CN107064646B - Power grid impedance real-time identification method for multi-input multi-output asymmetric system - Google Patents
Power grid impedance real-time identification method for multi-input multi-output asymmetric system Download PDFInfo
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Abstract
The invention discloses a real-time identification method for power grid impedance of a multi-input multi-output asymmetric system, which comprises the following steps: obtaining a three-phase network access current of a PCC point through a current sensor; respectively collecting three-phase current i in DSPa,ib,icCarrying out coordinate transformation to obtain current under a two-phase rotating coordinate; generating an orthogonal binary sequence having two signals; injecting the orthogonal binary sequence into reference currents of a d axis and a q axis of the power grid current through a digital obtaining unit; measuring and decomposing the grid voltage and current frequency responses into positive sequence and negative sequence components; and calculating to obtain the power grid impedance. The problem of the prior art in the off-line stability of discerning not enough is solved to measuring time has been shortened.
Description
Technical Field
The invention belongs to the technical field of power electronics, and relates to a real-time impedance identification method for a multi-input multi-output system.
Background
The real-time identification of the impedance can be roughly divided into a parametric method and a non-parametric method, and the non-parametric frequency domain method is widely applied to the identification and stability analysis of the impedance of the power grid. Unlike parametric methods, non-parametric methods require that a system model be assumed and only one excitation be selected. The excitation is injected into the grid and used to identify the grid impedance. In steady state, the current and voltage responses are measured and fourier transform analysis is performed.
With climate change and fossil fuel limitations, distributed generation is becoming more popular, and the mismatch between inverter output impedance and grid impedance, resulting in harmonic response between the two, is a major problem. Studies in recent years have shown that unstable situations can be avoided by measuring the impedances of the grid and the inverter, and that the design parameters of the inverter can be changed on the basis of these parameters. According to the identification technology, the method can be divided into off-line identification and on-line identification, and the off-line impedance identification cannot ensure the stability of the system because the impedance of the power grid and the inverter changes in real time. Recent research is also increasingly moving towards real-time impedance identification, which is faster than offline impedance identification. Accordingly, there is a need for improvements in the art.
Disclosure of Invention
The invention provides a real-time impedance identification method based on a multi-input multi-output system, which solves the problem of insufficient stability of off-line identification in the prior art and shortens the measurement time.
In order to achieve the technical purpose and achieve the technical effects, the invention is realized by the following technical scheme.
Step 2, three-phase current i collected in DSPa，ib，icCoordinate transformation is carried out to obtain current i under two-phase rotating coordinatesd,iq：
Step 3, generating an orthogonal binary sequence with two signals:
generation of raw signal (d-axis injection signal): DIBS signal amplitude is switched between two values of 1 and-1, let Ab (t) be a periodic real binary sequence. A amplitude, then the Fourier transform expression of its k harmonic is
The DIBS sequence signal is designed to be defined as:
and solving the optimization problem by means of Matlab simulation software, and storing the sequence under the specific frequency in the DSP in a code table form so as to be called when in injection.
Generation of q-axis injection reference signal sequence: the addition of the modulo-2 sequence 01010 … … to the original signal forms a perturbation of the q-axis input reference signal, which is stored in the DSP in the form of a code table for injection-time invocation.
Step 4, simultaneously injecting the orthogonal binary sequence into reference currents of a d axis and a q axis of the power grid current through a digital acquisition unit;
step 5, measuring the frequency response of the voltage and the current of the power grid and decomposing the frequency response into positive sequence components and negative sequence components, wherein the decomposition formula is as follows:
decomposition of current response:
wherein p, n represent positive and negative sequences, respectively, and a ═ ej2π/3
Because i isc＝-ia-ibIntroducing a matrix:
then there are:
decomposition of voltage response:
by vca＝-vab-vbcIntroducing a matrix:
thus, the positive and negative sequence of the voltage component can be simplified as:
and step 6, for positive and negative sequence coupled impedance components, the following steps are included:
obtaining the impedance of the power grid:
wherein x and y represent the response of two orthogonal binary sequence injections respectively.
The invention has the beneficial effects that: (1) the problem that the stability of the system cannot be guaranteed by the conventional offline identification is solved; (2) the design is reasonable, the usability is strong, and the operation is simple; (3) compared with the traditional single-input/single-output measurement technology, the method overcomes the defect of system dynamic characteristic change; the measurement time is shortened.
Drawings
FIG. 1 is a block diagram of a real-time impedance identification device.
FIG. 2 is a flowchart illustrating steps of an impedance identification method.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and examples.
As shown in fig. 1, a direct current voltage source passes through a three-phase grid-connected inverter, converts direct current into three-phase alternating current, passes through a filtering device, and is then incorporated into a power grid, a current sensor acquires network access current information, and a voltage sensor acquires network access voltage information.
The identification method of the invention transforms the acquired network access current into the current magnitude under the dq coordinate through carrying out coordinate transformation, simultaneously injects an orthogonal binary sequence into the reference current positions of d and q axes through a DSP, analyzes the current and voltage response of the power grid, respectively decomposes the current and voltage response into positive and negative sequence components, and obtains the impedance of the power grid according to the positive and negative sequence components of the current and voltage response.
Referring to fig. 2, the identification process of the present invention is detailed as follows:
Step 2, collecting three-phase current i in the DSPa,ib,icCoordinate transformation is carried out to obtain current i under two-phase rotating coordinatesd,iq；
Step 3, generating an orthogonal binary sequence with two signals:
generation of raw signal (d-axis injection signal): DIBS signal amplitude is switched between two values of 1 and-1, let Ab (t) be a periodic real binary sequence. And A is the amplitude, the Fourier transform expression of the k-th harmonic is as follows:
the DIBS sequence signal is designed to be defined as:
and solving the optimization problem by means of Matlab simulation software, and storing the sequence under the specific frequency in the DSP in a code table form so as to be called when in injection.
Generation of q-axis injection reference signal sequence: the addition of the modulo-2 sequence 01010 … … to the original signal forms a perturbation of the q-axis input reference signal, which is stored in the DSP in the form of a code table for injection-time invocation.
Step 4, simultaneously injecting the orthogonal binary sequence into reference currents of a d axis and a q axis of the power grid through a digital acquisition unit;
step 5, measuring the frequency response of the voltage and the current of the power grid and decomposing the frequency response into positive sequence components and negative sequence components (assuming that the zero sequence components of the three-phase inverter are infinite, namely the power grid connection mode is a three-wire system), wherein the decomposition formula is as follows:
decomposition of current response:
p and n respectively represent positive sequence and negative sequence;
because i isc＝-ia-ibIntroducing a matrix:
then there are:
decomposition of voltage response:
by vca＝-vab-vbcIntroducing a matrix:
thus, the positive and negative sequence of the voltage component can be simplified as:
and step 6, for positive and negative sequence coupled impedance components, the following steps are included:
obtaining the impedance of the power grid:
wherein x and y represent the response of two orthogonal binary sequence injections respectively.
The foregoing embodiments and description have been presented only to illustrate the principles and preferred embodiments of the invention, and various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (1)
1. A real-time identification method for power grid impedance of a multi-input multi-output asymmetric system is disclosed, wherein a direct current voltage source passes through an inverter and then is merged into a PCC point after passing through a filtering device, and the real-time identification method for the power grid impedance comprises the following steps:
step 1, obtaining a PCC point three-phase network access current i through a current sensora,ib,ic；
Step 2, respectively collecting three-phase network access current i in the DSPa,ib,icSit onStandard transformation to obtain current i under two-phase rotation coordinatedq0；
Step 3, generating an orthogonal binary sequence with two signals: generation of the original signal, i.e. the d-axis injection signal: the DIBS sequence signal amplitude value is switched between 1 and-1, and Ab (t) is a periodic real binary sequence; and A is the amplitude, the Fourier transform expression of the k-th harmonic is as follows:
the DIBS sequence signal is designed to be defined as:
solving the optimization problem by means of Matlab simulation software, and storing a sequence under a specific frequency in a DSP in a form of a code table so as to be called during injection;
generation of q-axis injection reference signal sequence: adding a modulus 2 sequence to an original signal to form disturbance of a q-axis input reference signal, and storing the disturbance in a DSP in a code table form so as to be called during injection;
the modulo-2 sequence is a "01" cyclic sequence;
step 4, simultaneously injecting the orthogonal binary sequence into reference currents of a d axis and a q axis of the power grid current through a digital acquisition unit;
and 5, measuring the frequency response of the voltage and the current of the power grid and decomposing the frequency response into a positive sequence component decomposition formula and a negative sequence component decomposition formula as follows:
decomposition of current response:
wherein p, n represent positive and negative sequences, respectively, and a ═ ej2π/3；
Because i isc＝-ia-ibIntroducing a matrix:
then there are:
decomposition of voltage response:
by vca＝-vab-vbcIntroducing a matrix
Thus, the positive and negative sequence components of the voltage component can be simplified as:
step 6, for positive and negative sequence coupled impedance components, there are
Obtaining the impedance of the power grid:
wherein x and y represent the response of two orthogonal binary sequence injections respectively.
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Inventor after: Chen Yanming Inventor after: Wu Huifang Inventor after: Li Hong Inventor after: Ding Xin Inventor after: Ren Xiaotong Inventor after: Han Chao Inventor after: Yang Zaixue Inventor after: Yang Liulin Inventor after: Huang Hongquan Inventor after: Mo Like Inventor before: Li Hong Inventor before: Wu Huifang Inventor before: Ding Xin Inventor before: Ren Xiaotong Inventor before: Han Chao Inventor before: Yang Zaixue Inventor before: Yang Liulin Inventor before: Huang Hongquan Inventor before: Chen Yanming Inventor before: Mo Like |