CN117310293B - Power grid impedance measurement method and device - Google Patents
Power grid impedance measurement method and device Download PDFInfo
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Abstract
The invention discloses a power grid impedance measurement method and a device, wherein the method comprises the steps of generating a characteristic harmonic current instruction according to a harmonic signal generation algorithm, and inputting the characteristic harmonic current instruction into a direct-current voltage feedback control loop; receiving a characteristic harmonic current instruction to generate a control signal, and performing current tracking control to obtain a harmonic current to be injected; converting the information electronic signal of the main circuit into a driving signal of a device control loop, and selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency; when the active harmonic injection is selected, controlling the switching-on and switching-off of a switching tube, and injecting current into a power grid; when passive harmonic injection is selected, the on-off of the IGBT is controlled, the time and the current flowing direction of the capacitor put into the power grid are controlled, and harmonic current is injected into the power grid; and selecting the voltage and current signals to perform signal decomposition processing analysis to obtain the power grid impedance. The embodiment effectively measures the impedance of the power grid, reduces disturbance of the impedance measurement process to the power grid, and improves measurement accuracy.
Description
Technical Field
The invention relates to the technical field of electronic measurement, in particular to a power grid impedance measurement method and device.
Background
Because of the large amount of application of the power electronic equipment in the power grid, especially the access of the new energy grid-connected power generation device, the impedance characteristic of the power grid is more complex than before, and in order to ensure the stability of a power system, various grid-connected devices need to adjust control parameters based on the impedance characteristic of the power grid, so that the on-line measurement of the impedance of the power grid becomes particularly important. In a power system, the performance and resonance suppression effect of the grid-connected inverter are closely related to the impedance characteristic of the power grid, and the impedance characteristic of the power grid can change along with the change of the running state of the power grid, so that accurate power grid impedance measurement is a key technology for realizing high-performance self-adaptive control of the grid-connected inverter in weak power grid occasions.
The existing power grid impedance measurement method comprises a passive method and an active method, and the active method is adopted for more actual power grid impedance measurement because the passive method has large calculated amount and lower precision. The active method is a method for injecting characteristic harmonic into a power grid, measuring voltage and current of the characteristic harmonic at a point of common coupling between the power grid and a converter, and further analyzing impedance of the power grid. The common active power grid impedance measurement method comprises a switched capacitor method, a switched thyristor branch method, a harmonic current source injection method and the like, and the three methods have the characteristics of respective advantages and disadvantages.
The switched capacitor method for measuring the harmonic impedance by using the switched capacitor is to measure the harmonic impedance by using the harmonic current injected into the power grid when the switched capacitor of a transformer substation or a transformer substation, and special harmonic current injection equipment is not required to be arranged, and in the measuring process, waveform recording is only required to be carried out on the harmonic current voltage before and after the switched capacitor. Since the switched capacitor is a regular operation of the grid, the interference to the grid is small. However, since the parallel capacitor of the transformer substation is used, the capacity of the capacitor is set for reactive compensation parameters, the situation that the capacitor capacity is insufficient and the induced harmonic current is injected too little may occur, and the harmonic current injection with insufficient amplitude may cause the accuracy of the result of the harmonic impedance measurement to be reduced. The switching thyristor branch method can conveniently control the on time through the thyristor controlled branch, so that the magnitude of the injected harmonic current, namely the magnitude of impact on a network, can be controlled. Therefore, the optimal junction point can be obtained in two aspects, the harmonic impedance measurement precision is ensured, and the disturbance to the power grid is as small as possible. And when the network condition is changed, the measuring condition is changed by changing the on time of the thyristor at any time. The switched capacitor method and the switched thyristor branch method both use harmonic current injected at the time of switching of a capacitor branch or a thyristor branch as a basic condition for measurement, and have the defect of incomplete and uncontrollable harmonic content.
The harmonic current source injection method enables the harmonic current source to be controllable in each subharmonic frequency and amplitude, so that the phenomenon of inaccurate harmonic measurement caused by small harmonic current flow in certain harmonic ranges does not occur, and extremely high measurement accuracy is achieved. However, this method requires a specific harmonic current source, and it is difficult to allow a large amount of harmonic current to be injected into a large-sized power system in operation, which interferes with the stable operation of the system. Meanwhile, the harmonic current content of passive harmonic injection is large in difference in different frequency ranges and influences measurement accuracy, and active harmonic injection is used for generating large interference to a power grid and causing the voltage distortion rate of the power grid to be increased.
Disclosure of Invention
The invention provides a power grid impedance measurement method and device, which can effectively measure the power grid impedance, reduce disturbance of an impedance measurement process to the power grid and improve measurement accuracy.
In order to solve the above technical problems, an embodiment of the present invention provides a method for measuring impedance of a power grid, including:
when the power grid impedance measuring device and the power grid perform grid-connected stable operation, generating a characteristic harmonic current instruction according to a harmonic signal generating algorithm, and inputting the characteristic harmonic current instruction into a direct-current voltage feedback control loop; the direct-current voltage feedback control loop detects direct-current voltage in real time and keeps the voltage stable;
Receiving a characteristic harmonic current instruction in a direct-current voltage feedback control loop, generating a control signal, and performing current tracking control on the direct-current voltage feedback control loop based on a voltage-current double-closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain harmonic current to be injected; the harmonic current to be injected is a current with a specific amplitude and a specific frequency;
according to the received control signal, converting the information electronic signal of the main circuit into a driving signal of a device control loop, and selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency; the harmonic injection mode comprises a passive harmonic injection mode and an active harmonic injection mode;
when an active harmonic injection mode is selected, controlling the switching on and off of a switching tube according to a driving signal and a first specific sequence, and injecting harmonic current to be injected into a power grid at a public coupling point;
when a passive harmonic injection mode is selected, according to the driving signal and a second specific sequence, the switching on and switching off of the IGBT are controlled, the time of putting the capacitor into the power grid to operate and the current flowing direction are controlled, and harmonic current to be injected is injected into the power grid at the public coupling point;
After the harmonic current to be injected is injected into the power grid, selecting voltage and current signals of ports of branches to be measured in multipoint voltage and current signals of the power grid, and performing signal decomposition processing analysis to obtain power grid impedance of the ports of the branches to be measured; the power grid multipoint voltage and current signals are obtained by collecting voltages and currents at different positions in the power grid in real time.
When the embodiment of the invention is implemented, the characteristic harmonic current instruction is generated according to the harmonic signal generation algorithm and is input into the direct-current voltage feedback control loop when the power grid impedance measuring device and the power grid perform grid-connected stable operation; the direct-current voltage feedback control loop detects direct-current voltage in real time and keeps the voltage stable; receiving a characteristic harmonic current instruction in a direct-current voltage feedback control loop, generating a control signal, and performing current tracking control on the direct-current voltage feedback control loop based on a voltage-current double-closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain harmonic current to be injected; the harmonic current to be injected is a current with a specific amplitude and a specific frequency; according to the received control signal, converting the information electronic signal of the main circuit into a driving signal of a device control loop, and selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency; the harmonic injection mode comprises a passive harmonic injection mode and an active harmonic injection mode; when an active harmonic injection mode is selected, controlling the switching on and off of a switching tube according to a driving signal and a first specific sequence, and injecting harmonic current to be injected into a power grid at a public coupling point; when a passive harmonic injection mode is selected, according to the driving signal and a second specific sequence, the switching on and switching off of the IGBT are controlled, the time of putting the capacitor into the power grid to operate and the current flowing direction are controlled, and harmonic current to be injected is injected into the power grid at the public coupling point; after the harmonic current to be injected is injected into the power grid, selecting voltage and current signals of ports of branches to be measured in multipoint voltage and current signals of the power grid, and performing signal decomposition processing analysis to obtain power grid impedance of the ports of the branches to be measured; the power grid multipoint voltage and current signals are obtained by collecting voltages and currents at different positions in the power grid in real time. The power grid impedance measuring device generates, receives and analyzes a signal, generates a specific harmonic signal according to a harmonic signal injection algorithm, then collects and separates the harmonic signal at other positions of the power grid, and analyzes and calculates the power grid impedance of a port of a branch to be measured according to the specific harmonic voltage and current signals obtained by separation, thereby realizing power grid impedance measurement. By selecting a preset passive harmonic injection mode and an active harmonic injection mode in real time, the power grid impedance measurement method with complementary impedance measurement advantages of the active and passive power distribution networks realizes the organic combination of passive harmonic injection and active harmonic injection, greatly reduces the disturbance of the impedance measurement process to the power grid, avoids the voltage distortion of the power grid, and improves the measurement accuracy in the full frequency range.
As a preferred scheme, a preset harmonic injection mode is selected according to a harmonic injection target frequency, specifically:
when the harmonic injection target frequency is lower than the first frequency, determining the current target frequency band as a low frequency band, and selecting an active harmonic injection mode;
when the harmonic injection target frequency is higher than the first frequency and lower than the second frequency, determining the current target frequency band as a medium frequency band, and selecting a passive harmonic injection mode;
when the harmonic injection target frequency is higher than the second frequency, determining the current target frequency band as a high frequency band, and selecting an active harmonic injection mode.
As a preferred scheme, according to the driving signal and the first specific sequence, the switching-on and switching-off of the switching tube is controlled, specifically:
performing voltage space vector pulse width modulation on the driving signal, determining an on-off sequence of a switching tube which changes along with time, and obtaining a first specific sequence; the voltage space vector pulse width modulation comprises coordinate transformation, sector and area judgment, basic voltage space vector action time calculation and basic voltage space vector action sequence planning;
the switching on and off of the switching tube is controlled based on the first specific sequence.
As a preferred scheme, according to the driving signal and the second specific sequence, the switching on and off of the IGBT is controlled, and the time and the current flowing direction of the capacitor put into the power grid are controlled, specifically:
Performing voltage space vector pulse width modulation on the driving signal, determining an on-off sequence of the IGBT changing along with time, and obtaining a second specific sequence;
controlling the turn-on and turn-off of the IGBT based on the second specific sequence;
according to the on time of the IGBT, controlling the running time of the capacitor in the power grid, and controlling the amplitude of the harmonic current to be injected;
and controlling the current flowing direction of the capacitor which is put into the power grid to operate according to the on-off state change condition of the IGBT.
Preferably, the characteristic harmonic current instruction is generated according to a harmonic signal generation algorithm, specifically:
in the SVPWM modulation of the inverter, the frequency of a harmonic signal is changed according to the harmonic injection target frequency, a modulation wave frequency spectrum with the amplitude of the harmonic gradually increasing from low frequency to the harmonic injection target frequency is generated by setting the change rule of a modulation wave, an injection harmonic signal is obtained, and a characteristic harmonic current instruction is generated according to the injection harmonic signal.
As a preferred scheme, selecting voltage and current signals of ports of branches to be measured in the multipoint voltage and current signals of the power grid to perform signal decomposition processing analysis to obtain power grid impedance of the ports of the branches to be measured, wherein the specific steps are as follows:
obtaining node data to be measured according to the voltage and current signals of the ports of the branches to be measured in the selected power grid multipoint voltage and current signals;
Performing fast Fourier transform on the node data to be detected to obtain frequency domain data;
extracting amplitude and phase information of frequency domain data, and converting three-phase voltage and current into positive and negative sequence voltage and current according to the amplitude and phase information of the frequency domain data;
carrying out power grid impedance calculation on the positive and negative sequence voltage and current to obtain power grid impedance of a port of a branch to be measured; wherein the grid impedance comprises a positive sequence impedance and a negative sequence impedance; the formula of the power grid impedance calculation is as follows:
wherein,is positive sequence impedance, < >>Is a negative sequence impedance>Is a positive sequence harmonic voltage->Is a negative sequence harmonic voltage and is used for generating a negative sequence harmonic voltage,is positive sequence harmonic current, +.>Is a negative sequence harmonic current, +.>Is a harmonic frequency.
As a preferable scheme, the current tracking control specifically includes:
multiplying the output result of the fuzzy PI parameter self-tuning controller with the three-phase voltage, superposing the multiplication result into a three-phase voltage control target, performing closed-loop control on the direct-current voltage, and tracking control current;
the fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting control parameters of the PI regulator through a fuzzy control algorithm; the three-phase voltage control target is obtained by carrying out coordinate transformation on the output result of the fuzzy PI parameter self-tuning controller.
As a preferable scheme, the fuzzy PI parameter self-tuning controller is constructed by online optimization and adjustment of control parameters of a PI regulator through a fuzzy control algorithm, and specifically comprises the following steps:
calculating a current error and a current error change rate, performing fuzzy reasoning by using a fuzzy rule, and outputting a control parameter change quantity of the PI regulator;
in the fuzzy reasoning process, dividing the current error, the current error change rate and the parameter change quantity of the fuzzy PI parameter self-tuning controller into a plurality of fuzzy subsets respectively, wherein a membership function adopts a triangle function;
correcting the control parameters of the PI regulator, and superposing the control parameter variation on the basis of the initial control parameters to obtain the control parameters of the current working condition.
In order to solve the same technical problem, an embodiment of the present invention further provides a power grid impedance measurement device, including: the system comprises a voltage and current acquisition module, a harmonic signal injection module, a direct current voltage control module, a current tracking control module, a driving module, a power electronic conversion module, a passive harmonic injection module and a power grid impedance calculation module;
wherein, the connection of each module is as follows: the voltage and current acquisition module is respectively connected with the current tracking control module, the power grid impedance calculation module and the power grid, the current tracking control module is respectively connected with the harmonic signal injection module, the driving module and the direct current voltage control module, the driving module is respectively connected with the power electronic conversion module and the passive harmonic injection module, and the output end of the power electronic conversion module is connected with the power grid through the three-phase LCL passive filter;
The harmonic signal injection module is used for generating a characteristic harmonic current instruction according to a harmonic signal generation algorithm when the power grid impedance measuring device and the power grid perform grid-connected stable operation, and inputting the characteristic harmonic current instruction into the direct-current voltage feedback control loop;
the direct-current voltage control module is used for constructing a direct-current voltage feedback control loop, and the direct-current voltage feedback control loop detects direct-current voltage in real time and keeps the voltage stable;
the current tracking control module is used for receiving the characteristic harmonic current instruction in the direct-current voltage feedback control loop, generating a control signal, and performing current tracking control on the direct-current voltage feedback control loop based on a voltage-current double-closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain harmonic current to be injected; the harmonic current to be injected is a current with a specific amplitude and a specific frequency;
the driving module is used for converting the information electronic signal of the main circuit into a driving signal of the device control loop according to the received control signal, and selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency; the harmonic injection mode comprises a passive harmonic injection mode and an active harmonic injection mode;
The power electronic conversion module is used for controlling the switching-on and switching-off of the switching tube according to the driving signal and the first specific sequence when the active harmonic injection mode is selected, and injecting harmonic current to be injected into the power grid at the public coupling point;
the passive harmonic injection module is used for controlling the on-off of the IGBT according to the driving signal and the second specific sequence when the passive harmonic injection mode is selected, controlling the running time and the current flowing direction of the capacitor put into the power grid, and injecting the harmonic current to be injected into the power grid at the public coupling point;
the power grid impedance calculation module is used for selecting voltage and current signals of the ports of the branches to be measured in the power grid multipoint voltage and current signals to perform signal decomposition processing analysis after the harmonic current to be injected is injected into the power grid, so as to obtain the power grid impedance of the ports of the branches to be measured;
the voltage and current acquisition module is used for acquiring voltage and current at different positions in the power grid in real time to obtain multi-point voltage and current signals of the power grid.
As a preferable scheme, the voltage and current acquisition module comprises a voltage transformer and a current transformer;
the harmonic signal injection module is constructed and generated by a harmonic signal generation algorithm;
the direct-current voltage control module is constructed and generated by a direct-current voltage feedback control loop;
The current tracking control module consists of a voltage-current double closed-loop structure, and the controller adopts a fuzzy PI parameter self-tuning controller;
the driving module is composed of a driving circuit;
the power electronic conversion module consists of a direct-current side energy storage capacitor and a voltage type inverter, wherein the voltage type inverter adopts a three-phase H-bridge structure;
the passive harmonic injection module is formed by connecting an IGBT and a capacitor;
the power grid impedance calculation module comprises an FFT calculation link, a positive and negative sequence conversion link and a positive and negative sequence impedance calculation link.
Drawings
Fig. 1: a flow diagram of an embodiment of a power grid impedance measurement method provided by the invention;
fig. 2: an embodiment diagram of an impedance measuring device of an embodiment of a power grid impedance measuring method is provided by the invention;
fig. 3: an impedance measuring device and a power grid connection diagram of one embodiment of the power grid impedance measuring method are provided by the invention;
fig. 4: the invention provides a schematic diagram of a fuzzy PI parameter self-tuning controller of one embodiment of a power grid impedance measurement method;
fig. 5: the invention provides a connection structure schematic diagram of an impedance measuring device of an embodiment of a power grid impedance measuring device.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Example 1
Referring to fig. 1, a flow chart of a power grid impedance measurement method according to an embodiment of the present invention is shown, and the power grid impedance measurement method according to the embodiment of the present invention is suitable for a power grid impedance measurement device, and the embodiment complements the advantages of active and passive power distribution network impedance measurement by combining an active harmonic injection mode and a passive harmonic injection mode, so as to effectively measure the power grid impedance, reduce disturbance of an impedance measurement process to the power grid, and improve measurement accuracy. The power grid impedance measurement method comprises steps 101 to 106, wherein the steps are as follows:
step 101: when the power grid impedance measuring device and the power grid perform grid-connected stable operation, generating a characteristic harmonic current instruction according to a harmonic signal generating algorithm, and inputting the characteristic harmonic current instruction into a direct-current voltage feedback control loop; the direct-current voltage feedback control loop detects direct-current voltage in real time and keeps the voltage stable.
In this embodiment, the power grid impedance measurement device performs a power grid impedance measurement method, and as shown in fig. 2, the power grid impedance measurement device is connected in parallel between a power grid and a load, and includes a voltage and current acquisition module, a harmonic signal injection module, a direct current voltage control module, a current tracking control module, a driving module, a power electronic conversion module, a passive harmonic injection module and a power grid impedance calculation module, where the modules cooperate with each other to complete power grid impedance measurement. The power grid active and passive complementary-advantage power distribution network impedance measuring device is equivalent to a signal generating-receiving-analyzing device, and can generate characteristic harmonic signals according to a harmonic signal generating algorithm, inject the characteristic harmonic signals into a power grid at a public coupling point, and then collect, process and decompose the harmonic signals at a plurality of impedance characteristic to-be-measured positions in the power grid. According to the characteristic harmonic voltage and current signals obtained through decomposition, the impedance characteristic of the node is obtained through analysis and calculation, and therefore impedance characteristic measurement of different branches in a power grid is achieved. For convenience in control, the whole control system is performed under a two-phase synchronous rotation coordinate system, so that coordinate transformation is required to be completed, and the module and the type of the coordinate transformation are shown in fig. 2, and angle information required by the coordinate transformation is calculated by a phase-locked loop (PLL). And a three-phase LCL passive filter is connected between the output end of the power electronic conversion module and the power grid and is used for filtering useless harmonic signals.
The grid impedance measuring device performs grid-connected stable operation with the power grid, specifically, the grid-connected stable operation is realized by completing the processes of grid voltage phase locking, direct-current side energy storage unit charging, dynamic balance exchange of energy of the power grid side and direct-current side, and the like, and the functions of current tracking control, direct-current voltage control, and the like are completed after the grid-connected stable operation. The connection diagram between the impedance measuring device and the power grid is shown in fig. 3, the topological structures and the current flowing directions of the power grid and the impedance measuring device are identified, the control process of the impedance measuring device is completed by selecting an FPGA chip based on the fact that the FPGA chip capacity, the combination logic, the working speed and the design flexibility are far superior to those of a singlechip, the impedance measuring device comprises an active impedance measurer and a passive impedance measurer, the connection mode of the device and the power grid adopts a parallel topological structure, and the device is integrally connected between the power grid and a load in parallel. The power grid and the impedance measuring device can respectively generate power grid current and harmonic injection current, and the power grid current and the harmonic injection current are converged at a common coupling point of the power grid and the impedance measuring device to form load current.
In this embodiment, the dc voltage control module detects the dc voltage of the power electronic conversion module at any time and ensures voltage stability by constructing a dc voltage control loop, and in the process of designing the dc voltage control module, based on the concept of the modulation degree of the inverter, optimizes the dc voltage control method, multiplies the output result of the fuzzy PI regulator by the three-phase voltage by performing closed-loop control on the dc voltage, and superimposes the result on the three-phase voltage obtained in the previous step, and changes the modulation degree of the inversion process in the power electronic conversion module by phase change, thereby simplifying the dc voltage control process and improving the control effect of the dc voltage. And the harmonic signal injection instruction is superimposed with the output results of the direct-current voltage control module and the coordinate transformation module.
Optionally, according to a harmonic signal generation algorithm, a characteristic harmonic current instruction is generated, specifically:
in the SVPWM modulation of the inverter, the frequency of a harmonic signal is changed according to the harmonic injection target frequency, a modulation wave frequency spectrum with the amplitude of the harmonic gradually increasing from low frequency to the harmonic injection target frequency is generated by setting the change rule of a modulation wave, an injection harmonic signal is obtained, and a characteristic harmonic current instruction is generated according to the injection harmonic signal.
In this embodiment, the harmonic signal injection module generates a characteristic harmonic current command of a specific waveform and frequency according to a harmonic signal generation algorithm and inputs the command to the dc voltage feedback control loop. As an example of this embodiment, the harmonic signal injection module is implemented by a DDS algorithm of the FPGA controller, according to which a harmonic signal with a specific waveform and frequency can be generated, and by outputting a command signal to the current tracking control module, the injection process of the harmonic signal into the power grid can be completed.
The harmonic injection signal frequency is usually fixed, and the technical scheme is to inject harmonic signals with variable frequency into the power grid by changing the harmonic injection signal frequency serving as a current tracking control target, and acquire and calculate the harmonic signals in the voltage and the current of the power grid to obtain the power grid impedance. Existing grid impedance measurement methods further analyze the grid impedance by injecting a characteristic harmonic into the grid and measuring the voltage and current of the characteristic harmonic across the grid. The characteristic harmonics of the injection include single harmonic injection, several harmonic injection and broadband harmonic injection. In the single harmonic injection method, typically, 75Hz harmonic current is selected for injection, the harmonic is close to 50Hz of the power frequency, interference signals of the harmonic are hardly generated in the power grid, and the detection precision of the power grid impedance of the power frequency is high. However, the power grid impedance is often not a first order system or even nonlinear, so that the relationship between the power grid impedance and the frequency can be obtained more accurately by adopting a harmonic injection method with a plurality of frequencies. The frequency of the injected harmonic signal is variable, and in the SVPWM modulation of the inverter, the frequency of the harmonic signal is changed to change near the set frequency, and the frequency spectrum of the modulated wave with the amplitude of the harmonic gradually increasing from the low frequency to the set frequency is obtained by setting the change rule of the modulated wave. The harmonic signal with the frequency change is used as a modulation wave of SVPWM modulation, and is modulated with a carrier wave generated by an inverter controller to generate a pulse signal to control an inverter switching tube, so that the output side of the controlled inverter also comprises harmonic signals with various frequencies. And sampling and analyzing the output voltage and current signals of the inverter to obtain the power grid impedance at various frequencies.
Step 102: receiving a characteristic harmonic current instruction in a direct-current voltage feedback control loop, generating a control signal, and performing current tracking control on the direct-current voltage feedback control loop based on a voltage-current double-closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain harmonic current to be injected; the harmonic current to be injected is a current with a specific amplitude and a specific frequency.
In this embodiment, the current tracking control module receives the characteristic harmonic current instruction of the harmonic signal injection module, generates a control signal of the power electronic conversion module, completes the tracking control process of the instruction current, and finally controls the power electronic conversion module to inject the characteristic harmonic current to the grid public coupling point. The characteristic harmonic current instruction (harmonic signal instruction) is used as a target of current tracking control and used for controlling the power electronic conversion module to inject harmonic current with specific amplitude and frequency into the power grid.
It should be noted that, in the open loop control, due to lack of observation and feedback of the control result, there are disadvantages of low control accuracy and the like, in order to improve control accuracy and response speed, closed loop control is adopted in the current tracking control module, and in order to improve anti-interference capability of the system, a PI regulator is introduced in the conventional control system, but since the constant-coefficient PI regulation is essentially a linear control, when the system is used for controlling a nonlinear complex power system, a good effect cannot be obtained. The fuzzy PI parameter self-tuning controller comprises a fuzzy reasoning module and a PI regulator, a fuzzy control algorithm is combined with the PI regulator, and the control parameters of the PI regulator are optimized and adjusted on line through the fuzzy control algorithm, so that the control system can resist disturbance better and obey a given state. The fuzzy reasoning module performs fuzzy reasoning by calculating the current error and the change rate thereof and utilizing a fuzzy rule to output the control parameter change quantity of the PI regulator, and the fuzzy PI parameter self-tuning controller principle is shown in fig. 4, and the fuzzy reasoning module performs fuzzy reasoning by calculating the current error and the change rate thereof and utilizing the fuzzy rule to output the control parameter change quantity of the PI regulator.
Optionally, the current tracking control specifically includes: multiplying the output result of the fuzzy PI parameter self-tuning controller with the three-phase voltage, superposing the multiplication result into a three-phase voltage control target, performing closed-loop control on the direct-current voltage, and tracking control current;
the fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting control parameters of the PI regulator through a fuzzy control algorithm; the three-phase voltage control target is obtained by carrying out coordinate transformation on the output result of the fuzzy PI parameter self-tuning controller.
In this embodiment, based on the concept of the modulation degree of the inverter, the direct-current voltage control method is optimized, by performing closed-loop control on the direct-current voltage, multiplying the output result of the fuzzy PI parameter from the tuning controller (fuzzy PI regulator) by the three-phase voltage, and superimposing the result on the three-phase voltage control target obtained in the previous step. And carrying out coordinate transformation on the output of the fuzzy PI parameter self-tuning controller to obtain a three-phase voltage control target under a three-phase static coordinate system, and on the basis, completing superposition of a direct-current voltage control signal and a harmonic injection signal, and outputting the superimposed direct-current voltage control signal and the harmonic injection signal to a voltage space vector pulse width modulation link to complete generation of a driving signal.
By implementing the embodiment of the invention, the modulation degree of the inversion process in the power electronic conversion module is changed by carrying out the fuzzy PI parameter self-tuning control process, so that the direct-current voltage control process is simplified, and the control effect of the direct-current voltage is improved; and combining the fuzzy control algorithm with the PI regulator, and optimizing and regulating the control parameters of the PI regulator on line through the fuzzy control algorithm.
Optionally, the fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting the control parameters of the PI regulator through a fuzzy control algorithm, and specifically comprises the following steps:
calculating a current error and a current error change rate, performing fuzzy reasoning by using a fuzzy rule, and outputting a control parameter change quantity of the PI regulator; in the fuzzy reasoning process, dividing the current error, the current error change rate and the parameter change quantity of the fuzzy PI parameter self-tuning controller into a plurality of fuzzy subsets respectively, wherein a membership function adopts a triangle function; correcting the control parameters of the PI regulator, and superposing the control parameter variation on the basis of the initial control parameters to obtain the control parameters of the current working condition.
In the embodiment, the fuzzy reasoning module performs fuzzy reasoning by calculating the current error and the change rate thereof and utilizing a fuzzy rule so as to output the control parameter change quantity of the PI regulator; in the process, the fuzzy PI parameter self-tuning controller input error E and error change rate EC and output PI parameter change quantity are divided into 7 fuzzy subsets { NB, NM, NS, Z, PS, PM, PB }, and a membership function adopts a triangle function, which can be a simple triangle function; the PI regulator corrects the control parameter, and superimposes the control parameter variation on the basis of the initial control parameter, so as to obtain the control parameter suitable for the current working condition.
Step 103: according to the received control signal, converting the information electronic signal of the main circuit into a driving signal of a device control loop, and selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency; the harmonic injection mode comprises a passive harmonic injection mode and an active harmonic injection mode.
In this embodiment, the driving module receives the control signal from the current tracking control module, converts the signal (the information electronic signal of the main circuit) transmitted from the information electronic circuit into the driving signal added to the device control loop, and determines whether to use the passive harmonic injection mode or the active harmonic injection mode according to the harmonic frequency suitable for injection by the passive harmonic injection module and the target frequency of the harmonic injection. When the harmonic wave is injected into the power grid in a sweep frequency mode from the low frequency band to the high frequency band, an active harmonic wave injection mode is adopted in the low frequency band, the intermediate frequency band is switched to an rimless harmonic wave injection mode, and the high frequency band is switched back to the active harmonic wave injection mode.
The driving module is composed of a driving circuit, and is used as an interface circuit between the main circuit and the control circuit to convert signals transmitted by the information electronic circuit into driving signals added in a device control loop, and the driving module receives the driving signals of the signal-level power electronic converter output by the current tracking control module, and the driving circuit is used for completing signal conversion to obtain driving-level driving signals capable of driving the power electronic converter.
According to the embodiment of the invention, aiming at the problems that the difference of the harmonic current content injected by the passive harmonic injection method in different frequency ranges is large, the measurement precision is influenced, the active harmonic injection method can generate larger interference on a power grid, the voltage distortion rate of the power grid is increased, and the like, the disturbance of the impedance measurement process on the power grid is greatly reduced and the measurement precision is improved in the full frequency range by organically combining the two methods through the power grid impedance measurement method with the complementary advantages of the impedance measurement of the active and passive power distribution networks.
Optionally, a preset harmonic injection mode is selected according to the harmonic injection target frequency, which specifically includes:
when the harmonic injection target frequency is lower than the first frequency, determining the current target frequency band as a low frequency band, and selecting an active harmonic injection mode;
when the harmonic injection target frequency is higher than the first frequency and lower than the second frequency, determining the current target frequency band as a medium frequency band, and selecting a passive harmonic injection mode;
when the harmonic injection target frequency is higher than the second frequency, determining the current target frequency band as a high frequency band, and selecting an active harmonic injection mode.
In this embodiment, when the harmonic is selectively injected into the power grid from a low frequency (e.g., 10 Hz) to a high frequency (e.g., 2000 Hz) in a frequency sweep manner, an active harmonic injection method is adopted in the low frequency band, a passive harmonic injection method is switched to the intermediate frequency band, and an active harmonic injection method is switched back to the high frequency band. Because of the difference of the impedance characteristics of the power grids in different areas, the frequency band division can comprehensively refer to the disturbance of different harmonic injection methods to the power grids and the impedance measurement precision, and the first frequency and the second frequency are determined with the minimum overall disturbance and the highest measurement precision as targets, so that the low frequency band, the medium frequency band and the high frequency band are divided.
By implementing the embodiment of the invention, the switching tube control branch can conveniently control the conduction time according to the control algorithm and the driving signal, so that the magnitude of the injected harmonic current, namely the magnitude of impact on a network, can be controlled. In general, the larger the injected harmonic current, the higher the measurement accuracy, and the higher the impact on the network. By gradually increasing the harmonic current, when the measurement accuracy reaches the target value, the harmonic current at the moment is the minimum current for network impact, so that the optimal junction point can be obtained in two aspects, the measurement accuracy of harmonic impedance is ensured, and the power grid is disturbed as little as possible. And when the network condition is changed, the measurement condition is changed by changing the on time of the switching tube at any time.
Step 104: when the active harmonic injection mode is selected, the switching-on and switching-off of the switching tube is controlled according to the driving signal and the first specific sequence, and the harmonic current to be injected is injected into the power grid at the public coupling point.
In this embodiment, when the power electronic conversion module operates in an active harmonic injection mode (active harmonic injection mode), the power electronic conversion module completes the switching on and off process of the switching tube according to a specific sequence according to a driving signal, and injects characteristic harmonic current with specific waveform and frequency into a point of common coupling between the power grid and the device. The power electronic conversion module consists of a direct-current side energy storage capacitor and a voltage type inverter, wherein the voltage type inverter adopts a three-phase H-bridge structure.
When the active harmonic injection method is needed, the power distribution network impedance measurement device is switched from a working mode to an active harmonic injection mode in the working process, and the power distribution network impedance measurement device firstly completes a grid-connected inversion process, wherein the grid-connected inversion process specifically comprises the following steps of: and (3) a process of power grid voltage phase locking, direct current side energy storage unit charging and power grid side and direct current side energy dynamic balance exchange. On the basis, the current tracking control module receives power grid operation parameters, control instructions, harmonic signals and the like from the voltage and current acquisition module, the direct current control module and the harmonic signal injection module, and the power electronic conversion module finishes the injection process of the harmonic signals by inputting the harmonic instructions into the control loop. The driving module is used as an interface circuit and plays a role in connection between the main circuit and the control circuit. Finally, the power grid impedance calculation module performs fast Fourier decomposition and impedance characteristic analysis and calculation on the characteristic harmonic voltage and current at the public coupling point of the power grid and the current transformer acquired by the voltage and current acquisition module, so that the purpose of measuring the impedance of the power distribution network is achieved.
Optionally, according to the driving signal and the first specific sequence, the switching tube is controlled to be turned on or off, specifically: performing voltage space vector pulse width modulation on the driving signal, determining an on-off sequence of a switching tube which changes along with time, and obtaining a first specific sequence; the voltage space vector pulse width modulation comprises coordinate transformation, sector and area judgment, basic voltage space vector action time calculation and basic voltage space vector action sequence planning; the switching on and off of the switching tube is controlled based on the first specific sequence.
In this embodiment, the inverter SVPWM modulation process utilizes an SVPWM control algorithm (voltage space vector pulse width modulation) of the inverter including: coordinate transformation, sector and area judgment, basic voltage space vector action time calculation and basic voltage space vector action sequence planning, wherein the total steps are 4; and obtaining the switching-on and switching-off sequence of the switching tube which changes along with time through a voltage space vector pulse width modulation process. At this time, the switching-on and switching-off sequence of the switching tube is of a signal level, and the energy is insufficient to effectively control the switching-on and switching-off of the switching tube.
In the inverter modulation scheme, the voltage utilization rate of the voltage Space Vector Pulse Width Modulation (SVPWM) is improved by 15% as compared with the Sinusoidal Pulse Width Modulation (SPWM), and a superior modulation effect can be obtained. The inverter modulation strategy of the method and the device selects an SVPWM mode.
Step 105: when the passive harmonic injection mode is selected, the switching on and off of the IGBT is controlled according to the driving signal and the second specific sequence, the running time and the current flowing direction of the capacitor in the power grid are controlled, and the harmonic current to be injected is injected into the power grid at the public coupling point.
Optionally, according to the driving signal and the second specific sequence, the IGBT is controlled to be turned on and off, and the time and the current flowing direction of the capacitor put into the power grid are controlled, specifically:
performing voltage space vector pulse width modulation on the driving signal, determining an on-off sequence of the IGBT changing along with time, and obtaining a second specific sequence; controlling the turn-on and turn-off of the IGBT based on the second specific sequence; according to the on time of the IGBT, controlling the running time of the capacitor in the power grid, and controlling the amplitude of the harmonic current to be injected; and controlling the current flowing direction of the capacitor which is put into the power grid to operate according to the on-off state change condition of the IGBT.
In this embodiment, when the passive harmonic injection module operates in a passive harmonic injection mode (passive harmonic injection mode), the passive harmonic injection module completes the switching-on and switching-off process of the switching tube according to a specific sequence according to a driving signal, controls the time and the current flowing direction of the capacitor put into the power grid to operate, and injects characteristic harmonic current with specific frequency into the point of common coupling between the power grid and the device.
When the passive harmonic injection method is needed, the control loop is operated according to the passive harmonic injection algorithm by switching the working mode to the passive harmonic injection mode in the working process of the device. In the running process, the time of the capacitor connected into the power grid to run can be controlled by controlling the on time of the IGBT, so that the amplitude of the injected harmonic current can be controlled, namely the impact on the network is controlled. Therefore, the optimal junction point can be obtained in two aspects, the harmonic impedance measurement precision is ensured, and the disturbance to the power grid is as small as possible. And when the network condition is changed, the measurement condition is changed by changing the on time of the switching tube at any time. The method can adjust the injection current according to the noise condition of the power grid, and can achieve balance between the harmonic impedance measurement precision and the injection interference of the power grid. Finally, the power grid impedance calculation module performs fast Fourier decomposition and impedance characteristic analysis and calculation on the characteristic harmonic voltage and current at the public coupling point of the power grid and the current transformer acquired by the voltage and current acquisition module, so that the purpose of measuring the impedance of the power distribution network is achieved.
The passive harmonic injection module is formed by connecting a push-pull IGBT inverter circuit and a capacitor in series, and the time and the direction of connecting the capacitor to a power grid and outputting harmonic current are controlled by phase change through controlling the conduction time and the conduction sequence of the IGBT, so that the amplitude of the harmonic current injected into the power grid by the impedance measuring device is controlled; the driving signal of the IGBT is provided by the driving module, and the control loop is designed according to the driving target of the IGBT corresponding to the driving signal of the IGBT. In the control loop design process, compared with an active harmonic injection module control loop, the control loop basic framework is kept unchanged, and only loop parameters are required to be adjusted.
Step 106: after the harmonic current to be injected is injected into the power grid, selecting voltage and current signals of ports of branches to be measured in multipoint voltage and current signals of the power grid, and performing signal decomposition processing analysis to obtain power grid impedance of the ports of the branches to be measured; the power grid multipoint voltage and current signals are obtained by collecting voltages and currents at different positions in the power grid in real time.
In this embodiment, the voltage and current acquisition module detects the voltage and current of the power grid in real time, and the acquisition module acquires three-phase voltage and three-phase current signals in the power grid by using a voltage and current sensor to obtain multi-point voltage and current signals of the power grid. The power grid impedance calculation module selects the voltage and current signals acquired by the voltage and current acquisition modules, selects the port voltage and current signals of the branch to be measured, then completes the processing and decomposition processes of the signals, and further analyzes and calculates the impedance characteristics of the branch in the power grid.
Optionally, selecting a voltage and current signal of a port of a branch to be measured in the multipoint voltage and current signals of the power grid to perform signal decomposition processing analysis to obtain the power grid impedance of the port of the branch to be measured, which specifically comprises:
obtaining node data to be measured according to the voltage and current signals of the ports of the branches to be measured in the selected power grid multipoint voltage and current signals;
performing fast Fourier transform on the node data to be detected to obtain frequency domain data;
extracting amplitude and phase information of frequency domain data, and converting three-phase voltage and current into positive and negative sequence voltage and current according to the amplitude and phase information of the frequency domain data;
carrying out power grid impedance calculation on the positive and negative sequence voltage and current to obtain power grid impedance of a port of a branch to be measured; wherein the grid impedance comprises a positive sequence impedance and a negative sequence impedance; the formula of the power grid impedance calculation is as follows:
wherein,is positive sequence impedance, < >>Is a negative sequence impedance>Is a positive sequence harmonic voltage->Is a negative sequence harmonic voltage and is used for generating a negative sequence harmonic voltage,is positive sequence harmonic current, +.>Is a negative sequence harmonic current, +.>For harmonic frequencies +.>Indicating phase angle, & lt + & gt>Representing the amplitude.
In this embodiment, the power grid impedance calculation module is capable of receiving the output signal from the voltage and current signal acquisition module and completing the selection, decomposition and processing of data; the implementation process of the power grid impedance calculation module comprises 3 links, namely: a Fast Fourier Transform (FFT) link, a positive and negative sequence conversion link and a positive and negative sequence impedance calculation link. After acquiring the three-phase voltage and current response of the measuring point during pulse injection, the power grid impedance calculation module performs FFT (fast Fourier transform) on the measuring point data (node data to be measured), extracts the amplitude and phase information of the measuring point data on a frequency domain, and converts the three-phase voltage and current into positive and negative sequence voltage and current, thereby calculating the positive and negative sequence power grid impedance.
By implementing the embodiment of the invention, the technology is innovated and improved in the aspects of direct-current voltage control, power electronic conversion, current tracking control and the like based on the power grid impedance measurement technology of the power electronic converter. Based on the characteristics of passive harmonic injection and active harmonic injection methods, particularly aiming at the problems that the difference of harmonic current content injected by the passive harmonic injection method in different frequency ranges is large, the measurement accuracy is influenced, the active harmonic injection method can generate large interference to a power grid, the voltage distortion rate of the power grid is increased, and the like, the power grid impedance measurement method with complementary impedance measurement advantages of an active power distribution network and a passive power distribution network is designed, and by organically combining the active method and the passive method, the disturbance of the impedance measurement process to the power grid is greatly reduced, and the measurement accuracy is improved in the full frequency range.
The invention furthermore has the following advantages: (1) The method for actively measuring the impedance of the power grid comprises the steps of generating disturbance by using a controller of an inverter, actively applying the disturbance to the power grid, and then acquiring corresponding response to perform signal processing so as to extract required information to calculate the impedance of the power grid; (2) The method and the device for complementarily measuring the impedance of the active power distribution network and the passive power distribution network are designed, and by organically combining the two methods, the disturbance of the impedance measurement process to the power grid is greatly reduced, and the measurement accuracy is improved in the full frequency range. (3) The control loop regulator adopts a fuzzy PI regulator, and can optimize loop control parameters on line according to different working conditions and disturbance, thereby realizing the self-adaption of the control parameters and greatly improving the dynamic performance and disturbance resistance of the system; (4) The push-pull IGBT inverter circuit is connected with the capacitor, so that the running time of the capacitor in a power grid and the flowing direction of current are controllable, and the magnitude of the injected harmonic current is controlled; (5) Based on the concept of the modulation degree of the inverter, the direct-current voltage control method is optimized, so that the direct-current voltage control process is simplified, and meanwhile, the control effect of the direct-current voltage is improved.
Example two
Correspondingly, referring to fig. 5, fig. 5 is a schematic diagram of a connection structure of an impedance measuring device according to a second embodiment of the power grid impedance measuring device. As shown in fig. 5, the grid impedance measuring apparatus includes: the system comprises a voltage and current acquisition module, a harmonic signal injection module, a direct current voltage control module, a current tracking control module, a driving module, a power electronic conversion module, a passive harmonic injection module and a power grid impedance calculation module;
wherein, the connection of each module is as follows: the voltage and current acquisition module is respectively connected with the current tracking control module, the power grid impedance calculation module and the power grid, the current tracking control module is respectively connected with the harmonic signal injection module, the driving module and the direct current voltage control module, the driving module is respectively connected with the power electronic conversion module and the passive harmonic injection module, and the output end of the power electronic conversion module is connected with the power grid through the three-phase LCL passive filter;
in this embodiment, as shown in fig. 5, the voltage-current acquisition module is connected with the current tracking control module and the power grid impedance calculation module, and the current tracking control module is connected with the current tracking control module and further comprises a harmonic signal injection module and a direct current voltage control module, the current tracking control module is connected with the driving module, the driving module is simultaneously connected with the power electronic conversion module and the passive harmonic injection module, and a three-phase LCL passive filter is further connected between the output end of the power electronic conversion module and the power grid.
The harmonic signal injection module is used for generating a characteristic harmonic current instruction according to a harmonic signal generation algorithm when the power grid impedance measuring device and the power grid perform grid-connected stable operation, and inputting the characteristic harmonic current instruction into the direct-current voltage feedback control loop;
the direct-current voltage control module is used for constructing a direct-current voltage feedback control loop, and the direct-current voltage feedback control loop detects direct-current voltage in real time and keeps the voltage stable;
the current tracking control module is used for receiving the characteristic harmonic current instruction in the direct-current voltage feedback control loop, generating a control signal, and performing current tracking control on the direct-current voltage feedback control loop based on a voltage-current double-closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain harmonic current to be injected; the harmonic current to be injected is a current with a specific amplitude and a specific frequency;
the driving module is used for converting the information electronic signal of the main circuit into a driving signal of the device control loop according to the received control signal, and selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency; the harmonic injection mode comprises a passive harmonic injection mode and an active harmonic injection mode;
The power electronic conversion module is used for controlling the switching-on and switching-off of the switching tube according to the driving signal and the first specific sequence when the active harmonic injection mode is selected, and injecting harmonic current to be injected into the power grid at the public coupling point;
the passive harmonic injection module is used for controlling the on-off of the IGBT according to the driving signal and the second specific sequence when the passive harmonic injection mode is selected, controlling the running time and the current flowing direction of the capacitor put into the power grid, and injecting the harmonic current to be injected into the power grid at the public coupling point;
the power grid impedance calculation module is used for selecting voltage and current signals of the ports of the branches to be measured in the power grid multipoint voltage and current signals to perform signal decomposition processing analysis after the harmonic current to be injected is injected into the power grid, so as to obtain the power grid impedance of the ports of the branches to be measured;
the voltage and current acquisition module is used for acquiring voltage and current at different positions in the power grid in real time to obtain multi-point voltage and current signals of the power grid.
Optionally, the voltage and current acquisition module comprises a voltage transformer and a current transformer, and can detect the voltage and current of the power grid and acquire the three-phase voltage and current of the power grid.
The harmonic signal injection module is constructed and generated by a harmonic signal generation algorithm, is a software module and is realized by an algorithm program, and can generate a harmonic current instruction signal with specific waveform and frequency according to the harmonic signal injection algorithm.
The direct-current voltage control module is constructed and generated by a direct-current voltage feedback control loop, and is used for detecting the direct-current voltage of the power electronic conversion module at any time and ensuring the voltage stability.
The current tracking control module adopts a voltage and current double-closed-loop structure, the controller adopts a fuzzy PI parameter self-tuning controller, the current tracking control module is a software module, is realized by an algorithm program, can receive a current tracking control instruction, and completes the tracking control process of the instruction current.
The driving module is composed of a driving circuit, and is used as an interface circuit between the main circuit and the control circuit to convert the signal transmitted by the information electronic circuit into a driving signal applied to the device control loop.
The power electronic conversion module consists of a direct-current side energy storage capacitor and a voltage type inverter, wherein the voltage type inverter adopts a three-phase H-bridge structure;
the passive harmonic injection module is formed by connecting an IGBT and a capacitor, can control the running time of the capacitor in a power grid and the flowing direction of current, and is preferably formed by connecting three groups of push-pull IGBT inverter circuits with the capacitor in series, and the time and the direction of the capacitor in the power grid and outputting harmonic current are controlled by phase change by controlling the conduction sequence and the conduction time of the two IGBTs, so that the amplitude of the harmonic current injected into the power grid by the impedance measuring device is controlled.
The power grid impedance calculation module comprises an FFT calculation link, a positive and negative sequence conversion link and a positive and negative sequence impedance calculation link. And decomposing and processing the voltage and the current acquired by the voltage and current acquisition module, and further calculating and analyzing to obtain the impedance characteristic of the power grid.
By implementing the embodiment of the invention, the impedance measuring device is equivalent to a signal generating-receiving-analyzing device, can generate specific harmonic signals according to a harmonic signal injection algorithm, then collect and separate the harmonic signals at other positions of the power grid, and analyze and calculate the power grid impedance of the node according to the specific harmonic voltage and current signals obtained by separation, thereby realizing the power grid impedance measurement. Based on the characteristics of passive harmonic injection and an active harmonic injection method, particularly aiming at the problems that the difference of the harmonic current content injected by the passive harmonic injection method in different frequency ranges is large, the measurement accuracy is influenced, the active harmonic injection method can generate larger interference to a power grid, the voltage distortion rate of the power grid is increased, and the like, the invention designs a method and a device for complementarily measuring the impedance of an active power distribution network and a passive power distribution network.
The power grid impedance measuring device can implement the power grid impedance measuring method of the method embodiment. The options in the method embodiments described above are also applicable to this embodiment and will not be described in detail here. The rest of the embodiments of the present application may refer to the content of the method embodiments described above, and in this embodiment, no further description is given.
The foregoing embodiments have been provided for the purpose of illustrating the general principles of the present invention, and are not to be construed as limiting the scope of the invention. It should be noted that any modifications, equivalent substitutions, improvements, etc. made by those skilled in the art without departing from the spirit and principles of the present invention are intended to be included in the scope of the present invention.
Claims (9)
1. A method for measuring impedance of a power grid, comprising:
when the power grid impedance measuring device and the power grid perform grid-connected stable operation, generating a characteristic harmonic current instruction according to a harmonic signal generating algorithm, and inputting the characteristic harmonic current instruction into a direct-current voltage feedback control loop; the direct-current voltage feedback control loop detects direct-current voltage in real time and keeps the voltage stable;
Receiving the characteristic harmonic current instruction in the direct-current voltage feedback control loop, generating a control signal, and performing current tracking control on the direct-current voltage feedback control loop based on a voltage-current double-closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain harmonic current to be injected; the harmonic current to be injected is a current with a specific amplitude and a specific frequency;
according to the received control signal, converting an information electronic signal of a main circuit into a driving signal of a device control loop, and selecting a preset harmonic injection mode in real time according to a harmonic injection target frequency; the harmonic injection mode comprises a passive harmonic injection mode and an active harmonic injection mode;
the method comprises the steps of selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency, wherein the method specifically comprises the following steps:
when the harmonic injection target frequency is lower than the first frequency, determining that the current target frequency band is a low frequency band, and selecting the active harmonic injection mode;
when the harmonic injection target frequency is higher than the first frequency and lower than the second frequency, determining the current target frequency band as a middle frequency band, and selecting the passive harmonic injection mode;
When the harmonic injection target frequency is higher than the second frequency, determining the current target frequency band as a high frequency band, and selecting the active harmonic injection mode;
when the active harmonic injection mode is selected, controlling the switching-on and switching-off of a switching tube according to the driving signal and a first specific sequence, and injecting the harmonic current to be injected into the power grid at a public coupling point;
when the passive harmonic injection mode is selected, controlling the on-off of the IGBT according to the driving signal and a second specific sequence, controlling the time of putting a capacitor into the power grid to operate and the current flowing direction, and injecting the harmonic current to be injected into the power grid at the public coupling point;
after the harmonic current to be injected is injected into the power grid, selecting voltage and current signals of ports of branches to be measured in multipoint voltage and current signals of the power grid, and performing signal decomposition processing analysis to obtain power grid impedance of the ports of the branches to be measured; the power grid multipoint voltage and current signals are obtained by collecting voltages and currents at different positions in the power grid in real time.
2. The power grid impedance measurement method according to claim 1, wherein the controlling the switching on and off of the switching tube according to the driving signal and the first specific sequence is specifically:
Performing voltage space vector pulse width modulation on the driving signal, determining an on-off sequence of the switching tube which changes along with time, and obtaining the first specific sequence; the voltage space vector pulse width modulation comprises coordinate transformation, sector and area judgment, basic voltage space vector action time calculation and basic voltage space vector action sequence planning;
and controlling the switching on and off of the switching tube based on the first specific sequence.
3. The method for measuring the impedance of the power grid according to claim 2, wherein the switching on and off of the IGBTs is controlled according to the driving signal and the second specific sequence, and the time and the current flowing direction of the capacitor put into the power grid are controlled, specifically:
performing voltage space vector pulse width modulation on the driving signal, determining an on-off sequence of the IGBT which changes along with time, and obtaining the second specific sequence;
controlling the turn-on and turn-off of the IGBT based on the second specific sequence;
according to the on time of the IGBT, controlling the running time of the capacitor in the power grid, and controlling the amplitude of the harmonic current to be injected;
And controlling the current flowing direction of the capacitor which is put into the power grid to run according to the on-off state change condition of the IGBT.
4. The method for measuring impedance of a power grid according to claim 1, wherein the generating the characteristic harmonic current command according to the harmonic signal generating algorithm comprises:
in the SVPWM modulation of the inverter, the frequency of a harmonic signal is changed according to the harmonic injection target frequency, a modulation wave frequency spectrum with the amplitude gradually increasing from low frequency to the harmonic injection target frequency is generated by setting the change rule of a modulation wave, an injection harmonic signal is obtained, and the characteristic harmonic current instruction is generated according to the injection harmonic signal.
5. The method for measuring the impedance of the power grid according to claim 1, wherein the step of selecting the voltage and current signals of the port of the branch to be measured from the voltage and current signals of the power grid multipoint is performed with signal decomposition processing analysis to obtain the impedance of the power grid of the port of the branch to be measured, specifically comprises the steps of:
obtaining node data to be measured according to the voltage and current signals of the ports of the branches to be measured in the selected power grid multipoint voltage and current signals;
performing fast Fourier transform on the node data to be detected to obtain frequency domain data;
Extracting amplitude and phase information of frequency domain data, and converting three-phase voltage and current into positive and negative sequence voltage and current according to the amplitude and phase information of the frequency domain data;
carrying out power grid impedance calculation on the positive and negative sequence voltage and current to obtain power grid impedance of the port of the branch to be measured; wherein the grid impedance comprises a positive sequence impedance and a negative sequence impedance; the formula of the power grid impedance calculation is as follows:
wherein,for the positive sequence impedance, < >>For said negative sequence impedance, < >>Is a positive sequence harmonic voltage->Is a negative sequence harmonic voltage, < >>Is positive sequence harmonic current, +.>Is negativeOrder harmonic current (S.C.)>For harmonic frequencies +.>Indicating phase angle, & lt + & gt>Representing the amplitude.
6. The grid impedance measurement method of claim 1, further comprising: the current tracking control is specifically as follows:
multiplying the output result of the fuzzy PI parameter self-tuning controller with the three-phase voltage, superposing the multiplication result into a three-phase voltage control target, performing closed-loop control on the direct-current voltage, and tracking control current;
the fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting control parameters of the PI regulator through a fuzzy control algorithm; and the three-phase voltage control target is obtained by carrying out coordinate transformation on the output result of the fuzzy PI parameter self-tuning controller.
7. The power grid impedance measurement method according to claim 6, wherein the fuzzy PI parameter self-tuning controller is constructed by online optimization and adjustment of control parameters of a PI regulator by a fuzzy control algorithm, specifically:
calculating a current error and a current error change rate, performing fuzzy reasoning by using a fuzzy rule, and outputting the control parameter change quantity of the PI regulator;
in the fuzzy reasoning process, dividing the current error, the current error change rate and the parameter change quantity of the fuzzy PI parameter self-tuning controller into a plurality of fuzzy subsets respectively, wherein a membership function adopts a triangle function;
correcting the control parameter of the PI regulator, and superposing the control parameter variation on the basis of the initial control parameter to obtain the control parameter of the current working condition.
8. A power grid impedance measurement device, comprising: the system comprises a voltage and current acquisition module, a harmonic signal injection module, a direct current voltage control module, a current tracking control module, a driving module, a power electronic conversion module, a passive harmonic injection module and a power grid impedance calculation module;
wherein, the connection of each module is as follows: the voltage and current acquisition module is respectively connected with the current tracking control module, the power grid impedance calculation module and the power grid, the current tracking control module is respectively connected with the harmonic signal injection module, the driving module and the direct current voltage control module, the driving module is respectively connected with the power electronic conversion module and the passive harmonic injection module, and the output end of the power electronic conversion module is connected with the power grid through a three-phase LCL passive filter;
The harmonic signal injection module is used for generating a characteristic harmonic current instruction according to a harmonic signal generation algorithm when the power grid impedance measuring device and the power grid perform grid-connected stable operation, and inputting the characteristic harmonic current instruction into the direct-current voltage feedback control loop;
the direct-current voltage control module is used for constructing the direct-current voltage feedback control loop, and the direct-current voltage feedback control loop detects direct-current voltage in real time and keeps the voltage stable;
the current tracking control module is used for receiving the characteristic harmonic current instruction in the direct-current voltage feedback control loop, generating a control signal, and performing current tracking control on the direct-current voltage feedback control loop based on a voltage-current double-closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain harmonic current to be injected; the harmonic current to be injected is a current with a specific amplitude and a specific frequency;
the driving module is used for converting the information electronic signal of the main circuit into a driving signal of a device control loop according to the received control signal, and selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency; the harmonic injection mode comprises a passive harmonic injection mode and an active harmonic injection mode;
The method comprises the steps of selecting a preset harmonic injection mode in real time according to the harmonic injection target frequency, wherein the method specifically comprises the following steps:
when the harmonic injection target frequency is lower than the first frequency, determining that the current target frequency band is a low frequency band, and selecting the active harmonic injection mode;
when the harmonic injection target frequency is higher than the first frequency and lower than the second frequency, determining the current target frequency band as a middle frequency band, and selecting the passive harmonic injection mode;
when the harmonic injection target frequency is higher than the second frequency, determining the current target frequency band as a high frequency band, and selecting the active harmonic injection mode;
the power electronic conversion module is used for controlling the on and off of a switching tube according to the driving signal and a first specific sequence when the active harmonic injection mode is selected, and injecting the harmonic current to be injected into the power grid at a public coupling point;
the passive harmonic injection module is used for controlling the on-off of the IGBT according to the driving signal and the second specific sequence when the passive harmonic injection mode is selected, controlling the time and the current flowing direction of the capacitor put into the power grid for operation, and injecting the harmonic current to be injected into the power grid at the public coupling point;
The power grid impedance calculation module is used for selecting voltage and current signals of the ports of the branches to be measured in the power grid multipoint voltage and current signals to perform signal decomposition processing analysis after the harmonic current to be injected is injected into the power grid, so as to obtain the power grid impedance of the ports of the branches to be measured;
the voltage and current acquisition module is used for acquiring voltage and current at different positions in the power grid in real time to obtain the power grid multipoint voltage and current signals.
9. The grid impedance measuring device of claim 8,
the voltage and current acquisition module comprises a voltage transformer and a current transformer;
the harmonic signal injection module is constructed and generated by the harmonic signal generation algorithm;
the direct-current voltage control module is constructed and generated by the direct-current voltage feedback control loop;
the current tracking control module consists of a voltage-current double closed-loop structure, and the controller adopts a fuzzy PI parameter self-tuning controller;
the driving module is composed of a driving circuit;
the power electronic conversion module consists of a direct-current side energy storage capacitor and a voltage type inverter, wherein the voltage type inverter adopts a three-phase H-bridge structure;
The passive harmonic injection module is formed by connecting the IGBT and a capacitor;
the power grid impedance calculation module comprises an FFT calculation link, a positive and negative sequence conversion link and a positive and negative sequence impedance calculation link.
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