CN104330627B - Automatic power grid resonance detection method based on active injection current - Google Patents

Abstract

The invention discloses an automatic detection method for grid resonance based on active injection current; aiming at the harmonic pollution when there is a nonlinear load in the power system and the problem that resonance is easily generated when a reactive power compensation capacitor is connected, the method is connected to the grid through parallel connection The active power quality adjustment device compensates harmonics, and at the same time, uses this power quality adjustment device to inject a specific waveform of reactive power and harmonic combined current into the grid at one time to automatically detect the resonant angular frequency and damping coefficient of the grid. For a single-phase system, inject 180 ° square wave current, injecting 120° square wave current into the three-phase system; according to the spectrum characteristics of grid harmonic voltage, harmonic current and harmonic power, the present invention establishes a resonance detection combination model of optimal weight algorithm, which can quickly and Accurately determine the grid resonance angular frequency and damping degree, provide an application basis for the damping control of grid resonance, and further improve the power quality of the power system.

Description

An automatic detection method of power grid resonance based on active injection current

technical field

The invention belongs to the field of electric energy quality control, and more specifically relates to an automatic detection method for grid resonance based on active injection current.

Background technique

Parallel capacitors for reactive power compensation is a very common power factor correction method, which occupies a dominant position in the field of reactive power compensation in the industrial field. In the power system, capacitors of corresponding capacity are usually put in according to different load levels to improve the power factor of the system, increase the voltage level of the transmission line, and at the same time reduce the loss of the power grid and eliminate overload.

With the wide application of nonlinear loads such as various power electronic devices, the harmonic current in the power grid is increasing day by day, and the harmonic voltage drop will be generated when the harmonic current flows through the line impedance. Harmonic current and harmonic voltage will cause parallel connection or series resonance between power factor correction capacitors and grid impedance. This situation often occurs in power systems and seriously endangers the stability, safety and reliability of parallel capacitors and power systems.

The detection of grid resonance frequency is a prerequisite for suppressing grid resonance. At present, the commonly used method for detecting grid resonance points is: first, use thyristor switching capacitors, reactors and other methods to generate harmonic currents. At the moment of thyristor switching capacitors/reactors, Harmonic current will be injected into the grid, and the harmonic current will have different effects on the grid at different harmonic frequencies, and then use the grid resonance detection tool to record the waveform of the harmonic current and voltage, and then analyze the spectrum through instruments and waveforms Analyze and calculate harmonic impedance to determine system parameters for grid resonance.

However, the method used above has the disadvantages of poor detection accuracy, slow detection speed and poor field adaptability; and the detection process needs to be divided into two processes to be completed independently by different detection equipment, the cost is high, and the implementation is more complicated and flexible. Poor performance, can not meet the needs of automatic detection. First of all, the thyristor switching capacitor/reactor method is a passive impedance switching method, which uses the harmonic current generated during the transient response of the switching capacitor/reactor to detect the grid resonance point, and this method produces The harmonic current of the harmonic current is a continuous spectrum, in which the large low-frequency harmonic content will endanger the normal operation of the power system, while the small high-frequency harmonic content will affect the accurate determination of the resonant frequency of the power system, and is affected by the grid voltage and switching capacitors/ Influenced by the impedance of the reactor, the harmonic content of each order cannot be flexibly selected, and the site adaptability is poor.

In addition, most of the current grid resonance detection tools use special detection equipment. The detection equipment is relatively expensive. After obtaining the harmonic current and voltage waveforms of the power grid, it is necessary to combine the upper computer software for waveform spectrum analysis. The entire process is complicated to operate. The speed is slow and the flexibility is poor, and online automatic detection cannot be realized. When the power system is connected with nonlinear loads and shunt capacitors at the same time, and the power system parameters change due to load changes, the resonant frequency of the power grid is also different, and the passive impedance switching process of the traditional resonance detection method and the algorithm analysis process of the detection equipment If it is carried out independently, it often has poor adaptability and cannot meet the needs of rapid and automatic detection.

Contents of the invention

Aiming at the defects of the prior art, the purpose of the present invention is to provide an automatic grid resonance detection method based on active injection current, aiming to solve the problems of poor accuracy of traditional detection methods and inability to realize fast online automatic detection function.

The invention provides a method for automatically detecting grid resonance based on active injection current, comprising the following steps:

S1: Obtain the grid voltage and grid current of the public coupling point, the output current of the power quality adjustment device, and the DC side voltage; and obtain the angular frequency ω and phase θ of the grid voltage through the digital phase-locked link PLL;

Wherein, the public coupling point refers to the connection point between the main circuit of the power quality adjustment device and the power grid, and the output current refers to the output current of the active power quality adjustment device;

S2: Perform recursive discrete Fourier transform on the grid voltage and the grid current respectively to obtain the effective value V _ph of the characteristic harmonic voltage of each order of the grid and the effective value I _Sh of the characteristic harmonic current; and according to the characteristic harmonic Voltage effective value V _ph and characteristic harmonic current effective value I _Sh obtain the first apparent power S _h of each harmonic;

S3: Obtain the corresponding reference current i ^* _ref according to the set current waveform; and perform recursive discrete Fourier transform on the reference current i ^* _ref to obtain the effective value I ^* _rh of the characteristic harmonic current of the reference current ;

S4: performing a static-difference-free double-closed-loop feedback regulation on the output current, so that the active power regulating device injects the same output current as the set current waveform into the grid;

S5: When the output current is equal to the set current waveform, recursive discrete Fourier transform is performed on the grid voltage and grid current at this moment respectively, and the effective value V of each characteristic harmonic voltage of the grid after injecting the reference current is obtained ' _ph and the effective value of the characteristic harmonic current _I'Sh , and obtain the second apparent power _S'h of each harmonic according to the effective value of the characteristic harmonic voltage _V'ph and the effective value of the characteristic harmonic current _I'Sh ;

S6: According to the effective value V _ph and V' _ph of the characteristic harmonic voltage of the power grid before and after injecting the reference current, the effective value of the characteristic harmonic current I _Sh and I' _Sh , the apparent power of the characteristic harmonic S _h and S' _h , the effective value of each characteristic harmonic current I ^* _rh of the reference current, and obtain the system characteristic parameters used to describe the grid resonance: grid resonance angular frequency ω _r and damping coefficient ζ.

Wherein, in step S3, the set current waveform is a 180° square wave current or a 120° square wave current.

Wherein, step S4 is specifically:

S41: Obtain the feedback current if according to the output current; proportionally integrate the DC side voltage V _dc to obtain the active current _i ^* _dc ;

S42: Obtain an error current according to the reference current _i ^* _ref , the feedback current if, and the active current, and then adjust the error current to obtain a reference modulation signal u ^* _r ;

Among them, the current regulation method is related to the control strategies in different coordinate systems. In the dq synchronous rotating coordinates, the proportional integral plus repetition (PI+RP) controller can be used, and the proportional resonance (PR) controller can be used in the static coordinate system. controller.

S43: Comparing the reference modulation signal u ^* _r with the high-frequency carrier signal to obtain a PWM switch signal; the PWM switch signal is used to control the on-off of the switch tube, so that the active power regulating device injects a set current into the grid waveform the same as the output current.

Wherein, in step S6, according to the formula Obtain the h-th harmonic combination judgment index F _h after injecting the h-th harmonic current, F _h integrates the change value of the h-th grid harmonic voltage, harmonic current and harmonic power after injecting the reference current of the specified waveform of the unit The sum of information is more accurate than using one of the three physical indicators to detect grid resonance; by comparing the comprehensive detection index F _h value of each harmonic, the harmonic order h _r with the largest F _h value can be obtained, then The resonant angular frequency of the power grid is ω _r =100πh _r , that is, the resonant frequency of the power grid is f _r =50h _r ; among them, w _uh is the weight of the voltage detection index in the combination model, and w _Ih is the weight of the current detection index in the combination model , w _Sh is the weight of the power detection index model in the combined model, is the value after linear dimensionless processing of _{Fu uh} , is the value after linear dimensionless processing of F _Ih , is the value after linear dimensionless processing of F _Sh , and its calculation formulas are: F _uh is the ratio of grid harmonic voltage change value to injected harmonic current, F _Ih is the ratio of grid harmonic current change value to injected harmonic current, F _Sh is the square of grid harmonic power change value and injected harmonic current ratio, and its calculation formula is:

Since the present invention only needs to inject a reference current of a specific waveform into the power grid at one time, and the function of injecting reference current and the analysis function of the resonance point detection algorithm are all realized in the power quality adjustment device, there is no need for multiple detection tools to cooperate with each other in steps of functions for detection , the operation is simple, the detection speed is improved, and the field adaptability is strong; the power quality adjustment device is usually connected to the power grid system for a long time to suppress reactive harmonics and harmonics. When the resonant frequency of the power grid changes, entering the resonance working mode can Realize the online automatic detection of grid resonance; at the same time, the injected reference current signal contains all the characteristic harmonics, the 180° square wave current contains all the 2k+1 characteristic harmonic currents of the single-phase system, and the 120° square wave current mode Contains all 6k±1 characteristic harmonic currents of the single-three system (where k is a positive integer), so after the currents of the two waveforms are injected into the grid, the voltage and current parameters of the grid system can be adjusted at different harmonic frequencies Get a response, so that the resonance point of the power system can be accurately detected.

Description of drawings

FIG. 1 is a schematic structural diagram of a main circuit of a power quality adjustment device for automatically detecting single-phase/three-phase grid resonance provided by an embodiment of the present invention.

Fig. 2 is a control block diagram of an active injection current type grid resonance automatic detection system of a power quality adjustment device suitable for single-phase/three-phase power systems provided by an embodiment of the present invention.

Fig. 3 is the 180 ° square wave reference current output waveform of the active injection of the single-phase power system power quality adjustment device provided by the embodiment of the present invention; wherein (a) the 180 ° square wave current output waveform injected in the single-phase system (b) The current content of each characteristic frequency.

Fig. 4 is the 120 ° square wave reference current output waveform of the active injection of the three-phase power system power quality control device provided by the embodiment of the present invention; wherein (a) the 120 ° square wave current output waveform injected in the three-phase system (b) The current content of each characteristic frequency.

FIG. 5 is a flow chart of a detection method for grid resonance characteristic parameters (ie, resonance angular frequency ω _r and resonance damping coefficient ξ) provided by an embodiment of the present invention.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

In view of the shortcomings of the traditional detection method of power grid resonance, such as poor accuracy, inability to realize rapid online automatic detection function, high cost of detection tools, and poor field adaptability, the present invention provides an active power quality adjustment device, By injecting a reference current with a specific waveform into the grid at one time, and establishing a detection model according to the spectrum characteristics of the operating parameters of the grid system, a fast and accurate automatic detection method for the resonance frequency and resonance damping coefficient of the grid is realized. The power quality adjustment device integrates the active injection current and the resonance analysis algorithm. When the grid parameters change, it can realize the online automatic detection of the grid resonance, thus providing the basis for the stable and reliable operation of the power system. The invention is applicable to both single-phase and three-phase power systems.

The grid resonance automatic detection method based on active injection current provided by the present invention includes using an active power quality adjustment device connected to the power system to detect operating parameters such as grid voltage and grid current in real time on the one hand, and then calculate through the RDFT algorithm The effective value of each characteristic harmonic voltage and current of the power grid, and thus calculate the apparent power of each characteristic harmonic of the power grid in real time; The harmonic voltage, harmonic current and the corresponding harmonic energy spectrum characteristics are different, and the resonance detection combination model of the optimal weight algorithm is established, so as to quickly and accurately detect the grid resonance frequency and damping coefficient automatically, which is the damping of the grid resonance. Control provides the application basis to further improve the power quality of the power system and improve the stability of the power system.

The power quality adjustment device is an active power electronic switch type compensation device, which can output a reference current with a specific waveform without static difference by controlling the on-off of the power electronic switch device in the main circuit.

The power quality adjustment device has grid voltage, grid current and its own output current detection module, which detects the operating parameters of the entire power system in real time. The central control unit of the power quality adjustment device has high data calculation and processing capabilities, and the peripheral circuit has a communication and display module, which can send the power grid operating parameters and resonance detection results to the panel for display.

The power quality adjustment device has two working modes, namely harmonic compensation mode and resonance detection mode. When working in the harmonic compensation mode, the characteristic harmonics of each order can be extracted through the RDFT algorithm, and no static error compensation of the specified harmonic can be realized; when working in the resonance detection mode, a reference current of a specific waveform can be injected into the power grid at one time , can quickly and accurately realize the automatic detection of the resonant frequency and damping coefficient of the power grid.

The reference current injection method of a specific waveform includes: for a single-phase system, a 180° square wave current injection method, and for a three-phase system, a 120° square wave current injection method. The fundamental wave of the injection current of the two different waveforms is reactive current, so that the DC bus voltage of the power quality adjustment device can be kept stable, and the content of each characteristic harmonic is complete, flexible and controllable, so it can quickly and accurately determine the power grid resonance point.

The tracking control of the reference current adopts the feedback control strategy of double closed-loop without static error, and the feedback control of proportional integral (PI) adjustment can be used for the voltage outer loop control mode; the current inner loop control mode is related to the control strategy under different coordinate systems, in dq Proportional-integral plus repetition (PI+RP) controllers can be used in synchronous rotating coordinates, and proportional resonant (PR) controllers can be used in stationary coordinates.

To achieve the above object, the present invention provides an automatic detection method for grid resonance based on active injection current, which is suitable for single-phase and three-phase power systems. The main circuit of the power quality adjustment device for automatic detection of grid resonance in a single-phase system is: a single-phase bridge-type power electronic converter connected in parallel between a single-phase grid and a single-phase nonlinear load. The main circuit of the power quality adjustment device for automatic detection of grid resonance in a three-phase system is: a three-phase bridge-type power electronic converter connected in parallel between the three-phase grid and the three-phase nonlinear load.

The present invention is applicable to the grid resonance automatic detection method based on the active injection current of the single-phase/three-phase power system, comprising the following steps:

S1: Obtain the grid voltage and grid current of the public coupling point (Point of Common Coupling, PPC point), the output current of the active power quality adjustment device, and the DC side voltage; and obtain the angular frequency ω of the grid voltage through the digital phase-locked link PLL and phase θ.

Wherein, step S1 is specifically:

S11: Perform AD conversion in real time to obtain the instantaneous value v _P of the grid voltage at the public coupling point, the instantaneous value i _S of the grid current, the instantaneous value i _C of the output current of the active power quality adjustment device, and the instantaneous value V _dc of the DC side voltage; in the present invention In the embodiment, the public coupling point refers to the connection point between the main circuit of the power quality adjustment device and the power grid.

S12: Obtain the angular frequency ω and phase θ of the grid voltage through a digital phase-locked link PLL.

S2: Recursive Discrete Fourier Transform (RDFT) is performed on the instantaneous value v _P of the grid voltage and the instantaneous value i _S of the grid current to obtain the effective value V _ph of the characteristic harmonic voltage and the characteristic harmonic current of the grid RMS value I _Sh ; and according to V _ph and I _Sh obtain the apparent power of each harmonic S _h ; where, h is the harmonic order, for single-phase system h=2k+1, for three-phase system h=6k±1 , k is a positive integer, k=1, 2, . . .

S3: Obtain the corresponding reference current i ^* _ref according to the specified current waveform; and perform recursive discrete Fourier transform on the reference current i ^* _ref to obtain the RMS value I ^* _rh of each characteristic harmonic current of the reference current. Among them, for the single-phase system reference current i ^* _ref , the specified current waveform is 180°square wave current, and for the three-phase system reference current i ^* _ref , the specified current waveform is 120°square wave current.

S4: Perform static error-free double closed-loop feedback regulation on the output current of the power quality regulating device, so that the active power regulating device injects the same output current as the specified current waveform into the grid.

Wherein, step S4 is specifically:

S41: Obtain the feedback current if from the instantaneous value of the output current _{i C} ; perform proportional integration (PI) on the instantaneous value of the DC side voltage V _dc to obtain the active current i ^* _dc .

S42: Summing i ^* _ref , -if and _i ^* _dc to obtain an error current, and then adjusting the error current to obtain a reference modulation signal u ^* _r .

S43: Comparing the reference modulation signal u ^* _r with the high-frequency carrier signal to obtain a PWM switch signal; the PWM switch signal controls the on-off of the switch tube, so that the active power regulating device injects the same output as the specified current waveform into the grid current.

According to the principle of sinusoidal pulse width modulation, the high-frequency carrier signal is a sawtooth wave or a triangular carrier signal with an amplitude of 1; within one carrier cycle, when the reference modulation signal u ^* _r is greater than the high-frequency carrier signal, a high-level signal is output PWM signal; when the reference modulation signal u ^* _r is smaller than the high-frequency carrier signal, a low-level PWM signal is output.

S5: After the repeated adjustment of step S4, the active power regulating device will stably output the same current as the specified current waveform. At this time, through RDFT operation, the effective value _V'ph of each characteristic harmonic voltage of the power grid after injecting the reference current is obtained And the effective value of the characteristic harmonic current I' _Sh , and calculate the apparent power S' _h of each harmonic.

S6: The effective values V _ph and V' _ph of the characteristic harmonic voltages of the power grid before and after the injection of the reference current, the effective values of the characteristic harmonic currents I _Sh and I' _Sh , the apparent powers of the characteristic harmonics S _h and S' _h , The RMS value I ^* _rh of each characteristic harmonic current of the reference current is substituted into the resonance detection combination model as an input for calculation and processing, so as to obtain the grid resonance angular frequency ω _r and the damping coefficient ζ, ω _r and ζ are used to describe the grid resonance system characteristic parameters.

In the embodiment of the present invention, the tracking control of the reference current in step S4 adopts the feedback control strategy of double closed-loop without static error, and the voltage outer loop control mode adopts the feedback control of proportional integral; the current inner loop control mode and the control under different coordinate systems It is related to the strategy. In the dq synchronous rotating coordinates, the proportional integral plus repetition (PI+RP) control method can be used, and the proportional resonance (PR) control method can be used in the stationary coordinate system.

The optimal weight algorithm in step S6 is specifically as follows: firstly, synchronous cumulative filtering is performed on the effective values of harmonic voltage V _ph and V' _ph , and the effective value of harmonic current I _Sh and I' _Sh , that is, the harmonics detected multiple times in succession Then, calculate the harmonic apparent power S _h and S' _h , calculate the harmonic voltage change value ΔV _ph before and after injecting the reference current of the specified waveform, and the harmonic current change value ΔI _Sh , the harmonic apparent power variation ΔS _h ; then, calculate the ratios _Fuh , F _Ih and F _Sh of the output variation of each harmonic voltage, harmonic current, and harmonic apparent power to the injected harmonic current . Finally, according to the spectrum characteristics of harmonic voltage, current and power, the resonant angular frequency ω _r and the damping coefficient ζ of the grid are judged.

The steps above are applicable to both single-phase and three-phase systems, and the control strategy adopted for the tracking control of the reference current is a double-closed-loop feedback control without static error; the difference lies in the specified current waveform of the reference current and The number of phases is different, and the three-phase system needs to generate three-phase 120° square wave reference currents i ^* _refa , i ^* _refb , i ^* _refc , where the phase of i ^* _refa is 120° ahead of the phase of i ^* _refb , and the phase of i ^* _refb Leading the phase of i ^* _refc by 120°, while the single-phase system only needs to generate a reference current i ^* _ref of a 180° square wave of one phase.

The invention provides an automatic detection method based on active injection current to realize the resonance frequency and resonance damping coefficient of the power grid. The automatic detection function can be realized by using single-phase and three-phase power quality adjustment devices. Therefore, in single-phase and three-phase Applicable to all phase power systems. The method integrates active injection of reference current and resonance analysis algorithm. It only needs to inject a reference current of a specific waveform into the power grid at one time, and can quickly and accurately realize the online automatic detection function of the resonant frequency and damping coefficient of the power grid, thereby ensuring The power system operates efficiently, stably and reliably. Since the function of injecting command harmonic current and the analysis function of resonance point detection algorithm are realized in the power quality adjustment device, there is no need for multiple detection tools to cooperate with each other to detect according to the function and step by step. The operation is simple, the detection speed is improved, and the field adaptability is relatively large. Strong; power quality adjustment devices are usually connected to the power grid system for a long time to suppress reactive harmonics and harmonics. When the resonant frequency of the power grid changes, entering the resonant working mode can realize online automatic detection of grid resonance; at the same time, the injected reference current The characteristic harmonic content of each order is complete, the 180° square wave current contains all the 2k+1 characteristic harmonic currents of the single-phase system, and the 120° square wave current mode contains all the 6k±1 characteristic harmonic currents of the single-three system ( Wherein, k is a positive integer), so after the currents of the two waveforms are injected into the grid, the voltage and current parameters of the grid system can be responded at different harmonic frequencies, so that the grid resonance frequency of the power system can be accurately detected and the degree of damping.

The present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

Fig. 1 shows the circuit structure of the power quality regulating device suitable for the active injection type grid resonance automatic detection of the single-phase/three-phase power system provided by the embodiment of the present invention. relevant part.

If the power quality control device is used for automatic detection of grid resonance in a single-phase power system, the main circuit includes: a single-phase bridge-type power electronic converter connected in parallel between the single-phase grid and the single-phase nonlinear load, the first straight The current bus capacitor C ₁ and the first AC reactor L _f ; the first DC bus capacitor C ₁ is connected in parallel to the DC terminal of the single-phase bridge power electronic converter, and the first AC reactor L _f is connected to the single-phase power grid and the single-phase between the AC sides of the phase bridge power electronic converter. If the power quality adjustment device is used for automatic detection of grid resonance in a three-phase power system, the main circuit is: a three-phase bridge-type power electronic converter connected in parallel between the three-phase grid and the three-phase nonlinear load, the first A DC bus capacitor C ₁ and the first AC reactor group L _f ; the first DC bus capacitor C ₁ is connected in parallel to the DC end of the three-phase bridge power electronic converter, and the first AC reactor group L _f is connected to the three-phase Between the phase grid and the AC side of the three-phase bridge power electronic converter.

As shown in Figure 2, in order to realize the system control block diagram of the above-mentioned grid resonance automatic detection method based on active injection current suitable for single-phase/three-phase power systems, the power quality adjustment device is connected in parallel on the PCC point of the AC grid, and the dotted line box It is the control system of the power quality adjustment device. The peripheral functions include harmonic voltage detection, harmonic current detection, load harmonic detection, output current detection and PWM drive, which are used to detect grid harmonic voltage v _P and grid harmonic current i _S , the load harmonic current i _L , the output current i _C and the on-off of the switch tube of the converter module of the main circuit drive; the functions realized by the control core include grid voltage phase-locked PLL, RDFT of peripheral detection signals, working mode selection (including switch control signal and S1), specified sub _- load harmonic compensation (realized by the specified sub-harmonic compensation control unit), specific waveform current generation (realized by the specified waveform current command generation unit), and the resonance of the optimal weight algorithm Detection and analysis (realized by command operation and resonance detection mode control, optimal weight algorithm combination model) and no static error double closed-loop control (including voltage outer loop control and current inner loop no static error control). When the power quality adjustment device works in the resonance detection mode, the command operation and resonance detection mode control module outputs the switch signal, and through the switch control, S1 is _switched to the reference current terminal, and the grid resonance point detection is started. On the one hand, specify the waveform current instruction generation module to output the reference current, compare it with the output current of the power quality adjustment device, and perform a double closed-loop feedback control without static error, so as to output a PWM signal to control the switching tube of the main circuit; on the other hand, the instruction operation The harmonic component of the current command is extracted with the resonance detection mode control module, and the effective value of the characteristic harmonic voltage, the effective value of the characteristic harmonic current and the effective value of the apparent power of each harmonic before and after the current injection are used as preprocessing Data, through the optimal weight algorithm combination model to analyze the grid resonance point, and identify the grid resonance angular frequency ω _r and damping coefficient ζ.

In the embodiment of the present invention, the reference current waveform for the active injection of the single-phase power system adopts the mode of injecting a 180° square wave current, and the mode of injecting a 120° square wave current for the reference current waveform of the active injection of the three-phase power system; The outer loop control method adopts the feedback control of the PI regulator; the current inner loop control strategy adopts the non-static closed-loop control strategy under the dq coordinate axis, and adopts PI+RP control under the dq coordinate pumping by performing dq coordinate transformation on the reference current The strategy can better realize the injection of the specified square wave current.

Fig. 5 shows the flow chart of the grid resonance characteristic parameters provided by the embodiment of the present invention, that is, the detection method of the grid resonance angular frequency ω _r and the resonance damping coefficient ξ, which specifically includes the following steps:

S1: Obtain the grid voltage and grid current of the PPC point, the output current of the active power quality adjustment device, and the DC side voltage; and obtain the angular frequency ω and phase θ of the grid voltage through the digital phase-locked link PLL.

S11: Perform AD conversion in real time to obtain the instantaneous value v _P of the grid voltage at the public coupling point, the instantaneous value i _S of the grid current, the instantaneous value i _C of the output current of the active power quality adjustment device, and the instantaneous value V _dc of the DC side voltage; in the present invention In the embodiment, the public coupling point refers to the connection point between the main circuit of the power quality adjustment device and the power grid.

S12: Obtain the angular frequency ω and phase θ of the grid voltage through the digital phase-locked link PLL;

S2: Perform recursive discrete Fourier transform on the grid voltage instantaneous value v _P and the grid current instantaneous value i _S respectively to obtain the effective value V _ph of the characteristic harmonic voltage and the effective value I _Sh of the characteristic harmonic current of the grid; and according to V _ph and I _Sh obtain the apparent power _Sh of each harmonic. Wherein, h is the harmonic order, for single-phase system h=2k+1, for three-phase system h=6k±1, k is a positive integer, k=1, 2,....

S3: Generate a corresponding reference current i ^* _ref according to the specified injected current waveform and its amplitude. In a single-phase system, the 180° square wave is used as the injected current waveform to generate the corresponding reference current i ^* _ref , which can be decomposed into:

Wherein, I _s is the effective value of the current, n=2k+1, and k is a positive integer. It can be seen that the 180° square wave current contains all odd harmonic components.

In the three-phase system, the 120° square wave is used as the injected current waveform to generate the corresponding three-phase current commands i ^* _refa , i ^* _refb , i ^* _refc , where the phase of i ^* _refa is ahead of i ^* _{refb by} 120°, i ^* The phase of _refb leads i ^* _{refc by} 120°. At this time, the 120° square wave current can be decomposed by Fourier into:

Among them, I _s is the effective value of the current, and it can be seen that the 120° square wave current contains all h=6k±1 harmonic components.

S4: Perform static error-free double closed-loop feedback regulation on the output current of the power quality regulating device, so that the active power regulating device injects the same output current as the specified current waveform into the grid.

S41: Obtain the feedback current if from the instantaneous value of the output current _{i C} ; perform proportional integration (PI) on the instantaneous value of the DC side voltage V _dc to obtain the active current i ^* _dc . The error current is obtained by summing i ^* _ref , -i _f and i ^* _dc . In the single-phase system, the single-phase error current is i ^* _er ; in the three-phase system error current, the three-phase error current is i ^* _era , i ^* _erb , i ^* _erc .

S42: For a single-phase system, transform the single-phase error current i ^* _er into the current i ^* _α and i ^* _β in the αβ static coordinate system, as shown in formula (2), and then transform the current signal through the first stage dq transformation The harmonic signals of different frequencies in streams i ^* _α and i ^* _β are transformed into signals i ^* _d and i ^* _q in the dq coordinate system, as shown in formula (3).

For a three-phase system, the three-phase error currents i ^* _era , i ^* _erb , i ^* _erc can be transformed into command signals i _d ^* , i _q ^* in the dq coordinate system through dq transformation, as shown in formula (5) Show.

S43: In the dq coordinate system, perform PI+RP control on the signals i _d ^* and i _q ^* to obtain the reference modulation signals u ^* _d and u ^* _q , and perform dq inverse transformation on the reference modulation signals u ^* _d and u ^* _q , get the final reference modulation signal u ^* _r , compare it with the high-frequency carrier signal, and generate a PWM signal according to the principle of sinusoidal pulse width modulation to control the on-off of the switch tube, so that the active power regulating device injects the reference current waveform into the grid same output current i _C .

As an embodiment of the present invention, the proportional resonance control method is adopted in the static coordinate system: first, the error current is obtained through S41 (the single-phase system error current is i ^* _er ; the three-phase system error current is i ^* _era , i ^* _erb , i ^* _erc .), and then adjust the error current with proportional resonance feedback, where the transfer function of the quasi-proportional resonance controller used for feedback adjustment is:

In the formula, K _p is the coefficient of the proportional term, K _R is the coefficient of the resonance term, ω _c is the bandwidth of the controller, and ω ₀ is the frequency of the maximum point of the controller gain. After the above feedback adjustment, the final reference modulation signal u ^* _r is obtained, which is compared with the high-frequency carrier signal to generate a PWM signal to control the on-off of the switch tube, so that the active power regulating device injects the same output as the reference current waveform into the grid current i _C .

Figure 3 shows the generated 180° square wave current waveform suitable for single-phase systems, and Figure 4 shows the generated 120° square wave current waveform suitable for single-phase systems. After the harmonic current is injected into the power grid, the resonance detection combination model based on the optimal weight algorithm needs to be used to determine the resonance point of the power grid.

S5: Obtain the effective value _V'ph of the characteristic harmonic voltage and the effective value I' _Sh of the characteristic harmonic current of the power grid after injecting the reference current through the RDFT operation, and calculate the apparent power _S'h of each harmonic. And perform RDFT operation on the reference current i ^* _ref to calculate the RMS value I ^* _rh of each characteristic harmonic current of the reference current.

S6: Perform synchronous cumulative filtering on harmonic voltage V _ph and V' _ph , harmonic current effective value I _Sh and I' _Sh , harmonic apparent power S _h and S' _h , that is, according to the harmonic Wave voltage, harmonic current and harmonic apparent power RMS are accumulated and summed and then averaged. The calculation formula is:

In the formula: is the average value of the same signal measured multiple times, and X _si is the effective value of the signal measured for the ith time.

According to the spectrum characteristics of harmonic voltage, current and energy, a resonance detection model is established to identify the grid resonance angular frequency ω _r . Calculate the ratio Fuh of the change value of the system hth harmonic voltage before and after injecting the reference current of the specified waveform to the injected _hth harmonic current:

Calculate the ratio F _Ih of the change value of the hth harmonic current of the system to the injected hth harmonic current:

Calculate the ratio F _Sh of the hth harmonic apparent power change value of the system to the injected hth harmonic current:

After _Fuh , F _Ih and F _Sh are obtained, the normalization method is used to perform linear dimensionless processing on _Fuh , F _Ih and F _Sh , and the calculation formula is:

Where Y represents voltage u, current I or apparent power S, for single-phase system h=2k+1, for three-phase system h=6k±1, k is a positive integer. Substituting the normalized results into the weighted sum of the detection model, the calculation formula is:

Among them, w _uh is the weight of the voltage detection index in the combination model, w _Ih is the weight of the current detection index in the combination model, and w _Sh is the weight of the power detection index model in the combination model. F _h is the judging index of the h-th harmonic combination after the h-th harmonic current is injected, from which the resonant angular frequency ω _r of the power grid can be judged. F _h is the judgment index of the hth harmonic combination after the injection of the hth harmonic current. By comparing the combined judgment index F _h value of each harmonic, the harmonic order h _r with the largest F _h value can be obtained, and the power grid can be judged The resonant angular frequency ω _r is ω _r =100πh _r , that is, the grid resonant frequency f _r is f _r =50h _r . The selection of weight values w _uh , w _Ih , w _Sh is related to the AD detection accuracy of grid system parameters, load and grid impedance. If the grid voltage AD detection accuracy is higher than the grid current detection accuracy, the value of w _uh is larger than w _Ih , otherwise the value of w _Ih is larger than w _uh ; when the AD detection accuracy of grid voltage and current is high, due to the power determination It is the product of the voltage judgment index and the current judgment index. Its spectrum curve is steeper, and it is more accurate to use the power judgment index to judge the resonance point of the power grid. At this time, w _Sh can take a relatively larger value, but when the AD detection accuracy is low, the power judgment The index error is the largest, and the value of w _Sh should be relatively small at this time; when the grid load is large, the spectrum curves of the voltage judgment index and the current judgment index are relatively gentle, but at this time the power judgment index spectrum curve is still steep, and w _Sh is desirable Relatively larger value; when the grid impedance is large, the spectrum curve of the voltage judgment index is steeper than the current judgment index, at this time the weights are w _Sh , w _uh , w _Ih in descending order, otherwise the weights are in descending order They are w _Sh , w _Ih , w _uh in turn.

In the power system connected to the load, let Z=L _SL s+R _SL be the sum of the equivalent impedance of the grid impedance and the load impedance, C is the parallel capacitor of the grid, since the equivalent resistance R _SL of the grid system is very small, then The resonant angular frequency of the grid system is close to its undamped resonant angular frequency, then the resonant angular frequency of the grid is The sum of grid current I _S and load current I _L is I _SL , then the transfer function of I _SL and the current I _C injected into the grid is:

When s= _jωr , let I _Cωr be the effective value of the grid current injected at the resonant frequency, and I _SLωr be the sum of the grid and the load current at the resonant frequency. The amplitude-frequency response at the resonant frequency of the power grid can be obtained:

From this, the resonance damping coefficient can be calculated as:

In the embodiment of the present invention, the resonance point of the single-phase grid is detected by injecting a 180° square wave current, and the resonance point of the three-phase grid is detected by injecting a 120° square wave current. The 180° square wave current contains all odd-order characteristic harmonics that may appear in single-phase circuits, and the 120° square wave current contains all (6k±1) order harmonics that may appear in three-phase three-wire systems. The accuracy of wave resonance detection is high; in addition, the functions of active injection reference current and resonance analysis algorithm are integrated on the power quality adjustment device, which is simple and convenient to operate, and can quickly and accurately realize the online automatic detection function of grid resonance. So as to ensure the stable and reliable operation of the power system.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention, All should be included within the protection scope of the present invention.

Claims (2)

1. A power grid resonance automatic detection method based on active injection current is characterized by comprising the following steps:

s1: acquiring the power grid voltage and the power grid current of a public coupling point, the output current of the power quality adjusting device and the voltage of a direct current side; acquiring the angular frequency omega and the phase theta of the power grid voltage through a digital phase-locked loop PLL;

the public coupling point refers to a connection point of a main circuit of the power quality adjusting device and a power grid, and the output current refers to the output current i of the active power quality adjusting device_C；

S2: performing recursive discrete Fourier transform on the power grid voltage and the power grid current respectively to obtain effective values V of each characteristic harmonic voltage of the power grid_phAnd the effective value of characteristic harmonic current I_Sh(ii) a And according to the characteristic harmonic voltage effective value V_phAnd the effective value of characteristic harmonic current I_ShObtaining the first apparent power S of each harmonic_h；

S3: obtaining a reference current i corresponding to the set current waveform according to the set current waveform^* _ref(ii) a And for the reference current i^* _refPerforming recursive discrete Fourier transform to obtain effective value I of each sub-characteristic harmonic current of the reference current^* _rh；

S4: performing non-static-error double closed-loop feedback regulation on the output current, so that the active electric energy regulating device injects the output current with the same waveform as the set current into the power grid;

s5: when the output current is equal to the set current waveform, performing recursive discrete Fourier transform on the power grid voltage and the power grid current at the moment respectively to obtain effective values V 'of each sub-characteristic harmonic voltage of the power grid after the reference current is injected'_phAnd characteristic harmonic current effective value I'_ShAccording to the characteristic harmonic voltage effective value V'_phAnd characteristic harmonic current effective value I'_ShObtaining second apparent power S 'of each subharmonic'_h；

S6: according to the effective value V of each sub-characteristic harmonic voltage of the power grid before and after the reference current is injected_phAnd V'_phEffective value of characteristic harmonic current I_ShAnd l'_ShApparent power S of characteristic harmonic wave_hAnd S'_hEffective value of each sub-characteristic harmonic current I of reference current^* _rhObtaining system characteristic parameters for describing the resonance of the power grid: grid resonant angular frequency omega_rAnd a damping coefficient ζ;

in step S3, the set current waveform is 180 ° square wave current or 120 ° square wave current;

in step S6, according to the formulaObtaining the h-th harmonic combination judgment index F after the h-th harmonic current is injected_hBy comparing the comprehensive detection indexes F of the harmonics_hValue obtaining F_hMaximum harmonic order h_rAnd according to the harmonic number h_rObtaining the resonant angular frequency omega of the power grid_r＝100πh_rI.e. the resonant frequency of the network is f_r＝50h_r(ii) a And according to the formulaA damping coefficient ζ is obtained, and,

wherein, w_uhFor the weight of the voltage detection indicator in the combined model, w_IhFor the weight of the current detection indicator in the combined model, w_ShWeights of the power detection index model in the combined model are obtained; f^* _uhIs pair F_uhValue after linear dimensionless processing, F^* _IhIs pair F_IhValue after linear dimensionless processing, F^* _ShIs pair F_ShValue after linear dimensionless processing, I_CωrFor injecting effective value of grid current at resonance frequency, I_SLωrIs the sum of the grid and load currents at the resonant frequency; n is 2k +1, k is a positive integer, F_uhIs the ratio of the h harmonic voltage variation value of the power grid to the h harmonic current injected, F_IhIs the ratio of the h harmonic current variation value of the power grid to the h harmonic current injected, F_ShIs the ratio of the power grid h harmonic power variation value to the square of the injected h harmonic current, wherein,

2. the method for automatically detecting the grid resonance according to claim 1, wherein the step S4 specifically comprises:

s41: obtaining a feedback current i from the output current_f(ii) a To the DC side voltage V_dcProportional integral is carried out to obtain active current i^* _dc；

S42: according to the reference current i^* _refFeedback current i_fObtaining an error current from the active current, and then carrying out current regulation on the error current to obtain a reference modulation signal u^* _r；

S43: modulating the reference modulation signal u^* _rComparing with the high-frequency carrier signal to obtain a PWM switching signal; the PWM switching signal is used for controlling the on-off of the switching tube, so that the active electric energy adjusting device injects output current with the same waveform as the set current into the power grid.