method, and obtains two pairs of mutually orthogonal filtered output sequence signals in two directions parallel to the reference axis of the rotating reference system, and separates two groups of error signals at the input end of the second-order generalized integrator.

[0040] The rotating voltage phasor v is respectively calculated by using a discrete second-order generalized integrator. a and v β Pre-filtering is performed. The traditional continuous second-order generalized integrator is Figure 2 , whose expression is

[0041]

[0042] The transfer function of the two orthogonal output signals of a continuous second-order generalized integrator filter is:

[0043]

[0044]

[0045] Where v represents the input signal, v' and qv' represent the orthogonal output signals, ε v represents the error signal, ω' is the estimated frequency of the input signal passed from the frequency locked loop, s is the Laplace transform operator, and k is the gain coefficient.

[0046] The traditional continuous second-order generalized integrator is realized by combining two integrators. However, for the actual signal processing chip, a discrete second-order generalized integrator model needs to be used for programming. The continuous second-order generalized integrator is discretized by zero-pole matching to obtain its discretized model as shown in the figure below. Figure 3 By calculating the closed-loop transfer function of the discrete second-order generalized integrator, we can obtain Figure 3 The corresponding discrete second-order generalized integrator transfer function expression is:

[0047]

[0048] Where U represents the input signal, Y1 represents the output signal, T s is the sampling period, k is the open-loop gain coefficient, z represents the operator of z-transform, and ω represents the estimated frequency transmitted from the outside.

[0049] Using the above discrete second-order generalized integrator transfer function expression, design and establish its corresponding open-loop and closed-loop state space expressions in the time domain:

[0050]

[0051]

[0052] Wherein equation (a.6) is the open-loop state space expression, and equation (a.7) is the closed-loop state space expression. It corresponds to the two orthogonal sequence fundamental signals observed by the second-order generalized integrator at the kth moment. The superscript k+1 represents the k+1th moment, ω represents the estimated frequency transmitted from the outside, and T s is the sampling period, k op is the open-loop gain coefficient, and u represents the state space input sequence signal.

[0053] Using the above discrete second-order generalized integrator state space expression, the sequence voltage signal on one axis can be filtered and a pair of filtered fundamental orthogonal sequence signals v and qv can be obtained. Replace the corresponding state space expression with Figure 3 The discrete second-order generalized integrator based on the zero-pole matching method and connected to a discrete accumulator forms Figure 4 Discrete single-phase second-order generalized integrator frequency-locked loop system, and the frequency-locked loop part of the three-phase discrete second-order generalized integrator frequency-locked loop needs to be further improved and designed to match the unique frequency of the three-phase power grid. At the same time, the frequency-locked loop part of the three-phase discrete second-order generalized integrator frequency-locked loop needs to assume two discrete second-order generalized integrators represented in two state spaces on the two axes of the rotating reference system, so as to obtain two pairs of filtered fundamental orthogonal sequence signals based on the two rotating reference axis directions, and at the same time, the error at the input end is discretized into a sequence.

[0054] In step 3, the present invention normalizes the filtered output sequence signal obtained in step 2, the filtered output sequence fundamental signal and the error signal, so that the frequency-locked loop system can adapt to three-phase circuits with different amplitudes. The normalized signal is used to calculate the error signal of the generated frequency, and the deviation between the estimated frequency and the actual frequency is set to zero through a discrete accumulator, so that the estimated frequency follows the fundamental frequency of the three-phase power grid.

[0055] The filtered output sequence signal obtained in step 2 is normalized with the filtered output sequence fundamental signal and the error signal, and the error signal in the two axis directions is multiplied by the corresponding orthogonal signal and then added to obtain the frequency error signal. Since the frequency error signal contains the amplitude information of the three-phase voltage, it will be additionally affected by the amplitude change. Therefore, it is necessary to divide the obtained frequency error signal by the amplitude signal of the three-phase voltage, and eliminate the influence of the introduced parameters in the discrete second-order generalized integrator in step 2, so as to obtain the normalized frequency error signal, which can be applied to the frequency estimation of the three-phase circuit with different voltage amplitudes. The normalized frequency error signal is passed to an accumulator with a negative gain coefficient, replacing the continuous state integrator, and the deviation between the estimated frequency and the actual frequency is set to zero, so that the estimated frequency follows the fundamental frequency of the three-phase power grid, thereby realizing unbiased estimation of the frequency.

[0056] Step 4, the present invention passes the sequence signal of the estimated frequency obtained in step 3 to the two second-order generalized integrator pre-filters in step 2 as the center frequency, forming a closed-loop estimation of the frequency signal.

[0057] The sequence signal of the estimated frequency obtained in step 3 is passed to the two discrete second-order generalized integrator prefilters in step 2 as the center frequency to form a closed-loop estimation of the frequency signal. Because the signal passed into the two second-order generalized integrator prefilters is the same frequency estimation sequence signal, the filtering characteristics of the two discrete second-order generalized integrators are exactly the same, so the closed-loop estimation of the frequency will not fail to converge due to the relative error of the filter, and the uniqueness of the estimated frequency tracking can be guaranteed.

[0058] Step 5, the present invention further calculates the filtered fundamental wave output sequence signal obtained in step 2 to obtain the positive and negative sequence components of the fundamental wave in the rotating reference system, and then transforms the positive and negative sequence components of the fundamental wave in the rotating reference system to the stationary three-phase reference system through Clark inverse transformation, thereby realizing the observation and tracking of the fundamental wave of the abc three-phase voltage.

[0059] In the above step 5, the two groups of orthogonal fundamental wave output sequence signals obtained in step 2 after filtering are passed to the positive and negative voltage calculator (PSCN). According to the basic relationship between the three-sequence voltage components and the Clark transformation matrix of formula (a.1), the following expression can be calculated:

[0060]

[0061]

[0062] In the formula It is a phase-shift time-domain operator used to represent an inverse quadrature version (90° delayed) of the original output. and represents the fundamental voltage of positive and negative sequence in the αβ reference frame, and Represents the fundamental voltage of positive sequence and negative sequence in the abc reference system, T αβ To transform the abc reference system into the αβ reference system, the Clark coefficient matrix [T + ] and [T - ] represents the coefficient matrix of positive sequence and negative sequence separated from the three-sequence voltage in the αβ reference frame.

[0063] Through the calculation of the above formula, the fundamental voltage of the positive sequence and negative sequence in the rotating reference frame can be obtained. Further through the Clark inverse transform, the fundamental voltage of the fundamental positive sequence and negative sequence in the stationary reference frame can be obtained, so as to observe whether there is a fault or imbalance in the power grid.

[0064] The present invention uses the S-function system written in C language in simulink-matlab to perform simulation tests to test the performance and design effectiveness of the system proposed in the present invention. The simulation system consists of three parts: a fault power input part, a discrete three-phase second-order generalized integrator frequency-locked loop implemented by the S-function in C language, and an estimated frequency and fundamental positive and negative sequence voltage display.

[0065] The overall sampling time of this simulation experiment is set to 1 microsecond. For the fault power input part, because it is necessary to introduce the known positive and negative sequence unbalanced components of the fault voltage, the transformation of high-order harmonics and the fault grid frequency to reflect the unbalanced state of the three-phase grid fault, so as to test the various performance indicators of the discrete three-phase second-order generalized integrator frequency-locked loop, it is necessary to set a sequence component method fault power supply as the simulated three-phase power input port. For the fault component of the general three-phase grid, because the third harmonic component does not exist in the line voltage, and the highest content of the high-order harmonic is the fifth harmonic. Therefore, the fault input voltage of this simulation experiment is set to a voltage amplitude of 220Vrms and a fundamental frequency of 50Hz for the three-phase voltage. Before the fault occurs, the output three-phase voltage is set to have only a positive sequence voltage, whose size is U0=0.943pu (the sum of the positive and negative sequence voltages after the fault) and a frequency of 50Hz. When t=0.1s, the grid fails, and the voltage at the common coupling point is unbalanced and waveform distorted. The positive sequence voltage of the fault is set to u + =0.733pu, the fault negative sequence voltage is u - =0.21pu, and the fault causes the positive and negative sequence voltages to generate 3.71% of the fifth harmonic component, and the frequency of the common coupling point rises from 50Hz to 60Hz. Then the input voltage waveform from t=0 to t=0.2s is as follows Figure 7 .

[0066] from Figure 7 It can be directly observed that the three-phase voltage waveform before the fault is normal, the amplitude is stable at about 293V, the three-order is symmetrical, and there is no obvious high-order harmonics and distortion. When the fault occurs, the three-phase power has an obvious asymmetric fault, in which the voltage amplitude of one phase increases significantly, and the voltage amplitudes of the other two phases decrease. And because the fifth harmonic is introduced, the three-phase power has peak waves and flat-top waves, indicating that the waveform has been seriously distorted. FFT analysis is performed on phase A (voltage increase phase) after the fault rises, and the result is Figure 8 .from Figure 8 It can be seen that the fundamental frequency is 60Hz, and the main harmonic component in the FFT analysis is the 5th harmonic, with a content of 3.71%, which is in line with the harmonic setting of the sequence component power supply.

[0067] For the discrete three-phase second-order generalized integrator frequency-locked loop implemented by the S-function of C language, in the corresponding C language file of S-function, set the open-loop gain coefficient k=1.414 of the three-phase discrete second-order generalized integrator and the negative gain coefficient -Γ=-50 of the discrete frequency-locked loop. At the same time, set the number of input port bits of the internal system to 3, representing the input three-phase abc voltage signal sequence, and the number of direct feedthrough channels to 1, indicating the direct influence of the voltage signal on the output. Set the number of output port bits of the system to 7, and output the positive and negative sequence components U of the three-phase fundamental voltage. abc+ and U abc- , and the estimated frequency signal. Set the number of internal state variables to 5, representing the two sets of internal orthogonal state signal sequences and the state signal sequence of the estimated frequency. The two sets of orthogonal state signals are updated in the iteration of the S-function, while the state signal sequence of the estimated frequency is processed by error accumulation in the iteration of the S-function, completing the approximate integration of the error frequency to obtain the estimated frequency.

[0068] The output estimated frequency and the fundamental positive and negative sequence voltage display are set as a three-layer interface, which respectively displays the positive and negative sequence components U of the three-phase voltage fundamental wave. abc+ and U abc- , and the estimated frequency signal f. Start the entire simulation system and get the output signal display image as follows Fig. 9 . It can be observed that the filter completed the startup after 0.04s, and the entire system output only positive sequence voltage, and its peak value is about 293.1V, which meets the input voltage setting before the fault. At the same time, the frequency f is also stable at 50Hz. When the fault occurs, the positive sequence voltage peak is reduced to about 228.2V, and the negative sequence voltage peak is increased to 65.1V, while the input positive sequence voltage is 0.733*311=228.0V, and the negative sequence voltage is 0.210*311=65.3V, which is basically error-free, and its estimation of positive and negative sequence voltages is accurate. In addition, the three phases in the waveform are completely symmetrical and the distortion is significantly reduced compared to the input voltage. It can be considered that the fifth harmonic filtering characteristics are good. The estimated frequency f of its output, after a rise time of about 0.02s after the fault occurs, rises from 50Hz to 60Hz and oscillates back and forth around 60Hz, and its oscillation range is about 60±0.1Hz. The analysis shows that the fifth harmonic is the cause, which causes the superposition of high-order frequency oscillations on the 60Hz base frequency. So the fifth harmonic of the fault is set to zero, and the output frequency can be stabilized to 60Hz, proving that the above analysis that the fifth harmonic is the cause is correct.

[0069] Perform FFT analysis on the output positive and negative sequence voltages to obtain the following Fig.10 and Fig.11. It can be observed from the figure that the total harmonic distortion is 3.71% for both positive and negative sequence inputs. After filtering by the discrete second-order generalized integrator frequency-locked loop, the total harmonic distortion of the positive sequence three-phase voltage is reduced to 0.56%, and the total harmonic distortion of the negative sequence three-phase voltage is reduced to 0.32%. The discrete second-order generalized integrator frequency-locked loop reduces the fifth harmonic of the positive sequence to 15.09% of its input after filtering, and reduces the fifth harmonic of the negative sequence to 8.63% of its input after filtering. Therefore, it is believed that the discrete second-order generalized integrator frequency-locked loop has a good filtering effect on high-order harmonics.

[0070] Summarizing the analysis of the output waveform and frequency in the above simulation experiment, it can be obtained that when the sampling frequency of the entire discrete system is 1 microsecond, the simulink connected to the three-sequence three-phase voltage source is simulated and verified. For the total harmonic distortion of 3.71% for both positive and negative sequences, after filtering by the discrete second-order generalized integrator frequency-locked loop, the total harmonic distortion of the positive sequence three-phase voltage is reduced to 0.56%, and the total harmonic distortion of the negative sequence three-phase voltage is reduced to 0.32%, indicating that its filtering performance is good. For the fault positive and negative sequence voltage components set by the three-sequence power supply, the discrete frequency-locked loop basically achieves error-free estimation and meets the requirements of accurately estimating the positive and negative sequence components of the power grid. Finally, the estimated frequency f of its output, after about one cycle of rise time after the fault occurs, rises from 50Hz to 60Hz and oscillates back and forth around 60Hz, and its oscillation range is around 60±0.1Hz, indicating that it has high steady-state accuracy and good dynamic performance. Finally, the feasibility of this discretization method and the excellent performance of the C language discrete three-phase second-order generalized integrator are proved.

[0071] In summary, for the measurement of the estimated frequency, the discrete three-phase second-order generalized integrator frequency-locked loop can almost achieve zero-static tracking when there is no influence of higher-order harmonics. When higher-order harmonics are introduced, the oscillation of the estimated frequency is also within an acceptable range, so it can be considered that it has a relatively high-precision steady-state performance. When the frequency changes, the estimated frequency can quickly achieve stable tracking, so it is considered that the discrete three-phase second-order generalized integrator frequency-locked loop has good dynamic performance. For its filtering characteristics, the fifth harmonic of the maximum component of a general fault can be filtered to less than 15% of the original, so it is considered that the filtering characteristics of the discrete second-order generalized integrator frequency-locked loop are good. Therefore, the simulation results prove that the discrete three-phase second-order generalized integrator frequency-locked loop system designed by the present invention can well complete its filtering and frequency tracking effects, and has practical application significance. At the same time, the present invention can also efficiently and accurately obtain the frequency conversion of the three-phase power grid and the positive and negative components of the voltage fundamental wave, thereby providing accurate information for the rapid response of the energy storage system.

[0072] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0073] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0074] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

Claims

1. A frequency-locked loop based on a discrete three-phase second-order generalized integrator, characterized in that: The invention comprises a Clark converter, two subtractors, two discrete second-order generalized integrators, a discrete frequency-locked loop with normalization, a positive / negative sequence calculator and a Clark inverse converter; the Clark converter is used to convert a three-phase sequence voltage into an αβ two-phase orthogonal voltage U under a two-phase stationary reference system. α and U β The two discrete second-order generalized integrators are used as pre-filters for the αβ two-phase orthogonal voltage, and a pair of mutually orthogonal sequence fundamental wave signals are obtained in the two directions of the reference axis of the parallel two-phase stationary reference system, that is, a pair of orthogonal sequence fundamental wave signals v are obtained in the α-axis direction. α and qv α , we get a pair of orthogonal sequence fundamental wave signals v in the β-axis direction α and qv α ;v α It represents the fundamental signal of the sequence parallel to the α reference axis after being filtered by the discrete second-order generalized integrator on the α reference axis, qv α represents the fundamental signal of the sequence orthogonal to the α reference axis after being filtered by a discrete second-order generalized integrator on the α reference axis, v β It represents the fundamental signal of the sequence parallel to the β reference axis after being filtered by the discrete second-order generalized integrator on the β reference axis, qv β represents the sequence fundamental signal orthogonal to the β reference axis after filtering by the discrete second-order generalized integrator on the β reference axis; the subtractor is used to subtract the αβ two-phase orthogonal voltage under the two-phase stationary reference system from the filtered sequence fundamental signal, respectively, to obtain two sets of error signals Ev α and Ev β ; Ev α is the two-phase orthogonal voltage U α And the two-phase orthogonal voltage U α The filtered sequence fundamental signal v α The error signal obtained by subtraction, Ev β is the two-phase orthogonal voltage U β And the two-phase orthogonal voltage U β The filtered sequence fundamental signal v β The error signal is obtained by subtracting the two sets of error signals; the two sets of error signals are respectively used as input signals of two discrete second-order generalized integrators; the discrete frequency-locked loop with normalization is used to obtain the frequency error signal by multiplying the error signals in the αβ directions with the corresponding sequence fundamental wave signals in the orthogonal directions respectively, and then adding them, and the frequency error signal is normalized and then passed through the accumulator of the negative gain coefficient to obtain the estimated frequency, and the estimated frequency is passed as the center frequency to the two discrete second-order generalized integrators to form a closed-loop estimation of the frequency signal; the product of the error signals in the αβ directions with the corresponding sequence fundamental wave signals in the orthogonal directions is: the error signal Ev in the α direction is α The corresponding sequence fundamental signal qv in the orthogonal direction α Multiply the error signal Ev in the β direction β The corresponding sequence fundamental signal qv in the orthogonal direction β The positive / negative sequence calculator is used to calculate the positive and negative sequence components of the sequence fundamental signal, and then obtain the positive and negative sequence components of the three-phase voltage fundamental through the Clark inverse converter; The open-loop and closed-loop state space expressions of the discrete second-order generalized integrator are: in, It corresponds to the two orthogonal sequence fundamental signals observed by the second-order generalized integrator at the kth moment. The superscript k+1 represents the k+1th moment, ω represents the estimated frequency transmitted from the outside, and T s is the sampling period, k op is the open-loop gain coefficient, u k Represents the state space input sequence signal at the kth moment.

2. The frequency locked loop based on discrete three-phase second-order generalized integrator according to claim 1, characterized in that: The positive / negative sequence calculator calculates the positive and negative sequence components of the sequence fundamental signal, and the expression is: In the formula, and Represents the fundamental voltage of the positive and negative sequences in the αβ reference frame.

3. A method for tracking power grid status using the frequency-locked loop of claim 1, applied to a three-phase power grid, characterized in that: The following steps are involved: Step 1: Clark converter transforms the three-phase sequence voltage into the αβ two-phase orthogonal voltage U in the two-phase stationary reference frame. α and U β ; Step 2: The discrete second-order generalized integrator performs filtering on the αβ two-phase orthogonal voltage, and obtains a pair of mutually orthogonal filtered sequence fundamental wave signals in the two directions of the reference axis of the parallel two-phase stationary reference system, that is, a pair of orthogonal sequence fundamental wave signals v are obtained in the α-axis direction. α and qv α , we get a pair of orthogonal sequence fundamental wave signals v in the β-axis direction α and qv α ;v α It represents the fundamental signal of the sequence parallel to the α reference axis after being filtered by the discrete second-order generalized integrator on the α reference axis, qv α represents the fundamental signal of the sequence orthogonal to the α reference axis after being filtered by a discrete second-order generalized integrator on the α reference axis, v β It represents the fundamental signal of the sequence parallel to the β reference axis after being filtered by the discrete second-order generalized integrator on the β reference axis, qv β It represents the sequence fundamental signal orthogonal to the β reference axis after filtering by the discrete second-order generalized integrator on the β reference axis; and at the input end of the second-order generalized integrator, the αβ two-phase orthogonal voltage is subtracted from the filtered sequence fundamental signal to obtain two sets of error signals Ev α and Ev β ; Ev α is the two-phase orthogonal voltage U α And the two-phase orthogonal voltage U α The filtered sequence fundamental signal v α The error signal obtained by subtraction, Ev β is the two-phase orthogonal voltage U β And the two-phase orthogonal voltage U β The filtered sequence fundamental signal v β subtract the error signals obtained; use the two sets of error signals as input signals of two discrete second-order generalized integrators respectively; Step 3, the normalized discrete frequency locked loop multiplies the error signals in the αβ directions with the corresponding sequence fundamental signals in the orthogonal directions respectively and then adds them to obtain a frequency error signal, and normalizes the frequency error signal and obtains an estimated frequency through an accumulator with a negative gain coefficient, so that the estimated frequency follows the fundamental frequency of the three-phase power grid; the multiplication of the error signals in the αβ directions with the corresponding sequence fundamental signals in the orthogonal directions respectively is: the error signal Ev in the α direction is α The corresponding sequence fundamental signal qv in the orthogonal direction α Multiply the error signal Ev in the β direction β The corresponding sequence fundamental signal qv in the orthogonal direction β multiply; Step 4, the normalized discrete frequency-locked loop transmits the estimated frequency to the discrete second-order generalized integrator as the center frequency, thereby forming a closed-loop estimation of the frequency signal; Step 5, the positive / negative sequence calculator calculates the positive and negative sequence components of the sequence fundamental signal obtained in step 2 to obtain the positive and negative sequence components of the fundamental wave in the rotating reference frame, and then transforms the positive and negative sequence components of the fundamental wave in the rotating reference frame to the stationary three-phase reference frame through the Clark inverse transformer to obtain the positive and negative sequence components of the fundamental wave of the three-phase voltage.

4. The power grid status tracking method according to claim 3, characterized in that: The positive / negative sequence calculator calculates the positive and negative sequence components of the sequence fundamental signal, and the expression is: In the formula, and Represents the fundamental voltage of the positive and negative sequences in the αβ reference frame.

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