Piotr Pura

The paper presents a decoupled direct power-controlled (DPC) of a doubly fed induction generator (DFIG) connected to an unbalanced power grid, executed in an orthogonal stationary reference frame related to the stator side. The control allows to maintain constant electromagnetic torque despite stator connection to the unbalanced power grid and to achieve controllable low-voltage ride through (LVRT) capability. The control does not require any signal decomposition into positive and negative sequence order, which allows to reduce its complexity and computational requirements. Then, the paper presents positive influence of rotor current feedback in the stator-controlled reference frame on control performance. It facilitates significant transient reduction and minor steady state control improvement. All the mentioned functionalities were validated both in simulation and in laboratory conditions and the obtained results are described in the paper.

The paper describes direct torque control of a doubly fed induction generator (DFIG) executed in the stator stationary reference frame. A full model of DFIG is presented, and relations for the rotor voltages are derived. The case with symmetrical and asymmetrical grid voltage operation is presented along with the corresponding simulation and experimental results. As a control target, the minimization of the torque oscillations at sinusoidal stator current is chosen. No decomposition into positive and negative voltage sequence for presented algorithm is necessary. The proportional‐resonance (PR) regulator tuned for fundamental harmonic of grid voltage frequency is applied. As the torque value is independent of the reference frame, the variables are not synchronized neither with the grid voltage nor with the stator flux vector. The control method is presented, and its performance for balanced and unbalanced voltage is given. To obtain the required target, the auxiliary scalar variable calculated as a dot product of stator flux and stator current is used.

This paper is focused on the unbalanced grid operation of a doubly fed induction generator. The main target taken into consideration for rotor converter control strategy is to reduce electromagnetic torque oscillations and simultaneously keep sinusoidal, but not necessarily symmetrical, stator current. The second, alternative target is to obtain symmetrical stator current, for which the generated stator power can be higher than for first target, but small torque oscillations occur. Synthesis of the reference signal calculation method for classic DPC with hysteresis controllers was preceded by analysis of mathematical model of the machine. The control method for unbalanced grid voltage operation of DFIG is designed in stationary αβ frame and does not require signal decomposition into positive and negative sequence for the first target, whereas for the second target determination of positive sequence of symmetrical stator current vector components is required. Additional part analyzed and described in the paper is the reference signal limitation method allowing overcurrent protection under unbalanced grid voltage.

The paper deals with the methods of either stator current or rotor current vector control of doubly fed induction machine connected to the unbalanced power network. Several alternative control targets, such as fixed electromagnetic torque, symmetrical stator current, or symmetrical rotor current, have been achieved. In spite of the selected target, the stator current is sinusoidal, however in some cases, it is unbalanced. The proposed vector control methods do not use separate negative sequence calculation and control. In some targets, filtration of signals is required in order to achieve symmetrical reference variables (positive sequence), which can be either stator or rotor current depending on the assumed target. Laboratory tests results have been presented in the steady state, as well as in transients. Short comparison of stator and rotor current controllers has been made on the basis of the laboratory test results.

The paper presents properties and control methods of grid-connected doubly-fed induction generator (DFIG). The paper is focused on unbalanced grid operation, because presently it is the biggest challenge for this type of generator. There are discussed three most popular methods - direct power control, direct torque control, and vector control with proportional-resonant PR controllers of rotor current in stationary alpha-beta frame. In each method, the reference signals of electromagnetic torque, instantaneous power components and rotor current components are calculated without sequence decomposition.

Purpose: – Doubly fed induction generator (DFIG) is widely used in wind energy conversion systems and it can operate with other primary movers. The purpose of this paper is to focus on the standalone operation of DFIG which may expand the area of possible applications and increase capabilities of the generation system in terms of power quality. Design/methodology/approach: – Synthesis of the control method was preceded by analysis of mathematical model of the machine. The control method based on the negative sequence and high harmonics extraction has been developed and verified in the laboratory unit. Control of the fundamental frequency component uses neither rotor speed nor position sensors. Findings: – The original method allows to compensate negative sequence and high harmonics of the generated voltage. At the same time, due to the active filtering capability of the grid side converter, the stator phase current shape is close to sine wave. Thus, it is seen by the machine as a linear load, what eliminates the electromagnetic torque ripples. Practical implications: – The system and control method can be applied in variable speed generation systems, e.g. wind turbines or diesel engines operating in the standalone mode. Originality/value: – Although the selective compensation of negative sequence and harmonics are known in the literature, until now the methods have been verified for the system with a rotor position sensor. Moreover, the stator current feed-forward improving the transient properties, as well as results of transient states caused by the load step change, have not been proposed in publications.

The paper presents a new type of rotor position observer using cross and dot products of the estimated and measured rotor current vector. A novel synchronization error function contains modified function atan2 and the vector products. The modification of error function is aimed at making the function monotonic, what provides faster response of the observer. Two rotor speed and position observers with the modified error function are presented. The former is based on the flux model of doubly fed induction machine DFIM and can be made in both αβ and dq frames, whereas the latter is based on the voltage steady state model of DFIM in dq frame.

Doubly-Fed Induction Machine (DFIM) still remains one of the most popular type of generation unit used in wind power conversion systems, due to variable-speed operation, price and reduced power of its bi-directional power converter. However, the machine connected to power grid is very sensitive to voltage unbalance. High DC-link voltage is required during unbalanced grid operation mode in order to sustain system stability. Without robust control excessive pulsation of electromagnetic torque occurs. This paper presents an effective and simple control of DFIM connected to unbalanced grid. The control is based on the simple direct power algorithm (DPC) with hysteresis controllers and does not require voltage and current decomposition into positive and negative sequence.

This paper presents a control strategy, which allows doubly-fed induction generator (DFIG) to generate a sinusoidal stator currents and minimize an electromagnetic torque oscillations under unbalanced grid conditions. The control does not require a positive and negative sequence decomposition of the control variables, so the independent control of positive and negative sequence is not used. Instead of it, the reference signal for reference power is calculated for the main control algorithm, which is the direct power control (DPC). Proposed strategy reduces the machine's torque oscillations and causes, that the machine stator and rotor current does not exceed the rated values, what may happen in a classic DPC control, when the positive sequence component of the grid voltage drops. Simulation results with 2MW doubly fed induction machine model are presented in the paper.