149 results about "Grid connected converter" patented technology

Disclosed are a grid-connected converter device of a direct-drive wind power system and a corresponding power coordination control method. The converter device is composed of basic converter units of a Boost circuit based on ac-side energy storage; the structure sufficiently utilizes the equivalent inductance of a power generator stator winding, avoids an external inductor in traditional dc voltage booster circuit, and reduces power loss and equipment cost. Two converter proposals, including a diode rectification mode and a PWM rectification mode are designed; converter topologies with two capacity grades, including medium and small power converter topology, megawatt grade high power converter topology, and relative extension structures are designed, such as a parallel connection converter and an H bridge cascade structure converter; wherein, the converter with the H bridge cascade structure is applicable to grids with high capacity and high voltage, and can obviously reduce the harmonic content in grid-connected current; meanwhile, the control method performing feedback through three closed loops is designed, so the input power and output power of the converter are balanced instantaneously, the dc bus voltage maintains stable, and the grid-connected current wave is sine.

An LVRT control method for a grid-connected converter of distributed energy resources comprises steps of: obtain a positive sequence electrical component and a negative sequence electrical component during an LVRT period; outputting a compensation signal according to a power threshold withstood by a grid-connected converter to undertake reactive power compensation; and constraining the output power lower than the power threshold. The present invention uses the positive and negative sequence electrical components to undertake reactive power compensation, whereby to improve balance and stability of voltage during the LVRT period, avoid reverse torque and mechanical resonance, and prolong the service life of the power generator. Moreover, the present invention constrains the sum of the compensation currents below the power threshold to prevent the circuit of the power generator from being overloaded and damaged, whereby is prolonged the service life of the power system.

The invention discloses an island detection method of a high-power photovoltaic grid-connected converter. The method comprises the following steps of: determining whether a system operates at a grid-connected power generation state, and starting up an island detection algorithm instantly if the system is at the grid-connected power generation state; detecting whether the frequency of the voltage of a port power grid is within the normal range in real time, if the frequency is not within the normal range, considering that the system is at the island state, and carrying out automatic safety stop on the system; adding reactive current Iqdisturb with the duration of td periodically within the normal range; and superposing the reactive current Iqdisturb and reactive control Icqset of the converter, thus an axis q component reference Icqref of grid-connected current of the converter is formed finally. The contradiction between two aspects of reliability of detection and the quality after grid-connected electric energy reduction for the traditional island detection algorithm is effectively solved by the method provided by the invention, and the implementation method is simple and effective.

A method of controlling the grid-side current of a single-phase grid-connected converter having an LCL filter connected between the output of the converter and the grid. The method includes measuring a grid voltage (vS) and at least one signal in a group of signals consisting of a grid-side current (i0), a converter-side current (i1) and a capacitor voltage (vC0). The method includes estimating the fundamental component (vS,1) of the grid voltage (vS), forming a grid-side current reference (i0*), a converter-side current reference (i1*) and a capacitor voltage reference (vC0*) for the grid-side current of the LCL filter using the fundamental component of the grid voltage (vS,1), forming estimates for the non-measured signals in said group of signals, forming a grid-side current difference term (ĩ0), a converter-side current difference term (ĩ1) and a capacitor voltage difference term ({tilde over (v)}C0) from the differences between the references and measured / estimated values of said signals, forming an injection term for damping the resonance of the LCL filter by using an active damping injection mechanism (ADI), in which the grid-side current difference term (ĩ0), the converter-side current difference term (ĩ1) and the capacitor voltage difference term ({tilde over (v)}C0) are used, forming an estimate of harmonic distortion term ({circumflex over (φ)}) using the grid-side current difference term (ĩ0), and controlling the output voltage (e) of the converter on the basis of the grid voltage, formed injection term and formed estimate of the harmonic distortion term ({circumflex over (φ)}) to produce a grid side (i0) current corresponding to the current reference.

The invention relates to a precise phase locking method based on a cross decoupling self-adaptive complex filter, which comprises the following steps: firstly carrying out the Clarke coordinate transformation on three-phase voltage input signals of Ua, Ub and Uc for obtaining voltage signals of Ualpha and Ubeta under an alpha-beta coordinate system; then leading the voltage signals to pass throughthe cross decoupling self-adaptive complex filter for extracting positive sequence voltage components *alpha<plus> and *beta<plus> under the alpha-beta coordinate system; further carrying out the Clarke inverse transformation on the *alpha<plus> and the *beta<plus> for obtaining three-phase voltage positive sequence components of *a<plus>, *b<plus> and *c<plus> under an a-b-c coordinate system; carrying out the Park transformation on the *alpha<plus> and the *beta<plus> for obtaining a d-axis component *d<plus> and a q-axis component *q<plus> under a d-q coordinate system, wherein, the q-axiscomponent *q<plus> reflects the positive sequence amplitude; leading the d-axis component *d<plus> to pass through a PI regulator to obtaining the frequency estimated value *0; feeding the *0 back tothe cross decoupling self-adaptive complex filter for realizing the frequency self-adaption; and carrying out integration on the *0 for 1 / s for obtaining the positive sequence phase *<plus>. The precise phase locking method is characterized by simpleness, easy realization, high phase-locking precision and fast speed, thereby being particularly applicable to the precise phase locking of grid-connected converters under the extreme industrial field situations of power grid voltage distortion, three-phase voltage imbalance, large fluctuation range of voltage frequency and the like.

The invention provides a method for controlling low-voltage ride-through of the grid-connected converter of a wind-driven generating set. On the basis of back-to-back voltage or current-typed grid-connected converter, a motor-side converter detects the middle direct current voltage; a voltage adjuster controls the middle direct current voltage to follow with the middle direct current voltage instruction value; a torque adjuster controls the practical torque to follow with the torque instruction value; an active power adjuster controls the grid-side converter and leads the grid-connected active power to follow with the active power instruction value. The transfer of the dump energy is realized by the coordinated control of the motor-side converter and the grid-side converter, elements such as additional resistance and the like are not required so as to consume the rest energy, thus ensuring that the wind-driven generating set does not run without the grids during the falling period of the network voltage at the access point. The method improves the efficiency of the unit, reduces the energy loss, simplifies the structure of the system, reduces the volume of the device, reinforces the compactness of the structure and reduces the cost of the device.

A method and an apparatus for controlling a grid-connected converter which includes a boost converter, a buck converter, and a current source inverter having an output CL filter. An input of the buck converter input is connected to an output of the boost converter, and an input of the current source inverter is connected to an output of the buck converter. The method includes controlling a boost converter input voltage, controlling a boost converter output voltage through control of a buck converter output voltage, and controlling the current source inverter to produce an AC current from the buck converter output voltage. The apparatus implements the method.

The invention provides a soft grid connection method of a grid-connected converter. The method is characterized in that the closed-loop control of the filtering capacitance and the filtering voltage is carried out synchronously; the current inter-loop control is added; and DC pre-charging return circuits are separated and boosted. According to the invention, the non-impact grid connection is realized without increasing great hardware complexity, thereby overcoming the defects of the traditional grid-connected method and improving the reliability of the grid-connected converter.

The invention provides a d-q axis parameter identification method for a grid-connected inverter of a photovoltaic power generation system. The d-q axis parameter identification method includes: an inverter control reference value Udc-ref and Qref jump are arranged on the grid-connected inverter of the photovoltaic power generation system, or a three-phase short circuit fault is manufactured on the side of a power grid so as to collect disturbance data required by parameter identification; d axis parameters of the grid-connected converter are calculated according to the collected data, and the d axis parameters comprise filter inductance, d axis inner ring current control ring proportionality coefficients, inner ring current control ring integral coefficients, outer ring active power control ring proportionality coefficients and outer ring active power control ring integral coefficients; and a filter inductance parameter is fixed to be a d axis identification result, q axis parameters of the grid-connected inverter are calculated according to the collected data, and the q axis parameters comprise q axis inner ring current control ring proportionality coefficients, inner ring current control ring integral coefficients, outer ring reactive power control ring proportionality coefficients and outer ring active power control ring integral coefficients. The d-q axis parameter identification method for the grid-connected inverter of the photovoltaic power generation system reduces order and identification difficulty of a model to be recognized and improves identification accuracy.

The invention discloses a marine wind power direct current collecting power transmission system and a design method. The marine wind power direct current collecting power transmission system comprises a wind farm inner electric energy collecting module, a wind farm direct current boost module, a high voltage direct current collecting and transmission module, and a land converter station grid-connected module, wherein the wind farm inner electric energy collecting module comprises a plurality of wind power generators and AC-DC rectifiers, each wind power generator converts wind energy to three-phase alternating current electric energy, the electric energy is converted to direct current electric energy via the AC-DC rectifiers, the wind farm direct current boost module comprises a plurality of DC-DC topology structures, the energy of fans in the wind farm is collected to a direct current boost station via direct current and boosted via DC-DC converters, the high voltage direct current collecting and transmission module is used for collecting high voltage direct current electric energy outputted by each wind farm, transmitting the energy to the land via a common submarine cable, and performing grid connection via a high-capacity converter station, and the land converter station grid-connected module comprises a grid-connected converter station arranged at the land side and a direct current power transmission converter station.

The invention discloses a compensation method for a pulse width modulation dead zone in a three-phase grid-connected converter. The compensation method has the advantages that the compensation method can solve the mis-compensation phenomenon caused by the oscillation of current in the vicinity of passing zero point, realize the real-time compensation for the dead zone and significantly improve the wave quality of the output current. The compensation method judges the dead-zone compensation phase and the dead-zone compensation direction by theta e which is defined by the invention, determines the compensation voltage amount by t e which is defined by the invention, then determines the final three-phase voltage control amount by combining the dead-zone compensation phase, the dead-zone compensation direction and the compensation voltage amount, and controls the three-phase grid-connected converter to transmit the power to a grid by a digital signal processor according to the three-phase voltage control amount.

The invention provides a positive and negative sequence impedance measuring module, a positive and negative sequence impedance measuring system and a positive and negative sequence impedance measuringmethod for a new energy grid-connected converter. The positive and negative sequence impedance measuring module comprises a disturbance voltage injection unit, a grid-connected point voltage / currentacquisition unit, an impedance calculation unit and an impedance bode map generating unit. The positive and negative sequence impedance measuring system comprises a controller, I / O interface equipmentand a real-time simulation system which are sequentially connected, wherein the real-time simulation system comprises the positive and negative sequence impedance measuring module. Compared with theprior art, the positive and negative sequence impedance measuring module, the positive and negative sequence impedance measuring system and the positive and negative sequence impedance measuring method for the new energy grid-connected converter are provided by the invention, the positive and negative sequence impedance measuring module can control and disturb phases, amplitude values and waveforms of a voltage signal flexibly; the controller, the I / O interface equipment and the real-time simulation system in the positive and negative sequence impedance measuring system form a hardware-in-loopcontrol real-time simulation system, real-time simulation and positive and negative sequence impedance measurement can be carried out on the new energy grid-connected converter, and the precision ofimpedance measurement of the new energy grid-connected converter is improved.

The invention discloses a DC micro grid hybrid energy storage system hierarchical control method comprising the following steps: 1, building a DC micro grid system structure that includes a distributed generating unit photovoltaic array, a hybrid energy storage unit, a DC load and a grid connected converter; 2, using a super capacitor and a lithium battery to form the hybrid energy storage system,and carrying out coordination cooperation in corresponding layers; 3, deriving a relation between DC bus voltage changes and a system power flow direction according to a DC micro grid system power equation, a bus capacitance energy storage change and voltage change equation; 4, dividing the system operation control into 5 layers according to DC bus voltage fluctuation sizes, wherein each operation layer has a converter as a balance point so as to adjust the DC bus voltage, thus ensuring the system power balance. The hierarchical coordination control strategy can reasonably use the clean energy formed by the distributed power supply and energy storage equipment, thus improving the energy utilization efficiency.

The invention provides a power flow calculation method for a complex topology active distribution network containing a direct current system in allusion to a problem of poor convergence in the AC-DC system hybrid power flow calculation process of the existing distribution network. The method is characterized in that the active distribution network is divided into a direct current system and an alternating current system according to a voltage source converter; whether weak loop running exists in the system or not is determined according to opening-closing conditions at a loop closing point so as to perform corresponding power flow calculation; and a power flow calculation result of the active distribution network hybrid system is acquired finally by combining a power flow calculation result of the direct current system. Corresponding adjustment is performed on the power flow calculation sequence of the direct current system and the alternating current system and selection for power flow calculation parameters of the converter under different control strategies of the grid-connected converter, and power flow calculation for the whole hybrid system can be realized by performing only one time of alternative iteration convergence on the direct current system and the alternating current system.

The invention discloses an output phase-locked loop system of a grid-connected converter and a control method. The system comprises a Clark transform module for Clark transform of a three-phase voltage signal outputted by a grid-connected converter, a delay elimination module for carrying out delay elimination on two phases of voltages in an alpha-beta coordinate system, a Park transform module for Park transform of the two phases of voltages in the alpha-beta coordinate system after delay elimination according to an initial output phase, a PI regulator for PI adjustment on a q-axis signal, an integration module for integration processing on a grid voltage estimation value to obtain an initial output phase, an amplification module for amplification of a frequency adjustment error to obtain a phase compensation value, and a superimposition module for superimposition of the initial output phase and the phase compensation value to obtain a final output phase. With the output phase-locked loop system and the control method, the fundamental frequency and phase information of the output voltage of the grid-connected converter can be obtained accurately when the system voltage is not balanced or distorts.

The invention provides an alternating-current / direct-current hybrid micro-grid layered control method based on electricity price. The control for an alternating-current / direct-current hybrid micro-grid is divided into three layers such as a converter control layer, a bus voltage control layer and a power scheduling layer. On the bottom converter control layer, all kinds of converters are divided into energy terminals and relax terminals according to control objectives thereof; on the middle bus voltage control layer, any unit in the alternating-current / direct-current hybrid micro-grid can smoothly adjust the output power of an interface converter thereof according to a direct-current bus voltage signal under a same control structure, so that dispersion autonomous operation of the alternating-current / direct-current hybrid micro-grid is realized; on the upper power scheduling layer, a grid-connected converter decides a specific operating state according to time-of-use electricity price information on an alternating-current grid side, and then the output power of an energy storage unit is changed so as to make full use of remaining capacity thereof and improve the economy and schedulability of system operation.

A method of controlling the grid-side current of a single-phase grid-connected converter having an LCL filter connected between the output of the converter and the grid includes measuring a grid voltage and at least one signal in a group of signals consisting of a grid-side current, a converter-side current and a capacitor voltage, estimating the fundamental component of the grid voltage, forming a grid-side current reference, a converter-side current reference and a capacitor voltage reference for the grid-side current of the LCL filter, forming estimates for the non-measured signals in the group of signals, forming a grid-side current difference term, a converter-side current difference term and a capacitor voltage difference term, and controlling the output voltage of the converter based on the grid voltage, a formed injection term and a formed estimate of the harmonic distortion term to produce a grid side current corresponding to the current reference.

The invention discloses a risk analysis method of sub-synchronous oscillation generated by connection of a converter and an equivalent grid by considering phase-locked loop influence, and belongs to the technical field of electric power systems and automation thereof. The method includes the steps: building an equivalent grid and grid-connected converter electrical appliance link and control linksmall-disturbance mathematical model, and analyzing influence of detection errors of phase-locked loops under small disturbance on grid-connected converter control variables; correcting the grid-connected converter control link small-disturbance mathematical model according to the influence; determining system input voltage variables and output current variables, and getting a system dynamic impedance transfer function under small disturbance; drawing a system frequency impedance curve, and analyzing system sub-synchronous oscillation risks according to characteristics of the curve. A novel stability analysis method of a joint of VSC converter and a grid is provided, and the stability and the oscillating characteristics of the system can be simply and efficiently judged.

The invention discloses a decoupling and delayed compensation method for a complex vector PI controller of a grid-connected converter. The decoupling and delayed compensation method comprises the steps of directly arranging a zero point of the controller at a pole point of a controlled object by taking a vector model of the controlled object as a basis to obtain a complex coefficient current controller model; calculating to obtain a system delay transfer function with regard to single delay of digital control and inherent zero-order maintaining characteristic of the converter; and achieving delay compensation of a current ring after connecting a delay compensation item in series with the complex vector PI controller. A single and effective method is designed for the current ring of the grid-connected converter, the problem of d-axis current and q-axis current coupling is prevented to the greatest extent, the phase margin of the system is expanded by delay compensation, the stability of the current ring is improved, and the dynamic response performance is improved.

The invention discloses an optimized power allocation method applied to parallel operation of grid-connected converters. The input power and output power of each grid-connected converter in independent operation are detected, and the item coefficients of an efficiency model based on the load characteristics of the converter are determined based on the input power and the output power. A system efficiency expression of n grid-connected converters during parallel operation is built according to the efficiency model of each grid-connected converter. By calculating the output power of each grid-connected converter, optimized output power allocation is carried out in order to improve the system efficiency of a parallel system under the condition that grid-connected converters with different energy conversion efficiency are in parallel operation. Therefore, optimized control during parallel operation of grid-connected converters such as grid-connected inverters and PWM rectifiers is realized.

The invention discloses an energy storage grid-connected converter based on reverse-blocking type three levels, and a control method therefor. The converter comprises a storage battery, a DC / DC converter, a DC-side capacitor, a reverse-blocking type three-level converter, and an LC filter, wherein the storage battery, the DC / DC converter, the DC-side capacitor, the reverse-blocking type three-level converter and the LC filter are sequentially cascaded. The control method comprises the steps: DC / DC end control and three-level converter control. A DC / DC end controller is used for the management of the charging and discharging of the storage battery, and the three-level converter control comprises the control of the output current of an inverter through a double-closed-loop decoupling control. The method achieves the functions of step-up / step-down, inversion, large-power energy storage and grid connection through power conversion, achieves the control from the storage battery to a power grid through a simpler structure, reduces the energy storage cost, achieves reactive compensation and power peak control, and improves the utilization rate and quality of the power grid.

The invention discloses a current control system and method for the grid-connected converter of a DC micro-grid. The method comprises pre-estimating the power of the grid-connected converter by the change of DC bus voltage; calculating the steady-state value of the inductive current according to the change of the DC bus voltage; and establishing a linear control relationship between the DC bus voltage and the inductive current to predict directive current. The control strategy improves the current control accuracy of the grid-connected converter, increases response speed, and improves the dynamic and static performance of the grid-side current of the grid-connected converter, reduces the influence of system parameter changes on the grid-side current, and enhances the robustness of the DC micro-grid. The DC micro-grid grid-connected converter current control strategy and device include a bidirectional converter topology circuit and its control device, and solve the problem that the control of the grid-connected converter in the DC micro-grid in the prior art cannot realize bidirectional power flow and the grid-connected converter has line impedance.

The invention provides a method for detecting and separating positive and negative sequence components of grid voltage of a three-phase neutral-line-free system. According to the method, grid voltage of each phase is sampled so as to acquire an instantaneous value and a corresponding differential value of the current grid voltage, thereby being capable of separating the positive and negative sequence components of the current voltage of the grid voltage of the phase, and calculating so as to acquire the sine and the cosine of a positive-negative sequence instantaneous phase angle of the current grid voltage. The method provided by the invention is simple in calculation, small in time delay, high in real-time performance, convenient for engineering realization, applied to a software phase-locked loop of a three-phase grid-connected converter control system, and capable of effectively avoiding overcurrent and fluctuation of DC-side voltage caused by asymmetry of three-phase voltage.

The invention discloses a control method for improving grid-connected voltage source inverter system stability. The method comprises the steps of (1) generating a grid-connected converter modulating wave initial order; (2) calculating a power grid side current control circuit gain T, obtaining corresponding cut-off frequency omega 1 and cut-off frequency omega 2 when an amplitude-versus-frequency curve of the T passes through 0dB twice before a resonant frequency omega r of an LCL filter, and obtaining an s-domain transfer function of a lag compensation link according to the cut-off frequency omega 1 and the cut-off frequency omega 2; (3) performing discretization on the s-domain transfer function of the lag compensation link, and bringing the grid-connected converter modulating wave initial order obtained in the step (1) into an express form, in a digital controller, of the s-domain transfer function of the lag compensation link obtained through the discretization, so as to obtain a final modulating wave order value of the converter; (4) obtaining a modulating signal according to the final modulating wave order value of the converter obtained in the step (3), and controlling various switching elements of the converter according to the modulating signal; the control method provided by the invention can effectively improve the system stability.

The invention discloses a method for detecting a fault in a relay of a three-level grid-connected converter. The method specifically comprises: under different states of a relay, opening auxiliary pipes of an inverter bridge arm of the converter, and detecting current passing through the relay in real time; if the relay is closed and the current is detected, determining the relay is normally closed, and otherwise, determining the relay has an open-circuit fault; and if the relay is in a disconnected state and the current isn't detected in a preset time, determining the relay is normally disconnected, and otherwise, determining the relay has a short-circuit fault. According to the method, open-circuit and short-circuit faults of the relay can be accurately detected, and addition of excess detection values isn't needed. The system is suitable for a single-phase, three-phase three-wire, or three-phase four-wire system, has characteristics of simplicity, accurate determination, and good versatility, and is suitable for detecting the fault in the relay of a grid-connected converter.

The invention relates to an integrated detection system and method for energy storage converter grid-connected and off-grid characteristics. A grid-connected converter unit and a simulation converter unit are comprised. The grid-connected converter unit comprises a rectifier module and an LCL filter. The alternating current side of the rectifier module is connected with a power grid through the LCL filter. The direct current side is connected with the direct current side of the simulation converter unit. The alternating current side of the simulation converter unit is connected with a detected energy storage converter. According to the technical scheme provided by the invention, the topology of the integrated detection system for energy storage converter grid-connected / off-grid characteristics is provided, which improves the effectiveness of the energy storage converter detection system and the power grid simulation fitting.

The invention discloses a voltage source converter grid-connected system stability analysis method considering the influence of a phase locked loop, The device and the system comprise: acquiring smalldisturbance state equations of an electric main loop unit of an equivalent power network and a grid-connected converter, and small disturbance state equations of a current inner loop, a feedforward link and a converter pulse width modulation link in the control unit; Based on the relationship between the variable increment of synchronous coordinate system of control unit and the variable increment of corresponding actual synchronous coordinate system caused by phase-locked loop detection error under small disturbance, the specific variable relation involving shaft system transformation is obtained. Obtaining a small disturbance state equation of a phase-locked loop in the control unit; Finally, the stability analysis of the grid-connected VSC system is completed by simultaneously establishing the above state equations, substituting the relevant parameters and solving the eigenvalues of the system. The invention can simply and efficiently judge the stability and oscillation characteristics of the grid-connected system of the voltage source converter.

The invention discloses an island detection method. The island detection method includes: controlling a switching module to be turned off so as to realize disconnection of a capacitor module composed of multiple capacitors and a point of common coupling of a distributed power generation system; controlling the switching module to be turned on so as to realize connection of the capacitor module composed of the capacitors and the point of public coupling of the distributed power generation system; acquiring an impedance value of the point of common coupling; comparing the impedance value with an impedance threshold, and if the impedance value is larger than the impedance threshold and duration exceeds preset duration, judging that island effect happens. The invention further discloses an island detection device, a grid-connected converter and the distributed power generation system. Occurrence of the island effect can be detected quickly and accurately, non-detection zones are eliminated basically, and increase of harmonic content and decrease of power generation efficiency of intra-system power-connected equipment are avoided.

The invention discloses a non-isolated grid-connected converter, an air-conditioning system and a converter control method, which belong to the field of efficient grid-connected converters, and are designed for solving the problems of large size of the existing devices and the like. The non-isolated grid-connected converter at least comprises a rectifying and inverting unit, and has the bidirectional functions of rectification and inversion. The air-conditioning system comprises the non-isolated grid-connected converter. The control method based on the non-isolated grid-connected converter comprises a rectification mode. The non-isolated grid-connected converter adopts a bidirectional DC / AC converter to realize rectification, PFC and grid-connection functions, omits an isolation transformer, reduces number of devices, and is small in size. The air-conditioning system provided by the invention is small in size and higher in energy efficiency. The control method of the non-isolated grid-connected converter achieves the effects of low leakage current and high grid access current, can increase the voltage withstand level through rectification, reduces AC harmonic voltage and current, and increases power factors.

The invention provides a subsynchronous oscillation suppression method based on grid-connected converter control. The method comprises the steps that three-phase alternating current voltage VPCC and current IPCC of a PCC point of a grid-connected converter are acquired; coordinate transformation is carried out on VPCC and IPCC to acquire active and reactive components id and iq of the current in arotating coordinate system; an active current control signal Ed is acquired according to an active current reference value idref, id and a voltage feed-forward control signal; according to a reactivecurrent reference value iqref, iq, an additional reactive current command optimization control signal Utcq and the voltage feed-forward control signal, a reactive current control signal Eq is acquired; a three-phase alternating current internal potential eabc is acquired from Eq, Ed and theta; and pulse width modulation is carried out on eabc to acquire a switching control signal to suppress thesubsynchronous oscillation of the shaft of an adjacent synchronous machine. According to the invention, the electrical damping of the synchronous machine and the suppression effect of the subsynchronous oscillation of a synchronous generator are increased by adding a control branch at the reactive current command of the grid-connected converter without increasing the system capacity and changing the parameters of a PI controller.