Research On Low Voltage Ride Through Strategies Of Nine-Switch-Converter-Driven DFIG-based Wind Generation System

Doubly-fed wind power generation system is widely used in the current wind power system,due to its mature manufacturing technology and low manufacturing cost.In this system,the stator of Doubly-Fed Induction Generator(DFIG)is connected to the grid directly,meanwhile the rotor is connected to the grid through the traditional back-to-back(B2B)converter.By controlling the B2 B converter,the variable-speed constant-frequency power is obtained.In the grid-normal condition,since the B2 B converter only handles the slip power of DFIG,its rating capacity could be only 20%~30% of the generation power,which is treated as one of the merit of the DFIG wind power system.However,the modern Grid Code demands the wind turbines be kept connected to the grid,and provide reactive power supported for the dipped grid voltage when the grid dips,i.e.,achieve Low Voltage Ride Through(LVRT).This presents new challenges to the converter design.On the one hand,the grid voltage dip induces a very high ElectroMotive Force(EMF)at the rotor-side of the DFIG,in order to protect the rotor-side converter away from being damaged by the over-voltage or over-current,the crowbar or active circuits should be added to absorb the inrush energy,or the larger capacity of the B2 B converter should be designed for the suppression of the over-voltage or over-current.On the other hand,the grid-side of B2 B converter also requires larger current capacity to keep the Dc-bus voltage stable and output reactive power to the grid during LVRT period.The aforementioned auxiliary devices and capacity margins can not be fully used in the grid-normal condition,which makes the waste of the capacity utilization and increases the cost,volume and complexity of the system.To reduce the number of the power devices,some researchers presented utilizing Nine Switch Converter(NSC)instead of the traditional B2 B converter,using in the DFIG wind power system.However,only the steady-state control scheme in grid-normal condition was proved,but the system design for the LVRT function of the NSC system in grid-dip condition is not discussed in the presented literatures.Therefore,focusing on the realization of the LVRT function of the NSC system,this thesis systematically presents the design method of the rating capacity,introduces a series LVRT strategies without any additional auxiliary circuits or margins,and proposes the comprehensive and quantitative comparisons between the NSC and B2 B systems.The innovations are: 1 Quantitative design of voltage and current capacitiesDue to the special time-shared modulation,the power capacity design of NSC is different from the traditional B2 B converter.At present,there is no literature presenting a design method for NSC using in the DFIG wind power system.Based on the time-shared modulation principle,this thesis reveals the relationship between the voltage and current requirements of NSC and the operation conditions of the grid and DFIG,so as to presents the quantitative design method of the voltage and current capacities for the NSC system in grid-normal condition.2 Dynamic assignment of voltage and current capacitiesAccording to the time-shared characteristic,this thesis presents the Dynamic Voltage Assignment(DVA)approach for NSC system.In the grid-dip condition,the Dc voltage is reassigned to the grid-and rotor-side of NSC,so that the rotor current of DFIG can be suppressed.This thesis also proposes the Dynamic Current Assignment(DCA)approach for NSC system.In grid-dip condition,the proportion of the grid-and rotor-side currents are reassigned,and thus the over-voltage in the rotor-side can be avoided and the reactive current can be injected into the grid for the grid-side.Whatever using the DVA strategy,or using the DCA strategy,the NSC system can achieve LVRT without any additional circuits or capacity margins.3 Optimized design of NSC systemFocusing on the high Dc voltage requirement of NSC,this thesis proposes using a small capacity transformer connected in series between the rotor terminal of DFIG and NSC,so that the equivalent ratio of the whole system can be adjusted and the NSC can work in the voltage and current capacities fully-used area.Furthermore,this thesis also presents “the most simplified” and “the most optimal” LVRT strategies.“The most simplified” LVRT strategy only uses one set of control system controlling NSC work in both grid-normal and grid-dip conditions,and thus the detection of the grid-dip moment and the switch of the control system can be avoided.“The most optimal” LVRT strategy organically integrate the aforementioned DVA and DCA strategies,so as to,the rating capacity of NSC can be maximally utilized to satisfy the grid code requirements.4 Advantages and application suggestions of NSC systemThis thesis presents an overall comparisons between the NSC and three typical B2 B systems,the comparison results point out that NSC system takes the advantages on current capacity,VA capacity and capacity utilization ratio.This thesis also compares analyze the impact of the turn ratio of DFIG in the B2 B and NSC systems,the comparisons indicate that NSC is more suitable for the applications in which the turn ratio lies between 0.84 and 1.18.In this case,with the proposed DVA and DCA strategies,NSC system shows better comprehensive performance,and gets better LVRT and reactive compensation abilities.