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Disclosed is a controller of a grid coupled type doubly-fed induction generator having a multi-level converter topology, which can control the doubly-fed induction generator having a high voltage specification and can perform a fault ride-through function, an anti-islanding function and a grid volta

Disclosed is a controller of a grid coupled type doubly-fed induction generator having a multi-level converter topology, which can control the doubly-fed induction generator having a high voltage specification and can perform a fault ride-through function, an anti-islanding function and a grid voltage synchronization function required for a dispersed power generation facility. The controller makes a H-bridge multi-level converter generate a three-phase voltage waveform resulted from the structure that single-phase converters each being composed of a 2-leg IGBT are stacked in a serial manner, and controls a rotor current so as to make the rotor coil of the doubly-fed induction generator in charge of a slip power only. The boost converter is composed of a 3-leg IGBT and a boost inductor generating a direct current voltage of its source required for the H-bridge multi-level converter.

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What is claimed is: 1. A controller for controlling a doubly-fed induction generator having a stator coil coupled to a three-phase grid coil through a switch and a rotor coil as a control coil, comprising: a H-bridge multi-level converter having a multi-level topology that single-phase converters e

What is claimed is: 1. A controller for controlling a doubly-fed induction generator having a stator coil coupled to a three-phase grid coil through a switch and a rotor coil as a control coil, comprising: a H-bridge multi-level converter having a multi-level topology that single-phase converters each composed of a 2-leg insulated-gate bipolar transistor (IGBT) are stacked in a serial manner, generating a three-phase voltage waveform, and controlling a rotor current so as to allow a rotor coil of the doubly-fed induction generator to be in charge of a slip power only; and a boost converter composed of a 3-leg IGBT and a boost inductor generating a direct current voltage of its source required for the H-bridge multi-level converter. 2. The controller according to claim 1, wherein the H-bridge multi-level converter performs a function of controlling a variable power factor in accordance with a magnitude of a grid voltage, and a function of synchronizing a voltage generated in the stator coil with a grid voltage without causing an inrush current. 3. The controller according to claim 2, wherein when the power factor of the stator coil is controlled for division by the H-bridge multi-level converter, an active power reference value determined by an existing maximum power point tracking method is set to a different and apparent power parameter (S*), which is completely converted to an active power reference value by making one a power factor reference value when the grid voltage is in a stable range, however, when the grid voltage becomes unstable, the power factor reference value is set to be lower using an equation η*=1-|slop*|×Δe in accordance with a degree of the unstableness so that a rate of converting into an active power is determined to be lower using an equation ps*=η*×S*, and the rest is made to have a reactive power reference value using an equation so as to restore the grid voltage. 4. The controller according to claim 2, wherein when the synchronization is performed by the H-bridge multi-level converter, in order to allow the rotor coil to be in charge of the slip power only, a slip angle (θslip) is used as a coordinate transformation angle and is calculated using an equation θslip=θs-θr+θoffset when a grid voltage phase angle is θs, a rotating position of the generator is θr, and a synchronization compensation angle is θoffset. 5. The controller according to claim 2, wherein when the synchronization is performed by the H-bridge multi-level converter, in order to make a phase of a voltage induced toward the stator coil equal to a phase of the grid voltage, a PI controller is used which receives a d-axis voltage error as an input when the d-axis voltage error measured from the stator coil of the generator and the grid coil is ΔVds, and an output signal of the PI controller is defined as a synchronization compensation angle θoffset determined by θoffset=PI(ΔVds). 6. The controller according to claim 2, wherein when the synchronization is performed by the H-bridge multi-level converter, in order to make a magnitude of a voltage induced toward the stator coil equal to a magnitude of the grid voltage, a PI controller is used which receives a q-axis voltage error as an input when the q-axis voltage error measured from the stator coil of the generator and the grid coil is ΔVqs, and an output signal of the PI controller is defined as a d-axis compensation current (idre--comp) for compensating for the existing d-axis current value, and the d-axis compensation current (idre--comp) is determined by idre--comp=PI(ΔVqs). 7. The controller according to claim 2, wherein when the synchronization is performed by the H-bridge multi-level converter, in order to make a magnitude of a voltage induced toward the stator coil equal to a magnitude of the grid voltage, an additional term, a feed-forward term idre--comp--ff is used to enhance a convergence speed when a d-axis compensation current is calculated so that an equation idre--comp=PI(ΔVqs)+i dre--comp--ff is calculated, where E/WeLo can have any value of the feed-forward term idre--comp--ff . 8. The controller according to claim 2, wherein when the synchronization is performed by the H-bridge multi-level converter, an error signal (Δidr) of the d-axis current controller is calculated from an equation Δidr=ie*dr-iedr+i dre--comp using the d-axis compensation current (idre--comp) required for making a d-axis reference current (ie*dr) obtained from a reactive power output equal to a d-axis current (iedr) measured from the stator coil and making a magnitude of a voltage of the stator coil equal to a magnitude of the grid voltage. 9. The controller according to claim 1, wherein the boost converter performs a function of controlling a power factor of the stator coil, a fault ride-through function of controlling a reactive power to be supplied when the grid voltage becomes unstable, and an anti-islanding function of inputting a white noise for easily preventing an islanding. 10. The controller according to claim 9, wherein when the power factor of the stator coil is controlled, a power factor reference value is determined by an equation η*=1-|slop*|×Δe in accordance with a magnitude (Δe) of the grid voltage varied from a reference value of the grid voltage, where |slop*| is an attenuation slope and has a predetermined value. 11. The controller according to claim 9, wherein, in order to perform the fault ride-through function, when an output of the boost converter for constantly controlling a direct current_link voltage is set as an apparent current reference value (ie*s), a q-axis current reference value (ie*q) is decreased using an equation ie*q=η*×ie* so as to have the fault ride-through function in a short interruption interval in consideration of a power factor (η*) of the stator coil variably determined in accordance with a magnitude of the grid voltage variation. 12. The controller according to claim 9, wherein, in order to perform the fault ride-through function by means of the boost converter, a reactive power is supplied in a short interruption interval by supplying a d-axis current component (iefrt) calculated from an equation using a q-axis current reference value (ie*q) and a power factor reference value (η*) to overcome the short interruption. 13. The controller according to claim 9, wherein a d-axis current value (iefrt) for having the fault ride-through function is limited to iefrt=ie*s when iefrt is greater than ie*s so as to prevent the d-axis current value from exceeding a maximum upper limit. 14. The controller according to claim 9, wherein, in order to perform the anti-islanding function by means of the boost converter, a method of using a sign of a voltage phase angle (θs) for obtaining signs periodically alternating between + and-, and a method of using a q-axis rated current (iq--rated) and an adjustment constant (kanti) to obtain a d-axis current (ieanti) from an equation ieanti=sign (θs)×Kanti×iq-- rated for anti-islanding capable of supporting the anti-islanding in an electrical interruption interval, determining that there exists a high possibility of an island mode when a frequency variation width (Δf) is greater than a predetermined reference value (ΔfUpper--Limit), increasing a signal currently being input by an increment (Δkanti) using an equation kanti=Kanti+ΔKanti, and inputting within an entire time band a d-axis current (ieanti) determined by re-adjusting an adjustment constant (kanti) to reduce the current to the initially set d-axis current when the frequency variation width (Δf) is not greater than a predetermined reference value (ΔfLower--Limit), are used to make the frequency variation (df/dt) significantly excited in the electrical interruption interval to rapidly get out of the islanding.