43 results about "Nyquist stability criterion" patented technology

The invention discloses a stability analysis method for a grid-connected inverter system under the coexistence condition of multiple frequency coupling factors. The method is applied to a grid-connected inverter system, wherein the situation that multiple frequency coupling factors coexist is considered. A frequency coupling characteristic analysis model of a grid-connected inverter is establishedin a phase sequence coordinate system, and a power grid impedance matrix is obtained through calculation under the condition that the frequency coupling is considered. The method is realized based onthe generalized Nyquist stability criterion, and the stability of the grid-connected inverter system is judged by utilizing the generalized impedance ratio matrix of the grid impedance matrix and thefrequency coupling characteristic matrix of the grid-connected inverter. Therefore, the method can be used for analyzing the system stability under the complex condition of the coexistence of multi-frequency coupling factors. Compared with an existing stability analysis method based on the impedance of the grid-connected inverter, the method is more complete. Therefore, analysis errors caused byignoring frequency coupling are avoided. The stability of the grid-connected inverter system under the complex condition can be analyzed more accurately.

The invention provides a quantitative analysis method for the subsynchronous oscillation stability of a wind turbine generator grid-connected system. The quantitative analysis method comprises the following steps: adding positive and negative sequence voltage disturbance signals to a wind turbine generator grid-connected point; establishing transfer functions of a wind turbine generator phase-locked loop, a current adjusting link and a PWM modulation link; obtaining positive and negative sequence impedance models Zp (s) and Zn (s) of a wind turbine generator, an equivalent impedance model Zg (s) of a power grid and a total impedance model Z (s) of a grid-connected system; determining proper fitting fractal polynomial orders m and n, and fitting the total impedance model Z (s) of the grid-connected system by using the fitting fractal polynomial orders m and n; solving a zero point of the fitting fractal polynomial in a fitting interval according to a fitting result, and determining theoscillation frequency and the damping level of the system according to the solved zero point. According to the invention, the method provided by the invention is provided based on the sequential impedance model in a static coordinate system. The method solves the problem that the traditional Nyquist stability criterion based on a sequential impedance model cannot quantitatively obtain the oscillation frequency and the damping level of the system in the prior art. The problem that the quantitative analysis method based on an aggregation RLC circuit model is narrow in application range is also solved.

The invention discloses an impedance modeling and stability analysis method of a current controlled virtual synchronous generator. In consideration of the influence of the active controller, the reactive controller, the current inner loop control and the phase locked loop of the current controlled virtual synchronous generator, a wideband small signal modeling method for current controlled virtualsynchronous generator based on sequence impedance model is provide to solve the problem of small signal impedance modeling of current controlled virtual synchronous generator. Based on a built wideband small signal impedance model, a grid impedance model and a Nyquist stability criterion, the influence of grid impedance, current controlled virtual synchronous generator grid-connected number and phase-locked loop control bandwidth on system stability is analyzed. The invention provides a model and method for analyzing the small disturbance stability in scenes of the current controlled virtualsynchronous generator connected to a micro grid and a new energy field station.

The invention discloses an LCL filter parameter design method of a grid-connected inverter for improving adaptability of a weak grid. The method comprises the following steps of firstly, calculating delay time of the grid-connected inverter, obtaining an open-loop transfer function according to a current control loop, obtaining system resonant frequency stability domains based on stability margin according to a generalized Nyquist stability criterion to be used as a selection basis of an LCL resonant frequency and a resonant frequency of a bridge arm side inductor and a filter capacitor, and limiting the two resonant frequencies within one of the stability domains, so that the system resonant frequency is enabled not to deviate from the stability domain when the impedance of the weak grid changes in a large range, and the system stability is improved; secondly, solving an inductance value of a bridge arm side according to maximum ripple requirement of the current of the bridge arm side inductor; and finally, solving the filter capacitance and the grid side inductance according to the two resonance frequencies. By the method, a parameter design method under different current control rings and different delay sizes is considered, a complicated and repeated trial and error step is omitted, and the calculation method is simple and intuitive.

The invention discloses a virtual synchronous generator parameter design method. The method comprises the following steps: to begin with, calculating delay time of digital control of a virtual synchronous generator; obtaining an LC resonant frequency stability domain based on stability margin according to an open-loop transfer function of a control loop and generalized Nyquist stability criterion; calculating resonant frequency through a weighted geometric mean calculation method; for outer ring parameters, parameters are designed independently through a droop coefficient and damping coefficient independent control method; and through combination of advantages of flexible control and fast dynamic response of a power electronic converter and adaptive droop and big inertia and the like of a synchronous generator, a quasi-synchronous generator outer-ring parameter design method is provided. The method saves tedious repeated trial and error steps, and the calculating method is simple and visual.

Approximating loci of eigenvalues or characteristic gains of a return ratio matrix of a model of a multi-phase power converter circuit by the loci of the d-d and q-q elements of said synchronous frame of reference applied to said model, allows determination and assessment of stability of the circuit or forbidden operational parameters of the combination of an AC power source and a power converter at an interface thereof by application of a standard Nyquist stability criterion.

The invention provides a method and system for analyzing the stability of a distributed DC grid. The method comprises a step of obtaining an open-loop equivalent transfer function based on the topology structure of the distributed DC grid and a pre-built stability analysis model, and a step of applying the Nyquist stability criterion to the open-loop equivalent transfer function to determine the stability of the distributed DC grid, wherein a power electronic transformer, a photovoltaic power generation system and an energy storage system in the distributed DC grid are equivalent to DC / DC circuits and the DC / DC circuits are constructed by using a superposition principle according to the stability analysis model. According to the invention, the stability of the DC grid constructed by a DC / DC converter is clearly provided, the grid system shutdown protection caused by faults such as load sudden change, power supply fluctuation or system disturbance of a grid system is avoided, and the stability of the power supply of the grid system is ensured.

The invention discloses an impedance modeling and stability analysis method for a voltage-control virtual synchronous generator. The influence of a virtual synchronous generator active controller is considered in order to provide a voltage-control virtual synchronous generator broadband small-signal modeling method based on a sequence impedance model, overcoming the difficulties in voltage-controlvirtual synchronous generator small-signal impedance modeling. On the basis of an established broadband small-signal sequence impedance model, a grid impedance model and Nyquist stability criterion,the influences of grid impedance and the number of voltage-control virtual synchronous generator grid-connected units upon system stability are analyzed. The results show that in any case of high gridimpedance and the large number of the virtual synchronous generator grid-connected units, a virtual synchronous generator grid-connected system can stably run. The models and method are provided forthe small disturbance stability analysis in the scenes, such as voltage-control virtual synchronous generators connected to a grid, and new energy stations; the popularization and application of the voltage-control virtual synchronous generators are benefited.

The invention discloses a stability analysis method of a rotor-side converter of a double-fed fan based on rotational speed control, Firstly, the circuit model of the rotor-side converter of the double-fed fan under the dq axis is established, then the current inner loop control and the speed control model are established according to the control block diagram, and the mathematical model of the coordinate transformation influence is established according to the relationship between the circuit and the control. Then the closed-loop output impedance and the back-ratio matrix of the rotor-side converter are derived. The influence of various parameters including wind speed on the stability of the doubly-fed fan converter is judged by the generalized Nyquist stability criterion. A method for analyze stability features that that connection mode of stator and rotor in doubly-fed fan and the influence of wind speed in the conversion of electric energy to mechanical energy are taken into account, Accurate impedance model is established and the influence of each parameter on system stability is analyzed. Harmonic instability and other harmful phenomena harming the power system can be avoidedby adjusting the parameters.

The invention discloses a stability criterion method suitable for a multi-voltage-grade direct-current power distribution system. The impedance criterion method comprises the following steps: firstly,according to the definitions of a control bus voltage type converter and a control bus current type converter, a unified expression form of the multi-voltage-grade direct-current power distribution system is obtained; secondly, small-signal modeling is performed on each converter, output voltage and output current control modes are adopted for a direct-current transformer, and a two-port small-signal model of the multi-voltage-level direct-current power distribution system in the unified form is built; thirdly, a transfer function from input variables to output variables of the system and anequivalent loop gain expression of the system are derived; and finally, the stability of the system is judged through whether the equivalent loop gain of the system meets a Nyquist stability criterionor not, and if the equivalent loop gain meets the Nyquist stability criterion, the system is stable, otherwise, the system is not stable. The impedance ratio criterion of an existing direct-current power distribution system is popularized to the multi-voltage-grade direct-current power distribution system, so that the application range is wider.

A harmonic instability evaluation method for DC transmitting and receiving end weak systems comprises the steps: at first, the current of the AC side of a converter and the positive sequence second harmonic current generated by the DC bias magnetic field are calculated, then the sum of the positive sequence second harmonic current generated by DC bias and the second harmonic current generated by converter modulation is calculated, then the second harmonic impedance from the converter transformer valve side to the AC system is calculated, and then the harmonic voltage at the outlet of the sending-end converter station in the port impedance network is calculated. Finally, the harmonic instability of the system is judged according to the phase angle difference between the AC and DC harmonic impedance equivalents. A method for judging the harmonic instability of a DC transmiting and a receiving end weak system based on a Nyquist stability criterion and the DC bias magnetic action of a converter transformer and considering the equivalent harmonic impedance of an AC system and a DC system, wherein the considered factors are comparatively identical with the conditions in actual application, so that more accurate analysis results can be obtained.

The invention discloses a doubly-fed fan grid-connected port output impedance modeling and stability analysis method. The method is characterized in that influence of a doubly-fed fan stator flux linkage component and phase-locked loop and current loop links in a fan control system on output impedance of a system is comprehensively considered, a doubly-fed fan grid-connected port overall output impedance model and a weak power grid impedance model are established, based on the two models and a generalized Nyquist stability criterion, influence of grid-connected doubly-fed fan output impedancecharacteristics on stability under the weak power grid condition is analyzed, and then optimization design of fan control system loop parameters is guided. The impedance model provided by the invention is simple in structure and has good accuracy; the stability analysis method is simple and effective, and the model and the method are provided for doubly-fed fan grid-connected small-interference stability analysis under the weak power grid condition.

The invention discloses an additional damping control doubly-fed fan grid-connected sub-super-synchronous oscillation suppression method. For the sub-super-synchronous oscillation phenomenon caused bygrid connection of a doubly-fed fan through a series compensation power transmission line, a second-order low-resistance filter is added to a doubly-fed fan rotor side converter control link to suppress the generation of the sub-super-synchronous oscillation phenomenon. The method comprises the following steps: firstly, establishing a mathematical impedance model under a dq coordinate system of the doubly-fed fan with additional damping control and an impedance model under a dq coordinate system of an equivalent power grid; and drawing a Nyquist curve graph of the system according to the doubly-fed fan and the impedance model under the dq coordinate system of the power grid, and enabling the Nyquist curve to meet a generalized Nyquist stability criterion by adjusting the damping coefficient of the second-order damper, thereby enabling the system to be stable. According to the invention, the second-order low-resistance filter is added to the control link of the rotor side converter ofthe doubly-fed fan, so that the sub-super-synchronous oscillation of the fan can be effectively suppressed.

The invention provides a method and a device for setting a low-excitation limitation parameter. The method comprises steps: a first characteristic curve for an equivalent transfer function of a generator system along with frequency changes in a set frequency range is acquired through a frequency scanning mode; under a low-excitation limitation parameter of the generator system, a second characteristic curve for a set transfer function of the generator system along with frequency changes in the set frequency range is acquired; the first characteristic curve and the second characteristic curve are used for calculating and acquiring a Nyquist curve for the generator system; and by using the Nyquist curve, the low-excitation limitation parameter is set according to a Nyquist stability criterion. The method and the device can be well applied to field low-excitation limitation parameter setting through the frequency scanning mode, and the stability of the generator system is not broken.

The invention belongs to the technical field of wind power generation, and particularly relates to a doubly-fed fan grid-connected system oscillation analysis method based on a Nyquist stability criterion. The method comprises the following steps: establishing a multi-machine grid-connected structure, enabling each DFIG to be equivalent to a current source, and establishing a multi-machine grid-connected system impedance network model; listing and writing a system voltage equation and a current equation according to the multi-machine grid-connected system impedance network model; converting the equation into a frequency domain equation to obtain a system open-loop transfer function; and carrying out oscillation analysis on the open-loop transfer function by utilizing the Nyquist stabilitycriterion to obtain a result. The method for analyzing an oscillation mechanism by utilizing the frequency domain Nyquist stability criterion comprises the following steps: determining a resonant frequency according to an intersection point of a semi-closed curve of the open-loop transfer function and a unit circle; judging a wind turbine generator acting on resonance according to a distance between the intersection point and a point (-1, j0); and determining a phase angle margin of resonance.

The invention discloses a stability analysis method of a DC-DC converter parallel system. The DC-DC converter parallel system comprises a plurality of DC-DC converts which are connected in parallel. The stability analysis method comprises the following steps of performing average modeling on the DC-DC converter parallel system to obtain a large signal equivalent model of the DC-DC converter parallel system; and judging the stability of the DC-DC converter parallel system by the large signal equivalent model of the DC-DC converter parallel system according to a Floquet theory. By the stability analysis method and the DC-DC converter parallel system, complicated derivation of a open-ring transfer function which is required by a traditional Nyquist stability criterion-based stability analysis method is prevented; and meanwhile, the modeling performed on the multi-DC-DC converter parallel system to obtain the large signal equivalent model and the stability criterion deviation based on the Floquet theory are simpler and more convenient, and better expandability is achieved.

The invention discloses a harmonic stability analysis method for a high-speed railway vehicle network system. The method comprises the following steps: firstly, establishing a mathematical model of adual-port network of a long-distance high-speed railway traction network; establishing an admittance model of the motor train unit converter in a complex frequency domain based on a harmonic linearization theory; and finally, solving the closed-loop stability of the vehicle network system based on the impedance analysis theory of the cascade system, and analyzing the network side parameters of thevehicle network system and the parameter stability of the motor train unit control system according to the generalized Nyquist stability criterion. According to the invention, an impedance model based on a harmonic linearization theory is applied, and a linear control theory can also be applied on the premise of a nonlinear vehicle network system, so that the system model is clear and concise; and obtaining harmonic characteristics of the high-speed railway vehicle network system based on a harmonic linearization theory, thereby performing harmonic stability analysis on the system.

The invention relates to a radial direct-current power grid stability judgment method and system. The method includes the following steps of: obtaining the type of an AC / AC converter; modeling the AC / AC converter according to the type of the AC / AC converter so as to obtain the output voltage of the AC / AC converter; judging the stability of the output voltage according to Nyquist stability criterion; and judging the stability of a radial direct-current power grid according to the judgment result of the stability of the output voltage. According to the radial direct-current power grid stability judgment method and system of the invention, the AC / AC converter is modeled, so that the output voltage of the AC / AC converter can be obtained; the stability of the output voltage is judged according to Nyquist stability criterion; and the stability of the radial direct-current power grid can be judged according to the judgment result of the stability of the output voltage. Modeling for the whole power grid is not required, and therefore, the judgment method of the stability of the radial direct-current power grid can be effectively simplified.

The invention discloses a three-phase LCL type SAPF parameter design method based on a double-loop current control strategy. The method comprises the steps of firstly building an equivalent admittance model of the interaction of an SAPF control system and a power grid based on a passivity theory under a double-loop current control framework, and defining two conditions which need to be satisfied for guaranteeing the stability of an interconnection system, and then, starting from the two stable conditions, designing LCL filter parameters and current controller parameters by using tools such as a Routh criterion, a Nyquist stability criterion, a root locus, a Bode diagram and the like. The parameters designed according to the method provided by the invention can enable the system to be stable, are not influenced by the impedance change of the power grid, are strong in robustness, and greatly widen the application scene of the SAPF.

The invention discloses a bandwidth design method for a grid-connected inverter controller under a weak power grid and belongs to the field of power control. The bandwidth design method comprises the following steps of sampling, firstly giving a grid-connected inverter current control loop open-loop transfer function, then solving the amplitude of the current control loop open-loop transfer function, and finally giving a constraint relation between the current control loop bandwidth and the phase-locked loop bandwidth under a weak power grid by combining a Nyquist stability criterion, and parameter design of a current control loop and a phase-locked loop controller being completed. According to the method, the constraint relation and the value range of the design bandwidth of the current control loop and the bandwidth of the phase-locked loop under the weak power grid condition are quantitatively given, and a problem that a grid-connected inverter system is unstable due to interaction under the weak power grid condition can be effectively solved.

The invention discloses a stability analysis method and device for a direct-driven wind power plant grid-connected system, and relates to the technical field of stability analysis of a power system. The method comprises the following steps: acquiring a state equation and an algebraic equation of a grid-connected system, and carrying out simultaneous merging by taking grid-side voltage as a carrier; representing a phase-locked loop and a network side control link in the form of equivalent impedance and an equivalent voltage source to obtain an equivalent circuit and equivalent impedance of the direct-driven wind power plant; combining the equivalent impedance to obtain a stability margin expression of the grid-connected system; obtaining the normalized sensitivity of different parameters in the grid-connected system to the stability margin of the system according to the stability margin expression; and quantifying the influence degree of different parameters on the normalized sensitivity of the system stability margin, and screening out dominant influence factors. According to the method, the analytical expression of the equivalent impedance of the direct-driven fan is constructed, and the Nyquist stability criterion is adopted, so that the stability margin of the grid-connected system of the direct-driven wind power plant can be deduced, and the influence degree of different parameters on the stability of the system can be quantified.

A design method of PID controllers for stabilizing a single-input single-output multiple-time-lag system comprises the following steps: firstly, an industrial control system is used for identifying a papermaking production process with low-concentration paper pulp delivered into a headbox serving as input and the thickness of manufactured finished product paper serving as output, so that a multiple-time-lag model of a controlled system is provided; secondly, a unit feedback control mechanism is adopted, a stabilization set of all the PID controllers which can guarantee stability of a closed-loop system is calculated in combination with a modified Nyquist stability criterion and a generalized Hermite theorem on the basis of model parameters of a controlled target, a frequency upper bound omega* which is large enough is selected first, the largest permitted stability range of a proportional gain kp is calculated, then, the largest permitted stability range of the proportional gain kp is traversed, a stability range of (kd, ki) is provided for each transversal point kp according to necessary and sufficient conditions for stabilizing the multiple-time-lag system, and therefore a stable set of (kp, ki, kd) is obtained. According to the design method of the PID controllers for stabilizing the single-input single-output multiple-time-lag system, the controller target can keep running in a stable state as long as control parameters are selected from the obtained stable set of the PID controllers and a PID control program is executed, and stable control over the system is realized.

The invention discloses a stability criterion method suitable for multi-voltage level DC power distribution systems. The impedance criterion includes: firstly, according to the definition of the control bus voltage type converter and the control bus current type converter, the multi-voltage level is obtained The unified expression form of the DC power distribution system; secondly, small-signal modeling is carried out for each converter, and two ports of the DC power distribution system with a unified form and multi-voltage levels are established for the two cases where the DC transformer adopts output voltage and output current control methods respectively. Small-signal model; then, deduce the transfer function from each input variable to each output variable of the system and the equivalent loop gain expression of the system; finally, judge the stability of the system by whether the equivalent loop gain of the system satisfies the Nyquist stability criterion, If the equivalent loop gain satisfies the Nyquist stability criterion, the system is stable; otherwise, the system is unstable. The invention extends the impedance ratio criterion of the existing DC power distribution system to the multi-voltage level DC power distribution system, and has a wider application range.

The invention discloses a method for improving the stability of an energy storage inverter grid-connected system based on current type control. The method comprises the following steps: analyzing the influence of energy storage inverter control parameters on the stability of the grid-connected system by adopting a Nyquist stability criterion of a cascade system based on a pre-constructed energy storage inverter output impedance model; determining and modifying energy storage inverter control parameters for improving the stability of the grid-connected system according to the influence of the energy storage inverter control parameters; judging whether the stability of the grid-connected system is effectively improved after the control parameters of the energy storage inverter are modified, and if not, outputting an impedance model based on the pre-constructed active damping control energy storage inverter; an energy storage inverter output impedance model based on active damping control analyzes the influence of active damping control parameters on the stability of a grid-connected system by adopting a Nyquist stability criterion of a cascade system; the active damping control parameters for improving the stability of the grid-connected system are determined and modified according to the influence of the active damping control parameters, and the stability of the grid-connected system can be effectively improved.

The invention provides an active damping method for basic voltage vector compensation (BVC). Firstly, a high-pass filter (HPF) and a delay link are used for extracting a direct-current link oscillation small signal; then, subtracting a small signal value from the direct current link voltage; and finally, the voltage difference value is used as a direct-current link voltage value for calculating a basic voltage vector of finite control set model prediction current control (FCS-MPCC). Besides, under a small signal model, the input impedance of the system after BVC compensation is deduced, and then the stability of the system before and after compensation is analyzed in detail according to an impedance matching criterion and a Nyquist stability criterion. The BVC active damping method not only has the advantages of FCS-MPCC high-speed dynamic response and good steady-state performance, but also can effectively suppress the DC link voltage oscillation phenomenon caused by a constant-power load. The method is not influenced by the bandwidth of a current loop; the influence on the motor performance is smaller; extra power electronic devices are not needed, and the influence on the efficiency of the inverter is small.