295 results about "Negative sequence current" patented technology

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 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 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 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.

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 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 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 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.

The invention discloses an autonomous power ripple suppression method based on a grid-connected multi-port converter, and the method comprises the steps: carrying out the negative-sequence current elimination at an AC port, carrying out the power ripple suppression at a DC port, automatically transferring a differential power ripple to a ripple port, and eliminating the dependence on the prior knowledge of the ripple; in addition, even under the unbalanced power grid working condition, balanced three-phase power grid current and ripple-free direct current output voltage can be obtained at the same time, and an additional power ripple controller does not need to be designed.

The invention relates to the technical field of power electronic application, in particular to a negative-sequence voltage compensation control method of a network-forming STATCOM (Static Synchronous Compensator). Comprising the steps of bridge arm current reference instruction unit phasor calculation, reactive voltage control, bridge arm positive sequence and negative sequence reactive current instruction calculation, bridge arm current reference instruction amplitude limiting logic, zero sequence voltage injection control and modulation wave instruction calculation. According to the method, the negative-sequence current output by the networking STATCOM in the asymmetric transient fault is always inductive, the proportion coefficient of the amplitude of the negative-sequence current instruction can be adjusted according to the requirements of an engineering project, and the cost is reduced; by dynamically solving a quadratic equation of unary of a reactive current amplitude limiting coefficient, an active current instruction is not limited by amplitude, balance control of direct current voltage is preferentially ensured, the reactive current is reasonably limited in amplitude, a total current reference instruction value is always in a maximum design value range of short-time overcurrent, and the stability and the safety of the device are improved.

The invention provides a double-layer low-voltage ride-through control method and system for a two-stage photovoltaic grid-connected system, and the method comprises the steps: carrying out the real-time detection of the voltage of a grid-connected point, and judging whether the voltage of the grid-connected point is qualified or not; when the voltage of the grid-connected point is not qualified, a current instruction is generated by taking the improvement of the positive sequence voltage of the grid-connected point and the reduction of the negative sequence voltage of the grid-connected point as targets, and the control of the voltage and the current of the inverter is realized through a positive and negative sequence current inner loop and a phase-locked loop; after the alternating current layer realizes positive and negative sequence voltage support, whether a post-stage inverter has residual capacity is judged based on a generated current instruction, and if yes, the residual capacity is calculated, and an active power instruction is generated and sent to the direct current layer; and after the back-stage inverter obtains the output active power instruction, the instruction is shared to the front-stage inverter, and the front-stage inverter compensates the current reference value of the direct current side in the low-pass period according to the photovoltaic output characteristic curve.

The invention discloses an unbalance compensation method and system for an energy storage converter, and belongs to the field of power electronics and electric energy quality treatment. The system comprises a power instruction analysis module, a current reference generation module, a negative sequence current control loop module, a zero sequence control loop module, a positive sequence control loop module, a pulse width modulation module, a first conversion module and a second conversion module, the current reference generation module is respectively connected with the negative sequence current control loop module and the positive sequence control loop module; the third end and the fourth end of the negative sequence current control loop module are connected with the pulse width modulation module through the second conversion module; the third end and the fourth end of the zero sequence control loop module and the third end and the fourth end of the positive sequence control loop module are connected with the pulse width modulation module through the first conversion module. According to the method, the energy storage and compensation functions of the PCS can be seamlessly integrated under a unified control framework, so that the control system is greatly simplified, and the performance is remarkably improved.

The present disclosure provides a method, a device and a system for detecting a single-phase broken conductor. The method includes: collecting a three-phase current at a measurement point; obtaining a positive sequence current and a negative sequence current from the collected three-phase current; calculating a change of the positive sequence current and a change of the negative sequence current within a first time interval; and when an amplitude of the positive sequence current decreases, an amplitude of the negative sequence current increases, a decrease of the amplitude of the positive sequence current is greater than a first predetermined value, and a ratio of a magnitude of change of the negative sequence current to a magnitude of change of the positive sequence current is greater than a second predetermined value, determining that a first set of faults occur, the first set of faults including a single-phase broken conductor.

The invention discloses a time domain distance protection method and system combined with a control strategy and distributed capacitance current, and the method comprises the steps: employing negative sequence current to suppress the adaptability of a control strategy based on converters at two ends of a line in a double-end weak feed AC system, and recognizing the problems that negative sequence networks at two sides in a composite sequence network are open-circuited and a non-fault phase generates overvoltage; aiming at the identification problem, an MMC negative-sequence current suppression control strategy is improved into a negative-sequence voltage suppression and voltage reduction V / F cooperative control strategy; based on a cooperative control strategy, a transition resistance item in the distance protection equation is split into an equivalent transition resistance and equivalent transition inductance complex model, and a line pi-type equivalent model is adopted to deduce a fault differential equation; and solving a differential equation containing a fault distance through a least square method to realize high-precision fault positioning.

The present application provides a power distribution system and a method for controlling its power distribution switches, which determines the load fluctuation entropy of the target power distribution system during the power distribution process through the load information when voltage jitter occurs during the power distribution process and the rated load of the distribution switch; determines the voltage disorder degree during the negative-sequence voltage jitter through the negative-sequence voltage record in the negative-sequence power data, and then determines the current symmetry of the negative-sequence current through the negative-sequence current record in the negative-sequence power data; determines the negative-sequence balance deviation of the three-phase power in the negative-sequence state based on the voltage disorder degree and the current symmetry; determines the critical threshold for the safe operation of the distribution switch when the distribution load changes through the load fluctuation entropy and the negative-sequence balance deviation, and then intelligently switches the opening and closing states of the distribution switches in the target power distribution system based on the critical threshold. By adopting the solution of the present application, adaptive switching of the distribution switch state during intermittent load jitter of the distribution system can be achieved, thereby improving the anti-interference performance of the distribution system during the power distribution process.

A phase current difference abrupt change quantity phase selection reliability analysis method considering new energy permeability and fault ride-through behavior comprises the steps of constructing a phase selection failure index based on a phase current difference abrupt change quantity amplitude ratio based on a phase current difference abrupt change quantity phase selection criterion; dividing new energy types according to an inversion type and a double-fed type, and determining fault equivalent impedance of different types of grid-connected inverters; establishing a new energy station equivalent impedance model of multi-type energy coupling; converting a phase selection failure index into a relation between positive and negative sequence current distribution coefficients, substituting the equivalent impedance of the new energy station into a sequence network for analysis, calculating a ratio of the positive and negative sequence current distribution coefficients, and comparing the relation between the ratio and a critical failure condition to finish the reliability analysis of the phase selection method. According to the method, the considered influence factors are comprehensive, the physical significance is clear, the phase selection failure risk of the line due to the high-proportion access of the new energy can be effectively and accurately judged, and certain guidance can be provided for the improvement of the phase selection method.

The invention relates to the technical field of modular multilevel converter control, in particular to an active-disturbance-rejection direct-current side fluctuation suppression method based on an MMC alternating-current side fault working condition. The method mainly aims at solving the problems that an existing suppression scheme based on a PI controller is insufficient in performance and difficult in parameter adjustment, and a control scheme based on an active disturbance rejection controller is complicated in parameter debugging and difficult in obtaining optimal parameters although the performance is better. The method comprises the following steps: step 1, suppressing a negative sequence current generated by an MMC system when an alternating current side has a fault; step 2, constructing a second-order LADRC model; and step 3, current inner loop control is carried out by using a second-order LADRC controller set by a particle swarm optimization algorithm. According to the method, a traditional PI controller is replaced by the second-order LADRC controller, parameters are set in combination with the particle swarm optimization algorithm, the ride-through capacity of the MMC system under the alternating current side fault is remarkably improved, and better dynamic response and interference immunity are achieved.

The invention belongs to the technical field of power system control, and discloses a direct-drive fan asymmetric fault ride-through control method and system based on negative sequence impedance, and the system comprises a phase-locked loop module PLL which achieves the separation of positive and negative sequence fundamental wave components and has a good dynamic response speed; the dynamic supporting point motion calculation module distributes positive and negative sequence reactive current and a boundary autonomous moving point strategy BAMP based on a fixed proportion of voltage change through a fixed positive and negative sequence proportion strategy FRPN; the maximum support point tracking module ensures that two constraints of peak current limit PCL and active power oscillation limit POL can be met; the selector module is used for switching the system from a normal operation mode to a support mode after fault detection, and activating the DSPM and MSPT modules; and the admittance current control module ACC receives the reference value of the negative sequence simulation admittance and obtains the reference value of the negative sequence current. The energy efficiency and stability of the whole system are improved, and the practical application value of the method is verified.