Operation and design issues of a doubly fed induction generator stator connected to a dc net by a diode rectifier

Abstract

The operation of a doubly fed induction generator (DFIG) connected to a dc net is considered in this study. The dc net is connected to the stator of the DFIG by a diode rectifier and simultaneously feeds the rotor-side inverter, which is used to control the machine. When the dc net operates at constant voltage, this solution preserves the possibility to work with a low flux at low speed, and, at the same time, reduces the cost of the power electronics. In fact, the only needed electronic converter is the rotor inverter designed for a fraction of the machine power. After introducing a simple control technique, the study proposes an analytic model for the steady-state operation and discusses the design specifications of the DFIG for given values of the dc voltage and of the power of the wind turbine. The most appropriate stator and rotor rated powers, rated stator voltage and turns ratio of the DFIG are deduced. Simulations and experimental results are presented in order to support the theory

Voltage control in a DFIG-DC system connected to a stand-alone dc load

“…A 104 kVA DFIG (50 Hz, four poles, 400 V, 150 A, stator-rotor turn ratio 0.5) has been considered: the complete data are reported in the Appendix. By considering the dc voltage set point V dc = 480 V, the maximum deliverable stator power can be estimated by [10] A low dc voltage source (V 0 =48 V) is connected to the dc bus via a diode in order to energize the system at the beginning of the operation: once the dc voltage increases to the set point, the diode blocks reverse current. A dc load resistance R dc = 6.15 Ω is connected to the dc bus in order to set the load power at 0.36 p.u.…”

“…By using the power invariant Clark transformation, by supposing a constant dc voltage v dc , and by approximating the phase stator voltage waveform with a square wave, the relation between the first harmonic amplitude of the stator voltage space vector and the average dc voltage is [10] dc s v v π ≅ 3 2 1 (6) where the factor √3 is introduced by the power invariant Clark transformation. By considering that across the rectifier Im( s s i v )≈0 and assuming that (6) also roughly holds for the slowly varying dc voltage component, the following approximated power balance across the diode bridge can be stated…”

“…Recently, some papers have proposed and investigated a DFIG-DC layout in which an uncontrolled diode bridge and a single reduced power VSI are used to connect the DFIG to a DC grid [7]- [10]. The frequency and torque regulation by using unconventional field oriented controls have been discussed in [7]- [9], whereas some design issues for this system have been investigated in [10].…”

“…The frequency and torque regulation by using unconventional field oriented controls have been discussed in [7]- [9], whereas some design issues for this system have been investigated in [10].…”

Optimized and coordinated model predictive control scheme for DFIGs with DC-based converter system

“…Torque ripple is reduced by resonant current controllers [8]. Operation and design issues are analyzed for a DFIG with a diode-based rectifier [9]. Self-sensing methods for regulating stator current and stator frequency are proposed for DFIG [10,11].…”