241 results about "Small-signal model" patented technology

The invention discloses a micro-grid multi-objective optimized operation control method, which comprises the steps of: constructing a micro-grid operation indicator function aiming at wind-light complementary and cold, heat and electricity combined supply micro-grid; then constructing a complete small signal model of the micro-grid for theoretical analysis to provide theoretical guidance for the multi-objective optimized operation control of the micro-grid; and finally on the basis of a multi-objective intelligent optimization theory, comprehensively using power system computer aided design (PSCAD) / electromagnetic transients including direct current (EMTDC) and MATLAB software to design the micro-grid multi-objective optimized operation control method, so as to enable the micro-grid to realize comprehensive optimization of multiple operation indicators such as economy, cleanness, reliability, stability, dynamic performance and the like in different operation modes such as a synchronized operation mode, a mode capable of being switched between the synchronized operation mode and an independent operation mode, an independent operation mode with sudden load change and a resynchronized operation mode.

The invention discloses an AC-DC hybrid micro grid bidirectional power converter virtual synchronization motor control method, and relates to the AC-DC hybrid micro grid virtual synchronization motorcontrol field; the bidirectional power converter is a three-phase voltage source type PWM converter formed by IGBT switch tubes; the AC side is connected to a AC bus via a LC filter and a line impedance Z ac; the DC side is connected to a DC bus via a DC capacitor Cdc and an line impedance Zdc; the energy bidirectional transmission can be realized respectively via a frequency control unit, a virtual excitation control unit and a bidirectional power transmission control unit; the AC frequency and DC voltage can be directly controlled so as to keep AC-DC subnet load balancing; throughput characteristics of various types of power supplies and loads of the AC-DC subnet can be fully utilized so as to provide inertia for the AC frequency and DC voltage, thus enabling the bidirectional power converter to present sagging characteristics under a stable state, and building a bidirectional power converter closed loop system small signal model so as to provide a key parameter design method.

The invention provides a modeling method for a dynamic model of a modular multi-level convector. The method comprises the following steps of establishing an upper bridge single submodule switch period average model and an upper bridge arm single submodule small signal communication model; establishing an upper bridge arm switch period average model and an upper bridge arm small signal communication model; establishing a lower bridge arm switch period average model and a lower bridge arm small signal communication model; establishing a lower bridge single submodule switch period average model and a lower bridge arm single submodule small signal communication model; and establishing a modular multi-level convector switch period average module and a modular multi-level convector small signal module. The established dynamic model of the modular multi-level convector facilitates analysis on a dynamic property and a frequency response characteristic of the modular multi-level converter, facilitates cascade control strategy design of the device, and achieves description of internal state quantity.

The invention provides a grid power quality control system and method based on a unified power quality conditioner. The system comprises a series MMC, a parallel MMC and a control circuit. The series MMC and the parallel MMC form a unified power quality conditioner (UPQC) topological structure based on MMC, and are connected to a grid side and a load side through a transformer respectively. The method comprises the following steps: carrying out transformation on an MMC module under an abc stationary coordinate system to obtain a differential equation matrix model under a dq rotating coordinate system; obtaining small-signal models of the series MMC and the parallel MMC under the dq rotating coordinate system; utilizing the parallel MMC to output an offset current and stabilize DC side voltage; and utilizing the series MMC to output an offset voltage. A fundamental wave positive sequence voltage component and a fundamental wave positive sequence current component adopt a PI regulator, and other components adopt a PR regulator to realize non-static-error control, so that compensation is carried out not only for the cases of distortion, harmonic wave and reactive power and the like, but also for the cases of unbalance power source voltage and unbalance load, and the load is allowed to obtain reliable power supply.

The invention provides alternating-current phasor analysis method and small signal model modeling method suitable for dual-active bridge (dual active H bridges, dual three-level half bridges or a three-level half bridge on one side and an active H bridge on the other side and the like) direct-current converters in all phase-shifting controls. The alternating-current phasor analysis method and small signal model modeling method comprise the specific steps as follows: (1) an alternating current side of each active bridge is equivalent to two square-wave voltage sources through equivalence, and square-wave voltage is decomposed into superposition of sinusoidal voltage through Fourier series decomposition to obtain phasor expressions, as shown in the specification, of (2n+1)th component voltage and inductive current; (2) phasor diagrams of control characteristics and control ranges under different phase-shifting controls are obtained according to the phasor expressions in the step 1; (3) complex power, as shown in the specification, of the (2n+1)th component is obtained according to the phasor expressions in the step 1, and active and reactive power characteristics of the dual-active bridge direct-current converters under different phase-shifting controls are analyzed; and (4) time domain Fourier series and expressions of the voltage and the current are obtained according to the phasor expressions in the step 1, so as to obtain a unified signal model suitable for all phase-shifting control methods.

The invention discloses a method of modeling a hybrid MMC state space model, a steady-state analytical model and a small signal model, belonging to the technical field of power system transmission anddistribution. The state space model includes deducing the modulated signals and output voltage reference values of the lower and upper legs of each phase to obtain the circuit dynamic model in the static coordinate system, and obtaining the dynamic phasor model of the hybrid MMC in the rotating coordinate system through the coordinate change. The steady-state analytical model includes based on the state space model, deriving the steady-state analytical model, and analyzing the solution of the steady-state analytical model under a variety of constraints. The small signal model includes deriving the small signal model of the total capacitance voltage of the bridge arm sub-module and the circulation current of the bridge arm, integrating the small signal model of the open-loop control, and deducing the small signal model of the closed-loop control on the basis of considering the control system. The method of the invention is applicable to the joint modeling of an AC system and a DC network, as well as the engineering design and operation characteristic analysis of a hybrid MMC.

The invention relates to a method for analyzing grid-connected inverter system stability by a modified generalized impedance method. The method for analyzing the grid-connected inverter system stability includes establishing a small-signal model of an inverter under polar coordinates according to a grid-connected inverter system, and then establishing a generalized admittance interface model of the inverter and a generalized admittance interface model of a power grid; using the generalized admittance interface models to obtain modified generalized impedance of the inverter and the power grid; acquiring modified generalized impedance criteria from a ratio of the modified generalized impedance of the inverter to the modified generalized impedance of the power grid so as to be used for system stability analysis. The method for analyzing the grid-connected inverter system stability can be used for grid-connected inverter stability analysis with voltage feed-forward filtering reckoned in, is capable of better solving the coupling problem as compared with a general impedance method and can be used for accurately analyzing stability of large-scale grid-connected problems.

The invention discloses a micro-grid small signal stability analyzing and parameter coordinated setting method, and belongs to the technical field of power system micro-grid operation and control. The micro-grid small signal stability analyzing and parameter coordinated setting method comprises the steps of establishing micro-grid mathematical models which comprise a network and load small signal model and an inverter small signal model and are in need of parameterized design, determining each parameter of the micro-grid according to the root-locus method and using the parameters as initial values of the particle swarm algorithm, conducting sensitivity analyzing through a characteristic value, determining a leading parameter, determining the value range of all the parameters or the mathematical relationships among all the parameters, solving the constraint conditions in the process by the utilization of the particle swarm algorithm, determining the initial values, the parameters in need of optimization and the constraint conditions, and conducting parameter optimization and coordinated setting by the utilization of the particle swarm algorithm. By means of the micro-grid small signal stability analyzing and parameter coordinated setting method, determining of each parameter can be realized quickly and accurately in a coordinated mode, the complexity of setting the parameters one by one is avoided, and the system is made to be more stable on the basis of small signal stability; target functions and boundary conditions are flexibly adjusted, the working time is greatly shortened and the efficiency is improved.

The invention relates to a control method for a micro-grid hybrid energy storage system. The control method comprises the following steps: selecting inductive current i L (lower coordinate) and capacitor voltage u c (lower coordinate) as state variables, and energy storage unit voltage u i (lower coordinate) and load current i Load (lower coordinate) as input variables for a buck / boost power converter in the micro-grid hybrid energy storage system; building a small signal model of buck / boost power converter based on PWM (Pulse-Width Modulation) complementary control; respectively designing control circuits for a storage battery and a super capacitor; controlling the charging and discharging of the storage battery and stabilize low-frequency component of synthetic load through a single current loop; stabilizing the difference between the synthetic load power and storage battery power through the upper capacitor; adding power feed-forward to the voltage-current double closed-loop control so as to inhibit the fluctuation of the voltage of a DC bus. According to the control method for the micro-grid hybrid energy storage system, each load power in a micro-grid is taken into full account, a plurality of control methods are carried out to stabilize the high-frequency fluctuation of power, thus the fluctuation of the voltage of the DC voltage is inhibited, and the stability of the voltage of the DC bus in working is maintained.

The present invention discloses a method for extracting efficient small-signal equivalent circuit model parameters based on a gallium nitride high-electron-mobility transistor (GaN HEMT) and belongs to the field of power devices. The method, for error accumulation problems in conventional parameter extraction methods, based on a GaN HEMT 20-element small-signal model, provides an iterative algorithm, which is implemented in Matlab by programming. In an iterative process, the algorithm uses a more accurate element value than a previous element value each time for calculation, thereby leading a result to tend to an optimal solution. Compared with the conventional parameter extraction methods, the method can accurately fit S-parameters of a GaN HEMT device under full bias by running a Matlab program once, thereby obtaining all parameters of an equivalent circuit model, greatly reducing human labor in a parameter extraction process, and greatly improving the efficiency of device modeling.

The invention provides an AlGaN / GaN HETM small-signal model and a parameter extraction method thereof. The small-signal model comprises a parasitic part and an intrinsic part; and the intrinsic part comprises intrinsic capacitors including Cgd, Cgs and Cds, a gate-source leakage resistor Rgsf, a gate-drain leakage resistor Rgdf, a channel resistor Ri, a source-drain resistor Rds and a transconductor Gm. According to the invention, on the basis of the original AlGaN / GaN HETM small-signal model, the gate-drain leakage resistor Rgsf applicable to representing electric leakage between the gate and the drain and the gate-source leakage resistor Rgdf applicable to representing electric leakage between the gate and the source are additionally arranged; because unavoidable leakage current exists below the gate and at two sides of the gate when a device is in a normal working state, the characteristics of the device can be reflected more precisely after the two components are added, and therefore, the accuracy rate of a device model is improved.

The invention discloses a PLL small signal modeling method in a double-end MMC-HVDC system. The method includes firstly obtaining the conversion relation of the electric quantity of the AC lines at both ends of the converter transformer; according to the control block diagram of a phase-locked loop, taking the voltage phase angle of the common node PCC and the output phase angle of the phase-locked loop as variables to construct a small-signal model of the phase-locked loop; according to the power current equation, obtaining the expression analysis formula of the voltage drop and the voltage phase angle difference at both ends of the AC branch connected with the converter; obtaining the linear expression of the voltage phase angle of the PCC point; and obtaining the small-signal mode complete expression of the phase-locked loop. Taking the voltage phase angle of the common point as the variable, and considering the common point voltage phase angle change caused by the equivalent impedance of the AC network connected with the converter, the change of two-phase synchronized rotation reference coordinate system rotating speed under the small disturbance can be reflected, and the model precision is improved.

A low-rate denial of service (LDoS) attack flow has an obvious periodical small-signal characteristic. By means of a small-signal detection theory, the invention provides an LDoS attack detection method based on a small-signal model. According to the LDoS attack detection method, the number of data packages reaching in certain time is counted through a structural characteristic value estimating matrix, the attack cycle of the LDoS is estimated, a statistic value is compared with a preset judgment characteristic value threshold for indicate differences between normal flow rate and mixed flow rate which includes the LDoS attack, and the differences serve as a basis for judging whether the LDoS attack exists or not. If the LDoS attack is judged to exist, the cycle value of the LDoS attack can be accurately calculated through the structural characteristic value estimating matrix. By means of the technical scheme, the LDoS attack can be effectively detected.

The invention relates to a multi-terminal flexible direct current system droop coefficient optimization method based on small signal modeling. The method comprises the steps of 1, determining a systemstructure; 2, modeling a transmitting terminal converter station; 3, modeling a receiving terminal converter station; 4, modeling a direct current network; 5, carrying out small signal model operation to obtain a state matrix A of a full system model, and calculating an eigenvalue lambda; 6, analyzing small signal parameters; and 7, carrying out optimization selection on the parameters.

The invention relates to a control method of a microgrid bidirectional converter based on a virtual synchronous machine and stability analysis thereof. An integrator is added in a damping link of a rotor motion equation, and an angular frequency deviation value is fed back and compensated, so that the error-free control of alternating current subnet frequency in a microgrid off-grid mode is achieved. In consideration of the bidirectional transmission characteristic of the converter, a control target is switched into a direct-current voltage from an alternating-current frequency in real time bychanging a virtual mechanical active power reference value given mode, and stable control of direct-current bus voltage is achieved. By establishing a power inner ring and a voltage / frequency outer ring small signal model, transfer functions of the converter in a frequency control mode and a direct current voltage control mode are respectively solved, and stability analysis is performed. The control method of the microgrid bidirectional converter based on a virtual synchronous machine and the stability analysis thereof can well achieve the frequency error-free adjustment of the frequency andthe stable control of the direct current voltage under the off-grid mode.

The invention discloses a parameter extraction method for InP HBT small-signal models, which mainly solves the problems of the prior art, i.e. complex extraction process and inaccurate extraction results. The technical scheme of the invention is as follows: an open circuit soldering point structure and a short circuit soldering point structure are adopted to analyze an equivalent circuit to extract parasitic parameters; a collector open circuit state is adopted to analyze the equivalent circuit to extract external resistance parameters; a circuit network theory is utilized to analyze the scattering parameter S, impedance parameter Z and admittance parameter Y of the intrinsic part of an InP HBT device; and a layer-by-layer stripping method is adopted to determine a fixed expression for each intrinsic model parameter to extract intrinsic parameters. The parameter extraction method has the advantage of accurate, rapid and visual parameter extraction. A simulation result shows that a small-signal model result extracted by the method can perfectly fit the scattering parameter S of an actual device-testing result and can be used for directing circuit design and the determination of a large-signal model peripheral circuit.

The invention discloses a method for judging subsynchronous oscillation induced by a direct drive fan, belonging to the technical field of power system security and stability. By acquiring the parameters of the electric network and the control system of the direct-drive fan, based on the structure of direct-drive fan control system, a real-time small-signal model is established, the response process of the control system to input harmonics is analyzed, the analytical expression of output harmonics is obtained, and the criterion for judging the stability of the system is obtained according to the phase relationship between harmonics in the expression. The dangerous frequency of subsynchronous oscillation can be obtained by analyzing the calculated criterion value in the subsynchronous frequency range. The stability of the system under a certain controller parameter can be predicted by using criterion W, which provides auxiliary information for decision-making and avoids the risk of subsynchronous oscillation.

The invention discloses a micro power grid time-delay margin calculating method based on critical characteristic root tracking, which belongs to the micro power grid operation control technology field. The method is based on the static input feedback to create a micro power grid closed-loop small signal model containing the communication time-delay voltage feedback control amount so as to obtain a characteristic equation containing a transcendental item; performing the critical characteristic root tracking of the transcendental item of the characteristic equation of the system; searching the possible purely virtual characteristic root so as to calculate the maximum delay time to stabilize the micro power grid; and carrying out research on the relationship between the controller parameters and the delay margin to guide the design of control parameters and to effectively improve the stability and dynamic performance of the micro power grid.

The invention discloses a method for directly extracting small-signal model parameters of an III-V family MOSFET, and belongs to the technical field of micro-electronics integrated circuits. The method includes the steps of conducting de-embedding through a bonding pad and a metal interconnection line, determining, by means of the transistor direct-current transfer characteristic and a cold state S parameter, a bias condition suitable for extracting a transistor parasitic resistor, then, removing the influences of a channel resistor under the bias condition, conducting linear fitting to extract the parameters of the parasitic resistor, finally, conducting de-embedding on the parasitic resistor, and extracting intrinsic parameters through an equivalent circuit of a small-signal model of the III-V family MOSFET and through linear fitting. The process for directly extracting the small-signal parameters completely conforms to the physical significance of a device, a necessary foundation for the application of the integrated circuit technology is provided, and meanwhile the method has a very good reference function for extraction of small-signal model parameters of other types of devices which are made of novel materials and are of novel structures.

The invention relates to a method for configuring inertia matched with energy storage capacity of a virtual synchronous machine, and belongs to the technical field of inertia matching. According to a virtual synchronous machine rotor motion equation and a virtual synchronous machine output active power expression, a small signal model between power grid frequency and output active power is obtained, and a maximum value of the output active power of the virtual synchronous machine when the power grid frequency changes, i.e., power that needs to be configured for energy storage is obtained. A method for matching the energy storage capacity of the virtual synchronous machine with virtual inertia is proposed. Optimized configuration of the energy storage capacity and the virtual inertia is realized, and an important method is provided for application and development of a virtual synchronous machine technology.

The invention discloses a power distribution and parameter adaptive control method of multi-machine parallel virtual-synchronous generators. The method includes: firstly, establishing a small signal model for parallel operation of n virtual-synchronous generators, acquiring a transfer function Gpli between output power and load power of the ith virtual-synchronous generator and then enabling parameters of the virtual-synchronous generators running in parallel to be mutually proportioned to acquire constraint conditions matched with parameters; by the aid of the constraint conditions matched with the parameters, acquiring control parameters of the virtual-synchronous generators of a microgrid system under different running modes. The power distribution and parameter adaptive control methodis applicable to an independent optical storage microgrid system, output power of a direct-current side is fully considered, reasonable and fair distribution of the power on the premise of priority utilization of photovoltaic power is realized, and utilization rate of renewable energy sources of photovoltaic is increased.

The invention discloses a dual-active-bridge resonant convert modeling, ord reduction, design method, device and system. Under the condition of controlling the output voltage of resonant double activebridge converter by single phase-shifting control method, by using the transmission power formula of resonant double active bridge converter, the order of resonant elements in resonant double activebridge converter is reduced, and the average value equivalent model is established. Based on the average mode equivalence, the small signal model of the resonant double active bridge converter is established by using small signal method, and the transfer function is obtained according to the control quantity and the control object. the zero-pole distribution of the transfer function is analyzed and the order of the transfer function is reduced; Aiming at the reduced-order transfer function, the controller parameter design method is used to design the typical type II system. The invention has the advantages of simple principle and clear organization, provides the theoretical basis for the analysis of the resonant double active bridge converter, reduces the difficulty of analysis and design,and is easy to adopt the traditional control system parameter design scheme to design the controller.

The invention relates to the technical field of semiconductor integrated circuit manufacture, and discloses a method for establishing a transistor noise model capable of scaling along with offset. Themethod comprises the following steps that: measuring the direct current IV features of a transistor, selecting a direct current IV model, and using the direct current IV model to fit tested direct-current IV data to obtain an accurate direct-current IV model; on the basis of the accurate direct-current IV model, calculating the small signal model parameter of the transistor to obtain the small signal model of the transistor; on the basis of the accurate direct current IV model, obtaining the noise current sources of the drain electrode and the grid electrode of the transistor, and correlationthereof; and importing the drain electrode noise current source and the grid electrode noise current source of the transistor and the correlation thereof into the small signal model of the transistor, and calculating four noise parameters of the transistor. Therefore, a modeling process is greatly simplified, modeling time is greatly shortened, the method is simple and efficient, is high in accuracy and is easy in expansion.

The invention provides a method and system for determining control parameters of a photovoltaic grid-connected inverter. According to the method and system, firstly, a system small-signal model is established for the photovoltaic grid-connected inverter; then, a root locus diagram of the system small-signal model is drawn; finally, according to the root locus diagram, a group of parameter values capable of enabling main system characteristic values to be away from an imaginary axis are selected as the control parameters. Due to the fact that according to the method and system, the control parameters are directly selected through a root locus method without repeated tests, the problems that repeated tests are needed in an existing method for determining the control parameters, time and labor are saved, and work efficiency is low can be solved.

The invention discloses a harmonic instability analysis method for a double-fed fan grid-side converter, comprising the following steps: firstly, building a harmonic model for the double-fed fan grid-side converter; secondly, only considering influence of a current control loop and grid-side impedance, and building an output admittance small signal model under a d axis according to the model in the step 1; thirdly, building a return ratio matrix under the influence of the d axis according to the model in the step 2; fourthly, obtaining an output impedance closed loop transfer function under adq axis and a return ratio matrix thereof according to the model in the step 2; and fifthly, respectively extracting characteristic values of the return ratio matrixes obtained in the step 3 and the step 4, and carrying out harmonic instability analysis on each parameter according to a generalized Nyquist criterion. The method disclosed by the invention can simply and visually judge and control influence of parameters as well as conversion trends of inductances and resistances at a grid side and an alternating current side on system stability.

The invention discloses a multi-machine parallel stability control and inertia matching method of virtual synchronous machines. In order to solve the problem of large-scale micro power supply access stability control in a new energy power system, a virtual synchronous machine multi-machine parallel connection stability control strategy is studied, and a VSG active-frequency and reactive-voltage controller is built, a simplified virtual inertia simulator is provided, so that a control system does not need to introduce a PLL link, and the influence of phase locking precision on VSG control characteristics is reduced; and establishing an accurate small signal model of a VSG multi-machine parallel operation system, and analyzing the change condition of the characteristic root of the system when parameters such as virtual rotational inertia, a droop coefficient, a line parameter and virtual impedance change, and the influence rule of the change condition on the stability of the small signal. The VSG multi-machine parallel system inertia matching method is provided, the virtual inertia of each VSG is configured according to the inertia matching principle, and it can be guaranteed that each VSG can achieve load distribution according to the capacity proportion in the steady-state and transient-state processes.

The invention discloses an inductive coupling type wireless energy transmission system and an active disturbance rejection control method thereof. The device comprises a rectification unit, an emission end inversion unit, an emission end resonant unit, a current detection unit, an active disturbance rejection control unit and a receiving end resonant unit, wherein a small signal model is built for the inductive coupling type wireless energy transmission device by means of a generalized state space averaging method and is used for describing an interior transient change process of the device, the active disturbance rejection control unit of the inductive coupling type wireless energy transmission system is designed according to an active disturbance rejection control principle on the basis of the small signal model, the active disturbance rejection control unit is used for effectively suppressing coil current change caused by disturbance due to reflection voltage, high-speed and stable control on emission end current of the device is further achieved, performance difference caused by device parameter change is effectively made up, the whole device system is enabled to be endowed with favorable dynamic process and high robustness, and communication between a receiving end and an emission end is not needed.

A small signal model based on a single-phase cascaded island ACHB inverter system and a stability analysis method based on a floquet theory are provided. The single-phase cascaded island ACHB invertersystem includes a digital controller with double closed loop control, an LC filter, a load and three single-phase H-bridge inverters operating at low voltage, medium voltage and high voltage respectively. The digital controller is connected to a filter capacitor. The small signal model based on the single-phase cascaded island ACHB inverter system, in consideration of a linear resistive load withsmall signal disturbance, separates a disturbance component, obtains a modulation signal expression by coordinate transformation, and analyzes the system stability by using the floquet theory. The beneficial effect is that a multi-level inverter reduces the harmonic components in the output voltage due to the increase of the output level of a main circuit, and the stability analysis of the smallsignal model solves the difficulty of modeling the nonlinear circuit of a converter, the dynamic characteristics of steady-state operating point of the converter are described by unified differentialequations, the stability domain of the system is accurately verified by the floquet theory, and a reliable criterion is provided for the stable operation of the whole single-phase cascaded island inverter system.

The invention discloses a method for controlling current instructions during a fault ride-through period of a doubly-fed fan in a weak grid environment. According to the method, the influence of gridconnection guideline requirements, the fan converter capacity and the system small interference stability is comprehensively considered, and active current and reactive current instructions are set. According to the invention, a small signal model of a doubly-fed wind generator grid-connected system is established, and the influence of active current and reactive current instructions on the systemstability is analyzed through the Lyapunov stability criterion, so that current instruction constraint conditions capable of enabling the system to stably operate in the grid drop fault period can beobtained. The small signal model is detailed and accurate, and the stability analysis method is simple and visual, so that a reliable reference can be provided for current instruction setting in thelow-voltage ride-through period of the doubly-fed wind power generation system in the weak grid environment.

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.