378 results about "Negative sequence current" patented technology

The invention discloses an asymmetric fault transient stability analysis method and system based on a network-forming inverter. Relates to the technical field of power system transient stability analysis. According to the scheme, a mathematical model of hybrid synchronous control before and after a fault is derived according to a power grid system model of a hybrid synchronous control inverter connected to a power grid, and a transient instability mechanism based on the hybrid synchronous control inverter under an asymmetric fault is analyzed from the aspect of legal quantity by means of a power angle curve and a phase plane. The influence rule of the positive and negative sequence current on the stability is disclosed; based on mathematical modeling, a power angle curve and a phase diagram method, the influence of positive and negative sequence component coupling under an asymmetric fault is considered, and transient instability mechanism analysis of hybrid synchronous control is realized; the boundary of instability caused by positive and negative sequence current can be determined, a converter transient stability analysis method considering positive and negative sequence coupling caused by asymmetric faults under network construction type control is perfected, and a reference is provided for improving the transient synchronization stability of the converter under large disturbance; meanwhile, the researched current loss stability boundary can provide reference for a current limiting reference value.

The invention provides a fault ride-through control method and a fault ride-through control system for a grid-forming converter, which are characterized in that the current of the grid-forming converter under an asymmetric fault is controlled by adopting a negative-sequence current reference generation strategy aiming at an inner ring of a controller, and negative-sequence reactive current is injected into a power grid; adopting a partial sequence current amplitude limiting strategy to control the maximum phase current amplitude of the injected asymmetric current not to exceed an amplitude limiting value; controlling the boundary of an active reference value by adopting an active reference value amplitude limiting strategy for an active synchronization ring; a synchronous damping link is adopted to enhance damping in power angle swing; for a reactive voltage loop, a reactive voltage variable coefficient control strategy is adopted to control the output of the maximum reactive current; and an internal potential locking strategy is adopted to suppress overvoltage. According to the invention, negative-sequence reactive power is provided, and overcurrent of each phase is avoided; the power angle characteristic is improved, and the transient stability is improved; stable transient voltage support is provided in a current output capability range; and after the fault is recovered, the voltage of the converter grid-connected point is smoothly recovered.

The invention discloses a three-phase imbalance compensation method and system based on negative sequence current and a medium, and relates to the technical field related to power imbalance compensation, and the method comprises the steps: executing the three-phase current, voltage and reactive power data collection of a power distribution network, and carrying out the three-phase imbalance recognition through negative sequence current analysis; carrying out additional acquisition of a load mode and environment data on the power distribution network; taking the data acquisition result and the additional data set as input data to execute negative-sequence current change trend prediction; a double-layer compensation device is configured for cooperative control optimization; power distribution network balance compensation is executed through a time sequence compensation strategy, and time sequence window feedback is established; and carrying out adaptive strategy correction. The technical problems that in the prior art, a three-phase imbalance compensation strategy lacks perspectiveness and self-adaptive capacity and cannot cope with a dynamically-changing power grid scene, and consequently compensation precision and stability are poor are solved, and the technical effects that perspectiveness and predictive compensation is achieved, and three-phase imbalance compensation precision and stability are improved are achieved.

The invention relates to the technical field of power system fault protection, in particular to a power distribution network distance protection method, system and device considering distributed power supply negative sequence currents, and aims at a medium-voltage power distribution network where a plurality of distributed power supplies are connected in a feeder line T-connection mode. Considering the influence of negative-sequence current output under negative-sequence current control of a distributed power supply in each feeder line on measured impedance, the feeder line with a two-phase short-circuit fault in the power distribution network is identified and positioned through cooperation of additional impedance compensation and distributed power supply negative-sequence reconstruction control, and then a protection device is controlled to cut off the fault feeder line. And implementing distance protection of two-phase short circuit of the power distribution network. The method realizes fault identification and positioning only by depending on locally acquired electrical quantity parameters, does not need to depend on a communication system, considers and ensures autonomy and reliability of distance protection, has the advantages of simple principle, low cost and high reliability, and can adapt to power distribution networks with different fault positions and different distributed power supply access numbers and capacities.

The invention discloses a fault upstream and downstream judgment method based on an instantaneous symmetric component method. The fault upstream and downstream judgment method comprises the following steps: S1, monitoring current data of each node of a power distribution network in real time; s2, calculating an instantaneous positive sequence current and an instantaneous negative sequence current of the real-time data by adopting an improved instantaneous symmetric component method; s3, performing primary difference on the instantaneous positive-sequence current and the instantaneous negative-sequence current respectively; s4, comparing the primary differential value of the instantaneous negative-sequence current with a set threshold value, and judging whether the directions of the instantaneous positive-sequence current and the instantaneous negative-sequence current are the same or not; and S5, performing fault section judgment by combining the direction of the instantaneous positive and negative sequence current and the topological structure of the power distribution network. The improved instantaneous symmetric component method is used to obtain the instantaneous positive-sequence current and the instantaneous negative-sequence current, the directions of the instantaneous positive-sequence current and the instantaneous negative-sequence current are analyzed, the fault section can be quickly and accurately positioned, and the cost is reduced.

The invention provides a new energy grid-connected converter fault ride-through method and system under an asymmetric fault, and the method comprises the steps: calculating the positive sequence voltage amplitude of a power grid and the voltage unbalance degree of the power grid according to the positive and negative sequence parameters of the power grid; judging whether the power grid is in a normal state or a fault state; when the power grid is in the asymmetric fault state, current amplitude limiting is carried out by adopting a calculation formula of a positive and negative sequence current instruction in the asymmetric fault state based on the related parameters of the power grid fault, and the amplitude limiting value of the positive and negative sequence current instruction of the new energy grid-connected converter in the asymmetric fault state is calculated; feedback control is carried out according to the amplitude limiting values of the positive and negative sequence current instructions, and fault ride-through is completed; according to the method, the current is subjected to amplitude limiting in the calculation formula of the positive and negative sequence current instructions, so that the positive and negative sequence active and reactive current control target can be met to the greatest extent on the premise of ensuring that overcurrent of the converter does not occur, and the power grid supporting capacity during the fault period is improved.

The invention provides a platform area power grid tail end voltage flexible regulation and control method and system. The method comprises the steps that a sampling module measures common coupling point voltage at the tail end of a platform area power grid; the impedance calculation unit obtains load end impedance; the current calculation unit calculates the current of the common coupling point through the voltage of the common coupling point and the impedance of the load end; the current resolving unit is used for resolving the common coupling point current into negative-sequence current and zero-sequence current; the compensation current unit calculates a compensation current component required for maintaining the stability of the voltage at the tail end of the transformer area power grid through the negative-sequence current and the zero-sequence current; the PWM unit generates a compensation current control signal; and the IGBT module outputs compensation current to the common coupling point. When three phases are unbalanced, the first disturbance generation circuit accurately injects a disturbance signal to effectively regulate and control the voltage at the tail end of a power grid in a transformer area; and the second disturbance generation circuit analyzes the stability of the tail end of the station area power grid by using coordinate transformation and a PI controller.

The invention discloses an active power distribution network negative-sequence impedance pilot protection method and system suitable for DG access, and the method comprises the steps: carrying out the constraint of the negative-sequence active current outputted by an inversion-type DG outputting the negative-sequence current after a fault occurs, and enabling the output value of the inversion-type DG to be not greater than 0; the three-phase voltage and the three-phase current at the protection installation position are obtained in real time, and the negative sequence impedance at the protection installation position is calculated; judging a fault direction by using the negative sequence impedance amplitude and the phase characteristics, and assigning a fault direction identifier; and judging whether an internal fault or an external fault occurs by using the fault direction identifiers at the protection mounting positions on the two sides of the protected section, and performing fault protection. According to the method, protection criteria are designed by using the negative sequence impedance angle and amplitude at the same time, so that the negative sequence impedance properties in the active power distribution network are uniform, the protection problems in all scenes can be solved, and meanwhile, the reliability and sensitivity of fault recognition by using the negative sequence impedance phase are improved.

The invention discloses a power maximization output-oriented multi-target double-sequence current injection method under a three-phase imbalance fault, and the method comprises the following steps: S1, obtaining a three-phase voltage # imgabs0 # and a current # imgabs1 # of a PCC point after a fault steady state, and correspondingly obtaining a positive and negative sequence voltage # imgabs2 # and a current # imgabs3 # of the PCC point under a dq coordinate system; s2, according to the # imgabs 4 # and the # imgabs 5 #, calculating to obtain positive and negative sequence voltage # imgabs 6 # and phase angle difference # imgabs 7 # of the power grid in the current fault scene; s3, according to # imgabs8 # and # imgabs9 #, finding out positive and negative sequence current reference values of a dq axis in a 3-D lookup table; and S4, generating an inverter modulation signal through positive and negative sequence separation control according to the found dq-axis positive and negative sequence current reference values.

The invention belongs to the technical field of new energy, and particularly relates to a fault voltage ride-through optimization method and system for a network construction type energy storage converter. Carrying out positive and negative sequence separation and Clark-Park coordinate transformation on the three-phase power grid voltage and the inversion current, calculating active power and reactive power of the network-forming type energy storage converter, and carrying out power loop control to obtain three-phase reference voltage and angular velocity; performing voltage loop control, and dynamically adjusting a positive sequence active current and a positive sequence reactive current according to whether a low-voltage-crossing indication signal exists or not; performing positive-sequence current loop control and negative-sequence current loop control, and then performing inverse Clark-park conversion to obtain positive-sequence three-phase reference voltage and negative-sequence three-phase reference voltage; and superposing the positive-sequence three-phase reference voltage and the negative-sequence three-phase reference voltage for PWM (Pulse Width Modulation). According to the invention, when the power grid voltage is unbalanced due to a fault, the oscillation of the direct current side voltage and power caused by the negative sequence component is eliminated, and the power angle stability is improved and the fault current is suppressed by dynamically adjusting the related reference value during the fault voltage ride-through period.

The invention relates to the technical field of alternating current electrified railway power supply, in particular to a negative sequence treatment method and system for an electrified railway traction substation, and the system comprises a step-down isolation transformer and an SVG compensation subsystem. The primary side of the step-down isolation transformer is used for being connected with the high-voltage wire inlet side of a three-phase power grid of a traction substation, and three-phase medium-voltage electricity is formed after step-down isolation and connected to a three-phase medium-voltage bus. The SVG compensation subsystem is used for being connected to a three-phase medium-voltage bus; the SVG compensation subsystem is used for outputting negative sequence compensation current; the direction of the negative-sequence compensation current is opposite to that of the negative-sequence current generated by the traction load, and the negative-sequence compensation current and the load current generated by the traction load counteract each other, so that the treatment of the negative-sequence current is realized.

The invention discloses a photovoltaic low-pass control strategy considering positive and negative sequence dynamic reactive current support and frequency multiplication power fluctuation suppression, and the strategy comprises the steps: obtaining an expression of instantaneous power outputted by an inverter according to an instantaneous power theorem; through Park transformation, positive and negative sequence power grid voltage and current components under the synchronous rotating coordinate system are obtained, and then the relation between each power component output by an inverter and the positive and negative sequence voltage and current components is obtained; designing q-axis positive and negative sequence current instruction values after the fault by considering the requirement of positive and negative sequence dynamic reactive current support; under the condition that q-axis positive and negative sequence current instruction values after the fault are known, d-axis positive and negative sequence current candidate instruction values after the fault are calculated; in consideration of the through-current capability of the inverter, under the condition that a q-axis positive sequence current instruction value after a fault is known, a d-axis positive sequence current instruction value after amplitude limiting is obtained so as to adjust the d-axis positive sequence current instruction value after the fault, and then the d-axis positive sequence current instruction value after the fault is obtained; and taking the post-fault d-axis negative-sequence current candidate instruction value as a post-fault d-axis negative-sequence current instruction value. According to the invention, a reference basis is provided for low-voltage ride-through control of the photovoltaic electric field.

The invention discloses a doubly-fed wind turbine generator active support capability improvement method under an asymmetric fault, and the method comprises the steps: building a doubly-fed wind turbine generator optimization model with the minimum voltage unbalance degree of a common connection point, and delimiting an optimization problem as two sub-optimization problems: a negative sequence voltage can be eliminated and a negative sequence voltage cannot be eliminated; according to the method, constraint is contracted by using a Kaush-Kuhn-Tuck condition and a geometric analysis method, and an analytical solution form of rotor positive and negative sequence current instructions is obtained, so that the problem of insufficient unbalanced voltage suppression capability under the asymmetric fault of the doubly-fed fan is solved, and the effect of greatly reducing the voltage unbalance degree of the common connection point under the asymmetric fault is achieved. And a reference is provided for improving the active support capability of the doubly-fed fan under the asymmetric fault.

Capacity-optimized grid-forming is provided. A controller includes a power-synchronization model (PSM). The PSM is configured to receive positive and negative sequence symmetrical components of an output voltage and an output current. The PSM is configured to generate current references including a positive and negative sequence current reference. The PSM is configured to generate an internal voltage phase reference and voltage magnitude reference. The controller includes a capacity constrainer to determine, based on the current references and a capacity limit terms, that a device is capacity constrained. The capacity constrainer is configured to determine optimum complex gains to scale the plurality of current references based on the capacity limit terms. The capacity constrainer is configured to, responsive to the determination of constraint, output scaled current reference to cause a power stage to adjust the output voltage / current, the scaled current reference scaled according to the complex gains.

The invention relates to the technical field of power system relay protection, in particular to a double-end weak system single-phase earth fault distance protection method, device and equipment and a medium, and the method comprises the steps: setting a protection starting criterion when a power transmission line has a single-phase earth fault; when the protection starting criterion is met, a single-phase earth fault composite sequence network diagram is obtained, and a closed curve is constructed in the composite sequence network diagram; on the basis of the Kirchhoff's current law, calculating the zero-sequence current of the new energy side by using the negative-sequence current and the zero-sequence current of the system side; the fault position is calculated through the current relation between the system side and the new energy power supply; fault identification is carried out on the fault position, and fault protection is carried out according to set conditions. According to the method, when a single-phase earth fault occurs, the influence of transition resistance on distance protection can be accurately eliminated only through the comprehensive sequence current on the side, and the fault distance is accurately calculated, so that the performance of distance protection is remarkably improved, and reliable and rapid action of distance protection in case of an internal fault is ensured.

The invention discloses a three-phase unbalanced power flow line loss analysis method, which comprises the following steps of: establishing a power flow model of a three-phase unbalanced system based on a sequence component method; the power flow model comprises admittance matrixes respectively corresponding to a positive sequence, a negative sequence and a zero sequence, and equivalent admittance corresponding to any two of the positive sequence, the negative sequence and the zero sequence; converting the power flow model, and determining a first relational expression among the coupled positive sequence current, the equivalent admittance and the uncoupled positive sequence current, and a second relational expression among the coupled negative sequence current, the equivalent admittance and the uncoupled negative sequence current; determining a positive sequence electrical component according to the first relational expression, and determining a negative sequence electrical component according to the second relational expression; and determining the line loss of the three-phase unbalanced system according to the positive sequence electrical component and the negative sequence electrical component. According to the method provided by the invention, the accuracy of line loss analysis can be improved.

According to the power distribution network weighted current differential protection method based on distributed photovoltaic fault current regulation and control, voltage and current signals on the two sides of a power distribution network line are collected in real time, and through fine signal processing steps including filtering, Fourier transform and sequence component decomposition, accurate recognition of fault current and rapid judgment of the fault direction are achieved; by adjusting the current reference value of the inverter, the distribution coefficients of the positive and negative sequence current are equal, and the influence of the negative sequence current on the protection performance is effectively eliminated; the Bergeron model is utilized to accurately deduce the current, the synchronization error is considered to perform weighting processing, and the weighting processing and the local actually measured current jointly form a weighting differential protection criterion, so that the selectivity and the reliability of the power distribution network protection are remarkably improved, and the correctness and the quickness of the protection action can also be ensured under the distributed photovoltaic high permeability; the problem that a traditional protection method is insufficient in protection selectivity in the distributed photovoltaic power distribution network is solved, and powerful technical support is provided for safe and stable operation of the power distribution network.

The invention discloses an improved flexible direct current fault control method for improving current differential protection adaptability, and belongs to the technical field of power system relay protection and new energy power generation system grid-connected control. The method comprises the following steps: detecting a voltage drop abrupt change signal and a negative sequence voltage signal of a flexible DC output end, and starting a control strategy when the signals are satisfied; detecting a terminal voltage signal and active power of a new energy sending-out terminal, and calculating a new energy side current phase angle and a power angle; calculating a flexible DC side current phase angle control value and a negative sequence dq axis current reference value based on the parameters; and controlling the flexible DC side to output a negative-sequence current corresponding to the negative-sequence dq-axis current reference value, so as to actively adjust the current phase of the flexible DC side, reduce the phase angle difference between the fault current of the flexible DC side and the fault current of the new energy side, and discriminate the internal and external faults of the region through traditional current differential protection. According to the invention, the degree of freedom is controlled and the current phase is adjusted by using the flexible direct-current side negative-sequence current to reduce the phase angle difference of the two sides, so that the problem that the current differential protection adaptability in the double-end weak feedback system is reduced is solved, additional hardware is not needed, and the power grid safety and the new energy stable grid connection are ensured.

The invention discloses a self-adaptive control method for transient stability of a hybrid synchronous control inverter, relates to the technical field of transient stability control of a new energy power generation system, and aims at a transient instability phenomenon based on the hybrid synchronous control inverter under a serious asymmetric fault, self-adaptively injecting positive sequence current and negative sequence current of the inverter, and improving the transient stability of the hybrid synchronous control inverter. The transient stability of the hybrid synchronous control inverter under the asymmetric fault is improved; according to the method, the balance of the system after a serious asymmetric fault occurs can be ensured, the transient stability of the flexible interconnection equipment is enhanced, the influence of positive and negative sequence current on the transient stability is analyzed based on a phase diagram, and the safe operation of the system is greatly improved by designing a self-adaptive current injection strategy; practical application is facilitated, and stable operation of new energy grid connection is guaranteed.

The invention discloses an active and reactive comprehensive oscillation quantity minimization cooperative control method under a current constraint condition, and the method comprises the steps: analyzing an inverter side current converter in a power grid voltage imbalance state, and quantifying the frequency doubling fluctuation quantity of active power and reactive power caused by a negative sequence component; a third harmonic voltage compensation method is adopted to suppress negative sequence current, the third harmonic voltage compensation amount of the converter is dynamically adjusted, and three-phase current balance is regulated and controlled; on the basis of active and reactive power fluctuation suppression of a negative-sequence current compensation method, an optimization function with the purpose of minimizing the comprehensive fluctuation quantity of active power and reactive power is constructed by utilizing controllable negative-sequence current compensation quantity and eliminating frequency doubling fluctuation quantity of active power and reactive power, and an optimal negative-sequence current compensation scheme is solved in real time by adopting a particle swarm optimization algorithm. Cooperative suppression of three-phase current balance and power oscillation is achieved, and the operation stability and the power transmission capacity of the inverter side converter under an asymmetric fault are remarkably improved under the current amplitude limiting condition.

The invention discloses a VSG current balance control method and system in a voltage imbalance state, and relates to the technical field of power electronics and power system application, and the method comprises the steps: taking current balance control as a core of integral control, building a VSG model, analyzing the output condition of the VSG in an imbalance power grid state, and obtaining a VSG current balance control result. VSG is controlled through active-frequency and reactive-voltage droop control, the instantaneous output power of the VSG is analyzed according to the instantaneous power theory, positive and negative sequence currents are decomposed when an unbalanced fault occurs in a power grid, and the targets of current balance and power constancy are achieved by controlling the positive and negative sequence currents and adding voltage correction. According to the invention, the current and electric energy quality control of the VSG is designed, and the output current balance, the active power constancy or the reactive power constancy are realized on the premise of not changing the characteristics of the VSG.

The invention provides a negative sequence voltage control method and a negative sequence voltage control system for a network-constructed converter, and relates to the technical field of network-constructed converter control. Calculating negative-sequence reactive power absorbed by the converter and the maximum negative-sequence reactive power which can be absorbed by the converter under the specified fault working condition, and comparing the negative-sequence reactive power and the maximum negative-sequence reactive power to obtain a negative-sequence active power instruction and a negative-sequence reactive power instruction; performing droop control by using the negative sequence active power instruction; pI control is carried out by using a negative sequence reactive power instruction; when a fault occurs, performing three-phase voltage and current sequence decomposition to obtain a negative sequence voltage amplitude and phase and a negative sequence current amplitude and phase of the converter; and performing power measurement according to the amplitude and phase of the negative-sequence voltage output by the converter and the amplitude and phase of the negative-sequence current to obtain negative-sequence active power and reactive power transmitted between the converter and the power grid.

The invention relates to an asymmetric fault ride-through cooperative control method and system for a virtual synchronous generator, and belongs to the technical field of power electronics and new energy power generation. Aiming at the problems of three-phase current asymmetry, overcurrent, transient instability and the like faced by a virtual synchronous generator when a power grid has an asymmetric fault, the method comprises the following steps: firstly, setting a negative sequence current reference to be zero through positive and negative sequence separation control so as to keep output current symmetry, and freezing a reactive power ring; secondly, comprehensively considering current amplitude limiting, voltage supporting capacity and system capacity, and dynamically calculating optimal solutions of positive sequence excitation reference amplitudes under different fault depths; finally, positive and negative sequence power components and excitation dynamic changes are considered, an active reference instruction real-time dynamic adjustment method is provided, and a power angle is controlled to be stabilized at a target value by adjusting a power reference value in real time. The method can effectively improve the transient stability during the fault ride-through period, maximally supports the power grid voltage while ensuring that the current is not out of limit, and improves the system capacity utilization rate.

The invention discloses a VSG negative sequence harmonic current control method under an asymmetric fault of a power grid, and the method comprises the steps: analyzing an instantaneous expression of VSG output power under an asymmetric working condition, and obtaining a quantitative relation between power frequency doubling fluctuation and positive and negative sequence voltage and current; introducing a direct current part of VSG reactive power output into a reactive power control ring, and constructing reactive power control ring control; positive sequence voltage current and negative sequence current are independently controlled in a positive and negative sequence separation mode; and unifying the positive sequence modulation signal and the negative sequence modulation signal to obtain a total PWM modulation wave signal, and setting a reference value for a negative sequence current loop, so that the VSG output current is balanced. According to the invention, the influence of power oscillation on the control effect of the VSG can be improved, the negative-sequence current harmonic waves can be suppressed, and the electric energy quality of the VSG is greatly improved.

The invention provides a low-voltage ride-through control method and system for power electronic equipment. The method comprises the steps that the control mode of the power electronic equipment is set to be double-closed-loop control; in the double-closed-loop control mode, according to outer-loop control parameters of the power electronic equipment, instruction values of positive and negative sequence currents of outer-loop control are calculated; carrying out amplitude limiting on the instruction values of the positive and negative sequence current, and carrying out vector fusion on the instruction values of the positive and negative sequence current after amplitude limiting to obtain the instruction values of the positive and negative sequence current after the maximum value is limited; performing positive and negative sequence current control according to the positive and negative sequence current instruction value after the maximum value is limited, and completing low-voltage ride-through; vector fusion is carried out on the instruction values of the positive sequence current and the negative sequence current after amplitude limiting, fault current suppression based on the low voltage ride through process can be achieved, the large overcurrent phenomenon is avoided, tripping conditions are reduced, and it is guaranteed that low voltage ride through can be achieved with high probability on the premise that normal control strategy logic is not affected.

The invention relates to an overvoltage risk assessment method, device and equipment for non-full-phase operation of a new energy access weak power grid system. The method comprises the following steps: acquiring electrical quantity data of the new energy access weak power grid system, and a voltage source, grid-connected equivalent impedance, series equivalent impedance and non-full-phase operation line impedance of a power transmission line of the new energy access weak power grid system with a single-phase fault; calculating n positive sequence current vectors and negative sequence current vectors according to each piece of electrical quantity data; calculating n positive sequence port voltage vectors and negative sequence port voltage vectors according to the fault data, each positive sequence current vector and each negative sequence current vector; according to the positive sequence voltage component rated value, each positive sequence port voltage vector and each negative sequence port voltage vector, actual positive sequence voltage and actual negative sequence voltage are obtained through calculation; and according to the actual positive sequence voltage, the actual negative sequence voltage and the positive sequence voltage component rated value, calculating to obtain a single-phase voltage maximum value, and according to the single-phase voltage maximum value, judging whether the system has an overvoltage risk in a non-full-phase operation state.

The invention discloses an AC / DC power distribution network protection method and device based on a negative sequence voltage ratio. The method comprises the following steps: constructing a fault identification criterion of a two-phase interphase short-circuit fault of the AC / DC power distribution network according to negative sequence voltage fault characteristics of internal and external faults of the AC / DC power distribution network; determining the fault type of the protection line according to the negative sequence current at the head end measuring point of the AC / DC power distribution network protection line and a pre-constructed fault type identification criterion; when the fault type is a three-phase fault, an overcurrent protection method is adopted to protect the protection line; and when the fault type is a two-phase interphase short-circuit fault, determining an internal and external fault identification result of the protection line according to the negative sequence voltage of the measuring points at the two ends of the protection line, the negative sequence comparison voltage and a fault identification criterion.

The invention belongs to the technical field of power grids, and particularly relates to a hydrogen fuel cell grid-connected inverter fault test and ride-through method. Comprising the following steps: S1, simulating three-phase symmetrical voltage drop, inter-phase voltage drop and single-phase voltage drop of a power grid through a low-voltage fault generation device; s2, collecting the voltage of a power grid through a high-precision voltage sensor, and when the voltage of the power grid drops, judging whether the system needs to carry out low-voltage ride through at the moment according to a distributed energy low-voltage ride through curve; and S3, when the system needs to carry out low-voltage ride-through, a dual-second-order generalized integrator pre-filtering phase-locked loop (DSOGI-PLL) is combined with grid-connected point voltage full-feedback compensation, negative-sequence components generated by symmetric and asymmetric drop of the power grid voltage are eliminated, and current impact suppression and negative-sequence current elimination are realized. The problem of power grid synchronization under the condition of power grid voltage drop is finally solved.

The invention discloses an impedance-adjustable series hybrid cascade static synchronous compensator and a control method thereof. The method comprises the following steps: acquiring grid-connected point voltage and load current information, and performing positive and negative sequence component analysis on the grid-connected point voltage and load current information; calculating a compensation line current component with the same size and opposite phase as the positive and negative sequence components of the load current so as to compensate the reactive power and the negative sequence current of the grid-connected point load; zero-sequence injection current used for in-phase active power balance is calculated, and reference operation current of each cascade branch is obtained; calculating the output reactive power of each phase based on the cascade branch voltage of each phase and the reference operation current, calculating the corresponding trigger angle of each phase thyristor under the condition that the adjustable impedance bears all phase voltages and generates all reactive power, and generating a pulse control signal to realize the control of the adjustable impedance; and each phase cascade converter adopts quasi-proportional resonance control to track the reference operation current, and generates a corresponding PWM signal to respectively control each phase cascade converter.

The invention discloses a permanent magnet synchronous motor turn-to-turn short circuit fault diagnosis method based on a high-frequency negative-sequence component, and the method comprises the steps: obtaining a high-frequency voltage injection signal according to the three-phase current of a motor, obtaining a high-frequency current signal through space vector pulse width modulation, injecting the high-frequency current signal into the motor, obtaining the three-phase voltage and three-phase current of the current motor, and extracting a high-frequency component in the three-phase voltage and three-phase current; high-frequency negative-sequence voltage and high-frequency negative-sequence current are obtained according to the extracted high-frequency components, and fault features are constructed; comparing the fault feature with a fault threshold value, judging that the permanent magnet synchronous motor is healthy if the fault feature is smaller than the fault threshold value, and judging that the permanent magnet synchronous motor is faulty if the fault feature is larger than or equal to the fault threshold value. According to the method, extra detection hardware is not needed, real-time detection and positioning of the turn-to-turn short circuit fault can be realized by using the negative sequence component of the motor in the high-frequency domain in the servo operation state of the permanent magnet synchronous motor, and the method has relatively high applicability and robustness.